Stranski-Krastanov Quantum Dots Research Articles

Temperature dependent behavior of sub-monolayer quantum dot based solar cell

This work investigates the temperature dependence of the performance of In(Ga)As-based solar cells made from sub-monolayer (SML) quantum dots (QDs), quantum wells (QWs), and Stranski-Krastanov QDs (SK-QDs). Well-defined sub-bandgap peaks are observed in external quantum efficiency (EQE) spectra for all samples. Apparently, changes in minority carrier lifetime affect both the sub-bandgap and the above bandgap EQE. Lateral confinement of carriers in the SMLs and QDs also affects the temperature dependence of the EQE. Finally, a “U” shaped curve was observed from the temperature dependence of the short-circuit current (ISC) from these nanostructure-based solar cells. This is due to the competition between the decreasing bandgap and the increasing carrier loss to recombination within the nanostructures as the temperature increases. In contrast, the solar cell's open-circuit voltage and power efficiency decrease monotonically with temperature.

Atomic-scale characterization of single and double layers of InAs and InAlAs Stranski-Krastanov quantum dots

We report a detailed structural characterization of single and double layers of InAs and InAlAs quantum dots (QDs) and their wetting layers (WLs) by atomic force microscopy (AFM) and cross-sectional scanning tunneling microscopy (X-STM). The X-STM analysis with atomic resolution showed that the InAlAs WL consists of two distinct layers: a bottom part where all the Al atoms of the InAlAs alloy settled, and a top part containing exclusively In and Ga atoms. The QDs formed from the InAlAs layer contains no Al atoms at all and lie on top of the Al-rich WL. In the double layers of QDs, the InAlAs QDs were used as a seed to influence the nucleation of the InAs QDs grown on top. A gradual decrease in the density of the top InAs QDs was observed in the AFM images with increasing thickness of the GaAs spacer. The X-STM images showed that both QDs layers were completely intermixed for a 2-nm-thick spacer, while effective strain-induced stacking of both types of QDs was observed for a GaAs spacer thickness of 4 nm. However, both QD layers were completely decoupled for a GaAs spacer thickness of 8 nm and could thus be treated as individual layers.

Near-infrared electroluminescence of AlGaN capped InGaN quantum dots formed by controlled growth on photoelectrochemical etched quantum dot templates

Near-infrared electroluminescence of InGaN quantum dots (QDs) formed by controlled growth on photoelectrochemical (PEC) etched QD templates is demonstrated. The QD template consists of PEC InGaN QDs with high density and controlled sizes, an AlGaN capping layer to protect the QDs, and a GaN barrier layer to planarize the surface. Scanning transmission electron microscopy (STEM) of Stranski–Krastanov (SK) growth on the QD template shows high-In-content InGaN QDs that align vertically to the PEC QDs due to localized strain. A high-Al-content Al 0.9 Ga 0.1 N capping layer prevents the collapse of the SK QDs due to intermixing or decomposition during higher temperature GaN growth as verified by STEM. Growth of low-temperature (830°C) p-type layers is used to complete the p-n junction and further ensure QD integrity. Finally, electroluminescence shows a significant wavelength shift (800 nm to 500 nm), caused by the SK QDs’ tall height, high In content, and strong polarization-induced electric fields.

Evaluation of In(Ga)As capping in a multilayer coupled InAs quantum dot system: Growth strategy involving the same overgrowth percentage

Strain-coupled multilayer Quantum Dot (QD) structures draw a great attention these days because of their superior optical and device performance. However, these coupled multilayer QD structures have limitations such as non-uniform QD size distribution, defects, and dislocations due to the propagation of strain in QD layers. Furthermore, the carrier relaxation lifetime must be improved in these coupled QD structures for sought-after device performance. This study serves for three main purposes. Firstly, we have utilized a growth strategy to maintain the overgrowth percentage such that the Stranski-Krastanov (SK) QD dimensions in every layer is same for all the multilayer structures. Secondly, these multilayer SK QDs are made to electronically couple with the Sub-monolayer (SML) QDs in such a way that the carrier relaxation lifetime can be increased, that could ameliorate the photoconductive gain and the responsivity. Lastly, the amount of cumulative strain generated inside QDs can be reduced by the incorporation of In0.15Ga0.85As strain reducing layer (capping layer) in a multilayer structure. Five different multilayer structures with single (x1), bi (x2), penta (x5), hepta (x7), and ten-layers (x10) of SK QDs electronically coupled to six stacks of SML QDs are used in this study. Photoluminescence (PL) studies reveal that the SK QD dimensions is maintained to be the same in all multilayer structures due to the proposed growth strategy, which is also observed through transmission electron microscopy (TEM) images. Photoluminescence excitation (PLE) analysis shows the match of excited states of SK QDs to the ground state of the SML QDs, aiding the carrier tunnel phenomena. The ω/2Ө measurements from high resolution X-ray diffraction (HRXRD) is carried out to calculate the overall strain in the grown heterostructures. The optical properties and the strain components obtained from the nextnano simulation tool are in good agreement with the experimental results. Thus, coupled multilayer QD structures with a growth strategy in the current study would be more suitable for the realization of quantum dot infrared photodetectors and intermediate band solar cell applications accommodated with high device efficiency.

