Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Knez M.

Atomic layer deposition (ALD) is a mature technology for the deposition of conformal thin films. During the ALD process or in a post-treatment, a variety of diffusion phenomena can occur which can not only deteriorate the desired product, but also can be used to fabricate materials or structures in a novel way. This special issue reviews some of the observed diffusion processes and strategies to make use of those. (Some figures may appear in colour only in the online journal)

Wiemer C., Lamagna L., Fanciulli M.

Atomic layer deposition (ALD) has been established as a powerful method for the growth of very thin and conformal films to be used in ultra-scaled conventional and novel microelectronic devices. We report the most recent advancements in the field of ALD of rare-earth-based oxides to be implemented as active dielectrics. The review is balanced between the development of new ALD processes and the assessment and the discussion of fundamental scientific issues related to the structural, chemical and electrical properties of thin films of rare-earth-based oxides. The deposition process of binary lanthanide oxides is critically reviewed focusing on the first (La) and last (Lu) element of the series. Concomitantly, the integration of rare earth elements as dopant atoms in HfO2?and ZrO2?is also systematically reported. A final overview is dedicated to the results obtained by ALD of more innovative lanthanum-based ternary oxides.

Elliott S.D.

Published papers on atomic-scale simulation of the atomic layer deposition (ALD) process are reviewed. The main topic is reaction mechanism, considering the elementary steps of precursor adsorption, ligand elimination and film densification, as well as reactions with substrates (particularly Si and SiO2) and CVD-like decomposition at the surface. Density functional theory is the first principles method generally applied to these mechanistic questions. The most popular subject for modelling is the ALD of oxides and nitrides, particularly the high-k dielectrics HfO2, ZrO2?and Al2O3, due to their importance in semiconductor processing.

van Delft J.A., Garcia-Alonso D., Kessels W.M.

Atomic layer deposition (ALD) is a vapour-phase deposition technique capable of depositing high quality, uniform and conformal thin films at relatively low temperatures. These outstanding properties can be employed to face processing challenges for various types of next-generation solar cells; hence, ALD for photovoltaics (PV) has attracted great interest in academic and industrial research in recent years. In this review, the recent progress of ALD layers applied to various solar cell concepts and their future prospects are discussed. Crystalline silicon (c-Si), copper indium gallium selenide (CIGS) and dye-sensitized solar cells (DSSCs) benefit from the application of ALD surface passivation layers, buffer layers and barrier layers, respectively. ALD films are also excellent moisture permeation barriers that have been successfully used to encapsulate flexible CIGS and organic photovoltaic (OPV) cells. Furthermore, some emerging applications of the ALD method in solar cell research are reviewed. The potential of ALD for solar cells manufacturing is discussed, and the current status of high-throughput ALD equipment development is presented. ALD is on the verge of being introduced in the PV industry and it is expected that it will be part of the standard solar cell manufacturing equipment in the near future.

Zaera F.

Atomic layer deposition (ALD) is one of the most promising methodologies available for the growth of solid thin films conformally on complex topographies and with atomic-level control on thickness. However, as a chemical process, ALD can lead to the incorporation of impurities and to the growth of poor-quality films. Here we discuss some possible complications associated with the chemistry of ALD, including its ill-defined stoichiometry, the stepwise and extensive surface conversion possible with the ligands of most ALD metalorganic precursors, the need for the reduction or oxidation of the deposited elements, the poor understanding of the role of the coreactants, the dominant activity of specific minority surface sites in starting ALD processes, and the development of complex layered or three-dimensional structures within the deposited films. The resolution of these issues should help with the development of a more systematic approach for the selection of ALD precursors and for the design of ALD processes.

Im H., Wittenberg N.J., Lindquist N.C., Oh S.

Although atomic layer deposition (ALD) has been used for many years as an industrial manufacturing method for microprocessors and displays, this versatile technique is finding increased use in the emerging fields of plasmonics and nanobiotechnology. In particular, ALD coatings can modify metallic surfaces to tune their optical and plasmonic properties, to protect them against unwanted oxidation and contamination, or to create biocompatible surfaces. Furthermore, ALD is unique among thin film deposition techniques in its ability to meet the processing demands for engineering nanoplasmonic devices, offering conformal deposition of dense and ultrathin films on high-aspect-ratio nanostructures at temperatures below 100 °C. In this review, we present key features of ALD and describe how it could benefit future applications in plasmonics, nanosciences, and biotechnology.

