GaSb Quantum Dots Research Articles

Characterization of Below-Bandgap Absorption in Type II GaSb Quantum Dots in GaAs Solar Cells

Characterization of Below-Bandgap Absorption in Type II GaSb Quantum Dots in GaAs Solar Cells

Unveiling the electronic structure of GaSb/AlGaSb quantum dots emitting in the third telecom window

Epitaxially-grown semiconductor quantum dots (QDs) provide an attractive platform for the development of deterministic sources of high-quality quantum states of light. Such non-classical light sources are essential for quantum information processing and quantum communication. QDs emitting in the telecom wavelengths are especially important for ensuring compatibility with optical fiber systems required to implement quantum communication networks. To this end, GaSb QDs fabricated by filling local-droplet etched nanoholes are emerging as a viable approach, yet the electronic properties of such nanostructures have not been studied in detail. In this article, an insight into the electronic structure and carrier dynamics in GaSb/AlGaSb QDs is provided through a systematic experimental analysis of their temperature-dependent photoluminescence behavior. A steady-state rate equation model is used to reveal the relevant energy barriers for thermally activated carrier capture and escape processes. Furthermore, results of detailed theoretical simulations of quantum-confined energy states using the multi-band k·p model and the effective mass method are presented. The purpose of the simulations is to reveal the direct and indirect energy states, carrier wavefunctions, and allowed optical transitions for GaSb QDs with different physical dimensions.

Statistical model of electronic structure in InAs, InP, GaSb, and Si quantum dots with surface roughness

Simulations based on the sp 3 d 5 s * empirical tight-binding method were performed to provide a statistical understanding of the electronic structures and bandgap distributions of III–V (InAs, InP, GaSb) and IV (Si) semiconductor quantum dots (QDs) with surface roughness. The electronic states and wavefunctions of QDs with surface roughness of different sizes, shapes, and materials were computed. The effects of surface roughness on the electronic structures and the bandgap distributions of QDs were investigated. The results show that the bandgaps of QDs of considered materials/sizes/shapes increase on average when introducing surface roughness. It is shown that the simulated bandgap distributions of QDs with surface roughness can be reproduced by a simple model formula, which can be applied to different materials, sizes, and shapes. The model formula was derived by assuming that removing and adding of one atom procedures are independent random processes.

Strain‐Free GaSb Quantum Dots as Single‐Photon Sources in the Telecom S‐Band

AbstractGenerating single photons in the telecommunication wavelength range from semiconductor quantum dots (QDs) and interfacing them with spins of electrons or holes is of high interest in recent years, with research mainly focusing on indium‐based QDs. However, there is not much data on the optical and spin properties of gallium antimonide (GaSb) QDs, despite it being a physically rich system with an indirect to direct bandgap crossover in the telecom wavelength range. This work investigates the (quantum‐) optical properties of GaSb QDs, which are fabricated by filling droplet‐etched nanoholes in an aluminum gallium antimonide (AlGaSb) matrix. Photoluminescence (PL) features from isolated and highly symmetric QDs are observed that exhibit narrow linewidth in the telecom S‐band and show an excitonic fine structure splitting of up to µeV. Moreover, time‐resolved measurements of the decay characteristics of an exciton are performed and the second‐order photon autocorrelation function of the charge complex is measured to , revealing clear antibunching and thus proving the capability of this material platform to generate non‐classical light.

Band-like transport in "green" quantum dot films: The effect of composition and stoichiometry.

Two-dimensional quantum dot (QD) arrays are considered as promising candidates for a wide range of applications that heavily rely on their transport properties. Existing QD films, however, are mainly made of either toxic or heavy-metal-based materials, limiting their applications and the commercialization of devices. In this theoretical study, we provide a detailed analysis of the transport properties of "green" colloidal QD films (In-based and Ga-based), identifying possible alternatives to their currently used toxic counterparts. We show how changing the composition, stoichiometry, and the distance between the QDs in the array affects the resulting carrier mobility for different operating temperatures. We find that InAs QD films exhibit high carrier mobilities, even higher compared to previously modeled CdSe (zb) QD films. We also provide the first insights into the transport properties of properly passivated InP and GaSb QD films and envisage how realistic systems could benefit from those properties. Ideally passivated InP QD films can exhibit mobilities an order of magnitude larger compared to what is presently achievable experimentally, which show the smallest variation with (i) increasing temperature when the QDs in the array are very close and (ii) an increasing interdot distance at low operating temperatures (70K), among the materials considered here, making InP a potentially ideal replacement for PbS. Finally, we show that by engineering the QD stoichiometry, it is possible to enhance the film's transport properties, paving the way for the synthesis of higher performance devices.

