Masamichi Akazawa

Single-electron logic device based on the binary decision diagram

The unit device consists of four tunnel junctions and operates as a two-way switch for single-electron transport. Any combinational logic can be implemented by connecting identical unit devices into a cascade to build the tree of a BDD graph. Several sample designs are presented for logic circuits of NAND, NOR, exclusive-OR, and AND-OR combinational logic. Computer simulation shows that the designed circuits perform the logic operations correctly.

Control of compound semiconductor–insulator interfaces by an ultrathin molecular‐beam epitaxy Si layer

Based on the disorder induced gap state (DIGS) model, an attempt is made to control the insulator–semiconductor (I–S) interfaces of GaAs and In0.53Ga0.47As by an ultrathin surface‐oxidized silicon (Si) interface control layer (ICL). A Si ICL is grown on GaAs or InGaAs by molecular beam epitaxy (MBE), and is partially oxidized. Then, a thick SiO2 or Si3N4 layer is deposited by in situ photo‐CVD processes. An in situ XPS analysis confirms formation of the intended structures. In the GaAs structure, the state density in the midgap region is remarkably reduced. However, the interface Fermi level is blocked by a high density of interface states near the conduction band minimum, contrary to the recent reports of ‘‘complete unpinning.’’ These states are Si‐derived intrinsic states, judging from the band line‐up. On the other hand, no such states exist in the InGaAs structure and completely ‘‘unpinned’’ behavior with a very small hysteresis is realized after annealing. The result is interpreted in terms of succes...

Effect of a thin dielectric layer on terahertz transmission characteristics for metal hole arrays.

We studied the role of surface-plasmon polaritons (SPPs) in a bandpass transmission property of two-dimensional metal hole arrays (2D-MHAs) by investigating the effect of thin dielectric layers on the 2D-MHA surfaces. We measured zero-order transmission spectra of the 2D-MHAs by changing the thickness of the dielectric layer and found that the bandpass transmission peak shifted to the lower-frequency side with increasing layer thickness, owing to the change of the resonant frequency of the SPP. This result shows that SPPs play a crucial role in the transmission property of 2D-MHAs in the terahertz region.

Binary-decision-diagram device

The device proposed here for future LSIs is based on a concept different from the Boolean equations usually used for representing digital functions. The unit function of this device is simple two-way switching and can be implemented utilizing various physical effects, such as optical switching, electron-wave modulation, and single-electron transport. Several possible device structures are presented, and a simulated result for a single-electron device is described. >

Measurement of valence-band offsets of InAlN/GaN heterostructures grown by metal-organic vapor phase epitaxy

The valence band offsets, ΔEV, of In0.17Al0.83N/GaN, In0.25Al0.75N/GaN, and In0.30Al0.70N/GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy (XPS). The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The ΔEV values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated ΔEV values were 0.2±0.2 eV for In0.17Al0.83N/GaN, 0.1±0.2 eV for In0.25Al0.75N/GaN, and 0.0±0.2 eV for In0.30Al0.70N/GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fr...

Control of GaAs and InGaAs Insulator-Semiconductor and Metal-Semiconductor Interfaces by Ultrathin Molecular Beam Epitaxy Si Layers

Removal or control of Fermi level pinning is attempted using an ultrathin molecular beam epitaxy (MBE) Si interface control layer (Si ICL) for insulator-semiconductor (I-S) and metal-semiconductor (M-S) interfaces of GaAs and InGaAs. For successful removal of Fermi level pinning at the I-S interface, the Si ICL should maintain an ordered pseudomorphic structure. The optimum thickness of the Si ICL is about 10 A. Formation of such a Si ICL alone does not remove pinning; subsequent deposition of a SiO2 film is necessary for unpinning. Pinning at the air-exposed surfaces can be removed by combining an HF surface treatment with the Si ICL technique. The Si ICL technique is promising for controlling barrier heights at M-S interfaces.

