Aqueous HCl

May 30, 2014 - Mikhail V. Lebedev*†, Wolfram Calvet‡, Thomas Mayer‡, and ... Sven Van Elshocht , Silvia Armini , Stefan De Gendt , and Christoph...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Photoelectrochemical Processes at n‑GaAs(100)/Aqueous HCl Electrolyte Interface: A Synchrotron Photoemission Spectroscopy Study of Emersed Electrodes Mikhail V. Lebedev,*,† Wolfram Calvet,‡ Thomas Mayer,‡ and Wolfram Jaegermann‡ † ‡

A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, St. Petersburg 194021, Russia Darmstadt University of Technology, Institute of Material Science, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany S Supporting Information *

ABSTRACT: High-resolution synchrotron photoemission spectroscopy has been applied to detail the electrochemical and photoelectrochemical corrosion reactions at the liquid junction n‑GaAs(100)/1 M aqueous HCl solution. Under anodic polarization of 1.8 eV, the main process initiated by the presence of holes in the Ga−As bonding states of the valence band is the formation of soluble gallium chloride complexes and insoluble elemental arsenic on the surface. In addition, arsenic hydroxide forms, which reacts further to soluble HAsO2. In toto, the As/Ga atomic ratio increases, which is accompanied by an increase of the work function. The anodic decomposition reaction is enhanced by illumination as more holes reach the n-semiconductor/electrolyte junction. Under cathodic polarization of 1.5 eV, only minor changes are observed in Ga and As core-level spectra, giving no indication of corrosion, but specific adsorption of hydrated HCl molecules and/or Cl− ions considerably modifies valence band spectra. cathodes.4 Such investigations are performed as a rule by electrochemical and optical methods, such as electrochemical impedance spectroscopy,5−8 second harmonic generation,7,9 and infrared spectroscopy.1,10 A more detailed view of contact formation at semiconductor/ electrolyte interfaces requires a precise knowledge of the microscopic structure of the phase boundary, e.g., the atomic and electronic structure of the semiconductor surface and interfacial composition of the electrolyte.11 In particular, the interface states both intrinsic and formed by interaction with the different species of a complex electrolyte solution should be taken into account in consideration of the reaction mechanisms at semiconductor/electrolyte interfaces. Photoemission spectroscopy, especially excited by synchrotron radiation, is a powerful tool for analyzing chemical composition and electronic structure of the surfaces and interfaces. However, ultrahigh vacuum (UHV) conditions are required, prohibiting analysis of highly volatile liquids. Some approaches were suggested for studying the processes occurring at semiconductor/electrolyte interfaces by means of photoemission spectroscopy. In particular, the study of the interface consisting of a thin electrolyte layer frozen-in on the semiconductor

1. INTRODUCTION Photoelectrochemical processes are widely used in semiconductor technology and recently became the subject of intensified interest in the context of water-splitting. Processes occurring at the semiconductor/electrolyte interfaces are very complex and include interrelated ion adsorption, surface bond breaking (corrosion), as well as charge transfer from semiconductor conduction and valence bands to species in the solution for reduction and oxidation reactions, respectively. GaAs is one of the most intensively studied semiconductors because of its suitability for fabrication of a large variety of electronic and optoelectronic devices, such as high-speed transistors, light-emitting diodes, diode lasers, and solar cells. Different processes can occur at a GaAs/electrolyte solution interface under various electrochemical conditions.1 Under anodic conditions with recombination of electrons from the solution with holes in the valence band, GaAs decomposes, while under cathodic conditions the reduction of protons (H2 evolution) takes place because of electron transfer from the conduction band of the semiconductor.2 Photoanodic dissolution has been investigated intensively in recent decades because it enables photoetching of n-type semiconductors2 but hampers the proper operation of photoelectrochemical solar cells.3 Hydrogen evolution has been studied as a possible process for hydrogen fuel production by photoelectrochemical water splitting at semiconductor photo© 2014 American Chemical Society

Received: January 17, 2014 Revised: May 26, 2014 Published: May 30, 2014 12774

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781

The Journal of Physical Chemistry C

Article

surface,12 as well as step-by-step coadsorption of the electrolyte solution components under vacuum conditions,13−15 were carried out. Nevertheless, a majority of studies of semiconductor/electrolyte interfaces by photoemission spectroscopy were performed on the samples emersed at room temperature from the electrolyte solution of interest giving access to the surface chemical composition after chemical and electrochemical treatment.14,16−18 With the recent development of near ambient pressure XPS, photoemission on solid/ liquid interfaces using high-energy excitation comes within reach.19 We analyze chemical reactions and accompanying chargetransfer processes occurring at the n-GaAs(100)/aqueous HCl solution interface in the course of applying different potentials to the semiconductor working electrode. The analysis of interface chemical processes and surface composition is performed by means of highly surface sensitive synchrotronradiation photoemission spectroscopy on emersed electrodes.

