Patent Publication Number: US-7713812-B2

Title: Method for manufacturing semiconductor thin film

Description:
RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 10/382,506, filed on Mar. 7, 2003, now U.S. Pat. No. 7,008,839, which in turn claims the benefit of Japanese Application No. 2002-063013, filed Mar. 8, 2002, the disclosures of which Applications are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for manufacturing a semiconductor thin film that can be used in short-wavelength light emitting diode devices, short-wavelength semiconductor laser devices and high-speed electronic devices. 
     A group III-V nitride semiconductor material, having a wide forbidden band, can be used in various light emitting devices, such as light emitting diode devices and short-wavelength semiconductor laser devices that are capable of emitting light of a color in a visible region such as blue, green or white. For its high electron saturated drift velocity, in addition to the wide forbidden band, it is also a promising material for electronic devices such as high-frequency devices and high-power devices. 
     Among others, light emitting diode devices using a nitride semiconductor have already been in practical use in large-size display apparatuses, traffic signals, etc. Particularly, white light emitting diode devices, which give white light by exciting a fluorescent substance with blue or ultraviolet light, can be used in lighting fixtures to replace electric bulbs and fluorescent lamps, and thus bears high expectations. On the other hand, the research and development of semiconductor laser devices has reached a point where samples are being shipped and products are being manufactured although in small quantities, aiming at application in optical disk apparatuses of larger storage capacities. 
     For the application in semiconductor laser devices, it is important to increase the operating lifetime. To this end, active research and development has been undertaken to reduce the crystal defects in a nitride semiconductor used in a laser device. At present, the most effective method for reducing defects is an epitaxial lateral overgrowth (ELO) method. Specifically, a thin film made of, for example, silicon oxide (SiO 2 ), or the like, is selectively formed on a base layer made of gallium nitride (GaN) so that the formed thin film has openings therein, and the nitride semiconductor is re-grown starting from the gallium nitride surface exposed through the openings. As a result, the crystal growth in the lateral direction (the direction parallel to the substrate surface), which proceeds starting from the openings, becomes dominant on the SiO 2  thin film. By promoting the lateral growth, it is possible to significantly reduce the crystal defects occurring in the nitride semiconductor layer, which is grown on the mask film. It has been reported that with this method, the dislocation density, which is about 1×10 9  cm −2  in the prior art, can be reduced to be on the order of 10 6  cm 2 . 
     A semiconductor laser device manufactured by the ELO method has a maximum operating lifetime exceeding 10000 hours, which is substantially sufficient for practical applications. Further reduction of the defect density is expected in order to realize a reliable laser device having an even longer operating lifetime. On the other hand, it has also been reported that when a GaN selective growth layer is grown by the ELO method to a thickness exceeding about 100 μm on a sapphire substrate, the substrate is separated from the selective growth layer while the substrate is being cooled after the crystal growth process, due to the stress caused by the difference between the coefficient of thermal expansion of the substrate and that of the GaN layer. 
     The separation of the GaN layer from the sapphire substrate provides various advantageous effects, including: the heat radiating property is improved as compared with a case where the sapphire substrate is not separated; an electrode can be formed on the reverse side of the substrate, thereby simplifying the process; and the substrate can be cleaved, thereby realizing a desirable mirror (cavity facet) while reducing the chip size. As a result, it is possible to increase the operating lifetime and the performance of a semiconductor laser device. 
     A conventional method for manufacturing a semiconductor thin film using the ELO method will now be described. 
       FIG. 25A  and  FIG. 25B  are cross-sectional views sequentially illustrating steps in the conventional method for manufacturing a semiconductor thin film. 
     First, as illustrated in  FIG. 25A , a first semiconductor layer  102  made of gallium nitride (GaN) and having a thickness of about 1 μm is grown as a base layer on a substrate  101  made of sapphire by a metal organic chemical vapor deposition (MOCVD) method, for example. Then, a mask-forming film made of silicon oxide (SiO 2 ) and having a thickness of about 200 nm is deposited on the first semiconductor layer  102  by a chemical vapor deposition (CVD) method, for example. Then, a resist pattern having a planar stripe pattern (not shown) is formed on the mask-forming film by a photolithography method, and then the mask-forming film is wet-etched with hydrogen fluoride using the formed resist pattern as a mask, thereby forming, from the mask-forming film, a mask film  103  having a stripe pattern. 
     Next, as illustrated in  FIG. 25B , a second semiconductor layer  104  having a thickness of about 100 μm is re-grown (through an ELO process) on the first semiconductor layer  102  with the mask film  103  formed thereon by a hydride vapor phase epitaxy (HVPE) method. In this process, gallium chloride (GaCl), which is a group III source obtained by reacting metal gallium (Ga) with a hydrogen chloride (HCl) gas, and ammonia, which is a nitrogen source, are used as material gases. With the HVPE method, the growth of the second semiconductor layer  104  made of gallium nitride starts from portions of the first semiconductor layer  102  that are exposed through the mask film  103 , and the growth proceeds also in the lateral direction so as to cover the top of the mask film  103 . After the second semiconductor layer  104  is flattened, the growth proceeds while retaining the flat surface. Since the lateral growth portions on the mask film  103  grow without being influenced by the crystal defects of the base layer, it is possible to significantly reduce the crystal defect density. 
     After the second semiconductor layer  104  is grown to an intended thickness, the structure is cooled to room temperature. In the cooling process, the substrate  101  is warped into a convex shape due to the difference between the coefficient of thermal expansion of the substrate  101  made of sapphire and that of the first and second semiconductor layers  102  and  104  made of gallium nitride. As a result, the substrate  101  and the first semiconductor layer  102 , or the mask film  103  and the second semiconductor layer  104 , are detached from each other at the interface therebetween, thereby separating the substrate  101  from the first semiconductor layer  102  or the second semiconductor layer  104 . 
     In the conventional method for manufacturing a semiconductor thin film, by decreasing the size of the openings of the mask film  103  as much as possible, it is possible to make the lateral growth of the second semiconductor layer  104  dominant without being influenced by the defect density of the base layer (the first semiconductor layer  102 ), whereby it is possible to reduce the defect density in the second semiconductor layer  104 . 
     However, in order to reduce the opening size to a deep submicron level, for example, it is necessary to use a micro pattern exposure system and a micro etching apparatus that have high precisions. Thus, further reducing the defect density in the second semiconductor layer  104  requires high-precision tools, thereby increasing the manufacturing cost. 
     Moreover, it is not possible to obtain openings of a size below the lower limit, which is dictated by the level of the state-of-the-art microprocessing technology. Thus, the cost of the process and the size of the openings can only be reduced to a certain degree. As a result, there is a limit to the reduction of the crystal defect density in the second semiconductor layer  104  through the miniaturization of the pattern of the mask film  103 . 
     Moreover, when the thickness of the second semiconductor layer  104  is increased, there is a problem occurring in the step of detaching the substrate  101 . That is, it is difficult to uniformly and reproducibly obtain a semiconductor layer of a large area due to problems relating to the thickness distribution across the plane of the second semiconductor layer  104  and the reproducibility of the interface condition thereof. 
     SUMMARY OF THE INVENTION 
     In view of these problems in the prior art, a first object of the present invention is to reduce, at a low cost, the crystal defect density in the formation of a semiconductor thin film on a single-crystal substrate through a growth process, and a second object of the present invention is to allow for the grown semiconductor thin film and the single-crystal substrate to be separated from each other reproducibly even if the semiconductor thin film has a large area. 
     In order to achieve the first object, the first invention provides a method for manufacturing a semiconductor thin film, in which a first semiconductor thin film selectively covered by a mask film is oxidized to form oxidized regions in the first semiconductor thin film, and a second semiconductor thin film is selectively grown on the first semiconductor thin film so as to cover the oxidized regions with the oxidized regions being used as a mask for selective growth. 
     Moreover, in order to achieve the second object, the second invention provides a method similar to that of the first invention, in which after the second semiconductor thin film is again oxidized selectively, a third semiconductor thin film is selectively grown on the second semiconductor thin film so that lower portions of the third semiconductor thin film are covered by the oxidized regions. 
     Specifically, a first method for manufacturing semiconductor thin film according to the present invention, which achieves the first object, includes: a first step of forming a first semiconductor thin film on a single-crystal substrate; a second step of selectively forming a mask film with a plurality of openings therein on the first semiconductor thin film; a fourth step of subjecting the first semiconductor thin film with the mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the first semiconductor thin film that are exposed through the openings of the mask film and a vicinity thereof, thereby forming a plurality of oxidized regions in the first semiconductor thin film; and a fifth step of removing the mask film to expose the first semiconductor thin film and then forming a second semiconductor thin film on the exposed first semiconductor thin film so as to cover the plurality of oxidized regions. 
     With the first method for manufacturing a semiconductor thin film, the oxidization process for the first semiconductor thin film whose upper surface is masked proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby the width dimension of the first semiconductor thin film interposed by the oxidized regions can be reduced as compared with a case where a mask pattern is used for the formation. As a result, the interval between oxidized regions to be a mask when forming the second semiconductor thin film can be reduced without using a high-precision pattern exposure system or an etching apparatus capable of performing minute processing, whereby the second semiconductor thin film to be selectively grown on the first semiconductor thin film being the base crystal is less influenced by the crystal defects in the first semiconductor thin film, which is the base layer, and the crystal defect density of the second semiconductor thin film can be reduced. Moreover, since the oxidization of the first semiconductor thin film mostly proceeds inward, the difference between the height of the surface of the first semiconductor thin film and that of the surface of the oxidized region can be reduced from that in the prior art where silicon oxide, or the like, is used. As a result, the lateral growth occurs in an early stage of the selective growth process, whereby direct growth on the upper surface of the oxidized region can be suppressed, and the crystallinity of the second semiconductor thin film on the oxidized region can be improved. 
     It is preferred that the first method for manufacturing a semiconductor thin film further includes, after the fifth step, a sixth step of separating the substrate from the first semiconductor thin film. 
     In such a case, it is preferred that the method further includes, between the fifth step and the sixth step, a seventh step of bonding a support substrate made of a material different from that of the second semiconductor thin film on an upper surface of the second semiconductor thin film. 
     In such a case, it is preferred that the sixth step includes a step of irradiating the substrate with irradiation light whose energy is smaller than a forbidden band width of a material of the substrate and is larger than a forbidden band width of a material of the first semiconductor thin film. 
     In such a case, it is preferred that the irradiation light is pulsed laser light. 
     Alternatively, in such a case, it is preferred that the irradiation light is an emission line of a mercury lamp. 
     Furthermore, it is preferred that the sixth step includes a step of heating the substrate. 
     Moreover, it is preferred that in the sixth step, the irradiation light is radiated so as to scan a surface of the substrate. 
     Moreover, it is preferred that the sixth step includes a step of removing at least one of the substrate and the oxidized regions by wet etching. 
     In such a case, it is preferred that the wet etching is performed by using an acidic solution or an alkaline solution. 
     A second method for manufacturing a semiconductor thin film according to the present invention, which achieves the first object, includes: a first step of forming a first semiconductor thin film on a single-crystal substrate; a second step of selectively forming a mask film with a plurality of openings therein on the first semiconductor thin film; a fourth step of subjecting the first semiconductor thin film with the mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the first semiconductor thin film that are exposed through the openings of the mask film and a vicinity thereof, thereby forming a plurality of oxidized regions in the first semiconductor thin film; a fifth step of removing the mask film to expose the first semiconductor thin film and then forming a plurality of second semiconductor thin films on the exposed first semiconductor thin film so that the plurality of oxidized regions have exposed portions; and a sixth step of removing the plurality of oxidized regions and then forming a third semiconductor thin film so that the third semiconductor thin film contacts and covers the second semiconductor thin films. 
