Flow system deposition

A long tube with the coupling coil at the middle was used for the plasma polymerization of TFE in an inductively coupled radio-frequency glow discharge using a flow system. Deposition rates and the chemical nature of the polymer were detected as a function of location in the reactor tube relative to the coupling coil and of applied... 

CVD reactions are most often produced at ambient pressure in a freely flowing system. The gas flow, mixing, and stratification in the reactor chamber can be important to the deposition process. CVD can also be performed at low pressures (LPCVD) and in ultrahigh vacuum (UHVCVD) where the gas flow is molecular. The gas flow in a CVD reactor is very sensitive to reactor design, fixturing, substrate geometry, and the number of substrates in the reactor, ie, reactor loading. Flow uniformity is a particulady important deposition parameter in VPE and MOCVD. 

In PECVD, the plasma generation region may be in the deposition chamber or precede the deposition chamber in the gas flow system. The latter configuration is called remote plasma-enhanced CVD (RPECVD). In either case, the purpose of the plasma is to give activation and partial reaction/reduction of the chemical precursor vapors so that the substrate temperature can be lowered and still obtain deposit of the same quaUty. 

The group of Stickney reported the first deposits of mercury selenide (HgSe) formed via ECALE, using a thin layer flow system [54, 55]. The solutions used were HgO (pH 2) and Se02 (pH 3) with Na2S04 as a supporting electrolyte, and the... 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

An automated flow system has also been used by Foresti et al. to form CdS layers, with up to 150 cycles, using pH 9.2 solutions for both elements on Ag(lll) electrodes [116], In their case, the deposits appeared stoichiometric, without the excess S previously observed by this group [111]. Their cycle produced relatively thin deposits, similar to this author s, or about 1/3 ML/cycle. 

The first electrodeposition of a compound superlattice appears to have been by Rajeshwar et al. [219], where layers of CdSe and ZnSe were alternately formed using codeposition in a flow system. That study was proof of concept, but resulted in a superlattice with a period significantly greater then would be expected to display quantum confinement effects. There have since been several reports of very thin superlattices formed using EC-ALE [152, 154, 163, 186], Surface enhanced Raman (SERS) was used to characterize a lattice formed from alternated layers of CdS and CdSe [163]. Photoelectrochemistry was used to characterize CdS/ZnS lattices [154, 186]. These EC-ALE formed superlattices were deposited by hand, the cycles involving manually dipping or rinsing the substrate in a sequence of solutions. 

However, surfactants have been used not only to enhance the signal of CL systems, but also to avoid problems of solubility in these systems. Thus, Klopf and Nieman have used SDS, at a submicellar concentration, to solubilize the product (TV-mcthylacridonc) of the CL reaction of lucigenin, due to its insolubility in water [9], In this way the appearance is avoided of solid deposits in the observation cell and in other components of the flow system. 

High shear forces are prevelant in the approach flow system to the paper machine (i.e. as the fibre suspension approaches the point of deposition on the wire), and these have a large impact upon the efficiency of retention aids (Figure 7.8). A study of the effect of shear can often be helpful in establishing the mechanism of retention. Bridging flocculation is irreversibly sensitive to shear (i.e. when the shear forces are removed the suspension does not reflocculate) whereas charge neutralisation is reversibly sensitive to shear. 

As mentioned above, initial work in automating ECALE involved use of a thin-layer flow cell-deposition system. A thin layer cell was chosen because it allowed very small amounts of solution to be used in each step, minimizing the total volume of solution used to form a deposit. Development of an automated thin layer flow deposition system began early on and proceeded by a long sequence of incremental improvements (Fig. 19) [158]. 

The images in Fig. 24 were all run on Au on Si wafers using the thin layer flow cell deposition system. Figure 24A is an optical micrograph of one of the best deposits formed with the thin layer flow cell system. 

A thin film of manganese oxides deposited over a glassy earbon electrode dramatically lowers the overpotential for oxidation of various hydrazines ad hydrogen peroxide, thereby facilitating their amperometric detection in flow systems. Sensors based on this principle are highly sensitive and provide... 

Pitts et al. (1986) exposed five individual PAHs, pyrene, fluoranthene, benz[a]anthracene, BeP, and BaP, deposited on glass fiber and Teflon-impregnated glass fiber filter (TIGF) substrates passively for 3 h in the dark in a 360-L Teflon environmental chamber to 50-300 ppb of 03 in air at several relative humidities. These experimental conditions more nearly resemble the actual exposure of ambient particles to 03 (in the dark) during transport than do exposures in Hi-Vol flow systems. Consistent with earlier studies, BaP, BaA,... 

Since the envisionaged application of a CD process in thin-fihn solar cells is a large-scale one, efforts have been made to optimize the deposition process used, particularly in minimizing the waste Cd-containing solutions. Dilute Cd solutions (ca. 1 mM), a flow system with filtration, and a heated substrate have been employed to this end. The heated substrate means that deposition occurs preferentially on the substrate rather than on the cooler walls of the deposition vessel. Also, ethylenediamine has been used as a complexant rather than the much more volatile ammonia. 

In an attempt to say something intelligent about these resistivities, there appears to be some correlation between the pH and resistivity, with low resistivity obtained when the pH is relatively low (only a few experiments have been carried out at relatively low values of pH also note Ref. 22, which describes an anomalously low resistivity even at normal values of pH). The bath described by Ito and Shiraishi [37] is very different from the previous ones, for three reasons the relatively low pH (= 8), the use of thioacetamide instead of thiourea, and the flow system used in this deposition. Very low values of dark resistivity were obtained with this bath and with an unusual temperature dependence (a minimum of 10 fi-cm was found at 63°C, which increased on either side of this temperature value). It was suggested that Cl, from the NH4CI buffer, acted as a dopant however, other chloride baths gave much higher resistivities. 

Fig. 4.2 Flow system for continuous deposition using a locally heated substrate. (After Ref. 74 with permission from Elsevier Science.)...

SnOi was deposited on hydrolyzed Si and on Si coated with sulphonate-terminated self-assembled monolayers from a solution of SnCU in dilute HCl at 80° [45]. The films, up to 65 nm thick and consisting of a dense-packed aggregate of SnOi nanocrystals (5-10 nm) together with some amorphous basic tin oxide, contained ca. 3 at.% Cl. They were adherent on all substrates, although the adherence and homogeneity on Si was less reproducible than on the monolayer-coated Si. Films were also deposited using a continuous flow system. The films were sim-... 

The same group also deposited (Cd,Pb)S using a flow system [24]. In this case, metal cyanamides were not detected by XRD, presumably because the flow system removed the cyanamide. The rate of flow affected the crystal size Larger flow rate resulted in finer-grained deposits. Elemental analysis and XRD showed the incorporation of Cd in the films, again up to ca. 10%, as a solid solution. 

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