InAs Dots Research Articles

Effect of capping rate on the performance of InAs/GaAs quantum dot solar cell

The GaAs capping layer significantly influences the structural and optoelectronic characteristics of the InAs quantum dot (QD). The capping rate modifies the essential parameters, such as size, shape, and composition, that determine the optical properties of the QDs. In this work, we present a theoretical model to study the effects of the capping rate on the absorption spectra of InAs dots embedded in the GaAs capping layer. The proposed model can be used for optimizing the structural and optical characteristics of InAs/GaAs QDs without using an annealing procedure or any capping material other than GaAs. In addition, the impact of three different GaAs capping layer growth rates on the performance of quantum dot solar cells is evaluated.

Detection of optical emissions from deep localized states in a self-assembled InAs/GaAs QD structure

This work reports on the optical properties of self-assembled InAs/GaAs quantum dots (QDs) grown by molecular beam epitaxy. A sigmoidal temperature-dependent variation of the bandgap energy of the dots is detected from the photoluminescence (PL) investigations of the studied sample. This sigmoidal dependence of the bandgap energy was accompanied by an anomalous increase in the integrated PL intensity (IPLI) over the temperature range of 10 K to 110 K. Such optical observations are related to the existence of deep localized states (LS) within the QD structure. To illustrate the presence of the LS, pump power-dependent PL measurements at different temperatures were performed. It is found that the sigmoidal dependence of the PL peak energy (PLPE) gradually converts into a monotonous variation with the gradual increase in the excitation density. These observations show a saturation behavior of a limited number of states within the bandgap of the QDs. Such optical results provide clear evidence for the existence of deep levels within the InAs dots.

Room temperature operation and low dark current of In0.15Ga0.85As/InAs/In0.15Ga0.85As dot-in-well short-wave infrared photodetector: Experimental and theoretical correlation

Photoluminescence (PL) and Photoluminescence excitation (PLE) measurements have been carried out to investigate the carrier recombination pathways in a GaAs capped InAs quantum dot (QD) and an InAs dot in InGaAs well (DWELL) structure. The presence of an InGaAs quantum well (QW) results in an anomalous behavior in the temperature dependent PL spectra. The carrier trapping in the InGaAs well at low temperature and the redistribution of these carriers as the temperature rises is well explored through the temperature dependent PL spectra. The existence of additional energy states due to the incorporation of an InGaAs QW has been observed through the PLE result and explained schematically. Cross-sectional transmission electron microscopy provides a direct evidence of the formation of hybrid thick QW structure due to the intermixing of InGaAs well and the InAs wetting layer (WL). The strain relaxation in the DWELL structure is substantiated from the simulated strain profile as well as the high-resolution X-ray diffraction (HRXRD) result. Moreover, the DWELL p-i-p quantum dot infrared detector (QDIP) exhibits a room temperature spectral response in the short-wavelength infrared (SWIR) region. A two-order reduced dark current and one order improved detectivity is obtained for the DWELL QDIP compared to the conventional GaAs capped InAs QDIP. • Infrared photodetector with InAs quantum dot in InGaAs well as the active layer. • The carrier recombination process along with temperature is discussed. • PL and PLE results show the dominance of the InGaAs wells at low temperature. • The device showed spectral response in short-wave IR range till room temperature. • The dark current obtained for the QDIP is also very low.

Spatial Optimization of Modulation Doping in P-I-P QDIPs: Towards Achieving Higher Operating Temperature

We report short wavelength infrared (SWIR) photodetectors using multi-stacked modulation doped Indium-Arsenide (InAs) self-assembled quantum dots (QDs). The detriments of direct doping on the energy levels of InAs QDs have been eliminated by choosing modulation doping. We have considered three P-I-P Indium Arsenide/Gallium Arsenide (InAs/GaAs) quantum dot infrared photodetector (QDIP) heterostructures in which a thin (3 nm) P-doped GaAs layer is introduced at a distance of 7, 12 and 17 nm away from each InAs dot layer respectively. The structures have been analyzed for their morphological and optical characteristics and an optimum separation between the active layer and modulation doped layer has been achieved. The fabricated single pixel detectors exhibit a broad spectral response from 1 to 3.5 μm in the SWIR region for all the samples. The infrared photocurrent survives up to a maximum temperature of 200K for the optimized device with a peak responsivity of 1.439 A/W at wavelength λ = 1.77 μm and temperature T = 200 K.

