Comparative study on InAs/InGaAs dots-in-a-well structure grown on GaAs(311) B and (100) substrates

The InAs/InGaAs dots-in-a-well (DWELL) structures were grown on both GaAs(311) B and (100) substrates by molecular beam epitaxy. Quantum dots (QDs) grown on GaAs(311) B substrate are of higher density and more uniform size distribution, yet QDs grown on GaAs(100) substrate demonstrate a bimodal size distribution. The growth mechanism of these surface morphologies was briefly discussed. We found that the photoluminescence (PL) linewidth of DWELL grown on GaAs(311) B substrate was much narrower than the linewidth of DWELL grown on GaAs(100), which suggests a promising advantage in many QD based devices. We also found the temperature had a stronger impact on the PL intensity of DWELL grown on GaAs(311) B, which was explained by the lower thermal active energy and higher density of interfacial point defects of DWELL grown on GaAs(311) B. These results provide people a rather comprehensive insight into the advantages and disadvantages of DWELL grown on GaAs(311) B substrates.     

Growth of Ga x In1 − x As y P1 − y /GaAs quantum dot arrays by ion beam deposition

Experimental data are presented that demonstrate the possibility of producing GaInAsP quantum dots on GaAs by ion beam deposition. The morphology of the quantum dots and the effect of GaAs substrate temperature on parameters of GaInAsP quantum dot arrays have been studied by atomic force microscopy and scanning electron microscopy. We have determined the elemental composition of the quantum dots and obtained photoluminescence spectra of the GaInAsP/GaAs heterostructures.     

The influence of post-growth annealing on the optical properties of InAs quantum dot chains grown on pre-patterned GaAs(100)

We report on the effect of post-growth thermal annealing of [011]- ,[011(-)]-, and [010]-oriented quantum dot chains grown by molecular beam epitaxy on GaAs(100) substrates patterned by UV-nanoimprint lithography. We show that the quantum dot chains experience a blueshift of the photoluminescence energy, spectral narrowing, and a reduction of the intersubband energy separation during annealing. The photoluminescence blueshift is more rapid for the quantum dot chains than for self-assembled quantum dots that were used as a reference. Furthermore, we studied polarization resolved photoluminescence and observed that annealing reduces the intrinsic optical anisotropy of the quantum dot chains and the self-assembled quantum dots.     

Piezoelectric effects in InAs/GaAs(N11) self-assembled quantum dots

The effects of a piezoelectric field on the spectroscopic properties of strained InAs/GaAs self-assembled quantum dot (QD) heterostructures grown on (N11) substrates with A or B termination are presented. An increasing blue shift of photoluminescence (PL) band was observed with increasing excitation density. The PL blue shift of (N11) quantum dots measured at the highest excitation grows with 1/N and shows an asymmetric dependence on whether the substrate has A or B termination. We attributed the blue shift of the photoluminescence band to the screening of the piezoelectric field by the photo-generated carriers, leading to a reduction of the piezoelectric induced quantum confined Stark effect.     

Photoluminescence and magnetophotoluminescence of circular and elliptical InAs/GaAs quantum dots

Photoluminescence, magnetophotoluminescence, and atomic force microscopy were used for the characterization of MOVPE prepared InAs/GaAs quantum dots. Significant differences in the behaviour of the first excited photoluminescence transition in magnetic field are explained by the different lateral shape of quantum dots. While the first excited luminescence peak of circular quantum dots splits with increasing magnetic field into two peaks, no splitting occurs for quantum dots with elliptic shape, only small red shift is observed. Theoretical calculations of energy levels in InAs/GaAs quantum dots with circular and elliptical shape with different elongations are presented and compared with experimental results.     

Photoluminescence study of strain-induced GaInNAs/GaAs quantum dots

Strain-induced Ga_0.8In_0.2N _y As_1–y quantum dots on GaAs were fabricated by metal organic vapor phase epitaxy. Nitrogen concentration y was varied between 0% and 1.3%. The effect of nitrogen concentration on the optical properties of the quantum dots was investigated by continuous-wave and time-resolved photoluminescence measurements. Carrier localization in the states below the band edge of the nitrogen-containing quantum well has been observed. These states are thought to originate from the variation of the quantum well width or from the fluctuation of the composition. Such variations have been identified in GaInNAs quantum wells on GaAs without stressor islands. The measured energy difference of the quantum well and quantum dot ground-state peak energies increase with increasing temperature.     

