Efficient Nitrogen Doping of Single-Layer Graphene Accompanied by

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Efficient nitrogen doping of single-layer graphene accompanied by negligible defect generation for integration into hybrid semiconductor heterostructures George Sarau, Martin Heilmann, Muhammad Y. Bashouti, Michael Latzel, Christian Tessarek, and Silke H. Christiansen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00067 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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ACS Applied Materials & Interfaces

Efficient nitrogen doping of single-layer graphene accompanied by negligible defect generation for integration into hybrid semiconductor heterostructures

George Sarau1,2, Martin Heilmann2, Muhammad Bashouti2,3, Michael Latzel2,4, Christian Tessarek1,2, and Silke Christiansen1,2,5*

1. Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner Platz 1, 14109 Berlin, Germany 2. Max Planck Institute for the Science of Light, Staudtstr. 2, 91058 Erlangen, Germany 3. Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 8499000, Israel 4. Institute of Optics, Information and Photonics, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Staudtstr. 7/B2, 91058 Erlangen, Germany 5. Physics Department, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany *Corresponding Author: E-mail: [email protected]

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Abstract

While doping enables application-specific tailoring of graphene properties, it can also produce high defect densities that degrade the beneficial features. In this work, we report efficient nitrogen doping of ~ 11 at% without virtually inducing new structural defects in the initial, large-area, low defective, and transferred single-layer graphene. To shed light on this remarkable high doping – low disorder relationship, a unique experimental strategy consisting in analyzing the changes in doping, strain, and defect density after each important step during the doping procedure was employed. Complementary micro-Raman mapping, X-ray photoelectron spectroscopy, and optical microscopy revealed that effective cleaning of the graphene surface assists efficient nitrogen incorporation accompanied by mild compressive strain resulting in negligible defect formation in the doped graphene lattice. These original results are achieved by separating the growth of graphene from its doping. Moreover, the high doping level occurred simultaneously with the epitaxial growth of n-GaN micro- and nanorods on top of graphene leading to the flow of higher currents through the graphene/n-GaN rod interface. Our approach can be extended towards integrating graphene into other technologically relevant hybrid semiconductor heterostructures and obtaining an ohmic contact at their interfaces by adjusting the doping level in graphene. Keywords: doping, graphene, hybrid, gallium nitride, metal-organic vapor phase epitaxy


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INTRODUCTION Doping of graphene-based materials adds up novel functionalities that are both intriguing for fundamental

studies

and

advantageous

for

emerging

applications

in

electronics,

optoelectronics, energy conversion and storage, catalysis, sensing, biomedicine and so on as detailed in recent reviews.1–6 Nevertheless, this comes at the cost of increased structural defects and distortions in the graphene planar structure depending mainly on the atomic weight of the inserted heteroatoms (B, N, P, O, S, Se, F, Cl, Br, I) and on the details of the doping proceseses.1–6 Among these elements, B and N are the closest to C with respect to size and number of valence electrons and therefore are simpler to accommodate into the graphene lattice leading to p- and n-type doping, respectively. Particularly, single-layer graphene exhibits the largest sensitivity of Fermi level to doping because of its one-atom thickness.7–9 To date, the chemical vapor deposition (CVD) technique is extensively used for simultaneously growing wafer-scale predominately monolayer graphene films and their in-situ doping. However, this approach can induce high defect densities in graphene through its doping,10–16 with defects acting as recombination centers for charge carriers, representing a critical issue that still has to be overcome in view of the graphene integration in hybrid semiconductor heterostructures.7,17– 19

Vertically aligned nanorods or nanowires based on group II – VI (ZnO) and III – V (GaAs, GaP, InP, InAs, GaN) compound semiconductors have been grown heteroepitaxially onto graphenelike surfaces standing for one of the most promising kind of nanoscale hybrid system.17–19 Recently, we showed that single-layer graphene preserves its electrical conductivity as well as its mechanical, thermal, and structural stability after the high temperature growth of GaN rods (up to 1200 oC) and can, in principle, be used as back contact to any semiconductor nanostructures even on insulating substrates like sapphire.20,21 Thus, graphene can act as both growth substrate and bottom (transparent, flexible, conductive) electrode in heterostructure devices such as nanorod-based solar cells and light emitting diodes.17,18 However, the changes in the graphene properties caused by different temperatures and gases during the various stages of the semiconductor growth procedure are up to now fully unexplored. These details



