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Photoluminescence enhancement and stabilisation of porous silicon passivated by iron
ARTICLE IN PRESS Journal of Luminescence 128 (2008) 1763– 1766
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence enhancement and stabilisation of porous silicon passivated by iron M. Rahmani a, A. Moadhen a, M.-A. Zaı¨bi a,b,, H. Elhouichet a, M. Oueslati a a b
´ des Sciences de Tunis, Unite´ de Recherche de Spectroscopie Raman, 2092 ElManar, Tunis, Tunisia De´partement de Physique, Faculte Ecole Supe´rieure des Sciences et Techniques de Tunis, 5 Avenue Taha Hussein, 1008 Tunis, Tunisia
a r t i c l e in fo
abstract
Article history: Received 10 November 2007 Received in revised form 29 February 2008 Accepted 8 April 2008 Available online 12 April 2008
Iron is incorporated in porous silicon (PS) by impregnation method using Fe(NO3)3 aqueous solution. The presence of iron in PS matrix is shown from energy-dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) measurements. The optical properties of PS and PS-doped iron are studied by photoluminescence (PL). The iron deposited in PS quenched the silicon dangling bonds then increased the PL intensity. The PL peak intensity of impregnated PS is seven times stronger than that in normal PS. Upon exposing iron-PS sample to ambient air, there is no significant change in peak position but the PL intensity increases during the first 3 weeks and then stabilises. The stability is attributed to passivation of the Si nanocrystallites by iron. & 2008 Elsevier B.V. All rights reserved.
Pacs: 78.30.j 78.55.m 78.55.Mb 75.50.Ss Keywords: Porous silicon Iron Passivation Enhancement Stability
1. Introduction The photoluminescence (PL) of porous silicon (PS) in the visible range and at room temperature has attracted much attention, especially to enhance and stabilise its emission. Many research efforts have been invested to realise an optical device with luminescent PS, but the inefficiency [1] and instability [2] of optical characteristic in PS still remains. The PL instability of PS has been known as originating from the transformation of the PS nanostructure and the surface chemical composition of PS under different formation conditions [3–6] and ambient atmosphere [7,8]. The ageing PS induces PL instability; under attack of oxygen the Si–H bonds of Si nanocrystallites (nc-Si) are broken then the oxygen could remain on or diffuse inside nc-Si, which leads a reduction of their size and consequently induces a blueshift of PL peak position [9,10]. To stabilise the PL intensity of PS and to introduce it in practice device, it is necessary to modify the PS surface structure. Corresponding author at: De´partement de Physique, Faculte´ des Sciences de Tunis, Unite´ de Recherche de Spectroscopie Raman, 2092 ElManar, Tunis, Tunisia. Tel.: +216 71496066; fax: +216 71391166. E-mail address: [email protected] (M.-A. Zaı¨bi).
0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.04.003
Iron can be easily introduced inside the silicon pores. In fact, Li et al. [11] have demonstrated that Si–H bonds can be substituted by Si–Fe ones. Zhu et al. [12] have impregnated PS in [Fe(NO3)3]/HF solution, to introduce Fe ions into the pores. They have found a strong PL intensity and the PL peak energy is centred at about 1.85 eV. In the same way, Chen et al. [13] have reported that, after hydrothermal erosion, silicon dangling bonds on porous layer are passivated by Fe ions, which has induced an increase of the PL intensity with storage oxidation in air while that corresponding to the PS decreases. They have reported, also, that the PL band is blue shifted only for PS within oxidation time. In the other work, they have reported that ironpassivated PS sample reaches stability during 4 months subsequent to preparation [14]. Recently, Lee et al. [15] have investigated the strong and thermally stable PL of PS with Fe contamination. At temperatures ranging from 14 to 120 K, the red PL of PS contaminated Fe is 10 times stronger than that of PS without Fe and the PL peak energy is kept at the same position. Beyond 120 K and up to 300 K, the PL peak position is blue shifted. In this work, the inside PS layer modification is performed by the incorporation of iron by impregnation. The enhancement and stability of PS containing iron are presented and discussed.
