Researchers in the UK and the US have discovered that indium nitride can support a large build-up of negative charge at its surface. Only one other semiconductor – indium arsenide – exhibits this property. The discovery could make it easier to fabricate hybrid devices made of metals and semiconductors (I Mahboob et al. 2003 Phys. Rev. Lett. to be published)
Almost all semiconductors have a near-surface region that is depleted of electrons. This means that any metal contacts to the semiconductor tend to have a high resistance, and that the electrons have to tunnel through a large Schottky barrier. This makes the fabrication of hybrid devices difficult. If semiconductors with an excess of electrons at their surface could be manufactured, it could be possible to overcome this problem.
Recently, indium nitride attracted attention when researchers found that its band gap is 0.7-0.8 eV, rather than 1.8-2.1 eV as previously believed. This means that alloys of indium nitride – such as indium gallium nitride – have band gaps that span the entire visible spectrum from the near infrared to the ultraviolet. Such materials could be used in a wide range of optoelectronics devices such as light-emitting diodes and solar cells.
Chris McConville and colleagues at Warwick University in the UK and Cornell University in the US analyzed the electron density profile of indium nitride using high-resolution electron spectroscopy and computer modelling. They studied samples that had been grown by molecular beam epitaxy and whose surfaces had been cleaned with hydrogen. The researchers found clear evidence for a layer of accumulated electrons on the indium nitride surface, and their results indicate that metal-semiconductor contacts would have low resistance and no Schottky barrier.
Potential applications for indium nitride include magnetoelectronic devices and semiconductor-superconductor hybrids, such as Josephson-junction field-effect transistors. “Coupling superconductors to semiconductors is problematic when the surface of the semiconductor is depleted of electrons,” said McConville. The team now plans to study the electronic properties of indium gallium nitride using the same technique.