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Abstract
Formation dynamics of laser-induced periodic surface structures (LIPSSs) on the SiC substrates were described with varying pulse numbers and pulse duration. As the number of laser pulses increases, two significant transformations become evident in the progression of structural formations. First from surface roughening with nanoparticles to LIPSS with the period that is slightly shorter than the laser wavelength. Second it turns to LIPSS with a period less than half the laser wavelength. It is found that maintaining the crystallinity is the key to changing the structures. In the cases of longer pulse width than sub-nanoseconds, no LIPSS formations are observed or LSFL does not change to HSFL because the irradiated area is poly-crystallized.
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1. Introduction
The comprehension of physical phenomena arising from the interaction between light and materials has played a pivotal role in advancing the field of optics. These lasers have not only greatly impacted material processing but have also advanced technology in a wide range of fields, including precision measurements and characterization [1], THz pulse generation [2,3], and medical applications [4,5]. In particular, lasers with ultra-short pulse durations have enabled the development of laser processing technology that minimizes thermal effects on the processing area [6,7].
It is known that periodic structures are spontaneously formed when ultrashort laser pulses are irradiated onto the material surface. These structures are called laser-induced periodic surface structures (LIPSSs) [8–13]. One of the interesting characteristics is that, in majority of cases, the period of LIPSS is shorter than the wavelength of the irradiating laser. Depending on its period, LIPSS is classified into low-spatial-frequency LIPSS (LSFL) and high-spatial-frequency LIPSS (HSFL) [14–16]. An LSFL has a period Λ slightly shorter than the laser wavelength λ (0.5λ < Λ < λ) and an HSFL has a period less than half of wavelength (Λ < 0.5λ). LIPSSs are expected to emerge as a new technique for fine processing, capitalizing on the simplicity of laser irradiation alone. Furthermore, it offers the potential for achieving miniaturization beyond the constraints of wavelength limitations. LIPSS has a broad range of applications due to its non-contact nature, making it applicable to most solid materials. For example, LIPSS has been applied to structural color design [17], formation of interfacial structures to improve the efficiency of solar cells [18], hydrophobicity control [19], and bacterial adhesion inhibition [20]. Our group has also reported the application of LIPSS to control the formation of crystal nuclei in semiconductor crystal growth [21].
For prospective applications of LIPSSs, structural control in nano-scale is required and it is important to understand their formation mechanism. There are still many phenomena that have not been explained, although some formation mechanisms have been proposed to shed light on this matter. For pulse durations in the femtosecond range, surface plasmon polariton (SPP) excitation or parametric decay process models have been proposed that contribute LIPSS to the change in plasma state at the material surface induced by laser irradiation [22–26]. On the other hand, for pulse durations longer than the picosecond range, the thermal distribution such as Marangoni convection and Rayleigh-Taylor instability [27] are proposed to be the main contributor in the formation of the LIPSS. To clarify the formation dynamics of LIPSS, it is important to understand the crystallinity of LIPSS. Previously, some research on the crystallinity evaluation of LIPSS has been reported [28–33], however, there is no report ever on the relationship between the formation dynamics of LIPSS and the crystallinity.
In this study, we systematically examined the dynamics of LIPSS formation on the 6H-SiC substrates by varying the laser pulse duration across femtosecond to nanosecond range within the proximity of 1 µm wavelength spectrum (1030, 1045, and 1064 nm). Our primary objective is to comprehend the intricacies of LIPSS formation by investigating modifications in crystal state and shape induced by superimposed laser pulses. This comprehensive and unified exploration of LIPSS formation dynamics aims to provide a deeper understanding of the interactive phenomena between light and materials. The insights gained from this research not only hold the potential to enhance the functional application of LIPSS but also contribute to a broader comprehension of light-material interactions.
