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Applications of a novel method for determining the rate of production of photochemical porous silicon
Materials Science and Engineering B69 – 70 (2000) 132 – 135 www.elsevier.com/locate/mseb
Applications of a novel method for determining the rate of production of photochemical porous silicon L. Koker, K.W. Kolasinski * School of Chemistry, Uni6ersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Abstract The rate of formation of photochemical porous silicon is measured in situ by studying the rate of increase in the radius of the circular interference patterns contained in the reflected laser beam. This technique is used to study the effects on etch rate of: (a) the composition and concentration of the etchant; and (b) the wavelength and flux of the laser irradiation. The technique is applicable over a wide range of reaction conditions and rates. The etch rate is found to be linearly dependent on the formal concentration of aqueous hydrogen fluoride (HF(aq)). The effect of increasing the flux of the incident radiation on the rate is more complex. In 48% HF(aq), the rate of etching increases up to a certain flux, the value of which is dependent on wavelength, and then exhibits saturation. In 25% HF the rate increases linearly with flux. It appears that the transport of reactive chemical species to the interface is responsible for the saturation of the etch rate. It is demonstrated that the method introduced here may be used in rate studies to investigate other factors that influence the etch rate and can be used to help elucidate the mechanism of etching. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Laser methods; Hydrogen fluoride; Reaction mechanisms; Reaction kinetics; Etching
1. Introduction The interest in porous silicon (por-Si) has increased greatly over the last decade, mainly due to its photoluminescence (PL) properties and the potential applications, which arise from these. There are, however, many other possible uses including chemical sensors [1], micromachining [2], microscale optical components, mirrors, filters and diffraction gratings [3 – 5], and as a biocompatible material [6]. Por-Si is usually prepared by anodization in aqueous hydrogen fluoride (HF(aq)). It is also possible to employ a photochemical technique, which does not involve an externally applied bias [4 – 14]. We have used a HeNe laser (l =633 nm) to form por-Si in aqueous hydrogen fluoride (HF(aq)). We have also reported that the reflected laser beam contains concentric circular interference patterns, which increase in radius as etching proceeds [15]. In the absence of a film, the radius of the pattern, r, is linearly proportional to the depth of the interface at constant laser beam waist. In the presence * Corresponding author. Tel.: +44-121-414-4418; fax: +44-121414-4426. E-mail address: [email protected] (K.W. Kolasinski)
of the film, there is a weak dependence of r on porosity and the depth of the upper film interface. Nonetheless, for the typical film thicknesses used in this study, to a good approximation the depth of the lower interface is linearly proportional to r. Even for a porosity change from 0.6 to 0.9, we expect only a 10% change in the radius leading to a maximum systematic error of 98% in the rate — a change that is insignificant compared to the observed rate changes. We have recently determined [16] the porosity of films made under similar conditions and have found a mean value of P= 0.83 90.08. Therefore, measurements of r provide an in situ method for measuring the rate of the por-Si formation reaction (etch rate) and it is this technique that we wish to illustrate here. The effects of (a) composition and concentration of etchants; and (b) irradiation wavelength and photon fluxes on the etch rate are investigated. The observed dependence of etch rates on concentration, wavelength and flux are briefly discussed with reference to concentration of active fluoride species, diffusion of reactants and products close to the surface and carrier density. We confirm a dependence of the rate on both laser intensity and wavelength but demonstrate, in addition, that the rate exhibits saturation effects at moderate laser intensities, which are insufficient to cause a significant temperature rise.
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 4 1 0 - 9
L. Koker, K.W. Kolasinski / Materials Science and Engineering B69–70 (2000) 132–135
2. Experimental details The method for both investigations is essentially the same. The por-Si is formed by irradiation of n-type Si(111), typically 4.5 – 6.4 V cm, with a laser while immersed in etchant. If the etchant is not pure HF(aq), the silicon is dipped in HF(aq) to remove the native oxide and rinsed in deionized water. The reflected laser beam is directed onto a screen at a fixed distance from the silicon and the time taken for the circular interference pattern to grow to a particular target size is recorded. This corresponds to a certain maximum depth of etching. It is not necessary to know the depth, since this has been shown to be constant when pattern radius and distance from silicon are fixed, but the depth for investigation: (a) is estimated to be 1.1 mm and for investigation; and (b) has been measured as 1.7 mm. The integral etch rate is taken as the reciprocal of this time and is reported in units of arbitrary length per unit time.
