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SiO2 layers
NanoStructured Materials, Vol. 11, No. 8, pp. 1239 –1243, 1999 Elsevier Science Ltd Copyright © 2000 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter
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PII S0965-9773(99)00414-6
VISIBLE PHOTOLUMINESCENCE IN ION BEAM MIXED SiO2/Si/SiO2 LAYERS K.H. Chae1, J.H. Son1, G.S. Chang1, H.B. Kim1, J.Y. Jeong2, S. Im2, J.H. Song3, K.J. Kim4, H.K. Kim4 and C.N. Whang1* 1
Atomic-Scale Surface Science Research Center and Department of Physics, Yonsei University, Seoul 120-749, Korea 2 Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea 3 Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 130-650, Korea 4 Surface Analysis Group, Korea Research Institute of Standards and Science, Taejon 305-600, Korea (Received September 3, 1999) (Accepted November 5, 1999) Abstract—Visible photoluminescence from silicon nanocrystals embedded in SiO2 matrix by ion beam mixing was investigated. Photoluminescence spectra of ion beam mixed SiO2/Si/SiO2 films excited by an Ar-laser (457.9 nm) showed more intense luminescence with a peak centered at 720 nm than that prepared by the conventional ion implantation method. The formation of nanocrystals in SiO2 matrix was confirmed by cross-sectional high resolution transmission electron microscopy. The red luminescence is attributed to the silicon nanocrystals produced by ion beam mixing. ©2000 Acta Metallurgica Inc.
1. Introduction Photoluminescence (PL) from silicon nanocrystals embedded in SiO2 has been under active investigation due to its potential application in Si-based optoelectronic devices. According to the quantum confinement model, PL is sensitive to the nanocrystal shape and size distribution [1]. Among various techniques such as selective-size precipitation [2], spark erosion [3], ion implantation [4,5], and chemical vapor deposition [6] to produce Si nanocrystals, ion implantation technique has been used by many researchers for studying PL in silicon nanocrystals, because it is a versatile technique where many parameters can be easily modified and controlled. However, high concentration of Si nanocrystals is hardly achieved by ion implantation due to its sputtering effect. The sputtered atoms are typically balanced with the incident ones about 10 atomic % of doping concentration [7]. Here we report the application of ion beam mixing to PL from Si nanocrystals formed in a certain depth of SiO2 layers with high efficiency. Ion beam mixing [8,9] has been known as a powerful technique to form nanometersized material clusters embedded in dielectric matrices, keeping most useful advantages of ion implantation. In addition, the ion beam mixing process can overcome the limitation of Si concentration, because alternately deposited films are intermixed by energetic heavy ions, and form a homogeneously mixed layer over the penetration depth of the irradiating ions. * Corresponding author.
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Figure 1. Photoluminescence spectra of the five samples; (a) annealed at 1100°C for 2 hours after ion beam mixing, (b) annealed at 1100 °C for 2 hours without ion beam mixing, (c) as-mixed, (d) as-deposited, and (e) Si⫺ implanted SiO2 sample followed by annealing at 1100 °C for 2 hours.
