- Email: [email protected]
Photoluminescence and optical absorption properties of silicon quantum dots embedded in Si-rich silicon nitride matrices
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1744–1746
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and optical absorption properties of silicon quantum dots embedded in Si-rich silicon nitride matrices Be´chir Rezgui , Abel Sibai, Tetyana Nychyporuk, Mustapha Lemiti, Georges Bre´mond Institut des Nanotechnologies de Lyon (INL), CNRS UMR-5270, Universite´ de Lyon, INSA LYON, 7 avenue Jean Capelle, 69621 Villeurbanne, France
a r t i c l e in f o
a b s t r a c t
Available online 7 May 2009
Silicon nitride (SiNx) films were prepared with a gas mixture of SiH4 and NH3 on Si wafers using the plasma-enhanced chemical vapor deposition (PECVD) method. High-resolution transmission electron microscopy and infrared absorption have been used to reveal the existence of the Si quantum dots (Si QDs) and to determine the chemical composition of the silicon nitride layers. The optical properties of these structures were studied by photoluminescence (PL) spectroscopy and indicate that emission mechanisms are dominated by confined excitons within Si QDs. The peak position of PL could be controlled in the wavelength range from 1.5 to 2.2 eV by adjusting the flow rates of ammonia and silane gases. Absorbance spectra obtained in the transmission mode reveal optical absorption from Si QDs, which is in good correlation with PL properties. These results have implications for future nanomaterial deposition controlling and device applications. & 2009 Elsevier B.V. All rights reserved.
Keywords: Silicon quantum dots Si-rich silicon nitride Photoluminescence Optical absorption
1. Introduction Silicon nanostructures have been widely studied due to their interesting luminescent properties [1,2]. Considerable effort has been spent in investigating the structural, electronic and optical properties of these nanostructures [3–5]. Particularly, lowdimensional composite structures, which consist of silicon quantum dots (Si QDs) embedded in silicon oxide matrix, have drawn much attention due to their property of providing robust, well-passivated Si QDs compound films which are compatible with the standard silicon processing technologies [6,7]. Recently, enormous attention has been paid to the light emission from silicon nitride (SiNx) films with silicon nanoparticles, which show relatively lower barrier (2.0 eV) for carriers than those of silicon oxide [8–10]. This opens up the possibility of fabricating siliconbased optoelectronic devices such as tandem solar cells [11,12]. The fabrication of silicon nanoparticles can be performed by many techniques which vary from silane decomposition in the gas phase to deposition through laser ablation or aerosol techniques. In these methods both the density and size of the nanoparticles can be controlled. However, these techniques are more easily found in research laboratories and are difficult to integrate with current silicon technology. In contrast, sputtering or chemical vapor deposition (CVD) techniques are more favorable although somewhat less controllable. In the present work, we will mainly refer to plasma-enhanced chemical vapor deposition (PECVD)
Corresponding author. Tel.: +33 4 72438066; fax: +33 4 72438531.
E-mail address: [email protected] (B. Rezgui). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.043
formed nanoparticles as a key example to study the properties of Si QDs. We have investigated the light emission features of Si-inSiNx compound film to clarify the luminescence mechanism of these composite structures by combining optical and structural characterization.
2. Experimental Si-rich silicon nitride films have been grown by low frequency (440 kHz) plasma-enhanced chemical vapor deposition, using pure silane (SiH4) and ammonia (NH3) as reactant gases. The total gas flow was maintained at a constant rate at 800 sccm. The ratio of NH3/SiH4 flows were varied from 2 to 10 by varying simultaneously the flow rates of SiH4 and NH3 to get different Si concentrations in films. The layers were deposited at 370 1C on 0.3-mm-thick n-type (1 0 0) Si substrates. The typical thickness of a silicon nitride film that contains Si QDs was 40 nm. The plasma power and the chamber pressure were fixed at 1000 W and 1500 m Torr, respectively. For the structural characterization, a high-resolution transmission electron microscopy (HRTEM) technique was used. The bonding structure of films was analyzed by means of a Fourier transform infrared (FTIR) spectrometer (Bruker 80). The optical absorption of films was deduced from UV-visible transmittance measurements using a Perkin Elmer spectrometer. Room temperature photoluminescence (PL) experiments were performed using a 458 nm Ar-ion laser and a single grating monochromator coupled to a GaAs cathode photomultiplier (Hamamatsu H5701-50)
ARTICLE IN PRESS B. Rezgui et al. / Journal of Luminescence 129 (2009) 1744–1746
for light detection. All the spectra were corrected for the spectral response of the system.
