Growth of nano-graphene on SrTiO3 (110) substrates by chemical vapour deposition

Accepted Manuscript Growth of nano-graphene on SrTiO3 (110) substrates by chemical vapour deposition S. Karamat, K. Çelik, A. Oral PII:

S0254-0584(17)30591-6

DOI:

10.1016/j.matchemphys.2017.07.074

Reference:

MAC 19881

To appear in:

Materials Chemistry and Physics

Received Date: 25 April 2017 Revised Date:

14 July 2017

Accepted Date: 21 July 2017

Please cite this article as: S. Karamat, K. Çelik, A. Oral, Growth of nano-graphene on SrTiO3 (110) substrates by chemical vapour deposition, Materials Chemistry and Physics (2017), doi: 10.1016/ j.matchemphys.2017.07.074. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Growth of Nano-Graphene on SrTiO3 (110) Substrates by Chemical Vapour Deposition S. Karamat1,2, K. Çelik3and A. Oral1 1

Department of Physics, Middle East Technical University, Ankara Turkey 06800 2

Department of Material Science & Engineering, İstanbul Technical University, İstanbul, Turkey, 34469

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Abstract

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COMSATS Institute of Information Technology, Islamabad Pakistan 54000

Transfer of graphene from metal catalyst to dielectrics is a complicated procedure which affects the quality of graphene. In the present work, direct growth of graphene was established on

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strontium titanate (SrTiO3) substrates with the means of chemical vapour deposition (CVD). The graphene growth on catalyst free dielectric substrates were carried out for 3, 4 and 7 hours at 1000 o

C. Raman spectrum showed D, G and 2D peaks of graphene for the samples. Scanning electron

microscope (SEM) was used to get an initial measurement about the morphological structure. Energy Dispersive X-ray spectrometer attached with SEM was also used to get information about the

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composition of carbon content which showed a considerable increase for the CVD grown sample as compare to bare substrate. Atomic force microscope images of the samples surface clearly showed

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multilayer graphene domains of different sizes and height for different growth time. AFM height profile showed an increase in vertical growth with the increase in growth time. X-ray photoelectron

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spectroscopy (XPS) was used to get further information about the presence of necessary elements like graphene (carbon), its bonding with STO substrates and the shift in position of fermi level of graphene layers. XPS mapping was also done to get information about the non-uniform growth of

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carbon surface grown on SrTiO3 surface.

Key words: Chemical vapour deposition, X-ray photoelectron spectroscopy, Raman spectroscopy, Graphene.

*Corresponding Author: [email protected] Introduction Graphene based electronics are the most promising candidates in the next generation of electronic devices [1–4]. To develop graphene based devices, high-quality graphene is required which can be grown directly on the dielectric material or it can be transferred on a dielectric surface. In order to make high quality graphene, different techniques like mechanical exfoliation

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[5], chemical vapour deposition [6], thermal decomposition of SiC [7], plasma enhanced chemical vapour deposition [8], reduction of graphene oxide [9,10] are utilized. Among these, CVD takes a leading role because it provides large area growth and high quality graphene, which is required at industrial scale. The catalyst material used for growth in CVD is metal surface

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which produces grain boundaries and multilayered grains at the nucleation sites, moreover, the mechanical transfer of graphene from metal to dielectric substrates like SiO2/Si produces wrinkles, contamination and breakage. The direct growth of graphene on dielectrics will give a way to minimize transfer complications, which affects the quality of graphene. The dielectric

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material has a strong impact on the properties of graphene, for example, the presence of interfacial traps, charge impurities, and lattice phonons reduce the carrier transport in graphene. The other concern in the graphene device fabrication is the selection of appropriate substrate,

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which can possess catalytic ability for growth and have high k-dielectric value. The high dielectric value substrates are expected to reduce gate leakage, improve gate capacitance, and provide better gate modulation in case of FETs as compare to the FETs made on low dielectric value substrates like SiO2 [11]. Upto now, various insulating substrates such as MgO [12], SiO2 [13-16], Al2O3 [17, 18], and Si3N4 [19] showed ability for the direct graphene growth by CVD.

