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Structural and optical properties of silicon quantum dots in silicon nitride grown in situ by PECVD using different gas precursors
Materials Science and Engineering B 159–160 (2009) 74–76
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Structural and optical properties of silicon quantum dots in silicon nitride grown in situ by PECVD using different gas precursors Lucia V. Mercaldo ∗ , Paola Delli Veneri, Emilia Esposito, Ettore Massera, Iurie Usatii, Carlo Privato ENEA - Portici Research Center - Piazzale E. Fermi, 80055 Portici (Napoli), Italy
a r t i c l e
i n f o
Article history: Received 9 May 2008 Received in revised form 21 November 2008 Accepted 25 November 2008 Keywords: Silicon quantum dots Silicon nitride Third generation photovoltaics
a b s t r a c t A low temperature in situ approach for growing silicon nanostructures in silicon nitride is investigated, as a powerful method of implementing third generation photovoltaic concepts within classical thin film silicon solar cell architectures on low cost substrates. Evidence of spontaneous aggregation of silicon quantum dots in silicon nitride films deposited by plasma enhanced chemical vapour deposition (PECVD) at low temperature (300 ◦ C) is reported. Two different types of samples are studied, grown using two gas mixtures, composed of silane and nitrogen with and without ammonia. The film microstructure is analysed through Raman spectroscopy. Visible photoluminescence (PL) is observed in all cases, and tuning of PL emission is demonstrated by adjusting the gas flow rates. As an effect of the extra hydrogen available through the dissociation of NH3 , much stronger PL is observed on samples grown with ammonia. Similar optical absorption spectra are found for the two types of samples, with the rising edge dominated by the absorption in Si nanoclusters. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The efficient light emission from silicon nanostructures has raised wide interest in view of adding optical functionality to silicon microelectronic chips [1]. The possibility of engineering new materials with tailored band gap, which absorb photons in a dedicated energy range, by using the quantum confinement effect, can be exploited also in the photovoltaic field within third generation tandem solar cells [2]. In this case Si quantum dots (QDs) would have to be closely spaced so that the overlap of the confined energy levels would form minibands. As a consequence of their luminescence properties, Si nanostructures have also potentialities for achieving down-conversion of high energetic photons, which are otherwise thermalised and wasted, to lower energy photons, better used by the solar cells [3,4]. In this work a low temperature in situ approach for growing silicon nanostructures in silicon nitride is investigated [4–7], as a powerful alternative to the typical techniques that require postdeposition high temperature treatments (usually at 1100 ◦ C) [2]. In principle, such method is compatible with use of low cost substrates and classical thin film silicon solar cell structures. Two gas mixtures have been used for realizing silicon rich silicon nitride films. Photo-
∗ Corresponding author. Tel.: +39 081 7723217 fax: +39 081 7723344. E-mail address: [email protected] (L.V. Mercaldo). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.11.049
luminescence phenomena, optical absorption and microstructure have been investigated for the two types of samples. In both cases evidence of quantum confinement effect is revealed. 2. Experimental Silicon nitride films have been grown by plasma enhanced chemical vapour deposition (PECVD) at 13.56 MHz on glass, quartz and c-Si substrates using two different gas mixtures: silane and nitrogen in one case and silane and ammonia diluted in nitrogen in the other. The two types of samples will be called nitrogentype and ammonia-type (or N2 -type and NH3 -type) respectively in this paper. The electrode configuration consists of a 13 cm × 13 cm powered electrode and a 11 cm × 11 cm substrate carrier as the grounded electrode. The inter-electrode distance is kept at 17 mm, and the gas supply is a simple cross-flow geometry. The effect of gas flow rates has been explored in low power regime (0.3 W), at substrate temperature of 300 ◦ C with pressure fixed at 0.5 Torr. For the N2 -type samples a 0.2 sccm SiH4 flow rate has been used, whereas the N2 flow rate has been varied in the range of 15–180 sccm. In the other case a fixed flow rate for SiH4 and N2 (2 and 40 sccm, respectively), and a variable flow rate for NH3 in between 1.5 and 9 sccm have been used. Depending on the deposition conditions, growth rate values in between 0.4 and 0.7 Å/s have been obtained for the samples grown with N2 . Larger values, in between 1.2 and 1.5 Å/s have been obtained in the other case, due to the larger silane flow rate and the use of ammonia, easy to
L.V. Mercaldo et al. / Materials Science and Engineering B 159–160 (2009) 74–76
Fig. 1. PL spectra for NH3 -type samples grown with different NH3 flow rates (reported in the figure).
