Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Impact of composition and ex-situ laser irradiation on the structure and optical properties of As-S-based films synthesized by PECVD Aleksey Nezhdanov a,* , Dmitry Usanov a , Mikhail Kudryashov a , Aleksey Markelov a , Vladimir Trushin a , Giovanni De Filpo b , Aleksandr Mashin a a Lobachevsky University, Nizhny Novgorod, Russia b University of Calabria, Rende, Italy ARTICLE INFO Keywords: Chalcogenide films PECVD Arsenic sulfide Optical properties Nanocrystals formation ABSTRACT Synthesis of amorphous chalcogenide As-S-based films with arsenic content from 35 to 55 at. % by a PECVD method is achieved. The composition-structure-optical properties relationship is revealed. Varying the compo- sition of the films from As 35 S 65 to As 55 S 45 is accompanied by a change of the dominant structural units: from AsS 3/2 pyramids to cage-like As 4 S 4 and As 4 S 3 units, causing a considerable decrease of the optical band gap from 2.42 to 1.87 eV. It has been found out that modification by a focused laser irradiation (473 nm) leads to for- mation of micro/nanocrystalline inclusions feasible for applications in medicine, optoelectronics and integrated optics. The type of inclusions depends on the dominant structural units of the initial films. In case of the As 55 S 45 film appearance of the dimorphite crystalline phase (α-As 4 S 3 ) is observed. The ex-situ laser modification of the As-S films leads to appearance of a photoluminescence emission, and its maximum position shifts from 1.8 to 2.05 eV depending on the initial film stoichiometry. 1. Introduction It is well-know fact, that the properties of amorphous chalcogenide glassy semiconductor (ChGS) bulk samples and thin films depend on two types of disorder: structural and compositional. Thus, it is possible to tune the properties of these materials in a wide range by adjusting the extent of disordering. The presence of non-equilibrium plasma in the PECVD method is a significant factor that influences the degree of disordering of the deposited films. This feature distinguishes our method of synthesis from the PVD methods (PLD [1] and thermal evaporation [2]), CVD [3–6] and the spin coating [7]. In the papers [8–11] we have shown the impact of plasma parameters on the struc- tural and optical properties of As-Se, As-Te, and As-Se-Te chalcogenide films. The properties of the ChGS films can be modified by an external impact, for example, by laser irradiation or by heating/annealing in vacuum, in air or in an inert gas atmosphere. In the works [12–17] it was shown that As-S films demonstrate the ability to change their structural and optical properties under light exposure, for example: photo-darkening, photo-bleaching, photo-crystallization and other ef- fects caused by rearrangements of the amorphous net. In addition, de- pending on the irradiation power, local heating thermal effects may also be observed. At sufficiently high temperatures (in the range ~ 200–320 °C), the As-S films melt (or soft) locally followed by a process of solidification. As a result, a partial crystallization of the films can be observed with formation of different type of crystalline na- noinclusions, depending on the composition, stoichiometry and struc- ture of the initial samples. In some cases, the appearance of such nano- inclusions leads to the emergence of photoluminescence [18,19]. The formation of such inclusions attracts detailed investigations since it is very important in terms of their potential applications in medicine [20], optoelectronics and integrated optics [21]. The purpose of this work is to synthesize amorphous As-S-based chalcogenide glassy semiconductors within a large compositional space by a PECVD-based approach, to study the composition-structure-optical property relationship of the chalcogenide films and to investigate the impact of ex-situ laser irradiation on the films structural and optical properties in terms of different practical applications. 2. Experimental A series of As-S films was synthesized using the PECVD approach and equipment described previously [8–11]. The initial chemical re- agents (solid state elemental arsenic and sulphur) were located in https://doi.org/10.1016/j.optmat.2019.109292 Received 20 June 2019; Received in revised form 21 July 2019; Accepted 4 August 2019 * Corresponding author. E-mail address: nezhdanov@phys.unn.ru (A. Nezhdanov). Optical Materials 96 (2019) 109292 0925-3467/ © 2019 Elsevier B.V. All rights reserved. T