Raman study of Zn_(1-x)Be_(x)Se/GaAs systems with low Be content (x≤0.20)

摘要

We present an experimental and theoretical investigation of the Raman line shape of long-wavelength phonons with longitudinal optical (LO) symmetry from both sides of Zn_(1-x)Be_(x)Se/GaAs (001) heterojunctions with special emphasis on samples with low Be content (x≤0.20). First the built-in p-type LO phonon-plasmon (LO-P) coupled mode at the near-interfacial substrate is used as a sensitive probe to investigate the interfacial quality. The corresponding hole gas is reinforced when the ZnBeSe layers are nominally p doped by nitrogen. This provides clear evidence for effective hole transfer across the junction, and thereby indicates a minimized density of interfacial defects. In the nitrogen-doped layers hole densities as high as 10~(17) cm~(-3), in accordance with capacitance-voltage measurements, are directly inferred from clear weakening of the ZnSe-like LO mode due to LO-P coupling. Concerning the intrinsic properties of the alloys, we demonstrate that the asymmetric broadening of the ZnSe-like LO mode is determined by topological disorder only, and not by other possible mechanisms such as structural disorder, nonhomogeneity in the alloy composition, a distribution of tensile strain, or a Fano-type interference. This reveals a high structural quality that parallels the high interfacial quality. We also show that a spatial correlation model with Gaussian distribution applies to Zn_(1-x)Be_(x)Se. Most of this study is supported by a quantitative treatment. We extend the phenomenological approach of Hon and Faust to equations of motion and polarization derived from the modified-random-element-isodisplacement model in order to achieve line shape analysis of the alloy-related LO and LO-P modes.