Microstructural and photoluminescence characterisation of germanium and silicon-germanium nanocrystalline materials

Abstract

The discovery of the strong room temperature visible photoluminescence (PL) emission from porous Si in 1990 has been the catalyst for much of the recent study on the visible PL emitting semiconductor nanocrystalline materials. Silicon, an indirect bandgap semiconductor, in the form of nanoparticles is thought to emit strong visible light due to quantum confinement effects and, in the near future, will replace GaAs (and the other direct bandgap III-IV semiconductors) as for the light emitting devices such as lasers. On the other hand, mainly due to its much larger exciton Bohr radius, Ge, in the form of nanocrystals, is expected show more pronounced quantum confinement effects compared to Si nanocrystals. SiGe alloys also constitute a more attractive material than Si in terms of both industrial applications and fundamental research: the lifetime of the 'porous Si-like' PL of porous SiGe is observed to be approximately two orders of magnitude faster than that of porous Si. Moreover, the bandgap of Si-Ge alloys can be intentionally varied between those of pure Si and Ge via the alloy composition. In this study, an investigation has been made of the microstructural properties of visible PL Group IV nanostructures (SiGe and Ge) that have been rather much less studied in the literature, for example, in comparison to Si nanocrystals. For the first time in the literature the confinement of phonons in SiGe nanocrystals has been shown, in anodised porous SiGe films, and variations in the film composition were estimated utilising Raman spectroscopy. Methods such as stain etching, ion-implantation, and spark processing, were employed to synthesise Ge nanostructures. Particle sizes were usually estimated by modelling the Raman spectra in line with a phonon confinement model. Properties of 2-10 nm Ge nanostructures, ranging in structure from partially amorphous to crystalline, and in various environments, e.g. oxide matrices, were studied. Typical PL spectra were observed from these samples. These spectra were determined to be originating either due to Ge nanocrystals or other chemical origins, such as defects in GeO sub x S or defects in host matrices (e.g. SiO sub 2 , GeO sub x). It is recommended that samples with a wider range of particle sizes must be prepared, preferably 'oxide-free', using the first two methods, and characterised optically from near UV to near IR in order to observe clearly the size dependence of the PL emission from Ge nanocrystals