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J. Phys. Chem. B 2001, 105, 5722-5729
Etching and Passivation of Silicon in Alkaline Solution: A Coupled Chemical/ Electrochemical System Xinghua Xia,*,† Colin M. A. Ashruf,‡ Patrick J. French,‡ Joerg Rappich,§ and John J. Kelly† Debye Institute, Utrecht UniVersity, P.O. Box 80000, 3508 TA Utrecht, The Netherlands, Delft UniVersity of Technology, DIMES Department of Electrical Engineering, Mekelweg 4, 2628 CD Delft, The Netherlands, and Hahn-Meitner-Institut, Abteilung Silizium-PhotoVoltaik, Kekulestrasse 5, D-12489 Berlin, Germany ReceiVed: September 12, 2000
Three types of experiments were used to study the surface chemistry of silicon in alkaline solution: minority carrier injection from a p-n junction electrode, in-situ photoluminescence, and electron transfer to a redox system in solution. The results lead to the conclusion that the surface chemistry and electrochemistry are determined to a large extent by an activated intermediate of the chemical etching reaction of silicon with water. This novel coupling of chemical and electrochemical steps can account for some unusual features of the system, such as a mechanism for anodic oxidation and passivation based on electron injection and the strong influence of a weak oxidizing agent on the surface morphology of chemically etched silicon.
Introduction The chemistry of silicon in alkaline solution has important industrial applications. Chemical etching of the semiconductor is strongly anisotropic,1-2 a feature which is exploited for making the three-dimensional structures required in microelectromechanical systems (MEMS).3-5 Silicon can be anodically passivated and the anodic oxide, by preventing access of water to the silicon-silicon back-bonds, suppresses chemical etching.6 This effect forms the basis for an “etch-stop mechanism” widely used in the fabrication of beams and membranes for mechanical sensors and actuators.7,8 While anisotropic etching and etch-stop mechanisms are very important in micromachining technology, the chemistry underlying these processes is not clear. In particular, the mechanism of anodic oxidation of silicon in alkaline solution is not straightforward. The semiconductor shows current-potential characteristics typical of a valve metal such as titanium whose oxide is partly soluble in the electrolyte solution. Contrary to what one would expect for a semiconductor, anodic oxidation of n-type silicon is observed in the dark. Although etching of p-type (100)Si proceeds via a potential-independent chemical mechanism, the surface morphology is sensitive to the potential applied to the semiconductor.9 The chemistry of redox systems at silicon in alkaline solution also shows quite unexpected features.10,11 In this paper we consider the surface chemistry of silicon at high pH and, in particular, the role played by chemical reactions in the electrochemistry of the semiconductor. Before discussing our results we briefly review the literature and point out a number of unresolved issues. These observations form the basis for some new experiments. In the discussion we consider mechanisms and show that anodic oxidation and chemical etching are coupled in a rather unique way. * Present address: Department of Chemistry, Nanjing University, Nanjing, P.R. China. † Debye Institute, Utrecht University. ‡ Delft University of Technology, DIMES Department of Electrical Engineering. § Hahn-Meitner-Institut, Abteilung Silizium-Photovoltaik.
Figure 1. Current-potential curves of p-type Si(100) (solid) and n-type silicon (dotted) in 2.0 M KOH solution at 45 °C in the dark. The potential was scanned from -2.0 V at a rate of 5 mV/s.
