Surface Chemistry and Topography - American Chemical Society

Article pubs.acs.org/JPCC

Initial Phase of Photoelectrochemical Conditioning of Silicon in Alkaline Media: Surface Chemistry and Topography Marika Letilly,*,†,‡ Katarzyna Skorupska,†,§ and Hans-Joachim Lewerenz†,‡ †

Helmholtz-Zentrum Berlin, Solar Fuel and Energy Storage Materials, Hahn-Meitner Platz 1, 14109 Berlin, Germany Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States § Brandenburg Technical University Cottbus, Platz der Deutschen Einheit 1, 03046 Cottbus, Germany ‡

ABSTRACT: Oxidation and dissolution phenomena of Si(111) in alkaline electrolyte are investigated by a combination of photoelectrochemistry, scanning probe microscopy (SPM), transmission electron microscopy (TEM) and in-system synchrotron radiation photoelectron spectroscopy (SRPES). The surface topography in the initial anodic potential regime shows the formation of mesoscale pores with widths in the range 300−500 nm and partial surface oxidation. The surface chemistry assessment by SRPES shows patchy silicon oxide growth, suboxides, and remnants of the former hydrogen terminated surface areas. The use of the obtained self-organized nanostructures for application in nanoemitter photocatalytic solar cells is discussed. The necessary requirements regarding the total surface area of electrocatalysts needed to sustain the current density due to light-induced excess minority carriers in conjunction with the exchange current density of the considered heterogeneous catalysts is discussed.

1. INTRODUCTION Self-organization is a process, ubiquitous in nature, and is observed in many physical, chemical, and biological systems.1−5 The phenomenon is characterized by a feedback in these nonlinear systems and is, in its simplest form, evident from the equations that describe the so-called Lotka−Volterra behavior6,7 It is cast into sets of coupled partial differential equations, and the evolution of each variable is separable into two contributions that account for the homogeneous reaction part dynamics and the superimposed lateral transport processes, respectively.8 Among the observed phenomena are Turing patterns,9 the occurrence of fractals,10−12 ordering processes in crystal growth and epitaxy,13 the functioning of a laser,14 and spintronics devices.15 In electrochemistry and photoelectrochemistry, self-organization has a long history dating back to the Belousov− Zhabotinsky reaction.16 More recently, the phenomenon has attracted interest within the semiconductor community where structures such as 2D photonic crystals17,18 and oxide nanopore arrays have been prepared in electrolytes.19−21 Such patterns have also been used for the preparation of solar energy converting structures22,23 that operate with local metallic or semiconducting nanodimensioned emitters24,25 which collect the light-induced minority carriers, generated in the semiconductor absorber. In the present work, we explore the behavior of Si electrodes in a novel parameter space, e.g., in the alkaline solutions in the anodic potential region under illumination. The system is characterized by competitive processes where silicon oxide is © 2013 American Chemical Society

formed from a hydrogen terminated surface and, simultaneously, Si etches chemically and electrochemically in alkaline media. In this respect, the situation bears a resemblance to (photo)current oscillations of Si in acidic fluoride containing solution.26 Due to the complexity of the chemical and (photo)electrochemical reactions, we restrict ourselves to the initial phase of the anodization process that occurs slightly anodic from open circuit potential (OCP). In the pursuit of preparing templates for solar energy converting devices, we employ experimental techniques that enable us to assess (i) the surface chemistry (synchrotron radiation photoelectron spectroscopy - SRPES), (ii) the surface morphology and topography (transmission electron microscopy - TEM and scanning probe microscopy, such as atomic force microscopy - AFM), (iii) the electronic behavior, and (iv) the influence of (photo)electrochemical conditioning on the incorporation of H into Si.

2. EXPERIMENTAL AND PROCEDURES Float-zone n-Si(111), P doped in the 1015 cm−3 range (Siltronix) was used. All solutions were made from Milli-Q water (18 MΩ) using ultrapure or analytical grade chemicals (NH4F, potassium hydrogen phthalate - KHP, H2SO4, NaOH, from Sigma Aldrich and Merck) and purged before in N2 (5N) for at least 30 min before use. Received: February 22, 2013 Revised: July 7, 2013 Published: July 16, 2013 16381

