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Observation of the nucleation kinetics of Si quantum dots on SiO2 by energy filtered transmission electron microscopy
Applied Surface Science 205 (2003) 304±308
Observation of the nucleation kinetics of Si quantum dots on SiO2 by energy ®ltered transmission electron microscopy G. Nicotraa,*, S. Lombardoa, C. Spinellaa, G. Ammendolab, C. Gerardib, C. Demurob a
Consiglio Nazionale delle Ricerche (CNR), Istituto per la Microelettronica e i Microsistemi (IMM), Stradale Primosole 50, 95121 Catania, Italy b STMicroelectronics, Central R&D, Catania Technology Center, Stradale Primosole 50, 95121 Catania, Italy Received 2 August 2002; accepted 30 September 2002
Abstract The formation of Si quantum dots on SiO2 by chemical vapour deposition of SiH4 is investigated by energy ®ltered transmission electron microscopy. It is demonstrated that this technique allows to measure size distributions down to dimensions of about 1 nm. This capability allows to put in evidence some important microscopic features of the nucleation process, whose consideration is fundamental to control the Si dot size. These aspects are shown and discussed. # 2002 Elsevier Science B.V. All rights reserved. PACS: 81.16.-c; 81.07.Ta; 85.35.-p Keywords: Nucleation and growth; Si quantum dots; CVD; Energy ®ltered TEM
The analysis of Si nanostructures on oxidized substrates is an important capability, given the possibility to exploit this system to obtain new functions in novel devices for microelectronics [1±3] and photonics [4]. One of the major issues in this ®eld is a strict control of the dot size distribution, since fundamental parameters ruling the electronic transport, such as the Coulomb blockade and the energy quantization due to the carrier con®nement are strong functions of the size and shape of the dots [5]. As a consequence, it is necessary to strictly control the dot size distribution *
Corresponding author. Tel.: 39-95-7351056; fax: 39-95-7139154. E-mail address: [email protected] (G. Nicotra).
and, therefore, to deeply understand the phenomena involved in the nucleation, growth, and coalescence of the dots. Several methods to synthesize the Si dots have been proposed and investigated: ion implantation [4,6], aerosol [7], and chemical vapor deposition (CVD) [1,8±10]. In this paper we show some essential features of the formation of Si dots by CVD of SiH4 on oxidized Si substrates. These features are evidenced through a suitable technique for the microscopic observation of the Si nanostructures. For this purpose, scanning electron microscopy (SEM) by using ®eld emission gun microscopes and atomic force microscopy (AFM) are widely used. However, these techniques show limits, due, in the former case, to the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 1 5 5 - 8
G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
resolution insuf®cient compared to the requirements of the system under analysis, while, in the latter case, to the convolution of the surface topography with the probe tip. Also the standard transmission electron microscopy shows limits, since it generally requires to have the Si dots in the crystalline phase, rather than in the amorphous one. To study the morphology of samples with Si dots deposited on oxidized Si substrates we have investigated the use of energy ®ltered TEM (EFTEM). In this paper we will also demonstrate that such technique is particularly suitable for this purpose. Si dots were deposited by CVD of SiH4 with H2 as carrier gas. The dot deposition has been carried out on 8 in. oxidized Si substrates at 550 8C. Deposition times are quite low, and therefore compatible with the high throughput required for ultra-large scale integration (ULSI) processing, and they range between 80 and 140 s. In some cases the samples have been annealed after deposition at 1000 8C for 40 s. EFTEM has been carried out by using a JEOL JEM 2010F TEM operating at 200 kV accelerating voltage and equipped with a ®eld emission gun. The energy ®ltering system is a GATAN GIF based on a magnetic sector and it has an energy resolution of about 0.8 eV.
