Visible light emission from Eu-containing Si-based materials

Abstract

Among the various rare earth ions, a particular attention is currently devoted to Eu, because of its peculiar chemical and optical properties. Indeed Eu is stable both in its trivalent and divalent oxidation state; Eu2+ ions are characterized by a very intense and broad emission band, tunable in all the visible range, which can be of interest for many technological fields, while Eu3+ ions present less intense and sharp emission peaks at arround 600 nm. In this thesis, two different approaches able to take advantage of the properties of Eu ions have been presented: i) the doping approach, which fully exploits the many advantages offered by a Si oxycarbide (SiOC) host matrix, and ii) the compound approach, which mainly focuses on Eu silicates. By centering the attention on host matrices compatible with Si technology, Eu-doped SiO2 is a widely studied system, but its perspectives are quite poor, since the low Eu solid solubility in SiO2 strongly limits the concentration of optically active Eu ions. On the other hand, it has been demonstrated in this thesis that a SiOC matrix is able to enhance the solubility of Eu ions by two orders of magnitude with respect to SiO2, and simultaneously acts as an efficient reducing ambient for Eu ions, which are indeed stabilized in their divalent oxidation state. As a result, Eu-doped SiOC shows a very strong room temperature PL emission in the visible range, which is more than two orders of magnitude more intense than that one of Eu-doped SiO2. Furthermore, the occurrence of an energy transfer mechanism between the SiOC matrix and Eu2+ ions increases the efficiency of photon absorption for excitation wavelengths shorter than 300 nm.

Due to this properties Eu-doped SiOC offers great potentialities for the fabrication of light sources. In this thesis a continuous redshift of the emission peak from 400 to 600 nm by increasing the Eu concentration has been reported, allowing to consider this material a tunable light source. Furthermore, Eu2+ properties have been also used to realize a white light source, through the synthesis of a bilayer consisting of two SiOC films doped with different Eu concentrations. Through a proper choice of the annealing temperature, and taking advantage of the dependence of the PL peak position on the Eu concentration, an intense white emission at room temperature has been obtained. The CIE chromaticity coordinates of the emission are (0.33, 0.36), to be compared with those relative to an ideal white emission which are (0.33, 0.33), and the calculated color rendering index is 91/100.

The second approach discussed in this thesis is based on the synthesis of Eu compounds by thermal processing of Eu2O3 thin films deposited by RF magnetron sputtering on Si substrates. It has been shown that annealing processes in N2 ambient lead to a complex reactivity at the Eu2O3/Si interface, which induces from Eu3+ to Eu2+ reduction and formation of stable Eu2+ silicates (Eu2SiO4 and EuSiO3). These materials show a very strong and broad room temperature emission peaked at 590 nm, with a very high external quantum efficiency arround 10%. This result is very impressive, especially if we consider previously reported data for Eu silicates grown with different methods, in which an external quantum efficiency of 0.1\% has been observed.

The results shown in this thesis demonstrate that Eu-doped SiOC and Eu silicates have the potentialities for becoming the building blocks of future devices for lighting and photonics. The development of suitable strategies for obtaining an electrical excitation in both kind of materials, coupled with a full comprehension of the dependence of their optical properties and structural features on growth conditions and post-growth processing will be able to drive a complete transition of these materials from research laboratories to devices for every-day life.