Dielectrics
Boron Nitride Film as a Buffer Layer in Deposition of Dielectrics on Graphene Qi Han, Baoming Yan, Teng Gao, Jie Meng, Yanfeng Zhang,* Zhongfan Liu, Xiaosong Wu,* and Dapeng Yu
As a two-dimensional material, graphene is highly susceptible to environmental influences. It is therefore challenging to deposit dielectrics on graphene without affecting its electronic properties. It is demonstrated that the effect of the dielectric deposition on graphene can be reduced by using a multilayer hexagonal boron nitride film as a buffer layer. Particularly, the boron nitride layer provides significant protection in magnetron sputtering deposition. It also enables growth of uniform and charge trapping free high-k dielectrics by atomic layer deposition. The doping effect of various deposition methods on graphene has been discussed.
1. Introduction The graphene fever started with the invention of the mechanical exfoliation method, as it has made graphene easily accessible to researchers.[1] By exfoliating graphene flakes on SiO2/Si substrates, it offers two additional benefits. One is that monolayer graphene can be conveniently identified using an optical microscope.[2] The other is that the carrier density of graphene can be tuned by the silicon back-gate. All these benefits greatly contribute to the research of this field, which has led to predictions of various applications. However, a few drawbacks of the method emerge when it comes to high carrier mobility and mass production compatibility. First, SiO2 is not a good surface to interface with graphene, as
Q. Han, B. M. Yan, J. Meng, Prof. X. S. Wu, Prof. D. P. Yu State Key Laboratory for Artificial Microstructure and Mesoscopic Physics Peking University Beijing 100871, China E-mail: [email protected] Q. Han, B. M. Yan, J. Meng, Prof. X. S. Wu, Prof. D. P. Yu Collaborative Innovation Center of Quantum Matter Beijing 100871, China T. Gao, Prof. Y. F. Zhang, Prof. Z. F. Liu College of Chemical and Molecular Engineering Peking University Beijing 100871, China E-mail: [email protected] DOI: 10.1002/smll.201303697 small 2014, 10, No. 11, 2293–2299
it introduces charge inhomogeneity and scattering, degrading the mobility.[3–6] Second, the silicon substrate back-gate is not compatible with integrated circuit technology because it cannot individually tune each device like a top-gate. On the other hand, a top-gate that does not reduce the quality of graphene is becoming increasingly important in graphene research, e.g., demonstration of Klein tunneling,[7,8] band gap tuning of bilayer graphene.[9] The key to these problems is to find a smooth and inert dielectric material that does not introduce many impurities to graphene. In this regard, hexagonal boron nitride (h-BN) has been proven to be so far the best substrate for graphene.[10–13] Naturally, it is an ideal topgate dielectric material, too.[14] Considerable efforts have been paid to search for methods of depositing dielectrics on graphene while maintaining its high quality.[15] Suspended top-gate by dedicated microfabrication has been attempted.[16–18] Over-exposed poly (methyl methacrylate) (PMMA) and hydrogen silsesquioxane (HSQ) have been tried.[7,8,19] Sputtering, having the advantage of producing dense and conformal film and being widely used in industry, was employed to deposit Al2O3 and MgO.[20] But, severe damage to graphene by energetic particles was found. Atomic layer deposition (ALD) is believed to be a superior technique in ultra-thin, uniform high-k dielectric coating. However, it turned out to be difficult to grow uniform dielectric layer on clean graphene, because growth only occurs at defects in graphene.[21,22] Although functionalization of graphene was demonstrated to be a solution for non-uniform growth, it also modifies graphene.[23–26] If
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h-BN is used as a buffer layer for dielectric deposition on graphene, it not only provides a clean interface to graphene, but also protects graphene from damage during deposition. As a result, a variety of deposition techniques may be chosen depending on particular applications. In this work, we demonstrate the use of multilayer hexagonal BN films as a buffer layer for dielectric deposition on graphene. Both the graphene and BN films were grown by chemical vapor deposition (CVD), as they can be grown in large area, which is crucial for future mass production. We have studied the damage and charge doping caused by deposition in the presence of a BN buffer layer for four common deposition techniques, i.e. thermal evaporation, e-beam evaporation, magnetron sputtering and ALD. It has been found that the BN film provides prominent protection for graphene in magnetron sputtering. For ALD, the film offers a surface for growth of uniform dielectrics.
2. Results and Discussion Figure 1(a) shows a schematic of our test structure for investigating the effect of dielectric deposition on graphene, while Figure 1(b) is the optical image of the structure. The detail of the device fabrication is described in the Experimental Section. The graphene film is partially covered by a few-layer BN film. The morphology of the BN film can be found in the Supporting Information Figure S1. Dielectrics are deposited on both surfaces. We first check the effect of deposition of a BN layer. Raman spectra were collected for both the exposed and BN covered areas, shown in
Figure 1. Deposition of multilayer BN film on graphene. (a) Schematic of the test structure for studying the effect of multilayer BN film on graphene. (b) Optical image of a graphene film (top-left) partially covered by a multilayer BN film (bottom-right). (c) Raman spectra for both the bare graphene area and the BN covered graphene area.
