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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.


Figure 1: (a) Photoemission spectra of In2O3(111) and ITO excited at 19 eV photons. (b) Expanded view of gap states (GS) and conduction band (CB) states. The intensity of the CB state is much stronger than expected given that the bulk carrier density is only 1.5x1019 cm-3 as compared to 4.5x1020 cm-3 for ITO.

In order to gain further insight on this intriguing system, the high brightness and high energy and momentum resolution of the BaDElPh beamline was used to acquire photoemission spectra from single crystal thin films of In2O3 grown by oxygen plasma assisted molecular beam epitaxy. The low photon energies used in the measurements ensured a probing depth comparable with the electron accumulation layer thickness. The measured photoemission intensity close to the Fermi energy for a sample with a bulk carrier density of only 1.5x1019 cm-3 is very much stronger than expected in comparison with ITO where the carrier density is 30 times higher (see Figure 1).

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.

Figure 2: (a) Angle-resolved photocurrent map of CB states of In2O3(111) excited with 9 eV photons, showing two sub-bands below the Fermi level; (b) Second-derivative image of (a); (c) Momentum distribution curve obtained by summing intensities over ±25 meV with respect to the Fermi level; (d) Downward band bending of 1.30 eV in the CB (upper curve) and 0.45 eV in the VB (lower curve) and corresponding quantized 2D states.

This research was conducted by the following research teams:

  • Kelvin Hongliang Zhang, Russell G. Egdell, Inorganic Chemistry Laboratory, University of Oxford, Oxford, United Kingdom
  • Francesco Offi, CNISM and Dipartimento di Fisica, Università Roma Tre, Roma, Italy
  • Stefano Iacobucci, CNR-IFN and Dipartimento di Fisica, Università Roma Tre, Roma, Italy
  • Luca Petaccia, Sergey Gorovikov, Elettra - Sincrotrone Trieste, Trieste, Italy
  • Philip D.C. King, School of Physics and Astronomy, University of St. Andrews, St. Andrews, United Kingdom

Reference

K.H. Zhang, R.G. Egdell, F. Offi, S. Iacobucci, L. Petaccia, S. Gorovikov and P.D.C. King,
Microscopic Origin of Electron Accumulation in In2O3”,
Phys. Rev. Lett. 110, 056803 (2013), DOI: 10.1103/PhysRevLett.110.056803
 
This paper was highlighted as an “Editor’s Suggestion”.
                               
Last Updated on Thursday, 09 May 2013 11:14