- Email: [email protected]
Light-emitting properties of amorphous Si:C:O:H layers fabricated by oxidation of carbon-rich a-Si:C:H films
Solid State Sciences 11 (2009) 1833–1837
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Light-emitting properties of amorphous Si:C:O:H layers fabricated by oxidation of carbon-rich a-Si:C:H films A.V. Vasin a, *, Y. Ishikawa b, S.P. Kolesnik a, A.A. Konchits a, V.S. Lysenko a, A.N. Nazarov a, G.Yu. Rudko a a b
Lashkaryov Institute of Semiconductor Physics, Pr. Nauki 41, 03028 Kiev, Ukraine Japan Fine Ceramics Center, Atsuta-ku, Nagoya 456-8587, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 May 2008 Received in revised form 10 December 2008 Accepted 27 May 2009 Available online 6 June 2009
Amorphous hydrogenated carbon-rich silicon–carbon alloy film (a-Si0.3C0.7:H) was deposited by reactive dc-magnetron sputtering of silicon target in argon–methane gas mixture. As-deposited film exhibits white photoluminescence at room temperature. After the deposition the samples were thermally annealed in dry Ar, wet Ar, or dry O2 flow at 450 C for 30 minutes that resulted in the enhancement of the photoluminescence intensity by a factor of about 5, 8 and 12 respectively. Spectral distribution of light emission was almost unchanged at the annealing in dry and wet argon while the oxidation in pure oxygen resulted in strong enhancement of a ‘‘blue’’ shoulder in the spectrum. EPR measurements at room temperature showed the decrease of spin concentration after thermal treatment in dry and wet argon and no EPR signal was detected after annealing in oxygen. FTIR and XPS measurements evidenced the formation of a-Si:O:C:H composite material after dry oxidation. Based on the measurements of photoluminescence in the temperature range 7–300 K it is suggested that light-emitting efficiency of a-Si0.3C0.7:H is determined by migration of the photo-excited carriers to non-radiative recombination centers. The physical mechanisms that can be involved in the strong enhancement of visible photoluminescence in Si:C:O:H layers are discussed. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: White photoluminescence a-SiC:H Annealing Oxidation
1. Introduction Display and lighting technologies demand low refractive index material that emits white light. From this viewpoint the carbonincorporated silicon oxide (a-SiO2:C) is one of the promising candidates. It was shown that photoluminescence (PL) of this material covers all visible spectral range of 400–700 nm and refractive index of silicon oxide matrix is low. To our knowledge, light-emitting SiO2:C layers have been fabricated by magnetron sputtering [1], Cþ implantation in SiO2 [2–4] and chemical vapor deposition techniques [5]. A new method for fabrication of whitelight-emitting carbon-rich silicon oxide layers by successive thermal carbonization of porous silicon in hydrocarbon atmosphere (650–1000 C) followed by high-temperature (800 C) oxidation in wet argon was recently proposed [6,7]. Unfortunately, this procedure involves several high-temperature treatment steps that are not desirable in some applications. In the present report we demonstrate that white-light-emitting a-Si:O:C:H layers can be fabricated by low-temperature (450 C) oxidation of amorphous
* Corresponding author. Lashkaryov Institute of Semiconductor Physics, pr. Nauki 41, 03028 Kiev, Ukraine. E-mail address: [email protected] (A.V. Vasin). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.05.030
carbon-rich a-Si1xCx:H films deposited at low substrate temperature (200 C).
2. Experimental a-Si1xCx:H films were deposited by reactive dc-magnetron sputtering of a silicon target in Ar/CH4 gas mixture. Details of the deposition procedure can be found elsewhere [8]. Si (100) wafers (p-type, 40 U cm), polished on both sides, were used as substrates. After the deposition the samples were thermally treated at 450 C for 30 minutes in dry Ar, wet Ar, or dry O2 flow at atmospheric pressure. Thickness of the a-SiC:H samples measured by interferometer was estimated to be about 500 nm. No noticeable change of the thickness was observed after thermal treatments. Composition, light-emitting properties, local structure, and paramagnetic defects were analyzed by Auger-electron spectroscopy (AES; Jump 10s, JEOL), photoluminescence spectroscopy (PL; excitation with 351 nm line of Arþ laser and 375 nm radiation of LED were used for room temperature and low-temperature measurements correspondingly), Fourier-transform infra-red spectroscopy (FTIR; 2000FT-IR, Perkin–Elmer), X-ray photoelectron spectroscopy (XPS; VGS ESCLAB MKII using non-monochromatic
1834
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837
Mg Ka X-ray source (hn ¼ 1253.6 eV)) and electron paramagnetic resonance (EPR; X-band spectrometer Radiopan SE/X-2244 with 100 kHz modulation of magnetic field). PL and EPR measurements were performed in the temperature ranges of 7–300 K and 15–300 K, correspondingly. 3. Results 3.1. Composition The composition of the a-Si1xCx:H was analyzed with Augerelectron spectroscopy using relative sensitivity factors of C(KVV) and Si(LVV) lines obtained from the bulk crystalline 6H–SiC standard. Silicon-to-carbon ratio was estimated to be about 30/70 (a-Si0.3C0.7:H). Residual contamination with oxygen and nitrogen in as-deposited film was found to be less than 7 at. %. 3.2. FTIR FTIR transmission spectra of as-deposited and thermally treated samples are presented in Fig. 1. In the range of 380–4000 cm1 the spectrum of the as-deposited sample is composed of five absorption bands typical for a-SiC:H layers: 800 cm1 (Si–C stretching), 1000 cm1 (Si:C–H2 bending, Si–C stretching), 1260 cm1 (Si–CH3 bending), 2100 cm1 (Si–Hn stretching), and 2800–3000 cm1 (C–Hn stretching) [9,10]. In the region of 1000–1100 cm1 some contribution to the absorption due to oxygen contamination is expected. We consider this contribution as a minor background. The presence of oxygen contamination is approved by the weak and broad but well detectable absorption band in range of 3000–3500 cm1 that is attributed to O–H interatomic vibration modes. After annealing in dry and wet argon the absorption bands at 2100 cm1 (Si–Hn) and 2900–3000 cm1 (C–Hn) decreased in both samples indicating that some of the corresponding interatomic bonds are broken. Another effect of annealing in dry and wet argon was increasing of absorption band at 1000 cm1 (Fig. 1, spectra 2 and 3). Similar effect was observed previously in near-stoichiometric and carbon-rich a-Si1xCx:H films after thermal annealing at 450 C in high vacuum [8,11]. The authors ascribed this effect to the creation of new Si:C–Hn bonds. They also suggested that formation of new Si:C–Hn bonds occurs due to interaction of carbon related
Si-Ox
Si-Hn
Si:C-Hn
C-H3
C-Hn
Si-C
Transmission, arb. un.
