  High-performance p-type oxide thin film transistors (TFTs) have great
potential for many semiconductor applications. However, these devices typically
suffer from low hole mobility and high off-state currents. We fabricated p-type
TFTs with a phase-pure polycrystalline Cu2O semiconductor channel grown by
atomic layer deposition (ALD). The TFT switching characteristics were improved
by applying a thin ALD Al2O3 passivation layer on the Cu2O channel, followed by
vacuum annealing at 300 C. Detailed characterisation by TEM-EDX and XPS shows
that the surface of Cu2O is reduced following Al2O3 deposition and indicates
the formation of 1-2 nm thick CuAlO2 interfacial layer. This, together with
field-effect passivation caused by the high negative fixed charge of the ALD
Al2O3, 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.
