The researchers described a technology to create a transistor design with the smallest transistor gate length ever reported. It is made from two atomically thin materials, and the gate is one carbon atom across.
The key component of the transistor is made from the edge of a graphene sheet. The discovery of atomically thin materials such as graphene and carbon nanotubes has eliminated the need to fabricate a 1-nanometer element from silicon if one can simply use a similarly wide carbon nanotube.
Thus, the gate of a 1 nanometer transistor is made from a single carbon nanotube. The challenge was to correctly place the atomically thin materials in the correct configuration to create a functional device.
The standard transistor design consists of two conductive electrodes—a source and a drain—separated by a semiconductor. The state of the semiconductor, i.e. whether it is conductive or insulating, is established by a third conductive electrode called the gate. While there are a number of transistor size criteria, gate length is one of the most important.
Silicon is probably the best known semiconductor, but atomically thin semiconductors also exist. The best known among these materials is molybdenum disulfide. In 2021, an international team of researchers has already announced the creation of new two-dimensional transistors from it with a thickness of one to several atoms. Later, researchers at Stanford University developed a technology for transferring monatomic semiconductors from molybdenum disulfide to a flexible substrate. They managed to get flexible transistors with a thickness of only 5 microns.
Although molybdenum disulfide is not as thin as a single atom due to the arrangement of its chemical bonds, it is compact. The researchers used molybdenum disulfide as the semiconductor material. The source and drain electrodes were strips of metal that were in contact with it.
In the previous 1nm device, the transistor gate was made from a single carbon nanotube. Achieving a smaller size is difficult, but possible. Graphene sheets are like flattened carbon nanotubes: they are sheets of carbonded together. Although the length and width of the sheet will be much greater than that of a nanotube, it will be only one carbon atom thick.
The secret of the new work lies in the orientation of the edge of the graphene sheet. The researchers noted that such a design is fairly easy to fabricate because it does not require extremely precise positioning of any of the atomically thin materials.
To create the transistor, the researchers started with layers of silicon dioxide. Silicon was purely structural - it is not in the transistor itself. A sheet of graphene was layered on top of silicon dioxide to create the gate material. On top, the researchers placed a layer of aluminum. Although aluminum is a conductor, it was left exposed to air for several days during which the surface oxidized to alumina. So the bottom surface of the graphene sheet was on silicon dioxide and the top was covered with alumina, both of which are insulators. This made it possible to isolate everything but the edge of the graphene from the rest of the transistor equipment.
To expose the graphene edge, the researchers simply etched the aluminum edge down to the underlying silicon dioxide. Eventually, they were able to use a sheet of graphene as a shutter. At this point, the entire device is coated with a thin layer of hafnium oxide, an insulator that provides a small space between the gate and the rest of the hardware. The edge of the graphene (now embedded in the wall of the vertical part of the device) was in close proximity to the molybdenum disulfide. And the length of the shutter was equal to the thickness of a sheet of graphene - one carbon atom or 0.34 nanometers.
After that, the researchers created dozens of transistors with a new design. Some have demonstrated that the equipment actually works like a transistor, although this requires a sufficiently high voltage.
Researchers have proposed many ways to improve the transistor.
What's really important is that they found a way to really take advantage of the smallest size of atomically thin material as part of a functional transistor. And they did it without requiring particularly precise positioning when graphene and molybdenum sulfide were added to the device. This is partly due to the fact that the part of the graphene that needed precise positioning - the edge - was created by etching. And the molybdenum disulfide just had to be positioned to cover the gate and reach where the source and drain connect.
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