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Photoluminescence spectroscopy of highly oriented Y2O3:Tb crystalline whiskers
Science and Technology of Advanced Materials 6 (2005) 215–218 www.elsevier.com/locate/stam
Photoluminescence spectroscopy of highly oriented Y2O3:Tb crystalline whiskers Yuko Satoh, Shigeo Ohshio, Hidetoshi Saitoh* Department of Chemistry, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-2188, Japan Received 15 September 2004; revised 20 October 2004; accepted 24 November 2004
Abstract The crystalline state of terbium-containing yttria is one of the important candidates for uses to ultraviolet- and electron-excited green phosphors. To increase the intensity of green emission, structural design of the polycrystalline Y2O3:Tb was carried out using a chemicalvapor-deposition technique operated under atmospheric pressure. The green luminescence intensity was strongly dependent upon the concentration of Tb. The intensity of the photoluminescence at 542 nm obtained from h100i oriented Y2O3:Tb whiskers was higher than that obtained from the uniform Y2O3:Tb polycrystalline film with random orientation. q 2005 Elsevier Ltd. All rights reserved. Keywords: Y2O3:Tb; CVD; Orientation; Whisker; Phosphor; Polycrystal; Photoluminescence
1. Introduction Photoluminescence of metal oxides has been widely investigated by numerous research groups. In particular, rare-earth-containing yttria (Y2O3:RE) is one of the most important metal oxides for luminescence study due to its advantages such as high luminescence efficiency, capability of doping for several rare-earths and ability of excitation by ultraviolet or electron irradiation. West et al. produced photoluminescent thin films of Y2O3:Eu by low-pressure metalorganic chemical-vapor-deposition (CVD). They measured the photoluminescent lifetimes of the films as a function of temperature using excimer laser excitation at 248 nm. Subsequent annealing at 1200 8C in air enhances and stabilizes the luminescence [1]. In addition, Cho et al. demonstrated that the increase in brightness is attributed to the rough surface morphology which results in reduced internal reflections using high quality Y2O3:Eu phosphor films deposited on the diamond-coated silicon substrates [2]. Furthermore, Goldburt et al. found that the luminescent efficiency of the nanocrystalline Y 2O 3:Tb phosphors increased with
˚. the decrease in the particle size from 100 to 40 A This correlation was obtained from microstructural studies performed using transmission electron microscopy and luminescent measurements [3]. We have developed a CVD technique operated under atmospheric pressure for growth of amorphous and crystalline metal oxide films such as ZnO [4,5], ZnO:Al [6] and TiO2 [7–10]. Our CVD setup is fundamentally a type of thermal CVD. However this setup possesses the abilities of design of microarchitechture of the metal oxide crystallites, epitaxy and non-epitaxy process with high growth rate, operation without vacuum system. For example, highly oriented ZnO epitaxial whiskers are the product of our CVD method [4,5]. In this study, random oriented Y2O3:Tb polycrystalline films and highly oriented Y2O3:Tb whiskers were synthesized using the CVD technique operated under atmospheric pressure. The analytical results for morphology, crystalline orientation and photoluminescence characteristics of the crystalline samples are described and discussed.
2. Experimental * Corresponding author. Tel.: C81 258 47 9316; fax: C81 258 47 9300. E-mail address: [email protected] (H. Saitoh).
1468-6996/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.stam.2004.11.017
The single crystalline wafer of (100) silicon substrate was cut into 10!10!0.5 mm3 and then washed with deionized water for 30 min. After the treatment, the single
Y. Satoh et al. / Science and Technology of Advanced Materials 6 (2005) 215–218
3. Results and discussion
Heater Fig. 1. Schematic diagram of atmospheric CVD apparatus used in this study.
