Ultrahigh vacuum studies of Pd metal-insulator-semiconductor diode H2 sensors

Abstract

Hydrogen sensitive Pd metal/insulator/semiconductor diodes provide an ambient temperature, low power electronic sensor for hydrogen as a result of hydrogen trapping at the Pd/insulator interface. Current kinetic models consider the rate limiting step to be adsorption at the Pd surface followed by rapid transport to the interface. We have obtained both steady‐state and kinetic results for diodes with clean Pd surfaces over hydrogen pressures ranging from 10⁻¹⁰ to 10⁻¹ Torr. The sensitivity limit (equivalent to 1¹ total H₂ impacts/cm² at the Pd surface) is set by our vacuum capabilities, and is at least seven orders of magnitude greater than that obtained for devices with contaminated surfaces. These results clearly show that the kinetic and sensitivity limitations reported for such devices are a result of surface contamination. For diodes with a clean Pd surface, analysis of the steady‐state results requires at least two binding states (9 and 6.8 kcal/mol of H relative to H_2(g)) for H at the Pd/SiO₂ interface. The kinetics of the diode response to H₂ are inconsistent with direct transfer to the bulk exclusively through strong chemisorbed surface states, but suggest with a ‘‘precursor’’ mechanism in which the initial step is adsorption into a weakly bound precursor state from which branching steps populate both strongly bound chemisorbed surface states and bulk states. The surface states act in part as traps for hydrogen in competition with transfer to the bulk (and therefore to the electronically active interface).