Kinetics of indirect photoluminescence in GaAs/AlxGa1−x As double quantum wells in a random potential with a large amplitude

The kinetics of indirect photoluminescence of GaAs/Al_xGa_1−x As double quantum wells, characterized by a random potential with a large amplitude (the linewidth of the indirect photoluminescence is comparable to the binding energy of an indirect exciton) in magnetic fields B≤12 T at low temperatures T≥1.3 K is investigated. It is found that the indirect-recombination time increases with the magnetic field and decreases with increasing temperature. It is shown that the kinetics of indirect photoluminescence corresponds to single-exciton recombination in the presence of a random potential in the plane of the double quantum wells. The variation of the nonradiative recombination time is discussed in terms of the variation of the transport of indirect excitons to nonradiative recombination centers, and the variation of the radiative recombination time is discussed in terms of the variation of the population of optically active excitonic states and the localization radius of indirect excitons. The photoluminescence kinetics of indirect excitons, which is observed in the studied GaAs/Al_xGa_1−x As double quantum wells for which the random potential has a large amplitude, is qualitatively different from the photoluminescence kinetics of indirect excitons in AlAs/GaAs wells and GaAs/Al_xGa_1−x As double quantum wells with a random potential having a small amplitude. The temporal evolution of the photoluminescence spectra in the direct and indirect regimes is studied. It is shown that the evolution of the photoluminescence spectra corresponds to excitonic recombination in a random potential. Zh. éksp. Teor. Fiz. 115, 1890–1905 (May 1999)     

Measurement of the radiative and nonradiative decay rates of single CdSe nanocrystals through a controlled modification of their spontaneous emission

We present a simple method to measure the radiative and nonradiative recombination rates of individual fluorescent emitters at room temperature. By placing a single molecule successively close and far from a dielectric interface and simultaneously measuring its photoluminescence decay and its orientation, both the radiative and nonradiative recombination rates can be determined. For CdSe nanocrystals, our results demonstrate that the fluorescence quantum efficiency, determined at the single-molecule level, is 98% in average, far above the value expected from conventional ensemble experiments. The bidimensional nature of the transition dipole is also directly evidenced from a single-particle measurement.     

Phosphorus doping as an efficient way to modify the radiative interband recombination in silicon nanocrystals

Experimental results on photoluminescence spectra and kinetics obtained in the studies of SiO₂ oxide layers nanostructured by ion implantation of silicon with subsequent doping and annealing are systematized. It is shown that phosporus doping by ion implantation leads to a severalfold increase in the intensity and decay time of nonstationary photoluminescence at λ = 750 nm because of an increase in the probability of radiative recombination at Si doped quantum dots and of passivation of dangling bonds at the nanocrystal-matrix interfaces.     

Phonon-assisted radiative electron-hole recombination in silicon quantum dots

The temperature dependence of the photoluminescence (PL) spectrum of silicon quantum dots (QDs) is studied both theoretically and experimentally, and the time of the corresponding electron-hole radiative recombination is calculated. The dependence of the recombination time on the QD size is discussed. The experiment shows that the PL intensity decreases by approximately 60% as the temperature increases from 77 to 293 K. The calculated characteristic recombination time has only a weak temperature dependence; therefore, the decrease in the PL intensity is associated primarily with nonradiative transitions. It is also shown that the phonon-assisted radiation is much more efficient than the zero-phonon emission. Moreover, the zero-phonon recombination time depends on the QD radius R as R⁸, whereas the phonon-assisted recombination time depends on this radius as R³.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Model calculations of diffusion limited trapping dynamics in quantum well laser structures

We present a phenomenological theoretical model to treat the trapping of carriers into quantum wells of semiconductor laser structures. We consider explicitely the transport within the barrier layers by solving the continuity equation with the appropriate boundary conditions taking into account surface recombination, radiative and nonradiative recombination in the barrier layers and trapping of carriers into the quantum wells. The experimental findings for the trapping dynamics in GaAs/AlGaAs quantum well structures can be consistently interpreted by the model calculations.     

Effect of the postimplantation-annealing temperature on the properties of silicon light-emitting diodes fabricated through boron ion implantation into <Emphasis Type="Italic">n</Emphasis>-Si

The effect of the temperature of postimplantation annealing on the electroluminescence and the electrophysical and structural properties of light-emitting diodes fabricated by the implantation of boron ions into n-Si with a resistivity of 0.5 and 500 Ω cm is studied. All spectra contain strong electroluminescence (EL) peaks associated with band-to-band radiative transitions. An increase in the annealing temperature from 700 to 1100°C is accompanied by a monotonic increase in the quantum efficiency for the dominating EL peak and in the effective minority-carrier lifetime in the base of the light-emitting diodes and by the transformation of extended structural defects. Analysis of the experimental data shows that the extended structural defects formed are most likely to affect the EL properties via the formation or gettering of the radiative or nonradiative recombination centers rather than via preventing the removal of charge carriers to nonradiative recombination centers. The maximum internal quantum efficiency is reached after annealing at 1100°C (where extended structural defects are absent) and is estimated to be 0.4% at 300 K.     

