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The effect of etching temperature on the photoluminescence emitted from, and the morphology of, p-type porous silicon
Electrochimica Acta 48 (2003) 623 /630 www.elsevier.com/locate/electacta
The effect of etching temperature on the photoluminescence emitted from, and the morphology of, p-type porous silicon D.J. Blackwood *, Y. Zhang Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, 119260 Singapore Received 23 October 2002
Abstract The temperature at which silicon is electrochemically etched has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature increased both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The fact that previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Porous silicon; Photoluminescence; Viscosity; Temperature
1. Introduction It has been known since the fifties that porous silicon can be produced by the treatment of Si wafers in hydrofluoric acid solutions [1 /3]. However, it was not until 1990, when Canham demonstrated the observation of bright red photoluminescence from the surface of electrochemically etched Si wafer [4], that intense interest focused on the material [5]. Much of the current work has been focus on methods to induce different colours from the porous silicon [6,7]. Although there have been some previous investigations into the influence of the etching temperature on the quality of the porous silicon [8,9] some of the conclusions from these have been contradictory and none have reported variations in the wavelength of the emitted light. For example, Ono et al.[8] found that
* Corresponding author. Fax: /65-776-3604 E-mail address: [email protected] (D.J. Blackwood).
decreasing the etching temperature resulted in a decrease in the intensity of the photoluminescence and a red-shift in its peak wavelength, whilst Setzu et al.[9] found the opposite trend for the intensity and peak wavelength to be independent of temperature. In this article, the influence of the etching temperature on the photoluminescence obtained from, and the porosity of, porous silicon has been investigated independent of other parameters. However, the observations made were not in agreement with either that of Setzu et al. or Ono et al, with a decrease in temperature resulting in blue-shifting of the peak wavelength of the photoluminescence. It is suggested that the differences between the observations made by the various authors is due to changes in the composition of etching solution, in particular the amount of ethanol used. The ethanol content effects both the solution’s ability to wet the hydrophobic porous silicon surface and the viscosity of the diffusion layer adjacent to the surface, which is known to control the extent of levelling in electrochemical polishing processes [10,11].
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2. Experimental Specimens were fabricated from a p-type (100) silicon wafer with a resistivity of 5 V cm. Gallium /indium eutectic was used to make ohmic contacts to the backside of the wafers whilst acid-resistant epoxy resin was applied to reduce the exposed surface area on the front face to 10 mm /8 mm. The specimens were weighed (without the epoxy resin) prior to and after the etching process. The porous silicon was obtained by electrochemical etching in a solution of 1:1:2 HF:H2O:ethanol held in a two-electrode PTFE closed-cell, which was in turn placed inside a temperature controlled bath. Etching was performed at a constant current of 20 mA cm 2 applied for a period of 20 min, with the surrounding bath being held at either of /10, 0, 25 or 378C. After etching the porous silicon films were rinsed with ethanol and dried with pentane, which helps reduce cracking [12]. The presence of porous silicon nanocrystallites (or at least structures with nanosized dimensions) was confirmed immediately after etching by using a handheld 4 W UV lamp to induce photoluminescence. The high level of porosity in the samples prepared in lower temperature caused these to have a very weak mechanical resistance, such that these had to be raised to room temperature slowly to prevent the films cracking due to thermal expansion. This was achieved by washing the specimens with, and then transferring these into a beaker of, ethanol that was at the same temperature as the etching solution, the beaker was then left to stand on the bench until it had reached room temperature. It was necessary to transfer the specimens to ethanol, rather than leaving these in the HF solution, to prevent any additional etching occurring during the warm-up period. No attempt was made to increase the rate of drying of the porous silicon layers as this was found to induce cracking. Low temperature films that were washed with room temperature ethanol not only cracked but were also found to emit red light rather than yellow light. The photoluminescence spectra were recorded via a Renishaw 2000 microRaman and photoluminescence system equipped with a 20 mW helium- /cadmium laser operating at 325 nm which was focused to a spot size of 1 mm diameter. The morphologies of the surfaces of the various specimens were examined under a Philips XL30 FEG scanning electron microscope (SEM) at an accelerating voltage of 10 keV and by a Shimadzu SPM9500J atomic force microscope (AFM) operated in the dynamic mode. The specimens were later cross-sectioned to allow the thickness of the porous silicon films to be measured by the SEM. The thickness of the samples was also determined by using a profilometer, as described previously by Setzu et al. [9]. Combining the thickness measurements from the profilometer with the weight
loss during the etching process yielded estimations for the film’s average porosity. Since the dimensions of the porous silicon structures were below the resolution of the SEM used, it was not possible to determine whether the observed photoluminescence was due to the presence of nanocrystals or columnar wire-like structures or other nanosized structures. However, the term nanocrystals has been used throughout this paper to be consistent with previous authors [13].
3. Results and discussions It was found that under stimulation from an UV light source the wavelength of the photoluminescence emitted from the porous silicon films, which was clearly visible to the naked eye in a dim environment, blue-shifted with decreasing etching temperature. With no visible photoluminescence, red light, orange light and yellow light being observed from films formed at 37, 25, 0 and / 108C, respectively. The absence of visible photoluminescence at 378C was expected by the authors, since it was the inability to form active porous silicon at this temperature (due to the failure of the laboratories airconditioning system) that first drew their attention to the possibility of temperature affects. Fig. 1 shows a series of normalized photoluminescence spectra, which confirmed the blue-shifting with decreased etching temperature. Although the wavelength dependence of the photoluminescence was found to be virtually independent of location sampled on any one specimen its intensity was found to vary. However, as the intensity appeared to vary randomly this was probably due to the
Fig. 1. Normalised photoluminescence spectra for porous silicon formed at different temperatures in a 1:1:2 solution of H2O:HF:C2H5OH. No luminescence observed at an etching temperature of 378C.