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2. Experimental Samples are a boron-doped p-type Si(1 0 0) substrate with 1–4 O cm resistivity. The PS is prepared using electrochemical anodisation in a solution of HF(40%)/C2H5OH/H2O (2:1:1), the current density is 10 mA/cm2 and the etching time is 10 mn. The PS layer is impregnated, during 5 mn, in different concentrations of ferric nitrate aqueous solution, maintained at room temperature. The solution solubility is total, i.e. no iron clusters are in suspension. At the impregnated time and to do not have any velocity effect in the reaction between PS and solution, no stirring of the solution has been applied. To eliminate the residual molecules into PS, the samples have been dried by nitrogen gas. These samples are labelled PS/Fe. We have checked that dry treatment does not have an important contribution in any analyses. The elaborated samples are analysed by scanning electron microscopy (SEM) (XL 30-Philips) equipped with energy-dispersive X-ray (EDX) spectrometer used for compositional investigations on the cross-section of the PS and Fourier transform infrared (FTIR) spectroscopy (Bruker IFS66v/s FTIR spectrometer). The FTIR analyses are taken on transmittance mode and investigated in the 400–4000 cm1 range with a 4 cm1 step. A triple monochromator and a GaAs photomultiplier were used to record the PL spectra. The samples were excited by Ar+ laser (2.54 eV). Aged experiments were carried out in ambient air at room temperature and natural light.
(3)
(1): (iron-oxygen)-Si (2): Si-Si + Si-Hn(n=1-2) (3): Si-H2 (4): Si-O-Si (5): Si-H (5)
(4)
Transmittance (arb. unit.)
(1) (2)
PS/Fe (0.7M) PS/Fe (0.3M)
PS/Fe (0.1M) PS 1000
500
2000
1500
2500
3000
Wavenumbers (cm-1) Fig. 2. FTIR spectra of PS and PS/Fe samples with different concentrations of Fe(NO3)3.
5
The SEM observations of PS and PS/Fe samples show that the porous layer thickness is about 5 mm. The EDX analysis performed on the cross-section of PS layer shows that atomic percentage of iron has increased from the top to the bottom of the layer (Fig. 1). This indicates that iron is deeply incorporated into the PS layer and it may take places over all the pore walls. FTIR spectra were obtained from PS sample and PS impregnated in some concentrations of Fe(NO3)3 aqueous solution (Fig. 2). The IR-spectrum of PS is in conformity with that found by Bisi et al. [16] and Gorbanyuk et al. [17]. The principal recorded vibration bands are 1100 cm1 corresponds to stretching mode Si–O–Si, 900 cm1 attributed to scissors mode Si–H2 and a large vibration absorption band at 610–660 cm1 which is a mixture of stretching wagging mode Si–Si and wagging mode Si–Hn (n ¼ 1 and 2). We also showed the presence of broad lines at 2100 cm1
Atomic percentage of Fe (%)
2.5 2.0 1.5 1.0 PS
Si Substrate
0.5 0.0 0
1
2
4 5 3 Thickness (µm)
6
7
8
Fig. 1. Atomic percentage of iron, deduced from EDX analyses, in different regions of the PS layer and Si substrate.
PL intensity (arb. unit.)
4 3. Results and discussion
3
(b)
2
(x3)
1 (a) 0 1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
Energy (eV) Fig. 3. PL spectra of as-prepared (a) PS and (b) PS/Fe (0.3 M) samples.
indicating the presence of Si–H bond where the Si atom is back bonded to another Si atom. After PS impregnation in Fe(NO3)3 aqueous solution, the principal modification is the decrease of the large vibration band while Si–O–Si vibration mode has been increased and a new vibration band at 460 cm1 has been appeared. The Si–H peak at 2100 cm1 showed a simultaneous decrease with higher concentration (Fig. 2). Many authors [18,19] have reported that metal–oxygen–silicon bonding is expected between 300 and 700 cm1. So, the new peak at 460 cm1 is due to the change in the surface bonds of PS/Fe nanocomposite and it can be attributed to (iron–oxygen)–silicon bonding. These results assume that Si–Hn bonds are substituted by iron and oxygen of the aqueous solution. PL investigations show that the PL peak position of PS is at E ¼ (1.7170.02) eV which is the same after iron incorporation (Fig. 3). The PL intensity of PS/Fe is almost seven times stronger than that in normal PS. The PL enhancement after Fe doping is due to the iron-passivation of nc-Si. This passivation should possess a surface electronic structure, which enhances the quantum confinement.
