Schottky Wrap Gate Research Articles

Novel Quantum Wire Branch-Switches for Binary Decision Diagram Logic Architecture Utilizing Schottky Wrap-Gate Control of GaAs/AlGaAs Nanowires

A novel binary decision diagram (BDD) node device based on wrap-gate (WPG) control of AlGaAs/GaAs nanowires is described and fabricated, and its basic operation is characterized. The node device consists of one entry-nanowire branch and two exit-nanowire branches where each of two exit-branches is switched by a Schottky WPG in a complimentary fashion so as to realize path switching for electrons. Basic branch switches as well as BDD node devices were fabricated on AlGaAs/GaAs nanowires formed by electron beam lithography and wet chemical etching of molecular beam epitaxy (MBE)-grown heterostructures. A detailed analysis of gate control characteristics was carried out by magnetotransport measurements. A three-dimensional potential simulation showed that Fermi level pinning on the semiconductor free surface should be carefully taken into consideration for accurate device design. The branch-switch exhibited excellent gate control from low temperatures up to room temperature, showing clear conductance quantization at low temperatures. The fabricated BDD node device realized clear path switching from low temperatures up to room temperature.

III–V quantum devices and circuits based on nanoscale Schottky gate control of hexagonal quantum wire networks

The concept, the present status, key issues and future prospects of a novel hexagonal binary decision diagram (BDD) quantum circuit approach for III–V quantum large-scale integrated circuits (QLSIs) are presented and discussed. In this approach, the BDD logic circuits are implemented on III–V semiconductor-based hexagonal nanowire networks controlled by nanoscale Schottky gates. The hexagonal BDD QLSIs can operate at delay-power products near the quantum limit in the quantum regime as well as in the many-electron classical regime. To demonstrate the feasibility of the present approach, GaAs Schottky wrap gate (WPG)-based single-electron BDD node devices and their integrated circuits were fabricated and their proper operations were confirmed. Selectively grown InGaAs sub-10 nm quantum wires and their hexagonal networks have been investigated to form high-density hexagonal BDD QLSIs operating in the quantum regime at room temperature.

Gate control characteristics in GaAs nanometer-scale Schottky wrap gate structures

Gate control characteristics of GaAs-based quantum wire transistors (QWRTrs) controlled by a nanometer-scale Schottky wrap gate (WPG) are investigated theoretically and experimentally. Gate bias dependence of the effective wire width of fabricated WPG QWRTrs determined theoretically as well as experimentally from Landau plots showed that the nanometer-scale WPG controls the potential very tightly near channel pinch-off and that the pinch-off threshold voltage is strongly dependent on the gate length, L G, when L G is shorter than 400 nm. The theory based on the three-dimensional (3D) potential simulation pointed out that Fermi level pinning on the semiconductor surface around the WPGs strongly affects the gate controllability in the nanometer-scale Schottky WPG structure.

GaAs-Based Single Electron Transistors and Logic Inverters Utilizing Schottky Wrap-Gate Controlled Quantum Wires and Dots

GaAs-based single electron transistors (SETs) and their logic inverters were successfully designed and fabricated using a Schottky wrap-gate (WPG) quantum wire and dot formation technology. Three-gate WPG SETs, which have two tunnel barrier gates and a center gate for a quantum dot-potential control, showed voltage gains larger than unity due to tight dot-potential control of the center WPG. The conductance peak position of the SET could be systematically controlled by changing the tunnel-barrier-control WPG voltages. A resistive-load single electron inverter utilizing a 3-gate WPG SET as a driver and a WPG quantum wire transistor as an active load was fabricated and it showed a proper inverter operation at 1.6 K and realized a logic transfer gain of larger than unity (1.3) for the first time in III-V semiconductor-based SET inverters. A III-V semiconductor-based complimentary inverter utilizing two 3-gate WPG SETs was also successfully fabricated for the first time and the inverter operation was also confirmed at 1.7 K.

