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High luminance YAG single crystal phosphor screen
Displays 21 (2000) 195±198
www.elsevier.nl/locate/displa
High luminance YAG single crystal phosphor screen Cheng Jianbo*, Chen Wenbin, Yang Kaiyu Department of Opto-electronic Technology, University of Electronic Science and Technology of China, Chengdu 610054, People's Republic of China Received 1 March 2000; accepted 31 May 2000
Abstract Epitaxial phosphors of Ce,Tb:YAG containing Lu 31 and Ga 31 on YAG (yttrium aluminum garnet) single crystal faceplates have properties which are suitable for projection CRTs. The external ef®ciency of the single-crystal YAG screen is relatively low. Truncated cone geometry can increase the external ef®ciency by a factor of 5.2, if such a shape can be formed in the phosphor layer. An anisotropy etching process of YAG has been developed for the fabrication of a YAG luminescent screen with truncated square mesas. A large fraction of the cathodoluminescent can be directed through the untextured front surface of the crystal. An improvement in phosphor ef®ciency by a factor of 2.3 has been achieved. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial phosphor screen; YAG single crystal; External ef®ciency; Anisotropic etching
1. Introduction Ce,Tb: YAG containing Lu 31and Ga 31 luminescent screen for projection CRT has been successfully developed by liquid phase epitaxial growth. This screen can be operated at extremely high electron beam power densities without thermal quenching because of the high melting point and good thermal conductivity. The luminance of a tube with a Ce,Tb:YAG phosphor screen amounts to 14 000 cd/m 2 at 25 kV anode voltage and 2 mA beam current in a 10 cm 2 raster size. It is also linearly proportional to excitation power and does not saturate. Because the light generated by the electron beam is produced within the crystal, large fraction of the light is trapped and not able to pass through the front surface to the viewer. In fact, all the rays that strike the air side of the faceplate at a half-cone angle greater than 338 are internally re¯ected and eventually exit the crystal at its edges or dissipate themselves by absorption, resulting in a relatively low external screen ef®ciency. Inclined facets can modify the path of some of the internally trapped light rays. The techniques used for fabricating facets include: the wet chemical etching [1], the reactive ion etching [2] and the multi-layer faceted screen growth [3]. The latter two processes have been found to be impractical for largescale production. The simpli®ed wet chemical etching process has been developed. The basic model, etching
* Corresponding author. Tel.: 186-283-201708; fax: 186-283-201745.
process and cathodoluminescence of the facet screen are described in detail. 2. Principle and model Fig. 1 schematically illustrates the structure of YAG single-crystal epitaxial phosphor ®lm. The external screen ef®ciency is relatively low. The major factor limiting the external ef®ciency of Ce,Tb:YAG phosphors is the high refractive index of the YAG substrate (n 1.84) which allows only rays less than a critical angle of uC 338 to be emitted from the faceplate. The corresponding solid angle is V 4p 1 2 cos u C : A fraction (F) can be de®ned as that portion (P 0 ) of the total photo emission (P0) from a small electron beam excited region inside the YAG, which does not undergo total internal re¯ection. The internal absorption of epitaxial ®lm and YAG substrate can be neglected for the mathematical derivations. To the isotropic light source, the light intensity is constant. Consequently, F P=P 00 VI=4pI 1 2 cos u C 16% This fraction can be signi®cantly increased by shaping the Ce,Tb:YAG luminescent surface into a series of truncated cones, as illustrated in Fig. 2. The surface, when aluminized, serves to re¯ect many of those rays toward the exit surface that otherwise would have been totally internally re¯ected [4]. In the truncated cone model, the photogeneration region is positioned at the apex. The rays from the light center O
0141-9382/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0141-938 2(00)00053-6
196
C. Jianbo et al. / Displays 21 (2000) 195±198
2 electrons
metal backing
YAG substrate Waveguided light
Etching rate (µm/min)
YAG epi. film
1.6 1.2 0.8 0.4 0 190
210
230
250
direct ray
Etching temperature (ºC)
Fig. 1. The waveguiding effect in single crystal epitaxial phosphor screen.