dissociate. The thickness of the films, evaluated with a step profiler, has been kept around 200 nm. The refraction index of the films has been evaluated through ellipsometry measurements. Absorption spectra have been extracted from optical reflectance and transmittance measured with a spectrophotometer. Raman spectra have been obtained using the 514 nm line of an Ar+ laser. Photoluminescence (PL) emission at room temperature has been excited by using the 325 nm line of a He–Cd laser source. The photoluminescence spectra were collected by an optic guide and registered in the visible range by a calibrated CCD spectrometer. PL and ellipsometry measurements have been performed on the samples grown on c-Si substrates, while the films deposited on glass or quartz plates have been devoted to structural characterization and UV–vis absorption spectroscopy. 3. Results and discussions For each type of samples (nitrogen-type and ammonia-type) PL and absorption properties have been analysed when changing N2 or NH3 flow rate. The refraction index at 633 nm of both types of samples increases from about 1.9, typical of stoichiometric PECVD grown hydrogenated silicon nitride, to 2.3, characteristic of silicon rich silicon nitride [8], when N2 or NH3 flow rate is reduced. Such variation is an evidence of increasing silicon content in the films. All the samples show clear room temperature PL emission. Tuning of PL emission with controlled changes of N2 flow rate for N2 -type samples is reported elsewhere [9]. The same evolution of the PL spectra is reproduced also with NH3 -type samples (Fig. 1). In this case much stronger intensity (roughly three times larger) is registered, so that the effect is visible in most cases under UV lamp by unaided eye. For both types of samples the PL peak shifts from the blue to the red region of the visible spectrum with larger refraction index, and then larger silicon content in the layers. Also the PL intensity changes, showing a maximum for some intermediate value of the varied deposition parameter. We ascribe the PL properties of our samples to the presence of silicon QDs spontaneously formed in the nitride matrix, through an inhomogeneous growth process favoured by the low deposition rate, since the freely controllable PL peak position should rule out an interpretation in terms of radiative defect centers, characterized by well defined energy levels [10]. Moreover, direct evidence of presence of quantum dots by TEM analysis in samples obtained with similar growth conditions has been reported in the literature [6,7]. It has been claimed that the inhomogeneous growth, in the low rate regime, is promoted by the formation of a large number of Si dangling bonds acting as nucleation sites: The increase of N2 or NH3 flow rates would influence size and density of Si QDs by enhancing the creation of such sites, and then favouring the realization of densely packed but smaller silicon clusters around them [11]. The PL
75
Fig. 2. PL peak position for the N2 (Ref. [9]) and NH3 -type samples grown with different gas flow rates.
