Electron and hole levels of InAs quantum dots in a GaAs matrix

Photoluminescence, capacitance-voltage and transmission electron microscopy studies have been carried out on structures containing a sheet of a self-assembled InAs quantum dots formed in GaAs matrices after the deposition of a 1.7 ML of InAs at 480°C. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the capacitance-voltage technique. From analysis of photoluminescence and capacitance-voltage measurements it follows that the quantum dots have electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 meV and 170 meV above the top of the GaAs valence band.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

Effects of Si-doped GaAs layer on optical properties of InAs quantum dots

We have investigated the optical properties of InAs self-assembled quantum dots (SAQDs) with the Si-doped GaAs barrier layer. Two types of samples are fabricated according to the position of the Si-doped GaAs layer. For type A samples the Si-doped GaAs layer is grown below the QDs, whereas for type B samples the Si-doped GaAs layer is grown above the QDs. For both types of samples the excited-state emissions caused by state filling are observed in photoluminescence (PL) spectra at high excitation power densities. The bandgap renormalization of QDs can be found from the shift of the PL peak energy. Particularly, for type A samples the Si atoms act as nucleation centers during the growth of InAs QDs on the Si-doped GaAs layer and affect the density and the size of the QDs. The Si-doped GaAs layer in type A samples has more effects on the properties of QDs, such as state filling and bandgap renormalization than those of type B samples.     

Strain effects on optical properties of pyramidal InAs/GaAs quantum dots

Strain distribution and optical properties in a self-assembled pyramidal InAs/GaAs quantum dot grown by epitaxy are investigated. A model, based on the theory of linear elasticity, is developed to analyze three-dimensional induced strain field. In the model, the capping material in the heterostructure is omitted during the strain analysis to take into account the sequence of the fabrication process. The mismatch of lattice constants is the driving source of the induced strain and is treated as initial strain in the analysis. Once the strain analysis is completed, the capping material is added back to the heterostructure for electronic band calculation. The strain-induced potential is incorporated into the three-dimensional steady-state Schrödinger equation with the aid of Pikus–Bir Hamiltonian with modified Luttinger–Kohn formalism for the electronic band structure calculation. The strain field, the energy levels and wave functions are found numerically by using of a finite element package FEMLAB. The energy levels as well as the wave functions of both conduction and valence bands of quantum dot are calculated. Finally, the transition energy of ground state is also computed. Numerical results reveal that not only the strain field but also all other optical properties from current model show significant difference from the counterparts of the conventional model.     

Effects of p-type doping on the optical properties of InAs/GaAs quantum dots

The effects of p-type doping on the optical properties of self-organized InAs/GaAs quantum dots (QDs) were investigated by both micro-photoluminescence and degenerated pump–probe reflection measurements. As compared to undoped InAs/GaAs QDs, it was observed that the transitions between the ground and the first excited states of electrons and holes levels appeared at higher energies for p-doped InAs/GaAs QDs. In addition, the PL intensities for both undoped and p-doped QDs were found to decrease when the excitation power exceeded a critical value. The critical excitation power for p-doped QDs appeared to be much lower than that for undoped ones. In the pump–probe experiments, it was revealed that the value and sign of the differential reflectivity depends strongly on excitation wavelength. P-doped QDs exhibited a response behavior that is different from that of undoped ones. It is believed that the large build-in population of holes plays a crucial role in determining the transient reflection spectrum.     

Wetting layers effect on InAs/GaAs quantum dots

FEM combining with the K·P theory is adopted to systematically investigate the effect of wetting layers on the strain-stress profiles and electronic structures of self-organized InAs quantum dot. Four different kinds of quantum dots are introduced at the same height and aspect ratio. We found that 0.5 nm wetting layer is an appropriate thickness for InAs/GaAs quantum dots. Strain shift down about 3%∼4.5% for the cases with WL (0.5 nm) and without WL in four shapes of quantum dots. For band edge energy, wetting layers expand the potential energy gap width. When WL thickness is more than 0.8 nm, the band edge energy profiles cannot vary regularly. The electron energy is affected while for heavy hole this impact on the energy is limited. Wetting layers for the influence of the electronic structure is obviously than the heavy hole. Consequently, the electron probability density function spread from buffer to wetting layer while the center of hole's function moves from QDs internal to wetting layer when introduce WLs. When WLs thickness is larger than 0.8 nm, the electronic structures of quantum dots have changed obviously. This will affect the instrument's performance which relies on the quantum dots' optical properties.     

