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Local structure of erbium–oxygen complexes in erbium-doped silicon and its correlation with the optical activity of erbium
Materials Science and Engineering B72 (2000) 173 – 176 www.elsevier.com/locate/mseb
Local structure of erbium–oxygen complexes in erbium-doped silicon and its correlation with the optical activity of erbium S. Pizzini a,*, S. Binetti a, D. Calcina a, N. Morgante a, A. Cavallini b a b
INFM and Department of Materials Science, Via Cozzi 53, Milan, Italy INFM and Department of Physics, Via Berti Pichat 6 /2, Bologna, Italy
Abstract It is well-known that the sharp luminescence emission at 1.54 mm from erbium-doped silicon has set off a great interest for this material in view of its applications in the third window of optical telecommunications. It is also known that the erbium luminescence is very poor in the absence of impurities like oxygen, carbon and nitrogen, but in spite of the large amount of research work devoted to this material, it is not yet completely clear what is the local structure of the optically active Er centre in oxygen-doped Er–Si alloys. The aim of this paper is to present and discuss the results of the analysis of the EXAFS spectra of two sets of Er-doped silicon samples, of which one was obtained by erbium and oxygen co-implantation and the other was grown by LPE (liquid phase epitaxy). The EXAFS spectra of these samples were satisfactorily fitted by assuming that Er sits in three different configurations, depending on the presence or the absence of oxygen and dislocations in the epi-layer. The relevance of these results in terms of optical and electrical activity of erbium is discussed in details. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Erbium – oxygen complexes; Optically active Er centre; EXAFS spectra
1. Introduction It is well-known that both erbium and dislocations in silicon present photo- and electroluminescence effects in the wavelength range of the third window of optical telecommunications. In fact, the D1 band of the dislocation multiplet falls almost exactly at 1.54 mm, which is also the peak wavelength of the Er luminescence and which also coincides with the absorption minimum of silica-based glass fibers. Although the background physics of these emission processes is obviously different, as the optical activity of Er is due to a radiative transition within the f-f manifold, while that of dislocations is due to an infragap transition, it should be noted that both Er and dislocations provide deep states, lying very close in energy, at which or through which (in the case of Er:Si) recombination could occur or the recombination energy of charge carriers could be channeled, thus excluding * Corresponding author. E-mail address: [email protected] (S. Pizzini)
the participation of phonons in the band to band recombination process. It is also known that metallic and non-metallic impurities in silicon do influence the dislocation luminescence As an example, the PL intensity of D1 and D2 dislocation bands in impurity free silicon is appreciable only at dislocation densities exceeding 107 cm − 2 but it could be enhanced by the presence of transition metals at concentrations lower than their solubility and by Mg [1] introduced by diffusion. It is also well-known that dislocations in silicon act as strong oxygen getters [2,3] and lead to the formation of oxygen dislocation complexes, which induce the presence of an additional recombination band [3]. It should be supposed that in every case dislocations behave as heterogeneous nucleation centres for the segregation of a second phase, but, apparently, the effect of TM and of those impurities which are strongly bonded with oxygen on PL is quite different. In fact, TM silicides segregated at dislocations quench the dislocation luminescence, while the heterogeneous segregation of erbium oxide at dislocations does not have the same affect.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 5 0 3 - 6
S. Pizzini et al. / Materials Science and Engineering B72 (2000) 173–176
174
In order to get more insight in this problem, it would be necessary to know the local configuration of Er in all these circumstances, to correlate the nature of the ligand field with the optical properties. To this scope, we already carried out EXAFS (Extended X-Rays Fine Structure) measurements on number of LPE grown and Ion Implanted (Er+O) samples and of erbium oxide and erbium silicide samples taken as model compounds [4]. From a qualitative comparison of the FT modules of the EXAFS spectra of our samples with those of the reference compounds a close similarity was inferred between the local structure of Er in the heat treated LPE sample and in the Er+O co-implanted sample with that of Er in erbium oxide [4,5]. This result was in qualitative good agreement with the results of Adler et al. [6] who carried out EXAFS measurements on Er implanted CZ silicon samples, and of Terrasi et al. [7] who carried out similar measurements on O and Er co-implanted samples, after a post-regrowth process at 620°C or at 620+ 900°C. Aim of this paper is report and discuss the results of full analysis of our EXAFS data, which was performed using the GNXAS package [8,9]. 2. Experimental details The characteristics of the samples which were used to collect the EXAFS spectra are reported in Tables 1 and 2. EXAFS measurements on the Er-LIII edge (8358 eV) were carried out with fluorescence yield detection collected at GILDA-CRG beamline (ESRF-Grenoble), employing a seven Ge diode detector (at 77 K) and double crystal silicon (311) monochromator. In addition, a Er2O3 and a non-stoichiometric erbium silicide (ErSi1.75) sample were used as model compounds. No independent structural investigation was performed on this last sample.
