Band-gap engineering

Molecular-beam epitaxy is a technique used to construct thin epitaxial films of materials ranging from oxides to semiconductors to metals. Different beams of atoms and molecules in an ultra-high vacuum environment are shot onto a nearly atomically clean crystal, creating a layering effect. This is a type of thin-film deposition. Semiconductors are the most commonly used material due to their use in electronics. Technologies such as quantum well devices, super-lattices, and lasers are possible with MBE. Epitaxial films are useful due to their ability to be produced with electrical properties different from those of the substrate, either higher purity, or fewer defects or with a different concentration of electrically active impurities as desired.[1] Varying the composition of the material alters the band gap due to bonding of different atoms with differing energy level gaps.

Semiconducting materials are able to be altered with strain-inducing from tunable sizes and shapes due to quantum confinement effects. A larger tunable bandgap range is possible due to the high elastic limit of semiconducting nanostructures.[2] Strain is the ratio of extension to original length, and can be used on the nanoscale.[3]

When lithographically generated graphene ribbons are laterally confined in charge it creates an energy gap near the charge neutrality point. The narrower the ribbons result in larger energy gap openings based on temperature dependent conductance. A narrow ribbon is considered a quasi one dimensional system in which an energy band gap opening is expected. Single sheets of graphene are mechanically extracted from bulk graphite crystals onto a silicon substrate and are contacted with Cr/Au metal electrodes. Hydrogen silsesquioxane is spun onto the samples to form an etch mask and then oxygen plasma is used to etch away the unprotected graphene.[4]