Showing papers by "Philippe M. Fauchet published in 1989"
TL;DR: In this article, the duration and coherence time of ultrashort laser pulses can be measured by a transient grating technique, where a self-diffracted signal is recorded as a function of delay between two identical pulses that induce the grating in an absorbing medium.
32 citations
TL;DR: In this article, femtosecond laser spectroscopy has been applied to study the dynamics of carriers in the neighborhood of the mobility edge in hydrogenated amorphous silicon.
Abstract: The techniques of femtosecond laser spectroscopy have been applied to study the dynamics of carriers in the neighborhood of the mobility edge in hydrogenated amorphous silicon. Just after injection of a dense electron–hole plasma with pump photons above the mobility edge, the index of refraction decreases by as much as 2%, and the induced absorption is as large as 104 cm−1. Ten picoseconds later, intraband absorption becomes negligible and the temperature rise dominates the optical properties through the band-gap shrinkage, which produces an increase in the index of refraction and an increase in the interband absorption. The recovery takes place with two characteristic times, ~1 psec and ~10 psec. Possible interpretations of these times are presented.
28 citations
TL;DR: In this paper, the authors proposed a spectral filter to simplify the intracavity shaping mechanisms in such a way that the nonlinear Schrodinger equation becomes a good approximation for pulse generation in the laser.
Abstract: The generation of solitons and other related periodic pulse evolutions in a passively mode-locked dye laser is controlled by adjustment of group velocity dispersion, self-phase modulation, and spectral filtering. Without spectral filtering, periodic pulse evolutions reminiscent of higher-order solitons are observed. The pulses differ from the classic solitons because of additional shaping mechanisms. With spectral filtering, pulses are generated that can be described analytically as true asymmetric N=2 solitons. The introduction of the filter appears to simplify the intracavity shaping mechanisms in such a way that the nonlinear Schrodinger equation becomes a good approximation for pulse generation in the laser. The remarkable stability achieved allows for accurate characterization and control. >
17 citations
TL;DR: In this paper, 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.
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.
16 citations
TL;DR: In this paper, a method for enhancing the sensitivity of time-resolved reflectivity measurements in semiconductors is discussed, by appropriate choice of the angle of incidence near Brewster's angle, the photoinduced reflectivity changes are easily increased by one order of magnitude.
Abstract: A method for enhancing the sensitivity of time-resolved reflectivity measurements in semiconductors is discussed. By appropriate choice of the angle of incidence near Brewster's angle, the photoinduced reflectivity changes are easily increased by one order of magnitude. Two examples, the lifetime of a dense electron-hole plasma and the picosecond laser-induced melting transition in silicon, illustrate the method. Extension to the case of thin films is possible, as demonstrated by experiments performed on thin polycrystalline silicon films. Possible complications due to the use of very tightly focused beams and ultrashort pulses near a strong resonance are considered and, together with other considerations of a more experimental nature, lead to general guidelines for signal-enhancement and easy implementation and data analysis. >
15 citations
TL;DR: In this article, the growth of amorphous (a-Si:H,F) and microcrystalline (μc-Si) silicon over trench patterns in crystalline silicon substrates was studied.
Abstract: We studied the growth of amorphous (a-Si:H,F) and of microcrystalline (μc-Si) silicon over trench patterns in crystalline silicon substrates. All deposited films show elbows at the trench mouth and are uniformly thick on the trench walls. Therefore, surface diffusion is not important. The results of a Monte-Carlo simulation suggest that film growth is controlled by a single growth species with a low sticking coefficient plus a highly reactive etching species.
15 citations
TL;DR: In this article, the crystallinity of films deposited over a range of substrate temperatures and SiF4/H2 flow ratios was studied by Raman spectroscopy and the boundary between microcrystalline and amorphous Si was determined.
Abstract: a-Si and μc-Si were grown from SiF4 with H2 dilution in a DC glow discharge. The crystallinity of films deposited over a range of substrate temperatures and SiF4/H2 flow ratios was studied by Raman spectroscopy and the boundary between microcrystalline and amorphous Si was determined. We find that μc-Si can be grown from SiF4 with less H2 dilution than from SiH4. In the SiF4/H2 system, the etching by of F atoms appears responsible for μc-growth; H atoms play an important role in balancing growth and etching reactions.
14 citations
TL;DR: In this paper, the etching of Si:H by H radicals in Hg-sensitized photo-CVD was studied as a function of deposition temperature, etch temperature and film hydrogen content.
Abstract: The etching of a-Si:H by H radicals in Hg-sensitized photo-CVD is studied as a function of deposition temperature, etch temperature and film hydrogen content. For films deposited at the same temperature, etch rates increased as etch temperature decreased from 230°C to 100°C and were temperature independent for temperatures between 100°C and 40°C. Etch rate increased with decreasing deposition temperature, but did not change when hydrogen content was varied by changing pressure and dilution. Under conditions producing high etch rates, microcrystalline films of Si:B:H and SiGe:H were prepared at temperatures as low as 100°C.
14 citations
TL;DR: In this paper, the authors measured the optical properties of free carriers in a-Si:H by pump and probe femtosecond spectroscopy and derived the upper bound for the mobility of free electrons approximately 100 meV above the mobility edge.
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.
