Surface limitations

As we have seen, the electron is the easiest probe to make surface sensitive. For that reason, a number of hybrid teclmiques have been designed that combine the virtues of electrons and of other probes. In particular, electrons and photons (x-rays) have been used together in teclmiques like PD [10] and SEXAFS (or EXAFS, which is the high-energy limit of XAES) [2, Hj. Both of these rely on diffraction by electrons, which have been excited by photons. In the case of PD, the electrons themselves are detected after emission out of the surface, limiting the depth of sampling to that given by the electron mean free path. 

Onishi, H., Fukui, K. and Iwasawa, Y. (1995) Atomic-scale surface structures of TiO2(110) determined by scanning tunneling microscopy A new surface-limited phase of titanium oxide. Bull. Chem. Soc. Jpn., 68, 2447—2458. 

Historically, EC-ALE has been developed by analogy with atomic layer epitaxy (ALE) [76-82], ALE is a methodology used initially to improve epitaxy in the growth of thin-films by MBE and VPE. The principle of ALE is to use surface limited reactions to form each atomic layer of a deposit. If no more than an atomic layer is ever deposited, the growth will be 2-D, layer by layer, epitaxial. Surface limited reactions are developed for the deposition of each component element, and a cycle is formed with them. With each cycle, a compound monolayer is formed, and the deposit thickness is controlled by the number of cycles. 

In techniques such as MBE and VPE, surface limited reactions are generally controlled by the temperatures of the reactants and substrate. In general, the temperature is kept high enough so that any deposition over a monolayer sublimes, leaving only the atomic layer, forming the compound. Problems are encountered when the temperatures needed to form atomic layers of different elements are not the same, as changing the temperature between layers is difficult. 

Surface limited reactions are well known in electrochemistry, and are generally referred to as underpotential deposits (UPD) [83-88], That is, in the deposition of one element on a second, frequently the first element will form an atomic layer at a potential under, or prior to, that needed to deposit the element on itself. One way of looking at UPD is that a surface compound, or alloy, is formed, and the shift in potential results from the free energy of formation of the surface compound. 

Of course, deposition times can be decreased by using a larger driving force, but that runs the risk of bulk deposition. It is easy to envision a cycle where overpotentials are used, and the deposition is simply stopped after a monolayer of charge has passed. Such a cycle would not involve surface limited reactions and 3D growth would be expected. 

EC-ALE studies of ZnTe using a TLEC were performed with up to 20 cycles of deposition [130], Coulometry was the only analysis performed on the deposits. A plot of coverage as a function of the number of cycles was linear, as expected for a surface limited process. No thicker films have as yet been formed using the flow deposition system. At present there is no reason to believe that the cycle developed for 20 cycle deposits will not produce good quality deposits of any given thickness, using the automated flow-cell system. 

The confidence intervals defined for a single random variable become confidence regions for jointly distributed random variables. In the case of a multivariate normal distribution, the equation of the surface limiting the confidence region of the mean vector will now be shown to be an n-dimensional ellipsoid. Let us assume that X is a vector of n normally distributed variables with mean n-column vector p and covariance matrix Ex. A sample of m observations has a mean vector x and an n x n covariance matrix S. 

One way to view UPD is as formation of a surface compound. In other words, deposition of the first atomic layer of an element on a second element involves a larger deposition driving force than subsequent layers, as it benefits from the AG of compound formation. For deposits formed at underpotential, once the substrate is covered the deposition stops because the reaction is surface limited. No more of the substrate element is available to react, unless it can quickly diffuse to the surface through or around the initially deposited monolayer (an example would be amalgam formation at a mercury electrode surface). Subsequent deposition is then only observed when the bulk deposition potential has been exceeded. 

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... 

The principle of ALE is that deposits are formed one atomic layer at a time using surface-limited reactions. That appears to be the case with ECALE. The name electrochemical atomic layer epitaxy, however, suggests that the deposits should be epitaxial, and the first 100 cycle deposits appeared anything but epitaxial (Eig. 21a). These initial deposits looked like they were composed of a large number of particles, which fell out of solution. 

An electrochemical analog of digital etching can easily be envisioned, where first some reactant is adsorbed in a surface-limited reaction, and then the potential is switched to one where a product species is produced, stoichiometric in the adsorbate and the substrate. The results of the CdTe etching study described above suggested still another scenario, where no adsorbate is involved, just two electrochemical potentials. Figure 68 is a... 

In addition to studies using photo-resist-covered substrates and AFM, atomic level studies were performed to help identify the nature of the surface-limited reactions used to form the electrochemical digital etching cycle [313]. In those studies the dependence of etched amounts on the potential used for Cd oxidation was investigated using a UHV-FC instrument (Fig. 39). 

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