peak blueshift with decreasing silicon content in the layers, attained with an increase of N2 or NH3 gas flow rates (Ref. [9], Figs. 1 and 2) in this picture is the direct evidence of the enlargement of the energy gap of the Si dots as their size decreases. On the other hand, the variation of dot density as the deposition conditions are varied (as an effect of varied number of nucleation sites) can furnish an explanation to the observed change of the PL peak intensity. Less intense luminescence is indeed found in Fig. 1 with the samples grown with low NH3 flow rate in the mixture (condition for having few nucleation sites during the film growth). Larger intensities are found for intermediate NH3 flow rates, corresponding to an optimal deposition regime for realizing high Si QDs density, as already reported by Lelievre et al. [4]. For high flow rates the response decreases again, most probably because with very low silane partial pressure and numerous Si dangling bonds there are not enough Si atoms that can aggregate around all the nucleation sites, thus resulting in a lower dot density. It is interesting to notice that with both types of samples the same region of the visible spectrum is spanned by the PL peak, when N2 or NH3 flow rate is varied (Fig. 2). This means that presumably the same energy gap enlargement has been reproduced using the two gas mixtures, which could mean same dot sizes only if the structural properties of the Si QDs is the same in the two cases. Crystalline structure, favoured by the extra hydrogen dissociated from the NH3 gas, has been reported for samples grown with ammonia [7]. With SiH4 and N2 as gas precursor mixture, spontaneous formation of crystalline Si dots has been reported for deposition rates below 0.3 Å/s [12], otherwise amorphous structure is obtained [6]. We have employed Raman spectroscopy to obtain information about the presence of silicon nanocrystals within the silicon nitride matrix (Fig. 3). A broad asymmetric band around the same wavenumbers is observed in both cases. The spectra can be deconvoluted with a contribution around 450 cm−1 , assigned to the asymmetric Si–N bond stretching mode, and another contribution slightly above 480 cm−1 , due to localized phonon modes in amorphous silicon [13]. Possible presence of amorphous dots can then be deduced in both cases, and as a consequence similar dot sizes should be expected. Comparing our PL data with Ref. [6] we can estimate values in between 1.4 (for samples with blue PL emission) and 3 nm (for samples with red PL emission). While the PL evolution is similar for both types of samples, the intensity is much stronger in the ammonia-type case. The enhanced luminescence could be related to larger Si QDs density and/or reduced amount of nonradiative defect centers in the material, in particular at the Si QDs surface. Such effect, already observed on samples grown in similar conditions, has been attributed to more efficient passivation of nonradiative defects at the dot-matrix interface, thanks to the extra hydrogen available during the growth
76
L.V. Mercaldo et al. / Materials Science and Engineering B 159–160 (2009) 74–76
in all cases (the average difference is E02 − EPL ∼ 250 meV). Theoretically, in case of phase separation into distinct regions of Si and Si3 N4 versus a random bonding model approach, where Si–Si and Si–N bonds are homogeneously dispersed throughout the alloy, the presence of strongly absorbing Si regions enhances the low energy optical absorption [16]. Our result can then be interpreted as a proof of presence of Si nanostructures. Moreover, as from the structural characterization, also from optical measurements no evidence of contribution from aggregates with crystalline structure can be inferred for the ammonia-type samples. 4. Conclusions
Fig. 3. Typical Raman spectra of N2 -type and NH3 -type samples with similar PL peak energy.
Fig. 4. Absorption coefficient for N2 (dashed lines) and NH3 -type samples (solid lines) characterized by similar refraction index, n, and PL peak energy, EPL . The absorption curves of a PECVD grown a-Si:H film, and tabulated c-Si and Si3 N4 [14] are also shown for reference (dotted lines).
process when NH3 is used in the gas mixture [7,14]. Our result seems to confirm this interpretation, however TEM analysis is planned as a future step to extract information about the dot density, together with a direct measurement of the dot sizes. The two types of samples are characterized also by similar absorption properties. In Fig. 4 we report two absorption spectra for each type of samples, comparing two cases characterized by similar refraction index (2.0 and 2.3) and photoluminescence response (PL peak energy, EPL , around 1.90 and 2.35 eV). Experimental data are shown together with the absorption curves of a 200 nm thick PECVD grown a-Si:H film and tabulated c-Si and Si3 N4 [15], shown for reference. The curves look almost the same, with a behaviour in between the a-Si:H and the tabulated Si3 N4 curves. With n ∼ 2.0, not far from stoichiometric case (n ∼ 1.9 for hydrogenated material), the absorption spectra are already very different from the Si3 N4 curve. The main difference is in the low energy side of the spectrum, where the curve seems to be pulled toward the a-Si:H spectrum. Using E02 , the energy value at which the absorption coefficient is equal to 102 cm−1 , as an estimate of the absorption rising edge of the material, a value very close to the PL peak energy is obtained
