- Email: [email protected]
Low-temperature deposition of hexagonal GaN films for UV electroluminescent devices by CS-MBE technique
ARTICLE IN PRESS
Journal of Crystal Growth 301–302 (2007) 424–428 www.elsevier.com/locate/jcrysgro
Low-temperature deposition of hexagonal GaN films for UV electroluminescent devices by CS-MBE technique Tohru Honda, Shinichi Egawa, Koichi Sugimoto, Masatoshi Arai Department of Electronic Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachiohji, Tokyo 192 0015, Japan Available online 24 January 2007
Abstract GaN-based electroluminescent devices (ELDs) operating in UV spectral region are proposed for the UV excitation source and their fabrication process using the compound-source molecular beam epitaxy (CS-MBE) technique. The trial substrates were (0 0 0 1)6H–SiC, Ta2O5/Al, and Ta2O5/glass substrates. In particular, the GaN buffer layer deposited at RT for the fabrication is discussed. The grown films were characterized by cathodoluminescence (CL), electroluminescence (EL), reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The UV-light emission observed from the GaN-based ELDs fabricated using CS-MBE technique is demonstrated under operating conditions of 340 V at 200 Hz (pulsed wave). Red, green, and blue (RGB) pixels using phosphors (Y2O2S:Eu, BaMgAl10O17:Eu+Mn, BaMgAl10O17:Eu, respectively) were also demonstrated. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 78.55.Cr; 78.60.Fi Keywords: A3. Molecular beam epitaxy; B1. Nitrides; B3. Light-emitting devices
1. Introduction Hexagonal GaN-based light-emitting diodes (LEDs) [1,2] have been applied for the application to out-door type large-scale flat panel displays (FPDs). Although the GaN and its related materials have large dislocation density, those are applied to the light-emitting devices. It is reported that a polycrystalline GaN films shows a strong photoluminescence [3]. This is expected the low-cost GaNbased light-emitting devices. For the home-use FPDs using GaN-based materials, we have to realize the low-cost lightemitting devices. Those applications require low-cost light-emitting pixels. In particular, the low-temperature and large-scale integration of GaN-based light-emitting devices are one of the crucial issues for development of them. The low-temperature deposition leads to the wide selectivity of substrates, because the growth of GaN and related materials sometimes requires a high temperature of approximately 1000 1C, which leads to limit the selectivity of the substrates. On the other hand, the Corresponding author. Tel.: +81 42 622 9291; fax: +81 42 625 8982.
E-mail address: [email protected] (T. Honda). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.206
integration of light-emitting devices is cost-effective for the fabrication of FPDs because approximately 1.4 mega LEDs for super video array systems (SVGA, 800 600 pixels) are required. In this study, compound-source molecular beam epitaxy (CS-MBE) [4] growth of hexagonal GaN films at low temperature using (0 0 0 1)6H–SiC, Ta2O5/Al and Ta2O5/ glass substrates was investigated as a preliminary study. GaN-based thin-film electroluminescent devices (TFLEDs) operating in UV to violet spectral region fabricated on Ta2O5/Al and Ta2O5/glass substrates are reported. Finally, red, green and blue (RGB) light-emitting pixels using the GaN-based TF-ELDs as excitation sources are discussed. 2. Compound source-molecular beam epitaxy 2.1. Low-temperature deposition GaN powder, which was synthesized using the reaction between gallium metal and ammonia, was used as a source for CS-MBE. GaN powder was annealed in vacuum at 800 1C for 5 h or longer before the deposition for high
ARTICLE IN PRESS T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
Intensity (Normalized)
GaN/Ta2O5 (RT)
325nm light
GaN/(0001)SiC (RT)
a
GaN/(0001)6HSiC
b
GaN/Ta2O5 e-beam direction: arbitary
Intensity (Relative)
(0 0)
(1 0)
− (1 0)
(2 0)
,y tio n
− (2 0)
ee Scr
,x
tion
een
i pos
Scr
osi
Direct spot
np
