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FTIR characterization of light emitting Si-rich nitride films prepared by low pressure chemical vapor deposition
Surface & Coatings Technology 201 (2007) 9359 – 9364 www.elsevier.com/locate/surfcoat
FTIR characterization of light emitting Si-rich nitride films prepared by low pressure chemical vapor deposition V. Em. Vamvakas ⁎, S. Gardelis Institute of Microelectronics, NCSR “Demokritos”, P. O. Box 60228, 15310 Aghia Paraskevi, Athens, Greece Available online 4 May 2007
Abstract We report on the infrared transmission and light emission of Si-rich nitride (SRN) films prepared by low pressure chemical vapor deposition (LPCVD) from dichlorosilane (SiH2Cl2, DCS) and ammonia (NH3) mixtures. The main absorption band at about 830 cm− 1, attributed to Si–N vibration mode and observed in stoichiometric silicon nitride, shifted to slightly higher wavenumbers with increasing Si content in the SRN films. Annealing at temperatures higher than the deposition temperature induced a further shift of the main band to higher wavenumbers. Additionally, a new band appeared as a “shoulder” at about 1080 cm− 1, attributed to partial oxidation of the silicon nanocrystals. Photoluminescence (PL) obtained from the SRN films increased considerably and shifted to shorter wavelengths as the Si content decreased whereas annealing caused further enhancement and a slight shift to shorter wavelengths in comparison with the as-grown films. © 2007 Elsevier B.V. All rights reserved. PACS: 78.30.-j; 78.55.-m; 78.67.Bf Keywords: Silicon nitride; Silicon nanoparticles; FTIR spectroscopy; Photoluminescence
1. Introduction Stoichiometric silicon nitride films are widely used in silicon based micro- and nanotechnology as barriers to sodium diffusion and as masking layers for the local oxidation of Si in ULSI technology [1,2]. In addition their excellent optical properties make them suitable for optical waveguides and for antireflective and protective coatings for solar cells [3–6]. The introduction of extra silicon in silicon nitride films was firstly used to lower or reverse the residual stresses of these films [7–9] making possible the development of suspending membranes in micro-mechanical systems. Other applications of Si-rich nitride (SRN) films include the typical Oxide–Nitride–Oxide (ONO) memory device [10–14] and photonic devices [15–22]. In this work SRN films were grown by low pressure chemical vapor deposition (LPCVD) using SiH2Cl2 (DCS) and NH3 mixtures in order to study their light emission properties in combination with FTIR analysis. All films were deposited at 800 °C and then annealed at 950 °C and 1100 °C in dry nitrogen for times ranging between 30 min and 4 h. FTIR analysis showed that ⁎ Corresponding author. Tel.: +30 210 6503117; fax: +30 210 6511723. E-mail address: [email protected] (V.Em. Vamvakas). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.069
annealing caused the formation of Si–O bonds even though precaution was taken to avoid the presence of oxygen during the annealing process. This indicated that SRN films could be very easily oxidized. The films emitted light in the visible at room temperature. The light emission characteristics depended on the Si content of the films and the post-annealing treatment. 2. Experimental details All depositions were carried out in a Tempress Systems Inc. (model omega junior), horizontal hot wall reactor at 800 °C, 230 mTorr. NH3 flow ratio was kept constant whereas DCS flow ratio varied in order to deposit films with different stoichiometries. The deposition parameters are summarized in Table 1. Before deposition Si substrates were cleaned in a 1:1 H2SO4: H2O2 solution followed by a dip in hydrofluoric acid (HF) solution, rinsed in de-ionized water and blown dry with dry nitrogen. All studied films had thickness of about 100 nm. Postannealings were performed at 950 °C and at 1100 °C for 30 min, 1, 2 and 4 h in a furnace in dry nitrogen (N2 99.996%, O2 b 1 ppm, H2O b1 ppm) flowing at a rate of 3.5 slm (standard liters per minute). Before annealing all samples were cleaned in 1:1 H2SO4:H2O2 solution, rinsed in de-ionized water, blown dry
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Table 1 Deposition parameters, refractive index n at 632.8 nm and Si/N ratio of the deposited films T (°C) P (mTorr) φDCS (sccm) φNH3 (sccm) n Stoichiometric 800 Silicon rich 800 800 800
230 230 230 230
20 13.8 27.5 55.0
60 5.5 5.5 5.5
2.02 2.15 2.28 2.51
Si/N 0.75 0.86 1.10 1.35
with dry nitrogen and then inserted in the furnace where they remained for 30 min at 300 °C. This procedure was followed in order to exclude the possibility of partial oxidation of the samples due to remaining humidity after the last rinse. Fourier transform infrared (FTIR) spectra were recorded in transmission mode using a Bruker (model Tensor 27) single beam spectrometer. Before recording the spectrum, the background was taken placing a freshly cleaned piece of silicon cut from the same silicon wafer used as the substrate. This was performed in order to eliminate absorption of the substrate caused by the vibration of the Si–Si bond which gives a peak at 611 cm− 1 and the interstitial oxygen which gives a peak at 1108 cm− 1 [23,24]. Photoluminescence was performed at room temperature using for excitation the 458 nm line of an Ar+-ion laser. The signal was analyzed by a Jobin-Yvon spex HR-320 spectrometer and detected by a photomultiplier tube. 3. Results and discussion The increase of the Si content of silicon nitride films results in an increase of the refractive index of the films compared to the stoichiometric ones [1]. The last two columns of Table 1 give the refractive index for our films at 632.8 nm and the expected Si/N ratio according to the literature [1]. Fig. 1 shows the FTIR transmission spectra of the as-grown SRN films investigated in this study. The strong absorption band located at about 830 cm− 1 corresponds to the asymmetric stretching mode of vibration of the Si–N bond. There is also a weak band located at about 480 cm− 1 which corresponds to the rocking mode of vibration of the Si–N bond. However, the study of this band is difficult with our equipment since it is located close to the lower band limit of our spectrometer. In addition the study of the low energy band does not offer any extra information thus we focus our study on the main absorption band. No Si–H or N–H or any other impurity related vibration mode was detected between 4000 cm− 1 and 400 cm− 1. The main absorption band of the stoichiometric silicon nitride film was located at 832 cm− 1. For the SRN films this band shifted slightly to higher wavenumbers. Specifically, in the film with Si/N ratio 1.1 this band appeared at 840 cm− 1 whereas in the film with Si/N ratio 1.35 it appeared at 842 cm− 1. For films with Si/N ratio 0.86 it appeared at almost the same position as that of the stoichiometric film, indicating that FTIR transmission measurements could not distinguish films with these stoichiometries. The observed shift of the Si–N vibration mode in the SRN films is expected to be due to the presence of the extra Si in the films.
