GaSb Quantum Dots Research Articles

Control of GaSb/GaAs Quantum Nanostructures by Molecular Beam Epitaxy

GaSb quantum wells (QWs) and quantum dots (QDs) were fabricated on GaAs(001) surfaces by molecular beam epitaxy. By scanning transmission electron microscopy and photoluminescence (PL) measurements, their nanostructures were evaluated. One-monolayer-thick GaSb was precisely formed by the surface exchange reaction between As and Sb atoms, which was monitored by reflection high-energy electron-beam diffraction. In the Stranski-Krastanov growth of the GaSb QDs, low growth rate and low Sb4 pressure were effective conditions for suppressing size fluctuation. As a result, we could obtain the narrowest PL linewidth of 67 meV for GaSb/GaAs QDs.

Growth of high-density InGaSb quantum dots on silicon atoms irradiated GaAs substrates

We discuss a technique that allows us to grow high-density GaSb and InGaSb quantum dots (QDs) on (0 0 1)-oriented GaAs substrates. We study the use of Si atom irradiation on the substrate surface as an anti-surfactant before the QDs fabrication. It is clear that the densities of GaSb and InGaSb QDs are drastically enhanced with the Si atom irradiation. Photoluminescence intensities from these QDs are also increased with the Si atom irradiation. These results indicate that the Si atom irradiation technique is useful to improve the properties of the Sb-based QDs.

Long-wavelength luminescence from GaSb quantum dots grown on GaAs substrates

Self-assembled GaSb quantum dots (QDs) with a photoluminescence wavelength longer than 1.3 μm were successfully grown by suppressing the replacement of As and Sb on the surface of the GaSb QDs. This result means that GaSb can thus join InAs or GaInAs as a suitable material for QD lasers for optical communications.

AFM ANALYSIS OF QUANTUM DOT STRUCTURES INDUCED BY ION SPUTTERING WITH DIFFERENT TIPS

With tips of different hardness, we analyzed the effect of the hardness and shape of the actual AFM tip on the measurement of the best GaSb quantum dot (QD) structures induced by low energy Ar + sputtering. The comparison indicated that the complete information on the detailed dot shape and order structure can be determined with the hard and good tip, while with the soft or worn tip some information on the dot height and shape cannot be obtained. Our results suggest that, in order to obtain the complete surface information, the high hardness and good AFM tip should be used for semiconductor QD structures.

450 meV hole localization in GaSb/GaAs quantum dots

The electronic properties of self-organized GaSb quantum dots (QDs) embedded in GaAs n+p diodes were investigated by capacitance–voltage and deep level transient spectroscopy. The localization energy of the hole ground state is 450 meV. State filling lowers the activation energy to 150 meV for completely charged QDs containing 15 holes. The hole retention time at room temperature for a single hole per QD is extrapolated to be in the microsecond range, about five orders of magnitude longer than in In(Ga)As/GaAs QDs. Hence, we consider GaSb/GaAs to be a suitable material system for future QD memory applications which require long storage times.

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.

Abnormal effect of growth interruption on GaSb quantum dots formation grown by molecular beam epitaxy

The growth interruption (GI) effect on GaSb quantum dot formation grown on GaAs by molecular beam epitaxy was investigated. The structure characterization was performed by reflection high-energy electron diffraction (RHEED), along with photoluminescence measurements. It is found that the GI can significantly change the surface morphology of GaSb QDs. During the GI, the QDs structures can be smoothed out and turned into a 2D-like structure. The time duration of the GI required for the 3D/2D transition depends on the growth time of the GaSb layer. It increases with the increase of the growth time. Our results are explained by a combined effect of the stress relaxation process and surface exchange reactions during the GI.

Formation of GaSb/GaAs quantum dots in MOCVD growth

The formation of GaSb quantum dots on a GaAs (0 0 1) substrate under metalorganic chemical vapor deposition conditions is investigated. Transmission electron microscopy images and photoluminescence measurements show the island density to be determined in an initial kinetically controlled phase. Subsequently, mass transfer from the two-dimensional wetting layer sustains the growth of the GaSb quantum dots (QDs). Many-particle effects in the type II QDs are investigated by excitation- and temperature-dependent photoluminescence measurements. The peculiar optical behavior is dominated by Coulomb charging and state filling of the localized holes.

