- Email: [email protected]
CaF2 nanostructures
Materials Science and Engineering B69 – 70 (2000) 340 – 344 www.elsevier.com/locate/mseb
Improvement in the luminescence properties of Si/CaF2 nanostructures F. Bassani a,*, S. Me´nard a, I. Berbezier a, F. Arnaud d’Avitaya a, I. Mihalcescu b a
Centre de Recherche sur les Me´canismes de la Croissance Cristalline – CNRS, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France b Laboratoire de Spectrome´trie Physique, Uni6ersite´ Joseph Fourier, BP 87, 38042 Saint Martin d’He`res, France
Abstract We report on the photoluminescence quantum efficiency and the lifetime of two Si/CaF2 heterostructures grown by molecular beam epitaxy. The first is a nanocrystalline Si/CaF2 multiquantum well which consists of interacting Si nanocrystallites within Si layers; the second is an annealed Si/CaF2 multiquantum well which can be described as a collection of non-interacting Si nanocrystallites embedded in the CaF2 matrix. While a photoluminescence efficiency of 0.01% has been found in the former, it reaches 1% in the latter. The energy dependence of the PL lifetime differs drastically between both sets of samples. These results are explained in terms of carrier localization. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Calcium fluoride; Low-dimensional structures; Optical properties; Silicon
1. Introduction Bulk crystalline silicon (c-Si) is by far the most important semiconductor used in the microelectronics industry. However, due to its indirect band gap of 1.1 eV, it is not suitable for emitting light. Nevertheless the surprising discovery of bright visible luminescence from porous silicon (PS) [1] has stimulated the fabrication of other light-emitting Si based materials and the study of the underlying mechanisms that govern the light emission process. For instance, nanocrystalline Si/CaF2 multiquantum wells [2] (nc-MQWs) and amorphous Si/SiO2 superlattices [3] synthesized by molecular beam epitaxy exhibit visible luminescence if the Si layer thickness is lower than 30 A, . In addition, a maximum PL intensity is obtained when the Si layer thickness is equal to 15 A, , which remains not understood. Thus, there is a lot of experimental evidence that quantum confinement and the passivation of non-radiative centers are the two main ingredients to get visible light from lowdimensional Si structures. However, the mechanism of relaxation of carriers after excitation is still under debate. * Corresponding author. Tel.: +33-491172872; fax: 491418916. E-mail address: [email protected] (F. Bassani)
+33-
The aim of this paper is to demonstrate that carrier localization is also an important parameter to get high PL efficiency. In this work, we want to emphasize the role of the barrier which strongly localizes the carriers inside Si crystallites and prevents them from diffusing towards non-radiative centers. We first describe the structure of an as-grown and annealed nc-MQW. We then compare static and dynamic optical properties of the two structures. Finally, a model based on carrier localization is proposed to explain the experimental data.
2. Experimental The samples were grown using molecular beam epitaxy. Growth rates for Si and CaF2 were typically around 0.5 A, /s and 0.7 A, /s, respectively. Substrate temperatures were calibrated using extinction wire pyrometry and a thermocouple fixed on the back of the sample holder. Further details of the experimental setup have been published elsewhere [4]. The main characteristics of the samples used in this paper are given in Table 1. Structural characterization of the samples has been performed by conventional transmission electron microscopy (TEM). Excitation of photoluminescence (PL) was provided by the 457.9-nm line of an Ar+ ion laser.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 2 3 3 - 0
F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
341
Table 1 Samples used in this work Sample
Nominal Si thickness (A, )
Nominal CaF2 thickness (A, )
Number of periods
Annealing temperature (°C)
A B C D E
15 3 3 3 3
20 20 20 20 20
50 100 100 100 100
396 477 585
Quantitative measurement of the quantum efficiency (QE) was obtained by comparing the sample luminescence with that of a lightly doped ruby sample under the same condition of excitation and detection. Timeresolved PL experiments were performed using a pulsed nitrogen laser (wavelength 337 nm, pulse width 3 ns, repetition rate 10 Hz). The time resolution of the analysis chain was about 5 ns.