irradiation by white light of an incandescent lamp with a power density of about 100 mW/cm2 directed to the semiconductor/ electrolyte interface using the glass pipe as waveguide (Scheme 1). The experimental procedure is shown schematically in Chart 1. We used three different samples from the same wafer. For conditioning, each sample was brought into contact with the 1 M aqueous HCl solution for 3 min without applied bias to remove the native oxide. Photoemission proved the similarity of the chemical composition after this etch step. Thereafter the samples were brought in contact with the electrolyte again. Sample 1 was subjected to cyclic voltammetry study. Samples 2 and 3 were maintained under open circuit voltage (OCV) for 30 min and subsequently characterized by photoemission. Samples 2 and 3 were then brought again into contact with the solution and either anodic (sample 2) or cathodic potential (sample 3) was applied in the dark for 30−40 min; when under applied voltage, the solution meniscus was blown off with an Ar jet (emersion) and the samples were characterized by SXPS again. Sample 2 was then brought into contact with the solution at the same anodic potential, but now under dedicated illumination. The photoemission studies were performed at the undulator beamline U49/2 of the BESSY II storage ring, which provides photons in the energy range between hν = 90 and 1400 eV. Photoemission spectra were measured with the highest possible surface sensitivity. The valence band as well as Ga 3d and As 3d core levels were measured with 90 eV excitation energy, whereas Cl 2p, C 1s, and O 1s core levels were measured using 250, 350, and 650 eV excitation energy, respectively. At these energies the electron inelastic mean free path λ of the respective photoelectrons is approximately 5 Å,21 implying that roughly 63% of the signal stems from the first atomic layer, 23% from the second, and 9% from the third. Ga 3d and As 3d core levels were also measured at 250, 350, and 650 eV, resulting in mean free path λ of 7, 9, and 15 Å, respectively. All spectra were measured at normal emission. Core-level spectra were fitted with Voigt functions after Shirley background subtraction using IGOR Pro software (WaveMetrics, Inc.). For As 3d and Ga 3d, similar parameters (Gaussian width for measurement uncertainty, Lorentzian width for lifetime broadening, spin− orbit splitting, branching ratio) as used in ref 22 were applied. The spectra were obtained using the Phoibos 150 (SPECS) energy analyzer of the experimental system SoLiAS.20 The work function of the samples is determined from the low kinetic energy cutoff of the secondary electron background (“secondary edge”) in the valence band spectra.23 A bias of −4 V was applied to the sample during the work function measurements to distinguish between the analyzer and the sample cutoff and to efficiently collect the low-energy electrons into the analyzer.

2. EXPERIMENTAL SECTION The semiconductor electrodes used in the experiments were made of single-crystalline n-GaAs(100) wafers with a dopant concentration of (3−5) × 1017 cm−3. The rear ohmic contact was provided by gluing the sample to the stainless-steel sample holder with silver paste. Cleaning of the electrode surface prior to the experiment was performed by ultrasonic rinsing with 2propanol and acetone. As electrolyte solution 1 M aqueous HCl solution was used prepared from reactant-grade concentrated HCl (37%) mixed with Milli-Q water. Prior to the experiment, the electrolyte solution was deaerated by continuous bubbling of Ar gas through the solution for 30 min. Experiments were carried out using a conventional threeelectrode scheme in the electrochemistry chamber (Scheme 1) Scheme 1. Electrochemical Setup Used in the Experiment

being part of the integrated solid/liquid analysis system (SoLiAS)20 permanently operated at BESSY. This glass chamber is purged with inert, dry, carbon-free Ar gas and is directly attached to a special buffer chamber to allow for the transfer of the sample into UHV for subsequent photoemission analysis without contact with ambient atmosphere. A special cell was introduced to the electrochemistry chamber to contact the sample with a meniscus of the solution in a three-electrode electrochemical setup (Scheme 1). The electrode potential was controlled with respect to the Ag/AgCl reference electrode (in saturated KCl), and a Pt wire was used as counter electrode. The sample voltage was applied to the insulated sample holder. Photoelectrochemical experiments were performed both in “dark” conditions, i.e., at low background light intensity of BESSY experimental hall, and under conditions of intensive

3. RESULTS After contact of the initial native-oxide-covered n-GaAs(100) surface with 1 M HCl(aq) solution, the oxygen content on the surface decreases considerably and the surface becomes As-rich. The cross-section-corrected24 ratio of integrated As 3d to Ga 3d emission as measured at 90 eV excitation energy becomes equal to ∼1.7. In addition, photoemission of the Cl 2p core level appears as well. The carbon C 1s photoemission intensity remains unaffected by treatment with 1 M HCl(aq) solution. These data are in agreement with the results of numerous studies25−27 indicating the effective removal of the native oxide 12775