2. Experiments Amorphous SiO2 (a-SiO2) and amorphous Si (a-Si) layers were deposited in an order of 100 nm SiO2, 3 nm Si, and 60 nm SiO2 on Si(001) substrate by ion sputtering at room temperature (RT). In these processes, a-Si layer was deposited by Ar⫹ ion sputtering of single crystal Si target at an Ar pressure of 5 ⫻ 10⫺5 Torr, while a-SiO2 layer was deposited by O2⫹ ion sputtering of single crystal Si target at an O2 pressure of 5 ⫻ 10⫺5 Torr. The Ar⫹ ions of 80 keV were irradiated into the SiO2/Si/SiO2 layered samples at RT with the dose of 5 ⫻ 1015 ions/cm2, followed by annealing at a temperature of 1100°C for various periods to remove the defects induced by ion irradiation and produce silicon nanocrystals. All thermal annealing was performed in an N2 ambient. Si⫺ implanted sample was also prepared for comparison purpose. Si ions were directly implanted into 300nm thick SiO2 layer on Si(001) substrate with a dose of 1 ⫻ 1017 ions/cm2 at RT. This sample was annealed in the same condition with ion beam mixed ones. For PL measurement, an Ar-laser (457.9 nm) with a power of 100 mW was used as an excitation source and the luminescence was detected by a cooled photomultiplier tube (PMT) employing the photon counting technique. A cutoff filter to pass only long waves above 475 nm was used to block the light scattered from the source. Cross-sectional high resolution transmission electron microscopy (HRTEM) was used to observe silicon nanocrystals in the ion beam mixed SiO2/Si/SiO2 layers. 3. Results and Discussion Figure 1 shows the PL spectra of SiO2/Si/SiO2 samples; (a) annealed at 1100°C for 2 hours after ion
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Figure 2. Cross sectional HRTEM images for the samples: (a) as-deposited, (b) annealed without ion beam mixing, and (c) annealed after ion beam mixing of SiO2/Si/SiO2.
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beam mixing, (b) annealed at 1100°C for 2 hours without ion beam mixing, (c) as-mixed, and (d) as-deposited. The PL spectrum (e) of Si⫺ implanted sample followed by annealing at 1100°C for 2 hours is also added for comparison. Both as-deposited and as-mixed samples have no PL behavior, because Si and SiO2 layers are probably amorphous states with high density of defects. On the other hand, the sample annealed at 1100°C for 2 hours without ion beam mixing shows a broad PL peak centered at 730 nm even though the intensity is weak. Recently, Tsybeskov et al. [1] reported that the nanocrystalline-silicons having nearly spherical shape and the size limited by the thickness of a-Si layer could be produced by controlled recrystallization of an a-Si/SiO2 superlattice. However, the PL spectra from the nanocrystalline-silicon superlattice at RT was found to be similar to bulk Si, because the sizes of silicon nanocrystals exceed the exciton Bohr diameter (dexc ⯝ 10 nm for Si). Shimizu-Iwayama et al. [4,10] reported a broad PL peak centered at 730 nm after annealing at 1100°C for the Si implanted SiO2. They attributed the 730 nm band to the presence of silicon nanocrystals. In our case, silicon nanocrystals smaller than the exciton Bohr diameter can be produced since the deposited a-Si layer is as thin as 3 nm. This suggests that the PL peak in Figure 1(b) seems to originate from some silicon nanocrystals smaller than the exciton Bohr diameter formed during the high temperature annealing. The sample annealed at 1100°C for 2 hours after ion beam mixing shows a broad PL peak centered at 720 nm at room temperature with strong intensity as in Figure 1(a). Since the amorphous silicon layer is effectively broken to pieces and mixed with SiO2 during the ion beam mixing process, the proper-sized silicon nanocrystals may increase in number, which results in an increased PL intensity compared with the peak intensity of PL spectrum in Figure 1(b). Moreover, the PL intensity of ion beam mixed sample was stronger than that of Si⫺ implanted sample. The 3 nm thickness of Si layer in SiO2 matrix is corresponding to 1 ⫻ 1016 Si⫺/cm2 in terms of Si⫺ implantation. Si concentration of ion beam mixed sample was smaller than that of Si⫺ implanted sample used in this experiment. However, PL intensity of ion beam mixed sample is stronger than that of Si⫺ implanted sample by a factor of 2. We suggest that this reason attributes to high density of Si nanocrystals of ion beam mixed sample compared to Si⫺ implanted sample. The formation of silicon nanocrystals is confirmed by cross-sectional HRTEM in Figure 2, where the cross sectional images for three samples are shown; (a) as-deposited, (b) annealed at 1100°C for 2 hours without ion beam mixing, and (c) annealed at 1100°C for 2 hours after ion beam mixing of SiO2/Si/SiO2 samples. The micrograph of annealed samples are magnified for reader’s convenience. As seen in Figure 2(a), a thin amorphous silicon layer is placed between SiO2 layers. After annealing without ion beam mixing, thin amorphous silicon layers turned into crystalline silicon layers as seen in Figure 2(b). However, this crystalline silicon layers can not be attributed to PL intensity since they exceed the exciton Bohr diameter. After ion beam mixing and subsequent annealing at 1100°C for 2 hours, the amorphous silicon layer turns into silicon nanocrystals. Silicon nanocrystals with clear {111} lattice planes are seen in Figure 2(c). One can see that the number of proper-sized silicon nanocrystal is significantly increased compared with the annealed sample without ion beam mixing. The average size of silicon nanocrystals are found to be ⬃5 nm. According to a model for the luminescence spectrum of silicon nanocrystals presented by Trwoga et al, [11] the 5 nm sized silicon nanocrystal shows a good agreement with the observed 720 nm PL band. Combining the results shown in Figure 1 and 2, it is suggested that the ion beam mixing seems to be an effective method to produce proper-sized silicon nanocrystals in a desired depth of SiO2 layer while the silicon nanocrystals should be the origin of the strong PL observed ion beam mixed SiO2/Si/SiO2 samples.