3.0
3. Results and discussion
2.5
1745
R=4
2.0 1.5 R=6
1.0 R = 10
0.5
R=8
0.0 1.4
1.6
1.8 2.0 Energy (eV)
2.5
2.6
1.98 eV
2.0
1.72 eV
1.5 1.0
1.54 eV
0.5 0.0 1.4
1.6
1.8 2.0 Energy (eV)
2.2
2.4
2.6
Fig. 2. (a) Room-temperature PL spectra of Si QDs in silicon nitride with different gas flow ratios. (b) PL spectrum of the sample with R ¼ 4, together with fitting curves.
0.08 0.06
R=2 R=4 R=6 R=8 R = 10
Absorbance (a.u)
0.10
Absorbance (a.u)
a-Si QDs
2.4
Sample R = 4
Si-N
Silicon substrate
2.2
3.0
PL intensity (a.u)
By controlling the gas flow ratio R (R ¼ NH3/SiH4), samples with various excess silicon contents were obtained. After deposition, formation of Si nanoparticles is achieved. Fig. 1 shows a highresolution transmission electron microscopy (TEM) image of the as-grown sample with gas flow ratio R ¼ 6. HRTEM micrograph of silicon nanoclusters reveals that they are essentially amorphous. A statistical analysis of HRTEM image in several zones of this sample was made to investigate more precisely the size distribution of Si QDs inclusions. The results of this analysis showed that the diameters of Si QDs vary in the range from 3 to 10 nm. Photoluminescence measurements have been performed at room temperature on the as-deposited samples. The PL emission peak energy of the composite structures was controlled by varying the gas flow ratio (NH3/SiH4). The systematic shift of the PL peak energy shown in Fig. 2a suggests that the Si QDs can be formed in silicon-rich silicon nitride (SRSN) films at low temperature when SiH4 and NH3 gases are used [13]. Two types of luminescent mechanisms, such as radiative defects in the film and the quantum confinement effect (QCE) in Si QDs have been proposed to explain the origin of light emission from these composite structures. We have decomposed the PL spectrum for R ¼ 4 into three Gaussian peaks ‘a’, ‘b’ and ‘c’ located at 1.54, 1.72 and 1.98 eV, respectively (Fig. 2b). The two peaks, ‘a’ and ‘b’, exhibit an obvious blueshift when the gas flow ratio R is increased, while the energy of the PL peak located at 1.98 eV (peak ‘c’) remains almost unchanged in all the samples. Therefore, we attribute the blueshift of PL peaks (‘a’ and ‘b’) to the QCE in Si QDs, since the higher flow ratio should correspond to the lower excess silicon content in SRSN films, leading to a smaller size for the Si QDs and consequently resulting in the higher photon energy of PL. The PL peak ‘c’ can be attributed to radiative defects in the SiNx films. In order to investigate the passivation state of the nc-Si provided by the matrix, the bonding configuration and composition of the films were investigated by Fourier transform infrared spectroscopy. The FTIR spectra for the as-grown samples are shown in Fig. 3. As can be seen, the FTIR spectra of the asdeposited samples are characteristic of silicon nitride films. The intense absorption band which continuously shifts from 825 cm1 for R ¼ 2 to 842 cm1 for R ¼ 10 (inset of Fig. 3) is assigned to the Si–N stretching mode. The two bands located at 1148 and 3330 cm1 can be attributed to the bending and stretching modes of the N–H bond. The small absorption peak located at
PL intensity (a.u)
R=2
Si-N stretching mode
Si-N 600 N-H
0.04
Si-H
700
800
1000
1100
N-H
R=2 R=4 R=6 R=8 R = 10
0.02
900
Wavenumber (cm-1)
0.00 0
500
1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm-1)
Fig. 1. HRTEM image showing silicon QDs for the as-grown sample with gas flow ratio R ¼ 6.
