In Si technology, the continuous process of downscaling leads to problems related to atomic steps on the Si substrate surface. For example, thickness variation due to atomic steps is expected to have a significant impact on the properties of metal-oxide-semiconductor devices, where the oxide is only a few monolayers thick. Regrettably, step-free Si substrates are not available. Preparation of step-free surfaces demands a step-flow regime during growth, but unfortunately, Si adatoms migration length cannot reach typically substrate dimensions. This problem can only be overcome by preparation of step-free regions on substrates having smaller lateral dimensions. To realize step-free regions on a desired location on a substrate, patterned surfaces with trenches (mesas) are essential, which can be produced by standard Si processing steps. In this way, steps appearing on the surface are restricted to a small number within the patterned area and, moreover, step-free areas are well located at the surface allowing an exact positioning of the device on the step-free area. In this context, the pattering of the surface is also of advantage, since the edges of the mesa allow effective accumulation of steps during the growth leaving the rest of the region free of steps.

For epitaxial growth, two-dimensional terrace nucleation must be completely suppressed to achieve an atomically step-free surface. Among the epitaxial techniques, molecular beam epitaxy (MBE) is a sophisticated, finely controlled method for growing single-crystalline epitaxial layers in high vacuum. In Si-MBE, the growth temperatures are usually in the range of 700 K to 900 K, where the migration length of Si adatoms is only in the range of up to 100 nm. Therefore, temperature must be higher than those typically used in Si-MBE to realize step-free areas in the range of several micrometers suitable for device application. However, the Si surface exhibits surface phase transition at higher temperature, what could be of influence on the epitaxial growth. Therefore, it is desirable to have an understanding of morphology evolution of surfaces during Si-MBE also at higher temperatures.

In our work, we performed studies of Si-MBE on mesa-structured Si(111) at temperatures around the (7x7)-"1x1" surface phase transition (1030 K - 1120 K), which was controlled by reflection high-energy electron diffraction (RHEED). The surface morphology was investigated ex situ by atomic force microscopy. Significant changes in surface morphology were found for an only small increase in temperature near the surface phase transition., accompanied by a strong increase in step-free area dimension. This behaviour cannot be explained within a simple step-flow growth model. In particular, the simultaneous appearance of the two surface phases under certain conditions and their specific influence on the growth behaviour are discussed with regard to this matter. We presume that even at temperatures down to 1060 K the (7x7) and "1x1" surface phase coexist under dynamic condition of Si growth, whereas under near equilibrium condition the transition to the "1x1" surface phase is usually observed at temperatures around 1100 K. The appearance of both surface phases results in a strong variation of Si island size and shape. Furthermore, strong step-bunching was observed at temperatures around 1080 K. Stable step-flow without step-bunching was obtained for MBE at 1120 K, where only the "1x1" surface phase was visible in RHEED. Atomically flat step-free areas are formed over 10x10 µm² mesas under this conditions.         

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