Electrical and optical properties of hydrogenated amorphous silicon-germanium (<i>a</i>-Si1−<i>x</i>Ge<i>x</i>:H) films prepared by reactive ion beam sputtering

Abstract

Thin films of hydrogenated amorphous silicon-germanium (a-Si1−xGex:H) alloys have been prepared by reactive ion beam sputtering of a composite target of silicon and germanium. The dependence of the deposition rate, conductivity-temperature variation, optical absorption coefficient, refractive index, imaginary part of the dielectric constant, hydrogen content, and infrared (IR) absorption spectra on germanium content (x) are reported and analyzed. For a typical composition—a-Si28Ge72:H (x=0.72), the effect of beam voltage, H2:Ar flow ratio, and substrate temperature on the material properties have also been investigated. For the films prepared with increasing x, the expected behavior of a decrease in both hydrogen content and band gap and an increase in the electrical conductivity have been observed. The films prepared at x&amp;gt;0.80 are found to be more homogeneous than the films deposited at 0.0&amp;lt;x&amp;lt;0.80, due to the compositional disorder introduced by the random mixing of Si and Ge atoms in the a-Si1−xGex:H network in the latter case. The a-Si28Ge72:H films exhibiting minimum conductivity (1.7×10−7 Ω−1 cm−1) have been obtained for an H2:Ar flow ratio of 10:1 and a beam voltage and substrate temperature of 1500 V and 300 °C, respectively. These films contain a hydrogen concentration of 10.2 at. % and show an optical band gap of 1.25 eV. The IR studies have shown that a-Si28Ge72:H films prepared both at low beam voltages and at low substrate temperatures show the unusual preferential attachment of hydrogen to Ge rather than to Si.