Size-dependent effect of energy transfer on photoluminescence from Si nanocrystals in close proximity with ZnO films

300 nm SiO _x layers grown on p-type (100) Si wafer have been annealed to form Si nanocrystals (NCs) within SiO ₂. 100 nm ZnO films have been then deposited on top of the SiO ₂:Si NC layers and annealed to form hybrid structures of ZnO/Si NCs. The PL _Si (photoluminescence from Si NCs) intensity of the hybrid structures increases almost linearly with decreasing size of Si NCs (d _Si) from 3.8 to 2.0 nm, whilst the PL _ZnO (PL from ZnO) gradually increases down to d _Si = ∼ 2.5 nm and then sharply decreases. In the SiO ₂:Si NC layers without ZnO, the PL _Si intensity sharply increases to a maximum at d _Si = ∼ 2.5 nm, and by further decrease of d _Si, it decreases. These results suggest that the energy transfer from Si NCs to ZnO occurred in the range of d _Si = 3.8 to ∼ 2.5 nm and vice versa below d _Si = ∼ 2.5 nm.