Fig. 1. FTIR transmission spectra of the as-grown SRN films with different Si content.
Annealing of SRN films with Si/N ratios 1.35 and 1.10 at temperatures higher than the deposition caused a slight shift of the main absorption band to higher wavenumbers followed by a broadening. In addition, a second band appeared, as a “shoulder” to the main band, at 1080 cm− 1. We note here that spectra obtained from stoichiometric silicon nitride films and SRN films with Si/N equal to 0.86 annealed together with the
Fig. 2. FTIR transmission spectra of SRN films with Si/N = 1.35 after deposition and after annealing for 4 h at 950 °C and 1100 °C.
V.Em. Vamvakas, S. Gardelis / Surface & Coatings Technology 201 (2007) 9359–9364
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Table 2 Position of the main absorption band for silicon nitride films of several stoichiometries Main absorption band (cm− 1) After deposition
Annealed 950 °C
Stoichiometric (Si/N) SRN (Si/N)
0.75 0.86 1.10 1.35
832 832 839 842
1100 °C
30 min
1h
2h
4h
30 min
1h
2h
4h
832 832 839 845
832 832 840 846
832 832 841 847
832 832 842 847
832 832 845 851
832 832 845 852
832 832 846 853
832 832 846 853
All films have a thickness of about 100 nm.
previously mentioned SRN films remained unchanged before and after annealing, showing that annealing affected slightly if not at all their chemical composition. In the following we focus on the main absorption band of the annealed SRN films with Si/ N ratios 1.35 and 1.10. In the film with Si/N ratio 1.10 the main absorption band shifted from 839 cm− 1 to 842 cm− 1 after annealing at 950 °C for 4 h. The same band shifted to 846 cm− 1 after annealing at 1100 °C for 4 h. In the film with Si/N ratio 1.35 the shift of the main absorption band after annealing was even more distinct. Specifically, annealing at 950 °C for 4 h resulted in a shift of the band from 842 cm− 1 to 847 cm− 1 while annealing at 1100 °C for 4 h caused the same band to appear at 853 cm− 1 (Fig. 2). Annealing at temperatures higher than the growth temperature, which promotes segregation of Si into nanoparticles and some of them into nanocrystals, as we have observed previously in similar SRN films [25], is expected to cause changes in Si–N vibration modes due to alterations in the arrangement of Si and N atoms in the films. The observed shifts of the main absorption band, summarized in Table 2, could be well explained by this effect.
Now we focus our discussion on the feature which appeared as a “shoulder” to the main absorption band of the SRN films at 1080 cm− 1 after annealing. In order to resolve the absorption band in the region where the shoulder appeared, we calculated the second derivative of transmission [26] between 1200 cm− 1 and 950 cm− 1. For the SRN film with Si/N ratio 1.10 annealing at 950 °C regardless of the duration showed a symmetric peak located at 1090 cm− 1 in the second derivative (Fig. 3a). However, for the SRN film with Si/N ratio 1.35 annealed at the same temperature for 1 h and longer, an extra weaker peak at 1070 cm− 1 was revealed. Annealing of both SRN films at 1100 °C showed both peaks in the second derivative. The strongest peaks in the second derivative were obtained for the film with Si/N ratio 1.35 when annealing was performed at 1100 °C for 4 h (Fig. 3b). In the past [27–29] both peaks have been attributed to the asymmetrical stretching mode of vibration of Si–O–Si bridges inside SiO2 films. Specifically, the peak located at 1090 cm− 1 has been attributed to Si–O–Si bridges located at the boundaries of the SiO2 films while the peak located at 1070 cm− 1 has been attributed to bridges located at
Fig. 3. Second derivative of transmission in the range where the “shoulder” to the main absorption band appears. (a) Si/N = 1.10 annealed at 950 °C, (b) Si/N = 1.35 annealed at 1100 °C.
– – y y – – y y – – y y – – y y – – y y – – y y – – y y – – – y – – y y – – – y All films have a thickness of about 100 nm (y: for presence of the corresponding peak, –: for lack of presence).
