H2O2 Electrochemical Interface Studied In Situ by Infrared

J. Phys. Chem. B 2000, 104, 5961-5973

5961

GaAs/H2O2 Electrochemical Interface Studied In Situ by Infrared Spectroscopy and Ultraviolet-Visible Ellipsometry Part I: Identification of Chemical Species B. H. Erne´ ,*,† F. Ozanam,† M. Stchakovsky,‡ D. Vanmaekelbergh,§ and J.-N. Chazalviel† Laboratoire de Physique de la Matie` re Condense´ e (U.M.R. 7643 du C.N.R.S.), EÄ cole Polytechnique, 91128 Palaiseau, France, I. S. A. Jobin-YVon-Spex, Groupe Instruments S. A., 7 route d’Egly, 91290 Arpajon, France, and Debye Institute, Utrecht UniVersity, P.O. Box 80000, 3508 TA Utrecht, The Netherlands ReceiVed: February 1, 2000; In Final Form: April 8, 2000

Chemical species at the electrochemical interface between n-GaAs(100) and H2O2 in 0.5 M H2SO4 are identified by in situ infrared spectroscopy and in situ ultraviolet-visible ellipsometry. Under anodic conditions, H2O2 dissolves GaAs chemically, and the surface appears rough but clean of other phases. The sulfate concentration near the surface increases to compensate for the charge of the Ga3+ ions produced by dissolution. Under cathodic conditions, H2O2 is reduced electrochemically, but chemical GaAs dissolution is never completely suppressed; both dissolution products are indirectly detected, the Ga3+ ions because they lead to an increase in the local sulfate concentration and HAsO2 because part of it is reduced cathodically, giving islands of a solid arsenic hydride, most likely As2H2. Variations in the amount of As2H2 and its surface coverage by adsorbed H and OH groups affect the H2O2 reduction rate. A kinetic model is proposed for the interfacial chemistry and electrochemistry. Numerical simulations reproduce the main features of experimental currentpotential scans, including the presence of a negative slope region related to oscillatory behavior.

Introduction Aqueous hydrogen peroxide (H2O2) solutions are widely used in optoelectronic device technology for etching III-V semiconductor materials such as gallium arsenide (GaAs).1-12 Until now, the complex interaction of H2O2 with semiconductor surfaces has been studied mainly by using classical electrochemical techniques and ex situ surface analysis. However, electrochemical methods alone do not yield enough information to understand the complicated behavior without making many conjectures, and ex situ surface analysis neglects species present under conditions of electrochemical current flow and crucial in the reaction mechanisms. Here, the potential-dependent chemical composition of GaAs surfaces in the presence of H2O2 is studied for the first time using in situ infrared (IR) spectroscopy and in situ ultraviolet-visible (UV-vis) ellipsometry. The electrochemistry of this system is complicated by current oscillations occurring under potentiostatic conditions and potential oscillations occurring under galvanostatic conditions.13,14 A timeresolved investigation of chemical changes occurring under potentiostatic oscillation conditions is presented in the following paper (part II). The present paper (part I) focuses on the identification of interfacial chemical species and their effects on the current-potential characteristics. A preliminary examination of the literature on GaAs in H2O2 solutions can be somewhat confusing because of the great variety of reported results: polishing, anisotropic etching, oxide growth, electroless photoetching, and so forth.1-12 The common * To whom correspondence should be addressed. E-mail: ben.erne@ chimie.uvsq.fr. Fax: +33 1 3925 4381. Present address: Institut Lavoisier (IREM, UMR CNRS C 8637), Universite´ de Versailles Saint-Quentin-enYvelines, 45 Avenue des Etats-Unis, 78035 Versailles, France. †E Ä cole Polytechnique. ‡ I. S. A. Jobin-Yvon-Spex. § Debye Institute.

point is that H2O2 always is the oxidizing agent. It can be added to an electrolyte solution in which the oxidation products of GaAs are soluble, in which case the surface is not passivated and chemical dissolution of GaAs occurs. The precise composition of the solution determines whether the resulting surface is smooth (diffusion-controlled etching) or whether dissolution is anisotropic (kinetically controlled etching).2,9 H2O2 solutions in which the oxidation products are insoluble can be used to grow high quality oxide layers.8 By alternating oxide growth in H2O2 solution and oxide dissolution in the absence of H2O2, stepwise dissolution, 15 Å at a time, has been achieved.15 One of the most attractive possibilities of H2O2 is as an electroless photoetchant.1-4,16-21 It requires conditions under which GaAs oxidation products are soluble. Highly anisotropic patterns can then be made under open-circuit conditions by light focusing. This makes it possible to obtain micron-scaled patterns with a high aspect ratio by photoetching without having to use an external current supply, an important advantage for the fabrication of photonic crystals and (opto-)electronic components.1-4,16-21 In light of this, we chose to restrict our study to pH 0 solutions, in which GaAs oxidation products are highly soluble.22 The electrochemistry of n-type GaAs electrodes in H2O2 solutions at pH 0 is illustrated in Figure 1. At positive potentials (Figure 1, region a), no current flows in the dark and H2O2 dissolves GaAs chemically. At moderately negative potentials (Figure 1, region b), the cathodic current density reaches a plateau value, but it is more than 1 order of magnitude lower than the diffusion-limited current corresponding to H2O2 reduction. This, and the observation that the plateau current density is nearly independent of stirring of the electrolyte solution, indicates that the H2O2 reduction rate is determined by surface kinetics.19,20 At even more negative potentials (Figure 1, region c), the rapidly increasing current is due to hydrogen gas

10.1021/jp000389c CCC: $19.00 © 2000 American Chemical Society Published on Web 06/03/2000

5962 J. Phys. Chem. B, Vol. 104, No. 25, 2000

Erne´ et al.

Figure 1. Current density versus applied potential curve for n-GaAs/ 0.1 M H2O2 + 0.5 M H2SO4 (scan rate ) 1 mV s-1). In potential range (a), GaAs is chemically oxidized by H2O2. In potential range (b), H2O2 is electrochemically reduced, and in potential range (c), hydrogen gas evolution takes place.

evolution. Product concentration analysis has revealed that the rate at which H2O2 is consumed in chemical dissolution of GaAs at positive potentials (Figure 1, range a) is approximately equal to that at which it is consumed during cathodic reduction.19-21 From this, Minks et al.19,20 concluded that the chemical dissolution and cathodic reduction mechanisms proceed via a common chemisorption intermediate, precursor for both reactions. The negative slope of current-potential curves during scans in positive direction (Figure 1, range b) reveals an inherent instability of the GaAs/H2O2 system. If the applied potential is fixed at a value in the negative slope region and the series resistance of the system is sufficiently large, periodic oscillations occur in the current density. The oscillations are more than just academically interesting; they provide important information about the interaction of GaAs with H2O2 because a complete model describing the interface should also be able to account for this behavior. The oscillations have been investigated by electrochemical methods13,14 and electrolyte electroreflectance spectroscopy,23,24 and it was proposed that the chemical origin of the instability resides in variations of the GaAs surface coverage by adsorbed hydrogen and in the resulting variation in interfacial potential distribution.14 However, this hypothesis was not tested using in situ surface analytical methods. Spectroscopic ellipsometry and IR spectroscopy are well suited to study the chemical composition of the GaAs electrochemical interface in situ. It was shown in other papers that elemental arsenic or gallium phases, formed during anodic dissolution or hydrogen gas evolution, respectively, can be distinguished unambiguously by in situ spectroscopic ellipsometry,25 and that hydrogen and hydroxyl groups adsorbed at the GaAs surface can be detected by in situ IR spectroscopy.25-27 Absolute surface coverages by adsorbed hydrogen and variations in surface dipole potential due to hydrogen adsorption were both determined quantitatively for GaAs at pH 0 and pH 14.26,27 Finally, by combining in situ spectroscopy with electrochemical measurements, the microscopic mechanism of hydrogen gas evolution at GaAs cathodes has been elucidated.26,27 We therefore chose to study the GaAs/H2O2 interface by in situ spectroscopic ellipsometry and in situ IR spectroscopy. In the present paper (part I), the chemical species present at different applied potentials are identified and quantitative variations occurring during dynamic potential scans are determined. The effects of the different surface species on current density are inferred from the measurements, leading to a kinetic model for the interfacial chemical changes and charge transfers. This model is tested by numerical simulation of the dynamic current-potential characteristics. In the following paper (part

Figure 2. Schematic representation of the electrochemical flow cell used for in situ ellipsometry.

