Abstract:
A low defect gallium nitride based semiconductor, and method for its production, is disclosed. A first gallium nitride based semiconductor layer overlying a substrate of a dissimilar material is grown. A trench is formed in the first gallium nitride based semiconductor layer. A material is deposited on a surface of the first gallium nitride based semiconductor layer to prevent a second gallium nitride based semiconductor layer, of a material different from the first gallium nitride based semiconductor layer, from nucleating thereon. The bottom surface of the trench is of a material such that the second gallium nitride based semiconductor layer will not nucleate thereon. The second gallium nitride based semiconductor material is grown, extending from at least one of the side walls of the trench, the second gallium nitride based semiconductor material having fewer defects than the first gallium nitride based semiconductor layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of patent application Ser. No. 09/263,654, filed Mar. 5, 1999, now U.S. Pat. No. 6,327,288 entitled “Buried Heterostructure For Lasers And Light Emitting Diodes,” having Shih-Yuan Wang and Yong Chen as inventors. This application is assigned to LumiLeds Lighting, U.S., LLC, the assignee of the present invention, and is hereby incorporated by reference in its entirety and for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor laser diodes and more particularly relates to edge-emitting semiconductor lasers. 
     BACKGROUND OF THE INVENTION 
     Blue laser diodes (LDs) have the potential for increasing the storage capacity of optical disks over the densities currently available in compact disk systems based on red laser diodes. Increased storage capacity will open new markets for compact disks in motion picture distribution. 
     One class of blue emitting elements is based on group III-V nitride films such as GaN epilayers grown on sapphire substrates. To fabricate a laser, a ridge structure is constructed to provide an appropriate optical cavity having parallel mirrors at each end of the cavity. The laser cavity is typically formed by sandwiching an active gain layer between two layers of GaN doped to form n-type and p-type semiconductors. The GaN layers are constructed so as to form a waveguide by depositing the various layers and then etching the stack to form a ridge structure whose vertical walls provide the waveguide. The ends of the waveguide are mirrors that reflect the light generated in the active region back and forth. In GaN based LDs the mirrors are typically formed by cleaving or etching the ends of the waveguide to provide the reflecting surface of the mirror. 
     The ridge structure discussed above has two problems. First, the structure has poor heat dissipation. The heat generated in the active region must either be dissipated through the substrate or the walls of the ridge structure. The path to the substrate is restricted by the width of the ridge structure; hence, removing heat by transferring the heat to the substrate, which is typically in thermal contact with a heat sink, is difficult. 
     The second problem with ridge structured devices is the high voltages needed to operate the devices. The p-contact is typically an ohomic contact on the top of the ridge. The resistance of this contact must be overcome to drive the device. To reduce this resistance, the contact needs to have as large an area as possible. However, the available area is limited by the area on the top of the ridge. 
     Broadly, it is the object of the present invention to provide an improved edge emitting laser diode. 
     It is a further object of the present invention to provide an edge emitting diode that does not utilize a ridge structure, and hence, avoids the above-described problems. 
     These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     A gallium nitride based semiconductor material, and method for its production, that has fewer defects than an underlying gallium nitride based semiconductor material is disclosed. This invention includes the steps of forming a first gallium nitride based semiconductor layer overlying a substrate of a dissimilar material, the first gallium nitride based semiconductor layer having defects due to a lattice mis-match between the substrate and the first gallium nitride based semiconductor layer, forming a trench in the first gallium nitride based semiconductor layer, the trench having a bottom surface and side walls, depositing a first material on a surface of the first gallium nitride based semiconductor layer to prevent a second gallium nitride based semiconductor layer from nucleating on the surface of the first gallium nitride based semiconductor layer, and growing the second gallium nitride based semiconductor layer, of a material different from the first gallium nitride based semiconductor layer, extending from at least one of the side walls, a bottom surface of the trench being of a material such that the second gallium nitride based semiconductor layer will not nucleate thereon, the second gallium nitride based semiconductor layer having fewer defects than the first gallium nitride based semiconductor layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are cross-sectional views of a typical prior art laser diode at two different stages in the fabrication thereof. 
         FIGS. 3-6  are cross-sectional views through a GaN-based laser according to the present invention at various stages in the fabrication process. 
