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The characteristics of inorganic electroluminescent devices with an amorphous diamond film as cathode material
Thin Solid Films 517 (2009) 1821–1824
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
The characteristics of inorganic electroluminescent devices with an amorphous diamond film as cathode material Sea-Fue Wang a,⁎, Jui-Chen Pu a, James C. Sung a,b a b
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan KINIK Company, Taipei, Taiwan
a r t i c l e
i n f o
Article history: Received 2 July 2007 Received in revised form 24 September 2008 Accepted 3 October 2008 Available online 14 October 2008 Keywords: Diamond like carbon Electroluminescence Luminosity Phosphor Electroluminescent devices Cathode materials Cathodic arc physical vapor deposition
a b s t r a c t Diamond like carbon (DLC) thin films were used as the cathode layers of inorganic alternating current driven thick dielectric electroluminescent devices. The results indicated that electroluminescent (EL) devices with DLC cathode has superior brightness over the EL with Al or Cr-doped DLC cathodes. Cr-doping in DLC thin film can increase the electrical conductivity, but degrades the EL properties. Also, the EL device with DLC cathode possesses the lowest decay rate among various cathodes, because of the high thermal conductivity and the inert nature of DLC film. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Among the mature display technologies, the rise of electroluminescent display (ELD) is justified by the advantages including wide viewing angle, wide operation temperature range and inherent ruggedness. Electroluminescent (EL) devices have a long history that traces back to 1936 when Destriau discovered that the ZnS powder dispersed in castor oil could be illuminated by applying an alternating current (AC) electrical field. According to the nature of phosphor materials, EL devices can be categorized into inorganic and organic. Inorganic EL technology is a dark horse in the display race because of its increasingly rapid progress and undeniable advantages in image quality and manufacturing simplicity [1]. Recently, AC driven inorganic EL technologies, including thin film EL, thick dielectric EL (TDEL), and black-dielectric EL, have attracted considerable attention. After mass production and research and development efforts, two conclusions are clearly emerging: (1) monochrome TFEL technology remains viable and profitable and (2) the quest for significant expansion of inorganic EL technology now depends on advances in variations of TDEL display. TDEL display with an easily manufactured screenprinted thick film dielectric layer (≈20 μm in thickness) possesses strong ⁎ Corresponding author. Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei, Taiwan, ROC. Tel.: +886 2 27712171x2735; fax: +886 2 27317185. E-mail address: [email protected] (S.-F. Wang). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.002
diffuse emission outcoupling, which substantially boosts the panel luminance and efficiency [2]. There are two major obstacles that EL has not overcome, viz., the high voltage required to trigger the illumination, and the rapid decay of the luminosity with time. Though, diamond-like carbon (DLC) has been used as an electron injection layer in organic light emitting diodes [3–5], as a cathode material in field emission display [6], as a buffer layer in polymeric electroluminescent devices [7], and as a nano-electrode in electrochemistry [8], up to now, it has not been employed in inorganic ELs yet. In this study, DLC and Cr doped-DLC thin films were selected as cathode material for the inorganic AC driven TDEL devices, because they possess relatively high thermal conductivity, which is expected to reduce the degradation caused by the poor thermal management. For comparison, a conventional TDEL device with Al electrode, which is a low work-function metal and commonly used for ELD's cathode, was also fabricated. PL spectra measurement, electroluminescent brightness, intensity, and aging characteristics of the EL devices were characterized. The effect of cathode materials on the performance of EL devices was discussed. 2. Experimental procedure EL devices with the cross-sectional structure shown in Fig. 1 were used in this study. Amorphous diamond films were deposited on Si substrate by a cathodic arc physical vapor deposition (PVD). The target size and the current density for cathodic arc PVD are 1 in. in diameter
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S.-F. Wang et al. / Thin Solid Films 517 (2009) 1821–1824
Fig. 1. Structure of EL device using DLC film as cathode material.
