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Possible explanation of the contradictory results on the porous silicon photoluminescence evolution after low temperature treatments
Applied Surface Science 108 Ž1997. 449–454
Possible explanation of the contradictory results on the porous silicon photoluminescence evolution after low temperature treatments B. Gelloz
)
Laboratoire de Spectrometrie ´ Physique, UniÕersite´ Joseph Fourier de Grenoble I, CNRS (UMR 5588), P.O. Box 87, 38402 St. Martin d’Heres ` Cedex, France Received 18 June 1996; accepted 24 September 1996
Abstract The modification of the porous silicon photoluminescence ŽPL. after a low level thermal oxidation at 3008C is studied by analyzing the surface chemical composition using infra-red spectroscopy. The amount of oxide is determined quantitatively. Two different porosities of porous silicon are studied: 70% and 80%. In the case of the 70% porosity porous layer, this thermal process leads to an important PL enhancement in the early stages of oxygen incorporation followed by a degradation at higher oxidation levels whereas the photoluminescence of the 80% porosity porous layer only decreases. It is shown how this new analysis can account for the different and often contradictory results on the PL behavior under various low-temperature oxidation processes.
1. Introduction Since the discovery of the porous silicon ŽPS. visible photoluminescence ŽPL. at room temperature w1x, a large amount of work has been achieved in order to investigate the stability of this luminescence upon ageing or upon thermal treatment under oxidizing or non-oxidizing ambiants. In particular, various oxidation treatments have been extensively studied. PS thermal oxidation up to 6008C generally leads to a decrease of the PL intensity w2–4x. This is due to the depassivation of the surface. Indeed, the break, in this temperature range, of the Si–H bonds which cover the as-formed PS surface generates some dan)
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gling bonds which act as non radiative centers so that the initial passivation is destroyed w3x. The passivation cannot be supplied by a low temperature Ž- 6008C. oxide which contains a large number of defects. However, high temperature oxidation above 8008C can lead to an enhancement of the PL intensity w2–4x. This behavior is explained by the fact that the SiO 2rSi interface obtained in the high temperature oxidation process is of good quality. Consequently, there is a rebuild of the Si nanocrystallites surface passivation. Other low temperature oxidation methods have been investigated and some of them can lead to an increase of the PL intensity. These methods are the oxidation in water at room temperature w5x, the oxidation in boiling water w6,7x, the photoinduced oxidation w5,8,9x and the anodic oxidation w10,11x. Some
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B. Gellozr Applied Surface Science 108 (1997) 449–454
results show an increase followed by a fall of the PL intensity as a function of the oxidation time w5,7x. Other results show only a degradation of the PL intensity w6,9x. Chang et al report a decrease followed by a rise of the PL intensity w8x. Finally, anodic oxidation enhances the PL intensity w10,11x. It is coming out that these low temperature oxidation experiments of PS lead to contradictory results even when they are achieved in very similar experimental conditions. For instance, Hou et al. w6x as well as Li et al. w7x have used oxidation in boiling water. The first ones have reported a degradation of the PL intensity whereas the others have observed an increase followed by a decrease of the PL intensity. Another example is the photoinduced oxidation. Baba et al. w5x have reported an increase followed by a decrease of the PL intensity, Tischler et al. w9x have only seen a degradation of the PL intensity and Chang et al. w8x have observed a decrease followed by a rise of the PL intensity. The aim of this article is to understand why so different, and even contradictory, results can be obtained upon PS low temperature oxidation.
