Graphene for True Ohmic Contact at MetalâSemiconductor Junctions

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Graphene for True Ohmic Contact at Metal-Semiconductor Junctions Kyung-Eun Byun, Hyun-Jong Chung, Jaeho Lee, Heejun Yang, Hyunjae Song, Jinseong Heo, David H. Seo, Seongjun Park, Sung Woo Hwang, In Kyeong Yoo, and Kinam Kim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl402367y • Publication Date (Web): 26 Aug 2013 Downloaded from http://pubs.acs.org on August 29, 2013

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Graphene for True Ohmic Contact at MetalSemiconductor Junctions Kyung-Eun Byun, Hyun-Jong Chung, Jaeho Lee, Heejun Yang†, Hyun Jae Song, Jinseong Heo, David H. Seo, Seongjun Park*, Sung Woo Hwang, InKyeong Yoo, and Kinam Kim Samsung Advanced Institute of Technology, Samsung Electronics Co., Yongin-si 446-712, Korea.

Corresponding Author *Seongjun Park (Address: Samsung Advanced Institute of Technology, Samsung Electronics Co., Yongin-si 446-712, Korea. Phone: +82-031-280-9479; Fax: +82-031-280-9308; E-mail: [email protected])

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Abstract The rectifying Schottky characteristics of the metal-semiconductor junction with high contact resistance have been a serious issue in modern electronic devices. Herein, we demonstrated the conversion of the Schottky nature of the Ni-Si junction, one of the most commonly used metal-semiconductor junctions, into an Ohmic contact with low contact resistance by inserting a single layer of graphene. The contact resistance achieved from the junction incorporating graphene was about 10-8 ~ -9 Ω cm2 at a Si doping concentration of 1017 cm3

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Keywords Graphene, metal-semiconductor junction, Ohmic contact, Schottky barrier, Ni-Si junction.


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When a metal forms a contact with a semiconductor, a potential barrier called a Schottky barrier can form as a result mainly due to the misalignment between the metal work function and the electron or hole affinity of the semiconductor. The Schottky barrier at the metalsemiconductor (M-S) junction prohibits current flow in one direction (Fig.1A-i) and causes a high contact resistance at the junction. The M-S junction and its Schottky barrier have been studied over the past 100 years for their interesting electrical, chemical and structural properties.1-7 The M-S junction is also a critical component in electronic devices which can determine device performance characteristics such as the switching speed in diodes, on-state currents in transistors, loss of the spin polarization in spin transport devices, or open circuit voltage in solar cells.8-12 For instance, the M-S junction has been one of the major issues in transistor performance because the resistance of the Schottky junction is comparable or superior to that of the channel in nano-scale devices.11 Ideally, metals would form Ohmic contacts without any Schottky barriers if their work functions aligned with the conduction or valance band edges of semiconductors. In practice, however, metals form Schottky contacts irrespective of work function since the Fermi-level of a metal is pinned at the a certain energy level at the semiconductor interface. This Fermi-level pinning arises from the surface states of the semiconductor and occurs for most commonly used semiconductors such as Si and GaAs.13 The current solution in semiconductor technology for a low-resistance junction is to increase the tunneling current (It) by reducing the barrier width using heavily doped semiconductors (Fig.1A-ii), rather than increasing the thermionic emission current (Ith) by reducing the barrier height.11 Even though the Schottky barrier is not completely reduced, this ‘Ohmic-like’ junction is often called an ‘Ohmic’ junction due to its lower contact resistance and symmetric tunneling current-voltage (I-V) behavior. However, this method is approaching its limit as devices are


