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Light emission from Si nanocrystals embedded in CaF2 epilayers on Si(1 1 1): Effect of rapid thermal annealing
Journal of Luminescence 80 (1999) 253—256
Light emission from Si nanocrystals embedded in CaF epilayers on Si(1 1 1): Effect of rapid thermal annealing Masahiro Watanabe*, Takeo Maruyama, Soichiro Ikeda Research Center for Quantum Effect Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Abstract Visible photoluminescence (PL) of nanocrystalline silicon (nc-Si) embedded in single crystal CaF formed on Si(1 1 1) has been studied and the influence of ex situ rapid thermal annealing (RTA) on the surface morphology and PL spectra has been studied. It has been found that the PL intensity and uniformity was improved by RTA with appropriate temperature and short annealing time. 1999 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline silicon; Epitaxial; CaF ; Photoluminescence; Rapid thermal annealing; Co-evaporation
1. Introduction Recently, visible light emission from silicon related materials such as porous silicon [1], microcrystalline silicon [2], Si/Ge superlattices [3], and Si/CaF multilayered suplerlattices [4] has been reported. For the last several years, we have studied the formation technique of nc-Si embedded in single-crystal CaF on Si(1 1 1) and found visible photoluminescence (PL) [5] and electroluminescence (EL) [6] from CaF epilayers including nc-Si at room temperature. On the other hand, we have demonstrated resonant tunneling diodes and transistors [7—9] using CaF /CoSi epitaxial superlattice on Si substrate. This implies that the single-crystal CaF -Si heterostructure has possibil
* Correspondig author. Tel.: #81-3-5734-3574; Fax: #81-35734-3574; e-mail: [email protected].
ities for integration in resonant tunneling electronic devices and photonic devices on an Si substrate. However, the PL intensity in the as grown sample is still inhomogeneous over the substrate, and this is probably due to a lack of migration energy for crystallization during the coevaporation of Si and CaF at a relatively low growth temper ature (300°C). In this paper, we have studied the influence of rapid thermal annealing (RTA) on PL and the surface morphology of the CaF layer including nc-Si. We have achieved drastic improvement in PL intensity and uniformity by RTA with appropriate temperature and annealing time.
2. Experiment CaF has fluorite lattice structure that is well matched to the lattice structure of Si with mismatches of #0.6% at room temperature. Si tends
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 1 0 7 - 0
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M. Watanabe et al. / Journal of Luminescence 80 (1999) 253—256
to form 3D nucleation during initial growth on CaF due to the small surface energy of CaF under appropriate conditions. When Si and CaF are simultaneously deposited on a CaF surface, Si atoms tend to agglomerate because Si and CaF have different bonding mechanisms (covalent bonding in Si, ionic bonding in CaF ). CaF flux suppresses and controls the nucleation of Si in the initial growth stages. As a result, the size and density of the nanocrystalline Si can be controlled by varying the growth temperature and the flux ratio of Si and CaF . Nanocrystalline Si of 5—10 nm in diameter embedded in epitaxial CaF layer can be obtained using this technique. CaF and Si were grown in a UHV ((1; 10\ Torr) chamber. The Si growth rate was approximately 1 nm/min, and CaF was controlled at 1—5 nm/min. A protective oxide layer was grown on a chemically cleaned, polished Si substrate with (1 1 1) orientation. The substrate was heated to 760°C with an Si flux of 0.6 nm/min to remove the protective oxide layer. This resulted in well-developed 7;7 reflection high-energy electron diffraction (RHEED). The nanocrystalline Si (nc-Si) layer was formed by coevaporation of Si and CaF at a substrate temperature (Ts) of 300°C, and the Si : CaF flux ratio was 1 : 3 to 1 : 5 because the PL peak intensity was maximized at this condition in the as grown sample [5]. Each nc-Si layer was 2 nm thick and a pure 2 nm thick CaF layer lacking Si was grown on the nc-Si layer. The CaF interlayer separates each nc-Si layer and suppresses nc-Si coalescence in a normal direction to the substrate; this is effective for minimizing fluctuations of nc-Si diameter and density. Five periods of 2 nm-thick nc-Si and 2 nm-thick CaF multilayered superlattice (SL) were prepared for the RTA sample. Ex situ RTA was carried out on the nc-Si/CaF using infrared rump radiation heating under H ambience at atmospheric pressure. The sample holder was made of graphite with a 50 mm diameter, and the typical sample size was 5 mm square. Temperature was increased from room temperature up to annealing temperature ¹ at the rate of approximately 100°C/s. The temperature was kept relatively constant using feedback control during the annealing process.