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are significant for understanding and controlling the interplay between dopants and defects in graphene/semiconductor hybrid systems. Here we demonstrate that efficient nitrogen doping of ~ 11 at% occurs in single-layer graphene during the epitaxial growth of n-GaN micro- and nanorods by metal-organic vapor phase epitaxy (MOVPE) on graphene transferred onto sapphire substrates. To the best of our knowledge, this is the highest ex-situ doping level reported so far for CVD large-area monolayer graphene that, furthermore, is accompanied by a negligible generation of new structural defects in the graphene lattice. Valuable insights are gained by following the evolution of the graphene films in terms of doping, strain, and defect density using complementary statistical Raman analysis and X-ray photoelectron spectroscopy (XPS) supported by optical microscopy. This experimental approach shows the importance of an effective intermediate cleaning of the as-transferred graphene surface through annealing for an efficient nitrogen doping at mild compressive strain and thus at low defect density. The positive effect of nitrogen doping on the graphene/n-GaN rod interface is the increase in the current flow through this contact. Other industry-relevant semiconductor materials that can be epitaxially grown on graphene for future hybrid nanoarchitectures can profit from these advancements.

EXPERIMENTAL SECTION Graphene production and transfer. The copper foils (99.8% purity, 25 µm thick, Alfa Aesar) were annealed in a self-made CVD reactor at 1000 oC for one hour in a mixture of 300 sccm Ar and 20 sccm H2 at 1.33 mbar. Predominantly single-layer graphene films were grown on the copper foils using a two-step process by adding 5 sccm CH4 for 6 min, stop CH4 for 1 min, and 30 sccm CH4 for 45 min. Then, the graphene on copper samples were removed from the CVD reactor, spin coated with poly(methyl methacrylate) (PMMA) (4 wt% in ethyl acetate) at 2500 rpm for 45 sec, and dried in air at 120 oC for 1 min. Oxygen plasma (Diener Zepto) was employed to etch graphene from one side of the copper foils. After that, the copper foils were etched using 2.5 wt% ammonium per oxo-disulfate in deionized water solution for 12 hours. The remaining PMMA-graphene layers were rinsed two times in deionized water and transferred on c-plane sapphire substrates, which were cleaned before by ultra-sonication in acetone and isopropanol baths each for 10 min and dried with nitrogen gas. Finally, most of the 4 ACS Paragon Plus Environment

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PMMA layer was removed by means of heating at 400 oC for one hour in 300 sccm Ar in the CVD reactor, followed by an ethyl acetate bath for 24 hours. The large-area single-layer graphene films were cut from the same Cu foil to avoid run-to-run variations in the graphene properties. Annealing, nitridation, and GaN rod growth. The pristine graphene/sapphire samples were loaded in an Aixtron 200RF horizontal flow MOVPE reactor for the growth of GaN micro- and nanorods on top of graphene. The pressure in the reactor was kept at 100 mbar for all processing stages. At room temperature, the reactor was evacuated and then purged with N2 to remove oxygen and water. Subsequently, the N2 flow was stopped and the reactor was heated up to 1200 oC under H2 atmosphere, followed by annealing for 5 min to thermally clean graphene from contaminates such as PMMA residues before starting the growth. Afterwards, 1500 sccm ammonia (NH3) was introduced for 10 min to perform the nitridation step that promotes the growth. To further support the growth, a GaN nucleation layer was deposited by adding 180 µmol/min trimethylgallium (TMGa) for 16 sec. In the end, 45 µmol/min TMGa, 25 sccm NH3, and 0.06 µmol/min silane (SiH4) at 1150 oC for 13 min were used to grow vertically aligned, n-doped GaN micro- and nanorods on graphene/sapphire substrates. The cooling down to room temperature was performed under 25 sccm NH3 (down to 500 oC), H2 (down to 100 oC), and N2 (between 100 – 20 oC). The exposure to NH3 resulted in efficient ex-situ nitrogen doping of monolayer graphene. Since the temperature was monitored by a thermocouple inside the susceptor, the real surface temperature of the samples on the susceptor was estimated to be ~ 50 to 100 °C lower than the value measured by the thermocouple. Raman spectroscopy and optical microscopy. The confocal Raman measurements were performed at room temperature under ambient conditions in a backscattering geometry employing a LabRam HR800 spectrometer (Horiba Scientific) with a laser excitation wavelength of 457 nm. A 100x objective (numerical aperture (NA) 0.9) was used to tightly focus the laser light down to a probing beam diameter of ~0.7 µm and to collect the Raman scattered light. To exclude damage or structural changes to graphene as well as material unrelated peak shifts, all likely to be induced by the local heating through the laser beam, the laser power on the sample surface was reduced to ~0.6 mW using a filter. Statistical Raman characterization was carried



