FREE-ELECTRONS AND DEFECTS IN MICROCRYSTALLINE SILICON STUDIED BY ELECTRON-SPIN-RESONANCE

Microcrystalline silicon prepared by plasma-enhanced chemical vapour d eposition has been investigated by electron spin resonance (ESR) in th e temperature range 10-300 K. In both undoped and residual-gas-doped m aterial, two resonances are observed: a resonance at g=2.0052 attribut ed to dangling bonds and a resonance at g=1.9983 attributed to free co nduction electrons. This result has important implications on the band structure of this composite material. It indicates that in some regio ns of the crystalline phase of the material the Fermi level is close t o the conduction band, whereas at the grain boundaries or in the remai ning amorphous phase the Fermi level is not high enough to fill dangli ng-bond states to give the negatively charged D- configuration. At tem peratures 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 wit h recombination times of more than 1 h. The results are related to ele ctrical transport measurements on microcrystalline silicon, where a ch aracteristic weakly activated transport behaviour can be observed at l ow temperatures, indicating the presence of free carriers and percolat ion paths.