Structure and method for index-guided inner stripe laser diode structure

Abstract: An inner stripe laser diode structure (100) for GaN laser diodes is disclosed. Inner stripe laser diode structures provide a convenient means of achieving low threshold, single mode laser diodes. The structure of an inner stripe laser diode is modified to produce lateral index guiding.

Description: The invention relates to index guided, inner stripe laser diode structures. Inner stripe laser diode structures have been commonly used in both red AlGaInP and infrared AlGaAs laser diodes. Inner stripe laser diode structures provide a convenient means of achieving low threshold, single mode laser diodes. An inner stripe laser diode structure requires two epitaxial growth steps. The first epitaxial growth step typically involves growth of the lower cladding layer, the active region, a portion of the upper cladding layer and an n-type blocking layer. Following the etching away of the n-blocking layer in a narrow stripe, the remaining portion of the n-cladding layer is grown. In operation, the injection current path is defined by the etched stripe opening in the n-blocking layer, even though the p-metal contact pad may be significantly wider than the stripe. The current-blocking layer is placed close to the active region, typically being at about a 100 - 200 nm separation from the boundary of the active region into the upper-cladding layer. Due to the relative ease of creating a 1 - 2 µm wide stripe in the n-blocking layer, very low threshold current lasers can be fabricated. It is much easier to form a 2 µm wide stripe in this self-aligned structure in comparison with, for example, the similarly narrow ridge waveguide laser because a ridge waveguide structure requires that a narrow contact stripe, typically 1 -1.5 µm, be carefully aligned on the top of the ridge structure. Very narrow inner stripe laser diodes offer improved heat dissipation because lateral heat spreading is enhanced as the width of the laser stripe is reduced. Hence, when a new semiconductor laser material system is developed, the inner stripe structure is often the first structure used to achieve a single mode laser. Although the inner stripe laser diode structure is advantageous to achieving low threshold, single mode operation, the resulting beam quality is relatively poor and unsuitable for many important applications. The beam quality is relatively poor because no lateral positive index guiding is provided by the inner stripe laser diode structure. Instead, a highly astigmatic beam is generated because the inner laser stripe structure is gain-guided. While astigmatism is correctable using cylindrical optics, the lateral beam divergence and astigmatism may vary with drive current. Nitride inner stripe laser diodes with a current blocking layer are disclosed in U.S. Patent No. 5,974,069 by Tanaka et al. Tanaka et al. disclose current blocking layers made from materials including those selected from a group consisting of Al y Ga 1-x N (0 < y = 1), SiO 2 , Si 3 N 4 and Al 2 O 3 . FIG. 1a shows the lateral index step as a function of the thickness of first upper cladding layer 5 for two cladding layer laser diode structure 11 shown in FIG. 1b which is similar to that of Tanaka et al. Note that Tanaka et al. disclose a first upper cladding layer with a thickness of at least 100 nm which limits the lateral index, Δn, to no greater than 4 x 10 -3 . FIG. 1b shows active region 4 beneath first upper cladding layer 5 which supports current blocking layer 6 covered by second upper cladding layer 7. Curve 50 shows that as the thickness of first upper cladding layer 5 increases for two cladding layer structure 11, the lateral index step (and the resulting optical confinement) drops from an initial lateral index step of about 11 x 10 -3 at zero thickness. EP-A-0788203 discloses an As-based laser diode structure having a GaAs quantum well structure and an AlGaAs current blocking layer inside a waveguide layer. The current blocking layers have a lower refractive index and higher Al-composition than that of the waveguide layer and are so formed as to sandwich therebetween a stripe-shaped active region. In such a structure the lattice constants for GaAs and AlGaAs are almost identical so an AlGaAs current blocking layer can be formed on top of a GaAs quantum well structure without experiencing lattice mismatch. According to a first aspect of the present invention there is provided an inner stripe laser diode structure comprising:- an active multiple quantum well layer; a waveguide layer having a lower layer and an upper layer, the lower layer defining a surface contacting said active multiple quantum well layer; a current blocking layer divided into two parts by a stripe region and sandwiched between facing surfaces of the upper and lower layers of the waveguide layer; and a cladding layer formed over waveguide layer such that a portion of the waveguide layer is located between the cladding layer and the current blocking