GROWTH AND CHARACTERIZATION OF TRANSITION METAL TELLURIDE THIN FILMS

THE HONG KONG POLYTECHNIC UNIVERSITY Chan Cheuk Ho Page III ABSTRACT Transition metal tellurides have attracted great deal of interests due to their non-trivial band structure leading to non-trivial electronic behaviors. As a member of the transition metal dichalcogenides (TMDs) family, zirconium tellurides have shown topological semimetal and Dirac fermion characteristics. In this project, the zirconium ditelluride thin films have been deposited on different substrates by pulsed laser deposition (PLD) system. It is found that the thin film quality can be well controlled by adjusting the deposition conditions such as substrate temperature. It is also found that the zirconium ditelluride thin films are extremely unstable under ambient atmosphere, therefore, tellurium and polycrystalline aluminum nitride (AlN) are used as capping layer to protect the thin films from reaction with moisture and air. Microstructure and crystallographic orientation of ZrTe2 thin films are characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The results reveal that within a range of growth temperatures from 500 oC to 750 oC, the hexagonal ZrTe2 thin films are epitaxially grown on (0001) sapphire substrate with high quality and uniformity; while the crystalline quality of thin films decreases as the grown temperature is decreased. The thicknesses of the ZrTe2 thin films are about 60 nm and there is no interfacial layer between the film and substrate. The orientation relationship of the film and substrate is (0001)ZrTe2//(0001)s and [112̅0]ZrTe2//[011̅0]s. For the ZrTe2 thin films grown on (110) STO substrates, the main growth orientation is also along its [0001] direction, but the films present more defects and grains. There is also an interfacial layer of about 5 nm thick. This interfacial layer could be ZrTe2 but with a different growth orientation along [011̅1] direction or a different phase such as ZrTe. ABSTRACT THE HONG KONG POLYTECHNIC UNIVERSITYTHE HONG KONG POLYTECHNIC UNIVERSITY Chan Cheuk Ho Page IV The dominant growth orientation along [0001] is attributed to the layered structure of ZrTe2. Electrical transport properties of the ZrTe2 films are characterized by a 9T-PPMS (Physical Property Measurement System) from Quantum Design. The ZrTe2 thin film on sapphire substrate presents metallic transport characteristics within temperature range from 300 K to 2 K, and weak magnetoresistance can also be observed. The thin film on STO (110) substrate also show typical R-T behavior of a metal or semi-metal, but with much larger change of resistance from room temperature to 2K. Moreover, large magnetoresistance is observed in the films when an external magnetic field is applied perpendicular to the film plane; while parallel magnetic field results in negative MR. More evidences suggest that the ZrTe2 film on (110) STO substrate could be topological semimetal. LIST OF PUBLICATIONS THE HONG KONG POLYTECHNIC UNIVERSITY Chan Cheuk Ho Page V LIST OF PUBLICATIONS Journal Publication [1] T. Zhang, Z. Zhang, C. H. Chan, L. Li, M. L. Wei, X. Meng, J. Y. Dai, X. Y. Qiu, "Visible-light enhanced charge storage characteristics of amorphous Ni-doped HfO2 films," Journal of Physics D: Applied Physics, 2018. [2] Z. Zhang, J. Qian, W. Lu, C. H. Chan, S. P. Lau, and J. Y. Dai, "In situ TEM study of the sodiation/desodiation mechanism of MnO2 nanowire with gel-electrolytes," Energy Storage Materials, vol. 15, pp. 91-97, 2018. [3] W. Z. Liang, M. Gao, C. Lu, Z. Zhang, C. H. Chan, L. J. Zhuge, J. Y. Dai, H. Yang, C. L. Chen, B. H. Park, "Enhanced Metal–Insulator Transition Performance in Scalable Vanadium Dioxide Thin Films Prepared Using a Moisture-Assisted Chemical Solution Approach," ACS applied materials & interfaces, vol. 10, no. 9, pp. 8341-8348, 2018 [4] Z. Zhang, F. Zhang, H. C. Wang, C. H. Chan, W. Lu, and J. Y. Dai, "Substrate orientation-induced epitaxial growth of face centered cubic Mo2C superconductive thin film," Journal of Materials Chemistry C, vol. 5, no. 41, pp. 10822-10827, 2017. [5] F. Zhang, Z. Zhang, H. C. Wang, C. H. Chan, N. Y. Chan, X. X. Chen, J. Y. Dai, "Plasma-enhanced pulsedlaser deposition of single-crystalline Mo2C ultrathin superconducting films," Physical Review Materials, vol. 1, no. 3, p. 034002, 2017. [6] C. H. Suen, D. L. Shi, Y. Su, Z. Zhang, C. H. Chan, X. D. Tang, Y. Li, K. H. Lam, X. X. Chen, B. L. Huang, X. Y. Zhou, J. Y. Dai, "Enhanced thermoelectric properties of SnSe thin films grown by pulsed laser glancing-angle deposition," Journal of Materiomics, vol. 3, no. 4, pp. 293-298, 2017. [7] X. Y. Qiu, R. X. Wang, G. Q. Li, T. Zhang, L. T. Li, M. L. Wei, X. S. Meng, H. Ji, Z. Zhang, C. H. Chan, "Oxygen-dependent epitaxial growth of Pt (001) thin films on MgO (001) by magnetron sputtering," Applied Surface Science, vol. 406, pp. 212-217, 2017. ACKNOWLEDGEMENTS THE HONG KONG POLYTECHNIC UNIVERSITY Chan Cheuk Ho Page VI ACKNOWLEDGEMENTS This project was supported by the Department of Applied Physics in The Hong Kong Polytechnic University. Without the help of my supervisor, Prof. J.Y. DAI, and my research team members, this project cannot be finished. Firstly, I would like to acknowledge Prof. J.Y. DAI for his guidance, encouragement and materials support. From the thin films deposition using L-MBE system to the structural study using TEM, Prof. DAI had provided lot of technical support to me. Secondly, I would like to thank for the helps from my research team members, especially Mr. SUEN Chun Hung, Dennis, who provided technical support of LMBE. Mr. Wong Chi Man and Ms. YAU Hei Man are thanked for their helpful advices and helps in TEM samples preparation using FIB. Dr. WANG HuiChao is thanked for providing technical help in electrical transport properties measurement using Physical Properties Measurement System (PPMS) and data analysis. LIST OF CONTENTS THE HONG KONG POLYTECHNIC UNIVERSITY Chan Cheuk Ho Page VII TABLE OF CONTENTS CERTIFICATE OF ORIGINALITY II ABSTRACT III LIST OF PUBLICATIONS V ACKNOWLEDGEMENTS VI TABLE OF CONTENTS VII LIST OF FIGURES IX LIST OF TABLES XII CHAPTER