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The origin of visible luminescencefrom “porous silicon”: A new interpretation
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Solid State Communications, Vol. 81, No. 4, PP. 307-312, 1992. Printed in Great Britain.
THE ORIGIN OF VISIBLE LUMINESCENCE
0038-1098/9255.00 +.00 Pergamon Press plc
FROM "POROUS SILICON":
A NEW INTERPRETATION M. S. Brandt, H. D. Fuehs, M. Stutzmann, J. Weber and M. Cardona Max-Planck-Institut ffir FestkSrperforschung, Heisenbergstrasse 1, D 7000 Stuttgart 80, Germany (Received 10 November 1991 by T. P. Martin)
Luminescence and vibrational properties (infrared and Raman) of anodically oxidized ("porous") silicon and of chemically synthesized siloxene (SisOzHs) and its derivates are compared. Based on the quantitative agreement of these two types of materials it is concluded that the origin of the strong room temperature luminescence in "porous" silicon can be traced to siloxene derivates present in "porous" silicon.
in anodically oxidized silicon is due to Si-O-H compounds derived from siloxene (Si603H6) [12,13]. The unique fluorescence properties of this class of materials are well known in silicon chemistry since 1922 [14]. In the following, we give a comparison between the optical and vibrational properties of PSL produced by standard etching and siloxene-derivates which were chemically synthesised from CaSi2.
I. I N T R O D U C T I O N THE RECENT observation of strong visible photoluminescence (PL) in anodically etched ("porous") silicon [1] has caused considerable interest. Lehmann and GSsele [2] observed a drastic shift of the fundamental absorption edge of free-standing "porous" silicon layers (PSL) to 1.76 eV at room temperature. Independently, Canham [3] found efficient photoluminescence in the energy region 1.4 to 1.8 eV from such layers at 300 K. Since then, many groups have reproduced these results. The current interpretation of the origin of the luminescence is a quantum size effect within crystalline silicon, i.e. radiative recombination occurs in small silicon particles of diameter < 20~ which are produced during the etching process [4-11]. Experimental evidence for this comes from the following studies: 1. Early Raman measurements [4] show the existence of a low frequency side band in PSL which was attributed to a shorter coherence length of optical phonons in the directions perpendicular to the pore axis. 2. Extensive etching studies [3,5] revealed a strong dependence of the peak position of the luminescence on porosity which could be controlled by treatments subsequent to the initial etching. 3. Proof for the presence of a quantum-size crystalline structure in PSL was obtained by TEM studies [6]. 4. Finally, quantum-size effects have also been invoked to explain the observation of visible luminescence in various systems containing silicon nanocrystals, which have been produced by techniques other than etching [7-11]. In this Communication we present an alternative interpretation of the visible luminescence in PSL which is not based on quantum confinement in crystalline silicon. Instead we demonstrate that the luminescence
II. S A M P L E P R E P A R A T I O N We have prepared a number of "porous" silicon layers using a variety of etching recipes as described in the literature. To exclude adverse effects of substrates on our measurements we have used predominantly freestanding PSL, produced by complete anodical oxidation of 20 #m thick p+-doped Si wafers. By variation of etch geometry, electrolyte composition, current density, substrate doping level and post-etching treatments we were able to produce strong visible photoluminescence in the spectral range from 530 to 900 nm with a typical width of 150 nm. Chemically synthesized siloxene and its derivates were prepared from a reaction of CaSi2 powder (Johnson Matthey, tech. grade) with concentrated hydrochloric acid, according to two different preparation methods described in the literature. Pure siloxene (SisO3H¢), prepared according to Kautsky [13], is a greenish white powder with a weak room temperature fluorescence in the green [15]. The structure of this material was decribed by Kautsky as consisting of layers formed by hexagonal silicon rings separated from each other by oxygen bridges. The remaining silicon dangling bonds are terminated by hydrogen (cf. Fig. la). Alternative 307
308
POROUS SILICON
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Vol. 81, No. 4 (a)
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structures with the same composition have been proposed based on X-ray measurements [16] and are shown in Fig. lb and c. The structure units here are linear Si chains interconnected by oxygen or pure silicon layers with alternating OH and H bond terminators. The second preparation method, which has been originally described by WShler in 1863 [12], leads to a bright yellow substance with a strong yellow fluorescence. The substance is believed to be a mixture of the different structural features in Fig. 1. In the following, we denote WShler's compound by SisOa+~Hs-m. The increase of the oxygen content (n) and the decrease of the hydrogen content (m) correspond to an increasing thrcedimensional crosslinking of the layered structures in Fig. 1 by oxygen bridges. Reviews on siloxene and its dcrivatcs can be found in [17-19]. The fluorescence properties of this class of materials are well documented in the early chemical literature and also in a few more recent studies [14,15,20-23]. Of particular importance in the present context is the fact that the fluorescence bands emitted
"'"
porous Si, HF etched
. . . . . Si603onH6.m (W6hler compound), i
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WAVELENGTH
oH
Structural models for siloxene (SiaOaHs). (a) Monolayers formed by hexagonal silicon rings interconnected by oxygen bridges. The fourth bond of silicon is terminated by hydrogen. (b) Monolayers formed by linear Si chains interconnected by oxygen, terminated by hydrogen. (c) Silicon monolayers with alternating OH and H bond terminators.
