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Formation of High-Density GaAs Hexagonal Nano-wire Networks by Selective MBE Growth on Pre-patterned (001) Substrates
Abstract
Attempts were made to grow high-density GaAs hexagonal nano-wire networks on (001) patterned substrates by selective molecular beam epitaxy (MBE). To form a hexagon, < -1 0 1 >- and < 5 1 0 >- directions were combined. By the growth of straight wire arrays in each direction, the growth mode, conditions and mechanism were investigated. The wire width was shown to be determined for both directions by the facet boundary planes resulting from the growth rate difference on different facets. By optimizing of growth condition, highly uniform and smoothly connected hexagonal nano-wire networks with a density of 3x10^8 cm^-2 were successfully formed.
Power consumption has become one of the primary challenges to meet the Moore's law. For reducing power consumption, Single-Electron Transistor (SET) at room temperature has been demonstrated as a promising device for extending Moore's law due to its ultra-low power consumption during operation. Prior work has proposed an automated mapping approach for SET arrays which focuses on minimizing the number of hexagons in an SET array. However, the area of an SET array is more related to the width. Consequently, in this work, we propose an approach for width minimization of the SET arrays. The experimental results show that the proposed approach saves 26% of width compared with the state-of-the-art for a set of MCNC and IWLS 2005 benchmarks while spending similar CPU time.
A coupled quantum disk, where two kinds of rectangular disks are connected by a narrow stripe, was fabricated on a patterned substrate with molecular beam epitaxy. The samples were characterized with cathode luminescence (CL) spectra. We found that the spectral peak wavelength depends on size of the square disk. Even if the large disk is excited, we observed the luminescence from not only the large disk but also from the small disk. This suggests the transport of carriers between the disks via the narrow stripe.
The basic feasibility of constructing hexagonal binary decision diagram (BDD) quantum circuits on GaAs-based selectively grown (SG) nanowires was investigated from viewpoints of electrical connections through embedded nanowires and electrical uniformity of devices formed on nanowires. For this, - and -oriented nanowires and hexagonal network structures combining these nanowires were formed on (001) GaAs substrates by selective molecular beam epitaxy (MBE) growth. The width and vertical position of the nanowires could be controlled by growth conditions for both - and -directions. By current-voltage (I-V) measurements, good electrical connection was confirmed at the node point where vertical alignment of embedded GaAs nanowire pieces was found to be important. SG quantum wire (QWR) switches formed on the nanowires showed good gate control over a wide temperature range with clear conductance quantization at low temperatures. Good device uniformities were obtained on the test chips, providing a good prospect for future integration. BDD node devices using SG QWR switches showed clear path switching characteristics. Estimated power-delay product values were very small, confirming the feasibility of ultra low-power operation of future circuits.
In view of applications to hexagonal binary decision diagram (BDD) LSIs, a first attempt is made to form quantum BDD node switches on selectively grown (SG) embedded quantum wires (QWRs) by molecular beam epitaxy (MBE). SG branch switches controlled by a Schottky wrap gate (WPG) were successfully fabricated by MBE growth and subsequent device processing. Gate control characteristics were studied by gate-dependent Shubnikov–de-Haas measurements, and the behavior was found to be similar to that of devices fabricated on wires by etching. The switch exhibited clear conductance quantization at low temperature, and temperature dependence of the voltage slope of conductance jump was clarified. A Y-branch BDD node device using two SG branch switches was successfully fabricated, and realized clear path switching characteristics.
Attempts were made to further elaborate our experimental growth method and the theoretical growth simulation method for formation of AlGaAs/GaAs QWRs on the (111)B substrates, paying attention to Al composition dependence of growth. A series of repeated growth experiments were carried out on simple one-sided mesa patterns, and from their analysis of the results led to determination of parameter values needed for computer simulation based on the continuum model. The experimental evolution of the cross-sectional structures was well reproduced by simulation, not only on one-side mesa, but also on mesa stripes actually used for wire growth. Finally, an optimum growth design was derived for growth of an array of GaAs triangular QWRs with 40 nm base width on GaAs (111)B substrate by the simulation, and the actual growth experiment confirmed its realization.
