We study the electrical properties of thin TiO2 films made by atomic layer deposition (ALD) on p-doped silicon in an electrolyte-oxide-silicon (EOS) configuration. The electrolyte contact of the TiO2/Si heterostructure allows measurements of the differential capacitance for a wide range of bias voltages as they cannot be performed in a metal-oxide-silicon (MOS) structure because of extensive leakage currents. In the accumulation region of p-silicon, we find a saturation of capacitance that decreases with oxide thickness, indicating an insulator with a dielectric constant of 34. In the inversion region of p-silicon, the capacitance increases in two steps far beyond the saturation capacitance. We assign this effect to the presence of electrons in TiO2 which is controlled by the bias voltage and by immobile positive charges at the TiO2/Si interface: When the Fermi energy in p-silicon is raised to the level of the low lying conduction band of TiO2, electrons accumulate in two layers near the TiO2/Si interface and at the electrolyte/TiO2 interface with a concomitantly enhanced differential capacitance. As a control, we study HfO2 films also made by ALD. We obtain a dielectric constant of 15 from the capacitance in the accumulation region of p-silicon. For HfO2 with a high lying conduction band, the capacitance decreases as expected in the inversion region for the high-frequency limit of silicon. The electrical characterization of TiO2 and HfO2 in EOS junctions opens future applications of high-K materials in bioelectronics for efficient capacitive interaction of silicon chips and living cells.
Area-specific differential capacitance cs versus voltage VES between electrolyte and silicon, measured at 100 Hz after application of voltage sweeps. (a) TiO2. (b) HfO2. The voltage sweeps start at +1 V for TiO2 and at +3 V for HfO2. The end voltages are indicated in the figures. The measurements are plotted for the third sweep at each end voltage.
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