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Low-temperature Al-induced crystallization of hydrogenated amorphous Si1−xGex (0.2 ≤ x ≤ 1) thin films
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Thin Solid Films 516 (2008) 2276 – 2279 www.elsevier.com/locate/tsf
Low-temperature Al-induced crystallization of hydrogenated amorphous Si1−xGex (0.2 ≤ x ≤ 1) thin films Shanglong Peng, Xiaoyan Shen, Zeguo Tang, Deyan He ⁎ Department of Physics, Lanzhou University, Lanzhou 730000, China Received 14 August 2006; received in revised form 20 June 2007; accepted 17 July 2007 Available online 25 July 2007
Abstract Low-temperature Al-induced crystallization of hydrogenated amorphous silicon–germanium thin films has been investigated by X-ray diffraction, Raman spectra and scanning electron microscopy measurements. It was shown that the Al-induced layer exchange significantly promotes the crystallization of the films. The influence of the annealing temperature and the Ge fraction on X-ray diffraction patterns and Raman spectra was analyzed. The increase in Raman peak intensity was observed with the increase of the annealing temperature, and the high-frequency shifts of Ge–Ge and Si–Ge peaks were found with the increase of the Ge fraction. There is an enhancement in film crystallinity and grain size with the increase of the Ge fraction and annealing temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: SiGe films; Al-induced crystallization; Raman spectra
1. Introduction Low-temperature formation of microcrystalline (mc-) or polycrystalline (poly-) Si1−xGex films on inexpensive substrates such as glass has been expected to realize advanced systems in displays and three-dimensional ultra large-scale integrated circuits. To suppress the diffusion of dopant atoms and prevent softening of the glass substrates, the formation temperature should be lower than 500 °C [1]. Some low-temperature approaches such as solid phase crystallization and laser annealing have been carried out to crystallize amorphous (a-) Si1−xGex films. However, poly-Si1−xGex films with small grain (∼1 μm) were often obtained by these techniques [2,3]. Although the melt-grown process could grow poly-Si1−xGex films with large grains (∼5 μm), Ge atoms are not distributed uniformly and ripples with ∼15 nm height were observed on the film surface [4]. Recently, a number of experiments have been performed on the crystallization of a-Si and a-SiGe in contact with certain metal species such as Al, Au or Ni [5]. The studies by G. Radnoczi et al. and I. Chambouleyron et al. showed that the ⁎ Corresponding author. Tel.: +86 931 8912546; fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.07.155
low-temperature crystallization takes place when the metal and the semiconductor form a system of alternating layer or they are alloyed together [6,7]. The Al-induced crystallization of a-Si below the eutectic temperature was reported to be a solid phase process [8]. There is an intermixing of Al and Si that occurs with the crystallization. The resulting supersaturation of Si within the Al layer is relieved through the nucleation and growth of poly-Si [9]. The general driving force in Al-induced crystallization of a-Si is the difference in the free energy between the amorphous and the crystalline phases [10]. D. Dimova-Malinovska et al. firstly demonstrated the possibility to prepare poly-SiGe films by Al-induced crystallization [11]. R. Lechner et al. have investigated the influence of Ge content on the Al-induced crystallization of a-Si1−xGex films, and found that the optical, electrical and structural properties of the prepared poly-SiGe films seem to be better than those obtained by other methods [12]. In this work, hydrogenated amorphous silicon–germanium (a-Si1−xGex) films were deposited on Al-coated Corning 7059 glass substrates by traditional plasma enhanced chemical vapor deposition (PECVD). X-ray diffraction (XRD), Raman spectra and scanning electron microscope (SEM) measurements were used to evaluate the structure and morphology of the Si1−xGex
S. Peng et al. / Thin Solid Films 516 (2008) 2276–2279
Fig. 1. XRD patterns of hydrogenated a-Si1−xGex (x = 0.2) film as-deposited and annealed at several temperatures for 3 h.
