1,720,967 research outputs found
Semiclassical and quantum transport in Si/SiO2 superlattices
No full textIn the last few years, many research groups have been trying to develop electroluminescent devices based on silicon. In particular, it has been shown that low-dimensional structures, such as silicon clusters, quantum wires and quantum wells, are suitable for this purpose. In this work we investigate transport properties of a particular superlattice using two approaches. The first method is a Monte Carlo simulation of electron transport in the biased superlattice. The band structure is calculated using the envelope function approximation, and the scattering mechanisms introduced in the simulator are confined optical phonons. Owing to the particularly flat band structure, drift velocities are very low, but it will be shown that a parallel component of the electric field can significantly increase the vertical drift velocity. Moreover, a superlattice based device is proposed in order to obtain high recombination efficiency. Finally, a quantum calculation is introduced
Monte Carlo simulation of electron transport in Si/SiO2 superlattices: vertical transport enhanced by a parallel field
No full textConsiderable effort is presently devoted to develop Si quantum structures for microelectronics and nanoelectronics. In particular, well-defined Si/SiO2 superlattices and quantum wells are under study. We investigate here the transport properties of a Si/SiO2 superlattice with a multiband one-particle Monte Carlo simulator. The band structure is obtained with an analytical model and the scattering mechanisms introduced in the simulator are confined optical phonons, both polar and nonpolar. Owing to the very flat shapes of the bands along the growth direction, very low drift velocities are obtained for vertical transport. However, the simulation shows that, for oblique fields, the transport properties along the vertical direction are strongly enhanced by the in-plane component of the electric field, consequently higher vertical drift velocities can be easily obtained
Adsorption of Indium on an InAs wetting layer deposited on the GaAs(001) surface
No full textn this work, we perform a first-principles study of the adsorption properties of an In adatom deposited on a 1.75 ML( monolayer) InAs, forming a wetting layer on GaAs(001) with the alpha2(2×4) or beta2(2×4) reconstruction. Thus, we are addressing here the important case of the wetting layer at a high In coverage, just before the occurrence of the two-dimensional to three-dimensional transition. The structural properties of these reconstructions have been studied: we determine the equilibrium geometry of the surfaces and their stability for various growth conditions. We have then carried out a detailed study of the potential energy surface (PES) for an In adsorbate, finding the minima and the saddle points. The main characteristics of the PES and the bonding configurations of the In adatom on the surface are analyzed by comparing them with analogous studies reported in the literature, trying to extract the effects due to (i) the compressive strain to which the InAs adlayer is subjected, (ii) the particular surface reconstruction, and (iii) the wetting layer composition. We found that, in general, stable adsorption sites are located at (i) locations beside the As in-dimers, (ii) positions bridging two As in-dimers, (iii) between two adjacent ad-dimers (only in beta2), and (iv) locations bridging two As ad-dimers. We find also other shallower adsorption sites which are more reconstruction specific due to the lower symmetry of the alpha2 reconstruction compared to the beta2 reconstruction. We point out that the alpha2 reconstruction has the lowest symmetry among the reconstructions studied so far for this system
Monte Carlo analysis of electron heating in Si/SiO2 superlattices
No full textIn this work, we investigate the transport properties of Si/SiO2 superlattices with a multiband one-particle Monte Carlo simulator. Using the envelope function approximation, we solve the Kronig-Penney potential along the growth direction z using a tight-binding-like analytical form; we also consider parabolic dispersion along the xy plane. The scattering mechanisms introduced in the simulator are confined optical phonons, both polar and nonpolar. Owing to the very flat shape of the bands along the growth direction, very low drift velocities are found for vertical transport. However, the simulation shows that, for oblique fields, the transport properties along the vertical direction are strongly influenced by the in-plane component of the electric field: in this way higher vertical drift velocities can be obtained. The results point out that in-plane carrier heating and multiband properties are responsible for this behavior. (C) 2002 Elsevier Science B.V. All rights reserved
Erratum: Step energy and step interactions on the reconstructed GaAs(001) surface (Physical Review B - Condensed Matter and Materials Physics (2014) 90 (115314) DOI: 10.1103/PhysRevB.90.115314)