Issues To Be Considered The anodic current-potential characteristics of p-type silicon in 2 M KOH solution at 45 °C are shown as a solid curve in Figure 1. As the potential is made positive with respect to the open-circuit value, the anodic current first increases to a maximum and then drops to a low value in the passive range. The current in the return sweep to negative potential is low since the surface is passivated. Anodic oxidation of a semiconductor should involve a surface reaction with valence band holes. It is therefore surprising that an anodic current of comparable magnitude is found for an n-type electrode in the dark (dotted curve, Figure 1). Under cathodic conditions the electrochemistry of silicon in alkaline solution conforms to the pattern expected for a semiconductor electrode; hydrogen evolution requiring conduction band electrons is observed for the n-type electrode but not for the p-type electrode in the dark (Figure 1). The electrochemistry of silicon in alkaline solution is complicated by a nonelectrochemical reaction, the chemical etching of the semiconductor by water.12 Allongue et al. suggested that the process is catalyzed by OH- ions.12 This idea
10.1021/jp003208f CCC: $20.00 © 2001 American Chemical Society Published on Web 05/30/2001
Etching and Passivation of Silicon was further worked out by Baum et al.13 The etching reaction occurs at potentials negative with respect to the peak; for p-type silicon the rate is essentially independent of potential in a broad range.10 With the formation of oxide at potentials positive with respect to the peak, water molecules no longer have access to Si-Si bonds and chemical etching ceases.12-14 Strong oxidizing agents are expected to be reduced via the valence band of a semiconductor, i.e., by hole injection. However, for p-type (100) silicon in alkaline solution results have shown that the reduction current for various oxidizing agents (Fe(CN)63-, MnO4-) is significant only at a lightly passivated electrode.10,11 Once the oxide is dissolved and the silicon etches chemically the current drops to a low value, despite the fact that the oxidizing agent continues to be consumed at a significant rate at the etching surface. The chemical etching of silicon in alkaline solution is often accompanied by severe surface roughening.9,15 For the (100) surface this is caused by the formation of crystallographically oriented pyramids with slow etching (111) facets. Obviously, such roughness cannot be tolerated in many applications. We have shown that pyramid formation can be prevented by polarizing the silicon at a potential between current onset and the peak;9 this does not change essentially the etch rate. The presence of certain oxidizing agents such as those mentioned above, (Fe(CN)63-, MnO4-), also prevents surface roughening.9,11 The results described above raise a number of issues. These include the nature of the process responsible for anodic current flow in n-type silicon. Previous authors have tacitly assumed that direct electrochemical oxidation of surface silicon hydride supplies electrons to the conduction band.2 However, theoretical calculations have shown that the energy levels of the hydride are in the valence band16 and this has been confirmed by photoelectron spectroscopy.17 Besides, there is no evidence for a significant direct oxidation of Si-H via the conduction band of silicon in indifferent electrolyte solution at lower pH. Similar arguments hold for electron injection as a result of the direct oxidation of Si-Si back-bonds. One must conclude that surface states are involved.18,19 The second issue relates to the type of reaction causing oxidation of p-type silicon. As indicated above this could be a straightforward valence band process. However, considering the large dark current observed with the n-type electrode we must also take into account the possibility of electron injection in this case. Results reported by Cattarin et al.,20 who used a dual-compartment cell, confirm a contribution from electron injection. The surface chemistry of the redox systems described above also needs to be elucidated. In particular, the nature of the reaction responsible for chemical reduction of oxidizing agents is unclear, as is the reason the oxidizing agents have such a strong effect on the morphology of the etched surface. To distinguish between oxidation reactions which occur via the conduction band and the valence band of a p-type semiconductor it is necessary to be able to detect minority carrier injection. The collection efficiency of electrons in the dualcompartment cell used by Cattarin and co-workers is not well defined. In this work we use instead a p-n junction electrode. The principle is described in the Experimental Section. These measurements indicate the importance of an activated surface state with an energy high in the band gap of silicon. To address this state we used a weak oxidizing agent CrO42- with acceptor levels well above the valence band edge of silicon. This redox system was first characterized by photoelectrochemical measurements and these results help with understanding the mech-