3. Results and discussion The temperature dependence of annealing on surface morphology has been observed using atomic force microscopy (AFM). The morphology is almost the same as that of the as-grown samples when annealed at lower than 750°C. The surface fluctuation was almost 1 nm in thickness. However, the surface morphology became extremely rough in the sample annealed at higher than 950°C. The mechanism of degradation in morphology has not yet been determined; however, it may occur by the complexity of reevaporation due to chemical reaction among Si, CaF and H and agglomeration of these materials. Controlling surface morphology is essential for device applications. Therefore, in the following experiment, the RTA temperature was fixed at 700—750°C to avoid degradation in surface morphology. Fig. 1 shows the room temperature PL spectra of samples (a) as-grown, (b) annealed at 700°C for 20 s. The PL intensity of the as-grown sample was relatively weak and inhomogeneous especially at room temperature. On the contrary, PL intensity and uniformity of annealed samples were drastically improved and the PL intensity was almost the same over the whole surface. PL peak wavelength is almost the same for both the PL spectra, although PL peak wavelength slightly shifts to longer direction by in situ annealing at 700°C, where the
Fig. 1. Room temperature PL spectra of samples (a) as-grown, (b) annealed at 700°C for 20 s.
M. Watanabe et al. / Journal of Luminescence 80 (1999) 253—256
temperature increasing rate was 0.5°C/s, as reported in Ref. [6]. One possible explanation of the red shift of the peak wavelength is the quantum size effect. In the case of non-rapid annealing process, Si nanocrystals formed in CaF crystallize and also agglomerate slowly during the relatively long annealing time, resulting in increase of diameter of the crystals. However, in the rapid annealing process, the annealing time is too short to increase the diameter of nc-Si. The time dependence of annealing on surface morphology and PL spectra were studied. Fig. 2
255
shows AFM images of the samples (a) as-grown, annealed at 750°C for (b) 10 s (c) 100 s (d) 1000 s. RTA for longer than 100 s drastically changed the surface morphology when compared with the asgrown-sample. This result indicates that RTA time is, in addition to temperature, essential for keeping surface morphology. Fig. 3 shows the PL spectra of the samples measured at room temperature. As the annealing time increased, PL intensity decreased. However, in samples annealed for 10 and 100 s, the PL was uniform over the whole surface even at room
Fig. 2. AFM images of the samples (a) as grown, annealed at 750°C for (b) 10 s (c) 100 s (d) 1000 s. Annealing for longer than 100 s led to drastic changes in the surface morphology when compared with the as-grown sample.
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M. Watanabe et al. / Journal of Luminescence 80 (1999) 253—256
was studied. Nanocrystalline silicon embedded in a CaF epilayer was prepared by coevaporation of Si and CaF flux in an ultra high vacuum on Si(1 1 1) substrate. Nc-Si with a diameter less than 10 nm was prepared with the appropriate flux ratio (Si : CaF "1 : 3-1 : 5) and growth temperature at 300°C. Ex situ RTA was performed on the nc-Si/CaF in H at a temperature range of 750°C—1050°C for annealing times of 10/100/ 1000 s. We have found that RTA at approximately 750°C for 10 s results in the improvement of PL intensity and uniformity as well as maintenance of surface morphology. However, a temperature higher than 950°C and a longer annealing time leads to degradation in the surface morphology and PL intensity. Fig. 3. PL spectra of the samples annealed at 750°C for 10, 100 s, 1000 s measured at room temperature. As the annealing time increased, PL intensity decreased. However, in samples annealed for 10 and 100 s, PL was uniform over the whole surface even at room temperature, and the peak intensity improved when compared with the as-grown sample.
temperature, and the peak intensity improved when compared with that of the as-grown-sample. This is probably because annealing for short time enhances crystallization of nc-Si and CaF resulting in an increase in PL intensity. However, annealing for a long time drastically changes surface morphology and decreases PL intensity. This is interpreted as the gradual damage of the nc-Si/CaF layer by chemical etching due to H ambience even at 750°C. PL intensity was not degraded when annealed at 10 s because the etching time is short enough to maintain the crystalline quality, and the improvement of crystal quality by thermal annealing and the passivation effect by H was clearly observable. On the contrary, annealing longer than 100 s damages crystals deeply resulting in degradation of PL intensity. These results indicate that temperature and annealing time are essential for improving the crystalline quality and maintaining surface morphology.
