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Site-selective excitation of Er3+ ions in oxygen-rich silicon
Physica B 308–310 (2001) 354–356
Site-selective excitation of Er3+ ions in oxygen-rich silicon a ! A. Kozaneckia,*, D. Kuritsyna, H. Przybylinska , W. Jantschb a
! 32/46, 02-668 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Al. Lotnikow b Institute fur . Linz A-4040, Austria . Halbleiterphysik, Johannes Kepler Universitat,
Abstract Photoluminescence excitation spectroscopy is used to study the location of Er3+ ions in oxygen-rich silicon and its excitation mechanisms. We find that Er luminescence excited within the range of the 4I13/2 excited manifold of Er3+ consists of two features: a broad band independent of excitation wavelength, lexc ; and a fluorescence narrowed line, dependent on lexc : These results show that Er3+ ions are excited via two energy transfer channels: the first one is due to non-radiative transfer of energy from excitons to the 4I13/2 state, whereas the second one, leading to line narrowing, is typically resonant, site selective excitation of Er ions in silica glass. The results show that excitable Er ions are located most probably within nano-precipitates of SiO2 d. r 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ww; 78.55.Hx Keywords: Luminescence; Excitation mechanisms; Silicon oxide; Erbium
1. Introduction Erbium-doped silicon is a promising material for light emitters operating at 1.54 mm, the most important wavelength for optical communication and prospective Si-based optoelectronics [1]. In spite of many efforts, however, the efficiency of the 1.54 mm luminescence of Er3+ at room temperature (RT) is still too low for practical applications. The main reason for this appears to be an indirect excitation mechanism for the 4f shell of Er3+, dominated by Auger-type interaction with charge carriers and excitons [2]. On the one hand, charge carrier mediated mechanisms of excitation ensure efficient energy transfer to Er centres at low temperatures. On the other they are also responsible for thermal quenching of the luminescence. The quenching can be substantially reduced by codoping with oxygen [3] and also by employing wide band gap semiconductor hosts [4]. Efficient photoluminescence (PL) of Er3+ was obtained at RT in disordered Sibased materials with high oxygen contents, such as *Corresponding author. Tel.: +48-22-843-68-61; fax: +4822-843-09-26. E-mail address: [email protected] (A. Kozanecki).
porous silicon [5], semi-insulating polycrystalline silicon [6,7], or oxygen-rich silicon grown by laser ablation [8]. Moreover, the only Si : Er-based light emitters operating at RT were shown to employ silica nano-precipitates containing the Er [9,10]. The weak temperature quenching of the PL of Er3+ and lack of sharp emission lines, typical of oxygenlean crystalline Si : Er [11], suggest that all the Er ions in oxygen-rich silicon layers are located within silica nano-precipitates. For such a location, direct optical excitation of Er3+ via absorption of photons should, in principle, be possible. It would result in fluorescence line narrowing (FLN), as different Er sites in silicon oxide precipitates would be selectively pumped [12]. In general, we should not expect direct optical excitation to be observed for pump wavelengths in the visible, because of the much higher absorption cross section of amorphous silicon (a-Si) than that of Er and, in effect, a dominant exciton mediated transfer of energy to the Er. On the other hand, resonant excitation of Er3+ ions should be easier to observe near 1 mm, where absorption in a-Si is relatively small. In this work, we show using PLE spectroscopy and selectively excited PL that those Er3+ ions that can be
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 8 - 6
A. Kozanecki et al. / Physica B 308–310 (2001) 354–356
excited optically in Si(O) : Er are indeed located within precipitates of SiOx (or SiO2) phase.
2. Experimental The composition of the investigated layers was approximately SiO, with an Er concentration of B1020 cm 3. PL and PLE measurements were performed at helium temperatures. PL was excited using a Ti : sapphire laser chopped at 18 Hz and detected with liquid nitrogen cooled Ge detector. The spectra were normalised to a constant excitation power. A quartz sample implanted with 800 keV Er ions and annealed at 8001C served us as a reference.
