(Invited) Naked Eye Blue Emission in Ce3+ Codoped SiOxNy: Toward Si-Based Light-Emitting Devices
Authors: Ehre Florian , Dufour Christian , Blázquez Oriol , Garrido Blas , Jadwisienczak Wojciech , Ingram David C. , Gourbilleau Fabrice , Labbé Christophe 
Published: February 2020

Rare earth (RE) doped silicon host matrices has been broadly investigated to procure light emitting sources for integrated optoelectronics devices using the RE inter-4f transitions. Classically, those transitions are partially allowed and result in a very low absorption cross section inducing a non-efficient excitation. Due to its 5d-4f transitions, Ce3+ ion is quite different from other RE3+ ions due to a large absorption cross section (10-19 cm-2) as compared to the other RE3+ ions (10-21 cm-2).[1] Furthermore, due to its single valence d-orbital electron, the 5d band is strongly dependent on the local environment, resulting typically in a large Stokes shift depending on the host matrix composition.[2]

Ce-doped SiOxNy films have been deposited by magnetron reactive sputtering from Si and CeO2 targets under nitrogen reactive gas atmosphere, with a typical thickness of 120 nm. Three investigation approaches were explored.
In the first one samples grown with nitrogen highly rich plasma and a low Ce concentration (0.3 at.%) were tested. Photoluminescence (PL) experiments show a wide blue emission band ranging from 400 to 650 nm under UV photons excitation. Reference samples grown with lower nitrogen content did not show any visible emission. To explain the blue emission origin, we have studied extensively the role of different RE ions emitting centers in Ce-doped SiOxNy films (e.g. band tails, CeO2, Ce clusters, Ce3+ ions), with different activation scenarios. The results confirmed that blue emission is mainly due to the Ce3+ ion. In addition, based on refractive index measurements, the Ce-doped SiOxNy films compositions were deduced from a Bruggeman effective medium model, confirming the presence of Si3N4 and SiO2 phases. Furthermore, the presence of those phases was confirmed independently by their bonding signatures identified by infrared spectroscopy (FTIR) analysis. By means of photoluminescence excitation spectroscopy (PLE), a wide excitation range from 250 to 400 nm was evidenced and various excitation channels of Ce3+ ions involving direct or indirect mechanisms were proposed.
In the second approach, we focused on samples grown at high nitrogen flow, where the effect of Ce3+ concentration variation was investigated. Under UV excitation, a strong blue emission is visible to the naked eyes for SiOxNy sample doped with high Ce3+ concentration (6 at. % as determined by RBS measurements). The external quantum efficiency was measured for the selected best emitting samples with help of integrating sphere. No saturation of the PL intensity was observed, demonstrating the absence of Ce clusters and/or silicate phase formation due do the nitrogen content.[3] We believe that this result is very promising for considering silicon based emitting applications.
Finally, in the third approach we measured electroluminescence (EL) from Ce-doped SiOxNy prototype devices. Signal evolution was investigated as the function of the N flow, the Ce concentration and the inclusion of Al dopants with the aim to improve the electrical conductivity. The influence of these factors on observed EL was studied through the conduction mechanisms.

[1] J.M. Ramírez, A. Ruiz-Caridad, J. Wojcik, A.M. Gutierrez, S. Estradé, F. Peiró, P. Sanchís, P. Mascher, B. Garrido, "Luminescence properties of Ce3+ and Tb3+ co-doped SiOxNy thin films: Prospects for color tunability in silicon-based hosts", J. Appl. Phys., 119 (2016) 113108.

[2] J. Li, O.H.Y. Zalloum, T. Roschuk, C.L. Heng, J. Wojcik, P. Mascher, "Light Emission from Rare-Earth Doped Silicon Nanostructures", Advances in Optical Technologies, 2008 (2008) 10.

[3] C. Labbé, Y.T. An, G. Zatryb, X. Portier, A. Podhorodecki, P. Marie, C. Frilay, J. Cardin, F. Gourbilleau, "Structural and emission properties of Tb3+-doped nitrogen-rich silicon oxynitride films", Nanotech., 28 (2017) 115710 (115714pp).