Rare earth doped silicon-based thin films can be used for light emitting applications, where the control of dopant type and concentration plays a significant role in the enhancement of optical functionalities . In this work, nanostructured terbium (Tb) doped oxygen-rich silicon oxide (ORSO) samples were fabricated using a novel fabrication technique: integrated sputtering and plasma enhanced chemical vapor deposition (PECVD) . The Tb dopant concentration in the ORSO host matrix was varied from 0.5 at. % to 17 at. % and post-deposition thermal annealing was performed in a wide range of temperatures in a N2 atmosphere. In addition, the presence of defects, one of the general physical problems limiting the emission efficiency of rare earth doped silicon-based materials, was investigated using a combination of photoluminescence measurements and positron annihilation spectroscopy (PAS).
We discuss the influence of the Tb dopant concentration on the silicon nanocrystal growth and the formation of Tb silicate nanocrystals. A precipitation mechanism as a function Tb content and annealing temperature is proposed and tested using X-ray diffraction, photoluminescence, and microscopy techniques. Following post-deposition annealing at 1200 °C, all Tb ions agglomerate and form large nanocrystals with diameters reaching 50 nm (Fig. 1). Scanning transmission electron microscopy (STEM) analysis shows that these large nanocrystals are composed of Tb, silicon, and oxygen, however, no Tb remains in the amorphous regions of the silicon oxide matrix. Despite the oxygen-rich content, small silicon nanocrystals (Si-ncs) are formed in the matrix with a size distribution between 2 and 5 nm.
The influence of hydrogen passivation and the contribution of trap states to the luminescence mechanism is discussed using the interdependency of the photoluminescence emission for the dominant Tb excited state (5D4 to 7F5) and the values of the S-parameter obtained from PAS. The values of the S- and W-parameters identify the traps at the interface between the amorphous silicon oxide host matrix and embedded nanocrystals.
Finally, the solubility level of Tb ions is extended using this novel processing method and an optimized Tb concentration for the most luminescent matrix is suggested. High-temperature annealing promoted the formation of a different Tb silicate crystal phase than the phases observed in samples with lower Tb content. A fiber texture with no preferred orientation in the plane but growth direction of <200> is observed [Fig. 2]; however, samples with lower Tb content are fully random and show no distinct texture. The correlation between the nanostructure and emission properties is an important step to fabricating Tb-doped silicon oxide materials which can be potentially used in a variety of light emitting applications such as displays, solar cells, optical communications, and other silicon optoelectronic devices.
Fig. 1 High-resolution transmission electron microscopy (HR-TEM) image of a focused ion beam (FIB) prepared Tb-doped ORSO thin film annealed at 1200 ℃ for 1 hour in N2 atmosphere.
Fig. 2 A fiber texture is evident by the ring of intensity shown in the pole figure and no preferred orientation in plane is observed.
Li, O. Zalloum, T. Roschuk, C. Heng, J. Wojcik, P. Mascher “The formation of light emitting cerium silicates in cerium-doped silicon oxides”, Appl. Phys. Lett. 94, 011112 (2009).
W. Miller, Z. Khatami, J. Wojcik, J. D. B. Bradley, P. Mascher “Integrated ECR-PECVD and magnetron sputtering system for rare-earth-doped Si-based materials”, Surf. Coat. Techn., 336, 99-105 (2018).