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being scaled down.14 These scaled devices incorporating highly doped semiconductors need to overcome difficulties such as the fine control in the dopant profile,15 high leakage current16 and power dissipation caused by a large contact resistance.11 To solve these problems, many researchers have been tried to increase Ith by alleviating the Fermi-level pinning effect using thin oxide or nitride films.17-19 However, these dielectric films can add additional tunneling barriers where even a few atomic layers can further reduce the tunneling currently rapidly. In addition, it is technically challenging to maintain interfacial uniformity across the junction. Thus, we need new and approachable routes to achieve an ideal M-S junction where the metal’s work function aligns with the conduction or valence band of the semiconductor. Herein, we successfully demonstrated an Ohmic junction by inserting graphene between the metal and semiconductor. In our strategy, graphene plays two roles: preventing Fermi-level pinning20 and modulating the work function21 to align with the conduction or valence band edges of the semiconductor. Thus, the metal-doped graphene can reduce or even potentially eliminate the Schottky barrier at the M-S junction and form a ‘true Ohmic’ contact. We were able to match the work function of graphene and the electron affinity of n-type Si (n-Si) by choosing appropriate metal, Ni (Fig.1A-iii).21 The Ni-Si junction is important in Si technology because it has often been used as the source/drain contact in Si devices. Moreover, the n-Si and Ni system is one of the most difficult systems in which to lower the Schottky barrier because Si has the strongest pinning effect and n-Si forms a higher Schottky barrier (~0.7 eV) than p-type Si (~0.3 eV). In the Ni-graphene-Si system, Ni can induce enough charge on graphene resulting in the proper work function for an Ohmic junction without Fermi-level pinning. First, we investigated the Ohmic or Schottky nature of metal-graphene-semiconductor (M-G-S) and M-S junctions. The M-G-S junction was formed when a Si probe tip used in conducting


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atomic force microscopy (cAFM) was moved into contact with the graphene on a Ni substrate as shown in Fig. 1B. The cAFM system allows us to make nanosized ideal M-G-S contacts without additional process, which could contaminate the graphene. To exclude potential confusion arising from It owing to its symmetric I-V behavior, we used a moderately doped Si cantilever. With a doping level of 1016 cm-3, we estimate a ratio of It /Ith ~ 10-5 at room temperature.13 Figure 2A shows the AFM topography image of graphene on a Ni substrate. We measured the I-V characteristics of the Ni-graphene-Si junctions at several points as shown in Fig. 2A (A~E). As we expected, the M-G-S junctions exhibited an Ohmic nature with symmetric I-V curves and low resistance (Fig. 2B). We compared the averaged I-V curves of graphene on the Ni substrate (M-G-S, red line in Fig. 2C) and a Ni-only substrate (M-S, black line in Fig. 2C). The Ohmic MG-S junction was distinguishable from the Schottky M-S junction with its asymmetric I-V and higher resistance. Considering that the Ith accounts for the majority (105 times of It) of the current, the symmetric I-V confirms that the Ohmic nature is caused not by a narrow barrier width but by a low barrier height. We would like to point out that the Ohmic nature of the junction supports the two main points of our strategy, the de-pinned Fermi level of graphene on a Si surface, and graphene doping from Ni. Without de-pinning, the Fermi level of graphene would still be pinned at the mid-gap of Si, and without proper doping, the Fermi level of graphene would be around 4.5 eV. In both case, the I-V characteristics would exhibit Schottky curves. Therefore, the Ohmic nature of the M-G-S junction was attributed to the work function modulation of graphene without Fermi-level pinning at the graphene-Si interface. Some M-G-S junctions exhibited a Schottky nature similar to that of the M-S junctions. This might have been caused by some part of the graphene separating from Ni owing to wrinkles and intrinsic ripples in graphene, which was consequently not fully doped by Ni.22, 23 The previous


