GaSb Quantum Dots Research Articles

Growth and Raman spectra of GaSb quantum dots in GaAs matrices by liquid phase epitaxy

The self-assembled type-II GaSb quantum dots (QDs) are successfully grown on semi-insulting GaAs (100) matrix by liquid phase epitaxy technique. The topography of QDs with high growth temperature is characterized by atomic force microscopy (AFM). The cap layer, which is needed for the device fabrication, is obtained for only some tens of nanometers. The non-resonant Raman spectra are applied to investigate the GaSb-like optical phonons localized in the QDs and to confirm convincingly the existence of GaSb QDs.

Anisotropic Scattering of Elongated GaSb/GaAs Quantum Dots Embedded Near Two-Dimensional Electron Gas

Geometrically anisotropic GaSb/GaAs quantum dots elongated along [110] direction, with a lateral aspect ratio of 2-3, are embedded in the vicinity of AIGaAs/GaAs two-dimensional electron gas, using the molecular beam epitaxy by carefully controlling the Sb4 beam flux and As4 background pressure. The electron mobilities micro parallel, micro perpendicular parallel and perpendicular to the direction of dot elongation axis are measured as functions of electron concentration n(2D) at 4.2 K, respectively. As the increase of n(2D), the mobility discrepancy (micro parallel-micro perpendicular) is found to be getting larger. This mobility difference is believed to be due to the anisotropic scattering potential of the elongated GaSb quantum dots. Under Born approximation, we propose a model with the constant squared-barrier potential to calculate the n(2D) dependence of mobility and reasonable agreement is achieved between the experimental results and theoretical simulation.

Three dimensional atom probe imaging of GaAsSb quantum rings

Unambiguous evidence of ring-shaped self-assembled GaSb nanostructures grown by molecular beam epitaxy is presented on the basis of atom-probe tomography reconstructions and dark field transmission electron microscopy imaging. The GaAs capping process causes a strong segregation of Sb out of the center of GaSb quantum dots, leading to the self-assembled GaAs(x)Sb(1-x) quantum rings of 20-30 nm in diameter with x ∼ 0.33.

Effects of Interface Grading on Electronic States and Optical Transitions in GaSb Type-II Quantum Dots in GaAs

Electronic states in type-II GaSb quantum dots (QDs) in GaAs are studied theoretically and compared with experiments to clarify how the spatial overlap of holes in the dot and electrons outside is affected by the interface grading caused by the interdiffusion of Sb and As. Cone-shaped QDs with the initial size of 6 nm height and 20 nm radius are analyzed. The wavefunctions of electrons and holes and their overlap Θ are calculated. In an initial stage of grading, when the grading length Lg is below 1.2 nm, the mixing occurs only near the dot/matrix interface, leading to a decrease in Θ, since holes are squeezed into a smaller volume whereas electrons are more repelled by the dot. In later stages where Lg exceeds 1.2 nm, however, the hole confinement and the electron repulsion by the dot both weaken, leading to an increase in Θ; this accounts for a recent finding that the annealing of GaSb QDs induces a blue shift and an intensity increase in photoluminescence spectra.

Influence of as on the Morphologies and Optical Characteristics of GaSb/GaAs Quantum Dots

The influence of As atoms on the morphologies of GaSb quantum dots (QDs) is investigated. Without any special treatment, GaSb quantum rings (QRs) are observed in the embedded GaSb layer even when the uncapped layer reveals QD like morphologies. With intentional As supply after the uncapped GaSb QD deposition, a QD to QR transition is observed. The phenomenon suggests that insufficient Sb atoms on the GaSb QDs would lead to the QD to QR transition as in the case of embedded GaSb layers. With extended Sb soaking time following GaSb deposition, QD structures could be well maintained for the embedded GaSb layers. A light-emitting diode operated at room temperature is fabricated based on the GaSb/GaAs QD structure. Identical peak positions in photoluminescence and electroluminescence (EL) spectra of the device show that type-II GaSb QDs are responsible for the observed EL.

GaSb quantum rings grown by metal organic molecular beam epitaxy

GaSb quantum rings (QRs) of different sizes and shapes were grown by metal organic molecular beam epitaxy (MOMBE). Partially capped InAs quantum dots (QDs) grown on a GaAs buffer layer were flattened by In-flushing, followed by regrowth of GaSb which forms ring structure around the InAs seed dots. With the assistance of such an InAs/GaAs groundwork layer, the GaSb QRs exhibit a better defined shape compared to the spontaneously formed rings. First, rims of QRs develop around the boundary of the InAs/GaAs, then additional GaSb QDs grow on top of them preferentially along the [0−11] direction compared to [011].

Compact microdisk cavity laser with type-II GaSb/GaAs quantum dots

Microdisk lasers with active region made of type-II GaSb/GaAs quantum dots on the GaAs substrate have been demonstrated. A microdisk cavity with diameter of 3.9 μm was fabricated from a 225-nm-thick GaAs layer filled with GaSb quantum dots. Lasing at wavelengths near 1000 nm at 150 K was achieved for this microdisk. A high threshold characteristic temperature of 77 K was also observed. It is found that the lasing wavelength matches closely with the first-order whispering-gallery mode of the cavity as obtained from the finite-element method simulation.