Silicon rich silicon nitride films have been deposited by plasma enhanced chemical vapour deposition (PECVD) at low temperature (300 ◦ C) using two different gas mixtures, composed of silane and nitrogen with and without ammonia. Visible PL is observed in all cases, and tuning of PL emission is demonstrated by adjusting the gas flow rates. Shift and intensity evolution of the PL spectra are ascribed to size and density change of silicon quantum dots spontaneously formed in the silicon nitride matrix, while the deposition parameters are varied. Presence of amorphous silicon aggregates can be inferred through Raman spectroscopy for both types of samples. Also optical absorption spectra are influenced by the inhomogeneous structure of the material with the strongly absorbing silicon nanostructures dominating at low energy and determining the rising edge. The main difference between the two types of samples is the much stronger PL observed for ammonia-type layers, most probably due to a better passivation of nonradiative defect centers at the dot-matrix interface. Moreover, in this case the films are grown at an interestingly higher deposition rate. Such results contribute to give strength to the low temperature in situ method for growing Si nanostructures, with some advantages when using silane, nitrogen and ammonia as process gases. We want to stress that the absence of a post-deposition annealing makes this growth technique very powerful when thinking at the application of nanostructured materials within multilayered devices, such as innovative solar cells. In particular third generation concepts would be feasible also on low cost substrates. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
L. Pavesi, et al., Nature 408 (2000) 440, and references therein. G. Conibeer, et al., Thin Solid Films 511–512 (2006) 654. V. Svrcek, et al., Thin Solid Films 451–452 (2004) 384. J.F. Lelièvre, et al., Proc. 21st European PV Solar Energy Conf., Dresden, 2006, p. 1456. J.F. Lelièvre, et al., Thin Solid Films 511–512 (2006) 103. N.-M. Park, et al., Phys. Rev. Lett. 86 (7) (2001) 1355. T.-W. Kim, et al., Appl. Phys. Lett. 88 (2006) 123102. E. Dehan, et al., Thin Solid Films 266 (1995) 14. L.V. Mercaldo, et al., Mater. Sci. Eng. B 159–160 (2009) 77. S.V. Deshpande, J. Appl. Phys. 77 (1995) 6534. N.-M. Park, et al., Chem. Vap. Depos. 8 (6) (2002) 254. T.-Y. Kim, et al., Appl. Phys. Lett. 85 (2005) 5355. V.A. Volodin, et al., Appl. Phys. Lett. 73 (1998) 1212. B.-H. Kim, et al., Appl. Phys. Lett. 86 (2005) 091908. E.D. Palik (Ed.), Handbook of Optical Constants of Solids, vol. I, Academic Press, 1985. Z. Yin, F.W. Smith, Phys. Rev. B 42 (1990) 3658.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Structural and optical properties of silicon quantum dots in silicon nitride grown in situ by PECVD using different gas precursors Lucia V. Mercaldo ∗ , Paola Delli Veneri, Emilia Esposito, Ettore Massera, Iurie Usatii, Carlo Privato ENEA - Portici Research Center - Piazzale E. Fermi, 80055 Portici (Napoli), Italy
a r t i c l e
i n f o
Article history: Received 9 May 2008 Received in revised form 21 November 2008 Accepted 25 November 2008 Keywords: Silicon quantum dots Silicon nitride Third generation photovoltaics
a b s t r a c t A low temperature in situ approach for growing silicon nanostructures in silicon nitride is investigated, as a powerful method of implementing third generation photovoltaic concepts within classical thin film silicon solar cell architectures on low cost substrates. Evidence of spontaneous aggregation of silicon quantum dots in silicon nitride films deposited by plasma enhanced chemical vapour deposition (PECVD) at low temperature (300 ◦ C) is reported. Two different types of samples are studied, grown using two gas mixtures, composed of silane and nitrogen with and without ammonia. The film microstructure is analysed through Raman spectroscopy. Visible photoluminescence (PL) is observed in all cases, and tuning of PL emission is demonstrated by adjusting the gas flow rates. As an effect of the extra hydrogen available through the dissociation of NH3 , much stronger PL is observed on samples grown with ammonia. Similar optical absorption spectra are found for the two types of samples, with the rising edge dominated by the absorption in Si nanoclusters. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The efficient light emission from silicon nanostructures has raised wide interest in view of adding optical functionality to silicon microelectronic chips [1]. The possibility of engineering new materials with tailored band gap, which absorb photons in a dedicated energy range, by using the quantum confinement effect, can be exploited also in the photovoltaic field within third generation tandem solar cells [2]. In this case Si quantum dots (QDs) would have to be closely spaced so that the overlap of the confined energy levels would form minibands. As a consequence of their luminescence properties, Si nanostructures have also potentialities for achieving down-conversion of high energetic photons, which are otherwise thermalised and wasted, to lower energy photons, better used by the solar cells [3,4]. In this work a low temperature in situ approach for growing silicon nanostructures in silicon nitride is investigated [4–7], as a powerful alternative to the typical techniques that require postdeposition high temperature treatments (usually at 1100 ◦ C) [2]. In principle, such method is compatible with use of low cost substrates and classical thin film silicon solar cell structures. Two gas mixtures have been used for realizing silicon rich silicon nitride films. Photo-
