Abstract
In2O3 is an n-type transparent semiconducting oxide possessing a surface electron accumulation layer (SEAL) like several other relevant semiconductors, such as InAs, InN, SnO2, and ZnO. Even though the SEAL is within the core of the application of In2O3 in conductometric gas sensors, a consistent set of transport properties of this two-dimensional electron gas (2DEG) is missing in the present literature. To this end, we investigate high quality single-crystalline as well as textured doped and undoped In2O3(111) films grown by plasma-assisted molecular beam epitaxy to extract transport properties of the SEAL by means of Hall effect measurements at room temperature while controlling the oxygen adsorbate coverage via illumination. The resulting sheet electron concentration and mobility of the SEAL are 1.5E13 cm^-2 and 150 cm^2/Vs, respectively, both of which get strongly reduced by oxygen-related surface adsorbates from the ambient air. Our transport measurements further demonstrate a systematic reduction of the SEAL by doping In2O3 with the deep compensating bulk acceptors Ni or Mg. This finding is supported by X-ray photoelectron spectroscopy measurements of the surface band bending and SEAL electron emission. Quantitative analyses of these XPS results using self-consistent, coupled Schroedinger-Poisson calculations indicate the simultaneous formation of compensating bulk donor defects (likely oxygen vacancies) which almost completely compensate the bulk acceptors. Finally, an enhancement of the thermopower by reduced dimensionality is demonstrated in In2O3: Seebeck coefficient measurements of the surface 2DEG with partially reduced sheet electron concentrations between 3E12 and 7E12 cm^-2 (corresponding average volume electron concentration between 1E19 and 2E19 cm^-3 indicate a value enhanced by 80% compared to that of bulk Sn-doped In2O3 with comparable volume electron concentration.
Highlights
Indium oxide (In2O3) is a transparent semiconducting oxide, which exhibits inherent n-type conductivity, commonly referred to as unintentional doping (UID)
An enhancement of the thermopower by reduced dimensionality is demonstrated in In2O3: Seebeck coefficient measurements of the surface 2DEG with partially reduced sheet electron concentrations between 3×1012 and 7×1012 cm−2 indicate a value enhanced by ≈80% compared to that of bulk Sn-doped In2O3 with comparable volume electron concentration
If the volume electron concentration dictated by the Boltzmann transport equation (BTE) curve, which is valid for bulk systems, is combined with the sheet electron concentration directly measured by Hall, the resulting surface electron accumulation layer (SEAL)
Summary
Indium oxide (In2O3) is a transparent semiconducting oxide, which exhibits inherent n-type conductivity, commonly referred to as unintentional doping (UID). A reduction of the SEAL in those films by oxygen adsorbates has been independently demonstrated by conductance and XPS measurements [4,33] These earlier works do not provide any information concerning the actual electron concentration at the In2O3 surface. The current work consistently determines the surface transport properties of In2O3 and demonstrates the intentional and controllable reduction of the sheet electron concentration at the surface of In2O3 by incorporation of the compensating bulk acceptors Ni and Mg or by oxygen surface adsorbates. This is accomplished through the combination of Hall effect transport measurements (with and without UV illumination) and x-ray photoelectron spectroscopy. As previously demonstrated for ZnO [40], the 2DEG at the In2O3 surface is shown to exhibit an increased thermopower in comparison to bulk Sn-doped films with comparable volume electron concentration
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