and 7.9 A/cm2, respectively. Si wafers were initially etched by Ar ions for 20 min. Subsequently, it was implanted with C ions under a high bias for 1 min. The wafer was then bombarded by C ions at a lower energy to deposit amorphous diamond. Cr doped-DLC thin film was prepared in such a way that Cr interlayer was deposited first, and then Cr doped-DLC was coated on the top of the Cr interlayer. Cr doped-DLC film and the Cr interlayer were deposited in the same chamber with a Cr target and a graphite target in Ar discharge under the same negative bias of 20 V. The arc current was 40 A for Cr and 80 A for graphite. Qualitative chemical analysis on the Cr-doped film was performed by an Auger Electron Spectroscopy (AES), to determine the concentration of Cr in the film. The qualitative sp2/sp3 ratio of the DLC films was further characterized by a Raman UV spectroscopy (Jobin Yvon HR800UV). Raman spectra were recorded by exciting with 514.5 nm line of an Ar laser. Al thin films were prepared by radio-frequency (RF) magnetron sputter deposition. Argon with at least 99.995% pure was used as the working gas. The target size and the power density for RF sputtering are 2 in. in diameter and 7.4 W/cm2, respectively. Prior to deposition, the target was sputter-cleaned. Working pressure was maintained at 2.67 Pa (2 × 10− 2 Torr) while RF power was 150 W. Films were sputtered onto an n type (100) silicon wafer. Nominal film thickness is 600 nm. Indium tin oxide (ITO) glass (AFC, Applied Films Company, USA) with the ITO thickness of 1800 Å and the sheet resistance of 7.5 Ω/□, was used to construct the EL device. ZnS:Cu,Cl phosphor layer with thickness of 10 μm was uniformly coated on the ITO glass using screen-printing method [9]. The phosphor paste contains the weight ratios of 50% phosphor and 50% binder of epoxy resins. Subsequently, the screen-printed phosphor film was dried at 80 °C for 5 h. After screen printing an additional epoxy resin (20 μm) on the phosphor films, it was glued to the cathode substrate. The multilayer structure was then dried at 80 °C for 5 h. The insulating layer of epoxy, inserted
Fig. 3. Raman spectra for (a) DLC film and (b) Cr doped-DLC film.
between the active phosphor layer and the DLC rear electrode, was used to prevent catastrophic breakdown during the current flow under a high field (N106 V/cm). Experiments were performed on the EL devices with various cathode thin films. The morphological features of the cross-sections and the thicknesses of the cathode thin films were observed by a field emission scanning electron microscopy (SEM, HITACHI S-4700), at an operation voltage of 15 V. Electrical characteristics of the cathode thin films were measured using a four-point-probe-resistance measurement system (Mitsubishi Chemical MCP-T600). Photoluminescence of the EL devices was measured with a Fluorescence Spectrophotometer (HITACHI F-4500) and a Chroma Meter (MINOLTA CS-100). Measurements were performed on a fixed frequency of 60 Hz and within voltages ranging from 0 to 130 V, using a sine-waveform AC power supplier. 3. Results and discussion Fig. 2 shows the SEM micrographs of the cross-sections of DLC film and Cr doped-DLC film. It is evident that a dense microstructure of DLC film was observed compared with those of Cr-DLC layer and Cr interlayer. The thickness of the DLC film is 200 nm, while the thicknesses of the Cr interlayer and Cr doped-DLC film are 70 and 600 nm, respectively. Surface roughness of all films, which would impact the intensity of AC field implemented and the life-time of the EL device, was minimized through careful monitoring of the deposition process. Based on the results of AES analysis, the average Cr and C contents in the Cr-doped DLC film are 35.8 at% and 64.2%, respectively.
Fig. 2. SEM micrographs of the cross-section for (a) DLC film and (b) Cr doped-DLC film.