– – y y – – – y – – y y – – – – – – y y – – – – – – – – 0.75 0.86 1.10 1.35
4h
1090 cm− 1 1070 cm− 1
2h
1090 cm− 1 1070 cm− 1
1h
1090 cm− 1 1070 cm− 1 1090 cm− 1 1070 cm− 1 1090 cm− 1
Stoichiometric (Si/N) SRN (Si/N)
4h
1090 cm− 1 1070 cm− 1
2h
1090 cm− 1 1070 cm− 1
1h
1090 cm− 1 1070 cm− 1
30 min
1090 cm− 1
30 min
1070 cm− 1
1100 °C Annealed 950 °C After deposition
2nd derivative of transmission (1200–950 cm− 1)
Table 3 Presence/lack of peaks of the 2nd derivative of transmission in the range 1200–950 cm− 1 of silicon nitride films with various stoichiometries
– – y y
V.Em. Vamvakas, S. Gardelis / Surface & Coatings Technology 201 (2007) 9359–9364 1070 cm− 1
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the bulk of the films, far away from their boundaries. Therefore, it is logical to assume that the formation of the “shoulder” at 1080 cm− 1 after annealing is due to the formation of Si–O–Si bridges caused by the partial oxidation of the extra silicon. When the silicon content was low or the annealing time was short then the oxidation of the films resulted in the formation of isolated Si–O–Si bridges surrounded by silicon nitride or silicon clusters. The asymmetrical stretching mode of vibration of these isolated bridges is similar to those boundary bridges located at the interfaces of SiO2 films thus resulting in the appearance of the peak at 1090 cm− 1 (Table 3). When annealing time is longer or/and the silicon content of the films is higher, the possibility of the formation of Si–O–Si bridges surrounded by other Si–O–Si bridges becomes higher. These isolated bridges which do not “feel” the silicon nitride matrix are vibrating similarly with those located in the bulk of SiO2 films thus giving a peak at 1070 cm− 1. However, the origins of the oxygen causing the partial oxidation of these films are not clear. It is possible that part of this oxygen enters during the deposition of the films, since the same furnace is also used for the deposition of SiO2 films, although great care was taken to avoid any oxygen contamination. We note here that FTIR transmission spectra obtained from stoichiometric silicon nitride films annealed together with the SRN films did not reveal the “shoulder”. In addition, second derivative of transmission spectra obtained from the as-grown SRN films did not clearly reveal the existence of Si–O bonds. It is possible that the inserted oxygen during deposition is making bonds with the silicon nitride matrix which are infrared inactive. During annealing oxygen is redistributed in the silicon nitride films and for the case of SRN films it is easy to oxidize the extra silicon and form Si–O–Si bridges. This most probably is not happening for stoichiometric silicon nitride films since before the formation of Si–O–Si bridges a break of Si–N bonds must occur. This is not thermodynamically favorable at temperatures as low as 950 °C while temperatures as high as 1100 °C must be considered close to the lowest limit of the oxidation in dry ambient of silicon nitrides. However, the scope of this discussion is not to find the origins of the oxygen which is responsible for the oxidation of the films, but it is the fact that SRN films can be oxidized relatively easy when traces of oxygen are present at temperatures starting from at least 950 °C. The light emission properties of all films were investigated. The light emission characteristics were mainly sensitive to the silicon content of the films. Significant improvement in the intensity of the emitted light was realized by post-annealing of the films at temperatures higher than the growth temperature. Specifically, the as-grown film with Si/N ratio 1.35 did not give any detectable luminescence in the region of measurements between 470 nm and 900 nm. The film with Si/N ratio 1.10 demonstrated a broad PL spectrum peaking at 600 nm (Fig. 4). The film with Si/N ratio 0.86 showed the most efficient luminescence peaking at 570 nm. (Fig. 4) We note that the PL spectra obtained from the as-grown films shifted to shorter wavelengths compared with the indirect band gap of bulk silicon (about 1100 nm). A considerable enhancement of the PL peak accompanied by a shift to higher energies with decreasing
V.Em. Vamvakas, S. Gardelis / Surface & Coatings Technology 201 (2007) 9359–9364
Fig. 4. Photoluminescence (PL) spectra of as-grown and annealed SRN films.
