III group nitride semiconductor substrate, substrate for group III nitride semiconductor device, and fabrication methods thereof
What is claimed is:
1. A method for fabricating a III group nitride semiconductor substrate, comprising the steps of: forming a metal film on a starting substrate, the metal film having a firstsurface contacting the starting substrate, and a second surface on an opposite side of the metal film from the first surface; conducting a heat treatment for the starting substrate on which the metal film is formed to form innumerable micro voids whichextend from the second surface to the first surface; and growing a III group nitride semiconductor crystal via the metal film on the heat treated starting substrate, wherein: a part of the III group nitride semiconductor crystal is cut out to provide aIII group nitride semiconductor self-standing substrate after growing the III group nitride semiconductor crystal to have a thickness of 1 mm or more.
2. A method for fabricating a III group nitride semiconductor substrate, comprising the steps of: forming a metal film on a starting substrate; conducting a heat treatment for the starting substrate on which the metal film is formed in an atmosphere of gas containing a nitrogen element at a temperature of 800.degree. C. or more to form a metal nitride film, the metal nitride film having a first surface contacting the starting substrate, and a second surface on an opposite side of the metal nitride film from the first surface, by nitriding the metal film and to form innumerable micro voids which extend from the second surface to the first surface; and growing a III group nitride semiconductor crystal via the metal nitride film on the heat treated starting substrate, wherein: a part of the III group nitride semiconductor crystal is cut out to provide a III group nitride semiconductor self-standing substrate after growing the III group nitride semiconductor crystal to have a thickness of 1 mm or more.
This application is based on Japanese patent application No. 2003-358350, and a whole content of this Japanese application is referred and introduced in this application.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a III group nitride semiconductor substrate, a substrate for a III group nitride semiconductor device and fabrication methods thereof.
2. Description of the Related Art
A GaN-based compound semiconductor such as gallium nitride (GaN), indium gallium nitride (InGaN), and aluminum gallium nitride (AlGaN) comes into the limelight as a material for a blue light emitting diode (LED) and a laser diode (LD). Inaddition, with making use of features such as heat resistance, environment characteristics resistance of the GaN-based compound semiconductor, the application development of an element for an electronic device using the GaN-based compound semiconductorbegins.
In general terms, as a method for fabricating a GaN-based compound semiconductor device, a method for epitaxially growing a GaN-based crystal on a sapphire substrate is used. However, a single crystal film of GaN cannot be directly grown on thesapphire substrate, since the sapphire substrate is different in lattice constant from GaN. So as to solve this problem, a method of growing a buffer layer of AlN or GaN at a low-temperature of around 500.degree. C. on a sapphire substrate once,relaxing a lattice strain by the low-temperature growth buffer layer, and growing GaN thereon at a high temperature of around 1000.degree. C., is invented and placed in practical use broadly (For example, Japanese Patent Laid-Open No. 63-188983 (Patentdocument 1)). The single crystal epitaxial growth of GaN can be achieved by using the low-temperature growth nitride layer as the buffer layer.
However, in this technique, the crystallinity of the GaN crystal grown at a high temperature is sensitive to the thickness and the crystallinity of the low-temperature growth buffer layer, so that it is difficult to grow the GaN crystal with agood repeatability. In addition, since a growth temperature must be varied in the crystal growth, there are disadvantages in that the increase/decrease and stabilization of the temperature take a longer time and so on. Further, the epitaxial growth ofthe GaN single crystal on the sapphire substrate can be realized by using the low-temperature growth buffer layer, however, it is assumed that innumerable defects occurring in the GaN due to the lattice-mismatch between the substrate and the crystalbecome the obstacle in fabricating a GaN-based LD.