3. Results and analysis
3.1. TEM obser6ations Fig. 1 shows cross-sectional TEM images of a 50 periods Si/CaF2 MQW (sample A) and a 100 periods Si/CaF2 MQW, subsequently annealed under vacuum at 477°C during 30 min (sample D). Note that the Si layer thickness is strongly reduced in sample D, since it is only 3 A, with respect to the 15 A, of sample A. As can be seen, the two structures present totally different morphologies. Sample A is a periodic layered structure as also evidenced by X-ray reflectivity measurements [4]
in which Si layers are composed of small Si grains with a mean size of 13 A, or less as estimated by extended X-ray absorption fine structure (EXAFS) [5]. On the contrary, there is a noteworthy modification of the structure after annealing as shown in Fig. 1b. At present, sample D is no longer bidimensional and periodic but consists of an array of columns perpendicular to the substrate. Note, however, that the first few periods close to the substrate interface are not strongly affected. This columnar type structure looks like the morphology of PS obtained from heavily doped p-type Si substrates. The columns have a lateral size of 200 A, or less and are composed of stacked grains with a mean diameter of 200 A, as shown in the inset of Fig. 1b. Transmission electron diffraction indicates that these grains are crystalline with both A-type and B-type orientations with respect to the substrate. Energy dispersive micro-analysis measurements performed on a grain reveals that each grain contains around 15% of Si and 85% of CaF2 which is the mean nominal ratio expected from the starting Si and CaF2 thicknesses. Up to now, it has not been possible to localize Si and CaF2 in the grains
Fig. 1. (a) Bright-field TEM image obtained on sample A. The sample was cleaved along two [110] directions at 90°, and TEM observation is made on the corner. (b) Cross-sectional TEM image of sample D. The inset is a magnification taken on the top of the columns.
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F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
alternated with nanocrystalline CaF2 barriers; the second (sample D) consists of Si dots embedded in crystalline CaF2 matrix. As we will see in the next section, these two structures result in different static and dynamic optical properties.
3.2. Static and dynamic PL
Fig. 2. Room temperature PL spectrum for a 50 periods nc-MQW with a 15 A, thick Si layer and a 20 A, thick CaF2 layer per each period. The inset shows the average lifetime as a function of the detection wavelength.
because the lattice mismatch between the two materials is too small to be revealed either by HREM or electron diffraction, and EDS nano-analysis spectra result from Si and CaF2 grains superimposition. Nevertheless, preliminary EXAFS measurements indicate that the minimum size of Si nanocrystallites is at least 13 A, . It should be also mentioned that only a few Si atoms are linked to oxygen as evidenced by the weak SiO peak with respect to the SiSi one in EXAFS and AUGER spectra. That means that Si nanocrystallites are surrounded by crystalline CaF2 barriers which reduce drastically the diffusion of oxygen towards them. In summary, we have two kinds of structures: the first (sample A) consists of nanocrystalline Si wells
Fig. 3. Room temperature PL spectra of a 100 periods nc-MQW with a 3 A, thick Si layer and a 15 A, thick CaF2 layer per each period annealed under vacuum at different temperatures for 30 min.