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Figure 1(c). Three well defined peaks can be seen, known as D, G and 2D peaks. The D peak is due to the breathing modes of sp2 atoms in rings and is related to lattice disorder in graphene.[27–29] Its height, relative to the G peak, is often used as an measure of the defect density. The G peak is an in-plane C–C bond stretching mode and present in all sp2 carbon system, while the 2D peak is the second order of the D peak. The 2D peak height is associated with the doping level of graphene. It can be seen that two spectra are almost identical. So, deposition of BN does not introduce appreciable defects or doping. Having known that the BN over layer has little effect on graphene, we can directly investigate the effect of dielectric deposition on graphene by comparing the bare graphene area and the BN covered area after deposition. Nominal 20 nm dielectrics were deposited by thermal evaporation, e-beam evaporation and ALD. The Raman spectra of graphene, dielectrics/graphene and dielectrics/BN/graphene are shown in Figure 2. For the thermal evaporation, the spectrum remains almost unchanged, with an exception that the D peak might be slightly enhanced after deposition for both bare graphene and BN/graphene. Overall, it seems, under our deposition condition, that thermal deposition is quite gentle and has little impact on graphene. The situation changed as the Al2O3 layer was deposited by e-beam evaporation. The bare graphene after deposition exhibits similar spectrum as the one after thermal deposition. However, the BN/graphene undergoes a few changes. Firstly, the 2D peak is significantly reduced. Secondly, a new feature at 1620 cm−1, known as the D′ peak, sets in. Lastly, the D peak is also broadened. It appears that the BN/graphene is less protected from the deposition process than bare graphene. This may look counterintuitive, but we will explain it later. Figure 2(c) displays the spectrum before and after ALD deposition of Al2O3. No change has been observed in the D peak. On the other hand, the 2D peak height is reduced for both BN/graphene and bare graphene. Besides, the peak positions of both the 2D and G peak are blue-shifted. All suggest that graphene is doped.[30–33] For the 2D peak for BN/graphene is more suppressed, the doping level is even higher even if it is supposed to be less affected by the deposition process. To compare the effect of the three deposition processes on graphene, we compile the evolution of the D peak and 2D peak intensity in Figure 3. The ratio of the D peak to G peak intensity, indication of structural defect density, is plotted in Figure 3(a). It is obvious that thermal deposition introduces little damage to the graphene lattice. Similar observation is made for ALD, because no functionalization of graphene has been carried out to improve the uniformity of the dielectric film, consistent with previous studies.[23–25] Among three methods only e-beam deposition causes apparent damage. For both thermal and e-beam evaporation, the energy of deposited particles is in the order of a fraction of an eV, determined by the Boltzmann distribution. This energy is too low to displace carbon atoms in the lattice, as evidenced in thermal deposition. Damage by local heating is unlikely, too, as heating effect for both deposition methods is similar. We also observed that the damage is independent of the stacking order, that is BN/graphene or graphene/BN, and
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Figure 2. Typical Raman spectra of graphene before and after deposition on bare and BN covered areas. (a) Thermal evaporation of SiO2. (b) E-beam evaporation of Al2O3. (c) ALD deposition of Al2O3.
more severe for BN/graphene than for bare graphene. All these observation are inconsistent with a local heating effect. Such a non-local effect suggests that penetrative radiation, i.e. X-ray and/or secondary electrons generated by e-beam hitting the target, is likely responsible for the damage. In fact, radiation damage to substrates in e-beam evaporation is well known. Moreover, not only e-beam radiation has been used to hydrogenate graphene,[34,35] but BN is susceptible to X-ray radiation in the presence of hydrogen.[36,37] Given abundant hydrogen coming from the transfer process, it is highly possible that with the assist of hydrogen and radiation, BN makes bonds to graphene, transforming some sp2 bonds to sp3-like, resulting in an enhancement of the D and D′ peak. Further study is needed to unveil the cause of the damage. Interestingly, the D′ peak intensity is slightly higher than that of the D peak, which has not been observed before. The D′ peak corresponds to an intravalley double resonance process. Little discussion has been made on this peak. Normally, the D′ peak is much weaker than the D peak. The only exception predicted by theory is that the D peak is not activated for zigzag edges while the D′ peak is.[29] But, we do not have a particular type of edges. Recently, a so-called R′ peak, also centered around 1620 cm−1 and due to an intravalley double resonance process, was found in twisted bilayer graphene superlattice.[38] Although it is unlikely that we have a superlattice here, the unusually strong D′ peak suggests that
