A DIRECT OBSERVATION OF THE DX CENTER IN GAAS USING OPTOELECTRONIC MODULATION SPECTROSCOPY ABOVE THE BANDEDGE

Optoelectronic modulation spectroscopy has been used to examine sulfur-doped VPE and tin-doped MBE GaAs using photon energies in excess of the fundamental energy gap. A peak in the modulated electrical admittance at 1.56 eV is attributed to the DX level and locates the level 1.56 eV above the valence band maximum, resonant with the conduction band. Transitions to it from the valence band seem to be indirect, and the level appears to have an optical cross section which increases with photon energy. This is the first direct measurement of its energy. The method avoids the necessity for high doping and the use of hydrostatic pressure to move the level into the energy gap.