silicon content in the films was observed. We note that decreasing silicon content in the films is expected to lead to formation of silicon clusters of smaller sizes. Considering all of the above, the most probable explanation for the light emission from the as-grown films is the quantum confinement of the carriers recombining in the silicon clusters which may be already formed during the growth of the films due to the high temperature of the growth reaction (800 °C). Similar light emission has been observed in SRN films grown by techniques other than the one used in this study [15–22]. There is though a debate as to what is the origin of the light emission. Some attribute the effect to the quantum confinement of carriers in the silicon nanocrystals [18, 22] whereas others suggest that the effect originates from nitrogen-related localized surface states introduced within the optical gap of silicon nanocrystals [19, 20]. In either case silicon nanocrystals are necessary to confine the electron and the hole which then recombine radiatively. All as-grown films were examined for their light emission properties after annealing. The sample with Si/N ratio 1.35 emitted light only after annealing at 1100 °C in which, as TEM images demonstrated, silicon nanocrystals of sizes between 1.5 nm and 5 nm were formed [25]. Annealing of the films with lower Si/N ratio resulted in PL enhancement accompanied by a shift of the PL peak to shorter wavelengths. Fig. 4 shows this effect in the case of the sample with Si/N ratio 1.10. This effect coincided with the appearance of the Si–O–Si vibration modes in the FTIR spectrum as discussed above in detail. The effect could be well explained by the enhancement of the localization of the carriers in the silicon nanocrystals as their size was reduced due to their oxidation. 4. Conclusions SRN films with different Si content were prepared by LPCVD from DCS and NH3 mixtures. Infrared transmission spectra of the SRN films revealed the existence of an absorption band at about 830 cm− 1 attributed to the Si–N asymmetrical stretching mode. This band slightly shifted to higher wavenumbers as the silicon
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content of the films increased. Annealing at temperatures higher than the growth temperature caused further shift of the main absorption band to higher wavenumbers whereas at the same time a new absorption band at about 1080 cm− 1 appeared as a “shoulder” to the main absorption band. Calculations of the second derivative of the “shoulder”, revealed the existence of Si– O–Si bridges due to partial oxidation of the extra Si from traces of oxygen which may be inevitably present during annealing. Light emission properties of the SRN films were sensitive to the Si content, the annealing temperature and the duration of the annealing. Enhancement of PL accompanied by a blue shift with decreasing Si content was observed in the as-grown SRN films. Similar effects were observed with increasing annealing temperature and annealing time which coincided with the partial oxidation of the Si nanocrystals resulting in a reduction in their sizes. All these effects could well be attributed to the confinement of the carriers in the Si nanocrystals which might recombine radiatively in the Si nanocrystals perhaps via surface states. References [1] A.C. Adams, in: S.M. Sze (Ed.), VLSI Technology, 2nd edn, McGraw-Hill, 1988, p. 233, International edition. [2] F.H.P.M. Habraken, A.E.T. Kuiper, Mater. Sci. Eng. R12 (3) (1994) 123. [3] D. Davazoglou, Thin Solid Films 437 (2003) 266. [4] K. Misiakos, E. Tsoi, E. Halmagean, S. Kakabakos, Technical Digest, International Electron Devices Meeting, 1998, p. 25. [5] P. Wu, P. Hogrebe, D.W. Grainger, Biosens. Bioelectron. 21 (2006) 1252. [6] O. Schultz, M. Hofmann, S.W. Glunz, G.P. Willeke, 31st IEEE PVSC Orlando, Florida, 2005, p. 872. [7] M. Sekimoto, H. Yoshihara, T. Ohkubo, J. Vac. Sci. Technol. 21 (4) (1982) 1017. [8] J.G.E. Gardeniers, H.A.C. Tilmans, C.C.G. Visser, J. Vac. Sci. Technol., A 14 (5) (1996) 2879. [9] E. Cianci, F. Pirola, V. Foglietti, J. Vac. Sci. Technol. B 23 (1) (2005) 168. [10] M.C. Poon, Y. Gao, T.C.W. Kok, A.M. Myasnikov, H. Wong, Microelectron. Reliab. 41 (2001) 2071. [11] J. Chan, H. Wong, M.C. Poon, C.W. Kok, Microelectron. Reliab. 43 (2003) 611. [12] T.C. Chang, S.T. Yan, P.T. Liu, M.C. Wang, S.M. Sze, Electrochem. SolidState Lett. 7 (7) (2004) G138. [13] K.-H. Wu, H.-C. Chien, C.-C. Chan, T.-S. Chen, C.-H. Kao, IEEE Trans. Electron. Devices 52 (5) (2005) 987. [14] S. Choi, H. Yang, M. Chang, S. Baek, H. Hwanga, S. Jeon, J. Kim, C. Kim, Appl. Phys. Lett. 86 (2005) 251901. [15] T.-Y. Kim, N.-M. Park, K.-H. Kim, G.Y. Sung, Y.-W. Ok, T.-Y. Seong, C.-J. Choi, Appl. Phys. Lett. 85 (22) (2004) 5355. [16] K.S. Cho, N.-M. Park, T.-Y. Kim, K.-H. Kim, G.Y. Sung, J.H. Shin, Appl. Phys. Lett. 86 (2005) 071909. [17] L.-Y. Chen, W.-H. Chen, F.C.-N. Hong, Appl. Phys. Lett. 86 (2005) 193506. [18] T.-W. Kim, C.-H. Cho, B.-H. Kim, S.-J. Park, Appl. Phys. Lett. 88 (2006) 123102. [19] L. Dal Negro, J.H. Yi, L.C. Kimerling, S. Hamel, A. Williamson, G. Galli, Appl. Phys. Lett. 88 (2006) 183103. [20] L. Dal Negro, J.H. Yi, J. Michel, L.C. Kimerling, T.-W.F. Chang, V. Sukhvatkin, E.H. Sargent, Appl. Phys. Lett. 88 (2006) 223109. [21] L.B. Ma, R. Song, Y.M. Miao, C.R. Li, Y.Q. Wang, Z.X. Cao, Appl. Phys. Lett. 88 (2006) 093102. [22] K. Ma, J.Y. Feng, Z.J. Zhang, Nanotechnology 17 (2006) 4650. [23] I.P. Herman, Optical Diagnostics for Thin Film Processing, Academic Press Inc., 1996. [24] H.R. Philipp, Properties of Silicon, INSPEC The Institution of Electrical Engineers, 1987, p. 1019, EMIS Datareview RN=16133.