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Fig. 5. (a) Normalised PL intensity evolution of PS/Fe within time oxidation. (b) Integrated PL intensity evolution of PS/Fe within time oxidation.
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Integrated PL intensity (arb. unit.)
Fig. 4 presents the integrated PL intensity (IInt. PL ) as function of iron concentration in solution, ranging from 0.1 to 1 M, of freshly PS/Fe samples (a) and after 15 days of exposure to air (b). The inset of Fig. 4 shows the atomic percentage of iron, deduced from EDX analysis, in different regions of PS layer and with various iron concentrations. As the amount of Fe ions incorporated in PS matrix increases the integrated PL intensity increased up to 0.3 M and then decreases. So, we can distinguish two regions (R1 and R2): (i) in R1 there is a substitution between hydrogen, bonded to nc-Si, and iron ions which induce an increase of the quantum yield hence the enhancement of the PL intensity. Furthermore, this quantum yield increases with the so-called substitution. (ii) R2 is characterised by the growth of incorporated iron quantity, confirmed by EDX measurements, which acts as an excitation energy trap and promotes the non-radiative energy transfer. This decrease of the PL intensity is always manifested in fluorescence spectroscopy and when there is a random bidimensional distribution of molecules. This behaviour is well known as autoextinction phenomenon [20–22]. The effect of natural oxidation on the PS/Fe has investigated. The PL peak position does not present a change during storing time; it is always recorded at the same position as that prepared PS/Fe (Fig. 5a). Chen et al. [13] have reported that iron-passivation plays a protect layer and prevents the PL blueshift. Then, the iron, passivated the majority of silicon dangling bonds, is oxidised during storing time. The integrated PL intensity increases during the first 3 weeks subsequent to preparation, and then is stabilised (Fig. 5b). The PL intensity enhancement is due to two essential effects: (i) The diffusion of oxygen (substituted to Si–H in impregnation process) in the inner of nc-Si therefore a Si dangling bonds are created and which are passivated by iron consequently a Si–Fe layer is formed; (ii) The oxidation of Si–Fe layer produces an (iron–oxygen)–Si layer in outer of nc-Si. Then iron has protected the nc-Si from the oxidation and has stopped the reduction of nc-Si size. Three weeks subsequent to preparation, the (iron–oxygen) layer wrapped, totally, nc-Si and then stabilised the PL intensity. Remember Zhang et al. [14] have found that the PL peak intensity is 2–2.5 times stronger than that in normal PS and the stabilised PL intensity is reached after 4 months. Fig. 6 exhibits the time evolution of the normalised PL intensity of normal PS with ageing time. As the surface passivation was gradually changing, the PL peak position was blue shifted.
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M. Rahmani et al. / Journal of Luminescence 128 (2008) 1763–1766
120 Days
85 Days 60 Days 45 Days 15 Days
0.0
0.2
0.4 0.6 Ferric nitrate concentration (M)
0.8
1.0
Fig. 4. Integrated PL intensity as function of Fe(NO3)3 concentration of freshly PS/ Fe (a) and after PL intensity stability (b). The inset shows the atomic percentage of iron in different regions of PS layer with various ferric nitrate concentrations: (’) 0.2 M, (K) 0.3 M and (%) 0.8 M.
0 Days 1.5
1.6
1.7
1.8
1.9
2.0
2.1
Energy (eV) Fig. 6. Normalised PL spectra of PS layer within ageing time.
2.2
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H
H
H H
Si
Si
Si
Si
Si
Si Si
Fe
O
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Si
Fe
Fe
H
Si
After ageing time
O
O Fe
nc-Si (iron-oxygen)-Si layer
Fe Si
O Fe
Si
O Si
Fe Si
Si
Fig. 7. Physical configuration of iron passivated PS after impregnation and upon exposing to ambient air. The inset is the schematic view of nc-Si wrapping by (iron–oxygen) layer formed during 21 days.
