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Formation of silicon nano-dots in luminescent silicon nitride
Applied Surface Science 212–213 (2003) 760–764
Formation of silicon nano-dots in luminescent silicon nitride Zingway Pei*, H.L. Hwang Department of Electrical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan, ROC
Abstract Strong room-temperature photoluminescence (PL) was observed in the hydrogenated silicon-rich silicon nitride (a-SiNx:H) thin films grown by plasma-enhanced chemical vapor deposition technique. After the thermal annealing process, silicon nano-dots were clearly observed in the Si-rich a-SiN0.56:H thin films. From the X-ray photoelectron spectroscopy (XPS), the Si 2p peak shows distinct Si–Si4 bonds after thermal annealing indicated the existence of Si clusters. In addition, the existence of nano-crystallized (nc) Si dots was revealed from the image of high-resolution cross-section transmission electron microscopy (XTEM) analysis. The PL spectra exhibited extensive red-shift from 1.97 to 1.33 eV along with the annealing temperature suggests that, the Si nano-dot related luminescence dominates in the luminescence centers of the a-SiN0.56:H layer. The reveal of Si nano-dots related luminescence and the high density of Si nano-dots suggest that the a-SiN0.56:H layer is suitable for the stable light-emitting device. # 2003 Elsevier Science B.V. All rights reserved. PACS: 71.20.Nr; 78.66 Keywords: Si nano-dots; Silicon nitride; Photoluminescence
1. Introduction The silicon-based luminescent material has attracted great attentions in recent years because it can combine the state-of-the-art silicon integrated circuits with the optoelectronic applications. However, the indirect nature of the silicon band structure prevents the efficient light emission as a result of small radiative dipole transition possibilities. The formation of near zerodimensional structure that breaking of the momentum conservation law and enabling strong radiative recombination is one of the most promising methods to the achieve stable light emissions. For this reason, extensive research efforts have been focused on the strong luminescence from silicon nano-structures [1–5]. *
Corresponding author. Tel.: þ886-3-5913339; fax: þ886-3-5917690. E-mail address: [email protected] (Z. Pei).
Recently, Si-rich silicon nitride layers were found to have intense multi-color emissions, from ultraviolet to infrared by varying the growth conditions [6], especially after the post-annealing process, the intensity of photoluminescence (PL) is largely enhanced. Moreover, the formation of amorphous silicon dots was found in this material [7]. As compared to the relations of emission wavelength to the dot size, the quantum confinement effect could be responsible for the emissions. Si-rich silicon nitride based electroluminescence (EL) device has been recently reported by our group that the spectrum of the emitting light was ranged from blue to near infrared, i.e. white light [8]. However, this white emission is originated from the gap states dominated luminescent centers, because of the inherent problems of high gap states concentrations for the amorphous materials [9], in which the emission from the amorphous silicon nano-dots is suppressed. Therefore, formation of the crystallized
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00369-6
Z. Pei, H.L. Hwang / Applied Surface Science 212–213 (2003) 760–764
silicon nano-dots in amorphous silicon nitride layer by thermal process could potentially enhance the radiative transition by having high-density Si dots and reducing the defect densities is a promising method to have stable luminescence in silicon based light-emitting device. In this paper, investigation on the formation of silicon nano-dots from amorphous to nano-crystal phase by thermal process in luminescent amorphous silicon-rich silicon nitride (a-SiNx:H) material is reported.
