Semiconductors of restricted area can be changed

A group of international physicists have for the first time created a semiconductor material with an energy gap between the valence band and the conduction band (band gap) that can be changed simply by put an external electric field. The material is a graphene bilayer ("graphene bilayer") made of carbon and has only 2 atomic layers of thickness. The team claims that this semiconductor can be used to create transistors, lasers and other components with extremely easy-to-adjust properties, much more so than semiconductor materials. Classics like Si (see details on Phys Rev Lett 99 216802).

Semiconductors are useful because they can be used to open and close electrical currents. This is done by placing a small voltage on the systems made of semiconductors, powering the electrons to bypass the empty energy gap between the valence band and the conduction band. However, this width of the energy gap (this width can be specified as a closure) is an intrinsic property of the semiconductor and can only be changed by changing the chemical composition or structure. of materials.

A semiconductor with a band gap adjusted for example by an external voltage can lead to the creation of a series of new electronic components, or most notably wavelength lasers. Adjustable with a great accuracy. And recently, the team of Antonio Castro (Boston University, USA) and colleagues in the US, Portugal, Spain and the United Kingdom, for the first time created a semiconductor of a nature. That's from graphene . This material is a thin sheet of carbon with a thickness of only one atomic layer, and usually there is no energy gap between the valence band and the conduction band. However, when replaced by two overlapping layers of graphene to form a double layer, the energy gap is formed if the material is placed between the positive and negative electrodes (placed in the electric field).

Figure 1. Graphene double layer (a) and dependence of energy gap width on conductive density and depend on external electric field (d) (Phys. Rev. Lett.99 216802).

According to the group's theoretical calculations, when the energy generated is due to the reverse voltage creating a difference in negatively charged electrons in one layer and thus creating positively charged holes in remaining class. These electrons and holes pair together, creating a particle standard, whose behavior is different from each individual particle. A unique feature of electrons and holes in graphene is that they can move in the material as if they have no rest mass, or in other words they give the material a very good conductivity. However, the standard particles have resting energy, and the Castro Neto, this mass leads to the creation of the energy gap that they must pass before the current can pass.

The team measured the mass of the grain standard in a graphene double layer tape about 1 micrometer in width and a few micrometres in length. This graphene layer is attached to an oxidized silicon blade and an external voltage is placed between the Si and an electrode above the graphene layer. An external magnetic field has also been placed on this double layer, making the particle standards move in circular orbit, creating a cyclotron resonance effect. The team measured the resonance cycle, depending on the mass of the particle standard. They discovered that this cyclotron mass increased as the external voltage increased from 0 to 100 V, allowing them to conclude that the energy gap also varies from 0 to 150 meV.

Castro Neto told Physicsworld.com that this graphene semiconductor could be used tomorrow to create a new type of transistor, or types of lasers and molecular sensors where change is needed. width of restricted areas to adjust properties. This property when combined with graphene has a small size, high mechanical strength, good conductivity, thermal conductivity, making it very attractive to replace the semiconductor semiconductors that Si is an example.

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According to PhysicsWeb.org & Physical Review Letters, Vietnamese Physics