Conformal Film Growth Research Articles

Modification of High-Surface-Area Carbons Using Self-Limited Atomic Layer Deposition

This study explores the application of Atomic Layer Deposition (ALD) to functionalize high-surface-area carbon supports with metal and metal oxide films and particles for applications in catalysis and electrocatalysis. The work reported here demonstrates that, through careful choice of precursors and absorption and reaction conditions, self-limited ALD growth on a high-surface-area carbon support can be achieved. Specific examples presented include the growth of conformal films of ZrO2 and SnO2 and the deposition of Ga2O3 and Pt particles on a carbon black support with a surface area of 250 m2·g−1. A novel strategy for controlling the Pt weight loading and producing sub-nanometer Pt particles on a carbon support using a single ALD cycle is also presented.

Electromagnetically Enhanced Atomic Layer Deposition for Modification of Thin Film Properties through in Situ Substrate Biasing

Atomic layer deposition (ALD) is broadly used for thin, conformal film growth in microelectronics, photovoltaics, and other industries. Enhanced film properties are required for advancement of device performance. Use of electrical current in situ during deposition to drive material properties of films produced by ALD toward a desired state (such as higher conductivity) will increase the utility of thin film materials for device applications. In an ALD reactor bias is applied directly to substrates during ALD precursor pulses. Higher conductivity and growth rates were observed for samples grown with ALD enhanced by in situ substrate biasing. Thicknesses were determined via spectroscopic ellipsometry, uniformity was investigated by atomic force microscopy, and crystallinity by x-ray diffraction. These systems are computationally investigated to probe the mechanism by which the ALD processes are enhanced.

Preparation of high-performance large-area perovskite solar cells by atomic layer deposition of metal oxide buffer layer

Perovskite solar cells have been widely recognized as the most promising new-type photovoltaic device due to its power conversion efficiency rapidly increasing from 3.8% to over 26% in merely fifteen years. However, the high performances are achieved mainly on small area cells with an active area lower than 0.1 cm<sup>2</sup>. When enlarging the active area of perovskite solar cells, the efficiency falls dramatically. So, how to reduce the gap between performances of small area cells and large area cells gradually becomes a critical point in the path towards the commercialization of perovskite photovoltaic technology. Herein, a strategy of pre-growing thin layer of TiO<sub>2</sub> on a rough FTO substrate by atomic layer deposition method before spin-coating SnO<sub>2</sub> nanoparticles is proposed. Due to the inherent conformal film growth mode of atomic layer deposition, the FTO substrate can be completely covered by TiO<sub>2</sub>, thus preventing the direct contact between local protrusions of FTO and perovskite layer and impeding the current leakage phenomenon, which can be verified by the measurements from X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy, and further proved by the dark current measurement. By using this method, the repeatability and consistency of the small area cell fabrication technology on the same substrate are improved obviously. The improved electron transport process revealed by photoluminescence results and incident light management process revealed by external quantum efficiency results also brings about better solar cell performances. More importantly, highly efficient 0.5 cm<sup>2</sup> large area perovskite solar cells are fabricated through optimization of TiO<sub>2</sub> thickness. When growing 200 cycles TiO<sub>2</sub> (~9 nm in thickness) by using atomic layer deposition technology, the champion large area perovskite solar cell possesses a power conversion efficiency as high as 24.8% (certified 24.65%). The device performances also show excellent repeatability between different fabrication batches. The perovskite solar cell with TiO<sub>2</sub> buffer layer grown by the atomic layer deposition method can still retain over 95% of its initial efficiency after having been stored in a nitrogen atmosphere for 1500 h. The technique proposed in this paper can be helpful in manufacturing perovskite solar cell modules in the realistic photovoltaic market and can be extended to the large area fabrication of other perovskite optoelectronic devices such as light emitting diode, laser and detector.

Synthesis of high-surface area tungstated zirconia by atomic layer deposition on mesoporous silica

Atomic layer deposition (ALD) was used to prepare ZrO 2 films on the surface of the mesoporous silica, SBA-15, and to modify the surface of these films with WO 3 in order to form tungstated zirconia. Adsorption-desorption isotherms, pore size distributions, and transmission electron microscopy demonstrated that the ALD synthesis produced zirconia films that were conformal to the SBA-15 pores. DRIFT spectroscopy of pyridine adsorbed on the tungstated-zirconia SBA-15 samples showed adsorbed pyridinium ions, confirming the presence of Brønsted-acid sites on this material, consistent with what has been reported for bulk tungstated zirconia. The ALD-synthesized, tungstated-zirconia SBA-15 was also shown to be active in the acid-catalyzed H-D exchange between toluene and D 2 O. • ALD growth of conformal films of zirconia and tungstated zirconia on SBA-15 was demonstrated. • Ultrathin ZrO 2 films significantly enhanced the thermal stability of the SBA-15. • ALD-grown tungstated zirconia on SBA-15 contained Brønsted acid sites similar to those on the corresponding bulk material.

