Search query: Quantum size effects on effective mass and band gap of semiconductor quantum dots

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abstract: In this research we calculate the band gap of ZnS nano particle and show that it increases with decreasing its size. The simplest way for discussing this phenomenon and finding the band gap for different sizes is effective mass approximation. We show, this method isn't accurate for particles smaller than 2.5 nm. This is due to the fact that the effective mass of particles at this size is dependent of size. To calculate the band gap of small size ZnS, the density functional theory was used. This theory has also used to determine the effective mass for various particle sizes. Our calculation indicates the appearance of blue shift by decreasing the size of particle.
author_names: Changiz Vatankhah

Alireza Ebadi
title: Quantum Size Effects on Effective Mass and Band gap of Semiconductor Quantum Dots
year: 2013

abstract: Dimensionality is an important factor to govern the electronic structures of semiconductor nanocrystals. The quantum confinement energies in one dimensional quantum wires and zero dimensional quantum dots are quite different. Using large scale first principles calculations, we systematically study the electronic structures of semiconductor (including group IV, III V, and II VI) surface passivated quantum wires and dots. The band gap energies of quantum wires and dots have the same scaling with diameter for a given material. The ratio of band gap increases between quantum wires and dots is material dependent, and slightly deviates from 0.586 predicted by effective mass approximation. Highly linear polarization of photoluminescence in quantum wires is found. The degree of polarization decreases with the increasing temperature and size.
author_names: Jingbo Li

Lin-wang Wang
title: Comparison of quantum confinement effects between quantum wires and dots
year: 2004

abstract: Light absorption and luminescence processes in nano sized materials can be modelled either by using computational approaches developed for quantum chemical calculations or by applying computational methods in the effective mass approximation (EMA) originally intended for solid state theory studies. An overview of the theory and implementation of an ab initio correlation EMA method for studies of luminescence properties of embedded semiconductor quantum dots is presented. The applicability of the method and the importance of correlation effects are demonstrated by calculations on InGaAs/GaAs quantum dot and quantum ring samples. Ab initio and density functional theory (DFT) quantum chemical studies of optical transitions in freestanding silicon nanoclusters are also discussed. The accuracy of the optical gaps and oscillator strengths for silicon nanoclusters obtained using different computational methods is addressed. Changes in the cluster structures, excitation energies and band strengths upon excitation are reported. The role of the surface termination and functional groups on the silicon nanocluster surfaces is discussed.
author_names: Olli Lehtonen

Dage Sundholm

Tommy Vanska
title: Computational studies of semiconductor quantum dots.
venue: Physical chemistry chemical physics PCCP
year: 2008

abstract: Coupled colloidal quantum dot molecules composed of two fused CdSe/CdS core/shell sphere monomers were recently presented. Upon fusion, the potential energy landscape changes into two quantum dots separated by a pretuned potential barrier with energetics dictated by the conduction and valence band offsets of the core/shell semiconductors and the width controlled by the shell thickness and the fusion reaction conditions. In close proximity of the two nanocrystals, orbital hybridization occurs, forming bonding and antibonding states in analogy to the hydrogen molecule. In this study, we examine theoretically the electronic and optical signatures of such a quantum dot dimer compared to its monomer core/shell building blocks. We examine the effects of different core sizes, barrier widths, different band offsets, and neck sizes at the interface of the fused facets on the system wave functions and energetics. Due to the higher effective mass of the hole and the large valence band offset, the hole still essentially resides in either of the cores, breaking the symmetry of the potential for the electron as well. We found that the dimer signature is well expressed in a red shift of the band gap both in absorption and emission, in slower radiative lifetimes and in an absorption cross section which is significantly enhanced relative to the monomers at energies above the shell absorption onset, while remains essentially at the same level near the band edge. This study provides essential guidance to predesign of coupled quantum dot molecules with specific attributes which can be utilized for various new opto electronic applications.
author_names: Yossef E Panfil