S.-F. Wang et al. / Thin Solid Films 517 (2009) 1821–1824
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Table 1 Resistivity and sheet resistance for cathodes including Al film, DLC film and Cr dopedDLC film Cathode materials
Resistivity (Ω-cm)
Sheet resistance (Ω/□)
Al film DLC film Cr doped-DLC film
1.212 × 10− 4 2.258 × 10− 2 8.066 × 10− 4
2.021 × 100 1.129 × 103 1.220 × 101
Fig. 3 shows the Raman spectra of DLC film and Cr doped-DLC film, which were used to identify the sp3 and sp2 bond conversion. The two distinct broad peaks located at about 1350 cm− 1 and 1550 cm− 1 agree well with those in the literature for the characteristic Raman D band, associated with disordered graphite, and G band, associated with crystalline graphite, of the DLC film [10]. Raman spectrum of the DLC shown in Fig. 3(a) indicates a strong and intense peak that confirms the existence of sp2 and sp3 bonds, while, Fig. 3(b) shows that the intensity of Raman spectra for Cr doped-DLC film is quite weak relative to the DLC film, which indicates a fairly weak bonding of graphitic phase. This is consistent with the reports of literature, that the C gradually looses its sp3 type bonding and the DLC network steadily breaks down as the Cr content in the film increases [11–13]. Table 1 shows the resistivities and the sheet resistances of Al, DLC and Cr-DLC films. As expected, the Al film possesses the lowest resistivity and sheet resistance, followed by the Cr-doped in DLC film and then DLC film. Doping with low level of chromium was reported to create a two-dimensional array of metal clusters within the DLC matrix and maintain a metallic bonding, which is of interest for uses such as nano-electrodes in electrochemistry [12]. The DLC film possesses the highest resistivity, because when higher sp3 structure exists in the DLC film, generally, more of the insulating property is obtained [14]. Fig. 4 shows the comparison of electroluminescent brightness against applied voltage for the EL devices with various cathode films. Among the three cathode materials, the EL device with DLC cathode yields the highest brightness intensity. EL brightness for device with Al cathode is similar to that with Cr doped DLC cathode. Though they possess lower resistivities, they have much lower brightness than that with DLC cathode. The results clearly indicate that DLC is a good candidate for the inorganic electroluminescent devices. Though it is known that DLC possesses a lower work function (1.1 eV) compared with those of Al (4.3 eV) and Cr-doped DLC [3,4], and a negative electron affinity which brought the electron injection enhancement in other devices, the exact contributing mechanism for the good performance requires further investigation.
Fig. 4. Comparison of electroluminescent brightness against applied voltage for ELDs with various cathodes.
Fig. 5. Electroluminescent spectra at various applied voltages for ELD with DLC film as cathode.
Fig. 5 shows the electroluminescent spectra of the ELD with DLC films used as cathode materials at various applied voltages. The EL spectra show the emission band at around 460–500 nm, which corroborates the presence of the ZnS:Cu,Cl phosphor. The ZnS:Cu,Cl EL is having hetero-junction that is comprised of n-type semiconductor ZnS and p-type CuxS [15]. When the ZnS lattice is activated with Cu (activator) and Cl (co-activator), donor (co-activator)–acceptor (activator) pairs are formed. The EL is caused by the radiative recombination of electron–hole pairs at the donor–acceptor sites. The combination of Cu and Cl gives blue (~ 460 nm) and green emission bands, their relative intensity depending on the relative amount of Cu to Cl [16]. The Cu forms thin embedded CuxS needles in the crystal matrix. CuxS is known to be a p-type semiconductor. As an applied electric field of 104 to 105 V/cm is applied on the phosphor, it can induce a local field of 106 V/cm or more inside. The electrons emitted from the cathode are captured in shallow traps, probably in Cl donor sites, while the holes are trapped by the Cu recombination centers. When the electric field is reversed, the emitted electrons recombine with trapped holes to produce EL. The required electric field in the phosphor layer is around 105 V/cm and hence a voltage drop across a 10 μm phosphor layer of 100 V is typical. The emission intensity of the EL devices increases with the applied voltage, whereas the increase in voltage does not affect the emission color. This suggests that the voltage has no influence on the spectral distribution of the emission. Fig. 6 shows the aging characteristics of the brightness for EL devices with various cathode films. Measurements were evaluated
Fig. 6. Aging characteristics of brightness for the EL devices with three kinds of cathodes including Al, DLC and Cr doped-DLC films (the driving frequency is 5 kHz).
