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Tunable bandgap heralds new photonic devices

06 Dec 2007

A new semiconductor material with a tunable bandgap could allow precise control over the wavelength of light emitted by a laser.

An international team of physicists has created the first semiconductor material in which the width of the energy gap between the valence and conduction bands can be changed by simply applying an external voltage. The team claims that the semiconductor could be used to make lasers, transistors and other devices with properties that could be much more easily tuned than devices based on traditional semiconductors such as silicon.

Antonio Castro Neto of Boston University, along with colleagues in the US, Portugal, Spain and the UK, made the tunable semiconductor from a "graphene bilayer", a material made of carbon that is only two atomic layers thick (Phys Rev Lett 99 216802). A single layer of graphene normally has no gap between its valence and conduction bands, but placing two layers of graphene on top of each other to create a bilayer generates an energy gap if the material is placed between positive and negative electrodes.

According to a theory developed by the team, the gap arises because the transverse voltage causes an excess of negatively charged electrons in one layer and an excess of positively charged holes in the other layer. These electrons and holes are believed to pair up to create "quasiparticles", which behave differently from their constituent particles.

A peculiar feature of electrons and holes in graphene is that they move through the material as if they have no rest mass – something that makes the material a very good conductor. However, Castro Neto says that the quasiparticles formed in the bilayer do have a rest mass that generates an energy gap that must be overcome before current can flow.

The team measured the quasiparticle mass in a graphene bilayer ribbon that was about one micron wide and several microns long. This was achieved by mounting the graphene on an oxidized silicon wafer, and then applying a voltage between the silicon and an electrode above the graphene.

A magnetic field was also applied to the bilayer, which caused the quasiparticles to move in circular orbits – an effect called cyclotron resonance. The period of this resonance depends on the mass of the quasiparticles, and the team discovered that the cyclotron mass increased as the applied voltage increased from zero to about 100 V. This suggests that the energy gap of the bilayer was also changing from zero to about 150 meV.

Castro Neto believes that graphene semiconductors could someday be used to make new types of transistors, lasers and molecular sensors in which the energy gap could be changed at will. This property, when combined with graphene’s small size, great mechanical strength and high thermal and electrical conductivity, make it look very attractive as a replacement for traditional semiconductors such as silicon.

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