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The local structure of porous silicon investigated by EXAFS
ELSEVIER
Physica B 208&209 (1995) 559-561
The local structure of porous silicon investigated by EXAFS G. Dalba a, P. Fornasini a, M. Grazioli a, R. Grisenti a'*, Y. Soldo a, F.
Rocca b
Dipartimento di Fisica dell' Universith degli Studi di Trento, 1-38050 Povo (Trento), Italy b Centro C N R - I T C di Fisica degli Stati Aggregati ed Impianto lonico, 1-38050 Povo (Trento), Italy
Abstract
X-ray absorption measurements at the K-edge of silicon have been done in transmission mode on three powdered samples of porous silicon (p-Si) with 50%, 65% and 80% porosity, prepared by decreasing the H F content in the anodic solution. The analysis of EXAFS reveals a crystal-like structure of p-Si at least up to the third coordination shell. A slight progressive reduction of coordination numbers and increase of Debye-Waller factors with respect to c-Si is found in passing from the first to the third coordination shell; this behaviour is connected to the limited range of homogeneous structural regions in p-Si. The Debye-Waller factors increase with decreasing porosity, indicating a more ordered local structure in samples prepared with lower H F content and corresponding to the highest porosity.
In the last years great scientific attention has been devoted to the luminescence properties of porous silicon (p-Si) in the visible region [1-3]; the promising perspectives that p-Si offers to optoelectronics and related technology are the reasons of such interest. The origin of photoluminescence is, however, not yet understood. A more detailed knowledge of the local environment of Si is strongly needed to give a structural basis to the proposed model for the origin of luminescence. X-ray absorption spectroscopy (XAS) in the low energy range (XANES) and in the higher range (EXAFS) is a particularly well suited technique to this purpose [4]. Previous XAS measurements performed by indirect techniques such as total electron yield (TEY) and X-ray induced optical luminescence (XIOL) already stated some steady points as the crystalline-like structure of p-Si and the unimportance of siloxene in the emitting process. Difference between c-Si and p-Si due to different preparation conditions, ageing and H F cleaning of the surface were pointed out as well. Nevertheless the relatively low * Corresponding author.
signal-to-noise ratio currently achievable by these methods prevented a quantitative analysis of the EXAFS signal. To overcome this difficulty, we have performed accurate direct transmission measurements on self-standing powders of pure p-Si. This allowed us to compare coordination numbers, Debye-Waller factors and interatomic distances of the first three coordination shells of Si in p-Si samples with different degree of porosity (50%, 65% and 80%). The EXAFS signal ~(k) was extracted by standard techniques, where k is the photoelectron wave vector. For sake of brevity we present here only Fig. 1 which gives the moduli of FT of the three porous samples and c-Si. The similarity between p-Si and c-Si extends in the R space at least up to the third shell. This is different from the case of a-Si, where only the first shell peak is present [5]. The magnitude reduction in the peaks amplitude of p-Si samples with respect to c-Si is worth noting. Such reduction is vanishing with increasing porosity. This feature can be attributed to changes either of static disorder and/or of coordination number related to small dimension of Si nanocrystals. In order to obtain quantitative information, the well defined peaks
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 5 0 - 0
560
G. Dalba et al. /Physica B 208&209 (1995) 559 561
20 ¸
- - ~ p
-Si
----
1 st ,
-Si(80%)
0.5
Shell
f
,
2 nd Shell j
,
l
3 rd Shell ,
r
i
,
f
,
p-Si
50°/~
15' 0
,-:, 73
,<
.,~ 005
5
.c
.........
~
6S°/o P-si
-0.5
0-
E
~--:~-~
-0.5
10-
b
~
a
t-
I::
0.5
p-Si
80°/*
i
20
fi
- Si
[ --e
i
i
80
i
,
0
i 40
~
i 80
i
i 0
i
40
:
I
80
i
120
Fig. 2. Logarithms of amplitudes' ratios: experiment (dashed line) and best fits (solid lines).
