Step bunching during Si(001) homoepitaxy caused by the surface diffusion anisotropy
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Step bunching during Si(001) homoepitaxy caused by the surface diffusion anisotropy J. Mysliveček1,2, C. Schelling2, F. Schäffler2, G. Springholz2, P. Šmilauer3, J. Krug4, B. Voigtländer1 1
Institut für Schichten und Grenzflächen, Forschungszentrum Jülich, D-52425 Jülich, Germany Institut für Halbleiterphysik, Johannes Kepler Universität, A-4040 Linz, Austria 3 Czech Academy of Sciences, Cukrovarnická 10, 162 53 Praha 6, Czech Republic 4 Fachbereich Physik, Universität Essen, D-45117 Essen, Germany 2
Abstract: Scanning tunneling microscopy experiments show that the unstable growth morphology observed during molecular beam homoepitaxy on slightly vicinal Si(001) surfaces consists of straight step bunches. The instability occurs under step-flow growth conditions and vanishes both during low-temperature island growth and at high temperatures. An instability with the same characteristics is observed in a 2D Kinetic Monte Carlo model of growth with incorporated Si(001)-like diffusion anisotropy. This provides strong evidence that the diffusion anisotropy destabilizes growth on Si(001) and similar surfaces towards step bunching. This new instability mechanism is operational without any additional step edge barriers. Morphological instabilities of thin films during growth and annealing are the basic mechanisms for the preparation of self-assembled quantum dots and wires, but they are detrimental to the interface quality of semiconductor heterostructures. Hence, a detailed understanding of the underlying mechanisms is essential for either application. Up to now, most investigations in this field assumed the dominance of strain-driven phenomena, but it becomes more and more obvious that kinetic step bunching is a widespread instability during homo- and heteroepitaxy [1,2]. In this paper we argue that the kinetic step bunching on vicinal Si(001) during molecular beam homoepitaxy (MBE) [3] is an inherent property of the 2×1 reconstructed Si(001) surface. In particular, kinetic Monte Carlo (KMC) simulations confirm that this step bunching instability appears in a growth model with Si(001)-like diffusion anisotropy without any additional step edge barriers. This new instability mechanism is expected to be relevant for all semiconductor surfaces of similar symmetry. In our experiment, 1000 Å thick silicon epilayers were deposited by MBE at 0.16 Å/s on Si(001) substrates miscut by 0.66º along [110]. After deposition, the samples were quenched to room temperature and transferred in situ to the STM. We tested that the thermal cleaning by direct current produces flat surfaces with equally distributed SA and SB steps [4]. Surface morphologies after growth at different temperatures are shown in Fig. 1. At 670 K and below, we observe a weak undulation of the surface (< 5 Å, Fig. 1a). Two dimensional (2D) islands grow on the terraces (Fig. 1d). With increasing temperature, a narrow temperature interval is reached (around 760 K for the miscut and the deposition rate employed), where the instability becomes most pronounced (Fig.
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