Ultrathin Chemical Vapor Deposition (CVD)-Grown Hexagonal Boron

Article pubs.acs.org/JPCC

Ultrathin Chemical Vapor Deposition (CVD)-Grown Hexagonal Boron Nitride as a High-Quality Dielectric for Tunneling Devices on Rigid and Flexible Substrates Carlo M. Orofeo, Satoru Suzuki, and Hiroki Hibino* NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan S Supporting Information *

ABSTRACT: We investigate the tunneling properties of large-area monolayer hexagonal boron nitride (BN) grown via chemical vapor deposition (CVD) by fabricating metal/BN/metal devices on rigid and flexible substrates and compare the properties to metal/exfoliated BN/graphite devices. The measured current of the tunneling devices sandwiched by metal electrodes is linear around zero bias and increases exponentially at higher biases, a behavior consistent with direct tunneling. We also investigate the effect of PMMA contamination on the tunneling current by comparing the zero-bias resistances of the BN devices that have undergone PMMA cleaning by acetone and by heat treatment. Annealing under Ar/O2 at 500 °C proves to be the best heat treatment for removing the PMMA contaminants introduced during BN transfer, though extra care must be given because this condition can also roughen the bottom electrodes. Further, from tunneling theory, we estimate the barrier height for tunneling to be ∼2.5 eV, and the dielectric strength to be 3.78 ± 0.83 GV m−1, which are comparable to those of exfoliated monolayer BN. Our results demonstrate that CVD-grown BN can be a perfect alternative to exfoliated BN for tunneling applications, such as vertical transistors and spintronics, with an advantage of being available in a large area.

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thickness. However, for large-scale applications, large area high-quality BN is needed, and so far, very few studies have investigated the electronic properties of chemical vapor deposited BN for tunneling applications. Here, we show that monolayer BN grown by the chemical vapor deposition (CVD) method can be a perfect alternative to exfoliated BN as a highquality dielectric for tunneling applications such as vertical transistors and spintronics, with the advantage of its being available in a large area. The current−voltage (I−V) characteristics of the devices follow the quantum tunneling behavior (e.g., linear at low voltages and increases exponentially at higher biases) with the current scaling correctly with the effective device area, similar to exfoliated BN. From the I−V data, the barrier height and dielectric strength are calculated and compared to the values obtained from exfoliated BN, following the formula for quantum tunneling developed by Simmons.11 We also present an effective recipe for removing PMMA contaminants introduced on the BN surface during transfer.

raphene has proven to be a leading candidate for postsilicon electronics, primarily because graphenebased devices have extremely high mobilities, which are several orders of magnitude higher than Si-based devices. However, graphene-based devices cannot be directly used as logic devices because of a very low on−off ratio caused by the absence of a band gap between its conduction and valence bands.1 Attempts to open a band gap in graphene include “trimming” graphene into nanoribbons,2,3 application of electric fields in bilayer graphene,4,5 chemical doping in a monolayer,6 or breaking of the structural geometry via stacking graphene with h-BN.7 In a variation to pure graphene-based devices, the creation of graphene-based heterostructures, aimed to circumvent the low on−off ratio of pure graphene-based transistors, has recently emerged as a subject of intense research.8,9 Specifically, the operation of the so-called “vertical tunneling field-effect transistors” has been successfully demonstrated using graphene stacked vertically on boron nitride (BN), which increases the on−off ratio by as much as 5 times at room temperature.9 BN, a wide band gap (6 eV) insulator, is a critical material for this type of configuration because the operation of this type of device is dependent on tuning the density of states in graphene and on the effective barrier height of BN. Furthermore, it was recently demonstrated that monolayer BN can be used as a tunneling barrier for spin injection into graphene−BN heterostructures.10 For the demonstrated devices, exfoliated BN proves to be a high-quality dielectric, sufficient as an effective tunneling barrier even at a single atomic layer © 2014 American Chemical Society

