Electronic Structure of Semiconductor Nanocrystals

This paper reviews our recent development of the use of the large-scale pseudopotential method to calculate the electronic structure of semiconductor nanocrystals, such as quantum dots and wires, which often contain tens of thousands of atoms. The calculated size-dependent exciton energies and absorption spectra of quantum dots and wires are in good agreement with experiments. We show that the electronic structure of a nanocrystal can be tuned not only by its size,but also by its shape. Finally,we show that defect properties in quantum dots can be significantly different from those in bulk semiconductors.     

Local tunneling spectroscopy of silicon nanostructures

The recharging of many-hole and few-electron quantum dots under the conditions of the ballistic transport of single charge carriers inside self-assembled quantum well structures on a Si (100) surface are studied using local tunneling spectroscopy at high temperatures (up to room temperature). On the basis of measurements of the tunneling current-voltage characteristics observed during the transit of single charge carriers through charged quantum dots, the modes of the Coulomb blockade, Coulomb conductivity oscillations, and electronic shell formation are identified. The tunneling current-voltage characteristics also show the effect of quantum confinement and electron-electron interaction on the characteristics of single-carrier transport through silicon quantum wires containing weakly and strongly coupled quantum dots.     

Inelastic light scattering from electronic excitations in deep-etched quantum dots and wires

Resonant Raman spectroscopy of modulation-doped GaAs/AlGaAs multiple quantum well dots and wires is reported. Deep etching with a SiCl₄ reactive ion etching process achieved an excellent aspect ratio (>10:1) and low surface damage for dots and wires of sizes in the range 60–250 nm. A rich spectrum of single particle excitations was observed at Raman shifts in the range 1–35 meV for both dots and wires. Sharp resonances were found for the Raman intensities. The electronic scattering in wires exhibits distinct polarization properties in agreement with theoretical predictions and the spin density excitation energies are in reasonable agreement with Hartree approximation calculations. The dispersion of the intrasubband plasmon collective mode in 60 nm wires has been determined. The excitations in dots show a systematic shift to higher energy with decreasing dot diameter consistent with increased confinement. Magneto-Raman scattering from dot samples was also investigated at magnetic fields up to 12 T and the excitation spectra show level splitting, level crossing and mode softening with increasing magnetic field.     

Polarization dependence of optoelectronic properties in quantumdots and quantum wires-consequences of valence-band mixing

The electronic bandstructure in quantum dots and quantum wires is studied, including the valence-band mixing effects. Based on the bandstructure, results for the polarization dependence of absorption and gain (at room temperature) are examined. The related differential gain and the α-parameter is also studied. Our results show that it is extremely important to include valence-band mixing effects in predicting the laser performance of such structures     

Lateral ordering of quantum dots and wires in the (In,Ga)As/GaAs(100) multilayer structures

The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the \([0\bar 11]\) direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed.     

Charging energy and spin polarization in artificial atoms and molecules

We investigate the electronic properties of single and coupled quantum dot systems by a self-consistent solution of Schrödinger and Poisson equations within the density functional theory. The single and coupled quantum dots show remarkable similarities to atoms and molecules. We observe that in the case of single quantum dots with cylindrical symmetry, the electrons in the dot form shells like in atoms. This sheel structure is slightly distorted due to the electron-electron interaction, as the number of electrons, N, increases. In the case of coupled quantum dots, we observe that the dots can be driven from a state wherein the individual dots are separate, akin to two isolated atoms, to one in which the dots couple forming an “artificial molecule.” By using the local spin density approximation, we observe spin polarization in the double dot for specific values of N.     

InP基应变自组装纳米材料及其光电子器件研究进展

半导体低维结构材料,如量子线(点)材料,由于其特殊的电子结构,在新一代光电子、微电子器件中有着重要的应用价值。本文对应变自组装InP基量子线(点)材料的生长制备、光学和电学特性及其在半导体激光器、红外探测器及其他光电子和微电子器件中的应用进行了综述,指出了目前需要改进的一些方面,并提出了一些相应的解决途径。     

Formation of InP-based quantum structures by selective MBE on patterned substrates having high-index facets

