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Materials Science and Engineering B69 – 70 (2000) 553 – 558 www.elsevier.com/locate/mseb
Mechanism of large oscillations of anodic potential during anodization of silicon in H3PO4/HF solutions V. Parkhutik a,*, E. Matveeva a, R. Perez b, J. Alamo b, D. Beltra´n b a
Technical Uni6ersity of Valencia, Camı´ de Vera s/n, 46071 Valencia, Spain b Uni6ersity of Valencia, Burjassot, 46007 Valencia, Spain
Abstract Effect of large oscillations of electrical potential during anodic polarization of silicon in electrolytes composed of phosphoric and hydrofluoric acids has been reported. The oscillations last hours without damping if experimental conditions are optimal. Changes of temperature, anodic current density, intensity of stirring, etc. quench them or convert into less periodic ones. The oscillations are of very high amplitude (typically 15 V) with a period ranging from 18 to 30 s. Scanning electron microscopy (SEM)-imaging of the samples experiencing the oscillatory kinetic behaviour shows unambiguously that the stage of the anodic voltage growth is assisted by the formation of a thin (50 – 80 nm) surface film, while the dropping of potential corresponds to its lifting-off. A mechanism responsible for the successive built-up and lifting-off of the surface passive film is assumed to be a triggered isotropic formation of micropores at the film/silicon interface. The present data are compared with other known cases of electrochemical oscillations, and a unified model is suggested. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Anodic oxide; Electrochemical oscillations; Electron microscopy; Silicon
1. Introduction Oscillations of voltage and current have been found in the case of electrochemical treatment of metals (f.i. pitting corrosion of iron, nickel, and other metals). These oscillations have been attentively studied both theoretically and experimentally in order to understand the mechanisms involved into the process [1 – 5]. Electrochemical treatment of silicon has also been shown to yield an oscillatory electrochemical kinetics. Earlier it has been shown that galvanostatic polarisation of Si in electrolytes containing strong acids (like sulfuric, phosphoric, oxalic, etc.) yields a specific oscillatory dependence of anodic potential on the anodization time [6]. The oscillatory kinetics is especially well resolved at 40–60°C and for sufficiently low anodic current densities (B1 mA cm − 2). The amplitude of these oscillations is about several volts and they are damped. This type of oscillatory kinetics has been ascribed to alternating stages of Si oxidation and localized dissolution of the oxide film. It has been shown by the X-ray scattering technique that this oscillatory pro* Corresponding author.
cess yields the growth of multi-layer porous silicon oxide films with a number of layers exactly equal to the number of experienced oscillations [7]. Lehmann [8] and Ozanam et al. [9] have given useful information on the properties of these films and the nature of the oscillatory anodic kinetics as well. Further on, the effect of giant oscillations of the anodic potential has been discovered when silicon is anodized in a mixture of hydrofluoric acid with other inorganic acids (like phosphoric, oxalic or sulfuric) [10]. These oscillations possess the amplitude of about 15 V and last days without any dumping. In the present paper a further insight into this new type of anodic oscillatory kinetics for silicon is presented. The observed time pattern of anodization kinetics is compared with that registered in the case of silicon anodized in NH4F solutions [11–15].
2. Experimental Silicon wafers used in the work were (100)-oriented p-Si (2–6 V cm − 1) and p+-Si (0.01 V cm − 1). The electrolytes were 0.01–0.1 M aqueous solutions of
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phosphoric acid with a small amount of HF (0.001– 0.01 M). The electrolyte temperature was varied within 20–45°C interval (precision 90.2°C). A three-electrode cell was used with both reference and counter electrodes made of platinum. The potentials measured in this cell were close to those obtained using a Standard Hydrogen Electrode. The high values (2 – 30 V) of voltage, indeed, made unnecessary any special precautions in measuring the anodic potential. The electrochemical set-up used in the work included a PAR-273A potentiostat-galvanostat controlled by CorrWare-2 software. The following types of experiments have been carried out: (1) dependence of the anodic potential on the anodization time during the galvanostatic polarisation of the sample; (2) dependence of the anodic potential on the stirring of the electrolyte; (3) dependence of the anodic potential on the temperature of the electrolyte. The morphology of the obtained samples was studied using scanning electron microscopy (SEM HITACHI S-4100) operating in a back-scattered electron imaging
Fig. 1. Oscillatory kinetics of Si anodization in aqueous H3PO4/HF solution.
mode and a transmission electron microscopy (TEM PHILIPS CM-10). Thin gold films were deposited onto the samples to enhance the contrast. TEM images were obtained by cleaving and bathing etched sample in ethanol that allowed to separate flakes of the surface oxide films and study them.