4. Conclusion In summary, silicon nanocrystals embedded in SiO2 layers have been produced by ion beam mixing of SiO2/Si/SiO2 layer at RT, followed by high temperature annealing in N2 ambient. The formation of
Vol. 11, No. 8
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nanocrystals in SiO2 matrices is confirmed by cross-sectional HRTEM. The average size of nanocrystals are found ⬃5 nm. The PL spectrum of the ion beam mixed sample shows an intense broad luminescence with a peak centered at 720 nm at room temperature, while that of the unmixed one does weaken luminescence near the same peak position. In comparing ion beam mixed sample with Si⫺ implanted sample, the former have higher PL intensity. According to quantum confinement model [11], this red luminescence corresponds to the silicon nanocrystal size of ⬃5 nm. The overall results suggest that the ion beam mixing method under proper irradiation condition can be a promising technique for PL of Si nanocrystal with high efficiency. When it is applied to Si/SiO2 multilayered film, the additional enhancement is expected with respect to our monolayered SiO2/Si/SiO2 film. Acknowledgments This work was supported in part by Korea Research Foundation made in the program year of 1998 and in part by grand (No. 1999-2-114-004-5) from the interdisciplonary research grogram of the Korea Science and Engineering Foundation (KOSEF). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
L. Tsybeskov, K. D. Hirschman, S. P. Duttagupta, M. Zacharias, P. M. Fauchet, J. P. McCaffrey, and D. J. Lockwood, Appl. Phys. Lett. 72, 43 (1998). W. L. Wilson, P. J. Szajowski, and L. E. Brus, Science. 262, 1242 (1993). R. E. Hummel and S.-S. Chang, Appl. Phys. Lett. 61, 1965 (1992). T. Shimazu-Iwayama, S. Nakao, and K. Saitoh, Appl. Phys. Lett. 65, 1814 (1994). P. Mutti, G. Ghislotti, S. Bertoni, L. Bonoldi, G. F. Cerofolini, L. Meda, E. Grilli, and M. Guzzi, Appl. Phys. Lett. 66, 851 (1995). H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, and T. Nakagiri, Appl. Phys. Lett. 56, 2379 (1990). C. M. Preece and J. K. Hiroven, Ion Implantation Metallurgy, p. 38, Metallurgic Society of AIME, Warrendale, PA (1979). S. Matteson and M. A. Nicolet, Annu. Rev. Mater. Sci. 13, 339 (1983). B. M. Paine, and R. S. Averback, Nucl. Instr. Methods. B7/8, 666 (1985). T. Shimazu-Iwayama, N. Kurumado, D. E. Hole, and P. D. Townsend, J. Appl. Phys. 83, 6018 (1998). P. F. Trwoga, A. J. Kenyon, and C. W. Pitt, J. Appl. Phys. 83, 3789 (1998).