films annealed at various temperatures. It was found that the Alinduced layer exchange leads to the formation of mc- or polySi1−xGex films over the composition range of 0.2 ≤ x ≤ 1 at low temperatures. 2. Experimental details Al films (200–300 nm thick) were firstly deposited by vacuum thermal evaporation on Corning 7059 glass substrates. Hydrogenated a-Si1−xGex films (1000–1200 nm thick) were then grown on the Al-coated glass substrates using a radio frequency (13.56 MHz) capacitively-coupled PECVD system. The reaction gases were SiH4 (Ar dilution), Ar and GeH4 (H2 dilution) with a total flow in the range of 40–50 sccm. The substrate temperature was fixed at 250 °C. The base pressure and the deposition pressure were 3.0 × 10− 4 Pa and 150 Pa, respectively. The applied radio frequency power was 30 W. The Ge fraction in the films was varied by changing the flow ratio of SiH4 to GeH4. Finally, the samples were annealed in an atmosphere of nitrogen. The annealing temperatures were 300, 350, 400 and 450 °C, respectively, and the annealing time was kept constant for 3 h. The structure and the morphology of the annealed Si1−xGex films were characterized with XRD, Raman spectra and SEM. All measurements were carried out at room temperature. XRD measurements were performed on a Rigaku RINT2400 X-ray diffractometer in reflection geometry and the diffraction patterns were acquired in θ–2θ coupled mode using Cu Kα radiation (λ = 0.154056 nm). The morphology of the samples was obtained using an S-4800 scanning electron microscope with the operating voltage of 5000 V. Micro-Raman spectra were measured with a Jobin-Yvon HK 800 with excitation wavelength of 535 nm. Great care was taken to limit the laser power in a low level in order to prevent a-Si1−xGex films from crystallization during the measurements.
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Fig. 1. Similar results were obtained for the films with other Ge fraction. The broad band centered at 2θ = 25° is attributed to the glass substrate. The absence of any characteristic peak for the as-deposited sample suggests that the structure of the sample is mostly amorphous. The peaks at 28°, 47° and 55° can be seen when the sample was annealed at a temperature of 350 °C, which correspond to the diffraction from (111), (220), and (311) planes of the crystallized Si1−xGex films, respectively. Further increase in the peak intensity and reduction in the full width at half maximum can be found with the increase of the annealing temperature, indicating an enhancement in the film crystallinity [13]. The fact that no dual peak could be detected confirms that the films consist of SiGe rather than Si and Ge clusters [14]. After the film was annealed at 450 °C for 3 h, a weak Al (111) peak is observed at 38.5°. Also, it can be seen that the film shows the preferential growth along the (111) orientation, which may be due to the growth along the (111) orientation having the lowest free energy [15]. Fig. 2 shows XRD patterns obtained for hydrogenated aSi1−xGex films with x = 0.5 (a) and x = 0.2 (b) annealed at 400 °C for 3 h. A strong and sharp (111) diffraction peak was detected in Fig. 2(a). Also, we found that the three XRD peaks of (111), (220), and (311) in Fig. 2(a) are much stronger than those in Fig. 2(b), confirming that the Ge-rich sample is easier to be crystallized at the same annealing temperature. The small shift of the peak position was observed, which results from the increase of the lattice constant with the increase of the Ge fraction. Fig. 3 shows the cross-sectional SEM images obtained for a typical crystallized Si1−xGex (x = 0.2) film, and they were respectively taken after the film was annealed for 3 h at 300 °C (a) and 400 °C (b). The two layers on the glass substrate, corresponding to Al coating and Si1−xGex film, can be clearly seen for the sample annealed at 300 °C, indicating that there is no layer exchange between the Al and SiGe layers under such a low annealing temperature. The columnar structure of the SiGe film is obvious. Increasing the annealing temperature to 400 °C,
3. Results and discussions Typical XRD patterns obtained for the as-deposited and annealed hydrogenated a-Si1−xGex (x = 0.2) films are shown in
Fig. 2. XRD patterns of hydrogenated a-Si1−xGex films with x = 0.5 (a) and x = 0.2 (b) annealed at 400 °C for 3 h.