n
Direct spot
ee Scr
y
Intensity (Relative)
e//<1120>
n, itio pos
crystallization. The GaN cell temperature was varied from 800 to 860 1C to keep the beam intensity for each deposition. The substrate temperatures of 450 and 500 1C were adopted in this study. The total deposition time was 5 h. A GaN buffer layer was deposited at RT for 15 min before emission layer deposition. No additional nitrogen sources, such as ammonia, were introduced during the deposition. Photoluminescence (PL) spectra of GaN films deposited on (0 0 0 1)6H–SiC and Ta2O5/Al at 450 1C are shown in Fig. 1. A 325 nm line of a He–Cd laser was used for their excitation source. At low temperature of 18 K, the nearband-edge (NBE) emission of GaN crystal was clearly observed in the spectrum of the GaN film deposited on the SiC substrate. On the other hand, the NBE emission component of the spectra observed at RT was somewhat weak compared with that observed at low temperature although its spectrum was starting from the near band edge of GaN. The shape of the spectrum of the GaN film deposited on Ta2O5/Al is similar to that of GaN film grown on (0 0 0 1)6H–SiC. RHEED patterns of the GaN film on (0 0 0 1)6H–SiC are shown in Fig. 2(a). Those indicate that hexagonal GaN crystals were grown on (0 0 0 1)6H–SiC at 450 1C. On the other hand, RHEED patterns of the film on Ta2O5/Al showed blunt ring patterns as shown Fig. 2(b), which indicated that the GaN film was amorphous. CS-MBE has been used for the fabrication of ZnSebased laser diodes (LDs) and the easy controllability of the composition in each of these layers was achieved [5]. In the case of GaN growth, this growth technique is able to use the low-temperature growth. A part of GaN powder heated in K-cell is decomposed and it changes Ga metals and N2 gas, although another part of GaN is sublimated. Thus the growth conditions are Ga rich. However, the vapor pressure of Ga metal is higher than that of GaN. The
425
,x
tion
een
i pos
Scr
Fig. 2. RHEED patterns of GaN films deposited on (a) (0 0 0 1)6H–SiC and (b) Ta2O5. Acceleration voltage of electron beam is 17 kV.
absorbed excess Ga tends to be re-evaporated by the thermal energy on the surface of substrates [6]. 2.2. Low-temperature buffer The introduction of a GaN low-temperature buffer deposited at RT was studied. Although the low-temperature buffer is well-known technique for the GaN growth [7], we had to confirm its effectiveness because the deposition temperature of conventional buffer layers was higher than the deposition temperature of GaN films in this study. The GaN buffer layer was deposited at RT prior to the GaN deposition at 500 1C. The K-cell temperature was kept at 860 1C. Its deposited time is 15 min. The images of atomic force microscopy (AFM) are shown in Fig. 3. The surface smoothness of the GaN film was improved by introducing the RT buffer layer on the Ta2O5 surface. Furthermore, cathodoluminescence (CL) intensity observed form the film with buffer was also increased compared with that without it. Those spectra observed at RT are shown in Fig. 4. The introduction of RT buffer layer for the GaN deposition on Ta2O5/Al is effective for the surface smoothness and the increase of CL intensity. Those are the same results as the case of GaN growth on sapphire substrate [8]. However, in this case, because the deposited films are amorphous, the introduction seems to affect the change of the surface free energy of the substrate surface. 3. Fabrication of electroluminescent devices 3.1. GaN-based UV ELDs
GaN/(0001)SiC (18 K)
2.0
2.5
3.0
3.5
Photon energy (eV) Fig. 1. PL spectra of GaN films deposited on (0 0 0 1)6H–SiC and Ta2O5. A 325 nm light from He–Cd laser was used for the excitation source.
Before the GaN deposition, a Ta2O5 layer, which was used as a bottom insulator, was formed on an Al substrate by spin coating for the fabrication of GaN-based TFELDs. After the GaN deposition, an organic insulator layer (cyanoethylpullulan) was spin-coated on the film.
