Quantization of the electronic spectrum and localization of electrons and holes in silicon quantum dots

The quantization of the electronic spectrum has been observed in photoluminescence experiments for silicon quantum dots prepared by implantation of silicon into silicon dioxide SiO2. The diameter of silicon quantum dots has been estimated as 1.8 nm. Injection of electron and holes is accompanied by the appearance of a paramagnetic resonance signal with the g factor equal to 2.006. This result unambiguously indicates that silicon clusters are the electron and hole traps in SiO2.     

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the quantum dots and the itinerant holes in the semiconductor valence band makes possible hole transfer between the DMS quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.Received: 17 Mars 2003, Published online: 30 January 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.30.Ds Spin waves - 75.50.Dd Nonmetallic ferromagnetic materials - 75.50.Pp Magnetic semiconductors     

Spin interactions,relaxation and decoherence in quantum dots

We review recent studies on spin decoherence of electrons and holes in quasi-two-dimensional quantum dots, as well as electron-spin relaxation in nanowire quantum dots. The spins of confined electrons and holes are considered major candidates for the realization of quantum information storage and processing devices, provided that sufficiently long coherence and relaxation times can be achieved. The results presented here indicate that this prerequisite might be realized in both electron and hole quantum dots, taking one large step towards quantum computation with spin qubits.     

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.     

Landé factors and orbital momentum quenching in semiconductor quantum dots

We show that electron and hole Landé g factors in self-assembled III-V quantum dots have a rich structure intermediate between that of paramagnetic atomic impurities and bulk semiconductors. Strain, dot geometry, and confinement energy modify the effective g factors, yet are insufficient to explain our results. We find that the dot's discrete energy spectrum quenches the orbital angular momentum, pushing the electron g factor towards 2, even when all the materials have negative bulk g factors. The approximate shape of a dot can be determined from measurements of the g factor asymmetry.     

Prediction of an excitonic ground state in InAs/InSb quantum dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.     

Hopping photoconduction and its long-time kinetics in a heterosystem with Ge quantum dots in Si

The effect of interband-transition-inducing illumination on the hole hopping conduction along a two-dimensional array of Ge quantum dots in Si was studied. It is found that the photoconductance has either positive or negative sign depending on the initial filling of quantum dots with holes. In the course of illumination and after switching off the light, long-time photoconduction kinetics was observed (10²?10⁴s at T=4.2 K). The results are discussed in terms of a model based on the spatial separation of nonequilibrium electrons and holes in a potential relief formed by positively charged dots. The effect of equalization of potential barrier heights as a result of photohole capture by the charged quantum dots during the process of illumination and relaxation is suggested as an additional factor for explaining the phenomenon of persistent conduction.     

Coulomb binding of electrons to multiply charged GaSb/GaAs self-assembled quantum dots

We explore the Coulomb binding of electrons to holes confined to type-II GaSb self-assembled quantum dots. We demonstrate that at low laser power electrons are more weakly bound to holes trapped by the dots than to holes in the wetting layer. On the other hand, at high laser power the hydrogenic binding energy of dot excitons increases by more than a factor of two, and so exceeds that of wetting layer excitons. We attribute this to the strong binding of ‘core’ electrons to dots that are highly charged with holes by optical pumping.     

基于应变锗空穴双量子点的可调耦合和自旋阻塞研究

应变锗空穴量子点是实现超大规模量子计算最有前景的平台之一.由于锗空穴不受超精细相互作影响，有着较长的自旋弛豫时间和量子退相干时间，且锗中本征的强旋轨道耦合和空穴载流子的低有效质量，使得全电场操控空穴自旋量子比特得以实现，极大地降低了器件加工难度，增加了量子点的可扩展性.本文介绍了一种使用应变锗异质结制备重叠栅空穴双量子点器件的方法，完成了应变锗异质结性质测量，空穴双量子点器件制作，单量子点输运性质和双量子点输运性质研究，双量子点耦合可研究调节性研究，以及外磁场存在下的漏电流性质研究和泡利自旋阻塞解除机制的研究.这些工作为未来实现高质量自旋量子比特制备和高保真度量子逻辑门操控提供了实验平台和基本参数.     

Phononless hopping conduction in two-dimensional layers of quantum dots

Regularities are studied in charge transport due to the hopping conduction of holes along two-dimensional layers of Ge quantum dots in Si. It is shown that the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the effective localization radius of charge carriers in quantum dots varies nonmonotonically upon filling quantum dots with holes, which is explained by the successive filling of electron shells. The preexponential factor of the hopping conductivity ceases to depend on temperature at low temperatures (T<10 K) and oscillates as the degree of filling quantum dots with holes varies, assuming values divisible by the conductance quantum e²/h. The results obtained indicate that a transition from phonon-assisted hopping conduction to phononless charge transfer occurs as the temperature decreases. The Coulomb interaction of localized charge carriers has a dominant role in these phononless processes.     

Exciton states and optical absorption in quantum wires under laser radiation

We analyze the exciton states in a quantum wire under intense laser radiation. Electrons and holes are confined by the parabolic potential of the quantum wire. An exactly solvable model is introduced for calculating the exciton binding energy, replacing the actual Coulomb interaction between the electron and the hole by a projective operator.     

Selective enhancement of the hole photocurrent by surface plasmon–polaritons in layers of Ge/Si quantum dots

It is found that the integration of Ge/Si heterostructures containing layers of Ge quantum dots with twodimensional regular lattices of subwave holes in a gold film on the surface of a semiconductor leads to the multiple (to 20 times) enhancement of the hole photocurrent in narrow wavelength regions of the mid-infrared range. The results are explained by the light wave excitation of the surface plasmon–polaritons at the metal–semiconductor interface effectively interacting with intraband transitions of holes in quantum dots.     

