The effect of metal–insulator interface interactions on electrical transport in granular metals

Abstract

We present an in-depth study of metal–insulator interfaces within granular metal (GM) films and correlate their interfacial interactions with structural and electrical transport properties. Nominally 100 nm thick GM films of Co and Mo dispersed within yttria-stabilized zirconia (YSZ), with volumetric metal fractions (φ) from 0.2–0.8, were grown by radio frequency co-sputtering from individual metal and YSZ targets. Scanning transmission electron microscopy and DC transport measurements find that the resulting metal islands are well-defined with 1.7–2.6 nm average diameters and percolation thresholds between φ = 0.4–0.5. The room temperature conductivities for the φ = 0.2 samples are several orders of magnitude larger than previously-reported for GMs. X-ray photoemission spectroscopy indicates both oxygen vacancy formation within the YSZ and band-bending at metal–insulator interfaces. The higher-than-predicted conductivity is largely attributed to these interface interactions. In agreement with recent theory, interactions that reduce the change in conductivity across the metal–insulator interface are seen to prevent sharp conductivity drops when the metal concentration decreases below the percolation threshold. These interface interactions help interpret the broad range of conductivities reported throughout the literature and can be used to tune the conductivities of future GMs.

Granular metals with SiN x dielectrics

“…The percolation threshold (j c ) is the metal fraction where the GM's electronic nature switches from insulating to metallic, and occurs in the j = 0.3-0.6 range. Below j c , the room temperature DC conduction is dominated by thermally-assisted electron tunneling, which depends on the prevalence of insulator defects and the separation distance between metal nanoparticles [4,5]. The capacitive transport dominates at high frequencies and scales according to Jonscher's universal power law [6,7] as confirmed by experiments [8,9].…”

“…While GMs with various metals have been investigated, nearly all GM studies employed oxide-based insulators, such as SiO 2 , Al 2 O 3 , or yttria-stabilized zirconia (YSZ), with notable non-oxides including MgF 2 [18], BN [19], and carbon-based [20,21] GMs. Recently, we found that GMs with non-noble metals and oxide insulators form metal-oxides, which deleteriously increase GM conductivity at j < j c [5]. Annealing of GMs causes nanoparticle coalescence and substantially alters the GM electrical properties [2]; high thermal stability transition metal-based GMs will enable applications at elevated temperatures and high current densities.…”

“…To evaluate the insulator quality, we consider GM compositions at j = 0.2, well within the insulating regime. To date, the GMs with the lowest conductivities all contained Au and Ag nanoparticles, which are resistant to oxidation, and these high-quality GMs can exhibit σ = 10 -4 -10 -6 S cm −1 at j = 0.2 [4,5,13,22,23]. For nonnoble metals, the metal nanoparticles absorb oxygen from the insulator.…”

“…For nonnoble metals, the metal nanoparticles absorb oxygen from the insulator. The resulting vacancies facilitate conduction through the insulator, thus σ at j = 0.2 is 2-4 orders of magnitude higher for non-noble transition metals compared to Au or Ag [5].…”

“…We investigated silicon nitride, frequently used as a passivation layer for power electronics, due to its high chemical stability [24][25][26]. Mo and Co were chosen for direct comparison to Mo-YSZ and Co-YSZ GMs [5]. We synthesized Mo-SiN x and Co-SiN x GMs by cosputtering Mo or Co with Si 3 N 4 .…”