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SiO2 superlattice prepared by magnetron sputtering
Applied Surface Science 255 (2009) 4547–4550
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Annealing temperature dependence of Raman scattering in Si/SiO2 superlattice prepared by magnetron sputtering Shihua Huang *, Hong Xiao, Sha Shou Physics Department, Zhejiang Normal University, Zhejiang 321004, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 May 2008 Received in revised form 30 September 2008 Accepted 26 November 2008 Available online 3 December 2008
Si/SiO2 superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals annealed at 850 8C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity of that at 520 cm1 increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm1 shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO2 superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images. ß 2008 Elsevier B.V. All rights reserved.
PACS: 61.72 80.15.C 81.40.E Keywords: Si/SiO2 superlattice Magnetron sputtering Raman spectroscopy
1. Introduction Since Lu et al. [1] discovered intense luminescence in Si/SiO2 superlattices (SLs) in 1995, they have attracted much interest due to their potential use in Si-based optoelectronic devices and photonic applications [2–4]. Unlike porous silicon, Si/SiO2 SLs have more stable chemical and mechanical properties. Also, compared to nanocrystalline Si (nc-Si) grains embedded in a silica matrix, the Si-sublayer thickness and the size of the nc-Si grains in Si/SiO2 SLs can be controlled more accurately. At present, many technologies are being employed for the deposition of Si/SiO2 SLs, such as molecular beam epitaxy (MBE) [5,6], radio-frequency (RF)/direct current (DC) magnetron sputtering (MS) [7–9], plasma-enhanced chemical vapor deposition (PECVD) [10,11], and thermally reactive evaporation (TRE) [12,13]. The standard RF-MS/DC-MS deposition method has high resource usability, high deposition velocity and a safe deposition process compared to the gas-source MBE and ultra-high-vacuum CVD methods, and large-area deposition capability compared to the solid-source MBE method. Many techniques, such as optical absorption, Raman scattering (RS), photoluminescence and photocurrent measurements, have been used to investigate the optical and vibrational properties of Si/SiO2 SLs.
RS, which is based on inelastic light scattering, is a powerful and non-destructive technique for the analysis of stress, crystal lattice disorder and homogeneity of materials. It is also an excellent method for studying nanostructured materials due to its high sensitivity to the crystal potential fluctuations and local atomic arrangement. Variations in the disorder and particle size change the shapes and positions of Raman lines. So in addition to giving qualitative information about the chemical structure, Raman spectroscopy has been shown to be useful in determining nanoparticle size, shape, size distribution etc. [14]. In this paper, we have employed the RF magnetron sputtering method to deposit Si/SiO2 SLs, the structures and properties of which can be controlled by changing the experimental parameters, such as the deposition and annealing temperatures. Raman spectra have been used to investigate the microstructural properties of the Si/SiO2 SLs. The size and distribution of the Si nanocrystals were both estimated by a theoretical analysis based on the phonon confinement model. Theoretical calculations were compared with the experimental results of Raman spectra. A comparison was made with the scanning electron microscope (SEM) analysis in order to verify the estimations deduced from the theoretical calculations. 2. Experimental details
* Corresponding author. Tel.: +86 579 82298661. E-mail address: [email protected] (S. Huang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.069
Si/SiO2 SLs were prepared on n-type Si (1 1 1) substrates with a resistivity of about 10 V cm. Si and SiO2 sputtering targets with a
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S. Huang et al. / Applied Surface Science 255 (2009) 4547–4550
diameter of 6 cm were used. The base pressure in the vacuum vessel was below 3 105 Pa and the working gas was Ar. The sputtering pressure was fixed at 1.3 Pa, and the sputtering power was 25 W. In order to deposit Si/SiO2 SLs, two cathodes were used. One cathode (the SiO2 target) was powered by a 13.56 MHz RF voltage and the other (Si) was powered by a DC voltage. The entire water-cooled cathodes could be moved up and down over a distance of 25 cm parallel to the deposition area. The deposition surface was positioned at a distance of 10 cm parallel to and opposite the cathode surface. The sample holder could be heated by a filament positioned behind the substrate. The temperature was measured using a thermocouple inserted into the sampleholder assembly. Before the substrates were placed in the