     With the second method for manufacturing a semiconductor thin film, a gap portion where no oxidized region exists is formed under the third semiconductor thin film, whereby the stress in the thin film can be reduced as compared with a case where an oxidized region is left unremoved, and the crystallinity of the third semiconductor thin film can be further improved. 
     It is preferred that the second method for manufacturing a semiconductor thin film further includes, after the sixth step, a seventh step of separating the substrate from the first semiconductor thin film. 
     It is preferred that the second method for manufacturing a semiconductor thin film further includes, between the sixth step and the seventh step, an eighth step of bonding a support substrate made of a material different from that of the third semiconductor thin film on an upper surface of the third semiconductor thin film. 
     In such a case, it is preferred that the seventh step includes a step of irradiating the substrate with irradiation light whose energy is smaller than a forbidden band width of a material of the substrate and is larger than a forbidden band width of a material of the first semiconductor thin film. 
     In such a case, it is preferred that the irradiation light is pulsed laser light. 
     Alternatively, in such a case, it is preferred that the irradiation light is an emission line of a mercury lamp. 
     Furthermore, it is preferred that the seventh step includes a step of heating the substrate. 
     Moreover, it is preferred that in the seventh step, the irradiation light is radiated so as to scan a surface of the substrate. 
     Moreover, it is preferred that the seventh step includes a step of removing at least one of the substrate and the oxidized regions by wet etching. 
     In such a case, it is preferred that the wet etching is performed by using an acidic solution or an alkaline solution. 
     A third method for manufacturing a semiconductor thin film according to the present invention, which achieves the first object, includes: a first step of forming a first semiconductor thin film on a single-crystal substrate; a second step of selectively forming a first mask film with a plurality of openings therein on the first semiconductor thin film; a fourth step of subjecting the first semiconductor thin film with the first mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the first semiconductor thin film that are exposed through the openings of the first mask film and a vicinity thereof, thereby forming a plurality of first oxidized regions in the first semiconductor thin film; a fifth step of removing the first mask film to expose the first semiconductor thin film and then forming a second semiconductor thin film on the exposed first semiconductor thin film so as to cover the plurality of first oxidized regions; a sixth step of forming a second mask film so that the second mask film masks regions of the second semiconductor thin film that are above portions of the first semiconductor thin film interposed by the first oxidized regions and has openings above the first oxidized regions; a seventh step of subjecting the second semiconductor thin film with the second mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the second semiconductor thin film that are exposed through the openings of the second mask film and a vicinity thereof, thereby forming a plurality of second oxidized regions in the second semiconductor thin film; and an eighth step of removing the second mask film to expose the second semiconductor thin film and then forming a third semiconductor thin film on the exposed second semiconductor thin film so as to cover the plurality of second oxidized regions. 
     With the third method for manufacturing a semiconductor thin film, each portion where the laterally-growing opposing end surfaces of the second semiconductor thin film are connected to each other can be turned into the second oxidized region, which is used as a mask for selective growth when growing the third semiconductor thin film. Normally, the crystal orientation does not match completely in a portion where the laterally-growing opposing end surfaces are connected to each other. The shift in orientation angle is more significant as the planar area of a mask region (the first oxidized region), e.g., the width dimension of the mask in a case where the mask has a stripe pattern, is larger. However, with the manufacturing method of the present invention, the third semiconductor thin film is re-grown with the connected portion of the second semiconductor thin film being again used as a mask region (the second oxidized region), whereby the planar area of the mask region (the second oxidized region) can be reduced in the second re-growth step. Therefore, the third semiconductor thin film can be re-grown without being influenced by the shift in the crystal orientation occurring when growing the second semiconductor thin film, whereby the crystallinity of the third semiconductor thin film can be improved. 
     It is preferred that the third method for manufacturing a semiconductor thin film further includes, between the seventh step and the eighth step, a step of removing the first oxidized regions and the second oxidized regions. 
     A fourth method for manufacturing a semiconductor thin film according to the present invention, which achieves the second object, includes: a first step of forming a first semiconductor thin film on a single-crystal substrate; a second step of selectively forming a first mask film with a plurality of openings therein on the first semiconductor thin film; a fourth step of subjecting the first semiconductor thin film with the first mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the first semiconductor thin film that are exposed through the openings of the first mask film and a vicinity thereof, thereby forming a plurality of first oxidized regions in the first semiconductor thin film; a fifth step of removing the first mask film to expose the first semiconductor thin film and then forming a second semiconductor thin film on the exposed first semiconductor thin film so as to cover the plurality of first oxidized regions; a sixth step of forming a second mask film so that the second mask film has a plurality of openings for exposing therethrough portions of the second semiconductor thin film that are above portions of the first semiconductor thin film interposed by the first oxidized regions; a seventh step of subjecting the second semiconductor thin film with the second mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the second semiconductor thin film that are exposed through the openings of the second mask film and a vicinity thereof, thereby forming a plurality of second oxidized regions in the second semiconductor thin film; and an eighth step of removing the second mask film to expose the second semiconductor thin film and then forming a third semiconductor thin film on the exposed second semiconductor thin film so as to cover the plurality of second oxidized regions. 
     With the fourth method for manufacturing a semiconductor thin film, the first oxidized regions and the second oxidized regions are connected together, whereby the substrate is separated from the second semiconductor thin film by removing the first oxidized regions and the second oxidized regions, thus realizing uniform separation of a substrate with a large area. Moreover, the separated substrate can be reused as a substrate for forming another semiconductor thin film, whereby the manufacturing cost can be reduced. 
     In the fourth method for manufacturing a semiconductor thin film, it is preferred that in the seventh step, the second oxidized regions are formed so that an entire surface of the first semiconductor thin film including the first oxidized regions is oxidized. 
     It is preferred that the fourth method for manufacturing a semiconductor thin film further includes, after the eighth step, a ninth step of separating the substrate from the first semiconductor thin film. 
     In such a case, it is preferred that the method further includes, between the eighth step and the ninth step, a tenth step of bonding a support substrate made of a material different from that of the third semiconductor thin film on an upper surface of the third semiconductor thin film. 
     In such a case, it is preferred that the ninth step includes a step of irradiating the substrate with irradiation light whose energy is smaller than a forbidden band width of a material of the substrate and is larger than a forbidden band width of a material of the first semiconductor thin film. 
     In such a case, it is preferred that the irradiation light is pulsed laser light. 
     Moreover, in such a case, it is preferred that the irradiation light is an emission line of a mercury lamp. 
     Furthermore, it is preferred that the ninth step includes a step of heating the substrate. 
     In such a case, it is preferred that in the ninth step, the irradiation light is radiated so as to scan a surface of the substrate. 
     Moreover, it is preferred that the ninth step includes a step of removing at least one of the substrate and the oxidized regions by wet etching. 
     In such a case, it is preferred that the wet etching is performed by using an acidic solution or an alkaline solution. 
     In the first to fourth methods for manufacturing a semiconductor thin film, it is preferred that the substrate is made of sapphire, silicon carbide, lithium aluminum oxide, lithium gallium oxide, lithium gallium aluminum oxide (LiGa x Al 1-x O 2  (where 0≦x≦1)), silicon, gallium arsenide, indium phosphide or gallium phosphide. 
     A fifth method for manufacturing a semiconductor thin film according to the present invention, which achieves the first object, includes: a first step of selectively forming a first mask film on a single-crystal substrate; a second step of subjecting the substrate with the first mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the substrate that are in a vicinity of, and around, the first mask film, thereby forming a plurality of first oxidized regions in the substrate; and a third step of removing the first mask film to expose the substrate and then forming a first semiconductor thin film so as to cover an upper surface of the substrate including the plurality of first oxidized regions. 
     With the fifth method for manufacturing a semiconductor thin film, even if a material that itself is oxidized is used for the substrate, the interval between oxidized regions is reduced, as in the first method for manufacturing a semiconductor thin film of the present invention, whereby the first semiconductor thin film can be laterally grown from an opening having a reduced size, thus reducing the crystal defect density of the first semiconductor thin film. 
     It is preferred that the fifth method for manufacturing a semiconductor thin film further includes, after the third step, a fourth step of separating the substrate from the first semiconductor thin film. 
     In such a case, it is preferred that the method further includes, between the third step and the fourth step, a fifth step of bonding a support substrate made of a material different from that of the first semiconductor thin film on the upper surface of the first semiconductor thin film. 
     Moreover, it is preferred that the fourth step includes a step of removing at least one of the substrate and the oxidized regions by wet etching. 
     In such a case, it is preferred that the wet etching is performed by using an acidic solution or an alkaline solution. 
     It is preferred that the fifth method for manufacturing a semiconductor thin film further includes, after the third step: a fourth step of forming a second mask film so that the second mask film has a plurality of openings for exposing therethrough portions of the first semiconductor thin film that are interposed by the first oxidized regions; a fifth step of subjecting the first semiconductor thin film with the second mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the first semiconductor thin film that are exposed through the openings of the second mask film and a vicinity thereof, thereby forming a plurality of second oxidized regions in the first semiconductor thin film; and a sixth step of removing the second mask film to expose the first semiconductor thin film and then forming a second semiconductor thin film on the exposed first semiconductor thin film so as to cover the plurality of second oxidized regions. 
     In this way, the first oxidized regions and the second oxidized regions are connected together, whereby the substrate is separated from the second semiconductor thin film by removing the first oxidized regions and the second oxidized regions. Therefore, the separated substrate can be reused as a substrate for forming another semiconductor thin film, thereby achieving the second object. 
     In the fifth method for manufacturing a semiconductor thin film, it is preferred that in the fifth step, the second oxidized regions are formed so that an entire surface of the substrate including the first oxidized regions is oxidized. 
     It is preferred that the fifth method for manufacturing a semiconductor thin film further includes, after the third step: a fourth step of forming a second mask film so that the second mask film masks regions of the first semiconductor thin film that are interposed by the first oxidized regions and has openings above the first oxidized regions; a fifth step of subjecting the first semiconductor thin film with the second mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the first semiconductor thin film that are exposed through the openings of the second mask film and a vicinity thereof, thereby forming a plurality of second oxidized regions in the first semiconductor thin film; and a sixth step of removing the second mask film to expose the first semiconductor thin film and then forming a second semiconductor thin film on the exposed first semiconductor thin film so as to cover the plurality of second oxidized regions. 
     It is preferred that the fifth method for manufacturing a semiconductor thin film further includes, between the fifth step and the sixth step, a step of removing the first oxidized regions and the second oxidized regions. 
     It is preferred that the fifth method for manufacturing a semiconductor thin film further includes, after the sixth step, a seventh step of separating the substrate from the first semiconductor thin film. 
     It is preferred that the fifth method for manufacturing a semiconductor thin film further includes, between the sixth step and the seventh step, an eighth step of bonding a support substrate made of a material different from that of the second semiconductor thin film on an upper surface of the second semiconductor thin film. 
     Moreover, it is preferred that the seventh step includes a step of removing at least one of the substrate and the oxidized regions by wet etching. 
     In such a case, it is preferred that the wet etching is performed by using an acidic solution or an alkaline solution. 
     A sixth method for manufacturing a semiconductor thin film according to the present invention, which achieves the second object, includes: a first step of selectively forming a first mask film on a single-crystal substrate; a second step of subjecting the substrate with the mask film formed thereon to a heat treatment in an oxidizing atmosphere to oxidize portions of the substrate that are in a vicinity of, and around, the mask film, thereby forming a plurality of oxidized regions in the substrate; a third step of removing the mask film to expose the substrate and then forming a plurality of first semiconductor thin films on the exposed substrate so that the plurality of oxidized regions have exposed portions; and a fourth step of removing the plurality of oxidized regions and then forming a second semiconductor thin film so that the second semiconductor thin film contacts and covers the first semiconductor thin films. 
