**3. Chemical deposition methods**

#### **3.1. Chemical bath deposition/dip coating**

Chemical bath deposition (CBD) or dip coating is the simplest and low-cost deposition method because this method does not need expensive and special equipment. In addition, it also has a simple deposition principle. The arrangement of equipment is shown in **Figure 3**.

At the precursor preparation, zinc oxide is prepared from zinc-salt compound such as zinc acetate dihydrate, zinc nitrate, zinc chloride, and zinc sulfate [17–21]. After these kinds of zinc salts were dissolved in the solution, zinc in the salt becomes a cation which can react with anion from basic compound, as well as form a seed of nuclei which adhere on the substrate. Besides that, zinc cation also reacts with other compounds in the precursor mixture such as surfactants which act as a binder and form zinc complex compound or in order to get desire properties of the mixture such as viscosity or homogeneity. **Table 3** shows various zinc salts and other compounds as precursor mixture.

Further, this seed of nuclei will develop as zinc-salt precipitation which adheres on the substrate, and after drying or heat treatment under various atmospheres, it becomes oxidized and forms a zinc oxide as thin film. The schematic of this preparation is shown in **Figure 4**.

This is the reaction mechanism of ZnO thin film fabrication.

 [ Zn (NH3) 4] <sup>2</sup><sup>+</sup> + H<sup>2</sup> O → Zn<sup>2</sup><sup>+</sup> + NH4+ + OH<sup>−</sup> Zn<sup>2</sup><sup>+</sup> + OH<sup>−</sup> → Zn (OH) 2 Zn (OH) <sup>2</sup> → ZnO + H<sup>2</sup> O

**Figure 3.** The arrangement of CBD equipment [16].

In the chemical preparation process, to adjust ZnO thin films into p or n semiconductor, it is done by adding some particular element of salt such as aluminum salt or boron salt into the

**Dopants Optimum content in target (%) Thickness (nm) Resistivity (Ωcm) Transmittance (%) References**

Al 2 500 4.5 × 10−4 88 [7] Ga 5 200 8.12 × 10−4 >90 [8] In 40 >1000 4.02 × 10−4 >85 [9] F 2 200 4.83 × 10−4 >90 [10] Si 2 ∼150 6.2 × 10−4 ∼80 [11]

**Figure 2.** A common preparation step to fabricate ZnO thin film by chemical processes.

Some of experiments only need a single step of heat treatment to develop the ZnO thin film, but in spin-coating method, several repetition steps of coating and heat treatment are required in order to get the desired thickness, and this repetition step is called as preheat treatment. Contrary with the spin coating, chemical vapor deposition (CVD) or spray pyrolysis method may not have specific drying or heat treatment steps because in this method, the heat treatment is done simultaneously with deposition. To adjust the thickness in CVD or spray pyroly-

precursor mixture.

sis, it is adjusted by the deposition time.

**Table 2.** Properties of ZnO films with different dopants.

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the increase of the concentration also affects the reaction rate of nucleation between zinc and anion, in which it becomes faster. However, besides concentration, the reaction rate is also

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The preparation of chemical vapor deposition (CVD) or spray pyrolysis is similar with CBD. The zinc precursor must be solved in a solution and has to be evaporated. Evaporation of precursor can be done by several ways. First, zinc precursor must be easy to evaporate or in other words it should be a volatile precursor. Second, by using high temperature, the zinc ingot as precursor can evaporate, and the last step is by using devices such as an atomizer or an ultrasonic transducer that can atomize the precursor or make a mist of it. The arrangement

In **Figure 5**, zinc precursor solution has changed into mist by ultrasonic apparatus and carried by inert gas into the heat furnace chamber with a substrate placed inside the chamber as a target to develop the ZnO thin film. The temperatures of CVD in several experiments are

After the precursor mist or vapor is carried and arrived at the heater chamber, some of the zinc precursor particles undergo a reaction with a molecule that contains oxygen in the chamber and forms a larger molecule. Then, this molecule is attached on to the substrate. Due to the high temperature or enough energy, this molecule decomposes into zinc that reacts with the

affected by the temperature reaction.

of CVD equipment is shown in **Figure 5**.

**Figure 5.** CVD using the ultrasonic transducer to produce a precursor mist [27].

shown in **Table 5**.

**3.2. Chemical vapor deposition/spray pyrolysis**

**Table 3.** Various zinc salts and other compounds as precursor mixture.

**Figure 4.** Mechanism of ZnO thin film growth [21].


**Table 4.** Various annealing temperatures and atmospheres/heat treatment step.

At drying or heat treatment step, the heat treatment can be done at various temperatures and atmospheres. These various processes have the effect to the shape, morphology, optical, and electrical properties of zinc oxide thin film [22]. **Table 4** shows various annealing temperatures and atmospheres or heat treatment step.

In addition to these, the concentration of precursor of zinc salt as zinc source and other additive concentrations also give significant role to ZnO thin film [22]. The effect of zinc and surfactant concentrations can be seen in **Figure 4**. The increase of zinc and surfactant concentration shows an increase of ZnO thin film crystallinity. Besides crystallinity, the effect of the increase of the concentration also affects the reaction rate of nucleation between zinc and anion, in which it becomes faster. However, besides concentration, the reaction rate is also affected by the temperature reaction.