II), these conclusions and new time-resolved measurements of interfacial chemical changes are taken as a basis to explain the oscillatory behavior of the GaAs/H2O2 electrochemical system. Experimental Section The experiments were performed on 〈100〉-oriented n-GaAs single crystals grown by the liquid encapsulated Czochralski method and purchased from MCP Wafer Technology, U.K. (ND ) 8 × 1015 cm-3) and from the Institute of Electronic Materials Technology, Poland (ND ) 4 × 1015 and 7 × 1016 cm-3). Twosided polished 500 µm thick wafers were used to make multiple total internal reflection prisms. Ohmic contacts were obtained by evaporating gold (+ 10% germanium) under reduced pressure (10-3 Pa) while the sample was being cooled from 400 °C to room temperature. The surface was pretreated with concentrated HCl to remove oxides, and according to a recipe by Aspnes and Studna28 to obtain ellipsometrically clean GaAs surfaces. Two different electrochemical flow cells were used. For the in situ IR measurements, the cell was of the type described in ref 29, with the vertically oriented prismatic electrode (45° angles) exposing an area of 0.8 cm2 to a circulating aqueous solution. Ten internal reflections were obtained at the electrochemical interface. For in situ ellipsometry, the cell was of the same type as that tested in ref 30, designed for uniform convection of the electrolyte toward the horizontally oriented GaAs working electrode (see Figure 2; W ) GaAs). The cell was fitted with two quartz optical windows perpendicular to the incoming and reflected light beams, allowing for experiments at a 65° angle of incidence. For both cells, circulation was realized by bubbling nitrogen into the cell, which simultaneously removed a large part of the dissolved oxygen. To prevent changes in electrolyte composition due to evaporation, nitrogen was first bubbled through an aqueous solution having the same composition as that in the cell. In Figure 2, the central tube is narrowed at the top to smoothen the flow rate by limiting the effect of the release of individual bubbles in the side tube. Comparison with measurements performed in the rotating-disk geometry indicates that the flow cell geometries have no significant effect on current-potential behavior. The counter electrode (C in Figure 2) was made of platinum. The potentials were measured against an Ag/AgCl (in saturated KCl) reference electrode (R in Figure 2). D2O2 was synthesized by reaction of BaO2 with D3PO4 in D2O, followed by filtration to remove Ba3(PO4)2,31 and the D2O2 concentration was verified by titration with a KMnO4 solution.31

GaAs/H2O2 Electrochemical Interface Part I

J. Phys. Chem. B, Vol. 104, No. 25, 2000 5963

Figure 4. Absorbance of (bi)sulfate ions in the in situ IR spectra of n-GaAs electrodes: (a) 0.5 M H2SO4 (same spectrum as Figure 3a, H2O baseline subtracted), (b) 0.5 M Na2SO4 (the difference between spectra for 1.0 M Na2SO4 and 0.5 M Na2SO4), (c) changes due to the addition of 20 mM Na2SO4 to 0.5 M H2SO4, and (d) changes due to the addition of 10 mM Ga2(SO4)3 to 0.5 M H2SO4.

Figure 3. (a) In situ IR spectrum of the n-GaAs/0.5 M H2SO4 interface, (b) immediate change in absorbance upon addition of H2O2, yielding a 1.0 M H2O2 solution, (c) change in absorbance when GaAs is kept at open circuit potential in a 0.5 M H2SO4 + 0.1 M H2O2 solution for (1) one minute and (2) five minutes without circulation of the electrolyte solution, and, for comparison, (d) absorbance change when pH increases from 0.3 to 0.5 in (0.5 - x) M HSO4- + x M SO42- (fixed total sulfate concentration).

The other chemicals were of analytical grade, and the water was distilled twice, deionized, and filtered. All of the measurements were performed at room temperature. Infrared absorbance was measured with a Bomem MB 100 Fourier transform IR spectrometer. The absorbance at wavenumber σ is defined as (1/N) ln[Iσ(U0)/Iσ(U)], where N ≈ 10 is the number of useful reflections at the electrochemical interface, Iσ(U) the light intensity at wavenumber σ reaching the detector at potential U, and Iσ(U0) the same as Iσ(U) but under reference conditions at potential U0. The ellipsometer was a spectroscopic phase-modulated ellipsometer of ISA Jobin-Yvon,32 operating in the 1.5 to 5 eV range (UV-vis). The spectral range in the presence of H2O2 is limited by the ultraviolet absorbance of H2O2.31 The ellipsometric light intensity was kept low to limit electrochemical changes due to holes photogenerated in GaAs. The photocurrent density measured at positive potentials due to the light of the ellipsometer was less than 50 µA cm-2 at the 1.3 mm2 illuminated spot. In Situ Infrared Spectroscopy Results. Anodic Conditions. The surface of GaAs was first investigated under conditions in which the current density is negligible, at open circuit potential and in the dark in the anodic range. Figure 3a gives the IR spectrum of 0.5 M H2SO4 without H2O2, measured by subtracting the absorbance of a dry GaAs multiple total internal reflection prism from the absorbance of such a prism in contact

with 0.5 M H2SO4. The other IR spectra are difference spectra as well. The absorbance in Figure 3a is that of the solution present within the probe depth of the evanescent wave (≈ 0.5 µm at 1000 cm-1 33). The signals at 800, 1650, 2200, and 3350 cm-1 are due to H2O, and the background absorbance, stronger at lower wavenumbers, is related to hydrogen bonding. HSO4ions absorb at 1050 and 1200 cm-1. Figure 3b shows the changes in absorbance due to the addition of an aqueous 30% H2O2 solution in the amount necessary to obtain 1.0 M H2O2; the spectrum was measured within seconds of the H2O2 addition, so that all signals are ascribed to changes in the composition of the electrolyte solution, for instance increased absorbance by H2O2. 31 The negative HSO4- signals are due to a simple dilution effect, resulting from the fact that the solution volume has increased by about 10%. In the anodic range, H2O2 dissolves GaAs chemically, and the main effects observed in the in situ IR spectra are due to rapid surface roughening. The rougher the surface, the more light is scattered and the less light is reflected. With 1 M H2O2 in 0.5 M H2SO4, transmission after 10 internal reflections at the electrolyte interface can drop by a factor of 2 in a few minutes if the initial surface is polished flat. The absorbance increases across the entire spectral range, especially at wavenumbers where H2O and electrolyte species absorb (Figure 3c), an optical effect of surface roughness. However, the increased absorbance by SO42- ions at 1100 cm-1 is not due to the same effect because increased absorbance by HSO4- would have been expected, its concentration being much higher than that of SO42at pH 0 (Figure 3a). The SO42- signal is not due to a rise in pH either. A simple rise in pH would not only have caused increased absorbance by SO42-, but also it would have caused decreased absorbance by HSO4- (Figure 3d). Absorbance in the (Bi)sulfate Range. Absorbance in the (bi)sulfate range (≈ 1000 to 1250 cm-1) is examined in greater detail in Figure 4. The objective is to understand the increase in SO42- absorbance in the presence of H2O2 (Figure 3c). Figure 4, parts a and b, gives the absorbances of HSO4- and SO42solutions. At constant pH, the HSO4- to SO42- concentration ratio is fixed by the equilibrium between the two species (acidbase pK ) 1.92 34). This is illustrated in Figure 4c; the addition of a slight amount of Na2SO4 to 0.5 M H2SO4 not only increases the concentration of SO42- but also that of HSO4-.


