Atomic Layer Deposition Research Articles

Silver CVD and ALD Precursors: Synthesis, Properties, and Application in Deposition Processes

This review summarized the developments in the field of volatile silver complexes, which can serve as precursors in gas-transport reactions for the production of thin films and metal nanoparticles via chemical vapor deposition (CVD) and atomic layer deposition (ALD). Silver-based films and nanoparticles are widely used in various high-tech fields, including medicine. For effective use in CVD and ALD processes, the properties of silver precursors must be balanced in terms of volatility, thermal stability, and reactivity. In this review, we focus on the synthesis and comprehensive analysis of structural and thermal characteristics for the most promising classes of volatile silver complexes, as well as organometallic compounds. Following the specifics of silver chemistry, some features of the use of precursors and their selection, as well as several key directions to improving the efficiency of silver material deposition processes, are also discussed.

Pt Nanocatalysts Modified with Carbon Dots as Non‐metallic Promoters for Enhanced Selective Hydrogenation Performance

Constructing non‐metal promoter‐decorated Pt‐based catalysts is a promising strategy for enhancing selective hydrogenation performance. Herein, a set of carbon dots (CDs)‐modified Pt/SBA‐15 nanocatalysts were synthesized using atomic layer deposition (ALD) and impregnation techniques. The catalysts were characterized using various techniques, including TEM, ICP‐MS, N2 adsorption‐desorption isotherms, XRD, XPS, H2‐TPD, H‐D exchange, CO‐DRIFTS, etc. The catalytic performance of the CDs‐Pt/SBA‐15 catalysts was evaluated by the selective hydrogenation reaction of cinnamaldehyde (CAL) to cinnamyl alcohol (COL). The results indicate that there is electron transfer between the CDs and Pt, and the addition of CDs can improve the hydrogen spillover effect, both of which significantly influence the selective catalytic hydrogenation performance of the CDs‐Pt/SBA‐15 catalysts. Among the tested catalysts, 0.44CDs‐Pt/SBA‐15 demonstrated the highest catalytic performance, with conversion and selectivity 2.8 times and 2.2 times higher, respectively, than Pt/SBA‐15. This study provides a new perspective on designing and preparing highly efficient non‐metallic promoter‐modified Pt‐based nanocatalysts and has significant implications for their application in catalytic reactions.

Sputter-Coated TiO2 Films as Passivation and Hole Transfer Layers for Improved Energy Conversion with Solar Fuel WO3/CuWO4 Photoanodes.

Atomic-layer-deposited (ALD) "leaky" TiO2 has gained interest as a charge-selective protection layer for semiconductor solar fuel electrodes. Here, the use of sputter-deposited TiO2 layers as hole-selective contacts for WO3/CuWO4 type-2 heterojunction water oxidation photoanodes is demonstrated for the first time. TiO2 protection layers with varying thicknesses (2 to 128 nm) were deposited by using the radio frequency (RF) magnetron sputtering technique. The resulting TiO2 films are amorphous as evidenced by Raman spectroscopy and powder X-ray diffraction (XRD). Photoelectrochemical scans and vibrating Kelvin probe photovoltage spectroscopy show that 2-8 nm TiO2 layers nearly double the photocurrent to 0.97 mA cm-2 under AM1.5 illumination (19% AQE at 350 nm), increase the surface photovoltage signal by 25%, and increase the WO3/CuWO4 effective band gap. These outcomes can be attributed to the selectivity of TiO2 for photoholes. Additionally, SPV data suggest that TiO2 overlayers suppress copper-based surface recombination defects. Reduced photocurrents and photovoltages are measured in thicker TiO2 films (16 to 128 nm) as a result of an increasing hole transfer resistance and because of light shading effects according to photoaction spectra. The TiO2 films also improve the stability of the WO3/CuWO4 photoelectrodes, allowing nearly constant O2 evolution over 3 h after an initial 20-35% loss. Overall, this work establishes RF magnetron sputtering as a useful method to install amorphous TiO2 passivation layers for improved WO3/CuWO4 solar fuel photoelectrodes. Furthermore, we show how the combination of PEC and SPV measurements provides insight into the function of the TiO2 coatings.

