3864
J. Phys. Chem. B 2001, 105, 3864-3871
Laser-Assisted Formation of Porous Si in Diverse Fluoride Solutions: Reaction Kinetics and Mechanistic Implications† Lynne Koker and Kurt W. Kolasinski* School of Chemistry, The UniVersity of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom ReceiVed: September 11, 2000; In Final Form: December 12, 2000
The formation rate of porous silicon by photoelectrochemical etching of n-type silicon is measured in situ by a novel technique. The reflection of the laser beam used to drive the reaction contains circular patterns, the outer radius of which bears a linear relationship to the depth of etching. The use of the radius to measure etch rate and the absolute depth of etching is described and justified. The rates are analyzed in terms of the calculated activities of the different species in solution. A rate equation, which is first order in HF and HF 2 , is deduced and rate coefficients calculated. It is concluded that the etching mechanism includes two parallel rate-determining steps involving HF and HF 2 and that the HF 2 channel is roughly 20 times faster than the HF channel. In light of our data, previously proposed mechanisms of Si dissolution in fluoride solutions should be reevaluated.
1. Introduction Porous silicon (por-Si) is conventionally prepared by anodization in aqueous hydrogen fluoride, HF(aq). One also can employ a photoelectrochemical technique without an externally applied bias.1 Alternative etchants to pure HF(aq) have been used such as mixtures of HF with nonaqueous acetonitrile,2 nitric acid,3 NaNO2, CrO3,4 and nonaqueous solutions of hexafluorophosphates,5 hydrogen fluoride, and fluoroborate6 in acetonitrile. Nonetheless, no studies have used solutions of diverse composition to vary comprehensively the concentrations of solution species that may be important in the etch mechanism. We refer to our method of porous silicon formation as laserassisted etching to differentiate it from the more common photoelectrochemical method in which the crystal is attached to an external electrode and biased. Both processes are photoelectrochemical in nature but differences do exist. Etching mechanisms found in the literature vary in their ability to explain a variety of experimental observations: (1) the silicon surface is hydrogen-terminated,7-9 (2) the photocurrent quantum efficiency, Q, drops from ∼4 to ∼2 in going from low (photon flux SiHF (where > indicates two Si-Si backbonds). The next step, cleavage of the first Si-Si back-bond, may be achieved by HF or HF 2 . Gerischer et al. left open the possibility of cleavage by H2O, but, as argued below, we consider its participation to be negligible. Gerischer et al. argue that cleavage of the second Si-Si back-bond should be rapid, as the departing Si atom is bonded to two F atoms. This renders the remaining back-bond highly polarized and weak. From the above reasoning, attack of the >SiHF by HF or HF 2 , breaking the first Si-Si back-bond, is the only step that may be rate determining. Since there is a steady and plentiful supply of >SiHF, the reaction should follow pseudo-first-order kinetics in the attacking species if this step is irreversible. All the solutions are dilute, therefore aH2O is constant, and since the rate changes with etchant and with formal concentration, we conclude that the contribution from H2O in the RDS is negligible. By inspection of the activity and rate curves, it is clear that aHF does not track the rate in all cases. aHF is far to low in KHF2 and NH4HF2 solutions to account for etching. aHF does not exhibit a maximum in NaF/HCl nor does it shape match the shape of the rate curve in NaF/HF. Another species must be involved. The activity of HF 2 exhibits a maximum in NaF/ HCl and is present in ample quantities in KHF2, NH4HF2, and NaF/HF. Therefore, from these considerations alone we conclude that both HF and HF 2 must be involved in the etch mechanism. This conclusion can be made more quantitative. The incorporation of the activities of HF and HF 2 allows a rate expression, eq 10, to be formulated that gives good agreement with the observed dependence of rate on activities. As expected, the observed rate law is pseudo-first-order and includes only those species involved in the attack of >SiHF. A fit to the data in Figure 4 yields
retch ) k1aHF + k2aHF-2
(10)
where k1 ) 0.15 ( 0.03 nm s-1 mol-1 kg and k2 ) 3.2 ( 0.3 nm s-1 mol-1 kg. The quality of the fit indicates that it is unnecessary to consider any other species in the concentration ranges studied. Figure 4 shows the observed rates, the calculated rates using the above rate expression, and the contributions to those rates from HF and HF 2 . Rates are calculated only up to the activities considered to be reliable (Table 1). The observed rate law implies that parallel reaction pathways are present, one which involves HF and one which involves HF 2 in the RDS. A perfect fit between the activities of the
Figure 5. Diagram of etching mechanisms that include the reaction of HF 2 . The presence of two parallel rate-determining steps augments the third step in the Gerischer et al. mechanism and the fourth step in the Kooij and Vanmaekelbergh mechanism.
