Effect of Sputtering Parameters on the Optical Properties of Luminescent Eu-Doped SiO_x Thin Films

Luminescent silicon (Si) based structures that are compatible with the existing complementary metal oxide semiconductor industry has drawn significant interest with the vision of developing an integrated optoelectronic platform on monolithic Si [1]. Such luminescent structures find several applications in inter- and intra-chip optical interconnects, microdisplay, biological detections, and solid-state lighting [2, 3]. The outstanding optical properties and chemical stability of rare earth materials make doping Si with rare earths a promising approach to obtain bright emission from Si-based structures [4]. In this work, we have investigated the influence of sputtering power and argon (Ar) partial pressure on the optical properties and the composition of europium (Eu) doped silicon oxide (SiOx) thin films. For this purpose, the thin films were fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) with in-situ magnetron sputtering on a p-type 3" Si (100) substrate. Silane (SiH4) and oxygen (O2) were used as precursor gases, and a solid 99.9% pure Eu as the sputtering target. Ar was used as the sputtering gas and brought into the chamber through a gas injection ring. A set of samples was prepared by changing the Ar flow rate ranging from 10 to 40 cubic centimeters per minute at standard temperature and pressure (sccm) in steps of 10 sccm. Another set of samples was fabricated by varying the sputtering power from 20 to 30W in steps of 5W. Post-deposition annealing was performed at a higher temperature from 900°C to 1200°C in ambient nitrogen and a mixture of nitrogen (95%) and hydrogen (5%) atmosphere. The effect of hydrogen passivation has also been studied in this work. The compositional analysis was performed by Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA). We found that the doping concentration was strongly influenced by the sputtering power, and it is slightly affected by the Ar partial pressure. The thickness, refractive index, and absorption coefficient of the thin films were obtained from variable angle spectroscopic ellipsometry (VASE) analysis. The dependency of the thickness and the optical parameters on sputtering power and Ar partial pressure was also analyzed. In addition, the luminescence properties of Eu doped SiOx films have been investigated. A bright white emission was observed due to the transition between 4f65d and 4f7energy levels of Eu2+ ion from all the films, which can be seen by the naked eye under daylight conditions. The relation between intensity and doping concentration was also discussed. [1] Lin, R. Huang, H. Wang, Y. Wang, Y. Zhang, Y. Guo, ... and H. Li, “Dense nanosized europium silicate clusters induced light emission enhancement in Eu-doped silicon oxycarbide films,” J. Alloys Compd., vol. 694, pp. 946–951, 2017, doi: 10.1016/j.jallcom.2016.10.132. [2] D. Thomson, A. Zilkie, J. E. Bowers, T. Komljenovic, G. T. Reed, L. Vivien, ... and J. M. Hartmann, , “Roadmap on silicon photonics,” J. Opt. (United Kingdom), vol. 18, no. 7, pp. 1–20, 2016, doi: 10.1088/2040-8978/18/7/073003. [3] H. Jeong, S. Lee, G. Y. Sung, and J. H. Shin, “Design and fabrication of Tb3+-doped silicon oxy-nitride microdisk for biosensor applications,” IEEE Photonics Technol. Lett., vol. 23, no. 2, pp. 88–90, 2011, doi: 10.1109/LPT.2010.2091268. [4] L. Li, J. Zheng, Y. Zuo, B. Cheng, and Q. Wang, “Strong Eu2+ light emission in Eu silicate through Eu3+ reduction in Eu2O3/Si multilayer deposited on Si substrates,” Nanoscale Res. Lett., vol. 8, no. 1, p. 194, 2013, doi: 10.1186/1556-276x-8-194.

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