F. Vega

Plasma expansion dynamics in reactive and inert atmospheres during laser ablation of Bi(2)Sr(2)Ca(1)Cu(2)O(7−y)

The dynamics of the species ejected by excimer laser ablation of a BiSrCaCuO target in different gas environments is studied by spatially resolved, real‐time optical emission spectroscopy. The evolution of the velocity and the emission intensity of the excited species versus the distance and the pressure of the foreign gas present a similar behavior both in reactive (oxygen) and inert (argon) environments. Furthermore, the results show that the plume expansion process is dominated by the interaction of the ejected species and the gas background atoms or molecules through collisional interactions rather than by reactions in the gas phase. The dynamics of the plume expansion is analyzed in the frame of the shock wave and drag models, the latter leading to a very good agreement with the experimental results and the dependence of the slowing coefficient with the gas pressure is established. Finally, the influence of the excitation energy of the considered transition on the observed emission features is discussed.

Real time optical diagnostics of the plume dynamics during laser ablation of germanium in an oxygen environment

The dynamics of the gas phase induced by excimer laser ablation of Ge is investigated by analyzing the light emitted by the plume. Space and time‐resolved optical spectroscopy measurements indicate the presence of both, neutral (Gei) and ionized (Geii) species. Two populations of neutrals with different velocities, which are related to the neutral atoms directly ejected from the target and those which are produced by recombination of ions, are observed. The velocities of the species remain unchanged for oxygen pressures up to 1 mbar, which suggest that the expansion of the plume occurs without further collisions with the foreign gas in this pressure range. The spectral emission characteristics are consistent with plume dynamics initiated by charged species.  The dynamics of the gas phase induced by excimer laser ablation of Ge is investigated by analyzing the light emitted by the plume. Space and time‐resolved optical spectroscopy measurements indicate the presence of both, neutral (Gei) and ionized (Geii) species. Two populations of neutrals with different velocities, which are related to the neutral atoms directly ejected from the target and those which are produced by recombination of ions, are observed. The velocities of the species remain unchanged for oxygen pressures up to 1 mbar, which suggest that the expansion of the plume occurs without further collisions with the foreign gas in this pressure range. The spectral emission characteristics are consistent with plume dynamics initiated by charged species.

Shifting of the thermal properties of amorphous germanium films upon relaxation and crystallization

The modification of the thermal conductivity and melting temperature of unrelaxed amorphous Ge films on Si substrates upon laser-induced relaxation and crystallization is presented. Real-Time Reflectivity (RTR) measurements are used to determine experimentally both the melting threshold and the melt durations, and the finite element method is used to simulate the laser-induced heat-flow process. A thermal conductivity ofk=0.010 W dem K is determined for the unrelaxed material by fitting the experimental melting thresholds of unrelaxed films of different thicknesses. A similar procedure applied to the amorphous relaxed and crystallized materials lead to a shift to higher values of both the thermal conductivity and the melting temperature. In order to achieve a good fit of the experimental melt durations, it was necessary to assume a large degree of undercooling prior to solidification. The role of undercooling in the solidification process is finally discussed in terms of its dependence on the faser energy density and the high thermal conductivity of the substrate.

RELAXATION AND CRYSTALLIZATION KINETICS OF AMORPHOUS GERMANIUM FILMS BY NANOSECOND LASER PULSES

Relaxation and crystallization of amorphous germanium films on silicon are induced by nanosecond laser pulses. Real time reflectivity measurements and Raman spectroscopy show that amorphous regrowth occurs upon melting and rapid solidification of the film because the thermal conductivity of the silicon substrate is high enough to extract the laser energy absorbed by the film in a very efficient way. The amorphous regrown film is in a relaxed state when compared to the as‐grown amorphous material. Further pulses induce fast crystallization of the film. An increase of the melting threshold is found upon relaxation and crystallization of the film.Relaxation and crystallization of amorphous germanium films on silicon are induced by nanosecond laser pulses. Real time reflectivity measurements and Raman spectroscopy show that amorphous regrowth occurs upon melting and rapid solidification of the film because the thermal conductivity of the silicon substrate is high enough to extract the laser energy absorbed by the film in a very efficient way. The amorphous regrown film is in a relaxed state when compared to the as‐grown amorphous material. Further pulses induce fast crystallization of the film. An increase of the melting threshold is found upon relaxation and crystallization of the film.

