Jani Hämäläinen

Antireflection (AR) coatings are crucial for a myriad of optical applications. The demand for conformal AR coatings over nonplanar substrates, such as a 3D glass or curved lenses, is increasing. In this study, atomic layer deposition (ALD) was utilized to deposit multilayer AR films composed of MgF2, SiO2, Al2O3, and HfO2. Notably, MgF2 thin films are desirable as the top layer due to their low refractive index (sub-1.4), wide spectral transparency from the ultraviolet to the infrared region, and considerable chemical stability. First, the thickness uniformity of the four distinct films was optimized using the thermal ALD method, and then their individual optical properties were investigated. This information supported the design of a seven-layer AR film, and subsequently, the AR layers were deposited onto substrates with highly curved lenses. Then, the optical performance of the coated lenses, with a focus on reflectance, was measured. Our measurements indicate that conformal AR coatings on lenses with pronounced curvature were successfully deposited. The research highlights the potential for achieving highly uniform AR coatings with precise optical control in areas such as medical optics, photonic integrated circuits, and metasurface devices. In conclusion, this study conclusively demonstrates the superiority of ALD-based AR coatings on complex geometric surfaces, offering an effective alternative to the traditional physical vapor deposition methods when confronted with their intrinsic constraints.

IrO₂ is an important material in numerous applications ranging from catalysis to the microelectronics industry, but despite this its behaviour upon annealing under different conditions has not yet been thoroughly studied. This work provides a detailed investigation of the annealing of IrO₂ thin films using in situ high-temperature X-ray diffraction and X-ray reflectivity (HTXRR) measurements from room temperature up to 1000°C in oxygen, nitrogen, forming gas and vacuum. Complementary ex situ scanning electron microscopy and atomic force microscopy measurements were conducted. The combined data show the dependencies of crystalline properties and surface morphology on the annealing temperature and atmosphere. The reduction of IrO₂ to Ir takes place at a temperature as low as 150°C in forming gas, but in oxygen IrO₂ is stable up to 800°C and evaporates as a volatile oxide at higher temperatures. The IrO₂ crystallite size remains constant in oxygen up to 400°C and increases above that, while in the more reducing atmospheres the Ir crystallites grow continuously above the phase-change temperature. The role of HTXRR in the analysis is shown to be important since its high sensitivity allows one to observe changes taking place in the film at temperatures much below the phase change.

Van der Waals epitaxy holds great promise in producing high-quality films of 2D materials. However, scalable van der Waals epitaxy processes operating at low temperatures and low vacuum conditions are lacking. Herein, atomic layer deposition is used for van der Waals epitaxy of continuous multilayer films of 2D materials HfS ₂ , MoS ₂ , SnS ₂ , and ZrS ₂ on muscovite mica and PbI ₂ on sapphire at temperatures between 75 °C and 400 °C. For the metal sulfides on mica, the main epitaxial relation is MS ₂ ‖ mica. Some domains rotated by 30° are also observed corresponding to the MS ₂ ‖ mica alignment. In both cases, the presence of domains rotated by 60° (mirror twins) is also expected. For PbI ₂ on sapphire, the epitaxial relation is PbI ₂ ‖ Al ₂ O ₃ with no evidence of 30° domains. For all of the studied systems there is relatively large in-plane mosaicity and in the PbI ₂ /Al ₂ O ₃ system some non-epitaxial domains are also observed. The study presents first steps of an approach towards a scalable and semiconductor industry compatible van der Waals epitaxy method.

Slight surface modifications of TiO₂(“insignificant” at first glance), such as exposing to reducing or oxidizing agents and deposition of small amounts of a semiconductor material at the surface, may strongly affect the photoactivity of materials.

Dissolution of elemental gold in organic solutions is a contemporary approach to lower the environmental burden associated with gold recycling. Herein, we describe fundamental studies on a highly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine‐4‐thiol (4‐PSH) as a reactive ligand and hydrogen peroxide as an oxidant. Dissolution of Au proceeds through several elementary steps: isomerization of 4‐PSH to pyridine‐4‐thione (4‐PS), coordination with Au⁰, and then oxidation of the Au⁰ thione species to Au^I simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is a Au^I complex bearing two 4‐PS ligands and SO₄²⁻ as a counterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.

