Atomic Layer Deposition Films Research Articles

Atomic Layer Deposition Films for Resistive Random‐Access Memories

AbstractResistive random‐access memory (RRAM) stands out as a promising memory technology due to its ease of operation, high speed, affordability, exceptional stability, and potential to enable smaller memory devices with sizes under 10 nm. This has drawn significant attention, with atomic layer deposition (ALD) emerging as an ideal technology to tackle the challenges of nanoscale fabrication in the micro‐ and nanomanufacturing industry. ALD offers technological advantages such as functional multiple‐layer stacking, doping capabilities, and incorporating oxygen reservoirs or reactive layers. These factors contribute to achieving more intriguing, stable, and reliable nonvolatile resistance switching behaviors in RRAM. Specifically, ALD greatly benefits RRAM, that relies on the valence change mechanism, where high‐k transition metal oxides are commonly used as switching materials, and precise control over oxygen vacancies is achievable. This review provides a comprehensive overview of ALD films used in RRAM, delves into resistive switching properties and microscopic mechanisms in binary and ternary oxides and nitrides, and explores the impact of ALD‐prepared electrodes. Furthermore, the current status and future prospects of ALD‐based RRAM are highlighted, which is poised to catalyze further advancements in the fields of information storage and neural networks.

Area-selective atomic layer deposition of palladium

We report area-selective deposition of palladium using sequential area-blocking and area-activation ALD processes. Thermal atomic layer deposition (ALD) of palladium is investigated at 100 °C using palladium (II) hexafluoroacetylacetonate and hydrogen. Palladium deposition does not proceed at such a low temperature unless catalytic sites are present to dissociate hydrogen during Pd film nucleation and before sufficient Pd has accumulated to serve as the catalyst for hydrogen dissociation. Ultrathin (<2 nm) nickel metal surfaces served as the initial catalytic sites and are prepared via low temperature (100 °C) reduction of NiO ALD films using a H-atom doser operating at 3 × 10−5 Torr. The Ni0 films are shown to seed the ALD of Pd, demonstrating a route to Pd ALD by area activation. Blanket NiO films are used to study Pd ALD growth and patterned NiO films, formed by area-blocking ALD, are used to demonstrate a bottom-up approach to patterned Pd films.

Influences of deposition conditions on atomic layer deposition films for enhanced performance in perovskite solar cells

Atomic layer deposition (ALD) is a key technology for fabricating functional layers in perovskite solar cells, as it can deposit pinhole-free films with atomic-level thickness and tunable composition on high-aspect-ratio surfaces. Various deposition conditions have significant effects on the growth, physical, and chemical properties of ALD films, which, in turn, critically influences the performance of associated devices. Here, we review the reaction mechanisms underlying ALD and summarize how variables, such as precursors, deposition temperatures, and substrates, impinge upon the quality of ALD films and the related devices. We emphasize the role of substrate in determining the nucleation and growth behavior of ALD films, which has been overlooked in previous reviews. Finally, we highlight the potential application of ALD in efficient perovskite solar cells in terms of carrier transport, encapsulated, and buffer layers, especially for tandem cells.

Oxygen tracer diffusion in amorphous hafnia films for resistive memory.

The oxygen diffusion rate in hafnia (HfO2)-based resistive memory plays a pivotal role in enabling nonvolatile data retention. However, the information retention times obtained in HfO2 resistive memory devices are many times higher than the expected values obtained from oxygen diffusion measurements in HfO2 materials. In this study, we resolve this discrepancy by conducting oxygen isotope tracer diffusion measurements in amorphous hafnia (a-HfO2) thin films. Our results show that the oxygen tracer diffusion in amorphous HfO2 films is orders of magnitude lower than that of previous measurements on monoclinic hafnia (m-HfO2) pellets. Moreover, oxygen tracer diffusion is much lower in denser a-HfO2 films deposited by atomic layer deposition (ALD) than in less dense a-HfO2 films deposited by sputtering. The ALD films yield similar oxygen diffusion times as experimentally measured device retention times, reconciling this discrepancy between oxygen diffusion and retention time measurements. More broadly, our work shows how processing conditions can be used to control oxygen transport characteristics in amorphous materials without long-range crystal order.

