This study presents a broad investigation on Al implantation in crystalline Ge. We show that up to 600 C, Al does not diffuse and a remarkable electrical activation of~ 1× 10 20 cm-3 is obtained. For higher annealing temperatures (from 700 to 800 C), Al shows a significant diffusion towards the bulk and an unexpected uphill diffusion next to the surface, where the electrical measurements indicate a significant deactivation of Al. Both these latter observations are explained in terms of the presence of dopant traps, able to make immobile and electrically inactive the dopant next to the surface.