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Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation
Applied Surface Science 142 Ž1999. 233–236
Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation Yoichiro Nakanishi a,b,) , Aki Miyake a , Hiroko Kominami b, Toru Aoki b, Yoshinori Hatanaka a,b, Goro Shimaoka a b
a Research Institute of Electronics, Shizuoka UniÕersity, 3-5-1 Johoku, Hamamatsu 432-8011, Japan Graduate School of Electronic Science and Technology, Shizuoka UniÕersity, 3-5-1 Johoku, Hamamatsu 432-8011, Japan
Abstract The dependence of the structural, photoluminescent and cathodoluminescent properties of ZnO thin films deposited by electron beam evaporation on the preparation conditions has been investigated. Both as-deposited and annealed thin films deposited at substrate temperatures higher than 2008C showed c-axis orientation, and their crystallinity was improved with increasing annealing temperature. The films showed the emission with a peak at around 510 nm in photoluminescence ŽPL. and cathodoluminescence ŽCL. except for the film annealed at 8008C in air. The emission seems to be well-known blue-green emission due to ZnO:Zn phosphor. The strong green emission with a peak at around 540 nm was obtained from the film annealed at 8008C in air. The origin of the emission is not understood. The film showed CL luminance of about 60 cdrm2 under excitation of 2 kV, 400 mArcm2. Moreover, it showed CL under excitation even at 250 V without charging-up. q 1999 Elsevier Science B.V. All rights reserved. PACS: 78.60.Hk; 78.55.m; 85.45.Fd; 68.55.a Keywords: ZnO; Phosphor; Thin film; Photoluminescence; Cathodoluminescence; Electron beam evaporation
1. Introduction Field emission displays ŽFEDs. are promising for the displays with high-contrast, wide-view angle and low power consumption. Accordingly, many phosphors are being developed for full color FEDs w1,2x.
) Corresponding author. Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan. Telefax: q81-53-478-1346; E-mail: [email protected]
The properties required for phosphors in FEDs are low resistivity to suppress charging-up on the phosphor surface, stable surface for electron irradiation to minimize outgassing from phosphor layer in operation, low temperature quenching, quick response under pulse operation and fine particle for high resolution. These subjects are overcome by making phosphor layer a thin film, because the thin film phosphor has advantages such as high resolution, high contrast, good thermal conduction and low electrical resistance in the direction along thickness, no binder removal and relatively smooth and uniform.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 5 4 - 0
234
Y. Nakanishi et al.r Applied Surface Science 142 (1999) 233–236
325 nm of He–Cd laser and electron beam lower than 2 keV, respectively, at room temperature.
3. Results and discussion 3.1. Structural properties
Fig. 1. The XRD curves of ZnO films as-deposited and annealed at various temperatures for 1 h in air, where the substrate temperature during the deposition was 3008C.
ZnO:Zn is a typical phosphor for monochrome FEDs which shows blue-green emission, because of its low resistivity and stability for electron irradiation w3x. ZnO thin films have normally been prepared by sputtering technique using Zn or ZnO target. However, the sputtering damages the growing film which may lower the display quality. In this experiment, the preparation of ZnO thin films was carried out by electron beam evaporation technique. Their structural and luminescent properties are described in this paper.
2. Experimental The ZnO thin films were deposited on quartz glass substrates by electron beam evaporation using 4N–ZnO pellets. The substrate temperatures during the deposition were 200, 300 and 4008C. Thicknesses of the films were from 50 to 100 nm. After the deposition, the films were annealed at 600, 700 and 8008C for 1 h in air or Ar atmosphere. The vacuum pressure during the deposition was in the order of 10y5 Torr. The structural and optical properties were characterized by X-ray diffraction ŽXRD., Auger electron spectroscopy ŽAES. and spectral transmission measurements. Luminescent properties were characterized by the measurements of photoluminescence ŽPL. and cathodoluminescence ŽCL. under excitation with
It is thought that the emission of ZnO:Zn is concerned with interstitial Zn or O vacancy, because the phosphor is prepared by firing in reducing atmosphere w4x. Therefore, the characterization of the structural and optical properties is important to investigate the luminescent properties. Fig. 1 shows XRD curves of the films deposited at a substrate temperature of 3008C, then annealed at several temperatures in air. Single peak at 2 u s 34.48 shows the diffraction of ZnO Ž0002.. Accordingly, all the films in this figure have the orientation along c-axis of wurtzite structure like as the case of sputtering w5x. It is also seen from the figure that the diffraction intensity increases with increasing annealing temperature. Nearly the same tendencies were obtained at substrate temperatures of 200 and 4008C. Fig. 2 shows full width at half maximum ŽFWHM. of Ž0002. peak of ZnO films deposited at 3008C and annealed in air or Ar as a function of the annealing temperature. It is seen that the FWHM reduces with an increase annealing temperature and attains a minimum at 7008C. In case of films deposited at 200 and
Fig. 2. The FWHM of Ž0002. peak of ZnO films annealed in air or Ar as a function of the annealing temperature, where the substrate temperature was 3008C.
