Supporting Information

The Supporting Information including the experimental methods, the AMR measurements of Co electrodes, the AFM characterizations of the GaN and AlN surfaces, the RA products of the Au/Co/Al2O3 and Au/Co/AlN tunnel junctions, determination of the effective channel thickness t, discussion on the intensities of the Hanle signals, investigation of spin selectivity of AlN by first-principle calculations, the dependence of spin polarization on bias, the comparison of local three-terminal and non-local four-terminal results, and the spin relaxation and scattering influenced by DP and EY mechanisms.