Miniaturized pH‐Sensitive Field‐Effect Capacitors with Ultrathin Ta2O5Films Prepared by Atomic Layer Deposition

Miniaturized electrolyte–insulator–semiconductor capacitors (EISCAPs) with ultrathin gate insulators have been studied in terms of their pH‐sensitive sensor characteristics: three different EISCAP systems consisting of Al–p‐Si–Ta 2 O 5 (5 nm), Al–p‐Si–Si 3 N 4 (1 or 2 nm)–Ta 2 O 5 (5 nm), and Al–p‐Si–SiO 2 (3.6 nm)–Ta 2 O 5 (5 nm) layer structures are characterized in buffer solution with different pH values by means of capacitance–voltage and constant capacitance method. The SiO 2 and Si 3 N 4 gate insulators are deposited by rapid thermal oxidation and rapid thermal nitridation, respectively, whereas the Ta 2 O 5 film is prepared by atomic layer deposition. All EISCAP systems have a clear pH response, favoring the stacked gate insulators SiO 2 –Ta 2 O 5 when considering the overall sensor characteristics, while the Si 3 N 4 (1 nm)–Ta 2 O 5 stack delivers the largest accumulation capacitance (due to the lower equivalent oxide thickness) and a higher steepness in the slope of the capacitance–voltage curve among the studied stacked gate insulator systems.