In‐plane coupling effect on absorption coefficients of InAs/GaAs quantum dots arrays for intermediate band solar cell

Coupled semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC), because of their ability to facilitate the formation of minibands. The quantum coupling effect that exists between states in QDs in an array influences the electronic and optical properties of such structures. So far, great experimental and theoretical efforts have been devoted to study the vertically coupled QD arrays. We present here a method based on multi‐band k ⋅ p Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs‐based lateral QD arrays. Formation of the intermediate band (IB) in all cases was achieved via delocalisation of the electron ground state (e0). We show that the IB in a laterally coupled QD‐IBSC is more robust against external electric field from the solar cell's pn junction than that in a vertically coupled arrangement. Because of symmetry of the QD array lattice and QD states itself, which are C_2v for the zinc blend QDs, the electronic and absorption structures were obtained via sampling throughout the reciprocal space in the first Brillouin zone of QD arrays. Copyright © 2014 John Wiley & Sons, Ltd.     

Realistic Detailed Balance Study of the Quantum Efficiency of Quantum Dot Solar Cells

An attractive but challenging technology for high efficiency solar energy conversion is the intermediate band solar cell (IBSC), whose theoretical efficiency limit is 63%, yet which has so far failed to yield high efficiencies in practice. The most advanced IBSC technology is that based on quantum dots (QDs): the QD‐IBSC. In this paper, k·p calculations of photon absorption in the QDs are combined with a multi‐level detailed balance model. The model has been used to reproduce the measured quantum efficiency of a real QD‐IBSC and its temperature dependence. This allows the analysis of individual sub‐bandgap transition currents, which has as yet not been possible experimentally, yielding a deeper understanding of the failure of current QD‐IBSCs. Based on the agreement with experimental data, the model is believed to be realistic enough to evaluate future QD‐IBSC proposals.     

Spectral-kinetic properties of heterostructures with GaAsSb/InGaAs/GaAs-based quantum wells emitting in the range of 1.0–1.2 μm

The spectral-kinetic properties of heterostructures with GaAs/GaAsSb-based and GaAsSb/InGaAs/GaAs-based quantum wells, emitting in the range of 1.0–1.2 μm are studied with picosecond and nanosecond temporal resolution. Intense photoluminescence in the GaAsSb/InGaAs/GaAs structure, as well as an increase in the photoluminescence wavelength by a factor of 2.5 and a shift of the location of the maximum of the peak (～100 meV) to the longer-wavelength region were observed up to room temperature. It is established that as the molar fraction of Sb and the thickness of the InGaAs layer increase, the energy of the fundamental transition decreases by a factor of 140 meV compared with the GaAsSb/InGaAs/GaAs structure with a lower Sb content and a smaller thickness of the InGaAs layer. At 300 K, the emission wavelength of such a structure was 1.18 μm. In addition, an increase in the thickness of the InGaAs layer led to an increase in the room-temperature photoluminescence intensity by a factor of 60, which is associated with a decrease in the energy of the fundamental state for electrons in the InGaAs layer and, consequently, to larger electron localization and smaller temperature quenching of photoluminescence.     

Below‐bandgap absorption in InAs/GaAs self‐assembled quantum dot solar cells

A new approach to derive the below‐bandgap absorption in InAs/GaAs self‐assembled quantum dot (QD) devices using room temperature external quantum efficiency measurement results is presented. The significance of incorporating an extended Urbach tail absorption in analyzing QD devices is demonstrated. This tail is used to evaluate the improvement in the photo‐generated current. The wetting layer and QD absorption contributions are separated from the tail absorption. Several absorption peaks due to QD excited states and potentially different size QDs are observed. An inhomogeneous broadening of 25 meV arising from the variance in the size of QDs is derived. Copyright © 2014 John Wiley & Sons, Ltd.     

