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Effect of patterned silicon nitride substrate on Raman scattering and stress of graphene
Materials and Design 198 (2021) 109338
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Effect of patterned silicon nitride substrate on Raman scattering and stress of graphene Daohan Ge a,b,⁎, Yuan Zhang a, Hui Chen a, Guangfu Zhen a, Minchang Wang a, Jiwei Jiao c, Liqiang Zhang a,d,⁎, Shining Zhu b a
Institute of Intelligent Flexible Mechatronics, School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, PR China SIWAVE, Inc., Shanghai 201800, PR China d Jiangsu Collaborative Innovation Centre of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, PR China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Si3N4 films with two patterns (holes and trenches) were produced on silicon wafers. • CVD-grown monolayer graphene were transferred onto the patterned Si3N4 substrates. • Peak red-shift in 2D and G bands was due to the strain induced by Si3N4 patterns. • Effect of pattern morphology is greater than that of roughness in graphene strain. • Depth effect of Si3N4 on Raman peak shift and the strain change of graphene exists.
a r t i c l e
i n f o
Article history: Received 26 July 2020 Received in revised form 9 November 2020 Accepted 9 November 2020 Available online 17 November 2020 Keywords: Patterned silicon nitride Graphene Surface morphology Surface roughness Raman scattering Tensile strain
a b s t r a c t Graphene is widely used for nano-devices due to its distinctive band structure and fascinating properties. The substrates could significantly affect the properties of graphene and related devices. In this work, we investigate the effect of surface morphology and roughness of patterned silicon nitride substrates on Raman scattering and stress of graphene. We find that the Raman scattering of graphene depend strongly on surface morphology and roughness of patterned substrates. It is concluded that the peak red-shifts in 2D and G bands was due to the strain induced by patterns with different surface morphology (holes and trenches) and roughness. Furthermore, the effects of morphology of patterned area are much greater than the role of surface roughness in the induced strain of graphene. Due to the larger surface area (about 1.65 times), the strain in the grooves is greater than in holes, in spite of greater surface roughness in holes. Our results also reveal that the effect of pattern depth should be taken into account to understand the Raman peak shift and the strain change of graphene. Our work is fundamentally important for understanding the graphene properties on dependence of surface morphology of substrates and enhancing the interfacial strength of graphene-based devices. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Ge), [email protected] (L. Zhang).
As a two-dimensional single atomic layer composed of sp2-bonded carbon atoms in a honeycomb lattice, graphene has attracted significant
https://doi.org/10.1016/j.matdes.2020.109338 0264-1275/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Materials and Design 198 (2021) 109338
tensile strain and Raman scattering of graphene. We find that the morphology and size of patterned area have much greater effect on tensile strain of graphene than the role of surface roughness. Moreover, our results reveal obvious depth effect of Si3N4 on the Raman peak shift and the strain change of graphene.
research interest since its first production in 2004 [1], due to its extraordinary structural features and desirable physical and chemical properties [2–4]. These highly appealing properties, especially with the development of high yield graphene fabrication technology via chemical vapor deposition (CVD) process [5,6] and epitaxial growth on silicon carbide (SiC) [7], offer a wide range of potential applications of graphene in varied fields including photodetectors [8], biosensors [9,10], energy-storage devices [11–13], and field effect transistors [14]. All these applications require graphene films to be transferred on a certain substrate, and varieties of materials, to date, have proved to be ideal substrates for graphene, ranging from metal layers (e.g., Cu, Ni and Fe) to dielectric materials (e.g., SiO2 and silicon nitride (Si3N4)). Si3N4 films have been widely used in the field of microelectromechanical system (MEMS) devices and beyond [15–18], owing to their low masses, ultrahigh mechanical quality factors [19], large fabrication tolerance, and excellent optical properties [20]. Recent development of high-quality graphene films growth on Si3N4 substrates by Liu et al. [21] has opened up the possibility for a series of applications of hybrid graphene-Si3N4 structures in microelectronics [22–25]. These hybrid graphene-Si3N4 devices, combining the desirable optical and electrical properties of graphene with the superior mechanical and optical properties of Si3N4, exhibit significantly enhanced performance and are expected to expand the range of electronic device applications. However, due to the sensitivity of graphene properties to the substrates, there are still lots of problems to be solved for practical graphene device applications, such as the interfacial structure [26] and adhesion between graphene and the substrate surface [27] and the effect of substrate morphologies and roughness on the properties of graphene. Much work has been done to study the effects of SiO2 substrate on graphene. The adhesion energy between graphene and SiO2 and the surface roughness effect on graphene were experimentally [28] and theoretically [29,30] investigated. Further research revealed that graphene morphology can be controlled by the surface roughness and wavelength instead of the surface amplitude [31]. However, few reports have been concentrated on the effects of Si3N4 substrates on graphene properties. The Si3N4 substrates could significantly affect the properties of graphene and related devices. Therefore, understanding the effects of the microscale shape features of patterned Si3N4 on the mechanical properties plays an important role in micro and nano devices. In this paper, we focused on the the effects of surface morphology and roughness of Si3N4 substrates on the Raman scattering and strain change of graphene. Hybrid graphene-Si3N4 structures were produced by photolithography and dry etching of Si3N4 thin films and wet transfer of CVD-grown monolayer graphene onto patterned Si3N4 substrates. The effects of surface morphology and roughness of Si3N4 on the strain change of graphene was demonstrated. Special attention was paid on exploring the roles of the patterned shape and depth in the change of
