Abstract
Undoped nanocrystalline n-type ZnO thin film was deposited by chemical deposition technique on a thermally oxidized p-Si (~5 Ω cm resistivity and orientation) substrate. Formation of stable zinc oxide thin film was confirmed by two-dimensional X-Ray Diffraction (XRD) and EDX analysis. The average crystallite size of the ZnO sample was evaluated as ~50 nm. The surface was characterized by Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) that confirm the formation of nanocrystalline (grain size ~50 nm) ZnO thin film with surface roughness of ~100 nm. Good conversion of precursor into ZnO thin film in the chemical deposition method was evident by Fourier Transform Infrared Spectroscopy (FTIR). A small peak at 479 cm−1was observed in the FTIR spectrum confirming the formation of quartzite structure of the ZnO. The band gap (~3.44 eV) of the material was calculated from the optical absorption spectroscopy. To prepare Pd–Ag/n-ZnO Schottky junction, Pd–Ag contacts were taken by electron beam evaporation method. I–V characteristics of the junction were studied at different temperatures in inert and reducing ambient (N2 and N2 + CH4) with turn on voltage of around 0.2 V. The parameters like ideality factor (η), saturation current (I 0), series resistance (Rs), and barrier height (Φ BO) of the junction were calculated in the temperature range 50–200 °C in N2 as well as in 1 % CH4 + N2 ambient. It was observed that the ideality factor decreases in the temperature range 50–200 °C (η = 12.34 at 50 °C and η = 1.52 at 200 °C) in N2 ambient and η = 1.18 in N2 +CH4 ambient at 200 °C. Schottky Barrier Height (Φ BO) of the Pd–Ag/n-ZnO junction was found to increase with temperature. A close observation of Pd–Ag/n-ZnO junction in the presence of methane was performed to appreciate its application as methane sensor. The sensing mechanism was illustrated by a simplified energy band diagram.
Highlights
Semiconducting metal oxide is receiving vast applications in toxic and inflammable gas sensing, optoelectronics like solar cells, ultraviolet laser emission, photodetectors, short wave length light-emitting diode, transparent electrode display, piezoelectricity, biosensors etc. [1,2,3,4,5,6,7]
The surface was characterized by Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) that confirm the formation of nanocrystalline Zinc Oxide (ZnO) thin film with surface roughness of *100 nm
Good conversion of precursor into ZnO thin film in the chemical deposition method was evident by Fourier Transform Infrared Spectroscopy (FTIR)
Summary
Semiconducting metal oxide is receiving vast applications in toxic and inflammable gas sensing, optoelectronics like solar cells, ultraviolet laser emission, photodetectors, short wave length light-emitting diode, transparent electrode display, piezoelectricity, biosensors etc. [1,2,3,4,5,6,7]. The second one is thermionic emission theory which is temperature dependent and proposes that the carriers which have energy equal to or larger than the conduction band energy at the metal–semiconductor interface contribute to the current flow across the Schottky junction; and the third one is quantum–mechanical tunneling through the Schottky barrier which suggests that as the doping concentration increases, the probability of tunneling through the barrier increases. The term nH is called tunneling probability which shows the dependency of total current on the carrier flux arriving at the tunnel barrier multiplied with the probability nH, at tunnel through the barrier In this case, Pd–Ag/n-ZnO junction characteristics were explained by thermionic emission theory. From the Schottky diode current–voltage relationship, the junction parameters of Pd–Ag/n-ZnO interface like ideality factor (g), saturation current (I0), series resistance (Rs), and barrier height (UB0) were calculated in the given temperature range (50–200 °C) in N2 ambient and CH4 ? The samples are exposed to N2 and to 1 % CH4 in N2 in the temperatures range 50–200 °C
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