Measurement of Barrier Heights of Semiconductor/Liquid Junctions Using a Transconductance Method: Evidence for Inversion at n-Si/CH₃OH-1,1'-Dimethylferrocene^(+/0) Junctions
Transconductance measurements have been used to characterize the space-charge regions of various n-Si/liquid contacts. To perform these measurements, Si electrodes were photolithographically processed to introduce p^+-contact areas into the surface of an n-type Si electrode. The electrical conductance between these p⁺ regions was then used to probe the minority carrier concentration in the near-surface region of the n-type Si. Unlike conventional differential capacitance or current-voltage measurements, these transconductance measurements can be performed under near-equilibrium conditions and can be performed in the presence of gaseous ambients or when the sample is in contact with ionically conducting electrolyte solutions. In contact with the electrolyte solutions, faradaic and solid-state conduction pathways were distinguished using ac impedance measurements. The impedance spectra provided clear evidence that contact with Me₂Fc (1,1'-dimethylferrocene)^(+/0) and Fc^(+/0) redox couples in CH₃OH(l)-1.0 M LiClO₄ formed an inversion layer in the n-Si, but that CH₃OH(l)-1.0 M LiClO₄-Me₁₀Fc^(+/0) solutions did not yield an inversion layer. These observations are consistent with prior current-voltage measurements on these junctions. The barrier heights of the n-Si/CH_3OH-Me₂Fc^(+/0) and n-Si/CH₃OH-Fc^(+/0) junctions were determined to be 1.01 and 1.02 V, respectively. These measurements provide new insight into the photoelectrochemical behavior of Si/CH₃OH contacts and provide an alternate method for characterizing the energetics of semiconductor/liquid contacts.
© 1994 American Chemical Society. Received: March 9, 1994; In Final Form: May 23, 1994. This research was supported by the National Science Foundation, Grant CHE-9221311. P.E.L. gratefully acknowledges the National Institutes of Health for a post-doctoral fellowship. We thank Prof. Harry Atwater of Caltech for the use of semiconductor fabrication facilities. This paper is contribution no. 8928 from the Division of Chemistry and Chemical Engineering at Caltech.