GaAsSb Barrier Research Articles

Characterisation, modelling and design of cut-off wavelength of InGaAs/GaAsSb type-II superlattice photodiodes

InGaAs/GaAsSb type-II superlattice (T2SL) photodiodes grown on InP substrates are an alternative detector technology for applications operating in the short wavelength infrared band. Their cut-off wavelengths are heavily influenced by the thickness and material composition of InGaAs and GaAsSb used in the T2SL. We present a single band k.p. model performed using a finite difference approach in nextnano validated against two T2SL photodiode wafers and results from literature. These photodiode wafers cover both lattice matched and strained GaAs1−x Sb x compositions (x = 0.40, wafer A and 0.49, wafer B). The validation data covers temperature dependence of cut-off wavelengths (obtained from phase-sensitive photo response data) from 200 K to room temperature. The cut-off wavelengths were found to reduce at 1.32 nm K−1 for wafer A and 1.07 nm K−1 for wafer B. Good agreement was achieved between the validation data and nextnano simulations, after altering the GaAs1−x Sb x valance band offset (VBO) bowing parameter to −1.06 eV. Using this validated model, we show that the wavefunction overlap drops significantly if the GaAsSb barrier is thicker than the InGaAs well layer, hence defining the upper limit of the barrier layer. This validated model is then used to demonstrate that there is a linear dependence between the maximum achievable wavefunction overlap and cut-off wavelength of a lattice matched InGaAs/GaAsSb T2SL. We also found that the adoption of a 5 nm/3 nm InGaAs/GaAsSb T2SL structure offers an improved wavefunction overlap over the more common 5 nm/5 nm InGaAs/GaAsSb T2SL designs. The data reported in this paper is available from doi: 10.15131/shef.data.20310591.

Suppressing Ge diffusion by GaAsSb barriers in molecular beam epitaxy of InGaAs on Ge

We have successfully mitigated the out‐diffusion of Ge during molecular beam epitaxy of InGaAs on Ge by using a GaAsSb barrier layer as evidenced by secondary ion mass spectroscopy. Compared to GaAs, this GaAsSb barrier layer also results in a smoother surface morphology with its root‐mean‐square roughness of 0.7 nm due to the surfactant effect of Sb. Using a step‐graded GaAsSb and AlGaAsSb metamorphic buffer layer, a 200‐nm p‐type In0.53Ga0.47As layer grown on Ge exhibits a hole mobility of 38 cm2 V−1 s−1 with a hole concentration of 2.6 × 1019 cm−3 at 300 K and 53 cm2 V−1 s−1 with a hole concentration of 1.2 × 1019 cm−3 at 77 K.

Combined vertically correlated InAs and GaAsSb quantum dots separated by triangular GaAsSb barrier

The aim of this work is to offer new possibilities for quantum dot (QD) band structure engineering, which can be used for the design of QD structures for optoelectronic and single photon applications. Two types of QDs, InAs and GaAsSb, are combined in self assembled vertically correlated QD structures. The first QD layer is formed by InAs QDs and the second by vertically correlated GaAsSb QDs. Combined QD layers are separated by a triangular GaAsSb barrier. The structure can be prepared as type-I, with both electrons and holes confined in InAs QDs, exhibiting a strong photoluminescence, or type-II, with electrons confined in InAs QDs and holes in GaAsSb QDs. The presence of the thin triangular GaAsSb barrier enables the realization of different quantum level alignment between correlated InAs and GaAsSb QDs, which can be adjusted by structure parameters as type-I or type-II like for ground and excited states separately. The position of holes in this type of structure is influenced by the presence of the triangular barrier or by the size and composition of the GaAsSb QDs. The electron-hole wavefunction overlap and the photoluminescence intensity alike can also be controlled by structure engineering.

Impact of stress relaxation in GaAsSb cladding layers on quantum dot creation in InAs/GaAsSb structures grown on GaAs (001)

We describe InAs quantum dot creation in InAs/GaAsSb barrier structures grown on GaAs (001) wafers by molecular beam epitaxy. The structures consist of 20-nm-thick GaAsSb barrier layers with Sb content of 8%, 13%, 15%, 16%, and 37% enclosing 2 monolayers of self-assembled InAs quantum dots. Transmission electron microscopy and X-ray diffraction results indicate the onset of relaxation of the GaAsSb layers at around 15% Sb content with intersected 60° dislocation semi-loops, and edge segments created within the volume of the epitaxial structures. 38% relaxation of initial elastic stress is seen for 37% Sb content, accompanied by the creation of a dense net of dislocations. The degradation of In surface migration by these dislocation trenches is so severe that quantum dot formation is completely suppressed. The results highlight the importance of understanding defect formation during stress relaxation for quantum dot structures particularly those with larger numbers of InAs quantum-dot layers, such as those proposed for realizing an intermediate band material.

Effect of Sb induced type II alignment on dynamical processes in InAs/GaAs/GaAsSb quantum dots: Implication to solar cell design

In order to improve the dynamical conditions for possible formation of quasi-Fermi level separation between states in the conduction band, upon external illumination of an quantum dot based solar cells, we employ methods of quantum engineering to design the type II alignment, using a GaAsSb barrier buffer underneath InAs/GaAs QD. By changing the Sb amount in the buffer region, we predict an increase of the interband radiative time to the same time scale as interband radiative time, with simultaneous increase of the Auger electron cooling to ∼0.1 ns.

