Abstract:
Included in the invention are laminates having layers of group III-V materials having low dislocation densities, semiconductor devices fabricated using low dislocation density group III-V layers, and methods for making these structures. Some of the inventions are concerned with GaN layers, GaN semiconductor devices, and semiconductor lasers fabricated from GaN materials. Detailed information on various example embodiments of the inventions are provided in the Detailed Description below, and the inventions are defined by the appended claims.

Description:
BACKGROUND OF THE INVENTIONS 
     The inventions disclosed herein relate to various GaN semiconductor devices, including those with a selective etching low-temperature buffer, and related methods for growing single crystal III-V compound semiconductor layers, including those where such layers include nitrides. 
     For a number of years high quality GaN semiconductor and related semiconductor layers have been sought for the production of semiconductor devices for optical and electronic devices. Recently the need for these layers has been amplified in light emitting devices, such as light emitting diodes (LEDs) and laser diodes (LDs). These devices have applications in displays, optical storage, and others. GaN and its related semiconductors have been used to produce UV, blue, green, red LEDs and UV and near UV lasers. The LEDs using GaN materials have been widely used in large indoor/outdoor displays, traffic signals, local lighting and others. Lasers using GaN materials tend to have life spans on the order of hundreds of hours. It is desirable that such devices have a lifespan of about 10,000 or more hours to be practicable in consumer electronics. The short lifetime in GaN lasers is due to the increased defects in the initial GaN layer which results in defects being transported to light emitting layers. 
     A usual GaN device structure consists of multiple layers of different type of materials and is built using epitaxial methods. Each layer requires near perfect crystal structure. Since there is no lattice matched substrate materials available for GaN material, GaN materials has to be grown in lattice mismatch substrate such as sapphire, SiC and etc. Thus, initial GaN layer grown has high defect density caused by lattice mismatch. The defects generated in initial GaN growth will propagate into sequential layers resulting in high defect density in critical layers for devices. The defects in critical layers of the device are detrimental to lifetime. Thus there is a need for initial GaN layer in mismatched substrate having low densities of defects being producible through conventional and reasonably inexpensive methods. 
     RELATED ART 
     U.S. Pat. No. 5,563,422 and related U.S. Pat. Nos. 5,767,581; 5,877,558 and 6,093,965 describe GaN based semiconductor devices, some of which are light producing. U.S. Pat. No. 5,578,839 and related U.S. Pat. Nos. 5,880,486 and 6,078,063 also describe GaN light-emitting semiconductor devices. U.S. Pat. No. 5,689,123 and related U.S. Pat. No. 5,689,123 describe III-V arsenide-nitride semiconductor devices. U.S. Pat. No. 5,739,554 and related U.S. Pat. No. 6,120,600 describe light emitting diodes having a GaN active layer. U.S. Pat. No. 5,751,752 and related U.S. Pat. No. 6,115,399 light emitting semiconductor devices using GaN based compounds. U.S. Pat. No. 5,886,367 describes a epitaxial wafer and a light emitting devices using the same. U.S. Pat. No. 5,987,048 and related U.S. Pat. No. 6,118,801 describe GaN based semiconductor lasers. U.S. Pat. No. 5,945,689 describes light emitting devices using group III nitride compounds. U.S. Pat. No. 5,970,080 describes GaN light emitting elements and methods of manufacture. U.S. Pat. No. 5,981,977 describes nitride compound semiconductor light emitters and methods of manufacture. U.S. Pat. No. 6,005,258 also describes GaN based light emitting semiconductor devices. U.S. Pat. No. 6,017,807 describes P-type GaN semiconductor devices and methods of manufacture. U.S. Pat. No. 6,020,602 also describes GaN based light emitting devices and methods of manufacture. U.S. Pat. No. 6,027,992 describes GaN semiconductor devices and methods of manufacture. U.S. Pat. No. 6,028,887 describes a GaN semiconductor laser having an AlGaN cladding layer. U.S. Pat. No. 6,033,490 describes methods of growing GaN layers on quartz substrates. U.S. Pat. No. 6,043,140 describes methods of manufacturing nitride compound