(Invited) Atomistic Insights into Continuous and Area-Selective ALD Processes: First-Principles Simulations of the Underpinning Surface Chemistry

Atomic layer deposition (ALD) is a highly advanced thin film deposition technology that has found widespread use in nanoelectronics, photovoltaics, fuel cell and recently all-solid-state battery applications. It is widely regarded as the state-of-the-art technique for producing thin films due to its exceptional conformality on high-aspect ratio structures, atomic level thickness control, and tuneable film composition. In recent years, area-selective atomic layer deposition (AS-ALD) has emerged as a promising technology for high-fidelity nano-patterning, and significant progress has been made in understanding and developing highly selective processes for various material systems. Along these lines, we have recently shown that high-throughput, low defectivity, highly selective SiO2 deposition can be achieved on various substrates by combining atmospheric-pressure spatial ALD with integrated area-deactivation using small molecule inhibitors and plasma-enhanced spatial etching [1].ALD is a surface-controlled technique that relies on the self-limiting reactions of the precursors and co-reactants with the deposition surface in alternating cycles. The initial step in ALD nucleation on the target substrate is a key factor in the deposited film quality, and efficient precursor binding on the surface is critical. Molecular dynamics (MD) simulations and minimum-energy reaction pathway analysis based on ab initio Density Functional Theory (DFT) calculations provide invaluable insights into the surface chemistry, enabling customisation of ALD processes to boost uniformity, conformality, and area-selectivity of the resulting films.In this talk, I will discuss how first-principles simulations can be used synergistically with experimental findings to achieve high-efficiency plasma-enhanced atomic-layer cleaning (ALC) of impurities from various ALD films and substrates, design proper chemical surface modifications that enable continuous and conformal ALD nucleation even on chemically-inert surfaces (like graphene), and unveil the particular surface chemistry governing high-fidelity area-selectivity with an atomic-level precision in ultrathin coatings [1-9]. By combining the insights gained from first-principles simulations with experimental techniques, we can push the frontiers of device manufacturing to new heights, opening up exciting possibilities for further technological advances in nanoelectronics, photovoltaics, fuel cell and energy storage applications.[1] Advanced Materials, Just Accepted, (DOI: 10.1002/adma.202301204); [2]Nanoscale, 2021, 13, 10092-10099; [3] Chem. Mater. 2019, 31 (4), 1250-1257.; [4] Adv. Mater. Interfaces 2018, 5 (13), 1800268.; [5] Chem. Mater. 2017, 29 (3), 921–925.; [6] ACS Nano 2017, 11 (9), 9303–9311.; [7] Sol. Energy Mater. Sol. Cells 2017, 163, 43–50.; [8] Chem. Mater. 2017, 29 (5), 2090–2100.; [9] Nanoscale 2016, 8 (47), 19829–19845. Figure 1

Atomic Layer Deposition of Lithium Borate and Borophosphate Thin Films for Lithium-Ion Battery Applications

