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Characterisation of Ge nanocrystals in co-sputtered Ge+SiO2 system using raman spectroscopy, RBS and TEM
Scripta mater. 44 (2001) 1291–1295 www.elsevier.com/locate/scriptamat
CHARACTERISATION OF Ge NANOCRYSTALS IN CO-SPUTTERED GeⴙSiO2 SYSTEM USING RAMAN SPECTROSCOPY, RBS AND TEM Y.W. Ho,1 V. Ng,1 W.K. Choi,1 S.P. Ng,1 T. Osipowicz,2 H.L. Seng,2 W.W. Tjui3 and K. Li3 1
Microelectronics Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 2Research Center for Nuclear Microscopy, Department of Physics, National University of Singapore, Lower Kent Ridge, Singapore 119260 3Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 (Received August 21, 2000) (Accepted in revised form December 13, 2000) Keywords: Rutherford backscattering; Germanium; Nanocrystal; Transmission electron microscopy; Raman spectroscopy
1. Introduction The attraction of silicon-based optoelectronics application has spurred research efforts in the areas of growth, synthesis and characterization of group IV (Si, Ge) nanocrystals. In this paper, germanium nanocrystals (nc-Ge) were synthesized by rapid thermal annealing (RTA) rf-cosputtered thin film composed of silicon dioxide and germanium. The synthesis of nc-Ge via this method was previously reported [1,2], where Raman spectroscopy was used to estimate the size of the nanocrystals synthesized. In this paper we present the results from Raman spectroscopy, analyzed with data from Rutherford backscattering (RBS) and HRTEM. Raman spectroscopy has been used extensively to estimate the size of the nanocrystals and the crystallinity of germanium because of its convenience and non-destructive nature. One interesting feature was the broadening of the Raman spectra for samples annealed at 900 and 1000°C. We correlated these results with HRTEM analysis and RBS experiments to fine tune the phonon confinement theory for our system.
2. Experiment The samples were prepared by rf-cosputtering of SiO2 and Ge (99.99%) on to a ⬍100⬎ n-type Si substrate. Six pieces of Ge (10 ⫻ 10 ⫻ 0.3 mm3 each) were attached to a 4“ SiO2 target and cosputtered in an Anelva sputtering system (SPH-210H) in argon ambient. The sputtering pressure was maintained at 3 ⫻ 10⫺3 Torr with rf sputtering power set to 100W. The process time of 30 minutes yielded film of approximately 3000Å. RTA was carried out in an A.S.T. rapid thermal processor (SHS 10). The annealing temperature used ranged from 600-1000°C with annealing time of 300s and ramp-up and ramp-down rates fixed at 30°C/s. Raman measurements were made on a Renishaw 2000 Micro-Raman spectroscopy system with a 514.5nm argon laser. The Raman spectrum had a resolution of 0.74cm⫺1. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00743-6
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Figure 1. TEM pictures of samples annealed at (a) 800°C and (b) 1000°C. In RTA 800°C sample (a), dark patches are Ge nanocrystals in the SiO2 matrix. Inset in (a) shows an enlarged image of the nanocrystal. In RTA 1000°C sample (b), the silicon substrate is labelled in the picture. The darker patches in the SiO2 matrix are the Ge nanocrystals. The large crystallites are found only at the Si-SiO2 interface, with smaller nanocrystals further from the surface.
