Thin GaAs Research Articles

Role of In desorption for formation of self-organized (In,Ga)As quantum wires on GaAs(1 0 0) during superlattice formation

We investigate the role of In desorption for (In,Ga)As self-organized quantum wire (QWR) formation on GaAs (1 0 0) based on elongated island formation, annealing, and stacking in (In,Ga)As/GaAs superlattice growth. We observe well-defined QWR formation only in the presence of thin GaAs capping of the elongated islands before annealing. Without thin GaAs capping, the In composition becomes too low for controlling the nucleation of (In,Ga)As islands in subsequent layers, necessary for QWR formation. Hence, thin GaAs capping is crucial to reproducibly balance In desorption for formation and vertical correlation of well-defined QWRs, serving as unique template for one-dimensional quantum dot ordering.

Breakdown probabilities for thin heterostructure avalanche photodiodes

The recurrence theory for the breakdown probability in avalanche photodiodes (APDs) is generalized to heterostructure APDs that may have multiple multiplication layers. The generalization addresses layer-boundary effects such as the initial energy of injected carriers as well as the layer-dependent profile of the dead space in the multiplication region. Reducing the width of the multiplication layer serves to both downshift and sharpen the breakdown probability curve as a function of the applied reverse-bias voltage. In structures where the injected carriers have an initial energy that is comparable to the ionization threshold energy, the transition from linear mode to Geiger-mode is more abrupt than in structures in which such initial energy is negligible. The theory is applied to two recently fabricated Al/sub 0.6/Ga/sub 0.4/As-GaAs heterostructure APDs and to other homostructure thin GaAs APDs and the predictions of the breakdown-voltage thresholds are verified.

GaSb quantum dot growth using InAs quantum dot stressors

Metalorganic vapor phase epitaxy of GaSb quantum dots (QDs) grown on top of a layer of InAs seed QDs shows a vertically aligned correlation if thin GaAs spacer layers deposited at low temperature are used. Introduction of an annealing step after spacer deposition strongly improves the crystalline quality of the spacer layer. Vertically anticorrelated ordering is found on annealed spacers in addition to aligned ordering. Good optical properties are achieved for stacked QD structures with an InGaAs QD layer on top of seeded GaSb QDs. The photoluminescence of these structures is red shifted by 75 meV with respect to the emission of a single InGaAs QD layer.

Terahertz optical properties of thin doped contact layers in GaAs device structures

We investigate the transmittance of thin doped GaAs layers in the terahertz (THz) frequency range taking into account multiple reflections. Our experimental and theoretical study aims at providing a guideline for designing the top-side contact layers for THz emitters and receivers with direct, antenna-free coupling of the radiation. It is shown that the surface conductivity of the contact layer is the determining factor for the THz transmittance.

As-mediated stacking fault in wurtzite GaN epilayers

Growth of a thin GaAs layer embedded in wurtzite GaN was performed on sapphire (0001) by rf-plasma assisted molecular-beam epitaxy. A well-confined As-rich layer with a tail in the cap layer was measured by secondary ion mass spectrometry. Transmission electron microscopy studies demonstrated the formation of basal plane stacking faults, which were correlated with the presence of As in the layer. High-resolution microscopy revealed the stacking fault as a thin platelike cubic inclusion in the hexagonal GaN.

Regrowth of high-quality Al0.3Ga0.7As/GaAs superlattices on laterally oxidized digital AlxGa1−xAs (x>0.80) alloys

Digital AlxGa1−xAs alloys consisting of a repeated structure of alternating layers of AlAs and GaAs were grown between layers of GaAs. Lateral oxidation characteristics of these digital AlxGa1−xAs (x>0.8) alloys were determined. Decreasing aluminum compositions resulted in slower oxidation rates, similar to true AlxGa1−xAs alloys. Further investigations of the oxidation characteristics of a digital Al0.98Ga0.02As alloy indicate that shorter periods (24.5 monolayers (ML) AlAs/0.5 ML GaAs) showed slower oxidation rates than the longer period alloys (49 ML AlAs/1 ML GaAs or 98 ML AlAs/2 ML GaAs). As the GaAs layers become closer together (that is a digital alloy with a shorter period), the material more closely resembles a true alloy showing slower oxidation rates. Regrowth of an Al0.3Ga0.7As/GaAs superlattice on the laterally oxidized samples, capped with a thin GaAs layer, was performed. The high-temperature regrowth process caused delamination problems for the oxidized conventional AlAs material, resulting in a rough surface morphology causing a decrease in photoluminescence (PL) intensity. Oxidized digital alloys allowed for a large increase in useable surface area and higher PL intensities, indicating the superiority of the digital AlxGa1−xAs alloys for regrowth applications.