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[25] V.Em. Vamvakas, N. Vourdas, S. Gardelis, Microelectron. Reliab. 47 (2007) 794. [26] M. Cardona, in: F. Seitz, D. Turnbull, H. Ehrenreich (Eds.), Modulation Spectroscopy, Academic Press, New York, 1969, p. 105. [27] D. Davazoglou, V.Em. Vamvakas, J. Electrochem. Soc. 150 (5) (2003) F90.
[28] V.Em. Vamvakas, D. Davazoglou, J. Electrochem. Soc. 151 (5) (2004) F93. [29] V.Em. Vamvakas, D. Davazoglou, J. Vac. Sci. Technol. B 23 (5) (2005) 1956.
FTIR characterization of light emitting Si-rich nitride films prepared by low pressure chemical vapor deposition V. Em. Vamvakas ⁎, S. Gardelis Institute of Microelectronics, NCSR “Demokritos”, P. O. Box 60228, 15310 Aghia Paraskevi, Athens, Greece Available online 4 May 2007
Abstract We report on the infrared transmission and light emission of Si-rich nitride (SRN) films prepared by low pressure chemical vapor deposition (LPCVD) from dichlorosilane (SiH2Cl2, DCS) and ammonia (NH3) mixtures. The main absorption band at about 830 cm− 1, attributed to Si–N vibration mode and observed in stoichiometric silicon nitride, shifted to slightly higher wavenumbers with increasing Si content in the SRN films. Annealing at temperatures higher than the deposition temperature induced a further shift of the main band to higher wavenumbers. Additionally, a new band appeared as a “shoulder” at about 1080 cm− 1, attributed to partial oxidation of the silicon nanocrystals. Photoluminescence (PL) obtained from the SRN films increased considerably and shifted to shorter wavelengths as the Si content decreased whereas annealing caused further enhancement and a slight shift to shorter wavelengths in comparison with the as-grown films. © 2007 Elsevier B.V. All rights reserved. PACS: 78.30.-j; 78.55.-m; 78.67.Bf Keywords: Silicon nitride; Silicon nanoparticles; FTIR spectroscopy; Photoluminescence
1. Introduction Stoichiometric silicon nitride films are widely used in silicon based micro- and nanotechnology as barriers to sodium diffusion and as masking layers for the local oxidation of Si in ULSI technology [1,2]. In addition their excellent optical properties make them suitable for optical waveguides and for antireflective and protective coatings for solar cells [3–6]. The introduction of extra silicon in silicon nitride films was firstly used to lower or reverse the residual stresses of these films [7–9] making possible the development of suspending membranes in micro-mechanical systems. Other applications of Si-rich nitride (SRN) films include the typical Oxide–Nitride–Oxide (ONO) memory device [10–14] and photonic devices [15–22]. In this work SRN films were grown by low pressure chemical vapor deposition (LPCVD) using SiH2Cl2 (DCS) and NH3 mixtures in order to study their light emission properties in combination with FTIR analysis. All films were deposited at 800 °C and then annealed at 950 °C and 1100 °C in dry nitrogen for times ranging between 30 min and 4 h. FTIR analysis showed that ⁎ Corresponding author. Tel.: +30 210 6503117; fax: +30 210 6511723. E-mail address: [email protected] (V.Em. Vamvakas). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.069
annealing caused the formation of Si–O bonds even though precaution was taken to avoid the presence of oxygen during the annealing process. This indicated that SRN films could be very easily oxidized. The films emitted light in the visible at room temperature. The light emission characteristics depended on the Si content of the films and the post-annealing treatment. 2. Experimental details All depositions were carried out in a Tempress Systems Inc. (model omega junior), horizontal hot wall reactor at 800 °C, 230 mTorr. NH3 flow ratio was kept constant whereas DCS flow ratio varied in order to deposit films with different stoichiometries. The deposition parameters are summarized in Table 1. Before deposition Si substrates were cleaned in a 1:1 H2SO4: H2O2 solution followed by a dip in hydrofluoric acid (HF) solution, rinsed in de-ionized water and blown dry with dry nitrogen. All studied films had thickness of about 100 nm. Postannealings were performed at 950 °C and at 1100 °C for 30 min, 1, 2 and 4 h in a furnace in dry nitrogen (N2 99.996%, O2 b 1 ppm, H2O b1 ppm) flowing at a rate of 3.5 slm (standard liters per minute). Before annealing all samples were cleaned in 1:1 H2SO4:H2O2 solution, rinsed in de-ionized water, blown dry
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Table 1 Deposition parameters, refractive index n at 632.8 nm and Si/N ratio of the deposited films T (°C) P (mTorr) φDCS (sccm) φNH3 (sccm) n Stoichiometric 800 Silicon rich 800 800 800
230 230 230 230
20 13.8 27.5 55.0
60 5.5 5.5 5.5
2.02 2.15 2.28 2.51
Si/N 0.75 0.86 1.10 1.35
with dry nitrogen and then inserted in the furnace where they remained for 30 min at 300 °C. This procedure was followed in order to exclude the possibility of partial oxidation of the samples due to remaining humidity after the last rinse. Fourier transform infrared (FTIR) spectra were recorded in transmission mode using a Bruker (model Tensor 27) single beam spectrometer. Before recording the spectrum, the background was taken placing a freshly cleaned piece of silicon cut from the same silicon wafer used as the substrate. This was performed in order to eliminate absorption of the substrate caused by the vibration of the Si–Si bond which gives a peak at 611 cm− 1 and the interstitial oxygen which gives a peak at 1108 cm− 1 [23,24]. Photoluminescence was performed at room temperature using for excitation the 458 nm line of an Ar+-ion laser. The signal