The role of faceting in biaxially textured thin films: Columnar morphology and abnormal tilting

Abstract

The groundbreaking properties of biaxially textured thin films have attracted increasing attention to the characterization and growth theory of their crystal morphologies. In particular, multi-faceted columnar structures developed during oblique angle deposition (OAD) show abnormal tilt angles that have not been previously captured by existing models. Current theories for the formation of biaxially aligned columnar structures overlook the fact that the surface diffusion on individual facets can be finite. In this work, a continuum model incorporating finite adatom mobility, flux-dependent sticking coefficient and material-specific surface energies is employed to study the growth of a well-known MgO-OAD system. Experimentally observed morphologies are reproduced by simulating the shadowing growth of an array of preferentially oriented single crystals. We show that the abnormal tilting is elusive considering only the effects of faceting and shadowing. A proposed sticking coefficient in our model, determined by the component of adatom momentum parallel to the surface, is responsible for the development of abnormal tilting. The role of faceting is demonstrated by its effect on the resulting columnar morphologies. Using the proposed model, the characteristic morphology and tilting behavior of a CaF 2-OAD system are also obtained which agree with experiments.

Faceting of Si and Ge crystals grown on deeply patterned Si substrates in the kinetic regime: phase-field modelling and experiments

“…With this respect, we notice that simulations including a more detailed description of flux distributions, e.g. shadowing effects 23,69 , may change the material supply within the same facets, thus slightly altering the material redistribution dynamics predicted by the present set of parameters. The implementation of flux shadowing in the phase-field approach 70 goes beyond the scope of this work as it would require computationally demanding calculations in 3D.…”

Phase field model of faceted anatase TiO2 dendrites in low pressure chemical vapor deposition

“…33 In the previous work, we proposed a phase field model incorporating detailed treatments of both process-specific shadowing growth and faceting driven by anisotropic surface energies to study the morphology of nanorods grown by oblique angle deposition. 34 It was found that the film morphology is influenced not only by the uni-directional flux due to ballistic transport but also by the strength of adparticle mobility in relation to asymmetric surface diffusion. We showed that shadowing and directional sticking coefficients were important for growing films with abnormal columnar tilting observed experimentally in two cubic crystals, MgO and CaF 2 .…”

“…In our previous work, a continuum model was formulated that describes an interfacial growth problem posed by two competing effects, i.e., the protuberances formed on the solid-gas interface following the ballistic trajectories of ad-particles and the stabilizing capillary forces due to surface diffusion. 34 The solid phase and the gas phase are treated as superimposed continua using the thermodynamically consistent diffuse-interface theory. Particle-size effects are ignored, because very little of the complex chemistry involved in CVD is known.…”

“…Based on the previous growth model 34 and by considering the ballistic flux distribution, we propose a formulation describing the transport of materials to the interface as well as the subsequent consumption and redistribution by surface diffusion. The first term in Eq.…”