**2. Semiconducting metal oxide thin-film transistors**

Semiconducting metal oxides are promising materials to replace semiconductors as amorphous silicon (a-Si) and polycrystalline silicon (poly-Si) in applications as drive circuits of active-matrix (AM) flat-panel displays commonly used in cell phones, notebooks and monitor screens. They present interesting features like high optical transmittance in the visible range, high electronic mobility, low fabrication cost and compatibility to large-area applications. In the past 15 years, substantial efforts have been made to achieve high-performance SMO TFTs which are suitable to transparent and flexible substrates, enabling the development of the next generation of thin-flat panel displays.

(CVD) and molecular-beam epitaxy (MBE), most of the technological applications use thin films which are polycrystalline or formed by large-size crystallites separated by grain boundaries (as obtained by RF sputtering), presenting limitations to the charge-carrier transport.

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides

http://dx.doi.org/10.5772/intechopen.78221

Zinc oxide is known as an unintentionally doped semiconductor, due to the presence of native (intrinsic) defects in the crystal lattice. These defects can be vacancies (missing atoms at regular lattice positions), interstitials (extra atoms occupying interstices in the lattice) and antisites (an anion occupying a cation position in the lattice or vice versa) [28]. Although controversial, oxygen vacancies and zinc interstitials have been often credited as the major source of the observed unintentional n-type conductivity in ZnO [9, 24, 29–32]. The oxygen vacancies (*Vo*

have the lowest formation energy among the native defects which act as donors in ZnO and are frequently associated in the literature to the n-type character of ZnO. However, density func-

donors and some authors affirm that they cannot be responsible for the n-type carrier transport

usually present in very low concentrations in n-type ZnO. An alternative explanation is that n-type conductivity is due to unintentional substitutional hydrogen impurities, which is supported by theoretical calculations [35, 36]. Therefore, the actual origin of n-type conductivity in ZnO still remains controversial since a great number of experimental reports on the electrical properties of thin-film ZnO electronic devices demonstrate a correlation between oxygen concentration (during deposition and/or device handling) and the electrical conductivity [23, 37, 38], disagreeing with results obtained from first-principle theoretical calculations in crystals.

Electronic and optoelectronic devices based on metal oxides usually comprise thin-films deposited on appropriate substrates. High-quality crystalline ZnO layers can be obtained by using extremely controlled deposition processes like pulsed laser deposition (PLD), molecular beam epitaxy (MBE), chemical vapor deposition (CVD), metal–organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) [12, 33, 39] and even using less

*'s* behave more as deep donor defects instead of shallow

), on the other hand, behave as shallow donors but are

tional calculations have shown that *Vo*

**Figure 1.** Representation of the ZnO unit cell with wurtzite structure.

in ZnO [33, 34]. Zinc interstitials (*Zni*

*2.1.1. Thin-film deposition methods*

sophisticated methods like RF magnetron sputtering [22–25].

)

137

As the active layer of thin-film transistors, SMOs usually present field-effect mobilities higher than 10 cm<sup>2</sup> .V−1.s−1 (with a reported values as high as 172 cm<sup>2</sup> V−1s−1) when deposited by techniques like RF sputtering and pulsed-laser deposition [22–27], which is a great advantage if we consider that a-Si can hardly present field-effect mobilities higher than 1 cm<sup>2</sup> V−1s−1. Compared to poly-Si, which presents carrier mobilities up to 100 cm<sup>2</sup> V−1s−1, SMOs present higher-film uniformity and considerably lower-processing temperatures, allowing large-area applications and low-production costs.
