Keiichiro Asai

High-electron-mobility AlGaN∕AlN∕GaN heterostructures grown on 100-mm-diam epitaxial AlN/sapphire templates by metalorganic vapor phase epitaxy

Al0.26Ga0.74N∕AlN∕GaN heterostructures with 1-nm-thick AlN interfacial layers were grown on 100-mm-diam epitaxial AlN/sapphire templates and sapphire substrates by metalorganic vapor phase epitaxy. It was found that AlN/sapphire templates significantly enhanced the electron mobility of the two-dimensional electron gas (2DEG) confined at the GaN channel. This can be explained by the high-crystal-quality GaN channel realized by the use of epitaxial AlN/sapphire templates as substrates. The very high Hall mobilities of approximately 2100cm2∕Vs at room temperature and approximately 17000cm2∕Vs at 77K with a 2DEG density of approximately 1×1013∕cm2 were uniformly obtained for AlGaN∕AlN∕GaN heterostructures on 100-mm-diam epitaxial AlN/sapphire templates. The Hall mobility of AlGaN∕AlN∕GaN heterostructures on epitaxial AlN/sapphire templates reached a very high value of 25500cm2∕Vs at 15K.

Improved dc characteristics of AlGaN/GaN high-electron-mobility transistors on AlN/sapphire templates

High-electron-mobility transistors (HEMTs) have been demonstrated on both AlN/sapphire templates and sapphire substrates, and the dc characteristics of the fabricated devices were examined at room temperature. Better dc characteristics with high extrinsic transconductances and drain current densities were observed in the HEMTs grown on AlN/sapphire templates when compared with the HEMTs on sapphire substrates. Extrinsic transconductances of 214 and 137 mS/mm for Wg/Lg=15/2 μm HEMTs on AlN/sapphire templates and HEMTs on sapphire substrates were achieved, respectively. The enhancement of dc characteristics with small variations in threshold voltage (⩽130 mV) is due to the reduction of dislocation density (1.5×108 cm−2). The decrease of dark spot density has been confirmed in the GaN grown on AlN/sapphire templates using cathodoluminescence measurements. The advantage of using AlN/sapphire templates is that low dislocation density GaN layers at a high temperature can be grown without using low-temperature-g...

Exciton spectra of an AlN epitaxial film on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy

Exciton resonance energies in an AlN epilayer on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral line shape was fitted considering A (Γ7vu→Γ7c) and BC (Γ9v,Γ7vl→Γ7c) exciton transitions. The fitting gave the values of them at 0 K to be 6.211 and 6.266 eV, giving the crystal- field splitting (Δcr) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature ΘE was estimated to be 580 K.

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.

Growth of high-quality GaN films on epitaxial AlN/sapphire templates by MOVPE

High-quality GaN films were grown on epitaxial AlN/sapphire templates by metal-organic vapor phase epitaxy. The Hall mobility as high as 790 cm2/V s with the carrier concentration of 7.6×1016 cm−3 at 300 K along with low dislocation density of 5×107 cm−2 have been achieved. The X-ray rocking curve full-width at half-maximum were 61 and 232 arcsec for the (0 0 0 4) and (2024) reflections, respectively. Atomic force microscopy images showed smooth surface morphology and clear step formation. Additionally, pit terminations could be hardly found. Photoluminescence measurement revealed a higher intensity near the band edge.

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.

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...

Growth and characterization of AlGaN/AlN/GaN HEMTs on 100-mm-diameter epitaxial AlN/sapphire templates

Al/sub 0.26/Ga/sub 0.74/N/AlN/GaN heterostructures with a 1-nm-thick AIN interfacial layer were grown on 100-mm-diameter epitaxial AIN/sapphire templates by metalorganic vapor phase epitaxy. They exhibited very high Hall mobilities of approximately 2100 cm/sup 2//Vs at room temperature and approximately 25000 cm/sup 2//Vs at 15 K with a sheet carrier density of approximately 1 /spl times/ 10/sup 13//cm/sup 2/. High-electron-mobility transistors were successfully fabricated on the epitaxial wafers. The device exhibited a high drain current density of 832 mA/mm and high extrinsic transconductance of 189 mS/mm.

Characterization of high-quality epitaxial AlN films grown by MOVPE

This paper presents the correlation between overall crystal mosaicities and dislocation behaviors of high-quality AlN epitaxial films grown on a C-plane sapphire substrate using a low-pressure metal organic vapor phase epitaxy (LP-MOVPE) method. Typical full width at half maximum (FWHM) values of x-ray rocking curves (XRC) for (0002) and (10-10) of the AlN are 80arcsec and 1800arcsec, respectively. From transmission electron microscopy (TEM) observations, edge-type dislocations thread the AlN layer, however, almost all screw-type dislocations disappear at an early AlN growth stage. The dislocation density of the AlN films in its surface region is as low as approximately 1x10 10 cm -2 . The distribution of the dislocations is considered to be caused by a large twisted mosaicity and a very small tilted mosaicity.

2.4 GHz SAW filters using AlN deposited on off-angle R-plane sapphire substrates by MOCVD

A-plane AlN films deposited on off-angle R-plane sapphire substrates, in which a surface plane was tilted towards [1-101] sapphire direction, were shown to be effective in restraining the generation of inverted twins. Surface acoustic wave (SAW) filters at 2.4 GHz were reliably made using the optimized AlN on the off-angle sapphire substrate. 2.4 GHz spread spectrum (SS) signals were able to be directly demodulated to baseband data using the optimized front-end SAW matched filter (MF) and delay line (DL) on the AlN.