D. Peruško

Femtosecond laser-induced periodic surface structure on the Ti-based nanolayered thin films

Laser-induced periodic surface structures (LIPSSs) and chemical composition changes of Ti-based nanolayered thin films (Al/Ti, Ni/Ti) after femtosecond (fs) laser pulses action were studied. Irradiation is performed using linearly polarized Ti:Sapphire fs laser pulses of 40 fs pulse duration and 800 nm wavelength. The low spatial frequency LIPSS (LSFL), oriented perpendicular to the laser polarization with periods slightly lower than the irradiation wavelength, was typically formed at elevated laser fluences. On the contrary, high spatial frequency LIPSS (HSFL) with uniform period of 155 nm, parallel to the laser light polarization, appeared at low laser fluences, as well as in the wings of the Gaussian laser beam distribution for higher used fluence. LSFL formation was associated with the material ablation process and accompanied by the intense formation of nanoparticles, especially in the Ni/Ti system. The composition changes at the surface of both multilayer systems in the LSFL area indicated the intermixing between layers and the substrate. Concentration and distribution of all constitutive elements in the irradiated area with formed HSFLs were almost unchanged.

Treatment of WTi Contacts on silicon with low energy argon ions

Abstract Thin layers of WTi were deposited by ion sputtering on p- and n-type Si(111) substrates. The initial target composition is 90: 10 W: Ti, the thicknesses of the layers were 200 and 350 nm, and the layers were grown at a rate of 10 nm min −1 . After deposition, the contacts were bombarded by backsputtering with Ar + ions at 3.5 keV. The as-deposited layers from a surface barrier on both substrates (0.62 V on n-Si and 0.66 V on p-Si). During ion bombardment, to doses from 0.5 × 10 18 to 5 × 10 18 ions cm −2 , the barrier is reduced until the contacts become non-rectifying. The main effect during bombardment is sputtering and development of topography (scanning electron microscopy analysis), but there is also a shallow mixing at the interface (as shown by Rutherford backscattering and X-ray analyses). During annealing at 800 °C, pronounced metal-silicon interdiffusion is observed in bombarded specimens and not in unbombarded specimens. So-formed silicides renew the rectifying properties of WTi contacts, with an effective barrier height of 0.66–0.69 V on n-Si.

Ion Beam Modification of Al/Ti Multilayers

Aluminum/titanium multilayers were sputter deposited on (100) Si wafers and irradiated with 200 keV Ar+ and N ions. Irradiation fluences were in the range from 5 · 1015 to 4 · 1016 ions cm−2, for argon ions, and 1 · 1017 to 2 · 1017 ions cm−2 for nitrogen. The samples were analyzed by Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Nano-hardness measurements were performed by Vickers method. Irradiation with argon ions induces mixing between Ti and Al layers, which is most pronounced in the vicinity of the projected range of implanted species. TEM cross-section analysis shows significant growth of grain size with the increase of Ar ion fluence. Implantation with nitrogen ions, up to these high fluences, causes a full intermixing of Al/Ti layers, resulting in multilayered structures with different content of Al, Ti, and N. In the latter case, nitrogen ion irradiation, probably, induces the formation of nitrides of titanium, aluminum, and AlTi. Nano-hardness measurements show significant increase of hardness for all applied ion fluences.

Semi-transparent, conductive thin films of electrochemical exfoliated graphene

The electrochemical exfoliation of graphite to give one-atom-thick graphene with desirable properties is a green, cost-effective method for high-yield graphene production. This paper presents the results of electrochemical exfoliation of two different graphite precursors under an applied direct current voltage of +12 V. The used characterization techniques (elemental analysis, Fourier transform infrared spectroscopy, X-ray diffraction, X-photoelectron spectroscopy, Raman spectroscopy, field emission scanning electron microscopy and atomic force microscopy) showed that the exfoliated powder is highly functionalized with a low carbon/oxygen content that is similar to graphene oxide. The exfoliated graphene sheets dispersed in N,N′-dimethylformamide were deposited on ano-discs by vacuum filtration and transferred to glass ceramic substrates. The thermal annealing of the as-deposited films at 600 °C for 30 minutes resulted in an increase in the carbon/oxygen ratio by more than 3 fold and a decrease in the sheet resistance by 25%. The lowest values for the sheet resistance of the annealed graphene thin films were in the range of 0.32 ± 0.04 to 0.84 ± 0.1 kohm sq−1 depending on the graphite source that was used.

