Abstract
n-type binary compound semiconductors such as InN, InAs, or In2O3 are especial because the branch-point energy or charge neutrality level lies within the conduction band. Their tendency to form a surface electron accumulation layer prevents the formation of rectifying Schottky contacts. Utilizing a reactive sputtering process in an oxygen-containing atmosphere, we demonstrate Schottky barrier diodes on indium oxide thin films with rectifying properties being sufficient for space charge layer spectroscopy. Conventional non-reactive sputtering resulted in ohmic contacts. We compare the rectification of Pt, Pd, and Au Schottky contacts on In2O3 and discuss temperature-dependent current-voltage characteristics of Pt/In2O3 in detail. The results substantiate the picture of oxygen vacancies being the source of electrons accumulating at the surface, however, the position of the charge neutrality level and/or the prediction of Schottky barrier heights from it are questioned.
Highlights
Semiconducting oxides (SO) find application in transparent electrodes for thin film solar cells, as rectifiers, as transistors, as UV-photodetectors, and as chemical, gas and pH-sensors
Especially the surface electron accumulation layer (SEAL) of SOs is difficult to modify and in some cases like for In2O3 not sufficiently tunable to allow the formation of Schottky contacts
A remote oxygen plasma treatment was capable of removing the electron accumulation at the surface of ZnO9,10 and allowed the fabrication of Schottky contacts with acceptable rectification thereon
Summary
Semiconducting oxides (SO) find application in transparent electrodes for thin film solar cells, as rectifiers, as transistors, as UV-photodetectors, and as chemical, gas and pH-sensors. King et al explained the surface electron accumulation as well as the ease of extrinsic n-type doping of In2O3 by the position of the branch point energy Ebp (sometimes referred to as charge neutrality level) within the conduction band and about 0.4 eV above the conduction band minimum (CBM).[5] Recently, a value of 0.35 eV above CBM was derived by quasi-particle calculations in close agreement with the experimental value.[6] The microscopic origin of surface electrons is likely the formation of oxygen vacancies at the topmost surface layer(s).[7,8] A remote oxygen plasma treatment was capable of removing the electron accumulation at the surface of ZnO9,10 and allowed the fabrication of Schottky contacts with acceptable rectification thereon.
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