Open Access
Add to BibSonomy
Abstract
In recent years, there has been increasing interest in optoelectronic applications of transparent conductive oxide (TCO) thin-film-based materials and devices fabricated using patterning techniques. Meanwhile, femtosecond laser processing is a convenient method that further improves the performance of TCO-based functional devices and expands their application prospects. In this study, we proposed a simple and effective strategy to determine the fluences required for laser processing TCOs. We investigated the modification of an indium tin oxide (ITO) film induced by a femtosecond laser (45/150 fs, 800 nm) at different pulse fluences. The results reveal that the laser modification of ITO films is highly dependent on the irradiated pulse fluences. Several distinct types of final micro/nanostructures were observed and may be attributed to superficial amorphization, spallation ablation, stress-assisted delamination, boiling evaporation, and phase explosion. The final micro/nanostructures were studied in detail using optical microscopy, scanning electron microscopy, transmission electron microscopy and a surface profiler. At a lower fluence above the melting but below the ablation threshold, a laterally parabolic amorphous layer profiled with maximum thicknesses of several tens of nanometers was quantitatively attained. At a higher fluence, stress-assisted delamination and superheated liquid-induced micro-honeycomb structures emerged. Furthermore, the electron and lattice temperature evolutions were also obtained using a two-temperature model to prove the ablation mechanism and ascertain the micro/nanostructure formation principle. The predicted surface temperatures confirmed film amorphization without ablation below 0.23 J/cm2. These results reveal the interaction mechanism between femtosecond laser pulse and ITO film including the competition between the free electron heating of intraband transition and the multiphoton absorption of the interband transition, which promotes the potential applications for femtosecond laser processing TCO films and other wide-band-gap semiconductors such as photodetectors, solar cells, UV-light-emitting diodes, and flat-panel displays.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Transparent conductive oxide (TCO) thin films are a form of semiconductor with a particular band structure that has attracted scientific interest in recent years owing to their unique optical [1,2], electronic, and mechanical properties [3–5]. TCO-based materials have also been extensively investigated as potential materials for application in photodetectors [6–8], sensors [9–11], solar cells [12–15], and transparent electrodes [16,17]. In addition to their outstanding linear optical and electrical properties, TCOs have recently exhibited an interesting nonlinear optical response over a wide spectral region from ultraviolet to mid-infrared [18–21]. TCOs have been exploited in broadband electro-absorption modulators [22–24], second-harmonic generation [25,26], third-harmonic generation [27–29], an ultrahigh nonlinear Kerr response [30,31], and all-optical switching [32,33].
Indium tin oxide (ITO) film, one of the most commonly used TCOs, is well known wide bandgap semiconductor that is often employed in studies of optoelectronic devices because of its low cost and high nonlinearity in the visible and near-infrared regions. However, the application of ITO to materials with -micro-nanoelectronics often requires complex patterning methods such as photolithography [34], electron beam lithography, and chemical wet etching [35]. These traditional photolithography methods have high spatial resolution while the multistage photolithography and wet chemical techniques consist of multiple processes, such as the coating of photoresist, soft bake, exposure, lithography, hard baking, and photoresist stripping. Ultrashort laser patterning is an interesting methodology to structure TCOs once it provides intensities high enough to induce nonlinear process interaction, melting, ablation, phase transition, or permanent structural changes [36–39]. Recently, femtosecond laser micromachining has shown itself to be an important material processing tool capable of generating complex geometries without the need for photomasks and clean rooms, enabling the processing of materials in distinct atmospheres as well as in a vacuum. Although the resolution of femtosecond laser micromachining is lower than that of traditional lithography, it has potential application for micro scale lithography. The energy of the femtosecond pulses is absorbed through nonlinear processes (i.e., two-photon or multiphoton absorption)—including avalanche ionization in a very short time—and can be precisely and rapidly transferred to the film, which does not allow for a fast heat-transfer process [40].
Micro/nanofabrication of ITO films by femtosecond laser pulses has been investigated, including applications in solar cells, flat-panel displays, enhanced electrical conductivity, anisotropic optical transmission, and performance of organic photovoltaic devices [41–43]. However, previous studies primarily focused on analyzing the final morphology of ITO films after femtosecond laser processing. When a material is irradiated by a femtosecond laser pulse, the laser-matter interaction is a nonlinear and nonequilibrium ultrafast process [44]. The photon–electron interaction between the femtosecond laser and the material directly determines the morphologies and properties of the final structures, thus affecting the performance of the device [45]. In addition, laser micromachining requires examination of the irradiated site before and after exposure or the use of sensitive in-line diagnostics to capture the changes near the modification threshold. Therefore, it is necessary to study the nonlinear ultrafast femtosecond laser–ITO film interaction process, especially electron dynamics and follow-up structure changes, to determine the corresponding modification threshold.