semiconductor including GaN. U.S. Pat. No. 6,051,849 discloses structures having an underlying gallium nitride layer, a patterned layer, and a vertical and lateral gallium nitride layer having a low disclocation density. U.S. Pat. No. 6,067,309 also describes a GaN light-emitting semiconductor device. U.S. Pat. No. 6,069,021 describes semiconductor devices including group III nitride crystal layers and methods of manufacture. U.S. Pat. No. 6,069,440 describes a nitride compound light emitting device having a garnet fluorescent phosphor. U.S. Pat. No. 6,072,189 describes group III light-emitting diodes and laser diodes on a sapphire substrate. U.S. Pat. No. 6,078,064 describes an InGaN light emitting diode. U.S. Pat. No. 6,091,083 describes a GaN semiconductor light emitting device having a non-flat buffer layer. U.S. Pat. No. 6,093,952 describes a high-power GaN schottky rectifier. U.S. Pat. No. 6,100,106 describes processes of the manufacture of nitride based semiconductor light emitting devices. U.S. Pat. No. 6,110,809 describes the manufacture of wafers with group III metal nitride epitaxial layers. U.S. Pat. No. 6,111,275 describes GaN compound semiconductor light emitting devices and methods of manufacture. U.S. Pat. No. 6,111,277 describes light emitting semiconductor devices having mask layers to prevent threading dislocations from propagating to successive semiconductor layers during manufacture. U.S. Pat. No. 6,113,985 describes the manufacture of group III nitride targets for use in sputtering and similar equipment. U.S. Pat. No. 6,121,636 describes a GaN light emitting diode having a reflective film to prevent the infiltration of light to mounting adhesive. U.S. Pat. No. 6,121,637 describes a packaged light emitting semiconductor device. All of the above U.S. patents in their entirety are hereby incorporated by reference. 
     BRIEF SUMMARY OF THE INVENTIONS 
     Included in the invention are laminates having layers of group III-V materials having low dislocation densities, semiconductor devices fabricated using low dislocation density group III-V layers, and methods for making these structures. 
     OBJECTS OF THE INVENTIONS 
     In the inventions, it is desired to provide methods for growing one or more group III-V compound semiconductor layers of nitrides having low dislocation densities on a substrate that has a large lattice mismatch compared with GaN and other group III-V compounds. Some of the inventions are concerned with GaN layers, GaN semiconductor devices, and semiconductor lasers fabricated from GaN materials. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a - 1   b  depict a method of creating a low dislocation density GaN layer on a substrate. 
     FIGS. 2 a - 2   c  depict a method of creating a low dislocation density GaN layer on a substrate. 
     FIGS. 3 a - 3   i  depict an improved method of creating a GaN layer having areas of reduced dislocations on a substrate. 
     FIG. 4 depicts a semiconductor laser fabricated on a low dislocation area of a GaN layer on a lattice mismatch substrate. 
    
    
     Reference will now be made in detail to some embodiments of the inventions, examples of which are illustrated in the accompanying drawings. 
     DETAILED DESCRIPTION 
     One first related method of creating GaN semiconductor layers uses a sapphire or SiC substrate and metalorganic chemical vapor deposition (MOCVD). Conventional methods utilize a first annealing step in which the substrate is heated to a high temperature, 1000° C. or higher, in a controlled atmosphere, such as pure H2. Because of the large lattice-mismatch between GaN and the substrate, it is necessary to introduce a very thin buffer layer of GaN or AlN grown at a low temperature, for example 400 to 600° C. for a sapphire substrate since the critical layer thickness to release the lattice mismatch between GaN (or AlN) and sapphire or SiC substrate to generate dislocations at low temperature will be thicker than higher growth temperature. As long as the GaN layer grown is less than a critical thickness, the GaN layer will be relatively free of defects. Next, a successive base layer of undoped GaN is grown at a high temperature, typically over 1000° C., to a specified thickness, for example 2 μm. Successive layers of doped GaN or other compatible materials may then be deposited on the base layer to form a semiconductor device. In practice this method yields layers of GaN with relatively high densities of dislocations. 
