Quantum Wire Heterostructures Research Articles

Control method for periodically faceted surfaces and application on AlGaAs/GaAs (6 3 1) heterostructures

Recently, the set of advantages of assembling quasi-one dimensional quantum wire heterostructures by molecular beam epitaxy over high-index orientations has been exhibited, including merging the benefits of both integer-dimension extremes while ruling out their flaws. Nonetheless, hitherto a shallow knowledge of their growth evolution has been barely gathered and thereupon a scarce design control could only be achieved. In this paper we impart a profound survey of the growth phenomena present on the GaAs (631) surface in addition of a method that enhances the reflection high-energy electron diffraction in situ characterization performed upon the mesoscopic periodic corrugation in the course of homoepitaxial growth by molecular beam epitaxy and for the first time in literature, during heteroepitaxy. The fundamentals of the unparalleled corrugation uniformity are expanded in terms of a novel perspective for the surface reconstruction and anisotropic diffusion process. Furthermore, an inverse Weissenberg-RHEED method is proposed to enhance the design control over the mesoscopic corrugation in situ, optimizing the production cycle of quasi-one dimensional QWRs and enabling a proper framework in pursuit for application into higher performance electro-optical devices.

Thermally stimulated conductivity in InGaAs/GaAs quantum wire heterostructures

Thermally stimulated conductivity in InGaAs/GaAs quantum wire heterostructures

Photoconductivity peculiarities in InGaAs quantum wire heterostructures: anisotropy and high photoresponsivity at room temperature

The anisotropy of photoconductivity was measured in InGaAs/GaAs quantum wire heterostructures. This anisotropy is the result of a one-dimensional band creating an effective channel for photo-generated carriers. High photoresponsivity of ∼2.3 A W−1 at 0.3 μW cm−2 incident light excitation was revealed at the InGaAs band edge absorption at room temperature. We show that the observation of this high photoresponsivity at lower power densities is due to the presence of both local electric fields at the InGaAs/GaAs interface caused by structural non-uniformities and a trap at ∼68 meV below the GaAs conductivity band edge. A proposed lateral one-dimensional photoconductive heterosystem is a good candidate for high sensitivity, low-bias infrared detectors.

Electrochemical fabrication of cadmium telluride quantum dots using porous anodized aluminum on a silicon substrate

A uniform array of uniformly sized cadmium telluride quantum dots is fabricated in this work. A practical multilayer template, composed of a silicon substrate, several metal layers and porous anodized aluminum is used as a mold to electrochemically grow II–VI semiconductor quantum wires. Only standard silicon processing and in situ electrochemical techniques are used to fabricate the template and quantum dots without the need for thin film handling procedures. Fabrication procedures are described and structural characterization using scanning electron microscopy and optical characterization using luminescence are performed. The extension of this technique to the fabrication of quantum wires and quantum wire heterostructures and superlattices, is discussed.

Design and numerical modeling of normal-oriented quantum wire infrared photodetector array

A design for an IR photodetector is proposed and described that uses an array of II–VI semiconductor quantum wire heterostructures and that uses intersubband transitions in the conduction band of the wires as the IR detection mechanism. The detection mechanism of these quantum wire infrared detectors (QRIP) is similar to that used in quantum well infrared photodetectors (QWIP) but important differences arise due to the further confinement of the electrons in an additional dimension. QWIPs are briefly described, including their undesirable aspects and how QRIPs offer solutions to these problematic issues. The electron quantum states, absorption and other important aspects of several QRIP designs are calculated using analytical and finite difference techniques. A potential design for a focal plane array using QRIPs is described that uses a nanopatterned alumina template for DC electrodeposition of II–VI semiconductor quantum wires oriented normal to the substrate.

Conductance anomalies in gated V-groove quantum wires

We present transconductance measurements on gated V-grooveGaAs/AlGaAs quantum wire heterostructures grown by metal-organicchemical vapour deposition. The results show anomalously largenegative magnetoresistance for a magnetic field perpendicular to thecurrent, and oscillations in conductancewith the field oriented along the width of the wire. Clear steps inconductance versus gate voltage are also observed and these areconsistent with the charging of a nearby `droplet' with each stepcorresponding to a change in the occupation of a single electron.

