Ultrafast Optical Properties of Dense Electron Gas in Silicon Nanostructures

Abstract

We investigate the ultrafast dynamics of carriers in a silicon nanostructure by performing spectrally resolved femtosecond spectroscopy measurements with a supercontinuum probe. The nanostructure consists of a 158-nm-thick crystalline Si layer on top of which a SiO2 passivation layer leads to a very high quality of the Si surface. In addition, a dielectric function approach, including contributions from a Drude part and interband transitions, combined with the Transition Matrix Approximation is used to model the photogenerated carrier dynamics. The spectrotemporal reflectivity reveals two mechanisms. First, an electron–hole plasma is created by the pump pulse and lasts for a few picoseconds. Importantly, its spectral signature is either a positive or a negative change of reflectivity, depending on the probe wavelength. This is complementary to the already reported results obtained with degenerate frequency measurements. The second mechanism is a thermal diffusion of carriers which occurs during several hundreds of picoseconds. The overall dynamics at short and long delays in the whole visible spectrum is well explained with our model which shows that the main contribution to the reflectivity dynamics is due to the Drude dielectric function. The observation of this predominance of free carriers requires both a long lived high density of carriers as well as a little influence of surface scattering as provided by our thin crystalline Si layer with passivated Si/SiO2 interface.

Carrier dynamics in silicon nanowires studied via femtosecond transient optical spectroscopy from 1.1 to 3.5 eV

“…(a) and 3(c)) and 4.5 eV (Figure 3(b) and 3(d)). The broad features around 2.5 eV reaches a negative maximum intensity much faster around 0.3-0.55 ps and change sign after 1-2 ps depending on the probe energy, similarly to what is observed in the transient reflectivity in the visible region in high quality thin filmSi layers[17]. Then, the positive signals persist for longer than 900 ps.…”

“…In the UV range, a negative signal is observed in the whole time range, particularly intense in the 1-10 ps region. Si layers [17]. Then, the positive signals persist for longer than 900 ps.…”

Effects of surface nanostructuring and impurity doping on ultrafast carrier dynamics of silicon photovoltaic cells: a pump-probe study

“…Optical pump-THz probe technique was firstly proposed in 1990s. During past decades [17,[20][21][22][23][24][25][26][27][28][29], this approach has been extensively used to study the nonequilibrium dynamical processes of photoexcited semiconductor systems. In these experiments, fundamental physical properties such as carrier scattering times, frequency-dependent complex dielectric response functions and photoconductivity, along with photo-excited carrier densities, were extracted from the THz signal as a function of time delay between the optical pump and the THz probe pulses.…”