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surface roughness of the specimens scattering both the incoming and emitted light. Whilst testing the Quantum confinement theory, Wilson et al. [13] and Schuppler et al. [14] clearly demonstrated that the peak luminescence emission energy increases as the average particle size decreases. The observed blue shifting of the photoluminescence with decreasing etching temperature, observed in the present work, is therefore consistent with the formation of smaller nanocrystals. The influence of the temperature of etching on the morphology of the resulting porous silicon can be clearly seen in the SEM photographs (Fig. 2), with decreasing temperature causing a large increase in the number of pores but these had smaller diameters. Fig. 3 shows high magnification images of the tops of the silicon columns for specimens formed at /10 and 378C. Although not fully resolvable, structures of only a few tens of nanometres, possibly even less than 10 nm, can be seen on the /108C specimen, whereas for the 378C sample there appears to be no features less than about 500 nm. The trend of decreasing crystal size with etching temperature was also observed in the AFM images (Fig. 4). Although, as with the SEM, the nanocrystals
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could not be fully resolved, there is a clear decrease in feature size as the etching temperature decreases from 37 to /108C. The AFM images also suggest that the average surface roughness decreased with decreasing temperature. The physical weight loss per unit area from the specimens during the etching process increased as the temperature was decreased, from 1.18 mg cm 2 at 378C to 1.71 mg cm 2 at /108C. The expected weight loss from the application Faraday’s law to the 24 coulombs per unit area passed during the etching process is 1.74 g cm 2 assuming that the dissolved silicon species was in the /4 oxidation state (probably SF2 6 ). Therefore, the current efficiency at the lowest temperature is /98% whilst at the highest temperature it is only about 68%. Presumably the unaccounted for charge at the higher temperatures is due to formation of lower oxidation state silicon fluorides, or possibly oxides, that remain attached to the lattice. The average depths of the porous silicon layers, determined from the profilometer measurements over different areas of the specimens’ surfaces, did not show any consistent variation with etching temperature. In all cases an averaged depth of about 11 microns (standard deviation of 2 microns) was found. However, as can be
Fig. 2. SEM photographs of porous silicon films etched at temperatures of (a) /108C, (b) 08C, (c) 258C and (d) 378C.
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Fig. 3. High resolution SEM photographs of porous silicon films etched at temperatures of (a) /108C and (b) /378C.
seen from Fig. 5, this value is approximately an order of magnitude greater than the thickness determined from the specimen cross sections in the SEM. The most likely reason for this is that most of the outer surface of the porous silicon gradually dissolved in the electrolyte (either by electrochemical polishing or chemical dissolution) during the course of the experiment and thus the SEM only reveals the remaining thickness.
The fact that the outer layers of porous silicon were completely dissolved into the porous silicon somewhat invalidates the values of average porosities calculated on the basis of profilometer depth readings and weight lost measurements, however, this procedure was still preformed as this allowed comparisons with the work of previous authors. Fig. 6 shows that the average porosity increased as the etching temperature was lowered. The
Fig. 4. AFM images of porous silicon films etched at temperatures of (a) /108C, (b) 08C, (c) 258C and (d) 378C.
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Fig. 5. SEM photographs of cross sections of porous silicon films etched at temperatures of (a) /108C and (b) /378C.
Fig. 6. Dependence of the average porosity of the porous silicon films on the temperature of the etching solution.
general trend in the temperature dependence of the porosity of the porous silicon films produced in the present work was similar to that previously reported by Setzu et al. [9]. The following explanation is similar to that postulated by these authors and is based on the observation that the production of porous silicon from p-type wafers in hydrofluoric acid is limited to a low potential region. At higher potentials the anodisation current is sufficiently high to cause electrochemical polishing [3]. The point at which the switch from the production of porous silicon to electrochemical polishing occurs is defined at the critical current density. It has also been reported that for porous silicon the surface roughness varies monotonically with anodisation current, such that the further the etching current is below the critical value for polishing the rougher the interface [15], this phenomenon is also observed in the electropolishing of metals [10]. The suggestion that the surface roughness should increase as the etching temperature increases is consistent with the series of AFM images shown in Fig. 4. Overall the results presented in this paper suggest that although the surface gets smoother as the critical current density for electrochemical polishing is approached, the average size of the nanocrystals produced decreases. Intuitively this observation makes sense, as electrochemical polishing can be envisaged as
the point at which the size of the nanocrystals shrinks to zero. It is well known in the electrochemical polishing that smooth and bright finishes are only obtained if a viscous layer in which the diffusion of reactive species is restricted develops at the metals surface [10,16,17]. Chemical attack is then favoured on surface asperities, explaining the smoothening effect that leads to polishing. It has been suggested that such a viscous layer forms with silicon when the anodizing current is sufficiently high to cause electrochemical polishing [2,9]. As early as 1957 Tuner [2] showed that increasing the viscosity of the etching solution, either by changing its chemical composition or by lowering the temperature, could reduce the critical current density for the onset of electrochemical polishing. Consequently, since in the present work the same etching current density was applied at all temperatures, as the etching temperature was reduced so the critical current density will have approached the applied current giving rise to the observed smoother interfaces. Again a similar phenomenon occurs in the electropolishing of metals, where increased mass transport across the viscous layer due to higher temperatures (or agitation of the solution) results in rough, uneven surfaces [10,18]. The high level of porosity of the porous silicon film produced at low temperatures can thus be explained by the convergence of the applied current and the critical current density for electrochemical polishing brought about by the increased viscosity of the etching solution. The high current efficiencies seen for the etching process at low temperatures is also consistent with electrochemical polishing of the silicon. However, as yet this viscosity argument does not explain the temperature dependence of the dimensions of the nanocrystals. One of the best mechanisms for explaining the chemistry behind anodic etching of silicon is that proposed by Allongue et al. [18], which is schematically represented in Fig. 7. In this mechanism it is proposed that a silicon hydroxide species is first formed at the
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Fig. 7. Schematic representation of the chemistry of pore formation based on the mechanism proposed by Allongue et al. [18].
