High precision in situ stress measurements were performed during doping of metal organic chemical vapour deposition (MOCVD) grown GaN films on 6H-SiC by implantation with Mg+, Ca+ and Si+ ions. The energy dependence and the effect of ion mass on the stress in the films were investigated and discussed on the basis of known models of ion-solid interaction. Further, the initial amorphisation in the centre of the film can explain the stress relaxation at higher fluences.