Two-step Atomic Layer Deposition Research Articles

Scalable Large-Area 2D-MoS2/Silicon-Nanowire Heterostructures for Enhancing Energy Storage Applications.

Two-dimensional (2D) transition-metal dichalcogenides have shown great potential for energy storage applications owing to their interlayer spacing, large surface area-to-volume ratio, superior electrical properties, and chemical compatibility. Further, increasing the surface area of such materials can lead to enhanced electrical, chemical, and optical response for energy storage and generation applications. Vertical silicon nanowires (SiNWs), also known as black-Si, are an ideal substrate for 2D material growth to produce high surface-area heterostructures, owing to their ultrahigh aspect ratio. Achieving this using an industrially scalable method paves the way for next-generation energy storage devices, enabling them to enter commercialization. This work demonstrates large surface area, commercially scalable, hybrid MoS2/SiNW heterostructures, as confirmed by Raman spectroscopy, with high tunability of the MoS2 layers down to the monolayer scale and conformal MoS2 growth, parallel to the silicon nanowires, as verified by transmission electron microscopy (TEM). This has been achieved using a two-step atomic layer deposition (ALD) process, allowing MoS2 to be grown directly onto the silicon nanowires without any damage to the substrate. The ALD cycle number accurately defines the layer number from monolayer to bulk. Introducing an ALD alumina (Al2O3) interface at the MoS2/SiNW boundary results in enhanced MoS2 quality and uniformity, demonstrated by an order of magnitude reduction in the B/A exciton photoluminescence (PL) intensity ratio to 0.3 and a reduction of the corresponding layer number. This high-quality layered growth on alumina can be utilized in applications such as for interfacial layers in high-capacity batteries or for photocathodes for water splitting. The alumina-free 100 ALD cycle heterostructures demonstrated no diminishing quality effects, lending themselves well to applications that require direct electrical contact with silicon and benefit from more layers, such as electrodes for high-capacity ion batteries.

Synergy of Ultrathin CoOx Overlayer and Nickel Single Atoms on Hematite Nanorods for Efficient Photo-Electrochemical Water Splitting.

To solve surface carrier recombination and sluggish water oxidation kinetics of hematite (α-Fe2 O3 ) photoanodes, herein, an attractive surface modification strategy is developed to successively deposit ultrathin CoOx overlayer and Ni single atoms on titanium (Ti)-doped α-Fe2 O3 (Ti:Fe2 O3 ) nanorods through a two-step atomic layer deposition (ALD) and photodeposition process. The collaborative decoration of ultrathin CoOx overlayer and Ni single atoms can trigger a big boost in photo-electrochemical (PEC) performance for water splitting over the obtained Ti:Fe2 O3 /CoOx /Ni photoanode, with the photocurrent density reaching 1.05mA cm-2 at 1.23V vs. reversible hydrogen electrode (RHE), more than three times that of Ti:Fe2 O3 (0.326 mA cm-2 ). Electrochemical and electronic investigations reveal that the surface passivation effect of ultrathin CoOx overlayer can reduce surface carrier recombination, while the catalysis effect of Ni single atoms can accelerate water oxidation kinetics. Moreover, theoretical calculations evidence that the synergy of ultrathin CoOx overlayer and Ni single atoms can lower the adsorption free energy of OH* intermediates and relieve the potential-determining step (PDS) for oxygen evolution reaction (OER). This work provides an exemplary modification through rational engineering of surface electrochemical and electronic properties for the improved PEC performances, which can be applied in other metal oxide semiconductors as well.

(Invited) ALD of Hexagonal Boron Nitride: Towards h-BN/Carbon Van Der Waals 1D Heterostructures

