Abstract:
A method for transferring a porous layer includes forming a porous layer on one side of a crystalline silicon member by anodization, fixing a supporting substrate onto the surface of the porous layer, and applying force to any one of the supporting substrate and the porous layer, whereby at least part of the porous layer is cleaved from the crystalline silicon member and is transferred onto the supporting substrate. The crystalline silicon member can be recycled and this method is suitable for mass production of semiconductor devices or solar batteries at low cost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for transferring a porous layer, to a method for making a semiconductor device, and to a method for making a solar battery. In particular, the present invention relates to a method for forming a porous layer on an inexpensive substrate and to a method for making semiconductor devices, such as a solar battery, by depositing crystalline thin-films on an inexpensive substrate with a porous layer provided therebetween. 
     2. Description of the Related Art 
     Solar batteries have been widely studied as driving energy sources for various devices and as electric power sources which are connected to commercial power systems. 
     Low-cost production of solar batteries requires forming semiconductor devices on an inexpensive substrate. A typical semiconductor material for solar batteries is silicon. In particular, single-crystal silicon is most advantageous in view of photoelectric conversion efficiency which is an efficiency for converting optical energy into electromotive force. On the other hand, amorphous silicon is advantageous in view of production using a large substrate at low cost. In recent years, the use of polycrystalline silicon has been examined in order to achieve a low cost comparable to that of amorphous silicon and a high energy conversion efficiency comparable to that of single-crystal silicon. 
     Since conventional methods for making solar batteries using single-crystal silicon or polycrystalline silicon use platelet substrates which are formed by slicing a crystal ingot, it is difficult to decrease the thickness to 0.3 mm or less. Since such substrates are larger than a thickness (approximately 20 μm to 50 μm) which is required for sufficiently absorbing light, the substrate material is not effectively used. Accordingly, low-cost production of solar batteries requires development of thinner single-crystal silicon or polycrystalline silicon substrates. 
     In recent years, a method for making a silicon sheet by a spinning process has been proposed in which melted silicon droplets are cast into a mold. The thickness of the crystal silicon substrate prepared by this method reaches approximately 0.1 mm to 0.2 mm at minimum, and is still larger than the above thickness (approximately 20 μm to 50 μm) which is required for sufficiently absorbing light. 
     An attempt to accomplish both a high energy conversion efficiency and a low production cost is proposed by using an epitaxial thin-layer which is deposited on a single-crystal silicon substrate and is cleaved from the substrate as solar batteries (Milnes, A. G. and Feucht, D. L., “Peeled Film Technology Solar Cells”, IEEE Photovoltaic Specialist Conference, p. 338, 1975). 
     In this method, however, a SiGe interlayer is provided on a single-crystal silicon substrate, a single-crystal silicon layer is heteroepitaxially deposited on the interlayer, and the interlayer is selectively melted to cleave the heteroepitaxially deposited single-crystal silicon layer. In general, defects readily occur at the interface between the underlayer and the heteroepitaxially deposited layer, since these layers have different lattice constants. Moreover, this method is not always advantageous in view of material cost since the method uses different materials. 
     U.S. Pat. No. 4,816,420 discloses a thin crystalline solar battery which is produced by a method including forming a crystalline sheet by selective epitaxial growth through a mask and then lateral crystal growth on a crystal substrate and cleaving the sheet from the substrate. 
     In this method, the mask has slits as line seeds, and a crystal sheet, which is formed by selective epitaxy from the slits and then lateral growing, is cleaved. That is, the crystal sheet is mechanically cleaved by means of crystal cleavage. When the line seeds have a size greater than a certain size, the crystal has a large contact area with the substrate, and the crystal sheet may be damaged during the cleaving process. In the production of solar batteries having a large area, the slit length may be several millimeters to several centimeters no matter how much the slit width is decreased (in practice, the slit width may be reduced to approximately 1 μm). Thus, this method is not practical. 
     In consideration of such problems, Japanese Unexamined Patent Application Publication No. 6-45622 discloses a method for making a crystalline thin-film solar battery having satisfactory characteristics. In this method, a porous silicon layer is formed on a silicon wafer by anodization and is then cleaved from the wafer. The cleaved porous silicon layer is bonded onto a metal substrate and an epitaxial layer is deposited thereon. This product is used as the solar battery. 
     In this method, however, the metal substrate is exposed in a high-temperature process. Such a process may causes inclusion of impurities in the epitaxial layer, resulting in deterioration of properties as the epitaxial layer. Moreover, a significantly thin porous layer is handled alone in this process. Thus, particular attention must be directed to transportation of the thin porous layer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method for transferring a porous layer which produces a semiconductor layer having satisfactory characteristics. 
     It is another object of the present invention to provide a method suitable for mass production of semiconductor devices having satisfactory characteristics. 
     It is still another object of the present invention to provide a method suitable for mass production of high-efficiency solar batteries. 
     It is a further object of the present invention to reuse a crystal substrate which was used for forming a porous layer thereon, after the substrate was separated therefrom, in order to reduce production costs. 
     An aspect of the present invention relates to a method for transferring a porous layer including an anodization step of forming by anodization a porous layer on one side of a crystalline silicon member, a fixing step of fixing a supporting substrate onto the surface of the porous layer, and a cleaving step of applying force to any one of the supporting substrate and the porous layer, whereby at least part of the porous layer is cleaved from the crystalline silicon member and is transferred onto the supporting substrate. 
     Other aspects of the present invention relate to methods for making a semiconductor device and a solar battery including the above steps for transferring the porous layer onto the supporting substrate and a step of forming a crystalline semiconductor layer onto the transferred porous layer. 
     The terms “at least part of the porous layer is cleaved” mean that the porous layer is cleaved at an interface between the porous layer and the other layer of the crystalline silicon member, that part of the porous layer is cleaved so that the porous layer remains on the crystalline silicon substrate. That is, the entire porous layer and the unanodized portion are separated from each other at the interface therebetween, or part of the porous layer and the unanodized portion are separated from each other at the interior of the porous layer. 
     The crystalline silicon member includes a substrate, a film, and the like. 
     In these methods, another porous layer is preferably formed by anodization onto the crystalline silicon after the prior porous layer is transferred. Moreover, the other porous layer is preferably transferred onto another supporting substrate which is different from the prior supporting substrate. 
    