fluoride species and the observed rates is not unexpected considering the uncertainties in the equilibrium constants and the activity coefficients. Nonetheless, it is clear that the rate constant for the HF 2 path is ∼20 times greater than that of the HF path. In Figure 5 we present a mechanism for the RDS, which replaces the third step in the Gerischer et al. mechanism and the fourth step in the Kooij and Vanmaekelbergh mechanism. For these steps HF and HF 2 produce the same surface intermediates, SiHF2(a) and SiH(a). HF 2 accomplishes this by simultaneously releasing F- into solution. The existence of parallel paths can therefore be understood in terms of the ability of HF and HF 2 to produce chemically equivalent surface products. The significantly larger rate constant for etching by HF 2 is quite remarkable. Intuitively the Si-Si back-bond should be attacked more readily by a less sterically constrained species. Infrared studies52 of HF(aq) indicate features assigned to H-F stretching in the complex, H3O+‚F-. This is formed in equilibrium with HF and H2O, with the equilibrium lying far to the right:
H2O + HF u F [H3O+‚F-]
(11)
HF in this form is more sterically hindered than HF 2 and hence would attack the Si-Si back-bonds at a slower rate, as confirmed by our data. Allongue et al.21 have identified two components to silicon etching, a chemical and an electrochemical reaction. The chemical reaction is capable of isotropic etching, but the rate is 0.0008-0.0017 nm s-1 in the pH range of our study. Note also that the chemical etch rate is insignificant compared to the porSi formation rates that we report (of order 1 nm s-1). Our results unequivocally show that both HF and HF 2 must be considered in the overall etch mechanism. We have interpreted our results in terms of two of the most widely accepted reaction mechanisms. While consistent with these mechanisms, our results do not prove either of them to be correct. An old adage of heterogeneous catalysis states that any important surface intermediate will never be isolated because its transient nature leads to prohibitively low coverage. While
Laser-Assisted Formation of Porous Si in Fluoride Solutions not wholly accurate, there is a grain of truth in this statement that must be borne in mind. The presence of intermediate SiFx species has never been observed by in situ infrared absorption measurements. The presence of F-containing species in por-Si, which has not been rinsed after formation, has been observed by secondary ion mass spectrometry (SIMS)53 and ex situ transmission infrared spectroscopy.54 X-ray photoelectron spectroscopy (XPS) of rinsed samples55 has failed to find more than trace amounts of fluorine, consistent with UHV experiments on Si(111)-(1 × 1) etched in HF(aq).56 The latter results suggest the possibility that the fluorine detected in the former experiments may result from physisorbed etch products that can be removed by rinsing. Peter et al.16 and Rao et al.8 have pointed out that the lack of an IR signature does not exclude the existence of Si-Fx intermediates. It only means that their lifetime must be sufficiently short to preclude the accumulation of sufficient coverage for them to be measurable. Similarly the lack of a signal from adsorbed oxygen species does not preclude their existence, it only puts an upper limit on their surface lifetime. We have no proof for the involvement of adsorbed oxygen species in the etching of Si in fluoride solutions. We simply note that HF and HF 2 are thought to be the active species in SiO2 etching and that, likewise, we have shown that they are involved in the photoelectrochemical etching of Si. Further research is required to determine whether this circumstance is mere coincidence or the expression of a fundamental link between these processes. 6. Conclusions The rate of photoelectrochemical etching of n-type Si without external bias to produce por-Si may be conveniently and reliably measured in situ by monitoring the radius of the pattern in the reflected laser beam. The effect of etchant composition and concentration on the por-Si formation rate shows that neither F- nor H+ activities correlate with the rate. While these species may be involved in the etching of silicon, they are not active in the RDS. Neither is it possible to account for the rates using only the activity of HF; the activity of HF 2 must also be included. We postulate a scheme, which we have incorporated into two previously proposed mechanisms, in which there are two parallel RDS involving HF and HF 2 . HF 2 is about 20 times more efficient at etching than HF. New electrochemical studies, which consider both HF and HF 2 in the kinetics, should be performed in light of our results. Acknowledgment. We acknowledge the financial support of the Royal Society and the provision of a studentship (L.K.) by the EPSRC. We also acknowledge A. Dodd for assistance in preliminary experimental work and Y. Ogata and P. Schmuki for helpful discussions. References and Notes (1) Noguchi, N.; Suemune, I. Appl. Phys. Lett. 1993, 62, 1429. (2) Rieger, M. M.; Kohl, P. A. J. Electrochem. Soc. 1995, 142, 1490. (3) Jones, L. A.; Yu¨kseker, O ¨ .; Thomas, D. F. J. Vac. Sci. Technol. A 1996, 14, 1505. (4) Fathauer, R. W.; George, T.; Ksendzov, A.; Vasquez, R. P. Appl. Phys. Lett. 1992, 60, 995. (5) Rieger, M. M.; Flake, J. C.; Kohl, P. A. J. Electrochem. Soc. 1999, 146, 4485. (6) Flake, J. C.; Rieger, M. M.; Schmid, G. M.; Kohl, P. A. J. Electrochem. Soc. 1999, 146, 1960. (7) Unagami, T. J. Electrochem. Soc. 1980, 127, 476. (8) Rao, A. V.; Ozanam, F.; Chazalviel, J.-N. J. Electrochem. Soc. 1991, 138, 153. (9) Peter, L. M.; Borazio, A. M.; Lewerenz, H. J.; Stumper, J. J. Electroanal. Chem. 1990, 290, 229.
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