D. Hulin

The properties of free carriers in amorphous silicon

The properties of free carriers photogenerated in the extended states of hydrogenated amorphous silicon have been investigated using the techniques of femtosecond time-resolved spectroscopy. The optical susceptibility of free carriers can be described by a Drude model with a relaxation time shorter than 1 fs. This relaxation time seems to remain constant for various experimental situations. It implies a very small mobility (∼ 6 cm 2 /V s) in the extended states. The hot carriers thermalize quickly to the mobility edge by emission of phonons. The thermalization rate is found to be ≥ 1 eV/ps. In addition, the decay time of optic phonons into acoustic phonons is ≤ 100 fs. The photogenerated carriers recombine non-radiatively in a time that can be as short as 1 ps at very large injected density ( N ≤ 10 21 cm −3 ). Several regimes are distinguished, depending on the value of N . In general, the characteristic times for all these processes are much shorter in a-Si:H than in a typical direct-gap crystalline semiconductor such as GaAs. The difference can be traced to the lack of momentum conservation in amorphous semiconductors.

Free‐carrier and temperature effects in amorphous silicon thin films

The electronic and thermal contributions to photoinduced changes in the optical properties of hydrogenated amorphous silicon (a‐Si:H) films can be distinguished in pump‐probe experiments by an appropriate choice of the probe wavelength. Intraband absorption decreases strongly with carrier localization.

Femtosecond spectroscopic determination of the properties of free carriers in a-Si:H

Abstract High intensity femtosecond pulses have been used to inject free carriers in the extended states of a-Si:H where they are maintained for a time of the order of one picosecond. Their optical properties are measured by pump and probe femtosecond spectroscopy. We find that the momentum scattering time that enters in the Drude model and in the expression for the mobility is less than one femtosecond for electrons. The upper bound for the mobility of electrons above the mobility edge is 6 cm2/V sec.

Femtosecond optical spectroscopy in a-Si:H and its alloys

Abstract The ultrafast dynamics of photogenerated carriers is monitored at room temperature by femtosecond pump-and-probe optical spectroscopy in undoped a-Si:H and its alloy with C. We find that above 5×1019cm−3 the recombination is bimolecular and non-radiative, with a coefficient B=3.5×10−9cm3sec−1 and below 5×10−10cm−3, it is monomolecular, with a time constant τ = 7 psec.

On-off resonance femtosecond non-linear absorption of chalcogenide glassy films

Abstract Non-linear absorption of chalcogenide thin films has been investigated with femtosecond pump-probe experiments. Different peculiarities of femtosecond pulse absorption have been found in the case of on- and off-resonance pumping and probing, due to either intraband or interband transitions. The physical mechanisms which can explain such different behaviours are discussed.

Femtosecond thermalization processes in a-Si:H

Abstract We report measurements in a-Si:H of the free carrier thermalization rate and the lattice heating rate following energy transfer from non-radiative carrier recombination. Both processes appear to be more rapid than in crystalline materials.

Extended state mobility in a-Si:H measured by femtosecond spectroscopy

Abstract We have measured the optical properties of free carriers in a-Si:H by pump and probe femtosecond spectroscopy. The Drude model is found to be applicable and the momentum scattering time that also enters in the expression for the mobility of the extended state carriers is of the order 0.6 femtosecond for electrons. The upper bound for the mobility of free electrons approximately 100 meV above the mobility edge is 6 cm 2 /V sec.

The mechanism of subnanosecond carrier recombination in a-Si:H

We report on the non-radiative picosecond recombination of photogenerated (free or trapped) carriers in a-Si:H and alloys. The mechanism is bimolecular multiphonon, with a limited number of recombining states. The recombination efficiency of tail states is discussed.

Femtosecond spectroscopy in amorphous silicon and silicon-germanium alloys

Abstract The methods of femtosecond optical spectroscopy have been used to study the ultrafast electronic processes in undoped alloys of amorphous hydrogenated silicon prepared by rf glow discharge. The transient changes in reflectivity and transmission are studied as a function of wavelength, carrier density and alloy composition. The results are explained by taking into account hot carrier relaxation in the extended states, trapping into weakly localized bandtail states and thermalization in those bandtail states.

Ultrafast Scattering Times in Amorphous Silicon

Disorder in amorphous semiconductors leads to unusual and interesting optical properties. Using femtosecond time-resolved spectroscopy, we find that the scattering time of carriers in the extended states is subfemtosecond and that the electronic susceptibility is well described by a Drude model. These free carriers recombine non-radiatively in a few picoseconds and the increase in lattice temperature follows the carrier recombination with no measurable delay.