Elsevier

Acta Materialia

Volume 220, November 2021, 117313
Acta Materialia

Full length article
Microstructure development and morphological transition during deposition of immiscible alloy films

https://doi.org/10.1016/j.actamat.2021.117313Get rights and content

Abstract

Complex three-dimensional microstructural patterns arise during deposition of immiscible alloys and their morphologies depend sensitively on alloy composition, deposition rates and substrate temperatures. Using phase field simulations, we construct a microstructure morphology map in a multi-dimensional space of material properties and processing parameters. We consider simultaneous effects from temperature-dependent surface and bulk diffusivities and thermodynamic driving force for phase separation, temperature- and composition-dependent interphase boundary and surface energies, as well as alloy composition, substrate temperature and deposition rate. The microstructural patterns and morphological transition sequences in as-deposited films revealed by the microstructure map are validated using experimental data from sputtered Cu-Mo alloy films as well as from other systems. Such a microstructural map can guide synthesis of three-dimensional compositionally modulated nanostructures via self-organization during deposition of immiscible alloy films.

Introduction

Thin films with periodic concentration modulations (CM) have attracted considerable attention lately because of technological applications found in, for example, field-effect transistors [1], photodetectors [2], energy storage [3], and phase change random access memory [4]. Films with vertical CM (VCM, where vertical refers to the direction that is perpendicular to the film, i.e., parallel to the growth direction) and lateral CM (LCM) have been shown to exhibit unique electronic band structures, electrical carrier mobility and phase transformation characteristics. They offer tremendous benefits for the design of functional devices with large excitonic effect, bandgap modulation, indirect to direct bandgap transition, piezoelectricity and valleytronics [[5], [6], [7]]. They also exhibit novel structural properties and radiation damage tolerance behavior [8,9].

Fabrication of CM films by alternating deposition can hardly control the modulation wavelength and film morphology in three dimensions (3D). On the other hand, phase separation during co-deposition of immiscible alloy films with a large positive enthalpy of mixing enables low-cost batch fabrication of self-organized 3D nanoscale morphological patterns [10,11] and has been utilized widely in producing films with various uniform spatial CM patterns having desired physical properties [[12], [13], [14], [15]]. Depending on the concave downward part of the free energy surface in either the compositional space, structural space, or both, the phase separation may occur by either the spinodal decomposition (isostructural) or de-mixing (heterostructural) mechanism. Previous studies have demonstrated that the development of directional CM patterns is determined by the interplay between phase separation kinetics and film deposition rate and substrate temperature [[16], [17], [18], [19], [20]], which have been utilized successfully in the design of self-organized CM films in various devices [21,22]. According to the theoretically predicted “isothermal microstructure map” [18], the spinodal decomposition process during co-deposition could be controlled in the processing parameter space to produce various microstructural patterns via morphological transition from one type to another.

A number of recent investigations have highlighted morphological control in co-deposited thin films at different deposition temperatures. Nano-lamellar TiN/AlN films and multilayer TiO2/VO2 films have been prepared via spinodal decomposition at 800 °C, at which bulk diffusion is relatively slow in these ceramic systems [15]. Derby et al. [16,23] showed that Cu-Mo films with all three types of CM microstructures, i.e., VCM, LCM and random concentration modulation (RCM) structures can be fabricated under the same deposition rate, e.g., 1.4 nm/s, but at different temperatures. Xie et al. reported [24] a series of self-organized binary immiscible Cu-X (X:W, Mo, Nb, V, Cr, Al) films with VCM structures prepared at low temperatures. These experimental studies represent an uncharted territory for theoretical modeling and computer simulations because the “isothermal microstructure map” developed in the literature [18], without considering the temperature effect, cannot be applied to these cases. For example, according to the isothermal microstructure map [18], the VCM formation requires a relatively fast rate of phase separation (compared to the deposition rate) via bulk diffusion [18,25], which is most unlikely to be the case at low temperatures because the sluggish bulk diffusion and faster surface diffusion may not be able to give rise to the precursory “chessboard structure” that leads to VCM after coarsening. Cu-X (where X is a BCC refractory metal) binary alloys have rather small interdiffusivities (10−16∼10−13 cm2/s from 500 to 700 °C) in bulk at low deposition temperatures [26]. Thus, the formation mechanisms of VCM in Cu-X films at low temperatures cannot be explained by the mechanisms revealed by the isothermal microstructure map either. Furthermore, the fast bulk diffusion at high temperatures could enhance coarsening and interrupt the continuous growth of LCM along the film growth direction. A more recent study [27] did consider the surface diffusion effect, but came short of predicting the VCM structure at low temperatures.

