Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A20: Focus Session: Quantum Dots and Semiconductor Surface Nanostructures |
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Sponsoring Units: DMP Chair: Kristen Fichthorn, Pennsylvania State University Room: Morial Convention Center 212 |
Monday, March 10, 2008 8:00AM - 8:12AM |
A20.00001: Phase-field modeling of solute precipitation and dissolution at solid-fluid interface Zhijie Xu, Paul Meakin Phase phase-field methods have been developed to simulate a variety of processes in which interface dynamics play a critical role. The mathematical formulation of a phase field approach to the dynamics of liquid solid interfaces that evolve due to precipitation and/or dissolution will be presented. For the purpose of illustration, and comparison with other methods, phase field simulations have been carried out assuming first order reaction dissolution/precipitation) kinetics. In contrast to solidification processes controlled by a temperature field that is continuous at the solid/liquid interface, with a discontinuous temperature gradient, precipitation/dissolution is controlled by a solute concentration field that is discontinuous at the solid/liquid interface. The sharp-interface asymptotic analysis of the phase-field equations for solidification by Karma and Rappel [Phys. Rev. \textbf{E57} (1998) 4342] have been extended to demonstrate that the phase-field equations converge to the proper sharp-interface limit for the precipitation/dissolution problem. The mathematical model has been validated for a one-dimensional precipitation/dissolution problem by comparison with the analytical solutions of the free-boundary problem. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A20.00002: Island Size Scaling and Evolution during Strained Film Epitaxy Zhi-Feng Huang, Ken Elder Strained film growth often gives rise to the self assembly of nanostructures such as quantum dots or islands. While there has been and continues to be much interest in such behavior the fundamental mechanisms that determine the precise morphologies remain unclear. In this work the phase field crystal model, which incorporates the atomic length and diffusive time scales, and the corresponding amplitude equations are used to examine this phenomenon. Direct numerical simulations of the model and a linear stability analysis of the amplitude equations are presented. The results predict that the average island size is inversely proportional to the strain. This result is consistent with recent experiments on SiGe, but inconsistent with the predictions of classic continuum elasticity theory (or the Asaro-Tiller-Grinfeld instability). Basic mechanisms identified in our study, which are associated with the crystalline nature but absent in all continuum approaches, are discussed. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A20.00003: Optimal Capping Layer Thickness for Stacked Quantum Dots Xiaobin Niu, Christian Ratsch, Young-Ju Lee, Russel Caflisch We study the effect of strain on the vertical and lateral self-organization of nanoscale patterns and stacked quantum dots during epitaxial growth. The computational approach is based on the level set method in combination with an atomistic strain code. Strain changes the energetics of microscopic parameters during growth, and thus determines the nucleation sites and the growth of islands and dots. Our results show that strain can lead to vertical alignment as well as lateral organization. Moreover, our simulations suggest that there is an optimal thickness of the capping layer to get the best alignment and most uniform size distribution of stacked quantum dots. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A20.00004: Modeling the complex evolution of self-assembled quantum dots Invited Speaker: In heteroepitaxy, misfit strain often leads to spontaneous formation of islands. Such islands have attracted great interest as ``self-assembled quantum dots". The growth of these quantum dots is remarkably rich, exhibiting alloy intermixing, island coarsening, trench formation, and even spontaneous {\it lateral} motion of islands. Islands also interact with topographic features on the substrate, providing a means for controlling the position of quantum dots. The diverse experimental observations provide an ideal opportunity to test and extend our theoretical understanding of growth at the nanoscale. We find that much of the complexity arises because there is a strong thermodynamic driving force for intermixing (to increase entropy and reduce strain energy) as well as for morphological evolution (to reduce strain energy); but this evolution must occur under the kinetic constraint of diffusion occurring only at the surface. Simulations of such constrained evolution directly reproduce many of the observed phenomena [Y. Tu and J. Tersoff, Phys. Phys. Lett. 98, 096103 (2007)]. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A20.00005: Effect of Elastic Inhomogeneity and Anisotropy on the Order of Epitaxial Self-Assembled Quantum Dots Chandan Kumar, Lawrence Friedman Growth of epitaxial self-assembled semiconductor quantum dots (SAQDs) is of particular interest in the development of quantum dot based devices such as quantum computing architectures, laser diodes, and other optoelectronic devices. The ordering of these SAQDs is critical for the development of these devices. Understanding what factors the order of these SAQDs depend on, is important for guiding both experiments and simulations. Most theoretical and numerical models approximate the film substrate system as a semi-infinite solid. Although models based on such an approximation have been able to predict some general behaviour in confirmation with the experimental results, predictions about a quantitative measure would be less approximate if the models could incorporate elastic inhomogeneity. The presented linear stochastic model for SAQD growth incorporates elastic inhomogeneity and anisotropy along with stochastic surface diffusion to produce a more refined quantitative model for SAQD order estimation. For the Ge/Si film-substrate system it is found that at the critical film height such an approximation could lead to an error of $\sim12$\% in the estimation of average spacing between SAQDs and an error of $\sim24$\% in the estimation of number of correlated dots for small height fluctuations. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A20.00006: Composition Maps in Strained Alloy Quantum Dots Nikhil Medhekar, Vishwanath Hegadekatte, Vivek Shenoy Knowledge of composition profiles within self-assembled SiGe and InGaAs quantum dots is critical for applications in optoelectronic and memory devices as variations in composition at the nanoscale can substantially influence their electronic properties. Obtaining the quantitative description of composition profiles in the quantum dot is a challenging task due to the coupling between composition variations, shape of the quantum dots and long-range elastic interactions. In this talk, we present an efficient scheme that combines the finite element analysis with an optimization scheme based on a quadratic programming method to determine equilibrium profiles in strained quantum dots. Composition profiles are found to strongly depend on the shape of the quantum dots, as strain relaxation in dots with steeper sidewalls allows for segregation of the larger alloy component in the regions near the apex. Based on these observations, we have developed a phase diagram that shows the degree of segregation of the alloy components in the phase space spanned by the temperature (which governs chemical mixing) and the shape of the dot. Further, we find that the segregation of the alloy components can substantially reduce the critical dot size for the transition between the shapes with different facets. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A20.00007: An Accelerated Molecular Dynamics Study of the GaAs (001) $\beta $2(2x4) Reconstruction Maria Mignogna, Kristen Fichthorn The GaAs (001) $\beta $2(2x4) reconstruction is the most commonly used substrate for growth in GaAs homoepitaxy by molecular beam epitaxy. While the atomic positions of the $\beta $2(2x4) unit cell have been determined, reflection high energy electron diffraction and scanning tunneling microscopy images show long range disorder on this surface[1]. It is hypothesized that domains of anti-phase $\beta $2(2x4) unit cells can be created by vacancies or As dimer shifts. Accelerated molecular dynamics (MD) allows us to examine atomic scale processes that can lead to this disorder. We have developed an adaptive accelerated MD scheme based on the bond boost method of Miron and Fichthorn[2]. The adaptive method is suitable for the rough energy landscape presented by GaAs (001). In the adaptive method, both the length thresholds for determining transition states and the magnitude of the boost are calculated on the fly. We are able to extend the physical timescale of the simulation by several orders of magnitude. We see events that lead to small domains of As dimers shifting. By simulating RHEED images of the surface, we link the disorder to experiment. [1] D.W. Pashley, J.H. Neave, B.A. Joyce, Surf. Sci., \textbf{582}, 189 (2005) [2] R.A Miron, K.A. Fichthorn, J. Chem. Phys., \textbf{119}, 6210 (2003) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A20.00008: Strain and Piezoelectric Effects on the Electronic Structure of Coupled In$_{x}$Ga$_{1-x}$As/GaAs Self-Assembled Quantum Dots Usman Muhammad, Shaikh Ahmed, Gerhard Klimeck In$_{x}$Ga$_{1-x}$As/GaAs coupled quantum dot systems have gained much attention for optical and quantum computing applications. Due to strain, originating from the assembly of lattice- mismatched semiconductors, the quantum dot arrays tend to grow in the vertical direction. These vertically stacked quantum dots are strongly coupled through the strain field, which is atomistically inhomogeneous and penetrates deep into the GaAs buffer layer surrounding the dots. Crystal symmetry and atomistic details of interfaces are extremely important in such systems. Also piezoelectric fields must be taken into account to properly model the experimentally observed symmetry breaking and the introduction of a global shift in the energy spectra of the system. In this work, we present a detailed description of strain and piezoelectric potential effects on the electronic structure of closely coupled identical and non-identical quantum dot systems using sp$^{3}$d$^{5}$s* nearest neighbor empirical tight binding model. We show that strain causes strong mixing of s- and p- electron energy levels in strongly coupled quantum dot, splits heavy hole and light hole bands and even reverses their order within dots. