ICMCTF2016 Session F4-2: Functional Oxide and Oxynitride Coatings

that rare-earth oxides are intrinsically hydrophobic because their electronic structure prohibits their bonding with water on the surface. Here we show that rare-earth oxide ceramics are intrinsically hydrophilic. The previously reportedhydrophobic behavior are likely due to airborne hydrocarbon contamination. Our results highlights the significant impact of environment on the wettability of oxide coating materials.

Different phases of the Bi₂O₃ – Nb₂O₅ pseudo-binary system have been reported with niobium oxide molar percentages up to 50%. These materials show optical and electrical properties interesting for applications such as solid electrolytes, photocatalysts and high-k dielectrics. They are commonly produced by solid state reaction varying the proportion of the binary oxides in the mixtures. Less works have reported their synthesis as thin films, particularly magnetron sputtering have been used with a single target having fixed composition. We proposed the synthesis of Bi-Nb-O phases by co-sputtering, starting from Bi₂O₃ and Nb targets. In this way, it could be possible to vary the composition into a wider range than the solid state reactions.

The aim is to correlate the structural and optical properties with the deposition conditions. The radio-frequency power applied to the Bi₂O₃ target was fixed at 30 W, while the DC power of the Nb target was varied; 20, 30, 50, 70, 100 and 150W. The films preparation was done under a reactive atmosphere of Ar:O₂ (16sccm:4sccm) and the substrates were silicon (100) and glass, heated at 150°C for 30 min before the deposition. The films structure was characterized by X-ray diffraction and Raman spectroscopy, and the morphology and composition were studied by scanning electronic microscopy and energy dispersion spectroscopy, respectively. The films resulted crystalline when the Nb target power was up to 50 W, above this value the films were amorphous. Also, the amorphous films were annealed at 600°C in air during 2 h, leading to structural changes showing Nb rich phases. Five crystalline structure have already being identified; BiNbO solid solution, Bi₃NbO₇, Bi₅Nb₃O₁₅, BiNbO₄ and Bi₈Nb₁₈O₅₇. The optical properties were obtained by UV-visible transmittance and reflectance spectroscopy observing that the band gap ranges between 2.7 to 3.3 eV depending on the composition and structure.

In the field of PVD techniques, the High Power Impulse Magnetron Sputtering (HiPIMS) process [1] is one of the newest and most promising development for low temperature depositions. Its understanding is still intensely investigated and reported in numerous articles and reviews every year [3]. Indeed, a better knowledge of the processes occurring within the plasma with precision is fundamental for its use to be effective during thin films depositions.

The study of the process is more complex than Direct Current Magnetron Sputtering (DCMS) and mid-frequency pulsed magnetron sputtering - due to a longer list of parameters and phenomena occurring - and need to be conducted with caution [2]. In particular, for the case of reactive sputtering: it has been demonstrated several times that the reactive gas has a huge impact on the plasma behaviour. It is also a very sensitive process. The hysteresis behaviour widely described in the literature [4] [5] implies that the reactive gas content should be regulated with precision along time during the deposition duration to keep the plasma stoichiometry constant. To do so, the Speedflo™ gas controller for HiPIMS systems [4] has been used for the work presented here. This gas flow controller measures the optical signal emitted by the energetic species of the plasma and regulates the reactive gas flux in order to keep the stoichiometry to a chosen value. The effectiveness of the device has been confirmed and used to conduct a more precise study on the influence of the oxygen content while sputtering a titanium target.

Engineering of functional oxide films and oxide-based coating systems requires comprehensive knowledge of the microstructure-property relationships of the developing oxide phases that form during synthesis and subsequent long-term operation. Namely, depending on the growth method (e.g. PVD, CVD, anodization, thermal oxidation) and the growth conditions, different single- or hetero-phase oxide phases with variable compositions and (defect) structures may form by competing nucleation and growth. Evidently, the applicability of synthesized oxide microstructures ultimately depend on their durability (i.e. the thermal, chemical and mechanical stability) during long-term operation in harsh environments (e.g. at high temperatures, in reactive liquids or gasses, during thermal cycling).

This talk will address advanced thermodynamic and kinetic model descriptions of oxide nucleation and growth on pure metals, alloys and their coating systems, while accounting for the crucial roles of free surfaces, internal interfaces and the relative mobilities of the reactant species on oxide phase stability [1-3]. As an example, model predictions on the thermodynamic stability of amorphous and metastable crystalline Al₂O₃ thin films (< 10 nm) on Al metal are compared with experiments [1]. Furthermore, a coupled thermodynamic–kinetic modelling approach for the high temperature oxidation of a (Ni,Co)CrAlY bondcoat at 1373 K is presented, which computes composition–depth profiles in the alloy, as well as the amount of each oxide phase developed as a function of oxidation time, including the formation of multiple oxide phases during the initial stage of fast oxidation [2]. Finally, the superior corrosion behaviour of pre-oxidized amorphous Al_1-xZr_x coating systems is highlighted, which from detailed experimental observations and thermodynamic assessments can be attributed to the thermodynamically-preferred formation of a structurally and chemically uniform amorphous (Al_0.33Zr_0.67)O_1.83 overlayer [3].

References

[1] L.P.H. Jeurgens, W.G. Sloof, F.D. Tichelaar, E.J. Mittemeijer, Phys Rev B 62 (2000) 4707.

