GaN high electron mobility transistor (HEMT) structures containing AlN nucleation layers were grown on SiC substrates by molecular beam epitaxy. Deleterious charge is observed near the GaN/AlN interface when the AlN layer is grown using aluminum-rich growth conditions which promote AlN material quality. The unwanted charge is correlated with nondestructive mercury probe buffer leakage measurements and degraded capacitance-voltage profiles. Secondary ion mass spectrometry measurements on a HEMT structure with a thick AlN layer grown aluminum rich confirm that the unintentional dopant is silicon which rapidly migrates through the AlN layer to the GaN buffer layer. Leakage current measurements on aluminum-rich AlN layers indicate that the conduction is in the initial GaN layers near the GaN/AlN interface. It is proposed that under aluminum-rich conditions the excess aluminum present on the growth surface in the liquid state is reacting with the substrate surface resulting in dissolved silicon that rapidly travels with the growth surface. Thermodynamic calculations and aluminum-silicon phase diagrams support this mechanism. By careful adjustment of the aluminum to nitrogen flux ratio, silicon outmigration is significantly reduced with a concomitant reduction in leakage current by four orders of magnitude. (c) 2006 American Vacuum Society.