Tetsu Kachi

Raman scattering from LO phonon‐plasmon coupled modes in gallium nitride

Raman spectra of n‐type gallium nitride with different carrier concentrations have been measured. The LO phonon band shifted towards the high‐frequency side and broadened with an increase in carrier concentration. Results showed that the LO phonon was coupled to the overdamped plasmon in gallium nitride. The carrier concentrations and damping constants were determined by line‐shape fitting of the coupled modes and compared to values obtained from Hall measurements. The carrier concentrations obtained from the two methods agree well. As a result, the dominant scattering mechanisms in gallium nitride are deformation‐potential and electro‐optic mechanisms.

Thermal stress in GaN epitaxial layers grown on sapphire substrates

Thermal stress in GaN epitaxial layers with different thicknesses grown on sapphire substrates by metalorganic vapor phase epitaxy using an AlN buffer layer was investigated. Biaxial compressive stress in the GaN layer, due to the difference in the thermal expansion coefficients between GaN and sapphire, was obtained by measuring the curvature of wafer bending, and the observed stress agreed with the calculated stress. In Raman measurements, the E2 phonon peak of GaN was found to shift and broaden with the stress as a consequence of the change of the elastic constants with strain. The frequency shift Δω (in cm−1) was obtained for the first time, given by the relation: Δω=6.2 σ, where the biaxial stress σ is expressed in GPa.

GaN-Based Trench Gate Metal Oxide Semiconductor Field-Effect Transistor Fabricated with Novel Wet Etching

A novel method for fabricating trench structures on GaN was developed. A smooth non-polar (1100) plane was obtained by wet etching using tetramethylammonium hydroxide (TMAH) as the etchant. A U-shape trench with the (1100) plane side walls was formed with dry etching and the TMAH wet etching. A U-shape trench gate metal oxide semiconductor field-effect transistor (MOSFET) was also fabricated using the novel etching technology. This device has the excellent normally-off operation of drain current–gate voltage characteristics with the threshold voltage of 10 V. The drain breakdown voltage of 180 V was obtained. The results indicate that the trench gate structure can be applied to GaN-based transistors.

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs.

A Vertical Insulated Gate AlGaN/GaN Heterojunction Field-Effect Transistor

We fabricated a vertical insulated gate AlGaN/GaN heterojunction field-effect transistor (HFET), using a free-standing GaN substrate. This HFET has apertures through which the electron current vertically flows. These apertures were formed by dry etching the p-GaN layer below the gate electrodes and regrowing n--GaN layer without mask. The HFET exhibited a specific on-resistance of as low as 2.6 mΩcm2 with a threshold voltage of -16 V. This HFET would be a prototype of a GaN-based high-power switching device.

A new buffer layer for high quality GaN growth by metalorganic vapor phase epitaxy

A new buffer layer to grow high-quality GaN films was proposed. The new buffer layer consisted of a thin (20–30 nm) InN layer deposited at low temperature (∼600 °C). GaN films were grown on (1120)-oriented (A-face) sapphire substrates using a conventional GaN buffer layer and an InN buffer layer by atmospheric pressure metalorganic vapor phase epitaxy. Dislocations in the GaN films were observed by cross-sectional transmission electron microscopy (TEM). The dislocation densities were measured from the TEM observation and were ∼4×109 and ∼6×108 cm−2 for epilayers with the GaN and the InN buffer, respectively. The low dislocation density by the InN buffer was attributed to relaxation of the stress in the GaN epilayers due to the low melting point of InN. GaN epilayers using the InN buffer also showed good electrical properties.

Dislocation etch pits in GaN epitaxial layers grown on sapphire substrates

Dislocations in GaN epitaxial layers grown on sapphire substrates have been studied by chemical etching. The authors have examined molten KOH as a defect etchant and characterized the etch pits on GaN layers. By use of molten KOH etching, etch pits were revealed on the surface of the GaN layer. All pits were hexagonal pyramids, which reflect the crystal symmetry of GaN. Results showed that molten KOH etching might be a useful method for the evaluation of the dislocations in GaN layers. The etch pit density (EPD) was typically 2 {times} 10{sup 7} cm{sup {minus}2}.

Recent progress of GaN power devices for automotive applications

Many power switching devices are used in hybrid vehicles (HVs) and electric vehicles (EVs). To improve the efficiency of HVs and EVs, better performance characteristics than those of Si power devices, for example, lower on-resistance, higher speed, higher operation temperature, are required for the power devices. GaN power devices are promising candidates for satisfying the requirements. A lateral GaN power device with a blocking voltage of 600 V and a vertical GaN power device with a blocking voltage of 1200 V are suitable for medium power applications for sub systems and high-power applications for the drive of main motors, respectively. Power device applications in HVs and EVs and the current status of the GaN power device are presented. The reliability of the GaN power device is also discussed.

Electrical activation characteristics of silicon-implanted GaN

Electrical activation studies of Si-implanted GaN layers on sapphire were made as a function of annealing temperature (1100–1400°C). For an ion dose of 1.0×1014cm−2, the optimum annealing temperature was 1400°C, exhibiting a nearly 100% electrical activation efficiency and a low sheet resistance of ∼450Ω∕square at room temperature. From variable temperature Hall-effect measurements, Si-implanted GaN films annealed below 1200°C displayed deep ionization levels of ∼280meV, whereas samples annealed above 1300°C had shallow ones of ∼11meV. For lateral Schottky diodes fabricated on Si-implanted GaN layers annealed below 1200°C, capacitance frequency and thermal admittance measurements showed a typical dispersion effect characteristic of a single deep donor with an activation energy of ∼133meV. These results illustrate that deep donor levels created by the Si implantation in GaN layers apparently annihilate and transit to shallow levels produced by the Si ion substitution for Ga in the GaN lattice (SiGa) by ann...

Oxygen content control for as‐deposited YBa2Cu3Ox thin films by oxygen pressure during rapid cooling following laser deposition

As‐deposited epitaxial YBa2Cu3Ox thin films have been prepared by ArF excimer laser deposition followed by a rapid cooling with controlled oxygen pressure. The best film after the rapid cooling has a Tc with a zero resistance of 88 K and a Jc (77 K) of 5.5×105 A/cm2. It has been observed that the c‐lattice parameter decreases, that is, the oxygen content x increases from ∼6 to 7, with increasing oxygen pressure during the rapid cooling. The relation between x and oxygen pressure is almost consistent with that for a powder sample being in a thermal equilibrium. We conclude that the oxygen content in the as‐deposited films is determined by the thermal equilibrium oxygen content during the cooling process.