Figure 5. In situ IR spectrum of n-GaAs under oscillation conditions in 0.5 M H2SO4 + 0.75 M H2O2 at -0.815 V vs Ag/AgCl: the absorbance just before a sudden drop in layer thickness minus the absorbance just after the drop in thickness (average j ≈ -6 mA cm-2, oscillation period ) 350 s, Rs ≈ 16 Ω cm2).

To see whether an increased sulfate concentration near the surface is also obtained during GaAs dissolution in 0.5 M H2SO4 without H2O2, in situ IR spectroscopy is carried out on n-GaAs electrodes under photoanodic conditions. In that case, it is well-known that one GaAs unit dissolves per 6 elementary charges measured in the external circuit.4 Absorbance in the sulfate range does indeed increase. The increase is proportional to the light intensity and much stronger when the electrolyte is not circulated than when it is circulated. This is as expected for the concentration of a photoanodic dissolution product near the surface. Qualitatively, the spectral change observed under photoanodic conditions is identical to that when Ga2(SO4)3 is added to 0.5 M H2SO4. Quantitatively, on the assumption that the spectrum of a 10 mM Ga2(SO4)3 solution in 0.5 M H2SO4 (Figure 4d) can be used as a standard, the Ga3+ concentration near the surface at 2 mA cm-2 photoanodic current density is estimated at 5 mM, the concentration expected under our circulation conditions (diffusion layer thickness ≈ 200 µm35). It is concluded that the increased absorbance in the sulfate range during our experiments is due to (bi)sulfate groups surrounding Ga3+ ions released as a product of GaAs dissolution. Other possible explanations for the increase in sulfate absorbance can be discarded. At pH 0, dissolved arsenic is present as (neutral) HAsO2 and is not expected to have a strong interaction with (bi)sulfate ions. It was seen that the addition of As2O3 to 0.5 M H2SO4, leading to HAsO2, only causes a slight decrease in pH (increased absorbance by HSO4- with no further specific spectral lines). Solid Ga2(SO4)3 and As2(SO4)3 are expected to dissolve or decompose in 0.5 M H2SO4.36,37 Moreover, solid hydrated Ga2(SO4)3 is known to absorb across a wider range than in Figure 4d.38 Cathodic Conditions. Under cathodic conditions, the GaAs surface roughens at a much lower rate than in the anodic range, depending on the applied potential. At fixed potential in the current density plateau (Figure 1, range b), absorbance incessantly changes at 1100 cm-1 (sulfate groups), at wavenumbers corresponding to H2O absorption, and at 2000 cm-1. This is illustrated in Figure 5 under potentiostatic oscillation conditions, when abrupt changes at these wavenumbers allow one to obtain a (differential) spectrum in which optical effects of surface roughening are negligible. Similar changes occur at these wavenumbers in the absence of oscillations, but the spectra are complicated by roughening effects. The absorbance at about 2000 cm-1 is absent at potentials positive of the current density plateau. This wavenumber is within the typical range where arsenic hydrides absorb (1950 to 2150 cm-1) and well outside the typical range where gallium hydrides absorb (1800 to 1950 cm-1).39,40 As shown in Figure 6, the signal observed during H2O2 reduction (Figure 6a) is in

Erne´ et al.

Figure 6. Changes in in situ IR absorbance of n-GaAs electrodes in the range of the As-H stretching mode, (a) under cathodic conditions in 0.5 M H2SO4 + 1 M H2O2, (b) after cathodic electrodeposition of arsenic hydride in 1 M HCl + 25 mM HAsO2, and (c) during cathodic hydrogen gas evolution in 1 M HCl.26,27

good agreement with the absorbance of electrodeposited arsenic hydride (Figure 6b). Closer examination reveals that the signals in Figure 6a and Figure 6b also contain a contribution at 2040 cm-1, the peak position for hydrogen atoms adsorbed at arsenic sites of a GaAs surface.26,27 The absorption of AsH species at a GaAs surface also comprises a second contribution at about 2100 cm-1 (Figure 6c). In Figure 6a, the absorbances at 2000 and 2040 cm-1 (Gaussians with an rms width of ∼30 cm-1) are stronger than the absorbance of a monolayer of adsorbed hydrogen at a flat GaAs surface26,27 by 2 orders of magnitude. It is concluded that both signals are associated with the presence of an arsenic hydride phase, plausibly As2H2.25 The 2000 cm-1 absorbance is ascribed to bulk arsenic hydride, and the 2040 cm-1 absorbance is ascribed to As-H bonds located at the arsenic hydride surface. The assignment of the 2000 cm-1 signal to a hydrogenated species was verified by isotopic substitution. Figure 7a was obtained under similar conditions as those of Figure 5 but with the hydrogen atoms replaced by deuterium atoms. As expected, not only the H2O signals at 3500 and 1650 cm-1 are shifted, but also the signal at 2000 cm-1 is shifted to lower wavenumbers by a factor of ∼x2, due to the higher reduced mass of the As-D oscillators. The absorbance at 1100 cm-1 does not shift, in agreement with the lack of bonds with hydrogen in SO42-. The CdO and C-H signals are artifacts from the rubber O-ring used to ensure a watertight seal between the GaAs and the electrochemical cell; in aqueous solutions, these signals are present as well but masked by H2O absorption. Special care was taken to avoid traces of hydrogen in the deuterated solution because HOD is obtained upon mixing H2O and D2O, and HOD has a strong absorbance at about 1450 cm-1,41 practically the same wavenumber at which As-D absorbs (see Figure 7c). The absence of a band near 3500 cm-1 in Figure 7b indicates that no O-H bonds, and therefore no HOD molecules, are present. The signal at 1450 cm-1 in Figure 7a can therefore be ascribed unambiguously to As-D. Thus, the spectrum in Figure 5 corresponds to an increase in the amount of the arsenic hydride phase. The increased absorbance by water indicates that the layer is porous. Hydrogenated arsenic is practically transparent to IR light in the spectral range of the measurements and has a high refractive index (about 3.5 to 4.542); as a result, enhanced absorbance of the electrolyte solution inside the porous arsenic hydride layer can be expected, in line with observations for porous silicon electrodes.43 The absorbance of H2O inside the porous layer (Figure 5) is not completely the same as that of H2O in the



Figure 8. (a) Current-potential curves and (b) integrated As-H absorbances at an n-GaAs electrode successively measured upon changing the electrolyte, first (1) in 1 M HCl, then (2) in 1 M HCl + 25 mM HAsO2, and finally (3) in 1 M HCl (scan rate ) 1 mV s-1).

Figure 7. (a) In situ IR spectrum of surface changes at n-GaAs electrodes under oscillation conditions in 0.5 M D2SO4 + 1 M D2O2 in D2O: the absorbance just before a sudden drop in layer thickness minus the absorbance just after the drop in thickness (-0.73 V vs deuterated Ag/AgCl reference electrode, average j ≈ -5 mA cm-2, oscillation period ) 270 s, Rs ≈ 17 Ω cm2); (b) in situ spectrum of the same interface with respect to a dry GaAs sample (electrolyte spectrum); and (c) transmission spectra of H2O, a 1:1 H2O + D2O mixture, and D2O.

bulk of the electrolyte solution (Figure 3a). The bending mode of H2O at 1640 cm-1 is at a slightly lower wavenumber than the corresponding signal in the electrolyte spectrum, and the O-H stretching mode of H2O at 3500 cm-1 has a different shape and is clearly shifted to higher wavenumbers than the corresponding signal in the electrolyte spectrum. These shifts correspond to a rise in pH,44 evidently due to the reduction of H2O2 at the surface. This does not contradict observations in the (bi)sulfate range, less sensitive to pH changes because pH is relatively far from the pK of HSO4-.34 The chemical stability of the arsenic hydride layer was tested in the absence of H2O2. It was found that the layer is stable in the absence of oxidants, even in concentrated hydrochloric acid, at least over a period of several hours. This agrees with other reports on the stability of solid arsenic hydrides.36 Quantification of the Amount of Arsenic Hydride. The absorbance of arsenic hydride obtained by electrodeposition from HAsO2 solutions without H2O2 can serve as a standard for the absorbance of arsenic hydride obtained during reduction of H2O2. Figure 8a shows the current-potential curve for n-GaAs in a 25 mM HAsO2 solution (curve 2), and Figure 8b shows the simultaneously measured changes in IR absorbance by arsenic hydride. The current density in the plateau is neither limited by HAsO2 diffusion toward the surface nor fully determined by kinetics; the circulation rate of the solution and the HAsO2 concentration have a sizable influence, but the plateau current is lower than if the reaction were diffusionlimited by about a factor of 10. In 1 M HCl solution, without HAsO2, cathodic current densities due to hydrogen gas evolution are much higher in the absence of electrodeposited arsenic hydride (Figure 8a, curve 1) than in its presence (Figure 8a, curve 3). One difference with arsenic hydride produced during

Figure 9. Infrared spectrum of the change due to electrodeposition of arsenic hydride during the current-potential scan shown in Figure 8 (curve 2).

