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Polarization behavior during porous silicon formation: effect of surfactant
Electrochimica Acta 46 (2001) 1013– 1018 www.elsevier.nl/locate/electacta
Polarization behavior during porous silicon formation: effect of surfactant Takashi Tsuboi 2, Tetsuo Sakka, Yukio H. Ogata *,1 Institute of Ad6anced Energy, Kyoto Uni6ersity, Uji, Kyoto 611 -0011, Japan Received 13 June 2000; received in revised form 7 August 2000
Abstract The effect of surfactant on the polarization behavior during porous silicon formation has been studied for p- and p+-type silicon wafers. The presence of surfactant showed strong effects on p-type silicon, whereas the p+-type silicon polarization curve changed negligibly with a surfactant. The anionic surfactant increased the overpotential; it was decreased by the cationic surfactant. The space charge layer mainly determines the dissolution process, since p-type silicon, which has a smaller capacity, was more affected than p+-type silicon. This is also supported by the capacity measurements of a p-type silicon: a more positive flatband potential in the anionic surfactant solution and a less positive flatband potential in the cationic one. The surface properties of p-type porous silicon have also been investigated by infrared spectroscopy. The absorption bands change in shape and in intensity, depending upon the surfactant. In particular, the cationic surfactant reduced the absorbance. The decrease is due to the chemical dissolution of the porous silicon layer. We can also see a correlation between the integrated absorbance and the electrode potential. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Capacity; Infrared spectroscopy; Polarization; Porous silicon; Surfactant
1. Introduction The anodization of a silicon wafer in hydrofluoric acid results in silicon dissolution: porous silicon formation or electropolishing. Considerable controversy exists as to how the applied potential is divided between the space charge layer of the semiconductor and the Helmholtz layer during pore formation, especially for p- and p+-type silicon wafers [1,2]. Ronga et al. reported on porous silicon formation on p-type silicon in concentrated hydrofluoric acid solutions and presented theoretical considerations [3]. The * Corresponding author. 2 Present address: Department of Chemistry, Faculty of Science, Niigata University, Ikarashi-ninocho, Niigata 9502181, Japan. 1 ISE member
interfacial impedance measurements provided evidence of hole depletion during the porous silicon formation at a current density below 100 mA cm − 2. The polarization curves were characterized by a linear regime with a slope of 60 mV decade − 1. They concluded that the anodic current for porous silicon formation is determined by the surface concentration of holes. At doping levels higher than 1019 cm − 3, however, the electrode polarization was shared between the space charge layer and the Helmholtz layer. The slope then deviated from 60 mV decade − 1 due to the variation of the potential drop in the Helmholtz layer. Zhang et al. found slopes of between 55 and 65 mV decade − 1 for porous silicon formation of p- and p+-type silicon wafers in diluted HF solutions [4]. They pointed out that the porous silicon formation of p+type silicon was controlled by the charge transfer in the Helmholtz layer. On the other hand, the carrier supply
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and/or transport in the semiconductor could also be considered to be the rate-limiting step for p-type silicon. Searson and Zhang performed impedance analysis of silicon in diluted HF solutions [5]. The porous silicon formation of p- and p+-type silicon wafers was controlled by the Helmholtz layer, although the space charge layer determined the dissolution at low overpotentials for p-type silicon. Kang and Jorne´ studied the polarization behavior of p- and p+-type silicon wafers in concentrated HF [6]. Linear regimes were observed and the slopes were reported to be 60–70 mV decade − 1. They analyzed the results on the basis of their theoretical model. They suggested that the total overpotential was partitioned between the space charge layer and the Helmholtz layer and that both layers controlled the anodic dissolution. Similar data have been reported by different workers, although they have drawn different conclusions using their own results. A different viewpoint should be adopted to clarify the dissolution mechanism. We thus develop the discussion on this issue through the modification of silicon–HF interface as follows. Surfactant can easily be adsorbed at the interface between solid and liquid. Addition of a surfactant to hydrofluoric acid is expected to change the interfacial structure and the electrochemical properties of the silicon electrode. The polarization behavior of silicon in hydrofluoric acid containing surfactants was investigated. We report the effect of the surfactant and look into the rate-controlling step involved in porous silicon formation. Sotgiu et al. reported on porous silicon prepared in HF-based solutions containing various surfactants [7]. Uniform porous silicon can be formed by a reduction of the surface tension, although the surfactant did not affect the microstructure characterized by the Raman scattering. However, there has been no report as to the effect on the porous silicon surface. We also investigate the surface structure using infrared spectroscopy.
