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Studies of the Vapor-Liquid-Solid STUDIES OF THE VAPOR-LIQUID-SOLID SYNTHESIS OF MgSnN2 by ZIHANG XU Submitted in partial fulfillment of the requirements for the degree of Master of Science Physics Program CASE WESTERN RESERVE UNIVERSITY August, 2021 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis of Zihang Xu Candidate for the degree of Master of Science Committee Chair Prof. Kathleen Kash Committee Member Prof. Corbin Covault Committee Member Prof. Walter Lambrecht Date of Defense June 4th, 2021 We also certify that written approval has been obtained for any proprietary material contained therein. 1 Table of Contents List of tables .......................................................................................................................4 List of figures .....................................................................................................................5 Acknowledgements ...........................................................................................................7 Abstract ..............................................................................................................................8 1 Introduction and overview .........................................................................................9 1.1 Overview ....................................................................................................................9 1.2 Scope of the thesis .....................................................................................................9 2 Background ...............................................................................................................11 2.1 III-Nitride semiconductors .......................................................................................11 2.2 II-IV-Nitride semiconductors ...................................................................................12 2.3 Previous research on MgSnN2 ..................................................................................13 2.3.1 Theoretical .......................................................................................................13 2.3.2 Experimental ....................................................................................................15 3 The growth experiments ............................................................................................17 3.1 Growth from Mg-Sn Alloys ...................................................................................17 3.1.1 The Mg-Sn phase diagram ...............................................................................17 3.1.2 Discussion of competing phases ......................................................................19 2 3.2 Description of the growth apparatus ......................................................................20 3.3 Growth procedure ..................................................................................................22 3.3.1 Preparation ......................................................................................................22 3.3.2 Growth experiments ........................................................................................23 4 Characterization results ..............................................................................................26 4.1 Optical microscopy ................................................................................................26 4.2 Scanning electron microscopy ...............................................................................31 5 Future work ..................................................................................................................38 Appendix Bibliography 3 List of Tables Table 1. Lattice parameters, lattice volume the average error with respect to experiment and lattice constant ratio in Mg-IV-N2 ...............................................................................14 Table 2. Calculated optoelectronic properties for the six cation ordered MgSnN2 structures ............................................................................................................................16 4 List of Figures Figure 1. Band gaps of II-IV-N2 and III-N compounds .....................................................12 Figure 2. Magnesium-Tin Binary Phase Diagram .............................................................17 Figure 3. Schematic of the growth instrument ...................................................................20 Figure 4. Experiment package. The crucible, not shown here, fits into the heater ............21 Figure 5. The thermocouple reading for the 350℃ growth. The point at which the nitrogen plasma was turned on is noted in the graph .........................................................23 Figure 6. The thermocouple reading for the 400℃ growth. The point at which the nitrogen plasma was turned on is noted in the graph .........................................................24 Figure 7. The alloys after the growth experiments. Note the boron nitride protector tubes embedded in the melts .......................................................................................................25 Figure 8. Optical microscope observation of the sample grown at 350℃ at 5X magnification .....................................................................................................................27 Figure 9. Optical microscope observation of the sample grown at 350℃ at 50X magnification. Circled areas are areas of particular interest, as discussed in the text .......28 Figure 10. Optical microscope observation of the sample grown at 400℃ at 5X magnification. Circled areas are particular areas of interest, as discussed in the text .......29 Figure 11. Optical microscope observation of the sample grown at 400℃ at 50X magnification. Circled areas are particular areas of interest, as discussed in the text .......30 5 Figure 12. SEM image of an area of the 350℃ sample surface with one example of a close-to-120-degree facet highlighted................................................................................31 Figure 13. SEM image of an area of the 400℃ sample surface with one example of a close-to-120-degree facet highlighted................................................................................32 Figure 14. EDX analysis of the selected area in Figure 13 on the 350℃ sample .............33 Figure 15. EDX analysis of the 400℃ sample ..................................................................35 Figure 16. EDX analysis of the 400℃ sample ..................................................................36 6 Acknowledgements I would like to express my gratitude to my advisor Prof. Kathleen Kash. Prof. Kathleen Kash provided inspiring guidance and support throughout my research at CWRU. I also want to thank other committee members of my thesis dissertation defense. I am grateful to Prof. Walter Lambercht from the Department of Physics, and Prof. Corbin Covault from the Department of Physics in CWRU. I am grateful to Dr. Dinushi Jayatunga for her help with the SEM/EDX measurements and support for materials from the NSF grant DMREF: SusChEM:1533957 and the U.S. Department of Energy award SSL: DE- EE0008718. Finally, I wish to express my deep and sincere gratitude to my family, especially my mother. Her encouragement always gives me confidence and strength to achieve my goals. Zihang Xu 7 Studies of the Vapor-Liquid-Solid Synthesis of MgSnN2 Abstract by ZIHANG XU MgSnN2 is a group II-IV nitride semiconductor. MgSnN2 has a wide band gap of 2.3 eV. It is also an affordable material because the tin element is abundant in the environment compared to the gallium and indium. This research focuses on the synthesis of the wurtzite structure of MgSnN2. In this work, the synthesis method is the vapor-liquid-solid method. Experiments on the growth of MgSnN2 from Mg/Sn pellets and Mg-Sn alloys were performed. The liquid Mg-Sn alloy was exposed to a plasma source in a vacuum chamber. According to observations using an optical microscope, the samples display structures that might be clusters of small crystallites. According to the scanning electron microscope observation, structures similar to crystallite are observed. Energy dispersive x-ray spectroscopy proves that the structures contain Mg, Sn, and N elements. There is a high possibility that MgSnN2 is successfully synthesized in these structures. 8 Chapter 1 Introduction and Overview 1.1 Overview MgSnN2 is a potentially promising semiconductor material because it is within the band 1 gap range for the green LED, as Sai Lyu et al. predicted. MgSnN2 is also promising because it is eco-friendly because of the availability and non-toxicity of Mg and Zn. In this thesis, experiments attempting to synthesize MgSnN2 using the vapor-liquid-solid method are described. The synthesis experiments took place in a low-pressure environment at temperatures lower than 500 degrees Celsius. Two different temperatures were chosen in the experiments: 350 degrees Celsius and 400 degrees Celsius. After growth the material was characterized by scanning electron microscopy and optical microscopy. 1.2 Scope of the Thesis Chapter 2 provides background information on nitride semiconductors. The chapter starts with group III-nitride semiconductors and compares them with group II-IV-nitride semiconductors. Previous theoretical and experimental studies of MgSnN2 are described. In Chapter 3, the growth experiments are described. This chapter discusses details of the Mg-Sn phase diagram
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