Abstract:
A method and structure for nitride based laser diode arrays on an insulating substrate is described. Various contact layouts are used to reduce electrical and thermal crosstalk between laser diodes in the array. A channel structure is used to make a surface emitting laser diode while maintaining a simple contact structure. Buried layers are used to provide a compact and low crosstalk contact structure for the laser diode array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to copending application “Structure for Nitride Based Laser Diode Arrays on a Conducting Substrate” by M. A. Kneissl, D. P. Bour, N. M. Johnson, and J. Walker Ser. No. 09/224,254, filed on the same day and assigned to the same assignee which is hereby incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OF DEVELOPMENT 
     The U.S. Government has a fully paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract no. 70NANB 2H-1241 awarded by the Department of Commerce. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of laser diodes, and more particularly to architecture for short-wavelength nitride based laser diode arrays. 
     Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Single-spot nitride laser diodes have applications in areas such as optical storage. 
     Laser diode arrays are desirable for application to high speed laser printing. Printing at high speeds and at high resolution requires laser arrays due to the fundamental limits of polygon rotation speed, laser turn-on times and laser power. Laser diode arrays have previously been employed using red and infrared laser diode structures. Dual-spot red lasers and quad-spot infrared lasers have been used for laser printers. 
     Laser diodes based on higher bandgap semiconductor alloys such as AlGaInN have been developed. Excellent semiconductor laser characteristics have been established in the near-UV to violet spectrum, principally by Nichia Chemical Company of Japan. See for example, A. Kuramata et al., “Room-temperature CW operation of InGaN Laser Diodes with a Vertical Conducting Structure on SiC Substrate”, Japanese Journal of Applied Physics, Vol. 37, L1373 (1998), S. Nakamura et al., “CW Operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates”, Applied Physics Letters, Vol. 72(6), 2014 (1998) and S. Nakamura and G. Fasol, “The Blue Laser Diode-GaN based Light Emitters and Lasers”, (Springer-Verlag, 1997) all of which are incorporated by reference in their entirety. 
     Extension of dual-spot lasers to shorter wavelengths enables printing at higher resolution. However, the architecture for short-wavelength laser diode arrays needs to be different when nitride based laser diodes are used in arrays because mirrors need to be formed by dry etching instead of cleaving and nitride based devices are mostly grown on insulating substrates such as sapphire. 
     SUMMARY OF THE INVENTION 
     Architectures using insulating substrates allow the economical construction of nitride based quad-spot diode laser and surface-emitting dual-quad-spot laser diode arrays. Currently, most advanced nitride based single laser structures are grown on insulating sapphire (Al 2 O 3 ) substrates. The use of insulating substrates for laser diode arrays presents a special problem in providing electrical contacts for the laser diodes. In contrast to the situation where conducting substrates are used, insulating substrates cannot provide a common contact for all laser diodes in an array. Hence, providing electrical contacts to laser diode arrays on insulating substrates requires the use of special architectures. 
     Dual spot and quad spot laser diodes built on an insulating substrate can be electrically contacted using an architecture with surface contacts for both anode and cathode. Two laser diodes may share a common n-contact or p-contact. Alternatively, each laser diode may have separate n-and p-contacts. Providing separate contacts for each laser diode greatly reduces electrical and thermal crosstalk but complicates the laser diode architecture. In quad spot laser diodes, two laser diodes may be aligned at an angle with respect to the other two laser diodes to achieve further reduction in electrical and particularly thermal crosstalk if necessary. 
     Alternatively, laser diodes built on an insulating substrate as an array may be contacted using multiple buried layers isolated from each other by blocking layers of opposite conductivity or by insulating layers. This allows good isolation of the conducting layers while still maintaining good conductivity. Alternating layers of opposite conductivities form p-n junctions that are reverse-biased under forward bias operation of the laser diode array. As a result, a buried isolated current channel is produced for each laser diode in the laser array. Alternating doped layers with insulating layers also forms a buried isolated current channel for each laser diode in the laser array. The insulating layers provide electrical blocking between the doped layers to isolate the current channel. Blocking layers may be epitaxially grown. 
