1. Introduction

Femtosecond laser micromachining has opened frontiers in numerous important fields in both scientific research and industry [1–3]. The advantage of nonlinear process during femtosecond laser interaction with transparent materials enables high precise, three dimensional, and self-assembled microstructuring for extensive applications [4–6]. Third-generation semiconductors such as SiC and GaN, are the cornerstones of future integrated circuits due to their wide band gap, excellent mechanical, electrical and chemical properties, which are highly desirable for applications in the cases of high-power, high-temperature, high-frequency, anti-radiation environment [7]. Micro processing of these materials is of great importance with the development of miniaturization and integration of microelectronic devices [8,9]. Nevertheless, the excellent mechanical and chemical properties bring in great challenges in traditional processing methods such as wet etching [10]. Therefore, the development of new processing technology for third-generation semiconductors is highly desired.

In the past decade, femtosecond laser has been recognized as a powerful tool in SiC processing owing to the superiorities including non-contact style, rapid removal rate, and minimum heat affected zone, and so forth [11–14]. Moreover, the nonlinear process during fs laser interaction with matter can lead to the formation of various periodic micro-nanostructures which are indispensable for applications including superhydrophobic surfaces, oil-resistant self-cleaning surfaces, and micro-electro-mechanical systems [15–18]. There have been numbers of publications reporting on the fs laser induced microstructures on the surface [12,13,19,20] and in the bulk [21] of 4H-SiC. Coarse ripples and fine ripples (nanogratings) were usually observed coexisting in laser modified areas where coarse ripples were located in the center and fine ripples were at the periphery [19–21]. This type of non-uniform structure may encounter great challenges in practical applications. Although the mechanism of nanograting formation on surfaces of (semi)transparent materials has been clarified by Juodkazis et al. [22], there are very few reports on the preparation of uniform nanogratings on 4H-SiC surface so far [13,23]. In addition, there is lack of systematic investigations of the influences of processing conditions on the morphology of nanogratings formed on 4H-SiC. Moreover, different types of large-area periodic structures based on 4H-SiC surface nanogratings and their applications remain to be explored. It is thus fairly important to investigate the influences of fabrication methods and optical parameters on the formation of uniform nanogratings and other complex periodic structures in more details for surface engineering and optoelectronic applications of 4H-SiC.

In this work, we demonstrate that uniform nanogratings and large-area nanoparticle array and nanograting-nanoparticle composite array structures can be created on the surface of 4H-SiC single crystal by direct laser writing with a low repetition rate (1 kHz) femtosecond laser in air. The influences of laser pulse energy, writing speed and polarization angle on the formation of uniform nanogratings are analyzed in detail. The suitable parameters for the formation of uniform nanogratings in a pulse energy-scan speed landscape are depicted. Moreover, a planar light attenuator for linear polarized light is fabricated by aligning the nanogratings by raster scanning. These results are expected to open up an avenue for nanostructuring on 4H-SiC.

2. Experiments

2.1 Structure writing

The experiments were carried out on a 1 mm thick n-type 4H-SiC single crystalline wafer which was well polished on both (0001) surfaces that are commonly used in device manufacturing with Mohs hardness up to 9.5. The sample was mounted onto a XYZ translation stage (Newport) controlled by a computer for laser writing. The laser writing process was conducted with a regeneratively amplified mode-locked Ti: sapphire laser system that delivers 800 nm, ∼35 fs laser pulses operated at 1 kHz repetition rate. The linearly polarized Gaussian beam was focused onto the sample surface via a 0.55 NA-objective lens. The pulse energy and polarization were controlled by an attenuator and a half-wave plate, respectively. The writing speed can be precisely controlled from a few micrometers to several millimeters per second. The beam spot diameter in the focus was estimated to be ∼2.28µm using the formula 2ω₀ = 1.22 λ / NA. In this configuration, the optical axis of the SiC sample was always along the Z axis, i.e. parallel to the laser propagation direction.