Y. Nakanishi et al.r Applied Surface Science 142 (1999) 233–236
235
Table 1 The OrZn compositional ratio of ZnO films shown in Fig. 2 obtained from AES measurement Atmosphere
Ar Air
Annealing temperature Ž8C. as-deposited
600
700
800
0.94 0.94
0.93 0.97
0.92 0.97
0.92 0.98
4008C substrate temperatures, the FWHM had minimum values when they were annealed at 8008C. This fact shows that the crystallinity of the ZnO film is improved by annealing. Moreover, it is also seen that the improvement of the crystallinity by the annealing at 8008C in air is better than that in Ar. Table 1 shows OrZn compositional ratio of ZnO films shown in Fig. 2 obtained from AES measurement. This result shows that the stoichiometry of the film is improved by the annealing at 8008C in air. Fig. 3 shows spectral transmittance of ZnO films as shown in Fig. 2. All the films show sharp absorption at around 370 nm which corresponds to the band edge of ZnO ŽEg s 3.37 eV.. In addition to this, low transmittance is also seen at the wavelength shorter than the absorption edge owing to too thin thickness Ž50 to 100 nm.. Furthermore, this sharp absorption suggests that impurities causing the absorption are very little in the films. Nearly the same results were also obtained at other substrate temperatures. The results from the above figures show that the ZnO film with good crystallinity and purity can be obtained by electron beam evaporation technique.
Fig. 3. Spectral transmittance of ZnO films shown in Fig. 2.
Fig. 4. The PL spectra of ZnO used as the evaporation source and ZnO:Zn.
3.2. Luminescent properties of ZnO thin films Fig. 4 shows PL spectra of ZnO used as the evaporation source in this experiment and ZnO:Zn phosphor used for vacuum fluorescent displays ŽVFDs.. The latter is produced in the Zn-rich form by firing in reducing atmosphere. Therefore, the emission peaked at around 510 nm is thought to have
Fig. 5. The PL spectra of ZnO films as shown in Fig. 2.
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Y. Nakanishi et al.r Applied Surface Science 142 (1999) 233–236
V at constant sample current. This result shows that the emission is obtained from the film without charging-up even at low excitation voltage of 250 V. The charging-up free might be due to the low resistivity of the ZnO thin film. The result shown in the figure is promising for FEDs by the excitation with low energy electrons although the luminance must be improved for practical use. The optimization of the preparation condition for high luminance and the characterization of the electrical properties are in progress. Fig. 6. Luminance vs. excitation voltage in cathodoluminescence of the ZnO film at 3008C and annealed at 8008C in air.