No full textWe report here the corrected values for the step energies and the step interactions calculated in our paper for a large set of different step configurations. We have found that one parameter, i.e., the smearing parameter governing the band occupation around the Fermi level, was not converged enough to predict the step properties with sufficient accuracy. Since the surfaces are metallic a smearing function had to be used for the state occupation around the Fermi level. We used the smearing function proposed by Marzari and Vanderbilt. We found that the parameter entering this expression is a particularly sensitive one. The previous paper used a commonly chosen value of 0.02 Ry. However, we found that a much smaller value for this parameter was necessary to obtain well converged values for the surface energies. The convergence of the smearing parameter has been carefully checked. Figure presented
Surface Effects on the Atomic and Electronic Structure of Unpassivated GaAs Nanowires
No full textOn the basis of accurate ab initio calculations, we propose a model for predicting the stability of III−V nanowires (NW) having different side walls and ridge configurations. The model allows us to obtain the NW formation energies by performing calculations only on relatively “small” systems, small diameter NWs and flat surfaces, to extract the contributions to the stability of each structural motif. Despite the idea illustrated here for the case of hexagonally shaped GaAs NWs grown along the [111]/[0001] direction, the method can also be applied generally to other differently shaped and oriented III−V NWs. The model shows that NW surfaces (side walls plus ridges) mainly determine the NW stability, so the changes to the surface structure (e.g., induced by defects or different growth conditions) would modify the final NW structure in a remarkable way. We find that wurtzite and zinc blende nanowires have similar energies over a wide diameter range, thus explaining the observed polytypism. Furthermore, new more stable ridge reconstructions are proposed for zinc blende nanowires. The surface-related structural motifs also have clear fingerprints on the NW electronic structure. We find that the more stable nanowires are all semiconducting. The band gaps are ruled by surface states and do not follow the trend mandated by the quantum confinement effect. Small diameter wurtzite nanowires have an indirect band gap, but for some of them, an indirect to direct transition can be foreseen to occur at larger diameters. Surface states have a larger impact on the zinc blende NW band gaps than on the wurtzite NW ones. Zinc blende nanowire band gaps reduce significantly with increasing nanowire radius, reaching the bulk value at a diameter of about 30 Å. The surface structure and the high surface related DOS below the conduction band are going to affect the nanowire dopant incorporation and efficiency when doping is carried out during the NW growth
Step energy and step interactions on the reconstructed GaAs(001) surface
Full text linkUsing ab initio total energy calculations we have studied the relation between the step atomic configuration and its properties (step energy, donor/acceptor behavior, and step interaction) on a beta(2)(2 x 4) reconstructed GaAs (001) surface. The results have been tested against the widely used elastic dipole model for the step energy and step interaction considered valid for stress-free surfaces. We have found that acceptor-behaving steps have an attractive interaction and donor-behaving steps have a repulsive interaction in contrast with the elastic dipole model which predicts always a repulsive interaction between like-oriented steps. To account for the attractive interaction we consider the electrostatic dipole interaction having the L-2 scaling with the step distance L and therefore compatible with the standard elastic model. Using a model charge distribution with localized point charges at the step based on the electron counting model we show that the electrostatic step interaction can indeed be generally attractive and of the same order of magnitude of the negative elastic dipole interaction. Our results show however that the usually employed dipole model is unable to account for the repulsive/attractive step interaction between donorlike/acceptorlike steps. Therefore, the ab initio results suggest an important electronic contribution to the step interaction, at least at the short step distances accessible to the first-principles study. Our results explain qualitatively many experimental observations and provide an explanation to the step bunching phenomenon on GaAs(001) induced by doping or by critical growth conditions as due to the stabilization of attractively interacting step structures. These ideas would lead to the development of a bottom-up surface step engineering
In adatom diffusion on InxGa1-xAs/GaAs(001): effects of strain, reconstruction and composition