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EXPERIMENTAL METHODS The growth of large-area monolayer BN on heteroepitaxial Co film by CVD using ammonia−borane as a precursor gas was reported by our group recently.12 Prior to device fabrication, Received: November 5, 2013 Revised: January 17, 2014 Published: January 18, 2014 3340

dx.doi.org/10.1021/jp410874z | J. Phys. Chem. C 2014, 118, 3340−3346

The Journal of Physical Chemistry C

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Figure 1. Steps in fabricating a BN-based tunnel device. (A) Scheme of the device fabrication. (B) LEEM image and reflectivity curves of the grown BN. From the reflectivity curve, the number of BN layer can be determined (see discussion in the main text). Some bilayer BN, which appears in patches, is also detected as labeled. The black spot in the upper left corner corresponds to the substrate. (C) AFM image with the corresponding (D) height profile showing the boundary between BN and SiO2. The AFM image in (C) was taken from one of the assembled tunneling devices as shown in (E).

the presence of BN after CVD growth was confirmed by UV− vis (JASCO V-650) absorption spectra (Figure S1 of Supporting Information), and the number of layers was determined by low-energy electron microscopy (LEEM, Elmitec LEEM III). This procedure was routinely done to monitor the presence of BN, as monolayer BN transferred on SiO2/Si is almost optically transparent under a microscope and can hardly be seen.12−14 To create tunneling devices, the grown BN was transferred onto suitable substrates. Figure 1A illustrates the process for creating a tunneling device based on CVD-grown BN on SiO2/Si. After spin coating with poly(methyl methacrylate) (PMMA, MicroChem Corp.) in anisole solution (50 vol %), the CVD-grown BN was detached from the surface of Co film by wet chemical etching of the metal film using an aqueous solution (5%) of HNO3, a standard method used to transfer graphene and BN on arbitrary substrates ((i) and (ii) of Figure 1A).12,14 After several washes with DI water (at least five times), the PMMA/BN is transferred onto SiO2 (285 nm)/Si prepatterned with metallic contacts (5 nm Cr/20 nm Au) that served as the bottom electrode, by using the SiO2 to scoop the floating BN/PMMA out of the DI water. The bottom electrodes are defined by standard electron beam (EB) lithography and were designed to have different widths to study the dielectric area dependence of the tunneling current. To remove the water between the BN and SiO2/Si, the substrate is tilted for several minutes right after transferring, or a piece of semiconductor-grade clean wipe is used to carefully remove the water on the substrate via capillary action. The newly transferred BN is then heated at 100 °C for 30 min, to enhance adhesion and evaporate any remaining water, before the PMMA was removed by dipping in acetone for several hours. However, we have found that dipping in acetone alone cannot fully remove the PMMA no matter how long the substrate is immersed in it. Thus, several heat treatments are performed on the transferred BN to clean the

BN surface as detailed below. Next, a top electrical contact is established by evaporating Cr/Au (5 nm/30 nm) over line openings of different widths (from 0.5 to 2.0 μm), which are defined by EB lithography and positioned perpendicular to the bottom electrodes ((iii) of Figure 1A). The result is a vertical tunneling device of Au/BN/Au with different areas ranging from 0.25 to 8 μm2. As a final step, contact pads are formed by depositing a 5 nm Cr/200 nm Au at the ends of the top and bottom electrodes. Because the BN covers the whole surface of the bottom electrode, photolithography followed by reactive ion etching (RIE) is used to expose a portion of the bottom electrode before the contact pads are deposited. Figure 1E shows a representative image of our device. All I−V measurements were done using an Agilent Parameter Analyzer (4156C) over a range of temperatures (50−300 K).

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RESULTS AND DISCUSSION The number of layers of the as-grown BN films was determined using LEEM. Figure 1B shows a LEEM image and the corresponding reflectivity curve of our grown BN film. The reflectivity curve was taken from the portion indicated by a black square. As previously reported, the oscillations from the LEEM signal can be used to identify the number of grown BN layers with the formula n = d + 1, where n is the number of layers and d is the number of dips in the 0−4.5 eV regime.12 The features in the LEEM reflectivity measurement shown in Figure 1B point to a BN film with monolayer thickness. Shown in Figure 1E is an example of the assembled devices used in this experiment. As seen, the transferred BN appears very transparent on SiO2 and can hardly be seen with an optical microscope. To verify the presence of the film, atomic force microscopy (AFM, Bruker FastScan) was performed at the boundary of the BN and substrate, which clearly shows the BN laid out continuously on the surface. Closer inspection of the AFM image reveals that the BN surface is rough and patchy in 3341



