Manato Deki

Single-shot picosecond interferometry with one-nanometer resolution for dynamical surface morphology using a soft X-ray laser

Using highly coherent radiation at a wavelength of 13.9 nm from a Ag-plasma soft X-ray laser, we constructed a pump-and-probe interferometer based on a double Lloyds mirror system. The spatial resolutions are evaluated with a test pattern, showing 1.8-mum lateral resolution, and 1-nm depth sensitivity. This instrument enables a single-shot observation of the surface morphology with a 7-ps time-resolution. We succeeded in observing a nanometer scale surface dilation of Pt films at the early stage of the ablation process initiated by a 70 fs near infrared pump pulse.

Highly elongated vertical GaN nanorod arrays on Si substrates with an AlN seed layer by pulsed-mode metal–organic vapor deposition

To extend the availability of nanostructure-based optoelectronic applications, vertically elongated nanorods with precisely controlled morphology are required. For group III nitrides, pulsed-mode growth has recently been reported as an effective method for growing nanorod arrays with geometric precision. Here, we demonstrated the growth of arrays of highly elongated nanorods on Si substrates by metal–organic chemical vapor deposition using a pulsed-mode approach. Unlike the thick and high (or middle)-quality GaN templates normally used, nanorod growth was performed on an ultrathin and low-quality AlN/Si platform. Using kinetically controlled growth conditions and a patterning process, exceptionally long GaN nanorods were achieved with high geometric precision. The grown nanorods showed considerably improved optical and structural properties while remaining in uniform arrays. This approach can be used with a variety of materials to obtain nanorods with high quality, high uniformity, and high aspect ratio, and it can also serve as an effective fabrication method for InAlGaN-alloyed core/shell nanostructures for optoelectronic nanodevices with ultrahigh efficiency.

Enhancement of local electrical conductivities in SiC by femtosecond laser modification

Enhancement of local electric conductivities induced by femtosecond laser modification in silicon carbide was studied. Current-voltage (I-V) characteristics of the laser-modified regions were measured between the ion-implanted metal contacts. Interestingly, the resistance sharply decreased in the fluence range from 5.0 to 6.7 J/cm2. The resistance at the irradiation fluence of 53 J/cm2 decreased by more than six orders of magnitude compared with the nonirradiated one. From the I-V characteristics and the scanning electron microscope observations, we conclude that the phase separation associate with the formation of classical laser induced periodic structure causes the drastic increase in electric conductivity.

Heavy-Ion Induced Anomalous Charge Collection From 4H-SiC Schottky Barrier Diodes

Heavy-ion induced anomalous charge collection was observed in 4H-SiC Schottky Barrier Diodes (SBDs). It is suggested that the range of the incident ions with respect to the thickness of the epi-layer, ion energy, and electric-field intensity of the SBD is the key to understanding this observation and understanding the SEB mechanism.

Selective-area growth of GaN microrods on strain-induced templates by hydride vapor phase epitaxy

In this paper, we discuss the influence of parameters such as type of carrier gas and NH3/HCl flow ratio on the growth of vertical GaN microstructures by selective-area growth (SAG) hydride vapor phase epitaxy (HVPE). On various strain-induced templates such as GaN/sapphire, GaN/Si, and AlN/Si, regular arrays of Ga-polar GaN microrods were properly achieved by adjusting the growth parameters. The photoluminescence and micro-Raman measurements reveal not only the crystal quality of the GaN microrods but also strain distribution. These results will give insight into the control of the morphology of GaN microrods in terms of the strain induced from templates in SAG-HVPE. The precisely controlled arrays of GaN microrods can be used for next-generation light-emitting diodes (LEDs) by realizing InGaN/GaN multi–quantum wells (MQWs) with a radial structure.

