Controlling surface morphology by nanocrystalline/amorphous competitive self-phase separation in thin films: Thickness-modulated reflectance and interference phenomena

Abstract

Controlling surface morphology is a key issue for obtaining functional materials with surface-based properties. In this paper, we explore the possibility of using the self-separation of phases as a way of controlling the surface morphology features. We demonstrate using X-ray diffraction and transmission electron microscopy that a competitive self-separation of a nanocrystalline and an amorphous phases occurs in co-sputtered Zr-Mo thin films with a Mo content of 60 at%, corresponding to a composition intermediate to those necessary to form single-phased amorphous and nanocrystalline films. The dependence of the residual stress with the thickness at the biphased composition is discussed in terms of the morphology evolution and a possible mechanism for the self-separation of phases is presented. We show that the self-separation of phases as presented here is not limited to Zr-Mo alloys and can be extended to other systems. By changing the film thickness, it is possible to change the surface morphology of the films at the biphasic composition, due to the competitive growth of the nanocrystalline phase in the amorphous phase. In this way, it was possible to control the surface roughness and, because of this, tuning the film reflectance at a determined wavelength. The occurrence of an interference pattern in the reflectance spectra was discussed and associated to the presence of two different height levels at the film surface.

Growth kinetics and origin of residual stress of two-phase crystalline–amorphous nanostructured films

“…This compositiondriven transition from a single-phased amorphous to a single-phased crystalline structure, shown here in the Zr-W system, is characteristic of many sputter-deposited binary alloys [37][38][39][40][41][42] . As we have recently shown 28,29 , twophase crystalline-amorphous nanostructures can be obtained by working in the transition zone between singlephased amorphous and crystalline films of the Zr-Mo and Zr-W systems. The X-ray diffractogram of films grown in this zone (Fig.…”

“…In previous works 28,29 , we reported on the mechanisms of competitive growth between amorphous and crystalline phases in thin films, leading to original microstructures consisting of a crystalline phase of featherlike morphology embedded in an amorphous matrix. By working at compositions in the transition zone between those needed to obtain single-phased amorphous or crystalline films, the desired microstructure is obtained as a consequence of the competitive growth of the amorphous and the crystalline phases.…”

“…Moreover, we have also shown that it occurs under a wide range of deposition conditions. For instance, the shape of the crystalline domains can be modified by selecting low or high pressure conditions 28 . In this crystalline-amorphous competitive growth process, the crystalline and amorphous regions can easily be identified and the kinetics of the competitive growth can be modeled through access to FIG.…”

Composition-driven transition from amorphous to crystalline films enables bottom-up design of functional surfaces

“…With the increase of the film thickness, the crystalline phase starts to nucleate, which implies that the energy barrier becomes of the order of . The reasons behind this modification could be thermal, chemical (a local change in composition due to the preferred segregation of one of the species to the grain boundaries, which favors the nucleation [32]), mechanical (local changes of the strain energy due to the heterogeneous distributions of the film stress [56,57] which can decrease the energy barrier), among others. Considering this, we propose to write as follows:…”

“…Yet, in most cases, single-phased coatings are considered and the mechanisms at the origin of multi-phased coatings and their potentialities remain essentially unexplored. In particular, the composition-driven transition between amorphous metal alloys and nanocrystalline films [32], at the core of the present study, has been overlooked so far as a possible way of surface functionalization and little is known about the nature of such transition, despite it occurs in many systems [31,[33][34][35][36][37].…”