Electronic Properties of GaN (0001) – Dielectric Interfaces

The characteristics of clean n- and p-type GaN (0001) surfaces and the interface between this surface and SiO2, Si3N4, and HfO2 have been investigated. Layers of SiO2, Si3N4, or HfO2 were carefully deposited to limit the reaction between the film and clean GaN surfaces. After stepwise deposition, the electronic states were measured with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). A valence band offset (VBO) of 2.0±0.2 eV with a conduction band offset (CBO) of 3.6±0.2 eV was determined for the GaN/SiO2 interface. The large band offsets suggest SiO2 is an excellent candidate for passivation of GaN. For the GaN/Si3N4, interface, type II band alignment was observed with a VBO of -0.5±0.2 eV and a CBO of 2.4±0.2 eV. While Si3N4 should passivate n-type GaN surfaces, it may not be appropriate for p-type GaN surfaces. A VBO of 0.3±0.2 eV with a CBO of 2.1±0.2 eV was determined for the annealed GaN/HfO2 interface. An instability was observed in the HfO2 film, with energy bands shifting ~0.4 eV during a 650°C densification anneal. The electron affinity measurements via UPS were 3.0, 1.1, 1.8, and 2.9±0.1 eV for GaN, SiO2, Si3N4, and HfO2 surfaces, respectively. The deduced band alignments were compared to the predictions of the electron affinity model and deviations were attributed to a change of the interface dipole. Interface dipoles contributed 1.6, 1.1 and 2.0±0.2 eV to the band alignment of the GaN/SiO2, GaN/Si3N4, and GaN/HfO2 interfaces, respectively. It was noted that the existence of Ga-O bonding at the heterojunction could significantly affect the interface dipole, and consequently the band alignment in relation to the GaN.