Ph.D Thesis


Ph.D StudentGuy Ankonina
SubjectElectronic Transport and Gas Sensing Properties of
Polycrystalline Titanium Oxide Thin Films with
Tailored Microstructures Obtained by
Laser Processing
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Rothschild Avner
Professor Emeritus Komem Yigal
Full Thesis textFull thesis text - English Version


Abstract

This research aimed at investigating the effect of grains and grain boundaries on the charge transport mechanism and gas sensing properties of polycrystalline TiO2 thin films. In order to achieve this, novel laser processing techniques were developed to control the grain size and texture.

Conventional thin film deposition methods such as sputtering offer limited control on the grain size and texture. To overcome this new processing methods were developed based on laser induced melting and recrystallization, using pulsed excimer laser. The high melting temperature of most metal oxides (e.g., TiO2), and their brittleness makes it difficult to apply these methods to these materials. In this research we applied, for the first time, laser processing methods to tailor the grain size of TiO2 thin films.

We developed two different processes: the flood irradiation, and the sequential lateral solidification (SLS). In the flood irradiation process, given areas are exposed to a single pulse of laser with no overlap between them. In the SLS process, the film is scanned by a sequence of partially overlapping translations of the laser.

The laser energy density plays an important role on the grain size. At low energy densities there is little effect on the grains. At intermediate energy densities we observed a bi-model distribution with small and large cells. At high energy densities we observed large cells. This observation is in agreement with an earlier model done on Si thin films. Temperature distribution simulations yield results that are in agreement with both the model and our observations. The SLS process creates films with elongated grains. The SLS processed films were found to be highly textured, with the long axis of the elongated grains being the c-axis of the rutile unit cell. The observations suggest that grain growth occurs primarily in the [001] and <111> directions.

In both types of processed films we observed SiO2 on the surface and at the grain boundaries of the TiO2 films which originated from the Si/SiO2 substrate.

Using impedance spectroscopy we resolved grain and grain boundary contributions to the electrical transport properties of the films. We combined the electrical and microstructural characterizations together with calculations of defect concentrations to analyze bulk and space charge effects on the charge carriers in the film. We also investigated their gas sensing mechanism, revealing a significant contribution of the grain boundaries to the gas sensing properties of dense polycrystalline TiO2 thin films.