2. Experimental The substrates used in this study were p- and p+-type (100)-oriented silicon wafers. The resistivities were 10– 20 V cm and 0.0015–0.003 V cm respectively. In order to make an ohmic contact, aluminum was evaporated on the back side of the wafer followed by the thermal annealing at 400°C for 30 min, or Ga–In alloy was deposited following the dissolution of native oxide in HF-based solution. The electrolytes were aqueous 20 wt% hydrofluoric acid containing various surfactants. The surfactants used were polyoxyethylene octylphenyl ether, sodium dodecyl sulfate, and dodecyltrimethylammonium bromide. These are nonionic, anionic, and cationic surfactants respectively. The concentrations were 100 ppm, 2000 ppm, and 400 ppm re-
spectively. Visual inspection confirmed the entire dissolution of the surfactants in the HF solutions. If these solutions contained no hydrofluoric acid, they would be around the critical micellar concentration and would have a surface tension of 25 – 40 dyn cm − 1 [8 – 10]. The effect of counter ions on porous silicon would be negligible, since they are much more difficult to adsorb and not likely to affect the properties of porous silicon. The electrochemical cell was a three-electrode system. The silicon wafer served as the working electrode. A platinum wire was used as the counter electrode. The reference electrode (RE) used was Ag/AgCl electrode; the electrolyte was 3.3 M KCl. The cell used was a plastic container with a round opening at its bottom, where the working electrode was fixed. The RE was isolated from the cell by a polyethylene Luggin capillary filled with agar containing KNO3. The linear-sweep voltammograms were recorded in the dark at room temperature, since the voltammograms were extremely sensitive to light. The scan rate was 1 mV s − 1. They hardly changed with sweep rates between 0.5 and 10 mV s − 1; thus the diffusions of chemical species were not important in the process. The current density– potential characteristics of the second and the third sweeps were adopted, because the voltammogram of the first sweep was different from that of second or subsequent sweeps and showed poor reproducibility. The ohmic drop between the working and reference electrodes was corrected. We also measured the capacity of the p-type silicon– electrolyte interface. It was analyzed at a frequency of 10 kHz by sweeping potentials from − 0.15 to −1.0 V versus RE. The voltammogram showed the negligible current density in this region; thus the Mott– Schottky plot allows us to estimate the flatband potential. It should be noted that the capacity depended on the measured frequency. The dependence was also reported by Ronga et al. [3]. The measurements were done after a waiting time of 20 min at a potential of − 0.15 V versus RE, after the formation of a porous silicon layer of about 10 mm thickness. This is because it takes 20 min to stabilize the interface [3]. Transmission infrared spectra of the p-type silicon electrode were measured after the formation of the porous silicon layer. In this case, the ohmic contact of aluminum or Ga– In alloy was not used, because a good contact is not necessary and because the coating causes the metallic absorption. A silicon wafer was used as a reference to remove the bulk absorption. The anodization current density was 2 mA cm − 2 and the time was 30 min. After the anodization, the sample was dried by blowing Ar gas without any rinse. Then the specimens were immersed for a few minutes in a 20 wt% ethanoic HF solution to remove the surfactants. The drying was done by the above procedure to avoid oxidation.
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3. Results
3.1. Polarization beha6iors of p- and p+-silicon electrodes
Fig. 1. Polarization behavior of p-type Si wafer in 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants.
Fig. 2. Polarization behavior of p+-type Si wafer in 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants.
Table 1 Summary of OCP and of slope of polarization curve. It should be noted that the surfactant changed the curve of p-type electrode drastically but that it did not strongly affect the p+-type electrode Surfactant
None Nonionic Anionic Cationic
OCP/V versus RE
Slope/mV decade−1
p-type
p+-type
p-type
−0.75 −0.85 −0.65 −0.90
−0.55 −0.65 −0.55 −0.75
60 70 40 80
p+-type 60 50 50 50
First, we measured the open circuit potential (OCP) of the working electrode. The p-type electrodes had OCPs of − 0.75 V, − 0.85 V, − 0.65 V, and −0.90 V versus RE in no, nonionic, anionic, and cationic surfactant solutions respectively. On the other hand, the OCPs of the p+-type electrodes were −0.55 V, − 0.65 V, −0.55 V, and −0.75 V versus RE respectively. The error was about 9 0.02 V. The p-type electrode showed a more negative OCP than the p+-type electrode. The nonionic and cationic surfactants shifted the OCP to a more negative potential and the cationic one caused the most negative OCP. A similar or less negative OCP was observed in anionic surfactant solution in comparison with the solution without any surfactant. The polarization characteristics of p- and p+-type silicon wafers are shown in Figs. 1 and 2 respectively. The anodic current density increased with the electrode potential. No current peak was observed in these regions. A higher current density flowed at the same potential for p+-type silicon than for p-type silicon, which is consistent with previous studies [1,3]. A linear region was seen in the solution without any surfactant. The slopes were about 60 mV decade − 1 for both silicon wafers, except for the low current density region. The surfactant drastically influenced the polarization curve for p-type silicon. The addition of the cationic surfactant shifted it to a more negative potential. On the other hand, silicon is harder to dissolve at the same electrode potential with the anionic surfactant. These curves also showed linear behavior. The slope varied depending on the surfactant type. The values were about 70 mV decade − 1, 40 mV decade − 1, and −1 80 mV decade for the solutions containing nonionic, anionic, and cationic surfactants respectively. However, the linear-sweep voltammogram was not strongly modified by the surfactant for p+-type silicon. A straight line characterized the curves and the slope was about 50 mV decade − 1 in all the solutions with the surfactant. Table 1 summarizes the OCPs and the slopes of the polarization curves.
3.2. Capacity characteristics of p-type silicon–electrolyte interface The interfacial capacity results from the addition of two different capacitors in series. One of them corresponds to the space charge layer of the silicon and the other to the Helmholtz layer. In general, the Helmholtz capacity is in the order of 10 mF cm − 2 in aqueous solutions [11], which is much larger than that of the
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Fig. 3. Mott– Schottky plot of p-type Si wafer in 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants. The plots were obtained after porous silicon formation.