--
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Si
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/
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i 1000
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Room temperature photoluminescence spectra of anodically oxidized "porous" silicon layers and of thermally annealed siloxene compounds.
by siloxene can be tuned in a well controlled way over a large spectral range via chemical substitution of hydrogen by other monovalent ligands such as halogens, OH or alcohol groups. Alternatively, tuning can also be achieved by annealing of siloxene in air [23]. III. R E S U L T S A N D D I S C U S S I O N In Fig. 2 we show a comparison of typical PL spectra obtained from "porous" silicon layers produced by different etching procedures and the fluorescence bands of the two chemical compounds described above after oxidation in air. In the case of Fig. 2a etching was performed in a HF/HCl/ethanol electrolyte resulting in a luminescence band centred at 690 nm. This agrees well with the fluorescence of Kautsky siloxene annealed at 400°C, the larger width of the band in the latter case being due to the rather uncontrolled substitution of hydrogen by OH groups during the anneal. In Fig. 2b we present the red luminescence with a maximum at 760 nm which was obtained from a PSL after etching in HF/ethanol. This luminescence agrees quantitatively with the fluorescence of the WShler compound after a similar annealing step at 400°C. The agreement between the light emission from PSL on the one hand and the Kautsky and WShler compounds on the other hand strongly suggests a common structural origin of the luminescence in both cases. This conclusion is further corroborated by infrared and Raman spectroscopy on these two types of materials.
Vol. 81, No. 4
POROUS SILICON l
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Tab. 1
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(a)
309 Assignment of the infrared modes observed in anodically oxidized "porous" silicon and Kautsky siloxene (cf. Fig. 3)[22,24,25].
porous Si .....
',,
Si603H 6 , ann. 400°C ~,lum =70Onto
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w m [cm-1] 460 640 835 870 1040-1160 2100 2250 3420-3580
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4000
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Fig. 3
Infrared absorption spectra of anodically oxidized "porous" silicon and Kautsky siloxene. The assignment of the modes is given in Tab. 1.
Figure 3 shows infrared (IR) vibrational spectra measured at 300 K on PSL and on annealed Kautsky siloxene'~'with luminescence spectra similar to those given in Fig. 2. Figure 3a contains the Si-Si and Si-O vibrations as well as the Si-H bending modes, whereas Fig. 3b shows the Si-H and O-H stretching modes. The different bands together with their structural assignments are given in Tab. 1 ([22,24,25] and references therein). The conclusion from the IR data is that the same chemical bonds are present in both types of samples, however with different concentrations depending on the details of the preparation. In particular, O-H bonds and Si-H stretch vibrations at 2100 cm -1 are expected for the structure in Fig. lc, whereas the structures shown in Fig. la and b require the presence of Si-O-Si and O-Si-H vibrational features. Coexistance of all bands as observed in Fig. 3 therefore implies a corresponding mixture of the different structural building blocks. In any case, the IR results clearly identify Si, H and O as the main constituents also of luminescent anodically oxidized "porous" silicon. However the IR data do not exclude the presence of small crystalline silicon particles
