NbO2 devices for subnanosecond transient protection

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Abstract
The feasibility of using ∼10‐μm‐thick polycrystalline NbOx layers on NbO and ∼25‐μm‐thick single‐crystal NbOx (x∼2.0) chips as fast‐response‐time low‐capacitance nonpermanent switching devices has been investigated. A packaging configuration with evaporated small‐area contacts exhibiting low insertion loss in high‐frequency applications has been developed. Both polycrystalline and single‐crystal NbOx materials exhibit switching at an electric field of ∼25 V/μm for a 3‐ns‐wide pulse with a delay time of less than 1 ns. For polycrystalline NbOx layers, device degradation (increased insertion loss) with repeated pulsing is observed for 3‐ns pulse widths and degradation is markedly accelerated for longer pulse width. Device stability in the thicker single‐crystal chips is greatly improved compared to the polycrystalline material. Switching and associated sparking phenomena in single‐crystal chips were investigated in air, Freon, helium, and vacuum environments, and only minor behavioral differences were observed. We have related this sparking to a volatilization of material from the NbOx upon pulsing and correlate it with the formation of pits ∼25 μm in diameter and ∼20 μm deep on the crystal surface around the periphery of the contact.

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