CHARACTERIZATION OF SURFACE STATES AT A SEMICONDUCTOR ELECTROLYTE INTERFACE BY ELECTROLYTE ELECTROREFLECTANCE SPECTROSCOPY

Abstract

Supra bandgap and subband gap Electrolyte Electroreflectance is being used to characterize surface states at semiconductor liquid interfaces. The surface states can manifest themselves either through direct optical transitions as in the case of n Ti02 aqueous electrolyte interface or through their effect on the response of the Fermi level to small changes in the electrode potential as in the case of single crystal CdT^Se/ in polysulfide solutions. We will present here, two modes in which Electrolyte Electroreflectance (EER) is being used to detect and characterize surface states at the semiconductor electrolyte interface. In the first mode, which is being demonstrated on single crystal Ti02, direct optical transitions between the surface states and the conduction band, are being observed. In the second mode, demonstrated here on single crystal CdIn2Se4 in polysulfide solution, the surface states are responsible for Fermi level pinning which quenches the EER signal. Sub-Bandgap EER -. Ti02: The potential distribution at the Ti02 aqueous electrolyte interface with particular emphasis on the surface states and their dependence on various electrolytes was investigated in detail in our laboratory, using a variety of techniques. These techniques include impedance spectroscopy (1-4). supra and sub-bandgap photorepsonse spectroscopies (2) ; photoelectrochemistry with single and double beam excitation) and EERTM) . Three main groups of surface states were identified: One state that tails from the conduction band edge and is primarily responsible for the recombination of light generated minority carriers(3), second state which resides 0.8 eV below the conduction band and is being controlled by hydrogen adsorption (2) and a third state 1.3 eV below the conduction band which can be observed only when an adsorbing anion penetrates into the inner Helmholtz layer) and is responsible for catalysis of water oxidation. This last state is the subject of the work that will be presented in this section. Whenever comparisons can be made, satisfactory agreement exist between UPS measurements under UHV(5) an(j the in situ measurements. Details about the electrode, cell and the experimental techniques were previously publishedC-^) . The doping level of the TiC>2 Is 5 x 10l9/cm3, Fig. 1 shows the EER spectra of TiC^ in various electrolytes in the sub-bandgap region. All spectra were taken at a potential which shows the maximum response. The broad peak is centered around 1.3 eV and the peak position does not change with the electrolyte but the intensity of the peak does. Within the halogen series it follows the same trend as the expected strength of adsorption, if the latter is dominated by substitution of the hydration shell of the ionsW. In all cases, the potential of the maximum response is 0.1 0.3 V more negative than the corresponding flatband potential. We interpret these results by concluding that the sub-bandgap EER originates from optical transitions between filled Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831032