Ferromagnet-semiconductor hybrid structures: Hall devices and tunnel junctions

For spintronics, both, the properties of small ferromagnetic particles as well as the properties of ferromagnet-semiconductor hybrid structures are of importance. Below we describe how micro-Hall magnetometry provides information about the magnetisation switching of small ferromagnetic particles. In the second part we demonstrate spin dependent transport through thin GaAs membranes. The latter experiments point to the important role of spin-flip scattering in ferromagnet-semiconductor hybrid structures.

High internal quantum efficiency, narrow linewidth InGaAs/GaAs/AlGaAs quantum wire light-emitting diodes

High internal quantum efficiency (∼60%), narrow linewidth (as narrow as 14 meV) exciton emission at room temperature has been obtained using strained InGaAs V-groove quantum wire (QWR) light-emitting diodes (LEDs). The high efficiency is achieved with the aid of selective carrier injection through self-ordered AlGaAs vertical quantum wells (VQWs), where the VQWs are separated from the InGaAs QWRs by thin GaAs spacer layers in order to reduce nonradiative recombination and inhomogeneous alloy broadening. Evidence for excitonic recombination in these LEDs up to RT is provided by measurements of the emission energy shifts at high magnetic fields.

Variations in critical coverage for InAs/GaAs quantum dot formation in bilayer structures

Reflection high-energy electron diffraction and scanning tunnelling microscopy (STM) have been used to study InAs/GaAs quantum dot (QD) formation in bilayer QD structures grown by molecular beam epitaxy on GaAs(0 0 1) substrates. For GaAs spacer layers thicknesses <200 Å, there is a reduction in the critical coverage ( θ crit) required to form the second layer QDs, the change in θ crit compared with first layer growth depending on the size of the QDs in the initial layer. STM measurements of the uncapped QDs show that second layer dots grown on thick GaAs spacer layers (>200 Å) are identical to those grown in the first layer. By contrast, QD formation on a thin spacer layer (<200 Å) results in a lower density of larger QDs that nucleate preferentially on top of the mounds that dominate the morphology of the relatively thin GaAs capping layer. The introduction of an annealing step before second layer QD growth smooths the spacer layer surface morphology but has no effect on θ crit. The implication is that strain is the dominant cause of the observed variation in second layer θ crit.

Arsenic surface segregation during the molecular-beam epitaxial growth of GaAs embedded in wurtzite GaN

We report on the growth of a thin GaAs layer embedded in wurtzite GaN prepared on (0001) sapphire substrate by rf-plasma molecular-beam epitaxy. It was found that the As surface segregation during the growth of the GaN cap layer can be controlled by the growth parameters, i.e., the growth interruption and subsequent in situ annealing process implemented in this study. The arsenic profile of the cap layer was accurately evaluated using an empirical segregation model. The surface morphology was also investigated using atomic force microscopy.

Large four-wave mixing of spatially extended excitonic states in thin GaAs layers.

We study the size dependence of the nonlinear response of weakly confined excitons for the size region beyond the long wavelength approximation regime. The observed degenerate-four-wave mixing signal of GaAs thin layers exhibits an anomalous size dependence, where the signal is resonantly enhanced at a particular thickness region. The theoretical analysis elucidates that this enhancement is due to the size-resonant enhancement of the internal field with a spatial structure relevant to the nondipole-type excitonic state. These results establish the formerly proposed new type of size dependence of nonlinear response due to the nonlocality induced double resonance.