supplied by an Argon laser with a 488.8 nm wavelength. The changes in the surface chemical structure during oxidation are studied by using a UNICAM Mattson 5000 Fourier transform infrared ŽFTIR. spectrometer. Fig. 1Ža. shows the absorption spectrum of an as-formed porous layer. The peaks between 2050 cmy1 and 2170 cmy1 represent the absorption due to the different vibrational modes of Si–H x Ž x s 1, 2, 3. bonds. It has been shown that the area under these peaks is proportional to the amount of hydrogen which covers the PS surface w12x. Fig. 1Žb. shows the absorption spectrum of a porous layer which has less than one oxide monolayer on its surface. The peaks stated before have less intensity and two new peaks have appeared between 2140 cmy1 and 2300 cmy1 . These new peaks are representative of the O–Si–H x bonds which means that oxygen is in fact incorporated in the silicon back bond without any perturbation of the initial hydrogen surface coverage w13x. The evolution of the total amount of hydrogen during oxidation is obtained by analyzing the changes in the spectrum area between 2050 cmy1 and 2300 cmy1 . This gives a measurement of the amount of hydrogen relatively
2. Experiment PS layers of two different porosities, namely 70 and 80%, are formed on boron doped Ž100.-oriented Si substrate of 3–6 V cm resistivity. The 70% porosity samples are obtained by anodization for 60 s under a constant current density of 20 mArcm2 , in a 50% HF–ethanol–water solution Ž25:50:25 vol%.. The 80% porosity layers are obtained by anodization for 180 s under a constant current density of 10 mArcm2 , in a 50% HF–ethanol–water solution Ž15:50:35 vol%.. The anodization time is monitored in order to obtain 1 mm thick layers. The as-formed samples show bright PL. They are thermally oxidized at 3008C in a quartz oven under a nitrogen flux containing some traces of oxygen. This method enables a very slow oxidation rate, and therefore the effects of the very first oxidation steps can be studied. Typically, it takes 120 min to build one oxide monolayer. During the oxidation treatment, the samples are taken out from the oven at regular intervals. PL is measured by using a Princeton optical multichannel analyzer ŽOMA.. The excitation light is
Fig. 1. FTIR spectra of an as-formed 1 m m thick 70% porosity p-type porous layer Ža. and of a partially oxidized 1 m m thick 70% porosity p-type porous layer Žb..
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to the amount of hydrogen present on the as-formed sample. The part of the spectrum between 950 cmy1 and 1250 cmy1 is much more intense for the partially oxidized porous layer than for the as-formed sample. This part of the spectrum is characteristic of the Si–O bonds. The more the porous layer is oxidized, the more this region of the spectrum is intense. The area under this part of the spectrum gives the amount of oxide covering the porous layer. Again, this is a relative measurement, but it is possible to deduce an absolute value using a calibration. This calibration is obtained from the anodic oxidation experiments on the same kind of porous layer as that used for thermal oxidation. Different porous layers were oxidized in H 2 SO4 1 M under a constant current of 5 mArcm2 at different exchanged charge Q which correspond to different oxidation levels. For each oxidation level, the number of moles n of SiO 2 is deduced from Q by the relation n s Qr4F where F is the Faraday. By performing FTIR measurements on the anodically oxidized samples it is then possible to relate the area of the FTIR spectrum between 950 cmy1 and 1250 cmy1 to the number of moles n. A calibration of the FTIR spectrum is thus obtained. In the case of the thermal oxidation experiments, the number of SiO 2 moles can then be deduced from FTIR measurements. Moreover, it is possible to have a rough evaluation of the oxide thickness t using t s n P MSiO2rŽ S r . where MSiO2 is the SiO 2 molar weight, and r is the volumic weight of the bulk SiO 2 . The internal surface areas S of the samples are those given in reference w14x.
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Fig. 2. Photoluminescence peak intensity for a 70% and a 80% porosity porous silicon layers as a function of the oxide thickness Ža. and the relative amount of hydrogen on the surface of the previous porous layers as a function of the oxide thickness Žb..
these conditions a result of the hydrogen desorption w3x. However, the behavior of the 70% porosity sample is different. The PL intensity increases approximately one order of magnitude. A maximum in the PL intensity is observed for about one oxide monolayer obtained after a treatment duration of 120 min. The PL intensity then shows a decrease. The
3. Results and discussion Fig. 2Ža. shows the PL peak intensity as a function of oxide thickness for a 70% porosity porous layer and for a 80% porosity porous layer. Fig. 2Žb. shows the evolution of the amount of hydrogen which covers the surface of PS for the two previous samples. A slight blue-shift Ž20 nm. is observed on both samples during oxidation as it can be seen in Fig. 3 for the 80% porosity example. The 80% porosity sample PL intensity decreases immediately upon the beginning of the treatment. At the same time, the quantity of hydrogen falls. As already mentioned, the decrease of the PL intensity is in
Fig. 3. Photoluminescence spectra of a 80% porosity sample, as-formed and after 120 min treatment.
