TiO2 : a study of electrical, dielectric and structural features as a route towards understanding a novel transparent conductive oxide

• TiO2 : eine Studie der elektrischen, dielektrischen und strukturellen Eigenschaften als Weg zum Verständnis eines neuartigen transparenten leitfähigen Oxids

Dorow-Gerspach, Daniel; Wuttig, Matthias (Thesis advisor); Mergel, Dieter (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

It is impossible to imagine our contemporary every day life without techniques like flat screens and touch displays or the energy revolution without solar cells. What do these technologies have in common? Among others, the necessity of transparent and conductive materials as top electrodes for addressing individual pixels and collect the generated energy respectively. Transparent conductive oxides (TCO) are a fascinating class of materials which are able to fulfill these needs. They are transparent like electrically insulating glass but at the same time nearly as conductive as shiny metals. The most common oxide in industry is indium oxide (In2O3) usually doped with tin (Sn)due to its superior electrical conductivity. The growing demand and the rarity of In requires the search for alternatives. Such candidates are tin dioxide (SnO2) and zinc oxide (ZnO) which are under investigation since several decades as well. Together with the first representative but toxic CdO (discovered 1907 [Cas11]), they form the group of the common binary oxides and are close to each other in the periodic table of the elements. One hundred years later titanium dioxide (TiO2) doped with niob (Nb) has been found to possess also very good TCO properties [FHY+05]. This was highly surprising as Ti is located in a completely different region of the periodic system.In this work TiO2 films with outstanding quality have been prepared by reactive magnetron sputtering which enables the large scale production and boosts the applicability of this novel TCO. A post-deposition annealing step at much lower temperatures than usually and without the need of a reducing atmosphere has been sufficient to produce an outstanding quality. Highly trans-parent layers with a metallic conductivity behavior could be deposited with various amounts of Nb. Moreover, for the first time a similar behavior could be achieved for undoped films as well by a precisely controlled small amount of oxygen vacancies. By combining various analysis techniques along the complete frequency spectrum of electrical fields, including low-temperature measurements, a profound understanding of the relevant factors and characteristics could be acquired.The comparison of a large number of TiO2 samples of different Nb levels, including undoped ones, reveal several intrinsic performance limits and various sometimes counteracting effects.Performing similar investigations on films of common oxides, especially ZnO:Al, and an additional broad-based literature study show several fundamental differences between TiO2 and the standard TCOs. They affect the maximal transparency, electron scattering mechanisms, doping and mobility limits, and phenomena near absolute zero temperature. These distinctions can be explained as a direct result of the disparity of their chemical nature. Together with several other unique properties of TiO2, these findings reinforce its high potential as a new TCO material for various applications and also point out the research issues for the future.

Institutions

• Chair of Experimental Physics I A and I. Institute of Physics [131110]
• Department of Physics [130000]