Correlation between morphology and electro-optical properties of nanostructured CdO thin films: Influence of Al doping
Highlights
► Physical properties of the CdO:Al nanostructured films at different Al% are studied. ► Maximum carrier concentration has been achieved for 1 at.% Al doped film. ► The undoped sample has maximum carrier mobility. ► The direct optical band gap shifted to the higher energy. ► The samples have relatively smooth surface with RMS roughness of about 4 nm.
Introduction
In recent years, metal oxide semiconductor materials have attracted much attraction due to their application in various fields ranging from electronic devices such as displays [1] solar cells [2] and thin film transistors [3] to gas sensors [4], [5]. Pure cadmium oxide (CdO) is an n-type degenerate semiconductor with high electrical conductivity (102–104 S/cm) [6]. With respect to its optical properties, however, CdO has an indirect band gap of 2.06 eV and a direct one of 2.59 eV, which are lower than that of ZnO, a fact that reduces the transmittance range of this material in the visible region [7]. Doping of CdO with metallic ions of smaller ionic radius than that of Cd2 +, like In, Sn and Al improves its electrical conduction and increases its optical energy gap [6]. Although Al is a popular dopant in ZnO thin films, reports on CdO:Al thin films are very limited [8], [9], [10], [11].
In this study, we have prepared CdO:Al thin films by the less reported sol–gel dip-coating process applying a precursor based on cadmium acetate along with methanol as the solvent, the rare-reported monoethanolamine as the stabilizer, glycerol, 2-methoxyethanol and aluminum nitrate. Maity and Chattopadhyay [11] have used cadmium acetate, isopropyl alcohol and acetic acid as the precursor material, solvent and pH controller respectively. Murali et al. [9] have used an acrylamide precursor based on cadmium chloride and Ilican et al. [10] have reported a precursor solution very similar to ours, but with higher molarities and triethylamine as the stabilizer. Here, we have utilized monoethanolamine as the stabilizer. It should be noted that while most of the existing works focus on the study of the structural, optical and/or electrical properties of the prepared films the present study mainly focuses on the correlation of structural, optical and electrical properties of Al-doped CdO films with surface morphology. SEM and AFM imaging have been applied to investigate the morphological features of CdO:Al thin films. The structural properties and surface topographies of the thin films were studied as a function of film composition for various Al concentrations.
Section snippets
Sample preparation
All samples considered in this study were deposited on soda-lime glass substrates by sol–gel dip-coating method at room temperature. Prior to dip coating, the glass substrates were first degreased by detergent, rinsed thoroughly by deionized water and then in boiling water. In order to remove macroscopic contaminations, the substrates were cleaned ultrasonically in a mixture of ethanol and acetone (each of 50% in volume) for 15 min. The latter procedure was repeated in deionized water. Finally,
Results and discussion
XRD patterns of CdO thin films doped with aluminum, whose concentration changes from 0 to 5 at.% are shown in Fig. 1. For undoped films the peaks associated to planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of the cubic face-centered structure (JCPDS No. 5-0640 [13]) are observed. It can be seen that the films have the (1 1 1) preferred orientation. The undoped thin film has shown the highest (111) diffraction peak intensity while the peak intensities of the aluminum doped films decreased with
Conclusions
Smooth and crack-free CdO:Al thin films with nano-grain size were successfully grown on glass substrates utilizing a new precursor, by sol–gel dip-coating method. XRD results showed that the films have (111) preferred orientation and the crystallite size decreased with Al doping. SEM images revealed a flat-grain surface and single mode size distribution of grains. AFM imaging indicated high smooth surfaces with small RMS roughness values. The fractal analysis based on AFM imaging data was
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2021, Optical MaterialsCitation Excerpt :Thus, these doped materials can display different characteristics which usually cannot be found in that host metal oxide. Previous studies report that the optoelectronic properties of CdO nanostructures are likely to be tuned by introducing different metallic ions to the prepared CdO samples [3,7–10]. Therefore, one can hope that Cr doping may generate some promising physical properties of CdO nanostructures.