    
     Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 G are schematic cross-sectional views illustrating an embodiment of a method for making a solar battery of the present invention; 
     FIGS. 2A to  2 G are schematic cross-sectional views illustrating another embodiment of a method for making a solar battery of the present invention; 
     FIGS. 3A to  3 G are schematic cross-sectional views illustrating another embodiment of a method for making a solar battery of the present invention; and 
     FIG. 4 is a schematic cross-sectional view of a thin-film GaAs/AlGaAs solar battery formed on a porous layer by the method shown in FIGS. 3A to  3 G. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A to  1 G are schematic cross-sectional views illustrating a preferred embodiment of a production process for a solar battery of the present invention. The process will be described below. 
     With reference to FIG. 1A, a p +  (or n + ) layer  102  is formed onto a crystal substrate  101  composed of crystalline silicon, e.g., a single-crystal silicon wafer, by thermal diffusion or ion implanting of a dopant or by incorporating a dopant into the wafer in a wafer production process. With reference to FIG. 1B, the doped wafer surface is anodized in a HF solution to form a porous layer  103 . When the anodization current is varied from a lower level to a higher level in the midway of the anodization process, the porous layer  103  has a gradient of porosity in the thickness direction, and the porous layer  103  will be readily cleaved later. For example, the porosity is increased at the lower side near the crystal substrate  101  of the porous layer  103  so that this portion becomes relatively fragile. Such a configuration facilitates transfer of major portions of the porous layer  103  in the subsequent transfer step. With reference to FIG. 1C, a supporting substrate  104 , for example, a metallurgical-grade silicon substrate is bonded onto the porous layer  103 . With reference to FIG. 1D, a force is applied to the supporting substrate  104  or the porous layer  103  to cleave the porous layer  103  along the fragile portion. As a result, a porous layer  103   a  is transferred onto the supporting substrate  104 . When the force is applied to the supporting substrate  104 , another force may be applied to the silicon wafer  101 . Alternatively, forces may be applied to both the supporting substrate  104  and the porous layer  103 , or all the supporting substrate  104 , the porous layer  103 , and the silicon wafer  101 . With reference to FIG. 1E, a single-crystal silicon layer  105  is formed by epitaxy on the transferred porous layer  103   a , and an n +  (or n + ) layer  106  is formed thereon. With reference to FIG. 1F, a surface electrode  107  is formed on the n +  (or n + ) layer  106 , an antireflective layer  108  is formed thereon, and a back electrode  109  is formed on another side, away from the porous layer, of the supporting substrate  104 . A solar battery is thereby formed. With reference to FIG. 1G, a porous layer  103   b  remaining on the silicon wafer after the cleaving step is removed by etching or the like to regenerate the silicon wafer  101 . The regenerated silicon wafer  101  is recycled to the first step shown in FIG.  1 A. 
     Since the metallurgical-grade silicon substrate is used instead of a metal substrate in the present invention, incorporation of impurities into the epitaxial layer can be significantly suppressed in the deposition process. Moreover, the porous layer has gettering effects for impurities for the epitaxial layer. In addition, the porous layer is bonded to the supporting substrate and then is cleaved from the porous layer before epitaxial growth in the present invention. This method is advantageous in operation over a series of steps compared to a method for cleaving and handling only the porous layer before epitaxial growth. 
     A method for making solar batteries will be described with reference to the following embodiments using various types of silicon as materials for forming porous layers. 
     First Embodiment 
     A method for making a thin-film single-crystal solar battery on a metallurgical-grade silicon substrate will be described with reference to FIGS. 1A to  1 G. 
     With reference to FIG. 1A, boron (B) is implanted into the surface layer of the silicon wafer  101  by thermal diffusion to form the p +  layer  102 . With reference to FIG. 1B, the single-crystal substrate provided with the surface p +  layer  102  is anodized in the HF solution to form the porous layer  103 . In this process, the anodization current may be set at a low level at the initial stage and then may be shifted to a higher level to form a gradient of porosity in the porous layer, whereby the porous layer can be cleaved at a desired portion. 
