Showing papers by "Takeyoshi Onuma published in 2002"
TL;DR: In this paper, a promising approach to obtain epitaxial films of oxide semiconductors was demonstrated, namely helicon-wave-excited-plasma sputtering epitaxy.
Abstract: A promising approach to obtain epitaxial films of oxide semiconductors was demonstrated, namely helicon-wave-excited-plasma sputtering epitaxy. Due to the surface-damage-free nature, completely a-axis-locked c(0001)-orientation ZnO epilayers were successfully grown on sapphire (0001) substrates having ultrasmooth surfaces with atomic steps. The ZnO epilayer exhibited a dominant near-band-edge photoluminescence (PL) peak at 300 K. Since the PL was considered to be due to the recombination of excitons bound to an impurity or a defect and certain tilting and twisting of the films were observed when Ar/O2 were used as sputtering gases, purification and optimization of the overall process are necessary to obtain improved epilayer qualities.
73 citations
TL;DR: In this article, the 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.
Abstract: 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.
65 citations
TL;DR: In this paper, the authors studied the recombination dynamics of localized excitons in In x Ga 1-x N/c-GaN multiple quantum wells (MQWs) and showed that the spontaneous emission is due to the radiative recombination of exciton localized in disordered quantum nanostructures even at 300 K.
Abstract: Recombination dynamics of localized excitons in strained cubic (c-)In x Ga 1-x N/c-GaN multiple quantum wells (MQWs) were studied. In contrast to hexagonal (h-)InGaN quantum wells (QWs), low-excitation photoluminescence (PL) peak energy increased moderately with decreasing well thickness L and the PL decay time did not depend on L. The results indicated that piezoelectric field had negligible influence on the transition processes. The time-resolved photoluminescence (TRPL) signal showed a stretched exponential decay up to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures even at 300 K. Decrease in internal quantum efficiency (η), which is related to the increase in quasi-radiative lifetime, with the increase in temperature was explained to be due to reduced localization rate.
2 citations