Optical Studies of Amorphous and Crystalline Magnesium-Bismuth Alloys

Abstract

Thin amorphous Mg-Bi alloy films were prepared by simultaneous deposition of the constituents on to substrates held near liquid nitrogen temperature. The deposition rates were monitored and fluctuations in the concentration of each alloy were typically less than 1 atomic percent Bi. The optical properties of these amorphous alloys have been studied in the near infrared and visible spectral regions. Amorphous Mg3Bi2, is found to be a semiconductor with a band gap of 0.152 ± 0.005 eV but with electron states tailing into the gap and a density of states at the Fermi level of order 10 20 eV-1cm-3. Excess Mg or Bi in amorphous Mg3Bi2 is shown to form localized states near the Fermi level which fill in this pseudogap. These optical results cannot be explained by Urbach's rule or by the rigid band model proposed by Ferrier and Herrell. Instead a variable density of states model is proposed with the Fermi level fixed near the centre of the pseudogap. The alloys were crystallised by heating and the optical properties of these crystalline alloys were also studied. The absorption spectrum of crystalline Mg3Bi2 is found to contain free carrier absorption with a free carrier density of 0.01 per atom, indirect interband absorption and exciton absorption. A rigid band semimetal model is proposed for crystalline alloys with compositions near Mg3Bi2. In this model the valence band is found to overlap the conduction band by 0.12 ± 0.06 eV while a second conduction band minimum with associated exciton bands lies 0.15 ± 0.015 eV above the valence band. The Fermi level in crystalline Mg3Bi2 is 0.06 ± 0.03 eV below the top of the valence band while excess Mg or Bi raises this level. The binding energy of the excitons in Mg3Bi2 was estimated as 0.07 ± 0.02 eV but this energy was particularly sensitive to excess Mg or Bi.