Electronic structure of indium-tungsten-oxide alloys and their energy band alignment at the heterojunction to crystalline silicon

Abstract

The electronic structure of thermally co-evaporated indium-tungsten-oxide films is investigated. The stoichiometry is varied from pure tungsten oxide to pure indium oxide and the band alignment at the indium-tungsten-oxide/crystalline silicon heterointerface is monitored. Using in-system photoelectron spectroscopy, optical spectroscopy and surface photovoltage measurements we show that the work function of indium-tungsten-oxide continuously decreases from 6.3 eV for tungsten oxide to 4.3 eV for indium oxide, with a concomitant decrease of the band bending at the hetero interface to crystalline silicon than indium oxide.

Evolution of Optical, Electrical, and Structural Properties of Indium Tungsten Oxide upon High Temperature Annealing

“…changing the material composition. [24] In that study, we found a strong Fermi-level pinning at the IWO x /(n)c-Si interface with a pinning factor of S ¼ 0.1. On the other hand, a high band bending of up to 600 meV toward (n)c-Si was observed for pure tungsten oxide, which should provide sufficient selectivity if applied as a hole extracting layer in silicon heterojunction solar cells.…”

“…The conductivity of the pure indium oxide could be increased by increasing the annealing temperature, up to 0.39 S cm À1 at T Ann ¼ 700 C. For T > T C , the pure tungsten oxide shows an increasing conductivity of up to 1.6 Â 10 À4 S cm À1 . However, for the thicknesses of about 10 nm, the estimated series resistance contribution and the low parasitic absorption for C In ¼ 22% and 26% of tungsten-rich mixtures in combination with the high band bending toward crystalline silicon [24] and, therefore, a probably reduced contact resistance could still make it interesting in applications as carrier selective contact for solar cells.…”

“…The IWO x layers were deposited on 1.2 mm thick, 1 Â 1 in corning glasses by thermal co-evaporation in an ultrahigh vacuum chamber (base pressure <10 À9 mbar) (see Menzel et al [24] for details). The substrate was not intentionally heated; however, a slightly elevated substrate temperature caused by the about 20 cm distant crucibles cannot be completely excluded.…”

“…For more detailed information about the fitting models, see also the previous study. [24] The In and W fractions of the metal oxide mixtures were calculated using sensitivity factors obtained from WO 3 and In 2 O 3 samples, which have been oxidized post-deposition by an oxygen plasma at 10 À5 mbar for 10 min and were, therefore, fully stoichiometric. The ratio of the metal peak area to the O 1s peak area, each divided by the known number of the respective atom in the chemical formula of the metal oxide, was used as a sensitivity factor.…”

“…The mixed oxide layers with C In ¼ 7%, 41%, 53% and also the one with the highest indium oxide fraction (C In ¼ 64%) are not conductive enough for this measurement method. The difference in composition from pure indium oxide to C In ¼ 64% is quite high; however, the main gain in terms of applicability as hole selective contact (high work function) is observed for C In < 40%, [24] which makes the indium-rich stoichiometries less interesting for our purpose. The pure indium oxide shows an overall increasing conductivity up to 0.39 S cm À1 after annealing at 700 C. For T > T C , the pure tungsten oxide also shows an increasing conductivity of up to 1.6 Â 10 À4 S cm À1 , whereas the conductivity of all mixed oxide layers stays below 10 À6 S cm À1 .…”