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Another perovskite-type dielectric which exhibits high transparency, high k-value and thermal stability is strontium titanate (SrTiO3) having a bandgap of about 3.2 eV [20-23]. In STO, low

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energy phonons brings the material close to a ferrolelctric instability, this softening of low energy mode causes the increase in the dielectric constant from room temperature to liquid

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helium temperature. It gives insight about the dielectric effect on charge carriers in graphene which influence the electron-electron interaction and long range electrostatic potentials [24-25].

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CVD grown graphene on STO showed directly bipolar FET behavior on the STO-gated FET, along with a low operation voltage [26]. Recently, voltage scaling was performed for a device structure using epitaxial STO thin film [27]. The direct growth of high quality graphene on STO substrates by CVD is very promising from the device point of view and a lot more need to be explored in this area especially graphene growth, its band structure and chemical bonding with the substrate. The present work is motivated by the possibility of direct growth of graphene on dielectrics using CVD technique. We grow multilayers graphene nano-islands by using common hydrocarbon gases and found the change in the shape from round domains to hexagons with the

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increasing growth time, additionally height profile also showed an increase in the number of graphene layers with growth time which showed lateral and vertical growth of graphene. Based on such behavior, it is sure that with the increase in growth time whole substrate would be covered with graphene layer. Different characterization tools were used to support the findings.

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Experiments

An ambient pressure CVD technique was used to grow graphene directly on SrTiO3 (110) substrates. The precursor gases of high purity CH4 (99.995% purity, Messer), H2 (99.999% purity, BOS) and Argon (99.999% purity, Messer) were used during growth. A standard cleaning

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procedure was used before loading the samples inside the quartz tube furnace. The quartz tube was evacuated down to ∼ 5x10-2 mbar pressure for 30 mins with a flush of Ar gas. Hydrogen (50 sccm) and argon (100 sccm) flow was maintained before ramping the furnace. The growth

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temperature was fixed at 1000 °C, CH4 (8 sccm) gas was introduced into the reaction chamber for 3, 4 and 7 hrs for different samples. The samples for 3, 4 and 7 hrs growth are labelled as GSTO 3hrs, G-STO 4hrs and G-STO 7hrs, respectively. After the growth, the samples were cooled down quickly by opening the furnace and CH4 flow was turned off at 650 °C. The samples were further cooled down in an Ar and H2 environment until the temperature reached 25°C. Renishaw

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in Via Reflex microRaman spectrometer with 532 nm laser source is used to measure the Raman spectra. JSM-6400 Electron Microscope (JEOL), equipped with NORAN System 6 X-ray

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Microanalysis System & Semafore Digitizer was also used to get initial information about the morphology and composition of the samples. hp-AFM from NanoMagnetics Instruments was

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employed for AFM imaging. A Thermo Fisher K-alpha electron spectrometer with a monochromatic Al Kα X-ray source (hν = 1486.6eV) was used to get information of different

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elements present in the samples. Results and Discussions

Raman spectroscopy is the basic characterization tool to get understanding about the different features of graphene. Different characteristic bands of graphene like D, G and 2D represents the defects, in-plane vibration of sp2 carbon atoms and the stacking orders, respectively [28, 29]. Raman spectroscopy was performed on bare and graphene grown samples.

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Figure1. (a) Raman spectra for bare STO substrates along with graphene grown samples, (b-d)

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Optical micrographs of the samples where Raman spectrum was taken.