pretreatment chamber, they were first processed by the standard RCA cleaning technology for silicon wafers, rinsed in deionized water and finally dried with nitrogen gas. In order to eliminate the oxide and water vapor absorbed on the surface of the Si substrate, it was anti-sputtered in the pretreatment chamber and then transported to the sputtering chamber by means of a magnetically coupled transfer rod. The typical Si/SiO2 SL structure consisted of 50 pairs of alternately stacked 3.0 nm thick Si layers and 3.0 nm thick SiO2 layers. The thickness of the Si or SiO2 layer was controlled by adjusting the sputtering power and sputtering time. The substrate temperature was varied from room temperature to 700 8C. The as-grown samples were annealed for about 30 min in a pretreatment chamber in nitrogen ambient at different temperatures (500–1100 8C). The Raman spectra were measured using a micro-Raman setup (Renishaw 2000) in backscattering configuration at room temperature. The 514.5 nm line from an Ar+ laser was used as the incident light. The power density of the incident light on the sample surface was about 1 mW/mm2. For the SEM observations, samples were prepared by standard procedures. 3. Results and discussion Fig. 1 shows the SEM images of typical Si/SiO2 SLs annealed at 850 and 1100 8C for 30 min. The distribution of Si nanocrystal size is not uniform, the value of the nanocrystal diameter varying in the range 3–20 nm. It is observed from these images that by increasing the annealing temperature, the difference in size between the smaller and larger nanocrystals increases. The mean size of Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals
Fig. 2. Raman spectra for Si/SiO2 SLs deposited at different temperatures: (a) room temperature; (b) 300 8C; (c) 500 8C; (d) 700 8C.
annealed at 850 8C. This can be understood by considering the growth and coarsening theory. During the annealing process, the growth of these tiny structures occurs via a diffusion mechanism, and the formation of many small crystals is kinetically favored. But small crystals have a larger surface area to volume ratio than large crystals; in other words, large crystals represent a lower energy state than small crystals, so many small crystals are transformed into a larger crystal. Comparing Fig. 1(a) and (b), it is clear that there are larger crystallites present at higher annealing temperatures. The different behaviors of the Raman spectra for samples deposited at different temperatures were investigated. Fig. 2 shows the Raman measurement results for typical Si/SiO2 SLs deposited at different temperatures. For the sample deposited at room temperature, only one broad Raman peak located near 470 cm1 is observed; it is attributed to the vibration of amorphous Si. In the case of disordered semiconductors, such as amorphous and nanocrystalline ones, the q = 0 selection rule that determines the Raman spectra of bulk crystals is either fully or partially violated (relaxed) so that phonons other than those that are zone centered can also be observed in their Raman spectra.
Fig. 1. SEM images for the typical Si/SiO2 SLs annealed at 850 8C (a) and 1100 8C (b).
S. Huang et al. / Applied Surface Science 255 (2009) 4547–4550
However, two peaks located near 470 and 520 cm1 were observed for the samples deposited at 300, 500 and 700 8C, the peak near 520 cm1 being interpreted as arising from the vibration mode of crystalline Si. With increasing deposition temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity of the 520 cm1 peak increases. The substrate temperature influences the surface mobility, revaporization and crystallization of surface adatoms on the substrate. At low temperature, sputtered atoms deposited on the substrate lose their kinetic energy quickly and condense on the substrate surface. Due to the low mobility of adatoms, the sputtered atoms tend to agglomerate as thin amorphous films. The kinetic energy and the probability of crossing the surface barrier of adatoms increase with increasing substrate temperature, which is in favor of superficially horizontal migration of sputtered atoms on the substrate. Atoms with weak adhesive strength will escape from the substrate surface because of revaporation, which is favorable for the nucleation, growth, crystallization and preferred orientation of thin films. However, very high substrate or annealing temperatures lead to the formation of large crystal grains and polycrystalline films are easily formed. When the substrate temperature increases from room temperature to 300 8C, the partial structure of the deposited film transforms from amorphous to crystalline. In other words, the amorphous and crystalline phases coexist. With an increased substrate temperature such as 500 and 700 8C, more of the amorphous structure in the deposited film is transformed into crystalline structure. The component of the crystal phase for