Figure 10. Integrated cathodic current against integrated As-H absorbance (2000 cm-1 band) for curve 2 in Figure 8.

H2O2 reduction is the weakness of the H2O signals in the IR spectrum of the electrodeposited arsenic phase (Figure 9). This indicates that arsenic hydride electrodeposited from a solution with HAsO2 is not porous but compact. From the data in Figure 8, a calibration curve can be constructed that relates IR absorbance by As-H to the quantity of electrodeposited arsenic hydride (Figure 10). It is assumed that during electrodeposition from HAsO2 solution, four electrons are measured per As-H bond inside the solid, absorbing at 2000 cm-1 (see the Discussion section, reaction 7). This neglects the charge used to produce the As-H bonds at the arsenic hydride surface, which absorb at 2040 cm-1 and account for about 25% of the total As-H absorbance. This neglect induces only a small error because the absorbance of surface As-H bonds is strongly enhanced relative to As-H bonds inside the solid, as a result of the lower dielectric constant of the ambient medium,33 and because fewer electrons are mea-


Figure 11. Two cyclic potential scans of n-GaAs/0.1 M H2O2 + 0.5 M H2SO4 (scan rate ) 1 mV s-1): (a) current density, (b) IR absorbance by As-H inside As2H2 (at 2000 cm-1) and at its surface (2040 cm-1), and (c) absorbance at 2500 cm-1, a measure of surface roughness. The scale on the right-hand side gives the thickness of the As2H2 layer calculated on the basis of As-H absorbance at 2000 cm-1, the calibration in Figure 10, and a layer porosity of 75%.

sured per surface As-H bond. The proportionality factor in Figure 10 is about -84 mC cm-1. On the basis of the calibration, it is possible to estimate the part of the cathodic current due to H2O2 reduction and the part that is due to arsenic hydride deposition in the cathodic current plateau. During the reduction of a 0.1 M H2O2 solution in 0.5 M H2SO4 at -0.7 V versus Ag/AgCl (series resistance insufficient for current oscillations), the total current density is about -2 mA cm-2, and the integrated absorbance per reflection at 2000 cm-1 increases at a rate of 9.7 10-5 cm-1 s-1; the latter rate was found to be constant for at least 6000 seconds and corresponds to the electrodeposition of As2H2 at ∼-10 µA cm-2, according to Figure 10, 0.5% of the total cathodic current density. The assumptions underlying this estimation are not strictly justified on a theoretical basis because the optical properties of porous layers are quite complex and not linear in the concentration of their constituents.45 Nevertheless, it will be seen that the result agrees well with that obtained by in situ ellipsometry. Dynamic Current-Potential CurVes. Figure 11a shows two consecutively measured current-potential scans of n-GaAs in 0.1 M H2O2, first to -0.77 V versus Ag/AgCl and then to -0.97 V versus Ag/AgCl. Subsequent scans to the same potentials gave practically the same curves. The simultaneous changes in IR absorbance by As-H bonds and in background absorbance at 2500 cm-1 are shown in Figure 11, parts b and c. The two contributions to the As-H absorbance, at 2000 cm-1 due to As-H bonds inside the solid arsenic hydride phase and at 2040 cm-1 due to As-H bonds at its surface, have been separated. The absorbance at 2500 cm-1 is a measure of the roughness of the interface. At potentials positive of the current density plateau (U > -0.45 V versus Ag/AgCl), chemical dissolution of GaAs by H2O2 causes rapid surface roughening. At more negative

Erne´ et al. potentials, surface roughening is slower by at least a factor of 20. It can be concluded that GaAs dissolution is much slower in the cathodic current plateau than in the anodic range, in agreement with results by other methods.19,21 During a potential scan in negative direction, As-H bonds first appear just below -0.4 V versus Ag/AgCl. The absorbances at 2000 and 2040 cm-1 increase more or less linearly in time while the scan direction remains negative. In principle, absorbance at 2040 cm-1 can be due to As-H bonds at the surface of the arsenic hydride phase or at the surface of GaAs. However, the values are too high to be due to hydrogen adsorbed at the surface of GaAs because a monolayer has an integrated absorbance of 0.01 cm-1.26,27 The absorbance at 2040 cm-1 is clearly due to As-H bonds at the surface of the arsenic hydride phase, and it masks the much weaker absorbance of As-H bonds at the surface of GaAs. During the positive scans, absorbance at 2040 cm-1 starts to decrease right away, whereas absorbance at 2000 cm-1 remains more or less constant until the applied potential has been scanned about 0.2 V. Then, As-H absorbance decreases until none is left at -0.4 V versus Ag/AgCl. In Situ UV-Vis Ellipsometry Results. The chemical composition of GaAs electrode surfaces in the presence of H2O2 was also investigated by in situ UV-vis ellipsometry. Ellipsometric measurements yield the ratio of the complex reflectances rp and rs for p- and s-polarized light, p-polarization indicates that it is parallel to the plane of incidence, and s-polarization indicates that it is perpendicular. This ratio is expressed by the angles Ψ and ∆, defined according to rp/rs ) tanΨ ei∆.46 Surface layer thickness and composition are determined by fitting the measured Ψ and ∆ spectra simultaneously to simulations performed on the basis of Bruggeman effective medium theory,47 using the optical properties of the expected possible components of the surface layers.25 The fit parameters are the surface layer thickness and the volume fractions of the possible constituents, whose optical properties are taken from the literature.25 We demonstrated elsewhere that the appreciable difference in spectral optical properties of possible constituents as elemental arsenic and metallic gallium enables one to discriminate between the presence of those components in GaAs surface layers.25 During reduction of 0.1 M H2O2 at a potential of the cathodic current plateau (-0.70 V versus Ag/AgCl), a layer is formed at the surface. The ellipsometric spectra can be fit as an effective medium mixture of GaAs, amorphous arsenic, and electrolyte solution, with a layer thickness of the order of hundreds of angstroms (Figure 12). It is assumed that the optical properties of the arsenic hydride in our surface layers are close to those of the amorphous arsenic reported by Greaves et al.,42 who suggest that their own arsenic may be hydrogenated too. It was verified by ellipsometry that the optical properties of the electrolyte solution in the energy range of our experiments are practically the same as those of H2O. Mixtures including metallic gallium, oxides of gallium arsenide, or simulating a clean but microscopically rough GaAs surface do not account for the experimental results in the cathodic range. The procedure of including the substrate (GaAs) as a component of the surface layer effective medium has been proposed as a convenient way to model a microscopically rough surface.46 The best fit of the data in Figure 12 is obtained by assuming that composition varies linearly from 30 vol % GaAs/8 vol % As/62 vol % H2O at the GaAs surface to 12 vol % GaAs/ 36 vol % As/52 vol % H2O on the bulk solution side of the layer. It seems quite reasonable that the percentage of GaAs included for structural features related to surface roughness is



GaAs + 3 H2O2(aq) + 3 H+(aq) f Ga3+(aq) + HAsO2(aq) + 4 H2O (1) At pH 0, cathodic H2O2 reduction is thermodynamically allowed below +1.58 V versus Ag/AgCl.22 At GaAs, the reaction is

H2O2(aq) + 2 H+(aq) + e- f 2 H2O + h+

(2)

where an electron and a hole are involved instead of two electrons.19,48 At the surface, holes which do not recombine with electrons can lead to the dissolution of GaAs4

GaAs + 6 h+ + 2 H2O f Ga3+(aq) + HAsO2(aq) + 3 H+(aq) (3)

Figure 12. In situ ellipsometric spectrum (angles Ψ and ∆, see text) of the n-GaAs/0.5 M H2SO4 + 0.1 M H2O2 interface during cathodic reduction of H2O2 at -0.70 V vs Ag/AgCl. The points are measured and the curves are fit based on the presence of a 930 Å thick layer with a composition varying linearly from 30 vol % GaAs/8 vol % As/ 62 vol % H2O at the GaAs surface to 12 vol % GaAs/36 vol % As/52 vol % H2O at the electrolyte interface.