### **3.2. Chemical vapor deposition/spray pyrolysis**

The preparation of chemical vapor deposition (CVD) or spray pyrolysis is similar with CBD. The zinc precursor must be solved in a solution and has to be evaporated. Evaporation of precursor can be done by several ways. First, zinc precursor must be easy to evaporate or in other words it should be a volatile precursor. Second, by using high temperature, the zinc ingot as precursor can evaporate, and the last step is by using devices such as an atomizer or an ultrasonic transducer that can atomize the precursor or make a mist of it. The arrangement of CVD equipment is shown in **Figure 5**.

In **Figure 5**, zinc precursor solution has changed into mist by ultrasonic apparatus and carried by inert gas into the heat furnace chamber with a substrate placed inside the chamber as a target to develop the ZnO thin film. The temperatures of CVD in several experiments are shown in **Table 5**.

After the precursor mist or vapor is carried and arrived at the heater chamber, some of the zinc precursor particles undergo a reaction with a molecule that contains oxygen in the chamber and forms a larger molecule. Then, this molecule is attached on to the substrate. Due to the high temperature or enough energy, this molecule decomposes into zinc that reacts with the

**Figure 5.** CVD using the ultrasonic transducer to produce a precursor mist [27].

At drying or heat treatment step, the heat treatment can be done at various temperatures and atmospheres. These various processes have the effect to the shape, morphology, optical, and electrical properties of zinc oxide thin film [22]. **Table 4** shows various annealing tempera-

**Temperature (°C) Duration (hour) Atmosphere References** 100 1 Air [23] 300 0.5 Oxygen [24] 300–600 1 Air [25] 400–800 1 Air [26]

**Zinc salt Other additive compounds References**

COO)2 2-methoxyethanol, mono-ethanolamine [17]

)<sup>2</sup> NaOH, sodium n-dodecyl sulfate, triethanolamine [18]

OH, NH4

**Table 3.** Various zinc salts and other compounds as precursor mixture.

OH, hexadecyl(trimethyl)azanium bromide [19]

HCO<sup>3</sup> [20]

In addition to these, the concentration of precursor of zinc salt as zinc source and other additive concentrations also give significant role to ZnO thin film [22]. The effect of zinc and surfactant concentrations can be seen in **Figure 4**. The increase of zinc and surfactant concentration shows an increase of ZnO thin film crystallinity. Besides crystallinity, the effect of

tures and atmospheres or heat treatment step.

**Table 4.** Various annealing temperatures and atmospheres/heat treatment step.

**Figure 4.** Mechanism of ZnO thin film growth [21].

Zn(CH<sup>3</sup>

Zn(NO3

ZnCl2 NH4

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ZnSO4 NH4


**Table 5.** Several ZnO thin film CVD condition.

oxygen and then forms a zinc oxide behind. The solvent is evaporated and leaves the chamber with the carrier gas, and at the same time, zinc oxide gets developed as zinc oxide thin film. Or the reaction mechanism could undergo another possibility, when zinc precursor is arrived at the chamber, and it will attach on the substrate and then undergo a reaction or bonding with another molecule which contains oxygen. Due to the high temperature and enough energy to decompose, the zinc precursor molecule and oxygen containing molecule react to the zinc oxide and release some decomposed solvents, which get carried out with gas, and finally the zinc oxide molecule becomes zinc oxide thin film. This schematic is shown in **Figure 6**.

After the precursor is attached on the substrate, the next stage is drying stage or evaporating the solvent. One thing that should be considered is that the evaporation of the solvent is affected by the viscosity of the precursor. Thus, it is really important to find out the optimum viscosity of the precursor. A high viscosity means difficult to spread well on the substrate surface, and it means that there is a high surface tension which makes it evaporate slowly. The evaporation of the precursor's solvent is called as preheat treatment. Once the solvent evaporates, it will leave the zinc particle on the substrate and makes the layer thinner. In this method, the desired thickness cannot be achieved by a single process. It should be done in several preheat treatment stages to get the desired thickness. After the preheat treatment, the next stage is to fabricate the zinc oxide from the attached zinc on the substrate by oxidizing heating process. The schematic of alignment process on spin coating is shown

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**Figure 8.** Schematic of the four stages of spin coating (a) Deposition, (b) Spin up, (c) Spin off, (d) Drying [34].

**Figure 7.** Schematic representation of a home-built spin coater (a) and details of the rotor (b) [33].

in **Figure 8**.

