Mechanisms of thermal equilibration in doped amorphous silicon

Abstract

Experimental and theoretical investigations of the mechanisms of thermal equilibration in n-type amorphous silicon are presented. The time, temperature, and doping dependence of the band-tail electron density is obtained from sweep-out experiments, and the dangling-bond and donor densities from photothermal deflection spectroscopy (PDS), bias annealing, and C-V characteristic measurements. An important new result is that donors participate in the equilibration, and that the doping efficiency can be greatly enhanced by a depletion bias. Numerical modeling of the transport allows us to deduce the changes in the density of states and in the position of the Fermi energy, both in equilibrium and in the frozen-in state. The equilibrium state is derived by minimizing the free energy of the doped a-Si:H using a simple density-of-states model. With this approach, the electronic properties (doping efficiency, conductivity, Fermi energy, etc.) can be computed and are shown to be in fairly good agreement with all the experimental results, although the observed lack of temperature dependence of the dangling-bond density remains a puzzle. Further evidence is presented that hydrogen motion is the underlying mechanism of equilibration, and we develop a qualitative model to describe the bonding and movement of hydrogen, based on a distribution of weak Si-Si bonds.