The influence of the Ga cell temperature on the quality of GaN films grown by MBE on p-Si(111) substrates was studied for cell temperatures (TGa) in the range from 865°C to 922°C using the RBS/Channeling technique. The films were in situ doped during growth with Er at a constant cell temperature. The films show a strong dependence of the crystalline quality on the Ga cell temperature with the best films grown at TGa=915°C. For temperatures TGa below 880°C the films showed no channeling effect. The thickness increases linearly with the temperature suggesting that changes in the Ga flux influence the growth process. The decrease of the Ga flux allows the incorporation of higher Er concentrations in the films. The data showed that a maximum value of about 0.35 at% was reached under the chosen growth conditions. The Er ions occupy mainly the Ga sublattice in the films with single crystalline quality. A comparison of the angular scans through the ⟨0001⟩ and the ⟨1010⟩ axes with Monte Carlo simulations leads to the conclusion that a majority (∼90%) of the Er ions occupies Ga sites.