semiconductor capacity. Thus we can neglect the contribution of the Helmholtz capacity to the overall one. The capacity obtained is related only to the space charge layer. Fig. 3 shows the Mott–Schottky plots of the p-type silicon electrode in various solutions. Straight lines were observed at potentials more negative than −0.6 V versus RE. The capacity varied with the electrolyte used. Those lines allowed us to estimate the flatband potentials. The calculated potentials were 0.36 V for no surfactant, 0.28 V for the nonionic surfactant, 0.45 V for the anionic surfactant, and 0.25 V for the cationic surfactant. The dopant concentration was also estimated to be 6 × 1014 cm − 3 from the slopes. This value
Fig. 4. Transmission infrared spectra of porous silicon samples prepared from p-type Si wafer. The solutions used were 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants. The anodization current density and time were 2 mA cm − 2 and 30 min respectively.
is consistent with the expected concentration [12]. However, the Mott– Schottky plot was dependent strongly upon the measured frequency. We measured the capacity at various frequencies. The flatband potential increased with anionic surfactant and decreased with cationic surfactant, although the value changed depending upon the measured frequency. The dependence was also reported by Ronga et al. [3]. The flatband potentials would thus show the qualitative tendency: the anionic surfactant led to the largest band bending in the space charge layer at the same potential and the smallest bending arose from the cationic one. The capacity measurements were carried out for various conditions; we obtained the same qualitative tendency. Linearity was not observed at potentials less negative than − 0.6 V versus RE. The capacity decreased with increasing potential in the solution without any surfactant. The inverse square of the capacity had a peak at − 0.4 V versus RE, as seen in Fig. 3, and then diminished at less negative potentials. However, the solutions containing a surfactant did not show the peak. The inverse square of the capacity reduced steeply with increasing potential.
3.3. Infrared spectra of porous silicon layer The large surface area of porous silicon enables investigation of the surface by infrared spectroscopy. Thus the passivation of a porous silicon surface has been studied extensively [13– 15]. The peak locations and the assignments are as follows: 630 cm − 1 due to SiH bending; 680 cm − 1 due to SiH2 wagging; 850 cm − 1 due to SiOSi symmetric stretching and/or OSiH deformation; 910 cm − 1 due to SiH2 scissors bending; 1100 cm − 1 due to SiOSi antisymmetric stretching; 2090 cm − 1 due to SiH stretching; 2100 cm − 1 due to SiH2 stretching and (SiH)2 stretching; 2140 cm − 1 due to SiH3 stretching; 2190 cm − 1 due to SiH stretching in O2SiH2; 2250 cm − 1 due to SiH stretching in O3SiH. Fig. 4 shows the infrared spectra of porous silicon layers obtained in various solutions. All spectra had a strong absorption at 620– 680 cm − 1, a sharp peak at 910 cm − 1, and a triplet at 2090– 2140 cm − 1. These absorption are due to SiHx species. No absorption appeared around 1100 cm − 1 and at 2200 cm − 1. These results meant that the porous silicon layer did not oxidize. A small peak could be seen at 810 cm − 1 due to SiFx for the porous silicon samples prepared in the solutions with no and anionic surfactants. We do not discuss this absorption further. The absorption intensities varied with the surfactant used. The intensities were comparable for porous silicon samples prepared in the solutions with no and anionic surfactants. The nonionic surfactant made the intensities weaker. The weakest absorptions were obtained in the solution containing the cationic surfac-
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tant. Furthermore, the cationic surfactant changed the shapes of the absorptions. In particular, the peak at 680 cm − 1 showed a stronger absorbance than that at 630 cm − 1, suggesting that the ratio of SiH2 to SiH was larger.
4. Discussion
4.1. Rate-determining step during porous silicon formation In general, the surfactant can have two effects on the silicon–liquid interface. One is the change of the Helmholtz layer due to the adsorption of the surfactant. The other is the modification of the space charge layer, which results from the adsorption of the charged particle. The voltammogram of p+-type silicon was not affected by the surfactant, whereas the surfactant showed a drastic effect on that of p-type silicon. In comparison with p+-type silicon, p-type silicon has a small capacity and thus little charge in the space charge layer. The space charge layer of p-type silicon is thus expected to be more strongly influenced by the surfactant than that of p+-type silicon. On the other hand, the Helmholtz-layer control of the silicon dissolution should result in a stronger, or at least similar, effect of the surfactant on p+-type silicon. This assumption is inconsistent with this result. Therefore, the space charge layer of the semiconductor plays a more important role in the dissolution, although the Helmholtz layer may have a small effect on p+-type silicon. The capacity data of p-type silicon further support the reaction control in the space charge layer. The flatband potential shifted more positive as the electrode potential became more positive, although they had only qualitative meaning in this system, as described in Section 3. For example, the anionic surfactant results in the most positive flatband potential and the largest overpotential. The space charge layer must be modified by the surfactants, which affects the polarization behavior. The anionic surfactant causes a larger hole depletion in the space charge layer, resulting in a more difficult dissolution. In contrast, a smaller overpotential is seen by the addition of the cationic surfactant, owing to an increased hole supply. The OCP would also be consistent with the change of the space charge layer by the surfactant. For example, the cationic surfactant may decrease the band bending, resulting in the lowest OCP and flatband potential. The above model cannot adequately explain the features of the polarization curves. The slopes deviated from 60 mV decade − 1, which was true for both p- and p+-silicon electrodes. A possible explanation is as follows. It becomes increasingly difficult for the surfactant to adsorb on the surface with increasing electrode po-
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tential, due to the enhanced dissolution of silicon. As a result, the surfactant should show a weaker effect on the space charge layer. This is supported by the result that the higher current density reduced the difference in the overpotential between the solutions. This effect would be stronger for p-type silicon, since the space charge layer is modified more by the surfactant, as already described. We should also consider the dependence of the adsorption state on the electrode potential. The Helmholtz layer may thus change in the structure with the electrode potential. These effects would cause the deviation of the slope from 60 mV decade − 1. The solution without any surfactant showed a peculiar behavior in the capacity. A peak was observed at − 0.4 V versus RE in Fig. 3. A similar peak was also presented by Ronga et al. [3]. The surfactants deleted the peak. It is clear that the condition of the silicon surface contributes to the capacity in this region. It should thus be noted that the surface conditions change by the addition of a surfactant.