layer; characterised in that the active multiple quantum well layer is comprised of nitrogen; and the current blocking layer comprises a short period superlattice structure. According to a second aspect of the present invention there is provided a method for making an inner-stripe laser diode structure comprising:- providing an active multiple quantum well layer comprising nitride; providing a wave guide layer having a first and second surface, said first surface of said wave guide layer contacting said multiple quantum well layer, wherein providing the wave guide layer includes growing a first portion of the wave guide layer; providing a current blocking layer penetrating into said second surface of said wave guide layer, said current blocking layer comprising a short period superlattice, wherein providing the current blocking layer includes forming the two parts of said current blocking layer, and then regrowing a second portion of the wave guide layer; and growing a cladding layer on the second portion of the wave guide layer. The structure of an inner stripe laser may be modified to produce lateral index guiding and provide the beam quality necessary for printing and optical storage. The modified inner stripe laser structure allows for excellent beam quality and modal discrimination while retaining the benefits of the basic inner stripe laser structure. The modified inner stripe structure is applicable to nitride laser diode structures which are relatively insensitive to regrowth interfaces close to the active region. The modified inner stripe laser structure involves an epitaxial growth of a conventional inner stripe laser structure including a partial upper waveguide layer. A current blocking layer is then grown on the partial upper waveguide layer with a narrow stripe subsequently opened in the blocking layer. Following definition of the narrow stripe, an epitaxial regrowth is performed to complete the upper waveguide layer along with the cladding and contact layers. Finally, the structure is processed in the standard manner, including contact metallization and mirror formation. Because regrowth for the modified inner stripe laser structure starts with the upper waveguide layer instead of proceeding directly to regrowth of the upper cladding layer, a positive lateral index guide may be created. By starting regrowth with the upper waveguide, the thickness of the waveguide in the narrow stripe region is made greater than the thickness of the wing regions of the waveguide. The wing regions of the upper waveguide are the waveguide regions beneath the blocking layer next to the active region. The difference in thickness functions to produce a lateral index step. The strength of the lateral index waveguiding depends in part on how close the blocking layer is placed to the active region with closer placement providing better lateral index guiding. The current blocking layer may be n-type or insulating material and materials that may be used for the current blocking layer include AlGaN, AlN, SiO 2 , SiON and Si 3 N 4 . Typical values of the lateral index step obtained in accordance with the invention are about 20 x 10 -3 . Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1a shows the lateral index step as a function of first upper cladding layer thickness in the prior art. FIG. 1b shows a laser diode structure in the prior art. FIGs. 2-4a show steps in the fabrication procedure for an index-guided inner stripe nitride laser diode structure. FIG. 4b shows a short period superlattice structure for use as a current blocking layer in accordance with the invention. FIG. 5 shows a laser diode structure. FIG. 6 shows the calculated tranverse effective index and optical confinement factor for an index guided inner stripe nitride laser diode structure. FIG. 7 shows the relationship between blocking layer thickness and lateral index step for embodiments in accordance with the invention. FIG. 8 shows the layer thickness required for a current blocking layer of a given refractive index to obtain a specific lateral index step in accordance with an embodiment of the invention. FIG. 9 shows the lateral optical confinement factor for 1 µm and 2 µm wide lasers as a function of lateral index step. An index-guided inner stripe laser diode structure is realizable if the regrowth interface can be placed close to the active region which is possible if the regrowth interface is relatively benign. In general, the regrowth interface is often structurally defective or chemically contaminated. However, material systems such as nitrides that may be used for laser diodes are relatively insensitive to regrowth interfaces near the active region and this insensitivity allows epitaxial regrowth to be performed within a waveguide of the laser diode structure. Regrowth allows the incorporation of a current blocking layer which has a low refractive index (typically less than n ~ 2.5) to enhance optical confinement. The current blocking layer material is selected to be thermally stable