Substrate interface states

The saturation of Si bonds by hydrogen and oxygen will, of course, be different at the substrate interface and at the interface between a-Si H and its native oxide. Weisfield et al. (1981) found, for instance, that an excessive number of substrate interface states, which prevented measurements of the field effect in sputtered a-Si H, could be eliminated by depositing first a 200-A-thick SiO ( layer and then the a-Si H film without extinguishing the sputtering plasma. 

Fig. 13. Energy-band diagram for a-Si H/a-SiN , H superlattice showing the effect of pinning the Fermi level Ef by the substrate interface states is the band-bending potential, Xo the depletion width, and the discontinuity of the conduction bands at the a-Si H/a-SiN, H interface.

The boundary conditions require knowledge of the interface concentration of hydrogen ChjL to compute E (see below). For hydrogenations, the equilibrium concentration (ChjL= CfJ L) can be used, albeit with the assumption of no mass transfer resistance on the gas side. Otherwise, it must be determined using Eq. (4). The boundary conditions for the substrate S state that it is not transferred to the gas phase - that is, S is not vaporized. This assumption is most often... 

When molecules adsorb to a flat substrate, their conformation is modified due to the geometric confinement between the two interfaces and the direct interaction to the substrate. This state can be far from equilibrium if the adsorption process has been fast and irreversible. In this case, the molecules do not have time to sample the whole assembly of thermodynamic states and get trapped kinetically at contact sites. The reversibility is difficult to achieve because of the great size of the molecules and strong adhesion which might exceeds kBT by far. In order to approach an equilibrium state, the sample has to be pre-... 

The values of the best fit parameters are < o 1-0 e V and x = 300 A. This value for Xn corresponds to a bulk density of states of 7 X 10 cm eV" or, equivalently, to 10 cm" eV states per interface. This density of states is more than sufficient to pin the Fermi level in high-quality a-Si H, where the density of gap states in the upper half of the gap is < 10 cm eV. The fit, shown in Fig. 12, is excellent given the range of samples covered and the expected sensitivity of the band bending at the substrate interface to substrate preparation conditions. 

It must be appreciated, that surfaces of Nb, Ta, V and Zr will be immediately reoxidized or otherwise contaminated upon re-exposure to air, albeit with thitmer layers (or in the limit a monolayer) relative to thick native films often encountered before abrasion. As a minimum, monolayer adsorption of contaminants is almost certainly assured in all but the best ultra-high vacuum (< 1 x torr or <1 X 10 Pa) or in systems which simultaneously sputter substrate surfaces with argon or other inert ion during catalyst deposition. Some interfacial impurities between the substrate and the catalyst layer are tolerated in practice. However, the state of the substrate surface immediately prior to catalyst deposition is critical for wetting and adherence of the catalyst layers and for prevention of delamination. Theoretical flux maxima will not be achieved if thick impurity layers at the cata-lyst/substrate interface hinder hydrogen diffusion. 

The key to inducing self-organization onto water-solid substrate interfaces is to achieve mild adsorption under controlled conditions. If adsorbate-substrate interactions are too strong, molecules cannot move around on the substrate surface. On the other hand, when adsorbate-substrate interactions are too weak, molecules desorb from surfaces. Relatively mild adsorption conditions between these extreme states leads to induction of 2D self-organization of molecules via rapid surface diffusion and acceleration of the adsorption/desorption equilibrium. Electrochemical potential management would be convenient for AISO, because it allows for precise control of adsorption strength in units of mV [11,13, 14]. 

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