Norikazu Nakamura

Improvements of lubricant performance in hard-disk media by vacuum ultraviolet irradiation

In order to increase the reliability of hard disk media especially for corrosion-resistant property, we have tried lowering the surface energy of a media with high coverage and high bonded ratio of lubricant. In this study for the UV treatment of a media we examined the wavelength of 172 nm that is shorter than that of the low-pressure mercury lamp normally used, which irradiates at wavelengths of 185 nm and 254 nm. We found a drastic improvement in corrosion-resistant property in the case of 172-nm irradiation. In this paper improved corrosion-resistant property and its mechanisms are discussed.

Effects of vacuum ultraviolet irradiation to lubricant layer on hard-disk media

Summary form only given. Adhesion between a lubricant and DLC (diamond-like carbon) used as a protective layer in magnetic recording media improves HDI (head disk interface) characteristics. Therefore, magnetic recording media coated with lubricant are treated with heat or ultraviolet (UV) to increase the thickness of the bonded layer on the DLC and to optimize friction, durability, and other characteristics of the media. Because there are many different photoreactions and photo-induced reactions are known, we believe that this UV treatment is an attractive technique to improve the characteristics of the media. Usually, UV treatment is carried out by using a low-pressure mercury lamp, which irradiates at a wavelength of 185 and 254 nm with an intensity ratio of one to five. Our research interest includes investigating applications of short-wavelength irradiation because it can produce more photoelectrons increasing the thickness of the bonded lubricant layer. To examine and determine the potential of ultraviolet treatment, we investigated the effects of 172 nm irradiation in the vacuum ultraviolet region.

Suppression of threshold voltage shift for normally-Off GaN MIS-HEMT without post deposition annealing

In this paper, we present a method of reducing threshold voltage shift for normally-off GaN MIS-HEMT by the optimization of dielectric deposition conditions. High-temperature deposition of Al2O3 insulator decreases the impurities in a dielectric film, leading to small C-V and I-V hysteresis under large positive gate voltage operation. Moreover, Al2O3 deposited at high temperature achieve high quality interface and bulk without post deposition annealing (PDA), preventing the degradation of electrodes and crystallization of insulator film. The fabricated device shows small C-V and I-V hysteresis, with a breakdown voltage of greater than 600 V.

Low dislocation density InAlN/AlN/GaN heterostructures grown on GaN substrates and the effects on gate leakage characteristics

This paper reports on the electrical characterization of Ni/Au Schottky diodes fabricated on InAlN high-electron-mobility transistor (HEMT) structures grown on low dislocation density free-standing GaN substrates. InAlN HEMT structures were grown on sapphire and GaN substrates by metal-organic vapor phase epitaxy, and the effects of threading dislocation density on the leakage characteristics of Ni/Au Schottky diodes were investigated. Threading dislocation densities were determined to be 1.8 × 104 cm−2 and 1.2 × 109 cm−2 by the cathodoluminescence measurement for the HEMT structures grown on GaN and sapphire substrates, respectively. Leakage characteristics of Ni/Au Schottky diodes were compared between the two samples, and a reduction of the leakage current of about three to four orders of magnitude was observed in the forward bias region. For the high reverse bias region, however, no significant improvement was confirmed. We believe that the leakage current in the low bias region is governed by a dislo...

New Model of Fe Diffusion in Highly Resistive Fe-Doped Buffer Layer for GaN High-Electron-Mobility Transistor

We demonstrated the suppression of Fe incorporation into undoped GaN layers grown on an Fe-doped GaN layer by metal organic chemical vapor deposition (MOCVD). Our systematic study of the depth profile of Fe doping by secondary-ion mass spectrometry (SIMS) revealed that Fe segregation on the growth surface is responsible for Fe incorporation into the upper undoped GaN layer. Moreover, we confirmed from the Fe doping profiles of strain-varied undoped GaN layers grown on an Fe-doped AlxGa1-xN layer that the compressive strain on the growth surface could effectively suppress Fe segregation. In this study, we propose a new model showing that the suppression is due to an increase in the extent of thermal desorption of Fe from the growth surface by compressive strain.

