Step - Step Interactions on the Vicinal GE(001) Surface
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BART A. G. KERSTEN, LIANDA SJERPS - KOOMEN, HAROLD J. W. ZANDVLIET, DAVE H. A. BLANK AND AREND VAN SILFHOUT, University of Twente, Dept. of Applied Physics, P.O. Box 217, 7500 AE Enschede, The Netherlands
ABSTRACT Ge(OO 1) surfaces misoriented towards the [011] direction have been investigated with a UHV STM. The vicinal Ge(0O1) surface consists of alternating straight SA steps and rough SB steps. The anisotropic surface stress tensor leads to interactions between the steps, which can be characterised by assigning a force monopole and a force dipole to each step. The monopolemonopole interaction describes the repulsion, the monopole-dipole interaction the attraction between adjacent steps. From STM images the distribution of the step-step separation was extracted. Using this distribution the value of the force monopole and the force dipole has been calculated. It was found that the short range monopole-dipole interaction attracts the SB step towards the lower lying SA step, which results in the formation of DB steps at a miscut angle of
50 1. INTRODUCTION The morphology of vicinal surfaces, i.e. surfaces slightly misoriented with respect to one of the low Miller indices, is under continuous interest. The misorientation is, in the case of Ge(001) and Si(001) surfaces, accommodated by either single or double height steps depending on the exact miscut angle [1,2]. Steps play an important role in phenomena as crystal growth, surface chemistry, catalysis and growth of epitaxial overlayers. Most attention, up to now, has been paid to Si(0O1) surfaces. The renewed interest in Ge surfaces is mainly due to the possibility to artificially create, using advanced methods as Molecular Beam Epitaxy (MBE), heterostructures of alternating Si and Ge layers. Furthermore, Ge(0O1) misoriented towards the [011] direction may serve as a template for the growth of GaAs and A1GaAs. The Ge(0O 1) surface, as its silicon counterpart, reconstructs by forming dimers which arrange in parallel rows [1]. Depending on the crystal cut, either a (2x1) or a (Ix2) reconstructed domain develops, related to each other by a 90' rotation. If the surface has a slight miscut towards the [011] direction, only single height steps are formed. In this configuration two types of steps, denoted by SA and SB, develop (fig. 1) [3]. The SA steps have the dimer rows of the upper terrace, running parallel to the step edge, whereas in the case of SB the dimer rows are oriented perpendicular to the edge. With increasing miscut angle the SB step shifts towards the lower lying SA step, until it catches up with it and double layer DB steps are formed. This transition from SA+SB steps to DB steps is determined by a number of factors, namely the formation energies of the edges, the kink energies, the temperature and the surface stress. It was first analysed by Chadi [3], who calculated the formation energies of the steps mentioned above. The calculations showed the formation energy of a DB step to be lower than that of a SA+SB step pair. Furthermore, the anisotropic surfac
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