Lithium-antimony co-doping induced morphology transition in spray deposited SnO2 thin films

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Abstract

Un-doped, Sb doped (ATO) and Li-Sb co-doped SnO2 (LATO) transparent conducting thin film electrodes have been deposited using facile spray pyrolysis method. X-ray diffraction study is carried out to confirm the crystal structure, phase purity, and textured growth of the films. Surface morphological transition from dendritic to cuboid shape is observed upon Li-Sb co-doping into the SnO2 system from the scanning microscopy measurements. The AFM analysis confirms that as Li doping concentration (1, 3, 5 wt.%) increases, the surface roughness of the films decreases. The UV-Vis transmittance spectra of the SnO2 film doped with 5 wt.% Li and 5 wt.% Sb (LATO5) shows maximum average transmittance of 87.9 % at 550 nm and the calculated direct band gap value is 4.06 eV. The LATO5 film exhibits high carrier concentration and high mobility of 7.54 × 1020 cm−3 and 7.221 cm2/Vs, respectively, due to co-dopant effect at the Sn sites of SnO2 lattice, resulting in enhanced electrical transport properties compared to un-doped SnO2 film. The LATO5 film also exhibits lowest sheet resistance of 32.38 Ω/□ and resistivity of 1.57 × 10−3 Ω cm. The temperature dependent electrical conductivity study is performed to understand the conduction mechanism of the film via estimation of activation energy from Arrhenius plots. The obtained results indicate that the Li-Sb co-doped SnO2 films are potential candidates for alternative transparent conducting electrode system.

Introduction

Transparent conducting oxides (TCOs) should have salient features like high conductivity, high optical transparency and good stability to be employed as transparent conducting electrodes (TCEs) in various optoelectronic devices [1,2]. Deposition of a TCE film with high electrical conductivity and optical transparency via facile deposition method is necessary for its usage as a component in efficient display and energy harvesting devices. To date, binary oxides like ZnO [3], SnO2 [4], WO3 [5], In2O3 [6], TiO2 [7], and ternary oxides like Zn2SnO4 [8], Cd2SnO4 [9], and BaSnO3 [10] have been explored for alternative TCO system by appropriate doping. Among the family of binary oxides, n-type tin oxide (SnO2) has been considered as one of the best host materials since its physical and chemical properties can be tuned by doping it with several metallic impurities [11]. The undoped SnO2 films show very low electrical conductivity and optical transmittance [12], whereas donor doped SnO2 films exhibit higher carrier concentration and improved mobility, thereby offering properties beneficial for optoelectronic device applications [11]. Donor doped SnO2 films have been extensively studied over the past two decades. Smaller crystallite size, smooth surface morphology, high optical transmittance and excellent electrical conductivity properties offer better device performance [13]. Precise control over surface features of donor doped SnO2 films also plays a critical role in device performance. In addition, conductivity of the donor doped SnO2 films is improved in the presence of optimal oxygen vacancy defects, which is also eventually necessary for better device performance [14]. The donor doped SnO2 materials such as fluorine-doped tin oxide (FTO), antimony (Sb) doped tin oxide (ATO), aluminum doped zinc oxide (AZO), along with indium zinc oxide (IZO), and gallium-doped zinc oxide (GZO) show promising TCO properties [11,[15], [16], [17]].