2. Experimental The schematic procedure for fabricating the hybrid graphene-Si3N4 structures is illustrated in Fig. 1. N-type silicon (3–8 Ω cm, 100oriented) wafers, cleaned by a standard RCA cleaning process [32], were used as the substrates in the experiments (Fig. 1a). First, a layer of Si3N4 film (500 and 600 nm thick) was deposited on the silicon substrates via plasma enhanced chemical vapor deposition (PECVD, Oxford Plasmalab System100) (Fig. 1b). Two kinds of lattices were prepatterned on the Si3N4 substrates using standard photolithography [33], containing spin coating photoresist, soft baking, alignment exposure (MA6, Karl Suss, Germany), post-baking, developing and hard baking steps. This photolithography step used exposure and development to depict geometrical structure on the photoresist layer, and then transferred the pattern on the photomask to the substrate through an etching process. One kind of pattern is the triangular array of points with 3 μm lattice constant and 1.5 μm point diameter (Fig. 1c), and the other is parallel lines with 2 μm width and 4 μm pitch (Fig. 1d). Ion-beam bombardment (Oxford Ionfab 300Plus) was employed to remove the Si3N4 in the patterned area to form holes and trenches respectively. After this step, the pattern was successfully transferred to the Si3N4 surface. Then, the monolayer graphene samples grown on the copper foil substrate by CVD method [6] were cut into 3 cm × 3 cm size. Finally, a standard wet transfer step [34] was carried out to transfer the graphene to the surface of Si3N4 substrates of different patterns and thickness (Fig. 1e and f). The first step of wet transfer was to coat a layer of polymethyl methacrylate (PMMA) on the surface of the graphenecoated copper foil, which served as a sacrificial layer to adsorb graphene. Then put the entire sample in a solution of ammonium persulfate with a concentration of 30 g/L. The corrosion solution can completely corrode the copper foil. After that, the graphene coated with PMMA was removed and transferred to the Si3N4 substrate, and finally PMMA was removed with an acetone solution to successfully transfer graphene. Samples were characterized by a field-emission scanning electron microscope (FESEM: SIGMA HD). The top view micrographs show the (100) plane. Atomic force microscopy (AFM: SHIMADZU SPM9700) was employed to analyze the surface morphology and roughness of the patterned Si3N4 substrates with the tapping mode. The Raman scattering of graphene on patterned Si3N4 was performed by a microRaman spectrometer (RENISHAW inVia) with the excitation wavelength of 488 nm, the spectral resolution of 1 cm−1 and the laser power no more than 2 mW to avoid damaging the graphene.
Fig. 1. A schematic showing the graphene formed on the patterned Si3N4 substrates: (a) silicon wafers, (b) Si3N4 film deposition, (c) hole-patterned Si3N4 substrate, (d) trench-patterned Si3N4 substrate, (e, f) graphene transferred to the surface of Si3N4 substrates. 2
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SEM picture in Fig. 3b shows the graphene grown on trench-patterned Si3N4 substrate (T-graphene). As one can see from both Fig. 3a and b, the graphene completely covers the substrates and exhibits a surface morphology consistent with the patterned Si3N4 substrates. That is to say, nearly perfect step coverage was obtained in our experiments, and the graphene was deposited into the hole and trench rather than suspended over it, except very few holes circled in red shown in Fig. 3a. These suspended graphene is probably due to the incompletely removal of PMMA during the de-glue step. As the surface tension of PMMA is much larger, there is a tendency for the PMMA-based graphene to be suspended over the hole instead of pulled into it. Raman spectra of the hybrid graphene-Si3N4 structures were recorded to discern the quality of graphene after the transfer process. Fig. 4 presents the Raman spectra of H-graphene and T-graphene in Fig. 3. G band, corresponding to ordered sp2–hybridized carbon atoms [35], is associated with highly ordered graphite, 2D band is characteristic of few layers graphene. The spectrum of the H-graphene (black line) shows two prominent characteristic peaks: G peak at ~1575 cm−1 and 2D peak at ~2690 cm−1, indicating a successful transfer of graphene onto the point-patterned Si3N4 substrates. In the Raman spectra of Tgraphene (blue line), the G and 2D peaks are also obvious, with their peak frequencies at ~1570 and ~ 2682 cm−1, respectively. This shows that graphene has also been transferred onto the line-patterned substrates, and both the G and 2D peak frequencies of T-graphene are lower than that of H-graphene (5 cm−1 for G band and 8 cm−1 for 2D band). Besides, the spectrum also reveals that the ratio of the intensity of G peak to 2D peak (IG/I2D) is less than 0.5, indicating the transferred monolayer graphene [36] on both hole- and trench-patterned
3. Results and discussion Characterization of the Si3N4 layers with different patterns was carried out using SEM and AFM methods (Fig. 2). Fig. 2a and b present the SEM images of the surface morphologies of two patterned Si3N4 samples. The top view in Fig. 2a shows triangularly arrayed holes with the pitch of 3 μm, while Fig. 2b shows the trench array with a pitch of 4 μm. Both patterns are about 500 nm in depth. The surface topography and roughness of the samples was analyzed by AFM with the tapping mode. AFM is a scanning probe microscope (SPM) that can scan the surface of the sample to obtain the surface morphology of the material. Using the force between the probe tip and the atoms on the surface of the sample, the cantilever beam is slightly displaced to determine the shape of the surface structure. Fig. 2c and d present the three-dimensional (3D) images on a 500 nm × 500 nm scan window, showing the bottom roughness of holes (Fig. 2c) and trenches (Fig. 2d), respectively. Here, the surface roughness was measured by the root-mean-square (rms) surface roughness parameter (Ra). The Ra of the hole bottom was measured to be 13.99 nm whereas the bottom of the trench had a surface with Ra of 7.12 nm, showing that the bottom of trench is much smoother with its rms surface roughness about half of that at the hole bottom. This difference in roughness will induce the peak red-shifts in 2D and G bands. After the wet transfer step, the CVD-grown monolayer graphene was transferred from copper foil onto the patterned Si3N4 substrates. A general view of the hybrid graphene-Si3N4 structures is presented in Fig. 3. The top view in Fig. 3a shows the graphene formed on 500 nm thick Si3N4 patterned with hole arrays (H-graphene), whereas the
Fig. 2. SEM and AFM images the patterned Si3N4 substrates: (a, c) point array with the triangular lattice with 3 μm lattice constant and 1.5 μm point diameter and (b, d) line array with 2 μm width and 4 μm pitch. The Ra of the hole bottom was measured to be 13.99 nm whereas the bottom of the trench had a surface with Ra of 7.12 nm, 3
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Materials and Design 198 (2021) 109338
Fig. 3. SEM images of graphene covering patterned Si3N4 substrates: (a) graphene on hole array (H-graphene); (b) graphene on trenches (T-graphene).
