Hydrophobic Surface Treatment and Interrupted Atomic Layer

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Hydrophobic surface treatment and interrupted atomic layer deposition for highly-resistive Al2O3 films on graphene Jae Ho Jeon, Sahng-Kyoon Jerng, Kamran Akbar, and Seung-Hyun Chun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09531 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

Hydrophobic surface treatment and interrupted atomic layer deposition for highly-resistive Al2O3 films on graphene

Jae Ho Jeon, Sahng-Kyoon Jerng, Kamran Akbar and Seung-Hyun Chun*

Department of Physics and Graphene Research Institute, Sejong University, Seoul 05006, Korea

* Corresponding Author: FAX: +82 2 3408 4316. E-mail address: [email protected] (S. H. Chun)

ACS Paragon Plus Environment

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ABSTRACT

The deposition of thin and uniform dielectric film on graphene is an important step for electronic applications. Here, we tackled this problem by combining a simple chemical treatment of graphene surface and a modification of standard atomic layer deposition (ALD). Instead of common approaches trying to convert hydrophobic graphene surface into hydrophilic one, we took the opposite way by applying a self-assembled-monolayer, hexamethyldisilazane (HMDS) to make defect-independent, more hydrophobic surface condition. In addition, Al2O3 ALD using trimethylaluminum (TMA) and water (H2O) was interrupted several times and the surface was air-exposed during the interruption to seed the following ALD processes. This combination greatly improved the uniformity of dielectric film and accomplished a successful deposition of 10 nm-thick Al2O3 on graphene with sub-nanometer roughness except for the locations of wrinkles and polymethyl methacrylate (PMMA) residues.

Electrochemical impedance

measurements revealed a 300-fold increase in the charge-transfer resistance by employing this modified ALD process. No change in the Raman spectra was observed after the dielectric film growth, demonstrating that the method proposed here is non-detrimental to the graphene quality.

KEYWORDS: graphene, Al2O3, atomic layer deposition, chemical vapor deposition, hexamethyldisilazane


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I. Introduction The discovery of graphene and its superb physical and chemical properties prompted a huge interest in this two-dimensional sp2-bonded carbon material.1–3 Especially, the successful growth of large area graphene by chemical vapor deposition (CVD) showed the potential of commercial applications because micro- and nano-scale devices utilizing the high carrier mobility and the tunable electrical characteristics of graphene could be fabricated by top-down processes.4–6 However, the use of graphene for high-performance field-effect transistors (FETs), for example, requires a variety of tasks to be solved. The growth of thin and uniform dielectric film on graphene is one of them.

An important constraint is that the growth process should not

deteriorate the graphene quality.

In addition, for practical reasons, the method should be

compatible with standard semiconductor fabrication processes so that it can be easily implemented. Since atomic layer deposition (ALD) has been successfully employed in the semiconductor industry for ultrathin high-k oxide film deposition7,8 and is more desirable for graphene because of the low energy involved, various recipes have been developed to grow metal oxides on graphene by ALD.9,10 However, most studies were done on exfoliated graphene, and their applicability to CVD graphene should be re-evaluated. The critical problem here is not the difficulty in thin film deposition by ALD, but the preferential growth on defects of graphene. The problem becomes more critical in large area CVD-grown graphene where high-density defects are inevitable during the growth on metal foil and the transfer processes. Since ALD is a highly selective process depending on the surface,11 an earlier study even utilized ALD to visualize defects in graphene.12 Typical schemes for uniform dielectric growth were chemical treatments or functionalization of chemically inert, hydrophobic graphene surface. Meric et al. treated graphene with polyvinyl alcohol (PVA) to


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make it hydrophilic and reported improved HfO2 growth.13 Shin et al. functionalized graphene by oxygen plasma and transferred onto pristine graphene to seed the following ALD.14 However, such attempts tend to invoke a reaction with graphene, resulting in undesirable doping or defect generation.

Metal-seeded, ozone-based, or plasma-enhanced ALD also suffer similar

risks.9,10,15,16 Here, we took a counter-intuitive approach: by applying hexamethyldisilazane (HMDS) to make the graphene surface fully hydrophobic, irrespective of microscopic defects. HMDS is a readily available chemical called “primer” or “adhesion promoter” in most fabrication centers, used to make the surface hydrophobic so that photoresist can adhere to the entire substrate.17 It has been also used in graphene FET before or after the transfer to provide an inert interface with graphene and to increase the mobility.18–20 Especially, the application of HMDS was not detrimental to graphene at all. We found that the application of HMDS prior to ALD resulted in a significant improvement of Al2O3 surface morphology. A similar improvement was observed by repeated interruption with air exposure during the ALD process.