     With the sixth method for manufacturing a semiconductor thin film, even if a material that itself is oxidized is used for the substrate, the second semiconductor thin film is formed so that the second semiconductor thin film contacts and covers the first semiconductor thin films after a plurality of oxidized regions are removed by, for example, etching, or the like, as in the second method for manufacturing a semiconductor thin film of the present invention, whereby a gap portion where no oxidized region exists is formed under the second semiconductor thin film. As a result, the stress in the thin film can be reduced as compared with a case where an oxidized region is left unremoved, and the crystallinity of the second semiconductor thin film can be further improved. 
     It is preferred that the sixth method for manufacturing a semiconductor thin film further includes, after the fourth step, a fifth step of separating the substrate from the first semiconductor thin film. 
     It is preferred that the sixth method for manufacturing a semiconductor thin film further includes, between the fourth step and the fifth step, a sixth step of bonding a support substrate made of a material different from that of the first semiconductor thin film on an upper surface of the first semiconductor thin film. 
     Moreover, it is preferred that the fifth step includes a step of removing at least one of the substrate and the oxidized regions by wet etching. 
     In such a case, it is preferred that the wet etching is performed by using an acidic solution or an alkaline solution. 
     In the fifth or sixth method for manufacturing a semiconductor thin film, it is preferred that the substrate is made of silicon carbide, silicon, gallium arsenide, indium phosphide or gallium phosphide. 
     In the first or sixth method for manufacturing a semiconductor thin film, it is preferred that at least one of the first semiconductor thin film and the second semiconductor thin film is made of a group III-V nitride semiconductor or silicon carbide. 
     In the second to fourth methods for manufacturing a semiconductor thin film, it is preferred that at least one of the first semiconductor thin film, the second semiconductor thin film and the third semiconductor thin film is made of a group III-V nitride semiconductor or silicon carbide. 
     In the fifth method for manufacturing a semiconductor thin film, it is preferred that the first semiconductor thin film is made of a group III-V nitride semiconductor or silicon carbide. 
     Thus, with a wide gap semiconductor, the crystal growth of which is generally considered difficult, such as a group III-V nitride semiconductor made of InGaAlN or silicon carbide (SiC), the crystallinity can be improved significantly by promoting the lateral growth (ELO) thereof. Moreover, where the semiconductor thin film of the present invention is used in a device, the parasitic capacitance can be reduced by providing the oxidized regions under the active layer such as the channel portion of a field effect transistor, for example. 
     In the first or sixth method for manufacturing a semiconductor thin film, it is preferred that at least one of the first semiconductor thin film and the second semiconductor thin film is deposited by a vapor deposition method, a metal organic chemical vapor deposition method, a hydride vapor phase epitaxy method or a molecular beam epitaxy method. 
     In the second to fourth methods for manufacturing a semiconductor thin film, at least one of the first semiconductor thin film, the second semiconductor thin film and the third semiconductor thin film is deposited by a vapor deposition method, a metal organic chemical vapor deposition method, a hydride vapor phase epitaxy method or a molecular beam epitaxy method. 
     In the fifth method for manufacturing a semiconductor thin film, it is preferred that the first semiconductor thin film is deposited by a vapor deposition method, a metal organic chemical vapor deposition method, a hydride vapor phase epitaxy method or a molecular beam epitaxy method. 
     In the first or sixth method for manufacturing a semiconductor thin film, it is preferred that the first semiconductor thin film and the second semiconductor thin film are deposited by different deposition methods. 
     In the second to fourth methods for manufacturing a semiconductor thin film, it is preferred that two of the first semiconductor thin film, the second semiconductor thin film and the third semiconductor thin film are deposited by different deposition methods. 
     In the first to sixth methods for manufacturing a semiconductor thin film, it is preferred that the oxidizing atmosphere is an oxygen gas. 
     In the first to sixth methods for manufacturing a semiconductor thin film, it is preferred that the oxidizing atmosphere contains water vapor. 
     In the first to sixth methods for manufacturing a semiconductor thin film, it is preferred that the support substrate is made of silicon, gallium arsenide, indium phosphide, gallium phosphide or a metal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  to  FIG. 1C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a first embodiment of the present invention. 
         FIG. 2A  and  FIG. 2B  each show the surface condition of an oxidized region obtained by using the method for manufacturing a semiconductor thin film according to the first embodiment of the present invention, wherein  FIG. 2A  is an AFM image taken where thermal oxidization is performed in a water-vapor-containing atmosphere, and  FIG. 2B  is an AFM image taken where thermal oxidization is performed in an oxygen atmosphere. 
         FIG. 3A  and  FIG. 3B  are cross-sectional views each showing an oxidized region obtained by using the method for manufacturing a semiconductor thin film according to the first embodiment of the present invention, wherein  FIG. 3A  is an SEM image taken where thermal oxidization is performed in a water-vapor-containing atmosphere, and  FIG. 3B  is an SEM image taken where thermal oxidization is performed in an oxygen atmosphere. 
         FIG. 4A  to  FIG. 4D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a second embodiment of the present invention. 
         FIG. 5A  and  FIG. 5B  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the second embodiment of the present invention. 
         FIG. 6A  to  FIG. 6E  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a third embodiment of the present invention. 
         FIG. 7A  to  FIG. 7D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a fourth embodiment of the present invention. 
         FIG. 8A  and  FIG. 8B  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the fourth embodiment of the present invention. 
         FIG. 9A  to  FIG. 9D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a fifth embodiment of the present invention. 
         FIG. 10A  to  FIG. 10C  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the fifth embodiment of the present invention. 
         FIG. 11A  to  FIG. 11D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a sixth embodiment of the present invention. 
         FIG. 12A  to  FIG. 12C  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the sixth embodiment of the present invention. 
         FIG. 13A  to  FIG. 13D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a seventh embodiment of the present invention. 
         FIG. 14A  to  FIG. 14D  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the seventh embodiment of the present invention. 
         FIG. 15A  to  FIG. 15C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to an eighth embodiment of the present invention. 
         FIG. 16A  to  FIG. 16D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a ninth embodiment of the present invention. 
         FIG. 17A  to  FIG. 17C  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the ninth embodiment of the present invention. 
         FIG. 18A  to  FIG. 18D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a tenth embodiment of the present invention. 
         FIG. 19A  and  FIG. 19B  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the tenth embodiment of the present invention. 
         FIG. 20A  to  FIG. 20D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to an eleventh embodiment of the present invention. 
         FIG. 21A  to  FIG. 21C  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the eleventh embodiment of the present invention. 
         FIG. 22A  to  FIG. 22E  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a twelfth embodiment of the present invention. 
         FIG. 23A  to  FIG. 23D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to a thirteenth embodiment of the present invention. 
         FIG. 24A  to  FIG. 24C  are cross-sectional views sequentially illustrating steps in the method for manufacturing a semiconductor thin film according to the thirteenth embodiment of the present invention. 
         FIG. 25A  and  FIG. 25B  are cross-sectional views sequentially illustrating steps in a conventional method for manufacturing a semiconductor thin film. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     The first embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 1A  to  FIG. 1C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the first embodiment of the present invention. 
     As illustrated in  FIG. 1A , first, a first semiconductor layer  12 A made of gallium nitride (GaN) and having a thickness of about 1 μm is grown on the principal surface of a sapphire (single-crystal Al 2 O 3 ) substrate  11  by a metal organic chemical vapor deposition (MOCVD) method, for example, using trimethylgallium (TMGa) as a group III source and ammonia (NH 3 ) as a nitrogen source. Herein, a so-called a low-temperature buffer layer may be provided between the substrate  11  and the first semiconductor layer  12 A for relieving the lattice mismatch between the substrate  11  and the nitride semiconductor grown on the substrate  11 . For example, the low-temperature buffer layer is made of aluminum nitride (AlN) that is deposited at a relatively low temperature of about 500° C. to a relatively small thickness of about 50 nm. Only a portion of the substrate  11 , which has a shape of a wafer, is shown in  FIG. 1A , as is also the case in other embodiments to follow. 
     Then, a mask-forming film made of silicon (Si) and having a thickness of about 100 nm is deposited on the first semiconductor layer  12 A by, for example, a chemical vapor deposition (CVD) method in which monosilane (SiH 4 ) is decomposed. Then, a resist pattern having a planar stripe pattern (not shown) is formed on the mask-forming film by a photolithography method, and then the mask-forming film is dry-etched by reactive ion etching (RIE) with a reactive gas containing hydrogen bromide (HBr) or chlorine (Cl 2 ) using the formed resist pattern as a mask, thereby forming, from the mask-forming film, a mask film  13  having a stripe pattern with an opening  13   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner apparatus or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the mask film  13 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 1B , the substrate  11  with the first semiconductor layer  12 A and the mask film  13  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxidizing atmosphere, e.g., an oxygen (O 2 ) gas or a water vapor (H 2 O)-containing atmosphere. Through the heat treatment, a plurality of oxidized regions  12 B made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the mask film  13 . As a result, a base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent oxidized regions  12 B. Herein, the oxidization of the oxidized regions  12 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the mask film  13  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base layer  12   a  after the oxidization process can be as small as about 0.25 μm. Moreover, since an oxygen gas or a water-vapor-containing atmosphere is used as an oxidizing atmosphere for selectively oxidizing the oxidized regions  12 B, a quick and uniform oxidization process can be performed reproducibly. 
     Next, as illustrated in  FIG. 1C , the mask film  13  is removed by hydrofluoric-nitric acid or an RIE method, and then a second semiconductor layer  14  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the base layer  12   a  that is exposed between the plurality of oxidized regions  12 B so as to cover the oxidized regions  12 B while flattening the upper surface thereof. 
     As described above, in the first embodiment, during the growth of the second semiconductor layer  14 , the lateral growth becomes dominant over the oxidized regions  12 B. Therefore, the second semiconductor layer  14  is not influenced by the surface of the base layer  12   a  made of the first semiconductor layer  12 A over the oxidized regions  12 B, whereby the crystal defect density of the second semiconductor layer  14  can be reduced. 
     As described above, the opening size of the base layer  12   a  made of gallium nitride can be made smaller than the width dimension of the mask film  13 . Therefore, if the mask film  13  is patterned by using a state-of-the-art pattern exposure system whose resolution is on the 0.1 μm level, the width dimension of the base layer  12   a  can be further reduced, whereby the crystallinity of the second semiconductor layer  14  to be re-grown is even less likely to be influenced by the surface of the base layer  12   a.    
     Moreover, the thickness of the oxidized regions  12 B can be reduced by shortening the oxidization time when forming the oxidized regions  12 B illustrated in  FIG. 1B . In this way, the difference between the height of the surface of the base layer  12   a  and that of the surface of the oxidized regions  12 B can be reduced, whereby when the second semiconductor layer  14  is re-grown, the lateral growth is dominant from the initial stage of the growth of the second semiconductor layer  14 . As a result, the crystal growth on the oxidized regions  12 B can be prevented, whereby it is possible to prevent the crystallinity of the second semiconductor layer  14  from deteriorating due to crystals produced on the oxidized regions  12 B. 
     Note that the growth method for the second semiconductor layer  14  is not limited to an MOCVD method. In order to obtain a larger thickness, a hydride vapor phase epitaxy (HVPE) method having a higher growth rate than an MOCVD method may be used. 