1 2 3 4
O-H Si-O-Si
500
dangling bonds and unsaturated carbon double bonds (Si:C ¼ C:Si) with hydrogen that is released from weakly bonded sites. Enhancement of the 1000 cm1 absorption band after wet oxidation is more pronounced than after dry annealing and can be ascribed most likely to partial oxidation i.e. to the creation of Si–O bonds. FTIR spectrum of the sample after oxidation in O2 atmosphere is presented in Fig. 1 (spectrum 4). No Si–Hn (2100 cm1) and C–Hn (2900–3000 cm1) stretching vibration bands were detected after dry oxidation. There are four pronounced absorption bands at 446 cm1, 805 cm1, 945 cm1, 1070 cm1 (with shoulder at 1200 cm1) and enhanced broad band at 3000–3600 cm1. All these bands except 945 cm1 band are known to be related to oxygen bonds. Typical infra-red absorption spectrum of a-SiO2 thermally grown by high-temperature oxidation of monocrystalline silicon is represented by three absorption bands at 457 cm1, 810 cm1 and 1070 cm1. These bands are attributed respectively to rocking, symmetrical stretching and asymmetrical stretching vibration modes of Si–O–Si bridges. Asymmetrical motion gives rise to two vibration modes in which adjacent oxygen atoms execute the asymmetrical motion (1) in phase with each other (1070 cm1) and (2) 180 out of phase (1200 cm1). Out of phase mode is responsible for the high frequency shoulder at 1200 cm1 [12]. The origin of the absorption peak at 945 cm1 is not clear. To our knowledge no absorption bands at such frequency were observed experimentally in a-SiC:H or thermally grown a-SiO2 layers. But the pronounced band at 930–950 cm1 was observed in FTIR spectra of amorphous hydrogenated silicon oxide films (a-SiOx:H) deposited by plasma enhanced chemical vapor deposition [13,14]. Authors attributed the band to Si–OH bonds but such identification was not strongly supported by both theoretical and experimental results. On the other hand strong absorption in the range of 915–938 cm1 was observed in IR spectra of siloxanes of type [H2SiO]n and assigned to SiH2 units [15]. Currently we are not able to suggest a comprehensive interpretation of this absorption band but most likely it is associated with hydrogen. 3.3. XPS C 1s and Si 2p core level XPS spectra of a-SiC:H film before and after thermal treatment in oxygen are presented in Fig. 2. Tabulated peak positions for interatomic bonds that are expected to be present in the samples are marked by arrows [16,17]. C 1s spectra of the as-deposited sample (solid line) are very broad due to the contribution of C–H, C–C and C–Si bonds. Moreover the energy of the interatomic bonds is expected to vary in broad range due to large number of structural defects that lead to the broadening of the spectrum. After oxidation C 1s spectrum is strongly narrowed due to the reduction of the contribution of C–H and C–Si that is in good agreement with FTIR data. Si 2p spectrum of as-deposited sample was also broad (Fig. 2, dotted line) due to contribution of Si–Si, Si–C and Si–O bonds as well as minor contribution of Si–H bonds. After dry oxidation Si 2p spectrum was strongly shifted towards higher binding energy that is explained by the formation of Si–O and O–Si–C bonds. 3.4. EPR
Si-O-Si 1000
1500
2000
2500
3000
3500
4000
Wavenumber, cm-1 Fig. 1. FTIR transmission spectra of as-deposited a-SiC:H (1), and thermally treated at 450 C in dry Ar (2), wet Ar (3), and O2 (4) flow.
EPR spectra of as-deposited and thermally treated samples measured at room temperature are presented in Fig. 3a. The value of the g-factor was calculated to be 2.0026 (0.0002) and was independent of the thermal treatments. Invariability of the g-factor indicates that the origin of the paramagnetic centers is the same in all samples. Earlier it was demonstrated that the paramagnetic
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837
1835
a C-H
Intensity, arb. un.