In view of applications to next-generation electronics based on quantum devices, this paper presents and discusses the results of a systematic study recently done by the authors' group on the growth conditions, characterization and device application of InP-based In_0.52Al_0.48As/In_0.53Ga_0.47As quantum wires and dots formed by selective M13E growth on mesa-patterned (001) InP substrates. First, selective MBE growth experiments of InAlAs/InGaAs wire and dot structures on patterned (001) InP substrates are discussed generally. Then, particular attention is paid to successful growth conditions and properties of (110) oriented InAlAs/InGaAs ridge quantum wires (QWRs), including the effect of misorientation of mesa stripes. Finally, device related issues such as successful surface passivation by a technique using a silicon interface control layer (Si ICL) and electronic transport characterization by QWR transistors are discussed. The QWR transistor exhibited excellent gate-controlled one-dimensional transport with the appearance of clear conductance oscillations near pinch-off, visible up to about 50K.     

Generation and Concentration of Terahertz Radiation in a Microcavity with an Open Quantum Dot

A scheme to generate terahertz radiation by an array of quantum dots localized at the center of a semiconducting heterostructure is developed. Electrons are injected into the active part due to the source and drain Fermi energy difference induced by a dc electric field. The structure is placed inside a microcavity stimulating the electronic transition in the quantum dots accompanied by the emission of a photon to the cavity mode. This process is optimized using the filters formed by the quantum wells, which facilitate the electron density concentration in a quantum dot. The electromagnetic field radiated by the cavity in the waveguide can be used for a local effect on the charge qubits. The parameters of such a source depend on the working characteristics of the quantum dots and cavity.     

Optical properties of supercrystals

We present a systematic study of the dependence of the optical properties on the geometry of the quantum dots, which form a quantum dots chain. We obtain electronic structure of 1D supercrystals by using a tight-binding method based on the s and p orbitals and wave functions of the quantum dots for the on-site parameters. For the hopping parameter we choose the extended Hückel method. We find a drastic dependence on this parameter especially on the electronic band structure. We also relate the tight-binding matrix elements with the momentum matrix elements taking the k-space gradient of the Hamiltonian for calculating the dielectric spectra of supercrystals made of cylindrical, spherical, and conical quantum dots. We compare the optical behavior of the chains in the three geometries and report anomalies for the conical quantum dots chain.     

Si/SiO2Structures with Quantum Size Effects: The Construction of a Low-Dimensional Nanoscale Electronic System in the Interface Layer of Si by Incorporating a Regularly Distributed Charge into SiO2

A novel approach to producing long-lived, reconfigurable, self-organized nanoscale electronic systems of a low dimension (quantum wells, wires, dots, superlattices, etc.) in a semiconductor is discussed. This approach consists of creating a regular charge distribution in an adjacent dielectric, so that a two-dimensional potential distribution is induced in the interface layer of the semiconductor. Advanced methods to generate a desired charge distribution in dielectrics are reviewed. They use (1) the local injection of electrons or ions into a thin dielectric layer by a precisely focused beam, (2) the local electronic or ionic polarization of dielectrics with a scanning tunneling microscope, or (3) nanoscale charge patterns that can be spontaneously formed by electrons or ions in dielectrics. The possible applications of the approach are considered in the context of the Si/SiO₂system.     

CdSe量子点的制备与荧光特性研究

主要讨论了CdSe量子点的制备及荧光特性。CdSe量子点由化学方法制备，通过选择不同的反应时间得到不同尺度的量子点样品。用荧光方法研究了量子点样品在石英衬底和有机溶剂中的荧光特性。实验表明，这些量子点都有良好的荧光特性。还用无限深球方势阱模型分析了量子点样品的电子态，并根据荧光参数估算了量子点的尺度．各样品荧光峰具有一致的半峰宽，表明CdSe量子点的成核过程在反应开始时同时完成。     

Photoconductivity of Si/Ge multilayer structures with Ge quantum dots pseudomorphic to the Si matrix

Longitudinal photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots grown pseudomorphically to the Si matrix are studied. Lines of optical transitions between hole levels of quantum dots and Si electronic states are observed. This allowed us to construct a detailed energy-level diagram of electron-hole levels of the structure. It is shown that hole levels of pseudomorphic Ge quantum dots are well described by the simplest “quantum box” model using actual sizes of Ge islands. The possibility of controlling the position of the long-wavelength photosensitivity edge by varying the growth parameters of Si/Ge structures with Ge quantum dots is determined.     

