Aluminum nitride growth via hydride vapor phase epitaxy has been investigated with respect to impurity uptake. The precursor aluminum chloride was identified to react with quartz glass and provide silicon into the reactor atmosphere and subsequently into grown AlN layers. Reactor parts made of quartz were consecutively replaced by carbon glass parts to prevent unintentional silicon incorporation into III-nitride layers during growth. Carbon glass substitution of quartz parts led to a reduction of silicon levels in AlN by three orders of magnitude to 4*10(16) cm(-3), while carbon concentration in grown AlN layers remained low in the range of 10(17) cm(-3).