Fig. 3. Change in etching rate with temperature for the (111) YAG surface.
can be separated into three beams. The ®rst one exits the faceplate after re¯ected by the cone left side; the second one exits directly; the third one is directed through the front surface by the cone right side re¯ecting. The calculated photometry ®gures of merit for this geometry are strongly dependent upon the half-angle b of the cone when a small photogeneration region at the apex center is assumed. If 908 2 uC =2 , b , uC ; and the truncated cone is high enough, all the rays will fall into the critical cone. When this truncated cone geometry is optimized,
b bm 908 2 uC =2 28:58 As discussed above, the corresponding solid angle is V 2p cos u C : Thus, the external power ef®ciency is
h FT h0
4n cos u0 h0 76:5%h0 n 1 12
where h 0 is the internal power ef®ciency and the transmission T for the rays partially re¯ected at the interface is about 4n= n 1 12 .Thus, a factor of 5.2 increase in external ef®ciency over that of a comparable ¯at YAG phosphor plate is obtained. If b $ uC or 0 , b , 908 2 uC =2; the improvement of the external ef®ciency is relatively low, because the incident angles of those rays re¯ected by the conic surface may be
2β O
direct ray reflected ray
Fig. 2. Cross-section schematic for truncated cone structure.
larger than the critical angle. These rays are wave guided to the edge, leading to a decrease of the useful conic surface. 3. Experimental 3.1. Epitaxial phosphor ®lm growth The procedure for the epitaxial growth of Ce,Tb: YAG phosphor ®lm by LPE was discussed in Ref. [5]. A single crystal of Lu 31 and Ga 31 containing Ce,Tb: YAG provides a better luminescence spectra [6]. In our laboratory, a preferred melt composition was: Y2O3:CeO2:Tb4O7:Lu2O7: Al2O3:Ga2O3:B2O3:PbO 10.50:4.00:0.40:8.50:17.50:17.00:30.00:12.00. B2O3, PbO was used as ¯ux, Lu 31/Ga 31 < 1.48. The melt was placed in a platinum crucible in air at ambient pressure. The starting materials were typically 99.999% purity. Growth temperature was 10608C, growth rate 5.2 mm/min, with substrate rotation at 55 rpm. As a result, a 6.34 mm thick epitaxial single crystal Ce,Tb:YAG layer on a 1-in. diameter (111)-oriented YAG substrate was produced. The composition of this green phosphor ®lm was: Y2.177Ce0.014Tb0.042Lu0.860Al4.321Ga0.582O12. 3.2. Screen etching Reticulation of the YAG phosphor layer allows a greater fraction of rays to enter the critical cone of 338 and exit the Ê thick) was faceplate. A plasma-deposited SiO2 ®lm (6000 A ®rst formed on the sample. A positive photoresist was spun onto the sample and exposed by contact photolithography. The optical mask consisted of a series of opaque squares, 10 mm on each side and separated by 3 mm. One side of the square pattern was aligned along the k110l direction. The photoresist was used as a mask for the etching of the SiO2 layer in a CF4 1 8% O2 plasma. The photoresist was then stipped in an O2 plasma. Finally, The Ce,Tb:YAG phosphor layer was etched in a 3:1 volume mixture of phosphoric and sulfuric acid at 2248C. The temperature dependence of the etching rate for the (111) YAG surface in this mixture is
C. Jianbo et al. / Displays 21 (2000) 195±198
197
4. Results and discussion
<211>
Fig. 4. SEM of the YAG epitaxial ®lm for 4 min etching at 2208C.
