Effect of Ge-nanodot incorporation on light-emission from ZnO thin films

ZnO films have been implanted with Ge^- ions to doses (n _Ge) of (1.5 ∼ 4) × 10¹⁶ cm^-2 and subsequently annealed at 700 ∼ 1000 °C for 20 min to form Ge nanodots (NDs) within the ZnO matrix. Transmission electron microscopy images demonstrate the existence of irregularly-shaped Ge NDs of ∼15 to ∼30 nm in the central region of the Ge^--implanted ZnO films (ZnO:Ge) after annealing. The photoluminescence (PL) spectra of the annealed ZnO:Ge films are observed in the infrared (IR) range of ∼0.75 to ∼0.95 eV and show a maximum intensity at n_Ge = 1.5 × 10¹⁶ cm ^-2, which is consistent with the dose-dependent relative intensity ratio of UV and visible PL emissions from near-band-edge and oxygen-related deep levels (DLs), respectively. These PL behaviors are attributed to the Ge suboxide states at the interfaces of Ge NDs/DL sites and Ge-related molecular complexes within the ZnO matrix, as confirmed by the X-ray diffraction patterns.