Fig. 3. FTIR spectra of the as-grown samples as a function of NH3/SiH4 flow ratio. The inset shows the shift of the intense band related to the stretching mode of Si–N bond.
ARTICLE IN PRESS 1746
B. Rezgui et al. / Journal of Luminescence 129 (2009) 1744–1746
4. Conclusion
Sample R = 4
Sample (R = 4)
0.8 (α.hv)
Absorbance (a.u)
1/2
In summary, the formation of silicon nanocrystals in Si-rich SiNx matrix which were grown by PECVD using SiH4 and NH3 were investigated. Structural characteristics of the films are shown by HRTEM and IR spectra. The broad PL band which was observed in the visible region from the as-grown samples could be controlled by varying the ratio of NH3/SiH4 flows. The PL spectra are very broad and can be decomposed into three peaks and it is reasonable to assume that there are different emission mechanisms responsible for these luminescence peaks, such as quantum confinement effect and radiative defects. The optical absorption contribution of Si QDs has been revealed. These results could have strong implications for the good control of such nanomaterials involved in photovoltaic applications.
0.6 opt
Eg SiNx 3.36 eV 0
0.4
1
2
3
4
5
6
7
Energy (eV)
0.2
QD's contribution
0.0 0
1
2
3
4
5
6
Energy (eV) Fig. 4. Absorption spectra of the sample with gas flow ratio R of 4. The insert shows the Tauc plot for the same sample.
482 cm1 can be associated to Si–N breathing vibrations in silicon nitride. Finally, the band at 2184 cm1 is assigned to the Si–H stretching mode [14]. Fig. 3 shows also that the N–H peaks increase and Si–H peaks decrease as the gas flow ratio R is increased. The increase in R appears to promote the dissociation of Si–H bands, resulting in an increase in silicon atoms having dangling bonds. The increase in dangling bonds of silicon atoms is believed to facilitate the creation of nucleation sites and the formation of silicon clusters in the silicon nitride film during the growth process [15,16]. Therefore, the size of the silicon clusters decreases with increasing the gas flow ratio. This result can explain the blueshift of the PL peak, since the creation of nucleation sites leads to the formation of small silicon clusters in SiNx films and, therefore, the shift of the PL peak emission to the higher energies can be attributed to the quantum confinement effect [17]. The absorbance spectrum for sample (R ¼ 4) grown onto fused silica is displayed in Fig. 4. The spectrum showed a typical feature of Si-in-SiNx structures. On the other hand, the data in the insert of Fig. 4 has been plotted as (ahu)1/2, as a function of the photon energy. As can be seen from this plot, the absorption edge of the SiN film is evaluated to be 3.36 eV. The low energy part (around 2 eV) of the absorbance spectrum clearly exhibits the contribution of the Si QDs. This is in good agreement with the measured PL energy. However, it is difficult to extract the absorption edge of the Si QDs from this spectrum. This is due to the large distribution of Si QDs size.
