# Patent

## Title
Facet on a gallium and nitrogen containing laser diode

## Abstract

Laser diode technology incorporating etched facet mirror formation and optical coating techniques for reflectivity modification to enable ultra-high catastrophic optical mirror damage thresholds for high power laser diodes.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 16/786,551, filed Feb. 10, 2020, which is a continuation of U.S. application Ser. No. 15/937,740, filed Mar. 27, 2018, now U.S. Pat. No. 10,559,939 B1, which is a continuation-in-part of U.S. application Ser. No. 15/789,413, filed Oct. 20, 2017, which is a continuation of U.S. application Ser. No. 15/153,554, filed May 12, 2016, which is a continuation-in-part of U.S. application Ser. No. 13/850,187, filed Mar. 25, 2013, which claims priority under 35. U.S.C. § 119(e) to U.S. Provisional Application No. 61/620,648, filed Apr. 5, 2012, the contents of each of which are incorporated herein by reference in their entirety for all purposes.


BACKGROUND
The present disclosure relates generally to optical techniques. More specifically, the present disclosure provides methods and devices using semi-polar oriented gallium and nitrogen containing substrates for optical applications.
In 1960, the laser was first demonstrated by Theodore H. Maiman at Hughes Research Laboratories in Malibu. This laser utilized a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nm. By 1964, blue and green laser output was demonstrated by William Bridges at Hughes Aircraft utilizing a gas laser design called an Argon ion laser. The Ar-ion laser utilized a noble gas as the active medium and produce laser light output in the UV, blue, and green wavelengths including 351 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, and 528.7 nm. The Ar-ion laser had the benefit of producing highly directional and focusable light with a narrow spectral output, but the wall plug efficiency was <0.1%, and the size, weight, and cost of the lasers were undesirable as well.
As laser technology evolved, more efficient lamp pumped solid state laser designs were developed for the red and infrared wavelengths, but these technologies remained a challenge for blue and green and blue lasers. As a result, lamp pumped solid state lasers were developed in the infrared, and the output wavelength was converted to the visible using specialty crystals with nonlinear optical properties. A green lamp pumped solid state laser had 3 stages: electricity powers lamp, lamp excites gain crystal which lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm. The resulting green and blue lasers were called “lamped pumped solid state lasers with second harmonic generation” (LPSS with SHG) had wall plug efficiency of about 1%, and were more efficient than Ar-ion gas lasers, but were still too inefficient, large, expensive, fragile for broad deployment outside of specialty scientific and medical applications. Additionally, the gain crystal used in the solid state lasers typically had energy storage properties which made the lasers difficult to modulate at high speeds which limited its broader deployment.
To improve the efficiency of these visible lasers, high power diode (or semiconductor) lasers were utilized. These “diode pumped solid state lasers with SHG” (DPSS with SHG) had 3 stages: electricity powers 808 nm diode laser, 808 nm excites gain crystal which radiates laser beam at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm. The DPSS laser technology extended the life and improved the wall plug efficiency of the LPSS lasers to 5% to 10%, and further commercialization ensue into more high end specialty industrial, medical, and scientific applications. However, the change to diode pumping increased the system cost and required precise temperature controls, leaving the laser with substantial size, power consumption while not addressing the energy storage properties which made the lasers difficult to modulate at high speeds.
As high power laser diodes evolved and new specialty SHG crystals were developed, it became possible to directly convert the output of the infrared diode laser to produce blue and green laser light output. These “directly doubled diode lasers” or SHG diode lasers had 2 stages: electricity powers 1064 nm semiconductor laser, 1064 nm goes into frequency conversion crystal, which converts to visible 532 nm green light. These lasers designs are meant to improve the efficiency, cost and size compared to DPSS-SHG lasers, but the specialty diodes and crystals required make this challenging today. Additionally, while the diode-SHG lasers have the benefit of being directly modulate-able, they suffer from severe sensitivity to temperature, which limits their application.
From the above, it can be appreciated that techniques for improving optical devices is highly desired.
SUMMARY
The present disclosure generally relates to optical techniques. More specifically, the present disclosure provides methods and devices using semi-polar oriented gallium and nitrogen containing substrates for optical applications.
In one or more examples, the present method and device provides the following:

    
    
        1. Wurtzite group III-Nitride laser diode composed of layers of various compositions of InGaAlN.
        2. Optical device consists of (AlIn)GaN p-type layers and n-type layers cladding a light emitting region comprised of light emitting layers and cladding layers of various compositions of AlInGaN.
        3. Device grown either homoepitaxially on a free-standing (AlIn)GaN crystal, hetero-epitaxially on a non (AlIn)GaN substrates with a wurtzite crystal structure (e.g. ZnO), or hetero-epitaxially on a non-wurtzite substrate.
        4. Epitaxial surface of device during growth of active region is oriented relative to the InAlGaN crystal such that the epitaxial surface coincides with a nonpolar m-plane or a (40-41), (40-4-1), (30-31), (30-3-1), (20-21), (20-2-1), (30-32), or (30-3-2) crystal plane where Small deviations (“offcut”) are acceptable between the above planes and the epitaxial surface.
        
            a. Acceptable component of offcut toward [0001] is <±3.5°;
            b. Offcut can be less than or equal to ≤±10° toward [11-20].
        
        
        5. LDs operating in the 410-500 nm or in the 500-540 nm range; preferably embodied as lasing between 430 nm and 470 nm or between 505 nm and 535 nm.
    
    


In a preferred embodiment, the present method and device includes a gallium and nitrogen containing substrate configured on a surface orientation of (30-3-1), which is offcut from m-plane by about −10 degrees toward (0001), which is desirable for a blue laser diode device operating in the 420 nm to 480 nm range. In one or more embodiments, the following features are included.
1. For active region designs sufficient for laser diode grown on (30-3-1), the EL and PL spectra are much narrower in the blue region compared to a broad m-plane configured device or a device on an orientation which is offcut from m-plane by less than about −8 degrees toward (0001). This indicates that the material is more homogenous and thus will provide a higher material gain than m-plane or even planes miscut by −5, −7, or −8 degrees off of m-plane toward (0001) such as (60-6-1) or (40-4-1).
2. The m-plane offers desirable characteristics because it allows the growth of thick p-cladding regions above the active region required in blue laser diodes at relatively high temperatures (e.g., 100 to 200 degrees Celsius higher than the active region) without substantial degradation to the active region. Additionally, the m-plane also enables growth of thick active regions in the blue emission region, which are desirable for high modal gain laser diodes. However, as explained above, the PL and EL emission spectra are broad in the blue regime indicating that the material is not homogenous and will not provide maximum gain.
As explained, by mis-cutting off of the m-plane toward (000-1) at about 9 or 10 degrees reaching the (30-3-1) or (30-31) plane, it is possible to achieve narrow emission spectra in the blue emission regime of 430 nm to 470 nm or 480 nm. However, if the miscut angle is increased much further than (30-3-1) toward (20-2-1) which is at about 15 degrees off of m-plane, it becomes difficult to grow thick active regions without degradation of the light emission or internal efficiency because defects form at lower strain values. Moreover, on the (20-2-1) plane the growth of the thick p-cladding regions at relatively higher temperature than the active region growth required in laser diodes results in substantial degradation to the light emission properties of the active region material. This characteristic indicates that growth of a highly optimized laser diodes may be difficult.
3. The orientations around the (30-3-1) plane offers an unexpected benefit of enabling bright emission in the blue regime using thick active regions designs and hot p-clad techniques while maintaining a narrow emission spectrum. Although the (30-31) plane offers narrow emission, the (30-3-1) plane offers brighter emission and higher gain. These characteristics are favorable for laser diodes.
The (30-3-1) plane is also highly polarized and very bright. It is desirable to grow the p-clad hot to achieve high crystal quality, low resistance cladding layers. In other embodiments, the present method and device includes (30-3-1) with offcut orientations ranging from plus 1-2 degrees to negative 2-3 degrees toward m-plane.
As used herein in an example, the term “substantially parallel to the c-projection” should be interpreted by ordinary meaning of one of ordinary skill in the art. As an example, this term indicates that the stripe should be aligned in the c-direction, like within +/−3 to 5 degrees, although there can be variations. In other examples, the stripe can be aligned away (but not opposite) from the projection of the c-direction by 5, 10, 15, 20, or 25 degrees toward the a-direction. In this example, the strips would be “more” aligned to the projection of the c-direction than to the a-direction. Additionally, the (30-3-1) plane refers also to any crystallographically equivalent planes with a minus c-orientation; i.e., (30-3-1), (−303-1), (03-3-1), (0-33-1), (3-30-1) and (−330-1).
Benefits are achieved over pre-existing techniques using the present invention. In particular, the present invention enables a cost-effective optical device for laser applications. In a specific embodiment, the present optical device can be manufactured in a relatively simple and cost effective manner. Depending upon the embodiment, the present apparatus and method can be manufactured using conventional materials and/or methods according to one of ordinary skill in the art. The present method and devices includes etched facets, thick QW layers, thin barrier layers, AlGaN cladding free for p-clad, n-clad, or both dads, operable in the 395-420 nm range, 420-440 nm range, 440-475 nm range, 475-505 nm range, and 505-535 nm range, but preferably in the 440-475 nm range. As used herein the thick active regions (e.g., summation of all light emitting layers within the device structures, including QW and double hetero-structures) can achieve higher model confinement leading to higher model gain, and an ability to grow laser diode structures that may be free from AlGaN cladding regions or aluminum containing cladding regions, which limits c-plane devices. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits may be described throughout the present specification and more particularly below.
In an alternative preferred embodiment, the present method and device includes a gallium and nitrogen containing substrate configured on a surface orientation of (20-21), which is offcut from m-plane by about 14.9 degrees toward (0001), which is desirable for a green laser diode device operating in the 500 nm to 540 nm range. In one or more embodiments, the following features are included. Further details of the (20-21) configuration can be found throughout the present specification and more particularly below.
Certain embodiments of the present disclosure provide a gallium and nitrogen containing laser device configured on either a non-polar or a semipolar surface orientation. The device includes a gallium and nitrogen containing substrate member and a cavity region formed overlying a cladding region and configured in alignment in substantially a c-direction or a projection of the c-direction of the substrate member. The device has a first optical coating formed overlying a first facet and a second optical coating formed overlying a second facet. The first coating overlying the first facet is configured to increase reflectivity and the second coating layer overlying the second facet is configured to reduce reflectivity. The device has an optical power density characterizing the laser device and the laser device is substantially free from catastrophic optical mirror damage (COMD) related failure.
Embodiments provided by the present disclosure achieve these benefits and others in the context of known process technology. However, a further understanding of the nature and advantages of the embodiments disclosed herein may be realized by reference to the specification and the attached drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an example of c-direction or projection of a c-direction oriented laser diode stripe on a nonpolar oriented substrate or semipolar oriented substrate in an example.
FIG. 2 shows a cross-sectional view of a waveguide design of a nonpolar or a semipolar laser diode that contains cladding regions that are substantially free from aluminum containing species in an example.
FIG. 3A shows a schematic diagram of a typical configuration of a laser facet with optical coating in an example.
FIG. 3B shows a graph of the reflectivity (%) of a laser diode front facet versus the wavelength (nm) for an optical coating configurations with and without the passivation layer shown in FIG. 3A in an example.
FIG. 4A shows a graph of laser output power (mW) versus input current (mA) for a laser diode that undergoes a COMD failure at >700 mW output power in an example.
FIG. 4B shows an image of the irreversible damage at the front facet of the laser that occurs with COMD. It can be seen that COMD results in a sudden drop in power. COMD is typically a thermal runaway process, consistent with the apparent burn mark/hole on the laser facet.
FIG. 5 shows an example from the literature (Tamikawa et al., Appl. Phys. Lett., 2009, 95, 031106) demonstrating how the COMD threshold power can be improved by ECR coating with AlON in conventional GaN laser diodes.
FIG. 6 shows experimental COMD level data for commercially available c-plane (−Plane) laser diodes having AlGaN claddings and ECR facet coatings, and for nonpolar/semipolar (NP/SP) laser diodes with cladding layers substantially free from Al-containing claddings and using conventional e-beam facet coating (E-Beam Coated) techniques in an example.
FIGS. 7A and 7B show examples of facet imperfections resulting from a cleaving process on nonpolar/semipolar laser diode facets in an example.
FIGS. 8A and 8B show an SEM image and an optical image, respectively, of facet failures related to imperfections from the cleaving process where the arrows depict the failure points and the imperfections in an example.
FIG. 9 shows an example taken from the literature (Behfar et al., www.binoptics.com) of etched facet technology that can yield smooth, vertical, and defect free etched facets to enable very high COMD levels in an example.
FIG. 10 shows example data demonstrating how etched facets technology can improve the COMD yield compared to etched facet technology on nonpolar or semipolar laser diodes with front facet coatings in an example.
FIG. 11 shows an example of an electron cyclotron resonance (ECR) plasma deposition tool, a model AFTEX-6200 manufactured by MES-AFTY.
FIG. 12 shows a TEM image of an AlON passivation layer epitaxially grown on a GaN interface according to an embodiment.
FIG. 13 shows a TEM image of an AlN passivation layer epitaxially grown on a GaN interface according to an embodiment.
FIG. 14 shows that COMD power can be improved by more than 2× with an AlN passivation layer in accordance with an embodiment.
FIG. 15A shows a cross-sectional view of a facet passivation layer below the reflectivity modification layer(s) in an example.
FIG. 15B shows a cross-sectional view of a facet passivation layer on a rough GaN facet in an example.
FIG. 16A shows a cross-sectional view of a facet passivation layer below a distributed Bragg reflector (DBR) of quarter lambda (quarter wave) layers that form the high-reflectivity (HR) coating in an example.
FIG. 16B shows a cross-sectional view of a facet passivation layer that is designed to affect the electric field in the HR coating in an example.
FIG. 17 shows a transmission electron microscopy (TEM) image of an etched facet whose etch conditions have been controlled to maintain the crystalline lattice all the way to the facet surface without any amorphous or contamination layers in an example.
FIG. 18 shows TEM-EDX line scan data of an etched facet which has been cleaned with an in-situ plasma clean, then coated with a single-crystalline AlN layer followed by an amorphous Al2O3 anti-reflectivity (AR) layer in an example.
FIG. 19 is a TEM image of an etched facet which maintains the crystalline lattice all the way to the semiconductor in an example.
FIG. 20 shows catastrophic optical damage (COD) test data of two etched facet laser diodes, the solid line represents the etched facet coated with a polycrystalline film, and the dotted line represents the etched facet coated with an amorphous film in an example.
FIG. 21 shows the lifetest result of similar etched facet laser devices coated with a single-crystalline passivation layer in an example.
FIG. 22 is a simplified flowchart of a method 22 of forming a robust laser diode on an etched facet laser according to an embodiment of the present disclosure.
FIGS. 23A through 23C are cross-sectional views illustrating a process flow and resulting structures of a laser diode grown on an epitaxial substrate according to an embodiment of the present disclosure.
FIG. 24A is a simplified perspective view illustrating a wafer having a back-side contact in an example.
FIG. 24B is a simplified perspective view illustrating the wafer of FIG. 24A that has been etched and separated into individual bars in an example.
FIG. 24C is a simplified perspective view illustrating a stacking of the individual bars where the facet coatings are then applied to the bars in an example.
FIG. 25A is a simplified perspective view illustrating such process flow where the facets are etched in the wafer, facet coating layers are then deposited on the etched facets in an example.
FIG. 25B is a simplified perspective view illustrating that the coated facets are singulated into individual bars in an example.
FIGS. 26A through 26C are cross-sectional views illustrating the key steps for forming a robust etched facet laser on a transferred epitaxy laser diode in an example.
FIG. 27A is a cross-sectional view illustrating ion assisted e-beam deposition (IAD) deposition of Al2O3 to a gallium and nitrogen containing substrate according to an embodiment of the present disclosure.
FIG. 27B is a cross-sectional view illustrating a mostly amorphous layer of Al2O3 in an example.
FIG. 27C is a cross-sectional view illustrating a polycrystalline layer at the facet interface and amorphous layers on the polycrystalline layer in an example.
FIG. 27D is a cross-sectional view illustrating a condition with a higher ion current density that has fully turned polycrystalline material Al2O3 in an example.
FIGS. 28A and 28B are cross-sectional views illustrating a schematic of how a passivation layer can be applied to an HR coating according to some embodiments of the present disclosure.
FIG. 29A shows a TEM micrograph of an example HR coating with a first layer of SiO2 on a GaN facet after a life test in an example.
FIG. 29B shows a passivation layer of Al2O3 has been added in an example.
FIG. 30A shows a calculation of the electric field distribution in the HR coating for a 20 nm passivation Al2O3 layer with a marker line at the coating semiconductor interface in an example.
FIG. 30B shows that, by using a half lambda (half wave) thickness coating, a null of the electric field can be generated at the coating interface in an example.
FIG. 31A shows a cross-sectional view of a scanning electron microscope (SEM) image of an etched facet.
FIG. 31B shows a cross-sectional view of an SEM image of a cleaved facet.
FIG. 32 shows a list of standard enthalpies of formation for SiO2, SiN, Ga2O3, GaN, Al2O3, and AlN.
FIG. 33 shows a diagram of the process for facet coating by ALD according to an embodiment of the present disclosure.
FIG. 34 shows a diagram of the process for facet coating by electron beam evaporation according to an embodiment of the present disclosure.
FIGS. 35A to 35C show diagrams of on-wafer laser facet coatings according to certain embodiments of the present disclosure.
FIG. 36A is a TEM image of such an on-wafer coating in an example.
FIG. 36B is an enlarged cross-sectional view of a portion of FIG. 35A in an example.
FIGS. 37A to 37C show cross-sectional views of on-wafer coatings by ALD according to some embodiments of the present disclosure.
FIG. 38 shows a high-resolution TEM image of the coating along with Fast-Fourier transform analysis of the AlN passivation layer and GaN surface showing polycrystalline AlN on top of the epitaxial GaN in an example.
FIG. 39A is a SEM image showing a portion of an etched facet of a gallium and nitrogen containing cavity region in an example.
FIG. 39B is a plot of atomic force microscopy (AFM) measurements across an area of the etched facet shown in FIG. 39A.
FIG. 39C is a three-dimensional view of an enlarged portion of the etched face shown in FIG. 39A.
FIG. 40 is graph showing the measured surface roughness along the lines 1, 2, and 3 in FIG. 39C.