Strain relaxation in InAs quantum dots through capping layer variation and its impact on the ultrafast carrier dynamics

Ultrafast carrier dynamics is found to be crucial for influencing the efficiency of quantum dot (QD)-based optoelectronic devices. Here, we have studied picosecond resolved dynamics in InAs semiconductor quantum dots forming various heterostructure geometries with their different capping layers. The carriers (electron-hole pairs) are optically injected into three dimensional heterostructure barriers after which, we monitor the carrier relaxation into the zero-dimensional InAs dots by using picosecond resolved time correlated single photon counting technique. We have explicitly investigated and analyzed the effect of three different capping layers, GaAs (binary), In0.15Ga0.85As/GaAs (ternary) and In0.21Al0.21Ga0.58As/GaAs (quaternary) offering various optical properties of InAs quantum dots. It has been observed that quaternary strain reducing layer effectively suppresses Indium migration from the QDs and enhances the confinement of carriers inside the QD heterostructures. In this study, we also find that thermal stability of the carriers is improved in the quaternary capped QDs more effectively compared to the conventional binary and ternary capped one. In brief, our experiments provide essential insight into carrier relaxation in the InAs QDs across various capping layers.

Controllable growth of InAs quantum dots on patterned GaAs (001) substrate

InAs/GaAs quantum dot (QD) is one of the promising material systems for the quantum information processing due to their atomic-like optical and electrical properties. There are many previous researches reporting the InAs QDs which can be implemented as solid-state single-photon sources for quantum information and quantum computing. However, the site-controlled growth of QDs is the prerequisite for addressability and integration. There are very few researches focusing on the systematic study of preferential nucleation of InAs QDs on a patterned GaAs (001) substrate. In this work, we study the preferential nucleation sites of InAs QDs on a patterned GaAs (001) substrate with different trench sidewall inclinations. With small inclination angle of the trench sidewalls, the InAs QDs nucleate preferentially inside the trenches, while with large inclination angle, the edges of the trenches appear to be the preferential nucleation sites. By utilizing the established method, a pair of InAs dots can be uniformly achieved in the patterned pits through tuning the inclination angle of the pits. The site-controlled single InAs QD and InAs QD molecules on the patterned substrates could have potential applications in quantum information processing and quantum computing.