Monolayer graphene films through nickel catalyzed transformation of fullerol and graphene quantum dots: a Raman spectroscopy study

In this paper we present synthesis of monolayer graphene islands. These films are deposited through nickel catalyzed transformation of fullerol and graphene quantum dots. Carbon doped nickel films are produced by autocatalytic chemical deposition. Upon rapid thermal annealing, graphene films are formed. Different characterization techniques are applied: Raman spectroscopy, scanning electron and atomic force microscopy. Raman spectroscopy analysis confirmed the formation of monolayer graphene films. Microscopy analysis revealed formation of monolayer islands.

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm2 were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.

Arsenic ion beam induced mixing in Ta/Pd thin films on silicon

Abstract In this paper we present a study of atomic mixing in Ta/Pd bilayers deposited on silicon, induced by As+ ion implantation. The films were deposited by dc sputtering to thicknesses of 60 nm (Pd) and 50 nm (Ta) on n-(111)Si wafers. After deposition the samples were implanted with As+ ions at 250 keV and 450 keV, to doses from 1 × 1015 ions cm−2 to 1 × 1016 ions cm−2, at room temperature (RT). Thermal treatments of samples were performed in vacuum (1 × 10−5 mbar) at 400 ° C and at 600 ° C, for 20 min. Structural changes were analysed by Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). We have studied the effects of the ion ranges and doses on the reactions in films, and also the effects of additional thermal treatments. The results indicate a dependence of the mixing processes on the film thickness to the ion range ratio ( d R p ). The pronounced structural changes and intermixing of components at the Ta/Pd interface are due to the high value of FD, damage energy deposited by the incident ions at the interface. Post-implantation annealing at 600 ° C initiates the reaction of Ta with Si at the Pd2Si/Ta interface, enabled by rapid silicon diffusion through the Pd2Si phase, which has already formed during annealing at 400 °C.

A protective TiN barrier layer for Ti and Pd silicides

Abstract This paper presents a study of the possibility of using a thin TiN layer as an overcoating to prevent contamination during growth of Ti and Pd silicides. Thin films of Ti (100 nm) and Pd (220nm) were deposited by evaporation and sputtering on (111) n-Si, 0.5–1 kΩ cm substrates. The TiN films were deposited up to 50 nm thick, at 5 nm min −1 , by reactive ac sputtering. Samples were treated in a vacuum furnace, at 1×10 −5 mbar and at temperatures from 400–700°C. The films were analysed by RBS, SEM, X-ray diffraction and their sheet resistivity was measured. It was found that bare Ti-layers were considerably contaminated with oxygen during thermal treatment but, despite this, intermixing with silicon occurred above 600°C. Contamination was almost prevented by a thin TiN overlayer, which remained stable during annealing. Silicides started to grow at 600°C, as confirmed by resistivity measurements. Growth of Pd-silicides was not influenced by the TiN overlayer. The role of the protective layer in this case can be to prevent silicon which has diffused through Pd to be contaminated by residual gases, when a thicker TiN overlayer should be used because a pronounced surface topography is developed during growth of Pd-silicides.

Pulse deposition of Au on graphite

Abstract In this paper we present pulse deposition as a new method to favor the self-assembly of nanostructured materials. Nucleation and growth of gold thin layers deposited on highly oriented pyrolytic graphite substrate have been studied using scanning tunneling microscopy in air. Depositions were performed in a pulsed regime and, for comparison, continuously in time. Much higher cluster density and better arrangement of clusters are obtained for pulse deposition than for gold deposited continuously. The formation of the channel structure of the deposited layer is favored and achieved earlier. The results indicate that the number of surface defect sites increases with subsequent pulses, behavior attributed to the previously formed clusters taking on the role of surface defect sites.

Ion beam induced mixing in Pd thin films on silicon

Ion beam mixing of thin palladium layers (50–130 nm) on n-Si(111) substrates, induced by 250–450 keV arsenic ions has been studied. Implantation has been performed at room temperature (RT) to the doses from 0.5–1 × 1016 ions cm−2. Structural characterization included Rutherford backscattering spectrometry and X-ray diffraction analysis. Our results show that the thickness of the polycrystalline Pd2Si phase induced by ion bombardment is proportional to the square root of the dose. The high value of FD, the linear energy deposition density at the Pd/Si interface, indicates that the formation of Pd2Si phase is a diffusion limited process.