Tailoring the optical properties of CdO nanostructures via barium doping for optical windows applications

Highlights

• Ba:CdO nanostructured films were synthesized using the spray pyrolysis technique.

• Ba contents play a great effect on the optical properties of the doped CdO film.

• Eg value is blue-shifted from 2.78 eV (neat CdO) to 3.03 eV (10 wt% of Ba:CdO).

• The increase in Eg value of Ba:CdO is interpreted in terms of the Burstein-Moss effect.

• Ba:CdO films could effectively compete well-known wide bandgap semiconductors.

Abstract

In this study, the optical properties of CdO nanostructured films have been tailored via barium doping for optical windows applications. Barium-doped cadmium oxide (Ba:CdO) nanostructured films (nFs) were synthesized using spray pyrolysis technique. The prepared nFs were examined using X-ray diffractometer, FT-IR and UV-Vis-NIR spectrophotometers. The XRD analysis reveals that the plain and Ba:CdO nanostructured films exhibit cubic crystalline structures. FT-IR analysis exposes that plain CdO absorption bands locate in the wavenumber range from 400 cm⁻¹ to 1200 cm⁻¹. The UV-Vis-NIR spectra analysis exhibits that Ba content plays a significant effect on the optical transmittance, bandgap ($E_g$), refractive index, dielectric constants and optical conductivity of CdO films in the Vis-NIR regions. The obtained $E_g$ value is blue-shifted from 2.78 eV (plain CdO) to 3.03 eV (10 wt% of Ba:CdO). This increase in $E_g$ value of Ba:CdO nanostructured films is interpreted in terms of the Burstein-Moss effect. The obtained novel results introduce Ba:CdO nanostructured films as serious competitors to the well-known wide bandgap semiconductors for different applications.

Introduction

Metal oxides have been gaining attention due to their effective role in various applications. This high level of concern has emerged as a result of their superior characteristics including their physical and chemical properties [1], [2], [3], [4], [5], [6]. Particularly, wide bandgap (>3.2 eV) metal oxides as titanium dioxide (TiO₂), zinc oxide (ZnO), indium oxide (In₂O₃) and tin oxide (SnO₂) occupy special attention because of their role in different fields [7], [8], [9], [10]. But, many applications require metal oxides with the aforementioned features with low energy bandgaps (<3.2 eV). Specifically, cadmium oxide (CdO) as an n-type semiconducting material and cubic crystalline structure is considered as an appropriate alternative metal oxide due to its novel properties [11]. Its intermediate direct optical bandgap (2.2 to 2.5 eV [2]), high transmittance of the visible spectrum, chemical stability and acceptable electrical conductivity (10² to 10⁴ S/cm) qualifies it as an active candidate in many fields [12], [13], [14], [15], [16]. These applications involve different optoelectronics as transistors, LEDs, solar cells, sensors, smart windows and optical devices [12], [13], [14], [17], [18], [19]. There are a lot of previous reports concerning plain CdO and its dopings. For example, plain CdO thin films were prepared for gas sensing applications using the SILAR technique [20]. Also, exploring CdO optical bandgap and its effective mass was performed by Jefferson et al. [21]. While other studies are mainly concerned with the effect of the dopant elements on the physical and chemical properties of the host CdO for different applications. Particularly, tuning the optical bandgap of the host CdO semiconductor and improving its electrical conductivity are some of the significant investigated parameters. For that, Zn-doped CdO nanoparticles were prepared using a chemical technique for nanotechnology applications [11]. They proved that CdO optical bandgap could be tailored from 2.67 to 2.61 eV via 5.0 wt% of Zn–doping. Also, La-doped CdO was prepared using the spin coating method by AlFaify et al. [12]. In their work, the optical bandgap of La-doped CdO films was tuned from 2.55 to 2.8 eV and the optical transparency of the prepared films was enhanced to 85%. Furthermore, Sn-doped CdO nanostructures were electrochemically prepared by Ganjiani et al. [2]. The optical bandgap was enhanced to 2.84 eV due to Sn-doping. The photoluminescent, electrical and optical bandgap of Cu-doped CdO nanostructures were explored via Benhaliliba et al. [22] for sensors applications. Moreover, SILAR method was utilized to prepare Ba-doped CdO for semiconductor device technologies [17]. Furthermore, the optical transmittance of the sprayed CdO thin films was greatly enhanced via fluorine doping [23], [24].

Many techniques are reported in the literature concerning the preparation of plain and doped CdO semiconductor. It is worth mentioning that the preparation technique could effectively control the physical and chemical properties of the final product. Some of these methods involve chemical and physical dealings as chemical bath deposition [17], [20], spin coating [22], thermal evaporation [13], magnetron sputtering [25], spray pyrolysis technique [23], [26] and others [15]. The last one (spray pyrolysis) involves many advantages over the others such as its simplicity, low fees and high area coverage [1], [3], [23], [25]. Moreover, it does not need any certain requirements and conditions to be carried out for metal oxide materials. Also, many previous works confirmed that the spray pyrolysis method could effectively employ to prepare nanostructured thin films [3], [25].

The current study aims to tailor the optical properties of CdO nanostructured thin film via barium (Ba) doping for optoelectronic applications. The effect of Ba concentration on the FT-IR, structural and optical properties of CdO as a host semiconductor has been explored. Since the majority of previous works available in the literature were concerned with doping CdO with small ionic radii elements as compared with that of Cd ions (0.95 Å [14]) [11], [15]. While limited studies are reported on doping with larger ionic radii as Ba ions (1.35 Å [27]) in the host CdO semiconductor. The obtained structural and FT-IR analyses reveal that Ba-doping causes a great effect on the structure of CdO nanostructured films (nFs). The UV-Vis-NIR measurements show that the optical transmittance of Ba:CdO nFs in the Vis-NIR spectra regions has been enhanced more than 2.5 times. Moreover, the optical properties (bandgap, refractive index, dielectric constant and optical conductivity) of Ba:CdO nFs have been determined in the UV-Vis-NIR spectra regions. The optical bandgap of the sprayed CdO nFs has been blue-shifted to shorter wavelengths via Ba-doping. The obtained results are interpreted in terms of the Burstein-Moss effect.