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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Evolution of Atomistic Topology at H2O/GaSb(100) Interface under Ambient Conditions and GaSb Surface Passivation Xueqiang Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04766 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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The Journal of Physical Chemistry

Evolution of Atomistic Topology at H2O/GaSb(100) Interface under Ambient Conditions and GaSb Surface Passivation Xueqiang Zhang#* Department of Chemistry & Biochemistry and Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA *Author

to whom correspondence should be addressed.

#Current

address: 1 Cyclotron Rd., Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94820, USA Electronic mail: [email protected]

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Abstract Gallium antimonide (GaSb) has emerged as a promising light-absorber in solar cells and optoelectronics because of its high hot carrier mobility. Here, the interfacial physiochemical process of H2O/GaSb(100) under series of isothermal and isobaric conditions was investigated using ambient pressure X-ray photoelectron spectroscopy (APXPS). At room temperature and elevated H2O vapor pressures, we observed the dissociative adsorption of H2O onto the GaSb (100) surface and the preferential formation of (HO)Ga-Sb(H) (atop) and Ga-O(H)-Ga-Sb(H) (bridge) over (H)Ga-O(H)-Sb(H) (bridge), i.e., the GaSb (100) surface was covered by hydroxyls instead of oxides. The oxyhydroxylation of the GaSb(100) surface was significantly enhanced at elevated temperatures, coupled with gradual desorption of surface Sb from 373 to 573 K in the form of SbH3. Meanwhile, large-scale oxyhydroxylation of GaSb (100) surface at 573 K was captured by on-line MS, where the increment in H2O consumption and the generation of H2 were observed. When temperature reaches 673 K and above, the surface oxyhydroxides formed under lower temperatures desorbs quickly, leaving behind a Sb-terminated GaSb surface, preventing any further H2O dissociative adsorption. The combinative isothermal and isobaric study offer a full picture of the H2O/GaSb (100) interface in terms of the atomistic topological and structural evolutions under various H2O pressures and temperatures. The present study provides a possible venue for low-cost surface passivation of GaSb and III-V semiconductor based electronic device.


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Introduction Gallium antimonide (GaSb) has attracted substantial interests in recent years because of its superb electrical properties, including much higher electron and hole mobility compared with classic semiconductors such as Si, GaAs, and InP1-4. GaSb has also emerged as a promising candidate for high-speed and low-power optoelectronics, infrared detectors, thermophotovoltaic and solar cells1,2,5-12. In semiconductor industry, since GaSb can provide a lattice-matched template, it has been frequently used as a substrate for the growth of other mid-infrared III-V materials5,13-15. During the growth of passivation oxides by atomic layer deposition (ALD), the chemical identity of the substrate surface exposed to alternating cycles of trimethyl aluminum (TMA) and H2O have significant influences on the final quality of the film grown on the substrate5. Further, because GaSb has been incorporated into high-performance tandem photoelectrochemical (PEC) solar cells16-18 and used for photocatalytic CO2 reduction3, a detailed understanding of the interfacial chemistry between small environmental molecules such as H2O and GaSb becomes practically meaningful. To date, most research in the field of III-V semiconductors has been focused on the study of GaAs, and other III-V-based material properties have been believed, rather than known, to be similar to those of GaAs5,10. Such type of fundamental research is particularly important since the stability and performance of GaSb-based device can be significantly influenced by the surface chemical and topological properties of the material. In earlier studies of small environmental molecules (H2O or O2) interactions with GaN, GaP, and GaAs under ambient conditions19-26, it was found that the chemical composition and electronic properties including band bending and work function at the H2O/III-V interface can be significantly modified by the adsorption of H2O or O2 and/or their reactions with the semiconductor surface. Under near ambient conditions, the surface chemistry of III-V 3 ACS Paragon Plus Environment


