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Dissociative Adsorption of Water on an H2O/GaAs(100) Interface: In Situ Near-Ambient Pressure XPS Studies Xueqiang Zhang†,‡ and Sylwia Ptasinska*,†,§ †

Radiation Laboratory, ‡Department of Chemistry and Biochemistry, and §Department of Physics, University of Notre Dame, 225 Nieuwland, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Dissociative water adsorption on a GaAs(100) surface is demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under near-ambient conditions. These in situ XPS studies enable us to monitor the evolution of water dissociation on GaAs under elevated pressures and temperatures for the first time. In pressure-dependent XPS studies, the GaAs surface was exposed to room-temperature water vapor at pressures ranging from UHV to 5 mbar. With an increase in H2O pressure, enhancement of oxygenation and hydroxylation of surface Ga atoms is observed. In addition, strong XPS signals are also detected that originate from water molecules that are molecularly adsorbed onto the GaAs surface and hydrogen-bonded to surface OH species. Peak broadening and shifts in the As 2p XPS spectra are correlated with the signal increase resulting from surface HO−Ga species, suggesting the formation of surface H−As bonds. In temperature-dependent XPS studies, the GaAs surface was annealed from room temperature up to 773 K at a water vapor pressure of 0.1 mbar. The temperature increase leads to desorption of weakly adsorbed water molecules or their dissociation, resulting in surface Ga oxidation. However, no changes in chemical state of the surface As atoms are observed at higher temperatures.

1. INTRODUCTION A clear trend in contemporary semiconductor-based technology is emerging for the design and fabrication of new combinatorial materials via the production of thin oxide layers or surfaces with different terminal groups to achieve specific well-defined properties and functionalities. Such technology is mainly driven by the possibility of its use in device applications. Before these modified systems can be used, they first need to be understood, refined, and controlled. Important fundamental knowledge can be obtained from investigations of adsorbed water molecules on semiconductor surfaces. Water from humidity in the air adsorbs easily onto a semiconductor surface due to the interaction of dangling bonds with the ambient air that can form a layer of native oxide on the surface. The ubiquitous presence of water in ambient air results in water coverage of material surfaces with thicknesses ranging from a few angstroms to bulk liquid layers.1−3 Therefore, much effort has been focused on the preparation of insulating native oxide films on semiconductor surfaces to provide a high electrical quality and chemically stable interface. Moreover, water is a common precursor for wet oxidation in semiconductor device fabrication and is present as a solvent in many reactions. For example, water has been used to produce a passivation layer on GaAs surfaces due to electrochemical reactions between semiconductor/electrolyte interfaces4 or to fabricate GaAs surfaces terminated with biologically relevant molecules.5 Water adsorption studies revealed that the sticking coefficient for H2O is several orders © 2014 American Chemical Society

of magnitude larger than that of O2 at 200 K, indicating stronger interactions between water and GaAs surfaces.6−8 Clearly, water is a good candidate as an oxidizing agent of semiconductor surfaces. Accordingly, understanding water−semiconductor interface chemistry under reaction conditions is a major objective for high-quality passivation layer preparation and surface functionalization. A large number of studies on water interactions with elemental semiconductors (e.g., silicon and germanium) have been reported, but there are relatively few investigations on compound semiconductors (e.g., III−V semiconductors).9−11 However, the importance of understanding the oxygenation and hydroxylation of III−V semiconductors is rapidly growing, with recent studies involving both GaP9,12−14 and GaSb.15 A systematic characterization of the H2O/GaAs(001) interface under ultrahigh vacuum (UHV) conditions was carried out by Chung et al.16 in which multiple techniques, such as temperature-programmed desorption (TPD), highresolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy (AES), were employed. In that study, initially, H2O was molecularly adsorbed on the GaAs (001) surface at 100 K and then dissociated promptly forming HO−Ga and H−As species upon annealing over a temperature Received: December 6, 2013 Revised: January 26, 2014 Published: January 31, 2014 4259

