**4. Properties of TiO2 thin films**

The performance of TiO2 thin-film based devices depends on its structural, surface morphological, compositional, optical and electrical properties. It is evident that the improvement of materials properties requires a closer inspection of preparative conditions and also the above said properties of the films.

The physical, optical, electrical and chemical properties of titanium dioxide (TiO2) depend greatly on the amorphous or crystalline phase of the material. TiO2 is a complex material with three crystalline phases, two of which are commonly observed in thin films-anatase and rutile. Anatase is commonly observed at film deposition temperatures of 350–700°C, while higher temperatures promote the growth of rutile. Deposition temperature lower than 300°C generally result in the formation of amorphous TiO2 and it has highest band gap (3.5 eV), low refractive index (1.9–2 at 600 nm) and extinction coefficient. Polycrystalline anatase thin films with an optical band gap (3.2 eV) exhibit a much higher refractive index and slightly increased absorption coefficient. The chemical resistance of amorphous TiO2 films is poor in many acidic and basic solutions as compared with crystalline structure because of anatase, which is insoluble in many acids and base. The TiO2 thin films with rutile phase are having extremely higher refractive indices (up to 2.7 at 600 nm) and lower the band gap absorption is still low. The chemical resistance of rutile is excellent, and after annealing at temperatures above 1000°C, it is insoluble in nearly all acids and bases.

Thin films of TiO2 are used in an extremely wide range of commercial applications and research areas, including the following:

TiO2 powders and nanopowders: as a white pigment in paint, plastic, inks, paper and cosmetics; in washing powder, toothpaste, sunscreen, foodstuffs, pharmaceuticals, photographic plates, for creating synthetic gemstones; and as a catalyst.

TiO2 thin films and their derivatives: for ultra-thin capacitors and MOSFETs due to their extremely high dielectric constant; as humidity and oxygen sensor due to the dependence of their electrical conductance on the gases present; as an optical coating and a material for waveguides due to their high refractive index; as a protective coating and corrosion resistant barrier; and as a photoanode in solar cells due their photoelectric activity.

#### **4.1 Semiconductor properties**

Solid materials are classified in three groups depending on their electrical conductivity σ. Highly conducting materials are metals (σ > 104 S m−1), material

**47**

**Figure 4.**

*The band diagram of a semiconductor.*

*Titanium Dioxide Versatile Solid Crystalline: An Overview*

(*EG*) inside which no electronic states are encountered.

*4.2.1 The amorphous-anatase-rutile phase transformations*

with very low electrical conductivity are insulators (σ < 10−8 S m−1), and in-between stand the semiconductors. The main difference between metal and semiconductor is the fact that for metals, the electrical conductivity decreases when temperature increases, while the reverse phenomenon usually occurs in the case of semiconductors. The energy band diagram of a pure semiconductor containing a negligible amount of impurities (intrinsic semiconductor) is characterized by an energy gap

When a semiconductor is doped with donor and/or acceptor impurities, impurity energy levels are introduced. A donor level is defined as being neutral if filled with an electron and positive if empty. An acceptor level is neutral if empty and negative if filled by an electron. The Fermi level for the intrinsic semiconductor lies close to the middle of the band gap (**Figure4**). When impurity atoms are introduced, the Fermi level must adjust itself to preserve charge neutrality, and the total negative charge (electrons and ionized acceptors) must equal the total positive charge (holes and ionized donors). N-type and p-type semiconductor band diagram is shown in **Figure4** [17, 18].