Enhancement of biocompatibility of anodic nanotube structures on biomedical Ti–6Al–4V alloy via ultrathin TiO2 coatings

This work aims to describe the effect of the surface modification of TiO2 nanotube (TNT) layers on Ti-6Al-4V (TiAlV) alloy by ultrathin TiO2 coatings prepared via Atomic Layer Deposition (ALD) on the growth of MG-63 osteoblastic cells. The TNT layers with two distinctly different inner diameters, namely ∼15 nm and ∼50 nm, were prepared via anodic oxidation of the TiAlV alloy. Flat, i.e., non-anodized, TiAlV alloy foils were used as reference substrates. Additionally, a part of the TNT layers and alloy foils was coated with ultrathin coatings of TiO2 by ALD. The number of TiO2 ALD cycles used was 1 and 5 leading to a nominal TiO2 thickness of ∼0.055 and ∼0.3 nm, respectively. The ultrathin TiO2 coating by ALD enabled to optimize the surface hydrophilicity for optimal cell growth. In addition, coatings shaded impurities of V- and F-based species (stemming from the alloy and the anodization electrolyte) that affect the biocompatibility of the tested materials while preserving the original structure and morphology. The evaluation of the biocompatibility before and after TiO2 ALD coating on TiAlV flat surfaces and TNT layers was carried out using MG-63 osteoblastic cells and compared after incubation for up to 96 h. The cell growth, adhesion, and proliferation of the MG-63 on TiAlV foils and TNT layers showed significant enhancement after the surface modification by TiO2 ALD.

Precise design of Cu-exchanged zeolites with diverse topologies via atomic layer deposition for boosting low-temperature NH3-SCR

Atomic layer deposition (ALD), as an advanced grafting technique, enables precise tailoring of the active sites using self-limiting growth at atomic level. The primary difficulties faced by this metal post-synthesis method are the low metal loading and expensive precursors due to the highly developed internal micropores of zeolite as a support. Cu(hfac)2 was deposited over zeolites with different topologies in order to clarify the matching principles as well as key factors between zeolites and ALD, and the method was applied for the first time to selective catalytic reduction (SCR) of NO by NH3. It was shown that Cu loading on EMC-2 zeolite can be reached to 1.58wt% after only twice cycles, and the Cu-EMC-2 exhibits excellent performance, achieving more than 90% NO conversion at 200-550 °C. The temperature-programmed reduction by hydrogen (H2-TPR) and electron paramagnetic resonance (EPR) demonstrated that Cu-based zeolites prepared by ALD were dominated by isolated Cu+ or [Cu(OH)]+ ions, which are combined with single Al sites. Kinetic diameter computation, molecular dynamic simulation, and X-ray photoelectron spectroscopy (XPS) together indicate that the copper precursors could successfully enter the zeolites with 12-member rings (MR), while having difficulty entering the zeolites with 10-MR or 8-MR because of pore diffusion limitation, making poorly dispersed copper ions on the zeolitic surface. This study guides how to improve metal precursor utilization and clarifies that the essentials in coupling ALD with zeolites is the pore structures or membered rings.

Effect of oxygen plasma treatment on the properties of ALD hafnium oxide films

In this work, we studied the effect of processing hafnium oxide samples prepared by atomic layer deposition in a gas discharge plasma of a mixture of argon and oxygen in a ratio of 1:10. It has been shown that such treatment preserves the amorphous structure of the film and reduces the content of oxygen vacancies in the film. This is important for the use of the film in the gate dielectric of a transistor. Processing in plasma leads to a change in its properties, namely: an increase in density and refractive index, as well as a decrease in the intensity of emission bands that are associated with an oxygen vacancy. The plasma temperature and ion energy were determined from the argon emission spectra. The activation energy for the diffusion of oxygen ions in hafnium oxide films has been calculated to be 0.43 eV.

