Subbandgap absorption spectra of slightly doped a-Si:H measured with constant photocurrent method (CPM) and photothermal deflection spectroscopy (PDS)

Abstract

The authors report on systematic measurements of the absorption spectra in the visible near-infrared range performed on a series of slightly phosphorus- and boron-doped amorphous hydrogenated silicon (a-Si:H) samples. Optical transmission spectra, Constant Photocurrent Method (CPM) and Photothermal Deflection Spectroscopy (PDS) have been used to measure the absorption coefficient (α) in differently doped samples. Variation of the gas phase doping levels in the ppm range is shown to lead to relevant differences between CPM and PDS spectra. The latter demonstrate that additional care has to be taken when evaluating deep defect density from CPM when the Fermi level is not constant, and especially so, if the Fermi level is below midgap.