Abstract
AbstractHighly anisotropic resistivity surfaces are produced in indium tin oxide (ITO) films by nanoscale self‐organization upon irradiation with a fs‐laser beam operating at 1030 nm. Anisotropy is caused by the formation of laser‐induced periodic surface structures (LIPSS) extended over cm‐sized regions. Two types of optimized structures are observed. At high fluence, nearly complete ablation at the valleys of the LIPSS and strong ablation at their ridges lead to an insulating structure in the direction transverse to the LIPSS and conductive in the longitudinal one. A strong diminution of In content in the remaining material is then observed, leading to a longitudinal resistivity ρL ≈ 1.0 Ω·cm. At a lower fluence, the material at the LIPSS ridges remains essentially unmodified while partial ablation is observed at the valleys. The structures show a longitudinal conductivity two times higher than the transverse one, and a resistivity similar to that of the pristine ITO film (ρ ≈ 5 × 10−4 Ω·cm). A thorough characterization of these transparent structures is presented and discussed. The compositional changes induced as laser pulses accumulate, condition the LIPSS evolution and thus the result of the structuring process. Strategies to further improve the achieved anisotropic resistivity results are also provided.
Highlights
(ITO) films by nanoscale self-organization upon irradiation with a fs-laser beam operating at 1030 nm
We report the production of surfaces with high anisotropic resistivity in ITO films, reaching electrical insulation along one axis and electrical conductance in the transversal one
The laser repetition rate used in this case was 500 kHz and the laser beam polarization was perpendicular to the scan direction
Summary
Fluences above 1 J cm−2 lead to splitting of the LSF-LIPSS in structures showing Λ ≈ λ/2 (cf Figure S1, Supporting Information) or the coexistence of both Λ ≈ λ and Λ ≈ λ/2 periods (cf Figure S2c, Supporting Information) Such a splitting phenomenon has been reported in ITO films upon irradiation with both ps- and fs-laser pulses,[22–24] and attributed to progressive compositional[22] or surface roughness changes[24,25] during the structure evolution. The narrow width and large depth of the valleys in the LF structures lead to tip-angle related artifacts yielding artificially low values in the modulation depth derived from AFM measurements This value has been estimated to be ≈350 nm by using interferometric microscopy and SEM cross-section analysis as shown in the insets, while the thickness of the film underneath the valleys is ≈150 nm (cf Figure S3, Supporting Information). The AFM measurements of the LF structures show the presence of a sub-wavelength pattern (Λ ≈ 400 nm) aligned
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