APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 16
22 APRIL 2002
White electroluminescence from hydrogenated amorphous-SiNx thin films Zingway Pei,a) Y. R. Chang, and H. L. Hwang Department of Electrical Engineering, National Tsing Hua University, Hsin-Chu, 300, Taiwan, Republic of China
共Received 11 July 2001; accepted for publication 25 February 2002兲 White electroluminescence 共EL兲 was observed from hydrogenated amorphous-SiNx -based light-emitting device. Silicon nitride thin films were deposited on the indium-tin-oxide 共ITO兲-coated glass substrate by plasma enhanced chemical vapor deposition method with a mixture of Ar-diluted 5% SiH4 and pure N2 gases, in the ratio 2 to 1. Measured x value of the film is 0.56, and the corresponding photoluminescence of a-SiN0.56 :H thin film exhibited a red-infrared spectrum, centered at 630 nm. The layer structure of the EL device is ITO/a-SiN0.56 :H 共80 nm兲/Al, with light emitting from the ITO layer, recognizable by the naked eye in the dark, under the 14 V forward bias conditions. White EL spectra from ⬃400 to 750 nm, with a central peak at 560 nm, were observed in the hydrogenated amorphous silicon nitride EL device. A carrier transport mechanism was suggested, and the EL was attributed to the recombination of carriers through the luminescent states. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1473230兴
The observation of strong photoluminescence 共PL兲 from silicon at room temperature has stimulated extensive studies in the light emission properties of silicon-based materials.1 The capacity of silicon based materials for optical emission makes low-cost and integrated circuit 共IC兲 compatible optoelectronic devices feasible. Several methods have been applied to produce these luminescent materials, such as electrochemical etching, silicon ion implanted into silicon oxide,2 and plasma deposition of silicon rich dielectric films.3 These silicon materials exhibited strong PL. The possibility of electroluminescence 共EL兲 was also explored to establish silicon based optoelectronic devices. The EL has been exhibited by porous silicon in the Au/p-PS/Si structure4 and Au/p ⫹ -PS/n-PS/n ⫹ -PS/n ⫹ -Si/Al vertical p-i-n diode structure.5 Furthermore, optoelectronic operation was demonstrated in a bipolar transistor operated light-emitting diode structure.6 However, the inherent problems of film instability in the porous silicon EL device prevent it from being used in actual applications. The electrical and structural properties of hydrogenated amorphous-SiNx has been widely studied which is applied in IC technology as an insulator and dielectric film.7 Our previous work revealed that hydrogenated amorphous-SiNx thin films exhibit strong photoluminescence at variable wavelengths from ultraviolet to infrared.8 Therefore, hydrogenated amorphous-SiNx is a candidate for active material in silicon based optoelectronic devices. The hydrogenated amorphous-SiNx was employed as the luminescent active layer in the indium-tin-oxide 共ITO兲 glass/p a-SiC:H/i a-SiN:H/n a-SiC:H/Al p-i-n structure for the EL operation.9 A silicon nitride light emission device was also fabricated in Au/ultrathin silicon nitride 共⬃40 Å兲/Si structure,10 which was directly nitrized the silicon surface by the electron cyclotron resonance chemical vapor deposition technique. Despite such progressive work, the luminescent and carrier a兲
