Invited Paper EM-TuM11
ZnO PEALD TFTs and Hybrid ZnO/Organic CMOS Circuits

Tuesday, October 21, 2008, 11:20 am, Room 210

ZnO and similar metal oxide semiconductors are of interest because of their wide band gap, transparency, and good electrical transport properties. ZnO thin-film transistors (TFTs) have potential as a higher performance, more stable alternative to amorphous silicon for use in displays and other large area electronics applications. Using ZnO semiconductor and Al₂O₃ dielectric layers deposited at 200 °C by plasma enhanced atomic layer deposition (PEALD) we have fabricated n-channel TFTs with field effect mobility > 15 cm²/V-s, subthreshold slope < 100 mV/decade, and current on/off ratio > 10⁸. Seven stage ring oscillators fabricated with 4 µm channel length ZnO PEALD TFTs have propagation delay < 40 nsec/stage for 15 V supply voltage (1.8 MHz oscillation frequency) and oscillate for supply voltage as low as 2 V. CMOS circuits are also of interest, but both p-type doping and p-channel TFTs have been problematic for ZnO. Hybrid inorganic/organic circuits using n-channel ZnO TFTs and p-channel organic thin film transistors provide an alternative path to CMOS circuits. Using Ti/Au contacts treated with a pentafluorobenzenethiol (PFBT) self-assembled monolayer as dual use contacts for both ZnO inorganic TFTs and spin cast difluoro 5,11-bis(triethylsilylethynyl) anthradithiophene (diF TES-ADT, synthesized by J. Anthony, U. Kentucky) organic TFTs, we have fabricated simple CMOS circuits. The diF TES-ADT is simply spin cast onto ZnO devices and Ti/Au contacts in a single step to complete the CMOS circuits. The diF TES-ADT develops a differential microstructure on and near PFBT-treated Au electrodes¹ and allows circuit operation with no direct patterning of the organic semiconductor layer and a simple, four mask CMOS process. The ZnO and diF TES-ADT TFTs used in the hybrid circuits have field effect mobility of 15 and 0.2 cm²/V-s, respectively, and hybrid seven stage ring oscillators with 3 µm channel length have propagation delay less than 200 nsec/stage for 30 V supply voltage. The low temperature processing and good performance of PEALD ZnO TFTs, and the ease of integration with organic TFTs and other devices, make these devices attractive candidates for large area electronic applications.

¹ D. Gundlach et al. Nature Materials, 7, 216–221 (2008).