the disorder is not typical disorder and is related to bonding between BN and graphene layers. We now turn to the charge doping effect of the deposition process by investigating the 2D peak. This peak has been used to infer the charging doping level of graphene.[30–33] As the doping level rises, the peak height is reduced and its position is blue-shifted. Figure 3(b) shows the intensity ratio of 2D/G. The ratio is not affected by thermal deposition. Similar observation can be made for e-beam deposition. But, the doping effect is strong in ALD deposition. The ratio I(2D)/I(G) decreases from 3.5 to 2.2 for bare graphene, accompanied by a 6 cm−1 blue-shift. The G peak is also shifted by 4 cm−1. From these numbers, we roughly estimate a doping level of 5 × 1012 cm−2. The doping likely comes from water, which is the oxidant precursors in our ALD and which is known to give rise to significant doping of graphene.[39,40] In comparison, the BN/graphene exhibits slightly higher doping level as the I(2D)/I(G) ratio goes from 3.5 to 1.6 and the G peak position is shifted by 10 cm−1. The reason is that water can diffuse into the interface between substrate and graphene.[41] The trapping effect of the BN film increases water absorption, hence the doping level, when the temperature is lowered from the ALD growth temperature. In ALD, one of the important advantages of the BN layer is that it provides a surface on which preprocesses can be performed to help growth of smooth dielectrics without
Figure 3. Comparison of the effect of the BN layer on graphene for three deposition methods. (a) I(D)/I(G) as an indication of lattice defect density. (b) I(2D)/I(G) as an indication of carrier doping. For each method, three samples were measured and the intensity ratios were the averages of three samples. small 2014, 10, No. 11, 2293–2299
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Figure 4. Surface morphology of Al2O3 deposited by ALD on bare graphene and BN/graphene. (a) Al2O3 on bare graphene exhibits formation of particles and the film is not continuous. (b) Al2O3 on BN/graphene displays smooth morphology. (c) Measurement of the thickness of the Al2O3 layer. The dark area is exposed BN film, which was covered by a Bi2Se3 nano-plate acting as a deposition mask. The nano-plate was removed after deposition, exposing the BN film. (d) A height profile taken from (a). The height variation is about ±10 nm. (e) A height profile taken from (b), where the height variation is only about ±0.5 nm. (f) A height profile taken from (c), which shows the thickness of the Al2O3 layer, 23 nm.
modifying graphene. It is found that uniform growth of top-gate dielectric, Al2O3, is free of charge trapping. To avoid Al2O3 can be achieved on BN/graphene even without any the back-gate hysteresis, the device was cooled down to 4 K preprocessing. Figure 4 shows the morphology of Al2O3 on and R as a function of the back-gate voltage VBG and the bare graphene and BN/graphene measured by atomic force top-gate voltage VTG is plotted in Figure 5. When VBG = 0 V, microscopy (AFM). On graphene, the dielectric layer is in a the Dirac point lies approximately at VTG = −0.7 V. As VBG is form of nanoparticles and not continuous. From the line pro- changed, the Dirac point voltage shifts linearly. The tunability file, flat regions that are 20 nm deep can be found, implying of the gate depends on the thickness and the dielectric conthat graphene in these regions is not covered by Al2O3 at stant εr of the gate dielectric. Knowing the thickness of the all. Two lines in the image are graphene pleats, where Al2O3 gate dielectrics, the dielectric constant of Al2O3 in our device nucleates. The rough and non-uniform dielectric layer agrees can be estimated by comparing the tunability of the backwith previous studies, which have shown that ALD growth and top-gates, i.e. the slope of the R maximum line in Figure of dielectrics on graphene took place mostly at defects and 5(a). Considering 285 nm SiO2 (εr = 3.9) for the back-gate, 3.3 pleats.[21,22] Special treatment for functionalization of gra- nm BN (εr = 4)/23 nm Al2O3 for the top-gate, we obtain εr = phene, e.g. NO2 or O3 treatment, is required for growth of 6.9 for Al2O3, consistent with previous study.[24] We have studied the effect of three deposition methods uniform dielectric films.[23–25] In sharp contrast, Al2O3 film on BN/graphene is very smooth. The root mean square rough- on BN/graphene. It would be also important to see how ness is only 0.71 nm, compared with 4.9 nm on graphene. The the BN layer holds up in other deposition methods where thickness of the deposited film is 23 nm, measured by AFM, deposited particles are more energetic, such as sputtering. as seen in Figure 4(c) and (f). Evidently, BN is a good buffer Figure 6 shows the Raman spectra of graphene and BN/ layer for ALD growth of dielectrics. The morphology of the dielectrics deposited by other methods is shown in the Supporting Information Figure S2. The dielectric property of the Al2O3 layer deposited by ALD is characterized by the field effect of dual-gated graphene devices. At 270 K, the resistance R of a graphene Hall bar exhibits significant hysteresis as the back-gate voltage VBG is swept up and down between −20 V and 20 V (See Supporting Information, Figure S3 and S4). This usually indicates charge trapping in the back-gate dielectric, SiO2, or in the interface between graphene and the Figure 5. Back-gate and top-gate field effects for a 2 µm by 6 µm dual-gated graphene Hall dielectric. Surprisingly, no apparent hys- bar. The top-gate dielectric is a 3.3 nm BN layer plus a 23 nm ALD deposited Al O layer. 2 3 teresis was observed when sweeping the (a) Resistance R of a graphene Hall bar as a function of VTG and VBG at T = 4 K. (b) Horizontal top-gate, which seems to suggest that the slice at VBG = 0 V, indicated by a dashed line in (a).