     Furthermore, a growth method that has a low growth rate but provides a high crystallinity may be used in combination. For example, a molecular beam epitaxy (MBE) method may be used for growing the first semiconductor layer  12 A to be selectively oxidized, while an HVPE method is used for growing the second semiconductor layer  14 . In this way, the crystallinity of the first semiconductor layer  12 A, which is the base layer for the second semiconductor layer  14 , is improved, thereby improving the crystallinity of the second semiconductor layer  14 . In addition, this makes it easier to reduce the thickness of the second semiconductor layer  14 . 
     Moreover, while sapphire (single-crystal Al 2 O 3 ) is used for the single-crystal substrate  11 , silicon carbide (SiC), lithium aluminum oxide (LiAlO 2 ), lithium gallium oxide (LiGaO 2 ), or a mixed crystal thereof, i.e., lithium gallium aluminum oxide (LiGa x Al 1-x O 2  (where 0≦x≦0.1)), may alternatively be used instead of sapphire. In this way, a group III-V compound semiconductor with a reduced lattice mismatch and a desirable crystallinity, specifically, a semiconductor thin film made of In x Ga y Al z N y As w  (where x+y+z=1, 0≦x, y, z≦1, v+w=1, 0≦v, w≦1) can be grown on the substrate  11 . 
     Furthermore, when silicon carbide is used for the substrate  11 , a silicon carbide thin film having a desirable crystallinity can be formed on the substrate  11 . Moreover, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP) is used for the substrate  11 , it is possible to obtain a substrate with a desirable crystallinity, and to form a semiconductor thin film having a desirable crystallinity on the substrate  11 . 
     The present inventors have confirmed that there is a difference in the surface condition of the obtained oxidized regions  12 B between when an oxygen (O 2 ) gas is used as the oxidizing atmosphere for the selective oxidization of the first semiconductor layer  12 A illustrated in  FIG. 1B  and when a mixed atmosphere of water vapor (H 2 O) and a nitrogen (N 2 ) gas is used. 
     For example, a mixed gas of water vapor and nitrogen is produced by bubbling water heated to 100° C. with a nitrogen gas, and the produced mixed gas is introduced onto the first semiconductor layer  12 A heated to about 1000° C. Through the heat treatment in a water-vapor-containing oxidizing atmosphere, the oxidized regions  12 B whose composition is represented as Ga 2 O 3  is formed in the exposed portions of the first semiconductor layer  12 A made of gallium nitride (GaN). 
       FIG. 2A  shows the surface condition of the oxidized region made of Ga 2 O 3 , which have been oxidized by a water-vapor-containing atmosphere as observed by an atomic force microscope (AFM). Moreover,  FIG. 3A  shows a cross section of the oxidized region, which have been oxidized by a water-vapor-containing atmosphere as observed by a scanning electron microscope (SEM). On the other hand,  FIG. 2B  and  FIG. 3B  show the surface condition and a cross section, respectively, of the oxidized region made of Ga 2 O 3 , which have been oxidized by an oxygen atmosphere. 
     As can be seen from  FIG. 2A  and  FIG. 3A , the flatness of the surface of the oxidized region made of Ga 2 O 3  is significantly improved by thermal oxidization with a water-vapor-containing atmosphere, as compared with thermal oxidization with an oxygen atmosphere. Moreover, it can be seen from  FIG. 3A  that in a part of the oxidized region, oxidization proceeds in a pillar-like (lump-like) pattern with the surface thereof protruding in a convex shape. Observation of the cross section of such a pillar-like portion with a transmission electron microscope (TEM) has confirmed that crystal dislocations (defects) are localized in the pillar-like oxidized portion. Therefore, it can be seen that the oxidized regions  12 B with a higher flatness can be obtained by selectively oxidizing regions where the crystal defect density has been reduced. 
     In view of the above, the flatness of the surface of the oxidized regions  12 B can be improved reliably by performing a selective thermal oxidization process with a water-vapor-containing atmosphere and on the first semiconductor layer  12 A with fewer crystal defects. As a result, in the step illustrated in  FIG. 1C , the second semiconductor layer  14  is selectively grown on the oxidized regions  12 B with a desirable surface flatness, and the abnormal growth of the second semiconductor layer  14  is suppressed, whereby it is possible to reliably form the second semiconductor layer  14  with a desirable flatness and a desirable crystallinity. 
     Second Embodiment 
     The second embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 4A  to  FIG. 4D  and  FIG. 5A  and  FIG. 5B  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the second embodiment of the present invention. 
     First, as illustrated in  FIG. 4A , the first semiconductor layer  12 A made of gallium nitride and having a thickness of about 1 μm is grown on the principal surface of the substrate  11  made of sapphire by an MOCVD method using trimethylgallium as a group III source and ammonia as a nitrogen source. Again, a low-temperature buffer layer may be provided between the substrate  11  and the first semiconductor layer  12 A. 
     Then, a mask-forming film made of silicon and having a thickness of about 100 nm is deposited on the first semiconductor layer  12 A by, for example, a CVD method in which monosilane is decomposed. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the mask-forming film by a photolithography method, and then the mask-forming film is dry-etched by an RIE method with a reactive gas containing hydrogen bromide or chlorine using the formed resist pattern as a mask, thereby forming, from the mask-forming film, the mask film  13  having a stripe pattern with the opening  13   a  therein. Then, the resist pattern is removed by ashing, or the like. Again, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the mask film  13 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 4B , the substrate  11  with the first semiconductor layer  12 A and the mask film  13  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of oxidized regions  12 B made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the mask film  13 . As a result, the base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent oxidized regions  12 B. Herein, the oxidization of the oxidized regions  12 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the mask film  13  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base layer  12   a  after the oxidization process can be as small as about 0.25 μm. 
     Next, as illustrated in  FIG. 4C , the mask film  13  is removed by hydrofluoric-nitric acid or an RIE method, and then the second semiconductor layer  14  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the base layer  12   a  that is exposed between the plurality of oxidized regions  12 B so as to cover the oxidized regions  12 B. 
     Next, as illustrated in  FIG. 4D , a support substrate  15  made of silicon (Si) whose principal surface is oriented along the (100) plane is bonded to the upper surface of the second semiconductor layer  14  by a bonding method. Note that in a case where the thickness of the second semiconductor layer  14  can be sufficiently increased to be 100 μm or more, the support substrate  15  may not need to be bonded. Moreover, while silicon is used for the support substrate  15 , gallium arsenide (GaAs) whose principal surface is oriented along the (100) plane, indium phosphide (InP), gallium phosphide (GaP) or a metal such as copper (Cu) may alternatively be used instead of silicon. 
     Next, as illustrated in  FIG. 5A , one surface of the substrate  11  made of sapphire that is away from the base layer  12   a  is irradiated with krypton fluoride (KrF) excimer laser light having a wavelength of 248 nm so as to scan the entire surface of the substrate  11 . Then, as illustrated in  FIG. 5B , the radiated excimer laser light is not absorbed by the substrate  11  but is absorbed by the base layer  12   a , whereby the base layer  12   a  is heated. This heat thermally decomposes gallium nitride, whereby the substrate  11  and the second semiconductor layer  14  including the support substrate  15  are separated from each other. Herein, the peak power density and the pulse width of the excimer laser light are set so that gallium nitride of the base layer  12   a  bound to the substrate  11  is thermally decomposed. Thus, by oscillating excimer laser light in a pulsed manner, the output power of the laser light can be increased significantly, whereby the substrate  11  can easily be separated from the second semiconductor layer  14 . In addition, since the excimer laser light is radiated so as to scan the surface of the substrate  11 , the substrate  11  can reliably be separated even if the substrate  11  has a relatively large area, irrespective of the beam diameter of the light source. 
     Moreover, in the second embodiment, after the growth of the second semiconductor layer  14 , the substrate  11  is heated with a temperature of about 500° C. for the purpose of relieving the stress caused by the difference between the coefficient of thermal expansion of the second semiconductor layer  14  and that of the substrate  11 . In this way, it is possible to prevent the occurrence of stress-induced cracks in the second semiconductor layer  14  when separating the substrate  11  with light irradiation. 
     Note that in a case where the separation between the oxidized regions  12 B and the substrate  11  is insufficient, the oxidized regions  12 B may be selectively removed by wet etching with an acidic aqueous solution including hydrogen fluoride (HF) or hydrogen chloride (HCl), or an alkaline aqueous solution including ammonia water (NH 4 OH), either before or after the laser light irradiation. 
     As described above, in the second embodiment, as in the first embodiment, the crystal defect density of the second semiconductor layer  14 , which laterally grows from the base layer  12   a  interposed between the oxidized regions  12 B, is reduced, thereby significantly improving the crystallinity thereof. In addition, since the substrate  11  made of sapphire for forming the base layer  12   a  is separated and removed, the substrate  11  can be reused if the substrate  11  can be separated without a damage thereto, thereby allowing for a reduction in the cost of a device manufacturing process using the second semiconductor layer  14 . 
     Moreover, when a semiconductor laser device is produced by bonding the support substrate  15  while choosing a material with a good heat radiating property therefor, the heat radiating property of the laser device is improved, whereby improvements in the operating characteristics of the laser device can be expected, such as an increase in the operating lifetime thereof. 
     Moreover, when an electrically conductive material is used for the support substrate  15 , the positive electrode and the negative electrode of a semiconductor laser device, for example, can be formed so as to oppose each other via the support substrate  15  therebetween, whereby the chip size can be reduced. 
     Furthermore, when silicon or gallium arsenide (GaAs) that is oriented along the (001) plane is used for the support substrate  15 , a desirable cleaved surface, i.e., a desirable cavity structure, can be formed with either material, thereby obtaining a desirable mirror characteristic in the active layer, and thus realizing improvements in the operating characteristics of a semiconductor laser device, such as a reduction in the threshold current thereof. 
     Alternatively, the substrate  11  may be separated by growing indium gallium nitride (In 0.1 Ga 0.9 N) whose forbidden band is equivalent to 3.0 eV, instead of using gallium nitride, for the first semiconductor layer  12 A, and irradiating the reverse surface of the substrate  11  with the emission line of a mercury (Hg) lamp having a wavelength of 365 nm (a forbidden band equivalent to 3.4 eV), instead of using KrF excimer laser light, so as to scan the reverse surface of the substrate  11 . Thus, when the emission line of a mercury lamp is used as the light source for irradiating the base layer  12   a , although the output power thereof is lower than that of laser light, the spot diameter can be increased from that of laser light, whereby the substrate  11  can be separated within a shorter period of time. 
     Moreover, as in the first embodiment, silicon carbide, silicon, gallium arsenide, indium phosphide or gallium phosphide may be used for the substrate  11 , instead of sapphire. Furthermore, lithium aluminum oxide, lithium gallium oxide, or a mixed crystal thereof, i.e., lithium gallium aluminum oxide (LiGa x Al 1-x O 2  (where 0≦x≦1)), may be used. 
     Third Embodiment 
     The third embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 6A  to  FIG. 6E  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the third embodiment of the present invention. 
     First, as illustrated in  FIG. 6A , the first semiconductor layer  12 A made of gallium nitride and having a thickness of about 1 μm is grown on the principal surface of the substrate  11  made of sapphire by an MOCVD method using trimethylgallium as a group III source and ammonia as a nitrogen source. Again, a low-temperature buffer layer may be provided between the substrate  11  and the first semiconductor layer  12 A. 
