The Effect of Strain on Intra- and Interlayer Mass Transport in Metal Epitaxy
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inding energies [9]. The additional barrier for interlayer diffusion can be measured by studying the nucleation probability on-top of preexistent islands as a function of their size [11, 121 which will be illustrated below. EXPERIMENT The experiments were performed with a variable-temperature STM (25 K-800 K) operating in UHV [131. The Ag films on Pt(1 11) are prepared by deposition of Ag at 450 K from an MBE Knudsen-cell at a background pressure better than 2x 10-10 mbar and subsequent annealing to 800 K. As a strained surface we use the first Ag monolayer which grows pseudomorphically and is thus under 4.2% compressive strain [14]. In order to prepare an unstrained Ag( 111) substrate we took advantage of the fact that very thick Ag films (>40 ML) grown on Pt(l 11) adopt the Ag(l 11) interplanar lattice constant and symmetry as characterized by He-diffraction [151. STM images revealed that both, the pseudomorphic Ag layer on Pt(l 11), and the Ag(l 11) surface consisted of extended flat terraces which were free of dislocations. For the study of nucleation kinetics, submonolayer coverages have been deposited (flux 1.lxl O-3 ML/s) onto these layers at various temperatures. Island densities are given in islands per Pt substrate atom, i.e., in ML. They were obtained on extended terraces to exclude the influence of steps and corrected for lateral drift. RESULTS A strong layer dependence of island densities becomes evident from inspection of Figure 1. It shows nucleation on the Pt( 111) substrate (a), the first (b) and second (c) Ag layer, as well as on a 50ML thick Ag layer which has adopted Ag(l 11) geometry (d). In all cases, except Fig. lc) (see below), the island density has saturated. These saturation island densities can be related to diffusion based on the following nucleation scenario. At the beginning of deposition the density of nuclei steadily increases as a function of coverage until it becomes more probable that diffusing atoms attach to existing islands rather than create new ones. At this state the island density stays nearly constant in a wide coverage range of G = 0.05 - 0.20 ML until it eventually decreases due to coalescence. Its maximum, the saturation island density, is determined by the ratio of the diffusivity to the deposition flux and therefore a measure for the adatom mobility at known deposition flux [81. It is important to note that nucleation of islands takes place at the time of deposition. After deposition, nucleation is then terminated and the resulting island density is stationary under isothermal conditions where it has been imaged by STM.
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0oo0 Fig. 1: STM images showing the nucleation of submonolayer coverages of Ag on Pt(l 11) (a), on IMLAg/Pt(l 11) (b), on 2MLAg/Pt(l 11) (c), and, on Ag(l 11) (d), respectively ((a) saturation island density N, = 1.7x10"2 islands per Pt(1 11) surface atom, 0 = 0.12 ML, (b) Nx = 6.6x10"4 , 0 = 0.05 ML, (c) N = 1.6x10"2 , 0 = 0.03 ML (note that the mean island size O/N is 2, i.e. saturation is not yet reached for this case), (d) Nx = 3.8x
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