Browsing by Author "Tomkiewicz M"

Now showing 1 - 8 of 8

Characterization of Surface States at a Semiconductor Electrolyte Interface by Electroreflectance Spectroscopy
(Journal De Physique, 1983) Tomkiewicz M; Siripala W
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 - TiO2 - 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 CdIn2Se4 in polysulfide solutions.
Direct Observation of Surface States at the TiO2 Electrolyte Interface
(Journal of Electrochemical Society, 1981) Siripala W; Tomkiewicz M
Relaxation Spectrum Analysis was suggested as a general technique for mearsurements of charge accumulation modes and their corresponding relaxation times at the space charge layer of a semiconductor with strong emphasis on semiconductor liquid junction interfaces. Among all the reported results(2-4),none was, as yet, confirmed by an independent technique.
Electrolyte Electroreflectance of Single-Crystal CdIn2Se4 in a Photoelectrochemical Solar Cell
(Journal of Electrochemical Society, 1984) Tomkiewicz M; Siripala W
Electroreflectance was used to evaluate the optical properties of in polysulfide solution, before and after photoetching. The variations of signal intensity with electrode potential were used to trace the band position as a function of potential. It was found that the optical transition fits a three?dimensional parabolic model of the density of states, with direct transition at 1.825 eV. When the crystal is photoetched, there is a shift of 98� in the phase factor and a decrease in the broadening parameter from 0.42 to 0.32 eV. By monitoring the signal intensity with potential, it was shown that, irrespective of photoetching, the Fermi level is pinned as reverse bias conditions are approached. The pinning is ascribed to surface states that most likely originate from the adsorption of the electrolyte. The variation of the flatband potential with electrode potentials was calculated and was determined to occur because of the changes in the potential of the Helmholtz layer; the energy distribution and the density of states, which are responsible for those changes in potential of the Helmholtz layer, were also calculated.
Electrolyte Electroreflectance Study of CdIn2Se4 Liquid Junction Solar cells
(American Physical Soc. Meeting, Los Angeles,USA, 1983) Tomkiewicz M; Siripala W
Interactions Between Photoinduced and Dark Charge Transfer across n-TiO[sub 2]Aqueous Electrolyte Interface
(Journal of Electrochemical Society, 1982) Siripala W; Tomkiewicz M
An intermediate of the dark reductive reaction of with an aqueous electrolyte was identified. This intermediate forms surface states on the semiconductor. The energy and surface concentration of these states were evaluated by impedance measurements. We report on sub?bandgap photoresponse due to excitation of electrons from the valence band to these states. The potential?photocurrent behavior of these states is unique and is being fully accounted for by the proposed mechanism of their dynamic formation and annihilation.
Observation of Intrinsic Surface States at the TiO2 Aqueous-Electrolyte Interface by Sub Band-Gap Electroreflectance Spectroscopy
(Physical Review Letters, 1983) Siripala W; Tomkiewicz M
Surface states were detected with sub?band-gap electroreflectance spectroscopy in the presence of electrolytes that can adsorb on the surface of TiO2. The energy of these states is located 1.3 eV below the conduction band and they can be detected only in the weak accumulation mode. The potential distribution at the interface as a function of the electrolyte was investigated by impedance spectroscopy. These results were interpreted in terms of "intrinsic" surface states of the unsolvated surface.
Surface Recombination at n-TiO2 Electrodes in Photoelectrolytic Solar Cells
(Journal of Electrochemical Society, 1983) Siripala W; Tomkiewicz M
The photocurrent?potential behavior of photoelectrodes in liquid junction configuration is investigated. The Gartner model is modified to include the surface recombination. Impedance and photocurrent?potential measurements reveal the presence of a high density of surface states at the interface, covering more than half a monolayer. A total of four parameters are used to characterize the surface states. Two of these parameters appear in the expression for both the quantum efficiency?potential dependence and the expression for the capacitance due to these states. The other two parameters appear separately in conjunction with each experiment. The model is tested by perturbating the steady state of the system by strong background illumination which induces higher injection rate of carriers. The effect of the background illumination is interpreted as the change in the equilibrium distribution of the surface states which will result in the increase in surface recombination of photocarriers.
The Interrelation Between the Potential Distribution and the Dark Charge Transfer Across n-TiO2 - Aqueous Electrolyte Interface
(Symposia on Photoelectrochemical Process and Measurements Techniques for Photoelectrochemical Solar Cells, 1981) Tomkiewicz M; Siripala W