Reducing Metal-Insulator interactions in Granular Metal High-Pass Filters

Several order of magnitude (OOM) on/off ratio improvements are shown for Mo-SiN_x granular metals (GMs) compared to GMs with oxide insulators. Granular metals are nanostructured materials in which metal islands are separated by an insulator. At volume metal fractions (φ) below the percolation threshold (φ_c), GMs are insulating; electrical conduction occurs via electron tunneling and capacitive transport. These conduction mechanisms enable the creation of GM high-pass filters that are insulating below 1 kHz and conductive above 1 MHz. For high-pass filters, the DC conductivity, including tunneling, should be minimized while maintaining the capacitive transport. Au- and Ag-based GMs exhibit a desirable sharp conductivity (σ) knee at φ_c, with σ decreasing 4-6 OOM with Δφ≈0.1. However, Au and Ag GMs are not suitable for high temperature applications, and non-noble metals exhibit metal-insulator interactions that dramatically reduce Δσ at φ_c. The metal-insulator effects are minimized in high thermal stability Mo-SiN_x by sputtering in a partial N₂ environment, which reduces σ 3-4 OOM for φ<φ_c. Furthermore, post-growth annealing increases the separation distance between islands and results in >6 OOM decreases in σ at φ_c. These advancements in material quality improve high-pass filter performance.