title: Direct Measurements of Fermi Level Pinning at the Surface of Intrinsically n-Type InGaAs Nanowires.
abstract: Surface effects strongly dominate the intrinsic properties of semiconductor nanowires (NWs), an observation that is commonly attributed to the presence of surface states and their modification of the electronic band structure. Although the effects of the exposed, bare NW surface have been widely studied with respect to charge carrier transport and optical properties, the underlying electronic band structure, Fermi level pinning, and surface band bending profiles are not well explored. Here, we directly and quantitatively assess the Fermi level pinning at the surfaces of composition-tunable, intrinsically n-type InGaAs NWs, as one of the prominent, technologically most relevant NW systems, by using correlated photoluminescence (PL) and X-ray photoemission spectroscopy (XPS). From the PL spectral response, we reveal two dominant radiative recombination pathways, that is, direct near-band edge transitions and red-shifted, spatially indirect transitions induced by surface band bending. The separation of their relative transition energies changes with alloy composition by up to more than ∼40 meV and represent a direct measure for the amount of surface band bending. We further extract quantitatively the Fermi level to surface valence band maximum separation using XPS, and directly verify a composition-dependent transition from downward to upward band bending (surface electron accumulation to depletion) with increasing Ga-content x(Ga) at a crossover near x(Ga) ∼ 0.2. Core level spectra further demonstrate the nature of extrinsic surface states being caused by In-rich suboxides arising from the native oxide layer at the InGaAs NW surface.
authors.affiliation: Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München , Am Coulombwall 4, Garching 85748, Germany.
authors.forename: Maximilian
authors.initials: M
authors.lastname: Speckbacher
authors.affiliation: Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München , Am Coulombwall 4, Garching 85748, Germany.
authors.forename: Julian
authors.initials: J
authors.lastname: Treu
authors.affiliation: Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool , Liverpool L69 7ZF, United Kingdom.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Whittles
authors.affiliation: Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool , Liverpool L69 7ZF, United Kingdom.
authors.forename: Wojciech M
authors.initials: WM
authors.lastname: Linhart
authors.affiliation: Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München , Am Coulombwall 4, Garching 85748, Germany.
authors.forename: Xiaomo
authors.initials: X
authors.lastname: Xu
authors.affiliation: Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München , Am Coulombwall 4, Garching 85748, Germany.
authors.forename: Kai
authors.initials: K
authors.lastname: Saller
authors.affiliation: Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool , Liverpool L69 7ZF, United Kingdom.
authors.forename: Vinod R
authors.initials: VR
authors.lastname: Dhanak
authors.affiliation: Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München , Am Coulombwall 4, Garching 85748, Germany.
authors.forename: Gerhard
authors.initials: G
authors.lastname: Abstreiter
authors.affiliation: Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München , Am Coulombwall 4, Garching 85748, Germany.
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Finley
authors.affiliation: Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool , Liverpool L69 7ZF, United Kingdom.
authors.forename: Tim D
authors.initials: TD
authors.lastname: Veal
authors.affiliation: Walter Schottky Institut, Physik Department, and Center of Nanotechnology and Nanomaterials, Technische Universität München , Am Coulombwall 4, Garching 85748, Germany.
authors.forename: Gregor
authors.initials: G
authors.lastname: Koblmüller
keywords: Fermi level pinning; InGaAs nanowires; X-ray photoemission spectroscopy; photoluminescence; surface states