1.0 eV GaAs based InAs quantum dot solar cells

There are great improvements in III-V semiconductor solar cells including GaAs single junction solar cell and tandem solar cells in last decades. The common commercially used triple-junction solar cell mostly utilize a Ge bottom cell with an (In)GaAs and InGaP middle and top cell respectively. The (In)GaAs middle cell absorbs limited solar spectrum and show minimum short-circuit current in multiple junction solar cell, which determine the overall current of the cell. Theoretically, higher energy conversion efficiency will be obtained when introducea subcell with 1.0 eV band gap in series to the InGaP/GaAs/Ge tandem structures. Lattice-matched GaInNAs and lattice-mismatched In0.3Ga0.7As are two common choices for achieving 1.0 eV band gap. However, the addition of N to GaAs has detrimental effects on the material quality, limiting its utility to approaches that required photocurrent. And the lattice mismatch makes thick In0.3Ga0.7As heterogeneous epitaxial difficult. The incorporation of low band gap nanoscale materials, such as quantum well and quantum dots, into current limiting junction of a multijunction solar cell can tuning the effective bandgap and enhance the low-energy photon absorption, thereby increasing the short-circuit current. The samples are grown by solid-source molecular beam epitaxy (V80H) on p-GaAs (001) substrates using an As4 source. Sample A is a standard GaAs solar cell with PIN structure. Sample B contains 10 period 8 nm In0.15Ga0.85As QW and sample C contains 10 period InAs dots-in-well structure. The intrinsic region of these samples was at the same length of 630 nm to obtain the same build-in electric field. Besides the instinct region, the three samples are designed the same. The ohmic contact layer’s doping concentration is N D/ N A=3×1018 cm−3 and its thickness is 300 nm. The emitter doping concentration is N D=5×1017 cm−3 and its thickness is 150 nm; The base doping concentration is N A=5×1017 cm−3 and its thickness is 300 nm. The As4 pressure and the InAs QDs growth rate are 1×10−5 Torr and 0.1 ML/s, respectively. The XRD pattern of sample B (QWSC) shows clearly defined and intense satellite peaks indicating steep interface and periodicity quantum well. However, the XRD pattern of sample C (the InAs dots in well solar cell) shows blurring diffraction peaks which means increased interface roughness. Uniform QD structure with density of 2×1010 cm−2 can be observed in SEM. From the STEM, we can see well-defined unrelated pyramidal quantum dots separated by a nominal 50 nm GaAs spacing. From the contrast, the pyramid InAs QDs are estimated to have the lateral dimension of 27 nm and the height of 12 nm. The PC response of sample A rise at 600 nm and fall off at 870 nm, which is corresponding to the GaAs bandgap according to the PL peak position; the PC response of QWSC (sample B) rise at 600 nm and fall off at 990 nm, which is corresponding to the In0.15Ga0.85As bandgap in PL spectrum. The incorporation of In0.15Ga0.85As quantum wells in GaAs standard solar extend the absorption of solar spectrum from 900 nm to 1000 nm. The wavelength region from 700 nm to 900 nm in PC of QWSC is weaker than the GaAs standard solar cell. The insertion of InAs QDs into QWSC can extend the infrared photon absorption to the wavelength of 1300 nm. The insertion of InAs quantum dots into In0.15Ga0.85As/GaAs quantum well solar cell extend the absorption of infrared light and the J sc reaches 13.68 mA/cm2, giving a 37.8% increase in short-circuit current over the baseline solar cell current. Because the voltage of multiple junction solar cell is the sum of the sub cell’s voltage. The voltage decreasement with the insertion of InAs quantum dots do not influence the overall voltage of multiple junction solar cell. In summary, the incorporation of quantum well and quantum dots in GaAs solar cell result in extended solar spectrum absorption and contribute to the sub-band gap current gains for GaAs solar cell. Especially, InAs dots-in-well solar cell can extend the absorption spectrum to the wavelength of 1300 nm, and increase by 37.8% in short-circuit current, which shows great potential in mitigating current mismatch of multiple junction solar cell. Furthermore, designing new multiple junction solar cell becomes possible.

Landau-level mixing, floating-up extended states, and scaling behavior in a GaAs-based two-dimensional electron system containing self-assembled InAs dots

Temperature-driven flow lines are studied in the conductivity plane in a GaAs-based two-dimensional electron system containing self-assembled InAs dots when Landau level filling factor ν = 2-4. In the insulator-quantum Hall (I-QH) transition resulting from the floating-up of the extended states, the flow diagram shows the critical behavior and we observed the expected semicircle in the strongest disorder case. By decreasing the effective disorder, we find that such flow lines can leave the I-QH regime and correspond to the plateau–plateau transition between ν = 4 and 2. The evolution of the conductivity curve at low magnetic fields demonstrates the importance of Landau-level mixing to the semicircle when the extended states float up.