C-Si
C1s
Counts
Hpp
C-C
1
2 33
288
286
284
282
280
34 3345
Si-C
104
3375
Hpp, Gauss
Si-Si
Si2p
106
3360
Magnetic field, Gauss
102
100
98
96
Binding energy, eV Fig. 2. C 1s and Si 2p core level XPS spectra of as-deposited a-SiC:H sample (solid line) and a-SiC:H sample after thermal treatment at 450 C in O2 flow (dotted line). Peak positions for interatomic bonds that are expected to be present in the samples are marked by arrows.
signal in these a-SiC:H films originated mainly forms carbon related defects with minor contribution from silicon dangling bonds [8,11]. The as-deposited samples exhibited spin concentration Ns of about 5 1019 cm3. Spin concentration was reduced down to 1.6 1019 cm3 and 0.9 1019 cm3 after the annealing in dry argon and wet argon, correspondingly. No EPR signal was detected after the annealing in O2 flow. EPR spectra and peak-to-peak DHpp EPR line width for asdeposited sample were measured in temperature range of 15–300 K (Fig. 3b). One can see that DHpp rapidly drops with the increase of the temperature up to 50 K followed by slower decrease at temperature above 50 K.
3.5. Photoluminescence The as-deposited sample exhibited white-light emission at room temperature under Arþ-laser irradiation. The PL spectrum is very broad and covered the spectral range from 400 nm to 700 nm (Fig. 4a, spectrum 1). Narrow peaks at 703 nm and 727 nm are a laser ‘‘satellite’’-lines. Broad PL spectrum is strongly modulated by interference features. To be sure that these features are due to the interference effect a surface of the piece of the a-SiC:H layer was mechanically roughened by diamond nano-powder suspension (20 nm grains) to reduce internal reflection from the surface. After this mechanical treatment peaks in PL spectra disappeared (Fig. 4a, spectrum 2) proving its interference origin. ‘‘True’’ PL spectrum is featureless with maximum intensity at about 540 nm. Unfortunately, uncontrollable removing of the film material during mechanical treatment results in uncontrollable reduction of the PL intensity and we did not use this procedure for other samples. Thermal treatment at 450 C in dry argon, wet argon and dry oxygen enhanced the PL intensity by a factor of about 5, 8 and 12, respectively (Fig. 4). Spectral distribution of PL intensity was almost unchanged after annealing in dry argon. Annealing in wet argon resulted in not large but noticeable ‘‘blue’’ shift in spectral
EPR linwidth
O-Si-O
C-Si-O
b 12
10
as deposited sample
8
0
50
100
150
200
250
300
Temperature T, K Fig. 3. (a) EPR spectra of as-deposited a-SiC:H (1), and thermally treated in dry Ar (2), wet Ar (3), and O2 (4) flow. Amplitude of the EPR signal is normalized to surface area of the samples; (b) peak-to-peak EPR line width DHpp of the as-deposited sample as a function of measurement temperature.
distribution while oxygen treatment resulted in strong enhancement of the ‘‘blue’’ shoulder. Low-temperature measurements of the integral PL intensity as a function of measurement temperature IPL(T) was performed in the temperature range from 7 K up to 300 K. IPL(T) curves for asdeposited and oxidized in O2 samples are presented in Fig. 5. As-deposited sample exhibited rapid drop of the IPL(T) as the temperature increased from 7 to 50 K followed by slower decrease at the temperatures higher than 50 K. No drop of the IPL(T) in the temperature range of 7–50 K was found after dry oxidation. On the contrary, a small increase of the IPL(T) mainly in the ‘‘blue’’ spectral region was observed (Fig. 5). Increase of the measurement temperature above 50 K resulted in a decrease of light emission similar to that observed in as-deposited sample. 4. Discussion Reduction of the spin concentration after annealing in high vacuum at 450 C accompanied by the enhancement of the room temperature PL was previously observed in near-stoichiometric a-SiC:H deposited by dc-magnetron sputtering [8]. A similar effect was found in carbon-rich a-SiC:H [11]. This phenomenon was attributed to the passivation of carbon related paramagnetic defects (non-radiative recombination centers) by hydrogen that is released from weakly bonded sites during the annealing. It was also suggested that the increase of localization of photo-excited electron-hole pairs by carbon-hydrogen bonds contributes to the PL enhancement [11]. In the present study we observed the same effect for carbon-rich a-SiC:H after annealing in argon at atmospheric pressure. Another new observation is that addition of water vapor to the argon flow or
1836
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837
300
a
Integral PL intensity IPL, arb. un.
0.04
0.02
PL inensity, arb. un.
1
2
0.00
0.4
O2,450°C as-deposited
200
b
0
50
100
150
200
250
300
Temperature, K 0.3
Fig. 5. Integral intensity of the photoluminescence of as-deposited a-SiC:H sample (C) and a-SiC:H sample after thermal treatment at 450 C in O2 flow (B) as a function of measurement temperature.
4 0.2 3 0.1 2 1
0.0 400
450
500
550
600
650
700
750
800
Wavelength, nm Fig. 4. (a) Room temperature PL spectra of as-deposited a-SiC:H sample (1) and the same sample after roughening of the surface (2); (b) room temperature PL spectra of as-deposited (1) and thermally treated at 450 C in dry argon (2), wet argon (3) and dry oxygen (4).
