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SiO2 multi-layer structure
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 179–181 www.elsevier.com/locate/jlumin
Preparation and photoluminescence of SiC/Si/SiO2 multi-layer structure L. Wang, Z.J. Liang, Z.B. Wang, F.L. Zhao, Z.H. He, Dihu Chen State Key Laboratory of Optoelectronic Materials and Technologies, Physics Department of School of Physics and Engineering, Zhongshan University, Guangzhou 510275, PR China Available online 6 March 2006
Abstract A multi-layer structure of SiC/Si/SiO2 was fabricated by successive deposition of SiC and Si using plasma-enhanced chemical vapor deposition and post-deposition thermal oxidation of the part as-deposited Si surface layer. The samples oxidized at 900 1C for various annealing times exhibit two strong photoluminescence peaks in the blue-light range, one is a stable PL peak at 433 nm and the other is an unstable PL peak at a range of 445–460 nm. A maximum PL intensity was obtained when oxidation time is 40 min over this oxidation time, the PL intensity decrease up to disappear with the increase of the oxidation time. Origin of the PL was discussed and a tight-binding theoretic calculation was used to study the relation of the excess Si defect and PL properties. Results show that the two PL peaks observed may be attributed to excess Si defect centers and SiC nanocrystalline with different size in SiC/Si/SiO2 multi-layer structure. r 2006 Elsevier B.V. All rights reserved. Keywords: SiC/Si/SiO2; Photoluminescence; PECVD; Thermal oxidation; Density of state (DOS)
Since the discovery of visible luminescence from porous Si at room temperature (RT) [1], fabrication of nanostructure Si-based materials and their photoluminescence (PL) properties induced by the quantum confine effect at RT, have attracted much interest [2–5]. Recently, lowtemperature or RT PL of multi-layer films has been widely reported [6–8]. But these multi-layers were mostly concerned about Si, GaN, SiO2. Little work on SiC has been reported. This work is devoted to fabrication of a SiC/Si/ SiO2 multi-layer structure, experiment and theoretic study at its PL properties and origin. Plasma-enhanced chemical vapor deposition (PECVD) was used to deposit SiC and Si layer with a mixed gases of SiH4+CH4+H2 and SiH4+H2. Thickness of SiC and Si layer is controlled by adjusting the deposition time at a stable deposition rate of 4 nm min1. In this work, thickness of SiC and Si layers is 3 and 10 nm. Part of deposited Si layer was oxidized to form SiO2 top layer by dry thermal oxidation at 900 1C, and crystallization of SiC and Si layer were performed going with oxidation. Finally a SiC/Si/SiO2 multi-layer was formed. The Si nanocrystalline (nc-Si) layer with different thicknesses in SiC/ Corresponding author. Tel.: +86 20 84113398; fax: +86 20 84035496.
E-mail address: [email protected] (D. Chen). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.078
Si/SiO2 multi-layer was obtained by adjusting different oxidation times. Table 1 gives a rough estimate of the expected nc-Si layer thickness as a function of the oxidation time according to standard dry oxidation process in microelectronic techniques [9]. The substrate used is ntype (100) silicon wafers with a resistivity of 5–10 O cm. An initial pad oxide thickness of 3 nm was grown on the silicon substrate by dry oxidation at 900 1C for 10 min. A second harmonic wave of locked Ti:sapphire laser (400 nm, 0.32 MW) was used for PL measurements. Fig. 1 shows the fourier transform infrared spectroscopy (FTIR) spectra of samples with different oxidation times, the peak at 800 cm1 represents stretching vibration of Si–C bonds in compounds with Si, C, and H, while peak at 1060 cm1 indicates Si–O–Si stretching [10]. Two luminescence peaks appeared in all samples in Fig. 2, peak position at 433 nm remains unchanged for the samples with various thickness of nanocrystalline silicon and silicon dioxide. It indicates that this blue PL peak does not originate from band-to-band recombination in the quantum-confined Si or SiC crystallites. It may be originate from the Si excess defect centers [11], which will be discussed later in theory. PL peak position at around 455 nm red-shift with the oxidation time, which is attributed to origin from SiC nano-particles [3]. Since crystallization of SiC layer
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Table 1 Rough estimate of the thickness of Si nanocrystalline (dSi) and SiO2 layers (dSiO2) prepared at 900 1C for different oxidation times (toxi) toxi (min) dSi (nm) dSiO2 (nm)
0 10.0 0
30 4.0 12.0
40 2.5 16.0
50 1.0 18.5
60 0.5 20.5
Fig. 1. FTIR spectra of the samples oxidized at 900 1C for various times indicated.