Structural and compositional analysis of (InGa)(AsSb)/GaAs/GaP Stranski\u2013Krastanov quantum dots

We investigated metal-organic vapor phase epitaxy grown (InGa)(AsSb)/GaAs/GaP Stranski–Krastanov quantum dots (QDs) with potential applications in QD-Flash memories by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT). The combination of X-STM and APT is a very powerful approach to study semiconductor heterostructures with atomic resolution, which provides detailed structural and compositional information on the system. The rather small QDs are found to be of truncated pyramid shape with a very small top facet and occur in our sample with a very high density of ∼4 × 1011 cm−2. APT experiments revealed that the QDs are GaAs rich with smaller amounts of In and Sb. Finite element (FE) simulations are performed using structural data from X-STM to calculate the lattice constant and the outward relaxation of the cleaved surface. The composition of the QDs is estimated by combining the results from X-STM and the FE simulations, yielding ∼InxGa1 − xAs1 − ySby, where x = 0.25–0.30 and y = 0.10–0.15. Noticeably, the reported composition is in good agreement with the experimental results obtained by APT, previous optical, electrical, and theoretical analysis carried out on this material system. This confirms that the InGaSb and GaAs layers involved in the QD formation have strongly intermixed. A detailed analysis of the QD capping layer shows the segregation of Sb and In from the QD layer, where both APT and X-STM show that the Sb mainly resides outside the QDs proving that Sb has mainly acted as a surfactant during the dot formation. Our structural and compositional analysis provides a valuable insight into this novel QD system and a path for further growth optimization to improve the storage time of the QD-Flash memory devices.

Green InGaN Quantum Dots Breaking through Efficiency and Bandwidth Bottlenecks of Micro‐LEDs

AbstractMicro‐LEDs are regarded as ideal light sources for next‐generation display and high‐speed visible‐light communication (VLC). However, the conventional micro‐LEDs based on InGaN quantum well (QW) active region suffer from a low efficiency under small injection (below 1 A cm−2) due to the size‐dependent effect and a limited 3 dB bandwidth (hundreds of MHz) due to quantum‐confined Stark effect. Here, InGaN quantum dots (QDs) are proposed as the active region of micro‐LEDs to address these challenges for their strong localization and low‐strain features. Green InGaN QDs are self‐assembled under Stranski–Krastanov (SK) and Volmer–Weber (VW) modes by using metal organic vapor phase epitaxy. The SK QDs can shift the peak efficiency of a micro‐LED to an extremely low current density of 0.5 A cm−2 (almost two orders of magnitude lower compared to QW ones) with an external quantum efficiency of 18.2% (nearly two times higher than present green micro‐LEDs). Besides, green micro‐LEDs based on VW QDs reach a 3 dB bandwidth of 1.3 GHz. These results indicate that InGaN QDs can provide an ultimate solution to micro‐LEDs for display and VLC applications, especially since they are fully compatible with current light‐emitting diode (LED) industrial technology.

InAs nanostructures for solar cell: Improved efficiency by submonolayer quantum dot

The effect of different quantum structures in the intrinsic region of a pin junction solar cell (SC) on the optical and electrical properties have been investigated. SCs with different quantum structures, such as, Stranski-Krastanov (SK) quantum dots (QDs), quantum well (QW), submonolayer (SML) QDs (0.25 ML, 0.5 ML and 0.75 ML) and a quasi-monolayer (1 ML) InAs stack, were fabricated while keeping the total InAs content the same in all SCs. In a comparison of performance, the SML-QD sample delivered superior performance (Almost 23% relative efficiency improvement compared to reference SC) compared to the other devices. Moreover, different coverages of SML InAs have been tested for optimum performance improvement of solar cell and near 0.25 ML InAs deposition was found best for solar cell application. These findings present a promising alternative to SK-QDs as intermediate band in photovoltaic applications.

