Digest: Japanese Journal of Applied Physics


Separation of Thin GaN from Sapphire by Laser Lift-Off Technique

Tetsuzo UedaMasahiro Ishida, and Masaaki Yuri

Laser lift-off of GaN from sapphire substrates has become a viable technique to increase the brightness of GaN-based light-emitting diodes (LEDs). The LEDs free from sapphire exhibit high luminous efficiency by placing highly reflective electrode on the back side. The devices serve low series resistance together with low thermal resistance taking advantages of the vertical structure. Thinner epitaxial structure is desired to serve better device performance, however, cracks in the film after the lift-off limits the minimum thickness. In this paper, successful laser lift-off of very thin GaN with the thickness down to 4 µm is described. The established laser lift-off system utilizes homogenized beam-profile of the employed neodymium-doped yttrium aluminium garnet (Nd:YAG) third harmonic laser in which optimization of the laser fluence minimizes the thickness of the decomposed GaN. It is also revealed by calculation that the compressive stress in the thin GaN is increased by reducing the thickness. It is demonstrated that the lattice of the GaN is relaxed after the laser lift-off, which is confirmed by photoluminescence (PL) and X-ray diffraction (XRD) measurements. In addition, reduction of the wafer bowing of GaN on sapphire is experimentally confirmed after the laser irradiation with the formation of metal Ga in between the interface.

High-Temperature Thermoelectric Property of Layered La2-2xCa1+2xMn2O7 Manganites (0.75 ≤x ≤1.0)

Takeshi ToyodaNaoya SasakiKazuhiro ShimadaMaki Okube, and Satoshi Sasaki

La2-2xCa1+2xMn2O7 manganites with a doping concentration range of 0.75 ≤x ≤1.0 have been investigated to evaluate their thermoelectric properties in the high-temperature range from 400 to 1200 K. The materials, which are well known as n = 2 Ruddlesden–Popper compounds, have been confirmed as n-type semiconductors from the observation of the temperature dependence of the electrical conductivity and Seebeck coefficients. The electrical conductivity increases with increasing La concentration, suggesting hopping conduction. The thermal conductivity of the materials examined in this study is less than 1.0 W·m-1·K-1 at room temperature. The dimensionless figure of merit (ZT) was drastically enhanced by La doping, and had the value of 0.018 at 1173 K for La0.05Ca2.95Mn2O7. This behavior is similar that of the well-known layered structure such as the p-type misfit-layered Ca3Co4O9 where high Seebeck coefficient and low thermal conductivity.

Effects of Gold Nanoparticles on Pentacene Organic Field-Effect Transistors

Keanchuan LeeMartin WeisWei Ou-YangDai TaguchiTakaaki Manaka, and Mitsumasa Iwamoto

The effect of gold nanoparticles (NPs) on pentacene organic field-effect transistors (OFETs) was being investigated by both DC and AC methods, which are current–voltage (IV) measurements in steady-state and impedance spectroscopy (IS) respectively. Here poly(vinyl alcohol) (PVA) and PVA blended with Au NPs as composite are spin-coated on SiO2 as gate-insulator for top-contact pentacene OFET. The characteristics of the device were being investigated based on the contact resistance, trapped charges, effective mobility and threshold voltage based on transfer characteristics of OFET. Results revealed that OFET with NPs exhibited larger hysteresis and higher contact resistance at high voltage region. IS measurements were performed and the fitting of results by the Maxwell–Wagner equivalent circuit showed that for device with NPs a series of capacitance and resistance which represents trapping must be introduced in order to have agreeable fitting. The fitting had helped to clarify the reason behind the higher contact resistance and bigger hysteresis which was mainly caused by the space charge field formed by the traps when Au NPs were introduced into the device.

