Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride

Amorphous silicon quantum dots (a-Si QDs) were grown in a silicon nitride film by plasma enhanced chemical vapor deposition. Transmission electron micrographs clearly demonstrated that a-Si QDs were formed in the silicon nitride. Photoluminescence and optical absorption energy measurement of a-Si QDs with various sizes revealed that tuning of the photoluminescence emission from 2.0 to 2.76 eV is possible by controlling the size of the a-Si QD. Analysis also showed that the photoluminescence peak energy E was related to the size of the a-Si QD, a (nm) by E(eV) = 1.56+2.40/a(2), which is a clear evidence for the quantum confinement effect in a-Si QDs.     

Spatial versus quantum confinement in porous amorphous silicon nanostructures

Light-emitting porous amorphous silicon has been produced by anodization in HF of hydrogenated amorphous silicon films. The maximal thickness of the porous films is limited by the onset of an instability which results in the formation of large channels short-circuiting the amorphous layer. This is due to the high resistivity of the amorphous silicon films as compared to that of the electrolyte. Confinement effects on the electron wavefunction are analyzed in situ using photoluminescence measurements in hydrofluoric acid and compared to those observed in porous crystalline silicon. For crystalline silicon, a huge blue shift of the photoluminescence is observable upon reducing the size of the structures by photo-etch, showing clear evidence of quantum confinement effects in this material. No shift has been observed when carrying out the same experiment with amorphous silicon. This indicates that the extent of the wavefunction in the bandtail states involved in luminescence is too small to be sensitive to confinement down to the minimum sizes of our porous material ( 3 nm). Measurements of the width and the temperature dependence of the photoluminescence demonstrate that the Urbach energy does not change upon increasing the porosity, i.e., upon decreasing the size of the a-Si:H nanostructures, in contradiction with what has been reported in ultrathin a-Si:H multilayers. Received: 3 August 1998     

Magnetic properties of epitaxial oxide heterostructures

Diluted Magnetic Semiconductors (DMS) are of great interest as injection sources for spin-polarized currents into semiconductors. Epitaxial devices have been synthesized with an intermediate spacer layer of the same semiconductor (zinc oxide, ZnO) used to produce the DMS material (ZnCoO) ensuring a homoepitaxial junction to help reduce the interface states and conduction mismatch. We observe a large magnetoresistance of about 32% in the devices at low temperatures. The present work suggests that spin polarized transport could be achieved with DMS materials acting as the source of injected spins into a non-magnetic host.     

Oxygen impact on quantum confinement effect for silicon clusters in different size regimes: ab initio investigations

The phenomena where the parameter like optical gap, HOMO-LUMO gap etc of a cluster will behave like a monotonic function with respect to its size is called quantum confinement effect. Here we are dealing with clusters having number of silicon atoms as 10, 16, 19, 20, 35, 54 and 78 which are representatives of different size regime clusters. For each clusters we have experimented with a number of isomers and the results we are showing are only of the stable most isomers. Then for each clusters we are capping them with oxygen atoms and calculating their optical response using DFT based calculations. We also calculate their HOMO-LUMO gap with and without oxygen atoms. The main essence of this work is mainly revolving around the variation of optical gap as well as the HOMO-LUMO gap for all the clusters with respect to their size and also whether oxygen insertion can induce any changes in quantum confinement effect. Also for Si₂₀, Si₁₉, Si₁₆ and Si₁₀ we have calculating the optical spectra with a variation of inserted oxygen on the surface of those clusters. All the calculation are performed using Vienna ab initio simulation package (VASP) and Car Parrinello molecular dynamics (CPMD) for geometry optimization. The optical spectra of the clusters are performed by real space PARSEC code and time dependent density functional theory implemented RGWBS code which takes into account the many body effects using GW and BSE equations.     

Raman investigation of silicon nanocrystals: quantum confinement and laser‐induced thermal effects

We present a detailed experimental and theoretical Raman investigation of quantum confinement and laser‐induced local thermal effects on hydrogenated nanocrystalline silicon with different nanocrystal sizes (3.6–6.2 nm). The local temperature was monitored by measuring the Stokes/anti‐Stokes peak ratio with the laser power density range from ~120 to 960 kW/cm². In combination with the three‐dimensional phonon confinement model and the anharmonic effect, which incorporates the three‐phonon and four‐phonon decay processes, we revealed an asymmetrical decay process with wavenumbers ~170 and 350 cm^–1, an increasing anharmonic effect with nanocrystal sizes, and a shortening lifetime with enhanced temperature and decreasing nanocrystal dimension. Furthermore, we demonstrated experimentally that for Si nanocrystals smaller than 6 nm, the quantum confinement effect is dominant for the peak shift and line broadening. Copyright © 2011 John Wiley & Sons, Ltd.     