Doaa Shamalia

Jiabin Cui

Somnath Koley

Uri Banin
title: Electronic coupling in colloidal quantum dot molecules; the case of CdSe/CdS core/shell homodimers.
venue: The Journal of chemical physics
year: 2019

abstract: Author(s) Gong, Ke Advisor(s) Kelley, David F Abstract: As it is well known, semiconductor nanocrystals (also called quantum dots, QDs) are being actively pursued for use in many different types of luminescent optical materials. These materials include the active media for luminescence downconversion in artificial lighting, lasers, luminescent solar concentrators and many other applications. Chapter 1 gives general introduction of QDs, which describe the basic physical properties and optical properties. Based on the experimental spectroscopic study, a semiquantitative method effective mass model is employed to give theoretical prediction and guide. The following chapters will talks about several topics respectively. A predictive understanding of the radiative lifetimes is therefore a starting point for the understanding of the use of QDs for these applications. Absorption intensities and radiative lifetimes are fundamental properties of any luminescent material. Meantime, achievement of high efficiency with high working temperature and heterostructure fabrication with manipulation of lattice strain are not easy and need systematic investigation.To make accurate connections between extinction coefficients and radiative recombination rates, chapter 2 will consider three closely related aspects of the size dependent spectroscopy of II VI QDs. First, it will consider the existing literature on cadmium selenide (CdSe) QD absorption spectra and extinction coefficients. From these results and fine structure considerations Boltzmann weighted radiative lifetimes are calculated. These lifetimes are compared to values measured on very high quality CdSe and CdSe coated with zinc selenide (ZnSe) shells. Second, analogous literature data are analyzed for cadmium telluride (CdTe) nanocrystals and compared to lifetimes measured for very high quality QDs. Furthermore, studies of the absorption and excitation spectra and measured radiative lifetimes for CdTe/CdSe Type II core/shell QDs are reported. These results are also analyzed in terms of a Boltzmann population of exciton sublevels and calculated electron and hole wave functions. Much of the absorption data and fine structure calculations are already in the literature. These results are combined with new measurements of radiative lifetimes and electron hole overlap calculations to produce an integrated picture of the II VI QD spectroscopic fundamentals. Finally, we adopt recent synthetic advances to make very monodisperse zincblende CdSe/CdS quantum dots having near unity photoluminescence quantum yields (PLQYs) Due the absence of nonradiative decay pathways, accurate values of the radiative lifetimes can be obtained from time resolved PL measurements. Radiative lifetimes can also be obtained from the Einstein relations, using the static absorption spectra and the relative thermal populations in the angular momentum sublevels. One of the inputs into these calculations is the shell thickness, and it is useful to be able to determine shell thickness from spectroscopic measurements. We use an empirically corrected effective mass model to produce a "map" of exciton wavelength as a function of core size and shell thickness. These calculations use an elastic continuum model and the known lattice and elastic constants to include the effect of lattice strain on the band gap energy. Radiative lifetimes calculated both experimentally and theoretically are checked and the size dependence is compared to previous studied Type I, II and single component particles. However, it is not enough to just understanding these basic photophysics of absorption and emission. The emission intensities (related to QYs) also change with changes of the temperature. The temperature dependent PLs of II VI QDs is extensively studied, but most of this work is at low temperatures. Temperatures well above ambient are of interest to lighting applications and in this regime both the reversible and irreversible loss of quantum yield (thermal quenching) are serious impediments to the implementation of QDs in commercial devices. Chapter 3 will elucidate the mechanism of static thermal quenching, in which the reduction of QYs does not affect the PL decay kinetics, on CdSe, CdTe and CdSe/ZnSe QDs as a function of particle sizes/shapes, surface composition and surface ligands. Through systematic experiments, this part of the dissertation discusses several possible mechanisms (e.g. structural, activated excited state, and electronic charging) and examines which the dominant cause for loss of QY at high temperature is. The more practical step is to develop the synthetic method of highly luminescent and stable core/shell QDs with minimum thermal quenching, which greatly enhance the energy efficiency of light emitting and photovoltaic devices. As