Electronic structures of c -plane and a -plane AlN/ZnO heterointerfaces determined by synchrotron radiation photoemission spectroscopy

c -plane and a -plane AlN films have been grown on single-crystal ZnO substrates by pulsed laser deposition at room temperature. The electronic structures of c -plane and a -plane AlN/ZnO heterojunctions have been characterized by synchrotron radiation photoemission spectroscopy. Based on the binding energies of core-levels and valence-band maximum values, the valence-band offsets have been found to be 0.4±0.1 and 0.1±0.1 eV for the c -plane and a -plane AlN/ZnO heterojunctions, respectively. Both heterojunctions show type-II band configurations with conduction band offsets of 3.0±0.1 and 2.7±0.1 eV, respectively. The potential on the ZnO side bends downward toward the interface for the a -plane AlN/ZnO heterojunction. However, that bends upward toward the interface for the c -plane AlN/ZnO heterojunction. This phenomenon is explained well by the effect of spontaneous polarization in AlN and ZnO.