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

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 present status and prospects for further development of polycrystalline or amorphous transparent conducting oxide (TCO) semiconductors used for practical thin-film transparent electrode applications are presented in this paper. The important TCO semiconductors are impurity-doped ZnO, In2O3 and SnO2 as well as multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2, including some ternary compounds existing in their systems. Development of these and other TCO semiconductors is important because the expanding need for transparent electrodes for optoelectronic device applications is jeopardizing the availability of indium-tin-oxide (ITO), whose main constituent, indium, is a very expensive and scarce material. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promising as alternatives to ITO for thin-film transparent electrode applications. In particular, AZO thin films, with a low resistivity of the order of 10−5 Ω cm and source materials that are inexpensive and non-toxic, are the best candidates. However, further development of the deposition techniques, such as magnetron sputtering or vacuum arc plasma evaporation, as well as of the targets is required to enable the preparation of AZO and GZO films on large area substrates with a high deposition rate.

https://iopscience.iop.org/article/10.1088/0268-1242/20/4/004/pdf

Transparent conducting oxide semiconductors for transparent electrodes

Optically transparent oxides tend to be electrical insulators, by virtue of their large electronic bandgap (⩾3.1 eV). The most notable exceptions are doped versions of the oxides In2O3, SnO2 and ZnO—all n-type (electron) conductors—which are widely used as the transparent electrodes in flat-panel displays1,2. On the other hand, no transparent oxide exhibiting high p-type (hole) conductivity is known to exist, whereas such materials could open the way to a range of novel applications. For example, a combination of the two types of transparent conductor in the form of a pn junction could lead to a ‘functional’ window that transmits visible light yet generates electricity in response to the absorption of ultraviolet photons. Here we describe a strategy for identifying oxide materials that should combine p-type conductivity with good optical transparency. We illustrate the potential of this approach by reporting the properties of thin films of CuAlO2, a transparent oxide having room-temperature p-type conductivity up to 1 S cm−1. Although the conductivity of our candidate material is significantly lower than that observed for the best n-type conducting oxides, it is sufficient for some applications, and demonstrates that the development of transparent p-type conductors is not an insurmountable goal.

P-type electrical conduction in transparent thin films of CuAlO2

Increasing demand for raw materials means that alternatives to indium-tin oxide are desired for optically transparent electrode applications. Carbon nanotube, metal nanowire networks and regular metal grids have been investigated as possible options. In this review, these materials and recently rediscovered graphene are compared with the usual transparent conductive oxides.

Past achievements and future challenges in the development of optically transparent electrodes

A working hypothesis for exploring optically transparent and electrically conducting amorphous oxides is proposed on the basis of simple considerations concerning chemical bonding. The hypothesis predicts that amorphous oxides composed of heavy metal cations with an electronic configuration of ( n − 1)d 10 ns 0 may be converted into transparent conducting amorphous oxides when doped by Li ion implantation or heating at temperatures below crystallization. Three new materials, amorphous Cd 2 GeO 4 , AgSbO 3 and Cd 2 PbO 4 , have been prepared as examples.

Working hypothesis to explore novel wide band gap electrically conducting amorphous oxides and examples

β-Ga2O3 single crystals were grown by the floating zone method and their conductivity along the b axis was controlled from <10−9 to 38 Ω−1 cm−1 by changing the growth atmosphere. By using feed rods doped with Sn, the grown crystal became highly conductive even under oxidative atmosphere. The optical transmission spectra showed that the β-Ga2O3 single crystal with 0.32 mm was transparent in the visible and ultraviolet region, with 20% transmittance at the fourth-harmonic wave of the Nd:YAG laser (266 nm). The band-gap widening was observed with the increasing of the carrier concentration. It is expected that the light of the KrF laser can be transmitted in the heavily doped β-Ga2O3.

Synthesis and control of conductivity of ultraviolet transmitting β-Ga2O3 single crystals

Anisotropy of electrical and optical properties in β-Ga2O3 single crystals has been investigated at room temperature. The conductivity and mobility of the degenerate sample along the direction of b and c axes are 38 Ω−1 cm−1, 46 cm2 V−1 s−1, and 2.2 Ω−1 cm−1, 2.6 cm2 V−1 s−1, respectively. The absorption edges of the insulating sample for light polarized E//b and E//c were 4.79 and 4.52 eV, respectively. The rate of the band gap widening with increasing carrier concentration was much larger for E//b than E//c. The origin of these properties are discussed by considering the crystal and electronic structure of β-Ga2O3.

Anisotropy of electrical and optical properties in β-Ga2O3 single crystals

ZnGa2O4 having the spinel structure was prepared, and optical and electrical properties of the material were measured. By the measurements of diffuse reflectance spectra, ZnGa2O4 was found to have a wider band gap (∼5 eV) than ITO (indium tin oxide). Electrical conductivity of the H2‐annealed ceramic of ZnGa2O4 was 3×101 S cm−1. Thus the ZnGa2O4 spinel was found to be a new UV‐transparent electronic conductor.

New ultraviolet‐transport electroconductive oxide, ZnGa2O4 spinel

CdO thin films were deposited on silica glass substrates by rf sputtering. The orientation of the crystal axis in the films was changed by varying the sputtering conditions. The (100)-oriented films were annealed under various conditions to change the carrier concentration. A blueshift of the optical absorption edge was observed as the carrier concentration increased, and band-gap widening was analyzed using the Burstein–Moss formula. We found that the intrinsic direct-band-gap Eg0 (Γ15–Γ1 gap) and the reduced effective mass mvc* are 2.22±0.01 eV and (0.274±0.013)m0, respectively, at room temperature. The results are discussed in relation to the band structure of CdO.

Naoyuki Ueda

Papers

Working hypothesis to explore novel wide band gap electrically conducting amorphous oxides and examples

Synthesis and control of conductivity of ultraviolet transmitting β-Ga2O3 single crystals

Anisotropy of electrical and optical properties in β-Ga2O3 single crystals

New ultraviolet‐transport electroconductive oxide, ZnGa2O4 spinel

Band-gap widening of CdO thin films