     Then, a mask-forming film made of silicon and having a thickness of about 100 nm is deposited on the first semiconductor layer  12 A by, for example, a CVD method in which monosilane is decomposed. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the mask-forming film by a photolithography method, and then the mask-forming film is dry-etched by an RIE method with a reactive gas containing hydrogen bromide or chlorine using the formed resist pattern as a mask, thereby forming, from the mask-forming film, the mask film  13  having a stripe pattern with the opening  13   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the mask film  13 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 6B , the substrate  11  with the first semiconductor layer  12 A and the mask film  13  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of oxidized regions  12 B made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the mask film  13 . As a result, the base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent oxidized regions  12 B. Herein, the oxidization of the oxidized regions  12 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the mask film  13  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base layer  12   a  after the oxidization process can be as small as about 0.25 μm. 
     Next, as illustrated in  FIG. 6C , the mask film  13  is removed by hydrofluoric-nitric acid or an RIE method, and then a second semiconductor layer  14 A made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the base layer  12   a  that is exposed between the plurality of oxidized regions  12 B so as to leave an exposed portion in the central portion of the upper surface of each oxidized region  12 B. Thus, the growth of the second semiconductor layers  14 A, which are grown in an ELO process, is stopped before the opposing end surfaces of adjacent second semiconductor layers  14 A that are growing toward each other come into contact with each other. 
     Next, as illustrated in  FIG. 6D , the plurality of oxidized regions  12 B are removed by an acidic aqueous solution such as hydrogen fluoride water or an alkaline aqueous solution such as ammonia water. Thus, each second semiconductor layer  14 A has overhang side portions. 
     Next, as illustrated in  FIG. 6E , a third semiconductor layer  16  made of gallium nitride is grown by an MOCVD method under growth conditions such that the ELO process with the second semiconductor layer  14 A being the base crystal layer is dominant. In this process, a gap portion  16   a  is formed between the lower surface of the third semiconductor layer  16  and the principal surface of the substrate  11 , whereby the third semiconductor layer  16  is grown without being influenced by either the surface of the oxidized regions  12 B or the surface of the substrate  11 . Again, an HVPE method may be used as the crystal growth method for the first semiconductor layer  12 A, the second semiconductor layer  14 A and the third semiconductor layer  16 . Furthermore, an MBE method may be used for the first semiconductor layer  12 A. 
     As described above, in the third embodiment, as in the first embodiment, the crystallinity of the second semiconductor layer  14 A and that of the third semiconductor layer  16  are both improved by the ELO process proceeding from the base layer  12   a  having a reduced exposed area. Moreover, in the third embodiment, the third semiconductor layer  16  is grown on the substrate  11  so that the gap portion  16   a  obtained by removing the oxidized regions  12 B is retained, whereby the growth of the third semiconductor layer  16  is not influenced by the surface of the substrate  11  or that of the oxidized regions  12 B. As a result, the crystallinity of the third semiconductor layer  16  is improved as compared with a case where the oxidized regions  12 B are not removed. In addition, the gap portion  16   a  also has an effect of reducing the stress in the third semiconductor layer  16  due to the lattice mismatch occurring while growing the third semiconductor layer  16 . 
     Thus, the crystallinity of the third semiconductor layer  16  obtained by using the second semiconductor layer  14 A as a new base layer can be further improved by removing the oxidized regions  12 B obtained by oxidizing the first semiconductor layer  12 A. 
     Note that as in the first embodiment, silicon carbide, silicon, gallium arsenide, indium phosphide or gallium phosphide may be used for the substrate  11 , instead of sapphire. Furthermore, lithium aluminum oxide, lithium gallium oxide, or a mixed crystal thereof, i.e., lithium gallium aluminum oxide (LiGa x Al 1-x O 2  (where 0≦x≦1)), may be used. 
     Fourth Embodiment 
     The fourth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 7A  to  FIG. 7D  and  FIG. 8A  and  FIG. 8B  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the fourth embodiment of the present invention. 
     First, as illustrated in  FIG. 7A , the first semiconductor layer  12 A made of gallium nitride and having a thickness of about 1 μm is grown on the principal surface of the substrate  11  made of sapphire by an MOCVD method using trimethylgallium as a group III source and ammonia as a nitrogen source. Again, a low-temperature buffer layer may be provided between the substrate  11  and the first semiconductor layer  12 A. 
     Then, a first mask-forming film made of silicon and having a thickness of about 100 nm is deposited on the first semiconductor layer  12 A by, for example, a CVD method in which monosilane is decomposed. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by an RIE method with a reactive gas containing hydrogen bromide or chlorine using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the mask film  13  having a stripe pattern with the opening  13   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  13 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 7B , the substrate  11  with the first semiconductor layer  12 A and the first mask film  13  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  12 B made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the first mask film  13 . As a result, the first base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent first oxidized regions  12 B. Herein, the oxidization of the first oxidized regions  12 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  13  is set to be 0.5 μm in the patterning process, for example, the width dimension of the first base layer  12   a  after the oxidization process can be as small as about 0.25 μm. 
     Next, as illustrated in  FIG. 7C , the first mask film  13  is removed by hydrofluoric-nitric acid or an RIE method, and then the second semiconductor layer  14  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the first base layer  12   a  that is exposed between the plurality of first oxidized regions  12 B so as to cover the first oxidized regions  12 B. 
     Next, as illustrated in  FIG. 7D , a second mask film  23  made of silicon and having a thickness of about 100 nm is formed by a method similar to that for the first mask film  13  so that each second mask film  23  masks a portion of the second semiconductor layer  14  above the first base layer  12   a  and has an opening above the first oxidized region  12 B. 
     Next, as illustrated in  FIG. 8A , the substrate  11  with the second semiconductor layer  14  and the second mask film  23  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, a plurality of second oxidized regions  14 B made of gallium oxide are formed through oxidization of the second semiconductor layer  14  in regions of the second semiconductor layer  14  that are not covered by the second mask film  23 . As a result, a second base layer  14   a  made of gallium nitride is formed in each region of the second semiconductor layer  14  that is interposed by adjacent second oxidized regions  14 B. 
     Next, as illustrated in  FIG. 8B , the second mask film  23  is removed by hydrofluoric-nitric acid or an RIE method, and then the third semiconductor layer  16  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the second base layer  14   a  that is exposed between the plurality of second oxidized regions  14 B so as to cover the second oxidized regions  12 B. 
     Thus, in the fourth embodiment, the junction between ELO surfaces of the second semiconductor layer  14  is oxidized to form the second oxidized region  14 B. At the junction between ELO surfaces, there is typically a shift in the crystal orientation, and the angle of the shift increases as the ELO distance, i.e., the width dimension of the second oxidized region  14 B, increases. 
     In the fourth embodiment, the junction in the second semiconductor layer  14  in which the orientation is shifted is oxidized to form the second base layer  14   a , and the ELO process is repeated using the second base layer  14   a  as a seed crystal, thereby reducing the orientation shift. As a result, the third semiconductor layer  16  growing from the second base layer  14   a  has an orientation with very little shift, whereby the crystallinity of the third semiconductor layer  16  is further improved as compared with that of the second semiconductor layer  14 . 
     Note that also in the fourth embodiment, an HVPE method may be used as the crystal growth method for the first semiconductor layer  12 A, the second semiconductor layer  14  and the third semiconductor layer  16 . Furthermore, an MBE method may be used for the first semiconductor layer  12 A. 
     Moreover, as in the first embodiment, silicon carbide, lithium aluminum oxide, silicon, gallium arsenide or indium phosphide may be used for the substrate  11 , instead of sapphire. 
     Fifth Embodiment 
     The fifth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 9A  to  FIG. 9D  and  FIG. 10A  to  FIG. 10C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the fifth embodiment of the present invention. 
     First, as illustrated in  FIG. 9A , the first semiconductor layer  12 A made of gallium nitride and having a thickness of about 1 μm is grown on the principal surface of the substrate  11  made of sapphire by an MOCVD method using trimethylgallium as a group III source and ammonia as a nitrogen source. Again, a low-temperature buffer layer may be provided between the substrate  11  and the first semiconductor layer  12 A. 
     Then, a first mask-forming film made of silicon and having a thickness of about 100 nm is deposited on the first semiconductor layer  12 A by, for example, a CVD method in which monosilane is decomposed. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by an RIE method with a reactive gas containing hydrogen bromide or chlorine using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the mask film  13  having a stripe pattern with the opening  13   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  13 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 9B , the substrate  11  with the first semiconductor layer  12 A and the first mask film  13  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  12 B made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the first mask film  13 . As a result, the first base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent first oxidized regions  12 B. Herein, the oxidization of the first oxidized regions  12 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  13  is set to be 0.5 μm in the patterning process, for example, the width dimension of the first base layer  12   a  after the oxidization process can be as small as about 0.25 μm. 
     Next, as illustrated in  FIG. 9C , the first mask film  13  is removed by hydrofluoric-nitric acid or an RIE method, and then the second semiconductor layer  14  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the first base layer  12   a  that is exposed between the plurality of first oxidized regions  12 B so as to cover the first oxidized regions  12 B. 
     Next, as illustrated in  FIG. 9D , a second mask film  23  made of silicon and having a thickness of about 100 nm is formed by a method similar to that for the first mask film  13  so that each second mask film  23  masks a portion of the second semiconductor layer  14  above the first base layer  12   a  and has an opening above the first oxidized region  12 B. 
     Next, as illustrated in  FIG. 10A , the substrate  11  with the second semiconductor layer  14  and the second mask film  23  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of second oxidized regions  14 B made of gallium oxide are formed through oxidization of the second semiconductor layer  14  in regions of the second semiconductor layer  14  that are not covered by the second mask film  23 . As a result, the second base layer  14   a  made of gallium nitride is formed in each region of the second semiconductor layer  14  that is interposed by adjacent second oxidized regions  14 B. 
     Next, as illustrated in  FIG. 10B , the plurality of first oxidized regions  12 B and the plurality of second oxidized regions  14 B are removed by an acidic aqueous solution such as hydrogen fluoride water or an alkaline aqueous solution such as ammonia water. Thus, each second base layer  14   a  has overhang side portions. 
     Next, as illustrated in  FIG. 10C , the third semiconductor layer  16  made of gallium nitride is grown under growth conditions such that the ELO process with the second base layer  14   a  being a seed crystal is dominant. Thus, the gap portion  16   a  is formed between the lower surface of the third semiconductor layer  16  and the principal surface of the substrate  11 , whereby the third semiconductor layer  16  is grown without being influenced by the surface of the second oxidized regions  14 B. Again, an HVPE method may be used as the crystal growth method for the first semiconductor layer  12 A, the second semiconductor layer  14  and the third semiconductor layer  16 . Furthermore, an MBE method may be used for the first semiconductor layer  12 A. 
     As described above, in the fifth embodiment, as in the fourth embodiment, the junction in the second semiconductor layer  14  in which the orientation is shifted through the ELO process is oxidized to form the second base layer  14   a , and the ELO process is repeated using the second base layer  14   a  as a seed crystal, thereby reducing the orientation shift in the third semiconductor layer  16 . 
     In addition, as in the third embodiment, the third semiconductor layer  16  is formed on the substrate  11  so that the gap portion  16   a  obtained by removing the oxidized regions  12 B and the second oxidized regions  14 B is retained, whereby the growth of the third semiconductor layer  16  is not influenced by the surface of the second oxidized regions  14 B, thus further improving the crystallinity of the third semiconductor layer  16 . In addition, the gap portion  16   a  reduces the stress in the third semiconductor layer  16  due to the lattice mismatch occurring while growing the third semiconductor layer  16 . 
     Moreover, in the third embodiment, since the growth process of growing the second semiconductor layer  14 A is stopped before the adjacent growth surfaces connect to each other, it is necessary to determine when to stop the growth process. In the fifth embodiment, the need to determine when to stop the growth process is eliminated, thereby simplifying the manufacturing process. 