DETAILED DESCRIPTION
The present disclosure relates generally to optical techniques. More specifically, the present disclosure provides methods and devices using semi-polar oriented gallium and nitrogen containing substrates for optical applications.
Lasers are useful and important. They are used in many applications and recently visible lasers have reached performance thresholds that make them useful in the field of video displays and video projectors. While there are several technologies that can produce visible lasers, solid-state diode lasers (LDs) emitting at the desired wavelength have the best chance of minimizing system size while maximizing wall-plug efficiency (WPE). While diode lasers emitting with high WPE in the red region of the visible spectrum have been available for years it is only recently that blue and green LDs have become good enough to consider for many high power and space and efficiency conscious applications.
Blue LDs currently available commercially are based on the semiconductor GaN grown with device layers oriented parallel to the (0001) plane of the GaN crystal. Recently, blue LDs based on other crystallographic orientations of GaN have been demonstrated. The so-called “non-polar” orientations [(10-10) or “m-plane” and (11-20) or “a-plane” offer an advantage over c-plane based devices in that the heterostructures that provide for electron and hole confinement do not result in piezoelectric or spontaneous electric fields due to differences in polarization between various AlInGaN alloys used in the devices. In c-plane devices the internal fields result in band-tilting that spatially separates electron and hole bound states and reduces the thickness of InGaN quantum wells that can be used. Smaller quantum wells result in less modal overlap, increased spontaneous emission lifetimes, which respectively reduce the gain and increase the threshold current densities of the devices. Growing in non-polar orientations results in better electron and hole overlap even for wide wells, resulting in higher gain and threshold current densities. Non-polar orientations also have the advantage that photon emission in the plane of the device is not isotropic, such that laser stripes can be oriented parallel to the direction of preferred emission to help decrease threshold currents even further.
While the (0001) and non-polar planes are orthogonal, there is a broad class of low-index planes that are not orthogonal to the (0001). This invention is based on the understanding that blue LD structures grown on a particular subset of semi-polar GaN substrate orientations offer a distinct set of advantages to both (0001) and non-polar oriented devices. Table 1 lists several of the orientations relevant to this invention along with the approximate tilt of these planes away from the (10-10) plane in both the [0001] and [11-20] directions.
m-Plane oriented InGaN/(AlIn)GaN devices exhibit a broadening of luminescence spectra in the range of wavelengths spanning from approximately 415 nm to 470 nm. This is not optimal for providing gain in laser diode active regions since when all other factors are equivalent using a gain medium with a narrower spontaneous emission spectrum will result in higher gain and consequently higher WPE. Growth on semi-polar planes that are derived from the m-plane results in devices with narrower full width at half maximum (FWHM) of luminescence spectra.
Commercially available GaN-based high power laser diodes fabricated on the conventional c-plane orientation typically require special facet coatings to modify the reflectivity of the front mirror/facet where the optical intensity is highest. Such facet coatings are required to reduce the reflectivity of the mirrored surface and thereby increase the slope efficiency and output power of the laser diode. Specialized coating techniques and methods can be used to raise the optical power threshold before the onset of catastrophic optical mirror damage (COMD) that occurs at the front facet where the dielectric coating layers are applied. The origin of the of the COMD failure is typically located at the semiconductor facet—dielectric coating interface and results from absorption induced by mechanisms such as surface states, trace contamination at the interface, surface damage, and/or from other facet imperfections such as roughness or shear lines in a cleaved facet. In conventional c-plane GaN based devices that utilize AlGaN cladding layers or other Al-containing cladding layers, the facet is very susceptible to COMD failure and expensive and/or highly specialized techniques must be used to increase the COMD threshold. As an example, commercial c-plane high power blue laser diodes operating with output powers of above 1 W typically require electron cyclotron resonance (ECR) facet coating. This technique requires a large capital expenditure and the equipment requires substantial amount of care and maintenance. An example of this technique is disclosed by Tamikawa et al, Appl. Phys. Lett., 95, 031106 (2009) where AlON interlayers are applied using ECR to increase the optical power level before the onset of COMD. However, to reduce cost and facilitate manufacturing it is desirable to use more conventional deposition techniques such as electron beam (e-beam) deposition, thermal evaporation, sputtering, or plasma enhanced chemical vapor deposition.
We have discovered that by using AlGaN-cladding free high power blue laser designs based on nonpolar/semipolar substrates we can achieve optical density COMD threshold levels using e-beam deposited anti-reflective coatings that are equivalent to the COMD threshold power (optical density) levels achieved in commercial high power blue c-plane laser didoes in which the anti-reflective coatings are deposited using specialized ECR coating process. By inserting an e-beam deposited SiO2 passivation layer between the laser facet and the e-beam deposited Al2O3 reflectivity modifying layer on nonpolar/semipolar laser diodes with facets that do not suffer from large imperfections resulting from the cleaving process, optical output power density threshold values for COMD are provided that equal those of commercially available AlGaN clad c-plane devices that utilize ECR coatings. Thus, certain embodiments provided by the present disclosure combine nonpolar/semipolar GaN technology with AlGaN cladding free waveguide designs and conventional dielectric deposition techniques to provide low cost, high-throughput methods for forming robust facets useful in high power GaN laser diodes. Additionally, embodiments provided by the present disclosure combine a passivation layer such as SiO2 to increase the optical power density COMD threshold.
Physical imperfections on a laser facet resulting from the cleaving process on non-polar or semi-polar laser diodes can serve as initiation sites for COMD and can greatly reduce the COMD power density threshold and thereby negatively impact high power laser yields. Examples of imperfections in such facets include scaling, bumps, chipping, shear lines, and any feature that deviates from a smooth surface. There can be many causes for these imperfections and although the number of imperfections can be reduced with optimized cleaving, this is challenging on nonpolar and semipolar substrates. The cause of COMD failure at these imperfections can be due to intra-cavity mode filamentation, which results in local spots of very high optical density. Alternatively, the imperfections can exhibit higher optical absorption, which the results in increased local temperature and increased optical absorption. Such absorption can result in a thermal runaway effect and causes COMD failure in conventional laser diodes. Regardless of the failure mechanism, failure occurs at these imperfections. Therefore, it is desirable to eliminate these imperfections on the laser diode facet to suppress COMD failure and increase the power density.
Forming high-quality cleaved facets on nonpolar or on semipolar laser diodes is challenging. On most all nonpolar or semipolar crystal orientations it is desirable to align the laser cavity in the c-direction or in the projection of the c-direction. For a single cavity section device fabricated on the nonpolar m-plane, this requires c-face facet formation since the c-plane is orthogonal to the c-direction. Since the c-plane is not charge neutral, forming high quality cleaved facets can be very challenging. If the surface orientation is rotated off of the c-plane toward semipolar planes or nonpolar planes such as the {30-31}, {30-3-1}, {20-21}, {20-2-1}, {30-32}, {30-32}, or the m-plane, the difficulty in achieving a high quality cleaved facet is great. This is due to the availability of the nonpolar a-face and m-face facets for c-plane surface oriented devices. This is because for a favorable stripe orientation in the projection of the c-direction where the gain is highest on such semipolar and nonpolar planes, the resulting candidate cleaved facet crystal orientations that are orthogonal to the surface plane are the polar c-plane or higher Miller index semipolar planes, which are not natural cleavage planes. Therefore, the challenge of achieving high-quality cleaved facets in such nonpolar or semipolar laser diodes is great and they are likely to suffer from a high number of imperfections such as scaling or shear stress mark that result from the scribing and breaking process. As discussed above, such imperfections can serve as failure sites for COMD at high optical densities. To increase the optical power density threshold for COMD failure it is necessary to have a facet formation technique that minimizes or eliminates such imperfections.
In an example, chemical assisted ion beam etching (CAIBE), inductively coupled plasma (ICP) etching, or reactive ion etching (RIE) can result in smooth and vertical etched sidewall regions, which could serve as facets in etched facet laser diodes. In the etched facet process a masking layer is deposited and patterned on the surface of the wafer. The etch mask layer could be comprised of dielectrics such as silicon dioxide (SiO2), silicon nitride (SixNy), a combination thereof or other dielectric materials. Further, the mask layer could be comprised of metal layers such as Ni or Cr, but could be comprised of metal combination stacks or stacks comprising metal and dielectrics. In another approach, photoresist masks can be used either alone or in combination with dielectrics and/or metals. The etch mask layer is patterned using conventional photolithography and etch steps. The alignment lithography could be performed with a contact aligner or stepper aligner. Such lithographically defined mirrors provide a high level of control to the design engineer. After patterning of the photoresist mask on top of the etch mask is complete, the patterns in then transferred to the etch mask using a wet etch or dry etch technique. Finally, the facet pattern is then etched into the wafer using a dry etching technique selected from CAIBE, ICP, RIE and/or other techniques. The etched facet surfaces must be highly vertical of between about 87 and 93 degrees or between about 89 and 91 degrees from the surface plane of the wafer. The etched facet surface region must be very smooth with root mean square roughness values of less than 50 nm, 20 nm, 5 nm, or 1 nm. Lastly, the etched must be substantially free from damage, which could act as nonradiative recombination centers and hence reduce the COMD threshold. In an example, CAIBE is provides very smooth and low damage sidewalls due to the chemical nature of the etch, while it can provide highly vertical etches due to the ability to tilt the wafer stage to compensate for any inherent angle in etch.
In an example, chemically assisted ion beam etching (CAIBE) characteristics of gallium nitride (GaN) are provided using a SOO-eVAr ion beam directed onto a sample in a Cl, ambient. Enhanced etch rates were obtained for samples etched in the presence of Cl, over those etched only by Ar ion milling at a substrate temperature of 20° C. The CAIBE etch rates were further enhanced at higher substrate temperatures whereas etch rates for Ar ion milling were not influenced by substrate temperature. Etch rates as high as 210 mm/min are reported. The etch rates reported here are the highest so far reported for GaN. Anisotropic etch profiles and smooth etched surfaces in GaN have been achieved with CAIBE. Further details of the CAIBE process can be found in “Characteristics of chemically assisted ion beam etching of gallium nitride,” in the names of Adesida, A. T. Ping, C. Youtsey, T. Dow, M. Asif Khan et al., Appl. Phys. Lett. 65, 889 (1994); dx.doi.org; apl.aip.org Published by the American Institute of Physics, which is hereby incorporated by reference herein.
Using the etched facet techniques as described above can yield high quality facets that are free from COMD-causing imperfections. The end yield will be improved compared to the yield obtained using the cleaving process which can lead to imperfections. By combining an etched facet technology to high power nonpolar and/or semipolar laser diodes with substantially Al-free cladding designs and multi-layer facet coating configurations containing a passivation layer and reflectivity modification layer, high power laser diodes with high power density COMD threshold values and high yield can be achieved using conventional facet coating deposition techniques. In addition to CAIBE, other techniques for etching mirrors include inductively coupled plasma (ICP) etching, reactive ion etching (ME) etching, and focused ion beam (FIB) etching. Embodiments provided by the present disclosure combine nonpolar or semipolar laser diode technology with etched facet mirror formation and conventional optical coating layer techniques for reflectivity modification to enable a method for ultra-high COMD thresholds for high power laser diodes.
Certain embodiments provided by the present disclosure include gallium and nitrogen containing laser device configured on either a nonpolar or a semipolar surface orientation, the device comprising: a gallium and nitrogen containing substrate member; a cladding region overlying the substrate member; a cavity region formed overlying the substrate member and configured in alignment in substantially a c-direction or a projection of the c-direction, the cavity region being characterized by a first end and a second end; the first end forming a first facet or mirror; the second end forming a second facet or mirror; a first optical coating formed overlying the first facet, wherein the first coating overlying the first facet is configured to increase a reflectivity; a second optical coating formed overlying the second facet, wherein the second coating layer overlying the second facet is configured to reduce a reflectivity; and an optical power density characterizing the laser device, the laser device being substantially free from COMD related failure.
In certain embodiments of a gallium and nitrogen containing laser device, the nonpolar or semipolar surface orientation comprises an m-plane, a (30-31) plane, a (20-21) plane, a (30-32) plane, a (30-3-1) plane, a (20-2-1) plane, a (30-3-2) plane, or an offcut of within +/−5 degrees of any of these planes toward an a-direction or a c-direction; the cladding region being substantially free from Al-containing material, the cladding region being characterized by an AlN mol fraction in the cladding region of less than about 2%.
In certain embodiments of a gallium and nitrogen containing laser device, the first optical coating is provided by a method selected from electron-beam deposition, thermal evaporation, PECVD, sputtering, and a combination of any of the foregoing.
In certain embodiments of a gallium and nitrogen containing laser device, the first facet and the second facet are formed using a scribing and breaking process.
In certain embodiments of a gallium and nitrogen containing laser device, the first facet and the second facet are formed using an etching process selected from reactive ion etching, inductively coupled plasma etching, and chemical assisted ion beam etching.
In certain embodiments of a gallium and nitrogen containing laser device, the second optical coating comprises Al2O3.
In certain embodiments of a gallium and nitrogen containing laser device, the second optical comprises at least of Al2O3 and SiO2.
In certain embodiments of a gallium and nitrogen containing laser device, the second optical coating comprises Al2O3 and SiO2, and the SiO2 is configured as a passivation layer and is configured between the second facet and the Al2O3.
In certain embodiments of a gallium and nitrogen containing laser device, the device comprises an output cavity width of greater than about 3 μm and less than about 20 μm, and is operable at over 1 W.
In certain embodiments of a gallium and nitrogen containing laser device, the device comprises an output cavity width of greater than about 3 μm and less than about 25 μm, and is operable at over 2 W.
In certain embodiments of a gallium and nitrogen containing laser device, the device comprises an output cavity width of greater than about 3 μm and less than about 35 μm, and is operable at over 3 W.
In certain embodiments of a gallium and nitrogen containing laser device, the device comprises an output cavity width of greater than about 3 μm and less than about 35 μm and is operable at over 4.5 W
In certain embodiments of a gallium and nitrogen containing laser device, the device comprises an output cavity width of greater than about 3 μm and less than about 50 μm, and is operable at over 3 W.
In certain embodiments of a gallium and nitrogen containing laser device, the device is operable at a wavelength from 430 nm to 480 nm.
In certain embodiments of a gallium and nitrogen containing laser device, the device is operable at a wavelength from 480 nm to 535 nm.
In certain embodiments of a gallium and nitrogen containing laser device, the device is operable at a wavelength from 390 nm to 430 nm.
FIG. 1 shows an example of c-direction or projection of a c-direction oriented laser diode stripe on a nonpolar or semipolar oriented substrate. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. As shown in an example, the optical device includes a gallium nitride substrate member having a semipolar crystalline surface region characterized by an orientation of about 9 degrees to about 12.5 degrees toward (000-1) from the m-plane. In a specific embodiment, the gallium nitride substrate member is a bulk GaN substrate characterized by having a semipolar crystalline surface region, but can be others. In a specific embodiment, the bulk GaN substrate has a surface dislocation density below 105 cm−2 or 105 to 107 cm−2. It should be noted that homoepitaxial growth on bulk GaN is generally better than hetero-epitaxy growth. The nitride crystal or wafer may comprise AlxInyGa1-x-yN, where 0≤x, y, x+y≤1. In one specific embodiment, the nitride crystal comprises GaN. In one or more embodiments, the GaN substrate has threading dislocations, at a concentration between about 105 cm−2 and about 108 cm−2, in a direction that is substantially orthogonal or oblique with respect to the surface. As a consequence of the orthogonal or oblique orientation of the dislocations, the surface dislocation density is below about 105 cm−2 or others such as those ranging from about 105-108 cm−2. Of course, there can be other variations, modifications, and alternatives.
In a specific embodiment, the device has a laser stripe region formed overlying a portion of the semipolar crystalline orientation surface region. In a specific embodiment, the laser stripe region is characterized by a cavity orientation is substantially parallel to the projection of the c-direction. In a specific embodiment, the laser stripe region has a first end and a second end. In a preferred embodiment, the device has a first facet provided on the first end of the laser stripe region and a second facet provided on the second end of the laser stripe region. In one or more embodiments, the first facet is substantially parallel with the second facet. Mirror surfaces are formed on each of the surfaces. The first facet comprises a first mirror surface. In a preferred embodiment, the first mirror surface is provided by an etching process. The etching process can use any suitable techniques, such as a chemical etching process using a CAIBE etching processor combinations. In a specific embodiment, the first mirror surface comprises a reflective coating. In a specific embodiment, deposition of the reflective coating occurs using, for example, e-beam evaporation, thermal evaporation, RF sputtering, DC sputtering, ECR sputtering, ion beam deposition, Ion Assisted Deposition, reactive ion plating, any combinations, and the like. In still other embodiments, the present method may provide surface passivation to the exposed surface prior to coating. The reflective coating is selected from silicon dioxide, hafnia, and titania, tantalum pentoxide, zirconia, including combinations, and the like. Preferably, the reflective coating is highly reflective and includes a coating of silicon dioxide and tantalum pentoxide, which has been deposited using electron beam deposition. Depending upon the embodiment, the first mirror surface can also comprise an anti-reflective coating. Additionally, the facets can be etched or a combination of them. Of course, there can be other variations, modifications, and alternatives. Also in a preferred embodiment, the second facet comprises a second mirror surface. The second mirror surface is provided by an etching techniques using etching technologies such as reactive ion etching (ME), inductively coupled plasma etching (ICP), or chemical assisted ion beam etching (CAIBE), or other method. In an example, (CAIBE), (ICP), or (ME) can result in smooth and vertical etched sidewall regions, which could serve as facets in etched facet laser diodes. In the etched facet process a masking layer is deposited and patterned on the surface of the wafer. The etch mask layer could be comprised of dielectrics such as silicon dioxide (SiO2), silicon nitride (SixNy), a combination thereof or other dielectric materials. Further, the mask layer could be comprised of metal layers such as Ni or Cr, but could be comprised of metal combination stacks or stacks comprising metal and dielectrics. In another approach, photoresist masks can be used either alone or in combination with dielectrics and/or metals. The etch mask layer is patterned using conventional photolithography and etch steps. The alignment lithography could be performed with a contact aligner or stepper aligner. Such lithographically defined mirrors provide a high level of control to the design engineer. After patterning of the photoresist mask on top of the etch mask is complete, the patterns in then transferred to the etch mask using a wet etch or dry etch technique. Finally, the facet pattern is then etched into the wafer using a dry etching technique selected from CAIBE, ICP, RIE and/or other techniques. The etched facet surfaces must be highly vertical of between about 87 and 93 degrees or between about 89 and 91 degrees from the surface plane of the wafer. The etched facet surface region must be very smooth with root mean square roughness values of less than 50 nm, 20 nm, 5 nm, or 1 nm. Lastly, the etched should be substantially free from damage, which could act as non-radiative recombination centers and hence reduce the COMD threshold. In an example, CAIBE is provides very smooth and low damage sidewalls due to the chemical nature of the etch, while it can provide highly vertical etches due to the ability to tilt the wafer stage to compensate for any angle in etch in an example.