Acknowledgments This work was supported by the National Agency of Research (DUOSIL project) and the Rhoˆne-Alpes region (PHOSIL project). The authors acknowledge Olivier Marty at the INL laboratory for preparing the HRTEM image. References [1] V. Vinciguerra, G. Franz‘o, F. Priolo, F. Iacona, C. Spinella, J. Appl. Phys. 87 (2000) 8165. [2] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [3] R. Tsu, A. Filios, C. Lofgren, K. Dovidenko, C.G. Wang, Electrochem. Solid-State Lett. 1 (1998) 80. [4] Q. Zhang, A. Filios, C. Lofgren, R. Tsu, Physica E 8 (2000) 365. [5] Y. Cui, L.J. Lauhon, M.S. Gudiksen, J. Wang, C.M. Lieber, Appl. Phys. Lett. 78 (2001) 2214. [6] L. Khriachtchev, M. Rasanen, S. Novikov, J. Sinkkonen, Appl. Phys. Lett. 79 (2001) 1249. [7] J. Ruan, P.M. Fauchet, L. Dal Negro, M. Cazzanelli, L. Pavesi, Appl. Phys. Lett. 83 (2003) 5479. [8] T.-W. Kim, C.-H. Cho, B.-H. Kim, S.-J. Park, Appl. Phys. Lett. 88 (2006) 123102. [9] L. Dal Negro, J.H. Yi, L.C. Kimerling, Appl. Phys. Lett. 88 (2006) 183103. [10] H.L. Hao, L.K. Wu, W.Z. Shen, Appl. Phys. Lett. 92 (2008) 121922. [11] G. Conibeer, et al., Thin Solid Films (2008). [12] J. De la Torre, G. Bremond, M. Lemiti, G. Guillot, P. Mur, N. Buffet, Thin Solid Films 511–512 (2006) 163. [13] J.-F. Lelie`vre, J. De la Torre, A. Kaminski, G. Bremond, R. Monna, M. Pirot, P.-J. Ribeyron, C. Jaussaud, M. Lemiti, Thin Solid Films 511–512 (2006) 103. [14] D.V. Tsu, G. Lukovsky, M.J. Mantini, Phys. Rev. B 33 (1986) 7069. [15] N.M. Park, C.J. Choi, T.Y. Seong, S.J. Park, Phys. Rev. Lett. 86 (2001) 1355. [16] N.M. Park, S.H. Kim, G.Y. Sung, S.J. Park, Chem. Vapor Deposition 8 (2002) 254. [17] B.-H. Kim, C.-H. Cho, T.-W. Kim, N.-M. Park, G.Y. Sung, S.-J. Park, Appl. Phys. Lett. 86 (2005) 091908.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and optical absorption properties of silicon quantum dots embedded in Si-rich silicon nitride matrices Be´chir Rezgui , Abel Sibai, Tetyana Nychyporuk, Mustapha Lemiti, Georges Bre´mond Institut des Nanotechnologies de Lyon (INL), CNRS UMR-5270, Universite´ de Lyon, INSA LYON, 7 avenue Jean Capelle, 69621 Villeurbanne, France
a r t i c l e in f o
a b s t r a c t
Available online 7 May 2009
Silicon nitride (SiNx) films were prepared with a gas mixture of SiH4 and NH3 on Si wafers using the plasma-enhanced chemical vapor deposition (PECVD) method. High-resolution transmission electron microscopy and infrared absorption have been used to reveal the existence of the Si quantum dots (Si QDs) and to determine the chemical composition of the silicon nitride layers. The optical properties of these structures were studied by photoluminescence (PL) spectroscopy and indicate that emission mechanisms are dominated by confined excitons within Si QDs. The peak position of PL could be controlled in the wavelength range from 1.5 to 2.2 eV by adjusting the flow rates of ammonia and silane gases. Absorbance spectra obtained in the transmission mode reveal optical absorption from Si QDs, which is in good correlation with PL properties. These results have implications for future nanomaterial deposition controlling and device applications. & 2009 Elsevier B.V. All rights reserved.
