B. Prévot

Pulsed laser crystallization and doping for the fabrication of high-quality poly-Si TFTs

Abstract We review the various applications of pulsed lasers, working in the nanosecond regime, to prepare high-quality poly-Si TFTs. It is shown that the best device performances (field-effect mobilities in excess of 140 cm2/V·s) are achieved by pulsed excimer laser crystallization of unhydrogenated amorphous Si thin films. In addition, for source and drain formation, we demonstrate that the excimer laser induced diffusion of dopant from a solid source (spin-on phosphorus-doped silicate glass) is very attractive to achieve good electrical properties of the n-channel TFTs.

Pulsed laser crystallization of hydrogen-free a-Si thin films for high-mobility poly-Si TFT fabrication

The possibility to fabricate high-mobility polysilicon TFTs by nanosecond pulsed laser crystallization of unhydrogenated amorphous Si thin films has been investigated. Two types of lasers have been used: a large area (≈ 1 cm2) single ArF excimer laser pulse and a small diameter (≈ 100 μm) frequency-doubled Nd:YAG laser beam, working in the scanning regime. Processed films have been characterized in detail by different optical and microscopic techniques. Device performances indicate that the best results are achieved with the excimer laser leading to high mobility values (up to 140 cm2/Vs) which are much larger than in polysilicon TFTs fabricated onto the same quartz substrates by low-temperature thermal (630° C) crystallization of amorphous Si films (μfe≈55 cm2/Vs).

Experimental and numerical analysis of surface melt dynamics in 200 ns-excimer laser crystallization of a-Si films on glass

Abstract In this work, the surface melt dynamics of amorphous Si films on glass was investigated experimentally and numerically for long pulse duration (200 ns) excimer laser crystallization. The melting threshold, melt duration, depth of fusion and critical fluence corresponding to the SLG regime were characterized by time-resolved reflectivity measurements, scanning electron microscopy and Raman spectroscopy. The numerical analysis of the laser melting–solidification process, including the explosive recrystallization phenomenon, was demonstrated to be consistent with the experimental results.

Multiple-Phonon Resonant Raman Scattering in CuGaS2.

Resonant enhancements of multiple-phonon Raman scattering in CuGaS2 excited by a wide range of laser energy are studied. At 8.6 K, the intensities of the multiple-phonon modes show a sharp resonance maximum at the scattered-photon energy just below that of the free exciton. Polarization characteristics of these modes have the same selection rule as the free exciton. The behavior is attributed to the intraband Frohlich interaction for longitudinal E modes of polar phonons. The resonant scattering by this mechanism exhibits a temperature-dependent enhancement correlated with the thermal shift and broadening of the free exciton energy. On the other hand, the temperature dependence of the resonance is greatly decreased for the nonpolar A1 mode which has a large deformation potential.

Evolution of implanted carbon in silicon upon pulsed excimer laser annealing

Formation of epitaxial Si1−yCy substitutional alloy layers on monocrystalline silicon surfaces with y≊1 at. % is reported. The preparation method was carbon ion implantation, followed by KrF excimer laser annealing. Results of Rutherford backscattering (RBS), secondary ion mass spectrometry (SIMS) and infrared absorption analyses are compared. The authors concluded that, up to ∼1 at. % carbon content, the dominant process is nonequilibrium trapping of carbon in substitutional lattice sites upon fast resolidification. Above this concentration the complex carbon redistribution processes are influenced by silicon carbide precipitation in the melt and segregation effects in the near‐surface region.

Preparation of Si1−xGex thin crystalline films by pulsed excimer laser annealing of heavily Ge implanted Si

Abstract Thin crystalline Si 1−x Ge x layers were obtained by irradiating heavily Ge implanted (10 17 atoms cm −2 ) 〈100〉 oriented Si substrates with a pulsed excimer (ArF) laser. The respective influences of the implantation conditions, laser energy density or number of successive laser pulses were investigated using Rutherford backscattering channeling analysis and Raman spectroscopy. In particular, it is shown that layers of good crystalline quality can be readily obtained, the width almost constant Ge concentration being controlled by the irradiation conditions. Optimizing these conditions leads to 200 nm thick alloy layers with a constant Ge content x = 0.14. These results were interpreted using a computer simulation based on the melting-resolidification process which occur during the pulsed laser irradiations.

Pulsed excimer and nd-yag laser crystallization of a-si-h - the specific role of hydrogen

Abstract We have investigated the possibility of preparing good-quality polycrystalline silicon by laser crystallization of hydrogenated and non-hydrogenated amorphous silicon thin films deposited onto glass substrates. The morphology and surface roughness of the crystallized layer is shown to be a function of the laser fluence and especially of the hydrogen content for both pulsed excimer and scanned Nd:YAG lasers used in this study.

Shallow beryllium implantation in GaAs annealed by rapid thermal annealing

Very shallow (40 nm) ion implants of Be in GaAs have been annealed with no noticeable redistribution using rapid thermal annealing (RTA). Differential Hall effect measurements, Raman Scattering, and spectroscopic ellipsometry have shown that a peak concentration of 3×1019 cm−3 is achieved, with an electrical activation of 80% and crystalline quality similar to unimplanted material. These results are interpreted in terms of diffusing (interstitial) atoms gettered by defects not yet annealed when electrical activation occurs in the RTA process.

Spectroscopic ellipsometry and Raman scattering study of the annealing behavior of Be‐implanted GaAs

The annealing of the lattice damage induced by Be‐ion implantation in GaAs has been studied by spectroscopic ellipsometry and Raman scattering after each step of an isochronal thermal treatment. These two optical (i.e., nondestructive) techniques are shown to be very sensitive both to the lattice recovery and to the electrical activation of the implants. It has been observed that the latter process occurs after the lattice perfection recovery at T=550 °C, while for the ultimate annealing temperatures (T>750 °C) a significant Be redistribution takes place resulting in a decrease of the electrical activity which is confirmed by differential Hall effect measurements.

Formation of poly-Si1−xGex using excimer-laser processing

Abstract Polycrystalline silicon-germanium (poly-SiGe) films can be an attractive alternative to poly-Si in several technologies like TFTs where thermal budget allowances may be limited. In this work, we investigate the formation of poly-Si 1− x Ge x films by using a XeCl pulsed excimer laser to induce either alloying (PLIA) or crystallization (PLIC) processes. In the first case, the starting material consisted of an amorphous Ge layer electron-evaporated onto poly-Si grown on quartz substrates. Both the Ge mole fraction and the alloyed junction depth can be controlled by varying the laser energy fluence and/or the number of laser pulses. As an example, poly-Si 1− x Ge x layers with x = 0.3 can be easily obtained by this technique. In the second case, thin amorphous Si/Ge layers, prepared by successive evaporations of the two elements onto oxidized Si substrates, were submitted to the XeCl laser irradiation and the resulting films were investigated using the TEM and Raman spectroscopy techniques. They showed that the processed layers (120 nm thick) are polycrystalline with grains as large as 200 nm and an almost equal Si and Ge content.