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under a driving frequency of 5 kHz and an applied voltage of 240 V. The electroluminescent spectral intensity of EL devices has reduced very rapidly in the initial 24 h. The EL device with DLC cathode possesses the lowest decay rate, while EL with Al cathode shows inferior stability when operating in air. This is due to the fact that the thermal conductivity of DLC (600–800 W/m K) is much larger than that of Al (210 W/m K). The high thermal conductivity and inert nature of DLC cathode can ensure that electrons flow more uniformly in the phosphor layer to avoid local heating, which lead to a better thermal stability as well as a smaller decay rate of luminosity. 4. Summary In this study, the EL devices with various cathodes including Al, DLC and Cr-DLC thin films were fabricated. The brightness efficiency for EL device with DLC cathode is better than with aluminum cathode at same applied voltage. Cr-doping in DLC thin film can increase the electrical conductivity, but degrades the EL efficiency. The aging rate of the EL device with the Al cathode is the largest among the devices due to the rapid oxidation and corrosion. The EL device with DLC cathode possesses the lowest decay rate because of its high thermal conductivity and inert nature.
References [1] A.N. Krasnov, Displays 24 (2003) 73. [2] J.C. Heikenfeld, A.J. Steckl, Inf. Disp. 12/03 (2003) 20. [3] G.E. Jabbour, Y. Kawabe, S.E. Shaheen, J.F. Wang, M.M. Morrell, B. Kippelen, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1762. [4] K. Lmimouni, C. Legrand, C. Dufour, A. Chapoton, C. Belouet, Appl. Phys. Lett. 78 (2001) 2437. [5] D.W. Han, S.M. Jeong, H.K. Baik, S.J. Lee, N.C. Yang, D.H. Suh, Thin Solid Films 420– 421 (2004) 190. [6] H. Ma, L. Zhang, N. Yao, B. Zhang, H. Hu, G. Wen, Diamond Relat. Mater. 9 (2000) 1608. [7] S.H. Choi, S.M. Jeong, W.H. Koo, S.J. Jo, H.K. Baik, S.J. Lee, K.M. Sung, D.W. Han, Thin Solid Films 483 (2005) 351. [8] T.N. Rao, B.H. Loo, B.V. Sarada, C. Terashima, A. Fujishima, Anal. Chem. 74 (2002) 1578. [9] G. Sharma, S.D. Han, J.D. Kim, S.P. Khatkar, Y.W. Rhee, Mater. Sci. Eng., B 131 (2006) 271. [10] C.L. Chang, J.Y. Jao, T.C. Chang, W.Y. Ho, D.Y. Wang, Diamond Relat. Mater.14 (2005) 2127. [11] X. Fan, E.C. Dickey, S.J. Pennycook, M.K. Sunkara, Appl. Phys. Lett. 75 (1999) 2740. [12] V. Singh, V. Palshin, R.C. Tittsworth, Carbon 44 (2006) 1280. [13] T.V. Murzina, R.V. Kapra, O.A. Aktsipetrov, V.F. Dorfman, Appl. Phys. Lett. 83 (2003) 4749. [14] M.C. Kan, J.L. Huang, J.C. Sung, K.H. Chen, D.F. Li, Diamond Relat. Mater. 12 (2003) 1691. [15] T. Kryshtab, V.S. Khomchenko, J.A. Andraca-Adame, L.V. Zavyalova, N.N. Roshchina, V.E. Rodionov, V.B. Khachatryan, Thin Solid Films 515 (2006) 513. [16] S. Shionoya, W.M. Yen, Phosphor Research Society, Phosphor Handbook, CRC Press, Athens, GA, 1998 U.S.A.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
The characteristics of inorganic electroluminescent devices with an amorphous diamond film as cathode material Sea-Fue Wang a,⁎, Jui-Chen Pu a, James C. Sung a,b a b
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan KINIK Company, Taipei, Taiwan
a r t i c l e
i n f o
Article history: Received 2 July 2007 Received in revised form 24 September 2008 Accepted 3 October 2008 Available online 14 October 2008 Keywords: Diamond like carbon Electroluminescence Luminosity Phosphor Electroluminescent devices Cathode materials Cathodic arc physical vapor deposition
a b s t r a c t Diamond like carbon (DLC) thin films were used as the cathode layers of inorganic alternating current driven thick dielectric electroluminescent devices. The results indicated that electroluminescent (EL) devices with DLC cathode has superior brightness over the EL with Al or Cr-doped DLC cathodes. Cr-doping in DLC thin film can increase the electrical conductivity, but degrades the EL properties. Also, the EL device with DLC cathode possesses the lowest decay rate among various cathodes, because of the high thermal conductivity and the inert nature of DLC film. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Among the mature display technologies, the rise of electroluminescent display (ELD) is justified by the advantages including wide viewing angle, wide operation temperature range and inherent ruggedness. Electroluminescent (EL) devices have a long history that traces back to 1936 when Destriau discovered that the ZnS powder dispersed in castor oil could be illuminated by applying an alternating current (AC) electrical field. According to the nature of phosphor materials, EL devices can be categorized into inorganic and organic. Inorganic EL technology is a dark horse in the display race because of its increasingly rapid progress and undeniable advantages in image quality and manufacturing simplicity [1]. Recently, AC driven inorganic EL technologies, including thin film EL, thick dielectric EL (TDEL), and black-dielectric EL, have attracted considerable attention. After mass production and research and development efforts, two conclusions are clearly emerging: (1) monochrome TFEL technology remains viable and profitable and (2) the quest for significant expansion of inorganic EL technology now depends on advances in variations of TDEL display. TDEL display with an easily manufactured screenprinted thick film dielectric layer (≈20 μm in thickness) possesses strong ⁎ Corresponding author. Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei, Taiwan, ROC. Tel.: +886 2 27712171x2735; fax: +886 2 27317185. E-mail address: [email protected] (S.-F. Wang). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.002
diffuse emission outcoupling, which substantially boosts the panel luminance and efficiency [2]. There are two major obstacles that EL has not overcome, viz., the high voltage required to trigger the illumination, and the rapid decay of the luminosity with time. Though, diamond-like carbon (DLC) has been used as an electron injection layer in organic light emitting diodes [3–5], as a cathode material in field emission display [6], as a buffer layer in polymeric electroluminescent devices [7], and as a nano-electrode in electrochemistry [8], up to now, it has not been employed in inorganic ELs yet. In this study, DLC and Cr doped-DLC thin films were selected as cathode material for the inorganic AC driven TDEL devices, because they possess relatively high thermal conductivity, which is expected to reduce the degradation caused by the poor thermal management. For comparison, a conventional TDEL device with Al electrode, which is a low work-function metal and commonly used for ELD's cathode, was also fabricated. PL spectra measurement, electroluminescent brightness, intensity, and aging characteristics of the EL devices were characterized. The effect of cathode materials on the performance of EL devices was discussed. 2. Experimental procedure EL devices with the cross-sectional structure shown in Fig. 1 were used in this study. Amorphous diamond films were deposited on Si substrate by a cathodic arc physical vapor deposition (PVD). The target size and the current density for cathodic arc PVD are 1 in. in diameter
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S.-F. Wang et al. / Thin Solid Films 517 (2009) 1821–1824
Fig. 1. Structure of EL device using DLC film as cathode material.
and 7.9 A/cm2, respectively. Si wafers were initially etched by Ar ions for 20 min. Subsequently, it was implanted with C ions under a high bias for 1 min. The wafer was then bombarded by C ions at a lower energy to deposit amorphous diamond. Cr doped-DLC thin film was prepared in such a way that Cr interlayer was deposited first, and then Cr doped-DLC was coated on the top of the Cr interlayer. Cr doped-DLC film and the Cr interlayer were deposited in the same chamber with a Cr target and a graphite target in Ar discharge under the same negative bias of 20 V. The arc current was 40 A for Cr and 80 A for graphite. Qualitative chemical analysis on the Cr-doped film was performed by an Auger Electron Spectroscopy (AES), to determine the concentration of Cr in the film. The qualitative sp2/sp3 ratio of the DLC films was further characterized by a Raman UV spectroscopy (Jobin Yvon HR800UV). Raman spectra were recorded by exciting with 514.5 nm line of an Ar laser. Al thin films were prepared by radio-frequency (RF) magnetron sputter deposition. Argon with at least 99.995% pure was used as the working gas. The target size and the power density for RF sputtering are 2 in. in diameter and 7.4 W/cm2, respectively. Prior to deposition, the target was sputter-cleaned. Working pressure was maintained at 2.67 Pa (2 × 10− 2 Torr) while RF power was 150 W. Films were sputtered onto an n type (100) silicon wafer. Nominal film thickness is 600 nm. Indium tin oxide (ITO) glass (AFC, Applied Films Company, USA) with the ITO thickness of 1800 Å and the sheet resistance of 7.5 Ω/□, was used to construct the EL device. ZnS:Cu,Cl phosphor layer with thickness of 10 μm was uniformly coated on the ITO glass using screen-printing method [9]. The phosphor paste contains the weight ratios of 50% phosphor and 50% binder of epoxy resins. Subsequently, the screen-printed phosphor film was dried at 80 °C for 5 h. After screen printing an additional epoxy resin (20 μm) on the phosphor films, it was glued to the cathode substrate. The multilayer structure was then dried at 80 °C for 5 h. The insulating layer of epoxy, inserted
Fig. 3. Raman spectra for (a) DLC film and (b) Cr doped-DLC film.
