Abstract
We present a study of the surface effects and optical properties of the self-assembled nanostructures comprised of vertically aligned 5 nm-diameter Al nanowires embedded in an amorphous Si matrix (a-Si:Al). The controlled (partial) removal of Al nanowires in a selective etching process yielded nanoporous a-Si media with a variable effective surface area. Different spectroscopy techniques, such as X-ray photoelectron spectroscopy (XPS), UV-Vis spectrophotometry and photoluminescence (PL), have been combined to investigate the impact of such nanostructuring on optical absorption and emission properties. We also examine long-term exposure to air ambient and show that increasing level of surface oxidation determines the oxide defect-related nature of the dominant PL emission from the nanoporous structures. The role of bulk, nanosize and surface effects in optical properties has been separated and quantified, providing a better understanding of the potential of such nanoporous a-Si:Al structures for future device developments.
Highlights
Silicon (Si) nanostructures are the focus of extensive research activities seeking to circumvent the fundamental limitation set by the indirect bandgap of Si that hinders many potential applications, in optoelectronics
The removal of Al NWs leaves behind hollow channels that are prone to surface oxidation once exposed to ambient air, as is observed for porous crystalline Si (c-Si) [24]
It is expected that X-ray photoelectron spectroscopy (XPS) provides information about the first 10 nm of the porous film, the results are not affected by a difference in Al concentration between the two porous samples, as the Al concentration is larger towards the substrate
Summary
Silicon (Si) nanostructures are the focus of extensive research activities seeking to circumvent the fundamental limitation set by the indirect bandgap of Si that hinders many potential applications, in optoelectronics. The significant developments in Si nanostructuring demonstrated in recent years indicate a gradual shift of focus from nanosize-related (quantum confinement) phenomena to surface/interface-related effects, including a variety of interactions with adsorbents, impurities, defects, etc. The nanostructuring of a-Si can be realized, for instance, by introducing a high-density nanopore network, as will be demonstrated in the present work. Such a nanoporous a-Si potentially represents a complementary constituent for all-Si based optical communication and can be envisaged as an emerging material in gas and biomedical sensing. The upsurge of surface-to-bulk ratio in nanoporous media makes surface-related effects the dominating factor in light interaction with matter, directly affecting optical scattering (Rayleigh and Mie), absorption and emission properties, and providing unique opportunities for novel optoelectronic and sensor applications
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