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rise of the PL intensity could be attributed to an enhancement of the passivation due to the oxidation. Nevertheless, this mechanism can be ruled out principally for two reasons. Firstly, the hydrogen coverage is known to lead to a very good passivation. Secondly, a 3008C oxide contains a lot of defects and can only degrade the passivation. Another possible mechanism, based on the quantum confinement model, may be an enhanced charge localization in the silicon crystallites due to the oxygen incorporation. Fig. 4 illustrates the charge localization mechanism. Fig. 4Ža. shows a Silicon crystallite linked to two narrowings associated with the energy diagram. Fig. 4Žb. represents the same crystallite with an uniform oxide layer on the surface and on the narrowings. The increase in the charge confinement due
to the oxide layer is more efficient on the narrowings than on the crystallite itself since the confinement energy varies as 1rr a , r being the radius of the considered silicon crystallite, and 1 - a - 2 w15,16x. As a result, the charge in the crystallite is more localized when it is oxidized. The same explanation has been given to account for the increase in PL intensity of the anodically oxidized PS w10x. The case of anodic oxidation is in fact rather similar to the present thermal oxidation since the maximum thickness of an anodically oxidized 70% porous layer does not exceed one monolayer w12,13x. Nevertheless, it is important to notice that another luminescence model based on the existence of extrinsic defects on the PS SiO x surface w17x could also explain the results. The initial increase in the lumi-
Fig. 4. Diagram of a silicon crystallite with the associated holes and electrons potential energies in the case of a non-oxidized silicon crystallite Ža. and for a partially oxidized one Žb.. The diagram shows the effect of the oxidation on the potential energy barriers. This effect increases the crystallite charge carriers localization.
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nescence intensity could then be attributed to an increase of such luminescent centers in the growing SiO x layer. On the other hand, the PL intensity decrease of the 70% porosity sample seems to be well correlated with the simultaneous decrease of the amount of hydrogen. This means that the hydrogen coverage still passivates the oxidized layer. This behavior is similar to what was observed in the case of anodic oxidation w11x. However, it is surprising to note that, in spite of the constant temperature, the hydrogen desorption takes place only after a large time delay Ž90 min. which moreover corresponds roughly to the incorporation of one oxide monolayer. A possible explanation for this time delay may be that the oxygen incorporated in the silicon back bonded configuration weakens the Si–H bond which could then be broken by a thermally activated process. Hydrogen on 80% porosity PS seems to desorb more easily than on the 70% porosity sample since the hydrogen desorption starts upon the very first stage of the experiment for the 80% porosity sample. Hydrogen desorption has been studied by using annealing of PS in vacuum. Robinson et al. w18x have found that hydrogen starts desorbing at about 2008C on p-type PS. More recently, Halimaoui et al. w19x have reported that this phenomenon occurs from 3008C on 58% p-type PS. These two different hydrogen desorption thresholds can be explained in the light of the present results. Indeed, the bright luminescent porous layers used by Robinson et al. had a higher porosity than that of Halimaoui et al. since 58% p-type PS is practically non luminescent. The same result was pointed out by Collins et al. w20x who studied the photoinduced hydrogen desorption on two kinds of porous layers. They have found that hydrogen desorbes more easily on a so called ‘green’ sample than on a so called ‘red’ one, the ‘green’ sample being more porous than the ‘red’ one. The authors suggested that the ratio of hydrogen on silicon would be higher for small crystallites than for larger ones so that the probability of hydrogen loss would be higher for the small crystallites. Another hypothesis suggested by Collins et al. stipulates that the smallest crystallites should be preferentially covered by Si–H 2 and Si–H 3 groupments rather than by Si–H groupments. As Si–H 2 and Si–H 3 bonds are weaker than Si–H bonds, hydrogen desorption
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should be more efficient on the smallest crystallites. This behavior is in agreement with the results of Robinson et al. w18x since they observed a red-shift while the PL intensity decrease due to hydrogen desorption when PS is annealed under vacuum. The present results cannot be used to check this hypothesis by looking at the FTIR spectra because, in addition to the hydrogen desorption, there is the oxidation phenomenon. Indeed, it is seen on Fig. 1 that the Si–H x vibrations peaks shift upon oxidation process. It is then hazardous to evaluate the relative quantity of Si–H, Si–H 2 and Si–H 3 bonds. In order to better understand the porosity-dependent hydrogen desorption and the hydrogen coverage nature of PS, studies including vacuum annealing, PL measurements and species desorption analysis would be of great interest. Our results have shown that for a 70% porosity porous layer, the PL intensity strongly depends on the oxidation level. This consideration could explain the contradictory results reported in the literature for non thermal oxidation which are stated in the introduction w5–11x, as in these cases, the oxidation level is not controlled. Dry thermal oxidation at 3008C has never been reported to enhance the PL intensity until now. This is probably due to the fact that, by using oxygen or air atmosphere, the resulting oxidation level is in all the reported cases Žthermal oxidation at 3008C., more important than in our experiments. Consequently, the effects of the first stage of the oxidation Ženhancement of the PL. cannot be seen. Indeed, some 70% porosity samples have been oxidized at 3008C and at higher temperatures in oxygen atmosphere and the results were similar to what is reported in these cases in the literature.
