Surface photovoltage spectroscopy of an epitaxial ZnO/GaP heterojunction

Abstract

The surface photovoltage generation mechanism is found to be the opposite for the epitaxial ZnO layer side compared to that of the GaP substrate side at the heterojunction due to the presence of an interfacial layer of GaOx. The low temperature (10 K) surface photovoltage spectroscopy spectrum for the ZnO layer with good crystalline quality is dominated by the free-A excitonic feature FXA. On the other hand, it is dominated by the donor bound excitonic feature DoXA for ZnO layers with relatively poorer crystalline qualities. We have also observed a feature in 10 K SPS data that can be related to the first excited state (n = 2) of free-A exciton (FX) and/or free-C exciton (FXC) features. Larger inhomogeneous broadening caused by strain and poorer crystalline quality hinders the observation of separate feature corresponding to free-B excitonic feature FXB. The temperature dependence of energy positions and broadening parameters are very well described by the scattering of an exciton with E2(low) and A1(LO) phonons present in ZnO. It also indicates that there is no dependence of exciton–phonon coupling strength on the crystalline quality of ZnO layers.