Investigations On Defect Structure, Magnetic Manipulation And Photoelectric Response In Cu-doped ZnO Diluted Magnetic Semiconductor

ZnO is a Ⅱ-Ⅵ group compound semiconductor with wide direct bandgap of 3.37 eV at room temperature. Based on theoretical prediction of semiconductors, it is expected to exhibit room temperature ferromagnetism more easily than other host materials. Like Ⅲ-Ⅴ group compounds, e.g. GaAs and GaN, by means of doping and bandgap engineering, the optical, electrical and magnetic properties of ZnO can be tailored and has bright prospect in the field of light-emitting diode, photo detection and spintronics. The features of charge and spin in carriers are both required in future spintronic devices and thus doping into ZnO by transition metal ions is a general method for the development of diluted magnetic semiconductors. Over the past 20 years, teams of researchers have devoted themselves to exploring the origin of ferromagnetism. Even so, this domain has still been confused in the following problems, like low reproducibility, poor controllability, and magnetic phase and clustering induced complexity. The appearence of ZnO:Cu, can overcome the above obstacles conveniently and possess some specific advantages, mainly containing four aspects. First, Cu and its oxides are not ferromagnetic and thus do not affect magnetic analysis. Second, as a valence-variable and deep-level center, Cu impurity is highly sensitive to fluorescence excitation and electron paramagnetic resonance techniques, which can provide real-time tracking into impurity states. Third, Cu2+ has the mostcomparable ion size with Zn among all transition metal ions. Combined with non-equilibrium doping process, the introduction of a large number of Cu ions into ZnO host becomes possbile. At last, the combination between Cu impurity and intrinsic defect, resulting the formation of Cu-defect complex, can help to study their synergistic effect and corresponding defect ferromagnetism.In this paper, we carry out the research from the preparation of high-quality ZnO:Cii nanocrystals to colloid spin-coating up to post-treating process, obtaining different ferromagnetic films. According to the microstructuxal and defect composition analyses, we found several new magnetic phenomena based on this traditonal doped ZnO material. By integrating optical and electrical attribute into these magnetic films, the mutual manipulation among optical, electrical and magnetic function can be realized and we discuss them in detail. The obtained results are given as follows.(1) ZnO:Cu nanocrystals with tunable doping amount (0-4.0%) were prepared by non-equilibrium colloidal chemistry process and the synergistic doping role between Cu and N atoms could activate ferromaganetism of spin-coated films. We discussed the mechanism of Cu2+ involving into ZnO from an energetic standpoint and confirmed the essence of light doping in the core part and heavy doping in the shell part of nanocrystals with the help of Vo (oxygen vacancies), highlighting chemistry method related non-equilibrium doping strategy. After adoping a two-step amine activation route, including amine bath and post rapid annealing, the coordination environment of Cu impurities changed from interstitial sites (Cui) to substitutional sites (Cnzn). The involvement of Cu-N bonds enhanced the stability of Cu impurities and induced the occurrence of ferromagnetism with the highest moment value of 1.58μB/Cu, corresponding to the case of 0.2% Cu doping. We compared the difference of hybridization mechanism between polarized Zni (interstitial zinc) donor and No (N on O sites) acceptor defect and discussed the relationship between magnetism and defect polarization type, well understanding acceptor defect mediated magnetic exchange process.(2) The donor defect mediated ferromagnetism in ZnO:Cu (0.89% doping) films was realized by high temperature and equilibrium post-annealing procedures. We tuned annealing temperatures (600 and 900℃) and atmospheres (vaccum and oxygen) and boosted the in-difussion process of Cu impurities. All films exhibited room temperature ferromagnetism and the optimal annealing parameters was 900℃ vaccum annealing, which corresponds to the moment of 1.59μB/Cu. Using temperature-dependent photoluminesence (PL) and lifetime spectra, the positions of Cuzn activation state, singly-charged Vo and Zni defect were confirmed at 0.43 eV above valence band maximum,0.87 and 0.15 eV below conduction band minimum, respectively. Defect analytic results indicated that, the variation of Vo and Zni defect concentration caused the distinction of magnetic results. Especially, even if moment values were equal, the corresponding magnetic origins were possibly different. We started from Zni defect mediated short-ranged magnetic ordering, developing a new senario based on Vo and Zni defects both participating exchange process. Additionally, we also found harmful charge-transfer ferromagnetism and grain ripening inhomogeneity induced magnetic instability, helping us understand the diversity of defect ferromagnetism.(3) We adopted three ways including defect polarity, electrical and optical control to realize the manipulation of magnetic "on/off" states. The detailed control steps are the alternative introduction of donor (Vo and Zn;) and acceptor (No) defects, the concentration tuning of Ho (H on O sites) shallow donors by changing H plasma treating time, and the "on/off" experiments of UV light, respectively. After evaluating the merit and demerit of every method, we considered the optical manipulation as the promising way partially due to the easy operation. Besides the above, we also revealed some abnormal phenomena, for example, donor defect distribution induced magnetic coupling discrepancy, Ho donor induced giant moment (3.26,μB/Cu), and optical demagnetization occuring in defect-rich ZnO:Cu films. We made some attempt to explain these phenomena based on the careful structural characterizations, not only consolidating the exsited opinions but also proposing several new viewpoints.(4) We fabricated ZnO:Cu based double-band photodetecting prototype devices aiming at UV and visible light range via the control of doping content. The highest responsivity of 11.7 A/W to 365 nm UV light could be obtained in 0.2% doped ZnO:Cu film devices. For the case of visible light (410～800 nm),1.5% doped ZnO:Cu device showed a responsivity of 0.3 A/W. In this regard, the external quantum efficiency and optical gain of UV light were determined to be 1.5x10-4 and 18000, respectively. In contrast, the external quantum efficiency and optical gain of visible light was 3.7×10-5 and 20150, respectively. Even though the external quantum efficiency of visible light was not high enough, its gain exceeded the case of UV light. We calculated the absorption cross-section of visible light in films and it was up to be on the order of 10-15 cm-2, far larger than that of intrinsic defect in ZnO (10-18～10-17 cm-2). Such big photon absorption cross-section, along with a competitive trap hybridization relationship between Cu traps and intrinsic donor traps, codetermined an abnormal space charge limited (SCL) photoconduction behavior. At last, we put forward to some ways to improve the device performance and demonstrated the rationality of ZnO:Cu photo response from different perspectives.