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report shows that the work function of graphene on metal can be varied by changing the distance between the graphene and metal.21 In order to verify this for our experimental situation, we simulated the work function and the formation energy of the graphene on Ni system while varying the distance between the graphene and the Ni surface. For this purpose, we performed density functional theory simulations without any geometry relaxations to maintain the distances. As shown in Fig. 3A, as graphene approaches the minimum formation energy distance, the work function of Ni-doped graphene can be as low as 3.5 eV. This value is about 1 eV lower than that of neutral graphene and low enough to form an Ohmic contact. This Ohmic contact could be converted back to a Schottky contact by a small increase in the graphene-Ni distance from the minimum energy distance, which subsequently increases the work function to form a Schottky barrier. Thus, wrinkles and ripples in graphene could be the obstacles to an Ohmic M-G-S junction. Monitoring the nature of the M-G-S junction, I-V curves were measured while varying the graphene-Ni distance, which can be controlled by applying a force on the tip. As shown in Fig. 3B, reverse-bias currents increased with contact force, and therefore, the asymmetry in the I-V curves were gradually removed which implies a lowering barrier height. Moreover, I-V curves at low contact force (0.5, 2 and 4 µN) were shifted under light illumination (dotted lines in Fig. 3B), which indicates the light-sensitive nature of a Schottky barrier.24 It should be noted that the I-V curve at high contact force (5.5 µN, red line in Fig. 3B) was symmetric with low resistance, and no shifts were observed under light illumination, which implies an Ohmic nature of the junction. Even at the M-G-S junction with Schottky characteristics, we confirmed that by applying a high contact force the M-G-S junction’s nature becomes Ohmic on the following two grounds


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mentioned earlier: I-V curve is symmetric without contribution from It; and the light response associated with Schottky characteristics of the M-G-S junctions disappeared. We also investigated the nature of M-S junctions with the same forces applied to M-G-S junctions (Fig. 3C). The I-V curves of the M-S junction exhibited rectifying Schottky characteristics and, more importantly, the curves did not modulate with contact force. Currents seem to be increased slightly with contact force (inset) because the tip-sample contact area could have increased because of plastic deformation of the tip and sample.25 We extracted Schottky barrier heights from the M-G-S and the M-S junctions for the various applied forces (Fig. 3D). The Schottky barrier heights were obtained from the thermionic current-voltage relationship of a Schottky diode.26 According to the thermionic current-voltage relationship, the Schottky barrier height is correlated with the diode current and the contact area. In this experiment, the increased tip-sample contact area at higher contact forces was estimated using the Derjaguin-Müller-Toporov (DMT) model.27 Detailed explanations can be found in Supporting Information. Because the Schottky barrier height of a nano sized junction has different value from that of a macroscopic junction,28 the obtained Schottky barrier heights were smaller than the well-known values. The Schottky barrier height of the Ni-Si junction (red circles) remained almost constant with increasing contact force. On the other hand, the Schottky barrier height (black squares) of the Ni-graphene-Si junction significantly decreased with increasing loading force. The Schottky barrier heights obtained from the diode equation confirm that the graphene could lower the barrier heights at the junction. In addition, we measured topography images to monitor the imperfections caused by the applied force and to rule out the following possibilities: deforming the substrate; tearing the graphene; and breaking the tip. Fig. 4A-i and 4A-ii show the topography images taken after the


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force-applied I-V measurements. As can be seen, the surface was not deformed and the graphene was not torn by the contact force applied to form an Ohmic contact. In addition, the same clear image could rule out the possibility of tip breakage (Fig. S3). However, as shown in Fig. 4A-iii, after applying a large contact force of about 14 µN, a trench (white circle) appeared because the tip scratched the graphene on the Ni substrate. Interestingly, the Ohmic I-V curves changed into Schottky curves again because a Ni-Si junction was formed after the graphene layer was torn open. It also confirms the role of graphene in the formation of a true Ohmic junction. The current was higher than the I-V characteristics using a 0.5 µN contact force due to a larger contact area caused by the deformed substrate. We next assessed the feasibility of the M-G-S junction from the following two engineering points of view. Firstly, there might be concerns whether a true Ohmic junction is feasible in the Si processes; whether the extra pressure is needed or not. As shown in Fig. 3A, the M-G-S junction can be Ohmic when the distance between Ni and graphene is less than 2.3 Å. It should be noted that the most stable distance between Ni and graphene, the minimum formation energy point, is around 2 Å, and that the graphene on Ni is stable within this distance, thereby making an Ohmic contact possible. Thus, a true Ohmic contact can be implemented by developing optimum graphene transfer and Ni deposition processes. Secondly, we compared the resistances between the M-G-S junctions (Fig. 1A-iii) and M-S junctions from current Si technology (Fig. 1A-ii). The contact resistance was extracted using the n-Si cantilever with a doping concentration of 1017cm-3 (Fig. S2) for the following reasons. In the previous case of 1016 cm-3, the Si tip resistance mainly contributed to the total junction resistance. On the other hand, in the case of a doping concentration of over 1018 cm-3, it is hard to confirm the barrier lowering effect because It/Ith is around 10-2 at room temperature. For the M-G-S junction with a Si doping concentration