shown in Fig. 3. Details of the etched surfaces were inspected with a scanning electron microscope. Finally, acid resist layer (SiO2) was stripped. 3.3. Measurement of cathodoluminescence properties The light output data of Ce,Tb: YAG phosphor screen containing Lu 31, Ga 31 was obtained using a SEM which had a modi®ed sample chamber to accept a sample of 25 mm diameter by 2 mm thickness, and also incorporate the optics required. The SEM was operated in stationary mode (continuous unscanned beam). The diameter of the circular light spot on the epitaxial ®lm was 30 mm. Screen high voltage was 25 kV. The total light intensity was monitored by a photometer and a Si-photocell. The input beam power density was changed by adjusting the beam current. For comparison, the CL properties of a ¯at Ce,Tb:YAG epitaxial ®lm was also measured.
A shaped Ce,Tb:YAG epitaxial ®lm for 2 min etching at 2208C is shown in Fig. 4. One pair of faces, parallel to the k110l direction, has asymmetrical etched pro®les. Another pair of faces, parallel to the k211l direction, has very steep pro®le near the surface and a much shallower pro®le near the mesa base. Around each corner, the high index planes show different pro®les. The overall mesa geometry has a relatively complicated form. Despite the great differences between the etching surface structure and the truncated cone, the improvement of CL should lie between a factor of 0±5.2. Increasing the etching depth increases the conic surface area and thus increases the re¯ected light yield. Table 1 illustrates the total light intensity at a 25 kV anode potential for a 1-in. Ce,Tb:YAG unreticulated screen (P2) and a reticulated screen (P3). The input beam power density (P1), the external ef®ciency (h ) and internal ef®ciency (h 0) for unreticulated are also presented. The unsaturation to a high power density is evident up to the tested limit of 7.1 £ 10 5 W/m 2. The beam is continuously not scanned. The measurement at an anode potential of 25 kV and 44 mA/cm 2 beam current density gave an internal ef®ciency of 9%. Because of the wave-guided effect, the external ef®ciency is only 1.44%. The improvement in the total light intensity of the facet screen by a factor of 2.3 is achieved. The inclined facets formed by the mesaetching act as ef®cient internal re¯ectors gives rise to an increase in light output. Apart from a simple consideration of external ef®ciency, the effect of surface etching upon CRT contrast and resolution must be assessed. If a small mesa size is used, such reticulation may not limit the faceplate resolution.
5. Conclusions Table 1 Cathodoluminescence of a reticulated and an unreticulated 1-in. Ce,Tb:YAG faceplate, excited by a stationary electron beam, 25 kV screen high voltage and a circular light spot of 30 mm diameter P1 (W/m 2)
P2 (W/m 2)
h (%)
h 0 (%)
P3 (W/m 2)
0.88 £ 10 4 0.11 £ 10 5 0.18 £ 10 5 0.21 £ 10 5 0.25 £ 10 5 0.35 £ 10 5 0.53 £ 10 5 0.71 £ 10 5 0.88 £ 10 5 0.11 £ 106 0.21 £ 106 0.25 £ 106 0.35 £ 106 0.53 £ 106 0.71 £ 106
0.14 £ 10 3 0.16 £ 10 3 0.24 £ 10 3 0.28 £ 10 3 0.31 £ 10 3 0.41 £ 10 3 0.55 £ 10 3 0.66 £ 10 3 0.82 £ 10 3 0.98 £ 10 3 0.17 £ 10 4 0.19 £ 10 4 0.23 £ 10 4 0.31 £ 10 4 0.39 £ 10 4
1.58 1.50 1.37 1.31 1.25 1.15 1.04 0.93 0.93 0.89 0.79 0.75 0.65 0.59 0.56
9.85 9.32 8.55 8.15 7.80 7.13 6.46 5.77 5.75 5.76 4.92 4.66 4.05 3.66 3.47
0.32 £ 10 3 0.37 £ 10 3 0.55 £ 10 3 0.67 £ 10 3 0.71 £ 10 3 0.95 £ 10 3 0.13 £ 10 4 0.15 £ 10 4 0.19 £ 10 4 0.22 £ 10 4 0.39 £ 10 4 0.44 £ 10 4 0.53 £ 10 4 0.71 £ 10 4 0.89 £ 10 4
A truncated cone model is studied in detail. An anisotropy etching process has been developed for the fabrication of an 1-in. Ce,Tb:YAG luminescent screen with truncated square mesas. An improvement in phosphor ef®ciency by a factor of 2.3 has been achieved. The luminance of Ce,Tb:YAG containing Lu 31 and Ga 31 is linearly proportional to excitation power and does not saturate. In the stationary operation mode, this screen can be used as light source of extreme luminance. A monochrome 28 000 cd/m 2 projection CRT can be achieved for a 2-in. Ce,Tb:YAG facet screen excited with a 25 kV anode voltage and 2 mA beam current in a scanned area of 10 cm 2. The reticulation will not limit the screen resolution and contrast if a small mesa size is used.