crystal was ultrasonically cleaned sequentially with acetone and methanol. The polycrystalline films and whiskers of Y2O3:Tb were prepared using an atmospheric CVD apparatus as shown in Fig. 1, that was previously employed for fabrication of the epitaxial anatase films [7–10], using titanium tetra-isopropoxide as the source complex. The reactants, yttrium dipivaloyl methanato (Y(DPM)3, yttrium tris(2,2,6,6-tetramethyl)-3,5-heptanedionate, C33H57O6Y, Showa Denko Co., quoted purity of 99.9%) and terbium dipivaloyl methanato (Tb(DPM)3, terbium tris(2,2,6,6tetramethyl)-3,5-heptane-dionate, C33H57O6Tb, Showa Denko Co., quoted purity of 99.7%) were loaded into a vaporizer and vaporized using an electric heater. The inside temperature of the vaporizer measured using a K-type thermocouple is defined as the vaporizing temperature. The vaporizing temperature was kept constant at 180–210 8C. The reactant vapor was first carried by nitrogen gas flowing at a rate of 1.2 dm3/min and then sprayed from the metallic nozzle directly onto the single crystalline (100) silicon substrate mounted on the electric heater. The surface temperature measured using the K-type thermocouple is defined as the substrate temperature. The reactant vapor was immediately decomposed by thermal energy from the substrate heater to form polycrystalline films. The deposition duration of Y2O3:Tb crystallites was varied for 20–30 min for each experiment to maintain a film thickness of 5 mm using a metallic shutter placed below the nozzle. The substrate was heated to 680 8C using the electric heater. The distance between the nozzle and the substrate was maintained at 15 mm throughout the experiments. X-ray diffraction analysis (using M03XHF, Mac Science Co.) was performed to reveal the crystal structure and the growth direction. The surface and cross-sectional morphology of the films was observed by field-emission scanning-electronmicroscopy (FE-SEM, using JSM 6700F, JEOL). The photoluminescence spectrum of the sample was measured at room temperature (using a spectrometer, FP 6500DS, JASCO Co.). The metal composition of the samples was determined using inductive coupled plasma-atomic emission spectroscopy (ICP–AES) with a spectrometer, SPS400, Seiko Instruments Co.
* Substrate (a)
2.5 at.% (800)
T.C. T.C.
* 2.0 at.% *
0.0 at.%
* 40 60 2 [degree]
20
(b)
80
2.8 at.% *
(800)
Slit Substrate:Si(100)
(622)
Vaporizer
Nozzle
T.C.
(622)
Y( Y(DPM) 3
(440)
Tb(DPM)3
To obtain the growth with preferential orientation, the Y2O3:Tb films were synthesized at the temperature region in which inhomogeneous gas reaction occurs. As the surface reaction was dominant, a few polycrystalline films demonstrated preferential orientation as shown in Fig. 2. All these samples formed on the silicon substrate had defective fluorite structure having relatively strong intensity at 29.2, 33.8, 48.5, 57.6 and 71.18 corresponding to (222), (400), (440), (622) and (800) diffraction lines. No diffraction lines indexed as the polycrystalline state of terbium oxide were observed. For the film grown at a vaporizing temperature of 180 8C, the intensity ratio of each diffraction line was nearly same as that of reference powder diffraction (ICDD Card No. 41–1105). The pattern obtained at a growth temperature of 210 8C was completely different from the powder pattern. The peak intensity of the (222) diffraction line decreased while the peak intensity of the (400) diffraction line increased, indicating preferential orientation of h100i crystalline direction of Y2O3:Tb. In addition, full width at half maximum for all diffraction lines of the sample grown at 210 8C was narrower than those of the sample deposited at 180 8C.
(440)
Heating tape
(400)
T.C.
(400)
1.2 dm 3 /min
(222)
L-N2Trap
(222)
Flow meter
Intensity [a.u]
N2
Intensity [a.u]
216
1.5 at.%
0.0 at.% *
20
40 60 2 [degree]
80
Fig. 2. XRD patterns of Y2O3:Tb films obtained at various temperatures: (a) 180 8C and (b) 210 8C.
