Facet Formation in Submicron Selective Growth of Si/SiGe
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Abstract The paper reviews the work in mostly Si and SiGe epitaxy and some rn-V work on patterned substrates. Results of metalorganic chemical vapor deposition (MOCVD), low pressure chemical vapor deposition (LPCVD), gas source molecular beam epitaxy (GSMBE), and solid source molecular beam epitaxy (MBE) were discussed in the context of facet formation and mass accumulation. A model was shown to explain the facet formation and its evolution in the process of growth. Further work on surface diffusion and nucleation processes as functions of temperature and other growth parameters will provide needed information for accurate modeling of the facet growth process.
I. Introduction As the scale of integration of electronic devices continues to increase, the feature size must be reduced. It will approach below 100 nm at the turn of the Century. Thus there is a great deal of interest in studying structures of reduced dimensions and in fabricating nanometer scale structures. In the past, advanced crystal growth techniques, such as molecular beam epitexy (MBE) and chemical vapor deposition (CVD), have made it possible to fabricate twodimensional structures such as heterojunctions, quantum wells and superlattices with layer thickness on the atomic scale. However further reduction of lateral dimensions in forms of onedimensional quantum wires, zero-dimensional quantum dots and nanometer scale structures in general has faced technological difficulties. Most widely used methods for the fabrication of nanostructures such as quantum wires and quantum dots are based on ex-situ lateral patterning techniques which combine lithography with dry or wet chemical etching to achieve desired patterns. Despite of the fact that e-beam lithography can achieve structures with dimensions smaller than 10 nm [1], there exist a lot of problems such as surface damage§ and irregularities in the shape and size of nanostructures. Therefore, there is an impetus for the development of new alternative techniques to form nanometer scale structures. Among them, the direct growth of nanostructures has a potential to form damage-free structures, making this technique desirable for studying the properties of nanostructures. To directly grow nanostructures, the substrate should be structured, i.e. there is a controlled starting surface pattern present on the substrate. There have been several kinds of structured substrates used and they are, for example, nonplanar patterned substrates[2], planar masked substrates [3], and planar high index surfaces [4]. When films are selectively grown on these
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Mat. Res. Soc. Symp. Proc. Vol. 448 © 1997 Materials Research Society
structured substrates, mesas are often formed on the substrates. Independent of growth techniques, the sidewalls of these mesas inevitably form facets. More than one facet may result This is because the adatoms migrate on the top surface of the mesa and the sidewalls to minimize the total free energy during growth. The possible facets and their development depend on the substrate orientation,
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