Deep-level optical spectroscopy in GaAs

Abstract

An experimental method which we call deep-level optical spectroscopy (DLOS) is described. It is based on photostimulated capacitance transients measurements after electrical, thermal, or optical excitation of the sample, i.e., a diode. This technique provides the spectral distribution of both ${\sigma }_n^0\mathrm{}\left(\mathrm{hv}\right)\mathrm{}$ and ${\sigma }_p^0\mathrm{}\left(\mathrm{hv}\right)\mathrm{}$, the optical cross sections for the transitions between a deep-level and the conduction and valence bands. Besides its sensitivity, DLOS is selective in the double sense that ${\sigma }_n^0\mathrm{}\left(\mathrm{hv}\right)\mathrm{}$ and ${\sigma }_p^0\mathrm{}\left(\mathrm{hv}\right)\mathrm{}$ are unambiguously separated, and that the signals due to different traps can be resolved from one another. As a result, the ${\sigma }^0\mathrm{}\left(\mathrm{hv}\right)\mathrm{}$ spectra are measured from their threshold up to the energy gap of the semiconductor, over a generally large temperature range. In addition, the straightforward coupling of DLOS with deep-level transient spectroscopy allows a clear identification of the optical spectra with known levels and the simultaneous determination of both thermal and optical properties for each defect. This experimental method has been used to analyze the most commonly observed deep levels in GaAs. For the well known "O" level, a comprehensive analysis of the results obtained through other techniques (as reported in the literature) is given, to compare with our DLOS data. The spectral shape of all ${\sigma }_n^0\mathrm{}\left(\mathrm{hv}\right)\mathrm{}$ curves appears to be strongly related to the density-of-states distribution in the conduction band, i.e., transitions towards $\Gamma$, $L$, and $X$ minima of this band are generally well resolved; this is a unique feature of DLOS. A simple theoretical model is proposed to take advantage of these newly available experimental data and to explain the sharpness of the ${\sigma }_p^0\mathrm{}\left(\mathrm{hv}\right)\mathrm{}$ curves, as compared with, e.g., Lucovsky's model. Phonon coupling is taken into account. A good fit of the DLOS results is obtained with a small number of adjustable parameters: the deep-level envelope wave function extent (in the $\delta$-potential approximation), the relative transition probabilities to the various conduction-band minima, and the Franck-Condon parameter. The values thus obtained for these physical parameters are discussed, and finally, all results concerning each trap are summarized on a configuration coordinate diagram.