4.2. Properties of porous silicon The surfactant is expected to affect the properties of porous silicon. The properties of porous silicon prepared with surfactants were reported by Sotgiu et al. [7]. The photoluminescence spectra of n+-type porous silicon decreased in intensity when the concentration of sodium dodecyl sulfate was greater than the critical micellar concentration; whereas hexadecyltrimethylammonium bromide brought about the maximum in the photoluminescence intensity and the peak shifted to a shorter wavelength. Raman scattering revealed that n+type porous silicon consisted of microcrystallites of size 2 – 4 nm and that the microstructure was not sensitive to the types and concentrations of the surfactant. These properties seem to be related to the different chemical structure. We also investigated the surface properties of porous silicon using infrared spectroscopy and found that surfactants affected the infrared spectra of porous silicon drastically. The cationic surfactant decreased the absorbance, whereas the anionic surfactant had a negligible effect on the absorption bands. The relative integrated absorbance due to SiHx correlates with potential at a current density of 2 mA cm − 2, as seen in Fig. 5. In the ordinate, the absorption of porous silicon from no surfactant solution is considered as unity. The integrated absorbance increases with the overpotential, except for the porous silicon prepared from the no surfactant solution. The exception would be because the contact with the solution by the surfactant facilitates the chemical dissolution of porous silicon, thus decreasing the surface area of porous silicon. The correlation would arise from the dissolution during porous silicon formation. We measured the weight loss after
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porous silicon formation and the effect of surfactants. Notable modifications were seen on p-type silicon electrodes. The polarization curves were shifted in more positive and more negative directions by the addition of anionic and cationic surfactants respectively. On the other hand, p+-type silicon was not strongly affected. Therefore, the anodization current is controlled mostly by the space charge layer in the semiconductor. The capacity and flatband potential also support this idea. A surfactant can change the surface properties of porous silicon. The shapes and intensities of infrared absorption bands depend upon the electrolyte composition. The chemical dissolution of porous silicon is facilitated by the cationic surfactant, resulting in a small absorbance. The integrated absorbance correlates with the electrode potential. Fig. 5. Correlation of relative integrated absorbance due to SiHx with potential at a current density of 2 mA cm − 2. Open circle corresponds to the porous silicon from no surfactant solution and the integrated absorbance is set unity. −2
the anodization at 2 mA cm for 8 h. The losses were 8.0 mg cm − 2, 9.4 mg cm − 2, 8.3 mg cm − 2, and 10.3 mg cm − 2 in the solutions with no, nonionic, anionic, and cationic surfactants respectively. The numbers of electrons per dissolved silicon were estimated to be 2.1, 1.8, 2.0, and 1.6 respectively. A value of two corresponds to porous silicon formation [1], and chemical dissolution of the porous silicon layer formed is expected from a value below two. The porous silicon dissolves in the nonionic or cationic surfactant solutions and the dissolution is more rapid in the cationic surfactant solution. Furthermore, the cationic surfactant also changes the shapes of the absorption bands around 620–680 and 2090– 2140 cm − 1. The properties of porous silicon thus vary with the surfactant used. We briefly mention the chemical dissolution mechanism of the porous silicon layer. The mechanism of chemical etching must be the same for porous silicon as for a silicon wafer, since silicon atoms dissolve chemically in both cases. The porous silicon is thus expected to dissolve chemically by the following process. The ligand exchange reactions occur first: SiHSiF and/or SiHSiOH [16]. This reaction involves the formation of molecular hydrogen. The SiOH groups are attacked by HF and change into SiF groups. The etching proceeds through the reaction of HF with SiSi backbonds polarized by SiF [17]. Then the SiF surface group is removed and the new SiH group is formed on the porous silicon surface [17]. The nonionic or cationic surfactant accelerates the above reaction and reduces the surface area of porous silicon.
5. Conclusion We have investigated the polarization behavior during
Acknowledgements We are grateful to Dr M. Mabuchi and Dr P. Allongue for useful discussions. This work was partly supported by a grant-in-aid for scientific research on priority area of Electrochemistry of Ordered Interfaces from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] R.L. Smith, S.D. Collins, J. Appl. Phys. 71 (1992) R1. [2] P.C. Searson, J.M. Macaulay, S.M. Prokes, J. Electrochem. Soc. 139 (1992) 3373. [3] I. Ronga, A. Bsiesy, F. Gaspard, R. He´rino, M. Ligeon, F. Muller, A. Halimaoui, J. Electrochem. Soc. 138 (1991) 1403. [4] X.G. Zhang, S.D. Collins, R.L. Smith, J. Electrochem. Soc. 136 (1989) 1561. [5] P.C. Searson, X.G. Zhang, J. Electrochem. Soc. 137 (1990) 2539. [6] Y. Kang, J. Jorne´, J. Electrochem. Soc. 144 (1997) 3104. [7] G. Sotgiu, L. Schirone, F. Rallo, Thin Solid Films 297 (1997) 18. [8] J.S. Jeon, S. Raghavan, R.P. Sperline, J. Electrochem. Soc. 142 (1995) 621. [9] R. Matsuura, H. Kimizuka, S. Miyamoto, R. Shimozawa, Bull. Chem. Soc. Jpn. 31 (1958) 532. [10] N.K. Adam, H.L. Shute, Trans. Faraday Soc. 34 (1938) 758. [11] J.O’M. Bockris, A.K.N. Reddy, Modern Electrochemistry, vol. 2, Plenum, New York, 1973. [12] S.M. Sze, Physics of Semiconductor Devices, 2nd edn, Wiley, New York, 1981. [13] Y. Ogata, H. Niki, T. Sakka, M. Iwasaki, J. Electrochem. Soc. 142 (1995) 195. [14] Y. Ogata, H. Niki, T. Sakka, M. Iwasaki, J. Electrochem. Soc. 142 (1995) 1595. [15] Y.H. Ogata, F. Kato, T. Tsuboi, T. Sakka, J. Electrochem. Soc. 145 (1998) 2439. [16] P. Jakob, Y.J. Chabal, J. Chem. Phys. 95 (1991) 2897. [17] P. Jakob, Y.J. Chabal, K. Raghavachari, R.S. Becker, A.J. Becker, Surf. Sci. 275 (1992) 407.