Photocurrent in PZT/TiOx composite film prepared via self-assembly of perovskite matrix and ALD of titania

A novel approach for the preparation of ferroelectric composite films has been successfully developed by combining sol–gel evaporation-induced self-assembly (EISA) of porous lead zirconate titanate (PZT) films with atomic layer deposition (ALD) of titania. The EISA process, which utilizes a Brij-type surfactant, facilitates the formation of large columnar perovskite grains with narrow (∼20 nm) interconnected pores. ALD, employing the thermal reaction of titanium isopropoxide with water, ensures uniform titania growth within the pores throughout the film thickness, as demonstrated by transmission electron microscopy and ellipsometric porosimetry. The resulting PZT-TiOx composite films exhibit a pronounced photovoltaic current under visible light illumination, attributed to electron excitation from the valence band to Ti3+ states, followed by movement via a hopping conduction mechanism. The photocurrent value varies with the direction of polarization. This behavior presents a potential method for controlling photoconductivity through polarization, with possible applications in electronic and photonic devices.

HgCdTe surface passivation with low-temperature plasma-enhanced atomic layer deposited HfO2

Mercury cadmium telluride (Hg1−xCdxTe) is widely utilized for infrared detection applications. However, the quality of the surface and the presence of interface defects significantly impact device performance, so further improvements in surface passivation are still necessary. In this study, we explore the use of plasma-enhanced atomic layer deposition (PE-ALD) to deposit HfO2 as a passivation layer for HgCdTe in the temperature range of 80–160 °C. The insulating film, semiconductor surface, and their interface are characterized using spectral ellipsometry, X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), atomic force microscopy (AFM), electron microscopy, and capacitance–voltage (C–V) methods. XPS analysis reveals that the fraction of impurities in the HfO2 films significantly decreases at higher deposition temperatures. Simultaneously, changes in the surface composition of HgCdTe become more pronounced. The bandgap width of HfO2 and the insulator–semiconductor interfacial band discontinuity are estimated using EELS and XPS. The surface morphology of the deposited coatings closely resembles that of the underlying HgCdTe. The permittivity of the insulator and the effective fixed charge strongly depend on the deposition temperature. Our findings demonstrate that low-temperature PE-ALD HfO2 effectively passivates the HgCdTe surface, and we propose an optimal deposition temperature for the further development of HgCdTe-based devices.

Synergistic impact of Al2O3 capping layer and deposition temperature for enhancing the ferroelectricity of undoped-HfO2 thin films

Synergistic effects of Al2O3 capping layer and deposition temperature, ranging from 220 to 280 °C, on the ferroelectric properties of undoped HfO2 thin films were investigated. The use of an Al2O3 capping layer during atomic layer deposition (ALD) process was verified to induce a ferroelectric orthorhombic phase in undoped HfO2 thin films, leading to a net remnant polarization (2Pr) of >20 μC/cm2 when deposited <260 °C. The HfO2 thin film deposited at 240 °C showed a low leakage current density of 5.7 × 10−6 A/cm2 at an electric field of 2 MV/cm, accompanied by a high 2Pr and an endurance of >107 cycles. Furthermore, the switching time for ferroelectric polarization reversal was estimated to range from 2.1 to 1.7 μs in the voltage range of 3.5–4.5 V. The results of this work demonstrated the impact of deposition temperature conditions on the ferroelectric properties of the HfO2 thin films by means of the Al2O3 capping process.