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out by scanning the samples under the objective with a motorized x-y table at a step size of 0.5 µm. More than 5000 spectra were acquired on average for each type of graphene sample and analyzed spectroscopically both as mean spectra and individually using the LabSpec 5 software. The fitting values of the D, G, and 2D Raman peaks arranged in frequency (position), intensity, and full-width at half-maximum (FWHM) column-like data sets were further examined by means of correlative 2D plots. For better visualization, the data corresponding to histogram bins with < 50 counts were removed, that is, < 3% from total. The optical microscopy images were taken under white-light illumination with a camera attached to the Raman spectrometer. Three objective (x100 NA 0.9, x50 NA 0.75, and x10 NA 0.25) were used to follow the sample transformation at different length scales. XPS. Core level spectra were excited by monochromatic Al K radiation (1487 eV) and photoelectrons were picked up at a take-off angle of 35° enhancing the surface sensitivity of the technique to about 10 - 15 Å depth. Scan times of up to ∼2 h were employed for all data collections. Data analysis was performed using the Sigma Probe Advantage software. Precise binding energy positions, full-width at half-maximum, and areas were calculated by peak fitting using the software package (XPSPEAK version 4.1). Peak fitting solutions were sought for χ2 < 1, where χ2stands for the standard deviation.

RESULTS AND DISCUSSION Selection of samples. While in-situ doping of single-layer graphene during the growth on copper foils by CVD is basically understood,13,16,22 extensive studies on ex-situ doping during the integration of graphene in hybrid semiconductor heterostructures after its transfer on other substrates are still missing. To fill this gap, we adopted a distinct experimental approach based on the careful investigation of each critical step influencing the ex-situ doping process of monolayer graphene. We prepared four types of graphene samples that were subject to different procedures: (1) standard PMMA-mediated transfer onto sapphire substrates, (2) annealing under H2 atmosphere, (3) doping using NH3, and (4) GaN rod growth employing TMGa, NH3, and SiH4 in H2 as schematically described in Fig. 1a. Note that each kind of sample underwent the previous steps except for the first type. The optimum experimental conditions (time, temperature profile, and gas flow rates) for concomitant high nitrogen doping of 6 ACS Paragon Plus Environment

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graphene and GaN rod growth on top of it in our MOVPE reactor are also shown. Optical microscopy images in Fig. 1b indicate the presence of residues attributed mainly to PMMA on as-transferred graphene, which are largely removed in case of annealed graphene,23,24 with no visible changes observed in doped graphene, followed by the growth of n-GaN micro- and nanorods20 (Supplementary Fig. 1). Evolution of the Raman peaks. Deeper insights into the structural and electronic properties of graphene are obtained using Raman spectroscopy, which has proven to be the ideal characterization technique for carbon-based materials being fast, non-destructive, and providing a large amount of information.7,25–28 The mean Raman spectra in Fig. 2a exhibit the characteristic Raman modes of graphene: the defect-induced peak (D), the Γ-point E2g phonon emission (G), and the near K-point transverse optical phonon emission (2D).7,27 These spectra are calculated by averaging over several thousands of individual Raman point spectra acquired on each sample by means of micro-Raman mapping accounting for a solid statistical analysis of doping, strain, and defect density in the graphene films.29–31 Figure 2b shows the mean values along with their standard deviations, extracted from the fitting of individual point spectra, for the frequencies (positions) of G (ωG) and 2D (ω2D) peaks as well as for the intensity ratios of I2D/IG and ID/IG (Supplementary Table 1). The ωG and ω2D of our ex-situ nitrogen-doped monolayer graphene (NMG) display an upshift of ~ 6 cm-1 and ~ 11 cm-1, respectively, compared to as-transferred monolayer graphene (TMG) (not to be confused with TMGa). A similar trend was reported for in-situ NMG,10–12,32–35 but a downshift was measured as well.13–15 The different sign and magnitude of these shifts are attributed to distinct doping and strain levels in single-layer graphene grown, doped, transferred, and annealed under various experimental conditions (Supplementary Table 2). Interestingly, opposite shifts of the G (~ -3.1 cm-1) and 2D (~ 5 cm-1) peaks are observed after the high-temperature annealing of TMG in H2. This points to an interplay between the PMMA decomposition (Fig. 1b and Supplementary Fig. 1) giving rise to a decrease in p-type doping36,37 and the thermally-induced strain caused by the different thermal expansion coefficients (TECs) of graphene and sapphire substrate.38 The former effect is further confirmed by the increase in I2D/IG from ~ 2.1 for TMG to ~ 2.98 for annealed graphene at comparable defect density as



