**2.1. Zinc oxide and related compounds**

Zinc oxide and related compounds and alloys are the most studied semiconducting metal oxides used as active materials in electronic and optoelectronic devices. Applications in thinfilm transistors, light-emitting diodes and UV photodetectors and photosensors are feasible due to their chemical stability and exceptional electronic and physical properties. ZnO is transparent in the whole visible spectrum, has a wide (direct) band gap (Eg ~ 3.37 eV), a high electron mobility and large exciton binding energy (~ 60 meV). It crystallizes in either cubic zinc blend or hexagonal wurtzite structure, with the latter being the most thermodynamically stable form at ambient conditions. At relatively high temperatures and pressures, the rocksalt (NaCl) crystalline structure can also be formed. **Figure 1** shows the representation of the ZnO unit cell with wurtzite structure. In this structure, each cation is surrounded by four anions at the corners of a tetrahedron and vice versa, with a typically sp<sup>3</sup> covalent bonding nature coordination.

Even though large single crystals of ZnO can be obtained using appropriate substrates via very controlled deposition techniques like pulsed laser deposition (PLD), chemical vapor deposition

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides http://dx.doi.org/10.5772/intechopen.78221 137

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

and more sophisticated logic circuits which can be used in memories, microcontrollers and processors. Recently, the manufacture of metal oxide thin films using organic precursor solutions or nanoparticle suspensions became widespread [1–10], permitting the use of low-cost and nonsophisticated deposition techniques as spin coating, ink-jet printing and spray pyrolysis [18–21]. These techniques produce very uniform and homogeneous nanoscaled films, with high control of thickness and of other physical properties. This chapter aims to describe briefly the manufacturing processes of TFTs using solution-processed metal oxides as the semiconducting active layer, focusing on the electrical characterization and the study of the

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

As the active layer of thin-film transistors, SMOs usually present field-effect mobilities higher

niques 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>

higher-film uniformity and considerably lower-processing temperatures, allowing large-area

Zinc oxide and related compounds and alloys are the most studied semiconducting metal oxides used as active materials in electronic and optoelectronic devices. Applications in thinfilm transistors, light-emitting diodes and UV photodetectors and photosensors are feasible due to their chemical stability and exceptional electronic and physical properties. ZnO is trans-

mobility and large exciton binding energy (~ 60 meV). It crystallizes in either cubic zinc blend or hexagonal wurtzite structure, with the latter being the most thermodynamically stable form at ambient conditions. At relatively high temperatures and pressures, the rocksalt (NaCl) crystalline structure can also be formed. **Figure 1** shows the representation of the ZnO unit cell with wurtzite structure. In this structure, each cation is surrounded by four anions at the corners of a

Even though large single crystals of ZnO can be obtained using appropriate substrates via very controlled deposition techniques like pulsed laser deposition (PLD), chemical vapor deposition

V−1s−1) when deposited by tech-

V−1s−1, SMOs present

~ 3.37 eV), a high electron

covalent bonding nature coordination.

V−1s−1.

.V−1.s−1 (with a reported values as high as 172 cm<sup>2</sup>

Compared to poly-Si, which presents carrier mobilities up to 100 cm<sup>2</sup>

parent in the whole visible spectrum, has a wide (direct) band gap (Eg

electrical properties relevant to the evaluation of device performance.

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

next generation of thin-flat panel displays.

136 Design, Simulation and Construction of Field Effect Transistors

applications and low-production costs.

**2.1. Zinc oxide and related compounds**

tetrahedron and vice versa, with a typically sp<sup>3</sup>

than 10 cm<sup>2</sup>

(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.

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 functional calculations have shown that *Vo 's* behave more as deep donor defects instead of shallow donors and some authors affirm that they cannot be responsible for the n-type carrier transport in ZnO [33, 34]. Zinc interstitials (*Zni* ), on the other hand, behave as shallow donors but are 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.

#### *2.1.1. Thin-film deposition methods*

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

However, with the recent increase on field-effect mobility of TFTs produced with the active layer deposited from solution processes [3, 5, 6, 17, 19, 20], more attention has been attracted to the possibility of using very simple and low-cost deposition methods to obtain high-performance TFTs. Solution-based deposition processes allow the use of techniques like dip coating, spin coating, spray coating, ink-jet printing, silk screen and numerous others which are compatible to large-area, flexible, affordable and scalable applications. **Figure 2** shows a schematic representation of the basic features of common low-cost deposition methods for fabrication of metal oxide TFTs. RF magnetron sputtering (**Figure 2a**) and PLD (**Figure 2b**) produce highly crystalline films with relatively good thickness and uniformity control; however, target materials, vacuum and partial gas pressure systems as well as RF source or laser beam are needed, making these techniques more sophisticated when compared to solution-based processes like spin coating (**Figure 2c**), airbrush spray pyrolysis (**Figure 2d**) or ultrasonic spray pyrolysis (**Figure 2e**).