2.2 Structure characterization

Optical images of the laser-inscribed structures were captured with a CCD camera attached to a Nikon microscope (Eclipse 80i) (transmission mode). Raman spectroscopy characterization of the laser-modified region was carried out by a laser confocal Raman spectrometer (Renishaw, inVia) with a 532 nm laser as the excitation source. For scanning electron microscopy (SEM) observation, the sample was further etched in 1 mol/L hydrofluoric (HF) solution for 30 s and ultrasonic cleaned afterwards.For the test of large-area nanogratings as an attenuator, a linearly polarized laser (λ=800 nm) was used as a testing light source and the experimental set up can be found elsewhere [24]. The laser power was measured after the testing laser passing through the fabricated planar structure. All the measurements were carried out at room temperature.

3. Results and discussion

Firstly, the influence of pulse energy on the modification of the surface of 4H-SiC was explored. Figure 1 shows the structures induced with various pulse energies by translating the sample at 5 µm/s. It is clearly noted that the color of the lines gradually turned black with increasing pulse energy in the optical microscope images (Fig. 1(a)), which reflects an enhanced absorption and scattering of light. The color change indicates transition of different types of modifications. The width of the lines increases approximately linearly with pulse energy from 30 nJ to 200 nJ, as shown in Fig. 1(b), which is in accordance with previous observations [25]. The structural change in laser modified areas was further characterized by Raman spectroscopy. The spectrum collected from unirradiated region shows the typical Raman peaks of single-crystal 4H-SiC [26]. The strongest peak at 778 cm⁻¹ can be assigned to the first-order transverse optical (TO) phonons. The sharp peak at 205 cm⁻¹ is originated from the transverse acoustic mode (FTA). The peak located at 975 cm⁻¹ belongs to the longitudinal optical folding mode (FLO). It is clearly seen that the intensities of all the three peaks decreased monotonically with increasing pulse energy (Fig. 1(e)), which could be caused by enhanced amorphization and scattering. Broadening of the Raman peaks with increasing pulse energy is observed, as shown in the insert of Fig. 1(d). The FWHM of the peak at 778 cm⁻¹ increases from 5 cm⁻¹ to 5.5 cm⁻¹ as pulse energy increases from 30 nJ to 200 nJ, indicating an enhanced amorphization. The Raman peaks at 205 cm⁻¹ and 975 cm⁻¹ almost disappeared at 200 nJ, which suggests that most of the crystalline 4H-SiC in the focus was converted into amorphous phase. A broad band appeared at ∼1325 cm⁻¹, which could be attributed to the D band activated in the first-order scattering process of sp² carbons when there are defects or SiC nanoparticles induced as the pulse energy increased to 200 nJ [27]. To identify the types of defects induced by laser processing, further studies such as luminescence characterization would be helpful [28], which is beyond the scope of this study.

 

Fig. 1. Structures formed with various pulse energy by translating the sample at 5 µm/s: (a) optical microscope images of the inscribed lines, (b) the line width as a function of pulse energy, (c) SEM images of the lines corresponding to (a) after etched for 30 s in 1 mol/L HF solution, (d) Raman spectra of the lines, and the insert shows the FWHM of the Raman peak at 778 cm⁻¹ as a function of pulse energy, (e) the intensities of the Raman peaks at 205, 778, and 976 cm⁻¹ as a function of pulse energy.

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The SEM images in Fig. 1(c) depicts the structural transformation from uniform nanogratings to severe ablation. The lines written with 30 nJ and 50 nJ are mainly covered by uniform nanogratings with a period of ∼150 nm. The relation between the period of the nanogratings and the laser wavelength approximately satisfies the formula Λ = λ/2n (here λ is 800 nm, n is ∼2.6 at 800 nm). The period coincides well with the formation mechanism of surface ripples proposed by Juodkazis [22]. Structural dissociation began to appear in the center of nanograting area at 80 nJ and became obvious at higher pulse energies. The grating structures in the center were gradually broken and removed by the HF solution, leaving a groove there. The result indicates that stability nanogratings decreases drastically with increasing pulse energy. Periodicity can still be observed at the bottom of the grooves and the periphery of the lines. These results prove that uniform nanogratings can be induced on the surface of 4H-SiC even with a low repetition rate fs laser in air compared to that induced by MHz laser [13], which means that femtosecond lasers operated at different repetition rates are applicable for the induction of uniform nanogratings on the surface of 4H-SiC.

