Far-infrared photothermal ionization spectroscopy of semiconductors in the presence of intrinsic light

Abstract

The equilibrium distribution of electrons and holes over shallow impurity states and energy bands of an ultrapure semiconductor is studied for the situation where the semiconductor is continuously illuminated with intrinsic light (i.e., radiation with energies of the order of the gap energy of the semiconductor). The response to additional injection of free minority or majority charge carriers into the energy bands—caused by photothermal ionization of minority or majority impurities, respectively—is separately investigated. The equilibrium and the response have theoretically been analyzed by means of a description with a set of rate of change equations. This analysis explains the usually observed behavior that photothermal ionization of minority impurities in ultrapure germanium under continuous illumination with intrinsic light gives rise to a decrease in electrical conductivity. The measured time evolution of the change in conductivity of an ultrapure germanium sample after the start of the photothermal process revealed a slow (∼5 ms) change, connected with minority impurities only, as well as a fast (<0.5 ms) change. The slow response time has been associated with the electron-hole recombination time, yielding a value 5×10−12 cm3 s−1 for the electron-hole recombination constant. It is demonstrated that in photothermal ionization spectroscopy, when using phase-sensitive detection techniques by means of a lock-in amplifier, such a simultaneous presence of a fast and a slow (i.e., of the order of magnitude of the chopping times applied) change in conductivity can cause artefacts in the spectra.