Chihoko Mizue

Effects of interface states and temperature on the C-V behavior of metal/insulator/AlGaN/GaN heterostructure capacitors

The impact of states at the insulator/AlGaN interface on the capacitance-voltage (C-V) characteristics of a metal/insulator/AlGaN/GaN heterostructure (MISH) capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiNx/AlGaN interface state density Dit(E) was increased. We attribute this behavior to the position of the Fermi level at the SiNx/AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300 °C. We analyzed the experimental C-V characteristics from a SiNx/Al2O3/AlGaN/GaN structure me...

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.

Simulations of Capacitance–Voltage–Temperature Behavior of Metal/Insulator/AlGaN and Metal/Insulator/AlGaN/GaN Structures

The effect of electronic states at insulator/AlGaN and AlGaN/GaN interfaces on capacitance–voltage (C–V) characteristics of a metal/insulator/semiconductor (MIS) capacitor with an AlGaN layer and a metal/insulator/semiconductor heterostructure (MISH) with an AlGaN/GaN heterojunction was studied theoretically taking into account extremely slow electron emission from the deep interface levels. The states at the insulator/AlGaN interface in MIS and at the AlGaN/GaN interface in MISH stretch out the C–V curves as usual, whereas the insulator/AlGaN interface states in MISH only shift the characteristics at room temperature and 300 °C (fixed-charge-like behavior). These effects were explained by the different positions of the Fermi level at the studied interfaces. Theoretical C–V curves were compared with experimental characteristics, which are available in the literature, measured at room and higher temperatures by the authors and others.

Deposition of aluminum oxide layer on GaN using diethyl aluminum ethoxide as a precursor

We performed a feasibility study on the deposition of aluminum oxides (AlOx) on GaN using diethyl aluminum ethoxide [(C2H5)2AlOC2H5] as a precursor by electron-cyclotron-resonance-assisted chemical vapor deposition. We determined the refractive index and permittivity of the deposited AlOx layer to be 1.59 and 8.3, respectively. An x-ray photoelectron spectroscopy (XPS) study showed that the energy positions of the Al- and O-core levels were very close to those of the reference crystalline Al2O3 and that there was no significant peak related to carbon in the AlOx film. XPS also indicated a bandgap of 7.0 eV and oxygen composition of 1.48 for the AlOx layer deposited on GaN. We found good capacitance-voltage (C-V) behavior for the Ni/AlOx/n-GaN diode including accumulation and depletion behavior at room temperature. Even at high temperatures, the C-V slope remained unchanged indicating relatively low interface state densities near the midgap.

Impact of Interface States and Bulk Carrier Lifetime on Photocapacitance of Metal/Insulator/GaN Structure for Ultraviolet Light Detection

The influence of interface state density and bulk carrier lifetime on the dependencies of photocapacitance versus wide range of gate bias (-0.1 to -3 V) and light intensity (109 to 1020 photon cm-2 s-1) was studied for metal/insulator/n-GaN UV light photodetector by means of numerical simulations. The light detection limit and photocapacitance saturation were analyzed in terms of the interface charge and interface Fermi level for electrons and holes and effective interface recombination velocity. It was proven that the excess carrier recombination through interface states is the main reason of photocapacitance signal quenching. It was found that the photodetector can work in various modes (on–off or quantitative light measurement) adjusted by the gate bias. A comparison between experimental data and theoretical capacitance-light intensity characteristics was made. A new method for the determination of the interface state density distribution from capacitance–voltage-light intensity measurements was also proposed.

UV-Induced Variation of Interface Potential in AlOx/n-GaN Structure

We performed a feasibility study of UV detection using an AlOx/n-GaN structure. The AlOx layer was deposited on n-GaN using diethyl aluminum ethoxide as a precursor. We obtained a refractive index of 1.59, a bandgap of 7.0 eV, and an oxygen composition of 1.48 for the AlOx layer. Even at higher temperatures, no significant change appeared in the capacitance–voltage curves, which were close to the ideal curve, indicating well-controlled and stable interface properties. From the photo response measurements, pronounced variation of the interface potential at the AlOx/GaN interface was observed under UV illumination, which was independent of temperature. The gate bias systematically modulated the depth of the interface potential well and thereby the density of accumulated holes resulting from UV illumination. The AlOx/GaN structure is sensitive to flame with a low power density.