peaks shown in Raman spectra of H-graphene is lower than that in T-graphene. This might be caused by the rupture of graphene when transferred to the hole-patterned substrate, which owns more complex morphology than trench-patterned substrate. As we know, the 2D peaks of the Raman spectra of graphene are located at ~2700 cm−1 on the smooth substrate. Therefore, from Fig. 5, we can find a red-shift in the 2D band in both H-graphene and T-graphene, and the frequency of H-graphene is a bit higher than that of T-graphene, in agreement with the results in Fig. 4. The zone-center LO phonon around 1580 cm−1 (G peak) and the two-phonon peak around 2700 cm−1 (2D peak), which corresponds to the double resonant excitation of two phonons close to the K point in the Brillouin zone. The phonon frequencies of graphene might be influenced by charge and mechanical strain [37]. However, due to the transfer process in our work, charge doping is a minor contribution to the observed frequency shifts. According to the previous report [38], the surrounding environment and the introduction of impurities in the graphene substrate will lead to the deviation of the graphene 2D peak. Water and oxygen molecules absorbed by the surface of the graphene can cause the graphene to become P-type dopant, resulting in a blue-shift of the graphene's Raman spectrum. However, the tensile stress generated by the transfer of graphene onto the substrate can lead to red shift. The strain produced in graphene is expressed by the formula
Fig. 4. Raman spectra of H-graphene (black line), T-graphene (blue line) and Si3N4 substrate (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Si3N4 substrates. However, a clear trend can be seen from the figure that both the H-graphene and T-graphene exhibit a rising baseline for Raman scattering with the increase of frequency, along with several bands not belonging to graphene. This phenomenon can be explained by a substrate effect as the rising trend and peaks are similar to that in the Raman signals of Si3N4 substrate (red line). With the aim to investigate the effects of pattern's morphology, roughness and depth on the graphene, we further analysis the Raman spectra of H-graphene and T-graphene in detail, especially the two characteristic peaks (G and 2D peaks) with red-shifts. The scan measures of 2D bands of Raman spectra in H-graphene were first performed in a region of 10 μm × 10 μm (Fig. 5a) with the distance between scanning points set to 0.5 μm during the scanning process to obtain enough test data. Meanwhile, for T-graphene, a series of Raman spectra 2D bands were obtained with the scanning length of 20 μm and step length of 0.2 μm (Fig. 5c). The 2D peaks of three representative points in H-graphene and T-graphene are presented in Fig. 5b and d, respectively, after eliminating the influence of the Si3N4 substrate. From Fig. 5b, we can see that the 2D peak frequency of H-graphene is distributed in a range between 2687 and ~ 2691 cm−1. The 2D band is also prominent in T-graphene with the peak frequency located between 2670 and 2682 cm−1 (Fig. 5d). Moreover, the intensity of 2D
εi ¼
Δωi ωi ð1−vÞγi
ð1Þ
where, γi is Grüneisen parameter. When i= 2D, Δω2Drefers to the deviation of the 2D peak frequency of graphene in a stress state, ν is material poisson's ratio, ε2D is the strain produced in graphene [39]. The 2D peak Grüneisen parameter we use is calculated by the magnitude of peak shift of Raman spectrum by Mohiuddin [40] with the calculated value of 2.7 and the Poisson's ratio of silicon nitride of 0.25. According to the formula, the strains produced in H-graphene and T-graphene are 0.2–0.24% and 0.32–0.54%, respectively. To verify the caculated strain in H-graphene and T-graphene, two random points were then selected to measure the Raman G peak of Hgraphene and T-graphene, respectively. As the G peaks of the Raman spectra of graphene are located at ~1580 cm−1 on the smooth substrate, the results in Fig. 6 show that both G peaks in H-graphene and Tgraphene redshift with the magnitude of the shift of 4.8 and 9.8 cm−1, respectively. Compared with the results in Fig. 5, it is found that the red-shift of G peak is much smaller than that of 2D peak. The internal strain in graphene films can also be calculated by the frequency shift of G peaks with the equation (equation 1). When i=G, εG is the amount of strain due to the graphene stretching, ω0G is the frequency of the Raman G peak of the graphene without tensile 4
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Materials and Design 198 (2021) 109338
Fig. 5. Differences in 2D band of Raman spectra: (a) the scanning area and (b) 2D peaks of three representative points in H-graphene; (c) the scanning area and (d) 2D peaks of three representative points in T-graphene.
deformation, ΔωGis the frequency shift of the Raman G peak of the graphene, νis the base material Poisson ratio, γGis the Grüneisen parameter for the corresponding G peak. We select the Grüneisen parameter calculated by Mohiuddin [40] asγG = 1.8, and the strain in Hgraphene and T-graphene is ~0.225% and ~ 0.459%, respectively, in accordance with the calculation results of 2D peaks. By comparing the red-shift of G and 2D peak in graphene on both substrates, it is found that the 2D peaks are more sensitive than G peaks when strain occurs. This result ties well with previous studies [40] wherein the 2D peak of graphene can feel ~0.01% to ~0.03% consistent strain.
