Properties of amorphous germanium-tellurium alloys

Evaporated amorphous GeₓTe₁_ₓ allov films have been produced across a compositional range x = 0.065 to x = 1.000 on a variety of substrates. These films have been studied using far-infrared and Raman spectroscopy, conductivity and thermopower measurements and electron spin resonance spectroscopy.

The vibrational spectra contain features which are explained by the presence of GeTe₄ tetrahedra over a wide compositional range. The compositional dependance of

these features is used to estimate the degree of chemical order in the alloy network. We find that the data are very closely predicted by a model consisting of four-fold

coordinated germanium and two-fold coordinated tellurium in a random network with a limited degree of local chemical order.

In germanium-rich alloys variable range hopping conduction dominates at low temperatures, with activation to the conduction band at, and above, room temperature.

The thermopower is negative and of the magnitude expected for activated conduction. As the tellurium concentration increases variable range hopping becomes less significant and n-type activated conductivity begins to dominate. The thermoporver changes sign at a tellurium concentration of 40 at.% and there is a shift to thermally activated p-type conductivity.

With the addition of tellurium to a-Ge the density of the dangling bond defect spin-signal drops rapidly and the g-value shifts significantly. This behaviour is analysed in terms of a model in which a proportion of the tellurium atoms enter the network as three-fold coordinated donor states and the Fermi level shifts into the conduction band tail. Normal two-fold coordinated tellurium also appears to relax the germanium network and significantly reduce the density of dangling bonds.