Enhancement mode Al0.25Ga0.75As/In0.2Ga0.8As nanowire HEMTs

Enhancement mode Al0.25Ga0.75As/In0.2Ga0.8As nanowire high electron mobility transistors (NW-HEMTs) are fabricated successfully by using selective wet etching and the depletion characteristic of a Schottky wrap gate (WPG). The devices exhibit very good modulation and saturation characteristics. For an NW-HEMT with an estimated channel width of 250 nm, the maximum transconductance is ~450 mS/mm at a drain voltage of 1.5 V.

Conductance oscillation characteristics of GaAs Schottky wrap-gate single-electron transistors

Conductance oscillation characteristics in GaAs-based Schottky wrap gate (WPG) single-electron transistors (SETs) were investigated both experimentally and theoretically in view of application as a key switching device in future quantum-integrated circuits. Fabricated WPG SETs showed that clear conductance oscillation characteristics with a small number of high conductance peaks. A simple theory based on a quantum mechanical treatment reproduced qualitatively the features of the experimentally observed conductance peaks, indicating that the resonant tunneling contributes to currents in the WPG SETs. However, quantitatively, a discrepancy existed between theory and experiment on the temperature dependence of peak heights.

A novel wrap-gate-controlled single electron transistor formed on an InGaAs ridge quantum wire grown by selective MBE

An attempt was made, for the first time, to fabricate a novel Schottky wrap gate (WPG) controlled single electron transistor (SET) directly on to an InGaAs stereographic ridge quantum wire grown by molecular beam epitaxy (MBE). To analyze the controllability of the quantum dot by Schottky WPG, a computer simulation of potential distribution was carried out. The fabricated SET showed clear conductance oscillation with an effective Coulomb gap of 10 meV. Comparison of the observed experimental behavior with simulation indicates that the SET is in the quantum confinement regime with resonant tunneling.

Computer Simulation and Experimental Characterization of Single Electron Transistors Based on Schottky Wrap Gate Control of 2DEG

Characteristics of GaAs-based single electron transistors (SETs) based on Schottky wrap gate (WPG) control of the two dimensional electron gas were studied. The two-gate and the newly proposed three-gate devices with different WPG sizes and channel widths were theoretically analyzed on a computer and experimentally fabricated and characterized. The computer simulation showed that dots and tunneling barriers could be formed in both the two- and the three-gate SETs. In the case of the two-gate SETs, both the dot size and the tunnel barriers were changed simultaneously by the gate voltage. On the other hand, in the case of the newly proposed three-gate SET, the dot size and the potential profile of the tunneling barrier could be separately controlled. Experimentally fabricated the two- and the three-gate SETs with narrow channel widths showed conductance oscillation characteristics which were consistent with the results of potential simulation.

Electrochemical formation and characterization of Schottky in-plane and wrap gate structures for realization of GaAs- and InP-based quantum wires and dots

In order to realize high temperature operating quantum devices, this paper attempts to form nm-size Schottky in-plane gate (IPG) and wrap gate (WPG) structures on GaAs- and InP-based nanostructures by an in-situ electrochemical process. Pulsed electrodeposition of Pt was found to be optimal, realizing nearly pinning-free high Schottky barrier heights as well as fine and highly uniform nm-size grains with minimal stress. Using the optimized process, quantum wire transistors and single electron transistors were successfully realized on GaAs etched 2DEG bars and on MBE grown InGaAs ridge quantum wires.

Basic Control Characteristics of Novel Schottky In-Plane and Wrap Gate Structures Studied by Simulation and Transport Measurements in GaAs and InGaAs Quantum Wires

To clarify the control properties of Schottky in-plane gates (IPGs) and Schottky wrap-gates (WPGs) recently employed in the high-temperature operation of compound semiconductor single electron devices, computer simulations and transport measurements were carried out for gated quantum wires (QWRs). Both types of QWRs showed clear Shubnikov-de Haas oscillations. Non-linear Landau plots confirmed gate-controlled 1D transport in the QWRs. In the GaAs IPG QWR, the effective wire width was found to change linearly with gate bias, whereas this was not observed in the InGaAs WPG QWR. These gate control behaviors are in excellent agreement with theory. Near pinch-off, clear conductance oscillation was seen in both QWRs.

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.