Precise Preparation And Physical Properties Modulation Of Chromium Nitride Thin Films

Transition metal nitrides（TMNs）exhibit wide applications in the field of energy,environment and microelectronic devices due to their outstanding thermoelectricity,catalysis and mechanical properties.The strong couplings among multiple degrees of freedom such as charge,orbital,lattice,and spin,provide an excellent route for externally modulating order parameters in the strongly correlated materials.However,the preparation of high-quality TMNs films and heterostructures is extremely challenging,therefore the researches on their intrinsic physical properties and interface effects are hilghly desired.This will not noly contribute to the construction of physical pictures on strongly correlated material,but also expand the functional applications of TMNs.In the first part,we investigate the effects of strain-induced electronic state transition of antiferromagnetic metal chromium nitride（CrN）thin films.CrN films with stoichiometric and high crystalline quality were prepared by active nitrogen-atom-source assisted pulsed laser deposition technique precisely.By changing the film thickness,we effectively modify the strain states of CrN films.By shrinking the film thickness down to 30 unit cells（u.c.）,CrN films undergo a metal-insulator transition（MIT）,accompanied with the lattice expanding and the carrier concentration decreasing.An CrN ultrathin film recovers from the insulating state to metallic state upon the removal of the as-grown tensile strain by water-soluble sacrificial layer method.Both first-principles calculations and synchrotron x-ray linear dichroism measurements indicate that the strain-induced orbital splitting customizes the relatively bandgap at the Fermi level effectively,leading to an exotic electronic phase transition in CrN film.In addition,we investigate the MIT of CrN films modulated by crystallographic orientation and crystalline symmetry of substrates.The CrN films grown on Mg O（001）substrates are oriented along the[001]direction,whereas the CrN films onα-Al2O3（0001）substrates are oriented along the（111）direction parallel to the surface normal.Transport properties of CrN films are remarkably different depending on crystallographic orientations.The critical thickness for the MIT in CrN（111）films is one order of magnitude larger than that of CrN（001）films.In contrast to CrN（001）films without apparent defects,scanning transmission electron microscopy results reveal that CrN（111）films exhibit strain-induced structural defects,e.g.,the periodic horizontal twinning domains,resulting in an increased electron scattering facilitating an insulating state.CrN films grown on different oriented Nd Ga O3 substrates exhibit significantly anisotropic transport properties.These studies not only reveal the intrinsic mechanism of strain-induced electronic state transition of CrN films,but also provide a new route for design of functional spintronic devices.The second part of this thesis,we report the successful epitaxial synthesis and characterization of chromium oxide（Cr2O3）-CrN superlattices.The structural characterizations demonstrate the high-quality superlattices with a high level of spatial uniformity.Using superconducting quantum interference device（SQUID）and diamond nitrogen-vacancy centers magnetometer,it confirms the ferromagnetic behavior with the Curie temperature up to～325 K,which is larger than the Néel temperature of both parent materials.X-ray magnetic circular dichroism measurements illustrate the origin of the magnetic peoperties of superlattices is trivalent Cr³⁺ions.The magnetic moments are oriented parallel to the in-plane direction and the total magnetic moment of Cr³⁺ions is determined to be～0.23μB/Cr.Polarized neutron reflectometry confirm that the magnetization uniformly distributed in the superlattices.First-principles calculations indicate that robust ferromagnetic spin interaction between Cr³⁺ions via anion-hybridization across the interfaces yields the lowest total energy.This work opens the door to fundamental understanding of the unexpected and exceptional properties of oxide-nitride interfaces and provides access to hidden phases at low-dimensional quantum heterostructures.In the last part of this thesis,the research results of ferromagnetic iron nitride（Fe3N）and superconducting vanadium nitride（VN）are briefly discussed.The precise preparation method is used for synthesizing the stoichiometric Fe3N and VN single-crystalline films.The transport prperties of Fe3N films were performed by physical property measurement system and SQUID.The MIT was observed when the thickness is reduced down to 10 u.c.The 30u.c.-thick Fe3N films show a clear magnetocrystalline anisotropy with the easy-axis along the in-plane direction.In addition,the CrN/Fe3N heterojunction exhibits the obvious exchange bias effects.Strain-dependent superconducting transport properties of VN films performed by transport measurements.Critical field for VN films decays with temperature increasing.In this thesis,we successful synthesis TMN films with extremely high crystallization quality and stoichiometry accurately.The strain-mediated electronic state transition in antiferromagnetic CrN films was observed.Besides,room-temperature ferromagnetism at an oxide/nitride interface is achieved at the interfaces between two antiferromagnets（Cr2O3 and CrN）.In addition,we explore the transport properties of ferromagnetic Fe3N and superconducting VN films preliminarily.This thesis not only promotes the basic research progress of TMNs,but also provides prototype materials for exploring multiple-stable-state magnetic memories and low dimensional quantum devices.