Strain-Engineered Arrays of InAs Quantum Dots on GaAs(001): Epitaxial Growth and Modeling

Working under critical conditions for dot nucleation in a Molecular Beam Epitaxy chamber, we were able to drive the formation of InAs dot chains to precise locations in multilayered samples grown on a rippled GaAs(001) surface. We discussed the role of the elastic field and the surface curvature in determining the dot arrangement at each stacked layer, proving a new mechanism of self-organization of the dots. In particular, we succeeded in controlling the interplay between elastic and curvature effects and we showed how a selection process is achievable in the chain formation. The role of the stress field was also studied by means of Finite Element Method simulations, and we gained a valuable understanding of the interlayer dot correlations for dot arrays with variable cap thicknesses. We proved the existence of an anisotropy in the cap formation of isolated dots, which appeared to be directly related to our peculiar growth geometry and experimental set-up.

Interaction-driven distinctive electronic states of artificial atoms at the ZnO interface

We have investigated the electronic states of planar quantum dots at the ZnO interface containing a few interacting electrons in an externally applied magnetic field. The electron–electron interaction effects are expected to be much stronger in this case than in traditional semiconductor quantum systems, such as in GaAs or InAs quantum dots. In order to highlight that stronger Coulomb effects in the ZnO quantum dots, we have compared the energy spectra and the magnetization in this system to those of the InAs quantum dots. We have found that in the ZnO quantum dots the signatures of stronger Coulomb interaction manifests in an unique ground state that has very different properties than the corresponding ones in the InAs dot. Our results for the magnetization also exhibits behaviors never before observed in a quantum dot for a realistic set of parameters. We have found a stronger temperature dependence and other unexpected features, such as paramagnetic-like behavior at high temperatures for a quantum-dot helium.

Wavelength extension beyond 1.5 µm in symmetric InAs quantum dots grown on InP(111)A using droplet epitaxy

By using a C3v symmetric (111) surface as a growth substrate, we can achieve high structural symmetry in self-assembled quantum dots, which are suitable for use as quantum-entangled-photon emitters. Here, we report on the wavelength controllability of InAs dots on InP(111)A, which we realized by tuning the ternary alloy composition of In(Al,Ga)As barriers that were lattice-matched to InP. We changed the peak emission wavelength systematically from 1.3 to 1.7 µm by barrier band gap tuning. The observed spectral shift agreed with the result of numerical simulations that assumed a measured shape distribution independent of the barrier choice.

The ground state properties of In(Ga)As/GaAs low strain quantum dots

We present theoretical studies on the confined states in low-strain In(Ga)As quantum dots (QDs). The 8-band k·p model together with the continuum elasticity theory and piezoelectric fields were employed to calculate the potential and confined electron and hole eigenstates. We focused on low-indium-content QDs with distinct in-plane asymmetry, which are naturally formed in the low strain regime of the Stranski-Krastanow growth mode. It has been found that the naturally thick wetting layer together with piezoelectric potential affect the total confinement potential to such extent that the hole eigenstates can get the spatial in-plane orientation orthogonal to the main axis of the dot elongation. This can influence both, qualitatively and quantitatively, many of the electronic and optical properties, as e.g. the polarization selection rules for the optical transition or the transitions oscillator strength. Eventually, importance of the degree of the shape asymmetry or the dots’ size, and differences between the low-strain (low-In-content) QDs and pure InAs dots formed in high strain conditions are discussed.

Comparison of Three Design Architectures for Quantum Dot Infrared Photodetectors: InGaAs-Capped Dots, Dots-in-a-Well, and Submonolayer Quantum Dots

