Study of the Density of Bulk Gap States in Undoped a-Si:H by Depletion Width Modulation Electron Spin Resonance

Abstract

ABSTRACTThe density of gap states in the bulk of intrinsic a-SiH has been probed using depletion width modulation electron spin resonance spectroscopy (DWM ESR) as well as complementary capacitance techniques. Our sample consists of a 2.85 micron thick layer of undoped a-Si:H sandwiched between a large area Pd Schottky barrier contact and a p+ c-Si substrate, allowing charge-counting capacitance and spin-counting ESR measurements to be performed on the same sample. DWM ESR measures the spin change that occurs when electrons are removed from gap states by increasing reverse bias on a Schottky barrier. The phase of the signal indicates whether unpaired spins are created or destroyed with increasing bias, thus revealing the defect charge state at the Fermi level The DWM ESR signal from our sample indicates that removing electrons from gap states leads to a reduction of the spin signal. This is qualitatively consistent with a band of Do states near EFo. However, for all metastable states and at all temperatures studied, the magnitude of the DWM ESR signal account for only about 14% of the responding charge as determined by junction capacitance. This would occur only if the majority of the filled bulk gap states were doubly occupied with a small correlation energy Ueff. In that case the majority of transitions caused by depletion width modulation would be D−/D+ which would produce no spin change. Model calculations show that the small observed Do/D+ DWM ESR signal would indeed result from the occupation statistics for a near zero value of Ueff at the moderate temperatures of the experiment. Comparison with our data sets the overall limits If −20meV < Ueff < 50 meV. Implication for recent models of the density of states in intrinsic a-Si:H are discussed.