Comparison of graphene growth on arbitrary non-catalytic substrates using low-temperature PECVD

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Comparison of graphene growth on arbitrary non-catalytic substrates using low-temperature PECVD Sunny Chugh a,b,1, Ruchit Mehta Zhihong Chen a,b,* a b c

a,b,1

, Ning Lu c, Francis D. Dios c, Moon J. Kim c,

School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, United States Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47906, United States Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, United States

A R T I C L E I N F O

A B S T R A C T

Article history:

Conventional Chemical Vapor Deposition (CVD) techniques require the use of a catalyst

Received 17 January 2015

surface and high temperature of growth (1000 C) to grow graphene, which renders the

Accepted 11 May 2015

process incompatible with arbitrary substrates. While post-synthesis transfer of graphene

Available online 21 May 2015

onto required substrates is widely used, it causes undesirable effects such as wrinkles/ folds/cracks and unintentional doping. Here, we report low-temperature growth of graphene at 650 C on non-catalytic SiO2 and quartz substrates using a one-step, rapid Plasma Enhanced Chemical Vapor Deposition (PECVD) process. We simultaneously study PECVD graphene growth on a traditional catalytic material such as copper and show that the growth substrate does not play any role in the dissociation of hydrocarbon precursor during PECVD, thus eliminating the possibility of a catalytic effect. Using several characterization techniques, we observe an increasing rate of growth from SiO2 to quartz to copper, which can be attributed to different adsorption and diffusion energies of plasma radicals on these substrates. As opposed to thermal CVD growth on copper, which is self-limiting, the PECVD method developed here is scalable in terms of number of layers, allowing its adept integration in commercial devices.  2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene has attracted immense interest in the scientific and technological community owing to its impressive properties and enormous potential for applications in electronics, photonics, sensing and many others [1–4]. Among various methods of synthesizing graphene, mechanical exfoliation [1] suffers from low throughput since it can only yield small size

flakes with random distribution, while SiC growth [5] poses difficulties in transferring graphene to other substrates. Chemical methods like graphene oxide reduction [6] have inadequate scalability and do not provide a good control over layer count. Chemical Vapor Deposition (CVD), on the other hand, is one of the most reliable, scalable and rapid methods for large-area graphene growth [7]. Thermal CVD generally involves growing graphene on a catalyst surface such as Cu

* Corresponding author at: School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, United States. E-mail address: [email protected] (Z. Chen). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.carbon.2015.05.035 0008-6223/ 2015 Elsevier Ltd. All rights reserved.

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[7,8], Ni [9], Ru [10], Pt [11], Ir [12], using a mixture of a hydrocarbon gas precursor and hydrogen at high temperatures (1000 C). After the growth, graphene can be transferred to any desired substrate by wet-etching of the metal catalyst [13]. While thermal CVD can consistently yield high quality graphene over a large area, the transfer step is not desirable since it can degrade the quality of the film by introducing defects and contamination. Besides requiring a catalyst, the high temperature of growth also limits the range of growth substrates. To enable facile graphene integration into commercial devices, it is imperative to reduce the growth temperature and grow graphene directly on technologically relevant substrates that may be non-catalytic. Plasma Enhanced Chemical Vapor Deposition (PECVD) allows lowering the growth temperature since the energy for breaking the precursor molecules is supplied through the plasma. There are several reports on PECVD growth of graphene at low temperatures [14–19]. However, most of the work so far has focused on catalytic approaches and little attention has been given to arbitrary non-catalytic substrates. The previous few attempts of growing graphene on noncatalytic materials either need growth time of several hours to obtain full coverage [15–17], which is commercially unviable, or lack a thorough analysis of the evolution of graphene growth during PECVD [18,19]. In this work, we achieve growth of few-layer graphene (FLG – less than 10 layers) and multilayer graphene (MLG – more than 10 layers) at 650 C using remote plasma-enhanced CVD on both non-catalytic and traditional catalytic substrates. Our process enables full coverage of graphene over large areas within only a few minutes. This work provides the first experimental comparison of graphene growth rates on different substrates using PECVD. We use Raman spectroscopy, Scanning Electron Microscopy (SEM) and optical measurements to analyze the progression of graphene growth with time, which is in striking contrast with the self-limiting scheme observed in thermal CVD. Our observations also provide insights into the mechanism of low-temperature PECVD growth where no catalytic action is present. This approach provides a one-step method towards catalyst-free, rapid growth of multilayer graphene at reduced temperatures.