Fig. 2. PL spectra of SiC/Si/SiO2 multi-layer structure oxidized at 900 1C for various times indicated.
happens during oxidation of Si layer, more and larger crystalline SiC nano-particles are formed with increases in oxidation time according to Fig. 1 [12]. The PL intensity reaches maximum when oxidation time is 40 min. When annealing time is sequentially prolonged, the SiC nanoparticles grow larger, which shows a bulk SiC behavior. Therefore, the PL intensity decreased and also the peak red-shift, but the FTIR peak intensity still increased. There is a third peak at 445 nm in PL spectra of samples oxidized for 50 and 60 min and PL peak position remains unchanged. Table 1 shows that the thickness of silicon layer for the samples oxidized for 50 and 60 min are below 2 nm, the PL peak may originate from nano-Si particles [10]. However, for a prolonged oxidation, the silicon layer was almost oxidized, the PL spectra got very weak. This behavior indicates that the crystalline silicon layer plays a key role in the PL process. A theoretical calculation and discussion of PL spectra was performed by building a model of nano-Si structure with some defects according to the method in Ref. [11], in which the radius r of nano-Si sphere and the defects density of D were used as the parameters. A typical density of state (DOS) plot of Si sphere (r ¼ 1.2 nm) without and with defects is shown in Fig. 3(a) which was calculated using the tight binding scheme proposed by Khan [13]. The dotted line is the curve for Si sphere without vacancies and the solid line is for the Si sphere with vacancy defects density D ¼ 0.3. It shows that some defect states appear in the band gap due to existence of vacancy defects, and the DOS sharp peak labeled by letter A at the edge of the value band gets higher indicating many defect states were pinned at this sharp peak nearby. We call them pinned states. Since the system relax to equilibrium state before recombination, electrons that have excited to the conduction band, relax to the bottom of conduction band according to the Fermi distribution. A momentum matrix element ccn Pcvm based on tight binding method can be calculated [14] by changing the cm at a fixed ccn, where ccn and cvm represent the system eigenstate
n and m in the conduction and value band. The ccn Pcvm is meaningful for PL emission only when the ccn is a nearby state of the conduction band bottom. Fig. 3(b) shows that transition probability from the lowest extent state ccn in the conduction band bottom to other state cvm, as a function of the energy of cvm. It also shows that extent state electron tends to transmit to the empty pinned state located in the edge of the value band. Fig. 3(c) shows that the transition probability from arbitrary fixed local state in the conduction band ccn to other state cvm, as a function of the energy of cvm. It also shows that local state electron in the conduction band tends to transmit to the defect state located in the middle of the band gap. Since the energy level of the pinned defect states are more stable than that of the defect states in the middle of band gap, the extent state electrons located at conduction band bottom have more chance to transmit to the pinned defect state in the value band. As shown in Fig. 3(b), the
ARTICLE IN PRESS L. Wang et al. / Journal of Luminescence 122–123 (2007) 179–181
181
nano-Si structure described above is shown in inset of Fig. 3(a). Since the pinned states are stable at near the sharp peak of the value band, and not sensitive to the size of the Si nanocrystalline, the PL peak position at 433 nm is unchanged with variation of the thickness of nanocrystalline silicon layer. In summary, we have prepared multi-layer structure of SiC/Si/SiO2 by PECVD, samples exhibit two strong PL peaks in the blue-light range. The PL intensity varies with Si layer thickness oxidized for various time, a maximum PL intensity of the two peaks was obtained at oxidation time of 40 min. Over this oxidation time, the PL intensity decreases and disappears with increase of the oxidation time. Theoretical study using a tight binding calculation shows that the two PL peaks are attributed to excess Si defect centers and SiC nanocrystalline in SiC/Si/SiO2 multi-layer structure. This work is supported by the National Science Foundation of China (no. NSFC30370410).
