InGaAs Ridge Quantum Wires Research Articles

Molecular beam epitaxy growth mechanism and wire width control for formation of dense networks of narrow InGaAs quantum wires

Precise wire width control is important in forming dense networks of narrow InGaAs ridge quantum wires (QWRs) by selective MBE growth on patterned InP substrates. In this paper, SEM, TEM, AFM, CL and PL measurements were carried out to clarify the QWR formation mechanism and to establish the method of wire width control. Formation of the arrow-head shaped QWR is due to evolution of (114)A facets caused by instability of (113)A facets during growth of the InAlAs layer. From PL peak energy positions, it has been found that the effective wire width can be controlled by precisely adjusting the growth time, or the grown thickness of the lower InAlAs layer.

Structural and Optical Properties of InGaAs Ridge Quantum Wire Arrays with Sub-micron Pitches Grown by Selective MBE on InP Substrates

A systematic microstructural and optical characterization has been carried out on linear arrays of InAlAs/InGaAs/InAlAs ridge quantum wires (QWRs) grown by selective MBE on patterned InP (001) substrates, by scanning electron microscopy (SEM), cross-sectional transmission microscopy (TEM), atomic force microscopy (AFM), and cathodoluminescence (CL) and photoluminescence (PL) techniques. The microstructural analysis proved high integrity and high uniformity of the top ridge region of the QWRs. It also showed poor uniformity in the bottom quantum well region which has to be improved in future studies. Spatially and spectrally resolved CL measurements identified the origin of the emission peaks. High photoluminescence efficiency and narrow spectral width from the ridge QWR proved the achievement of high quality and high uniformity in the ridge QWR arrays. From the PL peak position which showed a large blueshift, a linear array of QWRs with an effective width of 10 nm has been successfully realized at a small wire pitch of 800 nm. Excitation power-dependence and temperature-dependence of photoluminescence have shown that the present QWR arrays are very well behaved up to 250 K, being free from saturation, and from surface/interface recombination processes.

Molecular Beam Epitaxy Growth of High-Quality Linear Arrays of InGaAs Ridge Quantum Wires with Nanometer Wire Widths and Submicron Wire Pitches on Patterned InP Substrates

Systematic investigations have been carried out to grow InGaAs ridge quantum wire (QWR) linear arrays with nanometer wire widths and submicron wire pitches by selective molecular beam epitaxy (MBE) on patterned InP substrates. Uniformity of QWR arrays was markedly improved by employing pregrowth etching, removing native oxide by atomic-hydrogen-assisted cleaning, and optimizing the V/III ratio. The pitch of array was successfully reduced to the submicron range by scaling down both the mesa stripe patterns on the InP substrate and the epitaxial layer thicknesses properly. InGaAs ridge QWR arrays showed an intense and narrow single photoluminescence (PL) emission peak at 20 K with a full width at half maximum (FWHM) as small as 23 meV. The PL peak of the submicron-pitch QWR arrays showed a large blue shift of 134 meV with respect to that from a reference planar QW grown simultaneously, indicating achievement of an effective wire width of about 10 nm.

Control of morphology and wire width in InGaAs ridge quantum wires grown by atomic hydrogen-assisted selective molecular beam epitaxy

Growth controllabilities of morphology and wire width are systematically investigated for the InGaAs ridge quantum wires (QWRs) grown by the atomic hydrogen ( H ∗ )-assisted selective molecular beam epitaxy (MBE) growth. Detailed scanning electron microscopy (SEM), photoluminescence (PL) and magneto-transport measurements have been made. The optimal H ∗ cleaning remarkably improves the surface morphology of the InGaAs ridge structure on which QWR is formed. The geometrical width of the wire, W, can be controlled by changing the MBE growth time of the lower InAlAs barrier layer, t InAlAs. A simple relationship, W=9.5 nm/min t InAlAs min , has been obtained. Gate-dependent Shubnikov–de Haas (SdH) measurements on quantum wire transistor (QWRTr) structure having wrap gate (WPG) have indicated tight gate control with a maximum subband spacing of 15 meV .

Formation of high-density hexagonal networks of InGaAs ridge quantum wires by atomic hydrogen-assisted selective molecular beam epitaxy

Feasibility of growth of InGaAs ridge quantum wire (QWR) hexagonal network structures by atomic hydrogen (H ∗)-assisted selective molecular beam epitaxy (MBE) is investigated for use in novel hexagonal quantum circuits based on the binary-decision diagram (BDD) architecture. The fabricated structures were characterized in detail by SEM, AFM, PL and CL measurements. By using patterned substrates with mesa-pattern directions of 〈1 0 0〉– 〈 1 ̄ 1 0〉 and 〈5 1 0〉– 〈 1 ̄ 1 0〉 together with optimized H ∗-assisted selective MBE, hexagonal networks of the sharp and uniform InGaAs ridge structures were realized down to submicron pitches. Embedded InGaAs QWR hexagonal networks were successfully formed on the ridge structures, giving prospects of realizing a node device density lager than 10 8 cm −2.