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vacuum (at ∼2.3 Torr) is found to reduce the height of the measured film from 1.3 to ∼0.8 nm. The RMS value also decreases from ∼0.9 nm for the untreated sample to ∼0.6 nm for the BN annealed in Ar/H2 at 350 °C, signifying a cleaner surface (Figure 2B). Although this heat treatment generally eliminates the PMMA residue, a closer investigation in large scale showed that PMMA contamination, manifested as sparse and discontinuous, is still present on our BN surface, leaving some areas clean, and others slightly contaminated (Figure S2B of Supporting Information). These observations provide evidence that heat treatment at 350 °C can largely eliminate the PMMA residue but cannot fully eliminate it. Further, increasing the temperature to 500 °C under the same atmosphere has no effect on the remaining PMMA residues (Figure 2C). These results are consistent with a similar study done on exfoliated BN.17 A more effective heat treatment recipe, as systematically investigated by that group, involves annealing in Ar/O2. To explore this, we anneal our samples at varying concentrations of O2 in Ar at different temperatures (350 and 500 °C). Our results showed that 1% O2 content in an Ar/O2 (500 sccm/5 sccm) environment at 500 °C is enough to fully eliminate the PMMA contamination, as evidenced from our AFM measurements and visual inspection shown in Figure 2D. As can be seen, the height further decreased to ∼0.4 nm after annealing in Ar/O2, consistent with a monolayer BN. The RMS value also improves to ∼0.3 nm. One major drawback, however, is that at this annealing condition, the bottom contact roughened, leaving some areas with deep pits (Figure S2G-I of Supporting Information). Note that our bottom electrodes are deposited prior to BN transfer and annealing; therefore, any contact degradation can significantly affect the performance of the tunneling device. Annealing at lower temperature (350 °C) in the same Ar/O2 environment is not as effective as annealing at 500 °C, as evidenced in the AFM scans in Figure S2D-F (Supporting Information). On the basis of these results, it appears that annealing at 350 °C in either Ar/H2 or Ar/O2 can break down the PMMA but not fully eliminate it, and that the effects of these treatments are plainly thermal. In contrast, the gaseous environment of Ar/O2 when annealed at 500 °C

some areas (Figure 1C and Figure S2 of Supporting Information), which is contrary to what is expected for a monolayer BN.15,16 The rough and patchy areas are consistent with PMMA contamination, and this led us to explore the possibility of cleaning the BN surface by heat treatment, similar to the procedure done for cleaning graphene. Note that devices made from BN cleaned only with acetone have tunneling currents that are orders of magnitude lower than those of devices made of BN that have undergone heat treatment (Figure 3C). Table 1 summarizes the different heating Table 1. Annealing Conditions Used To Clean the Polymethylmethacrylate (PMMA) on the Transferred BN condition

temperature (°C)

PMMA removal

bottom contact

Ar/H2(100/50) Ar/H2(100/50) Ar/O2(500/50) Ar/O2(500/5) Ar/O2(500/5)

350 500 500 350 500

no no yes no yes

intact destroyed destroyed intact destroyed

conditions used in this experiment. The heating conditions are based on recipes used by other groups to complete graphene devices on BN or SiO2 and for exfoliated BN.17−19 The cleanness of the BN film is assessed on the basis of the AFM topographical features, namely, the BN thickness and the root-mean square (RMS) roughness of the surface, and by visual inspection. To gauge the effectiveness of the heat treatment, we first examined the untreated sample (i.e., only cleaned by acetone) through AFM and remeasured the same sample after heat treatment. The evolution of the BN topographical features before and after heat treatment is shown in Figure 2. The measured AFM height for the untreated sample is ∼1.3 nm. We found that the level of PMMA contamination depends where it is measured, and the measured height is subjective, making it prone to error. To this end, we made sure that our AFM scans before and after heat treatments are done on the same area, and that the height profiles are measured as close as possible to the same location. The heat treatment in Ar/H2 at 350 °C in