surface which then undergoes anion exchange with fluoride (step 3). In the event that this step is slow a hydroxy/oxide film forms such that electrochemical polishing ensues, whereas the anion exchange is rapid dissolution of the fluorinated silicon (step 4) results in the formation of porous silicon. Therefore whether electrochemical polishing or pore formation occurs depends on the relative rates of surface hydroxide formation (steps 1 and 2) to its replacement by fluoride (step 3). If it is assumed that the reaction between water and radical produced in step 1 is rapid then the rate of surface hydroxide produced is directly proportional to the flux of holes to the surface and thus for a given applied current density should be independent of temperature. On the other hand, step 3 is a purely chemical reaction, the rate which can be expected to have some Arrhenius type of temperature dependency. Also higher temperatures will favour the dissociation of HF such that more fluoride ions will be available to participate in step 3. Thus, increasing temperatures should favour surface fluoride formation and thus porous silicon formation. Likewise lower temperatures favour electrochemical polishing, which is consistent with the present results. However, the mechanism of Allongue et al. does not give a clear indication how temperature should influence the size of the resulting nanocrystals. There are several theories that have been presented to explain pore initiation and propagation in porous silicon, most of which can be broadly classified into one of three groups: surface instability, stationary pore growth or virtual passive films [19]. A good example of the surface instability theory is Kang and Jorne’s [20] growth model. This predicts that the dominate distance between pore tips should be proportional to the square root of temperature. Although qualitatively this is consistent with the formation of smaller nanocrystals at low temperatures observed in the present work, quantitatively the Kang and Jorne model only predicts about a 10% decrease over the
temperature range 310 /263 K. However, this model does predict that electrochemical polishing will occur if ionic diffusion in the electrolyte becomes the rate controlling step in the silicon dissolution reaction, which is more likely to occur at higher viscosities and thus at lower temperatures, i.e. it is consistent with argument of Setzu et al. [9] presented above. In models based on stationary pore growth, the original of which was that of Beale et al. [21], more random structures and smaller pore sizes are associated with low conductivity in nondegenerate silicon wafers. Although decrease temperatures would lower the extent of thermionic emission and thus favour small pore sizes the extent of temperature range in the present experiments (263 /310K) is not sufficient to have any significant effect and therefore cannot be used to explain the results found in the present work. Pore growth models based on virtual passive films [22,23] have apparently either not directly consider the influence of temperature or in the case of an early model developed by Parkhutik et al. [22] for porous aluminium specifically ruled out any temperature effects. Nevertheless temperature dependence could be envisaged in some of the later models as it would influence the magnitude of the electric field across the virtual passive film. Assuming the film to be a semiconductor means that higher fields will be required to pass a given current density as the temperature is lowered. Under these circumstances the models predict that this will lead to larger pore radii and eventually electrochemical polishing, however, it is not clear from these models whether this should be accompanied by a decrease in the dimensions of any nanocrystals. It therefore appears that none of the existing models fully explains the apparent decrease in the size of the nanocrystals formed with decreasing temperature observed in the present work. However, returning to the viscosity argument does yield a possible explanation. An increase in the viscosity within the porous silicon layer would impede the rate at which silicon ions could diffuse out into the bulk solution. Indeed, in electropolishing
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chemicals referred to as diffusion layer promoters are specifically added to form viscous layers adjacent to the surface in order control mass transport rates and thus obtain macroscopic levelling, with alcohols frequently being used for this purpose at low temperatures [10,24]. Thus, for a given rate of ion generation (i.e. for a given current density) the steady state concentration of silicon ions held within the porous layer should increase with a higher viscosity. From thermodynamic considerations the influence of the dimensions on the solubility (K) of crystal faces is expected to take the form [25]: K A(1(B=l))
(1)
where A and B are constants, representing the solubility of the bulk material and the capillary length scale for solubility shift respectively, whilst l is the perpendicular distance of the crystal face from the centroid. In the case of silicon nanoparticles l is very small, hence their solubility is significantly higher than that of the bulk and thus a much higher local concentration of silicon ions is required for equilibrium to be achieved. It therefore follows that the higher concentrations of silicon ions expected within the porous matrix at low temperatures, due to the increased viscosity, would help stabilize smaller nanocrystals. The blue-shifting in the photoluminescence spectra observed in the present work may thus in part be explained by the increase in viscosity, that accompanies a decrease in a solutions temperature, raising the steady-state concentration of silicon ions held within the porous layer and thereby reducing the solubility of the smaller nanocrystals. Furthermore, the solubility of the porous silicon (the A in Eq. (1)) is also likely to decrease with decreasing temperature and this will further stabilize the smallest nanocrystals. The major difference between the present work and that of previous authors, most notably Setzu et al. [9] and Ono et al. [8], is the observation of the temperature dependence of the energy of the light emitted during photoluminescence. Although Setzu et al. reported increased photoluminescence intensity at low temperatures, they did not observe any wavelength shifts in the spectra, which was predominantly in the near infra-red (700 /900 nm). However, these authors only related the temperature dependence of the photoluminescence spectra from rapidly grown films (166 mA cm 2). In addition Setzu et al. reported that some of their porous silicon films cracked, especially those of high porosity which is also where the smallest crystals are expected to be located, so it is possible that this was the reason these authors did not observe any temperature dependent blue-shifting of the photoluminescence. As mentioned above the current authors solved the problem of cracking by slowly bringing the films etched at low temperatures up to room temperature and allowing or the films to dry slowly.
