We report direct excitation of optically active Er centers in porous Si. Excitation spectroscopy performed close to the intracenter 4I15/2→4I11/2 and 4I15/2→4I9/2 transitions of Er3+ (4f11) ions allows us to identify two kinds of Er centers in porous Si: (i) Er diffused into porous nanograins with lower than cubic symmetry and (ii) Er centers incorporated in an amorphous silicalike matrix. The latter show much weaker thermal quenching of the Er3+ emission which decreases only by a factor of eight when the temperature is increased from 4.2 K up to 360 K.
Stepikhova, M., Jantsch, W., Kocher, G., Palmetshofer, L., & Schoisswohl, M. (1997). Direct Excitation Spectroscopy of Er Centers in Porous silicon. Applied Physics Letters, 71(20), 2975-2977. https://doi.org/10.1063/1.120234