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, 20 (3)

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

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

Soohwan Jang et al. Sensors (Basel).

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.

Keywords: Schottky diode; ZnO; gas sensor; humidity; hydrogen.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The relative w-rocking curve of c-plane (0001) ZnO single crystal wafer showing the full-width at half maximum (FWHM) value of 112 arcsec.
Figure 2
Figure 2
(a) Top-view optical microscope image of fabricated Schottky diode sensor; (b) cross-sectional schematic of polymethylmethacrylate (PMMA)-coated Pt/ZnO Schottky diode.
Figure 3
Figure 3
(a) Current–voltage (I–V) curve of ZnO diodes in log-scale before and after 4% hydrogen exposure in dry and wet ambient conditions; (b) corresponding I–V characteristics of ZnO diodes in linear-scale before and after 4% hydrogen exposure in dry and wet ambient conditions.
Figure 4
Figure 4
(a) I–V characteristics of PMMA-coated Pt/ZnO SBDs before and after wet 4% hydrogen exposure; (b) the transient change in Schottky barrier height (SBH) of Pt/ZnO SBDs in dry H2 and PMMA-coated Pt/ZnO diodes in wet H2 ambient conditions.
Figure 5
Figure 5
The current change as a function of bias voltage of ZnO diode sensors with and without the PMMA membrane layer in dry and wet H2 exposure. H2 (4%) was injected into the gas chamber for 30 s.
Figure 6
Figure 6
(a) Cyclic response curves of Pt/ZnO diode sensors without PMMA layer; (b) cyclic response curves of PMMA-coated Pt/ZnO diode sensors, which were measured at 0.2 V forward voltage when switching H2 on and off in dry and wet ambient conditions. The same level of sensitivity was observed in PMMA-coated diode sensors even in repeated cycles of switching dry and wet H2 ambient conditions in 100% RH.

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