This shift may be ascribed to the recombination of electrons trapped at the localised states due to SiQO bond of PS extended leading to the quantum confinement effect [23]. An irregular PL intensity evolution of PS was also observed during ageing time and traduces the chemical instability of the PS surface. The correlation of FTIR and PL results can be illustrated by physical configuration of iron-passivated PS as shown in Fig. 7. The high intensity and stable PL peak position are reported, also, by Mavi et al. [24] where they have used a n-type Si sample. Remember that the sizes of porous layer obtained by n-type Si are in the range of mm. Then, the iron is a good candidate to give a high and stable intensity emitted by any PS type.
4. Conclusion Iron is incorporated in PS layer by impregnation method. The correlation of EDX and FTIR indicates that iron is deeply incorporated in PS matrix. The PL intensity of PS/Fe nanocomposite is seven times stronger than that from PS one. This result is attributed to the large iron-passivation of Si nanocrystallites. The ageing time oxidation of the PS/Fe nanocomposite produces PL enhancement and stabilisation due to the (iron–oxygen) layer wrapping the nc-Si. A model for Si nanocrystallites termination is proposed to describe the passivation process. Our results support the quantum confinement model.
Acknowledgement The authors would like to thank Pr. H. Sidhom for providing SEM and EDX facilities and discussions.
References [1] H. Koyama, T. Nakagawa, T. Ozaki, N. Koshida, Appl. Phys. Lett. 65 (1994) 1656. [2] L.T. Canham, M.R. Houlton, W.Y. Leong, C. Pickering, J.M. Keen, J. Appl. Phys. 70 (1991) 422. [3] C.H. Lin, S.C. Lee, Y.F. Chen, J. Appl. Phys. 75 (1994) 7728. [4] C. Songyan, P.K. Kashkarov, V.Y. Timoshenko, L. Baolin, J. Bingxi, J. Cryst. Growth 247 (2003) 445. [5] Y. Zhao, D. Li, W. Sang, D. Yang, Sol. State Electron. 50 (2006) 1529. [6] D.A. Kim, S.I. Im, C.M. Whang, W.S. Cho, Y.C. Yoo, N.H. Cho, J.G. Kim, Y.J. Kwon, Appl. Surf. Sci. 230 (2004) 125. [7] T. Wadayama, T. Arigane, A. Hatta, Appl. Phys. Lett. 73 (1998) 2570. [8] H. Elhouichet, M. Oueslati, B. Bessais, H. Ezzaouia, J. Lumin. 71 (1997) 77. [9] S.H. Choi, H. Chung, G.S. Shin, Solid State Commun. 95 (6) (1995) 341. [10] M.A. Tischler, R.T. Collins, J.H. Stathis, J.C. Tsang, Appl. Phys. Lett. 60 (1992) 639. [11] X.J. Li, De L. Zhu, Q.W. Chen, Y.H. Zhang, Appl. Phys. Lett. 74 (1999) 389. [12] D. Zhu, Q. Chen, Y. Zhang, Appl. Surf. Sci. 191 (2002) 218. [13] Q.W. Chen, X. Li, Y. Zhang, Chem. Phys. Lett. 343 (2001) 507. [14] Y.H. Zhang, X.J. Li, L. Zheng, Q.W. Chen, Phys. Rev. Lett. 81 (1998) 1710. [15] D.Y. Lee, J.W. Park, J.Y. Leem, J.S. Kim, S.K. Kang, J.S. Son, H.B. Kang, Y.H. Mun, D.K. Lee, D.H. Kim, I.H. Bae, J. Cryst. Growth 260 (2004) 394. [16] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1. [17] T.I. Gorbanyuk, A.A. Evtukh, V.G. Litovchenko, V.S. Solnsev, E.M. Pakhlov, Thin solid films 495 (2006) 134. [18] C.M. Parler, J.A. Ritter, M.D. Amiridis, J. Non-Cryst. Solids 279 (2001) 119. [19] F. Hamadache, C. Renaux, J.-L. Duvail, P. Bertrand, Phys. Status Solidi (a) 197 (1) (2003) 168. [20] G.S. Beddard, G. Porter, Nature 260 (1976) 366. [21] J. Knoester, J.E. Van Himbergen, J. Chem. Phys. 86 (6) (1987) 3571. [22] A. Penzkofer, Y. Lu, Chem. Phys. 103 (2–3) (1986) 399. [23] S.M. Hossain, S. Chakraborty, S.K. Dutta, J. Das, H. Saha, J. Lumin. 91 (2000) 195. [24] H.S. Mavi, B.G. Rasheed, A.K. Shukla, R.K. Soni, S.C. Abbi, Mater. Sci. Eng. B 97 (2003) 239.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence enhancement and stabilisation of porous silicon passivated by iron M. Rahmani a, A. Moadhen a, M.-A. Zaı¨bi a,b,, H. Elhouichet a, M. Oueslati a a b
´ des Sciences de Tunis, Unite´ de Recherche de Spectroscopie Raman, 2092 ElManar, Tunis, Tunisia De´partement de Physique, Faculte Ecole Supe´rieure des Sciences et Techniques de Tunis, 5 Avenue Taha Hussein, 1008 Tunis, Tunisia
