Abstract
Laser pulse processing of surfaces and thin films is a useful tool for amorphous thin films crystallization, surface nanostructuring, phase transformation and modification of physical properties of thin films. Here we show the effects of nanostructuring produced at the surface and under the surface of amorphous GeTiO films through laser pulses using fluences of 10–30 mJ/cm2. The GeTiO films were obtained by RF magnetron sputtering with 50:50 initial atomic ratio of Ge:TiO2. Laser irradiation was performed by using the fourth harmonic (266 nm) of a Nd:YAG laser. The laser-induced nanostructuring results in two effects, the first one is the appearance of a wave-like topography at the film surface, with a periodicity of 200 nm and the second one is the structure modification of a layer under the film surface, at a depth that is related to the absorption length of the laser radiation. The periodicity of the wave-like relief is smaller than the laser wavelength. In the modified layer, the Ge atoms are segregated in spherical amorphous nanoparticles as a result of the fast diffusion of Ge atoms in the amorphous GeTiO matrix. The temperature estimation of the film surface during the laser pulses shows a maximum of about 500 °C, which is much lower than the melting temperature of the GeTiO matrix. GeO gas is formed at laser fluences higher than 20 mJ/cm2 and produces nanovoids in the laser-modified layer at the film surface. A glass transition at low temperatures could happen in the amorphous GeTiO film, which explains the formation of the wave-like topography. The very high Ge diffusivity during the laser pulse action, which is characteristic for liquids, cannot be reached in a viscous matrix. Our experiments show that the diffusivity of atomic and molecular species such as Ge and GeO is very much enhanced in the presence of the laser pulse field. Consequently, the fast diffusion drives the formation of amorphous Ge nanoparticles through the segregation of Ge atoms in the GeTiO matrix. The nanostructuring effects induced by the laser irradiation can be used in functionalizing the surface of the films.
Highlights
Laser pulse processing of surfaces and thin films is a useful tool for purposes such as the amorphous thin films crystallization [1,2,3,4,5,6], surface nanostructuring [7,8,9,10], laser-induced thin film dewetting [11,12], phase transformation and modification of physical properties of thin films [13,14,15,16]
The film structure remains amorphous after laser irradiation, but a small increase of the volume is observed at the film surface due to the laser pulse action
We investigated the effects of nanostructuring produced by the pulsed-laser irradiation at the surface and beneath the surface of nanoparticles beneath the film surface due to the fast diffusion of Ge atomic species
Summary
Laser pulse processing of surfaces and thin films is a useful tool for purposes such as the amorphous thin films crystallization [1,2,3,4,5,6], surface nanostructuring [7,8,9,10], laser-induced thin film dewetting [11,12], phase transformation and modification of physical properties of thin films [13,14,15,16]. If the irradiated film thickness is greater than the laser radiation absorption length, the laser annealing takes place only in a surface layer of the film, and a gradual modification of the nanostructure or a crystallization can be induced in the film [18,19]. The most important are the absorption coefficient of the laser radiation in the films and the heat diffusivity. Some of these parameters can be considered to be similar to those in bulk material, but in many cases of amorphous or multi-component films there are no corresponding bulk materials. High values of atomic diffusivity were observed for Ge diffusion in the case of laser crystallization annealing of SiGe amorphous film [18]
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