Optimization of Al2O3 shell thickness on SnO2 nanowires for realization of sensitive and selective H2 sensing

Detection of highly explosive H2 gas is a critical safety issue. Herein, using a vapor-liquid-solid growth mechanism, SnO2 nanowires (NWs) were prepared, after which Al2O3 shells (1.9, 3.3, 4.5, and 6.1 nm) were deposited on synthesized SnO2 NWs by atomic layer deposition. Based on characterization studies, C-S NW structures with a crystalline SnO2 core and amorphous Al2O3 shell were successfully formed. Based on H2 gas sensing measurement, SnO2-Al2O3 (4.5 nm) C-S NWs exhibited a high response of 152.54 to H2 (10 ppm) at 350°C. Furthermore, they showed excellent selectivity, where response to H2 gas (10 ppm) was more than 33 times higher than that to NO2 gas (10 ppm). Additionally, they displayed excellent repeatability as well as long-term stability in detection of low-concentration H2 gas. The improved H2 sensing capability was related to the high base resistance, the formation of the SnO2-Al2O3 heterojunction, the optimization of the Al2O3 shell thickness on SnO2 NWs, and the partial reduction of SnO2 and Al2O3 in an H2 gas atmosphere. This study sheds light on the development of H2 sensors based on present material.

Symmetric Dopant-Free Si Solar Cells Enabled by TiOx Nanolayers: An In-Depth Study on Bipolar Carrier Selectivity.

High-efficiency solar cells require two contact structures, engineered for efficient extraction of photogenerated holes and electrons at the respective electrodes. Herein, crystalline Si solar cell featuring hole- and electron-selective passivating contacts composed entirely of a single material, amorphous titanium oxide (TiOx), without extrinsic doping is demonstrated. The hole/electron selectivity of the TiOx layers (≈5 nm) is tailored by the oxidation process and the choice of Ti precursor in the atomic layer deposition (ALD). Ex situ and in situ X-ray photoelectron spectroscopy measurements elucidate that the hole-selective TiOx induces significant band bending in the Si (Φ≈0.7 eV), generating a p-type inversion layer in the n-Si absorber. The electron-selective TiOx induces a smaller band bending of Φ<0.35 eV. This clarifies that the bipolar carrier selectivity of TiOx is associated with the different amount of negative fixed charges generated during the ALD process, depending on the choice of Ti precursor and oxidant. In addition, the growth of a hydrogen-containing SiOy nanolayer (≈1-1.5 nm) at the Si/TiOx interface during postdeposition oxidation is crucial for providing chemical passivation in both types of TiOx. These findings pave the way for a deeper understanding of the charge generation mechanism and chemistry at the Si/metal oxide interfaces.

Directing SEI formation on Si-based electrodes using atomic layer deposition.

The design of artificial solid eletroctrolyte interphase is an important task to minimize capacity losses in Li-ion batteries. Herein, TiO2 created through atomic layer deposition was used as an artificial SEI on Si nanoparticles. Such coating led to substantial improvement of cycling stability when evaluated with FEC-free electrolyte.

Janus Electronic Devices with Ultrathin High-κ Gate Dielectric Directly Integrated on 1T'-MoTe2.

Integrating high-quality dielectrics with two-dimensional (2D) transition metal chalcogenides (TMDCs) is crucial for high-performance electronics. However, the lack of dangling bonds on 2D material surfaces complicates direct dielectric deposition. We propose using atomic layer deposition (ALD) to integrate ultrathin high-κ dielectric directly on 1T'-MoTe2 surfaces, facilitating the creation of high-performance back-gated field-effect transistors (FETs). Exploiting 1T'-MoTe2's natural oxidation in ambient conditions, we directly deposit dense and uniform HfO2 dielectric films below 5 nm, achieving an equivalent oxide thickness (EOT) of 0.97 nm. The resulting back-gate transistors, with a monolayer MoSSe on HfO2/1T'-MoTe2, show a current on/off ratio over 105 and operate at low voltages (<1 V), indicating high gating efficiency and a charge carrier mobility of 2.93 cm2V-1s-1. Additionally, we demonstrate a 6 × 5 bottom-gated array of MoSSe transistors using all-1T'-MoTe2 electrodes, achieving an 86.7% sample yield. Our approach also enables the creation of various integrated logic circuits such as inverters, NAND, and NOR gates. This research offers a feasible method for integrating high-κ dielectric films using industrially compatible ALD processes, providing excellent thickness control, uniformity, and scalability for 2D electronic devices.