Temporal evolution of GaSb/GaAs quantum dot formation

The formation of GaSb quantum dots in a GaAs matrix in the Stranski–Krastanow growth mode under metalorganic chemical vapor deposition conditions is investigated. Transmission electron microscopical images and photoluminescence measurements show the islands to nucleate during the GaSb deposition and to grow subsequently by mass transfer from the two-dimensional wetting layer. The evolving surface morphology indicates local equilibria between quantum dots and the surrounding wetting layer regions.

Ordered Quantum Dot Formation by Ion Sputtering

Ion sputtering with low energy ions at normal angle of incidence represents an alternative method for the self-organized formation of semiconductor quantum dots. Dot patterns are formed spontaneously on GaSb and InSb surfaces during sputtering and exhibit a uniform size distribution and a long-range hexagonal ordering. The diameter of the fabricated dots ranges from 15–80 nm with an aspect ratio of nearly unity and with dot densities of 3 × 1011 to 1 × 1010 cm—2. The size and density of the dots can be controlled separately by sputtering time and ion energy, respectively. GaSb quantum dots are prepared by this method through sputtering of GaSb layers in a multilayer sample down to the interface. Numerical integration of the Kuramoto-Sivashinsky equation which describes in a continuum model the dot formation are compared to the experimental findings.

Thin GaSb insertions and quantum dot formation in GaAs by MOCVD

Self-organized formation of GaSb quantum dots during MOCVD growth has been studied. Thin GaSb layers are deposited on GaAs (0 0 1), followed by a growth interruption before the deposition of a GaAs layer. The GaSb growth is performed using low V/III ratios and growth temperatures between 510°C and 550°C. Undercritical GaSb depositions lead to inhomogenities in the composition and thickness of such layers. Photoluminescence measurements suggest these quantum islands to provide zero-dimensional confinement for holes. The dependence of the luminescence on the growth temperatures indicates the formation of the quantum islands to be kinetically controlled.

Deep level transient capacitance measurements of GaSb self-assembled quantum dots

Deep level transient spectroscopy (DLTS) measurements have been made on GaAs n+p diodes containing GaSb self-assembled quantum dots and control junctions without dots. The self-assembled dots were formed by molecular beam epitaxy using the Stranski–Krastanov growth mode. The dots are located in the depletion region on the p side of the junction where they act as a potential well that may capture and emit holes. Spectra recorded for temperatures between 77 and 440 K reveal several peaks in diodes containing dots. A control sample with a GaSb wetting layer was found to contain a single broad high temperature peak that is similar to a line found in the GaSb quantum dot samples. No lines were found in the spectra of a control sample prepared without GaSb. DLTS profiling procedures indicate that one of the peaks is due to a quantum-confined energy level associated with the GaSb dots while the others are due to defects in the GaAs around the dots. The peak identified as a quantum-confined energy level shifts to higher temperatures and its intensity decreases on increasing the reverse bias. The activation energy for the quantum-confined level increases from 400 meV when measured at a low reverse bias to 550 meV for a large reverse bias. Lines with activation energies of 400, 640, and 840 meV are associated with defects in the GaAs based on the bias dependence of their peak positions and amplitudes.

Growth of stacked GaSb/GaAs self-assembled quantum dots by molecular beam epitaxy

A stacked structure of self-assembled GaSb/GaAs quantum dots (QDs) has been grown by molecular beam epitaxy in the Stranski–Krastanow growth mode. The stacked structure consists of alternate layers of GaAs, 10 nm thick, and GaSb QDs. The QDs, with nanometer-scale dimensions, were characterized by atomic force microscopy. Furthermore, we observed clearly separated photoluminescence peaks from the QDs and the wetting layer. This result indicates that the growth of the GaSb/GaAs heterostructure is well controlled despite the deposition of a few monolayers.

