Synchrotron-Induced Photoemission of Emersed GaAs Electrodes after Electrochemical Etching in Br 2 /H 2 O Solutions M. Beerbom, Th. Mayer,* and W. Jaegermann Department of Material Science, Darmstadt UniVersity of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany ReceiVed: March 27, 2000; In Final Form: June 30, 2000 GaAs(110) surfaces have been studied after etching in Br 2 /H 2 O solutions and emersion from the electrolyte using high-resolution synchrotron-induced photoelectron spectroscopy. High-quality spectra of the core lines and the valence band region have been obtained by using a specifically developed transfer procedure, which allows synchrotron-induced XPS analysis after several transfers between the solution and vacuum. After treatment in Br 2 /H 2 O solutions, the surface of GaAs is covered by a mixture of mostly Ga 2 O 3 , Ga(OH) 3 , As 2 O 3 , and As 2 O 5 . After rinsing in liquid H 2 O, the arsenic oxides are dissolved leaving the GaAs surface covered with a layer of elementary arsenic and gallium oxides and hydroxides. Introduction The photoelectrochemistry of GaAs electrodes in aqueous electrolytes turns out to be rather complex because of the formation of surface states that result from the interaction of the electrode surface with electrolyte components 1 . Especially H 2 O and oxygen are considered to be the main reactive species, and they can form several oxygen adsorption species, as well as bulklike oxides 2 . Several etching solutions have been suggested for the preparation of clean and oxygen-free surfaces 3 , which can subsequently be used for epitaxial growth of heterojunctions or for the study of fundamental (photo)- electrochemical properties. To check for surface cleanliness and/ or the composition of possibly formed surface layers, photo- electron spectroscopy (XPS or ESCA) with laboratory sources have often been used. 4-12 However, because of limited surface sensitivity and low energy resolution, only larger amounts of oxides are easily detected with this approach. So far, no systematic studies of emersed semiconductor electrodes have been performed using synchrotron-induced photoelectron spec- troscopy, which has developed into a standard characterization technique for adsorbate-induced changes of semiconductor surfaces. Because of the increased surface sensitivity, reasonable results with synchrotron-induced XPS can only be expected when emersion and transfer of the electrodes is realized in a very controlled and perfectly clean procedure. In this paper, we present our first results on the use of synchrotron-induced XPS for the characterization of GaAs(110) electrodes after treatment in wet etching solutions. For this study, Br 2 /H 2 O was used for etching as the involved chemical species can easily be identified with synchrotron-induced XPS by the available excitation energy. Br 2 -containing solutions, usually with methanol as the solvent, belong to the standard etches for GaAs. For these first experiments, we selected the GaAs(110) cleavage plane instead of the technologically more important (100) surfaces because we wanted to start with the very defined surface conditions obtained after UHV cleavage. The surface composition that is obtained after different steps of etching is analyzed and compared to results from adsorption experiments containing the same species. 12-15 Experimental Section We used a specifically designed electrochemical chamber for the etching process and a special buffer chamber for the transfer of the samples into UHV. A schematic sketch of the setup is shown in Figure 1. The base pressure in the measurement chamber is about 1 × 10 -10 mbar. After the electrode surface has been treated with wet etching solutions, it can (or cannot) be rinsed with deionized H 2 O, and the remainders of the solutions are blown off the surface with clean and dry N 2 . As etch solutions, Br 2 /H 2 O mixtures with a concentration ratio of about 10 -3 have been used. All solutions and the electrochem- istry chamber were kept in a N 2 atmosphere and were thus free of atmospheric O 2 . After transfer into UHV (a pressure in the range of 10 -9 mbar is usually reached within 20 min), the sample surface is characterized by photoelectron spectroscopy in the core and valence band regions with excitation energies in the minimum of the photoelectron escape curve. The spectra were recorded using an angle-resolving photoelectron spec- trometer (VG ADES 500) in normal emission mode. All experiments were performed at the TGM7 monochromator of the BESSY storage ring, which provides photons in the energy range between 10 and 120 eV. The overall resolution in the presented experiments is set at 0.3 eV. To avoid problems in the setting of the binding energy scale, we present the chemical shifts of reacted species as relative shifts from the main lines of bulk GaAs. For the fitting of the core level lines, a mixed Gaussian-Lorentzian line shape was used, 16 and the experi- mental spectra were fit with the minimum number of compo- nents. Experimental Results and Discussion The photoemission spectra of GaAs(110) surfaces after cleavage in UHV and after treatment in the Br 2 /H 2 O etching solution for different exposure times and subsequent rinsing in water are shown in Figures 2 and 3 for the core lines and Figure 4 for the valence band spectra. All spectra have been measured * Author to whom correspondence should be addressed. E-mail: mayerth@surface.tu-darmstadt.de. Fax: +49-6151-16-6308. 8503 J. Phys. Chem. B 2000, 104, 8503-8506 10.1021/jp0011342 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/12/2000