     Note that as in the first embodiment, silicon carbide, silicon, gallium arsenide, indium phosphide or gallium phosphide may be used for the substrate  11 , instead of sapphire. Furthermore, lithium aluminum oxide, lithium gallium oxide, or a mixed crystal thereof, i.e., lithium gallium aluminum oxide (LiGa x Al 1-x O 2  (where 0≦x≦1)), may be used. 
     Sixth Embodiment 
     The sixth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 11A  to  FIG. 11D  and  FIG. 12A  to  FIG. 12C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the sixth embodiment of the present invention. 
     First, as illustrated in  FIG. 11A , the first semiconductor layer  12 A made of gallium nitride and having a thickness of about 1 μm is grown on the principal surface of the substrate  11  made of sapphire by an MOCVD method using trimethylgallium as a group III source and ammonia as a nitrogen source. Again, a low-temperature buffer layer may be provided between the substrate  11  and the first semiconductor layer  12 A. 
     Then, a first mask-forming film made of silicon and having a thickness of about 100 nm is deposited on the first semiconductor layer  12 A by, for example, a CVD method in which monosilane is decomposed. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by an RIE method with a reactive gas containing hydrogen bromide or chlorine using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the mask film  13  having a stripe pattern with the opening  13   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  13 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 11B , the substrate  11  with the first semiconductor layer  12 A and the first mask film  13  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  12 B made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the first mask film  13 . As a result, the first base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent first oxidized regions  12 B. Herein, the oxidization of the first oxidized regions  12 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  13  is set to be 0.5 μm in the patterning process, for example, the width dimension of the first base layer  12   a  after the oxidization process can be as small as about 0.25 μm. 
     Next, as illustrated in  FIG. 11C , the first mask film  13  is removed by hydrofluoric-nitric acid or an RIE method, and then the second semiconductor layer  14  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the first base layer  12   a  that is exposed between the plurality of first oxidized regions  12 B so as to cover the first oxidized regions  12 B. 
     Next, as illustrated in  FIG. 1D , a second mask film  24  made of silicon and having a thickness of about 100 nm is formed by a method similar to that for the first mask film  13  so that the second mask film  24  has a plurality of openings for exposing therethrough portions of the second semiconductor layer  14  that are above the first base layers  12   a.    
     Next, as illustrated in  FIG. 12A , the substrate  11  with the second semiconductor layer  14  and the second mask film  24  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, a plurality of second oxidized regions  14 C made of gallium oxide are formed through oxidization of the second semiconductor layer  14  in regions of the second semiconductor layer  14  that are not covered by the second mask film  24 . As a result, the second base layer  14   a  made of gallium nitride is formed in each region of the second semiconductor layer  14  that is interposed by adjacent second oxidized regions  14 C. 
     Next, as illustrated in  FIG. 12B , the second mask film  24  is removed by hydrofluoric-nitric acid or an RIE method, and then the third semiconductor layer  16  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the second base layer  14   a  that is exposed between the plurality of second oxidized regions  14 C so as to cover the second oxidized regions  14 C. 
     Next, as illustrated in  FIG. 12C , the plurality of first oxidized regions  12 B and the plurality of second oxidized regions  14 C are removed by an acidic aqueous solution such as hydrogen fluoride water or an alkaline aqueous solution such as ammonia water. Thus, the substrate  11  and the third semiconductor layer  16  can be easily and uniformly separated from each other even if the substrate  11  has a relatively large area. 
     As described above, in the fourth or fifth embodiment, the second semiconductor layer  14  is processed to form the second oxidized regions  14 B so as to leave the first base layer  12   a . In the sixth embodiment, the second oxidized regions  14 C are formed in regions between the first oxidized regions  12 B in the second semiconductor layer  14 . Thus, in the sixth embodiment, the first base layer  12   a  is taken over by the second oxidized region  14 C, whereby the entire principal surface of the substrate  11  is covered by an oxide. Therefore, the third semiconductor layer  16  and the substrate  11  can be separated easily and reproducibly by wet etching even if the substrate  11  has a large area. As a result, the laser light irradiation process as illustrated in the second embodiment is no longer required in the step of separating the substrate  11 , thereby eliminating the thermal damage from the irradiation with laser light. Furthermore, the manufacturing cost of the third semiconductor layer  16  itself can be reduced, and thus the cost of manufacturing a device using the third semiconductor layer  16  can be reduced. 
     In addition, by separating the substrate  11  for forming the first base layer  12   a  from the third semiconductor layer  16 , the substrate  11  can be reused. Therefore, also by reusing the substrate  11 , the cost of manufacturing a device can be further reduced. 
     Note that as in the first embodiment, silicon carbide, silicon, gallium arsenide, indium phosphide or gallium phosphide may be used for the substrate  11 , instead of sapphire. Furthermore, lithium aluminum oxide, lithium gallium oxide, or a mixed crystal thereof, i.e., lithium gallium aluminum oxide (LiGa x Al 1-x O 2  (where 0≦x≦1)), may be used. 
     Seventh Embodiment 
     The seventh embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 13A  to  FIG. 13D  and  FIG. 14A  to  FIG. 14D  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the seventh embodiment of the present invention. 
     First, as illustrated in  FIG. 13A , the first semiconductor layer  12 A made of gallium nitride and having a thickness of about 1 μm is grown the principal surface of the substrate  11  made of sapphire by an MOCVD method using trimethylgallium as a group III source and ammonia as a nitrogen source. Again, a low-temperature buffer layer may be provided between the substrate  11  and the first semiconductor layer  12 A. 
     Then, a first mask-forming film made of silicon and having a thickness of about 100 nm is deposited on the first semiconductor layer  12 A by, for example, a CVD method in which monosilane is decomposed. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by an RIE method with a reactive gas containing hydrogen bromide or chlorine using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the mask film  13  having a stripe pattern with the opening  13   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  13 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 13B , the substrate  11  with the first semiconductor layer  12 A and the first mask film  13  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  12 B made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the first mask film  13 . As a result, the first base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent first oxidized regions  12 B. Herein, the oxidization of the first oxidized regions  12 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  13  is set to be 0.5 μm in the patterning process, for example, the width dimension of the first base layer  12   a  after the oxidization process can be as small as about 0.25 μm. 
     Next, as illustrated in  FIG. 13C , the first mask film  13  is removed by hydrofluoric-nitric acid or an RIE method, and then the second semiconductor layer  14  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the first base layer  12   a  that is exposed between the plurality of first oxidized regions  12 B so as to cover the first oxidized regions  12 B. 
     Next, as illustrated in  FIG. 13D , the second mask film  24  made of silicon and having a thickness of about 100 nm is formed by a method similar to that for the first mask film  13  so that the second mask film  24  has a plurality of openings for exposing therethrough portions of the second semiconductor layer  14  that are above the first base layers  12   a.    
     Next, as illustrated in  FIG. 14A , the substrate  11  with the second semiconductor layer  14  and the second mask film  24  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of second oxidized regions  14 C made of gallium oxide are formed through oxidization of the second semiconductor layer  14  in regions of the second semiconductor layer  14  that are not covered by the second mask film  24 . As a result, a second base layer  14   a  made of gallium nitride is formed in each region of the second semiconductor layer  14  that is interposed by adjacent second oxidized regions  14 C. 
     Next, as illustrated in  FIG. 14B , the second mask film  24  is removed by hydrofluoric-nitric acid or an RIE method, and then the third semiconductor layer  16  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the second base layer  14   a  that is exposed between the plurality of second oxidized regions  14 C so as to cover the second oxidized regions  14 C. 
     Next, as illustrated in  FIG. 14C , the support substrate  15  made of silicon (Si) whose principal surface is oriented along the (100) plane is bonded to the upper surface of the third semiconductor layer  16  by a bonding method. Note that in a case where the thickness of the third semiconductor layer  16  can be sufficiently increased to be 100 μM or more, the support substrate  15  may not need to be bonded. Moreover, while silicon is used for the support substrate  15 , gallium arsenide (GaAs) whose principal surface is oriented along the (100) plane, indium phosphide (InP), gallium phosphide (GaP) or a metal may alternatively be used. 
     Next, as illustrated in  FIG. 14D , the plurality of first oxidized regions  12 B and the plurality of second oxidized regions  14 C are removed by an acidic aqueous solution such as hydrogen fluoride water or an alkaline aqueous solution such as ammonia water. Thus, the substrate  11  and the third semiconductor layer  16  with the support substrate  15  bonded thereto can be easily and uniformly separated from each other even if the substrate  11  has a relatively large area. 
     As described above, in the seventh embodiment, the following effect can be obtained in addition to those of the sixth embodiment. Since the support substrate  15  made of silicon is bonded to the third semiconductor layer  16  with an improved crystallinity, when a semiconductor laser device is produced by using the third semiconductor layer  16 , the support substrate  15  made of silicon that is oriented along the (001) plane can easily be cleaved. As a result, a desirable mirror characteristic in the active layer is obtained, thus realizing improvements in the operating characteristics of the semiconductor laser device, such as a reduction in the threshold current thereof. 
     Moreover, when the support substrate  15  is bonded while choosing a material with a good heat radiating property therefor, the heat radiating property of the semiconductor device is improved, whereby improvements in the operating characteristics of the device can be expected, such as an increase in the operating lifetime thereof 
     Eighth Embodiment 
     The eighth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 15A  to  FIG. 15C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the eighth embodiment of the present invention. 
     A feature of this and other subsequent embodiments is that the substrate on which a semiconductor layer is to be grown is made of a material such that an oxidized region for selective growth can be formed through oxidization of the substrate itself First, as illustrated in  FIG. 15A , a mask-forming film made of silicon nitride (Si 3 N 4 ) and having a thickness of about 100 nm is deposited on the principal surface of a substrate  31 A made of silicon carbide (SiC) whose principal surface is oriented along the (0001) plane by, for example, a plasma CVD method. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the mask-forming film by a photolithography method, and then the mask-forming film is dry-etched by, for example, an RIE method with a reactive gas containing fluorocarbon using the formed resist pattern as a mask, thereby forming, from the mask-forming film, a mask film  25  having a stripe pattern with an opening  25   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner apparatus or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the mask film  25 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 15B , the substrate  31 A with the mask film  25  formed thereon is subjected to a heat treatment at a temperature of 1000° C. for about 5 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, a plurality of oxidized regions  31 B made of silicon oxide (SiO 2 ) are formed through oxidization of the substrate  31 A in regions of the substrate  31 A that are not covered by the mask film  25 . As a result, a base region  31   a  is formed in each region in the upper portion of the substrate  31 A that is interposed by adjacent oxidized regions  31 B. Herein, the oxidization of the oxidized regions  31 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the mask film  25  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base region  31   a  after the oxidization process can be as small as about 0.25 μm. Moreover, since an oxygen gas or a water-vapor-containing atmosphere is used as an oxidizing atmosphere for selectively oxidizing the oxidized regions  31 B, a quick and uniform oxidization process can be performed reproducibly. 
     Note that the (0001) plane of silicon carbide, which is the orientation thereof, typically has two polarities called the “Si plane” and the “C plane”, and the oxidization rate of the C plane is higher than that of the Si plane. Therefore, while the C plane is preferred in order to reduce the process time, it is preferred that the orientation of the principal surface of the substrate  31 A is determined in view of the crystallinity of the silicon carbide layer to be grown on the substrate  31 A. 
