Abstract:
Method and structure for nitride-based laser diode arrays on a conducting substrate are disclosed. Air-bridge structures are used to make compact laser diode arrays suitable for printer applications. The use of a channel structure architecture allows the making of surface emitting laser diode arrays.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to copending application “Structure for a Nitride Based Laser on an Insulating Substrate” by M. A. Kneissl, T. L. Paoli, D. P. Bour, N. M. Johnson, and J. Walker, Ser. No. 09/223,112, 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-Verdag, 1997) all of which are incorporated by reference in their entirety. 
     Extension of quad-spot laser diodes to shorter wavelengths enables printing at higher resolution. Growth of quad-spot laser diodes on conducting substrates allows a common backside contact for all laser diodes in the array. 
     SUMMARY OF THE INVENTION 
     Architectures using conducting substrates for nitride dual-spot laser diode arrays allow the use of common backside contacts for all devices in the laser diode array to permit a compact layout. Elimination of cleavage plane misalignment with GaN by using a conducting substrate enables the formation of high quality cleaved mirror facets without dry etching. 
     The metallization scheme on the frontside of the nitride based laser diode array structures is similar to that used in red and infrared lasers. Frontside contacts on quad-spot laser array structures are arranged to provide separate p-metal contacts for each laser diode. However, certain compact layouts result in small unpumped sections in two of the lasers making up the quad-spot structure due to necessary breaks in the contact pad structure to allow contact paths to the remaining laser pair. The presence of unpumped sections in a nitride based laser diode can have adverse effects such as raising the required threshold current density. 
     A layout that overcomes the problem of unpumped sections but still retains the compact structure needed in printing applications employs air-bridge contact structures to cross over intervening metal contact areas. This isolates the contacts to the laser diodes while avoiding having unpumped sections. Air-bridge contact structures also allow minimization of parasitic capacitance effects between contacts. 
     Surface emitting quad-spot laser diode arrays may be made on a conducting substrate using a channel structure between two dual-spot laser diode arrays. The channel structure contains mirrors to outcouple light at various angles to the laser cavity. This scheme is readily generalizable to produce surface emitting laser diode arrays containing an arbitrary number of laser diodes. 
     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 a  shows a top view of the layout of a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 1 b  shows a cross-sectional view of the embodiment in FIG. 1 a.    
     FIG. 2 shows a top view of the layout of a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 3 a  shows a top view of the layout of a quad-spot laser diode structure with air-bridge structures in an embodiment in accordance with the present invention. 
     FIG. 3 b  shows a cross-sectional view of the embodiment in FIG. 3 a.    
     FIG. 4 a  shows a top view of a dual dual-spot surface emitting laser diode structure in an embodiment in accordance with the present invention. 
     FIG. 4 b  shows a cross-sectional view of the embodiment in FIG. 4 a.    
     FIG. 5 shows the layers of a quad-spot laser diode structure in an embodiment in accordance with the present invention. 
     FIGS. 6 a - 6   e  show processing steps for 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 FIGS. 1 a  and  1   b . FIGS. 1 a  and  1   b  show quad-spot InGaAlN laser diode structure  100  grown on conducting substrate  150  which is typically SiC or GaN. Structure  100  shows laser diode  145  having two p-metal contacts  115  and  116  and laser diode  148  having two p-metal contacts  117  and  118 . Laser diode  146  is contacted by p-metal contact  110  and laser diode  147  is contacted by p-metal contact  120 . Note that the middle section of laser diodes  145  and  148  are unpumped due to the need to bring p-metal contact  110  to laser diode  146  and to bring p-metal contact  120  to laser diode  147 . Having an unpumped region in laser diodes  145  and  148  produces an absorption loss which raises the required threshold current densities for laser diodes  145  and  148 . Separation between individual laser diodes  145 ,  146 ,  147  and  148  is typically about 10 to 25 μm. 
     FIG. 1 b  shows a cross-sectional view of quad-spot InGaAlN laser diode structure  100 . P-GaN cap layer  155  is positioned atop p-AlGaN cladding layer  160 . The active region is InGaN layer  175  that has a multi-quantum well structure and is positioned above n-AlGaN cladding layer  170 . Layer  180  is n-GaN and resides on conducting layer  150  which is typically SiC or n-GaN. Insulating layer  196  separates p-metal contacts from n-type regions of laser diode structure  100 . Common backside n-contact  195  closes the current loop to lasers  145 ,  146 ,  147  and  148 . 
     FIG. 2 shows a layout of quad-spot InGaAlN laser diode structure  200  which is a variation of the metallization scheme shown in FIG. 1 a . Outer laser diodes  146  and  148  each have only one p-metal contact pad, p-metal contact pads  115  and  117 , respectively. 
     An embodiment in accordance with the present invention is shown in FIGS. 3 a  and  3   b . Quad-spot InGaAlN laser diode structure  300  is grown on conducting substrate  150 . Au-air-bridges  111  and  121  are used to contact laser diodes  146  and  147 , respectively. The use of Au-air-bridges  111  and  121  allows p-metal contacts  115  and  117  to provide uninterrupted contact to laser diodes  146  and  147 , respectively. This eliminates the unpumped regions of laser diodes  146  and  147  seen in FIGS. 1 a - 2  and reduces the level of the required threshold current. The regions beneath Au-air-bridges may be filled with dielectric material such as silicon-oxy-nitride, silicon dioxide or polyimide. However, using dielectric material as a filler increases parasitic capacitance versus using air and may not be desirable for certain applications, especially high-speed modulation. 