3.2.1. As-grown Si/CaF2 nc-MQWs Optical properties of nc-MQWs have been described in detail in [4], but the main characteristics are summarised here. Fig. 2 shows the room temperature PL spectrum of sample A after ageing in air. The peak luminescence is centered around 1.65 eV with a half width of 0.36 eV. After integration of the emitted photons over the full emission line and using the wellknown quantum efficiency of the ruby sample, we deduced a QE of 0.01% for this sample. The luminescence originated from the slow component of the PL decay which has been adjusted with a power law with an exponent close to 1 (see Fig. 5 of [4]) in analogy to the behavior encountered for hydrogenated amorphous silicon [6] or not well-passivated PS [7]. The room temperature energy dependence of the average lifetime detected in the PL line is displayed in the inset of Fig. 2. As can be seen, there is no significant dispersion of the PL lifetime in the interval 1.48–2.07 eV. It shows that the PL band is quite homogeneous, which is in marked contrast to well-passivated PS [8] where the lifetime is exponentially dependent on the emission wavelength. This can be interpreted as a consequence of the delocalization of the carriers over interconnected nanocrystallites within the Si layers. To be complete concerning the optical properties of nc-MQWs, it is worthwhile to notice that the luminescence is almost size independent, i.e. it does not vary so much with the Si layer thickness. In contrast, the effects of quantum confinement in such nc-MQWs have been clearly evidenced by absorption measurements [9] which show a strong blue-shift of the optical band gap (deduced from the Tauc plot) of more than 1 eV when decreasing the Si layer thickness from 50 to 10 A, . These observations lead us to conclude that the luminescence is related to deep luminescent centers located near the surface of Si nanocrystallites [10]. 3.2.2. Annealed Si/CaF2 nc-MQWs On the other hand, annealed nc-MQWs exhibit a completely different optical response. Fig. 3 shows the room temperature PL spectra of a 100 periods ncMQW annealed under vacuum during 30 min at different temperatures: not annealed (sample B) and annealed at 396°C (sample C), 477°C (sample D) and 585°C (sample E), respectively. No PL could be detected in the static PL spectrum for the as-grown (not annealed) MQW due to the small thickness (3 A, thick)
F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
of the Si layer. However, when the sample is annealed under vacuum, we observe a maximum PL intensity for an annealing temperature of 477°C. This value corresponds to the temperature at which the recrystallisation of CaF2 occurs as indicated by the modification of the reflection high energy electron diffraction pattern during the annealing. The PL is strongly shifted to the infrared with a peak position at 1.35 eV and is much broader. This can be attributed to larger Si nanocrystallites in size and a wider distribution resulting from the annealing, respectively. But we can not rule out the possibility that the luminescence originates from another deep luminescent center. Work is in progress to elucidate this point. Quantitative measurement of the QE for the nc-MQW annealed at 477°C gives a value of 1%, which is of the same order as that deduced on well-passivated porous silicon. Note that this QE is 100 times higher than that measured on a nc-MQW with a 15 A, thick Si layer (sample A). Fig. 4a shows the time evolution of the PL intensity for various detection wavelengths obtained on sample D. The slow component of the PL decay can be adjusted with a stretched exponential function: tb I(t)=I0 exp− (1) t where t is an effective decay time, b is a constant between 0 and 1 which marks the deviation to a simple exponential function, and I0 is a constant. By fitting the curves of Fig. 4a, we found b equal to 0.65. Note that this factor is similar to that measured on anodically oxidized porous silicon [11]. The decay time t can be determined using a least-squares fitting of the data. The value of t as a function of the detection wavelength is reported in Fig. 4b. As can be seen, the energy dependence of the PL lifetime is in marked contrast to that of nc-MQW described above. It can be fitted with an exponential of the form:
t= A exp−
E E0
343
(2)
with E0 $ 0.31 eV.
4. Discussion These experimental features, i.e. the PL efficiency and the energy dependence of the PL lifetime, can be well understood if we consider that the dynamic recombination of carriers is governed by the tunneling escape from bright crystallites towards dark crystallites where carriers can recombine nonradiatively, as suggested by Vial et al. [8]. Indeed, for a nc-MQW the carriers are delocalized over interconnected nanocrystallites within a Si layer and this is the reason why we do not observe any significant dispersion of the lifetime. On the other hand, for an annealed nc-MQW, the structure can be described as a collection of Si quantum dots surrounded by crystalline CaF2 barriers, and consequently excited carriers have more difficulties to transit from one Si crystallite to another one where they can be trapped on nonradiative recombination centers. This model is also consistent with measured PL efficiencies on both samples.
5. Conclusion In conclusion, we have described the structural an optical properties of two kind of Si/CaF2 nanostructures: nanocrystalline Si wells and Si dots. We suggest that localization of carriers is a crucial parameter to get high PL efficiency in low-dimensional Si based structures. Further studies are needed to confirm the role of the CaF2 barrier.
