**4. Indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO) films deposited by PLD on flat and nanopatterned glass substrates**

ITO is the most widely used TCO due to its exceptional properties, a large number of papers being focused on it [73–76]. Several works reported on the PLD deposition of ITO films and on the correlation between the experimental parameters and their optical, structural, morphological, and electrical properties, some results being well summarized by Yap and Kim [47, 77, 78]. The best properties achieved for the ITO films deposited by PLD had over 90% transparency and 7.2 × 10<sup>−</sup><sup>5</sup> Ωcm electrical resistivity [18].

In the last decade, many attempts were made to replace ITO due to the indium sources depletion [79]. An adequate alternative for ITO seems to be AZO, a nontoxic material that can be found at low cost—its precursors being abundant compounds, and already successfully applied in the OPV and OLED areas [80]. AZO transparent films characterized by an adequate electrical resistivity were deposited by different methods on both rigid and flexible substrates, proving its compatibility for wearable electronics [20, 81–83]. PLD technique was also used in the deposition of AZO

**Figure 3.** *FESEM images (at different magnifications) of the periodic pillars array obtained by UV-NIL method on glass substrates.*

layers on either rigid glass or plastic substrates with suitable optical and electrical properties [20, 26, 84].

In the following part, the preparation steps implied in the fabrication of ITO and AZO films by PLD on flat and UV-NIL nanopatterned substrates will be described [85, 86]. The patterns were fabricated on glass by UV-NIL (EVG 620 mask aligner) using the following procedure: (i) preheating of the glass substrate for 2 min at 150°C; (ii) spin coating of a primer to enhance the adherence of the polymeric photoresist film; (iii) deposition by spin coating of the UV-resist film that further is thermally treated for 30 s at 120°C; (iv) pressing the soft stamp (mold) with the pattern model over the photoresist film with an uniform contact pressure (100 mbar); (v) exposure of the photoresist layer at UV light for 90 s; and (vi) removal of the soft mold [87]. As can be seen in the field emission scanning electron microscopy (FESEM) images from **Figure 3**, a periodic array of pillars having ~350 nm in diameter and ~ 1100 nm distance between pillars were fabricated on glass substrate by this procedure. The height of the pillars was estimated at ~250 nm from the cross-sectional FESEM images given in **Figure 4**. The quality of the patterns (height, diameter, distance between pillars) imprinted onto photoresist depends on the experimental conditions mentioned above in the UV-NIL process.

Further, TCO layers were deposited on both flat and UV-NIL patterned glass substrates by a PLD system using an excimer laser with KrF (248 nm wavelength, 25 nm pulse duration, COMPex-Pro 205, Coherent Inc.) [85, 86]. The TCO solid targets (SCI Engineered Materials) were formed by In2O3:SnO2 = 90%:10% weight (ITO) and ZnO doped with 2% Al (AZO), the laser beam being directed on the target surface with a MgF2 lens having 300 mm focal length placed outside of the deposition chamber. During the deposition, the solid targets were rotated to avoid their local

*Pulsed Laser Deposition of Transparent Conductive Oxides on UV-NIL Patterned Substrates… DOI: http://dx.doi.org/10.5772/intechopen.105798*

#### **Figure 4.**

*Cross-sectional FESEM images (at two magnifications) of nanopatterned glass substrates.*

damage. For comparison, both types of substrates were coated with TCO layers in the same deposition cycle.

The ITO solid target placed at 5 cm distance toward substrate holder was irradiated with 7000 pulses under 45° incidence angle, the laser working at 10 Hz repetition rate into a deposition chamber filled with oxygen 6.0 at 1.5 Pa pressure and working with a low laser fluence of 1.2 J/cm2 [85]. The oxygen pressure was selected in order to obtain a low electrical resistivity, at room temperature (RT), as was mentioned in the reference [78]. The ITO layer thickness was estimated at ~340 nm as average media between the measurements made (with a profilometer) in three different points on the film deposited on flat glass substrate.

The AZO solid target placed at 8 cm distance toward substrate holder was ablated with 8000 laser pulse, a laser fluence of 2 J/cm2 , and an oxygen pressure of 1 Pa [86], the values being selected based on other preliminary results where films characterized by a high transmittance were fabricated using these experimental conditions [84]. The AZO layer thickness was estimated at ~300 nm from the interference fringes observed in the UV-VIS spectra considering two consecutive maxima and minima and the refractive index = 1.8 for AZO film with 2% Al content [88].