In a specific embodiment, the second mirror surface comprises a reflective coating, such as silicon dioxide, hafnia, titania, tantalum pentoxide, zirconia, combinations, and the like. In a specific embodiment, the second mirror surface comprises an anti-reflective coating, such alumina or aluminum oxide. In a specific embodiment, the coating can be formed using electron beam deposition, thermal evaporation, RF sputtering, DC sputtering, ECR sputtering, ion beam deposition, ion assisted deposition, reactive ion plating, any combinations, and the like. In still other embodiments, the present method may provide surface passivation to the exposed surface prior to coating. Of course, there can be other variations, modifications, and alternatives. In a specific embodiment, the laser stripe has a length and width. The length ranges from about 200 microns to about 3000 microns. The stripe also has a width ranging from about 0.5 microns to about 50 microns, but can be other dimensions. In a specific embodiment, the stripe can also be about 6 to 25 microns wide for a high power multi-lateral-mode device or 1 to 2 microns for a single lateral mode laser device. In a specific embodiment, the width is substantially constant in dimension, although there may be slight variations. The width and length are often formed using a masking and etching process, which are commonly used in the art. Further details of the present device can be found throughout the present specification and more particularly below.
In a specific embodiment, the device is also characterized by a spontaneously emitted light that is polarized in substantially perpendicular to the projection of the c-direction (in the a-direction. That is, the device performs as a laser or the like. In a preferred embodiment, the spontaneously emitted light is characterized by a polarization ratio of greater than 0.2 to about 1 perpendicular to the c-direction. In a preferred embodiment, the spontaneously emitted light characterized by a wavelength ranging from about 400 nanometers to yield a violet emission, a blue emission, a green emission, and others. In one or more embodiments, the light can be emissions ranging from violet 395 to 420 nanometers; blue 430 to 470 nm; green 500 to 540 nm; and others, which may slightly vary depending upon the application. In a preferred embodiment, the spontaneously emitted light is highly polarized and is characterized by a polarization ratio of greater than 0.4. In a specific embodiment, the emitted light is characterized by a polarization ratio that is desirable. Of course, there can be other variations, modifications, and alternatives. Further details of the laser device can be found throughout the present specification and more particularly below.
FIG. 2 shows a cross-sectional view of a waveguide design of a nonpolar or a semipolar laser diode that contains cladding regions that are substantially free from aluminum. Such laser diode designs can enable high COMD levels without the need for highly-specialized and costly mirror coating techniques such as electron cyclotron resonance (ECR). As shown in an example, the laser device includes gallium nitride substrate, which has an underlying n-type metal back contact region. In a specific embodiment, the metal back contact region is made of a suitable material such as those noted below and others. Further details of the contact region can be found throughout the present specification and more particularly below. FIG. 2 shows substrate 203, which has an underlying n-type metal back contact region 201, n-type gallium nitride layer 205 overlying substrate 205, n-side separate confinement heterostructure (SCH) 206, active region 207, p-side guiding layer or separate confinement heterostructure 208, p-type electron blocking layer 209, p++ type gallium nitride contact region 213, metal contact layer 215, and p-type gallium containing nitride layer configured as a laser stripe 211.
In a specific embodiment, the device also has an overlying n-type gallium nitride layer, an active region, and an overlying p-type gallium nitride layer structured as a laser stripe region. Additionally, the device also includes an n-side separate confinement heterostructure (SCH), p-side guiding layer or SCH, p-AlGaN EBL, among other features. In a specific embodiment, the device also has a p++ type gallium nitride material to form a contact region. In a specific embodiment, the p++ type contact region has a suitable thickness and may range from about 10 nm 50 nm, or other thicknesses. In a specific embodiment, the doping level can be higher than the p-type cladding region and/or bulk region. In a specific embodiment, the p++ type region has doping concentration ranging from about 10E19 to 10E21 Mg/cm3, and others. The p++ type region preferably causes tunneling between the semiconductor region and overlying metal contact region. In a specific embodiment, each of these regions is formed using at least an epitaxial deposition technique of metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial growth techniques suitable for GaN growth. In a specific embodiment, the epitaxial layer is a high quality epitaxial layer overlying the n-type gallium nitride layer. In some embodiments the high quality layer is doped, for example, with Si or O to form n-type material, with a dopant concentration between about 1016 cm−3 and 1020 cm−3.
In a specific embodiment, an n-type AluInvGa1-uvN layer, where 0≤u, v, u+v≤1, is deposited on the substrate. In a specific embodiment, the carrier concentration may lie in the range between about 1016 cm−3 and 1020 cm−3. The deposition may be performed using metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Of course, there can be other variations, modifications, and alternatives.
As an example, the bulk GaN substrate is placed on a susceptor in an MOCVD reactor. After closing, evacuating, and back-filling the reactor (or using a load lock configuration) to atmospheric pressure, the susceptor is heated to a temperature between about 900 to about 1200 degrees Celsius in the presence of a nitrogen-containing gas. As an example, the carrier can be hydrogen or nitrogen or others. In one specific embodiment, the susceptor is heated to approximately 1100 degrees Celsius under flowing ammonia. A flow of a gallium-containing metalorganic precursor, such as trimethylgallium (TMG) or triethylgallium (TEG) is initiated, in a carrier gas, at a total rate between approximately 1 and 50 standard cubic centimeters per minute (sccm). The carrier gas may comprise hydrogen, helium, nitrogen, or argon. The ratio of the flow rate of the group V precursor (e.g., ammonia) to that of the group III precursor (trimethylgallium, triethylgallium, trimethylindium, trimethylaluminum) during growth is between about 2000 and about 12000. A flow of disilane in a carrier gas, with a total flow rate of between about 0.1 and 10 sccm is initiated.
In a specific embodiment, the laser stripe region is made of the p-type gallium nitride layer 211. In a specific embodiment, the laser stripe is provided by an etching process selected from dry etching or wet etching. In a preferred embodiment, the etching process is dry, but can be others. As an example, the dry etching process is an inductively coupled plasma process using chlorine bearing species or a reactive ion etching process using similar chemistries or combination of ICP and RIE, among other techniques. Again as an example, the chlorine bearing species are commonly derived from chlorine gas or the like. The device also has an overlying dielectric region, which exposes 213 contact region, which is preferably a p++ gallium nitride region. In a specific embodiment, the dielectric region is an oxide such as silicon dioxide or silicon nitride, but can be others, such as those described in more detail throughout the present specification and more particularly below. The contact region is coupled to an overlying metal layer 215. The overlying metal layer is a multilayered structure containing gold and platinum (Ni/Au), but can be others such as gold and palladium (Pd/Au) or gold and nickel (Ni/Au). In an alternative embodiment, the metal layer comprises Ni/Au formed using suitable techniques. In a specific embodiment, the Ni/Au is formed via electron-beam deposition, sputtering, or any like techniques. The thickness includes nickel material ranging in thickness from about 50 to about 100 nm and gold material ranging in thickness from about 100 Angstroms to about 1-3 microns, and others. Of course, there can be other variations, modifications, and alternatives
In a preferred embodiment, the dielectric region can be made using a suitable technique. As an example, the technique may include reactively sputter of SiO2 using an undoped polysilicon target (99.999% purity) with O2 and Ar. In a specific embodiment, the technique uses RF magnetron sputter cathodes configured for static deposition; sputter target; throw distance; pressure: 1-5 mT or about 2.5 mT, power: 300 to 400 W; flows: 2-3.-9 sccm O2, 20-50 sccm, Ar, deposition thickness: 1000-2500 A, and may include other variations. In a specific embodiment, deposition may occur using non-absorbing, nonconductive films, e.g., Al2O3, Ta2O5, SiO2, Ta2O5, ZrO2, TiO2, HfO2, NbO2. Depending upon the embodiment, the dielectric region may be thinner, thicker, or the like. In other embodiments, the dielectric region can also include multilayer coatings, e.g., 1000 A of SiO2 capped with 500 A of Al2O3. Deposition techniques can include, among others, ebeam evaporation, thermal evaporation, RF Sputter, DC Sputter, ECR Sputter, Ion Beam Deposition, Ion Assisted Deposition, reactive ion plating, combinations, and the like. Of course, there can be other variations, modifications, and alternatives.
In a specific embodiment, the laser device has active region 207. The active region can include one to twenty quantum well regions according to one or more embodiments. As an example following deposition of the n-type AluInvGa1-u-vN layer for a predetermined period of time, so as to achieve a predetermined thickness, an active layer is deposited. The active layer may comprise a single quantum well or a multiple quantum well, with 1-20 quantum wells. Preferably, the active layer may include about 3-7 quantum wells or more preferably 4-6 quantum wells or others. The quantum wells may comprise InGaN wells and GaN barrier layers. In other embodiments, the well layers and barrier layers comprise AlwInxGa1-w-xN and AlyInzGa1-y-zN, respectively, where 0≤w, x, y, z, w+x, y+z≤1, where w<u, y and/or x>v, z so that the bandgap of the well layer(s) is less than that of the barrier layer(s) and the n-type layer. The well layers and barrier layers may each have a thickness between about 1 nm and about 30 nm. In a preferred embodiment, each of the thicknesses is preferably 3-8 nm. In a specific embodiment, each well region may have a thickness of about 5 nm to 7 nm and each barrier region may have a thickness of about 2 nm to about 5 nm, among others. In another embodiment, the active layer comprises a double heterostructure, with an InGaN or AlwInxGa1-w-xN layer about 10 nm to 100 nm thick surrounded by GaN or AlyInzGa1-y-zN layers, where w<u, y and/or x>v, z. The composition and structure of the active layer are chosen to provide light emission at a preselected wavelength. The active layer may be left undoped (or unintentionally doped) or may be doped n-type or p-type. Of course, there can be other variations, modifications, and alternatives.
In a specific embodiment, the active region can also include an electron blocking region, and a separate confinement heterostructure. In a specific embodiment, the separate confinement heterostructure (SCH) can include AlInGaN or preferably InGaN, but can be other materials. The SCH is generally comprised of material with an intermediate index between the cladding layers and the active layers to improve confinement of the optical mode within the active region of the laser device according to a specific embodiment. In one or more embodiments, the SCH layers have a desirable thickness, impurity, and configuration above and below the active region to confine the optical mode. Depending upon the embodiment, the upper and lower SCH can be configured differently or the same. The electron blocking region can be on either side or both sides of the SCH positioned above the active region according to a specific embodiment. In a preferred embodiment, the SCH can range from about 100 nm to about 1500 nm, and preferably about 40 to 100 nm for the lower SCH region. In the upper SCH region, the thickness ranges from about 20 to 50 nm in a specific embodiment. As noted, the SCH is preferably InGaN having about 2% to about 5% indium or 5% to about 10% by atomic percent according to a specific embodiment. Of course, there can be other variations, modifications, and alternatives.
In some embodiments, an electron blocking layer is preferably deposited. In a specific embodiment, the electron blocking layer comprises a gallium and nitrogen containing material including magnesium 10E16 cm−3 to about 10E22 cm−3. The electron-blocking layer may comprise AlsIntGa1-s-tN, where 0≤s, t, s+t≤1, with a higher bandgap than the active layer, and may be doped p-type. In one specific embodiment, the electron blocking layer comprises AlGaN with an Al composition ranging from 5 to 20%. In another embodiment, the electron blocking layer may not contain Al. In another embodiment, the electron blocking layer comprises an AlGaN/GaN super-lattice structure, comprising alternating layers of AlGaN and GaN, each with a thickness between about 0.2 nm and about 5 nm. Of course, there can be other variations, modifications, and alternatives.
As noted, the p-type gallium nitride structure, which can be a p-type doped AlqInrGa1-q-rN, where 0≤q, r, q+r≤1, layer is deposited above the active layer. The p-type layer may be doped with Mg, to a level between about 1016 cm−3 and 1022 cm−3, and may have a thickness between about 5 nm and about 1000 nm. The outermost 1 nm to 50 nm of the p-type layer may be doped more heavily than the rest of the layer, so as to enable an improved electrical contact. In a specific embodiment, the laser stripe is provided by an etching process selected from dry etching or wet etching. In a preferred embodiment, the etching process is dry, but can be others. The device also has an overlying dielectric region, which exposes 213 contact region. In a specific embodiment, the dielectric region is an oxide such as silicon dioxide, but can be others. Of course, there can be other variations, modifications, and alternatives.
In a specific embodiment, the metal contact is made of suitable material. The reflective electrical contact may comprise at least one of silver, gold, aluminum, nickel, platinum, rhodium, palladium, chromium, or the like. The electrical contact may be deposited by thermal evaporation, electron beam evaporation, electroplating, sputtering, or another suitable technique. In a preferred embodiment, the electrical contact serves as a p-type electrode for the optical device. In another embodiment, the electrical contact serves as an n-type electrode for the optical device. Of course, there can be other variations, modifications, and alternatives.
In a specific embodiment, a ridge waveguide is fabricated using a certain deposition, masking, and etching processes. In a specific embodiment, the mask is comprised of photoresist (PR) or dielectric or any combination of both and/or different types of them. The ridge mask is 1 to 2.5 microns wide for single lateral mode applications or 2.5 μm to 30 μm wide for multimode applications. The ridge waveguide is etched by ion-coupled plasma (ICP), reactive ion etching (ME), chemical assisted ion beam (CAIBE) etched, or other method. The etched surface is 20-250 nm above the active region. A dielectric passivation layer is then blanket deposited by any number of commonly used methods in the art, such as sputter, e-beam, PECVD, or other methods. This passivation layer can include SiO2, Si3N4, Ta2O5, or others. The thickness of this layer is 80 nm to 400 nm thick. An ultrasonic process is used to remove the etch mask which is covered with the dielectric. This exposes the p-GaN contact layer. P-contact metal is deposited by e-beam, sputter, or other deposition technique using a PR mask to define the 2D geometry. The contact layer can be Ni/Au but others can be Pt/Au or Pd/Au. Further details of the present method and resulting structure can be found throughout the present specification and more particularly below. Further details of the facets can be found throughout the present specification and more particularly below.
In one or more preferred embodiments, the present invention provides a laser structure without an aluminum bearing cladding region. In a specific embodiment, the laser device comprises a multi-quantum well active region having thin barrier layers. In one or more embodiments, the active region comprises three or more quantum well structures. Between each of the quantum well structures comprises a thin barrier layer, e.g., 7 nm and less, 6 nm and less, 5 nm and less, 4 nm and less, 3 nm and less, 2 nm and less. In a preferred embodiment, the combination of thin barrier layers configured in the multi-quantum well structures enables a low voltage (e.g., 6 volts and less) laser diode free from use of aluminum bearing cladding regions.
In a specific embodiment, the present invention provides an optical device. The optical device has a gallium and nitrogen containing substrate including a (30-3-1) or offcut crystalline surface region orientation, which may be off-cut. The device preferably has an n-type cladding material overlying the n-type gallium and nitrogen containing material according to a specific embodiment. The n-type cladding material is substantially free from an aluminum bearing material according to a specific embodiment. The device also has an active region comprising at least three quantum wells. Each of the quantum wells has a thickness of 3.0 nm and greater or 5.5 nm and greater, and one or more barrier layers. Each of the barrier layers has a thickness ranging from about 2 nm to about 5 nm or about 5 nm to about 9 nm and is configured between a pair of quantum wells according to a specific embodiment. At least one or each of the barrier layers has a thickness ranging from about 2 nm to about nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. At least one or each of the barrier layers has a thickness ranging from about 4 nm to about 6.5 nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. Preferably, the device has a p-type cladding material overlying the active region. Preferably, the p-type cladding material is substantially free from an aluminum bearing material according to a specific embodiment. In a preferred embodiment, the active region is configured operably for a forward voltage of less than about 6V or less than about 5V for the device for an output power of greater than 10 mW, 100 mW, 1 W, 10 W, or greater. Of course, there can be other variations, modifications, and alternatives.
In yet an alternative embodiment, the present invention provides an optical device. The device has a gallium and nitrogen containing substrate including a (30-3-1) or offcut crystalline surface region orientation. The device also has an n-type cladding material overlying the n-type gallium and nitrogen containing material. The n-type cladding material is substantially free from an aluminum bearing material. The device further has an active region comprising at least three quantum wells. Each of the quantum wells has a thickness of 3.5 nm and greater or 5 nm and greater and one or more barrier layers according to a specific embodiment. Each of the barrier layers has a thickness ranging from about 2 nm to about 4 nm or about 4 nm to about 8 nm according to a specific embodiment. Each of the barrier layers is configured between a pair of quantum wells according to one or more embodiments. At least one or each of the barrier layers has a thickness ranging from about 2 nm to about 5 nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. At least one or each of the barrier layers has a thickness ranging from about 4 nm to about 8 nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. The device also has a p-type cladding material overlying the active region. The p-type cladding material is substantially free from an aluminum bearing material according to a preferred embodiment. The device optionally has a p-type material overlying the p-type cladding material.
In other embodiments, the invention provides yet an alternative optical device, which has a gallium and nitrogen containing substrate including a (30-3-1) or offcut crystalline surface region orientation. An n-type cladding material is overlying the n-type gallium and nitrogen containing material. Preferably, the n-type cladding material is substantially free from an aluminum bearing material. The device has an active region comprising at least three quantum wells, each of which has a thickness of 4 nm and greater. The device has one or more barrier layers, each of which has an n-type impurity characteristic and a thickness ranging from about 2 nm to about 4 nm or about 4 nm to about 8 nm in one or more alternative embodiments. Preferably, each of the barrier layers is configured between a pair of quantum wells according to a specific embodiment. The device also has a p-type cladding material overlying the active region according to a specific embodiment. The p-type cladding material is substantially free from an aluminum bearing material according to a specific embodiment. The device also has a p-type material overlying the p-type cladding material.
In other embodiments, the invention provides a method of fabricating an optical device, which has a gallium and nitrogen containing substrate including a (30-3-1) or offcut crystalline surface region orientation. An n-type cladding material is overlying the n-type gallium and nitrogen containing material. Preferably, the n-type cladding material is substantially free from an aluminum bearing material. The method includes forming an active region comprising at least three quantum wells, each of which has a thickness of 3.5 nm and greater. The device has one or more barrier layers, each of which has an n-type impurity characteristic and a thickness ranging from about 2 nm to about 4 nm or about 4 nm to about 8 nm in one or more alternative embodiments. Preferably, each of the barrier layers is configured between a pair of quantum wells according to a specific embodiment. The method also includes forming a p-type cladding material overlying the active region according to a specific embodiment. The p-type cladding material is substantially free from an aluminum bearing material according to a specific embodiment. The method also includes forming a p-type material overlying the p-type cladding material.
In a specific embodiment, the present invention provides an optical device, such as a laser diode. The device has a gallium and nitrogen containing substrate including a (30-3-1) or offcut crystalline surface region orientation, which may be off-cut according to one or more embodiments. The device has an n-type cladding material overlying the n-type gallium and nitrogen containing material. In a preferred embodiment, the n-type cladding material is substantially free from an aluminum bearing material. The device also has an active region comprising at least three quantum wells. In a specific embodiment, each of the quantum wells has a thickness of 3.5 nm and greater and one or more barrier layers according to a specific embodiment. Each of the barrier layers has a p-type characteristic and a thickness ranging from about 2 nm to about 3.5 nm in a specific embodiment. Each of the barrier layers has a p-type characteristic and a thickness ranging from about 3.5 nm to about 7 nm in an alternative specific embodiment. In a preferred embodiment, each of the barrier layers is configured between a pair of quantum wells. The device also has a p-type cladding material overlying the active region. Preferably, the p-type cladding material is substantially free from an aluminum bearing material. And overlying p-type material is included. In a preferred embodiment, the active region is configured for a forward voltage of less than about 6V or less than about 7V for the device for an output power of 10 mW and greater. In other embodiments for nonpolar m-plane devices or semipolar (30-3-1) planes, operable in the blue (430-475 nm) and green (505-530 nm), the present method and structure include five (5) or more thick QWs of greater than 4 or 5 nm in thickness and thin barriers that are 2-4 nm in thickness.
