“…However, the reliable and reproducible fabrication of GeV − centers has remained a key challenge for practical development of GeV-based devices. In that respect, the formation yield of optically active GeV − following ion implantation was reported to be 0.4%-0.7% [11] and 1.9% [3], hence tentatively lower than in the case of SiV − (0.5%-1% [12], 0.5%-6% [13,14], ∼1%-3% [15], ∼2% [16], ∼2.5% [17], 2.5%-3.75% [18], 2.98% [19], 3.2% [3], 5% [20], 15% [21]) or SnV − (∼0.7% [22], >1% [23], 0.4%-3% [24], 1%-4% [25,26], ∼2.5% [17], ∼5% [27], 60% [28]). GeV centers have also been produced by means of recoil implantation via Xe-irradiation of thin Ge films deposited on diamond, followed by 950 • C annealing [29].…”