Research Project Highlight

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

aluminas oxide pictureHigh performance, large area solution-processible transistor circuits are key components in the application of printable, low-cost electronics, such as large area, inexpensive, low-voltage and transparent sensors.  Operating voltage and input power could be reduced by either increasing saturation field-effect mobility of semiconductors or the capacitance of gate dielectrics. During the last decade, novel high-capacitance gate dielectric materials and advanced processing techniques have been developed for macroelectronics, and low operating voltage, low leakage current, high on-off ratio transistors successfully fabricated.

The exceptional apparent dielectric constant and gate dielectric performance of sodium alumina (SA) formulations, nominally crystalline but amorphous on supermolecular scales, was discovered by our group.  SA-transistors with both inorganic and organic semiconductors, especially zinc tin oxide (ZTO) and pentacene, were successfully operated at very low voltage (2 V).  The sodium ions were experimentally proved to be necessary for the high capacitance of this gate dielectric. Most recently, we have studied the comparative properties of  K+, Li+, and Na+ aluminas, which we abbreviate PA, LA, and SA, respectively.  The dielectric behavior is consistent with an ionic conductor in series with an interfacial capacitive layer, in contrast to the bulk capacitive behavior of alumina itself.  Ion transport is also found to be more facile with K+ than with Li+, consistent with their anticipated binding strengths to oxide anion functional groups including water of hydration, and inconsistent with a model requiring ions to burrow through domains of nonintercalated (ion-free) alumina.  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 capacitive layer at low frequency, and are polarized on short length scales at high frequency.

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Using a combination of ab initio modeling and algorithms, we are investigating the structure of the amorphous form of an ionically conducting compound used in transparent electronic devices.

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