2. Experiments The a-SiNx:H thin film was deposited by plasma enhanced chemical vapor deposition (PECVD) technique on n-type (1 0 0) silicon substrate. After the standard clean procedures, the substrate was loaded to the reaction chamber. Hydrogen plasma treatment was applied to clean the silicon surface prior to the deposition of silicon nitride. The reacting gases for a-SiNx:H deposition was the mixture of 5% Ar-diluted SiH4 and pure N2, in the ratio 2:1, while substrate temperature of 300 8C and power density of 200 mW/cm2 was maintained during the deposition. The detailed compositions of the a-SiNx:H layer were determined and published elsewhere. The x-value of this nitride layer is 0.56 and, consequently, it is highly silicon-rich. After the deposition, samples were thermally treated by using AG model 610i rapid annealing furnace at the temperature of 900 8C for 5 min. The X-ray photoelectron spectroscopy (XPS) analysis was carried out by using Mg Ka (hn ¼ 1253:6 eV) X-ray source to determine the chemical structure of the silicon nano-dots. The kinetic energy of the photoelectrons was determined using a spherical capacitor analyzer (SCA) at a constant pass energy mode. The highresolution cross-section transmission electron microscopy (XTEM) was applied to investigate the size and structure of the silicon nano-dots. The room temperature photoluminescence was measured by using the excitation of a He–Cd laser (325 nm) with power of 1 mW focused to a 100 mm spot. The emitted lighted was collected by a 50 mm f/ 1.4 lens in the direction normal to the illuminated surface and was focused on a SPEX 500M monochromator equipped with an InGaAs photodetector, and the standard lock-in technique was used.
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3. Results and discussion The Si 2p binding energy revealed from X-ray photoelectron spectroscopy (XPS) analysis of aSiN0.56:H sample is shown in Fig. 1a, as-deposited and Fig. 1b, with 900 8C rapid thermal annealing. For the as-deposited sample, Si 2p peak is located at 101.4 eV where the control Si has 2p line at the energy of 99.7 eV is also presented. After 900 8C/5 min annealing, the spectra exhibit the multi-peak behavior and could be de-convoluted into five distinct peaks. Concerning the chemical structure of amorphous silicon nitride (a-SiNx:H) thin films, two models have been proposed. One model is the random bonding model proposed by Phillip [10], another one is the stoichiometric statistical model (SSM) proposed by Temkin [11]. The SSM model described the silicon 2p line to be an overlap of the two peaks corresponding to the a-Si and a-Si3N4 phase, with a fixed energy separation of 2.4 eV, and the peak position is a monotonic function of x. According to the XPS spectra of our result, the multi-peak in Si 2p line could not be explained by the SSM model. The a-SiNx network
Fig. 1. The XPS spectra of a-SiN0.56:H layer (a) as-deposited; and (b) 900 8C, 5 min annealed.
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is better described as a statistical distribution of the [Si–(SinN4n) (0 n 4)] components, where the Si 2p line is the superposition of the five components according to the random binding model. These five peaks corresponding to Si atoms in which zero, one, two, three or four of the tetrahedral Si–Si bonds were replaced by the Si–N bonds as denoted by Si0, Si1þ, Si2þ, Si3þ, and Si4þ. The emergence of the separated Si–Si4 binding appearing at 99.7 eV for the annealed sample strongly indicates the formation of silicon clusters in the a-SiN0.56:H films. The structure of silicon nano-dots was further characterized by the XTEM analysis. The XTEM image for a-SiN0.56:H sample is presented in Fig. 2a and b. In Fig. 2a, silicon nano-dots are spread in the silicon nitride matrix. The diameter is spread from 2 to 4 nm. However, Si nano-dots were not observed from the XTEM image for the as-deposited sample, which might be probably due to the intense background images of the SiNx matrix [12]. Two distinct silicon dots (guided by the circles) having different structure other than the surrounding silicon nitride matrix is shown in Fig. 2b, the enlarged part of the 800 K XTEM image. The size of the smaller one’s is about 2, and 4 nm is for the larger ones. As compared to the Si substrate, the atoms in the nano-dots has the same
Fig. 2. The 500 K (a); and enlarged part of 800 K (b) XTEM image of a-SiN0.56:H layers after 900 8C, 5 min annealed.
