Photoinduced Optical Absorption of Cr 4+ in Semi‐Insulating GaAs:Cr

The optical absorption of Cr 4+ created during illumination with hv > 0.75 eV in semi‐insulating GaAs:Cr is investigated at low temperatures (≤ 150 K). Analyzing the concentrations of Cr 2+ , Cr 3+ , and Cr 4+ during illumination and measuring the excitation spectrum of Cr 4+ , the creation mechanism of photoinduced Cr 4+ conclusively proved to be the two step process Cr 3+ + hv → Cr 2+ + hole vb and hole vb + Cr 3+ → Cr 4+ . The concentration of photoinduced Cr 4+ is measured in dependence on the contents of Cr 2+ and Cr 3+ present in the samples before illumination, on temperature, and on illumination intensity. Its time dependence of rise after switching on and of decay after switching off the illumination is measured under various conditions. The results are interpreted using dynamic equations for the concentration of Cr 2+ , Cr 3+ , and Cr 4+ during illumination basing on the assumption that only excitation and capture processes via the valence band contribute to the formation of photoinduced Cr 4+ and that the tunneling process Cr 4+ + Cr 2+ → 2 Cr 3+ is responsible for its decay.