to avoid decomposition in a metalorganic chemical vapor deposition (MOCVD) environment which typically has temperatures in excess of 900 °C and a reactive ambient. The current blocking layer may be insulating or semiconductor material, in the latter case a current blocking reverse bias junction is created to confine current flow to the active region by doping the current blocking layer. Semiconductor current blocking layers have a relatively high refractive index which limits the index step available for optical confinement. Advantages of semiconductor current blocking layers include thermal stability and straightforward regrowth. Conformal growth results in uniform surface coating whereas selective growth results in crystal nucleation only in select areas. Regrowth of semiconductor material performed at a temperature of about 900 °C is conformal; but for GaN growth no deposition occurs on Si 3 N 4 , SiO 2 or similar insulating material, i.e., the growth is selective. However, for AlGaN growth, polycrystalline material with rough surfaces is deposited on Si 3 N 4 , SiO 2 or similar insulating material. FIGs. 2- 5 show the fabrication procedure for index-guided inner stripe nitride laser structure 100. Standard epitaxial growth for nitride materials (see, for example, U.S. Pat. Application, Ser. No. 09/288,879, by Van de Walle et al. ) is performed on Al 2 O 3 substrate 110 to grow GaN:Si layer 120, AlGaN:Si cladding layer 130, GaN:Si waveguide 140, InGaN multiple active quantum well region 150, lower portion 160a, typically 50 nm thick but may be less to create a larger index step, of GaN:Mg waveguide 160 and n-type blocking 170 since n-type semiconducting material is used for current blocking layer 170. CAIBE (chemically assisted ion beam etching) etching typically involves using an argon ion beam and a Cl 2 /BCl 3 gas mixture to supply the reactive gas species. CAIBE etching of narrow stripe window 250 through n-type blocking layer 170 shown in FIG. 3 is performed to expose underlying GaN waveguide layer 160a after narrow stripe window 250 has been photolithographically defined. Typical widths for stripe window 250 are from about 1 -5 µm. Photoresist is stripped and acid cleaning is performed on partial laser diode structure 100 as shown in FIG. 3 prior to resumption of MOCVD growth. FIG. 4a shows epitaxial regrowth of upper portion 160b of GaN:Mg waveguide layer 160, AlGaN:Mg p-cladding layer 180 (typically 0.5 - 1 µm thickness) and GaN:Mg p-contact layer 190 (typically about 0.1 µm thickness). Note that the regrowth resumes with the addition of upper portion 160b of GaN:Mg waveguide instead of proceeding directly to growth of AlGaN:Mg p-cladding layer 180 so that the thickness of GaN:Mg waveguide 160 in stripe region 250 is thicker than layer 160a. To achieve a rapid, controlled initiation of Mg p-doping, care should be taken to flow the magnesium precursor into the MOCVD growth reactor before heatup to avoid doping turn on delay due to (C 5 H 5 ) 2 Mg affinity of the steel gas lines. Epitaxial overgrowth of AlGaN blocking layer 170 may be made difficult by blocking layer 170's native oxide. Overgrowth may be facilitated by capping AlGaN current blocking layer 170 with a thin n-GaN layer (not shown) or by grading the aluminum content of AlGaN blocking layer 170 down to GaN or a lower aluminum content alloy. AlGaN current blocking layer 170 presents a tradeoff associated with the aluminum content. A high aluminum composition (typically above 20%) is desirable for optimal lateral index guiding but high aluminum alloys are more prone to cracking as AlGaN current blocking layer 170 is thickened. Hence, AlGaN current blocking layer 170 is chosen to be thinner as the aluminum content is increased. Typically, as the aluminum content of a layer is increased by about a factor of two, the layer thickness needs to be reduced by about a factor of two to prevent cracking of the layer. In accordance with the invention, AlGaN current blocking layer 170 is replaced by AlGaN/GaN short period superlattice layer 175 shown in FIG. 4b having a period between 0.5 nm to 100 nm [5 Å to 1000 Å], with a typical superlattice period of 5 nm [50 Å] (2.5 nm AlGaN/2.5 nm GaN) [(25 Å AlGaN/25 Å Ga N)]. Growth conditions are similar to those for bulk AlGaN:Si layers. Aluminum content in AlGaN/GaN short period superlattice layer 175 may be varied in the range between 0 percent to 100 percent with a typical range between 40 percent to 100 percent with the average aluminum content in AlGaN/GaN short period superlattice 175 typically ranging from 20 - 50 percent. Use of the AlGaN/GaN short period superlattice allows increased current blocking layer thickness or increased average aluminum content, typically a factor of two, before onset of layer cracking. AlGaN/GaN short period superlattice layer 175 is typically doped with silicon throughout layer 175. Alternatively, AlGaN/GaN short period superlattice layer 175 may be doped only in GaN layers 182, 184, 186 ... 188 or AlGaN layers 181, 183, 185 ... 