annealing in dry oxygen results in further enhancement of the room temperature PL. The enhancement of the PL after wet annealing correlates with increase of absorption band at 1000 cm1 (Fig. 1) and decrease of spin concentration Ns (Fig. 3). These effects are more pronounced after wet annealing than after annealing in dry argon and we assume that this is due to partial oxidation of the films. The shapes of room temperature PL spectra of the samples annealed in dry and wet argon are more or less similar. This indicates that radiative recombination occurs mainly via the same electron states. Oxidation by dry oxygen converts the structure from a-SiC:H to a-Si:O:C:H and results in strong increase of the ‘‘blue’’ shoulder. It can be explained by the formation of new radiative recombination centers associated most likely with defect states in a-SiOx [18]. From Figs. 3b and 5 it is clearly seen that evolution of IPL(T) for as-deposited a-SiC:H film is well correlated with corresponding evolution of the EPR line width DHpp(T). Super-hyperfine (SHF) interaction of the electron spins with spins of hydrogen nuclei was previously shown to be a primary mechanism of the EPR line broadening in the as-deposited carbon-rich a-SiC:H film [11]. This mechanism is related to thermally activated jumps of electrons. The larger electron mobility the stronger is SHF-interaction and, as a result, the larger is DHpp. From this viewpoint we suggest that PL intensity in as-deposited sample is determined by the process of thermally activated migration of photo-excited electron-hole pairs to the non-radiative recombination centers. In the oxidized sample (a-Si:O:C:H) the development of IPL(T) at low temperature is quite different with puzzling increase of IPL(T) between 10 and 20 K (Fig. 5). Unfortunately EPR signal in this sample was undetectable and it was impossible to study the
development of the electron mobility by analyzing of DHpp(T). As it was shown above the concentration of non-radiative sites in this sample is much smaller and concentration of light-emitting sites is possibly higher in comparison to the as-deposited sample. It is reasonable to suggest that enhancement of the PL in the temperature range of 10–20 K involves some thermally activated mechanisms of transporting the photo-excited pairs to recombination sites responsible for the ‘‘blue’’ light emission. More detailed model of the mechanisms is currently under discussion and will be developed in forthcoming reports. At higher temperatures (above 50 K) a common mechanism of thermally activated migration of electron-hole pairs to non-radiative recombination centers is dominating and PL intensity decreases. 5. Conclusions Carbon-rich a-Si0.3C0.7:H films were deposited by reactive dc-magnetron sputtering of silicon target in argon/methane gas mixture. After the deposition the films were thermally treated at atmospheric pressure in Ar, (Ar þ H2O), or O2 flow at 450 C for 30 minutes that resulted in strong enhancement of room temperature photoluminescence. The most intense blue-white photoluminescence and no paramagnetic defects were observed after dry oxidation. It is suggested that enhancement of blue light emission is associated with the formation of light-emitting centers in SiOx and suppressing of non-radiative recombination by passivation of paramagnetic defects. Acknowledgments The present study was supported by Civilian Research & Development Foundation (project No. UKE2-2856). References [1] S. Hayashi, M. Kataoka, K. Yamamoto, Jpn. J. Appl. Phys., Part 2 32 (1993) L274. [2] Y.H. Yu, S.P. Wong, I.H. Wilson, Phys. Status Solidi A 168 (1998) 531. [3] J. Zhao, D.S. Mao, Z.X. Lin, B.Y. Jiang, Y.H. Yu, X.H. Liu, H.Z. Wang, G.Q. Yang, Appl. Phys. Lett. 73 (1998) 1838. [4] J. Zhao, D.S. Mao, Z.X. Lin, B.Y. Jiang, Y.H. Yu, X.H. Liu, G.Q. Yang, Mater. Lett. 38 (1999) 321. [5] S.Y. Seo, K.S. Cho, J.H. Shin, Appl. Phys. Lett. 84 (2004) 717. [6] A.V. Vasin, Y. Ishikawa, N. Shibata, J. Salonen, V.P. Lehto, Jpn. J. Appl. Phys. 19 (2007) L465. [7] S. Muto, A.V. Vasin, Y. Ishikawa, N. Shibata, J. Salonen, V.P. Lehto, Mater. Sci. Forum 561–565 (2007) 127.
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837 [8] A.V. Vasin, S.P. Kolesnik, A.A. Konchits, V.I. Kushnirenko, V.S. Lysenko, A.N. Nazarov, A.V. Rusavsky, S. Ashok, J. Appl. Phys. 99 (2006) 113520. [9] H. Wieder, M. Cardona, R. Guarnieri, Phys. Status Solidi B 92 (1979) 99. [10] J. Bullot, M.P. Schmidt, Phys. Status Solidi B 143 (1987) 345. [11] A.V. Vasin, S.P. Kolesnik, A.A. Konchits, A.V. Rusavsky, V.S. Lysenko, A.N. Nazarov, Y. Ishikawa, Y. Koshka, J. Appl. Phys. 103 (2008) 123710. [12] C.T. Kirk, Phys. Rev. B: Condens. Matter 38 (1988) 1255. [13] S. Sahli, Y. Segui, S. Ramdani, Z. Takkouk, Thin Solid Films 250 (1994) 206.
