Activation of Cr¹⁺ (3d⁵) Level in GaAs:Cr Induced by Hydrostatic Pressure

Abstract

The absorption spectrum of n-type GaAs:Cr, in which the Cr²⁺ EPR signal remains stable under illumination, was measured under hydrostatic pressure up to 10 kbar at liquid nitrogen temperature. The intracentre Cr²⁺ transition (⁵T₂→⁵E) disappears with increasing pressure; simultaneously, a rapid rise in resistivity is observed. These results can be explained by the existence of the Cr¹⁺ level in the conduction band. Under hydrostatic pressure, this level lowers relative to the conduction band minimum and its population increases, i.e. conduction electrons are trapped by Cr²⁺ centres. Above 9 kbar the saturation of the observed effects appears, indicating that at this pressure the Cr¹⁺ level is below the conduction band. This observation locates the Cr¹⁺ level no higher than 100 meV above the conduction band minimum at 77 K, i.e. about 1.6 eV above the valence band maximum.