Self-Assembling and Ordering of Ge/Si Quantum Dots on Flat and Nanostructured Surfaces
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Self-assembling and Ordering of Ge/Si Quantum Dots on Flat and Nanostructured Surfaces N. Motta1,2*, A. Sgarlata1, A.Balzarotti1, F. Rosei1,3 1 INFM Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy 2 INFM Dipartimento di Fisica, Università di RomaTre, via Vasca Navale 84, I-00146 Roma, Italy 3 Institute of Physics and Astronomy and CAMP, University of Århus, 8000 C Århus, Denmark ABSTRACT We have studied by Scanning Tunneling Microscopy (STM) the effect of step bunching on Ge/Si(111) epitaxy. We have verified that self-organization of Ge islands is greatly influenced by “step bunching” which arises from the flash-annealing procedure used to reconstruct the Si surface. Two different growth regimes arise: initially islands nucleate and evolve only at steps, up to complete ripening; subsequently the same evolution is observed on flat areas of the sample. The average distance between islands and steps is nearly constant, originating a single row of equally spaced islands, followed by other rows of islands in between. The exploitation of this phenomenon, which is governed by the surface diffusion length of Ge on Si (estimated from our data) and by the terrace width, constitutes one possible path to achieve self-organization of quantum dots. INTRODUCTION It has been shown that during resistive heating of Si(111) a bunching of natural surface steps can form, yielding a simple way to obtain nanostructured Si surfaces. Several authors have studied this phenomenon [1,2], demonstrating that the step configuration at a vicinal surface (with a small misorientation angle to the (111) plane) during sublimation of a Si crystal depends on the direction of the heating current flowing through the crystal as well as on temperature. The temperature dependence of this effect is not simple, but general agreement exists on the fact that, for Si sublimation at T > 1220° C, step bunching occurs in step-down direction, while a regular step distribution occurs in step-up direction. In this way we have obtained both regular (i.e. with steps naturally distributed) and step-bunched Si(111) surfaces on which we have grown epitaxially Ge at T = 450° C and 530° C. We have analyzed by STM and Atomic Force Microscopy (AFM) the evolution and distribution of the islands on different surfaces. An evident self-ordering on step-bunched surfaces exists and the parameters of this ordering have been studied. The amount of Ge-Si intermixing has been also investigated by means of X-Ray Absorption Fine Structure (XAFS), in connection with island shape modification. EXPERIMENTAL Ge/Si(111) samples were grown by Physical Vapour Deposition (PVD) in a UHV chamber (base pressure 4 x 10-11 mbar) equipped with an e-gun evaporator and a commercial Variable Temperature STM-AFM microscope [3]. Si(111) substrates (n-type ρ=10-3 Ω cm, miscut angle < 0.5°) were flashed at 1250 °C to form the 7x7 reconstruction. Both regular (R) and step bunched (SB) surfaces were obtained by using respectively a current flow oriented
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