     An embodiment in accordance with this invention of surface emitting quad-spot laser diode  400  is shown in FIG. 4 a . A two by two array configuration is shown in FIG. 4 a . Each side of channel structure  495  has a dual spot laser diode structure. This structure can be generalized to an arbitrary number of laser diodes, with half of the total number of laser diodes residing on one side of channel structure  495  and the remainder residing on the other side of channel structure  495 . P-metal contact  410  contacts laser diode  145 , p-metal contact  420  contacts laser diode  146 , p-metal contact  430  contacts laser diode  147  and p-metal contact  440  contacts laser diode  440 . All laser diodes  145 ,  146 ,  147  and  148  share n-metal contact  195  (see FIG. 4 b ) to complete the current loop. 
     FIG. 4 b  shows a cross section of surface emitting quad-spot laser diode structure  400  across channel structure  495 . Channel structure  495  contains mirrors  450  for outcoupling light from laser diodes  145 ,  146 ,  147  and  148  into a generally vertical direction as shown. Aluminum coated mirrors  450  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 mirrors  450  in FIG. 4 b is 45 degrees. The spacing between adjacent laser diodes  145  and  146  or  147  and  148  is typically from about 10 to 25 μm. 
     In an embodiment in accordance with this invention, FIG. 5 shows InGaAlN heterostructure wafer  500  grown by metalorganic chemical vapor deposition (MOCVD) on conducting substrate  150 . Conducting substrate  150  is typically SiC or GaN and has a thickness typically ranging on the order of 100 μm to 400 μM. GaN:Mg cap layer  510  is 0.1 μm thick and adjoins Al 0.08 Ga 0.92 N:Mg cladding layer  520  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  530  also 0.1 μm thick lies underneath cladding layer  520  and serves as a p-doped waveguide. Al 0.2 Ga 0.8 N:Mg layer  540  is typically 20 μm thick and serves as a tunnel barrier layer to prevent leakage of injected electrons. GaN:Si layer  550  functions as an n-doped waveguide for InGaN multi-quantum well active region  175 . Note that Si is added to produce an n-type conductivity material. Al 0.08 Ga 0.92 N:Si cladding layer  560  has a typical thickness from 0.5 to 1.5 μm. In 0.03 Ga 0.97 N:Si layer  565  has a typical thickness of 50 nm and functions as a defect reducing layer. 
     GaN:Si layer  180  with a typical thickness of 4 μm lies above conducting substrate  150  and serves to establish a good quality material for subsequent depositions (i.e., layer  180  serves as a buffer layer). Further details may be found in Nakamura and Fasol incorporated by reference above. Once structure  500  has been grown by MOCVD activation of Mg p-doping is performed in (Al)GaN:Mg layers  510 ,  520 ,  530  and  540 . Activation of dopants is accomplished by rapid thermal annealing at 850° C. for 5 minutes in N 2  ambient. 
     FIGS. 6 a - 6   e  show the major processing steps for a quad-spot laser diode structure in accordance with the present invention. Note that layers  540  and  565  are not shown in FIGS. 6 a - 6   f . FIG. 6 a  shows wafer  500  after p-metal deposition. P-metal layer  610  is typically nickel-gold (Ni—Au) and deposited using thermal evaporation with rapid thermal annealing (RTA) in an N 2  ambient. Dry-etching using CAIBE or reactive ion etching (RIE) in an Ar/Cl 2 /BCl 3  gas mixture creates the mesa structures shown in FIG. 6 b.    
     FIG. 6 c  shows wafer  500  after etching of ridge waveguides and trenches  699  in an Ar/Cl 2 /BCl 3  gas mixture using CAIBE or RIE. Subsequently, dielectric deposition of dielectric layer  196 , typically silicon-oxy-nitride, silicon dioxide or silicon nitride, using plasma enhanced chemical vapor deposition (PE-CVD) takes place. Polyimide may also be used for layer  196 . Contact windows are opened in the dielectric using radio frequency plasma etching in a CF 4 /O 2  ambient atmosphere. 
     FIG. 6 d  shows the result of depositing dielectric layer  196  and p-metal deposition by thermal evaporation to form p-metal pads  115 ,  333 ,  334  and  117 . Typically, p-metal pads  115 ,  333 ,  334  and  117  are made of titanium-gold (Ti—Au). Substrate  150  is thinned by mechanical polishing on diamond pads prior to deposition of n-metal layer  195  on the backside of substrate  150  by thermal evaporation. An RTA in an N 2  ambient serves to anneal n-metal layer  195  for lowest contact resistance. N-metal layer  195  is typically made of titanium-aluminum (Ti—Au). If the quad-spot laser diode structure utilizes Au-air-bridge structures, another Ti—Au deposition occurs to contact inner laser diodes  146  and  147  via Au-air bridge structures  111  and  121 , respectively. Au-air-bridge structures  111  and  121  may be constructed by first putting down a layer of photoresist (not shown) and subsequently depositing the Ti—Au metal on top of the photoresist. The photoresist is subsequently dissolved away to leave air under Au-air-bridges  111  and  121 . Alternatively, a second dielectric layer (silicon-oxy-nitride, silicon dioxide or silicon nitride, not shown) may be deposited using PE-CVD to isolate p-metal pads  333  and  334  of inner laser diodes  146  and  147 , respectively, with subsequent deposition of Ti—Au contacts on top of the dielectric layer (not shown). 
     The resulting laser diode structure using Au air-bridge structures  111  and  121  is shown in FIG. 6 a . Laser diode facets are cleaved and diced into individual structures. To reduce the laser threshold current, a SiO 2 /TiO 2  high reflective coating is deposited on the front and backside of laser diode facets (not shown) using e-beam evaporation. Typically, it is desirable to use a lower reflectivity for the front laser diode facets. 
     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 alterative, modifications, and variations that fall within the spirit and scope of the appended claims.