3. Results
3.1. Oscillatory kinetics of the anodic oxide growth Fig. 1 illustrates different cases of the anodic oscillatory kinetics when the polarization of p-Si is performed at constant current in H3PO4/HF electrolyte at room temperature. At appropriate experimental conditions regular oscillations of the potential are emerging after some induction period has passed (Fig. 1a). After certain induction period of time the oscillations become stable and each of them lasts about 20 s. If the experimental conditions are optimal, the amplitude of the oscillations is quite large (about 15 V) and they last days without any indication of damping. Among experimental variables (composition of the electrolyte, anodic current density, electrolyte temperature) the electrolyte composition is of great importance. Electrolytes which contain 0.1 M H3PO4 and 0.005 M HF aqueous solutions have yielded the best results at room temperature. Other important factor to adjust is the anodic current density. At low currents (typically 0.5–1 mA cm − 2) very regular oscillations of the potential are observed, while increasing current makes them less regular (secondary oscillations interfere) or even causes their disappearance. Fig. 1b shows the change of the anodization kinetics during sequental application of four different anodic current densities. Low current density (here 0.65 mA cm − 2) yields no oscillations. At higher anodic currents (0.7–1 mA cm − 2) large oscillations emerge in the anodization kinetics of the same type as in Fig. 1a (hereafter they will be called oscillations of the first type, T1). Oscillations get a rather complex shape with further increase of the current (above 1.3 mA cm − 2) where second type of oscillations (with lower amplitude and period) becomes visible in the oscillatory kinetics. Finally, when the current rises up to 2.6 mA cm − 2, these secondary oscillations become dominating while those of large amplitude disappear. These small and faster oscillations will be further referred to as T2 ones. Oscillation period is about 18–30 s for the T1 oscillations and decreases to 5–7 s for T2 ones. The current increase beyond 3 mA cm − 2 results in irregularity of the oscillations, damping of their amplitude and their disappearance in the long run.
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Fig. 2. Power density spectrum calculated for the experimental data on oscillatory anodization kinetics and fitted according to [20]. Frequency dependence Sv( f ): f − 3.6 means the existence of correlation links between individual events included into the kinetics.
Fig. 3. Temperature dependence of the kinetics of Si anodization of Si at 2 mA cm − 2.
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Generally, stirring of the electrolyte causes the drop of the amplitude and period. It causes the enhancement of the diffusion processes in the electrolyte. Turning the stirrer on can either destroy well-shaped oscillations or convert them into those of lower amplitude and period (Fig. 1c). The effect is reversible as switching off the stirrer restores the oscillatory behaviour corresponding to the non-stirred electrolyte. Fourier transform of the observed time series allows to distinguish sub-harmonic bifurcations (period doubling) of the fundamental frequency to frequencies of 1/2 and 1/4. This interesting behaviour, often found in systems with transitions to the chaos [16–19], is worth further studying using advanced methods of spectral analysis. The analysis of the obtained oscillating patterns has also been performed using Flicker–Noise Spectroscopy [20] which allows to reveal the possible correlation links existing among the sequential individual events in time series. Fig. 2 gives a so-called power density spectrum for the present case. Remarkable feature of this spectrum, apart of the presence of three characteristic resonant frequencies, is the decreasing curve slope that is the indication of the presence of the correlation links between the individual events during the oscillatory anodization kinetics (intensity of power spectrum depends on the frequency as f − 3.6). Time patterning described above looks similar to the anodic current oscillations when silicon is potentiostatically anodized in fluorite solutions. This type of treatment has been reported by Gerisher and Lubke [11] and more recently studied in a series of papers by Rappich et al. [12,13], Cattarin et al. [14], and Rausher et al. [15]. There are, however, some particular features in the present case making it different from the reported experimental data. The present time patterning, probably, can be related to a new case of chaos/order transitions in electrochemical systems of reaction/diffusion type, additional to known processes [21]. Fig. 3 shows the critical dependence of the oscillations on the value of the electrolyte temperature. A temperature increase within just 2°C is able to destroy the oscillations completely (compare curves corresponding to T= 44°C and T= 45.5°C in Fig. 3). Generally, increasing temperature works in a way opposite to what corresponds to the increasing current density. While irregular oscillations are characteristic for lower temperatures, higher ones yield well-shaped oscillations until they disappear if the temperature is sufficiently high. Higher temperature results in more active chemical diffusion kinetics at the Si/electrolyte interface and produces changes similar to those provoked by the electrolyte stirring.
