Trans. Nonferrous Met. Soc. China 29(2019) 2566−2576 Tuning optical properties of ITO films grown by rf sputtering: Effects of oblique angle deposition and thermal annealing L. G. DAZA 1 , M. ACOSTA 2 , R. CASTRO-RODRÍGUEZ 1 , A. IRIBARREN 3 1. Department of Applied Physics, CINVESTAV-IPN, Unity Mérida. 97310 Mérida, Yucatán, México; 2. Materials Science Laboratory, Faculty of Engineering, University of Yucatan, CP 97130 Mérida, Yuc., México; 3. Instituto de Ciencia y Tecnología de Materiales, Universidad de La Habana, Zapata s/n esq. G, Vedado, Plaza, La Habana 10400, Cuba Received 14 March 2019; accepted 17 October 2019 Abstract: Indium tin oxide (ITO) thin films were prepared using the technique of rf-sputtering with oblique angle deposition (OAD). The films were as-deposited and thermally treated at 250 °C. The combination of substrate inclination and annealing was used for modifying morphological and structural properties that lead to changes of the optical properties. The resulting films show morphology of tilted nanocolumn, fissures among columns, and structural changes. The as-deposited films are structurally disordered with an amorphous component and the annealed films are crystallized and more ordered and the film diffractograms correspond to the cubic structure of In 2 O 3 . The refractive index could be modified up to 0.3 in as-deposited films and up to 0.15 in annealed films as functions of the inclination angle of the nanocolumns. Similarly, the band gap energy increases up to about 0.4 eV due to the reduction of the microstrain distribution. It is found that the microstrain distribution, which is related to lattice distortions, defects and the presence of fissures in the films, is the main feature that can be engineered through morphological modifications for achieving the adjustment of the optical properties. Key words: oblique angle deposition; ITO thin films; nanocolumnar morphology; microstrain distribution; optical properties 1 Introduction Indium tin oxide (ITO) is a transparent conducting oxide (TCO). It is one of the more used TCO in multiple applications in optoelectronic and photovoltaic devices, due to its characteristics as wide band gap (about 3.50−3.75 eV) [1] and high transmittance in the range of visible light, low electrical resistivity and high conductivity [2]. ITO has been widely researched [3] for multiple applications and uses, such as windows layers in solar cells [4], UV-LED [2], touch screen [5], sensors and actuators [6], nanoelectronics [7] and others. A material with adjustable refractive index can improve the performance in photonic and optoelectronic applications, due to the possibility of a suitable optical adaptation [8], where either high or low refractive index can be useful and necessary [9,10]. The adjustment of the optical properties of ITO continues in the focus of investigation due to its multiple prospective applications [11−14]. There are several techniques for the growth of ITO as thin film. For example, sol−gel [15,16], spin coating [17,18], thermal evaporation or physical vapor deposition [19], electron beam irradiation [2], and magnetron sputtering [20−23], among others [24] have been used for obtaining ITO films. Magnetron sputtering technique is one of the most used because of the controllable deposition rate, the good stoichiometric reproducibility and low growth temperatures [25,26]. Oblique angle deposition (OAD) is a useful technique in which, by varying the angular position of the substrate respect to incident vapor flux, it is possible to obtain three-dimensionally nanostructured thin films [27] due to the growth of directionally tilted columns as a consequence of shading effect [28]. The formation and modification of such nanostructured films allow to adjust the optical properties in order to find better performance of optoelectronic devices [8,28]. The crystallinity, i.e., the conditions of crystalline, polycrystalline and amorphous phase of a material, influences the electrical, optical and other properties. Annealing can be used as a recrystallization method [29] Corresponding author: A. IRIBARREN; E-mail: augusto@imre.uh.cu, a_iribarren@yahoo.com DOI: 10.1016/S1003-6326(19)65164-2