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We investigated an early stage of laser-induced periodic surface structure (LIPSS) formation to elucidate the contribution of defects on the formation. 4H-SiC crystals were irradiated by multiple pulses of femtosecond laser with different laser spot sizes. We observed the decrease in formation thresholds of high-spatial-frequency LIPSS (HSFL) and low-spatial-frequency LIPSS (LSFL) with the increased irradiated laser spot size. For smaller laser spot size, HSFL was only formed at the periphery of LSFL formation area, whereas for larger spot size, HSFL was randomly distributed within the laser spot. Our results are coincident with the hypothesis that the existence of defects in crystal contributes to the early stage on the formation of LIPSS, in which the electron excitation via one or two photon absorption in a defect site cause local nanoablation at a laser fluence under the intrinsic ablation threshold, followed by the formation of a nanovoid, which act as a scatterer, and interference of scattered wave and laser pulses lead to HSFL formation.
© 2014 Optical Society of America
1. Introduction
Laser-induced periodic surface structures (LIPSS) are ripple nanostructures formed by laser irradiation at laser fluences under the ablation threshold for a single pulse. In 1965, the LIPSS formation on a Ge surface was first reported by Birnbaum [1]. Although LIPSS had been regarded as structural noises accompanied by laser processing, the development of Ti:sapphire laser enabled us to form stable periodic ripple structures with its advantage on high-throughput fabrication by dry process in air. Since then, LIPSS formation on various materials such as metals [2, 3], semiconductors [4,5], dielectrics [6, 7], and polymers [8, 9] has been reported. Applications of LIPSS for mechanical engineering such as friction reduction [10], enhancement of thin-film adhesion [11,12], and increase in water-repellent property [13] have been reported as well as for biomedical applications such as enhancement of cell adhesion and control of the cell growth orientation [14,15]. In the field of optics, LIPSS are applied to structural color [16], surface enhanced Raman spectroscopy (SERS) [17], and anti-reflection surface [18]. The further sufficient elucidation of LIPSS formation process could contribute to the improvement of the spatial controllability and the precise formation of a ripple structure, resulting in the acceleration of development in such applications.
Physics of LIPSS formation process has been discussed in many literatures. Sipe et al. have discussed the LIPSS formation process, by considering a surface scattered wave from the surface roughness of an irradiated substrate [19–21]. Bonse et al. discussed the change of optical properties of semiconductors induced by an electron excitation upon laser irradiation as well as a contribution of surface plasmon polaritons to the formation of Low-spatial-frequency LIPSS (LSFL) [22, 23]. Huang et al. reported that self-organized ripples were formed by an interference of initial surface plasmon and laser pulses followed by the grating-assisted surface plasmon-laser coupling [24]. At present, LSFL is considered to be formed by the interference of incident laser and surface plasmon polaritons. On the other hand, the mechanism of high-spatial-frequency LIPSS (HSFL) formation is still under debate. Many hypotheses of the underlying physics for HSFL formation have been proposed. Reif et al. reported that HSFL was self-organized during the electron relaxation after ion ejection caused by coulomb explosion [25]. Straub et al. suggested that the standing wave generated by the interaction between the counterpropagating transverse plasma and the co-propagating surface plasma, whose period is half the wavelength of surface plasmon, caused local ablation, resulting in a HSFL formation [26]. Dufft et al. and Harzic et al. explained the HSFL formation with the second harmonic wave generation from the surface of materials during laser irradiation [27, 28]. They proposed that the second harmonic wave arose out of the interaction between laser pulses and the second-order susceptibility generated by the surface roughness formed after initial several pulses irradiation. The period of HSFL was well explained by the induction of a surface-plasma wave through the parametric decay of laser beam which was reported by Okamuro et al. [29]. Although many hypotheses have been proposed, the formation process has not been fully explained in both theoretically and experimentally. On early stage of LIPSS formation, mechanisms with the scattering from cavities or craters were proposed and investigated by Buividat, et al. [30] and by Ionin, et al. [31]. In our recent work, we investigated the early stages of HSFL formation on the surface of LiNbO3 crystals [32]. The randomly formed nanovoids were observed in the laser spot after the initial pulses irradiation, suggesting the possibility of the hypothesis that defects randomly distributed in crystals may contribute to HSFL formation.
