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SiO2 layers
Optical Materials 17 (2001) 125±129
www.elsevier.nl/locate/optmat
Blue and red luminescence from Si ion-irradiated SiO2=Si=SiO2 layers J.H. Son a, T.G. Kim a, S.W. Shin a, H.B. Kim a, W.S. Lee b, S. Im b, J.H. Song c, C.N. Whang a, K.H. Chae a,* a
Atomic-scale Surface Science Research Center and Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, South Korea b Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, South Korea c Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 130-650, South Korea
Abstract Photoluminescence (PL) from the Si ion-irradiated SiO2 =Si=SiO2 layers on Si substrate at room temperature has been studied to elucidate the origins of the blue and red luminescence. A luminescence band around 450 nm was observed from as-irradiated sample, which was found to be originated from the diamagnetic defect known as B2 band generated by Si ion irradiation. The intensity of this band increases with the increase of annealing temperatures up to a critical temperature. After annealing at 1100°C, the defect-related PL peaks around 450 and 600 nm disappear and a new PL peak appears around 700 nm. This luminescence band is attributed to 5 nm-sized Si nanocrystals formed along the Si layer between SiO2 layers. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Si; Nanocrystals; Ion irradiation; SiO2 ; Luminescence; Radiative defects
1. Introduction Since the discovery of a visible luminescence from porous Si at room temperature (RT), [1] photoluminescence (PL) from silicon based materials consisting of nanoscale crystallites in an oxide environment has been under active investigation due to its potential application in Sibased optoelectronic devices. Among the various methods for nanocrystal formation, ion implantation [2] has the advantage that a given number
* Corresponding author. Tel.: +82-2123-4806; fax: +82-2-3127090. E-mail address: [email protected] (K.H. Chae).
of implanted ions can be placed at a predictable depth determined by the implantation conditions such as ion dose and energy. In the case of ion implantation, post-annealing at high temperature (HT) is the prerequisite for the formation of Si nanocrystals after ion implantation. The PL peak observed from as-implanted sample is attributed to the radiative defects generated during ion implantation, which is measured around 450 and 600 nm. Also, PL peak related to Si nanocrystals is observed around 700 nm. Liao et al. [3] reported that the blue emission, which is observed under an ultraviolet excitation of 5.0 eV, from Si implanted SiO2 ®lms was induced by neutral oxygen vacancies. Shimizu-Iwayama et al. [4] reported that the PL peak around 600 nm is
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 0 3 4 - 9
126
J.H. Son et al. / Optical Materials 17 (2001) 125±129
observed from as-implanted specimens under a 488 nm excitation and the luminescence band could be attributed to Si excess defects in the environment. In this study, the irradiation of Si ions into SiO2 /Si/SiO2 layers on Si substrate at RT has been performed to obtain a blue luminescence associated with neutral oxygen vacancies generated by Si ion implantation, and annealed at HT to obtain a red luminescence associated with Si nanocrystals. We present the PL spectra from SiO2 /Si/SiO2 layers on Si substrate that were irradiated at RT and subsequent annealing at various temperatures, and discuss the luminescence origins in terms of defectrelated luminescence center and quantum con®nement models. 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(0 0 1) substrate by ion sputtering at 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 Si ions of 70 keV were irradiated into the SiO2 /Si/SiO2 layered samples with a dose of 1:5 1016 ions/cm2 at RT. Irradiated samples were subsequently annealed in N2 ambient at 300°C, 500°C, 700°C, 900°C, and 1100°C for 2 h in order to see the variation of PL peak around 450 nm. For PL measurement, an Ar ion laser (351 nm) was used as an excitation source and a cooled photomultiplier tube employing the photon counting techniques as a detector for luminescence. A cuto ®lter to pass only long waves above 375 nm was used to block the lights scattered from the source. Electron spin resonance (ESR) spectra were taken to qualitatively evaluate the amounts of paramagnetic defects in irradiated samples. The ESR signal was measured at RT using Bruker EMX 300 ESR spectrometer operating at a microwave power of 2.002 mW, a modulation amplitude of 3.00 G. High-resolution transmission electron microscopy (HRTEM) was
performed to observe the Si nanocrystals in SiO2 / Si/SiO2 layers.