Correlation between optical properties, composition, and deposition parameters in pulsed laser deposited LiNbO3 films

The correlation between the optical properties and composition of lithium niobate films is investigated in films deposited by laser ablation of single‐crystal LiNbO3 and Li enriched LiNbO3 sintered targets under different oxygen pressures and laser fluences. The results show that the variation of the film refractive index is in many cases related to the deposition process itself, rather than to a variation of the film composition. The experimental conditions to produce stoichiometric films with optical properties similar to those of the bulk material are discussed.

Plasma properties and stoichiometry of laser-deposited BiSrCaCuO thin films

Abstract The composition of BiSrCaCuO films deposited by laser ablation in different gas environments (Ar or O 2 ) is related to the interaction processes observed in the plasma by means of optical emission spectroscopy. It is found that films deposited in Ar or O 2 exhibit an increase of Bi and a decrease of Sr, Ca and Cu contents with respect to films deposited in vacuum. These changes in composition are discussed in terms of the processes that may affect the film growth such as collisional interaction processes in the gas phase and re-sputtering.

Optical properties of Sb and SbOx films

The optical constants of Sb and SbOx films grown by d.c. sputtering are determined over a wide spectral (270–870 nm) and composition (x = 0.12−2.0) range. Spectroscopic ellipsometry is used to determine the film optical constants. The composition of the films is obtained by combining nuclear reaction analysis and Rutherford backscattering spectrometry. Optical properties of the films change drastically at a critical oxygen content (x = 0.19) which is related to the transition from a crystalline Sb-like film with optical properties similar to crystalline Sb to an amorphous Sb-like film. It will also be shown that amorphous SbOx films behave optically as a mixture of amorphous Sb-like material and a stoichiometric Sb oxide (Sb2O4, x = 2.0).

Optical properties of GeOx films obtained by laser deposition and dc sputtering in a reactive atmosphere

Optical constants of amorphous GeO(x) films as a function of wavelength are determined for the first time, to the best of our knowledge, in thin films grown by laser deposition and dc sputtering of Ge in an oxygen environment. We determined the oxygen content of the films by combining nuclear reaction analysis and Rutherford backscattering spectrometry. Spectroscopic ellipsometry is used to determine the film optical constants. Effective medium modeling is used to simulate the optical properties of the films assuming the films contain a mixture of amorphous Ge and GeO(2). The results show that substoichiometric GeO(x) films behave optically as a mixture of amorphous Ge and GeO(2). Films with low oxygen content (x < 1.0) seem to have inhomogeneous oxygen concentrations with depth. The effect of the deposition rate and oxygen pressure (and Ar pressure in sputtered films) on film stoichiometry and optical properties is also discussed.

Lithium niobate films grown by excimer laser deposition

Abstract Laser ablation of single-crystal LiNbO 3 targets is performed by means of an ArF laser. Films are grown onto glass, quartz and carbon-coated mica substrates held at room temperature. Amorphous and particulate-free films with the refractive index and optical energy gap close to those of bulk single-crystal LiNbO 3 are obtained depending on the oxygen pressure applied during deposition and the laser fluence. Optical diagnostics of the laser induced plume show that the complex dynamics followed by lithium species in the gas phase are responsible for the variation of the film refractive index as a function of the laser fluence.

Laser ablation of Ge in an oxygen environment: plasma and film properties

Abstract The growth of Ge oxide films by laser ablation of Ge in an oxygen atmosphere is reported for the first time. Time and space resolved optical measurements of the light emitted by the laser induced plume are performed in order to analyze the plasma reactivity. It is found that the velocity of the plasma species is not influenced by the presence of oxygen up to pressures of 10 -3 mbar, whereas the oxygen content of the films grown in this oxygen pressure range is always below 40%. This low oxygen content is discussed and compared to that obtained with other reactive deposition techniques. In the range of pressures studied, the stoichiometry of the films seems to be mainly determined by the ratio of Ge and oxygen atoms impinging on the substrates rather than by reactions in the plume.