1837
lez-Elipe, J. Cotrino, [14] A. Barranco, F. Yubero, J.P. Espinos, J. Benitez, A.R. Gonza J. Allain, T. Girardeau, J.P. Rivieˇr, Surf. Coat. Technol. 142–144 (2001) 856. [15] D. Seyferth, C. Prud’homme, G.H. Wiseman, Inorg. Chem. 22 (1983) 2163. [16] N.K. Huang, D.Z. Wang, Q. Xiong, B. Yang, Nucl. Instrum. Methods Phys. Res., Sect. B 207 (2003) 395. [17] A. Avila, I. Montero, L.L. Galan, J.P. Ripalda, R. Levy, J. Appl. Phys. 89 (2001) 212. [18] L. Roboble, J. von Borany, H. Fro¨b, W. Skorupa, Appl. Phys. B 70 (2000) 131.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Light-emitting properties of amorphous Si:C:O:H layers fabricated by oxidation of carbon-rich a-Si:C:H films A.V. Vasin a, *, Y. Ishikawa b, S.P. Kolesnik a, A.A. Konchits a, V.S. Lysenko a, A.N. Nazarov a, G.Yu. Rudko a a b
Lashkaryov Institute of Semiconductor Physics, Pr. Nauki 41, 03028 Kiev, Ukraine Japan Fine Ceramics Center, Atsuta-ku, Nagoya 456-8587, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 May 2008 Received in revised form 10 December 2008 Accepted 27 May 2009 Available online 6 June 2009
Amorphous hydrogenated carbon-rich silicon–carbon alloy film (a-Si0.3C0.7:H) was deposited by reactive dc-magnetron sputtering of silicon target in argon–methane gas mixture. As-deposited film exhibits white photoluminescence at room temperature. After the deposition the samples were thermally annealed in dry Ar, wet Ar, or dry O2 flow at 450 C for 30 minutes that resulted in the enhancement of the photoluminescence intensity by a factor of about 5, 8 and 12 respectively. Spectral distribution of light emission was almost unchanged at the annealing in dry and wet argon while the oxidation in pure oxygen resulted in strong enhancement of a ‘‘blue’’ shoulder in the spectrum. EPR measurements at room temperature showed the decrease of spin concentration after thermal treatment in dry and wet argon and no EPR signal was detected after annealing in oxygen. FTIR and XPS measurements evidenced the formation of a-Si:O:C:H composite material after dry oxidation. Based on the measurements of photoluminescence in the temperature range 7–300 K it is suggested that light-emitting efficiency of a-Si0.3C0.7:H is determined by migration of the photo-excited carriers to non-radiative recombination centers. The physical mechanisms that can be involved in the strong enhancement of visible photoluminescence in Si:C:O:H layers are discussed. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: White photoluminescence a-SiC:H Annealing Oxidation
1. Introduction Display and lighting technologies demand low refractive index material that emits white light. From this viewpoint the carbonincorporated silicon oxide (a-SiO2:C) is one of the promising candidates. It was shown that photoluminescence (PL) of this material covers all visible spectral range of 400–700 nm and refractive index of silicon oxide matrix is low. To our knowledge, light-emitting SiO2:C layers have been fabricated by magnetron sputtering [1], Cþ implantation in SiO2 [2–4] and chemical vapor deposition techniques [5]. A new method for fabrication of whitelight-emitting carbon-rich silicon oxide layers by successive thermal carbonization of porous silicon in hydrocarbon atmosphere (650–1000 C) followed by high-temperature (800 C) oxidation in wet argon was recently proposed [6,7]. Unfortunately, this procedure involves several high-temperature treatment steps that are not desirable in some applications. In the present report we demonstrate that white-light-emitting a-Si:O:C:H layers can be fabricated by low-temperature (450 C) oxidation of amorphous
* Corresponding author. Lashkaryov Institute of Semiconductor Physics, pr. Nauki 41, 03028 Kiev, Ukraine. E-mail address: [email protected] (A.V. Vasin). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.05.030
carbon-rich a-Si1xCx:H films deposited at low substrate temperature (200 C).
2. Experimental a-Si1xCx:H films were deposited by reactive dc-magnetron sputtering of a silicon target in Ar/CH4 gas mixture. Details of the deposition procedure can be found elsewhere [8]. Si (100) wafers (p-type, 40 U cm), polished on both sides, were used as substrates. After the deposition the samples were thermally treated at 450 C for 30 minutes in dry Ar, wet Ar, or dry O2 flow at atmospheric pressure. Thickness of the a-SiC:H samples measured by interferometer was estimated to be about 500 nm. No noticeable change of the thickness was observed after thermal treatments. Composition, light-emitting properties, local structure, and paramagnetic defects were analyzed by Auger-electron spectroscopy (AES; Jump 10s, JEOL), photoluminescence spectroscopy (PL; excitation with 351 nm line of Arþ laser and 375 nm radiation of LED were used for room temperature and low-temperature measurements correspondingly), Fourier-transform infra-red spectroscopy (FTIR; 2000FT-IR, Perkin–Elmer), X-ray photoelectron spectroscopy (XPS; VGS ESCLAB MKII using non-monochromatic
1834
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837
Mg Ka X-ray source (hn ¼ 1253.6 eV)) and electron paramagnetic resonance (EPR; X-band spectrometer Radiopan SE/X-2244 with 100 kHz modulation of magnetic field). PL and EPR measurements were performed in the temperature ranges of 7–300 K and 15–300 K, correspondingly. 3. Results 3.1. Composition The composition of the a-Si1xCx:H was analyzed with Augerelectron spectroscopy using relative sensitivity factors of C(KVV) and Si(LVV) lines obtained from the bulk crystalline 6H–SiC standard. Silicon-to-carbon ratio was estimated to be about 30/70 (a-Si0.3C0.7:H). Residual contamination with oxygen and nitrogen in as-deposited film was found to be less than 7 at. %. 3.2. FTIR FTIR transmission spectra of as-deposited and thermally treated samples are presented in Fig. 1. In the range of 380–4000 cm1 the spectrum of the as-deposited sample is composed of five absorption bands typical for a-SiC:H layers: 800 cm1 (Si–C stretching), 1000 cm1 (Si:C–H2 bending, Si–C stretching), 1260 cm1 (Si–CH3 bending), 2100 cm1 (Si–Hn stretching), and 2800–3000 cm1 (C–Hn stretching) [9,10]. In the region of 1000–1100 cm1 some contribution to the absorption due to oxygen contamination is expected. We consider this contribution as a minor background. The presence of oxygen contamination is approved by the weak and broad but well detectable absorption band in range of 3000–3500 cm1 that is attributed to O–H interatomic vibration modes. After annealing in dry and wet argon the absorption bands at 2100 cm1 (Si–Hn) and 2900–3000 cm1 (C–Hn) decreased in both samples indicating that some of the corresponding interatomic bonds are broken. Another effect of annealing in dry and wet argon was increasing of absorption band at 1000 cm1 (Fig. 1, spectra 2 and 3). Similar effect was observed previously in near-stoichiometric and carbon-rich a-Si1xCx:H films after thermal annealing at 450 C in high vacuum [8,11]. The authors ascribed this effect to the creation of new Si:C–Hn bonds. They also suggested that formation of new Si:C–Hn bonds occurs due to interaction of carbon related
Si-Ox
Si-Hn
Si:C-Hn
C-H3
C-Hn
Si-C
Transmission, arb. un.
