GaN-based semiconductors are promising materials for solid-state optoelectronic applications. However, the strong internal electrostatic field (IEF) along the [0001] direction is a serious problem that harms the efficiency of lighting devices based on GaNbased semiconductors due to the quantum confined Stark effect. Here we theoretically predict a method, reducing the dimensions from bulk to two-dimensional (2D) structures, to fundamentally remove the IEF. After thinning the materials to several nanometers, the wurtzite configuration (with strong IEF) spontaneously transform to the haeckelite (4 | 8) configuration (without IEF) due to the more stable neutral surface in the 4 | 8 configuration. Meanwhile, the 4 | 8 configuration maintain optoelectronic properties comparable to or even better than those of the wurtzite configuration. By carefully analyzing the interaction between 2D GaN and different types of substrates (SiC and graphene), we not only provide clear physical insights for experimental results but also address a "thickness-controlled" vdW epitaxy scheme to experimentally realize the 4 | 8 configuration. We believe that the 4 | 8 configuration without IEF is a prospective material for diverse optoelectronic applications. In addition, we propose a point of view in engineering the properties of GaN-based semiconductors.

Two-dimensional (2D) semiconductors exhibit great potential to minimize the size and drastically reduce the energy consumption of optoelectronic devices due to promising features induced by quantum confinement. It has achieved many successes in infrared and visible light optoelectronic devices. The study on ultrawide band gap 2D semiconductors except h-BN are still limited, however, the requirement is more and more urgent. Inspired by the progresses of III-nitride semiconductors in recent several decades, 2D AlN is highly expected to be a new member of ultrawide band gap 2D semiconductors. In this work, we employed the first-principles calculations to investigate the structural and electronic properties of 2D AlN. We revealed that few-layer AlN acquires a square-octagon (so-AlN) configuration in the vertical direction when the number of atomic layers n is smaller than 16. With increasing the thickness from 2 ML to 8 ML, the band gap decreased due to the weakening of quantum confinement effect. We demonstrated the intrinsic indirect band gap can be tuned to be direct by applying different direction strains for so-AlN. Our results open new avenues for their application in nanooptoelectronics.