Y. Satoh et al. / Science and Technology of Advanced Materials 6 (2005) 215–218
217
Fig. 3. SEM micrographs of (a) cross-section and (b) top view of Y2O3:Tb samples deposited on (100)Si at the vaporizing temperatures of 180 8C.
Fig. 4. SEM micrographs of (a) cross-section and (b) top view of Y2O3:Tb samples deposited on (100)Si at the vaporizing temperatures of 210 8C.
The degree of orientation estimated by the Lotgering orientation factor fhkl defined as
These results suggest that the intensity is strongly dependent upon the concentration of Tb. Fig. 6 shows relative intensity of the luminescence at 542 nm for random oriented films and h100i oriented whiskers. The film thickness and the whisker length were kept constant at 5 mm. The intensity of the photoluminescence obtained from h100i oriented Y2O3:Tb whiskers was higher than that obtained from the uniform polycrystalline film with random
(1)
where Phkl and P0 are the peak intensity ratio of oriented direction to all directions of oriented sample, and that of powder of the standard specimen, respectively. Therefore, f100 and f111 were calculated using P400 and P222, respectively. The pattern obtained from the sample at a growth temperature of 210 8C showed f 100Z0.85, while that obtained from the sample with a growth temperature of 180 8C showed f100Z0.38 indicating that the former one is preferentially oriented to h100i crystalline direction. Figs. 3 and 4 show a series of the SEM micrographs of the cross-section and the surface of the samples. The average grain size of the crystallites in the sample obtained at 180 8C is 5 mm. The growth orientation and shape of the crystallites are slightly random for each crystallite. However, the shape of the crystallite is uniform whisker. The size of the whisker of the sample obtained at 210 8C was determined as 1.5 mm and 5 mm in diameter and length, respectively. The size was confirmed by the top view and along to growth direction. The growth orientation is relatively aligned among whiskers. Fig. 5(a) and (b) illustrates the photoluminescence spectra of the Y2O3:Tb films obtained at 180 and 210 8C, respectively. The peaks at 542 nm assigned to 5D4/7F5 transition as well as the most intense one at 484 nm assigned to 5D4/7F6 transition were seen on the spectrum.
(a) 2.5 at.%
2.0 at.% PL Intensity [a. u. ]
fhkl Z ðPhkl K P0 Þ=ð1 K P0 Þ;
0.0 at.%
(b)
2.8 at.%
1.5 at.%
0.0 at.% 350
400
450
500 550 600 Wavelength [nm]
650
700
Fig. 5. Photoluminescence spectra of Y2O3:Tb samples with (a) random orientation and (b) preferential orientation to h100i.
218
Y. Satoh et al. / Science and Technology of Advanced Materials 6 (2005) 215–218
<100> orientated Y2 O 3 :Tb whiskers
100 Relative intensity [%]
that although random oriented film possesses the maximum intensity at the Tb concentration of 2.0 at.%, the intensity keeps increasing to 1.5 at.% for the h100i oriented whiskers.
80 60
Acknowledgements
40 20
Random oriented Y2 O 3 :Tb
0 0
1
2 3 Tb concentration [at.%]
4
5
This work was partially supported by a Grant-in Aid for Scientific Research (A) and Exploratory Research, the Ministry of Education, Science, Sports and Culture contract Nos. 16206068 and 14655269. The authors also thank to the support by the 21st century COE program at Nagaoka University of Technology.
