GB2435715A - Vapour phase epitaxial growth of thick II-VI semiconductor crystals - Google Patents

Vapour phase epitaxial growth of thick II-VI semiconductor crystals Download PDF

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GB2435715A
GB2435715A GB0526070A GB0526070A GB2435715A GB 2435715 A GB2435715 A GB 2435715A GB 0526070 A GB0526070 A GB 0526070A GB 0526070 A GB0526070 A GB 0526070A GB 2435715 A GB2435715 A GB 2435715A
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Arnab Basu
Ben Cantwell
Max Robinson
Andy Brinkman
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Durham Scientific Crystals Ltd
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Durham Scientific Crystals Ltd
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    • C30B23/00Single-crystal growth by condensing evaporated or sublimed materials
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    • C30B25/02Epitaxial-layer growth
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    • C30B25/00Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
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    • C30B29/00Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10Inorganic compounds or compositions
    • C30B29/46Sulfur-, selenium- or tellurium-containing compounds
    • C30B29/48AIIBVI compounds wherein A is Zn, Cd or Hg, and B is S, Se or Te
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    • H01L21/18Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer, carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/20Deposition of semiconductor materials on a substrate, e.g. epitaxial growth solid phase epitaxy
    • H01L21/205Deposition of semiconductor materials on a substrate, e.g. epitaxial growth solid phase epitaxy using reduction or decomposition of a gaseous compound yielding a solid condensate, i.e. chemical deposition
    • H01L21/2053Expitaxial deposition of elements of Group IV of the Periodic System, e.g. Si, Ge
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    • H01L21/18Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer, carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/20Deposition of semiconductor materials on a substrate, e.g. epitaxial growth solid phase epitaxy
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    • H01L21/2056Epitaxial deposition of AIIIBV compounds

Abstract

A series of lattice mismatch layers layer having lattice constants that changes from a value that equals the growth substrate to a value that equals the lattice constant of a II-VI semiconductor layer are deposited on the growth substrate. The lattice mismatch layers are grown to a total thickness of about 25-1000 microns to ensure that any misfit dislocations are grown out and that any strain is located in the substrate. The high deposition rate hydride vapour phase epitaxy method is then used to grow a thick ( 700 microns) bulk II-VI semiconductor crystal layer on the lattice mismatch layers.

Description

<p>DEVICE</p>

<p>Field of the invention</p>

<p>The present invention relates to a device structure.</p>

<p>Discussion of the prior art</p>

<p>Various techniques have been disclosed for the growth of bulk single crystal materials. For example, it is known to use vapour transport techniques using a simple linear system with a source and sink of single crystals of Il-VI compounds, such as CdS, ZnS and ZnSe which sublime easily from the solid phase to deposit the compounds onto a seed crystal. Traditionally, single crystals have also been formed using direct solidification techniques, such as by the Bridgman, travelling heater (THM) or gradient freeze (GF) methods in which the crystals are grown from the melt.</p>

<p>Such systems are not well-suited for depositing or growing other crystal materials, such as cadmium telluride or cadmium zinc telluride. Where these conventional methods are used to form such materials, it is difficult to form high quality crystals consistently, or to form crystals having a diameter greater than 25 or 50mm. In particular, with these known methods of crystal formation, dislocations, sub-grain boundaries, pipes and twins form easily.</p>

<p>Also, the process tends to form precipitates and inclusions due to the excess telluride in the melt. Tellurium inclusions can be tens of microns in size and this may be significant for detector applications. Further, there will be a dislocation cloud associated with each inclusion which will affect the performance of detectors formed from the crystal.</p>

<p>Other processes have been disclosed in the past which enable the growth of such compounds. One example of such a method is the vapour phase deposition method disclosed in the European Patent EP-B-I 019568.</p>

<p>In European Patent No EP-B-1019568 a method of forming crystals using a vapour phase technique is disclosed. This process is known as Multi-Tube Physical Vapour Phase Transport (MTPVT). According to this method, a sink or seed crystal of the same or similar material to be grown is provided.</p>