Raman spectroscopic stress evaluation of femtosecond-laser-modified region inside 4H-SIC

A femtosecond (fs)-laser-modified region inside single-crystal silicon carbide was studied by micro-Raman spectroscopy. Higher and lower peak energy shifts of the transverse optical (TO) phonon mode, which correspond to compressive and tensile stresses, were observed. Mappings of peak energies and spectral widths of the TO phonon mode showed a clear correspondence with the distributions of strained layers observed by transmission electron microscopy. The maximum compressive and tensile stresses were estimated to be 1.4 and 0.4 GPa, respectively. This result indicates that the periodic strained layers contain many nano-voids which are formed by nano-explosions induced by fs laser irradiation.

Structural and optical study of core–shell InGaN layers of nanorod arrays with multiple stacks of InGaN/GaN superlattices for absorption of longer solar spectrum

We report on the material and optical properties of core?shell InGaN layers grown on GaN nanorod arrays. The core?shell InGaN layers were well grown on polarization-reduced surfaces such as semipolar pyramids and nonpolar sidewalls. In addition, to compensate the biaxial strain between GaN and InGaN layers, we grew interlayers underneath a thick InGaN layer. Here, the interlayers were composed of multiple superlattice structures. We could observe that the indium composition of core?shell InGaN structures increased with the number of interlayers. This indicates that the absorption energy band of InGaN alloys can be better matched to the spectral irradiance of the solar spectrum in nature. We also implemented a simulation of Ga-polar and nonpolar InGaN-based solar cells based on the indium composition obtained from the experiments. The result showed that nonpolar InGaN solar cells had a much higher efficiency than Ga-polar InGaN solar cells with the same thickness of the absorption layer.

The interface analysis of GaN grown on 0° off 6H-SiC with an ultra-thin buffer layer

Previously, we reported a growth method by metalorganic vapor phase epitaxy using a single two-dimensional growth step, resulting in 1.2-µm crack-free GaN directly grown on 6H-SiC substrate. The introduction of Al-treatment prior to the standard GaN growth step resulted in improved surface wetting of gallium on the SiC substrate. Transmission electron microscope and energy dispersive spectrometer analysis of the epitaxial interface to the SiC determined that an ultra-thin AlGaN interlayer had formed measuring around 2–3 nm. We expect our growth technique can be applied to the fabrication of GaN/SiC high frequency and high power devices.

Correlation between dislocations and leakage current of p-n diodes on a free-standing GaN substrate

Dislocations that cause a reverse leakage current in vertical p-n diodes on a GaN free-standing substrate were investigated. Under a high reverse bias, dot-like leakage spots were observed using an emission microscope. Subsequent cathodoluminescence (CL) observations revealed that the leakage spots coincided with part of the CL dark spots, indicating that some types of dislocation cause reverse leakage. When etch pits were formed on the dislocations by KOH etching, three sizes of etch pits were obtained (large, medium, and small). Among these etch pits, only the medium pits coincided with leakage spots. Additionally, transmission electron microscopy observations revealed that pure screw dislocations are present under the leakage spots. The results revealed that 1c pure screw dislocations are related to the reverse leakage in vertical p-n diodes.

Preflow trimethylaluminum treatment effect on GaN growth on SiC with an ultrathin interlayer

Previously, we reported a growth method for GaN on SiC by metalorganic vapor phase epitaxy. By using a preflow trimethylaluminum treatment, the poor wetting problem of gallium on the SiC surface was alleviated, resulting in a 1.2-µm-thick crack-free GaN grown on an on-axis 6H-SiC(0001) substrate via an ultrathin AlGaN interlayer. In this study, the impact of the preflow trimethylaluminum treatment time is investigated to understand why a crack-free epilayer was realized. To demonstrate the electrical performance of devices formed by our technique, GaN/SiC vertical Schottky barrier diodes were fabricated and compared with GaN/AlN/SiC and GaN/GaN vertical Schottky barrier diodes. Compared with diodes including a high-resistance AlN interlayer, the series resistance of GaN/SiC Schottky barrier diodes incorporating the ultrathin interlayer with 5 s of TMAl treatment showed a marked reduction from 4.0 × 107 to 2.0 × 10−1 Ωcm2. The ultrathin interlayer growth technique is expected to be applied in future GaN/SiC hybrid high-power and high-frequency devices.