In recent years, some techniques for reducing the crystal dislocation density due to the difference in lattice constant between the sapphire and the GaN were reported, for example, an ELO (Epitaxial Lateral Overgrowth) method (Appl. Phys. Lett. 1997, Vol. 71, No. 18, p. 2638), and FIELO (Facet-Initiated Epitaxial Lateral Overgrowth) method (Jpn. J. Appl. Phys. 1999, Vol. 38, p. L184), as well as a Pendio-epitaxy (MRS Internet J. Nitride Semicond. Res. 1999, 4S1, G3, 38), and GaN epitaxialwafers with significantly improved crystallinity can be obtained. However, as for the ELO method, etc., there is a disadvantage in that complicated process such as photolithography process, etching process is required. Further, according to thesemethods, there is a disadvantage in that the dislocation distribution in the GaN becomes heterogeneous unless a GaN thick film having a thickness more than dozens of micrometers is grown.
In recent years, techniques of growing a GaN thick film on a substrate by HVPE (Hydride Vapor Phase Epitaxy) method and cutting out a self-standing substrate of GaN are disclosed (Japanese Patent Laid-Open No. 2000-012900, Japanese PatentLaid-Open No. 2000-022212, Japanese Patent Laid-Open No. 2000-252217 (patent documents 2 to 4), etc.). In these techniques, a technique for fabricating a GaN substrate once and growing a GaN crystal ingot by using this as a seed crystal thereafter isadopted, however, so as to simplify the process and to reduce the manufacturing cost of the GaN substrate, a technique for conducting a hetero epitaxial growth of a GaN thick film on a foreign substrate such as sapphire substrate and cutting out aself-standing substrate of the GaN directly therefrom is desirable. However, according to a conventional growth method using the low-temperature growth buffer layer, the relaxation of the strain due to the difference in lattice constant between thestarting substrate and the GaN is not enough, therefore, for example, when growing the GaN on the sapphire substrate to have a thickness of about 100 .mu.m, the GaN is naturally cracked during the growth, so that it is difficult to grow the GaN to havean enough thickness to cut out a self-standing substrate therefrom.
It has been known for a long time that micro voids are formed to have a mesh structure when a thin metal film is heated. For example, it is reported that micro voids with mesh structure are formed in a thin film of nickel, gold, copper, etc. byheating in M. L. Gimpl, A. D. McMaster, and N. Fuschillo, J. Appl. Phys. 1964, Vol. 35, p. 3572 (non-patent document 1) and L. Bachmann, D. L. Sawner, and B. M. Siegel, J. Appl. Phys. 1965, Vol. 36, p. 304 (non-patent document 2). Such mesh structureis conspicuously observed particularly in a film formed by the vacuum deposition method. However, a technique for conducting a crystal growth of a III group nitride semiconductor via a film with mesh structure thus obtained is not known.
Techniques for growing a III group nitride semiconductor crystal via titanium metal, titanium nitride, zirconium nitride, or hafnium nitride are disclosed in Japanese Patent Laid-open No. 11-195814, Japanese Patent Laid-Open No. 2000-049092, andJapanese Patent Laid-Open No. 2000-323753 (patent documents 5 to 7), and it is disclosed that the GaN crystal growth is possible even if the low-temperature growth buffer layer is not provided, in the Japanese Patent Laid-Open No. 2000-323753 (patentdocument 8). However, it is specified that it is preferable that the low-temperature growth buffer layer intervenes and it is difficult to obtain a high quality GaN crystal by the epitaxial growth, when only the titanium nitride, etc. intervenes.
Technique for growing III group nitride semiconductor crystal by providing a mask of high melting thin metal film is disclosed in Japanese Patent Laid-Open No. 2000-114178 (patent document 9). However, this mask is fabricated by making full useof the photolithography method, and an aperture width of a window in a mask is limited to several micrometers due to restrictions imposed by processing accuracy. In the case of growing the III group nitride semiconductor crystal by means of this mask, anumber of crystal growth nucleuses are generated from inside the windows in the mask, and effect for reducing the crystal dislocation can be obtained, however, the insertion of the low-temperature buffer layer becomes indispensable.
