A two-dimensional material with unique properties, graphene is very desirable in industry for a wide range of applications. Some applications have already been successful, despite the relatively short period of time since its discovery. For instance, it is perfect for composite materials due to its extreme mechanical and thermal properties, and thanks to its transparency and high conductivity, the material has found use in touch screen technology.
However, limitations have blocked its use in some industries, most notably in electronics where the lack of band gap and high contact resistances have stalled its application. Despite this, a lot of research has focused on these problems because of the rewards of using graphene.
Graphene could be used to fabricate considerably faster field effect transistors (FETs) than traditional materials, as it has the highest room temperature carrier mobility.
One-Dimensional (1D) Contact
Several potential solutions, such as using graphene nano ribbons, introducing a tunneling barrier or by modifying the graphene chemically, have been found to overcome the problem of having no band gap. However, very little progress had been made until recently to improve the contact resistances of graphene devices.
Indicial experiments have proved that there was minimal conduction through the Basel plane and further investigations illustrated the importance of the edge of graphene. The introduction of stamp transfer led to the possibility and necessity to contact the edge and only the edge of the graphene.
Using boron nitride (BN is an insulating two-dimensional (2D) crystal) as a stamp, graphene was encapsulated using van Der Waals interactions. The highest quality graphene developed to date had atomically clean interfaces, as the graphene did not touch any hydrocarbon polymers or solvents.
However, there was still no way to contact the graphene because both Basel planes were insulator-covered, and there was a need to form an edge to make contact to. Using a CHF3/O2 plasma it became possible to etch both the graphene and the BN at the same time, exposing a 1D line of graphene to make contact, as shown in Figures 1 and 2. It was shown that these 1D contacts were of better quality than anything achieved before.
Figure 1. A schematic demonstration of both 2D contacts (left) and 1D contacts (right).
Figure 2. a & b: SEMs of an early device with 1D contacts. c: Ion milling has been performed to create a cross-sectional
However, in this work, a second lithography step was required to perform metallization, as the etch masks could not be used for lift off after an etch had been performed. This meant that the 1D contact was contaminated with hydrocarbons. However, the latest techniques allow the same mask to be used for the etch as the lift off.
Effectively allowing the contacts to be placed before the geometry of the device is defined with a separate mask. A perfectly self-aligned atomically clean metal-graphene interface for low resistance ohmic contacts (Figure 3 shows the improvement of contact resistances) is obtained by this contact first technique.
It is important these are self-aligned to reduce capacitive coupling between the graphene and any overlapping metal through the BN. Capacitive contributions from the contacts are harmful to high frequency applications, and must be minimized.
Figure 3. A graph demonstrating the 2 point contact resistances divided by 2 for various graphene devices made using different techniques.
1. K. Novoselov, Nat. Mater., 2007. 6(10): p. 720.
2. K. Nagashio, T. Nishimura, K. Kita, and A. Toriumi. Metal/graphene contact as a performance Killer of ultra-high mobility graphene analysis of intrinsic mobility and contact resistance. in Electron Devices Meeting (IEDM), 2009 IEEE International. 2009.
3. J.T. Smith, A.D. Franklin, D.B. Farmer, and C.D. Dimitrakopoulos, ACS Nano, 2013. 7(4): p. 3661.
4. L. Wang, I. Meric, P.Y. Huang, Q. Gao, Y. Gao, H. Tran, T. Taniguchi, K. Watanabe, L.M. Campos, D.A. Muller, J. Guo, P. Kim, J. Hone, K.L. Shepard, and C.R. Dean, Science, 2013. 342(6158): p. 614.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
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