the nonradiative Auger processed are induced by surface charging described in chapter 3, static and time resolved fluorescence and high and low power transient absorption results on CdSe/CdS and CdSe/ZnSe core/shell particles are presented in chapter 4. Two CdS shell thicknesses were examined and all of the particles had either octadecylamine (ODA) and tributylphosphine (TBP) or just ODA ligands. The results can be understood in terms of a mechanism in which there is a thermal equilibrium between electrons being in the valence band or in chalcogenide localized surface states. Thermal promotion of a valence band electron to a surface state leaves the particle core positively charged. Photon absorption when the particle is in this state results in a positive trion, which undergoes a fast Auger recombination, making the particle nonluminescent. A lack of TBP ligands results in more empty surface orbitals and therefore shifts the equilibrium toward surface trapped electrons and hence trion formation. Low and high power transient absorption measurements give the trion and biexciton lifetimes and the ratio of the trion to biexciton Auger lifetimes are examined and compared to the degeneracies of Auger pathways. We also study the shell thickness and composition dependence of Auger times, which is compared to the scaling factors of effective volume and electron hole overlap considerations. Core/shell QDs often exhibit much higher luminescence quantum yields (QYs) more stability, and are depicted as having a nearly spherical core and a shell of very nearly uniform thickness, which results in a very simple picture of surface passivation. The uniformity of the shell is crucial in obtaining QDs with well passivated surfaces. However, transmission electron microscope (TEM) images disprove the ideal situation. Defects and thickness inhomogeneity in shell materials are treated qualitatively as an analog to film thickness inhomogeneity in epitaxially grown thin films. More quantitatively, the extent to which the shell thickness of core/shell particles is constant can be determined by time resolved PL studies that measure the dynamics of hole tunneling to acceptors that are adsorbed on the shell surface due that tunneling rates varies strongly with core acceptor separation. Careful analysis of the hole transfer kinetics reveals the extent of shell thickness inhomogeneity, however, it may be complicated by the distribution of numbers of adsorbed acceptors. All the considerations can be incorporated into a model we establish in Chapter 5for the distribution of measured hole tunneling rates. From this analysis the distribution of shell thicknesses can be extracted from the luminescence kinetic results. This approach is therefore a sensitive measure of the distribution of tunneling distances. Thus, any defects or structural irregularities that allow the hole acceptors to adsorb closer to the particle core increases the hole tunneling rate and can be detected and quantified.A quantitative treatment of the lattice strain energy in determining the shell morphology of CdSe/CdS core/shell nanoparticles is presented in chapter 5. We use the inhomogeneity in hole tunneling rates through the shell to adsorbed hole acceptors to quantify the extent of shell thickness inhomogeneity. The results can be understood in terms of a model based on elastic continuum calculations, which indicate that the lattice strain energy depends on both core size and shell thickness. This model assumes thermodynamic equilibrium, i.e. that the shell morphology corresponds to a minimum total (lattice strain plus surface) energy. Comparison with the experimental results indicates that CdSe/CdS nanoparticles undergo an abrupt transition from smooth to rough shells when the total lattice strain energy exceeds about 27eV or the strain energy density exceeds 0.59 eV/nm2. The predictions of this model are not followed for CdSe/CdS nanoparticles when the shell is deposited at very low temperature and therefore equilibrium is not established. The effects of lattice strain on the spectroscopy and photoluminescence quantum yields of zincblende CdSe/CdS core/shell quantum dots are examined. The quantum yields are measured as a function of core size and shell thickness. High quantum yields are achieved as long as the lattice strain energy density is below ~0.85 eV/nm2, which is considerably greater than the limiting value of 0.59 eV/nm2for thermodynamicstability of a smooth, defect free shell, as previously reported in chapter 5. Thus, core/shell quantum dots having strain energy densities between 0.59 and 0.85 eV/nm2 can have very high PL QYs, but are metastable with respect to surface defect formation. Such metastable core/shell QDs can be produced by shell deposition at comparatively low temperatures (l 140 degC).Annealing of these particles causes partial loss of core pressure, and a red shift of the spectrum.
author_names: Ke Gong
title: Photophysical Properties of II VI Semiconductor Nanocrystals
year: 2015

