Broadening Light Emission at the Telecommunications Wavelengths by Material Engineering of Rare-Earth Compounds on Silicon

Hiroo Omi and Takehiko Tawara*
Materials Science Laboratory, *Optical Science Laboratory

　Erbium silicates (e.g. Er₂SiO₅ and Er₂Si₂O₇) and erbium oxide (Er₂O₃) have great potential as optical amplifier materials in silicon photonics. Recent research has shown optical gain in waveguide erbium-doped amplifiers. However, the luminescence is limited by concentration quenching mechanisms, such as energy migration and up-conversion between Er³⁺ ions. Therefore, yttrium (Y), which has almost the same ionic radius as erbium (Y³⁺ of 0.9 Å, Er³⁺of 0.89 Å) and is optically inactive is incorporated to increase the luminescence efficiency of the Er ions in the silicates and oxides by forming Er_xY_2–xSiO₅ and Er_xY_2–xO₃. In addition, for further enhancement of the emission, ytterbium (Yb), with an ionic radius of 0.99 Å in Yb³⁺, has been incorporated in silicates and oxides. The incorporation of Yb effectively promotes the energy transfer from the Yb ions to Er ions, which results in high efficiency of the Er ion luminescence when pumped at a wavelength of 980 nm. In order for these materials to be used as gain media for optical amplifiers, they have to fulfill the requirement of broadband luminescence at the region of the C-band telecommunication transmission wavelengths (1530-1565 nm). Flat regions and broad peaks in the photoluminescence spectrum are required for broadband amplification. In this work, we grow Er_xYb_2–xSi₂O₇ and Er_xYb_2–xO₃ crystalline mixtures on Si(111) substrates by RF-sputtering of Er₂O₃, Yb₂O₃ on Si and subsequent annealing in Ar atmosphere, and characterize their optical properties [1].
　Thin films composed of polycrystalline Er_xYb_2–xO₃ grains and crystalline Er_xYb_2–xSi₂O₇layers were formed on a Si(111) substrate by RF-sputtering and subsequent thermal annealing in Ar gas ambient up to 1100^oC. The films were characterized by synchrotron radiation grazing incidence X-ray diffraction (Fig. 1), cross-sectional transmission microscopy, energy dispersive X-ray spectrometry and micro photoluminescence measurements. In the annealed film of 950^oC it is observed that the I_15/2 - I_13/2 Er³⁺ photoluminescence exhibits simultaneously maximum intensity and peak width at room temperature (Fig. 2). This effect satisfies the requirements for broadening the C-band of an optical amplifier on Si.


Fig. 1. θ–2θ X-ray powder diffraction pattern obtained at the incidence angle of 1.0^o from the samples (a) as-grown at room temperature and annealed at (b) 900 (c) 1000 and (d) 1100^oC in an Ar ambient. The X-ray wavelength was 0.124 nm.	Fig. 2. Photoluminescence spectra from samples annealed at T_a = 900, 950, 1000, and 1100^oC, obtained at 300 K with excitation wavelength at 980 nm.