Researchers from the University of Pennsylvania, the University of Illinois at Urbana-Champaign and University of California, Berkeley have demonstrated a new method for changing the quantity of electrons that exist in a particular region within a graphene piece. Using graphene, the team has provided a proof-of-principle for producing the basic building blocks of semiconductor devices.
The researchers suspended graphene over periodically poled lithium niobate. The "stripes" indicate different polar regions.
In this method, an application of an electric field can help tune this value. In the future, this feature may enable dynamical “rewiring” of graphene circuit elements, without altering the device physically.
The study was a collaboration between the groups of Andrew Rappe at Penn, Lane Martin at Berkeley and Moonsub Shim at Illinois.
Graphene is a 2D material. It is made up of an atom thick lattice of carbon atoms. It is considered to be a revolutionary material that could potentially replace silicon in electronics applications. Its 2D form factor and its ability to move electrons at very high speeds have made it a promising alternative. However, several hurdles have to be overcome in order to allow its adoption.
Addition of chemical impurities can help increase or decrease the number of free electrons contained by silicon, which is known as its charge-carrier density. This is called as a “doping” process and leads to formation of “p-type” and “n-type” semiconductors. These semiconductors have more positive and or negative charge carriers. The fundamental blocks of electronic devices are the junctions between p- and n-type semiconductors. When they are put in a sequence, transistors are formed by these p-n junctions. These transistors can be combined to form processors, microchips and integrated circuits.
Doping of graphene to attain the p- and n-type version is possible. However, it would affect its unique electrical properties. When local voltage changes are applied to the material, a similar effect can be obtained. However, placement of the required electrodes affects the advantages provided by the form factor of graphene.
We’ve come up with a non-destructive, reversible way of doping that doesn’t involve any physical changes to the graphene.
Andrew Rappe
The researchers used graphene and lithium niobate. A layer of graphene is deposited on lithium niobate. Graphene rests on lithium niobate but does not bond to the material. Lithium niobate is a polar material as it is ferroelectric. Hence, its surfaces may have a negative or positive charge. The polarity of the surface charges can be changed by the application of an electric field pulse.
That’s an unstable situation in that the positively charged surface will want to accumulate negative charges and vice versa. To resolve that imbalance, you could have other ions come in and bond or have the oxide lose or gain electrons to cancel out those charges, but we’ve come up with a third way.
Here we have graphene standing by, on the surface of the oxide but not binding to it. Now, if the oxide surface says, ‘I wish I had more negative charge,’ instead of the oxide gathering ions from the environment or gaining electrons, the graphene says ‘I can hold the electrons for you, and they’ll be right nearby.
Andrew Rappe
Lithium niobate was suggested by Rappe, as the material which allows for the creation of p-n junctions and is widely used in optical engineering. Periodically poled lithium niobate, a new material, is created in a manner so that it possesses “stripes” of polar regions that are positive and negative alternatively. The team utilized the advantages of this material.
Because the lithium niobate domains can dictate the properties, different regions of graphene can take on different character depending on the nature of the domain underneath. That allows, as we have demonstrated, a simple means of creating a p-n junction or even an array of p-n junctions on a single flake of graphene. Such an ability should facilitate advances in graphene that might be analogous to what p-n junctions and complementary circuitry has done for the current state-of-the-art semiconductor electronics.
What’s even more exciting are the enabling of optoelectronics using graphene and the possibility of waveguiding, lensing and periodically manipulating electrons confined in an atomically thin material.
Moonsub Shim at Illinois
Addition of a single gate to the device enabled further tuning of the overall carrier density by applying various voltages. Taking into consideration how the surface charges are balanced by the oxide by itself, or by ions binding from the aqueous solution, the researchers were able to demonstrate the relationship between the suspended graphene’s charge carrier density and the polarization of the oxide. This oxide polarization can be altered easily, and this allows alteration of the extent and type of supported graphene.
“You could come along with a tip that produces a certain electric field, and just by putting it near the oxide you could change its polarity,” Martin said. “You write an ‘up’ domain or a ‘down’ domain in the region you want it, and the graphene’s charge density would reflect that change. You could make the graphene over that region p-type or n-type, and, if you change your mind, you can erase it and start again.”
This unique property is an advantage when compared with chemically doped semiconductors. In order to change the carrier density, atomic impurities are mixed into the material. These can’t be removed after mixing. In the future, researchers plan to study the possibility of designing dynamic semiconducting using this technique.
“We can’t currently do that, but that’s the direction we want to take it,” Rappe said, “There are some oxides that can be repolarized on the timescale of nanoseconds, so you could make some really dynamic changes if you needed to. This opens up a lot of possibilities.”
The team of researchers has published this study in the journal, Nature Communications.
The Army Research Office, the Office of Naval Research, and the National Science Foundation and the Nanoelectronics Research Initiative have supported this study.
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