Figure 5: FIBSEM micrographs of Mo(1.5%):ZnO films deposited at a sputtering pressure of 1 × 10−2 mbar and at a substrate temperature of 473 K.
I looked at publications in the Indian Journal. Material Science. Generally, papers on plasma processes were quite rare. This is one of the few interesting one using RF plasma sputtering. The application was transparent conducting thin films. It is clear the researchers understood the applications and a detailed analysis was made of the deposited materials. The weakness for me was the lack of an explanation of the significance of their results. While properties of the film were stated I would have liked to see a better explanation of why the properties were significant. Was there an advantage to their films? This is the type of information that a new startup or a technology company will be looking to try to understand.
Indian Journal of Materials Science Volume 2013, Article ID 684730, 7 pages http://dx.doi.org/10.1155/2013/684730
Preparation and Characterization of R.F. Magnetron Sputtered Mo:ZnO Thin Films
1Department of Science and Humanities, Vignan University, Vadlamudi, Andhra Pradesh 522 213, India
2Postgraduate Department of Physics and Electronics, P.B. Siddhartha College of Arts and Science, Vijayawada, Andhra Pradesh 520 010, India
3Laboratory for Condensed Matter Physics, Satyendra Nath Bose National Centre for Basic Sciences, Salt Lake, Kolkata 700 098, India
Received 17 June 2013; Accepted 31 July 201
Abstract
Undoped and molybdenum doped ZnO thin films were deposited by r.f. magnetron sputtering, and their compositional, structural, surface morphological, and optical properties were studied. The composition study reveals that, at a fixed partial pressure, the substrate temperature and molybdenum concentration influence the composition, structure, surface morphology, and optical properties of the ZnO films.
The ZnO films deposited at 1 × 10−2 mbar and at 473 K have oxygen deficiency. The Zn/O ratio increased with increasing Mo at.%. The structure of ZnO films is hexagonal wurtzite with predominant (002) crystallographic orientation. There is no change in the crystal structure of the films with Mo doping. The compressive stress decreased with increasing Mo at.% in ZnO. The films deposited at 673 K have lower stress when compared to the films deposited at 473 K. The grain size of the films deposited at 473 K increased with increasing Mo at.% in ZnO and decreased when deposited at 673 K. The surface morphology indicates that the films contain lower particle size when deposited at lower substrate temperatures and higher particle size when deposited at higher substrate temperatures. The optical transmittance spectra reveal that the Mo effectively dopes to ZnO when the films are deposited at higher substrate temperatures than at lower temperatures. In conclusion, Mo can be effectively doped to ZnO at optimized deposition conditions.
Introduction
For the past few years, there has been a lot of interest shown to find new oxide materials which will act as transparent as well as conducting materials. Many metal oxides and their compounds have been found to be transparent and conductve. Among these oxides indium tin oxide (ITO) and aluminum doped zinc oxide (AZO) were proved to be transparent and conducting materials [1–3]. Still there has been a lot of interest shown to overcome the problem involved with those materials like abundance of material, stability, performance, reliability, and durability. So the search for new transparent and conducting oxides continues in the years to come. Various metallic elements doped to ZnO were tested for their transparency in the wavelength range from 450 nm to 750 nm for conductivity. The principal elements doped to ZnO were Al, In, Sn, Ga, Cd [4], and Mo. Among these, an extensive work was done on Al doped ZnO films, and they were proved as potential transparent conductors. Among these, Molybdenum doped ZnO was also found to be an attractive candidate as transparent conducting oxide (TCO) due to its ease to doping to ZnO.
Moreover, the difference between the valence electrons of Mo6+ and Zn2+ is 4. This difference can produce enough free carriers and reduce the ion scattering effect for a very small amount of Mo [5]. Moreover, Mo exhibits multiple valence states of +6, 5, 4, 3, 2. This enables the contribution of multiple carriers by a single Mo dopant atom [6].
The work reported on ZnO is extensive, and also the significant work was reported on Mo:ZnO [7, 8]. In TCOs, the concentration of dopant element to host compound decides the transparency and conductivity. In general, the conductivity increases with increasing dopant concentration, but at the same time it reduces the transparency. So, the optimization of deposition parameters like argon and oxygen pressure (in case of sputtering) during deposition and substrate temperature will decide the proper doping of dopant in the host lattice which in turn will result in the thin films with high transparency and conductivity. In the present investigation, the effect of Mo doping on various physical properties like structure, surface morphology, and optical properties of ZnO was studied in detail. Deposition of oxide films by r.f. sputtering has its own advantage over other physical vapour deposition techniques. The chief advantage is the control on deposition parameters like oxygen, Ar pressure, and substrate temperatures. The ZnO and Mo:ZnO films were deposited at various sputtering pressures (1 × 10−2 and 1 × 10−5 mbar) and substrate temperatures of 473 K and 673 K. In the present work, the physical properties of ZnO and Mo:ZnO thin films for various concentrations of Mo thin films deposited at the sputtering pressure of 1 × 10−2 mbar and at a substrate temperatures of 473 K and 673 K were reported
Conclusion
The ZnO and Mo:ZnO thin films were deposited by radio frequency magnetron sputtering on quartz and intrinsic silicon (100) substrates at a fixed combined partial pressure mbar of Ar + O2 and substrate temperatures of 473 K and 673 K. The effect of Molybdenum doping on ZnO thin films with different Molybdenum concentrations (1-2 atomic percent) was studied with the help of structural and microstructural characterization techniques. The films deposited at a substrate temperature of 473 K exhibited strong c-axis orientation with predominant (002) peak. At 673 K, along with (002) orientation, other orientations (100), (101), (220), and (103) were also observed. Among these, the (220) peak indicates the cubic phase of ZnO. With increasing Molybdenum concentration, the cubic phase of ZnO disappeared, and the (002) orientation became strong and intense. The composition analysis reveals that the undoped ZnO films deposited at 473 K have oxygen deficiency, and the ratio of Zn/O is improved with increasing Mo atomic percent in ZnO. The surface morphological features reveal that the undoped ZnO films were found to be uniform and have grain size of around 30 nm. The optical energy gap of the undoped ZnO films is 3.05 eV and increases with increasing Mo concentration. The thickness of the films is around 456 nm.
Mike B Hopkins 