Lithium-ion batteries (LiBs) are widely used in portable electronic devices and electric vehicles because of their high energy density and long cycle life. However, their performance is limited by unwanted side reactions that occur at the electrolyte-electrode interface during charge/discharge cycles.Previous studies have indicated that the application of a conformal and ultra-thin protective coating on the electrode surface through atomic layer deposition (ALD) is a promising strategy to prevent these undesired side reactions [1]. Due to their high stability, metal phosphates are deemed a promising class of materials for protective coatings, and several studies have focused on ALD of metal phosphates [2], with lithium phosphate as a particularly relevant material. However, the low ionic conductivity, as evidenced by the reported value of 1.4 x 10-10 S cm-1 with an activation energy of 0.92 eV for crystalline Li3PO4 films deposited by ALD [3] may hinder the flow of lithium ions through the coating. It has been shown, albeit using other deposition techniques, that the inclusion of borate species can enhance the ionic conductivity of phosphate materials, including lithium phosphate [4]. However, lithium borates exhibit a more narrow electrochemical stability window. Here, we investigate the combination of lithium phosphate and lithium borate into a new, intermixed ALD coating, i.e. lithium borophosphate, aiming to increase the ionic conductivity of the protective coating while preserving as much as possible its electrochemical stability.To date, only one ALD process for lithium borate has been reported by Kazyak et al. [5], which resulted in films with significant carbon incorporation. In this work, we have developed an ALD process for depositing lithium borate with minimal contaminants (< 5 at.% as determined by XPS) using LiHMDS, H2O and TMB as precursors. Through-plane impedance spectroscopy measurements were used to determine the ionic conductivity. For a lithium borate ALD film with a thickness of 81 nm deposited at 250°C, an ionic conductivity of 1.60 ± 0.03 x 10-7 S cm-1 was obtained at 30 °C, with an activation energy of 0.58 ± 0.01 eV.By combining this lithium borate ALD process with the well-known lithium phosphate ALD process [6], the composition of the film could be precisely controlled, resulting in tailored electrochemical properties. Cyclic voltammetry was employed to evaluate the electrochemical stability of the lithium borophosphate mixtures. The coatings were deposited on top of an ion-blocking material and cycled to high voltages. Initial results indicate that the stability of the coating material decreases as the amount of lithium borate to lithium phosphate cycles increases.These promising preliminary results demonstrate that ALD is a valuable tool in developing high-performance battery technologies, enabling the deposition of new protective coatings with controllable electrochemical properties. Finding the optimal lithium borophosphate coating will require a trade-off between ionic conductivity and electrochemical stability and will ultimately depend on the intended application. References Jung, Y. S., J. Electrochem. Soc. 157, A75 (2009).Henderick, L., Appl. Phys. Rev. 9, 011310 (2022).Put, B., J. Electrochem. Soc. 166, A1239 (2019).Magistris, A., J. Power Sources 14, 87 (1985).Kazyak, E., J. Mater. Chem. A 6, 19425 (2018).Hämäläinen, J., J. Electrochem. Soc. 159, A259 (2012). Figure 1

New ALD ZnO Precursors and Processes for Protective Coatings on Lithium-Ion Battery Cathodes

The ever-increasing demand for Li-ion batteries commands their performance to increase while at the same time size and mass to decrease. One promising cathode material to achieve this is the Ni-rich LiNi0.8Mn0.1Co0.1, (NMC811), a high-capacity layered oxide cathode material. However, NMC811 suffers from capacity fading during electrochemical cycling of the cells. This phenomenon is connected to the dissolution of the active transition metal ions in the cathodes, phase changes and to side reactions happening at the interface of the cathode and the electrolyte or with formed HF but remains poorly understood.[1] To mitigate these problems, many different coating processes have been employed. In the realm of atomic layer deposition (ALD), the predominant coating material is the very stable insulating Al2O3 prepared from trimethyl aluminum and H2O. However, ZnO is an alternative transition metal oxide coating that shows promise since it is an amphoteric conductor. Recently, amorphous ZnO coatings prepared by sol-gel methods were shown to significantly boost the capacity retention of NMC cathodes.[2] Taking inspiration from a recent study that compared different ALD Al2O3 precursors and their ability to precisely tune the protective coating, we set out to investigate the suitability of ZnO ALD films as protective coatings and the influence of different precursors used for that purpose.[3] We selected four Zn precursors for cathode surface treatments in this study with different coordination motifs, metal-to-ligand bonds and reactivities, (Fig. 1). The ZnO ALD processes for these precursors were established by in-situ spectroscopic ellipsometry, and the resulting films were subjected to compositional analysis by XPS and RBS/NRA proving high purity ZnO. Further, UV-VIS measurements were conducted to investigate the band gaps of the materials which showed interesting precursor dependent differences. To analyze the growth behavior of the different precursors, in-situ FTIR studies were conducted hinting at distinct decomposition/deposition behavior of each precursor, as shown by the FTIR difference spectra for ZnEt(HMDS) in Fig. 2. The in-situFTIR studies also revealed different nucleation behaviors for the ALD ZnO precursors on the NMC811 cathode surface. Next, we coated NMC811 powder with 1 and 5 ZnO ALD cycles and performed XPS to verify successful ZnO ALD, analyze the surface species, and probe for potential oxidation state changes caused by the applied precursors. Whereas the observed changes in the oxidation states were small, a reproducible reduction was seen for the Ni, Co, and Mn transition metals in the cathode material, see exemplarily the ratio of Co 2+ and 3+ species in Fig. 3. Coin cell cycling tests of cathodes fabricated from the ALD ZnO-coated NMC811 powders prepared using the different Zn precursors are ongoing. These results will be especially noteworthy and showcase how the choice of ALD precursor influences the properties of a battery protective coating and allows fine tuning of the cathode surface properties.[1] Z. Ruff, C. Xu, C. P. Grey, J. Electrochem. Soc. 2021, 168, 060518.[2] Y. Li, X. Li, J. Hu, W. Liu, H. Maleki Kheimeh Sari, D. Li, Q. Sun, L. Kou, Z. Tian, L. Shao, C. Zhang, J. Zhang, X. Sun, Energy Environ. Mater. 2020, 3, 522–528.[3] D. Kang, R. E. Warburton, A. U. Mane, J. Greeley, J. W. Elam, Applied Surface Science 2022, 599, 153329. Figure 1

