Mobility enhancement in wide bandgap semiconductor BaSnO3

Abstract

Wide bandgap semiconductors have received much attention for their potential applications in high-temperature and high-power electronics. Apart from typical wide bandgap semiconductors, e.g. SiC and GaN, there has been a growing interest in perovskite oxide BaSnO3, which is studied especially for transparent conducting oxides and transparent oxide semiconductors in transparent electronics, to date. In usual active devices in display, the transport property, e.g. mobility, is less important compared to its transparency because the reaction of human eye is slow compared to the speed of digital processing. On the other hand, BaSnO3 has a high mobility, which records the highest value among wide bandgap semiconductors in the degenerate doped regime, so that the study for enhancement of its mobility combined into devices is surely needed toward its application in high-temperature and high frequency area, not only constrained in transparent electronics. Another particular advantages using BaSnO3-based electronics is the development of all epitaxial heterostructures, that is, incorporating of perovskite oxides with large polarization, ferroelectricity, ferromagnetism and multiferroicity. This enables application toward non-volatile switching memory and spintronics, which give the opportunity for extending the amount and type of information in electronics. This dissertation focuses on the study of mobility enhancement of BaSnO3 in three aspects such as the excellent transport on MgO substrate, bandgap engineering by Hf doping for the possibility of modulation doped heterostructure, and modulation doped-polar interface by LaInO3/undoped BaSnO3/La-doped BaSnO3 heterostructures. All the epitaxial films are grown by using the pulsed laser deposition technique. And the crystallinity of films is investigated by X-ray diffraction analysis

single phase growth, well defined orientation. The transmission electron spectroscopy has been employed to study the microstructural property of crystalline BaSnO3 films such as misfit dislocations and threading dislocations. The transport property of films, e. g. resistivity, mobility, carrier concentration, was measured by Van der Pauw method and all the devices has a three terminal-field effect transistor structures with metal-insulator-semiconductor interfaces. Because the research of BaSnO3 is limited on films on perovskite oxide substrates such as SrTiO3, SmScO3, PrScO3, LaAlO3, and BaSnO3, which suffer from a small size, a small bandgap and oxygen instability, the effort to grow BaSnO3 films with excellent transport property on non-perovskite MgO substrate is very important toward wafer scale processing with oxygen stability. This opens commercially the area of BaSnO3-based electronics, too. In spite of non-perovskite oxide substrate, the maximum electron mobility of La-doped BaSnO3 films is 97.2 cm2/Vs, which is quite comparable value deposited on BaSnO3 single crystal substrate. And the field effect transistor with HfO2 dielectrics reveals successful carrier modulation of La-doped BaSnO3 channel with mobility of 43.9 cm2/Vs and on-off ratio of 3.0ⅹ107, which is slightly better compared to devices on SrTiO3 substrate. Another possibility to enhance mobility of devices based on BaSnO3 channel is modulation doping like GaAs system. The carrier modulation of BaSnO3 by field effect is limited in metal-insulator-semiconductor structure, to date. So intentional La doping or unintentional oxygen vacancies, source of electron carriers, may degrade the device performance. On the other hand, heterostructures with interfaces of doped large bandgap material and channel can increase mobility by reducing ionized impurity scattering. To employ this modulation doping in BaSnO3-based heterostructures, bandgap engineering is a necessary step. So, Hf substation for Sn in BaSnO3 is investigated, which significantly modifies the band structure, crystal structure, and the bandgap. Also La doping in BaSn1-xHfxO3 is investigated, displaying significant potentials for modulation doping in BaSnO3/BaSn1-xHfxO3 heterostructures. Lastly, enhancement of mobility of BaSnO3 in polar interface has been studied. The optimal condition of semiconductor devices is switching with low-on-state resistance. To manipulate highly conductive current, high sheet carrier concentration and high mobility is needed in field-induced confined well. Polarization doping helps to accumulate high density of mobile charges, which is impossible in usual solid-gate-dielectrics. Recently, conducting interface between LaInO3/La-doped BaSnO3 was reported with high sheet carrier density in the order of 1013 cm-2. Moreover, if the mobility of channel in this polar interface is enhanced, the two dimensional electron gas science in BaSnO3-based electronics will be more powerful. Because the slight La doping on BaSnO3 channel is needed to accumulate high carrier density, modulation doping technique is applied in smaller bandgap BaSnO3 compared to LaInO3 by inserting undoped BaSnO3 spacer layer between La-doped BaSnO3 and LaInO3. The enhanced mobility of this modulation doped-polar interface may lead further advance of LaInO3/BaSnO3 polar interface in high mobility and high power electronics applications.