abstract: This thesis consists of three parts: Quantum tunneling simulation, Schottky barrier induced injecting contact on wide band gap II VI materials, and excitons in semiconductor heterostructures. Part I (chapter 2, 3) deals with quantum transport and electronic band structure in semiconductor heterostructures. In chapter 2, we present a new method for quantum transport calculations in tunnel structures employing multiband k.p theory. This method circumvents the numerical instability problems that arise in the standard transfer matrix method. In addition to being numerically stable, efficient, and easy to implement, this method can also be easily generalized to include the magnetic field and strain effects. The development of this technique mainly consists of two parts, the discretization of effective mass Schrodinger's equation using finite difference method, and the formulation of boundary conditions. The treatment of boundary condition in quantum transport is similar to the Multiband Quantum Transmitting Boundary Method (MQTBM) for use with multiband tight binding models. The calculations of transmission coefficients reduce to a set of linear equations, which can be solved very easily. With appropriate formulation of boundary conditions, this technique can be readily extended to the calculations of electronic band structures in quantum confinement and superlattice structures. We have applied this new technique to magnetotunneling in interband tunnel structures in chapter 3, and studied two prototypical device structures: Resonant Interband Tunneling (RIT) devices and Barrierless Resonant Interband Tunneling (BRIT) devices. Effects of transverse magnetic field on the band structures, transmission spectrum, and I V characteristics are investigated. Evidence of heavy hole resonance contribution can be identified in the change of I V characteristics under applied magnetic field. The technique has also been illustrated for hole tunneling in p type GaAs/AlAs double barrier tunnel structures, and calculations of electronic band structures in lattice matched InAs/GaSb superlattices, and strained InAs/Ga1 xInxSb superlattices. Part II describes a novel approach to achieve ohmic injecting contact on wide bandgap II VI semiconductors. The problem of making good ohmic contact to wide bandgap II VI materials has been a major challenge in the effort of making visible light emitting diodes. The method we propose consists of forming the device structure in an electric field at elevated temperatures in the Schottky barrier region, to spatially separate the ionized dopants from the compensating centers. In this way, the ratio of dopants to compensating centers can be greatly increased at the semiconductor surface. Upon cooling, the dopant concentrations are frozen to retain a large net concentration of dopants in a thin surface layer, resulting in a depletion layer that is sufficiently thin to allow tunneling injection. Calculations of band profiles, distributions of dopant concentrations, and current voltage characteristics were performed. We have selected the case of Al doped ZnTe in our study, in which two Al donors complex with a doubly negatively ionized Zn vacancy to produce total compensation. The results show that the bulk doping concentration and the total band bending during the forming process are the crucial factors for achieving injecting contacts. For Schottky barrier heights above 1 eV, doping concentrations as high as 1020cm 3 are needed. In part III, we studied excitons in semiconductor heterostructures, consisting of two subjects: excitons in II VI heterostructures, and exciton coherent transfer process in quantum structures. Calculations of exciton binding energies and oscillator strengths are performed in both Type I strained CdTe/ZnTe superlattices with very small valence band offset and Type II strained ZnTe/ZnSe superlattices. A special variational approach was employed to take into account the effects of unusual band alignment, strain, and image charges at the heterojunction interface. It is found that the large enhancements of exciton binding energy and oscillator strength in the CdTe/ZnTe system are similar to what one finds in systems with a much larger valence band offset. For small CdTe layer thickness, however, the confinement of holes in the CdTe layer is weak, resulting in a lowering of the exciton binding energy. The oscillator strength in CdTe/ZnTe superlattice system shows the expected enhancement over the oscillator strengths in the bulk. For the ZnTe/ZnSe system, the Type II character of the heterojunction results in the confinement of the electrons and holes in different layers. It is found that strong confinement of electrons and holes by the large band offsets can give rise to a fairly large exciton binding energy for thin heterojunction layers. Also, the mismatch in dielectric constants induces an image charge at the interface, which modifies significantly the exciton Hamiltonian in an asymmetric superlattice structure and plays an important role in determining the degree of localization of the electron and hole at the interface. We have investigated exciton coherent transfer in semiconductor quantum structures. In systems where the typical dimensions of the semiconductor quantum structures and the spacings between them are significantly smaller than the photon wavelength, the resonant transfer of excitons between two identical quantum structures is accomplished through the interaction of near field dipole dipole transitions (exchange of virtual photons) The transfer matrix elements are calculated for three different geometries: quantum wells, quantum wires and quantum dots, respectively. The results show that the exciton transfer matrix element is proportional to exciton oscillator strength, and depends on exciton polarization. The transfer matrix element between quantum wells depends on the exciton wave vector in the plane of the wells, k, and vanishes when k 0. For quantum wire and quantum dot structures, the transfer matrix elements between two units separated by R vary as R 2 and R 3, respectively. For quantum structures with typical characteristic size of 50 A with separation about 100 A, the transfer matrix element is on the order of 10 3 meV. It corresponds to a resonant transfer time of 1 ns, comparable with the exciton lifetime. However, it is significantly smaller than the inhomogeneous broadening due to phonons and structural imperfection in most synthesized semiconductor quantum structures achievable today, which is typically on the order of a few meV, making the realization of experimental observation difficult. The study is to explore new ideas and potential technological applications based on excitonic devices.
author_names: Yixin Liu
title: Quantum Tunneling, Field Induced Injecting Contact, and Excitons
year: 1995