Fig. 4. (a) Wavelength dependence of the time evolution of the room temperature PL intensity obtained on a 100 periods Si/CaF2 MQW with a 0.3 nm thick Si layer and a 1.5 nm thick CaF2 layer per each period annealed under vacuum at 477°C. (b) Energy dependence of the PL lifetime measured at room temperature.
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F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
Acknowledgements We would like to thank A.M. Flank for supplying us with EXAFS results. This work was supported by the European Project ESPRIT MEL ARI No. 28741 SMILE (Silicon Modules for Integrated Light Engineering). References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] F. Arnaud d’Avitaya, L. Vervoort, F. Bassani, S. Ossicini, A. Fasolino, F. Bernardini, Eur. Phys. Lett. 31 (1995) 25. [3] D.J. Lockwood, Z.H. Lu, J.M. Baribeau, Phys. Rev. Lett. 76 (1996) 539.
.
[4] F. Bassani, L. Vervoort, I. Mihalcesu, J.C. Vial, F. Arnaud d’Avitaya, J. Appl. Phys. 79 (1996) 4066. [5] F. Bassani, L. Vervoort, I. Mihalcescu, J.C. Vial, F. Arnaud d’Avitaya, in: B. Gil, R.-L. Aulombard (Eds.), Semiconductor Heteroepitaxy, Growth, Characterization and Device Applications, World Scientific, 1995, pp. 286– 293. [6] R.A. Street, Adv. Phys. 30 (1981) 618. [7] I. Mihalcescu, Thesis, University J. Fourier, Grenoble, 1994. [8] J.C. Vial, A. Bsiesy, F. Gaspard, R. He´rino, M. Ligeon, F. Muller, R. Romestain, R.M. Macfarlane, Phys. Rev. B 45 (1992) 14171. [9] F. Bassani, I. Mihalcescu, J.C. Vial, F. Arnaud d’Avitaya, Appl. Surf. Sci. 117/118 (1997) 670. [10] F. Bassani, S. Me´nard, F. Arnaud d’Avitaya, Phys. Status Solidi A 165 (1997) 49. [11] I. Mihalcescu, M. Ligeon, F. Muller, R. Romestain, J.C. Vial, J. Luminesc. 57 (1993) 111.
Improvement in the luminescence properties of Si/CaF2 nanostructures F. Bassani a,*, S. Me´nard a, I. Berbezier a, F. Arnaud d’Avitaya a, I. Mihalcescu b a
Centre de Recherche sur les Me´canismes de la Croissance Cristalline – CNRS, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France b Laboratoire de Spectrome´trie Physique, Uni6ersite´ Joseph Fourier, BP 87, 38042 Saint Martin d’He`res, France
Abstract We report on the photoluminescence quantum efficiency and the lifetime of two Si/CaF2 heterostructures grown by molecular beam epitaxy. The first is a nanocrystalline Si/CaF2 multiquantum well which consists of interacting Si nanocrystallites within Si layers; the second is an annealed Si/CaF2 multiquantum well which can be described as a collection of non-interacting Si nanocrystallites embedded in the CaF2 matrix. While a photoluminescence efficiency of 0.01% has been found in the former, it reaches 1% in the latter. The energy dependence of the PL lifetime differs drastically between both sets of samples. These results are explained in terms of carrier localization. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Calcium fluoride; Low-dimensional structures; Optical properties; Silicon
1. Introduction Bulk crystalline silicon (c-Si) is by far the most important semiconductor used in the microelectronics industry. However, due to its indirect band gap of 1.1 eV, it is not suitable for emitting light. Nevertheless the surprising discovery of bright visible luminescence from porous silicon (PS) [1] has stimulated the fabrication of other light-emitting Si based materials and the study of the underlying mechanisms that govern the light emission process. For instance, nanocrystalline Si/CaF2 multiquantum wells [2] (nc-MQWs) and amorphous Si/SiO2 superlattices [3] synthesized by molecular beam epitaxy exhibit visible luminescence if the Si layer thickness is lower than 30 A, . In addition, a maximum PL intensity is obtained when the Si layer thickness is equal to 15 A, , which remains not understood. Thus, there is a lot of experimental evidence that quantum confinement and the passivation of non-radiative centers are the two main ingredients to get visible light from lowdimensional Si structures. However, the mechanism of relaxation of carriers after excitation is still under debate. * Corresponding author. Tel.: +33-491172872; fax: 491418916. E-mail address: [email protected] (F. Bassani)
+33-
The aim of this paper is to demonstrate that carrier localization is also an important parameter to get high PL efficiency. In this work, we want to emphasize the role of the barrier which strongly localizes the carriers inside Si crystallites and prevents them from diffusing towards non-radiative centers. We first describe the structure of an as-grown and annealed nc-MQW. We then compare static and dynamic optical properties of the two structures. Finally, a model based on carrier localization is proposed to explain the experimental data.