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of 1017 cm-3, we obtained a contact resistance in the range of 10-8 ~ 10-9 Ω cm2, which is about 10 times lower than the resistance of state-of-the-art Si technology even without a heavily-doped semiconductor layer.14 In conclusion, we have demonstrated a true Ohmic contact in Ni-Si junctions by lowering the Schottky barriers using graphene. The Ohmic nature is confirmed from the symmetry of the thermionic-emission current and the disappearance of the photo current. In addition, we also find that the contact resistance can be reduced by 90% without a heavily doped region. Therefore the Ni-graphene-Si junction could replace the highly doped Si source/drain junction, which has been causing high leakage currents and complicating the integration processes in current Si technology. This structure would be a key solution to the problems arising from M-S contact for the future device technology.


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Figure 1. Metal-graphene-Si (M-G-S) junction. A. Schematic band diagrams of (i) metal and nSi (reference), (ii) metal and heavily doped n-Si (current Si technology), and (iii) metal, graphene and n-Si junctions (new approach suggested in this article) under bias. The arrows of the upper images represent electron flows. The electrical current at the metal-semiconductor (MS) junction comprises thermionic-emission currents (Ith, blue) over the Schottky barrier and tunneling currents (It, red) through the barrier. Ith and It mainly depend on the barrier height and width, respectively. In our strategy, the Schottky barrier (the gray shaded area in the lower images) can be reduced by inserting graphene, Ith can then be increased. B. Experimental setup of conducting atomic force microscopy (cAFM).


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Figure 2. Characteristics of the M-G-S junction. A. AFM topography image of graphene on a Ni substrate. B. I-V characteristics of the M-G-S junctions with Si doping concentration of 1016 cm3

. The I-V curves were collected at points A~E in Fig. 2A. C. Averaged I-V characteristics of the

M-G-S junctions (red) and the M-S junction (black).


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Figure 3. Ni-graphene distance dependent characteristics of the M-G-S junction. A. Calculated work function (red) and formation energy (blue) of Ni-doped graphene as a function of Nigraphene distance. Tip-sample contact force dependent representative I-V characteristics of B. the M-G-S junction and C. the M-S junction with a Si doping concentration of 1016 cm-3. The dotted lines in Fig.3B represented the I-V curves measured under light illumination. D. Tipsample contact force-dependent Schottky barrier heights of Ni-graphene-Si (black squares) and Ni-Si (red circles) junctions. The error bars represent standard deviations of the mean values.


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Figure 4. Monitoring the imperfections caused by applied force. A. AFM topography images taken after the I-V measurements with contact force (i) 0.5 µN (ii) 7 µN (iii) 14 µN. The substrate was deformed (white circle) after contacting the tip with a 14 µN loading force. B. Tip-sample contact force dependent representative I-V characteristics of the M-G-S junction. Note that the Schottky curve at a 14 µN contact force came from the deformed Ni substrate.


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ASSOCIATED CONTENT Supporting Information Methods, AFM topography images taken after the I-V measurement with contact force, Raman spectra of graphene on a Ni substrate, Scanning electron microscopy (SEM) image of the Si tip after the I-V measurements, AFM topography image of the sharp Si grating, 3-D visualization image of the tip via the tip deconvolution method, and Tip-sample contact force dependent representative I-V characteristics with Si doping concentration of 1017 cm-3. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Seongjun Park (E-mail: [email protected]) Present Addresses † Unité Mixte de Physique CNRS/Thales, 91767 Palaiseau, France. ACKNOWLEDGMENT The authors are grateful to colleagues at Graphene center, Nano Fabrication group and AS group at Samsung Advanced Institute of Technology.


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