# Patent

## Title
Optical device structure using non-polar GaN substrates and growth structures for laser applications in 481 nm

## Abstract

An optical device. The optical device comprises a GaN substrate having a non-polar surface region, an n-type GaN cladding layer, an n-type SCH layer comprised of InGaN, a multiple quantum-well active region comprised of five InGaN quantum well layers separated by four InGaN barrier layers, a p-type guide layer comprised of GaN, an electron blocking layer comprised of AlGaN, a p-type GaN cladding layer, and a p-type GaN contact layer.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to U.S. Ser. No. 61/177,317, filed May 12, 2009; which is commonly assigned and hereby incorporated by reference.


BACKGROUND OF THE INVENTION
This invention is directed to optical devices and related methods. More particularly, the invention provides a method and device for emitting electromagnetic radiation using non-polar gallium containing substrates, such as GaN, MN, InN, InGaN, AlGaN, and AlInGaN, and others. By way of example, the invention can be applied to optical devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices.
In the late 1800's, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years for a variety of applications, including lighting and displays. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC power or DC power source. Such light bulbs can be found commonly in houses, buildings, and outdoor lighting displays, as well as elsewhere.
Unfortunately, there are several disadvantages to the conventional Edison light bulb. First, the conventional light bulb dissipates considerable energy as thermal energy. More than 90% of the energy used for the conventional light bulb is lost as thermal energy. Secondly, reliability is a concern. The conventional light bulb routinely fails from thermal expansion and contraction of the filament element. Furthermore, light bulbs emit light over a broad spectrum, much of which does not result in bright illumination due to the spectral sensitivity of the human eye. Another disadvantage is that light bulbs emit light in all directions. Therefore they are not ideal for applications requiring directionality or focus, such as projection displays, optical data storage, or specialized directed lighting.
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 Argon ion laser. The Ar-ion laser utilized a noble gas as the active medium and produced 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%. In addition, the size, weight, and cost of the lasers were undesirable.
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, with 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  ̃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 are now widely 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 lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm light. The DPSS laser technology extended the life and improved the wall plug efficiency of the LPSS lasers to 5-10%. Further commercialization resulted in more high end specialty industrial, medical, and scientific applications. The change to diode pumping, however, increases the system cost and requires precise temperature control. The resulting laser is large, consumes substantial power, yet does not address the energy storage properties which make 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 provide 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 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 modulated, they suffer from sensitivity to temperature, limiting their application.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, techniques related generally to optical devices are provided. More particularly, the present invention provides a method and device for emitting electromagnetic radiation using non-polar gallium containing substrates, such as GaN, MN, InN, InGaN, AlGaN, and AlInGaN, and others. The invention can be applied to optical devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices.
In a specific embodiment, the present invention provides an optical device. The optical device has a gallium nitride substrate member having a non-polar crystalline surface region. The device has an n-type GaN cladding layer overlying the surface region. The n-type GaN cladding layer preferably has a thickness from 100 nm to 3000 nm, with a silicon doping level of 5E17 to 3E18 cm-3. In a specific embodiment, the device has an n-side SCH layer overlying the n-type GaN cladding layer. Preferably, the n-side SCH layer is comprised of InGaN with a molar fraction of indium of between 4% and 6% and has a thickness from 45 to 60 nm. The device also has a multiple quantum well active region overlying the n-side SCH layer. In a preferred embodiment, the multiple quantum well active region is comprised of five 4.5-5.5 nm InGaN quantum wells separated by four 9.5-10.5 nm InGaN barriers with an indium molar fraction of between 1.5% and 3%. The optical device also has a p-side guide layer overlying the multiple quantum well active region. The p-side guide layer is comprised of GaN with molar a thickness from 10 nm to 20 nm. The device also has an electron blocking layer overlying the p-side guide layer. In a preferred embodiment, the p-side guide layer is comprised of AlGaN with molar fraction of aluminum of between 15% and 22% and has a thickness from 10 nm to 15 nm and doped with magnesium. The device further has a p-GaN cladding layer overlying the electron blocking layer. The p-GaN cladding layer has a thickness from 400 nm to 1000 nm with Mg doping level of 5E17 cm-3 to 1E19 cm-3. In a specific embodiment, the device has a p++-GaN contact layer overlying the p-GaN cladding layer. The p++-gaN contact layer has a thickness from 20 nm to 40 nm with a magnesium doping level of 1E20 cm-3 to 1E21 cm-3.
