# Structure and Optical Properties of Transparent Cobalt-Doped ZnO Thin Layers
## 1 Introduction  
Zinc oxide is an important semiconductor with a simple 3.37 eV direct gap band and a high exciton binding energy of 60 meV [1; 2]. The ZnO in the form of thin layers is primarily of interest as a transparent conducting oxide (TCOs). N-type ZnO thin layers have high visible transmission, excellent stability, and have an interesting combination of optical, electrical, piezoelectric, and thermal properties [3; 4; 5], which are already used in devices such as gas sensors [6], ultrasonic oscillators, and transparent electrodes in solar cells [7]. The favorable electrical and optical properties of zinc oxide have made thin layers for promising use in various fields of optoelectronics [8; 9; 10]. All of these achievements need stable and time-controlled electrical parameters of thin zinc oxide layers, which can be done without post-treatment of the applied layers [11]. One of the key properties of ZnO is the formation of photoexcited electron-hole pairs when exposed to light. ZnO shows visible luminescence even when excited at different UV wavelengths due to the formation of defects of various types [12; 13]. Native point defects in thin films of doped and undoped ZnO significantly affect optical and electrical properties [14; 15]. Based on the works [16; 17; 18], both the energy levels of the defects in the ZnO lattice (interstitial zinc (Zni), interstitial oxygen (Oi), and oxide antisite defect (OZn)) and Co\({}^{2+}\) impurity levels are located in the band gap. The defect levels OZn and Zni are below the conduction band, OZn is located at \(\sim\)2.38 eV and Zni is located \(\sim\)0.5 eV. The Oi level is located at \(\sim\)0.9 eV above the valence band. The energy levels of cobalt ions are located \(\sim\)1 eV above the valence band and about 0.3-0.7 eV below the conduction band [19]. Many conditions related to material production, such as structure, stoichiometry, impurities, annealing temperature, time, and so on, can result in the formation of various defects in ZnO thin films [14]. Dilute magnetic semiconducting (DMS) is currently of great interest due to the complementary properties of semiconductor systems and ferromagnetic materials. These materials represent a promising solution for spintronics devices. ZnO-based ferromagnetic semiconductors are noteworthy after the theoretical prediction of ferromagnetism at room temperature [20]. Many experimental studies have shown ferromagnetism at room temperature in ZnO doped with Mn, Fe, Co [21], Cu [22], and Ni [23]. In the field of spintronics, special attention is paid to oxygen-poor ZnO magnetic thin layers with substituted transition metal ions of the 3d configuration. Doping ZnO with transition metal ions increases the optical absorption, which leads to a change in the value of the band gap, making the semiconductor a more efficient photocatalyst [24], leading to the new magnetoelectronic devices [25]. Broadband oxides such as ZnO, TiO\({}_{2}\), and SnO\({}_{2}\) doped with 3d transition metal ions, compared to traditional type III-V matrices such as GaN, GaAs, InP, and GaP, are preferred for ferromagnetic applications at room temperature. Spontaneous magnetization is defined by bound band magnetic polarons. For example, the static permeability of ZnO increases three times in a ZnCoO solution [26]. This raises interest in sourcing materials for spintronics applications and contributes to enormous efforts in the synthesis of thin layers by using multiple methods, such as molecular beam epitaxy, sol-gel, reactive magnetron co-sputtering, and in particular pulsed laser deposition [27; 28; 29; 30]. The ZnO layer obtained by the pulsed laser deposition (PLD) process has advantages in terms of composition control, ease of processing, and lower material consumption. Therefore, the study of the optical properties of solid solutions with magnetic impurities based on zinc oxide, Zn\({}_{1-x}\)Co\({}_{x}\)O in particular, is an extremely important task, which is considered in the presented article.