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    Effect of Cd dopant on structural, optical and CO2 gas sensing properties of ZnO thin film sensors fabricated by chemical bath deposition method
    (Springer Science and Business Media Deutschland GmbH, 2021) Altun, Büşra; Karaduman Er, Irmak; Çağırtekin, Ali Orkun; Ajjaq, Ahmad; Sarf, Fatma; Acar, Selim
    Cadmium-doped zinc oxide (Cd-doped ZnO) films were produced by economic facile chemical bath deposition method. The Cd doping content was adjusted as 1%, 3%, 5% and 7%. The structural, morphological and optical properties of the films were characterized by XRD, Raman, SEM and UV–Vis. The response in a carbon dioxide atmosphere was measured by varying the concentration up to 100 ppm at different working temperatures (30–250 °C). XRD measurements demonstrated that all synthesized films have a good crystallite structure with hexagonal wurtzite dominant phase. A large variety of nanostructures are randomly distributed over the films’ surfaces depending on Cd doping content as was manifested by the corresponding SEM images. From the transmittance analysis, an ultraviolet absorption edge corresponding to pure ZnO film undergoes a redshift with the increase in Cd content. The results from Raman spectra are in good agreement with the XRD results. From the gas sensing measurements, a high response toward 100 ppm CO2 gas was detected by 3% Cd-doped ZnO sensor (88.24% at 125 °C) with an acceptable response of 8.36% at room temperature, which exhibited the lowest response/recovery times as well as highest selectivity, stability and reproducibility. Changes in the CO2 gas sensing response as a function of Cd doping content are explained based on particle size, optical bandgap and surface images.
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    Influence of isovalent Cd doping concentration and temperature on electric and dielectric properties of ZnO films
    (Elsevier Ltd, 2021) Altun, Büşra; Ajjaq, Ahmad; Çağırtekin, Ali Orkun; Karaduman Er, Irmak; Sarf, Fatma; Acar, Selim
    In this study, pure zinc oxide and lightly cadmium doped zinc oxide (Zn1-xCdxO; x = 0.01, 0.03, 0.05 and 0.07) films were synthesized by chemical bath deposition to mainly investigate the possible impact of isovalent (in particular Cd) doping ratio and temperature on their electric and dielectric features. X-ray diffraction patterns revealed that all produced films have a dominance of ZnO hexagonal wurtzite structure with the emergence of a minor CdO cubic phase at x ≥ 0.03, and predicted the decrease in average crystallite sizes with Cd doping. Cd content in the films was verified by energy dispersive X-ray analysis. Images of scanning electron microscopy revealed the formation of nanorods and spheres on the surface of pure ZnO film which changed to porous/agglomerative spheres with Cd doping. Then a comprehensive electric and dielectric analysis was carried out as a function of frequency in a wide temperature range (300–700 K) using two separate experimental data sets, (Z, θ) and (C, G). The results demonstrated the critical effect of temperature and Cd doping ratio on the electrical and dielectric properties of ZnO films. Among the investigated films, Zn0·97Cd0·03O film recorded highest conductivity and enhanced dielectric properties which was attributed to the equal activation of grains and grain boundaries in the film structure verified by the estimation of activation energies from impedance spectrum. However, the effect of Cd doping on electric and dielectric properties was prominent only below 500 K, beyond which the doping effect became negligible which might be correlated with the effective dominance of grain boundaries at high temperatures as was witnessed by modulus spectrum.