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Ho Soonmin,
Centre for Green Chemistry and Applied Chemistry, INTI International University, Negeri Sembilan, Malaysia E-mail: [email protected]
A SHORT REVIEW ON METAL OXIDE THIN FILMS
Abstract. Preparation and characterization of metal oxide thin films were extensively discussed by many researchers. In this work, nickel oxide, tin oxide, cuprous oxide and cupric oxide films were characterized by using various tools. The obtained band gap values are in the range of 1.75 to 4.4 eV.
Keywords: band gap, copper oxide, nickel oxide, thin films, tin oxide.
Introduction
Over the last decade, thin films were successfully deposited onto substrate by using various deposition techniques [1; 2]. These films have been considered attractive for utilizing in solar cell [3; 4], laser device, sensor device, switching device, Schottky barrier and IR detectors. Generally, deposition technique could be broadly divided into two groups, namely physical and chemical method [5; 6]. Selection of deposition technique is highly dependent on many factors such as easiest setup [7; 8], cheap equipment [9, 10], convenient for large area deposition [11; 12], controllable growth rate [13] and film thickness.
In this work, several metal oxides were prepared by using different techniques and obtained films were characterized by using various tools.
Literature survey
Nickel oxide thin films:
Spray pyrolysis technique has been used to produce nickel oxide (NiO) films [14]. X-ray diffraction (XRD) data indicated that amorphous structure at 280 °C, while cubic structure at 320-400 °C. Broad peak and polycrystalline structure could be observed in as-deposited NiO films and annealed films, prepared using sol gel technique [15]. Furthermore, XRD data showed intensity was increased with annealing temperature (300-600 °C). The band gap value (2.1 to 3.9 eV) was increased with increasing annealing temperature (300-473 K) for the chemical bath deposited NiO films [16]. The D. C. reactive magnetron sputtering method was used to prepare NiO films [17]. Irregular shape grains, uniform grain size distribution, worm like structure, and smooth structure could be seen when the thickness is 150, 250, 350, and 550 nm, respectively.
Tin oxide thin films:
The tin oxide films deposited onto glass substrate using atmospheric pressure chemical vapour deposition method [18] indicated major orientation along (110) plane. The films deposited onto fluorine doped tin oxide glass via electro deposition method [19] at different concentrations of butyl rhodamine B (BRhB) were studied. There are different morphologies could be observed including irregularly connected
nanoparticles (absence of BRhB), loose surface structure (100 ^M BRhB) and porous structure (150 ^M BRhB). The films prepared using pulsed laser deposition indicated pure polycrystalline phase, and almost stoichiometric [20]. Magnetron sputtering technique was used to prepare films [21] and, resistivity increases with reduce of oxygen pressure from 10 to 1%. The crystallite size (29.8 to 6.5 nm) reduced as the substrate temperature was reduced from 800 to 400 °C for the films prepared using spray pyrolysis [22]. Higher band gap value could be observed under higher anneal temperature for the films produced using thermal evaporation [23]. The annealed chemical bath deposited SnO2 films are highly transparent, highly stoichiometric [24] with a band gap of 4.4 eV
Cuprous oxide and cupric oxide thin films:
The cupric oxide (CuO) with monoclinic structure was produced using reactive radio frequency magnetron sputtering method [25]. Spray pyrolysis technique was used to synthesis CuO film. Its thickness (660-1250 nm) increased as the deposition time was increased from 30 to 80 minutes
[26]. Sol-gel spin coating was used to prepare Cu2O films
[27] in the presence of ethylene glycol. These films showed irregular morphology, band gap of 1.75 eV film thickness of 59 nm and lower absorption value. XRD data supported the existence of Cu2O at 250 °C, while formation of CuO at 350, 450 °C for the films prepared using thermal evaporation [28]. The obtained electro deposited Cu2O films [29] show cubic structure, crystal size about 210 nm and band gap is 1.98 eV. There are three peaks (627, 503 and 540 nm) could be observed in photoluminescence spectra for the chemical bath deposited Cu2O films [30]. The higher growth rate is achieved when using higher pressure during the preparation of Cu2O via magnetron sputtering [31].
Conclusion
Metal oxide thin films have been successfully prepared using various deposition techniques. The properties of nickel oxide, tin oxide, cuprous oxide and cupric oxide have been reported. Scanning electron microscopy studies indicate that a strong dependence of the surface texture, film thickness and grain size on the various experimental conditions.
_A SHORT REVIEW ON METAL OXIDE THIN FILMS
Acknowledgement:
INTI INTERNATIONAL UNIVERSITY is gratefully acknowledged for the financial support of this work.
References:
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