Hidemasa Tomozawa

Fabrication and Characterization of GaAs Single Electron Devices Having Single and Multiple Dots Based on Schottky In-Plane-Gate and Wrap-Gate Control of Two-Dimensional Electron Gas

Single-dot and multiple (2, 3, 18, and 37)-dot single electron transistors (SETs) based on the control of a two-dimensional electron gas (2DEG) with a recently proposed Schottky in-plane gate (IPG) and a newly introduced Schottky wrap gate (WPG) were successfully fabricated on AlGaAs/GaAs wafers using electron beam (EB) lithography and their transport properties were investigated. Each of the fabricated SETs showed Coulomb blockade-like conductance oscillation. In single-dot SETs, a strong correlation was found between the device dimensions and the temperature limit of the conductance oscillation. Conductance oscillation characteristics of multiple-dot SETs were complicated, and were not explained by the classical Coulomb blockade theory. Based on a simplified theoretical analysis using computer simulation, it was shown that quantized energy due to electron confinement and dot-coupling can dominate the charging effect in the fabricated SETs.

Reappraisal of Si-Interlayer-Induced Change of Band Discontinuity as GaAs-AlAs Heterointerface Taking Account of Delta-Doping

X-ray photoelectron spectroscopy reinvestigation is done for the recently reported Si-interlayer-induced change of the valence band discontinuity (ΔEv) at GaAs-AlAs interfaces. The XPS measurements reproduced the large apparent change of ΔEv caused by the Si interlayer. However, it also led to anomalous increases of separations between the core level peak and the valence band edge as well as anomalous increases of full width at half maximum of the core level spectra. It is concluded that the observed change of ΔEv is only an apparent one. The anomalies were explained quantitatively by a new model based on the surface Fermi level pinning and interface delta-doping.

A New N-Channel Junction Field-Effect Transistor Embedded in the i Layer of a Pin Diode

We experimentally confirmed a basic operation of the n-channel junction field-effect transistor (JFET) embedded in the i layer (n- substrate) of a pin diode for X-ray detectors, which was proposed in Jpn. J. Appl. Phys. 37 (1998) L115. To electrically isolate the n-channel from the n- substrate, a p+ ring is formed around the JFET and is reverse-biased, instead of a deep p layer under the n-channel (i. e. , the conventional structure). In the proposed structure, only one type of donor is ion-implanted in the n-channel, while in the conventional structure, both donor and acceptor are ion-implanted there. For the first time, the role of the p+ ring in the electrical characteristics of the n-channel JFET is experimentally elucidated.

Seedless Crystallization of Levitated Ge and GaSb Spherical Melts under Microgravity

Seedless crystallization of Ge and GaSb melts was achieved under microgravity occurring in a drop-shaft capsule. It was observed that melts levitated in Ar atmosphere, formed into a spherical shape and were solidified instantaneously when they collided with the container quartz wall. X-ray diffraction pattern of these samples showed clear Laue spots signifying a single-crystal structure, while Ge or GaSb solidified in crucibles under normal gravity did not show Laue spots. These results indicate that single crystal growth rapidly occurs without seed crystals from the spherical melt under microgravity. This can be understood by assuming that an ordered structure like a single crystal is already formed in a levitated melt, and assists rapid crystallization when the melt touches the crucible wall and latent heat is deprived.

Electrical Conductivity of TiO2 Thin Film on Insulator Induced by Radiation Exposure

We investigate the electrical conductivity of a TiO2 surface layer on insulators induced by radiation exposure. The surface layer is sufficiently thin to transmit radiation (typically γ-rays) without absorption, but exhibits a current signal indicating the interaction processes of the radiation with substrate insulators. We explain this effect arising from the layered structure and demonstrate a Monte Carlo simulation of photons and electrons to show that electrons generated by irradiation in the substrate travel and cause energy deposition onto the thin layer, envisaging a new type of radiation detector.