Yujin Hori

Capacitance–Voltage Characteristics of Al2O3/AlGaN/GaN Structures and State Density Distribution at Al2O3/AlGaN Interface

The potential modulation and interface states of Al2O3/Al0.25Ga0.75N/GaN structures prepared by atomic layer deposition (ALD) were characterized by capacitance–voltage (C–V) measurements. We observed the peculiar C–V characteristics with two capacitance steps in forward and reverse bias regions, corresponding to the electron accumulation or depletion behavior at the Al2O3/AlGaN and AlGaN/GaN interfaces. From the experimental and calculated C–V characteristics, it was found that the charging and discharging of interface states near the AlGaN conduction-band edge mainly caused the stretch-out and hysteresis of the C–V curve at the forward bias. On the other hand, it is likely that the interface states near the midgap or deeper in energies act as fixed charges. From the bias-dependent hysteresis voltage in the forward bias region and the photo-induced voltage shift at the reverse bias, we estimated the interface state density distribution at the Al2O3/AlGaN interface for the first time. The present ALD-Al2O3/AlGaN/GaN structure showed relatively high interface state densities with a minimum density of 1×1012 cm-2 eV-1 or higher.

Characterization of interface states in Al2O3/AlGaN/GaN structures for improved performance of high-electron-mobility transistors

We have investigated the relationship between improved electrical properties of Al2O3/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) and electronic state densities at the Al2O3/AlGaN interface evaluated from the same structures as the MOS-HEMTs. To evaluate Al2O3/AlGaN interface state densities of the MOS-HEMTs, two types of capacitance-voltage (C-V) measurement techniques were employed: the photo-assisted C-V measurement for the near-midgap states and the frequency dependent C-V characteristics for the states near the conduction-band edge. To reduce the interface states, an N2O-radical treatment was applied to the AlGaN surface just prior to the deposition of the Al2O3 insulator. As compared to the sample without the treatment, the N2O-radical treated Al2O3/AlGaN/GaN structure showed smaller frequency dispersion of the C-V curves in the positive gate bias range. The state densities at the Al2O3/AlGaN interface were estimated to be 1 × 1012 cm−2 eV−1 or less around the ...

Trap states in AlGaN/GaN metal-oxide-semiconductor structures with Al2O3 prepared by atomic layer deposition

Atomic layer deposition (ALD) of Al2O3 was used to prepare metal-oxide-semiconductor (MOS) devices on two different_AlGaN/GaN heterostructures, with and without a thin GaN cap layer. Their trapping effects were evaluated by the frequency dependent conductance measurement. The trap state density decreased sharply from ∼1×1012 cm−2 eV−1 at the energy of 0.27 eV to ∼3×1010 cm−2 eV−1 at 0.45 eV. The low trap state density and exactly exponential dependence of the trap state time constant on the gate voltage show a good quality of the gate oxide. The trap state density in the structure with a GaN cap is about 2–3 times lower than that in the structure without a cap, which might be due to the different Al2O3/GaN and Al2O3/AlGaN interface properties. The trap state density in the structures investigated is lower than those reported for the devices with the metal-organic chemical vapor deposition and Al-oxidized Al2O3 gate oxide. This shows an importance of the ALD technique for the preparation of high-performanc...

Characterization of electronic states at insulator/(Al)GaN interfaces for improved insulated gate and surface passivation structures of GaN-based transistors

This paper presents a systematic characterization of electronic states at insulators/(Al)GaN interfaces, particularly focusing on insulator/AlGaN/GaN structures. First, we review important results reported for GaN metal–insulator–semiconductor (MIS) structures. SiO2 is an attractive material for MIS transistor applications due to its large bandgap and high chemical stability. In-situ SiNx is effective for improving the operation stability of high electron mobility transistors (HEMTs). Meanwhile, Al2O3/GaN structures have high band offsets and low interface state densities, which are also desirable for insulated gate applications. We have proposed a calculation method for describing capacitance–voltage (C–V) characteristics of HEMT MIS structures for evaluating electronic state properties at the insulator/AlGaN interfaces. To evaluate near-midgap states at insulator/AlGaN interfaces, a photo-assisted C–V technique using photon energies less than the bandgap of GaN has been developed. Using the calculation in conjunction with the photo-assisted C–V technique, we estimate interface state density distributions at the Al2O3/AlGaN interfaces.

Formation of Recessed-Oxide Gate for Normally-Off AlGaN/GaN High Electron Mobility Transistors Using Selective Electrochemical Oxidation

A selective electrochemical oxidation has been applied to the AlGaN surface to fabricate a recessed-oxide-gate structure for normally-off AlGaN/GaN high-electron-mobility transistors (HEMTs). We observed bias-dependent oxidation current characteristics peculiar to the AlGaN/GaN heterostructure. A flat interface between the oxide and AlGaN was confirmed by cross-sectional transmission electron microscopy. The selective formation of the recessed oxide allowed the local depletion of two-dimensional electron gas at the AlGaN/GaN interface and thus the achievement of normally-off operation. The recessed-oxide-gate HEMT with the oxide thickness of 20 nm showed good gate control of drain current with the threshold voltage of +1.2 V.

Effects of Cl2-Based Inductively Coupled Plasma Etching of AlGaN on Interface Properties of Al2O3/AlGaN/GaN Heterostructures

The effects of the inductively coupled plasma (ICP) etching of AlGaN on the interface properties of the Al2O3/AlGaN/GaN structures prepared by atomic layer deposition were investigated. It was found from the photoassisted capacitance–voltage (C–V) results that the ICP etching of the AlGaN surface significantly increased the interface state density up to 8×1012 cm-2 eV-1 at the Al2O3/AlGaN interface. The transmission electron microscopy and X-ray photoelectron spectroscopy analyses indicated that the monolayer-level roughness, disorder of the chemical bonds at the AlGaN surface caused poor C–V characteristics due to high-density interface states at the Al2O3/ICP-etched AGaN interface.

Interface Properties of Al2O3/n-GaN Structures with Inductively Coupled Plasma Etching of GaN Surfaces

The effects of the Cl2-based inductively coupled plasma (ICP) etching of GaN on the interface properties of Al2O3/GaN structures prepared by atomic layer deposition (ALD) were investigated. We used n-GaN layers grown on freestanding n+-GaN substrates with low dislocation density. The ICP etching caused slight disorder of the chemical bonds at the GaN surface and monolayer-level interface roughness at the Al2O3/GaN interface, resulting in poor capacitance–voltage (C–V) characteristics due to high-density interface states including nitrogen-vacancy (VN) related levels. The postannealing process in N2 at 400 °C drastically improved the C–V characteristics, probably owing to the partial recovery of the VN-related defects and the increased ordering of chemical bonds in the GaN surface region.