2.1. In6estigation of different concentrations and etchants The silicon is irradiated with a 15 mW HeNe laser (l =633 nm, 7 W cm − 2) while immersed in etchant. The technique described above is used to study the etch rate in pure HF(aq) (concentration range 0 –46 mol kg − 1) and a mixture of HF(aq) (0 – 6 mol kg − 1) and HCl (1.0 mol kg − 1). The distance from the silicon crystal to the screen is 1135 mm, and the target radius is 10 mm.
Fig. 1. Dependence of etch rate on HF concentration in pure HF(aq).
133
2.2. In6estigation of irradiation wa6elength and photon flux Three different wavelengths, 685, 633 and 532 nm (maximum power densities 15.6, 4.0, 5.7 W cm − 2, respectively) are used to irradiate the silicon in 48% w/w (46.2 mol kg − 1) HF(aq). With the silicon in 25% w/w (17.4 mol kg − 1) HF(aq), 633 nm light is used. The power is varied using neutral density filters and measured with a power meter (Thorlabs, S20MM). The distance from the silicon crystal to the screen is 1200 mm and the target radius is 12 mm.
3. Results and discussion Por-Si is formed at all concentrations and with all wavelengths used. Upon removal from the etchant the irradiated area is visible as a dark spot, having the same diameter as the incident beam, within which concentric colored circles may be observed as reported by Noguchi and Suemune [7]. The spot is photoluminescent, showing bright red/orange PL on excitation with ultraviolet light in air. This is consistent with previously reports [17].
3.1. Different etchants As the concentration of pure HF(aq) is increased the etch rate increases as shown in Fig. 1. The increase in rate is linear over a wide range of molalities. A linear increase in rate is also observed with the HF(aq)/HCl mixture (Fig. 2) Examination of the equilibrium constants for the equilibria which exist in pure HF(aq) below 6 mol kg − 1 [18] shows that not only is the [F−] very low, it decreases with formal HF(aq) concentration. On the other hand [HF] (and [HF− 2 ]) are one to three orders of magnitude higher and increase with formal HF(aq) concentration. Hence although F− may be an active species in the etching of silicon, it is not involved in the rate determining step, but HF and HF− 2 may be. At concentrations higher than 1–6 mol kg − 1 different fluoride species, such as (HF)2 and H2F− 3 are present and may also have to be considered in the reaction kinetics. HCl does not form species such as HCl and HCl− in aqueous solution. This leads to a natural 2 explanation as to why photochemical or electrochemical production of por-Si cannot be carried out in HCl(aq).
3.2. Irradiation wa6elength and photon flux
Fig. 2. Dependence of etch rate on HF concentration in HF(aq)/HCl.
Two different modes of variation of rate with photon flux are observed with the two different [HF]. At higher concentration, as the photon flux is increased, for all
134
L. Koker, K.W. Kolasinski / Materials Science and Engineering B69–70 (2000) 132–135
changes to the effects of photochemical etching. They observe a sublinear dependence (and eventually saturation) of the rate of change on the laser irradiance for irradiances on the order of 0.1 W cm − 2 or greater. They suggest that at these irradiances, the number density of excited carriers is saturating as the result of Auger recombination. At first sight, the data in Fig. 3 would appear to be consistent with the proposition of Ozanam et al. [19]. However at lower concentration (25% HF), the rate increases linearly up to the maximum flux available, with no suggestion of a saturation region (Fig. 4). As would be expected from investigation (a) above, the rate is lower in less concentrated HF(aq) for any given laser flux. This suggests that the rate limiting process is diffusion of reactive species to, or products away from, the surface, and not the transport of holes to the surface. A series of etch rate studies to determine the relationship between concentration and the onset of a ‘saturation’ effect would give more information and will be carried out using our technique.