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range of 150−200 K. A further increase in the annealing temperature (600−700 K) of the crystal led to dehydrogenation of the OH species producing gallium oxides and desorption of hydrogen molecules. Simultaneously, depletion of surface As atoms occurred, leaving the surface primarily covered by gallium oxides. Previous investigations on GaAs4,7,8,10,11,16−18 were performed under UHV and relatively low-temperature conditions, which provided pioneering information on water−semiconductor interface chemistry. However, it is possible that the nature of the water−semiconductor interaction differs under more realistic ambient conditions. Such discrepancies in surface chemistry between UHV and higher pressures or in wide temperature regimes are often referred to as the “pressure gap” or “temperature gap”.19,20 Filling this knowledge gap is particularly important because water interactions with GaAs surfaces can be substantially influenced by changes in water vapor temperature and pressure. To obtain an atomistic-level understanding of H2O/GaAs interface chemistry under more realistic conditions, we have investigated the interfacial interactions using near-ambient pressure X-ray photoelectron spectroscopy (NAP XPS). NAP XPS allows us to track in situ surface chemistry at elevated pressures up to 5 mbar and temperatures up to 800 K. The core-level high-resolution XPS spectra identify the nature of the molecular interactions and allow the detection of changes in the electronic structure of surface Ga, As, and O atoms under the reaction conditions. Our results suggest that water molecules undergo dissociation, producing gallium oxides and hydroxides at the interface of H2O/GaAs. In addition, our findings are also compared with previous UHV studies of the H2O/GaAs interface.

Figure 1. Schematic view of Notre Dame Radiation Laboratory NAP XPS: (a) reaction cell attached to the first differential pumping stage; (b) sample stage with an electron beam heater; (c) Al Kα X-ray generator; (d) X-ray monochromator; (e) monochromized X-ray beam; (f) reaction cell xyz manipulator; (g) prelens of the first differential pumping stage; (h) gate valve that separates the first and second differential pumping stages; and (i) the quadruple mass spectrometer.

reaction cell operates under a continuous-flow mode, which can be adjusted by changing the flow rate of the gas being introduced into the cell. X-rays can penetrate into the reaction cell through a silicon nitride window that has a thickness of 100 nm. This window holds about 10 orders of magnitude pressure difference between the reaction cell (∼20 mbar) and the UHV analysis chamber (∼10−10 mbar). A nozzle that has an orifice with a diameter of ∼300 μm separates the near-ambient pressure environment in the reaction cell from the vacuum of the first differential pumping stage and is attached to the reaction cell. The sample stage can approach and retreat from the nozzle by means of an automatic motor. The distance between the sample and nozzle should not be shorter than twice the diameter of the orifice to ensure that the pressure around the sample is the same throughout the entire reaction cell and is unaffected by pumping in the first differential pumping stage.20 Additionally, this distance is usually within an inelastic mean free path (IMFP) of photoelectrons under a given reaction pressure. This enables the photoelectrons to escape through the nozzle orifice to the first differential pumping stage and to be collected by a detector at the end of the SPECS PHOIBOS 150 hemispherical analyzer. Along the differential pumping stages, photoelectrons are focused onto apertures by electrostatic lenses that can be tuned to achieve an optimized signal. Typically, the pressure in the prelens chamber is in the 10−5 mbar range when the reaction cell is under 1 mbar of N2 pressure; however, this depends on the type of pumped gas. The vacuum improves further in the following differential pumping chambers, and the detector is typically in the 10−9 to 10−10 mbar range. Additionally, a quadruple mass spectrometer (QMS) is mounted on the second stage of the differential pumping system to analyze the gas composition pumped away from the reaction cell (Figure 1). This can allow for a more complete understanding of the in situ surface chemistry by detection of species that are produced or desorbed from a sample surface under reaction conditions.