Hybrid strain-coupled multilayer SK and SML InAs/GaAs quantum dot heterostructure: Enabling higher absorptivity and strain minimization for enhanced optical and structural characteristics

The size and shape of self-assembled quantum dots (QDs) in the top layer of a strain-coupled multilayer Stranski-Krastanov (SK) type QD heterostructure is decided by the effect of strain from the bottom QD layers along with the compressive strain through InAs/GaAs interface. A novel approach is used in this article to grow the heterostructures such that QDs from one layer to the other are heterogeneously coupled resulting in reduced cumulative strain, which helps in formation of defect free and uniform dots even for 10 strain-coupled SK QD layers. Heterostructures with strain-coupled single- ( × 1), bi- ( × 2), tri- ( × 3), penta- ( × 5), hepta- ( × 7) and deca- ( × 10) layer SK QDs are grown employing a growth mechanism on six stack sub-monolayer (SML) type QDs, considering the individual advantages of SK and SML QDs. Full width half maximum (FWHM) calculated from the low temperature (19 K) photoluminescence (PL) are 50, 52, 54, 55, 56 and 50 nm for single, bi, tri, penta, hepta and deca-layer SK QD stack respectively, which shows uniform dot size distribution up to deca-layer SK QD structure because of the adapted growth mechanism. The activation energies calculated from temperature dependent PL are 244, 281, 299, 275, 288 and 397 meV for single, bi, tri, penta, hepta and deca-layers respectively, which aids that the strain-coupled deca-layer SK QD heterostructure has the highest activation energy compared to other strain-coupled multilayer SK QD structures. These properties make the deca-layer SK QD structure as the optimum device for quantum dot infrared photodetectors (QDIPs), which could operate at higher temperatures with improved performance parameters. High resolution X-ray diffraction (HRXRD) shows a defect free heterostructure with minimized hydrostatic strain and homogeneous dot size distribution for the strain-coupled deca-layer SK QD structure. Thus, the adapted growth mechanism along with heterogeneous coupling of SK and SML QDs facilitates higher absorption efficiency, increased carrier lifetime and better quantum confinement, making it a promising design to be used in intermediate band solar cells (IBSC) and QDIPs, where multilayer QDs are needed so as to improve the device performance.

Coherence in single photon emission from droplet epitaxy and Stranski–Krastanov quantum dots in the telecom C-band

The ability of two photons to interfere lies at the heart of many photonic quantum networking concepts and requires that the photons are indistinguishable with sufficient coherence times to resolve the interference signals. However, for solid-state quantum light sources, this can be challenging to achieve as they are in constant interaction with noise sources in their environment. Here, we investigate the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski–Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. We show that the droplet epitaxy growth mode is beneficial for a quiet environment, leading to 96% of exciton transitions having a coherence time longer than the typical detector resolution of 100 ps, even under non-resonant excitation. We also show that the decay profile indicates the presence of slow dephasing processes, which can be compensated for experimentally. We finally conduct Hong–Ou–Mandel interference measurements between subsequently emitted photons and find a corrected two-photon interference visibility of 98.6 ± 1.6% for droplet-epitaxy grown quantum dots. The understanding of the influence of their surroundings on the quantum optical properties of these emitters is important for their optimization and use in future quantum networking applications.

Internal quantum efficiency of AlGaN/AlN quantum dot superlattices for electron-pumped ultraviolet sources

In this paper, we describe the growth and characterization of ≈530 nm thick superlattices (100 periods) of AlxGa1-xN/AlN (0 ⩽ x ⩽ 0.1) Stranski–Krastanov quantum dots (QDs) for application as the active region of electron-beam pumped ultraviolet lamps. Highly dense (>1011 cm−2) QD layers are deposited by molecular beam epitaxy, and we explore the effect of the III/V ratio during the growth process on their optical performance. The study considers structures emitting in the 244–335 nm range at room temperature, with a relative linewidth in the 6%–11% range, mainly due to the QD diameter dispersion inherent in self-assembled growth. Under electron pumping, the emission efficiency remains constant for acceleration voltages below ≈9 kV. The correlation of this threshold with the total thickness of the SL and the penetration depth of the electron beam confirms the homogeneity of the nanostructures along the growth axis. Below the threshold, the emission intensity scales linearly with the injected current. The internal quantum efficiency (IQE) is characterized at low injection, which reveals the material properties in terms of non-radiative processes, and high injection, which emulates carrier injection in operation conditions. In QDs synthesized with III/V ratio <0.75, the IQE remains around 50% from low injection to pumping power densities as high as 200 kW cm−2, being the first kind of nanostructure that present such stable behaviour.