<p>Vapour phase material is provided to the sink or seed crystal, causing nucleation and subsequent deposition of the material to grow the crystal onto the sink or seed crystal. The resulting crystal will have the same or similar composition as the sink or seed crystal. In particular, EP-B-1019568 discloses a method in which the sink or seed crystal is provided in a sink zone which is connected to a source zone via a passage able to transport vapour from the source zone to the sink zone. The temperature in the source and sink zones are controllable independently, the zones being thermally isolated.</p>

<p>Single crystal compound semiconductors have a number of important applications. For example, cadmium telluride (CdTe) and cadmium zinc telluride (CZT) semiconductors are useful as x-ray and gamma-ray detectors which have application in security screening, medical imaging and space exploration amongst other things.</p>

<p>Summary of the invention</p>

<p>According to the present invention, there is provided a structure including a substrate, and interfacial region provided and formed directly onto the substrate, and a bulk crystal material provided and formed directly onto, or as an extension of, the interfacial region.</p>

<p>It has previously been considered possible to form a bulk crystal material only on a substrate having a complementary lattice structure. This is because crystal mismatches between the substrate and bulk crystal material prevent the formation of the bulk crystal material on the substrate, or would result in unacceptable stresses between the materials affecting the device unacceptably. For example, it is not generally possible to provide a cadmium telluride crystal material, which will have a lattice parameter a=6.481 A directly onto a silicon substrate which will have a lattice parameter a=5.4309 A due to the lattice mismatch. Accordingly, this limits the bulk crystal material that can be grown on any given substrate. However, the inventors have found that the inclusion of an interfacial region between the substrate and the bulk crystal material according to the present invention enables a gradual change in the crystal structure between the substrate and bulk crystal that can compensate for any mismatch in the lattice structure of the substrate and deposited crystal material.</p>

<p>In a preferred embodiment, the interfacial region includes a transition region in which there is a transition from the material of the substrate to an intermediate layer. The intermediate layer may have a lattice structure that is compatible with the lattice structure of the bulk crystal material.</p>

<p>Alternatively, the interfacial region may include a region in which there is a transition from an intermediate layer having a lattice structure compatible with the substrate to the material of the bulk crystal.</p>

<p>In a particularly preferred embodiment, the interfacial region includes a first transition region in which the material changes from the material of the substrate to an intermediate layer, and a second transition region in which there is a change from the intermediate layer to the material of the bulk crystal. In this way, there is a transition between the materials of the substrate and bulk crystal, with associated change in the lattice structure. In this case, the overall structure will comprise a substrate, a first transition region formed on the substrate, an intermediate layer, a second transition region formed on the intermediate layer and a bulk crystal material formed on the second transition region.</p>

<p>Where one or more transition regions are provided, these may be formed as regions in which there is a gradual change from the material of the type of an underlying layer to a material of the type of an overlayer, or maybe formed as one or more discreet layers of different materials.</p>

<p>With the structure of the present invention, it is possible to form bulk crystal materials which differ from the substrate on which they are formed, and in particular which have a different lattice structure from the underlying substrate. These composite layered materials may have better physical or structural properties than conventionally known materials, and therefore may have different applications.</p>

<p>In accordance with the present invention, particular examples of substrates include silicon, gallium arsenide, germanium, silicon carbide and sapphire substrates. The bulk crystal materials formed may include semiconductors such as cadmium tellurjde and cadmium zinc telluride, zinc selenide (ZnSe), cadmium sulphide (CdS), zinc telluride (ZnTe), gallium nitride (GaN), silicon carbide (SiC) and zinc sulphide (ZnS). The interfacial region in which there is a transition from the material of the underlying substrate to the bulk crystal material may include an intermediate layer of the same or different material to the bulk crystal material. For example, in addition to the use of cadmium telluride or cadmium zinc telluride for the intermediate layer, layers such as CdS, GaP, GaAs, CdSSei. and CdSe may be used.</p>

<p>In a preferred example, the interfacial region and bulk crystal can be deposited using the same growth technique, but with an initial variation in the growth parameters during the growth cycle to gradually change the composition and growth rate of the material deposited on the substrate.</p>

<p>During the initial transition, the interfacial region is formed. After completing the change to the material of the bulk crystal to be deposited, the growth rate can be accelerated to rapidly deposit the bulk crystal material.</p>

