J. Phys. Chem. 1995,99, 9472-9478
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Atomic Structure of Si Surfaces Etched in TritodNaOH Solutions P. Allongue*3t UPR 15 CNRS, Universit&P. & M. Curie, 4 Place Jussieu, Tour 22, F-75005 Paris, France
V. Kielingt LACOR, UFRGS, Av. Osvaldo Aranha 99, 7" andar, 90000 Porto Alegre, RS, Brazil
H. Gerischer Fritz-Haber Institut der Max Planck Gesselschafi, Faradayweg 4-6, 0-14195 Berlin, Germany Received: November 4, 1994; In Final Form: March 13, I995@
In situ real-time scanning tunneling microscopy is used to study Si etching in alkaline solutions modified by addition of Triton, a commonly used nonionic surfactant. On Si( 11l), time sequences of images, with the resolution of atomic steps, show that the rate of the nucleation of etch pits is decreased on terraces which reduces the etch rate. Results are interpreted in term of formation of a self-assembled micellar layer of Triton molecules whose disorder in the vicinity of atomic scale defects improves surface order compared to etching in NaOH. This layer being not bound to the surface, the (1 x 1)-H-terminated Si( 111) surface is always imaged at high resolution. Potential applications of Triton solutions to the preparation of flat surfaces of Si( 111) and (100) are discussed.
Introduction The advent of scanning tunneling microscopy (STM)and its recent use as in situ probe at the atomic level has opened new possibilities for surface science studies at the solid-liquid interface. In the case of semiconductors in situ STM observations can improve our knowledge about technological processes. Among them chemical etching is very important since wafers and chips of semiconductors are set in contact with aqueous solutions on many occasions in microelectronics industry. In the case of silicon HF-based solutions are generally used for oxide stripping. Recent studies have shown that perfectly atomically smooth surfaces could be obtained by using ammonium fluoride instead of HF.' Alkaline solutions are used for micromachining Si since etching is highly anisotropic (the ratio of the etch rate of the (100) face to the etch rate of the (111) face is as large as -100).293 The dissolution rate approaches 10 h n i n in 2 M NaOH for the (1 11) face$.5 which is 5 times larger than in 40% N&FS6 This probably explains that disordered arrays of terraces are observed with in situ STM in early stages of dissolution in NaOH unless the surface is cathodically biased.8 These phenomena have been accounted for by reaction models at a molecular level in NaOH,5 N h F solutions,6 and acidic HF.7 This paper examines whether alkaline solutions modified by organic additives could be used to prepare ideally flat Si surfaces. Corrosion inhibitors are widely used in metal ind~stry.~ In comparison their use is less developed with semiconductors.I0 Different additives such as hydrazine, ethylenediamine' ' . I 2 or alcohol^'^ have been used with Si etching in alkaline media. Isopropyl alcohol has been mainly used in HF solution^.'^*'^ Peiner and SchlachezkiI6 added Triton to aqueous ammonium bifluoride solutions to improve electrochemical carrier concentration profiling. To our knowledge 'We dedicate this work to the memory of Prof. H. Gerischer. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, May 1, 1995.
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there is however no STM report describing the role of such additives for dissolution at the atomic level on either metals or semiconductors. Only gross effects have been characterized ex situ. STh4 reports considering organic molecules in relation with an electrochemical process concemed only metal deposition (e.g., Cu on AU"-~~). Except for a preliminary study published recentlyz0this paper is a first in situ STM characterization, at the atomic level, of the effect of Triton molecules on Si(111) etching. This additive belongs to the family of non ionic surfactants. Here we present new results with quantitative determinationsrelative to the effect of molecules on the etch rate and reaction anisotropy on the basis of combined real-time in situ STM imaging and electrochemical characterizations. The role of the different functions of molecules, which consist of a hydrophobic saturated alkane chain and a long hydrophilic ethylene oxide ether tail separated by a benzene ring, is discussed. Results show that atomic scale defects are very critical to the action of the organic layer. Application of TritodNaOH solutions to etching of Si( 100) is also discussed.
