K. Wakatsuki

Optical properties of ZnO rods formed by metalorganic chemical vapor deposition

High-quality ZnO rods were formed directly on sapphire (0001) substrates by metalorganic chemical vapor deposition. The rods exhibited free exciton and very sharp bound exciton emissions at low temperatures. By increasing the excitation intensity, biexciton emission was observed. Temperature dependence of the emission spectra suggested that the emission peak at ∼3.315 eV, which had been attributed to neutral acceptor-bound exciton emission, is due to donor-acceptor pairs. The acceptor binding energy was determined to be about 107 meV, which agrees well with that estimated from a hydrogen-atom-like acceptor model.

Formation of highly aligned ZnO tubes on sapphire (0001) substrates

ZnO tubes were epitaxially grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. The tubes grew along the substrate normal and were characterized by hexagon-shaped cross sections. All of the tubes possessed the same epitaxial relationships with respect to the substrate. Both reactor pressure and growth temperature were found to play an important role in the formation of ZnO tubes. Spiral column growth mode was found to be responsible for the formation of ZnO tubes.

Low-temperature growth of ZnO nanostructure networks

Networks consisting of one-dimensional ZnO nanowires and two-dimensional ZnO nanowalls were synthesized using a catalyst-free low-temperature approach. The size of the nanostructure was much smaller than that obtained by the previous catalyst-assisted method. The nanostructures exhibited stable excitonic states at room temperature, and emission due to exciton-exciton scattering was observed.

High-Quality ZnO Layers Grown on 6H-SiC Substrates by Metalorganic Chemical Vapor Deposition

In this letter, the 6H-SiC substrate has been studied for the growth of the ZnO layer. The X-ray diffraction measurement clearly showed the c-axis oriented growth of ZnO layers on SiC(0001) substrates. The X-ray rocking curve measurement diffracted a smaller full-width at half maximum of the ZnO layers grown on SiC than that of the same layers grown on the conventional Al2O3 substrate with metalorganic chemical vapor deposition (MOCVD). A distinct free-exciton emission was dominantly observed even at room temperature (RT) while the donor-bound-exciton peaks were disappeared at around ~120 K. In addition, no deep-level emission was observed even at RT in the ZnO/SiC samples. These optical and crystalline properties have hardly been observed in the ZnO/Al2O3 samples grown by MOCVD. Therefore, the higher quality of the ZnO layers grown on SiC might be attributed to the smaller lattice mismatch of ~5% as well as the +c surface orientation in ZnO/SiC sample geometry.

Structures and Photoluminescence Properties of ZnO Films Epitaxially Grown by Atmospheric Pressure MOCVD

High-quality ZnO (1120) epitaxial films have been grown on sapphire (0112) substrate by atmospheric pressure MOCVD method using zinc acetylacetonate (Zn(C 5 H 7 O 2 ) 2 ) and oxygen. The crystallinity of the ZnO films was considerably improved with increasing the film thickness. The full width at half maximum (FWHM) obtained from the ω rocking curve of X-ray diffraction indicated a minimum value of 0.35° at the film thickness of 0.30 μm. The lattice constant estimated from the diffraction angle 20 was almost equivalent to the value of single crystalline bulk ZnO. The room temperature photoluminescence spectrum of the ZnO films consists of band edge emission at 380.08 nm. The low-temperature (4.2 K) photoluminescence spectrum indicates two broad lines of 3.31 and 3.32 eV from donor-acceptor pairs and one sharp line of 3.363 eV from exciton bound nature donor.

Structural and optical properties of ZnO films grown on R–Al2O3 substrates

Structural and optical properties of ZnO films grown on R–Al2O3 substrates by atmospheric pressure chemical-vapor deposition were investigated using x-ray diffraction and photoluminescence. The (1120) plane of the ZnO film tilted 0.3° with respect to the (1102) plane of the substrate and rotated about 7° around the normal of the sample surface. Symmetric (1120) and asymmetric (2022) x-ray reflection on ZnO films with different thicknesses were carried out. Comparison with photoluminescence measurements allowed us to conclude that the optical properties of the ZnO films are predominately determined by the in-plane, rather than out-of-plane, structural features.

Structural and optical properties of ZnO epitaxial films grown on Al2O3 (1120) substrates by metalorganic chemical vapor deposition

ZnO films of different thicknesses were grown on Al2O3 (110) substrates by metalorganic chemical vapor deposition. Characterizations using X-ray diffraction and scanning electron microscopy demonstrated the transition from two-dimensional growth to three-dimensional growth with an increase in film thickness. Photoluminescence spectra revealed the thickness dependence of the exciton peak and a stimulated emission due to exciton-exciton scattering was observed.

Temperature dependence of the exciton decay times of different quantum structures coexisting in a ZnSe/CdSe/ZnSe heterostructure

Study by time-resolved photoluminescence shows that two classes of quantum structures coexist in a ZnSe/CdSe/ZnSe heterostructure in which the CdSe coverage is less than the critical thickness. Excitons from class-A structures dominate the emission spectrum and exhibit temperature-independent decay times, demonstrating quantum-dot-like properties. On the other hand, excitonic transitions from class-B structures are characterized by decay times that depend linearly on temperature, indicating two-dimensional features. There is a sharp transition from class-A to class-B excitons as the emission energy goes from higher to lower energies across the emission band.

Nanostructures formed on CdSe/ZnSe surfaces

Nanometer scale islands were observed in air on CdSe/ZnSe surfaces grown by molecular beam epitaxy. Measurements by atomic force microscope revealed that the aspect ratio of the island increased first and then decreased with time. Surface analyses by Auger electron spectroscopy indicated clearly that the island was composed in large part of oxides and the outer shell of the island was Zn-enriched and Cd-depleted. These islands showed sharp emissions as quantum dots. This work demonstrates that the islands were formed mainly due to the oxidation of the surface and that it grew in a way similar to that generated from a strained film. In addition, intermixing of ZnSe and Zn segregation during the island formation occurred.

Growth and Characterization of ZnMgSe Alloys and ZnSe/ZnMgSe Multi-Quantum Wells

ZnMgSe alloys and ZnSe/ZnMgSe multi-quantum well (MQW) structures were grown on GaAs (001) substrates by solid-source molecular beam epitaxy (MBE). It was found that the content of Mg in ZnMgSe alloys can be well controlled by adjusting the beam fluxes of element sources. X-ray reflection spectra from MQWs revealed strong satellite peaks, indicating good structural quality. Due to the quantum confinement effect, the photoluminescence (PL) peaks from the MQWs were shifted with variation in well width. Small Stokes shifts were identified between the transition energies observed by PL and reflectance. The interaction between excitons and LO phonons in ZnSe/ZnMgSe MQWs was greatly reduced compared with that in ZnSe bulk.