Electrodeposition of Cu on GaAslOO surfaces

Figure 4 shows the cyclic vottammogram of p-GaAs in 9 mM HC1 + 1 mM CuCh solution. Cu deposition took place at potentials more negative than -0.1 V and anodic stripping peaks were observed at +0.01 V and +0.09 V. How the electrodeposition of Cu proceeded was strongly dependent on the structure of the substrate [23]. Typical examples are shown in Figs. 5 and 6. Figure 5 shows a series of AFM images taken (a) before and (b) - (f) during bulk deposition of Cu on a relatively flat surface of p-GaAs(l 00) in 9 mM HC1 + 1 mM CuCh solution. The potential was stepped from +0.1 V to -0.15 V at the time indicated by a thick arrow in Fig. 5(b). Figure 5(b) clearly shows that immediately after the potential had been stepped to -0. 15 V, a large number of small grains were generated with spacings of several tens of nanometer. These initial deposits of Cu on the surface acted as effective nucleation centers and the initial

Fig. 4. Cyclic voltammogram of p-GaAs(100) in 9 mM HC1 + 1 mM CuCl2.

growth of these grains seemed to be three-dimensional. AFM images were captured continuously at -0. 15 V and are shown in Fig. 5(c) - (e). As time progressed the grains of Cu overlapped with each other and finally truncated pyramidal structures of relatively uniform size were formed (Fig. 5(e)). The potential was pulsed back to +0.1 V, as indicated by a thick arrow in Fig. 5(f). Upon stepping the potential back to +0.1 V, Cu deposits were removed immediately within the time domain of AFM measurement and the surface was returned to a state similar to that before Cu deposition (Fig. 5(f)).

Fig. 5. Sequentially obtained AFM images of p-GaAs in the relatively flat region in 9 rnM HC1 + 1 mM CuCl2 solution (a) at +0.1 V vs. Ag/AgCl, (b) while the potential was pulsed to -0. 15 V, (c)-(e) at -0. 15 V, and (f) when the potential was taken back to the initial potential (+0.1 V). The time after the application of-0.15 V at the beginning of imaging was (c) 4 s, (d) 12 s, (e) 20 s, and (f) 74 s. Thick arrows indicate the onset of deposition and stripping. Arrows beside the figure indicate the scan direction of the tip.

Fig. 6. Sequentially obtained AFM images of a p-GaAs surface with preformed truncated pyramidal structures in 9 mM HC1 + 1 mM CuCl2 solution (a) at +0.1 V vs. Ag/AgCl, (b) while the potential was pulsed to -0. 15 V, (c)-(e) at -0. 15 V, and (f) when the potential was taken back to the initial potential (+0.1 V). The time after the application of-0.15 V at the beginning of imaging was (c) 36 s, (d) 116s,(e) 196 s, and (f) 204 s. Thick arrows indicate the onset of deposition and stripping. Arrows beside the figure indicate the scan direction of the tip.

Fig. 6. Sequentially obtained AFM images of a p-GaAs surface with preformed truncated pyramidal structures in 9 mM HC1 + 1 mM CuCl2 solution (a) at +0.1 V vs. Ag/AgCl, (b) while the potential was pulsed to -0. 15 V, (c)-(e) at -0. 15 V, and (f) when the potential was taken back to the initial potential (+0.1 V). The time after the application of-0.15 V at the beginning of imaging was (c) 36 s, (d) 116s,(e) 196 s, and (f) 204 s. Thick arrows indicate the onset of deposition and stripping. Arrows beside the figure indicate the scan direction of the tip.

When potentials more positive than 0 V were applied to the GaAs electrode in HC1 solution, truncated pyramidal structures were formed on the substrate surface as a result of the anodic dissolution of GaAs. Actually, it was hard to obtain an atomically flat surface over a wide range if the electrode was kept in the anodic potential region where no Cu deposition took place. The surface shown in Fig. 5(a) was one of the flattest surfaces observed. To examine the effect of the surface structure on Cu deposition, we also monitored the deposition process of Cu on the surface with the preformed truncated pyramids of relatively uniform size as shown in Fig. 6(a). Totally different time sequences were observed for the electrodeposition of Cu on this surface, as shown in Fig. 6(b) - (e). As soon as the potential had been stepped to the bulk deposition region (-0.15 V), Cu electrochemical deposition occurred along the preformed structure (Fig. 6(b)). Grains observed in Fig. 5 were not seen in this image. The truncated pyramids grew with the progress of deposition, mainly in the height direction.

Figure 7(a) shows AFM images of a Cu deposited surface in 1 mM CUSO4 + 10 mM H2SO4 solution. Many Cu islands with relatively uniform size were located on the GaAs surface. High-resolution AFM images on the top of a Cu island and the portion between Cu islands are shown in Fig. 7(b) and (c), respectively [221. Figure 7(b) shows that topmost atoms have a hexagonal structure with a nearest-neighbor distance of 0.26 ± 0.04 nm, which is almost equal to the known lattice constant (0.256 nm) of

Fig. 7. (a) An AFM image (1 pm x 1 pm) of the surface of Cu islands formed on the p-GaAs(100) surface. Atomically resolved AFM images of (b) the top of a Cu deposit and (c) the portion between the Cu deposits in 1 mM CuS04 + 10 mM H2SO4 solution at -0.2 V vs. Ag/AgCl.

bulk Cu in the (111) basal plane. Thus, Cu deposits seemed to have a closed-packed structure. On the other hand, the atomic arrangement of a square lattice with atomic distance of about 0.4 nm corresponding to the underlying GaAs(100)-(l x 1) structure was observed in the region between the deposited Cu. The atomic rows of the Cu(l 11) structure on the top of Cu islands were different from each Cu deposit. These results suggest that bulk Cu deposition on the GaAs surface proceeded without any strong influence of the orientation of the underlying GaAs(lOO) surface.

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