     Next, as illustrated in  FIG. 15C , the mask film  25  is removed by an RIE method, for example, and then a semiconductor layer  32  made of silicon carbide is selectively grown (through an ELO process) on the surface of the base region  31   a  that is exposed between the plurality of oxidized regions  31 B in the substrate  31 A so as to cover the oxidized regions  31 B by a CVD method in which monosilane (SiH 4 ) and ethylene (C 2 H 4 ) are reacted with each other under conditions such that the lateral growth is dominant. 
     Thus, in the eighth embodiment, as in the first embodiment, when the semiconductor layer  32  is grown, the lateral growth is dominant above the oxidized regions  31 B, whereby the semiconductor layer  32  is not influenced by the surface of the substrate  31 A to be the base thereof above the oxidized regions  31 B, thus reducing the crystal defect density of the semiconductor layer  32 . 
     In addition, the opening size of the base region  31   a  made of silicon carbide can be made smaller than the width dimension of the mask film  25 . Therefore, if the mask film  25  is patterned by using a pattern exposure system whose resolution is on the 0.1 μm level, the width dimension of the base region  31   a  can be further reduced, whereby the crystallinity of the semiconductor layer  32  to be re-grown is even less likely to be influenced by the surface of the base region  31   a.    
     Note that while silicon carbide (SiC) is used for the single-crystal substrate  31 A, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP) may alternatively be used instead of silicon carbide. 
     Alternatively, a support substrate made of silicon or the like may be bonded to the upper surface of the semiconductor layer  32 , which is the semiconductor thin film of interest, and then the substrate  31 A may be removed from the semiconductor layer  32  by a polishing method, or the like. 
     Ninth Embodiment 
     The ninth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 16A  to  FIG. 16D  and  FIG. 17A  to  FIG. 17C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the ninth embodiment of the present invention. 
     First, as illustrated in  FIG. 16A , a first mask-forming film made of silicon nitride and having a thickness of about 100 nm is deposited on the principal surface of the substrate  31 A made of silicon carbide whose principal surface is oriented along the (0001) plane by, for example, a plasma CVD method. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by, for example, an RIE method with a reactive gas containing fluorocarbon using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the first mask film  25  having a stripe pattern with the opening  25   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner apparatus or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  25 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 16B , the substrate  31 A with the first mask film  25  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  31 B made of silicon oxide are formed through oxidization of the substrate  31 A in regions of the substrate  31 A that are not covered by the first mask film  25 . As a result, the base region  31   a  is formed in each region in the upper portion of the substrate  31 A that is interposed by adjacent first oxidized regions  31 B. Herein, the oxidization of the first oxidized regions  31 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  25  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base region  31   a  after the oxidization process can be as small as about 0.25 μm. Moreover, since an oxygen gas or a water-vapor-containing atmosphere is used as an oxidizing atmosphere for selectively oxidizing the first oxidized regions  31 B, a quick and uniform oxidization process can be performed reproducibly. 
     Next, as illustrated in  FIG. 16C , the first mask film  25  is removed by an RE method, for example, and then the first semiconductor layer  32  made of silicon carbide is selectively grown (through an ELO process) on the surface of the base region  31   a  that is exposed between the plurality of first oxidized regions  31 B in the substrate  31 A so as to cover the first oxidized regions  31 B by a CVD method under conditions such that the lateral growth is dominant. 
     Next, as illustrated in  FIG. 16D , a second mask film  26  made of silicon nitride and having a thickness of about 100 nm is formed by a method similar to that for the first mask film  25  so that the second mask film  26  has a plurality of openings for exposing therethrough portions of the first semiconductor layer  32  that are above the base regions  31   a.    
     Next, as illustrated in  FIG. 17A , the substrate  31 A with the first semiconductor layer  32  and the second mask film  26  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, a plurality of second oxidized regions  32 B made of silicon oxide are formed through oxidization of the first semiconductor layer  32  in regions of the first semiconductor layer  32  that are not covered by the second mask film  26 . As a result, a base layer  32   a  made of silicon carbide is formed in each region in the first semiconductor layer  32  that is interposed by adjacent second oxidized regions  32 B. Herein, the base region  31   a  is taken over by the second oxidized region  32 B, whereby the entire principal surface of the substrate  31 A is covered by silicon oxide. 
     Next, as illustrated in  FIG. 17B , the second mask film  26  is removed by an RIE method, and then a second semiconductor layer  33  made of silicon carbide is selectively grown (through an ELO process) by a CVD method on the surface of the base layer  32   a  that is exposed between the plurality of second oxidized regions  32 B so as to cover the second oxidized regions  32 B. 
     Next, as illustrated in  FIG. 17C , the plurality of first oxidized regions  31 B and the plurality of second oxidized regions  32 B are removed by an acidic aqueous solution such as hydrogen fluoride water. Thus, the substrate  31 A and the second semiconductor layer  33  can be reproducibly and easily separated from each other even if the substrate  31 A has a large area. 
     Thus, in the ninth embodiment, the following effect can be obtained in addition to those of the eighth embodiment. By separating the substrate  31 A for forming the base region  31   a  and the base layer  32   a  from the second semiconductor layer  33 , the substrate  31 A can be reused. In this way, since the substrate  31 A made of silicon carbide is typically expensive, it is possible to reduce the manufacturing cost for manufacturing a device using the second semiconductor layer  33  by reusing the substrate  31 A. 
     Note that while silicon carbide (SiC) is used for the single-crystal substrate  31 A, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP) may alternatively be used instead of silicon carbide. 
     Tenth Embodiment 
     The tenth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 18A  to  FIG. 18D  and  FIG. 19A  and  FIG. 19B  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the tenth embodiment of the present invention. 
     First, as illustrated in  FIG. 18A , a first mask-forming film made of silicon nitride and having a thickness of about 100 nm is deposited on the principal surface of the substrate  31 A made of silicon carbide whose principal surface is oriented along the (0001) plane by, for example, a plasma CVD method. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by, for example, an RIE method with a reactive gas containing fluorocarbon using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the first mask film  25  having a stripe pattern with the opening  25   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner apparatus or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  25 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 18B , the substrate  31 A with the first mask film  25  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  31 B made of silicon oxide are formed through oxidization of the substrate  31 A in regions of the substrate  31 A that are not covered by the first mask film  25 . As a result, the base region  31   a  is formed in each region in the upper portion of the substrate  31 A that is interposed by adjacent first oxidized regions  31 B. Herein, the oxidization of the first oxidized regions  31 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  25  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base region  31   a  after the oxidization process can be as small as about 0.25 μm. Moreover, since an oxygen gas or a water-vapor-containing atmosphere is used as an oxidizing atmosphere for selectively oxidizing the first oxidized regions  31 B, a quick and uniform oxidization process can be performed reproducibly. 
     Next, as illustrated in  FIG. 18C , the first mask film  25  is removed by an RIE method, for example, and then the first semiconductor layer  32  made of silicon carbide is selectively grown (through an ELO process) on the surface of the base region  31   a  that is exposed between the plurality of first oxidized regions  31 B in the substrate  31 A so as to cover the first oxidized regions  31 B by a CVD method under conditions such that the lateral growth is dominant. 
     Next, as illustrated in  FIG. 18D , a second mask film  27  made of silicon nitride and having a thickness of about 100 nm is formed by a method similar to that for the first mask film  25  so that each second mask film  27  masks a portion of the first semiconductor layer  32  above the base region  31   a  and has an opening above the first oxidized region  31 B. 
     Next, as illustrated in  FIG. 19A , the substrate  31 A with the first semiconductor layer  32  and the second mask film  27  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, a plurality of second oxidized regions  32 C made of silicon oxide are formed through oxidization of the first semiconductor layer  32  in regions of the first semiconductor layer  32  that are not covered by the second mask film  27 . As a result, the base layer  32   a  made of silicon carbide is formed in each region in the first semiconductor layer  32  that is interposed by adjacent second oxidized regions  32 C. 
     Next, as illustrated in  FIG. 19B , the second mask film  27  is removed by an RIE method, and then a second semiconductor layer  33  made of silicon carbide is selectively grown (through an ELO process) by a CVD method on the surface of the base layer  32   a  that is exposed between the plurality of second oxidized regions  32 C so as to cover the second oxidized regions  32 C. 
     Thus, in the tenth embodiment, the junction in the first semiconductor layer  32  in which the orientation is shifted is oxidized to form the new base layer  32   a  from the first semiconductor layer  32 , and the ELO process is repeated using the formed base layer  32   a  as a seed crystal, thereby reducing the orientation shift. As a result, the second semiconductor layer  33  growing from the base layer  32   a  has an orientation with very little shift, whereby the crystallinity of the second semiconductor layer  33  is further improved as compared with that of the first semiconductor layer  32 . 
     Note that while silicon carbide (SiC) is used for the single-crystal substrate  31 A, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP) may alternatively be used instead of silicon carbide. 
     Eleventh Embodiment 
     The eleventh embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 20A  to  FIG. 20D  and  FIG. 21A  to  FIG. 21C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the eleventh embodiment of the present invention. 
     First, as illustrated in  FIG. 20A , a first mask-forming film made of silicon nitride and having a thickness of about 100 nm is deposited on the principal surface of the substrate  31 A made of silicon carbide whose principal surface is oriented along the (0001) plane by, for example, a plasma CVD method. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by, for example, an RIE method with a reactive gas containing fluorocarbon using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the first mask film  25  having a stripe pattern with the opening  25   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of minute processing with a resolution 0.5 μm or less, such as a contact mask aligner apparatus or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  25 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 20B , the substrate  31 A with the first mask film  25  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  31 B made of silicon oxide are formed through oxidization of the substrate  31 A in regions of the substrate  31 A that are not covered by the first mask film  25 . As a result, the base region  31   a  is formed in each region in the upper portion of the substrate  31 A that is interposed by adjacent first oxidized regions  31 B. Herein, the oxidization of the first oxidized regions  31 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  25  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base region  31   a  after the oxidization process can be as small as about 0.25 μm. Moreover, since an oxygen gas or a water-vapor-containing atmosphere is used as an oxidizing atmosphere for selectively oxidizing the first oxidized regions  31 B, a quick and uniform oxidization process can be performed reproducibly. 
     Next, as illustrated in  FIG. 20C , the first mask film  25  is removed by an RIE method, for example, and then the first semiconductor layer  32  made of silicon carbide is selectively grown (through an ELO process) on the surface of the base region  31   a  that is exposed between the plurality of first oxidized regions  31 B in the substrate  31 A so as to cover the first oxidized regions  31 B by a CVD method under conditions such that the lateral growth is dominant. 
     Next, as illustrated in  FIG. 20D , the second mask film  27  made of silicon nitride and having a thickness of about 100 nm is formed by a method similar to that for the first mask film  25  so that each second mask film  27  masks a portion of the first semiconductor layer  32  above the base region  31   a  and has an opening above the first oxidized region  31 B. 
     Next, as illustrated in  FIG. 21A , the substrate  31 A with the first semiconductor layer  32  and the second mask film  27  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of second oxidized regions  32 C made of silicon oxide are formed through oxidization of the first semiconductor layer  32  in regions of the first semiconductor layer  32  that are not covered by the second mask film  27 . As a result, the base layer  32   a  made of silicon carbide is formed in each region in the first semiconductor layer  32  that is interposed by adjacent second oxidized regions  32 C. 
     Next, as illustrated in  FIG. 21B , the plurality of first oxidized regions  31 B and the plurality of second oxidized regions  32 C are removed by an acidic aqueous solution such as hydrogen fluoride water. Thus, each base layer  32   a  has overhang side portions. 
