In this study, we investigated the temperature dependence of the initial deformation and cracks of indium tin oxide (ITO) thin films deposited on a fused silica substrate using a 1064-nm quasi-continuous-wave laser. We observed that the laser-induced morphology threshold of the film shows a dramatic thickness effect. The laser-induced morphology threshold of a 100-nm ITO film is four times that of a 300-nm ITO film. Initial laser-induced surface morphologies of the initial deformation and cracks will occur as long as temperature rises to about 520 K and 1250 K, respectively, irrespective of the thickness of a film. Experimental results indicate that a thin ITO film is more likely to tolerate laser irradiation because of lower absorptivity than a thicker ITO film. Studying the temperature effect helps clarify more about the laser annealing process, which is a promising process in improving the performance of the ITO films.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
As one of the most important transparent conductive electrode materials, indium tin oxide (ITO) is widely employed in optoelectronic devices like solar cells [1,2], electro-optic switches [3,4], liquid-crystal devices [5–8], and so on. Temperatures induced in depositing process, nanostructures fabrication process, and post-annealing process are one of the most important factors that influence performances of ITO films [9,10]. Temperature affects carrier characteristics and the microstructure of ITO films and thus results in their laser-induced damage and optoelectronic performance [11,12].
An appropriate annealing temperature can improve the crystallinity, surface roughness, and optical and electrical properties of the films [13,14]. It is evident that the thermal annealed films are more conductive and have higher carrier concentration compared to un-annealed films . In addition, as the annealing temperature increases, the optical losses decrease while the wavelength of epsilon-near-zero (ENZ) increases . Therefore, thermal annealed films are promising for the implementation of capacitive sensors, and promote the absorption in the optical telecom wavelength, enabling the fabrication of efficient electro-absorption modulators based on ITO films . Excessive temperature, specifically, a rapid temperature rise, leads to film damages like cracking, melting, evaporation, and ablation [12,17,18]. Besides, the laser-induced electron heating will also lead to nonlinear optical response due to an ultrafast transient of the free-electron temperature, which is described by means of a phenomenological two-temperature model . Materials with such a large nonlinear response are expected to develope optical nanostructures with large nonlinearity for applications in nanophotonics, plasmonics, and nonlinear nano-optics [20,21]. The phenomena of thermally-induced degradation suggests potential routes to improve performances of ITO films by optimizing peak and average temperatures of the films. Increasing heat dissipation is one strategy; it can be achieved by the selection of substrates with high thermal diffusivity or by the addition of capping-layer heatsinks. Yoo et al. have indicated that higher laser fluences can be tolerated by the ITO films on a sapphire substrate possessing high thermal conductivity (36.6 W/(mK)) than that on a common fused silica substrate (1.4 W/(mK)) . Another strategy to optimize peak and average temperatures is reducing film absorption by lowering carrier density. A film deposition process with low temperature can reduce carrier density and increase carrier mobility, while maintaining film conductance [22–24].
In this study, we focus on the influence of temperature on surface morphologies of ITO films. We found the temperature dependence of the initial deformation and cracks of ITO films using 1064-nm quasi-continuous-wave (quasi-CW) laser irradiations. We revealed that ITO films with various thicknesses have strikingly different laser-induced morphology thresholds and demonstrate that the primary reason for this is enhanced laser absorptivity with the increase of the films’ thicknesses. Enhanced laser absorptivity results in a higher temperature rise. Nevertheless, the growth of surface morphologies as a function of laser power density and film thickness shows that surface morphologies of initial deformations and cracks occur under same conditions as the temperature of a film surface rises to a specific threshold temperature. Experimental results indicate that the thinner an ITO film is, the more beneficial it is to maintain its surface morphology because of lower laser absorptivity compared with thicker films.
2. Materials and methods
2.1 Laser irradiation experiments
We conducted laser irradiation experiments using an Nd: YAG laser with the wavelength of 1064 nm. The quasi-CW laser used in this testing is a linear polarization and produced pulses with the pulse width of about 300 ns at the repetition rate of 10 kHz. We focused the laser using a single lens and delivered it to a film input surface at normal incidence. The spot diameter on the target plane was 726 µm (at 1/e2) with Gaussian beam profile, and the irradiation lasted for 60 s for each power density.
