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

Advances in physics

Publisher Summary This chapter deals with atomic layer deposition (ALD), which is a chemical gas phase thin film deposition method based on alternate saturative surface reactions. As distinct from the other chemical-vapor-deposition techniques, in ALD the source vapors are pulsed into the reactor alternately—one at a time—separated by purging or evacuation periods. Each precursor exposure step saturates the surface with a monomolecular layer of that precursor. This results in a unique self-limiting film-growth mechanism with a number of advantageous features—such as excellent conformality and uniformity and simple and accurate film-thickness control. As the current interest in ALD is largely centered on non-epitaxial films, the chapter focuses on these materials and the related chemistry. ALD is a unique process based on alternate surface reactions, which are accomplished by dosing the gaseous precursors on the substrate alternately. Under ideal conditions, these reactions are saturative, ensuring many advantageous features, such as excellent conformity, large-area uniformity, accurate and simple film-thickness control, repeatability, and large-batch processing capability.

Atomic layer deposition

Journal-of-physics-c - solid-state physics and semiconductor science and technology

Microcavities for organic light-emitting devices (OLED’s) with a metal mirror on one side and a distributed Bragg reflector (DBR) on the other side have been extensively studied in the literature. Usually the DBR is highly reflective, and the resulting emission of the microcavity depends strongly on angle and wavelength. With a thick metal mirror on one side and a semi-transparent metal mirror on the other side of the OLED, a microcavity can be obtained with an optical thickness of 1 half-wavelength. Because the emission is enhanced over a wide solid angle, with a small spectral dependence, this structure is very promising for display applications. For a TPD/ALq3 structure with a typical intrinsic emission spectrum, embedded in a microcavity with a thick and a semitransparent silver mirror, the integrated emission in air, the color variation with angle, and the change in the decay time are compared with those in a DBR-based microcavity.

Semitransparent metal or distributed Bragg reflector for wide-viewing-angle organic light-emitting-diode microcavities

Atomic layer deposition of ZnS thin films based on diethyl zinc and hydrogen sulfide

We present a simulation method for light emitted in uniaxially anisotropic light-emitting thin film devices. The simulation is based on the radiation of dipole antennas inside a one-dimensional microcavity. Any layer in the microcaviy can be uniaxially anisotropic with an arbitrary orientation of the optical axis. A plane wave expansion for the field of an elementary dipole inside an anisotropic medium is derived from Maxwell’s equations. We employ the scattering matrix method to calculate the emission by dipoles inside an anisotropic microcavity. The simulation method is applied to calculate the emission of dipole antennas in a number of cases: a dipole antenna in an infinite medium, emission into anisotropic slab waveguides and waveguides in liquid crystals. The dependency of the intensity and the polarization on the direction of emission is illustrated for a number of anisotropic microcavities.

https://biblio.ugent.be/publication/1899397/file/3145646.pdf

Dipole radiation within one-dimensional anisotropic microcavities: a simulation method.

It is shown that for calculating the effective surface recombination velocity, the drop of the minority carrier quasi-Fermi level in the space charge region should be taken into account. It is then found that a constant surface recombination velocity, independent of the excitation level, is valid up to a relatively high excitation level.

Comment on G.J. Rees “surface recombination velocity—a useful concept?”

This paper discusses contrast degradation in projection displays that use the Digital Micromirror DeviceTM (DMDTM) of Texas Instruments. Various possible mechanisms for the degradation of the contrast ratio are investigated by using optical ray-trace simulations and performing some experiments. It is found that the scattering of light at the DMD is the principal source of contrast degradation. This scattering is implemented in our simulations of the DMD projector in the optical ray-trace program ASAP. This enables us to simulate the contrast ratio in a realistic way. Two methods to improve the contrast ratio are proposed. Both methods, however, reduce the total light transmission of the projector. These methods are illustrated with our simulation model, and quantitative results are given.

Patrick De Visschere

Papers

Semitransparent metal or distributed Bragg reflector for wide-viewing-angle organic light-emitting-diode microcavities

Atomic layer deposition of ZnS thin films based on diethyl zinc and hydrogen sulfide

Dipole radiation within one-dimensional anisotropic microcavities: a simulation method.

Comment on G.J. Rees “surface recombination velocity—a useful concept?”

Contrast-improving methods for Digital Micromirror Device projectors