EXAFS and photoluminescence study on the Er-doped $SiO_{2}/nc-Si$ multilayers

Electrically pumped Si light source at the standard telecommunication wavelength (1535 nm) can be made by using a unique properties of Si nanostructures doped with rare earth ions. However, despite intensive research, highly efficient light sources based on silicon structures have not yet been obtained. One of the most crucial and still unresolved problems is how to achieve the efficient energy transfer from Si nanostructures to Er ions. The solution can be a structure constructed from the layers consisting of Si and SiO2:Er. The Er doped SiO2 and nc-Si multilayers, unlike homogeneous films, allow to precisely control the distance between the Er ions and the Si nanocrystals, which is a critical parameter for energy transfer. In this work the visible and infra-red photoluminescence (PL) as well as time-resolved photoluminescence (TR PL) supported with X-ray absorption spectroscopy studies will be presented. Er doped SiO2/nc-Si multilayers prepared in three different configurations were studied. The SiO2:Er/Si layers were fabricated by rf magnetron sputtering with or without pure SiO2 buffer between the Si and SiO2:Er layers. Samples were subjected to furnace annealing at 1050oC. The total Er concentration measured by Secondary Ion Mass Spectrometry was estimated to be 2•1020 atoms/cm3 for all multilayers. X-ray absorption fine structure technique was applied in order to investigate local structure around the Er ions. X-ray absorption fine structure measurements were performed at the Er L3 edge (PETRA III, beamline P65). All samples were measured in a fluorescence mode with 7-element Ge detector. Reference Er2O3 powder sample was measured in a transmission configuration. The PL spectra were carried out at room temperature using the Ar+ laser line at 514.5 nm, which is off resonance with Er3+. The time correlated photon-counting technique (TimeHarp200, PicoQuant) was used to collect the time-resolved PL under 405 nm femtosecond laser excitation from a frequency-doubled Ti:sapphire laser system with a 200 Hz repetition rate. Comparison of the samples' and reference XANES spectra confirmed that in all samples Er ions are at 3+ state. EXAFS analysis revealed that the first coordination sphere around the Er ions consists of oxygen atoms. It was found out that for the as grown samples there is a difference in the oxygen quantity and distance depending on the layers' configuration. The differences were also visible after annealing. All samples exhibit both visible and infrared emission. The strongest PL signal is observed for Er+SiO2 buffered by SiO2 layers. The TR PL studies show differences in the decay spectra. The visible (at 750 nm) TR PL spectra reveal two decay times (“fast” (nanosecond) and “slow” (millisecond)) from the sample with the closest Si-Er proximity, contrary to the samples with longer Si-Er distance, that show only one decay time – the “slow” one. The different TR PL results arise from two different energy transfer mechanisms from nc-Si to Er, depending on the Si-Er distance and neighborhood. EXAFS analysis allowed to point out structural differences in the Er ion neighborhood between the sample with pure SiO2 buffer and the samples without one. This result is correlated with the TR PL studies that show “fast” and “slow” ways of Er excitation in SiO2 matrix. This fact is reflected in the PL intensity - the sample with strong both fast and slow components shows the most effective infrared emission