     Since it is very difficult to obtain high quality mirror facets by cleaving because of the cleave plane mismatch between GaN and Al 2 O 3 , laser mirrors for laser diodes on insulating substrates are most often obtained by using either dry-etched vertical facets (i.e. chemically assisted ion -beam etching) or by integrating a distributed Bragg reflecting mirror into the laser device structure. 
     The ability to use insulating substrates for short wavelength nitride based lasers by employing special architectures offers a significant economic savings for laser diode array structures as well as allowing use of proven techniques for their manufacture. 
     Thus, the present invention and its various embodiments provide numerous advantages as will be described in further detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings. The drawings, briefly described below, are not to scale. 
     FIG. 1 shows a top view of the layout of a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 2 shows a cross-sectional view of the embodiment in FIG.  1 . FIG. 3 shows a top view of the layout of a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 4 shows a top view of the layout of a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 5 a  shows a top view of the layout of a dual quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 5 b  shows a cross-sectional view of the embodiment shown in FIG. 5 a.    
     FIG. 6 shows the layers of a quad-spot laser diode structure in an embodiment accordance with the present invention. 
     FIGS. 7 a - 7   e  show processing steps for a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 8 a  shows a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 8 b  shows a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 9 shows the layers of a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numeric ranges are provided for various aspects of the embodiments described. These recited ranges are to be treated as examples only, and are not intended to limit the scope of the claims hereof. In addition, a number of materials are identified as suitable for various facets of the embodiments. These recited materials are to be treated as exemplary, and are not intended to limit the scope of the claims hereof. 
     An embodiment in accordance with the present invention is shown in FIG.  1 . FIG. 1 shows quad-spot InGaAlN laser diode structure  100  typically grown on Al 2 O 3  insulating substrate  215  (see FIG.  2 ). Structure  100  shows laser diodes  145  and  146  sharing n metal contact  120  and laser diodes  147  and  148  sharing n-metal contact  125 . P-metal contact  110  connects to laser diode  145 , p-metal contact  135  connects to laser diode  146 , p-metal contact  130  connects to laser diode  147  and p-metal contact  115  connects to laser diode  148 . The lateral separation between adjacent laser diodes shown in FIG. 1 is about 25 μm. For example, the separation between laser diodes  145  and  146  is about 25 μm. The length of laser diode structures  145 ,  146 ,  147  and  148  is typically about 500 μm. Isolation layer  140  is made of a dielectric material, typically, silicon oxy-nitride, silicon dioxide, silicon nitride or polyimide. Notches  150  and  155  in p-metal contacts  135  and  130 , respectively, provide open space for subsequent evaporation of a dielectric high reflective coating, for example, TiO 2 /SiO 2 . 
     FIG. 2 shows a cross-sectional view of quad-spot InGaAlN laser diode structure  100 . P-GaN cap layer  220  is positioned atop p-AlGaN cladding layer  225 . The active region is InGaN layer  230  that has a multi-quantum well structure and is positioned on n-AlGaN cladding layer  235 . Layer  210  is n-GaN and resides on insulating substrate  215  which is typically Al 2 O 3 . 
     FIG. 3 shows an alternative quad-spot InGaAlN laser diode structure  300  typically grown on Al 2 O 3  insulating substrate  215  (see FIG.  2 ). Quad-spot InGaAlN laser diode structure  300  is similar to quad-spot InGaAlN laser diode structure  100  shown in FIG. 1 except that separate n-metal contacts  310 ,  315 ,  320  and  325  are provided for laser diode structures  145 ,  146 ,  147  and  148 , respectively. Providing separate n- and p-metal contacts for each device minimizes electrical and thermal crosstalk. Additional reduction in crosstalk can be achieved by etching very deep isolation grooves  340  (see FIG. 3) to separate laser diode  145  from laser diode  146 , to separate laser diode  146  from laser diode  147  and to separate laser diode  147  from laser diode  148 . Isolation grooves  340  may penetrate down to insulating substrate  215  (see FIG. 2) but at a minimum, grooves  340  should penetrate below active region  230 . This eliminates optical crosstalk and the electrical crosstalk caused by diffusion of injected carriers within active region  230 . 
