Electrical and Structural Properties of Doped BaSnO3 Single Crystals and Films

Abstract

Transparent conducting oxides (TCOs) and transparent oxide semiconductors (TOSs) have become necessary materials for a variety of applications in the information and energy technologies, ranging from transparent electrodes to active electronics components. Perovskite barium stannate (BaSnO3), a new TCO or TOS system, is a potential platform for realizing optoelectronic devices and observing novel electronic quantum states due to its high electron mobility, stable electrical properties, high transparency, structural versatility, and flexible doping controllability. Recently, we discovered that La doped BaSnO3 (BLSO) single crystals exhibited the highest electron mobility of 320 cm2V-1s-1 at electron carrier density of ~1020 cm-3 among perovskite oxides and stable electrical properties at high temperatures. However, most BLSO films grown on the SrTiO3(001) substrates have shown much lower electron mobility (10-70 cm2V-1s-1) due to the large lattice mismatch between the films and the substrates. While there is great potential to increase electron mobility in doped BaSnO3 films by defect minimization, heterostructure engineering provides another promising route for achieving a metallic state at the interface between the BaSnO3(001) substrates and the insulating perovskite oxide layers. Finding a suitable substrate to grow BLSO thin films has been one of major challenges. An ideal substrate would be the insulating BaSO3 single crystal itself, as its lattice constant matches well with those of BLSO within 0.08%. Chapter three of this dissertation discusses the growth of insulating BaSnO3(001) single crystals as a substrate. The insulating BaSnO3(001) substrates were grown by the Cu2O-CuO flux, in which the molar fraction of KClO4 was systematically increased to reduce electron carrier density and thus induce a doping induced metal-insulator transition, exhibiting a resistivity increase from ~10-3 to ~1012 Ω cm at room temperature. By optical transmission measurement in the mid-IR to UV light range, we observe that the crystals grown with KClO4 show significantly less free-carrier absorption. Through Raman spectroscopy, we remark that the oxygen vacant BaSnO3 crystals show first-order Raman scattering peaks at 138 cm-1 and 250 cm-1, which indicates it is locally distorted even though the X-ray diffraction patterns show a cubic perovskite structure. Chapter four introduces the enhanced electron mobility in epitaxial BLSO films grown on BaSnO3(001) substrates. We find that all the degenerate BLSO films turn out to be epitaxial, showing good in-plane lattice matching with the substrate as confirmed by X-ray reciprocal space mappings and transmission electron microscopy studies. The BLSO films show degenerate semiconducting behavior, and the μ at room temperature reached 100 and 85 cm2V-1s-1 at doping levels 1.3×1020 and 6.8×1019 cm3, respectively. We also show that a field-effect transistor with the homo-epitaxial BaSnO3 film as an active channel exhibits an on/off ratio of 1.2 × 106 and a high field-effect mobility of 48.7 cm2V-1s-1, being comparable to those of the well-known binary oxides and superior to those of the other perovskite materials. This work demonstrates that thin perovskite stannate films of high quality can be grown on the BaSnO3 (001) substrates for potential applications in transparent electronic devices. Chapter five discusses the oxygen diffusion phenomena in a BLSO thin film on SrTiO3(001) substrate by measurements of time-dependent Hall effect at high temperatures around 500 oC under different gas atmosphere. Under the Ar (O2) atmosphere, electron carrier density and electrical conductivity are increased (decreased) while electron mobility is slightly reduced (enhanced). This observation supports that although both electron carrier density and electron mobility are affected by the oxygen diffusion process, the change of electron carrier density is a major factor of determining electrical conductivity in the BLSO films. Detailed analyses of the time-dependent electron carrier density exhibit fast and slow dynamics that possibly correspond to the oxygen exchange reaction at the surface and oxygen diffusion into the BLSO grains, respectively. Fitting the time-dependence of electron carrier density reveals that the chemical diffusion coefficient of oxygen in the BLSO grains becomes ~10-16 cm2s-1. This coefficient marks the lowest value among perovskite oxides around 500 oC, directly proving stable electrical properties of BLSO. These results support that the donor-doped BaSnO3 system could be useful for realizing transparent semiconductor devices at high temperatures. Chapter six introduces the realization of atomically flat terraces with nearly single SnO2 layer-termination on the BaSnO3(001) substrate with lateral dimension about 3 x 3 mm2 by deionized water leaching and thermal annealing. The surface topography shows the evolution of the surface from chemically mixed termination to atomically flat single terminated surfaces with step-terrace structures of one unit cell step height of ~4.116 Å. The influence of out-of-plane and in-plane miscut angles on the surface, i.e., step bunching and kinked steps, is described. The single-terminated BaSnO3-δ(001) substrate has been studied by X-ray photoemission spectroscopy. From the Ba3d5/2 and Sn3d5/2 signals with its angle dependence, we confirmed that the topmost atomic layer of BaSnO3-δ(001) surface mostly consists of SnO2 rather than BaO and that deoxidized Ba sources exist on the surface of BaSnO3-δ(001) substrates. These observations facilitate the preparation of atomically aligned BaSnO3(001) substrates, which opens doors for realization of two-dimensional electron gases at the interface between BaSnO3(001) and other oxides. Chapter seven discusses the pathways to achieve two-dimensional electron gases at the interface between BaSnO3 and other perovskite oxides. Theoretical predictions for two-dimensional electron gases (2DEGs) at the heterojunction of BaSnO3 with KTaO3, LaAlO3, KNbO3, LaAlO3, SrTiO3, CaSnO3, SrSnO3, SrHfO3, SrZrO3, and LaInO3 are introduced. Experimental efforts on 2DEGs in BaSnO3 are presented and remaining challenges for observing novel quantum phenomena at the heterointerface between BaSnO3 and candidate barrier materials are described. By employing modulation or polar discontinuity doping on the SnO2-terminated BaSnO3(001) substrates, superconductivity or quantum Hall effects in the BaSnO3 system at ambient temperature may well be discovered.