Direct patterning of nitrogen-doped chemical vapor deposited graphene-based microstructures for charge carrier measurements employing femtosecond laser ablation

Chemical vapor deposited nitrogen-doped graphene, transferred onto a SiO2/Si substrate, was selectively patterned by femtosecond laser ablation for the formation of the topology dedicated to charge carrier measurements. Ultrashort 1030 nm wavelength Yb:KGW fs-laser pulses of 22 μJ energy,14 mJ cm−2 fluence, 96% pulse overlap, and a scanning speed of 100 mm s−1, were found to be the optimum regime for the high throughput microstructure ablation in graphene, without surface damage of the substrate in the employed fs-laser micromachining workstation. Optical scanning electron and atomic force microscopy, as well as Raman spectroscopy, were applied to clarify the intensive fs-laser light irradiation effects on graphene and the substrate, and to also verify the quality of the graphene removal. Measurements of magnetotransport properties of the fs-laser ablated nitrogen-doped graphene microstructure in the Hall configuration enabled the determination of the type, as well as concentration of charge carriers in a wide range of temperatures.


Introduction
Currently, graphene's two-dimensional (2D) nature and unique electronic structure with linear dispersion, make it a very attractive material. However, the great application potential of graphene still faces technological limitations, which result from its reduced dimensionality. Indeed, any impurities on the graphene surface such as adsorbed oxygen, trapped water, chemical residuals cause change of its electronic state mainly due to two mechanisms: charge transfer and charge scattering [1]. This may seriously affect the performance of designed electrical devices [2]. The top down approach widely used in modern electronics assumes application of conventional lithography, which is a multistep process and usually involves the utilization of electron or light irradiation sensitive layers. As a result, the inevitable residuals of the resist on the surface of graphene change its electronic state [3]. Tremendous effort is usually required to remove resist residuals completely [4,5].
The direct patterning technique of graphene via ultrashort pulse laser ablation, which is considered to be a prospective powerful tool in the micro and nanofabrication of graphenebased devices [6], can be a competitive alternative to the conventional lithography process. It should be noted that the 2D nature of graphene, with charge carriers confined within one atomic layer, causes its electrical, optical and sensing properties to be strongly influenced by the surrounding media or functionalization of graphene. In this sense, ultrashort pulse laser ablation appears to be an efficient technological solution for the production of planar structures excluding any impurities or residuals characteristic to different chemicals applied in conventional graphene patterning techniques. In contrast to the other serial exposure lithography techniques, such as focused ion beam [7] and electron beam lithography [8,9], microfabrication techniques based on the ultrashort pulse laser approach [10,11] are widely accessible and easily scalable. Fs-laser micromachining demonstrates a diffraction limited patterning resolution, or even sub-wavelength resolution via direct laser interference patterning [12,13]. At the same time promises a very competitive writing field area compared to conventional lithography techniques. One of the important parameters for the pulsed laser-assisted patterning is high peak power assured by the ultra-short pulse duration, which for the case of graphene ablation may vary from 10 −15 [14][15][16] to 10 −9 s [17]. Picosecond selective laser ablation of a wafer-scale graphene film was shown to be a flexible, high speed device fabrication method while avoiding the degradation of electrical properties associated with the conventional lithographic method [18]. Over the last decade, femtosecond (fs) lasers for micromachining technology have made their evolutionary step from scientific equipment to a reliable technological tool for industrial manufacturers. Due to their ability to process any material with a minimal number of heat-affected zones with the increased resolution of the desired structures, fs-lasers are being considered for a growing list of micromachining applications.
Graphene-based micro and nanostructures could be considered as one of the evolving areas for variety of applications. For example, micropatterning of chemical vapor deposition (CVD)-synthesized large-area graphene films employing fslaser cutting process could be mentioned [19]. It was shown that micro ribbon, or other patterned micro structures, can be fabricated without applying any resist. In [20], fs-laser ablation was reported as part of the technological process used to remove graphene from silicon-on-photonic integrated waveguides. Fs-laser ablation was shown to be an efficient solution in some more sophisticated applications, such as graphenebased gas/vapor sensors that have attracted significant attention in recent years due to their variety of structures, unique sensing performances [21].
Despite many papers [6,19,20,[22][23][24] addressing the issue of single layer and/or multilayer graphene ablation, in many cases directly applying the regimes reported in the literature may not lead to the expected patterning results because it is both a laser beam handling [25] and a substrate material sensitive method.