between the active phosphor layer and the DLC rear electrode, was used to prevent catastrophic breakdown during the current flow under a high field (N106 V/cm). Experiments were performed on the EL devices with various cathode thin films. The morphological features of the cross-sections and the thicknesses of the cathode thin films were observed by a field emission scanning electron microscopy (SEM, HITACHI S-4700), at an operation voltage of 15 V. Electrical characteristics of the cathode thin films were measured using a four-point-probe-resistance measurement system (Mitsubishi Chemical MCP-T600). Photoluminescence of the EL devices was measured with a Fluorescence Spectrophotometer (HITACHI F-4500) and a Chroma Meter (MINOLTA CS-100). Measurements were performed on a fixed frequency of 60 Hz and within voltages ranging from 0 to 130 V, using a sine-waveform AC power supplier. 3. Results and discussion Fig. 2 shows the SEM micrographs of the cross-sections of DLC film and Cr doped-DLC film. It is evident that a dense microstructure of DLC film was observed compared with those of Cr-DLC layer and Cr interlayer. The thickness of the DLC film is 200 nm, while the thicknesses of the Cr interlayer and Cr doped-DLC film are 70 and 600 nm, respectively. Surface roughness of all films, which would impact the intensity of AC field implemented and the life-time of the EL device, was minimized through careful monitoring of the deposition process. Based on the results of AES analysis, the average Cr and C contents in the Cr-doped DLC film are 35.8 at% and 64.2%, respectively.
Fig. 2. SEM micrographs of the cross-section for (a) DLC film and (b) Cr doped-DLC film.
S.-F. Wang et al. / Thin Solid Films 517 (2009) 1821–1824
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Table 1 Resistivity and sheet resistance for cathodes including Al film, DLC film and Cr dopedDLC film Cathode materials
Resistivity (Ω-cm)
Sheet resistance (Ω/□)
Al film DLC film Cr doped-DLC film
1.212 × 10− 4 2.258 × 10− 2 8.066 × 10− 4
2.021 × 100 1.129 × 103 1.220 × 101
Fig. 3 shows the Raman spectra of DLC film and Cr doped-DLC film, which were used to identify the sp3 and sp2 bond conversion. The two distinct broad peaks located at about 1350 cm− 1 and 1550 cm− 1 agree well with those in the literature for the characteristic Raman D band, associated with disordered graphite, and G band, associated with crystalline graphite, of the DLC film [10]. Raman spectrum of the DLC shown in Fig. 3(a) indicates a strong and intense peak that confirms the existence of sp2 and sp3 bonds, while, Fig. 3(b) shows that the intensity of Raman spectra for Cr doped-DLC film is quite weak relative to the DLC film, which indicates a fairly weak bonding of graphitic phase. This is consistent with the reports of literature, that the C gradually looses its sp3 type bonding and the DLC network steadily breaks down as the Cr content in the film increases [11–13]. Table 1 shows the resistivities and the sheet resistances of Al, DLC and Cr-DLC films. As expected, the Al film possesses the lowest resistivity and sheet resistance, followed by the Cr-doped in DLC film and then DLC film. Doping with low level of chromium was reported to create a two-dimensional array of metal clusters within the DLC matrix and maintain a metallic bonding, which is of interest for uses such as nano-electrodes in electrochemistry [12]. The DLC film possesses the highest resistivity, because when higher sp3 structure exists in the DLC film, generally, more of the insulating property is obtained [14]. Fig. 4 shows the comparison of electroluminescent brightness against applied voltage for the EL devices with various cathode films. Among the three cathode materials, the EL device with DLC cathode yields the highest brightness intensity. EL brightness for device with Al cathode is similar to that with Cr doped DLC cathode. Though they possess lower resistivities, they have much lower brightness than that with DLC cathode. The results clearly indicate that DLC is a good candidate for the inorganic electroluminescent devices. Though it is known that DLC possesses a lower work function (1.1 eV) compared with those of Al (4.3 eV) and Cr-doped DLC [3,4], and a negative electron affinity which brought the electron injection enhancement in other devices, the exact contributing mechanism for the good performance requires further investigation.