2.

Experimental

2.1.

PECVD graphene growth set-up

In our experiment, the PECVD process was carried out at 650 C using CH4 precursor for different durations of growth – 1 min, 5 min, 10 min and 15 min in a commercially available EasyTube 3000 First Nano system, with a 6-inch diameter quartz furnace with an RF plasma (13.56 MHz) generated remotely upstream. This design of the system allows plasma species to be created away from the sample to avoid ion bombardment damage and also provides an independent control of the substrate temperature. Samples for graphene growth were first cleaned using a triple sonication wash in toluene, acetone and isopropyl alcohol for 5 min each. After loading the sample, the chamber was completely evacuated and then heated to 650 C in about 20 min in a flow of 1 slpm of Ar at

300 mTorr. When the temperature reached the set point, Ar flow was shut off and the system was brought down to the base pressure. To start the growth, CH4 was injected into the system at a partial pressure of 1 mTorr (and flow rate of 6 sccm) and RF plasma was turned on instantly. The reflected RF power was minimized using a matching network for maximum plasma efficiency. After the growth period, CH4 flow was turned off and the chamber was evacuated. Subsequently, the system was cooled down at a rate of 15 C/min in a 100 sccm flow of Ar at a pressure of 1 Torr. Samples were unloaded from the CVD chamber at temperatures below 150 C. The process was repeated more than 15 times on all the substrates to check for reproducibility. Using 550 W of plasma power, we achieved FLG and MLG growth on non-catalytic substrates including SiO2 (90 nm thick, thermally grown on Si) and quartz (1 mm thick). In order to examine the function of a traditional metal catalyst (if any), we simultaneously studied growth on 500 nm thick physical-vapor-deposited copper (Cu) films on SiO2–Si substrates. 5 nm thick Ta underlayer was deposited to promote Cu adhesion.

2.2.

Characterization techniques

Raman measurements were carried out in ambient environment at room temperature. A 532 nm Diode Pumped Solid State (DPSS) green laser beam was focused on the sample by an Olympus 50· objective (NA = 0.75, WD = 0.38 mm). The excited Raman scattering was collected by a Horiba LabRAM HR800 Raman spectrometer with an 1800 mm1 grating (spectral resolution = 0.27 cm1). Transmission measurements were done using a commercially available Perkin Elmer Lambda950 spectrophotometer having an integrating sphere. Transmission Electron Microscopy (TEM) sample was made by FIB-SEM Nova 200 and SiO2 and Pt layers were deposited to protect the TEM sample area during ion-beam milling. Highresolution TEM was performed in a JEOL ARM200F operated at 200 kV. For X-ray Photoelectron Spectroscopy (XPS) studies, the photoelectrons were excited using an Mg-Ka (energy = 1253.6 eV) X-ray radiation source (SPECS XRC-1000) and analyzed using an Omicron Argus hemispherical electron analyzer with a round aperture of 6.3 mm. X-Ray Diffraction (XRD) measurements were carried out in a commercially available Bruker D8 XRD set-up using mixed Cu-Ka line (wavelength = 0.151838 nm).

3.

PECVD graphene growth results

3.1.

Graphene growth on SiO2

In Fig. 1(a), we plot the Raman spectra of as-grown graphene on SiO2 for growth time of 1, 5, 10 and 15 min. Raman spectrum of graphene has three dominant peaks [20], denoted by D, G and 2D at around 1350 cm1, 1600 cm1 and 2690 cm1, respectively. 1-min and 5-min growths on SiO2 give no Raman signals from the substrate, indicating no graphene growth. For longer growth time (P10 min), clear Raman signals are observed, providing concrete evidence that graphene growth can be realized without the presence of a

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Fig. 1 – (a) Raman spectra of PECVD graphene grown on SiO2 at 650 C for different durations of growth. Plots have been displaced vertically for clarity. (b) Top-view SEM image of SiO2 sample acquired after graphene growth for 10 min. Scale bar: 500 nm. (c) AFM image of SiO2 sample after 10-min graphene growth taken using a tapping-mode Veeco AFM. (d) Height variation along the white dotted line in (c), showing growth of few-layer graphene islands. (A color version of this figure can be viewed online.)