The main issues to be solved for any practical applications, where graphene acts as a functional layer, are targeted removal of graphene with minimum damage of the substrate and reasonable patterning speed. In the current research we have elucidated intensive light effects on the CVD grown bilayer nitrogen-doped graphene transferred on SiO 2 /Si substrate and optimized the fs-laser assisted patterning process. In particular, we found optimum relations between the fs-laser pulse energy, fluence and overlap of pulses. We successfully patterned the Hall structure with 80 μm width graphene strip employing fast galvanometric laser scanning setup. This gave us the opportunity, by conducting magnetotransport and electrical measurements at different temperatures and magnetic fields, to obtain information regarding the type and concentration of the charge carriers.

Experimental part
Graphene used in our work was grown by atmospheric pressure (AP) CVD from chemically pure n-decane on a copper foil (Alfa Aesar) at 1050 °C utilizing a custom-built setup. The samples were prepared in the presence of N 2 (99.95%) and H 2 (99.99%) gas flow with the rates of 100 and 6 cm 3 min −1 , respectively. This approach allowed to synthesize the bi-layer graphene which was proved by the exhaustive analysis of Raman data [26]. After deposition, graphene was transferred to the SiO 2 (~700 nm)/Si substrate by the wet transfer method. Detailed descriptions of the graphene synthesis, transfer approach and graphene characterization can be found elsewhere [26,27]. Comprehensive XPS analysis on identically synthesized samples revealed that graphene was N-doped with the nitrogen concentration of n N ≈ 1.5 × 10 13 cm −2 [26].
Laser patterning was performed, employing femtosecond Yb:KGW laser (λ = 1030 nm, τ = 270 fs, P = 4 W, Pharos 04-500-PP, Light Conversion) and micromachining workstation (FemtoLAB, Altechna R&D) controlled with the SCA software (Altechna R&D). Two modes of laser beam handling were utilized: (i) translation of the sample with respect to the tightly focused laser beam; (ii) steering of the beam with a galvo scanner with respect to the stationary sample. The former was exploited for elucidation of the intensive laser light interaction with the graphene on the SiO 2 /Si substrates and determination of the laser processing conditions necessary to remove the graphene. Laser micromachining was performed with N.A. = 0.42-and 4-mm focal length 50× objective lens (Plan Apo NIR Infinity-Corrected, Mitutoyo) and XYZ translation stage (ANT130-160-XY, ANT130-5-V, Aerotech, further noted as XYZ).
The latter was applied for upscaling the patterning to cm 2 range areas and was based on a galvanometer scanner (SCANcube III 14, ScanLab) and F-theta lens of 100 mm focal length and 115 mm working distance (150-1001, Eksma Optics). A detailed description of both modes applied can be found elsewhere [28][29][30][31]. The diffraction limited the spot sizes of the objective and F-theta lens focused laser beam were 2.54 μm and 14 μm, respectively. Using the first setup for evaluation of ablation parameters, we made test patterns of 10 × 10 μm in size. Two parameters were varied in our experiments, the fs-laser power (P = 0.69 mW-20.7 mW) and the intershot distance (0.5 μm-3.4 μm, or the pulse overlap from 80.3% to non-touching spots, respectively) resulting in an array of 30 × 30 spots. The power of the laser beam and the distance between the neighboring exposed spots were gradually changed along the vertical and horizontal lines, respectively ( figure 1(a)). When applicable graphene removal conditions were obtained, the microstructure design for the Hall effect measurements was patterned employing the second, high throughput patterning laser setup using the recalculated parameters in order to keep the same level of the laser pulse energy (see table 1). The differences in fluence per pulse, due to different setups used, (namely laser focusing conditions) were compensated, increasing the pulse overlap.
The patterned graphene structure was analyzed by optical microscopy (BXFM, Olympus), scanning electron microscopy (SEM, SU 9000, Hitachi), atomic force microscopy (AFM, Certus Light V, Nano Scan Technology) and Raman spectroscopy (Confotec NR 500, SOL Instruments, λ = 473 nm) to monitor structural changes under different fs-laser processing conditions. A 100× (numerical aperture NA = 0.95) objective and the laser power of 500 μW was used to collect the Raman signal. Mapping of the Raman signal was performed by employing the galvoscanner setup with sa patial resolution less than 1 μm.
Magnetotransport measurements of hte fs-laser ablated bridge structure were performed in Hall geometry with six silver contacts as electric probes. Two contacts, along the bridge, were used for supplying the current and measuring the voltage (V x ) and while the other two perpendicular to the bridge were employed for Hall voltage measurements (V H ). DC resistivity and Hall measurements were carried out using a cryogen free measuring system (Cryogenic Ltd, London), which allows setting the sample temperature in the range from 2 K to 300 K with an accuracy better than 0.05 K and magnetic field up to 8 T with central field homogeneity within 0.001% over 25 mm. The temperature was measured with a Cernox ™ CX-1030 thermometer located on the holder near the sample.