arrangement indicates that the structure of silicon nano-dots is nano-crystalline. The appearance of nano-crystallized (nc) Si dots in the annealed sample implies that the structure is transformed from a-Si to nc-Si during the thermal processes. For the as-deposited a-SiNx:H films that contain Si nano-dots, the chemical structure is a mixture of all the silicon nitride states, and it depends on the arrival species of the N and Si atoms and their combination on the substrate surface during the deposition. These random combinations make the five [Si–(SinN4n) (0 n 4)] binding elements spread and overlap as a broad band that the signal of silicon nano-crystals is hidden and could not be resolved for the as-deposited sample. From the XPS analysis, the nitride phase is mainly Si–Si2N2 and Si–Si1N3 in which Si–Si4 and Si–N4 phase rarely exist in the as-deposited a-SiN0.56:H sample. In addition, as regard to the concentrations in the gaseous phase of our PECVD deposited a-SiN0.56:H sample, large amounts of Si–H, H–N bonding and the unbounded Si and N atoms are contained which would not appear in the Si 2p spectrum from the XPS analysis. After the thermal process, the H atoms are greatly exhausted [6] which indicate that the Si–H and N–H bonding are broken and a room is left for the evolution of a-SiN0.56:H film to take place. The Si and N dangling bonds in the a-SiN0.56:H films might transform the Si–Sin, Si–Nn and their combination bonds after the annealing. The formation of these bonds will therefore increase the intensity of Si–Si4 and Si–N4 bindings in the XPS analysis. Furthermore, the unbonded Si and N atoms in the neighborhood of the a-Si dots might diffuse a very short distance to connect with Si nano-dots, which are then enlarged during the thermal annealing [13]. The increase in the size of a-Si dots is limited by the amount of surrounding Si atoms because the diffusivity of atoms in silicon nitride matrix is rather low. Moreover, the structure has changed from amorphous to nano-crystalline after the high-temperature thermal process (900 8C). Photoluminescence analysis is performed to further investigate the emission properties of a-SiN0.56:H films after the thermal process. The PL spectrum of the as-deposited and 900 8C/5 min thermal annealed a-SiN0.56:H film is presented in Fig. 3. The peak of PL spectrum for the as-deposited sample is originally centered at 630 nm with 200 nm in FWHM. After
Z. Pei, H.L. Hwang / Applied Surface Science 212–213 (2003) 760–764
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Fig. 4. The size distribution a-SiN0.56:H layer after 900 8C, 5 min annealing.
Fig. 3. The PL spectrum of a-SiN0.56:H layer (a) as-deposited and 900 8C, 5 min annealed; and (b) the dependence of peak position on annealing temperature.
the rapid thermal process, the peak is red-shifted to the 932 nm. In the previous work [5,8], the mechanism for the strong luminescence from a-SiNx:H is suggested from the combination of three main reasons. There are Si and silicon nitride interface luminescence, gap states’ luminescence from the imperfection of silicon nitride matrix and luminescence from Si nano-dots. Three origins are inherently coexisting in the a-SiNx:H layer and therefore they are named as the luminescence centers, and the photoluminescence spectrum depends on the contribution of each part. The red-shift in the photoluminescence spectrum might suggest the contribution from the part of Si nano-dot is enhanced. The red-shift of PL spectrum along with the annealing temperature is sketched in Fig. 3b. The peak exhibited a two-step shift from the as-deposited sample (denoted as 300 8C in the figure) at 1.97 eV (630 nm) to 1.79 eV (692 nm) for the 600 8C annealing, and it then rapidly declined to 1.33 eV (932 nm) for the 900 8C annealing. The Si nano-dots dominated luminescence was suggested [7] to be responsible for
the red-shifted luminescence in the a-SiNx:H layer. The PL of a-Si nano-dots follows the rule of EðeVÞ ¼ 1:56 þ 2:4=a2 which is modified by the effective mass model [14], while E is the emission energy, a the size of the dot in the unit of nm and 1.56 is the energy gap of the amorphous silicon. The redshift of PL in our film from the as-deposited sample to the 600 8C could thus be described by the increase in size of a-Si nano-dot after the thermal annealing. However, the emission energy is 1.33 eV for 900 8C annealing, which is far below the energy gap of a-Si that the a-Si model could not be well applied because the Si dots is nano-crystallized as described previously. The size distribution of the nc-Si dots was sketched in Fig. 4. The mean diameter for the Si nanodots is 3 nm with the density of 7 1011 /cm2 as revealed from the figure. Applying this value into the modified effective mass model and replacing the energy gap of a-Si (1.56 eV) by the energy gap of Si (1.12 eV), the emission energy is predicted to be 1.38 eV. This energy is closely to the PL energy of 1.33 eV. This result plus the observation of structure evolution for the Si nano-dots which is changed from amorphous to nano-crystalline under the thermal process, suggest the luminescence from the Si nano-dots dominates the luminescent centers of the a-SiN0.56:H layer.
