B. P. Zhang

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.

Naturally formed ZnCdSe quantum dots on ZnSe (110) surfaces

We successfully realized ZnCdSe quantum dots on a cleavage-induced ZnSe (110) surface by depositing a ZnSe/ZnCdSe/ZnSe heterostructure under growth conditions that cannot lead to layer-by-layer growth of ZnSe. This growth mode introduces surface roughness to the newly deposited ZnSe layer, and ZnCdSe quantum dots are then formed. Cathodoluminescence and microphotoluminescence measurements demonstrate the formation of quantum dots.

Temperature-dependent photoluminescence of ZnO layers grown on 6H-SiC substrates

This work was supported in part by the Special Postdoctoral Research Fellowship Program, Photodynamics Research Center, The Institute of Physical and Chemical Research, Japan.

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.

Epitaxial Growth and Polarity of ZnO Films on Sapphire (0001) Substrates by Low-Pressure Metal Organic Chemical Vapor Deposition

Epitaxial ZnO films are grown on sapphire (α-Al2O3) (0001) substrates by metal-organic chemical vapor deposition under 6 and 0.05 Torr using diethyl zinc (DEZn, (C2H5)2Zn) and O2 as precursors and nitrogen as carrier gas. Measurements by X-ray diffraction and photoluminescence reveal that films grown under 6 Torr have better crystal quality. In ZnO films grown under both pressures, free excitonic transitions are observed by reflection. However, the emission behaviors are different. Low-temperature photoluminescence of the ZnO film grown under 6 Torr is dominated by donor-bound exciton emission, whereas that grown under 0.05 Torr exhibits strong acceptor-bound exciton emission, indicating pronounced acceptor incorporation in the latter. In-plane orientations of the ZnO films are well controlled by adjusting the growth process. Measurements by coaxial impact collision ion scattering spectroscopy (CAICISS) demonstrate that the ZnO films have +c polarity, regardless of their in-plane orientation. This finding differs from the results obtained by laser-molecular beam epitaxy where the polarity was found to depend on the in-plane orientation.

Fabrication of SiGe-on-insulator through thermal diffusion of Ge on Si-on-insulator substrate

We report on the fabrication of a homogeneous SiGe-on-insulator as a substrate for strained Si-on-insulator (SOI) metal-oxide-semiconductor field-effect-transistors. The fabrication process includes the growth of a thin Ge film on a commercially available SOI substrate at 100°C using a molecular beam epitaxy system, the formation of a SiO2 cap layer by radio-frequency sputtering, and rapid thermal annealing (RTA) in an Ar atmosphere. After RTA at an appropriate temperature, the SiGe-on-insulator with a laterally homogeneous Si fraction was successfully obtained by the formation of epitaxial SiGe on a thin SOI as a seed and interdiffusion of Ge and Si atoms. However, inhomogeneous SiGe films were obtained when the annealing temperature was very high. The conditions for the realization of SiGe with a homogeneous Si fraction were found to be closely related to the phase diagram of the Si–Ge binary alloy.

A new approach to ZnCdSe quantum dots

Abstract We propose a new approach of fabricating ZnCdSe quantum dots (QDs) on ZnSe and GaAs (110) surfaces by simply depositing a ZnSe/ZnCdSe/ZnSe heterostructure. The growth conditions are selected to introduce surface roughness on the over grown ZnSe which allows the formation of ZnCdSe QDs. Optical studies unambiguously demonstrate the formation of QDs. Resolution-limited sharp emission lines are observed by micro-photoluminescences and the linewidths are much less than the thermal energy. As time goes on, intermittent behaviors of the QD emissions are observed. A proper selection of growth conditions is essential in obtaining ZnCdSe QDs by this method, especially on GaAs (110) surfaces.

Intermittent photoluminescence and thermal ionization of ZnCdSe/ZnSe quantum dots grown by molecular beam epitaxy

We report the intermittent photoluminescence of ZnCdSe quantum dots (QDs) embedded in a ZnSe matrix grown by molecular beam epitaxy. The luminous time of the QD is strongly dependent on temperature but not on excitation intensity. This indicates that the ionization of the QDs is determined predominantly by thermal excitation of carriers into the ZnSe matrix.

SiGe bulk crystal as a lattice-matched substrate to GaAs for solar cell applications

SiGe bulk crystal fabricated by a multicomponent zone-melting method was used as a substrate for epitaxial growth of GaAs. Compared with conventional GaAs/Ge heterostructure, the lattice mismatch of GaAs/Si0.022Ge0.978 was confirmed to be reduced by a decrease of the peak separation of (400) x-ray diffraction from the epitaxial GaAs layer and the substrate. Furthermore, the linewidth of the rocking curve of GaAs on SiGe was found to be narrower than that of GaAs on Ge. These results show that SiGe is promising material as an alternative substrate to Ge for realization of exactly lattice-matched GaAs/SiGe solar cells.