355
broad emission is observed. Its intensity depends slightly on the pump wavelength. For comparison, the PLE spectra in quartz implanted with Er ions at 800 keV and annealed at 8001C were measured (Fig. 3). Similarly as in Fig. 1, the peak positions in the spectra follow the changes in the detection energy. However, in contrast to Si(O) : Er, a long wavelength cut-off, which for increasing ldet shifts to shorter wavelengths, is clearly seen. The Er emission cannot be excited with pump wavelengths longer than 986 nm. As a result, it can be assumed that the low energy edge of the 4I11/2 state is at B1.2575 eV. In quartz, excitation of the Er3+ is achieved by optical pumpingFeach laser wavelength can selectively excite some Er centres having an energy of the 4I11/2 state at
3. Results and discussion PLE spectra, measured at 5 K, of Er3+ in our samples of SiO : Er are presented in Fig. 1 for different detection wavelengths, ldet : For an excitation wavelengths, lexc ; within the range of B975–986 nm the spectra depend on ldet and for larger ldet ; the peak positions shift towards longer wavelengths. On an energy scale, the shift follows exactly a 1 : 1 correspondence. For lexc beyond the 975–986 nm range the PLE spectra are flat and do not depend on ldet : Fig. 2 shows selectively excited PL spectra of Er3+ near 1.5 mm. The spectra depend in a characteristic way on the pump energy. For 975olexc o986 nm in each spectrum, a narrow PL can be distinguished whose spectral position shifts reflecting the changes of the pump wavelength. This narrow PL is superimposed on a broad band which does not change in shape with lexc : For excitation at lexc o975 nm and >990 nm only a
Fig. 1. PLE spectra of Er3+ in laser ablated silicon at 7 K.
Fig. 2. Site-selective PL spectra of Er3+ in laser ablated silicon.
Fig. 3. PLE spectra of Er3+ ions in quartz implanted and annealed at 8001C.
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A. Kozanecki et al. / Physica B 308–310 (2001) 354–356
resonance with the laser [12]. Then multi-phonon relaxation to the 4I13/2 state of the originally excited ion takes place followed by emission near 1.5 mm. Since at low temperatures phonon-absorption-assisted excitation of Er ions is not possible, the centres having energies of their 4I11/2 state higher than the pump light energy, by principle, cannot be excited. Therefore, a long-wavelength cut-off dependent on the ldet is observed in the PLE spectra (Fig. 3). The PLE spectra and selectively excited luminescence of erbium show that two excitation mechanisms of Er3+ are active. The first one is responsible for the dependence of the PL on lexc and ldet : This is typical of siteselective excitation of Er ions in glasses and quartz [12,13]. The similarities in the PLE spectra in Figs. 1 and 3 indicate that the local atomic environment around Er ions in quartz and in Si(O) : Er are similar. This finding supports our earlier suggestion that Er ions in Si(O) : Er are located inside oxygen-rich clusters, presumably within micro- or nano-precipitates of SiOx : Taking into account the composition of the layers (Si : OB1 : 1) and the absence of sharp lines typical for the luminescence of Er3+ in crystalline Si at low temperatures [11], this conclusion appears to be justified. A larger inhomogeneous broadening of the PL bands in the oxygen-rich silicon implies a much larger variety of Er centres as compared to quartz. Such a location explains also the low temperature quenching of the Er PL [10]. The second excitation process is responsible for the constant background in the PLE spectra (Fig. 1) and the broad emission observed for selectively excited PL (Fig. 2). It can be attributed to the excitation via the band edge of silicon. As excitons are formed in a silicon rich phase or in Si micro-precipitates, which have a band gap Eg equal to that of bulk Si, only an excitation to the 4 I13/2 of Er3+ state is possible due to non-radiative energy transfer from excitons. The efficiency of such a process practically does not depend on the excitation wavelength in this relatively narrow range of 960– 1000 nm, since the absorption cross-section of a-Si is almost constant there.
4. Summary The results of PLE and selectively excited PL measurements prove that Er ions in oxygen-rich silicon are located inside silicon oxide clusters as they reveal, at
least in part, the behaviour typical of rare earth dopants in glasses. We also show that for such a location two excitation mechanisms of Er ions are possible: (i) resonant optical excitation to the 4I11/2 state, which is responsible for fluorescence line narrowing, and (ii) excitation via the band edge of silicon to the 4I11/2 state. The latter process is responsible for broad luminescence in selectively excited PL and a constant background in the PLE spectra.
Acknowledgements This work is supported in Poland by the State Committee for Scientific Research (grant No. 7 T11B . 007 21) and in Austria by the FWF, GMe, and OAD.