2. Experimental The samples were grown using molecular beam epitaxy. Growth rates for Si and CaF2 were typically around 0.5 A, /s and 0.7 A, /s, respectively. Substrate temperatures were calibrated using extinction wire pyrometry and a thermocouple fixed on the back of the sample holder. Further details of the experimental setup have been published elsewhere [4]. The main characteristics of the samples used in this paper are given in Table 1. Structural characterization of the samples has been performed by conventional transmission electron microscopy (TEM). Excitation of photoluminescence (PL) was provided by the 457.9-nm line of an Ar+ ion laser.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 2 3 3 - 0
F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
341
Table 1 Samples used in this work Sample
Nominal Si thickness (A, )
Nominal CaF2 thickness (A, )
Number of periods
Annealing temperature (°C)
A B C D E
15 3 3 3 3
20 20 20 20 20
50 100 100 100 100
396 477 585
Quantitative measurement of the quantum efficiency (QE) was obtained by comparing the sample luminescence with that of a lightly doped ruby sample under the same condition of excitation and detection. Timeresolved PL experiments were performed using a pulsed nitrogen laser (wavelength 337 nm, pulse width 3 ns, repetition rate 10 Hz). The time resolution of the analysis chain was about 5 ns.
3. Results and analysis
3.1. TEM obser6ations Fig. 1 shows cross-sectional TEM images of a 50 periods Si/CaF2 MQW (sample A) and a 100 periods Si/CaF2 MQW, subsequently annealed under vacuum at 477°C during 30 min (sample D). Note that the Si layer thickness is strongly reduced in sample D, since it is only 3 A, with respect to the 15 A, of sample A. As can be seen, the two structures present totally different morphologies. Sample A is a periodic layered structure as also evidenced by X-ray reflectivity measurements [4]
in which Si layers are composed of small Si grains with a mean size of 13 A, or less as estimated by extended X-ray absorption fine structure (EXAFS) [5]. On the contrary, there is a noteworthy modification of the structure after annealing as shown in Fig. 1b. At present, sample D is no longer bidimensional and periodic but consists of an array of columns perpendicular to the substrate. Note, however, that the first few periods close to the substrate interface are not strongly affected. This columnar type structure looks like the morphology of PS obtained from heavily doped p-type Si substrates. The columns have a lateral size of 200 A, or less and are composed of stacked grains with a mean diameter of 200 A, as shown in the inset of Fig. 1b. Transmission electron diffraction indicates that these grains are crystalline with both A-type and B-type orientations with respect to the substrate. Energy dispersive micro-analysis measurements performed on a grain reveals that each grain contains around 15% of Si and 85% of CaF2 which is the mean nominal ratio expected from the starting Si and CaF2 thicknesses. Up to now, it has not been possible to localize Si and CaF2 in the grains
Fig. 1. (a) Bright-field TEM image obtained on sample A. The sample was cleaved along two [110] directions at 90°, and TEM observation is made on the corner. (b) Cross-sectional TEM image of sample D. The inset is a magnification taken on the top of the columns.
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F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
alternated with nanocrystalline CaF2 barriers; the second (sample D) consists of Si dots embedded in crystalline CaF2 matrix. As we will see in the next section, these two structures result in different static and dynamic optical properties.