The contribution of defects to laser ablation has been discussed to date. Emmert et al. have theoretically demonstrated the decrease in the ablation threshold after multiple pulses irradiation by solving the nonlinear equation [33]. The equation describes the increase in electron density with taking account of the shallow level in band structures induced by defects in crystal. The ablation threshold decreases because of the electron excitation via shallow defect levels. Since defects in a crystal are considered to be distributed randomly with a certain density, the number of defects in a laser spot differs depending on the spot size. Martin et al. experimentally investigated the ablation threshold of BBS glass under the irradiation of multiple femtosecond laser pulses and observed the decrease in the effective ablation threshold as the laser spot size enlarges [34]. This can be explained by the existence of defects inside the laser spot. If the laser spot include defects, the effective ablation threshold decreases to the ablation threshold at the defect site On the other hand, if the laser spot did not include any defects, the effective ablation threshold would be equal to the intrinsic threshold. To our knowledge, the discussion on the spot-size dependence has not been applied to LIPSS formation.
In this paper, we investigated the spot size dependence of HSFL formation threshold to evaluate the contribution of defects. If HSFL originates from defects in crystal, the effective HSFL formation threshold at a laser spot including defects should decrease. This method was also applied to the LSFL to show the contribution of defects in LSFL formation.
2. Experimental
100 successive pulses from Ti:sapphire chirped pulse amplification laser system (λ = 800 nm, f = 1 kHz, τ = 80 fs, Libra, Coherent) were focused on the surface of n-type 4H-SiC (0001) wafers (Sinyo Co.) at normal incidence using a plano-convex lens (focal length 200 mm or 50 mm) in air. The long axis and short axis of ellipsoidal initial beam before focusing were 6.0 mm and 4.9 mm, respectively. The wafer was placed before the focal spot. Surfaces of samples after laser irradiation were observed by scanning electron microscopy (SEM). Average of HSFL formation areas were determined by measuring the HSFL formation area of 5 samples irradiated at same laser conditions. The effective HSFL formation threshold was determined by extrapolating the plots of the averaged LIPSS (HSFL and LSFL) formation area versus the pulse energy. The threshold was determined when we could clearly observe the ripple formation. The spot size of an incident laser pulse was controlled by varying the distance between the lens and the crystal surface. Laser spot size was determined with experimentally obtained radius of modification area and incident laser pulse energy as below [35].
Here, r, ω0, E and Eth represent the radius of LIPSS formation area, the spot size of 1/e value of the peak intensity, the irradiated pulse energy and the pulse energy of modification threshold, respectively.3. Results and discussion
3.1. LIPSS formation on the surface of 4H-SiC crystal
Figure 1 shows the SEM images of the 4H-SiC crystal surface after laser irradiation. The average period of HSFL and LSFL is 220 nm and 615 nm, respectively as shown in Fig. 1(a). In Fig. 1(b), a crater is formed in the center of a laser spot and LIPSS is formed in periphery of the crater at higher laser fluence, Φ = 15.9 J/cm2. Figure 2 shows squared radius of the average LIPSS formation area, r2, as a function of the pulse energy, E (spot size ω0 = 26 μm). Radius, r, was estimated by assuming the actual elliptically shaped LIPSS formation area as an exact circle. Two regimes depending on the irradiated pulse energy were observed in Fig. 2, probably due to air plasma formation and initiation of filamentation.
Fig. 1 SEM images of the SiC surfaces. ω0 = 26 μm. (a) After laser irradiation at E = 11.3 μJ (laser fluence Φ = 530 mJ/cm2), (b) after laser irradiation at E = 340 μJ (Φ = 15.9 J/cm2), and (c) before laser irradiation.
