Evidence of two‐photon absorption in strain‐free quantum dot GaAs/AlGaAs solar cells

We report the fabrication procedure and the characterization of an Al_0.3Ga_0.7As solar cell containing high‐density GaAs strain‐free quantum dots grown by droplet epitaxy. The production of photocurrent when two sub‐bandgap energy photons are absorbed simultaneously is demonstrated. The high quality of the quantum dot/barrier pair, allowed by the high quality of nanostructured strain‐free materials, opens new opportunities for quantum dot based solar cells.

     

Insight into optical properties of strain-free quantum dot pairs

Self-assembled GaAs/AlGaAs quantum dot pairs (QDPs) are grown by molecular beam epitaxy using high temperature droplet epitaxy technique. A typical QDP consists of dual-size quantum dots as observed based on atomic force microscopy image. The average height of quantum dot is 5.7 nm for the large quantum dots and 4.6 nm for the small ones. The average peak-to-peak distance of the two dots is about 75 nm. The optical properties of GaAs QDPs are studied by measuring excitation power-dependent and temperature-dependent photoluminescence. Unique photoluminescence properties have been observed from both excitation power-dependent and temperature-dependent measurements. Excitation power-dependent as well as temperature-dependent PL measurements have suggested lateral exciton transfer in the QDPs.     

Self‐catalysed growth of InAs nanowires on bare Si substrates by droplet epitaxy

We demonstrate the self‐catalyst growth of vertically aligned InAs nanowires on bare Si(111) by droplet epitaxy. The growth conditions of indium droplets suitable for nucleation and growth of nanowires have been identified. We have then realized vertically aligned and non‐tapered InAs nanowires on bare Si(111) substrates through optimal indium droplets. It was found that the lateral dimensions and density of nano‐wires are defined by the indium droplets. This technique unravels a controllable, cost‐effective and time‐efficient route to fabricating functional monolithic hybrid structures of InAs nanowires on silicon.

     

The effect of post‐growth thermal annealing on the emission spectra of GaAs/AlGaAs quantum dots grown by droplet epitaxy

We fabricated GaAs/AlGaAs quantum dots by droplet epitaxy, and obtained the geometries of the dots by scanning transmission electron microscopy. Post‐growth thermal annealing is essential for the optical activation of quantum dots grown by droplet epitaxy. We measured the emission energy shifts of the dots and the underlying superlattice by post‐ growth thermal annealing, and specified the emission from dots by selectively etching the structure down to a low layer of quantum dots. We studied the influence of the degree of annealing on the optical properties of the dots from the peak shifts of the superlattice, since the superlattice has a uniform and well‐defined geometry. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unusual role of the substrate in droplet‐induced GaAs/AlGaAs quantum‐dot pairs

The structural complexity of GaAs quantum‐dot pairs has been revealed by cross‐sectional transmission electron microscopy. As a result of high‐temperature droplet epitaxy, the AlGaAs substrate beneath the quantum‐dot pairs is no longer immobile and its reconstruction is observed to define the crystallization of gallium droplets under an arsenic flux. The GaAs quantum‐dot pairs are immersed into the substrate and further confined by the re‐distributed AlGaAs materials above the substrate plane. There are two underlying mechanisms responsible for the final nanostructure configuration, melt‐back etching by the gallium droplets and preferential crystallization of gallium around reconstructed sidewalls. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimization of InAs/GaAs quantum-dot structures and application to 1.3-μm mode-locked laser diodes

The self-assembled growth of InAs/GaAs quantum dots by molecular beam epitaxy is conducted by optimizing several growth parameters, using a one-step interruption method after island formation. The dependence of photoluminescence on areal quantum-dot density is systematically investigated as a function of InAs deposition, growth temperature and arsenic pressure. The results of this investigation along with time-resolved photoluminescence measurements show that the com- bination of a growth temperature of 490℃, with a deposition rate of 0.02 ML/s, under an arsenic pressure of 1×10^-6 Torr （1 Torr = 1.33322×10^2 Pa）, provides the best compromise between high density and the photoluminescence of quantum dot structure, with a radiative lifetime of 780 ps. The applicability of this 5-layer quantum dot structure to high-repetition-rate pulsed lasers is demonstrated with the fabrication and characterization of a monolithic InAs/GaAs quantum-dot passively mode-locked laser operating at nearly 1300 nm. Picosecond pulse generation is achieved from a two-section laser, with a 19.7-GHz repetition rate.     