ARTICLE IN PRESS T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
426
Fig. 3. AFM surface images of GaN films (a) with no buffer and (b) with buffer. Observed areas are 20 20 mm.
4 RT
3 2
With buffer
1
Without buffer
Intensity (arb. units)
Intensity (Relative)
RT
0 200
400
600
800
Wavelength (nm) Fig. 4. CL spectra of GaN films with and without with buffer. Observed temperature was RT.
Finally, an Al upper electrode was evaporated onto the surface. The GaN-based TF-ELDs were operated with an AC voltage of 350 V at 200 Hz. UV light emission, with a peak located at 375 nm, was observed. Its electroluminescent (EL) spectrum is shown in Fig. 5. Here, UV light emission was observed around the top electrode. The shape of the EL spectrum looks like to be different from the CL spectrum of the GaN film. The EL spectrum includes all dynamic changes by a pulsed voltage. The EL spectrum corresponds to the NBE spectrum region in the CL spectrum. 3.2. Fabrication of RGB pixels A schematic of RGB pixels fabricated on GaN-based TF-ELDs is shown in Fig. 6. The RGB phosphors were positioned around the electrodes, as shown in Fig. 6. The RGB phosphors Y2O2S:Eu, BaMgAl10O17:Eu+Mn, and BaMgAl10O17:Eu (NP-2310-57, NP-108-62, NP-107-58, respectively, Nichia Chemical Industries) were used in this study. The RGB light emission peaks located at 457, 517 and 626 nm. They were excited by UV emissions from the GaN-
200
300
400
500
600
700
800
900
Wavelength (nm) Fig. 5. EL spectrum of GaN-based TF-ELD at RT. It was operated with a pulsed AC voltage of 350 V at 200 Hz.
based TF-ELD placed in each pixel. These spectra were shown in Fig. 7. These phosphors are used for the fabrication of cathode ray tubes (CRTs). The results clarified that the UV light radiated from the GaN-based TF-ELDs is enough to excite these RGB phosphors. 4. Discussion The low-cost GaN light-emitting devices using Mgdoped GaN combined with indium tin oxide (ITO) electrodes [9] and rare-earth-doped GaN diodes [10] have already been proposed. We consider that TF-ELDs using low-temperature GaN films by CS-MBE technique provide one of the technical routes to cost-effective FPDs. The advantage of a full-color system using TF-ELDs is that it is easy to integrate them. The system is composed of UV light-emitting devices and RGB phosphors. We can separate the integration of UV light-emitting devices and RGB phosphors. This leads to the cost-effective fabrication. However, some disadvantages were found. One disadvantage is the high operation voltage presently used.
ARTICLE IN PRESS T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
RGB pixels
427
Phosphor Electrode Insulator AC
GaN Substrate (Electrode)
Fig. 6. RGB light-emission system excited by UV GaN-based TF-ELDs.
RT 200 Intensity (relative)
Intensity (arb. units)
Y2O2S: Eu (Red)
Glass substrate 100 Al substrate
BaMgAl10O17: Eu+Mn (Green)
0 10
100
1000
Time (sec) BaMgAl10O17: Eu (Blue)
200
300
400
500
600
700
800
Fig. 8. Lifetime of GaN-based TF-ELDs using different substrates.
900
Wavelength (nm) Fig. 7. PL spectra of RGB light-emission system excited by UV GaNbased TF-ELDs. Y2O2S:Eu, BaMgAl10O17:Eu+Mn, and BaMgAl10O17:Eu were used as red, green and blue phosphors, respectively.
films. The UV light emission correspond to the NBE emission of hexagonal GaN was observed from the TF-ELDs, which were operated using AC voltage at RT. RGB lighting pixels were fabricated using UV GaN-based TF-ELDs combined with RGB phosphors, along with GaN deposition. Acknowledgments
The TF-ELDs require the application of high electric field. This field will be reduced to adopt a thin emission layer in a GaN film. A second disadvantage is the thermal generation in the films. This affects the device lifetime. The lifetimes of GaN-based TF-ELDs using different substrates are shown in Fig. 8. The lifetime of the TF-ELD deposited on Al substrate is longer than that on fused-glass substrate. It is due to the different of their thermal conductivities. Thus device design considering the thermal conductivity is required for the future fabrication of the RGB pixels.
