Photoemission investigation of the electronic properties of Ga-face GaN (0001)-dielectric interfaces

Abstract

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. Both n- and p-type Ga-face GaN (0001) surfaces have been cleaned via an 860° C anneal in an ammonia atmosphere, and carbon and oxygen contaminants were reduced to below the detection limits. Layers of SiO2, Si3N4, or HfO2 were carefully deposited to limit the reaction between the plasma and the GaN surface. 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 with 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 0.4 ± 0.2 eV with a CBO of 2.0 ± 0.2 eV was determined for the GaN/HfO2 interface. An instability was observed in the HfO2 film, with energy bands shifting ~0.5 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. Electron affinity measurements, along with band alignment data, allow a deviation from the electron affinity model due to a change of the interface dipole to be observed. Interface dipoles of 1.7, 1.1 and 1.9 ° 0.2 eV were observed for the GaN/SiO2, GaN/Si3N4, and GaN/HfO2 interfaces, respectively. The existence of Ga-O bonding at the heterojunction significantly increases the interface dipole, which raises the dielectric bands in relation to the GaN.