Coupled-stripe quantum-well-assisted AlGaAs–GaAs–InGaAs–InAs quantum-dot laser

Data are presented on the coupled-stripe laser operation (continuous wave, 300 K) of a single InAs quantum-dot (QD) layer coupled via a thin (5 Å) GaAs barrier to an auxiliary strained InGaAs quantum well (QW) grown (confined) in an AlGaAs–GaAs heterostructure. Because of strain-induced (QW strain) improvement of the QD growth and QD alignment along diagonal (reflecting) ridges, the InGaAs-QW+InAs-QD crystals exhibit high gain along and across laser stripes, which is advantageous for coupled-stripe laser operation. A twin-stripe single-QD-layer QW+QD laser (4 μm stripes on 6 μm centers) of usual cleaved length, &amp;lt; 500 μm, is capable of continuous 300 K operation, with only probe heat-sink clamping and testing, at &amp;gt;50 mW.

Strain relaxation and interfacial stability in III–V semiconductor strained-layer heteroepitaxy: atomistic and continuum modeling and comparisons with experiments

A systematic study is presented of interfacial stability and strain relaxation through misfit dislocation formation in III–V semiconductor layer-by-layer heteroepitaxy. A multiscale modeling strategy is developed that links continuum elasticity theory with atomistic structural relaxation and Monte Carlo simulations using a valence force field description of interatomic interactions. Results are presented for the energetics of the transition from a coherent to a semicoherent film/substrate interface consisting of a misfit dislocation network, the semicoherent interface structures, the associated strain fields, and the film surface morphological characteristics for InAs epitaxy on GaAs(1 1 1)A. The capability of continuum elasticity theory to provide a satisfactory description of the atomistic simulation results is discussed. In addition, using thin compliant substrates and grading the composition of the deposited film are demonstrated to have beneficial effects on film strain relaxation. Furthermore, the dynamics of strain relaxation is analyzed based on a phenomenological mean-field theoretical framework. Our theoretical results are in very good agreement with our experimental measurements on InAs/GaAs(1 1 1)A samples for films grown by molecular beam epitaxy on thick and thin GaAs buffer layers.

Controlling the electron tunneling through InAs self-assembled dots

We investigate electron tunneling through GaAs/(AlGa)As/GaAs, single-barrier tunnel diodes in which a layer of self-assembled InAs quantum dots is incorporated onto the center plane of the (AlGa)As tunnel barrier. We study the effect on the electrical conduction of substrate orientation and of the inclusion of thin GaAs cladding layers on either side of the dot layer. We find that the presence of the cladding layers increases the conduction. We attribute this to a raising of the energy of the electron states, probably due to the diffusion of Ga into the dots. Photoluminescence measurements confirm this hypothesis. We show that this effect can be used to tailor the transport characteristics for specific applications.

Height control of InAs/GaAs quantum dots by combining layer-by-layer in situ etching and molecular beam epitaxy

A technique for controlling the height of InAs quantum dots (QDs) on GaAs surfaces is proposed and investigated. This technique involves combining layer-by-layer in-situ AsBr3 etching and molecular beam epitaxy (MBE). One layer of InAs QDs in Stranski–Krastanow (SK) growth mode is initially deposited on a GaAs surface by MBE. After growth of a thin GaAs layer on the InAs SK QDs, in situ etching of this thin GaAs layer creates an array of nanoholes vertically aligned with the underlying SK QDs on the etched GaAs. The nanoholes offer highly selective nucleation centers for the subsequent growth of InAs QDs. Finally, a second-layer of QDs is grown by providing InAs in limited quantity of about 1 monolayer (ML). Atomic force microscopy surface observations show that the nanoholes are completely filled, and that a planar growth surface is obtained with ∼1 ML. Furthermore, cross-sectional structural observations with a transmission electron microscope and energy dispersive x-ray analysis reveal that vertically overlapping InAs QDs are achieved. These results demonstrate that this technique can be applied to form height-controlled InAs QDs.

