Ultraviolet and blue photoluminescence from sputter deposited Ge nanocrystals embedded in SiO2 matrix

Ge nanocrystals (NCs) embedded in silicon dioxide (SiO2) matrix are grown by radio-frequency magnetron sputtering and studied in order to understand the origin of ultraviolet (UV) and blue photoluminescence (PL) from the NC-SiO2 system. Ge NCs of diameter 7-8 nm are formed after postdeposition annealing, as confirmed by transmission electron microscopy and Raman scattering studies. Optical Raman studies indicate the presence of strain in the embedded Ge NCs. Polarization dependent low frequency Raman studies reveal surface symmetrical and surface quadrupolar acoustic phonon modes of Ge NCs. PL studies with 488 nm excitation shows a broad emission band peaked at ∼545 nm, which is attributed to oxygen deficient defects in the SiO2 matrix. PL studies with 325 nm excitation show additional strong peaks in the 377-400 nm region. Time resolved PL studies in the UV-blue range show double exponential decay dynamics in the nanosecond time scale, irrespective of the NC size. Comparative studies of PL emission from SiO2 layers with no Ge content and with Ge content show that the ∼400 nm PL emission is originated from a defective NC/SiO2 interface and the band is not unique to the presence of Ge. PL excitation spectroscopy measurements show large Stokes shift for the UV emission bands. We propose that the intense UV peaks at ∼377 nm is originated from the twofold coordinated silicon defect at the interface between NC and SiO2 matrix and it is not necessarily specific to the presence of Ge in the oxide matrix. It is believed that due to the influence of strain on the NCs and interface states, PL from quantum confined carriers may be partially quenched for the embedded Ge NCs