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.