Title Page
Abstract
Contents
Chapter 1. Introduction 18
1.1. Overview of oxide semiconductors 18
1.2. Oxide perovskite material, BaSnO₃ (BSO) 19
1.3. Wide bandgap semiconductor 21
1.4. High electron mobility 23
1.5. Thermal stability of BSO 24
1.6. Photoconductivity of BSO 25
References 29
Chapter 2. Overview of 2DEG: From Conventional Systems to Interfacial Polarization Model 31
2.1. 1. Introduction 31
2.2. Conventional 2DEG system: Modulation doping approach (AlGaAs/GaAs) 32
2.3. Conventional 2DEG system: Polarization-induced carrier accumulation(AlGaN/GaN and MgZnO/ZnO) 34
2.4. Polar catastrophe model (LaAlO₃ (LAO)/SrTiO₃ (STO)) 37
2.5. Interfacial polarization model of LIO/BSO 39
2.6. Analyzing the LIO/BSO interface with Poisson-Schrödinger simulation 40
Reference 43
Chapter 3. In-situ Study for Understanding the Role of Terminating Layer on 2DEG State Formation at the LIO/BSO Interface 46
3.1. Introduction 46
3.2. Growth rate measurement of SnO₂ using X-ray Reflectometry (XRR) 47
3.3. Electrical properties of LIO/BSO as a function of SnO₂ thickness 50
3.3.1. 2DEG conductance as a function of SnO₂ thickness on MgO 50
3.3.2. 2DEG conductance as a function of SnO₂ thickness on STO 51
3.3.3. Percolation threshold 53
3.4. Coaxial Impact Collision Ion Scattering Spectroscopy(CAICISS) 55
3.5. Surface morphology analysis using Atomic Force Microscopy (AFM) 60
3.6. Structure properties with Scanning Transmission Electron Microscopy (STEM) 61
3.7. Conclusion 63
References 64
Chapter 4. Ex-situ Study for the Terminating Layer Control at the LIO/BSO Interface 65
4.1. Introduction 65
4.2. Surface engineering with chemical treatment 67
4.3. Mask-heat treatment for SnO₂ termination of BSO and its role in forming 2DEG in LIO/BSO 77
4.3.1. Electrical properties for LIO/BSO with mask-heat treatment 79
4.3.2. Surface properties of mask-heat treated BSO with CAICISS 82
4.4. Conclusion 85
References 87
Chapter 5. Summary 89
국문초록 91
Chapter 2. 17
Table 2.1. Polarization of GaAs, AlGaAs, GaN, AlGaN, ZnO and MgZnO 37
Chapter 4. 17
Table 4.1. Chemical characteristics comparison of BaO and SnO₂. BaO reacts with water to form Ba(OH)₂, and Ba(OH)₂ not only dissolves in water but also... 68
Chapter 1. 9
Figure 1.1. BaSnO₃ Perovskite structure. 19
Figure 1.2. Extensive research and promising applications of perovskite materials. 20
Figure 1.3. Transmission spectra and absorption coefficient of BSO and BLSO single crystals. a) Transmission spectra of undoped BSO and BLSO single crystals. b)... 22
Figure 1.4. Electron mobility vs carrier density graph of reported BLSO. 23
Figure 1.5. a) The temperature profile and gas atmosphere were adjusted as shown in the upper panel, and the resulting resistance variation is plotted in the lower panel.... 25
Figure 1.6. The time-dependent behavior of the photoconductivity of BSO and STO on both short and long time scales. The wavelength of the illumination is fixed at... 26
Figure 1.7. Time-dependent increase in conductance of lightly doped BLSO films under yellow light. 27
Figure 1.8. Influence of thermal annealing conditions on photoconductivity under yellow light. a) A sample annealed at 750°C in vacuum for 1 hour. In this case, no... 28
Chapter 2. 10
Figure 2.1. Band structure of AlGaAs/GaAs 2DEG system with modulation doping. Schematic representation of the band structure in an AlGaAs/GaAs 2DEG system... 33
Figure 2.2. Polarization in AlGaN/GaN system with Ga-Face and N-Face orientations 34
Figure 2.3. one-dimensional band structure calculation of AlGaN/GaN 2DEG for Ga-face orientation 36
Figure 2.4. Polar catastrophe at interfaces between LAO and STO. 38
Figure 2.5. a) Schematic diagram of a LIO/BLSO 2DEG. b,c) The sheet conductance varies with the La concentration at the LIO/BLSO interfaces on an STO... 39
Figure 2.6. a) Schematic illustration of the (LaO)⁺/(SnO₂)⁰ interface (SnO₂ termination) and (InO₂)¯/(BaO)⁰ interface (BaO termination). At the (LaO)⁺/(SnO₂)⁰... 42