From these results it is clear that the stain in T-graphene is almost twice as much as that in H-graphene. It is worth discussing this interesting fact revealed by the results. According to the previous report [41], greater surface roughness will lead to higher surface energy, larger the surface tension of graphene and eventually larger surface stress in graphene. In our experiments, the surface roughness of hole is much greater than that of trench, as shown in Fig. 2c and d, and the strain in H-graphene would be larger than that in T-graphene. This conclusion is not in line with our result, which implies that there may be some other prime factors pertaining to the change of strain in graphene.
Fig. 6. G band of Raman spectra of (a) H-graphene and (b) T-graphene. The magnitude of the redshift is 4.8 and 9.8 cm−1 respectively. 5
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step coverage on the pore walls (Fig. 3). According to the calculation method of the elastic energy produced by graphene per unit area [29], the elastic energy U is as follows: D 2π 4 2 U δg ≈ δg 4 λ
ð2Þ
Where D is the graphene flexural modulus,λis the wavelength of the ripples andδis the amplitude of the ripples. From this equation, it can be deduced that the elastic energy of graphene per unit area is mainly affected byδ. Since the graphene transferred to the patterned substrate is completely covered on pore walls and bottoms, the elastic energy of the graphene is mainly affected by the surface roughness of the substrate. The deeper the hole depth, the larger is the area of hole walls. Therefore, the pattern with 600 nm hole depth has larger surface area than that with 500 nm, leading to higher elastic energy and therefore larger elastic strain of graphene. 4. Conclusion In this study, graphene was transferred on two kinds of patterned Si3N4 layers ((holes and trenches)) and the effects of surface morphology and roughness of patterned Si3N4 on Raman scattering and mechanical properties of graphene were explored. It was found that, Raman features, including the peak frequencies of 2D and G bands, significantly depend on the morphology and roughness of patterns. The Raman spectra analysis reveals that the strain of graphene induced on Si3N4 substrates with different patterns is the main cause of red-shifts both in 2D and G bands. In addition, the morphology and size of patterned area have much greater effect on tensile strain of graphene than the role of surface roughness. The strain in the grooves (0.32 to 0.54%) with larger surface area (1.65 times) is greater than in holes (0.2 to 0.24%), in spite of greater surface roughness in holes. Furthermore, the dependence of pattern depth on the Raman peak shift and stain change of graphene was investigated. The 2D peak is prominent in H-graphene 600 and shows an offset increased by nearly ~5 cm−1 with respect to Hgraphene 500. It can be calculated that the tensile strain of H-graphene 600 is between ~0.25% and ~ 0.42%, larger than that of H-graphene 500. The results reveal that the properties of graphene are controlled by the depth of silicon nitride and increasing the pattern depth leads to higher red-shift of 2D peak and larger strain in graphene. Understanding the effects of the microscale shape features of patterned Si3N4 on the mechanical properties plays an important role in micro and nano devices. The results of our work may provide certain guiding significance for the design of graphene-based devices.
Fig. 7. 2D peaks of three representative points in H-graphene 600. Scanning in an area of 10 μm × 10 μm with the distance between scanning points set to 0.5 μm.
We speculate that this might be due to the differences of morphologies and topographies between two substrates. According to NeekAmal’ work [42], graphene generates friction and energy when contacted with the substrate, and if the substrate is rough, nonuniform stress can be produced. Although the roughness in the hole is higher than that in the trench, the surface area of the bottom of trench is 1.65 times larger than that of the hole, resulting in a greater total tensile tress in T-graphene. In addition, the morphologies of these two patterns are different, with the shape similar to a circle and rectangle, respectively. T-graphene tends to produce localized stress concentration at sharp corners of trench, whereas the circle pattern makes homogeneous stress in H-graphene and avoids stress concentration at the edge. Overall the morphology of substrate pattern plays an important role in the transferred graphene and can cause a frequency shift of the 2D peak and G peak in the Raman spectrum. Then, the dependence of pattern depth on the Raman Scattering and mechanical properties of graphene was investigated. The Si3N4 layer with 600 nm thickness and hole array pattern was applied as the substrate to transfer graphene (H-graphene 600). The bottom roughness of holes in 600 nm thick pattern was measured to be 14.04 nm by AFM, similar to that in 500 nm thick pattern. The scan measures of 2D bands of Raman spectra in H-graphene 600 were performed in a region of 10 μm × 10 μm (Fig. 7 inset) with the distance between scanning points set to 0.5 μm during the scanning process. The 2D peaks of three representative points are presented in Fig. 7, after eliminating the influence of the Si3N4 substrate. As we can see, the 2D peak is prominent in H-graphene 600 and the peak frequency is distributed in a range between ~2677 cm−1 and ~ 2686 cm−1, an offset increased by nearly ~5 cm−1 with respect to H-graphene 500. According to equation (equation 1), it can be calculated that the tensile strain of H-graphene 600 is between ~0.25% and ~ 0.42%, which is larger than that of H-graphene 500. That is to say, increasing the depth of pattern leads to higher red-shift of 2D peak and larger strain in graphene. It is notable that the morphology and roughness of pattern in two samples are the same, so the results demonstrated that the Raman Scattering and mechanical properties of graphene are very sensitive to the pattern depth. As the hole wall cannot be very smooth after the photolithography and dry etching processes, the roughness of the hole wall is much larger than that of the hole bottom. After transferred onto the patterned Si3N4 substrate, the graphene was deposited into the holes with nearly perfect
Author statement YZ, HC and GZ carried out the experiments. MW and LZ created the figs. JJ and SZ participated in the discussion and gave useful suggestions. The manuscript was composed by DG and YZ. All authors read and approved the final manuscript.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by grants from the Natural Science Foundation of Jiangsu Province (BK20180098), Open Research Fund of National Laboratory of Solid State Microstructures (M32045, M33042) .