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A further difference between the experiments conducted for the current work and those of Setzu et al. [9] is the higher ethanol content of the etching solution used in the present case (50% compared to 30%). The purpose of the ethanol being to promote wetting of the silicon substrate thereby aided the electrolyte to better penetrate the porous layer, as would the lower surface tension associated with the higher ethanol content [26]. Surface wetting has recently been demonstrated to be particularly important in the etching of p-type silicon [27]. Furthermore, the additional ethanol should have increased the viscosity of the etching solution, which, as was discussed above, is an important parameter in controlling the quality of porous silicon films. The maximum viscosity for solutions of ethanol in water occurs at about 50% ethanol [28]. Hence higher ethanol content could help produce smaller nanocrystals. Ono et al. [8] etched their porous silicon in 46% HF, apparently without any ethanol content. The red shifting in the peak wavelength of the photoluminescence with decreasing temperature reported by these authors was from red ( /730 nm) to the near infrared (/800 nm). This suggests the lack of ethanol in the etching solution prevented the wetting of the porous silicon to the extent that it was not possible to form the nanocrystals with sufficiently small dimensions the give photoluminescence in the shorter wavelength yellow region observed in the present work. The fact that the manner in which the etching temperature influences the dimensions of the silicon nanocrystals formed appears to be dependent on the composition clearly supports the notion that the nature of the electrochemical interface plays a critical role in the development of the porous matrix.
4. Conclusions The etching temperature has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature at which the silicon is etched increases both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed, a conclusion that was supported by high resolution SEM photographs and AFM imaging. However, care must be taking when producing porous silicon at low temperatures to avoid cracking, due to thermal expansion, in the resultant films. Without these precautions films formed at low temperature not only cracked, but also emitted red light rather than yellow light. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation
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for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore, it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The importance of the viscosity of the diffusion layer in controlling the quality of a metal’s surface finish has been recognized in electropolishing for a long time, so it is not surprising that it influences porous silicon formation. It is proposed that as the critical current density for electrochemical polishing is approached the average size of the nanocrystals produced gets smaller, even though the surface on the macro-scale gets smoother. The fact that of previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced. In particular a high ethanol content is required to both wet the porous silicon and to raise the viscosity in order to produce nanostructures with dimensions low enough to blue-shift the photoluminescence out of the red region.
Acknowledgements The authors would like to thank Dr. Tok Eng Soon for useful discussions and providing the silicon wafer, also Prof. S.J. Chua and Dr. Li Peng as well as Ms. Agnes Lim photoluminescence and AFM measurements respectively. Y.Z. was supported by an NUS research scholarship.