InGaAs quantum dots formed in tetrahedral-shaped recesses on GaAs (111)B grown by metalorganic chemical vapor deposition

Novel semiconductor quantum dots (QDs), grown in tetrahedral-shaped recesses (TSRs) formed on a (111)B GaAs substrate, are described from both material science and device application points of view. After explaining the fabrication procedure for TSRs, growth of InGaAs QDs and their optical properties are explained. It is revealed that an InGaAs QD of indium-rich chemical composition is formed spontaneously at the bottom of each TSR. The mechanism of the QD formation is discussed in detail. It is proved from magneto-photoluminescence that the QDs actually have optical properties peculiar to zero-dimensional confinement. Several experimental results indicating excellent growth controllability of the QDs are presented. Finally, recent challenges to apply the QDs to electronic memory devices are reported. Two kinds of devices, where the position of individual QD is artificially controlled, are proposed for the first time and the preliminary experimental results are explained.     

Photoluminescence up to 1.6 μm of quantum dots with an increased effective thickness of the InAs layer

Multilayered InAs/GaAs quantum dot (QD) heterostructures are produced by metal-organic gas phase epitaxy. The structures exhibit photoluminescence around 1.55 μm at 300 K. The specific feature of the technology is the growth of an InAs layer with an increased effective thickness d _eff to form QDs, in combination with low-temperature overgrowth of the QDs with a thin (6-nm) GaAs layer and with the annelaing of defects. By X-ray diffraction analysis and PL studies, it is shown that, in a structure with the increased thickness d _eff, a secondary wetting InGaAs layer is produced on top of the QD layer from the growing relaxed large-sized InAs clusters on annealing. A new mechanism of formation of large-sized QDs characterized by a large “aspect ratio” is suggested. The mechanism involves the 2D–3D transformation of the secondary InGaAs layer in the field of elastic strains in previously formed QDs. The specific feature of the array of QDs is the coexistence of three populations of different-sized QDs responsible for the multimode photoluminescence in the range from 1 to 1.6 μm. The potentialities of such structures for infrared photoelectric detectors operating in the range from 1–2.5 μm at room temperature are analyzed.     

中间带太阳电池进展

中间带太阳电池是第三代光伏发电研究中很热门的研究领域之一。论述了中间带太阳电池的原理,以及实现中间带材料的三种方法,即量子点中间带电池、杂质带电池、高失配合金。量子点中间带太阳电池的红外吸收测量证实中间带太阳电池的基本原理是正确的。介绍了为提高短路电流,采用应力补偿技术,增加量子点层数,增大量子点的吸收系数。目前量子点中间带太阳电池的效率达到18%。阐述了杂质带的机理,研究表明,当Si中掺Ti浓度超过Mott相变浓度时,杂质抑制非辐射复合,有效载流子寿命增加。高失配合金具有不寻常的能带结构,AlGaN材料的带隙接近中间带的理想值,很可能成为下一个研究的热点。     

Functionalized Self‐Assembled InAs/GaAs Quantum‐Dot Structures Hybridized with Organic Molecules

Low‐dimensional III–V semiconductors have many advantages over other semiconductors; however, they are not particularly stable under physiological conditions. Hybridizing biocompatible organic molecules with advanced optical and electronic semiconductor devices based on quantum dots (QDs) and quantum wires could provide an efficient solution to realize stress‐free and nontoxic interfaces to attach larger functional biomolecules. Monitoring the modifications of the optical properties of the hybrid molecule–QD systems by grafting various types of air‐stable diazonium salts onto the QD structures surfaces provides a direct approach to prove the above concepts. The InAs/GaAs QD structures used in this work consist of a layer of surface InAs QDs and a layer of buried InAs QDs embedded in a wider‐bandgap GaAs matrix. An enhancement in photoluminescence intensity by a factor of 3.3 from the buried QDs is achieved owing to the efficient elimination of the dangling bonds on the surface of the structures and to the decrease in non‐radiative recombination caused by their surface states. Furthermore, a narrow photoluminescence band peaking at 1620 nm with a linewidth of 49 meV corresponding to the eigenstates interband transition of the surface InAs QDs is for the first time clearly observed at room temperature, which is something that has rarely been achieved without the use of such engineered surfaces. The experimental results demonstrate that the hybrid molecule–QD systems possess a high stability, and both the surface and buried QDs are very sensitive to changes in their surficial conditions, indicating that they are excellent candidates as basic sensing elements for novel biosensor applications.     