∗ Corresponding author. Tel.: +39 081 7723217 fax: +39 081 7723344. E-mail address: [email protected] (L.V. Mercaldo). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.11.049
luminescence phenomena, optical absorption and microstructure have been investigated for the two types of samples. In both cases evidence of quantum confinement effect is revealed. 2. Experimental Silicon nitride films have been grown by plasma enhanced chemical vapour deposition (PECVD) at 13.56 MHz on glass, quartz and c-Si substrates using two different gas mixtures: silane and nitrogen in one case and silane and ammonia diluted in nitrogen in the other. The two types of samples will be called nitrogentype and ammonia-type (or N2 -type and NH3 -type) respectively in this paper. The electrode configuration consists of a 13 cm × 13 cm powered electrode and a 11 cm × 11 cm substrate carrier as the grounded electrode. The inter-electrode distance is kept at 17 mm, and the gas supply is a simple cross-flow geometry. The effect of gas flow rates has been explored in low power regime (0.3 W), at substrate temperature of 300 ◦ C with pressure fixed at 0.5 Torr. For the N2 -type samples a 0.2 sccm SiH4 flow rate has been used, whereas the N2 flow rate has been varied in the range of 15–180 sccm. In the other case a fixed flow rate for SiH4 and N2 (2 and 40 sccm, respectively), and a variable flow rate for NH3 in between 1.5 and 9 sccm have been used. Depending on the deposition conditions, growth rate values in between 0.4 and 0.7 Å/s have been obtained for the samples grown with N2 . Larger values, in between 1.2 and 1.5 Å/s have been obtained in the other case, due to the larger silane flow rate and the use of ammonia, easy to
L.V. Mercaldo et al. / Materials Science and Engineering B 159–160 (2009) 74–76
Fig. 1. PL spectra for NH3 -type samples grown with different NH3 flow rates (reported in the figure).
dissociate. The thickness of the films, evaluated with a step profiler, has been kept around 200 nm. The refraction index of the films has been evaluated through ellipsometry measurements. Absorption spectra have been extracted from optical reflectance and transmittance measured with a spectrophotometer. Raman spectra have been obtained using the 514 nm line of an Ar+ laser. Photoluminescence (PL) emission at room temperature has been excited by using the 325 nm line of a He–Cd laser source. The photoluminescence spectra were collected by an optic guide and registered in the visible range by a calibrated CCD spectrometer. PL and ellipsometry measurements have been performed on the samples grown on c-Si substrates, while the films deposited on glass or quartz plates have been devoted to structural characterization and UV–vis absorption spectroscopy. 3. Results and discussions For each type of samples (nitrogen-type and ammonia-type) PL and absorption properties have been analysed when changing N2 or NH3 flow rate. The refraction index at 633 nm of both types of samples increases from about 1.9, typical of stoichiometric PECVD grown hydrogenated silicon nitride, to 2.3, characteristic of silicon rich silicon nitride [8], when N2 or NH3 flow rate is reduced. Such variation is an evidence of increasing silicon content in the films. All the samples show clear room temperature PL emission. Tuning of PL emission with controlled changes of N2 flow rate for N2 -type samples is reported elsewhere [9]. The same evolution of the PL spectra is reproduced also with NH3 -type samples (Fig. 1). In this case much stronger intensity (roughly three times larger) is registered, so that the effect is visible in most cases under UV lamp by unaided eye. For both types of samples the PL peak shifts from the blue to the red region of the visible spectrum with larger refraction index, and then larger silicon content in the layers. Also the PL intensity changes, showing a maximum for some intermediate value of the varied deposition parameter. We ascribe the PL properties of our samples to the presence of silicon QDs spontaneously formed in the nitride matrix, through an inhomogeneous growth process favoured by the low deposition rate, since the freely controllable PL peak position should rule out an interpretation in terms of radiative defect centers, characterized by well defined energy levels [10]. Moreover, direct evidence of presence of quantum dots by TEM analysis in samples obtained with similar growth conditions has been reported in the literature [6,7]. It has been claimed that the inhomogeneous growth, in the low rate regime, is promoted by the formation of a large number of Si dangling bonds acting as nucleation sites: The increase of N2 or NH3 flow rates would influence size and density of Si QDs by enhancing the creation of such sites, and then favouring the realization of densely packed but smaller silicon clusters around them [11]. The PL
75
Fig. 2. PL peak position for the N2 (Ref. [9]) and NH3 -type samples grown with different gas flow rates.