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5723 anism of chemical reduction of CrO42- at an etching silicon surface. In addition, in-situ photoluminescence measurements were used to obtain information about intermediates of the etching reaction and the influence of surface chemistry on these intermediates. The results of these various measurements allow us to speculate on the mechanism of chemical etching and anodic oxidation. Finally, the influence of CrO42- on the etched surface morphology is considered in the context of this reaction scheme. Experimental Section The p-type single-crystal silicon (100) wafers were borondoped with a resistivity in the range 1-15 Ω cm. The n-type samples had the same orientation; these were phosphorus-doped with a resistivity between 10 and 15 Ω cm. The electrodes were dipped in a solution of 1 M HF + 2 M NH4F for 1 min and rinsed with Millipore water (16 MΩ) before each measurement to remove native surface oxides. The silicon working electrode was mounted in a Kel-F holder with an O-ring, and electrical contact to a copper disk on the backside of the sample was made using Ga-In eutectic. All chemicals were of analytical grade (Merck), and used as received. Solutions were prepared with Millipore water. The electrochemical experiments were carried out with an EG&G Princeton Applied Research (PAR) 283 potentiostat which was controlled by a personal computer via a Labview interface. A conventional three-electrode electrochemical cell was used containing a Pt sheet as counter electrode and a saturated calomel electrode (SCE) as reference. All potentials are referred to SCE. For illumination a white light source (Schott KL 1500) was used. The etch rate was determined by measuring the etched depth as a function of etching time using a Tencor Alpha-Step 500 surface profiler. The surfaces of etched silicon samples were examined with a XL30FEG scanning electron microscope (SEM, Philips). The concentration of CrO42- in the etchant was followed during chemical etching of silicon at 45 °C by UV/visible absorption measurements with a Perkin-Elmer Lambda 16 UV/vis spectrophotometer with 2.0 M KOH solution as a reference. No reduction of the oxidizing agent by molecular hydrogen was observed. We used the “p-n junction technique” to determine if the anodic oxidation of p-type silicon in alkaline solution occurs via the valence band (hole capture) or the conduction band (electron injection). The principle of the technique is shown in Figure 2. The p-type face of a p-n junction forms the working electrode in a conventional electrochemical cell with counter and reference electrodes. The current passing through the cell (Icell) can be measured as a function of the Si electrode potential (USi). Electrons injected into the conduction band of the p-type region can be swept across the p-n junction by the built-in electric field if the electron diffusion length is comparable to or larger than the thickness of the p-type epi-layer. In that case, the injected electrons can be detected by measuring the shortcircuit current (Isc) flowing between n-type and p-type regions. By comparing Isc and Icell one can determine the relative contribution of the minority carrier (electron) process to the total (electron + hole) current. The p-n junction samples used in our experiments consisted of a 3 µm thick p-type silicon epilayer (2-5 Ω cm) on an n-type silicon substrate ((100) orientation, 500 µm thick). The p-n junction was isolated from the cleaved edges of the sample by a phosphorus diffusion through the p-type epilayer. A silicon nitride masking layer was deposited on the epilayer. Two windows were opened in the silicon nitride, one to serve as
5724 J. Phys. Chem. B, Vol. 105, No. 24, 2001
Xia et al.
Figure 3. Current-potential curves of a p-n junction with a p-type silicon epi-layer facing a solution of 2.0 M KOH at 45 °C in the dark. (a) Cyclic voltammogram of the p-type silicon epi-layer, and (b) the corresponding short-circuit current between p-type and n-type regions. The potential was scanned from -2.0 V at a rate of 5 mV/s. Figure 2. Schematic representation of the p-n junction technique, where the p-type silicon epilayer, used as the working electrode in an electrochemical cell, is in contact with the electrolyte. Electrons injected into the CB of the p-type layer can be detected as a short-circuit current (Isc) between p-type and n-type regions.
electrode area, the other for electrical contact to the epilayer. The backside of the p-n junction (n-type) was provided with an n+ diffusion. Aluminum was used to make contact to the front and backsides. The sample was mounted in a Kel-F holder. The short-circuit current was measured with a Digital Multimeter (HEWLETT 3478A). The time-integrated photoluminescence intensity of silicon (at 1.1 µm) was probed by pulsed excitation of a dye laser, pumped by a nitrogen laser. An InGaAs detector was used. An excitation wavelength of 360 nm guaranteed high absorption near the silicon/electrolyte interface. The duration and the energy of the laser pulse were 0.5 ns and 160 µJ, respectively. A detailed description of the setup is given in ref 21. Results Measurements with a p-n Junction. Figure 3 shows results obtained with the p-n junction in 2 M KOH solution at 45 °C. The general features of the current-potential curve measured in the external circuit (Figure 3a) are similar to those found with a conventional p-type electrode, as described above. The broadening of the peak is the result of ohmic drop caused by the resistance of the thin p-type epilayer which serves as the working electrode. From Figure 3b it is clear that, in the peak range, the potential dependence of the short-circuit current mirrors closely that of the current measured in the external circuit (Figure 3a). In the passive range the short-circuit current drops to a value considerably lower than the corresponding passive current in the forward scan. The short-circuit current remains low in the return scan to negative potentials. Similar results were obtained for a range of experimental conditions. For example, at room-temperature both anodic current and short-circuit current are much lower than at 45 °C.