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Effect of patterned silicon nitride substrate on Raman scattering and stress of graphene Daohan Ge a,b,⁎, Yuan Zhang a, Hui Chen a, Guangfu Zhen a, Minchang Wang a, Jiwei Jiao c, Liqiang Zhang a,d,⁎, Shining Zhu b a
Institute of Intelligent Flexible Mechatronics, School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, PR China SIWAVE, Inc., Shanghai 201800, PR China d Jiangsu Collaborative Innovation Centre of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, PR China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Si3N4 films with two patterns (holes and trenches) were produced on silicon wafers. • CVD-grown monolayer graphene were transferred onto the patterned Si3N4 substrates. • Peak red-shift in 2D and G bands was due to the strain induced by Si3N4 patterns. • Effect of pattern morphology is greater than that of roughness in graphene strain. • Depth effect of Si3N4 on Raman peak shift and the strain change of graphene exists.
a r t i c l e
i n f o
Article history: Received 26 July 2020 Received in revised form 9 November 2020 Accepted 9 November 2020 Available online 17 November 2020 Keywords: Patterned silicon nitride Graphene Surface morphology Surface roughness Raman scattering Tensile strain
a b s t r a c t Graphene is widely used for nano-devices due to its distinctive band structure and fascinating properties. The substrates could significantly affect the properties of graphene and related devices. In this work, we investigate the effect of surface morphology and roughness of patterned silicon nitride substrates on Raman scattering and stress of graphene. We find that the Raman scattering of graphene depend strongly on surface morphology and roughness of patterned substrates. It is concluded that the peak red-shifts in 2D and G bands was due to the strain induced by patterns with different surface morphology (holes and trenches) and roughness. Furthermore, the effects of morphology of patterned area are much greater than the role of surface roughness in the induced strain of graphene. Due to the larger surface area (about 1.65 times), the strain in the grooves is greater than in holes, in spite of greater surface roughness in holes. Our results also reveal that the effect of pattern depth should be taken into account to understand the Raman peak shift and the strain change of graphene. Our work is fundamentally important for understanding the graphene properties on dependence of surface morphology of substrates and enhancing the interfacial strength of graphene-based devices. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Ge), [email protected] (L. Zhang).
As a two-dimensional single atomic layer composed of sp2-bonded carbon atoms in a honeycomb lattice, graphene has attracted significant
https://doi.org/10.1016/j.matdes.2020.109338 0264-1275/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Materials and Design 198 (2021) 109338
tensile strain and Raman scattering of graphene. We find that the morphology and size of patterned area have much greater effect on tensile strain of graphene than the role of surface roughness. Moreover, our results reveal obvious depth effect of Si3N4 on the Raman peak shift and the strain change of graphene.
research interest since its first production in 2004 [1], due to its extraordinary structural features and desirable physical and chemical properties [2–4]. These highly appealing properties, especially with the development of high yield graphene fabrication technology via chemical vapor deposition (CVD) process [5,6] and epitaxial growth on silicon carbide (SiC) [7], offer a wide range of potential applications of graphene in varied fields including photodetectors [8], biosensors [9,10], energy-storage devices [11–13], and field effect transistors [14]. All these applications require graphene films to be transferred on a certain substrate, and varieties of materials, to date, have proved to be ideal substrates for graphene, ranging from metal layers (e.g., Cu, Ni and Fe) to dielectric materials (e.g., SiO2 and silicon nitride (Si3N4)). Si3N4 films have been widely used in the field of microelectromechanical system (MEMS) devices and beyond [15–18], owing to their low masses, ultrahigh mechanical quality factors [19], large fabrication tolerance, and excellent optical properties [20]. Recent development of high-quality graphene films growth on Si3N4 substrates by Liu et al. [21] has opened up the possibility for a series of applications of hybrid graphene-Si3N4 structures in microelectronics [22–25]. These hybrid graphene-Si3N4 devices, combining the desirable optical and electrical properties of graphene with the superior mechanical and optical properties of Si3N4, exhibit significantly enhanced performance and are expected to expand the range of electronic device applications. However, due to the sensitivity of graphene properties to the substrates, there are still lots of problems to be solved for practical graphene device applications, such as the interfacial structure [26] and adhesion between graphene and the substrate surface [27] and the effect of substrate morphologies and roughness on the properties of graphene. Much work has been done to study the effects of SiO2 substrate on graphene. The adhesion energy between graphene and SiO2 and the surface roughness effect on graphene were experimentally [28] and theoretically [29,30] investigated. Further research revealed that graphene morphology can be controlled by the surface roughness and wavelength instead of the surface amplitude [31]. However, few reports have been concentrated on the effects of Si3N4 substrates on graphene properties. The Si3N4 substrates could significantly affect the properties of graphene and related devices. Therefore, understanding the effects of the microscale shape features of patterned Si3N4 on the mechanical properties plays an important role in micro and nano devices. In this paper, we focused on the the effects of surface morphology and roughness of Si3N4 substrates on the Raman scattering and strain change of graphene. Hybrid graphene-Si3N4 structures were produced by photolithography and dry etching of Si3N4 thin films and wet transfer of CVD-grown monolayer graphene onto patterned Si3N4 substrates. The effects of surface morphology and roughness of Si3N4 on the strain change of graphene was demonstrated. Special attention was paid on exploring the roles of the patterned shape and depth in the change of