2. EXPERIMENTAL SECTION 2.1. Sample Pretreatment. Undoped GaAs(100) single crystals were purchased from AXT, USA. A crystal was mounted on a molybdenum sample holder and introduced into a UHV (∼10−9 mbar) preparation chamber. The crystal was cleaned using a typical procedure: a few cycles of Ar ion bombardment to remove native oxide layers and other contaminants, followed by annealing to 825 K for surfacestructure restoration.7,11,21 During Ar ion bombardment, the crystal was exposed to 1.0 keV ions at a pressure of 3 × 10−6 mbar for 10 min, while the sputtering angle was kept at 45°. The surface cleanness was monitored by XPS after each cycle of Ar bombardment until neither oxygen nor carbon species was detected. The crystal temperature was monitored by a K-type (chromel-alumel) thermocouple inserted between the crystal and sample holder. The Milli-Q ultrapure water was degassed via multiple freeze−pump−thaw cycles prior to use. The C 1s XPS spectra were frequently monitored throughout the entire experimental procedure and indicated that the crystal surface remained free of contamination. 2.2. Near-Ambient Pressure X-ray Photoelectron Spectroscopy. The in situ surface chemistry characterization of H2O/GaAs was performed by the custom-built NAP XPS system, which consists of several vacuum chambers: a load-lock chamber, preparation chamber, analysis chamber, reaction cell (Figure1), and analyzer chamber with a three-stage differentially pumped electrostatic lens system. A key part of the NAP XPS system is the miniature reaction cell with a volume of 15 mL assembled into an xyz manipulator. The pressure in the 4260

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All XPS spectra were recorded using an X-ray beam generated by an X-ray tube equipped with a Micro-FOCUS 600 X-ray monochromator. The angle between the incident Xray beam and the normal to the sample surface was kept at 54.7°. The aluminum Kα (1486.6 eV) X-ray source was operated at a power of 100 W with an anode voltage of 15 kV and a current of 6.7 mA. The base pressure in the analysis chamber was kept below 5 × 10−10 mbar. The full width at halfmaximum (fwhm) of the Au 4f7/2 peaks measured with 70 and 20 eV pass energies was 1.2 and 0.5 eV, respectively. Survey photoelectron spectra were measured with an energy step of 0.5 eV and pass energy of 70 eV to scan the entire photoelectron kinetic energy range, while binding energy spectra were used to obtain surface elemental information. High-resolution XPS spectra for the Ga 2p, Ga 3d, As 2p, As 3d, and O 1s photoelectrons were obtained using an energy step of 0.05 eV and a pass energy of 20 eV. The size of the X-ray beam was measured using a phosphor screen that was illuminated with a spot diameter of ∼2 mm. The photoelectron energy analyzer was operated in the fixed analyzer transmission (FAT) mode to keep the resolution independent of the kinetic energy of the photoelectrons. The energy calibration of the spectra was referenced to the method reported by Surdu-Bob et al.22 in which the As 3d5/2 peak was fixed at 41.1 eV. After subtracting a Shirley background, peak fitting of all XPS spectra was performed using the SpecsLab2 software and ComputerAided Surface Analysis for X-ray Photoelectron Spectroscopy (CasaXPS) with a combination of Gaussian/Lorentzian functions in the ratio of 70:30.

Figure 2. Comparison of high-resolution XPS spectra of the GaAs(100) surface under UHV (∼10−10 mbar) conditions and at a water vapor pressure of 5 mbar at RT (300 K).

inner layers of the GaAs surface. The 2p XPS spectra for both elements obtained at 5 mbar have lower photoelectron intensities and lower signal-to-noise ratios in comparison with those under UHV conditions (Figure 2). This attenuation of signals is due to photoelectron scattering with water molecules in the region between the crystal surface and the nozzle of the first differential pumping stage. In the case of the Ga XPS spectra, broadening of peaks at the higher binding energy side is observed at an H2O pressure of 5 mbar in comparison with those from a clean GaAs surface (Figure 2). This indicates changes in the electronic structure due to H2O interaction with Ga atoms, particularly on the outermost layers because broadening is much more pronounced for the Ga 2p peak. In contrast with these changes in the Ga XPS spectra, a relatively minor pressure effect is observed for surface As atoms in the presence of H2O. As shown in Figure 2, a slightly broadened As 2p peak is discernible once water was introduced while no changes occur in the As 3d XPS spectra. This observation is in good agreement with UHV XPS results obtained by Webb et al.11 in which a large dose of water vapor was adsorbed onto the GaAs surface, resulting in a large fraction of H2O monolayer coverage. Besides the slight broadening of the As 2p peak, we also observe a peak shift toward higher binding energies as the pressure was increased from UHV to 5 mbar. This shift remains even after pumping away water vapor from the reactor cell (Figure S1 in the Supporting Information). The broadening and peak shift can be due to molecular water adsorption or H− As hydrogen bonds that are formed due to water dissociation. Higher binding energy peaks in the As 2p XPS spectra were reported for surface AsHx (x = 1,2) species.27 Our assumption of H−As bond formation is also supported by HREELS studies that reported the formation of H−As bonds at the interface of H2O/GaAs under UHV conditions.16 To investigate the chemistry at the H2O/GaAs interface at different pressures, we chose the Ga 2p (because the 2p signal is the most surface sensitive) and O 1s peaks for the following detailed discussion. The Ga 2p signal is deconvoluted into three components at binding energies of 1117.2 ± 0.1 (peak A), 1118.2 ± 0.1 (peak B), and 1119.1 ± 0.1 eV (peak C) corresponding to Ga−As bonds (with a minor contribution from Ga−Ga bonds) and

