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One of the bottlenecks for the widespread application of Si photonics, and for the merging of electronic and optical functions on the same chip, is the lack of efficient light sources in Si. The material itself is known to be a poor light emitter because of its indirect fundamental band gap. In the last years many research efforts have been made to increase the efficiency of light emission in Si by several means, including the use of porous Si, Si nanocrystals (nc), and/or incorporation of rare earth (RE) ions. In particular, by doping silicon-based materials with REs, it is possible to obtain the radiative de-excitation of the RE and, by properly changing the RE, it is possible to tune the emission wavelength from the visible to the infrared.  For example, Er is a RE that shows a radiative transition at 1.54 µm, a strategic wavelength for telecommunication since it falls in a window of minimum losses for silica optical fibers. The Catania Unit of IMM has a twenty-years experience in this field and our activity has been focused on different ULSI compatible luminescent materials:

  • Er-doped Si nanocrystals (Si-nc): Er luminescence is enhanced by about a factor of 100 with respect to the Er/SiO2 system since Si-nc act as efficient sensitizers for Er. Efficient, stable and room temperature operating electroluminescent devices based on Er doped Si nc have been realized.

(Left hand side) Plan view transmission electron microscopy showing the Si-nc embedded in a SiO2 matrix.
(Right hand side) Emission microscopy image of a LED based on Er-doped Si-nc. The colours indicate different intensities of the emitted light, red being the highest.

 

  • Eu-doped Si based materials: Eu ions embedded in a SiOxCy matrix can be stabilized in the unusual oxidation state 2+ and exhibit an intense visible emission at room temperature which can be tuned by changing the Eu2+ concentration. This property can be exploited to fabricate SiOxCy multilayers in which the appropriate combination of layers with a different Eu2+ concentration leads to an intense white emission. Depth-resolved cathodoluminescence measurements have demonstrated that through a proper tuning of the energy of the exciting electron beam, Eu-doped SiOxCy multilayers are able to continuously tune their emission from blue to green and also to emit a high-quality white light. The above properties open the way to applications of Eu-doped materials in photonic, lighting and display technologies.

Photoluminescence spectra of Eu doped SiOxCy single layers with different Eu contents and of a bilayer sample that is a combination of the previous ones. The photographs of the sample under an UV illumination are also shown, demonstrating that it is possible to change the emission colour by only changing the Eu content. A proper combination of layers with different Eu contents gives an intense white emission. 

  • Mixed rare-earth compounds: these compounds permit to host up to 1022 Er/cm3 in optically active state, by overcoming the solubility limit of silica based materials. By the accurate control of the Er-Er distances and of eventual other active RE ions we obtain optically efficient system at 1.54 µm. Also, heavy metals, such as Bi, have been stabilized exploiting peculiar allowed transitions. By controlling the stabilized Bi oxidation states it is possible the tuning from the blue to the orange. Also the Bi role as efficient Er sensitizer is exploited by further increase the Er excitation cross section up to 3 orders of magnitude. The development of these innovative compounds is their implementation as active medium in photonic devices, such as slot waveguides and photonic crystal nanocavities, and for lighting and flat displays.

Photoluminescence spectra of a Bi-doped yttrium oxide sample obtained by exciting at different wavelengths in the 310-390 nm range. The emission varies from the blue to the green by only changing the excitation wavelength.

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