2.2 Indium tin oxide films characterization
ITO film samples with thicknesses of 100 (ITO.1 sample), 200 (ITO.2 sample), and 300 nm (ITO.3 sample) were grown on fused silica substrates by using a magnetron sputtering system with ITO targets (90 wt.% In2O3 and 10 wt.% SnO2 target, 99.99% purity). Depositions were carried out with a baking temperature of 200°C, and high purity Ar was used as the sputtering gas. The sizes of the samples used are Ф 30 mm × 3 mm. They are clamped on a programmable sample stage and held vertically with the film input surface facing the laser. We observed surface morphologies of all the samples after laser irradiation using a BX53M Olympus differential interference contrast optical microscope (OM) and an Zeiss Auriga scanning electron microscope (SEM). The root-mean-square (RMS) roughness of ITO samples of 20 × 20 µm2 are tested by Atomic Force Microscope (AFM). We measured the depth profiles with 3D laser scanning confocal microscope (LEXT, OLS5000). We acquired optical transmittance and reflectance using a Lambda 1050 spectrophotometer with a normal incident angle. In addition, we examined the absorptivity (A) of the ITO films at 1064 nm by the surface thermal lensing technique [25,26].
2.3 Laser-heating simulations
To understand the heat flow of the ITO films due to the quasi-CW laser irradiation, we investigated laser heating processes of the films based on finite element simulations. To simplify the temperature-rise modeling, the simulation parameters used for all the samples neglected any temperature dependence of thermo-optical parameters. Different from the two-temperature model of the heat induced by the femtosecond laser, the temperature induced by the quasi-CW laser follows the heat transport model and exhibits a slow transient [27,28]. Details of the heat transport model used were demonstrated in our previous study . The volumetric heat source deposited into the ITO films is closely related to absorption coefficients (α) and thicknesses of the films , and the relevant equation is as follows:31]:
3.1 Growing of laser-induced surface morphologies
Three samples showed similar laser-induced surface morpholgies. The ITO.2 sample is taken as an example to illustrate typical laser-induced surface morphology characteristics of the ITO samples. Figure 1(a), (c), (e), and (g) show surface morphologies of the ITO.2 sample induced by laser power density from 800 to 1500 W/cm2. The laser-induced morphology behaves as surface discoloration, which is visible to the naked eye and can be captured using the OM. The morphology depth profiles across the irradiated sites show that the discoloration is essentially bulge deformation. The similar surface morphology characteristics imply that the deforamtion occurs at the center of the irradiated spot at the position of the peak axial fluence of the Gaussian beam and then expands upon subsequent irradiation. With increase in laser power density, the discoloration becomes more obvious. In addition, the corresponding height and diameter of the deformation (Fig. 1(b), (d), (f), and (h)) increasement, and the surface RMS roughness of the center in the irradiated site is improved slightly, reducing from 8.4 nm to 6.3 nm irradiated by the laser power density from 1000 W/cm2 to 1500 W/cm2.
Additionally, cracks eventually developed with increasing laser power density, as shown in Fig. 2. When irradiated with the laser power density of 2000 W/cm2, we observed no crack morphology even in the center of the deformation, as shown in Fig. 2(b). However, when irradiated with the laser power density of 3000 W/cm2, cracks in the center of the irradiated spot were observed, at the same magnification compared with that when irradiated with the laser power density of 2000 W/cm2. We performed detailed views in the center of the irradiated site by SEM. Figure 2(d) is the magnified crack area marked by the black dotted box in Fig. 2(c). The inset in Fig. 2(d) is the cross section of the cracks marked by the white dotted box in Fig. 2(d), which is milled via sputter by focusing on the ion beam (FIB) and imaged by the SEM. The cross-section image of the cracks profile demonstrated that the cracks extended inside till the interface between the film and substrate and did not penetrate into the substrate. Besides, the cracks expanded from the center and grew outward, but were ultimately confined to the visible morphology, as shown in Fig. 2(c).
