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Melting and rapid solidification is induced in 50-nm-thick amorphous Ge films on glass substrates by single laser pulses at 583 nm with a duration of 10 ps. The solidification process is followed by means of reflectivity measurements with ns time resolution both at the air/film (front) and the substrate/film (back) interfaces. Due to interference effects between the light reflected at the film-substrate and film-liquid interfaces, the back side reflectivity measurements turn out to be very sensitive to the melt depth induced by the laser pulse and their comparison to optical simulations enables the determination of the solidification dynamics. For low fluences, only a thin layer of the film is melted and solidification occurs interfacially leading to reamorphization of the molten material. The results provide a critical interface velocity for amorphization of ∼4 m/s, much slower than the one that has widely been reported for elementary semiconductors. For high fluences, the molten layer depth approaches the film thickness and the results are consistent with a bulk solidification process. In this case, recalescence effects upon solid phase nucleation become important and lead to the formation of crystallites distributed throughout the whole resolidified volume. © 1998 American Institute of Physics.

Structural relaxation phenomena in the ps time scale have been observed upon pulsed laser irradiation of (GeSb)O amorphous films. The transformation of the irradiated surfaces has been temporally resolved by means of in situ optical techniques with ns and ps resolution. The results show the occurrence of structural relaxation phenomena in a time shorter than 600 ps in the as-deposited material or about 200 ps in thermally relaxed samples. In the latter case, a substantial amount of crystalline material is nucleated at the initial stage of the transformation. © 1998 American Institute of Physics.

Time resolved imaging has been used to analyze structural transformations induced by intense 100 fs laser pulses in amorphous GeSb films. Above a threshold of 19 mJ/cm2 the data show the formation of a transient nonequilibrium state of the excited material within 300 fs. The results are consistent with an electronically induced, amorphous-to-crystalline phase transition.

Matrix assisted laser desorption/ionisation (MALDI) mass spectrometry has been used to study the formation of metallic nanoclusters by pulsed laser deposition (PLD). Cu nanoclusters were deposited by ablating a Cu target, both in Ar and vacuum. MALDI analysis leads to well defined peaks corresponding to masses in the range 400-500 amu and therefore to clusters of 6-8 Cu atoms, the presence of an Ar pressure during deposition having no significant influence on the size of the clusters. High resolution electron transmission microscopy observation of similar specimens shows the presence of clusters of thousands of amu (diameters of 1.5 nm to 4 nm). The discrepancies between the two techniques are discussed in terms of both their applicability ranges and the differences in the quality of the surface of the substrates used to apply each technique. © 1998 Elsevier Science B.V.

Reversible phase transitions between the amorhphous and the crystalline states have been induced in Sb0.93Ge0.07 thin films by irradiation with picosecond laser pulses of different energy densities. It is shown for the first time that a material can be repetitively cycled between both states by means of ultrashort pulsed laser irradiation. The phase transformation involves optical contrast thus demonstrating that erasable phase change optical storage driven by ultrashort laser pulses is feasible.

Melting and rapid solidification has been induced in amorphous Ge films by irradiation with nano-and picosecond laser pulses. The degree of structural relaxation of the rapidly solidified amorphous material has been investigated both by determining the minimum fluence required for surface melting and by means of Raman spectroscopy. The results evidence that the degree of relaxation of the rapidly solidified material is controlled by several parameters such as the duration and fluence of the laser pulse, and the thermal conductivity of the substrate, all of them affecting the supercooling prior to solidification. It is demonstrated that both relaxation and derelaxation can be induced if the above mentioned parameters are properly selected. The degree of relaxation is observed to decrease as the supercooling increases. © 1997 American Institute of Physics.

Rapid solidification phenomena have been studied in amorphous germanium films on silicon substrates by means of real time reflectivity measurements performed during irradiation with nanosecond laser pulses. The influence of the thermal response of the film/substrate system has been investigated by comparing the behavior of films with thicknesses in the range of 30-180 nm. Two different solidification scenarios are observed depending on the ratio between film thickness (d) and the thermal diffusion length (l) of amorphous germanium (l≈80 nm for 12 ns laser pulses). In the thinner films (d

Melting has been induced in amorphous Ge films upon irradiation with both nano- and pico-second laser pulses. The role of undercooling and heat flow in the subsequent rapid solidification process has been investigated by analyzing the behavior of films with different thicknesses (30-180 nm) grown on Si(100) substrates by means of real time reflectivity measurements in the ns timescale. Recalescence is observed in films with a thickness above a threshold value which depends on the pulse duration. An additional solidification scenario, i.e. surface initiated solidification, is observed upon ps pulse irradiation in films with intermediate thicknesses.