in siloxene or PSL. Therefore we have also investigated both kinds of samples with Raman spectroscopy. Typical spectra are given in Fig. 4. Freestanding porous silicon shows a prominent Raman peak with a full width at half maximum of ~ 10 cm -1 and a peak position at 514 +1 cm -1. In addition, a low energy shoulder at about 480 cm -] is present. Also shown in Fig. 4 are the Raman spectra of crystalline silicon and amorphous quartz (a-SiO2) obtained under identical experimental conditions. The main observation is a shift of the principal peak due to Si-Si vibrations to lower energies by 7 cm -1 without a noticeable broadening. These results are in agreement with earlier Raman studies [4] where this behaviour was interpreted as a consequence of phonon confinement in small crystalline silicon particles. However this interpretation is in contradiction with the extensive literature on size effects in microcrystalline silicon [26-28], where shifts of the Raman peak position are always accompanied by a pronounced increase of the width of the Raman signal. According to modeling [28], a shift of the Raman peak by 7 cm -1 as in Fig. 4 should be correlated with a significant broadening of at least 20 cm -1, which is clearly not the case. On the other hand, a similar shift of the main Raman peak without a broadening is also observed in the WShler and Kautsky compounds for all states of oxidation. This is interesting because the oxidation changes the luminescence significantly as described above but does not alter the Raman peak at 514 cm -1. What is affected by the various preparation methods and annealing procedures is the low energy shoulder at ~ 480 cm -1, whose exact position and intensity can change considerably. A preliminary assignment of the two Raman features is as follows: 1. We attribute the sharp peak at 514 cm -1 to optical phonons in silicon monolayers such as shown in Fig. lc. Further evidence for this assignment comes from the observation of a similar mode in CaSi2 [29], a compound which is known to contain the same silicon layers interconnected by Ca atoms [30]. 2. The structural assignment of the mode which causes the low
310
POROUS SILICON i
]
SiO
2. Visible electroluminescence in PSL during anodic etching has been reported [33]. The equivalent observation in siloxene is a bright chemiluminescence during oxidation (e.g. with KMnO4) [17]. The electroand chemiluminescence can be observed at room temperature and the peak positions agree with that of the photoluminescence. The close correspondence between the electro- and the chemiluminescence is likely to be important for future application of PSL for optoelectronics. 3. Both PSL and siloxene exhibit a pronounced luminescence fatigue, i.e. the luminescence decays within hours even at low temperatures by up to an order of magnitude [34]. In both cases the effect can he completely reversed by annealing at temperatures slightly above 300 K. 4. The decay of the photoluminescence after pulsed excitation in both PSL and siloxene is strongly nonexponential [34-36] and also depends on the luminescence peak position, indicating a large distribution of carrier
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Raman spectra of free-standing anodically oxidized "porous" silicon, W6hler compound in the as prepared state and Kautsky siloxene after anneal at 400°C. Raman spectra of crystalline silicon and of amorphous quartz obtained under identical conditions are included for comparison.
.a,
1.5
1.0 energy shoulder at 480 cm -a is less clear. A possible origin could be hexagonal silicon-oxygen rings (cf Fig. la). Evidence for this assignment comes from the existence of a similar mode in a-SiO2, which is also assigned to six-fold rings ("91" peak, cf. [31,32]). More details of the vibrational spectra will be discussed elsewhere [29]. In addition to the striking similarities between structural and optical properties of PSL and siloxene compounds discussed so far, there are a number of observations in the literature which give further evidence for the siloxene derivates as the origin of strong luminescence in anodically oxidized silicon. 1. The luminescence peak position can be changed by pure chemical treatment after etching [3,5]. In siloxene the equivalent feature is the change of luminescence by substitution of H by other ligands at the silicon ring (Fig. la)[21].
0.5
0 2.0
25
3.0
Photon Energy (eV)
Fig. 5
Excitation spectra of room temperature luminescence in anodically oxidized "porous" silicon for different monitored luminescence wavelengths Aaet.
