Ultrathin Cu-Ti bilayer transparent conductors with enhanced figure-of-merit and stability

We propose a transparent conductor (TC) structure based on the combination of ultrathin Cu and Ti films. The conductive Cu layer becomes continuous, i.e., it reaches the percolation threshold, for thicknesses in the range of 5.5-6.5 nm. However, without any adequate countermeasure, such an ultrathin layer would unavoidably oxidize even at ambient conditions. In this letter, we show that Ti capping layer in situ treated by oxygen plasma can protect the underlying Cu ultrathin layer and enhances its figure-of-merit. The improved optical properties can be attributed to multiple reflection and refraction effects in Cu-Ti bilayer system. The obtained Cu-Ti bilayer ultrathin films show transparency of 86% at wavelengths around 630 nm and sheet resistance of 16 /sq, and exhibit excellent stability, as demonstrated by the fact that their properties do not significantly change after thermal treatment up to 120 degrees C for a dwell time of 45 min in ambient atmosphere.