Single-photon emission from single InGaAs/GaAs quantum dots grown by droplet epitaxy at high substrate temperature

ABSTRACT: The authors report single-photon emission from InGaAs quantum dots grown by droplet epitaxy on (100) GaAs substrates using a solid-source molecular beam epitaxy system at elevated substrate temperatures above 400 [DEGREE SIGN]C without post-growth annealing. High-resolution micro-photoluminescence spectroscopy exhibits sharp excitonic emissions with lifetimes ranging from 0.7 to 1.1 ns. The coherence properties of the emitted photons are investigated by measuring the first-order field correlation function.     

Energy state of InGaAs quantum dots on SiO2-patterned vicinal substrate

The optical properties of In_0.8Ga_0.2As self-assembled quantum dots (SAQDs) grown on GaAs wire structures formed by utilizing SiO₂-patterned exact and 5°-off (001) GaAs substrates have been studied with micro-photoluminescence (μ-PL). Single PL peak was occurred for In_0.8Ga_0.2As SAQDs grown on SiO₂-patterned exact (001) GaAs, whereas double PL peaks were showed for SAQDs grown on 5°-off (001) GaAs substrates as the width of the opening windows increased. The power-dependent μ-PL spectra show that the first and second peaks of these double peaks were originated from the well-defined ground and excited state, respectively. These results demonstrated that In_0.8Ga_0.2As SAQDs selectively grown by utilizing SiO₂-patterned 5°-off (001) GaAs substrates have well-defined zero-dimensional quantum states.     

Effect of In/Al ratios on structural and optical properties of InAlN films grown on Si(100) by RF-MOMBE

In _x Al_1-x N films were deposited on Si(100) substrate using metal-organic molecular beam epitaxy. We investigated the effect of the trimethylindium/trimethylaluminum (TMIn/TMAl) flow ratios on the structural, morphological, and optical properties of In _x Al_1-x N films. Surface morphologies and microstructure of the In _x Al_1-x N films were measured by atomic force microscopy, scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), respectively. Optical properties of all films were evaluated using an ultraviolet/visible/infrared (UV/Vis/IR) reflection spectrophotometer. XRD and TEM results indicated that In _x Al_1-x N films were preferentially oriented in the c-axis direction. Besides, the growth rates of In _x Al_1-x N films were measured at around 0.6 μm/h in average. Reflection spectrum shows that the optical absorption of the In _x Al_1-x N films redshifts with an increase in the In composition.     

Influence of substrate orientation on exciton fine structure splitting of InAs/InP nanowire quantum dots

ABSTRACT: : In this paper, we use an atomistic approach to investigate strain distributions, single particle and many body electronic properties of InAs/InP nanowire quantum dots with substrate orientation varying from [111] to high-index [119], and compared with [001] case. We show that single particle gap for high-index [11k] substrates is increased with respect to [111] and [001] cases, and oscillates with the substrate index due to faceting effects. Surprisingly, the overall shell-like structure of single particle states is preserved even for highly facetted, high-index substrates. On the contrary, we demonstrate that besides two limiting high-symmetry cases, [001] and [111], the bright exciton splitting varies strongly with substrate orientation. For [112]-oriented substrate, the fine structure splitting reaches maximum due to crystal lattice anisotropy despite fully cylindrical isotropic shape of nanowire quantum dot.     

Optical properties of as-grown and annealed InAs quantum dots on InGaAs cross-hatch patterns

InAs quantum dots (QDs) grown on InGaAs cross-hatch pattern (CHP) by molecular beam epitaxy are characterized by photoluminescence (PL) at 20 K. In contrast to QDs grown on flat GaAs substrates, those grown on CHPs exhibit rich optical features which comprise as many as five ground-state emissions from [1-10]- and [110]-aligned QDs, two wetting layers (WLs), and the CHP. When subject to in situ annealing at 700°C, the PL signals rapidly degrades due to the deterioration of the CHP which sets the upper limit of overgrowth temperature. Ex situ hydrogen annealing at a much lower temperature of 350°C, however, results in an overall PL intensity increase with a significant narrowing and a small blueshift of the high-energy WL emission due to hydrogen bonding which neutralizes defects and relieves associated strains.     