Keywords: Silicon quantum dots Si-rich silicon nitride Photoluminescence Optical absorption
1. Introduction Silicon nanostructures have been widely studied due to their interesting luminescent properties [1,2]. Considerable effort has been spent in investigating the structural, electronic and optical properties of these nanostructures [3–5]. Particularly, lowdimensional composite structures, which consist of silicon quantum dots (Si QDs) embedded in silicon oxide matrix, have drawn much attention due to their property of providing robust, well-passivated Si QDs compound films which are compatible with the standard silicon processing technologies [6,7]. Recently, enormous attention has been paid to the light emission from silicon nitride (SiNx) films with silicon nanoparticles, which show relatively lower barrier (2.0 eV) for carriers than those of silicon oxide [8–10]. This opens up the possibility of fabricating siliconbased optoelectronic devices such as tandem solar cells [11,12]. The fabrication of silicon nanoparticles can be performed by many techniques which vary from silane decomposition in the gas phase to deposition through laser ablation or aerosol techniques. In these methods both the density and size of the nanoparticles can be controlled. However, these techniques are more easily found in research laboratories and are difficult to integrate with current silicon technology. In contrast, sputtering or chemical vapor deposition (CVD) techniques are more favorable although somewhat less controllable. In the present work, we will mainly refer to plasma-enhanced chemical vapor deposition (PECVD)
Corresponding author. Tel.: +33 4 72438066; fax: +33 4 72438531.
E-mail address: [email protected] (B. Rezgui). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.043
formed nanoparticles as a key example to study the properties of Si QDs. We have investigated the light emission features of Si-inSiNx compound film to clarify the luminescence mechanism of these composite structures by combining optical and structural characterization.
2. Experimental Si-rich silicon nitride films have been grown by low frequency (440 kHz) plasma-enhanced chemical vapor deposition, using pure silane (SiH4) and ammonia (NH3) as reactant gases. The total gas flow was maintained at a constant rate at 800 sccm. The ratio of NH3/SiH4 flows were varied from 2 to 10 by varying simultaneously the flow rates of SiH4 and NH3 to get different Si concentrations in films. The layers were deposited at 370 1C on 0.3-mm-thick n-type (1 0 0) Si substrates. The typical thickness of a silicon nitride film that contains Si QDs was 40 nm. The plasma power and the chamber pressure were fixed at 1000 W and 1500 m Torr, respectively. For the structural characterization, a high-resolution transmission electron microscopy (HRTEM) technique was used. The bonding structure of films was analyzed by means of a Fourier transform infrared (FTIR) spectrometer (Bruker 80). The optical absorption of films was deduced from UV-visible transmittance measurements using a Perkin Elmer spectrometer. Room temperature photoluminescence (PL) experiments were performed using a 458 nm Ar-ion laser and a single grating monochromator coupled to a GaAs cathode photomultiplier (Hamamatsu H5701-50)
ARTICLE IN PRESS B. Rezgui et al. / Journal of Luminescence 129 (2009) 1744–1746
for light detection. All the spectra were corrected for the spectral response of the system.
3.0
3. Results and discussion
2.5
1745
R=4
2.0 1.5 R=6
1.0 R = 10
0.5
R=8
0.0 1.4
1.6
1.8 2.0 Energy (eV)
2.5
2.6
1.98 eV
2.0
1.72 eV
1.5 1.0
1.54 eV
0.5 0.0 1.4
1.6
1.8 2.0 Energy (eV)
2.2
2.4
2.6
Fig. 2. (a) Room-temperature PL spectra of Si QDs in silicon nitride with different gas flow ratios. (b) PL spectrum of the sample with R ¼ 4, together with fitting curves.
