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.     

The preferential formation site of dislocations in InAs/GaAs quantum dots

In this paper, taking elastic anisotropy into consideration, we use a dislocation position dependent model to calculate the preferential formation site of the pure edge and 60° mixed dislocation segment in different shaped InAs/GaAs quantum dots (QDs). From the result, it is clear that for the pure edge dislocations the most energy favorable position is always the base center of the quantum dots. While as to the 60° mixed dislocations, the positions near to the edge of the quantum dot base are the energy favorable area and the exact position is changed with different aspect ratio of the quantum dot.     

Self-assembly of InAs quantum dots on GaAs(001)by molecular beam epitaxy

Currently, the nature of self-assembly of three-dimensional epitaxial islands or quantum dots (QDs) in a lattice-mismatched heteroepitaxial growth system, such as InAs/GaAs(001) and Ge/Si(001) as fabricated by molecular beam epitaxy (MBE), is still puzzling. The purpose of this article is to discuss how the self-assembly of InAs QDs in MBE InAs/GaAs(001) should be properly understood in atomic scale. First, the conventional kinetic theories that have traditionally been used to interpret QD self-assembly in heteroepitaxial growth with a significant lattice mismatch are reviewed briefly by examining the literature of the past two decades. Second, based on their own experimental data, the authors point out that InAs QD self-assembly can proceed in distinctly different kinetic ways depending on the growth conditions and so cannot be framed within a universal kinetic theory, and, furthermore, that the process may be transient, or the time required for a QD to grow to maturity may be significantly short, which is obviously inconsistent with conventional kinetic theories. Third, the authors point out that, in all of these conventional theories, two well-established experimental observations have been overlooked: i) A large number of “floating” indium atoms are present on the growing surface in MBE InAs/GaAs(001); ii) an elastically strained InAs film on the GaAs(001) substrate should be mechanically unstable. These two well-established experimental facts may be highly relevant and should be taken into account in interpreting InAs QD formation. Finally, the authors speculate that the formation of an InAs QD is more likely to be a collective event involving a large number of both indium and arsenic atoms simultaneously or, alternatively, a morphological/structural transformation in which a single atomic InAs sheet is transformed into a three-dimensional InAs island, accompanied by the rehybridization from the sp²-bonded to sp³- bonded atomic configuration of both indium and arsenic elements in the heteroepitaxial growth system.     

InAs/GaAs量子点1.3 μm单光子发射特性

采用双层耦合量子点的分子束外延生长技术生长了InAs/GaAs量子点样品,把量子点的发光波长成功地拓展到1.3 μm.采用光刻的工艺制备了直径为3 μm的柱状微腔,提高了量子点荧光的提取效率.在低温5 K下,测量得到量子点激子的荧光寿命约为1 ns;单量子点荧光二阶关联函数为0.015,显示单量子点荧光具有非常好的单光子特性;利用迈克耳孙干涉装置测量得到单光子的相干时间为22 ps,对应的谱线半高全宽度为30 μeV,且荧光谱线的线型为非均匀展宽的高斯线型.     

Numerical investigation on threading dislocation bending with InAs/GaAs quantum dots

The threading dislocations (TDs) in GaAs/Si epitaxial layers due to the lattice mismatch seriously degrade the performance of the lasers grown on silicon. The insertion of InAs quantum dots (QDs) acting as dislocation filters is a pretty good alternative to solving this problem. In this paper, a finite element method (FEM) is proposed to calculate the critical condition for InAs/GaAs QDs bending TDs into interfacial misfit dislocations (MDs). Making a comparison of elastic strain energy between the two isolated systems, a reasonable result is obtained. The effect of the cap layer thickness and the base width of QDs on TD bending are studied, and the results show that the bending area ratio of single QD (the bending area divided by the area of the QD base) is evidently affected by the two factors. Moreover, we present a method to evaluate the bending capability of single-layer QDs and multi-layer QDs. For the QD with 24-nm base width and 5-nm cap layer thickness, taking the QD density of 10¹¹ cm^-2 into account, the bending area ratio of single-layer QDs (the area of bending TD divided by the area of QD layer) is about 38.71%. With inserting five-layer InAs QDs, the TD density decreases by 91.35%. The results offer the guidelines for designing the QD dislocation filters and provide an important step towards realizing the photonic integration circuits on silicon.     