The evolution of the shape of a stripe mesa during molecular-beam epitaxy of GaAs is studied under different growth conditions (e.g., growth temperature, As pressure). If the stripe mesa is along the axis on a (001) GaAs substrate, {11n}A facets (n=1, 3, 4, 5) are formed. We found that {111}A and {113}A predominate if the stripe width is less than 1.5 mum. If the stripe width is wider than 2.0 mum, however, we have a shallow groove made of {115}A facets on the top of the mesa. Thus, the cross-sectional shape (appearance/disappearance of each facet) depends on the growth time. The overall evolution is well explained numerically assuming diffusion of adatoms on the facets. We found that the mass transport is the dominant mechanism to determine the cross-sectional shape of the stripe. Slight discrepancy between the experimental and numerical results is attributed to the presence of a potential barrier for the diffusion across the boundary between different facets.
The mechanism of the cross-sectional evolution during the selective growth of GaAs quantum wires (QWRs) on (111)B-patterned substrates was studied in detail both experimentally and theoretically. For this purpose, growth experiments were carried out on <-1-12>-oriented wires by systematically changing growth conditions and pattern sizes. A detailed investigation on cross sections of wires has shown that the lateral wire width is determined by facet boundaries (FBs) within AlGaAs layers separating growth regions on top facets from those on side facets of mesa structures. FBs were found to be planar or curved, depending on initial pattern sizes and growth conditions. Computer simulation based on a phenomenological growth model was attempted, taking account of the facet-angle-dependent lifetime of adatoms. The simulation well reproduced the experimentally observed growth features including the evolution of FBs, indicating that the cross sections of wires grown on (111)B-patterned substrates are kinetically controlled by the pattern sizes and growth conditions.
The growth kinetics involved in the selective molecular beam epitaxial growth of GaAs ridge QWRs is investigated in detail experimentally and an attempt is made to model the growth theoretically. For this purpose, detailed experiments were carried out on the growth of <110>-oriented AlGaAs-GaAs ridge quantum wires on mesa-patterned (001) GaAs substrates.Aphenomenological modeling was done based on the continuum approximation including parameters such as group III adatom lifetime, diffusion constant and migration length. Computer simulation using the resultant model well reproduces the experimentally observed growth features such as the cross-sectional structure of the ridge wire and its temporal evolution, its temperature dependence and evolution of facet boundary planes. The simple phenomenological model developed here seems to be very useful for design and precise control of the growth process.
The growth kinetics involved in the selective molecular beam epitaxy growth of GaAs quantum wires (QWRs) on mesa-patterned substrates is investigated in detail experimentally, and an attempt is made to model the growth theoretically, using a phenomenological continuum model. Experimentally, <-110>-oriented QWRs were grown on (001) and (113)A substrates, and <-1-12>-oriented QWRs were grown on (111)B substrates. From a detailed investigation of the growth profiles, it was found that the lateral wire width is determined by facet boundaries (FBs) within AlGaAs layers separating growth regions on top facets from those on side facets of mesa structures. Evolution of FBs during growth was complicated. For computer simulation, measured growth rates of various facets were fitted into a theoretical formula to determine the dependence of a lifetime of adatoms on the slope angle of the growing surface. The continuum model using the slope angle dependent lifetime reproduced the details of the experimentally observed growth profiles very well for growth on (001), (113)A, and (111)B substrates, including the complex evolution of facet boundaries
This paper discusses feasibility of design and future implementation of ultrasmall-size and ultra-low-power digital logic systems by a hexagonal BDD (binary-decision diagram) quantum circuit approach. The discussion is based on various circuits formed on GaAs-based hexagonal nanowire networks controlled by nanometer scale Schottky wrap gates (WPGs). Starting from basic node devices and elementary logic function blocks, fabrication technology of hexagonal BDD quantum circuits up to 8-bit adders with node densities over 45 million nodes/cm2 has been successfully developed. Their correct operations at low temperatures and room temperature have been confirmed by experiments and simulation. Various circuit components in logic processors, including arithmetic logic unit (ALU), controller and decoders have been successfully designed as hexagonal BDD layouts without nanowire crossover. For sequential circuits, WPG-controlled nanowire FETs on hexagonal networks have been investigated, and registers and counters have been implemented using these nanowire FETs showing correct operation. Hexagonal BDD-based static 2-bit nano-processor unit (NPU) has been successfully designed completely on hexagonal nanowire network. Ultra high-density GaAs hexagonal nanowire networks with much smaller wire sizes than those of etched nanowire networks have been successfully formed by selective MBE growth, showing great promise for room temperature operation in quantum regime as well as reduction of system area and power consumption.
