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SYNK: Symposium Nanokristalline Materialien - Struktur und elektronische Eigenschaften

SYNK IX: HV IX

SYNK IX.1: Hauptvortrag

Mittwoch, 24. März 1999, 18:30–19:00, H3

Formation of low-dimensional semiconductor nanostructures on corrugated surfaces — •G. Biasiol — Institute of Micro Optoelectronics, Department of Physics, Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland

Molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (OMCVD) of GaAs/AlGaAs quantum wells on nonplanar substrates can be exploited for growth of one-dimensional quantum wire arrays. The resulting nonplanar growth profiles are commonly described in terms of orientation-dependent fluxes and surface diffusion. The growth rates on these profiles depend on the facet orientation and, if the diffusion length of the relevant species is larger than the typical facet size, are uniform and constant on each facet. These models can predict correctly the formation, evolution or disappearance of facets at the 100nm-µm size. However, they cannot account for the phenomena taking place at the 10-nm level at the top and bottom extrema of V-shaped substrates. In these regions nonconstant growth rates are observed, that self-adjust in order to form profiles whose size and shape are characteristic of the material and growth conditions. We have developed an analytic model that explains this self-limiting growth mode, and interprets the differences between MBE, where self-ordering is commonly observed at the top of the mesas, and OMCVD, where it takes place at the bottom of the grooves. Self-ordering is induced by local variations in the surface chemical potential, since this quantity defines the supersaturation, that is the (local, in this case) driving force for epitaxy. It can be shown that the chemical potential depends on the local surface curvature, and becomes lower as the concavity of the surface increases. This chemical potential gradient will generate diffusion fluxes towards the bottom of the grooves, which become larger as the sharpness of the groove increases. The growth rates at the bottom (top) facet will therefore be increased (decreased) by these capillarity fluxes. We will show formally that, if the incorporation rate on the sidewalls is lower (higher) than that on the extremal facets, self-limiting profiles can be attained at the top (bottom) region, while the bottom (top) will always expand, leading eventually to the planarization of the groove. These two different growth modes have been observed, respectively, in MBE on [011]-oriented mesas (where the sidewalls are slowly-growing 111B facets), and in OMCVD on [01-1] oriented mesas (where they are fast-growing, high index ones). We will show an experimental analysis of self-ordered OMCVD growth, proving quantitatively the validity of the model. We will also show that in the case of ternary alloys, entropy of mixing effects are essential in determining self-limiting shapes and compositions.