3.2 Effect of thickness on laser-induced surface morphologies
We defined the laser-induced morphology thresholds (W/cm2) shown in Fig. 3 as the maximum incident laser power density that could not cause observable modification. We could distinguish the observable modification from the pristine areas using the OM with the magnification of ×100. The thresholds of the ITO samples decreased as their film thicknesses increased, and the values were 1200, 500, and 300 W/cm2 for the ITO.1, ITO.2, and ITO.3 samples, respectively.
In addition, Fig. 4 shows the dependence of the film thicknesses on surface morphologies irradiated with the laser power density of 5000 W/cm2. The surface morphology of the ITO.1 sample appears as the deformation morphology within the range of the tested laser power density, and we observe no crack both at the center (Fig. 4(1)) and on the edge (Fig. 4(2)) of the deformation, as shown in Fig. 4(a). However, the ITO.2 and ITO.3 samples exhibit varying degrees of cracks in the center of the deformation, as shown in Fig. 4(b) and (c), respectively. At the magnification of ×100, we observed cracks in the center of the deformation morphology of the ITO.3 sample, as shown in Fig. 4(c). But cracks in the ITO.2 sample can be observed only at the higher magnification of ×500 (Fig. 4(3) and (5)) rather than the magnification of ×100, as shown in Fig. 4(b). Moreover, magnified images of the center (Fig. 4(3) and (5)) and edge (Fig. 4(4) and (6)) demonstrated more serious cracks morphologies of the ITO.3 sample than those of the ITO.2 sample.
Figure 5 shows statistical characteristics of the diameters of deformation and crack area for the film thickness of ITO samples and applied power density. As shown in Fig. 5(a), the areas of the deformation morphologies increased with the film thicknesses under the fixed power density laser irradiation. With increased laser power density, cracks began to appear in the center of the deformation morphologies. The areas of the cracks also increased with film thicknesses, as shown in Fig. 5(b). In summary, more serious modification morphology (both of the deformation and the cracks) can be obtained in the thicker film at a fixed laser power density.
To account for the initial deformation and cracks induced by the quasi-CW laser, we analyzed the temperature rise on the ITO.1, ITO.2, and ITO.3 samples. Durations of laser irradiation are 60 s in temperature-field simulations. The surface temperatures of the ITO samples rose sharply to their maximum value within about 2 s and were basically stable after about 30 s.
Figure 6(a) shows laser-induced temperature distribution along the radial direction of the ITO samples irradiated by different laser power densities near the laser-induced morphology thresholds. The temperature distribution along the radial direction is nearly the Gaussian distribution due to the Gaussian irradiation beam profile. The temperatures on the edge of the initial deformation morphology is defined as the deformation threshold temperature, which could be obtained by comparing the size of the initial deformation in the experiment with the temperature distribution curve in the simulation. The maximum and the minimum values of deformation threshold temperatures of different samples are summarized in Fig. 6(b). The bar represents the maximum and minimum deformation temperatures which can be obtained by comparing the measured maximum and minimum diameters of those initial deformations with the simulated temperature distribution curve. It was found that temperatures on the edge of the initial deformation morphologies summarized in Fig. 6(b) are nearly the same, which are about 520 K as represented schematically by the horizontal short-dashed line in Fig. 6(a).
This indicated that the initial deformation appeared once the temperature exceeds the deformation threshold temperature. As the ITO.1, ITO.2, and ITO.3 samples are irradiated using the laser with power densities of 1200, 500, and 300 W/cm2, respectively, even the maximum temperatures are below the deformation threshold temperature. As the ITO.1, ITO.2, and ITO.3 samples are further irradiated using lasers with power densities of 1500, 800, and 500 W/cm2, respectively, the temperatures of the three samples exceed the deformation threshold temperatures, and therefore results in the initial deformation morphologies as shown in Fig. 1. In addition, the high laser-induced morphology threshold of the thinner ITO sample shown in Fig. 3 is attributed to the lower absorptivity, as shown in Table 1. As irradiated with the fixed power density of the laser, the temperature rise of an ITO sample with small absorptivity is lower than that with large absorptivity.