1 2 3 4
O-H Si-O-Si
500
dangling bonds and unsaturated carbon double bonds (Si:C ¼ C:Si) with hydrogen that is released from weakly bonded sites. Enhancement of the 1000 cm1 absorption band after wet oxidation is more pronounced than after dry annealing and can be ascribed most likely to partial oxidation i.e. to the creation of Si–O bonds. FTIR spectrum of the sample after oxidation in O2 atmosphere is presented in Fig. 1 (spectrum 4). No Si–Hn (2100 cm1) and C–Hn (2900–3000 cm1) stretching vibration bands were detected after dry oxidation. There are four pronounced absorption bands at 446 cm1, 805 cm1, 945 cm1, 1070 cm1 (with shoulder at 1200 cm1) and enhanced broad band at 3000–3600 cm1. All these bands except 945 cm1 band are known to be related to oxygen bonds. Typical infra-red absorption spectrum of a-SiO2 thermally grown by high-temperature oxidation of monocrystalline silicon is represented by three absorption bands at 457 cm1, 810 cm1 and 1070 cm1. These bands are attributed respectively to rocking, symmetrical stretching and asymmetrical stretching vibration modes of Si–O–Si bridges. Asymmetrical motion gives rise to two vibration modes in which adjacent oxygen atoms execute the asymmetrical motion (1) in phase with each other (1070 cm1) and (2) 180 out of phase (1200 cm1). Out of phase mode is responsible for the high frequency shoulder at 1200 cm1 [12]. The origin of the absorption peak at 945 cm1 is not clear. To our knowledge no absorption bands at such frequency were observed experimentally in a-SiC:H or thermally grown a-SiO2 layers. But the pronounced band at 930–950 cm1 was observed in FTIR spectra of amorphous hydrogenated silicon oxide films (a-SiOx:H) deposited by plasma enhanced chemical vapor deposition [13,14]. Authors attributed the band to Si–OH bonds but such identification was not strongly supported by both theoretical and experimental results. On the other hand strong absorption in the range of 915–938 cm1 was observed in IR spectra of siloxanes of type [H2SiO]n and assigned to SiH2 units [15]. Currently we are not able to suggest a comprehensive interpretation of this absorption band but most likely it is associated with hydrogen. 3.3. XPS C 1s and Si 2p core level XPS spectra of a-SiC:H film before and after thermal treatment in oxygen are presented in Fig. 2. Tabulated peak positions for interatomic bonds that are expected to be present in the samples are marked by arrows [16,17]. C 1s spectra of the as-deposited sample (solid line) are very broad due to the contribution of C–H, C–C and C–Si bonds. Moreover the energy of the interatomic bonds is expected to vary in broad range due to large number of structural defects that lead to the broadening of the spectrum. After oxidation C 1s spectrum is strongly narrowed due to the reduction of the contribution of C–H and C–Si that is in good agreement with FTIR data. Si 2p spectrum of as-deposited sample was also broad (Fig. 2, dotted line) due to contribution of Si–Si, Si–C and Si–O bonds as well as minor contribution of Si–H bonds. After dry oxidation Si 2p spectrum was strongly shifted towards higher binding energy that is explained by the formation of Si–O and O–Si–C bonds. 3.4. EPR
Si-O-Si 1000
1500
2000
2500
3000
3500
4000
Wavenumber, cm-1 Fig. 1. FTIR transmission spectra of as-deposited a-SiC:H (1), and thermally treated at 450 C in dry Ar (2), wet Ar (3), and O2 (4) flow.
EPR spectra of as-deposited and thermally treated samples measured at room temperature are presented in Fig. 3a. The value of the g-factor was calculated to be 2.0026 (0.0002) and was independent of the thermal treatments. Invariability of the g-factor indicates that the origin of the paramagnetic centers is the same in all samples. Earlier it was demonstrated that the paramagnetic
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837
1835
a C-H
Intensity, arb. un.