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transport properties of a-SiNx based EL device have not yet been revealed. Moreover, the structure formation requires either an ultrathin layer or a long time high temperature annealing, incompatible with the silicon IC processing. Therefore, the simple making of a hydrogenated amorphous-SiNx EL device with a single active layer of moderate thickness 共⬃80 nm兲 is presented in this work. The device’s proper thickness is such that it is fully compatible with modern IC processes and appropriate to investigate light emitting property from a hydrogenated amorphous-SiHx layer. Additionally, this material and structure have the potential to produce full color emission based on its multiwavelength photoluminescence spectrum.8 The hydrogenated amorphous-SiNx thin film was deposited by the plasma enhanced chemical vapor deposition 共PECVD兲 technique on ITO-coated glass substrate. The substrate had been cleaned in de-ionized water and acetone prior to the film growth. The reacting gases for hydrogenated amorphous-SiNx deposition was the mixture of Ar-diluted 5% SiH4 and pure N2 , in the ratio 2 to 1, while the substrate temperature of 300 °C and the radio frequency 共13.56 MHz兲 power density of 200 mW/cm2 were maintained. The detail compositions of the hydrogenated amorphous-SiNx layer are determined and published elsewhere.8 The x value of this nitride layer was 0.56 and, consequently, was highly siliconrich. An aluminum metal layer was used as the top electrode deposited by vacuum evaporation, with a thickness of 3000 Å. The defined device area was a circle with 5 mm diameter. The thickness of the deposited hydrogenated amorphousSiNx thin film was about 80 nm. Accordingly, the layer structure of the EL device was therefore ITO glass/a-SiN0.56 :H 共80 nm兲/Al as depicted in Fig. 1. Figure 2 shows the current–voltage characteristics of the EL devices, measured under the forward and reverse bias. Forward bias is defined that positive voltage applied on the ITO as the same voltage polarity in Fig. 1, and reverse bias is opposite to the forward bias. When the forward bias exceeded a certain value 共14 V兲, white light emitting was ob-
0003-6951/2002/80(16)/2839/3/$19.00 2839 © 2002 American Institute of Physics Downloaded 21 Dec 2003 to 210.71.228.10. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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Appl. Phys. Lett., Vol. 80, No. 16, 22 April 2002
Pei, Chang, and Hwang
FIG. 1. a-SiN0.56 :H EL device structure.
served immediately through the ITO electrode, with the current density about 100 mA/cm2. This EL emission was recognizable by the naked eye in the dark. In contrast, the current density under a reverse bias was relatively small and EL emission was not observed. Moreover, the quantum efficiency remained low and efficiency is being improved. Figure 3 shows the EL spectrum of the hydrogenated amorphous-SiNx thin film light-emitting device at 22 V forward bias, along with the corresponding PL spectrum. The EL spectrum is centered at around 560 nm and is spread from ⬃400 to 750 nm. Four possible main emissions are marked in the emission spectrum with arrows at 498 nm 共2.48 eV兲, 560 nm 共2.21 eV兲, 590 nm 共2.10 eV兲, and 640 nm 共1.90 eV兲, respectively. However, the PL spectrum was centered at 630 nm 共1.97 eV兲 in the red-infrared region. A small current is recorded when the applied voltage under 5 V in our EL device as shown in Fig. 1 and is increased as the voltage increased. A further increase of the voltage over 10 V, luminescent was recognized and the current was increased extensively. These current–voltage characteristics indicate a transport and carrier recombination mechanisms that dominants the electrical and optical behavior. The suggested mechanism is as followed. Under low bias, the small current is usually called a leakage current in a metal–insulator–conductor structure and comes from the structure imperfection in the insulator films. Therefore, this milliampere range leakage current is the result of high defect concentration in our highly silicon-rich a-SiN0.56 :H thin film. Under higher bias, for the applied voltage large than 10 V, electric field is exceeding 1.25 MV/cm for 80 nm a-SiN0.56 :H EL device. At such high field, EL device is in
FIG. 3. Electroluminescence and photoluminescence spectra of the a-SiN0.56 :H thin film.