The authors thank Professors Emeriti Y. Suematsu and K. Iga of the Tokyo Institute of Technology for their encouragement. We also thank Professor H. Kawanishi of Kogakuin University for his support, and Nichia Chemical Industries, Ltd. For supplying RGB phosphors. This work was supported by a Grant-in-Aid (No. #18560344) from the Ministry of Education, Culture, Sports, Science and Technology. References
5. Summary The low-temperature deposition of GaN films was achieved by CS-MBE technique. UV GaN-based TF-ELDs were fabricated using the low-temperature deposited GaN
[1] I. Akasaki, H. Amano, I. Suemune, Inst. Phys. Conf. Ser. 142 (1995) 7. [2] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [3] K. Iwata, H. Asahi, K. Asami, R. Kuroiwa, S. Gonda, Jpn. J. Appl. Phys. 36 (1997) L661.
ARTICLE IN PRESS 428
T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
[4] T. Honda, K. Sato, T. Hashimoto, M. Shinohara, H. Kawanishi, Phys. Stat. Solidi (a) 188 (2001) 587. [5] K. Ohkawa, A. Tsujimura, T. Nishikawa, S. Yoshii, T. Yokogawa, M. Kubo, Y. Sasai, J. Crystal Growth 159 (1996) 632. [6] N. Obinata, K. Sugimoto, K. Ijima, M. Ishibiki, S. Egawa, T. Honda, H. Kawanishi, Jpn. J. Appl. Phys. 44 (2005) 8432.
[7] I. Akasaki, H. Amano, Jpn. J. Appl. Phys. 36 (1997) 5393. [8] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 31 (1992) 2883. [9] S. Yagi, S. Suzuki, T. Iwanaga, Jpn. J. Appl. Phys. 40 (2001) L1349. [10] A.J. Steckl, J.M. Zavada, MRS Bull. 24 (1999) 33.
Journal of Crystal Growth 301–302 (2007) 424–428 www.elsevier.com/locate/jcrysgro
Low-temperature deposition of hexagonal GaN films for UV electroluminescent devices by CS-MBE technique Tohru Honda, Shinichi Egawa, Koichi Sugimoto, Masatoshi Arai Department of Electronic Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachiohji, Tokyo 192 0015, Japan Available online 24 January 2007
Abstract GaN-based electroluminescent devices (ELDs) operating in UV spectral region are proposed for the UV excitation source and their fabrication process using the compound-source molecular beam epitaxy (CS-MBE) technique. The trial substrates were (0 0 0 1)6H–SiC, Ta2O5/Al, and Ta2O5/glass substrates. In particular, the GaN buffer layer deposited at RT for the fabrication is discussed. The grown films were characterized by cathodoluminescence (CL), electroluminescence (EL), reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The UV-light emission observed from the GaN-based ELDs fabricated using CS-MBE technique is demonstrated under operating conditions of 340 V at 200 Hz (pulsed wave). Red, green, and blue (RGB) pixels using phosphors (Y2O2S:Eu, BaMgAl10O17:Eu+Mn, BaMgAl10O17:Eu, respectively) were also demonstrated. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 78.55.Cr; 78.60.Fi Keywords: A3. Molecular beam epitaxy; B1. Nitrides; B3. Light-emitting devices
1. Introduction Hexagonal GaN-based light-emitting diodes (LEDs) [1,2] have been applied for the application to out-door type large-scale flat panel displays (FPDs). Although the GaN and its related materials have large dislocation density, those are applied to the light-emitting devices. It is reported that a polycrystalline GaN films shows a strong photoluminescence [3]. This is expected the low-cost GaNbased light-emitting devices. For the home-use FPDs using GaN-based materials, we have to realize the low-cost lightemitting devices. Those applications require low-cost light-emitting pixels. In particular, the low-temperature and large-scale integration of GaN-based light-emitting devices are one of the crucial issues for development of them. The low-temperature deposition leads to the wide selectivity of substrates, because the growth of GaN and related materials sometimes requires a high temperature of approximately 1000 1C, which leads to limit the selectivity of the substrates. On the other hand, the Corresponding author. Tel.: +81 42 622 9291; fax: +81 42 625 8982.