     Next, the metallurgical-grade silicon supporting substrate  104  is put into close contact with the porous layer  103  as shown in FIG. 1C, and the composite is heated in a furnace (not shown in the drawing) to bond the metallurgical-grade silicon supporting substrate  104  to the porous layer  103 . The metallurgical-grade silicon substrate  104  is generally prepared as follows. A silicon ingot is prepared using a melt of metallurgical-grade silicon by a Czochralski process as in a general semiconductor wafer or a polycrystalline silicon ingot is prepared by a casting process from melted silicon. These ingots are sliced to form supporting substrates. As disclosed in Japanese Unexamined Patent Application Publication No. 9-36403, a sheet substrate may be directly formed by placing, melting, and then solidifying powdered metallurgical-grade silicon into a mold having a thin rectangular groove. 
     In a method for bonding the metallurgical-grade silicon supporting substrate  104  to the porous layer  103 , these are put into close contact with each other and are annealed. Herein, it is preferable that a silicon oxide layer having a thickness as that of a native oxide or a trace amount of water be present on at least one of the surface of the metallurgical-grade silicon and the surface of the porous layer, in order to enhance the bonding strength therebetween. In another method for bonding the metallurgical-grade silicon substrate and the porous layer, a thin metal layer composed of Ni, Cr, Fe, Co, Ti, Mo, W, or the like is provided on a bonding surface of the metallurgical-grade silicon substrate or on a bonding surface of the porous layer by deposition or sputtering, the bonding surface of the metallurgical-grade silicon substrate and the bonding surface of the porous layer are put into close contact with each other and annealed to form a silicide layer. In these two methods, the porous layer  103  may be preliminarily annealed at 1,000° C. or more in a hydrogen atmosphere to planarize the surface of the porous layer. In a third method for bonding the metallurgical-grade silicon substrate to the porous layer, a metal layer having a low melting point, such as In, Sn, Ga, Bi, Al, or the like, is formed on one of a bonding surface of the metallurgical-grade silicon substrate or a bonding surface of the porous layer by evaporation or sputtering, and the both bonding surfaces are put into close contact with each other and are annealed. In this method, silicon was partially melted into the metal by heating and reprecipitated on the metallurgical-grade silicon substrate or the porous layer in the cooling step to partially bond the silicon substrate to the porous layer. 
     Among these, in bonding methods by providing metal layers, these metal layers also function as back reflective layers when optical devices, such as a solar battery, are formed on the porous layer. 
     With reference to FIG. 1D, a force is applied to the porous layer provided between the metallurgical-grade silicon substrate  104  and the single-crystal silicon substrate  101  to separate the porous layer  103   a  from the single-crystal silicon substrate  101 . With reference to FIG. 1E, a single-crystal silicon layer  105  is deposited by a crystal growing process, such as a thermal CVD process or a liquid-phase epitaxial process on the cleaved porous layer  103   a , until the thickness becomes adequate as an active layer of the solar battery. The applied force may be a pulling force, an unclenching force, ultrasonic waves, or a jet stream of liquid. A trace amount of dopant may be included when the single-crystal silicon layer  105  is formed in order to control the active layer to a p −  type (or n − type). An p +  (or n + ) layer  106  is formed on the active layer  105  by thermal diffusion of a dopant, by a plasma-enhanced CVD process, or by increasing the amount of the dopant at the final stage of the formation of the active layer  105 . 
     With reference to FIG. 1F, a grid collecting electrode  107  and an antireflective layer  108  are formed on the n +  (or n + ) layer  106 , and a back electrode  109  is formed on the back face of the metallurgical-grade silicon substrate  104  by vacuum deposition to complete the solar battery. 
     With reference to FIG. 1G, the porous layer  103   b  remaining on the single-crystal silicon substrate  101  after the cleaving step is removed by etching or the like. If the surface roughness is to great, the surface is planarized. The regenerated single-crystal silicon substrate  101  is recycled in the step shown in FIG.  1 A. 
