- Email: [email protected]
Influence of the spatial arrangement on the quantum confinement properties of Si nanocrystals
Optical Materials 17 (2001) 51±55
www.elsevier.nl/locate/optmat
In¯uence of the spatial arrangement on the quantum con®nement properties of Si nanocrystals Fabio Iacona a,*, Giorgia Franz o b, Vincenzo Vinciguerra b, Alessia Irrera a, Francesco Priolo b b
a CNR-IMETEM, Stradale Primosole 50, I-95121 Catania, Italy INFM and Dipartimento di Fisica e Astronomia, Universit a di Catania, Corso, Italia 57, I-95129 Catania, Italy
Abstract The room temperature photoluminescence of nanocrystalline Si/SiO2 superlattices (nc-SLs) and silicon nanocrystals (Si-nc) randomly dispersed inside an insulating matrix has been investigated. We have found that, for the same Si-nc mean size, photoluminescence (PL) spectra relative to nc-SLs are blueshifted with respect to those relative to Si-nc dispersed in SiO2 . Furthermore, the characteristic stretched exponential decay time exhibited by disordered Si-nc systems evolves towards a single exponential behavior, characterized also by longer decay times, when a low nanocrystal density is obtained by strongly reducing the excess of Si in the ®lms. As an extreme case, Si-nc arranged in ordered planar arrays, well separated by thin SiO2 layers, exhibit a single exponential behavior and decay times particularly long (about 200 ls). The above eects have been interpreted in terms of the lack of interaction among nanocrystals in ncSLs, due to their large reciprocal distance, and to the absence of relevant non-radiative decay processes. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon nanocrystals; Photoluminescence; Superlattices; SiOx ®lms
1. Introduction Carrier con®nement in Si nanostructures has been an extremely active ®eld of research in these last years [1±11]. This interest has been initiated by the observation by Canham [1] that silicon, in spite of its indirect bandgap, can eciently emit photons due to carrier recombination provided that low dimensionality is achieved. Emission occurs at shorter wavelengths with respect to bulk crystal-
* Corresponding author. Tel.: +39-095-591243; fax: +39-095713-9154. E-mail address: [email protected] (F. Iacona).
line silicon band edge as an eect of quantum con®nement. Among the dierent kinds of Si nanostructures, Si nanocrystals (Si-nc) have received a considerable attention due to their great stability [2±5]. Plasma-enhanced chemical vapor deposition (PECVD) of substoichiometric silicon oxide followed by high temperature annealing [6±8] represents a powerful method to produce Si-nc embedded within SiO2 . The nanocrystal size, and hence the photoluminescence (PL) properties, can be properly tuned either by varying the annealing temperature or the excess silicon content in the ®lm. In contrast with the disordered nature of the above described system, nanocrystalline Si/SiO2 superlattices (nc-SLs) have been prepared by
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 0 2 0 - 9
52
F. Iacona et al. / Optical Materials 17 (2001) 51±55
rf-magnetron sputtering [9] or by PECVD [10], and subsequent annealing. These systems exhibit peculiar PL properties related to the occurrence of very weak interaction among nanocrystals [10]. In this work, we compare the PL properties of nc-SLs with those relative to Si-nc uniformly distributed inside a SiO2 matrix. We will show that PL spectra of nc-SLs are blueshifted with respect to those relative to random distributions of Si-nc having the same mean size; furthermore, nc-SLs are characterized by decay times particularly long (up to 200 ls) and with a single exponential shape, while disordered Si-nc systems are able to approach this situation only by very carefully selecting the appropriate Si excess and annealing temperature conditions.
switching o the pump beam with an acoustooptic system and by detecting at a ®xed wavelength the PL intensity as a function of time. The overall time resolution of the system is 30 ns.