69.0µ

(c)

SiO2 - 15 min

68.0µ IDS (A)

catalyst. A visual feedback on the formation of graphene layers starting from the initial stages of growth was obtained through top-view SEM (Fig. 1(b)). Small graphene islands begin to appear on SiO2 after 10 min of growth. Atomic Force Microscopy (AFM) scan of the same sample (Fig. 1(c) and (d)) shows that these graphene islands have a thickness of 1–3 nm, indicating few-layer structures. Due to extremely low coverage, the corresponding Raman spectrum (Fig. 1(a)) is rather noisy. The D-to -G intensity ratio of the Raman peaks of the 10-min sample can be approximately used to find the grain size of graphene [21]. For the 532 nm Raman wavelength excitation, the grain size is calculated to be about 5–10 nm. This is much smaller than the size of the graphene islands observed under the SEM. Hence, these islands are not necessarily individual graphene grains but several grains merged together to form an arbitrarily sized island. As growth progresses, the island size is expected to increase. A statistical analysis of the island size, spacing between islands and island height has been added as Fig. S1 in the Supporting Information. It is found that majority of the islands formed after 10-min growth on the SiO2 substrate are few tens of nanometers in sizes, with sub-100 nm to 100 nm spacing. During the next 5 min, the already formed graphene islands coalesce to entirely cover the SiO2 surface (yet with visible boundaries between islands, Fig. 2(a)), achieving a complete coverage and yielding sharp Raman peaks. The layered structure of directly grown graphene was verified using cross-sectional TEM, which shows that the 15-min SiO2 sample has multiple graphene layers (12 layers) (Fig. 2(b)). We

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Fig. 2 – (a) Top-view SEM image of SiO2 sample acquired after graphene growth for 15 min. Scale bar: 500 nm. (b) High-resolution cross-sectional TEM image of MLG grown on SiO2 for 15 min, confirming the layered structure of graphene. (c) Measured IDS–VGS data from the same sample obtained by patterning graphene channels directly on the substrate. Shown in the inset is an SEM image of the 2-probe arrangement used for the measurement.

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also investigated the transport properties of directly grown MLG in a back-gated field-effect transistor configuration by patterning graphene channels directly on the 15-min SiO2 substrate and depositing Ti/Au source-drain electrodes in a 2-probe arrangement. Fig. 2(c) shows the source/drain current as a function of the Si back gate voltage with a drain voltage of 10 mV. The observed ambipolar nature of transport is a defining characteristic of graphene and the Dirac point appears around 8 V.

3.2.

Graphene growth on quartz

Identical growth recipes were used on quartz substrate and the corresponding Raman results are shown in Fig. 3(a). Interestingly, quartz starts to show Raman peaks at a shorter growth time of 5 min compared to 10 min for SiO2. SEM studies reveal that 10-min growth on quartz (Fig. 3(b)) already gives well-connected graphene islands covering a large fraction of the substrate, consistent with the distinct Raman peaks observed in Fig. 3(a). Increasing the growth time to 15 min leads to a complete coverage of graphene on quartz (Fig. 3(c)). Based on the SEM and Raman results, we conclude that quartz assists graphene growth faster than SiO2.

3.3. Optical characterization of graphene grown on SiO2 and quartz The evolution of graphene growth with time was further analyzed using optical measurements. Fig. 4(a) shows the measured reflectance of graphene on SiO2 (left axis) and the   RGrþSubstrate (right axis) for differextracted contrast C ¼ RSubstrate RSubstrate ent growth durations. 1–10 min of growth on SiO2 gives no appreciable change in contrast with respect to bare substrate in spite of the formation of small islands seen under the SEM. During the next 5 min, while the coverage becomes complete, a large change in contrast is observed, suggestive of MLG grown at that stage as confirmed by TEM (Fig. 2(b)). As a comparison, the measured transmittance and extracted

Quartz

15 min 10 min 5 min 1 min

Intensity (a. u.)

(a)

  T absorbance TGr ¼ GrþQuartz ; AGr ¼ 1  TGr ; RGr  0 of graphene TQuartz grown on quartz substrate is provided in Fig. 4(b). From the extracted absorbance, we notice that the 1-min sample gives no graphene growth as inferred from its Raman spectrum. Monolayer graphene absorbs about 2.3% of light in near-IR regime [22], hence, an absorption value of 9% shown by the 10-min sample would then correspond to 4 layers of graphene. However, since the growth is not yet complete at 10 min, we conclude that the PECVD process yields fewlayer graphene islands during the initial stage of the growth, which have transmission equivalent to 4 uniform layers of graphene. This observation suggests that absorption cannot be used to quantitatively analyze the number of layers unless growth is confirmed to be uniform across the surface. As the time increases to 15 min, these islands grow in size and eventually merge together to give a complete coverage. Absorption in near-IR range also increases from 9% to 27%, suggesting a large increase in layer count. Moreover, the 15-min sample shows a sheet resistivity of 1.3 kX sq1, which makes our directly grown PECVD graphene ideal for transparent conductive electrodes. Furthermore, in spite of the large number of layers, both SiO2 and quartz samples show very high 2D-to-G ratios and small 2D peak-widths in their Raman spectra. We suspect that the multi-layer graphene grown on non-catalytic substrates is turbostratic (random angle of orientation between layers), resulting in the high 2D-to-G ratio [23].