<p>In this case, it is preferred that the apparatus includes a means for introducing different source materials to be deposited onto the substrate.</p>

<p>In addition to the substrate, interfacjal region or layer and the bulk crystal material, additional layers may be deposited. For example, a metal layer such as a layer of indium, platinum, gold or aluminium may be formed for electrical contact. Alternatively or additionally a dielectric layer may be provided. This is especially useful where the structure is to be used as a radiation detector as the dielectric layer may act as a filter to block visible and near infra red light.</p>

<p>Brief description of the drawings</p>

<p>The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a suitable multi-tube physical vapour phase transport device for growing structures according to the present invention; and, Figure 2 shows a cross section of a material structure according to the present invention.</p>

<p>Detailed description of a Preferred example</p>

<p>A preferred apparatus for the formation of a structure according to the present invention is shown in Figure 1. The apparatus is suitable for forming bulk single crystal materials. Bulky materials are generally those with a thickness greater than that of a thin film, and so are generally at least 300 microns thick.</p>

<p>Generally bulk crystal materials will have a thickness of at least 500 microns.</p>

<p>The apparatus comprises an evacuated U-tube in the form of a quartz envelope 20 encased in a vacuum jacket 21. Two separate three zone vertical tubular furnaces are provided 22, 23 for the source 24 and the sink zone 25 respectively. The source and sink zones are connected by an optically heated horizontal crossmember 27 forming a passage 26. A flow restrictor 28 is provided in the passage 26. The passage comprises two separate points of deviation -in each case at an angle of 900 -providing respective junctions between diverging passages for in-situ monitoring and vapour transport from the source to the sink zone. Windows allowing optical access to source and sink respectively are provided. The temperature of the surface of growing crystal in the sink zone can be monitored by a pyrometer or other optical diagnostic apparatus 33 located external to the vacuum jacket and in optical communication with the surface of the growing crystal. The diagnostic apparatus is in communication with a suitable control system to vary the sink zone temperature. The apparatus also comprises means for in-situ monitoring of vapour pressure by access ports 33 to 36 in the region of the flow restrictor 28, through which vapour pressure monitoring lamps and optics may be directed from a position external to the vacuum jacket with detectors located as shown at a location 35, 36 diametrically opposed with respect to the passage for vapour transport 26. These are suitably linked to a control system providing for process control.</p>

<p>The source tube, growth tube and crossmember, in which transport takes place, are fabricated from quartz and the system is demountabje with ground glass joints between the crossmember and the two vertical tubes allowing removal of grown crystals and replenishment of source material. Radiation shields (not shown for clarity) together with the vacuum jacket which surrounds the entire system provide thermal insulation. A flow restrictor such as a capillary or a sintered quartz disc is located in the centre of the crossmember Growth takes place on a substrate located on a quartz block in the growth tube with the gap between this glass block and the quartz envelope forming the downstream flow restrictor. Provision is made for a gas inlet to the source tube and the growth tube may be pumped by a separate pumping system or by connection to the vacuum jacket via a cool dump tube.</p>

<p>The structure of the device according to the present invention is shown in Figure 2. As will be described in more detail below, the structure comprises a substrate 10, and interfacial region 12 and a bulk crystal material 14. In a preferred example, the overall structure can be defined by the formula a: xl,23.:b:y123:c where a is the substrate 10, c is the bulk crystal material 14 and xi,2,3..:b:y123 is the interfacial region 12. In this case, b is the intermediate layer, x123 is a transition region between the substrate 10 and the intermediate layer b comprising one or more layers x1, x2, x3 etc, and Y1,2,3... is a transition region between the intermediate layer b and the bulk crystal material C comprising one or more layers Yi, Y2, y3 etc. The substrate 10 is provided in the apparatus as described with respect to Figure 1. The substrate can be one of a number of different materials, including silicon, gallium arsenide, germanium, silicon carbide and sapphire.</p>

<p>The substrate 10 will typically have a thickness greater than 100 microns, preferably of at least 200 microns for mechanical stability and can have any available size. Silicon substrates with a diameter of up to 300mm are currently available.</p>