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3.2. SEM and TEM obser6ations The morphology of the samples which are experiencing the oscillatory kinetic behavior was studied using SEM and TEM. Examples of the surface and cross-section views are shown in Fig. 4 for the samples grown at 27°C by long-time (5000 s) anodization at the current density of 1 mA cm − 2. Thin layer of a surface substance (presumably silicon dioxide) is clearly seen in the cross-section view of the sample (Fig. 4a). TEM images were obtained by cutting and bathing etched sample in ethanol. The flakes of the surface oxide-like film were collected at Cu grid and then placed into TEM facility. The obtained images reveal a
Fig. 5. Scanning electron microscopy (SEM) images of the surface (a) and cross-section (b) of the sample after oscillatory anodization process (T2 oscillations).
Fig. 4. Electron microscopy images of the sample undergoing oscillatory anodization process (T1 oscillations); (a) scanning electron microscopy (SEM) image of the sample surface; (b) transmission electron microscopy (TEM) image of the surface film separated from the Si substrate; (c) SEM image of the stack of oxide layers at the sample surface.
porous film with specific features of 4–7 nm in diameter (Fig. 4b). Prolonged anodization process causes the formation of the stack of the oxide layers as its is seen in Fig. 4c. The thickness of these layers varies within the 30–90 nm interval. The internal layers are the most dense and adherent to the Si sample while external ones are rough and highly porous. The external layers are gradually dissolved in the electrolyte and wiped off from the sample surface. Type 2 oscillations are assisted by the formation of the cellular surface structures (Fig. 5). Each cell possesses a shape close to hemispheric and is covered by the film of oxide possessing a multilayer structure similar to the case of the T1 oscillations. The cell diameter varies from 0.7–1.5 mm depending on the duration of the polarization process. Obviously, there exist differences between the mechanisms of the T1 and T2 oscillations. Further work is under the way to study these mechanisms in more details. The SEM and TEM observations show unambigously that the oxide film is formed at the Si surface (the stage of the anodic potential growth) and then is lifted off from the surface (when the potential drops down). In the conditions of regular oscillations all points of the surface of the sample are involved altogether into oxidation or dissolution stages. Therefore, spatial synchronization of the reaction rates takes place
V. Parkhutik et al. / Materials Science and Engineering B69–70 (2000) 553–558
and the oscillations last undamped over long periods of time. Additional evidence for this is the observed correlation of individual events involved into the oscillatory kinetics. The whole surface of the sample is constantly renewed and the process repeats continuously for hours provided that the experimental conditions are optimal.
4. Discussion It has been suggested earlier [6] that the oscillatory kinetics is due to the alternating processes of the anodic oxide growth (the stage of the voltage growth) and its dissolution (voltage decrease) [6]. The oxide film grows due to inward migration of oxygen containing species (presumably atomic oxygen anions) through already existing oxide film to the interface Si/SiO2 where new oxide molecules are formed. This oxidation reaction thus needs the application of high electric field to the sample in order to enhance the migration of oxygen species. Another involved reaction is that of the oxide dissolution. There are two possible mechanisms for that. The first one is an electric field enhanced oxide dissolution due to the electric field-stimulated transfer of silicon cations into the solution enhanced by the presence of solvated protons. This mechanism is considered in details by Diggle [22]. The other mechanism is of chemical dissolution of SiOx film due to direct reaction of silicon oxide molecules with HF and formation of soluble silicon tetrafluoride molecular species. Both mechanisms work simultaneously to ensure an intriguing effect of the periodic detachment of the forming oxide layers from the Si substrate. Since oxidation and dissolution reactions depend differently on the anodization variables, it makes possible a de-synchronization of the reaction rates in a time domain, when oxidation is favorable at certain moments and dissolution at others. Non-linear