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Figure 6. The protection by BN layer during magnetron sputtering of SiO2. (a) Raman spectra for bare graphene and BN/graphene. The inset is a optical image of the test structure and the scalebar is 15 µm. The dashed rectangle indicates the area on which Raman mapping was performed. (b) (c) are the 2-dimentional mapping of the G peak and 2D peak, respectively.
graphene after radio frequency (RF) magnetron sputtering. The bare graphene film has experienced damages, which are so severe that the 2D peak completely disappears and the G peak becomes marginally discernible. Apparently, graphene is amorphized. For the graphene film under BN, although considerable damage occurs, all Raman features of graphene remain. In the 2-dimensional Raman mapping of Figure 6(b) and (c), we can see that the graphene structure is preserved despite bombardment of deposited particles. So the BN layer provides an essential protection for graphene underneath. It becomes clear that the effectiveness of BN film as a buffer layer for deposition of dielectrics on graphene depends on the characteristics of the deposition process. In our experimental condition, thermal deposition appears too gentle to induce damage to the graphene lattice, whether or not there is a BN buffer layer. In e-beam deposition, contrary to what is expected, the BN layer adds more lattice disorder and charge doping to graphene because of presence of hydrogen and radiation. Nevertheless, we want to point out that the effect can be largely avoided if better transfer technique is developed so as to leave little hydrogen. In ALD, deposition takes place via surface absorption, which is strongly non-uniform in graphene. BN layer however does not have this problem at all and uniform deposition can be achieved.[42] Moreover, functionalization of graphene is circumvented. Although we observed considerable charge doping due to interface diffusion of water, it is expected to be minimized if larger scale of BN film or other oxidant precursors are used, or the sides of the film are sealed. When the energy of deposited particles is high, as in sputtering deposition, direct damage of graphene by particle bombardment occurs. In this case, BN layer provide substantial protection by stopping the particle flux. The protection can be improved, for instance, by increasing the thickness of BN film. In a very recent study, it was shown that the particles small 2014, 10, No. 11, 2293–2299
that cause the damage are Argon ions, instead of deposited particles/clusters. This is because deposited particles experience much more collision events before they make their way to the substrate.[43] Therefore, by using a grazing-angle deposition configuration, damage to graphene can be significantly reduced. Combined with a BN buffer layer, the damage will be further reduced.
3. Conclusion We have investigated the effectiveness of the multilayer BN film as a buffer layer for four dielectric deposition methods. For physical vapor deposition, because low energy deposited particles impose little damage to graphene, the effect of the BN layer is mainly to act as a clean interface. In magnetron sputtering, the BN layer provides essential protection against energetic particles. In ALD, the BN layer works very well as a buffer layer for uniform growth of dielectrics and avoids functionalization of graphene. Electrical characterization suggests that the dielectric is free of charge trapping. Unfortunately, the BN layer traps dopants (water in this case) absorbed on graphene. Extra care needs to be taken to keep out dopants, such as using large area BN films, or sealing off the edges of the BN film. Over all, the BN buffer layer, is promising in dielectric deposition on graphene.
4. Experimental Section Growth and Transfer of Graphene and h-BN Films: Both of the graphene and hexagonal boron nitride films were synthesized via chemical vapor deposition on 25 µm thick Cu foils (Purchased from Alfa Aesar). Monolayer graphene was grown by this method with CH4 (35 sccm) and H2 (2 sccm) as the sources, at a pressure
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of 500 mTorr and at 1000 °C for 20 min. BN films of about 10 layers were grown by a low pressure CVD method with ammonia borane (NH3BH3, purchased from Sigma-Aldrich) as the precursor and 50 sccm Ar as the carrier gas, at a pressure of 110 Pa and at 90 °C for 10 min.[44] Graphene films were first transferred onto Si/SiO2 (285 nm) substrates, then BN films were transferred onto graphene. The detail of the transfer method can be found elsewhere.[45] Dielectric Layer Deposition: 20 nm Al2O3 were deposited onto graphene by e-beam evaporation and ALD. 20 nm SiO2 were deposited by thermal evaporation and RF sputtering. The deposition rate was 0.1 nm/s for both e-beam and thermal evaporation. RF sputtering was conducted using a Kurt J. Lesker system in 3 mTorr Ar with the substrate-target distance at 25–35 cm. ALD process was conducted using a Cambridge Nanotech Savannah S100 ALD system. Trimethylaluminum (TMA) and water were used as the precursors. 20 TMA pulses, each with an duration of 0.02 s, were initially applied to make sure BN films were fully covered by TMA. Then, 200 deposition cycles were performed. The substrate temperature was maintained at 350 °C during growth. Characterization: The Raman measurements were carried out using a Renishaw micro Raman spectrometer with a 514 nm wavelength laser. A low laser power of 5 mW was used. AFM topography was measured in a tapping mode using a SPI3800N AFM system. Dual-gated graphene Hall bar structures were fabricated using e-beam lithography. Electrical contacts were made of 5 nm Ti/80 nm Au. Electrical measurements were carried out in a He4 cryostat using a standard four-point method.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by National Key Basic Research Program of China (No. 2012CB933404, 2013CBA01603) and NSFC (project No. 11074007, 11222436, 11234001).