the sample deposited at 700 8C is larger than that for samples deposited at 300 and 500 8C. Fig. 3 shows Raman spectra for Si/SiO2 SLs annealed at different temperatures: (a) 700 8C; (b) 850 8C; (c) 1100 8C. The Raman spectra of the annealed Si/SiO2 SLs exhibit a sharp peak near 520 cm1, indicating the crystallization of Si in the SLs. The width and spectral shape of this peak are different from those of bulk Si, but similar to those of nc-Si. For the SLs annealed at 1100 8C, only a narrow and intense Raman peak near 520 cm1 is observed. However, for the SLs annealed at 700 or 850 8C, a very broad peak near 470 cm1 is observed as well as a narrow peak near 520 cm1. With increasing annealing temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity at 520 cm1 increases. Also, when the annealing temperature increases, the Raman peak near 520 cm1 shifts and becomes narrower, and asymmetry emerges. In these measurements, the zone-centered Si
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optical phonons peaked at 519.9 cm1 with a full width at half maximum (FWHM) of 4.5 cm1 in the spectrum for SLs annealed at 1100 8C (Fig. 3(a)), whereas the peaks in the spectra for the SLs annealed at 850 8C (Fig. 3(b)) and 700 8C (Fig. 3(c)) are downshifted to 519.1 and 518.2 cm1, and their FWHM values are 5.4 and 6.7 cm1, respectively. In Fig. 3, the peaks near 520 cm1 for the SLs annealed at 700, 850 and 1100 8C are asymmetrically broadened towards lower frequency with respect to the bulk Si peak. During the annealing process, with increasing annealing temperature, the formation of large crystals made up of many small crystals is thermodynamically favored. This behavior causes a red shift of the Raman frequency of and a narrowing of the peak. In fact, the experimental results show a blue shift of the Raman peak. This may be caused by the stress exerted on the Si nanocrystals, which increases with a decrease of the crystallite size. The mismatch of Si and SiO2 may result in compressive stress in the Si nanocrystals. This stress causes an upward shift of the Si crystalline peak, thereby basically compensating the downward shift caused by the confinement on the phonon frequency [15]. From the SEM images (Fig. 1), we can see that the mean size of the Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals annealed at 850 8C. The size and size distribution of the Si nanocrystals were both estimated by a theoretical analysis based on the phonon confinement model. This topic will be discussed in detail below. Variations in the size of the nanocrystals induce peak-frequency shift, peak broadening and peak asymmetry in the Raman spectra. A phonon can be described by a plane wave for a bulk crystal, but inside a nanocrystal it is instead described by a spatially confined wave packet. A finite particle size L would bring about an uncertainty in wavevector on the order of 2p/L, which will be larger for smaller crystal sizes. In a bulk crystal, this uncertainty is zero, and as only the phonons near the zone center can contribute to the first-order Raman spectrum, the Raman peak will be rather sharp. If the phonon dispersion curves are not flat near q = 0, then the spectral features of nanocrystals will shift, broaden and become asymmetric due to the phonon confinement. For nc-Si, the band-shape modification of the Raman peak has been explained using different theoretical models that quantify the phonon confinement. These models have become very popular because they allow the Si crystallite size to be determined [16–18]. The spherical shape of nanocrystals was confirmed by the SEM results (see Fig. 1). The q = 0 selection rule is a consequence of the full translational symmetry of a crystalline sample. In the presence of finite size effects, e.g., partial phonon confinement in nanoparticles, the Raman intensity can be written as a superposition of Lorentzians centered at v = v(q) (the dispersion relation of optical phonons) with q in the first Brillouin zone. This produces a symmetric broadening and red shift of the Raman peak. In particular this red shift is caused by the fact that in Si, v is a decreasing function of q. If the effective distributions of particle radii are also taken into account, the peak becomes asymmetric. Assuming the mean value of the size distribution function to be a Gaussian function, the Raman spectrum at frequency v, I(v), for nanocrystals can be written as [16]: IðvÞ /
Z
1 0
expðq2 L2 =4a2 Þ ½v vðqÞ2 þ ðG 0 =2Þ
3
2
d q
(1)
where q is expressed in units of 2p/a, a is the lattice constant, and G0 is the linewidth of the Si LO phonon in the Si bulk crystal (3.6 cm1 including instrument contributions). We consider the analytic form for the phonon dispersion of the LO phonons in nc-Si to be [19]: Fig. 3. Raman spectra for Si/SiO2 SLs annealed at different temperatures: (a) 1100 8C; (b) 850 8C; (c) 700 8C. The compression ratio of (a) is 1:10.