Fig. 6. Relative intensity of Y2O3:Tb at 542 nm.
orientation. Although the random oriented film possesses the maximum intensity at the Tb concentration of 2.0 at.%, the intensity keeps increasing to 1.5 at.% for the h100i oriented film. To our knowledge, the relationship between the green luminescence properties of Y2O3:Tb and Tb concentration has been still unknown. However, the optimum Eu concentrations in obtaining intense luminescence from the Y2O3:Eu whiskers is also different from that obtained at the uniform films [11]. The luminescence characteristic of the h100i oriented Y2O3 whisker with heavy dope of Tb may be different from that of the crystalline particle of Y2O3:Tb. First, the whiskers include more doping element into the lattice. For example, the ZnO whiskers have an Al solubility of more than 2 at.%, which is higher than that observed in the polycrystalline ZnO films [12]. Substitutional doping of Tb to the optimum lattice position is a possible reason to introduce intense luminescence. Second, microarchitecture of the aggregation of the Y2O3 crystallites may assist to increase photoluminescence. Although the total volume of the crystal of the aggregation of the whiskers is less than that of the uniform film, the surface area directly irradiated by the UV lamp on the whisker sample is larger than that of the film. Further investigation is required to clarify the luminescence mechanism of the whisker.
4. Conclusion The random oriented Y2O3:Tb polycrystalline films and highly oriented Y2O3:Tb whiskers were synthesized using the CVD technique operated under atmospheric pressure. The photoluminescence intensity of Y2O3:Tb polycrystalline films and highly oriented Y2O3:Tb whiskers indicated
References [1] G.A. West, K.W. Beeson, Low-pressure metalorganic chemical vapor deposition of photoluminescent Eu-doped Y2O3 films, J. Mater. Res. 5 (1990) 1573–1580. [2] K.G. Cho, D. Kumar, D.G. Lee, S.L. Jones, P.H. Holloway, R.K. Singh, Improved luminescence properties of pulsed laser deposited Eu:Y2O3 thin films on diamond coated silicon substrates, Appl. Phys. Lett. 71 (1997) 3335–3337. [3] E.T. Goldburt, B. Kulkarni, R.N. Bhargava, J. Taylor, M. Libera, Size dependent efficiency in Tb doped Y2O3 nanocrystalline phosphor, J. Lumi. 72 (1997) 190–192. [4] M. Satoh, N. Tanaka, Y. Ueda, S. Ohshio, S. Saitoh, Epitaxial growth of zinc oxide whiskers by chemical-vapor deposition under atmospheric pressure, Jpn. J. Appl. Phys. 38 (1999) L586–L589. [5] H. Saitoh, M. Satoh, N. Tanaka, Y. Ueda, S. Ohshio, Homogeneous growth of zinc oxide whiskers, Jpn. J. Appl. Phys. 38 (1999) 6873– 6877. [6] Y. Ohkawara, T. Naijo, T. Washio, S. Ohshio, H. Ito, H. Saitoh, Field emission properties of Al:ZnO whiskers modified by amorphous carbon and related films, Jpn. J. Appl. Phys. 40 (2001) 7013–7017. [7] S. Tokita, N. Tanaka, H. Saitoh, High-rate epitaxy of anatase fims by atmospheric chemical vapor deposion, Jpn. J. Appl. Phys. 39 (2000) L169–L171. [8] N. Tanaka, S. Ohshio, H. Saitoh, Preferential orientation of titanium dioxide polycrystalline films using atmospheric CVD technique, J. Ceram. Soc. Jpn 105 (1997) 551–554. [9] H. Saitoh, S.H. Sunayama, N. Tanaka, S. Ohshio, Initial growth process of epitaxially chemical-vapor-deposited titanium dioxide crystalline films, Mater. Sci. Eng. Serving Soc. (1998); 165–168. [10] H. Saitoh, H. Sunayama, N. Tanaka, S. Ohshio, Epitaxial growth mechanism of chemical-vapor-deposited anatase on strontium titanate substrate, J. Ceram. Soc. Jpn. 106 (1998) 1051–1055. [11] Y. Satoh, S. Ohshio, H. Saitoh, Photoluminescence properties of Y2O3:Eu whiskers with preferential h100i orientation, Jpn. J. Appl. Phys. 41 (2002) L1253–L1255. [12] H. Saitoh, Y. Namioka, H. Sugata, S. Ohshio, Nanoindentation analysis of Al:ZnO epitaxial whiskers, Jpn. J. Appl. Phys. 40 (2001) 6024–6028.
