Masakazu Ohishi

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.

Photo-assisted MBE growth of ZnSe crystals

Abstract MBE growth of ZnSe on GaAs under the irradiation of ultraviolet light has been performed to control native defects inherent in II–VI compounds. UV light irradiation enhances the surface migration of impinging atoms, which results in an improvement of the epilayer quality and in an increase of the growth rate. High growth rate, however, degrades surface morphology compared with that of unirradiated samples.

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.

Homo-epitaxial growth of ZnSe by MBE

Abstract Homo-epitaxial growth of ZnSe on melt-grown ZnSe single crystal with (110) or {111} surfaces orientation has been performed to obtain strain-free layers. Epitaxial growth was successful only on ( 1 1 1 ) and (110) substrate surfaces, which were prepared by chemical etching using a chromic acid mixture or a mixture of ammonia and hydrogen peroxide solution. Photoluminescence spectra measured at 4 K showed the strong Y and S lines and a broad band on the low energy side. While the layers of ZnSe grown on GaAs substrates with smooth surfaces showed only strong and sharp excitonic lines, growth on the rough ZnSe surface introduced disorder in the epitaxial layer degrading the luminescence response.

Time-Resolved Studies on Recombination Luminescence of Donor-Acceptor Pairs in ZnSe

Time-resolved studies of luminescence due to the recombination of donor-acceptor pairs in ZnSe are reported. Several sharp luminescence lines were observed under excitation below the band-gap energy, resulting in a determination of the binding energy of the Al donor to be 26.7 meV and that of Na acceptor to be 114 meV. It was also confirmed from analyses of the decay characteristics of the donor-acceptor pair luminescence that the hydrogenic model for energy levels as well as for the Bohr radius holds true for the Al donor. However, for the Na acceptor the Bohr radius which satisfied the experimental decay kinetics should be less than half of the value calculated using the effective-mass approximation.

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.

Investigation of the Luminescence in the SrS: Bi3+ Phosphor by Time-Resolved Emission Spectroscopy

The details of the luminescence spectra and the decay characteristics of the luminescence have been examined over a wide range of temperature. At low temperatures, the luminescence decay curve after the A -band excitation has two components, fast and slow. The difference between the decay times, about 10 ns and 4.1 ms at 4.2 K, enables one to decompose the emission spectrum into the 3 T 1u → 1 A 1g and 3 A 1u → 1 A 1g bands by time-resolved emission spectroscopy. The zero-phonon line at 445.3 nm of the former band and the zero-phonon line ( A 0 ) at 460.7 nm and the one-phonon line at 465.0 nm of the latter band are observed at 6 K. The A 0 line grows under an external magnetic field. The optical excitation mechanism has been also examined by observing the excitation spectra at various temperatures.

Molecular Beam Epitaxially Grown ZnSe(001) Surface Studied by the In Situ Observation of RHEED Intensity

The desorption of Zn and Se atoms or molecules on the (001) surface of a ZnSe/GaAs epitaxial layer were studied by means of reflection high-energy electron diffraction (RHEED). The temporal behavior of the specular intensity observed from two azimuths, [110] and [10], was well understood by taking the surface condition into consideration; i.e., the surface terrace is elongated toward the [10] direction and excessively adsorbed Se molecules exsist as well. We also confirmed that a higher-energy electron beam enhances the desorption of Se molecules and adatoms.

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.

Ultraviolet irradiation effect on the MBE growth of ZnSe/GaAs observed by RHEED

Abstract Ultraviolet light irradiation effects on the MBE growth of ZnSe/GaAs(001) were investigated by means of RHEED observation. One of the prominent effects induced by the UV irradiation is to enhance the desorption of Se molecules. The other is that the decay of the RHEED oscillations becomes faster. These results imply the enhanced possibility of three-dimensional growth, which leads to the inferior surface morphology of the ZnSe/GaAs epilayers grown under photo-irradiation. Possible speculation on photo-MBE growth mechanisms is also given.