References [1] D.T.C. Huo, T.W. Hou, Reticulated single-crystal luminescent screen, J. Electrochem. Soc. 133 (1986) 1492±1497.
198
C. Jianbo et al. / Displays 21 (2000) 195±198
[2] P.F. Bonger, M.W. Van Tol, J.M. Robertson, Luminescent screen, US Patent no. 4298 (1981) 820. [3] D.M. Gualtieri, Multi-layer faceted luminescent screens, US Patent no.4713 (1987) 577. [4] W.N. Carr, Photometric ®gures of merit for semiconductor luminescent sources operating in spontaneous mode, Infrared Phys. 6 (1966) 1±19.
[5] D.M. Gualtieri, Liquid phase epitaxy of yttrium aluminum garnet: reduction of growth rate by germanium oxide, Appl. Phys. Lett. 59 (1991) 650±652. [6] G.W. Berkstresser, D.T.C. Huo, J. Shmulovich et al., Visual display system utilizing high luminosity single crystal garnet material, Euro. Patent no. 0142 (1985) 931.
www.elsevier.nl/locate/displa
High luminance YAG single crystal phosphor screen Cheng Jianbo*, Chen Wenbin, Yang Kaiyu Department of Opto-electronic Technology, University of Electronic Science and Technology of China, Chengdu 610054, People's Republic of China Received 1 March 2000; accepted 31 May 2000
Abstract Epitaxial phosphors of Ce,Tb:YAG containing Lu 31 and Ga 31 on YAG (yttrium aluminum garnet) single crystal faceplates have properties which are suitable for projection CRTs. The external ef®ciency of the single-crystal YAG screen is relatively low. Truncated cone geometry can increase the external ef®ciency by a factor of 5.2, if such a shape can be formed in the phosphor layer. An anisotropy etching process of YAG has been developed for the fabrication of a YAG luminescent screen with truncated square mesas. A large fraction of the cathodoluminescent can be directed through the untextured front surface of the crystal. An improvement in phosphor ef®ciency by a factor of 2.3 has been achieved. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial phosphor screen; YAG single crystal; External ef®ciency; Anisotropic etching
1. Introduction Ce,Tb: YAG containing Lu 31and Ga 31 luminescent screen for projection CRT has been successfully developed by liquid phase epitaxial growth. This screen can be operated at extremely high electron beam power densities without thermal quenching because of the high melting point and good thermal conductivity. The luminance of a tube with a Ce,Tb:YAG phosphor screen amounts to 14 000 cd/m 2 at 25 kV anode voltage and 2 mA beam current in a 10 cm 2 raster size. It is also linearly proportional to excitation power and does not saturate. Because the light generated by the electron beam is produced within the crystal, large fraction of the light is trapped and not able to pass through the front surface to the viewer. In fact, all the rays that strike the air side of the faceplate at a half-cone angle greater than 338 are internally re¯ected and eventually exit the crystal at its edges or dissipate themselves by absorption, resulting in a relatively low external screen ef®ciency. Inclined facets can modify the path of some of the internally trapped light rays. The techniques used for fabricating facets include: the wet chemical etching [1], the reactive ion etching [2] and the multi-layer faceted screen growth [3]. The latter two processes have been found to be impractical for largescale production. The simpli®ed wet chemical etching process has been developed. The basic model, etching
* Corresponding author. Tel.: 186-283-201708; fax: 186-283-201745.