In this study, the photon–electron interaction between a femtosecond laser and ITO film and structure formation mechanisms under the irradiation of femtosecond laser pulses were investigated both experimentally and theoretically. The simple laser modification threshold determination method presented here involves the spatial deconvolution of the laser beam spatial fluence map. The experimental results suggest that after single-pulse irradiation, five types of final micro/nanostructures were found under different femtosecond laser fluences. Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a surface profiler were used to analyze the final structures. In addition, the composition changes of the ITO film before and after laser irradiation were analyzed using X-ray photoelectron spectroscopy (XPS). Based on these analyses, several different damage mechanisms were proposed and attributed to superficial amorphization, spallation ablation, stress-assisted delamination, boiling evaporation, and phase explosion of the ITO film. Simultaneously, a two-temperature model (TTM) was developed for the ITO film to prove the ablation mechanism and ascertain the micro/nanostructure formation principle. Our results provide more insight into the laser-induced modification mechanism of semiconductors, and may be of practical importance for laser micromachining photonic devices based on semiconductors.
2. Materials and experimental details
2.1 Experimental sample
In this study, an ITO film with a thickness of 300 nm was deposited on a fused-silica substrate via magnetron sputtering. Before deposition, the substrate was ultrasonically cleaned in acetone, ethanol, and deionized water for 15 min and subsequently dried under a flow of nitrogen. The ITO target was composed of 10 wt% SnO2 and 90 wt% In2O3 (purity 99.99%). The sputtering rate and thickness of the films were monitored using a quartz crystal oscillator (SQM-160).
2.2 Experimental setup
The laser processing setup consists of two stations: (1) a femtosecond-pulsed laser irradiation station and (2) a high-resolution monitoring station (see in Ref. [46]). The laser used for irradiation was a commercial chirped-pulse amplification-based Ti: sapphire regenerative amplifier (Spectra Physics, Spitfire) providing linearly polarized laser pulses at a wavelength of 800 nm and pulse duration of 45 fs. The laser pulse energy was varied using an energy attenuator consisting of a half-wave plate and a thin-film polarizer. The 1/e2 diameter of the beam focused on the ITO film was approximately 356 μm, achieved by a 75-cm focal length aspheric lens. To detect on-line damage, a microscope was focused on the test area, and any damage to radiation sites was detected by a charge-coupled device. The sample was placed on a three-dimensional micro-positioning stage with a surface perpendicular to the propagation direction of the laser beam. To determine the one-on-one laser-induced modification threshold, we irradiated 10 dependent sites per fluence and repeated the procedure for more than six fluences. All irradiations were performed in air.
2.3 Analysis method
The ellipsometry and Hall measurement were performed to characterize the optical and electrical properties of the ITO films. The linear optical permittivity of ITO film was investigated by spectroscopic ellipsometry (HORIBA, UVISEL 2 series, Japan) in the spectral region from 400 to 2000 nm. The Hall effect measurement (Quantum Design PPMS-9) was performed using the Van der Pauw method to evaluate the charge density and Hall mobility. The surface characteristics of the irradiated ITO films were inspected using OM (BX53M Olympus), SEM (ZEISS Auriga) and TEM (Thermo Scientific Glacios). The corresponding surface profiles were observed using a surface profiler (Bruker, Dektak XT). In addition, the composition changes of the ITO film before and after laser irradiation were analyzed using XPS (Thermo Scientifc).
3. Results
3.1 Optical and electrical properties
Figure 1 expresses the result of measured permittivity, which can reflect the linear optical properties of the ITO film. As a heavily doped semiconductor, the permittivity of ITO film can be well described using the Drude-Lorentz oscillator model [47]. The Drude model describes the interaction between electromagnetic field and conduction band electrons if we ignore the nonlocal effects. The Lorentz oscillator model is used to describe the absorption of photons by valence band electrons. From Fig. 1, we can see that near the 1200 nm wavelength, the real part of the permittivity of the ITO film is close to zero, which is related to the concentration of conduction band electrons and the electron damping in the ITO film. We used the Hall measurement method to obtain the electrical parameters of the ITO film and the results are used as the fitting parameters in the ellipsometry measurement.
3.2 Modification threshold fluence of the ITO film
To determine the different modification threshold conditions of the ITO film, single-pulse ablations were conducted at various laser fluences. Figure 2 shows optical micrographs of a few selected laser-irradiated spots on the ITO film at fluences of 0.23, 0.26, 0.29, 0.35, 0.44, and 0.51 J/cm2. Each picture shows a new spot produced at a different peak fluence, as noted in the upper right corner. Notably, several characteristic annular structures can be recognized in the series of laser-irradiated surface spots, as displayed in Fig. 2. The origin of the observed characteristic circular symmetry of the features can be found in the Gaussian distribution of fluences over the cross section of the laser beam. Four modification thresholds are apparent and identified by red, green, blue, and purple in Fig. 2(e) and (f). The threshold fluences for different modifications of the ITO film were determined experimentally using the Liu method [48]
Fig. 2. Comparison of bright-field optical microscopy (OM) images, scanning electron microscopy (SEM) images, and corresponding surface profile images of single-pulse irradiation at five different fluences.
Here, ${F_0}\textrm{ = }2{E_p}/\pi \omega _0^2$ and ${F_{\textrm{th}}}$ are the applied and threshold fluences, respectively; ${\omega _0}$ is the beam waist radius of the Gaussian-shaped beam at the focus; ${E_p}$ is the absorbed laser energy; and D is the measured crater diameter.
Figure 3(a) shows the transverse distribution of the focal spot on the beam cross section, which is the Gaussian profile. Figure 3(b) is a schematic diagram of the laser fluence distribution on the ITO film surface, and the dash rectangle is the region for analysis. According to Eq. (1), the experimentally measured squared ablation diameters should show a linear dependency with fluences in a semilogarithmic plot. By recording the size of the modification at different pulse energies, different threshold fluences can be determined. The results are presented in Fig. 3(c).
