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, Si₃N ₄, and HfO₂ have been investigated. Layers of SiO ₂, 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/SiO ₂ interface. The large band offsets suggest SiO₂ is an excellent candidate for passivation of GaN. For the GaN/Si₃N ₄ 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 Si₃N ₄ 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/Si₃N ₄, 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.