MBE unmasks the real indium oxide
Materials widely used in high-volume electronics manufacture do not always have to be well understood. Take In2O3, for example. Doped with tin, this conductive oxide provides the transparent contact in flat-panel displays and solar cells. However, despite its widespread deployment, the fundamental properties of this material are only just starting to emerge.
For years its character has been camouflaged by high defect densities that result from the high-speed techniques employed for its deposition, like sputtering or spray pyrolysis. At the UK universities of Oxford and Warwick, our team has recently addressed this by turning to oxygen plasma assisted MBE for high-quality crystal growth, which is a relatively radical shift in growth technology for metal oxides. Thanks to this switch, we re now starting to understand the true characteristics of this material.
We produce our high-quality In2O3 films at the University of Oxford by RF plasma assisted MBE on yttria-stabilized cubic zirconia substrates. This platform provides a good lattice match to the deposited layer and ensures the growth of body-centered cubic In2O3, which is known as the bixbyite phase. This has excellent crystal quality, according to transmission electron microscopy images (figure 1).
Armed with high-quality material, we have uncovered the fundamental properties of In2O3 for the first time. This revealed that the bandgap of In2O3 is not 3.7 eV, the value widely attributed to it for many years. In fact, it s actually less than 2.9 eV, according to independent X-ray photoemission spectroscopy measurements at Daresbury Laboratory and the European Synchrotron Radiation Facility. It may even be as low as 2.6 eV, which is a value previously assigned to an indirect bandgap within the material.
These findings agree with some ab initio band-structure calculations by our collaborators. Aron Walsh and colleagues at the US National Renewable Energy Labs, and Frank Fuchs and Friedhelm Bechstedt at Friedrich Schiller University of Jena, Germany, have found no evidence for any significant indirect nature of the fundamental bandgap, and their calculations suggest that the bandgap of In2O3 is significantly lower than 3.7 eV.
The lower bandgap might seem to contradict the transparent nature of In2O3. However, NREL s calculations suggest that the transition between the conduction band and the top-most valence band is very weak, which explains why this bandgap has not been revealed in optical measurements. Similarly, theorists at Friedrich Schiller University claim that optical transitions remain small up to 1 eV above the bandgap energy.
The dramatic improvement in material quality has also enabled us to determine the intrinsic electronic state of indium oxide. In the past, researchers have claimed that there are no carriers close to the material surface, but our recent measurements show the very opposite – In2O3 can actually sustain a large build-up of electronic charge at its surface.
The key to this discovery has been the growth of undoped In2O3 thin films with very low free electron concentrations. In these materials the surface Fermi level is pinned at around 0.4 eV above the conduction-band minimum, which allows electron accumulation in low carrier density In2O3. As expected, electron depletion kicks in as n-type doping levels increase.
Terahertz contender
There is a good chance that In2O3 will be an excellent material with which to exploit the terahertz spectrum. This is because InAs and InN, which are today s best materials for terahertz generation, have inherent surface electron accumulation. Terahertz generation in these – which is thought to result from surface field acceleration and the creation of an electric dipole at the surface – allows them to be used for applications as diverse as medical imaging, detecting concealed weapons and measuring water content in cheese.
In2O3 is also a promising material for making chemical and gas sensors, such as detectors of ozone and nitrous oxides. Prototypes have been fabricated at the Technical University of Ilmenau, Germany, and our efforts could improve detector performance through advances in understanding the semiconductor.
We ll continue to advance the growth technology for In2O3 and study this material s properties. But in addition we ll be branching out to other semiconducting metal oxides, such as SnO2 and copper-containing p-type oxides. It will be interesting to see what useful properties these materials might have in store.
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