305
To put in evidence the Si dots, we have used energy selected bright ®eld imaging with an energy loss tuned, within an energy window of 4 eV, to the value of the Si bulk plasmon (17 eV). Given the high energy resolution of the system, the Si plasmon loss is well separated from the SiO2 bulk plasmon (26 eV). So, we have used this clear energy separation to enhance the contrast between the Si dots and the oxide substrate. To fully exploit the potential of the energy ®lter, however, we have to eliminate the contribution to the image due to the Si substrate below the oxide. So it is necessary to analyze regions of the specimen suf®ciently thin, in which only the SiO2 ®lm is present and the underlying Si substrate has been etched off. These regions are particularly suitable for imaging the Si dots with high contrast with respect to the substrate. The contrast in this case is independent of the phase of the dots (i.e., whether crystalline or amorphous) and on their orientation. Fig. 1 shows a sequence of EFTEM micrographs of samples deposited for consecutive times, equal to 80 s (a), 90 s (b), 100 s (c), 110 s (d), 120 s (e), and 140 s (f). The white regions correspond to Si dots deposited on the oxidized surfaces. The nucleated dots show good wetting of the oxidized surface, with a contact
Fig. 1. EFTEM micrographs of samples deposited at 550 8C for consecutive times, equal to 80 s (a), 90 s (b), 100 s (c), 110 s (d), 120 s (e), and 140 s (f). The white regions correspond to Si dots deposited on the oxidized surfaces.
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G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
angle of about 258, evaluated by a cross-sectional TEM analysis. Such value of the contact angle has been reproducibly found in a number of cases, on asdeposited samples with Si coverage equal to or less than 10%. Above such coverage cross-sectional TEM does not allow to accurately measure the contact angle since the superposition of the images of different dots seen in transmission renders very dif®cult the measure of the sample. After annealing, the contact angle does not result anymore strictly constant, may be as a consequence of the dependence of the Si±SiO2 and the Si±vacuum energies on the grain orientation. This point is currently under study. From the images of Fig. 1, as the deposition time increases, it is evident that the increase in the amount of coverage of the oxidized surface with Si. Moreover, in correspondence with the lower deposition times (micrographs (a) and (b)), it is evident a process of nucleation of the Si dots, since the density of dots increases. The following period (micrographs (c)±(e)) corresponds to a coalescence phase up to the complete coverage with Si (f). It is, however, particularly important to note in micrographs (c)±(e) that, though a large fraction of the surface is covered with Si, in the uncovered regions new Si nuclei do continue to appear. This suggests that a continuous nucleation phenomenon, with no or a small limitation in the number of nucleation sites, is taking place. Such observation is in
Fig. 2. Time evolution of the Si dot size distributions relative to the depositions of durations of 80 s (triangles), 90 s (squares), and 100 s (circles). The corresponding EFTEM images are reported in Fig. 1(a)±(c). The dashed lines are best ®t curves calculated according to the model in Ref. [10].
agreement with previous observations based on SEM analyses on samples with lower dot densities [10]. Further information can be gained by an analysis of the grain size distributions. Fig. 2 reports the time evolution of the distributions relative to the deposition of times of 80 s (Fig. 1(a)), 90 s (Fig. 1(b)), and 100 s (Fig. 1(c)). We have concentrated our attention on such distributions since in these cases the coalescence effects play, at least at low grain size, a minor role, as discussed below. Each distribution is evaluated by using a sample of about 500 dots. The error bars have a length equal to twice the standard deviation, evaluated by assuming the Poisson statistics. As expected, as the deposition time increases the tail of the distribution shifts toward larger dot sizes. But in addition, the use of a high resolution microscopy technique allows us to show that at the low sizes the dot concentrations are very high and that they coincide within the experimental errors at various times. In fact, such conclusion concerning the distribution at low sizes may be affected by errors due to coalescence, in particular in the case of the sample relative to the 100 s deposition, where the Si coverage is higher. In general, coalescence produce errors because of two effects: one is the decrease in time of the area available for the nucleation of new dots, i.e., of the surface fraction not covered with Si. The second source of error is that when two dots coalesce, they are computed in the size distribution as a single dot of larger radius. However, when the surface fraction