Observation of band alignment transition in InAs/GaAsSb quantum dots by photoluminescence

The band alignment of InAs quantum dots (QDs) embedded in GaAsSb barriers with various Sb compositions is investigated by photoluminescence (PL) measurements. InAs/GaAsSb samples with 13% and 15% Sb compositions show distinct differences in emission spectra as the PL excitation power increases. Whilst no discernible shift is seen for the 13% sample, a blue-shift of PL spectra following a 1/3 exponent of the excitation power is observed for the 15% sample suggesting a transition from a type I to type II band alignment. Time-resolved PL data show a significant increase in carrier lifetime as the Sb composition increases between 13% and 15% implying that the transformation from a type I to type II band alignment occurs between 13% and 15% Sb compositions. These results taken together lead to the conclusion that a zero valence band offset (VBO) can be achieved for the InAs/GaAsSb system in the vicinity of 14% Sb composition.

Enhanced Rashba effect in transverse magnetic fields observed on InGaAs/GaAsSb resonant tunneling diodes at temperatures up to T = 180 K

A huge Rashba splitting enhanced by an in-plane magnetic field is observed in non-magnetic InGaAs resonant tunneling diodes with GaAsSb barriers. At T = 4 K, the current resonances split by the Rashba effect reveal peak to valley ratios up to 2.5:1 and the energy spacing between the split peaks reaches 30 meV at B = 5 T. The observed peak splitting can be observed at temperatures up to T = 180 K and higher. The Rashba parameters determined on four different samples are between α = 0.38 eVÅ and α = 0.78 eV Å, which are consistent with theoretical values reported for InAs quantum wells under external electric fields.

Controllability of the subband occupation of InAs quantum dots on a delta-doped GaAsSb barrier

Optical properties of InAs quantum dots (QDs) embedded in GaAsSb barriers with delta-doping levels equivalent to 0, 2, 4, and 6 electrons per dot (e/dot) are studied using time-integrated photoluminescence (PL). When the PL excitation power is increased the full width at half maximum (FWHM) of the 4 and 6 e/dot samples is found to increase at a much greater rate than the FWHMs for the 0 and 2 e/dot samples. PL spectra of the 4 e/dot sample show a high energy peak attributed to emission from the first excited states of the QDs, a result deduced to be due to preoccupation of states by electrons supplied by the delta-doping plane. When temperature dependent PL results are fitted using an Arrhenius function, the thermal activation energies for the 4 and 6 e/dot samples are similar and greater than the thermal activation energies for the 0 and 2 e/dot samples (which are similar to each other). This increased thermal activation energy is attributed to the enhanced Coulombic interaction in the InAs QD area by the delta-doping plane for higher doping levels. It is concluded that delta-doping of the barrier in QD systems is a feasible method for controlling the level of carrier occupation in a QD mediated intermediate band.

Variability of heterostructure type with thickness of barriers and temperature in the InAs/GaAsSb quantum dot system

We report photoluminescence studies of InAs quantum dots embedded in GaAsSb barrier layers of various thicknesses. Time resolved photoluminescence results suggest that the thicknesses of the GaAsSb barrier layers determines whether the quantum dot system acts as a type I (thicker) or type II (thinner) heterostructure due to strain induced changes in the band energies. Temperature dependent photoluminescence reveal that the transition type is also dependent on the temperature with one sample seeming to transform from type II to type I heterostructure as the temperature is increased. These results support previous assertions that it is possible to obtain a zero valence band offset in the InAs/GaAsSb quantum dot system, making it a system of interest for realization of a novel photovoltaic structure, the intermediate band solar cell. The results highlight some of the inherent issues in designing structures with specific band offsets. Some of the implications of these results on the design methodology for quantum dot based solar cells are briefly discussed.

A new aluminum-free material system for intersubband emitters and detectors

We report on the InGaAs/GaAsSb material system lattice-matched to InP for intersubband devices. Due to the much lower electron effective mass in the GaAsSb barrier material, this system is very promising for the realization of high performance intersubband devices, like quantum-cascade lasers and quantum well infrared photodetectors. Type I intersubband absorption in InGaAs/GaAsSb multi-quantum wells samples has been studied experimentally and the conduction band offset at the InGaAs/GaAsSb hetero-interface has been determined to be 360 meV. Additionally, we realized the first Al-free QCL based on the same material system, emitting at a wavelength of 11.3 μm.

Growth and characterization of GaAs1−xSbx barrier layers for advanced concept solar cells

The InAs∕GaAsSb material system is a promising medium for the implementation of a quantum dot solar cell due to a favorable valence band alignment. The quantum dot solar cell requires a highly dense, highly ordered array of quantum dots for overlap of wave functions to form a band in the band gap of the host material. Since the GaAsSb barriers are a III-V-V ternary the alloy composition is particularly sensitive to variations in temperature. We have studied the variation in Sb content for thin layers of GaAsSb in GaAs for various fluxes of Sb. The purpose was to be able to predict the required Sb flux at a particular temperature to obtain a desired Sb composition. The target Sb composition for this study was 12% with the composition obtained confirmed by x-ray diffraction. By also studying the reciprocal space maps of the 12% samples, it is inferred that the composition can be maintained for a large temperature range. The implications of these results for the growth of InAs quantum dots on GaAsSb barriers are briefly discussed.

Properties of photoluminescence in type-II GaAsSb/GaAs multiple quantum wells

The optical properties of type-II GaAsSb/GaAs multiple quantum wells were investigated by photoluminescence. It was found that the peak position of photoluminescence (PL) spectra shows a giant blueshift under a moderate optical excitation level. As the pumping intensity increases further, the saturation of the peak position was observed. The giant blueshift can be interpreted in terms of the band-bending effect due to the spatially photoexcited carriers in a type-II alignment. The saturation effect was attributed to the creation of an energy barrier induced by photocarriers, which prevents the charge transfer. The change of integrated photoluminescence intensity with increasing excitation intensity is also consistent with the band-bending model. In addition, the temperature dependence of the PL spectra reveals that the thermal escape of electrons from the GaAs well into the GaAsSb barrier is responsible for the PL quenching.