**2. Theory and process mechanism**

SILAR is widely used, simple technique to fabricate high-quality thin films [3, 15]. During deposition, successive ionic layer adsorption and reaction of the ions take place at the solid-solution interface of the substrate. Thus, the thin film of the compound, A*p*B*<sup>y</sup>* is deposited on to the substrate surface by dint of the adsorbed cations, xAy+ and anions, qBp� due to the following heterogeneous chemical reactions:

$$\mathbf{A\_{x}Q\_{\mathbf{y}}\left(\mathbf{s}\right) \rightarrow x \mathbf{A^{y+}}\left(\mathbf{aq}\right) + \mathbf{y} \,\mathrm{Q}^{x-}\left(\mathbf{aq}\right)}\tag{1}$$

$$\mathbf{P\_{p}B\_{q}}\left(\mathbf{s}\right) \to \mathbf{p}\,\mathbf{P}^{q+}\left(\mathbf{a}\mathbf{q}\right) + \mathbf{q}\,\mathbf{B}^{p-}\left(\mathbf{a}\mathbf{q}\right) \tag{2}$$

$$\mathbf{A}^{p+}\left(\mathbf{a}\mathbf{q}\right) + \mathbf{B}^{p-}\left(\mathbf{a}\mathbf{q}\right) \to \mathbf{A}\_p \,\mathbf{B}\_{\mathcal{Y}}\left(\mathbf{s}\right) \,\downarrow\tag{3}$$

*Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*

where x, y, p, q and y<sup>+</sup> , q<sup>+</sup> , x, p are the number and charges of the corresponding ions A (metal ions), P (cationic precursor), Q (anionic precursor), and B (anions) respectively [2, 16]. Sometimes, the ligands Ln are a necessity to complete the reaction [17–20]. The solution having the first element containing the final target material can be thought as the compound A*x*Q*<sup>y</sup>* fully dissociated in the chosen solvent such as in water (Reaction 3). Usually, A*x*Q*<sup>y</sup>* is a metal salt where A*y*<sup>+</sup> represents cations such as Zn2+, Cu2+, Mn2+, Cd2+, Bi3+, and B*<sup>p</sup>* represents anions such as NO<sup>3</sup>, Cl, SO4 2.

Hence, a basic SILAR cycle comprises four different steps, correlating alternate immersion of the substrate into cationic and anionic precursor solution followed by rinsing in every immersion cycle to eliminate loosely adhered particles as shown in **Figure 1** and described below:

#### **2.1 Adsorption**

First step of the SILAR process is the formation of the Helmholtz double layer, which is due to the initial adsorption of cationic precursor, *x*A*y*<sup>+</sup> , on the surface of the substrate. This layer is generally composed of two charged layers, the positively charged inner layer and negatively charged outer layers. The positive (+ve) layer consists of the cations, *x*A*y*<sup>+</sup> , while the negative (ve) layer, *<sup>y</sup>*Q*<sup>x</sup>*, is the counter ions of the cations.

#### **2.2 Rinsing I**

In the second step, excessive adsorbed ions, *x*A*y*<sup>+</sup> and *y*Q*<sup>x</sup>*, are rinsed away from the diffusion layer toward the bulk solution and a hypothetical monolayer is formed. This results in a saturated electrical double layer showing an ideal scenario of the process.

**Figure 1.** *Representation of different steps during a SILAR cycle.*

#### **2.3 Reaction**

In the reaction stage, the anions, qB*p*�, from anionic precursor solution are introduced into the system. A solid substance, A*p*B*y*, is formed on the interface due to the low stability of the material. This process employs the reaction of *x*A*<sup>y</sup>* <sup>+</sup> surface species with the anionic precursor, qB*p*�.

#### **2.4 Rinsing II**

In the final step of a SILAR cycle, the excess and unreacted species (*y*Q*x*�, pPq+) and the reaction by product from the diffusion layer are removed leaving expected films.