Highly uniform GaSb quantum dots with indirect–direct bandgap crossover at telecom range

We demonstrate a new quantum-confined semiconductor material based on GaSb quantum dots (QDs) embedded in a single-crystalline AlGaSb matrix by filling droplet-etched nanoholes. The droplet-mediated growth mechanism allows formation of low QD densities required for non-classical single-QD light sources. The photoluminescence (PL) experiments reveal that the GaSb QDs have an indirect–direct bandgap crossover at telecom wavelengths. This is due to the alignment of the Γ and L valleys in the conduction band as a result of the quantum confinement controlled by the dimensions of the nanostructure. We show that in the direct bandgap regime close to 1.5 µm wavelength, the GaSb QDs have a type I band alignment and exhibit excitonic emission with narrow spectral lines and very low inhomogeneous broadening of PL emission owing to the high material quality and dimensional uniformity. These properties are extremely promising in terms of applications in infrared quantum optics and quantum photonic integration.

Optical characteristics of type-II hexagonal-shaped GaSb quantum dots on GaAs synthesized using nanowire self-growth mechanism from Ga metal droplet

We report the growth mechanism and optical characteristics of type-II band-aligned GaSb quantum dots (QDs) grown on GaAs using a droplet epitaxy-driven nanowire formation mechanism with molecular beam epitaxy. Using transmission electron microscopy and scanning electron microscopy images, we confirmed that the QDs, which comprised zinc-blende crystal structures with hexagonal shapes, were successfully grown through the formation of a nanowire from a Ga droplet, with reduced strain between GaAs and GaSb. Photoluminescence (PL) peaks of GaSb capped by a GaAs layer were observed at 1.11 eV, 1.26 eV, and 1.47 eV, assigned to the QDs, a wetting-like layer (WLL), and bulk GaAs, respectively, at the measurement temperature of 14 K and excitation laser power of 30 mW. The integrated PL intensity of the QDs was significantly stronger than that of the WLL, which indicated well-grown GaSb QDs on GaAs and the generation of an interlayer exciton, as shown in the power- and temperature-dependent PL spectra, respectively. In addition, time-resolved PL data showed that the GaSb QD and GaAs layers formed a self-aligned type-II band alignment; the temperature-dependent PL data exhibited a high equivalent internal quantum efficiency of 15 ± 0.2%.

Temperature Dependence of Carrier Extraction Processes in GaSb/AlGaAs Quantum Nanostructure Intermediate-Band Solar Cells.

From the viewpoint of band engineering, the use of GaSb quantum nanostructures is expected to lead to highly efficient intermediate-band solar cells (IBSCs). In IBSCs, current generation via two-step optical excitations through the intermediate band is the key to the operating principle. This mechanism requires the formation of a strong quantum confinement structure. Therefore, we focused on the material system with GaSb quantum nanostructures embedded in AlGaAs layers. However, studies involving crystal growth of GaSb quantum nanostructures on AlGaAs layers have rarely been reported. In our work, we fabricated GaSb quantum dots (QDs) and quantum rings (QRs) on AlGaAs layers via molecular-beam epitaxy. Using the Stranski–Krastanov growth mode, we demonstrated that lens-shaped GaSb QDs can be fabricated on AlGaAs layers. In addition, atomic force microscopy measurements revealed that GaSb QDs could be changed to QRs under irradiation with an As molecular beam even when they were deposited onto AlGaAs layers. We also investigated the suitability of GaSb/AlGaAs QDSCs and QRSCs for use in IBSCs by evaluating the temperature characteristics of their external quantum efficiency. For the GaSb/AlGaAs material system, the QDSC was found to have slightly better two-step optical excitation temperature characteristics than the QRSC.

Epitaxial Growth of Optoelectronically Active Ga(As)Sb Quantum Dots on Al‐Rich AlGaAs with GaAs Capsule Layers

We present a study of optoelectronically active Ga(As)As quantum dots (QDs) on Al‐rich AlxGa1-xAs layers with Al concentrations up to x = 90%. So far, however, it has not been possible to grow optoelectronically active Ga(As)As QDs epitaxially directly on and in between Al‐rich barrier layers in the AlGaInAsSb material system. A QD morphology might appear on the growth front, but the QD‐like entities will not luminesce. Here, we use photoluminescence (PL) measurements to show that thin Al‐free capsule layers between Al‐rich barrier layers and the QD layers can solve this problem; this way, the QDs become optoelectronically active; that is, the dots become QDs. We consider antimonide QDs, that is, Ga(As)Sb QDs, either on GaAs for comparison or on AlxGa1-xAs barriers (x >10%) with GaAs capsule layers in between. We also discuss the influence of QD coupling both due to stress/strain from neighboring QDs and quantum‐mechanically on the wavelength of the photoluminescence peak. Due to their mere existence, the capsule layers alter the barriers by becoming part of them. Quantum dots applications such as QD semiconductor lasers for spectroscopy or QDs as binary storage cells will profit from this additional degree of design freedom.