187 or not doped. To maximize barrier height at the interface to GaN:Mg layer 160b, short period superlattice 175 is typically capped with AlGaN:Si layer 181. The thickness of short period superlattice 175 can be non-destructively and very accurately measured by x-ray diffraction. Precise knowledge of current blocking layer 175 is important for the chemically assisted ion beam etch (CAIBE) step defining window 250 in current blocking layer 175 to avoid etching through GaN:Mg layer 160a into multiple quantum well region 150. FIG. 5 shows metallization for p-contact 200 and n-contact 210. Palladium p-metal for p-contact 200 is alloyed at about 535°C for about 5 minutes in an N 2 ambient. The first mirror is photolithographically defined and etched using CAIBE. The deposited palladium p-metal layer is first chemically etched. CAIBE etching is performed to a depth of about 2 µm to penetrate into GaN:Si layer 120 under AlGaN:Si cladding layer 130. Hence, the CAIBE etching exposes the area for n-contact 210. The second mirror is similarly etched. Liftoff metallization, typically Ti-Al, is performed for n-contact 210. Subsequently, Ti-Au metallization builds up metal thickness on p-contact 200 and n-contact 210. The first and second mirrors are coated with SiO 2 /TiO 2 using an evaporative process. FIG. 6 shows effective transverse refractive index curve and optical confinement factor with curve 520 and curve 540, respectively, as a function of the thickness, t, of GaN:Mg waveguide 160 for an embodiment of a nitride laser diode structure 100. Al 0.07 Ga 0.93 N cladding layers 180 and 130 have a refractive index of 2.46 at 400 nm wavelength, 100 nm GaN waveguide 140 has a refractive index of 2.51 and multiple quantum well active region 150 has four 3.5 nm (35 Å) In 0.15 Ga 0.85 N quantum wells with a refractive index of 2.56 that are separated by 6.5 nm (65 Å) In 0.03 Ga 0.97 N barriers with a refractive index of 2.52. From the calculated transverse effective indices, a lateral index profile may be obtained for index-guided inner stripe nitride laser structure 100. N-blocking layer 170 is formed of Al 0.07 Ga 0.93 N which is the same alloy used in cladding layers 180 and 130. Higher aluminum content blocking layers may be used to produce larger index steps if cracking or overgrowth problems are avoided or a short period superlattice 175 in accordance with the present invention may be used instead. For the case where n-blocking layer 170 is grown 50nm above multiple quantum well active region 150, 100 nm of GaN:Mg is deposited after photolithographically defining narrow stripe window stripe 250 to complete waveguide layer 160 followed by a typical Al 0.07 Ga 0.93 N cladding layer having a thickness from about 0.4-0.5 µm and GaN:Mg capping layer 190. With reference also to FIG. 5, complete GaN waveguide 160 is about 100 nm thicker in the stripe region of waveguide layer 160 in comparison to the thickness of partial waveguide layer 160a. The resulting transverse effective refractive indices are shown in table 1 below: Region GaN:Mg waveguide 160 thickness transverse effective refractive index stripe region of layer 160 150nm 2.488 partial waveguide layer 160a 50nm 2.479 ΔN - 0.009 The transverse effective refractive index step, Δn, of 0.009 is greater than the refractive index step achieved in a conventional nitride ridge waveguide laser structure while allowing much easier fabrication of a narrow stripe structure. Furthermore, the optical confinement factor is not compromised since waveguide 160 thickness of 150 nm in the stripe region of layer 160 produces an optical confinement factor that is very nearly maximized (see FIG. 6). FIG. 7 shows simulated lateral index step, Δn, as a function of current blocking layer 170 or 670 thickness assuming current blocking layer 170 or 670 is positioned 50 nm from multiple quantum well region 150. Curve 810 shows lateral index step Δn as a function of SiO 2 current blocking layer 670 thickness assuming a typical SiO 2 refractive index of 1.5. Curve 820 shows lateral index step Δn as a function of SiON current blocking layer 670 thickness assuming a typical SiON refractive index of 1.8. Curve 830 shows lateral index step Δn as a function of Si 3 N 4 current blocking layer 670 thickness assuming a typical Si 3 N 4 refractive index of 2.0. Curve 840 shows lateral index step Δn as a function of AlN current blocking layer 170 thickness assuming a typical AlN refractive index of 2.1. Curve 850 shows lateral index step Δn as a function of Al 0.5 Ga 0.5 N current blocking layer 170 thickness assuming a typical Al 0.5 Ga 0.5 N refractive index of 2.3. Curve 860 shows lateral index step Δn as a function of Al 0.2 Ga 0.8 N current blocking layer 170 thickness assuming a typical Al 0.2 Ga 0.8 N refractive index of 2.4. Note that the index step for curves 810-840 plateaus for blocking layer thicknesses below about 0.08 µm while the index step for curve 850 requires a thickness of about 0.1 µm before achieving a plateau