Peng Q., Lewis J.S., Hoertz P.G., Glass J.T., Parsons G.N.

Clean renewable energy sources (e.g., solar, wind, and hydro) offers the most promising solution to energy and environmental sustainability. On the other hand, owing to the spatial and temporal variations of renewable energy sources, and transportation and mobility needs, high density energy storage and efficient energy distribution to points of use is also critical. Moreover, it is challenging to scale up those processes in a cost-effective way. Electrochemical processes, including photoelectrochemical devices, batteries, fuel cells, super capacitors, and others, have shown promise for addressing many of the abovementioned challenges. Materials with designer properties, especially the interfacial properties, play critical role for the performance of those devices. Atomic layer deposition is capable of precise engineering material properties on atomic scale. In this review, we focus on the current state of knowledge of the applications, perspective and challenges of atomic layer deposition process on the electrochemical energy generation and storage devices and processes.

Knoops H.C., Donders M.E., van de Sanden M.C., Notten P.H., Kessels W.M.

Nanostructuring is targeted as a solution to achieve the improvements required for implementing Li-ion batteries in a wide range of applications. These applications range in size from electrical vehicles down to microsystems. Atomic layer deposition (ALD) could be an enabling technology for nanostructured Li-ion batteries as it is capable of depositing ultrathin films (1–100 nm) in complex structures with precise growth control. The potential of ALD is reviewed for three battery concepts that can be distinguished, i.e., particle-based electrodes, 3D-structured electrodes, and 3D all-solid-state microbatteries. It is discussed that a large range of materials can be deposited by ALD and recent demonstrations of battery improvements by ALD are used to exemplify its large potential.

Parsons G.N., George S.M., Knez M.

This article reviews and assesses recent progress in atomic layer deposition (ALD) and highlights how the field of ALD is expanding into new applications and inspiring new vapor-based chemical reaction methods. ALD is a unique chemical process that yields ultra-thin film coatings with exceptional conformality on highly non-uniform and non-planar surfaces, often with subnanometer scale control of the coating thickness. While industry uses ALD for high-κ dielectrics in the manufacturing of electronic devices, there is growing interest in low-temperature ALD and ALD-inspired processes for newer and more wide-ranging applications, including integration with biological and synthetic polymer structures. Moreover, the conformality and nanoscale control of ALD film thickness makes ALD ideal for encapsulation and nano-architectural engineering. Articles in this issue of MRS Bulletin present details of several growing interest areas, including the extension of ALD to new regions of the periodic table, and molecular layer deposition and vapor infiltration for synthesis of organic-based thin films. Articles also discuss ALD for nanostructure engineering and ALD for energy applications. A final article shows how the challenge of scaling ALD for high rate nanomanufacturing will push advances in plasma, roll-to-roll, and atmospheric pressure ALD.

(Erwin) Kessels W.M., Putkonen M.

As applications of atomic layer deposition (ALD) in emerging areas such as nanoelectronics, photovoltaics, and flexible electronics expand beyond single-wafer semiconductor processing, there is a growing need for novel approaches to integrate new process designs, substrate materials, and substrate delivery methods. An overview is given of new means to extend the capabilities of ALD and to improve the speed and simplicity of ALD coatings using new reactor designs. These include energy-enhanced and spatial ALD schemes involving plasma, direct-write, atmospheric pressure, and roll-to-roll processing. The long-term goal of this work is to integrate viable high-throughput capabilities with ALD processes.

Bae C., Shin H., Nielsch K.

Atomic layer deposition (ALD) not only presents a direct way to prepare nanomaterials when combined with templates, but also allows surface engineering to fine-tune the properties of the material. Here, we review recent progress in the field of nanostructured materials and devices that have been fabricated by ALD. Various materials, including semiconducting, magnetic, noble metallic, and insulating materials, can be used to form three-dimensional (3D), complex nanostructures with controlled composition and physical properties. We begin this review with ALD nanomaterials that can be prepared from porous templates with a 2D pore arrangement, such as anodic aluminum oxide, and advance toward opal structures with a 3D pore arrangement. We also discuss surface engineering by ALD on existing nanowires/nanotubes, devices, and chemical patterns that has the potential for application in high-performance transistors, sensors, and green energy conversion. Finally, we provide perspectives for future device applications that could arise from ALD nanomaterials.

