The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications

Abstract

Transparent conducting oxides (TCO) have integral and emerging roles in photovoltaic, thermoelectric energy conversion, and more recently, photocatalytic systems. The functional properties of TCOs, and thus their role in these applications, are often mediated by the bulk electronic band structure but are also strongly influenced by the electronic structure of the native surface 2D electron gas (2DEG), particularly under operating conditions. This study investigates the 2DEG, and its response to changes in chemistry, at the (111) surface of the model TCO In2O3, through angle resolved and core level X‐ray photoemission spectroscopy. It is found that the itinerant charge carriers of the 2DEG reside in two quantum well subbands penetrating up to 65 Å below the surface. The charge carrier concentration of this 2DEG, and thus the high surface n‐type conductivity, emerges from donor‐type oxygen vacancies of surface character and proves to be remarkably robust against surface absorbents and contamination. The optical transparency, however, may rely on the presence of ubiquitous surface adsorbed oxygen groups and hydrogen defect states that passivate localized oxygen vacancy states in the bandgap of In2O3.

Do We Need “Ionosorbed” Oxygen Species? (Or, “A Surface Conductivity Model of Gas Sensitivity in Metal Oxides Based on Variable Surface Oxygen Vacancy Concentration”)

“…These findings were extended by elegant and visually beautiful experimental work by Jovic et al in which ARPES was used to directly image the electronic states forming the surface conductivity layer. These were shown to vary in intensity (i.e., surface conductivity varies) as a function of dynamic dosing of oxygen, as shown in Figure …”

“…(b) Variation in the two-dimensional electron gas intensity with sequential exposure to UHV and oxygen. A linear color scale of intensity is shown at the inset (reproduced with permission from ref . Copyright 2019 Wiley).…”

Two-dimensional electron gas of the In2O3 surface: Enhanced thermopower, electrical transport properties, and reduction by adsorbates or compensating acceptor doping

“…[21]. Moreover, angle-resolved photoelectron spectroscopy (ARPES) measurements have confirmed the twodimensional electron gas (2DEG) nature of the SEAL with sheet electron concentration of 4×10 13 cm −2 after surface preparation at a high temperature (by repeated cycles of Ar + sputtering (500 eV) and annealing at 600 • C in UHV for 1 h) [17] and 2×10 13 cm −2 after surface preparation at an intermediate temperature (annealing at 300 • C in UHV for ≈15 min) [22]. Both of the surface preparations employed within those studies are prone to reduce the surface-that is, increase the concentration of surface V O or In adatoms acting as surface donors-and thus result in a stronger SEAL (i.e., with a higher electron concentration) compared to that of an unprepared sample.…”

The electrical conductivity of cubic (In$_{1-x}$Ga$_x$)$_2$O$_3$ films ($x\le0.18$): Native point defects, Sn-doping, and the surface electron accumulation layer

“…8,9 In 2 O 3 also possesses a few-nanometers-thick, two-dimensional surface electron accumulation layer (SEAL), the existence of which has been demonstrated by a downward band bending at the surface and the presence of quantized subbands. [11][12][13] The conductivity of the SEAL is a particularly important property for applications of In 2 O 3 as gas-sensing material 14 , facilitates the forma-tion of ohmic contacts but makes the formation of Schottky contacts challenging. 15 Various methods allow the modulation of the SEAL: A reduction of the SEAL carriers can be achieved by an oxygen plasma treatment of the surface at elevated temperatures 14,16,17 , by adsorption of oxidizing reactive species, such as O 2 and O 3 18 or by intentional compensating acceptor doping.…”