4. Conclusion The influence of rapid thermal annealing on surface morphology and photoluminescence spectra
Acknowledgements The authors would like to thank Professors K. Furuya, S. Arai and Associate Professors M. Asada, Y. Miyamoto for providing fruitful discussion. This work was supported by the Ministry of Education, Science, Sports and Culture through a Scientific Grant-in-Aid, and by The Japan Society for the Promotion of Science (JSPS-RFTF 96P00101), and by the Research Center for UltraHigh-Speed Electronics.
References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] H. Takagi, H. Ogawa, Y. Yamazaki, T. Nakagiri, Appl. Phys. Lett. 56 (1990) 2379. [3] E.A. Montie, G.F.A. van de Walle, D.J. Gravesteijn, A.A. van Gorkun, C.W.T. Bulle-Lieuwma, Appl. Phys. Lett. 56 (1990) 340. [4] F. Bassani, L. Vervoort, I. Mihalcescu, J.C. Vial, F. Arnaud d’Avitaya, J. Appl. Phys. 79 (1996) 4066. [5] M. Watanabe, F. Iizuka, M. Asada, IEICE Trans. E79-C (1996) 1562. [6] M. Watanabe, T. Matsunuma, Y. Maeda, Jpn. J. Appl. Phys. 37 (1998) L591. [7] M. Watanabe, T. Suemasu, S. Muratake, M. Asada, Appl. Phys. Lett. 62 (1993) 300. [8] T. Suemasu, M. Watanabe, M. Asada, N. Suzuki, Electron. Lett. 28 (1992) 1432. [9] T. Suemasu, M. Watanabe, J. Suzuki, Y. Kohno, M. Asada, N. Suzuki, Jpn. J. Appl. Phys. 33 (1994) 57.
Light emission from Si nanocrystals embedded in CaF epilayers on Si(1 1 1): Effect of rapid thermal annealing Masahiro Watanabe*, Takeo Maruyama, Soichiro Ikeda Research Center for Quantum Effect Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Abstract Visible photoluminescence (PL) of nanocrystalline silicon (nc-Si) embedded in single crystal CaF formed on Si(1 1 1) has been studied and the influence of ex situ rapid thermal annealing (RTA) on the surface morphology and PL spectra has been studied. It has been found that the PL intensity and uniformity was improved by RTA with appropriate temperature and short annealing time. 1999 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline silicon; Epitaxial; CaF ; Photoluminescence; Rapid thermal annealing; Co-evaporation
1. Introduction Recently, visible light emission from silicon related materials such as porous silicon [1], microcrystalline silicon [2], Si/Ge superlattices [3], and Si/CaF multilayered suplerlattices [4] has been reported. For the last several years, we have studied the formation technique of nc-Si embedded in single-crystal CaF on Si(1 1 1) and found visible photoluminescence (PL) [5] and electroluminescence (EL) [6] from CaF epilayers including nc-Si at room temperature. On the other hand, we have demonstrated resonant tunneling diodes and transistors [7—9] using CaF /CoSi epitaxial superlattice on Si substrate. This implies that the single-crystal CaF -Si heterostructure has possibil
* Correspondig author. Tel.: #81-3-5734-3574; Fax: #81-35734-3574; e-mail: [email protected].
ities for integration in resonant tunneling electronic devices and photonic devices on an Si substrate. However, the PL intensity in the as grown sample is still inhomogeneous over the substrate, and this is probably due to a lack of migration energy for crystallization during the coevaporation of Si and CaF at a relatively low growth temper ature (300°C). In this paper, we have studied the influence of rapid thermal annealing (RTA) on PL and the surface morphology of the CaF layer including nc-Si. We have achieved drastic improvement in PL intensity and uniformity by RTA with appropriate temperature and annealing time.
