Stability of Ensembles of Ge/Si(100) Islands
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ABSTRACT We have investigated the stability of ensembles of Ge/Si(100) islands by annealing at their growth temperature. Islands grown by molecular beam epitaxy at temperatures of 450, 550, 600 and 650'C were annealed for times between 5 and 120 minutes. Small, pure Ge hut clusters, bound by {105} facets appear to be extremely stable structures, surviving the longest anneals with no apparent coarsening. Dome clusters, however, coarsen. Large alloyed hut clusters, apparent in as-grown samples only for growth temperatures greater than 600'C, appear during annealing at 450 and 550'C. During anneals at 550 and 650'C, we observe novel coarsening behavior. Arrays of crystallographically oriented, alloyed hut clusters are formed which result from the dissolution of large, alloyed dome clusters.
INTRODUCTION Ge/Si(100) islands have attracted attention both for the insights they provide into crystal growth fundamentals and for potential technological applications. These clusters form at the early stages of Ge heteroepitaxy because of the 4.2% misfit between the Ge and Si lattice. Since the surface energy of Ge is smaller than that of Si, Ge initially wets the Si substrate. Since the strain energy of the system increases proportionally to the Ge wetting layer thickness, after - 3 ML (monolayer=-6.78xl014 atoms/cm 2), a layer-to-island transition occurs. This is the StranskiKrastanov (SK) growth mode. Island formation occurs spontaneously. The surface energy increase as the film roughens is more than compensated by the strain energy reduction allowing the total free energy of the system to reduce. The first islands which form are rectangular-based huts [1, 2] or square-based pyramids [3] bound by four {105} facets making an 11.50 contact angle with the substrate. When huts (or pyramids) grow, they transform to octagonal-based domes [2, 4] which are bound by steeper {t113} or {102} facets with contact angles near 25'. Domes can relieve more strain energy than huts because more Ge atoms are located far from the Ge-Si interface. Further dome growth may result in island dislocation as a further strain relief mechanism. Here, we investigate the stability of the as-grown Ge/Si (100) island ensemble. Generally, during annealing, the length scale of an island ensemble coarsens. This coarsening results from chemical potential difference between islands. Usually, larger islands grow at the expense of smaller islands. Our results indicate that dome clusters and large, alloyed hut clusters exhibit conventional ripening behavior. Pure Ge hut clusters, however, survive the longest anneals.
EXPERIMENTAL DETAILS All samples were grown using standard molecular beam epitaxy (MBE) techniques. 25mm x 4mm strips were cleaved from ±1/2' p-type, B-doped Si wafers and transferred via a load-lock into the MBE chamber. After degassing and heating to -1250 ' C, the samples 123
Mat. Res. Soc. Symp. Proc. Vol. 618 ©2000 Materials Research Society
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