Thermodynamic and Kinetic Analysis of Hydrogen Sensing in Pt/AlGaN/GaN Schottky Diodes at High Temperatures

摘要

Schottky diodes on AlGaN/GaN heterostructures with Pt catalytic metal are fabricated and characterized for hydrogen sensing at a wide range of temperature and hydrogen concentration. The thermodynamic and kinetic processes of hydrogen adsorption/desorption at Pt/AlGaN are analyzed based on their steady and transient state sensing characteristics. The devices have great hydrogen detection capability even at sub-ppm level reliably at temperatures up to 800 $~^{circ}{rm C}$. Both forward and reverse currents of Schottky diodes increase with exposure to ${rm H}_{2}$ containing ambient, which is attributed to the reduction of Schottky barrier heights $(Phi_{B})$ resulted by hydrogen absorption at Pt/AlGaN. As temperature increases, the device sensitivity ($S$ ) is improved due to the more effective ${rm H}_{2}$ dissociation, but starts saturating from 600$~^{circ}{rm C}$. The coverage of hydrogen at the Pt/AlGaN interface exhibits the same trend as $DeltaPhi_{B}$ and $S$. The thermodynamic process of hydrogen adsorption in Pt/AlGaN is endothermic with an adsorption enthalpy of 21 ${rm kJ}cdot {rm mole}^{-1}$. The adsorption time constant shortens as the temperature or the ${rm H}_{2}$ concentration increases. On the other hand, the desorption time constant exhibits opposite trend on the temperature and ${rm H}_{2}$ concentration. The absolute magnitudes of the activation energies for hydrogen adsorption/desertion at Pt/AlGaN increase w-ith the ${rm H}_{2}$ concentration.