4. Conclusion Contrary to previously reported results of the literature w2–4x, it is possible to increase the quantum efficiency of the PL by using a 3008C thermal oxidation. Indeed, a PL intensity increase was obtained on a 70% porosity sample, the maximum of the PL intensity corresponding to an average oxide thickness of one monolayer. This rise in the PL intensity may be explained, in the quantum confinement model, by the charge carrier localization effect
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which has already been invoked in the case of the anodically oxidized PS w10x. However, the model which considers some luminescent centers in the SiO x or SiO 2 layer w17x may also explain the results. The immediate PL intensity decrease of the 80% porosity sample is explained by the depassivation of the PS internal surface due to hydrogen desorption. This process is in agreement with other literature results w3x. The PL intensity decrease of the 70% porosity PS, which occurs when one oxide monolayer is formed, is also explained by hydrogen desorption. This is also in agreement with the literature, since the fact that the hydrogen surface coverage has still a passivating role when an oxide thickness in the order of one monolayer is formed was reported on anodically oxidized PS w11x. PL intensity behavior during an oxidation treatment depends greatly on the as-formed PS porosity. A rise or a fall of the PL intensity can indeed be obtained under the same oxidation process with two different porosities. This result allows a better understanding of the various and sometimes contradictory PL intensity behaviors due to oxidation reported in the literature w5–11x. The hydrogen desorption process depends greatly on the PS porosity. The temperature threshold at which hydrogens start desorbing increases when porosity decreases. Some work remains to be done to understand this behavior.
Acknowledgements I am grateful to A. Bsiesy and F. Gaspard for critical discussion of this work.
References w1x L.T. Canham, Appl. Phys. Lett. 57 Ž1990. 1046–1048. w2x A. Takazawa, T. Tamura and M. Yamada, J. Appl. Phys. 75 Ž1994. 2489–2495. w3x M. Yamada and K. Kondo, Jpn. J. Appl. Phys. 31 Ž1992. L993–L996. w4x V. Petrova-Koch, T. Muschik, A. Kux, B.K. Meyer, F. Koch and V. Lehmann, Appl. Phys. Lett. 61 Ž1992. 943–945. w5x M. Baba, G. Kuwano, T. Miwa and H. Taniguchi, Jpn. J. Appl. Phys. 33 Ž1994. L483–L486. w6x X.Y. Hou, G. Shi, W. Wang, F.L. Zhang, P.H. Hao, D.M. Huang and X. Wang, Appl. Phys. Lett. 62 Ž1993. 1097–1098. w7x K.H. Li, C. Tsai, S. Shih, T. Hsu, D.L. Kwong and J.C. Campbell, J. Appl. Phys. 72 Ž1992. 3816–3817. w8x I.M. Chang, G.S. Chuo, D.C. Chang and Y.F. Chen, J. Appl. Phys. 77 Ž1995. 5365–5368. w9x M.A. Tischler, R.T. Collins, J.H. Stathis and J.C. Tsang, Appl. Phys. Lett. 60 Ž1992. 639–641. w10x J.C. Vial, S. Billat, A. Bsiesy, G. Fishman, F. Gaspard, R. Herino, M. Ligeon, F. Madeore, I. Mihalcescu, F. Muller and R. Romestain, Physica B 185 Ž1993. 593–602. w11x E.B. Vazsonyi, M. Koos, G. Jalsovszky and I. Pocsik, J. Lumin. 57 Ž1993. 121–124. w12x M.A. Hory, R. Herino, M. Ligeon, F. Muller, F. Gaspard, I. Mihalcescu and J.C. Vial, Thin Solid Films 255 Ž1995. 200–203. w13x J.L. Cantin, M. Schoisswohl, A. Grosman, S. Lebib, C. Ortega, H.J. Von Bardeleben, E. Vazsonyi, G. Jalsovszky and J. Erostyak, Thin Solid Films 276 Ž1996. 76–79. w14x A. Halimaoui, Surf. Sci. Lett. 306 Ž1994. L550–L554. w15x G. Fishman, I. Mihalcescu and R. Romestain, Phys. Rev. B 48 Ž1993. 1464–1467. w16x J.P. Proot, C. Delerue, G. Allan and M. Lannoo, Appl. Surf. Sci. 65–66 Ž1993. 423–425. w17x S.M. Prokes, J. Mater. Res. 11Ž2. Ž1996. 305–320. w18x M.B. Robinson, A.C. Dillon, D.R. Haynes and S.M. George, Appl. Phys. Lett. 61 Ž1992. 1414–1416. w19x A. Halimaoui, Y. Campidelli, A. Larre and D. Bensahel, Phys. Status Solidi Žb. 190 Ž1995. 35–40. w20x R.T. Collins, M.A. Tischler and J.H. Stathis, Appl. Phys. Lett. 61 Ž1992. 1649–1651.