Surprisingly, the

combination of these two variations resulted in a dramatic change in the dielectric growth and enabled thin Al2O3 layer deposition (~ 10 nm) on graphene with a factor of 300 increase in the charge-transfer resistance and an ultra-low effective porosity of 0.01 %, without any degradation of graphene quality.

II. Experimental Section Graphene films were synthesized on Cu foils by inductively-coupled-plasma (ICP) CVD system. Commercial Cu foils (0.025 mm thick, 99.8 % purity) from Alfa Aesar were used in this work.


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After introducing the Cu foil to a high vacuum (< 10-6 Torr) chamber, the substrate temperature was increased to 830 °C at a heating rate of 1.3 °C/s without gas flow into the chamber. To eliminate surface oxides on the Cu foil, H2 gas was discharged by radio frequency (RF) power of 50 W for 2 minutes. Then, the chamber was purged with Ar gas at a flow rate of 100 standard cubic centimeters per minute (sccm) for 2 minutes to remove the residual gas. For graphene synthesis, RF plasma was generated with a continuous flow of Ar (40 sccm) and methane (1 sccm) while the chamber pressure was maintained at 10 mTorr. Detailed processes about monolayer graphene synthesis can be found elsewhere.21 Graphene films were transferred onto SiO2/Si or sapphire substrates by etching Cu foils in an aqueous solution of ammonium persulfate (0.1 mole in water). Before wet etching, the surface of graphene/Cu was spin-coated with polymethyl methacrylate (PMMA: MicroChem 950 A2) which was dissolved by acetone after the transfer process. Thin films of Al2O3 were deposited at 125 °C by a conventional ALD system. The precursors of trimethylaluminum (TMA) and water was sequentially exposed with N2 gas. The deposition rate of Al2O3, ~0.145 nm/cycle, was determined by performing ALD on SiO2/Si, and the nominal thickness refers to the process cycle based on this rate. HMDS coating was performed by standard spin coating process at 2000 rounds per minute (RPM). The surface morphology was analyzed by an atomic force microscope (AFM: Nano Focus Inc., n-Tracer) operated in non-contact mode. The graphene quality was evaluated by Raman spectroscopy (Renishaw, inVia system) using a 514.5 nm laser operated at 0.75 mW. To obtain structural information and atomic distribution, high-resolution transmission electron microscopy (HR-TEM: JEOL, JEM-2100F) with electron dispersive spectroscopy (EDS: Oxford instruments) was employed at 200 keV accelerating voltage. PMMA was used to prevent Pt


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contamination during focused ion beam etching. The degree of surface hydrophobicity was estimated by taking photos of water droplets on the graphene surface. The effective porosity of Al2O3 film on graphene was calculated from the charge-transfer resistance as measured by electrochemical impedance spectroscopy (EIS).22

A 2-channel

potentiostat (SP-300, BioLogic) was used for EIS measurements from 3 MHz to 1 Hz with a sinusoidal amplitude of 10 mV. We used a platinum counter electrode and 0.5 M potassium sulfate aqueous electrolytes (pH = 5 ~ 6) prepared from potassium sulfate powder (Daejung, 99.0%) and deionized water (>15 MΩcm resistance) for these electrochemical measurements at room temperature.

III. Results and Discussion The surface morphology of conventional Al2O3 ALD (69 cycles for 10 nm nominal thickness) on pristine CVD-grown graphene is shown in Figure 1a. The height histogram (Figure 1b) reveals the problem of standard water-based ALD on graphene.

The bimodal height distribution

indicates that more than 30 % of the surface is not or barely covered, making this simple ALD process useless. Even the Al2O3 height of covered area has a wide distribution. We can also notice that most Al2O3 grows in the form of thin-line shapes. Dots or islands are concentrated at the intersections. These features are reminiscent of dense grain boundaries revealed by darkfield TEM,23 suggesting that the nucleation of ALD preferentially occurred at defects. These features have been observed in general.10,24 Figures 1c and 1d show the improved surface morphology after an interrupted ALD process (in a sequence of 17 ALD cycles – air exposure – 17 ALD cycles – air exposure – 34


