## 5 Deposition Methods
### Evaporation  
In evaporation, the arriving species to the substrate can be approximated to be strongly directional, see Figure 20a. Therefore, evaporation has the worst film conformality while it can provide the best directional deposition which is preferred by the lift-off patterning process. There are two approaches to deposit GST films in an evaporation system. One approach is to directly evaporation the Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) alloy (Single source evaporation) [114; 115], while the other method deals with the multi-source evaporation from the corresponding constituent elements [61].  
In the direct evaporation process of GST, association or dissociation of the GST alloy can happen individually or simultaneously. The association does not change the stoichiometry of the deposition film from the source but may introduce new constitute from the association. However, the stoichiometry in dissociation usually changes along the deposition process. [PERSON] et al. performed a mass-spectroscopic investigation of the vapors to show that the molecular species normally changes during the transition from solid to the gas phase in the evaporation of compounds [117]. In the case of dissociation during the evaporation process, the compound may dissociate into smaller components that the constituents of the compounds can evaporate individually. The more volatile species in GST source tend to evaporate first, which can result in a non-congruent stoichiometry from the source, see Figure 20b. Besides, the more volatile species can also tend to be rich in the bottom of the deposited films, which can end up with a refractive index variation along the depth [118]. Mono-atomic vapor may be observed for alloys even in those cases where the pure element can form molecules. In 1966, [PERSON] et al. showed the exist of mono-atomic antimony vapor in the Pt\(-\)Sb [119], then in 1978 [PERSON] et al. found the mono-atomic antimony vapor in Au\(-\)Sb [120]. However, it is worth pointing out that due to the change in the chemical potential in the solid compound source, the vapor pressure of a compound constitute is different from that of the pure material at the same temperature [121]. The constituents of the compound remain homogeneous throughout the evaporation process. The variation of the ratio during the evaporation process depends on the stoichiometry of the initial compound. The more volatile constituent evaporates preferentially which induces a reversed proportions after the evaporation. The prepared thin films normally have a vertical stoichiometry gradient, which can modify the refractive index along the vertical direction [118]. In the direct evaporation, one has to be more careful to choose the appropriate starting concentration of the constituent to compensate for the different volatilities in order to obtain the right composition from the evaporation [122].  
In the multi-source evaporation configuration, the constituents are evaporated from their corresponding sources at different temperatures and subsequently joint condenses on the substrate, which circumvents the decomposition problem as encountered in the direct evaporation of compounds. Furthermore, the multi-source evaporation technique also provides the possibility for doping.  
Figure 20: **(a)** Schematic of the evaporation technique for creating GST films. **(b)** Vapor pressure of Ge, Sb and Te [116].  
[PERSON] et al. reported indium-doped Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) thin films by thermal evaporating the single-element In, Ge, Sb, and Te sources [123].