In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点的形貌和光学性质

提出了利用分子束外延方法生长In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点，并分析量子点的形貌和光学性质随GaAs隔离层厚度变化的特点。实验结果表明，随着耦合量子点中的GaAs隔离层厚度从2 nm增加到10 nm，In0.5Ga0.5As量子点的密度增大、均匀性提高, Al原子扩散和浸润层对量子点PL谱的影响被消除，而且InAlAs材料的宽禁带特征使其成为InGaAs量子点红外探测器中的暗电流阻挡层。由此可见，选择合适的GaAs隔离层厚度形成InGaAs/InAlAs应变耦合量子点将有益于InGaAs量子点红外探测器的研究。     

Impact of spatial separation of type‐II GaSb quantum dots from the depletion region on the conversion efficiency limit of GaAs solar cells

The purpose of this work is to look for a practical structure for application of quantum dots (QDs) in solar cells. We focus on a stack of strain‐compensated GaSb/GaAs type‐II QDs. We propose a novel structure with GaSb/GaAs type‐II QD absorber embedded in the p‐doped region of ideal solar cell but spatially separated from the depletion region. We developed the model and used the detailed balance principle along with Poisson and continuity equations for calculating of the energy band bending and photovoltaic characteristics of the proposed solar cell. Our model takes into account both single‐photon and two‐photon absorption as well as non‐radiative processes in QDs and predicts that concentration from 1‐sun to 500‐sun increases the efficiency from 30% to 50%. We showed that accumulation of charge in the QD absorber is the clue to understanding of potentially superior performance of the proposed solar cell. An attractive feature of the proposed solar cell is that QDs do not reduce the open‐circuit voltage but facilitate generation of the additional photocurrent to the extent that photovoltaic characteristics reduce to that of ideal solar cell while the efficiency meets the Luque‐Marti limit. It should be noted that although non‐radiative processes like relaxation in QDs and recombination through QDs degrade photovoltaic characteristics of the proposed solar cell, its conversion efficiency is still predicted to be above the Shockley‐Queisser limit by 5% to 10%. This study is an important step toward producing practical solar cells that benefit from additional photocurrent generated by sub‐band gap photons. Copyright © 2014 John Wiley & Sons, Ltd.     

Fabrication and analysis of multijunction solar cells with a quantum dot (In)GaAs junction

InAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double‐junction solar cells and InGaP/(In)GaAs/Ge triple‐junction solar cells on 4‐in. wafers. One sun AM0 current–voltage measurement shows consistent performance across the wafer. Quantum efficiency analysis shows similar aforementioned bandgap performance of baseline and QD solar cells, whereas integrated sub‐band gap current of 10 InAs QD layers shows a gain of 0.20 mA/cm². Comparing QD double‐junction solar cells and QD triple‐junction solar cells to baseline structures shows that the (In)GaAs junction has a V_oc loss of 50 mV and the InGaP 70 mV. Transmission electron microscopy imaging does not reveal defective material and shows a buried QD density of 10¹¹ cm⁻², which is consistent with the density of QDs measured on the surface of a test structure. Although slightly lower in efficiency, the QD solar cells have uniform performance across 4‐in. wafers. Copyright © 2013 John Wiley & Sons, Ltd.     

