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.
Highlights
F ree-standing semiconductor nanowires (NW) are well known for their prominent surface effects due to their very large surface-to-volume ratio
The presence of surface states, adsorbed impurities, and their charge interactions can massively alter the physical properties of NWs; effects, which can be advantageous or detrimental depending on the given scenario
Label-free gas and chemical sensing using semiconductor NW-based electronic devices have been widely demonstrated.[1−4] In this respect, NW materials from low bandgap III−V and metal oxide semiconductors (e.g., InAs, InN, In2O3, etc.) are attractive due to their exceptional transport properties, straightforward formation of low-resistance Ohmic contacts,[5,6] and very high sensitivity to the adsorption of gaseous or liquid molecules[3,4,7,8] enabled by an intrinsic surface electron accumulation layer present in these materials.[9−12] This unusual surface electronic behavior and electron accumulation in such low-gap materials is commonly attributed to large densities of donor-type surface states, which pin the Fermi level above the conduction band minimum (CBM) at the surface and create strong surface band bending.[9−12]
Summary
Letter increased interface state defect densities (Dit) in InGaAs−NW metal−oxide−semiconductor (MOS)-FETs.[15,16] The situation becomes more complex when the energetic position of the surface defect level changes relative to the band edges as, for example, in band gap tunable III−V semiconductor materials. We anticipate a transition from downward bending (Fermi level pinned above the surface CBM, electron accumulation) in In-rich InGaAs NWs (0 < x(Ga) < ∼ 0.2) to upward bending (Fermi level pinned in the band gap at the surface, electron depletion) in more Ga-rich InGaAs NWs (x(Ga)> ∼ 0.3) This is supported by our XPS data below, semiquantitative space charge calculations by solving the Poisson equation (Supporting Information, Figure S2), as well as previous literature data on planar n-type InGaAs.[40] near flat-band conditions are expected in the region of ∼0.2 < x(Ga) < ∼0.3, as further verified by detailed XPS measurements (Figure 4).
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