Rutherford back-scattering (RBS) was performed using 2MeV alpha particles generated from a Van de Graaf Accelerator with scattering angle fixed at 160°. A Philips FEG300CM was used to obtain high resolution transmission electron microscopy (HRTEM) cross-sectional images of the samples at 300kV. 3. Results and Discussion High-resolution TEM of samples annealed at 800°C and 1000°C are presented here. The as-sputtered sample showed a featureless amorphous film before annealing. After RTA at 800°C, nanocrystals that were almost spherical with diameter ⬃5nm were formed. From Fig. 1(a), it can be seen that the distribution of the nanocrystals is relatively even and the size is also quite uniform. The crystal lattice fringe separation of 0.33nm corresponds to the {111} planes of Ge with diamond structure. The nanocrystals formed are relatively spherical. Fig. 1(b) shows the nanocrystals that were synthesized by RTA at 1000°C. The same fringe separation was observed for the particles as those RTA at 800°C. Another set of lattice fringes was observed as well which measured to be 0.20nm, corresponding to that of the {220} planes of Ge with diamond structure. It has been observed that away from the SiO2 and substrate interface, the number of nanocrystals decreases rapidly until there are no nanocrystals after 70nm from the interface. Further away from the interface, there were also smaller nanocrystals that resembled those observed in RTA 800°C samples. At the interface, we observed large nanocrystals (⬃20nm) with many different types of defects including twin boundaries, amorphous regions, lattice defects and grain boundaries. We attribute these to the rapid growth rate induced by high Ge concentration coupled with high annealing temperature. The short duration of RTA did not allow for the defects to be annealed out. Estimations of the size distributions were obtained from the TEM pictures. The size distribution of the nc-Ge formed by RTA 800°C can be approximated by a Gaussian distribution with average size (diameter) of 6.7nm and standard deviation of 1.7nm. On the other hand, the size distribution of the RTA 1000°C sample forms two Gaussian distributions with averages (diameters) at 5.2 nm and 25 nm respectively. Thus the distribution of nanocrystals throughout the film was not uniform and the size distribution of the nanocrystals was also uneven for sample annealed at RTA 1000°C. The smaller nanocrystals were observed to be similar to those formed after 800°C RTA. The larger size distribution represented the large crystallites at the Si-SiO2 interface. Fujii et al [3] also had reported the effect of Ge concentration on the size of the nanocrystals synthesized. Fig. 2 shows the RBS profile of the rapid thermal annealed samples showing the relative sample orientation. The profiles of samples annealed at 600 – 800°C are similar to that of the as-sputtered
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Figure 2. RBS spectra for as-sputtered sample and samples annealed at 600-1000°C. Films annealed at 600 – 800°C shows little difference from that of the as-sputtered sample. At 900 and 1000°C, significant out-diffusion occurs at the surface and also diffusion towards the Si-SiO2 interface resulting in a pile-up at the interface.
sample, indicating that there was no change in the film thickness after RTA. There is no preferential direction for migration of Ge atoms. The samples annealed at 900°C and 1000°C have a low Ge concentration at the bulk and a pile up at the Si-SiO2 interface. This suggested significant out-diffusion of Ge at the surface into the ambient and also diffusion of Ge to the Si-SiO2 interface. Similar observation was made by Heinig et al [4] who attributed the phenomenon to the lower concentration of Ge dissolved in the SiO2 at the Si-SiO2 interface compared to that at the bulk. The resulting concentration gradient led to a diffusion flux toward the Si-SiO2 interface. The reason for this to occur only for samples annealed at 900°C and 1000°C could be due to the higher mobility of Ge atoms at high temperatures. Note that the melting point of Ge is 937.8°C. The RBS results supported the TEM results very well. Fig. 3 shows the Raman spectra of the as-sputtered sample and samples annealed at annealing temperatures ranging from 600-1000°C. The data have been intentionally displaced for clarity. The as-sputtered sample shows a broad hump characteristic of amorphous Ge and small Ge clusters. The onset of Ge nanocrystals formation is shown by the relative increase of the Raman peak at 300cm⫺1 for the sample annealed at 600°C. As the annealing temperature is increased to 700 and 800°C, the Raman line-shapes narrow down. However, at higher annealing temperatures of 900 and 1000°C, the Raman line-shape broadens significantly. For the sample annealed at 1000°C, the Raman spectrum was similar to that of the as-sputtered sample.
Figure 3. Raman spectra for as-sputtered sample and samples annealed from 600-1000°C. The spectra are displaced vertically for clarity.