4. Conclusion
Fig. 3. (a – c) Dependence of etch rate on photon flux at three wavelengths: (a) 532 nm; (b) 633 nm; and (c) 685 nm in 48% HF(aq).
Applications of a novel technique for determining the photochemical etch rate of silicon based on the linear relationship between the radius of the interference pattern in the reflected laser beam and the maximum depth of the por-Si/crystalline silicon interface are described. The technique is used successfully to measure: (a) the rate of the etching reaction in the presence of reactive species at diverse concentrations; and (b) different irradiation wavelengths at various photon fluxes. From these measurements we have obtained insights into the etching reaction involved in por-Si formation. We venture that this novel and rapid in situ method for rate determination will prove a useful tool in these studies.
Acknowledgements Fig. 4. Dependence of etch rate on photon flux at 633 nm in 25% HF(aq).
three wavelengths, the etch rate increases up to a certain point after which little increase is observed (Fig. 2a–c). The photon flux has been corrected for reflectance at the different wavelengths. For the wavelengths used we find the reflectivity in the presence of the film to be roughly 90% of the reflectivity of crystalline Si. Note that the rates observed for the three wavelengths are not directly comparable because the laser beam waists were not the same for the three lasers used. Changes of PL spectra under laser illumination have been observed by Ozanam et al. [19]. They ascribed the
We acknowledge the financial support of the Royal Society and the provision of a studentship (L. Koker) by the EPSRC.
References [1] V.S.-Y. Lin, K. Motesharei, K.-P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840. [2] W. Lang, Mater. Sci. Eng. R 17 (1996) 1. [3] S. Frohnhoff, M.G. Berger, Adv. Mater. 6 (1994) 963. [4] A.V. Alexeev-Popov, S.A. Gevelyuk, Y.O. Roizin, D.P. Savin, S.A. Kuchinsky, Solid State Commun. 97 (1996) 591. [5] G. Le´rondel, R. Romestain, J.C. Vial, M. Tho¨nissen, Appl. Phys. Lett. 71 (1997) 196.
L. Koker, K.W. Kolasinski / Materials Science and Engineering B69–70 (2000) 132–135 [6] L.T. Canham, C.L. Reeves, D.J. Wallis, J.P. Newey, M.R. Houlton, G.J. Sapsford, R.E. Godfrey, A. Loni, A.J. Simons, T.I. Cox, M.C.L. Ward, Mat. Res. Soc. Symp. Proc. 452 (1997) 579. [7] N. Noguchi, I. Suemune, Appl. Phys. Lett. 62 (1993) 1429. [8] Z. Zhang, M.M. Lerner, T. Alekel III, D.A. Keszler, J. Electrochem. Soc. 140 (1993) L97. [9] K.W. Cheah, C.H. Choy, Solid State Commun. 91 (1994) 795. [10] C.H. Choy, K.W. Cheah, Appl. Phys. A Mater. Sci. Proc. 61 (1995) 45. [11] O.K. Andersen, T. Frello, E. Veje, J. Appl. Phys. 78 (1995) 6189. [12] W.B. Dubbelday, D.M. Szaflarski, R.L. Shimabukuro, S.D. Russell, M.J. Sailor, Appl. Phys. Lett. 62 (1993) 1694.
.
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[13] L.A. Jones, O8 . Yu¨kseker, D.F. Thomas, J. Vac. Sci. Technol. A 14 (1996) 1505. [14] D. Dimova-Malinovska, M. Tzolov, N. Malinowski, T. Marinova, V. Krastev, Appl. Surf. Sci. 96-98 (1996) 457. [15] L. Koker, K.W. Kolasinski, J. Appl. Phys. 86 (1999) 1800. [16] C. Lee, L. Koker, K.W. Kolasinski, Appl. Phys. A Mater. Sci. Process., submitted for publication. [17] A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909. [18] R. Braddy, P.T. McTigue, B. Verity, J. Fluorine Chem. 66 (1994) 63. [19] F. Ozanam, J.-N. Chazalviel, R.B. Wehrspohn, Thin Solid Films 297 (1997) 53.