At pH 0, cathodic hydrogen gas evolution is thermodynamically allowed below -0.20 V versus Ag/AgCl

2 H+(aq) + 2 e- f H2(g)

(4)

However, as long as the current density is significantly lower than in the H2O2 reduction plateau, no H2 is formed. The reason is that AsH sites are required for the formation of H2 (the reaction can be written:26,27 AsH + 2 H+ + 2 e- f AsH + H2), whereas the surface remains OH-terminated as long as the plateau current is not reached, as will be argued in the following section. Besides H+(aq) and H2O2 reduction, reduction of the GaAs oxidation product HAsO2 is also thermodynamically allowed, at potentials U < 0.05 V versus Ag/AgCl

HAsO2(aq) + 3 H+(aq) + 3 e- f As°(s) + 2 H2O

(5)

Figure 13. Evolution of layer thickness measured by in situ ellipsometry during cathodic reduction of H2O2 at n-GaAs in 0.5 M H2SO4 + 0.1 M H2O2 at -0.70 V vs Ag/AgCl. Thicknesses are based on the composition found in Figure 12.

However, in the presence of H2O2, As° can immediately be reconverted to HAsO2 by reaction with H2O2

the highest on the GaAs substrate side of the layer. In this and other single or multilayer fits of the data in Figure 12, the total GaAs content is about 20 vol %, the total As content is about 22 vol %, and the thickness of the layer is about 900 Å. Figure 13 shows how the arsenic hydride layer thickness evolves in time during reduction of a 0.1 M H2O2 solution at -0.70 V versus Ag/AgCl. Thicknesses are based on the fitting of spectra as in Figure 12. From an As2H2 concentration of 22 vol %, a density of 3.693 g cm-3 for amorphous (hydrogenated) arsenic,36 and 4 electrons required to deposit one arsenic atom (see the Discussion section), the rate at which As2H2 is formed corresponds to -12 µA cm-2, almost the same as what was found by in situ IR spectroscopy.

The sum of eqs 5 and 6 is equal to eq 2, suggesting HAsO2 may act as an intermediate species in cathodic reduction of H2O2. When the plateau current density is reached, we discovered that arsenic hydride, rather than As°, is electrodeposited, probably as a porous As2H2 phase.22,25,36

Discussion The diverse reactions possible at the GaAs/H2O2 interface are examined as a function of the applied potential, and the changes in chemical composition of the surface observed by in situ IR spectroscopy and in situ UV-vis ellipsometry are discussed. A kinetic model is proposed for the interfacial chemical changes and charge transfers, and it is tested by numerical simulation of dynamic current-potential behavior. Reactions as a Function of Applied Potential. Under anodic conditions, GaAs is dissolved chemically by H2O2, yielding gallium and arsenic products soluble at pH 022

As°(s) + 3 H2O2(aq) f 2 HAsO2(aq) + 2 H2O

HAsO2(aq) + 4 H+(aq) + 4 e- f 1/2 As2H2(s) + 2 H2O

(6)

(7)

The production of As2H2 during H2O2 reduction at GaAs means that HAsO2 continues to be produced by dissolution of GaAs. Reaction 7 was found to account for about 0.5% of the cathodic current, which makes it clearly a minor reaction compared to cathodic reduction of H2O2 (reaction 2). If As2H2 is in electrical contact with the GaAs electrode, it can be protected cathodically at negative potentials, or else it can be attacked by H2O2

As2H2(s) + 4 H2O2(aq) f 2 HAsO2(aq) + 4 H2O

(8)

Only at potentials below -0.8 V versus Ag/AgCl, the Ga°/Ga3+ redox potential, metallic gallium phase may appear.25 However, it is not observed here in the presence of H2O2. Chemical Surface Species as a Function of Applied Potential. Under anodic conditions, H2O2 dissolves GaAs chemically. Minks et al.19,20 and Plieth et al.17 have measured that under these conditions, the flatband potential is approximately the same as in the absence of H2O2. It was shown

5968 J. Phys. Chem. B, Vol. 104, No. 25, 2000 elsewhere by capacitance measurements combined with in situ IR spectroscopy that the flatband potential is very sensitive to the surface chemical composition and that anodic GaAs surfaces are covered by adsorbed OH-groups in the absence of H2O2.26,27 It is concluded that anodically polarized GaAs in the presence of H2O2 is OH-terminated as well. In the anodic range, sulfate absorbance increases, and this was ascribed to (bi)sulfate groups surrounding Ga3+ ions released as a product of GaAs dissolution. This conclusion is further argumented now. Reaction 1 suggests that the charge of Ga3+ product ions is compensated by the consumption of three H+ ions. However, all ions of the supporting electrolyte should participate in neutralizing the Ga3+ charge. Near the surface, the HSO4- concentration should increase more than the SO42- concentration because of the HSO4-/SO42- acid/base equilibrium. Nevertheless, in spectra as Figure 3c, it appears that especially the SO42- concentration increases. This could result from a decrease in activity coefficients associated with the increase in ionic strength (replacing three H+ ions by one Ga3+ ion raises the ionic strength), known to increase the ionization of HSO4-, which applies not only at concentrations sufficiently low for the Debye-Hu¨ckel theory49 to be valid but also at molar concentrations, as measured by Chen and Irish.50 Our observations support that this is the case here. In other words, the Coulombic attraction between Ga3+ ions and SO42is so strong that each Ga3+ ion forces at least one HSO4- ion to convert to SO42- and to remain in its direct vicinity. When cathodic current flows but current density is lower than in the kinetically limited plateau (Figure 1), in situ spectroscopy indicates that the main surface change occurring is surface roughening, as in the anodic range. Apparently, no new surface phase is produced yet. As soon as the current density attains the kinetic plateau, solid arsenic hydride is formed, indicating that chemical dissolution of GaAs continues even in the current plateau but at a much lower rate than in the anodic range. This new phase is hydrogenated (As2H2) and therefore clearly grows by electrodeposition from the arsenic-rich solution near the electrode surface; it should not be confused with the elemental arsenic (As°) phase observed during anodic decomposition of GaAs, a phase which grows from arsenic remaining after preferential dissolution of gallium.25 From the proportionality of As-H absorbance at 2000 and 2040 cm-1 during negative potential scans (Figure 11b), it is inferred that the As2H2 surface is fully covered with adsorbed H-atoms under As2H2 deposition conditions. The appearance of As2H2 is accompanied by increased absorbance of water (Figure 5), due to the porosity of the layer. Porosity is expected here because As2H2 must grow at the expense of GaAs, which implies dissolution of GaAs and, hence, accessibility of the GaAs surface to the electrolyte solution. In the presence of H2O2, As2H2 is present only at potentials below the onset of the cathodic current density plateau (Figure 11), whereas in the absence of oxidants, As2H2 is also stable at open circuit. This indicates that the amount of As2H2 is due to two competing processes, the cathodic formation of As2H2 and its dissolution due to chemical oxidation by H2O2. Because the rate at which H2O2 is consumed is well below the diffusion limit, the H2O2 concentration at the As2H2 surface is approximately the same as in the bulk of the electrolyte solution. It is concluded that As2H2 is cathodically protected in the potential range of the current plateau. Cathodic protection of As2H2 implies that it is in electrical contact with GaAs and that cathodic current flows through the As2H2 phase toward the

Erne´ et al.