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Figure 4. Theoretical Raman line-shape calculated with equations (1) and (2) plotted with experimental data for (a) RTA 800°C and (b) RTA 1000°C samples. Theory (1) and (2) uses equations (1) and (2) respectively. Theory (1) uses only a single size (diameter, L) whereas Theory (2) includes information of the size distribution.
Previously, the phonon confinement model developed by Nemanich et al [5], Richter et al [6] and Fauchet et al [7] was used to estimate the size of the nanocrystals [1] from the Raman spectra. From the model, the line intensity is given by I共 兲 ⫽
冕
兩C共0, q兲兩 2 ⌫2 关 ⫺ 共q兲兴 2 ⫹ 4
d3q
(1)
where C(0,q) is the Fourier transform of the confinement function, (q) is the phonon dispersion curve and ␥ is the natural line width of the Raman spectrum of bulk germanium (⬃3cm⫺1). The integral is taken over the entire Brillouin zone. Using the model without taking into consideration of the size distribution of the nanocrystals, the size of the nanocrystals for samples annealed at 900°C and 1000°C could be mistaken to be smaller than those prepared at 700°C and 800°C. This was contrary to the TEM observations which revealed that there were larger crystallites formed for samples annealed at 900°C and 1000°C. With estimations from TEM, the size distribution component was added to the phonon confinement model in (1), yielding I共 兲 ⫽
冕
共 ␦ 兲d␦ ⫻
冕
兩C共0, q兲兩 2 ⌫2 关 ⫺ 共q兲兴 2 ⫹ 4
d3q
(2)
where (␦) is the size distribution estimated from the TEM micrographs. Fig. 4a and b show the theoretical and experimental Raman spectra for samples annealed at 800 and 1000°C. A relatively good fit for both samples can be obtained from equation (2) that includes the effect of size distribution. In particular, it can be seen that the broadening on the left shoulder observed at 1000°C fits equation (2) better. This shows that the broadening was due to the uneven size distribution in the sample as observed by TEM. However, the fit of equation (2) with the experimental data is not as good in the 300 –310cm⫺1 region. We believe that this could be due to the presence of residual amorphous Ge dissolved in the SiO2 matrix and possibly also the strain and the disorder as observed in the TEM.
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4. Conclusions The HRTEM, RBS and Raman spectroscopy results of germanium nanocrystals embedded in SiO2 synthesized by RTA have been presented. HRTEM reveals that annealing temperatures below 800°C produce fewer and smaller Ge nanocrystals compared to those produced at 800°C. Above 800°C, the nanocrystals formed have a wide range of size distribution. Defects are also common. Therefore the optimum temperature is at 800°C to have uniformly sized and regularly spaced nanocrystals. RBS has been performed to establish the Ge distribution in the SiO2 matrix during annealing. Raman spectroscopy data has been fitted to a phonon confinement model which takes size distributions into account. Acknowledgments The authors would like to acknowledge the Data Storage Institute, Singapore for help in the Raman spectroscopy experiments and also the University for the provision of research scholarships for Y.W. Ho and S.P. Ng. We would also like to thank the National Science and Technology Board for the GR6471 research grant for this work. References 1. 2. 3. 4. 5. 6. 7.
W. K. Choi, V. Ng, S. P. Ng, H. H. Thio, Z. X. Shen, and W. S. Li, J. Appl. Phys. 86, 1389 (1999). W. K. Choi, H. H. Thio, S. P. Ng, V. Ng, and B. A. Cheong, Phil. Mag. B. 80, 729 (2000). M. Fujii, S. Hayashi, and K. Yamamoto, Jpn. J. Appl. Phys. 30, 687 (1991). K. H. Heinig, B. Schmidt, A. Markwitz, R. Grotzschel, M. Strobel, and S. Oswald, Nucl. Instrum. Methods Phys. Res. B. 148, 969 (1999). R. J. Nemanich, S. A. Solin, and R. M. Martin, Phys. Rev. B. 23, 6348 (1981). H. Richter, Z. P. Wang, and L, Ley, Solid State Commun. 39, 625 (1981). P. Fauchet and I. H. Campbell, Crit. Rev. Solid State Mater. Sci. 14(Suppl. 1), S79 (1988).