a r t i c l e in fo
abstract
Article history: Received 10 November 2007 Received in revised form 29 February 2008 Accepted 8 April 2008 Available online 12 April 2008
Iron is incorporated in porous silicon (PS) by impregnation method using Fe(NO3)3 aqueous solution. The presence of iron in PS matrix is shown from energy-dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) measurements. The optical properties of PS and PS-doped iron are studied by photoluminescence (PL). The iron deposited in PS quenched the silicon dangling bonds then increased the PL intensity. The PL peak intensity of impregnated PS is seven times stronger than that in normal PS. Upon exposing iron-PS sample to ambient air, there is no significant change in peak position but the PL intensity increases during the first 3 weeks and then stabilises. The stability is attributed to passivation of the Si nanocrystallites by iron. & 2008 Elsevier B.V. All rights reserved.
Pacs: 78.30.j 78.55.m 78.55.Mb 75.50.Ss Keywords: Porous silicon Iron Passivation Enhancement Stability
1. Introduction The photoluminescence (PL) of porous silicon (PS) in the visible range and at room temperature has attracted much attention, especially to enhance and stabilise its emission. Many research efforts have been invested to realise an optical device with luminescent PS, but the inefficiency [1] and instability [2] of optical characteristic in PS still remains. The PL instability of PS has been known as originating from the transformation of the PS nanostructure and the surface chemical composition of PS under different formation conditions [3–6] and ambient atmosphere [7,8]. The ageing PS induces PL instability; under attack of oxygen the Si–H bonds of Si nanocrystallites (nc-Si) are broken then the oxygen could remain on or diffuse inside nc-Si, which leads a reduction of their size and consequently induces a blueshift of PL peak position [9,10]. To stabilise the PL intensity of PS and to introduce it in practice device, it is necessary to modify the PS surface structure. Corresponding author at: De´partement de Physique, Faculte´ des Sciences de Tunis, Unite´ de Recherche de Spectroscopie Raman, 2092 ElManar, Tunis, Tunisia. Tel.: +216 71496066; fax: +216 71391166. E-mail address: [email protected] (M.-A. Zaı¨bi).
0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.04.003
Iron can be easily introduced inside the silicon pores. In fact, Li et al. [11] have demonstrated that Si–H bonds can be substituted by Si–Fe ones. Zhu et al. [12] have impregnated PS in [Fe(NO3)3]/HF solution, to introduce Fe ions into the pores. They have found a strong PL intensity and the PL peak energy is centred at about 1.85 eV. In the same way, Chen et al. [13] have reported that, after hydrothermal erosion, silicon dangling bonds on porous layer are passivated by Fe ions, which has induced an increase of the PL intensity with storage oxidation in air while that corresponding to the PS decreases. They have reported, also, that the PL band is blue shifted only for PS within oxidation time. In the other work, they have reported that ironpassivated PS sample reaches stability during 4 months subsequent to preparation [14]. Recently, Lee et al. [15] have investigated the strong and thermally stable PL of PS with Fe contamination. At temperatures ranging from 14 to 120 K, the red PL of PS contaminated Fe is 10 times stronger than that of PS without Fe and the PL peak energy is kept at the same position. Beyond 120 K and up to 300 K, the PL peak position is blue shifted. In this work, the inside PS layer modification is performed by the incorporation of iron by impregnation. The enhancement and stability of PS containing iron are presented and discussed.
