Hydrogen Sensing Performance of ZnO Schottky Diodes in Humid Ambient Conditions with PMMA Membrane Layer

Abstract

Enhanced hydrogen sensing performance of Pt Schottky diodes on ZnO single crystal wafers in humid ambient conditions is reported using a polymethylmethacrylate (PMMA) membrane layer. ZnO diode sensors showed little change in forward current when switching to wet ambient H2 conditions with 100% relative humidity. This sensitivity drop in the presence of water vapor can be attributed to surface coverage of hydroxyl groups on the Pt surface in humid ambient conditions. The hydrogen sensitivity of PMMA-coated diode sensors recovered up to 805% in wet H2 ambient conditions at room temperature. The PMMA layer can selectively filter water vapor and allow H2 molecules to pass through the membrane layer. It is clear that the PMMA layer can effectively serve as a moisture barrier because of low water vapor permeability and its hydrophobicity. In both dry and wet conditions, ZnO diodes exhibited relatively fast and stable on/off switching in each cycle with good repeatability.

Carbon Dioxide Sensing Characteristics of AlGaN/GaN High Electron Mobility Transistor with ZnO Nanorods

“…Under wet CO 2 conditions, H 2 O molecules were adsorbed on the surface of ZnO nanorods and prevented CO 2 molecules from reacting with oxygen ions on the active sites of the nanorods, reducing the change in drain current. (25) This deteriorating effect of humidity can be overcome by employing polyimide-based moisture barrier encapsulation with a high glass transition temperature, which enables CO 2 molecules to penetrate but blocks H 2 O. (6)(7)(8)(9)25)…”

“…ZnO is a direct-bandgap semiconducting material with a wide bandgap energy of 3.37 eV and a large exciton binding energy of 60 meV. (25)(26)(27) ZnO nanostructures are grown by a simple hydrothermal method, which is a low-cost, nontoxic, low-temperature, and scalable process. During hydrothermal growth, the growth rate of the c-plane is higher than that of the m-plane in the wurtzite crystal structure because of the higher surface energy of the c-plane; ZnO nanowires or bundles of nanorods along the c-axis are generally formed during the hydrothermal synthesis process.…”

Recent Advances and Challenges of Nanomaterials-Based Hydrogen Sensors

“…However, like some SnO2-based gas sensors, many ZnO gas sensors require high operating temperature (up to 400 °C) in sensing applications, which causes high power consumption and thus leads to instability during long-term gas detection. Therefore, more thoughtful strategies were developed to enhance their sensitivity and selectivity with lower power consumption [107,[121][122][123][124][125][126][127]. Another widely used metal oxide semiconductor is ZnO, an n-type semiconductor with a bandgap of 3.37 eV.…”

“…The sensitivity of Pt/ZnO Schottky diode-based hydrogen sensors is reduced when switching to humid ambient H conditions in the presence of water due to hydroxyl groups on the Pt surface. Consequently, Jang et al [124] spin coated the PMMA on Pt/ZnO Schottky diode sensors as a protective layer to enhance hydrogen sensing performance. The sensitivity of Pt/ZnO Schottky diode-based hydrogen sensors is reduced when switching to humid ambient H conditions in the presence of water due to hydroxyl groups on the Pt surface.…”

“…It was found that the Pd-loaded (0.6 wt%) ZnO nanofibers exhibited a high response of R a /R g = 74.7 at 350 • C for a low concentration of hydrogen at 100 ppb.The sensitivity of Pt/ZnO Schottky diode-based hydrogen sensors is reduced when switching to humid ambient H 2 conditions in the presence of water due to hydroxyl groups on the Pt surface. Consequently, Jang et al[124] spin coated the PMMA on Pt/ZnO Schottky diode sensors as a protective layer to enhance hydrogen sensing performance. Due to its hydrophobicity, the PMMA layer can effectively serve as a moisture barrier and selectively filter the water vapour during hydrogen sensing.…”