dx.doi.org/10.1021/jp401853p | J. Phys. Chem. C 2013, 117, 16381−16391

The Journal of Physical Chemistry C

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energies, 150 and 585 eV, with a pass energy of 10 eV. The deconvolution of the envelope curve of Si spin orbit split photoelectron core level spectra was done using Lorentz− Gauss profiles and Shirley background using the software Unifit.29 The applied scan rate during electrochemical preparation of samples varies for the topography (AFM, TEM) and chemistry study (XPS). To study the development of the surface topography, a lower scan rate was used (1 mV s−1). In order to avoid the stability problems in the electrochemical cell (Figure 1) attached to the UHV XPS system, a faster scan rate was applied (50 mV s−1). Mott−Schottky curves were recorded using a Zahner Elektrik GmbH impedance meter (software: Thales 3.14). The solution consisted of K2SO4 (0.1 M, pH 6.2), and the potential range was 0.2 to −0.2 V vs SCE with a scan rate of 2 mV s−1. Three different frequencies were tested: 600 Hz, 1 kHz, and 20 kHz with a modulation amplitude of 10 mV. H diffusion profile measurements have been performed at the Helmholtz-Zentrum Dresden-Rossendorf (Germany) by nuclear reaction analysis (NRA), also known as elastic recoil detection analysis (ERDA) using the reaction H(15N, γα)12C with a very sharp resonance at 6.385 MeV. The direct counting of emitted γ-rays allows the determination of the depthdependent H profile, defined by the incident ion energy and the stopping power of the ions in the material. The sample was tilted to an angle of 75° between sample normal and incident ion beam. The beam spot on the sample was about 2−3 mm in each direction. The concentrations have been derived from a comparison of the γ-ray counts for each measurement with the γ-ray counts from a sample with a known H concentration: an amorphous hydrogenated Si layer on Si.30 The position nearest to the surface has been measured twice at the beginning and the end of the series. The samples were only shortly exposed to air during mounting on the sample holder and immediately put into a vacuum. The depth scale in nm is based on the stopping power of 15N ions in Si and the bulk density of Si. The samples have been subjected to treatment in 4 M NaOH, from OCP to the potential value where the current was −0.1 A cm−2. Cathodic polarization of n-type Si results in the formation of an accumulation layer near the surface31 with very high electron concentration. According to the direction of the electric field across the semiconductor−electrolyte junction, increased cathodic currents lead to pronounced hydrogen evolution. In this hydrogenation experiment, the scan velocity was 10 mV/s and the sample was held for 40 s at the potential where the current reached −0.1 A cm−2.

Samples were prepared by a three-step electrochemical procedure in a glass sphere-electrochemical cell (Figure 1) to

Figure 1. Scheme of the electrochemical cell for the (photo)electrochemical sample preparation in N2 (5N) atmosphere. The sample, contacted with In−Ga, is pasted with silver epoxy to a molybdenum holder fixed on a transfer rod to allow a direct transfer to the UHV analysis chamber. Solutions are delivered to the system through a capillary, in which also the reference electrode is situated. The three-electrode experiments are carried out in a droplet of solution deposited at the sample surface. When needed, illumination is provided to the sample through the platinum counter electrode, which consists of a glass rod covered at its lower end with a cylindrical platinum foil.

obtain reproducible H-terminated Si surfaces (see ref 27). The procedure consists of first removing the native oxide (0.1 M NH4F, pH 4.0, dark conditions), second, photoelectrochemical surface oxidation in buffer solution (KHP), and, third, electrochemical etching of the photoanodically grown silicon oxide (0.1 M NH4F, pH 4.0 and then pH 4.9, and dark conditions). The measured dark current transient during the last step at pH 4.9 has been shown to be a direct indication of the surface quality (hydrogen termination, smoothness).28 For photoelectrochemical experiments on n-Si(111), white light illumination (50 mW cm−2) at very low anodic overpotential (+ 0.1 V positive of OCP) was used. A MI-150 SG high intensity illuminator (Dolan-Jenner Industries) was used to illuminate the sample. The electrolyte consisted of a 0.1 M NaOH solution (pH 13.3). Tapping-mode AFM (TM-AFM) images were recorded in ambient air with a Nanoscope IIIa (Digital Instruments) using Si-tips (NSC15/no Al, with stiffness in the 30−50 N/m range). High resolution (HR) TEM images were recorded using a Philips CM12 (cathode, LaB6; acceleration voltage, 120 kV; Super TWIN lens for high resolution). The sample holder used was a 3 mm molybdenum ring (Plano GmbH) with 1 mm holes. The pressure in the TEM sample chamber was ∼2 × 10−7 Torr, which assured that the sample did not oxidize during analysis. The samples used to record both the AFM and TEM images were prepared the same way both for the preparation of the H-terminated surface and the surface conditioning in alkaline solution. In addition, several samples have been identically prepared and analyzed to check the reproducibility of the process. Moreover, each sample was inspected very carefully and over a large area to make sure that we were not observing any random pattern at the surface. Synchrotron radiation photoelectron spectroscopy (SRPES) was performed at the U 49/2 beamline at BESSY II (Berlin, Germany). Spectra have been recorded for several photon