Figure 4. Current-potential curves of p-Si(100) electrodes in 2.0 M KOH solution in the presence (solid line) and in the absence of 5 mM CrO42- (dotted line) at 45 °C in the dark. The potential was scanned from -2.0 V at a rate of 5 mV/s.
From the results shown in Figure 3 we can conclude that in the potential range of the current peak electrons are injected into the conduction band of the p-type silicon and that this reaction accounts for at least 75% of the total anodic current. Electron injection ceases once a passive layer is formed. Electron Transfer to CrO42-. The presence of a low concentration of CrO42- (5 mM) does not alter the general features of the current-potential characteristics of p-type silicon in 2 M KOH solution at 45 °C (compare solid and dotted curves of Figure 4). The oxidizing agent suppresses the anodic current in the onset of the peak and gives rise to a slightly higher peak current. In addition, the passive current is higher and, in contrast to KOH solution, rises as a function of potential in the forward scan. These features are accentuated at higher CrO42- concentration. For example, in a 0.1 M solution the peak current is a factor of 2 higher than the value measured in chromate-free solution and the passive current is also higher. Results described
Etching and Passivation of Silicon
Figure 5. Current-potential curves of n-type (100) silicon in 2.0 M KOH solution in the presence (solid line) and in the absence of 0.1 M CrO42- (dotted line) at 45 °C in the dark. The potential was scanned from -2.0 V at a rate of 5 mV/s.
below suggest that CrO42- adsorbs on silicon and may adversely affect the quality of the protecting oxide as in the case of MnO4-.11 CrO42- does not lower the short-circuit current measured with the p-n junction electrode. We shall return to this point in the discussion. From the absence of cathodic current in the whole potential range of the solid curve of Figure 4, it is clear that CrO42- is not reduced electrochemically at p-type silicon, i.e., the oxidizing agent does not inject holes into the valence band of the semiconductor. That CrO42- can be reduced electrochemically at silicon is shown in Figure 5 for an n-type electrode in 2 M KOH solution at 45 °C. Addition of the oxidizing agent gives rise to a cathodic current at a potential almost 500 mV positive with respect to the onset potential for hydrogen evolution (solid line, Figure 5). A current plateau is observed in the forward scan and a peak in the reverse scan. Since the current was found to be proportional to the square root of the scan rate it is clear that CrO42- is being reduced at a diffusion-limited rate at negative potentials. Hydrogen evolution in chromate solution (solid line) occurs at a more negative potential than in KOH solution (dotted line, Figure 5). The results obtained with n-type silicon indicate that CrO42can be reduced by conduction band electrons. This is confirmed in experiments in which the p-type electrode is illuminated. In chromate solution a cathodic photocurrent is observed (solid curve, Figure 6) at a potential markedly positive with respect to the onset potential for photocathodic evolution of hydrogen in chromate-free solution (dotted curve, Figure 6). It is clear from the absence of a cathodic current in Figure 4 that CrO42- does not react electrochemically at p-type silicon in the dark. Nevertheless, at open-circuit potential CrO42- is reduced and its concentration decreases (Figure 7). At higher CrO42- content (not shown) the rate of the reaction does not depend on concentration while below 0.4 mM the rate is first order in CrO42- concentration (see inset of Figure 7). The zeroorder kinetics point to a step involving adsorption of the oxidizing agent prior to reaction; at higher concentration the surface coverage is complete and the reaction rate is independent of concentration. This reaction involving “chemical reduction” of the oxidizing agent occurs not only at open-circuit potential but also at more negative potentials. For example, at -2 V the kinetics of the reaction are exactly the same as those shown in Figure 7. When the temperature is lowered (e.g., to room temperature) the rate of the chemical reaction decreases markedly.