Following the graphene isolation, strong interest in two dimensional (2D) materials has been driven by their outstanding properties. Their typical intrinsic structure, including strong in-plane covalent bonding and weak out-of-plane Van der Waals interaction, makes them highly promising in diverse areas such as electronics, catalysis, and environment. Growth of 2D materials and staking of such materials, called van der Waals (VdW) heterostructures, require a synthesis approach able to control the deposition onto a support at the atomic scale. Thanks to their simplicity, versatility and ability to control thickness at the angstrom level, Atomic Layer Deposition (ALD) and its variant Atomic Layer Etching (ALET) appear as ones of the most suited techniques to synthesize 2D materials and based-heterostructures. In line with the spectacular improvement and emergence of new physical properties in 2D VdW heterostructures, the curvature and confinement effects provided by the reduced dimensionality of 1D VdW heterostructures make them highly attractive.Herein, ALD of hexagonal Boron Nitride (hBN) is discussed towards fabrication of VdW heterostructures. After introducing the two-step ALD approach for BN, based on polymer derived ceramic chemistry, deposition of hBN onto various carbon nanomaterials is presented. The influence of the degree of graphitization of the carbon substrate on BN growth (nucleation delay) and structure (amorphous, turbostratic, hexagonal phase) is discussed. Indeed, inertness of highly graphitic carbon might inhibit the initiation of ALD growth. The obtained BN-coated carbon materials are characterized by advanced analytic transmission electron microscopy. Particular attention is paid here to the successful fabrication of 1D VdW heterostructures made of few layer hBN coated-single wall carbon nanotubes. These new 1D materials are of particular interest for applications ranging from sensing and quantum optics, to biological labeling.

Precise growth of Al2O3/SnO2/CNTs composites by a two-step atomic layer deposition and their application as an improved anode for lithium ion batteries

SnO2 is considered as a high capacity anode material for lithium ion batteries. However, the poor cycling stability has seriously hindered its practical applications. In this work, we designed and prepared Al2O3/SnO2/CNTs composites by uniformly and conformally depositing SnO2 and Al2O3 on carbon nanotubes (CNTs) via a simple two-step atomic layer deposition (ALD) process. The thicknesses of SnO2 and Al2O3 were precisely controlled by tuning the number of ALD-cycle to optimize the cycling performance of Al2O3/SnO2/CNTs electrodes. The ultrathin SnO2 layer with a thickness of approximate 2.8 nm on CNTs exhibits a high capacity and an enhanced cycling stability due to the buffering effect of CNTs and limited SnO2 crystallization after the first step deposition. On this basis, Al2O3/SnO2/CNTs electrodes exhibit a significantly improved cycling stability, rate capability and Coulombic efficiency due to surface stability and the structural integrity brought by Al2O3 passivation after the second step deposition. Especially, 10-Al2O3/SnO2/CNTs electrode deliveries a discharge capacity of 581.6 mAh g−1 at a current density of 100 mA g−1 and capacity retention is 90.85% against 2nd cycle after 200 charge-discharge cycles. We believe that this work provides an effective way for the design of high performance anode materials in advanced batteries.

Encapsulation of Fe3O4 between Copper Nanorod and Thin TiO2 Film by ALD for Lithium-Ion Capacitors.

Lithium-ion capacitors (LICs) are considered to be promising power sources due to their combination of high-rate capacitors and high-capacity batteries. However, development of a high-performance LIC is still restricted by the sluggish intercalation reaction and unsatisfied specific capacities in battery-type bulk anodes. To overcome these issues, herein, we utilize two-step atomic layer deposition (ALD) to realize a uniform coating of FeO x and TiO2 on CuO nanorods, which results in the formation of ternary CuO@FeO x@TiO2 composite. After further treatment in H2/Ar atmosphere, the as-derived Fe3O4 is encapsulated between conductive Cu nanorod and hollow TiO2 shell (denoted as Cu@Fe3O4@TiO2). Owing to the rational design, the binder-free Cu@Fe3O4@TiO2 electrode exhibits an ultrahigh Li-ion storage capacity (1585 mA h g-1 at 0.2 A g-1), superior rate capability, and excellent cycle performance (no decay after 1000 cycles), which could efficiently boost the energy-storage capability of LICs. By employing an anode of Cu@Fe3O4@TiO2 and a cathode of activated carbon, the as-constructed full LIC device provides high energy//powder densities (154.8 Wh kg-1 at 200 W kg-1; 66.2 Wh kg-1 at 30 kW kg-1). These superior results demonstrate that ALD-enabled architectures hold great promise for synthesizing high-capacity anodes for LICs.

Bi-layer high-k dielectrics of Al2O3/ZrO2 to reduce damage to MoS2 channel layers during atomic layer deposition

To implement two-dimensional (2D) transition metal dichalcogenides (TMDCs) in electric devices, a top-gated device structure is desired. However, there has been possibility of the channel layer being damaged during the upper dielectric deposition process. Because several layers of 2D TMDCs are atomically thin, the damage may significantly degrade the overall electrical performance. In this study, we investigated the damage to molybdenum disulfide (MoS2) during the atomic layer deposition (ALD) of single dielectrics of Al2O3 and ZrO2. We observed the MoS2 layers were damaged, depending on the ALD process conditions; the kind of oxidant and the growth temperature. To reduce the damage, we formed a bi-layered Al2O3/ZrO2 dielectric structure by developing a two-step ALD process. It is notable that the electrical performance of the device was significantly improved compared to those using the single dielectrics, indicating this two-step process is a promising candidate to satisfy the requirements of future 2D TMDCs-based electronics.