13 citations
TL;DR: The second order Raman spectra are more sensitive to microcrystalline effects than first-order spectra as mentioned in this paper, and they offer the potential to measure crystal sizes greater than a few hundred angstroms but much work remains to be done to quantify the size dependence of the second order spectra.
Abstract: Raman scattering is becoming a widely used tool for the characterization of semiconductor microcrystals due to its sensitivity to crystal sizes below a few hundred angstroms. Through detailed analysis of the first order Raman spectrum it is possible to determine the size and shape of microcrystalline grains. First order spectra must be examined with care however, since they are sensitive to other factors including: stress/strain, surface vibrations, mixed amorphous/microcrystalline phases and intragrain defects. Second order Raman spectra are more sensitive to microcrystalline effects than first order spectra. They offer the potential to measure crystal sizes greater than a few hundred angstroms but much work remains to be done to quantify the size dependence of the second order spectra.
12 citations
TL;DR: The remarkable stability achieved allows for accurate characterization and control of solitons and solitonlike pulses in a passively mode-locked dye laser by adjustment of group-velocity dispersion, self-phase modulation, and spectral filtering.
Abstract: We control the generation of solitons and solitonlike pulses in a passively mode-locked dye laser by adjustment of group-velocity dispersion, self-phase modulation, and spectral filtering. Without spectral filtering, periodic pulse shaping reminiscent of higher-order solitons is observed. The pulses differ from the classic solitons because of additional shaping mechanisms. With spectral filtering, pulses are generated that can be described analytically as asymmetric N = 2 solitons. The results indicate an effect analogous to the soliton self-frequency shift observed in optical fibers. The remarkable stability achieved allows for accurate characterization and control.
TL;DR: In this article, the transport properties of intrinsic micro-crystalline and amorphous SiGe films are investigated and it is found that the electron lifetime is relatively independent of band gag while the electron mobility of a-Ge:H is a factor of ten less than that of a -Si:H.
Abstract: The transport properties of intrinsic micro-crystalline and amorphous SiGe films are investigated. In the amorphous system it is found that the electron lifetime is relatively independent of band gag while the electron mobility of a-Ge:H is a factor of ten less than that of a-Si:H. Micro-crystalline silicon films have greater excess photo-conductivity as compared to a-Si:H predominantly due to longer effective lifetimes. Films containing micro-crystalline silicon in a matrix of a-SiGe:H have been prepared and are found to have comparatively poor transport.
TL;DR: In this paper, the authors reported the first observation and full characterization of the weak photoluminescence signal at and above the Tauc gap in a-Si:H.
Abstract: We report the first observation and full characterization of the weak photoluminescence signal at and above the Tauc gap in a-Si:H. Detailed spectral dependence on temperature (from 10 K to 500 K), Tauc gap, deep defect density and laser power density is reported. The origin of the signal is discussed in terms of the carrier dynamics.
TL;DR: In this article, the optical absorption and electrical conductivity of microcrystalline Si films were studied in terms of a grain boundary model, showing that with increasing hydrogen content in the films, the dark conductivity decreases strongly and activation of the conductivity increases.
Abstract: Microcrystalline Si was grown from SiF 4 and H 2 by plasma-enhanced chemical vapor deposition. The films are almost completely crystalline with a crystallite size (determined from Raman spectra) of about 60 A. The optical absorption and the electrical conductivity of these films were studied. With increasing hydrogen content in the films, the dark conductivity decreases strongly and the activation of the conductivity increases. We explain the conductivity qualitatively in terms of a grain boundary model.
05 Jul 1989
TL;DR: In this paper, the relationship between structural, electronic, and optical properties of thin films made of alloys of Si, Ge, and C is investigated with Raman spectroscopy and other, mostly non-optical, techniques.
Abstract: The properties of thin films made of alloys of Si, Ge, and C are investigated with Raman spectroscopy and other, mostly non-optical, techniques. The focus is on the relationship between structural, electronic. and optical properties. Three topics are investigated: the microcrystallinity in A thick (> 1000 A) doped SiC films, the influence of the substrate on the properties of thin (< 1000 A ) films, and possible heterogeneities in SiGe alloys. This study is the first step towards a better understanding of the relationship between the growth parameters and the electronic and optical properties that are useful in devices.
TL;DR: A detailed study of the growth of amorphous hydrogenated fluorinated silicon (a-Si:H, F) from a DC glow discharge in SiF/sub 4/ and H/sub 2/ is discussed in this paper.
Abstract: A detailed study of the growth of amorphous hydrogenated fluorinated silicon (a-Si:H, F) from a DC glow discharge in SiF/sub 4/ and H/sub 2/ is discussed. The electrical properties of the films can be varied over a very wide range. The bulk properties of the best films that were measured included an Urbach energy E/sub u/=43 meV, a deep-level defect density N/sub s/=1.5*10/sup 15/ cm/sup -3/, and a hole drift mobility of 8*10/sup -3/ cm/sup 2/ V/sup -1/ s/sup -1/, which reflects a characteristic valence band energy of 36 meV. It was found that E/sub u/, N/sub s/, and the density of surface states N/sub ss/ are related to each other. Under the deposition condition of the films with the best bulk properties, N/sub ss/ reaches its highest value of 1*10/sup 14/ cm/sup -2/. It is suggested that in growth from SiF/sub 4//H/sub 2/, the density of dangling bonds at the growing surface is very sensitive to the deposition conditions. >