Enhanced properties of Zn-,Mg-incorporated CdO films through Cl doping

X-ray diffraction studies

Figure 1 a–f shows the XRD patterns of undoped CdO, ZMCO and Cl-doped ZMCO (Cl doping levels: 2, 4, 6 and 8 wt-%) thin films. All the films exhibit five diffraction peaks at 2θ values approximately equal to 33.022°, 38.243°, 55.172°, 65.990° and 69.272° indexed to (111), (200), (220), (311) and (222) planes of cubic CdO (JCPDS Card No. 73-2245). A close examination of the XRD patterns show that the preferential orientation is along the (111) plane for the undoped CdO and ZMCO thin films (Fig. 1 a and b), whereas for the Cl-doped ZMCO films it is along the (200) plane. Similar results have been reported earlier by Kumaravel et al. ² for In-doped CdO thin films. They reported a (200) preferential orientation for the undoped CdO which changed to (111) preferential orientation with In doping. Zheng et al. ¹³ reported a change in preferential orientation from (200) to (111) plane for Sn-doped CdO thin films which they attributed to the presence of internal stress induced by the doping of Sn which can alter the energetic balance between different crystal plane orientations. Also the incorporation of Sn might have changed the diffusion rate of Cd and O at the surface during deposition, thereby leading to the variation of growth direction. The invariance in the preferential orientation observed for the ZMCO thin film infer that Zn, Mg (metallic ions) incorporation does not affect the crystal structure of pure CdO. However when chlorine (a non-metallic ion) is added to the ZMCO film, the growth direction is altered due to the internal stress induced by the Cl ions which alter the energetic balance between different crystal plane orientations. In our previous work, ⁸ when CdO is doped with chlorine no change in grain growth direction is observed and for both the undoped and doped films the preferential orientation remained the same as (111). However, the crystal growth behaviour observed in the present study seems to be interesting in the sense that when CdO is doped simultaneously with metallic and non-metallic ions, the non-metallic ions dominate the structural property of CdO. It is observed that the 2θ values of the dominant (111) peak of the ZMCO film shift towards higher Bragg angle (Table 1) inferring a contraction in the unit cell volume, whereas for the Cl-doped ZMCO films it shift towards lower Bragg angles supporting an expansion in their unit cell volume. This variation in the unit cell volume might be due to the ionic size differences between the dopant ions [(Zn²⁺ (0.74 Å), Mg²⁺ (0.72 Å), Cl– (1.81  Å)] and the host ions [(Cd²⁺ (0.97 Å), O²⁻ (1.40 Å)]. The calculated lattice parameter values are given in Table 1.

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XRD patterns of a undoped CdO, b ZMCO, c 2 wt-% Cl, d 4 wt-% Cl, e 6 wt-% Cl and f 8 wt-% Cl-doped ZMCO thin films

Scherrer formula and Williamson–Hall (W–H) method are used to estimate the crystallite size (D) values of the films according to the formulae^14,15: (1) (2) where β is the full-width at half maximum of the strongest diffraction peak, θ is the Bragg angle, ε is the strain present in the films, λ is the wavelength of the X-rays used (1.5406 Å). In W–H method, crystallite size values are estimated from the reciprocal of the intercept on the Y-axis from the plots of vs. (Fig. 2). The calculated ‘D’ values are compiled in Table 1. (It is observed that the crystallite size value decreased for the ZMCO thin film which got enhanced with Cl doping. The increased D values observed for the Cl-doped ZMCO thin films favours that the crystalline quality improves with Cl doping.)

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Williamson–Hall plots of the undoped CdO, ZMCO and Cl-doped ZMCO thin films

The strain (ε) and dislocation density (δ) of the films calculated using the formulae: (3) (4) are presented in Table 1. The minimum values of ε and δ observed for the Cl-doped ZMCO thin films strongly favour for their enhanced crystalline quality.

SEM analysis

Figure 3 a–f shows the FESEM images of undoped CdO, ZMCO and Cl-doped ZMCO (Cl doping levels: 2, 4, 6 and 8 wt-%) films. All the film surfaces appear to be homogenous composed of densely packed grains of different sizes. The grains appear to be interconnected with no pinholes or cracks for the undoped film (Fig. 3 a). With Zn, Mg incorporation the surface appears to be modified with nano-sized grains with well-defined boundaries (Fig. 3 b). Grain size of the ZMCO film appears to be increased with 2 wt-% Cl incorporation (Fig. 3 c). Traces of few clustered grains are also visible. With increase in Cl concentration, the surface gets modified with tightly packed equally sized grains for the film coated with 4 wt-% Cl concentration (Fig. 3 d). Nano-sized grains are visible for the ZMCO film coated with 6 wt-% Cl concentration (Fig. 3 e). As the Cl concentration is increased to 8 wt-%, grains with irregular shapes and sizes are visible (Fig. 3 f). Thus Zn, Mg incorporation enhances the film morphology of the undoped CdO film which was further enhanced with chlorine incorporation.