Polarization behavior during porous silicon formation: effect of surfactant Takashi Tsuboi 2, Tetsuo Sakka, Yukio H. Ogata *,1 Institute of Ad6anced Energy, Kyoto Uni6ersity, Uji, Kyoto 611 -0011, Japan Received 13 June 2000; received in revised form 7 August 2000
Abstract The effect of surfactant on the polarization behavior during porous silicon formation has been studied for p- and p+-type silicon wafers. The presence of surfactant showed strong effects on p-type silicon, whereas the p+-type silicon polarization curve changed negligibly with a surfactant. The anionic surfactant increased the overpotential; it was decreased by the cationic surfactant. The space charge layer mainly determines the dissolution process, since p-type silicon, which has a smaller capacity, was more affected than p+-type silicon. This is also supported by the capacity measurements of a p-type silicon: a more positive flatband potential in the anionic surfactant solution and a less positive flatband potential in the cationic one. The surface properties of p-type porous silicon have also been investigated by infrared spectroscopy. The absorption bands change in shape and in intensity, depending upon the surfactant. In particular, the cationic surfactant reduced the absorbance. The decrease is due to the chemical dissolution of the porous silicon layer. We can also see a correlation between the integrated absorbance and the electrode potential. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Capacity; Infrared spectroscopy; Polarization; Porous silicon; Surfactant
1. Introduction The anodization of a silicon wafer in hydrofluoric acid results in silicon dissolution: porous silicon formation or electropolishing. Considerable controversy exists as to how the applied potential is divided between the space charge layer of the semiconductor and the Helmholtz layer during pore formation, especially for p- and p+-type silicon wafers [1,2]. Ronga et al. reported on porous silicon formation on p-type silicon in concentrated hydrofluoric acid solutions and presented theoretical considerations [3]. The * Corresponding author. 2 Present address: Department of Chemistry, Faculty of Science, Niigata University, Ikarashi-ninocho, Niigata 9502181, Japan. 1 ISE member
interfacial impedance measurements provided evidence of hole depletion during the porous silicon formation at a current density below 100 mA cm − 2. The polarization curves were characterized by a linear regime with a slope of 60 mV decade − 1. They concluded that the anodic current for porous silicon formation is determined by the surface concentration of holes. At doping levels higher than 1019 cm − 3, however, the electrode polarization was shared between the space charge layer and the Helmholtz layer. The slope then deviated from 60 mV decade − 1 due to the variation of the potential drop in the Helmholtz layer. Zhang et al. found slopes of between 55 and 65 mV decade − 1 for porous silicon formation of p- and p+-type silicon wafers in diluted HF solutions [4]. They pointed out that the porous silicon formation of p+type silicon was controlled by the charge transfer in the Helmholtz layer. On the other hand, the carrier supply
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and/or transport in the semiconductor could also be considered to be the rate-limiting step for p-type silicon. Searson and Zhang performed impedance analysis of silicon in diluted HF solutions [5]. The porous silicon formation of p- and p+-type silicon wafers was controlled by the Helmholtz layer, although the space charge layer determined the dissolution at low overpotentials for p-type silicon. Kang and Jorne´ studied the polarization behavior of p- and p+-type silicon wafers in concentrated HF [6]. Linear regimes were observed and the slopes were reported to be 60–70 mV decade − 1. They analyzed the results on the basis of their theoretical model. They suggested that the total overpotential was partitioned between the space charge layer and the Helmholtz layer and that both layers controlled the anodic dissolution. Similar data have been reported by different workers, although they have drawn different conclusions using their own results. A different viewpoint should be adopted to clarify the dissolution mechanism. We thus develop the discussion on this issue through the modification of silicon–HF interface as follows. Surfactant can easily be adsorbed at the interface between solid and liquid. Addition of a surfactant to hydrofluoric acid is expected to change the interfacial structure and the electrochemical properties of the silicon electrode. The polarization behavior of silicon in hydrofluoric acid containing surfactants was investigated. We report the effect of the surfactant and look into the rate-controlling step involved in porous silicon formation. Sotgiu et al. reported on porous silicon prepared in HF-based solutions containing various surfactants [7]. Uniform porous silicon can be formed by a reduction of the surface tension, although the surfactant did not affect the microstructure characterized by the Raman scattering. However, there has been no report as to the effect on the porous silicon surface. We also investigate the surface structure using infrared spectroscopy.
