Optical spectroscopy of a single InAs/GaAs quantum dot in high magnetic fields

We report on the measurements of the photoluminescence from the s-shell of a single InAs/GaAs quantum dot in magnetic fields up to 23 T. The observed multiline emission is attributed to different charge states of a single dot. Characteristic anticrossing of emission lines is explained in terms of hybridization of final states of a triply charged exciton (X⁻³).     

Molecular states observed in a single pair of strongly coupled self-assembled InAs quantum dots

Molecular states in a single pair of strongly coupled self-assembled InAs quantum dots are investigated using a sub-micron sized single-electron transistor containing just a few pairs of coupled InAs dots embedded in a GaAs matrix. We observe a series of well-formed Coulomb diamonds with charging energy of less than 5 meV, which are much smaller than those reported previously. This is because electrons are occupied in molecular states, which are spread over both dots and occupy a large volume. In the measurement of ground and excited state single-electron transport spectra with a magnetic field, we find that the electrons are sequentially trapped in symmetric and anti-symmetric states. This result is well explained by numerical calculation using an exact diagonalization method.     

Optical orientation and spin relaxation of resident electrons in n-doped InAs/GaAs self-assembled quantum dots

We report on optical orientation of electrons in n-doped InAs/GaAs quantum dots. Under non-resonant cw optical pumping, we measure a negative circular polarization of the luminescence of charged excitons (or trions) at low temperature (T=10 K). The dynamics of the recombination and of the circular polarization is studied by time-resolved spectroscopy. We discuss a simple theoretical model for the trion relaxation, that accounts for this remarkable polarization reversal. The interpretation relies on the bypass of Pauli blocking allowed by the anisotropic electron–hole exchange. Eventually, the spin relaxation time of doping electrons trapped in quantum dots is measured by a non-resonant pump–probe experiment.     

Vibrational spectroscopy of InAs and AlAs quantum dot structures

In this paper we present an experimental comparative study of InAs/AlAs periodical structures with InAs and AlAs quantum dots (QDs) by means of infrared and Raman spectroscopies. The first observation of optical phonons localized in InAs and AlAs QDs using infrared spectroscopy is demonstrated. Confined optical phonon frequencies of the QDs measured by means of Raman scattering are compared with those deduced from the analysis of infrared spectra performed in the framework of the dielectric function approximation.     

Redistribution of photogenerated carriers in neutral and charged InAs quantum dot systems

Time-integrated and time-resolved photoluminescence spectra of neutral and negatively charged self-assembled InAs quantum dots (QDs) were studied. Obtained spectra have indicated that the redistribution of carriers in QDs occurs in all samples, but the temperature dependence of spectra are quite different for neutral and charged QDs. To clarify the origin of these behaviors, a model calculation based on two possible redistribution mechanisms has been carried out, and compared with experiments to show that the carrier tunneling between neighboring QDs is suppressed in charged QDs.     

Single InAs/InP quantum dot spectroscopy in 1.3–1.55 μm telecommunication band

We present an optical spectroscopy and photon correlation measurement at telecommunication wavelengths performed on single InAs/InP quantum dots. Two main approaches brought high optical quality: an application of a ‘double-cap’ growth method to metalorganic chemical vapor deposition, and fabrication of a small mesa structure using low-damage wet chemical etching. Sharp and discrete exciton transition lines have been observed on the single quantum dots, which widely cover the spectral range of 1.3–1.55 μm. Using a pulsed excitation source and gated single-photon detection modules, we observed a photon antibunching behavior for an isolated exciton emission line, indicating nonclassical light emission near the wavelength of 1.3 μm.     

Giant optical anisotropy in single InAs quantum dots

We present the experimental evidence of giant optical anisotropy in single InAs QDs. Polarization-resolved photoluminescence spectroscopy in single QDs reveals a linear polarization ratio which fluctuates, from one dot to another, in sign and in magnitude with absolute values up to 82%. We do not observe any dependence of the linear polarization on incident power and temperature.     