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2. Experimental Samples are a boron-doped p-type Si(1 0 0) substrate with 1–4 O cm resistivity. The PS is prepared using electrochemical anodisation in a solution of HF(40%)/C2H5OH/H2O (2:1:1), the current density is 10 mA/cm2 and the etching time is 10 mn. The PS layer is impregnated, during 5 mn, in different concentrations of ferric nitrate aqueous solution, maintained at room temperature. The solution solubility is total, i.e. no iron clusters are in suspension. At the impregnated time and to do not have any velocity effect in the reaction between PS and solution, no stirring of the solution has been applied. To eliminate the residual molecules into PS, the samples have been dried by nitrogen gas. These samples are labelled PS/Fe. We have checked that dry treatment does not have an important contribution in any analyses. The elaborated samples are analysed by scanning electron microscopy (SEM) (XL 30-Philips) equipped with energy-dispersive X-ray (EDX) spectrometer used for compositional investigations on the cross-section of the PS and Fourier transform infrared (FTIR) spectroscopy (Bruker IFS66v/s FTIR spectrometer). The FTIR analyses are taken on transmittance mode and investigated in the 400–4000 cm1 range with a 4 cm1 step. A triple monochromator and a GaAs photomultiplier were used to record the PL spectra. The samples were excited by Ar+ laser (2.54 eV). Aged experiments were carried out in ambient air at room temperature and natural light.
(3)
(1): (iron-oxygen)-Si (2): Si-Si + Si-Hn(n=1-2) (3): Si-H2 (4): Si-O-Si (5): Si-H (5)
(4)
Transmittance (arb. unit.)
(1) (2)
PS/Fe (0.7M) PS/Fe (0.3M)
PS/Fe (0.1M) PS 1000
500
2000
1500
2500
3000
Wavenumbers (cm-1) Fig. 2. FTIR spectra of PS and PS/Fe samples with different concentrations of Fe(NO3)3.
5
The SEM observations of PS and PS/Fe samples show that the porous layer thickness is about 5 mm. The EDX analysis performed on the cross-section of PS layer shows that atomic percentage of iron has increased from the top to the bottom of the layer (Fig. 1). This indicates that iron is deeply incorporated into the PS layer and it may take places over all the pore walls. FTIR spectra were obtained from PS sample and PS impregnated in some concentrations of Fe(NO3)3 aqueous solution (Fig. 2). The IR-spectrum of PS is in conformity with that found by Bisi et al. [16] and Gorbanyuk et al. [17]. The principal recorded vibration bands are 1100 cm1 corresponds to stretching mode Si–O–Si, 900 cm1 attributed to scissors mode Si–H2 and a large vibration absorption band at 610–660 cm1 which is a mixture of stretching wagging mode Si–Si and wagging mode Si–Hn (n ¼ 1 and 2). We also showed the presence of broad lines at 2100 cm1
Atomic percentage of Fe (%)
2.5 2.0 1.5 1.0 PS
Si Substrate
0.5 0.0 0
1
2
4 5 3 Thickness (µm)
6
7
8
Fig. 1. Atomic percentage of iron, deduced from EDX analyses, in different regions of the PS layer and Si substrate.
PL intensity (arb. unit.)
4 3. Results and discussion
3
(b)
2
(x3)
1 (a) 0 1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
Energy (eV) Fig. 3. PL spectra of as-prepared (a) PS and (b) PS/Fe (0.3 M) samples.
indicating the presence of Si–H bond where the Si atom is back bonded to another Si atom. After PS impregnation in Fe(NO3)3 aqueous solution, the principal modification is the decrease of the large vibration band while Si–O–Si vibration mode has been increased and a new vibration band at 460 cm1 has been appeared. The Si–H peak at 2100 cm1 showed a simultaneous decrease with higher concentration (Fig. 2). Many authors [18,19] have reported that metal–oxygen–silicon bonding is expected between 300 and 700 cm1. So, the new peak at 460 cm1 is due to the change in the surface bonds of PS/Fe nanocomposite and it can be attributed to (iron–oxygen)–silicon bonding. These results assume that Si–Hn bonds are substituted by iron and oxygen of the aqueous solution. PL investigations show that the PL peak position of PS is at E ¼ (1.7170.02) eV which is the same after iron incorporation (Fig. 3). The PL intensity of PS/Fe is almost seven times stronger than that in normal PS. The PL enhancement after Fe doping is due to the iron-passivation of nc-Si. This passivation should possess a surface electronic structure, which enhances the quantum confinement.