Leskelä M., Ritala M., Nilsen O.

Over the past 10 years, the number of materials that can be processed by atomic layer deposition (ALD) has expanded rapidly. Significant progress has been seen in ALD of high-κ oxides, ternary oxides, and noble metals, which have been studied quite extensively. High-κ oxide processes are used today in various industrial applications. However, many new applications are pushing the need for less common compounds, and therefore new processes are being developed (e.g., for fluorides, Li containing compounds, and phosphates). New ALD processes require new designs for volatile precursors to deliver elements with ligands that ensure self-limiting surface reactions. In addition to inorganics, new polymeric and inorganic-organic hybrid materials are opening new frontiers for ALD, including expansion of the process to include molecular layer deposition. A combination of inorganic and organic parts in the deposited layers offers expanding opportunities for tailoring materials properties.

Elam J.W., Dasgupta N.P., Prinz F.B.

Atomic layer deposition (ALD) uses self-limiting chemical reactions between gaseous precursors and a solid surface to deposit materials in a layer-by-layer fashion. This process results in a unique combination of attributes, including sub-nm precision, the capability to engineer surfaces and interfaces, and unparalleled conformality over high-aspect ratio and nanoporous structures. Given these capabilities, ALD could play a central role in achieving the technological advances necessary to redirect our economy from fossil-based energy to clean, renewable energy. This article will survey some of the recent work applying ALD to clean energy conversion, utilization, and storage, including research in solid oxide fuel cells, thin-film photovoltaics, lithium-ion batteries, and heterogenous catalysts. Throughout the manuscript, we will emphasize how the unique qualities of ALD can enhance device performance and enable radical new designs.

George S.M., Lee B.H., Yoon B., Abdulagatov A.I., Hall R.A.

Hybrid organic-inorganic films can be deposited using atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques. A special set of hybrid organic-inorganic films based on metal precursors and various organic alcohols yields metal alkoxide films that can be described as "metalcones." Many metalcone films are possible such as the "alucones" and "zincones" based on the reaction of trimethylaluminum and diethylzinc, respectively, with various organic alcohols such as ethylene glycol (EG). This paper reviews the previous work on metalcone MLD and discusses a variety of new metalcone systems. "Titanicones" are grown using TiCl4 and glycerol or EG and "zircones" are grown using zirconium tetra-tert-butoxide and EG. In addition, the organic alcohol can also be varied to change the properties within one metalcone family. For example, the glycerol triol precursor allows for more cross-linking and higher toughness in alucones than the EG diol precursor. Alloys can also be formed by combining metalcone MLD and metal oxide ALD. By varying the relative number of cycles of MLD and ALD, the composition and properties of the hybrid organic-inorganic films can be tuned from pure metalcone MLD to pure metal oxide ALD.

Puurunen R.L., Sajavaara T., Santala E., Miikkulainen V., Saukkonen T., Laitinen M., Leskelä M.

The surface roughness of thin films is an important parameter related to the sticking behaviour of surfaces in the manufacturing of microelectomechanical systems (MEMS). In this work, TiO2 films made by atomic layer deposition (ALD) with the TiCl4-H2O process were characterized for their growth, roughness and crystallinity as function of deposition temperature (110-300 degrees C), film thickness (up to approximately 100 nm) and substrate (thermal SiO2, RCA-cleaned Si, Al2O3). TiO2 films got rougher with increasing film thickness and to some extent with increasing deposition temperature. The substrate drastically influenced the crystallization behaviour of the film: for films of about 20 nm thickness, on thermal SiO2 and RCA-cleaned Si, anatase TiO2 crystal diameter was about 40 nm, while on Al2O3 surface the diameter was about a micrometer. The roughness could be controlled from 0.2 nm up to several nanometers, which makes the TiO2 films candidates for adhesion engineering in MEMS.

Arandia A., Velasco J.A., Sajid A., Yim J., Shamshad H., Jiang H., Chahal A., Singh A.K., Gonsalves C., Karinen R., Puurunen R.L.