dependence of the oxidation/dissolution rates on the electric field strength has been suggested as the main reason for this desynchronization effect [23], although other reaction parameters like temperature, diffusion fluxes in the electrolyte, etc. can be also involved. A mechanism responsible for the lifting-off of the growing oxide is assumed to be isotropic formation of the micropores at oxide/silicon interface. The mechanism of detachment which is based on the earlier assumption of triggered local dissolution of the oxide film, first discussed in [23] has been proposed. At the stage of the barrier oxide growth some needle-like pores are propagating through the oxide film. The electrochemical conditions at the bottom of these pores become so unstable with time as to provoke massive pore formation in all directions that finally undermines the oxide layer. Beneath it a new oxide layer is formed and
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the process repeats. SEM measurements show the presence of several oxide layers (up to four in Fig. 4c), with the internal layer more dense and external ones gradually converted into gel-like disperse layer. The mechanism of oscillations through the formation and detachment of the oxide layer is somewhat different from that suggested by Cattarin et al. [14] where a periodic change of the oxide thickness is assumed as a main reason for the oscillations of the current in Si/ NH4F system. Without going into details of the cited paper it should be noted that its results could be quite reasonably explained in terms of the present model of the alternating stages oxide growth and detachment. The theoretical model for the mechanism of synchronized oscillations of the anodic current was recently developed by Carstensen et al. [24] based on the assumption of the localized ionic conductivity in the forming oxide layer. The model assumes a localized oxide growth with individual local events synchronized over the whole sample surface. While our assumption is based on the mechanism of the localized oxide dissolution. Probably both approaches are physically equivalent. Further studies will be necessary to verify the details of the mechanism of the oscillating oxide growth.
5. Conclusions Effect of large oscillations of electrical potential during anodic polarization of silicon in electrolytes composed of phosphoric and hydrofluoric acids has been reported. The oscillations last hours without damping if experimental conditions are optimal for this. Changes of temperature, anodic current density, intensity of stirring etc. quench them or convert into less periodic ones. The oscillations are of very high amplitude (typically 15 V) with a period ranging from 18 to 30 s. SEM-imaging of the sample surface experienced oscillatory kinetic behaviour shows unambiguously that the stage of the anodic voltage growth is assisted by the formation of thin (50–90 nm) surface film, while the dropping of potential corresponds to its lifting-off. A mechanism responsible for the successive built-up and lifting-off of the surface passive film is assumed to be a triggered isotropic formation of micropores at the film/silicon interface. The present data are compared with other known cases of electrochemical oscillations, and a unified model is suggested.
Acknowledgements The work was supported through the Contract MAT 98 0342 by the Ministry of Education and Culture of Spain. The help of the Council of Education and
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Culture of the Valencian Autonomous Community in assisting the costs of the participation in the 1999 E-MRS Meeting is greatly acknowledged. The authors are grateful to Prof. H.Fo¨ll for helpful discussions of the paper.
References [1] M. Koper, in: I. Prigogine, S. Rice (Eds.), Advances in Chemical Physics, Wiley, New York, 1996, p. 161. [2] J. Wojtowicz, in: J.O’M. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 8, Plenum, New York, 1972, p. 47. [3] O. Lev, M. Sheintuch, H. Yarnitsky, L. Pismeen, Nature 336 (1988) 488. [4] J.L. Hudson, T.T. Tsotsis, Chem. Eng. Sci. 49 (1994) 1493. [5] M. Wang, H.W. Pickering, Y. Xu, J. Electrochem. Soc. 142 (1995) 2986. [6] V. Parkhutik, Electrochim. Acta 36 (1991) 1611. [7] V. Parkhutik, Y. Chu, H. You, Z. Nagy, J. Porous Mater. (in press). [8] V. Lehmann, J. Eelectrochem. Soc 143 (1996) 1313. [9] F. Ozanam, J.L. Chazalviel, A. Radi, J. Electrochem. Soc. 39 (1992) 2491. [10] V. Parkhutik, E. Matveeva, Electrochem. Solid State Lett. 2 (8) (1999) 567.