Photoluminescence spectroscopy of highly oriented Y2O3:Tb crystalline whiskers Yuko Satoh, Shigeo Ohshio, Hidetoshi Saitoh* Department of Chemistry, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-2188, Japan Received 15 September 2004; revised 20 October 2004; accepted 24 November 2004
Abstract The crystalline state of terbium-containing yttria is one of the important candidates for uses to ultraviolet- and electron-excited green phosphors. To increase the intensity of green emission, structural design of the polycrystalline Y2O3:Tb was carried out using a chemicalvapor-deposition technique operated under atmospheric pressure. The green luminescence intensity was strongly dependent upon the concentration of Tb. The intensity of the photoluminescence at 542 nm obtained from h100i oriented Y2O3:Tb whiskers was higher than that obtained from the uniform Y2O3:Tb polycrystalline film with random orientation. q 2005 Elsevier Ltd. All rights reserved. Keywords: Y2O3:Tb; CVD; Orientation; Whisker; Phosphor; Polycrystal; Photoluminescence
1. Introduction Photoluminescence of metal oxides has been widely investigated by numerous research groups. In particular, rare-earth-containing yttria (Y2O3:RE) is one of the most important metal oxides for luminescence study due to its advantages such as high luminescence efficiency, capability of doping for several rare-earths and ability of excitation by ultraviolet or electron irradiation. West et al. produced photoluminescent thin films of Y2O3:Eu by low-pressure metalorganic chemical-vapor-deposition (CVD). They measured the photoluminescent lifetimes of the films as a function of temperature using excimer laser excitation at 248 nm. Subsequent annealing at 1200 8C in air enhances and stabilizes the luminescence [1]. In addition, Cho et al. demonstrated that the increase in brightness is attributed to the rough surface morphology which results in reduced internal reflections using high quality Y2O3:Eu phosphor films deposited on the diamond-coated silicon substrates [2]. Furthermore, Goldburt et al. found that the luminescent efficiency of the nanocrystalline Y 2O 3:Tb phosphors increased with
˚. the decrease in the particle size from 100 to 40 A This correlation was obtained from microstructural studies performed using transmission electron microscopy and luminescent measurements [3]. We have developed a CVD technique operated under atmospheric pressure for growth of amorphous and crystalline metal oxide films such as ZnO [4,5], ZnO:Al [6] and TiO2 [7–10]. Our CVD setup is fundamentally a type of thermal CVD. However this setup possesses the abilities of design of microarchitechture of the metal oxide crystallites, epitaxy and non-epitaxy process with high growth rate, operation without vacuum system. For example, highly oriented ZnO epitaxial whiskers are the product of our CVD method [4,5]. In this study, random oriented Y2O3:Tb polycrystalline films and highly oriented Y2O3:Tb whiskers were synthesized using the CVD technique operated under atmospheric pressure. The analytical results for morphology, crystalline orientation and photoluminescence characteristics of the crystalline samples are described and discussed.
2. Experimental * Corresponding author. Tel.: C81 258 47 9316; fax: C81 258 47 9300. E-mail address: [email protected] (H. Saitoh).
1468-6996/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.stam.2004.11.017
The single crystalline wafer of (100) silicon substrate was cut into 10!10!0.5 mm3 and then washed with deionized water for 30 min. After the treatment, the single
Y. Satoh et al. / Science and Technology of Advanced Materials 6 (2005) 215–218
3. Results and discussion
Heater Fig. 1. Schematic diagram of atmospheric CVD apparatus used in this study.
