I guess you are referring to doping by ion bombardment. In this method, ions are accelerated and bombarded onto some target material. For ions to be accelerated and directed along a specific direction they need to be charged otherwise it is not possible.  Atoms do not possess net charge and hence cannot be accelerated and directed along a specific direction.

Rare earths metals are heavy and they carry large momentum which is needed for ions to penetrate deeper, break bonds and get implanted.

 Dear Prof. Kishore,

thank you for your response! This is unexpectedly simple answer to my question.