The unintentional silicon incorporation during the metalorganic vapor phase epitaxy (MOVPE) of nominally undoped Al0.7Ga0.3N in a Planetary Reactor under various growth conditions was investigated. Dependent on growth temperature, pressure and V/III ratio, Si concentrations of below 1 x 10(16) up to 4 x 10(17) cm(-3) were measured. Potential Si sources are discussed and, by comparing samples grown in a SiC coated reactor setup and in a TaC coated setup, the SiC coatings in the reactor are identified as the most likely source for the unintentional Si doping at elevated temperatures above 1080 degrees C. Under identical growth conditions the background Si concentration can be reduced by up to an order of magnitude when using TaC coatings. (C) 2017 Published by Elsevier B.V.