Optical and structural characteristics of electrodeposited Cd _1-xZn_xS nanostructured thin films

Highlights

• Cd_1-xZn_xS (x ~ 0.5) nanostructured thin films were grown by the electrodeposition method.

• The crystalline structure of the deposited thin film was determined as cubic wurtzite.

• Temperature-dependent band gap energy characteristics were studied in the 10–300 K range.

• Direct band gap energy was found as 2.56 eV at 10 K and 2.51 eV at room temperature.

• The band gap energy vs. temperature dependency was studied applying Varshni model.

Abstract

The structural and optical characteristics of Cd_1-xZn_xS (CdZnS) thin films grown by the electrodeposition method were investigated in the present paper. The crystalline structure of the grown CdZnS thin film was determined as cubic wurtzite due to observed diffraction peaks associated with (111) and (220) planes. Atomic compositional ratios of the constituent elements were obtained using energy dispersive spectroscopy and doping concentration of the Zn was found as 5% (x ~ 0.05). Scanning electron microscopy image of the studied thin film indicated that grown film is nanostructured. Raman spectra of CdS and CdZnS thin films were measured and it was seen that observed longitudinal optical modes for CdZnS present a blue-shift. Temperature-dependent band gap energy characteristics of the thin films were studied performing transmission experiments in the 10–300 K temperature range. The analyses of the recorded transmittance spectra showed that direct band gap energy of the films decreases from 2.56 eV (10 K) to 2.51 eV (300 K) with the increase of temperature. The band gap energy vs. temperature dependency was studied applying well-known Varshni optical model and various optical parameters of the films were reported according to the results of the applied model.

Introduction

CdS is one of the attractive members of II-VI type semiconducting compounds. The wide range of optoelectronic device applications of the compound has kept the research interest on the CdS warm. The solar cells, light emitting diodes, nonlinear optics, transistors, sensors are some of the devices in which CdS has been used for years [[1], [2], [3], [4]]. CdS has been grown in various forms like single crystal, thin film, nanoparticle, nanowire. CdS exhibits two crystalline structures of cubic and hexagonal phases. The lattice constant of the cubic phase was reported as a = 5.832 Å while those of the hexagonal phase were obtained as a = 4.130 Å and c = 6.703 Å [5]. The direct band gap energy of the compound has been reported around 2.48 eV which is suitable especially for solar cell applications [6].

CdS compounds have been grown with various dopants like Mn, Cu, Al, Co, Gd, Ag, Pd and etc. [[7], [8], [9], [10]]. The addition of dopant may improve the structural and optical characteristics of the CdS for the purpose. One of the most studied and doped elements is zinc (Zn) which forms Cd_1-xZn_xS compounds in a wide range of composition (0 ≤ x ≤ 1) [11]. The significance of Zn among the other dopants come due to its large optical transmittance and closer ionic radius with Cd ($r_{Z{n}^{2+}}\approx r_{C{d}^{2+}}\approx 0.074nm$) [6]. The cheapness, abundance, and non-toxicity properties of the Zn are also appreciable advantages for technological applications [12]. The direct band gap energies of the prepared Cd_1-xZn_xS thin films were reported as increasing from 2.33 eV (x = 0) to 3.33 eV (x = 1). The remarkable shift of the band edge with composition provides the mixed compounds attractive position in optoelectronic devices. The role of the Zn substitution on nonlinear optical characteristics of the CdS was investigated and analyses showed that third-order nonlinear optical parameters of the compound improve [6].

One of the important optical parameters for compounds utilized in optoelectronic applications is band gap energy. Since temperature affects the band gap energy of the semiconducting materials, temperature-dependent band gap energy characteristics take an effective position in device fabrication. The temperature dependency of band gap energy of CdS thin films grown by thermal evaporation technique was reported in Ref. [13]. However, nanostructured Cd_1-xZn_xS thin films have not been investigated so far in a similar way. The present paper aims to grow nanostructured Cd_1-xZn_xS (x = 0.05) thin films by electrodeposition method and characterize its structural and temperature-tuned band gap energy properties. Since compositional dependence of nanostructured Cd_1-xZn_xS thin films was previously reported in the literature [[14], [15], [16], [17]], temperature-dependent band gap characteristic of the Cd_1-xZn_xS thin films was considered only for x = 0.05 to give a general view about this point. Structural properties of the films were studied utilizing x-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) methods. The crystalline structure, atomic compositional ratio and surface morphology of the nanostructured thin films were determined by applying these experimental techniques. Optical properties of the films were considered using Raman and temperature-dependent transmission measurements. The variation of the band gap energy with temperature was revealed for Cd_1-xZn_xS (x = 0.05) films and the band gap energy vs. temperature dependency was analyzed under the light of Varshni optical model.