was analyzed by a Jobin-Yvon spex HR-320 spectrometer and detected by a photomultiplier tube. 3. Results and discussion The increase of the Si content of silicon nitride films results in an increase of the refractive index of the films compared to the stoichiometric ones [1]. The last two columns of Table 1 give the refractive index for our films at 632.8 nm and the expected Si/N ratio according to the literature [1]. Fig. 1 shows the FTIR transmission spectra of the as-grown SRN films investigated in this study. The strong absorption band located at about 830 cm− 1 corresponds to the asymmetric stretching mode of vibration of the Si–N bond. There is also a weak band located at about 480 cm− 1 which corresponds to the rocking mode of vibration of the Si–N bond. However, the study of this band is difficult with our equipment since it is located close to the lower band limit of our spectrometer. In addition the study of the low energy band does not offer any extra information thus we focus our study on the main absorption band. No Si–H or N–H or any other impurity related vibration mode was detected between 4000 cm− 1 and 400 cm− 1. The main absorption band of the stoichiometric silicon nitride film was located at 832 cm− 1. For the SRN films this band shifted slightly to higher wavenumbers. Specifically, in the film with Si/N ratio 1.1 this band appeared at 840 cm− 1 whereas in the film with Si/N ratio 1.35 it appeared at 842 cm− 1. For films with Si/N ratio 0.86 it appeared at almost the same position as that of the stoichiometric film, indicating that FTIR transmission measurements could not distinguish films with these stoichiometries. The observed shift of the Si–N vibration mode in the SRN films is expected to be due to the presence of the extra Si in the films.
Fig. 1. FTIR transmission spectra of the as-grown SRN films with different Si content.
Annealing of SRN films with Si/N ratios 1.35 and 1.10 at temperatures higher than the deposition caused a slight shift of the main absorption band to higher wavenumbers followed by a broadening. In addition, a second band appeared, as a “shoulder” to the main band, at 1080 cm− 1. We note here that spectra obtained from stoichiometric silicon nitride films and SRN films with Si/N equal to 0.86 annealed together with the
Fig. 2. FTIR transmission spectra of SRN films with Si/N = 1.35 after deposition and after annealing for 4 h at 950 °C and 1100 °C.
V.Em. Vamvakas, S. Gardelis / Surface & Coatings Technology 201 (2007) 9359–9364
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Table 2 Position of the main absorption band for silicon nitride films of several stoichiometries Main absorption band (cm− 1) After deposition
Annealed 950 °C
Stoichiometric (Si/N) SRN (Si/N)
0.75 0.86 1.10 1.35
832 832 839 842
1100 °C
30 min
1h
2h
4h
30 min
1h
2h
4h
832 832 839 845
832 832 840 846
832 832 841 847
832 832 842 847
832 832 845 851
832 832 845 852
832 832 846 853
832 832 846 853
All films have a thickness of about 100 nm.
previously mentioned SRN films remained unchanged before and after annealing, showing that annealing affected slightly if not at all their chemical composition. In the following we focus on the main absorption band of the annealed SRN films with Si/ N ratios 1.35 and 1.10. In the film with Si/N ratio 1.10 the main absorption band shifted from 839 cm− 1 to 842 cm− 1 after annealing at 950 °C for 4 h. The same band shifted to 846 cm− 1 after annealing at 1100 °C for 4 h. In the film with Si/N ratio 1.35 the shift of the main absorption band after annealing was even more distinct. Specifically, annealing at 950 °C for 4 h resulted in a shift of the band from 842 cm− 1 to 847 cm− 1 while annealing at 1100 °C for 4 h caused the same band to appear at 853 cm− 1 (Fig. 2). Annealing at temperatures higher than the growth temperature, which promotes segregation of Si into nanoparticles and some of them into nanocrystals, as we have observed previously in similar SRN films [25], is expected to cause changes in Si–N vibration modes due to alterations in the arrangement of Si and N atoms in the films. The observed shifts of the main absorption band, summarized in Table 2, could be well explained by this effect.
Now we focus our discussion on the feature which appeared as a “shoulder” to the main absorption band of the SRN films at 1080 cm− 1 after annealing. In order to resolve the absorption band in the region where the shoulder appeared, we calculated the second derivative of transmission [26] between 1200 cm− 1 and 950 cm− 1. For the SRN film with Si/N ratio 1.10 annealing at 950 °C regardless of the duration showed a symmetric peak located at 1090 cm− 1 in the second derivative (Fig. 3a). However, for the SRN film with Si/N ratio 1.35 annealed at the same temperature for 1 h and longer, an extra weaker peak at 1070 cm− 1 was revealed. Annealing of both SRN films at 1100 °C showed both peaks in the second derivative. The strongest peaks in the second derivative were obtained for the film with Si/N ratio 1.35 when annealing was performed at 1100 °C for 4 h (Fig. 3b). In the past [27–29] both peaks have been attributed to the asymmetrical stretching mode of vibration of Si–O–Si bridges inside SiO2 films. Specifically, the peak located at 1090 cm− 1 has been attributed to Si–O–Si bridges located at the boundaries of the SiO2 films while the peak located at 1070 cm− 1 has been attributed to bridges located at
Fig. 3. Second derivative of transmission in the range where the “shoulder” to the main absorption band appears. (a) Si/N = 1.10 annealed at 950 °C, (b) Si/N = 1.35 annealed at 1100 °C.