     Second Embodiment 
     In this embodiment, a thin-film polycrystalline solar battery is formed on a metallurgical-grade silicon substrate. 
     With reference to FIG. 2A, boron (B) is implanted into the surface layer of a polysilicon substrate  201  by thermal diffusion. In the subsequent steps shown in FIGS. 2A to  2 F, the thin-film polycrystalline solar battery is prepared as in the first embodiment, wherein numeral  202  represents a diffused layer doped with boron, numerals  203 ,  203   a , and  203   b  represent porous layers, numeral  204  represents a metallurgical-grade silicon substrate, numeral  205  represents an active layer, numeral  206  represents an n +  (or p +  layer), numeral  207  represents a collecting electrode, numeral  208  represents an antireflective electrode (transparent electrode), and numeral  209  represents a back electrode. 
     Third Embodiment 
     In this embodiment, a thin-film compound semiconductor solar battery is formed on a metallurgical-grade silicon substrate. 
     With reference to FIG. 3A, boron (B) is implanted into the surface layer of a single-crystal silicon substrate  301  by thermal diffusion to form a p +  layer  302 . With reference to FIG. 3B, the single-crystal substrate provided with the surface p +  layer  302  is anodized in a HF solution by a gradient current mode in which the current is gradually increased from an initial low current after a predetermined time elapses, in order to form a porous layer  303 . 
     Next, a metallurgical-grade silicon supporting substrate  304  is put into close contact with the porous layer  303  as shown in FIG. 3C, and the composite is heated in a furnace (not shown in the drawing) as in the first embodiment to bond the metallurgical-grade silicon supporting substrate  304  to the porous layer  303 . 
     With reference to FIG. 3D, a force is applied to the porous layer provided between the metallurgical-grade silicon substrate  304  and the single-crystal silicon substrate  301  to separate a porous layer  303   a  from the single-crystal silicon substrate  301 . 
     With reference to FIG. 3E, for example, a p +  (or n +  layer), a p −  (or n − ) active layer, and an n +  (or p +  layer) are continuously deposited by a MOCVD process on the cleaved porous layer  303   a . These layers constitute a compound semiconductor layer  305 . 
     With reference to FIG. 3F, a grid collecting electrode  307  is formed on the compound semiconductor layer  305 , an antireflective layer  307  is formed thereon, and a back electrode  309  is formed on the back face of the metallurgical-grade silicon substrate by vacuum deposition to complete the solar battery. 
     With reference to FIG. 3G, the porous layer  303   b  remaining on the single-crystal silicon substrate  301  after the cleaving step is removed as in the first embodiment. If the surface roughness is too great, the surface is planarized. The regenerated single-crystal silicon substrate  301  is recycled in the step shown in FIG.  3 A. 
     As described above, the present invention relates to a method for forming a porous layer on a silicon substrate and then transferring the porous layer onto a supporting substrate, to a method for making a semiconductor layer prepared by depositing an epitaxial layer on the transferred porous layer, and to a method for making a thin-film crystalline solar battery. 
     The epitaxial layer deposited on the transferred porous layer has substantially the same characteristics as those of an epitaxial layer deposited on a wafer. Furthermore, this method allows the use of inexpensive substrates such as metallurgical-grade silicon. In addition, the silicon substrate used for forming the porous layer can be recycled. Thus, this method is advantageous in reducing material costs. Since a compound semiconductor layer can be formed on the transferred porous layer, this method is applicable to production of various semiconductor devices and solar batteries. 
     In the anodization process for forming the porous silicon layer of the present invention, a hydrofluoric acid (HF) solution is preferably used. The HF content in the solution is preferably 10% or more to facilitate pore formation. The current density during the anodization (anodization current density) is appropriately determined depending on the HF content, the target thickness of the porous layer, and the surface state of the porous layer, and is preferably in a range of 1 mA/cm 2  to 100 mA/cm 2 . 
