Recombination ina-Si: H: Spin-dependent effects

Spin-dependent recombination in $a$-Si: H is studied using optically detected magnetic resonance (ODMR). Measurements are reported of the dependence of ODMR on temperature, illumination intensity, luminescence energy, microwave power, and defect density. Time-resolved ODMR using both transient illumination and transient microwave pulses are shown to be particularly informative. Samples with high-defect density exhibit a quenching spin-dependent effect which is identified with nonradiative recombination between unthermalized spins. The recombination is attributed to tunneling of band-tail electrons to dangling bonds. Low-defect-density samples have an additional quenching nonradiative mechanism, tentatively identified with Auger recombination, and a radiative enhancing process which we attribute to band-tail luminescence. The results support previous luminescence studies. We discuss the effects of microwave power, spin relaxation, and exchange on the ODMR results. The spin-lattice relaxation time $T_1$ and its temperature dependence are obtained from time-resolved ODMR and from ESR saturation measurements. Relaxation mechanisms are identified from the data.