The present invention enables a cost-effective optical device for laser applications. In a specific embodiment, the 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. The present laser device uses a non-polar gallium nitride material capable of achieving a 481 nm light. These and other benefits may be described throughout the present specification and more particularly below by reference to the specification and attached drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified perspective view of a laser device fabricated on a non-polar substrate according to an embodiment of the present invention.
FIG. 2 is a detailed cross-sectional view of a laser device fabricated on a non-polar substrate according to an embodiment of the present invention.
FIGS. 3A and 3B are simplified diagrams illustrating a laser device for a laser device according to an embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, techniques related generally to optical devices are provided. More particularly, the present invention provides a method and device for emitting electromagnetic radiation using non-polar gallium containing substrates such as GaN, MN, InN, InGaN, AlGaN, and AlInGaN, and others. Merely by way of example, the invention can be applied to optical devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices. In a specific embodiment, the present laser device can be employed in a non-polar gallium containing substrate, as described below.
FIG. 1 is a simplified perspective view of a laser device fabricated on a non-polar substrate according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. As shown, the optical device includes a gallium nitride substrate member having a non-polar crystalline surface region. In a specific embodiment, the gallium nitride substrate member is a bulk GaN substrate characterized by having a non-polar crystalline surface region, but can be others. In a specific embodiment, the bulk nitride GaN substrate comprises nitrogen and has a surface dislocation density below 105 cm−2. 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.
In a specific embodiment on non-polar GaN, the device has a laser stripe region formed overlying a portion of the non-polar crystalline orientation surface region. In a specific embodiment, the laser stripe region is characterized by a cavity orientation is substantially parallel to the c-direction. In a specific embodiment, the laser strip region has a first end and a second end. In a specific embodiment, the non-polar crystalline orientation is configured on an m-plane, which leads to polarization ratios parallel to the a-direction. In one or more embodiments, the m-plane is the (10-10) family. Of course, there cavity orientation can also be substantially parallel to the a-direction as well. In the specific non-polar GaN embodiment having the cavity orientation substantially parallel to the c-direction is further described in “Laser Device and Method Using Slightly Miscut Non-Polar GaN Substrates,” in the names of Raring, James W. and Pfister, Nick listed as U.S. Provisional Ser. No. 61/168,926 filed Apr. 13, 2009, commonly assigned, and hereby incorporated by reference for all purposes.
In a preferred non-polar embodiment, the device has a first cleaved c-face facet provided on the first end of the laser stripe region and a second cleaved c-face facet provided on the second end of the laser stripe region. In one or more embodiments, the first cleaved c-facet is substantially parallel with the second cleaved c-facet. In a specific embodiment, the non-polar substrate is configured on (10-10) series of planes, which enables the formation of c-facets for laser cavities oriented in the c-direction. Mirror surfaces are formed on each of the cleaved surfaces. The first cleaved c-facet comprises a first mirror surface. In a preferred embodiment, the first mirror surface is provided by a scribing and breaking process. The scribing process can use any suitable techniques, such as a diamond scribe or laser scribe or combinations. In a specific embodiment, the first mirror surface comprises a reflective coating. The reflective coating is selected from silicon dioxide, hafnia, and titaniatantalum pentoxidezirconia, including combinations, and the like. Depending upon the embodiment, the first mirror surface can also comprise an anti-reflective coating. Of course, there can be other variations, modifications, and alternatives.