2. Experimental The schematic procedure for fabricating the hybrid graphene-Si3N4 structures is illustrated in Fig. 1. N-type silicon (3–8 Ω cm, 100oriented) wafers, cleaned by a standard RCA cleaning process [32], were used as the substrates in the experiments (Fig. 1a). First, a layer of Si3N4 film (500 and 600 nm thick) was deposited on the silicon substrates via plasma enhanced chemical vapor deposition (PECVD, Oxford Plasmalab System100) (Fig. 1b). Two kinds of lattices were prepatterned on the Si3N4 substrates using standard photolithography [33], containing spin coating photoresist, soft baking, alignment exposure (MA6, Karl Suss, Germany), post-baking, developing and hard baking steps. This photolithography step used exposure and development to depict geometrical structure on the photoresist layer, and then transferred the pattern on the photomask to the substrate through an etching process. One kind of pattern is the triangular array of points with 3 μm lattice constant and 1.5 μm point diameter (Fig. 1c), and the other is parallel lines with 2 μm width and 4 μm pitch (Fig. 1d). Ion-beam bombardment (Oxford Ionfab 300Plus) was employed to remove the Si3N4 in the patterned area to form holes and trenches respectively. After this step, the pattern was successfully transferred to the Si3N4 surface. Then, the monolayer graphene samples grown on the copper foil substrate by CVD method [6] were cut into 3 cm × 3 cm size. Finally, a standard wet transfer step [34] was carried out to transfer the graphene to the surface of Si3N4 substrates of different patterns and thickness (Fig. 1e and f). The first step of wet transfer was to coat a layer of polymethyl methacrylate (PMMA) on the surface of the graphenecoated copper foil, which served as a sacrificial layer to adsorb graphene. Then put the entire sample in a solution of ammonium persulfate with a concentration of 30 g/L. The corrosion solution can completely corrode the copper foil. After that, the graphene coated with PMMA was removed and transferred to the Si3N4 substrate, and finally PMMA was removed with an acetone solution to successfully transfer graphene. Samples were characterized by a field-emission scanning electron microscope (FESEM: SIGMA HD). The top view micrographs show the (100) plane. Atomic force microscopy (AFM: SHIMADZU SPM9700) was employed to analyze the surface morphology and roughness of the patterned Si3N4 substrates with the tapping mode. The Raman scattering of graphene on patterned Si3N4 was performed by a microRaman spectrometer (RENISHAW inVia) with the excitation wavelength of 488 nm, the spectral resolution of 1 cm−1 and the laser power no more than 2 mW to avoid damaging the graphene.
Fig. 1. A schematic showing the graphene formed on the patterned Si3N4 substrates: (a) silicon wafers, (b) Si3N4 film deposition, (c) hole-patterned Si3N4 substrate, (d) trench-patterned Si3N4 substrate, (e, f) graphene transferred to the surface of Si3N4 substrates. 2
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SEM picture in Fig. 3b shows the graphene grown on trench-patterned Si3N4 substrate (T-graphene). As one can see from both Fig. 3a and b, the graphene completely covers the substrates and exhibits a surface morphology consistent with the patterned Si3N4 substrates. That is to say, nearly perfect step coverage was obtained in our experiments, and the graphene was deposited into the hole and trench rather than suspended over it, except very few holes circled in red shown in Fig. 3a. These suspended graphene is probably due to the incompletely removal of PMMA during the de-glue step. As the surface tension of PMMA is much larger, there is a tendency for the PMMA-based graphene to be suspended over the hole instead of pulled into it. Raman spectra of the hybrid graphene-Si3N4 structures were recorded to discern the quality of graphene after the transfer process. Fig. 4 presents the Raman spectra of H-graphene and T-graphene in Fig. 3. G band, corresponding to ordered sp2–hybridized carbon atoms [35], is associated with highly ordered graphite, 2D band is characteristic of few layers graphene. The spectrum of the H-graphene (black line) shows two prominent characteristic peaks: G peak at ~1575 cm−1 and 2D peak at ~2690 cm−1, indicating a successful transfer of graphene onto the point-patterned Si3N4 substrates. In the Raman spectra of Tgraphene (blue line), the G and 2D peaks are also obvious, with their peak frequencies at ~1570 and ~ 2682 cm−1, respectively. This shows that graphene has also been transferred onto the line-patterned substrates, and both the G and 2D peak frequencies of T-graphene are lower than that of H-graphene (5 cm−1 for G band and 8 cm−1 for 2D band). Besides, the spectrum also reveals that the ratio of the intensity of G peak to 2D peak (IG/I2D) is less than 0.5, indicating the transferred monolayer graphene [36] on both hole- and trench-patterned
3. Results and discussion Characterization of the Si3N4 layers with different patterns was carried out using SEM and AFM methods (Fig. 2). Fig. 2a and b present the SEM images of the surface morphologies of two patterned Si3N4 samples. The top view in Fig. 2a shows triangularly arrayed holes with the pitch of 3 μm, while Fig. 2b shows the trench array with a pitch of 4 μm. Both patterns are about 500 nm in depth. The surface topography and roughness of the samples was analyzed by AFM with the tapping mode. AFM is a scanning probe microscope (SPM) that can scan the surface of the sample to obtain the surface morphology of the material. Using the force between the probe tip and the atoms on the surface of the sample, the cantilever beam is slightly displaced to determine the shape of the surface structure. Fig. 2c and d present the three-dimensional (3D) images on a 500 nm × 500 nm scan window, showing the bottom roughness of holes (Fig. 2c) and trenches (Fig. 2d), respectively. Here, the surface roughness was measured by the root-mean-square (rms) surface roughness parameter (Ra). The Ra of the hole bottom was measured to be 13.99 nm whereas the bottom of the trench had a surface with Ra of 7.12 nm, showing that the bottom of trench is much smoother with its rms surface roughness about half of that at the hole bottom. This difference in roughness will induce the peak red-shifts in 2D and G bands. After the wet transfer step, the CVD-grown monolayer graphene was transferred from copper foil onto the patterned Si3N4 substrates. A general view of the hybrid graphene-Si3N4 structures is presented in Fig. 3. The top view in Fig. 3a shows the graphene formed on 500 nm thick Si3N4 patterned with hole arrays (H-graphene), whereas the
Fig. 2. SEM and AFM images the patterned Si3N4 substrates: (a, c) point array with the triangular lattice with 3 μm lattice constant and 1.5 μm point diameter and (b, d) line array with 2 μm width and 4 μm pitch. The Ra of the hole bottom was measured to be 13.99 nm whereas the bottom of the trench had a surface with Ra of 7.12 nm, 3
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Fig. 3. SEM images of graphene covering patterned Si3N4 substrates: (a) graphene on hole array (H-graphene); (b) graphene on trenches (T-graphene).