– – y y – – y y – – y y – – y y – – y y – – y y – – y y – – – y – – y y – – – y All films have a thickness of about 100 nm (y: for presence of the corresponding peak, –: for lack of presence).
– – y y – – – y – – y y – – – – – – y y – – – – – – – – 0.75 0.86 1.10 1.35
4h
1090 cm− 1 1070 cm− 1
2h
1090 cm− 1 1070 cm− 1
1h
1090 cm− 1 1070 cm− 1 1090 cm− 1 1070 cm− 1 1090 cm− 1
Stoichiometric (Si/N) SRN (Si/N)
4h
1090 cm− 1 1070 cm− 1
2h
1090 cm− 1 1070 cm− 1
1h
1090 cm− 1 1070 cm− 1
30 min
1090 cm− 1
30 min
1070 cm− 1
1100 °C Annealed 950 °C After deposition
2nd derivative of transmission (1200–950 cm− 1)
Table 3 Presence/lack of peaks of the 2nd derivative of transmission in the range 1200–950 cm− 1 of silicon nitride films with various stoichiometries
– – y y
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the bulk of the films, far away from their boundaries. Therefore, it is logical to assume that the formation of the “shoulder” at 1080 cm− 1 after annealing is due to the formation of Si–O–Si bridges caused by the partial oxidation of the extra silicon. When the silicon content was low or the annealing time was short then the oxidation of the films resulted in the formation of isolated Si–O–Si bridges surrounded by silicon nitride or silicon clusters. The asymmetrical stretching mode of vibration of these isolated bridges is similar to those boundary bridges located at the interfaces of SiO2 films thus resulting in the appearance of the peak at 1090 cm− 1 (Table 3). When annealing time is longer or/and the silicon content of the films is higher, the possibility of the formation of Si–O–Si bridges surrounded by other Si–O–Si bridges becomes higher. These isolated bridges which do not “feel” the silicon nitride matrix are vibrating similarly with those located in the bulk of SiO2 films thus giving a peak at 1070 cm− 1. However, the origins of the oxygen causing the partial oxidation of these films are not clear. It is possible that part of this oxygen enters during the deposition of the films, since the same furnace is also used for the deposition of SiO2 films, although great care was taken to avoid any oxygen contamination. We note here that FTIR transmission spectra obtained from stoichiometric silicon nitride films annealed together with the SRN films did not reveal the “shoulder”. In addition, second derivative of transmission spectra obtained from the as-grown SRN films did not clearly reveal the existence of Si–O bonds. It is possible that the inserted oxygen during deposition is making bonds with the silicon nitride matrix which are infrared inactive. During annealing oxygen is redistributed in the silicon nitride films and for the case of SRN films it is easy to oxidize the extra silicon and form Si–O–Si bridges. This most probably is not happening for stoichiometric silicon nitride films since before the formation of Si–O–Si bridges a break of Si–N bonds must occur. This is not thermodynamically favorable at temperatures as low as 950 °C while temperatures as high as 1100 °C must be considered close to the lowest limit of the oxidation in dry ambient of silicon nitrides. However, the scope of this discussion is not to find the origins of the oxygen which is responsible for the oxidation of the films, but it is the fact that SRN films can be oxidized relatively easy when traces of oxygen are present at temperatures starting from at least 950 °C. The light emission properties of all films were investigated. The light emission characteristics were mainly sensitive to the silicon content of the films. Significant improvement in the intensity of the emitted light was realized by post-annealing of the films at temperatures higher than the growth temperature. Specifically, the as-grown film with Si/N ratio 1.35 did not give any detectable luminescence in the region of measurements between 470 nm and 900 nm. The film with Si/N ratio 1.10 demonstrated a broad PL spectrum peaking at 600 nm (Fig. 4). The film with Si/N ratio 0.86 showed the most efficient luminescence peaking at 570 nm. (Fig. 4) We note that the PL spectra obtained from the as-grown films shifted to shorter wavelengths compared with the indirect band gap of bulk silicon (about 1100 nm). A considerable enhancement of the PL peak accompanied by a shift to higher energies with decreasing
V.Em. Vamvakas, S. Gardelis / Surface & Coatings Technology 201 (2007) 9359–9364
Fig. 4. Photoluminescence (PL) spectra of as-grown and annealed SRN films.