originated from Zn interstitial or oxygen vacancy w4x. However, the detail of the luminescence center is not understood clearly. On the other hand, as the ZnO pellet used in this experiment is prepared basically in stoichiometric, the origin of the luminescence center of the emission around 540 nm is also unknown, although it is considered to be due to lattice defects or a trace of impurities incorporated during the preparation. Fig. 5 shows PL spectra of ZnO films as shown in Fig. 2. The films including as-deposited film except for the case annealed at 8008C in air show the emission peaked at around 510 nm. Therefore, this emission suggests that the Zn-rich state is formed in the ZnO films deposited in high vacuum as in this experiment; furthermore, the Zn-rich state cannot be changed by the annealing at temperatures lower than 7008C even in air. The film annealed at 8008C in air shows an emission at 540 nm with strong emission intensity. This result shows that the Zn-rich situation is compensated by annealing above 8008C in air. However, the structure of the luminescence center corresponding at 540 nm is not understood, and the investigation of the structure as well as that of 510 nm is important. Fig. 6 shows luminance vs. excitation voltage lower than 2 kV in CL of the ZnO film deposited at 3008C and annealed at 8008C in air, where a sample current density was kept at 400 mArcm2 . It can be seen that a luminance of about 60 cdrm2 is obtained at 2 kV excitation. The luminance decreases to nearly linear, with decrease of the excitation voltage to 250
4. Summary The preparation of ZnO thin film, which is aimed at the application for FEDs, has been carried out by electron beam evaporation technique. All the films prepared in this experiment showed the c-axis orientation, and their crystallinity was improved by the annealing in Ar or air. Two kinds of emission were observed from the ZnO films in both PL and CL. One is the emission with a peak at around 510 nm which is attributed to Zn interstitial or oxygen vacancy, another with a peak at around 540 nm of which the origin of the luminescence center is not understood. A luminance of about 60 cdrm2 was obtained from the ZnO film annealed at 8008C in air under excitation at 2 kV, 400 mArcm2 . Moreover, CL emission is obtained even at the excitation voltage of 250 V without charging-up. These results are promising for high-resolution FEDs.
References w1x R.O. Peterson, Extended Abstracts of the First International Conference on the Science and Technology of Display Phosphors, San Diego, CA, 1995, p. 11. w2x S. Itoh, H. Toki, F. Kataoka, K. Tamura, Y. Sato, Extended Abstracts of the Third International Conference on the Science and Technology of Display Phosphors, Huntington Beach, CA, 1997, p. 275. w3x C.H. Seager, N.A. Missert, D.R. Tallant, W.L. Warren, Extended Abstracts of the Third International Conference on the Science and Technology of Display Phosphors, Huntington Beach, CA, 1997, p. 279. w4x A. Pfanel, J. Electrochem. Soc. 109 Ž1962. 502. w5x S. Takada, J. Appl. Phys. 73 Ž1993. 4739.
Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation Yoichiro Nakanishi a,b,) , Aki Miyake a , Hiroko Kominami b, Toru Aoki b, Yoshinori Hatanaka a,b, Goro Shimaoka a b
a Research Institute of Electronics, Shizuoka UniÕersity, 3-5-1 Johoku, Hamamatsu 432-8011, Japan Graduate School of Electronic Science and Technology, Shizuoka UniÕersity, 3-5-1 Johoku, Hamamatsu 432-8011, Japan
Abstract The dependence of the structural, photoluminescent and cathodoluminescent properties of ZnO thin films deposited by electron beam evaporation on the preparation conditions has been investigated. Both as-deposited and annealed thin films deposited at substrate temperatures higher than 2008C showed c-axis orientation, and their crystallinity was improved with increasing annealing temperature. The films showed the emission with a peak at around 510 nm in photoluminescence ŽPL. and cathodoluminescence ŽCL. except for the film annealed at 8008C in air. The emission seems to be well-known blue-green emission due to ZnO:Zn phosphor. The strong green emission with a peak at around 540 nm was obtained from the film annealed at 8008C in air. The origin of the emission is not understood. The film showed CL luminance of about 60 cdrm2 under excitation of 2 kV, 400 mArcm2. Moreover, it showed CL under excitation even at 250 V without charging-up. q 1999 Elsevier Science B.V. All rights reserved. PACS: 78.60.Hk; 78.55.m; 85.45.Fd; 68.55.a Keywords: ZnO; Phosphor; Thin film; Photoluminescence; Cathodoluminescence; Electron beam evaporation
1. Introduction Field emission displays ŽFEDs. are promising for the displays with high-contrast, wide-view angle and low power consumption. Accordingly, many phosphors are being developed for full color FEDs w1,2x.