3. Results and discussion Fig. 1 shows PL spectra obtained from as-deposited and Si ion-irradiated SiO2 /Si/SiO2 layers, and SiO2 /Si/SiO2 layered samples annealed at 300°C, 500°C, 700°C, 900°C, and 1100°C for 2 h in N2 ambient after Si ion irradiation. As-deposited SiO2 /Si/SiO2 layer shows a weak PL peak around 450 nm under an excitation of 351 nm. Liao et al. [3] reported that this band was associated with the neutral oxygen vacancies called B2 band O3 BSiASiBO3 , which was a diamagnetic defect playing a radiative recombination center. Since a thin Si layer is embedded between SiO2 layers, the as-deposited SiO2 /Si/SiO2 layer may contain a few B2 band at the interfaces of Si layer and SiO2 layers. After Si ion irradiation, the intensity of PL peak around 450 nm was increased slightly. This means that the Si layer embedded between the two SiO2 layers can be broken into pieces and mixed with SiO2 by Si ion irradiation, the B2 band should be increased due to the increase of Si and SiO2 interfaces, and then the intensity of PL peak is increased after ion irradiation. Thus, it seems that
Fig. 1. PL spectra for SiO2 /Si/SiO2 layered samples of asdeposited ( ± ), as-irradiated (- -), annealed at 300 (-N-), 500 (--), 700 (--), 900 (-.-), and 1100°C (-r-) for 2 h in N2 ambient after Si ion irradiation with the dose of 1.5 1016 Si/ cm2 at the energy of 70 keV.
J.H. Son et al. / Optical Materials 17 (2001) 125±129
the Si layer embedded between SiO2 layers can play an important role for blue luminescence from ion irradiated SiO2 layer. When the irradiated sample was annealed at 300°C, the intensity of PL peak around 450 nm related to B2 band increases remarkably. The B2 band will be formed without diculty in the vicinity of Si layer since Si atoms are rich in this region and B2 band is related to SiSi bond. Thus, the intensity of PL band around 450 nm increases because of the B2 band increase after annealing at low temperature. The remarkable increase for the intensity of PL band around 600 nm was measured from the sample annealed at 500°C with the slight increase of the luminescence around 450 nm. When the sample is annealed at 500°C, radiative recombination defect centers related to PL band around 600 nm created in SiO2 layer may be activated. The intensity of defectrelated PL peak increases with the increase of annealing temperature up to a certain critical temperature, 700°C in our experiment. The results for variation of PL peak around 600 nm are in accordance with our previous one [5]. Thus, it can propose that the defects related to B2 band are activated at lower temperature than the defects related to 600 nm PL band. With annealing above the critical temperature, the peak intensity decreases abruptly. It implies that annealing above 700°C passivates defects. However, the luminescence is observed with the same intensity at all range of visible luminescence. This result implies the possibility of fabricating the white source from Si-based materials. When the annealing is performed at 1100°C for 2 h, the PL peak around 450 nm and 600 nm related to defects disappears, and the PL peak around 720 nm is only observed. This means that Si nanocrystals are generated after annealing at HT. Paramagnetic defects in Si-irradiated SiO2 /Si/ SiO2 layer were measured using ESR measurement as shown in Fig. 2. A large ESR signal appears from as-irradiated sample. The intensity of ESR signal grows with annealing at 300°C. However, the intensity of ESR signal from sample annealed above 700°C decreased signi®cantly. This implies that the defects generated during ion irradiation are passivated by the annealing above 700°C. The zero crossing g-values observed from the samples
127
Fig. 2. ESR spectra for SiO2 /Si/SiO2 layered samples of (a) asirradiated, annealed at (b) 300°C, (c) 500°C, (d) 700°C, (e) 900°C, and (f) 1100°C for 2 h in N2 ambient after Si ion irradiation with the dose of 1:5 1016 Si=cm2 at the energy of 70 keV.