silicon content in the films was observed. We note that decreasing silicon content in the films is expected to lead to formation of silicon clusters of smaller sizes. Considering all of the above, the most probable explanation for the light emission from the as-grown films is the quantum confinement of the carriers recombining in the silicon clusters which may be already formed during the growth of the films due to the high temperature of the growth reaction (800 °C). Similar light emission has been observed in SRN films grown by techniques other than the one used in this study [15–22]. There is though a debate as to what is the origin of the light emission. Some attribute the effect to the quantum confinement of carriers in the silicon nanocrystals [18, 22] whereas others suggest that the effect originates from nitrogen-related localized surface states introduced within the optical gap of silicon nanocrystals [19, 20]. In either case silicon nanocrystals are necessary to confine the electron and the hole which then recombine radiatively. All as-grown films were examined for their light emission properties after annealing. The sample with Si/N ratio 1.35 emitted light only after annealing at 1100 °C in which, as TEM images demonstrated, silicon nanocrystals of sizes between 1.5 nm and 5 nm were formed [25]. Annealing of the films with lower Si/N ratio resulted in PL enhancement accompanied by a shift of the PL peak to shorter wavelengths. Fig. 4 shows this effect in the case of the sample with Si/N ratio 1.10. This effect coincided with the appearance of the Si–O–Si vibration modes in the FTIR spectrum as discussed above in detail. The effect could be well explained by the enhancement of the localization of the carriers in the silicon nanocrystals as their size was reduced due to their oxidation. 4. Conclusions SRN films with different Si content were prepared by LPCVD from DCS and NH3 mixtures. Infrared transmission spectra of the SRN films revealed the existence of an absorption band at about 830 cm− 1 attributed to the Si–N asymmetrical stretching mode. This band slightly shifted to higher wavenumbers as the silicon
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content of the films increased. Annealing at temperatures higher than the growth temperature caused further shift of the main absorption band to higher wavenumbers whereas at the same time a new absorption band at about 1080 cm− 1 appeared as a “shoulder” to the main absorption band. Calculations of the second derivative of the “shoulder”, revealed the existence of Si– O–Si bridges due to partial oxidation of the extra Si from traces of oxygen which may be inevitably present during annealing. Light emission properties of the SRN films were sensitive to the Si content, the annealing temperature and the duration of the annealing. Enhancement of PL accompanied by a blue shift with decreasing Si content was observed in the as-grown SRN films. Similar effects were observed with increasing annealing temperature and annealing time which coincided with the partial oxidation of the Si nanocrystals resulting in a reduction in their sizes. All these effects could well be attributed to the confinement of the carriers in the Si nanocrystals which might recombine radiatively in the Si nanocrystals perhaps via surface states. References [1] A.C. Adams, in: S.M. Sze (Ed.), VLSI Technology, 2nd edn, McGraw-Hill, 1988, p. 233, International edition. [2] F.H.P.M. Habraken, A.E.T. Kuiper, Mater. Sci. Eng. R12 (3) (1994) 123. [3] D. Davazoglou, Thin Solid Films 437 (2003) 266. [4] K. Misiakos, E. Tsoi, E. Halmagean, S. Kakabakos, Technical Digest, International Electron Devices Meeting, 1998, p. 25. [5] P. Wu, P. Hogrebe, D.W. Grainger, Biosens. Bioelectron. 21 (2006) 1252. [6] O. Schultz, M. Hofmann, S.W. Glunz, G.P. Willeke, 31st IEEE PVSC Orlando, Florida, 2005, p. 872. [7] M. Sekimoto, H. Yoshihara, T. Ohkubo, J. Vac. Sci. Technol. 21 (4) (1982) 1017. [8] J.G.E. Gardeniers, H.A.C. Tilmans, C.C.G. Visser, J. Vac. Sci. Technol., A 14 (5) (1996) 2879. [9] E. Cianci, F. Pirola, V. Foglietti, J. Vac. Sci. Technol. B 23 (1) (2005) 168. [10] M.C. Poon, Y. Gao, T.C.W. Kok, A.M. Myasnikov, H. Wong, Microelectron. Reliab. 41 (2001) 2071. [11] J. Chan, H. Wong, M.C. Poon, C.W. Kok, Microelectron. Reliab. 43 (2003) 611. [12] T.C. Chang, S.T. Yan, P.T. Liu, M.C. Wang, S.M. Sze, Electrochem. SolidState Lett. 7 (7) (2004) G138. [13] K.-H. Wu, H.-C. Chien, C.-C. Chan, T.-S. Chen, C.-H. Kao, IEEE Trans. Electron. Devices 52 (5) (2005) 987. [14] S. Choi, H. Yang, M. Chang, S. Baek, H. Hwanga, S. Jeon, J. Kim, C. Kim, Appl. Phys. Lett. 86 (2005) 251901. [15] T.-Y. Kim, N.-M. Park, K.-H. Kim, G.Y. Sung, Y.-W. Ok, T.-Y. Seong, C.-J. Choi, Appl. Phys. Lett. 85 (22) (2004) 5355. [16] K.S. Cho, N.-M. Park, T.-Y. Kim, K.-H. Kim, G.Y. Sung, J.H. Shin, Appl. Phys. Lett. 86 (2005) 071909. [17] L.-Y. Chen, W.-H. Chen, F.C.-N. Hong, Appl. Phys. Lett. 86 (2005) 193506. [18] T.-W. Kim, C.-H. Cho, B.-H. Kim, S.-J. Park, Appl. Phys. Lett. 88 (2006) 123102. [19] L. Dal Negro, J.H. Yi, L.C. Kimerling, S. Hamel, A. Williamson, G. Galli, Appl. Phys. Lett. 88 (2006) 183103. [20] L. Dal Negro, J.H. Yi, J. Michel, L.C. Kimerling, T.-W.F. Chang, V. Sukhvatkin, E.H. Sargent, Appl. Phys. Lett. 88 (2006) 223109. [21] L.B. Ma, R. Song, Y.M. Miao, C.R. Li, Y.Q. Wang, Z.X. Cao, Appl. Phys. Lett. 88 (2006) 093102. [22] K. Ma, J.Y. Feng, Z.J. Zhang, Nanotechnology 17 (2006) 4650. [23] I.P. Herman, Optical Diagnostics for Thin Film Processing, Academic Press Inc., 1996. [24] H.R. Philipp, Properties of Silicon, INSPEC The Institution of Electrical Engineers, 1987, p. 1019, EMIS Datareview RN=16133.
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