3. Data analysis Standard EXAFS data analysis is commonly carried out on the previously extracted EXAFS signal. Assuming that EXAFS oscillations lay on a smooth atomic background, the signal extraction is typically performed modelling the atomic and instrumental background by means of splines. Any background feature not correctly accounted for is included in the EXAFS signal, introducing an error which propagates through the whole analysis. It is however clear that the efficiency of the extracting procedure critically depends on the quality of the experimental data. Particular care must be taken in fact when one deals with fluorescence data from a highly diluted photoabsorber, since instrumental contributions to the background may be far from negligible, affecting a wide range of frequencies. In such cases the usual extraction procedure may well lead to a significantly incorrect EXAFS signal. In fact, a preliminary analysis of our samples, where the concentration of the Er photoabsorber ranges around 1017 atoms cm − 3, showed the unsafeness of standard EXAFS data analysis due to problems in the extraction of the EXAFS signal. It was therefore decided to employ the GNXAS package [8,9], which allows to fit the raw data including the background in the model. This means that. the coefficients of the polynomial accounting for the background can be refined together with the structural parameters. Doing so, background will adjusted in order to lead to the best fit. Phase shifts and backscattering amplitudes where calculated using PHAGEN code (GNXAS package) employing Hedin–Lundqvist complex potentials within a muffin tin scheme. Only single scattering processes where included. Possible asymmetry effects in the radial distribution where accounted for.
Table 1 Properties of the LPE-grown samples Name
Substrate
A1 A1tt A2 A3
CZp CZp CZp FZn
r=4.4–6 V cm r = 4.4–6 V cm r= 4.4–6 V cm r =250 V cm
Post-growth annealing
Remarks
None 1100°C (24 h)+450°C (16 h)+750°C (24 h) None None
– Feeble Er PL at 1.54 mm Dislocation PL –
Table 2 Properties of the ion implanted samples Name
Substrate
Ions
Energy and dose
Post-implantation annealing
B1 B2
CZp r= 7.5 V cm CZp r= 7.5 V cm –
Er Er O
1 MeV, 3.2 · 1015 cm−2 1 MeV, 3.2 · 1015 cm−2 0.315 MeV, 1 · 1016 cm2
620°C (1 h)+900°C (30 min) 620°C (1 h)+900°C (30 min) –
S. Pizzini et al. / Materials Science and Engineering B72 (2000) 173–176 Table 3 Results of the EXAFS spectra fitting for the model compounds dEr–O (A, ) Er-oxide (exp) Ref. [10]
s2
dEr–Er (A, )
2.25 90.02 0.007 9 0.003 3.56 90.05
0.005 9 0.005
3.94 9 0.05 0.019 0.01 3.51 90.002 – 3.98 90.001 –
dEr–Si (A, )
DEr–Er (A, )
s2
Er-silicide 2.91 90.03 0.004 9 0.001 3.79 90.02 (exp)
Table 4 Results of the EXAFS spectra fitting for the implanted and LPE grown Er-doped samples
s2
– – 2.27 9 0.02 – – –
A1 A3 B1
s2 0.002 90.001
A1tt A2 B2
As the quality of the EXAFS spectra was relatively poor, the data analysis was performed on a small energy range (up to 8700 eV), in order to ensure the exclusions of non-physical spectral structures. It must be pointed out that the reduction of the energy range implies a loss of resolution in real space and enhances effects of correlation between structural parameters within the refinement.
