Growth and characterization of transition metal telluride thin films

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 °C to 750 °C, 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 [1120]ZrTe₂//[0110]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 [0111] direction or a different phase such as ZrTe. 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.