<p>A source is provided to supply a material to be deposited onto the substrate 10.</p>

<p>There are a number of factors which determine whether a particular material can suitably be deposited on an existing layer, or whether problems will arise from the mismatch between the adjacent layers. A mismatch may occur where there is a mismatch between parameters such as the lattice parameters, the thermal expansion coefficient and/or the coefficients of elasticity. Ideally, the parameters for the material of adjacent layers should be as close as possible to minimise mismatches. Where there is a large difference in the lattice parameters for adjacent layers, for example where the difference between lattice parameters is greater than 3%, misfit dislocations will occur as the subsequent layer is deposited. However, these misfit disclocatjons will in most cases grow out over the first few atomic layers -typically within 10 microns -so that the remainder of the material will be fully relaxed. However, this relaxation occurs only at the temperature of growth.</p>

<p>Where there is a difference between the thermal expansion coefficients of the adjacent layers, at temperatures other than the temperature of growth, there will be thermal strain. Such strain can be transmitted to other layers in the structure, for example to the substrate or crystal material. Where the crystal material is sufficiently thick, the strain will generally be located in the substrate. For example, it has been found that when a CdTe layer, with a thickness of about 250 microns, is formed on a 350 micron gallium arsenide substrate at 500 C, there will be substantially no strain in the CdTe layer when the device is held at a temperature of around 700 C during subsequent crystal formation.</p>

<p>According to the example of the present invention, the source is selected so as to initially deposit a first transition region x onto the substrate 10. The first transition region x should have similar lattice parameters to that of the substrate 10 and the intermediate layer 12. The first transition region x will have a thickness of between about 10 and 200 microns. As discussed above, a thickness of 10 microns will be sufficient for misfit dislocations to grow out, and a thicker layer will help ensure that any strain will be primarily located in the substrate. The region may comprise a number of layers x1, x2, x3 etc of different materials or properties to complete the transition from the substrate 10 to the intermediate layer 12. For example, where the substrate is a silicon substrate, this will have a lattice parameter a=5.4309 A. In this case, and the first transition region may comprise an initial layer of GaP deposited on the substrate. GaP has a lattice parameter a=5.4506A. This lattice parameter is sufficiently close to that of the underlying silicon substrate 10 that any lattice mismatch is minimised. The source material supplied to the growth chamber may be altered so as to deposit a gallium arsenide intermediate layer.</p>

<p>Gallium arsenide has a lattice parameter a=5.6533A. This is sufficiently close to the lattice parameter of the GaP layer as to minimise any lattice mismatch.</p>

<p>After forming the first transition region x, the intermediate layer 12 is deposited. This layer will typically have a thickness of about 25 to 1000 microns, preferably in the region of 100 to 700 microns. An intermediate layer of this thickness will withstand any initial sublimation of the layer during the initial stages of bulk crystal growth. The material will have a structure similar to that of both the substrate 10 and the bulk crystal material 14. In this case, the intermediate layer may be CdS which has a lattice parameter a=5.82A.</p>

<p>After depositing the intermediate layer 12, a second transition region y is formed. The second transition region y will have a thickness of between about 10 and 200 microns, preferably up to about 500, to achieve lattice and thermal matching. The region may comprise a number of layers y1, Y2, y3 etc of different materials or properties to complete the transition from the intermediate layer 12 to the bulk crystal material 14. In the particular example, the second transition layer may comprise a single layer of CdSe having a lattice parameter a=6.05A.</p>

<p>After forming the second transition layer y, the bulk crystal material 14 can be deposited by changing the source material. A preferred bulk crystal material include cadmium telluride which will have a lattice parameter a=6.481A. The bulk crystal material may be deposited to a thickness of about 700 microns.</p>

<p>This is important where the material is required to ensure effective absorption of high energy radiation. It has been found that to absorb 90% of x-rays at KeV, a thickness of 11mm is required.</p>

<p>During the formation of the interfacial region, the growth parameters are controlled such that the transitional region has a desired thickness. Once the transition has been made to the bulk crystal material to be deposited, the growth parameters can be adjusted so that the bulk crystal material can be deposited at a higher rate.</p>