abstract: This thesis deals with cross sectional scanning tunneling microscopy analyses on III/V and II/VI semiconductor nanostructures and single dopants in various III/V materials. A detailed atomic scale study of the structural properties of capped InAs quantum dots and ZnSeTe/ZnTe based quantum wells and the local charge distribution around acceptor type dopants in GaAs and GaP are presented. Self assembled InAs quantum dots are interesting objects from fundamental and technological points of view because they form nearly ideal zerodimensional systems in which quantum confinement effects become important. For example, InAs quantum dots are employed in quantum dot lasers, single electron transistors, midinfrared detectors, single photon sources, etc. For any device application, the quantum dots are capped after growth to prevent unwanted effects occurring at the surface of uncapped dots. However, the use of capping materials such as InP, GaAs and InGaAs strongly affects the structural properties of quantum dot size, shape and composition. In chapter 3, critical issues occurring during the capping process like dot decomposition, intermixing, segregation, As/P exchange, and compositional modulation in the dot/capping layer interface have been identified on the atomic scale. In chapter 4, II/VI semiconductor heterostructures grown by MBE are investigated on the atomic level. Some peculiar effects are found which are not observed in III/V semiconductors. For ZnTe, mono atomic vacancy chains are observed on the Te sublattice. These chains are found to be created during the cleavage process, pulling straight atomic rows out of the surface. The measured distances between these missing rows were found to vary between a few nanometers and tens of nanometers whereas their lengths go up to several hundreds of nanometers. It can be an interesting breakthrough to use these vacancy chains as templates to create 1D magnetic quantum wires with a cross section of only one atom by incorporating magnetic ions (e.g. Fe, Mn or Co) into these vacancy chains. Atom manipulation as a result of applying positive bias voltages to the sample is observed on ZnTe. In this process, atoms are randomly pulled out of the surface by STM tip. Moreover, the quantitative extent of the atom manipulation appears to increase with decreasing bias voltage while maintaining the constant tunnel current. Atomic vacancy chains in particular are determined to be highly sensitive to the change of the bias voltage. The most important result concerns with the observation of ZnSeTe/ZnTe quantum well structures. The compositional profile of these quantum wells is determined directly by atom counting and by numerical fitting of the cleavage induced outward relaxation of the ZnSeTe/ZnTe layered structure. In chapter 5, the identification of the spatial position of Mn acceptors in GaAs is studied with atomic layer precision at T 5 K. In the experiments, the STM tip is used not only to probe the hole distribution around the Mn acceptors, but also to manipulate the transition metal atoms and their adsorbate related complexes on the GaAs surface. The symmetry of Mn is studied experimentally both at GaAs(110) and GaAs(1 10) cleavage surface. In order to examine the electronic structure of the Mn state, we compared our measurements with a multi band tight binding model. The model considers the non spherical symmetry of the GaAs top most valence band structure and takes into account the spin orbit interaction. The calculations were performed for a bulk like (Mn2+3d5 hole) complex and excluded the possible effects caused by the presence of the surface and a vacuum half sphere. The results of the bulk calculations made it possible to distinguish the experimentally observed actual surface induced effects from the bulk properties of the Mn acceptor state located near the vacuum interface. In chapter 6, the wave functions of single Mn, and Cd acceptors at 10atomic layers below the (110) cleavage surface of GaAs and GaP are spatially mapped both at room and low temperature (T 5 K) In particular, the effect of the spin orbit interaction on the acceptor wave functions is investigated by comparing a set of measurements on Mn doped GaAs and Cd doped GaP samples. Although the binding energy of Cd (102 meV) in GaP is nearly identical to that of Mn (113 meV) in GaAs, the spin orbit interaction differs by a factor of about 4 between the two materials. Similar to the Mn hole wave function in GaAs, we found a highly anisotropic cross like shape of the hole bound to Cd acceptor at GaP(110) surface. The observed similarity of the symmetry properties of Mn:GaAs and Cd:GaP clearly showed that the anisotropic structure of the acceptor states in III/V semiconductors is found to be determined by the cubic symmetry of the host crystal. The role of the spinorbit interaction on the overall shape of acceptor wave functions is identified to be negligible. Nevertheless, the weak spin orbit coupling in GaP gives rise to additional components in the Cd acceptor wave function as compared to Mn:GaAs. Our experimental results are confirmed by two independent models based on tight binding and effective mass approaches. In chapter 7, a detailed depth dependence of the Mn acceptor wave function symmetry is characterized and quantified experimentally at T 5 K as a function of the depth of the Mn atom up to several layers below the surface. The experimental results are compared with the results of theoretical tightbinding calculations in the presence of an internal homogeneous strain and the strain induced by the surface relaxation, is identified to be the dominant cause of the symmetry breaking of both shallow and deep acceptors in III/V materials. To approximate the case of a relaxed GaAs surface within the tightbinding model, the Ga sub lattice of the GaAs crystal is displaced vertically relative to the As sub lattice. Such a shift of the Ga sub lattice created an internal strain which was found to break the Mn wave function symmetry along a particular direction as observed experimentally. Finally, the effect of the surface related strain on different acceptor species with different binding energies is investigated with an emphasis on their (001) reflection asymmetry. As the strain driven effect is found to be stronger for weakly bound acceptors, this work explains within a unified approach the long standing problem of the commonly observed triangular shaped STM images for all shallow acceptors and the cross like features of deeply bound acceptor wave functions in III/V semiconductors. The results are of general importance for the dopant atoms in a stain field as well as near a strained interface. Further analysis on the acceptor wave functions, as presented in this thesis, suggests that the deep acceptor wave functions can be used as atomic scale sensors to trace the strain profile in a strained semiconductors.
author_names: Cem Celebi
title: Atomic scale study of impurities and nanostructures in compound semiconductors
year: 2009

