External electric field effects on the electronic and hydrogenic impurity states in ellipsoidal and semi-ellipsoidal quantum dots

In the effective mass approximation, energy eigenvalues of an electron confined in ellipsoidal and semi-ellipsoidal quantum dots, with and without hydrogenic impurity, under the influence of an external electric field have been investigated, using the matrix diagonalization method. The lower-laying states of the electron as functions of the electric field strength, the dot size and its geometry are calculated. Our results show that the electronic states are strongly affected by the applied electric field, the size and the geometry of the dot.     

Excited states and multi-exciton complexes in single CdTe quantum dots

We present an optical study of excited states in single CdTe quantum dot (QDs). Using micro-photoluminescence excitation spectroscopy, absorption up to two confined excited levels have been observed in some dots. Power-dependent micro-photoluminescence is then used to study the occupation of excited states. The emission pattern is characteristic of the increase of the exciton number in the QD (shell-filling). A clear identification of the different multi-exciton complexes has been obtained in a highly symmetric dot. The evolution of the different multi-exciton intensities can then be reproduced by solving the rate equations for multi-exciton state occupancy and the fit by this simple model provides an estimate of the radiative lifetime of the different multi-exciton complexes.     

Far-infrared giant dipole resonances in neutral quantum dots

A resonance behaviour of the far-infrared absorption probability at a frequency N^1/4 is predicted for clusters of N electron–hole pairs (2N110) confined in disk-shaped quantum dots. For radially symmetric dots, the absorption is dominated by a giant dipole resonance, which accounts for more than 98% of the energy-weighted photoabsorption sum rule.     

Interband absorption and luminescence in small quantum dots under strong magnetic fields

Interband absorption and luminescence of quasi-two-dimensional, circularly symmetric, N_e-electron quantum dots are studied at high magnetic fields, 8B60 T, and low temperatures, T2 K. In the N_e=0 and 1 dots, the initial and final states of such processes are fixed, and thus the dependence on B of peak intensities is monotonic. For larger systems, ground state rearrangements with varying magnetic field lead to substantial modifications of the absorption and luminescence spectra. Collective effects are seen in the N_e=2 and 3 dots at “filling fractions” and .     

Dimensional effects and magnetic field influence on excitons in coupled quantum dots and coupled quantum wells

Direct and indirect excitons in coupled quantum wells and in coupled quantum dots are studied. We consider excitons with two-dimensional, quasi-two-dimensional and three-dimensional carriers. Problems were investigated for a wide range of characteristic parameters—confining to potential steepness, distances between quantum wells or dots, effective width of wells and magnetic fields. The mutual influence of the controlling parameters of the problem on exciton properties is analyzed. Energy and wave function spectra were calculated and dispersion law and effective masses were obtained.     

Exchange and Coulomb interactions of two electrons in double ellipsoidal quantum dots

The present study seeks to scrutinize the interactions of two electrons on the electronic properties of double ellipsoidal quantum dots (EQD). In this regard, the effective-mass approximation within a perturbation scheme is used and the Coulomb and exchange energies of the two electrons ellipsoidal quantum dot are calculated for GaAs/GaAlAs/AlAs structure. The results showed that the Coulomb and exchange energies depend not only on the thickness of the intermediate layer but also on the ellipticity constant.     

耦合锗量子点中空穴态对称特性研究

分别采用单带重空穴近似和六带Kronig-Penney模型, 对垂直耦合锗量子点在不同耦合距离下的空穴态特性进行了计算, 并探讨了自旋-轨道的相互作用对空穴态对称性的影响. 计算结果表明: 多带耦合的框架下, 随着量子点垂直间距的增大, 空穴基态从成键态转变为反键态, 而且价带基态能级和第一激发态能级发生反交叉现象, 这与单带模型下得到的相应结果存在较大差异. 通过分析六带模型计算得到的成、反键态波函数, 轻、重空穴态和自旋-轨道分裂态对特征空穴态波函数的贡献比例随着量子点垂直间距的增大发生了转变, 并最终导致量子点空穴基态波函数由成键态转变为反键态. 关键词： 耦合量子点 空穴态 成健态-反健态 自旋-轨道     

多孔硅量子点的受激辐射：陷阱态的作用

 经激光辐照和高温退火后能够在硅基上生成氧化多孔硅结构。用514 nm的激光泵浦,观测到该多孔硅的受激辐射。当激励强度超过阈值时,在650～750 nm区域有很强的受激发光峰。这些受激发光峰的半高宽小于0.5 nm。激光辐照和高温退火后,在样品上能形成某些特殊的氧化结构。在傅里叶红外光谱分析中,显示有硅氧双键或硅氧桥键在硅表面形成。计算结果表明：当硅氧双键或硅氧桥键形成时,电子的陷阱态出现在纳晶硅的带隙中。价带顶和陷阱态之间的粒子数反转是解释这种受激辐射的关键。     

Confined states in two-dimensional flat elliptic quantum dots and elliptic quantum wires

The energy spectrum and corresponding wave functions of a flat quantum dot with elliptic symmetry are obtained exactly. A detailed study is made of the effect of ellipticity on the energy levels and the corresponding wave functions. The analytical behavior of the energy levels in certain limiting cases is obtained.     