175
dEr–Si (A, )
s2
dEr–Er (A, )
s2
2.88 90.04 2.93 9 0.03 2.93 90.04
0.017 90.005 0.01 90.005 0.009 90.006
3.76 9 0.04 3.86 9 0.03 3.8 90.1
0.0017 9 0.005 0.01 9 0.01 0.02 9 0.02
dEr–O (A, )
s2
dEr–Er (A, )
s2
2.26 90.03 – 2.22 9 0.03 – 2.31 9 0.05
0.01 9 0.01 – 0.01 9 0.01 – 0.04 9 0.02
3.62 9 0.05 3.9790.07 3.52 90.03 3.8590.07 Absent
0.01 9 0.01 0.01 9 0.01 0.02 9 0.01 0.03 9 0.02 –
4. Experimental results EXAFS data analysis of the Er2O3 model compound allowed to extract structural data in nice agreement with X-ray diffraction studies [10] with regards to the first oxygen shell, as it is shown in Table 3. The effects of the reduction of the energy range are apparent on the Er–Er EXAFS signal. The crystal structure of Er2O3 contains two different Er sites, according to data in Ref. [9],the overall atomic radial distribution around Er peaks at 3.50 and 3.98 A, . A nice fit of the EXAFS signal is achieved provided two separate Er shells are introduced in the model. However, parameters describing both shells are highly correlated, yielding relative large errors for their distances while Debye Waller factors and coordination numbers result to be largely undetermined. Still, the average Er–Er distance well agrees with crystallographic data. Analysis on Er2O3 allows to state that no significant information can be extracted with regards to coordination numbers in the present study, while reliable data can be achieved on the chemical identity an mean distance of atoms around Er centers. The same procedure has been used for erbium silicide as model compound the erbium-doped samples, and the results reported in Tables 3 and 4. 5. Discussion and conclusions
Fig. 1. (a) EXAFS spectra of the samples A1tt, A2 and B2. (b) Dotted curves are experimental results; solid curves represent the best fits to the data.
We will consider first the results dealing with the LPE grown samples A1 and A3 and the sample B1. For all these samples the local structure of Er, within the experimental uncertainty, fits well with that of erbium silicide, as the model compound distances are fitted up to the Er–Er distance of the first Er shell. This result is well compatible with the absence of photoluminescence in the samples A1 and A3, but a problem arises with the Er-implanted, B1 sample, where a feeble but well
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S. Pizzini et al. / Materials Science and Engineering B72 (2000) 173–176
detectable photoluminescence indicates the presence of an optically active Er species. As we know from local density calculations [11] that the optically active Er(3+ ) ion should sit in the tetrahedral Ti position, with four silicon atoms at a distance of 2.48 A, (an expansion of 0.14 A, with respect to the normal Si–Si distance of 2.34 A, ) we would expect to see also a Si shell at this distance. However, in the absence of oxygen ligands, the solubility of Er is less than 1016 cm − 3, smaller than its total content in both LPE and ion implanted samples (\1017 cm − 3) and the excess erbium segregates as an inactive erbium silicide phase, leaving the optically active species at a concentration which is too low to be revealed by EXAFS experiments. Looking at the FT modules of the samples A1tt, A2 and B2 reported in Fig. 1 (see Table 1 for their characteristics) which reports the EXAFS spectra and the FT modules, it is immediately evident that the structure of the Er centre of the (Er +O) implanted sample is different from that of the LPE samples. This preliminary, qualitative conclusion is well supported by the inspection of the distances and variancies extracted from the EXAFS spectra reported in Table 4. In fact, only for the samples A1tt and A2 the local configuration of Er is that expected for Er in a erbium oxide environment. In the case of the co-implanted (Er+O) B2 sample, instead, we find a first shell of oxygen atoms at a distance averaging 2.31 A, , while the second Er shell typical of the oxide is absent. As it is very difficult to get from our data a physically significant value of the coordination number, we can not decide whether is a four or six coordinated (Er–O) system, although a feeble signal is observed at 4.44 A, , which is the average distance of the third next-nearest neighbours of a silicon atom in the silicon lattice. We could then only conclude that we deal with a true (Er–O) n complex, with n ]4, hosted in the silicon lattice without excessive stress, as the experimental Er–O distance lies very closely to a regular Si–Si distance. The formation of this complex implies the simultaneous generation of four or six silicon self-inter-
stitials, depending whether Er sits in a Sis or in a Ti position. The feeble photoluminescence at 1.54 mm exhibited by the heat treated LPE sample A1tt is a good, indirect evidence of the presence of an erbium oxide phase, where erbium is in fact optically active under light excitation. On the contrary, as expected for their purely structural nature, these present results could not offer any significant support to a direct effect of erbium on the dislocation luminescence exhibited by the sample A2, as already postulated in a previous paper [12] and proven in a forthcoming one.
