Abstract:We present femtosecond transient transmission (or absorbance) measurements in silicon nanowires in the energy range 1.1-3.5 eV, from below the indirect band-gap to above the direct band-gap. Our pumpprobe measurements allow us to give a complete picture of the carrier dynamics in silicon. In this way we perform an experimental study with a spectral completeness that lacks in the whole literature on carrier dynamics in silicon. A particular emphasis is given to the dynamics of the transient absorbance at the en… Show more
“…The peculiar behaviour of the optical transient reflectivity near the singularities in Si, Ge, and GaAs is clearly observed in present experimental data of Fig. 2 and compare well with similar results obtained in transient absorbance measurements of ZnSe and Si nanowires 8 , 11 . Figure 3 Time evolution of for Si (left panel), Ge (center panel) and GaAs (right-hand panel).…”
Section: Resultssupporting
confidence: 91%
“…www.nature.com/scientificreports/ The peculiar behaviour of the optical transient reflectivity near the E 1 singularities in Si, Ge, and GaAs is clearly observed in present experimental data of Fig. 2 and compare well with similar results obtained in transient absorbance measurements of ZnSe and Si nanowires 8,11 .…”
Section: And Refs Therein)supporting
confidence: 90%
“…Hardware and software of the spectrometer are conceived to reduce the amount of data to be stored for such time-resolved experiments, and in our case data for the normalized reflectivity were collected and arranged into an array for each time and wavelength in the selected time domain (typically from to 300 picoseconds for present experiments). Further experimental details on the ultrafast spectroscopy setup can be found in previous works 8 , 11 , 31 .…”
Section: Methodsmentioning
confidence: 99%
“…therein). Studies of the ultrafast carrier dynamics have been carried out for simple semiconductors 1,[4][5][6][7] and nano-sized systems relevant for devices and applications (see for example [8][9][10][11] and refs. therein).…”
Ultrafast optical reflectivity measurements of silicon, germanium, and gallium arsenide have been carried out using an advanced set-up providing intense subpicosecond pulses (35 fs FWHM, $$\lambda $$
λ
= 400 nm) as a pump and broadband 340–780 nm ultrafast pulses as a white supercontinuum probe. Measurements have been performed for selected pump fluence conditions below the damage thresholds, that were carefully characterized. The obtained fluence damage thresholds are 30, 20.8, 9.6 mJ/$$\hbox {cm}^2$$
cm
2
for Si, Ge and GaAs respectively. Ultrafast reflectivity patterns show clear differences in the Si, Ge, and GaAs trends both for the wavelength and time dependences. Important changes were observed near the wavelength regions corresponding to the $$E_1$$
E
1
, $$E_1+\Delta $$
E
1
+
Δ
singularities in the joint density of states, so related to the peculiar band structure of the three systems. For Ge, ultrafast reflectivity spectra were also collected at low temperature (down to 80 K) showing a shift of the characteristic doublet peak around 2.23 eV and a reduction of the recovery times.
