Atomic adsorption, semiconductor surfaces

In conclusion to this part it seems noteworthy that in contrast to the effect of adsorption of molecular particles on electrophysical properties of oxide semiconductors, the major peculiarity of this effect for such chemically active particles as the simplest free radicals or atoms of simple gases (H2, O2, N2, CI2, etc.) is that they are considerably more chemically active concerning the impurity centres [47]. The latter are responsible for dope conductivity of oxide semiconductors. As for the influence of electric fields on their adsorption due to adsorption-induced surface charge distribution, they are of minor importance which is proved by results of the experiments on assessing field effect on adsorp-... 

Fig. 11. Schottky diode device used for measurement of chemicurrents. Highly exoergic surface reactions like adsorption of an atom to the surface produce excited electrons and holes. Some of these electrons are able to surmount the Schottky barrier and arrive at the semiconductor conduction band. This results in a detectable chemicurrent. (From Ref. 64.)...

The NEB method has been applied successfiilly to a wide range of problems, for example studies of diffusion processes at metal surfaces,28 multiple atom exchange processes observed in sputter deposition simulations,29 dissociative adsorption of a molecule on a surface,25 diffusion of rigid water molecules on an ice Ih surface,30 contact formation between metal tip and a surface,31 cross-slip of screw dislocations in a metal (a simulation requiring over 100,000 atoms in the system, and a total of over 2,000,000 atoms in the MEP calculation),32 and diffusion processes at and near semiconductor surfaces (using a plane wave based Density Functional Theory method to calculate the atomic forces).33 In the last two applications the calculation was carried out on a cluster of workstations with the force on each image calculated on a separate node. 

All the inelastic scattering data discussed in this part of the review demonstrate the differences between H2 adsorption at a metallic and a semiconductor surface. They show the former preferentially occurs at multiply bonded sites located between the surface atoms whereas, in constrast, on MoS2 H2 adsorption is on top of a single atom. 

The surface topography is not only found to be important for metals, but also for elemental and compound semiconductors adsorption often occurs preferentially on the incompletely coordinated surface atoms of disordered surfaces, while ordered surfaces are relatively inert. 

Before a detailed presentation of the ab initio dynamics simulations, first the fundamental difference between atomic and molecular adsorption on the one hand and dissociative adsorption on the other hand has to be addressed. Then I will briefly discuss the question whether quantum or classical methods are appropriate for the simulation of the adsorption dynamics. This section will be followed by a short introduction into the determination of potential energy surfaces from first principles and their continuous representation by some analytical or numerical interpolation schemes. Then the dissociative adsorption and associative desorption of hydrogen at metal and semiconductor surfaces and the molecular trapping of oxygen on platinum will be discussed in some detail. 

This is different at semiconductor surfaces where the covalent bonds between the substrate atoms are often strongly perturbed by the presence of adsorbates. This can result in a significant surface restructuring. Hence the dynamics of the substrate atoms has to be explicitly taken into account which of course increases the complexity of the modelling of the adsorption/desorption dynamics, as will be shown below for the H2/Si system. 

Mui et al.36 report a comparative experimental - theoretical study of amines on both the Si(001)-(2x 1) and the Ge(001)-(2x 1) surface. Both substrates were modeled by X9H12 (X = Si, Ge) clusters, utilizing DFT at the BLYP/6-31G(d) level of theory. For both, the Si and the Ge substrate, formation of a X-N dative bond (X = Si, Ge) is the initial step of the reaction between the considered amine species and the semiconductor surface. Flowever, while primary and secondary amines display N-H dissociation when attached to Si(001)-(2 x 1), no such trend is observed for the Ge counterpart of this system. This deviating behavior may be understood in terms of the energy barrier that separates the physisorption from the chemisorption minimum, involving the cleavage of an H atom. For dimethylamine adsorption, this quantity turned out to be about 50% higher for the Ge than for the Si surface. The authors relate this characteristic difference between the two substrates to the different proton affinities of Si and Ge. 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

Atomic adsorption on elemental semiconductors 3.1.2.1. A tomic adsorption on (100) surfaces... 

Structural parameters for atomic adsorption on the (110) and (1010) surfaces of zincblende and wurtzile structure compound semiconductors. The bond length is that of the anion-cation dimer in the first layer. D is the tilt angle in the top layer. The other parameters arc defined by fig. 16. 

After 30 years of continuing investigation, the adsorption properties of the noble ses on metal and semiconductor surfaces have recently attracted renewed interest. On the one hand, some fundamental aspects have come within the reach of modem experimental and theoretical techniques, sueh as the very nature of physisorption and the noble gas - substrate interaction, the possibility to study growth and surface kinetics at the atomic scale, and the recent interest in nanoscale surface friction and related tribological issues, where noble gas adlayers serve as model systems [99P]. On the other hand, noble gas adsorption is being used as a non-destmctive and quantitative surface analytical tool as, for instance, in photoemission of adsorbed xenon (PAX) [97W] and for titration analysis of heterogeneous surfaees based on the site specificity of the interaction strength [96S, 98W]. 

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