abstract: The fundamental structure property relationship of semiconductor quantum dots has been investigated. For deposited germanium nanocrystals strong quantum confinement effects have been determined with synchrotron radiation based x ray absorption and photoemission techniques. The nanocrystals are condensed out of the gas phase with a narrow size distribution and subsequently deposited in situ onto various substrates. The particles are crystalline in the cubic phase with a structurally disordered surface shell and the resulting film morphology depends strongly on the substrate material and condition. The disordered surface region has an impact on the overall electronic structure of the particles. In a size dependent study, the conduction and valence band edge of germanium nanocrystals have been measured for the first time and compared to the bulk crystal. The band edges move to higher energies as the particle size is decreased, consistent with quantum confinement theory. To obtain a more accurate analysis of confinement effects in the empty states, a novel analysis method utilizing an effective particle size for the x ray absorption experiment, which allows a deconvolution of absorption edge broadening effects, has been introduced. Comparison of the present study to earlier studies on silicon reveals that germanium exhibits stronger quantum confinement effects than silicon. Below amore critical particle size of 2.3 0.7 nm, the band gap of germanium becomes larger than that of silicon even if it is the opposite for bulk materials. This result agrees phenomenologically with effective mass and tight binding theories but contradicts the findings of recent pseudopotential calculations. The discrepancy between theory and experiments is attributed to the differences in the theoretical models and experimental systems. The experimentally observed structural disorder of the particle surface has to be included in the theoretical models. less
author_names: Christoph Bostedt
title: Electronic Structure of Germanium Nanocrystal Films Probed with Synchrotron Radiation
year: 2002

