Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection

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Optical reflection, or in other words the loss of reflection, from a surface becomes increasingly crucial in determining the extent of the light-matter interaction. The simplest example of using an anti-reflecting (AR) surface is possibly the solar cell that incorporates an AR coating to harvest sunlight more effectively. Researchers have now found ways to mimic biological structures, such as moth eyes or cicada wings, which have been used for the AR purpose by nature herself. These nanoscopic biomimetic structures lend valuable clues in fabricating and designing gradient refractive index materials that are efficient AR structures. The reflectance from a selected sub-wavelength or gradient index structures have come down to below 1% in the visible region of the spectrum and efforts are on to achieve broader bands of such enhanced AR regime. In addition to the challenge of broader bands, the performance of AR structures is also limited by factors such as omnidirectional properties and polarization of incident light. This review presents selected state-of-the-art AR techniques, reported over the last half a century, and their guiding principles to predict a logical trend for future research in this field.

Anti-reflecting and photonic nanostructures

Multijunction solar cells are effective for increasing the power conversion efficiency beyond that of single-junction cells. Indeed, the highest solar cell efficiencies have been achieved using two or more subcells to adequately cover the solar spectrum. However, the efficiencies of organic multijunction solar cells are ultimately limited by the lack of high-performance, near-infrared absorbing organic subcells within the stack. Here, we demonstrate a tandem cell with an efficiency of 15.0 ± 0.3% (for 2 mm2 cells) that combines a solution-processed non-fullerene-acceptor-based infrared absorbing subcell on a visible-absorbing fullerene-based subcell grown by vacuum thermal evaporation. The hydrophilic–hydrophobic interface within the charge-recombination zone that connects the two subcells leads to >95% fabrication yield among more than 130 devices, and with areas up to 1 cm2. The ability to stack solution-based on vapour-deposited cells provides significant flexibility in design over the current, all-vapour-deposited multijunction structures. Organic tandem solar cells combining solution- and vapour-deposited layers are difficult to fabricate due to the degradation of the underlying subcell during the deposition of the next subcell. Here, Che et al. design a charge recombination layer that protects the underlying cell and leads to 15% average PCE for 2 mm2 devices and 11.5% PCE for 1 cm2 devices.

High fabrication yield organic tandem photovoltaics combining vacuum- and solution-processed subcells with 15% efficiency

Solid-state light sources are in the process of profoundly changing the way humans generate light for general lighting applications. Solid-state light sources possess two highly desirable features, which set them apart from most other light sources: (i) they have the potential to create light with essentially unit power efficiency and (ii) the properties of light, such as spectral composition and temporal modulation, can be controlled to a degree that is not possible with conventional light sources such as incandescent and fluorescent lamps. The implications are enormous and, as a consequence, many positive developments are to be expected including a reduction in global energy consumption, reduction of global-warming-gas and pollutant emissions and a multitude of new functionalities benefiting numerous applications. This review will assess the impact of solid-state lighting technology on energy consumption, the environment and on emerging application fields that make use of the controllability afforded by solid-state sources. The review will also discuss technical areas that fuel continued progress in solid-state lighting. Specifically, we will review the use of novel phosphor distributions in white light-emitting diodes (LEDs) and show the strong influence of phosphor distribution on efficiency. We will also review the use of reflectors in LEDs with emphasis on 'perfect' reflectors, i.e. reflectors with highly reflective omni-directional characteristics. Finally, we will discuss a new class of thin-film materials with an unprecedented low refractive index. Such low-n materials may strongly contribute to the continuous progress in solid-state lighting.

https://iopscience.iop.org/article/10.1088/0034-4885/69/12/R01/pdf

Solid-state lighting?a benevolent technology

The refractive-index contrast in dielectric multilayer structures, optical resonators, and photonic crystals is an important figure of merit that creates a strong demand for high-quality thin films with a low refractive index. A SiO2 nanorod layer with low refractive index of n = 1.08, to our knowledge the lowest ever reported in thin-film materials, is grown by oblique-angle electron-beam deposition of SiO2. A single-pair distributed Bragg reflector employing a SiO2 nanorod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-refractive-index films for applications in photonic structures and devices.

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Very low-refractive-index optical thin films consisting of an array of SiO 2 nanorods

In order to boost the performance of next-generation silicon integrated circuits, polymers with lower dielectric constant compared to silicon dioxide are required as interlayer dielectrics (ILDs). Ion penetration from surrounding interconnect metal (typically copper or aluminum) or from barrier materials (e.g., tantalum or tantalum nitride) under thermal and electrical stresses can lead to premature failure of the polymers. Hence, ion penetration was studied in hybrid organosiloxane polymer (HOSP) using bias-temperature stressing. Metal/polymer interface chemistry, particularly the presence of interfacial oxygen, was found to play a key role in aiding metal ionization, and subsequent mobile ion penetration into dielectrics.

The Effect of Interfacial Chemistry on Metal Ion Penetration into Polymeric Films

Pd reduction, while at the same time, enough hydrogen atoms are consumed in the plasma to ensure there is no visible degradation of the PPX. X-ray photoelectron spectroscopy (XPS) measurements of the substrate after hydrogen/nitrogen plasma treatment at 50 W clearly show the presence of nitrogen bound to the substrate surface. XPS measurements of the deposited Pd films indicate good quality for both substrates, suggesting that the substrate temperature was low enough to prevent dissociation of the hfac ligand and adequate scavenging of the hfac ligand by the available atomic hydrogen. The remote hydrogen/nitrogen plasma enables Pd film deposition on polymer surfaces, which do not typically react with the Pd precursor, and are not catalysts for the dissociation of molecular hydrogen.

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Plasma‐Enhanced Atomic Layer Deposition of Palladium on a Polymer Substrate

Dielectric barriers, pore sealing, and metallization

The introduction of porosity in dielectrics is desirable to reduce the dielectric constant; but it causes integration problems such as CVD∕ALD precursor penetration for barrier layer∕seed layer deposition. CVD Parylene-N has been shown to work as a pore sealant for porous low-κ materials but penetrates itself slightly into porous dielectric. The depth profile of Parylene-N in porous MSQ can be obtained using the Nuclear Reaction Analysis (NRA) of C12. The penetration of Parylene-N can be controlled by deposition at higher pressure where the deposition rate is also high. High deposition rate can also be attained by adding a carrier gas which also shows low Parylene-N penetration. The experimentally measured dielectric constants, after pore sealing, are compared to those calculated using the NRA data of Parylene-N penetration.

Jasbir S. Juneja

Papers

Very low-refractive-index optical thin films consisting of an array of SiO 2 nanorods

The Effect of Interfacial Chemistry on Metal Ion Penetration into Polymeric Films

Plasma‐Enhanced Atomic Layer Deposition of Palladium on a Polymer Substrate

Dielectric barriers, pore sealing, and metallization

Pressure dependent Parylene-N pore sealant penetration in porous low-κ dielectrics