peaks shown in Raman spectra of H-graphene is lower than that in T-graphene. This might be caused by the rupture of graphene when transferred to the hole-patterned substrate, which owns more complex morphology than trench-patterned substrate. As we know, the 2D peaks of the Raman spectra of graphene are located at ~2700 cm−1 on the smooth substrate. Therefore, from Fig. 5, we can find a red-shift in the 2D band in both H-graphene and T-graphene, and the frequency of H-graphene is a bit higher than that of T-graphene, in agreement with the results in Fig. 4. The zone-center LO phonon around 1580 cm−1 (G peak) and the two-phonon peak around 2700 cm−1 (2D peak), which corresponds to the double resonant excitation of two phonons close to the K point in the Brillouin zone. The phonon frequencies of graphene might be influenced by charge and mechanical strain [37]. However, due to the transfer process in our work, charge doping is a minor contribution to the observed frequency shifts. According to the previous report [38], the surrounding environment and the introduction of impurities in the graphene substrate will lead to the deviation of the graphene 2D peak. Water and oxygen molecules absorbed by the surface of the graphene can cause the graphene to become P-type dopant, resulting in a blue-shift of the graphene's Raman spectrum. However, the tensile stress generated by the transfer of graphene onto the substrate can lead to red shift. The strain produced in graphene is expressed by the formula
Fig. 4. Raman spectra of H-graphene (black line), T-graphene (blue line) and Si3N4 substrate (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Si3N4 substrates. However, a clear trend can be seen from the figure that both the H-graphene and T-graphene exhibit a rising baseline for Raman scattering with the increase of frequency, along with several bands not belonging to graphene. This phenomenon can be explained by a substrate effect as the rising trend and peaks are similar to that in the Raman signals of Si3N4 substrate (red line). With the aim to investigate the effects of pattern's morphology, roughness and depth on the graphene, we further analysis the Raman spectra of H-graphene and T-graphene in detail, especially the two characteristic peaks (G and 2D peaks) with red-shifts. The scan measures of 2D bands of Raman spectra in H-graphene were first performed in a region of 10 μm × 10 μm (Fig. 5a) with the distance between scanning points set to 0.5 μm during the scanning process to obtain enough test data. Meanwhile, for T-graphene, a series of Raman spectra 2D bands were obtained with the scanning length of 20 μm and step length of 0.2 μm (Fig. 5c). The 2D peaks of three representative points in H-graphene and T-graphene are presented in Fig. 5b and d, respectively, after eliminating the influence of the Si3N4 substrate. From Fig. 5b, we can see that the 2D peak frequency of H-graphene is distributed in a range between 2687 and ~ 2691 cm−1. The 2D band is also prominent in T-graphene with the peak frequency located between 2670 and 2682 cm−1 (Fig. 5d). Moreover, the intensity of 2D
εi ¼
Δωi ωi ð1−vÞγi
ð1Þ
where, γi is Grüneisen parameter. When i= 2D, Δω2Drefers to the deviation of the 2D peak frequency of graphene in a stress state, ν is material poisson's ratio, ε2D is the strain produced in graphene [39]. The 2D peak Grüneisen parameter we use is calculated by the magnitude of peak shift of Raman spectrum by Mohiuddin [40] with the calculated value of 2.7 and the Poisson's ratio of silicon nitride of 0.25. According to the formula, the strains produced in H-graphene and T-graphene are 0.2–0.24% and 0.32–0.54%, respectively. To verify the caculated strain in H-graphene and T-graphene, two random points were then selected to measure the Raman G peak of Hgraphene and T-graphene, respectively. As the G peaks of the Raman spectra of graphene are located at ~1580 cm−1 on the smooth substrate, the results in Fig. 6 show that both G peaks in H-graphene and Tgraphene redshift with the magnitude of the shift of 4.8 and 9.8 cm−1, respectively. Compared with the results in Fig. 5, it is found that the red-shift of G peak is much smaller than that of 2D peak. The internal strain in graphene films can also be calculated by the frequency shift of G peaks with the equation (equation 1). When i=G, εG is the amount of strain due to the graphene stretching, ω0G is the frequency of the Raman G peak of the graphene without tensile 4
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Fig. 5. Differences in 2D band of Raman spectra: (a) the scanning area and (b) 2D peaks of three representative points in H-graphene; (c) the scanning area and (d) 2D peaks of three representative points in T-graphene.
deformation, ΔωGis the frequency shift of the Raman G peak of the graphene, νis the base material Poisson ratio, γGis the Grüneisen parameter for the corresponding G peak. We select the Grüneisen parameter calculated by Mohiuddin [40] asγG = 1.8, and the strain in Hgraphene and T-graphene is ~0.225% and ~ 0.459%, respectively, in accordance with the calculation results of 2D peaks. By comparing the red-shift of G and 2D peak in graphene on both substrates, it is found that the 2D peaks are more sensitive than G peaks when strain occurs. This result ties well with previous studies [40] wherein the 2D peak of graphene can feel ~0.01% to ~0.03% consistent strain.