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Received: November 30, 2013 Revised: January 24, 2014 Published online: March 5, 2014
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Boron Nitride Film as a Buffer Layer in Deposition of Dielectrics on Graphene Qi Han, Baoming Yan, Teng Gao, Jie Meng, Yanfeng Zhang,* Zhongfan Liu, Xiaosong Wu,* and Dapeng Yu
As a two-dimensional material, graphene is highly susceptible to environmental influences. It is therefore challenging to deposit dielectrics on graphene without affecting its electronic properties. It is demonstrated that the effect of the dielectric deposition on graphene can be reduced by using a multilayer hexagonal boron nitride film as a buffer layer. Particularly, the boron nitride layer provides significant protection in magnetron sputtering deposition. It also enables growth of uniform and charge trapping free high-k dielectrics by atomic layer deposition. The doping effect of various deposition methods on graphene has been discussed.
1. Introduction The graphene fever started with the invention of the mechanical exfoliation method, as it has made graphene easily accessible to researchers.[1] By exfoliating graphene flakes on SiO2/Si substrates, it offers two additional benefits. One is that monolayer graphene can be conveniently identified using an optical microscope.[2] The other is that the carrier density of graphene can be tuned by the silicon back-gate. All these benefits greatly contribute to the research of this field, which has led to predictions of various applications. However, a few drawbacks of the method emerge when it comes to high carrier mobility and mass production compatibility. First, SiO2 is not a good surface to interface with graphene, as
Q. Han, B. M. Yan, J. Meng, Prof. X. S. Wu, Prof. D. P. Yu State Key Laboratory for Artificial Microstructure and Mesoscopic Physics Peking University Beijing 100871, China E-mail: [email protected] Q. Han, B. M. Yan, J. Meng, Prof. X. S. Wu, Prof. D. P. Yu Collaborative Innovation Center of Quantum Matter Beijing 100871, China T. Gao, Prof. Y. F. Zhang, Prof. Z. F. Liu College of Chemical and Molecular Engineering Peking University Beijing 100871, China E-mail: [email protected] DOI: 10.1002/smll.201303697 small 2014, 10, No. 11, 2293–2299
it introduces charge inhomogeneity and scattering, degrading the mobility.[3–6] Second, the silicon substrate back-gate is not compatible with integrated circuit technology because it cannot individually tune each device like a top-gate. On the other hand, a top-gate that does not reduce the quality of graphene is becoming increasingly important in graphene research, e.g., demonstration of Klein tunneling,[7,8] band gap tuning of bilayer graphene.[9] The key to these problems is to find a smooth and inert dielectric material that does not introduce many impurities to graphene. In this regard, hexagonal boron nitride (h-BN) has been proven to be so far the best substrate for graphene.[10–13] Naturally, it is an ideal topgate dielectric material, too.[14] Considerable efforts have been paid to search for methods of depositing dielectrics on graphene while maintaining its high quality.[15] Suspended top-gate by dedicated microfabrication has been attempted.[16–18] Over-exposed poly (methyl methacrylate) (PMMA) and hydrogen silsesquioxane (HSQ) have been tried.[7,8,19] Sputtering, having the advantage of producing dense and conformal film and being widely used in industry, was employed to deposit Al2O3 and MgO.[20] But, severe damage to graphene by energetic particles was found. Atomic layer deposition (ALD) is believed to be a superior technique in ultra-thin, uniform high-k dielectric coating. However, it turned out to be difficult to grow uniform dielectric layer on clean graphene, because growth only occurs at defects in graphene.[21,22] Although functionalization of graphene was demonstrated to be a solution for non-uniform growth, it also modifies graphene.[23–26] If
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h-BN is used as a buffer layer for dielectric deposition on graphene, it not only provides a clean interface to graphene, but also protects graphene from damage during deposition. As a result, a variety of deposition techniques may be chosen depending on particular applications. In this work, we demonstrate the use of multilayer hexagonal BN films as a buffer layer for dielectric deposition on graphene. Both the graphene and BN films were grown by chemical vapor deposition (CVD), as they can be grown in large area, which is crucial for future mass production. We have studied the damage and charge doping caused by deposition in the presence of a BN buffer layer for four common deposition techniques, i.e. thermal evaporation, e-beam evaporation, magnetron sputtering and ALD. It has been found that the BN film provides prominent protection for graphene in magnetron sputtering. For ALD, the film offers a surface for growth of uniform dielectrics.
2. Results and Discussion Figure 1(a) shows a schematic of our test structure for investigating the effect of dielectric deposition on graphene, while Figure 1(b) is the optical image of the structure. The detail of the device fabrication is described in the Experimental Section. The graphene film is partially covered by a few-layer BN film. The morphology of the BN film can be found in the Supporting Information Figure S1. Dielectrics are deposited on both surfaces. We first check the effect of deposition of a BN layer. Raman spectra were collected for both the exposed and BN covered areas, shown in
Figure 1. Deposition of multilayer BN film on graphene. (a) Schematic of the test structure for studying the effect of multilayer BN film on graphene. (b) Optical image of a graphene film (top-left) partially covered by a multilayer BN film (bottom-right). (c) Raman spectra for both the bare graphene area and the BN covered graphene area.