     Addition of alcohol, e.g., ethyl alcohol, to the HF solution facilitates instantaneous detachment of bubbles generated during the anodization process from the reacting surface without stirring and allows uniform production of the porous silicon layer effectively. The amount of the added alcohol is appropriately determined depending on the HF content, the target thickness of the porous layer, and the surface state of the porous layer, as long as the HF content is not noticeably decreased. 
     Examples of the epitaxial growing processes employed for forming the silicon layer on the porous layer include a thermal CVD process, a low pressure CVD (LPCVD) process, a sputtering process, a plasma-enhanced CVD process, and a photon-assisted CVD process. Examples of typical material gases used in a vapor phase epitaxial process, such as the thermal CVD process, the low pressure CVD (LPCVD) process, the plasma-enhanced CVD process, the photon-assisted CVD process, and the liquid-phase epitaxial process, include silanes and halogenated silanes, such as SiH 2 Cl 2 , SiCl 4 , SiHCl 3 , SiH 4 , Si 2 H 6 , SiH 2 F 2 , and Si 2 F 6 . 
     In addition to the material gas, hydrogen (H 2 ) gas is also added as a carrier gas in order to form a reducing atmosphere for facilitating the crystal growth. The ratio, the material gas:hydrogen, is appropriately determined depending on the growing process, the type of the material gas, and the deposition conditions, and is in a range of preferably 1:10 to 1:1,000 by fed flow rate, and more preferably 1:20 to 1:800. 
     The liquid-phase epitaxial process is preferably performed by dissolving silicon into a solvent, such as Ga, In, Sb, Bi, or Sn and by slowly cooling the solution or providing a temperature gradient in the solution to epitaxially grow silicon. 
     In the formation of the compound semiconductor layer on the porous layer, a MOCVD process, a MBE process, or a liquid-phase epitaxial process is employed. Materials used in these crystal growing processes are appropriately determined depending on the type of the compound semiconductor to be formed and the deposition process. Ga(CH 3 ) 3 , ASH 3 , Al(CH 3 ) 31  and the like are used in the MOCVD process. As or As and Al are dissolved into Ga as a solvent in the liquid-phase epitaxial process. 
     The temperature and the pressure in the epitaxial process used in the present invention are determined dependent on the type of the epitaxial process and the type of the material gas. The temperature is set at preferably 800° C. to 1,250° C. and more preferably 850 to 1,200° C. when silicon is deposited by a general thermal CVD process. The preferable temperature range varies with the type of the solvent in the liquid-phase epitaxial process. For example, the temperature is preferably set at 600° C. to 1,050° C. when silicon is deposited using Sn or In as a solvent. The temperature is preferably set at 650° C. to 850° C. when GaAs is deposited using Ga as a solvent. The temperature is preferably set at 650° C. to 900° C. when GaAs is deposited by a MOCVD process. Moreover, the temperature is set at preferably 200° C. to 600° C. and more preferably 200° C. to 500° C. in a low-temperature process such as a plasma-enhanced CVD process. 
     The pressure is in a range of preferably 1 Pa to 10 5  Pa and more preferably 10 Pa to 10 5  Pa in a process other than the MBE process. In the MBE process, the evacuation pressure is preferably 10 −3  Pa or less and more preferably 10 −4  Pa or less. 
     In the solar battery of the present invention, the surface of the semiconductor layer may be subjected to a texturing treatment to reduce reflective loss of the incident light. The texturing is performed using hydrazine, NaOH, or KOH. The height of the textured pyramid is preferably in a range of 1 μm to 100 μm. 
     The formation of preferable solar batteries by the method of the present invention will now be described in more detail with reference to the following non-limiting EXAMPLES. 
     EXAMPLE 1 
     A thin-film single-crystal solar battery was formed on a metallurgical-grade silicon substrate by a process shown in FIG.  1 . 
     A p +  layer  102  with a thickness of approximately 3 μm as a diffused layer was formed on a surface of a single-crystal silicon wafer  101  with a thickness of 600 μm by thermal diffusion of boron at 1,200° C. using a BCl 3  thermal diffusion source (FIG.  1 A). A porous silicon layer  103  was formed on the wafer by anodization in a HF solution under the conditions shown in Table 1 (FIG.  1 B). Anodization was performed at a low current density of 5 mA/cm 2  for 2.5 minutes at an initial stage, the current density was gradually increased for 30 seconds, and the anodization was completed when the current density reached 30 mA/cm 2 . 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Anodization Solution 
                 HF:H 2 O:C 2 H 5 OH = 1:1:1 
               
