Showing papers by "Shizuo Fujita published in 1999"

Growth of ZnO by Molecular Beam Epitaxy Using NO2 as Oxygen Source

Abstract: Nitrogen dioxide was applied for molecular beam epitaxy of ZnO on (0001) sapphire substrates. Equipped with liquid nitrogen cryoshrouds, the pressure inside the chamber was kept below 1×10-7 Torr during growth. The samples were characterized by reflection high energy electron diffraction, X-ray diffraction, photoluminesence and scanning electron microscopy. The ZnO crystals clearly showed excitonic optical properties up to room temperature. Effects of low-temperature buffer layers for better surface morphology and reproducibility were also investigated.

Energy states in ZnSe-GaAs heterovalent quantum structures

Abstract: We investigate the energy states in ZnSe-GaAs heterovalent quantum structures. In the ZnSe-GaAs heterovalent heterostructures, the band offsets are controllable, and therefore different offsets can artificially be put at both sides of the GaAs wells through the control of the growth sequence forming the interfaces, which brings unique characteristics on the heterovalent quantum structures. The characteristic features are examined by the optical-absorption measurement. Dependence of the absorption edge on the offsets and the well width reveals the presence of an electric field; by changing the valence-band offset at one side of the GaAs well from 0.6 to 1.1 eV, while keeping that at the other side constant at 0.6 eV, we observe that the absorption edge energies are shifted toward lower energy and that the degree of the redshift depends on the GaAs well width. These experimental results are well explained by the theoretical calculation of the Poisson and Schr\"odinger equations. On the other hand, the barrier width dependence exhibits the anomalous behavior that cannot be expressed by the theoretical analyses and is interpreted in terms of the charge transfer within the barrier region.

Tunable band offsets via control of interface atomic configuration in GaAs-on-ZnSe(001) heterovalent heterostructures

Abstract: The tunability of band offsets in GaAs/ZnSe(001) heterovalent heterostructures is investigated. The surface of GaAs grown on ZnSe by metalorganic vapor phase epitaxy is atomically flat under the condition employed in this study, which is indicative of the formation of the abrupt interface. Between the growth of ZnSe and GaAs, interval is introduced for purging a Se source precursor. After the interval, the GaAs growth begins with an initial exposure of ZnSe to an As precursor. The durations of the interval and the As exposure are the parameters to control the interface chemistry. During the interval, atoms on the ZnSe surface change from Se to Zn. As the duration of the As exposure lengthens, on the other hand, either Zn or Se atoms appear alternately on the ZnSe surface, because the As precursor etches ZnSe digitally. Using these characteristics, the valence band offsets in GaAs/ZnSe(001) heterostructures are controlled between 0.6 and 1.1 eV.

Photoluminescence Dynamics of Aluminumquinoline/Oxadiazole Multilayer Structures

Abstract: Photoluminescence (PL) dynamics of aluminumquinoline/oxadiazole (Alq3/PBD) multilayer structures, which may possess a carrier confinement type-I energy lineup, has been investigated by time-resolved PL spectroscopy with a mode-locked Ti-sapphire laser. The decay time of the emission from PBD in the multilayer structure was as fast as several picoseconds, in contrast to that from a PBD single layer, 1.7 ns, and tended to increase with an increase in PBD thickness. For the emission from Alq3, on the other hand, only a slight increase in the decay time was observed in the multilayer structure compared to a single layer. These results indicate the rapid energy transfer from PBD to Alq3, similar to that in semiconductor quantum well structures with the type-I energy lineup.

Electrical Characterization of MOVPE-Grown P-Type GaN:Mg Against Annealing Temperature

Abstract: Hall effect measurements have been applied for the electrical characterization of p-type Mg-doped GaN grown by metalorganic vapor-phase epitaxy on sapphire substrates in terms of annealing temperature for dehydrogenation (N2 annealing) and hydrogenation (H2 annealing) of the acceptors. With the N2 annealing temperature from 600 to 900 °C for dehydrogenation, both hole concentration and mobility increases, showing more activation of acceptors and less incorporation of unfavorable scattering centers probably originating from Mg-H bondings. The N2 annealing at higher than the growth temperature results in reduced hole concentration, but the mobility gets higher. Some defects compensating acceptors may be induced at high temperature annealing, but they seem to be no scattering centers and be inactivated by successive hydrogenation and re-dehydrogenation at the optimum dehydrogenation temperature 900 °C. The electrical degradation of GaN due to thermal damage is not very destructive and can be well recovered by annealing treatments.

Luminescence Dynamics of Alq3-Based Multilayer Structures in Terms of HOMO and LUMO Energy Discontinuity

Abstract: Optical properties of organic multilayer structures with type-I and type-II energy lineups with respect to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were investigated and compared. The fast energy transfer followed by carrier confinement in the material with lower HOMO-LUMO energy difference was proved for the structures with type-I energy lineup, while the tendency of separation of excited states, i.e., electrons and holes generated in the excited molecules, was demonstrated for those with type-II. These results indicate that the luminescence dynamics of organic multilayer structures are well discussed in terms of HOMO and LUMO energy lineups similarly to valence and conduction band energies in inorganic semiconductors.