Properties of amorphous germanium-tellurium alloys

<p>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.</p> <p><br></p> <p>The vibrational spectra contain features which are explained by the presence of GeTe₄ tetrahedra over a wide compositional range. The compositional dependance of</p> <p>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</p> <p>coordinated germanium and two-fold coordinated tellurium in a random network with a limited degree of local chemical order.</p> <p><br></p> <p>In germanium-rich alloys variable range hopping conduction dominates at low temperatures, with activation to the conduction band at, and above, room temperature.</p> <p>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.</p> <p><br></p> <p>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.</p>