Surface Electron Accumulation and the Charge Neutrality Level inIn2O3

Abstract

High-resolution x-ray photoemission spectroscopy, infrared reflectivity and Hall effect measurements, combined with surface space-charge calculations, are used to show that electron accumulation occurs at the surface of undoped single-crystalline In2O3. From a combination of measurements performed on undoped and heavily Sn-doped samples, the charge neutrality level is shown to lie approximately 0.4 eV above the conduction band minimum in In2O3, explaining the electron accumulation at the surface of undoped material, the propensity for n-type conductivity, and the ease of n-type doping in In2O3, and hence its use as a transparent conducting oxide material.

Conductivity in transparent oxide semiconductors

“…Zhang et al [136] employed energy-dependent photoemission, utilizing the inherent variation in surface specificity with photon energy to support this conclusion, providing spectroscopic confirmation of the increased electron density at In 2 O 3 surfaces. Via extrinsic Sn-doping to increase the bulk carrier concentration, it is possible to quench the electron accumulation [134,135]. This provides a mechanism to reconcile these recent results with earlier studies which suggested a depletion of electrons at the surface [131][132][133].…”

“…King et al [63,134] have argued that this can be understood within the concept of the charge neutrality level (CNL) [156,157]. Such localised impurities, defects, and surfaces are all examples of broken symmetries -that is, the perfect translational symmetry of the lattice is lost.…”

“…Investigating the (100) surface of nearly non-degenerate bixbyite In 2 O 3 films grown by MBE, King et al [134,135] used photoemission spectroscopy to show that the Fermi ‡ For p-type conventional semiconductors, opposite considerations apply, with a positive surface charge from unoccupied donor-like surface states leading to downward band bending. This again leads to a depletion of the majority carriers at the surface.…”

Band gap engineering of In2O3 by alloying with Tl2O3

“…6 The fundamental band gap is $0.8 eV smaller than the optical band gap of In 2 O 3 , as transitions from states within 0.8 eV of the valence band maximum (VBM) to the conduction band (CB) are symmetry disallowed. 6,7 This understanding helped to rationalize previous XPS band alignments, which had considered that the CBM of In 2 O 3 was 3.75 eV above the VBM. The ability to lower the CBM of In 2 O 3 relative to the vacuum level and also to simultaneously decrease the effective mass of the CBM would have a huge effect on its electronic conductivity and also on the ability to modulate the workfunction and open up the material for other applications, such as hole injection layers in organic photovoltaics.…”

“…6,24 The position of the valence band edge is influenced by a number of factors including experimental spectral broadening 25 and band bending at the surface, leading for undoped In 2 O 3 to formation of an electron accumulation layer. 7 Despite these complications changes in the position of the valence band edge upon alloying with Tl can be regarded as significant. It is therefore interesting to find that the valence band edge moves to lower binding energy with incorporation of Tl, with an onset at 2.49 eV for a surface x value 0.36. surface region probed by XPS assuming that there is no change in the band bending.…”