The Epitaxial Growth, Magnetism, Transport And Photoluminescence Properties Of ZnO-Based Diluted Magnetic Semiconductors

Spintronics is a science and technology which uses the electron's intrinsic properties of charge and spin as the information carriers and makes it able to interactively control the behaviors of them in order to obtain novel functional devices with higher efficiency and lower energy consumption than the ordinary electronic devices. Spintronics includes spin polarized current generation, spin injection, spin transport, spin detection, spin control and so on, involves the fundamental research and application development, and has great research value and broad application prospects. At present some spintronic devices, such as the magnetic read-head based on giant magnetoresistance effect and tunneling magnetoresistance effect, have been applied in the fields of information science. Some novel spintronic devices have been designed in the laboratory, but still far from the practical application. Today people have paid more attention to the efficiency and energy consumption problems, and the development of spintronics can not satisfy the expectations of them. In addition, the development of spintronics also helps people to understand the physical phenomena in-depth, which will promote the progress of science.During the past ten years, magnetic semiconductors were the most popular research direction in spintronics. The raise of magnetic semiconductors is mainly due to the higher spin-polarization-charge injection efficiency of them than that of the conventional ferromagnetic metal in the docking process between spintronics and present semiconductor industry and technologies. It will improve the performance of the device, and also can make the spintronic devices rapidly applied into practice. There are many ways to obtain magnetic semiconductors, and doping transition metal elements in II-VII group, III-V group or IV compounds is a usual method. Mn doped GaAs system has been meticulously studied in the magnetic semiconductor researches, and its Curie temperature (ferromagnetism-paramagnetism transition temperature) is up to200K at present. Both theoretical and experimental results have rigorously confirmed that the origin of the ferromagnetism in GaMnAs is from the ferromagnetic exchange interaction between Mn ions mediated by the carriers (holes). However, since the Curie temperature is still lower than room temperature, the GaMnAs-based spintronic devices can't be used in practice. Therefore, looking for the magnetic semiconductor materials with higher Curie temperature is an important problem.ZnO is an important material with excellent luminescence and piezoelectric properties. In2000, the theoretical results predicted that ZnO-based magnetic semiconductor materials can be obtained with Curie temperature above room temperature by introducing transition elements in ZnO. So a lot of people joined into the research about ZnO-based magnetic semiconductor materials and related new devices, and magnetic, electrical and optical methods were used from various aspects in these studies. Although people have a preliminary consensus in some issues, the controversies are still significant. According to the present work, a series of problems still exist in ZnO-based magnetic semiconductor research:the natures of the samples often strongly depends on the preparation methods and preparation conditions, the comparability of the magnetic, transport and other different behaviors is poor, and the interpretation of them is difficult and divergent. To solve these problems, using molecular beam epitaxy method, we prepared a series of ZnO-based magnetic semiconductor with good comparability, and their physical properties were studied. The single crystal samples prepared with molecular beam epitaxy method have good crystal structure and pure composition, and the growth conditions can be flexibly and accurately controlled in the preparation process. The basic structure and composition properties also can be rapidly characterized in situ, and it can effectively reduce the number of uncontrollable factors. The works presented in this paper include the preparation, magnetic, photoluminescence and transport properties of single crystal structure Mn doped ZnO, Co doped ZnO and pure ZnO epitaxial thin films on Al2O3(0001) substrates. The works about the studies of the growth, structural, photoluminescence properties of ZnO and ZnCoO thin film on STO (SrTiO3) substrate with different sections and pretreatments were also present in this paper:●Single crystal structure Mn doped ZnO thin films have been prepared on Al2O3(0001) substrates using molecular beam epitaxy method. The as-grown Zn0.96Mn0.04O thin film prepared with the same preparation condition and the same doping level showed different magnetisms. Room temperature ferromagnetism was observed for some samples, and only paramagnetism was observed for the others. In order to study the origin of the ferromagnetism in ZnMnO thin films, one of the paramagnetic Zn0.96Mn0.04O samples was cut into several pieces, and they were post annealed under different conditions. The magnetic property, Hall effect and photoluminescence spectrum of these annealed pieces and as-grown piece were measured. The sample annealed under oxygen plasma flux showed room temperature ferromagnetism, whereas these samples annealed under zinc flux and ultra high vacuum still showed paramagnetism. For the ferromagnetic sample, we found that the ferromagnetism and photoluminescence emission located at3.32eV at5K appeared together. While after annealing under Zn flux, this ferromagnetic sample was changed into paramagnetic. The above emission is also observed in another room temperature ferromagnetic as-grown ZnMnO sample. Through the effect of annealing to the emission, and the temperature dependence of the location and intensity, this emission at3.32eV was attributed to neutral acceptor bound exciton recombination luminescence. Using of Arrhenius law on the integrated intensity fitting of this emission, we derived the binding energy of the acceptor to exciton as about8meV, and then the acceptor ionization energy was estimated as about53-80meV. Hall effect results showed that all the samples are n-type conduction. The appearances of ferromagnetism and acceptor-type defects at the same time show that acceptor is a key factor in the induction of ferromagnetism in ZnMnO system, and is consistent with the theoretical results.