3.4.

Identical growth processes were simultaneously carried out on Cu films to study the impact of traditional metal catalysts. In order to verify complete coverage of graphene, Cu samples were heated at 180 C in ambient conditions for 30 min. Graphene, which acts as a passivation barrier, prevents the oxidation of Cu [24] (see Supporting Information for full description of heating test and XPS studies). The heating test was performed on all Cu samples and it was found that even 5 min of growth prevents Cu oxidation, indicating complete coverage of graphene on Cu. This result serves as a clear evidence of faster graphene growth on Cu than on quartz and, by extension, SiO2 too. Fig. 5(a) shows the Raman spectra of Cu samples after graphene growth. Measurements were acquired directly on the substrate to avoid any defects induced by graphene transfer. It can be seen that even 1 min of growth on Cu shows D and G peaks, albeit with very little intensity because of the incomplete growth.

4.

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1600 2000 2400 Raman Shift (cm-1)

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Fig. 3 – (a) Raman spectra of PECVD graphene grown on quartz at 650 C for different durations of growth. Plots have been displaced vertically for clarity. (b) and (c) Top-view SEM images of quartz sample acquired after graphene growth for 10 min and 15 min, respectively. Scale bar: 500 nm. (A color version of this figure can be viewed online.)

Graphene growth on copper films

Discussion

It has been established that catalytic CVD growth of graphene is governed by surface adsorption and is a complex process involving many steps [25] – (1) adsorption of hydrocarbon molecules on the surface, (2) decomposition of hydrocarbon to form active carbon, (3) diffusion of active carbon on the surface, (4) nucleation by forming carbon clusters of various sizes, (5) desorption of carbon and (6) continuous increase in size of graphene grains until they join together and cover the entire surface with graphene. Cu acts as a catalyst to dissociate hydrocarbon precursors at temperatures close to

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Fig. 4 – (a) Measured reflectance of graphene grown on SiO2 (solid lines, corresponding to the left vertical axis) and extracted contrast (dashed lines, corresponding to the right vertical axis) for different durations of growth. (b) Measured transmittance of graphene grown on quartz (solid lines, corresponding to the left vertical axis) and extracted absorbance of graphene after removing the quartz background (dashed lines, corresponding to the right vertical axis) for different durations of growth. (A color version of this figure can be viewed online.)

15 min 10 min 5 min 1 min

Copper

Intensity (a. u.)

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(c) Intensity (a. u.)

Cu(111)

Copper + Silicon Silicon

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40 50 60 Angle 2θ (Degrees)

Cu(220) absent

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Fig. 5 – (a) Raman spectra of PECVD graphene grown on Cu films at 650 C for different durations of growth. Plots have been displaced vertically for clarity. (b) Cross-sectional TEM image of MLG grown on the 15-min Cu sample. (c) XRD scan of a Cu sample after 1-min growth. The ‘copper + silicon’ scan is acquired with X-rays incident on top of the sample and the ‘silicon’ scan with X-rays incident on the backside. Cu(1 1 1) peak is found to be much higher than Cu(2 0 0) while Cu(2 2 0) is entirely absent, suggesting that Cu film is majorly (1 1 1). (A color version of this figure can be viewed online.)