5
t,
~
40
K~ (A~)
10-
f-
-0.5 0
,2
!
...........................
i
15'
"6
0
0 0
2
4
6
20
e-Si 15
I -
-
_P- s! (so%)
10
5
0 0
2
4
6
Interatomic Distance ( ~ )
Fig. 1. Moduli ofFT ofp-Si for (from bottom to top) 50%, 65% and 80% porosity and c-Si.
corresponding to the first three coordination shells were separately Fourier backtransformed, giving amplitude and phase signals corresponding to each shell. The ratio A J A r of the backscattering amplitude (Fig. 2) of a sample (s) and a reference (r) gives information about the coordination number ratio and the difference between the MSRDs Aa 2 within the single scattering and small-gaussian-disorder approximations [4, 6]:
In -As - ~ In N~ + 2k2(a~ - a~), A,
Ao~ = ( ~ - ~ ) .
Here the subscripts s, r refer to porous and crystalline samples, respectively. Our analysis shows that the Si-Si distances of first three coordination shells remain unchanged for all the porous Si samples with respect to c-Si. The relative coordination number is only slightly decreased for the first coordination shell in all the porous
samples, while for the outer shells the change is bigger going towards low porosity samples. Anyway, within our error bar (mainly induced by the H F etching procedure of the reference c-Si), we never obtained a relative coordination number less than 0.8. A more important change is shown by values of relative Debye-Waller factors, which are increasing with distance and decreasing with porosity. The parameter Atr2 represents the degree of structural and thermal disorder of the examined porous sample in comparison with the crystalline reference one. The difference Aa 2 observed should be attributed to higher static disorder present in our p-Si samples as the thermal contribution is expected to be comparable in c-Si and p-Si in view of their structure similarity. As a matter of fact, a very similar thermal behaviour was found in a-Ge and c-Ge by accurate temperature-dependent EXAFS measurements [6]. It is easy to explain why Aa 2 increases with R: the limited extension of crystalline-like regions, evidenced also by the slight reduction of coordination number with distance, produces distortions from crystalline-like behaviour, whose extent grows with growing interatomic distance. It is, instead, not trivial the increase of Aa 2 with decreasing porosity: the more porous the sample, the more similar to crystalline structure. This result could give some insight on the effects of anodic etching on the local structure. As a matter of fact, in the present case, the porosity was increased by decreasing the H F concentration, with constant current using identical Si wafers. The increasing of static disorder in p-Si can be attributed to H F concentration, resulting in a different equilibrium between chemical and electrochemical effects in the anodic process. We have shown EXAFS spectroscopy is a valuable tool for a quantitative characterisation of local structure of p-Si. In particular, interatomic distances, coordination numbers and MSRD have been compared for p-Si
G. Dalba et al. /Physica B 208&209 (1995) 559-561
samples with various degrees of porosity. Albeit a difficulty still exists in getting accurate absolute values, a clear trend of local structure is evidenced as a function of both interatomic distance and porosity. The most probable picture emerging from the present EXAFS measurements is that of a crystalline environment affected by a certain a m o u n t of disorder which is increasing with distance and H F concentration. This is a new very interesting result of the present work, and opens a discussion about the influence of H F concentration during the anodic process on the local structure of p-Si.
561
References [1] J.-C. Vial, L.T. Canham and W. Lang, eds., in: Proc. of Symp. on Light Emission from Silicon; J. Lumin. 57 (1993). [2] L.T. Canham, Appl. Phys. Lett. 56 (1990) 1046. [3] V. Lehmann and U. Grsele, Appl. Phys. Lett. 60 (1991) 856. [4] P.A. Lee, P.H. Citrin, P. Eisenberger and B.M. Kincaid, Rev. Mod. Phys. 53 (1981) 769. I-5] A. Filipponi, F. Evangelisti, M. Benfatto, S. Mobilio and C.R. Natoli, Phys. Rev. B 40 (1989) 9636. [6] G. Dalba, P. Fomasini, D. Diop, M. Grazioli and F. Rocca, J. Non-Cryst. Solids 159 (1993) 164.
