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

Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work. The fascinating wave phenomenon of ‘bound states in the continuum’ spans different material and wave systems, including electron, electromagnetic and mechanical waves. In this Review, we focus on the common physical mechanisms underlying these bound states, whilst also discussing recent experimental realizations, current applications and future opportunities for research.

Bound states in the continuum

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

The present invention provides for sequential chemical vapor deposition by employing a reactor operated at low pressure, a pump to remove excess reactants, and a line to introduce gas into the reactor through a valve. A first reactant forms a monolayer on the part to be coated, while the second reactant passes through a radical generator which partially decomposes or activates the second reactant into a gaseous radical before it impinges on the monolayer. This second reactant does not necessarily form a monolayer but is available to react with the monolayer. A pump removes the excess second reactant and reaction products completing the process cycle. The process cycle can be repeated to grow the desired thickness of film.

Sequential chemical vapor deposition

Optical bound states in the continuum (BICs) have recently been realized in photonic crystal slabs, where the disappearance of out-of-plane radiation turns leaky resonances into guided modes with infinite lifetimes. We show that such BICs are vortex centers in the polarization directions of far-field radiation. They carry conserved and quantized topological charges, defined by the winding number of the polarization vectors, which ensure their robust existence and govern their generation, evolution, and annihilation. Our findings connect robust BICs in photonics to a wide range of topological physical phenomena.

https://link.aps.org/accepted/10.1103/PhysRevLett.113.257401

Topological nature of optical bound states in the continuum.

The applications of surface-emitting lasers, in particular vertical-cavity surface-emitting lasers (VCSELs), are currently being extended to various low-power fields including communications and interconnections. However, the fundamental difficulties in increasing their output power by more than several milliwatts while maintaining single-mode operation prevent their application in high-power fields such as material processing, laser medicine and nonlinear optics, despite their advantageous properties of circular beams, the absence of catastrophic optical damage, and their suitability for two-dimensional integration. Here, we demonstrate watt-class high-power, single-mode operation by a two-dimensional photonic-crystal surface-emitting laser under room-temperature, continuous-wave conditions. The two-dimensional band-edge resonant effect of a photonic crystal formed by metal–organic chemical vapour deposition enables a 1,000 times broader coherent-oscillation area, which results in a high beam quality of M2 ≤ 1.1, narrowing the focus spot by two orders of magnitude compared to VCSELs. Our demonstration promises to realize innovative high-power applications for surface-emitting lasers. Researchers demonstrate a watt-class high-power, single-mode photonic-crystal laser operating continuously at room temperature. A beam quality of M2 ≤ 1.1 is achieved.

Watt-class high-power, high-beam-quality photonic-crystal lasers

Microscopic distributions of growth rates on GaAs(001) layers next to (111)A and (111)B surfaces were measured in real time during molecular beam epitaxy growth by scanning microprobe reflection high‐energy electron diffraction. The increase in the growth rate on the GaAs(001) surface near the edge of (111)A surfaces is observed and the decrease in the growth rate on the GaAs(001) surface near the edge of the (111)B surfaces is found out. The exponential variation of the growth rate as a function of the distance from the edge, reflects surface diffusion of Ga atoms. The diffusion lengths along the [110] and [110] directions are estimated to be about 1 and 8 μm at 560 °C, respectively.

Distributions of growth rates on patterned surfaces measured by scanning microprobe reflection high‐energy electron diffraction

Microscopic distribution of growth rates on mesa‐etched GaAs(001) wafers was measured in real time during molecular beam epitaxy growth by scanning microprobe reflection high‐energy electron diffraction. It has been observed that the growth rate on the GaAs(001) surface near the edge of (111)A surfaces becomes larger. The exponential variation of the growth rate as a function of the distance from the edge reflects surface diffusion of Ga atoms. The diffusion length on the (001) surface is estimated to be about 1 μm at 560 °C. The relatively larger diffusion length suggests that the incorporation rate of migrating Ga atoms by steps is much smaller than unity.

Real‐time observation of molecular beam epitaxy growth on mesa‐etched GaAs substrates by scanning microprobe reflection high‐energy electron diffraction

This light emitting element is provided with an electrode (8), an active layer (3), a photonic crystal layer (4), and an electrode (9) The conductivity type of the active layer (3) and that of the electrode (8) are different from each other, and the conductivity type of the active layer (3) and that of the electrode (9) are different from each other, the electrode (8) has an opening (8a), and the electrode (8), the active layer (3), the photonic crystal layer (4), and the electrode (9) are laminated in the X axis The X axis passes through a center portion (8a2) of the opening (8a) when viewed from the axis line direction of the X axis, the electrode (9) has, when viewed from the axis line direction of the X axis, an end portion (9e1) positioned in the direction opposite to the Y axis direction, and an end portion (9e2) positioned in the Y axis direction, the opening (8a) has, when viewed from the X axis, an end portion (8e1) positioned in the direction opposite to the Y axis direction, and an end portion (8e2) positioned in the Y axis direction, and the end portion (9e1) of the electrode (9) and the end portion (8e1) of the opening (8a) substantially match when viewed from the axis line direction of the X axis

Akiyoshi Watanabe

Papers

Watt-class high-power, high-beam-quality photonic-crystal lasers

Distributions of growth rates on patterned surfaces measured by scanning microprobe reflection high‐energy electron diffraction

Real‐time observation of molecular beam epitaxy growth on mesa‐etched GaAs substrates by scanning microprobe reflection high‐energy electron diffraction

Semiconductor light emitting element

Surface diffusion length observed by in situ scanning microprobe reflection high-energy electron diffraction