Flow rate modulation epitaxy (FME) is applied to the low‐temperature growth of AlGaAs/GaAs quantum wires (QWRs) on nonplanar substrates. The growth selectivity is found to be enhanced greatly by the use of FME, as compared with the conventional metalorganic chemical vapor deposition due to the enhanced migration of Ga species. An AlGaAs/GaAs QWR with a central thickness of about 9 nm and a lateral width of about 28 nm is grown at 600 °C on a V‐grooved substrate. Good photoluminescence properties are observed from the grown QWR, with the peak energy being in good agreement with the calculated energy level of a parabolic shape lateral confinement potential. © 1995 American Institute of Physics.
A ridge quantum wire structure has been successfully fabricated on a patterned (001) GaAs substrate by first growing a (111)B facet structure with a very sharp ridge and then depositing a thin GaAs quantum well on its top. Electron microscope study has shown that a GaAs wire with the effective lateral width of 17–18 nm is formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that one of the luminescence lines comes from the wire region at the ridge and its blue shift (∼60 meV) agrees with the quantum confined energy calculated for the observed wire structure.
A novel hexagonal binary-decision-diagram (BDD) quantum logic circuit approach for III-V quantum large scale integrated circuits is proposed and its basic feasibility is demonstrated. In this approach, a III-V hexagonal nanowire network is controlled by Schottky wrap gates (WPGs) to implement BDD logic architecture by path switching. A novel single electron BDD OR logic circuit is successfully fabricated on a GaAs nanowire hexagon and correct circuit operation has been confirmed from 1.5 K to 120 K, showing that the WPG BDD circuit can operate over a wide temperature range by trading off between the power-delay product and the operation temperature.
InP-based InGaAs/InAlAs ridge quantum wires were successfully fabricated by our new approach using selective molecular beam
epitaxy (MBE). As the starting structures, array of InGaAs ridge structures composed of smooth (311)A facets were formed by
MBE on mesa-patterned InP substrates. Prior to actual fabrication of the wires, MBE growth characteristics of In0.53Ga0.47As and In0.52Al0.48As layers on the starting structure were studied in detail. The results of growth experiments were then successfully applied
to the fabrication of InGaAs ridge quantum wires with high spatial uniformity. Low temperature cathodoluminescence spectrum
measured in response to spot excitation of wire region showed a strong light emission whose analysis indicated that it originates
from InGaAs ridge quantum wire itself. In photoluminescence measurements, the emission from the wire had strong intensity
even at room temperature, indicating that the wire crystal possesses excellent bulk and interface quality, and are largely
free from nonradiative recombination centers.
New GaAs quantum dot structures, called tetrahedral quantum dots (TQDs), are proposed to make a zero‐dimensional electron‐hole system. The TQDs are surrounded by crystallographic facets fabricated using selective area metalorganic chemical vapor deposition (MOCVD) on (111)B GaAs substrates. The calculated energy sublevel structures of zero‐dimensional electrons in a GaAs TQD show large quantum size effects, because electrons are confined three dimensionally. GaAs and AlGaAs tetrahedral facet structures on (111)B GaAs substrates partially etched into a triangular shape were grown using MOCVD. Tetrahedral growth with {110} facets occurs in the triangular areas. The cathodoluminescence intensity map for GaAs tetrahedrons buried in AlGaAs shows the tetrahedral dot array.
Previously quantum device research has been done on discrete device levels and lacks a clear vision for high density integration. This paper proposes a new, simple and realistic approach for quantum large scale integrated circuits (QLSIs) where a binary-decision diagram (BDD) logic architecture is implemented by BDD node devices based on quantum wire transistors (QWTrs) and single electron transistors (SETs) realized by the Schottky in-plane gate (IPG) and wrap-gate (WPG) control of III–V hexagonal nanowire networks. To investigate the feasibility of the proposed approach, BDD devices having QWTrs were formed on GaAs/AlGaAs etched nanowire patterns. They showed expected complimentary quantized conductance switching as required to achieve operation at delay-power products near the quantum limit. The use of embedded InGaAs honeycomb wire networks grown by selective MBE on InP substrates is proposed for constructing circuits operating in quantum regime at room temperature.
We report the first observation of stimulated emission in quasi-one-dimensional semiconductor quantum wires. Amplified spontaneous emission and stimulated emission spectra of the GaAs/AlGaAs quantum wires exhibit fine structure arising from transitions between lateral, one-dimensional electron and hole subbands. The observed subband separations, ~10 meV, are consistent with the calculated ones.
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