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781

The Journal of Physical Chemistry C

Article

Chart 1. Flowchart of the Experiment

Figure 1. Current−voltage characteristics of n-GaAs(100) in 1 M HCl(aq) solution measured in “dark” conditions, i.e., at low background light intensity of BESSY experimental hall, and under conditions of intensive irradiation by white light of an incandescent lamp with a power density of about 100 mW/cm2 directed to the semiconductor/electrolyte interface through a glass waveguide (a); cathodic portion of these polarization curves at reduced scale (b).

can vary within the range of ±0.15 V versus Ag/AgCl reference electrode for several hours of the experiment performance. The As 3d and Ga 3d core-level spectra measured after contact of the n-GaAs(100) surface to 1 M HCl(aq) solution under open circuit voltage for about 30 min are shown in Figure 2 (bottom curves). The As 3d core-level spectra can be fitted well using four different components (Figure 2a). Besides the bulk As−Ga component, there are components shifted to higher binding energies by 0.6, 0.9, and 1.25 eV. The intensities of these chemically shifted components decrease with excitation energy (Supporting Information, Figure S2), clearly indicating the surface origin of these components. The first chemically shifted component can be assigned to elemental arsenic As0,22 whereas the component with the highest chemical shift can be associated with As−OH bonds probably with some addition of As−Cl bonds.12,15,28−30 The assignment of the As

layer from GaAs surfaces with aqueous HCl solutions (Supporting Information, Figure S1). After the native-oxide layer was etched off with 1 M HCl(aq) solution and measurements of photoemission spectra were recorded, the n-GaAs(100) sample was again brought into contact with 1 M HCl(aq) solution for subsequent photoelectrochemical and photoemission studies. The results of the steady-state polarization measurements both under dark (background light of BESSY experimental hall) and intensive irradiation conditions are shown in Figure 1a,b, where Figure 1b illustrates the cathodic portion of polarization curves at reduced scale. The polarization curves look similar to the characteristics of the n-GaAs/acidic solution interfaces obtained previously.1,2,5−8 After cycling and stabilization of the polarization curve, the open circuit voltage value in the dark was determined to be about 0.1 V (Figure 1b). It should be noted that the OCV value 12776

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781

The Journal of Physical Chemistry C

Article

Figure 2. 90 eV excited As 3d (a) and Ga 3d (b) core-level spectra of a n-GaAs(100) electrode surface emersed from 1 M HCl(aq) solution under anodic polarization at +1.8 V versus Ag/AgCl with and without illumination by white light of an incandescent lamp with a power density of about 100 mW/cm2 directed to the semiconductor/electrolyte interface through a glass waveguide (top and middle spectra). The bottom spectra were obtained after emersion at OCV without illumination. Dashed red line is a fit with indicated components. Spectra are normalized to maximum intensity. Scaling factors are given with respect to the OCV, dark, bottom spectra.

Figure 3. Core-level Cl 2p (a) and O 1s (b) spectra measured on n-GaAs(100) emersed from 1 M HCl(aq) solution under OCV and anodic polarization of +1.8 V versus Ag/AgCl in dark and illuminated conditions, as well as the corresponding difference spectra. The excitation energies were 250 eV for Cl 2p and 650 eV for O 1s for respective highest surface sensitivity.

component with a chemical shift of 0.9 eV is not so straightforward. Considering the chemistry of GaAs interaction with aqueous HCl solution, one can expect the interaction of As with H,1,10 Cl, or H2O. As the chemical shift of As−H with respect to the bulk As−Ga component is +0.35 eV,31,32 we tentatively assign the 0.9 eV shifted component to As−Cl bonds. After interaction of the clean GaAs(100) surface with HCl in 2-propanol, a similar component with the chemical shift of about 1 eV was observed in As 3d spectra.33 This component was assigned to arsenic monochloride AsCl* intermixed with elemental arsenic, and we follow this assignment. Addition of a +0.35 eV shifted As−H component caused a little improvement of the fit, but the intensity of this component was only 3% compared to the bulk As−Ga emission and therefore is left out in the fits displayed in Figure 2. The Ga 3d core-level spectrum measured with 90 eV excitation energy can be fitted well using four different components (Figure 2b). Besides the bulk Ga−As component, there are surface components with chemical shifts of +0.4, +0.9, and +1.9 eV, the intensities of which increase with increasing surface sensitivity, i.e., by reducing the excitation photon energy (Supporting Information, Figure S2). The component with the

chemical shift of +0.4 eV can be associated with gallium suboxide GaxO.25 The assignment of the component with the chemical shift of 0.9 eV is more complex, as chemical shifts for gallium monochloride GaCl have been published in the range of 0.6−0.9 eV,29,30 but Ga−OH also falls in this range. The chemical shift of Ga−OH is +0.9 eV,17,33 and it can be expected to appear at the GaAs/HCl(aq) interface.1,10 Finally, the assignment of this component to Ga−H bonds cannot be ruled out as well.31,34 So, the component of the Ga 3d spectrum shifted from the bulk Ga−As component to higher binding energy by 0.9 eV and labeled GaCl in Figure 2b can also represent Ga−H, Ga−OH, or a Ga−Cl/Ga−H/Ga−OH mixture. The component with the chemical shift of +1.9 eV can be assigned to higher gallium chlorides as GaCl229,30 or GaCl3.12,30 Once the anodic bias is applied to the GaAs/electrolyte interface, the intensities of all chemically shifted components in As 3d and Ga 3d core-level spectra at open circuit voltage increase relative to the respective bulk components (Figure 2, middle curves) because of anodic decomposition of the GaAs(100) surface indicated by formation of elemental arsenic, As−OH, as well as gallium chlorides and suboxides. This is 12777