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indicated by close ID/IG values of ~ 0.15 and ~ 0.16, respectively.11,39 After the intermediate annealing step, the I2D/IG decreases to ~ 1.67 due to the n-type doping in NH3 leading to NMG.12,14,15,35,39 Remarkably, the ex-situ incorporation of N dopants into the graphene lattice has a minimal influence on the defect density as supported by the slight increase in ID/IG to ~ 0.17. This is in contrast to the high N doping-induced defect densities reflected in significant increments in ID/IG and decrements (below unity) in I2D/IG documented in case of in-situ NMG.10–16 Finally, the GaN rod growth on top of graphene results in additional n-type doping because of the electron transfer from n-GaN to graphene40 supported by the further decrease in I2D/IG to ~ 1.61 at nearly unchanged defect density as confirmed by ID/IG ~ 0.17. Strain and doping from Raman correlative analyses. Figure 3a displays the correlation plot (ωG, ω2D) used to separately quantify the doping concentration (p, n) as well as the type (tensile, compressive) and level (ɛ) of strain in graphene with similar defectiveness.38 Despite the concurrent dependence of ωG and ω2D on p, n, and ɛ, their fractional variations are highly dissimilar: linear (Δω2D/ΔωG)ɛbiaxial = 2.8 as illustrated by the black solid line at constant doping, quasi-linear (Δω2D/ΔωG)phole = 0.75 as shown by the red solid line, and nonlinear (Δω2D/ΔωG)nelectron as described by the blue solid line at constant strain.38,39,41,42 Thus, the magenta dashed lines, drawn parallel to the red (TMG) and brown (NMG, GaN rod growth) dashed lines, intersect with the black solid line providing the strain, while those parallel to the black solid line give the p or n doping. The mean values for biaxial, in-plane strain and doping are derived with respect to intrinsic graphene (Supplementary Table 1 and Supplementary Fig. 2). The initial, as-transferred single-layer graphene exhibits compressive strain ɛ ~ -0.082% and hole doping p ~ -4.67 x 1012 cm-2 (black points). When cooling down to room temperature (RT) after the thermal treatment at 400 oC in Ar to remove PMMA, the sapphire substrate (positive TEC) induces compressive strain in graphene (negative TEC) accompanied by the buckling of graphene at nanoscale.8,38,43 On the other hand, the TEC mismatch between graphene and the copper foil was found to account for a rather low residual strain (ɛ = -0.015%) measured after transfer without post-annealing.44 The p-type doping of TMG can be well explained by the presence of PMMA residues36–38 and the O2-induced hole doping, the latter being stabilized


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through the water molecules trapped during the transfer at the graphene-sapphire interface.8,9,38 Extra annealing of TMG at 1200 oC down to 100 oC in H2 (purple points) leads to a rise in compressive strain ɛ ~ -0.139% generated upon cooling down to RT and a striking drop in doping ~ -1012 cm-2 attributed to the effective removal of PMMA rests, water, and O2.3,5,22 It should be noted that the subsequent exposure to air did not lead to a doping recovery activated by the presence of water,8,36 which is in line with the predominate hydrophobic surfaces of both graphene and single-phase, O-terminated sapphire (0001) substrate.9 Moreover, the n-type doping due to H2 adsorption on graphene can be ruled out as supported by the absence of a Raman peak at ~ 1500 cm-1 (Fig. 2a) related to C-H vibrations.45 The virtually undoped and clean monolayer graphene was then exposed to NH3 at 1200 oC down to 500 oC (H2 still on) resulting in an increase in compressive strain ɛ ~ -0.167% and a boost in electron doping n ~ 9.56 x 1012 cm-2 (orange points), both originating from the nitrogen doping effect. As revealed later by XPS, our NMG accommodates mainly pyridinic and less pyrrolic C-N bonds, whose lengths of 1.337 Å and 1.37 Å, respectively, are shorter than the graphene C-C bond length of 1.42 Å.11,46 These differences cause a shortening of the C-C bonds around the N atoms and thus additional compressive strain in the graphene lattice. The n-type doping of single-layer graphene upon being in contact with nitrogen-containing environments incorporating also pyridinic and/or pyrrolic configurations was confirmed by numerous electrical studies including scanning tunneling microscopy10,16 and back-gating.12,14,15,22,35 Because of the thick insulating sapphire substrate, it was not yet possible to further characterize the structure of doped single-layer graphene at atomic resolution using scanning tunneling microscopy and high resolution transmission electron microscopy methods. New exposure of NMG to TMGa and SiH4 at 1200 oC and 1150 oC (NH3 and H2 still on) enhances further the compressive strain ɛ ~ -0.172% and electron doping n ~ 10.18 x 1012 cm-2 (green points). The added strain to the C-C bonds is tentatively attributed to the formation of C-N-Ga bonds during the GaN rod growth, while the extra doping is explained by the electron transfer from n-GaN to graphene.40 Figure 3b-e show more correlation plots of the Raman peak parameters, namely (I2D/IG, ωG), (ΓG, ωG), (I2D/IG, ω2D), and (Γ2D, ω2D). ΓG and Γ2D stand for the FWHM of the G and 2D peaks. Their



