Takeshi Karasawa

Characteristics of p-type ZnSe Layers Grown by Molecular Beam Epitaxy with Radical Doping

p-type ZnSe layers have been grown by molecular beam epitaxy using nitrogen radical doping. We have employed Pt as the electrode material for p-type ZnSe. The Pt electrodes markedly reduced contact resistances. The p-type conduction was confirmed by Hall measurement. Carrier concentration was 4.4×10 15 cm −3 . Hall mobility was a high as 86 cm 2 /V•s because of good crystallinity. The p-type ZnSe layers exhibited the 2.616-eV emission from recombination between free electrons and acceptor holes (FA) in room-temperature photoluminescence measurement. The FA emission provides evidence that the layers are p-type ZnSe

Doping of nitrogen acceptors into ZnSe using a radical beam during MBE growth

A new method of doping for ZnSe was attempted by using a neutral radical beam during the MBE growth. The radical beam dominantly consisted of N2 molecular radicals at A3σ+u state. The sticking coefficient of nitrogen was remarkably enhanced; thus this doping method was able to incorporate N into ZnSe by 1019 cm−3. The existence of shallow N acceptors was confirmed by photoluminescence measurements; recombination of free electrons and acceptor holes (FA) at room temperature and recombination of donor-acceptor pairs at low-temperature were observed. The FA emission was observed only for ZnSe layers with moderate doping level, which shows p-type conduction. The carrier concentration was the order of 1015 cm−3. The activation of N in ZnSe was less than 1%.

Hydrogen Gas Generation by Splitting Aqueous Water Using n-Type GaN Photoelectrode with Anodic Oxidation

Hydrogen gas generation from a counterelectrode was clearly observed for the first time using light-illuminated n-type GaN as a working photoelectrode in an electrolyte. The application of extra bias to a working electrode was required to obtain a sufficient volume of generated gas. The reactions at the GaN photoelectrode were both GaN decomposition and water oxidization, simultaneously.

Effects of Material Systems on the Polarization Behavior of Holographic Polymer Dispersed Liquid Crystal Gratings.

This study has investigated the effects of the material systems involved in the fabrication processes on the polarization behavior of a volume holographic grating. The diffraction efficiency of the gratings fabricated using prepolymer/liquid crystal mixtures shows strong dependence on the polarization of incoming light. Depending on the materials used in the formation of a grating, the diffraction properties are such that either p- or s-polarized light is strongly diffracted while the light with the other polarization is very weakly diffracted. The magnitude of the dependence on the polarization is greatly affected by the type of monomers, liquid crystals and substrates. The comparison of various types of monomers added to the base prepolymer mixtures, two distinctly different types of liquid crystals and glass slides and indium-tin oxide (ITO) coated glass as substrates was carried out using polyester-based and urethane-based oligomers.

Molecular-beam epitaxial growth and characterization of ZnS-ZnxCd1-xS strained-layer superlattices

ZnS‐ZnxCd1−xS (x=0.56) superlattices with the same barrier (ZnS) thickness (47 A) and varied well (ZnxCd1−xS) thicknesses (23–80 A) were grown by molecular‐beam epitaxy. Reflection high‐energy electron diffraction patterns during the growth showed streaks, and superlattice structure was confirmed by the presence of satellites in the x‐ray diffraction profile. Low‐temperature photoluminescence (PL) peaks from near‐band‐edge emission shift to lower energy as the well thickness increases, suggesting that the emission is related to confinement in the wells. The full width at half maximum of PL lines increased with the increase in well thickness. A Raman spectrum from a single alloy layer showed two peaks (ZnS‐ and CdS‐like) which also appeared clearly in superlattice spectra. The positions of these two superlattice peaks shift from the single alloy value to higher energy as the well thickness decreases.

Homoepitaxial growth of ZnSe on dry‐etched substrates

High quality ZnSe layers have been grown by molecular beam epitaxy on dry‐etched ZnSe substrates. Surface damage caused by cutting and polishing of the ZnSe substrate was removed by dry etching using BCl3 gas to 10 μm depth. The dry‐etched ZnSe substrates exhibited smooth surface morphology and showed excitonic emissions stronger than that from as‐polished substrates in photoluminescence (PL) measurements at 11 K. The low‐temperature PL spectra obtained from homoepitaxial ZnSe layers grown on the substrates dry etched at the optimum condition showed a strong free‐exciton emission at 2.804 eV and a dominant donor‐bound exciton emission at 2.798 eV. Since each excitonic emission shows a single peak, the homoepitaxial layers appear to be free from strain.

MBE-grown ZnTe-ZnS strained-layer superlattices

Abstract RHEED pattern of ZnTe-ZnS strained-layer superlattices showed streaks and reconstruction after only five periods of deposition. The surface lattice constant obtained from RHEED streak separation showed different values during ZnTe or ZnS deposition beyond a certain thickness of the superlattice. X-ray diffraction data showed satellite peaks around the central peak, indicating good superlattice structure.

Treatment of ZnSe substrates for homoepitaxy

We have developed a novel treatment of ZnSe substrates for homoepitaxy using dry etching with a BCl3 plasma. The dry‐etched substrates exhibited mirror‐like morphology even after 10‐μm etching in thickness to remove polishing damage, and the quality of its surface was remarkably improved. Subsequent thermal etching of ZnSe was found by reflection high‐energy electron diffraction observation to be valid in the temperature range from 440 to 650 °C. Highest‐quality ZnSe homoepitaxial layer has been obtained by growth on the substrate etched at the BCl3 pressure of 60 mTorr. Crystallinity of the layer was as good as that of the dry‐etched substrate. Low‐temperature optical analysis indicated the layer to be high purity and to be free from in‐plane biaxial strain.

Characterization of short-period ZnTe-ZnS superlattices grown by MBE

Structural properties of short period ( <30 A) ZnTe-ZnS strained-layer superlattices (SLSs) have been examined by Raman spectroscopy and low temperature photoluminescence (PL). The longitudinal optical (LO) phonon of ZnS shows a stress-induced frequency shift, which is in close agreement with theoretical calculation. Although ZnTe LO phonon frequency shifts depending on the SLS structure, there is a discrepancy between experiment and calculation, implying a relaxation in the layer. PL spectra are dominated by one intense peak, and the peak energy shows a trend to shifts according to layer thicknesses and SL period as expected from Kronig-Penney model, but the energy value is lower.

Gigahertz single-electron pumping in silicon with an accuracy better than 9.2 parts in 107

High-speed and high-accuracy pumping of a single electron is crucial for realizing an accurate current source, which is a promising candidate for a quantum current standard. Here, using a high-accuracy measurement system traceable to primary standards, we evaluate the accuracy of a Si tunable-barrier single-electron pump driven by a single sinusoidal signal. The pump operates at frequencies up to 6.5 GHz, producing a current of more than 1 nA. At 1 GHz, the current plateau with a level of about 160 pA is found to be accurate to better than 0.92 ppm (parts per million), which is a record value for 1-GHz operation. At 2 GHz, the current plateau offset from 1ef (∼320 pA) by 20 ppm is observed. The current quantization accuracy is improved by applying a magnetic field of 14 T, and we observe a current level of 1ef with an accuracy of a few ppm. The presented gigahertz single-electron pumping with a high accuracy is an important step towards a metrological current standard.