v2 ðqÞ ¼ A þ B cos
pq 2
(2)
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S. Huang et al. / Applied Surface Science 255 (2009) 4547–4550
The Raman peak red shifts, the FWHM and the asymmetry degree increase with a decrease of mean nanocrystal size, which can be explained by phonon confinement effects. 4. Conclusions
Fig. 4. Raman spectra for Si/SiO2 SLs deposited at room temperature is amorphous Si-like. The crystalline Si peaks in the spectrum of annealed films (dots) are fitted using a phonon confinement model considering spherical nanocrystals (solid lines). Calculated L, and annealing temperatures are indicated near the theoretical curves.
where A = 1.714 105 cm2, B = 1.000 105 cm2 (these values may be obtained experimentally by neutron scattering). Using Eqs. (1) and (2), we can calculate the Raman spectra of nanocrystals and estimate the average diameter of Si nanocrystal. In Fig. 4, the crystalline Si peaks in the spectrum of annealed films (dots) are fitted using a phonon confinement model considering spherical nanocrystals (solid lines). The calculated values of L and the annealing temperatures are indicated near the theoretical curves. The calculated spectra agree well with the experimental results. The spectrum of an as-grown SL deposited at room temperature is similar to that in Fig. 4(d) and shows that the Si in the SLs is in an amorphous phase. The Raman spectra for all of the annealed SLs, on the other hand, are characterized by a peak near the optical phonon mode of crystalline Si, i.e. 520 cm1, indicating the crystallization of Si and the formation of nanocrystals. By fitting the Si peak around 520 cm1, we can estimate L as 5.3, 7.5 and 9.8 nm in the SLs annealed at 700, 850 and 1100 8C, respectively (the corresponding FWHM values are 6.7, 5.4 and 4.5 cm1). The size of the crystal islands can be determined from the SEM image (see Fig. 1), and the nanocrystal size for SLs annealed at 1100 8C is estimated to vary between 3 and 20 nm. It can be seen from Fig. 1(b) that these nanocrystals fall into two groups: group 1 with small nanocrystals having an average grain size of 5 nm and group 2 with large nanocrystals having an average grain size of 16 nm. This can be explained by considering the growth and coarsening theory, in terms of which some nanocrystals grow while others shrink. We can confidently conclude that the average size of 9.8 nm calculated from the Raman spectrum is a good estimate of the average size of the nanocrystals. The mean size L of Si nanocrystals increases with the annealing temperature. We can see that the peak frequency is red shifted from the corresponding bulk value by 1.8, 0.9 and 0.1 cm1 for the SLs annealed at 700, 850 and 1100 8C, respectively.
Si/SiO2 superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals annealed at 850 8C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity of that at 520 cm1 increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm1 shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO2 superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images. Acknowledgement This work was supported by Program for Innovative Research Team in Zhejiang Normal University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Z.H. Lu, D.J. Lockwood, J.M. Baribeau, Nature 258 (1995) 378. E.C. Cho, J. Xia, A.G. Aberle, M.A. Green, Sol. Energy Mater. Sol. Cells 74 (2002) 147. X. Portier, C. Ternon, F. Gourbilleau, C. Dufour, R. Rizk, Physica E 16 (2003) 439. M. Modreanua, M. Gartnerb, E. Aperathitisc, N. Tomozeiud, M. Androulidakic, D. Cristeae, P. Hurleya, Physica E 16 (2003) 461. Q. Zhang, A. Filios, C. Lofgren, R. Tsu, Physica E 8 (2000) 365. G.G. Jernigan, C.L. Silvestre, M. Fatemi, M.E. Twigg, P.E. Thompson, J. Cryst. Growth 213 (2000) 299. N.N. Liu, J.M. Sun, S.H. Pan, Z.H. Chen, R.P. Wang, W.S. Shi, X.G. Wang, Superlattice Microstruct. 