Analyzing the Li-Al-O Interphase of Atomic Layer-Deposited Al2O3 Films on Layered Oxide Cathodes Using Atomistic Simulations.

Alumina surface coatings are commonly applied to layered oxide cathode particles for lithium-ion battery applications. Atomic layer deposition (ALD) is one such surface coating technique, and ultrathin alumina ALD films (<2 nm) are shown to improve the electrochemical performance of LiNixMnyCo1-x-yO2 materials, with groups hypothesizing that a beneficial Li-Al-O product is being formed during the alumina ALD process. However, the atomic structure of these films is still not well understood, and quantifying the interface of ultrathin (∼1 nm) ALD films is an arduous experimental task. Here, we perform molecular dynamics simulations of amorphous alumina films of varying thickness in contact with the (0001) LiCoO2 (LCO) surface to quantify the film nanostructure. We calculate elemental mass density profiles through the films and observe that the Li-Al-O interphase extends ∼2 nm from the LCO surface. Additionally, we observe layering of Al and O atoms at the LCO-film interface that extends for ∼1.5 nm. To access the short-range order of the amorphous film, we calculated the Al coordination numbers through the film. We find that while [4]Al is the prevailing coordination environment, significant amounts of [6]Al exist at the interface between the LiCoO2 surface and the film. Taken together, these principal findings point to a pseudomorphic Li-Al-O overlayer that approximates the underlying layered LiCoO2 lattice but does not exactly replicate it. Additionally, with sufficient thickness, the Li-Al-O film transitions to an amorphous alumina structure. We anticipate that our findings on the ALD-like, Li-Al-O film nanostructure can be applied to other layered LiNixMnyCo1-x-yO2 materials because of their shared crystal structure with LiCoO2. This work provides insight into the nanostructure of amorphous ALD alumina films to help inform their use as protective coatings for Li-ion battery cathode active materials.

Interfaces in Atomic Layer Deposited Films: Opportunities and Challenges

Atomic layer deposition (ALD) is an effective method for precise layer‐wise growth of thin‐film materials and has allowed for substantial progress in a variety of fields. Advances in the technique have instigated high‐level interpretations of the relationship between nanostructure architecture and performance. An inherent part in the ALD of films is the underlying interfaces between each material, which plays a significant role in advanced electronics. Considering the impact of sandwiched substructures, it is appropriate to highlight opportunities and challenges faced by applications that rely on these interfaces. This review encompasses the current prospects and obstacles to further performance improvements in ALD‐generated interfaces. 2D electron gas, high‐k materials, freestanding layered structures, lattice matching, and seed layers, as well as prospects for future research, are explored.

Dramatic Reduction of Silicon Surface Recombination by ALD TiOx Capping Layer from TiCl4 and H2O: The Role of Chlorine.