Light-induced circuit parameter variation in self-assembled quantum-dot photovoltaic cell

We investigate the variations of circuit parameters of photovoltaic (PV) cells containing self-assembled InSb and GaSb quantum dots. The solar cell samples are fabricated by conventional molecular beam epitaxy on (001) GaAs substrate. The current–voltage (I-V) characteristics and the frequency response (10 Hz – 100 kHz) of the fabricated cell is then extracted in dark and illuminated environment. The obtained I-V curves show typical PV properties. The static series and shunt resistances can be then extracted from this measurement result. From the Nyquist plot of the device impedance, both dynamic resistance and capacitance can be further extracted. Variations of these values are quantified. Light-induced lowering of the resistance values are explained by the presence of photogenerated carriers. This work provides a useful information for both the basic understanding of PV devices and the design of electronic circuit for energy harvesting and sensing with PV cells.

Growth and characterization of ferromagnetic Fe-doped GaSb quantum dots with high Curie temperature

We report the structural and magnetic properties of the Fe-doped GaSb quantum dots (QDs) (nominal Fe concentration x = 4.7%–16.6%) grown on GaAs (001) substrates by molecular beam epitaxy. The QDs with nanometer-scale dimensions consist of two areas with different crystal structures, a zinc-blende GaAsSb wetting layer and a new phase of FeGaSb alloy that has a simple cubic lattice. The size and distribution of the QDs depend on the Fe concentration, as revealed by atomic force microscopy. Magnetic force microscopy measurements at zero applied magnetic field show the presence of ferromagnetism in the QDs at room temperature with an easy axis in the 1¯10 direction, which is consistent with magnetometry measurements. The Curie temperature in these QDs is very high (>400 K), which is promising for spintronic applications at room temperature.

Dissolution suppression of self-assembled GaSb quantum dots on silicon by proper surface preparation

The temperature stability of high-density GaSb quantum dots grown on a Si(111) surface using solid-phase epitaxy is investigated during annealing at 450 °C. It is found that the bare silicon surface plays a critical role in the GaSb decomposition through the destruction of Sb–Ga bonds with the simultaneous formation of Sb–Si bonds. GaSb decomposition can be significantly suppressed if saturated monolayer coverage in the form of the Si(111)-(√3 × √3)-R30°-Sb surface reconstruction is preliminarily formed. This allows GaSb quantum dots to be embedded in the silicon matrix using molecular beam epitaxy at high temperature.

Type-II GaSb quantum dots grown on InAlAs/InP (001) by droplet epitaxy

GaSb quantum dots (QDs) have been grown by droplet epitaxy within InAlAs barrier layers on an InP (001) substrate. The droplet growth mode facilitates a larger size (average height ∼4.5 nm) and a lower density (∼6.3 × 109 cm-2) for the QDs than would be expected for the 4% lattice mismatch between GaSb and InAlAs. A type-II band alignment between the GaSb QDs and the InAlAs barriers is revealed by photoluminescence (PL) through a prominent blue-shift of ∼0.11 eV resulting from a six orders of magnitude increase in excitation power. Further confirmation of the type-II nature of these QDs is found through time-resolved PL studies showing a biexponential decay with a long carrier lifetime of ∼10.9 ns. These observations reveal new information for understanding the formation and properties of GaSb/InAlAs/InP QDs, which may be an optimum system for the development of both efficient memory cells and photovoltaic devices.

Mid-infrared electroluminescence from type-II In(Ga)Sb quantum dots

There exists significant interest in the demonstration and development of alternative mid-infrared emitters, with future applications for thermal scene projection, low-cost infrared sensing, and possible long-wavelength quantum communication applications. Type-II In(Ga)Sb quantum dots grown in InAs matrices have the potential to serve as a viable material system for wavelength-flexible, mid-infrared sources. Here, we dramatically expand the range of potential applications of these mid-infrared quantum emitters through the demonstration of surface-emitting electrically pumped mid-infrared light-emitting diodes with active regions utilizing type-II In(Ga)Sb quantum dots. Two device structures were studied, the first iteration being a single In(Ga)Sb insertion layer within a simple PIN structure and the second being a design engineered for improved room temperature emission with the addition of lattice matched AlAsSb cladding at the anode to block electrons and five layers of In(Ga)Sb dots to increase the effective volume of active material. Samples were grown by molecular beam epitaxy and the electrical and optical properties for each design were characterized as a function of temperature.

