Abstract

Deep donor centre, referred to as EL2, is the dominant deep level in melt-grown GaAs crystals. Photoionization of EL2, the subsequent space charge redistribution, and refractive index modulation through the electrooptic effect are responsible for what we call below the photorefractive (PR) gratings observed in GaAs semi-insulating crystals, see e.g. [1]. EL2 centre is also distinguished by a number of specific features including the existence of the excited metastable state EL2^* of the same charge. EL2 ⇒ EL2^* transitions take place tinder light excitation with photon energy hν between 1.0 and 1.3 eV. At low temperatures T≈100 K the corresponding cross section σ^* and lifetime τ^* are sufficiently large (σ^* ≈ 10⁻¹⁷sm², τ^* ≈ 10 s according to [2]) and hence the metastable state EL2 strongly affects the absorption and photocurrent spectrums, some photoelectrical properties of the material, etc. [3,4]. Nevertheless, the probable influence of EL2 ⇒ EL2^* transitions on the properties of GaAs crystals as a reversible holographic medium is still in question.

© 1991 Optical Society of America