Growth and photoluminescence study of ZnSe quantum dots

We report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.     

Exciton characteristics and exciton luminescence of silicon quantum dot structures

The exciton binding energy, the energies of the basic radiative exciton transition, and the zerophonon radiative lifetime of excitons in silicon quantum dots embedded in the SiO_x matrix are calculated in effective mass approximation with quadratic dispersion relation. In addition, the spectra of steady-state photoluminescence and of time-resolved photoluminescence of excitons in the silicon quantum dots are calculated, and the kinetics of the photoluminescence relaxation is considered. The theory is compared with the experiment. It is shown that, for nanostructures involving silicon quantum dots with diameters smaller than 4 nm, the governing factor in the broadening of the spectral photoluminescence bands is the effect of mesoscopic quantum fluctuations. In this case, either an even one dangling bond at the interface, or one intrinsic point defect, or one foreign atom located inside the small-sized nanocrystallite or in its close surroundings produces a pronounced effect on the energy of the exciton transition.     

Miniband properties of superlattice quantum dot arrays fabricated by the edge-defined nanowires

A new scheme to fabricate quantum dot arrays using the edge-defined nanowires is proposed. The three-dimensional quantum size effect due to the nanowire crossing was analyzed by using the finite-difference method taking into account the periodic nature of the superlattices. The quantum dot modes were graphically verified, and the miniband gap between quantum dot and quantum wire modes was quantitatively analyzed.     

Bright CdSe quantum dot inserted in single ZnSe nanowires

We report the evidence of CdSe quantum dot (QD) insertion in single defect-free ZnSe nanowire. These nanowires have been grown by molecular beam epitaxy in vapour-liquid-solid growth mode catalysed with gold particles. We developed a two-step process allowing us to grow very thin (from 15 to 5 nm) defect-free ZnSe nanowire on top of a nanoneedle, where all defects are localised. The CdSe QDs are incorporated to the defect-free nanowires part. Owing to the extraction efficiency of the nanowires and the reduced number of stacking fault defects in the two-step-process nanowires, a very efficient photoluminescence is observed even on isolated single nanowire. Time-resolved photoluminescence and correlation photon give evidences that the bright photon emission is related to the CdSe QD.     

Exciton binding energy in semiconductor quantum dots

In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass μ = μ(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy E _ex(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii a _ex, the exciton binding energy E _ex(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.     

Composite system based on CdSe/ZnS quantum dots and GaAs nanowires

The possibility of fabricating a composite system based on colloidal CdSe/ZnS quantum dots and GaAs nanowires is demonstrated and the structural and emission properties of this system are investigated by electron microscopy and photoluminescence spectroscopy techniques. The good wettability and developed surface of the nanowire array lead to an increase in the surface density of quantum dots and, as a consequence, in the luminosity of the system in the 600-nm wavelength region. The photoluminescence spectrum of the quantum dots exhibits good temperature stability in the entire range 10–295 K. The impact of surface states on energy relaxation and the role of exciton states in radiative recombination in the quantum dots are discussed.     

Photoluminescence properties of cadmium-selenide quantum dots embedded in a liquid-crystal polymer matrix

The photoluminescence properties of cadmium-selenide (CdSe) quantum dots with an average size of ～3 nm, embedded in a liquid-crystal polymer matrix are studied. It was found that an increase in the quantum-dot concentration results in modification of the intrinsic (exciton) photoluminescence spectrum in the range 500–600 nm and a nonmonotonic change in its intensity. Time-resolved measurements show the biexponential decay of the photoluminescence intensity with various ratios of fast and slow components depending on the quantum-dot concentration. In this case, the characteristic lifetimes of exciton photoluminescence are 5–10 and 35–50 ns for the fast and slow components, respectively, which is much shorter than the times for colloidal CdSe quantum dots of the same size. The observed features of the photoluminescence spectra and kinetics are explained by the effects of light reabsorption, energy transfer from quantum dots to the liquid-crystal polymer matrix, and the effect of the electronic states at the CdSe/(liquid crystal) interface.     