covered with Si is low (5% in the 80 s deposition, 10% for the 90 s, 18% for the 100 s), even though the coalescence effect cannot be neglected, some conclusions concerning the size distributions at low sizes, can be drawn. In fact, the probability P to have coalescence of a dot of size r with the other grains on the surface can be estimated as P Np R r2 , where R is the average dot radius and N the dot density. In our case (depositions from 80 to 100 s), for radii from 1 to 2 nm, P is between 10 and 20% in the sample at 80 s, and it arrives (worst case) up to 40% in the sample at 100 s. Most likely, such coalescence effects can be taken into account to correct the distributions. But in any case, we can safely conclude that at the low sizes the dot population does not change during time between 80 and 100 s. The steady state in the dot population at the low sizes re¯ects the balance between the number of dots per unit time which
G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
nucleate and the number of dots which grow towards large sizes. If so, to our knowledge, this is the ®rst experimental observation in Si of continuous nucleation in steady state, demonstrated by the behavior of the size distributions measured down to sizes very close to the critical radius. In the case of continuous nucleation in steady state, one expects to have the highest dot concentrations down to the low sizes and, as time increases, to observe a lateral ``diffusion'' of the distribution tail with a time independent plateau at the low sizes [11,12]. This is indeed what is experimentally observed, as reported in Fig. 2. As further proof, we have ®tted the size distributions by using an accurate analytical model of continuous nucleation in the approximation of capillarity [11]. This model solves the Frenkel±Zeldovich
307
nucleation equation in the supercritical region and it provides an analytical solution containing four free parameters. The best ®t curves, reported in Fig. 2 and all calculated by assuming the same set of plausible values for the free parameters, show good agreement with the data. The data have not been corrected for coalescence and this gives a good explanation for the deviation of the theoretical curves at the large sizes. The observation of continuous nucleation in steadystate conditions has an important implication concerning the accurate control of the grain size distributions: in fact, under these conditions, by increasing the deposition time one would expect to observe a worsening of the dispersion of the size distribution. Coalescence may, on the other hand, mitigate this negative effect.
Fig. 3. Sequence of EFTEM micrographs of samples as-deposited and after annealing at 1000 8C for 40 s. The micrographs (a) and (b) refer to samples subjected to 90 s deposition, before (a) and after annealing (b). Micrographs (c) and (d) refer to the corresponding sequence in the case of 100 s deposition.
308
G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
Another important aspect concerns two effects observed after thermal annealing on such nanostructures. The ®rst effect is the phase change of the dots, put in evidence through high resolution TEM, from amorphous after the deposition at 550 8C, to crystal after the rapid thermal annealing at 1000 8C for 40 s. The second concerns an effect of decrease in the amount of Si on the surface. Fig. 3 is a sequence of EFTEM micrographs of samples as-deposited and after annealing. The micrographs (a) and (b) refer to samples subjected to a 90 s deposition, before (a) and after annealing (b). Micrographs (c) and (d) refer to the corresponding sequence in the case of a 100 s deposition. The analysis shows a clear effect of decrease in the number of atoms from the Si nuclei: after annealing the dot density and size decrease. By the analysis of the difference between the dot size distributions before and after the annealing the amount of lost Si results in the order of 1 1014 atoms/cm2. It is interesting to note that in the case of the 80 s deposition, in which the amount of deposited Si is indeed about 1 1014 atoms/cm2, after annealing it is observed the complete loss of the Si. In summary, we have demonstrated that through the EFTEM technique one is able to directly measure grain size distributions for radii down to about 1 nm, i.e., down to cluster sizes of less than about 30 atoms. This ability has allowed to put in evidence major aspects of the kinetics of the formation of Si dots by CVD. One is the occurrence of continuous steady-state nucleation during the CVD deposition. Another is the observation of Si loss from the dots after rapid thermal annealing at 1000 8C.
Acknowledgements The authors would like to thank M. Vulpio for his fundamental initial contributions, C. Bongiorno for his precious technical assistance and R. Puglisi for useful discussions. This work is supported by the CNR program ``Nanotecnologie'' and by the project ``ADAMANT'' (contract no. IST-2001-34234).
