Interface characterization methods and properties of GaN metal-insulator-semiconductor structures

Abstract

This work presents a detailed study on the characterization methods of the interface properties of GaN-insulator structures for several dielectric materials. It is an pre-study for the purpose of fabricating inversion metal insulating semiconductor field-effective transistors (MISFETs) based on GaN, which are believe to have higher breakdown and current capability than those of other materials, due to the wide band gap related properties of GaN.<br />The focus is divided into a theoretical discussion about the characterization methods, the fabrication, the measurement and the analysis of MIS structures based on the interface properties and the field-effective mobility measured by Hall measurement and extract from Schottky transistors. As characterization methods, capacitive measurement with over band gap UV light has been used to study the interface charge induced flatband voltage shift and was compared to conductive AC measurement for extraction of the interface state density.<br />The investigation of p-type and n-type substrates allowed a characterization over the whole band gap. Due to the lack of inversion ability in the samples the field effective mobility was extracted from accumulation in enhancement device structures. As dielectric materials silicon nitride and silicon oxide deposited by plasma-enhanced chemical vapor deposition (PECVD) where compared to aluminum oxide from atomic layer deposition (ALD) under certain annealing conditions. Amorphous aluminum oxide has several advantages as a high dielectric constant, a high energy barrier and a high breakdown field, but shows a lower thermal stability only up to around 800°C.<br />The capacitive analysis was based on virtual-ground MIS devices in order to remove any distortion effects by not perfectly rectified ohmic contacts, which was necessary on p-type GaN. It resulted into symmetric interface state distribution over the band gap with a maximum at the band edges. Aluminum oxide showed the best performance with an interface state density below 2x1011 cm-2eV-1 in the middle of the band gap and below 1012 cm-2eV-1 at the band edge after thermal treatment in nitrogen ambient. Silicon oxide interfaces showed similar behavior but less effect on annealing. For p-type substrates high interface density were observed depending on the epitaxially grown substrate. The interface characteristics were generally less dependent on the dielectric or surface treatment due to the defects from the grown bulk material which could be shown by comparison to photoluminescence measurement. Inversion carriers were not observed using Schottky barrier MISFET devices. Further, capacitive study on p-type GaN, which is up to now rarely presented, is discussed with focus on the high series resistance due to the low mobility of holes in GaN. An accurate result was measured using silicon oxide with a minimum interface state density of 4x1011 cm-2eV-1 around the band edge.<br /> The Gated Hall mobility was extracted from the carriers in accumulation and separated from the bulk carriers. The Hall measurement resulted in a field-effective mobility of 150 cm2/Vs for silicon nitride in a bulk material with mobility around 300 cm2/Vs. Similar configuration was measured for transistors with Gate lengths of 4 to 30 mym. The channel length dependent resistance was extracted and led to a maximum field-effective conductivity mobility for silicon oxide of 50 cm2/Vs, while silicon nitride and aluminum oxide showed mobilities around 15 cm2/Vs.<br />