References
Fig. 3. (a) The DOS of Si nanocrystalline sphere with a radius of 1.2 nm and various vacancy defect density D as indicated. The dashed circle indicates the defect states deeply in band gap, the pinned states (labeled B) are at the top of the value band. The inset is calculated PL spectra. (b) Squared momentum matrix element of the extent state at bottom of conduction band as a function of the energy cm. (c) Squared momentum matrix element of the local state at bottom of conduction band as a function of energy of cm.
energy differences between the two pinned peaks and the extent states located at the bottom of the conduction band are 2.96 and 2.82 eV, corresponding to the PL peaks at 418 and 433 nm respectively. The calculated PL spectrum according to the scheme of Kengo [15] using a model of
[1] L.T. Canham, Appl. Phys. Lett. 56 (1990) 1046. [2] A.O. Konstantinov, A. Henry, C.I. Harris, E. Janzen, Appl. Phys. Lett. 66 (1995) 2250. [3] X.L. Wu, G.G. Siu, M.J. Stokes, D.L. Fan, Y. Gu, X.M. Bao, Appl. Phys. Lett. 77 (2000) 1292. [4] L.S. Liao, X.M. Bao, Z.F. Yang, N.B. Min, Appl. Phys. Lett. 66 (1995) 2382. [5] W.A. Nevin, H. Yamagishi, M. Yamaguchi, Y. Tawada, Nature (London) 368 (1994) 529. [6] S. Kalem, Phys. Rev. B 37 (1988) 8837. [7] T. Tiedje, B. Abeles, Appl. Phys. Lett. 45 (1984) 179. [8] J.K. Akalios, H. Fritzsche, Phys. Rev. Lett. 53 (1984) 1602. [9] D.N. Kouvatsos, M.K. Hatalis, R.J. Jaccodine, Appl. Phys. Lett. 61 (1992) 937. [10] W.L. Liu, M. Zhang, Ch.L. Lin, Appl. Phys. Lett. 78 (2001) 37. [11] X.L. Wu, Y. Gu, S.J. Xiong, J.M. Zhu, G.S. Huang, G.G. Siu, X.M. Bao, Appl. Phys. Lett. 94 (2003) 5247. [12] T. Kimura, S. Kagiyama, S. Yugo, Thin Solid Films 81 (1981) 319. [13] F.S. Khan, J.Q. Broughton, Phys. Rev. B 39 (1989) 3688. [14] J.B. Xia, Y.C. Chang, Phys. Rev. B 42 (1989) 1781. [15] K. Nishio, J. Koga, Phys. Rev. B 67 (2003) 195304.
Journal of Luminescence 122–123 (2007) 179–181 www.elsevier.com/locate/jlumin
Preparation and photoluminescence of SiC/Si/SiO2 multi-layer structure L. Wang, Z.J. Liang, Z.B. Wang, F.L. Zhao, Z.H. He, Dihu Chen State Key Laboratory of Optoelectronic Materials and Technologies, Physics Department of School of Physics and Engineering, Zhongshan University, Guangzhou 510275, PR China Available online 6 March 2006
Abstract A multi-layer structure of SiC/Si/SiO2 was fabricated by successive deposition of SiC and Si using plasma-enhanced chemical vapor deposition and post-deposition thermal oxidation of the part as-deposited Si surface layer. The samples oxidized at 900 1C for various annealing times exhibit two strong photoluminescence peaks in the blue-light range, one is a stable PL peak at 433 nm and the other is an unstable PL peak at a range of 445–460 nm. A maximum PL intensity was obtained when oxidation time is 40 min over this oxidation time, the PL intensity decrease up to disappear with the increase of the oxidation time. Origin of the PL was discussed and a tight-binding theoretic calculation was used to study the relation of the excess Si defect and PL properties. Results show that the two PL peaks observed may be attributed to excess Si defect centers and SiC nanocrystalline with different size in SiC/Si/SiO2 multi-layer structure. r 2006 Elsevier B.V. All rights reserved. Keywords: SiC/Si/SiO2; Photoluminescence; PECVD; Thermal oxidation; Density of state (DOS)
Since the discovery of visible luminescence from porous Si at room temperature (RT) [1], fabrication of nanostructure Si-based materials and their photoluminescence (PL) properties induced by the quantum confine effect at RT, have attracted much interest [2–5]. Recently, lowtemperature or RT PL of multi-layer films has been widely reported [6–8]. But these multi-layers were mostly concerned about Si, GaN, SiO2. Little work on SiC has been reported. This work is devoted to fabrication of a SiC/Si/ SiO2 multi-layer structure, experiment and theoretic study at its PL properties and origin. Plasma-enhanced chemical vapor deposition (PECVD) was used to deposit SiC and Si layer with a mixed gases of SiH4+CH4+H2 and SiH4+H2. Thickness of SiC and Si layer is controlled by adjusting the deposition time at a stable deposition rate of 4 nm min1. In this work, thickness of SiC and Si layers is 3 and 10 nm. Part of deposited Si layer was oxidized to form SiO2 top layer by dry thermal oxidation at 900 1C, and crystallization of SiC and Si layer were performed going with oxidation. Finally a SiC/Si/SiO2 multi-layer was formed. The Si nanocrystalline (nc-Si) layer with different thicknesses in SiC/ Corresponding author. Tel.: +86 20 84113398; fax: +86 20 84035496.