Effect of high energy proton irradiation on InAs/GaAs quantum dots: Enhancement of photoluminescence efficiency (up to ∼7 times) with minimum spectral signature shift

We demonstrate 7-fold increase of photoluminescence efficiency in GaAs/(InAs/GaAs) quantum dot hetero-structure, employing high energy proton irradiation, without any post-annealing treatment. Protons of energy 3–5 MeV with fluence in the range (1.2–7.04) × 10¹² ions/cm² were used for irradiation. X-ray diffraction analysis revealed crystalline quality of the GaAs cap layer improves on proton irradiation. Photoluminescence study conducted at low temperature and low laser excitation density proved the presence of non-radiative recombination centers in the system which gets eliminated on proton irradiation. Shift in photoluminescence emission towards higher wavelength upon irradiation substantiated the reduction in strain field existed between GaAs cap layer and InAs/GaAs quantum dots. The enhancement in PL efficiency is thus attributed to the annihilation of defects/non-radiative recombination centers present in GaAs cap layer as well as in InAs/GaAs quantum dots induced by proton irradiation.     

Improved photoluminescence efficiency of patterned quantum dots incorporating a dots-in-the-well structure

InAs quantum dots embedded in InGaAs quantum well (DWELL: dots-in-the-well) structures grown on nanopatterned GaAs pyramids and planar GaAs(001) surface are comparatively investigated. Photoluminescence (PL) measurements demonstrate that the DWELL structure grown on the GaAs pyramids exhibits a broad quantum well PL band (full width at half-maximum ～ 90?meV) and a higher quantum dot emission efficiency than the DWELL structure grown on the planar GaAs(001) substrate. These properties are attributed to the InGaAs quantum well with distributed thickness profile on the faceted GaAs pyramids, which introduces a tapered energy band structure and enhances carrier capture into the quantum dots.     

Synthesis and photoluminescence of ZnS quantum dots

Single-phase zinc sulphide (ZnS) quantum dots were synthesized by a chemical method. The influence of the pH value of the Zn(CH3COO)2 solution on the size and photoluminescence properties of the ZnS quantum dots was evaluated. X-ray power diffraction, transmission electron microscopy, and ultraviolet-visible spectroscopy were used to characterize the structure, size, surface states, and photoluminescence properties of ZnS quantum dots. The results showed that the crystal structure of ZnS quantum dots was a cubic zinc blende structure, and their average diameter was about 3.0 nm. ZnS quantum dots with good distribution and high purity were obtained. A strong broad band centered at about 320 nm was observed in the excitation spectrum of ZnS quantum dots. Their emission spectrum peaking at about 408 nm, was due mostly to the trap-state emission. The relative integrated emission intensity of ZnS quantum dots decreased as the pH value of the Zn(CH3COO)2 solution increased, which could be ascribed to the increase in average diameter of the ZnS quantum dots as the pH value of Zn(CH3COO)2 solution increased.     

Growth of InAs quantum dots on focussed ion beam implanted GaAs(100)

The growth of self-assembled InAs quantum dots on implantation doped GaAs was studied. Be and Si ions were implanted in a combined ion implantation/molecular beam epitaxy process to generate p- and n-type GaAs, respectively. The quality of the InAs quantum dots was investigated by photoluminescence spectroscopy and scanning electron microscopy. By employing an in situ annealing step before re-growth it was possible to fabricate high quality InAs quantum dots on ion doped GaAs for Be doses up to 1.4×10¹⁴ cm⁻². The sheet resistance of the Be doped GaAs was as low as 1 kΩ at 300 K and 0.6 kΩ at 4.2 K, respectively. Only for rather low Si doses up to 5×10¹³ cm⁻² acceptable photoluminescence could be detected. The sheet resistance for these doses was 1 kΩ at 300 K and 1.7 kΩ at 4.2K.     

