Abstract
Nanostructured CdS thin films co-doped with Mg and F were prepared on glass substrates maintained at 400°C by spray technique using perfume atomiser. Structural, morphological, optical and electrical properties of the films were investigated in order to study the effect of Mg, F doping concentration. XRD patterns confirm that both undoped and co-doped CdS films exhibit hexagonal crystal structure with preferential orientation along the (0 0 2) plane. Crystallite size decreased from 33.19 to 24.51 nm with increase in Mg, F concentration. Increased transparency and blue shift in optical band gap from 2.48 to 2.75 eV is observed with increase in Mg, F concentration. All the films have resistivity in the order of 10−2 Ω cm which decreased with co-doping. Improved optical transparency, widened band gap and decreased resistivity were observed for the CdS films simultaneously doped with Mg and F.
Introduction
Cadmium sulphide (CdS), a wide band gap II-VI semiconductor finds application in photovoltaics, gamma, X-ray and infrared detectors. 1 CdS thin films have been used as window layer in copper indium gallium selenide and cadmium telluride solar cells, 2 photo detectors, 3 gas sensors 4 and thin film transistors (TFTs). 5 In CdS based heterojunction solar cells, light penetrating through the n-type CdS layer, gets absorbed in its p-type heterojunction partner and the induced electron–hole pairs get divided in the depletion region of these cells. This makes CdS layer play vital role in improving the efficiencies of these solar cells. It has been reported earlier that the electrical and optical properties of the CdS layer drastically affect the performance of these heterojunction solar cells. The CdS layer must be highly transparent and more conductive in order to avoid the short circuit effects. Regarding its electrical properties, CdS films show n-type conductivity owing to the defects such as sulphur vacancies (Vs) and cadmium interstitials (Cdi). 6 These defects act as electron donors, thereby reducing the film resistivity and hence, by controlling these defects the electrical properties of CdS can be improved to a great extent. It has been reported earlier that the optoelectronic properties of CdS thin films can be improved by doping with metallic and non-metallic ions having ionic radii smaller than the host ions. Improved optical and electrical properties have been reported by Anbarasi et al. 7 and Sivaraman et al. 8 for Zn and Mg doped CdS thin films. Improved conductivity has also been reported for CdS films doped with copper. 9 Besides the metallic ions, non-metallic ions such as boron, 10 chlorine 11 and fluorine 12 also influence the optical and electrical properties of CdS thin films very much. The effects of incorporating simultaneously both metallic and non-metallic impurities into CdS thin film with the aim to optimise its properties were interesting. Recently, we reported the optoelectronic properties of CdS thin films simultaneously doped with magnesium and chlorine. 13 Besides this, a detailed report on the effect of both metallic and non-metallic dopant on the properties of CdS thin films is very scarce in the literature. Hence in this work a detailed investigation has been carried out on the structural, morphological, optical and electrical properties of spray deposited CdS thin films co-doped with magnesium and fluorine and the results are given in detail.
Experimental details
CdS thin films were deposited on glass substrates by spray technique using perfume atomiser from aqueous solution (50 ml in volume) containing cadmium chloride, CdCl2 (0.05 M) and thiourea, (NH2)2 CS (0.05 M). Mg and F doping was achieved by adding MgCl2 and NH4F with concentrations (0, 2, 4, 6 and 8 at.-%) to the starting solution. The thicknesses of the films were determined using profilometer (Surftest SJ-301). XRD and morphological studies were performed using X-ray diffractometer (PANalytical-PW 340/60 X'PERT PRO) with CuKα radiation as X-ray source and scanning electron microscope (HITACHI S-3000H). Electrical, optical and photoluminescence studies were performed using two point probe setup, Perkin Elmer double beam UV–Vis–NIR and Varian Cary Eclipse Fluorescence spectrophotometers, respectively.
Results and discussion
Structural analysis
Figure 1 shows the XRD patterns of undoped and (Mg, F) co-doped CdS thin films. In all the films, an intense peak is observed approximately at 26.5° which corresponds to the (0 0 2) plane of hexagonal CdS (JCPDS Card No. 41-1049). Besides this peak, six other peaks were observed approximately at 24.8, 28.2, 36.7, 43.7, 47.9 and 51.9° which corresponds to (1 0 0), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes. It can be observed from the XRD patterns, that the undoped and doped films had a strong (0 0 2) preferential orientation. The strong (0 0 2) preferential orientation observed here exactly matches with the results reported by Selvan et al.
14
for Zn-doped CdS thin films prepared from precursor solution having different aging periods. The occurrence of preferential orientation might be due to the minimisation of surface energy and internal stress.