●ZnO and Co doped ZnO thin films were prepared on STO substrates with different sections and pretreatments using molecular beam epitaxy equipment, and the structural, magnetic and photoluminescence properties of them were studied. We found that the growth orientations of ZnO films on STO(100) and (110) substrates were affected by the selective etch to the substrates. On as-received STO(110) substrates,(10-12) orientation ZnO and Co doped ZnO films were obtained with inclined c-axis. The epitaxial relationship between the film and substrate was determined using RHEED (Reflection High-energy Electron Diffraction) and XRD (X-ray diffraction):ZnCoO[100]//STO[1-10], ZnCoO[101]//STO[001]. According to the phi scan results, the c-axis of (10-12)-oriented ZnCoO patches is along unique direction and no twin structures were found. At the same time, the full width at half-maximum of Omega rocking curve near ZnCoO (10-12) peak was only0.20degrees, indicating a good crystal quality of (10-12)-oriented ZnO films after doping. Whereas on STO(110) substrate with selective etch by HF buffer solution, the obtained ZnO films showed (0001) orientation, and the determined epitaxial relationship is ZnO[100]//STO[001] and ZnO[1-10]//STO[1-10]. The photoluminescence spectra of the three samples were measured at low temperature. A strong emission peak between3.35eV and3.36eV was observed at5.8K for all samples and attributed to neutral donor bound exciton recombination luminescence. In addition, an emission at3.32eV was observed for the (10-12)-oriented ZnO and ZnCoO films, and was attributed to acceptor bound exciton recombination luminescence, which may be associated with the ZnO stacking faults of c-axis inclined ZnO and ZnCoO films. For the (10-12)-oriented ZnCoO film, the d-d transition related emissions were observed at low temperature around1.87eV, indicating that the Co2+ions have substituted part of the Zn ions successfully. The M-H cure of this sample was measured and weak ferromagnetism was observed at room temperature. The origin of the ferromagnetism was interpreted by bound magnetic polarons model.We also found that the ZnO film grown on as-received unetched STO(100) substrate showed (0001) orientation, but (0001) and (11-20) orientation coexist for the ZnO film grown on selectively etched STO(100) substrates by HF buffer solution.●A series of Co doped ZnO epitaxial thin films were prepared on Al2O3(0001) substrates with the same doping level and under different oxygen partial pressure in growth. According to the magnetic measurement, the magnetism of the samples showed close relationship with the oxygen partial pressure:the Zn0.95Co0.05O thin film prepared at low partial pressure showed room temperature ferromagnetism and only paramagnetism was observed for the high oxygen pressure condition. Three typical Zn0.95Co0.05O thin film samples were chosen for further study. According to the real-time RHEED patterns, XRD θ-2θ scan and ω rocking curves, the three samples showed high quality single crystal structure. XPS (X-ray Photoelectron Spectroscopy) results show that the introduced Co elements in ZnO showed+2chemical state. According to the high quality structure and chemical states of Co elements, the Co element showed substitutional doping to the Zn sites, and we conclude that the origin of the ferromagnetism is intrinsic rather than from the Co oxide or metallic state Co elements. The electrical transport and spin-dependent transport properties of the three samples were measured. Variable range hopping dominates at low temperature and thermal excitation conduction gradually becomes important at higher temperature for the three samples. Using a single level donor model, the shallow donor concentration was derived from the carrier concentration and the activation energy of the donor. According to the poor average magnetic moment for per Co2+element, the magnetization of the Zn0.95Co0.005O thin films should come from the common contribution of amount of tiny ferromagnetic regions. The origin of each local ferromagnetic region can be described with bound magnetic polarons model, and the local ferromagnetic regions come from the ferromagnetic coupling between bound magnetic polarons due to the fluctuational distribution of them. Anomalous Hall effect is an important spin dependent transport phenomenon. According to the temperature dependence of the optimal hopping distance, saturation magnetization and saturation anomalous Hall resistivity, we can found that the change of saturation anomalous Hall resistivities were mainly determined by the optimal hopping distance, rather than the magnetization. The optimal hopping distance will affect the transfer of the spin-polarized charges at different temperatures:at low temperature, the spin-polarized charges can directly hop among different ferromagnetic regions and the spin polarization can be maintained during the transfer process, where spin-polarized charge related to the anomalous Hall effect can be observed at such condition; when the temperature rises to some points, such as160K, the spin polarization characteristics may lose because the optimal hopping distance became smaller than the average distance between local ferromagnetic regions, and the anomalous Hall effect will disappeared. This result showed that homogeneous and continuous ferromagnetic samples are necessary to keep the macroscopical spin polarization properties of the carriers in magnetic semiconductors. The magnetoresistance of the films prepared at different oxygen partial pressures were measured. Positive magnetoresistance was observed for the samples prepared at high oxygen pressure and negative magnetoresistance was observed for low oxygen pressure condition. They belonged to different mechanisms and showed no magnetic dependence.●Pure ZnO epitaxial thin films were prepared using molecular beam epitaxy method. No ferromagnetism was observed for the samples prepared at high oxygen pressure, whereas room temperature ferromagnetism was observed for the ZnO film prepared at low oxygen partial pressure. They all showed good single crystal structure. The saturation magnetic moment of the pure ZnO thin film is rather weaker than the ZnCoO thin films prepared at the same oxygen partial pressure. We measured the transport properties of this ferromagnetic ZnO film prepared at low oxygen partial pressure and it showed similar behaviors as the ZnCoO films. Variable range hopping dominates at low temperature and thermal excitation conduction gradually becomes important at higher temperatures. At low temperatures anomalous Hall effect was observed for the ferromagnetic sample and disappeared above about160K. For the same sample, negative magnetoresistance was observed at low temperatures and interpreted by the existence of doublet occupied states case in the local states.