[2] T.J. Nijdam, L.P.H. Jeurgens, W.G. Sloof, Acta Materialia 53 (2005) 1643-1653

Zinc oxide (ZnO) thin films were deposited onto glass substrates by radio frequency (RF) magnetron sputtering using a metallic zinc target. Zinc oxide films were prepared in two different gas atmospheres; in the first set helium and oxygen gas flow ratio (He:O₂) was varied from 87.5% to 37.5%. In the second set of experiment, oxygen flow rate was kept constant at 2.5sccm while argon and helium gas flow ratio (Ar:He) was varied from 9.0% to 87.5%. The structural, wettability and optical properties of ZnO films were investigated by X-ray diffractometry (XRD), contact angle measuring system and UV-Vis-NIR spectrophotometer. The XRD results shows increased preferred orientation along (002) plane for deposited ZnO films in both cases. The average crystallite size of ZnO films increases with increase in the gas ratio for both set of experiments. The deposited films are hydrophobic by nature for water and ethylene glycol. Optical transmittances greater than 80% were observed in the wavelength interval from 450 nm to 650 nm for both cases.

Zinc oxide (ZnO) is a very wide-spread used material for surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices because of its excellent performance in piezoelectricity. Many researches have done by others in order to increase the piezoelectric coefficient (d₃₃) of ZnO by doping tiny amount of different elements, such as Vanadium (V), Chromium (Cr), Iron (Fe) by magnetron sputtering method. In our research, we chose Fe as a doping element out of consideration that V is too expensive for massive production and Cr holds human health and environmental concern. The smaller ions substitute Zn sites make the piezo effect respond greater while the dopant ions bigger than Zn do the opposite. This guide us that Fe³⁺ is smaller than Zn²⁺ and will exhibit a better properties compared with undoped ZnO films.

Among three formula types of iron oxide, Fe₂O₃ is the most stable one and is cheap enough to make it a candidate in this research. For a process costs reduction method, we use self-made target source from ceramics powder (ZnO and 0.6mole% Fe₂O₃, target tablet is made under calcined 600℃-900℃ for 3hr, sintered 1100℃ for 4hr). Iron oxideis selected to add into ZnO powder as Fe contributor better than Iron metal because Iron for most use is magnetic and it brings difficulty when the magnetron sputtering is carrying out. Therefore, we use e-beam evaporation method to deposit Fe-doped ZnO films on silicon substrate.

The X-ray diffraction results of powder mixture show a shift to higher angle and then shift to lower angle as the calcination temperature increase. The XRD results of the Fe-doped ZnO thin films are high c-axis orientated. The valence of Fe in ZnO was confirmed by XPS and the results reveal a correlation to the XRD results. The cross-section SEM image shows that the Fe-doped ZnO films are highly aligned columns with the c-axis perpendicular to the substrate.

Ultraviolet (UV) photodetection has been widely applied in various commercial and military purposes. To this end, ZnO has been regarded as one of the most promising candidates for UV photodetectors because of its wide band-gap energy (~3.3eV). Moreover, low cost in raw materials and fabrication makes ZnO even more appealing for commercialization compared with others semiconductor counterparts. To further extend the detection region from near UV to deep UV, Mg_xZn_1-xO ternary compound by incorporating Mg into ZnO is considered, where the fundamental band gap energy can be increased proportionally from 3.3 up to 7.7 eV.

In this research, high quality Mg_xZn_1-xO thin films with strong c-axis preferred orientations are grown on p-type Si(111) by magnetron sputtering using ZnO and Mg_0.3Zn_0.7O as targets. In this setup, we show that Mg content of Mg_xZn_1-xO alloys can still be varied in a large range by changing substrate temperature from 25°C to 250°C . The crystallinity and crystal orientation of the thin films are analyzed by x-ray diffraction, revealing that phase transformation takes place from hexagonal to cubic phase when substrate temperature is above 150°C. The surface morphology and microstructure are examined by using scanning electron microscopy. Accordingly, the absorption edge exhibits a blue shift from 380nm to 280nm measured by UV–vis–IR spectrophotometry. The piezo-phototronic effect is applied to boost the optoelectronic performance of devices by modulating charge-carriers generation, separation and transport. Finally, preliminary results on UV detectors based on selective MgZnO thin films are reported and discussed.

Since conventional wastewater treatment plants (WWTPs) effects only partial removal of alkylphenols, advanced oxidation process (AOP) involving titanium dioxide is a promising remediation technique. However, the low photodegradation efficiency of titanium dioxide limits its practical application. In this paper, it is proposed that the presence of graphene can enhance the photocatalytic efficiency of titanium dioxide. The titanium dioxide-graphene (TiO₂-G) composites were prepared via sonochemical and calcinations methods. The synthesized composite was characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), tunneling electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and fluorescence spectroscopy. The photocatalytic efficiency was evaluated by studying the degradation profiles of 3 alkylphenols using gas chromatography-flame ionization detector (GC-FID). It was found that the synthesized TiO₂-G composites exhibit enhanced photocatalytic efficiencies as compared to pristine TiO₂. The presence of graphene not only provides a large surface area support for the TiO₂ photocatalyst, but also stabilizes charge separation by trapping electrons transferred from TiO₂, thereby hindering charge transfer and enhancing its photocatalytic efficiency.