Figure 2. Effect of the various heat treatments on the PMMA residue. (A) AFM image with the corresponding height profile and RMS value of the transferred BN after using acetone to remove the PMMA. The BN and SiO2 surfaces are labeled. (B)−(D) AFM images with their corresponding height profiles and RMS roughness values of the same transferred BN after undergoing different heating conditions. The height profiles and the RMS roughness values were taken at the same area to monitor the effectiveness of the heat treatments. The RMS roughness measurement area: 1 μm × 1 μm. 3342



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Figure 3. Tunneling characteristic of the CVD-grown monolayer BN film. (A) Characteristic I−V curves at different device areas for monolayer BN sandwiched between two metallic electrodes. At a bias voltage around 0 V, the current is linear, as shown in the zoomed-in image in the inset of (B). (B) Zero-bias resistance, calculated from the linear part of the I−V curves, of several devices. The resistance is normalized to the area, which ranged from 0.25 to 8.0 μm2. (C) Comparison of the zero-bias resistances of the devices assembled from the BN film cleaned by acetone only and for those devices that have undergone annealing under Ar/H2 at 350 °C. The zero-bias resistance for exfoliated BN (ref 15) was also shown for comparison. (D) Temperature dependence of the tunneling current showing little change, as expected for BN having a barrier height much higher than the thermal energy. Device area: 3 μm2.

and can be understood using the well-established quantumtunneling model. When bias voltage Vb is applied between two metallic electrodes separated by a thin insulating film, a current can flow through the insulating film if (i) the barrier is thin enough to permit electron penetration by the electric tunnel effect and (ii) the electrons in the electrodes have sufficient thermal energy to surmount the potential barrier created by the insulating film.11 Note that in this configuration, the equilibrium condition positions the Fermi level of the metallic electrodes below the potential barrier ((i) of Figure 3A). The current flow across the barrier in the x-direction is given by11

proved to involve a chemical reaction with PMMA, consistent with the earlier report.17 Unfortunately, the high annealing temperature also leads to bottom contact degradation, presumably due to the diffusion of the bottom Au electrode onto the silicon substrate.20 The annealing in Ar/O2 provides the cleanest BN, but it cannot be used for devices with bottom electrodes, such as tunneling devices. Thus, for the duration of our experiment, we annealed our samples under Ar/H2 at 350 °C. Moreover, the average value of the measured tunneling current of the untreated and treated BN samples correlates well to the measured AFM height. The treated BN samples have an average tunneling current of about an order of magnitude higher than the untreated samples, which correspondingly results in the reduction of the zero-bias resistance by the same order of magnitude (Figure 3C). Figure 3A shows I−V curves of devices with different device areas. For most of our devices (50 in total), the magnitude of the current scales correctly with device area. However, for some devices, the measured current is not consistent with the expected device area (see inset of Figure 3B). This nonscalability leads to a wider spread in the normalized current readings. Several possible causes of the variation were identified as discussed below. As shown in Figure 3A, the I−V curves are linear around zero bias and increase exponentially at higher biases. This behavior is a characteristic of quantum tunneling

J∝

∫0

Em

T (Ex) dEx

∫0

∞

[f (E) − f (E + eV )] dEr

(1)

where J is the current normalized to the tunneling area (current density), T(E) is the transmission probability of the electrons across the barrier film, and f(E) is the Fermi function of the metallic electrodes. Em is the maximum energy of the electrons in the electrode, and Ex and Er are the energy components of the incident electrons in the directions normal and parallel to the interface, respectively. The Fermi function has the form f(E) = 1/(exp[(E − μ)/(kbT)] + 1), where μ is the chemical potential and kb is Boltzmann’s constant. In this work, analysis of the I−V characteristics can be done in two regimes: around the zero bias (Vb ≈ 0 V) regime, where the barrier is not deformed [(i) of Figure 3A], and at low-bias (0.1 V < |Vb| ≤ 1 3343