) Corresponding author. Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan. Telefax: q81-53-478-1346; E-mail: [email protected]
The properties required for phosphors in FEDs are low resistivity to suppress charging-up on the phosphor surface, stable surface for electron irradiation to minimize outgassing from phosphor layer in operation, low temperature quenching, quick response under pulse operation and fine particle for high resolution. These subjects are overcome by making phosphor layer a thin film, because the thin film phosphor has advantages such as high resolution, high contrast, good thermal conduction and low electrical resistance in the direction along thickness, no binder removal and relatively smooth and uniform.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 5 4 - 0
234
Y. Nakanishi et al.r Applied Surface Science 142 (1999) 233–236
325 nm of He–Cd laser and electron beam lower than 2 keV, respectively, at room temperature.
3. Results and discussion 3.1. Structural properties
Fig. 1. The XRD curves of ZnO films as-deposited and annealed at various temperatures for 1 h in air, where the substrate temperature during the deposition was 3008C.
ZnO:Zn is a typical phosphor for monochrome FEDs which shows blue-green emission, because of its low resistivity and stability for electron irradiation w3x. ZnO thin films have normally been prepared by sputtering technique using Zn or ZnO target. However, the sputtering damages the growing film which may lower the display quality. In this experiment, the preparation of ZnO thin films was carried out by electron beam evaporation technique. Their structural and luminescent properties are described in this paper.
2. Experimental The ZnO thin films were deposited on quartz glass substrates by electron beam evaporation using 4N–ZnO pellets. The substrate temperatures during the deposition were 200, 300 and 4008C. Thicknesses of the films were from 50 to 100 nm. After the deposition, the films were annealed at 600, 700 and 8008C for 1 h in air or Ar atmosphere. The vacuum pressure during the deposition was in the order of 10y5 Torr. The structural and optical properties were characterized by X-ray diffraction ŽXRD., Auger electron spectroscopy ŽAES. and spectral transmission measurements. Luminescent properties were characterized by the measurements of photoluminescence ŽPL. and cathodoluminescence ŽCL. under excitation with
It is thought that the emission of ZnO:Zn is concerned with interstitial Zn or O vacancy, because the phosphor is prepared by firing in reducing atmosphere w4x. Therefore, the characterization of the structural and optical properties is important to investigate the luminescent properties. Fig. 1 shows XRD curves of the films deposited at a substrate temperature of 3008C, then annealed at several temperatures in air. Single peak at 2 u s 34.48 shows the diffraction of ZnO Ž0002.. Accordingly, all the films in this figure have the orientation along c-axis of wurtzite structure like as the case of sputtering w5x. It is also seen from the figure that the diffraction intensity increases with increasing annealing temperature. Nearly the same tendencies were obtained at substrate temperatures of 200 and 4008C. Fig. 2 shows full width at half maximum ŽFWHM. of Ž0002. peak of ZnO films deposited at 3008C and annealed in air or Ar as a function of the annealing temperature. It is seen that the FWHM reduces with an increase annealing temperature and attains a minimum at 7008C. In case of films deposited at 200 and
Fig. 2. The FWHM of Ž0002. peak of ZnO films annealed in air or Ar as a function of the annealing temperature, where the substrate temperature was 3008C.
Y. Nakanishi et al.r Applied Surface Science 142 (1999) 233–236
235
Table 1 The OrZn compositional ratio of ZnO films shown in Fig. 2 obtained from AES measurement Atmosphere
Ar Air
Annealing temperature Ž8C. as-deposited
600
700
800
0.94 0.94
0.93 0.97
0.92 0.97
0.92 0.98
4008C substrate temperatures, the FWHM had minimum values when they were annealed at 8008C. This fact shows that the crystallinity of the ZnO film is improved by annealing. Moreover, it is also seen that the improvement of the crystallinity by the annealing at 8008C in air is better than that in Ar. Table 1 shows OrZn compositional ratio of ZnO films shown in Fig. 2 obtained from AES measurement. This result shows that the stoichiometry of the film is improved by the annealing at 8008C in air. Fig. 3 shows spectral transmittance of ZnO films as shown in Fig. 2. All the films show sharp absorption at around 370 nm which corresponds to the band edge of ZnO ŽEg s 3.37 eV.. In addition to this, low transmittance is also seen at the wavelength shorter than the absorption edge owing to too thin thickness Ž50 to 100 nm.. Furthermore, this sharp absorption suggests that impurities causing the absorption are very little in the films. Nearly the same results were also obtained at other substrate temperatures. The results from the above figures show that the ZnO film with good crystallinity and purity can be obtained by electron beam evaporation technique.