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Figure 1(c). Three well defined peaks can be seen, known as D, G and 2D peaks. The D peak is due to the breathing modes of sp2 atoms in rings and is related to lattice disorder in graphene.[27–29] Its height, relative to the G peak, is often used as an measure of the defect density. The G peak is an in-plane C–C bond stretching mode and present in all sp2 carbon system, while the 2D peak is the second order of the D peak. The 2D peak height is associated with the doping level of graphene. It can be seen that two spectra are almost identical. So, deposition of BN does not introduce appreciable defects or doping. Having known that the BN over layer has little effect on graphene, we can directly investigate the effect of dielectric deposition on graphene by comparing the bare graphene area and the BN covered area after deposition. Nominal 20 nm dielectrics were deposited by thermal evaporation, e-beam evaporation and ALD. The Raman spectra of graphene, dielectrics/graphene and dielectrics/BN/graphene are shown in Figure 2. For the thermal evaporation, the spectrum remains almost unchanged, with an exception that the D peak might be slightly enhanced after deposition for both bare graphene and BN/graphene. Overall, it seems, under our deposition condition, that thermal deposition is quite gentle and has little impact on graphene. The situation changed as the Al2O3 layer was deposited by e-beam evaporation. The bare graphene after deposition exhibits similar spectrum as the one after thermal deposition. However, the BN/graphene undergoes a few changes. Firstly, the 2D peak is significantly reduced. Secondly, a new feature at 1620 cm−1, known as the D′ peak, sets in. Lastly, the D peak is also broadened. It appears that the BN/graphene is less protected from the deposition process than bare graphene. This may look counterintuitive, but we will explain it later. Figure 2(c) displays the spectrum before and after ALD deposition of Al2O3. No change has been observed in the D peak. On the other hand, the 2D peak height is reduced for both BN/graphene and bare graphene. Besides, the peak positions of both the 2D and G peak are blue-shifted. All suggest that graphene is doped.[30–33] For the 2D peak for BN/graphene is more suppressed, the doping level is even higher even if it is supposed to be less affected by the deposition process. To compare the effect of the three deposition processes on graphene, we compile the evolution of the D peak and 2D peak intensity in Figure 3. The ratio of the D peak to G peak intensity, indication of structural defect density, is plotted in Figure 3(a). It is obvious that thermal deposition introduces little damage to the graphene lattice. Similar observation is made for ALD, because no functionalization of graphene has been carried out to improve the uniformity of the dielectric film, consistent with previous studies.[23–25] Among three methods only e-beam deposition causes apparent damage. For both thermal and e-beam evaporation, the energy of deposited particles is in the order of a fraction of an eV, determined by the Boltzmann distribution. This energy is too low to displace carbon atoms in the lattice, as evidenced in thermal deposition. Damage by local heating is unlikely, too, as heating effect for both deposition methods is similar. We also observed that the damage is independent of the stacking order, that is BN/graphene or graphene/BN, and
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Boron Nitride Film as a Buffer Layer in Deposition of Dielectrics on Graphene
Figure 2. Typical Raman spectra of graphene before and after deposition on bare and BN covered areas. (a) Thermal evaporation of SiO2. (b) E-beam evaporation of Al2O3. (c) ALD deposition of Al2O3.
more severe for BN/graphene than for bare graphene. All these observation are inconsistent with a local heating effect. Such a non-local effect suggests that penetrative radiation, i.e. X-ray and/or secondary electrons generated by e-beam hitting the target, is likely responsible for the damage. In fact, radiation damage to substrates in e-beam evaporation is well known. Moreover, not only e-beam radiation has been used to hydrogenate graphene,[34,35] but BN is susceptible to X-ray radiation in the presence of hydrogen.[36,37] Given abundant hydrogen coming from the transfer process, it is highly possible that with the assist of hydrogen and radiation, BN makes bonds to graphene, transforming some sp2 bonds to sp3-like, resulting in an enhancement of the D and D′ peak. Further study is needed to unveil the cause of the damage. Interestingly, the D′ peak intensity is slightly higher than that of the D peak, which has not been observed before. The D′ peak corresponds to an intravalley double resonance process. Little discussion has been made on this peak. Normally, the D′ peak is much weaker than the D peak. The only exception predicted by theory is that the D peak is not activated for zigzag edges while the D′ peak is.[29] But, we do not have a particular type of edges. Recently, a so-called R′ peak, also centered around 1620 cm−1 and due to an intravalley double resonance process, was found in twisted bilayer graphene superlattice.[38] Although it is unlikely that we have a superlattice here, the unusually strong D′ peak suggests that