28 (2000) 157. S.Y. Ma, Thin Solid Films 402 (2002) 222. C. Ternon, F. Gourbilleau, X. Portier, P. Voivenel, C. Dufour, Thin Solid Films 5 (2002) 419. Z. Ma, L. Wang, K. Chen, W. Li, L. Zhang, Y. Bao, X. Wang, J. Xu, X. Huang, D. Feng, J. Non-Cryst. Solids 302 (2002) 648. A.M. Mahajan, L.S. Patil, J.P. Bange, D.K. Gautam, Surf. Coat. Technol. 183 (2004) 295. S.V. Nguyen, D. Doubuzinnsky, D. Dopp, R. Gleason, M. Gibson, S. Friedmann, Thin Solid Films 194 (1990) 595. J. Heitmann, R. Scholz, M. Schmidt, M. Zacharias, J. Non-Cryst. Solids 299–302 (2002) 1075. X.L. Wu, T. Gao, X.M. Bao, F. Yan, S.S. Jiang, D. Feng, J. Appl. Phys. 82 (1997) 2704. L. Khriachtchev, O. Kilpela, S. Karirinne, J. Kera¨nen, T. Lepisto¨, Appl. Phys. Lett. 78 (2001) 323. I.H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1984) 739. H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. D.R. dos Santos, I.L. Torriani, Solid State Commun. 85 (1993) 307. R. Tubino, L. Piseri, G. Zerbi, J. Chem. Phys. 56 (1972) 1022.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Annealing temperature dependence of Raman scattering in Si/SiO2 superlattice prepared by magnetron sputtering Shihua Huang *, Hong Xiao, Sha Shou Physics Department, Zhejiang Normal University, Zhejiang 321004, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 May 2008 Received in revised form 30 September 2008 Accepted 26 November 2008 Available online 3 December 2008
Si/SiO2 superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals annealed at 850 8C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity of that at 520 cm1 increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm1 shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO2 superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images. ß 2008 Elsevier B.V. All rights reserved.
PACS: 61.72 80.15.C 81.40.E Keywords: Si/SiO2 superlattice Magnetron sputtering Raman spectroscopy
1. Introduction Since Lu et al. [1] discovered intense luminescence in Si/SiO2 superlattices (SLs) in 1995, they have attracted much interest due to their potential use in Si-based optoelectronic devices and photonic applications [2–4]. Unlike porous silicon, Si/SiO2 SLs have more stable chemical and mechanical properties. Also, compared to nanocrystalline Si (nc-Si) grains embedded in a silica matrix, the Si-sublayer thickness and the size of the nc-Si grains in Si/SiO2 SLs can be controlled more accurately. At present, many technologies are being employed for the deposition of Si/SiO2 SLs, such as molecular beam epitaxy (MBE) [5,6], radio-frequency (RF)/direct current (DC) magnetron sputtering (MS) [7–9], plasma-enhanced chemical vapor deposition (PECVD) [10,11], and thermally reactive evaporation (TRE) [12,13]. The standard RF-MS/DC-MS deposition method has high resource usability, high deposition velocity and a safe deposition process compared to the gas-source MBE and ultra-high-vacuum CVD methods, and large-area deposition capability compared to the solid-source MBE method. Many techniques, such as optical absorption, Raman scattering (RS), photoluminescence and photocurrent measurements, have been used to investigate the optical and vibrational properties of Si/SiO2 SLs.