The scan speed in scan writing regime also plays an important role in the formation of nanogratings. This is because the scan speed determines the number of pulses deposited in per unit length i.e. pulse density. And the pulse density can affect the formation of nanogratings from two aspects. Firstly, the local fluence, which highly dominates the type of laser modification, is proportional to the pulse density. Secondly, the incubation effect that plays an important role in the formation of uniform nanogratings is related to the pulse density [29]. Figure 2 shows the influence of scan speed on the formation of nanogratings at a fixed pulse energy of 50 nJ with laser polarization parallel to the scan direction. The scan speeds varying from 5 µm/s to 500 µm/s, corresponding to pulse densities from 200 pulses/µm to 2 pulses/µm, were adopted. The SEM images in Fig. 2(a) clearly depict the evolution (after HF etching) of nanogratings with increasing scan speed. It is notable that the periodicity of nanogratings is explicit when the scan speed is no more than 100 µm/s. However, the nanogratings induced at lower speeds (e.g. 5 and 10 µm/s) are much more vulnerable to HF solution. The center of the nanograting area was almost removed by HF while the periodicity is still preserved at the bottom, as shown in Fig. 2(a1 and a2). In Fig. 2(a3), the transition of well-preserved nanogratings and severely etched region can be observed at the scan speed of 40 µm/s. The uniform nanogratings with good tolerance to HF solution are formed at 100 µm/s, as shown in Fig. 2(a4). When the scan speeds were further increased to 200 and 500 µm/s, the grating structures became bended and ambiguous. In addition, the average period of nanogratings increased from 148 nm at 5 µm/s to 228 nm at 100 µm/s. This is mainly due to the decrease of pulse density at high scan speeds resulting in reduced incubation effect [29]. Raman spectra show that the intensities of the three representative Raman peaks of 4H-SiC monotonically decrease as the scan speed slows down, as shown in Fig. 2(b) and 2(c). The FWHM of the Raman peak at 778 cm⁻¹ broadens with decreasing writing speed, as shown in the insert of Fig. 2(b), indicating enhanced amorphization which could account for the poor stability of nanogratings formed at low speeds.

 

Fig. 2. Structures formed with various writing speeds at a fixed pulse energy of 50 nJ: (a) SEM images of the laser scanned lines after etched for 60 s in 1 mol/L HF solution, (b) Raman spectra of the laser induced lines and unirradiated area before HF etching, the insert shows the FWHM of the 778 cm⁻¹ peak as a function of scan speed, (c) the intensities of the Raman peaks at 205, 778, and 976 cm⁻¹ as a function of scan speed, (d) the types of modification at different pulse energy and scan speed. The scale bar is 1µm.

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Series experiments were carried out to clarify the suitable pulse energy-scan speed landscape for the formation of uniform nanogratings on the surface of 4H-SiC, as shown in the orange zone in Fig. 2(d). The parameter window for the nanograting formation is quite narrow as the speed and pulse energy are in the low regime. In this case, a small increase of the pulse energy will lead to a sharp increase of writing speed. When the pulse energy increases from 30 to 50 nJ, the maximum speed increases from 20 to 100 µm/s. The suitable speed range continues to extend until the pulse energy goes to 100 nJ. After that, the speed window starts to be saturated and keeps to be around 100 µm/s until the pulse energy goes to 200 nJ. The maximum speed is dependent on both the fluence threshold and incubation effect for nanograting formation, while the minimum speed is dependent on the ablation threshold. Parameters outside the orange region would lead to different types of modifications. For example, with pulse energy smaller than 30 nJ, no modifications could be observed regardless of scan speed. The combination of high pulse energy and low scan speed would lead to strong ablation, while the combination of low pulse energy and high scan speed would lead to the formation of random structures.