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A20.00009: Structure competition in growth of In island on Si{111} from first-principles calculations Cai-Zhuang Wang, Min Ji, J. Chen, M. Hupalo, M.C. Tringides, K.M. Ho We have carried out first principles calculations to understand the growth of indium island on Si{111} substrate which have been observed to have an interesting FCC and BCT structure competition. Our calculations show that quantum size effect (QSE) plays an important role in different island structure formation. Furthermore, the interface energy between In and Si substrate also controls the relative stabilily of different island structures. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A20.00010: Thermodynamic potentials in closed and open nanocrystalline systems: Si-Ge islands on Si(001). Marina S. Leite, Angelo Malachias, Stefan W. Kycia, Ted I. Kamins, R. Stanley Williams, Gilberto Medeiros-Ribeiro The driving forces for alloying in Si-Ge epitaxial nanocrystalline islands were quantified experimentally. Closed and open systems were emulated by controlling surface diffusion kinetics [1]. Grazing Incidence X-Ray Diffraction (GIXRD) experiments were performed to map the composition and the strain distribution within the Si-Ge:Si(001) islands, permitting the evaluation of the relevant thermodynamic potentials for alloying. For the closed system the elastic strain energy increased, which was more than compensated by the increase in the local mixing entropy [2]. In contrast, for the open system, the elastic energy decreased and the mixing entropy increased, driven by the intermixing originated from the inflow of Si from the reservoir. For both systems, the evolution of the composition leads to a lowering of the Gibbs free energy. The results were in full agreement with a theoretical prediction of the optimum concentration for epitaxial islands. [1] M. S. Leite \textit{et al}, Phys. Rev. Lett. \textbf{98}, 165901 (2007). [2] G. Medeiros-Ribeiro \textit{et al}, Nano Lett. \textbf{7}, 223 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A20.00011: Characterization of MBE grown PbSe Quantum Dots Nathaniel Becker, Dustin Klein, Tim Kidd Lead selenide (PbSe) has been shown to be an excellent candidate for solar cell research due to its ability to allow the possibility for multiple electronic carrier production by absorption of a single photon. These quantum dots (QDs) were created using molecular beam epitaxy (MBE) to evaporate PbSe onto clean and modified silicon and germanium substrates. Control of lattice strain was achieved by the deposition of buffer layers onto clean Si(111)in ultra-high vacuum. The MBE technique allows for structural control at the atomic level. We have investigated the samples using Auger spectroscopy, scanning probe and scanning electron microscopy to determine their suitability for solar cell applications. Specifically, we investigated structural properties such as size, distribution, and uniformity to correlate such features with the electronic properties of the sample. Our initial results indicate the structural properties can be controlled with careful tuning of the substrate surface properties. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A20.00012: Self Assembling Quantum Dot Aggregates in Liquid Crystal Matrices Christopher Ferri, M. Gallardo, Y. Verma, D. Kelley, S. Ghosh A system of colloidal quantum dots (QDs) embedded in a matrix of highly directional and ordered liquid crystal (LC) molecules at room temperature offers the novel potential of promoting controllable aggregation of the QDs. Photoluminescence (PL) of GaSe QDs embedded in a LC matrix, studied using a confocal-microscopy setup, shows considerable red-shift in the emission spectrum of the QD-LC composite. While bare QDs in solution emit at 485 nm, mixing with LC molecules results in an emission centered at 500 nm, system suggesting their aggregation into longer structures in the matrix. A high resolution two-dimensional spatial map of the PL on the sample provided evidence of the organization of QDs into these ordered domains. Application of in-plane electric fields further enhances the aggregation effect of the QDs and emission spectrum is red-shifted to around 525 nm. Furthermore, as the aligning electric field increases the degree of ordering of the liquid crystal molecules, the polarization (P) of the emission of the aggregated QDs rotates in step with that of the LCs' directionality. Unlike the disc-shaped GaSe QDs, investigations on LC and CdSe QD system failed to show such dramatic aggregation effects. [Preview Abstract] |
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