<p>Various possible material structures can be achieved in accordance with the present invention. In general, each of these structures will have a substrate, a transitional region from the substrate to an intermediate layer, the intermediate layer itself, a transition from the intermediate layer to the bulk crystal material, and the bulk crystal material itself. The transitional regions will typically be very small compared to the substrate, intermediate layer and bulk crystal material, and therefore the effects are considered negligible in the overall device.</p>

<p>The selection of the substrate will generally be determined by the availability of substrates of a required size, but other factors include the mechanical strength, thermal expansion and elasticity coefficients required for a desired application. Differences in the lattice parameters and elasticity and thermal expansion coefficients between the bulk crystal material and substrate can be compensated for in accordance with the present invention, although it will be appreciated that if the substrate can be chosen to minimise these differences, the overall structure may be improved.</p>

<p>Examples of possible structures, giving the substrate, intermediate layer and bulk crystal material are set out below.</p>

<p>Example Substrate Intermediate Bulk Overall Structure Layer + Crystal trace elements I Si CdTe -CdTe Si: CdTe: CdTe 2 Si CZT CZT Si:CZT:CZT 3 Si CZT CdTe Si: CZT: CdTe 4 Si CdTe CZT Si: CdTe: CZT GaAs CdTe CdTe GaAs: CdTe: CdTe 6 GaAs CZT CZT GaAs: CZT: CZT 7 GaAs CZT CdTe GaAs: CZT: CdTe 8 GaAs CdTe CZT GaAs: CdTe: CZT 9 Ge CdTe CdTe Ge: CdTe: CdTe Ge CZT CZT Ge: CZT: CZT 11 Ge CZT CdTe Ge: CZT: CdTe 12 Ge CdTe CZT Ge:CdTe:CZT 13 -Silicon CdTe CdTe Silicon Carbide: CdTe Carbide CdTe 14 Silicon CZT CZT Silicon Carbide: CZT: CZT Carbide Silicon CZT CdTe Silicon Carbide: CZT Carbide CdTe 16 Silicon CdTe CZT Silicon Carbide: CdTe Carbide CZT 17 Si ZnTe ZnTe Si:ZnTe:ZnTe 18 GaAs ZnTe ZnTe GaAs:ZnTe*znre 19 Si ZnTe ZnSe Si:ZnTe:ZnSe GaAs ZnTe ZnSe GaAs: ZnTe: ZnSe 21 Si CdS CdS Si:CdS:CdS 22 GaAs CdS CdS GaAS:CdS:CdS 23 SiC CdS CdTe SiC: CdS: CdTe 24 SIC CdS CZT SIC:CdS:CZT Si GaN GaN Si:GaN:GaN 26 GaAs GaN GaN GaAS:GaNGaN One particular advantage of devices made in accordance with the present invention is that the different materials used to form the substrate, intermediate layer and bulk crystal material may provide different functions in the final apparatus. For example, in the example of a silicon substrate, cadmium telluride bulk crystal material, the cadmium telluride material may be used to detect high-energy photons, whilst the silicon substrate may be able to detect lower energy photons.</p>

Claims (1)