Fig. 3. (a) The transverse distribution of the focal spot on the beam cross section (b) Schematic diagram of the fluence distribution on the ITO film surface. (c) Diameters squared of laser-induced modifications vs. logarithm of pulse fluences.
3.3 Structural characterization
The optical-changed and ablated regions were investigated in more detail using SEM and a surface profiler. Figure 4 shows images of five laser-irradiated spots on the ITO film generated by single femtosecond-laser pulses at different pulse fluences. The upper row shows the SEM micrographs, and the second row shows the corresponding horizontal surface profiles (indicated by a red dashed line) of the irradiated ITO film. With a single laser pulse at a low fluence, damage starts to appear on the surface, then at a fluence of 0.23 J/cm2. Notably, a dome-like structure appeared with a maximum dome height of ∼45 nm [see Fig. 4(a)]. Interestingly, the dome-like structures can be seen in the outermost locations of all laser-irradiated spots and may be attributed to the amorphization of the ITO film after laser irradiation. The dome-like features of the ITO film ablated at fluences of 0.23 to 0.35 J/cm2, as shown in Fig. 4(b)–(d). Both features (melting and ablation) can be identified in all images with different diameters because of the Gaussian laser beam. At a peak fluence of 0.44 J/cm2, several other characteristic features can be seen in the center of the ablation craters. These features include the rim, delamination, and voids [Fig. 4(e)].
Fig. 4. SEM images and corresponding surface profiles of ITO film surface after irradiation with different pulse fluences: (a)–(e) 0.23, 0.26, 0.29, 0.35, and 0.44 J/cm2, respectively.
A more detailed view of the femtosecond laser damage site in the ITO film [Fig. 4(e), 0.44 J/cm2] is shown in Fig. 5. Figure 5(b) shows the depth profile line of Fig. 5(a), taken along the black dotted line. The images in Fig. 5(c)–(h) show the enlargement of the colored rectangular area in Fig. 5(a). A stitched SEM image displays locations from the outside edge of the damage site all the way to the center. As shown in Fig. 5(b), starting at the boundary of the crater, the same dome-like feature and uniform ablation observed at lower fluences are present. This corresponds to Fig. 5(c) and (d)–(e). The dome-like feature may be associated with the physical processes of the laser-induced melting ITO film with subsequent superficial amorphization without ablation. The amorphous layer had a thickness of ∼50 nm.
Fig. 5. (a) SEM images and (b) surface profiles of laser irradiated ITO film at fluence of 0.44 J/cm2. (c)–(h) Enlarged SEM images where corresponding boxes from (a) are marked.
From an analysis of the diameters of the amorphous ring, a threshold fluence value of 0.21 J/cm2 is found here for melting and subsequent amorphization. Significant delamination and pitting are observed when moving inward to higher fluences, suggesting the formation of photomechanical stress and voids after laser excitation. Finally, small redeposited nanoparticles were observed at the center of the crater.
Based on the analysis of the damage craters, the first, weaker type of modification is proposed to be owing to the rapid melting of a thin surface layer, which has been known to result in the amorphization of the ITO film [49]. The formation of an amorphous ITO film in the outermost region indicates that the velocity of the cooling front exceeds the critical speed of amorphization in the ITO film. This region rises slightly above the sample surface and exhibits significant uniformity of the amorphous layer thickness, which is attributed to the Gaussian space distribution of fluence. The region adjacent to the amorphous ITO film demonstrates the features associated with the ablation of ITO films in which tens of nanometers of material are removed through a process known as photomechanical spallation [50].
Moving toward a higher laser fluence, a significant delamination of the ITO film and substrate was observed. Because of the flat crater bottom and lack of significant ITO ablation, we call this state damage-free delamination. The formation of voids, as shown in Fig. 5(g), suggests that at a higher laser fluence, the melted layer begins to boil. In this process, the surface region, heated to its limit of thermodynamic stability, is expected to undergo a rapid transition from an overheated liquid to a mixture of vapor and liquid droplets. Boiling involves heterogeneous nucleation. These are vapor bubbles, which, in the case of liquids, initiate heterogeneously from a variety of disturbances such as gas or solid impurities, defects, or an underlying or enclosing solid surface. At the highest fluence, the critical temperature is reached, leading to the explosive release of vapor in a process known as phase explosion, and redeposited vaporized droplets are observed, as shown in Fig. 5(h).
To verify the universality of the fluences dependence of the femtosecond laser-modified ITO films, we used a femtosecond laser with a wavelength of 800 nm and longer pulse width of 150 fs to perform laser irradiation experiments on the same ITO film. The experimental results are presented in Fig. 6. It is worth noting that in this experiment, - similar damage characteristics occurred as the previous experiment, such as photomechanical peeling, boiling evaporation, phase explosion. At the same time, the damage characteristics are also laser fluence dependent. By comparing the results of the two experiments, we can infer that our subsequent theoretical simulations are applicable to a certain range of laser pulse width (tens of femtosecond to hundreds of femtosecond).
