Photoconductivity Associated with Indium Acceptors in Silicon

Experiments are described in which excess free holes are generated in p-type silicon by the photo-ionization of neutral indium acceptors (Eₐ=0.155eV), and of the subsequent hole capture by ionized indium centers. Using chopped monochromatic radiation of small intensity for illumination, a comparison has been made of dc dark conductivity and of chopped photoconductive responsivity in order to determine the excess hole lifetime and the ionized-acceptor capture cross section as a function of temperature. For samples with an indium concentration in the range 5×{}10¹⁵ to 5×{}10¹⁷ cm^-3, the hole-capture cross section decreases by a factor of 100 in the temperature range 70-180^∘{}K. This cross section is {~}2×{}10^-13 cm² between 70 and 120^∘{}K, a range in which the sticking probability is appreciable for capture into the first excited states of the acceptor. A cross section of {~}2×{}10^-15 cm² at 180^∘{}K is found in agreement with predictions that capture at this temperature can occur only into the ground state of the acceptor. The spectral dependence of photoconductive response in "optically thin" samples is compared with the quantum efficiency expected from optical-absorption measurements and from theoretical models of indium acceptors; it is found that the photoconductive efficiency is anomalously small within the range 0.155<hν<0.24 eV at the lower temperatures.