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mean values are summarized in Supplementary Table 1. These dependences confirm the doping – strain evolution in (a) as well as the higher sensitivity of ωG to doping39 (b, c) and of ω2D to strain42 (d, e) in case of ex-situ NMG. Indeed, the G peak upshifts with increasing p-type (annealed → TMG) or n-type (annealed → NMG → GaN rod growth) doping. This is accompanied by a decrease in I2D/IG (b) and a sharpening of ΓG (c), in excellent agreement with doping experiments by electrostatic gating.39 Moreover, the narrow ΓG ~ 10.69 cm-1 in case of NMG indicates minor structural disorder47,48 produced by nitrogen doping, in line with the low ID/IG ~ 0.17. On the other hand, the 2D peak upshifts with incorporating more compressive strain (TMG → annealed → NMG → GaN rod growth) in the graphene lattice. Although the I2D/IG (d) and Γ2D (e) seem to follow different trends, they display basically (almost) the same doping dependences as (b, c), with the annealed and TMG points being reversed in (d, e) because of the stronger response of ω2D to strain. The raise in Γ2D upon doping is associated with increasing amplitudes of charge fluctuations caused by the presence of nanometer-sized electron-hole puddles in the graphene films.48 It should be noted here that nanometer-scale strain variations can also affect Γ2D.49 XPS characterization. Figure 4 displays the development of the XPS C1s (a, c, e) and N1s (b, d, f) line scan spectra following the important steps throughout the ex-situ doping procedure of single-layer graphene, with the fitting results listed in Supplementary Table 3. The initial, astransferred graphene is characterized by the main C1s peak at ~ 284.6 eV coexisting with three peaks at higher binding energies (BEs) and the N1s peak at ~ 400.1 eV (a, b). The C1s peak originates largely from sp2 C-C bonds indicating predominantly defect-free graphene lattice, but the fitting also includes sp3 C-C bonds from defective graphene and PMMA residues.23,36 Distinct carbon bonds in PMMA on graphene are known to give rise to the three peaks.23,36 However, the presence of the N1s peak in TMG represents a new result, which emphasizes that unintentional, transfer-induced charge doping may be difficult to separate from intentional heteroatom doping giving the similar BE intervals.11 A significant reduction of ~ 63% in the sum intensity of the PMMA peaks and a complete suppression of the N1s peak are detected subsequent to the high-temperature annealing of TMG (c, d). Furthermore, the C1s peak moves to lower BE at ~ 284.2 eV corresponding to mostly clean graphene50 and its FWHM decreases