Schmidl G., Pezoldt M., Jia G., Dellith A., Simon A., Ritter U., Voigt I., Plentz J.

Matsushita Y., Inoue G., Zhao Z., Ogawa N., Ishiguro H., Sunada K., Ishibashi K., Kojima H., Shimizu T., Shingubara S., Ito T.

Yoo J., Kwon Y., Seong N., Choi K., Yang J., Hwang C., Yoon S.

Sharma S., Rani M., Shanker U.

Cho Y., D'Acunto G., Nanda J., Bent S.F.

Abstract The use of atomic layer deposition (ALD) and molecular layer deposition (MLD) in energy sectors such as catalysis, batteries, and membranes has emerged as a growing approach to fine-tune surface and interfacial properties at the nanoscale, thereby enhancing performance. However, compared to the microelectronics field where ALD is well established on conventional substrates such as silicon wafers, employing ALD and MLD in energy applications often requires depositing films on unconventional substrates such as nanoparticles, secondary particles, composite electrodes, membranes with wide pore size distribution, and two-dimensional materials. This review examines the challenges and perspectives associated with implementing ALD and MLD on these unconventional substrates. We discuss how the complex surface chemistries and intricate morphologies of these substrates can lead to non-ideal growth behaviors, resulting in inconsistent film properties compared to those grown on standard wafers, even within the same deposition process. Additionally, the review outlines the strengths and limitations of several characterization techniques when employed for ALD or MLD films grown on unconventional substrates, and it highlights a few exemplary studies in which these growth methods have been applied for energy applications with a focus on energy storage. With ALD and MLD gaining increasing attention, this review aims to deepen the understanding of how to achieve controllable, predictable, and scalable deposition with atomic-scale precision, ultimately advancing the development of more efficient and durable energy devices.

Popov G., Mattinen M., Vihervaara A., Leskelä M.

In this review, we highlight new atomic layer deposition (ALD) precursors and process chemistries based on the ALD database found in atomiclimits.com. The aim was to compare the processes before and after 2010 and see possible changes. The motivations for process development and trends in the types of different metal precursors are discussed. The total number of published thermal ALD processes is 1711, of which more than half (942) were published after 2010. The number of materials deposited by thermal ALD is 539, and for 312 of these, the process was published after 2010. The most popular material group are binary oxides. After 2010, the share of nonoxide and ternary materials slowly increased. During the last years, a few material classes have come forth, viz., metals, 2D transition metal dichalogenides, and halides. The development of new ALD processes is clearly application-driven and visible in these material classes, motivated by the most important application areas of ALD: Microelectronics, energy technology, and catalysis. New elements added to the portfolio after 2010 are alkali metals (Na, K, and Rb), Be, Re, Os, Au, and Sb, the first two as oxides and the latter four as metals. The processes for Re, Os, Au, and Sb were different: Reductive for Re, oxidative for Os and Au, and exchange reaction for Sb. ALD of transition metals has been of interest because of their potential use in microelectronics. New metal precursors and novel reducing agents play an important role in their process development. Metal halides, alkoxides, alkyl compounds, β-diketonates, and amides/imides have been traditional metal precursors in ALD. After 2010, amides/imides have been the most applied precursors in new ALD processes, followed by cyclopentadienyl compounds. However, heteroleptic complexes containing two or more ligands are the largest precursor type, and they usually consist of a mixture of the above-mentioned ligands. The use of heteroleptic compounds enables tuning of precursor properties such as volatility, reactivity, and stability.

Maslar J.E., Kalanyan B.

