Anomalous electron-spin relaxation in amorphous silicon

Dangling-bond electron-paramagnetic-resonance spectra and relaxation rates have been measured in the (0.3--4)-K temperature range on samples of amorphous silicon produced by sputtering, vacuum evaporation, and ion implantation of silicon, argon, neon, oxygen, and nitrogen into crystalline silicon. Intensity measurements of the dangling-bond resonance associated with silicon made amorphous by Si implantation suggest that the Curie temperature is essentially zero (0{≤}{Θ}{≤}0.03 K) for temperatures down to 0.4 K. The relaxation rates follow unusual temperature dependencies that cannot be explained on the basis of conventional spin-phonon interactions. Instead, the relaxation rates obey a simple Tⁿ power law in temperature where n falls within two ranges: 2.09--2.36 and 3.26--3.47. A comparison of rates at microwave frequencies of 9.3 and 16.5 GHz indicates no magnetic field dependence. A relaxation model involving spin coupling to a distribution of two-level states is consistent with the observed Tⁿ dependence.