[Patent document 1] Japanese Patent Laid-Open No. 63-188983 bulletin
[Patent document 2] Japanese Patent Laid-Open No. 2000-012900 bulletin
[Patent document 3] Japanese Patent Laid-Open No. 2000-022212 bulletin
[Patent document 4] Japanese Patent Laid-Open No. 2000-252217 bulletin
[Patent document 5] Japanese Patent Laid-Open No. 11-195814 bulletin
[Patent document 6] Japanese Patent Laid-Open No. 2000-049092 bulletin
[Patent document 7] Japanese Patent Laid-Open No. 2000-323753 bulletin
[Patent document 8] Japanese Patent Laid-Open No. 2000-323753 bulletin
[Patent document 9] Japanese Patent Laid-Open No. 2000-114178 bulletin
[Non-patent document 1] M. L. Gimpl, A. D. McMaster, and N. Fuschillo, "Journal of Applied Physics" (Journal of Applied Physics), 1964, Vol. 35, p. 3572
[Non-patent document 2] L. Bachmann, D. L. Sawner, and B. M. Siegel, "Journal of Applied Physics" (Journal of Applied Physics), 1965, Vol. 36, p. 304
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to provide a method for fabricating a III group nitride semiconductor substrate without the need of forming a low-temperature growth buffer layer and changing the temperature during the growth.
Further, another object of the invention is to provide a III group nitride semiconductor crystal grown by hetero epitaxial growth on a starting substrate, in which the strain with the starting substrate is more relaxed and the crystal dislocationis less than conventional methods.
Still further, still another object of the invention is to provide a III group nitride semiconductor substrate which allows the relaxation of the strain with the starting substrate during the hetero epitaxial growth of the III group nitridesemiconductor crystal and allows the crystal growth of the thick film in which the self-standing substrate can be cut out.
In addition, a further object of the invention is to provide a III group nitride semiconductor substrate, a substrate for a III group nitride semiconductor device, and fabrication methods thereof, provided with a III group nitride semiconductorcrystal with high quality in which a surface is a nonpolar surface.
As a result of serious researches in view of the above objects, the Inventors of the present invention found that strain with a starting substrate can be relaxed by forming a metal film or a metal nitride film with mesh structure in whichinnumerable micro voids are provided on a starting substrate and growing III group nitride semiconductor crystal via the metal film or metal nitride film, so that a III group nitride semiconductor crystal with less dislocation can be obtained, therebyinventing the present invention. In other words, the first method for fabricating a III group nitride semiconductor substrate according to the present invention comprises the steps of forming a metal film on a starting substrate, conducting a heattreatment for the starting substrate on which the metal film is formed to form innumerable micro voids which extend from a surface of the metal film to a surface of the starting substrate in the metal film, and growing a III group nitride semiconductorcrystal via the metal film on the heat treated starting substrate.
The second method for fabricating a III group nitride semiconductor substrate according to the present invention comprises the steps of forming a metal film on a starting substrate, conducting a heat treatment for the starting substrate on whichthe metal film is formed in an atmosphere of gas containing a nitrogen element at a temperature of 800.degree. C. or more to form a metal nitride film by nitriding the metal film and to form innumerable micro voids which extend from a surface of themetal film to a surface of the starting substrate in the metal nitride film, and growing a III group nitride semiconductor crystal via the metal nitride film on the heat treated starting substrate.
It is preferable that the starting substrate comprises a sapphire single crystal. As a preferred starting substrate, a (0001) face of a hexagonal system single crystal, in concrete, a single crystal substrate of sapphire (Al.sub.2O.sub.3), zincoxide (ZnO), etc., or a (111) face of cubic system single crystal, in concrete, a single crystal substrate of silicon (Si), gallium arsenide (GaAs), etc are proposed. In addition, for obtaining the III group nitride semiconductor crystal in which asurface is a nonpolar surface, a surface of the starting substrate (a forming surface of a metal film or a metal nitride film) is used as a nonpolar forming surface. When the starting substrate is sapphire, a r-plane is the nonpolar forming surface. Herein, the nonpolar surface of the III group nitride semiconductor crystal is an a-plane or m-plane crystal plane.