During co-deposition, mechanically mixed alloy layers are continuously added on the surface of a growing film, which subsequently undergo phase separation. Thus, the deposition rate determines the thickness of freshly added surface layers. The kinetics of phase separation relative to the deposition rate is extremely sensitive to the substrate temperature. Using a regular solution model as an example, which characterizes an alloy system with of a miscibility gap, the temperature-dependent free energy hump between the two equilibrium compositions reflects the driving force for spinodal decomposition. The lower of the temperature, the higher of the free energy hump and hence the larger of the driving force for phase separation. However, both bulk and surface diffusivities are significantly lower at low temperatures, thereby limiting the kinetics of phase separation during deposition. Furthermore, the bulk diffusivity becomes significantly lower than the surface diffusivity at low temperatures, which may alter the evolution pathway for CM development. In previous models, the temperature dependences of the thermodynamic driving force and ratio of surface vs. bulk diffusivities were not considered. Therefore, these models may not be able to capture surface-diffusion-, bulk-diffusion- or mixed surface-diffusion + bulk-diffusion-controlled phase separation processes, and thus the true balance between phase separation kinetics and deposition rate that determines the experimentally observed CM patterns. At the same deposition rate, bulk-diffusion will dominate the phase separation kinetics at high temperatures while surface diffusion will make significant contributions to the phase separation process. In addition, when the two phases have different surface energies, the phase with lower surface energy will prefer to form at the film surface [28]. This effect drives interdiffusion of atoms in the vertical direction (perpendicular to the film) in the surface and subsurface layers and gives rise to a VCM structure [29].

In this study, using a combination of phase field simulation and experimental characterization of sputtered immiscible alloy films, we investigate morphological pattern formation and transition during film deposition in a multi-dimensional space of materials and processing parameters, including temperature-dependent surface and bulk diffusivities, temperature-dependent thermodynamic driving force for phase separation, as well as temperature- and composition-dependent interfacial and surface energies. Based on the simulation results, the relationships among the types of CM patterns and the deposition rate, deposition temperature and alloy composition are established and documented in a microstructure map that is then validated against the experimental data. Similar to phase diagrams and TTT/CCT diagrams for alloy microstructure design, such a microstructure map is useful for the design of self-organization of CM patterns during deposition of immiscible alloys.

Section snippets

Phase-field model

For simplicity and without losing generality, we consider an A-B binary system with a miscibility gap. A structural order parameter η is introduced to distinguish solid from vapor, with η= 0 and η= 1 representing the vapor and solid phases, respectively, and 0<η<1 representing the film surface. Assuming no lattice mismatch between the two co-existing phases (i.e., no coherency elastic strain), the total free energy of the system can be formulated on the basis of the gradient thermodynamics [30]:

Results

By introducing the temperature dependences of the thermodynamic parameters in the free energy model and the surface and bulk diffusivities, together with the dependence of surface energy on composition in the phase field model, we have simulated microstructure evolution during film deposition with different initial compositions at different deposition rates (v*= 0.01∼6) and temperatures (T*=T /Tc). The equilibrium solute concentrations of the two phases at different temperatures are symmetrical

Discussion

The formation of different types of CM patterns and transitions between them in the space of alloy composition, deposition rate and deposition temperature are revealed clearly in the computed microstructure map. According to the map, VCM, LCM and RCM structures can be obtained in a broad range of deposition temperatures. Previous studies [18,25,27] have focused on relatively high deposition temperature where bulk diffusion dominates the phase separation process. Without considering the

Summary

In summary, by considering the temperature-dependent driving force and surface and bulk diffusivities, and composition-dependent surface energy, we have constructed a CM microstructure map for thin film deposition in a multi-dimensional space of material properties and processing parameters using computer simulations based on the phase field method. The CM microstructure patterns and morphological transition sequences in as-deposited films predicted by the microstructure map agree well with

Declaration of Competing Interest

None.

Acknowledgements

YL would like to thank the financial support by National Key R&D Program of China (2017YFB0702401), Natural Science Foundation of Fujian Province of China (2019J01033) and the China Scholarship Council (Grant No.201806315068). AM and BD acknowledge support from the Center for Research Excellence on Dynamically Deformed Solids (CREDDS) sponsored by the Department of Energy—National Nuclear Security Administration (DOE-NNSA), Stewardship Science Academic Program under Award No. DE-NA0003857. YW

References (43)

  • M.A. Morris et al.

    Monolayer and multilayer surface diffusion, growth mode and thermal stability of indium on W {100}

    Surf. Sci.

    (1986)
  • I. Beszeda et al.

    Investigation of mass transfer surface self-diffusion on palladium

    Surf. Sci.

    (2003)
  • F.T.N. Vüllers et al.

    From solid solutions to fully phase separated interpenetrating networks in sputter deposited “immiscible” W–Cu thin films

    Acta Mater.

    (2015)
  • L. Li et al.

    Black phosphorus field-effect transistors

    Nat. Nanotechnol.

    (2014)
  • J.G. Song et al.

    Controllable synthesis of molybdenum tungsten disulfide alloy for vertically composition-controlled multilayer

    Nat. Commun.

    (2015)
  • S. Kim et al.

    Room-temperature metastability of multilayer graphene oxide films

    Nat. Mater.

    (2012)
  • R.E. Simpson et al.

    Interfacial phase-change memory

    Nat. Nanotechnol.

    (2011)
  • J.A. Rogers et al.

    Materials and mechanics for stretchable electronics

    Science

    (2010)
  • H. Ko et al.

    Ultrathin compound semiconductor on insulator layers for high-performance nanoscale transistors

    Nature

    (2010)
  • D.B. Mitzi et al.

    High-mobility ultrathin semiconducting films prepared by spin coating

    Nature

    (2004)
  • M.J. Demkowicz

    Does shape affect shape change at the nanoscale?

    Mrs Bull.

    (2019)
  • Cited by (0)

    View full text