The effect of the long-range order in a quantum dot array on the in-plane lattice thermal conductivity

Semiconductor quantum dot superlattices consisting of arrays of quantum dots have shown great promise for a variety of device applications, including thermoelectric power generation and cooling. In this paper we theoretically investigate the effect of long-range order in a quantum dot array on its in-plane lattice thermal conductivity. It is demonstrated that the long-range order in a quantum dot array enhances acoustic phonon scattering and, thus leads to a decrease of its lattice thermal conductivity. The decrease in the ordered quantum dot array, which acts as a phonon grating, is stronger than that in the disordered one due to the contribution of the coherent scattering term. The numerical calculations were carried out for a structure that consists of multiple layers of Si with layers of ordered Ge quantum dots separated by wetting layers and spacers.     

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.     

Self-assembling of Ge quantum dots in the CaF<Subscript>2</Subscript>/Ge/CaF<Subscript>2</Subscript>/Si heteroepitaxial system and the development of tunnel-resonance diode on its basis

A CaF₂/Ge/CaF₂/Si(111) heteroepitaxial structure with Ge quantum dots was grown by molecular-beam epitaxy. A negative differential conductivity and conductivity oscillations caused by resonant hole tunneling were observed at room temperature. The energy spacing between the levels in quantum dots, as determined from the oscillation period, is 40–50 meV depending on the Ge dot size.     

Infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices

We report the study of infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices. The superlattices, which were grown on (001) oriented Si substrates by a solid source molecular beam epitaxy system, are composed mainly of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of them and of the uncapped Ge dots grown on the surfaces of some of them were tested by cross-sectional transmission electron and atomic force microscopes, respectively. It is found that the Ge quantum dots have flat lens-like shapes. Infrared absorption signals peaking in the mid-infrared range were observed using Fourier transform infrared and Raman scattering spectroscopy techniques. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions within the valence band of the Ge quantum dots in the superlattices. The fact that the intraband absorption is strongly polarized along the growth axis of the superlattices signifies that the Ge quantum dots with flat lens-like shapes perform as Ge/Si-based quantum wells. This study demonstrates the application potential of these kinds of Ge/Si quantum dot superlattices for developing mid-infrared photodetectors.     

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.     

Heterostructures with self-organized quantum dots of Ge on Si for optoelectronic devices

In this paper an analysis of tendencies of Ge on Si quantum dots nanoheterostructures’ usage in different optoelectronic devices such as, for example, solar cells and photodetectors of visible and infra-red regions is carried out; a complex mathematical model for calculation of dependency on growth conditions of self-organized quantum dots of Ge on Si grown using the method of molecular beam epitaxy parameters is described. Ways of segregation effect and underlying layers’ influence are considered. It is shown that for realization of good device characteristics quantum dots should have high density, small sizes, uniformity, and narrow size distribution function. The desirable parameters of arrays of square and rectangular quantum dots for device application are attainable under certain growth conditions.     

水溶性CdTe:Mn量子点的双光子吸收

采用激发波长800 nm、脉宽50 fs、重复频率1 kHz的Ti:sapphire放大飞秒激光器作为激发光源,利用开孔Z扫描技术研究了不同粒径的CdTe:Mn量子点的非线性吸收性质。理论计算结果表明,同一生长时间CdTe:Mn量子点的双光子吸收系数是CdTe量子点的1.1倍,其双光子吸收系数随量子点尺寸的减小而增大,这是由于CdTe:Mn量子点非线性吸收属于反饱和吸收,掺杂了Mn元素,减小了表面缺陷浓度,表明掺杂量子点具有很好的双光子吸收现象。     

Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.     

Excitons in Ge/Si double quantum dots

The spatial structure of excitons and the oscillator strength characterizing the intensity of interband optical transitions in vertically coupled Ge/Si quantum dots have been theoretically studied. It has been found that the probability of the exciton transition under certain conditions (the sizes of the quantum dots, the separation of the dots) can be much larger (up to a factor of 5) than the value for the case of single quantum dots. It is expected that the results will make it possible to approach the creation of efficient light-emitting and photoreceiving devices based on Si and Ge indirect-band semiconductors.     

Charge accumulation in Ge quantum dots in a GaAs/ZnSe/QD-Ge/ZnSe/Ge floating gate transistor structure

A GaAs/ZnSe/QD-Ge/ZnSe/Ge germanium-quantum-dot floating-gate transistor structure is obtained and investigated by molecular beam epitaxy. It has been shown that a positive change in the channel current is observed upon light illumination with wavelengths longer than 0.5 μm and a negative change is observed for shorter wavelengths, which is associated with charging of quantum dots. Measurements of relaxation curves after switching off the illumination show that the decay of the current lasts for from tens of seconds to several hours, depending on the temperature of the sample. The changes in the channel current and relaxation curves indicated above are explained based on the existence of three types of transitions in quantum dots upon radiation absorption with allowance made for the variation of the channel state near the heteroboundary from depletion to inversion as a result of charge accumulation in quantum dots.     