#### **3.3. Spin coating**

Spin-coating method uses the energy of the substrate rotation to remove excess or unattached precursor and flatten the thin film. While the precursor drops on the substrate, it will attach on the substrate but the spread is uneven. When the substrate starts to spin, the precursor will spread along the substrate surface due to the centripetal force. A high spin velocity means a high centripetal force to remove the excess of precursor from the substrate. The amount of precursor in the substrate depends on the spin velocity and the precursor's ability to adhere or attach on the substrate, which is usually called as adhesivity or wet ability. It makes the precursor to attach strongly on the substrate. Besides that, the viscosity also has a role on it. If the precursor has a high viscosity, it will slowly move due to the centripetal force. Consequently, it takes a longer spinning time or a higher spin velocity. The schematic of the spin coater is shown in **Figure 7**.

**Figure 6.** ZnO thin film fabrication. Mechanism with CVD [32].

**Figure 7.** Schematic representation of a home-built spin coater (a) and details of the rotor (b) [33].

oxygen and then forms a zinc oxide behind. The solvent is evaporated and leaves the chamber with the carrier gas, and at the same time, zinc oxide gets developed as zinc oxide thin film. Or the reaction mechanism could undergo another possibility, when zinc precursor is arrived at the chamber, and it will attach on the substrate and then undergo a reaction or bonding with another molecule which contains oxygen. Due to the high temperature and enough energy to decompose, the zinc precursor molecule and oxygen containing molecule react to the zinc oxide and release some decomposed solvents, which get carried out with gas, and finally the zinc oxide molecule becomes zinc oxide thin film. This schematic is shown in **Figure 6**.

**Temperature (°C) Zinc source Atmosphere References**

135–235 Diethylzinc He-Diborane 2% [29] 120 Dimethylzinc O<sup>2</sup> [30]

, N<sup>2</sup> O, H<sup>2</sup>

O [28]

, O3 [31]

90 Zinc acetate O<sup>2</sup>

300–375 Zinc acetate N<sup>2</sup>

**Table 5.** Several ZnO thin film CVD condition.

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Spin-coating method uses the energy of the substrate rotation to remove excess or unattached precursor and flatten the thin film. While the precursor drops on the substrate, it will attach on the substrate but the spread is uneven. When the substrate starts to spin, the precursor will spread along the substrate surface due to the centripetal force. A high spin velocity means a high centripetal force to remove the excess of precursor from the substrate. The amount of precursor in the substrate depends on the spin velocity and the precursor's ability to adhere or attach on the substrate, which is usually called as adhesivity or wet ability. It makes the precursor to attach strongly on the substrate. Besides that, the viscosity also has a role on it. If the precursor has a high viscosity, it will slowly move due to the centripetal force. Consequently, it takes a longer spinning time or a higher spin velocity. The schematic of the spin coater is

**3.3. Spin coating**

shown in **Figure 7**.

**Figure 6.** ZnO thin film fabrication. Mechanism with CVD [32].

After the precursor is attached on the substrate, the next stage is drying stage or evaporating the solvent. One thing that should be considered is that the evaporation of the solvent is affected by the viscosity of the precursor. Thus, it is really important to find out the optimum viscosity of the precursor. A high viscosity means difficult to spread well on the substrate surface, and it means that there is a high surface tension which makes it evaporate slowly. The evaporation of the precursor's solvent is called as preheat treatment. Once the solvent evaporates, it will leave the zinc particle on the substrate and makes the layer thinner. In this method, the desired thickness cannot be achieved by a single process. It should be done in several preheat treatment stages to get the desired thickness. After the preheat treatment, the next stage is to fabricate the zinc oxide from the attached zinc on the substrate by oxidizing heating process. The schematic of alignment process on spin coating is shown in **Figure 8**.

**Figure 8.** Schematic of the four stages of spin coating (a) Deposition, (b) Spin up, (c) Spin off, (d) Drying [34].


**Author details**

Ersan Y. Muslih<sup>1</sup>

Indonesia

**References**

2005.12.055

10.1039/C3EE43865A

10.1016/j.tsf.2011.10.072

10.1016/j.solmat.2010.11.012

j.tsf.2006.01.051

04.196

\* and Badrul Munir2

\*Address all correspondence to: ersanmuslih@gmail.com

1 Department of Mechanical Engineering, Trisakti University, Jakarta, Indonesia

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taic devices. Thin Solid Films;**2017**:417-423. DOI: 10.1016/j.tsf.2017.11.021

films. Nature Materials. 2011;**10**:857-861. DOI: 10.1038/nmat3122

2 Department of Metallurgy and Materials Engineering, University of Indonesia, Depok,

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**Table 6.** Precursor conditions.


**Table 7.** Heat treatment condition.

The precursor solution in spin-coating method is different from the chemical bath deposition and chemical vapor deposition. Precursor preparation in spin coating is more varied. It is not only a solution precursor but also a sol–gel or a colloidal form. Additionally, in this method, the quality on the thin film is affected not only by the concentration of zinc source and other additives but also by the rotation speed, time of spin coating, viscosity, and adhesivity of the precursor solution as mentioned earlier. **Table 6** shows several conditions of ZnO spin coating.

Besides that, another thing that brings this method unique lies on the preheat treatment step. The function of this step is to make the coated film dried and attached on the substrate. The repetition of this step is to get a desired thickness. **Table 7** shows the condition of several preheat treatments and heat treatments.