The metal film to be formed on the starting substrate preferably comprises at least one element selected from a group composed of scandium (Sc), yttrium (Y), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta),chromium (Cr), molybdenum (Mo), tungsten (W), rhenium (Re), iron (Fe), ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), manganese (Mn), copper (Cu), platinum (Pt) and gold (Au). These metal films may bea multilayer film composed of a plurality of metals or an alloy film.
Heat treatment condition for forming the voids in the metal film to provide the mesh structure can be appropriately changed in accordance with the kind of the metal, since an optimum value for the heat treatment depends on the kind of the metal.
When the heat treatment is conducted simultaneously with the nitriding process, it is preferably conducted at a temperature of 800.degree. C. or more. The gas containing the nitrogen elements to be used for the nitriding process of the metalfilm comprises preferably at least one element selected from a group composed of nitrogen, ammonia, hydrazine and dimethylhydrazine. These gases may further comprise hydrogen.
It is required that the metal film or the metal nitride film is thinner than 200 nm so as to provide voids that penetrate to the surface of the starting substrate. On the other hand, if the metal film or the metal nitride film is thinner than 2nm, for example, a part of the metal film may be torn in the heat treatment or the like, so that it becomes difficult to homogeneously form the film with mesh structure on an entire surface of the substrate Therefore, the thickness of the metal film orthe metal nitride film is preferably 2 to 200 nm, and more preferably 10 to 100 nm.
It is preferable that the process for growing a III group nitride semiconductor crystal is conducted at a temperature of 900.degree. C. or more, and more preferably at a temperature of 950.degree. C. or more so as to keep the quality of the IIIgroup nitride semiconductor crystal high. When the crystal growth temperature is lower than 900.degree. C., the dislocation in the crystal increases, so that it becomes difficult to keep the high quality. While the high quality crystal is colorlessand transparent, when the dislocation in the crystal increases, a phenomenon that the crystal is colored by yellow to brown is observed. As for an upper limit of the crystal growth temperature, 1180.degree. C. is preferable to avoid the heatdecomposition of the crystal.
An area of each opening of the voids provided in the metal film or the metal nitride film is preferably 1.times.10.sup.-9 m.sup.2, and more preferably 1.times.10.sup.-15 to 1.times.10.sup.-12 m.sup.2. This void plays a role to provide a nucleigenerating site at an initial growth stage in the crystal growth of the III group nitride semiconductor, so that it is preferable to generate a crystal nuclei from one void as few as possible, and it is more preferable to generate a single crystal nucleifrom a single void. When the area of each opening is greater than 1.times.10.sup.-9 m.sup.2, numerous crystal nucleuses are generated from the void, so that the strain relaxation effect by virtue of a porous film is decreased, as a result, it is oftenthat a crack may occur in the crystal during the crystal growth or that the grown crystal becomes polycrystal. On the other hand, if the area of each opening is smaller than 1.times.10.sup.-16 m.sup.2, it does not function as the nuclei generating site,so that the strain relaxation effect cannot be obtained. It is preferable that the voids are substantially homogeneously distributed in the metal film or the metal nitride film. According to this, the III group nitride semiconductor crystal can behomogeneously grown on the starting substrate. When the distribution of the voids is heterogeneous, a thickness distribution of the III group nitride semiconductor crystal or a dislocation distribution of the crystal becomes heterogeneous at the surfaceof the substrate, so that the crack may occur in the crystal during the growth, and it may cause that the grown crystal becomes a polycrystal.
A total area of the opening of the void provided in the metal film or the metal nitride film is preferably 10 to 90 area %, and more preferably 30 to 70 area % for a surface area of the starting substrate. When the total area of the opening issmaller than 10 area % for the surface area of the starting substrate, even if the III group nitride semiconductor crystal is grown, the crystal surface may not be flattened, or the nuclei may be generated from a region other than the inside of the void,so that the crystal surface may be roughened or the grown crystal may become polycrystal. On the other hand, the area of the opening of the void is greater than 90 area %, it becomes difficult to maintain the configuration of the porous film, therebyproducing a factor obstructing the single crystal growth of the III group nitride semiconductor.