A Lakeshore 340 controller allowed stabilizing temperature with the accuracy of ±5 mK during the measurements. DC resistivity was measured by the four-probe method with a Keithley 6430 source-measure unit, which generated a DC current of 5 μA and measured the voltage. To measure the Hall voltage, a Keithley 2182А nanovoltmeter was used. The magnetic field was oriented perpendicular to the substrate surface and, hence, perpendicular to the current direction.

Results and discussion
An overview of whole fs-laser ablated 30 × 30 matrix is depicted in figure 1. From both, SEM ( figure 1(b)) and optical microscope ( figure 1(c)), micrographs one can directly observe a considerable correlation between the brightness or colors of the spots with respect to the laser power (along the vertical axis), and intershot distances (along the horizontal axis). In the area above the 9th row from the bottom (indicated with a 'SiO 2 ' arrow in figure 1(b)) significant destruction of the sample surface is clearly noticeable. The 9th row in 30 × 30 matrix was obtained applying fluence of 613 mJ cm −2 . At the lower laser fluences used (68-545 mJ cm −2 , the corresponding rows from 1 to 8), some modification of the substrate is still visible, especially at higher pulse densities. Considering the pulse energy (21 nJ) and the diffraction limited laser spot size, the fluence per pulse level of 409 mJ cm −2 was determined from SEM analysis as a good technological condition for selective graphene removal (indicated with a 'G' arrow in figure 1(c)) using micromachining setup, where the sample was translated with respect to the laser beam. The graphene ablation threshold value identified here is close to those reported in literature [14,24].
Zoomed-in SEM images of squares, patterned using different fs-laser processing conditions, are depicted in figure 1(d). Indeed, above the ablation threshold of SiO 2 as well for high and low level of the beam overlap, a significant destruction of the substrate surface is observed (figures 1(d)(ii)-(v)). For the square patterned, at power level just below the one indicated with a red arrow in figure 1(b), the substrate surface looks undamaged, whereas graphene layer is removed (figure 1(d) (i)). The estimated threshold fluence in the latter area of the array is smaller than the threshold of SiO 2 ablation but taking into account transparency of SiO 2 and approximately two times smaller ablation threshold of Si, ~300 mJ cm −2 [24], we may expect deterioration of top SiO 2 layer via ablation of Si underneath. One could also notice that the SiO 2 thickness value in our case was from two to six time thicker compared to [24,32], where fs-laser ablation thresholds were debated. From figure 1(d) one can see that the graphene layer seems intact only ~1.5-2 μm from the laser irradiated area indicating minimal heat affected zones achieved employing ultra-short pulse lasers. These findings are even more pronounced in the Raman analysis as discussed later on.
Since both Si and graphene have Raman active vibration modes, more quantitative information than just simple observation of fs-laser ablated graphene/SiO 2 /Si/ structure morphology can be obtained from Raman spectroscopy studies. One should keep in mind that the ratio of the intensity of the 2D peak to G peak, I 2D /I G , indicates the presence of graphene as well as its quality [33] while the position of the silicon peak at 520.7 cm −1 can be employed to disclose the structural changes in silicon [34,35]. In order to interpret the changes on the sample surface after the intensive laser light irradiation, systematic studies were performed employing Raman mapping of four characteristic laser ablated areas covering: (i) lowest, average and higher fluences at high pulse overlaps and (ii) area corresponding to minimal surface damage albeit visible changes according to microscopy study. This approach enabled us to reveal the laser effects on the sample at critical laser processing conditions and to relate the coverage of graphene together with Si destruction as a function of both fluence and pulse overlap. The summarized results are depicted in figure 2.
As expected, for the highest value of fluence (≫ 409 mJ cm −2 , determined from the microscopy analysis) I 2D /I G contour plots (see figures 2(a), (b) and (d)) indicate that no graphene is observed in the ablated area and at the same time the position of Si is strongly downshifted towards lower wave numbers compared to crystalline Si (521 cm −1 see figures 2(e) and (f)) indicating damage of the substrate [34,35]. The typical example of pristine and fs-laser ablated graphene spectra are depicted in figures 2(i) and (j). In particular, the red shift of Raman Si band is shown in figure 2(j) and spectrum of the spectral region characteristic to graphene corresponding to completely ablated graphene is presented in figure 2(i). For the intermediate energy density level (fluence 681 mJ cm −2 , overlap 80.3%), graphene is also removed, but the red shift of Si band position is still noticed, although it is less pronounced. Finally, for the lowest investigated energy density level (68 mJ cm −2 , 80.3%) no change in Si band position is visible ( figure 2(g)). However, in this case graphene is removed only partially as indicated by the I 2D /I G contour value of 0.59 (figure 2(c)). These discussed Raman results set us the range, where the energy density optimum condition for the graphene ablation is located i.e. fluence 409-681 mJ cm −2 and pulse overlap 33.1%-80.3%, still preserving the quality of the substrate. These values are fully consistent with those obtained from the microscopy analysis (see figure 1).