Applications of a novel method for determining the rate of production of photochemical porous silicon L. Koker, K.W. Kolasinski * School of Chemistry, Uni6ersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Abstract The rate of formation of photochemical porous silicon is measured in situ by studying the rate of increase in the radius of the circular interference patterns contained in the reflected laser beam. This technique is used to study the effects on etch rate of: (a) the composition and concentration of the etchant; and (b) the wavelength and flux of the laser irradiation. The technique is applicable over a wide range of reaction conditions and rates. The etch rate is found to be linearly dependent on the formal concentration of aqueous hydrogen fluoride (HF(aq)). The effect of increasing the flux of the incident radiation on the rate is more complex. In 48% HF(aq), the rate of etching increases up to a certain flux, the value of which is dependent on wavelength, and then exhibits saturation. In 25% HF the rate increases linearly with flux. It appears that the transport of reactive chemical species to the interface is responsible for the saturation of the etch rate. It is demonstrated that the method introduced here may be used in rate studies to investigate other factors that influence the etch rate and can be used to help elucidate the mechanism of etching. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Laser methods; Hydrogen fluoride; Reaction mechanisms; Reaction kinetics; Etching
1. Introduction The interest in porous silicon (por-Si) has increased greatly over the last decade, mainly due to its photoluminescence (PL) properties and the potential applications, which arise from these. There are, however, many other possible uses including chemical sensors [1], micromachining [2], microscale optical components, mirrors, filters and diffraction gratings [3 – 5], and as a biocompatible material [6]. Por-Si is usually prepared by anodization in aqueous hydrogen fluoride (HF(aq)). It is also possible to employ a photochemical technique, which does not involve an externally applied bias [4 – 14]. We have used a HeNe laser (l =633 nm) to form por-Si in aqueous hydrogen fluoride (HF(aq)). We have also reported that the reflected laser beam contains concentric circular interference patterns, which increase in radius as etching proceeds [15]. In the absence of a film, the radius of the pattern, r, is linearly proportional to the depth of the interface at constant laser beam waist. In the presence * Corresponding author. Tel.: +44-121-414-4418; fax: +44-121414-4426. E-mail address: [email protected] (K.W. Kolasinski)
of the film, there is a weak dependence of r on porosity and the depth of the upper film interface. Nonetheless, for the typical film thicknesses used in this study, to a good approximation the depth of the lower interface is linearly proportional to r. Even for a porosity change from 0.6 to 0.9, we expect only a 10% change in the radius leading to a maximum systematic error of 98% in the rate — a change that is insignificant compared to the observed rate changes. We have recently determined [16] the porosity of films made under similar conditions and have found a mean value of P= 0.83 90.08. Therefore, measurements of r provide an in situ method for measuring the rate of the por-Si formation reaction (etch rate) and it is this technique that we wish to illustrate here. The effects of (a) composition and concentration of etchants; and (b) irradiation wavelength and photon fluxes on the etch rate are investigated. The observed dependence of etch rates on concentration, wavelength and flux are briefly discussed with reference to concentration of active fluoride species, diffusion of reactants and products close to the surface and carrier density. We confirm a dependence of the rate on both laser intensity and wavelength but demonstrate, in addition, that the rate exhibits saturation effects at moderate laser intensities, which are insufficient to cause a significant temperature rise.
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 4 1 0 - 9
L. Koker, K.W. Kolasinski / Materials Science and Engineering B69–70 (2000) 132–135
2. Experimental details The method for both investigations is essentially the same. The por-Si is formed by irradiation of n-type Si(111), typically 4.5 – 6.4 V cm, with a laser while immersed in etchant. If the etchant is not pure HF(aq), the silicon is dipped in HF(aq) to remove the native oxide and rinsed in deionized water. The reflected laser beam is directed onto a screen at a fixed distance from the silicon and the time taken for the circular interference pattern to grow to a particular target size is recorded. This corresponds to a certain maximum depth of etching. It is not necessary to know the depth, since this has been shown to be constant when pattern radius and distance from silicon are fixed, but the depth for investigation: (a) is estimated to be 1.1 mm and for investigation; and (b) has been measured as 1.7 mm. The integral etch rate is taken as the reciprocal of this time and is reported in units of arbitrary length per unit time.