插入生长AlGaAs薄膜对InAs量子点探测器性能的影响

利用分子束外延生长获得的两个InAs量子点样品制备了n型的量子点红外探测器.对于其中一个器件，在InAs量子点有源区的底部和顶部分别插入生长了AlGaAs势垒层.利用透射电阻显微技术研究了两个样品的结构特性；利用光致发光光谱和光电流谱研究了两个器件的光电性质.实验结果表明，AlGaAs层的插入对器件的探测性质有显著的影响.利用有三维效质量近似模型的计算结果，指认了带内光电流谱中峰结构的起源.     

插入生长AlGaAs薄膜对InAs量子点探测器性能的影响

利用分子束外延生长获得的两个InAs量子点样品制备了n型的量子点红外探测器.对于其中一个器件,在InAs量子点有源区的底部和顶部分别插入生长了AlGaAs势垒层.利用透射电阻显微技术研究了两个样品的结构特性;利用光致发光光谱和光电流谱研究了两个器件的光电性质.实验结果表明,AlGaAs层的插入对器件的探测性质有显著的影响.利用有三维效质量近似模型的计算结果,指认了带内光电流谱中峰结构的起源. 关键词： InAs 量子点 AlGaAs薄膜 光致发光光谱 有效质量近似模型     

Resonant photoluminescence from modulation-doped InAs –GaAs quantum dots

Resonant luminescence studies of InAs quantum dots (QDs) embedded in a GaAs matrix grown by molecular beam epitaxy are presented, showing marked differences for modulation-doped and undoped QDs and indicating that the doping leads to different exciton formation and carrier relaxation mechanisms. The LO-phonon assisted relaxatioin of excitons between sub-levels is identified for the modulation-doped QDs.     

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs.     

Exchange interaction in InAs nanocrystal quantum dots

The near band-gap level structure in high-quality colloidal InAs nanocrystal quantum dots within the very strong confinement regime is investigated. Size-selective photoluminescence excitation and fluorescence line narrowing measurements reveal a size-dependent splitting between the absorbing and the emitting states. The splitting is assigned to the confinement-enhanced electron–hole exchange interaction. The size dependence of the splitting significantly deviates from the idealized 1/r³scaling law for the exchange splitting. A model incorporating a finite barrier which allows for wavefunction leakage is introduced. The model reproduces the observed 1/r²dependence of the splitting and good agreement with the experimental data is obtained. The smaller barriers for embedded InAs dots grown by molecular-beam epitaxy, are predicted to result in smaller exchange splitting as compared with colloidal dots with a similar number of atoms.     

Frequency-dependent C(V) spectroscopy of the hole system in InAs quantum dots

The hole system in InAs quantum dots was investigated by frequency-dependent capacitance–voltage spectroscopy. Up to eight distinct charging peaks could be observed and the energy difference between the individual peaks could be estimated. All charging peaks decrease with increasing measurement frequency; however, the lower the energy of the hole level the stronger the decrease. A comparison with the results of the electron system in similar quantum dots yields that for all hole levels the effective mass in the barrier is much larger than in the electron system.     

金纳米颗粒光散射提高InAs单量子点荧光提取效率

采用光学方法确定InAs/GaAs单量子点在样品外延面上的位置坐标, 利用AlAs牺牲层把含有量子点的GaAs层剥离并放置在含有金纳米颗粒或平整金膜上, 研究量子点周围环境不同对量子点自发辐射寿命及发光提取效率的影响. 实验结果显示, 剥离前后量子点发光寿命的变化小于13%, 含有金纳米颗粒的量子点发光强度是剥离前的7倍, 含有金属薄膜的量子点发光强度是剥离前的2倍. 分析表明在金纳米颗粒膜上的量子点荧光强度的增加主要来自于金纳米颗粒对量子点荧光的散射效应, 从而提高量子点发光的提取效率.     

Investigation of populated InAs/GaAs quantum dots by photoluminescence and photoreflectance

We studied self-assembled InAs/GaAs quantum dots by contrasting photoluminescence and photoreflectance spectra from 10 K to room temperature. The photoluminescence spectral profiles comprise contributions from four equally separated energy levels of InAs quantum dots. The emission profiles involving ground state and excited states have different temperature evolution. Abnormal spectral narrowing occurred above 200 K. In the photoreflectance spectra, major features corresponding to the InAs wetting layer and GaAs layers were observed. Temperature dependences of spectral intensities of these spectral features indicate that they originate from different photon-induced modulation mechanisms. Considering interband transitions of quantum dots were observed in photoluminescence spectra and those of wetting layer were observed in photoreflectance profiles, we propose that quantum dot states of the system are occupied up to the fourth energy level which is below the wetting layer quantum state.     