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from ~ 1.31 to ~ 1.12 eV, effects that can be explained by the reduced amount of PMMA charging and PMMA sp3 C-C bonds. All these changes upon annealing demonstrate the efficient removal of both PMMA and nitrogen physically adsorbed on the graphene surface. Many groups reported shifts of the C1s peak in the range 284.5 - 284.9 eV attributed to an increased structural disorder caused by disruptions of sp2 C-C bonds (implying more sp3 C-C bonds and thus larger C1s FWHM) during the incorporation of nitrogen atoms into the graphene network.11–16,32,35,51 Surprisingly, our NMG exhibits a constant C1s BE of ~ 284.2 eV and a comparable C1s FWHM of ~ 1.14 eV (e) pointing to a similar defect density following nitrogen doping as after annealing (c). The less intense peaks at higher BEs in NMG were assigned in literature mainly to sp2 C-N (285.5 – 285.9 eV) and sp3 C-N (286.5 – 288.9 eV) bonds.11,12,14,32,51,52 Such assignments are problematic here because of the overlap with the peaks from still remaining PMMA (c, e). The interaction between the carbon lattice, polymer residuals, and nitrogen doping can result in various, complex bonding configurations with different BEs and peak areas (peaks 2 - 4 in Supplementary Table 3) depending on the applied treatments.50,52–54 Moreover, the N1s peak recovers and can be decomposed into two components at ~ 396.3 and ~ 399.2 eV (f) ascribed to 83% pyridinic and 17% pyrrolic C-N covalent bonds.50,54 The atomic percentage of nitrogen (N/C) calculated as the ratio of the areas under the two N1s (f) and main C1s (e) peaks is ~ 11% corresponding to a high ex-situ nitrogen doping concentration obtained in case of single-layer graphene virtually without the generation of defects and no growth of GaN rods on top of it (NH3 provided, TMGa and SiH4 not provided). Thus, the XPS data corroborate the Raman results regarding the PMMA removal, nitrogen doping, and defect formation. This doped graphene free of GaN attached rods can be used for many other applications beyond its integration into hybrid semiconductor heterostructures).1–6 The reliable estimation of the doping level in graphene after GaN rod growth was not possible due to the absorption of both X-rays before reaching graphene (in excitation) and electrons escaped from graphene (in detection) by the vertically aligned GaN rods (XPS spectra not shown). According to the Raman data, the doping level is expected to be a little higher. Comparison with in-situ doping. The key findings of the current study are the efficient ex-situ nitrogen doping of ~ 11 at% in large-area monolayer graphene after the transfer on other



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substrate, the minimal generation of defects in graphene during the post-synthesis treatments including doping, and the simultaneous integration of graphene in hybrid semiconductor heterostructures. To understand the implications of these results, we start our discussion by summarizing the main downsides related to in-situ N doping during the CVD growth of predominantly single-layer graphene on copper foils. First, the supply of mixed carbon and nitrogen precursors triggers a competition between the incorporation of nitrogen atoms into the growing graphene lattice and its etching by -NHx groups. This leads to a clear increase in defect density upon N doping.10,11,13–16 Second, the nitrogen concentration was found to be the largest at low temperatures (600 - 880 oC), but here the graphene grows by itself with more defects. Low defective graphene can be produced apart at elevated temperatures (> 950 oC), however here its growth can be inhibited by the presence of -NHx groups.13,16,22,55 Third, the N doped graphene is brought into contact with various chemicals throughout the transfer procedure, which can contribute to the formation of additional defects as well because of the enhanced chemical reactivity at N sites.1,3 All these issues relevant for applications based on graphene/semiconductor hybrid materials are being solved by employing the novel ex-situ N doping method demonstrated in this work. In particular, we achieved low defective, cleaned, and virtually undoped transferred monolayer graphene based on the growth at high temperatures (1000 oC) using a carboncontaining gas (CH4) and the effective removal of PMMA residues. This graphene was then efficiently doped with nitrogen atoms by exposing it to a nitrogen-containing gas (NH3) also at high temperatures (mostly 1200 – 1150 oC). The N doping was shown to occur at intrinsic vacancies (mono and double vacancy defects) formed inevitably during the CVD growth, where the carbon atoms can easily be replaced by nitrogen atoms because of the defect-induced strain and the presence of dangling bonds. This resulted in mainly pyridinelike and less pyrrolelike configurations, at which several N atoms were accommodated.11,50,53,55 Meanwhile, weak C-C bonds can break giving rise to new post-growth vacancy defects that attract more N dopants by forming stable, covalent C-N bonds further enhancing the N content. Thus, the multiple N dopant aggregation near vacancies represents a straightforward explanation for the high N content measured despite the low defect density in the starting graphene. Moreover,