crystal was ultrasonically cleaned sequentially with acetone and methanol. The polycrystalline films and whiskers of Y2O3:Tb were prepared using an atmospheric CVD apparatus as shown in Fig. 1, that was previously employed for fabrication of the epitaxial anatase films [7–10], using titanium tetra-isopropoxide as the source complex. The reactants, yttrium dipivaloyl methanato (Y(DPM)3, yttrium tris(2,2,6,6-tetramethyl)-3,5-heptanedionate, C33H57O6Y, Showa Denko Co., quoted purity of 99.9%) and terbium dipivaloyl methanato (Tb(DPM)3, terbium tris(2,2,6,6tetramethyl)-3,5-heptane-dionate, C33H57O6Tb, Showa Denko Co., quoted purity of 99.7%) were loaded into a vaporizer and vaporized using an electric heater. The inside temperature of the vaporizer measured using a K-type thermocouple is defined as the vaporizing temperature. The vaporizing temperature was kept constant at 180–210 8C. The reactant vapor was first carried by nitrogen gas flowing at a rate of 1.2 dm3/min and then sprayed from the metallic nozzle directly onto the single crystalline (100) silicon substrate mounted on the electric heater. The surface temperature measured using the K-type thermocouple is defined as the substrate temperature. The reactant vapor was immediately decomposed by thermal energy from the substrate heater to form polycrystalline films. The deposition duration of Y2O3:Tb crystallites was varied for 20–30 min for each experiment to maintain a film thickness of 5 mm using a metallic shutter placed below the nozzle. The substrate was heated to 680 8C using the electric heater. The distance between the nozzle and the substrate was maintained at 15 mm throughout the experiments. X-ray diffraction analysis (using M03XHF, Mac Science Co.) was performed to reveal the crystal structure and the growth direction. The surface and cross-sectional morphology of the films was observed by field-emission scanning-electronmicroscopy (FE-SEM, using JSM 6700F, JEOL). The photoluminescence spectrum of the sample was measured at room temperature (using a spectrometer, FP 6500DS, JASCO Co.). The metal composition of the samples was determined using inductive coupled plasma-atomic emission spectroscopy (ICP–AES) with a spectrometer, SPS400, Seiko Instruments Co.
* Substrate (a)
2.5 at.% (800)
T.C. T.C.
* 2.0 at.% *
0.0 at.%
* 40 60 2 [degree]
20
(b)
80
2.8 at.% *
(800)
Slit Substrate:Si(100)
(622)
Vaporizer
Nozzle
T.C.
(622)
Y( Y(DPM) 3
(440)
Tb(DPM)3
To obtain the growth with preferential orientation, the Y2O3:Tb films were synthesized at the temperature region in which inhomogeneous gas reaction occurs. As the surface reaction was dominant, a few polycrystalline films demonstrated preferential orientation as shown in Fig. 2. All these samples formed on the silicon substrate had defective fluorite structure having relatively strong intensity at 29.2, 33.8, 48.5, 57.6 and 71.18 corresponding to (222), (400), (440), (622) and (800) diffraction lines. No diffraction lines indexed as the polycrystalline state of terbium oxide were observed. For the film grown at a vaporizing temperature of 180 8C, the intensity ratio of each diffraction line was nearly same as that of reference powder diffraction (ICDD Card No. 41–1105). The pattern obtained at a growth temperature of 210 8C was completely different from the powder pattern. The peak intensity of the (222) diffraction line decreased while the peak intensity of the (400) diffraction line increased, indicating preferential orientation of h100i crystalline direction of Y2O3:Tb. In addition, full width at half maximum for all diffraction lines of the sample grown at 210 8C was narrower than those of the sample deposited at 180 8C.
(440)
Heating tape
(400)
T.C.
(400)
1.2 dm 3 /min
(222)
L-N2Trap
(222)
Flow meter
Intensity [a.u]
N2
Intensity [a.u]
216
1.5 at.%
0.0 at.% *
20
40 60 2 [degree]
80
Fig. 2. XRD patterns of Y2O3:Tb films obtained at various temperatures: (a) 180 8C and (b) 210 8C.