3.2. Static and dynamic PL
Fig. 2. Room temperature PL spectrum for a 50 periods nc-MQW with a 15 A, thick Si layer and a 20 A, thick CaF2 layer per each period. The inset shows the average lifetime as a function of the detection wavelength.
because the lattice mismatch between the two materials is too small to be revealed either by HREM or electron diffraction, and EDS nano-analysis spectra result from Si and CaF2 grains superimposition. Nevertheless, preliminary EXAFS measurements indicate that the minimum size of Si nanocrystallites is at least 13 A, . It should be also mentioned that only a few Si atoms are linked to oxygen as evidenced by the weak SiO peak with respect to the SiSi one in EXAFS and AUGER spectra. That means that Si nanocrystallites are surrounded by crystalline CaF2 barriers which reduce drastically the diffusion of oxygen towards them. In summary, we have two kinds of structures: the first (sample A) consists of nanocrystalline Si wells
Fig. 3. Room temperature PL spectra of a 100 periods nc-MQW with a 3 A, thick Si layer and a 15 A, thick CaF2 layer per each period annealed under vacuum at different temperatures for 30 min.
3.2.1. As-grown Si/CaF2 nc-MQWs Optical properties of nc-MQWs have been described in detail in [4], but the main characteristics are summarised here. Fig. 2 shows the room temperature PL spectrum of sample A after ageing in air. The peak luminescence is centered around 1.65 eV with a half width of 0.36 eV. After integration of the emitted photons over the full emission line and using the wellknown quantum efficiency of the ruby sample, we deduced a QE of 0.01% for this sample. The luminescence originated from the slow component of the PL decay which has been adjusted with a power law with an exponent close to 1 (see Fig. 5 of [4]) in analogy to the behavior encountered for hydrogenated amorphous silicon [6] or not well-passivated PS [7]. The room temperature energy dependence of the average lifetime detected in the PL line is displayed in the inset of Fig. 2. As can be seen, there is no significant dispersion of the PL lifetime in the interval 1.48–2.07 eV. It shows that the PL band is quite homogeneous, which is in marked contrast to well-passivated PS [8] where the lifetime is exponentially dependent on the emission wavelength. This can be interpreted as a consequence of the delocalization of the carriers over interconnected nanocrystallites within the Si layers. To be complete concerning the optical properties of nc-MQWs, it is worthwhile to notice that the luminescence is almost size independent, i.e. it does not vary so much with the Si layer thickness. In contrast, the effects of quantum confinement in such nc-MQWs have been clearly evidenced by absorption measurements [9] which show a strong blue-shift of the optical band gap (deduced from the Tauc plot) of more than 1 eV when decreasing the Si layer thickness from 50 to 10 A, . These observations lead us to conclude that the luminescence is related to deep luminescent centers located near the surface of Si nanocrystallites [10]. 3.2.2. Annealed Si/CaF2 nc-MQWs On the other hand, annealed nc-MQWs exhibit a completely different optical response. Fig. 3 shows the room temperature PL spectra of a 100 periods ncMQW annealed under vacuum during 30 min at different temperatures: not annealed (sample B) and annealed at 396°C (sample C), 477°C (sample D) and 585°C (sample E), respectively. No PL could be detected in the static PL spectrum for the as-grown (not annealed) MQW due to the small thickness (3 A, thick)
F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
of the Si layer. However, when the sample is annealed under vacuum, we observe a maximum PL intensity for an annealing temperature of 477°C. This value corresponds to the temperature at which the recrystallisation of CaF2 occurs as indicated by the modification of the reflection high energy electron diffraction pattern during the annealing. The PL is strongly shifted to the infrared with a peak position at 1.35 eV and is much broader. This can be attributed to larger Si nanocrystallites in size and a wider distribution resulting from the annealing, respectively. But we can not rule out the possibility that the luminescence originates from another deep luminescent center. Work is in progress to elucidate this point. Quantitative measurement of the QE for the nc-MQW annealed at 477°C gives a value of 1%, which is of the same order as that deduced on well-passivated porous silicon. Note that this QE is 100 times higher than that measured on a nc-MQW with a 15 A, thick Si layer (sample A). Fig. 4a shows the time evolution of the PL intensity for various detection wavelengths obtained on sample D. The slow component of the PL decay can be adjusted with a stretched exponential function: tb I(t)=I0 exp− (1) t where t is an effective decay time, b is a constant between 0 and 1 which marks the deviation to a simple exponential function, and I0 is a constant. By fitting the curves of Fig. 4a, we found b equal to 0.65. Note that this factor is similar to that measured on anodically oxidized porous silicon [11]. The decay time t can be determined using a least-squares fitting of the data. The value of t as a function of the detection wavelength is reported in Fig. 4b. As can be seen, the energy dependence of the PL lifetime is in marked contrast to that of nc-MQW described above. It can be fitted with an exponential of the form:
t= A exp−
E E0
343
(2)
with E0 $ 0.31 eV.