Figure 1(a) showed Raman spectra for all G-STO samples, D and G peaks are clearly visible in all the samples. These peaks are fitted with Gaussian peak fittings to know the correct position, D and G peaks appear around 1277.54 and 1621.80 cm-1 for 3 hrs, 1282.99 and 1618.36 cm1

for4hrs,1300.21and

1616.18 cm-1 for 7 hrs (point 1) and 1285.51 and 1614.99 cm-1 for 7 hrs (point 2) samples. Interestingly, the sample grown for 7 hrs showed all the Raman active modes that are D, G and 2D peaks, which vary at different points, the spectrum in figure 1(a) for 7 hrs at point 1 showed the presence of G (1616.18

cm-1) and 2D (2430.33557 cm-1) peaks with very weak

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interactions between the carbon and oxygen bonds of STO [26]. The presence of 2D peak was also suppressed in the case of SiC [29]. It was reported that these type of differences appear due to formation of nano-size graphene because the graphene domains have a greater number of edge states as compare to the bulk graphene [12]. The presence of edge states which are basically

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defects give rise to D band with the broadening and shifting of G band from 1580 cm -1 to 1600 cm-1. Moreover, 2D peak which is an overtone of the D peak also lost [30]. We observed the similar effect in our present samples which showed the presence of nano-graphene. The intensity

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ratio of D to G peak is usually used to estimate the disorder found in the graphene, which is a direct measure of its quality. In our case the ID/IG is 0.97, 0.94, 0.79 for 3, 4 and 7 hrs, respectively, which is higher than the reported value [31, 32] and it showed the presence of nanographene, which has disorder. Moreover, bare STO substrate has shown a flat background in the region of interest, i.e., no D, G and 2D peak were observed, which showed that the appearance of

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peaks was possible only after successful carbonaceous growth on the substrate. 2D Raman peak also give information about the stacking sequence of the carbon layers, the peak width of 2D

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Raman peak is ~ 32.18 which hint towards twisted stacking. In order to get more precise information about the surface and composition of the samples AFM

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imaging and XPS with surface mapping was performed in detail for all the samples. AFM

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topography and 3D images of the scanned samples are shown in figure 2 and 3, respectively.

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Figure 2. Topography images of (a) bare STO substrate, (b) G-STO 3hrs, (c) G-STO 4hrs and (d)

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micrometer (horizontal) range.

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G-STO 7hrs obtained by hp-AFM in dynamic mode. The scales are in nanometer (vertical) and

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Figure 3. 3D view of AFM images of (a) bare STO substrate, (b) G-STO 3hrs, (c) G-STO 4hrs and (d) G-STO 7hrs samples.

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Figure 2(a) shows the surface morphology of the bare STO substrate prior to graphene growth, which is very smooth. The 3D view of the bare substrate in figure 3(a) shows the surface roughness of the substrate, which was invisible in topographical view. The CVD growth of

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graphene on metal surface take place when gas molecules decomposes by the catalytic ability of the metal surface followed by the dissolution or segregation of carbon atoms on the surface. In case of dielectrics, hydrocarbons also disassociate if the surface becomes chemically reactive. In STO, Sr and Ti vapour pressure is higher than 1000 °C (growth temperature of the graphene used in the experiment), so the surface becomes active due to desorption of oxygen content, which created pits [33]. These pits act like an active nucleation site where carbon atoms minimized their Gibbs free energy and nucleate at the surface. Previously, for other dielectrics, oxygen deficient surface was formed in vacuum above 1250 °C [33, 34]. In our case, the precursor gases were H2 and CH4, which causes reduction of the oxygen content from the STO surface due to

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chemical reaction at low temperature. The decrease in oxygen content is clearly visible from EDX results where atomic weight percentage of oxygen reduced from 42 to 30 in the G-STO 7hrs sample. It was expected that the surface under the steps and the pits became metallic after oxygen content reduced, which enhanced the growth of carbon species. The nucleation of carbon

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species would be higher at the step edges of the substrate because of pit formation near the edges as observed in the case of graphene growth on Al2O3 [33]. Figure 2(b-d) showed surface morphology of the graphene grown samples for 3, 4 and 7 hrs, respectively. Figure 2 (b-c) clearly shows the formation of round graphene domains, which beautifully covered the whole