References [1] S. Tiwari, F. Rana, K. Chan, H. Hana®, W. Chan, D. Buchanan, IEDM 521 (1995). [2] K. Yano, T. Ishii, T. Sano, T. Mine, F. Murai, T. Hashimoto, T. Kobayashi, T. Kure, K. Seki, Proc. IEEE 87 (1999) 633. [3] L. Guo, E. Leobandung, S.Y. Chou, Science 275 (1997) 649. [4] T. Gebel, L. Rebohle, J. Zhao, D. Borchert, H. FroÈb, J.V. Borany, W. Skorupa, Mater. Res. Soc. Symp. Proc. 638 (2001) F18.1. [5] A. Thean, J.P. Leburton, J. Appl. Phys. 90 (2001) 6384. [6] H.I. Hana®, S. Tiwari, I. Khan, IEEE Trans. Electron. Dev. 43 (1996) 1553. [7] J. De Blauwe, M. Ostraat, M.L. Green, G. Weber, T. Sorsch, A. Kerber, F. Klemens, R. Cirelli, E. Ferry, J.L. Grazul, F. Baumann, Y. Kim, W. Mans®eld, J. Bude, J.T.C. Lee, S.J. Hillenius, R.C. Flagan, H.A. Atwater, IEEE Int. Electron Devices Meeting (IEDM) Tech Dig., 2000, pp. 683±686. [8] J. Vizoso, F. Martin, X. Martinez, X. Aymerich, Microelectron. Eng. 48 (1999) 431. [9] T. Baron, F. Martin, P. Mur, C. Wyon, M. Dupuy, J. Cryst. Growth 209 (2000) 1004. [10] S. Madhukar, K. Smith, R. Muralidhar, D. O'Meara, M. Sadd, B.-Y. Nguyen, B. White, B. Jones, Mater. Res. Soc. Symp. Proc. 638 (2001) F5.2.1. [11] V.A. Schneidman, Sov. Phys. Technol. Phys. 33 (1988) 1338. [12] C. Spinella, S. Lombardo, F. Priolo, J. Appl. Phys. 84 (1998) 5383.
Observation of the nucleation kinetics of Si quantum dots on SiO2 by energy ®ltered transmission electron microscopy G. Nicotraa,*, S. Lombardoa, C. Spinellaa, G. Ammendolab, C. Gerardib, C. Demurob a
Consiglio Nazionale delle Ricerche (CNR), Istituto per la Microelettronica e i Microsistemi (IMM), Stradale Primosole 50, 95121 Catania, Italy b STMicroelectronics, Central R&D, Catania Technology Center, Stradale Primosole 50, 95121 Catania, Italy Received 2 August 2002; accepted 30 September 2002
Abstract The formation of Si quantum dots on SiO2 by chemical vapour deposition of SiH4 is investigated by energy ®ltered transmission electron microscopy. It is demonstrated that this technique allows to measure size distributions down to dimensions of about 1 nm. This capability allows to put in evidence some important microscopic features of the nucleation process, whose consideration is fundamental to control the Si dot size. These aspects are shown and discussed. # 2002 Elsevier Science B.V. All rights reserved. PACS: 81.16.-c; 81.07.Ta; 85.35.-p Keywords: Nucleation and growth; Si quantum dots; CVD; Energy ®ltered TEM
The analysis of Si nanostructures on oxidized substrates is an important capability, given the possibility to exploit this system to obtain new functions in novel devices for microelectronics [1±3] and photonics [4]. One of the major issues in this ®eld is a strict control of the dot size distribution, since fundamental parameters ruling the electronic transport, such as the Coulomb blockade and the energy quantization due to the carrier con®nement are strong functions of the size and shape of the dots [5]. As a consequence, it is necessary to strictly control the dot size distribution *
Corresponding author. Tel.: 39-95-7351056; fax: 39-95-7139154. E-mail address: [email protected] (G. Nicotra).