3. Results and discussion Fig. 1(a) reports the cross-sectional TEM image of a SiOx ®lm containing 44 at.% of Si after annealing at 1250°C for 1 h. A very high density of Si-nc, visible as bright spots randomly distributed inside the oxide matrix, can be seen. The Si-nc mean radius, obtained by analyzing the relative plan view TEM micrograph, is about 2.1 nm. By
2. Experimental Ten-period Si/SiO2 SLs and substoichiometric SiOx (x < 2) ®lms have been prepared by PECVD from SiH4 and N2 O precursors. Details about the deposition processes can be found elsewhere [6,10]. The thickness of the SiO2 layers has been ®xed at 8.5 nm in all the deposited SLs, while the thickness of the Si layers (DSi ) has been varied between 0.9 and 2.6 nm. SiOx ®lms about 200 nm thick with dierent Si content (from 35 to 44 at.%) have been obtained by properly varying the N2 O/SiH4 ¯ow ratio. To induce Si-nc formation, samples were annealed for 1 h at temperatures ranging between 1000°C and 1250°C in ultra-pure nitrogen atmosphere. The structural and optical properties of the ®lms were studied by transmission electron microscopy (TEM) and PL. Plan view and crosssectional TEM analyses were carried out with a 200 kV Jeol 2010 FX microscope. PL measurements were performed by pumping with the 488 nm line of an Ar±Kr laser. The luminescence signal was dispersed by a single-grating monochromator, revealed by a photomultiplier tube or a liquid nitrogen-cooled Ge detector and measured by a lock-in ampli®er. Spectra were measured at room temperature and have been corrected for the detector response. Time resolved PL measurements were performed by pumping to steady state,
Fig. 1. (a) Cross-sectional TEM image of a SiOx ®lm with 44 at.% of Si after annealing at 1250°C for 1 h; (b) cross-sectional TEM image of 10-period Si/SiO2 superlattice with a Si layer thickness of 2.6 nm and a SiO2 layer thickness of 8.5 nm after annealing at 1200°C for 1 h. The arrow indicates the sample surface.
F. Iacona et al. / Optical Materials 17 (2001) 51±55
decreasing the annealing temperature or the Si concentration, Si-nc distributions with a mean size around 1 nm can be obtained [6]. Si-nc can be also prepared by thermal annealing of amorphous Si/ SiO2 SLs. Fig. 1(b) shows the cross-sectional TEM micrograph of a Si/SiO2 SL annealed at 1200°C for 1 h. The annealing process induces the Si layer crystallization, and nanocrystals having a mean size depending on the thickness of the Si layer, as well as on the annealing temperature, are formed. In the sample shown in Fig. 1(b), starting from DSi 2:6 nm, we have obtained a nc mean radius of 2.3 nm, but smaller mean radii (1 nm) can be obtained by decreasing DSi and the annealing temperature [10]. The TEM image clearly demonstrates that Si-nc prepared from the Si/SiO2 SL are ordered in planar arrays, each plane being separated from the adjacent ones by a SiO2 layer. Oxide layers act as ecient diusion barriers, so essentially preserving the initial ordered structure. We have studied the room temperature PL properties of both kind of nanocrystalline systems shown in Fig. 1. We have observed a very intense light emission (visible to the naked eye) in the 650± 950 nm range; in agreement with the carrier quantum con®nement theory, the emission wavelength can be tuned by changing the Si-nc size through the variation of the annealing temperature and of the Si content of the ®lm (i.e., the Si layer thickness of the SLs or the Si excess of the SiOx ®lms). The intensity of the emitted light increases with the annealing temperature; the maximum emission is achieved at 1250°C for the SiOx ®lms and at 1200°C from the nc-SLs. This dierence can be explained by considering that Si atoms in ncSLs are not dispersed but they already form continuous layers, leading to a less critical dependence on the diusion processes. In both cases a further increase in the temperature leads to a marked decrease of the PL intensity due to the formation of a relevant fraction of Si-nc too large to exhibit quantum con®nement eects. Typical PL spectra observed from both types of systems are shown in Fig. 2. All spectra have been obtained at room temperature, and their intensities are normalized. In the ®gure we report the PL spectra relative to Si-nc formed from two dierent SiOx ®lms (37 and 42 at.% Si) annealed at 1250°C,
53
Fig. 2. Normalized PL spectra of nc-SLs with silicon layer thickness of 0.9 and 1.4 nm, annealed at 1200°C for 1 h, and SiOx ®lms with 37 and 42 at.% of Si, annealed at 1250°C for 1 h. Spectra were measured at room temperature, with a laser pump power of 10 mW.
