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Abstract
A Nd:YAG (λ = 355 nm) nanosecond laser is used to anneal a 45-nm-thick amorphous-Si (a-Si) thin film on a glass substrate. Via scanning with a laser beam having a Gaussian shape at a repetition rate of 14 kHz, the surface of the a-Si film is crystallized, and laser-induced periodic surface structures (LIPSSs) are formed within the fluence range of 30–35 mJ/cm2. The formation energy of surface ripples is significantly lower than the typical fluence of a few 100 mJ/cm2. Confocal Raman spectroscopy and atomic force microscopy reveal that the a-Si film is only crystallized near the top surface and that the surface ripples are aligned to the perpendicular direction of laser polarization, in accordance with the LIPSS model. For a laser fluence of >35 mJ/cm2, the surface texture loses its periodicity but forms randomly distributed Si grains with a surface roughness of >40 nm. The laser processing on an a-Si film achieved by scanning up to 20 × 20 mm2 shows uniform periodic surface textures, which can be employed in the display or photovoltaic applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-induced periodic surface structures (LIPSSs), i.e., the periodic ripples emerging on material surfaces after multiple irradiations of laser pulses, have been intensively studied for several decades [1–5]. The identification of the ripple formation mechanism has attracted considerable attention because it not only provides a fundamental understanding of the interaction of electromagnetic waves with materials [1,3,4] but also is useful for various industrial applications involving modification of material surfaces, such as semiconductors, metal, and organic materials [6–11]. Although several models have been suggested, no theoretical model has completely explained the mechanism of ripple generation [3,4,12–14]. LIPSSs are currently considered to originate from the interaction of incident laser light with the material surface, which leads to spatial field modulations that occur on the surface during laser irradiations and are transferred into the materials [1–5]. The ripple features usually depend on the laser energy, polarization, incident angle, and number of laser irradiations [4,5,9,12]. However, in most cases, the LIPSS process involves several parameters and the exact ripple formations are not characterized simply [6–8,11,12].
Recently, LIPSSs on crystalline Si (c-Si) and amorphous Si (a-Si) have attracted considerable attention because Si is a widely used material in the semiconductor or the display industry [6,12–21]. Using Gaussian-shaped pulses, various features of the surface texture are studied with respect to the polarization, laser fluence, and number of irradiations. It has been reported that periodic textures are formed at the average laser fluence of 200–600 mJ/cm2 for both c-Si [12–21] and a-Si [22–26], representing the two regions of the submicron-scale and micron-scale surface textures depending on the polarization and laser energy. Additionally, similar LIPSS patterns were obtained on a-Si and c-Si thin films at 30–60 mJ/cm2, which is as small as one order of magnitude [18,23,24]. However, most of the previous studies focused on the evaluation of the surface ripples in the local area of Si thin films. Practically, laser scanning over a large area to form a uniform ripple pattern is an important process in industrial applications.
Laser crystallization for converting a-Si into the poly-Si (p-Si) phase through laser scanning is a widely used technique in Si-based devices, such as organic light-emitting diode (OLED) displays and photovoltaic devices [22–33]. In several studies, surface textures of an a-Si thin film layer were obtained via ultrashort pulse laser irradiation or laser beam scanning, which increased the visible-light absorption, improving the light-conversion efficiency [22–24]. For OLED displays, the laser crystallization process is one of the major processes, which is called the low-temperature polycrystalline silicon (LTPS) process and is a well-established method for obtaining p-Si grains from a-Si [27–32]. Currently, the LTPS process is performed using an XeCl excimer laser (λ = 308 nm), which is transformed into a long and narrow line beam for large-area processing [27–32]. However, the LTPS process accompanies complicated processes such as light absorption, heat transfer, solid-to-liquid phase changes, and cooling within tens of nanoseconds, which hinders the formation of uniform p-Si grains [30–32]. Furthermore, the current LTPS process requires a high fabrication cost, owing to the excimer laser and the large optical parts for constructing a large line beam [31,32].
Herein, we report the results of periodic surface ripple formation on an a-Si film achieved by scanning an ultraviolet nanosecond laser up to 20 × 20 mm2. The surface ripples were regularly formed on the a-Si surface after the scanning of a Gaussian spot beam with laser fluences ranging from 30 to 35 mJ/cm2. Analysis of the surface ripples using an optical microscope, atomic force microscopy (AFM), and Raman spectroscopy indicated that a uniform surface texture can be achieved via laser scanning of a spot beam. This method can be employed in the LTPS process or photovoltaic applications.