0.08 0.06
R=2 R=4 R=6 R=8 R = 10
Absorbance (a.u)
0.10
Absorbance (a.u)
a-Si QDs
2.4
Sample R = 4
Si-N
Silicon substrate
2.2
3.0
PL intensity (a.u)
By controlling the gas flow ratio R (R ¼ NH3/SiH4), samples with various excess silicon contents were obtained. After deposition, formation of Si nanoparticles is achieved. Fig. 1 shows a highresolution transmission electron microscopy (TEM) image of the as-grown sample with gas flow ratio R ¼ 6. HRTEM micrograph of silicon nanoclusters reveals that they are essentially amorphous. A statistical analysis of HRTEM image in several zones of this sample was made to investigate more precisely the size distribution of Si QDs inclusions. The results of this analysis showed that the diameters of Si QDs vary in the range from 3 to 10 nm. Photoluminescence measurements have been performed at room temperature on the as-deposited samples. The PL emission peak energy of the composite structures was controlled by varying the gas flow ratio (NH3/SiH4). The systematic shift of the PL peak energy shown in Fig. 2a suggests that the Si QDs can be formed in silicon-rich silicon nitride (SRSN) films at low temperature when SiH4 and NH3 gases are used [13]. Two types of luminescent mechanisms, such as radiative defects in the film and the quantum confinement effect (QCE) in Si QDs have been proposed to explain the origin of light emission from these composite structures. We have decomposed the PL spectrum for R ¼ 4 into three Gaussian peaks ‘a’, ‘b’ and ‘c’ located at 1.54, 1.72 and 1.98 eV, respectively (Fig. 2b). The two peaks, ‘a’ and ‘b’, exhibit an obvious blueshift when the gas flow ratio R is increased, while the energy of the PL peak located at 1.98 eV (peak ‘c’) remains almost unchanged in all the samples. Therefore, we attribute the blueshift of PL peaks (‘a’ and ‘b’) to the QCE in Si QDs, since the higher flow ratio should correspond to the lower excess silicon content in SRSN films, leading to a smaller size for the Si QDs and consequently resulting in the higher photon energy of PL. The PL peak ‘c’ can be attributed to radiative defects in the SiNx films. In order to investigate the passivation state of the nc-Si provided by the matrix, the bonding configuration and composition of the films were investigated by Fourier transform infrared spectroscopy. The FTIR spectra for the as-grown samples are shown in Fig. 3. As can be seen, the FTIR spectra of the asdeposited samples are characteristic of silicon nitride films. The intense absorption band which continuously shifts from 825 cm1 for R ¼ 2 to 842 cm1 for R ¼ 10 (inset of Fig. 3) is assigned to the Si–N stretching mode. The two bands located at 1148 and 3330 cm1 can be attributed to the bending and stretching modes of the N–H bond. The small absorption peak located at
PL intensity (a.u)
R=2
Si-N stretching mode
Si-N 600 N-H
0.04
Si-H
700
800
1000
1100
N-H
R=2 R=4 R=6 R=8 R = 10
0.02
900
Wavenumber (cm-1)
0.00 0
500
1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm-1)
Fig. 1. HRTEM image showing silicon QDs for the as-grown sample with gas flow ratio R ¼ 6.
Fig. 3. FTIR spectra of the as-grown samples as a function of NH3/SiH4 flow ratio. The inset shows the shift of the intense band related to the stretching mode of Si–N bond.
ARTICLE IN PRESS 1746
B. Rezgui et al. / Journal of Luminescence 129 (2009) 1744–1746
4. Conclusion
Sample R = 4
Sample (R = 4)
0.8 (α.hv)
Absorbance (a.u)
1/2
In summary, the formation of silicon nanocrystals in Si-rich SiNx matrix which were grown by PECVD using SiH4 and NH3 were investigated. Structural characteristics of the films are shown by HRTEM and IR spectra. The broad PL band which was observed in the visible region from the as-grown samples could be controlled by varying the ratio of NH3/SiH4 flows. The PL spectra are very broad and can be decomposed into three peaks and it is reasonable to assume that there are different emission mechanisms responsible for these luminescence peaks, such as quantum confinement effect and radiative defects. The optical absorption contribution of Si QDs has been revealed. These results could have strong implications for the good control of such nanomaterials involved in photovoltaic applications.
0.6 opt
Eg SiNx 3.36 eV 0
0.4
1
2
3
4
5
6
7
Energy (eV)
0.2
QD's contribution
0.0 0
1
2
3
4
5
6
Energy (eV) Fig. 4. Absorption spectra of the sample with gas flow ratio R of 4. The insert shows the Tauc plot for the same sample.
