**3. Superior local conductivity in self-organized nanodots on indium-tin-oxide films induced by femtosecond laser pulses**

**Figure 2a**–**f** shows the surface morphology of ITO films after normal-incidence irradiation with different pulse numbers from 0 to 5 × 103 (fluence: 0.1 mJ/cm2 ). The film only exhibits few small dots for *N* = 5 × 103 shots (**Figure 2b**). Meanwhile, the laser-induced periodic structure is clearly observed on the surface of ITO films for *N* ≥ 2.5 × 104 shots (**Figure 2c**–**f**). In the inset of **Figure 2f**, the submicron ripple structure is composed of self-organized line dots (size: 20–500 nm). The periodic ripple pattern has long axis perpendicular to the direction of the laser polarization (the arrow in **Figure 2f**). The mixture of the dotted and ripple structures is attributed to the interference of scattered and diffracted waves at the grains and/or defects. This is similar to ordered YBCO array structures, formed by the solidification of melted dot patterns, under conditions of constructive interference and minimized surface energy [11, 19].

The present periodic ripple structures have spacing of approximately 798 ± 15, 420 ± 14, and 230 ± 15 nm. The formation of the two large ripple spacing cases (i.e., 798 ± 15 and 420 ± 14 nm) can be easily explained by classical scattering model [20]:

$$
\Lambda = \frac{\lambda}{1 \pm \sin \theta} \tag{1}
$$

where Λ is the ripple spacing, *λ* is the laser wavelength, and *θ* is the incident angle of the laser beam onto the target. However, the classical scattering model with Eq. (1) cannot be used to predict for the shorter ripple spacing of 230 ± 15 nm, which is much smaller than the laser wavelength of 800 nm. We noted that the spacing value is close to ∼200 nm, thus it could be induced by second harmonic generation (SHG) with a shorter wavelength of 400 nm around the surface of ITO film. It is reasonable for occurring SHG owing to the surface asymmetry when the subwavelength ripple with ∼200 nm is indeed observed in high intensity regions, especially in the center of laser Gaussian beam (**Figure 2f**).

**Figure 3** shows the effects of number of pulses (*N*) from 0 to 3 × 106 shots on the carrier concentration (*n*), carrier mobility (*μ*), and resistivity (*ρ*) of the ITO films. For *N* ≤ 103 , the laser-treated ITO films almost remain the same as that of the asdeposited ITO films. As further increasing *N* from 5 × 103 to 3 × 106 , the *n* increases noticeably from ∼1 × 1019 to ∼1.6 × 1019 cm−<sup>3</sup> . In contrast, the *μ* decreases correspondingly from 12.3 to 10.2 cm2 /V-s (i.e., 17% reduction), after fs laser annealing with 3 × 106 shots at a fluence of 0.1 mJ/cm2 . Reasonably, by using the Hall measurement, it is hard to detect a slight difference in *n* for *N* ≤ 103 . However, for larger *N* (>103 shots), the accumulated number of pulses induces the thicker laser-irradiated area for detecting the variation in *n* using the Hall measurement. Due to the aforementioned variation of *n* and *μ*, the resistivity presents a 14% reduction from 4.3 × 10<sup>−</sup><sup>2</sup> at *N* = 0 to 3.7 × 10<sup>−</sup><sup>2</sup> Ω-cm for *N* = 3 × 106 shots, which is primarily due to the increased *n*. Noticeably, the thickness the ITO films before and after irradiation are 30 ± 1.5 nm, and the thickness variation is not noticeable, and thus, the effect of thickness on the electrical properties can be eliminated.

The optical transmittance of the as-deposited ITO and fs-laser treated ITO films for 3 × 105 and 3 × 106 shots is shown in **Figure 3b**. Obviously, the optical transmittance of fs laser-treated ITO films remains about the same as that of the asdeposited ITO film, and they achieve a high transmittance of approximately 85% in the visible to near-infrared (NIR) range. The electrical and optical results indicate that fs laser annealing represents a new method to enhance the electrical properties of ITO films and retain their high optical transmittance.

