Thermal and plasma-enhanced atomic layer deposition of yttrium oxide films and the properties of water wettability

The atomic layer deposition (ALD) of yttrium oxide (Y2O3) is investigated using the liquid precursor Y(EtCp)2(iPr-amd) as the yttrium source with thermal (H2O) and plasma-enhanced (H2O plasma and O2 plasma) processes, respectively. Saturation is confirmed for the growth of the Y2O3 films with each investigated reactant with a similar ALD window from 150 to 300 °C, albeit with a different growth rate. All of the as-deposited Y2O3 films are pure and smooth and have a polycrystalline cubic structure. The as-deposited Y2O3 films are hydrophobic with water contact angles >90°. The water contact angle gradually increased and the surface free energy gradually decreased as the film thickness increased, reaching a saturated value at a Y2O3 film thickness of ∼20 nm. The hydrophobicity was retained during post-ALD annealed at 500 °C in static air for 2 h. Exposure to polar and nonpolar solvents influences the Y2O3 water contact angle. The reported ALD process for Y2O3 films may find potential applications in the field of hydrophobic coatings

Optimization of the annealing conditions for thin VO2 ALD films

Vanadium dioxide (VO2) is an intriguing material due to its semiconductor-metal transition (SMT). During this transition, which occurs near 67°C, electrical as well as optical properties change drastically. Possible applications include thermochromic windows, and memories or switches in micro- and optoelectronics. Although atomic layer deposition (ALD) is gaining importance for some of these applications, the growth of VO2 with this technique is not obvious, since in most cases V2O5 is obtained. In our previous work we presented ALD growth of VO2 by using Tetrakis[EthylMethylAmino]Vanadium and ozone at a temperature of 150°C [1]. XPS revealed the 4+ oxidation state of vanadium, indicating growth of VO2. Post-ALD thermal processing proved essential to crystallize the VO2 in the desired tetragonal phase (R). In this work we present the influence of the oxygen partial pressure on phase formation during such thermal processes. Additionally the influence of film thickness and annealing temperature on the post-annealing properties were studied, including morphology and SMT characteristics. During thermal processing a minimum oxygen partial pressure of approximately 1 Pa is indispensable to form crystalline VO2 (R) (figure 1). Oxygen partial pressures above 2 Pa show an intermediate monoclinic phase (B), which transforms to VO2 (R) at higher temperatures. At a value of 35 Pa this VO2 (B) phase finally transforms to V6O13 instead of VO2 (R). For very thin films, the thermal post-processing may result in agglomeration of the VO2 layers on the SiO2 substrate. Samples with a film thickness above 20nm show a typical resistivity ratio during the SMT of more than 2 orders of magnitude when annealed in the range 450°C to 500°C. For thinner films or higher annealing temperatures the resistivity ratio is suppressed and an overall increased resistivity is observed due to agglomeration (figure 2)