HfO2/Al2O3 Multilayer on Parabolic Cylinder Substrate to Monochromatize and Collimate Divergent X-Ray Beam

A monochromatic parallel X-ray beam is essential for some X-ray applications and a multilayer on a parabolic cylinder substrate is a good choice to obtain it. In this work, an HfO2/Al2O3 multilayer with a period of 3.80 nm and a bilayer number of 60 is grown on a smooth, flat Si substrate via atomic layer deposition for a monochromatizing Cu kα 0.154 nm X-ray and the first-order peak of the X-ray reflectivity is about 45%. The multilayer-coated Si substrate is then glued on a pre-made stainless steel body with a designed parabolic cylinder profile to convert divergent X-rays from a laboratory X-ray source into a parallel beam. The surface profiles before and after gluing Si on the stainless steel body are almost the same and basically consistent with the designed one. The results show that a monochromatic parallel X-ray (0.154 nm) beam can be acquired by an HfO2/Al2O3 multilayer on a parabolic cylinder substrate and the divergence angle of the reflected beam is 0.67 mrad.

Advanced Nested Coaxial Thin-Film ZnO Nanostructures Synthesized by Atomic Layer Deposition for Improved Sensing Performance

We report a new synthesis method for multiple-walled nested thin-film nanostructures by combining hydrothermal growth methods with atomic layer deposition (ALD) thin-film technology and sacrificial films, thereby increasing the surface-to-volume ratio to improve the sensing performance of novel ZnO gas sensors. Single-crystal ZnO nanorods serve as the core of the nanostructure assembly and were synthesized hydrothermally on fine-grained ALD ZnO seed films. Subsequently, the ZnO core nanotubes were coated with alternating sacrificial coaxial 3D wrap-around ALD Al2O3 films and ALD ZnO films. Basically, the center nanorod was coated with an ALD 3D wrap-around Al2O3 sacrificial layer to realize a nested coaxial ZnO thin-film nanotube. To increase the surface-to-volume ratio of the nested multiple-film nanostructure, both the front and backside of the nested coaxial ZnO films must be exposed by selectively removing the intermittent Al2O3 sacrificial films. The selective removal of the sacrificial films exposes the front and backside of the free-standing ZnO films for interaction with target gases during sensing operation while steadily increasing the surface-to-volume ratio. The sensing response of the novel ZnO gas sensor architecture with nested nanotubes achieved a maximum 150% enhancement at low temperature compared to a conventional ZnO nanorod sensor.

Covering a 1280–1495 nm (215 nm) wideband high-gain bismuth-doped fiber amplifier with only 1240 nm pumping

In this study, two bismuth-doped fibers were fabricated by atomic layer deposition (ALD) combined with modified chemical vapor deposition (MCVD) technology. For a dual-pump amplification configuration, 1240 and 1310 nm pumping the BDF simultaneously, when the 1240 nm pump power changes, it has little impact on the gain. The 1310 nm pump wavelength overlaps with the emission range of bismuth active centers associated with phosphorus (BACs-P), potentially suppressing its excitation. The experiment reveals that the emission range of BACs associated with silicon (BACs-Si) overlaps the absorption range of BACs-P, indicating that BACs likely exist for their reabsorption properties. Furthermore, enhancing the reabsorption properties can increase the luminescent intensity of BACs-Si in the 1420 nm region with only a 1240 nm pump. The gain of peaks at 1340 and 1420 nm exceeds 31.5 dB at a −23 dBm input signal. Notably, the gain within the 1280–1495 nm range exceeded 15 dB, achieving a bandwidth of 215 nm. This holds significant promise for applications in artificial intelligence, autonomous driving, and other fields with high-capacity communication demands.

