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Ferroelectric AlN ultrathin films prepared by atomic layer epitaxy

Wurtzite aluminum nitride (AlN) of space group P63mc has long been recognized as a non-ferroelectric material, lacking the polarization switching ability. This paper reports the induction of ferroelectricity in a single crystalline epitaxial AlN ultrathin film with a thickness of 8−10 nm. The ferroelectric AlN epilayer was grown on a single crystalline GaN layer, forming a [0001]-oriented AlN/GaN epitaxial heterostructure with two reversible polar variants: [000-1] and [0001]. The AlN epilayer exhibited soft ferroelectricity with large switching currents and a polarization value of ~3.0 μCcm-2 during a 180° polarization switch. The AlN epilayer was prepared by the atomic layer deposition technique at 300°C in conjunction with in-situ atomic layer annealing. The two-dimensional electron gas (2DEG) at the AlN/GaN interface could be manipulated by the ferroelectric switching in the AlN epilayer. Strain engineering via lattice mismatch at the AlN/GaN interface was the key to creating a ferroelectric AlN/GaN heterojunction. Based on the reciprocal space mapping analysis, the AlN ferroelectricity is believed to be stemming from the out-of-plane compressive strain and inplane tensile strain present in the [0001]-oriented AlN epilayer. The discovery of low-temperature prepared, CMOScompatible AlN ultrathin films with soft ferroelectric characteristics will undoubtedly spur new fundamental and applied research in low-dimensional ferroelectric systems based on the AlN/GaN heterojunction.