and to two dierent nc-SLs (DSi 0:9 and 1.4 nm) annealed at 1200°C. We remark that the SiOx ®lm with 37 at.% Si and the SL with DSi 0:9 nm, as well as the SiOx ®lm with 42 at.% Si and the SL with DSi 1:4 nm, are characterized by the same mean nc radius (1.1 and 1.7 nm, respectively); consequently, one should expect the same PL spectrum. Instead, we have found that the spectra of the SLs are in both cases blueshifted (peak position at 810 vs. 860 and at 880 vs. 910 nm) with respect to those of the SiOx ®lms with the same nc mean size. To gain more information on the characteristics of the two systems we have analyzed the decay time of their PL signals. In Fig. 3 we report the decay curves relative to four dierent SiOx samples (Si concentration ranging from 35 to 44 at.%) annealed at 1250°C, compared with that one relative to a nc-SL DSi 0:9 nm) annealed at 1200°C. All decay curves have been recorded at a wavelength of 700 nm, and therefore they have to be considered characteristic of the de-excitation properties of nc of the same size. The ®gure clearly shows that decay curves relative to the randomly
54
F. Iacona et al. / Optical Materials 17 (2001) 51±55
Fig. 3. Room temperature measurements of the decay time of the PL signals at 700 nm for four dierent SiOx ®lms (35, 37, 39 and 44 at.% Si) annealed at 1250°C for 1 h, and for a nc-SL with DSi 0:9 nm annealed at 1200°C for 1 h. Data are normalized to the initial PL intensities. The pump power of the excitation laser was 10 mW.
distributed Si-nc, for Si concentrations ranging from 37 to 44 at.%, exhibit a typical non-single (stretched) exponential behavior. To obtain the luminescence decay time for each sample, the experimental curves have been ®tted with the following expression: b t I t I0 exp ; s where I t and I0 are the PL intensity during the decay and at t 0, respectively, and s is the decay time. The dispersion factor b represents a measure of the deviation of the PL decay from a single exponential; in particular, b < 1 corresponds to a distribution of single exponentials, each one characterized by a dierent s value. Stretched exponential decay of the PL intensity is usually observed for Si-nc, and its appearance has been associated with the occurrence of migration of excitons from a nc to the adjacent ones [11]. The result of the ®t gives values of s ranging from 10 (44 at.% Si) to 35 ls (37 at.% Si), i.e., the decay time of the PL signal of these samples be-
comes shorter with increasing the silicon content. Since the decay times have been recorded at a ®xed wavelength, the observed dierence must depend on the dierent nc density; indeed, in SiOx ®lms a higher Si concentration involves also higher Si-nc densities, and this increases the probability that con®ned excitons may travel from a nc to another one, and ®nd a non-radiative decay channel. The occurrence of extensive energy transfer in Si-rich samples is con®rmed by the low b values (up to about 0.65) we have found. Another relevant feature shown by Fig. 3 is that the time decay of the less Si-rich sample (35 at.% Si), as well as that one relative to the nc-SL, are remarkably longer than the other ones (s 65 and 80 ls, respectively) and exhibit a single exponential behavior over two orders of magnitude. This means that these samples are characterized by a peculiar structure, in which the interaction among nc is very reduced, due to the very low Si excess (SiOx ®lm), and to the absence of interaction between the nanocrystalline layers (nc-SL). For both samples the decay time measured at longer detection wavelengths (i.e., by observing the PL signal emitted by larger nc, characterized by slower deexcitation processes) strongly increases, approaching values of about 200 ls for the nc-SL, demonstrating the absence of relevant non-radiative decay processes. Finally, the relevance of the nc density in determining the decay properties of the Si-nc systems is further demonstrated by Fig. 4, reporting the decay of the PL signal, recorded at 776 nm, relative to the nc-SL with DSi 0:9 nm, after thermal annealing at 1100°C and 1200°C. The dierent structure of the two samples strongly in¯uences the decay time. Indeed, in the sample annealed at 1100°C Si-nc are too small to be detected by TEM analysis (mean radius lower than 1 nm), but their density is very high and this implies a reduction of the decay time (from 120 to 70 ls) and also a slight deviation of the decay shape from the single exponential behavior. The same eect has also been observed in SLs with dierent DSi , as well as for randomly distributed Si-nc, but the variation is particularly marked in this case; indeed, due to the very low Si content, the increase of the Si-nc mean size implies, as a
F. Iacona et al. / Optical Materials 17 (2001) 51±55
55
almost-single exponential decay times in systems characterized by the presence of well isolated Si-nc and by the absence of relevant non-radiative decay processes. This result has been observed in nc-SLs, in which a strong Si-nc con®nement is obtained due to the presence of a SiO2 layer between the nanocrystalline Si layers, as well as in random Sinc distributions, in which a very low crystal density has been obtained by working with a very small Si excess.