         FIG. 7  is a cross-sectional view of a laser diode according to the present invention in which the active layer is confined by the trench used to form the waveguide. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention may be more easily understood with reference to  FIGS. 1 and 2 , which illustrate the fabrication of a typical prior art laser diode  100 . Refer now to  FIG. 1  which is a cross-sectional view of the layers deposited on a sapphire substrate  8  as the first step in fabricating a LD  100 . An AIN layer is deposited by metal-organic chemical vapor deposition (MOCVD). AIN is preferred as the buffer material because a flat film consisting of a GaN single crystal is easily formed on an AlN layer. 
     After depositing the AlN buffer layer  9 , a layer  10  of n-type GaN is deposited. The light-emitting part of the layer composed of GaN based cladding layers  11  and a GaN based active layer  12 , and a p-type GaN layer  13  are deposited on layer  10 . The cladding layers form the top and bottom walls of the laser waveguide. Referring to  FIG. 2 , the layers are then etched to create the side walls of the waveguide and to expose the n-type contact as shown at  101 . The exposed area is used to make a connection that becomes the cathode terminal of the LD. The positive terminal is deposited on p-type layer  13 . As pointed out above, the limited area available for the p-type contact imposes a minimum resistance on the contact, which, in turn, increases the operating voltage of the device. 
     The present invention avoids the need for a ridge structure that defines the light guide by burying a portion of the laser in a trench that is cut in a material having a substantially different index of refraction than the material from which the active region is fabricated. The portion of the device in the trench provides a side waveguide whose influence extends beyond the trench to provide the necessary containment of the light. In addition, the trench structure of the present invention provides a means for growing GaN layers that have a very low level of defects thereby providing additional benefits. Finally, the trench design of the present invention provides a self-aligning fabrication system with only one critical mask step. 
     The manner in which the present invention gains its advantages can be more easily understood with reference to  FIGS. 3-6 , which are cross-sectional views through a GaN-based laser  200  according to the present invention at various stages in the fabrication process. Refer first to FIG.  3 . Laser  200  is fabricated on a sapphire substrate  210  on which a buffer layer  212  of AlN and a base layer  213  of GaN have been epitaxially grown. A layer of AlGaN is grown on top of layer  213  as shown at  214  and a trench  216  is etched in layer  214 . The etching of trench  216  is the only precision masking step in the fabrication process. Trench  216  defines the location of the laser on the substrate. 
     As will be appreciated by those skilled in the art, there is a significant level of defects in GaN layer  213  that result from the lattice mis-match between sapphire and GaN. Even when buffer layer  212  is utilized, this level of defects degrades the performance of a laser constructed on GaN layer  213 . The AlGaN layer also has an undesirable level of defects that are seeded from the defects in GaN layer  213 . 
     One method for reducing the defect density is to epitaxially grow an additional GaN-based layer by extending an existing GaN surface over a material on which GaN will not nucleate. The present invention utilizes this technique to provide an essentially defect-free region on which the laser is fabricated. In addition, this region acts as the laser&#39;s lateral waveguide. 
     Referring again to  FIG. 3 , a layer of a material on which GaN will not nucleate is deposited on the bottom and one wall of trench  216  as shown at  217 . The preferred material is also an electrical conductor for reasons that will be discussed in detail below. In the preferred embodiment of the present invention TiN is used for this purpose. The TiN layer is deposited by a conventional shadow deposition technique. In addition, a dielectric layer  215  such as SiO 2  is deposited on the top surface of AlGaN layer  214 . It should be noted that GaN will not nucleate on SiO either. 
     The substrate is then placed in a deposition chamber and GaN is epitaxially grown in trench  216  by extending the wall of the trench as shown by arrow  218 . Any dislocations in the material grown from the wall will be lateral. Such a dislocation is shown at  219  in  FIG. 4 , which is a cross-sectional view of laser  200  after the extension growth of the GaN layer. The top surface  236  of the GaN layer is essentially defect free. Hence, any new layers grown seeded from this surface will also be substantially defect free. In addition, any layers grown from this layer that extend out over the SiO 2  layer will also be substantially defect free, since GaN will not nucleate on SiO 2 . 
