Ion Dependence of Gate Dielectric Behavior of Alkali Metal Ion-Incorporated Aluminas in Oxide Field-Effect Transistors

Abstract

The effect of different alkali metal ions (Li+, Na+, K+) incorporated into aluminas on the gate dielectric behavior of solution processed oxide field-effect transistors (FETs) was studied. High field-effect mobility (ca. 20 cm2·V–1·s–1), high saturation drain current (about 1 mA), and low subthreshold swing (ca. 200 mV/decade) were achieved in low-voltage (2 V), spin-coated zinc-tin-oxide (ZTO) FETs with potassium alumina (PA) and lithium alumina (LA) dielectrics, as we had previously demonstrated with sodium alumina (SA). To investigate the effect of alkali metal ion on the detailed alumina capacitance and AC conductivity response, the frequency, temperature, and thickness dependences of alumina capacitance were determined. Ion-incorporated alumina metal–insulator–metal (MIM) capacitors showed a possible electric double layer capacitor (EDLC) behavior, in contrast to what was observed for alumina itself. The frequency dependence of observed capacitance varied with the included ion. These dependences were consistent with expected ion–oxygen atom bonding and numbers of surrounding water molecules. Theoretical calculations gave a proposed structure for the amorphous phase of these aluminas, comprising dense ion-free alumina regions and more open, ion-intercalated channel regions, where ions appear to migrate to the double layer at low frequency and are polarized on short length scales at high frequency. The magnitudes of calculated ion migration activation energies indicate that the ions move through continuous pores or channels, rather than through domains of nonintercalated alumina, and that they migrate in hydrated forms.