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semiconductors can qualitatively be determined by two competing factors, the intrinsic III-V bonding characteristics and the interacting energetics at the adsorbate/semiconductor interface, i.e., the bonding energy between the III and V element (bond enthalpy) determines the intrinsic reactivity of the semiconductor surface, whereas the binding strength and therefore electronic communications between H2O (O2) and III-V semiconductors significantly influences the outcome of dissociative adsorption of H2O (O2) and the resulting electronic properties of III-V semiconductors (work function, band structures etc.)26,27. For instance, in the cases of H2O/GaN, H2O/InP and H2O/GaP interface, surface oxidation/hydroxylation of both Ga and V-group elements (N or P) were observed with no V-group element depletion at high temperatures detected, together with the preservation of V (P or N)-Ga bonding28. The strong V (P or N)-Ga bonds in GaN or GaP (compared with GaAs and GaSb) prevent the depletion of volatile V (P or N)-oxides at high temperatures. In the case of H2O/GaAs, changes were primarily observed in the photoemission spectra of Ga (2p and 3d) with no clear evidence for the oxidation/hydroxylation of As, which, depleted gradually at elevated temperatures. In the present study, investigating the H2O/GaSb interfacial chemistry offers a better understanding of the general trend of chemistry at H2O/III-V semiconductor interface and shed light on the surface passivation of electronically important semiconducting materials. Ongoing physiochemical processes at the H2O/III-V interface alter the surface chemical, morphological and electronic properties, which in return influence the further dissociative adsorption of H2O (O2) onto III-V semiconductors. For a long time, probing the complex chemistry under

continuous

evolving

conditions

at

the

electrolyte/electrode

interface

of

photoelectrochemical (PEC) solar cells has been a rather challenging task. Here we take advantage of an in-situ/operando characterization technique, ambient pressure X-ray photoelectron 4 ACS Paragon Plus Environment

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spectroscopy (APXPS), and track the dynamic chemical and structural evolutions at the H2O/GaSb (100) interface. We demonstrate the preferential formation of (HO)Ga-Sb(H) (OH at atop site of Ga) or Ga-O(H)-Ga-Sb(H) (OH at bridging site of two Ga atoms) over (H)Ga-O(H)-Sb(H) (OH at the bridging site of Ga and Sb) at room temperature (RT, 298 K), and an enhanced hydroxylation process at elevated temperatures accompanied by Sb-depletion process between 373 and 573 K, prior to an abrupt desorption of surface oxyhydroxides and the formation of a passivated Sbterminated GaSb surface. Experimental setup and procedures A 200-nm-thick GaSb film with (100) orientation was grown by molecular beam epitaxy (MBE) on a pre-cleaned commercial available GaSb (100) in a similar way as described elsewhere27. The newly grown GaSb (100) was capped by ~300 nm-thick Sb layer to prevent oxidation during transfer outside the vacuum chambers. The GaSb (100) crystal was then introduced into a preparation chamber of the APXPS system (SPECS Surface Nano Analysis GmbH, Germany) with a base pressure of 5×10-10 mbar, and followed by annealing at 725 K to evaporate the capping Sb layer and to reconstruct the GaSb (100) surface. Previous studies have shown that such treatment leads to the formation of a GaSb (100)-2×6 reconstructed surface28-30, representing the disordered GaSb (100)-3×4 surface5,31. The cleanness of the crystal surface was examined by XPS and was found to be free of any carbon and oxygen-containing contaminations. The crystal temperature was monitored by a K-type (chromel-alumel) thermocouple inserted between the GaSb substrate and a molybdenum sample holder. Simultaneously, a Pyrometer (LumaSense Technologies, USA) was used to monitor the surface temperature during the Sb decapping process. Ultra-pure water was degassed by multiple freeze–pump–thaw cycles prior to use. 5 ACS Paragon Plus Environment