Strain dependence on barrier material and its effect on the temperature stability of photoluminescence wavelength in self-assembled GaInAs quantum wires

The strain-induced lateral-layer ordering process was used to fabricate GaInAs quantum wire (QWR) heterostructures on InP whose QWR layers are separated by different barrier materials. Using cross-sectional transmission electron microscopy the microstructure of the different QWR samples was studied. It was found that GaInAs QWR heterostructures with nominally lattice matched AlGaInAs barriers resulted in heavily strained QWR regions relative to the same structure with nominally lattice matched AlInAs or InP barriers. Furthermore, photoluminescence studies demonstrate that these heavily strained QWR samples with AlGaInAs barriers have a wavelength-shift rate of 0.2 to 1.0 Å/°C in the 293–360 K range. This is in comparison to GaInAs QWR samples with ternary or binary barriers and a GaInAs quantum well control sample that have wavelength-shift rates of ∼2.5 and ∼6.0 Å/°C, respectively. It is shown empirically that a minimum amount of strain in the GaInAs QWR/AlGaInAs barrier region is required to achieve wavelength-shift rates below 1 Å/°C.

Self-assembled GaInAs quantum wire heterostructure design for temperature stabilized emission wavelength

The strain-induced lateral-layer ordering process is used to fabricate GaInAs quantum wire (QWR) heterostructures on (1 0 0)-InP via molecular beam epitaxy. Using photoluminescence (PL) spectroscopy, the effect of the barrier material between QWR layers on the peak PL behavior with respect to temperature is investigated. In addition, two types of growth interrupt schemes were used to optimize the PL characteristics of the QWR heterostructure. Growth pauses during the deposition of the QWR layer were explored as well as growth pauses in between the QWR layer and barrier material. Consequently, peak PL wavelengths stabilized to less than 1 Å/°C above room temperature have been obtained for GaInAs QWR heterostructures using nominally lattice-matched AlGaInAs barriers and QWR-barrier growth interrupts to improve PL intensity. It is shown that strain is the main factor behind this PL behavior with respect to temperature.

Temperature dependent polarization switching and band-gap anomalies in strained GaxIn1−xAs quantum wire heterostructures

We report on the polarized photoluminescence (PPL) properties of strained GaxIn1−xAs quantum wire (QWR) heterostructures formed in situ by the strain-induced lateral-layer ordering process. It is found that the PPL spectra of these QWRs have unique properties that depend on temperature and orientation of the pump polarization with respect to the QWR direction. In particular, the dominant polarization switches when the sample is warmed from 77 to 300 K provided the pump polarization is parallel to the QWRs. This indicates that the light-hole (LH) and heavy-hole (HH) bands cross with increasing temperature, which implies that the multiaxial strain in this material is a function of temperature. Furthermore, this effect is only observed in GaxIn1−xAs QWR heterostructures that display anomalous band-gap stability with respect to temperature. It is believed that the strain induced temperature dependent LH–HH crossing as evidenced by the polarization switching switching effect is responsible for this anomaly.

Growth of GaInAsP quantum wire heterostructures using the strain-induced lateral-layer ordering process

Abstract We present a generalized strain-balance theory on the strain-induced lateral-layer ordering (SILO) process, where various combinations of short-period superlattice (SPS) constituents are used to fabricate novel quantum wire (QWR) heterostructures. Based on the theory, we have fabricated two sets of QWRs in the GaInP and GaInAsP material systems with emission wavelengths of 6200 A and 1 μm, respectively. Growth of ( GaP ) 2 ( InP ) 1 SPS was used to create QWRs on GaAs0.66P0.34-based substrates. Also, QWRs were formed using ( GaP ) 2.2 ( InAs ) 1 SPS on GaAs substrates. In both these cases, the monolayer pairings of the SPS constituents were unequal. Transmission electron microscopy and polarized photoluminescence spectroscopy confirm the formation of QWRs. These results support the validity of the generalized theory of the SILO process.

Magnetic-field effects in the luminescence of V-shaped quantum wires

Quantum wire heterostructures, such as V- and T-shaped wires, are very promising candidates for low-threshold lasing. A crucial issue is the excitonic vs. free-carrier nature of the radiative recombination. Here, we report on magnetophotoluminescence studies of GaAs and InGaAs V-shaped wires that allow to discriminate different regimes of radiative recombination.