2. Experimental details
A 45-nm-thick a-Si film was deposited on a 300-nm-thick oxide layer and a 100-nm-thick nitride layer, which was deposited on a glass substrate via plasma-enhanced chemical vapor deposition. A Q-switched diode-pumped solid-state Nd:YAG laser (EdgeWave, IS-Series) with a wavelength of 355 nm was used for the surface texturing process. Figure 1(a) presents a schematic of the laser-scanning system, where the laser beam was steered in two dimensions using two Galvano-mirrors and applied to the a-Si film using an f-theta lens with a focal length of 255 mm. The laser pulse frequency and the pulse duration were 14 kHz and 5.5 ns, respectively.
Fig. 1. (a) Schematic of the laser-scanning system for nanosecond laser annealing. (b) Gaussian spatial profile of a single laser beam (λ = 355 nm) and cross-sectional profiles of the laser beam (solid lines) and Gaussian fit curves (dotted lines) at horizontal and vertical directions. The arrow indicates the polarization direction. (c) Schematic of two-dimensional laser scanning path.
Figure 1(b) shows a laser-beam profile and its cross sections in the horizontal and vertical directions, respectively. Because the laser beam was not isotropic, the beam profile was fitted to an asymmetric Gaussian function of the x- and y-axes, as follows:
3. Results and discussion
3.1 A single pulse irradiation on an a-Si thin film layer
Figures 2(a)–(e) show optical microscope images of the Si surface irradiated by a single laser beam, which were obtained in the reflective Nomarski differential interference contrast (DIC) mode. The laser powers, laser energies per pulse (Eo), average fluences (Iavg), and maximum fluences (Io) of the current experiments are listed in Table 1. The irradiated area of 4πσxσy, corresponding to 1/e2 of maximum intensity, Io was used. The maximum fluence, Io = Eo/(2πσxσy) was essentially two times larger than the average fluence, Iavg.
Fig. 2. Nomarski microscope images of a single pulse applied to an a-Si thin film with laser energies, Eo (max.fluence, Io) of 76 µJ (32 mJ/cm2) (a), 83 µJ (34 mJ/cm2) (b), 89 µJ (37 mJ/cm2) (c), and 94 µJ (39 mJ/cm2) (d). (See the Table 1 for more explanations of laser energy.) (e) Enlarged microscope image of rectangular in (d) and horizontal and vertical cross-sectional surface profiles.
Table 1. Values of laser power, a pulse energy, average fluence, and maximum fluence for laser annealing experiments.
The microscopic images clearly indicated a dark pink region where the laser intensity was maximized, and this region was enlarged as the laser intensity increased. In Fig. 2(a), the energy fluence corresponding to the dark pink region is approximately 30 mJ/cm2, implying that the surface properties changed after irradiation with a single laser beam. In Fig. 2(b), the dark pink area is further enlarged, because the laser energy (83 µJ) of (b) was 10% higher than that (76 µJ) of (a). Interestingly, another surface modification was observed at a higher laser energy, as shown in Figs. 2(c)–(e), where an orange region is observed within the dark pink area. In Fig. 2(c), the energy fluence of the orange region corresponds to approximately 35 mJ/cm2. In Fig. 2(d), the orange region extends further to a length of approximately 150 µm. Figure 2(e) presents an enlarged view of the rectangular area. The DIC microscope image of Fig. 2(e) reveals that the surface roughness increased from 2 nm in the orange and dark pink region to 37 nm in the orange region.
Recently, Differt et al. reported that a nanotextured surface was produced on an a-Si thin film via femtosecond laser processing (λ = 795 nm) with a peak fluence of 75 mJ/cm2 and suggested that the optical properties and ablation of an a-Si thin film could occur at approximately 30 and 40 mJ/cm2, respectively [23,33]. Although the laser processing, such as the laser type and the beam profile, were not the same as those in our experiments, the fact that the surface textures were obtained with a similar laser fluence (30–35 mJ/cm2) was interesting. Furthermore, our results indicate that the surface ripples were well aligned to the perpendicular direction of laser polarization, in accordance with the LIPSS model.
