BGaN ternary alloys with 0.7% and 1.7% boron grown on GaN/sapphire template substrates by metalorganic vapour phase epitaxy (MOVPE) have been investigated through HAADF-STEM. At low boron content, 0.7%, no compositional fluctuations were observed with XRD or STEM measurements. Photoluminescence at room temperature was measured. At higher boron content, cubic BGaN nano-sized clusters were identified. The clusters are 3nm wide, homogeneously distributed in size and in density and coherent with the surrounding wurtzite BGaN matrix. Their boron composition was estimated by EDX.