2. Methods
2.1 Laser irradiation
We used lasers with similar wavelengths and different pulse durations, where µ-jewel D-10 K (IMRA America Inc.; wavelength λ: 1045 nm, pulse duration τ: 450 fs, repetition frequency f: 100 kHz), TruMicro5050 (TRUMPF corp.; λ: 1030 nm, τ: 900 fs, f: 100 kHz), TruMicro5070FE (TRUMPF corp.; λ: 1064 nm, τ: 10 ps (τ<10 ps: vicinity of 10 ps), f: 100 kHz), and Giant-pulse duration tunable microchip laser (λ: 1064 nm, τ: 0.5, 1, 2.5 ns, f: 100 Hz) [34–36]. Laser irradiation was performed in air and focused the laser beam using plane-convex lens with a focal length of 10 cm. The laser fluences of laser irradiation were controlled using a half-wave plate and polarizing beam splitter (PBS). The irradiated laser fluence F at different pulse widths are [8.85 J/cm2 at τ=450 fs]; [6.62 J/cm2 at τ=900 fs and 10 ps]; and [4.07 J/cm2 at τ=500 ps, 1 ns and 2.5 ns]. The laser fluence at which LIPSS begins to form varies depending on the pulse duration. In this study, we used the laser fluence that can stably form LIPSS at each pulse duration. The LIPSS formation dynamics was investigated by varying the number of the laser pulses N. Here, N means the number of laser pulses irradiated to the same place on the substrate. We have controlled N in two ways. We have controlled N by changing the scanning speed for the lasers with τ=450 fs, 500 ps, 1 ns and 2.5 ns, and using galvanometer mirror for the lasers with τ=900 fs and 10 ps.
2.2 Materials and analytical methods
6H-SiC (0001) substrates were used as irradiated materials. A 6H-SiC is an attractive material for use in high-temperature and high-voltage devices. Moreover, SiC is useful for investigating the effect of laser irradiation on a material because it is physically and chemically stable. The bandgap of 6H-SiC is 2.93 eV, which is larger than the photon energy of the applied laser. SiC is a hard-to-process material. It is the third hardest material after diamond and boron carbide. Due to this, contact-based processes such as blade cutting are very difficult to execute, although laser processing techniques, such as those used in this research, allow the processing of materials. Before and after laser irradiation, the SiC substrate was cleaned using acetone, hydrofluoric acid (50 wt.%), ultrapure water, and isopropyl alcohol to remove contamination and redeposited debris. The shape and crystalline state of the LIPSS were evaluated using a scanning electron microscope (SEM; JSM-7800F, JEOL) and a transmission electron microscope (TEM; JEM-2100EX, JEOL).
The samples for TEM observation were prepared using a focused-ion-beam (FIB) processing machine (JEOL JEM-9320FIB).
3. Results
3.1 Structural change of LIPSS with superimposed laser pulses
The surface images of laser-irradiated 6H-SiC taken by a scanning electron microscope (SEM) are shown in Fig. 1. The upper images represent the whole irradiated area, and the lower images represent the enlarged area. Laser irradiation was performed using laser pulse duration τ=10 ps while N was varied from 3 to 200. The direction of the laser polarization is shown in the Fig. 1 as E. The laser fluence of the irradiated laser was 6.62 J/cm2. Following irradiation with 3 pulses, a circular ablation crater emerged, showcasing a particulate structure without specific periodicity and measuring approximately 100 nm in size. After 7 pulses irradiation, periodic linear structures perpendicular to the laser polarization started to form. The structures became highly periodic and clear with increasing the number of pulses from 7 to 12. At N = 12, the LIPSS was LSFL with a periodicity of approximately 900 nm. As shown in the surface SEM image of N = 50, the area of LSFL expanded and at the same time a HSFL with Λ=300 nm started to form. At N = 200, the area of the HSFL further expanded. It has been reported that LIPSS is formed at laser fluences that are slightly lower than the ablation threshold and that LIPSS changes from LSFL to HSFL with increasing the number of laser pulses [12,37]. Our group also has reported the shape change from LSFL to HSFL previously [38], in this research, we presented the detailed process of the formation of the particulate structures before LSFL formation, and the dynamics of structure change from LSFL to HSFL.
Fig. 1. Surface SEM images of the irradiated area of 6H-SiC using a laser with τ=10 ps. Surface structures changed with increase in the number of laser pulses.
