Reactive Sputtering

Sputtering is a thin film deposition process where the films are stoichiometric to the specific target. A schematic of the reactive sputtering system is shown in Figure 2.18. During the process, the atoms are ejected from the target by energized ions to form a plasma that is directed to the substrate under a high vacuum. Argon is the commonly used sputtering gas, and the sputtering is carried out with a DC power source, radio frequency alternating current, or ion-assisted deposition. In reactive sputtering, a reactive gas such as oxygen for oxides and nitrogen for nitrides is also passed to the reaction chamber along with argon. These reactive gases react with the target atoms in the plasma to form the desired composition.

While forming perovskite oxides films, multiple targets with different elements are simultaneously sputtered, which are reacted with oxygen and deposited as the desired film. Since the target elements and oxygen exhibit a large electronegativity difference, the formed ions can be negatively charged and accelerated towards the substrate due to the difference in the potential of negatively charged target and the grounded substrate. These ionic fluxes possibly act as sputtering ions to resputter the growing thin film on the substrate or modifying the composition of the films or etching the substrate. To avoid such problems, the following solutions are recommended. They are increasing the working gas pressures to reduce the kinetic energy of the ions striking the substrate, avoid placing the substrate directly facing the target, minimizing the discharge voltage, and designing the target composition to compensate for the resputtering effects (Habermeier 2007). The perovskite oxides synthesized by sputtering are presented in Table 2.6.

Reactive Molecular Beam Epitaxy

In molecular-beam epitaxy (MBE), the respective elements to be deposited are sublimed by heating in individual cells called effusion cells or furnaces. A typical MBE process setup is shown in Figure 2.19. The heating can also be carried out using a laser source or an electron-beam evaporator. The reaction chamber is always

FIGURE 2.18 Reactive sputtering of perovskite oxide.

Note: YSZ-Yllria stabilized zirconia.

FIGURE 2.19 MBE reaction chamber.

maintained under a very high vacuum. The elemental vapors are then condensed on the substrate, which is continuously rotated and heated. Precise control on the temperature of the target and the substrate is necessary to control the rate of material deposition on the substrate. Molecular beam constitutes the evaporated atoms that may or may not interact with each other depending on the composition of the thin film. A typical MBE system is composed of four main parts: (1) an ultra-high vacuum epitaxial chamber with a background pressure of 1.3xl0~⁸ Pa, containing a four-target holder and a substrate holder; (2) a device for heating the target; (3) a scanning device for the composition monitoring; and (4) a high-energy electron diffraction (RHEED) system and the charge-coupled device (CCD) camera.

RHEED accompanies the MBE set up for the continuous monitoring of the film growth. The shutters control the release of vaporized elements from the effusion cell. The oxide or nitride films are achieved by injecting the respective reactive gases into the chamber, as in the case of any other reactive deposition technique. In reactive MBE, the pressure is maintained sufficiently low to guarantee the reaction between the vaporized atoms and the reactive gases. A hybrid MBE technique is also used for the deposition of perovskite oxide films. In a hybrid MBE, А-site element is evaporated in the effusion cells, and a chemical beam produced by the thermal evaporation of the respective precursor is used as the source for the В-site element, which also acts as the source of the anion (Jalan et al. 2009). In reactive MBE, the pressure is reduced to К)-³ Pa; thus, the path of the atoms evaporated from the cells is much larger than the source-substrate distance. For the effective deposition of perovskite oxides, it is essential to retain the stoichiometric ratios of the molecular beams of the different constituent elements and the corresponding fluxes. To do the same spectroscopic techniques, such as atomic absorption spectroscopy, mass spectroscopy, quartz-crystal monitors, and electron microscopy, are often employed.

The major advantage of MBE is the fabrication of thin films with a few' unit cell thickness. Like other thin film deposition techniques, MBE requires a substrate, and thus the obtained films are with a heterostructure. However, MBE is used for depositing on any substrate, which can withstand high temperatures. MBE is used for the deposition of either a functional electrode on conductive substrates or a conductive layer on functional electrodes. The deposition of electroactive SrlrO, (100) films on DyScOj (110) (Tang et al. 2016) is an example for the former, and the deposition of conducting SrVO, on (LaAlO₃)₀₃(Sr₂AlTa₆)₀₇ (Moyer et al. 2013) is an example for the latter.

A laser source can replace the effusion cell to vaporize the targets, such a technique is called as laser MBE. Thin films of BaTiO,, SrTiO, (Lu et al. 2000), LaNiO,/ LaAlO, (Wrobel et al. 2017), etc. are few examples of perovskite oxides fabricated using laser MBE. GdTiO, (Moetakef et al. 2012), SrTiO, (Jalan et al. 2009), Gd,__A.Sr_v TiO, (Moetakef and Cain 2015), BaSnO, (Prakash et al. 2017), LaVO, (Zhang et al. 2015), SrVO, (Eaton et al. 2015), etc. are some examples of hybrid MBE deposited films using metal-organic precursors. Perovskite structured thin films with unique properties are deposited using reactive MBE, e.g., superconducting YBa₂Cu,0₇__v (Kwo et al. 1988) is successfully deposited on MgO (100) single crystals using oxygen as the reactive source.

Reactive Thermal Evaporation

In reactive thermal evaporation thin-film coating, the target material is physically evaporated by means of heat under a vacuum. The evaporated target material will condense directly to the substrate in the solid-state, similar to the condensation of water vapor on a lid. The thermal evaporation system consists of the sealed chamber connected to a vacuum pump with the provision to heat the target material resis- tively, and holders for the substrate. A schematic of the thermal evaporation system is shown in Figure 2.20. In typical thermal evaporation, the metallic target materials

FIGURE 2.20 Reactive thermal evaporation process.

are fed into evaporators called “boats,” due to their unique shape. The target materials are then heated above the melting temperature and subsequently evaporated to deposit on the substrates. Often, the boats themselves are highly resistive, and the adequate power supply allows the boats to be heated above the melting temperature of the targets. Alternatively, the target material in a crucible is heated radiatively by an electric filament, or it is fed continuously onto a heated element which allows the evaporation.

In the thermal evaporation process, a high vacuum is mandatory because the presence of gas molecules in the chamber can either redirect the travel of vaporized molecules towards the substrate or interact with the vapors to affect the purity of the films. However, in the reactive thermal deposition of oxide films, the evaporated metal atoms are allowed to interact with oxygen to form the respective oxide film, which obviously reduces the deposition rate and, ultimately, the control on the film thickness. The oxygen must be introduced only after attaining a high vacuum in the chamber, as the presence of water molecules in the moisture can quench the active functional properties of the films.