Possible explanation of the contradictory results on the porous silicon photoluminescence evolution after low temperature treatments B. Gelloz
)
Laboratoire de Spectrometrie ´ Physique, UniÕersite´ Joseph Fourier de Grenoble I, CNRS (UMR 5588), P.O. Box 87, 38402 St. Martin d’Heres ` Cedex, France Received 18 June 1996; accepted 24 September 1996
Abstract The modification of the porous silicon photoluminescence ŽPL. after a low level thermal oxidation at 3008C is studied by analyzing the surface chemical composition using infra-red spectroscopy. The amount of oxide is determined quantitatively. Two different porosities of porous silicon are studied: 70% and 80%. In the case of the 70% porosity porous layer, this thermal process leads to an important PL enhancement in the early stages of oxygen incorporation followed by a degradation at higher oxidation levels whereas the photoluminescence of the 80% porosity porous layer only decreases. It is shown how this new analysis can account for the different and often contradictory results on the PL behavior under various low-temperature oxidation processes.
1. Introduction Since the discovery of the porous silicon ŽPS. visible photoluminescence ŽPL. at room temperature w1x, a large amount of work has been achieved in order to investigate the stability of this luminescence upon ageing or upon thermal treatment under oxidizing or non-oxidizing ambiants. In particular, various oxidation treatments have been extensively studied. PS thermal oxidation up to 6008C generally leads to a decrease of the PL intensity w2–4x. This is due to the depassivation of the surface. Indeed, the break, in this temperature range, of the Si–H bonds which cover the as-formed PS surface generates some dan)
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gling bonds which act as non radiative centers so that the initial passivation is destroyed w3x. The passivation cannot be supplied by a low temperature Ž- 6008C. oxide which contains a large number of defects. However, high temperature oxidation above 8008C can lead to an enhancement of the PL intensity w2–4x. This behavior is explained by the fact that the SiO 2rSi interface obtained in the high temperature oxidation process is of good quality. Consequently, there is a rebuild of the Si nanocrystallites surface passivation. Other low temperature oxidation methods have been investigated and some of them can lead to an increase of the PL intensity. These methods are the oxidation in water at room temperature w5x, the oxidation in boiling water w6,7x, the photoinduced oxidation w5,8,9x and the anodic oxidation w10,11x. Some
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B. Gellozr Applied Surface Science 108 (1997) 449–454
results show an increase followed by a fall of the PL intensity as a function of the oxidation time w5,7x. Other results show only a degradation of the PL intensity w6,9x. Chang et al report a decrease followed by a rise of the PL intensity w8x. Finally, anodic oxidation enhances the PL intensity w10,11x. It is coming out that these low temperature oxidation experiments of PS lead to contradictory results even when they are achieved in very similar experimental conditions. For instance, Hou et al. w6x as well as Li et al. w7x have used oxidation in boiling water. The first ones have reported a degradation of the PL intensity whereas the others have observed an increase followed by a decrease of the PL intensity. Another example is the photoinduced oxidation. Baba et al. w5x have reported an increase followed by a decrease of the PL intensity, Tischler et al. w9x have only seen a degradation of the PL intensity and Chang et al. w8x have observed a decrease followed by a rise of the PL intensity. The aim of this article is to understand why so different, and even contradictory, results can be obtained upon PS low temperature oxidation.
