Abstract: Zinc oxide (ZnO) films were deposited onto glass substrates using reactive direct current magnetron sputtering technique by varying the substrate bias voltage (floating potential and -50 V) and retaining other parameters, viz., deposition time, substrate temperature, argon and oxygen flow, etc., as constant. The deposited ZnO films were doped with aluminium (Al) using thermal evaporation technique and subsequently annealed at 150°C for 2 h in ambient air. Pure and Al doped films were then investigated by X-ray diffraction (XRD), UV-Visible spectroscopy, Field-Emission Scanning Electron Microscopy (FE-SEM) and four probe method for their structural, optical, morphological and electrical properties, respectively. The films were polycrystalline in nature with hexagonal wurtzite structure. Increase in the grain size value was observed for the films deposited at -50 V substrate bias voltage as well as for the films doped with Al which in turn reduced the band gap value of the films. The band gap values of the films varied between 3.34 and 3.00 eV with respect to substrate bias voltage and Al doping. The variations in the properties of the films were correlated to the change in the micro-structural parameters due to the change in the ion energy variation influenced by the variation of substrate bias voltages.
INTRODUCTION
ZnO is a wide band gap (3.3 eV) semiconductor material which is widely used for different industrial applications due to its superior electrical and optical properties. It provides high transparency, high mobility and hence, it is used for optoelectronic applications, viz., transparent electrodes, thin film transistors, solar cells, LCD displays, antibacterial and sensing applications (Kim et al., 2000; Zhu et al., 2009; Saarenpaa et al., 2010; Lai et al., 2013; Koble et al., 2009; Badadhe and Mulla, 2011; Houng et al., 2008; Geetha et al., 2013; Iyer et al., 2014; Ponnusamy and Madanagurusamy, 2014; Vijayalakshmi et al., 2014). The electrical properties of ZnO can be enhanced by doping with metals like aluminium, indium and gallium (Lu et al., 2013). The resistivity of ZnO decreases with metal doping which is preferred in the fabrication of transparent conducting electrodes. Doped ZnO films can be synthesized by a variety of methods such as sputtering, sol-gel method, spray pyrolysis and thermal evaporation technique (Fu et al., 2004; Seeber et al., 1999; Gu et al., 2011; Ataev et al., 1995). Though there are several methods available, magnetron sputtering is preferred due to better growth rate and large area deposition (Calnan et al., 2008). In magnetron sputtering technique various parameters such as doping concentration, oxygen pressure, sputtering power, target to substrate distance, the substrate temperature is varied and the film properties had been studied. But only a few works are reported on substrate bias. So, in the present work we investigated the influence of substrate bias voltage on the properties of ZnO:Al films.
MATERIALS AND METHODS
Experimental details: ZnO films were deposited on glass substrates by reactive dc magnetron sputtering technique. The substrates were ultrasonically cleaned with deionized water for 10 min followed by in acetone and ethanol for 5 min. Zinc target of 99.99% purity was used and the substrate to target distance was kept constant at 5 cm. Initially the deposition chamber was pumped down to a base pressure of 4 ×10-5 mbar. The zinc target was sputter-cleaned in argon atmosphere for 10 min, to remove the contamination on the target surface. Argon and oxygen gas was allowed inside the chamber and the working pressure was 3.5×10-3 mbar. Sputtering was carried out for 15 min with the cathode power of 30 W at room temperature. Substrate bias was varied from floating potential to -50 V. ZnO films deposited at different bias voltage was doped with a known quantity of Al = 0.010 g by thermal evaporation technique. The substrates coated with ZnO films were placed inside the chamber and pumped down to a pressure of 10-5 mbar. High potential was applied to the electrodes where the tungsten filament was kept. As a result the filament turns red hot and the Al which is placed over the filament is vaporized and it is deposited over ZnO films. Then the samples were annealed at 150°C for 2 h and 300°C for 3 h in a tubular furnace to diffuse Al into ZnO thin films. The ZnO:Al films were characterized by XRD (Rigaku Ultima III) for its structural properties, UV-visible spectroscopy (Perkin Elmer, Lambda 35) for its optical properties, FE-SEM (JEOL-JSM 6701F) for its morphological properties and four probe method for its electrical properties.
RESULTS AND DISCUSSION
Structural properties: XRD patterns of pure and Al doped ZnO thin films are shown in Fig. 1. XRD pattern of the ZnO film deposited for the substrate bias of floating potential was found to be amorphous. But the film deposited at -50 V was polycrystalline in nature with hexagonal wurtzite structure. The grains were well oriented c-axis (002) plane (presence of the single peak 2θ = 34.08°) the peak was indexed by JCPDS 36-1451 (Verma et al., 2010). With increase in the bias voltage from FP to -50 V, we observed the increase in film crystallinity due to the bombardment of energized Ar+ ions on the film surface.