In one or more embodiments, the present invention includes a laser diode substantially free from an aluminum containing cladding region. To form the laser diode without an aluminum containing cladding region, the present laser diode includes three or more quantum wells to provide enough confinement of the optical mode for sufficient gain to reach lasing. However, when the number of quantum wells increases in the active region, the forward voltage of the diode can increase, as a tradeoff. We have determined that the forward voltage of the diode can be reduced in multi-quantum well active regions by way of the use of thin barriers on the order of 3-4 nm, which are much thinner than conventional lasers such as those in Yoshizumi et al., “Continuous-Wave operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {20-21} GaN Substrates,” Applied Physics Express 2 (2009) 092101. We have also determined that the forward voltage can be reduced in multi-quantum well active regions by adding p or n-type dopant species to the active region according to one or more other embodiments. Although any one or combination of these approached can be used, we believe it would be preferable to use the thin barrier approach to avoid adding impurities to the active region. The impurities may change optical losses and alter the electrical junction placement according to one or more embodiments. Accordingly, the present invention provides a laser device and method that is free from aluminum-containing cladding regions with low voltage on (30-3-1) or offcut substrates. Of course, there can be other variations, modifications, and alternatives.
Moreover, the present invention provides an optical device that is substantially free from aluminum bearing cladding materials. The device has a gallium and nitrogen containing substrate member having a (30-3-1) or offcut crystalline surface region. The device has an n-type gallium and nitrogen containing cladding material. In a specific embodiment, the n-type gallium and nitrogen containing cladding material is substantially free from an aluminum species, which leads to imperfections, defects, and other limitations. The device also has an active region including multiple quantum well structures overlying the n-type gallium and nitrogen containing cladding material. In one or more preferred embodiments, the device also has thin barrier layers configured with the multiple well structures. The device has a p-type gallium and nitrogen containing cladding material overlying the active region. In a preferred embodiment, the p-type gallium and nitrogen containing cladding material is substantially free from an aluminum species. The device preferably includes a laser stripe region configured from at least the active region and characterized by a cavity orientation substantially parallel to a projection in a c-direction. The laser stripe region has a first end and a second end. The device also has a first etched or etched facet provided on the first end of the laser stripe region and a second etched or etched facet provided on the second end of the laser stripe region. Depending upon the embodiment, the facets may be etched, etched, or a combination of cleaved and etched. In yet other embodiments, the present device includes a gallium and nitrogen containing electron blocking region that is substantially free from aluminum species. In yet other embodiments, the device does not include any electron blocking layer or yet in other embodiments, there is no aluminum in the cladding layers and/or electron blocking layer, although other embodiments include aluminum containing blocking layers. In still other embodiments, the optical device and method are free from any aluminum material, which leads to defects, imperfections, and the like. Further details of these limitations can be found throughout the present specification and more particularly below.
In preferred embodiments, the present method and structure is substantially free from InAlGaN or aluminum bearing species in the cladding layers as conventional techniques, such as those in Yoshizumi et al., “Continuous-Wave operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {20-21} GaN Substrates,” Applied Physics Express 2 (2009) 092101. That is, the present laser structure and method are substantially free from any aluminum species in the cladding region. Aluminum is generally detrimental. Aluminum often leads to introduction of oxygen in the reactor, which can act as non-radiative recombination centers to reduce the radiative efficiency and introduce other limitations. We also determined that oxygen can compensate p-type dopants in the p-cladding to cause additional resistivity in the optical device. In other aspects, we also determined that aluminum is detrimental to the MOCVD reactor and can react or pre-react with other growth precursors. Use of aluminum cladding layers is also cumbersome and can take additional time to grow. Accordingly, it is believed that the aluminum cladding free laser method and structure are generally more efficient to grow than conventional laser structures. Further benefits are described throughout the present specification and more particularly below.
In alternative example, the present invention provides a green laser diode configured using a semipolar gallium and nitrogen containing bulk substrate member, as described in more detail below, which has etched facets.
In preferred embodiments, the invention provides a laser structure without an aluminum bearing cladding region. In a specific embodiment, the laser device comprises a multi-quantum well active region having thin barrier layers, with the active region comprising three or more quantum well structures. Between each of the quantum well structures is a thin barrier layer, e.g., 8 nm and less, 7 nm and less, 6 nm and less, 5 nm and less, 4 nm and less, 3 nm and less, 2 nm and less, 1.5 nm and less. In a preferred embodiment, the combination of thin barrier layers configured in the multi-quantum well structures enables a low voltage (e.g., 7 volts and less, 6 volts and less) laser diode free from use of aluminum bearing cladding regions.
In one embodiment, the optical device has a gallium and nitrogen containing substrate including a {20-21} crystalline surface region orientation, which may be off-cut. The device preferably has an n-type cladding material overlying the n-type gallium and nitrogen containing material according to a specific embodiment. The n-type cladding material is substantially free from an aluminum bearing material. The device also has an active region comprising at least three quantum wells. Each of the quantum wells has a thickness of 2.5 nm and greater or 3.5 nm and greater and one or more barrier layers. Each of the barrier layers has a thickness ranging from about 2 nm to about 4 nm or about 3 nm to about 6.5 nm and is configured between a pair of quantum wells according to a specific embodiment. At least one or each of the barrier layers has a thickness ranging from about 2 nm to about 4 nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. At least one or each of the barrier layers has a thickness ranging from about 3 nm to about 6.5 nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. Preferably, the device has a p-type cladding material overlying the active region. Preferably, the p-type cladding material is substantially free from an aluminum bearing material according to a specific embodiment. In a preferred embodiment, the active region is configured operably for a forward voltage of less than about 7V or less than about 6V for the device for an output power of 10 mW and greater.
In yet an alternative embodiment, the present invention provides an optical device. The device has a gallium and nitrogen containing substrate including a {20-21} crystalline surface region orientation. The device also has an n-type cladding material overlying the n-type gallium and nitrogen containing material. The n-type cladding material is substantially free from an aluminum bearing material. The device further has an active region comprising at least two quantum wells. Each of the quantum wells has a thickness of 2.5 nm and greater or 3.5 nm and greater and one or more barrier layers according to a specific embodiment. Each of the barrier layers has a thickness ranging from about 2 nm to about 5 nm or about 3 nm to about 8 nm according to a specific embodiment. Each of the barrier layers is configured between a pair of quantum wells according to one or more embodiments. At least one or each of the barrier layers has a thickness ranging from about 2 nm to about 5 nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. At least one or each of the barrier layers has a thickness ranging from about 3 nm to about 8 nm and is configured between a pair of quantum wells or adjacent to a quantum well according to a specific embodiment. The device also has a p-type cladding material overlying the active region. The p-type cladding material is substantially free from an aluminum bearing material according to a preferred embodiment. The device optionally has a p-type material overlying the p-type cladding material.
In other embodiments, the invention provides yet an alternative optical device, which has a gallium and nitrogen containing substrate including a {20-21} crystalline surface region orientation. An n-type cladding material is overlying the n-type gallium and nitrogen containing material. Preferably, the n-type cladding material is substantially free from an aluminum bearing material. The device has an active region comprising at least two quantum wells, each of which has a thickness of 2.5 nm and greater. The device has one or more barrier layers, each of which has an n-type impurity characteristic and a thickness ranging from about 2 nm to about 5 nm or about 3 nm to about 8 nm in one or more alternative embodiments. Preferably, each of the barrier layers is configured between a pair of quantum wells according to a specific embodiment. The device also has a p-type cladding material overlying the active region according to a specific embodiment. The p-type cladding material is substantially free from an aluminum bearing material according to a specific embodiment. The device also has a p-type material overlying the p-type cladding material.
In other embodiments, the invention provides a method of fabricating an optical device, which has a gallium and nitrogen containing substrate including a {20-21} crystalline surface region orientation. An n-type cladding material is overlying the n-type gallium and nitrogen containing material. Preferably, the n-type cladding material is substantially free from an aluminum bearing material. The method includes forming an active region comprising at least two quantum wells, each of which has a thickness of 2.5 nm and greater. The device has one or more barrier layers, each of which has an n-type impurity characteristic and a thickness ranging from about 2 nm to about 5 nm or about 3 nm to about 8 nm in one or more alternative embodiments. Preferably, each of the barrier layers is configured between a pair of quantum wells according to a specific embodiment. The method also includes forming a p-type cladding material overlying the active region according to a specific embodiment. The p-type cladding material is substantially free from an aluminum bearing material according to a specific embodiment. The method also includes forming a p-type material overlying the p-type cladding material.
In a specific embodiment, the present invention provides an optical device, such as a laser diode. The device has a gallium and nitrogen containing substrate including a {20-21} crystalline surface region orientation, which may be off-cut according to one or more embodiments. The device has an n-type cladding material overlying the n-type gallium and nitrogen containing material. In a preferred embodiment, the n-type cladding material is substantially free from an aluminum bearing material. The device also has an active region comprising at least two quantum wells. In a specific embodiment, each of the quantum wells has a thickness of 2.5 nm and greater and one or more barrier layers according to a specific embodiment. Each of the barrier layers has a p-type characteristic and a thickness ranging from about 2 nm to about 3.5 nm in a specific embodiment. Each of the barrier layers has a p-type characteristic and a thickness ranging from about 3.5 nm to about 7 nm in an alternative specific embodiment. In a preferred embodiment, each of the barrier layers is configured between a pair of quantum wells. The device also has a p-type cladding material overlying the active region. Preferably, the p-type cladding material is substantially free from an aluminum bearing material. And overlying p-type material is included. In a preferred embodiment, the active region is configured for a forward voltage of less than about 6 v or less than about 7V for the device for an output power of 10 mW and greater.
In one or more embodiments, the present invention includes a laser diode substantially free from an aluminum containing cladding region. To form the laser diode without an aluminum containing cladding region, the present laser diode includes three or more quantum wells to provide enough confinement of the optical mode for sufficient gain to reach lasing. However, when the number of quantum wells increases in the active region, the forward voltage of the diode can increase, as a tradeoff. We have determined that the forward voltage of the diode can be reduced in multi-quantum well active regions by way of the use of thin barriers on the order of 5 nm, which are much thinner than conventional lasers such as those in Yoshizumi et al., “Continuous-Wave operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {20-21} GaN Substrates,” Applied Physics Express 2 (2009) 092101. We have also determined that the forward voltage can be reduced in multi-quantum well active regions by adding p or n-type dopant species to the active region according to one or more other embodiments. Although any one or combination of these approached can be used, we believe it would be preferable to use the thin barrier approach to avoid adding impurities to the active region. The impurities may change optical losses and alter the electrical junction placement according to one or more embodiments. Accordingly, the present invention provides a laser device and method that is free from aluminum-containing cladding regions with low voltage on {20-21) substrates.
Moreover, the present invention provides an optical device that is substantially free from aluminum bearing cladding materials. The device has a gallium and nitrogen containing substrate member having a {20-21} crystalline surface region. The device has an n-type gallium and nitrogen containing cladding material. In a specific embodiment, the n-type gallium and nitrogen containing cladding material is substantially free from an aluminum species, which leads to imperfections, defects, and other limitations. The device also has an active region including multiple quantum well structures overlying the n-type gallium and nitrogen containing cladding material. In one or more preferred embodiments, the device also has thin barrier layers configured with the multiple well structures. The device has a p-type gallium and nitrogen containing cladding material overlying the active region. In a preferred embodiment, the p-type gallium and nitrogen containing cladding material is substantially free from an aluminum species. The device preferably includes a laser stripe region configured from at least the active region and characterized by a cavity orientation substantially parallel to a projection in a c-direction. The laser stripe region has a first end and a second end. The device also has a first etched facet provided on the first end of the laser stripe region and a second etched facet provided on the second end of the laser stripe region. In yet other embodiments, the present device includes a gallium and nitrogen containing electron blocking region that is substantially free from aluminum species. In yet other embodiments, the device does not include any electron blocking layer or yet in other embodiments, there is no aluminum in the cladding layers and/or electron blocking layer, although other embodiments include aluminum containing blocking layers. In still other embodiments, the optical device and method are free from any aluminum material, which leads to defects, imperfections, and the like.
In preferred embodiments, the present method and structure is substantially free from InAlGaN or aluminum bearing species in the cladding layers as conventional techniques, such as those in Yoshizumi et al., “Continuous-Wave operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {20-21} GaN Substrates,” Applied Physics Express 2 (2009) 092101. That is, the present laser structure and method are substantially free from any aluminum species in the cladding region. Aluminum is generally detrimental. Aluminum often leads to introduction of oxygen in the reactor, which can act as non-radiative recombination centers to reduce the radiative efficiency and introduce other limitations. We also determined that oxygen can compensate p-type dopants in the p-cladding to cause additional resistivity in the optical device. In other aspects, we also determined that aluminum is detrimental to the MOCVD reactor and can react or pre-react with other growth precursors. Use of aluminum cladding layers is also cumbersome and can take additional time to grow. Accordingly, it is believed that the aluminum cladding free laser method and structure are generally more efficient to grow than conventional laser structures.
In a specific embodiment on the {20-21} GaN, the device has a laser stripe region formed overlying a portion of the off-cut crystalline orientation surface region. In a specific embodiment, the laser stripe region is characterized by a cavity orientation substantially in a projection of a c-direction, which is substantially normal to an a-direction. In a specific embodiment, the laser stripe region has a first end and a second end, each of which is etched. In a preferred embodiment, the device is formed on a projection of a c-direction on a {20-21} gallium and nitrogen containing substrate having a pair of etched mirror structures, which face each other.
In a preferred embodiment, the device has a first etched facet provided on the first end of the laser stripe region and a second etched facet provided on the second end of the laser stripe region. In one or more embodiments, the first etched is substantially parallel with the second etched facet. Mirror surfaces are formed on each of the etched surfaces. The first etched facet comprises a first mirror surface. In a specific embodiment, the first mirror surface comprises a reflective coating. The reflective coating is selected from silicon dioxide, hafnia, and titania, tantalum pentoxide, zirconia, including combinations, and the like. Depending upon the embodiment, the first mirror surface can also comprise an anti-reflective coating.
Also in a preferred embodiment, the second etched facet comprises a second mirror surface. In a specific embodiment, the second mirror surface comprises a reflective coating, such as silicon dioxide, hafnia, and titania, tantalum pentoxide, zirconia, combinations, and the like. In a specific embodiment, the second mirror surface comprises an anti-reflective coating.
In a specific embodiment, the laser stripe has a length and width. The length ranges from about 50 microns to about 3000 microns or preferably from about 400 microns to about 650 microns or about 650 microns to about 1200 μm microns. The strip also has a width ranging from about 0.5 microns to about 50 microns or preferably between 1 microns to about 1.5 microns, about 1.5 microns to about 2.0 microns, or about 2.0 microns to about 4 microns, but can be other dimensions. In a specific embodiment, the width is substantially constant in dimension, although there may be slight variations. The width and length are often formed using a masking and etching process, which are commonly used in the art.
In a specific embodiment, the present invention provides an alternative device structure capable of emitting 501 nm and greater light in a ridge laser embodiment having etched facets. The device is provided with one or more of the following epitaxially grown elements, but is not limiting.
an n-GaN cladding layer with a thickness from 100 nm to 3000 nm with Si doping level of 5E17 to 3E18 cm−3;
an n-side SCH layer comprised of InGaN with molar fraction of indium of between 3% and 10% and thickness from 20 to 150 nm;
multiple quantum well active region layers comprised of at least two 2.0-5.5 nm InGaN quantum wells separated by thin 2.5 nm and greater, and optionally up to about 8 nm, GaN barriers;
a p-side SCH layer comprised of InGaN with molar a fraction of indium of between 1% and 10% and a thickness from 15 nm to 100 nm;
an electron blocking layer comprised of AlGaN with molar fraction of aluminum of between 5% and 20% and thickness from 5 nm to 20 nm and doped with Mg;
a p-GaN cladding layer with a thickness from 400 nm to 1000 nm with Mg doping level of 2E17 cm-3 to 2E19 cm−3; and
a p++-GaN contact layer with a thickness from 20 nm to 40 nm with Mg doping level of 1E19 cm-3 to 1E21 cm−3.
Of course there can be other embodiments such as the use of p-side GaN guiding layer in place of the p-SCH, the use of multiple different layers in the SCH regions, or the omission of the EBL layer. Again, there can be other variations, modifications, and alternatives.
In an example, a laser device is fabricated on a {20-21} substrate according to an embodiment of the present invention. The laser device includes gallium nitride substrate, which has an underlying n-type metal back contact region. In a specific embodiment, the metal back contact region is made of a suitable material such as those noted below and others. Further details of the contact region can be found throughout the present specification, and more particularly below.
In a specific embodiment, the device also has an overlying n-type gallium nitride layer, an active region, and an overlying p-type gallium nitride layer structured as a laser stripe region. In a specific embodiment, each of these regions is formed using at least an epitaxial deposition technique of metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial growth techniques suitable for GaN growth. In a specific embodiment, the epitaxial layer is a high quality epitaxial layer overlying the n-type gallium nitride layer. In some embodiments the high quality layer is doped, for example, with Si or O to form n-type material, with a dopant concentration between about 1016 cm−3 and 1020 cm−3.
In a specific embodiment, an n-type AluInvGa1-u-vN layer, where 0≤u, v, u+v≤1, is deposited on the substrate. In a specific embodiment, the carrier concentration may lie in the range between about 1016 cm−3 and 1020 cm−3. The deposition may be performed using metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
As an example, the bulk GaN substrate is placed on a susceptor in an MOCVD reactor. After closing, evacuating, and back-filling the reactor (or using a load lock configuration) to atmospheric pressure, the susceptor is heated to a temperature between about 1000 and about 1200 degrees Celsius in the presence of a nitrogen-containing gas. In one specific embodiment, the susceptor is heated to approximately 900 to 1100 degrees Celsius under flowing ammonia. A flow of a gallium-containing metalorganic precursor, such as trimethylgallium (TMG) or triethylgallium (TEG) is initiated, in a carrier gas, at a total rate between approximately 1 and 50 standard cubic centimeters per minute (sccm). The carrier gas may comprise hydrogen, helium, nitrogen, or argon. The ratio of the flow rate of the group V precursor (ammonia) to that of the group III precursor (trimethylgallium, triethylgallium, trimethylindium, trimethylaluminum) during growth is between about 2000 and about 12000. A flow of disilane in a carrier gas, with a total flow rate of between about 0.1 and 10 sccm, is initiated.