     Refer now to  FIG. 5 , after growing GaN layer  220  by extending the wall of the trench, the additional layers needed to complete the laser are grown. The first layer is an n-type cladding layer  221  consisting of an n-AlGaN sub-layer and a n-GaN sub-layer that are grown on the top surface of  236  of GaN in the trench. To simplify the drawing, the individual sub-layers making up the cladding layers are shown as a single layer. The cladding layer is grown out over the SiO 2  layer  215 . The active layer  222  is then grown over the cladding layer. A p-type cladding layer  223  consisting of a p-GaN sub-layer and a p-AlGaN sub-layer is then grown over the active layer. A p-GaN layer  224  is then grown over the p-cladding layer. Finally, a p-ohmic contact layer  225  is deposited on layer  224 . 
     After the various layers discussed above have been deposited, the stack is etched back to GaN layer  213  using standard photolithographic techniques for masking. Either wet chemical etching or reactive ion etching is used for this etching operation. The n-ohmic contacts  226  are then deposited as shown in FIG.  6 . 
     As noted above, the material used to coat the walls of trench  216  is preferably an electrical conductor. While the device will still operate if an insulator is utilized, such embodiments may suffer from current crowding along the wall of the trench that is not insulated. Hence, a conductor is preferred. 
     It should also be noted that the SiO 2  layer  215  provides current confinement for the device. Hence, the device does not need to be etched to provide the current confinement. As a result, a larger p-contact may be utilized, thereby reducing the resistance of the p-contact. As noted above, p-contact resistance is a significant problem in prior art devices. 
     It should also be noted that the difference in index of refraction between the material in the trench  220  and the surrounding layer  214  from which the trench is etched provides the light guiding normally provided by the edges of conventional ridge-structure. A difference in index of refraction of 1% or greater will provide the necessary light guide even though a significant portion of the device is constructed outside of the trench. The material in the trench must have an index of refraction that is greater than that of the material in which the trench is cut. Accordingly, a much larger ridge structure can be utilized. Hence, the area available for the p-contact is further enhanced relative to prior art devices. 
     A laser according to the present invention does not require any critical mask alignment operations during the fabrication process. The position of the trench mask determines the position of the laser on the die. The alignment of the final stack etch mask relative to the trench is not critical. The edges left by the stack etch no longer act as a wave guide, and hence, the positions of these etches relative to the trench are not critical provided there is sufficient distance between the trench and the edges to allow the light guiding provided by the trench to dominate the light guiding process. 
     The above-described embodiments of the present invention had only a portion of the n-cladding layer in the trench itself. However, embodiments of the present invention in which other layers are confined in the trench may be constructed by increasing the thickness of the SiO 2  dielectric layer such that these layers are confined by the sides of the dielectric layer. Such an embodiment is shown in  FIG. 7 , which is a cross-sectional view of a laser  300  according to another embodiment of the present invention. Laser  300  differs from laser  200  in that a much thicker dielectric layer  335  is used in place of layer  215 . This thicker layer allows the entire n-cladding layer  321 , the active layer  322 , and a portion of the p-cladding layer  323  to be grown in the trench. The remaining layers and fabrication steps are the same as discussed above with reference to laser  200 . Those layers that serve the same function as the layers shown in  FIGS. 3-6  have been given the same numeric designations in FIG.  7 . By constructing a greater portion of the layers in the trench, stronger light guiding is provided by the trench. It should be noted that device  300  can also be constructed by depositing the dielectric layer  215  shown in  FIGS. 3-6  such that the dielectric layer overlaps the upper part of trench  218  on both sidewalls. This embodiment does not require a thick dielectric layer, since the dielectric in the upper part of the trench is provided by the overlap. 
     The above-described embodiments of the present invention utilized an n-GaN layer in the trench. However, it will be obvious to those skilled in the art from the preceding discussion that embodiments in which the p-type layers are deposited in a trench in a p-type material having a different index of refraction from the layers grown in the trench may also be constructed. 
     Furthermore, the method of the present invention may also be applied to laser diodes constructed from other material systems. While other material systems may not benefit from all of the advantages of the present invention, the lack of critical masking steps will provide a significant benefit in other laser diode systems. 
     Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.