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5725
Figure 6. Current-potential curves of p-type (100) silicon in a solution containing 2.0 M KOH + 0.1 M CrO42- at 45 °C under illumination with low light intensity (solid curve). For comparison, the curve in 2.0 M KOH solution at the same light intensity is shown (dotted curve). The potential was scanned from -2.0 V at a rate of 5 mV/s.
Figure 7. The concentration of chromate as a function of time during chemical etching of (100) Si at open-circuit in 2.0 M KOH + 0.25 mM CrO42- at 45 °C in the dark. Inset shows the logarithm of the ratio of the concentration of chromate normalized with respect to the initial concentration as a function of time. Area of silicon: 14.4 cm2; and volume of solution: 100 mL.
The presence of CrO42- in solution does not influence significantly the chemical etch rate of silicon; at higher concentration (0.1 M) a slight decrease in the rate is observed. Photoluminescence. Photoluminescence could be detected in-situ from both (100) and (111) Si samples polarized at 3.0 V and then etched-back in 0.1 M NH4F solution (pH 4). This pretreatment is known to give a hydrogen-terminated surface. In the case of (111) Si the defect density after such a treatment is low, typically 1011/cm2 or less.22 When the electrodes were left to stand in the NH4F solution, the photoluminescence intensity decreased to a steady-state level. On increasing the pH from 4 to 13.5 we observed a further decrease in photoluminescence intensity. These changes could be followed in-situ at room temperature but not at elevated temperatures, for which the emission intensity was too low. Figure 8 (open circles) shows the dependence of the luminescence intensity from a (100) Si sample on the pH in the range 9-13.5. These measurements were carried out at 22 °C close to the open-circuit potential of the p-type electrode (a cathodic current density of 1 µA/cm2 was passed during the experiment). The decrease in intensity with increasing alkalinity is clear. The emission increases when the potential is increased from near open-circuit to the peak potential (closed circles, Figure 8).
5726 J. Phys. Chem. B, Vol. 105, No. 24, 2001
Xia et al. has the same effect on the surface morphology as strong oxidizing agents such as Fe(CN)63- and MnO4-.9,11 Suppression of pyramid formation occurs not just at the open-circuit potential but also at negative potentials (e.g., -2.0 V). Discussion
Figure 8. Plot of the logarithm of the photoluminescence emission intensity (IPL) of a hydrogen-terminated (100) Si surface during etching near the open-circuit potential (open circles) and the peak potential (closed circles) as a function of pH.
Figure 9. SEM micrographs of p-type (100) silicon samples etched for 30 min at 70 °C in 2.0 M KOH in the absence (a) and in the presence (b) of 10 mM CrO42-.