and, therefore, to deeply understand the phenomena involved in the nucleation, growth, and coalescence of the dots. Several methods to synthesize the Si dots have been proposed and investigated: ion implantation [4,6], aerosol [7], and chemical vapor deposition (CVD) [1,8±10]. In this paper we show some essential features of the formation of Si dots by CVD of SiH4 on oxidized Si substrates. These features are evidenced through a suitable technique for the microscopic observation of the Si nanostructures. For this purpose, scanning electron microscopy (SEM) by using ®eld emission gun microscopes and atomic force microscopy (AFM) are widely used. However, these techniques show limits, due, in the former case, to the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 1 5 5 - 8
G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
resolution insuf®cient compared to the requirements of the system under analysis, while, in the latter case, to the convolution of the surface topography with the probe tip. Also the standard transmission electron microscopy shows limits, since it generally requires to have the Si dots in the crystalline phase, rather than in the amorphous one. To study the morphology of samples with Si dots deposited on oxidized Si substrates we have investigated the use of energy ®ltered TEM (EFTEM). In this paper we will also demonstrate that such technique is particularly suitable for this purpose. Si dots were deposited by CVD of SiH4 with H2 as carrier gas. The dot deposition has been carried out on 8 in. oxidized Si substrates at 550 8C. Deposition times are quite low, and therefore compatible with the high throughput required for ultra-large scale integration (ULSI) processing, and they range between 80 and 140 s. In some cases the samples have been annealed after deposition at 1000 8C for 40 s. EFTEM has been carried out by using a JEOL JEM 2010F TEM operating at 200 kV accelerating voltage and equipped with a ®eld emission gun. The energy ®ltering system is a GATAN GIF based on a magnetic sector and it has an energy resolution of about 0.8 eV.
305
To put in evidence the Si dots, we have used energy selected bright ®eld imaging with an energy loss tuned, within an energy window of 4 eV, to the value of the Si bulk plasmon (17 eV). Given the high energy resolution of the system, the Si plasmon loss is well separated from the SiO2 bulk plasmon (26 eV). So, we have used this clear energy separation to enhance the contrast between the Si dots and the oxide substrate. To fully exploit the potential of the energy ®lter, however, we have to eliminate the contribution to the image due to the Si substrate below the oxide. So it is necessary to analyze regions of the specimen suf®ciently thin, in which only the SiO2 ®lm is present and the underlying Si substrate has been etched off. These regions are particularly suitable for imaging the Si dots with high contrast with respect to the substrate. The contrast in this case is independent of the phase of the dots (i.e., whether crystalline or amorphous) and on their orientation. Fig. 1 shows a sequence of EFTEM micrographs of samples deposited for consecutive times, equal to 80 s (a), 90 s (b), 100 s (c), 110 s (d), 120 s (e), and 140 s (f). The white regions correspond to Si dots deposited on the oxidized surfaces. The nucleated dots show good wetting of the oxidized surface, with a contact
Fig. 1. EFTEM micrographs of samples deposited at 550 8C for consecutive times, equal to 80 s (a), 90 s (b), 100 s (c), 110 s (d), 120 s (e), and 140 s (f). The white regions correspond to Si dots deposited on the oxidized surfaces.
306
G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
angle of about 258, evaluated by a cross-sectional TEM analysis. Such value of the contact angle has been reproducibly found in a number of cases, on asdeposited samples with Si coverage equal to or less than 10%. Above such coverage cross-sectional TEM does not allow to accurately measure the contact angle since the superposition of the images of different dots seen in transmission renders very dif®cult the measure of the sample. After annealing, the contact angle does not result anymore strictly constant, may be as a consequence of the dependence of the Si±SiO2 and the Si±vacuum energies on the grain orientation. This point is currently under study. From the images of Fig. 1, as the deposition time increases, it is evident that the increase in the amount of coverage of the oxidized surface with Si. Moreover, in correspondence with the lower deposition times (micrographs (a) and (b)), it is evident a process of nucleation of the Si dots, since the density of dots increases. The following period (micrographs (c)±(e)) corresponds to a coalescence phase up to the complete coverage with Si (f). It is, however, particularly important to note in micrographs (c)±(e) that, though a large fraction of the surface is covered with Si, in the uncovered regions new Si nuclei do continue to appear. This suggests that a continuous nucleation phenomenon, with no or a small limitation in the number of nucleation sites, is taking place. Such observation is in
Fig. 2. Time evolution of the Si dot size distributions relative to the depositions of durations of 80 s (triangles), 90 s (squares), and 100 s (circles). The corresponding EFTEM images are reported in Fig. 1(a)±(c). The dashed lines are best ®t curves calculated according to the model in Ref. [10].