and the index step for curve 860 increases with increasing thickness beyond a 0.1 µm thickness. FIG. 8 shows the layer thickness required for a current blocking layer 170 or 670 of a given refractive index to obtain lateral index step 910 of 5 x 10 -3 , lateral index step 920 of 10 x 10 -3 , lateral index step 930 of 15 x 10 -3 and lateral index step 940 of 20 x 10 -3 . If short period superlattice structure 175 is used for current blocking, the thickness will be the same as for a current blocking layer 170 of the same aluminum content. The refractive indices of several representative materials (SiO 2 , SiON, Si 3 N 4 , AlN, Al 0.5 Ga 0.5 N and Al 0.2 Ga 0.8 N) are denoted on the horizontal axis. It is apparent from FIG. 8 that as the refractive index of the material used for blocking layer 170 or 670 increases, the thickness of the blocking layer 170 or 670 must be increased to maintain the same lateral index step. For lateral index step 940 of 20 x 10 -3 , the thickness of the blocking layer 170 or 670 is required to be thicker than 100 nm for representative materials other than SiO 2 . FIG. 9 shows the lateral confinement factor in percent, G lateral , versus the lateral index step for a 2 µm wide stripe laser plotted as curve 1010 and a 1 µm wide stripe laser plotted as curve 1020. FIG. 9 shows that improvement of the lateral confinement factor, G lateral , is marginal for a 2 µm wide stripe laser with a lateral index step greater than about 10 x 10 -3 and for a 1 µm wide stripe laser with a lateral index step greater than about 20 x 10 -3 .

Claims: An inner stripe laser diode structure (100) comprising:- an active multiple quantum well layer (150); a waveguide layer (160) having a lower layer (160a) and an upper layer (160b), the lower layer (160a) defining a surface contacting said active multiple quantum well layer (150); a current blocking layer (175) divided into two parts by a stripe region and sandwiched between facing surfaces of the upper and lower layers (160a, 160b) of the waveguide layer (160); and a cladding layer (180) formed over waveguide layer such that a portion of the waveguide layer (160) is located between the cladding layer (180) and the current blocking layer (175); characterised in that the active multiple quantum well layer (150) is comprised of nitrogen; and the current blocking layer (175) comprises a short period superlattice structure (175). The structure of Claim 1, wherein said short period superlattice is silicon doped. The structure of Claim 1 or 2, wherein the short period superlattice has a period in the range 0.5 nm to 100 nm (5 Å to 1000 Å). The structure of Claim 1, 2 or 3, wherein said short period superlattice has a period of 5 nm (50 angstroms). The structure of Claim 1, 2, 3 or 4, wherein said short period superlattice is comprised of AlGaN/GaN. The structure of Claim 5 wherein said short period superlattice has an average aluminum percent composition selected from the range of 20 percent to 50 percent. The structure of any preceding Claim wherein a thickness of a first part (160a) of the waveguide layer located in the stripe region between the two parts of the current blocking layer (175) is greater than a thickness of a second part (160b) of the waveguide layer located between one part of the current blocking layer (175) and the multiple quantum well layer (150). A method for making an inner-stripe laser diode structure comprising:- providing an active multiple quantum well layer comprising nitride; providing a waveguide layer having a first and second surface, said first surface of said waveguide layer contacting said multiple quantum well layer, wherein providing the waveguide layer includes growing a first portion of the waveguide layer; providing a current blocking layer penetrating into said second surface of said waveguide layer, said current blocking layer comprising a short period superlattice, wherein providing the current blocking layer includes forming the two parts of said current blocking layer, and then regrowing a second portion of the waveguide layer; and growing a cladding layer on the second portion of the waveguide layer. The method of Claim 8 wherein said short period superlattice is silicon doped. The method of Claim 8 or 9 wherein the short period superlattice has a period in the range 0.5 nm to 100 nm (5 Å to 1000 Å). The method of Claim 8, 9 or 10 wherein said short period superlattice has a period of 5 nm (50 angstroms). The method of Claim 8, 9 or 10 wherein said short period superlattice is comprised of AlGaN/GaN. The method of Claim 12 wherein said short period superlattice has an average aluminum percent composition selected from the range extending from approximately 20 percent to 50 percent.

Inventors: BOUR DAVID P, JOHNSON NOBLE M, KNEISSL MICHAEL A, KRUSOR BRENT S, ROMANO LINDA T, BOUR, DAVID P., JOHNSON, NOBLE M., KNEISSL, MICHAEL A., KRUSOR, BRENT S., ROMANO, LINDA T.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 2000
Main classification: H01S5
IPC codes: H01S5