Quantum wires and quantum dots defined by lithography with an atomic force microscope

Semiconductor nanostructures are realized by patterning AlGaAs/GaAs heterostructures with an atomic force microscope. Steep potential walls, precise pattern transfer and a combination of in-plane and top gates can be achieved with this technique. The electronic properties of nanostructures defined in this way are discussed on two examples, namely a quantum point contact and a single electron transistor. For the quantum point contact we demonstrate quantized conductance at temperatures of 4 K and above. This indicates the strong confinement energy in this system. For the single electron transistor, the realization of special potential shapes and the observation of high-quality Coulomb blockade is shown.     

Quantum dots top the pyramid in Egypt

From self-assembly to FETs, micromachining to transport in superlattices and quantum wires, the 11th ‘International Conference on Superlattices, Microstructures and Microdevices’ (ICSMM-11) covered a wealth of topics. The meeting was held in Hurgada, Egypt from 27–31 July 1998 and attracted more than 100 participants from some 20 countries.     

Quantum dots keep their distance

Probes developed by a team of researchers at Sandia Laboratories have enabled real-time observation of the formation of quantum dots, revealing that the dots repel each other. Quantifying the repulsion may help in turning assemblages of quantum dots into future solid-state lasers.     

On way to ideal quantum dots

Temperature and concentration self-assembling conditions of tetrahedral 1Sb4Ga clusters in multi-valley zinc blende AlN:(Ga, Sb) in the ultra dilute Sb limit are represented. Tetrahedral clusters should occur as a result of the second-order transition. The final stage of self-assembling when all Sb atoms are in 1Sb4Ga clusters also has to be reached as a result of the second-order transition at finite temperatures.     

Polarization of the photoluminescence of quantum dots incorporated into quantum wires

The photoluminescence spectra of individual quantum dots incorporated into a quantum wire are studied. From the behavior of the spectra in a magnetic field, it is possible to estimate the exciton binding energy in a quantum dot incorporated into a quantum wire. It is found that the exciton photoluminescence signal emitted from a quantum dot along the direction of the nanowire axis is linearly polarized. At the same time, the photoluminescence signal propagating in the direction orthogonal to the nanowire axis is practically unpolarized. The experimentally observed effect is attributed to the nonaxial arrangement of the dot in the wire under conditions of a huge increase in the exciton binding energy due to the effect of the image potential on the exciton.     

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.     

Electron pumping through quantum dots defined in parallel etched quantum wires

We present the first results on electron transport through a tuneable-barrier double quantum dot electron pump. This device could yield twice the current to a single-dot pump previously reported [M.D. Blumenthal, B. Kaestner, L. Li, S. Giblin, , T.J.B.M. Janssen, M. Pepper, D. Anderson, G. Jones, D.A. Ritchie, Gigahertz quantized charge pumping, Nat. Phys. 3 (2007) 343], at the same operating frequency. The two quantum wires forming the device were not sufficiently homogeneous for pumping to be observed through both dots simultaneously, but sufficient homogeneity should be attainable in future batches of devices.     

Molecular beam epitaxy of quantum wires and quantum dots on patterned high-index substrates

Using a novel growth mechanism on patterned high-index GaAs (311)A substrates we have developed a new concept to fabricate quantum wires and quantum dots as well as coupled quantum wire-dot arrays by molecular beam epitaxy. The combination of self-organized growth with lithographic patterning and the assistance of atomic hydrogen produces these quasi-planar lateral nanostructure arrays with unprecedented uniformity in size and composition and with controlled positioning on the wafer. The sought for one- and zero-dimensional nature of these quantum wire and quantum dot arrays manifests itself in the superior optical properties. To functionalize our lateral semiconductor quantum wire and quantum dot arrays with the properties of magnetic thin films, epitaxial Fe layers have been grown on GaAs (311)A. Defect free Fe layers are obtained on As-saturated GaAs surfaces. The large electrical conductivity of thin Fe layers indicates reduced Fe-GaAs interface compound formation. An unusual in-plane spontaneous Hall-effect is observed in these epitaxial Fe layers of reduced symmetry.     