Nucleation and growth behavior of aluminum nitride film using thermal atomic layer deposition

Aluminum nitride (AlN) film possesses wide band gap energy (~6.2 eV) and a high dielectric constant (~9.2), and is resilient to thermal and chemical stimuli. It also exhibits several functionalities, such as piezoelectricity and pyroelectricity. Therefore, AlN film has been used for electronic and optoelectronic devices and micro-electromechanical systems (MEMSs). Among the various methods of AlN thin film growth, atomic layer deposition (ALD) can control film thickness at the nanoscale. Uniform and conformal film growth is possible at temperatures lower than that of chemical vapor deposition or molecular beam epitaxy. Because the ALD process relies on surface chemical reactions, it shows substrate dependency. To control film uniformity from the beginning, an understanding of nucleation and growth behavior on the substrate is necessary. Therefore, the nature of nucleation and growth behaviors on different substrates is investigated. In this study, AlN films are grown on bare Si and TiN substrates at 295–342 °C by thermal ALD using trimethyl aluminum (TMA) and ammonia. Facile nucleation and linear growth on the TiN substrate, and substrate-inhibited nucleation on the Si substrate, are observed. NH3 pretreatment may enhance the growth rate at the nucleation stage. Therefore, the dissociation of NH3 on the substrate is crucial to making uniform nuclei for the subsequent growth of AlN film.

Conformal BaTiO3 Films with High Piezoelectric Coupling through an Optimized Hydrothermal Synthesis.

Two-dimensional (2D) ferroelectric films have vast applications due to their dielectric, ferroelectric, and piezoelectric properties that meet the requirements of sensors, nonvolatile ferroelectric random access memory (NVFeRAM) devices, and micro-electromechanical systems (MEMS). However, the small surface area of these 2D ferroelectric films has limited their ability to achieve higher memory storage density in NVFeRAM devices and more sensitive sensors and transducer. Thus, conformally deposited ferroelectric films have been actively studied for these applications in order to create three-dimensional (3D) structures, which lead to a larger surface area. Most of the current methods developed for the conformal deposition of ferroelectric films, such as metal-organic chemical vapor deposition (MOCVD) and plasma-enhanced vapor deposition (PECVD), are limited by high temperatures and unstable and toxic organic precursors. In this paper, an innovative fabrication method for barium titanate (BaTiO3) textured films with 3D architectures is introduced to alleviate these issues. This fabrication method is based on converting conformally grown rutile TiO2 nanowire arrays into BaTiO3 textured films using a simple two-step hydrothermal process which allows for thickness-controlled growth of conformal films on patterned silicon wafers coated with fluorine-doped tin oxide (FTO). Moreover, the processing parameters have been optimized to achieve a high piezoelectric coupling coefficient of 100 pm/V. This high piezoelectric response along with high relative dielectric constant (εr = 1600) of the conformally grown textured BaTiO3 films demonstrates their potential application in sensors, NVFeRAM, and MEMS.

Development of atomic layer deposition-activated microchannel plates for single particle detection at cryogenic temperatures

Atomic layer deposition (ALD) technology is used to nanoengineer functional films inside the pores of microchannel plate (MCP) electron multipliers, enabling a novel MCP manufacturing technology that substantially improves performance and opens novel applications. The authors have developed custom tools and recipes for the growth of conformal films, with optimized conductance and secondary electron emission inside very long channels (∼6–20 μm diameter and >600 μm length, with tens of millions of channels per single MCP) by ALD. The unique ability to tune the characteristics of these ALD films enables their optimization to applications where time-resolved single particle imaging can be performed in extreme conditions, such as high counting rates at cryogenic temperatures. Adhesion of the conductive and emissive nanofilms to the 20 μm pore MCP glass substrates and their mechanical stability over a very wide range of temperatures (10–700 K) were confirmed experimentally. Resistance of ALD MCPs was reproducible during multiple cool-down cycles with no film degradation observed. Optimizing resistance of novel MCPs for operation at cryogenic temperature should enable high count rate event detection at temperatures below 20 K.

Analysis of high- Q, gallium nitride nanowire resonators in response to deposited thin films

Gallium nitride nanowires (GaN-NWs) are systems of interest for mechanical resonance-based sensors due to their small mass and, in the case of c-axis NWs, high mechanical quality ( Q) factors of 10,000–100,000. We report on singly-clamped NW mechanical cantilevers of roughly 100 nm diameter and 15 μm length that resonate near 1 MHz and describe the behavior of GaN-NW resonant frequencies and Q factors following coating with various materials deposited by atomic layer deposition (ALD), including alumina (Al 2O 3), ruthenium (Ru), and platinum (Pt). Changes in the GaN-NW resonant frequencies with ALD deposition clearly distinguish conformal film growth versus island film growth. Conformal films lead to a stiffening of the NW and typically increase resonant frequency, whereas island films simply increase the NW mass and cause decreased resonant frequencies. We find that conformal growth of ALD alumina leads to stiffening of ∼4 kHz per nm of alumina, in agreement with previously measured material properties. Conformal growth of Ru and Pt, respectively, qualitatively confirm our analytical predictions of positive and negative resonant frequency shifts. Island growth of ALD Ru has demonstrated a decrease in resonant frequency consistent with mass loading of ∼0.2 fg for a 150 ALD-cycle film, also consistent with analytical predictions. Resonant Q factors are found to decrease with ALD film growth, offering the additional possibility of studying mechanical dissipation processes associated with the ALD-NW composite structures.

