Intra-center and recombination luminescence of bismuth defects in fused and unfused amorphous silica fabricated by SPCVD

Abstract

Photoluminescence (PL) of bismuth doped silicon dioxide excited by UV excimer lasers (ArF — 193nm, KrF — 248nm) and a green light laser diode (532nm) is studied in a wide spectral band at temperatures ranging from 12 to 750K. Two types of samples are investigated: unfused, 100μm in thickness amorphous layer immediately deposited on the inner surface of silica substrate tube, and the same material after profusion resulted from tube collapsing to a rod by external heating. PL bands centered at 620–650nm, 820nm and 1400nm wavelengths are observed in both fused and unfused samples. Under excitation by the green laser diode decay time constants for 650nm (orange) and 1400nm (NIR) PL bands measured at room temperature amount 3μs and 600μs respectively. These rather long decay times point to partly forbidden intra-center electron transitions. PL intensities of the orange and NIR bands are not temperature dependent within 12–450K range. At higher temperatures the orange band manifests an intra-center thermal quenching, activation energy and frequency factor being 0.42±0.04eV and 5·109s−1 respectively, while intensity of the NIR band weakly depends on temperature up to 700K. Electron-hole recombination excitation mechanism is found to contribute to the orange, but not to the NIR PL band under UV laser pumping. The specific feature of orange PL excited by recombination is the magnitude of decay time constant being about milliseconds at temperature of 12K and decreasing with the temperature increase. Localized state ionization and two-photon absorption of intense UV light excite electron-hole pairs in silica host. Some bismuth defects serve as traps for the electrons, while holes transmute into self-trapped state thus generating self-trapped holes (STHs). Thermally activated escape of the STHs followed by their subsequent recombination with trapped electrons forms the mechanism to transfer the excitation to particular bismuth defects responsible for orange PL. At the same time no signature of the impact of such recombination process on the excitation of NIR PL is observed. This permits one to conclude that the nature of bismuth defects responsible for these two PL bands varies, and these two types of defects are available in both fused and infused silicon dioxides.