The same trends are observed on increasing pH and potential for (111) Si samples. Surface Morphology. The surface of a (100) silicon wafer, etched chemically at open-circuit in 2 M KOH solution at 45 °C, is rough. This roughness, also observed with p-type silicon at negative potentials, is caused by crystallographically oriented pyramids, as shown in Figure 9a. From Figure 9b it is clear that addition of a low concentration of CrO42- to the etchant
It is clear that an anodic current from an n-type semiconductor can only be due to electron injection into the conduction band. The results obtained in this work with the p-n junction show surprisingly that the anodic current from p-type silicon in alkaline solution is also to a large extent the result of a conduction band process. Since there is no species in solution with electron donor levels corresponding to the conduction band of silicon, one must conclude that a surface entity is involved. As pointed out above, electrons from Si-H bonds or Si-Si back-bonds cannot be directly excited thermally into the conduction band. Previous work has shown that the anodic current, including the maximum, scales with the chemical etch rate. This leads us to conclude that the electron-injecting state arises during chemical etching. Electrochemical reduction of CrO42- at silicon requires conduction band electrons, being observed at the n-type electrode in the dark and at the p-type electrode only under illumination. Clearly, the acceptor levels of the redox couple overlap with the conduction band of the semiconductor or with band gap states high in the gap. This would agree with the energy corresponding to the redox potential of the CrO42-/Cr(OH)4- couple (-0.96 V vs SCE).23 The conduction band edge of silicon is at -1.3 V vs SCE.14 The CrO42- ion is also reduced at p-type silicon in the dark via a “chemical” reaction which does not involve free charge carriers (Figure 7); there is no hole injection from this oxidizing agent. CrO42- reduction occurs not only at open-circuit potential but also at negative potentials corresponding to strong depletion. The rate of this chemical reaction decreases when the etching temperature is lowered, i.e., when the chemical etch rate is decreased. If chemical etching generates an intermediate with an energy level high in the band gap, as suggested above, then it seems plausible to expect direct electron transfer from such a state to the CrO42- acceptor levels at the surface. A similar explanation has been given by Gorostiza et al.25 for electroless deposition of nickel on silicon in NH4F solution. Electrons from activated states cause reduction of the metal ions. The photoluminescence measurements show a considerable drop in emission intensity from p-type silicon under conditions corresponding to chemical etching. This quenching of the photoluminescence becomes stronger as the pH of the solution is increased (Figure 8), i.e., as the etch rate increases. We conclude that during chemical etching bond rupture gives rise to surface species which are responsible for nonradiative recombination of photogenerated carriers. The emission recovers as the potential is increased from the open-circuit to the peak value, indicating a decrease in surface recombination as a result of a change in surface chemistry. From three separate pieces of evidence (electron injection, electron transfer to CrO42- and photoluminescence) we can conclude that an intermediate of the etching reaction must be important for the surface chemistry of silicon in alkaline solution. Under the present chemical etching conditions the surface of silicon remains hydrophobic and largely hydrogenterminated.10-13 The etching reaction can be represented simplistically by two main steps.10,12-14 In the first a Si-H surface
Etching and Passivation of Silicon bond is converted to Si-OH. This reaction is catalyzed by OHions
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5727 levels have an energy high in the band gap. The electrochemical results show this to be the case for CrO42-. These processes can be represented schematically by eqs 5 and 6
For the sake of clarity we only show one Si-Si back-bond. The presence of OH on the surface atom polarizes and destabilizes the Si-Si back-bond which is attacked by water
The OH from water adds to the “positively” charged surface atom to which an OH is already bonded. This further destabilizes the remaining back-bonds and the surface atom is dissolved, leaving behind an Si-H site at the surface. Reactions 1 and 2 are clearly complex. Schiffrin and co-workers26 noted an analogy between reaction 1 and the homogeneous OH--catalyzed decomposition of silanes. The latter occurs via an SN-type mechanism and involves a pentacoordinated silicon atom. Schiffrin et al. have shown that a similar reaction can be expected at the solid surface, although steric constraints will be much more severe in this case. One can visualize an activated state (intermediate I in eq 3) in which the incoming OH- ion drives out the “hydride” which, in the process, reacts with water to form hydrogen26
The course of the reaction in which water reacts with the Si-Si back-bond is less clear. Very likely the lone pair on the oxygen attaches to the surface silicon atom which is positively charged. This leads to a rearrangement of electrons, with the Si-Si bonding pair being pushed toward the subsurface silicon atom and the electron pair in one of the O-H bonds in water contracting toward the oxygen (eq 4). This gives rise to a transition state II in which, in effect, a “proton” is being transferred from H2O to a subsurface Si atom, and an Si-H bond is formed.