Acknowledgements
Fig. 4. Room temperature measurements of the decay time of the PL signals at 776 nm for a nc-SL with DSi 0:9 nm annealed at 1100°C and 1200°C for 1 h. Data are normalized to the initial PL intensities. The pump power of the excitation laser was 10 mW. The continuous lines are single exponential ®ts to the experimental data.
direct consequence, a strong decrease in their density, so reducing also the interaction between nc lying on the same plane. A clear picture emerges from the data we have just reported. Indeed, systems with a higher nc density have been demonstrated to have an energy ¯ow among Si-nc, leading to faster PL decay times, also characterized by a stretched exponential shape. Clearly, the energy can only be transferred from smaller to larger nc. Hence, in systems with a high nc density one expects to have a redshift in the PL emission since the energy is always transferred towards larger nc sizes (smaller energy gap). This observation clearly explains the data in Fig. 2 where, in samples having similar nc size distributions, a redshift in the PL was observed for those materials having a higher nc density. On the other hand, we have been able to obtain long and
C. Spinella and S. Pannitteri (CNR-IMETEM) are acknowledged for the assistance with TEM measurements. This work has been supported in part by the INFM Project RAMSES.
References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [3] J.G. Zhu, C.W. White, J.D. Budai, S.P. Withrow, Y. Chen, J. Appl. Phys. 78 (1995) 4386. [4] K.S. Min, K.V. Shcheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 69 (1996) 2033. [5] L.N. Dihn, L.L. Chase, M. Balooch, L.J. Terminello, Appl. Phys. Lett. 65 (1994) 3111. [6] F. Iacona, G. Franz o, C. Spinella, J. Appl. Phys. 87 (2000) 1295. [7] A.J. Kenyon, P.F. Trwoga, C.W. Pitt, G. Rehm, J. Appl. Phys. 79 (1996) 9291. [8] T. Inokuma, Y. Wakayama, T. Muramoto, R. Aoki, Y. Kurata, S. Hasegawa, J. Appl. Phys. 83 (1998) 2228. [9] L. Tsybeskov, K.D. Hirschman, S.P. Duttagupta, M. Zacharias, P.M. Fauchet, J.P. McCarey, D.J. Lockwood, Appl. Phys. Lett. 72 (1998) 43. [10] V. Vinciguerra, G. Franz o, F. Priolo, F. Iacona, C. Spinella, J. Appl. Phys. 87 (2000) 8165. [11] J. Linnros, N. Lalic, A. Galeckas, V. Grivickas, J. Appl. Phys. 86 (1999) 6128.
www.elsevier.nl/locate/optmat
In¯uence of the spatial arrangement on the quantum con®nement properties of Si nanocrystals Fabio Iacona a,*, Giorgia Franz o b, Vincenzo Vinciguerra b, Alessia Irrera a, Francesco Priolo b b
a CNR-IMETEM, Stradale Primosole 50, I-95121 Catania, Italy INFM and Dipartimento di Fisica e Astronomia, Universit a di Catania, Corso, Italia 57, I-95129 Catania, Italy
Abstract The room temperature photoluminescence of nanocrystalline Si/SiO2 superlattices (nc-SLs) and silicon nanocrystals (Si-nc) randomly dispersed inside an insulating matrix has been investigated. We have found that, for the same Si-nc mean size, photoluminescence (PL) spectra relative to nc-SLs are blueshifted with respect to those relative to Si-nc dispersed in SiO2 . Furthermore, the characteristic stretched exponential decay time exhibited by disordered Si-nc systems evolves towards a single exponential behavior, characterized also by longer decay times, when a low nanocrystal density is obtained by strongly reducing the excess of Si in the ®lms. As an extreme case, Si-nc arranged in ordered planar arrays, well separated by thin SiO2 layers, exhibit a single exponential behavior and decay times particularly long (about 200 ls). The above eects have been interpreted in terms of the lack of interaction among nanocrystals in ncSLs, due to their large reciprocal distance, and to the absence of relevant non-radiative decay processes. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon nanocrystals; Photoluminescence; Superlattices; SiOx ®lms
1. Introduction Carrier con®nement in Si nanostructures has been an extremely active ®eld of research in these last years [1±11]. This interest has been initiated by the observation by Canham [1] that silicon, in spite of its indirect bandgap, can eciently emit photons due to carrier recombination provided that low dimensionality is achieved. Emission occurs at shorter wavelengths with respect to bulk crystal-
* Corresponding author. Tel.: +39-095-591243; fax: +39-095713-9154. E-mail address: [email protected] (F. Iacona).