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D.R. Turner, J. Electrochem. Soc. 105 (1958) 402. R. Memming, G. Schwandt, Surf. Sci. 4 (1966) 109. L. Canham, Appl. Phys. Lett. 57 (1990) 1046. A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909. J.M. Rehm, G.L. McLendon, L. Tsybeskov, P.M. Fauchet, Appl. Phys. Lett. 66 (1995) 3669. R.B. Bjorklund, S. Zangooie, H. Arwin, Appl. Phys. Lett. 69 (1996) 3001. H. Ono, H. Gomyou, H. Morisaki, S. Nozaki, Y. Show, M. Shimasaki, M. Iwase, T. Izumi, J. Electrochem. Soc. 140 (1993) L180. S. Setzu, G. Lerondel, R. Romenstain, J. Appl. Phys. 84 (1988) 3129. D.R. Gabe, in: L.L. Shrier, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, Ch. 11.3, vol. 2, 3rd ed., Butterworth-Heinemann, Oxford, 1994. D. Landolt, Electrochim. Acta 32 (1987) 1. D. Bellet, L. Canham, Adv. Mater. (Weinheim) 10 (1998) 487. W.L. Wilson, P.F. Szajowski, L.E. Brus, Science 262 (1993) 1242. S. Schuppler, S.L. Friedman, M.A. Marcus, D.L. Adler, Y.H. Xie, F.M. Ross, T.D. Harris, W.L. Brown, Y.J. Chabal, L.E. Brus, P.H. Citrin, Phys. Rev. Lett. 72 (1994) 2648. G. Lerondel, R. Romestain, S. Barret, J. Appl. Phys. 81 (1997) 6171. I.L. Analis, D.J. Schiffrin, Electrochim. Acta 27 (1982) 837. L.E.A. Berlousis, D.J. Schiffrin, Trans. Inst. Met. Fin. 63 (1985) 547. P. Allongue, V. Kieling, H. Gerischer, Electrochim. Acta 40 (1995) 1353. V. Parkhutik, Solid-State Electron. 43 (1999) 1121. Y. Kang, J. Jorne, J. Electrochem. Soc. 140 (1993) 2258. M.I.J. Beale, J.D. Benjamin, M.J. Uren, N.G. Chew, A.G. Cullis, J. Cryst. Growth 73 (1985) 622. V.P. Parkhutik, V.I. Shershulsky, J. Phys. D. Appl. Phys. 25 (1992) 1258. Y.E. Makushok, V.P. Parkhutik, J.M. Martinez0Duart, J.M. Albella, J. Phys. D. Appl. Phys. 27 (1994) 66. D.R. Gabe, Metallography 5 (1972) 415. R. DeHoff, Thermodyanic of Materials, McGraw-Hill, New York, 1993, p. 383. W.J. Moore, Physical Chemistry, 5th ed., Longman, London, 1972, p. 487. T. Tsuboi, T. Sakka, Y.H. Ogata, Electrochim. Acta 46 (2001) 1013. D.R. Lide (Ed.), Handbook of Chemistry and Physics, 74th ed., CRC Press, London, 1993, pp. 6 /198.
The effect of etching temperature on the photoluminescence emitted from, and the morphology of, p-type porous silicon D.J. Blackwood *, Y. Zhang Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, 119260 Singapore Received 23 October 2002
Abstract The temperature at which silicon is electrochemically etched has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature increased both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The fact that previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Porous silicon; Photoluminescence; Viscosity; Temperature
1. Introduction It has been known since the fifties that porous silicon can be produced by the treatment of Si wafers in hydrofluoric acid solutions [1 /3]. However, it was not until 1990, when Canham demonstrated the observation of bright red photoluminescence from the surface of electrochemically etched Si wafer [4], that intense interest focused on the material [5]. Much of the current work has been focus on methods to induce different colours from the porous silicon [6,7]. Although there have been some previous investigations into the influence of the etching temperature on the quality of the porous silicon [8,9] some of the conclusions from these have been contradictory and none have reported variations in the wavelength of the emitted light. For example, Ono et al.[8] found that
* Corresponding author. Fax: /65-776-3604 E-mail address: [email protected] (D.J. Blackwood).
decreasing the etching temperature resulted in a decrease in the intensity of the photoluminescence and a red-shift in its peak wavelength, whilst Setzu et al.[9] found the opposite trend for the intensity and peak wavelength to be independent of temperature. In this article, the influence of the etching temperature on the photoluminescence obtained from, and the porosity of, porous silicon has been investigated independent of other parameters. However, the observations made were not in agreement with either that of Setzu et al. or Ono et al, with a decrease in temperature resulting in blue-shifting of the peak wavelength of the photoluminescence. It is suggested that the differences between the observations made by the various authors is due to changes in the composition of etching solution, in particular the amount of ethanol used. The ethanol content effects both the solution’s ability to wet the hydrophobic porous silicon surface and the viscosity of the diffusion layer adjacent to the surface, which is known to control the extent of levelling in electrochemical polishing processes [10,11].
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2. Experimental Specimens were fabricated from a p-type (100) silicon wafer with a resistivity of 5 V cm. Gallium /indium eutectic was used to make ohmic contacts to the backside of the wafers whilst acid-resistant epoxy resin was applied to reduce the exposed surface area on the front face to 10 mm /8 mm. The specimens were weighed (without the epoxy resin) prior to and after the etching process. The porous silicon was obtained by electrochemical etching in a solution of 1:1:2 HF:H2O:ethanol held in a two-electrode PTFE closed-cell, which was in turn placed inside a temperature controlled bath. Etching was performed at a constant current of 20 mA cm 2 applied for a period of 20 min, with the surrounding bath being held at either of /10, 0, 25 or 378C. After etching the porous silicon films were rinsed with ethanol and dried with pentane, which helps reduce cracking [12]. The presence of porous silicon nanocrystallites (or at least structures with nanosized dimensions) was confirmed immediately after etching by using a handheld 4 W UV lamp to induce photoluminescence. The high level of porosity in the samples prepared in lower temperature caused these to have a very weak mechanical resistance, such that these had to be raised to room temperature slowly to prevent the films cracking due to thermal expansion. This was achieved by washing the specimens with, and then transferring these into a beaker of, ethanol that was at the same temperature as the etching solution, the beaker was then left to stand on the bench until it had reached room temperature. It was necessary to transfer the specimens to ethanol, rather than leaving these in the HF solution, to prevent any additional etching occurring during the warm-up period. No attempt was made to increase the rate of drying of the porous silicon layers as this was found to induce cracking. Low temperature films that were washed with room temperature ethanol not only cracked but were also found to emit red light rather than yellow light. The photoluminescence spectra were recorded via a Renishaw 2000 microRaman and photoluminescence system equipped with a 20 mW helium- /cadmium laser operating at 325 nm which was focused to a spot size of 1 mm diameter. The morphologies of the surfaces of the various specimens were examined under a Philips XL30 FEG scanning electron microscope (SEM) at an accelerating voltage of 10 keV and by a Shimadzu SPM9500J atomic force microscope (AFM) operated in the dynamic mode. The specimens were later cross-sectioned to allow the thickness of the porous silicon films to be measured by the SEM. The thickness of the samples was also determined by using a profilometer, as described previously by Setzu et al. [9]. Combining the thickness measurements from the profilometer with the weight
loss during the etching process yielded estimations for the film’s average porosity. Since the dimensions of the porous silicon structures were below the resolution of the SEM used, it was not possible to determine whether the observed photoluminescence was due to the presence of nanocrystals or columnar wire-like structures or other nanosized structures. However, the term nanocrystals has been used throughout this paper to be consistent with previous authors [13].
