Preparations And Physical Properties Of Titanium Oxide Thin Films

Due to the diversity of the outer electrons in transition metals,their oxides possess rich physical properties and are widely used in fields of semiconductors,catalysis,sensors,magnetic storage,luminescent materials,solar energy,fuel cells,lithium batteries,supercapacitors,biosensing,inorganic pigment,thermoelectric,and superconductivity,etc.Especially in the field of superconductivity,a variety of superconducting systems and peculiarly superconducting properties have emerged in transition metal oxides,which play a crucial role in promoting superconducting research.Among them,titanium-based oxide superconductors formed on the basis of titanium are prominent particularly.For example,the first discovered oxide superconductor and interface superconductor are both titanium-based oxides.Recently,titanium-based oxide superconductors have attracted attention of researchers,including TiO,Ti2O3,?-Ti3O5,and Ti4O7,which deserve further study due to the superconductivity with such a wide range of oxygen content.Moreover,the superconductor composed of only titanium and oxygen elements eliminates the interference of other elements,and is more conducive to fundamentally understanding the various phenomena and mechanisms of titanium-based oxide superconducting systems.In this dissertation,we take titanium oxide superconductors as the research object,and conducts deep investiigations on the superconducting properties and superconducting phenomena.In the Chapter 1,we focused on the physical properties of different kinds of titanium oxide superconductors,such as the fundamental phenomena of superconductivity,interplay between superconductivity and ferromagnetism,quantum phase transitions in superconductors,and finally outlined the development history and application value of laser induced transverse voltage.In the Chapter 2,the titanium oxide films(Ti2O,TiO1+?,and ?-Ti3O5)with different oxygen contents on ?-Al2O3(0001)single crystal substrates were prepared by pulsed laser deposition technique using a titanium metal target,and their structures and electrical transport properties were systematically studied.It is found that,Ti2O is metallic,TiO1+?exhibits superconductivity,and ?-Ti3O5 is semiconducting,and their electrical transport behaviors follow the Bloch-Gruneisen model,variable range hopping model,and small polaron hopping model,respectively.Moreover,the superconductivity of TiO1+? films gradually decreases with increasing oxygen content,which is due to the decrease of carrier density and the increase of disorder strength caused by the increase of oxygen content,thus the formation of superconducting Cooper pairs is suppressed.In the Chapter 3,we observed that TiO1+? films exhibit quantum superconductor-insulator transition(SIT)behavior with increasing oxygen content,which critical sheet resistance is close to the quantum resistance RQ?h/4e2=6.45 k?/?,following the dirty boson model.In addition,the quantum SIT of TiO1+? films was also realized by the external magnetic fields,and the scale analyses indicated that the TiO1+? film is a disordered system,which is consistent with the prediction of the dirty boson model.Finally,the two-dimensional superconductivity of TiO1+? films was characterized,and the quantum SIT phase diagram of TiO1+? films was constructed,describing physical mechanisms during the phase transition.In the Chapter 4,the Ti1-xMgxO(x=0,0.05,0.1,0.2,and 0.4)films on ?-Al2O3(0001)single crystal substrates were prepared by pulsed laser deposition technique.It is noted that the superconducting transition temperatures show a domed shape with the increase of Mg doping content,and the enhancement of superconductivity is related to carrier density and disorder strength.Simultaneously,Mg doping enhances the ferromagnetism of Ti1-xMgxO films,and a rare coexistence of superconductivity and ferromagnetism was observed.In addition,we performed first-principles calculations and achieved a consistent interpretation of all experimental results,revealing the possible origin of the variations of carrier density,disorder strength,and ferromagnetism with Mg content,where the ferromagnetism is mainly from the spin polarization of 3d states of Ti atoms surrounding Mg atoms.In the Chapter 5,we detected a significant laser induced transverse voltage(LITV)signal in the(111)-oriented TiO1+? film,and systematically studied the effect of in-plane measurement angle,oxygen content,laser energy density,sample temperature,and laser wavelength on its LITV.The LITV signal of TiO1+? films gradually weakened with increasing oxygen content,probably due to the weakening of thermoelectric efficiency and anisotropy of Seebeck coefficient.In addition,when the photon energy of irradiated laser is less than the band gap of materials,the LITV signal is induced by the transverse thermoelectric effect,and when the photon energy of irradiated laser is greater than the band gap of materials,the LITV signal is contributed by both transverse photovoltaic effect and transverse thermoelectric effect.