2. Experimental The substrates used in this study were p- and p+-type (100)-oriented silicon wafers. The resistivities were 10– 20 V cm and 0.0015–0.003 V cm respectively. In order to make an ohmic contact, aluminum was evaporated on the back side of the wafer followed by the thermal annealing at 400°C for 30 min, or Ga–In alloy was deposited following the dissolution of native oxide in HF-based solution. The electrolytes were aqueous 20 wt% hydrofluoric acid containing various surfactants. The surfactants used were polyoxyethylene octylphenyl ether, sodium dodecyl sulfate, and dodecyltrimethylammonium bromide. These are nonionic, anionic, and cationic surfactants respectively. The concentrations were 100 ppm, 2000 ppm, and 400 ppm re-
spectively. Visual inspection confirmed the entire dissolution of the surfactants in the HF solutions. If these solutions contained no hydrofluoric acid, they would be around the critical micellar concentration and would have a surface tension of 25 – 40 dyn cm − 1 [8 – 10]. The effect of counter ions on porous silicon would be negligible, since they are much more difficult to adsorb and not likely to affect the properties of porous silicon. The electrochemical cell was a three-electrode system. The silicon wafer served as the working electrode. A platinum wire was used as the counter electrode. The reference electrode (RE) used was Ag/AgCl electrode; the electrolyte was 3.3 M KCl. The cell used was a plastic container with a round opening at its bottom, where the working electrode was fixed. The RE was isolated from the cell by a polyethylene Luggin capillary filled with agar containing KNO3. The linear-sweep voltammograms were recorded in the dark at room temperature, since the voltammograms were extremely sensitive to light. The scan rate was 1 mV s − 1. They hardly changed with sweep rates between 0.5 and 10 mV s − 1; thus the diffusions of chemical species were not important in the process. The current density– potential characteristics of the second and the third sweeps were adopted, because the voltammogram of the first sweep was different from that of second or subsequent sweeps and showed poor reproducibility. The ohmic drop between the working and reference electrodes was corrected. We also measured the capacity of the p-type silicon– electrolyte interface. It was analyzed at a frequency of 10 kHz by sweeping potentials from − 0.15 to −1.0 V versus RE. The voltammogram showed the negligible current density in this region; thus the Mott– Schottky plot allows us to estimate the flatband potential. It should be noted that the capacity depended on the measured frequency. The dependence was also reported by Ronga et al. [3]. The measurements were done after a waiting time of 20 min at a potential of − 0.15 V versus RE, after the formation of a porous silicon layer of about 10 mm thickness. This is because it takes 20 min to stabilize the interface [3]. Transmission infrared spectra of the p-type silicon electrode were measured after the formation of the porous silicon layer. In this case, the ohmic contact of aluminum or Ga– In alloy was not used, because a good contact is not necessary and because the coating causes the metallic absorption. A silicon wafer was used as a reference to remove the bulk absorption. The anodization current density was 2 mA cm − 2 and the time was 30 min. After the anodization, the sample was dried by blowing Ar gas without any rinse. Then the specimens were immersed for a few minutes in a 20 wt% ethanoic HF solution to remove the surfactants. The drying was done by the above procedure to avoid oxidation.
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3. Results
3.1. Polarization beha6iors of p- and p+-silicon electrodes
Fig. 1. Polarization behavior of p-type Si wafer in 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants.
Fig. 2. Polarization behavior of p+-type Si wafer in 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants.
Table 1 Summary of OCP and of slope of polarization curve. It should be noted that the surfactant changed the curve of p-type electrode drastically but that it did not strongly affect the p+-type electrode Surfactant
None Nonionic Anionic Cationic
OCP/V versus RE
Slope/mV decade−1
p-type
p+-type
p-type
−0.75 −0.85 −0.65 −0.90
−0.55 −0.65 −0.55 −0.75
60 70 40 80
p+-type 60 50 50 50
First, we measured the open circuit potential (OCP) of the working electrode. The p-type electrodes had OCPs of − 0.75 V, − 0.85 V, − 0.65 V, and −0.90 V versus RE in no, nonionic, anionic, and cationic surfactant solutions respectively. On the other hand, the OCPs of the p+-type electrodes were −0.55 V, − 0.65 V, −0.55 V, and −0.75 V versus RE respectively. The error was about 9 0.02 V. The p-type electrode showed a more negative OCP than the p+-type electrode. The nonionic and cationic surfactants shifted the OCP to a more negative potential and the cationic one caused the most negative OCP. A similar or less negative OCP was observed in anionic surfactant solution in comparison with the solution without any surfactant. The polarization characteristics of p- and p+-type silicon wafers are shown in Figs. 1 and 2 respectively. The anodic current density increased with the electrode potential. No current peak was observed in these regions. A higher current density flowed at the same potential for p+-type silicon than for p-type silicon, which is consistent with previous studies [1,3]. A linear region was seen in the solution without any surfactant. The slopes were about 60 mV decade − 1 for both silicon wafers, except for the low current density region. The surfactant drastically influenced the polarization curve for p-type silicon. The addition of the cationic surfactant shifted it to a more negative potential. On the other hand, silicon is harder to dissolve at the same electrode potential with the anionic surfactant. These curves also showed linear behavior. The slope varied depending on the surfactant type. The values were about 70 mV decade − 1, 40 mV decade − 1, and −1 80 mV decade for the solutions containing nonionic, anionic, and cationic surfactants respectively. However, the linear-sweep voltammogram was not strongly modified by the surfactant for p+-type silicon. A straight line characterized the curves and the slope was about 50 mV decade − 1 in all the solutions with the surfactant. Table 1 summarizes the OCPs and the slopes of the polarization curves.