No full textBy using density functional theory (DFT) calculations of the potential energy surface in conjunction with the analytical solution of the master equation for the time evolution of the adatom site distribution, we study the diffusion properties of an isolated In adatom on InxGa1-xAs wetting layers (WL) deposited on the GaAs(001). The WL reconstructions considered in this study are, listed in the order of increasing In coverage: c(4 x 4), (1 x 3), (2 x 3), alpha(2)(2 x 4) and beta(2)(2 x 4). We analyze the dependence of the diffusion properties on WL reconstruction, composition and strain, and find that: (i) diffusion on the (2 x N) reconstructions is strongly anisotropic, owing to the presence of the low barrier potential in-dimer trench, favoring the diffusion along the [(1) over bar 10] direction over that along the [110] direction; (ii) In diffusion at a WL coverage theta = 2/3 monolayers (ML; with composition x = 2/3) is faster than on clean GaAs(001) c(4 x 4), and decreases at theta = 1.75 ML (x = 1; e. g. InAs/GaAs(001)); (iii) diffusion and nucleation on the (2 x 4) WL is affected by the presence of adsorption sites for indium inside the As dimers; (iv) the approximation used for the exchange-correlation potential within DFT has an important effect on the description of the diffusion properties
Monte Carlo simulation of electron transport in Si/SiO2 superlattices
No full textABSTRACT In this work we investigate the transport properties of Si/SiO2 superlattices with a multiband one-particle Monte Carlo simulator. The band structure of the system is obtained analytically by solving the Kronig-Penney potential in a tight binding approximation along the growth direction z while we have assumed parabolic dispersion in the in-plane directions. We have introduced in the simulator confined optical phonons, both polar and non polar, as scattering mechanisms. Owing to the very flat shape of the bands along the growth direction, very low drift velocities are obtained for vertical transport. However it turns out that for oblique fields, the in-plane component of the electric field strongly influences the transport properties along the vertical direction as effect of carrier heating. In particular higher vertical drift velocities can be obtained
Indium surface diffusion on InAs (2x4) reconstructed wetting layers on GaAs(001)
No full textIn this paper we present a study of In surface diffusion on InAs wetting layers deposited on the (001) surface of GaAs. The alpha2(2×4) and beta2(2×4) reconstructions stabilized by a high In concentration are considered. The low symmetry of the alpha2(2×4) reconstruction allowed us to understand the effect of the wetting-layer symmetry on the adsorbate diffusion. We find that (i) the diffusion coefficient value is larger for In motion on the alpha2 reconstruction than on the beta2 reconstruction. This is due to the presence on beta2 of an additional As dimer that rises locally the potential energy surface and offer an additional site to which the In adatom can bind strongly. (ii) The In adsorption sites located within the As dimers have to be taken into account properly for these specific reconstructions, since they greatly affect the value of the diffusion coefficient. This is in contrast to what happens for the other reconstructions reported in the literature. (iii) The adsorbate diffusion is highly anisotropic with the [[overline 1]10] direction favored over the [110] direction, due to the presence of low-potential channels along [[overline 1]10]. (iv) The anisotropy is slightly smaller on the alpha2 reconstruction than on the beta2 reconstruction because on the beta2 there is an additional diffusion channel along the [[overline 1]10] direction.In this paper we present a study of In surface diffusion on InAs wetting layers deposited on the (001) surface of GaAs. The α2 (2×4) and β2 (2×4) reconstructions stabilized by a high In concentration are considered. The low symmetry of the α2 (2×4) reconstruction allowed us to understand the effect of the wetting-layer symmetry on the adsorbate diffusion. We find that (i) the diffusion coefficient value is larger for In motion on the α2 reconstruction than on the β2 reconstruction. This is due to the presence on β2 of an additional As dimer that rises locally the potential energy surface and offer an additional site to which the In adatom can bind strongly. (ii) The In adsorption sites located within the As dimers have to be taken into account properly for these specific reconstructions, since they greatly affect the value of the diffusion coefficient. This is in contrast to what happens for the other reconstructions reported in the literature. (iii) The adsorbate diffusion is highly anisotropic with the [1̄ 10] direction favored over the [110] direction, due to the presence of low-potential channels along [1̄ 10]. (iv) The anisotropy is slightly smaller on the α2 reconstruction than on the β2 reconstruction because on the β2 there is an additional diffusion channel along the [1̄ 10] direction. © 2009 The American Physical Society
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