Transport mechanism in lightly doped hydrogenated microcrystalline silicon thin films

Abstract

Boron-doped microcrystalline silicon films have been deposited in a plasma-enhanced chemical vapor deposition system using silane diluted in hydrogen, and diborane sB2H6d as a dopant gas. The temperature dependence of the dark conductivity has been measured from 120 to 420 K in all samples. In the high-temperature range above room temperature, the carrier transport is found to be thermally activated, with a single activation energy that changes with the B2H6 compensation degree. In the low-temperature range s300–120 Kd, variable range hopping sVRHd was established as a predominant electronic transport mechanism for all samples, with the exception of the sample with a diborane concentration of 12.5 ppm. A model for Mott’s VRH, referred to as the “diffusional model,” which yields a relation between the conductivity and the localized density of gap states, is presented. Using classical equations from the percolation theory and the diffusional model, the density of states near the Fermi level, as well as the hopping parameters, are calculated. A correlation between the hopping parameters for both models is deduced. A numerical factor that improves the value of each parameter is calculated.