Films for capping GaAs during annealing of implant damage have been deposited by magnetron sputtering of an Al target in Ar/N2 mixtures in two systems. The N2 flow was adjusted to obtain nitride formation on the target; this required a flow rate of 5 sccm at sputtering pressures of 0.2–0.5 Pa. The AlN deposition rate in N2 was approximately 10% of the Al rate in an argon discharge but increased by 30% as the fraction of Ar in the gas flow increased from 0 to 0.5. The film stress also varied with this fraction; compressive stress decreased and became tensile at a value of approximately 0.4. X-ray analysis showed all the films were single phase AlN with a lattice constant c=4.970 Å and had a preferred orientation with the c axis near the substrate normal. Undoped, semi-insulating GaAs wafers showed no decrease in resistance after rapid thermal anneal with an AlN cap deposited in one sputtering system. Annealing of AlN caps from the other system caused p-type conductivity in the GaAs which were shown by photoluminescence, SIMS, and Hall measurements to result from a Zn concentration of 1014 cm−2 arising from Zn vapor pressure in the sputtering system due to earlier depositions of ZnO. The sticking coefficient of Zn to the GaAs surface increased 1000 times by exposure of the GaAs to an oxygen plasma prior to deposition of the capping layer.