     Next, as illustrated in  FIG. 21C , the second semiconductor layer  33  made of silicon carbide is grown under growth conditions such that the ELO process with the base layer  32   a  being a seed crystal is dominant. Thus, the gap portion  33   a  is formed between the lower surface of the second semiconductor layer  33  and the principal surface of the substrate  31 A, whereby the second semiconductor layer  33  is grown without being influenced by the surface of the second oxidized regions  32 C. Again, an HVPE method may be used as the crystal growth method for the first semiconductor layer  12 A, the second semiconductor layer  14  and the third semiconductor layer  16 . Furthermore, an MBE method may be used for the first semiconductor layer  12 A. 
     As described above, in the eleventh embodiment, the following effect can be obtained in addition to those of the tenth embodiment. Since the second semiconductor layer  33  is formed on the substrate  11  so that the gap portion  33   a  obtained by removing the first oxidized regions  12 B and the second oxidized regions  14 B is retained, whereby the growth of the second semiconductor layer  33  is not influenced by the surface of the second oxidized regions  32 C, thus further improving the crystallinity of the second semiconductor layer  33 . In addition, the gap portion  33   a  reduces the stress in the second semiconductor layer  33  due to the lattice mismatch occurring while growing the second semiconductor layer  33 . 
     Note that while silicon carbide (SiC) is used for the single-crystal substrate  31 A, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP) may alternatively be used instead of silicon carbide. 
     Twelfth Embodiment 
     The twelfth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 22A  to  FIG. 22E  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the twelfth embodiment of the present invention. 
     First, as illustrated in  FIG. 22A , a mask-forming film made of silicon nitride and having a thickness of about 100 nm is deposited on the principal surface of the substrate  31 A made of silicon carbide whose principal surface is oriented along the (0001) plane by, for example, a plasma CVD method. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the mask-forming film by a photolithography method, and then the mask-forming film is dry-etched by, for example, an RIE method with a reactive gas containing fluorocarbon using the formed resist pattern as a mask, thereby forming, from the mask-forming film, a mask film  25  having a stripe pattern with an opening  25   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner apparatus or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the mask film  25 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 22B , the substrate  31 A with the mask film  25  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of oxidized regions  31 B made of silicon oxide are formed through oxidization of the substrate  31 A in regions of the substrate  31 A that are not covered by the mask film  25 . As a result, a base region  31   a  is formed in each region in the upper portion of the substrate  31 A that is interposed by adjacent oxidized regions  31 B. Herein, the oxidization of the oxidized regions  31 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the mask film  25  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base region  31   a  after the oxidization process can be as small as about 0.25 μm. Moreover, since an oxygen gas or a water-vapor-containing atmosphere is used as an oxidizing atmosphere for selectively oxidizing the oxidized regions  31 B, a quick and uniform oxidization process can be performed reproducibly. 
     Next, as illustrated in  FIG. 22C , the mask film  25  is removed by an RIE method, and then a first semiconductor layer  32 A made of silicon carbide is selectively grown (through an ELO process) by a CVD method on the surface of the base region  31   a  that is exposed between the plurality of oxidized regions  12 B so as to leave an exposed portion in the central portion of the upper surface of each oxidized region  31 B. Thus, the growth of the first semiconductor layers  32 A, which are grown through an ELO process, is stopped before the opposing end surfaces of adjacent first semiconductor layers  32 A that are growing toward each other come into contact with each other. 
     Next, as illustrated in  FIG. 22D , the plurality of oxidized regions  31 B are removed by an acidic aqueous solution such as hydrogen fluoride water. Thus, each first semiconductor layer  32 A has overhang side portions. 
     Next, as illustrated in  FIG. 22E , the second semiconductor layer  33  made of silicon carbide is grown by a CVD method under growth conditions such that the ELO process with the first semiconductor layer  32 A being the base crystal layer is dominant. In this process, the gap portion  33   a  is formed between the lower surface of the second semiconductor layer  33  and the principal surface of the substrate  31 A, whereby the second semiconductor layer  33  is grown without being influenced by either the surface of the oxidized regions  31 B or the surface of the substrate  31 A. 
     As described above, in the twelfth embodiment, as in the eighth embodiment, the crystallinity of the first semiconductor layer  32 A and that of the second semiconductor layer  33  are both improved by the ELO process proceeding from the base region  31   a  having a reduced exposed area. Moreover, in the twelfth embodiment, the second semiconductor layer  33  is grown on the substrate  31 A so that the gap portion  33   a  obtained by removing the oxidized regions  31 B is retained, whereby the growth of the second semiconductor layer  33  is not influenced by the surface of the substrate  31 A or that of the oxidized regions  31 B. As a result, the crystallinity of the second semiconductor layer  33  is improved as compared with a case where the oxidized regions  31 B are not removed. In addition, the gap portion  33   a  also has an effect of reducing the stress in the second semiconductor layer  33  due to the lattice mismatch occurring while growing the second semiconductor layer  33 . 
     Note that while silicon carbide (SiC) is used for the single-crystal substrate  31 A, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP) may alternatively be used instead of silicon carbide. 
     Thirteenth Embodiment 
     The thirteenth embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 23A  to  FIG. 23D  and  FIG. 24A  to  FIG. 24C  are cross-sectional views sequentially illustrating steps in a method for manufacturing a semiconductor thin film according to the thirteenth embodiment of the present invention. 
     First, as illustrated in  FIG. 23A , a first mask-forming film made of silicon nitride and having a thickness of about 100 nm is deposited on the principal surface of the substrate  31 A made of silicon carbide whose principal surface is oriented along the (0001) plane by, for example, a plasma CVD method. Then, a resist pattern having a planar stripe pattern (not shown), for example, is formed on the first mask-forming film by a photolithography method, and then the first mask-forming film is dry-etched by, for example, an RIE method with a reactive gas containing fluorocarbon using the formed resist pattern as a mask, thereby forming, from the first mask-forming film, the first mask film  25  having a stripe pattern with the opening  25   a  therein. Then, the resist pattern is removed by ashing, or the like. Herein, an optical pattern exposure system capable of performing minute processing with a resolution of about 0.5 μm or better in terms of pattern width, such as a contact mask aligner apparatus or a g-ray stepper, is used. Note that while a stripe pattern is used as the planar shape of the first mask film  25 , it may alternatively be a dotted (island-like) pattern. 
     Next, as illustrated in  FIG. 23B , the substrate  31 A with the first mask film  25  formed thereon is subjected to a heat treatment at a temperature of 1000° C., for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, the plurality of first oxidized regions  31 B made of silicon oxide are formed through oxidization of the substrate  31 A in regions of the substrate  31 A that are not covered by the first mask film  25 . As a result, the base region  31   a  is formed in each region in the upper portion of the substrate  31 A that is interposed by adjacent first oxidized regions  31 B. Herein, the oxidization of the first oxidized regions  31 B proceeds also in the direction parallel to the substrate surface (the lateral direction), whereby if the width dimension of the first mask film  25  is set to be 0.5 μm in the patterning process, for example, the width dimension of the base region  31   a  after the oxidization process can be as small as about 0.25 μm. Moreover, since an oxygen gas or a water-vapor-containing atmosphere is used as an oxidizing atmosphere for selectively oxidizing the first oxidized regions  31 B, a quick and uniform oxidization process can be performed reproducibly. 
     Next, as illustrated in  FIG. 23C , the first mask film  25  is removed by an RIE method, and then the first semiconductor layer  12 A made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the base region  31   a  that is exposed between the plurality of first oxidized regions  31 B so as to cover the first oxidized regions  31 B. 
     Next, as illustrated in  FIG. 23D , a second mask film  28  made of silicon and having a thickness of about 100 nm is formed by, for example, a CVD method in which monosilane is decomposed so that the second mask film  28  has a plurality of openings for exposing therethrough portions of the first semiconductor layer  12 A that are above the base regions  31   a.    
     Next, as illustrated in  FIG. 24A , the substrate  31 A with the first semiconductor layer  12 A and the first mask film  28  formed thereon is subjected to a heat treatment at a temperature of 900° C. for about 4 hours, for example, in an oxygen gas or a water-vapor-containing atmosphere. Through the heat treatment, a plurality of first oxidized regions  12 C made of gallium oxide are formed through oxidization of the first semiconductor layer  12 A in regions of the first semiconductor layer  12 A that are not covered by the first mask film  28 . As a result, the base layer  12   a  made of gallium nitride is formed in each region of the first semiconductor layer  12 A that is interposed by adjacent second oxidized regions  12 C. Herein, the base region  31   a  is taken over by the second oxidized region  12 C, whereby the entire principal surface of the substrate  31 A is covered by an oxide made of silicon oxide and gallium oxide. 
     Next, as illustrated in  FIG. 24B , the second mask film  28  is removed by hydrofluoric-nitric acid or an RIE method, and then the second semiconductor layer  14  made of gallium nitride is selectively grown (through an ELO process) by an MOCVD method on the surface of the base layer  12   a  that is exposed between the plurality of second oxidized regions  12 C so as to cover the second oxidized regions  12 C. 
     Next, as illustrated in  FIG. 24C , the plurality of first oxidized regions  31 B and the plurality of second oxidized regions  32 B are removed by wet etching. Thus, the substrate  31 A and the second semiconductor layer  33  can be separated from each other easily, uniformly and reproducibly even if the substrate  31 A has a large area. 
     Herein, the etchant for the first oxidized regions  31 B made of silicon oxide may be, for example, hydrogen fluoride water or buffered hydrogen fluoride, and the etchant for the second oxidized regions  12 C made of gallium oxide may be hydrogen chloride water or ammonia water. 
     Thus, in the thirteenth embodiment, the following effect can be obtained in addition to those of the eighth embodiment. By separating the substrate  31 A for forming the base region  31   a  and the base layer  12   a  from the second semiconductor layer  14 , the substrate  31 A can be reused. In this way, since the substrate  31 A made of silicon carbide is typically expensive, it is possible to reduce the manufacturing cost for manufacturing a device using the second semiconductor layer  33  by reusing the substrate  31 A. 
     Alternatively, as in the second embodiment, the substrate  31 A may be separated after bonding a support substrate made of silicon (Si) whose principal surface is oriented along the (100) plane to the upper surface of the second semiconductor layer  14 . 
     Note that while silicon carbide (SiC) is used for the single-crystal substrate  31 A, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or gallium phosphide (GaP) may alternatively be used instead of silicon carbide. 
     Moreover, the orientation of the principal surface of the substrates  11  and  31  used in the embodiments described above is not limited to the (0001) plane. Furthermore, the substrates  11  and  31  may be provided with a so-called “off-angle” by offsetting it slightly from the typical orientation such as the (0001) plane. 
     Moreover, while gallium nitride is used for the second semiconductor layer  14 , which is the semiconductor thin film of interest, in the first, second and thirteenth embodiments, a mixed crystal to which indium or aluminum is added may alternatively be used. Furthermore, a device structure including a hetero junction such as a quantum well structure, for example, using these mixed crystals may be formed in the second semiconductor layer  14 . 
     Similarly in the third to seventh embodiments, a device structure including a hetero junction using a mixed crystal of InGaAlNAs may be formed in the third semiconductor layer  16 . Moreover, also in the eighth to twelfth embodiments, a device structure may be formed in the first semiconductor layer  32  or the second semiconductor layer  33 . 
     Moreover, in embodiments where the substrate  11  or the substrate  31 A is not separated from the grown semiconductor layer, the substrates  11  and  31 A may be separated from the semiconductor layer by irradiating the substrates  11  and  31 A with irradiation light whose energy is smaller than the forbidden band width of the material of the substrates  11  and  31 A and is larger than the forbidden band width of the semiconductor. In such a case, oxidized regions may be selectively removed by heating the substrates  11  and  31 A to about 500° C. or by using an acidic or alkaline aqueous solution.