It is clear that intermediates I and II correspond to activated states with an energy considerably higher than those of the Si-H and Si-Si parent states. Consequently, one could expect electron transfer from the excited state to the conduction band of the semiconductor. In the case of n-type silicon this will result in an anodic current if the band bending at the surface allows the electrons to escape to the back contact. Current is observed in a p-type electrode if the hole concentration at or near the surface is sufficient to ensure recombination of holes with the injected electrons. Alternatively, the intermediate can transfer an electron directly to a species at the surface or in solution if its acceptor
In eq 6 two fast consecutive electron injection steps occur. In reactions 5 and 6 the electrons are transferred either to the conduction band or to the oxidizing agent. Under etching conditions the surface silicon atom is rapidly removed as Si(OH)4 and an Si-OH site is left at the surface
Equation 7 has two important implications. As a result of this reaction the surface becomes OH-terminated, in contrast to H-terminated in the case of a purely chemical etching mechanism (see eq 2). Equation 7 combined with eq 6 reduces the surface concentration of intermediate II. It is not immediately clear if only one of the intermediates (I or II) is responsible for anodic oxidation of silicon and chemical reduction of CrO42- or whether both are active in these processes. The passivation of the semiconductor points to the importance of intermediate II. Obviously, the electron-injection step which is responsible for the anodic current must lead directly or indirectly to the formation of a passivating oxide. It seems unlikely that reaction 5 will give rise to oxide growth. On the other hand, formation of a blocking oxide layer via intermediate II is easy to visualize
Whether passivation will occur depends on whether the surface atom dissolves (see eq 7) before the oxygen bridge is formed (eq 8), leading to an oxide island. The latter will be favored by a positive potential in an electrochemical experiment. Passivation can also be achieved by the chemical reaction; in this case the rate of reduction of the oxidizing agent must be equivalent to the passivation current in an electrochemical experiment. This requires favorable kinetics and a relatively high concentration of the oxidizing agent. This occurs with Fe(CN)639,10 and MnO -,11 but not with CrO 2-. As reported in the 4 4 Results section, the reduction of CrO42- is kinetically limited at higher concentration. From Figure 3 it is clear that the mechanism of oxide formation changes when the potential enters the passive range. Electron injection is no longer possible since chemical etching stops. It is known that the oxide layer continues to grow as the
5728 J. Phys. Chem. B, Vol. 105, No. 24, 2001 potential is made positive (the thickness depends on the potential).6,10,27 On the other hand, the oxide on n-type silicon does not thicken in the passive range unless the electrode is illuminated.27 Clearly, a reaction involving valence band holes is responsible for oxide growth in this range. The sharp drop in the photoluminescence intensity from a HF-pretreated (hydrogen-terminated) silicon surface upon immersion in alkaline solution can be attributed to the creation of nonradiative recombination centers. These are very likely to be surface states such as intermediates I and II formed as a result of bond rupture during chemical etching (eqs 3 and 4). The increase in emission intensity as the potential of a p-type silicon electrode is raised from the open-circuit to the peak potential (Figure 8) is consistent with the electronic passivation of recombination centers by conversion of the reactive surface to Si-OH or Si-O-Si. In the present mechanism intermediate II can react in three different ways: (i) via the chemical etching path in which an Si-H bond is formed (eq 4), (ii) by electrochemical oxidation with the electron being removed via the external circuit (this gives rise to anodic current), and (iii) by chemical oxidation with the electron being transferred to the CrO42- ion. Since the third reaction competes for electrons with the second, one might expect the oxidizing agent to decrease the peak current and the short-circuit current of the p-n junction. This is not the case (see, for example Figure 4). To understand this effect one should realize that the rate of chemical etching (expressed as a current density) is a factor of more than 13 higher than the peak current density, which in turn is higher than the maximum rate of chemical oxidation of the intermediate (0.08 mA/cm2). Since most of the intermediates react to give Si-H species (the chemical etching path), the small contribution of electron transfer to the oxidizing agent has only a slight influence on the rate of the other slow process, electron transfer to the back contact. That CrO42- in fact increases the peak current is due to its influence on oxide formation. The Fe(CN)63- ion is not adsorbed on silicon and the steady-state peak current is not changed by the oxidizing agent.10,11 As