A film thickness of the III group nitride semiconductor crystal is preferably 500 nm or more, and it is desirable that the surface thereof is substantially flattened. By providing the film thickness of 500 nm or more, adjacent ones of theinitial growth nucleuses in the III group nitride semiconductor crystal, that are generated from the voids provided in the metal film or the metal nitride film, are bound to each other, so that the entire surface can be flattened. An upper limit of thefilm thickness of the III group nitride semiconductor crystal is not limited in particular. When the III group nitride semiconductor crystal is thinner than 500 nm, a lot of pits or steps appear on the crystal surface, and it may become a great obstaclefor fabricating a device by using the obtained crystal.
Additionally, in case where the surface of the III group nitride semiconductor crystal is the nonpolar surface, it is also a preferred embodiment that such III group nitride semiconductor crystal constitutes a device structure comprising aplurality of III group nitride semiconductor epitaxial layers, and in that case, the III group nitride semiconductor crystal has a thickness of an ordinary epitaxial layer for the device.
The crystal growth of the III group nitride semiconductor is preferably conducted by using any one of MOVPE (Metal Organic Vapor Phase Epitaxy) method and HVFE (Hydride Vapor Phase Epitaxy) method, or the combination thereof. Particularly in theinitial stage of the crystal growth, it is preferable to use the MOVPE method in which the crystal growth speed is relatively slow and that the generation of the initial growth nuclei can be obtained enough, and when the crystal growth is once begun, itis preferable to use the HVPE method in which high crystal growth speed can be obtained. In addition, it is also preferable to use MOHVPE method in which the advantages of both methods are combined.
It is preferable to selectively generate the nuclei from the inside of the voids formed in the metal film or the metal nitride film at the initial growth stage in the process for growing the III group nitride semiconductor crystal. When themicro voids provided in the metal film or the metal nitride film play a role to provide the nuclei generating site at the initial growth stage when conducting the crystal growth of the III group nitride semiconductor. Namely, since there is a surfaceenergy difference between the metal film or the metal nitride film and the starting substrate, materials that extend to a surface of the metal film or the metal nitride in the crystal growth of the III group nitride semiconductor move on the startingsubstrate within the void because of the surface migration, so that crystal growth nuclei is selectively formed there. By conducting the crystal growth of the III group nitride semiconductor via the metal film or the metal nitride film provided with themicro voids, it is possible to control the density of the initial nuclei generation of the III group nitride semiconductor crystal without depending on the lattice constant of the starting substrate, so that it is possible to conduct the crystal growthin which the occurrence of dislocation can be significantly suppressed compared with the case where the metal film or metal nitride film is not provided. In addition, the metal film or the metal nitride film having the micro voids has an effect ofrelaxing the strain due to the lattice mismatch between the starting substrate and the III group nitride semiconductor crystal and due to a difference in linear expansion coefficients therebetween, as a result, it is possible to provide the III groupnitride semiconductor crystal substrate in which the occurrence of the crystal dislocation is low and warping is small. In addition, even if a thick film crystal of 1 mm or more is grown, the crack will not occur in the crystal. Accordingly, it ispossible to fabricate a III group nitride semiconductor self-standing substrate by growing a large-sized crystal of 1 mm or more and cutting out a part of the III group nitride semiconductor crystal which is grown.
According to material of the starting substrate or material of the metal, it is expected that the initial growth nuclei of the III group nitride semiconductor crystal may be selectively generated on the metal film or metal nitride film. For thiscase, the generation of the nuclei will not occur from the inside of the voids contrary to the aforementioned case, and the crystal nuclei is generated from a limited region on the metal film or the metal nitride film, and the crystal growth advances inthe manner that the III group nitride semiconductor crystal is over-grown on the micro voids of the metal film or the metal nitride film, as result, a high quality crystal can be grown without using the low-temperature buffer layer similarly to the casewhere the nuclei is selectively generated within the void. For this case, the voids provided in the metal film or the metal nitride film functions to relax the strain due to the lattice mismatch between the starting substrate and the III group nitridesemiconductor crystal and due to the difference in the linear expansion coefficients therebetween.
