Observation of long-lived excitons in InAs quantum dots under thermal redistribution temperature

We study the temperature-dependent time-resolved photoluminescence (TRPL) of self-assembled InAs quantum dots (QDs). Under low excitation power, a surprisingly long PL decay time is observed at about 60 K, under the thermal redistribution temperature. The long decay time decreases with increasing excitation power but is nearly independent of the detection energy of TRPL measurements. A model considering the spin relaxation through the excited excitonic state is proposed to quantitatively explain the unusual phenomena. The rate equation analysis indicates that the observation of long-lived excitons is caused by the shortened spin-flip time.     

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.     

One-electron spin-dependent transport in split-gate structures containing self-organized InAs quantum dots

The paper presents the results obtained in a study of electron transport in split-gate structures prepared from heterostructures with self-organizing InAs quantum dots situated close to a two-dimensional electron gas. Coulomb oscillations of current through InAs quantum dots depending on the voltage on the gate were observed. Coulomb current oscillations persisted up to about 20 K. The Coulomb energy ΔE _C = 12.5 meV corresponding to theoretical estimates for the p-states of quantum dots in our structures was determined.     

电场调谐InAs量子点荷电激子光学跃迁

在低温5 K下, 采用光致发光光谱及外加偏压调谐量子点电荷组态研究了InAs单量子点的精细结构和对应发光光谱的偏振性、不同带电荷激子的圆偏振特性. 得出如下结果: 1) 指认InAs单量子点中不同荷电激子的发光光谱和对应的激子本征态的偏振特性; 2) 外加偏压可以调谐量子点的荷电激子的发光光谱; 3) 伴随着电子、空穴的能量弛豫, 电子的自旋弛豫时间远大于空穴的自旋弛豫时间. 关键词： InAs量子点 激子 荧光光谱 电场调谐     

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.     

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.     

Phonon-assisted polar exciton–transitions in self-organized InAs/GaAs quantum dots

Phonon-assisted exciton transitions are investigated for self-organized InAs/GaAs quantum dots (QDs) using selectively excited photoluminescence (PL) and PL excitation spectroscopy. The results unambiguously demonstrate intrinsic recombination in the coherent InAs/GaAs QDs and the absence of a Stokes shift between ground state absorption and emission. Phonon-sidebands corresponding to a phonon energy of 34 meV are resolved and Huang–Rhys parameters of 0.015 and 0.08 are found for phonon-assisted emission and absorption, respectively, which are about one order of magnitude larger than in bulk InAs. Calculations of the exciton–LO–phonon interaction based on an adiabatic approximation and realistic wave functions for ideal pyramidal InAs/GaAs QDs show this enhanced polar coupling to result from the particular confinement and the strain-induced piezoelectric potential in such strained low-symmetry QDs.     

Cracking self-assembled InAs quantum dots

We present a cross-sectional scanning tunneling microscopy (X-STM) investigation of InAs quantum dots in a GaAs matrix. The structures were grown by molecular beam epitaxy (MBE) at a low growth rate of 0.01 ML/s and consist of five layers of uncoupled quantum dot structures. Detailed STM images with atomic resolution show that the dots consist of an InGaAs alloy and that the indium content in the dot increases towards the top. The analysis of the height versus base-length relation obtained from cross-sectional images of the dots shows that the shape of the dots resembles that of a truncated pyramid and that the square base is oriented along the [010] and [100] directions. Using scanning tunneling spectroscopy (STS) we determined the onset for electron tunneling into the conduction and out of the valence band, both in the quantum dots and in the surrounding GaAs matrix. We found equal voltages for tunneling out of the valence band in GaAs or InGaAs whereas tunneling into GaAs occurred at higher voltages than in InGaAs.     

Rabi oscillations of excitons in single quantum dots

Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.     

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.     

Optical orientation and alignment of excitons in quantum dots

Optical orientation and alignment of excitons in InAlAs quantum dots in the AlGaAs matrix have been studied both theoretically and experimentally. Experiments performed in a longitudinal magnetic field (Faraday geometry) reveal transformation of optical orientation to alignment and alignment to orientation, which is caused by exchange splitting of the dipole-active exciton doublet and allowed by the quantum-dot low symmetry. A comparison of theory with experiment made with inclusion of the anisotropy of exciton generation and recombination along the and [110] axes permits one to determine the character of dipole distribution in direction for resonant optical transitions in the self-organized quantum-dot ensemble studied. Fiz. Tverd. Tela (St. Petersburg) 40, 858–861 (May 1998)     

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.     

Magnetic and optical properties of self-organized InMnAs quantum dots

Ten layers of self-assembled InMnAs quantum dots with InGaAs barrier were grown on high resistivity (1 0 0) p-type GaAs substrates by molecular beam epitaxy (MBE). The presence of ferromagnetic structure was confirmed in the InMnAs diluted magnetic quantum dots. The ten layers of self-assembled InMnAs quantum dots were found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T_C=80 K. It is likely that the ferromagnetic exchange coupling of sample with T_C=80 K is hole mediated resulting in Mn substituting In and is due to the bound magnetic polarons co-existing in the system. PL emission spectra of InMnAs samples grown at temperature of 275, 260 and 240 °C show that the interband transition peak centered at 1.31 eV coming from the InMnAs quantum dot blueshifts because of the strong confinement effects with increasing growth temperature.     

Radiative lifetime of resonantly excited excitons in quantum dots

Il Nuovo Cimento D - We report on the intrinsic radiative lifetime of excitons in InP quantum dots embedded in an In0.48Ga0.52 P matrix. This lifetime is about 400 ps as measured by time-resolved...