Nature of Localized States in Amorphous Semiconductors—A Study by Electron Spin Resonance

Abstract

The paramagnetic behavior of localized states in amorphous chalcogenide alloys and the tetrahedrally coordinated amorphous germanium has been studied by ESR experiments. No signal has been detected in any of the chalcogenide glasses, prepared by quenching from the melt, with a sensitivity of about ${10}^{11}$ spins/G near $g=2$. This enables us to estimate ${10}^{14}$ spins/${\mathrm{cm}}^{3}$ as an upper limit to the density of unpaired spins. In some samples a small signal is detectable whose magnitude is \ensuremath{\approx} ${10}^{15}$ spins/${\mathrm{cm}}^{3}$ and depends sensitively on heat treatment. In contrast, and in agreement with Brodsky and Title, thin films of evaporated amorphous germanium ($a$-Ge) are found to yield a large ESR signal at $g=2.022\ifmmode\pm\else\textpm\fi{}0.001$, corresponding to a spin density of approximately ${10}^{20}$ spins/${\mathrm{cm}}^{3}$. In $a$-Ge, the temperature dependence of the linewidth indicates that two relaxation mechanisms are operative. For temperatures below $T\ensuremath{\approx}200$ K, the width is temperature independent, probably due to spin-spin relaxation. One obtains a spin-spin relaxation time ${T}_{2}\ensuremath{\approx}{10}^{\ensuremath{-}9}$ sec. Above 200 K the linewidth is proportional to $\frac{1}{T}$ in accordance with a direct relaxation process. It is suggested that this direct relaxation process is associated with the high density of low-energy excitations which may be responsible for large linear specific heat in amorphous materials at low temperatures observed by Zeller and Pohl.