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

Highly transparent ZnO-based thin-film transistors (TFTs) are fabricated with optical transmission (including substrate) of ∼75% in the visible portion of the electromagnetic spectrum. Current–voltage measurements indicate n-channel, enhancement-mode TFT operation with excellent drain current saturation and a drain current on-to-off ratio of ∼107. Threshold voltages and channel mobilities of devices fabricated to date range from ∼10 to 20 V and ∼0.3 to 2.5 cm2/V s, respectively. Exposure to ambient light has little to no observable effect on the drain current. In contrast, exposure to intense ultraviolet radiation results in persistent photoconductivity, associated with the creation of electron-hole pairs by ultraviolet photons with energies greater than the ZnO band gap. Light sensitivity is reduced by decreasing the ZnO channel layer thickness. One attractive application for transparent TFTs involves their use as select-transistors in each pixel of an active-matrix liquid-crystal display.

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ZnO-based transparent thin-film transistors

Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 297 eV and 371 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 325 eV Different magnetoresistance behavior was observed for the samples codoped with n-type carriers Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K

High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: A series of oxide-diluted magnetic semiconductors and their properties

Adsorbed layer and thin film growth modes monitored by Auger electron spectroscopy

Wide bandgap oxide semiconductors constitute a unique class of materials that combine properties of electrical conductivity and optical transparency. They are being widely used as key materials in optoelectronic device applications, including flat-panel displays, solar cells, OLED, and emerging flexible and transparent electronics. In this article, an up-to-date review on both the fundamental understanding of materials physics of oxide semiconductors, and recent research progress on design of new materials and high-performing thin film transistor (TFT) devices in the context of fundamental understanding is presented. In particular, an in depth overview is first provided on current understanding of the electronic structures, defect and doping chemistry, optical and transport properties of oxide semiconductors, which provide essential guiding principles for new material design and device optimization. With these principles, recent advances in design of p-type oxide semiconductors, new approaches for achieving cost-effective transparent (flexible) electrodes, and the creation of high mobility 2D electron gas (2DEG) at oxide surfaces and interfaces with a wealth of fascinating physical properties of great potential for novel device design are then reviewed. Finally, recent progress and perspective of oxide TFT based on new oxide semiconductors, 2DEG, and low-temperature solution processed oxide semiconductor for flexible electronics will be reviewed.

Wide Bandgap Oxide Semiconductors: from Materials Physics to Optoelectronic Devices

Quantum properties of strong accumulation layers on ZnO surfaces

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

ELS studies of quantized accumulation layers on ZnO surfaces

Implantation of 100‐eV hydrogen ions on the (0001) face of ZnO produces extremely strong electron accumulation layers on the surface. Calibrated argon‐ion sputtering measurements indicate that the implanted ions (consisting mostly of protons) penetrate to a depth of 10–20 A beneath the surface. They act as fully ionized donors giving rise to a free electron layer of comparable depth and of surface electron densities up to 2×1014 cm−2. The narrow width, combined with detailed transport measurements, show that the implanted accumulation layers closely resemble ordinary accumulation layers on ZnO surfaces obtained, for example, by exposing the surface to atomic hydrogen. Thus the implanted layers, just as the ordinary accumulation layers, constitute a quantized, two‐dimensional electron gas system. The important advantage of the implanted over the ordinary accumulation layers, however, is that they are completely inert to oxygen or even to room air. Annealing experiments provide estimates for the activation...

Quantized electron accumulation layers on ZnO surfaces produced by low‐energy hydrogen‐ion implantation

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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