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

We have studied the dependence of the absorption edge and the refractive index of wurtzite AlxGa1−xN films on temperature and composition using transmission and photothermal deflection spectroscopy. The Al molar fraction of the AlxGa1−xN films grown by plasma induced molecular beam epitaxy was varied through the entire range of composition (0⩽x⩽1). We determined the absorption edges of AlxGa1−xN films and a bowing parameter of 1.3±0.2 eV. The refractive index in the photon energy range between 1 and 5.5 eV and temperatures between 7 and 295 K was deduced from the interference fringes. The static refractive index n(0) changed from 2.29 for GaN to 1.96 for AlN at room temperature. A variation of temperature from 295 to 7 K resulted in a decrease of refractive index (at photon energies close to the band gap) by 0.05±0.01 and in an energy shift of the absorption edge of about 64±5 meV independent of the Al content of the films. Using the Kramers–Kronig dispersion relation and an approximation for the dispersion coefficient for photon energies near the band gap, the refractive index could be described as a function of photon energy, Al content, and temperature.

Optical constants of epitaxial AlGaN films and their temperature dependence

We examine the effect of growth temperature in the 150−300 °C range on the structural and morphological properties of Al2O3 films deposited using atomic layer deposition, contrasting the effect of H2O and O3 as oxygen sources. Trimethylaluminum (TMA) is the metal source. A mechanism for the O3 reaction is investigated using ab initio calculations and provides an explanation for the observed temperature dependence. The simulations show that hydroxyl groups are produced at the surface by the oxidation of adsorbed methyl groups by O3. This is confirmed by the measured rates; both H2O and O3 processes show molar growth rates per cycle that decrease with increasing reactor temperature, consistent with a decrease in hydroxyl coverage. At no temperature does the O3 process deposit more Al2O3 per cycle than the H2O process. Morphological characterization shows that O3 as the oxygen source yields lower-quality films than H2O; the films are less dense and rougher, especially at low growth temperatures. The existenc...

Ozone-Based Atomic Layer Deposition of Alumina from TMA: Growth, Morphology, and Reaction Mechanism

A reliable and reproducible deposition process for the fabrication of Al2O3 waveguides with losses as low as 0.1 dB/cm has been developed. The thin films are grown at ~ 5 nm/min deposition rate and exhibit excellent thickness uniformity within 1% over 50times50 mm2 area and no detectable OH- incorporation. For applications of the Al2O3 films in compact, integrated optical devices, a high-quality channel waveguide fabrication process is utilized. Planar and channel propagation losses as low as 0.1 and 0.2 dB/cm, respectively, are demonstrated. For the development of active integrated optical functions, the implementation of rare-earth-ion doping is investigated by cosputtering of erbium during the Al2O3 layer growth. Dopant levels between 0.2-5times1020 cm-3 are studied. At Er3+ concentrations of interest for optical amplification, a lifetime of the 4I13/2 level as long as 7 ms is measured. Gain measurements over 6.4-cm propagation length in a 700-nm-thick Al2O3:Er3+ channel waveguide result in net optical gain over a 41-nm-wide wavelength range between 1526-1567 nm with a maximum of 5.4 dB at 1533 nm.

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Reliable Low-Cost Fabrication of Low-Loss $\hbox{Al}_{2}\hbox{O} _{3}{:}\hbox{Er}^{3+}$ Waveguides With 5.4-dB Optical Gain

Elastic recoil detection analysis (ERD) with swift heavy ions has been improved in view of depth resolution. Profiles of light elements could be measured in carbon near the surface with single atomic layer resolution using 58 Ni, 127 I or 197 Au beams with energies between 0.5 A MeV and 1 A MeV and a Q3D magnetic spectrograph. Due to the specific ion optical arrangement of the Q3D a relative energy resolution of about 7 × 10 −4 was achieved in ERD experiments although a large solid angle of 5 msr was used. This large acceptance guarantees good statistics even with low ion fluences. In this way the elemental concentration profiles are not essentially disturbed by irradiation damage effects and can be measured in the 0.1% level on an atomic depth scale.

Elastic recoil detection analysis with atomic depth resolution

Slackened carbon stripper foils of about 4 μg/cm2, produced by vacuum evaporation-condensation and used for heavy ion stripping in the terminal of the Munich MP tandem, were investigated in order to get more information about irradiation damages. Electron diffraction and electron transmission muscope investigations related with theoretical considerations revealed a clear picture of the irradiation damage mechanism in carbon stripper foils. Due to these results it can be predicted, how the crystallographic structure of a carbon foil should be constituted in order to show best stripper performance.

Heavy-ion irradiation damage in carbon stripper foils

Investigations about the irradiation damage in carbon foils [1] led to the prediction that carbon stripper foils with a structure of randomly oriented crystallites should adduce an optimum strength against damages caused by swift heavy ions. As a result of these investigations new stripper foils were developed, which are prepared by laser plasma ablation-deposition in high vacuum. Such foils are provided with the preferred crystalline structure and show the predicted improvement in tests using an 127I-beam at the Munich tandem accelerator.

Carbon stripper foils with isotropic structure for optimum strength against irradiation damage

The aim to develop irradiation resistant carbon stripper foils for swift heavy ions led to the new technique of laser plasma ablation-deposition of carbon, in which a pulsed high power Nd:YAG laser induces a hot carbon plasma for deposition on suitable substrates in high vacuum. The essential physical property of the new technique is the ablation of monoatomic carbon with an average charge state of 4+ and a broad spectrum of energies of up to 1 keV of the monomers. These conditions influence the nucleation and growth of a carbon film in such a way that a nearly isotropic orientation of the nanocrystals in the condensed film is achieved. This results, as postulated, in a much better strength against irradiation damage caused by heavy ions, compared to all other types of stripper foils.

Development of laser plasma ablation-deposition techniques for carbon stripper foils

Carbon foils produced by laser plasma ablation-deposition (lpa-foils), by the glow discharge process (gd-foils) and by evaporation-condensation (ec-foils) were investigated by high resolution transmission electron microscopy and electron diffraction. A quantitative analysis is presented for the degree of anisotropic orientation of the quasi-graphitic nanocrystals in the polycrystalline foils. In the new type lpa-foils the nanocrystals are nearly isotropically oriented, in contrast to the anisotropic alignment in the ec-and gd-foils. This random orientation is responsible for a longer lifetime of lpa-foils when irradiated with heavy ions.

Structure investigations of thin carbon foils

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