process and cathodoluminescence of the facet screen are described in detail. 2. Principle and model Fig. 1 schematically illustrates the structure of YAG single-crystal epitaxial phosphor ®lm. The external screen ef®ciency is relatively low. The major factor limiting the external ef®ciency of Ce,Tb:YAG phosphors is the high refractive index of the YAG substrate (n 1.84) which allows only rays less than a critical angle of uC 338 to be emitted from the faceplate. The corresponding solid angle is V 4p 1 2 cos u C : A fraction (F) can be de®ned as that portion (P 0 ) of the total photo emission (P0) from a small electron beam excited region inside the YAG, which does not undergo total internal re¯ection. The internal absorption of epitaxial ®lm and YAG substrate can be neglected for the mathematical derivations. To the isotropic light source, the light intensity is constant. Consequently, F P=P 00 VI=4pI 1 2 cos u C 16% This fraction can be signi®cantly increased by shaping the Ce,Tb:YAG luminescent surface into a series of truncated cones, as illustrated in Fig. 2. The surface, when aluminized, serves to re¯ect many of those rays toward the exit surface that otherwise would have been totally internally re¯ected [4]. In the truncated cone model, the photogeneration region is positioned at the apex. The rays from the light center O
0141-9382/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0141-938 2(00)00053-6
196
C. Jianbo et al. / Displays 21 (2000) 195±198
2 electrons
metal backing
YAG substrate Waveguided light
Etching rate (µm/min)
YAG epi. film
1.6 1.2 0.8 0.4 0 190
210
230
250
direct ray
Etching temperature (ºC)
Fig. 1. The waveguiding effect in single crystal epitaxial phosphor screen.
Fig. 3. Change in etching rate with temperature for the (111) YAG surface.
can be separated into three beams. The ®rst one exits the faceplate after re¯ected by the cone left side; the second one exits directly; the third one is directed through the front surface by the cone right side re¯ecting. The calculated photometry ®gures of merit for this geometry are strongly dependent upon the half-angle b of the cone when a small photogeneration region at the apex center is assumed. If 908 2 uC =2 , b , uC ; and the truncated cone is high enough, all the rays will fall into the critical cone. When this truncated cone geometry is optimized,
b bm 908 2 uC =2 28:58 As discussed above, the corresponding solid angle is V 2p cos u C : Thus, the external power ef®ciency is
h FT h0
4n cos u0 h0 76:5%h0 n 1 12
where h 0 is the internal power ef®ciency and the transmission T for the rays partially re¯ected at the interface is about 4n= n 1 12 .Thus, a factor of 5.2 increase in external ef®ciency over that of a comparable ¯at YAG phosphor plate is obtained. If b $ uC or 0 , b , 908 2 uC =2; the improvement of the external ef®ciency is relatively low, because the incident angles of those rays re¯ected by the conic surface may be
2β O
direct ray reflected ray
Fig. 2. Cross-section schematic for truncated cone structure.