     In a second related method, two stages being depicted in FIG. 1, a laminate is produced following the procedure of the first method having a substrate  101 , a buffer layer  102 , a base layer  103 . A series of strips of SiO2, Si x N 1−x , or other material is deposited to the base layer in the GaN &lt;1{overscore (1)}00&gt; direction forming a mask layer  104 , as shown in FIG. 1 a . The width of the strips compared to the separation between the strips  105  is usually made to be 1:2. From the openings between the strips  105 , a new high temperature GaN layer  106  is re-grown at a high temperature of more than 1000° C., first vertically and then laterally over the mask strips until the lateral growth fronts coalesce to form a continuous layer, as in FIG. 1 b . The material grown laterally over the mask strips is the overgrowth region  107 . The growth of the high temperature GaN layer is generally thought to proceed by an islanding and coalescence mechanism. The initial growth of the high temperature GaN layer appears in the form of islands with a truncated hexagonal pyramid shape, the islands coalescing where they meet. After coalescence and further growth the layer eventually becomes flat. The dislocation density of the GaN layer in the overgrowth region will be greatly reduced, and can be used as a new high-quality substrate for producing successive GaN compound layers also having a reduced dislocation density. 
     In a third related method, stages of which being shown in FIG. 2, a laminate is produced following the procedure of the first method having a substrate  201 , a buffer layer  202 , a base layer  203 . A patterned mask layer is applied  204  leaving apertures  205 , as shown in FIG. 2 a . The GaN is then selectively etched down to the substrate through apertures  205  leaving troughs  206 , as shown in FIG. 2 b . The pattern mask is then removed. The laminate is then introduced into a MOCVD apparatus for growth of a new high temperature GaN layer  207 , with overgrowth regions  208  having improved dislocation density, as shown in FIG. 2 c.    
     The reader will note that all of the above methods require the step of depositing a several micrometer GaN layer at high temperature over a buffer layer. 
     In the invention, it is desired to provide methods for growing one or more high-quality single crystal III-V compound semiconductor layers of nitrides on a substrate that has a large lattice mismatch compared with GaN, such as sapphire, sapphire/quartz, SiC, Si, GaAs, glass, diamond and others, and to provide devices structures that result from implementation of the methods. 
     FIG. 3 demonstrates steps of a method of the invention by showing a laminate material in cross-section at various stages of the method. The first step of the method is to obtain a substrate  300  such as one of those listed above or another appropriate substrate, as shown in FIG. 3 a . Next, a vapor phase deposition technique is used to grow a low temperature buffer layer  302   a  on substrate  300 , as shown in FIG. 3 b . MOCVD, sputtering deposition, MBE (molecular beam epitaxy) and HVPE (hydride vapor phase epitaxy) are among suitable techniques for growing a low temperature buffer layer. The temperature for growing a low temperature buffer layer according to the invention will preferably be in the range of from about 200° C. to about 600° C., but may be outside that range in some embodiments of the invention. The material for the low temperature buffer layer may include AlN, GaN, or ZnO and other suitable materials. An example of appropriate thickness for the low temperature buffer layer is about 10-50 nm, although low temperature buffer layers of greater or less thickness may also be constructed according to the inventive concepts. 
     After the growth of the low-temperature buffer, a pattern mask layer  304   a  is deposited to the laminate, as shown in FIG. 3 c . Selected portions of the mask are removed leaving islands of mask  304   b  with apertures  306  in between. In some methods the islands are strips, intended to produce strips of low temperature buffer material. Apertures  306  should generally run in the GaN &lt;1{overscore (1)}00&gt; direction in order to achieve low dislocation densities. In a specific method of the invention, the mask layer is a photo-sensitive resist layer deposited in liquid form using spin-coat technology, and baked or cured to harden. This layer is typically at least several micrometers thick to provide structural strength. The photo-resist mask is then exposed to light through a photo mask having a predetermined pattern. The non-exposed portions of the photo-resist are then removed. 
     The low-temperature buffer  302   a  is selectively etched down in the direction of the substrate  300 , by which material from the low temperature buffer layer is removed. The result is a number of low temperature buffer material islands  302   b  separated by troughs  308 , as shown in FIG. 3 e . One skilled in the art will recognize that these islands may be features of a larger mass of low temperature buffer material, such as peninsulas, so long as they are separated by empty troughs. As shown in the drawing, the walls of the islands  302   b  may not necessarily be perpendicular to the substrate, but may have a slope which is a side effect of many etchant processes. A dry etching technology such as ICP (intensed coupled plasma) and RIE (reactive ion etching) can be used for the etching step. Following etching the remaining mask  304   b  is removed in a cleaning step, with the resulting structure of FIG. 3 f . In the troughs  308 , there is preferably no appreciable low temperature buffer material. The ratio of the width of the low temperature buffer islands to the troughs may be as desired, although it will be seen that a wider trough may produce proportionately more areas of low dislocation density material, but may also require longer successive growth periods. 
