Characterization of Eu-Doped SnO2 Thin Films Deposited by Radio ...

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bDepartment of Chemistry, Kookmin University, 862-2 Chongnung-dong, Seongbuk-gu, Seoul 136-702,. Korea. Eu-doped SnO2 thin films deposited by ...
Journal of The Electrochemical Society, 153 共4兲 H63-H67 共2006兲

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Characterization of Eu-Doped SnO2 Thin Films Deposited by Radio-Frequency Sputtering for a Transparent Conductive Phosphor Layer Do Hyung Park,a Yang Hwi Cho,a Young Rag Do,b and Byung Tae Ahna,*,z a

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea b Department of Chemistry, Kookmin University, 862-2 Chongnung-dong, Seongbuk-gu, Seoul 136-702, Korea Eu-doped SnO2 thin films deposited by radio-frequency 共rf兲 magnetron sputtering have been studied for the transparent conductive phosphor layer, which is transparent in visible light, electrically conductive, and luminescent. The resistivity of the SnO2 film increased as the firing temperature and Eu concentration increased. The film showed an excitation peak at 300 nm and an emission peak at 588 nm. The maximum photoluminescence and cathodoluminescence intensity was observed under conditions of 1.0 atom % Eu doping and a 1200°C firing temperature; the resistivity was 0.5 ⍀ cm and the transmittance was above 70%. The relation between the resistivity and cathodoluminescence intensity has been discussed. © 2006 The Electrochemical Society. 关DOI: 10.1149/1.2167953兴 All rights reserved. Manuscript submitted August 8, 2005; revised manuscript received September 26, 2005. Available electronically February 9, 2006.

Recently, rare-earth ion-doped semiconductors have been focused on because of their potential applications in thin-film electroluminescent devices and cathodoluminescent devices.1,2 In the advent of carbon-nanotube field emitters, there is a need to develop phosphors in which electron irradiation causes no accumulation of charges. Thus, phosphors should be deposited on an indium-tin oxide 共ITO兲 electrode, which, as shown in Fig. 1a, is a transparent and conductive material. In Fig. 1a, the light reflection is expected to an extent at the phosphor/ITO and air/ITO interfaces. We propose a transparent conductive phosphor 共TCP兲 layer that has similar luminescent properties to the phosphor and transparentconductive properties of transparent conductive oxide 共TCO兲 as shown in Fig. 1b. Using a TCP layer can eliminate a phosphorpowder printing process. In addition to process simplification, no less light reflection is expected because, as shown in Fig. 1b, the phosphor and TCO is one material. The reflection of the quartz/air, quartz/SnO2, and SnO2 /air interface is 0.035, 0.024, and 0.111, respectively. The TCP layer can also be utilized in a conventional phosphor/ TCO structure, as shown in Fig. 1c. Using the printing or sedimentation method with phosphor powder, the phosphor layer can have some void because phosphor powders vary in size and the powder layer varies in thickness. When a phosphor layer is irradiated by electrons, the luminescent light penetrates the ITO and the substrate. Generally, ITO is used only for a transparent electrode that has transparency in visible light and electrical conductivity. Some of the electrons that do not hit the phosphors disappear through ITO penetration 共Fig. 1a兲. These electrons are called damping electrons, and, as shown in Fig. 1c, with the TCP layer the damping electrons can be used for light emission. In Fig. 1c, luminescences 1 and 2 are emitted by the phosphor and TCP layer, respectively. This type of emission means that the TCP layer can increase the brightness, color purity, and rendering index. The TCP layer consists of a host material which contains the electrode, and a dopant material which contains the luminescent color. The host material was selected among TCOs and the dopant material was selected among color activators. In this study, SnO2 was selected as the host material and Eu2O3 was selected as the activator. We chose SnO2 because it is transparent in the visible wavelength range and electrically conductive. In addition, SnO2 films are chemically inert and scratch-resistant. The electrical conductivity in SnO2 film originates from lattice imperfections and oxygen

deficiencies.3 Moreover, SnO2 is an n-type semiconductor with a bandgap of 3.6 eV. There is also growing interest in SnO2 because it is a luminescent phosphor material by doping.4-6 The luminescence of SnO2 activated by Eu was first studied by Crabtree and later by Matsuoka et al.7-9 The TCP layer was fabricated by radio-frequency 共rf兲 magnetron sputtering with the target consisting of SnO2 and Eu2O3 pellets.