3. Results and discussions It was found that under stimulation from an UV light source the wavelength of the photoluminescence emitted from the porous silicon films, which was clearly visible to the naked eye in a dim environment, blue-shifted with decreasing etching temperature. With no visible photoluminescence, red light, orange light and yellow light being observed from films formed at 37, 25, 0 and / 108C, respectively. The absence of visible photoluminescence at 378C was expected by the authors, since it was the inability to form active porous silicon at this temperature (due to the failure of the laboratories airconditioning system) that first drew their attention to the possibility of temperature affects. Fig. 1 shows a series of normalized photoluminescence spectra, which confirmed the blue-shifting with decreased etching temperature. Although the wavelength dependence of the photoluminescence was found to be virtually independent of location sampled on any one specimen its intensity was found to vary. However, as the intensity appeared to vary randomly this was probably due to the
Fig. 1. Normalised photoluminescence spectra for porous silicon formed at different temperatures in a 1:1:2 solution of H2O:HF:C2H5OH. No luminescence observed at an etching temperature of 378C.
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surface roughness of the specimens scattering both the incoming and emitted light. Whilst testing the Quantum confinement theory, Wilson et al. [13] and Schuppler et al. [14] clearly demonstrated that the peak luminescence emission energy increases as the average particle size decreases. The observed blue shifting of the photoluminescence with decreasing etching temperature, observed in the present work, is therefore consistent with the formation of smaller nanocrystals. The influence of the temperature of etching on the morphology of the resulting porous silicon can be clearly seen in the SEM photographs (Fig. 2), with decreasing temperature causing a large increase in the number of pores but these had smaller diameters. Fig. 3 shows high magnification images of the tops of the silicon columns for specimens formed at /10 and 378C. Although not fully resolvable, structures of only a few tens of nanometres, possibly even less than 10 nm, can be seen on the /108C specimen, whereas for the 378C sample there appears to be no features less than about 500 nm. The trend of decreasing crystal size with etching temperature was also observed in the AFM images (Fig. 4). Although, as with the SEM, the nanocrystals
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could not be fully resolved, there is a clear decrease in feature size as the etching temperature decreases from 37 to /108C. The AFM images also suggest that the average surface roughness decreased with decreasing temperature. The physical weight loss per unit area from the specimens during the etching process increased as the temperature was decreased, from 1.18 mg cm 2 at 378C to 1.71 mg cm 2 at /108C. The expected weight loss from the application Faraday’s law to the 24 coulombs per unit area passed during the etching process is 1.74 g cm 2 assuming that the dissolved silicon species was in the /4 oxidation state (probably SF2 6 ). Therefore, the current efficiency at the lowest temperature is /98% whilst at the highest temperature it is only about 68%. Presumably the unaccounted for charge at the higher temperatures is due to formation of lower oxidation state silicon fluorides, or possibly oxides, that remain attached to the lattice. The average depths of the porous silicon layers, determined from the profilometer measurements over different areas of the specimens’ surfaces, did not show any consistent variation with etching temperature. In all cases an averaged depth of about 11 microns (standard deviation of 2 microns) was found. However, as can be
Fig. 2. SEM photographs of porous silicon films etched at temperatures of (a) /108C, (b) 08C, (c) 258C and (d) 378C.
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Fig. 3. High resolution SEM photographs of porous silicon films etched at temperatures of (a) /108C and (b) /378C.
seen from Fig. 5, this value is approximately an order of magnitude greater than the thickness determined from the specimen cross sections in the SEM. The most likely reason for this is that most of the outer surface of the porous silicon gradually dissolved in the electrolyte (either by electrochemical polishing or chemical dissolution) during the course of the experiment and thus the SEM only reveals the remaining thickness.
The fact that the outer layers of porous silicon were completely dissolved into the porous silicon somewhat invalidates the values of average porosities calculated on the basis of profilometer depth readings and weight lost measurements, however, this procedure was still preformed as this allowed comparisons with the work of previous authors. Fig. 6 shows that the average porosity increased as the etching temperature was lowered. The
Fig. 4. AFM images of porous silicon films etched at temperatures of (a) /108C, (b) 08C, (c) 258C and (d) 378C.
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Fig. 5. SEM photographs of cross sections of porous silicon films etched at temperatures of (a) /108C and (b) /378C.