References [1] S. Coffa, G. Franzo, F. Priolo, Mater. Res. Soc. Bull. 23 (1998) 2325. [2] G. Franzo, F. Priolo, S. Coffa, A. Carnera, Phys. Rev. B 57 (1998) 4443. [3] J. Michel, J.L. Benton, R.F. Ferrante, D.C. Jacobson, D.J. Eaglesham, E.A. Fitzgerald, Y.-H. Xie, J.M. Poate, L.C. Kimerling, J. Appl. Phys. 70 (1991) 2672. [4] P.N. Favennec, H. l’Haridon, D. Moutonnet, M. Salvi, M. Gauneau, Material Research Society Symposium Proceedings, Vol. 301, 1993, p. 181. [5] T. Kimura, A. Yokoi, H. Horiguchi, R. Saito, T. Ikoma, A. Sato, Appl. Phys. Lett. 75 (1999) 3989. [6] G. van den Hoven, J.H. Shin, A. Polman, S. Lombardo, S.U. Campisano, J. Appl. Phys. 78 (1995) 2642. [7] S. Lombardo, S.U. Campisano, G.N. van den Hoven, A. Cacciato, A. Polman, Appl. Phys. Lett. 63 (1993) 1942. [8] Wai Lek Ng, M.P. Temple, P.A. Childs, F. Wellhofer, K.P. Homewood, Appl. Phys. Lett. 75 (1999) 97. [9] S. Lanzerstorfer, L. Palmetshofer, W. Jantsch, J. Stimmer, Appl. Phys. Lett. 72 (1998) 809. [10] W. Jantsch, S. Lanzerstorfer, L. Palmetshofer, M. Stephikova, H. Preier, J. Lumin. 80 (1999) 9. ! [11] H. Przybylinska, W. Jantsch, Yu. Suprun-Belevitch, M. Stephikova, L. Palmetshofer, G. Hendorfer, A. Kozanecki, R.J. Wilson, B.J. Sealy, Phys. Rev. B 54 (1996) 2532. [12] A.M. Jurdyc, B. Jacquier, J.C. G#acon, J.F. Bayon, E. Delavaque, J. Lumin. 6061 (1994) 89. [13] A. Kozanecki, M. Stephikova, S. Lanzerstorfer, W. Jantsch, L. Palmetshofer, B.J. Sealy, C. Jeynes, Appl. Phys. Lett. 66 (1995) 115.
Site-selective excitation of Er3+ ions in oxygen-rich silicon a ! A. Kozaneckia,*, D. Kuritsyna, H. Przybylinska , W. Jantschb a
! 32/46, 02-668 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Al. Lotnikow b Institute fur . Linz A-4040, Austria . Halbleiterphysik, Johannes Kepler Universitat,
Abstract Photoluminescence excitation spectroscopy is used to study the location of Er3+ ions in oxygen-rich silicon and its excitation mechanisms. We find that Er luminescence excited within the range of the 4I13/2 excited manifold of Er3+ consists of two features: a broad band independent of excitation wavelength, lexc ; and a fluorescence narrowed line, dependent on lexc : These results show that Er3+ ions are excited via two energy transfer channels: the first one is due to non-radiative transfer of energy from excitons to the 4I13/2 state, whereas the second one, leading to line narrowing, is typically resonant, site selective excitation of Er ions in silica glass. The results show that excitable Er ions are located most probably within nano-precipitates of SiO2 d. r 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ww; 78.55.Hx Keywords: Luminescence; Excitation mechanisms; Silicon oxide; Erbium
1. Introduction Erbium-doped silicon is a promising material for light emitters operating at 1.54 mm, the most important wavelength for optical communication and prospective Si-based optoelectronics [1]. In spite of many efforts, however, the efficiency of the 1.54 mm luminescence of Er3+ at room temperature (RT) is still too low for practical applications. The main reason for this appears to be an indirect excitation mechanism for the 4f shell of Er3+, dominated by Auger-type interaction with charge carriers and excitons [2]. On the one hand, charge carrier mediated mechanisms of excitation ensure efficient energy transfer to Er centres at low temperatures. On the other they are also responsible for thermal quenching of the luminescence. The quenching can be substantially reduced by codoping with oxygen [3] and also by employing wide band gap semiconductor hosts [4]. Efficient photoluminescence (PL) of Er3+ was obtained at RT in disordered Sibased materials with high oxygen contents, such as *Corresponding author. Tel.: +48-22-843-68-61; fax: +4822-843-09-26. E-mail address: [email protected] (A. Kozanecki).