3. RESULTS 3.1. Photoelectrochemistry. Figure 2 shows the photocurrent−voltage characteristic of H-terminated n-Si(111) in alkaline solution. The potential scan started at OCP and was stopped at the potential marked by the arrow, i.e., before the photocurrent maximum, for analysis of the initially induced surface changes that are described in the Results and the Discussion sections. The inset in Figure 2 shows the electrode behavior for a considerably larger anodic potential range where one observes a passivation (small photocurrents) in the range between −0.7 and 2 V vs SCE, followed by a current increase that can be attributed to the formation of a porous silicon oxide layer. The curve shows some similarity to that observed in dilute ammonium fluoride solutions where also a passive region is followed by a current increase.32 However, in the latter case, 16382



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Figure 2. Photocurrent−voltage behavior. Scan velocity 1 mV s−1 (originally H-terminated n-Si(111)); illumination 50 mW cm−2; solution: 0.1 M N2-purged NaOH, pH 13.3. The inset shows the photocurrent−voltage behavior for larger anodic potentials.

also photocurrent oscillations are observed.19 The surface condition for larger anodic potentials in alkaline electrolyte is presently investigated.33 Figure 3 shows Mott−Schottky measurements for a pH of 6.2, before electrochemical treatment in alkaline solution. For

Figure 4. Energy band diagram of n-Si(111) for (a) the flat band situation (adjusted at pH 13.3, based on the Mott−Schottky plot recorded at pH 6.2) and (b) the electronic surface condition at the anodic potential limit of the electrochemical conditioning, e.g., at −0.95 V.

3.2. Surface Topography. In Figure 5a, an AFM image of an H-terminated Si(111) surface is presented; it exhibits the expected terrace structure. The AFM image of the sample after electrochemical conditioning (Figures 2 and 4) is presented in Figure 5b. The surface exhibits a rms (root-mean-square) roughness of 1.5 nm (Figure 5c), and also, dominant pores are observed, which show an average diameter of 350 nm and an average depth of 5 nm (Figure 5d). The pore size distribution appears to be characterized by two sets of rather monodisperse diameters: the larger pores of 350 nm size and smaller pores of about 100 nm diameter. The TEM image for the surface before conditioning is not presented here because it does not show discernible features. Figure 6a shows a HR-TEM cross-section image after the electrochemical conditioning according to Figures 2 and 4. One sees on the bottom side the Si anode and the Si lattice planes are visible. The step-like indentation on the left part indicates obviously the border of one of the larger mesopores seen in Figure 5. This pore exhibits a depth of around 2.9 nm and a visible length of around 72 nm; hence, it is larger than imaged here. Figure 6b shows an enlargement of the right side of the cross-section of the pore presented in Figure 5d (see inset), at the same scale as the TEM image. The two profiles match one another, attesting that the pore observed by AFM, where some convolution of the tip radius with the actual surface topographical features has to be considered, is revealing the true shape of the pores. This aspect will be discussed later on in the article. 3.3. Surface Chemistry. One of main advantages of running the electrochemistry in the setup presented in Figure 1 is the very low level of surface contamination obtained after the

Figure 3. Mott−Schottky plots of the sample before electrochemical conditioning, in a solution of 0.1 M K2SO4 electrolyte, pH 6.2; scan rate 2 mV s−1.

the three used frequencies, linear regions of 0.4 V can be used to determine the flatband potential that is located at −0.65 V. For a pH of 13.3, the flat band potential is shifted to −1.07 V.34 The evaluation of the slope of the Mott−Schottky plots yields a doping level of 8 × 1014 cm−3 which corresponds well with that given by the supplier of the samples, e.g., 1015 cm−3. Also, the small modulation frequency dispersion of the Mott−Schottky plots indicates the absence of surface states under these (pH 6.2, 0.1 M K2SO4) conditions. The energetic distance between the conduction band and the Fermi level is calculated using the Boltzmann approximation of the Fermi−Dirac integral35 to be located at 0.27 eV below the conduction band edge. A comparison of the electrode potentials and the electron energy scale is presented in Figure 4. For the flatband condition, the dashed line shows the position of the conduction band relative to the Fermi level as a straight line. At the potential where the photoelectrochemical conditioning was terminated, at −0.95 V, the Si bands show a slight band bending due to a depletion condition of about 0.1 eV as indicated. 16383



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Figure 5. AFM images obtained for the sample (a) before and (b) after the electrochemical conditioning, (c) surface profile, and (d) pore profile (profiles are indicated in part b).

