Inverse energy transfer process during electroluminescence in Er-doped SiO2 layers containing Ge nanocrystals

For a comparative study of the energy transfer mechanism during room temperature electroluminescence (EL), two sets of samples have been prepared by implanting either Ge and Er ions (type-I) or Si and Er ions (type-II) into a 200 nm SiO2 layer combined with rapid-thermal-annealing. Three reference samples have been prepared by implanting only Er, Ge or Si ions in SiO2 followed by post-implantation annealing. The formation of Si and Ge nanocrystals (NCs) was confirmed by transmission electron microscopy. The metal-oxide-semiconductor dot structure was prepared by depositing indium-tin-oxide and Al films on the front and rear sides of the structure. Quantum-confinement mediated recombination of carriers in Si-nanocrystals (Si-NCs) as well as the triplet (T1) → singlet (S0) transition in the O3-Si-Si-O3 oxygen deficient centre (ODC) provides the 750 and 475 nm bands, respectively, in the only Si-implanted sample. Conversely, the 525, 550, 660 and 1532 nm emissions corresponding to the deexcitation of the 2H11/2, 4S3/2, 4F9/2 and 4I13/2 states to the ground state 4I15/2 of the Er3+, respectively, were recorded for the Er-doped SiO2 layer. A blue-violet light at ~400 nm, representing the T1→S0 transition in the O3-Si-Ge-O3 ODC, was observed in the Ge-NCs embedded SiO2 layer. Although the intensity of the Er-related visible/infrared EL signals in type-I sample was found disappearing/decreasing with a concomitant enhancement of the 400 nm band, a sharp rise in intensity of the 1532 nm Er EL at the expense of the Si-NC related 750 nm band was evidenced in type-II sample. While the later result confirms the energy transfer mechanism from Si-NCs to the nearest Er3+, the former finding can be explained in terms of the energy transfer from Er3+ to the Ge-related ODC.