Influence of MBE growth parameters on GaSb/GaAs quantum dot morphology

Abstract In this paper, growth of self-assembled type-II GaSb/GaAs quantum dots by molecular beam epitaxy technique is presented. The effect of various growth parameters such as substrate temperature, Sb/Ga beam equivalent pressure ratio, growth rate and total mono-layer coverage on surface morphology is analyzed. The GaSb quantum dots exhibit high anisotropy along and directions under specific growth conditions. The anisotropy is discussed on the basis of incorporation of background Arsenic and anisotropic diffusion of Gallium adatoms on the GaSb surface. The low temperature photoluminescence measurements in GaAs capped samples show recombination in the type-II quantum dots and the wetting layer.

Phonon replicas of type-II GaSb/GaAs quantum dot structure grown by liquid phase epitaxy

Type-II GaSb/GaAs quantum dot (QD) structures have attracted a great deal of attention owing to their unique optical and electrical properties associated with type-II band alignment. In this study, GaSb QDs were grown on GaAs substrates using liquid phase epitaxy (LPE) technique. The morphology of the obtained QDs was studied by scanning electron microscope and atom force microscope. The optical properties of the QDs were investigated by photoluminescence (PL) spectra. Strong first- and second-order longitudinal optical-phonon replicas were observed on the low-energy side of the wetting layer peak in the PL spectra.

Dynamical properties and charge accumulation in solar cells with embedded GaSb quantum dots

We studied the dynamical properties of quantum dot solar cells in which GaSb quantum dots (QDs) were embedded in the middle part of an undoped layer sandwiched by n- and p-layers. Three kinds of GaSb QDs with GaAs or AlGaAs as a barrier material for QDs were prepared for comparison of their properties. The capacitance C was measured as a function of the frequency f in the ranges between 5 kHz and 3 MHz under different illumination conditions. A rise in capacitance in response to illumination was observed in all the samples, showing that photogenerated carriers (or holes) contribute to the dynamical properties of solar cell devices. Differences in capacitance change among the three samples are observed in magnitude and its frequency dependence. We discuss them in terms of the difference in dynamical processes, specifically the thermal escape rate of holes in QDs.

Photoluminescence investigation of type-II GaSb/GaAs quantum dots grown by liquid phase epitaxy

GaSb quantum dots (QDs) with an areal density of ∼1 × 1010 cm−2 are successfully grown by the modified (rapid slider) liquid phase epitaxy technique. The morphology of the QDs has been investigated by scanning electron microscope (SEM) and atom force microscope (AFM). The power-dependence and temperature-dependence photoluminescence (PL) spectra have been studied. The bright room-temperature PL suggests a good luminescence quality of GaSb QDs/GaAs matrix system. The type-II alignment of the GaSb QDs/GaAs matrix system is verified by the blue-shift of the QDs peak with the increase of excitation power. From the temperature-dependence PL spectra, the activation energy of QDs is determined to be 111 meV.

Atmospheric pressure-MOVPE growth of GaSb/GaAs quantum dots

This study focuses on the growth of GaSb/GaAs quantum dots (QD) using an atmospheric pressure MOVPE system. For the best uncapped dots, the average dot height, base diameter and density are 5nm, 45nm and 4.5×1010cm−2, respectively. Capping of GaSb QDs at high temperatures caused flattening and formation of thin inhomogeneous GaSb layer inside GaAs resulting in no obvious QD PL peak. Capping at low temperatures lead to the formation of dot-like features and a wetting layer (WL) with distinct PL peaks for QD and WL at 1097nm and 983nm respectively. Some of the dot-like features had voids. An increase in excitation power caused the QD and WL peaks to shift to higher energies. This is attributed to electrostatic band bending leading to triangular potential wells, typical of type-II alignment between GaAs and strained GaSb. Variable temperature PL measurements of the QD sample showed the decrease in the intensity of the WL peak to be faster than that of the QD peak as the temperature increased.

Morphology and valence band offset of GaSb quantum dots grown on GaP(001) and their evolution upon capping

This work presents a detailed study of GaSb quantum dot (QD) epitaxy on (001) GaP substrates by means of molecular beam epitaxy. Despite the large mismatch between GaP and GaSb, we show that in the nucleation–diffusion regime, the QD size distribution follows the predictions of the scaling theory. Scanning transmission electron microscopy analysis of grown QDs reveal that they are plastically relaxed by 60° pairs of misfit dislocations and the valence band offset measured by x-ray photoelectron spectroscopy on such QDs amounts to 0.5 eV. After capping, the QD morphology is strongly modified with a large P/Sb exchange–segregation reaction, which even leads to the formation of core–shell nanostructures. Remarkably the resulting QD layer is coherent to the substrate without any remaining misfit dislocation and exhibits still strong composition modulations.