|
Abstract:
Controlling pattern formation at the surface of epitaxially growing
crystals is an important technical and fundamental topic. In particular,
morphological instabilities have to be understood to be avoided, or,
possibly, exploited. The existing theoretical framework describing such
instabilities will be reviewed, and various novel mechanisms leading to
spontaneous surface patterning during epitaxial growth on vicinal substrates,
will be presented. One of them [1-3] is based on the coupling
between the surface densities of several diffusing species, in the presence
of Ehrlich-Schwoebel barriers at step edges. Such diffusing species may
be precursor molecules (e.g. trimethylgallium) and adatoms (e.g. Ga
adatoms) in chemical vapour deposition (CVD), or adatoms (e.g. Ga and
As) and molecules (e.g. GaAs), or even adatoms and dimers, in molecular
beam epitaxy (MBE). I will discuss in some details a simple two-particle
model accounting for this coupling, and I will show that, depending just
on growth conditions, step flow growth in one and the same system may
be unstable against either meandering or step bunching, as observed for
instance in GaAs deposition on vicinals of GaAs(110).
Other instability-inducing mechanisms, are atom diffusion along step
edges, and diffusion anisotropies. For such situations, Monte Carlo simulations
of epitaxial growth will be discussed, and the resulting spontaneous
surface patterning will be investigated. In particular, I will show that
the growing surface exhibits anomalous scaling when step-edge diffusion
creates the pattern.
The scaling properties of the surface are the only means for discriminating
which instability-inducing mechanism is at work in a given system.
The recently proposed idea of the existence of universality classes for step
bunching [4] will be checked against real and numerical experiments.
Once created, nanostructures evolve in time, according to the temperature.
At high enough temperature, they decay within a typical lifetime.
The latter has been found, in a number of experiments with nanopyramids,
to scale as a power of the size of the nanostructure. I will claim that simple
power-laws do not exist, and that crossovers between different scaling
regimes are the rules. A simple analytic formulation for such a crossover will be presented and compared with kinetic Monte Carlo simulations of
a model for nanopyramids on a Si(001) surface.
1. A Pimpinelli and A. Videcoq, Novel mechanism for the onset of morphological
instabilities during chemical vapour epitaxial growth, Surface Sci.
Letters 445, L23-L28 (2000)
2. M. Vladimirova, A Pimpinelli and A. Videcoq, A new model of morphological
instabilities during epitaxial growth: from step bunching to mounds
formation, J. Crystal Growth 220, 631-636 (2000)
3. A. Videcoq, M. Vladimirova and A Pimpinelli, Kinetic surface structuring
during homoepitaxy of GaAs(110): a model study, Appl. Surf. Sci.
175-176, 141-146 (2001)
4. A. Pimpinelli, V. Tonchev, A. Videcoq and M. Vladimirova, Scaling and
universality of self-organized patterns on unstable vicinal surfaces, Phys.
Rev. Lett. 88, 206103 (2002)
|