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ALD cycles). Clearly, more dots or islands are formed and the surface coverage increases significantly. Furthermore, the voids become smaller in width and the height distribution of covered area becomes narrower. We intended to investigate the early stages of ALD growth by taking AFM images after a few ALD cycles, and it turned out that the final products were different even after the same number of total cycles. It was found that the air exposures are critical in obtaining improved surface morphology, since just cooling in the ALD chamber does not make a difference. Moreover, the number of air-exposures affects uniformity. The effect of interruption and air exposure during the ALD process is not clear; however, influences of air exposure have been attributed to adventitious carbon adsorption previously.25,26 Despite the improvement by interrupted ALD, Al2O3 growth is far from satisfactory. Since the uneven ALD growth comes from the uneven seeding on the surface, finding a way to prepare a homogenous graphene surface is required. Instead of hydrophilic chemical treatments, which have been investigated so far, we took the opposite approach of hydrophobic treatment. HMDS is one kind of self-assembled-monolayer (SAM), primarily used on substrates before the spin coating of photoresist. It makes the surface hydrophobic by elimination of hydroxyl (-OH) groups on the surface or defective graphene edges.27 This property has been already utilized to suppress the substrate effect on graphene. Lafkioti et al. reported the increase of carrier mobility and the reduction of unintentional doping for graphene flakes transferred on HMDS treated substrates.18 Kim et al. reported a four-fold enhancement of mobility for CVD graphene using HMDS as a buffer layer.19 The HMDS treatment can be done simply in the fabrication room by spin-coating and soft-bake. Our approach may sound unreasonable, since the problem of ALD on graphene has been attributed to hydrophobicity. However, HMDS treatment can make the following ALD process less dependent on graphene defects where water vapors are adsorbed and


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make them preferential nucleation sites. Indeed, the application of HMDS makes the graphene surface more hydrophobic, as revealed by the increase of water droplet contact angle (Figure S1 in Supporting Information, SI). The surface morphology and the roughness average, Ra, are the same after HMDS treatment (Figure S3). Figure 1e and 1f show that HMDS treatment alone yields an improvement better than that of interrupted ALD. The increase of surface coverage can be inferred from the height histogram and, as in the case of interrupted ALD, the covered area is dominated by dots or islands instead of lines (see Figure S2 for line profiles). A striking change occurs when the two methods are combined. The HMDS treatment followed by interrupted ALD produces a greatly improved surface morphology (Figure 1g); an almost fully covered surface is observed and Ra is drastically reduced from 4.23 nm (Figure 1a) to 0.66 nm (Figure 1g) over the 5 um x 5 um area (see Figure S3 for a thinner case). Furthermore, the bimodal height distribution changes to a single peak of 2.5 nm full width at half maximum (FWHM), as shown in the inset of Figure 1g. The Al2O3 film becomes noticeably pinhole-free except for the locations of PMMA residues that remain after the transfer process (the upper panel of Figure 1h, corresponding to the blue line in Figure 1g). The large-area transfer process also produces many wrinkles that are the source of sharp and long protrusions (2-5 nm high), as shown in the middle panel of Figure 1h. If the PMMA residues and the wrinkles were avoided by the introduction of dry transfer or other advanced transfer methods,28,29 our modified ALD would be able to produce uniformly-thick, pinhole-free dielectric films on graphene (the lower panel of Figure 1h). The most important advantage of our modified ALD method is that the high quality of underlying graphene is conserved without unintended doping and/or damages. As shown in Figure 2, Raman spectra show identical shapes after HMDS treatment and/or Al2O3 ALD. The


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intensity of 2D peak (~ 2690 cm-1) relative to that of G peak (~ 1590 cm-1) is larger than 2.88 and the FWHM of 2D peak is less than 33.86 cm-1, which prove that high-quality monolayer graphene is prepared and preserved after the dielectric film deposition. This contrasts to other methods, where a reduction of 2D peak intensity and/or an increase of D peak intensity has been observed after chemical treatment, metal-precursor deposition or ozone ALD.15,30 There is a possibility that ammonia, generated by HMDS reaction with silanol, could dope the graphene, but Raman spectra do not show any sign of doping (Raman peak positions are tabulated in Table S1). Sheet resistance measurements on graphene before and after ALD also prove that the graphene remains intact during HMDS treatment and the interrupted ALD process (Figure 2b). The inert nature of graphene/HMDS interface is expected from the previous studies.18–20. Furthermore, HMDS treatment and standard water-based ALD are readily available in most facilities, making the present method more attractive. The structure of Al2O3 thin film deposited by our modified method was further examine by cross-sectional TEM. We doubled the total ALD cycles for a better resolution (in a sequence of 17 ALD cycles – air exposure – 17 ALD cycles – air exposure – 34 ALD cycles – air exposure – 69 ALD cycles). As can be seen in Figure 3a, an amorphous Al2O3 layer of uniform thickness is identified. It turns out that the thickness is larger by ~40 % than the nominal thickness. We also checked the actual thickness of thinner layers, and obtained a deposition rate of ~0.208 nm/cycle for interrupted ALD on HMDS treated graphene (Figure S4). Although different deposition rates on different surfaces are common in ALD process, the variation seems a bit large. More studies on growth kinetics need to be conducted. Figure 3b and 3c show the EDS data taken along the growth direction. The amount of carbon within Al2O3 layer is below resolution limit, implying that no significant contamination was occurred during the interrupted