C-Si
C1s
Counts
Hpp
C-C
1
2 33
288
286
284
282
280
34 3345
Si-C
104
3375
Hpp, Gauss
Si-Si
Si2p
106
3360
Magnetic field, Gauss
102
100
98
96
Binding energy, eV Fig. 2. C 1s and Si 2p core level XPS spectra of as-deposited a-SiC:H sample (solid line) and a-SiC:H sample after thermal treatment at 450 C in O2 flow (dotted line). Peak positions for interatomic bonds that are expected to be present in the samples are marked by arrows.
signal in these a-SiC:H films originated mainly forms carbon related defects with minor contribution from silicon dangling bonds [8,11]. The as-deposited samples exhibited spin concentration Ns of about 5 1019 cm3. Spin concentration was reduced down to 1.6 1019 cm3 and 0.9 1019 cm3 after the annealing in dry argon and wet argon, correspondingly. No EPR signal was detected after the annealing in O2 flow. EPR spectra and peak-to-peak DHpp EPR line width for asdeposited sample were measured in temperature range of 15–300 K (Fig. 3b). One can see that DHpp rapidly drops with the increase of the temperature up to 50 K followed by slower decrease at temperature above 50 K.
3.5. Photoluminescence The as-deposited sample exhibited white-light emission at room temperature under Arþ-laser irradiation. The PL spectrum is very broad and covered the spectral range from 400 nm to 700 nm (Fig. 4a, spectrum 1). Narrow peaks at 703 nm and 727 nm are a laser ‘‘satellite’’-lines. Broad PL spectrum is strongly modulated by interference features. To be sure that these features are due to the interference effect a surface of the piece of the a-SiC:H layer was mechanically roughened by diamond nano-powder suspension (20 nm grains) to reduce internal reflection from the surface. After this mechanical treatment peaks in PL spectra disappeared (Fig. 4a, spectrum 2) proving its interference origin. ‘‘True’’ PL spectrum is featureless with maximum intensity at about 540 nm. Unfortunately, uncontrollable removing of the film material during mechanical treatment results in uncontrollable reduction of the PL intensity and we did not use this procedure for other samples. Thermal treatment at 450 C in dry argon, wet argon and dry oxygen enhanced the PL intensity by a factor of about 5, 8 and 12, respectively (Fig. 4). Spectral distribution of PL intensity was almost unchanged after annealing in dry argon. Annealing in wet argon resulted in not large but noticeable ‘‘blue’’ shift in spectral
EPR linwidth
O-Si-O
C-Si-O
b 12
10
as deposited sample
8
0
50
100
150
200
250
300
Temperature T, K Fig. 3. (a) EPR spectra of as-deposited a-SiC:H (1), and thermally treated in dry Ar (2), wet Ar (3), and O2 (4) flow. Amplitude of the EPR signal is normalized to surface area of the samples; (b) peak-to-peak EPR line width DHpp of the as-deposited sample as a function of measurement temperature.
distribution while oxygen treatment resulted in strong enhancement of the ‘‘blue’’ shoulder. Low-temperature measurements of the integral PL intensity as a function of measurement temperature IPL(T) was performed in the temperature range from 7 K up to 300 K. IPL(T) curves for asdeposited and oxidized in O2 samples are presented in Fig. 5. As-deposited sample exhibited rapid drop of the IPL(T) as the temperature increased from 7 to 50 K followed by slower decrease at the temperatures higher than 50 K. No drop of the IPL(T) in the temperature range of 7–50 K was found after dry oxidation. On the contrary, a small increase of the IPL(T) mainly in the ‘‘blue’’ spectral region was observed (Fig. 5). Increase of the measurement temperature above 50 K resulted in a decrease of light emission similar to that observed in as-deposited sample. 4. Discussion Reduction of the spin concentration after annealing in high vacuum at 450 C accompanied by the enhancement of the room temperature PL was previously observed in near-stoichiometric a-SiC:H deposited by dc-magnetron sputtering [8]. A similar effect was found in carbon-rich a-SiC:H [11]. This phenomenon was attributed to the passivation of carbon related paramagnetic defects (non-radiative recombination centers) by hydrogen that is released from weakly bonded sites during the annealing. It was also suggested that the increase of localization of photo-excited electron-hole pairs by carbon-hydrogen bonds contributes to the PL enhancement [11]. In the present study we observed the same effect for carbon-rich a-SiC:H after annealing in argon at atmospheric pressure. Another new observation is that addition of water vapor to the argon flow or
1836
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837
300
a
Integral PL intensity IPL, arb. un.
0.04
0.02
PL inensity, arb. un.
1
2
0.00
0.4
O2,450°C as-deposited
200
b
0
50
100
150
200
250
300
Temperature, K 0.3
Fig. 5. Integral intensity of the photoluminescence of as-deposited a-SiC:H sample (C) and a-SiC:H sample after thermal treatment at 450 C in O2 flow (B) as a function of measurement temperature.
4 0.2 3 0.1 2 1
0.0 400
450
500
550
600
650
700
750
800
Wavelength, nm Fig. 4. (a) Room temperature PL spectra of as-deposited a-SiC:H sample (1) and the same sample after roughening of the surface (2); (b) room temperature PL spectra of as-deposited (1) and thermally treated at 450 C in dry argon (2), wet argon (3) and dry oxygen (4).
