Study on growth of optoelectronic materials (LiNbO3 and Si) by directional solidification method based on numerical simulation

다국어 초록 (Multilingual Abstract)

Recently, certain materials have attracted attention for a new generation of high speed, efficient, multi-functional optical devices. Among these materials, small-diameter long-length bulk crystals are of considerable interest for miniaturization and high efficiency. In particular, fiber single crystals have already received attention as attractive materials for a variety of electro-optical application because of their extended interaction length and high optical intensity. There is increasing interest in the growth of rod or fiber-like micro-single crystals, suitable for application in non-linear optical and electro-optical devices, such as second harmonic generation (SHG), micro-laser sources or optical memory arrangements. An improved efficiency can be expected because of the high surface-volume ratio, the long interaction length and the high crystal quality. Fiber single crystals were grown by variously modified melt growth like the laser heated pedestal growth (LHPG) method, the drawing down method and the micro-Czochralski (µ-CZ) method. For micro-single crystals the habit is of special importance because the dimension of the cross section approaches the diameter of the stimulating laser mode. Therefore, the correlation between surface morphology and growth parameters, like starting composition of the melt and temperature field, are of considerable interest for optimization of the SHG properties. LiNbO3 is a widely studied optoelectronic material because of its technological applications. LiNbO3 is a material that combines excellent electro-optic, acousto-optic and non-linear properties with the possibility of rare earth or transition metal doping. Doping with foreign ions modifies the optical properties of the matrix and makes the system useful for a great variety of application such as photorefractive devices, solid-state lasers or optical waveguides.
In this study, rare-earth elements-doped LiNbO3 single crystals from congruent and stoichiometric melts composition were simulated and grown by the micro-pulling down (µ-PD) method. The characteristics of this method has a high pulling rate, a low thermal strain compared with other growth methods and it is possible to grow a crystal from incongruent melt composition. The grown fiber single crystals were investigated for the change of the photoluminescence (PL) properties in various wavelength according to influence on adding of rare-earth elements (Er, Nd and Tm).

To guarantee the growth of the photovoltaic industry, the manufacturing process of solar silicon (Si) ingots must be further developed in order to lower the manufacturing costs of ingots, wafers, solar cells and solar modules. In the photovoltaic (PV) industry, more than 90% of the annual solar cell production is based on mono-crystalline, multi-crystalline wafer and thin film module Si. In recent years, the market share of multi-crystalline silicon has increased remarkably. Multi-crystalline Si wafers constitute more than 60% of the photovoltaic market because of the cost advantages compared to mono-crystalline Si wafers. Since the first paper that focused on the casting process in 1976, several solidification processes industry, including casting, heat exchanger method (HEM), and electromagnetic casting have been developed. However, the rapid market growth of mono and multi-crystalline Si wafers is being suppressed by the shortage of Si feedstock. Therefore, two of the main goals in the PV industry today are to increase the weight of ingots and to accelerate the growth rate of solar ingots. The directional solidification (DS) process is a cost-effective technique for producing multi-crystalline Si ingots. A detailed understanding of the DS process is important to producing high-quality multi-crystalline Si ingots and wafers.
In this study, an open and closed heat insulation system with a heat exchanger was used to improve the DS process and to satisfy the above-mentioned goals. The improved DS process was beneficial because of its small heat loss, short cycle time and efficient directional solidification. Based on the FD model, a numerical simulation was performed on the thermal characteristics of the improved DS process. Using a commercial CFD code, Fluent, the heat transfer characteristics in the DS system were calculated, and the results were graphically depicted. From simulation results, multi-crystalline Si ingot was grown using the improved DS process, and the characteristic resistivity and life time of the ingot were investigated.

목차 (Table of Contents)

• CHAPTER 1. Introduction 1
• CHAPTER 2. Literature Review 4
• 2.1. Optoelectronics 4
• 2.1.1 Photonic Applications of Rare Earth-doped Materials 5
• 2.1.2 UV-Visible Lasers based on Rare Earth Ions 6
• 2.1.2.1. Alternative Laser Sources for Visible and UV Wavelengths 7
• 2.1.2.2. Basics 9
• 2.1.3. Display Applications of Rare Earth-doped Materials 12
• 2.1.4. Luminescence from Rare Earth-doped Materials 13
• 2.2. LiNbO3 Single Crystal 18
• 2.3. Light-absorbing materials 20
• 2.4. Crystalline silicon 23
• 2.5. Directional solidification 24
• 2.5.1. Micro-pulling Down method 24
• 2.5.2. Directional Solidification method 29
• 2.6. Numerical Simulation 30
• CHAPTER 3. Rare Earth-doped LiNbO3 Single Crystal Growth
• by Micro-pulling Down Method 33
• 3.1. Numerical simulation of heat transfer in micro-pulling down method 33
• 3.1.1. Experimental Procedure 33
• 3.1.2. Results and Discussion 37
• 3.1.3. Summary 37
• 3.2. Growth of Erbium-doped LiNbO3 Single Crystals 40
• 3.2.1. Experimental Procedure 41
• 3.2.2. Results and Discussion 41
• 3.2.3. Summary 51
• 3.3. Growth of Neodymium and Zinc co-doped LiNbO3 Single Crystals 52
• 3.3.1. Experimental Procedure 53
• 3.3.2. Results and Discussion 54
• 3.3.3. Summary 63
• 3.4. Growth of Thulium and Zirconium co-doped LiNbO3 Single Crystals 67
• 3.4.1. Experimental Procedure 68
• 3.4.2. Results and Discussion 69
• 3.4.3. Summary 77
• CHAPTER 4. Large-sized Polycrystalline Silicon Ingot Growth by Improved Directional Solidification based on Numerical Simulation 82
• 4.1. Experimental Procedure 84
• 4.2. Results and Discussion 91
• 4.3. Summary 104
• CHAPTER 5. Conclusion 109
• REFERENCE 115
• CHAPTER 6. 국문요약 124
• PUBLICATIONS 128