Gallium nitride (GaN) has severed as one of the most crucial photoelectronic and electronic materials during past few decades, promoting numerous advances for light emitting diodes, photodetectors and high electron mobility transistors. The wurtzite structure of GaN makes it in an inversed symmetry along the [0001] direction. Thus, GaN processes two different polarities named Ga-polar and N-polar. The modulation of GaN polarity exhibits great meaning for flexible device design with high performance.

Recently, Dr. CHEN Yang and his collaborators proposed a new strategy for the direct epitaxy of high crystalline quality and stress-free N-polar GaN, which was simply achieved by inserting a two-dimensional (2D) MoS₂ layer. Furthermore, the modulation of GaN polarity resulted in obvious responsivity enhancement for the GaN-based ultraviolet photodetector, also showed great potential for other devices. This work was published in Advanced Electronic Materials.

The epitaxy structure was designed as sapphire substrate, 2D MoS₂ insert layer, Aluminum nitride (AlN) layer and GaN epilayer from bottom to top. The low-temperature AlN nucleation layer play a key role in preventing the decomposition of MoS₂ layer and reducing the dislocation density in GaN epilayer. Researchers applied chemical etching, surface potential and Hall measurement to identify the N-polar nature of as-grown GaN epilayer, also demonstrated the uniform modulation of GaN polarity over a large area. 
The N-polar GaN epilayer possesses enhanced crystalline quality and less residual strain due to the weak van der Waals force in 2D MoS₂ layer. For instance, the edge dislocation density of the N-polar GaN epitaxy on the MoS₂ layer has decreased one order compared to the Ga-polar density on the bare sapphire substrate. The mechanism for GaN polarity modulation with the MoS₂ insert layer was proved  by the first-principle calculation. The atom deposition sequence and bonding type on MoS₂ layer induced the tendency for the N-polar GaN epitaxy. Therefore, through the integration of N-polar GaN and graphene carrier transport channel, the responsivity of ultraviolet photodetector reached 86.3 A/W.

In summary, this study provided a novel method to modulate the polarity of GaN by inserting a 2D MoS₂ layer, which might be extended into other nitride alloys including AlGaN and InGaN.

Moreover, the van der Waals epitaxy has alleviated the strong bonding between the epilayer and substrate, resulting in better crystal property. The ultraviolet photodetector based on N-polar GaN had a 7 times enhancement in the responsivity.  Structure designs through nitride polarity modulations would promote the performance evolution of various photoelectronic and electronic devices.