Vol, 81, No. 4
311
POROUS SILICON
lifetimes. In particular, the photoluminescence decay in PSL is several orders of magnitude slower than in single crystalline silicon, showing that non-radiative processes are negligible in PSL. These long lifetimes would be dillicult to understand for the luminescence from small quantum wells. 5. The peak position of the luminescence in both materials shifts to higher energies when the excitation energy is increased [34]. This can also be seen in the luminescence excitation spectra of Fig. 5. Excitation of luminescence in the red occurs mainly via a distinct band centred around 2.3 eV, whereas light emission in the yellow is excited predominantly through a broad band between 2.7 and 3.2 eV. Given the close behaviour of porous silicon and chemically prepared siloxene, one is led to the conclusion that the origin of the luminescence is the same in both materials. At this stage the following model for the luminescence in PSL emerges. Sixfold silicon rings (Fig. la) play the essential role in determining the luminescence properties. Luminescence peak positions can be varied over a large spectral range by changing the ring ligands [37]. The rate of radiative recombination is determined by the intrinsic lifetime of excited states within the rings and also by the rate with which excited carriers can be transfered from other parts of the network into these rings. As shown in Fig. 1, the remaining network can consist of a mixture of silicon chains or layers interconnected by oxygen. This two-phase structure povides a simple explanation for the large distribution of radiative lifetimes, the distinct features appearing in the luminescence excitation spectra and the two peak structure of the Raman spectra. Clearly, further work is necessary to understand structure and recombination in PSL in more detail. IV. CONCLUSION We have compared optical and structural properties of anodically oxidized silicon with those of siloxene and its related compounds. This study leads us to the conclusion that the entity causing the luminescence in porous silicon layers is identical to that in siloxene. In particular, we have found full agreement between vibrational properties and the behaviour of the luminescence (lifetimes, intensities, positions, shifts, ...) in both materials. One main conclusion is that visible light emission from PSL is not an intrinsic property of crystalline silicon (quantum confinement effects) but depends on specific chemical reactions of silicon with hydrogen and oxygen. The knowledge of silicon chemistry can provide valuable input for future work on light emitting silicon. ACKNOWLEDGEMENTS We are grateful to H. J. Queisser, A. Breitschwerdt, C. I. Harris, A. LSbert and K. Weronek for stimulating discussions and experimental support and to P. England (Bellcore) for providing a sample.
REFERENCES [1] A. Uhlir, Bell System Tech. J. 35,333 (1956). [2] V. Lehmann and U. GSsele, Appl. Phys. Lett. 58, 856 (1991). [3] L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990). [4] S. R. Goodes, T. E. Jenkins, M. I. J. Beale, J. D. Benjamin, and C. Pickering, Semicond. Sei. Teehnol. 3, 483 (1988). [5] A. Bsiesy, J. C. Vial, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain, A. Wasiela, A. Halimaoui, and G. Bomchil, Surface Science 254, 195 (1991). [6] A. G. Cullis and L. T. Canham, Nature 353, 335 (1991). [7] D. J. DiMaria, J. R. Kirtley, E. J. Pakulis, D. W. Dong, T. S. Kuan, F. L. Pesavento, T. N. Theis, J. A. Cutro, and S. D. Brorson, J. Appl. Phys. 56, 401 (1984). [8] S. Furukawa and T. Miyasato, Phys. Rev. B 38, 5726 (1988). [9] S. Furukawa and T. Miyasato, Jap. J. Appl. Phys. 27, L2207 (1988). [10] H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, and T. Nakagiri, Appl. Phys. Lett. 56, 2379 (1990). [11] F. Kozlowski, V. Petrova-Koch, A. Kux, W. Stadler, A. Fleischmann, and H. Sigmund, J. NonCryst. Solids (1992), in print. [12] F. WShler, Lieb. Ann. 127, 275 (1863). [13] H. Kautsky, Z. anorg. Chemie 117, 209 (1921). [14] H. Kautsky and H. Zocher, Z. Phys. 9,267 (1922). [15] E. Hengge, Fortschr. Chem. Forsch. 9, 145 (1967). [16] A. Weiss, G. Beil, and H. Meyer, Z. Naturforsch. 34b, 25 (1979). [17] Silicon, vol. B of Gmelin Handbuch der anorganischen Chemie (Verlag Chemie, Weinheim, 1959), p. 591. [18] E. Hengge, Fortschr. Chem. Forsch. 51, 92 (1974). [19] Silicon, vol. B1 of Gmelin handbook of inorganic chemistry (Springer, Heidelberg, 1982), p. 231. [20] H. Kautsky and G. Herzberg, Z. anorg. Chemie 139, 135 (1924). [21] E. Hengge and K. Pretzer, Chem. Ber. 96, 470 (1963). [22] H. Ubara, T. Imura, A. Hiraki, I. Hirabayashi, and K. Morigaki, J. Non-Cryst. Solids 59&60, 641 (1983).
[23] I. Hirabayashi and K. Morigaki, J. Non-Cryst. Solids 59&60, 645 (1983). [24] M. Cardona, phys. stat. sol. (b) 118, 463 (1983). [25] G. Lucovsky and W. B. Pollard, in Hydrogenated Amorphous Silicon II, edited by J. D. Joannopoulos and G. Lucovsky (Springer, Berlin, 1984), p. 301.