RS, which is based on inelastic light scattering, is a powerful and non-destructive technique for the analysis of stress, crystal lattice disorder and homogeneity of materials. It is also an excellent method for studying nanostructured materials due to its high sensitivity to the crystal potential fluctuations and local atomic arrangement. Variations in the disorder and particle size change the shapes and positions of Raman lines. So in addition to giving qualitative information about the chemical structure, Raman spectroscopy has been shown to be useful in determining nanoparticle size, shape, size distribution etc. [14]. In this paper, we have employed the RF magnetron sputtering method to deposit Si/SiO2 SLs, the structures and properties of which can be controlled by changing the experimental parameters, such as the deposition and annealing temperatures. Raman spectra have been used to investigate the microstructural properties of the Si/SiO2 SLs. The size and distribution of the Si nanocrystals were both estimated by a theoretical analysis based on the phonon confinement model. Theoretical calculations were compared with the experimental results of Raman spectra. A comparison was made with the scanning electron microscope (SEM) analysis in order to verify the estimations deduced from the theoretical calculations. 2. Experimental details
* Corresponding author. Tel.: +86 579 82298661. E-mail address: [email protected] (S. Huang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.069
Si/SiO2 SLs were prepared on n-type Si (1 1 1) substrates with a resistivity of about 10 V cm. Si and SiO2 sputtering targets with a
4548
S. Huang et al. / Applied Surface Science 255 (2009) 4547–4550
diameter of 6 cm were used. The base pressure in the vacuum vessel was below 3 105 Pa and the working gas was Ar. The sputtering pressure was fixed at 1.3 Pa, and the sputtering power was 25 W. In order to deposit Si/SiO2 SLs, two cathodes were used. One cathode (the SiO2 target) was powered by a 13.56 MHz RF voltage and the other (Si) was powered by a DC voltage. The entire water-cooled cathodes could be moved up and down over a distance of 25 cm parallel to the deposition area. The deposition surface was positioned at a distance of 10 cm parallel to and opposite the cathode surface. The sample holder could be heated by a filament positioned behind the substrate. The temperature was measured using a thermocouple inserted into the sampleholder assembly. Before the substrates were placed in the pretreatment chamber, they were first processed by the standard RCA cleaning technology for silicon wafers, rinsed in deionized water and finally dried with nitrogen gas. In order to eliminate the oxide and water vapor absorbed on the surface of the Si substrate, it was anti-sputtered in the pretreatment chamber and then transported to the sputtering chamber by means of a magnetically coupled transfer rod. The typical Si/SiO2 SL structure consisted of 50 pairs of alternately stacked 3.0 nm thick Si layers and 3.0 nm thick SiO2 layers. The thickness of the Si or SiO2 layer was controlled by adjusting the sputtering power and sputtering time. The substrate temperature was varied from room temperature to 700 8C. The as-grown samples were annealed for about 30 min in a pretreatment chamber in nitrogen ambient at different temperatures (500–1100 8C). The Raman spectra were measured using a micro-Raman setup (Renishaw 2000) in backscattering configuration at room temperature. The 514.5 nm line from an Ar+ laser was used as the incident light. The power density of the incident light on the sample surface was about 1 mW/mm2. For the SEM observations, samples were prepared by standard procedures. 3. Results and discussion Fig. 1 shows the SEM images of typical Si/SiO2 SLs annealed at 850 and 1100 8C for 30 min. The distribution of Si nanocrystal size is not uniform, the value of the nanocrystal diameter varying in the range 3–20 nm. It is observed from these images that by increasing the annealing temperature, the difference in size between the smaller and larger nanocrystals increases. The mean size of Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals
Fig. 2. Raman spectra for Si/SiO2 SLs deposited at different temperatures: (a) room temperature; (b) 300 8C; (c) 500 8C; (d) 700 8C.
annealed at 850 8C. This can be understood by considering the growth and coarsening theory. During the annealing process, the growth of these tiny structures occurs via a diffusion mechanism, and the formation of many small crystals is kinetically favored. But small crystals have a larger surface area to volume ratio than large crystals; in other words, large crystals represent a lower energy state than small crystals, so many small crystals are transformed into a larger crystal. Comparing Fig. 1(a) and (b), it is clear that there are larger crystallites present at higher annealing temperatures. The different behaviors of the Raman spectra for samples deposited at different temperatures were investigated. Fig. 2 shows the Raman measurement results for typical Si/SiO2 SLs deposited at different temperatures. For the sample deposited at room temperature, only one broad Raman peak located near 470 cm1 is observed; it is attributed to the vibration of amorphous Si. In the case of disordered semiconductors, such as amorphous and nanocrystalline ones, the q = 0 selection rule that determines the Raman spectra of bulk crystals is either fully or partially violated (relaxed) so that phonons other than those that are zone centered can also be observed in their Raman spectra.
Fig. 1. SEM images for the typical Si/SiO2 SLs annealed at 850 8C (a) and 1100 8C (b).