2. Experiment CaF has fluorite lattice structure that is well matched to the lattice structure of Si with mismatches of #0.6% at room temperature. Si tends
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 1 0 7 - 0
254
M. Watanabe et al. / Journal of Luminescence 80 (1999) 253—256
to form 3D nucleation during initial growth on CaF due to the small surface energy of CaF under appropriate conditions. When Si and CaF are simultaneously deposited on a CaF surface, Si atoms tend to agglomerate because Si and CaF have different bonding mechanisms (covalent bonding in Si, ionic bonding in CaF ). CaF flux suppresses and controls the nucleation of Si in the initial growth stages. As a result, the size and density of the nanocrystalline Si can be controlled by varying the growth temperature and the flux ratio of Si and CaF . Nanocrystalline Si of 5—10 nm in diameter embedded in epitaxial CaF layer can be obtained using this technique. CaF and Si were grown in a UHV ((1; 10\ Torr) chamber. The Si growth rate was approximately 1 nm/min, and CaF was controlled at 1—5 nm/min. A protective oxide layer was grown on a chemically cleaned, polished Si substrate with (1 1 1) orientation. The substrate was heated to 760°C with an Si flux of 0.6 nm/min to remove the protective oxide layer. This resulted in well-developed 7;7 reflection high-energy electron diffraction (RHEED). The nanocrystalline Si (nc-Si) layer was formed by coevaporation of Si and CaF at a substrate temperature (Ts) of 300°C, and the Si : CaF flux ratio was 1 : 3 to 1 : 5 because the PL peak intensity was maximized at this condition in the as grown sample [5]. Each nc-Si layer was 2 nm thick and a pure 2 nm thick CaF layer lacking Si was grown on the nc-Si layer. The CaF interlayer separates each nc-Si layer and suppresses nc-Si coalescence in a normal direction to the substrate; this is effective for minimizing fluctuations of nc-Si diameter and density. Five periods of 2 nm-thick nc-Si and 2 nm-thick CaF multilayered superlattice (SL) were prepared for the RTA sample. Ex situ RTA was carried out on the nc-Si/CaF using infrared rump radiation heating under H ambience at atmospheric pressure. The sample holder was made of graphite with a 50 mm diameter, and the typical sample size was 5 mm square. Temperature was increased from room temperature up to annealing temperature ¹ at the rate of approximately 100°C/s. The temperature was kept relatively constant using feedback control during the annealing process.
3. Results and discussion The temperature dependence of annealing on surface morphology has been observed using atomic force microscopy (AFM). The morphology is almost the same as that of the as-grown samples when annealed at lower than 750°C. The surface fluctuation was almost 1 nm in thickness. However, the surface morphology became extremely rough in the sample annealed at higher than 950°C. The mechanism of degradation in morphology has not yet been determined; however, it may occur by the complexity of reevaporation due to chemical reaction among Si, CaF and H and agglomeration of these materials. Controlling surface morphology is essential for device applications. Therefore, in the following experiment, the RTA temperature was fixed at 700—750°C to avoid degradation in surface morphology. Fig. 1 shows the room temperature PL spectra of samples (a) as-grown, (b) annealed at 700°C for 20 s. The PL intensity of the as-grown sample was relatively weak and inhomogeneous especially at room temperature. On the contrary, PL intensity and uniformity of annealed samples were drastically improved and the PL intensity was almost the same over the whole surface. PL peak wavelength is almost the same for both the PL spectra, although PL peak wavelength slightly shifts to longer direction by in situ annealing at 700°C, where the
Fig. 1. Room temperature PL spectra of samples (a) as-grown, (b) annealed at 700°C for 20 s.
M. Watanabe et al. / Journal of Luminescence 80 (1999) 253—256
temperature increasing rate was 0.5°C/s, as reported in Ref. [6]. One possible explanation of the red shift of the peak wavelength is the quantum size effect. In the case of non-rapid annealing process, Si nanocrystals formed in CaF crystallize and also agglomerate slowly during the relatively long annealing time, resulting in increase of diameter of the crystals. However, in the rapid annealing process, the annealing time is too short to increase the diameter of nc-Si. The time dependence of annealing on surface morphology and PL spectra were studied. Fig. 2
255
shows AFM images of the samples (a) as-grown, annealed at 750°C for (b) 10 s (c) 100 s (d) 1000 s. RTA for longer than 100 s drastically changed the surface morphology when compared with the asgrown-sample. This result indicates that RTA time is, in addition to temperature, essential for keeping surface morphology. Fig. 3 shows the PL spectra of the samples measured at room temperature. As the annealing time increased, PL intensity decreased. However, in samples annealed for 10 and 100 s, the PL was uniform over the whole surface even at room
Fig. 2. AFM images of the samples (a) as grown, annealed at 750°C for (b) 10 s (c) 100 s (d) 1000 s. Annealing for longer than 100 s led to drastic changes in the surface morphology when compared with the as-grown sample.