3.2 The ripple formations on an a-Si thin film layer via 2D scanning
The laser spot beam was scanned in a 20 mm × 20 mm area of the a-Si surface in the horizontal and vertical directions with the same laser intensities used for Figs. 2(a)–(d). The scanning speed was set by applying pulse-to-pulse intervals of 1.5 µm and 62 µm in the x and y directions, respectively. The laser fluence of 30 mJ/cm2 was chosen as the triggering energy of the surface modification, and the dark pink region of Fig. 2(a) was approximately 30 µm × 180 µm in the x and y directions. Therefore, the average number of irradiations (N) was approximately 60. It should be noted that the large area scanning with small irradiation number is crucially important to improve the productivity [30–32] and the current experiments show the potential validation of productivity. Figures 3(a)–(d) show photographs of the scanned Si substrate, where the laser pulse energies were used as listed in Table 1. All the photographs of Figs. 3(a)–(d) were taken at oblique angle of 45° from normal direction under ambient white light. The arrow indicates the polarization direction. As the laser energy increased beyond 89 µJ, the Si surface layer exhibited a hazy feature, as shown in Figs. 3(c) and (d), which is attributed to the overlap area of the rough orange region in Figs. 2(c) and (d), resulting in surface light scattering.
Fig. 3. Photographic images of Si thin film, scanned over a 20 mm x 20 mm by laser spot beam with laser energies, Eo (maximum fluence, Io) of 76 µJ (32 mJ/cm2) (a), 83 µJ (34 mJ/cm2) (b), 89 µJ (37 mJ/cm2) (c), and 94 µJ (39 mJ/cm2) (d). The images were obtained at oblique angle of 45° under ambient white light. (e)–(h) Photographs of the same area of (a)–(d), taken backward illumination of the white light at the same oblique angle. The arrow indicates the polarization direction.
Figures 3(e)–(h) present images taken at the same oblique angle. However, in this case, the white light is illuminated backward at the position, where the images were taken. The blue color is clearly observed in Figs. 3(e)–(g). This is attributed to the 1st-order diffraction of visible light caused by surface periodic ripples, as can be expressed by Eq. (2) [34]:
where θo, θi, Λ, λ, and m are incident angle, diffraction angle, period of ripples, and diffracted wavelength, respectively. For θo = θi ∼ −45°, λ = 450 nm (blue color), and m = −1, the surface ripple period, Λ is estimated to be about 320 nm that is close to the measured data in Fig. 6. For a higher laser energy, the diffraction was deteriorated by the surface scattering, as shown in Figs. 3(g) and (h). The formation of the surface ripples as a result of the laser annealing and their alignment to the perpendicular direction of laser polarization are consistent with the LIPSS theory and previous works [4,6,15,30]. However, the surface ripples were formed at a lower laser fluence (30–35 mJ/cm2) than those in previous studies (approximately 200–600 mJ/cm2) [22–26,30].3.3 The crystallinity of the 2D scanned Si thin film layer
The crystallinity of the Si layer was analyzed via confocal Raman spectroscopy performed with 3 µm spot diameter of 532 nm excitation wavelength. Figure 4 shows the Raman spectra of annealed surfaces with different laser energies and the non-annealed a-Si surface. Each Raman spectrum was obtained by averaging 200 measurements at 3 µm spacing in the y-axis direction of the Si surface. The maximum peak of a-Si was normally observed at 480 cm−1 [35]. For the laser-annealed surfaces, the maximum peaks were observed at 510 cm−1, and the peak intensity increased in proportion to the laser power [35].
Fig. 4. Raman spectra of the a-Si surface irradiated with various laser powers. The laser energies for S1-S4 are used as listed in Table 1.
However, for the Raman peaks of laser energies of 76 µJ and 83 µJ, the peak intensity at 510 cm−1 is relatively low and the peak of a-Si phase is still dominant. Considering that the a-Si has strong absorption at the laser wavelength of 355 nm and the irradiated laser energy of 76 and 83 µJ (or peak fluence of 32 and 34 mJ/cm2) is small, these results indicate that the surface ripples were formed near the top surface layer through the partial melting process.