the disorder is not typical disorder and is related to bonding between BN and graphene layers. We now turn to the charge doping effect of the deposition process by investigating the 2D peak. This peak has been used to infer the charging doping level of graphene.[30–33] As the doping level rises, the peak height is reduced and its position is blue-shifted. Figure 3(b) shows the intensity ratio of 2D/G. The ratio is not affected by thermal deposition. Similar observation can be made for e-beam deposition. But, the doping effect is strong in ALD deposition. The ratio I(2D)/I(G) decreases from 3.5 to 2.2 for bare graphene, accompanied by a 6 cm−1 blue-shift. The G peak is also shifted by 4 cm−1. From these numbers, we roughly estimate a doping level of 5 × 1012 cm−2. The doping likely comes from water, which is the oxidant precursors in our ALD and which is known to give rise to significant doping of graphene.[39,40] In comparison, the BN/graphene exhibits slightly higher doping level as the I(2D)/I(G) ratio goes from 3.5 to 1.6 and the G peak position is shifted by 10 cm−1. The reason is that water can diffuse into the interface between substrate and graphene.[41] The trapping effect of the BN film increases water absorption, hence the doping level, when the temperature is lowered from the ALD growth temperature. In ALD, one of the important advantages of the BN layer is that it provides a surface on which preprocesses can be performed to help growth of smooth dielectrics without
Figure 3. Comparison of the effect of the BN layer on graphene for three deposition methods. (a) I(D)/I(G) as an indication of lattice defect density. (b) I(2D)/I(G) as an indication of carrier doping. For each method, three samples were measured and the intensity ratios were the averages of three samples. small 2014, 10, No. 11, 2293–2299
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Figure 4. Surface morphology of Al2O3 deposited by ALD on bare graphene and BN/graphene. (a) Al2O3 on bare graphene exhibits formation of particles and the film is not continuous. (b) Al2O3 on BN/graphene displays smooth morphology. (c) Measurement of the thickness of the Al2O3 layer. The dark area is exposed BN film, which was covered by a Bi2Se3 nano-plate acting as a deposition mask. The nano-plate was removed after deposition, exposing the BN film. (d) A height profile taken from (a). The height variation is about ±10 nm. (e) A height profile taken from (b), where the height variation is only about ±0.5 nm. (f) A height profile taken from (c), which shows the thickness of the Al2O3 layer, 23 nm.
modifying graphene. It is found that uniform growth of top-gate dielectric, Al2O3, is free of charge trapping. To avoid Al2O3 can be achieved on BN/graphene even without any the back-gate hysteresis, the device was cooled down to 4 K preprocessing. Figure 4 shows the morphology of Al2O3 on and R as a function of the back-gate voltage VBG and the bare graphene and BN/graphene measured by atomic force top-gate voltage VTG is plotted in Figure 5. When VBG = 0 V, microscopy (AFM). On graphene, the dielectric layer is in a the Dirac point lies approximately at VTG = −0.7 V. As VBG is form of nanoparticles and not continuous. From the line pro- changed, the Dirac point voltage shifts linearly. The tunability file, flat regions that are 20 nm deep can be found, implying of the gate depends on the thickness and the dielectric conthat graphene in these regions is not covered by Al2O3 at stant εr of the gate dielectric. Knowing the thickness of the all. Two lines in the image are graphene pleats, where Al2O3 gate dielectrics, the dielectric constant of Al2O3 in our device nucleates. The rough and non-uniform dielectric layer agrees can be estimated by comparing the tunability of the backwith previous studies, which have shown that ALD growth and top-gates, i.e. the slope of the R maximum line in Figure of dielectrics on graphene took place mostly at defects and 5(a). Considering 285 nm SiO2 (εr = 3.9) for the back-gate, 3.3 pleats.[21,22] Special treatment for functionalization of gra- nm BN (εr = 4)/23 nm Al2O3 for the top-gate, we obtain εr = phene, e.g. NO2 or O3 treatment, is required for growth of 6.9 for Al2O3, consistent with previous study.[24] We have studied the effect of three deposition methods uniform dielectric films.[23–25] In sharp contrast, Al2O3 film on BN/graphene is very smooth. The root mean square rough- on BN/graphene. It would be also important to see how ness is only 0.71 nm, compared with 4.9 nm on graphene. The the BN layer holds up in other deposition methods where thickness of the deposited film is 23 nm, measured by AFM, deposited particles are more energetic, such as sputtering. as seen in Figure 4(c) and (f). Evidently, BN is a good buffer Figure 6 shows the Raman spectra of graphene and BN/ layer for ALD growth of dielectrics. The morphology of the dielectrics deposited by other methods is shown in the Supporting Information Figure S2. The dielectric property of the Al2O3 layer deposited by ALD is characterized by the field effect of dual-gated graphene devices. At 270 K, the resistance R of a graphene Hall bar exhibits significant hysteresis as the back-gate voltage VBG is swept up and down between −20 V and 20 V (See Supporting Information, Figure S3 and S4). This usually indicates charge trapping in the back-gate dielectric, SiO2, or in the interface between graphene and the Figure 5. Back-gate and top-gate field effects for a 2 µm by 6 µm dual-gated graphene Hall dielectric. Surprisingly, no apparent hys- bar. The top-gate dielectric is a 3.3 nm BN layer plus a 23 nm ALD deposited Al O layer. 2 3 teresis was observed when sweeping the (a) Resistance R of a graphene Hall bar as a function of VTG and VBG at T = 4 K. (b) Horizontal top-gate, which seems to suggest that the slice at VBG = 0 V, indicated by a dashed line in (a).