supplied by an Argon laser with a 488.8 nm wavelength. The changes in the surface chemical structure during oxidation are studied by using a UNICAM Mattson 5000 Fourier transform infrared ŽFTIR. spectrometer. Fig. 1Ža. shows the absorption spectrum of an as-formed porous layer. The peaks between 2050 cmy1 and 2170 cmy1 represent the absorption due to the different vibrational modes of Si–H x Ž x s 1, 2, 3. bonds. It has been shown that the area under these peaks is proportional to the amount of hydrogen which covers the PS surface w12x. Fig. 1Žb. shows the absorption spectrum of a porous layer which has less than one oxide monolayer on its surface. The peaks stated before have less intensity and two new peaks have appeared between 2140 cmy1 and 2300 cmy1 . These new peaks are representative of the O–Si–H x bonds which means that oxygen is in fact incorporated in the silicon back bond without any perturbation of the initial hydrogen surface coverage w13x. The evolution of the total amount of hydrogen during oxidation is obtained by analyzing the changes in the spectrum area between 2050 cmy1 and 2300 cmy1 . This gives a measurement of the amount of hydrogen relatively
2. Experiment PS layers of two different porosities, namely 70 and 80%, are formed on boron doped Ž100.-oriented Si substrate of 3–6 V cm resistivity. The 70% porosity samples are obtained by anodization for 60 s under a constant current density of 20 mArcm2 , in a 50% HF–ethanol–water solution Ž25:50:25 vol%.. The 80% porosity layers are obtained by anodization for 180 s under a constant current density of 10 mArcm2 , in a 50% HF–ethanol–water solution Ž15:50:35 vol%.. The anodization time is monitored in order to obtain 1 mm thick layers. The as-formed samples show bright PL. They are thermally oxidized at 3008C in a quartz oven under a nitrogen flux containing some traces of oxygen. This method enables a very slow oxidation rate, and therefore the effects of the very first oxidation steps can be studied. Typically, it takes 120 min to build one oxide monolayer. During the oxidation treatment, the samples are taken out from the oven at regular intervals. PL is measured by using a Princeton optical multichannel analyzer ŽOMA.. The excitation light is
Fig. 1. FTIR spectra of an as-formed 1 m m thick 70% porosity p-type porous layer Ža. and of a partially oxidized 1 m m thick 70% porosity p-type porous layer Žb..
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to the amount of hydrogen present on the as-formed sample. The part of the spectrum between 950 cmy1 and 1250 cmy1 is much more intense for the partially oxidized porous layer than for the as-formed sample. This part of the spectrum is characteristic of the Si–O bonds. The more the porous layer is oxidized, the more this region of the spectrum is intense. The area under this part of the spectrum gives the amount of oxide covering the porous layer. Again, this is a relative measurement, but it is possible to deduce an absolute value using a calibration. This calibration is obtained from the anodic oxidation experiments on the same kind of porous layer as that used for thermal oxidation. Different porous layers were oxidized in H 2 SO4 1 M under a constant current of 5 mArcm2 at different exchanged charge Q which correspond to different oxidation levels. For each oxidation level, the number of moles n of SiO 2 is deduced from Q by the relation n s Qr4F where F is the Faraday. By performing FTIR measurements on the anodically oxidized samples it is then possible to relate the area of the FTIR spectrum between 950 cmy1 and 1250 cmy1 to the number of moles n. A calibration of the FTIR spectrum is thus obtained. In the case of the thermal oxidation experiments, the number of SiO 2 moles can then be deduced from FTIR measurements. Moreover, it is possible to have a rough evaluation of the oxide thickness t using t s n P MSiO2rŽ S r . where MSiO2 is the SiO 2 molar weight, and r is the volumic weight of the bulk SiO 2 . The internal surface areas S of the samples are those given in reference w14x.