E-mail address: [email protected] (D. Chen). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.078
Si/SiO2 multi-layer was obtained by adjusting different oxidation times. Table 1 gives a rough estimate of the expected nc-Si layer thickness as a function of the oxidation time according to standard dry oxidation process in microelectronic techniques [9]. The substrate used is ntype (100) silicon wafers with a resistivity of 5–10 O cm. An initial pad oxide thickness of 3 nm was grown on the silicon substrate by dry oxidation at 900 1C for 10 min. A second harmonic wave of locked Ti:sapphire laser (400 nm, 0.32 MW) was used for PL measurements. Fig. 1 shows the fourier transform infrared spectroscopy (FTIR) spectra of samples with different oxidation times, the peak at 800 cm1 represents stretching vibration of Si–C bonds in compounds with Si, C, and H, while peak at 1060 cm1 indicates Si–O–Si stretching [10]. Two luminescence peaks appeared in all samples in Fig. 2, peak position at 433 nm remains unchanged for the samples with various thickness of nanocrystalline silicon and silicon dioxide. It indicates that this blue PL peak does not originate from band-to-band recombination in the quantum-confined Si or SiC crystallites. It may be originate from the Si excess defect centers [11], which will be discussed later in theory. PL peak position at around 455 nm red-shift with the oxidation time, which is attributed to origin from SiC nano-particles [3]. Since crystallization of SiC layer
ARTICLE IN PRESS L. Wang et al. / Journal of Luminescence 122–123 (2007) 179–181
180
Table 1 Rough estimate of the thickness of Si nanocrystalline (dSi) and SiO2 layers (dSiO2) prepared at 900 1C for different oxidation times (toxi) toxi (min) dSi (nm) dSiO2 (nm)
0 10.0 0
30 4.0 12.0
40 2.5 16.0
50 1.0 18.5
60 0.5 20.5
Fig. 1. FTIR spectra of the samples oxidized at 900 1C for various times indicated.
Fig. 2. PL spectra of SiC/Si/SiO2 multi-layer structure oxidized at 900 1C for various times indicated.
happens during oxidation of Si layer, more and larger crystalline SiC nano-particles are formed with increases in oxidation time according to Fig. 1 [12]. The PL intensity reaches maximum when oxidation time is 40 min. When annealing time is sequentially prolonged, the SiC nanoparticles grow larger, which shows a bulk SiC behavior. Therefore, the PL intensity decreased and also the peak red-shift, but the FTIR peak intensity still increased. There is a third peak at 445 nm in PL spectra of samples oxidized for 50 and 60 min and PL peak position remains unchanged. Table 1 shows that the thickness of silicon layer for the samples oxidized for 50 and 60 min are below 2 nm, the PL peak may originate from nano-Si particles [10]. However, for a prolonged oxidation, the silicon layer was almost oxidized, the PL spectra got very weak. This behavior indicates that the crystalline silicon layer plays a key role in the PL process. A theoretical calculation and discussion of PL spectra was performed by building a model of nano-Si structure with some defects according to the method in Ref. [11], in which the radius r of nano-Si sphere and the defects density of D were used as the parameters. A typical density of state (DOS) plot of Si sphere (r ¼ 1.2 nm) without and with defects is shown in Fig. 3(a) which was calculated using the tight binding scheme proposed by Khan [13]. The dotted line is the curve for Si sphere without vacancies and the solid line is for the Si sphere with vacancy defects density D ¼ 0.3. It shows that some defect states appear in the band gap due to existence of vacancy defects, and the DOS sharp peak labeled by letter A at the edge of the value band gets higher indicating many defect states were pinned at this sharp peak nearby. We call them pinned states. Since the system relax to equilibrium state before recombination, electrons that have excited to the conduction band, relax to the bottom of conduction band according to the Fermi distribution. A momentum matrix element ccn Pcvm based on tight binding method can be calculated [14] by changing the cm at a fixed ccn, where ccn and cvm represent the system eigenstate