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781

The Journal of Physical Chemistry C

Article

Figure 4. Excited (90 eV) As 3d (a) and Ga 3d (b) core-level spectra of an n-GaAs(100) surface emersed from 1 M HCl(aq) solution after exposure under OCV (bottom curves) and after cathodic polarization at −1.5 V versus Ag/AgCl in dark conditions (top curves). Dots are data points after background subtraction. Dashed red line is a fit with indicated components.

which is evidenced by the decrease in the gallium chlorides surface content after rinsing with water.38 Gallium chloride can be also transferred as a complex to the solution after capturing another hole.6 In contrast, elemental arsenic is insoluble in aqueous HCl solution and thus accumulates at the surface.25,38 Under illumination with visible light, additional holes are generated and the process of Ga dissolution and elemental arsenic accumulation is enhanced correspondingly. This is accompanied by the increase in the content of higher gallium chlorides (Figure 2b) and in the increase in the total chlorine amount on the surface (Figure 3a). In addition, an n-GaAs sample emersed from HCl solution and cooled to liquid N2 temperature before evacuation showed increased gallium chloride surface content because of frozen-in reaction products, which desorbed under synchrotron light exposure,12 whereas without synchrotron light the desorption of GaCl3 in UHV starts at 350 K.39 Under anodic bias, the formation of arsenic hydroxides is increased as well (Figure 2a) because of adsorption and dissociation of water molecules. The As−OH component increases along with the high-binding-energy part of the O 1s photoemission (Figure 3b). Therefore, the process of arsenic hydroxide formation is considered to be electrochemical as well. The detected As−OH can be considered as a precursor for the HAsO2 compound that can be formed in accordance with the anodic decomposition process.1

accompanied by the decrease in a high-binding-energy part of the Cl 2p photoemission (Figure 3a) and the increase in the high-binding-energy part of the O 1s photoemission with the binding energy of about 533.3 eV (Figure 3b), indicating adsorption of OH groups in accordance with the increase of the As−OH component (Figure 2a). The anodic decomposition process becomes stronger under illumination (Figure 2, upper curves) and is accordingly accompanied by the increase in the Cl 2p photoemission on the high-binding-energy side (Figure 3a) and decrease of the O 1s photoemission (Figure 3b). In the latter case, higher gallium chlorides are formed that obviously can be transferred to a large extent to the solution in the form of chloride complexes causing intense formation of elemental arsenic on the surface. The overall As/Ga atomic concentration ratio increases by a factor of 2 from 2.4 (bias +1.8 V, darkness) to 4.7 (bias +1.8 V, illumination). The anodic decomposition process results in slight shift of the bulk components in As 3d and Ga 3d spectra toward lower binding energies (Figure 2), indicating an increase in the surface band bending by 50 and 100 meV at +1.8 eV under darkness and illumination, respectively. The anodic decomposition also causes a work function increase from 4.15 eV after OCV to 4.35 eV after +1.8 V versus Ag/AgCl reference electrode and to 4.9 eV after anodic bias of +1.8 V and illumination. In studies of the atomic structure of GaAs(100) surfaces with different reconstructions, increasing work function with increasing surface concentration of As was observed.35,36 Therefore, we relate the increased work function after anodic decomposition to the observed increased As surface concentration. Using the displayed results, the following mechanism of GaAs etching in aqueous HCl solution under anodic bias in dark conditions and exposed to visible light can be derived. Formation of elemental arsenic proceeds by the reaction1,37 GaAs + 3h+ → Ga 3 + + As0

GaAs + 2H 2O + 6h+ → Ga 3 + + HAsO2 + 3H+

(2)

When the anodic voltage is applied in dark conditions, the number of holes at the liquid junction is rather small, reactions 1 and 2 take place in parallel, and both elemental arsenic and HAsO2 (As−OH) are formed in similar amounts (Figure 2a). Under exposure to light, more holes are provided and reactions 1 and 2 are intensified to a large extent. Because of high solubility,10 most of the formed HAsO2 (As−OH) is dissolved while elemental arsenic is continuously formed. It should be noted that no arsenic oxides and very little gallium suboxide is formed when the anodic current is passed through the GaAs/HCl(aq) interface (Figure 2). This is different from the previous studies on anodic decomposition of GaAs surfaces in aqueous HCl solutions.16,40,41 We relate the