Spectrum of electron-hole states of the Si/Ge structure with Ge quantum dots

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots of various sizes are investigated. We observed optical transition lines between the hole levels of quantum dots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantum dots are successfully described by the “quantum box” model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantum dots can be controlled by changing the growth parameters.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.     

Anomalous quantum-confined Stark effects in stacked InAs/GaAs self-assembled quantum dots

Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs) are predicted to exhibit strong hole localization even with vanishing separation between the dots, and a nonparabolic dependence of the interband transition energy on the electric field, which is not encountered in single SAD structures. Our study based on an eight-band strain-dependent k x p Hamiltonian indicates that this anomalous quantum confined Stark effect is caused by the three-dimensional strain field distribution which influences drastically the hole states in the stacked SAD structures.     

Enhancement of the hole photocurrent in layers of Ge/Si quantum dots with abrupt heterointerfaces

The photoconductive gain, hole photocurrent spectra in the mid-infrared range, and band-to-band photoluminescence spectra in arrays of Ge/Si quantum dots with different elemental compositions of the heterointerface are measured. The diffusive mixing of the materials of the matrix and the dots is controlled by varying the temperature at which the Ge layers are overgrown with Si. It is found that the formation of abrupt heterointerfaces leads to the enhancement of the hole photocurrent and quenching of photoluminescence. The results are explained by an increase in the lifetime of nonequilibrium holes owing to the suppression of their capture into the bound states of quantum dots.     

Quantum dots formed in InSb/AlAs and AlSb/AlAs heterostructures

The crystal structure of new self-assembled InSb/AlAs and AlSb/AlAs quantum dots grown by molecularbeam epitaxy has been investigated by transmission electron microscopy. The theoretical calculations of the energy spectrum of the quantum dots have been supplemented by the experimental data on the steady-state and time-resolved photoluminescence spectroscopy. Deposition of 1.5 ML of InSb or AlSb on the AlAs surface carried out in the regime of atomic-layer epitaxy leads to the formation of pseudomorphically strained quantum dots composed of InAlSbAs and AlSbAs alloys, respectively. The quantum dots can have the type-I and type-II energy spectra depending on the composition of the alloy. The ground hole state in the quantum dot belongs to the heavy-hole band and the localization energy of holes is much higher than that of electrons. The ground electron state in the type-I quantum dots belongs to the indirect X_XY valley of the conduction band of the alloy. The ground electron state in the type-II quantum dots belongs to the indirect X valley of the conduction band of the AlAs matrix.     

Influence of the thickness of CdTe and ZnTe layers on cathodoluminescence spectra of strained CdTe/ZnTe superlattices with quantum-dot layers

The influence of the thickness of ZnTe barrier layers on the cathodoluminescence spectra of strained CdTe/ZnTe superlattices containing layers of quantum dots with an average lateral size of approximately 3 nm has been investigated. In samples with thick barrier layers (30, 15 nm), the cathodoluminescence spectra of quantum dots exhibit one band with a maximum at E = 2.03 eV. It has been revealed that, at a barrier layer thickness of ∼3 nm, the luminescence band is split. However, at a ZnTe layer thickness of 1.5 nm, the luminescence spectrum also contains one band. The experimental results have been interpreted with allowance made for the influence of elastic biaxial strains on the energy states of light and heavy holes in the CdTe and ZnTe layers. For the CdTe/ZnTe heterostructure with quantum dots in which the thickness of the deposited CdTe layer is 1.5 monolayers and the thickness of the barrier layer is 100 monolayers, the cathodoluminescence spectrum contains 2LO-phonon replicas. This effect has been explained by the resonance between two-phonon LO states and the difference between the energy states in the electronic spectrum of wetting layer fragments.     

Hole states in artificial molecules formed by vertically coupled Ge/Si quantum dots

In the tight binding approximation, the spatial configuration of the ground state and the binding energy of a hole in a “diatomic” artificial molecule formed by vertically coupled Ge/Si(001) quantum dots are studied. The inhomogeneous spatial distribution of elastic strain arising in the medium due to the lattice mismatch between Ge and Si is taken into account. The strain is calculated using the valence-force-field model with a Keating interatomic potential. The formation of the hole states is shown to be determined by the competition of two processes: the appearance of a common hole due to the overlapping of “atomic” wavefunctions and the appearance of asymmetry in the potential energy of a hole in the two quantum dots because of the superposition of the elastic strain fields from the vertically aligned Ge nanoclusters. When the thickness of the Si layer separating the Ge dots (t _Si) is greater than 2.3 nm, the binding energy of a hole in the ground state of the two-dot system proves to be lower than the ionization energy of a single quantum dot because of the partial elastic stress relaxation due to the coupling of the quantum dots and due to the decrease in the depth of the potential well for holes. For the values of the parameter t _Si, an intermediate region is revealed, where the covalent molecular bond fails and the hole is localized in one of the two quantum dots, namely, in the dot characterized by the highest strain values.     

Antibonding ground state of holes in double vertically coupled Ge/Si quantum dots

The existence of the antibonding ground state of holes in artificial molecules, which are formed by the vertically coupled Ge/Si quantum dots, has been proved experimentally. This phenomenon is absent in natural molecules and double quantum dots containing electrons. It is a consequence of spin-orbit interaction and deformation effects in the valence band of vertically aligned quantum dots.