Results and Discussion

3.1 Substrate Preparation and Structural Characterization

Immediately prior to multilayer deposition, the Cu substrates were typically immersed in 10 mmol/1 HCl for 5-30 min. The solution was open to the atmosphere (i.e., oxygen),

Fig. 2. A 13 nm x 13 nm STM image of a (a/2 x V2)R45° chlorine adlattice on Cu(100) at -0.169 V vs. Cu/Cu+ in 10 mmol/1 HC1 with Elip = 0.120 V, /, =9.0 nA, and a frame time of 32 s. A schematic of the proposed chloride adlayer structure is also shown.

which results in etching of the surface [4, 5] and the formation of large (100) terraces on the miscut Cu(100). For short pretreatment times this results in a root mean square (rms) surface roughness on the order of ~1.4 nm over a 100 pm2 area. The terraces are completely covered by a (a/2 x V2)R45° CI adlattice as determined by low-energy electron diffraction (LEED) and in-situ STM [6, 7, 8]. As shown in Fig. 2, a highresolution constant-current STM image obtained at -0.25 V vs. Cu/Cu+ reveals a series of terraces bounded by <100> oriented steps. The <100> step orientation is in sharp contrast to the close-packed <110> orientation associated with clean copper surfaces in ultrahigh-vacuum (UHV) systems [9]. The <100> step edge corresponds to the close packed direction of the chlorine adlattice.

Similar observations of the influence of a (V2 x V2)R45° oxygen adlayer on the step structure of Ni(100) were recently reported [10]. In the present case, the chloride adlattice has a dominant influence on the binding energy of an adatom to a step as well the activation energy for migration along a step edge and thereby controls the evolution of the step morphology on Cu(100). Interestingly, the importance of enhanced step edge mobility relative to terrace mobility has been previously noted for Cu atoms on clean Cu(100) in UHV [11]. This results in kink-saturated <100> metal steps beneath the adlattice. The monatomic step height of -0.18 nm corresponds to copper (a0 = 0.361). The (V2 x V2)R45° CI adlattice allows the coexistence of two possible domains within a terrace. Unambiguous imaging of such domain boundaries with atomic resolution proved difficult, however; numerous mesoscopic images were obtained of the migration of individual line defects across the (100) terraces as shown in Fig. 3. On this scale the domain boundaries generally tend to be aligned in the <110> which corresponds to the kink-saturated directions for the chlorine adlattice. The chemical potential driving the process may be related to the differing registry between (V2 x V2)R45° CI adlayers on two neighboring terraces leading to chlorine-rich and -poor step edges. The different energetics of these steps may also account for the tendency of the small island to adopt a rectangular shape. This argument is analogous to that used for the rectangular growth habit of nickel islands that form during homoepitaxial growth on Ni(100) which is covered with a 0/2 x V2)R45° oxygen adlattice [10]. In a similar vein, the movement of the domain boundary in the CI (V2 x V2)R45° adlattice were somewhat hindered at steps due, presumably, to the phase shift of the adlattice between neighboring terraces. The domain boundaries were most often noted for substrates which had first been exposed to 10 mmol/1 HCIO4 prior to immersion in 10 mmol/1 HC1.

Fig. 3. A sequence of 75 nm x 75 nm images revealing the movement of a domain boundary in the chlorine adlattice across the Cu(100) surface at -0.250 V vs. Cu/Cu with Ehp= 0.056 V, /,= 9 nA, and a frame time of 35 s.

In contrast to halide media, immersion in 10 mmol/1 sulfuric or perchloric acid results in an ill-defined surface step structure and obtaining reliable atomically resolved images of Cu(100) proved difficult. The recent observations [12] of the influence of chloride on the kinetic anisotropy of copper deposition compared to the isotropic nature of deposition from sulfate solution may be, at least partially, related to oxidative halide adsorption. As noted above, compared to perchlorate and sulfate solutions, adsorbed chloride leads to a distinct anisotropy in the step morphology of the evolving Cu(100) surface. The impact of halide adsorption on the electrochemical processing of copper is further highlighted by its ubiquitous presence as an additive in most commercial copper electroplating baths [13]. In addition, the recent observation of the formation of well-ordered organic monolayers on gold which are mediated by an adsorbed iodine layer should stimulate much interest in the study of organic copper plating additives in the presence of chloride [14].

0 0

Post a comment