Er-Er interactions in yttrium-erbium compounds thin films

Description

Er-doped materials are extensively studied in Si-based photonics, owing to the capability of Er ions to emit and absorb photons at discrete wavelengths, extending from the ultraviolet to the infrared. However the maximum Er amount that can be introduced as a dopant in a solid host is limited to about 1020 at/cm3 owing to the low solid solubility. Such a limit can be overcome in mixed Y-Er compounds: given the strong similarities between Y and Er compounds, the Er amount can be varied with continuity in a wide range, extending from the low values typical of a doping condition to the extreme values of Er compounds (about 1022 at/cm3). 
However the Er content increase certainly leads to the occurrence of Er-Er interactions that determine a strong modification of the emitting properties of the Er ions. We have studied the influence of such interactions in both oxide Y(2-x)ErxO3 and disilicate Y(2-x)ErxSi2O7 thin films, grown on c-Si substrates by rf magnetron co-sputtering. In both compounds the existence of two well-defined Er concentration regimes (defined as Er-doping and Er compound) has been demonstrated, with a threshold value of about 5*1021 Er/cm3. Above this limit, the interactions between the excited Er ions and the Er population in the ground state lead to a fast depletion of the high-energy levels with a consequent refilling of the low-energy ones. Although the interactions occurring in both materials are exactly the same, their effects are different. Y(2-x)ErxO3 is a low phonon energy host, and then non-radiative phononic decays have low rates: Er-related optical emission both in the visible and in the infrared regions is then observed. In the doping regime a population inversion condition between the first two excited levels is achieved, opening the route for the realization of optical amplifiers, operating at 2.75 um. However when the Er amount is increased (compound regime) the interactions have a detrimental effect, since the condition of population inversion is lost.
In the high-phonon energy host Y(2-x)ErxSi2O7 only Er-related emission at 1.54 um is observed for any x value. In this material the Er-Er interactions are demonstrated to produce a quantum cutting process in which it is possible to excite several Er ions with a single incoming excitation photon. In particular, in the Er-richest film (Er2Si2O7) maximum quantum cutting efficiencies of 400 % have been reached. In this regime this material can be exploited therefore in Ge solar cells, thanks to the generation of several infrared photons at expenses of only one incident visible photon.

Organised by Dr. Lo Savio Roberto