Improvement in the Sensing Response of Nano-Crystalline ZnO-Based Hydrogen Sensor: Effect of Swift Heavy Ion Irradiation

Abstract

Hydrogen gas sensing response of RF sputtered nano-crystalline ZnO thin film is improved by ~34.73% using swift heavy ion irradiation technique. X-ray diffraction reveals single crystalline wurtzite structure of ZnO thin films. Full width half maximum (FWHM) of (0002) diffraction peak is decreased with increase in ion fluences. However, at very high fluence 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , crystallinity is degraded while FWHM of diffraction peak increases. The ZnO films were grown under highly compressive stress, which was relaxed with incremental ion fluences up to 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Cathodoluminescence spectroscopy shows intense predominant peak around ~3.23 eV, which indicates good optical quality of the films. As ion fluence increases, a blue shift appears in near band emission peak from 3.23 to 3.33 eV, which indicates that in-plane stress is decreased. Surface morphology shows generation of self-affine nanostructure and grain fragments as ion fluences were increased. Grain fragmentations had enhanced the surface reaction, and as a result, gas sensor's relative response has improved. It was observed that the gas sensor's sensitivity is strongly dependent on ion fluences and operating temperature. The sensitivity was enhanced from 66.68% to 89.84% with fluence variations from pristine to 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 175 °C operating temperature.