Crystallization in the limit of ultra thin layers- A new crystallization model
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Crystallization in the limit of ultra thin layers- A new crystallization model Margit Zacharias1 and Peter Streitenberger2 1 2
Max- Planck- Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany Institut für Experimentelle Physik, Otto-von- Guericke Universität, PF 4120, 39016 Magdeburg
ABSTRACT Annealing of amorphous Si/SiO2 multilayer produces nanocrystals embedded between oxide interfaces with the size of the nanocrystals depending on the Si layer thickness. It is found that the crystallization temperature is strongly enhanced by the presence of the oxide interface and follows an exponential law. A model is presented, which derives the exponential dependence taking into account the interface energies, the thickness of the layers, the melting point of the system, and the bulk amorphous crystallization temperature. The critical crystallization radius and the critical thickness of the Si layer are discussed. INTRODUCTION Today, polycrystalline Si (poly-Si) is applied in ultra large scale integration technology both for active and passive components. For device application precise engineering of the Si grain size with focus on control in grain boundary and defect density is essential. In view of the visible luminescence of nanocrystalline Si (nc-Si) the control of size, the passivation and arrangement of Si nanocrystals embedded in oxide matrix is mandatory. The preparation of Si/SiO2 superlattices (SL) represents a way to control the arrangement as well as the size of the nanocrystals. However, the kinetics of the amorphous to polycrystalline transition has to be reviewed to get a qualitative understanding of the crystallization process in the limit of very thin films. The solid phase crystallization of chemical vapor deposited amorphous Si films for thickness above 50 nm has been reviewed extensively [1]. However, there are only a few data at hand for the crystallization behavior for a thickness below 50 nm. Also, the existing models for the kinetic mechanisms of crystal grain growth are not applicable in the presence of multiple stacks of Si/SiO2 periods. Here, the amorphous-to-crystalline phase transition occurs through random nucleation of crystalline clusters surrounded by amorphous material under the strain field of the superlattice structure. Compared to a solid bulk phase crystallization the process involves several additional phenomena such as the creation of the crystalline seeds themselves, the influence of the oxide interface, the influence of strain, the influence of extended defects at the grain surface etc. Also, a prevented nucleation of Si near the SiO2 interface concerning the first adjacent 0.5-1.0 nm of the Si layer was reported [2]. A strong increase of the crystallization temperature was shown for amorphous Si/SiO2 superlattices [3], Si/SiOx [4], Ge/SiO2 [5], and Ge:H/GeNx [6]. In this work we will show the general character of the increase in crystallization temperature for reduced layer thickness and discuss the origin of this retarded crystallization.
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