High-performance p-type oxide thin film transistors (TFTs) have great potential for many semiconductor applications. However, these devices typically suffer from low 1 hole mobility and high off-state currents. We fabricated p-type TFTs with a phasepure polycrystalline Cu 2 O semiconductor channel grown by atomic layer deposition (ALD). The TFT switching characteristics were improved by applying a thin ALD Al 2 O 3 passivation layer on the Cu 2 O channel, followed by vacuum annealing at 300 • C. Detailed characterisation by TEM-EDX and XPS shows that the surface of Cu 2 O is reduced following Al 2 O 3 deposition and indicates the formation of 1-2 nm thick CuAlO 2 interfacial layer. This, together with field-effect passivation caused by the high negative fixed charge of the ALD Al 2 O 3 , leads to an improvement in the TFT performance by reducing the density of deep trap states as well as by reducing the accumulation of electrons in the semiconducting layer in the device off-state.

High-performance p-type oxide thin film transistors (TFTs) have great potential for many semiconductor applications. However, these devices typically suffer from low