Experimental Eu-doped tin oxide thin films were deposited on quartz substrates by rf magnetron sputtering with an SnO2 pellet 共3 in. diam兲 from Kojundo Chemical Lab. Co. Ltd. with a purity of 99.99% and an Eu2O3 pellet 共0.5 cm diam兲. The doping concentration was controlled by varying the number of Eu2O3 pellets attached to the SnO2 pellet from one to six. Sputtering was carried out at room temperature with an rf power of 150 W. During the sputtering, the working pressure was 1 ⫻ 10−3 Torr and the Ar flow rate was 10 sccm. The substrate was rotated during the deposition to improve the film uniformity and was adjusted to room temperature by a water-cooled substrate holder. The thickness of the films was fixed to 300 nm. The firing temperature was varied from 1100 to 1300°C, and the firing time was 2 h in air. The phase and crystallographic structure were characterized by X-ray diffraction 共XRD兲 operated at 30 kV and at 60 mA with Cu K␣ 共␭ = 1.5405 Å兲 radiation. The 20 scanning ranged from 20 to 70°. The Eu concentration of Eu was measured by Auger electron spectroscopy 共AES兲. The optical transmission spectra were recorded on a UV-visible spectrometer in wavelengths ranging from 380 to 780 nm. The resistivity and sheet resistance were measured with a four-point probe. To investigate the luminescent properties, the photoluminescence 共PL兲 and PL excitation of the TCP layers were measured using a photon-counting spectrometer 共ISS PC1兲 operated at 500 W. The cathodoluminescence 共CL兲 was measured at a beam current density of 100 ␮A/cm2 and irradiation energy of 1.0 keV using a Kimball Physics FRA-2X1-2/EGPS-2X1 electron gun 共E-gun兲 system. The electron gun system was installed in a demountable ultrahigh vacuum chamber equipped with an inhouse CL spectrophotometer.

Results and Discussion * Electrochemical Society Active Member. z

E-mail: [email protected]

Figure 2a shows the XRD patterns of 1.0 atom % Eu-doped SnO2 thin films fired at 1100, 1200, and 1300°C for 2 h. All the

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Journal of The Electrochemical Society, 153 共4兲 H63-H67 共2006兲

Figure 1. Schmatic diagram of 共a兲 phosphor/ITO structure, 共b兲 new TCP layer structure, and 共c兲 phosphor/TCP layer structure 共luminescences 1 and 2 are emitted by phosphor and TCP layer, respectively兲.

diffraction lines are assigned to tetragonal rutile SnO2 phases of tin oxide. No characteristic peaks of impurities, such as europium oxide or other tin oxides, are observed. Moreover, the patterns at 1100 and 1200°C are almost the same, indicating that significant crystallization occurs above 1100°C. When the firing temperature is 1300°C, the SnO2 peaks become more sharp, possibly because the crystallinity of the films improves. Figure 2b shows the XRD patterns of the SnO2 thin films with various Eu concentrations after firing at 1200°C. The Eu concentrations in the thin films were examined by using Auger electron spectroscopy. The XRD patterns confirm that the SnO2 films have a tetragonal rutile structure. When the Eu concentration is above 3.0 atom %, the 共222兲 diffraction peak from the Eu2Sn2O7 phase, which has cubic structure, appears, indicating that the film contains a second phase. Figure 3 shows the surface morphologies of 共a兲 the as-deposited film, and of the films fired at 共b兲 1100°C, 共c兲 1200°C, and 共d兲 1300°C. The grain size changed from 50 to 300 nm as firing temperature changed from 1100 to 1300°C. The Eu concentration in the film is 1.0 atom %. Furthermore, although the grain size increases as the firing temperature increases, the film fired at 1300°C is opaque and has relatively low transmittance because of the rough surface and interface between the SnO2 and quartz. The film thickness is very similar 共about 300 nm兲 before and after firing. The Eu concentration had no effect on the morphology of the thin film. Figure 4a shows the transmittance spectra of the films as a function of the firing temperature in the visible wavelength range. The transmittance in the visible range is above 70%, though, as the temperature increases to 1300°C, the transmittance of the film drops

