Surface-controlled deposition

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

ECALE is then ALE where the elements are deposited by controlling the substrate s electrochemical potential, so that atomic layers are formed at underpotentials [Eq. (1)]. The underpotentials are used in order to obtain surface-limited deposition reactions. Compounds are deposited using a cycle where a first solution containing a precursor to one of the elements is introduced to the substrate and an atomic layer is electrodeposited at its underpotential. The cell is then rinsed, a solution containing a precursor to... 

Solid Particulates These contaminants can originate from a variety of sources, and their presence is a major concern because they can block gas passages and/or the anode surface. Carbon deposition and conditions that can be used to control its formation have been discussed earlier in this section. Solid particles such as ZnO, which is used for sulfur removal, can be entrained in the fuel gas leaving the desulfurizer. The results by Pigeaud (72) indicate that the tolerance limit of MCFCs to particulates larger than 3 pm diameter is <0.1 g/1. 

In many supported catalytic systems, it is nearly impossible to determine either the specific species, responsible for the observed catalytic activity, or the mechanistic pathway of the reaction. Using a defined SAM system in which careful molecular design is followed by controlled deposition into a solid-supported catalyst of known morphology, surface coverage, mode of binding and molecular orientation, allows direct correlation of an observed catalytic activity with the structure on the molecular scale. SAM and LB-systems allow detailed and meaningful studies of established surface bound catalysts to understand their behavior in heterogeneous... 

As an example let us consider the pentacene/samarium interface (Koch et al, 2002). Samarium has a low work function ((/>m — 2.7 eV), which is comparable to E a from pentacene (— 2.7 eV). Thus, if A 0, the condition Ep, should provide efficient electron injection because in this case fp and LUMO are nearly aligned. In order to avoid contamination that may alter the instrinsic m, h and homo values, such heterostructures have to he prepared in ideally clean conditions, imposing the use of UHV. The UPS experiments performed with synchrotron radiation are shown in Fig. 4.24. After measuring (pM of the clean samarium surface (2.7 eV) as described above, increasing amounts of pentacene are controllably deposited onto the samarium surface. The survey spectra of the valence states and a close-up view of the energy region near E are shown in Figs. 4.24(a) and (b). 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

One of the ultimate goals in organic functionalization is to be able to control deposition of multiple organic layers so that precisely tailored surfaces may be... 

In using CVD for microelectronics applications, the deposit thickness must be as uniform as possible. This can best be achieved by conducting the deposition in a surface-controlled, not a diffusion-controlled, regime. 

The Damkohler numbers are useful measures of the characteristic transport time relative to the reaction time. If the surface Damkohler number (sometimes referred to as the CVD number see reference 7) is large, mass transfer to the surface controls the growth. For small Damkohler numbers, surface kinetics governs the deposition. Similarly, if the gas-phase Damkohler number is large, the reactor residence time is an important factor, whereas if it is small, gas-phase reactions control the deposition. 

Almost all hydroconversion processes use a catalyst or additive to control the formation of coke, to serve as a surface for deposition of metals, and to enhance hydrogenation reactions. 

In the mid-1970s, it was realized that low-pressure CVD processing could have significant advantages over atmospheric pressure systems. By reducing the pressure, it was found that the diffusion coefficient was sufficiently enhanced that deposition became surface controlled (see Chapter 1). In this case, wafers could be stacked closely and placed in a diffusion furnace to be processed... 

In Chapter 2, we reviewed the concept of carrying out CVD processes at low pressure so that deposition becomes surface controlled. When the only thing controlling the uniformity of deposition is the temperature of the wafer surface, all we have to do is ensure that the wafer is in a uniform temperature furnace. Again, at low pressures, the diffusion coefficient is so large that we can stack wafers up next to each other so 50 to 100 can be placed in a long tubular furnace. 

Pitner and Hussey studied the electrochemistry of tin in acidic and basic AICI3/I-ethyl-3-methyl-imidazolium chloride-based ionic liquids by using voltammetry and chronoamperometry at 40 °C [15]. They reported that the Sn(II) reduction process is uncomplicated at a platinum substrate, where in the atidic ionic liquid the reduction wave was observed at +0.46 V on the Pt electrode and the oxidation at +0.56 V. When they used a gold electrode instead of platinum, they observed an underpotential deposition of a tin monolayer and an additional underpotential deposition process that was attributed to the formation of tin-gold alloy at the surface. The deposition of tin on glassy carbon was controlled by nudeation. 

Experimental determinations of barrier heights on oxide semiconductor interfaces using photoelectron spectroscopy are rarely found in literature and no systematic data on interface chemistry and barrier formation on any oxide are available. So far, most of the semiconductor interface studies by photoelectron spectroscopy deal with interfaces with well-defined substrate surfaces and film structures. Mostly single crystal substrates and, in the case of semiconductor heterojunctions, lattice matched interfaces are investigated. Furthermore, highly controllable deposition techniques (typically molecular beam epitaxy) are applied, which lead to films and interfaces with well-known structure and composition. The results described in the following therefore, for the first time, provide information about interfaces with oxide semiconductors and about interfaces with sputter-deposited materials. Despite the rather complex situation, photoelectron spectroscopy studies of sputter-deposited... 

PLD in UHV (laser-MBE) MBE-like background pressure and in situ RHEED to ensure clean and controlled deposition of high-quality nucleation layers and films. For particular systems as SrTiC>3 and BaTiC>3, atomically smooth surface and interface were obtained [128,132]... 

A crucial omission in the research on residual insecticides for bark beetle control has been the absence of corollary studies relating control effectiveness to the physical structure of insecticide deposits on and in bark. The importance of deposit structure has been well documented (3, 21, 36). The structure of the deposit affects its availability to the insect contacting it and governs its toxicity. On an absorbent surface like bark, two major types of residues can be created deposits on the surface and deposits in the bark tissue. Henceforth, these will be referred to as surface and tissue deposits, respectively. Dusts and wettable powders form surface deposits. Solutions and emulsions penetrate and form mainly tissue deposits, though they may not remain in the tissue. The insecticide may crystallize out of solution, forming a deposit of fine crystals on the bark surface. 

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