Nanocomposite films comprising metal Cu nanocrystals embedded in an Al2O3 matrix were deposited by alternating pulsed laser ablation from metallic Cu and ceramic Al2O3 targets. The films were grown in vacuum on glass substrates held at room temperature. The as-grown films contain 4 nm Cu nanocrystals in an amorphous Al2O3 matrix, with a total thickness of 190 nm. The films show a substantial third-order susceptibility with an electronic nonlinear refractive index of (2.93 ± 1.08) · 10–10 cm2 W – 1 and a nonlinear saturation of –(2.34 ± 0.18) · 10–5 cm W – 1.

Melting and rapid solidification are induced in amorphous Ge films upon irradiation with 10 ps laser pulses at 583 nm. The role of heat flow during the solidification process was investigated by comparing the behavior of films grown on substrates with different thermal properties. The melting and solidification kinetics are followed in real time by reflectivity measurements in the nanosecond time scale and the induced structural changes are analyzed by means of Raman spectroscopy in micro-Raman configuration. If the thermal diffusivity of the substrate is high enough, the film reamorphizes via bulk nucleation of the amorphous phase from the melt. When the thermal diffusivity of the substrate is reduced, the initial nucleation of the solid phase leads to an increase in the liquid temperature (recalescence) and in the melt duration, thus promoting the formation of the crystalline phase. © 1996 American Institute of Physics.

Planarization of large grain diamond films has been induced by 193 nm excimer laser irradiation. Secondary emission images and cathodoluminescence (CL) in the scanning electron microscope have been used to characterize the irradiated area. Irradiation causes changes in the structure of defects involving nitrogen and vacancies. Evolution of the CL signal with the number of pulses indicates that the luminescence intensity tends to stabilize when a smooth film surface is obtained. © 1996 American Institute of Physics.

We report an experimental investigation of self-starting operation of picosecond and femtosecond Kerr-lens mode-locked Ti : sapphire lasers. Pulses as short as 5 ps duration have been generated from self-starting lasers with no intracavity prisms to optimise group velocity dispersion. Self-starting dispersion-compensated lasers have generated pulses as short as 43 fs duration with mode-locking self-starting within 0.7 ms of the onset of laser action.

The energy density crystallization threshold of amorphous GeSb films has been studied for the first time as a function of the laser pulse duration in the range from 170 fs to 8 ns. The results obtained provide evidence of the existence of enhanced crystallization upon irradiation with pulses shorter than 800 fs, which is most likely related to electronic excitation effects.

The process of oxide removal in crystalline Ge using a pulsed ultraviolet laser has been studied by means of real-time reflectivity measurements with nanosecond resolution. The interaction of laser radiation with a clean, oxide-free surface has been characterized and the inhomogeneous and homogeneous energy density melting thresholds of c-Ge for 193 nm radiation have been determined. The values are 180 and 370 mJ/cm2, respectively. We have demonstrated that it is possible to remove an oxide overlayer by irradiation in vacuum and to produce a surface that shows the same response to laser radiation as a smooth, oxide-free, chemically cleaned surface. Under certain specific irradiation conditions it is even possible, after removing the oxide overlayer, to produce an enhanced crystalline quality in the near-surface region compared to that obtained upon chemical cleaning as evidenced by Rutherford backscattering/channeling measurements.

Structural relaxation processes are induced by pulsed laser irradiation in amorphous SbGe thin films with compositions richer in Sb than the eutectics. These processes are studied by means of real-time optical measurements and transmission electron microscopy analytical techniques. The influence of the laser pulse length and the film composition is analysed. The results show that the relaxed amorphous material exhibits different optical properties, density and lower-energy density threshold for crystallisation. A qualitative change in the relaxation mechanism is observed when the composition of the films is slightly changed.

The growth of heteroepitaxial Si0.8Ge0.2 films on Si(100) by a novel all laser-assisted technique using only ArF excimer laser radiation is demonstrated. Amorphous 30 nm thick films are grown by pulsed laser deposition from alternating pure Si and Ge targets on clean Si substrates. Melting and rapid solidification is then induced by pulsed irradiation (0.54 J/cm2), promoting epitaxial growth.

The crystallization process in SbxGe1-x alloy films has been observed during in situ annealing in a transmission electron microscope. Results are presented for two films with x=0.89 (89 at. % Sb) and x=0.71 (71 at. % Sb), which lie on either side of the eutectic composition (x=0.85). In the former films radial crystals are observed to grow rapidly from discrete nuclei, whereas in the latter films the crystallization process occurs through a near-planar front. In addition, quantitative data obtained from these experiments show that the Sb0.89Ge0.11 films have a higher activation energy for crystal growth and a lower temperature for the nucleation of crystals. Significant differences are observed between the crystallization processes for the two films studied, with the Sb0.89Ge0.11 film showing better potential for development as an ultrafast optical phase-change storage medium. © 1995 American Institute of Physics.