Fig. 3. Spectral transmittance of ZnO films shown in Fig. 2.
Fig. 4. The PL spectra of ZnO used as the evaporation source and ZnO:Zn.
3.2. Luminescent properties of ZnO thin films Fig. 4 shows PL spectra of ZnO used as the evaporation source in this experiment and ZnO:Zn phosphor used for vacuum fluorescent displays ŽVFDs.. The latter is produced in the Zn-rich form by firing in reducing atmosphere. Therefore, the emission peaked at around 510 nm is thought to have
Fig. 5. The PL spectra of ZnO films as shown in Fig. 2.
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Y. Nakanishi et al.r Applied Surface Science 142 (1999) 233–236
V at constant sample current. This result shows that the emission is obtained from the film without charging-up even at low excitation voltage of 250 V. The charging-up free might be due to the low resistivity of the ZnO thin film. The result shown in the figure is promising for FEDs by the excitation with low energy electrons although the luminance must be improved for practical use. The optimization of the preparation condition for high luminance and the characterization of the electrical properties are in progress. Fig. 6. Luminance vs. excitation voltage in cathodoluminescence of the ZnO film at 3008C and annealed at 8008C in air.
originated from Zn interstitial or oxygen vacancy w4x. However, the detail of the luminescence center is not understood clearly. On the other hand, as the ZnO pellet used in this experiment is prepared basically in stoichiometric, the origin of the luminescence center of the emission around 540 nm is also unknown, although it is considered to be due to lattice defects or a trace of impurities incorporated during the preparation. Fig. 5 shows PL spectra of ZnO films as shown in Fig. 2. The films including as-deposited film except for the case annealed at 8008C in air show the emission peaked at around 510 nm. Therefore, this emission suggests that the Zn-rich state is formed in the ZnO films deposited in high vacuum as in this experiment; furthermore, the Zn-rich state cannot be changed by the annealing at temperatures lower than 7008C even in air. The film annealed at 8008C in air shows an emission at 540 nm with strong emission intensity. This result shows that the Zn-rich situation is compensated by annealing above 8008C in air. However, the structure of the luminescence center corresponding at 540 nm is not understood, and the investigation of the structure as well as that of 510 nm is important. Fig. 6 shows luminance vs. excitation voltage lower than 2 kV in CL of the ZnO film deposited at 3008C and annealed at 8008C in air, where a sample current density was kept at 400 mArcm2 . It can be seen that a luminance of about 60 cdrm2 is obtained at 2 kV excitation. The luminance decreases to nearly linear, with decrease of the excitation voltage to 250
4. Summary The preparation of ZnO thin film, which is aimed at the application for FEDs, has been carried out by electron beam evaporation technique. All the films prepared in this experiment showed the c-axis orientation, and their crystallinity was improved by the annealing in Ar or air. Two kinds of emission were observed from the ZnO films in both PL and CL. One is the emission with a peak at around 510 nm which is attributed to Zn interstitial or oxygen vacancy, another with a peak at around 540 nm of which the origin of the luminescence center is not understood. A luminance of about 60 cdrm2 was obtained from the ZnO film annealed at 8008C in air under excitation at 2 kV, 400 mArcm2 . Moreover, CL emission is obtained even at the excitation voltage of 250 V without charging-up. These results are promising for high-resolution FEDs.
References w1x R.O. Peterson, Extended Abstracts of the First International Conference on the Science and Technology of Display Phosphors, San Diego, CA, 1995, p. 11. w2x S. Itoh, H. Toki, F. Kataoka, K. Tamura, Y. Sato, Extended Abstracts of the Third International Conference on the Science and Technology of Display Phosphors, Huntington Beach, CA, 1997, p. 275. w3x C.H. Seager, N.A. Missert, D.R. Tallant, W.L. Warren, Extended Abstracts of the Third International Conference on the Science and Technology of Display Phosphors, Huntington Beach, CA, 1997, p. 279. w4x A. Pfanel, J. Electrochem. Soc. 109 Ž1962. 502. w5x S. Takada, J. Appl. Phys. 73 Ž1993. 4739.