we have shown, the chemistry of silicon in alkaline solution is determined by the instability of the Si-H bond. The OH--catalyzed reaction of the surface hydride sets off a chain of reactions which include chemical rupture of back-bonds, electron injection into the conduction band, and electron transfer to solution. Silicon hydride is much more stable at lower pH; the chemical etch rate of the semiconductor decreases markedly as the pH is decreased. As a result no anodic current is observed from hydrogen-terminated n-type Si in HF solution. However, addition of Br2 to a HF solution changes the surface chemistry of silicon dramatically.28 Both (100) and (111) surfaces are etched via a chemical mechanism and etching is accompanied by electron injection into the CB. The latter is revealed as an anodic current from an n-type electrode at positive potentials. It has been shown28 that Br2 adds across the back-bond in a manner analogous to that of water in eq 4. It was suggested that an activated intermediate state of this reaction provides electrons in the conduction band, which can be detected as an anodic current. The presence of a high density of pyramids, essentially constant in size, on a rapidly etching (100) surface suggests that both the pyramids and the (100) base are being removed at approximately the same rate. Factors which enhance the rate of attack on the pyramids with respect to the etch rate of the (100) surface will tend to remove the pyramids. In previous work we have shown that the polarization of the silicon at a potential
Xia et al. between current onset and the peak can completely remove the pyramids.9 It is quite striking that a weak oxidizing agent such as CrO42- has an effect on the surface morphology of chemically etched silicon similar to that of anodic polarization. Suppression of pyramids is observed with CrO42- not only at the open-circuit potential but also at considerably more negative potentials. According to the mechanism proposed above, the main distinction between etching at open-circuit and under applied potential is a difference in surface chemistry. Open-circuit etching leaves the silicon surface mainly hydrogen-terminated while electron injection resulting from anodic polarization produces Si-OH sites on the surface. The chemical reduction of CrO42- will also lead to a hydroxylated surface. The etch rate of (100) silicon is independent of the applied potential up to the peak potential and is also not influenced by oxidizing agents in solution at lower concentration. On the other hand, polarization at a potential positive with respect to the open-circuit value has been shown to accelerate the dissolution of the slow-etching (111) surface.12,29-30 Etch pits are nucleated on terraces and the etch rate is increased with respect to that of the (100) surface. As a result the (111) facets of pyramids formed on the (100) surface during chemical etching are “peeled away” and the pyramids disappear.31,32 These ideas are supported by Monte Carlo simulations.33 Since oxidizing agents have the same effect on surface chemistry as anodic polarization (they remove electrons from surface intermediates (eqs 5 and 6) to give Si-OH sites) they should also influence the surface morphology in the same way. Since, however, electrons can be transferred to the oxidizing agent even at negative potentials, an improvement in the surface morphology is observed in a wide potential range. Conclusions It is proposed that activated states resulting from the attack by water on surface bonds play a pivotal role in the surface chemistry of silicon in alkaline solution. From such a state an electron can be injected into the conduction band giving rise to anodic current (and passivation) for both n-type and p-type silicon. Alternatively, an electron can be transferred to an acceptor in solution with energy levels high in the band gap, thus explaining the chemical reduction of weak oxidizing agents at an etching surface. The intermediate can act as a nonradiative recombination center, quenching the photoluminescence from the semiconductor. Transfer of an electron from the intermediate to the back contact or to solution changes the chemistry of the surface from that observed under chemical etching conditions. The conversion of a hydrogen-terminated to a hydroxylated or oxidized surface can explain the increase in emission intensity in photoluminescence measurements and the marked changes in surface morphology. While the reaction series described in this paper is schematic and the nature of the reactive intermediates somewhat speculative, we believe that the ability of the model to account for some surprising features of silicon surface chemistry justifies the assumptions we have made. Acknowledgment. This work was carried out as part of a Brite/EuRam project BE97-4371. References and Notes (1) Kendall, D. L. Appl. Phys. Lett. 1975, 26, 195. (2) Seidel, H.; Csepregi, L.; Heuberger, A.; Baumga¨rtel, H. J. Electrochem. Soc. 1990, 137, 3612. (3) Vellenkoop, M. J.; van Rhijn, A. J.; Lubking, G. W.; Venema, A. Sensors Actuators A 1991, 25-27, 699.
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