E-mail address: [email protected] (T. Honda). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.206
integration of light-emitting devices is cost-effective for the fabrication of FPDs because approximately 1.4 mega LEDs for super video array systems (SVGA, 800 600 pixels) are required. In this study, compound-source molecular beam epitaxy (CS-MBE) [4] growth of hexagonal GaN films at low temperature using (0 0 0 1)6H–SiC, Ta2O5/Al and Ta2O5/ glass substrates was investigated as a preliminary study. GaN-based thin-film electroluminescent devices (TFLEDs) operating in UV to violet spectral region fabricated on Ta2O5/Al and Ta2O5/glass substrates are reported. Finally, red, green and blue (RGB) light-emitting pixels using the GaN-based TF-ELDs as excitation sources are discussed. 2. Compound source-molecular beam epitaxy 2.1. Low-temperature deposition GaN powder, which was synthesized using the reaction between gallium metal and ammonia, was used as a source for CS-MBE. GaN powder was annealed in vacuum at 800 1C for 5 h or longer before the deposition for high
ARTICLE IN PRESS T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
Intensity (Normalized)
GaN/Ta2O5 (RT)
325nm light
GaN/(0001)SiC (RT)
a
GaN/(0001)6HSiC
b
GaN/Ta2O5 e-beam direction: arbitary
Intensity (Relative)
(0 0)
(1 0)
− (1 0)
(2 0)
,y tio n
− (2 0)
ee Scr
,x
tion
een
i pos
Scr
osi
Direct spot
np
n
Direct spot
ee Scr
y
Intensity (Relative)
e//<1120>
n, itio pos
crystallization. The GaN cell temperature was varied from 800 to 860 1C to keep the beam intensity for each deposition. The substrate temperatures of 450 and 500 1C were adopted in this study. The total deposition time was 5 h. A GaN buffer layer was deposited at RT for 15 min before emission layer deposition. No additional nitrogen sources, such as ammonia, were introduced during the deposition. Photoluminescence (PL) spectra of GaN films deposited on (0 0 0 1)6H–SiC and Ta2O5/Al at 450 1C are shown in Fig. 1. A 325 nm line of a He–Cd laser was used for their excitation source. At low temperature of 18 K, the nearband-edge (NBE) emission of GaN crystal was clearly observed in the spectrum of the GaN film deposited on the SiC substrate. On the other hand, the NBE emission component of the spectra observed at RT was somewhat weak compared with that observed at low temperature although its spectrum was starting from the near band edge of GaN. The shape of the spectrum of the GaN film deposited on Ta2O5/Al is similar to that of GaN film grown on (0 0 0 1)6H–SiC. RHEED patterns of the GaN film on (0 0 0 1)6H–SiC are shown in Fig. 2(a). Those indicate that hexagonal GaN crystals were grown on (0 0 0 1)6H–SiC at 450 1C. On the other hand, RHEED patterns of the film on Ta2O5/Al showed blunt ring patterns as shown Fig. 2(b), which indicated that the GaN film was amorphous. CS-MBE has been used for the fabrication of ZnSebased laser diodes (LDs) and the easy controllability of the composition in each of these layers was achieved [5]. In the case of GaN growth, this growth technique is able to use the low-temperature growth. A part of GaN powder heated in K-cell is decomposed and it changes Ga metals and N2 gas, although another part of GaN is sublimated. Thus the growth conditions are Ga rich. However, the vapor pressure of Ga metal is higher than that of GaN. The
425
,x
tion
een
i pos
Scr
Fig. 2. RHEED patterns of GaN films deposited on (a) (0 0 0 1)6H–SiC and (b) Ta2O5. Acceleration voltage of electron beam is 17 kV.