In a specific embodiment, the laser stripe region is made of the p-type gallium nitride layer. In a specific embodiment, the laser stripe is provided by an etching process selected from dry etching or wet etching. In a preferred embodiment, the etching process is dry, but can be others. As an example, the dry etching process is an inductively coupled process using chlorine bearing species or a reactive ion etching process using similar chemistries. Again as an example, the chlorine bearing species are commonly derived from chlorine gas or the like. The device also has an overlying dielectric region, which exposes contact region. In a specific embodiment, the dielectric region is an oxide such as silicon dioxide or silicon nitride, but can be others. The contact region is coupled to an overlying metal layer. The overlying metal layer is a multilayered structure containing gold and platinum (Pt/Au), nickel gold (Ni/Au), but can be others.
In a specific embodiment, the laser device has active region. The active region can include one to twenty quantum well regions according to one or more embodiments. As an example following deposition of the n-type AluInvGa1-u-vN layer for a predetermined period of time, so as to achieve a predetermined thickness, an active layer is deposited. The active layer may be comprised of multiple quantum wells, with 2-10 quantum wells. The quantum wells may be comprised of InGaN with GaN barrier layers separating them. In other embodiments, the well layers and barrier layers comprise AlwInxGa1-w-xN and AlyInzGa1-y-zN, respectively, where 0≤w, x, y, z, w+x, y+z≤1, where w<u, y and/or x>v, z so that the bandgap of the well layer(s) is less than that of the barrier layer(s) and the n-type layer. The well layers and barrier layers may each have a thickness between about 1 nm and about 20 nm. The composition and structure of the active layer are chosen to provide light emission at a preselected wavelength. The active layer may be left undoped (or unintentionally doped) or may be doped n-type or p-type.
In a specific embodiment, the active region can also include an electron blocking region, and a separate confinement heterostructure. In some embodiments, an electron blocking layer is preferably deposited. The electron-blocking layer may comprise AlsIntGa1-s-tN, where 0≤s, t, s+t≤1, with a higher bandgap than the active layer, and may be doped p-type. In one specific embodiment, the electron blocking layer comprises AlGaN. In another embodiment, the electron blocking layer comprises an AlGaN/GaN super-lattice structure, comprising alternating layers of AlGaN and GaN, each with a thickness between about 0.2 nm and about 5 nm.
In a specific embodiment, the action region structure does not include an AlGaN EBL layer. That is, the laser device is free from any electron blocking layer, which is optional in such embodiment.
As noted, the p-type gallium nitride structure is deposited above the electron blocking layer and active layer(s). The p-type layer may be doped with Mg, to a level between about 1016 cm−3 and 1022 cm−3, and may have a thickness between about 5 nm and about 1000 nm. The outermost 1-50 nm of the p-type layer may be doped more heavily than the rest of the layer, so as to enable an improved electrical contact. In a specific embodiment, the laser stripe is provided by an etching process selected from dry etching or wet etching. In a preferred embodiment, the etching process is dry, but can be others. The device also has an overlying dielectric region, which exposes a contact region. In a specific embodiment, the dielectric region is an oxide such as silicon dioxide.
In a specific embodiment, the metal contact is made of suitable material. The reflective electrical contact may comprise at least one of silver, gold, aluminum, nickel, platinum, rhodium, palladium, chromium, or the like. The electrical contact may be deposited by thermal evaporation, electron beam evaporation, electroplating, sputtering, or another suitable technique. In a preferred embodiment, the electrical contact serves as a p-type electrode for the optical device. In another embodiment, the electrical contact serves as an n-type electrode for the optical device.
In an example, a laser device includes a starting material such as a bulk nonpolar or semipolar GaN substrate, but can be others. In a specific embodiment, the device is configured to achieve emission wavelength ranges of 390 nm to 420 nm, 420 nm to 440 nm, 440 nm to 470 nm, 470 nm to 490 nm, 490 nm to 510 nm, and 510 nm to 530 nm, but can be others.
In a preferred embodiment, the growth structure is configured using between 3 and 5 or 5 and 7 quantum wells positioned between n-type GaN and p-type GaN cladding layers. In a specific embodiment, the n-type GaN cladding layer ranges in thickness from 500 nm to 2000 nm and has an n-type dopant such as Si with a doping level of between 1E18 cm−3 and 3E18 cm−3. In a specific embodiment, the p-type GaN cladding layer ranges in thickness from 500 nm to 1000 nm and has a p-type dopant such as Mg with a doping level of between 1E17 cm−3 and 7E19 cm−3. In a specific embodiment, the Mg doping level is graded such that the concentration would be lower in the region closer to the quantum wells.
In a specific preferred embodiment, the quantum wells have a thickness of between 2.5 nm and 4 nm, 4 nm and 5.5 nm or 5.5 nm and 8 nm, but can be others. In a specific embodiment, the quantum wells would be separated by barrier layers with thicknesses between 2 nm and 3.5 nm or 3.5 nm and 6 nm or 6 nm and 8 nm. The quantum wells and the barriers together comprise a multiple quantum well (MQW) region.
In a preferred embodiment, the device has barrier layers formed from GaN, InGaN, AlGaN, or InAlGaN. In a specific embodiment using InGaN barriers, the indium contents range from 0% to 5% (mole percent), but can be others. Also, it should be noted that % of indium or aluminum is in a molar fraction, not weight percent.
An InGaN separate confinement heterostructure layer (SCH) could be positioned between the n-type GaN cladding and the MQW region according to one or more embodiments. Typically, such separate confinement layer is commonly called the n-side SCH. The n-side SCH layer ranges in thickness from 10 nm to 60 nm or 60 nm to 150 nm and ranges in indium composition from 1% to 12% (mole percent), but can be others. In a specific embodiment, the n-side SCH layer may be doped with an n-type dopant such as Si.
In yet another preferred embodiment, an InGaN separate confinement heterostructure layer (SCH) is positioned between the p-type GaN cladding and the MQW region, which is called the p-side SCH. In a specific embodiment, the p-side SCH layer ranges in thickness from 10 nm to 40 nm or 40 nm to 150 nm and ranges in indium composition from 0% to 10% (mole percent), but can be others. The p-side SCH layer may be doped with a p-type dopant such as Mg.
In another embodiment, the structure would contain both an n-side SCH and a p-side SCH. In another embodiment the p-side SCH would be replaced with p-side GaN guiding layer. In another embodiment the n-side and/or p-side SCH regions would contain multiple layers.
In another embodiment, the structure would contain a GaN guiding layer on the p-side positioned between the p-type GaN cladding layer and the MQW region. This GaN guiding layer could range in thickness from 10 nm to 60 nm and may or may not be doped with a p-type species such as Mg.
In a specific preferred embodiment, an AlGaN electron blocking layer, with an aluminum content of between 5% and 20% (mole percent), is positioned between the MQW and the p-type GaN cladding layer either between the MQW and the p-side SCH, within the p-side SCH, or between the p-side SCH and the p-type GaN cladding. The AlGaN electron blocking layer ranges in thickness from 5 nm to 20 nm and is doped with a p-type dopant such as Mg from 1E17 cm−3 and 1E21 cm−3 according to a specific embodiment. In other embodiments, the electron blocking layer is free from any aluminum species and/or may be eliminated altogether. In yet another embodiment, the device would be substantially free from an electron blocking layer.
Preferably, a p-contact layer positioned on top of and is formed overlying the p-type cladding layer. The p-contact layer would be comprised of GaN doped with a p-dopant such as Mg at a level ranging from 1E20 cm−3 to 1E22 cm−3.
In an example, a laser device has a gallium and nitrogen containing substrate member (e.g., bulk gallium nitride) having a {20-21} crystalline surface region or other surface configuration. The device has an n-type gallium and nitrogen containing cladding material. In a specific embodiment, the n-type gallium and nitrogen containing cladding material is substantially free from an aluminum species, which leads to imperfections, impurities, and other limitations. In one or more preferred embodiment, the cladding material has no aluminum species and is made of a gallium and nitrogen containing material. Of course, there can be other variations, modification, and alternatives.
In a specific embodiment, the device also has an active region including multiple quantum well structures overlying the n-type gallium and nitrogen containing cladding material. In one or more embodiments, the active regions can include those noted, as well as others. That is, the device can include InGaN/InGaN and/or InGaN/GaN active regions, among others. In a specific embodiment, the optical can include seven MQW, six MQW, five MQW, four MQW, three MQW, more MQW, or fewer, and the like. Of course, there can be other variations, modification, and alternatives.
In a specific embodiment, the device has a p-type gallium and nitrogen containing cladding material overlying the active region. In a preferred embodiment, the p-type gallium and nitrogen containing cladding material is substantially free from an aluminum species, which leads to imperfections, defects, and other limitations. In one or more preferred embodiment, the cladding material has no aluminum species and is made of a gallium and nitrogen containing material.
In a specific embodiment, the device preferably includes a laser stripe region configured from at least the active region and characterized by a cavity orientation substantially parallel to a projection in a c-direction. Other configurations may also exist depending upon the specific embodiment. The laser stripe region has a first end and a second end or other configurations. In a specific embodiment, the device also has a first etched facet provided on the first end of the laser stripe region and a second etched facet provided on the second end of the laser stripe region.
In yet other embodiments, the present device includes a gallium and nitrogen containing electron blocking region that is substantially free from aluminum species. In yet other embodiments, the device does not include any electron blocking layer or yet in other embodiments, there is no aluminum in the cladding layers and/or electron blocking layer.
In preferred embodiments, the present method and structure is substantially free from InAlGaN or aluminum bearing species in the cladding layers as conventional techniques, such as those in Yoshizumi et al., “Continuous-Wave operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {20-21} GaN Substrates,” Applied Physics Express 2 (2009) 092101. That is, the present laser structure and method are substantially free from any aluminum species in the cladding region. Aluminum is generally detrimental. Aluminum often leads to introduction of oxygen in the reactor, which can act as non-radiative recombination centers to reduce the radiative efficiency and introduce other limitations. We also determined that oxygen can compensate p-type dopants in the p-cladding to cause additional resistivity in the optical device. In other aspects, we also determined that aluminum is detrimental to the MOCVD reactor and can react or pre-react with other growth precursors. Use of aluminum cladding layers is also cumbersome and can take additional time to grow. Accordingly, it is believed that the aluminum cladding free laser method and structure are generally more efficient to grow than conventional laser structures. Further details of the present laser configured on {20-21} can be found in U.S. application Ser. No. 12/883,652 filed on Sep. 16, 2010, which is incorporated by reference herein.
FIG. 3A shows a schematic diagram of a typical configuration of a laser facet with optical coating configured in two dielectric layers where layer 1 functions as the passivation layer to increase the COMD level and layer 2 functions as the reflectivity modification layer.
FIG. 3B shows a graphs of the reflectivity (%) of a laser diode front facet versus the wavelength (nm) for optical coating configurations with and without the passivation layer shown in FIG. 3A, and for designs targeting  ̃5% and  ̃10% reflectivity in the blue region. The curves show that a thin SiO2 passivation layer offsets the reflectivity at a given wavelength.
FIG. 4A shows a graph of laser output power (mA) versus input current (mA) for a laser diode that undergoes a COMD failure at >700 mW output power. Upon the onset of COMD the output power abruptly drops. FIG. 4B shows an image of the irreversible damage at the front facet of the laser that occurs with COMD. It can be seen that COMD results in a sudden drop in power. COMD is typically a thermal runaway process, consistent with the apparent burn mark/hole on the laser facet.
FIG. 5 shows an example from the literature demonstrating how the COMD threshold power can be improved by ECR coating in conventional GaN laser diodes.
FIG. 6 shows experimental COMD level data for commercially available c-plane laser diodes having AlGaN claddings and ECR facet coatings, and for nonpolar/semipolar laser diodes with cladding layers substantially free from Al-containing claddings and using conventional e-beam facet coating techniques. As seen in the figure, with good facet quality, the e-beam coated nonpolar/semipolar devices can achieve the same power level before COMD as the c-plane or nonpolar/semipolar devices using the more specialized ECR coating technique. However, when imperfections are introduced to the facet, the COMD level is substantially reduced.
FIGS. 7A and 7B shows examples of facet imperfections resulting from a cleaving process on nonpolar/semipolar laser diode facets. Such imperfections can act as initiation sites for catastrophic failure and lead to low COMD levels and hence device failures. FIG. 7A shows a top view image of an imperfect facet and FIG. 7B shows an end view image of an imperfect facet.
FIGS. 8A and 8B show an SEM image and an optical image, respectively, of facet failures related to imperfections from the cleaving process where the arrows depict the failure points and the imperfections.
FIG. 9 shows an example taken from literature of etched facet technology which can yield smooth, vertical, and defect free etched facets to enable very high COMD levels.
FIG. 10 shows example data demonstrating how etched facets technology could improve the COMD yield compared to etched facet technology on nonpolar or semipolar laser diodes with front facet coatings. Because the quality of a cleaved varies, the COMD power threshold is scattered due to the varied occurrences of imperfections. With the etched facet process, the COMD threshold power levels are tightly distributed due to the lack of cleave associated imperfections, and meet the COMD power requirements for certain applications as indicated by the dashed line of 400 mW/μm.
Catastrophic optical damage (COD) takes places suddenly during operation which is concomitant with high optical power density. Although COD could occur at defects in the bulk of the laser resonator, it occurs mainly at laser facet surfaces (catastrophic optical mirror damage—COMD). The physical origin of this mirror facet failure is due to the fact that when a laser facet is formed by conventional cleaving or other chemical methods, a set of the unsaturated bonds appear at the exposed surface. These dangling bonds interact strongly with atoms and molecules of the ambient, forming non-negligible surface states. Strong facet heating occurs through surface nonradiative recombination and surface current effects mediated by surface states, and can be detrimental to the performance and reliability of diode lasers, in particular regarding the optical integrity of their facet mirrors.
Although the surface recombination velocity of GaN is about two orders lower than that of a GaAs facet, GaN lasers are still prone to facet failure when operated at high optical power density. In order to mitigate this facet failure and improve the performance and reliability of GaN lasers, mirror facet surface passivation is necessary to reduce the number of surface states and hence the recombination velocity.
There are several methods of formation of high quality passivation layers, such as atomic layer deposition (ALD), electron cyclotron resonance (ECR) plasma deposition, EMOF (epitaxial mirror on facet) deposition by molecular beam epitaxy (MBE), and MOCVD.
ALD is a vapor phase technique capable of producing thin films of a variety of materials. Based on sequential, self-limiting reactions, ALD offers exceptional conformality on high-aspect ratio structures, thickness control at the Angstrom level, and tunable film composition. With these advantages, ALD has emerged as a tool for many industrial and research applications. It consists of sequential alternating pulses of gaseous chemical precursors that react with the substrate. Typically, ALD processes are conducted at temperatures of <350° C. Although ALD has many promising features, it also suffers from slow deposition rates. Because of the long cycle times involved in pulsing and purging precursors and the layer-by-layer nature of the deposition, most ALD rates are on the order of 100-300 nm/h.
EMOF process is a process that involves a facet cleaving process under vacuum, followed by mirrors grown from a wide-bandgap semiconductor that is lattice-matched to the laser material to provide an interface with minimal defects. This wide-bandgap material is grown by MBE at low temperatures. MBE growth enables control of the mirror thickness, which translates to control of reflectivity. Because the EMOF deposition is a batch process, and because the MBE provides good process control, EMOF deposition can enable inexpensive volume production of lasers materials with facet passivation technology that yielded high COMD power and long term reliability.
Another method is ECR plasma deposition. FIG. 11 shows an example of an ECR deposition tool, a model AFTEX-6200 manufactured by MES-AFTY. High speed electrons collide with gas molecules, which generate a high density plasma. Low energy and high density ion irradiation can provide high quality films. Prior to film deposition, a plasma cleaning process can be used in some embodiments either in-situ or in another tool.
Several materials could be used for the passivation layer for GaN laser diodes, such as AlN, AlON, and/or AlO. The passivation layer typically has a thickness of 5-15 nm. With optimized plasma cleaning and film deposition processes, high quality crystalline or polycrystalline passivation layers have been demonstrated on GaN facets. FIG. 12 shows a TEM image of an AlON passivation layer epitaxially grown on a GaN interface according to an embodiment, and FIG. 13 shows a TEM image of an AlN passivation layer epitaxially grown on a GaN interface according to another embodiment. Device performance can be improved. FIG. 5 shows an example demonstrating how COMD threshold power can be improved by ECR coating with AlON passivation layer in accordance with an embodiment, and FIG. 14 shows that COMD power can be improved by more than 2× with an AlN passivation layer in accordance with another embodiment. Improved long term reliability has been demonstrated as well.
Laser facets are a frequent point of failure in solid state lasers due to the high optical intensity they must sustain over the operating life of the laser. Facet formation typically leaves dangling bonds and other defects on the free surface. These defects can cause both non-radiative surface recombination and optical absorption at the facets during the operation of laser devices. Both of these can generate heat, which often leads to thermal degradation of the device. Catastrophic optical mirror damage (COMD) is a typical example of device degradation caused by rapid heat generation at the facet surface. Any absorption in the facet area can lead to a runaway heating effect which can cause COMD. The sources of this absorption can be in the surface of the semiconductor itself near the facet, the interface of the semiconductor and coating, or in the coating itself.
To prevent optical, thermal, or chemical damage at the facet and sustain the robustness of the device, facet coating techniques can be performed. To maximize lifetime and robustness to damage it is critical to pick compatible coating materials that are deposited with high quality. This can mean a coating has multiple layers deposited by different deposition techniques to achieve the required performance. A properly designed coating not only can passivate the surface to reduce optical absorption and minimize surface recombination, but can also isolate the laser devices from the ambient such as moisture and oxygen to provide mechanical protection for lifetime improvement. In some embodiments, a specific ‘passivation layer’, distinct from the AR or HR coatings, can be used for the purpose of enhancing lifetime. With proper engineering of the contrast of refractive indices, AR and HR coatings can be implemented on top of the passivation layer to enhance the performance of the laser device in some embodiments. Alternatively, the AR and HR coatings themselves can be engineered to serve the passivation layer functions. For the purposes of this disclosure, all of these layers will be referred to as ‘facet coatings’ and the present disclosure can apply to any one or combination of these layers. Facet coating films on the etched facets are preferred in some embodiments to be poly-crystalline or single-crystalline for optimum adhesion, thermal transport, and minimization of potentially optically absorbing interfacial defects. These films can also be amorphous depending on the application and film properties. Epitaxial single- or poly-crystal facet coating layers are desirable in some embodiments for lifetime since these may have the best adhesion, thermal transport, and may leave the fewest potentially absorbing dangling bonds or other defects.
