Principal Investigator Jeehwan Kim
The performance of advanced GaN-based electronics and optoelectronics can rely heavily on the structural quality of the epilayer used in its fabrication. The layer’s characteristics, such as dislocation density or surface roughness, are largely inherited from the initial GaN growth. Due to the limited availability and the cost of high-quality bulk GaN substrates, heteroepitaxy of GaN on foreign substrates such as Al2O3, SiC, and Si is conventionally used. The lattice and thermal-expan- sion-coefficient mismatch of these substrates to GaN unavoidably lead to the formation of dislocations, as well as potential cracks and wafer bow.
In addition, the majority of the substrate material is usually removed from state-of-the-art devices to lower the thermal resistance of the packaged devices and improve performance. The removal of GaN devices from bulk/foreign substrates is very challenging and is an ongoing subject of research. Existing removal processes involving photoelectrochemical etching, mechanical spalling, and laser interface decomposition suffer from slow processing speed and/or significant surface roughening and cracking, limiting the process yield and practicality of substrate reusing.
Recently, we discovered that the epitaxial registry of adatoms could be determined by the underlying substrate remotely without direct contact with the substrate, but through a narrow gap defined by monolayer graphene. Therefore, homoepitaxial growth can be performed remotely through the single-atom-thickness gap, with the dislocation density of the epitaxial thin film at the same level as the high-quality substrate. In addition, because of the van der Waals interaction at the graphene interface, the epitaxial thin film can be precisely and rapidly exfoliated from the substrate, demonstrating the atomic flatness at the released surface mimicking the morphology of graphene surface. We performed the remote epitaxy of GaN on GaN/sapphire substrate with monolayer graphene as an interlayer to demonstrate high-quality, low dislocation density GaN thin films. We obtained GaN epilayer with material quality identical to the GaN/ sapphire substrate in terms of surface morphology and dislocation density. We further exfoliated the GaN epitaxial thin film from the substrate achieving free-standing GaN of 300nm thick.
Ultimately, we will develop the process of GaN remote epitaxy on bulk GaN substrate with minimal defects, enabling the GaN-based electrical and optoelectronic devices approaching intrinsic performance without the limitation from material quality. On the other hand, the cost of such high-performance devices will be significantly reduced since expensive substrates will be reused.