Hydrogen-sensitive sensor with stabilized Pd-mixture forming sensing nanoparticles on an interlayer

Pd-based mixtures comprising silicon dioxide (SiO₂) were used as sensing materials in fabrication of GaN-based hydrogen sensors. The mixture as-deposited has a rough surface with many pores. After wet selectively etching to remove SiO₂, the mixture turns into Pd nanoparticles with a size of ∼ 30 nm on an interlayer with oxygen, as indicated by SEM, EDX, and SIMS methods. A careful study of the Pd-mixture on a metal-semiconductor-metal type of hydrogen sensor provides significant information on the roles of oxygen and the interlayer. Experimental results reveal that hydrogen atoms trapped inside the mixture as-deposited cannot contribute to changes in barrier height as an applied voltage is not large enough. Improved sensing properties such as hydrogen dissociation rate, diffusion rate, and storage capability were obtained when Pd nanoparticles were formed by selectively etching the mixture. The situation that hydrogen atoms were blocked and disturbed by oxygen will exist no more. Uniform sensing responses of higher than 10⁵ (defined as (J_H2-J_N2)/J_N2, J_H2 and J_N2 are current densities measured in H₂/N₂ and N₂ ambiences, respectively), voltage shifts of larger than 20 V were obtained at 2.13 ppm H₂/N₂. In addition, hydrogen transport through grain boundaries of Pd nanoparticles is much faster than diffusion through a Pd-mixture layer. A much shorter response time was obtained from the sensors with the Pd-mixture wet etched. Furthermore, stable and reliable sensing characteristics were also expected.