Acknowledgements The authors would like to thank Prof. S. Mobilio and Dr F. D’Acapito for their technical and scientific assistance during the measurements carried out at ESRF.
.
References [1] E.A. Steinman, H.S. Leipner, H.G. Grimmeis, Solid State Phenom. 57 – 58 (1997) 313. [2] S. Acerboni, S. Pizzini, S. Binetti, M. Acciarri, B. Pichaud, J. Appl. Phys. 76 (1994) 2703. [3] E.A. Steinman, V.V. Kveder, H.G. Grimmeis, Solid State Phenom. 47 – 48 (1996) 217. [4] A. Cavallini, B. Fraboni, S. Binetti, S. Pizzini, L. Lazzarini, G. Salviati, Phys. Stat. Sol. (a) 171 (1999) 347. [5] S. Binetti, A. Cavallini, A. Dellafiore, B. Fraboni, E. Grilli, M. Guzzi, S. Pizzini, S. Sanguinetti, J. Lumin. 80 (1999) 351–374. [6] D.L. Adler, D.C. Jacobson, D.J. Eaglesham, M.A. Marcus, J.L. Benton, J.M. Poate, P.H. Citrin, Appl. Phys. Lett. 61 (1992) 2181. [7] A. Terrasi, G. Franzo, S. Coffa, F. Priolo, F. D’Acapito, S. Mobilio, Appl. Phys. Lett. 101 (1991) 712. [8] A. Filipponi, A. Di Cicco, Phys. Rev. B 52 (1995) 15122. [9] A. Filipponi, A. Di Cicco, Phys. Rev. B 52 (1995) 15135. [10] Yu. Malinowskii, O.S. Bondareva, Kristallografiya 36 (1991) 1558. [11] M. Lanoo, C. Delerue, MRS Proc. 301 (1993) 385. [12] A. Cavallini, B. Fraboni, S. Pizzini, S. Binetti, S. Sanguinetti, L. Lazzerini, G. Salviati, J. Appl. Phys. 85 (1999) 1582.