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not only the Al layer disappeared, but also the structure of the SiGe film changed dramatically. We believe that Al atoms diffuse into the SiGe film and induce the film to crystallize by forming a mixture of Al and SiGe. As shown in Fig. 3(b), the relative coherent layer just on the substrate is poly-SiGe. The
rough layer with scattered bright spots under the film surface is a mixture of Al and SiGe. It has been shown that, for a-Si films, the Al-induced layer exchange starts with the formation of Si nuclei within the Al layer at the Al/a-Si interface. At the beginning, these Si nuclei grow in all directions within the Al layer until they are confined between the original glass/Al and Al/a-Si interfaces. In the following phase the Si grains go on growing laterally only until they touch adjacent grains and form a continuous poly-Si film on the glass substrate. A layer of both segregated Al and Si, which was not incorporated into the polySi film is formed on top of the film [16]. It was reported that crystallization of a-SiGe needs much longer annealing time at low annealing temperature below 420 °C (the eutectic temperature of binary of Al–Ge). We cannot observe the segregated Al on top of the films, because the annealing time is not long enough to complete this process at low annealing temperature (∼ 400 °C). In order to evaluate crystal quality of the resulted SiGe films, micro-Raman spectra were measured at room temperature. The typical spectra of the as-deposited hydrogenated a-Si1−xGex films with various Ge fractions are shown in Fig. 4. The three Raman broad peaks located at 300, 400 and 500 cm− 1 can be clearly seen, which respectively correspond to the Ge–Ge, Si– Ge and Si–Si stretching modes. The Raman peaks of the Ge–Ge and Si–Ge modes shift to higher frequency (blue shift) with the increase of the Ge fraction, however, the peak of the Si–Si mode shifts to low frequency (red shift) in the Ge composition range under study. Fig. 5 shows the Raman spectra of hydrogenated a-Si1−xGex films annealed at 350 °C for 3 h. The shifts for the Ge–Ge, Si– Ge and Si–Si phonon modes are also observed with the increase of the Ge fraction. It was reported that the increase of the crystalline phase leads to high-frequency shifts of Ge–Ge and Si–Ge peaks. So the results shown in Figs. 4 and 5 demonstrate that the crystallinity of the films increases with increase of the Ge fraction. Furthermore, it is known that the intensities of the
Fig. 4. Raman spectra of the as-deposited hydrogenated a-Si1−xGex films with x = 1, 0.5, 0.4, 0.33 and 0.2.
Fig. 5. Raman spectra of hydrogenated a-Si1−xGex films with x = 0.5, 0.4, 0.33 and 0.2 annealed at 350 °C for 3 h.
Fig. 3. Cross-section SEM images of hydrogenated a-Si1−xGex (x = 0.2) film annealed at 300 °C (a) and 400 °C (b) for 3 h.
S. Peng et al. / Thin Solid Films 516 (2008) 2276–2279
peaks for crystallized Si1−xGex alloys depend on the composition x because the number of Si–Si, Si–Ge and Ge–Ge bonds scales like (1 − x)2, 2x(1 − x) and x2 and therefore the relative intensities [17]. The dependence of the Raman peak frequencies for crystalline Si1−xGex on both composition and stress has been the subject of many reports [18,19]. The Raman shift of the Si–Si, Si–Ge and Ge–Ge phonon modes for the unstrained Si1−xGex layer varies linearly with the Ge fraction according to the following relationships [19–21]: xSiSi ðxÞ ¼ 520:2 29:7x
ð1Þ
xSiGe ðxÞ ¼ 400 þ 29x
ð2Þ
xGeGe ðxÞ ¼ 282:5 þ 12x
ð3Þ
The numerical fitting of the phonon modes shown in Fig. 5 illustrates the peak frequencies as a function of the composition. The fitting results are shown in Fig. 6. It can be seen that the experimental data approximately fit the relationships described as Eqs. (1)–(3) for the crystallized Si1−xGex films. The little deviations may be due to the presence of tensile strain and the interaction of lattice phonons caused by residual Al-doping (Fano interaction) [22].
Fig. 6. Experimental frequency shifts (squares and dashed lines) of the Si–Si (a), Si–Ge (b) and Ge–Ge (c) modes as a function of Ge fraction x for the hydrogenated a-Si1−xGex films annealed at 350 °C for 3 h. The corresponding calculated frequency shifts (lines) for fully unstrained SiGe films (Eqs. (1)–(3)) are presented in comparison with the experimental data.
