| Summary: | 碩士 === 國立清華大學 === 物理系 === 104 === In this work, the growth of topological insulators Bi2Se3 and Bi2Te3 thin films on 2D materials of graphene/SiC, MoS2/Al2O3 was studied in details. The advantage of using 2D materials as substrates is that they do not have dangling bonds on the surface. This provides us a good template for the growth of layered materials such as Bi2Se3 and Bi2Te3. Using molecular beam epitaxy, high-quality thin films have been achieved with a single-step growth method at a substrate temperature (Ts) 280℃ with the Se (or Te) to Bi flux ratio kept at 15-20.
For Bi2Se3 and Bi2Te3 films grown on graphene/SiC, sharp and streaky RHEED patterns showed up at the first layer of growth, suggesting that the crystallinity of the films were good and they were almost strain free. This is the main difference between Bi2Se3 and Bi2Te3 on other non van der Waals type substrate surface. The crystal quality and structure were further checked by synchrotron radiation X-ray diffraction. Clear triangular domain with the size of a few hundreds nm to 1μm for Bi2Se3 and a few hundreds nm to 2μm for Bi2Te3 were observed. Sharp surface states were found in ARPES measurement.
Further study of TI growth is focused on using the MoS2 monolayer as a growth buffer on Al2O3. The result is compared to the one directly grown on Al2O3.¬ MoS2 is a layered material which has van der Waals type surface without dangling bonds, having a hexagonal symmetry surface. Large and continuous MoS2 films can be grown using CVD. At the initial growth stage of Bi2Se3 on MoS2/Al2O3, streaky diffraction rods showed up which was similar to the ones on graphene/SiC. For Bi2Se3 directly on Al2O3, it has a blur polycrystalline ring and the diffraction is not as sharp as the one on MoS2 buffer at the first few QLs. Larger triangular domain size up to 1.5μm, less spiral defects, about 2-3 time enhancement in mobility and stronger SdH oscillation amplitude were observed in films with MoS2 buffer. Moreover, topological surface state transport was found by fitting the SdH oscillation curve. These all indicate that the film quality can be improved by using 2D materials with van der Waals type substrate surface.
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