High Frequency Carbon Nanotube/Graphene Transistor

Transistors made from nanocarbon materials have the potential to revolutionize electronics. Carbon nanotubes are perfect semiconducting or metallic 1-dimensional wires and graphene sheets are showing record breaking electron mobility.

The challenge in building high performance electronic devices from carbon nanotubes or graphene lies in integrating them into circuits without destroying their unique properties. In particular, when creating nanocarbon transistors for wireless applications making an efficient gate electrode has proven difficult.

One way to improve performance is to create freestanding 3-dimensional gate structures. Unfortunately, this is quite difficult using conventional photolithography or electron beam lithography. This instructable will show a fast and easy way how to create a suspended gate electrode for high frequency nanoelectronics.

Step 1: Preparations

What you need to get started:
- Chunk of graphite
- Scotch tape
- Silicon wafer
- Microscopes: optical, AFM, SEM
- Clean room facilities
- Focused ion beam (handy but not absolutely necessary)

Making graphene is fairly straightforward: take a piece of graphite and rub it over the silicon wafer. Then use the scotch tape to peel of layers until you end up with a single layer left on the substrate. You can check the process with an optical microscope: if the layer is only barely visible anymore you have (probably) succeeded.

Step 2: Lithography

The source/drain and the gate electrodes can be patterned in a resist layer either by optical lithography or electron beam lithography. In any case after developing the resist you should end up with structure similar to the one shown below.

Step 3: Metallization

The sample with the developed resist mask is covered with a metal layer. Typical several different metals are sputtered or evaporated in a vacuum apparatus (e.g. first a 5 nm thick titanium adhesion layer and on top a 50 nm gold layer).

Step 4: Bridging

The gate electrode is going to be formed from the metal covering the resist on top of the graphene sheet. In order to support it a thick bridge like structure needs to be created. This can be done conveniently by focused ion beam (FIB) induced metal deposition:

A organic precursor gas is decomposed by the focused Ga-ion beam resulting in local deposition of a conducting compound. The metal covered resist layer protects the sensitive graphene sheet from any gallium ion implantation or irradiation damage.

(In principle it is also possible, yet more difficult, to achieve this in a second lithography process.)

Step 5: Release

In order to create a well defined gate electrode the to-be-suspended part of the metal layer is separated from the adjacent parts. This is done using the high energy ion beam to mill cuts through the metal layer.

Step 6: Lift-off

The resist and surplus metal is removed using a suitable solvent; e.g. acetone (so called [http://en.wikipedia.org/wiki/Lift-off_(microtechnology) lift-off] process). The result is a graphene field effect transistor (FET) for radio frequency applications.

Note:
- Special precaution stepsprecaution steps might be necessary to prevent capillary forces during drying from collapsing the bridge
- Performance can be increased further when the graphene sheet itself is subsequently suspended by etching away the sacrificial layer

Step 7: Proof of Concept

The micrographs shows a suspended structure demonstrating the feasibility of the method. It is notable, that the structure withstood dicing and lift-off without any special precautions.