3. RESULTS Prior to ion bombardment and annealing, the GaAs(100) crystal is largely oxidized due to exposure to ambient air conditions. Under Ar ion bombardment in the preparation chamber at a base pressure of 3 × 10−6 mbar, surface hydrocarbons and oxides are totally removed from the surface. Moreover, preferential As depletion is observed due to the lower surface binding energy of As atoms compared with Ga atoms, leaving a Ga-rich surface.23,24 Subsequently, the crystal is contamination-freely transferred to the reactor chamber that has a base pressure of 5 × 10−10 mbar. Two types of XPS experiments are performed on the cleaned GaAs(100) crystal: exposure to different (1) H2O pressures and (2) temperatures. (1) The pressure dependence investigation was carried out in an H2O vapor pressure range of UHV (5 × 10−10 mbar) to 5 mbar at room temperature (RT, ∼300 K) (see Supporting Information, Figure S1) and (2) the temperature dependence study was carried out in a temperature range from RT to 773 K under a constant H2O vapor pressure environment (0.1 mbar) (see Supporting Information, Figure S2). Water interactions with the GaAs surface and the electronic structures of the surface atoms were evaluated from the Ga (2p and 3d), As (2p and 3d), and O (1s) XPS spectra. 3.1. Pressure Dependence. Figure 2 displays the corresponding 2p and 3d level photoelectron features in Ga and As XPS spectra obtained under UHV conditions and at the maximum pressure introduced into the reactor cell, that is, 5 mbar of H2O, at RT. Because the 2p photoelectrons possess higher binding energies, they are ejected with lower kinetic energies, implying shorter IMFPs compared with those of the 3d photoelectrons.22,25,26 Therefore, the 2p XPS peaks represent photoelectrons from the outermost layers of the GaAs surface, whereas the 3d XPS peaks also originate from 4261

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O−Ga−OH and Ox−Ga−(OH)y (1 < x ≤ 3, y = 3 − x) surface species, respectively (Figure 3). These assignments are

Figure 3. Evolution of high-resolution Ga 2p XPS spectra of the GaAs(100) surface at different water vapor pressures and after H2O was pumped away from the reaction cell (evacuated). Peak assignments: (A) 1117.2 ± 0.1 eV (Ga−As bonds), (B) 1118.2 ± 0.1 eV (O−Ga−OH), and (C) 1119.1 ± 0.1 eV (Ox−Ga−(OH)y).

Figure 4. Evolution of high-resolution O 1s XPS spectra of the GaAs(100) surface at different water vapor pressures and after H2O was pumped away from the reaction cell (evacuated). Peak assignments: (D) 530.4 ± 0.1 eV (O*−Ga−OH), (E) 531.4 ± 0.1 eV (O−Ga−O*H), (F) 531.8 ± 0.2 eV (H2O···HO*−Ga), and (G) 532.5−533 eV (molecularly adsorbed H2O).

