Hydrogen gas sensors from polysilicon nanobelt devices selectively modified with sensing materials

Double-junction n⁺/n^-/n⁺ polysilicon nanobelts featuring selectively deposited sensing materials have been investigated for application as H₂ gas sensors. The selective modification of the devices was performed through a combination of localized ablation of a resist and lift-off of a previous catalyst material deposited through e-beam evaporation. Four nanobelt devices, differentiated by their doping concentrations at the n^- region (from 2.5 × 10¹³ to 2.5 × 10¹⁴ cm^-2), were analyzed in terms of the responses to H₂ and their self-heating effects. A low doping concentration improved the response at room temperature, owing to a longer Debye length. The variation in the H₂-induced surface potential associated with temperature, accounting for degradation in the response of the nanobelts with Joule heating bias, was analyzed in terms of the I-V characteristics of the double-junction device. Among various catalysts (Pt, Pd, Pt/Pd) evaluated for their H₂ sensing characteristics, an ultrathin film of Pt/Pd was most favorable.