[en] Recently it has been demonstrated that by using the ion beam assisted deposition (IBAD) the cluster size selection in growth can be controlled. Particularly interesting is the Ge/Si system where IBAD leads to enhanced transition from two-dimensional (2D) to three-dimensional (3D) growth clusters, resulting to narrowing of island size distribution as compared with conventional deposition methods. In this report, size selection of nanoclusters in IBAD is studied by using a Fokker-Planck type growth model, where growth rates depend on the cluster size through the formation energy of clusters. We find that such system in IBAD may become metastable against growth with a result of narrow size distributions