Si impurity concentration in nominally undoped Al0.7Ga0.3N grown in a planetary MOVPE reactor

•Measured Si background of up to 4 × 1017 cm−3 in undoped AlGaN grown in MOVPE reactor.•Analyzed influence of T, p, V/III, and reactor coating materials on Si concentration.•SiC coatings clearly identified as main source for Si background.•TaC coatings reduce Si background by up to an order of magnitude.