Compositional dependence of the electronic properties of amorphous Te-Tl

The behavior associated with chemical ordering in thin amorphous films of ${\mathrm{Te}}_x{\mathrm{Tl}}_{1-x}$ deposited and measured on 77-K substrates has been studied by transport, optical, Auger, and other experiments. It was determined from the high-temperature conductivity and the transmission in the near infrared, that the principal bonding in ${\mathrm{Te}}_m{\mathrm{Tl}}_2$ is independent of composition over the range studied ($1.8<m<\mathrm{}3.8\mathrm{}$). The dc conductivity at integral $m$ has typical semiconducting values (∼ ${10}^{-8\mathrm{}}$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ at 77 K) but at half-integral $m$ the samples are as many as seven orders of magnitude more conductive. This strong compositional dependence is interpreted to be due to a peak in the density of electronic gap states near the Fermi level that occurs in films of half-integral $m$. These states may be annealed from oxygen-free films but are locked into samples containing a few atomic percent of oxygen.