The shallow acceptor states in gallium arsenide have been studied by far-infrared Fourier transform spectroscopy. The energy level structure observed in photoconductivity is compared with the predictions of recent acceptor state theory and the agreement is found to be generally good. The groundstate ionisation energies are found to be 26.9 meV (carbon), 28.7 meV (magnesium), 30.6 meV (zinc) and 34.8 meV (silicon). The general features of the GaAs spectra are also compared with results previously obtained for Ge by Jones and Fisher (1965) for InSb by Kaplan (1973) and deep manganese acceptors in GaAs by Chapman and Hutchinson (1967). There is strong similarity between GaAs and the other materials. This similarity extends to the Zeeman spectra and is exploited in interpreting the Zeeman components observed in GaAs in terms of theoretical predictions.

The high sensitivity of photoconductive techniques is exploited to study the central cell structure on the extrinsic donor spectra of pure n-type epitaxial films of GaAs. Experiments with back-doped samples have enabled the chemical shifts of most of the common donors to be determined with some confidence. The residual contaminants are identified in material from different sources prepared without intentional doping by conventional vapour and liquid phase techniques and by the recently developed 'alkyl' vapour epitaxial process. Each growth process produces a different set of residual contaminants, but samples grown by the same technique in different laboratories have a very similar donor spectrum.

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