From these results it is clear that the stain in T-graphene is almost twice as much as that in H-graphene. It is worth discussing this interesting fact revealed by the results. According to the previous report [41], greater surface roughness will lead to higher surface energy, larger the surface tension of graphene and eventually larger surface stress in graphene. In our experiments, the surface roughness of hole is much greater than that of trench, as shown in Fig. 2c and d, and the strain in H-graphene would be larger than that in T-graphene. This conclusion is not in line with our result, which implies that there may be some other prime factors pertaining to the change of strain in graphene.
Fig. 6. G band of Raman spectra of (a) H-graphene and (b) T-graphene. The magnitude of the redshift is 4.8 and 9.8 cm−1 respectively. 5
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step coverage on the pore walls (Fig. 3). According to the calculation method of the elastic energy produced by graphene per unit area [29], the elastic energy U is as follows: D 2π 4 2 U δg ≈ δg 4 λ
ð2Þ
Where D is the graphene flexural modulus,λis the wavelength of the ripples andδis the amplitude of the ripples. From this equation, it can be deduced that the elastic energy of graphene per unit area is mainly affected byδ. Since the graphene transferred to the patterned substrate is completely covered on pore walls and bottoms, the elastic energy of the graphene is mainly affected by the surface roughness of the substrate. The deeper the hole depth, the larger is the area of hole walls. Therefore, the pattern with 600 nm hole depth has larger surface area than that with 500 nm, leading to higher elastic energy and therefore larger elastic strain of graphene. 4. Conclusion In this study, graphene was transferred on two kinds of patterned Si3N4 layers ((holes and trenches)) and the effects of surface morphology and roughness of patterned Si3N4 on Raman scattering and mechanical properties of graphene were explored. It was found that, Raman features, including the peak frequencies of 2D and G bands, significantly depend on the morphology and roughness of patterns. The Raman spectra analysis reveals that the strain of graphene induced on Si3N4 substrates with different patterns is the main cause of red-shifts both in 2D and G bands. In addition, the morphology and size of patterned area have much greater effect on tensile strain of graphene than the role of surface roughness. The strain in the grooves (0.32 to 0.54%) with larger surface area (1.65 times) is greater than in holes (0.2 to 0.24%), in spite of greater surface roughness in holes. Furthermore, the dependence of pattern depth on the Raman peak shift and stain change of graphene was investigated. The 2D peak is prominent in H-graphene 600 and shows an offset increased by nearly ~5 cm−1 with respect to Hgraphene 500. It can be calculated that the tensile strain of H-graphene 600 is between ~0.25% and ~ 0.42%, larger than that of H-graphene 500. The results reveal that the properties of graphene are controlled by the depth of silicon nitride and increasing the pattern depth leads to higher red-shift of 2D peak and larger strain in graphene. Understanding the effects of the microscale shape features of patterned Si3N4 on the mechanical properties plays an important role in micro and nano devices. The results of our work may provide certain guiding significance for the design of graphene-based devices.
Fig. 7. 2D peaks of three representative points in H-graphene 600. Scanning in an area of 10 μm × 10 μm with the distance between scanning points set to 0.5 μm.
We speculate that this might be due to the differences of morphologies and topographies between two substrates. According to NeekAmal’ work [42], graphene generates friction and energy when contacted with the substrate, and if the substrate is rough, nonuniform stress can be produced. Although the roughness in the hole is higher than that in the trench, the surface area of the bottom of trench is 1.65 times larger than that of the hole, resulting in a greater total tensile tress in T-graphene. In addition, the morphologies of these two patterns are different, with the shape similar to a circle and rectangle, respectively. T-graphene tends to produce localized stress concentration at sharp corners of trench, whereas the circle pattern makes homogeneous stress in H-graphene and avoids stress concentration at the edge. Overall the morphology of substrate pattern plays an important role in the transferred graphene and can cause a frequency shift of the 2D peak and G peak in the Raman spectrum. Then, the dependence of pattern depth on the Raman Scattering and mechanical properties of graphene was investigated. The Si3N4 layer with 600 nm thickness and hole array pattern was applied as the substrate to transfer graphene (H-graphene 600). The bottom roughness of holes in 600 nm thick pattern was measured to be 14.04 nm by AFM, similar to that in 500 nm thick pattern. The scan measures of 2D bands of Raman spectra in H-graphene 600 were performed in a region of 10 μm × 10 μm (Fig. 7 inset) with the distance between scanning points set to 0.5 μm during the scanning process. The 2D peaks of three representative points are presented in Fig. 7, after eliminating the influence of the Si3N4 substrate. As we can see, the 2D peak is prominent in H-graphene 600 and the peak frequency is distributed in a range between ~2677 cm−1 and ~ 2686 cm−1, an offset increased by nearly ~5 cm−1 with respect to H-graphene 500. According to equation (equation 1), it can be calculated that the tensile strain of H-graphene 600 is between ~0.25% and ~ 0.42%, which is larger than that of H-graphene 500. That is to say, increasing the depth of pattern leads to higher red-shift of 2D peak and larger strain in graphene. It is notable that the morphology and roughness of pattern in two samples are the same, so the results demonstrated that the Raman Scattering and mechanical properties of graphene are very sensitive to the pattern depth. As the hole wall cannot be very smooth after the photolithography and dry etching processes, the roughness of the hole wall is much larger than that of the hole bottom. After transferred onto the patterned Si3N4 substrate, the graphene was deposited into the holes with nearly perfect
Author statement YZ, HC and GZ carried out the experiments. MW and LZ created the figs. JJ and SZ participated in the discussion and gave useful suggestions. The manuscript was composed by DG and YZ. All authors read and approved the final manuscript.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by grants from the Natural Science Foundation of Jiangsu Province (BK20180098), Open Research Fund of National Laboratory of Solid State Microstructures (M32045, M33042) .
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