larger than the critical angle. These rays are wave guided to the edge, leading to a decrease of the useful conic surface. 3. Experimental 3.1. Epitaxial phosphor ®lm growth The procedure for the epitaxial growth of Ce,Tb: YAG phosphor ®lm by LPE was discussed in Ref. [5]. A single crystal of Lu 31 and Ga 31 containing Ce,Tb: YAG provides a better luminescence spectra [6]. In our laboratory, a preferred melt composition was: Y2O3:CeO2:Tb4O7:Lu2O7: Al2O3:Ga2O3:B2O3:PbO 10.50:4.00:0.40:8.50:17.50:17.00:30.00:12.00. B2O3, PbO was used as ¯ux, Lu 31/Ga 31 < 1.48. The melt was placed in a platinum crucible in air at ambient pressure. The starting materials were typically 99.999% purity. Growth temperature was 10608C, growth rate 5.2 mm/min, with substrate rotation at 55 rpm. As a result, a 6.34 mm thick epitaxial single crystal Ce,Tb:YAG layer on a 1-in. diameter (111)-oriented YAG substrate was produced. The composition of this green phosphor ®lm was: Y2.177Ce0.014Tb0.042Lu0.860Al4.321Ga0.582O12. 3.2. Screen etching Reticulation of the YAG phosphor layer allows a greater fraction of rays to enter the critical cone of 338 and exit the Ê thick) was faceplate. A plasma-deposited SiO2 ®lm (6000 A ®rst formed on the sample. A positive photoresist was spun onto the sample and exposed by contact photolithography. The optical mask consisted of a series of opaque squares, 10 mm on each side and separated by 3 mm. One side of the square pattern was aligned along the k110l direction. The photoresist was used as a mask for the etching of the SiO2 layer in a CF4 1 8% O2 plasma. The photoresist was then stipped in an O2 plasma. Finally, The Ce,Tb:YAG phosphor layer was etched in a 3:1 volume mixture of phosphoric and sulfuric acid at 2248C. The temperature dependence of the etching rate for the (111) YAG surface in this mixture is
C. Jianbo et al. / Displays 21 (2000) 195±198
197
4. Results and discussion
<211>
Fig. 4. SEM of the YAG epitaxial ®lm for 4 min etching at 2208C.
shown in Fig. 3. Details of the etched surfaces were inspected with a scanning electron microscope. Finally, acid resist layer (SiO2) was stripped. 3.3. Measurement of cathodoluminescence properties The light output data of Ce,Tb: YAG phosphor screen containing Lu 31, Ga 31 was obtained using a SEM which had a modi®ed sample chamber to accept a sample of 25 mm diameter by 2 mm thickness, and also incorporate the optics required. The SEM was operated in stationary mode (continuous unscanned beam). The diameter of the circular light spot on the epitaxial ®lm was 30 mm. Screen high voltage was 25 kV. The total light intensity was monitored by a photometer and a Si-photocell. The input beam power density was changed by adjusting the beam current. For comparison, the CL properties of a ¯at Ce,Tb:YAG epitaxial ®lm was also measured.
A shaped Ce,Tb:YAG epitaxial ®lm for 2 min etching at 2208C is shown in Fig. 4. One pair of faces, parallel to the k110l direction, has asymmetrical etched pro®les. Another pair of faces, parallel to the k211l direction, has very steep pro®le near the surface and a much shallower pro®le near the mesa base. Around each corner, the high index planes show different pro®les. The overall mesa geometry has a relatively complicated form. Despite the great differences between the etching surface structure and the truncated cone, the improvement of CL should lie between a factor of 0±5.2. Increasing the etching depth increases the conic surface area and thus increases the re¯ected light yield. Table 1 illustrates the total light intensity at a 25 kV anode potential for a 1-in. Ce,Tb:YAG unreticulated screen (P2) and a reticulated screen (P3). The input beam power density (P1), the external ef®ciency (h ) and internal ef®ciency (h 0) for unreticulated are also presented. The unsaturation to a high power density is evident up to the tested limit of 7.1 £ 10 5 W/m 2. The beam is continuously not scanned. The measurement at an anode potential of 25 kV and 44 mA/cm 2 beam current density gave an internal ef®ciency of 9%. Because of the wave-guided effect, the external ef®ciency is only 1.44%. The improvement in the total light intensity of the facet screen by a factor of 2.3 is achieved. The inclined facets formed by the mesaetching act as ef®cient internal re¯ectors gives rise to an increase in light output. Apart from a simple consideration of external ef®ciency, the effect of surface etching upon CRT contrast and resolution must be assessed. If a small mesa size is used, such reticulation may not limit the faceplate resolution.