482 cm1 can be associated to Si–N breathing vibrations in silicon nitride. Finally, the band at 2184 cm1 is assigned to the Si–H stretching mode [14]. Fig. 3 shows also that the N–H peaks increase and Si–H peaks decrease as the gas flow ratio R is increased. The increase in R appears to promote the dissociation of Si–H bands, resulting in an increase in silicon atoms having dangling bonds. The increase in dangling bonds of silicon atoms is believed to facilitate the creation of nucleation sites and the formation of silicon clusters in the silicon nitride film during the growth process [15,16]. Therefore, the size of the silicon clusters decreases with increasing the gas flow ratio. This result can explain the blueshift of the PL peak, since the creation of nucleation sites leads to the formation of small silicon clusters in SiNx films and, therefore, the shift of the PL peak emission to the higher energies can be attributed to the quantum confinement effect [17]. The absorbance spectrum for sample (R ¼ 4) grown onto fused silica is displayed in Fig. 4. The spectrum showed a typical feature of Si-in-SiNx structures. On the other hand, the data in the insert of Fig. 4 has been plotted as (ahu)1/2, as a function of the photon energy. As can be seen from this plot, the absorption edge of the SiN film is evaluated to be 3.36 eV. The low energy part (around 2 eV) of the absorbance spectrum clearly exhibits the contribution of the Si QDs. This is in good agreement with the measured PL energy. However, it is difficult to extract the absorption edge of the Si QDs from this spectrum. This is due to the large distribution of Si QDs size.
Acknowledgments This work was supported by the National Agency of Research (DUOSIL project) and the Rhoˆne-Alpes region (PHOSIL project). The authors acknowledge Olivier Marty at the INL laboratory for preparing the HRTEM image. References [1] V. Vinciguerra, G. Franz‘o, F. Priolo, F. Iacona, C. Spinella, J. Appl. Phys. 87 (2000) 8165. [2] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [3] R. Tsu, A. Filios, C. Lofgren, K. Dovidenko, C.G. Wang, Electrochem. Solid-State Lett. 1 (1998) 80. [4] Q. Zhang, A. Filios, C. Lofgren, R. Tsu, Physica E 8 (2000) 365. [5] Y. Cui, L.J. Lauhon, M.S. Gudiksen, J. Wang, C.M. Lieber, Appl. Phys. Lett. 78 (2001) 2214. [6] L. Khriachtchev, M. Rasanen, S. Novikov, J. Sinkkonen, Appl. Phys. Lett. 79 (2001) 1249. [7] J. Ruan, P.M. Fauchet, L. Dal Negro, M. Cazzanelli, L. Pavesi, Appl. Phys. Lett. 83 (2003) 5479. [8] T.-W. Kim, C.-H. Cho, B.-H. Kim, S.-J. Park, Appl. Phys. Lett. 88 (2006) 123102. [9] L. Dal Negro, J.H. Yi, L.C. Kimerling, Appl. Phys. Lett. 88 (2006) 183103. [10] H.L. Hao, L.K. Wu, W.Z. Shen, Appl. Phys. Lett. 92 (2008) 121922. [11] G. Conibeer, et al., Thin Solid Films (2008). [12] J. De la Torre, G. Bremond, M. Lemiti, G. Guillot, P. Mur, N. Buffet, Thin Solid Films 511–512 (2006) 163. [13] J.-F. Lelie`vre, J. De la Torre, A. Kaminski, G. Bremond, R. Monna, M. Pirot, P.-J. Ribeyron, C. Jaussaud, M. Lemiti, Thin Solid Films 511–512 (2006) 103. [14] D.V. Tsu, G. Lukovsky, M.J. Mantini, Phys. Rev. B 33 (1986) 7069. [15] N.M. Park, C.J. Choi, T.Y. Seong, S.J. Park, Phys. Rev. Lett. 86 (2001) 1355. [16] N.M. Park, S.H. Kim, G.Y. Sung, S.J. Park, Chem. Vapor Deposition 8 (2002) 254. [17] B.-H. Kim, C.-H. Cho, T.-W. Kim, N.-M. Park, G.Y. Sung, S.-J. Park, Appl. Phys. Lett. 86 (2005) 091908.