The invention concerns a method for forming a growth mask on the surface of an initial crystalline substrate, comprising the following steps: formation of a layer of second material on one of the faces of the initial substrate of first material, formation of a pattern in the thickness of the layer of second material so as to expose the zones of said face of the initial substrate, said zones forming growth windows on the initial substrate, the method being characterized in that the formation of the pattern is obtained by ion implantation carried out in the surface layer of the initial substrate underlying the layer of second material, the implantation conditions being such that they cause, directly or after a heat treatment, on said face of the initial substrate, the appearance of exfoliated zones of first material leading to the localized removal of the zones of second material covering the exfoliated zones of first material, thereby locally exposing the initial substrate and forming growth windows on the initial substrate.The invention further concerns methods for forming a crystalline thin film and transferring this thin film onto a host substrate.