CHARACTERISATION OF Ge NANOCRYSTALS IN CO-SPUTTERED GeⴙSiO2 SYSTEM USING RAMAN SPECTROSCOPY, RBS AND TEM Y.W. Ho,1 V. Ng,1 W.K. Choi,1 S.P. Ng,1 T. Osipowicz,2 H.L. Seng,2 W.W. Tjui3 and K. Li3 1
Microelectronics Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 2Research Center for Nuclear Microscopy, Department of Physics, National University of Singapore, Lower Kent Ridge, Singapore 119260 3Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 (Received August 21, 2000) (Accepted in revised form December 13, 2000) Keywords: Rutherford backscattering; Germanium; Nanocrystal; Transmission electron microscopy; Raman spectroscopy
1. Introduction The attraction of silicon-based optoelectronics application has spurred research efforts in the areas of growth, synthesis and characterization of group IV (Si, Ge) nanocrystals. In this paper, germanium nanocrystals (nc-Ge) were synthesized by rapid thermal annealing (RTA) rf-cosputtered thin film composed of silicon dioxide and germanium. The synthesis of nc-Ge via this method was previously reported [1,2], where Raman spectroscopy was used to estimate the size of the nanocrystals synthesized. In this paper we present the results from Raman spectroscopy, analyzed with data from Rutherford backscattering (RBS) and HRTEM. Raman spectroscopy has been used extensively to estimate the size of the nanocrystals and the crystallinity of germanium because of its convenience and non-destructive nature. One interesting feature was the broadening of the Raman spectra for samples annealed at 900 and 1000°C. We correlated these results with HRTEM analysis and RBS experiments to fine tune the phonon confinement theory for our system.
2. Experiment The samples were prepared by rf-cosputtering of SiO2 and Ge (99.99%) on to a ⬍100⬎ n-type Si substrate. Six pieces of Ge (10 ⫻ 10 ⫻ 0.3 mm3 each) were attached to a 4“ SiO2 target and cosputtered in an Anelva sputtering system (SPH-210H) in argon ambient. The sputtering pressure was maintained at 3 ⫻ 10⫺3 Torr with rf sputtering power set to 100W. The process time of 30 minutes yielded film of approximately 3000Å. RTA was carried out in an A.S.T. rapid thermal processor (SHS 10). The annealing temperature used ranged from 600-1000°C with annealing time of 300s and ramp-up and ramp-down rates fixed at 30°C/s. Raman measurements were made on a Renishaw 2000 Micro-Raman spectroscopy system with a 514.5nm argon laser. The Raman spectrum had a resolution of 0.74cm⫺1. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00743-6
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Figure 1. TEM pictures of samples annealed at (a) 800°C and (b) 1000°C. In RTA 800°C sample (a), dark patches are Ge nanocrystals in the SiO2 matrix. Inset in (a) shows an enlarged image of the nanocrystal. In RTA 1000°C sample (b), the silicon substrate is labelled in the picture. The darker patches in the SiO2 matrix are the Ge nanocrystals. The large crystallites are found only at the Si-SiO2 interface, with smaller nanocrystals further from the surface.