Figure 14. Schematic illustration of the chemical species present at the surface of n-GaAs in 1.0 M H2O2 + 0.5 M H2SO4 under cathodic conditions.

surface. The large cathodic currents measured during electrodeposition of As2H2 from an HAsO2 solution in the absence of H2O2 (Figure 8) clearly indicate that large cathodic currents are able to flow at the surface of the As2H2 layer. The chemical composition of the interface under cathodic conditions is summarized schematically in Figure 14. The As2H2 phase that partially covers the rough GaAs surface is formed by reduction of HAsO2, a dissolution product of GaAs. Ga3+ ions, the other dissolution product, are surrounded by (bi)sulfate groups, which neutralize the charge. Both H and OH groups are adsorbed on GaAs at the electrolyte interface,26,27 and the As2H2 surface is covered with adsorbed H atoms under conditions in which the As2H2 layer thickness increases (OH groups are present when layer thickness decreases). The interfacial region typically has a thickness of a few hundred Å and contains about 22 vol % arsenic hydride. The amount of As2H2 and the presence of H or OH groups adsorbed on As2H2 and GaAs have different effects on the current. These effects can be deduced from Figure 11. At -0.5 to -0.45 V, currents are higher during the potential scan in positive direction than during the scan in negative direction; the higher currents coincide with the presence of As2H2, whose surface is covered with OH-groups (strong absorbance at 2000 cm-1 and weak absorbance at 2040 cm-1). Cathodic H2O2 reduction apparently occurs at a high rate on OH-covered As2H2. At more negative potentials, for example -0.7 V, the rate of cathodic H2O2 reduction is lower even though more As2H2 is present, but now it is covered with adsorbed H atoms (strong absorbance at 2040 cm-1). Cathodic H2O2 reduction apparently occurs at a lower rate on H-covered As2H2 than on OH-covered As2H2. It is assumed that this is because H2O2 molecules adsorb at a lower rate at AsH sites than at AsOH sites; this should then not only be the case at the surface of As2H2, but it should also be the case at the surface of GaAs. The experimental evidence for the roles of As2H2 and H or OH groups adsorbed on As2H2 and GaAs makes it possible to present a detailed explanation of the dynamic behavior of the GaAs/H2O2 system. Kinetic Model. A kinetic model is proposed for the changes in interfacial chemical composition and current density observed during the potential scans. The main elements of the model are summarized schematically in Figure 15. H2O2 adsorbs at sites where GaAs or As2H2 is in direct contact with the electrolyte solution. A key assumption deduced from the experiments (see the previous section) is that H2O2


J. Phys. Chem. B, Vol. 104, No. 25, 2000 5969 ohmic drop across the series resistance Rs (electrolyte resistance, externally added resistance, and so forth):

Uint ) U - j Rs

(14)

where U is the applied potential, and j is the total current density. For As2H2, ξ* is taken to depend on the potential U* at the As2H2 surface as follows:

ξ* ) {1 + exp[e(U* - U°)/kBT]}-1

(15)

where U° is a constant that resembles a redox potential; negative of this value, ξ* tends to 1, and positive of this value, ξ* tends to 0. The surface potential U* is an average value because the potential is different at different sites due to ohmic drop inside As2H2. U* is assumed to depend on the average thickness δ* of the As2H2 layer according to

U* ) Uint - j* F* δ* Figure 15. Schematic illustration of the proposed model for chemical changes and charge transfers at the GaAs/H2O2 interface. Solid arrows represent electrochemical reactions, and shaded arrows represent chemical reactions. A similar scheme applies to the As2H2/H2O2 interface (ξ must then be replaced by ξ* and hydrogen evolution is neglected).

adsorption on GaAs and As2H2 occurs at a much higher rate at OH sites than at H sites. The site specific adsorption rates are expressed as anodic current densities jadsOH and jadsH and are assumed to be the same for GaAs and As2H2. The fractional surface coverages by adsorbed H and OH groups are symbolized by θH and θOH for GaAs and by θH* and θOH* for As2H2. Competition between chemical attack by H2O2 and cathodic reduction of H2O2 is described by a factor ξ for GaAs and a factor ξ* for As2H2: ξ ) 1 or ξ* ) 1 when H2O2 is reduced cathodically (cathodic protection of GaAs or As2H2, respectively), and ξ ) 0 or ξ* ) 0 when H2O2 is consumed in chemical attack of GaAs or As2H2 (ξ and ξ* are assumed to be site-independent). The local current densities corresponding to cathodic H2O2 reduction on GaAs and As2H2 are referred to as jH2O2 and jH2O2*, respectively

jH2O2 ) - (jadsOH θOH + jadsH θH ) ξ

(9)

jH2O2* ) - (jadsOH θOH* + jadsH θH*) ξ*

(10)

For GaAs, ξ depends on the surface concentration ns of electrons in the conduction band of the semiconductor

ξ ) (ke ns)/(ke ns + kch)

(11)

where ke and kch are constants. The surface electron concentration is given by53

ns ) ND exp[ -e(Uint - Ufb)/kBT ]

(12)

where ND is the dopant concentration, e the positive elementary charge, Uint the potential at the GaAs/electrolyte interface, Ufb the flatband potential, kB the Boltzmann constant, and T the absolute temperature. As previously found, the flatband potential is not a constant but depends on hydrogen surface coverage in the following way (β ≈ -0.4 V):27

Ufb(θH) ) Ufb(θH ) 0) + β θH

(13)

The potential at the GaAs/electrolyte interface is affected by

(16)

where j* is the current density flowing through As2H2 and F* the effective resistivity of (porous) As2H2. A model that accounts for the geometric dependence of the potential at the As2H2 surface will be proposed in the following paper (part II). The fractional coverages θOH and θH of the GaAs surface were previously found to result from a competition between cathodic hydrogenation of AsOH sites and chemical or anodic oxidation of AsH sites, both processes occurring through a common arsenic radical intermediate, As•.26,27 However, it is found that surface concentration in As• is negligible, and we will consider that θH ≈ 1 - θOH. Similarly, we will assume that when H2O2 molecules adsorb at AsH sites and are not cathodically reduced, they oxidize AsH into AsOH directly, without going into details of the elementary reaction steps. The time dependencies of the surface coverages of GaAs are then ruled by

dθH/dt ) k1θOH ns - jadsH θH (1 - ξ)/ 2eNs

(17)

where k1 is a rate constant describing cathodic hydrogenation of AsOH sites,27 and Ns is the surface concentration of As sites at a GaAs surface. The partial current density due to hydrogen gas evolution at GaAs (reaction 4) is given by

jH2 ) -2eNsk2θHns

(18)

where k2 is a constant. The rate at which GaAs dissolves is

jdiss ) (jadsOH θOH ) (1 - ξ)

(19)

The fractional surface coverages of As2H2 by adsorbed H and OH groups are assumed to change in a similar way: θH* ≈ 1 - θOH* and

dθH*/dt ) k1*θOH* exp[-eU*/kBT ] jadsH θH*(1 - ξ*)/ 2eNm (20) where k1* is a constant and Nm is the concentration of surface sites at a flat As2H2 surface. Under conditions in which HAsO2 is deposited as As2H2, the rate at which the As2H2 layer grows appears to be affected both by the rate at which the solution is circulated (Figure 8) and by the electron concentration at the semiconductor surface (see the next section). If deposition had just been limited by diffusion, the rate would have corresponded to jdiss (eq 19). If the As2H2 deposition rate had been determined by only the electron concentration, the rate would have been proportional to ns. The


Erne´ et al.

