## 3 Interfacial Phenomena between Solid-State Electrolytes and Electrodes
### Deposition Techniques
#### 3.1.2 Chemical Deposition Methods  
Sol-gel deposition which is a wet chemical method was originally used for the synthesis of metal oxide nanoparticles [267]. The conventional sol-gel process undergoes three stages, namely the hydrolysis, the condensation and the drying stage. In the hydrolysis stage, a metal precursor undergoes a chemical reaction, forming a hydroxide colloidal suspension (sol). In the condensation stage, centrifugation or normal sedimentation can be used to form a gelatinous network (gel). In the drying stage, the remaining solvent in the \"gel\" network is removed by a thermal treatment. For thin-film deposition, the \"sol\" matter was deposited onto the substrate via dip-coating/spin coating process, followed by the condensation at room temperature than a firing process [268]. Owing to its simplicity, flexibility and low deposition temperature, the sol-gel process has gained much attention with applications in different fields, including piezoelectric devices [269], superconductors [270] and synthesis of nanoparticles [271].  
Chemical vapor deposition (CVD) is a chemical deposition technique that utilizes a chemical reaction of vapor phase precursors (Figure 10) [272]. In the CVD process, the chemical reactions of precursors occur both in the gas phase and on the substrate. A precursor is controlled by balancing flow regulators and control valves. Precursor molecules are drawn into the boundary layer and then deposited on the surface of the substrate. The deposition process occurs in three successive stages: (i) introduction of the volatile precursor by carrier gas to the reactor chamber; (ii) adsorption of precursor vapors on the substrate surface and the formation of intermediate products; (iii) decomposition of these products on the heated substrate followed by nucleation and growth of the solid layer/grains and the formation of volatile byproducts and their removal from the chamber by the carrier gas [273]. CVD can be applied for obtaining diverse thin-film materials such as metals, semiconductor III-V compounds, dielectric oxides, perovskite heterostructures, magnetics and conductors [274].  
Figure 10: Schematic of a chemical vapor deposition (CVD) system with the basic components and operation.  
Figure 9: Schematic diagram of a sputtering system showing the basic components and operation.  
Atomic layer deposition (ALD) is a surface controlled thin-film deposition technique based on the sequential pulsing of the vapors of precursor chemicals in which one atomic layer is formed for each pulse. Generally, two precursors (A and B) are used in the ALD process [275]. Each of the precursors makes up half of the reactions that yields the desired materials. First, precursor A is pumped into the chamber for their own half-reaction for a certain amount of time to fully cover the substrate surface with a monolayer of precursor A. It is worth mentioning that the precursor A does not stack on top of deposited A molecules due to the surface termination, which makes the single monolayer coverage possible. Then, the unreacted precursor A is purged out of the chamber via inert gas. Lastly, the precursor B is pumped into the chamber, forming a layer of the desired material, followed by another purging to remove the byproduct from the reaction between precursor A and B. This deposition cycle of precursor A and B continues until the film reaches the desired thickness [275]. Therefore, precise thickness control can be enabled by controlling the number of reaction cycles. It is also possible to initiate the growth of materials by ALD on the specific position of the substrate, such as defect sites that can have a significant impact on the device performance improvement. In addition, the conformality of thin films is also a unique feature of ALD, which leads to the formation of thin films with high aspect ratios and three-dimensional substrates.