Demonstration and STEM Analysis of Ferroelectric Switching in MOCVD-Grown Single Crystalline Al_(0.85)Sc_(0.15)N

Wurtzite-type Al$_{1-x}$Sc$_x$N solid solutions grown by metal organic chemical vapour deposition are for the first time confirmed to be ferroelectric. The film with 230 nm thickness and x = 0.15 exhibits a coercive field of 5.5 MV/cm at a measurement frequency of 1.5 kHz. Single crystal quality and homogeneous chemical composition of the film was confirmed by X-ray diffraction spectroscopic methods such as time of flight secondary ion mass spectrometry. Annular bright field scanning transmission electron microscopy served to proof the ferroelectric polarization inversion on unit cell level. The single crystal quality further allowed to image the large-scale domain pattern of a wurtzite-type ferroelectric for the first time, revealing a predominantly cone-like domain shape along the c-axis of the material. As in previous work, this again implies the presence of strong polarization discontinuities along this crystallographic axis, which could be suitable for current transport. The domains are separated by narrow domain walls, for which an upper thickness limit of 3 nm was deduced, but which could potentially be atomically sharp. We are confident that these results will advance the commencing integration of wurtzite-type ferroelectrics to GaN as well as generally III-N based heterostructures and devices.