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4. Conclusion We have applied the Al-induced crystallization technique to hydrogenated a-Si1−xGex films in order to extend the technique for preparing mc- or poly-Si1−xGex films on glass. It is shown that the crystallization of hydrogenated a-Si1−xGex films begins at the Al/a-Si1−xGex interface and the Al-induced layer exchange significantly promotes the crystallization of the films. An increase in peak intensity and a reduction in the full width at half maximum are observed with the increase of the annealing temperature. The Ge–Ge and Si–Ge peaks shift to a higher frequency with the increase of the Ge fraction. A linear relationship was obtained for the dependence of the peak frequency of the Ge–Ge and Si–Ge phonon modes on the Ge fraction. With the increase of the Ge fraction and annealing temperature, there is an enhancement in film crystallinity and grain size. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No.10175030), and the Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China. References [1] T. Aoki, H. Kanno, A. Kenio, T. Sadoh, M. Miyao, Thin Solid Films 508 (2006) 44. [2] J. Olivares, A. Rodriguez, J. Sangrador, T. Rodriguez, C. Ballesteros, A. Kling, Thin Solid Films 337 (1999) 51. [3] C. Eisele, M. Berger, M. Nerding, H.P. Strunk, C.E. Nebel, M. Stutzmann, Thin Solid Films 427 (2003) 176. [4] M. Miyao, T. Sadoh, S. Yamaguchi, S.K. Park, Tech. Rep. IEICE 101 (2001) 1. [5] M. Gjukic, M. Buschbeck, R. Lechner, M. Stutzmann, Appl. Phys. Lett. 85 (2004) 2134. [6] G. Radnoczi, A. Robertsson, H.T.G. Hentzell, S.F. Gong, M.A. Hasan, J. Appl. Phys. 69 (1991) 6394. [7] I. Chambouleyron, F. Fajardo, A.R. Zanatta, Appl. Phys. Lett. 79 (2001) 3233. [8] T.J. Konno, R. Sinclair, Philos. Mag., B 66 (1992) 749. [9] M.S. Ashtikar, G.L. Sharma, J. Appl. Phys. 78 (1995) 913. [10] L. Hultman, A. Robertsson, H.T.G. Hentzell, I. Engstrom, P.A. Psaras, J. Appl. Phys. 62 (1987) 3647. [11] D. Dimova-Malinovska, O. Angelov, M. Sendova-Vassileva, M. Kamenova, J.C. Pivin, Thin Solid Films 451 (2004) 303. [12] R. Lechner, M. Buschbeck, M. Gjukic, M. Stutzmann, Phys. Status Solidi, C 1 (2004) 1131. [13] E.V. Jelenkovic, K.Y. Tong, Z. Sun, C.L. Mak, W.Y. Cheung, J. Vac. Sci. Technol., A, Vac. Surf. Films 15 (1997) 2836. [14] W.K. Choi, L.K. The, L.K. Bera, W.K. Chim, A.T.S. Wee, Y.X. Jie, J. Appl. Phys. 91 (2002) 444. [15] R.J. Jaccodin, J. Electrochem. Soc. 110 (1963) 524. [16] S. Gall, M. Muske, I. Sieber, O. Nast, W. Fuhs, J. Non-Cryst. Solids 299 (2002) 741. [17] M.A. Renucci, J.B. Renucci, M. Cardona, in: M. Balkanski (Ed.), Proceedings of the Second International Conference on Light Scattering in Solids, Paris, France, July 19–23, Flammarion, Paris, 1971, p. 326. [18] M.I. Alonso, K. Winer, Phys. Rev., B 39 (1989) 10056. [19] W.J. Brya, Solid State Commun. 12 (1973) 253. [20] J.C. Tsang, P.M. Mooney, F. Dacol, J.O. Chu, J. Appl. Phys. 75 (1994) 8098. [21] A. Perez-Rodrigue, A. Cornet, J.R. Morante, Microelectron. Eng. 40 (1998) 223. [22] N. Nakano, L. Marville, R. Reif, J. Appl. Phys. 72 (1992) 3641.