Suppression of gate leakage current in in-situ grown AlN/InAlN/AlN/GaN heterostructures based on the control of internal polarization fields

This paper investigates the gate leakage characteristics of in-situ AlN capped InAlN/AlN/GaN heterostructures grown by metal-organic vapor phase epitaxy. It was revealed that the leakage characteristics of AlN capped InAlN/AlN/GaN heterostructures are strongly dependent on the growth temperature of the AlN cap. For an AlN capped structure with an AlN growth temperature of 740 °C, the leakage current even increased although there exists a large bandgap material on InAlN/AlN/GaN heterostructures. On the other hand, a large reduction of the gate leakage current by 4–5 orders of magnitudes was achieved with a very low AlN growth temperature of 430 °C. X-ray diffraction analysis of the AlN cap grown at 740 °C indicated that the AlN layer is tensile-strained. In contrast to this result, the amorphous structure was confirmed for the AlN cap grown at 430 °C by transmission electron microscopy. Furthermore, theoretical analysis based on one-dimensional band simulation was carried out, and the large increase in two...

Advantages of the AlGaN spacer in InAlN high-electron-mobility transistors grown using metalorganic vapor phase epitaxy

We demonstrate the advantages of an AlGaN spacer layer in an InAlN high-electron-mobility transistor (HEMT). We investigated the effects of the growth parameters of the spacer layer on electron mobility in InAlN HEMTs grown by metalorganic vapor phase epitaxy, focusing on the surface roughness of the spacer layer and sharpness of the interface with the GaN channel layer. The electron mobility degraded, as evidenced by the formation of a graded AlGaN layer at the top of the GaN channel layer and the surface roughness of the AlN spacer layer. We believe that the short migration length of aluminum atoms is responsible for the observed degradation. An AlGaN spacer layer was employed to suppress the formation of the graded AlGaN layer and improve surface morphology. A high electron mobility of 1550 cm2 V−1 s−1 and a low sheet resistance of 211 Ω/sq were achieved for an InAlN HEMT with an AlGaN spacer layer.

Origin of unintentional gallium incorporation into AlN layers grown by metalorganic vapor phase epitaxy

This paper presents the origin of gallium (Ga) incorporation into AlN layers grown by metalorganic vapor phase epitaxy (MOVPE). We systematically investigated Ga incorporation from GaN deposits on the inner walls of a growth reactor and an underlying GaN layer. The Ga incorporation is not affected by the GaN deposits on the inner walls of the reactor, but is strongly affected by the underlying GaN layer. We found that the AlN layer incorporates Ga atoms which are desorbed from the underlying GaN layer, and the Ga incorporation decreases with the growth temperature. We conclude that the dominant source of unintentional Ga incorporation into an AlN layer is the underlying GaN layer. Furthermore, we successfully achieved an aluminum composition of over 0.95, almost without unintentional Ga incorporation, by growing the AlN layers at a low temperature.

Coverage Analysis of Molecularly Thin Lubricant Films Using Molecular Dynamics Simulations and CAICISS Measurements

In order to examine the coverage of molecularly thin lubricant films on magnetic recording media, a method using coaxial impact collision ion scattering spectroscopy (CAICISS) is proposed. In this method, it is presumed that if a lubricant film of perfluoropolyether completely covers the surface, the fluorine-to-carbon atomic ratio (F/C ratio) of the surface should become 2. In this paper, we reproduce molecularly thin lubricant films on a carbon overcoat by full-atom molecular dynamics simulations, and calculate their coverage and F/C ratios. assuming the CAICISS measurement, the F/C ratios were computed considering the shadow cones formed behind the surface atoms. By comparing the F/C ratios with the coverage, we conclude that the coverage measurement method using CAICISS can accurately evaluate the coverage of molecularly thin lubricant films.