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[11] H. Gerisher, M. Lu¨bke, Ber. Bunsenges. Phys. Chem. 92 (1988) 573. [12] J. Rappich, V.Yu. Timoshenko, Th. Dittrich, J. Electrochem. Soc. 144 (1997) 493. [13] J. Rappich, V.Yu. Timoshenko, Th. Dittrich, Ber. Bunsenges. Phys.Chem. 101 (1997) 139. [14] S. Cattarin, J.N. Chazalviel, C. Da Fonseca, F. Ozanam, L.M. Peter, G. Schlichtho¨rl, J. Stumper, J. Electrochem. Soc. 145 (1998) 498. [15] S. Rausher et al., In: Proc. 5th International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, The Electrochemical Society, Pennington, 1998, p. 439. [16] J.P. Gollub, S.V. Benson, J. Fluid Mech. 100 (1980) 449–470. [17] J. Testa, J. Perez, C. Jeffries, Phys. Rev. Lett. 48 (1982) 714– 717. [18] Mitchell J. Feigenbaum, J. Stat. Phys. 19 (1978) 1. [19] H.M. Gibbs, F.A. Hopf, D.L. Kaplan, R.L. Shoemaker, Phys. Rev. Lett. 46 (1981) 474 – 477. [20] S.F. Timashev, In: C. Rossi, S. Bastianoni (Eds.), Annals of the New York Academy of Science, vol. 879, 1999, pp. 129– 142. [21] N. Mazouz, K. Kricher, G. Fla¨tgen, G. Ertl, J. Phys. Chem. B 101 (1997) 2403. [22] J.W. Diggle, in: A.K. Vijh (Ed.), Oxides and Oxide Films, vol. 2, Marcel Dekker, New York, 1973, p. 281. [23] Yu. Makushok, V. Parkhutik, J.M. Martı´nez Duart, J.M. Albella, J. Phys. D Appl. Phys. 27 (1994) 661. [24] J. Carstensen, R. Prange, H. Fo¨ll, J. Electrochem. Soc. 146 (1999) 1134.
Mechanism of large oscillations of anodic potential during anodization of silicon in H3PO4/HF solutions V. Parkhutik a,*, E. Matveeva a, R. Perez b, J. Alamo b, D. Beltra´n b a
Technical Uni6ersity of Valencia, Camı´ de Vera s/n, 46071 Valencia, Spain b Uni6ersity of Valencia, Burjassot, 46007 Valencia, Spain
Abstract Effect of large oscillations of electrical potential during anodic polarization of silicon in electrolytes composed of phosphoric and hydrofluoric acids has been reported. The oscillations last hours without damping if experimental conditions are optimal. Changes of temperature, anodic current density, intensity of stirring, etc. quench them or convert into less periodic ones. The oscillations are of very high amplitude (typically 15 V) with a period ranging from 18 to 30 s. Scanning electron microscopy (SEM)-imaging of the samples experiencing the oscillatory kinetic behaviour shows unambiguously that the stage of the anodic voltage growth is assisted by the formation of a thin (50 – 80 nm) surface film, while the dropping of potential corresponds to its lifting-off. A mechanism responsible for the successive built-up and lifting-off of the surface passive film is assumed to be a triggered isotropic formation of micropores at the film/silicon interface. The present data are compared with other known cases of electrochemical oscillations, and a unified model is suggested. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Anodic oxide; Electrochemical oscillations; Electron microscopy; Silicon
1. Introduction Oscillations of voltage and current have been found in the case of electrochemical treatment of metals (f.i. pitting corrosion of iron, nickel, and other metals). These oscillations have been attentively studied both theoretically and experimentally in order to understand the mechanisms involved into the process [1 – 5]. Electrochemical treatment of silicon has also been shown to yield an oscillatory electrochemical kinetics. Earlier it has been shown that galvanostatic polarisation of Si in electrolytes containing strong acids (like sulfuric, phosphoric, oxalic, etc.) yields a specific oscillatory dependence of anodic potential on the anodization time [6]. The oscillatory kinetics is especially well resolved at 40–60°C and for sufficiently low anodic current densities (B1 mA cm − 2). The amplitude of these oscillations is about several volts and they are damped. This type of oscillatory kinetics has been ascribed to alternating stages of Si oxidation and localized dissolution of the oxide film. It has been shown by the X-ray scattering technique that this oscillatory pro* Corresponding author.
cess yields the growth of multi-layer porous silicon oxide films with a number of layers exactly equal to the number of experienced oscillations [7]. Lehmann [8] and Ozanam et al. [9] have given useful information on the properties of these films and the nature of the oscillatory anodic kinetics as well. Further on, the effect of giant oscillations of the anodic potential has been discovered when silicon is anodized in a mixture of hydrofluoric acid with other inorganic acids (like phosphoric, oxalic or sulfuric) [10]. These oscillations possess the amplitude of about 15 V and last days without any dumping. In the present paper a further insight into this new type of anodic oscillatory kinetics for silicon is presented. The observed time pattern of anodization kinetics is compared with that registered in the case of silicon anodized in NH4F solutions [11–15].