Temperature dependent synthesis and optical properties of CdSe quantum dots

The CdSe quantum dots (QDs) were synthesized at various temperatures in aqueous solution. The as-synthesized QDs were characterized by X-ray diffraction (XRD) method and transmission electron microscopy (TEM), and their optical properties were assessed via ultraviolet–visible (UV–vis) absorption spectrum and fluorescence spectrum. The results showed that the reaction temperature could significantly influence the band gap, particle size, and spectral behaviors of the QDs. With the increase of the temperature from room temperature to 90 °C, the band gap of the QDs linearly decreased, corresponding to an essential increase of particle size as well as variable spectral behaviors. In particular, the starting temperature had an important effect on the QDs synthesis and their optical properties. From a viewpoint of wide controls of fluorescence color and intensity, a reaction temperature range of 17–90 °C was appropriate for the synthesis of the CdSe QDs.     

Morphology control and optical properties of SiGe nanostructures grown on glass substrate

With the rapid progress of nanotechnology, nanostructures with different morphologies have been realized, which may be very promising to enhance the performance of semiconductor devices. In this study, SiGe nanostructures with several kinds of configurations have been synthesized through a chemical vapor deposition process. By controlling growth conditions, different SiGe nanostructures can be easily tuned. Structures and compositions of the nanostructures were determined by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The optical properties of various SiGe nanostructures revealed some dependence with their morphologies, which may be suitable for solar cell applications. The control of the SiGe morphology on nanoscale provides a convenient route to produce diverse SiGe nanostructures and creates new opportunities to realize the integration of future devices.     

谷氨酰胺为稳定剂制备硒化镉纳米晶及光谱性质研究

以谷氨酰胺(Gln)为稳定剂合成了硒化镉纳米晶,利用X-射线粉末衍射(XRD)和透射电镜(TEM)对纳米晶结构进行了表征,粒径约为20 nm。通过紫外-可见吸收光谱、激发光谱与发射光谱研究了纳米晶光谱特性。实验结果表明,反应温度过高、反应时间过长都会破坏谷氨酰胺(Gln)的稳定作用,使CdSe聚集,影响其荧光性质。而聚乙二醇(PEG)的加入会使纳米晶的荧光发射明显加强,而且发射峰峰形尖锐。     

The influence of temperature on the photoluminescence properties of single InAs quantum dots grown on patterned GaAs

ABSTRACT: We report the temperature-dependent photoluminescence of single site-controlled and self-assembled InAs quantum dots. We have used nanoimprint lithography for patterning GaAs(100) templates and molecular beam epitaxy for quantum dot deposition. We show that the influence of the temperature on the photoluminescence properties is similar for quantum dots on etched nanopatterns and randomly positioned quantum dots on planar surfaces. The photoluminescence properties indicate that the prepatterning does not degrade the radiative recombination rate for the site-controlled quantum dots.     

Excitonic effects on the second-order nonlinear optical properties of semi-spherical quantum dots

We study the excitonic effects on the second-order nonlinear optical properties of semi-spherical quantum dots considering, on the same footing, the confinement potential of the electron-hole pair and the Coulomb interaction between them. The exciton is confined in a semi-spherical geometry by means of a three-dimensional semi-parabolic potential. We calculate the optical rectification and second harmonic generation coefficients for two different values of the confinement frequency based on the numerically computed energies and wavefunctions of the exciton. We present the results as a function of the incident photon energy for GaAs/AlGaAs quantum dots ranging from few nanometers to tens of nanometers. We find that the second-order nonlinear coefficients exhibit not only a blue-shift of the order of meV but also a change of intensity compared with the results obtained ignoring the Coulomb interaction in the so-called strong-confinement limit.     

Temperature dependent optical properties of single, hierarchically self-assembled GaAs/AlGaAs quantum dots

We report on the experimental observation of bright photoluminescence emission at room temperature from single unstrained GaAs quantum dots (QDs). The linewidth of a single-QD ground-state emission (≈ 8.5 meV) is comparable to the ensemble inhomogeneous broadening (≈ 12.4 meV). At low temperature (T  ≤  40 K) photon correlation measurements under continuous wave excitation show nearly perfect single-photon emission from a single GaAs QD and reveal the single photon nature of the emitted light up to 77 K. The QD emission energies, homogeneous linewidths and the thermally activated behavior as a function of temperature are discussed.     