Temperature dependence of the electron distribution in a GaAs matrix with embedded InAs quantum dots

We report on the effect of the Debye averaging process on the CV characteristics of a sample containing four coupled planes of InAs self-assembled quantum dots. The observed electron distribution presented a dynamical dependence of the temperature during the C–V measurements which was explained in terms of the screening length dependence on the temperature. In addition, using the C–V data, we calculated the electron density at the planes containing the InAs dots and we have observed a high-temperature stability: the electron density at the quantum dots remained constant over a large range of temperature.     

Photoluminescence and lasing properties of InAs/GaAs quantum dots grown by metal-organic chemical vapour deposition

Photoluminescence （PL） and lasing properties of InAs/GaAs quantum dots （QDs） with different growth procedures prepared by metalorganic chemical vapour deposition are studied. PL measurements show that the low growth rate QD sample has a larger PL intensity and a narrower PL line width than the high growth rate sample. During rapid thermal annealing, however, the low growth rate sample shows a greater blueshift of PL peak wavelength. This is caused by the larger InAs layer thickness which results from the larger 2-3 dimensional transition critical layer thickness for the QDs in the low-growth-rate sample. A growth technique including growth interruption and in-situ annealing, named indium flush method, is used during the growth of GaAs cap layer, which can flatten the GaAs surface effectively. Though the method results in a blueshift of PL peak wavelength and a broadening of PL line width, it is essential for the fabrication of room temperature working QD lasers.     

Optical nonlinearities of Au nanoparticles embedded in a zinc oxide matrix

Optical nonlinearities of Au nanoparticles embedded in zinc oxide (ZnO) matrix have been investigated by the Z-scan method at the wavelength of 532 nm using nanosecond Nd³⁺:YAG laser radiation. The nonlinear refractive index has been measured and the real part of the third-order nonlinear susceptibility is deduced. The results of the investigation of nonlinear refraction using the off-axis Z-scan configuration are presented and the mechanisms responsible for the nonlinear response are discussed. The nonlinear refraction is found to be negative (self-defocusing) in the vicinity of the surface plasmon resonance. Moreover, its strength is shown to be larger for materials having higher gold concentration. Finally, the prevailing influence of the electronic Kerr effect over the possible thermo-optical contribution is demonstrated.     

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.     

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.     

Raman study of InAs quantum dots on GaAs/InP grown by low pressure metal-organic chemical vapor deposition

Raman spectra of InAs quantum dots (QDs) on InP substrate were investigated. Both longitudinal-optic (LO) and transverse-optic (TO) frequency of InAs QDs showed a large blue-shift comparing to its bulk due to the compressive strain in InAs QDs. Raman scattering of InAs QDs with a thin GaAs interlayer was studied. We obtained that the peak position of LO and TO mode of InAs QDs became larger blue-shifted when we inserted the GaAs layer. At the same time, we found a red-shift of the frequency of GaAs LO mode because of tensile strain. Theoretical calculation was performed and its prediction coincided with our experiment results well. They both showed that strain played an important role in formation of InAs QDs.     

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.     

Broadband light emitting from multilayer-stacked InAs/GaAs quantum dots

We report the effect of the GaAs spacer layer thickness on the photoluminescence(PL) spectral bandwidth of InAs/GaAs self-assembled quantum dots(QDs).A PL spectral bandwidth of 158 nm is achieved with a five-layer stack of InAs QDs which has a 11-nm thick GaAs spacer layer.We investigate the optical and the structural properties of the multilayer-stacked InAs/GaAs QDs with different GaAs spacer layer thicknesses.The results show that the spacer thickness is a key parameter affecting the multi-stacked InAs/GaAs QDs for wide-spectrum emission.     

