Flat band voltage (VFB) drift by integrated reference electrode work function in Electrolyte Insulator Semiconductor (EIS) devices

Electrolyte-insulator-semiconductor (EIS) devices were fabricated using titanium nitride (TiN), gold (Au) and platinum (Pt) as integrated reference electrodes. Here, Au and Pt were deposited by RF sputtering and TiN was deposited by DC sputtering. The devices were developed with different geometries and using silicon (Si) n and p type wafers. A titanium oxide (TiO2) film was used as a pH sensitive membrane in all EIS devices. Capacitance versus voltage (C-V) measurements were used to determine the sensitivity of the devices from their interaction with solutions of pH: 1, 3, 4, 7, 9, 10 and 12. From the analysis of the C-V curves for the EIS capacitors with integrated Pt and Au reference electrodes, two well-defined sensitivity regions are observed: one for acidic and neutral pH solutions, and one for basic pH solutions. This type of behavior is due to the difference between the values of the work functions of Au and Pt, in relation to that of Si. This difference between the work functions causes an increase in the curvature of the energy bands, as the pH value of the solution to be analyzed increases. As a result, leakage currents are generated by Schottky emission and Fowler-Nordheim (F-N) tunneling, causing incorrect displacements in C-V responses when interacting with different pH solutions, and with this, the formation of two regions of sensitivity. For EIS capacitors with TiN as an integrated reference electrode, this effect is not observed because the value of its work function is remarkably close to that of Si.