Thin Solid Films 516 (2008) 2276 – 2279 www.elsevier.com/locate/tsf
Low-temperature Al-induced crystallization of hydrogenated amorphous Si1−xGex (0.2 ≤ x ≤ 1) thin films Shanglong Peng, Xiaoyan Shen, Zeguo Tang, Deyan He ⁎ Department of Physics, Lanzhou University, Lanzhou 730000, China Received 14 August 2006; received in revised form 20 June 2007; accepted 17 July 2007 Available online 25 July 2007
Abstract Low-temperature Al-induced crystallization of hydrogenated amorphous silicon–germanium thin films has been investigated by X-ray diffraction, Raman spectra and scanning electron microscopy measurements. It was shown that the Al-induced layer exchange significantly promotes the crystallization of the films. The influence of the annealing temperature and the Ge fraction on X-ray diffraction patterns and Raman spectra was analyzed. The increase in Raman peak intensity was observed with the increase of the annealing temperature, and the high-frequency shifts of Ge–Ge and Si–Ge peaks were found with the increase of the Ge fraction. There is an enhancement in film crystallinity and grain size with the increase of the Ge fraction and annealing temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: SiGe films; Al-induced crystallization; Raman spectra
1. Introduction Low-temperature formation of microcrystalline (mc-) or polycrystalline (poly-) Si1−xGex films on inexpensive substrates such as glass has been expected to realize advanced systems in displays and three-dimensional ultra large-scale integrated circuits. To suppress the diffusion of dopant atoms and prevent softening of the glass substrates, the formation temperature should be lower than 500 °C [1]. Some low-temperature approaches such as solid phase crystallization and laser annealing have been carried out to crystallize amorphous (a-) Si1−xGex films. However, poly-Si1−xGex films with small grain (∼1 μm) were often obtained by these techniques [2,3]. Although the melt-grown process could grow poly-Si1−xGex films with large grains (∼5 μm), Ge atoms are not distributed uniformly and ripples with ∼15 nm height were observed on the film surface [4]. Recently, a number of experiments have been performed on the crystallization of a-Si and a-SiGe in contact with certain metal species such as Al, Au or Ni [5]. The studies by G. Radnoczi et al. and I. Chambouleyron et al. showed that the ⁎ Corresponding author. Tel.: +86 931 8912546; fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.07.155
low-temperature crystallization takes place when the metal and the semiconductor form a system of alternating layer or they are alloyed together [6,7]. The Al-induced crystallization of a-Si below the eutectic temperature was reported to be a solid phase process [8]. There is an intermixing of Al and Si that occurs with the crystallization. The resulting supersaturation of Si within the Al layer is relieved through the nucleation and growth of poly-Si [9]. The general driving force in Al-induced crystallization of a-Si is the difference in the free energy between the amorphous and the crystalline phases [10]. D. Dimova-Malinovska et al. firstly demonstrated the possibility to prepare poly-SiGe films by Al-induced crystallization [11]. R. Lechner et al. have investigated the influence of Ge content on the Al-induced crystallization of a-Si1−xGex films, and found that the optical, electrical and structural properties of the prepared poly-SiGe films seem to be better than those obtained by other methods [12]. In this work, hydrogenated amorphous silicon–germanium (a-Si1−xGex) films were deposited on Al-coated Corning 7059 glass substrates by traditional plasma enhanced chemical vapor deposition (PECVD). X-ray diffraction (XRD), Raman spectra and scanning electron microscope (SEM) measurements were used to evaluate the structure and morphology of the Si1−xGex
S. Peng et al. / Thin Solid Films 516 (2008) 2276–2279
Fig. 1. XRD patterns of hydrogenated a-Si1−xGex (x = 0.2) film as-deposited and annealed at several temperatures for 3 h.