The III group nitride semiconductor substrate according to the present invention can be fabricated by the aforementioned fabrication method. In other words, the first III group nitride semiconductor substrate according to the present inventionis characterized by that a III group nitride semiconductor crystal is formed via a metal film with mesh structure in which micro voids are provided. In addition, the second III group nitride semiconductor substrate according to the present invention ischaracterized by that a III group nitride semiconductor crystal is formed via a metal nitride film with mesh structure in which micro voids are provided.
As described above, the patent documents 5 to 8 disclose methods for growing a III group nitride semiconductor crystal via a titanium nitride or the like. However, this titanium nitride is used for the purpose of reducing the lattice mismatchbetween the intervening layer and the III group nitride semiconductor crystal. On the other hand, the fabrication method according to the present invention is characterized by that the micro voids are provided in the metal film or the metal nitride filmand the voids are provided with the nuclei generating site for the III group nitride semiconductor crystal.
In other words, the metal film or the metal nitride film and the III group nitride semiconductor crystal are not continuously lattice matched at the interface therebetween. Therefore, the fabrication method according to the present inventiondoes not depend upon the lattice matching between the metal film or the metal nitride film and the III group nitride semiconductor crystal, that is not likely to the methods described in the aforementioned patent documents, and the function and effect ofthe metal film or the metal nitride film are also different therefrom.
Further, as for a mask disclosed in the patent document 9, an effect of providing the nuclei generating site to a limited extent like the fabrication method according to the present invention cannot be expected. In the fabrication methodaccording to the present invention, the metal film or the metal nitride film having the micro voids can be self-assembled by a heat treatment of the metal film, which is a simple and easy method without the need for the photolithography process, so thatinsertion of the low-temperature buffer layer is not necessary, and the nuclei generating site for the III group nitride semiconductor crystal can be provided to a limited extent.
According to the present invention, the metal film or the metal nitride film in which the micro voids are formed is formed on the starting substrate, so that the crystal growth of the III group nitride semiconductor is started from the voids. The micro voids formed in the metal film or the metal nitride film function as the nuclei generating site at the initial growth of the III group nitride semiconductor crystal, and relax the strain due to the lattice mismatch between the startingsubstrate and the III group nitride semiconductor crystal and due to the difference in the linear expansion coefficients therebetween. Therefore, the III group nitride semiconductor substrate with low dislocation density and less warping can be obtainedwithout using the low-temperature growth buffer layer.
The III group nitride semiconductor substrate according to the present invention can be broadly used as a substrate for a GaN-based device. Particularly, when used as the substrate for a laser diode or a high power light emitting diode device, ahigh quality GaN-based crystal with low dislocation density can be provided, so that it is possible to fabricate a device with high reliability and high performance.