Contact correlations in one-dimensional systems of interacting fermions and the photoluminescence from quantum wires

We calculate the dependence of the carriers lifetime with the wire width in quantum wires by considering a strictly one-dimensional system of interacting electrons and holes. Confinement effects are taken into account through a width-dependent pair-potential proposed by Hu and Das Sarma. The carriers lifetime is then obtained from the inverse of the contact electron–hole correlations. We explain the change in the sign of the derivative at a critical temperature, as it is observed in photoluminescence experiments from In Ga As/InP quantum wires, by taking into account the carriers density dependence with temperature and assuming that the contact correlations are either just a two-body quantity or a many-body one for the lower and higher densities, respectively. In the former case, the system is viewed as an ionized excitonic gas, the pair correlation being the square of the two-body wave function for unbound states. In the latter, we have a metallic electron–hole system and we calculate the contact pair correlation in the many-body ladder approximation.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

多孔硅量子点的受激辐射：陷阱态的作用

经激光辐照和高温退火后能够在硅基上生成氧化多孔硅结构。用514 nm的激光泵浦,观测到该多孔硅的受激辐射。当激励强度超过阈值时,在650～750 nm区域有很强的受激发光峰。这些受激发光峰的半高宽小于0.5 nm。激光辐照和高温退火后,在样品上能形成某些特殊的氧化结构。在傅里叶红外光谱分析中,显示有硅氧双键或硅氧桥键在硅表面形成。计算结果表明：当硅氧双键或硅氧桥键形成时,电子的陷阱态出现在纳晶硅的带隙中。价带顶和陷阱态之间的粒子数反转是解释这种受激辐射的关键。     

Strain distribution and electronic states in stacked InAs/GaAs quantum dots with dot spacing 0–

We have calculated the strain distribution and electronic structures in stacked InAs/GaAs quantum dots (QDs) with the dot spacing 6–. We used the elastic continuum theory for the strain distribution, and the 8-band k·p theory for the electronic structures. For the triply stacked QDs, the light-hole (LH) component of the hole ground state increases with decreasing the dot spacing. The LH component in the columnar QD (dot spacing ) reaches 21.1% which is 4.8 times larger than that in the single QD due to the reduction of the biaxial strain. Further increase of the LH component (up to 28.6%) is obtained in the fivefold-stacked columnar QD. This result suggests a possibility of increase in the TM-mode transition in the columnar QDs.     

Effect of dielectric mismatch on impurity binding energy in double ellipsoidal quantum dots

The present study seeks to scrutinize the effect of polarization charges on the electronic properties of double ellipsoidal quantum dots. In this regard, the effective-mass approximation within a variational scheme is used and the binding energy of hydrogenic impurity located at the center of ellipsoidal quantum dot (EQD) is calculated for GaAs/GaAlAs/AlAs structure. The effect of surface polarization charges due to impurity and self-polarization charges on the binding energy is considered. The results showed that the binding energy depends not only on the thickness of the intermediate layer but also on the ellipticity constant.     

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.     

Influence of the quantum dot shape on the determination of the electronic structure and electron decoherence

Theoretical calculations of electron–phonon scattering rates in AlGaN/GaN quantum dots (QDs) have been performed by means of effective mass approximation in the frame of finite element method. The influence of a symmetry breaking of the carrier's wave function on the electron dephasing time is investigated for various QDs shapes. In a QD system the electron energy increases when the QD shape changes from a spherical to a non-spherical form. In addition, the influence of the QD shape upon the electronic structure can be modulated by external magnetic fields. We also show that the electron–acoustic phonon scattering rates strongly depend upon both the QD shape and the applied magnetic field. As an additional parameter, the QD shape can be used to modify the electron–acoustic phonon interaction in a wide range. Moreover, the scattering rate of different transitions, such as Δm=0(1), presents distinct magnetic field dependency.     

Electron–hole complexes in self-assembled quantum dots in strong magnetic fields

We present results of calculations and experiments on electron–hole complexes in InGaAs/GaAs self-assembled quantum dots in high magnetic field (B). Due to hidden symmetries, the chemical potential of an N-exciton system at special B fields becomes insensitive to the exciton number as well as the magnetic field. This results in plateau regions of high intensity in measured magneto-PL spectrum. Theoretical calculations using exact diagonalization techniques successfully explain the measured magneto-photoluminescence spectrum with B fields up to 28 T.     

Transport through quasi-degenerate states in coupled quantum dots

By means of a general method for treating mesoscopic systems with strong internal correlations, transport properties through a set of quasi-degenerate transitions in the interacting region, or active element (AE), are considered. It is shown that the behaviour of the AE drastically changes as the couplings to the contacts are varied from the strong to the weak coupling limit. These changes strongly influence the transport properties of the system, from a single increase of the current to a staircase form with unequally large steps. In the present study, kinematic interactions, non-equilibrium populations numbers and dependence on the bias voltage has been included in the treatment of the local properties of the AE. Analytical results for the equilibrium situation are presented as well as a derivation of the corresponding non-equilibrium quantities. Results from self-consistent numerical calculations of the considered case are presented.     

Millisecond fluorescence in InAs quantum dots embedded in AlAs

The temperature dependence of steady-state and time-resolved photoluminescence from self-assembled InAs quantum dots embedded in AlAs has been studied. Millisecond-long nonexponential photoluminescence decay is observed in the temperature range of 4.2–50 K. At higher temperatures, the decay time decreases to a few nanoseconds. The experimental results are interpreted using a model of singlet–triplet splitting of exciton levels in small dots in a dense quantum dot system with local carrier transfer between dots.