Structural and optical properties of porous silicon prepared from a $p^+$-epitaxial layer on $n$-Si(111)

The structural and optical properties of porous silicon prepared by anodic etching of an $n$-Si(111) wafer with a $p^+$-homoepitaxial layer on one side are studied by scanning electron microscopy and multiple-crystal X-ray diffraction. A considerable difference between the microstructures on the sides of the wafer is found. Upon aging for 4.5 months, diffraction peaks of the por-Si structures shift from that of the substrate by $\delta\theta$ = -42" for the $n$-Si porous layer and -450" for the $p^+$-Si porous layer. The photoluminescence band associated with the $p^+$-layer is twice as narrow as the band associated with the n-layer and is shifted toward shorter wavelengths (higher energies) by 0.4 eV, with the intensities of the bands being the same.