(2012) IRG 2: Controlling the Conductivity of ZnO Thin Films using Surface Chemistry

By: J. W. Spalenka, P. Gopalan, H. E. Katz, and P. G. Evans

Solution deposited ZnO films have the opportunity for wide application in large-area electronics on flexible substrates, as transparent circuit elements, and as a transparent electrode material.  However, at small film thicknesses (<50 nm), the regime where transparency and mechanical flexibility are greatest, solution deposited ZnO films typically have high resistivity.  This work has shown that the attachment of organic molecules using carboxylic acid binding chemistry increases the field-effect electron mobility of ZnO thin films by more than an order of magnitude, and also increases the zero gate bias conductivity of the films.  The improvement is consistent with the passivation of defects via covalent bonding of the carboxylic acid and is reversible by exposing the film to a UV-ozone treatment that removes the organic surface layer.  The process of attaching molecules to nanoscale ZnO is challenging because ZnO is sensitive to etching in most acidic and basic conditions.  Therefore, the properties of the solvent used for the attachment are crucial because solvents with high acid dissociation constants (Ka) for carboxylic acids lead to high proton activities and etching of the nanometers-thick ZnO films, masking the electronic effect.

A measurement of ZnO FET deposit before and after stearic acid application

Reference: J. W. Spalenka, P. Gopalan, H. E. Katz and P. G. Evans, “Electron mobility enhancement in ZnO thin films via surface modification by carboxylic acids,” Appl. Phys. Lett., 102, 041602 (2013).