Abstract:
An inner stripe laser diode structure 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:
FIELD OF INVENTION 
     The invention relates to index guided, inner stripe laser diode structures. 
     BACKGROUND OF INVENTION 
     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. Pat. 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 AlyGa 1−x N (0&lt;y=1), SiO 2 , Si 3 N 4  and Al 2 O 3 . 
     FIG. 1 a  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. 1 b  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, An, to no greater than 4×10 −3 . FIG. 1 b  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 11×10 −3  at zero thickness. 
     SUMMARY OF THE INVENTION 
     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 and other material systems which are relatively insensitive to regrowth interfaces close to the active region. In AlGaAs and AlGaInP laser systems, for example, the defect states associated with a regrowth interface close to the active region are sufficient to inhibit lasing. 
     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 20×10 −3 . 
    
    
     The advantages and objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention, its preferred embodiments, the accompanying drawings, and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows the lateral index step as a function of first upper cladding layer thickness in the prior art. 
     FIG. 1 b  shows a laser diode structure in the prior art. 
     FIGS. 2-4 a  show steps in the fabrication procedure for an index-guided inner stripe nitride laser diode structure in accordance with the invention. 
     FIG. 4 b  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 in accordance with the invention. 
     FIG. 6 shows the calculated tranverse effective index and optical confinement factor for an index guided inner strip nitride laser diode structure in accordance with the invention. 
     FIG. 7 shows an index guided inner strip nitride laser diode structure with insulating current blocking layers in accordance with an embodiment of the invention. 
     FIG. 8 shows the calculated tranverse effective index for an index guided inner strip nitride laser diode structure with SiO 2  current blocking layer in accordance with an embodiment of the invention. 
     FIG. 9 shows the relationship between blocking layer thickness and lateral index step for embodiments in accordance with the invention. 
     FIG. 10 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. 11 shows the lateral optical confinement factor for 1 μm and 2 μm wide lasers as a function of lateral index step. 
     FIG. 12 shows a dual-spot inner stripe laser structure using a semiconductor current blocking layer in accordance with an embodiment of the invention. 
     FIG. 13 shows a dual-spot inner stripe laser structure using an insulating current blocking layer in accordance with an embodiment of the invention. 
     FIG. 14 shows a dual-spot inner stripe laser structure using an air gap current blocking layers in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     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  in accordance with an embodiment of the invention. Standard epitaxial growth for nitride materials (see, for example, U.S. Pat. No. 6,430,202 by Van de Walle et al. incorporated by reference in its entirety) 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 quantum well region  150 , lower portion  160   a , 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  160   a  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. 
     In accordance with an embodiment of the invention, FIG. 4 a  shows epitaxial regrowth of upper portion  160   b  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  160   b  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  160   a . 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 &#39;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 an embodiment of the invention, AlGaN current blocking layer  170  may be replaced by AlGaN/GaN short period superlattice layer  175  shown in FIG. 4 b  having a period between 5 Å to 1000 Å, with a typical superlattice period of 50 Å (25 Å AlGaN/25 Å GaN). 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  160   b , 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  160   a  into multiple quantum well region  150 . 
     If AlN is chosen for current blocking layer  170 , deposition of an amorphous or polycrystalline film by sputtering may be preferable to epitaxial growth. This retains a low refractive index for current blocking layer  170  while avoiding lattice strain that leads to cracking. Care should be taken to avoid poor structural quality that contributes to large scattering and absorption losses. 
     In accordance with an embodiment of the invention, 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 nitride laser diode structure  100  in accordance with the present invention. 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 35 Å In 0.15 Ga 0.85 N quantum wells with a refractive index of 2.56 that are separated by 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 short period superlattice  175  may be used instead. For the case where n-blocking layer  170  is grown  50 nm 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  160   a . The resulting transverse effective refractive indices are shown in table 1 below: 
     
       
         
               
               
               
             
           
               
                   
               
               
                   
                 GaN:Mg 
                 transverse 
               
               
                   
                 waveguide 160 
                 effective 
               
               
                 Region 
                 thickness 
                 refractive index 
               
               
                   
               
             
             
               
                 stripe region of layer 160 
                 150 nm 
                 2.488 
               
               
                 partial waveguide layer 160a 
                  50 nm 
                 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 ). 
     In accordance with an embodiment of the invention, current blocking layers may also be formed from lower refractive index insulating materials such as SiON, Si 3 N 4 , or SiO 2  to provide a larger refractive index step and greater lateral index waveguiding. For example, referring to index-guided inner stripe nitride laser structure  600  in FIG. 7, using SiO 2  with a refractive index of 1.50 for current blocking layer  670  significantly increases the lateral refractive index step in comparison to Al 0.07 Ga 0.93 N blocking layer  170  placed a comparable distance from multiple quantum well active region  150 . Current blocking layer  670  is deposited after the first epitaxy in contrast to current blocking layer  170  in FIG. 3 which is deposited during the first epitaxy. Insulating current blocking layer  670  may be deposited by sputtering, evaporation or high temperature CVD process. Narrow stripe window  650  is photolithographically defined followed by CAIBE or plasma etching with CF 4 /O 2  into, but not through, waveguide layer  660   a.    
     The transverse effective refractive indices for SiO 2  blocking layer  670  are summarized in table 2 below: 
     
       
         
               
               
               
             
           
               
                   
               
               
                   
