Study of 2-dimensional systems based on BaSnO3

Abstract

Oxide semiconductors have been widely studied because of their optical transparency and great electrical properties. In particular, oxides with a perovskite structure showed additional novel characteristics such as ferromagnetism, ferroelectricity, multiferroicity, and superconductivity. However, the oxygen instability at high temperature and low mobility at room temperature of oxides have been problems for device applications. BaSnO3 is a perovskite oxide with the highest electron mobility of 320 cm2/Vs among oxides at a carrier density of about 1020 cm-3, and it has high oxygen stability that makes the p-n junction possible. BaSnO3 has been and can be applied to various fields such as power electronics, high frequency device, solar cell, etc. This dissertation focuses on the study of BaSnO3-based two-dimensional systems to investigate the electrical characteristics of quantum wells made by the structures of BaSnO3/(Ba,La)SnO3/BaSnO3 (delta-doped BaSnO3) and LaInO3/BaSnO3. Delta-doped BaSnO3 has quantum well at the La-doped BaSnO3 layer, which is made by conduction band bending at the BaSnO3/(Ba,La)SnO3 interface. LaInO3/BaSnO3 has quantum well on the BaSnO3 side due to the large conduction band offset between the two materials and polarization of LaInO3. In the delta-doped BaSnO3, two-dimensional carrier densities were measured at various thicknesses and doping levels, and exhibited two unpredictable electrical properties; too low conductance in thin (Ba,La)SnO3 sample and the conductance increase as the undoped BaSnO3 capping layer thickens. Analysis using Poisson-Schrödinger simulation shows that these macroscopic properties are physically well explained by continuous band bending and changing surface boundary conditions. Temperature dependent resistance has also been investigated in delta-doped BaSnO3 films and will be the basis for quantum phenomenon measurements. LaInO3/BaSnO3 showed conductance enhancement at the interface even though both LaInO3 and BaSnO3 have insulating properties. The interface has been thought of as a two-dimensional electron gas, and I measured electrical properties of the interface with varing doping level of BaSnO3 and LaInO3 thicknesses using epitaxially well grown films confirmed by XRD and STEM. And the field effect transistor was fabricated using a two-dimensional electron gas as a channel layer and LaInO3 as the high dielectric oxide, and it operated well. The temperature dependent resistance has also been investigated at the LaInO3/BaSnO3 interface, and still requires lower dislocation density than now to see the quantum phenomena. Experimental results of the LaInO3/BaSnO3 interface were analyzed using Poisson-Schrödinger simulation to understand how quantum well with high two-dimensional carrier density is formed. 13 kinds of material parameters of LaInO3 and BaSnO3 (polarization, concentration and activation energy of donor, deep donor, acceptor, and deep acceptor, effective mass, dielectric constant, band gap, and conduction band offset between two materials) were analyzed to understand their effect on quantum well. High polarization of LaInO3, appropriate concentrations and activation energies of carriers, not too small effective mass, not too high dielectric constant, and large conduction band offset make quantum well with high two-dimensional carrier density compared to conventional two-dimensional electron gases. Based on these calculational analysis, I suggest methods for improvement of LaInO3/BaSnO3 two-dimensional electron gas and predict another BaSnO3-based two-dimensional electron gas interface. These studies of delta-doped BaSnO3 and two-dimensional electron gas at LaInO3/BaSnO3 have led to a physical understanding of the macroscopic electrical characteristics in the two-dimensional system, and the analysis results predict another advanced BaSnO3-based two-dimensional systems. Furthermore, it will develop into the observation of quantum phenomena by solving the current problem of dislocation density.