Local structure of erbium–oxygen complexes in erbium-doped silicon and its correlation with the optical activity of erbium S. Pizzini a,*, S. Binetti a, D. Calcina a, N. Morgante a, A. Cavallini b a b
INFM and Department of Materials Science, Via Cozzi 53, Milan, Italy INFM and Department of Physics, Via Berti Pichat 6 /2, Bologna, Italy
Abstract It is well-known that the sharp luminescence emission at 1.54 mm from erbium-doped silicon has set off a great interest for this material in view of its applications in the third window of optical telecommunications. It is also known that the erbium luminescence is very poor in the absence of impurities like oxygen, carbon and nitrogen, but in spite of the large amount of research work devoted to this material, it is not yet completely clear what is the local structure of the optically active Er centre in oxygen-doped Er–Si alloys. The aim of this paper is to present and discuss the results of the analysis of the EXAFS spectra of two sets of Er-doped silicon samples, of which one was obtained by erbium and oxygen co-implantation and the other was grown by LPE (liquid phase epitaxy). The EXAFS spectra of these samples were satisfactorily fitted by assuming that Er sits in three different configurations, depending on the presence or the absence of oxygen and dislocations in the epi-layer. The relevance of these results in terms of optical and electrical activity of erbium is discussed in details. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Erbium – oxygen complexes; Optically active Er centre; EXAFS spectra
1. Introduction It is well-known that both erbium and dislocations in silicon present photo- and electroluminescence effects in the wavelength range of the third window of optical telecommunications. In fact, the D1 band of the dislocation multiplet falls almost exactly at 1.54 mm, which is also the peak wavelength of the Er luminescence and which also coincides with the absorption minimum of silica-based glass fibers. Although the background physics of these emission processes is obviously different, as the optical activity of Er is due to a radiative transition within the f-f manifold, while that of dislocations is due to an infragap transition, it should be noted that both Er and dislocations provide deep states, lying very close in energy, at which or through which (in the case of Er:Si) recombination could occur or the recombination energy of charge carriers could be channeled, thus excluding * Corresponding author. E-mail address: [email protected] (S. Pizzini)
the participation of phonons in the band to band recombination process. It is also known that metallic and non-metallic impurities in silicon do influence the dislocation luminescence As an example, the PL intensity of D1 and D2 dislocation bands in impurity free silicon is appreciable only at dislocation densities exceeding 107 cm − 2 but it could be enhanced by the presence of transition metals at concentrations lower than their solubility and by Mg [1] introduced by diffusion. It is also well-known that dislocations in silicon act as strong oxygen getters [2,3] and lead to the formation of oxygen dislocation complexes, which induce the presence of an additional recombination band [3]. It should be supposed that in every case dislocations behave as heterogeneous nucleation centres for the segregation of a second phase, but, apparently, the effect of TM and of those impurities which are strongly bonded with oxygen on PL is quite different. In fact, TM silicides segregated at dislocations quench the dislocation luminescence, while the heterogeneous segregation of erbium oxide at dislocations does not have the same affect.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 5 0 3 - 6
S. Pizzini et al. / Materials Science and Engineering B72 (2000) 173–176
174
In order to get more insight in this problem, it would be necessary to know the local configuration of Er in all these circumstances, to correlate the nature of the ligand field with the optical properties. To this scope, we already carried out EXAFS (Extended X-Rays Fine Structure) measurements on number of LPE grown and Ion Implanted (Er+O) samples and of erbium oxide and erbium silicide samples taken as model compounds [4]. From a qualitative comparison of the FT modules of the EXAFS spectra of our samples with those of the reference compounds a close similarity was inferred between the local structure of Er in the heat treated LPE sample and in the Er+O co-implanted sample with that of Er in erbium oxide [4,5]. This result was in qualitative good agreement with the results of Adler et al. [6] who carried out EXAFS measurements on Er implanted CZ silicon samples, and of Terrasi et al. [7] who carried out similar measurements on O and Er co-implanted samples, after a post-regrowth process at 620°C or at 620+ 900°C. Aim of this paper is report and discuss the results of full analysis of our EXAFS data, which was performed using the GNXAS package [8,9]. 2. Experimental details The characteristics of the samples which were used to collect the EXAFS spectra are reported in Tables 1 and 2. EXAFS measurements on the Er-LIII edge (8358 eV) were carried out with fluorescence yield detection collected at GILDA-CRG beamline (ESRF-Grenoble), employing a seven Ge diode detector (at 77 K) and double crystal silicon (311) monochromator. In addition, a Er2O3 and a non-stoichiometric erbium silicide (ErSi1.75) sample were used as model compounds. No independent structural investigation was performed on this last sample.