312
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[26} H. Richter, Z. P. Wang, and L. Ley, Solid State Comm. 39, 625 (1981). [27] Z. lqbal and S. Veprek, J. Phys. C 15,377 (1982). [28] I. IL Campbell and P. M. Fa.uchet, Solid St,at(, Comm. 58,739 (1986). [29] H. D. Fuchs et. al., unpublished. [30] J. Evers and A. Weiss, Mat. Res. Bull. 9, 549 (1974). [al] F. L. Galeener, R. A. Barrio, E. Martinez, and R. J. Elliott, Phys. Rev. Lett. 53, 2429 (1984). [32] R. J. Hemley, H. K. Mao, P. M. Bell, and B. O.
[33]
[34] [35]
[36] [37]
Vol. 81, No. 4 Mysen, Phys. Rev. Left. 57, 747 (1986). A. Halimaoui, C. Oules, G. Bomchil, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, and F. Muller, Appl. Phys. Lett. 59, 304 (1991). I. Hirabayashi, K. Morigaki, and S. Yamanak~, J. Phys. Soc. Jap. 52, 671 (1983). A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romesatain, and J. C. Vial, unpublished. J. Weber et. al., unpublished. E. Hengge and H. Grupe, Chem. Ber. 97, 1783 (1964).
Solid State Communications, Vol. 81, No. 4, PP. 307-312, 1992. Printed in Great Britain.
THE ORIGIN OF VISIBLE LUMINESCENCE
0038-1098/9255.00 +.00 Pergamon Press plc
FROM "POROUS SILICON":
A NEW INTERPRETATION M. S. Brandt, H. D. Fuehs, M. Stutzmann, J. Weber and M. Cardona Max-Planck-Institut ffir FestkSrperforschung, Heisenbergstrasse 1, D 7000 Stuttgart 80, Germany (Received 10 November 1991 by T. P. Martin)
Luminescence and vibrational properties (infrared and Raman) of anodically oxidized ("porous") silicon and of chemically synthesized siloxene (SisOzHs) and its derivates are compared. Based on the quantitative agreement of these two types of materials it is concluded that the origin of the strong room temperature luminescence in "porous" silicon can be traced to siloxene derivates present in "porous" silicon.
in anodically oxidized silicon is due to Si-O-H compounds derived from siloxene (Si603H6) [12,13]. The unique fluorescence properties of this class of materials are well known in silicon chemistry since 1922 [14]. In the following, we give a comparison between the optical and vibrational properties of PSL produced by standard etching and siloxene-derivates which were chemically synthesised from CaSi2.
I. I N T R O D U C T I O N THE RECENT observation of strong visible photoluminescence (PL) in anodically etched ("porous") silicon [1] has caused considerable interest. Lehmann and GSsele [2] observed a drastic shift of the fundamental absorption edge of free-standing "porous" silicon layers (PSL) to 1.76 eV at room temperature. Independently, Canham [3] found efficient photoluminescence in the energy region 1.4 to 1.8 eV from such layers at 300 K. Since then, many groups have reproduced these results. The current interpretation of the origin of the luminescence is a quantum size effect within crystalline silicon, i.e. radiative recombination occurs in small silicon particles of diameter < 20~ which are produced during the etching process [4-11]. Experimental evidence for this comes from the following studies: 1. Early Raman measurements [4] show the existence of a low frequency side band in PSL which was attributed to a shorter coherence length of optical phonons in the directions perpendicular to the pore axis. 2. Extensive etching studies [3,5] revealed a strong dependence of the peak position of the luminescence on porosity which could be controlled by treatments subsequent to the initial etching. 3. Proof for the presence of a quantum-size crystalline structure in PSL was obtained by TEM studies [6]. 4. Finally, quantum-size effects have also been invoked to explain the observation of visible luminescence in various systems containing silicon nanocrystals, which have been produced by techniques other than etching [7-11]. In this Communication we present an alternative interpretation of the visible luminescence in PSL which is not based on quantum confinement in crystalline silicon. Instead we demonstrate that the luminescence
II. S A M P L E P R E P A R A T I O N We have prepared a number of "porous" silicon layers using a variety of etching recipes as described in the literature. To exclude adverse effects of substrates on our measurements we have used predominantly freestanding PSL, produced by complete anodical oxidation of 20 #m thick p+-doped Si wafers. By variation of etch geometry, electrolyte composition, current density, substrate doping level and post-etching treatments we were able to produce strong visible photoluminescence in the spectral range from 530 to 900 nm with a typical width of 150 nm. Chemically synthesized siloxene and its derivates were prepared from a reaction of CaSi2 powder (Johnson Matthey, tech. grade) with concentrated hydrochloric acid, according to two different preparation methods described in the literature. Pure siloxene (SisO3H¢), prepared according to Kautsky [13], is a greenish white powder with a weak room temperature fluorescence in the green [15]. The structure of this material was decribed by Kautsky as consisting of layers formed by hexagonal silicon rings separated from each other by oxygen bridges. The remaining silicon dangling bonds are terminated by hydrogen (cf. Fig. la). Alternative 307
308
POROUS SILICON
(a)
Vol. 81, No. 4 (a)
,/,'/
/ /'
/
i;
// /
-
porous Si,
~ x
/
HF+HCI etched .