3.2. Capacity characteristics of p-type silicon–electrolyte interface The interfacial capacity results from the addition of two different capacitors in series. One of them corresponds to the space charge layer of the silicon and the other to the Helmholtz layer. In general, the Helmholtz capacity is in the order of 10 mF cm − 2 in aqueous solutions [11], which is much larger than that of the
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Fig. 3. Mott– Schottky plot of p-type Si wafer in 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants. The plots were obtained after porous silicon formation.
semiconductor capacity. Thus we can neglect the contribution of the Helmholtz capacity to the overall one. The capacity obtained is related only to the space charge layer. Fig. 3 shows the Mott–Schottky plots of the p-type silicon electrode in various solutions. Straight lines were observed at potentials more negative than −0.6 V versus RE. The capacity varied with the electrolyte used. Those lines allowed us to estimate the flatband potentials. The calculated potentials were 0.36 V for no surfactant, 0.28 V for the nonionic surfactant, 0.45 V for the anionic surfactant, and 0.25 V for the cationic surfactant. The dopant concentration was also estimated to be 6 × 1014 cm − 3 from the slopes. This value
Fig. 4. Transmission infrared spectra of porous silicon samples prepared from p-type Si wafer. The solutions used were 20 wt% HF containing (a) no, (b) nonionic, (c) anionic, and (d) cationic surfactants. The anodization current density and time were 2 mA cm − 2 and 30 min respectively.
is consistent with the expected concentration [12]. However, the Mott– Schottky plot was dependent strongly upon the measured frequency. We measured the capacity at various frequencies. The flatband potential increased with anionic surfactant and decreased with cationic surfactant, although the value changed depending upon the measured frequency. The dependence was also reported by Ronga et al. [3]. The flatband potentials would thus show the qualitative tendency: the anionic surfactant led to the largest band bending in the space charge layer at the same potential and the smallest bending arose from the cationic one. The capacity measurements were carried out for various conditions; we obtained the same qualitative tendency. Linearity was not observed at potentials less negative than − 0.6 V versus RE. The capacity decreased with increasing potential in the solution without any surfactant. The inverse square of the capacity had a peak at − 0.4 V versus RE, as seen in Fig. 3, and then diminished at less negative potentials. However, the solutions containing a surfactant did not show the peak. The inverse square of the capacity reduced steeply with increasing potential.
3.3. Infrared spectra of porous silicon layer The large surface area of porous silicon enables investigation of the surface by infrared spectroscopy. Thus the passivation of a porous silicon surface has been studied extensively [13– 15]. The peak locations and the assignments are as follows: 630 cm − 1 due to SiH bending; 680 cm − 1 due to SiH2 wagging; 850 cm − 1 due to SiOSi symmetric stretching and/or OSiH deformation; 910 cm − 1 due to SiH2 scissors bending; 1100 cm − 1 due to SiOSi antisymmetric stretching; 2090 cm − 1 due to SiH stretching; 2100 cm − 1 due to SiH2 stretching and (SiH)2 stretching; 2140 cm − 1 due to SiH3 stretching; 2190 cm − 1 due to SiH stretching in O2SiH2; 2250 cm − 1 due to SiH stretching in O3SiH. Fig. 4 shows the infrared spectra of porous silicon layers obtained in various solutions. All spectra had a strong absorption at 620– 680 cm − 1, a sharp peak at 910 cm − 1, and a triplet at 2090– 2140 cm − 1. These absorption are due to SiHx species. No absorption appeared around 1100 cm − 1 and at 2200 cm − 1. These results meant that the porous silicon layer did not oxidize. A small peak could be seen at 810 cm − 1 due to SiFx for the porous silicon samples prepared in the solutions with no and anionic surfactants. We do not discuss this absorption further. The absorption intensities varied with the surfactant used. The intensities were comparable for porous silicon samples prepared in the solutions with no and anionic surfactants. The nonionic surfactant made the intensities weaker. The weakest absorptions were obtained in the solution containing the cationic surfac-
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tant. Furthermore, the cationic surfactant changed the shapes of the absorptions. In particular, the peak at 680 cm − 1 showed a stronger absorbance than that at 630 cm − 1, suggesting that the ratio of SiH2 to SiH was larger.