absorbed excess Ga tends to be re-evaporated by the thermal energy on the surface of substrates [6]. 2.2. Low-temperature buffer The introduction of a GaN low-temperature buffer deposited at RT was studied. Although the low-temperature buffer is well-known technique for the GaN growth [7], we had to confirm its effectiveness because the deposition temperature of conventional buffer layers was higher than the deposition temperature of GaN films in this study. The GaN buffer layer was deposited at RT prior to the GaN deposition at 500 1C. The K-cell temperature was kept at 860 1C. Its deposited time is 15 min. The images of atomic force microscopy (AFM) are shown in Fig. 3. The surface smoothness of the GaN film was improved by introducing the RT buffer layer on the Ta2O5 surface. Furthermore, cathodoluminescence (CL) intensity observed form the film with buffer was also increased compared with that without it. Those spectra observed at RT are shown in Fig. 4. The introduction of RT buffer layer for the GaN deposition on Ta2O5/Al is effective for the surface smoothness and the increase of CL intensity. Those are the same results as the case of GaN growth on sapphire substrate [8]. However, in this case, because the deposited films are amorphous, the introduction seems to affect the change of the surface free energy of the substrate surface. 3. Fabrication of electroluminescent devices 3.1. GaN-based UV ELDs
GaN/(0001)SiC (18 K)
2.0
2.5
3.0
3.5
Photon energy (eV) Fig. 1. PL spectra of GaN films deposited on (0 0 0 1)6H–SiC and Ta2O5. A 325 nm light from He–Cd laser was used for the excitation source.
Before the GaN deposition, a Ta2O5 layer, which was used as a bottom insulator, was formed on an Al substrate by spin coating for the fabrication of GaN-based TFELDs. After the GaN deposition, an organic insulator layer (cyanoethylpullulan) was spin-coated on the film.
ARTICLE IN PRESS T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
426
Fig. 3. AFM surface images of GaN films (a) with no buffer and (b) with buffer. Observed areas are 20 20 mm.
4 RT
3 2
With buffer
1
Without buffer
Intensity (arb. units)
Intensity (Relative)
RT
0 200
400
600
800
Wavelength (nm) Fig. 4. CL spectra of GaN films with and without with buffer. Observed temperature was RT.
Finally, an Al upper electrode was evaporated onto the surface. The GaN-based TF-ELDs were operated with an AC voltage of 350 V at 200 Hz. UV light emission, with a peak located at 375 nm, was observed. Its electroluminescent (EL) spectrum is shown in Fig. 5. Here, UV light emission was observed around the top electrode. The shape of the EL spectrum looks like to be different from the CL spectrum of the GaN film. The EL spectrum includes all dynamic changes by a pulsed voltage. The EL spectrum corresponds to the NBE spectrum region in the CL spectrum. 3.2. Fabrication of RGB pixels A schematic of RGB pixels fabricated on GaN-based TF-ELDs is shown in Fig. 6. The RGB phosphors were positioned around the electrodes, as shown in Fig. 6. The RGB phosphors Y2O2S:Eu, BaMgAl10O17:Eu+Mn, and BaMgAl10O17:Eu (NP-2310-57, NP-108-62, NP-107-58, respectively, Nichia Chemical Industries) were used in this study. The RGB light emission peaks located at 457, 517 and 626 nm. They were excited by UV emissions from the GaN-
200
300
400
500
600
700
800
900
Wavelength (nm) Fig. 5. EL spectrum of GaN-based TF-ELD at RT. It was operated with a pulsed AC voltage of 350 V at 200 Hz.
based TF-ELD placed in each pixel. These spectra were shown in Fig. 7. These phosphors are used for the fabrication of cathode ray tubes (CRTs). The results clarified that the UV light radiated from the GaN-based TF-ELDs is enough to excite these RGB phosphors. 4. Discussion The low-cost GaN light-emitting devices using Mgdoped GaN combined with indium tin oxide (ITO) electrodes [9] and rare-earth-doped GaN diodes [10] have already been proposed. We consider that TF-ELDs using low-temperature GaN films by CS-MBE technique provide one of the technical routes to cost-effective FPDs. The advantage of a full-color system using TF-ELDs is that it is easy to integrate them. The system is composed of UV light-emitting devices and RGB phosphors. We can separate the integration of UV light-emitting devices and RGB phosphors. This leads to the cost-effective fabrication. However, some disadvantages were found. One disadvantage is the high operation voltage presently used.