agreement with previous observations based on SEM analyses on samples with lower dot densities [10]. Further information can be gained by an analysis of the grain size distributions. Fig. 2 reports the time evolution of the distributions relative to the deposition of times of 80 s (Fig. 1(a)), 90 s (Fig. 1(b)), and 100 s (Fig. 1(c)). We have concentrated our attention on such distributions since in these cases the coalescence effects play, at least at low grain size, a minor role, as discussed below. Each distribution is evaluated by using a sample of about 500 dots. The error bars have a length equal to twice the standard deviation, evaluated by assuming the Poisson statistics. As expected, as the deposition time increases the tail of the distribution shifts toward larger dot sizes. But in addition, the use of a high resolution microscopy technique allows us to show that at the low sizes the dot concentrations are very high and that they coincide within the experimental errors at various times. In fact, such conclusion concerning the distribution at low sizes may be affected by errors due to coalescence, in particular in the case of the sample relative to the 100 s deposition, where the Si coverage is higher. In general, coalescence produce errors because of two effects: one is the decrease in time of the area available for the nucleation of new dots, i.e., of the surface fraction not covered with Si. The second source of error is that when two dots coalesce, they are computed in the size distribution as a single dot of larger radius. However, when the surface fraction covered with Si is low (5% in the 80 s deposition, 10% for the 90 s, 18% for the 100 s), even though the coalescence effect cannot be neglected, some conclusions concerning the size distributions at low sizes, can be drawn. In fact, the probability P to have coalescence of a dot of size r with the other grains on the surface can be estimated as P Np R r2 , where R is the average dot radius and N the dot density. In our case (depositions from 80 to 100 s), for radii from 1 to 2 nm, P is between 10 and 20% in the sample at 80 s, and it arrives (worst case) up to 40% in the sample at 100 s. Most likely, such coalescence effects can be taken into account to correct the distributions. But in any case, we can safely conclude that at the low sizes the dot population does not change during time between 80 and 100 s. The steady state in the dot population at the low sizes re¯ects the balance between the number of dots per unit time which
G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
nucleate and the number of dots which grow towards large sizes. If so, to our knowledge, this is the ®rst experimental observation in Si of continuous nucleation in steady state, demonstrated by the behavior of the size distributions measured down to sizes very close to the critical radius. In the case of continuous nucleation in steady state, one expects to have the highest dot concentrations down to the low sizes and, as time increases, to observe a lateral ``diffusion'' of the distribution tail with a time independent plateau at the low sizes [11,12]. This is indeed what is experimentally observed, as reported in Fig. 2. As further proof, we have ®tted the size distributions by using an accurate analytical model of continuous nucleation in the approximation of capillarity [11]. This model solves the Frenkel±Zeldovich
307
nucleation equation in the supercritical region and it provides an analytical solution containing four free parameters. The best ®t curves, reported in Fig. 2 and all calculated by assuming the same set of plausible values for the free parameters, show good agreement with the data. The data have not been corrected for coalescence and this gives a good explanation for the deviation of the theoretical curves at the large sizes. The observation of continuous nucleation in steadystate conditions has an important implication concerning the accurate control of the grain size distributions: in fact, under these conditions, by increasing the deposition time one would expect to observe a worsening of the dispersion of the size distribution. Coalescence may, on the other hand, mitigate this negative effect.
Fig. 3. Sequence of EFTEM micrographs of samples as-deposited and after annealing at 1000 8C for 40 s. The micrographs (a) and (b) refer to samples subjected to 90 s deposition, before (a) and after annealing (b). Micrographs (c) and (d) refer to the corresponding sequence in the case of 100 s deposition.