Photoluminescence kinetics of structures with InAs quantum dots for IR photodetectors

The experimental results of a photoluminescence kinetics study of InAs/GaAs structures with quantum dots grown by metal-organic vapor-phase epitaxy are shown. The measurements have revealed the fast capture of excited carriers from the GaAs barrier to quantum dots and slow interlevel relaxation inside the quantum dots.     

硅衬底生长的InGaN/GaN多层量子阱中δ型硅掺杂n-GaN层对载流子复合过程的调节作用

为了解决在单晶硅衬底上生长的InGaN/GaN多层量子阱发光二极管器件发光效率显著降低的问题,使用周期性δ型Si掺杂的GaN取代Si均匀掺杂的GaN作为n型层释放多层界面间的张应力。采用稳态荧光谱及时间分辨荧光谱测量,提取并分析了使用该方案前后的多层量子阱中辐射/非辐射复合速率随温度（10~300 K）的变化规律。实验结果表明引入δ-Si掺杂的n-GaN层后,非辐射复合平均激活能由（18±3）meV升高到（38±10）meV,对应非辐射复合速率随温度升高而上升的趋势变缓,室温下非辐射复合速率下降,体系中与阱宽涨落有关的浅能级复合中心浓度减小,PL峰位由531 nm左右红移至579 nm左右,样品PL效率随温度的衰减受到抑制。使用周期性δ型Si掺杂的GaN取代Si均匀掺杂的GaN作为生长在Si衬底上的InGaN/GaN多层量子阱LED器件n型层,由于应力释放,降低了多层量子阱与n-GaN界面、InGaN/GaN界面的缺陷密度,使得器件性能得到了改善。     

The temperature dependence of the photoluminescence from overgrown GaAs/Al0.3Ga0.7As quantum dots and wires

Photoluminescence measurements on GaAs/Al_0.3Ga_0.7As quantum dots and wires fabricated using electron bears lithography and reactive ion etching are reported both before and after regrowth with a layer of Al_0.4Ga_0.6As. Dots exhibit little change in luminescence efficiency from the bulk with a reduction in diameter either before or after regrowth. Surface recombination therefore appears to be suppressed. In wires, however, luminescence intensity is very sensitive to wire width, decreasing rapidly with this parameter, but recovers and becomes independent of size after overgrowth. The temperature dependence of the photoluminescence from the dots and wires showed that dots and wires less than 150nm in width luminesced to higher temperatures than the larger diameter structures and dots liminesced to higher temperatures than wires of comparable width. This suggests that there is a finite coherence area effect which increases the radiative lifetimes of excitons in the quantum structures due to the geometric constraint, in the lateral direction in the wires and in all three directions in the dots. Below 20K bound exciton luminescence dominates in the dots but not in the wires. In wires it is still possible for the excitons to diffuse to nonradiative sites within the exciton lifetime. Regrowth at 750°C causes migration of aluminium into the quantum well and causes the shape of the well to become parabolic resulting shifts in the exciton emission to shorter wavelengths, making it difficult to separate the effect of processing from those due to quantum confinement.     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

量子阱中光生载流子的瞬态衰减过程与发光效率

提出了量子阱系统中包括非辐射复合效应和载流子屏蔽效应在内的光生载流子瞬态复合过程的唯象模型．结果表明，荧光衰退时间与样品质量、掺杂浓度以及激发光强度有着密切的联系 关键词：     

ZnCuInS/ZnS量子点光致发光温度依赖性研究

ZnCuInS/ZnS量子点是一种无重金属"绿色"半导体纳米材料。制备出了直径为2.9nm的ZnCuInS/ZnS核壳量子点。从ZnCuInS/ZnS量子点的吸收及光致发光光谱中可以看到,量子点的斯托克斯位移为410meV。这样大的斯托克斯位移表明,ZnCuInS/ZnS量子点的复合机制与缺陷能级有关。研究并计算了在辐射及非辐射驰豫过程的(Huang-Rhys)因子及平均声子能量。结果表明在50~373K范围内,能量带隙的变化以及光致发光光谱的增宽是分别由光从能带边缘向缺陷能级跃迁及载流子声子耦合导致的。     