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Fig. 2. Photoluminescence peak intensity for a 70% and a 80% porosity porous silicon layers as a function of the oxide thickness Ža. and the relative amount of hydrogen on the surface of the previous porous layers as a function of the oxide thickness Žb..
these conditions a result of the hydrogen desorption w3x. However, the behavior of the 70% porosity sample is different. The PL intensity increases approximately one order of magnitude. A maximum in the PL intensity is observed for about one oxide monolayer obtained after a treatment duration of 120 min. The PL intensity then shows a decrease. The
3. Results and discussion Fig. 2Ža. shows the PL peak intensity as a function of oxide thickness for a 70% porosity porous layer and for a 80% porosity porous layer. Fig. 2Žb. shows the evolution of the amount of hydrogen which covers the surface of PS for the two previous samples. A slight blue-shift Ž20 nm. is observed on both samples during oxidation as it can be seen in Fig. 3 for the 80% porosity example. The 80% porosity sample PL intensity decreases immediately upon the beginning of the treatment. At the same time, the quantity of hydrogen falls. As already mentioned, the decrease of the PL intensity is in
Fig. 3. Photoluminescence spectra of a 80% porosity sample, as-formed and after 120 min treatment.
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rise of the PL intensity could be attributed to an enhancement of the passivation due to the oxidation. Nevertheless, this mechanism can be ruled out principally for two reasons. Firstly, the hydrogen coverage is known to lead to a very good passivation. Secondly, a 3008C oxide contains a lot of defects and can only degrade the passivation. Another possible mechanism, based on the quantum confinement model, may be an enhanced charge localization in the silicon crystallites due to the oxygen incorporation. Fig. 4 illustrates the charge localization mechanism. Fig. 4Ža. shows a Silicon crystallite linked to two narrowings associated with the energy diagram. Fig. 4Žb. represents the same crystallite with an uniform oxide layer on the surface and on the narrowings. The increase in the charge confinement due
to the oxide layer is more efficient on the narrowings than on the crystallite itself since the confinement energy varies as 1rr a , r being the radius of the considered silicon crystallite, and 1 - a - 2 w15,16x. As a result, the charge in the crystallite is more localized when it is oxidized. The same explanation has been given to account for the increase in PL intensity of the anodically oxidized PS w10x. The case of anodic oxidation is in fact rather similar to the present thermal oxidation since the maximum thickness of an anodically oxidized 70% porous layer does not exceed one monolayer w12,13x. Nevertheless, it is important to notice that another luminescence model based on the existence of extrinsic defects on the PS SiO x surface w17x could also explain the results. The initial increase in the lumi-
Fig. 4. Diagram of a silicon crystallite with the associated holes and electrons potential energies in the case of a non-oxidized silicon crystallite Ža. and for a partially oxidized one Žb.. The diagram shows the effect of the oxidation on the potential energy barriers. This effect increases the crystallite charge carriers localization.
