Low-temperature photoemission measurements of valence-band discontinuities at buried heterojunctions

Synchrotron-radiation photoemission studies of CdTe-GaAs(110) heterojunctions prepared in situ were performed at room temperature and after the interface was cooled to 35 K. At room temperature we observe a valence-band offset Δ${\mathit{E}}_{\mathit{v}}$=0.20±0.07 eV, together with band bending in opposite directions of the substrate and overlayer bands. After cooling to 35 K the synchrotron-radiation-induced saturation photovoltage yields flat-band conditions for the two semiconductors in the whole CdTe-coverage range explored (1–50 Å). A valence-band offset Δ${\mathit{E}}_{\mathit{v}}$=0.21±0.05 eV could then be measured directly from the valence-band spectra. We conclude that photoinjection compensates the long-range component of the electrostatic potential across the junction, while the local interface dipole remains essentially invariant with temperature and photoinjected-carrier density. In these conditions, low-temperature photoemission becomes an ideal tool to study valence-band offsets at buried heterojunctions.