Optical studies of amorphous magnesium-antimony alloys

Abstract

Amorphous Magnesium-Antimony alloys undergo a transition between metallic and semiconducting behaviour as the alloy composition is varied. Measurements of the dc conductivity as a function of temperature and calculations of the electrical activation energy had been previously made at representative compositions. In order to obtain further information about the behaviour of the alloy system, and in particular, knowledge of how the electronic density of states varies with composition, optical studies were carried out on thin film samples of the alloy. Samples were produced by coevaporation of Mg and Sb onto sodium chloride substrates which were held at liquid nitrogen temperature. Measurements of dc conductivity and optical reflectance and transmittance were carried out on the samples while they were still in vacuum. The optical measurements were used to calculate n and k, the real and imaginary parts of the refractive index, as a function of photon energy over the measured range (photon energies between 0.1 and 2.5 eV). Calculation of n and k proved to be more difficult than expected and two methods of calculation (one of which utilized a Kramers-Kronig technique) were required and were used in combination. From n and k, the optical band gap of each sample was determined and these values were compared to the previously measured activation energies. It was found that the optical band gap varied with composition and that for compositions on the Mg side of the intermetallic compound Mg3Sb2, excess Mg caused the band gap to be filled with electron states causing the alloy to become metallic. On the Sb side of Mg3Sb2 Sb - Sb bonding meant that states were introduced into the gap only for compositions near Mg3Sb2, the band gap decreasing gradually as the amount of excess Sb was increased. Some comparisons with the Mg-Bi alloy system were drawn.