4. Conclusions Silicon nano-dots were observed in the a-SiN0.56:H thin film by the XPS spectra and XTEM image. The
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size of the Si nano-dots after the rapid thermal annealing was between 2 and 4 nm has a mean diameter of 3 nm with density of 7 1011 /cm2. The structure of silicon nano-dots in a-SiN0.56:H layer is proposed to change from amorphous to the nano-crystalline state. The emission in the photoluminescence spectrum that is extensively red-shifted after the rapid thermal annealing. This shift is described as the Si nano-dot related luminescence that the increase in the dot size would reduce the emission energy. Moreover, the emission energy of PL peak (1.33 eV) is close to the calculated value from the effective mass model by using the XTEM observed dots size. The luminescence from the nano-crystallized Si dominates in a-SiN0.56:H layer is therefore further confirmed. The reveal of the Si nano-dots related luminescence and the high density of Si nano-dots suggest that the a-SiN0.56:H layer is suitable for the stable light-emitting device. Acknowledgements The assistance of photoluminescence measurement by the Materials Research Center at the National Tsing-Hua University is gratefully acknowledged.
References [1] K.D. Hirschman, L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Nature 384 (1996) 338. [2] R.T. Collins, P.M. Fauchet, M.A. Tischler, Phys. Today 50 (1997) 24. [3] H.Z. Song, X.M. Bao, N.S. Li, X.L. Wu, Appl. Phys. Lett. 72 (3) (1998) 356. [4] J.F. Tong, H.L. Hsiao, H.L. Hwang, Appl. Phys. Lett. 74 (16) (1999) 2316. [5] G.G. Qin, A.P. Li, B.R. Zhang, B.C. Li, J. Appl. Phys. 78 (3) (1995) 2006. [6] Z. Pei, Y.R. Chang, H.L Hwang, Advanced Lumincescent Materials and Quantum Confinement, The Electrochemical Society, Pennington, 1999, p. 474. [7] N.M. Park, C.J. Choi, T.Y. Seong, S.J. Park, Phys. Rev. Lett. 7 (86) (2001) 1355. [8] Z. Pei, Y.R. Chang, H.L. Hwang, Appl. Phys. Lett 80 (2002) 2839. [9] C. Mo, L. Zhang, C. Xie, T. Wang, J. Appl. Phys. 73 (10) (1993) 5185. [10] H.R. Phillip, J. Non-Cryst. Solids 8/10 (1972) 627. [11] R.J. Temkin, J. Non-Cryst. Solids 17 (1975) 215. [12] S. Takeoka, M. Fujii, S. Hayashi, K. Yamamoto, Phys. Rev. B 58 (1998) 7921. [13] C.S. Yang, C.J. Lin, P.Y. Kuei, S.F. Horng, C.C.H. Hsu, M.C. Liaw, Appl. Surf. Sci. 113–114 (1997) 116. [14] P.F. Trwoga, A.J. Kenyon, C.W. Pitt, J. Appl. Phys. 83 (1998) 3789.
Formation of silicon nano-dots in luminescent silicon nitride Zingway Pei*, H.L. Hwang Department of Electrical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan, ROC
Abstract Strong room-temperature photoluminescence (PL) was observed in the hydrogenated silicon-rich silicon nitride (a-SiNx:H) thin films grown by plasma-enhanced chemical vapor deposition technique. After the thermal annealing process, silicon nano-dots were clearly observed in the Si-rich a-SiN0.56:H thin films. From the X-ray photoelectron spectroscopy (XPS), the Si 2p peak shows distinct Si–Si4 bonds after thermal annealing indicated the existence of Si clusters. In addition, the existence of nano-crystallized (nc) Si dots was revealed from the image of high-resolution cross-section transmission electron microscopy (XTEM) analysis. The PL spectra exhibited extensive red-shift from 1.97 to 1.33 eV along with the annealing temperature suggests that, the Si nano-dot related luminescence dominates in the luminescence centers of the a-SiN0.56:H layer. The reveal of Si nano-dots related luminescence and the high density of Si nano-dots suggest that the a-SiN0.56:H layer is suitable for the stable light-emitting device. # 2003 Elsevier Science B.V. All rights reserved. PACS: 71.20.Nr; 78.66 Keywords: Si nano-dots; Silicon nitride; Photoluminescence
1. Introduction The silicon-based luminescent material has attracted great attentions in recent years because it can combine the state-of-the-art silicon integrated circuits with the optoelectronic applications. However, the indirect nature of the silicon band structure prevents the efficient light emission as a result of small radiative dipole transition possibilities. The formation of near zerodimensional structure that breaking of the momentum conservation law and enabling strong radiative recombination is one of the most promising methods to the achieve stable light emissions. For this reason, extensive research efforts have been focused on the strong luminescence from silicon nano-structures [1–5]. *
Corresponding author. Tel.: þ886-3-5913339; fax: þ886-3-5917690. E-mail address: [email protected] (Z. Pei).