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Figure 6. The protection by BN layer during magnetron sputtering of SiO2. (a) Raman spectra for bare graphene and BN/graphene. The inset is a optical image of the test structure and the scalebar is 15 µm. The dashed rectangle indicates the area on which Raman mapping was performed. (b) (c) are the 2-dimentional mapping of the G peak and 2D peak, respectively.
graphene after radio frequency (RF) magnetron sputtering. The bare graphene film has experienced damages, which are so severe that the 2D peak completely disappears and the G peak becomes marginally discernible. Apparently, graphene is amorphized. For the graphene film under BN, although considerable damage occurs, all Raman features of graphene remain. In the 2-dimensional Raman mapping of Figure 6(b) and (c), we can see that the graphene structure is preserved despite bombardment of deposited particles. So the BN layer provides an essential protection for graphene underneath. It becomes clear that the effectiveness of BN film as a buffer layer for deposition of dielectrics on graphene depends on the characteristics of the deposition process. In our experimental condition, thermal deposition appears too gentle to induce damage to the graphene lattice, whether or not there is a BN buffer layer. In e-beam deposition, contrary to what is expected, the BN layer adds more lattice disorder and charge doping to graphene because of presence of hydrogen and radiation. Nevertheless, we want to point out that the effect can be largely avoided if better transfer technique is developed so as to leave little hydrogen. In ALD, deposition takes place via surface absorption, which is strongly non-uniform in graphene. BN layer however does not have this problem at all and uniform deposition can be achieved.[42] Moreover, functionalization of graphene is circumvented. Although we observed considerable charge doping due to interface diffusion of water, it is expected to be minimized if larger scale of BN film or other oxidant precursors are used, or the sides of the film are sealed. When the energy of deposited particles is high, as in sputtering deposition, direct damage of graphene by particle bombardment occurs. In this case, BN layer provide substantial protection by stopping the particle flux. The protection can be improved, for instance, by increasing the thickness of BN film. In a very recent study, it was shown that the particles small 2014, 10, No. 11, 2293–2299
that cause the damage are Argon ions, instead of deposited particles/clusters. This is because deposited particles experience much more collision events before they make their way to the substrate.[43] Therefore, by using a grazing-angle deposition configuration, damage to graphene can be significantly reduced. Combined with a BN buffer layer, the damage will be further reduced.
3. Conclusion We have investigated the effectiveness of the multilayer BN film as a buffer layer for four dielectric deposition methods. For physical vapor deposition, because low energy deposited particles impose little damage to graphene, the effect of the BN layer is mainly to act as a clean interface. In magnetron sputtering, the BN layer provides essential protection against energetic particles. In ALD, the BN layer works very well as a buffer layer for uniform growth of dielectrics and avoids functionalization of graphene. Electrical characterization suggests that the dielectric is free of charge trapping. Unfortunately, the BN layer traps dopants (water in this case) absorbed on graphene. Extra care needs to be taken to keep out dopants, such as using large area BN films, or sealing off the edges of the BN film. Over all, the BN buffer layer, is promising in dielectric deposition on graphene.
4. Experimental Section Growth and Transfer of Graphene and h-BN Films: Both of the graphene and hexagonal boron nitride films were synthesized via chemical vapor deposition on 25 µm thick Cu foils (Purchased from Alfa Aesar). Monolayer graphene was grown by this method with CH4 (35 sccm) and H2 (2 sccm) as the sources, at a pressure
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of 500 mTorr and at 1000 °C for 20 min. BN films of about 10 layers were grown by a low pressure CVD method with ammonia borane (NH3BH3, purchased from Sigma-Aldrich) as the precursor and 50 sccm Ar as the carrier gas, at a pressure of 110 Pa and at 90 °C for 10 min.[44] Graphene films were first transferred onto Si/SiO2 (285 nm) substrates, then BN films were transferred onto graphene. The detail of the transfer method can be found elsewhere.[45] Dielectric Layer Deposition: 20 nm Al2O3 were deposited onto graphene by e-beam evaporation and ALD. 20 nm SiO2 were deposited by thermal evaporation and RF sputtering. The deposition rate was 0.1 nm/s for both e-beam and thermal evaporation. RF sputtering was conducted using a Kurt J. Lesker system in 3 mTorr Ar with the substrate-target distance at 25–35 cm. ALD process was conducted using a Cambridge Nanotech Savannah S100 ALD system. Trimethylaluminum (TMA) and water were used as the precursors. 20 TMA pulses, each with an duration of 0.02 s, were initially applied to make sure BN films were fully covered by TMA. Then, 200 deposition cycles were performed. The substrate temperature was maintained at 350 °C during growth. Characterization: The Raman measurements were carried out using a Renishaw micro Raman spectrometer with a 514 nm wavelength laser. A low laser power of 5 mW was used. AFM topography was measured in a tapping mode using a SPI3800N AFM system. Dual-gated graphene Hall bar structures were fabricated using e-beam lithography. Electrical contacts were made of 5 nm Ti/80 nm Au. Electrical measurements were carried out in a He4 cryostat using a standard four-point method.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by National Key Basic Research Program of China (No. 2012CB933404, 2013CBA01603) and NSFC (project No. 11074007, 11222436, 11234001).
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Received: November 30, 2013 Revised: January 24, 2014 Published online: March 5, 2014
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