In addition, according to the present invention, by providing a surface of the starting substrate (a forming surface of the metal film or the metal nitride film) as a nonpolar forming surface, it is possible to provide a III group nitridesemiconductor substrate, a substrate for a III group nitride semiconductor device and fabrication methods thereof, each of which is provided with a high quality III group nitride semiconductor crystal having a nonpolar surface at its surface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view showing an example of III group nitride semiconductor substrate according to the present invention;
FIG. 2 is a process cross-sectional view showing an example of a method for fabricating the III group nitride semiconductor substrate according to the present invention;
FIG. 3 is a process cross-sectional view showing another example of the method for fabricating the III group nitride semiconductor substrate according to the present invention;
FIG. 4 is a scanning electron microscope (SEM) photograph showing a surface of a Pt film with mesh structure in the EXAMPLE 1;
FIG. 5 is a scanning electron microscope (SEM) photograph showing a surface of a TiN film with mesh structure in the EXAMPLE 2;
FIG. 6 is a scanning electron microscope (SEM) photograph showing a surface of a TiN film with mesh structure in the EXAMPLE 3;
FIG. 7 is a schematic cross-sectional view showing a substrate for LED which is an example of a substrate for a III group nitride semiconductor device according to the present invention; and
FIG. 8 is a schematic cross-sectional view showing a LD device fabricated by using a substrate for a LD which is an example of the substrate for a III group nitride semiconductor device according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an example of III group nitride semiconductor substrate of the present invention. A metal film or a metal nitride film 2' with mesh structure in which micro voids are provided is formed on a sapphire substrate 1, and a III groupnitride semiconductor crystal layer 3 is formed via the metal film or the metal nitride film 2'. The method of fabricating a III group nitride semiconductor substrate according to the present invention may adopt various arrangements for respectiveprocesses or materials to be used for the III group nitride semiconductor substrate. It is preferable that the metal film or the metal nitride film 2' is formed on an entire surface of a starting substrate (sapphire substrate 1). According to thisstructure, the micro voids can be formed over the entire surface of the substrate, thereby growing the III group nitride semiconductor crystal layer 3 homogeneously as well as relaxing the strain more effectively.
It is preferable that the metal film or the metal nitride film satisfies the following conditions.
(i) A melting point or a decomposition starting temperature of the metal film or the metal nitride film is higher than a growth temperature of the III group nitride semiconductor formed thereon, and a form of the film can be kept at the growthtemperature.
(ii) A vapor pressure at the growth temperature for the III group nitride semiconductor layer is low enough, and sublimation does not occur in the growth temperature.
(iii) In the growth temperature for the III group nitride semiconductor layer, the metal film or the metal nitride film does not react with a nitride semiconductor and source gas thereof, or the gas (ammonia gas or hydrogen gas) in a growthatmosphere, so that the crystal orientation is not perturbed.
(iv) The metal film or the metal nitride film can transmit the orientation of the starting substrate, and can be oriented in the [0001] axis direction if it is a hexagonal system on the starting substrate and can be oriented in the [111] axisdirection if it is a cubic system.
The III group nitride semiconductor substrate having an excellent crystal quality can be obtained by forming the metal film or the metal nitride film satisfying the above conditions.
The metal film or the metal nitride film may be a single film or a composite film in which more than two kinds of layers are laminated. For the metal nitride film, a metal film comprising metallic element which is easy to be nitrided is used,and the nitriding treatment and the heat treatment can conducted simultaneously by exposing it to the growth gas atmosphere for the III group nitride semiconductor layer. For the metallic element composing the metal film, titanium, tantalum, tungsten orthe like are preferable. For this case, it is not absolutely necessary to provide a process for nitriding, and a process for controlling a degree of the nitriding may be provided independently. For the method for forming the metal film, vacuumdeposition, sputtering method, various chemical vapor depositions or the like can be used.
When titanium is selected as the material of the metal film, the heat treatment is preferably conducted at 700 to 1400 t to nitride the metal film and form substantially homogeneous voids, and more preferably at 800 to 1200 t. If it is lower than700.degree. C., the nitriding reaction will not progress enough and the substantially homogeneous voids cannot be formed. In addition, if the temperature exceeds 1400.degree. C., the metal nitride film will not be flat. It is preferable to conductthe heat treatment at 700 to 1200.degree. C., when a nitride semiconductor substrate or a nitride semiconductor epitaxial wafer is used as the starting substrate. If the temperature exceeds 1200.degree. C., the heat decomposition of the single crystalgallium nitride layer progresses in excess, and there is a risk that metal nitride film may be exfoliated.