Amorphous thin-film TiOx prepared via atomic layer deposition (ALD) has been identified as one of the most promising materials for use in transparent passivating contacts in high-efficiency and low-cost crystalline silicon (c-Si) solar cells. As highlighted in this work, the passivation performance of ALD TiOx layers strongly depends on the metal precursor used, with films prepared using TiCl4 recently showing the best results. However, a full understanding of how such films achieve their high level of surface passivation has not yet been demonstrated. This study provides a clear demonstration that a key part of this passivation mechanism is due to chlorine (Cl) accumulation at the Si surface. This mechanism is demonstrated to be quite general in nature by showing how 2 nm of ALD TiOx (TiCl4 + H2O) can be applied as a capping layer for either ZnO or Al2O3 interlayers to dramatically reduce silicon surface recombination. Cl depth profiles obtained using secondary ion mass spectrometry confirm the presence of Cl extending through the depth of the interlayers with a peak at the silicon interface. Remarkably, this diffusion of Cl is observed following low-temperature (75 °C) deposition of the TiOx capping layer, without any subsequent thermal treatment. Contrary to earlier studies that treated residual Cl in ALD films as a general contamination issue, these findings reveal unequivocally that chlorine plays a crucial role in Si surface passivation and can be classed as an effective passivation element, similar to hydrogen in its ability to passivate Si dangling bonds. The outcomes of this research emphasize the importance of residual chlorine in enhancing the passivation of buried interfaces and provide additional motivation for employing metal chloride precursors for silicon surface passivation applications.

The growth mechanisms of TiO2 film onto PET surfaces by atomic layer deposition

Atomic layer deposition (ALD) was used to coat a polyethylene terephthalate (PET) polymer substrate with TiO2 film. The TiO2 was grown onto the surface with better film coverage by using thicker ALD deposition. Further evaluation on the coated substrates indicated that the reactive sites of –C=O that existed on PET surface played a significant contribution to facilitating the initial ALD growth of the TiO2 thin film. The chemical composition of the coated substrates was characterised using energy dispersive X-ray spectroscopy, which showed that increasing the TiO2 film thickness increased the Ti element content. Two growth mechanisms, namely, diffusion growth and direct coordination through the precursor coordination with surface reactive sites of –C=O with product release occurred simultaneously in the initial growth of TiO2 coating onto PET polymer by ALD. The surface alteration of the coated ALD was characterised by Fourier transform infrared spectroscopy, which showed that the hydroxyl –OH groups emerged in the TiO2 ALD film.

Atomic Level Chemical and Structural Properties of Silicon Surface and Initial Stages of Oxidation

This work aims to summarize previous results reported in literature on atomic level properties of the wet chemically treated hydrogen-terminated silicon surfaces and of the Si oxidation, in comparison to a model system of ultraclean Si surfaces prepared in ultrahigh vacuum (UHV) conditions. A literature review shows that a proper wet chemical treatment of Si(111) provides an atomically smooth, high-quality surface, similar to the model template obtained in UHV conditions after high temperature heating. However, it seems that Si(111) is an exception among semiconductor surfaces concerning the effects of wet chemistry. Although the insulator films grown by the atomic layer deposition (ALD) have replaced the thermal oxide of SiO2 in many applications, still an intermediate SiO2 layer is formed and often grown intentionally beneath the ALD film to improve the device performance. However, a detailed atomic structure of the SiO2/Si interface is still debatable, which might be due to differences in atomic level smoothness of the used Si(100) starting surfaces.

Extending atomic layer deposition for use in next-generation piezoMEMS: Review and perspective

The objective of this work is to describe the current state of the rapidly evolving field of 3D piezoelectric microelectromechanical systems (piezoMEMS), and where it needs to go to fully leverage the potential performance benefits offered by atomic layer deposition (ALD). We define 3D piezoMEMS as the application of piezoelectric ALD films to 3D, high aspect-ratio, mechanically pliable structures. Since there are so few existing reports of 3D piezoMEMS, a literature review of ALD films applied to conventional microelectromechanical system (MEMS) devices is given. ALD processes for piezoelectric thin films are reviewed in the context of relevant applications such as transducers and actuators. Examples include aluminum nitride, hafnium zirconate, doped-hafnia, lead zirconate-titanate, lead hafnate, and lead hafnate-titanate. New concepts for ALD-enabled 3D piezoMEMS actuators are presented with supporting theoretical calculations that show that chip-scale mechanical work densities could be improved by ≫10× compared to conventional planar piezoMEMS. 3D fabrication methods are also discussed, while the future needs of atomic layer processing are highlighted.