Cytotoxicity of InP/ZnS quantum dots related to reactive oxygen species generation

Indium phosphide (InP) quantum dots (QDs) have emerged as a presumably less hazardous alternative to cadmium-based particles, but their cytotoxicity has not been well examined. Although their constituent elements are of very low toxicity to cells in culture, they nonetheless exhibit phototoxicity related to generation of reactive oxygen species by excited electrons and/or holes interacting with water and molecular oxygen. Using spin-trap electron paramagnetic resonance (EPR) spectroscopy and reporter assays, we find a considerable amount of superoxide and a small amount of hydroxyl radical formed under visible illumination of biocompatible InP QDs with a single ZnS shell, comparable to what is seen with CdTe. A double thickness shell reduces the reactive oxygen species concentration approximately two-fold. Survival assays in five cell lines correspondingly indicate a distinct reduction in toxicity with the double-shell InP QDs. Toxicity varies significantly across cell lines according to the efficiency of uptake, being overall significantly less than what is seen with CdTe or CdSe/ZnS. This indicates that InP QDs are a useful alternative to cadmium-containing QDs, while remaining capable of electron-transfer processes that may be undesirable or which may be exploited for photosensitization applications.     

TiO2/CdS/CdSe quantum dots co-sensitized solar cell with the staggered-gap (type-II) heterojunctions for the enhanced photovoltaic performance

The effect of co-sensitization and ZnS passivation on the photovoltaic performance of CdS quantum dot sensitized solar cells (QDSSCs) were investigated. The deposition of CdS, CdSe quantum dots (QD) and ZnS passivation on TiO₂ photoanode was carried out by successive ionic layer adsorption and reaction (SILAR) method. CdS/CdSe co-sensitization developed two staggered type-II heterojunctions at TiO₂/CdS and CdS/CdSe interfaces and resulted a cascade energy band structure. This suitable band alignment facilitated the double charge transfer mechanism at each heterojunction and transported the electrons easily into the photoanode. The narrow bandgap sensitizers CdS and CdSe significantly improved the potential utilization of solar spectrum with more charge carrier generation. ZnS passivation on QD surface suppressed electrode/electrolyte interfacial charge recombination and facilitated more electron injection from QDs into TiO₂ photoanode. The EDAX elemental mapping results inferred that CdS, CdSe and ZnS have efficiently covered the TiO₂ surface. TiO₂/CdS and CdS/CdSe interfaces and the amorphous nature of ZnS could be verified with HRTEM images. Hence, the co-sensitization and surface passivation played a significant role to enhance the PCE of CdS QDSSC from 1.9% to 4.05%.     

Effects of growth variables on structural and optical properties of InGaN/GaN triangular-shaped quantum wells

Structural and optical properties of InGaN/GaN triangular-shaped multiple quantum well (QW) structures were investigated under various conditions of growth parameters such as growth temperature, flow rate of Ga and/ or In composition, and well and barrier widths. The optical properties affected by the growth parameters were well correlated with an In band gap, which is determined by the potential depth and the In composition in the well region. The emission peak energy was almost independent of the barrier width due to the relaxation of the piezoelectric fields in the triangular-shaped QWs. Photoluminescence spectra of the InGaN/GaN multiple QW structures showed a parabolic curve centered at 2.66 eV. The optical property of the triangular-shaped multiple QWs was substantially improved due to formation of quantum dot-like In composition fluctuations.     

铜离子掺杂CsPbCl3量子点光学性能及稳定性探究

近年来,钙钛矿因其特殊的结构受到广泛的关注。其中,全无机钙钛矿量子点作为下一代发光材料更因其优异的发光性能得到了广泛的研究和关注。但是因为其本身的铅（Pb）元素带来的毒性和较差的稳定性,钙钛矿量子点在生产和应用方面依然面临着诸多阻碍。为了解决这些难题,介绍了一种铜离子（Cu²⁺）B位掺杂CsPbCl₃钙钛矿量子点。采用了热注射的方法成功地将Cu²⁺引入CsPbCl₃钙钛矿量子点中。研究发现,Cu²⁺掺杂CsPbCl₃量子点能够保持初始四方晶体结构。由于Cu²⁺的掺杂,有效地消除了CsPbCl₃量子点的表面缺陷,从而通过辐射途径促进了激子复合,提高了CsPbCl₃量子点的发光性质。通过稳定性对比测试发现,一段时间内,Cu²⁺掺杂CsPbCl₃量子点在水中的发光强度明显高于CsPbCl₃量子点。     