Figure 6. (a) HR-TEM image obtained for a sample after emersion at the potential indicated by the arrows in Figures 2 and 4, i.e., after photoelectrochemical conditioning. (b) Comparison of image a with the cross-section of the pore seen in Figure 5d.

Figure 7. Survey spectrum of n-Si(111) for the photoelectrochemical conditioning shown in Figure 2, recorded at a photon energy of 585 eV.

electrochemical conditioning. Indeed, as can be seen in Figure 7, the sample presents a coverage of C smaller than 1 Å, attesting to the very clean surface preparation. Figure 8 shows the SRPES data obtained after emersion of the sample at −0.95 V. Figure 8a shows the Si 2p core level line measured for different photon energies which correspond to different elastic photoelectron escape depths. For an excitation energy of 150 and 585 eV, the escape depths are λesc = 4 and 15 Å, respectively.36 The spin−orbit split between the Si 2p3/2 and 2p1/2 levels of 0.6 eV is clearly visible in the spectra. The Si 2p bulk line at 99.5 eV exhibits an additional signal shifted to higher binding energy by ΔE = 3.9 eV whose height increases with increasing surface sensitivity. Figure 8b shows XP valence band spectra recorded for the two photon energies. For higher surface sensitivity, the energetic distance between Fermi level and valence band is about 1 eV, which is larger than expected from the doping (compare to Figure 4). For decreased surface sensitivity, the

value reduces to 0.84 V, which is in excellent agreement with the value expected from the doping level and the result of the Mott−Schottky experiments, indicating a flatband surface condition. Figure 9 shows the O1s core level line, recorded at hν = 585 eV, where the envelope curve shows a slight asymmetry that will be discussed further below.

4. DISCUSSION 4.1. Oxidation and Dissolution Mechanisms. The chemistry on the studied n-Si(111) surface, due to performed conditioning under illumination (Figure 2), is quite complex. Indeed, on one hand, during the scan from OCP to −0.95 V vs SCE, electrochemical as well as chemical processes will take place, leading to the removal of Si atoms by dissolution. On the other hand, as the conditioning is realized under illumination, 16384



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Figure 8. SRPES of anodized Si(111) as shown in Figure 2: (a) Si 2p1/2−3/2 core level photoelectron spectra for two photon energies as indicated in the figure; (b) valence band onset measurements for two photon energies. The binding energy EB = 0 eV refers to the Fermi level position of the sample.

The electrochemical reaction route is based on the assumption that a kink site on a Si(111) surface represents a surface state with energy within the band gap of the semiconductor.39 Consequently, light-induced excess minority holes are able to oxidize the Si surface atom, as shown in the first reaction step where the interaction with OH− leaves a surface radical Si atom that, in the second step, is energetic enough to dissociate water. This results in a penta-coordinated SN2 transition state40,41 which transforms via electron injection into the conduction band to OH-terminated surface atoms under H2 evolution. Further dissolution follows the chemical route. Photoanodic oxide formation presumably takes place at fully coordinated Si atoms on the (111) surface where the top layer consists of H atoms (H-terminated surface). The scheme in Figure 10c shows deflected light-induced excess minority holes that interact with the Si atoms that are exclusively connected to other Si atoms. This presumption is based on the consideration of partial charge shifts at the topmost Si atoms toward H, which leaves the underlying Si−Si bond somewhat polarized, as indicated in Figure 10c.42,43 The polarization of the bond is due to the larger electronegativity of H compared to that of Si, which results in a preferential migration of the holes to the indicated Si atoms. The presence of positive charge (holes) results in an attack of the polarized bond by hydroxyl groups, leading to radical formation and a Si−OH bond. At the Si radical, water splitting occurs under H2 evolution and hydroxyl bridge formation. Finally, an oxo-bridge in the Si back-bond network is formed under release of a water molecule. 4.2. Influence of Hydrogen. In both the chemical and electrochemical reaction schemes above, H2 is evolved and the influence of hydrogen diffusion into Si has been discussed intensively in the literature on electronic properties of Si.44,45 Mostly, however, the influence of hydrogen as an amphoteric dopant in passivating the original dopants has been investigated.46,47 In silicon electrochemistry, an established influence is the change of p-type samples toward more intrinsic behavior due to an energetically upward shift of the Fermi level.48 Theoretically, several hydrogen species have been proposed to influence the carrier concentration in Si,45,49 but so far, an accurate assessment of electrochemically conditioned samples has not yet been performed. Therefore, we have performed experiments to determine H profiles in Si, related to H2 evolution. Since sample transfer was in ambient air, we selected an experimental condition where H2 evolution is known to be pronounced (see caption of Figure 12). The ERDA nuclear resonance analysis data are displayed in