Figure 2. XRD patterns of SnO2:Eu thin films with various 共a兲 firing temperatures 共Eu = 1.0 atom %兲 and 共b兲 doping concentrations 共fired at 1200°C for 2 h兲.

significantly. Figure 4b shows the transmittance spectra of the films as a function of the Eu concentration after firing at 1200°C. The transmittance is unaffected by the variation in the Eu concentration and is only affected only by the firing temperature. Figure 5a shows the resistivity change of the film with 1.0 atom % Eu as a function of the firing temperature. As the firing temperature increases, the resistivity increases rapidly, thereby indicating that the SnO2 film becomes more stoichiometric. As the substrate temperature increases, the resistivity of undoped SnO2 thin films increases.10 Figure 5b shows the resistivity of the Eu-doped thin films after firing at 1200°C as a function of the Eu concentration. As the Eu concentration increases, the resistivity increases. The doping of a lower valency cation produces an acceptor level in the n-type semiconductor and increases the resistivity. In contrast, the replacement of a higher valence cation in the oxide films produces a donor level that increases the n-type conductivity. Figure 6 shows the PL excitation and PL spectra of 共a兲 and Eudoped SnO2 powder and 共b兲 the Eu-doped SnO2 thin film fired at 1200°C for 2 h. The Eu concentration in the powder and thin film is 1.0 atom %. The excitation peaks are at 330 nm for the SnO2 powder and at 300 nm for the thin film. The excitation peak of the thin film is 30 nm shorter than that of the SnO2 powder. The difference in the wavelength comes from the difference in crystallinity between the thin film and the powder. The thin film appears to have more bulk properties than the powder. As shown in Fig. 6, it has been observed that the thin film and powder showed very sharp luminescent spectra in the red region.

Journal of The Electrochemical Society, 153 共4兲 H63-H67 共2006兲

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Figure 3. SEM micrographs of SnO2:Eu thin films with various firing temperature: 共a兲 asdeposition, 共b兲 1100°C, 共c兲 1200°C, and 共d兲 1300°C.

Figure 4. Transmittance spectra of SnO2:Eu thin films with various 共a兲 firing temperatures 共Eu = 1.0 atom %兲 and 共b兲 doping concentrations 共asdeposited兲.

Figure 5. Resistivity of SnO2:Eu thin films with various 共a兲 firing temperatures 共Eu = 1.0 atom %兲 and 共b兲 doping concentrations 共fired at 1200°C for 2 h兲.

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Journal of The Electrochemical Society, 153 共4兲 H63-H67 共2006兲

Figure 6. Excitation spectrum and PL spectrum of 共a兲 SnO2:Eu powder and 共b兲 SnO2:Eu thin film 共fired at 1200°C for 2 h兲.