To identify the cause of surface conductivity enhancement for fs laser-treated ITO films, we performed the X-ray photoelectron spectroscopy (XPS) experiments *Nanostructuring Indium-Tin-Oxide Thin Films by Femtosecond Laser Processing DOI: http://dx.doi.org/10.5772/intechopen.82790*

#### **Figure 2.**

*(a–f) SEM images of periodic surface structures induced by 800 nm fs laser pulses at a fluence of 0.1 mJ/cm2 and with various pulse numbers (N = 0, 5 × 103 , 2.5 × 104 , 1 × 105 , 3 × 105 , and 3 × 106 , respectively). The black-square inset shows the enlarged surface features at corresponding locations in (f). The arrow indicates the direction of the laser polarization [11].*

on the as-deposited and fs laser-treated ITO films in comparison with In2O3 powder. **Figure 4a** shows Donley's model of surface composition of an ITO film, which has In2O3-like sites, oxygen-deficient sites, and hydroxide and oxyhydroxide groups [21]. **Figure 4d**–**g** shows the O1s XPS spectra of fs laser-treated ITO films with *N* from 5 × 103 to 3 × 106 . **Figure 4c** presents the O1s XPS spectrum of

**Figure 3.**

*(a) The carrier concentration, mobility, and resistivity in the fs laser-treated ITO films as a function of the pulse numbers (the solid lines are a guide to the eyes). (b) The transmittance in the fs laser-treated ITO films as a function of wavelengths with various pulse numbers (N = 0, 3 × 105 , and 3 × 106 , respectively) at a fluence of 0.1 mJ/cm2 [11].*

as-deposited ITO films. It can be fitted well with three peaks, namely the In2O3-like oxygen at 529.6 ± 0.1 eV [21, 22], oxygen adjacent to the oxygen-deficient sites at 531.0 ± 0.1 eV [21, 22], and hydroxide and/or oxyhydroxide peak at 532.0 ± 0.1 eV [21–23]. Compared with a reference sample of In2O3 powder (**Figure 4b**), the oxygen peak (at 531.0 ± 0.1 eV) that is adjacent to the oxygen-deficient sites of as-deposited ITO film (**Figure 4c**) increases because of the formation of oxygen vacancies during the thin film growth using sputtering [23].

Furthermore, the XPS spectra of In 3d5/2, for In metals, In2O3 powders, and fs laser-treated ITO films, are shown in **Figure 5a**. The In 3d5/2 peak of In metal locates at a lower binding energy of 443.7 eV, corresponding to the In bonding state of In-In bonds [23]. However, the In 3d5/2 peak of the In2O3 powder shifts to the higher binding energy of 444.6 eV, corresponding to the In3+ bonding state of In2O3 [22, 24]. Thus, the valence states of In2O3 for the as-deposited ITO film can be demonstrated by the In 3d5/2 peak at 444.4 eV. As increasing the number of fs laser pulses, there is a gradual shift in the In 3d5/2 peak from the In-O bonding state to In-In bonding state. Particularly, the peak of In 3d5/2 for *N* = 3 × 106 is located at 443.9 eV, which is close to the binding energy of In-In bonds. This result strongly indicates that the existence of In metal-like clusters on the surfaces of fs laser-treated ITO films, particularly inside the self-organized nanodots that result in the enhanced electrical conductivity for the films. Similarly, upon fs laser irradiation, the aggregation of metal nanoparticles was observed in thin plasma polymer films due to the strong interaction via dipolar forces of the metals [25].

**Figure 5b** summarizes the findings that periodic self-organized nanodots are formed on the surfaces of the ITO films treated by fs laser pulses because of constructive interference of fs laser. The nanodots are In-like clusters, which induce superior local surface conductivity, and consequently offer reducing the electrical resistivity for the films. The approach for finding the self-organized nanodots with superior local surface conductivity may be suitable for applications such as nanolithography, nanophotoelectrons, and nanomechanics, in large-area nanotechnology.

In brief, periodic ripple structures with a multiperiodic spacing of ∼800, ∼400, and ∼200 nm were successfully fabricated by fs laser pulse irradiation. The ripple structure is composed of self-organized nanodots with a size of 20–500 nm, which are presumably formed by the solidification of melted dot patterns, under conditions of constructive interference and minimized surface energy. The ITO films with ripple structures on the surfaces exhibited significant enhancement in electrical conductivity, which is attributed to the formation of In metal-like nanodots/clusters.

*Nanostructuring Indium-Tin-Oxide Thin Films by Femtosecond Laser Processing DOI: http://dx.doi.org/10.5772/intechopen.82790*

#### **Figure 4.**

*(a) Schematic representation of as-deposited ITO surface composition based on Donley's model [21]. (b) The O1s XPS spectra of In2O3 powder and fs laser-treated ITO films with various pulse numbers (N = 0, 5 × 103 , 2.5 × 104 , 3 × 105 , and 3 × 106 , respectively) [11].*

#### **Figure 5.**

*(a) The In 3d5/2 XPS spectra of In2O3 powder, In metal, and fs laser-treated ITO films with various pulse numbers (N = 0, 5 × 103 , 1 × 105 , 3 × 105 , and 3 × 106 , respectively). (b) A schematic illustration for the formation of self-organized nanodots induced by the constructive interference of fs laser at near-surface region. The dot is composed of In-rich clusters with a height of* ∼*5 nm as a result of In-O bonding breaking into In-In under local-field enhancement [11].*
