u00d7 10 17 cm 3 in the case of the n type si the concentration is preferably 10 13 cm 3 or more and more preferably 10 16 to 10 13 cm 3 the present invention is described below with reference to preferred embodiments an n type si epitaxial layer was grown to a thickness of 1 micron on a p type 100 si substrate having a thickness of 200 microns by the cvd process the conditions for deposition were the following ______________________________________reaction gas flow rate sih sub 2 cl sub 2 1000 sccm h sub 2 230 l min ph sub 3 50 ppm 72 sccmtemperature 1080 u00b0 c pressure 80 torrtime 2 min ______________________________________ the substrate was then subjected to anodic etching in a 50 hf solution during the oxidation the current density was 100 ma cm 2 during this process the rate with which the substrate was made porous was 8 4 u03bcm min and the whole p type si substrate having a thickness of 200 microns was made porous within 24 minutes as described above during the anodic etching only the p type 100 si substrate was made porous and the n type si epitaxial layer was not changed si 3 n 4 was then deposited on the surface of the epitaxial layer to have a thickness of 0 1 u03bcm.