Yusuke Kumazaki

Large photocurrents in GaN porous structures with a redshift of the photoabsorption edge

Photoresponse and photoabsorption properties of GaN porous structures were investigated by measuring photocurrent and spectroscopic photoabsorption under monochromatic light with various wavelengths. The measured photocurrents on the porous GaN electrodes were larger than those on the planar electrodes due to the unique features of the former electrode, such as large surface area and low photoreflectance properties. Moreover, the photocurrents were observed even under illumination with wavelength of 380 nm, corresponding to photon energy of 3.26 eV, which is 130 meV lower than the bandgap energy of bulk GaN. A potential simulation revealed that a high-electric field was induced at the pore tips due to modification of the potential in the porous structures. The observed redshift of the photoabsorption edge can be qualitatively explained by the Franz–Keldysh effect.

Formation of GaN porous structures with improved structural controllability by photoassisted electrochemical etching

We aimed to develop a photoassisted electrochemical etching process for the formation of GaN porous structures. Pore linearity and depth controllability were strongly affected by the anode voltage. In addition, the use of light with an energy below the band gap played an important role in controlling the pore diameter. Spectro-electrochemical measurements revealed that the high electric field induced at the GaN/electrolyte interface caused a redshift of the photoabsorption edge. This specific phenomenon can be explained by a theoretical calculation based on the Franz–Keldysh effect. On the basis of the results of our experimental and theoretical analyze, we propose a formation model for GaN porous structures. We also note that the application of the Franz–Keldysh effect is useful in controlling the structural properties of GaN porous structures.

Electrochemical formation of N-type GaN and N-type InP porous structures for chemical sensor applications

The feasibility of liquid-phase sensors based on n-type GaN and n-type InP porous structures was investigated. The response currents to the addition of H2O2 increased on the porous electrodes. The source-drain currents of ion-sensitive field-effect transistors having a porous channel changed with good response to the pH values. The sensitivities of two kinds of chemical sensors to ions were drastically enhanced by the implementation of the porous structures having large surface areas with good conductivity.

Interface control technologies for high-power GaN transistors: Self-stopping etching of p-GaN layers utilizing electrochemical reactions

The selective and low-damaged etching of p-type GaN or AlGaN layer is inevitable process for AlGaN/GaN high-power transistors. We have investigated an electrochemical etching of p-GaN layer grown on AlGaN/GaN heterostructures, consisting of an anodic oxidation of p-GaN surface and a subsequent dissolution of the resulting oxide. The p-GaN layer was electrochemically etched by following the pattern of the SiO2 film that acted as an etching mask. Etching depth was linearly controlled by cycle number of triangular waveform at a rate of 25 nm/cycle. The AFM, TEM and μ-AES results showed that the top p-GaN layer was completely removed after 5 cycles applied, and the etching reaction was automatically sopped on the AlGaN surface. I-V and C-V measurements revealed that no significant damages were induced in the AlGaN/GaN heterostructures.

Precise thickness control in recess etching of AlGaN/GaN hetero-structure using photocarrier-regulated electrochemical process

The photocarrier-regulated electrochemical (PREC) process was developed for fabricating recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) for normally off operation. The PREC process is based on photo-assisted electrochemical etching using low-energy chemical reactions. The fundamental photo-electrochemical measurements on AlGaN/GaN heterostructures revealed that the photo-carriers generated in the top AlGaN layer caused homogeneous etching of AlGaN with a smooth surface, but those generated in the GaN layer underneath caused inhomogeneous etching that roughens the surface. The concept of the PREC process is to supply the photo-carriers generated only in the AlGaN layer by selecting proper conditions on light wavelength and voltage. The phenomenon of self-termination etching has been observed during the PREC process, where the etching depth was controlled by light intensity. The recessed-gate AlGaN/GaN HEMT fabricated with the PREC process showed positive threshold voltage and improvement...