are ranged from 2.0023 to 2.0042. According to Stesmans et al., [6] the zero crossing g-values of Pb center, depicted as SiBSi , vary from 2.0015 to 2.0087. This defect center is well known as a nonradiative defect and exists mainly at the interface between Si layer and SiO2 layer, and in Si substrate. Due to Si ion irradiation, the shape of the interface between Si layer and SiO2 layer is damaged and the area of damaged interface between Si layer and SiO2 layers is increased. It can be supposed that this phenomenon is generally enhanced with annealing up to 500°C in our experiments. In the case of annealing above 700°C, the Pb centers in the interface between Si layer and SiO2 layer seem to be passivated by oxygen and/or interstitial Si. Fig. 3 shows PL intensities from as-deposited SiO2 /Si/SiO2 layer and Si-irradiated SiO2 /Si/SiO2 layer after annealed at 1100°C for 2 h. After annealing, the PL peaks around 450 nm and 600 nm related to defects disappeared, and the PL peak around 720 nm appears. This PL band is attributed to 5 nm-sized Si nanocrystals. This means that Si nanocrystals are produced after annealing at HT. When as-deposited SiO2 /Si/SiO2 layer is annealed in N2 ambient at 1100°C for 2 h, the PL peak associated with Si nanocrystals is observed
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J.H. Son et al. / Optical Materials 17 (2001) 125±129
carried to con®rm the existence of Si nanocrystals in SiO2 /Si/SiO2 layer. However, we could not ®nd any Si nanocrystals in SiO2 /Si/SiO2 layers using HRTEM measurements. Therefore, it can be concluded that blue luminescence is associated with B2 band in SiO2 /Si/SiO2 layer irrespective of Si nanocrystals. On the other hand, Si nanocrystals was observed along the Si layer between SiO2 layers from the sample annealed at HT as shown in Fig. 4. The sizes of Si nanocrystals are nearly accordance with the value calculated using quantum con®nement eects [7]. Fig. 3. PL spectra from the SiO2 /Si/SiO2 layered sample annealed at 1100°C for 2 h in N2 ambient of as-deposited (--) and Si ion irradiated at RT (-j-).
4. Conclusions
weakly. However, the PL peak from ion irradiated SiO2 /Si/SiO2 layer is increased signi®cantly. Moreover, the luminescence related to Si nanocrystals in Si-irradiated SiO2 /Si/SiO2 layer could be seen with naked eyes. This implies that ion irradiation can break the Si layer embedded between SiO2 layers into pieces and mix with SiO2 , accordingly it helps the production of proper sized Si nanocrystals for PL. Therefore, ion irradiation should be a useful technique for obtaining intense PL as producing a high density of Si nanocrystals in SiO2 layers. Blue luminescence from Si based material can be achieved by Si nanocrystals with the size of 2 nm in diameter or neutral oxygen vacancies called B2 band in SiO2 layer. HRTEM measurements were
PL from Si ion-irradiated SiO2 /Si/SiO2 layers on a Si substrate at RT has been studied to elucidate the origins of the luminescence. Blue luminescence around 450 nm was observed from the as-deposited and as-irradiated samples. This luminescence is attributed to neutral oxygen vacancy in SiO2 /Si/SiO2 layer and not Si nanocrystals. The intensity of the luminescence increases after low temperature annealing of 300°C. After annealing at 500°C, the luminescence around 600 nm is increased remarkably. The intensity of PL peak related to defects increases with the increase of annealing temperature up to the critical temperature. As annealed at HT, the sample contains Si nanocrystals along the Si layer embedded between SiO2 layers, and this nanocrystals attribute to the PL peak around 720 nm.
Fig. 4. HRTEM image of (a) as-deposited sample, (b) annealed at 1100°C for 2 h without ion irradiation, and (c) post-annealed at 1100°C for 2 h after Si ion irradiation. Si nanocrystals are observed along the Si layer between SiO2 layers.
J.H. Son et al. / Optical Materials 17 (2001) 125±129
Acknowledgements This work was supported in part by the KOSEF through the ASSRC at Yonsei University, the grants from KOSEF (1999-2-114-004-5), and the Pusan branch of Korea Basic Science Institute (KBSI). References [1] L.T. Canham, Appl. Phys. Lett. 56 (1990) 1046.
129
[2] E. Rimini, Ion Implantation: Basic to Device Fabrication, Kluwer, Dordrecht, 1995, p. 19. [3] L.S. Liao, X.M. Bao, X.Q. Zheng, N.S. Li, N.B. Min, Appl. Phys. Lett. 68 (1996) 850. [4] T. Shimizu-Iwayama, Y. Terao, A. Kamiya, M. Takda, S. Nakao, K. Saitoh, Nanostruct. Mater. 5 (1995) 307. [5] J.Y. Jeong, M.S. Oh, S. Im, H.B. Kim, K.H. Chae, C.N. Whang, J.H. Song, J. Lumin. 80 (1999) 285. [6] A. Stesmans, J. Barett, J. Witters, R.F. Dekeersmaecker, Surf. Sci. 141 (1984) 225. [7] T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15578.