Experiments and modeling of alloying in self-assembled quantum dots

We review the recent advances in the experimental and theoretical investigation of alloy distribution in semiconductor quantum dots (QDs). X-ray diffraction analysis, as well as wet chemical etching, represent two powerful techniques that are able to measure the alloy distribution inside the dots. From a theoretical point of view, determination of the alloy distribution follows from consideration of the thermodynamic quantities involved in the formation and stability of the QD: strain energy, surface energy, internal energy and entropy. Starting from the alloy distribution, the investigation of its role in influencing the electronic and optical properties of QDs is possible. Tight binding and ab initio calculation show the band structure of non-uniform alloyed Ge/Si and InAs/GaAs quantum dots. While for Ge/Si the indirect bandgap does not offer a strong photoluminescence spectra, direct-bandgap materials offer intense light emission, including the range for telecom applications (1.77–1.37 μm). Control of alloying inside the QDs allows for the tailoring of their band structure and photoluminescence spectra, where high alloy gradients induce a blue-shift of the spectra, compared to a more uniform composition.     

Molecular beam epitaxy growth methods of wavelength control for InAs/(In)GaAsN/GaAs heterostructures

We discuss the molecular beam epitaxy (MBE) growth methods of emission wavelength control and property investigations for different types of InAs/(In)GaAsN/GaAs heterostructures containing InGaAsN quantum-size layers: (1) InGaAsN quantum wells deposited by the conventional mode in a GaAs matrix, (2) InAs quantum dots deposited in a GaAsN matrix or covered by an InGaAs(N) layer, and (3) InAs/InGaAsN/GaAsN strain-compensated superlattices with quantum wells and quantum dots. The structures under investigation have demonstrated photoluminescence emission in a wavelength range of ～1.3-1.8?μm at room temperature without essential deterioration of the radiative properties.     

Alignment and optical polarization of InGaAs quantum wires on GaAs high index surfaces

Highly ordered three-dimensional periodic arrays of In_0.40Ga_0.60As quantum wires (QWRs) on GaAs (311)A and GaAs (331)A have been achieved by molecular beam epitaxy and revealed by high-resolution X-ray diffraction. Polarization dependent photoluminescence measurements demonstrated high optical anisotropy of 40% in (331)A QWRs and 16% in (311)A QWRs. Such a difference in polarization value could be caused by the differences in geometry, ordering, and high piezoelectric field between (331)A and (311)A samples.     

圆偏振光诱导CdTe量子点再生长及其对光致发光的影响

实验研究了以3-巯基丙酸为配体合成的水溶性CdTe量子点经过非偏振光与圆偏振光照射处理后, 量子点的再生长变化规律。采用光致发光谱、紫外-可见吸收光谱、透射电子显微镜与X射线衍射等表征手段分析表明: 非偏振光会促进CdTe量子点的光氧化, 导致量子点尺寸缩小, 荧光发光峰位蓝移, 且发光效率降低; 而圆偏振光增强了配体的光氧化, 在量子点表面形成CdS层, 导致量子点尺寸进一步增大, 荧光发光峰红移, 且发光效率提升。进一步讨论了CdTe量子点与配体之间的键合作用, 相关光化学反应机制及其对量子点光致发光性质的影响。     

Quantum dots in InAs layers of subcritical thickness on GaAs(100)

Ensembles of InAs quantum dots formed at the GaAs(100) surface have been studied by the methods of reflection high-energy electron diffraction and photoluminescence. The amount of deposited InAs corresponds to a wetting layer thickness smaller than the critical value necessary for the transition from two-to three-dimensional growth. It is experimentally shown that, at a deposited film thickness of 1.5 and 1.6 monolayers, islands are formed after keeping the sample in a flow of As₄. The influence of the substrate temperature on the kinetic characteristics of the formation of InAs/GaAs islands has been studied.     

Self-assembled InAs/GaAs quantum dots covered by different strain reducing layers exhibiting strong photo- and electroluminescence in 1.3 and 1.55 microm bands

The effect of different InGaAs and GaAsSb strain reducing layers on photoluminescence and electroluminescence from self-assembled InAs/GaAs quantum dots grown by metal-organic vapour phase epitaxy was investigated. The aim was to shift their luminescence maximum towards optical communication wavelengths at 1.3 or 1.55 microm. Results show that covering by InGaAs strain reducing layer provides stronger shift of photoluminescence maximum (up to 1.55 microm) as compared to GaAsSb one with similar strain in the structure. This is caused by the increase of quantum dot size during InGaAs capping and reduction of quantum confinement of the electron wave function which spreads into the cap. Unfortunately, the weaker electron confinement in quantum dots is a reason of a considerable blue shift of electroluminescence from these InGaAs structures since optical transitions move to InGaAs quantum well. Although strong electroluminescence at 1300 nm was achieved from quantum dots covered by both types of strain reducing layers, the GaAsSb strain reducing layer is more suitable for long wavelength electroluminescence due to higher electron confinement potential allowing suppression of thermal carrier escape from quantum dots.     