films annealed at various temperatures. It was found that the Alinduced layer exchange leads to the formation of mc- or polySi1−xGex films over the composition range of 0.2 ≤ x ≤ 1 at low temperatures. 2. Experimental details Al films (200–300 nm thick) were firstly deposited by vacuum thermal evaporation on Corning 7059 glass substrates. Hydrogenated a-Si1−xGex films (1000–1200 nm thick) were then grown on the Al-coated glass substrates using a radio frequency (13.56 MHz) capacitively-coupled PECVD system. The reaction gases were SiH4 (Ar dilution), Ar and GeH4 (H2 dilution) with a total flow in the range of 40–50 sccm. The substrate temperature was fixed at 250 °C. The base pressure and the deposition pressure were 3.0 × 10− 4 Pa and 150 Pa, respectively. The applied radio frequency power was 30 W. The Ge fraction in the films was varied by changing the flow ratio of SiH4 to GeH4. Finally, the samples were annealed in an atmosphere of nitrogen. The annealing temperatures were 300, 350, 400 and 450 °C, respectively, and the annealing time was kept constant for 3 h. The structure and the morphology of the annealed Si1−xGex films were characterized with XRD, Raman spectra and SEM. All measurements were carried out at room temperature. XRD measurements were performed on a Rigaku RINT2400 X-ray diffractometer in reflection geometry and the diffraction patterns were acquired in θ–2θ coupled mode using Cu Kα radiation (λ = 0.154056 nm). The morphology of the samples was obtained using an S-4800 scanning electron microscope with the operating voltage of 5000 V. Micro-Raman spectra were measured with a Jobin-Yvon HK 800 with excitation wavelength of 535 nm. Great care was taken to limit the laser power in a low level in order to prevent a-Si1−xGex films from crystallization during the measurements.
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Fig. 1. Similar results were obtained for the films with other Ge fraction. The broad band centered at 2θ = 25° is attributed to the glass substrate. The absence of any characteristic peak for the as-deposited sample suggests that the structure of the sample is mostly amorphous. The peaks at 28°, 47° and 55° can be seen when the sample was annealed at a temperature of 350 °C, which correspond to the diffraction from (111), (220), and (311) planes of the crystallized Si1−xGex films, respectively. Further increase in the peak intensity and reduction in the full width at half maximum can be found with the increase of the annealing temperature, indicating an enhancement in the film crystallinity [13]. The fact that no dual peak could be detected confirms that the films consist of SiGe rather than Si and Ge clusters [14]. After the film was annealed at 450 °C for 3 h, a weak Al (111) peak is observed at 38.5°. Also, it can be seen that the film shows the preferential growth along the (111) orientation, which may be due to the growth along the (111) orientation having the lowest free energy [15]. Fig. 2 shows XRD patterns obtained for hydrogenated aSi1−xGex films with x = 0.5 (a) and x = 0.2 (b) annealed at 400 °C for 3 h. A strong and sharp (111) diffraction peak was detected in Fig. 2(a). Also, we found that the three XRD peaks of (111), (220), and (311) in Fig. 2(a) are much stronger than those in Fig. 2(b), confirming that the Ge-rich sample is easier to be crystallized at the same annealing temperature. The small shift of the peak position was observed, which results from the increase of the lattice constant with the increase of the Ge fraction. Fig. 3 shows the cross-sectional SEM images obtained for a typical crystallized Si1−xGex (x = 0.2) film, and they were respectively taken after the film was annealed for 3 h at 300 °C (a) and 400 °C (b). The two layers on the glass substrate, corresponding to Al coating and Si1−xGex film, can be clearly seen for the sample annealed at 300 °C, indicating that there is no layer exchange between the Al and SiGe layers under such a low annealing temperature. The columnar structure of the SiGe film is obvious. Increasing the annealing temperature to 400 °C,
3. Results and discussions Typical XRD patterns obtained for the as-deposited and annealed hydrogenated a-Si1−xGex (x = 0.2) films are shown in
Fig. 2. XRD patterns of hydrogenated a-Si1−xGex films with x = 0.5 (a) and x = 0.2 (b) annealed at 400 °C for 3 h.