An uncoated laser facet can lead to poor device lifetimes or low COMD levels. This is partly because the semiconductor surface can react with atmospheric or packaging gases or the surface states on the semiconductor can lead to optical absorption. A facet passivation layer can be used to increase stability to atmospheric gases and to passivate the surface states of the semiconductor which can prevent optical absorption. Good passivating coatings will have high COMD threshold as deposited and will maintain this high threshold over the operating life of the laser. Some things that reduce the COMD threshold over time are the reaction of the coating with the semiconductor material underneath due to the high optical intensity and temperature. While a passivation layer can be amorphous, a polycrystalline or single crystal layer can often provide longer lifetimes. This is partially because a crystalline material can be thermodynamically stable while amorphous materials can typically be densified or crystallize over time with enough heat and energy. This means that even though an amorphous coating can have high COMD thresholds initially it can slowly degrade until the threshold approaches the operating condition and the coating fails. An example of this is Al2O3 on a GaN facet which can be deposited as an amorphous or crystalline layer depending on the deposition technique and conditions. If it is amorphous the material can crystallize over time which changes its density and chemical bond structure. This can cause defects and lead to absorption in the coating or semiconductor interface. If the material is deposited as a crystalline it will not change structurally over time and can have a longer lifetime. The method of depositing crystalline coatings is critical though as a low quality crystalline coating can have low COMD threshold initially if defects are present in the crystal or interface. FIG. 15A shows a cross-sectional view of a facet passivation layer below a reflectivity modification layer(s) in an example. The facet passivation layer can be amorphous, polycrystalline, or single crystal. FIG. 15B shows a cross-sectional view of a facet passivation layer on a rough GaN facet in an example. The passivation layer then produces a smooth interface with the reflectivity modification layer(s).
FIG. 16A shows a cross-sectional view of a facet passivation layer below a distributed Bragg reflector (DBR) of quarter lambda (quarter wave) layers that form the HR coating in an example. The facet passivation layer passivates the surface and separates the DBR from the semiconductor surface. The stack of quarter lambda layers can end on a high or low index material. FIG. 16B shows a cross-sectional view of a facet passivation layer that is designed to affect the electric field in the HR coating in an example. By changing the thickness of the passivation layer or combining with different thickness of high or low index material the electric field peaks can be shifted in the coating which can increase the COMD threshold.
Etched Facets
Etched facets provide a number of benefits for laser diode performance and fabrication but also introduce a special set of challenges for lifetime and reliability. Facets formed using dry etch techniques are exposed to a variety of chemical reactions and ion bombardment as part of the etching process. This exposure can lead to a variety of undesirable defects which can impact device performance, lifetime, and reliability. Some examples include point defects, extended crystalline defects, and amorphous regions in an otherwise crystalline material, implanted ions, disrupted surface bonds, and surface residue layers. These defects can directly cause failure through mechanisms such as non-radiative recombination or optical absorption. They can also indirectly cause poor reliability by disrupting facet passivation and coating steps. Controlling these defects is critical to the successful formation of high quality facet coating layers which lead to reliable parts. The present disclosure includes methods of etching facets such that the resulting surface is suitable for the formation of high quality coating layers, which are critical to performance, lifetime, and reliability.
Defects related to dry etching laser facets can be primarily controlled by careful selection of etch parameters. The etching mechanism in plasma based dry etching works through the physical bombardment of the material to be etched with high energy ions. The mechanical impact of impinging ions can cause atoms in the target material to be sputtered off. Chemically reactive species can also be included to aid in the removal of the material. Two parameters which must be controlled to prevent undesirable damage to the semiconductor are ion voltage and ion density. The higher the energy of colliding ions, the greater the likelihood that ions can become implanted in the material creating point defects. The series of collisions caused by the impinging ions also cause disruptions to the crystal lattice, creating amorphous regions near the surface. These amorphous regions can hamper later growth of high-quality epitaxial facet coating layers. By carefully controlling the incoming ion energy and density, the formation of a defective layer in the semiconductor can be avoided. This is critical to forming an high-quality epitaxial facet coating film later in the process, which in turn is critical to device performance and lifetime. Ions are accelerated at the target material through an electric potential. Keeping this forward accelerating potential below 2000V is preferable in some embodiments for the prevention of damage to the semiconductor material.
Another critical parameter for leaving a high quality, contamination free, crystalline etched surface is temperature. It is desirable to perform the etch at temperatures above 100 degrees C. in some embodiments to aid in the removal of etching species and other byproducts of the etch process.
In the formation of laser diodes from an InAlGaN material, it is desirable to include chlorine in the etch chemistry in some embodiments to provide a chemical component to the etch process. This chemical component may be less prone to leaving physical damage than the physical ion bombardment which can disrupt later growth of facet coating layers.
In addition to the already mentioned parameters, a number of other factors must be controlled to maintain vertical, smooth, and defect free facet surfaces suitable for the later formation of a coherent facet coating layer. These parameters can include, but are not limited to, ion element selection, etch rates, thermal annealing after etch, presence of chemically reactive species, chamber pressure, and thermal coupling of wafer to etch tool. Etch tools which can be used include, but are not limited to, reactive ion etching (RIE), inductively coupled plasma (ICP) etching, ion beam etching (IBE), chemically assisted ion beam etching (CAME), reactive ion beam etching (RIBE), and electron cyclotron resonance (ECR) etching.
FIG. 17 shows a transmission electron microscopy (TEM) image of an etched facet whose etch conditions have been controlled to maintain the crystalline lattice all the way to the facet surface without any amorphous or contamination layers in an example. Layer A shows the etched semiconductor, layer B is a coating layer used in the preparation of the sample for TEM analysis and would not be present in a manufactured laser diode. Careful control of the etched facet process for the purpose of leaving a surface suitable for the epitaxial growth of a facet coating layer represents a key and novel technology which enables the manufacture of highly reliable laser diodes with an etched facet process.
With a properly controlled etched facet containing a crystalline surface, one can form single- or poly-crystalline films as well as amorphous films to passivate the surface through various deposition or growth techniques such as ion beam deposition, ECR plasma sputtering, RF-magnetron sputtering, DC sputtering, thermal evaporation deposition, e-beam evaporation deposition with or without assistance from ions, PECVD, MOCVD, MBE, aqueous-solution-based deposition, and so on. Facet coating films can be a single layer or multiple layers of materials such as, but not limited to, AlN, AlON, SiO2, SiON, Ga2O3, GaN, Al2O3, ITO, ZnO, etc. These materials can be deposited at room temperature or elevated temperature depending on the deposition or growth condition and technique mentioned above.
Surface Cleans and Coatings on Etched Facets
Dry etching facets can introduce impurities, etch damage, and re-deposition of the material on the etched surface which are detrimental to the film quality of facet coatings, which may impact roughness, absorbance, uniformity, adhesion, and lifetime. To improve facet robustness, a facet coating may be preceded by a surface cleaning procedure in some embodiments to remove impurities and re-deposited materials as well as to cure damage from the dry etch. This can be done by various chemical and mechanical methods, either in-situ or ex-situ, such as wet etching, plasma clean, ion bombardment clean, UV-ozone photoreaction, etc. The surface cleaning may be done at elevated substrate temperatures in some embodiments to enhance the cleaning reaction for more efficient cleaning. Furthermore, the surface cleaning may be done before the deposition of the first layer in the coating or in the middle of the multilayer depositions to improve the quality of the film in terms of the aforementioned criteria.
As mentioned above, properly controlled dry etching can provide a crystalline facet surface, which may be good enough to form nucleation of crystal. However, even well controlled dry etch processes can leave impurities and etch damage on the surface. Also, any dangling bonds formed right after the etching will be prone to oxidation from the ambient oxygen or moisture. Therefore, to enhance the surface cleanliness and the crystal quality, one can use plasma formed from gasses such as Ar, Xe, N2, O2, or other gases and any mixture of those gases in some embodiments. This plasma clean process is preferably done in-situ without exposing the facet to air between cleaning and facet coating formation, and also can be done at an elevated temperature to expedite surface cleaning reactions. It is preferable in some embodiments to keep the facet in a controlled environment of vacuum or non-reactive gases between the cleaning and coating steps.
FIG. 18 shows the TEM-EDX line scan data of an etched facet which has been cleaned with an in-situ plasma clean, then coated with a single-crystalline AlN layer followed by an amorphous Al2O3 AR layer in an example. As shown in FIG. 18, the controlled etch and pre-deposition clean leave no etch byproduct or other contaminant at the interface. Some trace Ar was detected, but is to be expected as this gas is the sputtering agent in the deposition process. As a result, epitaxial AlN was formed on the etched facet as shown in the FIG. 19, with minimal potentially absorbing impurities at the interface. The inset diffraction patterns demonstrate that the etched material remains crystalline to the surface and the first coating layer is epitaxially grown with a coherent crystal structure. FIG. 19 is a TEM image of an etched facet which maintains the crystalline lattice all the way to the semiconductor in an example. Layer A shows the etched semiconductor, layer B is an epitaxial passivation layer, and layer C is an AR coating. The cutout shows diffraction patterns from Layer A and Layer B, indicating a coherently grown single crystal passivation layer. In other words, the etched facet has a surface that is substantially free of contaminants.
FIG. 20 shows catastrophic optical damage (COD) test data of two etched facet laser diodes, the solid line represents the etched facet coated with a polycrystalline film in accordance with an embodiment, and the dotted line represents the etched facet coated with an amorphous film. As shown in FIG. 20, the laser diode with polycrystalline coating (the solid line) shows more robust operation under higher current injection (surviving a high COD power of about 30 W, whereas the laser diode with amorphous coating (the dotted line) has poor COD power resulted in a device failure below 6 W.
FIG. 21 shows the lifetest result of similar etched facet laser devices coated with a single-crystalline passivation layer in accordance with some embodiments. With the single crystal film passivation, very stable light output has been demonstrated with no degradation over nearly 1000 hours.
Methods of Manufacturing an Etched Facet Laser
FIG. 22 is a simplified flowchart of a method 22 of forming a robust laser diode on an etched facet laser according to an embodiment of the present disclosure. Method 22 may include, at step 221, forming a facet with an etch process that minimizes damage and contamination. Method 22 may also include, at step 223, proper cleaning of any contamination which may have occurred between facet etching and facet coating. Method 22 may further include, at step 225, depositing a single or polycrystalline passivation layer on the etched facets, and, at step 227, depositing an optional AR or HR coating on the single or polycrystalline passivation layer. In an embodiment, the surface cleaning can be done on both facets simultaneously or it can be done on each side individually. In a similar manner, the facet coating can be performed on both facets at the same time or it can be performed at different times. The facet coatings may consist of multiple layers of different materials with different degrees of crystallinity. The facet cleaning can be performed before the formation of the first layer or in the middle of the formation of multiple layers on the etched facets. The exact process flow and resulting structures can vary depending on whether the final laser device is formed on the epitaxial substrate on which the laser diode is grown or deposited, or whether it is integrated onto a secondary substrate. In an embodiment, method 22 may also include forming an intermediate protective layer after forming the facet but prior to depositing the passivation layer to prevent exposure of the facet to intervening process steps.
FIG. 23A through 23C are cross-sectional views illustrating a process flow and resulting structures of a laser diode grown on an epitaxial substrate according to an embodiment of the present disclosure. In an embodiment, the laser diode has a total thickness of less than 10 μm. FIG. 23A is a cross-sectional view illustrating the formation of a facet by a low damage and low residue etch process. FIG. 23B is a cross-sectional view illustrating a proper surface clean prior to facet coating. FIG. 23C is a cross-sectional view illustrating a single-crystalline or polycrystalline facet coating step. Other coating steps may also be performed following the facet coating. The facet coating can include multiple layers for each facet and can be distinct between the front and back facets. One of ordinary skill in the art will appreciate that FIGS. 23A through 23C are not intended to show all steps necessary or possible for the formation of an etched facet laser diode, but rather to call out specific steps required to forming robust facets on an etched facet laser diode.
The key steps of forming a laser diode and robust facet coating can be done in many orders. One example would be an etched facet laser diode where the facet coatings are applied after the lasers have been separated into individual bars. This has the advantage that the bars can be stacked and coated with incident material depositing from a direction orthogonal to the plane of the facet. FIG. 24A is a simplified perspective view illustrating a wafer having a back-side contact in an example. FIG. 24B is a simplified perspective view illustrating the wafer of FIG. 24A that has been etched and separated into individual bars. FIG. 24C is a simplified perspective view illustrating a stacking of the individual bars where the facet coatings are then applied to the bars. An alternative process flow would form the facets by etching, then depositing the facet coating layers prior to singulation into laser bars. FIG. 25A is a simplified perspective view illustrating such process flow where the facets are etched in the wafer and facet coating layers are then deposited on the etched facets in an example. FIG. 25B is a simplified perspective view illustrating that the coated facets are singulated into individual bars in an example. This option eliminates highly serial laser bar processing steps with a more parallel wafer level approach, reducing manufacturing costs. Both of these options are enabled by the above methods for forming a highly reliable facet coating on an etched facet.
In the manufacture of edge emitting etched facet lasers, there can often be processing steps between facet formation and facet coating. These intervening process steps can potentially cause physical or chemical damage to the facet, or alternatively can deposit contamination layers. In order to preserve the high quality facet interface critical to the formation of robust poly or single crystalline facet coatings, it can be desirable to deposit a facet protect layer immediately after facet etch in some embodiments, and then remove this layer prior to the deposition of permanent facet coatings. This protect layer can be formed of SiO2, SiN, Al2O3, TiN, Ge, or any other material and can be deposited by many deposition methods such as RF sputtering, DC sputtering, PECVD, ALD, e-beam, ECR deposition, thermal deposition, etc.
Another desirable method to manufacture etched facet semiconductor laser diodes can include a full or partial passivation and/or HR and/or AR deposition or facet protect layer immediately after facet etch without exposing the freshly formed facet to atmosphere. This can be accomplished using either the same vacuum chamber for etch and deposition, or transferring via connected vacuum chambers to a separate deposition chamber.
Facet Coating on Transferred Epitaxial Layers
For some applications, it is preferable to form diode lasers using a process by which the active light generation and waveguide layers are transferred from a substrate on which they are grown to a second substrate for purposes of improving performance, reducing manufacturing cost, or other reasons. See, for example, U.S. Pat. Nos. 9,362,715; 9,368,939; 9,379,525; 9,520,695; and 9,666,677, which are incorporated herein by reference. In some embodiments, the transferred laser diode, the transferred light generation and waveguide layers, or the transferred gallium and nitrogen containing material have a total thickness of less than 10 μm. This transfer process presents special challenges for producing high performance, reliable semiconductor laser diodes. Some of these additional challenges can include: limits on the ability to cleave the active material, increased thermal resistance between a heat sink and facets, potential for contamination from the second substrate getting on the facet before or after coating, and/or other challenges. In order to mitigate the impact of these additional challenges, it is preferable in some embodiments to use the above facet coating methods to improve facet reliability on these types of laser diodes. In particular, the combination of transferred epitaxy semiconductor laser diodes with a low damage and low residue facet etch, followed by a crystalline or polycrystalline facet coating is desirable. This facet coating layer or layers can be deposited by any of the methods described above. It can also be beneficial in some embodiments to combine transferred epitaxial layer laser diodes with facet coating pre-deposition preparation methods which leave a contamination free and coherent crystal free surface. This cleaning or other preparation can be done by any of the methods described above. It can also be beneficial to combine transferred epitaxy laser diodes with etched facet processes which are optimized for leaving a coherent epitaxial free surface to provide the substrate for epitaxial growth of a subsequent facet coating layer. This facet etch process can be done by any of the methods described above.
FIG. 26A through 26C are cross-sectional views illustrating the key steps for forming a robust etched facet laser on a transferred epitaxy laser diode in an example. These key steps are the formation of a facet by a low damage and low residue etched facet process (FIG. 26A), proper surface cleaning prior to facet coating (FIG. 26B), such that the etched facet has a surface that is substantially free of contaminants, and a crystalline or polycrystalline facet coating step (FIG. 26C). This facet coating can include multiple layers for each facet and can be distinct between the front and back facets. The transfer process occurs prior to the beginning of this diagram shown in FIG. 26A. The laser diode is transferred to a gallium-free carrier wafer that may include one of Si, SiC, Al, AlN, diamond, or Cu. It is not the intention to show all steps necessary or possible to the formation of an etched facet laser diode, but rather to call out specific steps required to forming robust facets on an etched facet laser diode.
In an alternative process to the flow described in FIGS. 26A to 26C, the facets can be etched and either partial or full facet coatings can be applied prior to epitaxy transfer. This method can be particularly desirable when the cost of processing steps prior to epitaxy transfer is lower than the cost of process steps after epitaxy transfer.
Additionally, laser diodes made using an epitaxy transfer process step can be combined with the optional process steps described above. This includes the facet coating step taking place either before or after singulation of devices into bars, the inclusion of a facet protect layer to prevent intervening process steps from contaminating or damaging the facet, or the full or partial deposition of facet coating layers after facet etch while maintaining the wafers in vacuum to prevent contamination or damage to the facet surface.
Electron Cyclotron Resonance (ECR)
In an embodiment of the present disclosure, the film coating on the etched facet may be implemented with an ECR plasma sputtering system. Since the ECR source gently delivers sputtered particles and reactive gas ions in a circular motion in the stream of the plasma, ECR sputtering systems can provides superior uniformity and smoothness of films. This is of particular importance for etched facets, where surfaces can be rougher and contain additional defects. The inclusion of ECR coatings on etched facets can improve facet robustness and device lifetime. ECR systems can use inert gases such as Ar and Xe as sputtering gas. These gas ions will bombard with the target material and the sputtered materials are integrated with reactive gas ions such as N2, O2 or any mixture of those to form nitrides, oxides, and/or oxynitrides, respectively. With ECR sputtering, there is no limit on the kind of target material, but it is preferred to use a metal or metallic compound, such as Al, Si, and so on, whose nitride or oxide compound has little absorption at the operating wavelength of the laser devices.
Ion Assisted E-Beam Deposition (IAD) polycrystalline coating
Because of the necessity of a high quality passivation layer, many coating techniques have been developed including E-beam deposition (E-Beam), ion assisted E-Beam deposition (IAD), magnetron sputtering, ion beam deposition (IBD), and electron cyclotron resonance sputtering (ECR). Coating techniques such as IAD can produce a denser amorphous film than E-Beam but more advanced sputtering techniques are typically needed to produce a higher quality polycrystalline or even epitaxial quality film.
In an embodiment, the inventors operated an IAD system far from typical operating parameters in order to obtain a polycrystalline Al2O3 coating. This is not reported in scientific literature sources which state that IAD does not produce polycrystalline films and high temperature anneals must be done to crystallize the material. Typical deposition rates in E-Beam or IAD deposition are between 2 and 10 Å/s. The difference between the two is that IAD adds an ion source which helps densify the film and reduce point defects. Often, oxygen is a component of the ion beam and helps reduce oxygen vacancies in the film which can reduce optical absorption. The process described here uses high ion current density, high deposition temperature, and reduces the deposition rate to less than 1 Å/s which allows the Al2O3 film to crystallize during deposition. An increase in refractive index is observed as the film transitions from amorphous to partially polycrystalline and then to fully polycrystalline. FIG. 27A is a cross-sectional view illustrating IAD deposition of a passivation layer on a gallium and nitrogen containing substrate according to an embodiment of the present disclosure. FIGS. 27B to 27D are cross-sectional views illustrating the transformation from an amorphous passivation layer to a polycrystalline passivation layer. FIG. 27B is a cross-sectional view illustrating a mostly amorphous passivation layer 271b. FIG. 27C is a cross-sectional view illustrating a polycrystalline passivation layer 272a at the facet interface and amorphous passivation layer 272b on the polycrystalline layer 272a. FIG. 27D is a cross-sectional view illustrating a condition with a higher ion current density that has a fully turned polycrystalline passivation layer 273a. The technique could also be extended to epitaxial films under the right deposition conditions.