For ITO samples, mismatched thermal expansion coefficients of the substrate and ITO films as well as the temperature rise play important roles in thermal stress generation . As irradiated by the external laser, externally applied stresses exceed the critical limit, and then generate the cracks . Figure 7(a) shows laser-induced temperature distribution along the radial direction of the three ITO samples irradiated using laser with power densities of 5000, 3000, and 2000 W/cm2. Same as the definition of the deformation threshold temperature (Fig. 6), the cracks threshold temperature is about 1250 K, as shown in Fig. 7(b). The initial cracks appear whenever temperature exceeds the cracks threshold temperature.
At the cracks threshold temperature of 1250 K, the calculated threshold stress induced cracks is 1.23 GPa, which approximates the ITO failure stress threshold of 1.2 GPa at room temperature . Figure 8 shows surface thermal stress distributions of the ITO.1, ITO.2, and ITO.3 samples under laser irradiation with power densities of 5000, 3000, and 2000 W/cm2, respectively. When the ITO.2 and ITO.3 are irradiated by laser power density with 3000 and 2000 W/cm2 respectively, their temperature fields are almost the same (shown in Fig. 7(a)), which result in the similar heat-induced thermal stress distribution shown in Fig. 8(b) and Fig. 8(c). But the maximum surface thermal stress of the ITO.1 sample induced within the range of the tested laser power density is below the crack threshold stress (1.23 GPa), resulting in the phenomenon of no cracks formation. The maximum surface thermal stresses of ITO.2 and ITO.3 samples induced with laser power density of 3000 W/cm2 and 2000 W/cm2, are 1.25 GPa and 1.26 GPa, respectively. Both exceed the induced crack threshold stress, resulting in the cracks morphology. However, the maximum surface thermal stress of the ITO.1 sample (1.17 GPa) induced within the range of the tested laser power density is below the cracks threshold stress, resulting in the phenomenon of no cracks formation. This indicates that initial deformation and cracks will occur whenever the temperature of a film surface rises to a certain threshold temperature.
In addition, for the deformation and cracks morphologies induced by the laser power density from 500 W/cm2 to 5000 W/cm2, the simulated and experimental diameter are summarized in Fig. 9. The results in Fig. 9(a), (b), and (c) indicate the simulation results are good in agreement with the experimental results under the irradiation of laser power density that are insufficient to produce cracks, while the simulation results gradually deviate from the experimental result with the increase of the irradiated power density. This phenomenon should be ralated with the occurrence of cacks as well the influence of high tempratue on thermo-optical parameters. Similarly, the comparison in Fig. 9(d) indicates that the simulated diameter of crack area and the experimental diameter of crack area are comparable under the laser power density from 2000 W/cm2 to 5000 W/cm2.
With the 1064-nm quasi-CW laser, we investigated the temperature dependence of the initial deformation and cracks of the ITO thin films deposited by magnetron sputtering deposition on fused silica substrates. We observed that the laser-induced morphology threshold of the films shows a dramatic thickness effect. The laser-induced morphology threshold of a 100-nm ITO film is four times larger than that of a 300-nm ITO film, and the effect of absorptivity by increasing the film thickness is considered to be the primary reason for this result. Initial laser-induced morphologies of deformation and cracks will occur as long as the temperature of the film surface rises to a certain temperature, whatever the thickness of the film is. The threshold temperature that is sufficient to cause the initial deformation morphology or cracks is about 520 K or 1250 K, respectively. Our experiment findings indicate that a thin ITO film is more beneficial to tolerate laser irradiation because of low radiation absorptivity. In addition, studying temperature effect on ITO films helps to further clarify the laser annealing process, which has the potential for the efficient enhancement of the performance of ITO films.
Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology; Strategic Priority Research Program of the Chinese Academy of Sciences (XDB1603); National Natural Science Foundation of China (11874369, U1831211).
The authors declare no conflicts of interest.
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