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not only the Al layer disappeared, but also the structure of the SiGe film changed dramatically. We believe that Al atoms diffuse into the SiGe film and induce the film to crystallize by forming a mixture of Al and SiGe. As shown in Fig. 3(b), the relative coherent layer just on the substrate is poly-SiGe. The
rough layer with scattered bright spots under the film surface is a mixture of Al and SiGe. It has been shown that, for a-Si films, the Al-induced layer exchange starts with the formation of Si nuclei within the Al layer at the Al/a-Si interface. At the beginning, these Si nuclei grow in all directions within the Al layer until they are confined between the original glass/Al and Al/a-Si interfaces. In the following phase the Si grains go on growing laterally only until they touch adjacent grains and form a continuous poly-Si film on the glass substrate. A layer of both segregated Al and Si, which was not incorporated into the polySi film is formed on top of the film [16]. It was reported that crystallization of a-SiGe needs much longer annealing time at low annealing temperature below 420 °C (the eutectic temperature of binary of Al–Ge). We cannot observe the segregated Al on top of the films, because the annealing time is not long enough to complete this process at low annealing temperature (∼ 400 °C). In order to evaluate crystal quality of the resulted SiGe films, micro-Raman spectra were measured at room temperature. The typical spectra of the as-deposited hydrogenated a-Si1−xGex films with various Ge fractions are shown in Fig. 4. The three Raman broad peaks located at 300, 400 and 500 cm− 1 can be clearly seen, which respectively correspond to the Ge–Ge, Si– Ge and Si–Si stretching modes. The Raman peaks of the Ge–Ge and Si–Ge modes shift to higher frequency (blue shift) with the increase of the Ge fraction, however, the peak of the Si–Si mode shifts to low frequency (red shift) in the Ge composition range under study. Fig. 5 shows the Raman spectra of hydrogenated a-Si1−xGex films annealed at 350 °C for 3 h. The shifts for the Ge–Ge, Si– Ge and Si–Si phonon modes are also observed with the increase of the Ge fraction. It was reported that the increase of the crystalline phase leads to high-frequency shifts of Ge–Ge and Si–Ge peaks. So the results shown in Figs. 4 and 5 demonstrate that the crystallinity of the films increases with increase of the Ge fraction. Furthermore, it is known that the intensities of the
Fig. 4. Raman spectra of the as-deposited hydrogenated a-Si1−xGex films with x = 1, 0.5, 0.4, 0.33 and 0.2.
Fig. 5. Raman spectra of hydrogenated a-Si1−xGex films with x = 0.5, 0.4, 0.33 and 0.2 annealed at 350 °C for 3 h.
Fig. 3. Cross-section SEM images of hydrogenated a-Si1−xGex (x = 0.2) film annealed at 300 °C (a) and 400 °C (b) for 3 h.
S. Peng et al. / Thin Solid Films 516 (2008) 2276–2279
peaks for crystallized Si1−xGex alloys depend on the composition x because the number of Si–Si, Si–Ge and Ge–Ge bonds scales like (1 − x)2, 2x(1 − x) and x2 and therefore the relative intensities [17]. The dependence of the Raman peak frequencies for crystalline Si1−xGex on both composition and stress has been the subject of many reports [18,19]. The Raman shift of the Si–Si, Si–Ge and Ge–Ge phonon modes for the unstrained Si1−xGex layer varies linearly with the Ge fraction according to the following relationships [19–21]: xSiSi ðxÞ ¼ 520:2 29:7x
ð1Þ
xSiGe ðxÞ ¼ 400 þ 29x
ð2Þ
xGeGe ðxÞ ¼ 282:5 þ 12x
ð3Þ
The numerical fitting of the phonon modes shown in Fig. 5 illustrates the peak frequencies as a function of the composition. The fitting results are shown in Fig. 6. It can be seen that the experimental data approximately fit the relationships described as Eqs. (1)–(3) for the crystallized Si1−xGex films. The little deviations may be due to the presence of tensile strain and the interaction of lattice phonons caused by residual Al-doping (Fano interaction) [22].
Fig. 6. Experimental frequency shifts (squares and dashed lines) of the Si–Si (a), Si–Ge (b) and Ge–Ge (c) modes as a function of Ge fraction x for the hydrogenated a-Si1−xGex films annealed at 350 °C for 3 h. The corresponding calculated frequency shifts (lines) for fully unstrained SiGe films (Eqs. (1)–(3)) are presented in comparison with the experimental data.