Superconductor/ferroelectric epitaxial heterostructures for tunable microwave devices

The formation of epitaxial heterostructures including YBa₂Cu₃O_7−δ and SrTiO₃ thin films on R-plane sapphire substrates has been studied. The thin CeO₂ layer and the CeO₂/SrBi₂Nb₂O₉ epitaxial bilayer served as buffers. The temperature dependence of the dielectric permittivity ɛ of the SrTiO₃ film behaved in essentially different ways when incorporated in the YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ and in Ag/SrTiO₃/YBa₂Cu₃O_7−δ capacitor structures. An external bias U _b=±2.5 V applied to a plane-parallel YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ capacitor structure (T<100 K) reduced the ɛ of the SrTiO₃ film to one half its original value. Fiz. Tverd. Tela (St. Petersburg) 39, 222–227 (February 1997)     

量子受限效应和对称性效应对硅光子晶体禁带的影响

光子晶体不仅可以用来调控自发辐射, 还可以用来控制光的传输和局域. 采用平面波展开法进行模拟计算, 分析硅背景下的二维正方、三角晶格光子晶体散射基元的形状和空间取向对光子禁带的影响. 计算结果表明: 对称性和量子受限效应之间的竞争是导致光子晶体禁带宽度发生变化的原因.     

Semiconductor heterostructures for spintronics and quantum information

A selection of optical experiments is presented, demonstrating the utility of semiconductors in two novel areas of research: spintronics and quantum information. First we show examples of spin manipulation in semiconductor quantum wells. The light is used to generate a spin polarization and to detect it. Next we discuss application of optical methods in studies of carrier-induced ferromagnetism in quantum wells. Finally, we present examples of single quantum dot spectroscopy related to perspectives of application of quantum dots in quantum information, and, in particular, the use of photon correlation measurements as a tool to study the quantum dot excitation mechanisms. To cite this article: J.A. Gaj et al., C. R. Physique 8 (2007).     

Mechanisms for spontaneous and stimulated recombination in multiple quantum wells of InGaN/GaN heterostructures on silicon substrates

With the aim of establishing the mechanisms for spontaneous recombination and lasing, we have studied InGaN/GaN multiple quantum well heterostructures emitting in the 450 nm region and grown by organometallic vapor-phase epitaxy on silicon substrates using several mechanical stress-reducing AlN/AlGaN inserts. Photoluminescence (PL) excitation spectroscopy, the non-monoexponential nonequilibrium carrier relaxation kinetics, and x-ray diffractometry data indicate significant inhomogeneity of the InGaN solid solution in quantum wells of these structures. The dependences of the position of the photoluminescence spectra on the excitation level and the temperature, the large shift in the photoluminescence, gain, and lasing spectra relative to the absorption edge allow us draw the conclusion that the dominant contribution to spontaneous and stimulated recombination comes from nonequilibrium charge carriers localized in indium-rich InGaN clusters. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 94–101, January–February, 2008.     

Correlation effects in quantum spin-Hall insulators: a quantum Monte Carlo study

We consider the Kane-Mele model supplemented by a Hubbard U term. The phase diagram is mapped out using projective auxiliary field quantum Monte Carlo simulations. The quantum spin liquid of the Hubbard model is robust against weak spin-orbit interaction, and is not adiabatically connected to the spin-Hall insulating state. Beyond a critical value of U>U(c) both states are unstable toward magnetic ordering. In the quantum spin-Hall state we study the spin, charge, and single-particle dynamics of the helical Luttinger liquid by retaining the Hubbard interaction only on a ribbon edge. The Hubbard interaction greatly suppresses charge currents along the edge and promotes edge magnetism but leaves the single-particle signatures of the helical liquid intact.     

Modeling of confinement potential for cylindrical quantum dot

Quantum states and energy levels of an electron in a cylindrical quantum dot with different models of confinement potentials are studied. Two models of confinement potentials, Morse potential and modified Pöschl-Teller potential, are considered. It is shown that due to distinction between symmetric and asymmetric nature of potentials, there is a fundamental difference in behavior of the ground levels of charge carriers in these potentials. At small values of the width of Morse potential, quantum emission of electron occurs which is not observed in case of the modified Pöschl-Teller potential.     

