Photocapacitance study of bulk deep levels in ZnSe grown by molecular-beam epitaxy

The influence of initial growth conditions and lattice matching on the deep level spectrum of $\mathrm{n-}\mathrm{ZnSe}$ grown on GaAs by molecular-beam epitaxy is investigated by means of deep level optical spectroscopy. A detailed study of both the steady-state and transient photocapacitance allows us to measure optical threshold energies, concentrations, and emission rates of electronically active defects in the ZnSe layer. Several deep levels are found in the ZnSe layer at $E_c{\mathrm{-E}}_t\text{=1.15,}$ 1.46, 1.90, and 2.25 eV with concentrations in the ${10}^{12}{\text{–10}}^{14}$ ${\mathrm{cm}}^{\text{−3}}$ range. When a 2-nm-thick composition controlled interface layer is grown at different beam pressure ratios prior to the ZnSe growth, a distinct decrease in the 1.46 eV level concentration with increasing Se content is found. Deposition of a lattice-matched ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{As}$ buffer layer prior to the ZnSe growth reduces the concentration of both the 1.15 and 1.46 eV levels by over an order of magnitude, indicating the role of lattice matching in the ZnSe overlayer. We also perform depth profiling of the defect distributions within the ZnSe overlayer to see the effect of the ZnSe thickness on the concentration of these levels as well as their possible association to the ZnSe/GaAs interface. We find that only the 2.25 eV level concentration shows a dependence on depth, increasing as the II–VI/III–V interface is approached.