Metallic Glass Nano-composite Thin Films for High-performance Functional Applications
Metallic glass nanocomposite thin films were synthesized for an immiscible Ag-Cu alloy system by magnetron sputtering. The structure of the films was unique, consisting of homogeneously dispersed nanocrystallites in an amorphous matrix. The size and volume fraction of the nanocrystallites increased with increasing film thickness resulting in increased elastic modulus and hardness. The high electrical conductivity of the nanocomposite films was examined by a valence-band study, which showed that exchange interaction between Ag and Cu in the nanocomposite structure resulted in enhanced charge carrier concentration. The inverse correlation between electrical conductivity and film thickness was explained by surface and interface scattering of electrons with increasing volume fraction of nanocrystallites. The small temperature dependence of conductivity was attributed to the distorted Fermi surface of the nanocomposite films resulting in a greater contribution from structure scattering, which is temperature-independent.
Individually, silver (Ag) and copper (Cu) exhibit the highest electrical conductivity among metals and are widely used for numerous scientific and industrial applications. However, Ag and Cu are mutually immiscible due to their positive heat of mixing.1 In an immiscible system, the mixing of the constituent elements even in the liquid state may be unfavorable,2,3 and long-range chemical partitioning may be suppressed by quenching or other non-equilibrium processing. However, some degree of energy minimization results in the development of local structures on extremely fine scales.4 Formation and stabilization of amorphous phases in immiscible systems have also been reported,5,6 along with a strong clustering tendency leading to non-uniform spinodal-like structures.5 Vapor deposition and other far-from-equilibrium processing techniques may be employed to extend solid solubility in immiscible systems.5, 6, 7, 8, 9, 10, 11, 12, 13 Sputtering represents a versatile technique for producing homogeneous thin films of immiscible alloys with varying composition and tunable properties.14,15 While there are a few studies on the structure as a function of composition for silver-copper alloys by far-from-equilibrium processing,13,16, 17, 18 there are no reports on the physics of electrical transport in this immiscible system of immense technological importance. Fundamental understanding of electrical transport properties in Ag-Cu alloys has the potential to open up a new paradigm of highly conducting metals for deeply scaled interconnects, energy conversion technologies, and micro-/nano-electromechanical systems (MEMS/NEMS).
Here, we demonstrate a highly conductive nanocrystallite dispersed amorphous structure synthesized from an immiscible Ag-Cu alloy system. Nanocomposite films of nominal composition ~60 at.% Ag–40 at.% Cu were synthesized by direct current (DC) magnetron sputtering. This particular composition was chosen because it is close to a eutectic and therefore glass formation is likely to be favored. The structure of the nanocomposite films was unique, consisting of homogeneously dispersed nanocrystallites in an amorphous matrix. The size and volume fraction of the nanocrystallites increased with increasing film thickness resulting in increased elastic modulus and hardness. These nanocomposite films showed high electrical conductivity, with a very small temperature dependence. The high conductivity of the nanocomposite films was attributed to the d–d band interaction of Ag and Cu, leading to an increased density of charge carriers as seen from their valence band structure.
Nanocomposite Ag-Cu thin films of composition 60.1 at.% Ag–39.9 at.% Cu were sputter-deposited onto thermal oxide-grown (thickness of ~300 nm) Si substrates. Three different film thicknesses of 50 nm, 100 nm, and 200 nm were deposited by varying deposition time at a constant deposition rate (1.39 Å/s). Substrate temperature during the deposition processes was kept at around 16°C. A 2-nm coating of Cr was used to ensure good film-substrate adhesion. The film thicknesses were measured using a profilometer (Vecco, USA) to be: 50 nm ± 1.6 nm, 100 nm ± 4.9 nm, and 200 nm ± 11.1 nm, respectively. More experimental details and characterization of the thin ilms are discussed in the supplementary information Section S1.
Results and Discussion
Figures S1a and S1b (supplementary information) show the characteristic Cu 2p and Ag 3d peaks measured using XPS for the 50-nm-thick film. Similar peak positions were also obtained for the 100-nm- and 200-nm-thick films. The energy difference (i.e. core-level spin orbital splitting) between 3d5/2 and 3d3/2 for Ag (6.0 eV) as well as between 2p3/2 and 2p1/2 for Cu (19.8 eV) were found to be unchanged, which shows the metallic states of the respective elements in the nanocomposite films. However, a slight shift of the Ag 3d5/2 peak to higher binding energy and a slight shift of the Cu 2p3/2 peak to lower binding energy may be attributed to nearest neighbors of different kinds in the alloy, in contrast to pure metals. After baseline correction followed by XPS peak fitting, the composition of each nanocomposite film (10-point average) was calculated to be: Ag ~60 at.% ± 0.4 at.%, Cu 39.7 at.% ± 0.28 at.%, and 0.3% others. This suggests no surface segregation of either Cu or Ag in the alloy.
The structure of the thin films and the corresponding sputtering target were analyzed by XRD as shown in Fig. 2. The increase in nanocrystallite size and volume fraction with increasing film thickness are also evident from the XRD patterns. For the 50-nm film, the XRD pattern shows a broad hump, indicating that the structure is almost entirely amorphous. However, with increasing film thickness, the XRD patterns for the 100-nm and 200-nm films show a decreasing trend for full-width half-maxima, suggesting nucleation of crystalline phases in the amorphous matrix.20,21 XRD characterization of the binary-alloy (60.12 at.% Ag–39.88 at.% Cu) sputtering target was also carried out (Fig. 2d). The XRD pattern shows a clear signature of pure Ag and pure Cu peaks, indicative of an immiscible binary eutectic alloy.
High-conductivity nanocrystalline dispersed amorphous thin films were synthesized for a Ag-Cu immiscible binary alloy system. The 50-nm film exhibited high electrical conductivity of 5.09 × 105 S cm−1 and the conductivity decreased with increasing film thickness. The decrease in conductivity with increasing volume fraction of nanocrystallites is attributed to increased surface and grain boundary scattering of electrons. However, all the films exhibited very small temperature dependence of conductivity. Valence band studies showed that interaction between outer d-bands of Ag and Cu resulted in enhanced charge carrier concentration and increased density of states. This study sets the stage for developing high-performance conductors from immiscible systems with outstanding electrical and mechanical properties at a reduced length scale. Possible applications include nano-electromechanical systems (NEMS) and nanomechanical actuators, which demand tailoring of both electrical and mechanical properties.
This work was partly supported by funding from Semiconductor Research Corporation (SRC/CEMPI Task ID: 2071.031). The authors also gratefully acknowledge the Center for Advanced Research and Technology (CART) at UNT for some of the characterization.
- 15.I. Atanasov, R. Ferrando, and R.L. Johnston, J. Phys.: Condens. Matter 26, 275301 (2014).Google Scholar