The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon-based electronics cannot provide a solution. Examples of active flexible applications include paper displays and wearable computers1. So far, mainly flexible devices based on hydrogenated amorphous silicon (a-Si:H)2,3,4,5 and organic semiconductors2,6,7,8,9,10 have been investigated. However, the performance of these devices has been insufficient for use as transistors in practical computers and current-driven organic light-emitting diode displays. Fabricating high-performance devices is challenging, owing to a trade-off between processing temperature and device performance. Here, we propose to solve this problem by using a novel semiconducting material—namely, a transparent amorphous oxide semiconductor from the In-Ga-Zn-O system (a-IGZO)—for the active channel in transparent thin-film transistors (TTFTs). The a-IGZO is deposited on polyethylene terephthalate at room temperature and exhibits Hall effect mobilities exceeding 10 cm2 V-1 s-1, which is an order of magnitude larger than for hydrogenated amorphous silicon. TTFTs fabricated on polyethylene terephthalate sheets exhibit saturation mobilities of 6–9 cm2 V-1 s-1, and device characteristics are stable during repetitive bending of the TTFT sheet.

Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors

We report the fabrication of transparent field-effect transistors using a single-crystalline thin-film transparent oxide semiconductor, InGaO 3 (ZnO) 5 , as an electron channel and amorphous hafnium oxide as a gate insulator. The device exhibits an on-to-off current ratio of ∼10 6 and a field-effect mobility of ∼80 square centimeters per volt per second at room temperature, with operation insensitive to visible light irradiation. The result provides a step toward the realization of transparent electronics for next-generation optoelectronics.

http://science.sciencemag.org/content/sci/300/5623/1269.full.pdf

Thin-Film Transistor Fabricated in Single-Crystalline Transparent Oxide Semiconductor

Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper.

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

The surface and materials science of tin oxide

We have investigated carrier transport in a crystalline oxide semiconductor InGaO3(ZnO)5 using single-crystalline thin films. When carrier concentration is less than 2×1018cm−3, logarithm of electrical conductivity decreases in proportion to T−1∕4 and room-temperature Hall mobility was as low as ∼1cm2(Vs)−1. When carrier concentration was increased to 4×1018cm−3, the conduction mechanism changed to degenerate conduction and room-temperature Hall mobility was steeply increased to >10cm2(Vs)−1, showing metal–insulator transition behavior. These results are explained by percolation conduction over distribution of potential barriers formed around conduction band edge. The potential distribution is a consequence of potential modulation originating from random distribution of Ga3+ and Zn2+ ions in the crystal structure of InGaO3(ZnO)5.

Carrier transport in transparent oxide semiconductor with intrinsic structural randomness probed using single-crystalline InGaO3(ZnO)5 films

A natural superlattice homologous single crystal thin film, characterized in that it comprises a composite oxide which is represented by the formula M1M2O3(ZnO)m, wherein M1 is at least one of Ga, Fe, Sc, In, Lu, Yb, Tm, Er, Ho and Y, M2 is at least one of Mn, Fe, Ga, In and Al, and m is a natural number of 1 or more, and has been grown epitaxially on an epitaxial thin film formed on a single crystal substrate, or on said single crystal substrate from which said epitaxial thin film has disappeared, or on a ZnO single crystal; a method for preparing the natural superlattice thin film which comprises depositing the composite oxide, and diffusing the resultant laminated film by heating it. The natural superlattice homologous single crystal thin film is suitably used in an optical device, an electronic device, an X-ray optical device and the like.

Natural-superlattice homologous single crystal thin film, method for preparation thereof, and device using said single crystal thin film

Inorganic solids with wide bandgaps are usually classified as electrical insulators and are used in industry as insulators, dielectrics, and optical materials. Many metallic oxides have wide bandgaps because of the significant contribution of ionic character to the chemical bonds between metallic cations and oxide ions. Their ionic nature simultaneously suppresses the formation of easily ionizable shallow donors or acceptors and enhances the localization of electrons and positive holes. Thus it is understandable that interest in these wide-gap oxides as conductive materials has not been strong.

Transparent p -Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions

Electrical and optical properties of CuAlO2, a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique. The indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively. The conductivity at 300 K was ∼3×10−1 S cm−1 and its temperature dependence is of the thermal-activation type (activation energy ≈0.2 eV) at temperatures >220 K but is of the variable-range hopping type (log σ∝T−1/4) at <220 K. It was inferred that an admixed state of Cu 3d and O 2p primarily constitutes the upper valence band, which controls transport of positive holes, from a combined information on ultraviolet photoemission spectrum with x-ray photoemission spectrum. An energy band calculation by full-potential linearized augmented plane wave method substantiated the experimental findings. The present results gave a solid basis for our working hypothesis [Nature (London) 389, 939 (1997)] for chemical design of p-type conducting transpar...

Electronic structure and optoelectronic properties of transparent p-type conducting CuAlO2

Transparent p-type conducting CuGaO2 thin films were prepared on α-Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60° with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (∼80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3×10−2 S cm−1, 1.7×1018 cm−3, and 0.23 cm2 V−1 s−1, respectively.

Kazushige Ueda

Papers

Carrier transport in transparent oxide semiconductor with intrinsic structural randomness probed using single-crystalline InGaO3(ZnO)5 films

Natural-superlattice homologous single crystal thin film, method for preparation thereof, and device using said single crystal thin film

Transparent p -Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions

Electronic structure and optoelectronic properties of transparent p-type conducting CuAlO2

Epitaxial growth of transparent p-type conducting CuGaO2 thin films on sapphire (001) substrates by pulsed laser deposition