Among the donor doped SnO2 materials, Sb doped SnO2 (ATO) thin film is explored extensively for alternative TCO application due to its better optoelectronic properties [18]. It should also be noted that ATO film usually has a grey tinge which degrades its optical properties. To overcome this issue, ATO film is co-doped with suitable elements like F, Mn, Ba, etc. [19], [20], [21], [22], [23]. Low resistivity values of 4.0 × 10−3 Ω cm and 3.4 × 10−3 Ω cm have been reported for ATO films deposited using co-sputtering and plasma-assisted molecular beam epitaxy methods, respectively [24,25]. However, the reported transmittance values of those films are relatively lower than those for commercially available FTO/ITO materials. The co-doping effect of SnO2 sample has a strong correlation to its TCE properties [26]. An early study by Shanthi et al. showed that the F:Sb co-doped SnO2 sample has better transmittance and good conductivity when compared to Sb-doped SnO2 sample [27]. They proposed that this is mainly caused by the improved mobility of the sample due to co-doping effect of F and Sb into the SnO2 lattice. Recently, Ba has been co-doped with Sb into SnO2 host to overcome the grey tinge problem and is found to enhance the electrical and optical properties [22]. However, few research groups have explored Li doping into SnO2 lattice, and it has been reported that the spray deposited Li-doped SnO2 films exhibit low sheet resistance and good optical transmittance due to better surface and structural properties [28], [29], [30]. It is reported that alkali metals (Li, Na) effectively promote oxygen vacancies in the metal oxide system [31]. It has also been reported that Li occupying the interstitial sites of ZnO leads to change in the conduction type, i.e., from n- to p-type as a function of increasing Li concentration [32]. However, at high concentration, reversal of conduction type is observed which is attributed to self-compensation effect [32]. Moreover, presence of oxygen vacancies leading to improved electrical conductivity is also reported [33]. It is also identified that Li can improve the ionic conductivity due to 2s1 electron in its valence shell [31].

Doping SnO2 thin films with either Li or Sb for TCE applications has been investigated by researchers [18,30]. However, there are no reports available on the investigation of Li (varying concentration) and 5 wt.% Sb co-doped SnO2 thin films deposited using spray pyrolysis for alternative TCE applications. With this motivation, it is intended to explore the effect of Li-Sb co-doping into the SnO2 host material. In the present work, thin films of undoped SnO2, 5 wt.% Sb doped SnO2 (ATO), and Sb (5 wt.%): Li (1, 3, and 5 wt.%) co-doped SnO2 (LATO) are deposited onto glass substrates using spray pyrolysis method. The deposited films are studied for their structural, surface morphological, optical and temperature dependent electrical properties. The obtained results are discussed in detail in the forthcoming sections.

Section snippets

Spray deposition of un-doped, Sb doped and Li-Sb co-doped SnO2 thin films

Stoichiometric amount of SnCl2.2H2O, SbCl3 and LiCl precursors were dissolved thoroughly in 250 mL of ethylene glycol. De-ionized water (50 mL) along with 2 mL of HCl (to make a transparent solution) was also added to the above mixture. Microscopic glass slides [75 mm (l) × 25 mm (w) × 1.2 mm (t)] were used as substrates after cleaning ultrasonically in mild acid and de-ionized water to remove traces of grease or oil adhered onto the surface. The substrate temperature maintained at 350 ˚C was

X-ray diffraction analysis

The XRD patterns of TO, ATO, LATO1, LATO3, and LATO5 thin films show polycrystalline nature (Fig. 1). All the peaks can be indexed to the tetragonal SnO2 structure matching well with the joint committee on powder diffraction standards (JCPDS) card No. 41-1445 [37]. The plausible secondary phase peaks of SnO, Sn2O3, Sb2O3, and Li2O are not observed in the XRD patterns, indicating phase purity of the films. However, significant variation in intensity as a function of doping is observed in the XRD

Conclusion

In summary, undoped SnO2, ATO, and LATO thin films were deposited on glass substrates by facile spray pyrolysis method. The XRD patterns confirm the polycrystalline nature with tetragonal crystal structure. The films are textured along the (110) plane direction. The SEM analysis clearly indicates the change in surface morphology from dendritic to cuboid shape upon Sb-Li co-doping into the SnO2 system. The observed changeover of surface morphology from dendritic to cuboid shape is the main

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.

Acknowledgements

The author UD acknowledges the Dr. D. S. Kothari Post-Doctoral Fellowship (Project No: F.4-2/2006 (BSR)/PH/17-18/0098) for the financial support. The author MK thanks the Science & Engineering Research Board (SERB), Department of Science and Technology, Government of India, for the award of National Post Doctoral Fellowship (File Number: PDF/2019/002766).

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