Ultraviolet (UV) laser pulses are used to irradiate c-Ge and c-Sb both in vacuum and in an oxygen pressure while a HeNe laser beam is used to record the real time reflectivity (RTR) transients. It will be shown that the removal of the native oxide and therefore surface cleaning is produced when irradiating in vacuum while an oxidation process occurs when irradiating in an oxygen environment. The optical coupling between the growing oxide layer with the bulk material underneath is responsible of the high sensitivity of the RTR measurements to the presence or transformation of small thickness oxide layers at the surface. The RTR transients can be used to optically control pulse-to-pulse the laser energy density in order to induce surface oxidation at a constant rate and to minimize material losses during the oxidation process. © 1994.

A systematic study is presented on the optical and structural transformations induced in amorphous Ge1-xSbx alloys with Sb contents in the 0.71≤x≤0.91 range by irradiation with 250 ns laser pulses. The results show that only those films richer in Sb than the eutectics (x=0.85) show appreciable optical contrast upon irradiation. The dominant role of Sb in the performances of this family of alloys is analyzed. The transition from a low-reflectivity amorphous phase to a high-reflectivity extended solid solution of Ge in crystalline Sb is shown to be the key for a new generation of fast reversible optical storage materials in which amorphous-crystalline cycling using ultrashort laser pulses is feasible.

Laser-induced oxidation of single-crystalline antimony and polycrystalline Sb films is studied. The samples are irradiated in an oxygen atmosphere (1.2 bar) by means of UltraViolet (UV) laser pulses while reflectivity measurements are used to monitor the oxidation process in real time. It is shown that there is an optical coupling between the growing oxide layer and the bulk material underneath which produces a dynamical change of the optical properties and leads to a non-constant growth rate. Depending on the laser energy density used, there is a critical oxide thickness above which a material-loss process starts, limiting the ultimate growth of the oxide layer. © 1994 Springer-Verlag.

Crystallization is triggered in Sb-rich GeSb and SbO amorphous films by irradiation with short (nanosecond) and ultrashort (picosecond) laser pulses. Crystallization is accompanied by optical contrast and the process ends within the nanosecond time scale, independently of the irradiation pulse length (nano- or picosecond). In GeSb alloys the observed process always involves melting and rapid solidification. The observed changes in the film optical properties both during and after laser irradiation are consistent with the formation of Sb crystals with no evidence of other crystalline phases. © 1993.

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.

Resonance ionization spectroscopy combined with time of flight mass spectrometry is used to analyze the ejection of ions and neutrals from the surface of a Ge target irradiated with ultraviolet XeCl excimer laser pulses. The ejection of ions and neutrals has been observed to occur at temperatures well below the melting point of the target and, therefore, through a nonthermal mechanism that may be dominant for fluences below the threshold for visible plasma formation. Within this regime, the velocities of the ejected neutrals in the ground state are in the order of 3×104 cm/s.

The dynamics of the plume produced during excimer laser deposition of germanium oxide films by excimer laser ablation of a Ge target in the presence of an oxygen atmosphere is studied by real time emission spectroscopy. It is found that the velocity of the species in the plume (both neutral and ionized) remains unaffected under the presence of an oxygen pressure ranging from 10-4 up to 1 mbar, whereas the oxygen content of the films increases with the oxygen pressure up to the stoichiometric value (GeO2) for pressures higher than 5 × 10-3 mbar. The spectral emission features show no evidence of oxidized species in the gas phase even at high oxygen pressures. It is then concluded that the incorporation of oxygen to the growing film occurs mainly by reactions at the substrate site. © 1993.

Time resolved reflectivity and transmission measurements with nanosecond time resolution are used to follow the evolution of ablation and material transport during Ar+ and ArF excimer laser induced blow-off of W and Cr films deposited onto glass substrates. The analysis of the optical transients combined with heat flow calculations provides evidence that solid phase material removal is the dominant mechanism for the ablation of W films, while Cr is ablated in the form of vapor and small molten droplets. © 1993.

Amorphous-to-crystalline and crystalline-to-amorphous transformations are triggered in GeSb thin films by irradiation with femtosecond and picosecond laser pulses. Phase changes are accompanied with optical contrast and therefore the feasibility of phase-change optical recording at ultrafast rates is demonstrated for the first time. The phase reversal by ultrashort pulses seems to be related to the dependence of the degree of undercooling prior to solidification on the irradiation energy density.

We have studied the particulates formed on single-phase copper oxide thin films obtained by laser deposition in a variable oxygen atmosphere. Two different targets have been used: a thick copper foil and a sintered copper powder. We conclude that there are two very different kinds of particulates. The first have very irregular morphology and very similar average size. They are due to solid debris coming from the surface target. The second kind of particulate observed has a round perimeter and is formed in the liquid phase. The morphology of the latter round particulates, which varies considerably, is dependent on the substrate and the oxygen content. Thus, we have been able to determine the conditions for growing particulate-free single-phase copper oxides. © 1992.