(1)

First, a hole is captured by a gallium site, causing breaking of the surface Ga−As bond.6,37 Next, a chlorine anion adsorbs at the Ga+ cation, forming a Ga−Cl bond (Figure 2b). Capture of the next holes results in breaking of further Ga−As bonds and formation of GaCl2 and GaCl3 species, as well as elemental arsenic As0. The gallium chlorides are highly soluble in water, 12778

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781

The Journal of Physical Chemistry C

Article

Figure 5. Core-level Cl 2p (a) and O 1s (b) spectra obtained from the n-GaAs(100) surface emersed from 1 M HCl(aq) solution after exposure under OCV and after cathodic polarization of −1.5 V versus Ag/AgCl in dark. The Cl 2p difference spectrum shows an additional Cl species at OCV. The O 1s difference spectrum shows an additional species at −1.5 V. The excitation energies were 250 eV for Cl 2p and 650 eV for O 1s for respective high surface sensitivity.

absence of oxides in our experiment to absence of dissolved oxygen because of thorough deaeration of the solution. Also, we used low impact etching instead of sputter cleaning to prepare the initial oxide-free surface and we use lower pH and low anodic bias, as compared to those in the experiments in refs 16 and 40; thus, the conditions for oxide formation41 are not met. After a cathodic current was transferred at a bias of −1.5 V versus Ag/AgCl reference electrode, the Ga/As atomic ratio and the shape of As 3d and Ga 3d core levels appear very similar to that of the sample emersed at OCV (Figure 4). In particular, the elemental arsenic and gallium chloride components of the OCV sample somewhat decrease, whereas the AsCl* and As−OH component slightly increases. This is accompanied by the decrease in a high-binding-energy part of the Cl 2p photoemission (Figure 5a) and the increase in the high-binding-energy part of the O 1s photoemission (Figure 5b). Therefore, the high-binding-energy part of the Cl 2p core level can be associated with the Ga−Cl bonds, whereas the lowbinding-energy part can be associated with the hydrated HCl molecules and/or Cl− ions physisorbed at the surface. In the O 1s spectra (Figure 5b) on the other hand, the feature with a binding energy of about 534.5 eV increases, indicating adsorbed H2O molecules and/or OH-groups bonded to As atoms. The chemical modification of the surface is accompanied by electronic changes: the surface Fermi level shifts toward the valence band by 250 meV while the work function decreases from 4.15 eV after OCV to 4.0 eV, indicating a decrease of the electron affinity by 400 meV after −1.5 V versus Ag/AgCl reference electrode. The valence band spectrum is modified significantly after the bias voltage of −1.5 V has been applied (Figure 6). As evidenced by the difference spectrum in Figure 6, two additional features with binding energies of about 6 and 4 eV are formed under cathodic bias. These emissions should reflect the species we found with increased intensity in the core levels, i.e., AsCl*, HCl, Cl−, and H2O or OH. The emission at 6 eV has been related to combinations of the hydrogen orbital with Ga-derived s-like states localized in the second atomic plane,34 but we found a decrease in the intensity of the Ga 3d core-level component with the chemical shift of 0.9 eV that can be partly assigned to Ga−H bonds (Figure 4b). Because H2O and OH also show valence band emissions in this binding energy range, we rather relate the emission at 6 eV to these species. The binding energy of the 1b1 nonbonding state of the water molecule can be found at about 6 eV,12,42 but the valence spectra of water molecules should also contain the bondingstate features 3a1 and 1b2.12,42 Very small features that probably

Figure 6. Valence band spectra obtained from the n-GaAs(100) surface emersed from 1 M HCl(aq) solution after exposure under OCV and after cathodic polarization of −1.5 V versus Ag/AgCl reference electrode in dark conditions, as well as the corresponding difference spectra. Arrows indicate possible positions of 3a1 and 1b2 states of water molecule. The excitation energy was 90 eV.

can be associated with these water orbitals are visible in the difference spectrum at about 9.5 and 12 eV binding energy (Figure 6). However, the intensities of these features are too small in relation to the 6 eV emission indicating that the latter feature cannot be assigned solely to physically adsorbed water molecules. The binding energy of the 1π orbital of the OH group can be also about 6 eV,42,43 but the valence band spectra of the OH layer should contain an additional feature related to the 3σ orbital with a binding energy of about 10 eV. Finally, the binding energy of the Cl 3p core level is close to 6 eV, as well.44 Therefore, only chlorine-related species are left and we relate the 6 eV emission to GaCl and AsCl*, whereas the 4 eV emission is related to hydrated Cl− ions. In summary, the emersion of the n-GaAs(100) surface from aqueous HCl solution under cathodic bias causes formation of a chemical surface composition different from that of emersion under anodic bias, which has been evidenced in detail by comparing the surface sensitive core-level spectra displayed in Figures 4 and 2. As the processing is ultraclean and has been integrated to the UHV system, even differences in the electronic structures could be derived. The respective values of the valence band maximum and the work function derived from the valence band spectra are collected in Figure 7. The changes of the valence band maximum position and ionization energy are mostly due to variations of the surface composition and to a smaller extent to the variation of the Fermi level 12779

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781

The Journal of Physical Chemistry C

Article

level shifts down in the energy gap by 50 meV and with additional exposure to light by 100 meV.