256
M. Watanabe et al. / Journal of Luminescence 80 (1999) 253—256
was studied. Nanocrystalline silicon embedded in a CaF epilayer was prepared by coevaporation of Si and CaF flux in an ultra high vacuum on Si(1 1 1) substrate. Nc-Si with a diameter less than 10 nm was prepared with the appropriate flux ratio (Si : CaF "1 : 3-1 : 5) and growth temperature at 300°C. Ex situ RTA was performed on the nc-Si/CaF in H at a temperature range of 750°C—1050°C for annealing times of 10/100/ 1000 s. We have found that RTA at approximately 750°C for 10 s results in the improvement of PL intensity and uniformity as well as maintenance of surface morphology. However, a temperature higher than 950°C and a longer annealing time leads to degradation in the surface morphology and PL intensity. Fig. 3. PL spectra of the samples annealed at 750°C for 10, 100 s, 1000 s measured at room temperature. As the annealing time increased, PL intensity decreased. However, in samples annealed for 10 and 100 s, PL was uniform over the whole surface even at room temperature, and the peak intensity improved when compared with the as-grown sample.
temperature, and the peak intensity improved when compared with that of the as-grown-sample. This is probably because annealing for short time enhances crystallization of nc-Si and CaF resulting in an increase in PL intensity. However, annealing for a long time drastically changes surface morphology and decreases PL intensity. This is interpreted as the gradual damage of the nc-Si/CaF layer by chemical etching due to H ambience even at 750°C. PL intensity was not degraded when annealed at 10 s because the etching time is short enough to maintain the crystalline quality, and the improvement of crystal quality by thermal annealing and the passivation effect by H was clearly observable. On the contrary, annealing longer than 100 s damages crystals deeply resulting in degradation of PL intensity. These results indicate that temperature and annealing time are essential for improving the crystalline quality and maintaining surface morphology.
4. Conclusion The influence of rapid thermal annealing on surface morphology and photoluminescence spectra
Acknowledgements The authors would like to thank Professors K. Furuya, S. Arai and Associate Professors M. Asada, Y. Miyamoto for providing fruitful discussion. This work was supported by the Ministry of Education, Science, Sports and Culture through a Scientific Grant-in-Aid, and by The Japan Society for the Promotion of Science (JSPS-RFTF 96P00101), and by the Research Center for UltraHigh-Speed Electronics.
References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] H. Takagi, H. Ogawa, Y. Yamazaki, T. Nakagiri, Appl. Phys. Lett. 56 (1990) 2379. [3] E.A. Montie, G.F.A. van de Walle, D.J. Gravesteijn, A.A. van Gorkun, C.W.T. Bulle-Lieuwma, Appl. Phys. Lett. 56 (1990) 340. [4] F. Bassani, L. Vervoort, I. Mihalcescu, J.C. Vial, F. Arnaud d’Avitaya, J. Appl. Phys. 79 (1996) 4066. [5] M. Watanabe, F. Iizuka, M. Asada, IEICE Trans. E79-C (1996) 1562. [6] M. Watanabe, T. Matsunuma, Y. Maeda, Jpn. J. Appl. Phys. 37 (1998) L591. [7] M. Watanabe, T. Suemasu, S. Muratake, M. Asada, Appl. Phys. Lett. 62 (1993) 300. [8] T. Suemasu, M. Watanabe, M. Asada, N. Suzuki, Electron. Lett. 28 (1992) 1432. [9] T. Suemasu, M. Watanabe, J. Suzuki, Y. Kohno, M. Asada, N. Suzuki, Jpn. J. Appl. Phys. 33 (1994) 57.