UV-laser-induced oxidation of crystalline Ge (c-Ge) is analyzed by means of real-time optical measurements. Irradiations are performed in a controlled oxygen atmosphere with laser fluences above the melting threshold. The melt duration determined from the reflectivity transients depends strongly on the oxygen pressure and on the number of laser pulses. The reflectivity of the material tends to decrease as the number of pulses is increased. The results indicate that the irradiation induces the growth of a surface oxide layer at a non-constant rate. The optical coupling between the oxide layer and the bulk material underneath at the irradiation wavelength explains the results and is found responsible for an enhancement of the oxide growth rate. © 1992.

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. © 1992.

This work reports a systematic study of the growth of copper oxide thin films by laser evaporation from a Cu foil or a Cu sintered powder target in an oxygen atmosphere. The experimental conditions to deposit Cu2O and CuO thin films are determined. Particle-free single-phase films are obtained from the foil target. The laser-deposited films have good optical properties. For comparison, DC-sputtered films deposited from the same Cu foil were studied. To obtain the same phase the former method requires higher oxygen pressure during deposition. The microstructure formed in the laser-deposited samples is always polycrystalline with a homogeneous grain size whereas sputtering can form columnar growth. © 1992.

Real time optical measurements are used to analyse two different kinds of phase changes which generate optical contrast in (In43Sb57)87Ge13 thin films. Amorphous to crystalline and amorphous to amorphous structural transformations are induced by pulsed laser irradiation in micron-sized regions. A two beam configuration is used to follow the evolution of the optical properties of the films in real time. It is shown that real time optical measurements provide a unique tool to analyse laser-induced fast structural transformations leading to optical contrast. Processes occurring via relaxation, solid state crystallization or melting-solidification are clearly distinguished. From the analysis of the optical transients the minimum transformation times are directly determined. © 1992 Springer-Verlag.

Time-resolved reflectivity (TRR) measurements are performed in crystalline Si under UV and visible wavelength irradiation. The former are carried out with ArF excimer laser pulses whereas the latter are performed in micron-sized areas irradiated with Ar+ laser pulses by means of a novel experimental setup. It is the first time that TRR measurements in the nanosecond regime are performed in micron-sized irradiated areas although they are very suitable to characterize processes in phase change optical storage and microelectronics applications. The energy density melting thresholds at both Ar+ and ArF laser wavelengths are determined. The reflectivity values obtained for pulse fluences just above the melting threshold show that melting proceeds inhomogeneously being the near-surface region formed by a mixture of solid and liquid phases without a well-defined interface. The comparison of the results obtained with uv and visible irradiation indicates that inhomogeneous melting is a general phenomenon which does not depend on the irradiation wavelength. It is present in the early stages of the melting process and its origin is related to the phase nucleation process itself. As the laser fluence is increased, the evolution of the melt duration exhibits a >different> behavior which is related to the formation of a homogeneous molten layer on top of the surface.

Pulsed laser irradiation is used to induce melting and rapid solidification of micron-sized areas of initially amorphous GeAl films. The kinetics of the laser-induced process are followed by using time-resolved optical techniques and the microstructures formed upon solidification are studied by transmission electron microscopy. The results show that, depending on the irradiation power, mainly amorphous or crystalline meta- stable phases are formed and conclusions related to the role of undercooling in their formation are achieved. The laser-induced amorphous phase is found to be unstable and its isothermal crystallization at room temperature is kinetically studied. It is found that crystallization is nucleated by aluminium and it is a diffusion-controlled proces with a decreasing nucleation rate. © 1990.

Time-resolved reflectivity (TRR) and transmission (TRT) measurements are used to characterize the influence of the thermal properties of the substrate on the processes induced in Si by laser irradiation. The irradiations and time-resolved optical measurements are performed in a pump and probe configuration with both beams focused on micron-sized areas. The results show changes in the solidification times and cooling rates of three orders of magnitude when selecting substrates with different thermal conductivities. It is also concluded that the microstructures induced in thin Si films by short laser pulses are different depending on the nature of the substrate and are consistent with the measured solidification parameters. © 1990.

Time-resolved optical measurements have been performed on micron-sized laser irradiated areas of GeAl thin films. The results show that transmission transients are suitable to calculate kinetical parameters of the melting-solidification process and that reflectivity transients depend strongly on the surface topography. In the present experimental conditions an amorphous phase together with an Al crystalline network is observed and an excellent agreement is found when comparing the calculated solidification parameters with the observed microstructures. © 1989.