Luminescence of GaN Layers Grown on GaAs Substrates by the Method of Radical‐Beam Epitaxy

We have investigated the properties of GaN films obtained by the method of radicalbeam gettering epitaxy. At room temperature the photoluminescence spectra display a weak peak at 3.37 eV, with the fundamental radiation band being located in the range 1.7–2.0 eV. At the temperature 4.2 K, the band with an energy of 3.47 eV is present. The donoracceptor transition with a maximum at 3.26 eV and its phonon repetition dominate in the spectrum. A wide yellow band with a maximum at 2.10 eV and a weak peak at 2.88 eV are also observed. It has been established by Auger electron spectroscopy that gallium and nitrogen are the basic elements that enter into the composition of the film. Xray structural investigations have shown that the films possess a hexagonal structure.     

Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously (n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.     

Properties of InGaAs/GaAs quantum wells with a δ〈Mn〉-doped layer in GaAs

A method of formation of two-dimensional structures containing a δ〈Mn〉-doped layer in GaAs and an In_xGa_1?x As quantum well (QW) separated by a GaAs spacer of thickness d = 4–6 nm is developed using laser evaporation of a metallic target during MOS hydride epitaxy. It is shown that, up to room temperature, these structures have ferromagnetic properties most likely caused by MnAs clusters. At low temperatures (T _m ～ 30 K), the anomalous Hall effect is revealed to occur. This effect is related to hole scattering by Mn ions in GaAs and to the magnetic exchange between these ions and QW holes, which determines the spin polarization of the holes. The behavior of the negative magnetoresistance of these structures at low temperatures indicates the key role of quantum interference effects.     

Micro-photoluminescence spectroscopy of hierarchically self-assembled quantum dots

Novel, self-assembled quantum dot (QD) structures suitable for single-dot optical spectroscopy are fabricated by combining III–V molecular beam epitaxy and in situ, atomic layer precise etching. Several growth and etching steps are used to produce lateral InAs/GaAs QD bimolecules and unstrained GaAs/AlGaAs QDs with low surface density . Micro-photoluminescence is used to investigate the ensemble and single-QD properties of GaAs QDs. Single-QD spectra show resolution-limited sharp lines at low excitation and broad “shell-structures” at high excitation intensity.     

Investigation of various optical transitions in GaAs/Al0.3Ga0.7As double quantum ring grown by droplet epitaxy

This work examines the optical transitions of a GaAs double quantum ring (DQR) embedded in Al_0.3Ga_0.7As matrix by photoreflectance spectroscopy (PR). The GaAs DQR was grown by droplet epitaxy (DE). The optical properties of the DQR were investigated by excitation‐intensity and temperature‐dependent PR. The various optical transitions were observed in PR spectra, whereas the photoluminescence (PL) spectrum shows only the DQR and GaAs band emissions. The various optical transitions were identified for the GaAs near‐band‐edge transition, surface confined state (SCS), DQR confined state, wetting layer (WL), spin–orbital split (E_GaAs + Δ_o), and AlGaAs band transition. PR spectroscopy can identify various optical transitions that are invisible in PL. The PR results show that the GaAs/AlGaAs DQR has complex electronic structures due to the various interfaces resulting from DE.     

Type II GaSb/GaAs quantum dot/ring stacks with extended photoresponse for efficient solar cells

We report on the fabrication of GaAs based p–i–n solar cells containing 5 and 10 layers of type II GaSb quantum rings grown by molecular beam epitaxy. Solar cells containing quantum rings show improved efficiency at longer wavelengths into the near-IR extending up to 1500 nm and show enhanced short-circuit current under 1 sun illumination compared to a GaAs control cell. A reduction in the open-circuit voltage is observed due to the build-up of internal strain. The MBE growth, formation and photoluminescence of single and stacked layers of GaSb/GaAs quantum rings are also presented.     

Efficient and long‐life green light‐emitting diodes comprising tridentate thiol capped quantum dots

In this paper, the green quantum dots capped with the ligand, tris(mercaptomethyl)nonane (TMMN), are fabricated as the light‐emitting layer for efficient and bright light‐emitting diodes. These TMMN‐capped quantum dots exhibit well‐preserved photoluminescence properties with quantum yields of ∼90% after ligand exchange. The light‐emitting diodes based on TMMN‐capped quantum dots are reported with a maximum external quantum efficiency of 16.5% corresponding to a power efficiency and current efficiency of 57.6 lm W^–1 and 70.1 cd A^–1, respectively. The devices exhibit high color stability that is not markedly affected by the increase of applied voltage, thus leading to a high color reproducibility. Most importantly, the devices exhibit high environmental stability. For the highest luminance devices (with emitting layer thickness of 25 nm) and the highest power efficiency devices (with emitting layer thickness of 38 nm), the lifetimes are > 480 000 h and > 110 000 h, respectively.