peak blueshift with decreasing silicon content in the layers, attained with an increase of N2 or NH3 gas flow rates (Ref. [9], Figs. 1 and 2) in this picture is the direct evidence of the enlargement of the energy gap of the Si dots as their size decreases. On the other hand, the variation of dot density as the deposition conditions are varied (as an effect of varied number of nucleation sites) can furnish an explanation to the observed change of the PL peak intensity. Less intense luminescence is indeed found in Fig. 1 with the samples grown with low NH3 flow rate in the mixture (condition for having few nucleation sites during the film growth). Larger intensities are found for intermediate NH3 flow rates, corresponding to an optimal deposition regime for realizing high Si QDs density, as already reported by Lelievre et al. [4]. For high flow rates the response decreases again, most probably because with very low silane partial pressure and numerous Si dangling bonds there are not enough Si atoms that can aggregate around all the nucleation sites, thus resulting in a lower dot density. It is interesting to notice that with both types of samples the same region of the visible spectrum is spanned by the PL peak, when N2 or NH3 flow rate is varied (Fig. 2). This means that presumably the same energy gap enlargement has been reproduced using the two gas mixtures, which could mean same dot sizes only if the structural properties of the Si QDs is the same in the two cases. Crystalline structure, favoured by the extra hydrogen dissociated from the NH3 gas, has been reported for samples grown with ammonia [7]. With SiH4 and N2 as gas precursor mixture, spontaneous formation of crystalline Si dots has been reported for deposition rates below 0.3 Å/s [12], otherwise amorphous structure is obtained [6]. We have employed Raman spectroscopy to obtain information about the presence of silicon nanocrystals within the silicon nitride matrix (Fig. 3). A broad asymmetric band around the same wavenumbers is observed in both cases. The spectra can be deconvoluted with a contribution around 450 cm−1 , assigned to the asymmetric Si–N bond stretching mode, and another contribution slightly above 480 cm−1 , due to localized phonon modes in amorphous silicon [13]. Possible presence of amorphous dots can then be deduced in both cases, and as a consequence similar dot sizes should be expected. Comparing our PL data with Ref. [6] we can estimate values in between 1.4 (for samples with blue PL emission) and 3 nm (for samples with red PL emission). While the PL evolution is similar for both types of samples, the intensity is much stronger in the ammonia-type case. The enhanced luminescence could be related to larger Si QDs density and/or reduced amount of nonradiative defect centers in the material, in particular at the Si QDs surface. Such effect, already observed on samples grown in similar conditions, has been attributed to more efficient passivation of nonradiative defects at the dot-matrix interface, thanks to the extra hydrogen available during the growth
76
L.V. Mercaldo et al. / Materials Science and Engineering B 159–160 (2009) 74–76
in all cases (the average difference is E02 − EPL ∼ 250 meV). Theoretically, in case of phase separation into distinct regions of Si and Si3 N4 versus a random bonding model approach, where Si–Si and Si–N bonds are homogeneously dispersed throughout the alloy, the presence of strongly absorbing Si regions enhances the low energy optical absorption [16]. Our result can then be interpreted as a proof of presence of Si nanostructures. Moreover, as from the structural characterization, also from optical measurements no evidence of contribution from aggregates with crystalline structure can be inferred for the ammonia-type samples. 4. Conclusions
Fig. 3. Typical Raman spectra of N2 -type and NH3 -type samples with similar PL peak energy.