annealing in dry oxygen results in further enhancement of the room temperature PL. The enhancement of the PL after wet annealing correlates with increase of absorption band at 1000 cm1 (Fig. 1) and decrease of spin concentration Ns (Fig. 3). These effects are more pronounced after wet annealing than after annealing in dry argon and we assume that this is due to partial oxidation of the films. The shapes of room temperature PL spectra of the samples annealed in dry and wet argon are more or less similar. This indicates that radiative recombination occurs mainly via the same electron states. Oxidation by dry oxygen converts the structure from a-SiC:H to a-Si:O:C:H and results in strong increase of the ‘‘blue’’ shoulder. It can be explained by the formation of new radiative recombination centers associated most likely with defect states in a-SiOx [18]. From Figs. 3b and 5 it is clearly seen that evolution of IPL(T) for as-deposited a-SiC:H film is well correlated with corresponding evolution of the EPR line width DHpp(T). Super-hyperfine (SHF) interaction of the electron spins with spins of hydrogen nuclei was previously shown to be a primary mechanism of the EPR line broadening in the as-deposited carbon-rich a-SiC:H film [11]. This mechanism is related to thermally activated jumps of electrons. The larger electron mobility the stronger is SHF-interaction and, as a result, the larger is DHpp. From this viewpoint we suggest that PL intensity in as-deposited sample is determined by the process of thermally activated migration of photo-excited electron-hole pairs to the non-radiative recombination centers. In the oxidized sample (a-Si:O:C:H) the development of IPL(T) at low temperature is quite different with puzzling increase of IPL(T) between 10 and 20 K (Fig. 5). Unfortunately EPR signal in this sample was undetectable and it was impossible to study the
development of the electron mobility by analyzing of DHpp(T). As it was shown above the concentration of non-radiative sites in this sample is much smaller and concentration of light-emitting sites is possibly higher in comparison to the as-deposited sample. It is reasonable to suggest that enhancement of the PL in the temperature range of 10–20 K involves some thermally activated mechanisms of transporting the photo-excited pairs to recombination sites responsible for the ‘‘blue’’ light emission. More detailed model of the mechanisms is currently under discussion and will be developed in forthcoming reports. At higher temperatures (above 50 K) a common mechanism of thermally activated migration of electron-hole pairs to non-radiative recombination centers is dominating and PL intensity decreases. 5. Conclusions Carbon-rich a-Si0.3C0.7:H films were deposited by reactive dc-magnetron sputtering of silicon target in argon/methane gas mixture. After the deposition the films were thermally treated at atmospheric pressure in Ar, (Ar þ H2O), or O2 flow at 450 C for 30 minutes that resulted in strong enhancement of room temperature photoluminescence. The most intense blue-white photoluminescence and no paramagnetic defects were observed after dry oxidation. It is suggested that enhancement of blue light emission is associated with the formation of light-emitting centers in SiOx and suppressing of non-radiative recombination by passivation of paramagnetic defects. Acknowledgments The present study was supported by Civilian Research & Development Foundation (project No. UKE2-2856). References [1] S. Hayashi, M. Kataoka, K. Yamamoto, Jpn. J. Appl. Phys., Part 2 32 (1993) L274. [2] Y.H. Yu, S.P. Wong, I.H. Wilson, Phys. Status Solidi A 168 (1998) 531. [3] J. Zhao, D.S. Mao, Z.X. Lin, B.Y. Jiang, Y.H. Yu, X.H. Liu, H.Z. Wang, G.Q. Yang, Appl. Phys. Lett. 73 (1998) 1838. [4] J. Zhao, D.S. Mao, Z.X. Lin, B.Y. Jiang, Y.H. Yu, X.H. Liu, G.Q. Yang, Mater. Lett. 38 (1999) 321. [5] S.Y. Seo, K.S. Cho, J.H. Shin, Appl. Phys. Lett. 84 (2004) 717. [6] A.V. Vasin, Y. Ishikawa, N. Shibata, J. Salonen, V.P. Lehto, Jpn. J. Appl. Phys. 19 (2007) L465. [7] S. Muto, A.V. Vasin, Y. Ishikawa, N. Shibata, J. Salonen, V.P. Lehto, Mater. Sci. Forum 561–565 (2007) 127.
A.V. Vasin et al. / Solid State Sciences 11 (2009) 1833–1837 [8] A.V. Vasin, S.P. Kolesnik, A.A. Konchits, V.I. Kushnirenko, V.S. Lysenko, A.N. Nazarov, A.V. Rusavsky, S. Ashok, J. Appl. Phys. 99 (2006) 113520. [9] H. Wieder, M. Cardona, R. Guarnieri, Phys. Status Solidi B 92 (1979) 99. [10] J. Bullot, M.P. Schmidt, Phys. Status Solidi B 143 (1987) 345. [11] A.V. Vasin, S.P. Kolesnik, A.A. Konchits, A.V. Rusavsky, V.S. Lysenko, A.N. Nazarov, Y. Ishikawa, Y. Koshka, J. Appl. Phys. 103 (2008) 123710. [12] C.T. Kirk, Phys. Rev. B: Condens. Matter 38 (1988) 1255. [13] S. Sahli, Y. Segui, S. Ramdani, Z. Takkouk, Thin Solid Films 250 (1994) 206.
1837
lez-Elipe, J. Cotrino, [14] A. Barranco, F. Yubero, J.P. Espinos, J. Benitez, A.R. Gonza J. Allain, T. Girardeau, J.P. Rivieˇr, Surf. Coat. Technol. 142–144 (2001) 856. [15] D. Seyferth, C. Prud’homme, G.H. Wiseman, Inorg. Chem. 22 (1983) 2163. [16] N.K. Huang, D.Z. Wang, Q. Xiong, B. Yang, Nucl. Instrum. Methods Phys. Res., Sect. B 207 (2003) 395. [17] A. Avila, I. Montero, L.L. Galan, J.P. Ripalda, R. Levy, J. Appl. Phys. 89 (2001) 212. [18] L. Roboble, J. von Borany, H. Fro¨b, W. Skorupa, Appl. Phys. B 70 (2000) 131.