www.elsevier.nl/locate/optmat
Blue and red luminescence from Si ion-irradiated SiO2=Si=SiO2 layers J.H. Son a, T.G. Kim a, S.W. Shin a, H.B. Kim a, W.S. Lee b, S. Im b, J.H. Song c, C.N. Whang a, K.H. Chae a,* a
Atomic-scale Surface Science Research Center and Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, South Korea b Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, South Korea c Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 130-650, South Korea
Abstract Photoluminescence (PL) from the Si ion-irradiated SiO2 =Si=SiO2 layers on Si substrate at room temperature has been studied to elucidate the origins of the blue and red luminescence. A luminescence band around 450 nm was observed from as-irradiated sample, which was found to be originated from the diamagnetic defect known as B2 band generated by Si ion irradiation. The intensity of this band increases with the increase of annealing temperatures up to a critical temperature. After annealing at 1100°C, the defect-related PL peaks around 450 and 600 nm disappear and a new PL peak appears around 700 nm. This luminescence band is attributed to 5 nm-sized Si nanocrystals formed along the Si layer between SiO2 layers. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Si; Nanocrystals; Ion irradiation; SiO2 ; Luminescence; Radiative defects
1. Introduction Since the discovery of a visible luminescence from porous Si at room temperature (RT), [1] photoluminescence (PL) from silicon based materials consisting of nanoscale crystallites in an oxide environment has been under active investigation due to its potential application in Sibased optoelectronic devices. Among the various methods for nanocrystal formation, ion implantation [2] has the advantage that a given number
* Corresponding author. Tel.: +82-2123-4806; fax: +82-2-3127090. E-mail address: [email protected] (K.H. Chae).
of implanted ions can be placed at a predictable depth determined by the implantation conditions such as ion dose and energy. In the case of ion implantation, post-annealing at high temperature (HT) is the prerequisite for the formation of Si nanocrystals after ion implantation. The PL peak observed from as-implanted sample is attributed to the radiative defects generated during ion implantation, which is measured around 450 and 600 nm. Also, PL peak related to Si nanocrystals is observed around 700 nm. Liao et al. [3] reported that the blue emission, which is observed under an ultraviolet excitation of 5.0 eV, from Si implanted SiO2 ®lms was induced by neutral oxygen vacancies. Shimizu-Iwayama et al. [4] reported that the PL peak around 600 nm is
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 0 3 4 - 9
126
J.H. Son et al. / Optical Materials 17 (2001) 125±129
observed from as-implanted specimens under a 488 nm excitation and the luminescence band could be attributed to Si excess defects in the environment. In this study, the irradiation of Si ions into SiO2 /Si/SiO2 layers on Si substrate at RT has been performed to obtain a blue luminescence associated with neutral oxygen vacancies generated by Si ion implantation, and annealed at HT to obtain a red luminescence associated with Si nanocrystals. We present the PL spectra from SiO2 /Si/SiO2 layers on Si substrate that were irradiated at RT and subsequent annealing at various temperatures, and discuss the luminescence origins in terms of defectrelated luminescence center and quantum con®nement models. 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(0 0 1) substrate by ion sputtering at 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 Si ions of 70 keV were irradiated into the SiO2 /Si/SiO2 layered samples with a dose of 1:5 1016 ions/cm2 at RT. Irradiated samples were subsequently annealed in N2 ambient at 300°C, 500°C, 700°C, 900°C, and 1100°C for 2 h in order to see the variation of PL peak around 450 nm. For PL measurement, an Ar ion laser (351 nm) was used as an excitation source and a cooled photomultiplier tube employing the photon counting techniques as a detector for luminescence. A cuto ®lter to pass only long waves above 375 nm was used to block the lights scattered from the source. Electron spin resonance (ESR) spectra were taken to qualitatively evaluate the amounts of paramagnetic defects in irradiated samples. The ESR signal was measured at RT using Bruker EMX 300 ESR spectrometer operating at a microwave power of 2.002 mW, a modulation amplitude of 3.00 G. High-resolution transmission electron microscopy (HRTEM) was
performed to observe the Si nanocrystals in SiO2 / Si/SiO2 layers.