Coherent spectroscopy of a strongly driven triple quantum dot molecule

Based on a calculation model,we study the interference phenomena of serially coupled V-type and Λ-type triple quantum dots (CTQDs) driven simultaneously by a strong driving field and a weak probe field.Strongly depending on the configuration of the three-level CTQD,the probe absorption spectra,which are shown in the tunneling current,exhibit various quantum coherence properties.In the case where the two pairs of transitions of the CTQD have a small eigenfrequency difference △ω,the double-coupling effect of the driving field results in two Autler-Townes doublets and one weak Mollow triplet in one spectrum.With the value of △ω increasing,only one Autler-Townes splitting remains due to the single-coupling of the field.We also find that the effect of spontaneous emission of phonons may lead to an obvious background current,which can be used to distinguish which transition is driven by the driving field in experiment.The interesting quantum property of a CTQD revealed in our results suggests its potential applications in quantum modulators and quantum logic devices.     

Theoretical study of quantum confined Stark shift in InAs／GaAs quantum dots

The quantum confined Stark effect (QCSE) of the self-assembled InAs/GaAs quantum dots has been investigated theoretically. The ground-state transition energies for quantum dots in the shape of a cube, pyramid or “truncated pyramid” are calculated and analysed. We use a method based on the Green function technique for calculating thestrain in quantum dots and an efficient plane-wave envelope-function technique to determine the ground-state electronic structure of them with different shapes. The symmetry of quantum dots is broken by the effect of strain. So the properties of carriers show different behaviours from the traditional quantum device. Based on these results, we also calculate permanent built-in dipole moments and compare them with recent experimental data. Our results demonstrate that the measured Stark effect in self-assembled InAs/GaAs quantum dot structures can be explained by including linear grading.     

Mn-including InAs quantum dots fabricated by Mn implantation

Mn-including InAs quantum dots (QDs) were fabricated by Mn-ion implantation and subsequent annealing. The optical, compositional, and structural properties of the treated samples were analyzed by photoluminescence (PL) and microscopy. Energy dispersive X-ray (EDX) results indicate that Mn ions diffused from the bulk GaAs into the InAs QDs during annealing, and the diffusion appears to be driven by the strain in the InAs QDs. The temperature dependence of the PL of Mn-including InAs QD samples exhibits QDs PL characteristics. At the same time, the heavy Mn-including InAs QD samples have ferromagnetic properties and high T_c.     

Magnetic field dependence of hole levels in InAs quantum dots

We have studied the charging of InAs quantum dots with holes in perpendicular fields up to 16 T by capacitance—voltage spectroscopy. The first two charging peaks show almost no shift with magnetic field which is consistent with the filling of a twofold degenerate s-like state with no orbital angular momentum. The next four charging peaks shift towards lower and higher energy in an alternating fashion. Peaks 5 and 6 shift approximately twice as strong as peaks 3 and 4. This behavior cannot be explained by the charging of a fourfold degenerate p-shell according to Hund′s first rule. We speculate that the p-shell is not completely filled before the filling of the d-shell starts.     

Investigation of the effect of varying growth pauses on the structural and optical properties of InAs/GaAs quantum dot heterostructures

Effect of growth pause or ripening time on structural and optical properties of self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE ) technique with two different growth rates of InAs (0.032 MLs⁻¹ and 0.197 MLs⁻¹) has been investigated. The QD heterostructures were grown at 520 ^°C with InAs monolayer coverage of 2.7 ML. The results were explained on the basis of high angle annular dark field scanning transmission electron microscope (HAADF-STEM), scanning electron microscope (SEM) and photoluminescence (PL) measurements. Introduction of growth pause leads the QD system towards a thermodynamic equilibrium state which in turn makes interesting changes on the morphology of the samples. Coagulation of some smaller dots occurs because of ripening to produce evolved QDs and the dot density reduces with growth pause.