Thinfilm (Norway)

Self‐heating of high power GaN devices during their operation is a major drawback that limits the performance. Integration of sheets with very high thermal conductivity material could help in this matter. After some unsuccessful GaN growth experiments carried out directly on graphene, we succeeded to grow nitride layers on patterned graphene/6H‐SiC by Metalorganic Chemical Vapour Deposition (MOCVD). The growth is similar to the well‐known Epitaxial Lateral Overgrowth method in which the graphene buried stripes are overgrown laterally from the window regions, where AlN could grow on bare SiC with epitaxy. An AlN buffer layer was first deposited on patterned graphene/6H‐SiC surface followed by a deposition of ~ 300 nm thick Al 0.2 Ga 0.8 N and ~ 1.5 µm thick GaN layer. The AlN buffer deposited onto the graphene stripe was grown in a 3D way (Fig.1a). The heterostructure was studied using aberration‐corrected transmission electron microscopy (TEM) methods in combination of electron energy‐loss X‐ray spectroscopy (EDXS) and electron energy‐loss spectroscopy (EELS). TEM specimens were prepared using both conventional and focused ion beam methods. The most surprising details of this study is the appearance of the AlN/GaN superlattices, which were formed in a self‐organised way over the buffer layer. Instead the ternary AlGaN we have superlattice (Fig. 1.b and c) in which the thickness of the AlN/GaN is determined by the available elements from the Al 0.2 Ga 0.8 N which we wanted to grow. The control sample (without graphene) showed a much more flat AlN buffer and a ternary Al 0.2 Ga 0.8 N on that without any phase separation. EDXS mapping and also superlattice reflections show however, clearly the complete phase separation in the case the nitride layers are grown on graphene. We suppose, that some excess carbon induced the phase separation. The detailed TEM studies revealed the AlN nucleation directly on SiC and lateral overgrowth of graphene island as shown in Fig.2a. The high resolution image in Fig.2.b shows three layers of graphene and the AlN that is in epitaxy with SiC. Both interfaces are sharp and no interdiffusion of the elements are observed according to the Si, C (not shown) and Al maps in Fig. 2c The results show that high quality GaN layer over graphene/SiC can be grown with MOCVD that can serve as templates for high power GaN devices.