Recently, Si-rich silicon nitride layers were found to have intense multi-color emissions, from ultraviolet to infrared by varying the growth conditions [6], especially after the post-annealing process, the intensity of photoluminescence (PL) is largely enhanced. Moreover, the formation of amorphous silicon dots was found in this material [7]. As compared to the relations of emission wavelength to the dot size, the quantum confinement effect could be responsible for the emissions. Si-rich silicon nitride based electroluminescence (EL) device has been recently reported by our group that the spectrum of the emitting light was ranged from blue to near infrared, i.e. white light [8]. However, this white emission is originated from the gap states dominated luminescent centers, because of the inherent problems of high gap states concentrations for the amorphous materials [9], in which the emission from the amorphous silicon nano-dots is suppressed. Therefore, formation of the crystallized
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00369-6
Z. Pei, H.L. Hwang / Applied Surface Science 212–213 (2003) 760–764
silicon nano-dots in amorphous silicon nitride layer by thermal process could potentially enhance the radiative transition by having high-density Si dots and reducing the defect densities is a promising method to have stable luminescence in silicon based light-emitting device. In this paper, investigation on the formation of silicon nano-dots from amorphous to nano-crystal phase by thermal process in luminescent amorphous silicon-rich silicon nitride (a-SiNx:H) material is reported.
2. Experiments The a-SiNx:H thin film was deposited by plasma enhanced chemical vapor deposition (PECVD) technique on n-type (1 0 0) silicon substrate. After the standard clean procedures, the substrate was loaded to the reaction chamber. Hydrogen plasma treatment was applied to clean the silicon surface prior to the deposition of silicon nitride. The reacting gases for a-SiNx:H deposition was the mixture of 5% Ar-diluted SiH4 and pure N2, in the ratio 2:1, while substrate temperature of 300 8C and power density of 200 mW/cm2 was maintained during the deposition. The detailed compositions of the a-SiNx:H layer were determined and published elsewhere. The x-value of this nitride layer is 0.56 and, consequently, it is highly silicon-rich. After the deposition, samples were thermally treated by using AG model 610i rapid annealing furnace at the temperature of 900 8C for 5 min. The X-ray photoelectron spectroscopy (XPS) analysis was carried out by using Mg Ka (hn ¼ 1253:6 eV) X-ray source to determine the chemical structure of the silicon nano-dots. The kinetic energy of the photoelectrons was determined using a spherical capacitor analyzer (SCA) at a constant pass energy mode. The highresolution cross-section transmission electron microscopy (XTEM) was applied to investigate the size and structure of the silicon nano-dots. The room temperature photoluminescence was measured by using the excitation of a He–Cd laser (325 nm) with power of 1 mW focused to a 100 mm spot. The emitted lighted was collected by a 50 mm f/ 1.4 lens in the direction normal to the illuminated surface and was focused on a SPEX 500M monochromator equipped with an InGaAs photodetector, and the standard lock-in technique was used.