Fig. 6. (a) SEM images and (b) surface profiles of laser irradiated ITO film at fluence of 0.70 J/cm2. (c)–(e) Enlarged SEM images where corresponding boxes from (a) are marked.
3.4 Determination of the amorphous layer
To further investigate the formation mechanism of the final structures, especially the amorphous layer, XPS analysis of the pristine and modified regions was performed. We investigated the influence of femtosecond laser irradiation (at 0.23 J/cm2) on the nature of chemical bonding for both oxygen and indium, specifically from the core-level spectra. Curve-fitting of the core levels (In 3 d5/2, O 1s) was performed, as shown in Fig. 7. The spectral features of O 1s are composed of three peaks centered at 530.0 eV (OI), 531.0 eV (OII), and 532.2 eV (OIII) for the pristine ITO film. The intensity of the component at 530.0 eV is attributed to crystalline, while the medium-binding energy component at 531.0 eV is attributed to amorphous ITO [51]. The OIII peaks are primarily associated with chemisorbed hydroxyl or COx species [52].
Fig. 7. XPS spectra of (a) O and (c) In for pristine, and (b) O and (d) In for modified regions irradiated by femtosecond laser with fluence of 0.23 J/cm2.
It was noted that the atomic percentage of crystalline ITO (OI) decreased from 62.86% to 52.43%, and that of amorphous ITO (OII) increased from 21.07% to 29.33% after the laser irradiation of the sample with a fluence of 0.23 J/cm2. This is consistent with the amorphization of the ITO surface observed in Section 3.2. Similarly, the spectral features of In 3d5/2 are composed of three peaks centered at 444.3 eV (InI), 444.8 eV (InII), and 446.1 eV (InIII) for the pristine ITO film with a minor peak shifting from 446.1 eV to 445.7 eV for InIII. The InI and InII peaks were identified as crystalline and amorphous ITO, respectively and the higher InIII peaks can be attributed to metal In and In(OH)x [53]. The InIII component in the In 3d5/2 peak at 445.7 eV is associated with the many-body screening effect of core holes in the process of photoionization. Regardless of the various spectral features observed, our data indicate that an amorphous layer appeared on the surface of ITO film after femtosecond laser irradiation.
In addition, to verify assumptions made in the surface profiler and to verify the corresponding amorphous layer thickness results, cross-sectional TEM analyses were performed on the spot irradiated at 0.23 J/cm2. Figure 8(b) presents a TEM cross-section through the lamella prepared by FIB at the position marked by the red square in Fig. 8(a). From Fig. 8(c), below the amorphous carbon protection layer, an amorphous a-ITO layer of thickness (40∼45 nm) was measured at this specific position on the top on the crystalline ITO material, which is consistent with the result of the surface profiler test. Higher magnifications of the a-ITO /c-ITO interface is presented in Fig. 8(d), revealing a transition zone with an extent of several nanometer and no indications of crystal defects formed upon re-solidification.
Fig. 8. TEM images of fs-laser irradiated ITO film with fluence at 0.23 J/cm2. (a) The position of TEM image observation; (b) and (c) Overview; (d) high-resolution images of the a-ITO / c-ITO interface.
4. Discussion
To examine the effects of laser irradiation, it is necessary to know how energy is absorbed, relaxed, and transported in the system. A multiscale model was used to describe all underlying physical mechanisms that couple modules related to energy absorption, carrier excitation, carrier–phonon relaxation, phase transition, and re-solidification processes. Because of the ultrafast duration of the femtosecond laser pulse, the laser–material interaction process occurs over an ultrashort time scale and induces transient out-of-equilibrium plasma [54].
We consider that the interactions between the femtosecond laser and ITO film consist of three processes. First, laser-induced ionization and free electron heating—two-photon ionization was considered because of the ∼3.5 eV band gap of the ITO film. Second, high-energy electron-induced lattice heating involves the transfer of energy from free electrons to the lattice, which causes lattice vibration and an increase in the lattice temperature. Third, high-energy electron-induced ionization—free electrons with high kinetic energy in the conduction band collide with bound electrons in the valence band to ionize more free electrons. During laser pulse irradiation, the lattice does not have enough time to effectively respond to the electromagnetic field and remains cool. After laser irradiation, the hot electrons interact with the cool lattice and valence electrons. Finally, the system relaxes toward temperature equilibrium within tens of picoseconds.
The dynamics of a free electron in ITO are metal-like and can be described by the Drude model. The ionized free electrons are heated by an intense electromagnetic field during the laser irradiation. After the femtosecond laser transfers energy to electrons, the electron temperature increases rapidly, and the lattice temperature increases solely during the lattice heating process. To describe lattice heating, a two-temperature model (TTM) is used to calculate the energy transfer between high-energy electrons and the lattice, as follows [55,56]:
Here, ${F_0}$ is the laser fluence, R is the reflectivity of the material, ${t_p}$ is the laser pulse duration, ${\omega _0}$ is the beam radius, t is time, $\alpha $ is the material absorption coefficient, and z is the depth component of the material.
