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A semiconductor substrate includes a porous semiconductor having: a porous layer, with an impurity concentration on varying in the depth direction, or having a porous semiconductor containing an impurity with a content of 1×10 18 cm −3 or more, or provided by pore formation in an epita

A semiconductor substrate includes a porous semiconductor having: a porous layer, with an impurity concentration on varying in the depth direction, or having a porous semiconductor containing an impurity with a content of 1×10 18 cm −3 or more, or provided by pore formation in an epitaxial growth layer. A method of making a semiconductor substrate; includes forming a variant impurity layer with an impurity concentration varying in the depth direction on one surface of a supporting substrate, and converting the variant impurity layer into a porous layer having a variant porosity in the depth direction. A method of making a thin-film semiconductive member; includes forming a semiconductive thin film on the supporting substrate and separating it by cleavage in the porous phase, in addition to the method for making the semiconductor substrate.

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1. A method of making a semiconductor substrate comprising:forming a variant impurity layer with an impurity concentration varying in a depth direction on one surface of a supporting substrate by means other than anodic oxidation, wherein a variant impurity layer including at least two sublayers hav

1. A method of making a semiconductor substrate comprising:forming a variant impurity layer with an impurity concentration varying in a depth direction on one surface of a supporting substrate by means other than anodic oxidation, wherein a variant impurity layer including at least two sublayers having different impurity concentrations is formed in said variant layer forming step; andforming a porous layer by providing pores in the variant impurity layer by anodic oxidation so that a porosity in the porous layer varies in the depth direction, wherein a porous layer including at least two sublayers having different porosities is formed in said porous layer forming step. 2. A method of making a semiconductor substrate according to claim 1, wherein at least two sublayers having different impurity concentrations are formed on one surface of the supporting substrate in said variant layer forming step. 3. A method of making a semiconductor substrate according to claim 1, wherein, in said variant layer forming step, a growth layer is deposited on one surface of the supporting substrate, and then an impurity is diffused into the growth layer so as to form at least two sublayers having different impurity concentrations. 4. A method of making a semiconductor substrate comprising:forming a variant impurity layer with an impurity concentration varying in a depth direction on one surface of a supporting substrate by means other than anodic oxidation, wherein, in said variant layer forming step, a low-impurity sublayer comprising a semiconductor having a low impurity concentration is formed and a high-impurity sublayer comprising a semiconductor having a high impurity concentration is formed on the surface, away from the supporting substrate, of the low-impurity sublayer; andforming a porous layer by providing pores in the variant impurity layer by anodic oxidation so that a porosity in the porous layer varies in the depth direction. 5. A method of making a semiconductor substrate according to claim 4, wherein, in said variant layer forming step, each of said supporting substrate and said variant impurity layer comprises p-type silicon containing a p-type impurity, the low-impurity sublayer has a p-type impurity concentration of 1×10 19 cm −3 or more, and the high-impurity sublayer has a p-type impurity concentration of less than 1×10 19 cm −3 . 6. A method of making a semiconductor substrate comprising:forming a variant impurity layer with an impurity concentration varying in a depth direction on one surface of a supporting substrate by means other than anodic oxidation;forming a porous layer by providing pores in the variant impurity layer by anodic oxidation so that a porosity in the porous layer varies in the depth direction; andforming a semiconductive thin film on the surface, away from the supporting substrate, of the porous layer. 7. A method of making a semiconductor substrate according to claim 6, wherein said semiconductive thin film is formed of a single crystal provided by epitaxial growth. 8. A method of making a semiconductor substrate according to claim 6, wherein said semiconductive thin film comprises a semiconductor selected from the group consisting of a semiconductor containing at least one of silicon and germanium, a semiconductor containing gallium and arsenic, a semiconductor containing gallium and phosphorus, and a semiconductor containing gallium and nitrogen. 9. A method of making a semiconductor substrate comprising:forming a variant impurity layer with an impurity concentration varying in a depth direction on one surface of a supporting substrate by means other than anodic oxidation;forming a porous layer by providing pores in the variant impurity layer by anodic oxidation so that a porosity in the porous layer varies in the depth direction; andheating the porous layer for recrystallization. 10. A method of making a semiconductor substrate comprising:forming a high-impurity layer com prising a semiconductor having an impurity concentration of 1×10 18 cm −3 or more on one surface of a supporting substrate by means other than anodic oxidation, wherein in said high-impurity layer forming step, the high-impurity layer is formed by epitaxial growth; andforming pores in the high-impurity layer by anodic oxidation to form a porous layer having different porosities in a depth direction. 11. A method of making a thin-film semiconductive member comprising:forming a variant impurity layer with an impurity concentration varying in a depth direction on one surface of a supporting substrate by means other than anodic oxidation;forming a porous layer by providing pores in the variant impurity layer by anodic oxidation so that a porosity in the porous layer varies in a depth direction;forming a semiconductive thin film on the surface, away from the supporting substrate, of the porous layer; andseparating the semiconductive thin film from the supporting substrate by cleavage in a porous phase. 12. A method of making a thin-film semiconductive member comprising:forming a high-impurity layer comprising a semiconductor having an impurity concentration of 1×10 18 cm −3 or more on one surface of a supporting substrate by means other than anodic oxidation;forming pores in the high-impurity layer by anodic oxidation to form a porous layer having different porosities in a depth direction;forming a semiconductive thin film on the surface, away from the supporting substrate, of the porous layer; andseparating the semiconductive thin film from the supporting substrate by cleavage in a porous phase.