consistent with previous UHV XPS studies on different model Ga-based or As-based compounds and GaAs crystals exposed to O2 or H2O.17,25,28,29 The binding energies of peaks B and C coincidently fit to Ga 2p peaks reported in UHV XPS studies on Ga2O and Ga2O3 powder samples, respectively.22 However, the formation of Ga2O and Ga2O3 would require a deep penetration of oxygen atoms into the lattice of GaAs crystals and the splitting of Ga−As bonds on a large scale. Previous UHV studies on O2 interactions with the GaAs surface eliminated the possibility of formation of stoichiometric Ga2O3 at room temperature.30 Therefore, we assume that formation of stoichiometric Ga2O3 or even Ga2O at room temperature and water pressures up to 5 mbar is even more difficult to be achieved. Therefore, the peaks B and C (Figure 3) are assigned to O−Ga−OH and Ox−Ga−(OH)y in the present study, where Ox−Ga−(OH)y indicates a mixed phase consisting of oxides and hydroxides. The Ga 2p XPS spectrum for the clean GaAs surface under UHV conditions is composed of only one peak (A) located at the lowest binding energy. Despite the cleaning procedure used in our studies, which leads to a Ga-rich surface, the majority of the signal represented by the peak A originates from Ga−As bonds with a neglected contribution from Ga−Ga bonds. This contribution is noticeable as a tail at the lower binding energy side of the peak A. The two higher binding energy peaks (B and C) appear due to water interactions with surface Ga atoms. Both species (i.e., O−Ga−OH and Ox−Ga− (OH)y) are products of water dissociation on the GaAs surface and will be discussed later. The O 1s signal is deconvoluted into four components at binding energies of 530.4 ± 0.1 eV (peak D), 531.4 ± 0.1 eV (peak E), 531.8 ± 0.2 eV (peak F), and 532.5−533 eV (peak G) corresponding to O*−Ga−OH, O−Ga−O*H, and OH species attached to H2O molecules via hydrogen bonds (H2O··· HO*−Ga) and molecularly adsorbed H2O, respectively (Figure 4), and the asterisk indicates an oxygen atom from which photoelectrons are ejected. This assignment is consistent with previous in situ XPS work on water dissociation on metal surfaces17,31,32 and UHV XPS studies on a GaAs surface

covered with a monolayer of water.9,11,17 In addition to the photoelectron signals from the above species, a very intense peak is observed at a binding energy of 535.6 eV at 5 mbar (Figure 4). This contribution is assigned to photoelectrons from gas-phase H2O molecules. The O 1s spectral evolution obtained with increasing pressures displays the enhancement of photoelectron signals at higher binding energies originating from molecularly adsorbed H2O and H2O···HO*−Ga, peaks G and F, respectively. However, the O 1s XPS spectrum recorded under UHV conditions after evacuation of water vapor from the reactor cell indicates a significant increase in three contributions (peaks D−F, Figure 4). The ratio of total peak areas of these contributions in comparison with the higher binding energy peak (G) is approximately 4:1, which indicates a strong dissociation of H2O molecules and subsequent oxidation and hydroxylation of Ga atoms. 3.2. Temperature Dependence. Figure 5 displays the corresponding 2p and 3d level photoelectron features for Ga and As at RT and 773 K at a water vapor pressure of 0.1 mbar. As shown in Figure 5, the Ga 2p and 3d XPS spectra obtained at the higher temperature exhibit larger broadening of peaks on the higher binding energy side in comparison with the spectra taken at RT. The spectrum of surface-sensitive Ga 2p photoelectrons exhibits a much stronger oxidation effect than the Ga 3d photoelectrons, while only a slight change in the width of the As 2p peaks was observed. Additionally, the peak area ratio of As:Ga, which was estimated by taking into account the relative sensitivity factor, decreased from 3:7 to 1:9 as the surface Ga oxidation increased. Similarly, As atom depletion at higher temperatures was observed for UHV AES studies of H2O interacting with the GaAs surface.11,16 To investigate the chemistry at the H2O/GaAs interface at different temperatures, we again chose the Ga 2p and O 1s peaks for detailed discussion (Figures 6 and 7). Deconvolution and assignment of specific peaks in the Ga 2p and O 1s XPS spectra have been done in the same manner as for those in Figures 3 and 4. From a comparison of pressure and 4262

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Figure 5. Comparison of high-resolution XPS spectra of the GaAs(100) surface at RT (300 K) and 773 K at a water vapor pressure of 0.1 mbar.