The model assumes that optical energy is first absorbed by the electronic subsystem and then transferred to the lattice through electron–phonon coupling. The electron–photon coupling factor G describes the strength of the energy transfer between the excited electrons and lattice vibrational modes of ITO and can be estimated as
where $m_e^\ast $ is the effective mass of the electron; ${\tau _e}$ is the temperature-dependent relaxation time; ${n_e}$ is the density of electrons; ${T_e}$ is the temperature of electrons; ${c_s}$ is the speed of sound given as ${c_s} = \sqrt {\phi /\rho } $, where $\phi $ is the bulk modulus; and $\rho $ is the material density. The thermophysical parameters used in the simulations are listed in Table 1.
Table 1. Thermodynamic and optical property parameters of ITO and silica used in modeling.
In femtosecond laser interactions, electrons reestablish the Fermi distribution after the absorption of energy from the laser pulse, while the lattice remains at the same temperature. The purpose of the simulation was to guide the interpretation of the results. The simulation provided a reasonable approximation to the laser process; simulating values for ${T_e}$ and ${T_l}$ allowed for the determination of overall temperature evolution across a range of applied fluences, which can then be compared to the surface topography observed experimentally. The simulation results describing the evolution of the electron and lattice temperatures are presented in Fig. 9. As shown in Fig. 9(a), the electron temperature increased by a large margin during the early stage of the intense laser irradiation and then transferred the energy to the lattice system through electron–phonon scattering because of the significant temperature difference between the electron and lattice systems. The lattice was strongly heated by the electrons, and thermal electron-lattice equilibrium was established after 30 ps. It is worth noting that increasing the incident laser fluence causes a higher temperature of the electrons and a longer time for hot electrons and lattice to reach thermal equilibrium.
Fig. 9. (a) Electron and lattice temperatures after onset of laser irradiation with different laser fluences. (b) Change in peak lattice temperature with different applied fluence from 0.23 to 0.55 J/cm2.
The black solid line in Fig. 9(b) represents the melting point of the ITO film (1900K) from previous studies [60]. For 0.23 J/cm2, the lattice temperature was just above the melting point, which indicated the presence of melting without ablation. However, for 0.23 to 0.37 J/cm2, the lattice temperature is above the melting point and ablation begins to occur. When the fluence was greater than 0.37 J/cm2, the lattice was heated to a temperature much higher than the melting point, which confirmed the analysis regarding the superheating phenomenon. The surface region is heated to its limit of thermodynamic stability and undergoes a rapid transition from an overheated liquid to a mixture of vapor and liquid droplets.
In addition, we calculated the electron and lattice temperatures of the ITO film under laser irradiation with a pulse width of 150 fs. Figure 10 shows the changes in the electron and lattice temperatures with time under laser irradiation at a 0.4 J/cm2 fluence. As the figure shows, when the electron and lattice temperatures reach thermal equilibrium, the lattice temperature is approximately 2600 K, accompanied by boiling and evaporation. This result is consistent with our previous observations in Fig. 9(b), indicating that the femtosecond laser-modified ITO film is highly dependent on the lattice temperature.
Fig. 10. Electron and lattice temperatures after onset of laser irradiation with laser fluence at 0.40 J/cm2.
5. Conclusion
An investigation of different pulse widths of femtosecond pulse laser ablation properties for 300-nm ITO films on silica substrates was undertaken. The surface characteristics for laser ablation analyzed by OM, SEM, TEM, and surface profiles revealed multiple stages of surface damage, which were found to be highly dependent on the irradiated laser fluences. For well-defined fluences below the ablation threshold in the melting region, a superficial amorphous layer of several tens of nanometers was found. Meanwhile, for a radial Gaussian laser beam irradiating ITO film at fluences up to 0.26 J/cm2, a weak spallation ablation occurred. This process is believed to be due to the ultrafast melting of the surface layer of the ITO film, ultimately leading to photomechanical spallation. For a higher fluence, microvoid structures composed of nanoridges and nanocracks were observed, which involved a rapid transition from an overheated liquid to a mixture of vapor and liquid droplets. A theoretical model was developed for the ITO film to simulate electron and lattice dynamics and temperature. The calculation results confirmed the emergence of the superheating phenomenon. The results are helpful for understanding the mechanism of the interaction between ITO film and intense femtosecond lasers. This can promote potential applications of intense ultrafast laser processing TCOs and other wide-bandgap semiconductors.