ZnO一维纳米线及其异质结构的外延生长

一维纳米结构因其优异的光、电特性，在纳米电子学，光电子学器件等方面有重要的应用价值而倍受关注．在一维半导体纳米材料中，ZnO因激子束缚能大（60meV），可在室温获得高效的紫外发光而成为近年来继GaN材料后的又一研究热点．外延生长一维纳米结构ZnO及其量子阱材料除因量子尺寸效应更适宜做室温紫外发光、激光材料与器件外，还因界面和量子限制效应而具有许多新奇的光、电、和力学特性，可应用于纳米光电子学器件，传感器及存储器件，纳米尺度共振隧道结型器件和场效应晶体管的研制和开发．文章着重介绍了目前ZnO一维纳米结构制备，一维ZnO纳米异质结构和一维ZnO／Zn1-xMgxO多量子阱结构的外延生长和研究进展．     

Screw misfit dislocations in soft substrates of epitaxial heterostructures

We derive compact analytical formulae for the elastic field induced by an anti-plane mismatch deformation in a heterostructure with different elastic moduli of the constituents. Unlike previous studies, we consider the possibility that the misfit dislocations may appear in the substrate, not in the epilayer. We show that this situation can be realized in heterostructures where the substrate is softer than the epilayer. In order to avoid cumbersome calculations, we consider screw misfit dislocations. The misfit dislocations emerge with zero density away from the interface in the body of the substrate when the epilayer reaches its critical thickness. Thus the epilayer remains free from dislocations if it is grown on a softer substrate. This property, which was recently observed experimentally, may find numerous applications in electronics, where epilayers are widely used as active elements.     

Electric-field control of exchange bias in multiferroic epitaxial heterostructures

The magnetic exchange between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the FM layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an electric field allows control of the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to paving the way towards a new generation of electric-field controlled spintronic devices.     

Recent progresses of quantum confinement in graphene quantum dots

Graphene quantum dots (GQDs) not only have potential applications on spin qubit, but also serve as essential platforms to study the fundamental properties of Dirac fermions, such as Klein tunneling and Berry phase. By now, the study of quantum confinement in GQDs still attract much attention in condensed matter physics. In this article, we review the experimental progresses on quantum confinement in GQDs mainly by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Here, the GQDs are divided into Klein GQDs, bound-state GQDs and edge-terminated GQDs according to their different confinement strength. Based on the realization of quasi-bound states in Klein GQDs, external perpendicular magnetic field is utilized as a manipulation approach to trigger and control the novel properties by tuning Berry phase and electron–electron (e–e) interaction. The tip-induced edge-free GQDs can serve as an intuitive mean to explore the broken symmetry states at nanoscale and single-electron accuracy, which are expected to be used in studying physical properties of different two-dimensional materials. Moreover, high-spin magnetic ground states are successfully introduced in edge-terminated GQDs by designing and synthesizing triangulene zigzag nanographenes.     

Strain-induced quantum confinement of electron gases

Zero-dimensional electron gases have been fabricated by the strain-patterning of a GaAs/AlAs heterojunction using amorphous carbon stressors. We have used steady-state, time-resolved and temperature-dependent photoluminescence measurements to probe the occupied density of states of the electron gases. We observe a novel lateral confinement mechanism and efficient transfer of modulation-doped electrons from the regions between the stressors to the quantum dots. In finite magnetic fields we have mapped the evolution of the electron states from which we estimate the number of electrons per dot to be 15.     

Exciton-induced enhancement of optical waveguide confinement in (Zn,Cd)(Se,S) quantum well laser heterostructures

Room-temperature waveguide properties have been studied experimentally in ZnSe-based multiple-quantum-well laser structures. The spectra of waveguide transmission show distinct spectral features associated with heavy- and light-hole quantum well excitons. Refractive index dispersion in the multiple-quantum-well region is derived from interference spectra measured in short waveguide samples. The refractive index increases with approaching the exciton resonances, resulting in perfect confinement and low losses of the fundamental TE waveguide mode in a narrow spectral region below the heavy-hole exciton resonance energy. The spectra of laser generation have been measured under the conditions of optical pumping, showing the position of the lasing line to correlate with the spectral region of the waveguide transparency.     

Electrical properties of epitaxial heterostructures on a zinc sulfide base

An investigation is performed on the volt-ampere, volt-faraday, and photoelectrical characteristics of the epitaxial heterostructures n·ZnS-p-Ge, n-ZnS-p-GaAs, and n-ZnS-n-Si obtained by the method of vacuum evaporation in a quasiclosed volume. The heterotype structures are rectifying, and the current passage therein is described by the theories of space-charge-limited current. The quantity of surface states N_s.s=1.2·10¹²cm^–2 on the interface of n-ZnS-p-GaAs is computed, this being less than the theoretical value by almost two orders of magnitude. The heterostructures n-ZnS-n-Si were of two kinds. The volt-ampere characteristics of the structure of type I are rectifying with a rectification factor of 10⁴ at the 1 V level. The direct current had an exponential dependence and depended weakly on the temperature. The heterostructures had photosensitivity. Their no-load voltage was up to 0.2 V. The structures of type II had volt-ampere characteristics with double saturation. Starting from the model of two series-connected Schottky diodes, the heights of the barriers were computed for such heterostructures: Eg_ZnS=0.8–0.91 eV and E_Si=0.66–0.77 eV.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 104–108, July, 1978.