Temperature-tuned optical bandgap of Al-doped ZnO spin coated nanostructured thin films
Graphical abstract
Introduction
ZnO is one of the very promising semiconducting compounds due to its wide range of device applications [1], [2]. ZnO is effectively utilized in applications such as solar cells, LEDs, lasers, transistors, UV photodetectors, biosensors and similar devices due to its fascinating structural, optical and electrical characteristics [3], [4], [5], [6]. ZnO crystallizes as hexagonal wurtzite structure with lattice parameters of a = 3.252 Å and c = 5.313 Å [2]. The wide bandgap energy and large exciton binding energy have been reported as 3.37 eV and 60 meV, respectively [2].
Aluminum doped ZnO (AZO) thin film have been studied due to its applications like visible luminescence [7], transparent heater [8], solar cells [9], and transistors [10]. The low resistivity, high optical transparency, low cost, direct bandgap energy and many other effective characteristics of AZO provide the compound a remarkable position in optoelectronic device applications. Temperature dependency of bandgap of the undoped ZnO has been previously studied in few papers [11], [12], [13]. The spectroscopic ellipsometry and photoluminescence studies were reported on the ZnO films at temperatures above room temperature [11]. The decrease of bandgap from 3.43 to 3.33 eV was observed as temperature was increased to 620 from 300 K. The analyses of absorption measurements on electron-beam deposited ZnO thin films in the 78–300 K showed that bandgap decreases from 3.51 to 3.48 eV as temperature was increased [12]. The present paper aims expanding studies on temperature-tuned bandgap properties of AZO films. For that purpose, transmission experiments were performed on the AZO nanostructured films in the 10–300 K region which has not been applied on AZO thin films up to now. The measured spectra were analyzed to get bandgap values at each applied temperatures and next temperature-dependent bandgap energy graph was investigated considering the Varshni and Bose-Einstein expressions.
Section snippets
Experimental details
AZO nanoparticle ink with ∼5% Al-concentration was bought from Sigma-Aldrich (No: 807729). AZO thin films were produced by spin coating of the ink on soda-lime glass substrates. The coating dispersion was dropped onto the substrate rotating at 3000 rpm for 20 s. After this coating process, film was dried at 300 °C for 10 min using muffle furnace to remove possible residuals. The coating-drying processes were repeated for 10 times and the resulting film was annealed at 400 °C for half an hour.
Results and discussions
Fig. 1 indicates XRD plot of the AZO films. The observed peaks are consistent with those of previously reported [14], [15]. The pattern does not present peaks related with Al, Al2O3 and similar secondary phases. This shows that Al-atoms are replaced with host atoms in the films. The Miller indices of hexagonal wurtzite crystalline structure were represented on the peaks. Inset of Fig. 1 presents the SEM image and as seen from the image, AZO film is in the nanostructured form in which the size
Conclusion
AZO nanostructured films produced by spin-coating method of its ink form were structurally and optically investigated. SEM images indicated the forms of coated films as nanostructured with sizes around 20–30 nm. XRD graph exhibited well-defined diffraction peaks corresponding to hexagonal lattice structure. The room temperature bandgap energies of undoped and Al-doped (5% concentration) ZnO nanostructured films were obtained as 3.32(7) and 3.35(3) eV, respectively. The bandgap-temperature
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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