Comparative Study of Thermal and Plasma-Enhanced Atomic Layer Deposition of Iron Oxide Using Bis(N,N'-di-butylacetamidinato)iron(II).

Only a few iron precursors that can be used in the atomic layer deposition (ALD) of iron oxides have been examined thus far. This study aimed to compare the various properties of FeOx thin films deposited using thermal ALD and plasma-enhanced ALD (PEALD) and to evaluate the advantages and disadvantages of using bis(N,N'-di-butylacetamidinato)iron(II) as an Fe precursor in FeOx ALD. The PEALD of FeOx films using iron bisamidinate has not yet been reported. Compared with thermal ALD films, PEALD films exhibited improved properties in terms of surface roughness, film density, and crystallinity after they were annealed in air at 500 °C. The annealed films, which had thicknesses exceeding ~ 9 nm, exhibited hematite crystal structures. Additionally, the conformality of the ALD-grown films was examined using trench-structured wafers with different aspect ratios.

In situ x-ray photoelectron spectroscopy analysis of the atomic layer deposition of Al2O3 on SiO x /Si: Interface dipole and persistent surface groups

Atomic layer deposition (ALD) has become an essential technology in many areas. To better develop and use this technology, it is of the pivot to understand the surface chemistry during the ALD film growth. The growth of an ALD oxide film may also induce an electric dipole at the interface, which may be further tuned to modulate the flat band voltage for electronic device applications. To understand the associated surface chemistry and interface dipole formation process, we herein employ an in situ x-ray photoelectron spectroscopy technique to study the ALD growth of Al2O3, from trimethylaluminum and H2O, on the SiO x /Si surface. We find that an electric dipole is formed at the Al2O3/SiO x interface immediately after the first Al2O3 layer is deposited. We also observe persistent surface methyl groups in the H2O half-cycle during ALD, and the amount of the persistent methyls is particularly higher during the initial Al2O3 ALD growth, which suggests the formation of Si−CH3 on the surface. These findings can provide useful routes and insights toward interface engineering by ALD.

Tunable Sulfur Incorporation into Atomic Layer Deposition Films Using Solution Anion Exchange

Metal sulfide and oxysulfide thin films deposited by atomic layer deposition (ALD) are important functional materials for a range of applications including solar cells, catalysts, and electronic devices. However, ALD of sulfur-containing films typically requires H2S, a toxic, corrosive, and flammable gas. To circumvent these challenges, we propose a method of tuning sulfur incorporation into ALD films by first growing a metal oxide, using CuOx as a model system, followed by a solution-phase sulfur anion-exchange process. By controlling the reaction time, solution molarity, and temperature, we demonstrate tunable sulfur incorporation into the films, which is described using a coupled reaction-diffusion model. The evolution of the film crystallinity, composition, and morphology was quantified as a function of the anion-exchange process parameters. Conformal anion exchange on a ZnO@TiO2@CuO core–shell–shell nanowire is shown, demonstrating the ability to maintain the conformality of the initial ALD process on high-aspect-ratio structures. Additionally, area-selective anion exchange was performed on micro-patterned substrates, illustrating a pathway toward device fabrication. Finally, the electrical properties of the converted films were evaluated, indicating a tunable reduction in sheet resistance of up to 4 orders of magnitude. In the future, the combination of ALD and anion-exchange chemistry can be used to incorporate sulfur without the need for H2S gas, while maintaining the atomically precise and conformal properties of the original ALD process.

Super-conformal TiN thin film deposition by carrier pulse purge atomic layer deposition system: Chamber design optimization with computational fluid dynamics

Owing to the increase in demand for the shrinkage of semiconductor design rules and the advent of the three-dimensional structure of semiconductor devices, atomic layer deposition (ALD) has emerged as an attractive alternative to conventional chemical vapor deposition techniques. Although the development of precursors and characterization of ALD films have been widely studied, few studies have been conducted on the modification of ALD parameters and optimization of the ALD process. In this study, an advanced ALD titanium nitride (TiN) system was designed using a pumping and purging simulationBased on the simulation results, we adopted a carrier pulse purge method, where TiCl4 fed, purge N2, pulse purge N2, purge N2, NH3 fed, purge N2, pulse purge N2, and purge N2 were alternatively injected into the chamber in the ALD cycle. Compared to the conventional purge method, the carrier pulse purge method can reduce the ALD cycle time by 18.27% and yields a high-quality (high step coverage, 98 %) TiN film with lower electrical resistivity and chlorine impurity. This carrier pulse purge method can ultimately lead to improved throughput and productivity in the semiconductor industry by reducing the ALD cycle time.