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Fig. 5. (a) Normalised PL intensity evolution of PS/Fe within time oxidation. (b) Integrated PL intensity evolution of PS/Fe within time oxidation.
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0 Day
Integrated PL intensity (arb. unit.)
Fig. 4 presents the integrated PL intensity (IInt. PL ) as function of iron concentration in solution, ranging from 0.1 to 1 M, of freshly PS/Fe samples (a) and after 15 days of exposure to air (b). The inset of Fig. 4 shows the atomic percentage of iron, deduced from EDX analysis, in different regions of PS layer and with various iron concentrations. As the amount of Fe ions incorporated in PS matrix increases the integrated PL intensity increased up to 0.3 M and then decreases. So, we can distinguish two regions (R1 and R2): (i) in R1 there is a substitution between hydrogen, bonded to nc-Si, and iron ions which induce an increase of the quantum yield hence the enhancement of the PL intensity. Furthermore, this quantum yield increases with the so-called substitution. (ii) R2 is characterised by the growth of incorporated iron quantity, confirmed by EDX measurements, which acts as an excitation energy trap and promotes the non-radiative energy transfer. This decrease of the PL intensity is always manifested in fluorescence spectroscopy and when there is a random bidimensional distribution of molecules. This behaviour is well known as autoextinction phenomenon [20–22]. The effect of natural oxidation on the PS/Fe has investigated. The PL peak position does not present a change during storing time; it is always recorded at the same position as that prepared PS/Fe (Fig. 5a). Chen et al. [13] have reported that iron-passivation plays a protect layer and prevents the PL blueshift. Then, the iron, passivated the majority of silicon dangling bonds, is oxidised during storing time. The integrated PL intensity increases during the first 3 weeks subsequent to preparation, and then is stabilised (Fig. 5b). The PL intensity enhancement is due to two essential effects: (i) The diffusion of oxygen (substituted to Si–H in impregnation process) in the inner of nc-Si therefore a Si dangling bonds are created and which are passivated by iron consequently a Si–Fe layer is formed; (ii) The oxidation of Si–Fe layer produces an (iron–oxygen)–Si layer in outer of nc-Si. Then iron has protected the nc-Si from the oxidation and has stopped the reduction of nc-Si size. Three weeks subsequent to preparation, the (iron–oxygen) layer wrapped, totally, nc-Si and then stabilised the PL intensity. Remember Zhang et al. [14] have found that the PL peak intensity is 2–2.5 times stronger than that in normal PS and the stabilised PL intensity is reached after 4 months. Fig. 6 exhibits the time evolution of the normalised PL intensity of normal PS with ageing time. As the surface passivation was gradually changing, the PL peak position was blue shifted.
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M. Rahmani et al. / Journal of Luminescence 128 (2008) 1763–1766
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0.8
1.0
Fig. 4. Integrated PL intensity as function of Fe(NO3)3 concentration of freshly PS/ Fe (a) and after PL intensity stability (b). The inset shows the atomic percentage of iron in different regions of PS layer with various ferric nitrate concentrations: (’) 0.2 M, (K) 0.3 M and (%) 0.8 M.
0 Days 1.5
1.6
1.7
1.8
1.9
2.0
2.1
Energy (eV) Fig. 6. Normalised PL spectra of PS layer within ageing time.
2.2
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H
H
H H
Si
Si
Si
Si
Si
Si Si
Fe
O
After iron deposition Si
Si
Fe
Fe
H
Si
After ageing time
O
O Fe
nc-Si (iron-oxygen)-Si layer
Fe Si
O Fe
Si
O Si
Fe Si
Si
Fig. 7. Physical configuration of iron passivated PS after impregnation and upon exposing to ambient air. The inset is the schematic view of nc-Si wrapping by (iron–oxygen) layer formed during 21 days.
This shift may be ascribed to the recombination of electrons trapped at the localised states due to SiQO bond of PS extended leading to the quantum confinement effect [23]. An irregular PL intensity evolution of PS was also observed during ageing time and traduces the chemical instability of the PS surface. The correlation of FTIR and PL results can be illustrated by physical configuration of iron-passivated PS as shown in Fig. 7. The high intensity and stable PL peak position are reported, also, by Mavi et al. [24] where they have used a n-type Si sample. Remember that the sizes of porous layer obtained by n-type Si are in the range of mm. Then, the iron is a good candidate to give a high and stable intensity emitted by any PS type.