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4. Conclusion We have applied the Al-induced crystallization technique to hydrogenated a-Si1−xGex films in order to extend the technique for preparing mc- or poly-Si1−xGex films on glass. It is shown that the crystallization of hydrogenated a-Si1−xGex films begins at the Al/a-Si1−xGex interface and the Al-induced layer exchange significantly promotes the crystallization of the films. An increase in peak intensity and a reduction in the full width at half maximum are observed with the increase of the annealing temperature. The Ge–Ge and Si–Ge peaks shift to a higher frequency with the increase of the Ge fraction. A linear relationship was obtained for the dependence of the peak frequency of the Ge–Ge and Si–Ge phonon modes on the Ge fraction. With the increase of the Ge fraction and annealing temperature, there is an enhancement in film crystallinity and grain size. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No.10175030), and the Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China. References [1] T. Aoki, H. Kanno, A. Kenio, T. Sadoh, M. Miyao, Thin Solid Films 508 (2006) 44. [2] J. Olivares, A. Rodriguez, J. Sangrador, T. Rodriguez, C. Ballesteros, A. Kling, Thin Solid Films 337 (1999) 51. [3] C. Eisele, M. Berger, M. Nerding, H.P. Strunk, C.E. Nebel, M. Stutzmann, Thin Solid Films 427 (2003) 176. [4] M. Miyao, T. Sadoh, S. Yamaguchi, S.K. Park, Tech. Rep. IEICE 101 (2001) 1. [5] M. Gjukic, M. Buschbeck, R. Lechner, M. Stutzmann, Appl. Phys. Lett. 85 (2004) 2134. [6] G. Radnoczi, A. Robertsson, H.T.G. Hentzell, S.F. Gong, M.A. Hasan, J. Appl. Phys. 69 (1991) 6394. [7] I. Chambouleyron, F. Fajardo, A.R. Zanatta, Appl. Phys. Lett. 79 (2001) 3233. [8] T.J. Konno, R. Sinclair, Philos. Mag., B 66 (1992) 749. [9] M.S. Ashtikar, G.L. Sharma, J. Appl. Phys. 78 (1995) 913. [10] L. Hultman, A. Robertsson, H.T.G. Hentzell, I. Engstrom, P.A. Psaras, J. Appl. Phys. 62 (1987) 3647. [11] D. Dimova-Malinovska, O. Angelov, M. Sendova-Vassileva, M. Kamenova, J.C. Pivin, Thin Solid Films 451 (2004) 303. [12] R. Lechner, M. Buschbeck, M. Gjukic, M. Stutzmann, Phys. Status Solidi, C 1 (2004) 1131. [13] E.V. Jelenkovic, K.Y. Tong, Z. Sun, C.L. Mak, W.Y. Cheung, J. Vac. Sci. Technol., A, Vac. Surf. Films 15 (1997) 2836. [14] W.K. Choi, L.K. The, L.K. Bera, W.K. Chim, A.T.S. Wee, Y.X. Jie, J. Appl. Phys. 91 (2002) 444. [15] R.J. Jaccodin, J. Electrochem. Soc. 110 (1963) 524. [16] S. Gall, M. Muske, I. Sieber, O. Nast, W. Fuhs, J. Non-Cryst. Solids 299 (2002) 741. [17] M.A. Renucci, J.B. Renucci, M. Cardona, in: M. Balkanski (Ed.), Proceedings of the Second International Conference on Light Scattering in Solids, Paris, France, July 19–23, Flammarion, Paris, 1971, p. 326. [18] M.I. Alonso, K. Winer, Phys. Rev., B 39 (1989) 10056. [19] W.J. Brya, Solid State Commun. 12 (1973) 253. [20] J.C. Tsang, P.M. Mooney, F. Dacol, J.O. Chu, J. Appl. Phys. 75 (1994) 8098. [21] A. Perez-Rodrigue, A. Cornet, J.R. Morante, Microelectron. Eng. 40 (1998) 223. [22] N. Nakano, L. Marville, R. Reif, J. Appl. Phys. 72 (1992) 3641.