               
                   
                 Current Density 
                 5 mA/cm 2  → 30 mA/cm 2   
               
               
                   
                 Anodization Time 
                 2.5 min → (30 sec) → 0 sec 
               
               
                   
                   
               
             
          
         
       
     
     An ingot was pulled up using 98% metallurgical-grade silicon by a Czochralski (CZ) method and was sliced into wafers having a thickness of 0.5 mm. The wafer was mirror-polished to prepare a metallurgical-grade silicon substrate  104 . Table 2 shows the results of an elemental analysis on the surface of the resulting metallurgical-grade silicon substrate  104  as a supporting substrate. The crystal grain size of the metallurgical-grade silicon substrate was several millimeters to several centimeters, and the resistivity was 0.05 Ω·cm (p-type). 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Impurities 
                 Content 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 B 
                 8 
                 ppm 
               
               
                   
                 Al 
                 2 
                 ppm 
               
               
                   
                 Ni 
                 &lt;5 
                 ppm 
               
               
                   
                 Fe 
                 1 
                 ppm 
               
               
                   
                 Cr 
                 0.6 
                 ppm 
               
               
                   
                 Mn 
                 &lt;0.2 
                 ppm 
               
               
                   
                 Ti 
                 &lt;1 
                 ppm 
               
               
                   
                   
               
             
          
         
       
     
     After a small amount of water was applied to a bonding surface of the metallurgical-grade silicon substrate  104 , the bonding surface of the metallurgical-grade silicon substrate  104  was put into close contact with the porous layer  103 , and the composite was heated in a furnace (not shown in the drawing) to bond the metallurgical-grade silicon substrate  104  to the porous layer  103  (FIG.  1 C). 
     A sharp edged tool (not shown in the drawing) was inserted into the porous layer  103  between the metallurgical-grade silicon substrate  104  and the single-crystal silicon wafer  101  to cleave a porous layer  103   a  from the single-crystal silicon wafer  101  by a force applied to the tool (FIG.  1 D). 
     A single-crystal silicon layer  105  with a thickness of 30 μm was formed on the cleaved porous layer  103   a  by epitaxy under the conditions shown in Table 3 using a conventional thermal CVD system. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 Ratio of Gas Flow Rate 
                 SiH 2 Cl 2 /H 2  = 0.5/100 (liter/min) 
               
               
                   
                 Substrate Temperature 
                 1,050° C. 
               
               
                   
                 Pressure 
                 Atmospheric Pressure 
               
               
                   
                 Deposition Time 
                 30 min 
               
               
                   
                   
               
             
          
         
       
     
     In this growing step, a trace amount (in the order of 0.1 ppm to several ppm) of B 2 H 6  was added so that the deposited silicon layer became a p −  type, and in the final stage of the growing step, the gas was changed to PH 3  (several ppm) so as to form an n +  layer  106  (FIG.  1 E). 
     According to the cross-section observation by transmittance electron microscopy, no more crystal defects were introduced, and high crystallinity was still maintained in these silicon layers  105  and  106 . 
     Finally, a collecting electrode  107  of Ti/Pd/Ag (400 nm/200 nm/1 μm) and then an ITO transparent conductive film  108  with a thickness of 82 nm were deposited on the n +  layer  106  by an electron beam deposition process. An aluminum back electrode with a thickness of 2 μm was formed on the back face of the metallurgical-grade silicon substrate  104  to complete the solar battery (FIG.  1 F). 
     The I—V characteristics of the single-crystal silicon solar battery formed on the metallurgical-grade silicon substrate which was irradiated with light of AM 1.5 (100 mW/cm 2 ) were measured. In the cell area of 6 cm 2 , the open-circuit voltage was 0.59 V, the short-circuit photocurrent density was 33 mA/cm 2 , the fill factor was 0.79, and the photoelectric conversion efficiency was 15.4%. 
     The silicon wafer  101  after cleaving having the residual porous layer  103   b  was immersed into a mixed solution containing hydrofluoric acid, hydrogen peroxide, and deionized water with stirring for selective etching. Most of the nonporous portion of the wafer  101  remained while the porous layer was completely etched (FIG.  1 G). 
     The etching rate of the nonporous single-crystal silicon is extremely low in the above etching solution, and the selective etching ratio of the porous layer to the nonporous single-crystal silicon is as high as the order of 10 5 . Accordingly, a decrease in thickness of the nonporous single-crystal silicon layer during the etching is several nm which is a negligible level in practical use. 
     The above steps were repeated several times using the resulting recycled wafer, and several thin-film single-crystal solar batteries having high-quality semiconductor layers were prepared. 
     EXAMPLE 2 
     A thin-film polycrystalline solar battery was formed on a metallurgical-grade silicon substrate by a process shown in FIG.  2 . 
     A p +  layer  202  with a thickness of approximately 3 m as a diffused layer was formed on a surface of a cast silicon (polycrystalline silicon) wafer  201  with a thickness of 1 mm by thermal diffusion of boron at 1,200° C. using a BCl 3  thermal diffusion source (FIG.  2 A). A porous silicon layer  203  was formed on the polycrystalline silicon wafer  201  by anodization in a HF solution under the conditions shown in Table 4 (FIG.  2 B). Anodization was performed at a low current density of 5 mA/cm 2  for 2.5 minutes, and the current density was abruptly increased to 100 mA/cm 2  and was maintained at this current density for 8 seconds. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 Anodization Solution 
                 HF:H 2 O:C 2 H 5 OH = 1:1:1 
               
