Low temperature sputter-deposited ZnO films with enhanced Hall mobility using excimer laser post-processing

Abstract

We report the low temperature (T < 70 °C) fabrication of ZnO thin films (~140 nm) with Hall mobility of up to 17.3 cm2 V−1 s−1 making them suitable for thin film transistor (TFT) applications. The films were deposited by rf magnetron sputtering at T < 70 °C and subsequently laser processed in ambient temperature in order to modify the Hall mobility and carrier concentration. Medium-to-low energy laser radiation densities and a high number of pulses were used to avoid damaging the films. Laser annealing of the films after aging in the lab under 25%–35% relative humidity and at an average illuminance of 120 lux resulted in an overall higher mobility and relatively low carrier concentration in comparison to the non-aged films that were laser processed immediately after deposition. A maximum overall measured Hall mobility of 17.3 cm2 V−1 s−1 at a carrier density of 2.3 × 1018 cm−3 was measured from a 1 GΩ as deposited and aged film after the laser treatment. We suggest that the aging of non-processed films reduces structural defects mainly at grain boundaries by air species chemisorption, with concomitant increase in thermal conductivity so that laser processing can have an enhancing effect. Such a processing combination can act synergistically and produce suitable active layers for TFT applications with low temperature processing requirements.