Experimental Section Low-doped (1 11) and (100) Si samples, respectively 0.5 and 3-6 S'2 cm, were cut from as-received wafers (Siltronix,France).
The misorientation was 0.7" and 4" for the (111) face and nominal for the (100) face. Samples were first cleaned in hot trichloroethylene,acetone, and methanol before the native oxide was removed in 40% HF (for 1 min). A final etching in 40% m F (3 min) was then performed with the (111) samples. This last step was omitted with (100) surfaces to avoid facetting. A rear-side ohmic contact was made with an In-Ga alloy. Electrochemical measurements were conducted in a classical three-electrode electrochemical cell, with a Pt counter electrode and a mercury electrode in saturated K2S04 as reference (hereafter SSE). Solutions were bubbled with Ar to remove dissolved oxygen and prepared with reagent-grade chemicals using bidistilled water. X- 100 Triton solutions were prepared 0 1995 American Chemical Society
Si Surfaces hepared in Triton/NaOH Solutions
J. Phys. Chem., Vol. 99, No. 23, 1995 9473
(1111
1-
i
-1.5
b
7-\
(111)
-
V I SSE
-1io
- 0.5
-
VlSSE
-0.5
VlSSE
-1.0 Figure 1. Initial I-U scans of well oriented n-Si(ll1) and (100) showing the effect of oxygen in 2 M NaOH. Curves a: with Ar bubbling. Curves b: with 0 2 dissolved. See text for exact experimental procedure.
by addition of 2 drops to 800 cm3 of 2 M NaOH, which nearly corresponds to 5 x h4ll of Triton. In the following this solution is referred to as nominal solution. Further dilution have been used by adding NaOH. The STM was a home built microscope, coupled to a potentiostat in a four-electrode configuration. The tips were etched in NaOH and covered with Apiezon wax, leaving the very end free. The reference electrode was a quasi-reference electrode made with a Pd wire loaded with hydrogen, by evolving H2 in NaOH for 15-30 min. Its potential was nearly constant around -1.4 V against SSE over several hours. The tunnel current ranged between 0.2 and 0.5 nA, and images could be acquired within 30-50 s (typical x-scan frequency 3-4 Hz). Images are presented as top views and are unfiltered data. Heights are decreasing from white to black.
Results Potentiodynamic Characterizations. Before the potential was scanned, from a potential slightly negative of the rest potential, freshly prepared electrodes were held at the rest potential (also abbreviated by OCP for open-circuit potential in the following) for 2 min. The scan rate was 10 mVls. Figure 1 examines the role of dissolved oxygen in 2 M NaOH. At anodic bias all curves pass through a broad maximum whose shape depends on the presence of 0 2 and whose amplitude is also highly orientation dependent. With the well oriented (1 11) electrode two shallow peaks centered around - 1.4 and - 1.1 VlSSE are resolved. On the (100) face there is a unique peak, centered around -1.25 V and with an intensity much greater than for the (1 11) face. The presence of 0 2 significantly alters curves in two ways. For Si( 100) a shoulder appears on the left side of the anodic peak. The intensity of the left shallow peak is decreased in the case of the (111) face. A small negative shift of OCP is also observed with both orientations. The effect of Triton concentration is shown in Figure 2 for deaerated solutions although no effect of 0 2 was observable in TritodNaOH, except in the most dilute solution. Curves (a) are identical to curves (a) of Figure 1 and serve as reference. With increasing Triton concentration the anodic peak becomes narrower for the (100) and a cathodic shift of H2 evolution is
- 0.5
Figure 2. Initial I-U scans of well oriented n-Si(ll1) and (100) in deoxygenated T r i t o f l a O H solutions. The Triton concentration in 2 M NaOH is 0, 5 x and 5 x lo-' M, respectively for curves (a)-(c). Curve (d) has been recorded after curve (c), after the OCP has returned to its initial value. In the other solutions, the second scans were identical to initial ones after oxide dissolution at OCP.
