Modification of the chemical composition, morphology, and antireflection properties of WSe x films formed by pulsed laser deposition

Abstract

793Thin films of transition metal dichalcogenide (TMDs) including tungsten diselenide (WSe 2 ) have drawn considerable interest due to the layered (lamel lar) structure of these materials, which accounts for a number of extraordinary properties. A weak chemical interaction between lamellae ensures weak resistance to shear, which can be used in tribology for obtaining solid lubricant coatings possessing very low friction coefficients in the absence of traditional lubricants [1]. In addition, TMD films possess semiconductor prop erties, which are retained (and sometimes modified in an unusual manner) in the case of formation of ultrathin or even monolayer films. The semiconductor properties of TMD films are used in creating func tional layers for promising photovoltaic, thermoelec tric, and nanoelectronic devices [2, 3].Thin film TMD materials are most widely obtained using ion plasma sputtering and chemical deposition techniques. Each of these methods has both advantages and drawbacks, which stimulate the search for new methods capable of depositing TMD films with improved characteristics. In particular, the method of pulsed laser deposition (PLD) also allows thin film TMD materials to be obtained with quite good properties for tribological applications [4]. The use of PLD techniques for depositing functional TMD films for nano and optoelectronics is hindered by the fact that laser ablation of TMD targets is accompanied by the formation of micro and nanosized particles violating the homogeneity of structure and chemical composition of deposited films [5]. Deposition of these particles can be avoided by mounting an anti droplet shield in front of the substrate, so that material behind the shield is deposited from a flow of atoms scattered on a buffer gas [6].The presence of a shield leads to significant changes in the chemical composition of TMD films, which are related to differences in the dynamics of expansion and the kinematics of collisions of metal and chalcogen atoms with buffer gas molecules. In the case of significantly different atomic weights of the metal and chalcogen (e.g., W versus Se), the shadowed region is characterized by extremely high chalcogen content [6]. Therefore, it is necessary to develop methods ensuring flexible control over the chemical composition and structure of thin film TMD materi als obtained by PLD with an anti droplet shield. In particular, for the functional layers of photovoltaic devices, it is very important to decrease the reflectance of the TMD film surface, which can exceed 30% for the films manufactured by traditional methods. The same problem is encountered in the creation of sili con based solar cells. In order to decrease the reflec tion coefficient, it is a commonly accepted approach to apply special antireflection coatings [7] or to make the Si matrix surface profiled by using special process ing methods [8].The present work was aimed at studying features of the growth and chemical composition of thin WSe x films obtained by PLD under specific conditions, including t...