\ ' . k....
/
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// // . . . .
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structures with the same composition have been proposed based on X-ray measurements [16] and are shown in Fig. lb and c. The structure units here are linear Si chains interconnected by oxygen or pure silicon layers with alternating OH and H bond terminators. The second preparation method, which has been originally described by WShler in 1863 [12], leads to a bright yellow substance with a strong yellow fluorescence. The substance is believed to be a mixture of the different structural features in Fig. 1. In the following, we denote WShler's compound by SisOa+~Hs-m. The increase of the oxygen content (n) and the decrease of the hydrogen content (m) correspond to an increasing thrcedimensional crosslinking of the layered structures in Fig. 1 by oxygen bridges. Reviews on siloxene and its dcrivatcs can be found in [17-19]. The fluorescence properties of this class of materials are well documented in the early chemical literature and also in a few more recent studies [14,15,20-23]. Of particular importance in the present context is the fact that the fluorescence bands emitted
"'"
porous Si, HF etched
. . . . . Si603onH6.m (W6hler compound), i
I 700
i
WAVELENGTH
oH
Structural models for siloxene (SiaOaHs). (a) Monolayers formed by hexagonal silicon rings interconnected by oxygen bridges. The fourth bond of silicon is terminated by hydrogen. (b) Monolayers formed by linear Si chains interconnected by oxygen, terminated by hydrogen. (c) Silicon monolayers with alternating OH and H bond terminators.
--
/// ///
_J
Si
/
/
Z
Fig. 1
"~,
/
ann. 400°C I i I 800 900
i 1000
(nm)
Room temperature photoluminescence spectra of anodically oxidized "porous" silicon layers and of thermally annealed siloxene compounds.
by siloxene can be tuned in a well controlled way over a large spectral range via chemical substitution of hydrogen by other monovalent ligands such as halogens, OH or alcohol groups. Alternatively, tuning can also be achieved by annealing of siloxene in air [23]. III. R E S U L T S A N D D I S C U S S I O N In Fig. 2 we show a comparison of typical PL spectra obtained from "porous" silicon layers produced by different etching procedures and the fluorescence bands of the two chemical compounds described above after oxidation in air. In the case of Fig. 2a etching was performed in a HF/HCl/ethanol electrolyte resulting in a luminescence band centred at 690 nm. This agrees well with the fluorescence of Kautsky siloxene annealed at 400°C, the larger width of the band in the latter case being due to the rather uncontrolled substitution of hydrogen by OH groups during the anneal. In Fig. 2b we present the red luminescence with a maximum at 760 nm which was obtained from a PSL after etching in HF/ethanol. This luminescence agrees quantitatively with the fluorescence of the WShler compound after a similar annealing step at 400°C. The agreement between the light emission from PSL on the one hand and the Kautsky and WShler compounds on the other hand strongly suggests a common structural origin of the luminescence in both cases. This conclusion is further corroborated by infrared and Raman spectroscopy on these two types of materials.
Vol. 81, No. 4
POROUS SILICON l
I
'
I
'
i
I
I
Tab. 1
I
(a)
309 Assignment of the infrared modes observed in anodically oxidized "porous" silicon and Kautsky siloxene (cf. Fig. 3)[22,24,25].
porous Si .....
',,
Si603H 6 , ann. 400°C ~,lum =70Onto
U.I
///
o
\,.
