Computer simulation of thin amorphous Si films on crystalline Si substrates (original) (raw)
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A Structural Analysis of Thin Amorphous Silicon Films
MRS Proceedings, 1987
Using a semiempirical potential energy function for Si, thin amorphous films of Si have been simulated on crystalline Si by a partial melting and subsequent quenching process. After relaxation at 450°K, the dominant structural feature was a dense free surface skin with a void layer underneath for both c-Si(100) and c-Si(111) substrates and for film thicknesses up to 16Å. Tetrahedral coordination was maintained throughout the amorphous region and structural differences were noted in the α-Si/c-Si interface region for the two orientations considered.
Journal of Materials Science, 2008
Polycrystalline silicon obtained by the crystallization of thin amorphous silicon films has been an important material for microelectronics technology during the last decades. Many properties are improved in crystallized amorphous silicon compared to the as-deposited polysilicon such as larger grain size, smoother surface, and higher-carrier mobility. In this work, the crystallization of amorphous silicon is investigated by combining transmission electron microscopy (TEM) observations and molecular dynamics calculations. TEM observations on a series of specimens have shown that the majority of the silicon grains are oriented with a \( {\left\langle {110} \right\rangle} \) zone axis normal to the surface. In order to understand the crystallization mechanism molecular dynamic simulations were performed. It is found that the \( {\left\langle {110} \right\rangle} \) c/amorphous interface exhibits the lowest reduced interfacial energy density while the \( {\left\langle {111} \right\rangle} \) c/amorphous has the lowest reduced energy differences per unit interfacial area. The most energetically unfavorable interface is \( {\left\langle {001} \right\rangle} \) c/amorphous.
Amorphous-crystal interface in silicon: A tight-binding simulation
1998
The structural features of the interface between the cystalline and amorphous phases of Si solid are studied in simulations based on a combination of empirical interatomic potentials and a nonorthogonal tight-binding model. The tight-binding Hamiltonian was created and tested for the types of structures and distortions anticipated to occur at this interface. The simulations indicate the presence of a number of interesting features near the interface. The features that may lead to crystallization upon heating include 110 chains with some defects, most prominently dimers similar to those on the Si(001) 2 × 1 reconstructed free surface. Within the amorphous region order is lost over very short distances. By examining six different samples with two interfaces each, we find the energy of the amorphous-crystal interface to be 0.49 ± 0.05 J/m 2 .
Electronic properties of amorphous/crystalline Si interface from atomic-scale simulations
The band alignment of the amorphous/crystalline Si heterostructures is studied by means of a direct atomistic simulation of the interface morphology. To this aim we have adopted a hierarchical combination of various computational schemes, ranging from classical model-potential and tight-binding molecular dynamics to ab-initio methods. The different tools have been used to provide reliable atomistic modelling of interface structure and to evaluate electronic properties, respectively. Remarkably, the band alignment is found to be almost the same for all those samples with a non negligible concentration of coordination defects in the amorphous region, whereas it is sizeably different for a defect-free sample. The explanation of this result is ascribed to the semi-metallic behaviour of the defect-rich a-Si in the former case, and to the establishment of a ``genuine'' semiconductor/semiconductor type of interface in the latter. The role of global structural properties in the elec...
Role of defects in the electronic properties of amorphous/crystalline Si interface
Physical Review B, 2001
The mechanism determining the band alignment of the amorphous/crystalline Si heterostructures is addressed with direct atomistic simulations of the interface performed using a hierarchical combination of various computational schemes ranging from classical model-potential molecular dynamics to ab-initio methods. We found that in coordination defect-free samples the band alignment is almost vanishing and independent on interface details. In defect-rich samples, instead, the band alignment is sizeably different with respect to the defect-free case, but, remarkably, almost independent on the concentration of defects. We rationalize these findings within the theory of semiconductor interfaces.
Physical Review B
Thin silicon films of varying thickness were deposited on foreign substrates by electron-cyclotron resonance chemical vapor deposition from SiH 4-H 2 mixtures at 600 K. Optical thickness measurements, Rutherford backscattering, and transmission electron microscopy reveal that a thin amorphous interlayer of some 10 nm thickness has formed upon the substrate, before the growth of a microcrystalline layer begins. The amorphous layer is found to be deposited with a higher rate than the crystalline phase. Since similar effects have been observed for a large variety of deposition techniques, the amorphous-crystalline phase transition is considered as an inherent property of the growth of thin silicon films on foreign substrates at low homologous temperatures. The change in growth mode is interpreted in terms of Ostwald's rule of stages, which predicts the evolution of film growth to proceed via a set of phases of descending metastability and nucleation rate. In applying capillarity theory a criterion is derived from the ratio of amorphous-phase and crystalline-phase nucleation rates J a /J c. This ratio is developed into basic thermodynamic functions and is shown to govern the formation of either the stable or metastable phase. The approach is of general validity for thin-film deposition processes. In the case of microcrystalline silicon, experimental measures can be derived from the developed model to directly design the evolution of film structure.
Large and realistic models of amorphous silicon
Journal of Non-Crystalline Solids
Amorphous silicon (a-Si) models are analyzed for structural, electronic and vibrational characteristics. Several models of various sizes have been computationally fabricated for this analysis. It is shown that a recently developed structural modeling algorithm known as force-enhanced atomic refinement (FEAR) provides results in agreement with experimental neutron and x-ray diffraction data while producing a total energy below conventional schemes. We also show that a large model (∼ 500 atoms) and a complete basis is necessary to properly describe vibrational and thermal properties. We compute the density for a-Si, and compare with experimental results.
Characterization of amorphous and nanostructured Si films by differential scanning calorimetry
Thin Solid Films, 2009
Over the last 5 years, we have successfully applied differential scanning calorimetry (DSC) to study silicon thin films. The aim of this paper is to review our main results to give an overview of the possibilities offered by this technique, which is widely used to characterize solid state transformations. We will address some classical subjects related to the structure of pure and hydrogenated a-Si, such as the strength of the Si-H bond, hydrogen diffusion and the contribution of structural defects and strained bonds to the enthalpy of structural relaxation and crystallization. Special attention will be paid to the crystallization kinetics of films with structures ranging from pure amorphous to highly crystalline. Despite previous expectations, in films deposited at low temperature, the presence of nanocrystals embedded in the amorphous phase does not promote crystallization. In fact, the crystallization temperature is much higher than expected from a simple solid state epitaxy mechanism, which indicates low coupling between the amorphous and crystalline phases.