     

Localized states emission in type‐I GaAsSb/AlGaAs multiple quantum wells grown by molecular beam epitaxy

As an important candidate for novel infrared semiconductor lasers, the optical properties of GaAsSb‐based multiple quantum wells (MQWs) are crucial. The temperature‐ and excitation power‐dependent photoluminescence (PL) spectra of the GaAs_0.92Sb_0.08/Al_0.2Ga_0.8As MQWs, which were grown by molecular beam epitaxy, were investigated and are detailed in this work. Two competitive peaks were observed from 40 K to 90 K. The peak located at the low‐energy shoulder was confirmed to be localized states emission (LE) and the high‐energy side peak was confirmed to be free‐carrier emission by its temperature‐dependent emission peak position. It is observed that the LE peak exhibited a blueshift with the increase of laser excitation power, which can be ascribed to the band filling effect of localized states. Our studies have great significance for application of GaAsSb‐based MQWs in infrared semiconductor lasers.

     

GaAs integrated quantum photonics: Towards compact and multi‐functional quantum photonic integrated circuits

The recent progress in integrated quantum optics has set the stage for the development of an integrated platform for quantum information processing with photons, with potential applications in quantum simulation. Among the different material platforms being investigated, direct‐bandgap semiconductors and particularly gallium arsenide (GaAs) offer the widest range of functionalities, including single‐ and entangled‐photon generation by radiative recombination, low‐loss routing, electro‐optic modulation and single‐photon detection. This paper reviews the recent progress in the development of the key building blocks for GaAs quantum photonics and the perspectives for their full integration in a fully‐functional and densely integrated quantum photonic circuit.

     

Surface mediated control of droplet density and morphology on GaAs and AlAs surfaces

We present a comparative study of gallium (Ga) and aluminium (Al) droplets fabricated on GaAs (100) and AlAs (100) surfaces. Higher density of Ga droplets is achieved on AlAs surface compared with GaAs surface. Similarly, the density of Al nanostructures is higher on AlAs surface than on GaAs surface, even though different morphologies are obtained on each surface. Further, while uniform Ga droplets are formed on both GaAs and AlAs surfaces, Al rings and dots, with big inhomogeneity, are observed on GaAs and AlAs surface, respectively. This investigation suggests that size and shape of nanostructures grown by the droplet epitaxy method can be designed by employing different surfaces. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Extremely Low Density InAs Quantum Dots with No Wetting Layer

Extremely low density InAs quantum dots （QDs） are grown by molecular beam droplet epitaxy. The gallium deposition amount is optimized to saturate exactly the excess arsenic atoms present on the GaAs substrate surface during growth, and low density InAs/GaAs QDs （4× 10^6 cm^-2） are formed by depositing 0.65 monolayers （MLs） of indium. This is much less than the critical deposition thickness （1.7 ML）, which is necessary to form InAs/GaAs QDs with the conventional Stranski-Krastanov growth mode. The narrow photoluminescence linewidth of about 24 meV is insensitive to cryostat temperatures from IO K to 250K. All measurements indicate that there is no wetting layer connecting the QDs.     

Al/GaAs表面量子阱界面层的原位光调制反射光谱研究

用分子束外延（MBE）方法在GaAs表面量子阱上外延生长不同厚度的Al层，以超高真空下的原位光调制光谱（PR）作为测量手段，研究Al扩散形成的表面势垒层对于GaAs表面量子阱中带间跃迁峰位和峰形的影响.根据跃迁峰的变化，采用有效质量近似理论计算出了Al和GaAs 的互扩散长度为0.5nm，这是半导体工艺中的一个重要常数. 关键词： Al GaAs 原位光调制反射光谱（PR） 分子束外延（MBE）     

Study of post-growth annealing and Ga coverage effects in low-density GaAs/AlGaAs quantum dots grown by modified droplet epitaxy

We present a detailed analysis of the Ga coverage and of the post-growth annealing effects on the optical properties of very-low-density self-assembled GaAs/AlGaAs quantum dots grown by modified droplet epitaxy. Through theoretical calculation of the QD electronic states, including thermally activated Al–Ga interdiffusion processes, we were able to relate our spectroscopic observations to QD structural properties.     

双层堆垛长波长InAs/GaAs量子点发光性质研究

研究了双层堆垛InAs/GaAs/InAs自组织量子点的生长和光致发光(PL)的物理性质。通过优化InAs淀积量、中间GaAs层厚度以及InAs量子点生长温度等生长条件,获得了室温光致发光1391～1438nm的高质量InAs量子点。研究发现对量子点GaAs间隔层实施原位退火、采用Sb辅助生长InGaAs盖层等方法可以增强高密度(2×1010 cm-2)InAs量子点的发光强度,减小光谱线宽,改善均匀性和红移发光波长。