Anisotropic transport of two-dimensional electron gas modulated by embedded elongated GaSb/GaAs quantum dots

Geometrically anisotropic GaSb/GaAs quantum dots (QDs) that are elongated along the [110] direction are embedded in the vicinity of a modulation-doped AlGaAs/GaAs heterointerface. At 4.2 K, the electron mobilities μ∥ and μ⊥, which are parallel and perpendicular to the QD elongation axis, respectively, are systematically investigated as a function of the electron concentration NS by both experimental measurements and theoretical simulations. In the experiments, the mobility ratio μ∥/μ⊥ increased with increasing NS. The mobility anisotropy, attributed to the anisotropic scattering potential of the elongated GaSb QDs, is calculated under the theoretical model of a rectangular scattering potential of GaSb QDs.

Effects of Sb/As intermixing on optical properties of GaSb type-II quantum dots in GaAs grown by droplet epitaxy

Optical properties of GaSb type-II quantum dots (QDs) in GaAs were studied and compared with a theoretical model to clarify how the spatial overlap of holes in the dot and electrons outside is affected by the interdiffusion of Sb and As. GaSb QDs were grown in a GaAs substrate by droplet epitaxy and annealed at the temperature Ta=650–850 °C to induce the Sb/As intermixing. Photoluminescence (PL) studies showed that the integrated PL intensity I decreases to less than 1/10 as Ta is raised from 650 to 750 °C, while I increases by three orders of magnitude with the increase of Ta from 750 to 850 °C. This behavior is explained by the overlap Θ between electron and hole wave functions; in an initial stage of the interdiffusion, the mixing occurs only near the dot/matrix boundary, leading to the decrease in the overlap Θ, since electrons are more repelled by the dot. In later stages, however, the hole confinement and the electron repulsion in the dot both weaken, leading to the increase in the overlap Θ.

The QD-Flash: a quantum dot-based memory device

We demonstrate the large potential of III–V compound semiconductors for a novel type of Flash memory. The concept is based on self-organized III–V quantum dots (QDs). Here the advantages of the most important semiconductor memories, the dynamic random access memory and the Flash are merged. A non-volatile memory with fast access times (<10 ns) and good endurance (>1015 write/erase cycles) as an ultimate solution seems possible. A storage time of 1.6 s at 300 K in InAs/GaAs QDs with an additional Al0.9Ga0.1As barrier was demonstrated and a retention time of 106 years is predicted by us for GaSb QDs in an AlAs matrix. In addition, a minimum write time of 6 ns is obtained for InAs/GaAs QDs. First prototypes with all-electrical data access prove the feasibility of the concept. The stored information is read-out by a two-dimensional hole gas underneath the QD layer. Time-resolved drain-current measurements demonstrate the memory operations.

EFFECTS OF ANTIMONY FLUX ON MORPHOLOGY AND PHOTOLUMINESCENCE SPECTRA OF GaSb QUANTUM DOTS FORMED ON GaAs BY DROPLET EPITAXY

We investigated effects of the antimony flux on GaSb quantum dots (QDs) formed by droplet epitaxy. Ga droplets were first formed on GaAs and exposed to Sb4 molecular beam at 200 °C, where the flux PSb of Sb beam was varied from 2.4 to 12.8 × 10-7 Torr. The samples were then annealed for 2 minutes under the Sb flux. An atomic microscope study showed that the diameter of GaSb QDs increases and the density decreases, as the Sb flux PSb is increased. This indicates that the coalescence process of GaSb QDs occurs and is accelerated by the increase of the Sb flux. In a photoluminescence (PL) study, we observed a broad peak of GaSb QDs in all samples, while a strong luminescence of a wetting layer (WL)-like structure was found only in the samples prepared with the high Sb flux. This suggests that the PL of WL is controllable by adjusting the flux PSb of Sb beam.

Growth of GaSb and InSb quantum dots on GaAs (311)A by droplet epitaxy

AbstractWe have studied the growth of GaSb quantum dots (QDs) on GaAs (311)A surfaces by droplet epitaxy. By atomic force microscopic studies, it was found that smaller and denser GaSb QDs are formed on GaAs (311)A than (100). We also tried to grow InSb QDs by droplet epitaxy. On a GaAs (100) substrate, a large ring‐like structure is formed instead of a QD structure. In contrast, InSb QDs were successfully grown on a GaAs (311)A substrate. Photoluminescence properties of these QDs were also investigated. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Confined States of Individual Type-II GaSb/GaAs Quantum Rings Studied by Cross-Sectional Scanning Tunneling Spectroscopy

Combined cross-sectional scanning tunneling microscopy and spectroscopy results reveal the interplay between the atomic structure of ring-shaped GaSb quantum dots in GaAs and the corresponding electronic properties. Hole confinement energies between 0.2 and 0.3 eV and a type-II conduction band offset of 0.1 eV are directly obtained from the data. Additionally, the hole occupancy of quantum dot states and spatially separated Coulomb-bound electron states are observed in the tunneling spectra.