When the optimized graphene ablation conditions were obtained, a topology for Hall measurements was imposed for verification of fs-laser ablated graphene applicability for practical electrical measurements. Therefore, we used graphene grown and transferred on SiO 2 /Si substrate at the same conditions as for the experiment described above. The determined fs-laser ablation conditions were adapted for the galvoscanner-based beam steering setup preserving the necessary fluence level. The latter setup was chosen considering the macroscopic size of the graphene sample used in the  electrical measurements (2 × 2 cm 2 ) and necessity of the fast fs-laser processing. An optical micrograph of the central part of the resulting microstructure is depicted in figure 3(a). The continuity of the fs-laser patterned graphene strip was verified by Raman mapping technique as well. The I 2D /I G contour plot of (figure 3(b)) revealed that, graphene in the bridge does not contain any holes or gaps, whereas no graphene remained outside the bridge. The AFM scans of the virgin SiO 2 surface, SiO 2 after graphene ablation under optimized conditions and graphene transferred on SiO 2 , were acquired in tapping mode. The roughness analysis revealed that the morphology of SiO 2 after graphene ablation (average surface roughness R a = 0.55 nm) is determined by the overall morphology of the virgin surface-R a = 0.57 nm. Finally, the roughness of graphene was evaluated as R a = 0.56 nm. The AFM study is in line with the Raman mapping results and indicates that the surface of the substrate is preserved. In addition to the demonstration of the ability for the direct fs-laser patterning technique of graphene, the present paper also illustrates the applicability of such structure for the evaluation of the charge carrier concentration. In this work we measured Hall resistance R H as a function of B at different T. These results are depicted in figure 4(a). The experimental R H (B) dependencies are well fitted by linear approximation. Applying the standard relation for the carrier concentration n from the Hall effect measurements, n = q −1 (B/R H ), q being the elementary charge, one can evaluate the n values at different temperatures. This result is shown in figure 4(c). It follows that the carrier concentration slightly varies between 5.1 × 10 12 cm −2 at T = 50 K and 5.55 × 10 12 cm −2 at T = 300 K, which is in good agreement with what it should be expected for N-doped graphene [36]. Considering the mutual directions of the external magnetic field and biasing current used in our configuration, we may conclude that the main type of carriers in our nitrogen-doped graphene are holes (p -type).
The main advantage of the structure produced in our case is that graphene was not contaminated by polymer [5] as usually occurs in patterning utilizing conventional lithography. Current-voltage characteristics measured at different temper atures T and constant magnetic inductance B revealed a linear relationship ( figure 4(b)), which confirms the absence of leakage currents through the oxide layer into the substrate. It is generally accepted that the type of charge carriers in N-doped graphene depends on the configuration that the embedded nitrogen atoms form. It is known that the graphitic and pyrolic configuration lead to n-type, whereas the pyridinic and nitrilic configuration leads to p -type [37]. In our earlier work [26] we have demonstrated that according to XPS analysis of graphene synthesized at identical conditions, it contained N atom concentration of n N ≈ 1.5 × 10 13 cm −2 . Comparing this value with the charge carrier concentration value obtained from the Hall measurements we may conclude that the average number of holes transferred by nitrogen, n/n N , is 0.37 per atom at room temperature. This value is very close to what is known for pyridinic N-bond type, 0.45 [37]. For nitrilic bonding this value is higher, 0.66 [37]. From this we one could suppose that the probable chemical N-bonding is pyridinic. However, we need to emphasize that evaluation of exact n/n N value for graphene requires the elimination of charged transfer effect caused by substrate, trapped water during the graphene transfer, etc which is difficult to achieve and is out of the scope of this work.

Conclusions
It was demonstrated that bilayer graphene can be effectively removed by fs-laser ablation (fluence per pulse14.1 mJ cm −2 ) at relatively high laser beam positioning speed of 100 mm s −1 in cm 2 areas employing a galvoscanner beam steering without damaging the SiO 2 /Si substrate.
Micro Raman analysis elucidated typical effects, correspondent to femtosecond laser irradiation energy densities of the graphene/SiO 2 /Si samples, that were also visible as morph ology changes under microscopy analysis.  The fs-laser patterned graphene microstructures demonstrated themselves as applicable for the electrical Hall resistivity measurements that were used in order to obtain the carrier type and their concentration: p -type and 5.5 × 10 12 cm −2 , respectively.