Amorphous TiO2 thin films can be deposited at temperatures as low as 100*–*150*°*C [19, 20]. Amorphous TiO2 does not have a strict crystallographic structure, often incorporates voids within the material, and has a relatively low density. For TiO2 thin films formed by chemical reaction, the lowest temperature crystalline phase of TiO2 that can be obtained is anatase. To obtain polycrystalline anatase, the film can be either deposited as amorphous TiO2 and then crystallized by annealing at a higher temperature or deposited as polycrystalline material directly. The results indicate that the transition from an amorphous to anatase film occurs at about 300–365*°*C, regardless of whether this is the deposition or annealing temperature. Rutile films are initially observed on silicon substrates at deposition temperatures above 700*°*C, and more typically from 900 to 1100°C. It should be noted that anatase is a metastable phase of TiO2, and the

*DOI: http://dx.doi.org/10.5772/intechopen.92056*

**4.2 Physical properties**

*Titanium Dioxide Versatile Solid Crystalline: An Overview DOI: http://dx.doi.org/10.5772/intechopen.92056*

with very low electrical conductivity are insulators (σ < 10−8 S m−1), and in-between stand the semiconductors. The main difference between metal and semiconductor is the fact that for metals, the electrical conductivity decreases when temperature increases, while the reverse phenomenon usually occurs in the case of semiconductors. The energy band diagram of a pure semiconductor containing a negligible amount of impurities (intrinsic semiconductor) is characterized by an energy gap (*EG*) inside which no electronic states are encountered.

When a semiconductor is doped with donor and/or acceptor impurities, impurity energy levels are introduced. A donor level is defined as being neutral if filled with an electron and positive if empty. An acceptor level is neutral if empty and negative if filled by an electron. The Fermi level for the intrinsic semiconductor lies close to the middle of the band gap (**Figure4**). When impurity atoms are introduced, the Fermi level must adjust itself to preserve charge neutrality, and the total negative charge (electrons and ionized acceptors) must equal the total positive charge (holes and ionized donors). N-type and p-type semiconductor band diagram is shown in **Figure4** [17, 18].

#### **4.2 Physical properties**

*Assorted Dimensional Reconfigurable Materials*

**4. Properties of TiO2 thin films**

**Figure 3.**

The performance of TiO2 thin-film based devices depends on its structural, surface morphological, compositional, optical and electrical properties. It is evident that the improvement of materials properties requires a closer inspection of pre-

The physical, optical, electrical and chemical properties of titanium dioxide (TiO2) depend greatly on the amorphous or crystalline phase of the material. TiO2 is a complex material with three crystalline phases, two of which are commonly observed in thin films-anatase and rutile. Anatase is commonly observed at film deposition temperatures of 350–700°C, while higher temperatures promote the growth of rutile. Deposition temperature lower than 300°C generally result in the formation of amorphous TiO2 and it has highest band gap (3.5 eV), low refractive index (1.9–2 at 600 nm) and extinction coefficient. Polycrystalline anatase thin films with an optical band gap (3.2 eV) exhibit a much higher refractive index and slightly increased absorption coefficient. The chemical resistance of amorphous TiO2 films is poor in many acidic and basic solutions as compared with crystalline structure because of anatase, which is insoluble in many acids and base. The TiO2 thin films with rutile phase are having extremely higher refractive indices (up to 2.7 at 600 nm) and lower the band gap absorption is still low. The chemical resistance of rutile is excellent, and after annealing

parative conditions and also the above said properties of the films.

*Crystallographic structures of TiO2 (a) anatase, (b) brookite, and (c) rutile.*

at temperatures above 1000°C, it is insoluble in nearly all acids and bases.

tions and research areas, including the following:

their photoelectric activity.

**4.1 Semiconductor properties**

Thin films of TiO2 are used in an extremely wide range of commercial applica-

TiO2 powders and nanopowders: as a white pigment in paint, plastic, inks, paper and cosmetics; in washing powder, toothpaste, sunscreen, foodstuffs, pharmaceuticals, photographic plates, for creating synthetic gemstones; and as a catalyst.