Increasing the quantum efficiency of InAs/GaAs QD arrays for solar cells grown by MOVPE without using strain‐balance technology

Research into the formation of InAs quantum dots (QDs) in GaAs using the metalorganic vapor phase epitaxy technique is presented. This technique is deemed to be cheaper than the more often used and studied molecular beam epitaxy. The best conditions for obtaining a high photoluminescence response, indicating a good material quality, have been found among a wide range of possibilities. Solar cells with an excellent quantum efficiency have been obtained, with a sub‐bandgap photo‐response of 0.07 mA/cm² per QD layer, the highest achieved so far with the InAs/GaAs system, proving the potential of this technology to be able to increase the efficiency of lattice‐matched multi‐junction solar cells and contributing to a better understanding of QD technology toward the achievement of practical intermediate‐band solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Structural and optical properties of InAs quantum dots in AlGaAs matrix

Structural and optical properties of InAs quantum dots (QDs) grown in a wide-bandgap Al_0.3Ga_0.7As matrix is studied. It is shown that a high temperature stability of optical properties can be achieved owing to deep localization of carriers in a matrix whose band gap is wider than that in GaAs. Specific features of QD formation were studied for different amounts of deposited InAs. A steady red shift of the QD emission peak as far as ∼1.18 μm with the effective thickness of InAs in Al_0.3Ga_0.7As increasing was observed at room temperature. This made it possible to achieve a much higher energy of exciton localization than for QDs in a GaAs matrix. To obtain the maximum localization energy, the QD sheet was overgrown with an InGaAs layer. The possibility of reaching the emission wavelength of ~1.3 μm is demonstrated. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 5, 2003, pp. 578–582. Original Russian Text Copyright ? 2003 by Sizov, Samsonenko, Tsyrlin, Polyakov, Egorov, Tonkikh, Zhukov, Mikhrin, Vasil’ev, Musikhin, Tsatsul’nikov, Ustinov, Ledentsov.     

[1 1 1]B-oriented GaAsSb grown by gas source molecular beam epitaxy

We report the GaAsSb bulk layers and GaAsSb/GaAs quantum wells (QWs) grown on (1 1 1)B GaAs substrates by gas source molecular beam epitaxy. We found that Sb composition in the GaAsSb epilayers is very sensitive to the substrate temperature. The composition drops from 0.35 to 0.16 as the substrate temperature increases from 450 to 550 °C. The [1 1 1]B-oriented GaAsSb epilayers show phase separation when the substrate temperature is lower than 525 °C. For a GaAsSb/GaAs multiple quantum wells (MQWs) structure composed of five periods of 5 nm GaAs_0.73Sb_0.27 QW and 30 nm GaAs barrier, the room temperature photoluminescence emission is located at 1255, 80 nm longer than the [1 0 0]-oriented sample with the same Sb composition. The peak wavelength shows significant blue shift as the excitation level increases, which evidences the type-II band alignment in this heterostructure.     

InGaAs/GaAs quantum dot heterostructures for 3–5 μm IR detectors

Specific features in the formation of InAs quantum dots (QD) by MOCVD were studied in relation to the growing time or equivalent thickness of the InAs layer. TEM and photoluminescence studies have shown that, as the growing time of QDs in a GaAs matrix becomes longer, both the size and shape of the QDs are modified; namely, the aspect ratio increases. Selectively doped multilayer InGaAs/GaAs QD structures were fabricated, and photoconductivity in the IR range was studied for lateral and vertical electron transport. Under a normal incidence of light, intraband photoconductivity in the mid-IR range, 2.5–5 μm, was observed at temperatures of up to 110 K.     

1.3μm自组织InGaAs/InAs/GaAs量子点激光器分子束外延生长

报道了分子束外延生长的1.3μm多层InGaAs/InAs/GaAs自组织量子点及其室温连续激射激光器.室温带边发射峰的半高宽小于35meV,表明量子点大小比较均匀.原子力显微镜图像显示,量子点密度可以控制在(1～7)×1010cm-2范围之内,而面密度处于4×1010cm-2时有良好的光致发光谱性能.含有三到五层1. 3μm量子点的激光器成功实现了室温连续激射.     

