Optical and Structural Properties of CaF 2 :Nd Films on Si-Based Substrates
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OPTICAL AND STRUCTURAL PROPERTIES OF CaF2:Nd FILMS ON Si-BASED SUBSTRATES C.-C. CHO, W.M. DUNCAN and H.-Y. LIU Central Research Laboratories, Texas Instruments, P.O. Box 655936, MS 147, Dallas, Texas 75265. ABSTRACT By thermally evaporating CaF2 and NdF3, we have grown Nd-doped CaF2 films on Si(l 11), AI/Si(l 11) and quarter-wavelength Ta2Os/SiO2 multilayer Bragg reflectors. The optical and structural properties of the CaF2:Nd films are characterized by photoluminescence spectroscopy (PL) and x-ray diffraction. The effects of different Nd concentration, growth temperatures and post-annealing were studied. Regardless of the substrates, the as-grown films show emission lines at wavelengths similar to bulk CaF2 :Nd. Annealing the films at 700'C in forming gas results in a new emission pattern. Little difference between the PL spectra of polycrystalline and single crystal CaF2 :Nd films is observed, indicating that the luminescence efficiency is insensitive to the crystalline quality of the films. INTRODUCTION Since CaF2 can be grown epitaxially on Si, and vice versa, the epitaxial growth and the electrical properties of CaF2 on Si-based substrates have been widely studied for their possible applications in high speed electronics devices. 1,2 In addition to their usage in electronics devices, several recent papers have reported the luminescence properties of 3 6 rare earth doped fluorides - and discussed the feasibility of employing these materials as the optical sources for Si-based optoelectronics. Optical devices made of these materials may also provide light sources at wavelengths where semiconductor lasers do not exist or perform poorly. These studies have shown that rare earth ions can be incorporated into epitaxial 4 6 3 6 fluoride films by using molecular beam epitaxy (MBE). - For Nd-doped CaF2 films, strong emission intensities with luminescence at wavelengths similar to bulk CaF2:Nd were observed. In contrast to the low quenching concentration, charge compensation and Nd aggregation problems observed in bulk CaF 2 :Nd, high Nd concentrations are obtainable in the MBE grown films with less Nd aggregation and no charge compensation. In this paper, we report the effects of post-annealing, different growth temperatures and crystalline quality on the photoluminescence (PL) properties of these films. The PL spectra from three different substrates, i.e., Si(l 11), AI(l 11) and Ta205/SiO2, were studied, and found to exhibit similar emission patterns but with Mat. Res. Soc. Symp. Proc. Vol. 301. ©1993 Materials Research Society
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0 0 different intensities. Growing the films at various temperatures from 100 C to 700 C did not change the emission pattern either. In contrast, when the films were annealed at 700'C
in forming gas after they were deposited, the PL spectra of the post-annealed films are very different from the PL spectra of the as-grown films. By comparing the films grown at different temperatures, we found the crystalline quality of the films do not noticeably affect their PL properties. EXPERIMENT Th
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