                 GaN:Mg 
                 transverse 
               
               
                   
                 waveguide 660 
                 effective 
               
               
                 Region 
                 thickness 
                 refractive index 
               
               
                   
               
             
             
               
                 stripe region of layer 660 
                 150 nm 
                 2.480 
               
               
                 partial waveguide layer 660a 
                  50 nm 
                 2.460 
               
               
                   
                   
                 Δn = 0.020 
               
               
                   
               
             
          
         
       
     
     The values plotted in FIG. 8 correspond to index-guided, inner stripe nitride laser structure  600  in FIG.  7 . In index-guided, inner stripe nitride laser structure  600 , waveguide layer  660  is epitaxially laterally grown over SiO 2  current blocking layer  670  followed by deposition of Al 0.07 Ga 0.93 N cladding layer  180 . Inner stripe  650  is opened in current blocking layer  670  to allow for current flow to multiple quantum well active region  150 . Waveguide layer  660  is made up of layers  660   a  and  660   b . However, waveguide layer  660   b  typically only partially laterally overgrows current blocking layer  670  before the desired thickness for waveguide layer  660   b  is achieved resulting in part of current blocking layer  670  being uncovered prior to growth of AlGaN cladding layer  180 . Hence, instead of epitaxial overgrowth, polycrystalline AlGaN regions  666  form over the exposed portion of current blocking layers  670  whereas epitaxial overgrowth occurs on waveguide layer  660   b.    
     If SiO 2  blocking layer  670  is placed closer than 50 nm to multiple quantum well active region  150  the lateral index step would be increased. The value of 50 nm for the separation of blocking layer  670  from multiple quantum well active region  150  is conservatively selected and less separation is possible. 
     However, SiO 2  may not have sufficient thermal stability to function as current blocking layer  670  if the SiO 2  is exposed to high temperature MOCVD process conditions. SiO 2  at high MOCVD temperatures may act as a source of oxygen or silicon donors which are n-type and this could make p-type doping difficult and result in degraded performance of the inner stripe laser diode. High quality (dense, perfectly stoichiometric) Si 3 N 4  may be deposited at high temperatures by CVD which indicates excellent thermal stability. Hence, if high temperature processing is involved, Si 3 N 4  is an alternative dielectric material for current blocking layer  670  even though its refractive index is higher producing a smaller transverse refractive index step, Δn. 
     FIG. 9 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 02 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. 10 shows the layer thickness required for current blocking layer  170  or  670  of a given refractive index to obtain lateral index step  910  of 5×10 −3 , lateral index step  920  of 10×10 −3 , lateral index step  930  of 15×10 −3  and lateral index step  940  of 20×10 −3 . If short period superlattice structure  175  is used for current blocking, the thickness will be the same as for current blocking layer  170  for 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 axis. It is apparent from FIG. 10 that as the refractive index of the material used for blocking layer  170  or  670  increases, the thickness of blocking layer  170  or  670  must be increased to maintain the same lateral index step. For lateral index step  940  of 20×10 −3 , the thickness of blocking layer  170  or  670  is required to be thicker than 100 nm for representative materials other than SiO 2 . 
     FIG. 11 shows the lateral confinement factor in percent, ┌ 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. 11 shows that improvement of the lateral confinement factor, ┌ lateral , is marginal for a 2 μm wide stripe laser with a lateral index step greater than about 10×10 −3  and for a 1 μm wide stripe laser with a lateral index step greater than about 20×10 −3 . 
     FIG. 12 shows dual spot, inner-stripe nitride laser diode structure  1100  using semiconductor current blocking layers  170 . Quadspot lasers or other multispot configurations are also realizable using the inner-stripe structure disclosed. The two lasers making up dual spot, inner-stripe laser diode  1110  individually have essentially the same layer structure as laser  100  shown in FIG.  5 . Semiconductor growth over AlGaN current blocking layers  170  is epitaxial and conformal. In comparison, FIG. 13 shows dual spot, inner-stripe nitride laser diode structure  1200  using current blocking layer  1670  made of insulating material such as, for example, SiO 2 , SiON or Si 3 N 4 . Current blocking layers  1670  have partial lateral overgrowth by waveguide layer  1660   b  that occurs during regrowth. However, waveguide layer  1660   b  typically only partially laterally overgrows current blocking layer  1670  before the desired thickness for layer  1660   b  is achieved resulting in part of current blocking layer  1670  being uncovered prior to growth of AlGaN cladding layer  1180 . Hence, instead of epitaxial overgrowth, polycrystalline AlGaN regions  1666  form over the exposed portion of current blocking layers  1670  whereas epitaxial overgrowth occurs on waveguide layer  1660   b . Note that overgrowth in the vicinity of the stripe is epitaxial. 
     Air having a refractive index of about  1  is an alternative material for current blocking layers  670  and  1670 . FIG. 14 shows dual spot, inner-stripe nitride laser diode structure  1200  in accordance with the invention having air as the current blocking material. SiO 2 , SiON or other selectively etchable material is used for current blocking layers  670  and  1670 . Following fabrication of inner-stripe nitride laser diode structure  600  or dual spot, inner-stripe nitride laser diode structure  1200 , the respective structure is treated with hydrofluoric acid, for example, to etch away the selectively etchable material by undercutting from the exposed sidewalls and leaving air gaps in to function as current blocking layer  670  or  1670 . 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.