Tomohiko Shibata

AlN epitaxial growth on off-angle R-plane sapphire substrates by MOCVD

A-plane (1210) AlN films were deposited on the off-angle R-plane (1 102) sapphire substrates, in which the surface plane was tilted towards the [1101] sapphire direction, by metalorganic chemical vapor deposition in order to clarify off-angle effects. The polar direction of the AlN film was inverted by changing the sign of the off-angle and the off-angle was effective in restraining the generation of inverted twins in the AlN. A minus off-angle was found to improve total crystal quality of the AlN from X-ray diffraction results. High-resolution transmission electron microscopy images in the vicinity of interfaces between the AlN and the sapphire indicated that atomic arrangement at an initial AlN growth stage influenced the total crystal quality.

Impact of strain on free-exciton resonance energies in wurtzite AlN

The strain dependence of the free-exciton resonance energies in AlN epilayers is presented and the values are analyzed using an appropriate Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. Based on the results, we study the strain dependence of the valence band ordering, optical transition probability, and free-exciton binding energy. As a result of these calculations, the following strain-free values are obtained for the energy gap, averaged dielectric constants, and ordinary and extraordinary dielectric constants: Eg=6.095 eV at T=11 K, ϵ=7.87, ϵ⊥=7.33, and ϵ∥=8.45, respectively. A brief discussion of the valence band ordering in bulk AlxGa1−xN is also presented.

Growth of Thick AlN Layer by Hydride Vapor Phase Epitaxy

Thick AlN crystals were grown by conventional hydride vapor phase epitaxy (HVPE) on AlN/sapphire templates under low pressure (~15 Torr) at high temperature (1100°C–1200°C). Colorless, mirror-like AlN films were obtained at the growth rates of up to 20.6 µm/h. The best root mean square (RMS) value of atomic force microscope (AFM) observations for the AlN surface was 2.34 nm. The typical values of full width half maximum (FWHM) of X-ray rocking curves for (0002) and (1012) diffraction of AlN films were 173–314 arcsec and 1574–1905 arcsec, respectively. We also investigated the influences of carrier gas, growth temperature and growth rate on the crystal quality.

Transmission electron microscopic observation of AlN/α-Al2O3 heteroepitaxial interface with initial-nitriding AlN layer

The AlN/α-Al2O3 heteroepitaxial interface is investigated using a transmission electron microscope. The epitaxial AlN film is deposited by metalorganic chemical vapor deposition with and without initial nitriding. The initial nitriding method is to convert the α-Al2O3 substrate surface to a nanometer-thick AlN single-crystal buffer layer in NH3 ambient just before AlN deposition. The (1\bar210)AlN/(1\bar102)α-Al2O3 interface with initial nitriding is found to be atomically flat, and no dislocation inside the postdeposited AlN is observed. We have confirmed that initial nitriding is excellent in improving the crystal quality of the AlN epitaxial layer.

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Grown on Epitaxial AlN/Sapphire Templates

AlGaN films and deep ultraviolet light-emitting diode (UV-LED) structures with AlGaN multi-quantum wells (MQWs) were grown directly on 2-in.-diameter epitaxial AlN/sapphire template (AlN template) by metalorganic chemical vapor deposition (MOCVD). It was confirmed that crack-free and good-crystal-quality AlGaN films and UV-LED structures can be grown on AlN template without the use of any buffer layer technique such as superlattice structures or pulsed atomic layer deposition. This seemed to be mainly due to the in-plane compressive stress caused by using AlN template as underlying substrates. Deep UV-LED devices were successfully fabricated using MOCVD-grown wafers. Electroluminescence spectra for those LEDs exhibited sharp near-band emissions ranging from 265 to 277 nm, depending on their Al compositions in MQWs. It was also confirmed that the insertion of a thin AlN layer between the active region and the p-type cladding layer can drastically suppress a parasitic sub-band emission around 320 nm.

DC Characteristics in High-Quality AlGaN/AlN/GaN High-Electron-Mobility Transistors Grown on AlN/Sapphire Templates

High-crystal-quality Al0.26Ga0.74N/AlN/GaN structures with a very high mobility, such as over 2100 cm2/(V s) with a two-dimensional-electron gas (2DEG) density of approximately 1?1013/cm2, were grown on epitaxial AlN/sapphire templates by metalorganic vapor phase epitaxy and adopted for the fabrication of high-electron-mobility transistors (HEMTs). The fabricated devices showed good DC characteristics compared with those in conventional AlGaN/GaN HEMTs and in devices grown on sapphire substrates. Also, it was confirmed that further improvements in DC characteristics are achieved by applying recessed ohmic contacts to AlGaN/AlN/GaN HEMTs. A high extrinsic transconductance of approximately 220 mS/mm and a high drain current density of over 1 A/mm with a threshold voltage of -4.08 V were obtained for 1.5-?m-gate-length recessed ohmic AlGaN/AlN/GaN HEMTs on AlN/sapphire templates. In addition, it was found that the gate leakage current of HEMTs can be suppressed by using epitaxial AlN/sapphire templates as substrates.

