1. EXPERIMENTAL PROCEDURE MAJOR FINDINGS & DISCUSSIONS
EEP
Effect of Sn Doping on the Properties of Nano-structured ZnO Thin Films Deposited by Co-sputtering Technique
M. A. Islam1, K. S. Rahman1, F. Haque1, N. A. Khan1, M. Akhtaruzzaman1, M. M. Alam 3, K. Sopian1, N. Amin1,2, 3
1Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, 43600, Bangi, Malaysia 2 Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600, Bangi, Malaysia 3King Saud University, Riyadh 11421, Saudi Arabia
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INTRODUCTION
Zinc oxide (ZnO) nanostructures have drawn much research attention in recent years due to their unique opto-electronic properties. It is non-toxic, highly transparent, and shows a wide range of conductivity from metallic to insulating, depending on the preparation techniques and doping material types and concentration. ZnO seems to have the richest family of semiconductors among all applied materials as it is using in various devices, such as in antireflection coatings, UV range detecting devices, chemical sensors, light emitting diodes, laser diodes, liquid crystal displays, solar cells, and photocatalysis. Various technologies have been reported to prepare ZnO-based thin films, such as the sol-gel method, pulsed laser deposition (PLD) chemical vapor deposition (CVD) and RF/DC magnetic sputtering. However, with the advantages of low deposition temperature, simple processing, high growth rate, low-cost equipment and suitability for large areas deposition, magnetron sputtering is one of the most promising deposition techniques. In this study, ZnO:Sn films were prepared with different Sn dopant concentration (0-10 at.%) by co-sputtering and were annealed at the temperature 500°C in vacuum. The effects of Sn concentration on crystallinity, microstructure, optical and electrical properties of the prepared as-deposited and annealed films were investigated. It is found that, as the Sn dopant is increase in a certain limit (to 8 at.%), the crystallinity and optical and electrical properties of ZnO thin films are improved. Also, ZnO films with Sn dopant showed strong exciton confinement effect.
EXPERIMENTAL PROCEDURE
The Sn doped ZnO thin films of nm were deposited on top of the glass substrate by co-sputtering of Sn and ZnO target at a 300 oC substrate temperature. The deposition rate of ZnO was fixed at 3.0 watt/cm2. The concentration of Sn ions in ZnO:Sn films were controlled by changing the RF power ranging from 0.5 to 1.5 watt/cm2 and Sn concentration varied from 0 to 10 at.%. The samples were annealed in a vacuum ambient with 50 mTorr pressure at 500 oC for 30 min. The elemental analysis has been done by Energy-dispersive X-ray spectroscopy (EDX) characterized by Oxford Instruments INCA x-sight SEM detector (Model: 7317). The structural and morphological properties were investigated using XRD, and FESEM. The XRD patterns were recorded by using ‘BRUKER aXS-D8 Advance Cu-Kα diffractometer. The surface morphology and roughness by FESEM ‘ZEISS SUPRA 55Vp’ & AFM ‘INTERA PRIMA, NT-MDT’ brand. The electrical properties were measured at room temperature by van der Pauw Hall measurements using ‘ECOPIA HMS 5300’. The optical properties are observed from ‘Perkin Elmer Instruments Lambda 35’ UV-Vis spectrophotometer.
Fig.1 X-ray diffraction patterns of undoped and Sn doped ZnO films, (a) as-deposited and (b) annealed films
MAJOR FINDINGS & DISCUSSIONS
The most prominent peak along with (002) plane indexed to a polycrystalline hexagonal wurtzite structure of ZnO, but, the films with 8 at.% of Sn and above are found in amorphous form. In addition, (002) peak is slightly shifted to the higher diffraction angles (2θ with addition of Sn (inset of Fig. 1(a)), indicating that Sn incorporated into the ZnO lattice and leads to imperfection in crystals. The (002) peak are shifted to a high angle, because, the dopant (Sn4+) ions have smaller radii than Zn2+ ions and dopant ions only substituted for Zn ions. The peak shift to the higher angle in the annealed films indicates that films are in a uniform state of stress with tensile components parallel to the c-axis.
The inhomogeneous nano-grains with the average grain size of less than 10 nm are observed in the films (Fig. 2). The root-mean-square (RMS) roughness decreased with Sn concentrations of up to 8 at.% but increased further for the Sn concentration 10 at% (Fig. 3). Precisely, the 8 at.% Sn-doped ZnO thin film exhibited the lowest RMS value of 2.62 nm among all of the as-depoited and RMS value of 2.17 nm found for the 10 at.% Sn-doped ZnO thin among all of the annealed ZnO:Sn thin films investigated in this study.
All ZnO.Sn films are shown a sharp absorption edges in the UV region and these absorption edges slightly shifted to shorter wavelengths (blueshift) for the Sn doped ZnO thin films indicating the poor crystallinity. This shift of absorption edge is associated with Burstein-Moss effect. It can be been seen that Sn doped films are possessed higher bandgap than the undoped ZnO.
Among the as-deposited films, the highest carrier concentration is observed for 6 at.% of Sn (Fig. 5(b)). In this case, Sn dopant atoms into the ZnO lattice act as a donor by supplying two free electrons when Zn2+ ion sites are occupied by Sn4+ ions. This is consecutively increases the free electron concentration in the films and reduce film’s resistivity. The lowest mobility is shown by the film with 6 at.% of Sn (as-deposited). This film showed higher carrier concentration, that has an adverse effect on the electron Mobility. mobility is sharply increased for the annealed ZnO:Sn films. Vacuum annealing may be reduces the point defect, and/or, dislocation densities and grain boundary heights by annealing
Fig. 2 SEM images of Sn doped (6 at.% and 8 at.%) as-deposited and annealed ZnO:Sn thin films
CONCLUSION
The (002) peak of the ZnO:Sn films are shifted to higher 2θ angle with Sn doping as well as vacuum annealing indicating the uniform stress parallel to the c-axis. Thus, the films with 8 at.% and 10 at.% of Sn were found in amorphous form. The nano-sized grains of less than 10 nm were observed from SEM images.. All the Sn doped films were possessed low surface roughness and higher bandgap than undoped ZnO thin films and bandgap was increased with the vacuum annealing. The highest bandgap 3.74 eV for as-deposited and 3.84 eV for vacuum annealing were found for the film doped with 8 at.% of Sn. The Sn dopent as well as vacuum annealing induced the thermal instability in the optical band of the films. Also, optical blue shift with excitonic confinement effect is observed in the nano-structured films. The vacuum annealed films were shown the lesser resistivity and higher mobility indicating the improvement of crystal structure of the films. Among the ZnO:Sn films have been investigated in this work, the film with 8 Sn at.% exhibited best surface morphology, best transparency and highest mobility with resistivity of 2.36 ohm-cm. From the results have found in this study, it can be assumed that the limit of Sn doping in co-sputtering is 8 at.%, higher concentration may reduce the optical and electrical properties of the films.
Fig. 3 : Surface roughness and average transmittance of ZnO:Sn thin films as a function of Sn concentration.
Fig. 4 Absorption spectra of annealed ZnO:Sn films (inset-bandgap evolution curves [(αhv)2 vs. hv])
Fig. 5 Electrical properties of as-deposited and annealed ZnO:Sn thin films as a function of Sn concentration