Surface Functionalization of Two-Dimensional Vertically Aligned Semiconductor Heterojunctions

Large-scale fabrication of two-dimensional (2D) nanomaterials by vapor phase depostion enabled the establishment of vertically aligned semiconductor herterojunctions. However, the property modulation of 2D semiconductor heterojunctions remains chanlleging within such thin layers. Herein, we proposed a general strategy towards the surface functionlization of 2D semiconductor heterojunctions simply by two-step atomic layer deposition (ALD) process with following post-annealing. TiO2-WO3 heterojunction was taken as a typical case in this work and its electrochemical properties were significantly improved via the proposed strategy. This strategy may open a new pathway for facile functionalization of 2D nanomaterials for the energy conversion and storage devices.

TiO2 nanoparticles-functionalized two-dimensional WO3 for high-performance supercapacitors developed by facile two-step ALD process

TiO2 nanoparticles-functionalized two-dimensional (2D) WO3 (TiO2NP-WO3) films fabricated by the facile two-step atomic layer deposition (ALD) process with following post-annealing at 380°C were investigated as electrode material for supercapacitors. The surface functionalization of 2D WO3 electrode by TiO2 nanoparticles significantly enhanced the specific capacitance by ∼1.5 times over the pure 2D WO3 electrode. Transition of electric double-layer capacitive behavior to the pseudo-capacitive behavior was also observed. Besides, the TiO2NP-WO3 electrode exhibited faster charge transfer than the pure 2D WO3 electrodes, due to the conduction modulation from the development of TiO2-WO3 heterojunctions. The decoration of TiO2 nanoparticles on 2D WO3 electrodes by this two-step ALD process with the following post-annealing has been solidly proven to a facile and effective method to enhance supercapacitor performances along with advantage of ALD technique for fabrication of large-scale, ultrathin films over other conventional methods. The fabrication strategy of the 2D TiO2NP-WO3 electrodes presented in this work may open a new pathway for the design and functionalization of the other 2D nanomaterials for the energy conversion and storage devices.

Controllable Synthesis of TiO2@Fe2O3 Core-Shell Nanotube Arrays with Double-Wall Coating as Superb Lithium-Ion Battery Anodes

Highlighted by the safe operation and stable performances, titanium oxides (TiO2) are deemed as promising candidates for next generation lithium-ion batteries (LIBs). However, the pervasively low capacity is casting shadow on desirable electrochemical behaviors and obscuring their practical applications. In this work, we reported a unique template-assisted and two-step atomic layer deposition (ALD) method to achieve TiO2@Fe2O3 core-shell nanotube arrays with hollow interior and double-wall coating. The as-prepared architecture combines both merits of the high specific capacity of Fe2O3 and structural stability of TiO2 backbone. Owing to the nanotubular structural advantages integrating facile strain relaxation as well as rapid ion and electron transport, the TiO2@Fe2O3 nanotube arrays with a high mass loading of Fe2O3 attained desirable capacity of ~520 mA h g−1, exhibiting both good rate capability under uprated current density of 10 A g−1 and especially enhanced cycle stability (~450 mA h g−1 after 600 cycles), outclassing most reported TiO2@metal oxide composites. The results not only provide a new avenue for hybrid core-shell nanotube formation, but also offer an insight for rational design of advanced electrode materials for LIBs.

Supercritical Fluid Atomic Layer Deposition: Base-Catalyzed Deposition of SiO2.

