Abstract
To exploit the optoelectronic properties of silicon nanostructures (SiNS) in real devices, it is fundamental to study the ultrafast processes involving the photogenerated charges separation, migration and lifetime after the optical excitation. Ultrafast time-resolved optical measurements provide such information. In the present paper, we report on the relaxation dynamics of photogenerated charge-carriers in ultrafine SiNS synthesized by means of inductively-coupled-plasma process. The carriers’ transient regime was characterized in high fluence regime by using a tunable pump photon energy and a broadband probe pulse with a photon energy ranging from 1.2 eV to 2.8 eV while varying the energy of the pump photons and their polarization. The SiNS consist of Si nanospheres and nanowires (NW) with a crystalline core embedded in a SiOx outer-shell. The NW inner core presents different typologies: long silicon nanowires (SiNW) characterized by a continuous core (with diameters between 2 nm and 15 nm and up to a few microns long), NW with disconnected fragments of SiNW (each fragment with a length down to a few nanometers), NW with a “chaplet-like” core and NW with core consisting of disconnected spherical Si nanocrystals. Most of these SiNS are asymmetric in shape. Our results reveal a photoabsorption (PA) channel for pump and probe parallel polarizations with a maximum around 2.6 eV, which can be associated to non-isotropic ultra-small SiNS and ascribed either to (i) electron absorption driven by the probe from some intermediate mid-gap states toward some empty state above the bottom of the conduction band or (ii) the Drude-like free-carrier presence induced by the direct-gap transition in the their band structure. Moreover, we pointed up the existence of a broadband and long-living photobleaching (PB) in the 1.2–2.0 eV energy range with a maximum intensity around 1.35 eV which could be associated to some oxygen related defect states present at the Si/SiOx interface. On the other hand, this wide spectral energy PB can be also due to both silicon oxide band-tail recombination and small Si nanostructure excitonic transition.
Highlights
Crystalline silicon is one of the most used material in electronics and photovoltaics
Our results reveal a photoabsorption (PA) channel for pump and probe parallel polarizations with a maximum around 2.6 eV, which can be associated to non-isotropic ultra-small silicon nanostructures (SiNS) and ascribed either to (i) electron absorption driven by the probe from some intermediate mid-gap states toward some empty state above the bottom of the conduction band or (ii) the Drude-like free-carrier presence induced by the direct-gap transition in the their band structure
Our results reveal a PA behavior for pump and probe parallel polarizations located between 2.4 and 2.8 eV, which can be associated to non-isotropic ultrafine quantum confinement (QC) SiNS and ascribed either (i) to the electron promotion from some mid-gap state Si/SiOx interface level to some empty level above the bottom of the conduction band or (ii) to the Drude-like free-carrier presence induced by the direct-gap transition in their band structure
Summary
Crystalline silicon is one of the most used material in electronics and photovoltaics. Because of its indirect bandgap and the conservation of momentum restriction, bulk silicon has inefficient bulk phonon-assisted emission and absorption of photons. Quantization-related effects increase the bandgap energy and produce a gradual relaxation of the momentum conservation [2], with the consequent increase of the probability of radiative excitonic emission, even if the transition may remain indirect. This means that QC SiNS promise to overcome the bulk crystalline Si poor optical properties due to its indirect bandgap which reduces the efficiency of light emission. Better understanding of the QC effects and charge carrier dynamics in SiNS is essential for the development of nano-silicon based optoelectronic devices
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