Development of luminescent europium-based phosphor films.
Chong, Mun Kit.
Date of Issue2010
School of Electrical and Electronic Engineering
Photonics Research Centre
School of EEE, NTU
Phosphor material is a light-emitting material that shows great potential in many photonic applications such as electronic display devices, optoelectronic devices, and fluorescent lamps. Yttrium oxide doped with europium ions (Y2O3:Eu3+ material) is a well-known red-emitting phosphor material with the peak wavelength at about 613nm (5D0-7F2 emission transition). However, the emission efficiency of Y2O3:Eu3+ phosphor films is still low. Therefore, there is a demand to increase the efficiency of Y2O3:Eu3+ phosphor films. The low efficiency of phosphor films is associated with the process conditions, the composition of materials, and the morphology of films. The first objective of this work is to develop high efficiency Y2O3:Eu3+-based phosphor films by optimizing the process conditions and by using the co-dopants, namely Mg2+ and Al3+. Recently, zinc oxide doped with europium ions (ZnO:Eu3+ material) has attracted considerable interest for future photonic applications. The widespread use of the ZnO:Eu3+ material in practical applications relies on the energy transfer process from the ZnO to the Eu3+ ions. The energy transfer process in the ZnO:Eu3+ materials is usually mediated by the defect states. However, the efficiency of energy transfer process is still low and the mechanism is not well understood. Another problem related to the practical applications of ZnO:Eu3+ material is that the defect states in the ZnO:Eu3+ material that are involved in the energy transfer process are hard to control and reproduce. Therefore, the second objective of this work is to develop ZnO-based material systems which show efficient radiative energy transfer process from the ZnO films to the Eu3+ ions without the involvement of the defect states. This work began with the development of Y2O3:Eu3+ phosphor films by using the sol-gel process. The sol-gel process has three main advantages compared to other thin film preparation methods. These advantages include excellent material stoichiometry, inexpensive raw materials and equipment, and the flexibility to control the composition of materials. The development of efficient Y2O3:Eu3+ phosphor films by using the sol-gel process involved the optimization of several important process parameters such as concentration of Eu3+ dopants, annealing temperature, annealing environment, and annealing duration. The optimum concentration of Eu3+ dopant is found to be 12mole%Eu3+ (or Y1.76Eu0.24O3). The optimum annealing temperature has been received at 750°C. It has been found that annealing in the rapid thermal processor (RTP) and in vacuum environment is more efficient than that in oxygen and nitrogen atmosphere to eliminate H2O impurities and hence yields the Y2O3:Eu3+ phosphor films with higher photoluminescent (PL) intensity at wavelength of about 613nm (5D0-7F2 emission transition). The H2O impurities have been effectively eliminated after longer annealing time both in the RTP and in the furnace. Furthermore, two sol-gel precursors, namely yttrium 2-methoxyethoxide and yttrium (III) isopropoxide, were used in the fabrication of Y2O3:Eu3+ phosphor films. It has been found that the yttrium 2-methoxyethoxide is better than the yttrium (III) isopropoxide in terms of higher stability against moisture sensitivity and the ease of handling during the preparation. Subsequently, the sol-gel derived Y2O3:Eu3+ phosphor films were incorporated with the Mg2+ and Al3+ co-dopants in order to enhance the emission efficiency. The effect of Mg2+ and Al3+ co-dopants on the PL intensity of the Y2O3:Eu3+ phosphor films were investigated. Both the Mg2+ and Al3+ co-dopants have been found to further enhance the PL intensity of the Y2O3:Eu3+ phosphor films at wavelength of about 613nm (5D0-7F2 emission transition) at the optimum concentration of 7mole%Mg2+ and 2mole%Al3+, respectively. The enhancement of PL intensity by the Mg2+ and Al3+ co-dopants is explained in terms of the creation of defect states near the Y(4d+5s) conduction band, which overlap with the charge-transfer state (CTS) of Eu3+ ions. The overlap leads to CTS broadening and consequently induces higher absorption and hence the increase of the PL intensity. From the experiment results, a schematic energy band diagram of Y2O3:Eu3+ phosphor films has been constructed. Having achieved the first objective, the work continued to realize the second objective. In the second part of this work, the author developed the ZnO-based material systems which show efficient radiative energy transfer process from the ZnO films to the Eu3+ ions without the involvement of the defect states. The Zn1-xCdxO films (ZnO films with varied concentration of cadmium (Cd) dopants) and the ZnO films were used in two different experiments. In one experiment, the Y2O3:Eu3+ phosphor films were deposited on top of the Zn1-xCdxO films and this resulted in Y2O3:Eu3+/Zn1-xCdxO structure. By varying the concentration of Cd dopants, the peak wavelength of Zn1-xCdxO films was tuned from 381nm (x value is 0mole%) to 394nm (x value is 8mole%) so that the emission spectrum of Zn1-xCdxO films overlapped with the absorption spectrum of Eu3+ ions peaked at 394nm (7F0-5L6 absorption transition). The peak wavelength shifts because the bandgap of Zn1-xCdxO films takes an intermediate value between the bandgap of the ZnO and CdO, which is influenced by the concentration of Cd dopants (i.e. the x value of Zn1-xCdxO films). When the peak wavelength of Zn1-xCdxO films shifts towards the absorption wavelength of Eu3+ ions at 394nm, this results in larger spectral overlap and hence an increase of the PL intensity of Y2O3:Eu3+ phosphor films at about 611nm (5D0-7F2 emission transition). This is because the larger spectral overlap leads to more efficient radiative energy transfer process from the Zn1-xCdxO films to the Eu3+ ions of Y2O3:Eu3+ phosphor films. In another experiment, the Y2O3:Eu3+ phosphor films were deposited on top of the ZnO films to form the Y2O3:Eu3+/ZnO structure. The ultra-violet (UV) random lasing spectrum in the wavelength range of about 385–397nm is produced from the polycrystalline ZnO films with an optical excitation (Nd:YAG laser) at room temperature. In this experiment, the UV random lasing spectrum of ZnO films was tuned to overlap with the absorption spectrum of Eu3+ ions peaked at 394nm (7F0-5L6 absorption transition) by increasing the pump power of Nd:YAG laser. It has been found that increasing the pump power of Nd:YAG laser results in larger spectral overlap between the UV random lasing spectrum of ZnO films and the absorption spectrum of Eu3+ ions. The large spectral overlap leads to very efficient radiative energy transfer from the ZnO films to the Eu3+ ions of Y2O3:Eu3+ phosphor films. Hence, the UV random laser pumped red emission centered at about 611nm (5D0-7F2 emission transition) has been observed. A schematic energy diagram has been constructed to explain the radiative energy transfer process in the Y2O3:Eu3+/Zn1-xCdxO structure and the Y2O3:Eu3+/ZnO structure. As an additional work, the indium tin oxide (ITO) films were also developed by using radio-frequency (RF) magnetron sputtering. The process parameters such as substrate temperature, oxygen gas flow rate, post-annealing temperature, and post-annealing ambient for forming the ITO films were optimized. The ITO film post-annealed at 500°C in vacuum has a low resistivity of about 3.49×10-4 Ωcm. Comparison of preferred crystal growth orientation in the X-ray diffraction (XRD) characterization supports the model of oxygen diffusion from air into the ITO films during post-annealing. The diffusion of oxygen atoms from air results in (400) preferred crystal growth orientation. Moreover, the atomic force microscope (AFM) images reveal that the ITO films post-annealed in vacuum have larger grain size compared to those post-annealed in air. The resistivity of ITO films post-annealed in vacuum is lower because of reduced grain boundary scattering.
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