line silicon band edge as an eect of quantum con®nement. Among the dierent kinds of Si nanostructures, Si nanocrystals (Si-nc) have received a considerable attention due to their great stability [2±5]. Plasma-enhanced chemical vapor deposition (PECVD) of substoichiometric silicon oxide followed by high temperature annealing [6±8] represents a powerful method to produce Si-nc embedded within SiO2 . The nanocrystal size, and hence the photoluminescence (PL) properties, can be properly tuned either by varying the annealing temperature or the excess silicon content in the ®lm. In contrast with the disordered nature of the above described system, nanocrystalline Si/SiO2 superlattices (nc-SLs) have been prepared by
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 0 2 0 - 9
52
F. Iacona et al. / Optical Materials 17 (2001) 51±55
rf-magnetron sputtering [9] or by PECVD [10], and subsequent annealing. These systems exhibit peculiar PL properties related to the occurrence of very weak interaction among nanocrystals [10]. In this work, we compare the PL properties of nc-SLs with those relative to Si-nc uniformly distributed inside a SiO2 matrix. We will show that PL spectra of nc-SLs are blueshifted with respect to those relative to random distributions of Si-nc having the same mean size; furthermore, nc-SLs are characterized by decay times particularly long (up to 200 ls) and with a single exponential shape, while disordered Si-nc systems are able to approach this situation only by very carefully selecting the appropriate Si excess and annealing temperature conditions.
switching o the pump beam with an acoustooptic system and by detecting at a ®xed wavelength the PL intensity as a function of time. The overall time resolution of the system is 30 ns.
3. Results and discussion Fig. 1(a) reports the cross-sectional TEM image of a SiOx ®lm containing 44 at.% of Si after annealing at 1250°C for 1 h. A very high density of Si-nc, visible as bright spots randomly distributed inside the oxide matrix, can be seen. The Si-nc mean radius, obtained by analyzing the relative plan view TEM micrograph, is about 2.1 nm. By
2. Experimental Ten-period Si/SiO2 SLs and substoichiometric SiOx (x < 2) ®lms have been prepared by PECVD from SiH4 and N2 O precursors. Details about the deposition processes can be found elsewhere [6,10]. The thickness of the SiO2 layers has been ®xed at 8.5 nm in all the deposited SLs, while the thickness of the Si layers (DSi ) has been varied between 0.9 and 2.6 nm. SiOx ®lms about 200 nm thick with dierent Si content (from 35 to 44 at.%) have been obtained by properly varying the N2 O/SiH4 ¯ow ratio. To induce Si-nc formation, samples were annealed for 1 h at temperatures ranging between 1000°C and 1250°C in ultra-pure nitrogen atmosphere. The structural and optical properties of the ®lms were studied by transmission electron microscopy (TEM) and PL. Plan view and crosssectional TEM analyses were carried out with a 200 kV Jeol 2010 FX microscope. PL measurements were performed by pumping with the 488 nm line of an Ar±Kr laser. The luminescence signal was dispersed by a single-grating monochromator, revealed by a photomultiplier tube or a liquid nitrogen-cooled Ge detector and measured by a lock-in ampli®er. Spectra were measured at room temperature and have been corrected for the detector response. Time resolved PL measurements were performed by pumping to steady state,
Fig. 1. (a) Cross-sectional TEM image of a SiOx ®lm with 44 at.% of Si after annealing at 1250°C for 1 h; (b) cross-sectional TEM image of 10-period Si/SiO2 superlattice with a Si layer thickness of 2.6 nm and a SiO2 layer thickness of 8.5 nm after annealing at 1200°C for 1 h. The arrow indicates the sample surface.