The large separation between the three strongest emission lines, which are assigned to the 5D0 → 7F1 magnetic-dipole transition, corresponds to a large 7F1 level splitting.7,11 According to Blasse, this large separation indicates that the Eu+3 site makes a considerable electrostatic contribution to the crystal field.11 A PL peak is also observed at the same wavelength of 588 nm for both the thin film and the powder. The PL intensity changed by firing temperature and doping concentration.12 Figure 7a shows the PL intensity of the 1.0 atom % Eu-doped SnO2 films as a function of the firing temperature. The PL intensity increases as the temperature increases, though at 1300°C the relative intensity decreases because of the chemical reaction between the SnO2 and the quartz substrate. Figure 7b shows the relative PL intensity of the SnO2 film as a function of the Eu concentration. Initially, the PL intensity increases as a result of the Eu doping, and the PL intensity approaches its maximum at an Eu concentration of 1.0 atom %. When the Eu concentration is greater than 1.0 atom %, the PL intensity decreases rapidly as the Eu concentration increases. Although the exact mechanism of the low PL intensity by high Eu doping is not yet clear, the decrease in the PL intensity above the 1.0 atom % Eu doping seems to be related to the concentration quenching. Figure 7a shows the CL intensity of the 1.0 atom % Eu-doped SnO2 film as a function of the firing temperature. The CL intensity increases as the temperature increases, though at 1300°C the relative intensity decreases because of the reaction between the SnO2 and the quartz substrate. Figure 7b shows the CL intensity as a function of the doping concentration. The CL intensity reaches its maximum at an Eu doping concentration of 1.0 atom %. The CL intensity decreases as the doping concentration increases. Note that the variation trend of the

Figure 7. PL and CL maximum intensity of SnO2:Eu thin films with various 共a兲 firing temperatures 共Eu = 1.0 atom %兲 and 共b兲 doping concentrations 共fired at 1200°C for 2 h兲.

CL intensity is similar to that of the PL intensity. Therefore, the optimum Eu concentration for the best CL intensity in SnO2 is 1.0 atom %. As shown in Fig. 5, the resistivity increases as the Eu doping increases. The excited electrons and holes from the SnO2 matrix cannot be transferred to Eu sites fast enough, most likely because of the high resistivity, and they recombine in the matrix rather than at the Eu sites. When we compare the resistivity trend in Fig. 5 with the PL and CL intensity trend in Fig. 7, we can see that the PL and CL intensity decrease as the resistivity increases, thereby suggesting that a further reduction of resistivity might be necessary to achieve a higher PL intensity. To reduce the resistivity of SnO2, the annealing of SnO2 in H2 or doping by atoms with higher valence atoms can be considered. However, the H2 annealing has an adverse effect on the resistivity and PL and CL intensity. To reduce the resistivity in SnO2, 1.0 atom % Sb or V atoms are additionally doped into the SnO2. The defects formed by replacing the Sn+4 site with the Sb+5 or V+5 ion are equal to a monovalent cation bound by an electron that forms the donor level.13,14 With the Sb or V codoping with Eu, the resistivity is lowered 共Fig. 5a兲, though the CL intensity of the Sb or V codoped SnO2 decreases to less than half the CL intensity of the Eu-doped SnO2. The doping of Sb or V adversely affects the point of luminescence. With n-type doping, the reduced resistivity fails to produce the expected high PL intensity. Although the phenomena of conductivity and luminescence correlate quite strongly, we cannot clearly describe the exact relation at this stage.

Journal of The Electrochemical Society, 153 共4兲 H63-H67 共2006兲 Conclusions In this work, the concept of the TCP layer has been introduced for the first time. Eu-doped SnO2 thin films were deposited by rf magnetron sputtering for the TCP layer. As the Eu doping concentration and firing temperature increase, the resistivity increases. The excitation and the emission peaks of the film were at 300 and 588 nm, respectively. The maximum PL/CL intensity was obtained when the Eu concentration was 1.0 atom % and when the firing temperature was around 1200°C, in which case the resistivity was 0.5 ⍀ cm and the optical transmittance was greater than 70%. The PL and CL intensity decrease as the Eu concentration increases above 1.0 atom %, probably due to the concentration quenching. Moreover, lowering the resistivity by Sb or V codoping fails to improve the PL and CL intensity.

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Korea Advanced Institute of Science and Technology assisted in meeting the publication costs of this article.

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