observed although the rest potential was continuously shifted positively (see curves (a)-(c)). In the case of Si(ll1) no such significant effects were noticed. Immediately after completion of the first anodic scan, the OCP of Si electrodes was systematically more positive than initially. It returned however to its original value after sufficient time. This process was always much faster with the (1 11) specimen. With (100) electrodes, the process lasted few minutes and the time constant was longer in Triton/NaOH than in NaOH. Except for the most dilute (by a factor of 100) Triton solution, the subsequent scans were then identical to initial ones, meaning that the oxide layer formed anodically had been dissolved by holding the sample at OCP, just as in NaOH.5 In Figure 2 curve (d) was recorded after the OCP was stabilized following the initial scan (c). Even a cathodic treatment of the surface was unable to restore curve (c). HF etching was necessary to recover curves (c). Note that the above effects were also observed if the potential scan was stopped slightly negative of the peak maximum. In Situ STM Investigations. Since n-Si samples were used and since the junction semiconductorlelectrolyteltip behaves similarly to a diode,2' in situ imaging was restricted to cathodic potentials to tunnel electrons from the conduction band edge into the tip. Figures 3-5 are time sequences of STM images recorded on well-oriented (tilted by 0.7") n-Si( 111) in the nominal Triton solution under different electrochemical conditions. Before each of the series the surface was prepared so as to present only atomically smooth terraces, separated by monoatomic steps (3.1 A high), which are descending from left to right in all series. Unless otherwise specified images were recorded about 45 s apart in these series. Under strong H2 evolution (also abbreviated as HER for hydrogen evolution reaction in the following), Figure 3 shows that dissolution occurs according to a perfect step-flow mechanism as in NaOH under the same conditions.8 Terrace edges migrate laterally toward the upper left corner of images and stay almost parallel to each other. No nucleation of etch pits
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d d
e
C
C P
1
500i
Figure 3. Time sequence of in situ STM images taken on 0.7" misoriented n-Si(ll1j in the nominal Triton solution. The cathodic current is 150 ,uA/cm2. Frames are 1400 x 1400 A2 and images are recorded 45 s apart.
is seen in the sequence. Notice that some small impurities (see arrows) may pin the motion of steps (images a and b). Etching of Si is here completely anisotropic. Decreasing the H2 current accelerates the nucleation of etch pits (3.1 8, deep) on terraces. This is seen in Figures4 and 5, where imaging was found to be less difficult than in NaOH. For a hydrogen current decreased to 50 pA/cm2, Figure 4 shows that all triangular pits, except one, are appearing close to the top of terrace edges. This is remarkably different from the behavior in pure NaOH where pits nucleate much faster under the same electrochemicalconditions (see ref 8, Figure 5) and regardless of step position. The origin of the large pit (one double layer deep) created in the middle of the widest terrace is unusual since no defect is visible. The perfect triangular shape of this pit reflects the 3-fold symmetry of the Si( 111) surface. Its exceptional size is interesting to evaluate the effect of Triton on the reaction rates at the atomic scale. Using a recent Monte Carlo simulation,22we found that molecules are efficiently reducing the reaction rate on terrace sites (vertical monohydrides) while the effect is less pronounced at step edges. This point will be published separately. In Figure 5 the sample was polarized cathodically to reach an HER current of 150 pA/cm2 in the initial image. The potential was then stepped positively between images (a) and (b) so as to decrease the current to 10 pA/cm2 during images (b) and (c). That steps are predominantly roughened in the top of image (b) is simply due to the upward y-scan direction of the tip. Pits are again appearing preferentially close to terrace edges, either on the top or on the descending terrace. In the
5ooA Figure 4. As in Figure 3 for a cathodic current of 50 pA/cm2. Pits initiation is marked by arrows. Frames are 1400 x 1400 A2.