Quantum dots: Paradigm changes in semiconductor physics

Deposition of one or a few monolayers of a semiconductor having a lattice constant largely different from the underlying substrate leads to formation of coherent “quantum dot arrays” of densities beyond 10¹¹ cm⁻² in a matter of seconds. Self-organization effects govern their massively parallel formation. Fundamental paradigms of semiconductor physics must be changed in describing such quantum dots or their ensembles. Fiz. Tekh. Poluprovodn. 33, 1044–1048 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

量子点标记及其在植物细胞生物学中的应用

量子点是一种直径在1-100 nm,能接受激发光产生荧光的半导体纳米晶粒,由于其独特的物理、化学特性,在生物学中的应用不断扩大。本文介绍了量子点的合成及功能方面的进展,同时也对其在细胞生物学中的应用进展,特别是对其在植物细胞生物学中的应用进行了评述。此外,对量子点在单分子检测、高分辨率细胞成像及活体动态长时追踪方面的应用前景进行了展望。     

Quantum dots in photonic molecules and quantum informatics Part II.

The paper continues to consider hybrid systems that are based on quantum dots (QDs) integrated into solid-state optical structures—photonic molecules (PMs). It presents the main results that are directly or indirectly related to processing quantum information by the above systems, in particular, by an elementary cell of a solid-state quantum register—a QD coherently interacting with delocalized photonic modes. The properties and the manufacturing technology of large PMs—coupled resonator optical waveguides (CROWs) serving as transport channels during the control of qubits—are considered in detail.     

Quantum dots in photonic molecules and quantum informatics. Part I

The main types, fabrication technologies, and physical characteristics of photonic molecules are reviewed. The spectra and spatial configurations of molecular eigenmodes are analyzed within the tight-binding model. Experimental methods for studying the properties of photonic molecules and controlling their states with the use of various external devices are described in detail.     

CdTe和CdTe/CdS量子点的量子尺寸效应

利用水热法合成了三种不同尺寸的单核CdTe量子点和核壳CdTe/CdS量子点。应用Top-hat Z-scan技术在纳秒、皮秒、飞秒激光脉冲作用下研究了三种不同尺寸单核CdTe量子点的非线性吸收特性。实验结果表明:在不同激光脉冲作用下三种不同尺寸的CdTe量子点的非线性吸收特性均表现为饱和吸收,并且均呈现出随着量子点尺寸的减小,其非线性吸收特性增大的趋势。为了进一步研究量子点尺寸的变化对非线性吸收特性的影响,又在飞秒激光脉冲作用下研究了核壳CdTe/CdS量子点的非线性吸收特性;随着包壳时间的增加,壳层厚度增加,量子点尺寸增加,其非线性吸收特性呈减小趋势,并且核壳CdTe/CdS量子点的非线性吸收特性明显优于单核CdTe量子点;分析讨论了单核CdTe量子点与核壳CdTe/CdS量子点的非线性吸收特性和量子尺寸效应机制,实验结果表明合成的量子点样品均具有良好的量子尺寸效应。     

Coherently strained InAs insertions in GaAs: do they form quantum wires and dots?

Coherently strained InAs insertions in a GaAs matrix are expected to exhibit ‘quantum-wire-like’ or ‘quantum-dot-like’ behavior for two regimes of InAs coverage. Firstly, when depositing submonolayer InAs films on vicinal GaAs surfaces forming regular steps, an ordered array of InAs aggregates is obtained whose lateral size is given by the step density and whose size distribution is determined by the homogeneity of the step distribution of the initial surface. Secondly, when exceeding a certain InAs coverage ( 2 monolayers), a morphological phase transformation occurs which results in the formation of three-dimensional InAs islands having sizes compatible with the quantum-confinement of excitons. In this review, we present experimental results for InAs insertions in GaAs for both coverage regimes. Particular emphasis is put on the evaluation of the actual structural and compositional configuration of the InAs insertions in the final heterostructure. It is demonstrated that such an evaluation is stringently required for a correct interpretation of the optical properties of the structure.     

Quantum confinement of carriers in heterostructured GaAs/GaP quantum wires

We present theoretical calculations on the confinement of electrons and holes in a heterostructured GaAs/GaP quantum wires within the effective mass approximation. Energy levels (ground state and excited states), as well as electron-hole recombination energies for some transitions, are calculated for different combinations of wire radius, well widths, and interface thickness.