Tuning the structural and optical characteristics of heterogeneously coupled InAs Submonolayer (SML) quantum dots grown on InAs Stranski-Krastanov (SK) quantum dots

Abstract A complete investigation on the strain and structural properties of the heterogeneously coupled InAs Submonolayer (SML) on InAs Stranski-Krastanov (SK) quantum dot (QD) heterostructure has been done. This study shows the tunability of structural and optical characteristics of this coupled SML on SK QDs by varying the strain field through growth rate and barrier thickness. Both experimental and theoretical investigations were done to understand the strain distribution within the heterostructures. The low growth rate samples exhibited lesser in- and out-plane strain, which were also reflected in the transmission electron microscopy (TEM) images. This study would help the researchers to design devices (utilizing advantages of both SK and SML QDs) for high performance solar photovoltaics.

Improved carrier transfer in vertically coupled surface and buried InAs Stranski-Krastanov quantum dot system via ex-situ surface state passivation

In this work, the authors have investigated the optical and morphological characteristics of uncapped InAs quantum dots (QDs) and their sensitivity to the surface passivation. The temperature dependent carrier transfer mechanism between self-assembled vertically coupled InAs surface quantum dot (SQD) and underneath buried quantum dot (BQD), separated by a thin spacer layer is investigated. Oxide layer etching followed by surface passivation has been carried out to deplete the surface states and improve the structural/optical behaviour of uncapped InAs SQDs. The passivation time has been varied from 0.5 to 2.5 h and its effect on the optical and morphological properties is investigated. The larger SQDs are resized to smaller ones and show lower size dispersion after the etching process. Additionally, an increased energy level overlapping between SQD and BQD is obtained after the etching process. This partial overlap of energy levels further corroborates the enhanced electronic communication achieved between the surface and underneath buried dots. The sample, with 1 h of surface passivation, unveils as the best among all in terms of stronger luminescence from the SQD. Through this study, the SQDs are projected as the primary receptor, whereas the BQDs behave like carrier reservoirs. Hence, the BQDs would provide the carriers to enhance the sensitivity of the SQDs, which would impact the performance of QD based sensors.

Vertically Coupled Hybrid InAs Sub-Monolayer on InAs Stranski–Krastanov Quantum Dot Heterostructure: Toward Next Generation Broadband IR Detection

In the present article, we are introducing a novel heterogeneously coupled InAs Submonolayer (SML) on Stranski-Krastanov (SK) quantum dot (QD) heterostructure with reduced strain accumulation. Theoretical comparison manifests the superiority of this hybrid SML-SK QDs over bilayer SK QDs. Photoluminescence and photoluminescence excitation spectroscopy are employed to characterize the electronic interaction and carrier tunneling in between this hybrid quantum dot assembly. Growth rate is optimized at 0.1 ML/sec. To tune the coupling, the barrier layer thickness is varied from 5 nm to 10 nm. Even up to the highest barrier thickness, no signature of SML peak is obtained in the PL response. But, PLE depicts the presence of SML peak, which is partially overlapped with the SK 2nd excited peak. The sample with 7.5 nm GaAs barrier shows perfect resonance between SML ground state and SK 2nd excited state. Based on experimental and analytical results, this SML on SK (SML-SK) configuration is compared with SK on SML (SK-SML) configuration; where SML-SK exhibits better inter dot electronic interaction than its reciprocal configuration. For ex situ tailoring of this heterogeneous electronic coupling, rapid thermal annealing has been done. It shows tunability of inter dot interaction with improved crystalline quality. Finally, we have shown a SML-SK quantum dot infrared photodetector with a broad spectral response (from SWIR to near LWIR).

Role of the wetting layer in the crystallization stage during droplet epitaxy of InAs/GaAs nanostructures

In this paper, we present the results of experimental study of the role of the wetting layer in the crystallization stage of droplet nanostructures in the lattice-mismatched InAs/GaAs material system. We demonstrate that the thickness of the wetting layer can significantly affect the characteristics of nanostructures formed during crystallization in the arsenic flux. While the surface density of InAs nanostructures does not increase relatively to the initial droplet density obtained at 300 °C, a significant increase of the nanostructure density is observed after crystallization of droplets obtained at 200 °C. This behaviour is attributed to the additional stress-driven Stranski-Krastanov quantum dot formation and confirmed by the 2D-3D transformation of indium layer obtained at 100 °C.

