Microscopic origin of electron accumulation in In₂O₃

There are only a handful of materials combining optical transparency in the visible region with high electrical conductivity. This unusual conjunction is exploited in liquid crystal and electroluminescent displays, as well as in photovoltaic cells. One of the mostly widely used so-called transparent conducting oxides is Sn-doped In₂O₃, aka ITO. Despite its almost ubiquitous application, several aspects of the physics of this material are still controversial. A striking example is provided the band gap of In₂O₃. By using photoemission spectroscopy, a team of scientists of Elettra and the University of Oxford have recently determined a value of 2.7 eV, almost 1 eV lower than previous estimates. This important finding points to the existence of a pronounced downward band bending, giving rise to an electron accumulation layer, contrary to the established picture of an upward band bending generating a depletion layer. The cause of such electron accumulation has so far remained elusive.

This qualitatively demonstrates the existence of an accumulation layer. Interestingly, we found that the intensity of this feature is strongly suppressed by annealing in oxygen, suggesting that doubly ionized oxygen vacancies in the outermost ionic layer act as the source of the electrons in the accumulation layer. Theoretical calculations support this picture and demonstrate that surface oxygen vacancies have a much reduced formation energy than bulk vacancies and act as shallow donors.
The angular resolved photoemission map shown in Figure 2 reveals that the electrons confined in the In₂O₃(111) surface by band bending are quantized into two sub-band states with dispersion parallel to the surface. A detailed analysis using non-parabolic coupled Poisson-Schrödinger calculations indicates that the band bending associated with the conduction band is much larger than the valence band bending, leading to a strong reduction of the band gap at the surface. Whereas it is well known that band gap shrinkage takes place in the bulk of highly degenerate semiconductors due to many-body interactions, the effects found here for  In₂O₃(111) thin films are much larger than those observed in the bulk at comparable carrier densities, suggesting that dimensionality plays an important role in determining the band gap width.
The implications of electron accumulation for device applications are even more striking than might initially be thought, with not just the presence of a near-surface layer that is highly electron rich, but one that even has an electronic structure fundamentally altered from that of the bulk. However at the same time the ability to tune the surface oxygen vacancy concentration suggests the potential to control the surface electronic properties of ITO and other transparent conducting oxides for applications in electronics and sensors.