mixed dependence of the cathodic deposition rate jdep on diffusion and ns is taken into account as follows

jdep ) -jdiss k3ns (k3ns + jdiss/e)-1

(21)

where k3 is a constant. The rate at which As2H2 dissolves is given by the rate at which H2O2 molecules adsorb at OH sites of the porous surface without being cathodically reduced. The rate at which As2H2 layer thickness changes results from the difference between cathodic deposition rate and chemical dissolution rate

dδ*/dt ) -jdep V - jadsOH θOH* (1 - ξ*) S* δ* V (22) where δ* is the thickness of the As2H2 layer, S* its specific surface area (it corresponds to the surface area of the porous layer in cm2 per layer thickness in angstroms, for a 1 cm2 electrode), and V is the volume of As2H2 per cathodic deposition charge

V ) M* [8F d* (1 - P*)]-1

(23)

where M* is the molar mass of As2H2, F the Faraday constant, d* the density of As2H2, and P* the porosity of the As2H2 layer; the factor 8 is based on reaction 7. The total current density j* at As2H2 is the sum of cathodic As2H2 deposition and cathodic H2O2 reduction at As2H2

j* ) jdep + jH2O2* S* δ*

(24)

No hydrogen evolution is taken into account at the As2H2 surface, the kinetics of the reaction being experimentally found to be very low (see Figure 8). The total current density combines hydrogen gas evolution at GaAs, H2O2 reduction at GaAs, and H2O2 reduction plus As2H2 electrodeposition at As2H2

j ) jH2 + jH2O2 + j*

(25)

Numerical Simulation of Dynamic Current-Potential Behavior. On the basis of the proposed model, numerical simulations are carried out (same numerical approach as in ref 27). The H2O2 adsorption rates at OH and H sites are chosen to correspond to jadsOH ) 1 mA cm-2 and jadsH ) 0.1 mA cm-2. The series resistance Rs is taken zero to avoid oscillatory behavior. The choice of the other parameter values is presented in the Appendix. Figure 16a shows simulated dynamic current-potential behavior, Figure 16b gives the simultaneous changes in the As2H2 layer thickness. Figure 16c shows the fractional surface coverages of GaAs and As2H2 by adsorbed H atoms. Partial or equivalent current densities are presented in Figure 17, parts a and b, corresponding to the rate of GaAs dissolution (jdiss), H2O2 reduction at the GaAs surface (jH2O2), current flowing through As2H2 (j*), and hydrogen gas evolution at GaAs (jH2). Figure 17c traces the parameters ξ and ξ* (eqs 11 and 15), and Figure 17d shows the variations in electron concentration at the surface ns (eq 12). The simulations reproduce the main features of the experimental current-potential curves and the data from in situ IR spectroscopy (Figure 11). This provides one with a comprehensive picture of interfacial behavior during a potential scan. In the anodic range, there is no As2H2, and GaAs is covered with OH groups. H2O2 adsorbs at GaAs and oxidizes surface bonds. As the potential is scanned in negative direction, H2O2 starts to be reduced cathodically at GaAs, causing a decrease in the rate at which GaAs dissolves chemically. When GaAs

Figure 16. Simulated cyclic potential scan of n-GaAs/0.1 M H2O2 (pH 0) (scan rate ) 1 mV s-1): (a) current density, (b) thickness of the As2H2 layer, and (c) fractional hydrogen surface coverage of GaAs (θH) and As2H2 (θH*). The simulation parameters are given in the Appendix.

dissolution has dropped to a sufficiently low value, cathodic hydrogenation of AsOH sites starts to compete efficiently with chemical oxidation of AsH, so that adsorbed H atoms start to appear at the GaAs surface. During a negative potential scan at a fixed rate, the amount of As2H2 increases at a more or less linear rate. The reason is essentially that the increasing coverage of GaAs by adsorbed H atoms causes a shift in the flatband potential (eq 13), so that the surface electron concentration (eq 12) changes much more slowly than if the flatband potential were constant. The amount of As2H2 continually increases during the negative potential scans, and it also increases when a fixed potential is chosen in the cathodic range (Figure 13); it is clear that the amount of As2H2 is a function of not only the potential but also the time spent in the cathodic range. At the most negative potentials of the scan, the increase in current density is due to hydrogen gas evolution. As soon as the direction of potential scanning is reversed, current density quickly drops due to a decrease in the hydrogen gas evolution rate. The hysteresis in the hydrogen gas evolution rate upon potential scanning was investigated in detail in refs 26 and 27. This occurs because θH does not begin to drop immediately when scan direction is reversed; hence, variations in applied potential fall at first across the semiconductor space charge layer. Current density is now lower than it was at the positive limit of the current plateau, even though the GaAs dissolution rate is not much higher. This means that the rate of H2O2 adsorption is now lower than it was at the positive limit of the current plateau. The model assumes that this is due to a lower H2O2


Figure 17. (a,b) Equivalent current density corresponding to GaAs dissolution (jdiss) and partial current densities due to H2O2 reduction at GaAs (jH2O2), cathodic current at As2H2 (j*), and hydrogen gas evolution at GaAs (jH2); (c) factors ξ and ξ*; and (d) conduction band electron concentration at the GaAs surface (same simulation parameters as Figure 16).

adsorption rate at H sites than that found at the OH sites of GaAs and As2H2. For As2H2, the assumption that H2O2 adsorbs at a high rate at OH sites explains why, at the positive edge of the current plateau (at -0.5 to -0.45 V in Figure 11), the current density is higher in the presence of OH-covered As2H2 than it is in its absence. The increased current density at the positive edge of the current plateau during the positive scan is not due to increased roughness of the electrode because it was found experimentally that a subsequent potential scan yields the same cyclic voltammogram. In the experiments, the amount of As2H2 stops rising as soon as the scan is switched in the positive direction, when the surface electron concentration drops abruptly. This indicates that the As2H2 growth rate depends on the conduction band electron concentration at the GaAs surface. The amount of As2H2 starts to decrease when the potential at the As2H2 surface is no longer sufficiently negative for preserving H coverage and ensuring cathodic H2O2 reduction. As U approaches the positive edge of

J. Phys. Chem. B, Vol. 104, No. 25, 2000 5971 the j plateau, θH and θH* decrease to zero, and GaAs and As2H2 are no longer cathodically protected. It is now possible to discuss the origin of the negative slope of the dynamic current-potential curve, associated with the oscillatory behavior (see the following paper, part II). Without any As2H2, there would be a negative slope, because the H-surface coverage of GaAs increases at negative potentials, leading to a decrease in the H2O2 adsorption rate on GaAs (see jH2O2 in Figure 17a). For a fixed amount of As2H2, an extra negative slope appears at intermediate potentials because the H-surface coverage of As2H2 steeply increases in the potential range from -0.4 to -0.5 V, leading to a decrease in H2O2 adsorption rate on As2H2. However, the amount of As2H2 is always varying, and this only favors a negative slope in certain cases, for instance, when the amount of As2H2 increases while the potential is scanned in positive direction. A last effect on the slope is the rate of hydrogen gas evolution on GaAs, whose potential dependence always gives a positive contribution to the slope but which is lower when the potential is scanned in positive direction27 (see jH2 in Figure 17b). To summarize, the negative current-potential slope for GaAs/H2O2 results from a balance between the various contributions to the complex kinetics of the system, depending for instance on the direction of the potential scan, but the major role is played by the distinct kinetics of H2O2 adsorption on H and OH covered As2H2. The model accounts for the essential features of experimental behavior, despite several oversimplifications. One simplification is that it does not consider that As2H2 growth decreases the available surface area of GaAs; however, this does not affect the main conclusion from the experimental results in Figure 11, i.e., that H-covered As2H2 yields a lower rate of cathodic H2O2 reduction than OH-covered As2H2 (a decreased GaAs surface area is even an additional argument in favor of this conclusion). Another simplification is the crude description of the arsenic hydride layer. The potential at the As2H2 surface is now described by a single, average value, whereas the local potential at the As2H2 surface should be different depending on the distance from GaAs. This aspect of the model will be refined in the following paper. Nevertheless, the ability of the present model to reproduce simultaneously current-potential curves (including the negative slope region) and experimental measurements of hydrogen coverages and arsenic hydride thickness gives a comprehensive picture of the physical chemistry of the interface and a firm ground for analyzing its complex oscillating behavior. Conclusions The chemistry of the GaAs/H2O2 interface at pH 0 has been examined during electrochemical current flow using in situ IR spectroscopy and in situ spectroscopic ellipsometry. Under anodic conditions, the GaAs surface is covered by adsorbed OH groups and appears to be clean of secondary phases. When H2O2 is reduced under cathodic conditions, chemical dissolution of GaAs is nonetheless not completely suppressed. A large part of the HAsO2 produced by GaAs dissolution is deposited as a solid arsenic hydride phase, As2H2. Quantities of As2H2 determined by in situ IR spectroscopy are in good agreement with in situ UV-vis ellipsometry. The presence of solid arsenic hydride at the GaAs/H2O2 (pH 0) interface under cathodic conditions was previously unknown. In contrast with prior assumptions,14 it now appears that coverages by H and OH groups adsorbed to GaAs are not the only variables in surface composition. Changes in the amount of As2H2 and its own coverage by adsorbed H and OH groups

