Surface Plasmon Resonance based optical temperature sensor using ZnO:N thin film
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Surface Plasmon Resonance based optical temperature sensor using ZnO:N thin film Kajal Jindal1, Monika Tomar2 and Vinay Gupta1 1 Department of Physics and Astrophysics, University of Delhi, Delhi-110007, INDIA 2 Department of Physics, Miranda House, University of Delhi, Delhi, INDIA E-mail: [email protected] ABSTRACT Temperature dependent optical properties of RF-sputtered c-axis oriented ZnO:N thin film have been investigated. Surface Plasmon modes are excited at the metal-dielectric interface in the Kretschmann-Reather configuration using prism coupling technique. Effect of ZnO:N thin film deposited over Prism-Au structure on the SPR reflectance is studied over a wide range of temperature from 300–500 K at 633 nm wavelength. The value of dielectric constant of ZnO:N film obtained by fitting the experimentally obtained data with the theoretically generated SPR curve at the optical frequency is found to increase linearly with temperature. The increase in dielectric constant (4.03 to 4.11) with increase in temperature from 300 K to 500 K indicates a promising application of the system as an efficient low-cost temperature sensor. INTRODUCTION Surface Plasmon resonance (SPR) is a highly sensitive and noninvasive surface-sensing technique for detection of chemically or physically related phenomena that can induce refractive index variations of the medium in contact with the metal surface using an electromagnetic (EM) field. SPR is highly sensitive toward fine changes at the metal-dielectric interface and finds great application as optical sensors1-4. Recently, doped wide band gap oxides have gathered increasing research interest for their potential applications in optoelectronics5, optical waveguides and spintronics6. ZnO, has been considered as an excellent material due to its wide band gap of 3.37 eV and large exciton binding energy of 60 meV7. Owing to its semiconducting, piezoelectric and photoconducting properties, ZnO finds potential applications in optoelectronic devices, such as ultraviolet light emitting diodes (LEDs), transparent thin-film transistors, lasers and sensor devices including gas sensors, biosensors, chemical sensors8, UV-detectors, acoustic sensor and pressure sensor. Nitrogen (N) has been identified as a promising dopant in ZnO since it has almost the same ionic radii (0.75 Å) as that of O (0.72 Å) and is not expected to perturb the wurtzite structure of ZnO. It has been reported that N doping in ZnO (ZnO:N) may result in introducing p-type conductivity in ZnO9 but the reports are controversial10,11 and its reproducibility is still an issue. ZnO:N has also been exploited for inducing ferromagnetic properties12. Additionally, introduction of N in ZnO can reduce the threshold optical energy, which would enhance the photocatalysis13 or solar energy conversion effectiveness of ZnO. However, it is essential to have a clear determination of the optical parameters such as refractive index and dielectric constant for the simulation and fabrication of ZnO based devices such as laser diodes and LEDs.
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