     An embodiment in accordance with the present invention is shown in FIG.  4 . Quad-spot InGaAlN laser diode structure  400  is grown on insulating substrate  215 . Laser diodes  145  and  148  and associated n-metal and p-metal contacts  410 ,  435  and  415 ,  430 , respectively, are aligned at an angle to provide more separation from laser diodes  146  and  147 . The angle is chosen so that the resulting separation achieves a further reduction in electrical and especially thermal crosstalk. Laser diode  145  is provided electrical contact using n-metal contact  410  and p-metal contact  415  and laser diode  148  is provided electrical contact using n-metal contact  435  and p-metal contact  430 . Laser diode  146  is provided electrical contact using p-metal contact  420  and n-metal contact  440 . N-metal contact  440  is shared with laser diode  147  which has separate p-metal contact  425 . N-metal contact  440  may be divided into two separate contacts to provide separate n-metal contacts to laser diodes  146  and  147  for a further reduction of electrical and thermal crosstalk. 
     An embodiment in accordance with this invention of dual-quad-spot laser diode structure  501  is shown in FIGS. 5 a  and  5   b . FIG. 5 a  shows the placement of quad-spot laser diode structure  100  on one side of channel structure  595  and corresponding mirror image laser diode structure  500  on the other side of channel structure  595  resulting in dual-quad-spot laser diode structure  501 . Structure  500  has laser diodes  545  and  546  sharing n metal contact  520  and laser diodes  547  and  548  sharing n-metal contact  525 . P-metal contact  510  connects to laser diode  545 , p-metal contact  535  connects to laser diode  546 , p-metal contact  530  connects to laser diode  547  and p-metal contact  515  connects to laser diode  548 . The lateral separation between adjacent laser diodes shown in FIG. 1 is about 25 μm. For example, the separation between laser diodes  545  and  546  is about 25 μm. The length of laser diode structures  545 ,  546 ,  547  and  548  is typically about 500 μm. Structure  100  has been described above with reference to FIGS. 1 and 2. 
     FIG. 5 b  shows a cross-section of dual-quad-spot laser diode structure  501  along laser diode structures  147  and  547  and channel structure  595 . Channel structure  595  contains tilted mirrors  575  (see FIG. 5 b ) for outcoupling light from laser diodes  145 ,  146 ,  147 ,  148 ,  545 ,  546 ,  547  and  548  into the vertical direction. Aluminum coated mirrors  575  are dry etched using, for example, chemically assisted ion-beam etching (CAIBE) and the inclination angle may be adjusted by varying etching parameters. A suitable inclination angle for mirror facets  575  in FIG. 5 b  is 45 degrees. The architecture shown in FIGS. 5 a  and  5   b  allows a closely spaced dual-quad-spot laser diode structure to be achieved. The spacing between adjacent laser diodes such as laser diodes  145  and  146  or  545  and  546  is typically 25 μm. 