Figure 9. O 1s core level SRPE spectrum of Si(111) after electrochemical conditioning recorded at a photon energy of 585 eV.

the minority carriers will be able to play a role in the surface modification leading to Si−O−Si bridge formation. The basic mechanisms for the chemical and electrochemical anodic dissolution of Si and the oxygen bridge formation under illumination are displayed in Figure 10. The chemical route has been extensively investigated during the last decades37 and is here summarized. It is characterized by solvolytic attack of the Si surface atoms at kink sites which releases two H2 molecules in the process of successive formation of Si−OH, leading to dissolution of the silicon atom and to formation of Si(OH)4, soluble in alkaline solution (Figure 10A). The reaction leaves an H-terminated surface, as schematically shown in the figure. The surface is then characterized by one kink site Si atom and in the next step by two kink site Si atoms (Figure 11), resulting in the transformation of a zigzag type atomic terrace appearance into a truncated one, as has been observed also in acidic solutions.38 The dissolution process can follow four possible routes, as indicated in Figure 10A. In the first step, the substitution of the H atom at the kink site by the hydroxyl group takes place. The driving force for this process is the electronegativity difference between the silicon at the kink site (1.9) and the bonded hydrogen (2.2). This makes the attack of the kink site silicon by the water molecule the most favorable step. In the next steps, different sequences of reactions can be observed. Because the much more electronegative −OH group polarizes the Si kink site atom already at this stage, the splitting of the Si−Si backbond, marked in Figure 10A as a, can take place. However, the dissolution can also go via substitution of the −OH group at the Si−H bond; this is marked as b. Splitting of the back-bond, c, can take place when the Si kink site atom is terminated with two or even more probably with three hydroxyl groups. 16385



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Figure 10. Chemical (A) and electrochemical (B) dissolution processes and oxygen bridge formation (C) reaction mechanisms for the illuminated n-Si(111) surface in alkaline solution. The relative partial charge on particular atoms, due to electronegativity differences between neighbors, is marked as σ+ and σ−. The black circles (●) represent electrons. The small letters in image A indicate the solvolytic attack of water molecules on a dangling bond (b) or on one of the two back bonds (a and c). The dissolution process always starts from the water attack on the dangling bond at the kink atom which leads to substitution of the hydrogen by a hydroxyl group. Then, the process can follow four routes which differ by the sequence of bound dissolution: (1) ac, (2) abc, (3) bac, and (4) bca.

Figure 11. The terrace dissolution on the Si(111) surface, starting from left to right. After dissolution of the first kink site atom, the next kink site atom is available for reaction (middle image). The dissolution of this atom leads to the presence of two kink site atoms at the terrace.

smaller currents flow, this influence is weaker, but nevertheless, it shows that irreversible and substantial loading of Si by H occurs. In view of the SRPES experiments, to be discussed below, the main interdiffused hydrogen species and their influence on the electronic properties of Si need to be considered. With the high H concentrations observed by ERDA, the influence of complexes that are related to hydrogen−donor interactions50 appears negligible, since the relative concentrations differ by several orders of magnitude (doping level 1015 cm−3). Therefore, the abundant data that relate to donor and acceptor passivation effects by H48,51 do not apply for interpretation of our results. Also, because of the

Figure 12 and compared to experimental results from the literature, obtained by temperature-induced deuterium diffusion. Both the ranges of concentration and distance differ because of the differences between our experiments and the ones from the literature, i.e., temperature and pressure. However, the same trend of diffusion profile is observed. We make use of the very high surface sensitivity of the ERDA method and of the ability to obtain absolute concentration levels. We observe that, in an ultrathin surface layer of about 15 nm, the H concentration reaches values that correspond to about 5 at % of H species in Si. We expect that, under our experimental conditions where 16386



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Figure 12. Concentration profiles obtained from ERDA using the γ-quanta counts from the nuclear reaction H(15N,γα)12C (▲) compared to data excerpted from the literature (■ and ●).45,49 Prior to the measurement, the samples have been conditioned: (▲) in 4 M N2 purged-NaOH, by applying a potential from OCP in the cathodic range until a potential where the current density reached −0.1 A cm−2, then the sample was held at that potential for 40 s, (■) by deuteration at 150 °C from a gas discharge (70 W - 2 Torr) for 60 min, and (●) by deuteration at 120 °C from a gas discharge (0.12 W cm−2 - 1 mbar) for 120 min. All samples are P doped n-type Si in the (▲) 1015 cm−3, (■) 1017 cm−3, and (●) 1016 cm−3 range.

