Measurement of Barrier Heights of Semiconductor/Liquid Junctions Using a Transconductance Method: Evidence for Inversion at n-Si/CH_3OH-1,1'-Dimethylferrocene^(+/0) Junctions

Abstract

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 Me_2Fc (1,1'-dimethylferrocene)^(+/0) and Fc^(+/0) redox couples in CH_3OH(l)-1.0 M LiClO_4 formed an inversion layer in the n-Si, but that CH_3OH(l)-1.0 M LiClO_4-Me_(10)Fc^(+/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-Me_2Fc^(+/0) and n-Si/CH_3OH-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/CH_3OH contacts and provide an alternate method for characterizing the energetics of semiconductor/liquid contacts.