Formation of vanadium dioxide nanocrystal arrays via post-growth annealing for stable and energy-efficient switches

The abrupt and reversible semiconductor-metal phase transition in vanadium dioxide nanocrystals has attracted considerable attention for potential applications in oxide electronics, including neuromorphic systems. This study presents a systematic investigation of post-growth annealing conditions for the formation of single VO2 M-phase nanocrystals arrays from VOx films synthesized by atomic layer deposition. The composition of the initial VOx films and the annealing parameters were found to significantly affect the morphology, phase composition and electrical properties of the obtained single nanocrystal arrays. Our results demonstrate that the formation of VO2 M-phase nanocrystal arrays occurs at annealing temperatures of 650 °C and above, irrespective of the initial film composition. More homogeneous in size nanocrystals are formed from initial VOx films with higher V+4 content. The structures with the initial V+4 content of 60 % annealed at 650 °C for 2 h demonstrates the resistive switching with an energy less than 150 fJ, and a total number of stable switching cycles more than 101⁰. Our results pave the way for the novel energy-efficient nanoelectronic and nanophotonic devices based on VO₂ nanoparticles.

Eliminating chemo-mechanical degradation of lithium solid-state battery cathodes during >4.5 V cycling using amorphous Nb2O5 coatings.

Lithium solid-state batteries offer improved safety and energy density. However, the limited stability of solid electrolytes (SEs), as well as irreversible structural and chemical changes in the cathode active material, can result in inferior electrochemical performance, particularly during high-voltage cycling (>4.3 V vs Li/Li+). Therefore, new materials and strategies are needed to stabilize the cathode/SE interface and preserve the cathode material structure during high-voltage cycling. Here, we introduce a thin (~5 nm) conformal coating of amorphous Nb2O5 on single-crystal LiNi0.5Mn0.3Co0.2O2 cathode particles using rotary-bed atomic layer deposition (ALD). Full cells with Li4Ti5O12 anodes and Nb2O5-coated cathodes demonstrate a higher initial Coulombic efficiency of 91.6% ± 0.5% compared to 82.2% ± 0.3% for the uncoated samples, along with improved rate capability (10x higher accessible capacity at 2C rate) and remarkable capacity retention during extended cycling (99.4% after 500 cycles at 4.7 V vs Li/Li+). These improvements are associated with reduced cell polarization and interfacial impedance for the coated samples. Post-cycling electron microscopy analysis reveals that the Nb2O5 coating remains intact and prevents the formation of spinel and rock-salt phases, which eliminates intra-particle cracking of the single-crystal cathode material. These findings demonstrate a potential pathway towards stable and high-performance solid-state batteries during high-voltage operation.

Adjustable repetition rate mode-locked fiber laser using a ZnO nanolaminate

This paper experimentally demonstrates the use of a zinc oxide (ZnO) nanolaminate as a polarization-dependent loss element to control the frequency repetition rate in a passive mode-locked fiber laser. The ZnO nanolaminate was deposited via the atomic layer deposition technique. The system achieves repetition frequencies ranging from 2.2 to 6.6 MHz, with emission peaks occurring at wavelengths between 1569.1 nm and 1570.8 nm, exhibiting three to four peaks. The fiber exhibits a minimal pulse duration of around 9.78 ns; the maximal pulse-to-pulse separation was 452 ns. The fiber laser demonstrates excellent stability in both power and wavelength emission. The compact experimental setup leverages the properties of the thin film of zinc oxide, offering a versatile laser system capable of quickly altering pulse repetition rates by simply adjusting the polarization state.

Monolith Catalyst Design by Combining 3D Printing and Atomic Layer Deposition: Toward Green Palladium‐Catalyzed Cross‐Coupling Reactions

Monolith Catalyst Design by Combining 3D Printing and Atomic Layer Deposition: Toward Green Palladium‐Catalyzed Cross‐Coupling Reactions