Polarization Effects on Heterojunction Band Offset Measurements of Oxide-GaN Interfaces

XPS has been used to determine the band offsets of both polar oxide-polar nitride and non-polar oxide-polar nitride heterojunctions for both N-face and Ga-face GaN. In the case of polar oxide-polar nitride heterojunctions (specifically, ZnO-GaN), it is expected that the spontaneous polarizations will align across the interface. ZnO epitaxial layers were prepared by MBE on unintentially doped GaN films grown on sapphire substrates. The valence band heterojunction band offset was deduced from the difference in the core level positions where the valence band of each material was determined from prior XPS and UPS measurements of clean GaN layers and bulk ZnO. We found the ZnO valence band to be positioned below the GaN valence band at 1.85 and 1.05 for the Ga-face and N-face heterojunctions, respectively. Assuming Eg = 3.4 for GaN and Eg = 3.3 eV for ZnO, the Ga- and N-face conduction band offset are 1.95 and 1.15 eV, respectively. The interface dipole for each configuration is deduced based on the deviation from the electron affinity model, and the band offsets are analyzed in terms of the interface bonding and polarity. It is found that polarization effects play an significant role in the band alignment and interface electronic structure.