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Highlights • High benefits in-situ SiN capping layer on AlN to prevent from surface degradation. • Partial pressure of ammonia strongly impact Rsheet mobility and charge density. • We find Gallium content into AlN layer according to process condition. Abstract In this work we have studied the growth of AlN barriers on GaN channels by Metal-Organic Vapor Phase Epitaxy (MOVPE). We have shown that an SiN in-situ capping layer is critical on AlN barrier layers. In addition, we have shown that an extreme reduction of NH 3 partial pressure results in gallium incorporation into the layers around 22%. However, we have shown that lesser reductions of NH 3 partial pressure allow us to achieve thin (3 nm) AlN layers capped with SiN which have a high quality crack free surface and state of the art Rsheet values <330 Ohm/sq for such thin layers. [ABSTRACT FROM AUTHOR]