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Infrared photoluminescence from holmium ions in single-crystal silicon and holmium oxide

Infrared photoluminescence (PL) has been studied in Ho-doped single-crystal silicon structures fabricated by solid-phase epitaxy and in crystalline holmium oxide in the temperature range from 4.2 to 300 K. PL arises from the internal 4f-shell transitions in Ho3+ ions. The maximum intensity of the PL associated with the transitions from the first excited state (5I7) to the ground state (5I8) in Ho-implanted silicon is observed at ~1.96 µm. Oxygen co-implantation enhances the intensity by an order of magnitude at 77 K. The luminescence intensity in Si:Ho:O structures increases by an order of magnitude upon 15 min annealing at 900 °C. The temperature quenching of the Ho-related PL is similar to that for Er-related PL in single-crystal Si: the intensity decreases by a factor of 25 on raising the temperature from 4.2 to 150 K, and above 130 K the activation energy is ~160 meV. Significant differences are observed between the PL spectra from Ho-doped silicon and holmium-oxide: (i) in addition to the PL lines related to the 5I7→5I8 transitions, there are lines due to transitions from the second excited state of Ho3+ ions (5I6) to the ground state (5I8) in Ho2O3; (ii) for the 5I7→5I8 transitions the positions of the PL lines with a maximum intensity differ for Ho2O3 and Ho-doped silicon; and (iii) the PL intensity in Si:Ho:O is lower only by a factor of ~3, as compared with that in Ho2O3, despite the Ho concentration being ~4 orders of magnitude lower in the former. The second distinction is due to the crystal field splitting of the Ho3+ 4f-states.