     Referring again to FIG. 3 f , the substrate  300  with the above-described low temperature buffer islands  302   b  and intervening troughs  306  is next re-introduced into a vapor deposition growth chamber for re-growth of a vertically grown layer. The substrate temperature is raised to a high temperature, preferably greater than 1000° C. At the beginning of the re-growth process, GaN forms only on the low-temperature buffer strips  302   b  but not on the substrate  300  in the troughs  306 . In one preferred method the aforementioned temperature and V/III ratio (the V/III ratio is from 50000 to 500) is considered optimal for the enhancement of vertical growth. A certain thickness of material  309  is grown, preferably about 0.7 μm to about 1 μm, as shown in FIG. 3 g . The V/III ratio is changed (10000 to 100) for the enhancement of lateral growth. Referring to FIG. 3 h , the growth of material continues generally in the direction of the arrows. So called threading dislocations are continued in that direction of growth, leading to material which is relatively high in dislocations. As the growth of material is continued the lateral growth fronts  310  originating from the low temperature buffer islands coalesce at divisions  312  to form a continuous layer of GaN  314 . The growth of the GaN layer is allowed to continue until a desired thickness is achieved in accordance with flatness, structural, thermal, and other requirements. FIG. 3 i  shows the resulting laminate with divisions  312  where growth fronts have met, regions having high dislocation density  316 , and regions of low dislocation density  318 . The surface of the resulting GaN layer includes areas  320  with reduced dislocation density on which other III/V structures may be grown inheriting the low dislocation density of the underlying layer. 
     In practice, the second growth temperature can be ranged from about 800 to about 130° C., V/III ratio can be about 50,000 to about 100, spacing between islands can be ranged from about 10 to about 500 μm, and the layer thickness can be ranged from about 2 to about 100 μm. 
     On a laminate structure as described, a semiconductor device with improved characteristics can be manufactured. Using the techniques described above, a longer life laser device can be grown. An example of such a laser is depicted in the perspective view of FIG. 4. A substrate  401  is provided. The substrate  401  may be of any of the previously mentioned substrate materials or another suitable material. Then low temperature buffer strips  402  with intervening troughs are formed as already described. Next, GaN is re-grown on the strips in a new layer  403   a , and that new layer eventually coalesces over the troughs to form a continuous and uninterrupted layer  403   b . Region  404  provides a low dislocation density substrate for the remaining semiconductor layers. 
     In one example of a laser, an n-type GaN contact layer  405  from 1 to 5 μm in thickness is grown followed by a n-type AlGaN/GaN strain-layer supperlattice (SLS)  406  from 0.3 μm to 1.5 μm in thickness as a cladding layer. Then a 0.1 to 0.2 μm n-type GaN as a guiding layer  407  is deposited. Next, 1 to 10 periods of InGaN/GaN multiple-quantum-well structures, shown as five layers  408   a - 408   e , that are 0.01-0.02 μm in thickness are deposited. These layers  408   a - 408   e  are the active layers from which light is produced. Next, a p-AIGaN electron blocking layer  409  is deposited. Then a 0.1 to 0.2 μm thick p-type GaN layer  410  is formed to serve as a top guiding layer. The top guiding layer  410  is followed by a 0.3 u to 1.5 μm thick p-type AlGaN/GaN SLS as a top cladding layer  411 . Finally, a 0.01 to 0.2 μm p-GaN contact layer  412  is formed. 
     Based on such technology, high performance InGaN/GaN-based LED&#39;s and lasers, AlGaN/GaN-based UV-LED&#39;s and lasers, and GaN-based electron devices can be formed. 
     Those skilled in the art will recognize that the above method is superior than that disclosed in U.S. Pat. Nos. 6,051,849 and 6,111,277 in that fewer steps are required to produce a high-quality gallium nitride layer. More specifically, the steps of creating a surface wide buffer layer and a gallium nitride underlayment are eliminated from the methods of U.S. Pat. No. 6,051,849. Steps to create additional mask layers, as in the methods of U.S. Pat. No. 6,111,277, are not performed. 
     While the present devices and methods have been described and illustrated in conjunction with a number of specific configurations, those skilled in the art will appreciate that variations and modifications may be made without departing from the principles herein illustrated, described, and claimed. The present invention, as defined by the appended claims, may be embodied in other specific forms without departing from its spirit or essential characteristics. The configurations described herein are to be considered in all respects as only illustrative, and not restrictive. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.