Effect of surface ligands on the optical properties of aqueous soluble CdTe quantum dots

We investigate systematically the influence of the nature of thiol-type capping ligands on the optical and structural properties of highly luminescent CdTe quantum dots synthesized in aqueous media, comparing mercaptopropionic acid (MPA), thioglycolic acid (TGA), 1-thioglycerol (TGH), and glutathione (GSH). The growth rate, size distribution, and quantum yield strongly depend on the type of surface ligand used. While TGH binds too strongly to the nanocrystal surface inhibiting growth, the use of GSH results in the fastest growth kinetics. TGA and MPA show intermediate growth kinetics, but MPA yields a much lower initial size distribution than TGA. The obtained fluorescence quantum yields range from 38% to 73%. XPS studies unambiguously put into evidence the formation of a CdS shell on the CdTe core due to the thermal decomposition of the capping ligands. This shell is thicker when GSH is used as ligand, as compared with TGA ligands.     

Growth kinetics and optical properties of PbSe quantum dots in dual-phase lithium-aluminum-silicate glass ceramic

Lead selenide quantum dots (QDs) were precipitated in lithium-aluminum-silicate glass-ceramics. Their optical properties and the crystallization behavior of both glass host and PbSe QDs were investigated. When the glasses heat-treated at 480–520 °C for 10 h, the absorption bands from PbSe QDs were tuned from 1051 to 2016 nm, and the PL bands from 1165 to 2095 nm. Upon the dwelling time 10 h, the crystallization of glass host required relatively higher heating temperature (∼490 °C) than the formation of PbSe QDs (∼480 °C). With the heat treatment at 490 °C for10 h, the PLQY from PbSe QDs in glass-ceramics reached ∼38.7 %. Phase separation induced the glass crystallization and had a significant effect on the distribution of PbSe QDs. To study the growth kinetics of PbSe QDs, the glasses were heat treated at 480 °C and 500 °C for various durations. The average diameters of PbSe QDs (D) showed exponential relationship to the growth time (t), described by the equation D(t)∼ t ^x. In rapid growth stage, the growth exponents x is ∼0.56 at 480 °C and ∼0.47 at 500 °C, and then decrease to ∼0.13 and ∼0.10, respectively. The formation of a large number of LiAlO₂ crystals in glass hindered the diffusion of semiconductor ions, which is the main reason for the decreasing in the growth rate of PbSe QDs.     

Multi-scale ordering of self-assembled InAs/GaAs(001) quantum dots

Ordering phenomena related to the self-assembly of InAs quantum dots (QD) grown on GaAs(001) substrates are experimentally investigated on different length scales. On the shortest length-scale studied here, we examine the QD morphology and observe two types of QD shapes, i.e., pyramids and domes. Pyramids are elongated along the [1–10] directions and are bounded by {137} facets, while domes have a multi-facetted shape. By changing the growth rates, we are able to control the size and size homogeneity of freestanding QDs. QDs grown by using low growth rate are characterized by larger sizes and a narrower size distribution. The homogeneity of buried QDs is measured by photoluminescence spectroscopy and can be improved by low temperature overgrowth. The overgrowth induces the formation of nanostructures on the surface. The fabrication of self-assembled nanoholes, which are used as a template to induce short-range positioning of QDs, is also investigated. The growth of closely spaced QDs (QD molecules) containing 2–6 QDs per QD molecule is discussed. Finally, the long-range positioning of self-assembled QDs, which can be achieved by the growth on patterned substrates, is demonstrated. Lateral QD replication observed during growth of three-dimensional QD crystals is reported.     

Composition and growth of thin anodic oxides formed on InP (100)

Thin anodic oxides (<100 Å) were formed on p-InP (100) in phosphate solution (0.3 M NH₄H₂PO₄) and in sodium tungstate solution (0.1 M Na₂WO₄·2H₂O) at different temperatures (25 and 80 °C) and potentials (1-8 V). Thickness and composition were determined by different surface-analytical techniques including Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. In general, it has been observed that double-layered films are obtained with an outer In-rich layer. The thickness of the outer layer, oxide morphology and roughness as well as the composition of the duplex structure are strongly dependent on the temperature and the composition of the electrolyte. It has been found that oxides formed in phosphate exhibit a higher stability against dissolution compared with oxides formed in tungstate. The latter contain a large amount of In₂O₃, which leads to poor electrical properties.