Y. Satoh et al. / Science and Technology of Advanced Materials 6 (2005) 215–218
217
Fig. 3. SEM micrographs of (a) cross-section and (b) top view of Y2O3:Tb samples deposited on (100)Si at the vaporizing temperatures of 180 8C.
Fig. 4. SEM micrographs of (a) cross-section and (b) top view of Y2O3:Tb samples deposited on (100)Si at the vaporizing temperatures of 210 8C.
The degree of orientation estimated by the Lotgering orientation factor fhkl defined as
These results suggest that the intensity is strongly dependent upon the concentration of Tb. Fig. 6 shows relative intensity of the luminescence at 542 nm for random oriented films and h100i oriented whiskers. The film thickness and the whisker length were kept constant at 5 mm. The intensity of the photoluminescence obtained from h100i oriented Y2O3:Tb whiskers was higher than that obtained from the uniform polycrystalline film with random
(1)
where Phkl and P0 are the peak intensity ratio of oriented direction to all directions of oriented sample, and that of powder of the standard specimen, respectively. Therefore, f100 and f111 were calculated using P400 and P222, respectively. The pattern obtained from the sample at a growth temperature of 210 8C showed f 100Z0.85, while that obtained from the sample with a growth temperature of 180 8C showed f100Z0.38 indicating that the former one is preferentially oriented to h100i crystalline direction. Figs. 3 and 4 show a series of the SEM micrographs of the cross-section and the surface of the samples. The average grain size of the crystallites in the sample obtained at 180 8C is 5 mm. The growth orientation and shape of the crystallites are slightly random for each crystallite. However, the shape of the crystallite is uniform whisker. The size of the whisker of the sample obtained at 210 8C was determined as 1.5 mm and 5 mm in diameter and length, respectively. The size was confirmed by the top view and along to growth direction. The growth orientation is relatively aligned among whiskers. Fig. 5(a) and (b) illustrates the photoluminescence spectra of the Y2O3:Tb films obtained at 180 and 210 8C, respectively. The peaks at 542 nm assigned to 5D4/7F5 transition as well as the most intense one at 484 nm assigned to 5D4/7F6 transition were seen on the spectrum.
(a) 2.5 at.%
2.0 at.% PL Intensity [a. u. ]
fhkl Z ðPhkl K P0 Þ=ð1 K P0 Þ;
0.0 at.%
(b)
2.8 at.%
1.5 at.%
0.0 at.% 350
400
450
500 550 600 Wavelength [nm]
650
700
Fig. 5. Photoluminescence spectra of Y2O3:Tb samples with (a) random orientation and (b) preferential orientation to h100i.
218
Y. Satoh et al. / Science and Technology of Advanced Materials 6 (2005) 215–218
<100> orientated Y2 O 3 :Tb whiskers
100 Relative intensity [%]
that although random oriented film possesses the maximum intensity at the Tb concentration of 2.0 at.%, the intensity keeps increasing to 1.5 at.% for the h100i oriented whiskers.
80 60
Acknowledgements
40 20
Random oriented Y2 O 3 :Tb
0 0
1
2 3 Tb concentration [at.%]
4
5
This work was partially supported by a Grant-in Aid for Scientific Research (A) and Exploratory Research, the Ministry of Education, Science, Sports and Culture contract Nos. 16206068 and 14655269. The authors also thank to the support by the 21st century COE program at Nagaoka University of Technology.
Fig. 6. Relative intensity of Y2O3:Tb at 542 nm.