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the elevated temperatures can enable atomic rearrangements implying C-N bond reconstructions so that the substitution of C with N at vacancies caused only small structural disorder in the graphene lattice.53 This last point is supported by the mild compressive strain ɛ ~ -0.028% induced by doping only, being calculated as the strain difference between NMG and annealed samples. Such atomic rearrangements during doping and annealing are expected to be less effective or even inhibited in highly disordered samples as those obtained after NH3 plasma treatment of transferred monolayer graphene12,56,57 leading to higher defective doped graphene as compared to our ex-situ doping method. A comparison in terms of nitrogen doping concentration and defect density (ID/IG) between this work and other reports is presented in Fig. 5a based on our literature research on nitrogendoped, large-area CVD single-layer graphene including mostly in-situ doping (Supplementary Table 4). Complete Raman – XPS data sets are still scarce in case of ex-situ doping of CVD monolayer graphene, which is mainly synthesized by a NH3 plasma treatment after transfer resulting in highly defective graphene and low nitrogen doping.12,57 For a reliable comparison, few-layer graphene produced by CVD and other graphene-based materials obtained from different types of graphite or graphene oxide using various methods are not included.1–6 The simultaneous growth of single-layer graphene directly on the sapphire substrate and its doping was not yet possible because of technical issues related to our MOVPE reactor (no carboncontaining gas line and higher temperatures above 1425 oC).58,59 Notably, a comparable doping level does not necessarily lead to a similar structural modification (constant ID/IG) indicating that in addition to the actual incorporation of the nitrogen atoms into the carbon lattice, the variety of growth, doping, transfer, and annealing conditions impact differently the properties of monolayer graphene (Supplementary Table 2). By varying one parameter (precursor ratio,14 nitrogen flow rate,52 or temperature13,22), while keeping the other ones unchanged, significant increases in ID/IG (up to ~ 1.7) with doping (up to ~ 5.6 at%) were mainly observed (the points connected by lines). In this context, our approach to separate the growth from doping after the transfer onto the substrate of choice20,21 resulting in the lowest ID/IG ~ 0.17 and the largest exsitu N-doping of ~ 11 at% reported so far represents a substantial progress on doping in CVD single-layer graphene, also in combination with its integration in hybrid GaN heterostructures.



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Another less common way to express nitrogen doping in graphene is to treat NMG as a carbon nitride (CN) alloy,60 with ~ 11 at% corresponding to ~ C8N. Graphene doping in hybrid semiconductor heterostructures. Lastly, we focus on the influence of doping in monolayer graphene when acting as a bottom contact to semiconductor nanostructures grown epitaxially on top of it. In our case, the electron doping concentration n ~ 10.18 x 1012 cm-2 estimated by Raman after the GaN rod growth can be translated into an upshift of the Fermi level of ~ 0.4 eV as given by = ℏ| |√, where | | = 1.1 x 106 ms-1 is the Fermi velocity.39 This changes the work function of graphene from Φ ~ 4.6 eV (neutral) to Φ ~ 4.2 eV (n doped) and consequently reduces the Schottky-barrier height at the graphene/n-GaN rod contact to Φ ~ 0.1 eV as given by Φ = Φ − χ , where χ = 4.1 eV is the electron affinity of GaN (left inset in Fig. 5b).61 Indeed, our previously published electrical measurements in an scanning electron microscope using tungsten nanoprobes showed that higher currents flow through the graphene/GaN interface20 (up to two orders of magnitude) confirming the decrease in Φ caused by graphene n-doping. These results indicate that, in principle, one can think of tuning the n- or p-type doping in graphene to match its work function with the electron affinity or electron affinity plus band gap, respectively, of semiconductor materials in order to achieve an ohmic contact. Figure 5b and the right inset illustrate this outlook for important group II – VI, IV, and III – V semiconductor nanostructures that have been or can potentially be grown in an epitaxial relationship to graphene providing orientation values for future experiments (Supplementary Table 5).17–19 However, extensive research has to be devoted to each type of graphene/semiconductor hybrid system, especially with regard to the maximum doping concentration in conjunction with specific doping and growth requirements that can be sustained by single-layer graphene before structural instability occurs.60

CONCLUSION In summary, this work showcase the importance of dividing the growth and doping phases of graphene that led us to an efficient ex-situ nitrogen doping of ~ 11 at% without virtually causing further structural defects in transferred CVD single-layer graphene. The high doping level was explained by an effective cleaning of the graphene surface followed by the covalent bonding of 14 ACS Paragon Plus Environment

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multiple nitrogen atoms at intrinsic and post-growth vacancies in the graphene lattice. A moderate compressive strain was estimated to be produced by doping indicating C-N bond reconstructions by atomic rearrangements at high temperatures. Moreover, the N doping of graphene appearing together with the epitaxial growth of n-GaN micro- and nanorods by MOVPE on top of it caused a reduction in the Schottky-barrier height at the graphene/n-GaN rod interface permitting the flow of higher currents. Since the MOVPE growth of GaN rods is not a necessary step for a high N doping efficiency, the clean doped graphene can be further employed in numerous applications. Compared to the lower nitrogen concentrations accompanied by higher defect densities published to date on in-situ doping of single-layer graphene during the CVD growth, our results constitute a significant advancement in the field of graphene doping towards its combination with industry-relevant semiconductor nanostructures in novel hybrid systems.