2. Experimental Silicon wafers used in the work were (100)-oriented p-Si (2–6 V cm − 1) and p+-Si (0.01 V cm − 1). The electrolytes were 0.01–0.1 M aqueous solutions of
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 3 2 3 - 2
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phosphoric acid with a small amount of HF (0.001– 0.01 M). The electrolyte temperature was varied within 20–45°C interval (precision 90.2°C). A three-electrode cell was used with both reference and counter electrodes made of platinum. The potentials measured in this cell were close to those obtained using a Standard Hydrogen Electrode. The high values (2 – 30 V) of voltage, indeed, made unnecessary any special precautions in measuring the anodic potential. The electrochemical set-up used in the work included a PAR-273A potentiostat-galvanostat controlled by CorrWare-2 software. The following types of experiments have been carried out: (1) dependence of the anodic potential on the anodization time during the galvanostatic polarisation of the sample; (2) dependence of the anodic potential on the stirring of the electrolyte; (3) dependence of the anodic potential on the temperature of the electrolyte. The morphology of the obtained samples was studied using scanning electron microscopy (SEM HITACHI S-4100) operating in a back-scattered electron imaging
Fig. 1. Oscillatory kinetics of Si anodization in aqueous H3PO4/HF solution.
mode and a transmission electron microscopy (TEM PHILIPS CM-10). Thin gold films were deposited onto the samples to enhance the contrast. TEM images were obtained by cleaving and bathing etched sample in ethanol that allowed to separate flakes of the surface oxide films and study them.
3. Results
3.1. Oscillatory kinetics of the anodic oxide growth Fig. 1 illustrates different cases of the anodic oscillatory kinetics when the polarization of p-Si is performed at constant current in H3PO4/HF electrolyte at room temperature. At appropriate experimental conditions regular oscillations of the potential are emerging after some induction period has passed (Fig. 1a). After certain induction period of time the oscillations become stable and each of them lasts about 20 s. If the experimental conditions are optimal, the amplitude of the oscillations is quite large (about 15 V) and they last days without any indication of damping. Among experimental variables (composition of the electrolyte, anodic current density, electrolyte temperature) the electrolyte composition is of great importance. Electrolytes which contain 0.1 M H3PO4 and 0.005 M HF aqueous solutions have yielded the best results at room temperature. Other important factor to adjust is the anodic current density. At low currents (typically 0.5–1 mA cm − 2) very regular oscillations of the potential are observed, while increasing current makes them less regular (secondary oscillations interfere) or even causes their disappearance. Fig. 1b shows the change of the anodization kinetics during sequental application of four different anodic current densities. Low current density (here 0.65 mA cm − 2) yields no oscillations. At higher anodic currents (0.7–1 mA cm − 2) large oscillations emerge in the anodization kinetics of the same type as in Fig. 1a (hereafter they will be called oscillations of the first type, T1). Oscillations get a rather complex shape with further increase of the current (above 1.3 mA cm − 2) where second type of oscillations (with lower amplitude and period) becomes visible in the oscillatory kinetics. Finally, when the current rises up to 2.6 mA cm − 2, these secondary oscillations become dominating while those of large amplitude disappear. These small and faster oscillations will be further referred to as T2 ones. Oscillation period is about 18–30 s for the T1 oscillations and decreases to 5–7 s for T2 ones. The current increase beyond 3 mA cm − 2 results in irregularity of the oscillations, damping of their amplitude and their disappearance in the long run.
V. Parkhutik et al. / Materials Science and Engineering B69–70 (2000) 553–558
Fig. 2. Power density spectrum calculated for the experimental data on oscillatory anodization kinetics and fitted according to [20]. Frequency dependence Sv( f ): f − 3.6 means the existence of correlation links between individual events included into the kinetics.
Fig. 3. Temperature dependence of the kinetics of Si anodization of Si at 2 mA cm − 2.