Improvement of Growth Process to Achieve High Geometrical Uniformity in InGaAs Ridge Quantum Wires Grown by Selective MBE on Patterned InP Substrate

Attempts were made to identify and remove the major sources of structural nonuniformity in the InGaAs ridge quantum wires (QWRs) grown by selective molecular beam epitaxy (MBE) on patterned substrates. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and cathodoluminescence (CL) were used to characterize the uniformity of ridge QWR. Nonuniformity of the initial InGaAs ridge structure prior to QWR growth was found to be a key factor to be controlled. Using sufficient pre-growth etching and atomic-hydrogen-assisted cleaning in ultra-high-vacuum (UHV), a defect-free, oxide-free and flat InGaAs/InP interface was realized. By further optimizing the growth parameters using a low V/III beam-equivalent pressure ratio between 30 to 45 at a growth temperature of 500°C, an InGaAs ridge structure with an atomically flat facet having a standard deviation of the ridge top height of 0.25 nm for a 600 nm-long ridge stripe was realized, for the first time. InGaAs QWRs with various effective widths down to 6 nm were grown on this ridge structure, and they exhibited excellent optical properties with enhanced luminescence intensity and narrow linewidth.

Characterization and Optimization of Atomic Hydrogen Cleaning of InP Surface for Selective Molecular Beam Epitaxial Growth of InGaAs Quantum Structure Arrays

An oxide removal process for InP surface using atomic hydrogen cleaning was characterized and optimized for use in the selective molecular beam epitaxial (MBE) growth of InGaAs quantum structures. In-situ X-ray photoelectron spectroscopy (XPS), reflection high energy electron diffraction (RHEED), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used. Detailed XPS measurements clarified the effects of atomic hydrogen on the planar InP surfaces. The cleaning efficiently removed oxygen. However, the P/In atomic ratio on the surface depended sensitively on the cleaning procedure. Under the optimum low-temperature atomic hydrogen cleaning, the surface stoichiometry could be maintained and realized an atomically flat InGaAs/InP interface. This has led to the formation of uniform InGaAs ridge quantum wires (QWRs) with good optical properties.

Origin of non-uniformity in MBE grown nanometer-sized InGaAs ridge quantum wires and its removal by atomic hydrogen-assisted cleaning

The origin of non-uniformity of MBE-grown InGaAs ridge quantum wires (QWRs) in sub 10-nm wire width range was investigated in detail by SEM, in situ XPS, TEM and PL measurements. InAlAs/InGaAs/InAlAs wires were selectively grown on InGaAs ridge structures prepared also by MBE on 〈 1 ̄ 10〉 stripe patterned (001) InP substrates. The main source of non-uniformity was thermal cleaning of InP in As 4 done prior to InGaAs ridge formation which produced ridges having irregular sized InGaAs islands due to an initial As–P exchange reaction. Low temperature atomic hydrogen cleaning removed this problem, and led to successful formation of sub 10-nm QWRs.

Vertical barrier layer formation during selective MBE growth of InGaAs ridge quantum wires on InP patterned substrates

Transmission electron microscope (TEM) observations and energy dispersive X-ray (EDX) analyses were carried out to investigate the microscopic structure and local variation of alloy composition in InGaAs/InAlAs ridge quantum wires (QWRs) grown by selective molecular beam epitaxy (MBE) on patterned InP substrates. In the QWR structure, formation of the vertical barrier layer having a width of 5–15 nm, which bisects whole InGaAs/InAlAs QWR structure including the QWR itself, was observed for the first time. Particularly, marked reduction of In composition down to xIn=0.45 was observed for the InGaAs layer grown at 500°C. The formation of the vertical barrier layer and temperature dependence of its composition can be explained in terms of a curvature-induced capillarity and a free energy of alloy mixing competing with the adatom diffusion effect.

Extra-Side-Facet Control in Selective Molecular Beam Epitaxial Growth of InGaAs Ridge Quantum Wires for Improvement of Wire Uniformity

For InP-based InGaAs ridge quantum wires fabricated by selective molecular beam epitaxy (MBE) on <110>-oriented InP mesa-stripes, the origin of wire inhomogeneity and the possible methods to improve the wire uniformity were investigated in detail. The appearance of extra-side-facets on (111)A sidewalls was found to be the major reason for ridge waving, leading to wire inhomogeneity. Use of mis-oriented mesa-stripes and high-temperature growth of InGaAs buffer layer were found to be significantly effective for reducing the width of the extra-facets, resulting in the suppression of ridge waving and a marked improvement of wire uniformity. The narrow photoluminescence (PL) peak of the InGaAs ridge quantum wire formed with the present extra-side-facet control indicated the realization of a wire having the best uniformity of all the InP-based InGaAs quantum wires reported so far and comparable or better uniformity than most of the GaAs-based quantum wires reported so far.

