Epitaxial Growth And Physical Properties Of Zno-based Magnetic Semiconductor And Multiferroic Oxide Heterostructures

An electron has two degrees of freedom:charge and spin. In conventional information technology we only took advantage of the charge property of electrons, while the spin degree of freedom was ignored. Adding the spin degree of freedom to conventional semiconductor charge-based electronics or using the spin degree of freedom alone will add substantially more capability and performance to electronic products. The advantages of these new devices would be nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices.In the past50-plus years, it is mainly external magnetic field that controls the magnetization orientation. Recently, new enabling phenomena blossom that permit spintronic functionality to prevailing in the absence of external magnetic fields. The new ways to control the magnetization orientation include control via electric fields and photonic fields. Multi-field control makes multifunctional materials more and more important. In this thesis, we mainly focus on multifunctional materials such as magnetic semiconductor and multiferroics.Magnetic field can be used to switch the magnetization of a memory element. This is an example of a tunneling magnetoresistance (TMR) effect. Magnetic tunnel junction (MTJ) devices are nonvolatile and non-destructive. Magnetic semiconductor based MTJ devices are easy to integrate with conventional semiconductor devices. One of the most popular diluted magnetic semiconductor (DMS) materials,(Ga,Mn)As, has already been investigated in the context of spintronics applications. Large TMR of a few hundred percent at4K, which can be amplified to1500-fold at1.7K, has been observed in a (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel junction. However, the curie temperature Tc of (Ga,Mn)As (～191K) is far below room temperature (RT). This inhibits its application in RT spintronics devices. For practical application, DMS materials with RT ferromagnetism are very demanding. As one of the most promising candidates to obtain RT ferromagnetism,(Zn,Co)O is intensely studied. In this thesis, high quality (Zn,Co)O films were grown on Al2O3(0001) substrates by oxygen plasma-assisted molecular beam epitaxy (OPAMBE). The effect of oxygen pressure and concentration of Co dopant on the crystal quality, morphology, magnetism and MR were studied. It shows that (Zn,Co)O films grown at substrate temperature of450°and oxygen partial pressure of～-8.0×10-7mbar exhibit flat surfaces and robust magnetization. Fully epitaxial ZnO-based (Zn,Co)O/(Zn,Mg)O/(Zn,Co)O MTJs were grown under optimal growth condition. The MR behavior and spin injection through (Zn,Mg)O barrier were investigated. An enhanced positive tunnel magnetoresistance (TMR) ratio of85.6%is observed at1.8T at5K, which can be attributed to the high quality of two epitaxial (Zn,Co)O/(Zn,Mg)O interfaces in our MTJs. The MR can be explained by the resultant contributions of both positive MR and the reduction of spin-flip scattering at interfaces. The junction resistance at zero magnetic field is linear with respect to temperature power law T-4/3between5K and70K, indicating that carriers tunnel through (Zn,Mg)O barrier via two localized states.Another new mean of manipulating magnetic moments and electron spins is electric field. One of the ways by which electric fields have recently been demonstrated to control magnetic properties is in multiferroic systems. Multiferroic systems couple magnetic and ferroelectric order parameters and are useful because electric fields can potentially be used to switch the magnetization. Multiferroics have aroused ever increasing interest worldwide. However, the preparation, physical properties and magneto-electric coupling of multiferroics are still under investigation. Multiferroic materials are classified into two main groups:single phase multiferroics and composite multiferroics. Until now, BiFeO3(BFO) is perhaps the only material that is both magnetic and strong ferroelectric at room temperature. For practical application, alternative artificial multiferroic composites are being extensively studied, which can be obtained by combining a ferroelectric phase and a ferromagnetic phase relying on strain, charge or magnetic interaction through the interface. These materials exhibit stronger ME coupling than single phase materials. Moreover, the ME coefficient is tunable by modulating the composite structures and component ratio. In this thesis, we prepared BFO thin films of different thickness on exact and miscut SrTiO3(STO)(001) and (111) substrates and systemically studied the structural and physical properties of BFO films. In addition, lead-free piezoelectric Na0.5K0.5NbO3(KNN) and CoFe2O4(CFO)/KNN films were deposited by radio-frequency magnetron sputtering. Their structural and physical properties have been studied.High quality BFO films can only be obtained in a narrow window, out of which Bi2O3, Bi2Fe4O9, Fe2O3may form. These impurity phases hinder the characterization of the intrinsic properties of BFO. In this thesis, High quality epitaxial BFO films were fabricated by OPAMBE on SrTiO3(STO)(001),(111) and vicinal STO (001) substrates at growth temperature of450°and oxygen partial pressure of～8.0×10-7mbar with a fixed Bi:Fe flux ratio of8:1. The structural, magnetic, ferroelectric and piezoelectric properties were systematically studied.For BFO films grown on TiO2-terminated STO (001) substrates, when the thickness is less than50nm, BFO is fully strained tetragonal phase with orientation relationship (001)[100]BFO||(001)[100]STO. In thicker BFO films (thickness>80nm), there is a little amount of tetragonal-like BFO phase with large c/a ratio (c/a-1.2) besides the rhombohedral-like BFO phase (c/a～1.04). BFO films have complex domain structure with four polarization variants and can be electrically switched by a voltage of10V. The leakage current of these BFO films is remarkably reduced. At an electric field of-100kV/cm, the leakage current density is～3.4×10-5A/cm2. Besides, bipolar resistance switching effect without forming process was observed in Pt/BFO/Nb-STO capacitors.The BFO growth on STO (111) substrate was three-dimensional as can be expected given the high bond density of the (111)p pseudocubic faces. XRD results indicate that BFO films grown on STO (111) are rhombohedral. Comparing to BFO/STO(001), BFO/STO(111) has smaller leakage current density, which is4.5×10-6A/cm2At an electric field of-100kV/cm, weaker frequency dependent dielectric constant and lower dielectric loss due to less space charge which induced by defects in BFO film, such as oxygen vacancies and/or Bi vacancies. Moreover, BFO/STO(111) exhibits a single domain behavior. Epitaxial BFO thin films are monoclinic MA when grown on TiO2-terminated vicinal STO (001) substrates. Leakage current measurements indicate that the leakage current density is largely reduced. At an electric field of-100kV/cm, the leakage current density is～2.81×10-7A/cm2The domain structure of BFO films is simplified by employing vicinal STO substrates, which only show two polarization variants. Weak room temperature ferromagnetism and periodic ordering doubled along the [001] direction are observed, which result from rotation of oxygen octahedral.Lead-free piezoelectric KNN films were deposited by radio-frequency magnetron sputtering at different termperatures. It indicates that KNN hardly crystallize below800℃. Perovskite phase can be obtained at growth temperature of800℃. The annealing treatment in air remarkably enhances the dielectric constant due to denser grains and recombination of oxygen vacancies. CFO/KNN multiferroic bilayers with different volume ratio were deposited on conductive Nb-STO (001) single-crystal substrates. Their structure, magnetic, dielectric and magnetodelectric propreties were studied. Results indicate that the CFO phase maintains its good magnetic properties in CFO/KNN bilayers, while the dielectric constant of CFO/KNN decreases with increasing content of CFO ascribed to lower dielectric constant and higher conductivity nature of CFO comparing with KNN. Noticeable magnetodielectric effect was observed, indicating the presence of magnetoelectric coupling in CFO/KNN films. The magnetodielectric ratio increases with increasing volume fraction of CFO layer, showing a maximum of about7%for0.6CFO/0.4KNN at10kOe at2kHz. The magnetodielectric phenomenon can be attributed to strain-mediated effect between ferromagnetic CFO layer and dielectric KNN layer.