An in situ FTIR thin film technique was used to study the sequential atomic layer deposition (ALD) reactions of SiCl4, tetraethyl orthosilicate (TEOS) precursors, and water on nonporous silica powder using supercritical CO2 (sc-CO2) as the solvent. The IR work on nonporous powders was used to identify the reaction sequence for using a sc-CO2-based ALD to tune the pore size of a mesoporous silica. The IR studies showed that only trace adsorption of SiCl4 occurred on the silica, and this was due to the desiccating power of sc-CO2 to remove the adsorbed water from the surface. This was overcome by employing a three-step reaction scheme involving a first step of adsorption of triethylamine (TEA), followed by SiCl4 and then H2O. For TEOS, a three-step reaction sequence using TEA, TEOS, and then water offered no advantage, as the TEOS simply displaced the TEA from the silica surface. A two-step reaction involving the addition of TEOS followed by H2O in a second step did lead to silica film growth. However, higher growth rates were obtained when using a mixture of TEOS/TEA in the first step. The hydrolysis of the adsorbed TEOS was also much slower than that of the adsorbed SiCl4, and this was overcome by using a mixture of water/TEA during the second step. While the three-step process with SiCl4 showed a higher linear growth rate than obtained with two-step process using TEOS/TEA, its use was not practical, as the HCl generated led to corrosion of our sc-CO2 delivery system. However, when applying the two-step ALD reaction using TEOS on an MCM-41 powder, a 0.21 nm decrease in pore diameter was obtained after the first ALD cycle whereas further ALD cycles did not lead to further pore size reduction. This was attributed to the difficulty in removal of the H2O in the pores after the first cycle.

Homogeneous transparent conductive ZnO:Ga by ALD for large LED wafers

Highly conductive and uniform Ga doped ZnO (GZO) films were prepared by atomic layer deposition (ALD) as transparent conductive layers for InGaN/GaN LEDs. The optimal Ga doping concentration was found to be 3 at%. Even for 4” wafers, the TCO layer shows excellent homogeneity of film resistivity (0.8 %) according to Eddy current and spectroscopic ellipsometry mapping. This makes ALD a favourable technique over concurrent methods like MBE and PLD where the up-scaling is problematic. In agreement with previous studies, it was found that by an annealing treatment the quality of the GZO/p-GaN interface can be improved, although it causes the degradation of TCO conductivity. Therefore, a two-step ALD deposition technique was proposed and demonstrated: a “buffer layer” deposited and annealed first was followed by a second deposition step to maintain the high conductivity of the top layer.

Control of Al2O3/InAlN interface by two-step atomic layer deposition combined with high-temperature annealing

An attempt was made to control the Al2O3/InAlN interface by the phase change of the Al2O3 layer formed by atomic layer deposition (ALD). The electrical properties of an InAlN metal–oxide–semiconductor (MOS) diode with a sufficiently thick ALD-Al2O3 layer deteriorated following conventional postdeposition annealing (PDA) at 850 °C, which is sufficiently high for microcrystallization of the ALD-Al2O3 layer. However, X-ray photoelectron spectroscopy showed no evidence of an interface disorder in the ultrathin ALD-Al2O3/InAlN structure annealed at 850 °C. Two-step ALD interrupted by annealing at 850 °C right after the formation of the initial ultrathin Al2O3 layer improved the electrical properties of the MOS diode with reduced interface state density (Dit) and leakage current. A weak crystallization of the ultrathin Al2O3 layer was confirmed by transmission electron microscopy. Improvement of the interface disorder by high-temperature annealing is discussed as the origin of the Dit reduction.

Controlling spatial density and size of nanocrystals by two-step atomic layerdeposition

Two-step atomic layer deposition (ALD) is proposed in order to control both thespatial density and size of nanocrystals (NCs) via modulation of the nucleationrate during deposition. In this process, two different deposition conditions aresequentially used: a high nucleation rate condition for the formation of high density NCsand a low nucleation rate condition with a slow growth rate for the subsequentgrowth of pre-formed NCs. To control the nucleation rate of Ru during ALD,pulsing time and carrier flow rate of the Ru precursor are varied. By controllingthose factors, both the film growth rate and a nucleation rate of Ru are decreasedconsiderably. Two-step ALD of Ru NCs using the surface-saturated conditionfollowed by the reduced condition allows for variation of the spatial density from7.9 × 1011 to3.2 × 1012 cm − 2 and variation of the average diameter from 1.9 to 3.3 nm.

Low Temperature Two-Step Atomic Layer Deposition of Tantalum Nitride for Cu Diffusion Barrier

A cubic thin film with an electrical resistivity of was successfully obtained by suppressing the formation of using two-step atomic layer deposition independent of dosage. The deposition cycle involved two chemical reaction steps: The formation of elemental tantalum (Ta) by reducing tantalum pentafluoride with hydrogen plasma and the subsequent nitridation of the preformed Ta with at . The microstructure of the preformed Ta was phase with an electrical resistivity of , which was formed without regard to the deposition temperature. At a deposition temperature of less than , cubic with a ratio of 1 was achieved independent of the dosage. However, at a deposition temperature of greater than , the resistivity of Ta–N-based thin film increased abruptly as the dosage exceeded due to the formation of .