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SEM images of a undoped CdO, b ZMCO, c 2 wt-% Cl, d 4 wt-% Cl, e 6 wt-% Cl and f 8 wt-% Cl-doped ZMCO thin films

Optical studies

The transmittance spectra of undoped CdO, ZMCO and Cl-doped ZMCO thin films (Fig. 4) showed that ZMCO film exhibit a better transparency compared to that of the undoped film which was further improved through chlorine doping. Increased transparency for the doped films might have been resulted from less scattering defects or from improved structural homogeneity. It is also observed that the absorption edge shifts towards lower wavelength side for the doped films suggesting an increase in their band gap values. From the fundamental absorption corresponding to electron excitation from the valence to conduction bands, optical band gap (E_g) values of the films were calculated. Using the relation between the absorption coefficient (α) and the incident photon energy (hυ), the E_g values are calculated by extrapolating the linear portion of the plots of ((αhυ)² vs. hυ) ¹⁶ (Fig. 5) to α = 0, and the calculated values are provided against each film in Fig. 5. It is seen that the undoped film has a band gap of 2.47 eV which increased to 2.54 eV for the Zn-, Mg-incorporated CdO (ZMCO) film. The increased band gap value observed for the ZMCO film may be due to any of the following reasons: (i) sp-d spin-exchange interaction between the band electrons and localised spin of the metal ions as reported by Nagaraja et al. ¹⁷ for Mn-doped ZnO films, (ii) increased transition tail width and shift effect as reported by Benhaoua et al. ¹⁸ for Al-doped ZnO thin films and (iii) consequence of size effect as reported by Usharani and Balu ¹² for Zn- and Mg-co-doped CdO thin films. Optical band gap is found to be blue shifted for Cl-doped ZMCO thin films, which can be attributed to Burstein–Moss (BM) effect. According to BM effect, increased free carrier concentration through Cl doping might have lifted the Fermi level into the conduction band which causes an increase in the band gap values. Similar blue shift in the optical band gap values with doping has been reported earlier by Kul et al. ¹⁹ and Ferro et al. ²⁰ for F-doped CdO thin films. Besides BM effect, electron localisation within the crystallites, disorder and defects at grain boundaries might have also played a role in widening the band gap values. ²¹

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Transmittance spectra of the undoped CdO, ZMCO and Cl-doped ZMCO thin films

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Plots of (αhν)² vs. hν of the undoped CdO, ZMCO and Cl-doped ZMCO thin films

PL studies

Emission peaks at 340, 361, 420 and 461 nm are evinced from the room temperature PL spectra of undoped CdO, ZMCO and Cl-doped ZMCO thin films (Cl doping levels: 2 and 6 wt-%) (Fig. 6). A small intense peak observed at 393 nm for the ZMCO and Cl-doped ZMCO films might be due to the transition of electrons to the valence band from the localised level slightly above the conduction band which can be assigned to a shallow donor originated from the complex defect (Zn_i) as reported by Wang et al. ²² for Mg and Al co-doped ZnO thin films. This peak corresponds to pure CdO as reported in the literature. ²³ Transitions due to excitons at higher energy levels caused by recombination of excitons and/or shallowly trapped electron/hole pairs might have attributed to the emission peak observed at 340 nm. Near band edge attributed to free exciton annihilation is responsible for the peak at 361 nm. ²⁴ The formation of new energy levels in the band gap of CdO resulting from the reconstruction of the photo-generated hole with an electron occupying the oxygen vacancy is responsible for the emission peak at 420 nm. The blue emission peak at 461 nm may be due to the electronic transitions to the top of the valence band from cadmium interstitials and/or oxygen vacancies.

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PL spectra of the undoped CdO, ZMCO and Cl-doped ZMCO thin films

Electrical studies

The electrical properties (resistivity and carrier concentration) of undoped CdO, (Zn, Mg)-incorporated CdO (ZMCO) and Cl-doped ZMCO thin films are shown in Fig. 7. All the films have resistivity in the order of 10⁻³  Ωcm which exactly matches with the values reported by Rajammal et al. ²⁵ for sol–gel spin-coated CdO thin films. It is observed that free carrier concentration determines the resistivity variation of the as deposited films. The carrier concentration of undoped film was found to be equal to 1.189 × 10²⁰ cm⁻³ which increased to 1.604 × 10²⁰ cm⁻³ for ZMCO thin film. Increased carrier concentration observed for the ZMCO thin film might be due to increased oxygen vacancies created as Zn²⁺ and Mg²⁺ ions substitutionally replaced Cd²⁺ ions in the host lattice. The carrier concentration of ZMCO films is further increased with Cl doping, which might be due to improved crystalline quality achieved through successful substitution of Cl ions into the CdO lattice replacing O² ions.

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Variation of carrier concentration and electrical resistivity of the undoped CdO, ZMCO and Cl-doped ZMCO thin films