Browsing by Author "Johnson, N."

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    Characterization of cuInSe2/CdS thin-film photovoltaics by x-ray photoelectron spectroscopy
    (Lognor, 2016) Aydogan, Pinar; Johnson, N.; Rockett, A. A.; Suzer, Sefik
    CuInSe2/CdS are promising photovoltaic materials due to their low production costs, chemical flexibility, and their acceptable conversion efficiencies. Although basic understanding of the device performance in terms of charge collection and recombination mechanism(s) are identified, there is still room for better understanding the effects of various defects and imperfections on device performance. X-Ray Photoelectron Spectroscopy has long been utilized for investigating nature of the chemical and physical parameters effecting the performance of these materials and devices made out of them. One particular advantage of XPS is its ability to identify and quantify charge accumulation and/or depletion through analysis of the shifts in the binding energy of the elemental peaks as a result of the local electrical potentials developed, since the kinetic energy of the photoelectrons emitted is directly influenced by them. This work focuses on measuring the photoshifts individually for Cd, In and Cu via illumination by lasers with three different colors towards assessing the presence and the effect of atomic defects and/or imperfections.
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    Electrical properties from photoinduced charging on Cd-doped (100) surfaces of CuInSe2 epitaxial thin films
    (AVS Science and Technology Society, 2016) Johnson, N.; Aydogan, P.; Süzer, Şefik; Rockett, A.
    The photoresponse of Cd-doped CuInSe2 (CIS) epitaxial thin films on GaAs(100) was studied using x-ray photoelectron spectroscopy under illumination from a 532 nm laser between sample temperatures of 28-260 °C. The initial, air-exposed surface shows little to no photoresponse in the photoelectron binding energies, the Auger electron kinetic energies or peak shapes. Heating between 50 and 130 °C in the analysis chamber results in enhanced n-type doping at the surface and an increased light-induced binding energy shift, the magnitude of which persists when the samples are cooled to room temperature from 130 °C but which disappears when cooling from 260 °C. Extra negative charge trapped on the Cu and Se atoms indicates deep trap states that dissociate after cooling from 260 °C. Analysis of the Cd modified Auger parameter under illumination gives experimental verification of electron charging on Cd atoms thought to be shallow donors in CIS. The electron charging under illumination disappears at 130 °C but occurs again when the sample is cooled to room temperature.