Fig. 1. (a) Cross-section HRTEM image of well-grown nc-Ge 5 nm in size in 600°C annealed Ge:Er:ZnO (Er∼0.6 at. %) films. (b) HAADF image of the above film.

Fig. 2. Visible PL spectra of as-deposited, 600°C annealed Ge:Er:ZnO and 600°C annealed Er:ZnO films (all with Er∼0.6 at. %).

Fig. 3. 1.54 μm PL spectra of the as-deposited, 600°C annealed Ge:Er:ZnO and Er:ZnO films (all with Er∼0.6 at. %) under different excitation wavelengths. The arrows denoted in the diagram indicate each corresponding transition level to the ground state in Er3+ ions. The inset in (a) shows the energy diagram of Er3+ ions.

Fig. 4. (a) PLE spectra observed at 1.54 μm for 600°C annealed Ge:Er:ZnO and Er:ZnO (all with Er∼0.6 at. %) films. Arrows in the diagram indicate the different excitation ways of Er3+ ions. (b) Visible and NIR PL spectra of 600°C annealed Ge:Er:ZnO films containing nc-Ge with different Er concentrations (0.6 at. % and 0.3 at. %).