Facetting of the self-assembled droplet epitaxial GaAs quantum dot

In this work, droplet epitaxially grown GaAs quantum dots on AlGaAs surface are studied. The quantum dots are investigated in situ with RHEED and ex situ with TEM method. The TEM picture shows that the quantum dot is perfectly crystalline and fits very well to the crystal structure of the substrate. Furthermore, the side of the quantum dot shows stepped facet shape. Here, we show, how the stepped side shape forms from the droplet during crystallization. The RHEED picture shows broadened chevron-tail, which can be explained by the shape of the quantum dot.     

Synthesis and characterization of ZnS-based quantum dots to trace low concentration of ammonia

In the present work,a solution-based co-precipitation method has been adopted to synthesize pure and cobalt-doped ZnS quantum dots and characterized by XRD,SEM,TEM with EDX,FTIR and gas sensing properties.XRD analysis has shown a single phase of ZnS quantum dots having a zinc blend structure.TEM and XRD line broadening indicated that the aver-age crystallite size in the sample is in the range of 2 to 5 nm.SEM micrographs show spherical-shaped quantum dots.FTIR stud-ies show that cobalt has been successfully doped into the ZnS cubic lattice.EDX spectra have analyzed the elemental pres-ence in the samples and it is evident that the spectra confirmed the presence of cobalt (Co),zinc (Zn),oxygen (O),and sulphur(S) elements only and no other impurities are observed.The ZnS-based quantum dot sensors reveal high sensitivity towards 50 ppm of ammonia vapors at an operating temperature of 70 ℃.Hence,ZnS-based quantum dots can be a promising and quick traceable sensor towards ammonia sensing applications with good response and recovery time.     

Land of Ancient myths,super-microchips & quantum devices part I

Semiconductor companies in Japan are making major investments in quantum devices technology. The interest in quantum devices by Japanese firms is largely due to the commercial success of the HEMT, and quantum well lasers in satellite broadcasting and optical communications. The key issue all researcher face is: how to fashion these novel materials, quantum wells, wires and dots into working devices. In this article I will show how Japanese workers are very serious about quantum devices, warn that European and American manufacturers are in real danger of being left out by their failure to back fundamental research, and report on the research activities of the laboratories I visited.     

Land of ancient myths,super-microchips & quantum devices part II

Semiconductor companies in Japan are making major investments in quantum devices technology. The interest in quantum devices by Japanese firms is largely due to the commercial success of the HEMT, and quantum well lasers in satellite broadcasting and optical communications. The key issue all researchers face is: how to fashion these novel materials, quantum wells, wires and dots into working devices. In this two-part article I show how Japanese workers are very serious about quantum devices, warn that European and American manufacturers are in real danger of being left out by their failure to back fundamental research, and report on the research activities of the laboratories I visited.     

Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.     

Semiconductor science: the fourth generation

The author reviews work on quantum wires and quantum dots in the context of a generational framework. The first generation centered on the quantum physics of solids. The second generation saw the commercial and technical application of these new physical concepts. The third generation yielded the creation of man-made quantum systems and low-dimensional systems: inversion layers and quantum wells, superlattices, quantum wires, and quantum dots. The author discusses work on low-dimensional systems as a part of this progression. There is a clear connection between the work done during the first generation and the creation of new semiconductor structures, novel band structures, and the associated fundamental excitations. He also discusses the current directions in semiconductor research and speculates on the potential applications of semiconductor physics in the fourth generation     

Lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots

Spectra of lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots, fabricated by molecular-beam epitaxy are studied. The photoresponse caused by optical transitions between hole levels of quantum dots and Si electronic states was observed in the energy range of 1.1–0.3 eV at T = 78 K. It was shown that the electronic states localized in the region of Si band bending near the Ge/Si interface mainly contribute to lateral photoconductivity. The use of the quantum box model for describing hole levels of quantum dots made it possible to understand the origin of peaks observed in the photoconductivity spectra. A detailed energy-level diagram of hole levels of quantum dots and optical transitions in Ge/Si structures with strained Ge quantum dots was constructed.