2.1. In6estigation of different concentrations and etchants The silicon is irradiated with a 15 mW HeNe laser (l =633 nm, 7 W cm − 2) while immersed in etchant. The technique described above is used to study the etch rate in pure HF(aq) (concentration range 0 –46 mol kg − 1) and a mixture of HF(aq) (0 – 6 mol kg − 1) and HCl (1.0 mol kg − 1). The distance from the silicon crystal to the screen is 1135 mm, and the target radius is 10 mm.
Fig. 1. Dependence of etch rate on HF concentration in pure HF(aq).
133
2.2. In6estigation of irradiation wa6elength and photon flux Three different wavelengths, 685, 633 and 532 nm (maximum power densities 15.6, 4.0, 5.7 W cm − 2, respectively) are used to irradiate the silicon in 48% w/w (46.2 mol kg − 1) HF(aq). With the silicon in 25% w/w (17.4 mol kg − 1) HF(aq), 633 nm light is used. The power is varied using neutral density filters and measured with a power meter (Thorlabs, S20MM). The distance from the silicon crystal to the screen is 1200 mm and the target radius is 12 mm.
3. Results and discussion Por-Si is formed at all concentrations and with all wavelengths used. Upon removal from the etchant the irradiated area is visible as a dark spot, having the same diameter as the incident beam, within which concentric colored circles may be observed as reported by Noguchi and Suemune [7]. The spot is photoluminescent, showing bright red/orange PL on excitation with ultraviolet light in air. This is consistent with previously reports [17].
3.1. Different etchants As the concentration of pure HF(aq) is increased the etch rate increases as shown in Fig. 1. The increase in rate is linear over a wide range of molalities. A linear increase in rate is also observed with the HF(aq)/HCl mixture (Fig. 2) Examination of the equilibrium constants for the equilibria which exist in pure HF(aq) below 6 mol kg − 1 [18] shows that not only is the [F−] very low, it decreases with formal HF(aq) concentration. On the other hand [HF] (and [HF− 2 ]) are one to three orders of magnitude higher and increase with formal HF(aq) concentration. Hence although F− may be an active species in the etching of silicon, it is not involved in the rate determining step, but HF and HF− 2 may be. At concentrations higher than 1–6 mol kg − 1 different fluoride species, such as (HF)2 and H2F− 3 are present and may also have to be considered in the reaction kinetics. HCl does not form species such as HCl and HCl− in aqueous solution. This leads to a natural 2 explanation as to why photochemical or electrochemical production of por-Si cannot be carried out in HCl(aq).
3.2. Irradiation wa6elength and photon flux
Fig. 2. Dependence of etch rate on HF concentration in HF(aq)/HCl.
Two different modes of variation of rate with photon flux are observed with the two different [HF]. At higher concentration, as the photon flux is increased, for all
134
L. Koker, K.W. Kolasinski / Materials Science and Engineering B69–70 (2000) 132–135
changes to the effects of photochemical etching. They observe a sublinear dependence (and eventually saturation) of the rate of change on the laser irradiance for irradiances on the order of 0.1 W cm − 2 or greater. They suggest that at these irradiances, the number density of excited carriers is saturating as the result of Auger recombination. At first sight, the data in Fig. 3 would appear to be consistent with the proposition of Ozanam et al. [19]. However at lower concentration (25% HF), the rate increases linearly up to the maximum flux available, with no suggestion of a saturation region (Fig. 4). As would be expected from investigation (a) above, the rate is lower in less concentrated HF(aq) for any given laser flux. This suggests that the rate limiting process is diffusion of reactive species to, or products away from, the surface, and not the transport of holes to the surface. A series of etch rate studies to determine the relationship between concentration and the onset of a ‘saturation’ effect would give more information and will be carried out using our technique.
