Analysis of photothermal ionization spectra of shallow impurities in silicon

Abstract

A general model has been developed for calculating the photoconductive response in a multiply doped semiconductor. With consideration of the effects of sample temperature and thickness, as well as impurity parameters such as concentration and the frequency-dependent photoabsorption cross section and thermal ionization probability as obtained from existing experimental data, rate equations have been developed for the excess photogenerated carrier density Δp(ω) per unit angular frequency ω of incident light. The model is formulated to include illumination by both band-edge light and the modulated far-infrared light used in Fourier-transform spectrometry. Our calculations so far assume no band-edge light. The model has been applied to doubly doped Si(In,Al) and Si(Ga,B), with the effects of compensation taken into account. The temperature and concentration dependence of the discrete lines of the deeper acceptor, which are superimposed on the continuum background of the shallower acceptor, agree well with experimental spectra, indicating that the model holds promise for making photothermal ionization spectroscopy a more quantitative characterization technique.