Growth of GaxIn1−xAs quantum wire heterostructures by the strain-induced lateral-layer ordering process

GaxIn1−xAs quantum wire (QWR) arrays were grown in situ by molecular beam epitaxy. A lateral composition modulation was achieved in the GaxIn1−xAs material system via the strain-induced lateral layer ordering process occurring spontaneously when (GaAs)n/(InAs)n short-period superlattices are grown on (100)-oriented on-axis InP substrates. The resulting quasiperiodic modulation of the GaxIn1−xAs composition, with a periodicity of 300 Å, offers a lateral quantum confinement along the growth plane. The quantum confinement in the growth direction is provided by sandwiching the thin (∼100 Å) laterally ordered GaxIn1−xAs layer between layers of bulk Al0.24Ga0.24In0.52As. The formation of the QWR array is confirmed by cross-sectional transmission electron microscopy and polarized photoluminescence measurements.

Strain and piezoelectric fields in arbitrarily oriented semiconductor heterostructures. I. Multiple quantum wells.

In this paper, we investigate the strain and piezoelectric fields of semiconductor quantum-wire heterostructures. The quantum-wire structures considered here are those which are fabricated from superlattices grown on a thick substrate crystal and are laterally confined by vacuum or air. The strain-tensor components are calculated by minimization of the strain-energy density and by imposing the commensurability constraint at the wire/substrate interface only along the wire direction. In fact, due to the finite lateral dimension of the structure, a relaxation of the crystal lattice perpendicular to the wire occurs until the minimum of the strain energy is reached. A detailed study of the symmetry of the distorted crystallographic unit cell as a function of the substrate orientation and the wire direction is presented. We find that due to the anisotropic elastic lattice relaxation of the quantum wires a lower-symmetry lattice deformation than a tetragonal one can occur also for the [001]-substrate orientation. We show that, for zinc-blende heterostructure quantum wires, these strain fields can generate high piezoelectric fields that can be different from zero even for the high-symmetry [001]-interface orientation. Due to the presence of these high piezoelectric fields in quantum-wire structures, a one-dimensional electron gas can be produced.

Strained GaxIn1−xP multiple quantum wire light-emitting diodes: A luminescence polarization study

Strongly polarized photoluminescence and electroluminescence spectra have been obtained from strained GaxIn1−xP quantum wire heterostructures grown on (100) oriented, on-axis GaAs substrates by an in situ epitaxial technique. The phenomenon of strain-induced lateral layer ordering has been exploited in order to create lateral superlattices of GaxIn1−xP compositionally modulated in the [110] direction with a modulation period of 96 Å. The previous and subsequent growth of lattice-matched Ga0.51In0.49P ternary alloy epilayers results in the formation of compressively strained quantum wires. Transmission electron microscopy shows the wire cross sections to be ∼48×200 Å. These structures exhibit 77 K photoluminescence spectra at 1.79 eV that are strongly (96%) polarized parallel to the wires due to strain resulting from the lateral compositional modulation. The intensity of this emission depends critically on the polarization of the incident excitation. Electroluminescence spectra from multiple quantum wire light-emitting diodes display anisotropic polarization as well. The energies and optical anisotropies of these luminescence bands are consistent with a simple theoretical analysis.

Formation of multiple quantum wires by strain-induced lateral-layer ordering process

Quantum wire heterostructures have been grown in situ using gas source molecular beam epitaxy. Lateral composition variation was achieved in the Ga x In 1- x P material system via a strain-induced lateral-layer ordering process occurring spontaneously when (GaP) n/(InP) n short-period superlattices are grown on (100)-oriented on-axis GaAs substrates. This results in a quasi-periodic modulation of the Ga x In 1- x P composition along the [110] direction with a periodicity of 95 °A. Thin (≈200°A) layers of these laterally ordered regions imbedded between bulk lattice-matched Ga 0.51In 0.49P epilayers in the longitudinal direction form quantum wire arrays. The formation of multiple quantum wires is confirmed by cross-sectional transmission electron microscopy. The Ga x In 1- x P multiple quantum wire heterostructure exhibits highly polarized photoluminescence spectra with peak intensity ratios of over 24 for the two orthogonal polarizations as a result of strain-induced lateral-layer ordering.

Removal of accidental degeneracies in semiconductor quantum wires

We obtain the energy levels of carriers in rectangular GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum-wire heterostructures with finite barriers using finite-element analysis. The energy spectra obtained are dramatically different from those determined analytically with use of the infinite-barrier approximation. The infinite-barrier approximation also introduces extra degeneracies in the energy spectra as a consequence of the separability of the potential. These accidental degeneracies are removed when the barrier height corresponding to the band offset of the surrounding medium is used in calculating the energy levels. Group-theory considerations of the square are used to explain the removal of these accidental degeneracies.