■

ASSOCIATED CONTENT

S Supporting Information *

As 3d, Ga 3d, and O 1s core-level spectra of the native-oxidecovered GaAs(100) surface before and after etching with 1 M HCl(aq) solution for 3 min (Figure S1); excitation-energydependent As 3d and Ga 3d core-level photoemission spectra of the GaAs(100) surface exposed to 1 M HCl(aq) solution under OCV (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

Figure 7. Binding energies of the valence band onsets and work function values obtained from the n-GaAs(100) surface emersed from 1 M HCl(aq) solution under different photoelectrochemical conditions. The values of ionization energy are shown as well.

*E-mail: [email protected]ffe.ru. Fax: +7(812) 297 10 17. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported in part by Russian Foundation for Basic Research (Project 13-02-00540).

positions indicated by shifts of the bulk Ga−As and As−Ga components.

■

4. CONCLUSION Photoelectrochemical processes occurring at n-GaAs(100)/1 M HCl(aq) solution interface at different bias voltages applied to semiconductor electrode were studied in detail by highresolution surface sensitive synchrotron-radiation photoemission spectroscopy that enables the identification of many reaction products, which form in the photoelectrochemical processes. In particular, it was found that at the n-GaAs(100)/ HCl(aq) solution interface the gallium chlorides, as well as elemental arsenic and arsenic hydroxide, are initially formed under open circuit voltage. Under anodic bias, the surface concentration of gallium chlorides, as well as arsenic hydroxide and elemental arsenic, increases. In addition, the As/Ga ratio increases because of enrichment of elemental As. The processes of gallium chlorides and elemental arsenic formation are accelerated under irradiation with visible light, which clearly illustrates the participation of photo holes in the photoelectrochemical etching process. The light-induced accumulation of elemental As under anodic conditions is accompanied by a work function increase of around 0.75 eV. When cathodic bias voltages are applied to the nGaAs(100)/HCl(aq) interface, the As and Ga core-level spectra change to a much smaller degree than under anodic bias, in comparison to the respective initial OCV condition. In particular, the concentrations of the elemental arsenic and gallium chloride components decrease somewhat, whereas the arsenic hydroxide component increase. However, the valence band spectrum changes considerably and two additional features with binding energies of about 6 and 4 eV are formed, which in accord with the core-level findings have been assigned to arsenic chlorides and hydrated Cl− ions. The variation of the composition of the n-GaAs(100) surface emersed from the aqueous HCl solution under different photoelectrochemical conditions is accompanied by variations in the electronic surface parameters. Compared to OCV values, the work function and ionization energy decrease under cathodic and increase under anodic bias, and much stronger so with additional light exposure. Under anodic bias, a shift of the bulk Ga−As and As−Ga components indicates the Fermi

REFERENCES

(1) Erné, B. H.; Stchakovsky, M.; Ozanam, F.; Chazalviel, J.-N. Surface Composition of n-GaAs Cathodes During Hydrogen Evolution Characterized by In-Situ Ultraviolet-Visible Ellipsometry and in situ Infrared Spectroscopy. J. Electrochem. Soc. 1998, 145, 447− 456. (2) Memming, R. Semiconductor Electrochemistry; Wiley-VCH: Weinheim, Germany, 2001. (3) Abshere, T. A.; Richmond, J. L. Corrosion, Passivation, and the Effect of Water Addition on an n-GaAs(100)/Methanol Photoelectrochemical Cell. J. Phys. Chem. B 2000, 104, 1602−1609. (4) Nozik, A. Photoelectrochemistry: Applications to Solar Energy Conversion. Annu. Rev. Phys. Chem. 1978, 29, 189−222. (5) Allongue, P.; Cachet, H. Band-Edge Shift and Surface Charges at Illuminated n-GaAs/Aqueous Electrolyte Junctions. Surface-State Analysis and Simulation of Their Occupation Rate. J. Electrochem. Soc. 1985, 132, 45−52. (6) Huang, Y.; Luo, J.; Ivey, D. G. Steady-State and Impedance Study of n-GaAs in H2SO4 Solution: Mechanism Analysis. Thin Solid Films 2006, 496, 724−734. (7) Lazarescu, V.; Lazarescu, M. F.; Santos, E.; Schmickler, W. Second Harmonic Generation and Impedance Spectroscopy at nGaAs(100) Electrodes. Electrochim. Acta 2004, 49, 4231−4238. (8) Lebedev, M. V.; Masuda, T.; Uosaki, K. Charge Transport at the Interface of n-GaAs(100) with an Aqueous HCl Solution: Electrochemical Impedance Spectroscopy Study. Semiconductors 2012, 46, 471−477. (9) Yagi, I.; Idojiri, S.; Awatani, T.; Uosaki, K. Electrodeposition of Flattened Cu Nanoclusters on a p-GaAs(100) Electrode Monitored by In Situ Optical Second Harmonic Generation. J. Phys. Chem. B 2005, 109, 5021−5032. (10) Erné, B. H.; Ozanam, F.; Chazalviel, J.-N. The Mechanism of Hydrogen Gas Evolution on GaAs Cathodes Elucidated by In Situ Infrared Spectroscopy. J. Phys. Chem. B 1999, 103, 2948−2962. (11) Jaegermann, W. The Semiconductor/Electrolyte Interface: A Surface Science Approach. In Modern Aspects of Electrochemistry; White, R. E., Ed.; Plenum Press: New York, 1996; Vol. 30. (12) Mayer, T.; Lebedev, M. V.; Hunger, R.; Jaegermann, W. Synchrotron Photoemission Analysis of Semiconductor/Electrolyte Interfaces by Frozen Electrolyte Approach: Interaction of HCl in 2Propanol with GaAs(100). J. Phys. Chem. B 2006, 110, 2293−2301.