n and m in the conduction and value band. The ccn Pcvm is meaningful for PL emission only when the ccn is a nearby state of the conduction band bottom. Fig. 3(b) shows that transition probability from the lowest extent state ccn in the conduction band bottom to other state cvm, as a function of the energy of cvm. It also shows that extent state electron tends to transmit to the empty pinned state located in the edge of the value band. Fig. 3(c) shows that the transition probability from arbitrary fixed local state in the conduction band ccn to other state cvm, as a function of the energy of cvm. It also shows that local state electron in the conduction band tends to transmit to the defect state located in the middle of the band gap. Since the energy level of the pinned defect states are more stable than that of the defect states in the middle of band gap, the extent state electrons located at conduction band bottom have more chance to transmit to the pinned defect state in the value band. As shown in Fig. 3(b), the
ARTICLE IN PRESS L. Wang et al. / Journal of Luminescence 122–123 (2007) 179–181
181
nano-Si structure described above is shown in inset of Fig. 3(a). Since the pinned states are stable at near the sharp peak of the value band, and not sensitive to the size of the Si nanocrystalline, the PL peak position at 433 nm is unchanged with variation of the thickness of nanocrystalline silicon layer. In summary, we have prepared multi-layer structure of SiC/Si/SiO2 by PECVD, samples exhibit two strong PL peaks in the blue-light range. The PL intensity varies with Si layer thickness oxidized for various time, a maximum PL intensity of the two peaks was obtained at oxidation time of 40 min. Over this oxidation time, the PL intensity decreases and disappears with increase of the oxidation time. Theoretical study using a tight binding calculation shows that the two PL peaks are attributed to excess Si defect centers and SiC nanocrystalline in SiC/Si/SiO2 multi-layer structure. This work is supported by the National Science Foundation of China (no. NSFC30370410).
References
Fig. 3. (a) The DOS of Si nanocrystalline sphere with a radius of 1.2 nm and various vacancy defect density D as indicated. The dashed circle indicates the defect states deeply in band gap, the pinned states (labeled B) are at the top of the value band. The inset is calculated PL spectra. (b) Squared momentum matrix element of the extent state at bottom of conduction band as a function of the energy cm. (c) Squared momentum matrix element of the local state at bottom of conduction band as a function of energy of cm.
energy differences between the two pinned peaks and the extent states located at the bottom of the conduction band are 2.96 and 2.82 eV, corresponding to the PL peaks at 418 and 433 nm respectively. The calculated PL spectrum according to the scheme of Kengo [15] using a model of
[1] L.T. Canham, Appl. Phys. Lett. 56 (1990) 1046. [2] A.O. Konstantinov, A. Henry, C.I. Harris, E. Janzen, Appl. Phys. Lett. 66 (1995) 2250. [3] X.L. Wu, G.G. Siu, M.J. Stokes, D.L. Fan, Y. Gu, X.M. Bao, Appl. Phys. Lett. 77 (2000) 1292. [4] L.S. Liao, X.M. Bao, Z.F. Yang, N.B. Min, Appl. Phys. Lett. 66 (1995) 2382. [5] W.A. Nevin, H. Yamagishi, M. Yamaguchi, Y. Tawada, Nature (London) 368 (1994) 529. [6] S. Kalem, Phys. Rev. B 37 (1988) 8837. [7] T. Tiedje, B. Abeles, Appl. Phys. Lett. 45 (1984) 179. [8] J.K. Akalios, H. Fritzsche, Phys. Rev. Lett. 53 (1984) 1602. [9] D.N. Kouvatsos, M.K. Hatalis, R.J. Jaccodine, Appl. Phys. Lett. 61 (1992) 937. [10] W.L. Liu, M. Zhang, Ch.L. Lin, Appl. Phys. Lett. 78 (2001) 37. [11] X.L. Wu, Y. Gu, S.J. Xiong, J.M. Zhu, G.S. Huang, G.G. Siu, X.M. Bao, Appl. Phys. Lett. 94 (2003) 5247. [12] T. Kimura, S. Kagiyama, S. Yugo, Thin Solid Films 81 (1981) 319. [13] F.S. Khan, J.Q. Broughton, Phys. Rev. B 39 (1989) 3688. [14] J.B. Xia, Y.C. Chang, Phys. Rev. B 42 (1989) 1781. [15] K. Nishio, J. Koga, Phys. Rev. B 67 (2003) 195304.