Stimulated emission in heterostructures with double InGaAs/GaAsSb/GaAs quantum wells,grown on GaAs and Ge/Si(001) substrates

We report the observation of stimulated emission in heterostructures with double InGaAs/GaAsSb/GaAs quantum wells, grown on Si(001) substrates with the application of a relaxed Ge buffer layer. Stimulated emission is observed at 77 K under pulsed optical pumping at a wavelength of 1.11 μm, i.e., in the transparency range of bulk silicon. In similar InGaAs/GaAsSb/GaAs structures grown on GaAs substrates, room-temperature stimulated emission is observed at 1.17 μm. The results obtained are promising for integration of the structures into silicon-based optoelectronics.     

Room-temperature optical absorption in the InAs/GaAs quantum-dot superlattice under an electric field

Electroluminescence and absorption spectra of a ten-layer InAs/GaAs quantum dot (QD) superlattice built in a two-section laser with sections of equal length is experimentally studied at room temperature. The thickness of the GaAs spacer layer between InAs QD layers, determined by transmission electron microscopy, is ∼6 nm. In contrast to tunnel-coupled QDs, QD superlattices amplify the optical polarization intensity and waveguide absorption of the TM mode in comparison with the TE mode. It is found that variations in the multimodal periodic spectrum of differential absorption of the QD superlattice structure are strongly linearly dependent on the applied electric field. Differential absorption spectra exhibit the Wannier-Stark effect in the InAs/GaAs QD superlattice, in which, in the presence of an external electric field, coupling of wave functions of miniband electron states is suppressed and a series of discrete levels called the Wannier-Stark ladder states are formed.     

Injection lasers based on InGaAs quantum dots in an AlGaAs matrix

Arrays of vertically coupled InGaAs quantum dots (QDs) in an AlGaAs matrix have been used in injection lasers. Increase in the band gap of a matrix material by replacement of a GaAs matrix with an AlGaAs one led to dramatic increase in quantum dot localization energy. By using this approach, we reduced the thermal population of the matrix and wetting layer states and thus decreased room temperature threshold current density to 63 A/cm², increased differential efficiency up to 65%, and achieved room temperature continuous wave operation with output power of 1 W. Negative characteristic temperature has been observed in temperature dependence of threshold current density of these lasers in some temperature range. A qualitative explanation assuming a transition from non-equilibrium to Fermi population of QD states is proposed.     

Progress in Epitaxial Growth and Performance of Quantum Dot and Quantum Wire Lasers

We report on interplay of epitaxial growth phenomena and device performance in quantum dot (QD) and quantum wire (QWW) lasers based on self-organized nanostructures. InAs QDs are the most explored model system for basic understanding of "near-ideal" QD devices. Vertically-coupled growth of QDs and activated phase separation allow ultimate QD wavefunction engineering enabling GaAs lasers beyond 1400 nm and polarization-insensitive optical amplification. A feasibility of QD semiconductor optical amplifiers at terabit frequencies using InAs QDs is manifested at 1300 and 1500 nm. 1250-1300 nm QD GaAs edge emitters and VCSELs operate beyond 10 Gb/s with ultimate temperature robustness. Furthermore, temperature-insensitive operation without current or modulation voltage adjustment at >20 Gb/s is demonstrated up to ~90 degC. Light-emitting devices based on InGaN-QDs cover ultraviolet (UV) and visible blue-green spectral ranges. In these applications, InN-rich nanodomains prevent diffusion of nonequilibrium carries towards crystal defects and result in advanced degradation robustness of the devices. All the features characteristic to QDs are unambiguously confirmed for InGaN structures. For the red spectral range InGaAlP lasers are used. Growth on misoriented surfaces, characteristic to these devices, leads to nano-periodi- cally-step-bunched epitaxial surfaces resulting in two principal effects: 1) step-bunch-assisted alloy phase separation, leading to a spontaneous formation of ordered natural super lattices; 2) formation of quantum wire-like structures in the active region of the device. A high degree of polarization is revealed in the luminescence recorded from the top surface of the structures, in agreement with the QWW nature of the gain medium. QD and QWW lasers are remaining at the frontier of the modern optoelectronics penetrating into the mainstream applications in key industries.