Simulation of conformality of ALD growth inside lateral channels: comparison between a diffusion-reaction model and a ballistic transport-reaction model.

Atomic layer deposition (ALD) has found significant use in the coating of high-aspect-ratio (HAR) structures. Approaches to model ALD film conformality in HAR structures can generally be classified into diffusion-reaction (DR) models, ballistic transport-reaction (BTR) models and Monte Carlo simulations. This work compares saturation profiles obtained using a DR model and a BTR model. The saturation profiles were compared qualitatively and quantitatively in terms of half-coverage penetration depth, slope at half-coverage penetration depth and adsorption front broadness. The results showed qualitative agreement between the models, except for a section of elevated surface coverage at the end of the structure, 'trunk', observed in the BTR model. Quantitatively, the BTR model produced deeper penetration into the structure, lower absolute values of the slope at half-coverage penetration depth and broader adsorption fronts compared to the DR model. These differences affect the values obtained when extracting kinetic parameters from the saturation profiles.

Band alignment of sputtered and atomic layer deposited SiO2 and Al2O3 on ScAlN

The band alignments of two candidate dielectrics for ScAlN, namely, SiO2 and Al2O2, were obtained by x-ray photoelectron spectroscopy. We compared the effect of deposition method on the valence band offsets of both sputtered and atomic layer deposition films of SiO2 and Al2O3 on Sc0.27Al0.73 N (bandgap 5.1 eV) films. The band alignments are type I (straddled gap) for SiO2 and type II (staggered gap) for Al2O3. The deposition methods make a large difference in relative valence band offsets, in the range 0.4–0.5 eV for both SiO2 and Al2O3. The absolute valence band offsets were 2.1 or 2.6 eV for SiO2 and 1.5 or 1.9 eV for Al2O3 on ScAlN. Conduction band offsets derived from these valence band offsets, and the measured bandgaps were then in the range 1.0–1.1 eV for SiO2 and 0.30–0.70 eV for Al2O3. These latter differences can be partially ascribed to changes in bandgap for the case of SiO2 deposited by the two different methods, but not for Al2O3, where the bandgap as independent of deposition method. Since both dielectrics can be selectively removed from ScAlN, they are promising as gate dielectrics for transistor structures.

Improvement of Mechanical Characteristics on Ultra-High Molecular Weight Polyethylene Surface through Zinc Oxide Atomic Layer Deposition Film

Wear failures in ultra-high molecular weight polyethylene (UHMWPE) compartments are known to limit the life of a joint implant. In this study, we applied atomic layer deposition (ALD) technique to produce wear-resistant zinc oxide (ZnO) films to improve mechanical and wear characteristics on the surface of UHMWPE. Organic material layers in molecular units were mixed with ALD ZnO films to minimize surface cracks through a molecular layer deposition (MLD) technique. We aimed to examine (1) the effect of organic layers on minimizing surface cracks and (2) the mechanical properties of ZnO and inorganic/organic hybrid films. We prepared UHMWPE samples with pure ZnO and five different hybrid films with ZnO to organic layer ratios of 1 : 1, 2 : 1, 3 : 1, 4 : 1, and 5 : 1. Surface cracks were observed by using a field-emission scanning electron microscope. Hardness and elastic modulus of an ALD-coated UHMWPE were measured by nano-indentation examinations. Severe cracks were found in the samples with pure ZnO films. However, no cracks were found in the samples with all hybrid films except the samples with a ZnO to organic layer ratio of 5 : 1. The hardness and elastic modulus of the samples with pure ZnO and hybrid films significantly increased compared to those of the native UHMWPE. The hardness to elastic modulus ratios, indicating the wear resistance, increased over 50% for all films compared to the values of native UHMWPE. These results suggested that ALD ceramic coatings with organic layers may become a potential solution for realizing a wear-resistant protection coating for the polymer compartment of joint prostheses.