12780

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781

The Journal of Physical Chemistry C

Article

(13) Mayer, T.; Jaegermann, W. A Photoemission Study of Solute− Solvent Interaction: Coadsorption of Na and H2O on WSe2 (0001). J. Phys. Chem. B 2000, 104, 5945−5952. (14) Beerbom, M.; Henrion, O.; Klein, A.; Mayer, T.; Jaegermann, W. XPS Analysis of Wet Chemical Etching of GaAs(110) by Br2− H2O: Comparison of Emersion and Model Experiments. Electrochim. Acta 2000, 45, 4663−4672. (15) Lebedev, M. V.; Mankel, E.; Mayer, T.; Jaegermann, W. SXPS Study of Model GaAs(100)/Electrolyte Interface. Phys. Status Solidi C 2010, 7, 193−196. (16) Solomun, T.; McIntyre, R.; Richtering, W.; Gerischer, H. Surface Stoichiometric Changes of n-GaAs After Anodic Treatment: An XPS Study. Surf. Sci. 1986, 169, 414−424. (17) Beerbom, M.; Mayer, Th.; Jaegermann, W. SynchrotronInduced Photoemission of Emersed GaAs Electrodes after Electrochemical Etching in Br2/H2O Solutions. J. Phys. Chem. B 2000, 104, 8503−8506. (18) Traub, M. C.; Biteen, J. S.; Michalak, D. J.; Webb, L. J.; Brunschwig, B. S.; Lewis, N. S. High-Resolution X-ray Photoelectron Spectroscopy of Chlorine-Terminated GaAs(111)A Surfaces. J. Phys. Chem. B 2006, 110, 15641−15644. (19) Axnanda, S.; Crumlin, E. J.; Mao, B.; Rani, S.; Chang, R.; Ross, P. N.; Hussain, Z.; Liu, Z. Ambient-Pressure “Tender” X-Ray Photoelectron Spectroscopy (APXPS) for in situ Study of LiquidSolid Interface of Pt Foil in 6 M KF. 224th ECS Meeting, San Francisco, CA, October 27−November 1, 2013; The Electrochemical Society; abstract 921. (20) Mayer, T.; Lebedev, M.; Hunger, R.; Jaegermann, W. Elementary Processes at Semiconductor/Electrolyte Interfaces: Perspectives and Limits of Electron Spectroscopy. Appl. Surf. Sci. 2005, 252, 31−42. (21) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of Electron Inelastic Mean Free Paths. III. Data for 15 Inorganic Compounds Over the 50−2000 eV Range. Surf. Interface Anal. 1991, 17, 927−939. (22) Le Lay, G.; Mao, D.; Kahn, A.; Hwu, Y.; Margaritondo, G. HighResolution Synchrotron-Radiation Core-Level Spectroscopy of Decapped GaAs(100) Surfaces. Phys. Rev. B 1991, 43, 14301−14304. (23) Beerbom, M. M.; Lägel, B.; Cascio, A. J.; Doran, B. V.; Schlaf, R. Direct Comparison of Photoemission Spectroscopy and In Situ Kelvin Probe Work Function Measurements on Indium Tin Oxide Films. J. Electron Spectrosc. Relat. Phenom. 2006, 152, 12−17. (24) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1 ≤ Z ≤ 103. At. Data Nucl. Data Tables 1985, 32, 1−155. (25) Song, Z.; Shogen, S.; Kawasaki, M.; Suemune, I. X-Ray Photoelectron Spectroscopic and Atomic-Force Microscopic Study of GaAs Etching with a HCl Solution. Appl. Surf. Sci. 1994, 82−83, 250− 256. (26) Ishikawa, Y.; Ishii, H.; Hasegawa, H.; Fukui, T. Macroscopic Electronic Behavior and Atomic Arrangements of GaAs Surfaces Immersed in HCl Solution. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.Process., Meas., Phenom. 1994, 12, 2713−2719. (27) Osakabe, S.; Adachi, S. Study of GaAs(001) Surfaces Treated in Aqueous HCl Solutions. Jpn. J. Appl. Phys. 1997, 36, 7119−7125. (28) Lebedev, M. V.; Mankel, E.; Mayer, T.; Jaegermann, W. Etching of GaAs(100) with Aqueous Ammonia Solution: A SynchrotronPhotoemission Spectroscopy Study. J. Phys. Chem. C 2010, 114, 21385−21389. (29) Simpson, W. C.; Tong, W. M.; Weare, C. B.; Shuh, D. K.; Yarmoff, J. A. The Temperature Dependence of the Cl2/GaAs(110) Surface Product Distribution. J. Chem. Phys. 1996, 104, 320−325. (30) Hung, W. H.; Wu, S. L.; Chang, C. C. Low-Temperature Chlorination of GaAs(100). J. Phys. Chem. B 1998, 102, 1141−1148. (31) Petravic, M.; Deenapanray, P. N. K.; Usher, B. F.; Kim, K.-J.; Kim, B. High-Resolution Photoemission Study of Hydrogen Interaction with Polar and Nonpolar GaAs Surfaces. Phys. Rev. B 2003, 67, 195325.