recoil character of the ERDA experiment, species such as H2 with their different mass are not plotted in Figure 12. Although H2 interdiffusion cannot be assessed, the incorporation of H is so pronounced that it is likely the most influential species. Accordingly, we focus on the influence of hydrogen as a shallow donor in Si. The uncertainties in assigning energy positions to H-related shallow donors range from EH = EC − 0.16 eV to EC − 0.22 eV52 and, more recently, EC − 0.035 eV.53 In ref 53, even a doping type conversion has been observed upon H exposure of p-type Si, which indicates the strong influence of H as a donor. H diffusion into Si occurs as a precursor reaction of H2 evolution of the three reactions shown in Figure 10. From the XP valence band spectra, a Fermi level shift in the surface region is noted for hν = 150 eV. The energetic distance between the conduction band edge and EF is 0.14 eV. This corresponds to a doping level in the 1017 cm−3 range, e.g., much less than the hydrogen concentrations shown in Figure 11. This difference can be attributed to the considerably reduced hydrogen generation in this experiment anodic from open circuit compared to the excessive hydrogen generation at −1.2 V from OCP used in the ERDA analysis. For an electron escape depth of 15 Å (hν = 585 eV), the Fermi level position is close to that calculated from the original P doping level, indicating that the H profile in Si is limited to the near surface region, as would be expected for a considerably lower hydrogen generation rate and, consequently, a lower interdiffusion into Si at the surface. This is supported by comparison of the H2 evolution rate in the experiment that led to step bunched surfaces (Figure 12) and the H2 formation due to chemical and electrochemical etching and anodic oxide formation (Figures 2 and 10). In the former, the negative applied potential results in an inhibition of the chemical etch rate,42 and therefore, the current passed can be used to calculate the H2 evolution. The hydrogen evolutions of the two experiments are considerably different. The uncertainty here is, however, the assessment of the relative amount of hydrogen that is actually interdiffusing into the Si host, but presently, we assume that the same relative amount of hydrogen is incorporated. 4.3. Surface Chemistry. In Figures 13 and 14, the deconvoluted envelope signals of Figures 8 and 9 are shown. The Si 2p core level can be decomposed into six components. The feature negative from the Si bulk line (ΔE = −0.4 eV vs Si bulk) is characteristic of compressive stress, related to silicon oxide formation where the volume mismatch between oxide

Figure 13. Deconvoluted Si 2p core level for the photoelectrochemical conditioning shown in Figure 2; the binding energy shifts ΔEB are given with respect to the bulk line (see text).

Figure 14. Deconvoluted O 1s core level for the photoelectrochemical conditioning shown in Figure 2 but for a larger scan velocity. Dashed line, signal attributed to O in SiO2; dotted line, O in OH (see text).

and Si is 2.27.54 Accordingly, the signal shifted with respect to the bulk line by ΔEB = 3.9 eV is that of Si in SiO2. From the signal height, the oxide thickness can be calculated on the basis of eq 1:55 ⎛ I I∞ ⎞ dox = λf ln⎜ ox Si∞ + 1⎟ ⎝ ISi Iox ⎠

(1)

where λf is the mean inelastic length of photoexcited electrons ∞ with kinetic energy E, I∞ Si and Iox are the intensities of pure elements, ISi and Iox are the measured line intensities, and dox is the overlayer thickness. An oxide thickness of around 1.5 Å has been calculated. This corresponds to less than a monolayer, 16387