3.2. Spot size dependence of HSFL formation threshold
Figure 3 shows spot size dependence of effective HSFL formation threshold. The actual laser spot was elliptically shaped, but we assumed the spot size as round shape in order to apply to the Eq. (1). Error bars show two-sided 95% confidence interval of standard error. Effective HSFL formation threshold changed drastically in spot sizes of 15 - 30 μm. With increasing the spot size, effective HSFL formation threshold decreased from approximately 0.6 J/cm2 to 0.3 J/cm2. With spot sizes larger than 30 μm, effective HSFL formation threshold showed no significant change. This result indicates that the effective HSFL formation threshold decreases if the laser spot includes defects, while the threshold becomes equal to the intrinsic formation threshold when laser spot does not include any defect. As shown in Fig. 3, error bars of spot sizes are shorter for spot size smaller than 20 μm, reflecting the smaller dispersion in the probability of defect existence inside the laser spot. Longer error bars for larger spot size are probably due to the increase in dispersion of the number of defects inside the laser spot which act as an origin of HSFL formation.
Fig. 3 Dependence of HSFL formation threshold on spot size. The solid line indicates the theoretical curve of the defect model. Fitting parameters: ΦiH = 0.7 J/cm2, ΦdH = 0.32 J/cm2, d0 = 12 μm.
The probability P of collision of incident laser with a defect randomly distributed on a crystal surface can be written as [34]
where d0 is mean defect distance and ω0 is spot size of incident laser. The effective damage threshold can be written as below by using damage threshold at defect sites Φd and an intrinsic damage threshold Φi [36].Here, the effective HSFL formation threshold, the defect HSFL formation threshold, and intrinsic HSFL formation threshold are defined as ΦthH, ΦdH and ΦiH, respectively. We substituted Φth, Φd, and Φi for ΦthH, ΦdH, and ΦiH, respectively. The fitting curve of the experimental plots according to the defect model is also shown in Fig. 3. The fitting parameters are ΦiH = 0.7 J/cm2, ΦdH = 0.32 J/cm2, and d0 = 12 μm, respectively.Figure 4(a) shows SEM images of the crystal surface after laser irradiation (Φ = 450 mJ/cm2, ω0 = 28 μm). The random formation of HSFL and nanovoids in the laser spot can be observed in Fig. 4(a). Taking these results into the consideration, the formation process of HSFL can be explained as follows. The initial pulses ablate at randomly distributed defect sites in crystal, resulting in the formation of nanovoids. The subsequent pulses would interfere with the scattered waves from the nanovoid to engender HSFL formation [30–32]. Point defects, dislocation, and stacking defects in SiC crystals are known to generate the intermediate levels at 1.2 eV [36], at 1.8 eV [37], and at 2.9 eV [38], respectively in band structure. Although further study is needed to identify which defect is the main origin, at the defect sites, the electron excitation via one or two photon absorption induces local nanoablation at the laser fluence under the intrinsic ablation threshold. In addition, the electrons excited to conduction band would be trapped at defect sites and form bound excitons. The bound excitons would be excited to conduction band via one or two photon absorption and cause avalanche ionization. As a result, local ablation could occur at the irradiated defect sites. The subsequent laser irradiation would generate an enhanced scattered near field at the edge of voids and grooves formed by local ablation. The high intensity of near field induces the ablation and the growth of HSFL perpendicular to the polarization direction as demonstrated in our previous study [39]. The electromagnetic field calculation by 3D-FDTD method estimated that the peak of the optical intensity locates at about 240 nm away from the center of the groove when the surface of SiC crystal is irradiated by 800 nm laser. The interference of incident wave with a scattered wave from the groove could generate an optical enhanced field, and new grooves would be formed at both sides of the groove where the peak intensity locates, resulting in the HSFL formation.
Fig. 4 SEM image of the SiC surface after laser irradiation. (a) Φ = 450 mJ/cm2, ω0 = 28 μm. The HSFL and nanorod-shaped craters were randomly distributed in the laser spot, (b) Φ = 1400 mJ/cm2, ω0 = 8 μm.
The distance between randomly formed HSFL in the laser spot was shorter than the mean defect distance derived from the defect model, d0 = 12 μm as shown in Fig. 4(a). This may be due to the decrease in a damage threshold induced by multiple pulse irradiations. Since the multiple pulse irradiations induce excitons and Frenkel pairs, the damage threshold decreases [40]. This results in forming high-dense defects in the laser spot, which contributes to the HSFL formation. In Fig. 4(a), nanoparticles, ablation debris, are observed on the surface of the SiC. Nanoparticles may scatter laser pulses and thus it might be expected that scattering from nanoparticles contributed to the formation of HSFL. However, nanoparticles are distributed with high density on the surface which has inter-nanoparticle distances shorter than 1μm, suggested that the scattering from nanoparticles has little relation on the significant change in the threshold shown in Fig. 3.