Small valence-band offset of In0.17Al0.83N/GaN heterostructure grown by metal-organic vapor phase epitaxy

The valence-band offset of a lattice-matched In0.17Al0.83N/GaN heterostructure grown by metal-organic vapor phase epitaxy (MOVPE) was investigated by x-ray photoelectron spectroscopy (XPS). Atomic force microscopy and angle-resolved XPS indicated that a thin In0.17Al0.83N (2.5 nm) layer was successfully grown by MOVPE on GaN. The XPS result showed that the valence band offset was 0.2±0.3 eV. This result indicates that the conduction-band offset at the In0.17Al0.83N/GaN interface is large, i.e., 0.9 to 1.0 eV, and occupies a large part of the entire band discontinuity.

Preparation of carbon nanoparticles by plasma-assisted pulsed laser deposition method : size and binding energy dependence on ambient gas pressure and plasma condition

Abstract Nanometer-size carbon particles were prepared on a Si substrate using pulsed laser deposition (PLD) assisted by radio frequency (RF) Ar plasma and were compared with ones prepared by PLD in vacuum and Ar gas. In both the plasma and gas ambiences, experiments were carried out in Ar pressure p Ar ranging from 0.13 to 13 Pa. The particle size increased as p Ar increased. However, the size obtained in the RF Ar plasma was approximately 1.5 times larger than that prepared in the Ar gas. An X-ray photoelectron spectroscopy (XPS) analysis revealed that the carbon film covered by the particles was in an amorphous state. The sp 3 /sp 2 carbon ratio of the film was evaluated by deconvolution of XPS carbon (1 s ) spectra into three components, which are attributed to diamond (sp 3 ), graphite (sp 2 ) and carbon oxide components. The highest sp 3 /sp 2 ratio was 0.4 in the Ar gas and Ar plasma at p Ar =0.13 Pa. The sp 3 /sp 2 ratio decreases monotonously, as the particle size increases. The ratio obtained in the Ar plasma is larger than that in the Ar gas. The effects of p Ar and plasma for nanoparticle characteristics are discussed.

Boltzmann machine neural network devices using single-electron tunnelling

We proposed a method of implementing the Boltzmann machine neural network on electronic circuits by making use of the single-electron tunnelling phenomenon. The single-electron circuit shows stochastic behaviour in its operation because of the probabilistic nature of the electron tunnelling phenomenon. It can therefore be successfully used for implementing the stochastic neuron operation of the Boltzmann machine. The authors developed a single-electron neuron circuit that can produce the function required for the Boltzmann machine neuron. A method for constructing Boltzmann machine networks by combining the neuron circuits was also developed. The simulated-annealing operation can be performed easily by regulating an external control voltage for the network circuits. A sample network was designed that solves an instance of a combinatorial optimization problem. Computer simulation demonstrated that, through the simulated-annealing process, the sample network can converge to the global minimum energy state that represents the correct solution to the problem.

Characterization of electronic states at insulator/(Al)GaN interfaces for improved insulated gate and surface passivation structures of GaN-based transistors

This paper presents a systematic characterization of electronic states at insulators/(Al)GaN interfaces, particularly focusing on insulator/AlGaN/GaN structures. First, we review important results reported for GaN metal–insulator–semiconductor (MIS) structures. SiO2 is an attractive material for MIS transistor applications due to its large bandgap and high chemical stability. In-situ SiNx is effective for improving the operation stability of high electron mobility transistors (HEMTs). Meanwhile, Al2O3/GaN structures have high band offsets and low interface state densities, which are also desirable for insulated gate applications. We have proposed a calculation method for describing capacitance–voltage (C–V) characteristics of HEMT MIS structures for evaluating electronic state properties at the insulator/AlGaN interfaces. To evaluate near-midgap states at insulator/AlGaN interfaces, a photo-assisted C–V technique using photon energies less than the bandgap of GaN has been developed. Using the calculation in conjunction with the photo-assisted C–V technique, we estimate interface state density distributions at the Al2O3/AlGaN interfaces.