Electronic structure and photoemission spectra of thin (GaAs) n(AlAs) n superlattices

Increased translational period of crystalline superlattices gives rise to qualitative modifications of the electron band structure E( k): minigaps appear at new Brillouin zone boundaries and band dispersions are reduced into narrower allowed energy bands. Modifications of the wave functions, consisting in electron confinement into one of the two components of a superlattice can affect the intensities of photoemitted electrons. The layer-resolved contributions from a few topmost layers to the photoelectron intensity are evaluated in the one-step model and the importance of the related optical matrix elements is shown. Large number of surface states and resonances connected with superlattice can be expected. This expectation is confirmed by evaluation of the local densities of electron states for the unreconstructed (1 0 0) surface of the 2×2 superlattice. The space distribution of localized states is presented. Energy distribution curves for normally photoemitted electrons are analyzed from the above viewpoints.

Fabrication of height-controlled InAs quantum dots on GaAs surfaces by in situ AsBr 3 etching and molecular beam epitaxy

A technique for controlling the height of InAs quantum dots (QDs) on GaAs surfaces has been investigated by combining layer-by-layer in situ AsBr 3 etching and molecular beam epitaxy (MBE). One layer of InAs QDs in the Stranski–Krastanow (SK) growth mode is first formed on a GaAs surface by MBE. After a thin GaAs film is deposited on the InAs SK QDs, in situ etching creates an array of nanoholes on the etched GaAs surface. The nanoholes offer highly selective nucleation sites for the subsequent growth of InAs QDs. Finally, second-layer QDs are grown by providing InAs of only about 1 monolayer (ML). Atomic force microscopy surface observations indicate that a planar growth surface is obtained with the supplied amount of ∼1 ML of InAs. Cross-sectional structural observations conducted using a scanning transmission electron microscope and energy dispersive X-ray analysis reveal that vertically overlapping InAs QDs are achieved. These results demonstrate the formation of height-controlled InAs QDs.

Photoluminescence and the structure of heterointerfaces of (311)A-and (311)B-oriented GaAs/AlAs superlattices

GaAs/AlAs superlattices grown simultaneously on GaAs substrates with the (311)A and (311)B orientations have been studied by photoluminescence and high-resolution transmission electron microscopy with a Fourier analysis of images. A periodic interface corrugation is observed for (311)B superlattices. A comparison of the structure of (311)A and (311)B superlattices indicates that the corrugation occurs in both cases and its period along the \([01\overline 1 ]\)direction is equal to 3.2 nm. The corrugation is less pronounced in (311)B superlattices, wherein it exhibits an additional modulation (long-wavelength disorder) with the characteristic lateral size exceeding 10 nm. The vertical correlation of regions rich in GaAs and AlAs, which is well observed in (311)A superlattices, is weak in (311)B superlattices due to the occurrence of long-wavelength disorder. The optical properties of (311)B superlattices are similar to those of (100) ones and differ radically from those of (311)A superlattices. As distinct from (311)B, strong photoluminescence polarization anisotropy is observed for (311)A superlattices. It is shown that it is the interface corrugation rather than the crystallographic (311) surface orientation that determines the optical properties of (311)A corrugated superlattices with thin GaAs and AlAs layers.

Nucleation and ripening of seeded InAs/GaAs quantum dots

We have investigated the nucleation and ripening of pairs of InAs/GaAs quantum dot layers separated by thin (2–20 nm) GaAs spacer layers. Reflection high energy electron diffraction (RHEED) measurements show that the 2D–3D transition in the second layer can occur for less than 1 monolayer deposition of InAs. Immediately after the islanding transition in the second layer chevrons were observed with included angles as low as 20° and this angle was seen to increase continuously to 45±2° as more material was deposited. Atomic force microscopy showed the dot density in both layers to be the same. It is proposed that surface morphology can radically alter processes that determine the nucleation and ripening of the 3D islands.

Dispersion and lifetimes of electromagnetic modes attached to strongly textured slab waveguides

The influence of strong, two-dimensional (2D) periodic texture on the dispersion and lifetimes of resonant electromagnetic modes localized in the vicinity of thin (80 to 155 nm) GaAs membranes supported on thick (1 to 2 μm) wet-oxidized AlAs cladding layers is reported. A method of extracting this information by fitting Fano-like features in broadband specular reflectivity spectra is described in detail, and applied to samples containing both triangular and square 2D lattices. A new and easier method of probing the properties of these modes is also demonstrated. It involves incorporating a superlattice of ‘defects’ in the principal crystal lattice, and monitoring the defect diffraction spectrum, rather than the specular reflectivity.