761
3. Results and discussion The Si 2p binding energy revealed from X-ray photoelectron spectroscopy (XPS) analysis of aSiN0.56:H sample is shown in Fig. 1a, as-deposited and Fig. 1b, with 900 8C rapid thermal annealing. For the as-deposited sample, Si 2p peak is located at 101.4 eV where the control Si has 2p line at the energy of 99.7 eV is also presented. After 900 8C/5 min annealing, the spectra exhibit the multi-peak behavior and could be de-convoluted into five distinct peaks. Concerning the chemical structure of amorphous silicon nitride (a-SiNx:H) thin films, two models have been proposed. One model is the random bonding model proposed by Phillip [10], another one is the stoichiometric statistical model (SSM) proposed by Temkin [11]. The SSM model described the silicon 2p line to be an overlap of the two peaks corresponding to the a-Si and a-Si3N4 phase, with a fixed energy separation of 2.4 eV, and the peak position is a monotonic function of x. According to the XPS spectra of our result, the multi-peak in Si 2p line could not be explained by the SSM model. The a-SiNx network
Fig. 1. The XPS spectra of a-SiN0.56:H layer (a) as-deposited; and (b) 900 8C, 5 min annealed.
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is better described as a statistical distribution of the [Si–(SinN4n) (0 n 4)] components, where the Si 2p line is the superposition of the five components according to the random binding model. These five peaks corresponding to Si atoms in which zero, one, two, three or four of the tetrahedral Si–Si bonds were replaced by the Si–N bonds as denoted by Si0, Si1þ, Si2þ, Si3þ, and Si4þ. The emergence of the separated Si–Si4 binding appearing at 99.7 eV for the annealed sample strongly indicates the formation of silicon clusters in the a-SiN0.56:H films. The structure of silicon nano-dots was further characterized by the XTEM analysis. The XTEM image for a-SiN0.56:H sample is presented in Fig. 2a and b. In Fig. 2a, silicon nano-dots are spread in the silicon nitride matrix. The diameter is spread from 2 to 4 nm. However, Si nano-dots were not observed from the XTEM image for the as-deposited sample, which might be probably due to the intense background images of the SiNx matrix [12]. Two distinct silicon dots (guided by the circles) having different structure other than the surrounding silicon nitride matrix is shown in Fig. 2b, the enlarged part of the 800 K XTEM image. The size of the smaller one’s is about 2, and 4 nm is for the larger ones. As compared to the Si substrate, the atoms in the nano-dots has the same
Fig. 2. The 500 K (a); and enlarged part of 800 K (b) XTEM image of a-SiN0.56:H layers after 900 8C, 5 min annealed.
arrangement indicates that the structure of silicon nano-dots is nano-crystalline. The appearance of nano-crystallized (nc) Si dots in the annealed sample implies that the structure is transformed from a-Si to nc-Si during the thermal processes. For the as-deposited a-SiNx:H films that contain Si nano-dots, the chemical structure is a mixture of all the silicon nitride states, and it depends on the arrival species of the N and Si atoms and their combination on the substrate surface during the deposition. These random combinations make the five [Si–(SinN4n) (0 n 4)] binding elements spread and overlap as a broad band that the signal of silicon nano-crystals is hidden and could not be resolved for the as-deposited sample. From the XPS analysis, the nitride phase is mainly Si–Si2N2 and Si–Si1N3 in which Si–Si4 and Si–N4 phase rarely exist in the as-deposited a-SiN0.56:H sample. In addition, as regard to the concentrations in the gaseous phase of our PECVD deposited a-SiN0.56:H sample, large amounts of Si–H, H–N bonding and the unbounded Si and N atoms are contained which would not appear in the Si 2p spectrum from the XPS analysis. After the thermal process, the H atoms are greatly exhausted [6] which indicate that the Si–H and N–H bonding are broken and a room is left for the evolution of a-SiN0.56:H film to take place. The Si and N dangling bonds in the a-SiN0.56:H films might transform the Si–Sin, Si–Nn and their combination bonds after the annealing. The formation of these bonds will therefore increase the intensity of Si–Si4 and Si–N4 bindings in the XPS analysis. Furthermore, the unbonded Si and N atoms in the neighborhood of the a-Si dots might diffuse a very short distance to connect with Si nano-dots, which are then enlarged during the thermal annealing [13]. The increase in the size of a-Si dots is limited by the amount of surrounding Si atoms because the diffusivity of atoms in silicon nitride matrix is rather low. Moreover, the structure has changed from amorphous to nano-crystalline after the high-temperature thermal process (900 8C). Photoluminescence analysis is performed to further investigate the emission properties of a-SiN0.56:H films after the thermal process. The PL spectrum of the as-deposited and 900 8C/5 min thermal annealed a-SiN0.56:H film is presented in Fig. 3. The peak of PL spectrum for the as-deposited sample is originally centered at 630 nm with 200 nm in FWHM. After
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Fig. 4. The size distribution a-SiN0.56:H layer after 900 8C, 5 min annealing.
