Recently GaN layers grown on graphene have been getting a lot of attention due to potentially being high quality while lowering manufacturing costs and complexity. Here, we analyze said structures using combined scanning electron (SEM) and cathodoluminescence (CL) microscopies and compare them to GaN layers grown without the graphene interlayer.
In this work, we focus on two samples, both having GaN/graphene and reference GaN/sapphire areas, grown identically using MOCVD process, with one key difference: final GaN layer in one sample was grown for 1800 s (resulting in non-uniform GaN layer, made of semi fused islands), while the other – 4000 s (resulting in homogenous GaN layer). The use of hybrid SEM-CL microscopy lets us see direct correlation between the topography of a given sample and CL signal in the same area.
We analyze the difference in CL spectra between GaN/graphene and reference GaN structures, showing no immediate differences in near band edge (NBE) peak CL luminescence signal despite the presence or absence of graphene layer. However, we observed a small (up to 3 nm) CL peak wavelength blueshift, appearing at dislocations seen in CL pictures. Also, yellow band region CL intensity differs when comparing both structures, most likely due to change in point defect density depending on existence of graphene layer.
Another key characteristic of GaN is its high defect density. Having calculated it in both samples, we demonstrate that CL intensity is inversely proportional to the defect density of examined samples, with higher defect density areas having lower average CL intensity, although this dependence is minimal. More substantial dependence was found in fully grown GaN/graphene layer, where lower amount of point defects resulted in CL intensity roughly 4 times larger than reference GaN.