4. Discussion
4.1. Rate-determining step during porous silicon formation In general, the surfactant can have two effects on the silicon–liquid interface. One is the change of the Helmholtz layer due to the adsorption of the surfactant. The other is the modification of the space charge layer, which results from the adsorption of the charged particle. The voltammogram of p+-type silicon was not affected by the surfactant, whereas the surfactant showed a drastic effect on that of p-type silicon. In comparison with p+-type silicon, p-type silicon has a small capacity and thus little charge in the space charge layer. The space charge layer of p-type silicon is thus expected to be more strongly influenced by the surfactant than that of p+-type silicon. On the other hand, the Helmholtz-layer control of the silicon dissolution should result in a stronger, or at least similar, effect of the surfactant on p+-type silicon. This assumption is inconsistent with this result. Therefore, the space charge layer of the semiconductor plays a more important role in the dissolution, although the Helmholtz layer may have a small effect on p+-type silicon. The capacity data of p-type silicon further support the reaction control in the space charge layer. The flatband potential shifted more positive as the electrode potential became more positive, although they had only qualitative meaning in this system, as described in Section 3. For example, the anionic surfactant results in the most positive flatband potential and the largest overpotential. The space charge layer must be modified by the surfactants, which affects the polarization behavior. The anionic surfactant causes a larger hole depletion in the space charge layer, resulting in a more difficult dissolution. In contrast, a smaller overpotential is seen by the addition of the cationic surfactant, owing to an increased hole supply. The OCP would also be consistent with the change of the space charge layer by the surfactant. For example, the cationic surfactant may decrease the band bending, resulting in the lowest OCP and flatband potential. The above model cannot adequately explain the features of the polarization curves. The slopes deviated from 60 mV decade − 1, which was true for both p- and p+-silicon electrodes. A possible explanation is as follows. It becomes increasingly difficult for the surfactant to adsorb on the surface with increasing electrode po-
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tential, due to the enhanced dissolution of silicon. As a result, the surfactant should show a weaker effect on the space charge layer. This is supported by the result that the higher current density reduced the difference in the overpotential between the solutions. This effect would be stronger for p-type silicon, since the space charge layer is modified more by the surfactant, as already described. We should also consider the dependence of the adsorption state on the electrode potential. The Helmholtz layer may thus change in the structure with the electrode potential. These effects would cause the deviation of the slope from 60 mV decade − 1. The solution without any surfactant showed a peculiar behavior in the capacity. A peak was observed at − 0.4 V versus RE in Fig. 3. A similar peak was also presented by Ronga et al. [3]. The surfactants deleted the peak. It is clear that the condition of the silicon surface contributes to the capacity in this region. It should thus be noted that the surface conditions change by the addition of a surfactant.
4.2. Properties of porous silicon The surfactant is expected to affect the properties of porous silicon. The properties of porous silicon prepared with surfactants were reported by Sotgiu et al. [7]. The photoluminescence spectra of n+-type porous silicon decreased in intensity when the concentration of sodium dodecyl sulfate was greater than the critical micellar concentration; whereas hexadecyltrimethylammonium bromide brought about the maximum in the photoluminescence intensity and the peak shifted to a shorter wavelength. Raman scattering revealed that n+type porous silicon consisted of microcrystallites of size 2 – 4 nm and that the microstructure was not sensitive to the types and concentrations of the surfactant. These properties seem to be related to the different chemical structure. We also investigated the surface properties of porous silicon using infrared spectroscopy and found that surfactants affected the infrared spectra of porous silicon drastically. The cationic surfactant decreased the absorbance, whereas the anionic surfactant had a negligible effect on the absorption bands. The relative integrated absorbance due to SiHx correlates with potential at a current density of 2 mA cm − 2, as seen in Fig. 5. In the ordinate, the absorption of porous silicon from no surfactant solution is considered as unity. The integrated absorbance increases with the overpotential, except for the porous silicon prepared from the no surfactant solution. The exception would be because the contact with the solution by the surfactant facilitates the chemical dissolution of porous silicon, thus decreasing the surface area of porous silicon. The correlation would arise from the dissolution during porous silicon formation. We measured the weight loss after
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porous silicon formation and the effect of surfactants. Notable modifications were seen on p-type silicon electrodes. The polarization curves were shifted in more positive and more negative directions by the addition of anionic and cationic surfactants respectively. On the other hand, p+-type silicon was not strongly affected. Therefore, the anodization current is controlled mostly by the space charge layer in the semiconductor. The capacity and flatband potential also support this idea. A surfactant can change the surface properties of porous silicon. The shapes and intensities of infrared absorption bands depend upon the electrolyte composition. The chemical dissolution of porous silicon is facilitated by the cationic surfactant, resulting in a small absorbance. The integrated absorbance correlates with the electrode potential. Fig. 5. Correlation of relative integrated absorbance due to SiHx with potential at a current density of 2 mA cm − 2. Open circle corresponds to the porous silicon from no surfactant solution and the integrated absorbance is set unity. −2
the anodization at 2 mA cm for 8 h. The losses were 8.0 mg cm − 2, 9.4 mg cm − 2, 8.3 mg cm − 2, and 10.3 mg cm − 2 in the solutions with no, nonionic, anionic, and cationic surfactants respectively. The numbers of electrons per dissolved silicon were estimated to be 2.1, 1.8, 2.0, and 1.6 respectively. A value of two corresponds to porous silicon formation [1], and chemical dissolution of the porous silicon layer formed is expected from a value below two. The porous silicon dissolves in the nonionic or cationic surfactant solutions and the dissolution is more rapid in the cationic surfactant solution. Furthermore, the cationic surfactant also changes the shapes of the absorption bands around 620–680 and 2090– 2140 cm − 1. The properties of porous silicon thus vary with the surfactant used. We briefly mention the chemical dissolution mechanism of the porous silicon layer. The mechanism of chemical etching must be the same for porous silicon as for a silicon wafer, since silicon atoms dissolve chemically in both cases. The porous silicon is thus expected to dissolve chemically by the following process. The ligand exchange reactions occur first: SiHSiF and/or SiHSiOH [16]. This reaction involves the formation of molecular hydrogen. The SiOH groups are attacked by HF and change into SiF groups. The etching proceeds through the reaction of HF with SiSi backbonds polarized by SiF [17]. Then the SiF surface group is removed and the new SiH group is formed on the porous silicon surface [17]. The nonionic or cationic surfactant accelerates the above reaction and reduces the surface area of porous silicon.
5. Conclusion We have investigated the polarization behavior during
Acknowledgements We are grateful to Dr M. Mabuchi and Dr P. Allongue for useful discussions. This work was partly supported by a grant-in-aid for scientific research on priority area of Electrochemistry of Ordered Interfaces from the Ministry of Education, Science, Sports and Culture, Japan.
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