Chapter 3. 11
Figure 3.1. a) Schematic illustration of PLD and image of BSO plume. b) Photograph of the PLD system 47
Figure 3.2. SnO₂ X-ray spectrum and growth rate measurement using XRR. SnO₂ is grown by 525 laser pulses with the laser energy fluence of 1.4 J cm¯² in 100 mTorr... 48
Figure 3.3. Crystal structure of SnO₂ in the rutile structure. The lattice parameters are a=b=4.7374 Å and c=3.1864 Å. Each unit volume of 71.512... 49
Figure 3.4. LIO/BSO 2DEG conductance as a function of additional SnO₂. a) Schematic illustration of in-situ LIO/BSO 2DEG on MgO with additional SnO₂... 50
Figure 3.5. LIO/BSO 2DEG on STO conductance as a function of additional SnO₂. a) Schematic illustration of in situ LIO/BSO 2DEG on STO with SnO₂ dusting on... 52
Figure 3.6. Illustration of the percolation phenomenon in 2-dimensional n x n lattice. Each site can be either occupied by a conducting site with a probability p... 53
Figure 3.7. The percolation threshold pc in two-dimentional lattice. a) Percolations probabilities of two phase composition, for a 2D hexagonal (triangular) lattice for...[이미지참조] 54
Figure 3.8. CAICISS spectrum of three different samples (as-grown BSO / additional 1.1 Å SnO₂ deposition on BSO / additional 1.9 Å SnO₂ deposition on... 58
Figure 3.9. The schematic image of the exposure of Ba atoms despite the SnO₂ termination through CAICISS by the Ne ion beam. The incomplete... 60
Figure 3.10. AFM morphology images of (a) as-grown BSO, (b) after 3-pulse (1.1 Å) dusting on BSO, (c) after 5-pulse (1.9 Å) dusting on BSO. 61
Figure 3.11. STEM images of BSO/LIO. a,b) HAADF-STEM image of LIO on optimally SnO₂ terminated BSO (3-pulse dusting). c) Schematic diagram of LIO on... 62
Chapter 4. 13
Figure 4.1. Schematic illustration of the ex-situ method for SnO₂ terminated BSO and LIO/BSO 2DEG. a) Deposition of BSO film onto substrate. b) The ex-situ... 69
Figure 4.2. LIO/BSO 2DEG on MgO conductance as a function of water leaching time. a) Schematic illustration of LIO/BSO 2DEG on MgO with water leaching on... 70
Figure 4.3. LIO/BSO 2DEG conductance as a function of leaching time. a) Schematic illustration of LIO/BSO 2DEG on STO with ex-situ treatment. b) The... 71
Figure 4.4. a) The schematic of principle of ARXPS. As take-off angle θ decreases, measurement is more sensitive to the surface. b) The ratio of Ba and Sn in as-... 73
Figure 4.5. ARXPS spectra of as-grown and ex-situ treated BSO. The signal decreases as the photoelectron take-off angle decreases. a) ARXPS image of Ba... 74
Figure 4.6. a) The CAICISS spectrum of BSO measured after annealing at 750 °C in an oxygen atmosphere. Compared to the as-grown BSO in Figure 4.7, the Sn peak... 75
Figure 4.7. a) CAICISS spectrum of the As-grown BSO shows that the intensity of both the Ba and Sn peaks are large. The CAICISS spectrum of BSO treated with... 75
Figure 4.8. AFM morphology images of (a,b) As-grown BSO and (c,d) after ex-situ treated BSO. 77
Figure 4.9. Schematic deposition process using a mask in four steps. a) Deposition of BSO buffers on a substrate. b) Use of a stencil mask to deposit lightly BLSO as... 78
Figure 4.10. LIO/0.2 % BLSO 2DEG conductance with mask heat-treatment as a function of additional SnO₂ deposition. a) Schematic illustration of the LIO/0.2 %... 80
Figure 4.11. LIO/0.2% BLSO 2DEG on STO conductance as a function of additional SnO₂. a) Schematic illustration of in situ LIO/0.2 %BSO 2DEG on STO with SnO₂... 81
Figure 4.12. CAICISS sample preparation and annealing process. a) Schematic depiction of the preparation for the CAICISS sample, featuring the placement of an... 82
Figure 4.13. CAICISS spectra of two different samples. a) CAICISS spectrum of as-grown BSO, which is identical to the spectrum shown in Figure 3.8a. b)... 83
Figure 4.14. Comparison of 2DEG conductance in LIO/BSO: As-grown BSO vs. mask heat treated BSO. Direct LIO deposition on BSO exhibits insulating, while... 84