F. Iacona et al. / Optical Materials 17 (2001) 51±55
decreasing the annealing temperature or the Si concentration, Si-nc distributions with a mean size around 1 nm can be obtained [6]. Si-nc can be also prepared by thermal annealing of amorphous Si/ SiO2 SLs. Fig. 1(b) shows the cross-sectional TEM micrograph of a Si/SiO2 SL annealed at 1200°C for 1 h. The annealing process induces the Si layer crystallization, and nanocrystals having a mean size depending on the thickness of the Si layer, as well as on the annealing temperature, are formed. In the sample shown in Fig. 1(b), starting from DSi 2:6 nm, we have obtained a nc mean radius of 2.3 nm, but smaller mean radii (1 nm) can be obtained by decreasing DSi and the annealing temperature [10]. The TEM image clearly demonstrates that Si-nc prepared from the Si/SiO2 SL are ordered in planar arrays, each plane being separated from the adjacent ones by a SiO2 layer. Oxide layers act as ecient diusion barriers, so essentially preserving the initial ordered structure. We have studied the room temperature PL properties of both kind of nanocrystalline systems shown in Fig. 1. We have observed a very intense light emission (visible to the naked eye) in the 650± 950 nm range; in agreement with the carrier quantum con®nement theory, the emission wavelength can be tuned by changing the Si-nc size through the variation of the annealing temperature and of the Si content of the ®lm (i.e., the Si layer thickness of the SLs or the Si excess of the SiOx ®lms). The intensity of the emitted light increases with the annealing temperature; the maximum emission is achieved at 1250°C for the SiOx ®lms and at 1200°C from the nc-SLs. This dierence can be explained by considering that Si atoms in ncSLs are not dispersed but they already form continuous layers, leading to a less critical dependence on the diusion processes. In both cases a further increase in the temperature leads to a marked decrease of the PL intensity due to the formation of a relevant fraction of Si-nc too large to exhibit quantum con®nement eects. Typical PL spectra observed from both types of systems are shown in Fig. 2. All spectra have been obtained at room temperature, and their intensities are normalized. In the ®gure we report the PL spectra relative to Si-nc formed from two dierent SiOx ®lms (37 and 42 at.% Si) annealed at 1250°C,
53
Fig. 2. Normalized PL spectra of nc-SLs with silicon layer thickness of 0.9 and 1.4 nm, annealed at 1200°C for 1 h, and SiOx ®lms with 37 and 42 at.% of Si, annealed at 1250°C for 1 h. Spectra were measured at room temperature, with a laser pump power of 10 mW.
and to two dierent nc-SLs (DSi 0:9 and 1.4 nm) annealed at 1200°C. We remark that the SiOx ®lm with 37 at.% Si and the SL with DSi 0:9 nm, as well as the SiOx ®lm with 42 at.% Si and the SL with DSi 1:4 nm, are characterized by the same mean nc radius (1.1 and 1.7 nm, respectively); consequently, one should expect the same PL spectrum. Instead, we have found that the spectra of the SLs are in both cases blueshifted (peak position at 810 vs. 860 and at 880 vs. 910 nm) with respect to those of the SiOx ®lms with the same nc mean size. To gain more information on the characteristics of the two systems we have analyzed the decay time of their PL signals. In Fig. 3 we report the decay curves relative to four dierent SiOx samples (Si concentration ranging from 35 to 44 at.%) annealed at 1250°C, compared with that one relative to a nc-SL DSi 0:9 nm) annealed at 1200°C. All decay curves have been recorded at a wavelength of 700 nm, and therefore they have to be considered characteristic of the de-excitation properties of nc of the same size. The ®gure clearly shows that decay curves relative to the randomly
54
F. Iacona et al. / Optical Materials 17 (2001) 51±55
Fig. 3. Room temperature measurements of the decay time of the PL signals at 700 nm for four dierent SiOx ®lms (35, 37, 39 and 44 at.% Si) annealed at 1250°C for 1 h, and for a nc-SL with DSi 0:9 nm annealed at 1200°C for 1 h. Data are normalized to the initial PL intensities. The pump power of the excitation laser was 10 mW.