Physica B 208&209 (1995) 559-561
The local structure of porous silicon investigated by EXAFS G. Dalba a, P. Fornasini a, M. Grazioli a, R. Grisenti a'*, Y. Soldo a, F.
Rocca b
Dipartimento di Fisica dell' Universith degli Studi di Trento, 1-38050 Povo (Trento), Italy b Centro C N R - I T C di Fisica degli Stati Aggregati ed Impianto lonico, 1-38050 Povo (Trento), Italy
Abstract
X-ray absorption measurements at the K-edge of silicon have been done in transmission mode on three powdered samples of porous silicon (p-Si) with 50%, 65% and 80% porosity, prepared by decreasing the H F content in the anodic solution. The analysis of EXAFS reveals a crystal-like structure of p-Si at least up to the third coordination shell. A slight progressive reduction of coordination numbers and increase of Debye-Waller factors with respect to c-Si is found in passing from the first to the third coordination shell; this behaviour is connected to the limited range of homogeneous structural regions in p-Si. The Debye-Waller factors increase with decreasing porosity, indicating a more ordered local structure in samples prepared with lower H F content and corresponding to the highest porosity.
In the last years great scientific attention has been devoted to the luminescence properties of porous silicon (p-Si) in the visible region [1-3]; the promising perspectives that p-Si offers to optoelectronics and related technology are the reasons of such interest. The origin of photoluminescence is, however, not yet understood. A more detailed knowledge of the local environment of Si is strongly needed to give a structural basis to the proposed model for the origin of luminescence. X-ray absorption spectroscopy (XAS) in the low energy range (XANES) and in the higher range (EXAFS) is a particularly well suited technique to this purpose [4]. Previous XAS measurements performed by indirect techniques such as total electron yield (TEY) and X-ray induced optical luminescence (XIOL) already stated some steady points as the crystalline-like structure of p-Si and the unimportance of siloxene in the emitting process. Difference between c-Si and p-Si due to different preparation conditions, ageing and H F cleaning of the surface were pointed out as well. Nevertheless the relatively low * Corresponding author.
signal-to-noise ratio currently achievable by these methods prevented a quantitative analysis of the EXAFS signal. To overcome this difficulty, we have performed accurate direct transmission measurements on self-standing powders of pure p-Si. This allowed us to compare coordination numbers, Debye-Waller factors and interatomic distances of the first three coordination shells of Si in p-Si samples with different degree of porosity (50%, 65% and 80%). The EXAFS signal ~(k) was extracted by standard techniques, where k is the photoelectron wave vector. For sake of brevity we present here only Fig. 1 which gives the moduli of FT of the three porous samples and c-Si. The similarity between p-Si and c-Si extends in the R space at least up to the third shell. This is different from the case of a-Si, where only the first shell peak is present [5]. The magnitude reduction in the peaks amplitude of p-Si samples with respect to c-Si is worth noting. Such reduction is vanishing with increasing porosity. This feature can be attributed to changes either of static disorder and/or of coordination number related to small dimension of Si nanocrystals. In order to obtain quantitative information, the well defined peaks
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 5 0 - 0
560
G. Dalba et al. /Physica B 208&209 (1995) 559 561
20 ¸
- - ~ p
-Si
----
1 st ,
-Si(80%)
0.5
Shell
f
,
2 nd Shell j
,
l
3 rd Shell ,
r
i
,
f
,
p-Si
50°/~
15' 0
,-:, 73
,<
.,~ 005
5
.c
.........
~
6S°/o P-si
-0.5
0-
E
~--:~-~
-0.5
10-
b
~
a
t-
I::
0.5
p-Si
80°/*
i
20
fi
- Si
[ --e
i
i
80
i
,
0
i 40
~
i 80
i
i 0
i
40
:
I
80
i
120
Fig. 2. Logarithms of amplitudes' ratios: experiment (dashed line) and best fits (solid lines).
5
t,
~
40
K~ (A~)
10-
f-
-0.5 0
,2
!
