Free electrons and defects in microcrystalline silicon studied by electron spin resonance

Abstract

Abstract Microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition has been investigated by electron spin resonance (ESR) in the temperature range 10–300 K. In both undoped and residual-gas-doped material, two resonances are observed: a resonance at g = 2.0052 attributed to dangling bonds and a resonance at g= 1.9983 attributed to free conduction electrons. This result has important implications on the band structure of this composite material. It indicates that in some regions of the crystalline phase of the material the Fermi level is close to the conduction band, whereas at the grain boundaries or in the remaining amorphous phase the Fermi level is not high enough to fill dangling-bond states to give the negatively charged D− configuration. At temperatures below T=60 K, we observe a light-induced ESR signal with a considerable enhancement of the free-electron resonance. The decay of the light-induced ESR signal has a fast and a very slow component with recombination times of more than 1 h. The results are related to electrical transport measurements on microcrystalline silicon, where a characteristic weakly activated transport behaviour can be observed at low temperatures, indicating the presence of free carriers and percolation paths.