Kenta Watanabe

Comprehensive study on initial thermal oxidation of GaN(0001) surface and subsequent oxide growth in dry oxygen ambient

Initial oxidation of gallium nitride (GaN) (0001) epilayers and subsequent growth of thermal oxides in dry oxygen ambient were investigated by means of x-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy, and x-ray diffraction measurements. It was found that initial oxide formation tends to saturate at temperatures below 800 °C, whereas the selective growth of small oxide grains proceeds at dislocations in the epilayers, followed by noticeable grain growth, leading to a rough surface morphology at higher oxidation temperatures. This indicates that oxide growth and its morphology are crucially dependent on the defect density in the GaN epilayers. Structural characterizations also reveal that polycrystalline α- and β-phase Ga2O3 grains in an epitaxial relation with the GaN substrate are formed from the initial stage of the oxide growth. We propose a comprehensive model for GaN oxidation mediated by nitrogen removal and mass transport and discuss the model on the basis of ex...

Improved interface properties of GaN-based metal-oxide-semiconductor devices with thin Ga-oxide interlayers

The impact of thin Ga-oxide (GaOx) interlayers on the electrical properties of GaN-based metal-oxide-semiconductor (MOS) devices was systematically investigated. Thin thermal oxides formed at around 900 °C were found to be beneficial for improving the electrical properties of insulator/GaN interfaces, despite the fact that thermal oxidation of GaN surfaces at high temperatures proceeds by means of grain growth. Consequently, well-behaved capacitance-voltage characteristics of SiO2/GaOx/n-GaN stacked MOS capacitors with an interface state density (Dit) as low as 1.7 × 1011 cm−2 eV−1 were demonstrated. Moreover, the Dit value was further reduced for the SiO2/GaOx/GaN capacitor with a 2-nm-thick sputter-deposited GaOx interlayer. These results clearly indicate the intrinsically superior nature of the oxide/GaN interfaces and provide plausible guiding principles for fabricating high-performance GaN-MOS devices with thin GaOx interlayers.

Design and control of interface reaction between Al-based dielectrics and AlGaN layer in AlGaN/GaN metal-oxide-semiconductor structures

Important clues for achieving well-behaved AlGaN/GaN metal-oxide-semiconductor (MOS) devices with Al-based gate dielectrics were systematically investigated on the basis of electrical and physical characterizations. We found that low-temperature deposition of alumina insulators on AlGaN surfaces is crucial to improve the interface quality, thermal stability, and variability of MOS devices by suppressing Ga diffusion into the gate oxides. Moreover, aluminum oxynitride grown in a reactive nitric atmosphere was proven to expand the optimal process window that would improve the interface quality and to enhance immunity against charge injection into the gate dielectrics. The results constitute common guidelines for achieving high-performance and reliable AlGaN/GaN MOS devices.

L10-ordered MnAl thin films with high perpendicular magnetic anisotropy

L10-ordered MnAl thin films were epitaxially grown by sputtering. The film composition dependences of structural and magnetic properties were systematically investigated in the MnAl thin films. Both the L10-ordered parameter and the perpendicular magnetic anisotropy energy strongly depended on the composition of the MnAl thin films. The MnAl thin films with a Mn composition of 53–54 at. % showed both the highest L10-ordered parameter and the perpendicular magnetic anisotropy. The substrate and annealing temperatures were optimized to improve the magnetic properties and surface morphology. We have fabricated MnAl thin films with both a very high K u of 12 × 106 erg/cm3 and a small surface roughness of ca. 0.2 nm by optimizing the film composition and substrate and annealing temperatures. These results are useful guidelines for the fabrication of highly L10-ordered MnAl thin films with a large perpendicular magnetic anisotropy.

Fabrication of thin-film fresnel optics by combining diamond turning and photolithographic processes

A novel fabrication process is proposed for manufacturing thin-film metal Fresnel lenses for X-ray applications. This process combines diamond turning technology and photolithographic processes. To prevent thin-film lens substrates from deflection during diamond turning, films were prepared on single crystalline silicon wafers by electrolytic plating. After the Fresnel lens structure is generated on the metal thin films by diamond turning, the silicon substrate was then removed selectively by reactive ion etching. Experimental results demonstrated that the proposed hybrid fabrication process achieves submicron form accuracy and nanometer surface roughness.

Control of Ga-oxide interlayer growth and Ga diffusion in SiO2/GaN stacks for high-quality GaN-based metal?oxide?semiconductor devices with improved gate dielectric reliability

A simple and feasible method for fabricating high-quality and highly reliable GaN-based metal–oxide–semiconductor (MOS) devices was developed. The direct chemical vapor deposition of SiO2 films on GaN substrates forming Ga-oxide interlayers was carried out to fabricate SiO2/GaO x /GaN stacked structures. Although well-behaved hysteresis-free GaN-MOS capacitors with extremely low interface state densities below 1010 cm−2 eV−1 were obtained by postdeposition annealing, Ga diffusion into overlying SiO2 layers severely degraded the dielectric breakdown characteristics. However, this problem was found to be solved by rapid thermal processing, leading to the superior performance of the GaN-MOS devices in terms of interface quality, insulating property, and gate dielectric reliability.

Design and control of interface reaction between Al-based dielectrics and AlGaN layer for hysteresis-free AlGaN/GaN MOS-HFETs

We have demonstrated hysteresis-free recessed gate AlGaN/GaN metal-oxide-semiconductor heterojunction field-effect transistor (MOS-HFET) by implementing AIGN gate insulator and selective AlGaN regrowth technique. High thermal stability and excellent electrical properties of AIGN gate dielectrics will provide a large process window for further optimization of AlGaN/GaN MOS-HFET.

Cobalt substituted L10-MnAl thin films with large perpendicular magnetic anisotropy

The Co composition dependences of the structural and magnetic properties of L10-(MnAl)1− x Co x alloy films were investigated. The lattice constants of (MnAl)1− x Co x films gradually changed with increasing Co content while maintaining the L10-ordered structure below x = 0.08. The saturation magnetization gradually decreased with increasing Co content, and perpendicular magnetic anisotropy was observed below x = 0.08. In addition, Co substitution markedly improved the surface roughness of the films by decreasing the substrate temperature of (MnAl)1− x Co x films. We found that both a high magnetic anisotropy and a small surface roughness can be obtained by the substitution of Co atoms into MnAl films.