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| Home > Publications database > Die Untersuchung der Morphologie einer wachsenden Platin(111)-Fläche mittels der Streuung thermischer He-Atome |
| Book/Report | FZJ-2018-03631 |
1991
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/19041
Report No.: Juel-2526
Abstract: The homoepitaxial growth of Pt(111) has been studied using He-diffraction as a function of surface temperature and deposition rate. At a constant deposition rate ofR $\simeq$ 1 ML/min different growth modes are observed: step-flow (T$_{S}$ $\gtrsim$ 850 K), layer-by-layer (2D$_{h^{-}}$) growth at 460 K $\lesssim$ T$_{S}$ $\lesssim$ 800 K, multilayer (3D-) growth at 340 K $\lesssim$ TS $\lesssim$ 460 K, and, surprisingly, reentrant layer-by-layer (2D$_{e}^{-}$) gKrowth at T$_{S}$ $\lesssim$ 340 K. During deposition at very high temperatures an approximately constant He-reflectivity was observed. This behavior is attributed to growth via step-flow, which corresponds to the ideal layer-by-layer growth of the real surface. Here, all adatoms are incorporated into pre-existing atomic steps which prevent nucleation on the terraces. The surface is growing simply by propagation of the pre-existing steps. At surface temperatures between T$_{S}$ $\simeq$ 460 K and T$_{s}$ $\simeq$ 800 K damped oscillations of the reflected He-signal are observed. The oscillation period corresponds to the monolayer deposition time. In this temperature range the Pt(111) surface shows a nearly perfect layer-by-layer growth caused by a massive mass transport between layers. Due to finite adatom mobility, nucleation takes place on the terraces. In this mode, basically one layer is growing at a time, except for a small time interval close to monolayer completion where nucleation in the next layer already starts. It has been shown that most of the damping of the oscillations at high temperatures is caused by minute inhomogeneities of the Pt deposition rate over the macroscopic spot area of the probing He-beam. Under certain circumstances more than 150 oscillations can be observed. Upon decreasing the surface temperature the damping of the oscillations is increased due to accumulation of surface roughness. At intermediate temperatures (340 K $\lesssim$ T$_{S}$ $\lesssim$ 460 K) 3D-structures develop, i. e. several (two dimensional) layers grow simultaneously. This growth mode is evidenced by a monotonic decay of the specular peak height with deposition. In agreement with previous FIM-results, the 3D-growth is attributed to the existence of an activation barrier, which suppresses the descent of adatoms from the top of the growing adatom islands onto the lower terraces. This barrier can be overcome at TS $\gtrsim$ 460 K enabling interlayer mass transport which is a prerequisite to 2D-growth. The transition from 3D- to 2D$_{h}$-growth takes place in spite of the still substantial thermal mobility of adatoms. This is in contrast to the conventional picture of homoepitaxial growth in which 3D-growth is attributed to a lack of adatom mobility. Surprisingly, at T$_{s}$ $\lesssim$ 340 K and a deposition rate of R $\simeq$ 1 ML/min the reentrance of a layer-by-layer (2D$_{e}^{-}$) growth was observed. Careful experiments have been performed to prove that this growth mode is characteristic of the clean Pt(111)-surface rather than caused by minute amounts of contaminations. Upon reduction of the deposition rate a transition to 3D-growth occurs, which implies that the activation barrier can be overcome only in the presence of small and/or irregularly shaped islands enabling interlayer mass transport. The main result of our investigations is the observation of the reentrant layer-by-layer growth mode. Comparing the experimental data to Monte-Carlo simulations shows that 2D$_{e}^{-}$ growth cannot be explained by transient mobility arguments. It must rather be attributed to a break down of the activation barrier for the descent of mobile adatoms from the top of small and/or irregularly shaped islands onto the lower terrace. This finding is in line with previous FIM-results.
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