Near-Surface Channel Impedance Measurements, Open-Circuit Impedance Spectra, and Differential Capacitance vs Potential Measurements of the Fermi Level Position at Si/CH_3CN Contacts
Near-surface channel impedance measurements, open-circuit impedance spectra, and differential capacitance vs potential measurements have been used to determine the barrier height of liquid contacts formed with n-type and p-type Si electrodes. Barrier heights were measured as the redox potential, E(A/A^-), of a metallocene-based, one-electron, outer-sphere, acceptor/donor (A/A^-) pair was varied in CH_3CN solvent. The barrier heights of p-Si(111) electrodes in contact with CH_3CN−Me_(10)Fc^(+/0) (where Me_(10)Fc is decamethylferrocene) or CH_3CN-CoCp_2^(+/0) (where CoCp_2 is cobaltocene) were 0.69 ± 0.1 and 1.1 ± 0.1 V respectively. In contrast, barrier heights for n-Si(111)/CH_3CN−Me_(10)Fc^(+/0) and n-Si(111)/CH_3CN-CoCp_2^(+/0) contacts were 0.66 ± 0.1 and 0.09 ± 0.01 V, respectively. These measurements indicate that the barrier heights closely track changes in the electrochemical potential of the contact, instead of being relatively invariant to changes in the Fermi level of the contacting phase, as is observed for Si/metal Schottky barriers. These measurements also demonstrate that the low effective surface recombination velocity, S, for silicon in contact with CoCp_2^(+/0) is primarily the result of an accumulation layer rather than solely being due to a low density of surface electrical defects.
© 2007 American Chemical Society. Received 22 September 2006. Published online 15 May 2007. Published in print 1 June 2007. We acknowledge the National Science Foundation, Grant CHE-0604894, for support of this work and the Link Energy Foundation for a graduate fellowship to D.J.M.