the early breakdown regime and occupied gap states are likely to be impact ionized by injected carriers under high electric field.11 The generated carriers would impact other occupied states and then make it ionized. Finally, the multiplications of impact ionized process causes the current is increased extensively. The behavior of the EL emission intensity as a function of the forward bias, shown in Fig. 4共a兲, indicates the integrated emission intensity is increased rapidly as the forward voltage increased support the suggestion of impact ionization mechanism. Some energy states in the hydrogenated amorphous-SiNx thin films are called luminescence centers13 and optical emission is the results of electrons and holes radiative recombined in these centers. Therefore, the intensity of EL device being strong depends on the number of luminescent centers and number of carriers. The plot of integrated EL intensity, shown in Fig. 4共b兲, is increased linearly relative to the current density supports this assertion. We now turn to the optical emission property of EL device. Our plasma deposited hydrogenated amorphous-SiNx : thin films exhibit a much higher density of defects than those of the stoichiometric silicon nitride. The existence of gap states is an inherent property of nonstoichiometric thin films.12 Many mechanisms are responsible for the optical emission. The radiative recombination through a quasidirect energy level as a result of the quantum effect in Si small clusters and recombination through the level between states
FIG. 2. Current 共I兲–voltage (V) characteristics of the a-SiN0.56 :H EL deFIG. 4. 共a兲 EL intensity as a function of the applied voltage and 共b兲 EL vice under forward bias. intensity as a function of the current density. Downloaded 21 Dec 2003 to 210.71.228.10. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
Appl. Phys. Lett., Vol. 80, No. 16, 22 April 2002
in the band gap are the main causes. However, such mechanisms are not easily elucidated. Therefore, the concept of a luminescent center was suggested to explain the optical recombination in the Si-based thin film.10 The three possible main emissions at 2.48, 2.21, and 1.90 eV appeared in our EL device has been reported previously from defect luminescence studies on hydrogenated amorphous-SiNx : thin film. The calculated14 and measured12 emissions between 2.4 and 2.5, 2.2 and 2.3, and 1.9 and 2.0 eV were considered as emissions from the transition between the E c , ⬅Si⫺ , ⬅Si0 , and ⫽N⫺ states. Therefore, the gap states related luminescent centers might have contributed to the light emission in our system. The linear behavior of integrated EL intensity with current density in Fig. 4共b兲 is further support this assertion. Where the number of gap states’ related luminescent centers is also increased during carrier generation process that have more centers for optical recombination, as a result, increase the EL intensity. As a consequence, the electroluminescence spectrum can be explained by optical recombination from the luminescent centers. To summarize, the silicon based EL device has been shown to be successfully made by employing PECVD deposited hydrogenated amorphous-SiNx :thin films as the active layer in the ITO glass/hydrogenated amorphous-SiNx : 共80 nm兲/Al structure. The EL spectrum was to have a wide distribution from 400 to 750 nm and was extensively blueshifted as compared to the PL spectrum. The electroluminescence was suggested as being due to gap states’ impact ionized as a result of high electric field and recombination in the luminescent centers in hydrogenated amorphous-SiNx :thin
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films. And compare to the calculated and measured results, transitions between the E c , ⬅Si⫺ , ⬅Si0 , and ⫽N⫺ states are responsible for the EL Spectrum. The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC87-2215-E-007-031. The Materials Research Center at National Tsing Hua University is also appreciated for its assistance. Professor S. A. Chen is also commended for his assistance in taking the EL measurements. L. T. Canham, Appl. Phys. Lett. 57, 1046 共1990兲. H. Z. Song, X. M. Bao, N. S. Li, and J. Y. Zhang, J. Appl. Phys. 82, 4028 共1997兲. 3 J. F. Tong, H. L. Hsiao, and H. L. Hwang, Appl. Phys. Lett. 74, 2316 共1999兲. 4 P. Steiner, F. Kozlowski, and W. Lang, Appl. Phys. Lett. 62, 2700 共1993兲. 5 J. Linnros and N. Lalic, Appl. Phys. Lett. 66, 3048 共1995兲. 6 K. D. Hirschman, L. Tsybeskov, S. P. Duttagupta, and P. M. Fauchet, Nature 共London兲 384, 338 共1996兲. 7 P. Knapek, B. Rezek, D. Muller, J. J. Grob, R. Levy, K. Luterova, J. Kocka, and I. Pelant, Phys. Status Solidi A 167, R5 共1998兲. 8 Z. Pei, Y. R. Chung, H. L. Hsiao, and H. L. Hwang, ECS proceedings 99-22 共1999兲. 9 W. Boonkosum, D. Kruangam, and S. Panyakeow, Jpn. J. Appl. Phys., Part 1 32, 1534 共1993兲. 10 A. P. Li, L. Zhang, Y. X. Zhang, G. G. Qin, X. Wang, and X. W. Hu, Appl. Phys. Lett. 69, 4 共1996兲. 11 C. T. Sah, Fundamentals of Solid-State Electronics 共World Scientific, Singapore, 1991兲. 12 C. H. Mo, L. Zhang, C. Xie, and T. Wang, J. Appl. Phys. 73, 5185 共1993兲. 13 J. M. Shannon, S. C. Deane, B. McGarvey, and J. N. Sandoe, Appl. Phys. Lett. 65, 2978 共1994兲. 14 J. Robertson and M. J. Powell, Appl. Phys. Lett. 44, 415 共1984兲. 1 2
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