,.-/
I
i
w m [cm-1] 460 640 835 870 1040-1160 2100 2250 3420-3580
f
z
mode Si-O-Si bending Si-H bending Si-H2 wagging Si-H2 scissors Si-O-Si stretching Si-H stretching O-Si-H stretching O-H stretching
./
¢r O 09 300
I
500
I
I
700
900
I
I
L
I
i
t
J
f
1100
1300
123
< n,-
I
I
i
E
i
I
i
(b)
2000
2400
I
2800
i
3200
I
3600
4000
WAVENUMBERS (cm-1)
Fig. 3
Infrared absorption spectra of anodically oxidized "porous" silicon and Kautsky siloxene. The assignment of the modes is given in Tab. 1.
Figure 3 shows infrared (IR) vibrational spectra measured at 300 K on PSL and on annealed Kautsky siloxene'~'with luminescence spectra similar to those given in Fig. 2. Figure 3a contains the Si-Si and Si-O vibrations as well as the Si-H bending modes, whereas Fig. 3b shows the Si-H and O-H stretching modes. The different bands together with their structural assignments are given in Tab. 1 ([22,24,25] and references therein). The conclusion from the IR data is that the same chemical bonds are present in both types of samples, however with different concentrations depending on the details of the preparation. In particular, O-H bonds and Si-H stretch vibrations at 2100 cm -1 are expected for the structure in Fig. lc, whereas the structures shown in Fig. la and b require the presence of Si-O-Si and O-Si-H vibrational features. Coexistance of all bands as observed in Fig. 3 therefore implies a corresponding mixture of the different structural building blocks. In any case, the IR results clearly identify Si, H and O as the main constituents also of luminescent anodically oxidized "porous" silicon. However the IR data do not exclude the presence of small crystalline silicon particles
in siloxene or PSL. Therefore we have also investigated both kinds of samples with Raman spectroscopy. Typical spectra are given in Fig. 4. Freestanding porous silicon shows a prominent Raman peak with a full width at half maximum of ~ 10 cm -1 and a peak position at 514 +1 cm -1. In addition, a low energy shoulder at about 480 cm -] is present. Also shown in Fig. 4 are the Raman spectra of crystalline silicon and amorphous quartz (a-SiO2) obtained under identical experimental conditions. The main observation is a shift of the principal peak due to Si-Si vibrations to lower energies by 7 cm -1 without a noticeable broadening. These results are in agreement with earlier Raman studies [4] where this behaviour was interpreted as a consequence of phonon confinement in small crystalline silicon particles. However this interpretation is in contradiction with the extensive literature on size effects in microcrystalline silicon [26-28], where shifts of the Raman peak position are always accompanied by a pronounced increase of the width of the Raman signal. According to modeling [28], a shift of the Raman peak by 7 cm -1 as in Fig. 4 should be correlated with a significant broadening of at least 20 cm -1, which is clearly not the case. On the other hand, a similar shift of the main Raman peak without a broadening is also observed in the WShler and Kautsky compounds for all states of oxidation. This is interesting because the oxidation changes the luminescence significantly as described above but does not alter the Raman peak at 514 cm -1. What is affected by the various preparation methods and annealing procedures is the low energy shoulder at ~ 480 cm -1, whose exact position and intensity can change considerably. A preliminary assignment of the two Raman features is as follows: 1. We attribute the sharp peak at 514 cm -1 to optical phonons in silicon monolayers such as shown in Fig. lc. Further evidence for this assignment comes from the observation of a similar mode in CaSi2 [29], a compound which is known to contain the same silicon layers interconnected by Ca atoms [30]. 2. The structural assignment of the mode which causes the low
310
POROUS SILICON i
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2. Visible electroluminescence in PSL during anodic etching has been reported [33]. The equivalent observation in siloxene is a bright chemiluminescence during oxidation (e.g. with KMnO4) [17]. The electroand chemiluminescence can be observed at room temperature and the peak positions agree with that of the photoluminescence. The close correspondence between the electro- and the chemiluminescence is likely to be important for future application of PSL for optoelectronics. 3. Both PSL and siloxene exhibit a pronounced luminescence fatigue, i.e. the luminescence decays within hours even at low temperatures by up to an order of magnitude [34]. In both cases the effect can he completely reversed by annealing at temperatures slightly above 300 K. 4. The decay of the photoluminescence after pulsed excitation in both PSL and siloxene is strongly nonexponential [34-36] and also depends on the luminescence peak position, indicating a large distribution of carrier
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Raman spectra of free-standing anodically oxidized "porous" silicon, W6hler compound in the as prepared state and Kautsky siloxene after anneal at 400°C. Raman spectra of crystalline silicon and of amorphous quartz obtained under identical conditions are included for comparison.