TiO2 thin films and their derivatives: for ultra-thin capacitors and MOSFETs due to their extremely high dielectric constant; as humidity and oxygen sensor due to the dependence of their electrical conductance on the gases present; as an optical coating and a material for waveguides due to their high refractive index; as a protective coating and corrosion resistant barrier; and as a photoanode in solar cells due

Solid materials are classified in three groups depending on their electrical

S m−1), material

conductivity σ. Highly conducting materials are metals (σ > 104

**46**

#### *4.2.1 The amorphous-anatase-rutile phase transformations*

Amorphous TiO2 thin films can be deposited at temperatures as low as 100*–*150*°*C [19, 20]. Amorphous TiO2 does not have a strict crystallographic structure, often incorporates voids within the material, and has a relatively low density. For TiO2 thin films formed by chemical reaction, the lowest temperature crystalline phase of TiO2 that can be obtained is anatase. To obtain polycrystalline anatase, the film can be either deposited as amorphous TiO2 and then crystallized by annealing at a higher temperature or deposited as polycrystalline material directly. The results indicate that the transition from an amorphous to anatase film occurs at about 300–365*°*C, regardless of whether this is the deposition or annealing temperature. Rutile films are initially observed on silicon substrates at deposition temperatures above 700*°*C, and more typically from 900 to 1100°C. It should be noted that anatase is a metastable phase of TiO2, and the

**Figure 4.** *The band diagram of a semiconductor.*

conversion to rutile involves a collapse of the anatase structure, which is irreversible [21, 22]. Although rutile and anatase are both of tetragonal crystallographic structure, rutile is more densely packed and thus possesses a greater density.

The deposition of TiO2 thin films is formed by chemical reaction, using chemical vapor deposition (CVD), and spray pyrolysis and hydrolysis systems. In this scenario, the substrate temperature is the primary means of controlling the deposited phase of the material. In contrast, physical vapor deposition (PVD) systems, such as evaporation, sputtering, and ion-beam deposition, are used to determine the structure with its phase primarily by the kinetic energy of the impinging atoms.

Therefore, the progression through the amorphous, anatase and rutile phases may not necessarily be expected. This is confirmed by the occurrence of rutile films at low deposition temperatures (*<* 450*°*C) by carefully optimized deposition methods, [23, 24] ion-assisted deposition [25] and reactive evaporation [26]. The TiO2 films are formed by a chemical reaction, where the substrate temperature dominates film growth characteristics. Several researchers observed that the processing temperatures required to convert an anatase film into a rutile one are much higher than temperature required depositing a rutile film directly [26–28]. The variation in physical and chemical properties of the films is determined solely by the maximum processing temperature, whether the deposition temperature or a subsequent annealing temperature was observed by researchers [20].

The mechanism for the sintering and transformation of anatase into rutile involves several steps. Initially, the smallest particles coalesce, forming bigger particles. The fractions of particles that are already large have been shown not to undergo sintering. The heat evolved from the exothermic sintering process causes the local nucleation of the rutile phase. Finally, as the conversion to rutile is also an exothermic process, this results in the transformation of the whole particle to rutile.

#### *4.2.2 The effect of impurities on the anatase-rutile phase transformation*

In many research works, researchers have observed that the inclusion of a certain amount of impurities into TiO2 can drastically alter the physical properties of the film. It has been shown that silicon and phosphorus inhibit the transformation from anatase to rutile, with 100% anatase phase being retained at temperatures as high as 870*°*C for up to 3 h for thin films 80 K and 1500 K for bulk samples [29]. The retardation of the anatase-rutile transformation can be achieved with impurities [29]. Most researchers agree that oxygen vacancies are responsible for the overall transformation mechanism [30]. Thus, the oxides and fluorides that assist the transformation can substitute for Ti+4 in the anatase lattice, resulting in the creation of oxygen vacancies. On the other hand, the inhibiting effect of other impurities involves the reduction of oxygen vacancies due its substitution into the anatase lattice.

Titanium alkoxides are common TiO2 precursors, with the most frequently used being titanium tetra isopropoxide (TTIP) (also called tetra isopropyl titanate). The residue of the organic binders results in carbon contamination of typically a few atomic weight percentage (at. wt.%), but as high as 13 at. wt.% being observed [31]. It is likely that carbon incorporation could be higher at low growth temperatures, as when higher temperatures were used the carbonate species decomposed, resulting in the removal of hydrocarbon fragments [32]. Titanium tetrachloride (TiCl4) is another common TiO2 precursor, and this results in chlorine contamination of the deposited film.