The TCO layers deposited by PLD were labeled taking into account the substrates type, flat (glass) or nanopatterned (NP-glass), as follows: ITO/glass, AZO/glass and ITO/NP-glass and AZO/NP-glass. The morphology and optical properties of the samples were investigated by field emission scanning electron microscopy (FESEM, Zeiss Merlin Compact field emission scanning electron microscope), atomic force microscopy (AFM, Nanonics Multiview 4000), and UV-VIS spectroscopy (Carry 5000 Spectrophotometer).

The FESEM images from **Figure 5** disclose that the ITO/glass (**Figure 5 left**) has a smooth surface while the AZO/glass (**Figure 5 right**) has a granular morphology, some particles being also presented on the surface of this sample. The results are in accordance to data already reported for ITO and AZO layers deposited by PLD [18, 84] or by other deposition techniques [89, 90].

The AFM topographic images from **Figure 6** were collected on ITO/glass (**Figure 6 left**) and AZO/glass (**Figure 6 right**), interpolated root mean square (RMS) having a low value in both cases, 1 nm and 2.8 nm, respectively. As was expected, in the case of ITO film, the RMS value is lower in comparison with ITO layers deposited by other techniques but in agreement with those calculated for ITO layers previously deposited by PLD [23, 78, 91]. It has to be mentioned that in the PLD, the resultant smooth surface is associated to the low energy density implied in the deposition process [91]. As was already

#### **Figure 5.**

#### **Figure 6.**

emphasized, such low roughness value is necessary for various applications being known that this parameter has a significant influence on optical and electrical properties of the deposited films. Thus, it was demonstrated that using a low laser fluence, ITO and AZO films characterized by a small roughness can be obtained by PLD at room temperature, making them suitable and compatible with flexible (plastic) substrates.

Analyzing the FESEM images of the ITO/NP-glass and AZO/NP-glass from **Figures 7** and **8**, respectively, it can be clearly seen that the patterns imprinted onto glass substrate are preserved during the TCO deposition by PLD. Considering that the TCO films are relatively thin (ITO ~ 340 nm and AZO ~300 nm), they tend to copy the topography of the substrate.

However, attention must be paid when the TCO layers are deposited on a patterned surface by PLD because the interaction between the ablated species, presented in the plasma plume, characterized by high kinetic energy and the deposition substrate can affect the growth of the film during the laser deposition [35, 92]. Thus, point defects can be formed due to species kinetic energy transfer toward the surface atoms [92]. In the PLD deposition on nanopatterned substrates, the first encountered layer is that based on photoresist (polymer) nanopillars. Nevertheless, the pillars are clearly observed in the FESEM images of the TCO deposited of nanopatterned glass substrates, only a small change in their shape being noted (in the case of ITO/NP-glass from cylindrical into a pyramid trunk-like one). Both TCO films seem similar at lower *Pulsed Laser Deposition of Transparent Conductive Oxides on UV-NIL Patterned Substrates… DOI: http://dx.doi.org/10.5772/intechopen.105798*

**Figure 7.**

*FESEM images (at different magnifications) of ITO films deposited by PLD on nanopatterned glass substrates.*

#### **Figure 8.**

*FESEM images (at different magnifications) of AZO films deposited by PLD on nanopatterned glass substrates.*

**Figure 9.** *UV-VIS spectra of TCO layers (ITO or AZO) deposited by PLD on flat (left) and nanopatterned (right) glass substrates.*

magnification, some differences due to the film thickness and the specific morphology being visible only at higher magnification. Thus, in comparison to the nanopatterned glass substrates, an enlargement in the pillars width and a narrowing in the distance between pillars are remarked, the TCO films tending to fill the space between pillars. Although the TCO films have thickness appropriate to the pillars' height, these are not hidden by the deposited layers.

The optical transmittance is an essential criterion for the selection of the TCO films for their use in the field of OPV and OLED. Hence, the UV-VIS spectra of the prepared samples were presented in **Figure 9**. The TCO layers deposited on flat glass substrates are characterized by a transmittance over 80% for ITO and 75% for AZO in the visible part of the solar spectrum. Interference maxima are visible for both analyzed materials, their presence being associated with the uniformity of the deposited films [23]. This is not surprising, as it is known that high-quality layers can be obtained by PLD [93]. The refractive index (*n*) value was estimated from the interference maxima and minima with the Swaneopel method [94] as being ~1.9 for ITO/glass, value characteristic for this TCO films deposited by PLD. Kim reported that the *n* value depends on the Sn concentration in the targets and on the substrate temperature, for example, ITO films with low *n* were obtained increasing the substrate temperature [78]. Additionally, the band gap value was estimated from the UV-VIS spectra. Thus, for ITO/glass, the band gap was estimated at ~3.65 eV, in agreement with other results reported for ITO layers deposited by PLD or by magnetron sputtering, other technique frequently used to deposit commercially available ITO layers, with the same SnO2 content (10%) [23, 91]. It is well known that this oxide is a semiconductor characterized by a direct wide band gap that can exceed 3.0 eV [95]. According to the Burstein-Moss theory, the increase in the band gap is linked to the increase in the film's carrier concentration [78].