Rutherford back-scattering (RBS) was performed using 2MeV alpha particles generated from a Van de Graaf Accelerator with scattering angle fixed at 160°. A Philips FEG300CM was used to obtain high resolution transmission electron microscopy (HRTEM) cross-sectional images of the samples at 300kV. 3. Results and Discussion High-resolution TEM of samples annealed at 800°C and 1000°C are presented here. The as-sputtered sample showed a featureless amorphous film before annealing. After RTA at 800°C, nanocrystals that were almost spherical with diameter ⬃5nm were formed. From Fig. 1(a), it can be seen that the distribution of the nanocrystals is relatively even and the size is also quite uniform. The crystal lattice fringe separation of 0.33nm corresponds to the {111} planes of Ge with diamond structure. The nanocrystals formed are relatively spherical. Fig. 1(b) shows the nanocrystals that were synthesized by RTA at 1000°C. The same fringe separation was observed for the particles as those RTA at 800°C. Another set of lattice fringes was observed as well which measured to be 0.20nm, corresponding to that of the {220} planes of Ge with diamond structure. It has been observed that away from the SiO2 and substrate interface, the number of nanocrystals decreases rapidly until there are no nanocrystals after 70nm from the interface. Further away from the interface, there were also smaller nanocrystals that resembled those observed in RTA 800°C samples. At the interface, we observed large nanocrystals (⬃20nm) with many different types of defects including twin boundaries, amorphous regions, lattice defects and grain boundaries. We attribute these to the rapid growth rate induced by high Ge concentration coupled with high annealing temperature. The short duration of RTA did not allow for the defects to be annealed out. Estimations of the size distributions were obtained from the TEM pictures. The size distribution of the nc-Ge formed by RTA 800°C can be approximated by a Gaussian distribution with average size (diameter) of 6.7nm and standard deviation of 1.7nm. On the other hand, the size distribution of the RTA 1000°C sample forms two Gaussian distributions with averages (diameters) at 5.2 nm and 25 nm respectively. Thus the distribution of nanocrystals throughout the film was not uniform and the size distribution of the nanocrystals was also uneven for sample annealed at RTA 1000°C. The smaller nanocrystals were observed to be similar to those formed after 800°C RTA. The larger size distribution represented the large crystallites at the Si-SiO2 interface. Fujii et al [3] also had reported the effect of Ge concentration on the size of the nanocrystals synthesized. Fig. 2 shows the RBS profile of the rapid thermal annealed samples showing the relative sample orientation. The profiles of samples annealed at 600 – 800°C are similar to that of the as-sputtered
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Figure 2. RBS spectra for as-sputtered sample and samples annealed at 600-1000°C. Films annealed at 600 – 800°C shows little difference from that of the as-sputtered sample. At 900 and 1000°C, significant out-diffusion occurs at the surface and also diffusion towards the Si-SiO2 interface resulting in a pile-up at the interface.
sample, indicating that there was no change in the film thickness after RTA. There is no preferential direction for migration of Ge atoms. The samples annealed at 900°C and 1000°C have a low Ge concentration at the bulk and a pile up at the Si-SiO2 interface. This suggested significant out-diffusion of Ge at the surface into the ambient and also diffusion of Ge to the Si-SiO2 interface. Similar observation was made by Heinig et al [4] who attributed the phenomenon to the lower concentration of Ge dissolved in the SiO2 at the Si-SiO2 interface compared to that at the bulk. The resulting concentration gradient led to a diffusion flux toward the Si-SiO2 interface. The reason for this to occur only for samples annealed at 900°C and 1000°C could be due to the higher mobility of Ge atoms at high temperatures. Note that the melting point of Ge is 937.8°C. The RBS results supported the TEM results very well. Fig. 3 shows the Raman spectra of the as-sputtered sample and samples annealed at annealing temperatures ranging from 600-1000°C. The data have been intentionally displaced for clarity. The as-sputtered sample shows a broad hump characteristic of amorphous Ge and small Ge clusters. The onset of Ge nanocrystals formation is shown by the relative increase of the Raman peak at 300cm⫺1 for the sample annealed at 600°C. As the annealing temperature is increased to 700 and 800°C, the Raman line-shapes narrow down. However, at higher annealing temperatures of 900 and 1000°C, the Raman line-shape broadens significantly. For the sample annealed at 1000°C, the Raman spectrum was similar to that of the as-sputtered sample.
Figure 3. Raman spectra for as-sputtered sample and samples annealed from 600-1000°C. The spectra are displaced vertically for clarity.
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Figure 4. Theoretical Raman line-shape calculated with equations (1) and (2) plotted with experimental data for (a) RTA 800°C and (b) RTA 1000°C samples. Theory (1) and (2) uses equations (1) and (2) respectively. Theory (1) uses only a single size (diameter, L) whereas Theory (2) includes information of the size distribution.