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Generally, stirring of the electrolyte causes the drop of the amplitude and period. It causes the enhancement of the diffusion processes in the electrolyte. Turning the stirrer on can either destroy well-shaped oscillations or convert them into those of lower amplitude and period (Fig. 1c). The effect is reversible as switching off the stirrer restores the oscillatory behaviour corresponding to the non-stirred electrolyte. Fourier transform of the observed time series allows to distinguish sub-harmonic bifurcations (period doubling) of the fundamental frequency to frequencies of 1/2 and 1/4. This interesting behaviour, often found in systems with transitions to the chaos [16–19], is worth further studying using advanced methods of spectral analysis. The analysis of the obtained oscillating patterns has also been performed using Flicker–Noise Spectroscopy [20] which allows to reveal the possible correlation links existing among the sequential individual events in time series. Fig. 2 gives a so-called power density spectrum for the present case. Remarkable feature of this spectrum, apart of the presence of three characteristic resonant frequencies, is the decreasing curve slope that is the indication of the presence of the correlation links between the individual events during the oscillatory anodization kinetics (intensity of power spectrum depends on the frequency as f − 3.6). Time patterning described above looks similar to the anodic current oscillations when silicon is potentiostatically anodized in fluorite solutions. This type of treatment has been reported by Gerisher and Lubke [11] and more recently studied in a series of papers by Rappich et al. [12,13], Cattarin et al. [14], and Rausher et al. [15]. There are, however, some particular features in the present case making it different from the reported experimental data. The present time patterning, probably, can be related to a new case of chaos/order transitions in electrochemical systems of reaction/diffusion type, additional to known processes [21]. Fig. 3 shows the critical dependence of the oscillations on the value of the electrolyte temperature. A temperature increase within just 2°C is able to destroy the oscillations completely (compare curves corresponding to T= 44°C and T= 45.5°C in Fig. 3). Generally, increasing temperature works in a way opposite to what corresponds to the increasing current density. While irregular oscillations are characteristic for lower temperatures, higher ones yield well-shaped oscillations until they disappear if the temperature is sufficiently high. Higher temperature results in more active chemical diffusion kinetics at the Si/electrolyte interface and produces changes similar to those provoked by the electrolyte stirring.
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3.2. SEM and TEM obser6ations The morphology of the samples which are experiencing the oscillatory kinetic behavior was studied using SEM and TEM. Examples of the surface and cross-section views are shown in Fig. 4 for the samples grown at 27°C by long-time (5000 s) anodization at the current density of 1 mA cm − 2. Thin layer of a surface substance (presumably silicon dioxide) is clearly seen in the cross-section view of the sample (Fig. 4a). TEM images were obtained by cutting and bathing etched sample in ethanol. The flakes of the surface oxide-like film were collected at Cu grid and then placed into TEM facility. The obtained images reveal a
Fig. 5. Scanning electron microscopy (SEM) images of the surface (a) and cross-section (b) of the sample after oscillatory anodization process (T2 oscillations).
Fig. 4. Electron microscopy images of the sample undergoing oscillatory anodization process (T1 oscillations); (a) scanning electron microscopy (SEM) image of the sample surface; (b) transmission electron microscopy (TEM) image of the surface film separated from the Si substrate; (c) SEM image of the stack of oxide layers at the sample surface.
porous film with specific features of 4–7 nm in diameter (Fig. 4b). Prolonged anodization process causes the formation of the stack of the oxide layers as its is seen in Fig. 4c. The thickness of these layers varies within the 30–90 nm interval. The internal layers are the most dense and adherent to the Si sample while external ones are rough and highly porous. The external layers are gradually dissolved in the electrolyte and wiped off from the sample surface. Type 2 oscillations are assisted by the formation of the cellular surface structures (Fig. 5). Each cell possesses a shape close to hemispheric and is covered by the film of oxide possessing a multilayer structure similar to the case of the T1 oscillations. The cell diameter varies from 0.7–1.5 mm depending on the duration of the polarization process. Obviously, there exist differences between the mechanisms of the T1 and T2 oscillations. Further work is under the way to study these mechanisms in more details. The SEM and TEM observations show unambigously that the oxide film is formed at the Si surface (the stage of the anodic potential growth) and then is lifted off from the surface (when the potential drops down). In the conditions of regular oscillations all points of the surface of the sample are involved altogether into oxidation or dissolution stages. Therefore, spatial synchronization of the reaction rates takes place
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and the oscillations last undamped over long periods of time. Additional evidence for this is the observed correlation of individual events involved into the oscillatory kinetics. The whole surface of the sample is constantly renewed and the process repeats continuously for hours provided that the experimental conditions are optimal.