     In an embodiment in accordance with this invention, FIG. 6 shows InGaAlN heterostructure wafer  600  grown by metalorganic chemical vapor deposition (MOCVD) on insulating substrate  215 . Insulating substrate  215  is typically Al 2 O 3  and has a thickness typically ranging on the order of 100 μm to 400 μm. GaN:Mg cap layer  610  is 0.1 μm thick and adjoins Al 0.08 Ga 0.92 N:Mg cladding layer  620  which has a typical thickness in the range of 0.5 to 1.0 μm. Note that Mg is added to produce a p-type conductivity. A second GaN:Mg layer  630  also 0.1 μm thick lies underneath cladding layer  620  and serves as an p-doped waveguide. Al 0.2 Ga 0.8 N:Mg layer  640  is typically 20 nm thick and serves to create a tunnel barrier to prevent leakage of injected electrons. GaN:Si layer  650  functions as an n-doped waveguide for active region  230 . Note that Si is added to produce an n-type conductivity. Al 0.08 Ga 0.92 N:Si cladding layer  660  has a typical thickness from 0.5 to 1.5 μm. In 0.03 Ga 0.97 N:Si layer  665  has a typical thickness of 50 nm and functions as a defect reducing layer. 
     GaN:Si layer  210  with a typical thickness of 4 μm lies above insulating substrate  215  and serves to establish a good quality material for subsequent depositions and to provide a lateral contact layer. Further details may be found in Nakamura and Fasol incorporated by reference above. Once structure  600  has been grown by MOCVD activation of Mg p-doping is performed in (Al)GaN:Mg layers  610 ,  620 ,  630  and  640 . Activation of dopants is accomplished by rapid thermal annealing at 850° C. for 5 minutes in N 2  ambient. 
     FIGS. 7 a - 7   e  show the major processing steps for a quadspot ridge waveguide laser diode structure in accordance with this invention. Note that layers  640  and  665  are not shown in FIGS. 7 a - 7   e . FIG. 7 a  shows wafer  600  after p-metal deposition. P-metal layer  710  is typically nickel-gold (Ni—Au) and deposited using thermal evaporation and rapid thermal annealing in an N 2  ambient. Dry etching is performed using CAIBE or reactive ion etching (RIE) to etch the mesa structure shown in FIG. 7 b  in an Ar/Cl 2 /BCl 3  gas mixture. The mirrors (not shown) are also dry etched using a CAIBE or RIE process. FIG. 7 c  shows wafer  600  after etching of ridge waveguides  707  and trenches  711  in an Ar/Cl 2 /BCl 3  gas mixture using CAIBE or RIE. FIG. 7 d  shows the result of depositing n-metal  720  which is typically titanium-aluminum (Ti—Al) using thermal evaporation and rapid thermal annealing in a N 2  ambient. 
     Dielectric isolation deposition is then performed using plasma enhanced chemical vapor deposition (PECVD) using, for example, silicon-oxy-nitride, silicon oxide or silicon nitride as the dielectric. Polyimide may also be used as the dielectric. Contact windows are opened in dielectric isolation layer  755  using radio frequency (RF) plasma etching in CF 4 /O 2  ambient prior to deposition of titanium/gold p-metal contact pads using thermal evaporation. FIG. 7 e  shows wafer  600  after p-metal contact pad  730  and n-metal contact pad  720  deposition. Substrate  215  is then thinned by mechanical polishing to prepare wafer  600  for cleaving of laser diodes into individual devices. A final step involves using electron beam evaporation for deposition of a SiO 2 /TiO 2  high reflective coating on the front and backside of the laser diode mirrors (not shown) to reduce the laser threshold current and protect the mirror surfaces. 
     FIG. 8 a  shows an embodiment in accordance with the present invention of quadspot laser diode structure  800  built on insulating substrate  215  using GaN/AlGaN. Quadspot laser diode structure  800  uses n-GaN buried layer  210  separated from n-GaN buried layer  885  by p-GaN or AlGaN blocking layer  890  to form buried current channels  850  and  855 . Buried current channels  850  and  855  go from n-metal contacts  815  and  820  to contact laser diodes  830  and  836 , respectively. Buried current channels  870  and  875  go from n-metal contacts  810  and  825  to contact laser diodes  832  and  834 , respectively, using n-GaN buried layer  885  which is sandwiched between p-GaN or AlGaN blocking layer  890  and insulating substrate  215 . Typically, insulating substrate  215  is made of Al 2 O 3 . Laser diode pairs  830 ,  832 ;  832 ,  834 ; and  834 ,  836  are separated from each other both optically and electrically by grooves  831 ,  833  and  835 , respectively. Groove  833  is etched down through lowest GaN layer  885  while grooves  831  and  835  are etched only through p-GaN layer  890  to allow current to flow to inner laser diodes  832  and  834 , respectively. GaN or AlGaN layer  890  may be made an insulating layer if desired. 