Improved near infrared energy harvesting through heterogeneously coupled SK on SML quantum dot heterostructure

Near Infra Red (NIR) energy harvesting is one of the advanced strategies to improve the overall performance of new generation solar photovoltaics. We introduce here a new heterogeneously coupled Stranski–Krastanov (SK) on Submonolayer (SML) quantum dot (QD) heterostructure, which is capable of enhanced energy conversion efficiency in that typical spectral range. The coupling in the proposed structure has been optimized by varying the growth rate and the barrier thickness between seed SML and top SK QDs. The optimal configuration has been incorporated into a quantum dot solar cell (QDSC). The carrier transitions in this hybrid QD assembly have been investigated through luminescence spectroscopy. An existence of carrier tunnelling between SML and SK QDs has been revealed from the photoluminescence and the photoluminescence excitation spectroscopy. The unique configuration of such coupled QDs shows the presence of resonating energy states. These phenomena ameliorate the absorption efficiency in the NIR regime with increased photoexcited carrier lifetime, which has been reflected in the external quantum efficiency.

Stacked Self‐Assembled Cubic GaN Quantum Dots Grown by Molecular Beam Epitaxy

We have investigated the stacking of self‐assembled cubic GaN quantum dots (QDs) grown in Stranski–Krastanov (SK) growth mode. The number of stacked layers is varied to compare their optical properties. The growth is in situ controlled by reflection high energy electron diffraction to prove the SK QD growth. Atomic force and transmission electron microscopy show the existence of wetting layer and QDs with a diameter of about 10 nm and a height of about 2 nm. The QDs have a truncated pyramidal form and are vertically aligned in growth direction. Photoluminescence measurements show an increase of the intensity with increasing number of stacked QD layers. Furthermore, a systematic blue‐shift of 120 meV is observed with increasing number of stacked QD layers. This blueshift derives from a decrease in the QD height, because the QD height has also been the main confining dimension in our QDs.

The coupling between two heterogeneous InAs quantum dot families and its effect into optical properties

Abstract:In this study, we present the coupling between InAs submonolayer (SML) and stranski krastanov (SK) quantum dots (QDs). Interaction between these two different dot families has been manipulated by changing the capping layer thickness. Significant shift in photoluminescence (PL) peak is observed due to the coupling effect. The dynamics of the carriers in this mixed dot matrix has also been modified, which is evident from the increasing activation energy with increasing thickness of the capping layer. Moreover, an ex situ annealing study at different temperatures has been done to check the thermal stability of the as-grown samples. Annealing at lower temperatures, improves the crystal quality a bit, but higher annealing temperatures accelerate the In-Ga interdiffusion and form smaller dots, which is visible from a blue shift in the PL peak of annealed samples. Also, this thermal process improves the dot size distribution.

Ultrafast gain recovery and large nonlinear optical response in submonolayer quantum dots

Submonolayer quantum dots combine the zero-dimensional charge-carrier confinement of self-assembled quantum dots with the large density of states of a quantum well. Electroluminescence and pump-probe experiments on a submonolayer-based optical amplifier show that the system exhibits a high gain of $90{\mathrm{cm}}^{\ensuremath{-}1}$ and an ultrafast gain recovery. We propose a rate equation system describing the microscopic carrier dynamics which quantitatively reproduces the observed behavior and provides deeper theoretical understanding of the material system. In contrast to Stranski-Krastanov quantum dots, the fast gain recovery is enhanced by a strong interdot coupling. Optically inactive submonolayer states form an efficient carrier reservoir and give rise to a large nonlinear optical response.

Surface photovoltage spectroscopy study of InAs quantum dot in quantum well multilayer structures for infrared photodetectors

Inter-band optical transitions in InAs submonolayer and Stranski-Krastanov quantum dot (QD) in quantum well (QW) nanostructures are studied by means of room temperature surface photovoltage (SPV) spectroscopy taking advantage of its high sensitivity and contactless nature. The QD optical transitions are identified by the combined analysis of SPV amplitude and phase spectra and are in agreement with photoluminescence results. The SPV spectra have further revealed the optical transitions in all other relevant layers in the structures - wetting layer, QWs, and AlGaAs barriers. The analysis of the SPV phase spectra has revealed that the carrier separation and transport in the QD structure is determined by the energy band bending, resulting from the slight residual p-type doping. The complicated interaction between the SPV signals from the nanostructure and the semi-insulating GaAs substrate is discussed and clarified. The advantages of the SPV spectroscopy for characterizing complicated nanostructures at room temperature are highlighted.

The initial stage of growth of self-induced GaN nanowires

The initial growth stage of self-induced GaN nanowires (NWs) on an AlN(0001)/Si(111) substrate is studied theoretically. Calculations are carried out within the model of Stranski-Krastanov quantum dot formation. The surface density of GaN islands is calculated, the formation of which precedes NW formation. GaN NW density is found as a function of gallium flux and deposition time for the case of molecular beam epitaxy growth.