A. Mourchid

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.

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.

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.

Ultrafast thermal nonlinearities in amorphous silicon

We report the observation in a-Si:H of a large thermal nonlinearity with a picosecond response time. The spectral dependence of the refractive and absorptive parts of this unusually fast thermal nonlinearity reveals that it arises from a picosecond nonradiative recombination of electrons and holes across the band gap. The optical and electronic processes that make this thermal nonlinearity possible and observable are discussed.

Hot Carrier Dynamics In Amorphous Semiconductors

We report novel experimental and theoretical results of our investigation of hot carrier dynamics in disordered materials. Femtosecond optical spectroscopy and cw photoluminescence appear to be very promising techniques for the study of carrier relaxation in the extended states and carrier trapping in the weakly localized states. In this paper, we focus on undoped a-Si:H.