InAs自组装量子点GaAs肖特基二级管的电学特性研究

分析研究了GaAsInAs自组装量子点的电输运性质,通过对实验数据的分析,讨论了Schottky势垒对InAs量子点器件的影响和IV曲线中迟滞回路以及电导曲线中台阶结构产生的原因.迟滞回路和台阶的出现与电场中量子点的充放电过程相关:迟滞回路反映了量子点充电后对载流子的库仑作用,而电导台阶则反映了量子点因共振隧穿的放电现象 关键词： 迟滞现象 自组装量子点 共振隧穿     

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.     

«Self-organized» InAs/GaAs quantum dots

Il Nuovo Cimento D - We discuss some specific properties of self-organized InAs-GaAs quantum dots grown by Molecular Beam Epitaxy. We report on the optical spectra obtained under resonant...     

Disorder-induced natural quantum dots in InAs/GaAs nanostructures

Properties of excitons confined to potential fluctuations due to indium distribution in the wetting layer which accompany self-assembled InAs/GaAs quantum dots are reviewed. Spectroscopic studies are summarized including time-resolved photoluminescence and corresponding single-photon emission correlation measurements. The identification of charge states of excitons is presented which is based on results of a theoretical analysis of interactions between the involved carriers. The effect of the dots’ environment on their optical spectra is also shown.     

GaSb/GaAs复合应力缓冲层上自组装InAs量子点的生长

研究了GaSb/GaAs复合应力缓冲层上自组装生长的InAs量子点.在2ML GaSb/1ML GaAs复合应力缓冲层上获得了高密度的、沿［100］方向择优分布量子点.随着复合应力缓冲层中GaAs层厚度的不同,量子点的密度可以在1.2×10¹⁰cm^-2和8×10¹⁰cm^-2进行调控.适当增加GaAs层的厚度至5ML,量子点的发光波长红移了约25nm,室温下PL光谱波长接近1300nm. 关键词： 自组装量子点 分子束外延 Ⅲ-Ⅴ族化合物半导体     

离子注入对InAs/GaAs量子点光学效质的影响

研究了低能质子注入诱导的界面混合和快速热退火对量子点发光效率的影响，对其光致发光 峰强进行了拟合计算.研究发现量子点的发光峰强度主要由载流子俘获时间和非辐射复合寿 命决定.由于后退火处理能够部分的消除因质子注入造成的缺陷，量子点中非辐射复合中心 浓度与注入剂量成亚线性关系；退火温度越高，非辐射复合中心被消除越多，亚线性程度越 高.界面混合导致的俘获效率的增加和注入损伤引起的非辐射复合是相互竞争过程，存在一 个临界的注入剂量N_C，当注入剂量N小于N_C，界面混合作 用较为明显，量子点 发光峰强随注入剂量增加而增强；当N大于N_C时，质子注入引起了大量的非 辐射复合 中心，主要表现为注入损伤，量子点的发光峰强随注入剂量增加而迅速减弱.退火温度越高 ，N_C越大. 关键词： 量子点 离子注入 峰强     

Effect of the electron population on intraband absorption in InAs/GaAs self-assembled quantum dots

We present a comprehensive study of the intraband transitions in n-type InAs/GaAs quantum dots (QDs) with a filling varying from 0.5 to 4 electrons per dot, using both polarization-dependent absorption and photocurrent spectroscopy. Applying these complementary mid- and far-infrared spectroscopies over a wide energy range allows us to obtain a detailed picture of the intraband transitions and energy levels in self-assembled QDs.     

