PRESSURE-INDUCED TRANSITIONS IN AMORPHOUS SILICON AND GERMANIUM

Abstract
Pressure-induced transitions from semiconductor to metal in a-Si, a-Ge and their alloys have been investigated by measurements of the optical absorption edge, electrical resistivity, superconductivity, X-ray diffraction and Raman scattering as a function of pressure. The transitions, accompanied by discontinuous changes in optical gap and resistivity occur in evaporated a-Si and a-Ge and in silane-decomposed a-Si : H while in reactvely sputtered a-Si : H and a-Ge : O they show a continuous decrease in resistivity. It is shown by X-ray diffraction experiments that these transitions are accompanied by a change of structure which is heterogeneous with a mixture of two metastable phases. The metallic modification of a-Si under pressures of 100 to 150 kbar is reversible to the amorphous state after the compression. New metastable polymorphs of hexagonal Ge and tetragonal Si : H are recovered.