Al doped ZnO films showed good crystalline property compared to pure ZnO films. The intensity of the peak is increased when compared to pure ZnO films. The enhancement in the crystallinity is due to the annealing effect carried out for the diffusion of Al into ZnO films. Shift of peaks towards the higher 2 theta value indicates the decrease in that of interplanar distance which is due to the doping of aluminium.
| Fig. 1(a-b): | XRD patterns of (a) Pure and (b) Al doped ZnO films |
The substitution of Al3+ ions in the interstitial sites of Zn2+ ions causes the change in that of d-value because the ionic radius of Al3+ions (0.54 Å) is less when compared to Zn2+ ions (0.74 Å) (Wang et al., 2005). The average crystallite size was calculated using the Scherrer's relation:
where, k is the shape factor and it takes the value of 0.9 for this case, λ is the wavelength of X-ray 1.54 Å and β is the full width half maximum. The lattice constant value was calculated using the relation:
The microstructural parameter values are shown in the Table 1.
The FE-SEM micrographs of pure and Al doped ZnO for the substrate bias of FP and -50 V is shown in Fig. 2. The film deposited at FP shows irregular morphology without distinguishing grain and grain boundaries are seen.
| Table 1: | Micro-structural properties of pure and Al doped ZnO films |
| Fig. 2(a-b): | FE-SEM micrographs of (a) Pure and (b) Al doped ZnO films |
In the case of films deposited at -50 V shows uniform spherical shape grains with size about 25 nm. Similarly to Al doped ZnO film, the nanorod formation was less for the film deposited at FP whereas, the film deposited at -50 V shows the enhanced nanorod formation. For both the cases the grain sizes were increased to 30 and 45 nm for FP and -50 V when compared to pure ZnO films.
Optical properties: The optical properties of pure and Al doped ZnO were studied using UV-Visible spectrophotometer in the wavelength range of 300-1100 nm.
The absorption coefficient was used to calculate the optical band gap by using the Tauc’s relation (Tauc, 1982) shown in Fig. 3:
| Fig. 3(a-b): | Plot of (αhν)2 vs. (hν) for (a) Pure and (b) Al doped ZnO films |
where, hν is the photon energy, α is the absorption coefficient. By extrapolating the slope of the curve in the Tauc’s plot to the X-axis gives the optical band gap. The optical band gaps for pure ZnO and ZnO:Al films are shown in the Fig. 3a-b. The band gap energy for pure ZnO at floating potential and -50 V was 3.34 and 3.28 eV respectively. Similarly, for Al doped ZnO the band gap is 3.14 and 3.00 eV. The decrease in the band gap is due to the doping of Al. On doping the increase in grain size decreases the grain boundaries, thereby it decreases film resistance (Serio et al., 2011).
Electrical properties: The resistivity of the films was measured using the four probe method. Pure ZnO films exhibits very high resistivity whereas, the Al doped ZnO films exhibit very less resistivity. Generally, Al doped ZnO films have n-type conductivity due to the generation of electrons from Al3+ ions on substitutional sites of Zn2+ ions and Al interstitial atoms. Thus Al doped ZnO exhibits better conductivity than pure ZnO. These ZnO:Al films with tunable properties will find potential applications in optoelectronic devices, sensors, solar cells, etc.
CONCLUSION
ZnO:Al thin films were synthesized using reactive direct current magnetron sputtering at different substrate bias voltages and also the films were doped with Al using thermal evaporation technique. XRD analysis indicated the increase in the crystallinity of the films due to substrate bias voltage and Al doping and also peak shift in higher angle was observed while doping with Al, it is due to the difference in the ionic radius of ZnO and Al. FE-SEM analysis indicated the formation of nanorod structures on the film and the increase in the grain size (15-27 nm). Optical studies revealed the shift in the band gap to lower levels, confirming the presence of Al. The band gap values of pure and Al doped ZnO films were 3.34 and 3.14 eV respectively. When Al was incorporated, the band gap value reduced to 3.00 eV. Electrical studies showed a drastic decrease in the, resistivity after Al doping.
ACKNOWLEDGMENTS
The authors sincerely thank SASTRA University for providing necessary infrastructure and experimental facilities. AST thanks SASTRA University for permitting him to carry out research work at Functional Nanomaterials and Devices Lab, CeNTAB, SASTRA University through the SASTRA-UPC Student Exchange Programme.