A schematic presentation of a single cycle for the fabrication of Cu2SnS3 film is shown in **Figure 2** [21]. In the case of Cu2SnS3 film fabrication, ion-by-ion type of deposition takes place through nucleation spots of the adsorbed surfaces [22]. Nucleation occurs due to the surface condensation of the ions and outcomes of, that is, an dense adherent thin film [23]. The substrate was firstly dipped into the cationic precursor containing mixed CuCl2 and SnCl2 solutions, where Cu2+ and Sn2+ species were available. Sn2+ ion in solution is good reducing agents, and thus, Cu2+ reduces to Cu<sup>+</sup> and Sn2+ is oxidized to Sn4+ in cationic solution as shown by the following reaction:

$$\text{2CuCl}\_2 + \text{SnCl}\_2 + \text{6H}\_2\text{O} = 2\text{Cu}^+ + \text{Sn}^{4+} + \text{6HCl} + \text{6OH}^- \tag{4}$$

The substrate was then rinsed off with DI H2O to eliminate the loosely bounded reactants. Then, it was dipped into an anionic precursor containing Na2S.xH2O solution, which gave sulfide ions (S2�) to react with the cations Cu<sup>+</sup> and Sn4+. Finally, the reaction occurred between the pre-adsorbed Cu<sup>+</sup> , Sn4+ cations, and the S2�anion to form a solid Cu2SnS3 thin film as,

$$\rm Na\_2S + H\_2O \to 2Na^+ + HS^- + OH^- \tag{5}$$

$$\rm H\rm S^{-} + H\_{2}\rm O \rightarrow H\_{3}\rm O^{+} + S^{2-} \tag{6}$$

$$\text{2Cu}^{+} + \text{Sn}^{4+} + \text{3S}^{2-} \rightarrow \text{Cu}\_2\text{SnS}\_3 \tag{7}$$

**Figure 2.** *Schematic representation of Cu2SnS3 thin film fabrication by SILAR technique [21].*

In the last step of the process, the substrate was again dipped into the DI H2O to remove the unwanted excessive particles to provide a uniform surface containing Cu2SnS3 thin film.

### **3. SILAR-facilitated material deposition: A summary**

The deposition of the series of chalcogenide mainly metal oxides, sulfides, selenides, and tellurides films has been always on numerous attentions in the advancement of the SILAR since its launch. Currently, SILAR has become a broadly functional technique in the deposition of a huge variety of semiconductor thin films. For the simplicity of discussion, we have summarized most of the metals still synthesized as metal compounds by SILAR in **Table 1**.

A list of materials deposited using SILAR technique with their growth conditions with the required raw materials for the growth is summarized in **Table 2**. For the simplicity, the discussion is divided into four parts as of **Table 1**, as specified below:

#### **3.1 Metal oxides**

An increasing number of oxide materials deposited by SILAR have demonstrated high chemical, thermal, and expected stability that is one of the reasons to increase the popularity of oxide synthesis by SILAR. However, the technique of oxide synthesis is somehow difficult compared to sulfides, selenides, and tellurides due to the unavailability of the anionic precursors, which is the direct source of O<sup>2</sup> to form oxides. For example, in case of the synthesis of most of the binary metal oxides, H2O, NaOH, and NH4OH are used as anionic precursors with a mild thermal treatment of around (70 90) <sup>0</sup> C to activate the precipitation of hydroxides. On the other hand, the most common cationic precursors are mainly of metal thiosulfates, sulfates, chlorides, nitrates, etc., to provide metal ion adsorption on the substrate surface. Until today, CuxO, ZnO, TiO2, and CdO are the most examined materials by SILAR. The investigation of Mn3O4, NiO, and Bi2O3 is also increasing [31–34]. Recently, both nanostructured Fe2O3 and Fe3O4 have been fabricated applying sulfate and chloride salts using NaOH as the anionic precursor *via* SILAR [30, 66]. But research on WO3 [32], MgO [34], and SnO [67] fabrication is still rare. In case of ternary metal oxides, the SILAR deposition has been widely increased due to their ability to the additional modulate characteristics by controlling the composition of the materials. The synthesis of CST is discussed in the theory and mechanism section, which can be again done by two ways—a combined solution of both the deposited metal cations or, an alternating (one by one) fabrication of the two cations. A good technique to produce ternary metal oxides with excessive control on stoichiometry is to react one of the two metal ions by its own oxyanion. For example, Bi(NO3)3 and NH4VO3 react to fabricate BiVO4 [37], as ammonium vanadates are extremely soluble in water, while the anticipated metal vanadates are not. Consequently, they precipitate out of solution as the


#### **Table 1.**

*List of the metals still grown by SILAR technique.*


#### *Thin Films - Deposition Methods and Applications*