Photoluminescence study on coarsening of self-assembled InAlAs quantum dots on GaAs (001)

Red-emission at ∼640 nm from self-assembled In_0.55Al_0.45As/Al_0.5Ga_0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy (MBE) has been demonstrated. We obtained a double-peak structure of photoluminescence (PL) spectra from quantum dots. An atomic force micrograph (AFM) image for uncapped sample also shows a bimodal distribution of dot sizes. From the temperature and excitation intensity dependence of PL spectra, we found that the double-peak structure of PL spectra from quantum dots was strongly correlated to the two predominant quantum dot families. Taking into account quantum-size effect on the peak energy, we propose that the high (low) energy peak results from a smaller (larger) dot family, and this result is identical with the statistical distribution of dot lateral size from the AFM image.     

The effect of misorientation of the GaAs substrate on the properties of InAs quantum dots grown by low-temperature molecular beam epitaxy

The structural and optical properties of arrays of InAs quantum dots grown on GaAs substrates at relatively low temperatures (250 and 350°C) and with various degrees of misorientation of the surface are studied. It is shown that low-temperature growth is accompanied by the formation of quantum dot clusters along the dislocation loops on the singular surface and along the steps caused by the surface vicinality on the misoriented surface. The formation of quantum dot clusters leads to the appearance of a new long-wavelength band in the exciton photoluminescence (PL) spectra. It is found that the degrees of polarization of the PL spectral band for clusters of various shapes are different.     

Exciton photoluminescence and energy in a percolation cluster of ZnSe quantum dots as a fractal object

The results of studies of samples containing ZnSe quantum dots with a density corresponding to or considerably higher than the exciton percolation threshold, at which quantum dots form conglomerates, are reported. Excitonic emission from a percolation cluster of bound quantum dots as a fractal object is observed for the first time. Analysis of the structure of the photoluminescence spectra shows that the spectra are determined by the contribution of exciton states that belong to different structural elements of the percolation cluster, specifically, to the skeleton (backbone), dangling (dead) ends, and internal hollow spaces. A qualitative model is proposed to interpret the dependence of the exciton energy in these structural elements on the concentration of quantum dots in the material.     

Exciton states and photoluminescence of silicon and germanium nanocrystals in an Al2O3 matrix

This paper deals with the theoretical and experimental study of radiative processes in zero-dimensional Si and Ge nanostructures consisting of a system of Si or Ge nanocrystals embedded in an Al₂O₃ matrix. The Al₂O₃ films containing Si or Ge quantum dots were produced by pulsed laser-assisted deposition. The timeresolved photoluminescence spectra of the films were recorded in the energy range from 1.4 to 3.2 eV in the range of photoluminescence relaxation times between 50 ns and 20 μs. The exciton binding energy and the energy of radiative excitonic transitions are calculated, taking into account the finite barrier height and the polarization of heterointerfaces. In addition, the excitonic photoluminescence spectra are calculated, taking into account the effect of quantum mesoscopic fluctuations and the possible nonmonotonically varying dependence of the radiative zero-phonon lifetime of excitons on the dimensions of the quantum dots. The observed agreement between the calculated and recorded photoluminescence spectra confirms the excitonic nature of photoluminescence and provides a means for the determination of the model parameters of photoluminescence in the nanostructures.     

基于化学溶液法制备的氧化锌量子点发光二极管研究

氧化锌（ZnO）量子点是一种宽直接带隙半导体纳米颗粒,具有激子束缚能大、绿色环保、量子效应等优点,引起广泛关注。近期,将通过化学溶液法制备的ZnO量子点应用到发光二极管的研究成为热点。文章综述了近几年ZnO量子点发光二极管研究进展,重点介绍了各种结构的ZnO量子点发光二极管最新研究成果,并对ZnO量子点发光二极管的发展趋势进行了展望。     

Exciton states in quasi-zero-dimensional semiconductor nanosystems

The variational method in the context of the modified effective mass approximation is used to calculate the dependence of exciton ground-state energy for a quantum dot embedded in a borosilicate glassy matrix on the quantum dot radius. It is shown that the peaks in the absorption and low-temperature luminescence spectra of such a nanosystem are shifted to shorter wavelengths due to size quantization of the exciton ground-state energy in the quantum dot.     

Excitons in single and double GaAs/AlGaAs/ZnSe/Zn(Cd)MnSe heterovalent quantum wells

Exciton photoluminescence spectra, photoluminescence excitation spectra, and magnetophotoluminescence spectra of single (GaAs/AlGaAs/ZnMnSe) and double (GaAs/AlGaAs/ZnSe/ZnCdMnSe) heterovalent quantum wells formed by molecular beam epitaxy are studied. It is shown that the exciton absorption spectrum of such quantum wells mainly reproduces the resonant exciton spectrum expected for usual quantum wells with similar parameters, while the radiative exciton recombination have substantial distinctions, in particular the additional localization mechanism determined by defects generated by heterovalent interface exists. The nature of these localization centers is not currently clarified; their presence leads to broadening of photoluminescence lines and to an increase in the Stokes shift between the peaks of luminescence and absorption, as well as determining the variation in the magnetic g factor of bound exciton complexes.     