4. Discussion These experimental features, i.e. the PL efficiency and the energy dependence of the PL lifetime, can be well understood if we consider that the dynamic recombination of carriers is governed by the tunneling escape from bright crystallites towards dark crystallites where carriers can recombine nonradiatively, as suggested by Vial et al. [8]. Indeed, for a nc-MQW the carriers are delocalized over interconnected nanocrystallites within a Si layer and this is the reason why we do not observe any significant dispersion of the lifetime. On the other hand, for an annealed nc-MQW, the structure can be described as a collection of Si quantum dots surrounded by crystalline CaF2 barriers, and consequently excited carriers have more difficulties to transit from one Si crystallite to another one where they can be trapped on nonradiative recombination centers. This model is also consistent with measured PL efficiencies on both samples.
5. Conclusion In conclusion, we have described the structural an optical properties of two kind of Si/CaF2 nanostructures: nanocrystalline Si wells and Si dots. We suggest that localization of carriers is a crucial parameter to get high PL efficiency in low-dimensional Si based structures. Further studies are needed to confirm the role of the CaF2 barrier.
Fig. 4. (a) Wavelength dependence of the time evolution of the room temperature PL intensity obtained on a 100 periods Si/CaF2 MQW with a 0.3 nm thick Si layer and a 1.5 nm thick CaF2 layer per each period annealed under vacuum at 477°C. (b) Energy dependence of the PL lifetime measured at room temperature.
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F. Bassani et al. / Materials Science and Engineering B69–70 (2000) 340–344
Acknowledgements We would like to thank A.M. Flank for supplying us with EXAFS results. This work was supported by the European Project ESPRIT MEL ARI No. 28741 SMILE (Silicon Modules for Integrated Light Engineering). References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] F. Arnaud d’Avitaya, L. Vervoort, F. Bassani, S. Ossicini, A. Fasolino, F. Bernardini, Eur. Phys. Lett. 31 (1995) 25. [3] D.J. Lockwood, Z.H. Lu, J.M. Baribeau, Phys. Rev. Lett. 76 (1996) 539.
.
[4] F. Bassani, L. Vervoort, I. Mihalcesu, J.C. Vial, F. Arnaud d’Avitaya, J. Appl. Phys. 79 (1996) 4066. [5] F. Bassani, L. Vervoort, I. Mihalcescu, J.C. Vial, F. Arnaud d’Avitaya, in: B. Gil, R.-L. Aulombard (Eds.), Semiconductor Heteroepitaxy, Growth, Characterization and Device Applications, World Scientific, 1995, pp. 286– 293. [6] R.A. Street, Adv. Phys. 30 (1981) 618. [7] I. Mihalcescu, Thesis, University J. Fourier, Grenoble, 1994. [8] J.C. Vial, A. Bsiesy, F. Gaspard, R. He´rino, M. Ligeon, F. Muller, R. Romestain, R.M. Macfarlane, Phys. Rev. B 45 (1992) 14171. [9] F. Bassani, I. Mihalcescu, J.C. Vial, F. Arnaud d’Avitaya, Appl. Surf. Sci. 117/118 (1997) 670. [10] F. Bassani, S. Me´nard, F. Arnaud d’Avitaya, Phys. Status Solidi A 165 (1997) 49. [11] I. Mihalcescu, M. Ligeon, F. Muller, R. Romestain, J.C. Vial, J. Luminesc. 57 (1993) 111.