               
                   
                 Current Density 
                 5 mA/cm 2  → 100 mA/cm 2   
               
               
                   
                 Anodization Time 
                 2.5 min → 8 sec 
               
               
                   
                   
               
             
          
         
       
     
     After a Ni layer (not shown in the drawing) with a thickness of 50 nm was deposited on a bonding surface of a metallurgical-grade silicon substrate  204  by a deposition process, the bonding surface of the metallurgical-grade silicon substrate  204  was put into close contact with the porous layer  203 , and the composite was heated in a furnace (not shown in the drawing) to bond the metallurgical-grade silicon substrate  204  to the porous layer  203 , as in Example 1 (FIG.  2 C). In this case, a silicide layer (not shown in the drawing) was formed between the metallurgical-grade silicon substrate  204  and the porous layer  203 . 
     A pulling force was applied to the porous layer  203  between the metallurgical-grade silicon substrate  204  and the polycrystalline silicon wafer  201  to separate a porous layer  203  from the polycrystalline silicon wafer  201  (FIG.  2 D). 
     A polycrystalline silicon layer  205  with a thickness of 30 μm was formed on the porous silicon layer  203   a  by an epitaxial process using a slider-type liquid-phase deposition system using indium as a solvent under the conditions shown in Table 5. In this process, a trace amount of B (in the order of 0.1 ppm to several ppm to the amount of the dissolved silicon) was added so that the deposited silicon layer  205  was a p −  type, and an n +  type layer  206  with a thickness of 200 nm was formed on the deposited silicon layer  205  using another In melt containing phosphorus (several thousands ppm to the amount of the dissolved silicon) (FIG.  2 E). 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
             
               
                   
                 H 2  Flow Rate 
                 5 liter/min 
               
               
                   
                 Solvent (In) Temperature 
                 950° C. → 830° C. 
               
               
                   
                 Cooling Rate 
                 1° C./min 
               
               
                   
                   
               
             
          
         
       
     
     According to the cross-section observation by transmittance electron microscopy, no more crystal defects were introduced, and high crystallinity was still maintained in these silicon layers  205  and  206 . 
     Finally, a collecting electrode  207  of Ti/Pd/Ag (400 nm/200 nm/1 μm) and then an ITO transparent conductive film  208  with a thickness of 82 nm were deposited on the n +  layer  206  by an electron beam deposition process. An aluminum back electrode with a thickness of 2 μm was formed on the back face of the metallurgical-grade silicon substrate  204  to complete the solar battery (FIG.  2 F). 
     The I—V characteristics of the polycrystalline silicon solar battery formed on the metallurgical-grade silicon substrate which was irradiated with light of AM 1.5 (100 mW/cm 2 ) were measured. In the cell area of 6 cm 2 , the open-circuit voltage was 0.58 V, the short-circuit photocurrent density was 32.5 mA/cm 2 , the fill factor was 0.76, and the photoelectric conversion efficiency was 14.3%. 
     The silicon wafer  201  after cleaving having the residual porous layer  203   b  was immersed into an organic alkaline solution containing tetramethylammonium hydroxide (TMAH) with stirring to remove the residual porous layer  203   b  by selective etching (FIG.  2 G). 
     The above steps were repeated several times using the resulting recycled wafer, and several thin-film polycrystalline solar batteries having high-quality semiconductor layers were prepared. 
     EXAMPLE 3 
     A thin-film compound semiconductor solar battery was formed on a metallurgical-grade silicon substrate by a process shown in FIG.  3 . 
     A p +  layer  302  with a thickness of approximately 3 μm as a diffused layer was formed on a surface of a single-crystal silicon wafer  301  with a thickness of 500 μm by thermal diffusion of boron at 1,200° C. using a BCl 3  thermal diffusion source (FIG.  3 A). A porous silicon layer  303  was formed on the single-crystal silicon wafer  301  by anodization in a HF solution under the conditions shown in Table 6 (FIG.  3 B). Anodization was performed at a low current density of 1 mA/cm 2  for 2 minutes and then at another low current density of 5 mA/cm 2  for 2.5 minutes, the current density was gradually increased for 20 seconds, and the anodization was completed when the current density reached 40 mA/cm 2 . 
     After an indium layer with a thickness of 1,000 nm (not shown in the drawing) was deposited on a bonding surface of a metallurgical-grade silicon substrate  304  by a deposition process, the bonding surface of the metallurgical-grade silicon substrate  304  was put into close contact with the porous layer  303 , and the composite was heated in a furnace (not shown in the drawing) to bond the metallurgical-grade silicon substrate  304  to the porous layer  303 , as in Examples 1 and 2 (FIG.  3 C). In this case, silicon was partially melted into indium by heating and reprecipitated on the metallurgical-grade silicon substrate or the porous layer in the cooling step to partially bond the silicon substrate to the porous layer. 
     The composite was placed into a water bath and ultrasonic waves were applied to the composite to cleave the porous layer  303  between the metallurgical-grade silicon substrate  304  and the single-crystal silicon wafer  301  and to form a porous layer  303   a  on the metallurgical-grade silicon substrate  304  (FIG.  3 D). 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
             