For the method for growing the III group nitride semiconductor layer, various kinds of methods including MOCVD method (Metalorganic chemical vapor deposition method), MBE method (molecular beam epitaxy method), HVPE method (Hydride vapor phaseepitaxy method) or the like may be used. It is preferable to use the HVPE method for growing a thick film of the III group nitride semiconductor so as to obtain an ingot of the III group nitride semiconductor. It is because that the crystal growthspeed is high and the thick film can be easily obtained. However, the present invention is not limited to the HVPE method, and other methods including the MOCVD methods or the like may be used, further, the combination of a plurality of growth methodsmay be used (the III group nitride semiconductor may be grown by using the MOCVD method halfway, and the III group nitride semiconductor may be grown to be thick by using the HVPE method thereafter).
For the growth of the III group nitride semiconductor layer, inert gas or mixed gas of the inert gas and hydrogen or the like may be used as a carrier gas. For the inert gas, at least one kind of gas selected from a group composed of N.sub.2,He, Ne, Ar, Kr and Xe may be used. In the case of forming the III group nitride layer, the inert gas of nitrogen or the like may be used as the carrier gas at the initial growth, and the carrier gas may be changed to the hydrogen thereafter to grow alayer with excellent crystallinity.
For the starting substrate, a substrate comprising various kinds of materials may be used. As a preferred substrate, a different material substrate such as sapphire, silicon, SiC, Langasite, Al, GaAs, .gamma.-LiAlO.sub.2 or the like, and asubstrate comprising a III group nitride semiconductor such as GaN, AlN, AlGaN or the like may be proposed. When the sapphire is used as the substrate material, C-plane, A-plane, R-plane or the like may be used. It does not matter if the startingsubstrate has the off-angle, however, it is preferable that the off-angle is within 1.degree.. By keeping the off-angle within 1.degree., the orientation of the metal film or the metal nitride film can be kept well and the III group nitridesemiconductor layer can be grown well.
In addition, by providing the surface of the starting substrate (the forming surface of the metal film or the metal nitriding film) as the nonpolar forming surface, a high quality III group nitride semiconductor crystal having the nonpolarsurface at its surface can be obtained. A crystal plane to be used for the nonpolar forming surface may be changed in accordance with the substrate material for the starting substrate. For example, when the starting substrate is sapphire, the nonpolarforming surface will be the r-plane, when the starting substrate is .gamma.-LiAlO.sub.2, the (100) plane will serve as the nonpolar forming surface, when the starting substrate is SiC, the a-plane or the m-plane will serve as the nonpolar formingsurface, and when the starting substrate is a III group nitride semiconductor, the nonpolar surface (i.e. the a-plane or the m-plane) will serve as the nonpolar forming surface.
The III group nitride semiconductor layer may comprise various kinds of semiconductor layers. For example, a semiconductor layer such as GaN, AlGaN, InGaN, InAlGaN or the like may be used. In addition, the self-standing substrate of the IIIgroup nitride semiconductor can be obtained by exfoliating and removing the starting substrate after forming the III group nitride semiconductor layer. In the present specification, the word "self-standing substrate" means a substrate having thestrength enough to hold its proper configuration as well as the strength in which any inconvenience does not occur for handling. For a method for removing the starting substrate, a method of mechanically exfoliating the starting substrate by applying astress to a portion having a gap in a wafer after growth, a method of exfoliating the starting substrate by removing a metallic element-containing film or a region provided with a gap in the III group nitride semiconductor layer by etching, or the likemay be used.
The method for fabricating a III group nitride semiconductor substrate according to the present invention may be modified to provide any variations other than the aforementioned configurations, within a range which does not go beyond the objectof the present invention. For example, it may be applied to the fabrication of a single crystal self-standing substrate of a ternary mixed crystal such as AlGaN, InGaN, or the like, and the fabrication of a p-type GaN substrate doped with Mg or thelike. In the III group nitride semiconductor substrate according to the present invention, the micro voids provided in the metal film or the metal nitride film may be formed by the photolithography or electron beam lithography after the film formationas well as the aforementioned self-assembling technique which is simple and easy.
Marcos Pinto D´derlee
C.I. 17862728
EES
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