Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models

We present a detailed study of the effect of local chemical ordering on the structural, electronic, and dynamical properties of amorphous gallium arsenide. Using the recently-proposed ``activation-relaxation technique'' and empirical potentials, we have constructed two 216-atom tetrahedral continuous random networks with different topological properties, which were further relaxed using tight-binding molecular dynamics. The first network corresponds to the traditional, amorphous, Polk-type, network, randomly decorated with Ga and As atoms. The second is an amorphous structure with a minimum of wrong (homopolar) bonds, and therefore a minimum of odd-membered atomic rings, and thus corresponds to the Connell-Temkin model. By comparing the structural, electronic, and dynamical properties of these two models, we show that the Connell-Temkin network is energetically favored over Polk, but that most properties are little affected by the differences in topology. We conclude that most indirect experimental evidence for the presence (or absence) of wrong bonds is much weaker than previously believed and that only direct structural measurements, i.e., of such quantities as partial radial distribution functions, can provide quantitative information on these defects in a-GaAs.Comment: 10 pages, 7 ps figures with eps

Topology of amorphous tetrahedral semiconductors on intermediate lengthscales

Using the recently-proposed ``activation-relaxation technique'' for optimizing complex structures, we develop a structural model appropriate to a-GaAs which is almost free of odd-membered rings, i.e., wrong bonds, and possesses an almost perfect coordination of four. The model is found to be superior to structures obtained from much more computer-intensive tight-binding or quantum molecular-dynamics simulations. For the elemental system a-Si, where wrong bonds do not exist, the cost in elastic energy for removing odd-membered rings is such that the traditional continuous-random network is appropriate. Our study thus provides, for the first time, direct information on the nature of intermediate-range topology in amorphous tetrahedral semiconductors.Comment: 4 pages, Latex and 2 postscript figure

Characterization of optical properties and surface roughness profiles: The Casimir force between real materials

The Lifshitz theory provides a method to calculate the Casimir force between two flat plates if the frequency dependent dielectric function of the plates is known. In reality any plate is rough and its optical properties are known only to some degree. For high precision experiments the plates must be carefully characterized otherwise the experimental result cannot be compared with the theory or with other experiments. In this chapter we explain why optical properties of interacting materials are important for the Casimir force, how they can be measured, and how one can calculate the force using these properties. The surface roughness can be characterized, for example, with the atomic force microscope images. We introduce the main characteristics of a rough surface that can be extracted from these images, and explain how one can use them to calculate the roughness correction to the force. At small separations this correction becomes large as our experiments show. Finally we discuss the distance upon contact separating two rough surfaces, and explain the importance of this parameter for determination of the absolute separation between bodies.}Comment: 33 pages, 14 figures, to appear in Springer Lecture Notes in Physics, Volume on Casimir Physics, edited by Diego Dalvit, Peter Milonni, David Roberts, and Felipe da Ros

STABILITY AND ELECTRONIC PROPERTIES OF VACUUM-EVAPORATED AMORPHOUS METALLIC Au-Ge ALLOY FILMS

On discute les conditions d'existence et la stabilité d'alliages métalliques amorphes Au-Ge préparés par co-évaporation sous ultra-vide. Leurs propriétés optiques sont analysées en contribution de Drude et contribution interbande. On en déduit la présence d'hybridation sp-d dans ces alliages.The existence and stability of amorphous metallic Au-Ge alloys prepared by co-evaporation under ultra-high vacuum are discussed. Their optical properties are analysed in terms of a Drude and an interband contribution. Occurrence of sp-d hybridization is inferred

Band tails and deep-defect density of states in hydrogenated amorphous germanium

International audienceA correlation between the midgap-defect-state density and the Urbach energy is experimentally observed in undoped hydrogenated amorphous germanium (a-Ge:H) thin films prepared by the glow-discharge technique under different conditions. Since these samples show thermally induced metastable effects, the data are analyzed with a thermal-equilibration model recently proposed for a-Si:H. This yields a neutral Ge dangling-bond (D0) energy at 0.50–0.55 eV above the valence-band edge

Hydrogen, microstructure and defect density in hydrogenated amorphous silicon

It is well established that by bonding with the dangling bonds of silicon, hydrogen reduces the density of states of amorphous silicon and renders this material suitable to electronic applications. For so-called “standard” a-Si : H films deposited by the RF glow discharge decomposition of silane at low deposition rates ($\approx 1$ Å/s) and over a large range of deposition temperatures, we observed the usual correlation between the hydrogen bonding and the defect density in the as-deposited material only. It clearly appears that the widely accepted correlation between the hydrogen bonding, the microstructure and the defect density only applies to a limited set of deposition parameters. Moreover we found that the deposition of a-Si : H from mixtures of ................Il est admis qu'en s'attachant aux liaisons pendantes du silicium, l'hydrogène réduit la densité d'états du silicium amorphe et fait de ce matériau un semi-conducteur apte aux applications électroniques. Pour des échantillons '“`standard”, élaborés à partir de la décomposition du silane dans un plasma de décharge RF à faible vitesse de dépôt ($\approx 1$ Å/s), nous observons que la présence d'une bande d'absorption à 2 090 cm$^{-1}$ dans le spectre infrarouge est corrélée avec une densité de défauts élevée dans le matériau. Cependant, cette corrélation n'est valable que dans une plage réduite dans l'espace des paramètres de dépôt. De plus, nous avons observé que le a-Si : H élaboré à partir de mélanges silane-hélium à des vitesses élevées (8-16 Å/s) présente une forte bande d'absorption à 2 090 cm$^{-1}$, bien que la densité de défauts soit faible. Nous présentons ici les résultats d'une étude sur les propriétés opto-électroniques de couches minces de a-Si : H préparées sous différentes conditions de plasma. Nos résultats montrent que l'on peut élaborer des matériaux a-Si : H très différents avec une faible densité d'états. Il est donc envisageable de choisir les conditions d'élaboration de façon à obtenir les propriétés opto-électroniques souhaitées pour différentes applications