Ge quantum dot molecules and crystals: Preparation and properties

Templated self-organization has been used to prepare two-dimensional arrays as well as three-dimensional quantum dot crystals (QDC) containing Ge dots in a Si host crystal. Si(1 0 0) substrates have been patterned with two-dimensional hole gratings using extreme ultra-violet interference lithography (EUV-IL) and reactive ion etching. The EUV-IL was realized by multiple beam diffraction using Cr gratings on SiN_x membranes fabricated by e-beam lithography. Si/Ge overgrowth was performed by molecular beam epitaxy. The impact of the microscopic shape and size of the prepattern using the mask design and the EUV-IL exposure dose as parameters on the Ge dot nucleation has been studied with atomic force microscopy, transmission electron microscopy and photoluminescence measurements. Adjusting the growth parameters in multiple layer deposition the initial two-dimensional configuration was transferred into three-dimensional QDC.     

Raman study of Si–Ge intermixing in Ge quantum rings and dots

The Ge/Si (1 0 0) nanostructures have been studied by atomic force microscopy (AFM) and Micro Raman optical spectroscopy. Two layers of Ge of total thickness 0.75 nm and Si cap with thickness 2.5 nm were deposited by the method of molecular beam epitaxy at the temperature range 640–700 °C. AFM shows both quantum dots and ring-shape Ge nanostructures. From the analysis of the intensity and energy shift of the Raman signal we have found that the average concentration of Ge decreases considerably from 44% to 27%, when the growth temperature increases, whereas the degree of strain relaxation remains roughly the same. This allows us to conclude that intermixing is a dominating mechanism for strain relaxation in processes of transformation of Ge quantum dots to quantum rings.     

Spatial separation of electrons in Ge/Si(001) heterostructures with quantum dots

It is shown experimentally that the excitation of interband optical transitions in arrays of Ge/n-Si(001) quantum dots leads to a decrease in the concentration of electrons in the conduction band. The phenomenon observed is due to the formation of negatively charged exciton complexes in Ge islands and represents the first experimental confirmation of the spatial separation of electrons in the silicon matrix surrounding the islands.     

Synchrotron investigations of the specific features in the electron energy spectrum of silicon nanostructures

The Si L _{2, 3} x-ray absorption near-edge structure (XANES) spectra of porous silicon nanomaterials and nanostructures with epitaxial silicon layers doped by erbium or containing germanium quantum dots are measured using synchrotron radiation for the first time. A model of photoluminescence in porous silicon is proposed on the basis of the results obtained. According to this model, the photoluminescence is caused by interband transitions between the energy levels of the crystalline phase and oxide phases covering silicon nanocrystals. The stresses generated in surface silicon nanolayers by Ge quantum dots or clusters with incorporated Er atoms are responsible for the fine structure of the spectra in the energy range of the conduction band edge and can stimulate luminescence in these nanostructures.     

Intraband absorption and emission of light in quantum wells and quantum dots

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.     

Absorption spectra of small semiconductor quantum dots

A density matrix approach has been employed to study analytically the absorption spectra of small semiconductor quantum dots under the strong confinement regime. The results are obtained for a single quantum dot (SQD) as well as for inhomogeneous distribution of quantum dots (IQDs) with Gaussian distribution of quantum dot sizes. A numerical analysis has been made for a SQD and IQDs in a CdS crystal with data taken from recent experimental work. A negative change in the absorption coefficient occurs in the shorter pump wavelength side of the spectrum due to the biexcitonic contribution. The wavelength at which crossover from positive to negative values of the change in absorption coefficient occurs is found to depend upon both the QD size as well as the excitation intensity. The results agree satisfactorily with the experimental observations in small CdS quantum dots.     

Hall effect in hopping conduction in an ensemble of quantum dots

The Hall effect in heterostructures with a two-dimensional array of tunneling-coupled Ge quantum dots grown by molecular-beam epitaxy on Si is investigated. The conductivity of these structures in zero magnetic field at 4.2 K varies in the range of 10^?12?10^?4 Ω^?1, which includes both the diffusive transport under weak localization conditions and hopping conduction. It is shown that the Hall effect can be discerned against the magnetoresistance-related background in both high- and low-conductivity structures. The Hall coefficient in the hopping regime exhibits a nonmonotonic dependence on the occupancy of quantum dots by holes. This behavior correlates with that of the localization length of the hole wavefunctions.     

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.     

Barrier height and tunneling current in Schottky diodes with embedded layers of quantum dots

Electrical characteristics of silicon Schottky diodes containing Ge quantum dot (QD) arrays are investigated. It has been found that the potential barrier height at the metal-semiconductor contact can be controlled by introducing dense QD layers, which is a consequence of the formation of a planar electrostatic potential of charged QDs. When the applied voltage is varied, the ideality factors of Schottky barriers exhibit oscillations due to the tunneling of holes through discrete levels in quantum dots.     

Role of dynamic depolarization for the intersubband resonance in the localization regime

We investigate the electronic intraband absorption in quantum wells with a strong lateral random potential, realized for example by modulation doping with a thin spacer layer. In such systems, electrons become in-plane localized in isolated potential minima and behave like an inhomogeneous array of natural quantum dots. When excited with a coherent light field, the dots respond as individual oscillators, which are however coupled by dynamic dipole–dipole interactions. The absorption spectrum is then determined by the interplay of the single dot properties (related to the disorder potential) and the many-particle Coulomb interactions. Using a simple model for the single-particle states, we calculate the absorption spectrum as a function of the electron density. In the case of light polarized perpendicular to the layer, we find with increasing density a dramatic line narrowing (associated with a collective excitation of the electrons) and a depolarization blue shift. For in-plane polarized light, the peak is shifted to the red. Our theory also applies to far-infrared absorption experiments in artificial quantum dot arrays.