13
The preferential orientation factor f (h k l) values of the plane (0 0 2) for the undoped and (Mg, F) doped CdS thin films estimated by the method adopted by Suganya et al.
15
are compiled in Table 1.
XRD patterns of Mg, F doped CdS thin films Structural parameters of Mg, F doped CdS thin films
It can be observed that f(0 0 2) decreases linearly suggesting that the crystalline quality of pure CdS deteriorates with doping. No peaks from impurities such as MgS, MgO and phases related to fluorine were detected in the X-ray patterns suggesting the purity of the as deposited films. The 2θ value of the (0 0 2) plane of the doped films shifted towards higher Bragg angles, inferring a contraction in their lattice volumes. The lattice parameter values ‘a’ and ‘c’ calculated for the (0 0 2) plane are presented in Table 1. It is observed that the lattice parameter values decreases with doping which could be ascribed to the change of intrinsic strain of the crystals resulting from the creation of defects and lattice distortions due to the smaller ionic radii of the dopant ions (Mg2+ (0.72 Å), F−(1.33 Å)) as compared with that of the host ions (Cd2+ (0.97 Å), S2− (1.84 Å)). The change of intrinsic strain might also be due to the mismatch thermal coefficients between the film and substrate.
16
The crystallite size (D) values of the films calculated for the (0 0 2) plane using the Scherrer formula
17
:
, where λ is the wavelength of the X-ray used (1.5406 Å), β is the full width at half maximum (FWHM) in radians and θ is the Bragg angle are presented in Table 1. It is observed that the crystallite size show a decreasing trend with doping. When CdS is codoped with magnesium and fluorine, Mg2+ and F− ions uniformly substitute Cd2+ and S2− ions in the host lattice and due to lattice distortion resulting from the ionic radii mismatch between the dopant and host ions, grain growth is affected. Crystallite size also decreases due to the drag force exerted by the dopant ions on boundary motion which is in accordance with Abdolahzadeh Ziabarai et al.
18
for Al-doped CdO thin films. The length of the dislocation lines per unit volume of the crystal called the dislocation density (δ) was estimated using the relation:
17
and the calculated values are complied in Table 1. It is observed that the dislocation density increases with Mg, F doping. As dislocation density and crystallite size show opposite trend behaviour, increased δ and decreased D values observed for the doped films confirms that the crystalline quality of pure CdS deteriorates with Mg and F doping.
SEM analysis
Figure 2
a–e shows the SEM images of CdS thin films simultaneously doped with magnesium and fluorine. All the films surfaces appear to be uniform composed of tightly packed grains with no pin holes and cracks. Grains appear to be interconnected for the undoped film (Fig. 2
a). With an increase in Mg, F concentration, the surface gets modified with irregular shaped grains for the film coated with 2 at.-% doping concentration (Fig. 2
b). For 4 at.-% (Mg, F) concentration, the surface gets fully modified with equally sized grains (Fig. 2
c). As the doping concentration is increased further to 6 at.-%, the surface gets modified with nanoneedle shaped grains (Fig. 2
d). Few interconnected grains are also visible. Elongated needled grains are evinced for the CdS film coated with 8 at.-% Mg, F concentration (Fig. 2
e). Cracks and few honey comb structures are also visible for this film. Thus the film morphology gets modified from irregular shaped grains to nano needled grains with Mg, F doping.
SEM images of a 0 at.-%, b 2 at.-%, c 4 at.-%, d 6 at.-% and e 8 at.-% Mg, F doped CdS thin films
Elemental analysis
The EDX spectra of the undoped CdS and CdS films doped with Mg and F are shown in Fig. 3
a–e. The EDX spectra of the doped films confirm the presence of magnesium and fluorine. The atomic proportions of Cd, S, Mg and F in the films are presented in the insets of Fig. 3. It is observed that the (Mg, F) co-doped films are found to be sulphur deficient i.e. Cd-enriched (S/Cd ratio <1), confirming their non-stoichiometric nature. This non-stoichiometric nature make (Mg, F) doped CdS films more beneficial, as the stoichiometric CdS films are more resistive in nature than the Cd-enriched film. It is also observed that Mg and F content in the co-doped films increases with increase in doping concentration, inferring that Mg2+ and F− ions successfully replaced Cd2+ and S2− ions in the host CdS lattice and the decreased content of Cd and S observed strongly favours for this supposition.
EDX spectra of Mg, F doped CdS thin films
Optical studies
Figure 4 shows the transmittance spectra of CdS thin films simultaneously doped with magnesium and fluorine. The average transmittance in the visible region was found to be 80% for the undoped CdS film. It is observed that the film transparency increases with doping and the CdS film coated with 6 at.-% (Mg, F) concentration exhibited a maximum transparency of nearly equal to 90%. The increased transmittance observed for the doped films might be due to decreased defect density, structural homogeneity and less scattering effects.