“…The peculiar behaviour of the optical transient reflectivity near the singularities in Si, Ge, and GaAs is clearly observed in present experimental data of Fig. 2 and compare well with similar results obtained in transient absorbance measurements of ZnSe and Si nanowires 8 , 11 . Figure 3 Time evolution of for Si (left panel), Ge (center panel) and GaAs (right-hand panel).…”
Section: Resultssupporting
confidence: 91%
“…www.nature.com/scientificreports/ The peculiar behaviour of the optical transient reflectivity near the E 1 singularities in Si, Ge, and GaAs is clearly observed in present experimental data of Fig. 2 and compare well with similar results obtained in transient absorbance measurements of ZnSe and Si nanowires 8,11 .…”
Section: And Refs Therein)supporting
confidence: 90%
“…Hardware and software of the spectrometer are conceived to reduce the amount of data to be stored for such time-resolved experiments, and in our case data for the normalized reflectivity were collected and arranged into an array for each time and wavelength in the selected time domain (typically from to 300 picoseconds for present experiments). Further experimental details on the ultrafast spectroscopy setup can be found in previous works 8 , 11 , 31 .…”
Section: Methodsmentioning
confidence: 99%
“…therein). Studies of the ultrafast carrier dynamics have been carried out for simple semiconductors 1,[4][5][6][7] and nano-sized systems relevant for devices and applications (see for example [8][9][10][11] and refs. therein).…”
Ultrafast optical reflectivity measurements of silicon, germanium, and gallium arsenide have been carried out using an advanced set-up providing intense subpicosecond pulses (35 fs FWHM, $$\lambda $$
λ
= 400 nm) as a pump and broadband 340–780 nm ultrafast pulses as a white supercontinuum probe. Measurements have been performed for selected pump fluence conditions below the damage thresholds, that were carefully characterized. The obtained fluence damage thresholds are 30, 20.8, 9.6 mJ/$$\hbox {cm}^2$$
cm
2
for Si, Ge and GaAs respectively. Ultrafast reflectivity patterns show clear differences in the Si, Ge, and GaAs trends both for the wavelength and time dependences. Important changes were observed near the wavelength regions corresponding to the $$E_1$$
E
1
, $$E_1+\Delta $$
E
1
+
Δ
singularities in the joint density of states, so related to the peculiar band structure of the three systems. For Ge, ultrafast reflectivity spectra were also collected at low temperature (down to 80 K) showing a shift of the characteristic doublet peak around 2.23 eV and a reduction of the recovery times.
“…This agreement supports our attribution of the highenergy peak to the -X (indirect) transition. As indirect band gap transitions do not show a sharp absorption edge, a clear absorption bleaching peak is generally not observed for indirect transitions, see, e.g., the case of Si [33]. In our case of the ternary GaAsP alloy, the relatively strong absorption bleaching peak observed at the -X (indirect) transition is made possible by the disorder induced by the local potential fluctuations of the alloy composition that break down the translational symmetry of the crystal Hamiltonian.…”
Semiconductor nanowires (NWs) often present different structural and opto-electronic properties than their thin film counterparts. The thinner they are, the larger these differences are. Here, we present femtosecond transient absorbance measurements on GaAs 0.8 P 0.2 NWs of two different diameters, 36 and 51 nm. The results show that thinner NWs sustain a higher carrier temperature for longer times than thicker NWs. This observation suggests that, in thinner NWs, the buildup of a hot-phonon bottleneck is easier than in thicker NWs because of the increased phonon scattering at the NW sidewalls, which facilitates the buildup of a large phonon density. Moreover, the important observation that the carrier temperature in thin NWs is higher than in thick NWs already at the beginning of the hot carrier regime suggests that the phonon-mediated scattering processes in the nonthermal regime play a major role at least for the carrier densities investigated here (8 × 10 18 to 4 × 10 19 cm -3 ). Our results also suggest that the phonon scattering at crystal defects is negligible compared with the phonon scattering at the NW sidewalls.
In this study, the electronic band structure and the dynamics of the excited carriers in the formamidinium lead bromide (FAPbBr3) perovskite are investigated by combining the information obtained from steady‐state absorption, photoluminescence, femtosecond transient absorption spectroscopy, photoelectron spectroscopy, and density functional theory calculations. A detailed description of the electronic transitions in the UV–vis energy range has been provided, giving an estimation of the exciton binding energy (40 ± 5 meV), the transition energy from the first valence band (VB1) to the conduction band (CB1) (electronic bandgap at 2.37 ± 0.01 eV) and assigning the broad peak at ≈3.4 eV to the transition from the second innermost valence band (VB2) to CB1. The temporal dynamics of the excited carriers involved in these transitions are investigated using different excitation energies and carrier densities. Different trends are observed in the dynamics of the transient signals associated with the VB1→CB1 (PB1) and VB2→CB1 (PB2) transitions. As the carrier density increased, PB1 exhibited a slowing down of its rise time, while PB2 showed an acceleration attributed to the thermalization of the excited holes in VB2. These valuable findings have the potential to unlock new strategies aimed at maximizing the efficiencies and performance of perovskite‐based solar cell devices.
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