Studies on Electrical and Structural Properties in Oxygen Level Controlled BaSnO3-d Single Crystals

Abstract

Transparent conducting oxides are gaining more interest because of its importance in display-related applications, photovoltaic, energy efficient windows and so on. Indium tin oxide, fluorine doped tin oxide and aluminium doped zinc oxides are the most widely used compounds in this field. However, they have drawbacks as well, such as high cost and oxygen instability issues. Our group previously succeeded in growing the single crystal of a possible replacement, BaSnO3. The physical properties of perovskite structured barium stannate single crystals, BaSnO3, (Ba,La)SnO3, and Ba(Sn,Sb)O3 were studied thoroughly. Especially, (Ba,La)SnO3 has exhibited high mobility, 320 cm2V- 1S-1, while maintaining its transparency. Also, there were efforts to grow (Ba,La)SnO3 film with high mobility, but due to the lattice mismatch between (Ba,La)SnO3 and the substrate, the mobility remained low, 70 cm2V-1s-1. Thus, undoped BaSnO3 comes in fore as a lattice-matched substrate. However, previous data show that undoped BaSnO3 has a carrier concentration of ~1018 cm-3 by the presence of oxygen vacancy. In this thesis, we present that we successfully reduced oxygen vacancy in undoped BaSnO3 crystals by adding an oxidizer during flux growth. The crystals are confirmed to be an identical cubic perovskite with a lattice parameter of a=b=c=4.117 Å. The reduction of oxygen vacancy led to a metal-insulator transition where the resistivity changed drastically from ~10-3 Ω∙cm to ~1011 Ω∙cm at room temperature. The crystals with low resistivity were blue in color while the crystals with high resistivity were yellow. By optical spectroscopy ranging from mid-IR to UV, we observed that crystals grown with an oxidizer have significantly lower free carrier absorption. Moreover, through Raman spectroscopy, we noticed that the BaSnO3-d crystals show first-order Raman scattering which means that it is locally distorted even though the x-ray diffraction pattern shows a cubic perovskite structure. This may indicate that the presence of carrier in BaSnO3-d single crystal is closely related to local structure distortion. Through polarized Raman spectroscopy, we attempted to assign the observed first order Raman peaks. Additionally, we have investigated the external effects, such as polishing and forming gas annealing, on the structure of BaSnO3-d single crystals through Raman spectroscopy. In all, this dissertation focuses on the physical properties of BaSnO3-d single crystals while modifying the oxygen vacancy level by the use of an oxidizer.