Funding
National Key Research and Development Program of China (2018YFE0115900); National Natural Science Foundation of China (11874369, U1831211); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB1603).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. O. Reshef, I. De Leon, M. Z. Alam, and R. W. Boyd, “Nonlinear optical effects in epsilon-near-zero media,” Nat Rev Mater 4(8), 535–551 (2019). [CrossRef]
2. G. V. Naik, V. M. Shalaev, and A. Boltasseva, “Alternative plasmonic materials: beyond gold and silver,” Adv. Mater. 25(24), 3264–3294 (2013). [CrossRef]
3. M. Thirumoorthi and J. T. J. Prakash, “Structure, optical and electrical properties of indium tin oxide ultra thin films prepared by jet nebulizer spray pyrolysis technique,” Journal of Asian Ceramic Societies 4(1), 124–132 (2016). [CrossRef]
4. C. Guillen and J. Herrero, “Structure, optical, and electrical properties of indium tin oxide thin films prepared by sputtering at room temperature and annealed in air or nitrogen,” J. Appl. Phys. 101(7), 073514 (2007). [CrossRef]
5. H. Kim, C. M. Gilmore, A. Pique, J. S. Horwitz, H. Mattoussi, H. Murata, Z. H. Kafafi, and D. B. Chrisey, “Electrical, optical, and structural properties of indium-tin-oxide thin films for organic light-emitting devices,” Journal of Applied Physics 86(11), 6451–6461 (1999). [CrossRef]
6. M. Purica, E. Budianu, and E. Rusu, “ZnO thin films on semiconductor substrate for large area photodetector applications,” Thin Solid Films 383(1-2), 284–286 (2001). [CrossRef]
7. S. Dias, S. Chirakkara, N. Patel, and S. B. Krupanidhi, “Gallium and indium co-doped ZnO as a transparent conducting oxide for Cu2SnS3 photodetectors,” J Mater Sci: Mater Electron 29(3), 2131–2139 (2018). [CrossRef]
8. X. X. Li, Z. Deng, J. Li, Y. F. Li, L. B. Guo, Y. Jiang, Z. G. Ma, L. Wang, C. H. Du, Y. Wang, Q. B. Meng, H. Q. Jia, W. X. Wang, W. M. Liu, and H. Chen, “Hybrid nano-scale Au with ITO structure for a high-performance near-infrared silicon-based photodetector with ultralow dark current,” Photon. Res. 8(11), 1662–1670 (2020). [CrossRef]
9. M. N. Rumyantseva, E. A. Makeeva, S. M. Badalyan, A. A. Zhukova, and A. M. Gaskov, “Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption,” Thin Solid Films 518(4), 1283–1288 (2009). [CrossRef]
10. Z. Q. Zheng, J. D. Yao, B. Wang, and G. W. Yang, “Light-controlling, flexible and transparent ethanol gas sensor based on ZnO nanoparticles for wearable devices,” Sci Rep 5(1), 11070 (2015). [CrossRef]
11. P. P. Sahay and R. K. Nath, “Al-doped ZnO thin films as methanol sensors,” Sensors and Actuators B: Chemical 134(2), 654–659 (2008). [CrossRef]
12. M. A. Martinez, J. Herrero, and M. T. Gutierrez, “Deposition of transparent and conductive Al-doped ZnO thin films for photovoltaic solar cells,” Solar Energy Materials and Solar Cells 45(1), 75–86 (1997). [CrossRef]
13. O. K. Varghese, M. Paulose, and C. A. Grimes, “Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells,” Nature Nanotech 4(9), 592–597 (2009). [CrossRef]
14. C. K. Xu, P. H. Shin, L. L. Cao, J. M. Wu, and D. Gao, “Ordered TiO2 Nanotube Arrays on Transparent Conductive Oxide for Dye-Sensitized Solar Cells,” Chem. Mater. 22(1), 143–148 (2010). [CrossRef]
15. H. Sung, N. Ahn, M. S. Jang, J. K. Lee, H. Yoon, N. G. Park, and M. Choi, “Transparent Conductive Oxide-Free Graphene-Based Perovskite Solar Cells with over 17% Efficiency,” Adv. Energy Mater. 6(3), 1501873 (2016). [CrossRef]
16. B. Bissig, T. Jager, L. Ding, A. N. Tiwari, and Y. E. Romanyuk, “Limits of carrier mobility in Sb-doped SnO2 conducting films deposited by reactive sputtering,” APL Materials 3(6), 062802 (2015). [CrossRef]
17. Y. G. Bi, Y. F. Liu, X. L. Zhang, D. Yin, W. Q. Wang, J. Feng, and H. B. Sun, “Ultrathin Metal Films as the Transparent Electrode in ITO-Free Organic Optoelectronic Devices,” Advanced Optical Materials 7(6), 1800778 (2019). [CrossRef]
18. D. C. Adams, S. Inampudi, T. Ribaudo, D. Slocum, S. Vangala, N. A. Kuhta, W. D. Goodhue, V. A. Podolskiy, and D. Wasserman, “Funneling light through a subwavelength aperture with epsilon-near-zero materials,” Phys. Rev. Lett. 107(13), 133901 (2011). [CrossRef]
19. M. H. Javani and M. I. Stockman, “Real and Imaginary Properties of Epsilon-Near-Zero Materials,” Phys. Rev. Lett. 117(10), 107404 (2016). [CrossRef]
