Slide 1: University of Cyprus Research Center of Ultrafast Science Ultrafast Carrier Dynamics in Si nanocrystals Department of Physics University of Cyprus and IMEL/NCSR Demokritos Surface-related states in oxidized silicon nanocrystals enhance carrier relaxation and inhibit Auger recombination Andreas Othonos, Emmanouil Lioudakis, A. G. Nassiopoulou http://ultrafast.ucy.ac.cy/ Slide 2: Initial Relaxation in a Photoexcited Semiconductor When a semiconductor in thermodynamic equilibrium is excited by an ultrashort pulse, it undergoes several stages of relaxation before it returns to the thermodynamic equilibrium. Slide 3: What do we expect to observe on short time scales ? Following above band gap excitation and the generation of carriers some of the factors that will affect the dielectric function of the material are The induced changes in the dielectric function can be detected as changes in Time-Resolved Reflectivity and Transmission measurements following ultrashort pulse excitation. Slide 4: Pump-Probe Experimental Techniques Time resolved measurements with femtosecond resolution may be accomplished using pump-probe techniques. A simplified pump-probe reflection setup is shown in the diagram below The time evolution is investigated by varying the time delay Δt between the pump and the probe pulses Slide 5: Short Pulsed Amplifier Laser System The source of excitation in our pump-probe experiments is an Ultrafast Amplifier Laser System. A schematic of the laser system is shown below. Slide 6: Pump-probe Reflectivity and Transmission Measurements Slide 7: Time-resolved absorption measurements State filling Free carrier absorption/
Photo-induced absorption 5 nm ultrathin nanocrystalline silicon film Substrate - transparent quartz Oxidation at 900oC 2.8 nm size nanocrystals Slide 8: Time-resolved absorption in 2.8 nm Si nanocrystals Degenerate measurements over a broad range of excitations Contributions to the Induced absorption
• State-filling (negative)
• Free-carrier absorption (positive) SF is the main contribution for photon energies between 4.27 eV and 3.42 eV, where direct excitation occurs. Slide 9: Time-resolved absorption in 5 nm polysilicon thin film Degenerate measurements over a broad range of excitations Similar behavior with the nanocrystals
Longer recovery Slide 10: Non-degenerate absorption in 2.8 nm Si nanocrystals Non-Degenerate measurements at 4.12 eV (300 nm) Slide 11: Intensity measurements – Auger recombination Intensity measurements Auger recombination is negligible Photoexcitation of the Si-NCs with 4.13 eV femtosecond pulses and probing with 2.75 eV photons.
The fluence shown corresponds to an estimated carrier generation of 20, 10, 2 and 1 carriers/NC. Slide 12: Conclusions There is a non trivial contribution from; free carriers, state filling, carrier recombination to Transient Reflectivity and Transmission depending on the probing position on the bandgap intensity measurements revealed a linear dependence of the photoinduced absorption signal on the photon flux suggesting that Auger recombination does not play a significant role in these nanostructures for fluences generating up to 20 carriers/NC. We have investigated femtosecond carrier dynamics in oxidized 2.8 nm Si-NCs using transient degenerate and non-degenerate absorption measurements