3. Data analysis Standard EXAFS data analysis is commonly carried out on the previously extracted EXAFS signal. Assuming that EXAFS oscillations lay on a smooth atomic background, the signal extraction is typically performed modelling the atomic and instrumental background by means of splines. Any background feature not correctly accounted for is included in the EXAFS signal, introducing an error which propagates through the whole analysis. It is however clear that the efficiency of the extracting procedure critically depends on the quality of the experimental data. Particular care must be taken in fact when one deals with fluorescence data from a highly diluted photoabsorber, since instrumental contributions to the background may be far from negligible, affecting a wide range of frequencies. In such cases the usual extraction procedure may well lead to a significantly incorrect EXAFS signal. In fact, a preliminary analysis of our samples, where the concentration of the Er photoabsorber ranges around 1017 atoms cm − 3, showed the unsafeness of standard EXAFS data analysis due to problems in the extraction of the EXAFS signal. It was therefore decided to employ the GNXAS package [8,9], which allows to fit the raw data including the background in the model. This means that. the coefficients of the polynomial accounting for the background can be refined together with the structural parameters. Doing so, background will adjusted in order to lead to the best fit. Phase shifts and backscattering amplitudes where calculated using PHAGEN code (GNXAS package) employing Hedin–Lundqvist complex potentials within a muffin tin scheme. Only single scattering processes where included. Possible asymmetry effects in the radial distribution where accounted for.
Table 1 Properties of the LPE-grown samples Name
Substrate
A1 A1tt A2 A3
CZp CZp CZp FZn
r=4.4–6 V cm r = 4.4–6 V cm r= 4.4–6 V cm r =250 V cm
Post-growth annealing
Remarks
None 1100°C (24 h)+450°C (16 h)+750°C (24 h) None None
– Feeble Er PL at 1.54 mm Dislocation PL –
Table 2 Properties of the ion implanted samples Name
Substrate
Ions
Energy and dose
Post-implantation annealing
B1 B2
CZp r= 7.5 V cm CZp r= 7.5 V cm –
Er Er O
1 MeV, 3.2 · 1015 cm−2 1 MeV, 3.2 · 1015 cm−2 0.315 MeV, 1 · 1016 cm2
620°C (1 h)+900°C (30 min) 620°C (1 h)+900°C (30 min) –
S. Pizzini et al. / Materials Science and Engineering B72 (2000) 173–176 Table 3 Results of the EXAFS spectra fitting for the model compounds dEr–O (A, ) Er-oxide (exp) Ref. [10]
s2
dEr–Er (A, )
2.25 90.02 0.007 9 0.003 3.56 90.05
0.005 9 0.005
3.94 9 0.05 0.019 0.01 3.51 90.002 – 3.98 90.001 –
dEr–Si (A, )
DEr–Er (A, )
s2
Er-silicide 2.91 90.03 0.004 9 0.001 3.79 90.02 (exp)
Table 4 Results of the EXAFS spectra fitting for the implanted and LPE grown Er-doped samples
s2
– – 2.27 9 0.02 – – –
A1 A3 B1
s2 0.002 90.001
A1tt A2 B2
As the quality of the EXAFS spectra was relatively poor, the data analysis was performed on a small energy range (up to 8700 eV), in order to ensure the exclusions of non-physical spectral structures. It must be pointed out that the reduction of the energy range implies a loss of resolution in real space and enhances effects of correlation between structural parameters within the refinement.