3. Results and discussion Fig. 1 shows PL spectra obtained from as-deposited and Si ion-irradiated SiO2 /Si/SiO2 layers, and SiO2 /Si/SiO2 layered samples annealed at 300°C, 500°C, 700°C, 900°C, and 1100°C for 2 h in N2 ambient after Si ion irradiation. As-deposited SiO2 /Si/SiO2 layer shows a weak PL peak around 450 nm under an excitation of 351 nm. Liao et al. [3] reported that this band was associated with the neutral oxygen vacancies called B2 band O3 BSiASiBO3 , which was a diamagnetic defect playing a radiative recombination center. Since a thin Si layer is embedded between SiO2 layers, the as-deposited SiO2 /Si/SiO2 layer may contain a few B2 band at the interfaces of Si layer and SiO2 layers. After Si ion irradiation, the intensity of PL peak around 450 nm was increased slightly. This means that the Si layer embedded between the two SiO2 layers can be broken into pieces and mixed with SiO2 by Si ion irradiation, the B2 band should be increased due to the increase of Si and SiO2 interfaces, and then the intensity of PL peak is increased after ion irradiation. Thus, it seems that
Fig. 1. PL spectra for SiO2 /Si/SiO2 layered samples of asdeposited ( ± ), as-irradiated (- -), annealed at 300 (-N-), 500 (--), 700 (--), 900 (-.-), and 1100°C (-r-) for 2 h in N2 ambient after Si ion irradiation with the dose of 1.5 1016 Si/ cm2 at the energy of 70 keV.
J.H. Son et al. / Optical Materials 17 (2001) 125±129
the Si layer embedded between SiO2 layers can play an important role for blue luminescence from ion irradiated SiO2 layer. When the irradiated sample was annealed at 300°C, the intensity of PL peak around 450 nm related to B2 band increases remarkably. The B2 band will be formed without diculty in the vicinity of Si layer since Si atoms are rich in this region and B2 band is related to SiSi bond. Thus, the intensity of PL band around 450 nm increases because of the B2 band increase after annealing at low temperature. The remarkable increase for the intensity of PL band around 600 nm was measured from the sample annealed at 500°C with the slight increase of the luminescence around 450 nm. When the sample is annealed at 500°C, radiative recombination defect centers related to PL band around 600 nm created in SiO2 layer may be activated. The intensity of defectrelated PL peak increases with the increase of annealing temperature up to a certain critical temperature, 700°C in our experiment. The results for variation of PL peak around 600 nm are in accordance with our previous one [5]. Thus, it can propose that the defects related to B2 band are activated at lower temperature than the defects related to 600 nm PL band. With annealing above the critical temperature, the peak intensity decreases abruptly. It implies that annealing above 700°C passivates defects. However, the luminescence is observed with the same intensity at all range of visible luminescence. This result implies the possibility of fabricating the white source from Si-based materials. When the annealing is performed at 1100°C for 2 h, the PL peak around 450 nm and 600 nm related to defects disappears, and the PL peak around 720 nm is only observed. This means that Si nanocrystals are generated after annealing at HT. Paramagnetic defects in Si-irradiated SiO2 /Si/ SiO2 layer were measured using ESR measurement as shown in Fig. 2. A large ESR signal appears from as-irradiated sample. The intensity of ESR signal grows with annealing at 300°C. However, the intensity of ESR signal from sample annealed above 700°C decreased signi®cantly. This implies that the defects generated during ion irradiation are passivated by the annealing above 700°C. The zero crossing g-values observed from the samples
127
Fig. 2. ESR spectra for SiO2 /Si/SiO2 layered samples of (a) asirradiated, annealed at (b) 300°C, (c) 500°C, (d) 700°C, (e) 900°C, and (f) 1100°C for 2 h in N2 ambient after Si ion irradiation with the dose of 1:5 1016 Si=cm2 at the energy of 70 keV.