porous silicon [5], semi-insulating polycrystalline silicon [6,7], or oxygen-rich silicon grown by laser ablation [8]. Moreover, the only Si : Er-based light emitters operating at RT were shown to employ silica nano-precipitates containing the Er [9,10]. The weak temperature quenching of the PL of Er3+ and lack of sharp emission lines, typical of oxygenlean crystalline Si : Er [11], suggest that all the Er ions in oxygen-rich silicon layers are located within silica nano-precipitates. For such a location, direct optical excitation of Er3+ via absorption of photons should, in principle, be possible. It would result in fluorescence line narrowing (FLN), as different Er sites in silicon oxide precipitates would be selectively pumped [12]. In general, we should not expect direct optical excitation to be observed for pump wavelengths in the visible, because of the much higher absorption cross section of amorphous silicon (a-Si) than that of Er and, in effect, a dominant exciton mediated transfer of energy to the Er. On the other hand, resonant excitation of Er3+ ions should be easier to observe near 1 mm, where absorption in a-Si is relatively small. In this work, we show using PLE spectroscopy and selectively excited PL that those Er3+ ions that can be
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 8 - 6
A. Kozanecki et al. / Physica B 308–310 (2001) 354–356
excited optically in Si(O) : Er are indeed located within precipitates of SiOx (or SiO2) phase.
2. Experimental The composition of the investigated layers was approximately SiO, with an Er concentration of B1020 cm 3. PL and PLE measurements were performed at helium temperatures. PL was excited using a Ti : sapphire laser chopped at 18 Hz and detected with liquid nitrogen cooled Ge detector. The spectra were normalised to a constant excitation power. A quartz sample implanted with 800 keV Er ions and annealed at 8001C served us as a reference.
355
broad emission is observed. Its intensity depends slightly on the pump wavelength. For comparison, the PLE spectra in quartz implanted with Er ions at 800 keV and annealed at 8001C were measured (Fig. 3). Similarly as in Fig. 1, the peak positions in the spectra follow the changes in the detection energy. However, in contrast to Si(O) : Er, a long wavelength cut-off, which for increasing ldet shifts to shorter wavelengths, is clearly seen. The Er emission cannot be excited with pump wavelengths longer than 986 nm. As a result, it can be assumed that the low energy edge of the 4I11/2 state is at B1.2575 eV. In quartz, excitation of the Er3+ is achieved by optical pumpingFeach laser wavelength can selectively excite some Er centres having an energy of the 4I11/2 state at
3. Results and discussion PLE spectra, measured at 5 K, of Er3+ in our samples of SiO : Er are presented in Fig. 1 for different detection wavelengths, ldet : For an excitation wavelengths, lexc ; within the range of B975–986 nm the spectra depend on ldet and for larger ldet ; the peak positions shift towards longer wavelengths. On an energy scale, the shift follows exactly a 1 : 1 correspondence. For lexc beyond the 975–986 nm range the PLE spectra are flat and do not depend on ldet : Fig. 2 shows selectively excited PL spectra of Er3+ near 1.5 mm. The spectra depend in a characteristic way on the pump energy. For 975olexc o986 nm in each spectrum, a narrow PL can be distinguished whose spectral position shifts reflecting the changes of the pump wavelength. This narrow PL is superimposed on a broad band which does not change in shape with lexc : For excitation at lexc o975 nm and >990 nm only a
Fig. 1. PLE spectra of Er3+ in laser ablated silicon at 7 K.
Fig. 2. Site-selective PL spectra of Er3+ in laser ablated silicon.
Fig. 3. PLE spectra of Er3+ ions in quartz implanted and annealed at 8001C.