(32) Lebedev, M. V.; Mankel, E.; Mayer, T.; Jaegermann, W. Interaction of 2-Propanol with the GaAs(100) Surface. J. Phys. Chem. C 2009, 113, 20421−20428. (33) Lebedev, M. V.; Mankel, E.; Mayer, T.; Jaegermann, W. Wet Etching of GaAs(100) in Acidic and Basic Solutions: A SynchrotronPhotoemission Spectroscopy Study. J. Phys. Chem. C 2008, 112, 18510−18515. (34) Santoni, A.; Sorba, L.; Shuh, D. K.; Terminello, L. J.; Franciosi, A.; Nannarone, S. Initial Stages of Atomic Hydrogen Chemisorption on GaAs(110): A High Resolution Photoemission Study. Surf. Sci. 1992, 269/270, 893−901. (35) Vitomirov, I. M.; Raisanen, A.; Finnefrock, A. C.; Viturro, R. E.; Brillson, L. J.; Kirchner, P. D.; Pettit, G. D.; Woodall, J. M. Geometric Ordering, Surface Chemistry, Band Bending, and Work Function at Decapped GaAs(100) Surfaces. Phys. Rev. B 1992, 46, 13293−13302. (36) Chen, W.; Dumas, M.; Mao, D.; Kahn, A. Work Function, Electron Affinity, and Band Bending at Decapped GaAs(100) Surfaces. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.Process., Meas., Phenom. 1992, 10, 1886−1890. (37) Allongue, P.; Blonkowski, S. Corrosion of III−V Compounds: A Comparative Study of GaAs and InP: II. Reaction Scheme and Influence of Surface Properties. J. Electroanal. Chem. Interfacial Electrochem. 1991, 317, 77−89. (38) Song, Z.; Shogen, S.; Kawasaki, M.; Suemune, I. X-Ray Photoelectron Spectroscopy and Atomic Force Microscopy Surface Study of GaAs(100) Cleaning Procedures. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.Process., Meas., Phenom. 1995, 13, 77−82. (39) Bond, P.; Brier, P. N.; Fletcher, J.; Gorry, P. A. Reactive Scattering of Cl2 on GaAs(100). Cl2 and GaCl Product Distributions. Chem. Phys. Lett. 1993, 208, 269−275. (40) Finnie, C. M.; Li, X.; Bohn, P. W. Production and Evolution of Composition, Morphology, and Luminescence of Microcrystalline Arsenic Oxides Produced during the Anodic Processing of (100) GaAs. J. Appl. Phys. 1999, 86, 4997−5003. (41) Park, S.-M.; Barber, M. E. Thermodynamic Stabilities of Semiconductor Electrodes. J. Electroanal. Chem. 1979, 99, 67−75. (42) Henderson, M. A. The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1−308. (43) Krischok, S.; Höfft, O.; Kempter, V. Interaction of Alkali Atoms with Water Multilayers Adsorbed on TiO2(110): A Study with MIES and UPS. Surf. Sci. 2003, 532−535, 370−376. (44) Lou, C.-T.; Li, H.-D.; Chung, J.-Y.; Lin, D.-S.; Chiang, T.-C. Electronic Reconstruction at a Buried Ionic-Covalent Interface Driven by Surface Reactions. Phys. Rev. B 2009, 80, 195311.

12781

dx.doi.org/10.1021/jp500564c | J. Phys. Chem. C 2014, 118, 12774−12781