are ranged from 2.0023 to 2.0042. According to Stesmans et al., [6] the zero crossing g-values of Pb center, depicted as SiBSi , vary from 2.0015 to 2.0087. This defect center is well known as a nonradiative defect and exists mainly at the interface between Si layer and SiO2 layer, and in Si substrate. Due to Si ion irradiation, the shape of the interface between Si layer and SiO2 layer is damaged and the area of damaged interface between Si layer and SiO2 layers is increased. It can be supposed that this phenomenon is generally enhanced with annealing up to 500°C in our experiments. In the case of annealing above 700°C, the Pb centers in the interface between Si layer and SiO2 layer seem to be passivated by oxygen and/or interstitial Si. Fig. 3 shows PL intensities from as-deposited SiO2 /Si/SiO2 layer and Si-irradiated SiO2 /Si/SiO2 layer after annealed at 1100°C for 2 h. After annealing, the PL peaks around 450 nm and 600 nm related to defects disappeared, and the PL peak around 720 nm appears. This PL band is attributed to 5 nm-sized Si nanocrystals. This means that Si nanocrystals are produced after annealing at HT. When as-deposited SiO2 /Si/SiO2 layer is annealed in N2 ambient at 1100°C for 2 h, the PL peak associated with Si nanocrystals is observed
128
J.H. Son et al. / Optical Materials 17 (2001) 125±129
carried to con®rm the existence of Si nanocrystals in SiO2 /Si/SiO2 layer. However, we could not ®nd any Si nanocrystals in SiO2 /Si/SiO2 layers using HRTEM measurements. Therefore, it can be concluded that blue luminescence is associated with B2 band in SiO2 /Si/SiO2 layer irrespective of Si nanocrystals. On the other hand, Si nanocrystals was observed along the Si layer between SiO2 layers from the sample annealed at HT as shown in Fig. 4. The sizes of Si nanocrystals are nearly accordance with the value calculated using quantum con®nement eects [7]. Fig. 3. PL spectra from the SiO2 /Si/SiO2 layered sample annealed at 1100°C for 2 h in N2 ambient of as-deposited (--) and Si ion irradiated at RT (-j-).
4. Conclusions
weakly. However, the PL peak from ion irradiated SiO2 /Si/SiO2 layer is increased signi®cantly. Moreover, the luminescence related to Si nanocrystals in Si-irradiated SiO2 /Si/SiO2 layer could be seen with naked eyes. This implies that ion irradiation can break the Si layer embedded between SiO2 layers into pieces and mix with SiO2 , accordingly it helps the production of proper sized Si nanocrystals for PL. Therefore, ion irradiation should be a useful technique for obtaining intense PL as producing a high density of Si nanocrystals in SiO2 layers. Blue luminescence from Si based material can be achieved by Si nanocrystals with the size of 2 nm in diameter or neutral oxygen vacancies called B2 band in SiO2 layer. HRTEM measurements were
PL from Si ion-irradiated SiO2 /Si/SiO2 layers on a Si substrate at RT has been studied to elucidate the origins of the luminescence. Blue luminescence around 450 nm was observed from the as-deposited and as-irradiated samples. This luminescence is attributed to neutral oxygen vacancy in SiO2 /Si/SiO2 layer and not Si nanocrystals. The intensity of the luminescence increases after low temperature annealing of 300°C. After annealing at 500°C, the luminescence around 600 nm is increased remarkably. The intensity of PL peak related to defects increases with the increase of annealing temperature up to the critical temperature. As annealed at HT, the sample contains Si nanocrystals along the Si layer embedded between SiO2 layers, and this nanocrystals attribute to the PL peak around 720 nm.
Fig. 4. HRTEM image of (a) as-deposited sample, (b) annealed at 1100°C for 2 h without ion irradiation, and (c) post-annealed at 1100°C for 2 h after Si ion irradiation. Si nanocrystals are observed along the Si layer between SiO2 layers.
J.H. Son et al. / Optical Materials 17 (2001) 125±129
Acknowledgements This work was supported in part by the KOSEF through the ASSRC at Yonsei University, the grants from KOSEF (1999-2-114-004-5), and the Pusan branch of Korea Basic Science Institute (KBSI). References [1] L.T. Canham, Appl. Phys. Lett. 56 (1990) 1046.
129
[2] E. Rimini, Ion Implantation: Basic to Device Fabrication, Kluwer, Dordrecht, 1995, p. 19. [3] L.S. Liao, X.M. Bao, X.Q. Zheng, N.S. Li, N.B. Min, Appl. Phys. Lett. 68 (1996) 850. [4] T. Shimizu-Iwayama, Y. Terao, A. Kamiya, M. Takda, S. Nakao, K. Saitoh, Nanostruct. Mater. 5 (1995) 307. [5] J.Y. Jeong, M.S. Oh, S. Im, H.B. Kim, K.H. Chae, C.N. Whang, J.H. Song, J. Lumin. 80 (1999) 285. [6] A. Stesmans, J. Barett, J. Witters, R.F. Dekeersmaecker, Surf. Sci. 141 (1984) 225. [7] T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15578.