Figure 4(b) shows the SEM images of the crystal surface after laser irradiation with smaller spot size of ω0 = 8 μm (Φ = 1400 mJ/cm2). Nanovoids and HSFL are randomly distributed with larger laser spot (Fig. 4(a)), whereas HSFL are not dispersed but locally formed in the periphery of LSFL in Fig. 4(b). This difference in the location of HSFL formation with the laser spot size can be explained by the hypothesis that the HSFL originates from nanovoids at defect site as well as surface roughness and the craters induced by initial laser pulses. Tomita et al. reported that effective HSFL formation threshold is lower on rough surface than on flat surface because the surface roughness worked as scatterers, which is a core of HSFL formation [41]. Liang et al. reported the expansion of HSFL formation area with the increase in number of pulses [42]. They explained that first several pulses take a role in forming craters in the center of laser spot, and the subsequent pulses in the interaction with the crater as scatters induce HSFL. These previous works and our results are compatible with supporting the hypothesis of random HSFL formation. Therefore, it can be concluded that defects is highly related to the decrease in the effective HSFL formation threshold with larger laser spot size in the surface of crystals and can be considered as an origin of HSFL formation.
3.3. Spot size dependence of LSFL formation threshold
Figure 5 shows the spot size dependence of effective LSFL formation threshold, ΦthL, determined by the same method used for HSFL as described with Eq. (3). The effective LSFL formation threshold also decreased with increasing spot size. This indicates that defects in crystal could contribute to LSFL formation as explained in the HSFL formation. Defects in crystal form the intermediate levels in band structure [36–38]. The conduction electrons excited through the defect levels would become seeds for avaranche ionization [33]. An electron density increases at a periphery of defects via avaranche process, resulted in the excitation of surface plasmon polaritons which is responsible for LSFL formation. Therefore, defects in crystal could cause the decrease in effective LSFL formation threshold. The fitting parameters of the defect model in Fig. 5 were determined as the intrinsic LSFL formation threshold ΦiL = 1 J/cm2, the formation threshold of LSFL originating from defects ΦdL = 0.34 J/cm2, and the mean defect distance d0 = 12 μm. The value of the mean defect distance derived from Fig. 5 was equal to the mean distance derived from the spot size dependence of the HSFL formation threshold. This correspondence of the mean defect distances suggests that LSFL and HSFL formation could have derived from the same type of defects. The ablation at defects, which is one of the origins of HSFL, and the excitation of surface plasmon polaritons, which is the origin of LSFL, are phenomena which occur via the same process in terms of the contribution of electron excitation by multiphoton absorption and avaranche ionization. The electron excitation occurs at lower pulse energy due to weak multiphoton ionization through defect levels. LSFL would be formed when a laser fluence and the conduction electron density around defect sites are high enough to excite surface plasmon polaritons. At lower laser fluence, however, HSFL could be induced from nanovoids formed at defect sites interfering with the laser irradiation.
Fig. 5 Spot size dependence of LSFL formation threshold fluence. The solid line indicates the theoretical curve of the defect model. Φi = 1 J/cm2, Φd = 0.34 J/cm2, d0 = 12 μm.
4. Conclusion
The effective HSFL formation threshold decreased drastically with increasing laser spot size. Nanovoids and HSFL were formed randomly with larger spot sizes, whereas HSFL were only formed at a periphery of LSFL with smaller spot sizes. At the defect sites, the electron excitation via one or two photon absorption cause local nanoablation at laser fluences under the intrinsic ablation threshold. These results suggest that local absorption at defects induces nanoablation at defect sites, resulting in the HSFL formation. The effective LSFL formation threshold also decreased with the increase in the laser spot size. The fitting parameters of the mean defect distance derived from the spot size dependence of HSFL and LSFL corresponded, which suggests that the same type of defects contribute to the formation of HSFL and LSFL.
Acknowledgment
This study was supported in part by a grant from the Casio Science Promotion Foundation. Authors are grateful to Prof. Minoru Obara for fruitful comments on this paper. G. Obara is grateful for the JSPS Fellowship for Young Scientists.
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