HR Facet Passivation
For most applications laser diodes typically require power to only be emitted from one side. This requires one laser facet to be highly reflective and the other facet to have a controlled reflectivity based on the laser diode design. Due to the high optical intensity, reflectors in LEDs or solar cells based on reflective metals such as silver cannot be used. Instead, the reflectivity must come from alternating high and low refractive index layers of transparent materials in structures called distributed Bragg reflectors. The low refractive index material used in these stacks is typically SiO2 due to ease of deposition and low absorption over a wide range of wavelengths. The high refractive index material used varies depending on the target wavelength but can include Ta2O5, ZrO2, or HfO2. These materials do not necessarily make good passivation layers so they can be combined with a different passivation material to give good reliability in some embodiments. FIGS. 28A and 28B are cross-sectional views illustrating a schematic of how a reflectivity modification layer can be applied according to some embodiments of the present disclosure.
FIG. 29A shows a TEM micrograph of an example HR coating with a first layer on a GaN facet after a life test. The interface is not stable and over time a layer of GaO is formed. FIG. 29B shows a passivation layer 291 has been added as an intermediate layer on the GaN surface in an example. The passivation layer does not mix with the GaN semiconductor layer or reflectivity modification layer. An additional use of the passivation layer could be to shift the electric field in the coating and at the interface so that the peaks or nulls of the field are at strong or weak points in the coating structure. These weak points could be the coating and semiconductor interface or in the coating stack itself.
FIG. 30A shows a calculation of the electric field distribution in the HR coating for a 20 nm passivation layer with a marker line at the coating semiconductor interface in an example. The position is defined in optical thickness units so a quarter wavelength layer represents 0.25 in position no matter what the physical thickness is. With a thin layer, the electric field has a peak at the coating/semiconductor interface. FIG. 30B shows that, by using a half lambda (wavelength) thickness coating, a null of the electric field can be generated at the coating interface. Depending on what layers are most absorbing or susceptible to high optical intensity, the nulls and peaks of the electric field can be shifted by changing the passivation layer thickness.
Facet Roughness
One challenge of etched facets is that they can have a higher surface roughness when compared to cleaved facets. Etched facets show different surface morphology and angle relative to cleaved facets. Cleaved facets show smoother surfaces and have angles that are defined primarily by crystal planes. The smoother surface of cleaved facets facilitates an easy deposition of passivation layers and optical coatings. Etched facets have striations and roughness so that it is difficult to deposit a passivation layer on the etched facets. In addition, etched facets may have a wider variation of angles in the range greater than 89 to 91 degrees, e.g., the sidewall angle of etched facets may have an angle in the range of 86 to 94 degrees or greater. The sidewall angle is defined as the angle inclined relative to the surface normal of the surface of the semiconductor (or the substrate). FIG. 31A shows a cross-sectional view of a scanning electron microscope (SEM) image of an etched facet. FIG. 31B shows a cross-sectional view of a SEM image of a cleaved facet. Referring to FIG. 31A, the etched facet shows a plurality of parallel striations arranged along a vertical direction, i.e., the direction substantial perpendicular to the lateral surface of the semiconductor. In contrast, the cleaved facet has a smooth surface, as shown in FIG. 31B. Furthermore, etching can leave surface damage so it is important to control the etch process to reduce damage as well as the facet cleaning process to reduce any residual damage. Some embodiments combine an etched facet cleaning process that leaves an RMS roughness ranging from 2 to 50 nm with a multitude of striations across the facet with a high quality passivation layer deposited by IAD or ECR which produces a COMD power threshold over 1 W/μm. Without passivation this could lead to more light scattering or a higher number of surface states causing lower laser lifetimes. This makes a high-quality passivation layer especially critical. One component of a passivation layer not typically discussed is that it can create a smoother surface after deposition than the semiconductor/passivation layer interface. An example is shown in FIG. 15A and FIG. 15B. This is especially true for epitaxial coatings which have the same crystal structure as the semiconductor. In this case, if the atoms are deposited in the correct lattice sites and the properties of the passivation layer are similar to the semiconductor there can be less discontinuity of refractive index and electronic states. This causes less light scattering at the interface which can also reduce optical absorption and heat generation. An example of this is AlN deposited on an InAlGaN based laser in which both have a wurtzite crystal structure and a high quality interface can be produced.
Thermodynamic Stability of Facet Coatings
One way that the facet coating can degrade over time is by mixing with the semiconductor itself. The high optical intensity and high temperatures of a laser facet can accelerate this intermixing and can happen when the enthalpy of formation of the mixture is lower than each component individually. An example of this is an SiO2 coating on a GaN facet. FIG. 32 shows a list of standard enthalpies of formation for SiO2, SiN, Ga2O3, GaN, Al2O3, and AlN. Because the sum of the enthalpies of formation are lower for a SiN/Ga2O3 system than a SiO2/GaN system, there is a thermodynamic drive towards the SiO2 reacting with the GaN to form SiN and Ga2O3. This reaction does not take place at any noticeable rate at room temperature, but under the high optical intensity and temperature of a laser the rate increases. FIG. 29A and FIG. 29B each show this effect in a life tested GaN laser where the Ga2O3 growth is evident under TEM. When the same analysis is done for an Al2O3/GaN interface, it is found that there is not a chemically driven intermixing. A similar device with a passivation layer is shown in FIG. 29B where the reaction is reduced or prevented.
Atomic Layer Deposition [ALD] Coating Process
Some embodiments apply wafer deposition methods including atomic layer deposition (ALD) and other deposition methods such as ECR or electron beam evaporation to form coatings for passivation, anti-reflectivity, and high-reflectivity purposes on laser facets on a wafer prior to bar singulation. In contrast to traditional laser facet coating methods where lasers are singulated into bars and loaded with facets perpendicular to a coating source, some embodiments of the present invention take advantage of facets etched on-wafer to deposit coatings prior to singulation using either angled source geometry in the case of electron beam evaporation or isotropic deposition in the case of ALD to conformally coat high-aspect vertical features. By coating on-wafer prior to singulation, more devices may be coated at once resulting in higher throughput and decreased cost.
Description of ALD Processes
Load and Clean
FIG. 33 shows a diagram of the process for facet coating by ALD according to an embodiment of the present disclosure. Referring to FIG. 33, in the case of ALD, the (a) wafer, (b) bar, or (c) die is placed into an ALD chamber and heated to raise surface temperature between 50 and 500° C. FIG. 34 shows a diagram of the process for facet coating by electron beam evaporation according to an embodiment of the present disclosure. Referring to FIG. 33 and FIG. 34, the wafer may be directly transferred from facet etch chamber under vacuum to prevent impurity contamination or loaded from atmosphere. The chamber is pumped to vacuum then optionally exposed to plasma consisting of argon, nitrogen, and/or oxygen gas to clean the surface of impurities.
Coating Deposition
In some embodiments, after pumping to vacuum, a chemical precursor is applied to the wafers carried by argon gas and chamber pressure is held above 40 mT for 0-10 seconds to complete the surface reaction. Chemical precursors include but are not limited to trimethylaluminium (TMA), tetrakis-dimethylamido titanium (TDMAT), tetrakis-ethylmethylamido hafnium (TEMAH), tetrakis-ethylmethylamido zirconium (TEMAZ), and tris-dimethylamino silane (TDMAS). The chamber may be purged and pumped to vacuum. Nitrogen, oxygen, and/or water vapor may be pumped into the chamber to form an oxide or nitride. In the case of nitrogen and/or oxygen gas, a plasma may be applied for 2-30 seconds. The chamber may then be pumped to vacuum and the chemical precursor and oxidation or nitridation steps may be repeated until desired coating thickness is achieved.
Coating Applications
There are many types of coatings that may be deposited by on-wafer facet coating. In one embodiment, an anti-reflective coating is formed using TMA precursor and oxygen plasma and/or water vapor to form a single amorphous aluminum oxide (Al2O3) layer. In another embodiment, a passivation layer is formed using TMA precursor and nitrogen plasma to form a continuous polycrystalline aluminum nitride (AlN) that may be capped by amorphous Al2O3 to seal the passivation layer from oxidation. FIGS. 35A to 35C show diagrams of on-wafer laser facet coating according to certain embodiments of the present disclosure. Referring to FIGS. 35A to 35C, a passivation layer of continuous polycrystalline material 100 may be deposited on a gallium nitride (GaN) 101 facet and capped by an amorphous reflectivity modification layer 102 to form an anti-reflective coating. In another embodiment, an anti-reflective coating is formed using TEMAH precursor and oxygen plasma and/or water vapor to form a hafnium oxide layer. In another embodiment, an anti-reflective coating is formed using TEMAZ precursor and oxygen plasma and/or water vapor to form a zirconium oxide layer. In another embodiment, alternating layers of high and low index oxides are applied to form a high-index coating.
There are many manufacturing processes that may be used with on-wafer coating. Referring to FIG. 35A, on-wafer coatings may be applied to cleaved facets. Referring to FIG. 35B, on-wafer coatings may be applied to etched facets. Referring to FIG. 35C, on-wafer coatings may be applied to facets etched on an epitaxial material transferred from one substrate to a carrier substrate. FIG. 36A is a TEM image of such an on-wafer coating in an example. Here the coating has been deposited on the etched facet of a transferred GaN laser by ALD. The passivation layer is roughly 25 nm thick and continuously polycrystalline from the GaN to the capping layer. Another layer roughly 95 nm thick protects the passivation layer from oxidation. The two layers combine to form an anti-reflective coating. FIG. 36B is an enlarged cross-sectional view of a portion of FIG. 36A.
FIGS. 37A to 37C show cross-sectional views of on-wafer coatings by ALD according to some embodiments of the present disclosure. Here the images show a similar coating to FIGS. 36A and 36B with conformal deposition both in the air gap below the transferred laser and on the masking material directly above the facet. FIG. 38 shows a high-resolution TEM image of the coating along with Fast-Fourier transform analysis of the AlN passivation layer and GaN surface showing polycrystalline AlN on top of the epitaxial GaN.
FIG. 39A is a SEM image showing a portion of an etched facet of a gallium and nitrogen containing cavity region. FIG. 39B is a plot of AFM measurements across an area of the etched facet shown in FIG. 39A. The area has an approximate an x-dimension of 1.2 μm and y-dimension of 1.2 μm. Surface roughness across the area is as high as about 52 nm. FIG. 39C is a three-dimensional view of an enlarged portion of a portion of the etched facet in FIG. 39A.
FIG. 40 is graph showing the measured surface roughness along the lines 1, 2, and 3 in FIG. 39C. The root mean square (RMS) roughness Rq is about 10 nm and the average roughness Ra is about 8 nm. In this example the surface roughness Rq varies across the portion of the etched facet by as much as about 50 nm or more.
In an embodiment, a GaN laser device may include a gallium and nitrogen containing surface having a crystalline lattice facet, a facet passivation layer on the crystalline surface, and one or more reflectivity modification layers on the facet passivation layer, as shown in FIG. 15A. In an embodiment, the one or more reflectivity modification layers may include a stack of quarter lambda layers having a high-index material and a low-index material, where lambda is a laser light wavelength. In an embodiment, the stack of quarter lambda layers includes two to twenty alternating layers of the high-index material and the low-index material with a high-index material layer as a top layer, as shown in FIG. 16A.
In an embodiment, the GaN laser device is grown on an epitaxy substrate (FIG. 23A) or bonded to a substrate (a carrier wafer) as shown in FIG. 26A.
Some embodiments of the present disclosure also provide methods of fabricating a gallium and nitrogen containing laser device. The methods may include providing a gallium and nitrogen containing surface by etching a lateral surface of the laser device in a direction perpendicular to a direction of light propagation of the laser device to obtain a crystalline-lattice facet, submitted the gallium and nitrogen containing surface to a surface cleaning process to obtain a cleaned surface, forming a passivation layer on the cleaned surface, and forming one or more reflectivity modification layers on the passivation layer. In an embodiment, etching the lateral surface includes a reactive ion etching (RIE) process, an inductively coupled plasma etch (ICP) process, an ion beam etch (IBE) process, a chemically assisted ion beam etch (CAIRE) process, a reactive ion beam etch (RIBE) process, or an electron cyclotron resonance (ECR) etch process.
In an embodiment, the gallium and nitrogen containing surface of the laser device being etched is formed from an InAlGaN material system, and the etching includes chlorine species for promoting a crystalline facet surface that is suitable for forming the passivation layer thereon.
In an embodiment, the surface cleaning process is performed using an ion beam deposition, ECR plasma sputtering, RF-magnetron sputtering, DC sputtering, thermal evaporation deposition, e-beam evaporation deposition with or without assistance from ion, PECVD, MOCVD, ALD, or MBE tool.
In an embodiment, forming the passivation layer comprises an ion beam deposition, ECR plasma sputtering, RF-magnetron sputtering, DC sputtering, thermal evaporation deposition, e-beam evaporation deposition with or without ion assistance, PECVD, MOCVD, MBE, ALD, or aqueous-solution-based deposition.
In an embodiment, the etching is performed at a temperature above 100 degrees C.
In an embodiment, the one or more reflectivity modification layers are formed after singulation of a wafer containing the laser diode into a plurality of bars. In another embodiment, the one or more reflectivity modification layers are formed before singulation of a wafer containing the laser diode into a plurality of bars.
In an embodiment, the one or more reflectivity modification layers are formed before the laser diode is bonded to a carrier wafer. In another embodiment, the one or more reflectivity modification layers are formed after the laser diode is bonded to a carrier wafer.
In an embodiment, the passivation layer comprises one material selected from the group consisting of AlN, AlON, SiO2, SiON, Ga2O3, GaN, Al2O3, ITO, and ZnO.
In an example, a gallium and nitrogen containing laser device configured on either a nonpolar or a semipolar surface orientation. The device has a gallium and nitrogen containing substrate member and a cladding region overlying the substrate member. In an example, the device has a cavity region formed overlying the cladding region and configured in alignment in substantially a c-direction or a projection of the c-direction. Preferably, a cavity region is characterized by a first end and a second end. In an example, the device has a first optical coating formed overlying the first facet, wherein the first coating overlying the first facet is configured to increase a reflectivity and a second optical coating formed overlying the second facet, wherein the second coating layer overlying the second facet is configured to reduce a reflectivity. The device has an optical power density characterizing the laser device, the laser device being substantially free from COMD related failure.
In an example, the nonpolar or semipolar surface orientation comprises an m-plane, a (30-31) plane, a (20-21) plane, a (30-32) plane, a (30-3-1) plane, a (20-2-1) plane, a (30-3-2) plane, or an offcut of within +/−5 degrees of any of these planes toward an a-direction or a c-direction; the cladding region being substantially free from Al-containing material, the cladding region being characterized by an AlN mol fraction in the cladding region of less than about 2%. In an example, the first optical coating is provided by a method selected from electron-beam deposition, thermal evaporation, PECVD, sputtering, and a combination of any of the foregoing. In other examples, the present invention also includes related methods reciting the same or similar elements.
In an example, the device comprises an output cavity width of greater than about 3 μm and less than about 25 μm, and is operable at over 1 W or wherein the device comprises an output cavity width of greater than about 3 μm and less than about 25 μm and is operable at over 2 W or wherein the device comprises an output cavity width of greater than about 3 μm and less than about 35 μm, and is operable at over 3 W or wherein the device comprises an output cavity width of greater than about 3 μm and less than about 35 μm, and is operable at over 4.5 W or wherein the device comprises an output cavity width of greater than about 3 μm and less than about 50 μm and is operable at over 3 W. In an example, the device is substantially free from COMD for power levels greater than 100 mW per micron of output cavity width, 200 mW per micron of output cavity width, or 400 mW per micron of output cavity width.
Finally, it should be noted that there are alternative ways of implementing the embodiments disclosed herein. Accordingly, the present embodiments are to be considered as illustrative and not restrictive. Furthermore, the claims are not to be limited to the details given herein, and are entitled their full scope and equivalents thereof.
As used herein, the term GaN substrate is associated with Group III-nitride based materials including GaN, InGaN, AlGaN, or other Group III containing alloys or compositions that are used as starting materials. Such starting materials include polar GaN substrates (i.e., substrate where the largest area surface is nominally an (h k 1) plane wherein h=k=0, and 1 is non-zero), non-polar GaN substrates (i.e., substrate material where the largest area surface is oriented at an angle ranging from about 80-100 degrees from the polar orientation described above toward an (h k l) plane wherein l=0, and at least one of h and k is non-zero) or semi-polar GaN substrates (i.e., substrate material where the largest area surface is oriented at an angle ranging from about +0.1 to 80 degrees or 110-179.9 degrees from the polar orientation described above toward an (h k l) plane wherein l=0, and at least one of h and k is non-zero).
As shown, the present device can be enclosed in a suitable package. Such package can include those such as in TO-38 and TO-56 headers. Other suitable package designs and methods can also exist, such as TO-9 or flat packs where fiber optic coupling is required and even non-standard packaging. In a specific embodiment, the present device can be implemented in a co-packaging configuration such as those described in U.S. application Ser. No. 12/789,303 filed on May 27, 2010, which is incorporated by reference in its entirety.
In other embodiments, the present laser device can be configured in a variety of applications. Such applications include laser displays, metrology, communications, health care and surgery, information technology, and others. As an example, the present laser device can be provided in a laser display such as those described in U.S. Ser. No. 12/789,303 filed on May 27, 2010, which claims priority to U.S. Provisional Application Nos. 61/182,105 filed on May 29, 2009, and 61/182,106 filed May 29, 2009, each of which is incorporated by reference in its entirety.
Merely by way of example, the embodiments described herein can be applied to applications such as white lighting by way of phosphor pumping or mixing wavelengths, white spot lighting, flash lights, automobile headlights, all-terrain vehicle lighting, flash sources such as camera flashes, light sources used in recreational sports such as biking, surfing, running, racing, boating, light sources used for drones, planes, robots, other mobile or robotic applications, safety, counter measures in defense applications, multi-colored lighting, lighting for flat panels, medical, metrology, beam projectors and other displays, high intensity lamps, spectroscopy, entertainment, theater, music, and concerts, analysis fraud detection and/or authenticating, tools, water treatment, laser dazzlers, targeting, communications, LiFi, visible light communications (VLC), sensing, detecting, distance detecting, Light Detection And Ranging (LIDAR), transformations, transportations, leveling, curing and other chemical treatments, heating, cutting and/or ablating, welding, pumping other optical devices, other optoelectronic devices and related applications, and source lighting and the like.
All of the methods and structures described herein can be applied to a super luminescent diode devices as well as laser diode devices.
While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. As an example, the packaged device can include any combination of elements described above, as well as outside of the present specification. As used herein, the term “substrate” can mean the bulk substrate (a growth substrate or a carrier substrate) or can include overlying growth structures such as a gallium and nitrogen containing epitaxial region, or functional regions such as n-type GaN, combinations, and the like. Additionally, the examples illustrates multiple waveguide structures in some configurations, there can be variations, e.g., other angles and polarizations. For semi-polar, the present method and structure includes a stripe oriented perpendicular to the c-axis, an in-plane polarized mode is not an Eigen-mode of the waveguide. The polarization rotates to elliptic (if the crystal angle is not exactly 45 degrees, in that special case the polarization would rotate but be linear, like in a half-wave plate). The polarization will of course not rotate toward the propagation direction, which has no interaction with the Al band. The length of the a-axis stripe determines which polarization comes out at the next mirror. Although the embodiments above have been described in terms of a laser diode, the methods and device structures can also be applied to any light emitting diode device. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.