Photoluminescence studies of self-assembled InSb, GaSb, and AlSb quantum dot heterostructures

Photoluminescence (PL) spectroscopy has been performed on a set of self-assembled InSb, GaSb, and AlSb quantum dot (QD) heterostructures grown on GaAs. Strong emission bands with peak energies near 1.15 eV and linewidths of ∼80 meV are observed at 1.6 K from 3 monolayer (ML) InSb and GaSb QDs capped with GaAs. The PL from a capped 4 ML AlSb QD sample is weaker with peak energy at 1.26 eV. The PL bands from these Sb-based QD samples shift to lower energy by 20–50 meV with decreasing excitation power density. This behavior suggests a type II band lineup. Support for this assignment, with electrons in the GaAs and holes in the (In,Ga,Al)Sb QDs, is found from the observed shift of GaSb QD emission to higher energies when the GaAs barrier layers are replaced by Al0.1Ga0.9As.

Self-assembled InSb and GaSb quantum dots on GaAs(001)

Quantum dots of InSb and GaSb were grown on GaAs(001) by molecular-beam epitaxy. In situ scanning tunneling microscopy measurements taken after 1–2 monolayers of InSb or GaSb growth reveal the surface is a network of anisotropic ribbon-like platelets. These platelets are a precursor to quantum dot growth. Transmission electron microscopy measurements indicate that the quantum dots are coherently strained. Quantum dots of InSb and GaSb capped by GaAs exhibit strong luminescence near 1.1 eV.

Optical investigations of the dynamic behavior of GaSb/GaAs quantum dots

Time-resolved radiative recombination measurements on GaSb quantum dots have been performed. The GaSb quantum dots are grown by molecular beam epitaxy on (100) GaAs through a self-assembly process. Time-resolved measurements show that, after a rapid hole capture process, the photoluminescence decays with a fast and a slow component. The fast component is shortened significantly with higher excitation intensity while the slow component is roughly constant. The radiative lifetimes are much longer than the lifetimes of ordinary GaSb quantum wells with a straddling band lineup. These results support a staggered band lineup and space charge induced band-bending model.

Radiative recombination in type-II GaSb/GaAs quantum dots

Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self-organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan-view transmission electron microscopy studies reveal well developed rectangular-shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Type II broken-gap quantum wires and quantum dot arrays: A novel concept for self-doping semiconductor nanostructures

A novel concept for creating self-doping quantum wires and quantum dot arrays based upon the InAs-GaSb material system is proposed. The unusual type II, broken-gap band line-up in this system allows charge transfer across the InAs-GaSb interfaces. We employ a recently developed coupled band formalism to examine analytically the band structure of InAs-GaSb quantum dots and wires. The analysis shows that appropriately engineered nanostructures which contain high free-carrier densities are possible without intentional impurity doping. Quantum dots in this system behave as artificial quasiatoms, with ionization energy and valence determined by fabricationally determined parameters. Synthetic p-(n-)type semiconductors may therefore be formed from arrays of InAs(GaSb) quantum dots embedded in GaSb(InAs). InAs-GaSb quantum wires are also investigated and found to exhibit self-doping behavior. Possible fabrication schemes utilizing recently developed technologies are discussed.

In-mole-fraction of InGaAs Insertion Layers Effects on the Structural and Optical Properties of GaSb Quantum Dots Grown on (001) GaAs Substrate

GaSb quantum dots (QDs) have been grown by solid-source molecular beam epitaxy on a 4-monolayer (ML) InxGa1-xAs (x = 0.07, 0.15, 0.20, and 0.25) to investigate the effects of In-mole-fraction of InGaAs insertion layers on the structural and optical properties of the GaSb QDs. The density of grown GaSb QDs is approximately 1.2-2.8109 cm−2 on In-GaAs insertion layers which depends on the In-molefraction. Dot shape and size change substantially when In-mole-fraction of InGaAs insertion layers is varied. The uniformity of GaSb QDs improves when the indium content increases. The change in freestanding QD morphology is likely due to the modified strain at different values of indium compositions in InGaAs insertion layers. The effects of In-molefraction of InGaAs insertion layer on optical properties of the QDs are studied by photoluminescence (PL). PL results show the blueshift of the emission when the indium content in InGaAs insertion layer increases.