Dislocations and high electron mobility in epitaxial Ba1-xLaxSnO3 and SnO2-x

Abstract

Perovskite-type oxides have an advantage of a simple and flexible structure that enables ionic substitution on both A and B sites which can form a vast set of important materials for a wide variety of academic and industrial research. Our research group have recently published about high mobility in Ba0.96La0.04SnO3 (BLSO) single crystal: as high as 320 cm2/Vs at the carrier concentration 8.0×〖10〗^19 cm^(-3) in single crystals. On the other hand, epitaxial BLSO films were found to exhibit lower mobility, 27 cm2/Vs at the carrier concentration 6.0×〖10〗^19 cm^(-3). The large difference in mobility between single crystals and epitaxial films was attributed to the existence of the dislocations in the epitaxial films from the mobilitys dependence on the carrier density. In this dissertation, we have investigated the influence of dislocation for electrical properties of BLSO films on SrTiO3 (STO) substrates in detail. In our research, we studied the relations between the mobility and the dislocation density of the recently discovered high mobility Ba0.96La0.04SnO3 thin films. The effect of dislocations on the electrical transport properties have hardly been studied for perovskite films. We have found that the carrier density and the mobility, as high as 4.0×〖10〗^20 cm^(-3) and 70 cm2/Vs, decrease as the dislocation density increases. We provide the values for the density of dislocations by TEM micrographs as well as AFM images after surface etching. Furthermore, we found the effect of dislocations on the mobility to be large, when compared with GaN with similar density of dislocations. The importance of dislocation scattering in the perovskite structure is emphasized for the first time, especially in the low carrier density regime. We, also, investigated the electronic transport properties of epitaxial SnO2-x thin films on r-plane sapphire substrates. The films were grown by pulsed laser deposition technique and its epitaxial growth direction was [101] and the in-plane alignment was of SnO2-x [010] // Al2O3 [12 ̅10]. The mobility of SnO2 films has delicate sensitivity of thickness due to the high density of the dislocations between the tin dioxide film and the sapphire substrate. Therefore, we studied the effect of buffer layer on the mobility of SnO2, intensively. We have searched the various buffer layers to reduce the interfacial defects and it is found that fully oxidized SnO2 at grown 800 °C and Ruthenium-doped SnO2 (Ru-doped SnO2) buffer layer demonstrates good insulating and crystalline properties. From the X-ray diffraction analysis, phase pure SnO2 film is grown on Ru-doped SnO2 on r-plane sapphire substrate. Through the omega scan, SnO2 and Ru-doped SnO2 shows good crystallinity. As a result of using 1um Ru-doped SnO2 buffer layer, we can get a mobility of almost about 80 cm2/Vs at carrier density of 3.7×1018 cm-3. On the other hand, the SnO2-x films were grown with fully oxidized SnO2 buffer layer, we have found the mobility of the 30 nm thick SnO2-x thin films strongly dependent on the thicknesses of the fully oxidized insulating SnO2 buffer layer. When the buffer layer thickness increased from 100 nm to 700 nm, the mobility values increased from 23 to 106 cm2/Vs and the carrier density increased from 9×1017 cm-3 to 3×1018 cm-3, which we attribute to reduction of large density of dislocations as the buffer layer thickness increases. In addition, we studied the doping dependence of the mobility of SnO2-x thin films grown on top of 500 nm thick insulating SnO2 buffer layers. The oxygen vacancy doping level was controlled by the oxygen pressure during deposition. As the oxygen pressure increased to 45 mTorr, the carrier density were found to decrease to 1.8×1017 cm-3 and the mobility values to 22 cm2/Vs, which is consistent with the dislocation limited transport properties. We also checked the conductance change of the SnO2-x during thermal annealing cycles, demonstrating unusual stability of its oxygen. The correlation between the electronic transport properties and microstructural defects investigated by the transmission electron microscopy were drawn. The excellent oxygen stability and high mobility of low carrier density SnO2-x films demonstrates its potential as a transparent oxide semiconductor. In applicative works, the author demonstrated a field effect transistor made with an un-doped layer of SnO2 on r-plane Al2O3 substrate, with the gate dielectrics as HfO2. The field effect mobility, the Ion/Ioff ratio, and the subthreshold swing of the device are 72.1 cm2/Vs, 6.0×106, and 0.48 Vdec-1, respectively.