An absorption imaging technique was described for visualizing molybdenum pentachloride (MoCl 5 ) flow during an atomic layer deposition/pulsed chemical vapor deposition process. The imaging system was composed of a telecentric lens and a commercial 7.1 megapixels (MP) complementary metal oxide semiconductor (CMOS) camera. The light source was a fiber-coupled light emitting diode operating at a peak emission wavelength of 443  nm. Flow images of MoCl 5 vapor entrained in a carrier gas were recorded at approximately 93 frames per second in a research-grade vapor deposition chamber. The utility of this technique was illustrated by comparing the MoCl 5 flow patterns for two precursor injection conditions, conditions consisting of different argon carrier gas flow rate and chamber pressure. For a low flow rate and chamber pressure, the flow images showed a gradual expansion of the MoCl 5 concentration front through the field of view with a relatively short MoCl 5 residence time. These flow patterns result in a relatively uniform precursor concentration front impinging on the wafer surface with the precursor being efficiently exhausted from the chamber, making these conditions desirable for thin film deposition in this chamber. For a high carrier gas flow rate and elevated chamber pressure, the flow images showed a high gas velocity jet impinging on the wafer chuck surface and the formation of gas recirculation zones, resulting in a relatively long residence time. These flow conditions would make it difficult to reproducibly deposit uniform thin films in this chamber. This comparison demonstrated the utility of this technique for qualitative characterization of precursor flow fields with minimal data processing. However, the two-dimensional data obtained from this technique can also provide the basis for training and validating computational fluid dynamics models. Furthermore, the addition of duplicate optical systems would provide the basis for determining the three-dimensional precursor distribution through tomographic analysis.

Guo K., Yang X., Han J., Lu G., Wei L., Li P., Gao M.

Bayramlı H.M., Genç M., Yücel O., Bulut B., Bek A., Demirtaş M.

Abstract InGaN-based light-emitting diodes (LEDs) are at the forefront of solid-state lighting technologies due to their superior efficiency and broad spectral emission. However, their performance is often compromised by leakage currents, which lead to reduced external quantum efficiency. Passivation of surface defect, the need of which arises from either epitaxial growth or mesa etching, emerges as a promising strategy to mitigate leakage currents and enhance LED performance. This study compares the effects of different sidewall passivation using two dielectric materials, Al2O3 and SiO2, on the reliability and long-term stability performance of InGaN LEDs. The study conducts a comprehensive analysis to evaluate the impact of each material on reducing leakage current and improving overall device efficiency. The experimental findings of our study indicate that the LEDs with Al2O3 sidewall passivation have better long-term stability performance, lower series resistance, higher breakdown voltages, significantly lower leakage current, and up to a 19% increase in light output power compared to SiO2 sidewall passivation. These superior properties of Al2O3-passivated LEDs increase device reliability and stability. Conversely, SiO2-passivated LEDs demonstrate relatively higher leakage currents, which can be attributed to lower dielectric constant, non-uniform film deposition and incomplete defect passivation.

Tomašiūnas R., Mandl M., Reklaitis I., Malinauskas T., Stanionytė S., Paipulas D., Ritasalo R., Taeger S., Strassburg M., Sakoda K.

Merenda A., Gangadoo S., Johannessen B., Wilson K., Chapman J., Lee A.F.

Jones R., Kokkonen E., Eads C., Küst U.K., Prumbs J., Knudsen J., Schnadt J.

Dores B.S., Maciel M.J., Correia J.H., Vieira E.M.

In this work, we developed nanostructured Bi2Te3 and Sb2Te3 thin films by thermal co-evaporation of their alloys with corresponding pure elements (Bi, Sb, and Te). The films were fabricated on borosilicate glass at different substrate temperatures and deposition rates. At 300 °C, enhanced thermoelectric performance was demonstrated for n-type Bi2Te3:Bi and p-type Sb2Te3:Te, with Seebeck coefficients of 195 µV K−1 and 178 μV K−1, along with electrical conductivities of 4.6 × 104 (Ω m)−1 and 6.9 × 104 (Ω m)−1, resulting in maximum power factor values of 1.75 mW K−2 m−1 and 2.19 mW K−2 m−1, respectively. These values are found to be higher than some reported works in the literature, highlighting the advantage of not introducing additional elements to the system (such as extra doping, which induces complexity to the system). The structural properties, film morphology, and chemical composition of the optimized films were investigated using X-ray diffraction (XRD) and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS). The films were found to be polycrystalline with preferred (0 0 6) and (0 1 5) orientations for Bi2Te3 and Sb2Te3 films, respectively, and stable rhombohedral phases. Additionally, a ring-shaped p-n thermoelectric device for localized heating/cooling was developed and a temperature difference of ~7 °C between the hot and cold zones was obtained using 4.8 mA of current (J = 0.068 mA/mm2).

Melhem H., Hallais G., Amiri G., Patriarche G., Findling N., Van den Berg T., Ameziane H., Renard C., Sallet V., Vincent L.