Fig. 6. Dependence of the average porosity of the porous silicon films on the temperature of the etching solution.
general trend in the temperature dependence of the porosity of the porous silicon films produced in the present work was similar to that previously reported by Setzu et al. [9]. The following explanation is similar to that postulated by these authors and is based on the observation that the production of porous silicon from p-type wafers in hydrofluoric acid is limited to a low potential region. At higher potentials the anodisation current is sufficiently high to cause electrochemical polishing [3]. The point at which the switch from the production of porous silicon to electrochemical polishing occurs is defined at the critical current density. It has also been reported that for porous silicon the surface roughness varies monotonically with anodisation current, such that the further the etching current is below the critical value for polishing the rougher the interface [15], this phenomenon is also observed in the electropolishing of metals [10]. The suggestion that the surface roughness should increase as the etching temperature increases is consistent with the series of AFM images shown in Fig. 4. Overall the results presented in this paper suggest that although the surface gets smoother as the critical current density for electrochemical polishing is approached, the average size of the nanocrystals produced decreases. Intuitively this observation makes sense, as electrochemical polishing can be envisaged as
the point at which the size of the nanocrystals shrinks to zero. It is well known in the electrochemical polishing that smooth and bright finishes are only obtained if a viscous layer in which the diffusion of reactive species is restricted develops at the metals surface [10,16,17]. Chemical attack is then favoured on surface asperities, explaining the smoothening effect that leads to polishing. It has been suggested that such a viscous layer forms with silicon when the anodizing current is sufficiently high to cause electrochemical polishing [2,9]. As early as 1957 Tuner [2] showed that increasing the viscosity of the etching solution, either by changing its chemical composition or by lowering the temperature, could reduce the critical current density for the onset of electrochemical polishing. Consequently, since in the present work the same etching current density was applied at all temperatures, as the etching temperature was reduced so the critical current density will have approached the applied current giving rise to the observed smoother interfaces. Again a similar phenomenon occurs in the electropolishing of metals, where increased mass transport across the viscous layer due to higher temperatures (or agitation of the solution) results in rough, uneven surfaces [10,18]. The high level of porosity of the porous silicon film produced at low temperatures can thus be explained by the convergence of the applied current and the critical current density for electrochemical polishing brought about by the increased viscosity of the etching solution. The high current efficiencies seen for the etching process at low temperatures is also consistent with electrochemical polishing of the silicon. However, as yet this viscosity argument does not explain the temperature dependence of the dimensions of the nanocrystals. One of the best mechanisms for explaining the chemistry behind anodic etching of silicon is that proposed by Allongue et al. [18], which is schematically represented in Fig. 7. In this mechanism it is proposed that a silicon hydroxide species is first formed at the
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Fig. 7. Schematic representation of the chemistry of pore formation based on the mechanism proposed by Allongue et al. [18].
surface which then undergoes anion exchange with fluoride (step 3). In the event that this step is slow a hydroxy/oxide film forms such that electrochemical polishing ensues, whereas the anion exchange is rapid dissolution of the fluorinated silicon (step 4) results in the formation of porous silicon. Therefore whether electrochemical polishing or pore formation occurs depends on the relative rates of surface hydroxide formation (steps 1 and 2) to its replacement by fluoride (step 3). If it is assumed that the reaction between water and radical produced in step 1 is rapid then the rate of surface hydroxide produced is directly proportional to the flux of holes to the surface and thus for a given applied current density should be independent of temperature. On the other hand, step 3 is a purely chemical reaction, the rate which can be expected to have some Arrhenius type of temperature dependency. Also higher temperatures will favour the dissociation of HF such that more fluoride ions will be available to participate in step 3. Thus, increasing temperatures should favour surface fluoride formation and thus porous silicon formation. Likewise lower temperatures favour electrochemical polishing, which is consistent with the present results. However, the mechanism of Allongue et al. does not give a clear indication how temperature should influence the size of the resulting nanocrystals. There are several theories that have been presented to explain pore initiation and propagation in porous silicon, most of which can be broadly classified into one of three groups: surface instability, stationary pore growth or virtual passive films [19]. A good example of the surface instability theory is Kang and Jorne’s [20] growth model. This predicts that the dominate distance between pore tips should be proportional to the square root of temperature. Although qualitatively this is consistent with the formation of smaller nanocrystals at low temperatures observed in the present work, quantitatively the Kang and Jorne model only predicts about a 10% decrease over the
temperature range 310 /263 K. However, this model does predict that electrochemical polishing will occur if ionic diffusion in the electrolyte becomes the rate controlling step in the silicon dissolution reaction, which is more likely to occur at higher viscosities and thus at lower temperatures, i.e. it is consistent with argument of Setzu et al. [9] presented above. In models based on stationary pore growth, the original of which was that of Beale et al. [21], more random structures and smaller pore sizes are associated with low conductivity in nondegenerate silicon wafers. Although decrease temperatures would lower the extent of thermionic emission and thus favour small pore sizes the extent of temperature range in the present experiments (263 /310K) is not sufficient to have any significant effect and therefore cannot be used to explain the results found in the present work. Pore growth models based on virtual passive films [22,23] have apparently either not directly consider the influence of temperature or in the case of an early model developed by Parkhutik et al. [22] for porous aluminium specifically ruled out any temperature effects. Nevertheless temperature dependence could be envisaged in some of the later models as it would influence the magnitude of the electric field across the virtual passive film. Assuming the film to be a semiconductor means that higher fields will be required to pass a given current density as the temperature is lowered. Under these circumstances the models predict that this will lead to larger pore radii and eventually electrochemical polishing, however, it is not clear from these models whether this should be accompanied by a decrease in the dimensions of any nanocrystals. It therefore appears that none of the existing models fully explains the apparent decrease in the size of the nanocrystals formed with decreasing temperature observed in the present work. However, returning to the viscosity argument does yield a possible explanation. An increase in the viscosity within the porous silicon layer would impede the rate at which silicon ions could diffuse out into the bulk solution. Indeed, in electropolishing