20. E. G. Carnemolla, L. Caspani, C. DeVault, M. Clerici, S. Vezzoli, V. Bruno, V. M. Shalaev, D. Faccio, A. Boltasseva, and M. Ferrera, “Degenerate optical nonlinear enhancement in epsilon-near-zero transparent conducting oxides,” Opt. Mater. Express 8(11), 3392–3400 (2018). [CrossRef]
21. H. Ma, Y. A. Zhao, Y. C. Shao, Y. F. Lian, W. L. Zhang, G. H. Hu, Y. X. Leng, and J. D. Shao, “Principles to tailor the saturable and reverse saturable absorption of epsilon-near-zero material,” Photon. Res. 9(5), 678–686 (2021). [CrossRef]
22. J. Yoon, M. Zhou, M. A. Badsha, T. Y. Kim, Y. C. Jun, and C. K. Hwangbo, “Broadband Epsilon-Near-Zero Perfect Absorption in the Near-Infrared,” Sci Rep 5(1), 12788 (2015). [CrossRef]
23. Q. Gao, E. W. Li, and A. X. Wang, “Comparative analysis of transparent conductive oxide electro-absorption modulators,” Opt. Mater. Express 8(9), 2850–2862 (2018). [CrossRef]
24. A. Anopchenko, L. Tao, C. Arndt, and H. W. H. Lee, “Field-Effect Tunable and Broadband Epsilon-Near-Zero Perfect Absorbers with Deep Subwavelength Thickness,” ACS Photonics 5(7), 2631–2637 (2018). [CrossRef]
25. A. Capretti, Y. Wang, N. Engheta, and L. Dal Negro, “Comparative Study of Second-Harmonic Generation from Epsilon-Near-Zero Indium Tin Oxide and Titanium Nitride Nanolayers Excited in the Near-Infrared Spectral Range,” ACS Photonics 2(11), 1584–1591 (2015). [CrossRef]
26. N. C. Passler, I. Razdolski, D. S. Katzer, D. F. Storm, J. D. Caldwell, M. Wolf, and A. Paarmann, “Second Harmonic Generation from Phononic Epsilon-Near-Zero Berreman Modes in Ultrathin Polar Crystal Films,” ACS Photonics 6(6), 1365–1371 (2019). [CrossRef]
27. T. S. Luk, D. de Ceglia, S. Liu, G. A. Keeler, R. P. Prasankumar, M. A. Vincenti, M. Scalora, M. B. Sinclair, and S. Campione, “Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films,” Appl Phys Lett 106(15), 151103 (2015). [CrossRef]
28. A. Capretti, Y. Wang, N. Engheta, and L. Dal Negro, “Enhanced third-harmonic generation in Si-compatible epsilon-near-zero indium tin oxide nanolayers,” Opt. Lett. 40(7), 1500–1503 (2015). [CrossRef]
29. W. D. Tian, F. Liang, D. Z. Lu, H. H. Yu, and H. J. Zhang, “Highly efficient ultraviolet high-harmonic generation from epsilon-near-zero indium tin oxide films,” Photon. Res. 9(3), 317–323 (2021). [CrossRef]
30. M. Z. Alam, I. De Leon, and R. W. Boyd, “Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region,” Science 352(6287), 795–797 (2016). [CrossRef]
31. P. J. Guo, R. D. Schaller, J. B. Ketterson, and R. P. H. Chang, “Ultrafast switching of tunable infrared plasmons in indium tin oxide nanorod arrays with large absolute amplitude,” Nature Photon 10(4), 267–273 (2016). [CrossRef]
32. N. Kinsey, C. DeVault, J. Kim, M. Ferrera, V. M. Shalaev, and A. Boltasseva, “Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths,” Optica 2(7), 616–622 (2015). [CrossRef]
33. Z. Chai, X. Y. Hu, F. F. Wang, C. Li, Y. T. Ao, Y. Wu, K. B. Shi, H. Yang, and Q. H. Gong, “Ultrafast on-Chip Remotely-Triggered All-Optical Switching Based on Epsilon-Near-Zero Nanocomposites,” Laser & Photonics Reviews 11(5), 1700042 (2017). [CrossRef]
34. Y. C. Chung, Y. H. Chiu, H. J. Liu, Y. F. Chang, C. Y. Cheng, and F. C. N. Hong, “Ultraviolet curing imprint lithography on flexible indium tin oxide substrates,” J. Vac. Sci. Technol. B 24(3), 1377–1383 (2006). [CrossRef]
35. S. S. Mammana, A. Greatti, F. H. Luiz, F. I. da Costa, A. P. Mammana, G. A. Calligaris, L. P. Cardoso, C. I. Z. Mammana, and D. den Engelsen, “Study of wet etching thin films of indium tin oxide in oxalic acid by monitoring the resistance,” Thin Solid Films 567, 20–31 (2014). [CrossRef]
36. O. Yavas, C. Ochiai, and M. Takai, “Substrate-assisted laser patterning of indium tin oxide thin films,” Applied Physics A: Materials Science & Processing 69(7), S875–S878 (1999). [CrossRef]
37. M. F. Chen, Y. P. Chen, W. T. Hsiao, and Z. P. Gu, “Laser direct write patterning technique of indium tin oxide film,” Thin Solid Films 515(24), 8515–8518 (2007). [CrossRef]
38. Z. Kuang, W. Perrie, D. Liu, P. Fitzsimons, S. P. Edwardson, E. Fearon, G. Dearden, and K. G. Watkins, “Ultrashort pulse laser patterning of indium tin oxide thin films on glass by uniform diffractive beam patterns,” Applied Surface Science 258(19), 7601–7606 (2012). [CrossRef]
39. H. J. Kim, K. W. Seo, Y. H. Kim, J. Choi, and H. K. Kim, “Direct laser patterning of transparent ITO-Ag-ITO multilayer anodes for organic solar cells,” Applied Surface Science 328, 215–221 (2015). [CrossRef]