308
G. Nicotra et al. / Applied Surface Science 205 (2003) 304±308
Another important aspect concerns two effects observed after thermal annealing on such nanostructures. The ®rst effect is the phase change of the dots, put in evidence through high resolution TEM, from amorphous after the deposition at 550 8C, to crystal after the rapid thermal annealing at 1000 8C for 40 s. The second concerns an effect of decrease in the amount of Si on the surface. Fig. 3 is a sequence of EFTEM micrographs of samples as-deposited and after annealing. The micrographs (a) and (b) refer to samples subjected to a 90 s deposition, before (a) and after annealing (b). Micrographs (c) and (d) refer to the corresponding sequence in the case of a 100 s deposition. The analysis shows a clear effect of decrease in the number of atoms from the Si nuclei: after annealing the dot density and size decrease. By the analysis of the difference between the dot size distributions before and after the annealing the amount of lost Si results in the order of 1 1014 atoms/cm2. It is interesting to note that in the case of the 80 s deposition, in which the amount of deposited Si is indeed about 1 1014 atoms/cm2, after annealing it is observed the complete loss of the Si. In summary, we have demonstrated that through the EFTEM technique one is able to directly measure grain size distributions for radii down to about 1 nm, i.e., down to cluster sizes of less than about 30 atoms. This ability has allowed to put in evidence major aspects of the kinetics of the formation of Si dots by CVD. One is the occurrence of continuous steady-state nucleation during the CVD deposition. Another is the observation of Si loss from the dots after rapid thermal annealing at 1000 8C.
Acknowledgements The authors would like to thank M. Vulpio for his fundamental initial contributions, C. Bongiorno for his precious technical assistance and R. Puglisi for useful discussions. This work is supported by the CNR program ``Nanotecnologie'' and by the project ``ADAMANT'' (contract no. IST-2001-34234).
References [1] S. Tiwari, F. Rana, K. Chan, H. Hana®, W. Chan, D. Buchanan, IEDM 521 (1995). [2] K. Yano, T. Ishii, T. Sano, T. Mine, F. Murai, T. Hashimoto, T. Kobayashi, T. Kure, K. Seki, Proc. IEEE 87 (1999) 633. [3] L. Guo, E. Leobandung, S.Y. Chou, Science 275 (1997) 649. [4] T. Gebel, L. Rebohle, J. Zhao, D. Borchert, H. FroÈb, J.V. Borany, W. Skorupa, Mater. Res. Soc. Symp. Proc. 638 (2001) F18.1. [5] A. Thean, J.P. Leburton, J. Appl. Phys. 90 (2001) 6384. [6] H.I. Hana®, S. Tiwari, I. Khan, IEEE Trans. Electron. Dev. 43 (1996) 1553. [7] J. De Blauwe, M. Ostraat, M.L. Green, G. Weber, T. Sorsch, A. Kerber, F. Klemens, R. Cirelli, E. Ferry, J.L. Grazul, F. Baumann, Y. Kim, W. Mans®eld, J. Bude, J.T.C. Lee, S.J. Hillenius, R.C. Flagan, H.A. Atwater, IEEE Int. Electron Devices Meeting (IEDM) Tech Dig., 2000, pp. 683±686. [8] J. Vizoso, F. Martin, X. Martinez, X. Aymerich, Microelectron. Eng. 48 (1999) 431. [9] T. Baron, F. Martin, P. Mur, C. Wyon, M. Dupuy, J. Cryst. Growth 209 (2000) 1004. [10] S. Madhukar, K. Smith, R. Muralidhar, D. O'Meara, M. Sadd, B.-Y. Nguyen, B. White, B. Jones, Mater. Res. Soc. Symp. Proc. 638 (2001) F5.2.1. [11] V.A. Schneidman, Sov. Phys. Technol. Phys. 33 (1988) 1338. [12] C. Spinella, S. Lombardo, F. Priolo, J. Appl. Phys. 84 (1998) 5383.