Previously, the phonon confinement model developed by Nemanich et al [5], Richter et al [6] and Fauchet et al [7] was used to estimate the size of the nanocrystals [1] from the Raman spectra. From the model, the line intensity is given by I共 兲 ⫽
冕
兩C共0, q兲兩 2 ⌫2 关 ⫺ 共q兲兴 2 ⫹ 4
d3q
(1)
where C(0,q) is the Fourier transform of the confinement function, (q) is the phonon dispersion curve and ␥ is the natural line width of the Raman spectrum of bulk germanium (⬃3cm⫺1). The integral is taken over the entire Brillouin zone. Using the model without taking into consideration of the size distribution of the nanocrystals, the size of the nanocrystals for samples annealed at 900°C and 1000°C could be mistaken to be smaller than those prepared at 700°C and 800°C. This was contrary to the TEM observations which revealed that there were larger crystallites formed for samples annealed at 900°C and 1000°C. With estimations from TEM, the size distribution component was added to the phonon confinement model in (1), yielding I共 兲 ⫽
冕
共 ␦ 兲d␦ ⫻
冕
兩C共0, q兲兩 2 ⌫2 关 ⫺ 共q兲兴 2 ⫹ 4
d3q
(2)
where (␦) is the size distribution estimated from the TEM micrographs. Fig. 4a and b show the theoretical and experimental Raman spectra for samples annealed at 800 and 1000°C. A relatively good fit for both samples can be obtained from equation (2) that includes the effect of size distribution. In particular, it can be seen that the broadening on the left shoulder observed at 1000°C fits equation (2) better. This shows that the broadening was due to the uneven size distribution in the sample as observed by TEM. However, the fit of equation (2) with the experimental data is not as good in the 300 –310cm⫺1 region. We believe that this could be due to the presence of residual amorphous Ge dissolved in the SiO2 matrix and possibly also the strain and the disorder as observed in the TEM.
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4. Conclusions The HRTEM, RBS and Raman spectroscopy results of germanium nanocrystals embedded in SiO2 synthesized by RTA have been presented. HRTEM reveals that annealing temperatures below 800°C produce fewer and smaller Ge nanocrystals compared to those produced at 800°C. Above 800°C, the nanocrystals formed have a wide range of size distribution. Defects are also common. Therefore the optimum temperature is at 800°C to have uniformly sized and regularly spaced nanocrystals. RBS has been performed to establish the Ge distribution in the SiO2 matrix during annealing. Raman spectroscopy data has been fitted to a phonon confinement model which takes size distributions into account. Acknowledgments The authors would like to acknowledge the Data Storage Institute, Singapore for help in the Raman spectroscopy experiments and also the University for the provision of research scholarships for Y.W. Ho and S.P. Ng. We would also like to thank the National Science and Technology Board for the GR6471 research grant for this work. References 1. 2. 3. 4. 5. 6. 7.
W. K. Choi, V. Ng, S. P. Ng, H. H. Thio, Z. X. Shen, and W. S. Li, J. Appl. Phys. 86, 1389 (1999). W. K. Choi, H. H. Thio, S. P. Ng, V. Ng, and B. A. Cheong, Phil. Mag. B. 80, 729 (2000). M. Fujii, S. Hayashi, and K. Yamamoto, Jpn. J. Appl. Phys. 30, 687 (1991). K. H. Heinig, B. Schmidt, A. Markwitz, R. Grotzschel, M. Strobel, and S. Oswald, Nucl. Instrum. Methods Phys. Res. B. 148, 969 (1999). R. J. Nemanich, S. A. Solin, and R. M. Martin, Phys. Rev. B. 23, 6348 (1981). H. Richter, Z. P. Wang, and L, Ley, Solid State Commun. 39, 625 (1981). P. Fauchet and I. H. Campbell, Crit. Rev. Solid State Mater. Sci. 14(Suppl. 1), S79 (1988).