third image the reaction becomes already generalized and the terraced structure becomes hardly visible. Note also the rough edges of the nearly triangular 3.1 8, deep pits visible in the same image. Stepping the potential back to its initial value, slowly restores a good resolution in images (d)-(f), suggesting that the loss of resolution in image (c) was probably due to fast removal of Si atoms from the surface. Also note that the etch rate is larger in images (d)-(f) than in Figure 3 despite a similar HER rate. The attack slows down when terrace edges become smoother and pits have straight edges. Comparing images (a) and (f) reveals that the long-range stepped structure of the surface has been almost preserved despite fast etching. With a higher resolution, terraces in images (d)-(f) were not as smooth as in image (a) and presented many shallow 0.5 8, deep depressions attributed to isolated Si-OH groups formed during etching close to OCP.23 Such depressions were slowly removed with time after new topmost atom layers are exposed to solution by the lateral flow of steps. Figure 6 focuses on the very initial stages of etching. The three sequences consist of two images, the first image recorded with a H2 current of 150 pAlcm2 (left column), while the potential was stepped closer to OCP just before recording the second image (right column). In series (a)-(c) the cathodic current was respectively decreased to 22, 10, and 5 pA/cm2. In all cases the y-scan direction of tip was upward in the second image, which explains the apparent faster attack in the top of images. Note the very rapid roughening at step edges, forming round-shaped nanostructures, and the preferential nucleation of etch pit close to atomic scale defects (steps, preexisting pits), which was not observed in NaOH. Our attempt to image Triton molecules were unsuccessful. Under the tunneling conditions used in this study Figure 7 shows
J. Phys. Chem., Vol. 99, No. 23, 1995 9475
Si Surfaces Prepared in TritodNaOH Solutions
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b
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500 A c-------( Figure 5. As in Figure 3. The cathodic current is 10 ,uA/cm2 during images (b)-(c). It is 150 ,uA/cm2 in other images. The potential was stepped without stopping tip scanning between frames (a) and (b) or between frames (c) and (d). Frames are 1400 x 1400
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the ideal (1 x 1) atomic structure of Si(111) with an inter atomic distance of 3.8 8, and an atomic corrugation of -0.3 A as in NaOH.8 The cathodic current on the sample was ca. 100 PA/ cm2. This observation is characteristic of a Si(ll1) surface saturated by Si-H monohydrides. This image probably means that molecules are not tightly bound to Si and/or that the tip exercises too strong an interaction and is able to push them away from the surface. Last Figures 8 and 9 are two typical large-scale images respectively taken on a vicinal Si( 111) surface in NaOH (no Triton added) and on n-Si(100) in TritodNaOH (nominal solution). Both surfaces are cathodically polarized for imaging, but were shortly, ca. 1 min, etched at OCP. The image of the vicinal (111) surface (Figure 8) show narrow terraces whose width is in agreement with the 4" tilt of the sample. Note that terraces are perfect terraces, with no pits and straight edges which is impossible to achieve with better oriented (111) surfaces in NaOH.8 The (100) surface presents anisotropic surface structures (Figure 9) with a long-range modulation and nearly 90" angles, suggesting that very narrow (100) terraces are exclusively exposed to solution. Atomic resolution has however not been achieved on this surface. The gray scale is 20 8, high and would be much larger in the absence of Triton, even if the sample had been polarized negatively from the very beginning of the experiment.
Discussion Etch Rate Determination. Since the dissolution of Si(111) is practically a layer-by-layerprocess the local etch rate R n can
Figure 6. Three series of two images. Left images were recorded at cathodic bias (150 ,uA/cm2). Right images were recorded under a HZ currents of 22, 10, and 5 ,uA/cm2, respectively. The potential was stepped between frames without stopping tip scanning. The y-scan direction is upward in right images. Frames are 1400 x 1400 A'.