4. Conclusion
Fig. 3. (a – c) Dependence of etch rate on photon flux at three wavelengths: (a) 532 nm; (b) 633 nm; and (c) 685 nm in 48% HF(aq).
Applications of a novel technique for determining the photochemical etch rate of silicon based on the linear relationship between the radius of the interference pattern in the reflected laser beam and the maximum depth of the por-Si/crystalline silicon interface are described. The technique is used successfully to measure: (a) the rate of the etching reaction in the presence of reactive species at diverse concentrations; and (b) different irradiation wavelengths at various photon fluxes. From these measurements we have obtained insights into the etching reaction involved in por-Si formation. We venture that this novel and rapid in situ method for rate determination will prove a useful tool in these studies.
Acknowledgements Fig. 4. Dependence of etch rate on photon flux at 633 nm in 25% HF(aq).
three wavelengths, the etch rate increases up to a certain point after which little increase is observed (Fig. 2a–c). The photon flux has been corrected for reflectance at the different wavelengths. For the wavelengths used we find the reflectivity in the presence of the film to be roughly 90% of the reflectivity of crystalline Si. Note that the rates observed for the three wavelengths are not directly comparable because the laser beam waists were not the same for the three lasers used. Changes of PL spectra under laser illumination have been observed by Ozanam et al. [19]. They ascribed the
We acknowledge the financial support of the Royal Society and the provision of a studentship (L. Koker) by the EPSRC.
References [1] V.S.-Y. Lin, K. Motesharei, K.-P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840. [2] W. Lang, Mater. Sci. Eng. R 17 (1996) 1. [3] S. Frohnhoff, M.G. Berger, Adv. Mater. 6 (1994) 963. [4] A.V. Alexeev-Popov, S.A. Gevelyuk, Y.O. Roizin, D.P. Savin, S.A. Kuchinsky, Solid State Commun. 97 (1996) 591. [5] G. Le´rondel, R. Romestain, J.C. Vial, M. Tho¨nissen, Appl. Phys. Lett. 71 (1997) 196.
L. Koker, K.W. Kolasinski / Materials Science and Engineering B69–70 (2000) 132–135 [6] L.T. Canham, C.L. Reeves, D.J. Wallis, J.P. Newey, M.R. Houlton, G.J. Sapsford, R.E. Godfrey, A. Loni, A.J. Simons, T.I. Cox, M.C.L. Ward, Mat. Res. Soc. Symp. Proc. 452 (1997) 579. [7] N. Noguchi, I. Suemune, Appl. Phys. Lett. 62 (1993) 1429. [8] Z. Zhang, M.M. Lerner, T. Alekel III, D.A. Keszler, J. Electrochem. Soc. 140 (1993) L97. [9] K.W. Cheah, C.H. Choy, Solid State Commun. 91 (1994) 795. [10] C.H. Choy, K.W. Cheah, Appl. Phys. A Mater. Sci. Proc. 61 (1995) 45. [11] O.K. Andersen, T. Frello, E. Veje, J. Appl. Phys. 78 (1995) 6189. [12] W.B. Dubbelday, D.M. Szaflarski, R.L. Shimabukuro, S.D. Russell, M.J. Sailor, Appl. Phys. Lett. 62 (1993) 1694.
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135
[13] L.A. Jones, O8 . Yu¨kseker, D.F. Thomas, J. Vac. Sci. Technol. A 14 (1996) 1505. [14] D. Dimova-Malinovska, M. Tzolov, N. Malinowski, T. Marinova, V. Krastev, Appl. Surf. Sci. 96-98 (1996) 457. [15] L. Koker, K.W. Kolasinski, J. Appl. Phys. 86 (1999) 1800. [16] C. Lee, L. Koker, K.W. Kolasinski, Appl. Phys. A Mater. Sci. Process., submitted for publication. [17] A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909. [18] R. Braddy, P.T. McTigue, B. Verity, J. Fluorine Chem. 66 (1994) 63. [19] F. Ozanam, J.-N. Chazalviel, R.B. Wehrspohn, Thin Solid Films 297 (1997) 53.