Enhanced photoluminescence of nanostructured Er3+-doped a-Si/a-Al2O3 thin films prepared by PLD

Enhancement of the 1540 nm emission in nanostructured Er3+-doped a-Si/a-Al2O3 thin films prepared by pulsed laser deposition is studied. This paper is focused on a symmetric film structure formed by a central Er3+-doped Al2O3 layer sandwiched between two a-Si layers, which are isolated from the silicon substrate and the ambient medium by non-doped Al2O3 buffer layers. The layer thicknesses have been chosen in order to minimize the reflectance of the film at the pumping wavelength of 514.5 nm and thus to increase the effective excitation of the Er3+ ions. The Er3+ ions within the central Er3+-doped Al2O3 layer have been distributed in layers separated by about 5 nm in order to minimize clustering and concentration quenching effects. Upon excitation at 514.5 nm, the film annealed at 700 °C shows the characteristic emission of Er3+ ions at 1540 nm in addition to a continuous emission band, which is also observed upon excitation at 476 nm while no Er3+ related emission can be evidenced. This band may thus be linked to emission from the Si layers. It has been determined that the net Er3+ emission intensity enhancement at 1540 nm is of a factor 1.5 when compared to a film prepared with no Si layers. A two-component luminescence decay is observed upon excitation at 514.5 nm: first, a fast decay regime that can be associated to the broad emission band from the Si layers, and then a slower decay regime linked to the Er3+ emission and characterized by a time constant of 5.7 ms. This lifetime value is 2.5 times higher than for the film without silicon layers (2.2 ms).