40. F. Chen and J. R. V. de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond- laser micromachining,” Laser Photonics Rev 8(2), 251–275 (2014). [CrossRef]
41. C. Wang, H. I. Wang, W. T. Tang, C. W. Luo, T. Kobayashi, and J. P. Leu, “Superior local conductivity in self-organized nanodots on indium-tin-oxide films induced by femtosecond laser pulses,” Opt. Express 19(24), 24286–24297 (2011). [CrossRef]
42. C. Wang, H. I. Wang, C. W. Luo, and J. Leu, “Anisotropic optical transmission of femtosecond laser induced periodic surface nanostructures on indium-tin-oxide films,” Appl Phys Lett 101(10), 101911 (2012). [CrossRef]
43. M. H. Chen, Y. H. Tseng, Y. P. Chao, S. Y. Tseng, Z. R. Lin, H. H. Chu, J. K. Chang, and C. W. Luo, “Effects on Organic Photovoltaics Using Femtosecond-Laser-Treated Indium Tin Oxides,” Acs Appl Mater Inter 8(38), 24989–24993 (2016). [CrossRef]
44. B. Guo, J. Sun, Y. Lu, and L. Jiang, “Ultrafast dynamics observation during femtosecond laser-material interaction,” International Journal of Extreme Manufacturing 1(3), 032004 (2019). [CrossRef]
45. Y. L. Lian, L. Jiang, J. Y. Sun, H. Jin, M. M. Wang, J. Liu, C. J. Pan, Q. S. Wang, and Z. C. Chen, “Asymmetric Response Optoelectronic Device Based on Femtosecond-Laser-Irradiated Perovskite,” Acs Appl Mater Inter 12(14), 17070–17076 (2020). [CrossRef]
46. Y. C. Shao, Y. A. Zhao, H. Ma, C. Li, D. W. Li, and J. D. Shao, “Refining multi-photon polymerization feature size by optimizing solvent content in SU-8 photoresist,” Opt. Mater 112, 110800 (2021). [CrossRef]
47. L. Rodriguez-Sune, M. Scalora, A. S. Johnson, C. Cojocaru, N. Akozbek, Z. J. Coppens, D. Perez-Salinas, S. Wall, and J. Trull, “Study of second and third harmonic generation from an indium tin oxide nanolayer: Influence of nonlocal effects and hot electrons,” Apl Photonics 5(1), 010801 (2020). [CrossRef]
48. J. M. Liu, “Simple Technique for Measurements of Pulsed Gaussian-Beam Spot Sizes,” Opt. Lett. 7(5), 196–198 (1982). [CrossRef]
49. J. Bonse, S. M. Wiggins, and J. Solis, “Dynamics of femtosecond laser-induced melting and amorphization of indium phosphide,” J. Appl. Phys. 96(4), 2352–2358 (2004). [CrossRef]
50. L. V. Zhigilei, Z. B. Lin, and D. S. Ivanov, “Atomistic Modeling of Short Pulse Laser Ablation of Metals: Connections between Melting, Spallation, and Phase Explosion,” J Phys Chem C 113(27), 11892–11906 (2009). [CrossRef]
51. T. Szorenyi, L. D. Laude, I. Bertoti, Z. Kantor, and Z. Geretovszky, “Excimer-Laser Processing of Indium-Tin-Oxide Films - an Optical Investigation,” J. Appl. Phys. 78(10), 6211–6219 (1995). [CrossRef]
52. A. Thogersen, M. Rein, E. Monakhov, J. Mayandi, and S. Diplas, “Elemental distribution and oxygen deficiency of magnetron sputtered indium tin oxide films,” J. Appl. Phys. 109(11), 113532 (2011). [CrossRef]
53. J. H. Park, C. Buurma, S. Sivananthan, R. Kodama, W. Gao, and T. A. Gessert, “The effect of post-annealing on Indium Tin Oxide thin films by magnetron sputtering method,” Applied Surface Science 307, 388–392 (2014). [CrossRef]
54. S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B. Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, “Laser-induced microexplosion confined in the bulk of a sapphire crystal: Evidence of multimegabar pressures,” Phys. Rev. Lett. 96(16), 166101 (2006). [CrossRef]
55. J. K. Chen, D. Y. Tzou, and J. E. Beraun, “A semiclassical two-temperature model for ultrafast laser heating,” Int J Heat Mass Tran 49(1-2), 307–316 (2006). [CrossRef]
56. M. Z. Alam, S. A. Schulz, J. Upham, I. De Leon, and R. W. Boyd, “Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material,” Nat Photonics 12(2), 79–83 (2018). [CrossRef]
57. T. Yagi, K. Tamano, Y. Sato, N. Taketoshi, T. Baba, and Y. Shigesato, “Analysis on thermal properties of tin doped indium oxide films by picosecond thermoreflectance measurement,” J Vac Sci Technol A 23(4), 1180–1186 (2005). [CrossRef]
58. J. J. Lin and Z. Q. Li, “Electronic conduction properties of indium tin oxide: single-particle and many-body transport,” J Phys-Condens Mat 26(34), 343201 (2014). [CrossRef]
59. T. Ashida, A. Miyamura, N. Oka, Y. Sato, T. Yagi, N. Taketoshi, T. Baba, and Y. Shigesato, “Thermal transport properties of polycrystalline tin-doped indium oxide films,” J. Appl. Phys. 105(7), 073709 (2009). [CrossRef]
60. O. Yavas and M. Takai, “Effect of substrate absorption on the efficiency of laser patterning of indium tin oxide thin films,” J. Appl. Phys. 85(8), 4207–4212 (1999). [CrossRef]
