Fabrication and characterization of active and sequential circuits utilizing Schottky-wrap-gate-controlled GaAs hexagonal nanowire network structures

For realization of hexagonal BDD-based digital systems, active and sequential circuits including inverters, flip flops and ring oscillators are designed and fabricated on GaAs-based hexagonal nanowire networks controlled by Schottky wrap gates (WPGs), and their operations are characterized. Fabricated inverters show comparatively high transfer gain of more than 10. Clear and correct operation of hexagonal set-reset flip flops (SR-FFs) is obtained at room temperature. Fabricated hexagonal D-type flip flop (D-FF) circuits integrating twelve WPG field effect transistors (FETs) show capturing input signal by triggering although the output swing is small. Oscillatory output is successfully obtained in a fabricated 7-stage hexagonal ring oscillator. Obtained results confirm that a good possibility to realize practical digital systems can be implemented by the present circuit approach.

Fabrication of high quality two-dimensional photonic crystal mask layer patterns

This work discusses the fabrication of two-dimensional photonic crystal mask layer patterns. Photonic crystal patterns having holes with smooth and straight sidewalls are achieved by optimizing electron beam exposure doses during electron beam lithography process. Thereafter, to precisely transfer the patterns from the beam resist to the SiO2 mask layer, we developed a pulse-time etching method and optimize various reaction ion etching conditions. Results show that we can obtain high quality two-dimensional photonic crystal mask layer patterns.

Formation of total-dose-radiation hardened materials by sequential oxygen and nitrogen implantation and multi-step annealing

Separation by implantation of oxygen and nitrogen (SIMON) silicon-on-insulator (SOI) materials were fabricated by sequential oxygen and nitrogen implantation with annealing after each implantation. Analyses of SIMS, XTEM and HRTEM were performed. The results show that superior buried insulating multi-layers were well formed and the possible mechanism is discussed. The remarkable total-dose irradiation tolerance of SIMON materials was confirmed by few shifts of drain leakage current-gate source voltage (I-V) curves of PMOS transistors fabricated on SIMON materials before and after irradiation.

Metal alloy and monoelemental nanoclusters in silica formed by sequential ion implantation and annealing in selected atmosphere

The preparation of metal alloy and monoelemental nanoclusters in silica by Ag, Cu ion sequential implantation and annealing in selected oxidizing or reducing atmosphere is studied. The formation of metastable Ag-Cu alloy is verified in the as-implanted samples by optical absorption spectra, selected area electron diffraction and energy dispersive spectrometer spectrum. The alloy is discomposed at elevated annealing temperature in both oxidizing and reducing atmospheres. The different effects of annealing behaviors on the Ag Cu alloy nanoclusters are investigated. (C) 2004 Elsevier B.V. All rights reserved.

Cathodoluminescence on GaN hexagonal pyramids on submicron dot-patterns via selective MOVPE

Three-dimensional GaN pyramids have been successfully obtained on submicron dot-patterned (0001) sapphire substrates by using the selective metalorganic vapor phase epitaxy (MOVPE) technique. The dot-pattern is a hexagon arranged with a 0.5-mu m width and 1.0-mu m spacing. The GaN structure comprises a hexagonal pyramid covered with six {1 (1) over bar 01} pyramidal facets on the side of a hexagonal pyramid having a (0001) facet on the top. Cathodoluminescence (CL) measurement was carried our. on the hexagonal pyramid at low temperature. Two distinct spectra were observed to occur at about 359 and 329 nm. The higher energy is thought to be related to GaN dot, and the lower one is due to GaN dot band edge emission. The intensities of the two spectra were investigated as a function of temperature in the range of 135-150 K. (C) 2000 Published by Elsevier Science B.V.

Anomaly of the current self-oscillation frequency in the sequential tunneling of a doped GaAs/AlAs superlattice

An anomalous behavior of the current self-oscillation frequency is observed in the dynamic de voltage bands, emerging from each sawtoothlike branch of the current-voltage characteristic of a doped GaAs/A1As superlattice in the transition process from static to dynamic electric field domain formations. Varying the applied de voltage at a fixed temperature, we find that the frequency increases while the averaged current decreases. Inside each voltage band, the frequency has a strong voltage dependence in the temperature range where the averaged current changes with the applied de voltage. This dependence can be understood in terms of motion of the system along a limit cycle.

Evidence of multimodal patterns of self-organized quantum dots

We have used Deep Level Transient Spectroscopy to investigate self-organized InAs/GaAs quantum dots. The existence of different dot families is confirmed by the deconvolution of the spectra in Gaussian components with full width at half maximum of 60-70meV. The strain of quantum dots is responsible for the relaxation of large quantum dots leading to generation of dislocations. (C) 1998 Academic Press.

Sequential resonant tunnelling through Landau levels in GaAs/AlAs superlattices

Electron transport in heavily-doped GaAs/AlAs superlattices in parallel electric and magnetic fields is reported. The current-voltage (I-V) characteristic exhibited the feature of negative differential velocity (NDV) and high electric field domain effect at different biases. Under strong magnetic fields, sequential resonant tunnelling through Landau levels in the negative differential velocity regime is observed, which are manifested as oscillations in the conductance-voltage characteristics. (C) 1998 Elsevier Science B.V. All rights reserved.

Suppression of sequential tunneling current by a perpendicular magnetic field in a three-barrier, two-well heterostructure

When an intersubband relaxation is involved in vertical transport in a tunneling heterostructure, the magnetic suppression of the intersubband LO or LA phonon scattering may also give rise to a noticeable depression of the resonant tunneling current, unrelated to the Coulomb correlation effect. The slowdown of the intersubband scattering rate makes fewer electrons able to tunnel resonantly between two adjacent quantum wells (QWs) in a three-barrier, two-well heterostructure. The influence of the magnetic field on the intersubband relaxation can be studied in an explicit way by a physical model based on the dynamics of carrier populations in the ground and excited subbands of the incident QW. (C) 1998 American Institute of Physics. [S0003-6951(98)00925-5].

On equations for regular languages, finite automata, and sequential networks

We consider systems of equations of the form where A is the underlying alphabet, the Xi are variables, the Pi,a are boolean functions in the variables Xi, and each δi is either the empty word or the empty set. The symbols υ and denote concatenation and union of languages over A. We show that any such system has a unique solution which, moreover, is regular. These equations correspond to a type of automation, called boolean automation, which is a generalization of a nondeterministic automation. The equations are then used to determine the language accepted by a sequential network; they are obtainable directly from the network.