4. Conclusion Iron is incorporated in PS layer by impregnation method. The correlation of EDX and FTIR indicates that iron is deeply incorporated in PS matrix. The PL intensity of PS/Fe nanocomposite is seven times stronger than that from PS one. This result is attributed to the large iron-passivation of Si nanocrystallites. The ageing time oxidation of the PS/Fe nanocomposite produces PL enhancement and stabilisation due to the (iron–oxygen) layer wrapping the nc-Si. A model for Si nanocrystallites termination is proposed to describe the passivation process. Our results support the quantum confinement model.
Acknowledgement The authors would like to thank Pr. H. Sidhom for providing SEM and EDX facilities and discussions.
References [1] H. Koyama, T. Nakagawa, T. Ozaki, N. Koshida, Appl. Phys. Lett. 65 (1994) 1656. [2] L.T. Canham, M.R. Houlton, W.Y. Leong, C. Pickering, J.M. Keen, J. Appl. Phys. 70 (1991) 422. [3] C.H. Lin, S.C. Lee, Y.F. Chen, J. Appl. Phys. 75 (1994) 7728. [4] C. Songyan, P.K. Kashkarov, V.Y. Timoshenko, L. Baolin, J. Bingxi, J. Cryst. Growth 247 (2003) 445. [5] Y. Zhao, D. Li, W. Sang, D. Yang, Sol. State Electron. 50 (2006) 1529. [6] D.A. Kim, S.I. Im, C.M. Whang, W.S. Cho, Y.C. Yoo, N.H. Cho, J.G. Kim, Y.J. Kwon, Appl. Surf. Sci. 230 (2004) 125. [7] T. Wadayama, T. Arigane, A. Hatta, Appl. Phys. Lett. 73 (1998) 2570. [8] H. Elhouichet, M. Oueslati, B. Bessais, H. Ezzaouia, J. Lumin. 71 (1997) 77. [9] S.H. Choi, H. Chung, G.S. Shin, Solid State Commun. 95 (6) (1995) 341. [10] M.A. Tischler, R.T. Collins, J.H. Stathis, J.C. Tsang, Appl. Phys. Lett. 60 (1992) 639. [11] X.J. Li, De L. Zhu, Q.W. Chen, Y.H. Zhang, Appl. Phys. Lett. 74 (1999) 389. [12] D. Zhu, Q. Chen, Y. Zhang, Appl. Surf. Sci. 191 (2002) 218. [13] Q.W. Chen, X. Li, Y. Zhang, Chem. Phys. Lett. 343 (2001) 507. [14] Y.H. Zhang, X.J. Li, L. Zheng, Q.W. Chen, Phys. Rev. Lett. 81 (1998) 1710. [15] D.Y. Lee, J.W. Park, J.Y. Leem, J.S. Kim, S.K. Kang, J.S. Son, H.B. Kang, Y.H. Mun, D.K. Lee, D.H. Kim, I.H. Bae, J. Cryst. Growth 260 (2004) 394. [16] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1. [17] T.I. Gorbanyuk, A.A. Evtukh, V.G. Litovchenko, V.S. Solnsev, E.M. Pakhlov, Thin solid films 495 (2006) 134. [18] C.M. Parler, J.A. Ritter, M.D. Amiridis, J. Non-Cryst. Solids 279 (2001) 119. [19] F. Hamadache, C. Renaux, J.-L. Duvail, P. Bertrand, Phys. Status Solidi (a) 197 (1) (2003) 168. [20] G.S. Beddard, G. Porter, Nature 260 (1976) 366. [21] J. Knoester, J.E. Van Himbergen, J. Chem. Phys. 86 (6) (1987) 3571. [22] A. Penzkofer, Y. Lu, Chem. Phys. 103 (2–3) (1986) 399. [23] S.M. Hossain, S. Chakraborty, S.K. Dutta, J. Das, H. Saha, J. Lumin. 91 (2000) 195. [24] H.S. Mavi, B.G. Rasheed, A.K. Shukla, R.K. Soni, S.C. Abbi, Mater. Sci. Eng. B 97 (2003) 239.