Site‐Controlled InGaAs Quantum Dots with Tunable Emission Energy

Semiconductor quantum‐dot (QD) systems offering perfect site control and tunable emission energy are essential for numerous nanophotonic device applications involving spatial and spectral matching of dots with optical cavities. Herein, the properties of ordered InGaAs/GaAs QDs grown by organometallic chemical vapor deposition on substrates patterned with pyramidal recesses are reported. The seeded growth of a single QD inside each pyramid results in near‐perfect (<10 nm) control of the QD position. Moreover, efficient and uniform photoluminescence (inhomogeneous broadening <10 meV) is observed from ordered arrays of such dots. The QD emission energy can be finely tuned by varying 1) the pyramid size and 2) its position within specific patterns. This tunability is brought about by the patterning of both the chemical properties and the surface curvature features of the substrate, which allows local control of the adatom fluxes that determine the QD thickness and composition.     

Shape and size control of InAs/InP (113)B quantum dots by Sb deposition during the capping procedure

The role of Sb atoms present on the growth front during capping of InAs/InP (113)B quantum dots (QDs) is investigated by cross-sectional scanning tunnelling microscopy, atomic force microscopy, and photoluminescence spectroscopy. Direct capping of InAs QDs by InP results in partial disassembly of InAs QDs due to the As/P exchange occurring at the surface. However, when Sb atoms are supplied to the growth surface before InP capping layer overgrowth, the QDs preserve their uncapped shape, indicating that QD decomposition is suppressed. When GaAs(0.51)Sb(0.49) layers are deposited on the QDs, conformal growth is observed, despite the strain inhomogeneity existing at the growth front. This indicates that kinetics rather than the strain plays the major role during QD capping with Sb compounds. Thus Sb opens up a new way to control the shape of InAs QDs.     

InAs quantum dots capped by GaAs,In0.4Ga0.6As dots,and In0.2Ga0.8As well

We have fabricated and characterized three types of InAs quantum dots (QDs) with different InxGa1-xAs capping layers. Post-growth atomic force microscopy measurements show that the In0.2Ga0.8As/InAs structure has a smooth surface (dot-in-well structure), whereas the In0.4Ga0.6As/InAs structure revealed large QDs with a density similar to that underneath InAs QDs on GaAs (dot-in-dot). With increasing In mole fraction of the capping layer and increasing In0.4Ga0.6As thickness, the energy position of the room-temperature photoluminescence (PL) peak is red-shifted. The quantum dot-in-dot structure emits stronger room-temperature PL than does the quantum dot-in-well structure. With a spatially distributed strain in the InAs quantum dot, we have solved the three-dimensional Schr?dinger equation by the Green's function theory for the eigenvalues and eigen wave functions. It is concluded that the ground state increases its wave function penetration into the low-barrier InxGa1-xAs capping layer so that its energy position is red-shifted. The reduced PL peak intensity of the dot-in-well (compared with GaAs covered dots) is due to the reduced overlapping between the ground state and the extended states above the GaAs barrier. The overlapping reduction in the dot-in-dot is over compensated for by the reduced relaxation energy (full width at half-maximum), indicating the importance of the sample quality in determining the PL intensity.     

Stacked GaAs quantum dots fabricated by refilling of self-organized nanoholes: optical properties and post-growth annealing

We study the photoluminescence and impact of post-growth annealing of stacked, strain-free GaAs quantum dots fabricated by refilling of self-organized nanoholes using molecular beam epitaxy. Temperature- and power-dependent photoluminescence studies reveal an excellent optical quality of the quantum-dot stack. After high-temperature post-growth annealing only slight blueshifts and an increase in full width at half-maximum of the photoluminescence peak are observed, indicating very high-temperature stability and crystalline quality of the stacked GaAs quantum-dot structure.