Electron paramagnetic resonance experiments applied to Sl-GaAs

In this paper we present electron paramagnetic resonance experiments applied to liquid encapsulated Czochralski grown semiinsulating GaAs. We show that deep paramagnetic acceptors, FR1, GR2 and FR4, with concentrations of around 10¹⁵cm^-3 exist in this material. We show further, that these acceptors, despite their low concentrations, are relevant for the compensation of present-day-material. The chemical natures of these acceptors are not clear. It was, however, possible to locate their energetic position from photoresponse curves. The kinetics of the photoexcited holes are shown to be a natural consequence of the known optical and thermal properties of the EL2 midgap level.