.a,
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1.0 energy shoulder at 480 cm -a is less clear. A possible origin could be hexagonal silicon-oxygen rings (cf Fig. la). Evidence for this assignment comes from the existence of a similar mode in a-SiO2, which is also assigned to six-fold rings ("91" peak, cf. [31,32]). More details of the vibrational spectra will be discussed elsewhere [29]. In addition to the striking similarities between structural and optical properties of PSL and siloxene compounds discussed so far, there are a number of observations in the literature which give further evidence for the siloxene derivates as the origin of strong luminescence in anodically oxidized silicon. 1. The luminescence peak position can be changed by pure chemical treatment after etching [3,5]. In siloxene the equivalent feature is the change of luminescence by substitution of H by other ligands at the silicon ring (Fig. la)[21].
0.5
0 2.0
25
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Excitation spectra of room temperature luminescence in anodically oxidized "porous" silicon for different monitored luminescence wavelengths Aaet.
Vol, 81, No. 4
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POROUS SILICON
lifetimes. In particular, the photoluminescence decay in PSL is several orders of magnitude slower than in single crystalline silicon, showing that non-radiative processes are negligible in PSL. These long lifetimes would be dillicult to understand for the luminescence from small quantum wells. 5. The peak position of the luminescence in both materials shifts to higher energies when the excitation energy is increased [34]. This can also be seen in the luminescence excitation spectra of Fig. 5. Excitation of luminescence in the red occurs mainly via a distinct band centred around 2.3 eV, whereas light emission in the yellow is excited predominantly through a broad band between 2.7 and 3.2 eV. Given the close behaviour of porous silicon and chemically prepared siloxene, one is led to the conclusion that the origin of the luminescence is the same in both materials. At this stage the following model for the luminescence in PSL emerges. Sixfold silicon rings (Fig. la) play the essential role in determining the luminescence properties. Luminescence peak positions can be varied over a large spectral range by changing the ring ligands [37]. The rate of radiative recombination is determined by the intrinsic lifetime of excited states within the rings and also by the rate with which excited carriers can be transfered from other parts of the network into these rings. As shown in Fig. 1, the remaining network can consist of a mixture of silicon chains or layers interconnected by oxygen. This two-phase structure povides a simple explanation for the large distribution of radiative lifetimes, the distinct features appearing in the luminescence excitation spectra and the two peak structure of the Raman spectra. Clearly, further work is necessary to understand structure and recombination in PSL in more detail. IV. CONCLUSION We have compared optical and structural properties of anodically oxidized silicon with those of siloxene and its related compounds. This study leads us to the conclusion that the entity causing the luminescence in porous silicon layers is identical to that in siloxene. In particular, we have found full agreement between vibrational properties and the behaviour of the luminescence (lifetimes, intensities, positions, shifts, ...) in both materials. One main conclusion is that visible light emission from PSL is not an intrinsic property of crystalline silicon (quantum confinement effects) but depends on specific chemical reactions of silicon with hydrogen and oxygen. The knowledge of silicon chemistry can provide valuable input for future work on light emitting silicon. ACKNOWLEDGEMENTS We are grateful to H. J. Queisser, A. Breitschwerdt, C. I. Harris, A. LSbert and K. Weronek for stimulating discussions and experimental support and to P. England (Bellcore) for providing a sample.
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[26} H. Richter, Z. P. Wang, and L. Ley, Solid State Comm. 39, 625 (1981). [27] Z. lqbal and S. Veprek, J. Phys. C 15,377 (1982). [28] I. IL Campbell and P. M. Fa.uchet, Solid St,at(, Comm. 58,739 (1986). [29] H. D. Fuchs et. al., unpublished. [30] J. Evers and A. Weiss, Mat. Res. Bull. 9, 549 (1974). [al] F. L. Galeener, R. A. Barrio, E. Martinez, and R. J. Elliott, Phys. Rev. Lett. 53, 2429 (1984). [32] R. J. Hemley, H. K. Mao, P. M. Bell, and B. O.
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