A comparative study of the vibrational and luminescence properties of embedded Ge nanocrystals prepared by ion implantation and sputter deposition methods: role of strain and defects
We present a comparative study of the vibrational and luminescent properties of Ge nanocrystals (NCs) prepared by two different techniques, namely ion implantation (process I) and radio frequency (RF) sputter deposition (process II). Ge NCs of size 6–13 nm are formed embedded in a SiOx matrix as a result of post-deposition annealing. Optical Raman studies reveal the presence of strain in the embedded Ge NCs in both cases. Polarization dependent low frequency Raman scattering studies show that process I yields NCs with surface symmetrical Raman modes only, whereas process II yields additional surface quadrupolar Raman modes. Photoluminescence (PL) studies using 488 nm excitation show a broad emission band that can be deconvolved into two peaks centered at ~535 nm and ~594 nm. PL studies on Ar implanted and similarly annealed SiOx layers confirm that the 535 nm and 594 nm emission bands originate from oxygen deficient defects (ODCs) in the SiOx matrix. PL studies with 325 nm excitation show additional strong peaks in the ultraviolet–blue region. Analyses of the PL spectra from various samples show that different PL emission bands are related to various forms of ODCs having different environments dependent on different defect formation processes. It is concluded that due to the poor quality of interface between NC and defective SiOx matrix, recombination at defects dominates the room temperature light emission from embedded Ge NCs, while light emission due to quantum-confined carriers in the NCs is quenched, perhaps due to inherent strain in the embedded NCs.