Figures 5(a)–(d) show AFM images of 5 µm × 5 µm Si surfaces, each corresponding to Figs. 3(a)–(d). The laser polarization is indicated by an arrow, but 90° rotated. In Figs. 5(a) and (b), periodic ripples are formed vertically, perpendicular to the polarization. The ripples in Fig. 5(b) are clearly observed, which is consistent with the results of the LIPSS model [4,15,30]. However, in Figs. 5(c) and (d), the surface textures lose periodic feature and are randomly distributed with round-shape grains. The abrupt surface changes are attributed from the orange region in Figs. 2(c)–(e) at which the surface morphological variations are observed significantly. As laser energy increases, the rough orange regions are enlarged further and more overlapped under the multiple irradiations, resulting in formations of aggregated surfaces as shown in Figs. 5(c) and (d). The surface roughness, Ra, in Figs. 5(a)–(d) is 9.7, 10.3, 39.0, and 62.6 nm, respectively and the abrupt increase of the surface roughness in Fig. 5(c) and (d) also supports the hazy features in Figs. 3(c), (d), (g), and (h).
Fig. 5. AFM images of the annealed Si surfaces with laser energies, Eo (max. fluence, Io) of 76 µJ (32 mJ/cm2) (a), 83 µJ (34 mJ/cm2) (b), 89 µJ (37 mJ/cm2) (c), and 94 µJ (39 mJ/cm2) (d). The arrow indicates the polarization direction.
Figures 6(a)–(d) show 2D Fourier-transformed images of Figs. 5(a)–(d). Figures 6(a) and (b) show sickle-shaped patterns in the horizontal direction, whereas in Figs. 6(c) and (d), these patterns are weak or hardly observed. The intensity around the center gradually increased with the increasing laser energy. Figure 6(e) presents a horizontal cross-sectional plot of Figs. 6(a)–(d), separated vertically for comparison. The peak position of plots (a)–(c) is 3.12 ± 0.1 µm−1, which corresponds to a ripple period (Λ) of 321 ± 10 nm. This value corresponds to approximately 90% of the laser wavelength (λ = 355 nm). In contrast, plot (d) exhibits no peak but is centered with a Gaussian-like distribution, which indicates relatively large grain size distributions on the surface. In fact, Fig. 5(d) shows that the grain sizes are about 100 nm ∼ 400 nm.
Fig. 6. 2D Fourier transformed images corresponding to Figs. 5(a)-(d) and (e) horizontal cross-sectional plot.
During the laser irradiation, the melt Si transformed into the conducting state, and the quasi-free carriers inside the melt Si formed the surface plasmon (SP) mode, oscillating along the surface at the air and melt Si interface, and surface ripples with the same period as the SP mode were generated along the SP propagating direction [4,5,13]. The ripple period (Λ) was affected by the coupling between the SP mode and the grating-like ripples and decreased as the number of laser irradiations increased. In general, the ripple period is approximately 0.6λ–0.9λ, depending on the number of irradiations, N [4,5,13]. Considering that N ≈ 60 in our experiments, the measured ripple period of 0.9λ is consistent with the foregoing analysis, and the surface texture should be further studied for different laser energies and numbers of irradiations.
4. Conclusions
We performed the crystallization of an a-Si thin film by scanning a Gaussian beam over a 20 mm × 20 mm area and examined the surface texture induced by the laser irradiation. The 355-nm nanosecond laser formed periodic surface ripples on the a-Si thin film, which comprised submicron sized Si grains, depending on the laser energy. The laser energy corresponding to a peak energy fluence of 30–35 mJ/cm2 produced aligned surface ripples perpendicular to the polarization, whereas the laser energy corresponding to a peak energy fluence of >35 mJ/cm2 produced a random surface. The surface morphology was analyzed via Raman spectroscopy, and AFM, confirming that the a-Si surface was textured with a lower laser energy compared with previous results. The formation of periodic ripples surface on the a-Si thin film with less energy can be applied to the LTPS process in OLED display or photovoltaic devices as an alternative technology.
Funding
Ministry of Trade, Industry and Energy (20184030201910, 10079974).
Acknowledgments
This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and financial resources were granted by the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20184030201910). This work was also supported by the Future Growth Engine Program (10079974, Development of core technologies on materials, devices, and processes for TFT backplane and light emitting front plane with enhanced stretchability above 20%, with application to stretchable display) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).
Disclosures
The authors declare no conflicts of interest.
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