Fig. 4. Absorption coefficient for N2 (dashed lines) and NH3 -type samples (solid lines) characterized by similar refraction index, n, and PL peak energy, EPL . The absorption curves of a PECVD grown a-Si:H film, and tabulated c-Si and Si3 N4 [14] are also shown for reference (dotted lines).
process when NH3 is used in the gas mixture [7,14]. Our result seems to confirm this interpretation, however TEM analysis is planned as a future step to extract information about the dot density, together with a direct measurement of the dot sizes. The two types of samples are characterized also by similar absorption properties. In Fig. 4 we report two absorption spectra for each type of samples, comparing two cases characterized by similar refraction index (2.0 and 2.3) and photoluminescence response (PL peak energy, EPL , around 1.90 and 2.35 eV). Experimental data are shown together with the absorption curves of a 200 nm thick PECVD grown a-Si:H film and tabulated c-Si and Si3 N4 [15], shown for reference. The curves look almost the same, with a behaviour in between the a-Si:H and the tabulated Si3 N4 curves. With n ∼ 2.0, not far from stoichiometric case (n ∼ 1.9 for hydrogenated material), the absorption spectra are already very different from the Si3 N4 curve. The main difference is in the low energy side of the spectrum, where the curve seems to be pulled toward the a-Si:H spectrum. Using E02 , the energy value at which the absorption coefficient is equal to 102 cm−1 , as an estimate of the absorption rising edge of the material, a value very close to the PL peak energy is obtained
Silicon rich silicon nitride films have been deposited by plasma enhanced chemical vapour deposition (PECVD) at low temperature (300 ◦ C) using two different gas mixtures, composed of silane and nitrogen with and without ammonia. Visible PL is observed in all cases, and tuning of PL emission is demonstrated by adjusting the gas flow rates. Shift and intensity evolution of the PL spectra are ascribed to size and density change of silicon quantum dots spontaneously formed in the silicon nitride matrix, while the deposition parameters are varied. Presence of amorphous silicon aggregates can be inferred through Raman spectroscopy for both types of samples. Also optical absorption spectra are influenced by the inhomogeneous structure of the material with the strongly absorbing silicon nanostructures dominating at low energy and determining the rising edge. The main difference between the two types of samples is the much stronger PL observed for ammonia-type layers, most probably due to a better passivation of nonradiative defect centers at the dot-matrix interface. Moreover, in this case the films are grown at an interestingly higher deposition rate. Such results contribute to give strength to the low temperature in situ method for growing Si nanostructures, with some advantages when using silane, nitrogen and ammonia as process gases. We want to stress that the absence of a post-deposition annealing makes this growth technique very powerful when thinking at the application of nanostructured materials within multilayered devices, such as innovative solar cells. In particular third generation concepts would be feasible also on low cost substrates. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
L. Pavesi, et al., Nature 408 (2000) 440, and references therein. G. Conibeer, et al., Thin Solid Films 511–512 (2006) 654. V. Svrcek, et al., Thin Solid Films 451–452 (2004) 384. J.F. Lelièvre, et al., Proc. 21st European PV Solar Energy Conf., Dresden, 2006, p. 1456. J.F. Lelièvre, et al., Thin Solid Films 511–512 (2006) 103. N.-M. Park, et al., Phys. Rev. Lett. 86 (7) (2001) 1355. T.-W. Kim, et al., Appl. Phys. Lett. 88 (2006) 123102. E. Dehan, et al., Thin Solid Films 266 (1995) 14. L.V. Mercaldo, et al., Mater. Sci. Eng. B 159–160 (2009) 77. S.V. Deshpande, J. Appl. Phys. 77 (1995) 6534. N.-M. Park, et al., Chem. Vap. Depos. 8 (6) (2002) 254. T.-Y. Kim, et al., Appl. Phys. Lett. 85 (2005) 5355. V.A. Volodin, et al., Appl. Phys. Lett. 73 (1998) 1212. B.-H. Kim, et al., Appl. Phys. Lett. 86 (2005) 091908. E.D. Palik (Ed.), Handbook of Optical Constants of Solids, vol. I, Academic Press, 1985. Z. Yin, F.W. Smith, Phys. Rev. B 42 (1990) 3658.