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ALD. Minute inclusions of carbon, however, could affect the dielectric property of Al2O3 layer. We fabricated Au-Al2O3-Au metal-insulator-metal structures with Al2O3 films grown by conventional ALD and interrupted ALD, and deduced the dielectric constants from standard capacitance measurements. The results show that interrupted ALD does not affect the dielectric property of Al2O3 significantly (Figure S5). EIS measurements provide a useful information on the insulating property of ALD film, not attainable from structural analysis. Figure 4a shows the Nyquist plot of EIS data from the various Al2O3 covered-graphene samples transferred on sapphire substrates. Roughly speaking, the diameters of semicircles in the plot correspond to the charge-transfer resistances of Al2O3 covered-graphene.31 One can notice the giant enhancement of charge-transfer resistance due to the improved surface morphology by HMDS treatment and/or interrupted ALD (EIS data related HMDS treatment is available in Figure S6).

A simple equivalent-circuit model can

quantitatively analyze the data (Figure 4b). Here, R1 is the resistance of electrolyte, which should be the same for all the samples since the same electrolyte was used. R2 is the ionic charge-transfer resistance of Al2O3 covered graphene and Q2 is a constant phase element describing the electrochemical double-layer capacitance at the electrode/electrolyte interface. We find that the charge-transfer resistance increases by a factor of 300, from 8.5 kΩ (by simple ALD) to 2.7 MΩ (by a combination of HMDS treatment and interrupted ALD as the modified ALD method). This proves that the modified process helps not only the surface flatness but also the insulating property of the dielectric layer. We can also estimate the effective porosity of the dielectric layer by comparing the charge-transfer resistances of bare graphene and Al2O3 covered graphene,32 since R2 is inversely proportional to the charge transfer area where the electrolyte meets the graphene surface via


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through-film pores (EIS measurements on bare graphene is shown in Figure S7). Figure 4c shows that modified ALD yields an effective porosity of 0.01 %, which is smaller by an order of magnitude than those of recent results by NO2/TMA pretreated ALD.22

IV. Conclusions We propose here the spin-coating of HMDS and the following interrupted ALD as a scalable method of thin Al2O3 deposition on ICP-CVD-synthesized graphene. This method is harmless to graphene quality and readily applicable in most fabrication facilities. EIS measurements confirm the giant enhancement of charge-transfer resistance and the ultra-low effective porosity of the dielectric layer. With the improvement of graphene transfer method, our modified ALD is expected to produce pinhole-free dielectric films on graphene. Other variations such as pre-H2O or NO2 treatment to seed the growth may be combined with HMDS pre-treatment for better uniformity.22,33

ACKNOWLEDGMENTS We thank N. J. Kim and H. J. Cha for their help in graphene growth. This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future

Planning

(Nos.

2010-0020207,

2011-0030786,

2012M3A7B4049888,

2014R1A2A2A01005963).


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Supporting Information Contact angle measurements, additional line profiles, additional AFM images, Raman peak positions, deposition rate of Al2O3 ALD on graphene, dielectric constants of Al2O3 grown by conventional ALD and interrupted ALD, EIS data of HMDS treated graphene, EIS data of pristine graphene. This material is available free of charge via the Internet at http://pubs.acs.org.


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Figure 1. AFM images and height histograms of nominally 10 nm-thick Al2O3 grown by (a, b) conventional ALD, (c, d) interrupted ALD, (e, f) HMDS treatment and conventional ALD. (g) AFM image of Al2O3 grown by HMDS treatment and interrupted ALD (inset: height histogram). (h) Cross-sectional profile along the colored line in (g). The dotted blue and red lines in (b, d, f) correspond to uncovered region and the most populated region, respectively.


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Figure 2. (a) Raman spectra of pristine graphene and Al2O3 covered graphene where Al2O3 were grown by various methods as in Figure 1. The data are almost identical, indicating the processes are non-detrimental to graphene. Dotted grey lines indicate G peak (~ 1590 cm-1) and 2D peak (2690 cm-1). (b) Sheet resistance of graphene before and after modified ALD.


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Figure 3. TEM and EDS data for nominally 20 nm-thick Al2O3 grown by interrupted ALD on HMDS treated graphene. (a) Cross-sectional TEM image at high magnification, (b) low magnification image with EDS data along the growth direction, and (c) EDS data for C/Si/O/Al elemental components.


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Figure 4. (a) Nyquist plots of EIS data from Al2O3 covered graphene where Al2O3 were grown by conventional ALD (A), HMDS treatment and conventional ALD (B), interrupted ALD (C), HMDS treatment and interrupted ALD (D). (b) A simple equivalent-circuit model to interpret EIS data. (c) The effective porosity calculation of dielectric layers.


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Table of contents

Hydrophobic treatment of graphene followed by modified atomic layer deposition yields thin and flat Al2O3 with highly-enhanced charge-transfer resistance and reduced effective porosity.


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Hydrophobic Surface Treatment and Interrupted Atomic Layer

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