Electron-spin-lattice relaxation in amorphous silicon and germanium

Electron-spin-lattice relaxation times have been measured using adiabatic-passage techniques in doped and undoped $a$-Si: H and $a$-Ge: H. When scaled by the square of the atomic spin-orbit coupling constants, the magnitudes of the relaxation times are found to be surprisingly similar. The temperature dependence of the dangling bonds and band-tail states in all cases shows a nearly quadratic behavior to temperatures far below the Debye temperature. The results are shown to be consistent with relaxation via two-level systems (TLS's). The TLS's make transitions between levels by phonon-assisted tunneling and flip spins by modulating the spin-orbit interaction. Hydrogen evolution studies allow us to vary the TLS distribution and show that the TLS density of states is related to the degree of departure of the network from its thermal-equilibrium value.