Figure 7. Evolution of the high-resolution O 1s XPS spectra of the GaAs(100) surface at different temperatures at a water vapor pressure of 0.1 mbar and the O 1s spectrum under UHV conditions at RT (UHV, RT). Peak assignments: (D) 530.4 ± 0.1 eV (O*−Ga−OH), (E) 531.4 ± 0.1 eV (O−Ga−O*H), (F) 531.8 ± 0.2 eV (H2O··· HO*−Ga), and (G) 532.5−533 eV (molecularly adsorbed H2O).

In the case of the O 1s XPS spectra shown in Figure 7, two higher binding energy peaks that correspond to molecularly adsorbed H2O and H2O···HO*−Ga species (peaks G and F) exhibit strong quenching at higher temperatures, and then vanish above 673 K. A similar temperature behavior was reported for the O 1s XPS spectra in previous in situ XPS studies of water dissociation on metal surfaces at near-ambient pressures.1,32 A reversed trend to peaks G and F, that is, the increase with temperatures, is observed for the two lower binding energy peaks (D and E) that are assigned to O*−Ga− OH and O−Ga−O*H surface species. In particular, the former peak drastically increases with a rise in temperature, indicating a strong oxidation of the GaAs surface.

4. DISCUSSION Pressure and temperature dependencies obtained from the XPS studies allowed us to determine the surface chemistry evolution at the H2O/GaAs interface. The molecularly adsorbed water with two lone electron pairs can bind as a Lewis base to an empty hybrid p orbital of Ga-rich surfaces.8,9 However, this adsorption mode is not stable enough, and water molecules can desorb with a low activation energy that has been estimated to be 0.7 eV (16 kcal/mol).8 This was also verified in UHV TPD studies in which molecularly adsorbed and fully or partially hydrogen bonded H2O molecules desorbed mostly from the GaAs surface below RT.16 In contrast with oxygen interactions with the GaAs surface, in which oxygen is at first exclusively adsorbed onto As atoms,11 H2O preferentially interacts with Ga atoms, and only a minor effect is observed on the surface As atoms over the investigated pressure and temperature ranges. The water dissociation is preferentially located on surface Ga atoms, while As atoms act as “neighbors” that can eventually assist in water dissociation into OH and H on the GaAs surface. The OH dissociation product interacts with Ga atoms forming hydroxide species and oxides, whereas the H interacts with an As atom. Similar final

Figure 6. Evolution of high-resolution Ga 2p XPS spectra of the GaAs(100) surface at different temperatures at a water vapor pressure of 0.1 mbar and the Ga 2p spectrum under UHV conditions at RT (‘UHV, RT’). Peak assignments: (A) 1117.2 ± 0.1 eV (Ga−As bonds), (B) 1118.2 ± 0.1 eV (O−Ga−OH), and (C) 1119.1 ± 0.1 eV (Ox− Ga−(OH)y).

temperature evolutions of the Ga 2p XPS spectra, a much higher efficiency of Ga oxidation is observed for elevated temperatures. Figure 6 displays a strong increase in both higher energy peaks (B and C) attributed to O−Ga−OH and Ox− Ga−(OH)y surface species as the GaAs crystal temperature is increased, whereas a relative decrease in photoelectron contributions from Ga−As and Ga−Ga bonds (peak A) is observed. If the formation of Ga2O3 would occur at higher temperatures, this would be accompanied by changes in As XPS spectra. Besides a slight decrease in the As/Ga ratio due to gradual As depletion, no changes in the shape of As peaks are observed, which would indicate the production of As−O or As−As bonds if stoichiometric Ga2O3 phase would be formed. 4263

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Figure 8. Schematic representation of different species on a GaAs surface formed due to water adsorption and dissociation at elevated pressures and temperatures. The GaAs surface in this scheme does not represent a realistic GaAs(100) crystal.