19
From the transmittance spectra, it is observed that the absorption edge of the doped films shift towards lower wavelength side suggesting an increase in their band gap values. From the Tauc's plots (Fig. 5), the band gap values of the films are estimated by extrapolating the linear portion to the energy axis at α = 0 and the values are given in the inset table of Fig. 5. Undoped CdS film has a band gap of 2.48 eV which exactly matches with the value reported by Mahdi et al.
20
for CdS film coated using microwave assisted chemical bath deposition technique. The Eg value of the undoped film was found to be slightly higher than the energy for bulk CdS (2.42 eV). This blue shift in the band gap might be probably due to quantum size effect as expected for the nanocrystalline nature of the film. The band gap values of the CdS films with 2, 4, 6 and 8 at.-% Mg, F doping concentrations were found to be equal to 2.54, 2.6, 2.69 and 2.75 eV, respectively and this blue shift in the optical band gap values can be related to Burstein–Moss (BM) effect, according to which intentionally doped elements lead to an increase of free charge carriers in the conduction band causing a shift in the Fermi level thereby increasing the band gap energies of the doped films. Another reason for this blue shift might be due to quantum confinement effect.
21
Quantum confinement contributes to the widening of band gap at smaller crystallites, due to the shift of absorption threshold to shorter wavelength due to individual confinement of electrons and holes. The decreased crystallite size values observed for the doped films (Table 1) strongly favours for this supposition.
Transmittance spectra of Mg, F doped CdS thin films Tauc's plots of Mg, F doped CdS thin films
PL studies
The PL spectra of the undoped and (Mg, F) co-doped CdS thin films (Fig. 6) showed emission peaks at 360, 475, 486, 495, 520, 536, 572 and 595 nm, respectively. The yellow emission peak observed at 595 nm is the result of de-excitation of the electron via the surface/defect states present in the CdS nanocrystals. The electrons, transfer to the surface states non-radiatively after excitation extending into the band gap region.
22
The PL peak at 572 nm corresponds to radiative recombination involving shallow levels in the band gap due to native impurities. The green peak observed at 536 nm may be related to sulphur interstitial acceptors (Is
−) compensated by a close ionised donor centre.
23
The peak at 520 nm is attributed to the band-to-band transition which might have been originated from the recombination of electrons bound to donors with holes bound to acceptors (“bound to bound” or donor-acceptor pair DAP).
24
The peaks at 475 and 495 nm can be attributed to the transitions of trapped electrons to the valence band from donor levels.
8
The peak at 486 nm occurs due to the recombination of impurity-bound exciton.
25
The peak at 360 nm may be attributed to a higher level excitonic emission related to quantum confinement. This peak may be originated from S vacancies which act as deep level traps for electrons extracting them from the valence band.
PL spectra of Mg, F doped CdS thin films
Electrical studies
The electrical resistivity values of the CdS films coated with 0, 2, 4, 6 and 8 at.-% Mg, F concentrations were found to be equal to 10.232 × 10−2 Ω cm, 0.602 × 10−2 Ω cm, 0.414 × 10−2 Ω cm, 0.024 × 10−2 Ω cm and 0.392 × 10−2 Ω cm, respectively. It is observed that film resistivity decreases with doping which might be due to the substitutional incorporation of Mg2+ and F− ions in the CdS structure, which increases the free carrier concentration. The increased free carriers observed for the doped films might also be due to the presence of sulphur deficiencies.
Conclusion
To improve the opto-electrical properties of spray deposited CdS thin films towards solar cell applications, Mg and F are simultaneously doped. All the doped films exhibited hexagonal crystal structure with a strong (0 0 2) preferential orientation similar to that of the undoped CdS. A high transparency of almost 90% in the visible region is achieved for undoped CdS through Mg, F codoping. The limitation of using CdS in optoelectronic device applications was throttled due to the high band gap values obtained for the doped films. A high band gap value of 2.75 eV was obtained for the CdS film coated with 8 at.-% Mg, F doping concentration. From the electrical studies it was observed that the resistivity of pure CdS drastically decreased with Mg, F doping and a minimum value of 0.024 × 10−2 ohm cm was obtained for the film coated with 6 at.-% Mg, F doping concentration. The high transparency, more conductive nature obtained confirmed that CdS films codoped with Mg and F when used as window layer in heterojunction solar cells high efficiency could be obtained due to the minimisation of short circuit effects.
Footnotes
Acknowledgements
The authors thank the Head, Department of Physics and Mr Karthik, Alagappa University for the XRD measurements.