5. Conclusions Table 1 Cathodoluminescence of a reticulated and an unreticulated 1-in. Ce,Tb:YAG faceplate, excited by a stationary electron beam, 25 kV screen high voltage and a circular light spot of 30 mm diameter P1 (W/m 2)
P2 (W/m 2)
h (%)
h 0 (%)
P3 (W/m 2)
0.88 £ 10 4 0.11 £ 10 5 0.18 £ 10 5 0.21 £ 10 5 0.25 £ 10 5 0.35 £ 10 5 0.53 £ 10 5 0.71 £ 10 5 0.88 £ 10 5 0.11 £ 106 0.21 £ 106 0.25 £ 106 0.35 £ 106 0.53 £ 106 0.71 £ 106
0.14 £ 10 3 0.16 £ 10 3 0.24 £ 10 3 0.28 £ 10 3 0.31 £ 10 3 0.41 £ 10 3 0.55 £ 10 3 0.66 £ 10 3 0.82 £ 10 3 0.98 £ 10 3 0.17 £ 10 4 0.19 £ 10 4 0.23 £ 10 4 0.31 £ 10 4 0.39 £ 10 4
1.58 1.50 1.37 1.31 1.25 1.15 1.04 0.93 0.93 0.89 0.79 0.75 0.65 0.59 0.56
9.85 9.32 8.55 8.15 7.80 7.13 6.46 5.77 5.75 5.76 4.92 4.66 4.05 3.66 3.47
0.32 £ 10 3 0.37 £ 10 3 0.55 £ 10 3 0.67 £ 10 3 0.71 £ 10 3 0.95 £ 10 3 0.13 £ 10 4 0.15 £ 10 4 0.19 £ 10 4 0.22 £ 10 4 0.39 £ 10 4 0.44 £ 10 4 0.53 £ 10 4 0.71 £ 10 4 0.89 £ 10 4
A truncated cone model is studied in detail. An anisotropy etching process has been developed for the fabrication of an 1-in. Ce,Tb:YAG luminescent screen with truncated square mesas. An improvement in phosphor ef®ciency by a factor of 2.3 has been achieved. The luminance of Ce,Tb:YAG containing Lu 31 and Ga 31 is linearly proportional to excitation power and does not saturate. In the stationary operation mode, this screen can be used as light source of extreme luminance. A monochrome 28 000 cd/m 2 projection CRT can be achieved for a 2-in. Ce,Tb:YAG facet screen excited with a 25 kV anode voltage and 2 mA beam current in a scanned area of 10 cm 2. The reticulation will not limit the screen resolution and contrast if a small mesa size is used.
References [1] D.T.C. Huo, T.W. Hou, Reticulated single-crystal luminescent screen, J. Electrochem. Soc. 133 (1986) 1492±1497.
198
C. Jianbo et al. / Displays 21 (2000) 195±198
[2] P.F. Bonger, M.W. Van Tol, J.M. Robertson, Luminescent screen, US Patent no. 4298 (1981) 820. [3] D.M. Gualtieri, Multi-layer faceted luminescent screens, US Patent no.4713 (1987) 577. [4] W.N. Carr, Photometric ®gures of merit for semiconductor luminescent sources operating in spontaneous mode, Infrared Phys. 6 (1966) 1±19.
[5] D.M. Gualtieri, Liquid phase epitaxy of yttrium aluminum garnet: reduction of growth rate by germanium oxide, Appl. Phys. Lett. 59 (1991) 650±652. [6] G.W. Berkstresser, D.T.C. Huo, J. Shmulovich et al., Visual display system utilizing high luminosity single crystal garnet material, Euro. Patent no. 0142 (1985) 931.