Comparison of Tantalum Nitride Films for Different NH3/H2/Ar Reactant States in Two-Step Atomic Layer Deposition

A two-step atomic layer deposition (ALD) process, applying either thermal or plasma-assisted Ta nitridation with a NH3/H2/Ar gas mixture, was conducted to discern the formation conditions of the cubic-TaN phase versus those of the Ta3N5 phase at an extremely low deposition temperature of 200 °C. Metallic cubic-TaN films with an electrical resistivity of 400 µΩ cm were successfully grown by thermal nitridation at 200 °C, while the incorporation of the Ta3N5 phase in TaN films could not be suppressed in the plasma nitridation process owing to large amounts of reactive radicals inherent to plasma nitridation. The dielectric Ta3N5 phase led to a significant increase in electrical resistivity, up to approximately 2500 µΩ cm. The relative concentration of cubic TaN and Ta3N5 phases was precisely controlled by changes in the nitridation time and the NH3 flow rate during the plasma-assisted nitridation process.

Two-Stage Atomic Layer Deposition of Aluminum Oxide on Alkanethiolate Self-Assembled Monolayers Using n-Propanol and Water as Oxygen Sources

Aluminum oxide (AlOx) was deposited onto strong hydrophobic surfaces of alkanethiolate self-assembled monolayers (SAMs) by a two-stage atomic layer deposition (ALD) process; the first stage utilized n-propanol as the oxygen source, and the second stage proceeded with water. The resulting AlOx layers were characterized with spectroscopic ellipsometry, reflection−absorption infrared spectroscopy, low-energy X-ray emission spectroscopy, and atomic force microscopy. The optimized two-step ALD process significantly improved the surface morphology of AlOx layers and effectively protected the structural integrities of underlying SAMs.

Ferromagnetic nanotubes by atomic layer deposition in anodic alumina membranes

In this paper, two methods for the synthesis of magnetic nanotubes inside the pores of anodic alumina membranes by atomic layer deposition (ALD) are compared. The precursors were nickelocene or cobaltocene, and H2O or O3. The first method consists of a three-step ALD cycle: First, the sample is exposed to the metal-organic precursor, subsequently to water, and finally, to hydrogen. In the second method, metal oxide is deposited by a conventional two-step ALD cycle. After the ALD process, the sample is reduced under hydrogen atmosphere. The magnetic nanotubes obtained by the second method have a smaller grain size and improved magnetic properties. The magnetic nanotubes with diameters ranging from 35to60nm exhibit a preferential magnetization direction along the nanowire axis. The Ni or Co nanotubes with larger diameters (around 160nm) show a nearly isotropic magnetic behavior, with the magnetic moments arranged in a vortex state at zero field.

Two-step Atomic Layer Deposition for Tantalum Nitride by Nitridation of Tantalum with Ammonia

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A Chemical Reaction Path Design for the Atomic Layer Deposition of Tantalum Nitride Thin Films

The two-step atomic layer deposition of tantalum nitride (TaN) in which the deposition cycle involved two chemical reaction steps, the formation of elemental tantalum (Ta) by reducing tantalum-pentafluoride (TaF5) with hydrogen plasma and the subsequent nitridation of the preformed Ta with NH 5 , was performed at 350°C. Cubic-TaN with a Ta/N ratio of 1:1 was achieved independently of the nitridation time, and the fluorine content of the films was below the detection limit using Auger electron spectroscopy. As a result, the electrical resistivity of the TaN film was reduced below 500 μΩ cm by suppressing the formation of Ta 3 N 5 .

Characteristics of copper films produced via atomic layer deposition

Properties of copper films produced using atomic layer deposition (ALD) were characterized. Composition, morphology, and electrical properties of these films grown on glass, as well as Ta, TiN, and TaN on silicon wafers were examined. The resistivity of films thicker than about 60 nm was near bulk value. Films were deposited using a two-step ALD process in which copper(II)-1,1,1,5,5,5,-hexafluoroacetylacetonate hydrate and water vapor were introduced in the first step and a reducing agent was introduced in a subsequent step. Five reducing agents were evaluated, with the best results obtained using isopropanol or formalin. The optimum deposition temperature with isopropanol was about 260 °C, whereas it was about 300 °C with formalin. These films were also investigated as seed layers for electrodeposition of thicker Cu layers for possible interconnect applications. Excellent fills in high aspect ratio trenches were demonstrated.