(Invited) Rare Earth Luminescence in Nanostructured Amorphous Silicon Alloys

Amorphous silicon and its alloys are widely used in semiconductor industry. Due to short range order, doping can be achieved by substituting phosphorus or boron for silicon. Moreover, the optical bandgap can be increased by alloying with C, N or O or decreased by alloying with Ge. Due to its amorphous nature the network can accommodate a variety of elements up to relatively high concentrations at the expense of an increased density of defects in the bandgap. Rare earth elements (RE) have been introduced in the form of trivalent ions in amorphous silicon or its wider bandgap alloys. They present characteristic intra-4f luminescence if their chemical neighborhood is non-centrosymetric and the electric dipole forbidden transitions become partially allowed. In the amorphous alloys excitation through the network can be efficiently achieved when a RE transition corresponds to roughly half the bandgap. The RE act as nucleation centers to induce formation of Si nanocrystals under high temperature annealing.

Samples were prepared by reactive RF sputtering from a Si target partially covered with metallic RE platelets using appropriate reactive atmospheres. Annealing was used either to optimize the photoluminescence or to induce the formation of Si nanocrystals.

We will present characterization by photoluminescence and EXAFS of Er, Nd, Eu and Tb in a-Si:H, a-SiO_x:H and a-SiN_x:H. 

The RE ions tend to occupy low coordination low symmetry sites in the network in the amorphous samples. This favors relatively strong wide photoluminescence lines. The RE present two luminescence lifetimes, the fast component determined by the host and the slow associated to the local symmetry of the RE ions.

Models for the excitation of luminescence and recombination processes and their relation with the local nanostructure around the RE ions will be presented.