Fabrication of 5-GHz-band SAW filter with atomically-flat-surface AlN on sapphire

5-GHz-band surface acoustic wave (SAW) filters for mobile communications were fabricated on an atomically-flat-surface (0001)aluminum nitride/(0001)sapphire (AlN/Al/sub 2/O/sub 3/) combination. The SAW devices were fabricated using electron beam lithography and a lift-off method. Atomically-flat-surface AIN films were used to reduce SAW propagation loss. The center frequency of the fabricated SAW filter was 5.18 GHz. SAW velocity was 5688 m/s at normalized thickness by wave number (kH) of 9.9. The effective coupling coefficient was 0.1% and temperature-coefficient of delay was 9 ppm//spl deg/C at kH of 5.9. The SAW propagation loss was 0.0053 dB at 5.18 GHz. The atomically-flat-surface (0001)AlN/(0001)Al/sub 2/O/sub 3/ combination is promising for use in 5-GHz-band SAW filters for application to mobile communications.

Low-frequency vibrational properties of crystalline and glassy indomethacin probed by terahertz time-domain spectroscopy and low-frequency Raman scattering.

In order to clarify the intermolecular vibrations, the low-frequency modes of the glassy and crystalline states of model pharmaceutical indomethacin have been studied using broadband terahertz time-domain spectroscopy and low-frequency Raman scattering. In the crystalline γ-form, the center of symmetry was suggested by the observation of the exclusion principle of the infrared (IR) and Raman selection rules in the frequency range between 0.2 and 6.5 THz. In addition, a boson peak of the glassy state was observed in both IR and Raman spectra and their frequency showed apparent discrepancy. The intermediate correlation length of the glassy structure was estimated to be about 2.5 nm. The existence of hydrogen bonded cyclic dimers in a glassy state was suggested by the observation of the infrared active intermolecular vibrational mode of the hydrogen bonded cyclic dimers as a broad peak at 3.0 THz in the IR spectrum.

Broadband Terahertz Time-Domain and Low-Frequency Raman Spectroscopy of Crystalline and Glassy Pharmaceuticals

The applications of the broadband Terahertz Time-Domain Spectroscopy (THz-TDS) and the low-frequency Raman scattering spectroscopy to glassy and crystalline pharmaceuticals were reviewed. The real and imaginary parts of a complex dielectric constant were measured by the transmission of THz-TDS using a pure pellet without mixing polyethylene in the frequency range from 0.2 to 6.5 THz. The low-frequency Raman spectra were measured down to 0.3 THz using a double-grating spectrometer. In crystalline indomethacin, indapamide, and racemic ketoprofen, the clear difference in peak frequencies was observed between THz dielectric and low-frequency Raman scattering spectra. It can be attributed to the mutual exclusion principle between Raman and IR activities of optical vibrational modes in centrosymmetric crystals. In glassy indomethacin the broad peak at 3.0 THz in the IR spectrum is attributed to the infrared active intermolecular vibrational mode of the hydrogen bonded cyclic dimers, which is centrosymmetric. The boson peak is the well-known low-energy excitation reflecting vibrational density of states in glassy or amorphous materials. The boson peaks of glassy indomethacin were clearly observed at about 0.3 THz and 0.5 THz in THz-TDS and Raman scattering spectra, respectively. The difference in peak frequencies is attributed to the different frequency dependence between IR-vibration and Raman-vibration coupling constants.

Electrical characterization of Pt∕AlGaN∕GaN Schottky diodes grown using AlN template and their application to hydrogen gas sensors

Pt Schottky diodes were fabricated on high-crystal-quality Al0.2Ga0.8N∕GaN epitaxial films grown using epitaxial AlN/sapphire template (AlN template), and their current transport characteristics were measured and analyzed. It was confirmed that reverse leakage currents in Schottky diodes formed on AlN template were drastically reduced compared with samples on sapphire formed with a low-temperature buffer layer. The current transport characteristics in Schottky diodes formed on AlN template were found to be almost explicable using the thermoionic emission and tunneling current components, unlike with samples formed on sapphire. This indicates that the amount of unintentional impurity levels near the surfaces is extremely small for those high-crystal-quality Schottky diodes. Hydrogen-sensing characteristics were also investigated for a Pt∕AlGaN∕GaN Schottky diode formed on AlN template. The diode sensor exhibited sufficient changes in the reverse current even under exposure to extremely slight H2 concentration of 50ppm and exhibited clear reversible responses. These current changes seemed to be due to the reduction of the Schottky barrier height with H2 exposure.Pt Schottky diodes were fabricated on high-crystal-quality Al0.2Ga0.8N∕GaN epitaxial films grown using epitaxial AlN/sapphire template (AlN template), and their current transport characteristics were measured and analyzed. It was confirmed that reverse leakage currents in Schottky diodes formed on AlN template were drastically reduced compared with samples on sapphire formed with a low-temperature buffer layer. The current transport characteristics in Schottky diodes formed on AlN template were found to be almost explicable using the thermoionic emission and tunneling current components, unlike with samples formed on sapphire. This indicates that the amount of unintentional impurity levels near the surfaces is extremely small for those high-crystal-quality Schottky diodes. Hydrogen-sensing characteristics were also investigated for a Pt∕AlGaN∕GaN Schottky diode formed on AlN template. The diode sensor exhibited sufficient changes in the reverse current even under exposure to extremely slight H2 concentrat...