     P-metal contact pads  816  can be arranged in several different ways as shown above in FIGS. 1-5. In FIG. 8 a , individually addressable p-metal contact pads  816  are connected to laser diodes  830 ,  832 ,  834  and  836  through a window (not shown) in isolation layer (not shown) applied to p-AlGaN cladding layer  225 . An alternative embodiment in accordance with the present invention is shown in FIG. 8 b . FIG. 8 b  shows quadspot laser diode structure  801  having common p-metal contact  817  formed by filling grooves  831 ,  833  and  835  and areas surrounding p-GaN contact layer  818  (FIG. 8 a ) with an insulator such as polyimide. Addressability of individual laser diodes  830 ,  832 ,  834  and  836  is preserved through n-metal contacts  815 ,  810 ,  825  and  820 , respectively. The common p-metal contact structure can be extended to more than four closely spaced laser diodes by the addition of a pair of n- and p-GaN layers for every two laser diodes added to laser diode structure  801  shown in FIG. 8 b.    
     For some laser diode driver circuits it is more convenient to have a common n-metal contact structure for quadspot laser diode structure  801 . This is readily achieved by reversal of the polarities of all layers (see FIG. 9) in quadspot laser diode structure  801 . This makes buried current channels  850 ,  855 ,  870  and  875  p-type channels. However, this arrangement is not presently preferred for nitride based lasers because carrier mobility and achievable doping levels in p-type GaN are significantly lower than in n-type GaN. 
     In an embodiment in accordance with this invention, FIG. 9 shows InGaAlN heterostructure wafer  900  grown by metalorganic chemical vapor deposition (MOCVD) on insulating substrate  215 . The layer structure of wafer  900  is identical to wafer  600  of FIG. 6 except for the addition of p-(Al)GaN:Mg isolation layer  890 , typically several hundred nm in thickness and second n-GaN:Si layer  885 , typically at least 1-2 μm in thickness. Isolation layer  890  can also be grown as an insulator since its purpose is to provide electrical isolation of GaN layer  885  from GaN layer  210 . Following growth of n-GaN layer  885  and p-GaN isolation layer  890 , wafer  900  is removed from the growth reactor and isolation layer  890  is selectively removed in the regions where laser diodes  832  and  834  (see FIG. 8 a ) will be located. After selective removal of isolation layer  890 , etched wafer  900  is returned to the growth reactor for growth of GaN:Si layer  210  and subsequent layers as in FIGS. 6 and 9. In this embodiment, GaN:Si layers  210  and  885  are doped to achieve an electron concentration of the order of 10 18 /cm 3  for high electrical conductivity. Individual laser diodes  830 ,  832 ,  834  and  836  are etched similarly as described above and shown in FIGS. 7 a - 7   e.    
     N-metal contacts  810 , 815 ,  820  and  825  (see FIGS. 8 a  and  8   b ) are formed by selectively removing the upper layers. Specifically, formation of n-metal contacts  815  and  820  requires selective removal of all layers down to n-GaN layer  210  and formation of n-metal contacts  810  and  825  requires selective removal of all layers down to n-GaN layer  885 . Deposition of n-metal contacts  810 ,  815 ,  820  and  825  is performed by masking wafer  900  for contact metallization and lift-off patterning. N-metal contact pads  810 ,  815 ,  820  and  825  are typically Ti—Al while p-metal contact pads  816  or pad  817  are typically Ni—Au. 
     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 other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.