Compared with the ZnO band gap value (3.3 eV [96]), the AZO/glass band gap was estimated at ~3.7 eV, similar to the value reported for AZO grown by PLD at room temperature and 1 Pa oxygen pressure [97]. Depending on the experimental conditions, especially by the oxygen pressure and the substrate temperature, the band gap of the AZO films deposited by PLD can take value between 3.32 and 3.77 eV [98].

In the case of the TCO layers deposited on nanopatterned glass substrate, a lowering in the transmittance is noticed in the UV-VIS spectra compared with the ones deposited on flat glass substrates. Moreover, the pillars introduced additional

#### *Pulsed Laser Deposition of Transparent Conductive Oxides on UV-NIL Patterned Substrates… DOI: http://dx.doi.org/10.5772/intechopen.105798*

absorptions and reflections at interfaces [35]. The light couples to waveguide modes via diffraction and thus is trapped in the nanostructures, the pattern characteristics (mainly the period) affecting the optical properties of the films deposited on it [99]. Also, a shift of the absorption edge is visible for both transparent electrodes. A possible explanation for the peculiar behavior observed in the absorption edge shift of nanopatterned TCO (ITO/NP-glass to long wavelength region and AZO/NP-glass to short wavelength region) can be linked to the arrangement of the molecules inside the cavities determined by the nanostructuration. Thus, the interaction between the neighboring molecules can modify differently the energy levels of nanopatterned TCO with effect on their band gap.

Electrical properties of the prepared TCO layers are considered key features since, in the field of optoelectronic applications, conductive films are required. Hall measurements were performed on ITO/glass and ITO/NP-glass samples in order to analyze their electrical parameters, the obtained values being presented in **Table 1**.

In principle, the electrical resistivity values of ITO films deposited on flat and nanopatterned glass substrates are lower than ~4 × 10<sup>−</sup><sup>4</sup> Ωcm reported for ITO films deposited at room temperature by PLD [100] in the same conditions (laser wavelength, target composition, and repetition rate) with those used in our study. Interesting, the electrical resistivity value of ITO film deposited on flat glass substrate is nearly to that of ITO films deposited by PLD from targets with different SnO2 content (5 or 10%) but with a heated substrate [18, 91, 101]. Kim carried on a comprehensive study regarding the influence of various experimental parameters such as oxygen pressure, SnO2 content, and deposition temperature on the resistivity of ITO films deposited by PLD [78]. Hence, this work shows that the resistivity of ITO film is influenced by the oxygen pressure through the number of the oxygen vacancies presented in the TCO layer. Also, the resistivity of ITO films is sensitive to the SnO2 content, an increase up to 5% leads to the resistivity decrease while an increase above this percent results in the increase of resistivity because the concentration of the electron traps expands due to Sn excess [91].

The carrier concentration values of ITO films deposited on flat and nanopatterned glass substrates are in concordance with those reported usually on ITO films deposited by PLD [78]. The refractive index of ITO films is influenced by the carrier density, a reduction of this parameter being possible by increasing the electron density, which can be achieved by enlarging the Sn content from the deposition target up to a certain value [78].

The extracted Hall mobility values of ITO films deposited on flat and nanopatterned glass substrates are just a little smaller than other value reported for ITO films deposited by PLD [91] utilizing the same deposition target with that implied in our work. The low Hall mobility values of ITO films can be related to the carrier-carrier scattering [44].


#### **Table 1.**

*Electrical parameters of ITO films deposited by PLD on flat and nanopatterned glass substrates evaluated from Hall investigations.*

In the case of AZO film deposited on flat glass substrates, the resistivity was evaluated to be 2.4 × E<sup>−</sup><sup>4</sup> Ωcm using a Jandel four-point probe, the value being in the same range with others obtained for the AZO layers deposited by PLD on glass substrates [20, 102] using the same oxygen pressure with that applied in our study. A thoroughgoing study regarding the influence of the oxygen pressure on the optical and electrical properties of some AZO layers deposited by PLD was carried on in Ref. [102] pointing out that the films grown at a low oxygen pressure (under 3 Pa) have a compact structure characterized by a low resistivity.