356
A. Kozanecki et al. / Physica B 308–310 (2001) 354–356
resonance with the laser [12]. Then multi-phonon relaxation to the 4I13/2 state of the originally excited ion takes place followed by emission near 1.5 mm. Since at low temperatures phonon-absorption-assisted excitation of Er ions is not possible, the centres having energies of their 4I11/2 state higher than the pump light energy, by principle, cannot be excited. Therefore, a long-wavelength cut-off dependent on the ldet is observed in the PLE spectra (Fig. 3). The PLE spectra and selectively excited luminescence of erbium show that two excitation mechanisms of Er3+ are active. The first one is responsible for the dependence of the PL on lexc and ldet : This is typical of siteselective excitation of Er ions in glasses and quartz [12,13]. The similarities in the PLE spectra in Figs. 1 and 3 indicate that the local atomic environment around Er ions in quartz and in Si(O) : Er are similar. This finding supports our earlier suggestion that Er ions in Si(O) : Er are located inside oxygen-rich clusters, presumably within micro- or nano-precipitates of SiOx : Taking into account the composition of the layers (Si : OB1 : 1) and the absence of sharp lines typical for the luminescence of Er3+ in crystalline Si at low temperatures [11], this conclusion appears to be justified. A larger inhomogeneous broadening of the PL bands in the oxygen-rich silicon implies a much larger variety of Er centres as compared to quartz. Such a location explains also the low temperature quenching of the Er PL [10]. The second excitation process is responsible for the constant background in the PLE spectra (Fig. 1) and the broad emission observed for selectively excited PL (Fig. 2). It can be attributed to the excitation via the band edge of silicon. As excitons are formed in a silicon rich phase or in Si micro-precipitates, which have a band gap Eg equal to that of bulk Si, only an excitation to the 4 I13/2 of Er3+ state is possible due to non-radiative energy transfer from excitons. The efficiency of such a process practically does not depend on the excitation wavelength in this relatively narrow range of 960– 1000 nm, since the absorption cross-section of a-Si is almost constant there.
4. Summary The results of PLE and selectively excited PL measurements prove that Er ions in oxygen-rich silicon are located inside silicon oxide clusters as they reveal, at
least in part, the behaviour typical of rare earth dopants in glasses. We also show that for such a location two excitation mechanisms of Er ions are possible: (i) resonant optical excitation to the 4I11/2 state, which is responsible for fluorescence line narrowing, and (ii) excitation via the band edge of silicon to the 4I11/2 state. The latter process is responsible for broad luminescence in selectively excited PL and a constant background in the PLE spectra.
Acknowledgements This work is supported in Poland by the State Committee for Scientific Research (grant No. 7 T11B . 007 21) and in Austria by the FWF, GMe, and OAD.
References [1] S. Coffa, G. Franzo, F. Priolo, Mater. Res. Soc. Bull. 23 (1998) 2325. [2] G. Franzo, F. Priolo, S. Coffa, A. Carnera, Phys. Rev. B 57 (1998) 4443. [3] J. Michel, J.L. Benton, R.F. Ferrante, D.C. Jacobson, D.J. Eaglesham, E.A. Fitzgerald, Y.-H. Xie, J.M. Poate, L.C. Kimerling, J. Appl. Phys. 70 (1991) 2672. [4] P.N. Favennec, H. l’Haridon, D. Moutonnet, M. Salvi, M. Gauneau, Material Research Society Symposium Proceedings, Vol. 301, 1993, p. 181. [5] T. Kimura, A. Yokoi, H. Horiguchi, R. Saito, T. Ikoma, A. Sato, Appl. Phys. Lett. 75 (1999) 3989. [6] G. van den Hoven, J.H. Shin, A. Polman, S. Lombardo, S.U. Campisano, J. Appl. Phys. 78 (1995) 2642. [7] S. Lombardo, S.U. Campisano, G.N. van den Hoven, A. Cacciato, A. Polman, Appl. Phys. Lett. 63 (1993) 1942. [8] Wai Lek Ng, M.P. Temple, P.A. Childs, F. Wellhofer, K.P. Homewood, Appl. Phys. Lett. 75 (1999) 97. [9] S. Lanzerstorfer, L. Palmetshofer, W. Jantsch, J. Stimmer, Appl. Phys. Lett. 72 (1998) 809. [10] W. Jantsch, S. Lanzerstorfer, L. Palmetshofer, M. Stephikova, H. Preier, J. Lumin. 80 (1999) 9. ! [11] H. Przybylinska, W. Jantsch, Yu. Suprun-Belevitch, M. Stephikova, L. Palmetshofer, G. Hendorfer, A. Kozanecki, R.J. Wilson, B.J. Sealy, Phys. Rev. B 54 (1996) 2532. [12] A.M. Jurdyc, B. Jacquier, J.C. G#acon, J.F. Bayon, E. Delavaque, J. Lumin. 6061 (1994) 89. [13] A. Kozanecki, M. Stephikova, S. Lanzerstorfer, W. Jantsch, L. Palmetshofer, B.J. Sealy, C. Jeynes, Appl. Phys. Lett. 66 (1995) 115.