4. Discussion It has been suggested earlier [6] that the oscillatory kinetics is due to the alternating processes of the anodic oxide growth (the stage of the voltage growth) and its dissolution (voltage decrease) [6]. The oxide film grows due to inward migration of oxygen containing species (presumably atomic oxygen anions) through already existing oxide film to the interface Si/SiO2 where new oxide molecules are formed. This oxidation reaction thus needs the application of high electric field to the sample in order to enhance the migration of oxygen species. Another involved reaction is that of the oxide dissolution. There are two possible mechanisms for that. The first one is an electric field enhanced oxide dissolution due to the electric field-stimulated transfer of silicon cations into the solution enhanced by the presence of solvated protons. This mechanism is considered in details by Diggle [22]. The other mechanism is of chemical dissolution of SiOx film due to direct reaction of silicon oxide molecules with HF and formation of soluble silicon tetrafluoride molecular species. Both mechanisms work simultaneously to ensure an intriguing effect of the periodic detachment of the forming oxide layers from the Si substrate. Since oxidation and dissolution reactions depend differently on the anodization variables, it makes possible a de-synchronization of the reaction rates in a time domain, when oxidation is favorable at certain moments and dissolution at others. Non-linear dependence of the oxidation/dissolution rates on the electric field strength has been suggested as the main reason for this desynchronization effect [23], although other reaction parameters like temperature, diffusion fluxes in the electrolyte, etc. can be also involved. A mechanism responsible for the lifting-off of the growing oxide is assumed to be isotropic formation of the micropores at oxide/silicon interface. The mechanism of detachment which is based on the earlier assumption of triggered local dissolution of the oxide film, first discussed in [23] has been proposed. At the stage of the barrier oxide growth some needle-like pores are propagating through the oxide film. The electrochemical conditions at the bottom of these pores become so unstable with time as to provoke massive pore formation in all directions that finally undermines the oxide layer. Beneath it a new oxide layer is formed and
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the process repeats. SEM measurements show the presence of several oxide layers (up to four in Fig. 4c), with the internal layer more dense and external ones gradually converted into gel-like disperse layer. The mechanism of oscillations through the formation and detachment of the oxide layer is somewhat different from that suggested by Cattarin et al. [14] where a periodic change of the oxide thickness is assumed as a main reason for the oscillations of the current in Si/ NH4F system. Without going into details of the cited paper it should be noted that its results could be quite reasonably explained in terms of the present model of the alternating stages oxide growth and detachment. The theoretical model for the mechanism of synchronized oscillations of the anodic current was recently developed by Carstensen et al. [24] based on the assumption of the localized ionic conductivity in the forming oxide layer. The model assumes a localized oxide growth with individual local events synchronized over the whole sample surface. While our assumption is based on the mechanism of the localized oxide dissolution. Probably both approaches are physically equivalent. Further studies will be necessary to verify the details of the mechanism of the oscillating oxide growth.
5. Conclusions Effect of large oscillations of electrical potential during anodic polarization of silicon in electrolytes composed of phosphoric and hydrofluoric acids has been reported. The oscillations last hours without damping if experimental conditions are optimal for this. Changes of temperature, anodic current density, intensity of stirring etc. quench them or convert into less periodic ones. The oscillations are of very high amplitude (typically 15 V) with a period ranging from 18 to 30 s. SEM-imaging of the sample surface experienced oscillatory kinetic behaviour shows unambiguously that the stage of the anodic voltage growth is assisted by the formation of thin (50–90 nm) surface film, while the dropping of potential corresponds to its lifting-off. A mechanism responsible for the successive built-up and lifting-off of the surface passive film is assumed to be a triggered isotropic formation of micropores at the film/silicon interface. The present data are compared with other known cases of electrochemical oscillations, and a unified model is suggested.
Acknowledgements The work was supported through the Contract MAT 98 0342 by the Ministry of Education and Culture of Spain. The help of the Council of Education and
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Culture of the Valencian Autonomous Community in assisting the costs of the participation in the 1999 E-MRS Meeting is greatly acknowledged. The authors are grateful to Prof. H.Fo¨ll for helpful discussions of the paper.
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