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surface. The increase in growth time increased the size of domains, for 3hrs growth the domain size was in the range of 0.10 to 0.13 microns, for 4hrs it was in the range of 0.13 to 0.16 clearly seen in the figure 2(c) and for 7 hrs growth it was in the range of 0.28 to 0.35 microns. The shape

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of the round domains also changed into hexagonal shape domains for 7 hrs growth. The shape of hexagonal graphene domains for 7 hrs growth have rough splashy edges. Previous reports showed that hydrogen gas has an etching nature which controls the shape of graphene domains grown on metal [35] and boron nitride [36, 37]. In case of strontium titanate substrates, with the increase of growth time, the surface oxygen deceases and hydrogen content promotes hexagonal

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domains with different edges (Arm Chair and Zigzag) like the case of h-BN [37] where silane (SiH4) was able to control the graphene edges. Edges are reactive part of graphene domains and

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growth parameters (pressure, temperature) easily influence them. Moreover, the control over the edges of graphene on dielectric substrates is still challenging due to less understanding of

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surface reactivity during CVD and a lot more is needed to understand these systems. Figure 3(bd) give 3D view of the graphene domains in the form of islands. The increase in domain size

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occurred due to coalescence of the small round domains. Figure 4(a-c) showed the height profile for different growth times, consistent increase in height of domains with the time showed vertical growth. With the increase in time there is lateral and vertical growth in our samples and it is expected it will continue with the increase in time until the whole substrate will be covered.

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Figure 4(a). Height profile with corresponding topography image of graphene grown for 3hrs.

Figure 4(b). Height profile with corresponding topography image of graphene grown for 4hrs.

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Figure 4(c). Height profile with corresponding topography image of graphene grown for 7hrs. The grain height obtained from topographic images of AFM for 3, 4 and 7 hrs are ~ 2.35, 3.94 and 10.20 nm, respectively. Figure 5 shows one of the graphene domain for G-STO 7 hrs sample scanned at lower step size for high resolution. The image clearly showed boundaries of the two

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domains, which merged together to form a bigger domain.

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Figure 5. Boundary between two graphene domains for G-STO 7hrs sample, the white spot on

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the top is reflected as dark spot in SEM image of 7hrs growth.

Figure 6(a) SEM picture of bare STO substrate, and (b) G-STO 7 hrs sample. SEM analysis was also performed on the same sample to get an idea about the morphology and composition of the sample. The surface image of the bare substrate showed no growth while the G-STO 7 hrs showed small round islands of darker colour decorated on the graphitic surface formed on the top of STO in figure 6. By comparing SEM image with AFM image of 7 hrs growth, we observe that the graphene domains are round in shape and scattered which is due to limited magnification of SEM. These dark spots in SEM image corresponds to white spots lies

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on the top of graphene islands in AFM image. AFM height variation confirms that these spots are on the top position. With the help of EDX analysis, we conclude that there is a carbon growth on the surface along with small structures. EDX spectra represents Al peak which basically belongs to Sr. Due to limited accuracy about the surface composition Al appears in the spectrum.

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The presence of Al was further checked with XPS which is more sensitive tool and it was not present. The atomic and weight percentage obtained from EDX analysis is showed in Table 1 which clearly represents the increase in carbon content after growth. Bare Substrate

G-STO 4 hrs

Elements

Atomic Wt %

Wt %

Atomic Wt %

Wt %

Atomic Wt %

C

5.46

1.50

21.15

6.38

23.55

O

41.99

15.40

35.35

16.74

36.85

Sr

29.01

57.25

24.74

Ti

23.54

25.85

18.76

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G-STO 3 hrs

G-STO 7hrs

Wt %

Atomic Wt %

Wt %

7.88

28.29

9.46

17.24

30.06

13.39

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Elemental %

52.05

22.34

50.55

22.15

51.12

24.83

17.26

24.33

19.50

26.03

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Table1: Elemental composition obtained from EDX for bare substrate and G-STO samples.