Also in a preferred non-polar embodiment, the second cleaved c-facet comprises a second mirror surface. The second mirror surface is provided by a scribing and breaking process according to a specific embodiment. Preferably, the scribing is diamond scribed or laser scribed or the like. In a specific embodiment, the second mirror surface comprises a reflective coating, such as silicon dioxide, hafnia, and titaniatantalum pentoxidezirconia, combinations, and the like. In a specific embodiment, the second mirror surface comprises an anti-reflective 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 50 microns to about 3000 microns. The strip also has a width ranging from about 0.5 microns to about 50 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. Further details of the present device can be found throughout the present specification and more particularly below
In a specific non-polar embodiment, the device is also characterized by a spontaneously emitted light that is polarized parallel to 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 about 0.1 and less than about 1 parallel to the projection of the c-direction. 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 is a detailed cross-sectional view of a laser device 200 fabricated on a non-polar substrate according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. As shown, the laser device includes gallium nitride substrate 203, which has an underlying n-type metal back contact region 201. 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 205, an active region 207, and an overlying p-type gallium nitride layer structured as a laser stripe region 209. 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). 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 1000 and about 1200 degrees Celsius in the presence of a nitrogen-containing gas. 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 (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 209. 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 213 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 215. The overlying metal layer is a multilayered structure containing gold and platinum (Pt/Au), but can be others. 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. 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 40 nm. 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 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. 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-4 N, 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 1-50 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.
EMBODIMENT
In a specific embodiment, the present invention provides an alternative device structure capable of emitting 481 nm light, among others, in a ridge laser embodiment. The device is provided with one or more of the following elements, as also referenced in FIGS. 3A and 3B.
n-GaN cladding layer with a thickness from 100 nm to 3000 nm with Si doping level of 5E17 to 3E18 cm-3
n-side SCH layer comprised of InGaN with molar fraction of indium of between 4% and 6% and thickness from 45 to 60 nm
Multiple quantum well active region layers comprised of five 4.5-5.5 nm InGaN quantum wells separated by four 9.5-10.5 nm InGaN barriers with an indium molar fraction of between 1.5% and 3%
p-side guide layer comprised of GaN with molar a thickness from 10 nm to 20 nm.
Electron blocking layer comprised of AlGaN with molar fraction of aluminum of between 15% and 22% and thickness from 10 nm to 15 nm and doped with Mg.
p-GaN cladding layer with a thickness from 400 nm to 1000 nm with Mg doping level of 5E17 cm-3 to 1E19 cm-3
p++-GaN contact layer with a thickness from 20 nm to 40 nm with Mg doping level of 1E20 cm-3 to 1E21 cm-3
In a specific embodiment, the laser device is fabricated on a non-polar oriented surface region (m-plane). In a preferred embodiment, the non-polar substrate has a miscut or off cut of −0.8 to −1.2 degrees towards (0001) and −0.3 to 0.3 degrees towards (11-20). In a specific embodiment, the non-polar oriented surface region has an overlying n-GaN/p-GaN grown with H2/N2 subflow and H2 carrier gas. Of course, there can be other variations, modifications, and alternatives.
Although the above has been described in terms of a specific embodiment, other variations, modifications, and alternatives can exist. While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. 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. An optical device comprising:
a gallium nitride substrate having a non-polar crystalline surface region;
an n-type GaN cladding layer overlying the surface region, the n-type GaN cladding layer having a thickness from 100 nm to 3000 nm with a silicon doping level of 5E17 cm-3 to 3E18 cm-3;
an n-type SCH layer overlying the n-type GaN cladding layer, the n-type SCH layer comprised of InGaN with a molar fraction of indium of between 4% and 6% and having a thickness from 45 nm to 60 nm;
a multiple quantum well active region overlying the n-type SCH layer, the multiple quantum well active region comprised of five 4.5 nm-5.5 nm-thick InGaN quantum wells separated by four 9.5 nm-10.5 nm-thick InGaN barriers with an indium molar fraction of between 1.5% and 3%;
a p-type guide layer overlying the multiple quantum well active region, the p-type guide layer comprised of GaN with a thickness from 10 nm to 20 nm;
an electron blocking layer overlying the p-type guide layer, the electron blocking layer comprised of AlGaN with a molar fraction of aluminum of between 15% and 22% and having a thickness from 10 nm to 15 nm and doped with magnesium;
a p-type GaN cladding layer overlying the electron blocking layer, the p-type GaN cladding layer having a thickness from 400 nm to 1000 nm with a Mg doping level of 5E17 cm-3 to 1E19 cm-3; and
a p-type GaN contact layer overlying the p-type GaN cladding layer, the p-type GaN contact layer having a thickness from 20 nm to 40 nm with a magnesium doping level of 1E20 cm-3 to 1E21 cm-3.