Short range scattering mechanism of type-II GaSb/GaAs quantum dots on the transport properties of two-dimensional electron gas

We have studied the scattering process of AlGaAs/GaAs two-dimensional electron gas with the nearby embedded GaSb/GaAs type-II quantum dots (QDs) at low temperature. Quantum Hall effect and Shubnikov–de Haas oscillation were performed to measure the electron density n2D, the transport lifetime τt, and the quantum lifetime τq under various biased gate voltage. By comparing measured results of QDs sample with that of reference sample without embedded QDs, mobilities (transport mobility μt and quantum mobility μq) dominated by GaSb QDs scattering were extracted as functions of n2D. It was found that the ratios of τt to τq were varying within the range of 1–4, implying the scattering mechanism belonging to the sort of short-range interaction. In the framework of Born approximation, a scattering model considering rectangular-shaped potential with constant barrier height was successfully applied to explain the transport experimental data. In addition, an oscillating ratio of τt/τq with the increasing n2D was predicted in the model.

GaSb quantum dot morphology for different growth temperatures and the dissolution effect of the GaAs capping layer

We compare the characteristics of GaSb quantum dots (QDs) grown by molecular beam epitaxy on GaAs at temperatures from 400 to 490 °C. The dot morphology, in terms of size, shape and density, as determined by atomic force microscopy (AFM) on uncapped QDs, was found to be highly sensitive to the growth temperature. Photoluminescence (PL) spectra of capped QDs are also strongly dependent on growth temperature, but for samples with the highest dot density, where the QD luminescence would be expected to be the most intense, it is absent. We attribute this to dissolution of the dots by the capping layer. This explanation is confirmed by AFM of a sample that is thinly capped at 490 °C. Deposition of the capping layer at low temperature resolves this problem, resulting in strong QD PL from a sample with a high dot density.

Thermal annealing of GaSb quantum dots in GaAs formed by droplet epitaxy

Abstract We investigate effects of annealing on GaSb quantum dots (QDs) formed by droplet epitaxy. Ga droplets grown on GaAs are exposed to Sb molecular beam and then annealed at T a =340–450 °C for 1 min to form GaSb QDs. An atomic force microscope study shows that with the increase of T a , the average diameter of dots increases by about 60%, while their density decreases to about 1/3. The photoluminescence (PL) of GaSb QDs is observed at around 1 eV only for those samples annealed above T a =380 °C, which indicates that the annealing process plays an important role in forming high quality GaSb QDs.

GaSb/GaAs type-II quantum dots grown by droplet epitaxy

We demonstrate the formation of GaSb quantum dots (QDs) on a GaAs(001) substrate bydroplet epitaxy using molecular beam epitaxy. The high crystal quality and bimodal sizedistribution of the QDs are confirmed using atomic force and transmission electronmicroscope images. A staggered type-II QD band structure is suggested by aphotoluminescence peak that is blue shifted with increasing excitation intensity, alarge emission polarization of 60%, and a long carrier decay time of 11.5 ns. Ourresearch provides a different approach to fabricating high quality GaSb type-II QDs.

Strain-induced Stranski–Krastanov three-dimensional growth mode of GaSb quantum dot on GaAs substrate

The growth dynamics of self-assembled GaSb quantum dots (QDs) on GaAs substrate was investigated using kinetic Monte Carlo method. The strain induced by the lattice mismatch between the epitaxial material and the substrate was shown to be directly responsible for the three-dimensional QD formation. Different geometries of the initial seeds on the surface which are equally favorable from an energy point of view can result in different GaSb nanostructures (nanostrips and nanoring).

Contrast mechanisms in cross-sectional scanning tunneling microscopy of GaSb/GaAs type-II nanostructures

Cross-sectional scanning tunneling microscopy results on GaSb quantum wells and dots in GaAs are found to exhibit a narrow, sharply defined contrast of the nanostructure at negative sample bias, but a smoothly broadened contrast at positive sample bias. This contrast is related to the specific type-II band alignment of GaSb/GaAs heterostructures in combination with tip-induced band bending. The corresponding model is quantitatively verified by numerical simulations of band bending and tunnel current profiles combined with calculations of cleavage-induced strain relaxation.

Growth of GaSb dots on GaAs(100) by droplet epitaxy

AbstractWe have studied the growth of GaSb quantum dots (QDs) on GaAs by droplet epitaxy. First, Ga droplets are formed on GaAs and then exposed to Sb flux to be clad by large granular crystals of Sb; by annealing this wafer at 380 °C, GaSb QDs are successfully formed. By atomic force microscopic studies, it is found that GaSb QDs thus formed are on average 9.2 nm in height, 74 nm in diameter, and 7.8 × 109 cm2 in density. Photoluminescence properties of these QDs have been also investigated. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)