Type of presentation: Poster

MS-3-P-3179 Microstructural characterization of AlGaN/GaN heterostructures grown on low angle-off 4H-SiC substrates

Gkanatsiou A.¹, Lioutas C. B.¹, Frangis N.¹, Prystawko P.^2,3, Leszczynski M.^3,2

¹Department of Physics, Aristotle University of Thessaloniki 54124, Greece, ²Institute of High Pressure Physics “Unipress”, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland, ³TopGaN Ltd, Sokolowska 29/37, 01-142 Warsaw, Poland

alexgkan@auth.gr

The present work concerns the microstructural characterization of a multi-component (based on GaN and related materials) and multi-layered (5 layers) film, grown on 4H-SiC substrate (with a misorientation of ±0.5-2 degrees off from (0001) plane), using High Resolution Transmission Electron Microscopy (HRTEM). A typical sequence of the epilayers from bottom to top is: AlN nucleation layer-GaN layer-AlN spacer layer-AlGaN layer-GaN cap layer (Fig. 1).
The layers are grown epitaxially, as it is confirmed from the corresponding electron diffraction patterns (with the [0001] SiC direction parallel to the [0001] GaN direction, as seen in Fig. 1 inset a). Conventional TEM images allow the measure of the layers’ thicknesses. Sharp interfaces are observed between the layers. However, the AlN spacer layer is not uniform in thickness and in several areas the GaN/AlN and AlN/AlGaN interfaces are not too clear. Moreover, in the AlN nucleation layer V-shaped formations are observed (Fig.1 inset b). It is remarkable that the surface of the ±0.5 deg. sample appears a characteristic roughness, which is not present in the other sample.
HRTEM micrographs clarify the quality of the interfaces and of the defects observed in the layers. In the case of the 2 deg. off sample, multiple steps are observed at the interface between the AlN nucleation layer and the 4H-SiC substrate (Fig.1 inset c). The height of the steps is one atomic layer (about 0.25 nm) and their period is about 5-6 nm. Moreover, characteristic contrast (in conventional TEM and HRTEM images) suggests the growth of threading dislocations in the AlN layer that begin from the SiC/AlN interface at the steps’ positions. On the other hand, as shown in fig. 2, no steps are observed in the case of the ±0.5 degrees off sample. Additionally, Fast Fourier Transforms (FFT), performed on HRTEM micrographs taken from the AlN layer, confirm the GaN diffusion from the epilayer forming the V-shapes.

This work was supported by the JU ENIAC Project LAST POWER Grant agreement no. 120218 and the Greek G.S.R.Τ., contract SAE 013/8 - 2009SE 01380012.

Fig. 1: A multilayer structure grown on 4H-SiC substrate. Five layers were grown epitaxially on the substrate. a) An electron diffraction pattern taken close to SiC/GaN interface, revealing the very good epitaxial growth of GaN on the 4H-SiC substrate, b) V-shaped formations in the AlN nucleation layer, c) Steps in the SiC/AlN interface.

Fig. 2: An HRTEM image of the ±0.5 deg. off sample revealing no steps at the SiC/AlN interface showing the very good growth of AlN on the SiC substrate.