Ayumu Tsujimura

Suppression of Inhomogeneous Segregation in Graphene Growth on Epitaxial Metal Films

Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films.

Optical Properties of CdS Microcrystallite-Doped SiO2 Glass Thin Films

The optical properties of SiO2 glass thin films containing CdS microcrystallites have been investigated. The thin films were deposited onto SiO2 and GaAs substrates using an rf-sputtering technique. X-ray photoelectron spectroscopy and X-ray diffraction measurement indicated that the Cd and S atoms were doped as CdS with a hexagonal system in SiO2 glass matrixes. In the optical absorption spectra, the absorption edge of the films with 18.5 at% of CdS clearly exhibited a blue shift by about 0.13 eV in comparison with that of the bulk CdS. In accordance with this result, the photoluminescence spectra of the films exhibited a blue band-edge emission at a higher energy than that of the bulk CdS. The shift to higher energy seems to be a low-dimensional quantum size effect due to confinement of electrons and holes in CdS microcrystallites.

Room Temperature 339 nm Emission from Al0.13Ga0.87N/Al0.10Ga0.90N Double Heterostructure Light-Emitting Diode on Sapphire Substrate

Room-temperature deep-ultraviolet emission has been observed from Al0.13Ga0.87N/Al0.10Ga0.90N double heterostructure light-emitting diodes (LEDs) on (0001)-oriented sapphire substrate. By introducing undoped barrier layers, which sandwich the active layer, the LED was operated at a peak emission wavelength of 339 nm with a narrow linewidth of 5.6 nm. The dependence of emission intensity on injection current suggests that the nonradiative recombination was suppressed and the diffusion current for the recombination process was dominant at the injection current of over 20 mA.

ZnSe-based laser diodes and p-type doping of ZnSe

Abstract Highly conductive p-type ZnSe layers have been grown by molecular beam epitaxy with nitrogen radical doping. Active nitrogens responsible for doping are N 2 metastables in the A 3 Σ + u state. The free-hole concentration of N-doped ZnSe is of the order of 10 17 cm −3 at room temperature. Laser diode action has been observed from ZnCdSe single quantum well structures grown on GaAs substrates without GaAs buffer layers. Coherent light was observed at 490–520 nm at 77 K. The minimum threshold current density was as low as 160 A/cm 2 under pulsed operation.

Improvement of Crystalline Quality in GaN Films by Air-Bridged Lateral Epitaxial Growth

Air-bridged lateral epitaxial growth (ABLEG), a new technique of lateral growth of GaN films, has been developed using low-pressure metalorganic vapor phase epitaxy. A previously grown 1-µm-thick GaN film is grooved along the GaN direction, and the bottoms of the trenches and the sidewalls are covered with a silicon nitride mask. A free-standing GaN material is regrown from the exposed (0001) surface of the ridged GaN seed structure. Cross-sectional transmission electron microscopy analysis reveals that the dislocations originating from the underlying seed GaN extend straight in the direction and dislocations do not propagate into the wing region. The wing region also exhibits a smooth surface and the root mean square roughness is found to be 0.088 nm by atomic force microscopy measurement of the (0001) face of the wing region.

Electrically Pumped CdZnSe/ZnSe Blue-Green Vertical-Cavity Surface-Emitting Lasers

We demonstrate an electrically pumped CdZnSe/ZnSe blue-green vertical-cavity surface-emitting laser (VCSEL) with a SiO2/TiO2 dielectric multilayer mirror. Electrically pumped lasing was achieved at 77 K with a threshold current of 3 mA (pulsed). A low far-field radiation angle of 7° was observed above the threshold in a 10-µ m-diameter device, which indicates the spatial coherence expected for lasing.

Zn1-XCdXSe (X=0.2-0.3) Single-Quantum-Well Laser Diodes without GaAs Buffer Layers

Laser diode action has been observed from a ZnSe-based single-quantum-well structure grown on GaAs substrates without GaAs buffer layers. The lasers emit coherent light under pulsed current injection at 77 K in a wide wavelength range from 490 nm to 520 nm depending on the quantum well composition ratio. In spite of uncoated facets, the lowest threshold current density of the lasers was as low as 160 A/cm2. The output power from the lasers exceeded 100 mW per facet.

Dynamic Observation of Brain-Like Learning in a Ferroelectric Synapse Device

A brain-like learning function was implemented in an electronic synapse device using a ferroelectric-gate field effect transistor (FeFET). The FeFET was a bottom-gate type FET with a ZnO channel and a ferroelectric Pb(Zr,Ti)O3 (PZT) gate insulator. The synaptic weight, which is represented by the channel conductance of the FeFET, is updated by applying a gate voltage through a change in the ferroelectric polarization in the PZT. A learning function based on the symmetric spike-timing dependent synaptic plasticity was implemented in the synapse device using the multilevel weight update by applying a pulse gate voltage. The dynamic weighting and learning behavior in the synapse device was observed as a change in the membrane potential in a spiking neuron circuit.

Complex Flow and Gas-Phase Reactions in a Horizontal Reactor for GaN Metalorganic Vapor Phase Epitaxy

Three-dimensional fluid simulations are performed in a horizontal reactor for GaN epitaxy. Attention is paid to the effect of gas flow velocity at the inlet and gas pressure. It is found that the gas flow rate rather than the velocity or the pressure is a key parameter which controls the spatial distribution of streamlines, temperature and gas-phase species. As the gas flow rate increases, the size of return-flow or flow-separation appearing near the gas entrance of the expansion region with a tapering angle increases. This causes velocity peaking near the reactor symmetry plane and complicated transport of gas-phase species along the streamlines of the return-flow. If an optimum gas flow rate which gives minimum return-flow and uniform macroscopic spatial distribution for flow pattern and gas-phase species can be determined, then it is desirable to change the gas flow velocity and the gas pressure on the condition that the gas flow rate is maintained.

Compound source molecular beam epitaxy for II–VI laser structures

Abstract Compound source molecular beam epitaxy (CSMBE) for II–VI laser structures is developed. This method employs compounds as source materials instead of elements. The surface of ZnSe during CSMBE at 100–350°C is Se-stabilized, in spite of the VI II ratio of unity. Study of defects observed on ZnCdSe/ZnSSe/ZnMgSSe laser structures indicates that defects are pairs of the stacking faults which form an unique hillock. The composition modulation was not observed for ZnMgSSe layers grown by CSMBE. These results are owing to hot and group-VI diatomic molecular beam generated from compound sources.