...........................
i
15'
"6
0
0 0
2
4
6
20
e-Si 15
I -
-
_P- s! (so%)
10
5
0 0
2
4
6
Interatomic Distance ( ~ )
Fig. 1. Moduli ofFT ofp-Si for (from bottom to top) 50%, 65% and 80% porosity and c-Si.
corresponding to the first three coordination shells were separately Fourier backtransformed, giving amplitude and phase signals corresponding to each shell. The ratio A J A r of the backscattering amplitude (Fig. 2) of a sample (s) and a reference (r) gives information about the coordination number ratio and the difference between the MSRDs Aa 2 within the single scattering and small-gaussian-disorder approximations [4, 6]:
In -As - ~ In N~ + 2k2(a~ - a~), A,
Ao~ = ( ~ - ~ ) .
Here the subscripts s, r refer to porous and crystalline samples, respectively. Our analysis shows that the Si-Si distances of first three coordination shells remain unchanged for all the porous Si samples with respect to c-Si. The relative coordination number is only slightly decreased for the first coordination shell in all the porous
samples, while for the outer shells the change is bigger going towards low porosity samples. Anyway, within our error bar (mainly induced by the H F etching procedure of the reference c-Si), we never obtained a relative coordination number less than 0.8. A more important change is shown by values of relative Debye-Waller factors, which are increasing with distance and decreasing with porosity. The parameter Atr2 represents the degree of structural and thermal disorder of the examined porous sample in comparison with the crystalline reference one. The difference Aa 2 observed should be attributed to higher static disorder present in our p-Si samples as the thermal contribution is expected to be comparable in c-Si and p-Si in view of their structure similarity. As a matter of fact, a very similar thermal behaviour was found in a-Ge and c-Ge by accurate temperature-dependent EXAFS measurements [6]. It is easy to explain why Aa 2 increases with R: the limited extension of crystalline-like regions, evidenced also by the slight reduction of coordination number with distance, produces distortions from crystalline-like behaviour, whose extent grows with growing interatomic distance. It is, instead, not trivial the increase of Aa 2 with decreasing porosity: the more porous the sample, the more similar to crystalline structure. This result could give some insight on the effects of anodic etching on the local structure. As a matter of fact, in the present case, the porosity was increased by decreasing the H F concentration, with constant current using identical Si wafers. The increasing of static disorder in p-Si can be attributed to H F concentration, resulting in a different equilibrium between chemical and electrochemical effects in the anodic process. We have shown EXAFS spectroscopy is a valuable tool for a quantitative characterisation of local structure of p-Si. In particular, interatomic distances, coordination numbers and MSRD have been compared for p-Si
G. Dalba et al. /Physica B 208&209 (1995) 559-561
samples with various degrees of porosity. Albeit a difficulty still exists in getting accurate absolute values, a clear trend of local structure is evidenced as a function of both interatomic distance and porosity. The most probable picture emerging from the present EXAFS measurements is that of a crystalline environment affected by a certain a m o u n t of disorder which is increasing with distance and H F concentration. This is a new very interesting result of the present work, and opens a discussion about the influence of H F concentration during the anodic process on the local structure of p-Si.
561
References [1] J.-C. Vial, L.T. Canham and W. Lang, eds., in: Proc. of Symp. on Light Emission from Silicon; J. Lumin. 57 (1993). [2] L.T. Canham, Appl. Phys. Lett. 56 (1990) 1046. [3] V. Lehmann and U. Grsele, Appl. Phys. Lett. 60 (1991) 856. [4] P.A. Lee, P.H. Citrin, P. Eisenberger and B.M. Kincaid, Rev. Mod. Phys. 53 (1981) 769. I-5] A. Filipponi, F. Evangelisti, M. Benfatto, S. Mobilio and C.R. Natoli, Phys. Rev. B 40 (1989) 9636. [6] G. Dalba, P. Fomasini, D. Diop, M. Grazioli and F. Rocca, J. Non-Cryst. Solids 159 (1993) 164.