Kinetic Monte Carlo simulations of nanocrystalline film deposition

Abstract

Simulations of chemical vapor deposition diamond film growth using a kinetic Monte Carlo model and twodimensional models of microwave plasma and hot filament chemical vapor deposition reactors A full diffusion kinetic Monte Carlo algorithm is used to model nanocrystalline film deposition, and study the mechanisms of grain nucleation and microstructure formation in such films. The major finding of this work is that new grain nucleation occurs predominantly on surface peaks. Consequently, development of a nanocrystalline structure is promoted by a growth surface with nanoscale roughness, on which new grains can nucleate and grow separately from one another. The grain minor dimension ͑in the plane of the film͒ is primarily dictated by surface peak spacing, which in turn is reduced at low temperatures and high deposition rates. The grain major dimension ͑in the growth direction͒ is related to the probability of nucleating new grains on top of pre-existing ones, with finer grains being formed at low temperatures and low grain boundary energies. Because vacancies kinetically pin grain boundaries, high vacancy content, which is obtained at high deposition rate, also favors nanograins. Consistent with empirical observations common in the experimental literature, it is found that as grains shrink, they transition from elongated to equiaxed.

Morphology evolution of thermally annealed polycrystalline thin films

“…This rate is two orders of magnitude higher than the typical rates (∼0.1Å/s), 10 which gives rise to a film microstructure ruled by kinetic limitations that hinder both the structural and morphological relaxations during deposition. 11 Once deposited, the flux was stopped, and the samples were held at the growth temperature for an in-situ annealing (which enables the film relaxation) during t a = 1.2 × 10 2 − 1.2 × 10 5 s under an Ar flux (P Ar = 1 atm). Afterwards, the samples were cooled quickly down to room temperature and investigated by atomic force microscopy (AFM) using ultrasharp silicon tips (with a nominal radius of 2 nm and estimated aspect ratio on calibration samples of 2m tip ≈ 1.5) and by x-ray diffraction (XRD) in θ -2θ and φ-scan geometries measured in an X'Pert four-circle diffractometer using Cu K α radiation.…”

From x-ray telescopes to neutron focusing

“…Note that commercial neutron guides have nominal roughness of 5 Å. Recent computer simulations demonstrated the connection between the substrate roughness and the grain size of Ni crystallites [18]. These results show the existing polishing technique used at the MSFC is sufficient to suppress the size of Ni crystallites, the major contributor to the multilayersubstrate interface roughness.…”

A compound unit method for incorporating ordered compounds into lattice models of alloys

“…This method of including multi-body terms has been useful for developing bulk phase diagrams of alloys [26][27][28][29]. Increasingly, however, lattice models are being extended outside of classical bulk thermodynamic behavior, for instance to study the effects of interfaces [16][17][18][31][32][33][34][35]. In such cases, while using a cluster expansion to define the interatomic potential may still be feasible, it is not optimal because (i) the stability of compounds in bulk systems is already known [30], and thus ''predicting" the stable compounds through simulation is unnecessary and (ii) the multi-body terms are calculated for each alloy system and thus too specific to probe a large number of alloy combinations and often not easily extendable to non-bulk environments.…”

“…In such cases, while using a cluster expansion to define the interatomic potential may still be feasible, it is not optimal because (i) the stability of compounds in bulk systems is already known [30], and thus ''predicting" the stable compounds through simulation is unnecessary and (ii) the multi-body terms are calculated for each alloy system and thus too specific to probe a large number of alloy combinations and often not easily extendable to non-bulk environments. In some of our group's work on nanostructured alloys we have found a need for a lattice-based method that can be rapidly calibrated to known bulk thermodynamics and then used to explore, e.g., driven processing through ballistic mixing [34] or deposition [35], or simply to explore the space of accessible structures in nanostructured systems [18,[36][37][38]. It is our purpose in this paper to present such a method that overcomes the limitations of the pairwise model in describing negative enthalpy of mixing systems, specifically for cases where the structure and formation energy of equilibrium compounds are already known, and a full cluster expansion would be redundant.…”