The influence of laser polarization direction on the formation of nanogratings was further studied. The structures induced with three different polarizations respectively angled 90°, 45°, and 0° to the writing direction are compared. Firstly, we confirmed that the orientation of the nanogratings formed on 4H-SiC surface is strictly perpendicular to the laser polarization direction, as shown in Fig. 3(a), which is in accordance with the observations in other materials [29,30]. Interestingly, more polarization dependent features on the morphology and stability of nanogratings are observed. The uniformity of the nanogratings, which can usually be evaluated by the distortion of nanograting orientation, integrity of grating structure, and fluctuation of periods, gradually declined with the decreasing angle between laser polarization and the writing direction. The ripples formed at 90° are almost straight while those formed at 0° and 45° have obvious bending deformations. Some debris or big hollows can be observed in the center of the grating area for the latter two angles. The average periods of nanogratings at 90°, 45°, and 0° are respectively 166 ± 10 nm, 162 ± 40 nm, and 152 ± 60 nm. It is seen that the average period slightly decreases, while the period fluctuation sharply increases with decreasing angle.

 

Fig. 3. (a) SEM images of the lines written with different laser polarizations with pulse energy of 30 nJ and scan speed of 5 µm/s, (b) Schematic diagram of interaction between chevron-shaped stress and nanogratings at different polarizations, (c) the transmission of the testing laser as a function of the angle between the testing laser and the writing laser. The scale bar is 2 µm.

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The dependence of stability and period fluctuation of nanogratings on the angle could be associated with the chevron-shaped stress distribution in the modified area, as indicated in Fig. 3(b) and previously observed in other materials [31,32]. At 90°, the nanogratings orient parallel to the writing direction (Fig. 3(b), left) and thus the chevron-shaped stress has minimum impact on the alignment of nanogratings. However, the nanogratings orient perpendicular to the writing direction when polarization is 0° (Fig. 3(b), right). In this case, the chevron-shaped stress may result in nonnegligible impact on the transverse nanogratings, as depicted in the red box in Fig. 3(b), which leads to the reduction of stability and increase of period fluctuations. Another possible explanation for the uniformity and period dependence on polarization could be due to the enhanced heat diffusion along the electric field of a linearly polarized laser [33]. The anisotropic heat diffusion creates an elliptical heat-affected zone with the presence of defects along laser polarization. The defects induced by the former laser pulses will enhance the absorption of the latter pulses in the nanograting area. When the angle is turned to 0°, the writing direction is the same as the polarization direction. Thus the defects are mostly distributed in the writing direction, which will lead to a maximum energy deposition as compared to other angles. Excess energy deposition reduces the stability and uniformity of nanogratings. Comparatively, the 90° angle is the most suitable for the preparation of uniform grating structure in our experiment. In addition, the nanograting period increases with increasing polarization angle. This indicates that the increased energy deposition enhanced the modification of the nanograting structure. Similar dependence of nanograting period on laser polarization was also reported in Ref. [33]. These results suggest that geometrically complicated structures can be achieved by control of laser polarization direction.

We further demonstrated the function of the large-area nanogratings as a light attenuator for polarized light, as shown in Fig. 3(c). The large-area nanogratings with a dimension of 5×5 mm² were fabricated by a simple raster scan strategy, as shown in the insert. The pulse energy and the scan speed adopted here were 80 nJ and 100 µm/s, respectively. The line width is ∼3µm with an interval space of 2 µm. The testing laser was normally incident into the large-area nanogratings. When we rotated the polarization plane azimuth of the testing laser, the detected power varied regularly with a period of 180°, which is similar to the function of a polarizer. The extinction ratio is about 30% for the current structure. As carefully seen in the insert, there are some dislocations of the nanogratings, which could be the main reason that leads to a low extinction ratio. Another reason could be that the wavelength of testing laser is not the best wavelength to the response of the structure, which needs to be further verified. For practical applications, further strategy such as coating and optimization of processing conditions will be necessary.