ARTICLE IN PRESS T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
RGB pixels
427
Phosphor Electrode Insulator AC
GaN Substrate (Electrode)
Fig. 6. RGB light-emission system excited by UV GaN-based TF-ELDs.
RT 200 Intensity (relative)
Intensity (arb. units)
Y2O2S: Eu (Red)
Glass substrate 100 Al substrate
BaMgAl10O17: Eu+Mn (Green)
0 10
100
1000
Time (sec) BaMgAl10O17: Eu (Blue)
200
300
400
500
600
700
800
Fig. 8. Lifetime of GaN-based TF-ELDs using different substrates.
900
Wavelength (nm) Fig. 7. PL spectra of RGB light-emission system excited by UV GaNbased TF-ELDs. Y2O2S:Eu, BaMgAl10O17:Eu+Mn, and BaMgAl10O17:Eu were used as red, green and blue phosphors, respectively.
films. The UV light emission correspond to the NBE emission of hexagonal GaN was observed from the TF-ELDs, which were operated using AC voltage at RT. RGB lighting pixels were fabricated using UV GaN-based TF-ELDs combined with RGB phosphors, along with GaN deposition. Acknowledgments
The TF-ELDs require the application of high electric field. This field will be reduced to adopt a thin emission layer in a GaN film. A second disadvantage is the thermal generation in the films. This affects the device lifetime. The lifetimes of GaN-based TF-ELDs using different substrates are shown in Fig. 8. The lifetime of the TF-ELD deposited on Al substrate is longer than that on fused-glass substrate. It is due to the different of their thermal conductivities. Thus device design considering the thermal conductivity is required for the future fabrication of the RGB pixels.
The authors thank Professors Emeriti Y. Suematsu and K. Iga of the Tokyo Institute of Technology for their encouragement. We also thank Professor H. Kawanishi of Kogakuin University for his support, and Nichia Chemical Industries, Ltd. For supplying RGB phosphors. This work was supported by a Grant-in-Aid (No. #18560344) from the Ministry of Education, Culture, Sports, Science and Technology. References
5. Summary The low-temperature deposition of GaN films was achieved by CS-MBE technique. UV GaN-based TF-ELDs were fabricated using the low-temperature deposited GaN
[1] I. Akasaki, H. Amano, I. Suemune, Inst. Phys. Conf. Ser. 142 (1995) 7. [2] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [3] K. Iwata, H. Asahi, K. Asami, R. Kuroiwa, S. Gonda, Jpn. J. Appl. Phys. 36 (1997) L661.
ARTICLE IN PRESS 428
T. Honda et al. / Journal of Crystal Growth 301–302 (2007) 424–428
[4] T. Honda, K. Sato, T. Hashimoto, M. Shinohara, H. Kawanishi, Phys. Stat. Solidi (a) 188 (2001) 587. [5] K. Ohkawa, A. Tsujimura, T. Nishikawa, S. Yoshii, T. Yokogawa, M. Kubo, Y. Sasai, J. Crystal Growth 159 (1996) 632. [6] N. Obinata, K. Sugimoto, K. Ijima, M. Ishibiki, S. Egawa, T. Honda, H. Kawanishi, Jpn. J. Appl. Phys. 44 (2005) 8432.
[7] I. Akasaki, H. Amano, Jpn. J. Appl. Phys. 36 (1997) 5393. [8] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 31 (1992) 2883. [9] S. Yagi, S. Suzuki, T. Iwanaga, Jpn. J. Appl. Phys. 40 (2001) L1349. [10] A.J. Steckl, J.M. Zavada, MRS Bull. 24 (1999) 33.