Supporting Information. Supplementary Figure 1 - Optical microscopy images at three magnifications of the analyzed four types of graphene samples. Supplementary Table 1 – Evolution of mean values for important Raman peak parameters, doping, and strain. Supplementary Table 2 – Illustration of the wide variety of experimental conditions used for insitu CVD growth and doping. Supplementary Figure 3 - 2D phonon frequency dependence on laser energy. Supplementary Table 3 – XPS fitting results. Supplementary Table 4 - Comparison between doping and ID/IG values. Supplementary Table 5 - List of industry-relevant semiconductor materials and graphene doping level for achieving an ohmic contact.



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FIGURE 1

Figure 1. (a) Schematic representation of the optimized experimental parameters for simultaneous ex-situ nitrogen doping of transferred CVD predominately single-layer graphene and epitaxial growth of GaN rods by MOVPE on top of it. The time indicated by the black line at 1200 oC was multiplied by 10 and corresponds to real 16 sec. (b) Optical microscopy images of the four types of large-area graphene samples investigated in this work by Raman and XPS analyses. Similar images at lower magnifications can be found in Supplementary Fig. 1. The residues attributed mainly to PMMA visible on as-transferred graphene are effectively removed by annealing in H2. The clean graphene is then efficiently doped with nitrogen when exposed to NH3 without noticeable optical changes, followed by the growth of n-GaN micro- and nanorods using TMGa and SiH4. The scale bars are 5 µm.


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FIGURE 2

Figure 2. (a) Mean Raman spectra obtained by averaging the point spectra from micro-Raman mappings on the four graphene films described in Fig. 1. (b) Mean values of relevant Raman peak parameters calculated by averaging the data from the fitting of individual point spectra used to monitor the development of the graphene properties in terms of doping, strain, and defect density. The means and their standard deviations are summarized in Supplementary Table 1.



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FIGURE 3

Figure 3. (a) Correlation analysis of the 2D and G frequencies (ωG, ω2D) employed to separately quantify the doping concentration as well as the type and level of strain in our graphene samples. The solid lines show published dependences of (ωG, ω2D) on strain at constant doping (black) and on doping at constant strain (red for hole and blue for electron doping, respectively). The red and brown dashed lines are (local) linear fits to the experimental solid lines being used to obtain the doping at the intersection with the magenta dashed lines. Likewise, the crossing with the black solid line is utilized to read the strain. (b - e) Additional correlation plots confirm the doping – strain evolution presented in (a). The mean values and standard deviations for these Raman peak features along with the doping and strain data are listed in Supplementary Table 1.


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FIGURE 4

Figure 4. XPS C1s (a, c, e) and N1s (b, d, f) line scan spectra. The graphene C1s peak (1) is accompanied by three peaks (2 – 4) at higher binding energies associated with PMMA residues. Both PMMA (a) and nitrogen (b) physically adsorbed on the graphene surface are efficiently cleaned by annealing (c, d) enabling high ex-situ nitrogen doping of ~ 11 at% in transferred single-layer graphene virtually without the generation of new structural defects (e, f), in very good agreement with the Raman results. The two peaks in (f) are assigned to mainly pyridinic and less pyrrolic C-N configurations. The fitting values are shown in Supplementary Table 3.



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FIGURE 5

Figure 5. (a) Comparison between this work and other nitrogen doping studies with respect to the interplay between N concentration and defect density (indicated by the D/G intensity ratio) in CVD monolayer graphene (Supplementary Table 4). Our procedure to separate the growth from doping after transfer results in the highest ex-situ N content at the lowest doping-induced defect density in the graphene lattice. (b) Electron concentration decreases the Schottkybarrier height Φ at the graphene/n-GaN rod interface by means of a positive Fermi level shift and a reduction in the work function of graphene (left inset). By tuning the electron or hole concentration in graphene through doping, it would be possible, in principle, to obtain an ohmic contact between graphene and technologically relevant group II – VI, IV, and III – V semiconductor nanostructures that can be grown epitaxially on top of it (Supplementary Table 5). The right inset shows the materials located outside the range of the main plot along with those in the main graph inside the rectangle.


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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the German Research Foundation (DFG) within the research projects “Dynamics and Interactions of Semiconductor Nanowires for Optoelectronics” (FOR 1616), “Hybrid Inorganic/Organic Systems for OptoElectronics” (HIOS, SFB 951), "In-Situ Microscopy with Electrons, X-rays and Scanning Probes" (GRK 1896) as well as the cluster of excellence "Engineering of Advanced Materials" at the Friedrich-Alexander-Universität Erlangen-Nürnberg.

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Efficient Nitrogen Doping of Single-Layer Graphene Accompanied by

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