β‐Ga2O3 thin films are deposited by radiofrequency (RF)‐magnetron sputtering on quartz and c‐sapphire substrates using a ceramic stoichiometric Ga2O3 target and a constant flux of argon process gas. Oxygen flux, heater power, and sputtering power are varied in the synthesis of the layers. The resulting Ga2O3 layers are analyzed in terms of their structural and optical properties. Based on this analysis, the process parameters leading to the formation of an optimized β‐Ga2O3 layer are identified. The main challenge in obtaining the stoichiometric β‐Ga2O3 thin films by sputter deposition is to overcome the influence of a strong preferential sputtering of Ga from the ceramic target. This can be achieved by adding a suitable fraction of oxygen to the argon process gas used in the deposition process. Furthermore, it is demonstrated that the refractive index dispersion of β‐Ga2O3 depends strongly on its composition. Thus, a combined analysis of refractive index dispersion and optical bandgap position may serve as a valuable preliminary probe of the thin film's composition.

As known, the rechargeable lithium-ion battery (LIB) is the most representative candidate when it comes to powering portable, electrical devices such as cell phones or laptops. LiCoO 2 proposed by Goodenough et al., was the first commercially used cathode material in such a LIB. Even today, LiCoO 2 is one of the most important cathode materials due to its high energy density (125 Wh kg À1 , 440 Wh l À1), the high reversible capacity (140 mAh g À1), and an achievable battery voltage of over 4 V. [1] However, due to the ever increasing energy needs of humanity, it is important to further increase the energy density and to continue to ensure or even improve the safety of the batteries. These ideas should be satisfied by the so-called all-solid-state-battery (solid-state battery), a battery made entirely of solid-state materials. Easy miniaturization, no leakage of electrolyte or ignition, and prolonged lifetime are the major advantages that result from the use of a solid electrolyte. In recent years, there has been a severe interest in the application of thin-film cathodes in microelectromechanical systems (MEMS), smart cards, or implantable medical devices. For example, a miniaturized solid-state battery should serve as a so-called "on-chip power supply element" and enable a direct emergency power supply. [2] LiCoO 2 is an extremely promising cathode material due to its excellent electrochemical properties. To date, many studies have focused on LiCoO 2 thin-film cathodes fabricated using common techniques such as radio frequency (RF) magnetron sputtering, [3-5] pulsed laser deposition, [6,7] spray methods, [8] sol-gel coating, [9] and chemical vapor deposition. [10,11] LiCoO 2 crystallizes in the rhombohedral high temperature (HT) or cubic low temperature (LT) phase, depending on the manufacturing conditions. In battery applications, the HT phase is preferred because of its suitable electrochemical properties such as increased capacity and better cycle stability compared with the LT phase. Furthermore, there is a desire to obtain the HT-LiCoO 2 film in a suitable orientation in growth direction, as the efficiency of the Li þ diffusion can be further enhanced. An overview of the properties of RF-sputtered LiCoO 2 is given by Julien et al. [12] In this article, we will investigate the deposition of LiCoO 2 thin films via RF magnetron sputtering on heated platinum-coated c-sapphire substrates. Compared with the parameters given by Julien et al., we gain additional information about the thin film using a different substrate and deposition temperature. Structural characterization of the films was carried out as a function of the substrate temperature and the oxygen-to-argon ratio during the deposition. Elemental distribution of atomic compounds near the surface of optimized films was analyzed. Also the electrochemical performance of a LiCoO 2 film was evaluated.