distributed Si-nc, for Si concentrations ranging from 37 to 44 at.%, exhibit a typical non-single (stretched) exponential behavior. To obtain the luminescence decay time for each sample, the experimental curves have been ®tted with the following expression: b t I t I0 exp ; s where I t and I0 are the PL intensity during the decay and at t 0, respectively, and s is the decay time. The dispersion factor b represents a measure of the deviation of the PL decay from a single exponential; in particular, b < 1 corresponds to a distribution of single exponentials, each one characterized by a dierent s value. Stretched exponential decay of the PL intensity is usually observed for Si-nc, and its appearance has been associated with the occurrence of migration of excitons from a nc to the adjacent ones [11]. The result of the ®t gives values of s ranging from 10 (44 at.% Si) to 35 ls (37 at.% Si), i.e., the decay time of the PL signal of these samples be-
comes shorter with increasing the silicon content. Since the decay times have been recorded at a ®xed wavelength, the observed dierence must depend on the dierent nc density; indeed, in SiOx ®lms a higher Si concentration involves also higher Si-nc densities, and this increases the probability that con®ned excitons may travel from a nc to another one, and ®nd a non-radiative decay channel. The occurrence of extensive energy transfer in Si-rich samples is con®rmed by the low b values (up to about 0.65) we have found. Another relevant feature shown by Fig. 3 is that the time decay of the less Si-rich sample (35 at.% Si), as well as that one relative to the nc-SL, are remarkably longer than the other ones (s 65 and 80 ls, respectively) and exhibit a single exponential behavior over two orders of magnitude. This means that these samples are characterized by a peculiar structure, in which the interaction among nc is very reduced, due to the very low Si excess (SiOx ®lm), and to the absence of interaction between the nanocrystalline layers (nc-SL). For both samples the decay time measured at longer detection wavelengths (i.e., by observing the PL signal emitted by larger nc, characterized by slower deexcitation processes) strongly increases, approaching values of about 200 ls for the nc-SL, demonstrating the absence of relevant non-radiative decay processes. Finally, the relevance of the nc density in determining the decay properties of the Si-nc systems is further demonstrated by Fig. 4, reporting the decay of the PL signal, recorded at 776 nm, relative to the nc-SL with DSi 0:9 nm, after thermal annealing at 1100°C and 1200°C. The dierent structure of the two samples strongly in¯uences the decay time. Indeed, in the sample annealed at 1100°C Si-nc are too small to be detected by TEM analysis (mean radius lower than 1 nm), but their density is very high and this implies a reduction of the decay time (from 120 to 70 ls) and also a slight deviation of the decay shape from the single exponential behavior. The same eect has also been observed in SLs with dierent DSi , as well as for randomly distributed Si-nc, but the variation is particularly marked in this case; indeed, due to the very low Si content, the increase of the Si-nc mean size implies, as a
F. Iacona et al. / Optical Materials 17 (2001) 51±55
55
almost-single exponential decay times in systems characterized by the presence of well isolated Si-nc and by the absence of relevant non-radiative decay processes. This result has been observed in nc-SLs, in which a strong Si-nc con®nement is obtained due to the presence of a SiO2 layer between the nanocrystalline Si layers, as well as in random Sinc distributions, in which a very low crystal density has been obtained by working with a very small Si excess.
Acknowledgements
Fig. 4. Room temperature measurements of the decay time of the PL signals at 776 nm for a nc-SL with DSi 0:9 nm annealed at 1100°C and 1200°C for 1 h. Data are normalized to the initial PL intensities. The pump power of the excitation laser was 10 mW. The continuous lines are single exponential ®ts to the experimental data.
direct consequence, a strong decrease in their density, so reducing also the interaction between nc lying on the same plane. A clear picture emerges from the data we have just reported. Indeed, systems with a higher nc density have been demonstrated to have an energy ¯ow among Si-nc, leading to faster PL decay times, also characterized by a stretched exponential shape. Clearly, the energy can only be transferred from smaller to larger nc. Hence, in systems with a high nc density one expects to have a redshift in the PL emission since the energy is always transferred towards larger nc sizes (smaller energy gap). This observation clearly explains the data in Fig. 2 where, in samples having similar nc size distributions, a redshift in the PL was observed for those materials having a higher nc density. On the other hand, we have been able to obtain long and
C. Spinella and S. Pannitteri (CNR-IMETEM) are acknowledged for the assistance with TEM measurements. This work has been supported in part by the INFM Project RAMSES.
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