Evidence for a dominant midgap trap in n-ZnSe grown by molecular beam epitaxy

A midgap deep level in n-type ZnSe grown by molecular beam epitaxy (MBE) on In_0.04Ga_0.96As/GaAs is detected and investigated by deep level optical spectroscopy and cathodoluminescence spectroscopy. The deep level has an optical threshold energy of 1.46 eV below the conduction band edge, and its concentration strongly depends on the Zn:Se beam pressure ratio during initial nucleation of the ZnSe layer. The concentration of this level decreases by a factor of ∼8 for Se rich vs Zn rich nucleation conditions, correlating with a decrease in the Se vacancy concentration for Se-rich nucleation. The investigation of photocapacitance transients revealed a strong interaction of the 1.46 eV level with both the conduction and the valence bands. Moreover, this level showed the largest optical cross section (emission rate of ∼ 10³ s^-1) of all of the levels found in the ZnSe layer. Taken together, these observations suggest this level may be an important recombination-generation center in MBE-grown ZnSe devices on GaAs substrates.