               
                   
                 Anodization Solution 
                 HF:H 2 O:C 2 H 5 OH = 1:1:1 
               
               
                   
                 Current Density 
                 1 mA/cm 2  → 5 mA/cm 2  → 40 mA/cm 2   
               
               
                   
                 Anodization Time 
                 2 min → 2.5 min → (20 sec) → 0 sec 
               
               
                   
                   
               
             
          
         
       
     
     After the surface of the porous silicon layer  303   a  was annealed at 1,050° C. for 7 minutes in a hydrogen atmosphere, a tandem-type (single crystal) GaAs/AlGaAs layer  305  shown in FIG. 4 was deposited using a metal-organic chemical vapor deposition (MOCVD) system (FIG.  3 E). In FIG. 4, numeral  403  represents a p-GaAs layer, numeral  404  represents an n-GaAs layer, numeral  405  represents an n +  Al 0.9  Ga 0.1  As layer, numeral  407  numeral  406  represents an n-Al 0.37 Ga 0.63 As layer, numeral  407  represents an n + -Al x Ga 1-x As layer, numeral  408  represents a p-Al 0.37 Ga 0.63 As layer, numeral  409  represents a p + -Al x Ga 1-x As layer, numeral  410  represents a p-Al 0.37 Ga 0.63 As layer, numeral  411  represents an n-Al 0.37 Ga 0.63 As layer, numeral  412  represents an n + -Al x Ga 1-x As layer, and numeral  413  represents an n + -GaAs layer. According to the cross-sectional observation by transmittance electron microscopy, no more crystal defects were introduced, and high crystallinity was still maintained in the GaAs/AlGaAs layer  305 . 
     The uppermost layer of the resulting GaAs/AlGaAs layer  305  was etched to form a grid of the n + -GaAs layer  413  and to expose the n + -Al x Ga 1-x As layer  412 , and a (Au/Ge/Ni/Au) surface electrode  306  was formed only on the grid of the n + -GaAs layer  413  by an electron beam (EB) deposition process and a photolithographic process. A TiO 2 /MgO antireflective layer  307  was deposited by a plasma-enhanced CVD process, and an aluminum back electrode  308  with a thickness of 2 μm was deposited by evaporation on the back surface of the metallurgical-grade silicon substrate to complete the solar battery (FIG.  3 F). 
     The I—V characteristics of the resulting silicon solar battery was irradiated with light of AM 1.5 (100 mW/cm 2 ) were measured. In the cell area of 4 cm 2 , the open-circuit voltage was 2.3 V, the short-circuit photocurrent density was 13.2 mA/cm 2 , the fill factor was 0.80, and the photoelectric conversion efficiency was 24.3%. 
     The residual porous layer  303   b  was removed from the silicon wafer after cleaving by etching as in Example 1 or 2 to form a smooth surface on the silicon wafer. The above steps were repeated several times using the resulting recycled wafer, and several thin-film compound semiconductor solar batteries having high-quality semiconductor layers were prepared. 
     The present invention is not limited to the above EXAMPLES and may include a variety of modifications. For example, a porous layer is formed on one side (first side) of a substrate. Instead, another porous layer may be formed on the other side (second side) of the substrate by reversing the direction of the current in the anodization step after the porous layer is formed on the first side. The subsequent steps may be repeated for the sides so that twice the number of crystal solar batteries are produced at the same time. 
     In the present invention, the supporting substrate may be a graphite or carbon substrate instead of the above metallurgical-grade silicon substrate. 
     According to the present invention, a SOI wafer can also be produced by transferring a porous layer on an insulating substrate of, for example, mulite (3Al 2 O 3-2 SiO 2 ) instead of the metallurgical-grade silicon. 
     According to the present invention, the method for transferring the porous layer provides a semiconductor layer having satisfactory characteristics. By employing this method in the production of a semiconductor device or a solar battery, a high-quality epitaxial layer can be formed on an inexpensive substrate such as a metallurgical-grade silicon substrate. Moreover, by recycling the silicon substrate for the formation of the porous layer, the semiconductor devices and thin-film crystalline solar batteries having high performance can be produced at low costs. 
     While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.