Manifestation of size-related quantum oscillations of the radiative exciton recombination time in the photoluminescence of silicon nanostructures

The spectra of steady-and nonsteady-state photoluminescence in silicon nanostructures are calculated using the envelope-function method. The distinguishing feature of this calculation is that it takes into account an additional size-related quantum effect that manifests itself in the nonmonotonic (oscillatory) dependence of the time of radiative pseudodirect (phononless) exciton transitions on the nanocrystallite size. This is due to the oscillating overlap integral of the electron wave functions of the X valleys in the conduction band and the hole wave functions of the Γ valley in the valence band. The mesoscopic effect, which manifests itself in an appreciable broadening of the energy spectrum of small nanocrystals because of the fact that the fluctuations of both the structure of heteroboundaries and the atomic arrangement of nanocrystals become significant, is also taken into account. The potential application of this approach in interpreting experimental photoluminescence spectra in low-dimensional silicon is analyzed.     

Highly circular-polarized single photon generation from a single quantum dot at zero magnetic field

Origin of sharp photoluminescence lines observed from an InAlAs quantum dot was identified with the measurements of excitation-power dependences and polarization correlations, together with photon correlation measurements. Single photon emission with high degree of circular polarization (DCP) up to 60% was observed from a positively charged exciton (trion) state in the single quantum dot under non-resonant excitation at zero magnetic field.     

量子限制效应对限制在多量子阱中受主束缚能的影响

从实验和理论上,研究了量子限制效应对限制在GaAs/AIAs多量子阱中受主对重窄穴束缚能的影响.实验中所用的样品是通过分子束外延技术生长的一系列GaAs/AIAs多量子阱,量子阱宽度从3nm到20nm,并且在量子阱中央进行了浅受主铍(Be)原子的δ掺杂.在4,20,40,80和120K不同温度下,分别对上述系列样品进行了光致发光谱(PL)的测量,清楚地观察到了受主束缚激子从ls3/2(Г6)基态到同种宇称2s3/2(Г6)激发态的两空穴跃迁,并且从实验上测得了在不同量子阱宽度下受主的束缚能.理论上应用量子力学中的变分原理,数值计算了受主对重空穴束缚能随量子阱宽度的变化关系,比较发现理论计算和实验结果符合较好.     

Quantum size effect on excitons in zinc-blende GaN/AlN quantum dot

Within the framework of effective-mass approximation, exciton states confined in zinc-blende GaN/AlN quantum dot (QD) are investigated by means of a variational approach, including three-dimensional confinement of the electron and hole in the QD and finite band offsets. Numerical results show that the exciton binding energy and the interband emission energy are both decreased when QD height (or radius) is increased. Our theoretical results are in agreement with the experimental measurements.     

纳米微晶结构氧化锌中激子发光的研究进展

由于ZnO具有大的激子束缚能,有利于在室温下获得高效稳定的紫外辐射.因此,对ZnO激子发光的研究极有希望解决半导体照明、紫外半导体激光器等多种技术面临的瓶颈问题.综述了ZnO中激子的典型光致发光谱、激子的结构及复合发光过程.重点介绍了温度、激发功率、量子限域效应以及激光辐照等因素对ZnO激子发光的影响.     

A study of the excitonic characteristics in heterostructures with quantum wells and corrugated surface

The excitonic characteristics of In_xGaAs_1−x -GaAs heterostructures in quantum wells in the presence of a corrugated surface has been investigated. The corrugated surface gives a substantial (up to 20%) polarization of the excitonic spectra even when the exciting light is incident in the direction of the normal. The exciton binding energies are calculated on the basis of data on the phonon repetitions of the photoluminescence spectra and the results agree well with the theoretical calculations. The parameters of the surface microrelief of the experimental layered structures are estimated on the basis of the degree of polarization of the photoluminescence and the magnitudes of the reflection and transmission. Fiz. Tekh. Poluprovodn. 31, 875–879 (July 1997)