## Claims

1. A laser device, comprising:
a substrate having a surface;
a gallium and nitrogen containing cavity region overlying the surface, the gallium and nitrogen containing cavity region characterized by a first end and a second end, the first end comprising a first etched facet and the second end comprising a second etched facet; and
a passivation layer comprising a polycrystalline layer of Al2O3 directly contacting the first etched facet, wherein an interface between the passivation layer and the first etched facet is substantially contaminant free.

2. The laser device of claim 1, wherein the passivation layer further comprises a single-crystalline film of Al2O3 on the polycrystalline layer of Al2O3.

3. The laser device of claim 1, wherein the first etched facet has a surface roughness that varies by greater than at least one of about 10 nm, 30 nm, or 50 nm.

4. The laser device of claim 1, further comprising one or more reflectivity modification layers on the passivation layer, the one or more reflectivity modification layers comprising a plurality of alternating layers of high refractive-index material and low refractive-index material.

5. The laser device of claim 1, further comprising two to twenty alternating layers of high refractive-index material and low refractive-index material on the passivation layer of Al2O3 with a top-most layer being a high refractive-index material layer.

6. The laser device of claim 1, further comprising two to twenty alternating layers of high refractive-index material and low refractive-index material on the passivation layer of Al2O3 with a top-most layer being a low refractive-index material layer.

7. The laser device of claim 1, wherein the laser device is on a gallium and nitrogen containing epitaxial substrate.

8. The laser device of claim 1, wherein the laser device is bonded to a gallium-free substrate.

9. An apparatus comprising the laser device of claim 1, wherein the apparatus is a lighting apparatus, an automotive apparatus, a display apparatus, a LIDAR apparatus, or a materials processing apparatus.

10. A laser device, comprising:
a gallium-free substrate comprising at least one of silicon (Si), silicon carbide (SiC), aluminum (Al), aluminum nitride (AlN), diamond, or copper (Cu);
a gallium and nitrogen containing material having a total thickness of less than 10 μm overlying the gallium-free substrate, the gallium and nitrogen containing material having a cavity region characterized by a first end and a second end, the first end comprising a first facet, and the second end comprising a second facet; and
a passivation layer comprising a polycrystalline layer of Al2O3 directly contacting the first facet.

11. The laser device of claim 10, wherein the first facet comprises a plurality of etched striations along a direction substantially perpendicular to the gallium-free substrate.

12. The laser device of claim 10, wherein the first facet has a root mean square surface roughness of about 10 μm or more.

13. The laser device of claim 10, wherein the first facet has a sidewall angle greater than 88 degrees to 92 degrees inclined relative to a surface normal of the gallium-free substrate.

14. The laser device of claim 10, further comprising an interface region between the gallium and nitrogen material and the gallium-free substrate.

15. The laser device of claim 10, wherein the first end comprises an etched surface substantially free of contaminants.

16. An apparatus comprising the laser device of claim 10, wherein the apparatus is a lighting apparatus, an automotive apparatus, a display apparatus, a LIDAR apparatus, or a materials processing apparatus.

17. A laser device, comprising:
a substrate having a surface;
a gallium and nitrogen containing cavity region overlying the surface, the gallium and nitrogen containing cavity region characterized by a first end and a second end, the first end comprising a first facet and the second end comprising a second facet; and
a passivation layer comprising a polycrystalline layer of Al2O3 directly contacting the first facet, wherein an interface between the passivation layer and the first facet is substantially contaminant free.

18. The laser device of claim 17, wherein the passivation layer further comprises a single-crystalline film of Al2O3 on the polycrystalline layer of Al2O3.

19. The laser device of claim 17, further comprising one or more reflectivity modification layers on the passivation layer, the one or more reflectivity modification layers comprising a plurality of alternating layers of high refractive-index material and low refractive-index material.

20. An apparatus comprising the laser device of claim 17, wherein the apparatus is a lighting apparatus, an automotive apparatus, a display apparatus, a LIDAR apparatus, or a materials processing apparatus.