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Figure 7. De-convoluted C-1s spectra for (a) G-STO 3hrs, (c) G-STO 4hr and (d) G-STO 7hrs samples

The information about graphene was further probed by XPS. C-1s core level spectra showed asymmetrical behavior for all the samples. In order to get the clear picture about the sp2 and sp3 carbon content, it was de-convoluted and fitted with multiple peaks. C1s core peak spectrum for G-STO 3hrs, shown in figure 7(a) was fitted with four Gaussian peaks centered at 285.02, 285.39, 285.77 and 286.89 eV. Similarly, C1s core level spectrum for G-STO 4hrs sample,

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shown in figure 7(b), was fitted with four peaks centered at 285.38, 285.83, 286.35 and 286.90 eV. G-STO 7hrs sample, shown in figure 7(c) was fitted with five Gaussian peaks centered at 284.41, 285.31, 285.63, 286.04 and 286.21 eV. In literature the sp2 bonded carbon (nonoxygenated) atoms exhibits binding energy around 284.6 ± 0.2 eV while the sp3 bonded carbon

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atoms (C-O) showed binding energy values around 286.5 ± 0.2 eV [32, 33] . Also, the hydrogen bonded carbon appeared around 285.3 eV. With the help of deconvolution, we can conclude that

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in our samples three type of carbon bonding (C-C, C-O, C-H) existed 38, 39].

Figure 8. XPS valence band spectra for (a) bare STO substrate, (b) G-STO 3hrs, (c) G-STO 4hr and (d) G-STO 7hrs samples.

The information about the Fermi level was also deduced from the XPS data shown in figure 8. The Fermi level for bare substrate has value around -0.66 eV. The samples after growth showed

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an increase of ~ 2 eV from the bare substrate, which clearly showed carbon growth on STO substrates. The elemental distribution or mapping of grown material can be revealed by XPS imaging. The compositional imaging derived from the XPS spectra provides a valuable way to analyze the uniformity or segregation of elements present on the surface. The images constructed

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from the area profile of carbon peak for all the samples are showed in figure 9. The darker regions showed less carbon content as compare to brighter regions. The change in color at different points hints non-uniformity of graphene which is well supported by AFM

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characterizations.

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Figure 9. C1s distribution for (a) G-STO 3hrs, (b) G-STO 4hrs and (c) G-STO 7hrs obtained by XPS imaging.

Conclusions The present study is about the growth of nano-graphene on SrTiO3 (110) substrates by CVD. The graphene growth was carried out for different time duration at 1000 oC. Raman spectrum was varied

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at different points on the same sample due to formation of nano-size graphene. The nano-size

graphene domains have a large number of edge states which are basically defects and give rise to D band, suppression of 2D band along with broadening and shifting of G band. SEM and EDX gave initial information about the morphology and composition of carbon content present on the

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substrate. The detailed surface imaging was obtained by AFM which clearly showed graphene domains of different sizes. The size of graphene domains increased from 0.10 microns to 0.35 microns with the increase in growth time. XPS was used to get further information about the presence of graphene which has sp2 bonding of carbon atoms. Carbon core peak spectrum was

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deconvoluted for all the samples to get information about the different carbon bonding present in the samples. XPS mapping was also done to get information about the surface uniformity.

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Acknowledgements

One of us, S. Karamat would like to thank TÜBİTAK and European Union Marie-Curie CoFunded 2236 Fellowship for accomplishing this work. We would like to extend thanks to Prof Sefik Suzer from Bilkent University for the invaluable discussion and XPS measurements, and Prof Arshad Saleem Bhatti from COMSATS for the dielectric substrates and his suggestions. Further, we would like to thanks Ugur Inkaya and Selda Sonusen for their help during

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• SrTiO3substrates were used to grow nano-graphene directly via CVD. • Purpose is to avoid graphene transfer process from metal catalyst. • Graphene domains of different sizes and shapes were observed. • XPS showed carbonaceous growth.