The electronic properties of inversion and accumulation layers at semiconductor-insulator interfaces and of other systems that exhibit two-dimensional or quasi-two-dimensional behavior, such as electrons in semiconductor heterojunctions and superlattices and on liquid helium, are reviewed. Energy levels, transport properties, and optical properties are considered in some detail, especially for electrons at the (100) silicon-silicon dioxide interface. Other systems are discussed more briefly.

Electronic properties of two-dimensional systems

TiO2 sol–gel films doped with gold nanoparticles are used as both optical and conductometric sensors for the detection of CO and H2. Tests performed at 360 °C show for the first time CO- and H2-induced reversible variations in the optical absorption of thin TiO2/Au sol–gel films. The absorbance changes are strongly dependent on both the testing wavelength and the film microstructure. Together with the sensing dynamics observed through an electric interface, it is possible to obtain a better understanding of the mechanisms involved in the detection of both gases. The presence of H2 elicits almost ideal, step-like dynamic responses using conductometric detection.

/pdf/gold-nanoparticle-doped-tio2-semiconductor-thin-films-gas-q1xozao5sj.pdf

Gold Nanoparticle-Doped TiO2 Semiconductor Thin Films: Gas Sensing Properties†

In this investigation, the growth, structure, and electronic properties of Pt films on the polar surfaces of ZnO were examined using high‐resolution electron energy‐loss spectroscopy (HREELS) and low‐energy, electron diffraction (LEED) The growth mode of vapor‐deposited Pt films on ZnO(0001) and ZnO(0001) at 300 K was found to be nearly layer‐by‐layer The surfaces of Pt films produced in this manner exhibited hexagonal symmetry and were stable up to 600 K At higher temperatures, the Pt agglomerated into particles which remained oriented with respect to the ZnO substrate HREELS results indicate that there are only weak interactions at the Pt/ZnO(0001) interface, while charge transfer and Schottky barrier formation occurs at the Pt/ZnO(0001) interface

/pdf/interaction-of-platinum-films-with-the-0001-and-0001-56au864veg.pdf

Interaction of platinum films with the (0001̄) and (0001) surfaces of ZnO

Angle-resolved photoelectron spectroscopy utilizing synchrotron radiation is used to investigate the surface electronic structures of nonpolar $\text{ZnO}(10\overline{1}0)$ surfaces covered with hydrogen, methanol, and water. Adsorption of these species leads to the semiconductor-to-metal transition by forming single partially filled metallic bands with a $\text{Zn}\text{ }4s$ character. The surface charge densities are in the order of ${10}^{12}--{10}^{13}\text{ }{\text{cm}}^{\ensuremath{-}2}$. The bottom positions of the metallic bands are shallower than the expected positions of the ZnO conduction-band minima for all three adsorption systems. This implies that the metallic bands should be quantized states along the surface-normal direction.

Metallization of ZnO ( 10 1 ¯ 0 ) by adsorption of hydrogen, methanol, and water: Angle-resolved photoelectron spectroscopy

We have investigated the stability of O-polar $\text{ZnO}\text{ }(000\overline{1})\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces with photoemission spectroscopy and low-energy electron diffraction. We present direct evidence of quasi-two-dimensional electron gas (2DEG) formation $({N}_{2D}\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }{\text{cm}}^{\ensuremath{-}2})$ at the O-face from our angle-resolved photoemission spectroscopy studies. These findings are discussed in terms of hydrogenation of the O-face by chemisorptions process and the energetic location of the charge-neutrality level of ZnO. Our results reveal insight into the role of hydrogen in forming the quasi-2DEG and stabilizing the O-polar nonreconstructed surface.

Direct evidence of metallicity at ZnO ( 000 1 ¯ ) − ( 1 × 1 ) surfaces from angle-resolved photoemission spectroscopy

Low-energy\char22{}electron-loss spectroscopy is used to study collective surface excitations on accumulation layers on ZnO. A prominent loss peak is observed which shifts in energy with electron concentration. The results are well accounted for theoretically and provide direct evidence for the existence of two-dimensional plasmons.

Plasmas on Quantized Accumulation Layers on ZnO Surfaces

Angle-resolved electron-energy-loss spectroscopy from solid-state surfaces is studied both theoretically and experimentally, with particular emphasis on the ZnO surface. The plasmonlike loss peak is found to shift as a function of detector angle. This is due to the highly dispersive nature of the plasmon mode. There is also evidence for a phononlike model which remains unshifted as a function of detector angle. The role of both collective and single-particle excitations are considered, and the theory is found to be in good agreement with the experiment.

Angle-resolved electron-energy-loss spectroscopy of two-dimensional plasmons

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Plasmas on Quantized Accumulation Layers on ZnO Surfaces

Angle-resolved electron-energy-loss spectroscopy of two-dimensional plasmons