5972 J. Phys. Chem. B, Vol. 104, No. 25, 2000 affect current density and are partly responsible for the presence of a negative slope region in the dynamic current-potential curve. A kinetic model is proposed for the interfacial chemical changes and charge transfers at the GaAs/H2O2 interface. The parameters of the model are the adsorption rates of H2O2 at H and OH sites of GaAs and As2H2, the surface coverages of GaAs and As2H2 by adsorbed H and OH groups, the influence of the H-surface coverage of GaAs on the flatband potential, the specific resistivity of As2H2, and the kinetics of the competition between chemical attack by H2O2 and cathodic reduction of H2O2 at GaAs and As2H2. This model forms a basis for investigating the origin of the oscillatory behavior of this system, examined in the following paper (part II). Acknowledgment. The authors thank R. Benferhat and F. Maroun for helpful discussions and the European Commission for a TMR Research Training Grant (B.H.E.). List of Symbols d* e F j j* jadsOH

jadsH jdep jdiss jH2 jH2O2, jH2O2*

kB ke,kch,k1,k2,k3,k1* M* ns ND Nm Ns P* Rs S* T U U* U° Ufb Uint V β δ*

density of the As2H2 layer (eq 23) the positive elementary charge the Faraday constant total current density (eq 25) current density flowing through As2H2 (eq 24) H2O2 adsorption rate at OH sites of GaAs and As2H2 surfaces expressed as an anodic current density H2O2 adsorption rate at H sites of GaAs and As2H2 surfaces expressed as an anodic current density cathodic As2H2 deposition rate (eq 21) GaAs dissolution rate expressed as an anodic current density (eq 19) partial current density corresponding to cathodic hydrogen gas evolution at GaAs (eq 18) local current densities corresponding to cathodic H2O2 reduction at GaAs and As2H2, respectively (eqs 9 and 10) the Boltzmann constant reaction rate constants (eqs 11, 17, 18, 20, and 21) molar mass of As2H2 (eq 23) conduction band electron concentration at the GaAs surface (eq 12) donor concentration (eq 12) surface concentration of surface sites at a flat As2H2 surface (eq 20) surface concentration of As sites at the GaAs As2H2 surface (eq 17) porosity of the As2H2 layer (eq 23) series resistance of the electrochemical system (eq 14) specific surface area of As2H2 (eq 22) absolute temperature applied potential potential at the surface of As2H2 (eq 16) switch potential for H or OH coverage of the As2H2 surface (eq 15) flatband potential (eq 13) potential at the GaAs/electrolyte interface (eq 14) volume of As2H2 per cathodic deposition charge (eq 23) coefficient for the effect of H coverage on flatband potential (eq 13) thickness of the As2H2 layer (eq 22)

Erne´ et al. θH, θOH θH*, θOH* F* ξ, ξ*

fractional surface coverages of GaAs by adsorbed H and OH groups fractional surface coverages of As2H2 by adsorbed H and OH groups effective resistivity of (porous) As2H2 (eq 16) the factor describing the competition between chemical attack by H2O2 and cathodic reduction of H2O2 at GaAs and As2H2, respectively (eqs 11 and 15)

Appendix: Parameter Values of the Numerical Simulations The numerical simulations of dynamic current-potential behavior are carried out on the basis of the proposed model, following the same numerical approach as in ref 27. The H2O2 adsorption rates at OH and H sites are chosen to correspond to jadsOH ) 1 mA cm-2 and jadsH ) 0.1 mA cm-2. Parameters affecting the coverage of GaAs by adsorbed H and OH groups (eq 17) and hydrogen gas evolution (eq 18) are of the same order as those found in previous simulations of cathodic hydrogen gas evolution27 (k1 ) 5 × 10-12 cm3 s-1, k2 ) 3 × 10-9 cm3 s-1, β ) -0.4 V, Ufb(θH ) 0) ) -1.00 V versus Ag/AgCl, ND ) 7 × 1016 cm-3, Ns ) 1 × 1015 cm-2). The competition between cathodic reduction of H2O2 at GaAs and chemical dissolution of GaAs by H2O2 (eq 11) is described using kch / ke ) 107 cm-3. The dependence of the As2H2 deposition rate on the conduction band electron concentration at the GaAs surface (eqs 12 and 21) is calculated using k3 ) 5 × 105 cm s-1. The switch from an OH to an H covered As2H2 surface (eqs 15 and 20) is described using U° ) -0.40 V versus Ag/ AgCl and k1* ) 10-9 s-1. The porosity P* of the As2H2 layer is assumed to be 75%. The density of massive As2H2 is assumed to be that of amorphous arsenic.36 The proportionality constant S* (eq 22) is chosen to be 0.03 Å-1; this value corresponds to one flat surface monolayer equivalent per 33 Å layer thickness, which is in line with the nanometric dimensions of structural features estimated in the following paper (part II). The density of surface sites at a flat As2H2 surface is taken to be Nm ) 1015 cm-2. The series resistance Rs is taken to be zero to avoid oscillatory behavior, and the effective resistivity F* of (porous) As2H2 is chosen to be 105 Ω cm. References and Notes (1) Kelly, J. J.; van den Meerakker, J. E. A. M.; Notten, P. H. L.; Tijburg, R. P. Philips Tech. ReV. 1988, 44, 61. (2) Notten, P. H. L.; van den Meerakker, J. E. A. M.; Kelly, J. J. Etching of III-V Semiconductors, An Electrochemical Approach; Elsevier: Oxford, 1991. (3) Kelly, J. J.; Vanmaekelbergh, D. In Semiconductor Micromachining. Fundamentals and Technology; Campbell, S. A., Lewerenz, H. J., Eds.; Wiley: Chichester, 1998; Vol. 1. (4) Gomes, W. P.; Goossens, H. H. In AdVances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH: Weinheim, 1994; Vol. 3. (5) Kern, W. RCA ReV. 1978, 39, 278. (6) Tuck, B. J. Mater. Sci. 1975, 10, 321. (7) Iida, S.; Ito, K. J. Electrochem. Soc. 1971, 118, 768. (8) Logan, R. A.; Schwartz, B.; Sundburg, W. J. J. Electrochem. Soc. 1973, 120, 1385. (9) Mori, Y.; Watanabe, N. J. Electrochem. Soc. 1978, 125, 1510. (10) Lu, Z. H.; Lagarde, C.; Sacher, E.; Currie, J. F.; Yelon, A. J. Vac. Sci. Technol. A 1989, 7, 646. (11) Flemish, J. R.; Jones, K. A. J. Electrochem. Soc. 1993, 140, 844. (12) Takebe, T.; Yamamoto, T.; Fuji, M.; Kobayashi, K. J. Electrochem. Soc. 1993, 140, 1169. (13) Koper, M. T. M.; Meulenkamp, E. A.; Vanmaekelbergh, D. J. Phys. Chem. 1993, 97, 7337. (14) Koper, M. T. M.; Vanmaekelbergh, D. J. Phys. Chem. 1995, 99, 3687.

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H2O2 Electrochemical Interface Studied In Situ by Infrared

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