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nescence intensity could then be attributed to an increase of such luminescent centers in the growing SiO x layer. On the other hand, the PL intensity decrease of the 70% porosity sample seems to be well correlated with the simultaneous decrease of the amount of hydrogen. This means that the hydrogen coverage still passivates the oxidized layer. This behavior is similar to what was observed in the case of anodic oxidation w11x. However, it is surprising to note that, in spite of the constant temperature, the hydrogen desorption takes place only after a large time delay Ž90 min. which moreover corresponds roughly to the incorporation of one oxide monolayer. A possible explanation for this time delay may be that the oxygen incorporated in the silicon back bonded configuration weakens the Si–H bond which could then be broken by a thermally activated process. Hydrogen on 80% porosity PS seems to desorb more easily than on the 70% porosity sample since the hydrogen desorption starts upon the very first stage of the experiment for the 80% porosity sample. Hydrogen desorption has been studied by using annealing of PS in vacuum. Robinson et al. w18x have found that hydrogen starts desorbing at about 2008C on p-type PS. More recently, Halimaoui et al. w19x have reported that this phenomenon occurs from 3008C on 58% p-type PS. These two different hydrogen desorption thresholds can be explained in the light of the present results. Indeed, the bright luminescent porous layers used by Robinson et al. had a higher porosity than that of Halimaoui et al. since 58% p-type PS is practically non luminescent. The same result was pointed out by Collins et al. w20x who studied the photoinduced hydrogen desorption on two kinds of porous layers. They have found that hydrogen desorbes more easily on a so called ‘green’ sample than on a so called ‘red’ one, the ‘green’ sample being more porous than the ‘red’ one. The authors suggested that the ratio of hydrogen on silicon would be higher for small crystallites than for larger ones so that the probability of hydrogen loss would be higher for the small crystallites. Another hypothesis suggested by Collins et al. stipulates that the smallest crystallites should be preferentially covered by Si–H 2 and Si–H 3 groupments rather than by Si–H groupments. As Si–H 2 and Si–H 3 bonds are weaker than Si–H bonds, hydrogen desorption
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should be more efficient on the smallest crystallites. This behavior is in agreement with the results of Robinson et al. w18x since they observed a red-shift while the PL intensity decrease due to hydrogen desorption when PS is annealed under vacuum. The present results cannot be used to check this hypothesis by looking at the FTIR spectra because, in addition to the hydrogen desorption, there is the oxidation phenomenon. Indeed, it is seen on Fig. 1 that the Si–H x vibrations peaks shift upon oxidation process. It is then hazardous to evaluate the relative quantity of Si–H, Si–H 2 and Si–H 3 bonds. In order to better understand the porosity-dependent hydrogen desorption and the hydrogen coverage nature of PS, studies including vacuum annealing, PL measurements and species desorption analysis would be of great interest. Our results have shown that for a 70% porosity porous layer, the PL intensity strongly depends on the oxidation level. This consideration could explain the contradictory results reported in the literature for non thermal oxidation which are stated in the introduction w5–11x, as in these cases, the oxidation level is not controlled. Dry thermal oxidation at 3008C has never been reported to enhance the PL intensity until now. This is probably due to the fact that, by using oxygen or air atmosphere, the resulting oxidation level is in all the reported cases Žthermal oxidation at 3008C., more important than in our experiments. Consequently, the effects of the first stage of the oxidation Ženhancement of the PL. cannot be seen. Indeed, some 70% porosity samples have been oxidized at 3008C and at higher temperatures in oxygen atmosphere and the results were similar to what is reported in these cases in the literature.
4. Conclusion Contrary to previously reported results of the literature w2–4x, it is possible to increase the quantum efficiency of the PL by using a 3008C thermal oxidation. Indeed, a PL intensity increase was obtained on a 70% porosity sample, the maximum of the PL intensity corresponding to an average oxide thickness of one monolayer. This rise in the PL intensity may be explained, in the quantum confinement model, by the charge carrier localization effect
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which has already been invoked in the case of the anodically oxidized PS w10x. However, the model which considers some luminescent centers in the SiO x or SiO 2 layer w17x may also explain the results. The immediate PL intensity decrease of the 80% porosity sample is explained by the depassivation of the PS internal surface due to hydrogen desorption. This process is in agreement with other literature results w3x. The PL intensity decrease of the 70% porosity PS, which occurs when one oxide monolayer is formed, is also explained by hydrogen desorption. This is also in agreement with the literature, since the fact that the hydrogen surface coverage has still a passivating role when an oxide thickness in the order of one monolayer is formed was reported on anodically oxidized PS w11x. PL intensity behavior during an oxidation treatment depends greatly on the as-formed PS porosity. A rise or a fall of the PL intensity can indeed be obtained under the same oxidation process with two different porosities. This result allows a better understanding of the various and sometimes contradictory PL intensity behaviors due to oxidation reported in the literature w5–11x. The hydrogen desorption process depends greatly on the PS porosity. The temperature threshold at which hydrogens start desorbing increases when porosity decreases. Some work remains to be done to understand this behavior.
Acknowledgements I am grateful to A. Bsiesy and F. Gaspard for critical discussion of this work.
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