an increase in the H2O···HO*−Ga species (peak F), which implies water dissociation on the surface (Figure 4). In the case of oxide formation (peak D), the surface OH recombination reaction, OHads + OHads ↔ H2Ogas + Oads, can also occur even at RT.1,40 This process, in which the OH species are converted to oxides assisted by water molecule desorption, has not been reported for UHV XPS studies of H2O/GaAs interfaces at RT. We note that the water molecule dissociation and subsequent Ga oxidation is limited by several factors, for example, the barrier of the OH recombination reaction, the recombination frequency of H and OH, which was reported to be relatively high at RT,16 and energy dissociation of surface OH species into O and H products. Moreover, the existence of H−As bonds along with the occurrence of O−Ga−OH bond formation indicates that dissociative water adsorption results initially in the formation of OH and H products that react with surface Ga and As atoms, respectively. A similar mechanism was postulated for water interactions with a GaP surface from DFT calculations.9 In our temperature-dependent study, greater oxidation of the GaAs surface is observed (Figures 5 and 6) at a constant water vapor pressure of 0.1 mbar. Moreover, weakly bound water molecules quickly desorb from the surface because their desorption temperatures are much lower in comparison with oxygen or hydroxyl groups bonded to Ga atoms.16 However, there is always a small amount of molecularly adsorbed H2O and H2O···HO*−Ga below 673 K. This can be further verified by the fitted O 1s XPS spectrum that shows an approximately 1:1 peak area ratio for both contributions (Figure 7). Interestingly, the chemical state of surface As atoms remains nearly “intact” in the studied temperature range. There is a negligible effect on widening of the fwhm, and no observable peak shifts occur in the As 2p and As 3d XPS spectra at a given pressure with a rise in temperature (Figure 5). We suggest that this broadening effect results from chemical-state changes in neighboring Ga atoms. The combined results of the pressure and temperature dependencies suggest that the As 2p peak

results were observed in previous UHV studies of H2O/GaAs interfaces.11,16,33 In our pressure-dependent study, although at RT, we believe that water molecules actually are adsorbed on the GaAs surface (Figure 4, peak G) when the adsorption−desorption equilibrium kinetics are considered.18 The water molecule tends to first adsorb on the atop site, which is predicted by DFT calculations to be the most stable adsorption site of metal surfaces.34−36 However, water monomers are not typically observed in experiments, even at low surface temperatures.18 The possibility of surface diffusion facilitates clustering of neighboring water molecules and formation of H bonds.37 The calculated water dissociation barrier on various materials is lower for clusters or a packed monolayer of water with respect to a monomeric water molecule.32,38,39 Thus, the molecularly adsorbed water cluster can be a precursor for dissociation processes promoted by H bonding. A build-up of water coverage as either water clusters or water molecules bonded to surface OH species is evident in Figure 4 by an increase in higher binding energy contributions (peaks F and G) at higher pressures. After pumping the water vapor out of the reaction cell, the O 1s XPS spectrum (Figure 4, top panel) displays an oxygen and hydroxyl-rich Ga surface. On one hand, the photoelectron signals for these species are highly suppressed due to a large amount of molecularly adsorbed water. On the other hand, both the O−Ga−OH and Ox−Ga−(OH)y surface species show a clear increase in the corresponding signals in the Ga 2p spectra (Figure 3). Moreover, molecularly adsorbed water molecules (peak G) and OH anchoring water molecules via H bonds (peak F)32,38 are still present with a 1:1 peak area ratio after H2O evacuation. In general, there are two possible reaction pathways for water dissociative adsorption on the surface: (1) reversible dissociation: H2Oads → OHads + Hads → H2Ogas and (2) irreversible dissociation: H2Oads → OHads + Hads → Oads + H2gas.18 The fitted O 1s XPS spectra display a decrease in the HO− Ga species (peak E) as the water vapor pressure increases but 4264

