Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full length article Physical properties of rf-sputtered ZnS and ZnSe thin films used for double- heterojunction ZnS/ZnSe/CdTe photovoltaic structures O. Toma a , L. Ion a, ⁎ , S. Iftimie a , V.A. Antohe a , A. Radu a , A.M. Raduta a , D. Manica a , S. Antohe a,b, ⁎ a University of Bucharest, Faculty of Physics, 077125 Magurele, Ilfov, Romania b Academy of Romanian Scientists, 030167 Bucharest, Romania ARTICLE INFO Keywords: ZnS ZnSe CdTe rf-Magnetron sputtering Protons irradiation ABSTRACT Polycrystalline ZnX (X = S, Se) and CdTe thin films were prepared by rf magnetron sputtering and by thermal vacuum evaporation (CdTe films), respectively. The structural properties were studied by X-ray diffraction (XRD), which revealed that ZnX films are polycrystalline with a marked (111) texture. After irradiation with protons crystallite sizes decreased while mechanical strains increased. Thicknesses of ZnX films and surface roughness have been measured by X-ray reflectometry (XRR) with thickness values between 58 nm and up to 163 nm and with surface roughness between 1.7 nm and 2.4 nm. Morphological investigations were made by scanning electron microscopy (SEM), drops - free surfaces with compact and uniform aspect being deposited. Absorption and transmission measurements were carried out for all samples deposited on optical glass sub- strates. Experimentally determined bandgap energies were between 2.31–2.75 eV for ZnSe layers, respectively, 3.10–3.65 eV for ZnS films. Optical transmissions in VIS-NIR regions are higher than 60% for both ZnSe and ZnS films. Glass/ITO/ZnS/ZnSe/CdTe/Cu:Au structures in superstrate configuration were produced by depositing CdTe absorber layers by thermal vacuum evaporation (TVE). Action spectra of the external quantum energy (EQE) and the current-voltage (I-V) characteristics in AM 1.5 conditions (the density power of the incident light is equal with 100 mW/cm 2 ) corresponding for double-heterojunction ZnS/ZnSe/CdTe photovoltaic structures were investigated before and after irradiations with high energy protons (3 MeV). 1. Introduction A II – B VI binary semiconducting compounds have attracted con- siderable attention due to their excellent optoelectronic properties. Among the wide bandgap semiconductors, ZnS and ZnSe constitute a specific family of A II – B VI compounds, crystallizing in the cubic zinc- blende structure at ambient pressure. ZnSe has a large direct bandgap of 2.67 eV at room temperature, high optical transmittance over a large spectral range between 0.6 and 20 μm and good photosensitivity. Visible and infrared applications based on ZnSe thin films include: blue and mixed color light-emitting devices [1,2], solar cells [3–6], lasers [7], thin film transistors [8], sensors [9] and photodetectors [10]. Different deposition methods were used to obtain ZnSe thin films, in- cluding thermal vacuum evaporation [11–13], chemical bath deposi- tion [14,15], pulsed laser deposition [16], molecular beam epitaxy [17] and close-spaced vacuum sublimation [18]. The use of ZnSe as replacement material for classical CdS n-type window layers in the preparation of p-CdTe thin film solar cells still remains an interesting problem, especially for environmental protection, ZnSe being a less toxic alternative as buffer material. Cell efficiencies around 11% were obtained using ZnSe thin films in n-ZnSe/ p-CdTe devices [19]. We decided to combine ZnSe films with ZnS films as a wider (3.66 eV) bandgap material obtaining a mixed buffer ZnS/ ZnSe window layers. ZnS also owns great photoelectric and luminescent properties, recommending it for use as a window layer in thin film photovoltaics industry [20]. Both types of ZnX (X = S, Se) layers were deposited by r. f. magnetron sputtering due to the excellent control of film thickness uniformity proved by this deposition method [21]. In this paper, after the preparation and structural, morphological and optical characterization of the ZnS, ZnSe and CdTe thin films, we report also a study on the ZnS/ZnSe/CdTe double-heterojunction properties and how these properties are influenced by irradiations with high energies protons. 2. Material and methods Thin films of ZnX (X = S, Se) were deposited by rf-magnetron sputtering; the frequency of rf generator was fixed at 13.56 MHz. The https://doi.org/10.1016/j.apsusc.2019.02.032 Received 11 December 2018; Received in revised form 31 January 2019; Accepted 5 February 2019 ⁎ Corresponding authors at: University of Bucharest, Faculty of Physics, 405 Atomistilor Street, PO BOX MG-11, 077125 Magurele, Ilfov, Romania. E-mail addresses: lucian@solid.fizica.unibuc.ro (L. Ion), santohe@solid.fizica.unibuc.ro (S. Antohe). Applied Surface Science 478 (2019) 831–839 Available online 06 February 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved. T