Minoru Yoneta

Low temperature molecular beam epitaxial growth of ZnS/GaAs(001) by using elemental sulfur source

Abstract We report successful growth of ZnS on GaAs(001) using elemental sulfur and zinc sources. In order to use S, which has a very high vapor pressure for molecular beam epitaxy, a new type of effusion cell is developed. The cell is composed of the effuser and post-heating zones. Not only the control of vapor pressure, but also bakeout of the MBE chamber could be achieved. ZnS/GaAs could be grown at temperatures as low as 150°C using S beam post-heated at 210°C and conventional Zn beam. Low temperature photoluminescence shows that high quality ZnS films are grown.

Optical Characterization of the ZnTe Pure-Green LED

The photoluminescence (PL), piezoelectric photothermal (PPT) and transmittance measurements were carried out at room temperature for ZnTe light emitting diode fabricated by the thermal diffusion of Al on p-type substrates. The same transmittance spectra were observed when the light was illuminated to both the Al-diffused side and the other sides without Al. No signals due to impurities were present in the transmittance spectra. However, a signal due to impurities was clearly observed at 2.15 eV in the PPT spectrum. This signal which had an activation energy of 110 meV was not seen in the PL spectrum. Therefore, it was deduced that the defect acted as a nonraditive recombination center.

Low temperature growth of ZnSe/GaAs using hot molecular beams

Abstract Epitaxial layers of ZnSe/GaAs were grown successfully at the growth temperature as low as of 100°C using hot molecular beams generated by post-heating at 600°C both for Zn and Se. Although having rather inferior surface morphology, ZnSe epilayers grown at 100–150°C showed good low-temperature photoluminescence properties. The relation between the surface morphology and the growth rate, and also the role of the post-heated Se beam are discussed.

Li-acceptor doping in ZnS/GaAs by post-heated molecular beam epitaxy

Abstract Li-doping of ZnS/GaAs layers by means of molecular beam epitaxial (MBE) growth using post-heated molecular beams is reported. Low-temperature photoluminescence measurements and secondary ion mass spectrometry have clarified that the Li atoms act as acceptors and donor Ga is incorporated from the substrate. It is also found that under higher doping levels the growth rate is considerably reduced. Together with the reflection high-energy electron diffraction (RHEED) results, this behavior is attributed to some interaction between Li and S atoms on the growing surface.

Optical characterization of native defects in ZnSe substrate

The photoluminescence (PL) and piezoelectric photothermal (PPT) spectroscopy of the nondoped and Al-doped ZnSe single crystals were carried out between liquid helium and room temperatures. Two signals were obtained in the Al-doped ZnSe crystal in the PPT spectrum at liquid nitrogen temperature. These signals were due to the Al-related defects because the signals were not observed in the nondoped ZnSe crystal. The emission band which was due to the Al-related defects were observed in the PL spectrum at liquid nitrogen temperature. Therefore, it was deduced that the Al atom in the ZnSe crystal acted as the nonradiative and radiative carrier recombination centers at liquid nitrogen temperature. At room temperature, one broad band, Al-related defects, was clearly observed in the PL spectrum of the Al-doped ZnSe crystal. On the other hand, no emission peak was present in the nondoped ZnSe crystal. Therefore, the PL emission, the radiative carrier recombination process, could be observed by the Al-doping. In addition, it was clear that the PPT signal, the nonradiative carrier recombination process, decreased by the Al-doping.

On the growth mechanism of Li- and Na-doped Zn chalcogenides on GaAs(001) by means of molecular beam epitaxy

Sharp and semicircular patterns were observed in RHEED during the MBE growth of Li- or Na-acceptor doped ZnSe and ZnS on GaAs(001). The radius and the separation between the diffraction circles vary with the change of the azimuth of the incident electron beam. Calculated diffraction patterns assuming that Li or Na atoms are arrayed one-dimensionally along the [110] direction of the crystal axis are in good agreement with the experimental results. We conclude that Li or Na atoms are incorporated at the [110] terrace steps, which prevents the further growth from the step edge.

Growth and characterization of SPR-ZnS bulk crystal

The solid-phase recrystallization (SPR) method was used to grow ZnS bulk crystals. Scanning electron microscopy, polarized microscope and photoluminescence (PL) assessed the characterization of solid-phase recrystallized ZnS. A larger single grain has been grown by higher temperature annealing under sulfur atmosphere, and the grain growth mechanism was discussed based on both the annealing temperature and the atmosphere. The low-temperature PL indicated that the ZnS crystals grown by SPR was of high quality.

Auto-doping of Ga in ZnSe/GaAs layers grown at low temperatures by post-heated molecular beam epitaxy

Abstract The donor species in Li acceptor-doped ZnSe epilayers grown on GaAs substrate are determined by means of the secondary ion mass spectroscopy (SIMS). The compositional depth profile by the SIMS points out that a large amount of Ga atoms are incorporated into the ZnSe epilayer regardless of doping. In doped epilayers, the Ga atoms are concentrated at the region where the Li atoms are doped. We conclude that Ga atoms originating from the droplets due to the thermal cleaning of the substrate are most likely the major donor species.

Nature of oxygen donor in Czochralski-grown silicon

Changes in the substitutional carbon and interstitial oxygen concentrations due to the formation of oxygen donors were studied on carbon-rich Czochralski-grown silicon (7-9×1017 carbon atoms/cm3). The result suggests that the new donor formation is controlled not by the substitutional carbon concentration directly, but by a density of some unknown embryo. A donor formation was observed in a 450°C annealing which was preceded by a 650°C annealing without new donor formation. This donor does not exhibit a 0.767 eV line in a measurement of the photoluminescence spectrum. However,it exhibits an infrared absorption band which is characteristic to the thermal donor.

Growth and Annealing of ZnO Films Grown by Spray Pyrolysis

Undoped and In-doped ZnO films on glass substrate were successfully grown by spray pyrolysis at 500 °C. The samples were annealed under N2 atmosphere between 100 and 600 °C for 5 min. The lattice constants and grain sizes of both samples were unchanged with increasing annealing temperature. However, the photoluminescence intensity of the In-doped sample markedly decreased in comparison with that of the undoped sample with increasing annealing temperature. The carrier concentration increased with increasing annealing temperature. These findings indicated that the number of nonradiative recombination centers of the In-doped ZnO samples markedly increased.