Paper TC+EM+AS-WeA3
Ion Dependence of Gate Dielectric Behavior of Beta-Aluminas in Transparent Oxide Field-Effect Transistors

Wednesday, October 31, 2012, 2:40 pm, Room 007

Sodium beta-alumina (SBA) is an excellent gate dielectric material which can be used in low-voltage (2 V), solution-processed transparent oxide field-effect transistors (FETs). Sodium ions have been experimentally proved to be the origin of the high capacitance observed in SBA gate dielectric. With this discovery, the investigation of dielectric properties of alumina with the incorporation of other alkali metal ions (for example K⁺, Li⁺) becomes compelling.

High field-effect mobility (about 20 cm²·V^-1·s^-1), high saturation drain current (about 1 mA), and small subthreshold swing (about 200 mV/decade ) were achieved in low-voltage (2 V), spin-coated zinc-tin-oxide (ZTO) FETs with potassium beta-alumina (PBA) and lithium beta-alumina (LBA) dielectrics. This proves that the incorporation of alkali metal ions in beta-aluminas is a general route to reduce operation voltage of transistors while achieve excellent electrical performance.

To investigate the effect of alkali metal ions on beta-alumina capacitance, beta-alumina Metal-Insulator-Metal (MIM) capacitors (PBA, LBA, and SBA) were analyzed in a frequency range from 100 Hz to 1 MHz. A tendency for beta-alumina capacitance to increase with increasing atomic number of alkali metal ions was observed. Besides, beta-alumina capacitance was found to decrease as temperature increases and LBA showed the strongest temperature dependence of capacitance. Moreover, capacitance of beta-aluminas with different thickness was measured and they were independent of thickness. With these results, electric double layer (EDL) structure was proposed as one way to explain the high capacitance of beta-alumina dielectrics. Ion exchange experiments showed significant diffusion of both lithium ion and potassium ion between PBA and LiNO₃ solution; however, a high concentration difference did not seem to cause obvious diffusion of either lithium ion or potassium ion between LBA and KNO₃ solution. This selective ion exchange behavior in beta-aluminas showed that the Al₂O₃ matrix structure would be affected by the alkali metal ions incorporated, and/or that Li ions are much more strongly bound. Thus, varying intercalated ion types and concentrations can be a means of tuning frequency-dependent capacitance of alumina films.