Phase-Change Kinetics of Bi–Fe–(N) Layer for High-Speed Write-Once Optical Recording

Sung-Hsiu HuangYu-Jen HuangHung-Chuan Mai, and Tsung-Eong Hsieh

In this work, we present the phase-change kinetics of Bi–Fe–(N) layers for write-once optical recording. In situ reflectivity measurement indicated that the phase-change temperature (Tx) of the Bi–Fe–(N) layers is strongly related to the heating rate. The Tx's were about 170 °C at low heating rates and approached the melting point of the Bi phase (i.e., 271.4 °C) at high rate of heating provided by laser heating. For a 100-nm-thick Bi–Fe–(N) layer, Kissinger's analysis showed that the activation energy of phase transition (Ea) = 1.24 eV, while the analysis of isothermal phase transition in terms of the Johnson–Mehl–Avrami (JMA) theory showed that the average Avrami exponent (m) = 2.2 and the appropriate activation energy (ΔH) = 5.15 eV. With the aid of X-ray diffraction (XRD) analysis, a two-dimensional phase transition behavior in the Bi–Fe–(N) layers initiated by the melting of the Bi-rich phase was confirmed. For optical disk samples with optimized disk structure and write strategy, the signal properties far exceeding the write-once disk test specifications were achieved. Satisfactory signal properties indicated that the Bi–Fe–(N) system is a promising alternative for high-speed write-once recording in the Blu-ray era.

Effect of Heat Treatment on Ion Conductivity of Hydrated ZrO2 Thin Films Prepared by Reactive Sputtering Using H2O Gas

Ning LiYoshio AbeMidori KawamuraKatsutaka SasakiHidenobu Itoh, and Tsutomu Suzuki

Hydrated ZrO2 thin films were prepared by reactive sputtering using H2O gas, and these films were heat treated in air at temperatures from 100 to 350 °C. Absorbance peaks due to hydrogen-bonded OH groups for these samples were observed by Fourier transform infrared spectroscopy. The peak intensities were nearly the same before and after heat treatment below 200 °C, but began to decrease at 250 °C, and the absorption peak disappeared at 350 °C. Ion conductivity of the films was evaluated by AC impedance measurements and was found to be about 3 ×10-6 S/m before and after heat treatment at 200 °C; it also decreased after heat treatment above 250 °C. From these results, we considered that protons of OH and/or H2O in the films are the dominant ionic species that contribute to the ion conductivity of the films.

Electromagnetic Imprint Technique Combined with Electrophoretic Deposition Technique in Forming Microelectrode Structures

Yung Chun WengYung Jin WengHuang Sheng Fang, and Sen Yeu Yang

In this study, we integrate the electromagnetic soft mold imprint technique with the electrophoretic deposition technique, and apply them to forming microelectrode structures. The compound casting technology is used to produce a magnetic soft mold of a microelectrode structure, which can effectively reduce the time and cost of molding. The use of an electromagnetic imprint device can apply more evenly distributed imprint pressure, thus, the microelectrode structure can be entirely imprinted onto an indium tin oxide (ITO) soft substrate, and then the electrophoretic deposition technique is employed to deposit titanium dioxide (TiO2) nanopowder on the ITO soft substrate of the microelectrode structure. In addition to the key techniques and processes of electromagnetic soft mold imprinting, In this study, we explore the application of electrophoretic deposition and imprinting to prove that combining these techniques to form a microelectrode structure is a simple, low-cost, high duplication, and high-speed process. It is proven a good choice for producing micro-nanocomponents

Analysis of High-Intensity Focused Ultrasound Source Using Time Reversal: Effect of Lamb-Like Waves

Yasuhiro KaneshimaShin Yoshizawa, and Shin-ichiro Umemura

The pressure distribution of a focused ultrasound source was reconstructed from a measured pressure field on the focal plane using time-reversal. The two-dimensional Fourier transform of the distribution in the time and space domain showed a peak with a finite phase velocity, which corresponds to a mode of waves propagating from the circumference to the center of the transducer similar to Lamb waves. It was numerically confirmed that the propagating waves significantly enhanced the secondary lobe on the near side of the main focal lobe. The near side lobe was markedly reduced by increasing the thickness of the transducer by three times in the experiment. This significant change is consistent with the hypothesis that the near side lobe was formed by Lamb-like waves.

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