Fig. 3. The PL spectrum of a-SiN0.56:H layer (a) as-deposited and 900 8C, 5 min annealed; and (b) the dependence of peak position on annealing temperature.
the rapid thermal process, the peak is red-shifted to the 932 nm. In the previous work [5,8], the mechanism for the strong luminescence from a-SiNx:H is suggested from the combination of three main reasons. There are Si and silicon nitride interface luminescence, gap states’ luminescence from the imperfection of silicon nitride matrix and luminescence from Si nano-dots. Three origins are inherently coexisting in the a-SiNx:H layer and therefore they are named as the luminescence centers, and the photoluminescence spectrum depends on the contribution of each part. The red-shift in the photoluminescence spectrum might suggest the contribution from the part of Si nano-dot is enhanced. The red-shift of PL spectrum along with the annealing temperature is sketched in Fig. 3b. The peak exhibited a two-step shift from the as-deposited sample (denoted as 300 8C in the figure) at 1.97 eV (630 nm) to 1.79 eV (692 nm) for the 600 8C annealing, and it then rapidly declined to 1.33 eV (932 nm) for the 900 8C annealing. The Si nano-dots dominated luminescence was suggested [7] to be responsible for
the red-shifted luminescence in the a-SiNx:H layer. The PL of a-Si nano-dots follows the rule of EðeVÞ ¼ 1:56 þ 2:4=a2 which is modified by the effective mass model [14], while E is the emission energy, a the size of the dot in the unit of nm and 1.56 is the energy gap of the amorphous silicon. The redshift of PL in our film from the as-deposited sample to the 600 8C could thus be described by the increase in size of a-Si nano-dot after the thermal annealing. However, the emission energy is 1.33 eV for 900 8C annealing, which is far below the energy gap of a-Si that the a-Si model could not be well applied because the Si dots is nano-crystallized as described previously. The size distribution of the nc-Si dots was sketched in Fig. 4. The mean diameter for the Si nanodots is 3 nm with the density of 7 1011 /cm2 as revealed from the figure. Applying this value into the modified effective mass model and replacing the energy gap of a-Si (1.56 eV) by the energy gap of Si (1.12 eV), the emission energy is predicted to be 1.38 eV. This energy is closely to the PL energy of 1.33 eV. This result plus the observation of structure evolution for the Si nano-dots which is changed from amorphous to nano-crystalline under the thermal process, suggest the luminescence from the Si nano-dots dominates the luminescent centers of the a-SiN0.56:H layer.
4. Conclusions Silicon nano-dots were observed in the a-SiN0.56:H thin film by the XPS spectra and XTEM image. The
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Z. Pei, H.L. Hwang / Applied Surface Science 212–213 (2003) 760–764
size of the Si nano-dots after the rapid thermal annealing was between 2 and 4 nm has a mean diameter of 3 nm with density of 7 1011 /cm2. The structure of silicon nano-dots in a-SiN0.56:H layer is proposed to change from amorphous to the nano-crystalline state. The emission in the photoluminescence spectrum that is extensively red-shifted after the rapid thermal annealing. This shift is described as the Si nano-dot related luminescence that the increase in the dot size would reduce the emission energy. Moreover, the emission energy of PL peak (1.33 eV) is close to the calculated value from the effective mass model by using the XTEM observed dots size. The luminescence from the nano-crystallized Si dominates in a-SiN0.56:H layer is therefore further confirmed. The reveal of the Si nano-dots related luminescence and the high density of Si nano-dots suggest that the a-SiN0.56:H layer is suitable for the stable light-emitting device. Acknowledgements The assistance of photoluminescence measurement by the Materials Research Center at the National Tsing-Hua University is gratefully acknowledged.
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