The ability to grow inorganic thin films with highly controllable structural and optical properties at low substrate temperature enables the manufacturing of functional devices on thermo-sensitive substrates without the need of material postprocessing. In this study, the authors report on the growth of zinc oxide films by direct plasma-enhanced atomic layer deposition at near room temperature. Diethyl zinc and oxygen plasma were used as the precursor and coreactant, respectively. The process was optimized with respect to the precursor and coreactant dosing as well as to the purging times, which ultimately resulted in saturated atomic layer deposition growth. The so-obtained films exhibit a polycrystalline pattern with a (100) texture and low amount of incorporated carbon. Furthermore, the possibility to tune crystallite size, refractive index, and bandgap of the films by adapting the plasma radio-frequency power is demonstrated.

Zinc oxide (ZnO) thin films are deposited by plasma‐enhanced atomic layer deposition (PE‐ALD). This deposition method allows depositing stoichiometric and highly resistive ZnO films at room temperature. Despite such important requirements for piezoelectricity being met, not much is known in literature about the piezoelectric properties of ZnO thin films (<70 nm) deposited by PE‐ALD. The films are grown at different substrate temperatures to investigate the effect on crystalline and piezoelectric properties. Films deposited on flexible poly(ethylene terephthalate) (PET) generated a higher piezoelectric current (>1.8 nA) and charge (>80 pC) compared with films deposited on glass (>0.3 nA and >30 pC) due to bending effects of the substrate when mechanically excited. Furthermore, increasing the substrate temperature, during deposition, enhances the growth along the (002) crystallographic orientation, which further strengthens the generated piezoelectric current signal for mechanical excitations along the ZnO film's c‐axis.

ZnO thin films and nanostructures are applied in various devices due to their interesting optical and electrical properties. Atomic layer deposition (ALD) of ZnO offers unique advantages such as precise thickness control, uniformity, and conformality. Using reactive plasma species as the co-reactant (PE-ALD) allows further enhancement of the material characteristics and tunable properties. The substrate temperature has been reported to be the most influential parameter in this technique, as it affects the growth per cycle (GPC) and material properties. However, an investigation on how the film properties are linked to the GPC is lacking in the literature. Herein, the temperature dependence of several material properties is found closely related to the GPC. The preferential crystal orientation switches from (100) to (002) up to the constant region of the GPC versus temperature, the so-called ALD window. Refractive index and mass density show different slopes in temperature regions outside and within the ALD window. Excitonic absorption is only found for films prepared within the ALD window, and the resistivity drops rapidly above the ALD window. Following these results, more insights can be gained on the ALD growth (especially the role of the ALD window) and ideal temperature ranges for specific applications.

The synthesis of nanoporous ZnO thin films is achieved through annealing of zinc-alkoxide (“zincone”-like) layers obtained by plasma-enhanced atomic layer deposition (PE-ALD). The zincone-like layers are deposited through sub-saturated PE-ALD adopting diethylzinc and O2 plasma with doses below self-limiting values. Nanoporous ZnO thin films were subsequently obtained by calcination of the zincone-like layers between 100–600 °C. Spectroscopic ellipsometry (SE) and X-ray diffraction (XRD) were adopted in situ during calcination to investigate the removal of carbon impurities, development of controlled porosity, and formation and growth of ZnO crystallites. The layers developed controlled nanoporosity in the range of 1–5%, with pore sizes between 0.27 and 2.00 nm as measured with ellipsometric porosimetry (EP), as a function of the plasma dose and post-annealing temperature. Moreover, the crystallinity and crystallite orientation could be tuned, ranging from a powder-like to a (100) preferential growth in the out-of-plane direction, as measured by synchrotron-radiation grazing incidence XRD. Calcination temperature ranges were identified in which pore formation and subsequent crystal growth occurred, giving insights in the manufacturing of nanoporous ZnO from Zn-based hybrid materials.

No abstract