Fig. 4. Comparison of electroluminescent brightness against applied voltage for ELDs with various cathodes.
Fig. 5. Electroluminescent spectra at various applied voltages for ELD with DLC film as cathode.
Fig. 5 shows the electroluminescent spectra of the ELD with DLC films used as cathode materials at various applied voltages. The EL spectra show the emission band at around 460–500 nm, which corroborates the presence of the ZnS:Cu,Cl phosphor. The ZnS:Cu,Cl EL is having hetero-junction that is comprised of n-type semiconductor ZnS and p-type CuxS [15]. When the ZnS lattice is activated with Cu (activator) and Cl (co-activator), donor (co-activator)–acceptor (activator) pairs are formed. The EL is caused by the radiative recombination of electron–hole pairs at the donor–acceptor sites. The combination of Cu and Cl gives blue (~ 460 nm) and green emission bands, their relative intensity depending on the relative amount of Cu to Cl [16]. The Cu forms thin embedded CuxS needles in the crystal matrix. CuxS is known to be a p-type semiconductor. As an applied electric field of 104 to 105 V/cm is applied on the phosphor, it can induce a local field of 106 V/cm or more inside. The electrons emitted from the cathode are captured in shallow traps, probably in Cl donor sites, while the holes are trapped by the Cu recombination centers. When the electric field is reversed, the emitted electrons recombine with trapped holes to produce EL. The required electric field in the phosphor layer is around 105 V/cm and hence a voltage drop across a 10 μm phosphor layer of 100 V is typical. The emission intensity of the EL devices increases with the applied voltage, whereas the increase in voltage does not affect the emission color. This suggests that the voltage has no influence on the spectral distribution of the emission. Fig. 6 shows the aging characteristics of the brightness for EL devices with various cathode films. Measurements were evaluated
Fig. 6. Aging characteristics of brightness for the EL devices with three kinds of cathodes including Al, DLC and Cr doped-DLC films (the driving frequency is 5 kHz).
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under a driving frequency of 5 kHz and an applied voltage of 240 V. The electroluminescent spectral intensity of EL devices has reduced very rapidly in the initial 24 h. The EL device with DLC cathode possesses the lowest decay rate, while EL with Al cathode shows inferior stability when operating in air. This is due to the fact that the thermal conductivity of DLC (600–800 W/m K) is much larger than that of Al (210 W/m K). The high thermal conductivity and inert nature of DLC cathode can ensure that electrons flow more uniformly in the phosphor layer to avoid local heating, which lead to a better thermal stability as well as a smaller decay rate of luminosity. 4. Summary In this study, the EL devices with various cathodes including Al, DLC and Cr-DLC thin films were fabricated. The brightness efficiency for EL device with DLC cathode is better than with aluminum cathode at same applied voltage. Cr-doping in DLC thin film can increase the electrical conductivity, but degrades the EL efficiency. The aging rate of the EL device with the Al cathode is the largest among the devices due to the rapid oxidation and corrosion. The EL device with DLC cathode possesses the lowest decay rate because of its high thermal conductivity and inert nature.
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