Growth Kinetics and Structure of Ultrathin Copper Films on the W(110) Surface Studied by Leem
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Mat. Res. Soc. Symp. Proc. Vol. 399 0 1996 Materials Research Society
W(1 10) are therefore understood to be a sequential relief of strain in the Cu film. The competition of film strain energy with the potential energy gained by placing overlayer atoms in the substrate potential minima (pseudomorphy) has been considered theoretically by Bauer and van der Merwe for fcc(1 11)/bcc(1 10) interfaces [6]. At temperatures approaching 1000K where our experiments were carried out, the (lxi1) to (15xl) transformation and subsequent (15xl) to (1x8) transformation also occur. However, there have been no detailed investigations of the coverage dependence of these transformations at high temperature. As described below, our experimental investigations indicate that the onset of the (lxl) to (15xl) transformation at high temperature is delayed until about 2 ML prior to its completion at 2.13 ML. The (lxi) to (15xl) transformation plays a particularly important role in our work. Direct observation of this transformation with LEEM provides an extremely accurate, local probe of coverage which allows quantitative modelling of the epitaxial growth process. These observations also give valuable insight into the relationship of growth morphology and supersaturation. EXPERIMENTAL RESULTS The W sample was oriented to within 0.050 from the (110) direction. It was cleaned by annealing to 1000K in an oxygen pressure of lxl0 7 torr and flashing to 2000K. Copper was deposited from a resistively heated alumina crucible with a base pressure during deposition of 9 Ixl0- torr. The experiments were carried out in a low energy electron microscope (LEEM). Contrast, resolution and the operational principle of LEEM have been described previously [8,9]. In summary, LEEM images surfaces with elastically backscattered, low energy electrons. The lateral resolution of the instrument used in this work was 15 nm. The capability to image atomic surface steps gives LEEM atomic resolution perpendicular to the surface [9]. The high reflectivity of electrons with low energy (typically 0-30 eV) permits imaging in real-time. The images presented in this paper were reproduced from video tape. The experimental measurements concentrated on the (lxi) to (1 5xl1) transformation of the Cu double layer. In addition to direct observations of the growth morphology, we measured the dependence of the completion time for the (15xl) transformation, tc , upon the sample temperature, T, and the incident flux, F. The completion time is equivalently the time required for the coverage to reach the (15x 1) Cu double layer completion coverage, 0(to) = 0c= 2.13 ML. This time is related to the net flux, F - OD, where the desorption flux, OD, is the coverage times the desorption rate, D = v e-Eacr. LEEM observations of the (lxi) to (15xl) transformation at 900K and 730 K and incident flux of 0.23 ML per minute are shown in figure l(a)-(c) and l(g)-(i), respectively. The transformations at these two temperatures are typical of the growth behaviour in low and high temperature regimes. In
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