Shape and Stability of Epitaxial Nanostructures Evolving under Growth or Annealing

Abstract

Nanostructures may become useful components of future electronic devices. In our first study, The growth dynamics of Ge islands on Si(001) and Si(113) surfaces is studied in situ using ultra-violet photoelectron emission microscopy (UV-PEEM) with tunable UV light from the free electron laser at Duke University. In situ Ge deposition and real-time monitoring of the growing islands allowed observation of the evolution of the size, shape and density of the surface structures. Following deposition, the transition from layer by layer to 3D growth was detected with the presence of islands with a density of Τ1x109 cm-2. Continuous deposition of Ge led to the enlargement of the islands, but further island nucleation was not detected. Upon annealing, the average size of the islands increased while the island density slightly decreased. AFM measurements performed on the islands grown on the Si(001) surface showed dome structures while the islands on Si(113) substrates display flat tops with multiple steep facets on their sides. Reduction of the deposition rate on Si(113) resulted in the formation of metastable elongated structures aligned along the [33 ] direction.

Next, narrow and wide nanowires of DySi2 were formed on a Si (001) substrate through high temperature deposition of few monolayers of dysprosium and annealing at 700 C. The formation, growth and decay of the silicide nanowires were observed by real time imaging using photo electron emission microscopy (PEEM). We report on the decay of the nanowires at different temperatures between 700 and 800 C. Upon annealing, we observe that the nanowires width remains constant while the length decreases with time. Narrow nanowires decay only from the ends while wide nanowires may also break in fragments before they eventually disappear. A linear decay rate suggests an attachment/detachment dominant mechanism.

We also report on the shape transition and migration of TiSi2 nanostructures embedded in a Si matrix. Grown multifaceted TiSi2 were exposed to a Si flux under different growth conditions forming a thick capping layer. AFM and XREM have been used to study the shape, geometry and evolution of the buried structures. We establish that under conditions of epitaxial Si deposition, Ti-silicide nanostructures undergo a shape transition and "migrate" to the surface.

We have also studied the shape evolution of Si/Ge quantum dots (QDs) superlattices. Using a two-temperature procedure that has been found to prevent Si/QDs intermixing and the truncation of QDs, we have been grown dense, uniformly sized and distributed QDs. The dome-shaped Ge QDs were first capped by a cold Si layer which allows them to retain their shape and their functionality. They were then covered by a hot layer of Si before depositing another layer of Ge at high temperature. This process was repeated 20 times and terminated by a layer of Ge. The surface morphology was studied by AFM. Cross-sectional TEM was used to analyze the growth sequence and the shape of the buried QDs.

The observed structural changes in these experiments are explained in terms of the interplay between thermodynamics and kinetics, solid state capillarity, and the roughening transition.