The present document discloses a heterostructure for a high electron mobility transistor (HEMT). The heterostructure comprises a SiC substrate, an InxAlyGa1-x-yN nucleation layer (12), wherein x=0-1, y=0-1, preferably x 0.50, more preferably x 0.70 and most preferably x 0.90, formed on the SiC substrate. The heterostructure further comprises a GaN channel layer formed on the InxAlyGa1-x-yN nucleation layer. A thickness of the GaN channel layer is 50 to 500 nm, preferably 100 to 450 nm, most preferably 150 to 400 nm. The GaN channel layer presents a rocking curve with a (002) peak having a FMHW below 300 arcsec, and a rocking curve with a (102) peak having a FMHW below 400 arcsec as determined by X-ray diffraction, XRD. A surface of an uppermost layer of the heterostructure (1) exhibits an atomic step-flow morphology with rms roughness over a 10 μm2 scan area of below 1.8 nm, preferably below 1.4 nm, most preferably below 1 nm, over a 3 μm2 scan area of below 1 nm, preferably below 0.7 nm, most preferably below 0.4 nm, as determined by atomic force microscopy, AFM.