Pergamon
PII S0965-9773(99)00414-6
VISIBLE PHOTOLUMINESCENCE IN ION BEAM MIXED SiO2/Si/SiO2 LAYERS K.H. Chae1, J.H. Son1, G.S. Chang1, H.B. Kim1, J.Y. Jeong2, S. Im2, J.H. Song3, K.J. Kim4, H.K. Kim4 and C.N. Whang1* 1
Atomic-Scale Surface Science Research Center and Department of Physics, Yonsei University, Seoul 120-749, Korea 2 Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea 3 Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 130-650, Korea 4 Surface Analysis Group, Korea Research Institute of Standards and Science, Taejon 305-600, Korea (Received September 3, 1999) (Accepted November 5, 1999) Abstract—Visible photoluminescence from silicon nanocrystals embedded in SiO2 matrix by ion beam mixing was investigated. Photoluminescence spectra of ion beam mixed SiO2/Si/SiO2 films excited by an Ar-laser (457.9 nm) showed more intense luminescence with a peak centered at 720 nm than that prepared by the conventional ion implantation method. The formation of nanocrystals in SiO2 matrix was confirmed by cross-sectional high resolution transmission electron microscopy. The red luminescence is attributed to the silicon nanocrystals produced by ion beam mixing. ©2000 Acta Metallurgica Inc.
1. Introduction Photoluminescence (PL) from silicon nanocrystals embedded in SiO2 has been under active investigation due to its potential application in Si-based optoelectronic devices. According to the quantum confinement model, PL is sensitive to the nanocrystal shape and size distribution [1]. Among various techniques such as selective-size precipitation [2], spark erosion [3], ion implantation [4,5], and chemical vapor deposition [6] to produce Si nanocrystals, ion implantation technique has been used by many researchers for studying PL in silicon nanocrystals, because it is a versatile technique where many parameters can be easily modified and controlled. However, high concentration of Si nanocrystals is hardly achieved by ion implantation due to its sputtering effect. The sputtered atoms are typically balanced with the incident ones about 10 atomic % of doping concentration [7]. Here we report the application of ion beam mixing to PL from Si nanocrystals formed in a certain depth of SiO2 layers with high efficiency. Ion beam mixing [8,9] has been known as a powerful technique to form nanometersized material clusters embedded in dielectric matrices, keeping most useful advantages of ion implantation. In addition, the ion beam mixing process can overcome the limitation of Si concentration, because alternately deposited films are intermixed by energetic heavy ions, and form a homogeneously mixed layer over the penetration depth of the irradiating ions. * Corresponding author.
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Figure 1. Photoluminescence spectra of the five samples; (a) annealed at 1100°C for 2 hours after ion beam mixing, (b) annealed at 1100 °C for 2 hours without ion beam mixing, (c) as-mixed, (d) as-deposited, and (e) Si⫺ implanted SiO2 sample followed by annealing at 1100 °C for 2 hours.
2. Experiments Amorphous SiO2 (a-SiO2) and amorphous Si (a-Si) layers were deposited in an order of 100 nm SiO2, 3 nm Si, and 60 nm SiO2 on Si(001) substrate by ion sputtering at room temperature (RT). In these processes, a-Si layer was deposited by Ar⫹ ion sputtering of single crystal Si target at an Ar pressure of 5 ⫻ 10⫺5 Torr, while a-SiO2 layer was deposited by O2⫹ ion sputtering of single crystal Si target at an O2 pressure of 5 ⫻ 10⫺5 Torr. The Ar⫹ ions of 80 keV were irradiated into the SiO2/Si/SiO2 layered samples at RT with the dose of 5 ⫻ 1015 ions/cm2, followed by annealing at a temperature of 1100°C for various periods to remove the defects induced by ion irradiation and produce silicon nanocrystals. All thermal annealing was performed in an N2 ambient. Si⫺ implanted sample was also prepared for comparison purpose. Si ions were directly implanted into 300nm thick SiO2 layer on Si(001) substrate with a dose of 1 ⫻ 1017 ions/cm2 at RT. This sample was annealed in the same condition with ion beam mixed ones. For PL measurement, an Ar-laser (457.9 nm) with a power of 100 mW was used as an excitation source and the luminescence was detected by a cooled photomultiplier tube (PMT) employing the photon counting technique. A cutoff filter to pass only long waves above 475 nm was used to block the light scattered from the source. Cross-sectional high resolution transmission electron microscopy (HRTEM) was used to observe silicon nanocrystals in the ion beam mixed SiO2/Si/SiO2 layers. 3. Results and Discussion Figure 1 shows the PL spectra of SiO2/Si/SiO2 samples; (a) annealed at 1100°C for 2 hours after ion
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Figure 2. Cross sectional HRTEM images for the samples: (a) as-deposited, (b) annealed without ion beam mixing, and (c) annealed after ion beam mixing of SiO2/Si/SiO2.
