Energy Transfer from the Er3+ to Ge Nanocrystals During Electroluminescence in MOSLEDs.

It is well established that Si nanocrystals (NCs) can act as sensitizers in Er-doped SiO2 during optical pumping [1-4]. In fact, following the recombination of excitons (electron-hole pairs) in optically excited Si NCs transfer their energy to the higher energy levels of the nearest Er3+ ions and subsequently decay to the ground state by intra-4f transitions. Among those, the 4I13/2 → 4I15/2 radiative transition has attracted substantial interest since the respective luminescence at ~ 1.53 m corresponds to the maximum transparency of silica-based optical fibers [2]. Ge is another group-IV element with similar electronic properties to that of Si. We have examined the impact of Ge NCs in Er-doped SiO2 layers by investigating electroluminescence (EL) of the metal-oxide semiconductor (MOS) structures, where the Er-doped Ge-rich SiO2 layers have been prepared by ion implantation technique combined with rapid thermal annealing (RTA). The samples have been prepared by two steps: (i) 130 keV Ge ions have been implanted with a dose of 2  1016 ions/cm2 in a 200 nm thick thermally grown SiO2 layers followed by RTA at 1050 oC for 180 s, and subsequently (ii) 250 keV Er ions have been implanted with a dose of 1  1015 ions/cm2 followed by RTA in the range of 850-1050 oC for 6-150 s in nitrogen ambience. Transmission electron microscopy experiments reveal formation of randomly oriented Ge NCs with average size ~4 nm. The MOS structures have been fabricated by depositing indium-tin-oxide (ITO) and aluminium in the front and rare sides of the samples, respectively, and patterning the ITO layer using photolithography. During EL measurements, in absence of the visible range band correlated to the quantum confinement in Ge NCs a band appears at ~400 nm in Ge-rich SiO2 layer as a consequence of hot electron mediated impact excitation in Ge-related oxygen-deficiency centres (GeODCs) during electrical pumping [5]. We find an increase of the 400 nm EL intensity with a concomitant reduction of the Er-related emission, and discuss the observed phenomenon on the ground of an inverse energy transfer process [4] from excited Er3+ to the GeODCs.

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