organic phase of the precursor material) to produce thin and highly uniform metal-oxide films. Difference between airbrush and ultrasonic spray deposition is that, in the former, a mechanically actuated needle is responsible to release the precursor solution, which flows due to gravity through the spray nozzle whereas in the latter, a piezoelectric nozzle driven by an ultrasonic power supply is used to nebulize the precursor solution which is injected by a micro-syringe pump. In both cases, compressed dry air or inert gas is used to carry the nebulized material. Ultrasonic spray has the advantage to provide higher droplet size control (in the order of tens of microns) and to economize precursor solution whereas airbrush is a low-cost and simple alternative to obtain very uniform films which can be used to produce high-performance TFTs [13].

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Solution-processed metal oxide thin films used as the active layer of transistors can be deposited from any of the techniques mentioned previously. An important key to obtain high performance and reproducible devices is the film uniformity, which can be macroscopically inspected by visual observation (translucent and shiny films usually represent superior quality films) or microscopically from techniques like profilometry (which can measure the surface roughness), atomic force microscopy (AFM) or scanning electron microscopy (SEM). The solution preparation method plays a significant role in the film formation and must be meticulously planned to obtain improved performance devices. The most commonly used solution processing techniques used to produce SMO TFTs are based on: (i) the calcination or pyrolysis of an organic precursor of the desired metal oxide which is soluble in an organic solvent or (ii) on the physical agglomeration or chemical reaction of previously synthesized nanoparticles which can form a uniform suspension in water or in other polar protic solvents.

The solutions used to prepare SMO films deposited by spin-coating and spray technique are frequently obtained by the dissolution of organic salts containing the metallic atom which forms the metal oxide. **Figure 3** shows the scheme that most common organic salts used metal oxide precursors. Special attention is devoted to zinc acetate dihydrate (**Figure 3a**), which is the basic compound used to obtain ZnO and is soluble in water and other polar protic solvents like ethanol, isopropanol and methanol. Very often, 2-methoxyethanol is used as the solvent with ethanolamine as stabilizer [11, 15, 26] due to the credited better precursor

Spin-coating deposition is commonly carried out using solution concentrations in the (0.03–0.3 M) range to originate organic precursor films which undergo a pyrolysis process by heating up the substrates in a hotplate or in an oven at temperatures above the degradation temperature of the organic compound (usually above 300°C). After the thermal decomposition of the precursor organic phase, oxide agglomerates intermediated by voids, but which can be interconnected along macroscopic distances (superior to few millimeters), are formed. To form a continuous and uniform oxide film, the multiple deposition of precursor layers by spin coating is performed, intercalated by the thermal treatment process to promote the oxide

formation and to avoid the dissolution of the previous layers [14].

**2.2. Solution-processed metal oxide thin films**

*2.2.1. Organic precursor pyrolysis*

dissolution and film formation.

Spin coating is a widespread used deposition technique which yields very thin (ranging from few nanometers up to micrometers) and uniform films by spreading a solution of the desired material onto cleaned substrates and making them spin at high rotation speeds (about 1000–8000 rpm) promoting solvent evaporation. The technique is very successful in the formation of organic or polymeric films but can be used to deposit inorganic materials solutions or suspensions as well. Spray pyrolysis is based on spraying a solution of an organic precursor onto a preheated substrate (usually at a temperature above the degradation temperature of the

**Figure 2.** Summary of the frequently used low-cost deposition techniques used to produce electronic devices based on semiconducting metal oxides. (a) RF magnetron sputtering; (b) pulsed laser deposition (PLD); (c) spin-coating; (d) airbrush spray-pyrolysis (ASP); and (e) ultrasonic spray-pyrolysis (USP).

organic phase of the precursor material) to produce thin and highly uniform metal-oxide films. Difference between airbrush and ultrasonic spray deposition is that, in the former, a mechanically actuated needle is responsible to release the precursor solution, which flows due to gravity through the spray nozzle whereas in the latter, a piezoelectric nozzle driven by an ultrasonic power supply is used to nebulize the precursor solution which is injected by a micro-syringe pump. In both cases, compressed dry air or inert gas is used to carry the nebulized material. Ultrasonic spray has the advantage to provide higher droplet size control (in the order of tens of microns) and to economize precursor solution whereas airbrush is a low-cost and simple alternative to obtain very uniform films which can be used to produce high-performance TFTs [13].