175
dEr–Si (A, )
s2
dEr–Er (A, )
s2
2.88 90.04 2.93 9 0.03 2.93 90.04
0.017 90.005 0.01 90.005 0.009 90.006
3.76 9 0.04 3.86 9 0.03 3.8 90.1
0.0017 9 0.005 0.01 9 0.01 0.02 9 0.02
dEr–O (A, )
s2
dEr–Er (A, )
s2
2.26 90.03 – 2.22 9 0.03 – 2.31 9 0.05
0.01 9 0.01 – 0.01 9 0.01 – 0.04 9 0.02
3.62 9 0.05 3.9790.07 3.52 90.03 3.8590.07 Absent
0.01 9 0.01 0.01 9 0.01 0.02 9 0.01 0.03 9 0.02 –
4. Experimental results EXAFS data analysis of the Er2O3 model compound allowed to extract structural data in nice agreement with X-ray diffraction studies [10] with regards to the first oxygen shell, as it is shown in Table 3. The effects of the reduction of the energy range are apparent on the Er–Er EXAFS signal. The crystal structure of Er2O3 contains two different Er sites, according to data in Ref. [9],the overall atomic radial distribution around Er peaks at 3.50 and 3.98 A, . A nice fit of the EXAFS signal is achieved provided two separate Er shells are introduced in the model. However, parameters describing both shells are highly correlated, yielding relative large errors for their distances while Debye Waller factors and coordination numbers result to be largely undetermined. Still, the average Er–Er distance well agrees with crystallographic data. Analysis on Er2O3 allows to state that no significant information can be extracted with regards to coordination numbers in the present study, while reliable data can be achieved on the chemical identity an mean distance of atoms around Er centers. The same procedure has been used for erbium silicide as model compound the erbium-doped samples, and the results reported in Tables 3 and 4. 5. Discussion and conclusions
Fig. 1. (a) EXAFS spectra of the samples A1tt, A2 and B2. (b) Dotted curves are experimental results; solid curves represent the best fits to the data.
We will consider first the results dealing with the LPE grown samples A1 and A3 and the sample B1. For all these samples the local structure of Er, within the experimental uncertainty, fits well with that of erbium silicide, as the model compound distances are fitted up to the Er–Er distance of the first Er shell. This result is well compatible with the absence of photoluminescence in the samples A1 and A3, but a problem arises with the Er-implanted, B1 sample, where a feeble but well
176
S. Pizzini et al. / Materials Science and Engineering B72 (2000) 173–176
detectable photoluminescence indicates the presence of an optically active Er species. As we know from local density calculations [11] that the optically active Er(3+ ) ion should sit in the tetrahedral Ti position, with four silicon atoms at a distance of 2.48 A, (an expansion of 0.14 A, with respect to the normal Si–Si distance of 2.34 A, ) we would expect to see also a Si shell at this distance. However, in the absence of oxygen ligands, the solubility of Er is less than 1016 cm − 3, smaller than its total content in both LPE and ion implanted samples (\1017 cm − 3) and the excess erbium segregates as an inactive erbium silicide phase, leaving the optically active species at a concentration which is too low to be revealed by EXAFS experiments. Looking at the FT modules of the samples A1tt, A2 and B2 reported in Fig. 1 (see Table 1 for their characteristics) which reports the EXAFS spectra and the FT modules, it is immediately evident that the structure of the Er centre of the (Er +O) implanted sample is different from that of the LPE samples. This preliminary, qualitative conclusion is well supported by the inspection of the distances and variancies extracted from the EXAFS spectra reported in Table 4. In fact, only for the samples A1tt and A2 the local configuration of Er is that expected for Er in a erbium oxide environment. In the case of the co-implanted (Er+O) B2 sample, instead, we find a first shell of oxygen atoms at a distance averaging 2.31 A, , while the second Er shell typical of the oxide is absent. As it is very difficult to get from our data a physically significant value of the coordination number, we can not decide whether is a four or six coordinated (Er–O) system, although a feeble signal is observed at 4.44 A, , which is the average distance of the third next-nearest neighbours of a silicon atom in the silicon lattice. We could then only conclude that we deal with a true (Er–O) n complex, with n ]4, hosted in the silicon lattice without excessive stress, as the experimental Er–O distance lies very closely to a regular Si–Si distance. The formation of this complex implies the simultaneous generation of four or six silicon self-inter-
stitials, depending whether Er sits in a Sis or in a Ti position. The feeble photoluminescence at 1.54 mm exhibited by the heat treated LPE sample A1tt is a good, indirect evidence of the presence of an erbium oxide phase, where erbium is in fact optically active under light excitation. On the contrary, as expected for their purely structural nature, these present results could not offer any significant support to a direct effect of erbium on the dislocation luminescence exhibited by the sample A2, as already postulated in a previous paper [12] and proven in a forthcoming one.
Acknowledgements The authors would like to thank Prof. S. Mobilio and Dr F. D’Acapito for their technical and scientific assistance during the measurements carried out at ESRF.
.
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