Electron spin resonance investigation of electronic states in hydrogenated microcrystalline silicon

Phosphorus-doped microcrystalline silicon with high-crystalline volume fraction was prepared by very high-frequency plasma enhanced chemical vapor deposition. The material is studied by electron spin resonance and transport measurements as a function of doping and temperature. In all samples a resonance at $g=1.998\mathrm{}$ is found with spin densities very similar to the phosphorus dopant density and also the carrier density at high doping levels. This resonance is related to doping-induced excess electrons. Its spin density is largely temperature independent, and the corresponding electrons occupy dopant or conduction band tail states at low temperatures, while they are excited into the conduction band at high T. This gradual transition is accompanied by changes in linewidth, g value and spin-lattice relaxation time. Hyperfine interaction with P nuclei is only observed for intermediate doping levels and has very small intensity. From the value of the hyperfine splitting, the effective Bohr radius of the impurity wave function is estimated to 12 Å. Transport at low temperatures $(T<\mathrm{}20\mathrm{}\mathrm{K})$ proceeds via hopping between donor states and/or conduction-band tail states. A thermal activation energy of 3.5 meV and similar localization lengths as from the hyperfine data are found for this process. At temperatures above 20 K electronic transport is governed by a wide distribution of activation energies.