Ioanna Fasaki (2006-2009)
Study of NiO, TiO2 and ZnO thin films for gas sensing and optoelectronic applications
PhD thesis, Chem. Eng. Dept./National Technical University of Athens.
In the frame of this thesis, ZnO, NiO and TiO2 thin films, their characterization and the possibilities of their applications to optoelectronics and hydrogen sensing was studied. In this sense, the development of a gas test system for the detection of gases and the employment of an innovative deposition method (Pulsed laser Deposition with 2-lasers and 2-targets) for the controlled doping of thin films was the main goal of this work.
Initially, for the study of the thin films as gas sensors, an apparatus was developed, consisting of: an Al vacuum chamber, a home-made heater to achieve the operating sensor temperature, a constant current power supply for the heater with digital temperature controller, and an electronic circuit to acquire the sensor response, digitize it and send it to a pC for recording in real time. For testing, NiO microsensors, grown by sputtering and a photolithographic process were successfully tested.
In addition, a device for application of the 4-point van der Pauw measurement of resistivity was constructed, as the resistivity values are crucial for the thin film applications to optoelectronics.
NiO (p-type) thin films were grown by PLD by an excimer laser (248 nm), for their application as gas sensors. The effect of the deposition parameters on the hydrogen sensing was studied. In particular, NiO samples were grown at different substrate temperatures (RT, 200 400 oC). For the 400 oC with the highest sensitivity to hydrogen, a set of NiO films was deposited at 5, 10, 20, 30 and 40 Pa reactive oxygen pressure during deposition. After the optimization of the deposition parameters, the effect of Au nanoparticles to the structural, optical morphological and hydrogen sensing properties was studied. The controlled incorporation of the Au nanoparticles was realized by the 2-laser, 2-target method, varying the fluence of a Nd:YAG laser that was used to ablated the Au target. It was proved that the fluence significantly affected the properties of the Au:NiO nanocomposite material and at the same time the Au nanoparticles enhances the hydrogen detection.
In order to confirm the operation of the sensing setup for other thin film-based sensors too, sputtered TiO2 (n-type) films were tested. The response was as expected, and thus it proved that the setup works correct, independently of the sensing material, semiconductor type and method of growth.
Further, ZnO thin films were grown by PLD in order to be tested for optoelectronic applications. The samples were grown on PET substrates to test their properties on this material. The effect of oxygen pressure in the deposition chamber during growth as well as the effect of Al and In doping with the 2-laser, 2-target method, on the optical, electrical structural and morphological properties were studied. It was found that oxygen is an important parameter and there is an optimum value for the growth of good quality thin films.