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beam mixing, (b) annealed at 1100°C for 2 hours without ion beam mixing, (c) as-mixed, and (d) as-deposited. The PL spectrum (e) of Si⫺ implanted sample followed by annealing at 1100°C for 2 hours is also added for comparison. Both as-deposited and as-mixed samples have no PL behavior, because Si and SiO2 layers are probably amorphous states with high density of defects. On the other hand, the sample annealed at 1100°C for 2 hours without ion beam mixing shows a broad PL peak centered at 730 nm even though the intensity is weak. Recently, Tsybeskov et al. [1] reported that the nanocrystalline-silicons having nearly spherical shape and the size limited by the thickness of a-Si layer could be produced by controlled recrystallization of an a-Si/SiO2 superlattice. However, the PL spectra from the nanocrystalline-silicon superlattice at RT was found to be similar to bulk Si, because the sizes of silicon nanocrystals exceed the exciton Bohr diameter (dexc ⯝ 10 nm for Si). Shimizu-Iwayama et al. [4,10] reported a broad PL peak centered at 730 nm after annealing at 1100°C for the Si implanted SiO2. They attributed the 730 nm band to the presence of silicon nanocrystals. In our case, silicon nanocrystals smaller than the exciton Bohr diameter can be produced since the deposited a-Si layer is as thin as 3 nm. This suggests that the PL peak in Figure 1(b) seems to originate from some silicon nanocrystals smaller than the exciton Bohr diameter formed during the high temperature annealing. The sample annealed at 1100°C for 2 hours after ion beam mixing shows a broad PL peak centered at 720 nm at room temperature with strong intensity as in Figure 1(a). Since the amorphous silicon layer is effectively broken to pieces and mixed with SiO2 during the ion beam mixing process, the proper-sized silicon nanocrystals may increase in number, which results in an increased PL intensity compared with the peak intensity of PL spectrum in Figure 1(b). Moreover, the PL intensity of ion beam mixed sample was stronger than that of Si⫺ implanted sample. The 3 nm thickness of Si layer in SiO2 matrix is corresponding to 1 ⫻ 1016 Si⫺/cm2 in terms of Si⫺ implantation. Si concentration of ion beam mixed sample was smaller than that of Si⫺ implanted sample used in this experiment. However, PL intensity of ion beam mixed sample is stronger than that of Si⫺ implanted sample by a factor of 2. We suggest that this reason attributes to high density of Si nanocrystals of ion beam mixed sample compared to Si⫺ implanted sample. The formation of silicon nanocrystals is confirmed by cross-sectional HRTEM in Figure 2, where the cross sectional images for three samples are shown; (a) as-deposited, (b) annealed at 1100°C for 2 hours without ion beam mixing, and (c) annealed at 1100°C for 2 hours after ion beam mixing of SiO2/Si/SiO2 samples. The micrograph of annealed samples are magnified for reader’s convenience. As seen in Figure 2(a), a thin amorphous silicon layer is placed between SiO2 layers. After annealing without ion beam mixing, thin amorphous silicon layers turned into crystalline silicon layers as seen in Figure 2(b). However, this crystalline silicon layers can not be attributed to PL intensity since they exceed the exciton Bohr diameter. After ion beam mixing and subsequent annealing at 1100°C for 2 hours, the amorphous silicon layer turns into silicon nanocrystals. Silicon nanocrystals with clear {111} lattice planes are seen in Figure 2(c). One can see that the number of proper-sized silicon nanocrystal is significantly increased compared with the annealed sample without ion beam mixing. The average size of silicon nanocrystals are found to be ⬃5 nm. According to a model for the luminescence spectrum of silicon nanocrystals presented by Trwoga et al, [11] the 5 nm sized silicon nanocrystal shows a good agreement with the observed 720 nm PL band. Combining the results shown in Figure 1 and 2, it is suggested that the ion beam mixing seems to be an effective method to produce proper-sized silicon nanocrystals in a desired depth of SiO2 layer while the silicon nanocrystals should be the origin of the strong PL observed ion beam mixed SiO2/Si/SiO2 samples.
