Molecular Beam Epitaxial Growth And Physical Properties Of MnTe₂ Thin Films

Manganese ditelluride（MnTe₂）is an important transition metal chalcogenide semiconductor,which crystal structure is pyrite structure.MnTe₂is an antiferromagnetic semiconductor.Its Neel temperature T_Nis about 86 K.At this temperature,the second-order antiferromagnetic transition occurs;MnTe₂also has a high Seebeck coefficient（～400μV/k）and low lattice thermal conductivity,showing high conversion efficiency in thermoelectric device applications.At present,the growth technology and basic physical properties of MnTe₂films,such as electronic band structure,work function,electronic transition behavior and magnetic structure,have not been systematically studied.In this paper,we successfully fabricate polycrystalline MnTe₂thin films on the two-dimensional layered mica surface by van der Waals via molecular beam epitaxy technology.By optimizing the flux ratio and substrate temperature,we continuously eliminated the impurities in the polycrystalline MnTe₂thin films,optimized the surface morphology of the thin films,and overcome the difficulties of large-area epitaxy of thin films without hanging bonds on the surface of mica substrates and particularly low surface energy.The optical,magnetic and electrical properties of MnTe₂thin films were systematically studied.Based on the weak bonding force between van der Waals epitaxial MnTe₂films and substrates,the stripping and transfer of van der Waals epitaxial MnTe₂films on mica substrates were realized,and MnTe₂and Mo S₂heterostructures were prepared.The surface work function of MnTe₂was determined by UPS,I-V and KPFM.The main contents and results of this paper are as follows:1.By optimizing the van der Waals epitaxial growth process,the optimal process conditions for the growth of pure MnTe₂thin films on mica substrates were obtained.Using XRD,AFM and Raman spectroscopy,it is found that the growth temperature,Mn/Te flux ratio,and the phase structure,growth direction and surface morphology of Mn-Te epitaxial system can be adjusted,so as to achieve a relatively flat and high-quality MnTe₂polycrystalline film（RMS:～3.904 nm）.High-resolution transmission electron microscopy shows that the interface between the film and the substrate is clear and the crystal epitaxy is of high quality.2.Through the characterization of UV-IR transmission spectrum and the Tauc relation,it is determined that MnTe₂is a semiconductor with direct electron transition,and its band gap is1.39 e V.Using the modified Curie-Weiss relationship to fit the M-T curve,combined with the M-T relationship under field-cooling and zero-field-cooling conditions,it is found that MnTe₂has a non-collinear antiferromagnetic structure,and its antiferromagnetic transition temperature is about 86.2 K.In addition,we also found a sudden change in the resistance of the MnTe₂film in the 160 K region,indicating that there may be a structural phase transition around this temperature,which was further confirmed by temperature-varying Raman spectroscopy.3.Using different work functions metals（Pt,Cr,and Ti）,the metal/MnTe₂contact behavior and transport properties were investigated.The study found that the temperature range from160 K to 300 K,the I-V relationship showed good linearity,indicating that MnTe₂formed ohmic contact with these metals,and the reasons why different metals and the prepared MnTe₂films always maintained ohmic contact were analyzed.The work function of MnTe₂was determined to be about 5.07 e V according to X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy.4.Based on the weak binding energy between the MnTe₂/mica van der Waals epitaxy system,the exfoliation and transfer of MnTe₂thin films were realized,and the MnTe₂/Mo S₂heterostructure was prepared,which exhibited typical diode rectification characteristics（RF～40）,the heterointerface barrier inhomogeneity,which is consistent with the Werner-Gutler model.Through the fitting analysis of the I-V curves at different temperatures,the height of the MnTe₂/Mo S₂interface barrier is about 0.1 e V,and the work function of MnTe₂is calculated to be 5.1 e V,which is confirmed by the KPFM.