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surface or anchored by hydrogen in HO−Ga bonds, as verified in the fitted O 1s spectra. The presence of H2O molecules on the GaAs surface at room and higher temperatures was not observed in the previous UHV studies and indicates a different temperature for water desorption at elevated pressures. This needs to be taken into account especially when performing industrial tests with semiconductor devices. Moreover, the XPS spectra of GaAs surfaces obtained from previous studies in which surfaces were exposed to water vapor pressures and then evacuated exhibit a different relative ratio of adsorbed species in comparison with those in present XPS spectra at elevated pressures. This highlights the importance of in situ studies for obtaining a comprehensive picture of the surface chemistry of semiconductor/molecule systems. Higher temperature NAP XPS studies reveal substantial Ga oxidation and hydroxylation on the surface with desorption of intact water molecules. However, the formation of hydrogen molecules, suggested in recent theoretical studies for GaP9 and GaSb,15 is not observed in our studies. This discrepancy can be explained by low sensitivity of the employed QMS or by the absence of a mechanism leading to H2 formation under the investigated conditions. However, the exact mechanism would need to be confirmed by computational calculations of H2O/ GaAs interfaces.

broadening and slight shift toward a higher binding energy is correlated with the increase in water vapor pressure due to hydrogen atom attachment on surface As atoms. According to the irreversible dissociation reaction that is previously mentioned, the final products are the oxygen atom adsorbed on the surface forming gallium oxides and the H2 molecule released into the environment. Because we observe a strong oxidation of the GaAs, the formation of H2 as a byproduct is expected. Moreover, previous experimental11,16 and theoretical15 studies on water interaction with Ga-based III−V semiconductors reported evidence of H2 desorption to vacuum at a temperature around 600 K. However, in our studies, no H2 formation due to water dissociation on the GaAs surface is detected with the QMS located in the second stage of the differential pumping system (Figure 1). Assuming that the first layer of water molecules dissociates and a totally oxidized GaAs surface forms along with hydrogen molecules that are released into the environment of the reactor cell, the partial pressure due to H2 should be in the range of 10−3 mbar (∼1.5 × 10−10 mol of H2), which is below the detection limit of the QMS. For example, in a blank experiment (without the GaAs surface) when water vapor is continuously introduced to the reactor cell below the pressure of 5 × 10−3 mbar, which corresponds to 9 × 10−8 mol of H2O, no signal from water is detected by the mass spectrometer. Under such conditions, the pressure remains in the range of 10−10 mbar in the differential pumping stage where the mass spectrometer is mounted. Perhaps, if any H2 molecules are formed, they gradually desorb into the environment at lower temperatures that were previously reported7,16 as the sample is heated or due to a continuous gas flow in the reactor cell that causes the effective H2 partial pressure to be much lower and thus well below the detection limit. Another possibility is that there is indeed no formation of H2 in our system under the experimental conditions. In such a case, part of the surface should be terminated with hydrogen atoms that can contribute to broadening of the As 2p peak recorded at higher temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

High-resolution XPS spectra of the GaAs(100) surface at RT and at the different water vapor pressures. High resolution XPS spectra of the GaAs(100) surface at different temperatures and at 0.1 mbar water pressure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: + 1 (574) 631-4506. Notes

5. CONCLUSIONS The characterization of molecular interactions between gasphase molecules and semiconductor surfaces and the identification of specific chemical bonds under more realistic conditions can lead to better designs of current optoelectronic technology. Therefore, in situ NAP XPS methodology was applied for the first time to study the interactions of water molecules with the GaAs(100) surface under near-ambient water vapor pressures and at elevated temperatures. We observed that water molecules undergo dissociation on the GaAs surface at elevated pressures and room temperature. This process is further enhanced by increasing the water vapor pressure and surface temperature, which leads to significant oxidation and hydroxylation of the surface Ga atoms and possible hydrogenation of surface As atoms. Figure 8 schematically summarizes the results obtained from the NAP XPS studies carried out at higher pressures and higher temperatures. We suggest that molecularly adsorbed water or its clusters facilitate H2O dissociation due to the lowering of the water dissociation barrier because large quantities of O−Ga and HO−Ga bonds along with H−As bonds are formed under elevated pressures. Moreover, in contrast with UHV H2O/ GaAs interface studies reported previously,4,7,8,10,11,16−18 our spectra at higher pressures exhibit a large quantity of intact water molecules that are physisorbed directly onto the GaAs

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through grant number DE-FC02-04ER15533. This is contribution number NDRL 5005 from the Notre Dame Radiation Laboratory. The authors gratefully acknowledge the technical support of Edward Lamere during the initial stages of the project.

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