  1. <p>CLAIMS</p>
    <p>1. A structure including a substrate, and interfacial region provided and formed directly onto the substrate, the interfacial region including an intermediate layer, and a bulk crystal material provided and formed directly onto, or as an extension of, the interfacjal region.</p>
    <p>2. A structure according to Claim 1, in which the substrate comprises a substrate of silicon, gallium arsenide, germanium, silicon carbide or sapphire.</p>
    <p>3. A structure according to Claim 1 or Claim 2, in which the substrate has a thickness of at least 100 microns, preferably at least 200 microns.</p>
    <p>4. A structure according to any one of the preceding claims, in which the substrate has a diameter greater than 25mm.</p>
    <p>5. A structure according to any one of the preceding claims, in which the bulk crystal material comprises cadmium telluride, cadmium zinc tefluride, zinc selenide (ZnSe), cadmium suiphide (CdS), zinc telluride (ZnTe), gallium nitride (GaN), silicon carbide (SiC) or zinc sulphide (ZnS).</p>
    <p>6. A structure according to any one of the preceding claims, in which the bulk crystal material has a thickness of at least 700 microns.</p>
    <p>7. A structure according to any one of the preceding claims, in which the intermediate layer comprises CdTe, CZT, CdS, GaP, GaAs, CdSSei.. or CdSe.</p>
    <p>8. A structure according to any one of the preceding claims, in which the intermediate layer has a thickness of between 25 and 1000 microns.</p>
    <p>9. A structure according to any one of the preceding claims, in which the interfacial region includes a transition region in which there is a transition from the material of the substrate to the intermediate layer. S 13</p>
    <p>10. A structure according to Claim 9, in which the transition region between the substrate and the intermediate layer has a thickness of between 10 and 250 microns.</p>
    <p>11. A structure according to Claim 9 or Claim 10, in which the transition region between the substrate and the intermediate layer comprises a plurality of layers of material having different structure and/or composition.</p>
    <p>12. A structure according to any one of the preceding claims, in which the interfacial region includes a transition region from the intermediate layer to the material of the bulk crystal.</p>
    <p>13. A structure according to Claim 12, in which the transition region between the intermediate layer to the material of the bulk crystal has a thickness of between 10 and 500 microns.</p>
    <p>14. A structure according to Claim 12 or Claim 13, in which the transition region between the intermediate layer to the material of the bulk crystal comprises a plurality of layers of material having different structure and/or composition.</p>
    <p>15. A method of forming a structure according to any one of the preceding claims, in which the interfacial region and the bulk crystal are deposited using the same growth technique, using a variation in the growth parameters during the growth cycle to form the interfacial region and the bulk crystal.</p>
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PCT/GB2006/004864 WO2007072023A1 (en) 2005-12-21 2006-12-21 Semiconductor device and method of manufacture thereof
US12158114 US20090053453A1 (en) 2005-12-21 2006-12-21 Semiconductor device and method of manufacture thereof
EP20060831438 EP1969622A1 (en) 2005-12-21 2006-12-21 Semiconductor device and method of manufacture thereof
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Citations (6)

* Cited by examiner, † Cited by third party
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JP2001200366A (en) * 1999-12-30 2001-07-24 Samsung Electro Mech Co Ltd Method for producing crack-free gallium nitride thick film by hydride vapor phase epitaxial growth method
US20020028564A1 (en) * 2000-07-10 2002-03-07 Kensaku Motoki Method of producing a single crystal gallium nitride substrate and single crystal gallium nitride substrate
US6733591B2 (en) * 1998-06-18 2004-05-11 University Of Florida Research Foundation, Inc. Method and apparatus for producing group-III nitrides
US6784085B2 (en) * 2000-11-30 2004-08-31 North Carolina State University MIIIN based materials and methods and apparatus for producing same
US20050247260A1 (en) * 2004-05-07 2005-11-10 Hyunmin Shin Non-polar single crystalline a-plane nitride semiconductor wafer and preparation thereof
US20060011135A1 (en) * 2001-07-06 2006-01-19 Dmitriev Vladimir A HVPE apparatus for simultaneously producing multiple wafers during a single epitaxial growth run

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6733591B2 (en) * 1998-06-18 2004-05-11 University Of Florida Research Foundation, Inc. Method and apparatus for producing group-III nitrides
JP2001200366A (en) * 1999-12-30 2001-07-24 Samsung Electro Mech Co Ltd Method for producing crack-free gallium nitride thick film by hydride vapor phase epitaxial growth method
US20020028564A1 (en) * 2000-07-10 2002-03-07 Kensaku Motoki Method of producing a single crystal gallium nitride substrate and single crystal gallium nitride substrate
US6784085B2 (en) * 2000-11-30 2004-08-31 North Carolina State University MIIIN based materials and methods and apparatus for producing same
US20060011135A1 (en) * 2001-07-06 2006-01-19 Dmitriev Vladimir A HVPE apparatus for simultaneously producing multiple wafers during a single epitaxial growth run
US20050247260A1 (en) * 2004-05-07 2005-11-10 Hyunmin Shin Non-polar single crystalline a-plane nitride semiconductor wafer and preparation thereof

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