CdSeS合金结构量子点的多激子俄歇复合过程

多激子效应通常是指吸收单个光子产生多个激子的过程,该效应不仅可以为研究基于量子点的太阳能电池开拓新思路,还可以为提高太阳能电池的光电转换效率提供新方法.但是,超快多激子产生和复合机制尚不明确.这里以CdSeS合金结构量子点为研究对象,研究了其多激子生成和复合动力学.稳态吸收光谱显示, 510, 468和430 nm附近的稳态吸收峰,分别对应1S_(3/2)(h)-1S(e)(或1S), 2S_(3/2)(h)-1S(e)(或2S)和1P_P(3/2)(h)-1P(e)(或1P)激子的吸收带.通过飞秒时间分辨瞬态吸收光谱和纳秒时间分辨荧光光谱两种时间分辨光谱技术对CdSeS合金结构量子点的超快动力学进行了探究,结果显示, 1S激子的双激子复合时间大概是80 ps,这一时间比传统量子点的双激子复合时间(小于50 ps)延长了近一倍,结合最近发展的超快界面电荷分离技术,在激子湮灭之前将其利用起来,这一时间的延长将有很大的应用前景;其中,在2S和1P激子中除上述双激子复合外,还存在一个通过声子耦合路径的空穴弛豫过程,时间大概是5—6 ps.最后,利用纳秒时间分辨荧光光谱得到该样品体系单激子复合的时间约为200 ns.     

Linearly polarized photoluminescence from an ensemble of wurtzite GaN/AlN quantum dots

Microphotoluminescence from GaN/AlN quantum dots grown by molecular beam epitaxy on sapphire substrates along the (0001) axis has been studied. To produce quantum dots of different average sizes and densities, the nominal amount of deposited GaN has been varied from 1 to 4 ML. The density of the quantum dots was about 10¹¹ cm⁻², which corresponded to about 10³ quantum dots excited in the experiments. The photo-luminescence from the quantum dots was linearly polarized and the maximum polarization degree (15%) has been observed for the sample with the lowest amount of deposited GaN. The photoluminescence intensity from this sample under continuous laser excitation decreased by more than two orders of magnitude for about 30 min and then stabilized. The photoluminescence intensity from other samples under continuous excitation remained constant. We suggest that a rather high polarization degree is caused by anisotropy in the strain and shape of the quantum dots formed near the dislocations, which also act as the centers of nonradiative recombination.     

Two-photon-pumped lasing from colloidal nanocrystal quantum dots

Upconverted lasing in the nonlinear, two-photon absorption regime has been demonstrated, for the first time to the best of our knowledge in colloidal nanocrystal quantum dots (NQDs). Upon pulse excitation at sub-bandgap photon energies the radiative recombination of excitons in close-packed CdSe/CdS/ZnS core-shell NQDs was found to be sufficiently fast to compete with the intrinsic nonradiative Auger recombination, as confirmed by the presence of a fast decay (approximately 7 ps) in the time-resolved photoluminescence.     

无序GaInP光致发光谱的温度依赖关系

研究了无序ＧａＩｎＰ样品的温度依赖关系，大低温ＰＬ谱中，谱线呈单峰结构。随着温度从１５Ｋ升高到２５０Ｋ，说地宽从１６ｍｅＶ增大到３１ｍｅＶ，并且发生红移，同时强度减小两个数数量级。     

Dynamics of single InGaN quantum dots

Decay dynamics for single InGaN quantum dots are presented using time-resolved photoluminescence. The recombination is shown to be characterized by a single exponential decay, in contrast to the non-exponential recombination dynamics seen in the 2D wetting layer. The lifetimes of single dots in the temperature range 4–60 K decrease with increasing temperature. Different dots show similar lifetimes of 2 ns.     

Direct synthesis of luminescent SiC quantum dots in water by laser ablation

Direct synthesis of luminescent SiC quantum dots in pure deionized water by laser ablation is reported. A zeta‐potential of –49 mV, higher than for nanoparticles produced by electrochemical etching, suggests a higher colloidal stability of the laser‐produced nanoparticles. IR spectroscopy shows that surface charges related to carboxylate anions ensure long‐term stability of the colloidal solutions based on the as‐prepared SiC quantum dots. Main radiative channel at room temperature corresponds to the recombination of quantumly confined photogenerated charge carriers. Emission from the nanoparticles is mainly centred at 2.75 eV regardless of laser power used for ablation. According to photoluminescence spectra, a partial transformation of the SiC quantum dots from cubic to hexagonal crystalline arrangement is supposed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

光致发光谱研究自组织InAs双模量子点态填充

采用固态源分子束外延技术在GaAs(100)衬底上,制备了InAs量子点,对样品进行原子力显微镜测试,统计结果表明量子点尺寸呈双模分布。光致发光谱研究表明,在室温和77 K下,小量子点的发光峰均占主导地位,原因可能是：(1)大量子点的态密度小于小量子点;(2)捕获载流子速率,大量子点小于小量子点;(3)大量子点与盖层存在较大的应变势垒和可能出现的位错和缺陷,导致温度变化引起载流子从小尺寸量子点转移到大尺寸的量子点中概率很小。