20 A
Figure 7. Atomically resolved in situ STM image of n-Si( 1 1 1) in 2M NaOH in presence of Triton. The frame is 91 x 21 A2. The cathodic current is ca. 100 pA/cm2.
be derived from sequences of STM images by measuring the volume of material removed.8 This volume is given by (AS/ S)h [AS is the area of terraces removed during the time A?, h = 3.14 8, is the step height and S the area of the surface under observation]. Rn (1) was simply calculated by dividing this quantity by time. On a given series the accuracy of measurements is better than 10%. The etch rates below have been derived from data recorded on the same day in each of the solutions to avoid problems with temperature variations which affect the rate of chemical reactions. Figure 10a compares plots of the etch rate vs the hydrogen current obtained in the nominal TritodNaOH solution and in NaOH. This representation is more convenient than plotting the etch rate vs the potential for several reasons. First, values of potentials may vary by a few tens of millivolts from day to day for a given current due to variations of the reference potential. Second, impregnation of the surface by hydrogen had the tendency to increase the cathodic current with time at a defined potential. The difference between the two curves is
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9476 J. Phys. Chem., Vol. 99, No. 23, 1995
100 li
500
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20 1
0
0 200
0
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Figure 8. In situ STM image (470 x 480 A*) taken in 2 M NaOH on a vicinal n-Si( 11 1) surface tilted by 4' The cathodic current is 100 ,uA/cm2for imaging, but the surface was etched at OCP for ca 60 s.
most visible as the sample becomes polarized close to OCP (Le., for small hydrogen currents). The etch rate is nearly 3 times smaller in TritodNaOH, a value which approaches the one in 40% NH4F.6 The reaction anisotropy can be defined as the fraction of Rll R, with RI the averaged speed with which steps are laterally migrating in initial stages. The larger this ratio, the better the etching anisotropy. RI was estimated on places where the step motion was not affected by the growth of pits. Figure 10b shows that the ratio RID?, is significantly enhanced by triton which quantitatively accounts for earlier observations concerning the large size of the triangular etch pits (see Figure 5 ) and Monte Carlo simulation.22 Results also show also that the frequency of pit formation is typically 1 order of magnitude smaller in TritodNaOH than in NaOH. Mechanism of Dissolution and Inhibition. To qualitatively understand STM observations in Triton/NaOH solutions, the mechanism of Si etching in alkaline bases and the structure of organic molecules need be considered. Figure 1 1 focuses on the initial (rate-determining)hydrolysis reaction Si-H Si-OH of the reaction of dissolution, the following removal of the Si-OH group being common to the two reaction paths. Complete explanations about Si etching reactions in NaOH are given in ref 5. Figure 11 shows the reaction Si-H Si-OH is either purely chemical or electrochemical. The chemical path is a one-step process involving OH- ions as catalyst and molecular water as reactant (Figure 1 la). The electrochemical hydrolysis (Figure 1lb) is a twostep process which begins with the reversible acid-base dissociation of the Si-H bond and continues with a reaction
-
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I
1000
Figure 9. In situ STM image (1400 x 1400 A*) taken on n-Si( 100) in the nominal Triton solution. The cathodic current is 150pA/cm2 for imaging but the surface was etched ca. 60 s at OCP.The gray scale corresponds to 20 A. The cross-sectional scan line correspond to cut AA'.