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Quantum Wires and Ballistic Point Contacts: M. B*aduttiker, When is the Hall Resistance Quantized? G. Timp, R. Behringer, S. Sampere, J.E. Cunningham, and R.E. Howard, When Isn't the Conductance of an Electron Waveguide Quantized? H. van Houten and C.W.J. Beenakker, Electron Beams and Waveguide Modes: Aspects of Quantum Ballistic Transport. B.J. van Wees, L.P. Kouwenhoven, E.M.M. Willems, and C.J.P.M. Harmans, Quantum Ballistic Transport in High Magnetic Fields. A.D. Stone, A. Szafer, and H.U. Baranger, New Theoretical Results on Ballistic Quantum Transport: Quenching of the Hall Resistance and Quantized Contact Resistance. Y. Imry, Theoretical Considerations for Some New Effects in Narrow Wires. C.J.B. Ford, The Low Field Hall Effect in Quasi Ballistic Wires. A. Sachrajda, D. Landheer, R. Boulet, J. Stalica, and T. Moore, Evidence for an Inhomogeneity Size Effect in Micron Size GaAs/A1GaAs Constrictions. S.D. Berger, H.A. Huggins, A.E. White, K.T. Short, and D. Loretto, A New Technique to Produce Single Crystal Epitaxial Nanostructures. M. Cahay, S. Bandyopadhyay, and H.L. Grubin, Doubled Frequency of the Conductance Minima in Electrostatic Aharonov Bohm Oscillations in One Dimensional Rings. A. Pruisken and Z. Wang, Mesoscopic Conductance Fluctuations in Disordered Metals. Related Fabrication and Phenomena: A.N. Broers, Resolution Limits of Electron Beam Lithography and Methods for Avoiding These Limits. A. Scherer, M.L. Roukes, and B.P. Van der Gang, Quantum Device Microfabrication at the Resolution Limit of Ion Beam Processing. E.E. Ehrichs and A.L. de Lozanne, Fabrication of Nanometer Features with a Scanning Tunneling Microscope. A.E. Owen, P.J.S. Ewen, A. Zakery, M.N. Kozicki, and Y. Khawaja, Metal Chalcogenide Photoresists for High Resolution Lithography and Sub Micron Structures. D.C. Ralph, K.S. Ralls, and R.A. Buhrman, Defect Motion, Electromigration and Conductance Fluctuations in Metal Nanocontacts. C.R. Martin, M.J. Tierney, I.F. Cheng, L.S. Van Dyke, Z. Cai, J.R. McBride, and C.J. Brumlik, Nano and Microstructures in Chemistry, Electrochemistry, and Materials Science. C. Van Haesendonck, H. Vloeberghs, Y. Bruynseraede, and R. Jonckheere, Electronic Transport in Mesoscopic AuFe Spin Glasses. N. Giordano, Nature of the Superconducting Transition in Very Thin Wires. L. Reggiani and V. Kozlov, Population Inversion in Superconducting Quantum Wells Under Ballistic Conditions. A. Badakhshan, C. Durbin, A. Giordana, R. Glosser, S.A. Lambert, and J. Liu, Photoreflectance of Semiconductors Beyond the Band Gap. M. Cahay, M.A. Osman, H.L. Grubin, and M. McLennan, Space Charge Effects in Compositional and Effective Mass Superlattices. G.Y. Hu and R.F. O'Connell, 1/f Noise in Two Dimensional Mesoscopic Systems from a Generalized Quantum Langevin Equation Approach. NPF Symposium Participants. Author Index.
author_names: Mark A Reed