Control of structural and excitonic properties of self-organized InAs/GaAs quantum dots

A systematic dependence of excitonic properties on the size of self-organized InAs/GaAs quantum dots is presented. The bright exciton fine-structure splitting changes from negative values to more than 0.5 meV, and the biexciton binding energy varies from antibinding to binding, as the height of truncated pyramidal dots increases from 2 to above 9 InAs monolayers. A novel mode of metalorganic vapor phase epitaxy was developed for growing such quantum dots with precise shape control. The dots consist of pure InAs and feature heights varying in steps of complete InAs monolayers. Such dot ensembles evolve from a strained, rough two-dimensional layer with a thickness close to the critical value for the onset of the 2D–3D transition. Dots with a common height represent subensembles with small inhomogeneous broadening. Tuning of subensemble emission energy is achieved by varying the mean lateral extension of the respective QDs. Detailed knowledge of the structural properties of individual dots enable realistic k·p calculations to analyze the origin of the observed excitonic properties. The binding energies of charged and neutral excitons increase due to correlation by the gradually increasing number of bound states for increasing dot size. The monotonously increasing magnitude of the fine-structure splitting with dot size is shown to be caused by piezoelectricity. The identification of key parameters allows to tailor exciton properties, providing a major step towards the development of novel applications.     

AlGaAs capping effect on InAs quantum dots self-assembled on GaAs

The effects of AlGaAs capping on InAs quantum dots self-assembled on GaAs are investigated. It is observed that, the photoluminescence intensity becomes stronger up to twice when Al is incorporated into the cap layer. In the mean time, the full width at half maximum of the photoluminescence spectrum becomes narrower, the peak splitting between the ground and first excited exciton levels becomes wider, and the photoluminescence peak wavelength becomes longer. With considerations of the increased barrier height and the changed microstructures of the quantum dots induced by AlGaAs capping, the mechanisms of the observed improvements are discussed.     

Observation of monolayer-splitting for InAs/GaAs quantum dots

A pronounced modulation is observed in the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots (QDs), recorded at low excitation densities. The clearly distinguishable peaks are identified as a multimodal distribution of the ground state transition energy, originating from a discrete, stepwise variation of the structural properties of the QDs, which is associated with an increase of the QD height in monolayer (ML) steps. The observation of a ML splitting implies a flat QD shape with well-defined upper and lower interfaces as well as negligible indium segregation. The electronic properties of the InAs/GaAs QDs were investigated by PL and PL-excitation spectroscopy and are discussed based on realistic calculations for flat InAs/GaAs QDs with a truncated pyramidal shape based on an extended 8-band k·p model. The calculations predict a red shift of the ground state transition with each additional ML, which saturates for heights above 9 ML, is in good agreement with experiment.     

Control of structural and excitonic properties of self-organized InAs/GaAs quantum dots

A systematic dependence of excitonic properties on the size of self-organized InAs/GaAs quantum dots is presented. The bright exciton fine-structure splitting changes from negative values to more than 0.5 meV, and the biexciton binding energy varies from antibinding to binding, as the height of truncated pyramidal dots increases from 2 to above 9 InAs monolayers. A novel mode of metalorganic vapor phase epitaxy was developed for growing such quantum dots with precise shape control. The dots consist of pure InAs and feature heights varying in steps of complete InAs monolayers. Such dot ensembles evolve from a strained, rough two-dimensional layer with a thickness close to the critical value for the onset of the 2D–3D transition. Dots with a common height represent subensembles with small inhomogeneous broadening. Tuning of subensemble emission energy is achieved by varying the mean lateral extension of the respective QDs. Detailed knowledge of the structural properties of individual dots enable realistic k·p calculations to analyze the origin of the observed excitonic properties. The binding energies of charged and neutral excitons increase due to correlation by the gradually increasing number of bound states for increasing dot size. The monotonously increasing magnitude of the fine-structure splitting with dot size is shown to be caused by piezoelectricity. The identification of key parameters allows to tailor exciton properties, providing a major step towards the development of novel applications.     

Extremely low density self-assembled InAs/GaAs quantum dots

The self-assembled InAs/GaAs quantum dots (QDs) with extremely low density of 8×106 cm-2 are achieved using higher growth temperature and lower InAs coverage by low-pressure metal-organic chemical vapour deposition (MOVCD). As a result of micro-photoluminescence (micro-PL), for extremely low density of 8×106 cm-2 InAs QDs in the micro-PL measurements at 10 K, only one emission peak has been achieved. It is believed that the InAs QDs have a good potential to realize single photon sources.     

Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from to as much as with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.