with H20. There are two electrons in excess which are injected into the conduction band from an activated surface state, similarly to which has been described Given the hard-sphere diameter of H20 (about 2.9 A24)and the Si-Si bond length (2.35 A), strong steric hindrance is expected in Figure 1 la, which explains that the chemical path is at the origin of the remarkable orientation dependence of Si etching in NaOH (this reaction path represents 90% of the dissolution at OCP in NaOH5). The electrochemical component is in contrast relatively isotropic, because the initial Si-H dissociation may occur at steps and on terraces as well (Figure 1lb). This path is responsible for pit initiation. As soon as the Si-OH group is formed, it is removed from the surface because the adjacent Si-Si back bonds are weakened. This is very rapid at steps. On terraces the necessity to disrupt three Si-Si back bonds underneath is slower and requires that several neighboring Si-OH bonds are formed to initiate a pit.22 The structure of Triton X-100 (formula C(CH3)y CH2C(CH3)2C6H4(0CH2CH2),0H) with n = 9- 10) presents three important functions shown in Figure 12 (top). The saturated alkane chain and the long ethylene oxide ether tail are separated by a benzene ring. Above the critical micellar c~ncentration?~ which is our situation in the STM cell (nominal solution), we tentatively draw in Figure 12 (bottom) the structure of the organic layer at the interface. This organization is anticipated for bias negative of the anodic peak (Figures 1 and 2) because both the H-terminated Si surface and saturated hydrocarbon chains are hydrophobic. The ethylene oxide chain is in contrast hydrophilic, which favors interactions with the solution and pulls it outward of the surface. This view is
Si Surfaces Prepared in TritodNaOH Solutions
J. Phys. Chem., Vol. 99, No. 23, 1995 9477
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in interactions between benzene rings are indeed anticipated when molecules are localized on adjacent terraces (the step in Figure 12 is drawn on scale). Below a critical concentration in Triton, formation of an ordered layer will however become impossible since benzene rings will loosely interact with each other. Depending on steric space on the surface, Triton molecules may even bend and probably attach themselves to the surface, forming Si-0-R
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9478 J. Phys. Chem., Vol. 99, No. 23, 1995
groups. The irreversible alterations of I-U curves after the initial scan in the most (deaerated) dilute TritodNaOH solution are indeed very similar to those induced by dissolved oxygen in NaOH (compare curve (d) in Figure 2 with curve (b) in Figure 1). In NaOH changes are due to enhanced Si-OH bond formation during 0 2 reduction since the reaction produces H202 which can chemically hydrolyse Si-H surface bonds forming Si-OH and H20. Note that the Si-0-R bonds formed in dilute TritodNaOH seems quite resistant to cathodic reduction since J3F etching is necessary to restore completely the initial surface, presumably by breaking the Si-0-R group between Si and 0. Application to Surface Preparation. From the above discussion, the adsorbed Triton layer can be described as a thin organic membrane floating on the top of the surface. It sufficiently sticks to the surface (van der Waals forces) to protect it on its flat parts while any protrusion is etched preferentially by local disorder in the film. This is a self-repairing mechanism, which very much resembles the role of the oxide layer during surface electropolishing. Our observations on Si( 111) show that extended atomic scale defects are sufficient to induce this disorder. On stepped (111) surfaces the organic layer induces a longrange structuring process due to preferential etching at terrace and etch pit edges, leaving an ordered surface with regularly spaced steps. This can be seen in Figure 5, where step edges remain regularly spaced before and after the sequence of images despite fast dissolution during two images. There the dissolution is nearly 2-D. In Figure 4, the spacing of terraces becomes also more regular in image (f) than in image (a) (bottom), demonstrating again the ability of the Triton layer to remove surface inhomogeneities. In the absence of Triton fast terrace pitting occurs.8 When terraces are narrow, they can however be left ideally unpitted by etching because the flow of steps is faster than the rate of pit nucleation and growth. This is well verified when etching a vicinal surface in NaOH, as in Figure 8. With Si(100) surfaces, which are more relevant in technology, it is interesting to compare etching in TritodNaOH and in 40% N&F. After treatment in 40% N&F, Si(100) surfaces are disordered and present flat areas and a large density of small pyramids. Etching in TritodNaOH leads in comparison to an improved surface structure which is flatter. There are in particular no pyramid visible in Figure 9 and the surface is also much better ordered. For a 10 s exposure to 40% W F (see Figure 1 of ref 27) the gray scale is comparable with that in Figure 9. For longer exposures, surfaces etched in 40% W F are however much rougher than in TritodNaOH. This is encouraging to prepare flat (100) surfaces.
