Digital etching

Initially, a thin layer flow cell (Fig. 19) was used in this group to study the EC ALE formation of compounds [158] and in studies of electrochemical digital etching [312,313], Wei and Rajeshwar [130] used a flow cell system to deposit compound semiconductors as well, however, the major intent of that study was to form superlattices by modulating the deposition of CdSe and ZnSe. Their study appears to be the first example of the use of a flow electrodeposition system to form a compound semiconductor superlattice. 

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... 

FIG. 68. Schematic illustrating the electrochemical digital etching process on CdTe(lOO). A) initial surface, B) after oxidative stripping of a Cd atomic layer, C) after reductive stripping of the Te atomic layer. 

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). 

FIG. 69. AFM images of CdTe(lOO) surfaces (A) before etching and (B) after 150 cycles of electrochemical digital etching. 

Prom the previous sections of this chapter, it should be clear that the substrate is very important and will continue to be a major area of study. Improvements in the quality of Au substrates can be made by switching to Au that is vapor-deposited on mica or glass. However, a very important direction of study will be towards using lattice-matched compound semiconductor substrates, and that work is closely tied with studies of electrochemical digital etching [44,312,313,400]. 

One of the most intriguing directions for electrochemical digital etching involves the selective etching of device structures composed of multiple layers of different compounds. Some compounds contain less noble elements, which will be the first to oxidize, possibly using conditions that leave behind the compound containing the more noble element, while the other compound might contain an element that is more easily reduced then... 

The STM result that dissolution of Pd atoms occurs selectively at step (disordered) sites provided the inqietus for an additional stuify which demonstrated that the I(ads)-catalyzed anodic dissolution process is able to regenerate an ordered Pd(l 11) surface from one that had been subjected to extensive Ar -ion bombardment [8]. This particular reordering reaction is unique because it occurs (i) in the absence of bulk conosive reagent, and (ii) only if a chemisorbed layer of iodine is present. This process may be viewed similarly to digital etching [9] under electrochemical conditions [10] except that (a) bulk material is not needed to replenish the adsorbed iodine that activates the surface, and (b) the dissolution process does not cease even after the atomically smooth surface has been regenerated. 

Electrochemical Digital Etching Atomic Level Studies of CdTe(lOO)... 

Figure 1. Schematic illustrating the electrochemical digital etching process on CdTe(100).

An electrochemical method for digitally etching CdTe(lOO) is being investigated. In principle, two potentials are needed, one where surface atoms of Cd are oxidatively... 

Atomic layer epitaxy, 117 Atomic level studies of CdTe(lOO), electrochemical digital etching, 115-123 Auger electron spectrometers, 106 Auger electron spectroscopy bisulfate anion adsorption on Au(l 11), Pt(l 11), and Rh(l 11) surfaces, 126-138 to monitor composition of acidified water surfaces, 106-114 underpotential deposition of lead on Cu(lOO) and Cu(lll), 142-154 use for bisulfate anion adsorption, 127... 

---