All photoemission spectra were recorded using the APXPS system. The spectrometer consists of three major vacuum chambers, the load-lock, preparation, and analysis chambers. The analysis chamber is equipped with an Al Kα X-ray tube (1486.7 eV) coupled to a Micro-FOCUS 600 monochromator (XR-MF). In the X-ray source, an anode voltage of 15 kV and an emission current of 6.7 mA were applied to achieve a power of 100 W. A detailed description of the experimental setup was reported previously24,25. The binding energy (BE) was calibrated using both the Fermi edge and the Sb 4d5/2 at 31.94 eV originated from Sb-Ga32. All spectra were fitted with a multiple Gaussian/Lorentzian peak combination using SpecsLab2 and CasaXPS software. Fitting parameters in the range of 0.1-0.2 eV for the BE and full width at half-maximum (FWHM) were adjusted in our peak-fitting analysis. The FWHM of the Au 4f7/2 peak was 0.5 eV at a pass energy of 20 eV, while the inherent lifetime broadening of this peak has been reported to be ~0.3 eV33. We performed two types of experiments: the isothermal study was conducted at RT and the H2O pressure was adjusted from ultra-high vacuum (UHV, 5×10-10 mbar) to 5 mbar. Photoemission spectra were recorded at UHV, 0.005, 0.05, 0.50, and 5 mbar. The isobaric study was performed at the H2O pressure of 0.1 mbar while the temperature was varied from RT to 773 K. Photoemission spectra were recorded at 373, 473, 573, 673, and 773 K. During the isobaric experiments, photoemission spectra were recorded after heating the crystal at a rate of 5 K/min, followed by 20 min of stabilization time. Results and Discussion I. Isothermal study at room temperature The photoemission spectra of Ga 2p3/2 and Sb 3d3/2 recorded under two conditions, i.e., UHV and 5 mbar of H2O at RT, are presented in Figures 1a and b. Note that Sb 3d3/2, instead of Sb 3d5/2, 6 ACS Paragon Plus Environment

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was chosen for the interpretation of the core-level electronic states at the H2O/GaSb (100) interface, because of a BE overlap between O 1s and Sb 3d5/2 (Figure S1), especially when the GaSb surface is oxyhydroxylated. Upon the introduction of H2O vapor, a shoulder appears at the higher BE side of the Ga 2p3/2 photoemission spectrum (Figure 1a), suggesting a modified electronic structure of surface Ga by H2O adsorption and/or dissociative adsorption that forms OH, H and possibly O. A less pronounced shift in the Sb 3d3/2 spectrum towards higher BE (Figure 1b) stems from the formation of the Sb-H bond. Note that the electronegativity of H (2.1) is higher than that of Ga (1.6) and similar to Sb (1.9)34, and the formation of Sb-H bond, by applying a model of initial state effect, would lead to a contribution in the photoemission spectra with BE higher than Sb-Ga and close to Sb-Sb. The formation of Ga-O-Sb is unlikely as a peak in the 540542 eV range is anticipated if Sb-O bonds were formed, as demonstrated in our control study after a pristine GaSb (100) surface was exposed to 5 mbar of O2 at RT (Figure 1b). The isothermal study suggests that H2O experiences dissociative adsorption onto the GaSb (100) surface, where OH interacts preferentially with Ga and H interacts with Sb. Therefore, the shoulder observed in Ga 2p3/2 photoemission spectra in an isothermal study originates mainly from Ga-O(H)-Ga-Sb-H and/or (HO)Ga-Sb(H) surface chemical species rather than from (H)Ga-O(H)-Sb(H). Due to low signal intensities, the O 1s spectra are overwhelmed by the intensity of Sb 3d5/2 spectra. As shown in Figure S1b, the O 1s photoemission spectra obtained in the isothermal study are featureless and cannot provide physically meaningful information. However, O 1s spectrum recorded after the evacuation of H2O shows a broad peak between 531 and 533 eV (Figure S1b), suggesting the dominant presence of OH-based species, Ga-O(H)-Ga-Sb-H or (HO)Ga-Sb(H), instead of Ga-O, which would otherwise contribute at ~530.3 eV21,26,35-39.



Based on our understanding of the atomistic topology of surface Ga and our previous studies of H2O/III-V semiconductor interface21,24,25, we fit the Ga 2p3/2 into three components (Figure 1c): Ga-Sb (A), Ga-O(H)-Ga or (HO)Ga-Sb(H) (B), and Ox‒Ga‒(OH)3-x (C, 0

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