S. Huang et al. / Applied Surface Science 255 (2009) 4547–4550
However, two peaks located near 470 and 520 cm1 were observed for the samples deposited at 300, 500 and 700 8C, the peak near 520 cm1 being interpreted as arising from the vibration mode of crystalline Si. With increasing deposition temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity of the 520 cm1 peak increases. The substrate temperature influences the surface mobility, revaporization and crystallization of surface adatoms on the substrate. At low temperature, sputtered atoms deposited on the substrate lose their kinetic energy quickly and condense on the substrate surface. Due to the low mobility of adatoms, the sputtered atoms tend to agglomerate as thin amorphous films. The kinetic energy and the probability of crossing the surface barrier of adatoms increase with increasing substrate temperature, which is in favor of superficially horizontal migration of sputtered atoms on the substrate. Atoms with weak adhesive strength will escape from the substrate surface because of revaporation, which is favorable for the nucleation, growth, crystallization and preferred orientation of thin films. However, very high substrate or annealing temperatures lead to the formation of large crystal grains and polycrystalline films are easily formed. When the substrate temperature increases from room temperature to 300 8C, the partial structure of the deposited film transforms from amorphous to crystalline. In other words, the amorphous and crystalline phases coexist. With an increased substrate temperature such as 500 and 700 8C, more of the amorphous structure in the deposited film is transformed into crystalline structure. The component of the crystal phase for the sample deposited at 700 8C is larger than that for samples deposited at 300 and 500 8C. Fig. 3 shows Raman spectra for Si/SiO2 SLs annealed at different temperatures: (a) 700 8C; (b) 850 8C; (c) 1100 8C. The Raman spectra of the annealed Si/SiO2 SLs exhibit a sharp peak near 520 cm1, indicating the crystallization of Si in the SLs. The width and spectral shape of this peak are different from those of bulk Si, but similar to those of nc-Si. For the SLs annealed at 1100 8C, only a narrow and intense Raman peak near 520 cm1 is observed. However, for the SLs annealed at 700 or 850 8C, a very broad peak near 470 cm1 is observed as well as a narrow peak near 520 cm1. With increasing annealing temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity at 520 cm1 increases. Also, when the annealing temperature increases, the Raman peak near 520 cm1 shifts and becomes narrower, and asymmetry emerges. In these measurements, the zone-centered Si
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optical phonons peaked at 519.9 cm1 with a full width at half maximum (FWHM) of 4.5 cm1 in the spectrum for SLs annealed at 1100 8C (Fig. 3(a)), whereas the peaks in the spectra for the SLs annealed at 850 8C (Fig. 3(b)) and 700 8C (Fig. 3(c)) are downshifted to 519.1 and 518.2 cm1, and their FWHM values are 5.4 and 6.7 cm1, respectively. In Fig. 3, the peaks near 520 cm1 for the SLs annealed at 700, 850 and 1100 8C are asymmetrically broadened towards lower frequency with respect to the bulk Si peak. During the annealing process, with increasing annealing temperature, the formation of large crystals made up of many small crystals is thermodynamically favored. This behavior causes a red shift of the Raman frequency of and a narrowing of the peak. In fact, the experimental results show a blue shift of the Raman peak. This may be caused by the stress exerted on the Si nanocrystals, which increases with a decrease of the crystallite size. The mismatch of Si and SiO2 may result in compressive stress in the Si nanocrystals. This stress causes an upward shift of the Si crystalline peak, thereby basically compensating the downward shift caused by the confinement on the phonon frequency [15]. From the SEM images (Fig. 1), we can see that the mean size of the Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals annealed at 850 8C. The size and size distribution of the Si nanocrystals were both estimated by a theoretical analysis based on the phonon confinement model. This topic will be discussed in detail below. Variations in the size of the nanocrystals induce peak-frequency shift, peak broadening and peak asymmetry in the Raman spectra. A phonon can be described by a plane wave for a bulk crystal, but inside a nanocrystal it is instead described by a spatially confined wave packet. A finite particle size L would bring about an uncertainty in wavevector on the order of 2p/L, which will be larger for smaller crystal sizes. In a bulk crystal, this uncertainty is zero, and as only the phonons near the zone center can contribute to the first-order Raman spectrum, the Raman peak will be rather sharp. If the phonon dispersion curves are not flat near q = 0, then the spectral features of nanocrystals will shift, broaden