Void formation during film growth: A molecular dynamics simulation study

Abstract

Two-dimensional, nonequilibrium molecular dynamics simulations have been applied to study the structure of thin films grown on single-crystal Lennard-Jones substrates. The principal microstructural features to develop within these films are single vacancies and small voids which tend to be slightly elongated and to be aligned in the growth direction. Both the void volume and the mean surface roughness of the films are found to be decreasing functions of substrate temperature and deposition kinetic energy. Voids are shown to form as a consequence of both surface roughness and shadowing effects. The attraction between deposited atoms and the sides of surface depressions lead to the formation of outgrowths from the sidewalls of the surface depression. These outgrowths shadow the open void beneath them and continue to grow across the voids by interaction with the depositing atoms until a continuous bridge is formed that closes off the void. Since this bridging mechanism leaves behind a surface depression above the closed-off void, new voids tend to form above it. This leads to the alignment of voids along the film growth direction. The spacing of the resultant void tracks is correlated with the wavelength of the surface roughness. Increasing temperature and deposition kinetic energy enhancing surface mobility leads to an increase in the wavelength of the surface roughness and hence an increase in the spacing between void tracks. Edge dislocations tend to form within voids as a natural consequence of the void bridging process, however nondislocated voids are also observed.

Nanovoid formation through the annealing of amorphous Al2O3 and WO3 films

“…2(a). The microstructure of the as-deposited a-WO 3 is consistent with the general tendency that as-deposited films grown at very low temperature tend to be porous [14]. The growth of the existing voids is not clear at 573 K ( Fig.…”

The effect of ion-surface and ion-bulk interactions during hydrogenated amorphous silicon deposition

“…25 Two possible mechanisms, which can reduce the nanosized void incorporation, are the enhancement of the surface diffusion or sputtering of the overhangs. Both mechanisms are plausible since they also have been observed in molecular dynamics calculations 44,45 of the deposition of an arbitrary film from high energetic particles ͑corresponding to ions͒ and low energetic particles ͑corresponding to neutral or radicals͒. Enhancement of the diffusion of surface species results in a more efficient filling of the surface valleys and as a consequent less voids are incorporated.…”

“…A valley is incorporated as a nanosized void by overhangs that are formed during growth. 25,44,45 As mentioned earlier, nanosized void incorporation is controlled by a competition between the growth flux and the surface diffusion for non-ionassisted growth conditions. 25 Two possible mechanisms, which can reduce the nanosized void incorporation, are the enhancement of the surface diffusion or sputtering of the overhangs.…”

Relationship of thermal boundary conductance to structure from an analytical model plus molecular dynamics simulations

“…5(a). Voids at the horizontal interfaces were studied because (1) they effectively test the analytical model on the role of the horizontal interfaces; and (2) due to a shadowing effect, voids are most likely to form at the growth surfaces that are oblique to the deposition flux 63,64 , which is likely to be the case for the horizontal GaN surface when Al is deposited on the roughened GaN. The same set of simulations as those explored in Fig.…”