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chemicals referred to as diffusion layer promoters are specifically added to form viscous layers adjacent to the surface in order control mass transport rates and thus obtain macroscopic levelling, with alcohols frequently being used for this purpose at low temperatures [10,24]. Thus, for a given rate of ion generation (i.e. for a given current density) the steady state concentration of silicon ions held within the porous layer should increase with a higher viscosity. From thermodynamic considerations the influence of the dimensions on the solubility (K) of crystal faces is expected to take the form [25]: K A(1(B=l))
(1)
where A and B are constants, representing the solubility of the bulk material and the capillary length scale for solubility shift respectively, whilst l is the perpendicular distance of the crystal face from the centroid. In the case of silicon nanoparticles l is very small, hence their solubility is significantly higher than that of the bulk and thus a much higher local concentration of silicon ions is required for equilibrium to be achieved. It therefore follows that the higher concentrations of silicon ions expected within the porous matrix at low temperatures, due to the increased viscosity, would help stabilize smaller nanocrystals. The blue-shifting in the photoluminescence spectra observed in the present work may thus in part be explained by the increase in viscosity, that accompanies a decrease in a solutions temperature, raising the steady-state concentration of silicon ions held within the porous layer and thereby reducing the solubility of the smaller nanocrystals. Furthermore, the solubility of the porous silicon (the A in Eq. (1)) is also likely to decrease with decreasing temperature and this will further stabilize the smallest nanocrystals. The major difference between the present work and that of previous authors, most notably Setzu et al. [9] and Ono et al. [8], is the observation of the temperature dependence of the energy of the light emitted during photoluminescence. Although Setzu et al. reported increased photoluminescence intensity at low temperatures, they did not observe any wavelength shifts in the spectra, which was predominantly in the near infra-red (700 /900 nm). However, these authors only related the temperature dependence of the photoluminescence spectra from rapidly grown films (166 mA cm 2). In addition Setzu et al. reported that some of their porous silicon films cracked, especially those of high porosity which is also where the smallest crystals are expected to be located, so it is possible that this was the reason these authors did not observe any temperature dependent blue-shifting of the photoluminescence. As mentioned above the current authors solved the problem of cracking by slowly bringing the films etched at low temperatures up to room temperature and allowing or the films to dry slowly.
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A further difference between the experiments conducted for the current work and those of Setzu et al. [9] is the higher ethanol content of the etching solution used in the present case (50% compared to 30%). The purpose of the ethanol being to promote wetting of the silicon substrate thereby aided the electrolyte to better penetrate the porous layer, as would the lower surface tension associated with the higher ethanol content [26]. Surface wetting has recently been demonstrated to be particularly important in the etching of p-type silicon [27]. Furthermore, the additional ethanol should have increased the viscosity of the etching solution, which, as was discussed above, is an important parameter in controlling the quality of porous silicon films. The maximum viscosity for solutions of ethanol in water occurs at about 50% ethanol [28]. Hence higher ethanol content could help produce smaller nanocrystals. Ono et al. [8] etched their porous silicon in 46% HF, apparently without any ethanol content. The red shifting in the peak wavelength of the photoluminescence with decreasing temperature reported by these authors was from red ( /730 nm) to the near infrared (/800 nm). This suggests the lack of ethanol in the etching solution prevented the wetting of the porous silicon to the extent that it was not possible to form the nanocrystals with sufficiently small dimensions the give photoluminescence in the shorter wavelength yellow region observed in the present work. The fact that the manner in which the etching temperature influences the dimensions of the silicon nanocrystals formed appears to be dependent on the composition clearly supports the notion that the nature of the electrochemical interface plays a critical role in the development of the porous matrix.
4. Conclusions The etching temperature has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature at which the silicon is etched increases both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed, a conclusion that was supported by high resolution SEM photographs and AFM imaging. However, care must be taking when producing porous silicon at low temperatures to avoid cracking, due to thermal expansion, in the resultant films. Without these precautions films formed at low temperature not only cracked, but also emitted red light rather than yellow light. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation
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for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore, it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The importance of the viscosity of the diffusion layer in controlling the quality of a metal’s surface finish has been recognized in electropolishing for a long time, so it is not surprising that it influences porous silicon formation. It is proposed that as the critical current density for electrochemical polishing is approached the average size of the nanocrystals produced gets smaller, even though the surface on the macro-scale gets smoother. The fact that of previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced. In particular a high ethanol content is required to both wet the porous silicon and to raise the viscosity in order to produce nanostructures with dimensions low enough to blue-shift the photoluminescence out of the red region.
Acknowledgements The authors would like to thank Dr. Tok Eng Soon for useful discussions and providing the silicon wafer, also Prof. S.J. Chua and Dr. Li Peng as well as Ms. Agnes Lim photoluminescence and AFM measurements respectively. Y.Z. was supported by an NUS research scholarship.
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