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processes start at kink site atoms. Since one process provides the dissolution and the other the passivation, the relative rates determine the surface topography. Since, at the potential of −0.95 V, where the conditioning process has been interrupted, passivation has not yet set in, this potential regime is characterized by a slower oxidation rate compared to dissolution. However, since silicon oxide is very slowly etching in alkaline media,61 the successive formation of oxide will eventually result in complete surface coverage and inhibit further etching, resulting, for a thicker oxide layer, in the wellknown passivation observed in Figure 2. The situation at the increasing branch of the photocurrent is characterized by initial oxidation and simultaneously by etching at those sites where oxidation has not yet taken place. Using the relative capacitance of Si space charge layers and that of the electrolyte double layer on Si,62 the electric field strength in the double layer corresponds to that in a diffuse Gouy−Chapman type layer. Accordingly, at the small applied potential, anion (OH−) counterions will be laterally separated and the standard microcapacitor model for the Helmholtz layer using the approximation of smeared out charge and counter charge on both sides of the interface will not apply. Therefore, the ionic counter charge is assumed to be located at surface sites where the electric field is enhanced, for instance, due to roughness (Ez = V/rc, where rc is the curvature radius at a protrusion). This results in local anodic currents and explains the island type initial oxide formation. Since the dissolution occurs preferably at kink sites, this process, too, is local on an atomic scale. The scheme for chemical dissolution in Figure 10A shows a predominantly lateral mechanism which also would explain the rather large diameter of the observed pores and their relatively small depth (compare Figure 5). After etching in alkaline solution, the Si(111) surface developed pores that are triangular or circular in shape, as can be observed in Figure 5.63 The shape of pits depends on the removal ratio of a double bonded silicon atom (kink site Si) versus triple bonded Si at the step site. The straight steps are constructed by three coordinated Si atoms, and they propagate toward [21̅1̅] and two other equivalent directions on the 111 surface. The triangular shape of pores comes by connection of the three straight steps parallel to the [21̅1̅], [1̅1̅2], and [1̅21̅] directions, respectively. If removal of the kink site Si atoms is much faster than the monohydride Si atoms, the triangular pit with straight edges is formed due to decrease of kink density. If the velocities of both dissolution processes are in a similar range, then the kink density is large and circular pits can be observed. For shape analysis of the pores, the a and b radius were introduced (Figure 15A), where a is the distance projected on the (111) plane, between the pit center and the pit edges ⟨21̅1̅⟩, and b is the distance projected on the (111) plane, between the pit center and the pit corners opposite to the ⟨2̅11⟩ edges. When the ratio a/b changes from 1 to 0.5, the pit shape changes from circular to triangular, respectively, as can be seen in Figure 15B. Figure 16 shows an AFM image of the surface after the (photo)electrochemical conditioning, where a zoom on several pores can be seen. The pores exhibit either a transitory shape between circular and triangular (Figure 15B, 0.5 < a/b < 1) or a triangular shape (Figure 15B, a/b = 0.5). The pure circular shape of the pore, where a/b = 1, is difficult to find on the image.

which means that the oxide does not completely cover the surface, indicating oxide island growth under these photoelectrochemical conditions. The component shifted by +0.2 eV can be attributed either to the surface core level shift that results from H-terminated surface parts or to tensile stress in Si underneath an ultrathin oxide layer.56,57 It is unlikely, however, that this signal originates from hydrogen that is incorporated into the topmost surface region (compare Figure 8b), since its concentration would be too low to be detected by a photoemission experiment. On one hand, it has been seen that the oxide growth is of Volmer−Weber type, leading to island formation. This would mean that, on the surface free of oxide, H-terminated Si is expected. Moreover, based on the chemical and electrochemical mechanisms written above (Figure 10), both reactions end up by letting the surface be covered by H termination.37 On the other hand, since the influence of the oxide islands on the line position for compressive stress is rather pronounced, the presence of strained Si underneath the oxide patches is also expected. However, at this very surface sensitive energy, it is very possible that the strained Si is not seen as the signal has to cross the monolayer of oxide (4.5 Å) to be detected. We can conclude that the signal shifted by +0.2 eV is due to both the surface core level shift that results from H-terminated surface parts and to tensile stress in Si underneath an ultrathin oxide layer. However, very probably, we only detect the Hterminated Si contribution. Besides the SiO2 signal, we observe lines, shifted by +0.5, +0.8, and +2.3 eV. The line shifted by +0.5 eV is attributed to silicon bonded to 2 H at the surface (a kink site), and the signal at +0.8 eV is attributed to Si(I)-type surface atoms that are OH-terminated58 and, for kink sites at a (111) surface, the signal results from Si− H−OH. The shift of +2.3 eV is characteristic for a Si(III) species with one remaining back-bond to the Si lattice.36 The existence of oxide and hydroxide species is also visible from the deconvolution of the O 1s line in Figure 9, shown in Figure 14. The OH related component is attributed to a precursor state of the oxidation process34 and/or OHterminated surface atoms in the alkaline electrolyte. From the comparison of the signal intensities of the two components, the OH related one is clearly in the lower monolayer range, since already the oxide signal is due to less than a monolayer as deduced above. In the potential region investigated here, oxide formation has not been observed by, for example, ellipsometry.59 Figures 13 and 14, however, clearly show a contribution of SiO2 in the XP spectra. We attribute this to two effects: first, SRPES with the ultrahigh surface sensitivity for the Si 2p line at a photon energy of 150 eV and a very high surface sensitivity of the O 1s line allows identification of minute submonolayer amounts of oxidized Si species. Second, the interpretation of ellipsometric data is not straightforward and typically based on several assumptions, including, for instance, roughness and the use of effective medium theories.60 4.4. Topography. The energy band diagram in Figure 4 shows that the sample is in slight depletion during the conditioning which allows light-generated holes from the valence band to reach the sample surface and oxidize n-Si, as shown in the scheme in Figure 10. Two holes are consumed to form an oxygen bridge and to remove the H termination of the Si surface atoms. In the chemical etching and anodic oxidation mechanism shown in Figure 10, it was proposed that the 16388