Conclusions Si etching in TritodNaOH solutions has been studied by in situ STM. Observations suggest the formation of a selfassembled micellar organic layer which inhibits etching. The most interesting effect is however the surface smoothing induced
by preferential etching on places where the organic layer is disordered, which occurs close to protrusions, even those at the atomic scale. On the (1 11) face the initial staircase structure can be preserved by addition of Triton. Preliminary results with the (100) specimen are promising since the etched surface exposes only (100) planes to solution and is flatter than after etching in 40% N&F. With respect to technology, the conclusions derived from this study should be also applicable to any etching solutions of Si as long as the surface remains H-terminated [this chemical state of the surface is responsible for the arrangement of molecules at the interface]. It is further important to note that Triton does not adsorbed on the surface by forming chemical bonds. This should avoid organic contamination after rinsing.
References and Notes (1) For a review, see: Higashi, G. S.; Chabal, Y. J. In Handbook of Semiconductor Wafer Cleaning Technology; Kem, W., Ed.; Noyes Publications: Park Ridge, NJ, 1993. (2) Kendall. D. L. A'D. D ~Phvs. . , Lett. 1975. 26. 195: Annu. Rev. Mater sci. 1979, 9, 373. (3) Seidel, H.: Csepreni, L.; Heuberger, H. J. - A,; Baumgiirtel, Electrochem. SOC.1990, i37,3612, 3626. (4) Glembocki, 0. J.; Stahlbush, R. E.; Tomkiewicz, M. J. Electrochem. SOC. 1989, 132, 145. ( 5 ) Allongue, P.; Kieling, V.; Gerischer, H. J. Electrochem. SOC. 1993, 140, 1018. (6) Allongue, P.; Kieling, V.; Gerischer, H. Electrochim. Acta, in press. (7) Gerischer, H.; Allongue, P.; Kieling, V. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 753. (8) Allongue, P.; Kieling, V.; Gerischer, H. J. Electrochem. SOC. 1993, 140, 1009. (9) Rozenfeld, I. L. In Corrosion Inhibitors; Mac Graw Hill: New York, 1981. (10) Kern, W. RCA Rev. 1978, 39, 278. (11) Finne, R. M. J. Electrochem. SOC. 1967, 114, 965. (12) Camubell. S. A.: Shiffrin. D. J.: Tufton. P. J. J. Electroanal. Chem. 1963,344, 2 i i . (13) Price. J. B. In Semiconductor Silicon; Huff, H. R.. Burgess, R. R., Eds.; 'Electrochem. SOC. Soft Bounds; The Electrochemicil Society: Princeton: NJ, 1973. (14) Verkaverbeke, S.; Schmidt, H. F.; Meuris, M.; Martens, P. W.; Heyns, M. M.; Werkhoven, C.; de Blank, R.; Philipossian, A. Techn. Con$ Semiconductor/Europe '93. (15) Vatel, 0. Ph.D Thesis, Universiti de Marseille-Luminy, 1993. (16) Peiner, E.; Schlachetzki, A. J. Electrochem. SOC. 1992, 139, 552. (17) Batina, N.; Will, T.; Kolb, D. M. Faraday Discuss. 1992, 94, 1992. (18) Nichols, R. J.; Bach, C. E.; Meyer, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 1012. (19) Rynders, R. M.; Alkire, R. C. J. Electrochem. SOC. 1994, 141, 1166. (20) Allongue, P.; Bertagan, V.; Kieling, V.; Gerischer, H. J. Vac. Sci. Technol. 1994, 812, 1539. (21) Allongue, P. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; Weinheim; Vol4, in press. (22) Allongue, P.; Kasparian, J. Microscop. Microstruct. Microanal. 1994, 5, 257. (23) Allongue, P. To be published. (24) Thiel, P. A.; Madey, T. E. Surf: Sci. Rep. 1987, 7, 211. (25) Hiemenz, P. C. In Principles of colloids and surface chemistry, 2nd ed.; Marcel Dekker: New York, 1986. (26) Ward, R. N.; Davis, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 7141. (27) Neuwald, U.; Hessel, H. E.; Feltz, A,; Memmert, U.; Behm, R. J. Surf: Sri. Lett. 1993, 296, L8. Jp9429964