Highly conformal film growth by chemical vapor deposition. I. A conformal zone diagram based on kinetics

The authors present a unified description of conformal film growth in recessed features by low pressure chemical vapor deposition. Experimental data on step coverage and growth rate are interpreted in terms of a kinetic model for the elementary surface processes. This model is combined with the continuity equation for gas transport and consumption to predict the values of the precursor pressure and substrate temperature (p,T) that are necessary for conformal coverage to occur for a given precursor. They introduce a conformal zone diagram that considers the gas phase limitations (maximum precursor pressure and onset of secondary reactions), the surface limitations (minimum reaction temperature, precursor adsorption and desorption rates, and film growth rate), and the aspect ratio of the feature to be coated; the allowed intersection between the resulting boundaries defines the (p,T) zone for conformal growth. Sufficient precursor pressure is identified as a major requirement for conformal film growth, one that is not met by many commercially available molecules. Finally, they derive an approximate analytical solution to the inverse problem: what are the experimental conditions necessary to afford a desired degree of step coverage on a given aspect ratio feature at a desired growth rate. The solution depends on a set of rate constants in the kinetic model that can be extracted from measurements of the film growth rate as a function of precursor pressure and substrate temperature on trench or planar substrates.

Highly conformal film growth by chemical vapor deposition. II. Conformality enhancement through growth inhibition

The authors present a novel strategy for enhancing conformality in chemical vapor deposition (CVD) based on the concept of growth inhibition. In Part I, they showed how surface site blocking was responsible for the increase in conformality observed at higher pressures in high vapor pressure precursors. In this work, they apply this concept to enhance conformality by considering a secondary species that acts as an inhibitor of the precursor. The experimental results obtained for the growth of TiB2 from Ti(BH4)3dme (dme=dimethoxyethane), with dme acting as the growth inhibitor, agree well with the models, based on a first order adsorption/desorption kinetics, and show how this strategy greatly enhances the conformality of nonconformal precursors. Finally, they show how multiple surface mechanisms including surface site blocking, associative desorption, long surface residence time of by-products, or coadsorption of two reactants can induce growth inhibition. Consequently, this strategy can be potentially applied to many existing CVD systems, provided suitable molecular inhibitors are identified.

Highly Conformal Film Growth by Chemical Vapor Deposition: A Zone Diagram Based on Kinetics

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Large Scaled ALD/PECVD Reactor for Flat Panel Display Application

High quality SiO2 gate insulators are one of the key technologies for improving performance of Low Temperature Poly Silicon TFTs. We have developed an ALD/PECVD reactor, which handles 370 mm×470 mm glass substrates. The reactor has a unique plasma source of the monopole antenna, which is the electromagnetic wave coupled type and operates at VHF range. A PE-ALD SiO2 film was deposited at 200{degree sign} with alternating exposures of aminosilane and radical oxygen generated by the plasma. Growth rate of the film and thickness variation were 0.1 nm cycle-1 and less than {plus minus}5 %, respectively. Conformal film growth was observed on patterned substrates. Uniform current-voltage performance was confirmed on the glass and its electrical field was higher than 8.0 MVcm-1 at a leakage current of 1×10-6 Acm-2. The film had an excellent Si / SiO2 interface trap density of lower than 1×1011 cm-2eV-1 after forming gas annealing.

High filling fraction gallium phosphide inverse opals by atomic layer deposition

High filling fraction gallium phosphide (GaP) inverse opals were fabricated by atomic layer deposition within the void spaces of silica colloidal crystal templates. Depositions were performed from 400to500°C using trimethylgallium and tris(dimethylamino)phosphine precursors. The resulting films were characterized by optical reflectance, which indicated infiltration as high as 100% of the conformal film growth maximum, corresponding to a volume filling fraction of 0.224. X-ray diffraction measurements confirmed the crystallinity of the film. These results indicate the fabrication of three-dimensional photonic crystals using a III-V optoelectronic material with sufficient dielectric contrast to form a full photonic band gap in the visible.

A predictive tool in micro- and nanoengineering: Straightforward estimation of conformal film growth efficiency

It is shown that the conformal film growth (CFG) efficiency (εCFG) of micro- and nanoscale cavities can be rationalized in terms of relevant physical and geometric parameters. Based on extensive numerical simulations and using the dynamic scaling theory of interface growth, it is concluded that the relevant quantity for the evaluation of εCFG is the so-called physical aspect ratio SP=L/Mβ/α, where α (β) is the roughness (growth) exponent that governs the dynamic evolution of the system, and L (M) is the typical depth (width) of the cavity. The theoretical predictions are in excellent agreement with recently reported experimental data for the super conformal filling of electrodeposited copper and chemically deposited silver in confined geometries [J. Electrochem. Soc. 147, 4524 (2000)], thus providing the basis of a tool to manage micro- and nano-engineering-related problems not completely resolved so far.