Stepped terrace domains

Fig. 1. STM image of a chemically polished Ag(l 11) electrode in 0.01 M HCIO4, showing stepped terrace domains with monoatomic steps, a monoatomic island, and a monoatomic pit [4].

The largest part of the surface consists of stepped terrace domains composed of "stacks" of monoatomic terraces. The width of the terraces in these stacked parts varies between ca. 2 nm and more than 20 nm. Exceptionally, terrace widths up to 100 nm have been observed. 

Fig.2. Schematic presentation of the local progress of Pb underpotential deposition at monoatomic pits, monoatomic islands, and stepped terrace domains of non-ideal chemically polished Ag(l 11) electrodes [4]. For fiarther explanation see the text.

At the stepped terrace domains, the adsorbate-free peripheral parts are completely covered, leading to a. complete" hep adsorbate that is stable over several hours. 

Desorption of the complete Pb adlayer within the three distinct desorption peaks D3, D2 and D1 (see Fig. 2) by step polarization proceeds in an analogous way to the adsorption sequence, except on the monoatomic islands in contrast to the complete adsorbate formation at the islands in peak A3, desorption in peak D3 only involves the outermost part of the monolayer at the island periphery, whereas the remaining adsorbate coverage is completely desorbed in peak D2. Desorption on the monoatomic islands occurs thus in the same way as at the stepped terrace domains, except for the missing step decoration coverage desorbed in D1. 

In a recent STM study by Carnal et al. [3], these assumptions have been confirmed by the observation that a hexagoiuilly close-packed adlayer with slightly compressed Tl-Tl interatomic distances is formed at more anodic potentials, followed by the formation of a second hep adlayer with slightly disordered domains at small undervoltages. The progress of the formation of the first adsorbate layer was studied in that work by more conventional STM imaging techniques and was restricted to investigations at file stepped terrace domains. As shown in detail in [3], the formation of the first adsorbate layer at the stepped terrace domains follows the same scheme as shown in Fig. 2 for the system Pb/Ag(l 11), i.e. 

The Saddle Point Features of the 2-D Gratings For an ideal 2-D sinewave the saddle point features should appear to have 4-fold symmetry when viewed in LEEM images. From the sketch of figure 13 it can be seen that the hypocycloid shaped terrace at the saddle point has the same type of monoatomic step on all four sides due to the difference in the free energies of the two steps, Sa and Sb, on Si(OOl) there should be a strong preference for Sa steps and hence each maximum would prefer to be flanked by two white domains and two "black ones as is the case in figure 10. (A similar conclusion follows if the the saddle point terrace is surrounded by two Sa steps and two double steps of Db type[31]). 

Due to the presence of reconstruction not all possible configurations of clockwise steps will actually occur on the surface. As illustrated in Fig. 4(a) a clockwise step going up followed by another clockwise step going down induces a shift in the reconstruction of the lower terrace. For this reason, a closed clockwise step cannot be formed on a given terrace unless it is accompanied by domain boundaries separating regions of different reconstruction order (Fig. 4(b)). 

The process of irreversibly adsorbed Sb on Au(lll) at the open-circuit potential (close to 0.2 V) has been investigated using in situ STM [474]. The oxygenated Sb adlayer was nucleated and grown on terraces and at step edges. The oxygenated Sb domains present on terraces were roundshaped islands of a diameter ranging between 3 and 6 nm. 

The slice through a bulk crystal can differ from both the 111 plane and the 100 plane by small angles. This produces a kink in the face of the step. By an extension of the analysis that leads to step characterization, these kinks can also be characterized. For example, a plane with Miller indices 10,8,7 has 111 terraces seven atoms wide, 110 steps one atom high, and kinks of 100 orientation every two atoms. Because of the greater thermodynamic stability of the planes of low Miller index, these surfaces of ordered roughness are stable and can be prepared and studied. Since it is sensitive to periodicity over a domain about 20 nm in diameter, LEED sees the pattern associated with terraces of various widths and may be used to characterize these surfaces. Satisfactory LEED patterns do not require absolute uniformity of terrace width but may be obtained with experimental surfaces that display a distribution of widths. 

There can be any number of types of sites on a surface. For example, in the simulation of a crystal growth process we might specify that a surface consists of step sites and terrace sites. The number of sites of each type may be characteristic of the crystal surface, for example, the mis-cut orientation of a crystal face. We denote each surface site type as a phase these phases reside in a particular surface (2D) domain. Surface species occupy the surface sites (i.e., populate the surface phases), which is the next step down the hierarchy. 

SiC terraces separated by a step. Unlike the case for sapphire, the polarity of the SiC 0001 surfaces and the bonding at the film/substrate interface prevents die formation of inversion domain boundaries (IDBs) in GaN grown on SiC 0001 [11,12,19]. Vermaut et al [10] identified the SMBs in GaN and AIN grown on SiC as prismatic stacking faults with three possible Burgers vectors / <2023>, V3 <10l0> and V2 <0001>. 

As a first step toward overcoming the above problems, a hybrid diffusion-adsorption model for the terrace linked with a KMC model near the steps was developed (Schulze, 2004 Schulze et al., 2003). This domain decomposition stems from a natural separation of scales. The continuum terrace model between steps is... 

Figure 3.67 Stepwise formation of 2D Meads UPD overlayers, (a) stepped bare substrate surface (b) kink site decoration (c) step decoration (d) coexistance of domains of expanded Meads overlayer and bare substrate on atomically flat terraces (e) formation of a condensed 2D Meads overlayer.

Fig. 4 a) STM image of the NiAl(100)-c(V2 x 3V2)R45° surface. Two domains A and B could be seen on the upper terrace, which is separated by a double step from the lower terrace on the right side (from ref [47]). b) Hard sphere model of the corresponding surface, (from ref [37]). 

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