Based on the orientation dependence of nanogratings on polarization, we find that it is possible to fabricate different types of large-area nanostructures (arrays) on the 4H-SiC surface by a cross-scan strategy. The schematic of the experimental set up is shown in Fig. 4(a). The maximum patterning speed can reach to around 200 µm/s since the repetition rate is only 1 kHz, as indicated in Fig. 2(d). Higher speeds will lead to less uniform pattern structures. Here a 100 µm/s is chosen for the balance of processing time and structure uniformity. The patterning area is optional within the motion range of the XYZ translation stage (125 mm×125 mm on the XY plane). The pulse energies applied in the two rounds were adjusted to control the type of structures. Figure 4(b) shows the nanoparticle arrays produced with pulse energies of 70 nJ in the first scanning and 50 nJ in the second scanning. The nanoparticles are uniformly distributed in the scanning area. The average diameter of the nanoparticles is ∼180 nm. In this case, the pulse energy applied in the second scanning round is smaller than that in the first round. This is because the first scanning led to the formation of defects and amorphous phases, as indicated in the Raman spectra, and thus reduced the modification threshold of the material. In order not to erase the periodicity formed in the first scan round, the pulse energy applied in the second round should be considerably lower. Figure 4(c) shows the total rewriting of nanogratings with pulse energy of 70 nJ in the first round and 80 nJ in the second round. It is seen that the nanogratings formed in the first round are totally erased and new nanogratings determined by the second scan round were formed. The new nanogratings are less continuous than that formed in the first round. This phenomenon can be applied in rewritable data storage [34]. Furthermore, by adjusting the spacing of adjacent scan traces, a new type of nanograting-nanoparticle hybrid array structures can be obtained, as demonstrated in Fig. 4(d). The nanogratings are mainly located in the center of the scan trace of the second round and their orientation is determined by the polarization in the second scan round. The nanoparticles are mainly distributed in between two adjacent scan traces where the laser intensity is lower than that in the center. These sub-micro array structures may have great potential in surface engineering and integrated optoelectronics.

 

Fig. 4. Fabrication of large-scale nanostructures by tailoring the nanogratings: (a) schematics of raster scanning and cross scanning setups, (b) SEM image of large-scale nanoparticles fabricated by cross scanning with pulse energy of 70 nJ in the first scan round and 50 nJ in the second scan round, (c) rewriting of nanogratings with pulse energy of 70 nJ in the first round and 80 nJ in the second round, (d) nanograting-nanoparticle composite structure fabricated with pulse energy of 70 nJ in both scan rounds and the scan spacing of 4 µm. The scale bar is 1 µm.

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4. Conclusions

In conclusion, new types of large-area periodic structures including nanoparticle array and nanoparticle-nanograting hybrid structures were induced on the surface of n-type 4H-SiC by tailoring the surface nanogratings induced by femtosecond laser. The proper range of pulse energy and scan speed for the formation of nanogratings is depicted. The polarization direction perpendicular to the writing direction is preferred for the uniformity of nanogratings. In addition, the period of nanogratings slightly decreases with increasing angle between polarization direction and scan direction. Raman analysis of the laser-modified region indicates an enhanced amorphization process with increasing laser fluence. The fabricated large-area nanogratings show an extinction efficiency of up to 30% to 800 nm linearly polarized laser. The formation of different large-area nanostructures can be realized by control of pulse energy and scan interval using a cross scan strategy. This work provides important prospects to create and manipulate periodic nanostructures on SiC crystals for photonic applications.