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considered heterogeneous catalysts are discussed. Although the minority carrier diffusion length in Si is in the order of several 100 μm, the use for electrocatalytic processes depends on the relation between exchange current density, j0,cat, of the considered catalyst emitter material and the necessary area to sustain, for example, a photocurrent, j, of 10 mA cm−2 at an overvoltage η of 150 mV. Assuming that the pores cover 5% of the surface, and using the Tafel approximation for an estimate of the exchange current density j0,cat ∼ j/exp(αnFη/RT), where α is the charge transfer coefficient, taken here to be 0.5, and n is the number of electrons transferred to complete the desired reaction, taken here to be 2 for a divalent process, the required exchange current density is approximately 0.5 mA cm−2. Comparing this value with the volcano plots for HER (hydrogen evolution reaction) and OER (oxygen evolution reaction)66−68 shows that, for these rather stringent conditions, only highly active electrocatalysts (Pt, Rh, and Ru) can be used. For Pt, Rh, or Ru, however, the nanoemitter design would work, using only amounts of noble metals that are in the submonolayer range, and in future experiments, the structure formation identified here will be used as a template for preparation of photoelectrochemical solar cells.

Figure 15. (A) Schematic drawing of the side view of the pore. (B) Schematic drawing of the top view of pit shapes depending on the changing ratio between a and b. Adapted from ref 63.

5. CONCLUSION The initial phase of light-induced anodization of Si(111) in alkaline electrolyte has been investigated. Both the surface chemistry and topography have been described and modeled on the basis of SRPES and SPM, and the related oxidation and dissolution mechanisms have been proposed. The often neglected influence of hydrogen evolution during the photoelectrochemical conditioning of the sample on electronic properties has been determined experimentally with ERDA, and absolute values for H incorporation have been obtained. The formation of self-organized nanostructures has been observed in the rather narrow initial anodic potential range, as evidenced by scanning probe and transmission electron microscopy. The possible application of the prepared nanostructures, formed in situ at room temperature in a scalable manner, for light-induced water splitting half cells has been discussed.

Figure 16. AFM image obtained for the sample after the electrochemical experiment (Figure 2) was performed.

At this point, the influence of reaction products on the shape of pores should be emphasized. Silicates formed during dissolution have a tendency to polymerize into siloxanes that are characterized by high viscosity (Figure 17). If stirring is not

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AUTHOR INFORMATION

Corresponding Author

* E-ma il: m arik a.letilly@hemlholtz -ber lin.de. Fax: 004930806242331. Phone: 004930806242434. Figure 17. Polymerization reaction of surface dissolution products. The reaction takes place in the solution and leads to the formation of siloxanes.

Notes

applied during the etching process, the dissolution products will block the surface, influencing the etch rate64 and hence the pore shape. The observed imperfection of pore shapes in Figure 16 could be attributed to the described effect. 4.5. Possible Use for Photocatalytic and Photovoltaic Electrochemical Nanoemitter Solar Cell. An important aspect of these nanostructures is the possible use for photocatalytic and photovoltaic electrochemical nanoemitter solar cell structures.65 The necessary requirements regarding the total surface area of electrocatalysts needed to sustain the current density due to light-induced excess minority carriers in conjunction with the exchange current density of the

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, as follows: Part of the electrochemical experiments were supported though the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993; the SRPES and ERDA measurements and the AFM and TEM images were supported by the Deutsche Forschungsgemeinschaft-DFG under Award No. LE 1192/4-1/2. The authors are grateful to M. Kanis and H. Jungblut for contributing to the SRPES experiments, to M. Aggour for his help regarding electrochemical measurements, to U. Bloeck for recording of the TEM images, and to F. Munnik and K. Saravanan for performing the

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS



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ERDA measurements. M.L. is grateful for financial support of the DFG (Award No. LE 1192/4-1/2).

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