To observe the crystalline state of the irradiated area or LIPSS, cross-sectional images taken using transmission electron microscope (TEM) are represented in Fig. 2. The observation area is represented in the Fig. 1 with a yellow square. The laser pulse duration (τ) used for irradiation is 10 ps, and the number of laser pulses varied from N = 3 to 200. The top row images present the bright-field images and the bottom row presents the dark-field images while the selected area electron diffraction (SAED) patterns taken at the position are presented by green dashed circles in the bright-field images, respectively. Gallium and carbon were deposited on the surface during the Focused Ion Beam (FIB) process of the sample preparation for TEM observation. At the initial stage before LIPSS formation (N = 3), the transmission and diffraction images of the irradiated area exhibit uniformity, and the SAED shows a well-oriented dot pattern with slight spread, which indicates that the irradiated area kept crystalline, while the uppermost surface (down to approximately 20 nm) including the particulate nanostructures was non-crystalline. Similar crystallinity for the uppermost surface has been reported previously [28,29]. As represented in the TEM images and the SAED pattern, the LSFLs formed at N = 7, 12 and 50 were crystalline and increased the height with laser pulses while maintaining their crystallinity. The HSFLs formed at N = 200 also kept crystalline. In this regard, it can also be shown that dislocation-like defects are generated locally at the valleys of LSFL and HSFL. During the formation process of LIPSS with laser pulses with τ=10 ps, which changes its shape from particulate structure, LSFL then HSFL, the crystalline state is maintained and not changed to amorphous or polycrystalline. This is while even some dislocation-like defects are generated locally.
Fig. 2. Cross-sectional TEM images of the irradiated area using a laser with τ=10 ps. Irradiated area kept crystalline throughout the entire process.
3.2 Effects of laser pulse duration on the LIPSS formation
Figure 3 represented the surface SEM images of irradiated SiC substrates by lasers with pulse durations between τ=450 fs to 2.5 ns and pulse number ranging between N 1-500. Different types of LIPSS (only LSFL, both LSFL and HSFL, or only HSFL) are categorized under each image and are highlighted using colored boxes. The LSFL and HSFL areas are highlighted with blue and yellow squares for the LIPSS induced at τ=450 fs, N = 50 as an example. LSFLs were formed at τ=450 fs to 1 ns, however, for the case of τ=2.5 ns formation of LIPSS was not observed. Furthermore, in the case at τ=1 ns and a part of irradiated area at τ=500 ps, the period of LIPSS did not change from LSFL to HSFL. As the tendency is shown in Fig. 1, the LIPSS changed the shape from LSFL to HSFL with the increase in the number of laser pulses, regardless of the pulse width. Moreover, these results indicate that the shorter the pulse width is, the smaller the number of pulses that are required to form the LSFL and transit from LSFL to HSFL will get.
Fig. 3. Surface SEM images of LIPSS with varying laser pulse duration τ from 450 fs to 2.5 ns. With laser pulses, the LIPSS changed the shape from LSFL to HSFL. Kinds of LIPSS, where LSFL, LSFL and HSFL, and HSFL, are categorized by box color.
The period of the LIPSSs is plotted as shown in Fig. 4. The period of the LSFL was 900–1000 nm for the cases where small number of pulses were applied. When the number of applied pulses increased, it then decreased to approximately 600 nm. On the other hand, the period of the HSFL is approximately 200–300 nm and does not change with the number of pulses. Similar behavior in which a period change in LSFL and a constant period in HSFL with the number of pulses, have been reported previously [39]. Moreover, the period shows the same tendency regardless of the pulse duration.
Fig. 4. The period of LIPSS with laser pulses varying laser pulse duration τ. The period’s change in LIPSS shows same tendency, which is that the LSFL period decreases whereas the HSFL period does not change significantly with pulse numbers, regardless the pulse duration.
Figure 5 shows the cross-sectional dark-field TEM images and the SAED patterns of the initial stage of LIPSS formation, LSFL and HSFL with varying the pulse duration. Here, the term “initial stage” is used for the stage before the formation of LIPSS despite the number of laser pulses. In the cases where the pulse duration ranges from τ=450 fs to less than 10 ps, the crystalline state is maintained in the whole step from the initial stage to LSFL and then HSFL. In the case of τ=500 ps, the irradiated area in the initial stage is crystalline as shown with a well-oriented dot-like SAED pattern, however, LSFL has both crystalline and polycrystalline area. The SAED pattern of the polycrystalline area shows a halo pattern. Most of the convex area of the LSFL maintains crystalline, while most groove area is polycrystallized. The polycrystalline area is aggregate of crystal particles with sizes in the range of several tens of nm. Such a polycrystalline state has been reported to occur when crystals are recrystallized after melting once [40], and we believe that this process at τ=500 ps is influenced by thermal effects. In case where the LSFL maintained crystalline, it would change to HSFL by subsequent pulses irradiation, and the resulting HSFL also maintains crystalline. On the other hand, the polycrystalline LSFL would not change to HSFL even if they are irradiated by larger number of laser pulses. The same conclusions are also obtained for the case of τ= 1 ns. For the τ= 2.5 ns, LIPSS was not formed even after irradiation by 500 or more pulses, and the irradiated area is already polycrystallized at the small number of pulses and maintains this status after the subsequent pulses. Similar to the previously mentioned case, the crystal state of this polycrystal area is also aggregated of nanoparticles that their size distribution ranges from several tens to a hundred nm. The polycrystalline area would not form LIPSS even if laser pulses are irradiated.