Rhenium is both a refractory metal and a noble metal that has attractive properties for various applications. Still, synthesis and applications of rhenium thin films have been limited. We introduce herein the growth of both rhenium metal and rhenium nitride thin films by the technologically important atomic layer deposition (ALD) method over a wide deposition temperature range using fast, simple, and robust surface reactions between rhenium pentachloride and ammonia. Films are grown and characterized for compositions, surface morphologies and roughnesses, crystallinities, and resistivities. Conductive rhenium subnitride films of tunable composition are obtained at deposition temperatures between 275 and 375 °C, whereas pure rhenium metal films grow at 400 °C and above. Even a just 3 nm thick rhenium film is continuous and has a low resistivity of about 90 μΩ cm showing potential for applications for which also other noble metals and refractory metals have been considered.

Rhenium is both a refractory metal and a noble metal that has attractive properties for various applications. Still, synthesis and applications of rhenium thin films have been limited. We introduce herein the growth of both rhenium metal and rhenium nitride thin films by the technologically important atomic layer deposition (ALD) method over a wide deposition temperature range using fast, simple, and robust surface reactions between rhenium pentachloride and ammonia. Films are grown and characterized for compositions, surface morphologies and roughnesses, crystallinities, and resistivities. Conductive rhenium subnitride films of tunable composition are obtained at deposition temperatures between 275 and 375 °C, whereas pure rhenium metal films grow at 400 °C and above. Even a just 3 nm thick rhenium film is continuous and has a low resistivity of about 90 μΩ cm showing potential for applications for which also other noble metals and refractory metals have been considered.

2D materials research is advancing rapidly as various new “beyond graphene” materials are fabricated, their properties studied, and materials tested in various applications. Rhenium disulfide is one of the 2D transition metal dichalcogenides that has recently shown to possess extraordinary properties such as that it is not limited by the strict monolayer thickness requirements. The unique inherent decoupling of monolayers in ReS₂ combined with a direct bandgap and highly anisotropic properties makes ReS₂ one of the most interesting 2D materials for a plethora of applications. Here, a highly controllable and precise atomic layer deposition (ALD) technique is applied to deposit ReS₂ thin films. Film growth is demonstrated on large area (5 cm × 5 cm) substrates at moderate deposition temperatures between 120 and 500 °C, and the films are extensively characterized using field emission scanning electron microscopy/energy‐dispersive X‐ray spectroscopy, X‐ray diffractometry using grazing incidence, atomic force microscopy, focused ion beam/transmission electron microscopy, X‐ray photoelectron spectroscopy, and time‐of‐flight elastic recoil detection analysis techniques. The developed ReS₂ ALD process highlights the potential of the material for applications beyond planar structure architectures. The ALD process also offers a route to an upgrade to an industrial scale.

Nucleation and conformality are important issues, when depositing thin films for demanding applications. In this study, iridium and iridium dioxide (IrO₂) films were deposited by atomic layer deposition (ALD), using five different processes. Different reactants, namely, O₂, air, consecutive O₂ and H₂ (O₂ + H₂), and consecutive O₃ and H₂ (O₃ + H₂) pulses were used with iridium acetylacetonate [Ir(acac)₃] to deposit Ir, while IrO₂ was deposited using Ir(acac)₃ and O₃. Nucleation was studied using a combination of methods for film thickness and morphology evaluation. In conformality studies, microscopic lateral high-aspect-ratio (LHAR) test structures, specifically designed for accurate and versatile conformality testing of ALD films, were used. The order of nucleation, from the fastest to the slowest, was O₂ + H₂ > air ≈ O₂ > O₃ > O₃ + H₂, whereas the order of conformality, from the best to the worst, was O₃ + H₂ > O₂ + H₂ > O₂ > O₃. In the O₃ process, a change in film composition from IrO₂ to metallic Ir was seen inside the LHAR structures. Compared to the previous reports on ALD of platinum-group metals, most of the studied processes showed good to excellent results.