1000 C and enables graphene growth. However, in the low-temperature regime used in this work, while thermal dehydrogenation of CH4 on Cu is not attainable [26], dehydrogenation is realized due to the energetic plasma, thus allowing graphene growth to occur at reduced temperatures. To validate this hypothesis, we executed a test run with the same process conditions but without the plasma and found that no graphene growth occurs on any of the samples. As there is no catalytic action required for low-temperature PECVD, this process can enable graphene growth on

substrates that are traditionally non-catalytic. Furthermore, the large change in measured reflectance for SiO2 and transmittance for quartz with increasing growth time from 10 to 15 min implies that the growth rate increases sharply with time. It is worthwhile mentioning that if there was a catalytic action present due to the substrate, the growth rate would rather be expected to slow down with time as less and less of substrate area is made available. However, in the case of the non-catalytic growth, we believe that the graphene islands formed during the initial stages can act as seeds to

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enhance the rate of graphene growth in the later stages. Furthermore, the host substrates used in the growth process have negligible solubility of carbon in them, particularly at low temperatures. Hence, the formation of multi-layer graphene also supports our conclusion that no catalytic action due to the substrate is present in our low-temperature PECVD growth. This is also true for the case of Cu, which is generally self-limiting for thermal CVD, but gives more than 20 layers after 15-min PECVD growth (Fig. 5(b)). Consequently, our results prove that PECVD growth is not self-limiting and the number of layers can be tuned by simply changing the duration of growth. Moreover, our results show that as far as the growth rate is concerned, Cu is the most favorable among the substrates used and SiO2 is the least. On the contrary, a previous study on low-temperature graphene growth on quartz [18] attributes the growth to in-situ formation of SiC (from SiO2 and plasma radicals) fulfilling the role of a catalyst. Their work reported the presence of a faint SiC Raman peak at 800 cm1 after the PECVD process. In an effort to verify so, we performed Raman measurements (Fig. S4) and found that the peak was already present on fresh quartz samples and it marginally reduced in intensity after 15 min of graphene growth, signifying that it is a characteristic peak for quartz [27]. SiO2, on the other hand, did not show such a Raman peak before or after the process. Secondly, we performed identical growth processes on non-silicon based materials such as sapphire and obtained similar results for graphene growth, thus excluding the SiC hypothesis. Having established that there is no catalytic action present during low-temperature PECVD process, we can analyze our results to gain a deeper understanding of the growth mechanism. The significant difference observed between the growth rates on different substrates implies that dissociation of CH4 is not the rate-limiting step during the deposition process as all substrates experienced identical process conditions and they do not play any role in the breakdown of precursor molecules. Thus, for PECVD growth, adsorption of carbon species followed by their diffusion on the growth substrate determines the rate of graphene growth. Interestingly, XRD measurements carried out on the Cu sample just after 1-min growth revealed that Cu’s crystal orientation is mostly (1 1 1) even during the initial stages (Fig. 5(c)). As a result, enhanced adsorption and high diffusion of carbon species on Cu(1 1 1) could be a major factor for the high growth rate of graphene when compared to arbitrary substrates [28]. A quantitative description of the impact of substrate on the kinetics of growth process, however, warrants future studies on plasma species and their adsorption/diffusion energy barriers on different substrates. The high D-peak observed in the Raman spectra of our samples signifies the nano-crystalline nature of PECVD graphene and is a direct consequence of the high carbon flux in CH4 plasma and the low growth temperature [29], thus leading to high nucleation density and small grain sizes. This is followed by further adsorption of carbon species on the host substrate that can attach to the edges of the islands or direct adsorption on top of graphene islands to induce multi-layer growth. In a study on vertical graphene growth with PECVD, diffusion of carbon adatoms on graphene walls has already been observed [30]. Once the substrate is entirely covered with graphene, interaction of incoming

carbon species with substrate vanishes that leads to them being directly adsorbed on existing graphene layers and facilitating growth of top layers. Improvement in graphene quality can likely be achieved by introducing additional H2 during the growth process to induce edge-growth [16], albeit at the cost of reduced growth rates.

5.

Conclusion

In conclusion, we present an approach to achieve rapid multilayer graphene growth on arbitrary non-catalytic materials at a low temperature of 650 C. Our results demonstrate that while a traditional metal catalyst like Cu can speed up the growth process, it is not a necessary condition to grow graphene using PECVD. The method presented in this paper eliminates the need to transfer graphene, a condition strictly imposed by conventional CVD techniques. SEM studies, coupled with optical measurements, exhibit interesting information about the dynamic growth rate and evolution of multilayer graphene islands. Our observations also exclude the possibility of any catalytic effect during lowtemperature PECVD, thus allowing tunability of number of layers that might be critical for several applications. Lowtemperature, rapid, non-catalytic synthesis of graphene is the key to paving way for industrial mass production of graphene devices and we believe that our findings are a crucial step along this direction that will allow direct integration of graphene in fields such as flexible electronics and transparent conductive electrodes.

Acknowledgements The work at UT Dallas was supported by the Center for Low Energy Systems Technology (LEAST), one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2015.05.035.

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