Wiley P Kirk
title: Nanostructure physics and fabrication proceedings of the international symposium, College Station, Texas, March 13 15, 1989
year: 1989

abstract: This thesis describes growth, processing, characterization and photoluminescence (PL) spectroscopy of nanowire heterostructures. The nanowires were made of III V semiconductor materials and were produced by Au particle assisted growth. Axial heterostructures in the form of quantum dots (QDs) in the nanowires as well as radial heterostructures in the form of core shell nanowires were studied. Axial heterostructures in the InAs GaAs and InAs InP material systems were studied and it was found that the latter material system gave the best control of the QD dimensions and spectrum. In this system wurtzite InAs0.85P0.15 QDs with well defined dimensions were grown in wurtzite InP nanowires. QDs with diameters, d, between 9 and 22 nm were produced. By growing the heterostructures using only the In stored in the Au seed particle and AsH3 from the gas phase a well controllable QD height of 0.35d was achieved for nanowires with diameters larger than the critical diameter for the Gibbs Thomson effect, which was estimated to be 9 nm under the growth conditions used. The size dependence of the QD PL spectrum was studied. We found that the largest QDs were luminescing at the telecommunication wavelength of 1300 nm. With decreasing QD size the emission blue shifted and level splittings as well as the biexciton binding energy increased, illustrating strong confinement. A comparison of the experimental data to a strain dependent k*p model indicated that the band gap of InAs0.85P0.15 is 190 meV larger and the effective mass is a factor two larger in the wurtzite polytype compared to the zinc blende polytype. Radial heterostructures, core shell nanowires, were grown using low temperature, kinetically limited growth for the core to suppress lateral growth and high temperature growth for the shell. Core shell nanowires in two material systems, GaAs GaInP and GaAs AlInP, were investigated. It was found that the shell increases the emission efficiency of the core two to three orders of magnitude by moving the surface states away from the core. The effect of strain, caused by a lattice mismatched shell, on the PL spectrum was investigated and the core emission was tuned over a range of 240 meV. It was also found that planar defects such as twins and stacking faults in the nanowire propagate into the shell during growth and that new twins and stacking faults are created due to {1,1,1} facetted corrugations on the nanowire surface. A defect free shell therefore requires a defect free core without {1,1,1} micro facets. Furthermore, phase segregation due to capillarity effects during shell growth was studied. Cross sectional transmission electron microscopy showed that Al rich domains are formed in the directions in the shell. However, PL investigations indicated that In rich domains also exist, these are possibly associated with the corrugated nanowire surface. Two other applications for the core shell nanowires were also investigated. A growth scheme was developed where the shell growth was extended in order to completely embed the nanowires which could then be cleaved to enable cross sectional scanning tunneling microscopy of the interior of the nanowires. Such studies of axial heterostructures revealed an asymmetry in the sharpness of heterostructure interfaces as a consequence of large amounts of group III material being stored in the Au particle causing memory effects following material switching. In the second application a process for conversion of core shell nanowires into tubular nanowires which can be used for cell injection was developed. It was shown that these tubular nanowires can be used to efficiently deliver cell membrane impermeable molecules to macrophages. (Less)
author_names: Niklas Skold
title: Nanowire Heterostructures Growth, Characterization and Optical Physics
year: 2008