Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

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

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Single-Atom Catalysts across the Periodic Table.

The effect of annealing on the upconversion luminescence of ZnO:Er3+ nanocrystals was investigated in detail. The green and the red upconverted emissions under infrared 978-nm light excitation were remarkably enhanced with an increase of annealing temperature. Moreover, for the sample annealed at 500 °C, the ratio of the intensity of 2H11/2 → 4I15/2 emission to that of 4S3/2 → 4I15/2 emission increased from less than to more than unity with an increase of the excitation density. However, the same case did not occur to the sample annealed at 700 °C, where the ratio was independent of excitation density except when the excitation density was higher than 42 700 W/cm2. This distinction was attributed mainly to the difference in energy gap between the 2H11/2 and 4S3/2 states in the two samples, originating from the local microstructure variation around Er3+ ions. In addition, a high thermal sensitivity of 0.0062/°C was obtained in the ZnO:Er3+ nanocrystals based on the temperature-dependent fluorescence intens...

Effect of annealing on upconversion luminescence of ZnO : Er3+ nanocrystals and high thermal sensitivity

Superhydrophobic properties of ultrathin rf-sputtered Teflon films coated etched aluminum surfaces

The local structure of an optically active center in erbium-doped zinc oxide (ZnO:Er) thin film produced by a laser ablation technique and its optical activation process are investigated by Er LIII-edge x-ray absorption fine structure analysis using a synchrotron radiation as an x-ray source. In as-ablated ZnO:Er thin film, Er has an approximately five-fold coordination of O surrounded by eight other O atoms as second-nearest neighbors. The high-order coordination of O decreases the Er-related photoluminescence (PL) intensity due to an undesirable crystal field for 4f radiation transition. After annealing in O2 ambient, the local structure of Er changes to a pseudo-octahedral structure with C4v symmetry, similar to the optically activated Er-doped Si (Si:Er), resulting in strong PL. The bond lengths of Er–O are evaluated, and differences in the optical activation processes between ZnO:Er and Si:Er thin films are discussed.

Local structure analysis of an optically active center in Er-doped ZnO thin film

The local structure of erbium-doped silicon produced by the laser ablation technique is investigated by Er LIII-edge x-ray absorption fine structure analysis. The combined analysis of extended x-ray absorption fine structure analysis and an x-ray absorption near-edge structure simulation based on multiple-scattering theory reveals the most probable atomic coordination of the optically active center; Er bonded with six oxygen atoms has a C4v symmetry. The optical activation process of this system is also discussed. The Si target with 10 wt% Er2O3 has two kinds of local structures, C-rare-earth Er2O3 grain and another Er phase incorporated in Si. The laser ablation homogenizes these phases, and deposits a new single-phase structure of the octahedron (Oh point group) on the substrates. In this phase, the optical transition probability is low due to the forbidden 4f transition of Er in the crystal field originating from the higher-order symmetry of O. After annealing, degradation of the symmetry from Oh to C4...

The optically active center and its activation process in Er-doped Si thin film produced by laser ablation

Alternative Er doping into anatase (A–) or rutile (R−)TiO2 can be achieved by O2 pressure control during Er-doped TiO2 deposition using laser ablation. An x-ray absorption fine structure analysis of the atomic coordination around Er revealed the dynamics of Er–O clustering in A– and R−TiO2. The Er local structure in A−TiO2 can be determined by a Ti–O arrangement of the host crystal surrounding Er, while that in R−TiO2 can be decided by the Er–O chemical properties rather than the Ti–O arrangement. Moreover, a substitution of Er for Ti induced O deficit in A−TiO2 and induced Ti removal in R−TiO2. These differences can be explained by the ErO6 stabilization mechanism based on the spontaneous symmetry reduction (SSR) theory. The Ti–O arrangement in A−TiO2 prevents the SSR, resulting in ErO6 instability. However, ErO6 is stabilized in the host crystal because the SSR is smoothly induced in R−TiO2.

Study on atomic coordination around Er doped into anatase– and rutile– TiO2: Er–O clustering dependent on the host crystal phase

This paper reviews charges that locally functionalize materials. Microscopic analyses and operation of charges using various scanning probe microscopy (SPM) techniques have revealed static, quasi-static/quasi-dynamic and dynamic charge behaviours. Charge-sensitive SPM has allowed for the visualization of the distribution of functionalized charges in electronic devices. When used as bit data in a memory system, the charges can be operated by SPM. The behaviour of quasi-static/quasi-dynamic charges is discussed here. In the data-writing process, spatially dispersive charges rather than a fast injection rate are introduced, but the technical problems can be solved by using nanostructures. Careful charge operations using SPM should realize a memory with a larger density than Tbit/inch2. Dynamic charges have been introduced in physical analyses and chemical processes. Although the observable timescale is limited by the SPM system response time of the order of several seconds, dynamics such as photon-induced charge redistributions and probe-assisted chemical reactions are observed.

http://iopscience.iop.org/article/10.1088/0953-8984/22/17/173001/pdf

Static states and dynamic behaviour of charges: observation and control by scanning probe microscopy

In order to discuss the local structure of an optically active center in Er-doped Si thin film, site-selective x-ray absorption fine structure (XAFS) analysis using x-ray-excited optical luminescence was performed. The XAFS spectrum at the Er LIII edge was obtained from the x-ray photon energy dependence of the peak intensity of infrared luminescence due to Er intra-4f transition. Although conventional XAFS measurement analyzes the average structure of all of the Er, this method intrinsically provides structural information for only optically active Er. A broad 2p–5d resonant peak in the site-selective XAFS spectrum is reproduced by a density-of-state calculation of a distorted ErO6 cluster, assuming an Er transformation from an octahedral center of 0.25 A.

Site-selective x-ray absorption fine structure analysis of an optically active center in Er-doped semiconductor thin film using x-ray-excited optical luminescence

Masashi Ishii

Papers

Local structure analysis of an optically active center in Er-doped ZnO thin film

The optically active center and its activation process in Er-doped Si thin film produced by laser ablation

Study on atomic coordination around Er doped into anatase– and rutile– TiO2: Er–O clustering dependent on the host crystal phase

Static states and dynamic behaviour of charges: observation and control by scanning probe microscopy

Site-selective x-ray absorption fine structure analysis of an optically active center in Er-doped semiconductor thin film using x-ray-excited optical luminescence