In this paper, we report the possibility of phase-selective growth of Ge nanocrystals by changing the kinetic energy of the clusters in an ionised cluster beam deposition system. Typically, the films are of mixed phase of normal cubic and high energy tetragonal structures, the relative proportion of which could be controlled by controlling the ionisation and applied accelerating potential as has been confirmed from Raman spectroscopic study. The films deposited using neutral clusters showed higher yield of the tetragonal phase with nanocrystallites of diameter ~7 nm as evidenced from HRTEM data. The optical bandgap of the nanocrystals were observed to be blue shifted upto 1.75 eV compared to the bulk Ge attributing to the presence of Ge tetragonal ST-12 phase and the resulted quantum confinement effect inside the nanocrystals. The tetragonal-rich films were further studied by controlled photo-oxidation to tune their optical band gap. A visible photoluminescence due to excitonic transitions have been observed from the as-grown Ge film enriched in tetragonal phase with average crystallite size ~7 nm. The photoluminescence peak was further blue shifted after the course of photo-oxidation due to reduced nanocrystallite size.