Fig. 5. Cross-sectional dark field TEM images and the SAED patterns. The images represent the crystallinity of irradiated area of initial stage of LIPSS formation, LSFL and HSFL induced by each pulse duration.
4. Discussion
The structure changes, starting from particulate nanostructures to LSFL, then turning to HSFL if the number of pulses is increased. Meanwhile LIPSS are not formed, or LSFL does not change to HSFL, at pulse duration longer than 500 ps. The TEM observation indicates that the crystalline state was maintained the entire time from the initial stage to LSFL and later to HSFL for τ < 10 ps. In the case of τ = 500 ps or 1 ns, some of the irradiated regions remained crystalline and followed the process to HSFL formation, while the regions that became polycrystalline during the process did not exhibit any subsequent crystal structural changes. The region irradiated by the laser with τ=2.5 ns became polycrystalline in the initial stage and LSFL was not formed afterwards. This suggests that maintaining the crystallinity in a prior process is the key to forming LIPSS, or changing the structure from LSFL to HSFL. The crystalline state is a must, for the material surface excitations such as the excitation of the electric field distribution that leads to LIPSS formation. On the other hand, it has been reported that LIPSS was formed on amorphous materials such as fused silica or glass [41]. Our group has also formed LSFL on amorphous materials, but we were not able to form HSFL on such materials.
Based on our observations, maintaining the crystalline state is a must to form HSFL which is formed by excitation of the plasma state, but may not be a must to form LSFL which is formed by electric field distribution induced by wavelength component. In fact, there are also reports of LSFL formation using nanosecond lasers [42,43]. The phenomenon occurring at the material surface may be different from LIPSS formation in crystalline materials and amorphous materials, such as the heating effects also might affect the LIPSS formation. In this regard, it has some possibility that another factor, such as the difference in the repetition frequency of each laser, also may have an effect on the LIPSS formation. Therefore, it is necessary to investigate the effect of original crystallinity in detail.
Figure 6 represents the schematic diagram of the formation dynamics of LIPSS with varying pulse duration. The LIPSS formation proceeds in the process of structure changes starting from surface roughening with nano-particles to LSFL, then turning to HSFL as we mentioned above. However, polycrystallization during the process prevents the process into the next structure. In our case, the irradiated area was polycrystallized at τ > 10 ps, where 1 µm laser irradiation onto 6H-SiC is utilized. Moreover, for the case where the pulse durations are longer than nanoseconds the polycrystallization becomes significant even at the initial stage. We believe the main reason for the polycrystallization was thermal effects, so the results also can conclude that the boundary between thermal and non-thermal effect on SiC, which is the processed material, is between 10 ps and 500 ps. This boundary is in accord with the previous reports, in which it is considered that lattice vibrations are initiated after few picoseconds [21,25,26].
Fig. 6. Schematic diagram of the formation dynamics of LIPSS with varying the pulse duration. In the process of LIPSS formation, maintaining the crystallinity is essential for the further process to make LSFL or HSFL.
5. Conclusion
We demonstrated the formation process of LIPSS by varying the number of laser pulses and the laser pulse width. As the number of pulses increases, the structure shape undergoes a transformation, evolving from particulate nanostructures to LSFL and eventually transitioning to HSFL. In the case of a pulse width longer than τ = 500 ps, LIPSS did not form, and the shape remained unchanged after the irradiated material polycrystallized. This finding emphasizes the importance of maintaining crystallinity for the formation of LIPSS or any shape alterations.
Funding
Japan Society for the Promotion of Science (18H05338, JP16H06415, JP17K14111); Amada Foundation (AF2017235).
Acknowledgements
The authors would like to acknowledge TRUMPF corp. for providing the opportunity to use their laser sources.
Disclosures
All authors declare no competing interests.
Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.
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