Size-Controlled Formation of Decananometer InGaAs Quantum Wires by Selective Molecular Beam Epitaxy on InP Patterned Substrates

Detailed scanning electron microscopy (SEM), photoluminescence (PL) and magnetoresistance measurements were made for InGaAs ridge quantum wires (QWRs) grown by selective molecular beam epitaxy (MBE) in order to establish a method for size-controlled formation of high-quality decananometer wires. The width of our InGaAs ridge QWRs was found to be proportional to the growth time of the bottom InAlAs layer with a rate of about 9.5 nm/min. A minimum wire width of about 35 nm was achieved. The wire widths measured by SEM, PL and magnetoresistance methods agreed reasonably well with each other as well as with the design values. The results of PL and magnetotransport measurements indicated that the present decananometer wires possess good crystalline and interface qualities as well as strong one-dimensional electron confinement. The present method was concluded to be powerful for size-controlled formation of high-quality decananometer InGaAs QWRs.

Formation of highly uniform InGaAs ridge quantum wires by selective molecular beam epitaxy on novel InP patterned substrates

For InP-based InGaAs ridge quantum wires by selective molecule beam epitaxial (MBE) growth on 〈 1 10〉 oriented InP mesa-stripes, the origin of wire inhomogeneity and possible methods to improve the wire uniformity were investigated in detail. Appearance of extra-side-facets on (111)A sidewalls was the major reason for ridge waving, leading to the inhomogeneity of the wire. By introducing mis-orientation into the mesa-direction, the width of the extra-facets were drastically reduced, leading to the suppression of the ridge waving. On the other hand, growth on a mesa-stripe having wide and narrow width regions led to complete removal of the extra-side-facets and the ridge waving from the wide mesa region, resulting in formation of a highly uniform InGaAs wire segment having a length of several microns.

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.

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.

Effect of mis-orientation of mesa-stripes on the growth of InGaAs quantum wires by selective molecular beam epitaxy

In this paper, the effect of mis-orientation of mesa-stripes on the In 0.53Ga 0.47As ridge quantum wire formation in our selective MBE growth procedure is studied. Scanning electron microscope (SEM), atomic force microscope (AFM), and cathodoluminescence (CL) observations revealed that the InGaAs ridge quantum wires were modulated into segments along the wires by the growth on the mis-oriented mesa-stripes. Particularly, in the case of the mis-orientation angle of 13°, pseudo-periodic segment formation was successfully achieved in the wires, resulting in the formation of a periodic array of 3-dimensional carrier confining dot structures.

Observation of Coulomb Blockade Type Conductance Oscillations up to 50 K in Gated InGaAs Ridge Quantum Wires Grown by Molecular Beam Epitaxy on InP Substrates

InP-based quantum wire (QWR) transistors were successfully fabricated for the first time directly on InAlAs/InGaAs ridge QWRs grown by selective molecular beam epitaxy (MBE) on patterned (001) InP substrates. Each wire showed strong photoluminescence, cathodoluminescence and Shubnikov-de Haas (SdH) oscillations. The behavior of the SdH oscillations confirmed the presence of gate-controlled one-dimensional transport. Near pinch-off, QWR transistors showed Coulomb blockade type conductance oscillations up to 50 K. The effective value of the Coulomb gap was estimated to be 28 mV. Possible mechanisms for quantum dot formation are discussed.

Fabrication of InP-based InGaAs ridge quantum wires utilizing selective molecular beam epitaxial growth on (311)A facets

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.

Photoluminescence and Cathodoluminescence Investigation of Optical Properties of InP-Based InGaAs Ridge Quantum Wires Formed by Selective Molecular Beam Epitaxy

Optical properties of InP-based InGaAs ridge quantum wires formed by selective molecular beam epitaxy (MBE) growth were characterized in detail by using cathodoluminescence (CL) and photoluminescence (PL). Both the CL and PL spectra had strong emission peaks with narrow widths. Spot-excited CL images confirmed that these peaks originated from ridge quantum wires. Depending on the excitation power, emission peaks showed energy shift and half-width change due to band filling. Temperature dependence of the PL emission peak followed that of the energy gap. The PL intensity decreased with increasing temperature, but was still fairly intense at room temperature. The behavior of PL intensity change with temperature was found to be the same for wires and reference wells, indicating the presence of a common nonradiative recombination mechanism.

Fabrication of InGaAs ridge quantum wires by selective molecular beam epitaxy and their characterization

InGaAs/InAlAs ridge quantum wires were successfully fabricated by selective molecular beam epitaxy (MBE) for the first time. Prior to wire fabrication, detailed data on selective growth characteristics were taken by using test structures. Then, triangular shaped InGaAs ridge quantum wires with a width of 300 Å were fabricated, using the selectivity data. Photoluminescence (PL) measurements detected a strong and narrow peak from the wires which showed a blue shift of 159 meV with respect to the InGaAs band-gap. This value agrees excellently with the calculation.

Fabrication of InGaAs ridge quantum wires by selective molecular beam epitaxy and their characterization

Fabrication of InGaAs ridge quantum wires by selective molecular beam epitaxy and their characterization