Synthesis And Optoelectronic Properties Of Fe And Bi Co-doped H-YMnO₃ Films

Multiferroic materials is one of hot spots in the current study about materials due to the potential application of spintronics and memory devices.It often has more than one simultaneous ferror order,ferroelectricity,ferromagneticity,ferroelasticity and so on.Recently,hexagonal YMnO3(h-YMO),as a multiferroric material attracted a great deal of research interest due to the coupling of the antiferromagnetic and ferroelectric.h-YMO is different from the general perovskite structure.Each Mn3+ ion is surrounded by three in-plane and two apical oxygen ions,submitting naturally to a trigonal crystal field.For h-YMO,antiferromagnetic is derived from a triangular lattice of Mn3+ ions,and ferroelectricity is derived from the distortion of Y3+ ions.h-YMO is room temperature with the Curie temperature of about 900 K and low temperature antiferromagnetic with the Neer temperature of about 70 K.There are the corresponding ferroelectric domain and antiferromagnetic domain structure,simultaneously.The coupling of ferroelectric domain and antiferromagnetic domain and composite domain walls occurred in lower temperature.The band gap and dielectric constant of h-YMO is about 1.5 eV and 20,respectively,which approach those of Si(-1.1 eV and?12).As a suitable ferroelectric material in a metal/ferroelectric/insulator/semiconductor,it provided the possibility for better developing the ferroelectric memory based on a silicon integrated circuit.The electronic band structure as a function of temperature often has an important referent value for the research on the phase transformation.Recently,Katsufuji et al reported that the dielectric abnormality was caused by the magnetic-ordering-dependent electronic excitation gap in the ab-plane due to the coupling of the antiferromagnetism and the dielectric properties.According to optical conductivity spectra and theoretical calculation,the Eg1 and Eg2 of YMO could be attributed to the gaps between two hybridized bands.One is from the hybridization between in-plane O 2p orbitals and the related Mn E2g/E1g orbitals,and the other is from the hybridization between apical O 2p orbitals and the related Mn a1g orbitals.Therefore,antiferromagnetic phase transformation has a possible influence on Eg1 and Eg2,which can be attributed to the possible modulation of hybridization between in-plane O 2p orbitals and the relevant Mn E2g/E1g orbitals.However,the influence was ignored for Katsufuji's mode.It is hoped that the direct evidence of modulation on the band gap and other energy gaps can be found.Moreover,there has been few studies of temperature dependent double energy gaps of h-YMO.Getting a higher uniaxial ferroelectric orientational h-YMO is a challenging for the potential applications of h-YMO.The higher uniaxial ferroelectric orientational h-YMO can be required by the pulsed laser deposition(PLD),floating zone melting process and molecular beam epitaxy(MBE).But,it is not helpful to be popularized and applied due to the exorbitant prices of these methods.At the process of the solid phase sintering,the poor surface and electrical deterioration occurred,especially,for larger leakage current,due to the micron grade crack at the high temperature heat treatment.According to Choi's report,there is the highly c-axis-oriented growth of Bi doped h-YMO films at lower temperature.However,the effective concentration of Bi doped is above 5%,which brings out the significant Bi2O3 surface layer.The influence of Fe doped on antiferromagnetic phase transformation of h-YMO has been studied.However,the modulation of Fe doped on crystal and surface layer was rarely mentioned,especially,for Fe and Bi co-doped.Note that the sol-gel method is a well-accepted method due to some evident advantages.It is hoped that the sol-gel films of Fe and Bi co-doping can realize the controlled growth of h-YMO,especially,for the significant modulation of crystal orientation and morphology.In this work,crystallographic excellent(1-x)YMnO3-xBiFeO3(x=0-2.5%)polycrystalline films were successfully obtained by the sol-gel technique.There are several substrates,including quartz,silicon,and Pt.The effects of Bi and Fe co-doping on the microstructure,morphology,ferroelectricity and optical properties at the room temperature and the electronic band structure at various temperatures have been investigated by X-ray diffraction spectroscopy(XRD),X-ray photoelectron spectroscopy(XPS),atomic force microscopy(AFM),scanning electron microscopy(SEM),ferroelectric test system and electrometer and the various temperature transmission.The main works are listed as following:(1)Synthesis and basic characterization of YBMFx films:The crystalline structure,surface morphology,element composition,ferroelectricity and volt-ampere characteristics of YBMFx films have been investigated by X-ray diffraction(XRD),atomic force microscopy(AFM),sanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),X-ray photoelectron spectroscopy(XPS),ferroelectric test system and electrometer.The crystalline structure,the element valence and the room ferroelectricity of 0%h-YMO were confirmed.The elemental mapping of EDS can demonstrate where the Bi and Fe located on the surface and the homogeneous distribution of elements.The result of XRD indicated that all films exhibit a single phase with the hexagonal structure and no impurity phase was observed.The crystal orientation of YBMFx films on the n+ silicon substrate is c-axis-orientation(as the films of testing electrical properties).However,there are two groups,highly c-axis-orientation(HCO)and no preferred orientation(NPO),corresponding to three lower doping samples and three higher doping samples for YBMFx films on the quartz substrates.Moreover,For the NPO,there is greater ups and downs of morphology than the regular and flatter surface morphology of HCO series.For h-YMO,the crystal orientation is c-axis-orientation,the valence of manganese ions is +3 and ferroelectricity at room temperature was verified.Peak positions of Mn 2p level move to higher energy with increasing concentration.Besides,there is an optimal doping concentration of x=1.5%for the YBMFx films by taking the shapes of P-E loops,the values of 2P,and 2Ec into consideration.The space charge limited current(SCLC)is considered as the dominating leakage mechanism of YBMFx films on the n+ silicon substrates in the high-electric-field region.(2)Spectra study on YBMFx films at room temperature:For YBMFx films on the quartz substrate,the transmittance spectra at room temperature pointed out that double energy gaps(-1.25 eV and-3.3 eV)corresponded to significant absorption edges,the band gap(Eg1)and subband gap(Eg2),respectively.The influences of different substrates on the electronic band structure have been investigated by spectroscopic ellipsometry(SE).It contributes to different constraint dielectric properities due to the superposition of dielectric constant about substrates.The vibration of films on Pt substrates is weaken than that on Si and quartz substrates.The SE spectra were studied with Adachi model with three-phase-layered structure(air/YBMFx/Pt)to obtained the dielectric functions and optical constants of YBMFx films deposited on Pt/TiO2/SiO2/Si substrates.The double energy gaps were further verified.UV-near infrared reflectance spectrum reflects significant vibration,which is difficult to distinguishing absorption and vibration.Lattice vibration behaviors of YBMFx films derived from far infrared reflectance spectra at room temperature.(3)Variable temperature transmission and temperature dependence of double energy gap:For YBMFx films on the quartz substrates,temperature dependent double energy gaps were investigated by the variable temperature transmission spectra.The Eg1 meets Katsufuji's model,but Eg2 does not meet Katsufuji's model.The abnormal shrinkage of Eg2 below 100 K can be explained by the pinning effect of composite domain walls and the accumulation of discrete oxygen vacancies at composite domain walls.The antiferromagnetic phase transformation(or relaxation)possibly has a significant influence on the Eg2.