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V), where the barrier is slightly deformed by a magnitude of eVb [(ii) of Figure 3A]. In the zero-bias regime, the tunneling current density just depends linearly on V and can be described as11 J (V ) =

e 2 2mϕ V 2

hd

⎤ ⎡ ⎛ 4πd ⎞ ⎟ 2mϕ ⎥ exp⎢ −⎜ ⎦ ⎣ ⎝ h ⎠

use a shadow mask to thermally deposit the top electrodes to ensure a cleaner interface. And last, some of the devices may have a BN thickness of more than a monolayer. As seen from our LEEM and AFM measurements, bilayer BN is grown in patches, with sizes of less than 1 μm, across the BN sheet (Figure S2H, Supporting Information). Devices made of bilayer BN can have resistance an order of magnitude higher than that of devices made from monolayer BN.15,16 In addition, we measured the temperature dependence of the tunneling current of our devices. Figure 3D shows the I−V characteristics of one of the devices, measured over a range of temperatures. The zero-bias resistance changed little, as expected for BN whose barrier height is much greater than the thermal energy.8,15,16 We estimate the barrier height of our BN by calculating the zero-bias resistance as a function of BN thickness from eq 2 and superimposing our measured zero-bias resistances at its measured BN thickness (Figure 4A). The estimated barrier height is ∼2.5 eV. This value falls around the previously reported values of exfoliated BN.15,16 However, our barrier height may be overestimated due to the higher resistance values, and also because eq 2 assumes the ideal case where the BN and electrodes are in intimate contact with each other. In a real case, the presence of impurities and the space in between

(2)

where e, m, and h are electronic charge, electron mass, and Planck’s constant, respectively. The d and ϕ are the separation distance between the two electrodes (i.e., BN thickness) and barrier height, respectively. The linear dependence can be clearly seen in the zoomed in data around 0 V of Figure 3A as displayed in the inset of Figure 3B. Meanwhile, in the low-bias regime, the barrier is reshaped and the current is described by field emission tunneling across the barrier, which has an exponential dependence on V.8,11,15,16 Note that due to the atomic thickness of our film, the application of a small bias (as small as ∼|Vb| > 0.1 V) rapidly increases the current toward dielectric breakdown of the BN film in the high-bias regime. Thus, we focus our analyses of the electronic properties of BN in the zero-bias regime (Vb ∼ 0 V). The normalized zero-bias resistance of our samples annealed at 350 °C under Ar/H2, as calculated from the linear portion of the I−V curves, ranged from 800 Ω μm2 to 8 kΩ μm2, with an average value of ∼4.5 ± 1.9 kΩ μm2 (Figure 3B). This average value is about on the same order of magnitude as that of exfoliated monolayer BN.15,16 Further analysis of our measured resistance values showed that our devices have higher spread in resistances compared to exfoliated-BN that used graphite as the bottom electrode (ref 15, Figure 3C). We have identified several possible sources of the variation in the resistance of exfoliated BN on graphite, and our CVD-grown BN on metallic electrode. First, the differences could arise from the variation in the effective contact area of the electrodes and BN interface. For BN on graphite, the quality of the interface between the BN and graphite electrode is guaranteed due to the atomic flatness of the graphite; therefore, it is expected that the whole area of BN is at all times in contact with the graphite electrode. However, for BN on Au, the BN may delaminate mechanically from the metallic contact, leading to a reduction of the effective contact area and therefore the tunneling current. Thus, some of the devices showed high current readings, but for others, a lower current value is registered. One should note that the magnitude of the tunneling current is directly proportional to the zero bias resistance. Another source of higher resistance is PMMA contamination, as obviously manifested in the resistance of some of the devices made from the untreated BN sample. The devices showed resistances that can go as high as an order of magnitude compared to those devices made of heat-treated BN (Figure 3C). The untreated sample also exhibited a wider spread in resistance, probably due to the patchy PMMA layer, where a higher current reading is registered for devices with less contamination and lower readings for those that are heavily contaminated. For the heat-treated sample, the possibility of PMMA contamination cannot be eliminated. Although our heating treatment generally eliminates the PMMA residue, PMMA contamination can still be reintroduced on top of the BN sample during EB patterning for the top-electrode deposition. Even though the effect on the tunneling current is not as pronounced as that for the untreated sample, this possibility cannot be neglected. A possible solution to this problem is to

