Sensitivity enhancement of EPR spectroscopy on single crystalline surfaces under UHV conditions by employing an electrical detection scheme

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The sensitivity of conventional electron paramagnetic resonance (EPR) spectroscopy on single crystal surfaces under ultrahigh vacuum (UHV) conditions is often insufficient to detect the limited number of defect sites available on high quality surfaces. Electrical detection of the signal known as EDMR spectroscopy offers the possibility to enhance the sensitivity of conventional cw-EPR spectroscopy by several orders of magnitude and was shown to be able to detect less than a thousand spins. In the current project this detection scheme will be implemented into an UHV apparatus to characterize paramagnetic defects at semiconductor surfaces. Paramagnetic defects play a crucial role for the functional properties many silicon-based devices such as solar cells, due to the fact that these defects create states in the band gap of the material, which act as recombination sites for charge carriers. The properties of the defect and hence the performance of the device can be significantly improved by chemical and thermal modifications of the system. Due to a lack of in-situ EPR analytics with sufficient sensitivity the impact of these modifications on the surface defects become very difficult to assess. On the one hand, this project focuses on the investigation of defects on the low index silicon surfaces (001) and (111) under ultrahigh vacuum conditions. The paramagnetic defects of the pristine surfaces will be characterized and subsequently modified under UHV conditions, which offers the possibility to correlate the observed changes in the defect structure to modification by subsequent thermal or chemical treatments such as hydrogenation or oxidation.On the other hand this strategy will be applied to characterize paramagnetic defects on single crystalline cubic SiC(001) surfaces. Apart from the characterization of defects on the pristine surface as a function of the termination and the adsorption of simple gases such as oxygen and hydrogen it will be of particular interest to explore the modification of the defect structure upon adsorption of molecules such as oligo(3-hexylthiophene) (P3HT), which are used in hybrid organic/inorganic solar cells.