In this letter, we compare three design architectures for quantum dot infrared photodetectors—InGaAs-capped InAs dots, InAs dot-in-a-well (DWELL), and InAs submonolayer (SML) heterostructures—in terms of optical and spectral behavior. The photoluminescence (PL) intensity measured of the SML sample at 8 K was 20 times stronger than that of the other samples, and its full-width at half-maximum value broader than the rest. Activation energy was calculated using temperature-dependent PL and dark current measurements, which showed the same trend. Peak spectral responses for the InGaAs-capped InAs dot and DWELL samples were observed at 4.1 and 7.3 μm and at 4.1 and 8.5 μm, respectively; however, only a single transition was observed for the SML sample because of the absence of a wetting layer. Spectral response of DWELL sample exhibited bias tunability at 87 K, and the SML sample exhibited high temperature of operation till 110 K. One order increment in responsivity was observed in the SML sample compared to others. The peak detectivity of InGaAs-capped InAs dot, DWELL, and SML samples was $4.1 \times 10^{9}$ , $4.99 \times 10^{9}$ , and $3.89 \times 10^{9}$ Jones, respectively, at 87 K.

Molecular Beam Epitaxy growth and characterization of silicon – Doped InAs dot in a well quantum dot infrared photo detector (DWELL-QDIP)

We report the growth by Molecular Beam Epitaxy (MBE), fabrication and characterization of silicon doped 20 layer InAs dot in a well quantum dot infrared photo detector (DWELL-QDIP) device structures. Two structures with InAs dots of vertical heights of 50Å and 40Å were compared. A 2–8μm band normal incidence photo response of the detector with polarization and bias dependence was obtained at 77K. The specific peak detectivity D∗ be 0.8×109Jones for one of the detectors.

InAs quantum dot growth on AlxGa1−xAs by metalorganic vapor phase epitaxy for intermediate band solar cells

InAs quantum dot multilayers have been grown using AlxGa1−xAs spacers with dimensions and compositions near the theoretical values for optimized efficiencies in intermediate band photovoltaic cells. Using an aluminium composition of x = 0.3 and InAs dot vertical dimensions of 5 nm, transitions to an intermediate band with energy close to the ideal theoretical value have been obtained. Optimum size uniformity and density have been achieved by capping the quantum dots with GaAs following the indium-flush method. This approach has also resulted in minimization of crystalline defects in the epilayer structure.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Kinetically driven selective growth of InAs quantum dots on GaAs

Abstract

Spin-orbit effects in nanowire-based wurtzite semiconductor quantum dots

We study the effect of the Dresselhaus spin-orbit interaction on the electronic states and spin relaxation rates of cylindrical quantum dots defined on quantum wires having wurtzite lattice structure. The linear and cubic contributions of the bulk Dresselhaus spin-orbit coupling are taken into account, along with the influence of a weak external magnetic field. The previously found analytic solution for the electronic states of cylindrical quantum dots with zincblende lattice structures with Rashba interaction is extended to the case of quantum dots with wurtzite lattices. For the electronic states in InAs dots, we determine the spin texture and the effective g-factor, which shows a scaling collapse when plotted as a function of an effective renormalized dot-size dependent spin-orbit coupling strength. The acoustic-phonon-induced spin relaxation rate is calculated and the transverse piezoelectric potential is shown to be the dominant one.

Investigation of thermal interdiffusion in InAs/In0.15Ga0.85As/GaAs quantum dot-in-a-well heterostructures

An analysis of the effects of rapid thermal annealing on the photoluminescence spectrum of InAs/In0.15Ga0.85As/GaAs Quantum dot-in-a-well heterostructures is presented. The variation of ground state interband transition energies with annealing has been modeled and the effect of varying dot aspect ratios, well indium concentrations and well thicknesses analysed on both peak transition energy and interdiffusion driven blueshift in the photoluminescence spectrum. Our simulations indicate that at low annealing temperatures indium diffusion from InAs dot to InGaAs well dominates, while at higher temperatures indium diffusion from well to GaAs barrier is predominant. Further, we see an increase in strain relaxation as the coverage of dots with InGaAs is increased, and as the indium concentration and/or thickness of InGaAs well is increased. Our model is useful in engineering dot-in-a-well heterostructures with the dual aim of achieving a desired peak wavelength and minimizing blueshift due to annealing.