Figure 4. (A) Estimation of the barrier height for tunneling of our CVD-grown BN by plotting the measured zero-bias resistances for a given thickness and comparing the theoretically computed resistance at each barrier height as defined by eq 2. The estimated barrier height is ∼2.5 eV. (B) Sample I−V curve for measuring dielectric breakdown voltage (black curve). A positive voltage is applied to the devices until a sharp increase in current is seen (indicated by an arrow), which designates the dielectric breakdown voltage. Remeasurement of the device results in a straight line (red curve), which indicates a successful dielectric breakdown. 3344



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the length of the PEN film was altered in such a way that the devices are located at the center (Figure 5B). The amount of applied strain, ε, is related to the device’s geometry such as the bending radius, r, and the thickness of the PEN, t, as ε = t/ 2r.23,24 The measured I−V characteristics before and after application of the strain are very similar to those obtained for devices mounted on SiO2 substrate, though the current is about an order of magnitude lower. The lower current output is due to the thicker PMMA left during device fabrication as no annealing was done. To study the integrity of the created devices, a cyclic strain was applied to the devices by repeatedly bending the substrate to achieve a 1% strain. After the application of 4000 bending cycles, we found little change (5 V). The breakdown voltage is the voltage at which dielectric breakdown occurs, characterized by a sharp increase in current. Such a sharp increase in current is depicted in Figure 4B (indicated by an arrow). Subsequent measurements of the same sample yielded a straight line, which indicates contact between the bottom and upper electrodes, confirming the breakdown. Breakdown voltages measured in more than 20 devices varied by ∼20% for different device areas, and the average breakdown voltage was 2.95 ± 0.65 V (Figure 4B, inset). This value corresponds to a dielectric strength of 3.78 ± 0.83 GV m−1, assuming the separation distance of 0.78 nm as measured from AFM (i.e., the measured thickness of the BN), which is a slightly higher value than those reported for exfoliated BN.15,16 The discrepancy is attributed to the surface roughness of our contact, where a space between the metallic contact and BN could contribute to the increase in dielectric strength constant, or to PMMA contamination on top of the BN. With these, further qualitative analysis is difficult to perform because the sources of variation are compounded within the assembled devices. The ultrathin nature, the superior mechanical strength,22 and the vertical geometry of the tunneling device are attractive for making devices on flexible substrate. We therefore prepared a vertical tunneling device on a flexible 100 μm thick polyethylene naphthalate (PEN, Dupont Teijin Films) substrate using the same transfer procedure described above but without the annealing process (Figure 5A,B). It is expected that these devices, to some extent, will be resilient to strain caused by bending. Bending the substrate inward introduces a certain amount of elastic strain into the devices, assuming that no sliding between the devices and the PEN substrate occurs, which is likely in this case. To ensure a uniform and maximum strain along the direction of the bend is applied on the devices,

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CONCLUSIONS In summary, we have demonstrated that CVD-grown monolayer BN can replace exfoliated BN as a tunneling dielectric for tunneling devices. The resulting I−V characteristics of the created Au/BN/Au devices, on both rigid and flexible substrates, consistently show characteristics similar to those made out of exfoliated materials. We estimated the dielectric constant and barrier height to be ∼3.78 ± 0.83 GV m−1 and ∼2.5 eV, respectively, which are comparable to those of exfoliated BN. These results indicate that CVD-grown BN is a promising ultrathin insulator and can be used as a dielectric barrier for large-scale spintronics or vertical transistors on rigid and flexible substrates.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis absorption spectroscopy and band gap estimation of the grown BN, and AFM images of the transferred BN cleaned at different annealing conditions. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*H. Hibino: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.M.O. acknowledges Dr. Shengnan Wang and Dr. Kazuaki Furukawa for their valuable help in obtaining the AFM images and Dr. Shinichi Tanabe for his valuable help in the I−V measurements.

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Ultrathin Chemical Vapor Deposition (CVD)-Grown Hexagonal Boron

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