and become asymmetric due to the phonon confinement. For nc-Si, the band-shape modification of the Raman peak has been explained using different theoretical models that quantify the phonon confinement. These models have become very popular because they allow the Si crystallite size to be determined [16–18]. The spherical shape of nanocrystals was confirmed by the SEM results (see Fig. 1). The q = 0 selection rule is a consequence of the full translational symmetry of a crystalline sample. In the presence of finite size effects, e.g., partial phonon confinement in nanoparticles, the Raman intensity can be written as a superposition of Lorentzians centered at v = v(q) (the dispersion relation of optical phonons) with q in the first Brillouin zone. This produces a symmetric broadening and red shift of the Raman peak. In particular this red shift is caused by the fact that in Si, v is a decreasing function of q. If the effective distributions of particle radii are also taken into account, the peak becomes asymmetric. Assuming the mean value of the size distribution function to be a Gaussian function, the Raman spectrum at frequency v, I(v), for nanocrystals can be written as [16]: IðvÞ /
Z
1 0
expðq2 L2 =4a2 Þ ½v vðqÞ2 þ ðG 0 =2Þ
3
2
d q
(1)
where q is expressed in units of 2p/a, a is the lattice constant, and G0 is the linewidth of the Si LO phonon in the Si bulk crystal (3.6 cm1 including instrument contributions). We consider the analytic form for the phonon dispersion of the LO phonons in nc-Si to be [19]: Fig. 3. Raman spectra for Si/SiO2 SLs annealed at different temperatures: (a) 1100 8C; (b) 850 8C; (c) 700 8C. The compression ratio of (a) is 1:10.
v2 ðqÞ ¼ A þ B cos
pq 2
(2)
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The Raman peak red shifts, the FWHM and the asymmetry degree increase with a decrease of mean nanocrystal size, which can be explained by phonon confinement effects. 4. Conclusions
Fig. 4. Raman spectra for Si/SiO2 SLs deposited at room temperature is amorphous Si-like. The crystalline Si peaks in the spectrum of annealed films (dots) are fitted using a phonon confinement model considering spherical nanocrystals (solid lines). Calculated L, and annealing temperatures are indicated near the theoretical curves.
where A = 1.714 105 cm2, B = 1.000 105 cm2 (these values may be obtained experimentally by neutron scattering). Using Eqs. (1) and (2), we can calculate the Raman spectra of nanocrystals and estimate the average diameter of Si nanocrystal. In Fig. 4, the crystalline Si peaks in the spectrum of annealed films (dots) are fitted using a phonon confinement model considering spherical nanocrystals (solid lines). The calculated values of L and the annealing temperatures are indicated near the theoretical curves. The calculated spectra agree well with the experimental results. The spectrum of an as-grown SL deposited at room temperature is similar to that in Fig. 4(d) and shows that the Si in the SLs is in an amorphous phase. The Raman spectra for all of the annealed SLs, on the other hand, are characterized by a peak near the optical phonon mode of crystalline Si, i.e. 520 cm1, indicating the crystallization of Si and the formation of nanocrystals. By fitting the Si peak around 520 cm1, we can estimate L as 5.3, 7.5 and 9.8 nm in the SLs annealed at 700, 850 and 1100 8C, respectively (the corresponding FWHM values are 6.7, 5.4 and 4.5 cm1). The size of the crystal islands can be determined from the SEM image (see Fig. 1), and the nanocrystal size for SLs annealed at 1100 8C is estimated to vary between 3 and 20 nm. It can be seen from Fig. 1(b) that these nanocrystals fall into two groups: group 1 with small nanocrystals having an average grain size of 5 nm and group 2 with large nanocrystals having an average grain size of 16 nm. This can be explained by considering the growth and coarsening theory, in terms of which some nanocrystals grow while others shrink. We can confidently conclude that the average size of 9.8 nm calculated from the Raman spectrum is a good estimate of the average size of the nanocrystals. The mean size L of Si nanocrystals increases with the annealing temperature. We can see that the peak frequency is red shifted from the corresponding bulk value by 1.8, 0.9 and 0.1 cm1 for the SLs annealed at 700, 850 and 1100 8C, respectively.
Si/SiO2 superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 8C is larger than that of nanocrystals annealed at 850 8C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm1 decreases, and the intensity of that at 520 cm1 increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm1 shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO2 superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images. Acknowledgement This work was supported by Program for Innovative Research Team in Zhejiang Normal University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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