等离子体增强InAs单量子点荧光辐射的研究

通过测量光致发光(PL)谱、PL时间分辨光谱及不同激发功率下PL发光强度,研究了低温(5 K)下等离子体对InAs单量子点PL光谱的增强效应.采用电子束蒸发镀膜技术在InAs量子点样品表面淀积了5 nm厚度的金膜,形成纳米金岛膜结构.实验发现,金岛膜有利于量子点样品发光强度的增加,最大PL强度增加了约5倍,其主要物理机理是金岛膜纳米结构提高了量子点PL光谱的收集效率.     

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.     

Tunnelling current in Schottky diodes containing InAs quantum dots

Current transport mechanism in Schottky diode containing InAs quantum dots (QDs) is investigated using temperature-varying current-voltage characteristics. We found that the tunnelling emission has obvious effects on the I-V characteristics. The I-V-T measurements revealed clear effects of QDs on the overall current flow. Field emission (FE, pure tunnelling effect) was observed at low temperature and low voltages bias region. The zero-bias barrier height decreases and the ideality factor increases with decreasing temperature, and the ideality factor was found to follow the T₀-effect. When the reverse bias is varied, the ideality factors of Schottky barriers exhibit oscillations due to the tunnelling of electrons through discrete levels in quantum dots. The traps distributed within InAlAs layer can also act as a transition step for reverse bias defect-assisted tunnelling current which can phenomenologically explain the decrease of the effective barrier height with measurement temperature.     

Size dependence of biexciton binding energy in single InAs/GaAs quantum dots

This paper studies the size dependence of biexciton binding energy in single quantum dots (QDs) by using atomic force microscopy and micro-photoluminescence measurements. It finds that the biexciton binding energies in the QDs show ``binding' and ``antibinding' properties which correspond to the large and small sizes of QDs, respectively. The experimental results can be well interpreted by the biexciton potential curve, calculated from the exciton molecular model and the Heitler--London method.     

Mn离子注入InAs/GaAs量子点结构材料的光电性质研究

用高能离子注入(160keV)的方法对InAs/GaAs量子点结构进行掺杂，研究了不同退火工艺处理后量子点的光致发光和电学性能.相对于长时间退火，快速退火处理后的量子点发光通常较强.在相同的退火条件下，量子点发光峰位随着Mn注入剂量的增加，先是往高能量端快速移动，而后发光峰又往低能方向移动.后者可能是由于Mn原子进入InAs量子点，释放了InAs量子点中的应变所致.对于高注入剂量样品和长时间退火样品，变温电阻曲线在40 K附近会出现反常行为. 关键词： 离子注入 InAs/GaAs量子点 光致发光 团簇     

InAs quantum dots in GaAs holes: island number dependence on hole diameter and conduction-band coupling estimates

We studied the formation of InAs islands in holes defined by electron-beam lithography on GaAs substrates. The islands grew selectively in the holes, with one to nine islands per hole. The number of islands depends simply on the hole diameter, filling the holes at a constant effective two-dimensional density. We define the ratio of this effective density to the density on an unpatterned control sample to be the selectivity ratio, and we find a selectivity ratio of greater than 1000 for the present samples. We estimated the lateral conduction-band coupling for closely spaced islands and conclude them to be plausible candidates for weakly coupled device building blocks.     

Electronic characteristics of InAs self-assembled quantum dots

Deep-level transient spectroscopy and photoluminescence studies have been carried out on structures containing self-assembled InAs quantum dots formed in GaAs matrices. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the deep-level transient spectroscopy technique. From analysis of deep-level transient spectroscopy measurements it follows that the quantum dots have electron levels 130 meV below the bottom of the GaAs conduction band and heavy-hole levels at 90 meV above the top of the GaAs valence band. Combining with the photoluminescence results, the band structures of InAs and GaAs have been determined.