Avoiding water reservoir effects in ALD of functional complex alkali oxides by using O3 as the oxygen source.

Toxic Pb-containing piezo-, pyro- and ferroelectrics continue to dominate the market even though they were banned from use in consumer products more than a decade ago. There is a strong need for sustainable alternatives, but the lack of facile synthesis routes for thin films exhibiting suitable functional properties have limited the transition from Pb workhorse materials like Pb(Zr,Ti)O3 and Pb(Mg,Nb)O3 - PbTiO3. Atomic layer deposition has proven capable of the deposition of possible successors, such as LiNbO3, (K,Na)NbO3 and K(Ta,Nb)O3, albeit with limited control due to water reservoir effects resulting from the hygroscopicity of intermediate products. In this article, we show that replacing H2O with O3 in the deposition of complex alkali oxides provides an alternative and much more controlled process. We exemplify this by deposition of crystalline K(Ta,Nb)O3 with high compositional control and over a larger composition range than previously reported. This opens new doors to a simplified synthesis of polar functional lead-free alternatives.

Understanding KOtBu in atomic layer deposition - in situ mechanistic studies of the KNbO3 growth process.

Functional coatings based on alkali metals have become increasingly attractive in the current shift towards sustainable technologies. While lithium-based compounds have a natural impact on batteries, other alkali metal compounds are important as replacements for toxic materials in a range of electronic devices. This is especially true for potassium, being a major component in e.g. KxNa1-xNbO3 (KNN) and KTaxNb1-xO3 (KTN), with hope to replace Pb(ZrxTi1-x)O3 (PZT) in piezo-/ferroelectric and electrooptic devices. ALD facilitates functional conformal coatings at deposition temperatures far below what is reported using other techniques and with excellent compositional control. The ALD growth of potassium-containing films using KOtBu has, however, been unpredictable. Untraditional response to the pulse composition and precursor dose, severe reproducibility issues, and very high growth per cycle are some of the puzzling features of these processes. In this article, we shed light on the growth behavior of KOtBu in ALD by in situ quartz crystal microbalance and Fourier transform infrared spectroscopy studies. We study the precursor's behavior in the technologically interesting KNbO3-process, showing how the potassium precursor strongly affects the growth of other cation precursors. We show that the strong hygroscopic nature of the intermediary potassium species has far-reaching implications throughout the growth. This helps not only to enhance the understanding of alkali metal containing compounds' growth in ALD, but also to provide the means to control the growth of novel sustainable technological materials.

A foundation for complex oxide electronics -low temperature perovskite epitaxy.

As traditional silicon technology is moving fast towards its fundamental limits, all-oxide electronics is emerging as a challenger offering principally different electronic behavior and switching mechanisms. This technology can be utilized to fabricate devices with enhanced and exotic functionality. One of the challenges for integration of complex oxides in electronics is the availability of appreciable low-temperature synthesis routes. Herein we provide a fundamental extension of the materials toolbox for oxide electronics by reporting a facile route for deposition of highly electrically conductive thin films of LaNiO3 by atomic layer deposition at low temperatures. The films grow epitaxial on SrTiO3 and LaAlO3 as deposited at 225 °C, with no annealing required to obtain the attractive electronic properties. The films exhibit resistivity below 100 µΩ cm with carrier densities as high as 3.6 · 1022 cm-3. This marks an important step in the realization of all-oxide electronics for emerging technological devices.

Chemical Uniformity in Ferroelectric K x Na1- x NbO3 Thin Films.

Potassium sodium niobate (KNN) has long been considered a viable candidate for replacing lead-based materials in piezo- and ferroelectric devices. The introduction of KNN on an industrial scale is highly awaited; however, processing challenges still remain to be solved. The main obstacle is lack of reproducible growth of uniform boules or thin films at temperatures that facilitate monolithic device integration. Herein, atomic layer deposition (ALD) of KNN thin films, exhibiting high chemical uniformity over large areas, is reported. The cation composition can be controlled at a 1% level, enabling fine-tuning of the film stoichiometry across the morphotropic phase boundaries of the KNbO3-NaNbO3 solid solution. The films are obtained as highly oriented on Pt (111)||Si (100)-substrates after annealing at temperatures as low as 550 °C. They exhibit converse piezoelectric effects with magnitudes in accordance with literature. It is believed that the successful development of the described ALD process represents a major step toward achieving lead-free piezo- and ferroelectrics on an industrial scale.

Biocompatible organic-inorganic hybrid materials based on nucleobases and titanium developed by molecular layer deposition.

We have constructed thin films of organic-inorganic hybrid character by combining titanium tetra-isopropoxide (TTIP) and the nucleobases thymine, uracil or adenine using the molecular layer deposition (MLD) approach. Such materials have potential as bioactive coatings, and the bioactivity of these films is described in our recent work [Momtazi, L.; Dartt, D. A.; Nilsen, O.; Eidet, J. R. J. Biomed. Mater. Res., Part A 2018, 106, 3090-3098. doi:10.1002/jbm.a.36499]. The growth was followed by in situ quartz crystal microbalance (QCM) measurements and all systems exhibited atomic layer deposition (ALD) type of growth. The adenine system has an ALD temperature window between 250 and 300 °C, while an overall reduction in growth rate with increasing temperature was observed for the uracil and thymine systems. The bonding modes of the films have been further characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction, confirming the hybrid nature of the as-deposited films with an amorphous structure where partial inclusion of the TTIP molecule occurs during growth. The films are highly hydrophilic, while the nucleobases do leach in water providing an amorphous structure mainly of TiO2 with reduced density and index of refraction.

Rubidium containing thin films by atomic layer deposition.

The application range for atomic layer deposition (ALD) has now been extended to include the deposition of rubidium-containing films, enabling the deposition of new and exploratory types of compounds by ALD. The properties of rubidium t-butoxide as an ALD precursor are promising, revealing similar behavior as its lithium, sodium and potassium counterparts. The deposition of rubidium containing films is reported as proof of concept through the Rb-Ti-O and Rb-Nb-O systems. Rubidium content in the doping level range of Rb is controllably achieved in Rb:TiOx up to 20%, whereas Rb can be introduced as a major component in Rb:NbOx. Perovskite RbNbO3, otherwise unattainable in bulk systems under ambient conditions, is shown to be stabilized on SrTiO3 (100) substrates. This report opens up the investigation of thin films of new and unexplored systems, not only in the world of ALD, but in materials chemistry in general.