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UBC Theses and Dissertations

Impurity band photoconductivity in Boron-doped silicon Scott, Myrsyl Walter


The effect of impurity concentration on the photoconductive spectrum of boron-doped silicon at helium temperatures was investigated. Photoconductivity was observed for excitations of the bound hole into the impurity excited states. The photoconductivity in this region depends strongly on impurity concentration and was interpreted as being conduction through excited state impurity bands. Two bands were observed to form, with excited states 3 and 4 in the boron spectrum forming one band and state 2 forming the other. A qualitative description of the mobility in the impurity bands was obtained using Baltensperger's theory and the additional assumption that the holes are scattered by "randomness" in the impurity array. Photoconductivity of holes in the valence band was also measured in order to obtain lifetimes and capture cross sections. Assuming the mobility in this region to be determined by neutral impurity scattering, the hole lifetime was estimated to be ~ 10⁻⁹ sec and the capture cross section of ionized boron ~ 3 x 10⁻¹⁰ cm². Assuming similar lifetimes for the holes in impurity bands, the peak mobility in band 3 and 4 was found to be ~ 150 cm² /volt-sec. The d.c. characteristics of the various samples, while at low temperature and exposed to room temperature radiation, were measured during the course of this investigation. All samples were observed to have a non-linear dependence between the current and applied field, terminated by a non-destructive low field breakdown.

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