4. Conclusion In summary, silicon nanocrystals embedded in SiO2 layers have been produced by ion beam mixing of SiO2/Si/SiO2 layer at RT, followed by high temperature annealing in N2 ambient. The formation of
Vol. 11, No. 8
PHOTOLUMINESCENCE IN SiO2/Si/SiO2 NANOLAYERS
1243
nanocrystals in SiO2 matrices is confirmed by cross-sectional HRTEM. The average size of nanocrystals are found ⬃5 nm. The PL spectrum of the ion beam mixed sample shows an intense broad luminescence with a peak centered at 720 nm at room temperature, while that of the unmixed one does weaken luminescence near the same peak position. In comparing ion beam mixed sample with Si⫺ implanted sample, the former have higher PL intensity. According to quantum confinement model [11], this red luminescence corresponds to the silicon nanocrystal size of ⬃5 nm. The overall results suggest that the ion beam mixing method under proper irradiation condition can be a promising technique for PL of Si nanocrystal with high efficiency. When it is applied to Si/SiO2 multilayered film, the additional enhancement is expected with respect to our monolayered SiO2/Si/SiO2 film. Acknowledgments This work was supported in part by Korea Research Foundation made in the program year of 1998 and in part by grand (No. 1999-2-114-004-5) from the interdisciplonary research grogram of the Korea Science and Engineering Foundation (KOSEF). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
L. Tsybeskov, K. D. Hirschman, S. P. Duttagupta, M. Zacharias, P. M. Fauchet, J. P. McCaffrey, and D. J. Lockwood, Appl. Phys. Lett. 72, 43 (1998). W. L. Wilson, P. J. Szajowski, and L. E. Brus, Science. 262, 1242 (1993). R. E. Hummel and S.-S. Chang, Appl. Phys. Lett. 61, 1965 (1992). T. Shimazu-Iwayama, S. Nakao, and K. Saitoh, Appl. Phys. Lett. 65, 1814 (1994). P. Mutti, G. Ghislotti, S. Bertoni, L. Bonoldi, G. F. Cerofolini, L. Meda, E. Grilli, and M. Guzzi, Appl. Phys. Lett. 66, 851 (1995). H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, and T. Nakagiri, Appl. Phys. Lett. 56, 2379 (1990). C. M. Preece and J. K. Hiroven, Ion Implantation Metallurgy, p. 38, Metallurgic Society of AIME, Warrendale, PA (1979). S. Matteson and M. A. Nicolet, Annu. Rev. Mater. Sci. 13, 339 (1983). B. M. Paine, and R. S. Averback, Nucl. Instr. Methods. B7/8, 666 (1985). T. Shimazu-Iwayama, N. Kurumado, D. E. Hole, and P. D. Townsend, J. Appl. Phys. 83, 6018 (1998). P. F. Trwoga, A. J. Kenyon, and C. W. Pitt, J. Appl. Phys. 83, 3789 (1998).