Substrate atom diffusion

Autodopiag occurs whea dopants are unintentionally released from a substrate through diffusion and evaporation, and subsequently reiacorporated during the deposition layer. Epitaxial layers are typically doped at concentrations of lO " -10 atoms/cm. The higher levels of doping are used in bipolar technology where the epilayer forms the transistor base. The epitaxial layer can be up to several hundred micrometers, and as thin as 0.05—0.5 p.m. Uniformities of 5% are common. 

There was therefore a clear need to assess the assumptions inherent in the classical kinetic approach for determining surface-catalysed reaction mechanisms where no account is taken of the individual behaviour of adsorbed reactants, substrate atoms, intermediates and their respective surface mobilities, all of which can contribute to the rate at which reactants reach active sites. The more usual classical approach is to assume thermodynamic equilibrium and that surface diffusion of reactants is fast and not rate determining. 

In an effort to understand silicon surface diffusion, NoorBatcha, Raff and Thompson have employed molecular dynamics to model the motion of single silicon atoms on the Si(001) and Si(lll)surfaces. Morse functions are used for the pair forces, with the parameters being determined by the heat of sublimation. Because different forces were used for the diffusing and substrate atoms, the incorporation of gas-phase species into the crystal could not be directly modeled. Nonetheless, they were able to explore the characteristics of adsorption and diffusion for single atoms. 

Two specific illustrative cases of the extreme limits of behavior are given by Al deposition on CH3 and -COOCH3 terminated hexadecanethiolate/Au SAMs [20, 21]. The -CH3 terminated SAM case shows a spectrum of deposition modes, including penetration to the Au-SAM interface and ambient surface overlayer formation. The penetration was explained in terms of Al atoms diffusing into dynamically formed temporal vacancies in the SAM (see Sect. 2) caused by fluctuations of Au thiolate moieties around their equilibrium positions on the Au substrate [11-13]. Once the Al atoms arrive at the substrate, energetically favorable insertion into S Au bonds can occur. This in turn can result in strongly decreased... 

In a molecular dynamic simulation147 of bulk atomic diffusion by a vacancy mechanism, two atoms may occasionally jump together as a pair. The temperature of the simulation is close to the melting point of the crystal. In FTM studies of single atom and atomic cluster diffusion, the temperature is only about one tenth the melting point of the substrate. All cluster diffusion, except that in the (1 x 1) to (1 x 2) surface reconstruction of Pt and Ir (110) surfaces already discussed in Section 4.1.2(b), are consistent with mechanisms based on jumps of individual atoms.148,149 In fact, jumps of individual atoms in the coupled motion of adatoms in the adjacent channel of the W (112) surface can be directly seen in the FTM if the temperature of the tip is raised to near 270 K.150... 

Compositional analysis involves the determination of three quantities. The most fundamental of these is the elemental identity of surface species, i.e., the atomic number of each species. It also is desirable to know, however, the chemical identities of these species. For example, is CO adsorbed as a molecule or is it dissociated into separate C and 0 complexes with the substrate. Finally, it is necessary to determine the approximate spatial location of the various chemical species. Are they "on top" an otherwise undisturbed substrate Do they reconstruct the substrate or diffuse into it, e.g., along grain boundaries Or perhaps they form localized islands or even macroscopic segregated phases at various positions across the surface. An important trend in modern compositional analysis is the increasing demand for spatial resolution laterally across the surface on a scale (d 0.1 u m = 10 A) comparable to the dimensions of modern integrated circuits (10-12). Compositional analysis is by far the most extensively used form of surface analysis and is the subject of most of the papers in this symposium as well as of numerous reviews in the literature (5-9., 13, 14). 

The chapter is organized as follows. In Section 2, we give a brief overview of the silicon and germanium (0 01) surfaces. Sections 3-7 cover various topics related to the dimer diffusion studies including the stability of various ad-dimer adsorption sites (Section 3), the existence of various diffusion pathways (Section 4), rotation of an on-top ad-dimer (Section 5), diffusion driven concerted motion of substrate atoms (Section 6) and intermixing (Section 7). In Section 8, we will briefly address the influence of the STM tip on the experimentally obtained activation barriers for diffusion and rotation. Finally, Section 9 contains a summary and the most important conclusions. 

DIFFUSION DRIVEN CONCERTED MOTION OF SUBSTRATE ATOMS... 

In a very recent study the lattice calculations have been generalized to biased diffusion [44]. The difference between the tracer atom and the substrate atoms was taken into account by having different vacancy-tracer and vacancy-substrate exchange probabilities, while the rate of vacancy moves was kept constant. A repulsive interaction reduces, while a moderately attractive interaction increases the spreading of the tracer distribution. 

In this subsection we describe a discrete model for vacancy-mediated diffusion of embedded atoms, solve it numerically for the case of In/Cu(0 0 1), and present the results. Our model is defined on a two-dimensional simple square lattice of size / x / (typically, l = 401) centered around the origin. This corresponds to the top layer of a terrace of the Cu(00 1) surface, with borders representing steps. The role of steps in the creation/annihilation of vacancies will be discussed in more detail in the next section. All sites but two are occupied by substrate atoms. At zero time the two remaining sites are the impurity (or tracer) atom, located at the origin, and a vacancy at position (1,0). This corresponds to the situation immediately after the impurity atom has changed places with the vacancy. 

An additional advantage of molecular beam epitaxy over chemical vapor deposition is that lower substrate temperatures are used in molecular beam epitaxy. The high temperatures required to effect a chemical reaction in chemical vapor deposition are not needed in molecular beam epitaxy. Given the extremely thin nature of the films, atomic diffusion is kept to small distances, and hence the small diffusion coefficients do not seriously retard the overall reaction rate. The difficulty presented by small diffusion coefficients with respect to chemical reactions between bulk solids is discussed in Chapter 5. 

Mordenite is a naturally occurring zeolite with a Si/Al ratio of ca. 10 and a structure composed of 12-ring and 8-ring tunnels with diameters of 0.39 nm and ca. 0.7 nm, respectively, extending through the entire framework (Fig. 2.7). Every framework atom forms a part of the walls of these tunnels and is accessible to substrate molecules diffusing through them. 

The continued oxidation of the metal substrate beneath the protective oxide layer must become a diffusion-controlled process for thick enough oxide films in which either metal atoms or oxygen atoms diffuse through the metal oxide layer to the appropriate interface where reaction proceeds. Let us assume a thick enough oxide layer on a plane metal surface where a steady state has been achieved. Then we can write for the rate of formation of metal oxide, MO, per unit area (assuming metal ion diffusion) ... 

As far as the effect of substrate temperature was concerned, it was suggested that at temperatures lower than 70°C, impingement of low energy carbon ions creates immobile carbon interstitials and therefore the sp rich phase is formed. With increasing temperature the mobility increases and the atoms diffuse to the surface releasing the internal stress to form the thermodynamically stable sp phase. 

Current fluctuations from clean (310), (211), and (100) tungsten surfaces have been studied by Swanson and attributed to the earliest stages in surface self-diffusion in which surface atoms diffuse on the terraces between plane edges. The observed threshold temperatures for noise on (310), (211), and (100) surfaces which are respectively 300 K, 600 K, and 1000 K, serve as a salutary reminder of the ease with which substrate atoms may become mobile and hence involved in adsorbate behaviour. 

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