The Investigation Of MBE Preparation And Electrical Transport Properties Of MnTe Based Single Crystalline Films

Nickel arsenide type MnTe compounds are an important class of intermediate temperature thermoelectric materials.Compared with Pb Te and Ge Te,which belong to the same intermediate temperature material system,MnTe has the advantages of non-toxic,harmless,and abundant raw materials.At the same time,the high Seebeck coefficient（500～600μV/K）in the intrinsic bulk samples also lays an important foundation for its excellent thermoelectric performance.In addition,due to their unique magnetic and optical properties,and special MnTe and its composite thin film materials have also received extensive attention in the field of magnetic transport research such as anomalous Hall effect and quantum Hall effect.The current low carrier concentration(10¹⁸～10¹⁹cm^-3)has always been the main factor restricting the thermoelectric performance of MnTe.However,there is no experimental study on the origin of its intrinsic carrier concentration and the microscopic mechanism of carrier concentration optimization.There are few reports on the electrical transport properties of MnTe-based composite thin films.In this paper,we achieved in situ growth and characterization of high-quality intrinsic MnTe single-crystal thin films by MBE-STM-ARPES ultra-high vacuum system.At the same time,the electrical transport properties of the intrinsic MnTe films were improved by adjusting the growth process（substrate temperature,Mn:Te beam flux ratio）.Based on the performance optimization of MnTe intrinsic samples,We designed and fabricated[（MnTe）_x/（Sb₂Te₃）_y]_n superlattice thin films using MnTe and Sb₂Te₃ as the basic elements,and studied and elucidated the laws and rules for optimizing the electrical properties of superlattice thin films by heterointerface and interlayer interactions.Finally,we obtained the following conclusions.（1）We successfully obtained high-quality MnTe（00l）oriented single crystal thin films by MBE.At the same time,we explored the method of suppressing twinning in MnTe by pre-growing three buffer layers of Sb₂Te₃,Bi₂Te₃,and Bi_0.5Sb_1.5Te₃ on Al₂O₃（000l）substrate and Ba F₂（111）substrate.The lattice mismatch between the substrate and the film is very sensitive,and only slightly reduced twinning results were obtained between the Ba F₂（111）substrate without lattice mismatch and the Bi₂Te₃ buffer layer.（2）After realizing the epitaxial preparation of MnTe single-crystal films with a high crystalline quality,we used STM to characterize the surface atomic structure of MnTe single-crystal films grown under different substrate temperatures.The results comprehensively confirmed that there are two main defect structures of p-type V_Mn and n-type V_Te in MnTe films,among which V_Mn defects dominate.This is also the microscopic mechanism by which intrinsic MnTe exhibits p-type conduction in terms of electrical transport properties.which is one of the reasons why intrinsic MnTe exhibits p-type conduction in electrical transport properties.At the same time,we explored the electrical transport performance under different substrate temperatures T_sub and Mn:Te beam ratios.Increasing T_sub and decreasing Mn:Te beam current ratio are beneficial to the generation of p-type V_Mn point defects,thereby significantly increasing the hole concentration of the film,with the highest carrier concentration reaching 21.5×10¹⁹ cm^-3,compared with the unregulated intrinsic The MnTe sample improved by an order of magnitude.Finally,the MnTe film grown at T_sub=280℃,Mn:Te=1:12 obtained the highest power factor among all samples,reaching 1.3μWcm^-1K^-2 at 483 K This study provides an important reference for the regulation of electrical transport properties of MnTe-based materials through point defect structures.（3）Based on the research on high-quality MnTe epitaxial single-crystal films,we further designed and fabricated[（MnTe）_x/（Sb₂Te₃）_y]_n superlattice films.High-resolution XRD measurements confirm that the as-prepared[（MnTe）_x/（Sb₂Te₃）_y]_nsuperlattice films have excellent crystalline quality,and the MBE process can achieve precise control of the superlattice period.Due to the difference in work function between MnTe and Sb₂Te₃ elements,hole carriers at the interface will be injected from MnTe to Sb₂Te₃,causing the Sb₂Te₃ energy band structure to bend downwards and causing the hole concentration in the superlattice film to reach 1～2 an order of magnitude improvement.ARPES electronic structure characterization verifies the downward bending of electronic energy bands near the heterointerface;for example,superlattice films with superlattice period x=3,y=3 have a valence-oriented Fermi level relative to the intrinsic Sb₂Te₃ The band moved～330 me V.Heterointerface structure and interfacial effects studies have shown that charge transfer,modulated doping and energy filtering effects are important mechanisms for optimizing the electrical transport properties of[（MnTe）_x/（Sb₂Te₃）_y]_n superlattice thin films.Therefore,the electrical properties of the superlattice film with x=0.1,y=12 are greatly improved,and the best power factor at room temperature is 25.0μWcm^-1K^-2,which is better than that of the intrinsic Sb₂Te₃ film in this experiment(21.5μWcm^-1K^-2)by 16%.The preparation rules of MnTe-based superlattice films and the discovery of various new effects of heterointerfaces provide important references and new ideas for the regulation of physical properties and optimization of thermoelectric properties of MnTe-based superlattice films.