Atomic layer deposition: an overview.

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

/pdf/science-and-technology-roadmap-for-graphene-related-two-4q9m7czlkc.pdf

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

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

The scaling of complementary metal oxide semiconductor transistors has led to the silicon dioxide layer, used as a gate dielectric, being so thin (14?nm) that its leakage current is too large It is necessary to replace the SiO2 with a physically thicker layer of oxides of higher dielectric constant (?) or 'high K' gate oxides such as hafnium oxide and hafnium silicate These oxides had not been extensively studied like SiO2, and they were found to have inferior properties compared with SiO2, such as a tendency to crystallize and a high density of electronic defects Intensive research was needed to develop these oxides as high quality electronic materials This review covers both scientific and technological issues?the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure and reactions, their electronic structure, bonding, band offsets, electronic defects, charge trapping and conduction mechanisms, mobility degradation and flat band voltage shifts The oxygen vacancy is the dominant electron trap It is turning out that the oxides must be implemented in conjunction with metal gate electrodes, the development of which is further behind Issues about work function control in metal gate electrodes are discussed

https://iopscience.iop.org/article/10.1088/0034-4885/69/2/R02/pdf

High dielectric constant gate oxides for metal oxide Si transistors

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

A chemical approach to atomic layer deposition (ALD) of oxide thin films is reported here. Instead of using water or other compounds for an oxygen source, oxygen is obtained from a metal alkoxide, which serves as both an oxygen and a metal source when it reacts with another metal compound such as a metal chloride or a metal alkyl. These reactions generally enable deposition of oxides of many metals. With this approach, an alumina film has been deposited on silicon without creating an interfacial silicon oxide layer that otherwise forms easily. This finding adds to the other benefits of the ALD method, especially the atomic-level thickness control and excellent uniformity, and takes a major step toward the scientifically challenging and technologically important task of replacing silica as the gate dielectric in the future generations of metal oxide semiconductor field effect transistors.

https://science.sciencemag.org/content/sci/288/5464/319.full.pdf

Atomic Layer Deposition of Oxide Thin Films with Metal Alkoxides as Oxygen Sources

Dielectric thin films applicable, for instance, as insulating layers in electroluminescent display devices have been studied. In order to improve dielectric characteristics HfO2–Ta2O5 nanolaminates were prepared by atomic layer epitaxy at 325 °C. The nanolaminates were evaluated in capacitance and current–voltage measurements. By optimizing the layer thicknesses in the nanolaminate structures the dielectric properties, especially leakage current densities, could be tailored remarkably. The best nanolaminates showed charge storage factors improved up to 8 times when compared with those of the single oxide films. The presence of nanosize crystallites of monoclinic and metastable tetragonal HfO2 was observed by x‐ray diffraction analysis.

Tailoring the dielectric properties of HfO2–Ta2O5 nanolaminates

Ta{sub 2}O{sub 5} thin films have been used in a number of applications such as dielectric materials for storage capacitors and thin film transistors, antireflection coatings for solar cells, optical waveguides, and filters chemical sensors, as well as corrosion resistant materials. The deposition of thin Ta{sub 2}O{sub 5} films by atomic layer epitaxy (ALE) was investigated in the temperature range of 150 to 450 C using Ta(OC{sub 2}H{sub 5}){sub 5} and water as precursors. Because of the thermal self-decomposition of Ta(OC{sub 2}H{sub 5}){sub 5} the self-limiting ALE growth was achieved only below 350 C. All the films grown were amorphous as examined by X-ray diffraction analysis. The films grown at 250 and 325 C were stoichiometric within the accuracy of Rutherford backscattering spectrometry and contained 4 and 0.6 atom percent (a/o) hydrogen as determined by nuclear reaction analysis, respectively. Except for the outermost surface, the content of carbon residues was below 3 a/o as analyzed by X-ray photoelectron spectroscopy. The films exhibited smooth surfaces as observed by scanning electron microscopy and relatively uniform thicknesses with 7% deviation in the gas flow direction. The refractive index of the films increased with deposition temperature stabilizing at 2.23 at temperatures higher than 300more » C. The permittivities for the films grown at 250 and 325 C were 21 and 25, respectively, and leakage current densities at 1 MV/cm electric field were 4.0 and 2.3 mA/cm{sup 2}, respectively.« less

https://iopscience.iop.org/article/10.1149/1.2048637/pdf

Atomic Layer Epitaxy Growth of Tantalum Oxide Thin Films from Ta ( OC 2 H 5 ) 5 and H 2 O

Polycrystalline monoclinic HfO2 films were atomic layer deposited on Si(100) substrates by a nonhydrous carbon-free process of HfI4 and O2. The oxygen to hafnium ratio corresponded to the stoichiometric dioxide within the limits of accuracy of ion beam analysis. A 1.5–2.0 nm thick SiO2 interface layer formed between the HfO2 films and Si substrates. Hysteresis of the capacitance–voltage curves was observed in Al/HfO2/p-Si(100) structures with oxide grown in the substrate temperature range of 570–755 °C. The hysteresis ceased with an increase in O2 pressure. The effective permittivity of the dielectric layers varied between 12 and 16. The breakdown voltages were found to be lower in the case of higher oxygen doses and higher HfO2 deposition temperatures.

Properties of hafnium oxide films grown by atomic layer deposition from hafnium tetraiodide and oxygen

TaN, Ta3N5, and TaOxNy films were deposited by the atomic layer deposition technique. The alternate surface reactions between TaCl5 and NH3 resulted in Ta3N5 films, but when elemental zinc, serving as an additional reducing agent, was supplied on the substrates between the TaCl5 and NH3 pulses, TaN with a resistivity of 9 × 10-4 Ω cm was obtained. TaOxNy films were grown by depositing first thin Ta3N5 layers which were then partially oxidized by single water pulses. By varying the number of the Ta3N5 deposition cycles between the water pulses, the oxygen-to-nitrogen ratio of the films was controlled. The permittivity of the TaOxNy films was around 30 which is somewhat higher than that of Ta2O5.

Kaupo Kukli

Papers

Atomic Layer Deposition of Oxide Thin Films with Metal Alkoxides as Oxygen Sources

Tailoring the dielectric properties of HfO2–Ta2O5 nanolaminates

Atomic Layer Epitaxy Growth of Tantalum Oxide Thin Films from Ta ( OC 2 H 5 ) 5 and H 2 O

Properties of hafnium oxide films grown by atomic layer deposition from hafnium tetraiodide and oxygen

Controlled Growth of TaN, Ta3N5, and TaOxNy Thin Films by Atomic Layer Deposition