orientation. Although the random oriented film possesses the maximum intensity at the Tb concentration of 2.0 at.%, the intensity keeps increasing to 1.5 at.% for the h100i oriented film. To our knowledge, the relationship between the green luminescence properties of Y2O3:Tb and Tb concentration has been still unknown. However, the optimum Eu concentrations in obtaining intense luminescence from the Y2O3:Eu whiskers is also different from that obtained at the uniform films [11]. The luminescence characteristic of the h100i oriented Y2O3 whisker with heavy dope of Tb may be different from that of the crystalline particle of Y2O3:Tb. First, the whiskers include more doping element into the lattice. For example, the ZnO whiskers have an Al solubility of more than 2 at.%, which is higher than that observed in the polycrystalline ZnO films [12]. Substitutional doping of Tb to the optimum lattice position is a possible reason to introduce intense luminescence. Second, microarchitecture of the aggregation of the Y2O3 crystallites may assist to increase photoluminescence. Although the total volume of the crystal of the aggregation of the whiskers is less than that of the uniform film, the surface area directly irradiated by the UV lamp on the whisker sample is larger than that of the film. Further investigation is required to clarify the luminescence mechanism of the whisker.
4. Conclusion The random oriented Y2O3:Tb polycrystalline films and highly oriented Y2O3:Tb whiskers were synthesized using the CVD technique operated under atmospheric pressure. The photoluminescence intensity of Y2O3:Tb polycrystalline films and highly oriented Y2O3:Tb whiskers indicated
References [1] G.A. West, K.W. Beeson, Low-pressure metalorganic chemical vapor deposition of photoluminescent Eu-doped Y2O3 films, J. Mater. Res. 5 (1990) 1573–1580. [2] K.G. Cho, D. Kumar, D.G. Lee, S.L. Jones, P.H. Holloway, R.K. Singh, Improved luminescence properties of pulsed laser deposited Eu:Y2O3 thin films on diamond coated silicon substrates, Appl. Phys. Lett. 71 (1997) 3335–3337. [3] E.T. Goldburt, B. Kulkarni, R.N. Bhargava, J. Taylor, M. Libera, Size dependent efficiency in Tb doped Y2O3 nanocrystalline phosphor, J. Lumi. 72 (1997) 190–192. [4] M. Satoh, N. Tanaka, Y. Ueda, S. Ohshio, S. Saitoh, Epitaxial growth of zinc oxide whiskers by chemical-vapor deposition under atmospheric pressure, Jpn. J. Appl. Phys. 38 (1999) L586–L589. [5] H. Saitoh, M. Satoh, N. Tanaka, Y. Ueda, S. Ohshio, Homogeneous growth of zinc oxide whiskers, Jpn. J. Appl. Phys. 38 (1999) 6873– 6877. [6] Y. Ohkawara, T. Naijo, T. Washio, S. Ohshio, H. Ito, H. Saitoh, Field emission properties of Al:ZnO whiskers modified by amorphous carbon and related films, Jpn. J. Appl. Phys. 40 (2001) 7013–7017. [7] S. Tokita, N. Tanaka, H. Saitoh, High-rate epitaxy of anatase fims by atmospheric chemical vapor deposion, Jpn. J. Appl. Phys. 39 (2000) L169–L171. [8] N. Tanaka, S. Ohshio, H. Saitoh, Preferential orientation of titanium dioxide polycrystalline films using atmospheric CVD technique, J. Ceram. Soc. Jpn 105 (1997) 551–554. [9] H. Saitoh, S.H. Sunayama, N. Tanaka, S. Ohshio, Initial growth process of epitaxially chemical-vapor-deposited titanium dioxide crystalline films, Mater. Sci. Eng. Serving Soc. (1998); 165–168. [10] H. Saitoh, H. Sunayama, N. Tanaka, S. Ohshio, Epitaxial growth mechanism of chemical-vapor-deposited anatase on strontium titanate substrate, J. Ceram. Soc. Jpn. 106 (1998) 1051–1055. [11] Y. Satoh, S. Ohshio, H. Saitoh, Photoluminescence properties of Y2O3:Eu whiskers with preferential h100i orientation, Jpn. J. Appl. Phys. 41 (2002) L1253–L1255. [12] H. Saitoh, Y. Namioka, H. Sugata, S. Ohshio, Nanoindentation analysis of Al:ZnO epitaxial whiskers, Jpn. J. Appl. Phys. 40 (2001) 6024–6028.