Atomic Layer Deposition Of TiO2 Research Articles

Growth of Atomic layer-deposited Monoclinic Molybdenum Dioxide Films Stabilized by Tin Oxide Doping for DRAM Capacitor Electrode Applications.

Dynamic random-access memory (DRAM) capacitor electrodes, exemplified by TiN, face performance limitations owing to their relatively low work functions in addition to the formation of a low-k interfacial layer caused by their insufficient chemical stability. With recent advances in device scaling, these issues have become increasingly problematic, prompting the exploration of alternative electrode materials to replace TiN. Molybdenum dioxide (MoO2) has emerged as a promising candidate for this application, outperforming TiN due to its low resistivity, high work function (>5 eV), and excellent chemical stability. Moreover, monoclinic MoO2 exhibits a distorted rutile structure, enabling the in situ growth of high-k rutile TiO2 on MoO2 at low deposition temperatures. However, MoO2 deposition poses challenges because of its metastable nature compared to the more stable molybdenum oxide (MoOx) phases, such as MoO3 and Mo4O11. In this work, we successfully fabricated Sn-doped MoOx (TMO) films by atomic layer deposition (ALD) at 300 °C. A stabilized monoclinic MoO2 phase was achieved using ALD by incorporating SnOx into MoOx on both SiO2 and TiN substrates. The ALD TMO process comprised MoOx and SnOx subcycles, and the MoOx:SnOx subcycle ratio was varied from 100:1 to 20:1. High growth rates ranging from 0.19 to 0.34 nm/cycle were achieved for ALD TMO with varying the MoOx:SnOx subcycle ratio from 20:1 to 100:0. After post-deposition annealing at 500 °C, polycrystalline TMO films were obtained with smooth surface morphology. ALD TMO exhibited excellent interface quality with ALD TiO2, possessing a negligible low-k interfacial layer. Moreover, a rutile TiO2 film with a high dielectric constant of 136 was successfully grown on a 20% Sn-TMO electrode. Overall, this study provides a strategy to stabilize metastable MoO2 films using ALD, and it demonstrates the superiority of ALD TMO as a promising DRAM capacitor electrode material.

Low‐Temperature Dual‐Material Area‐Selective Deposition: Molybdenum Hexafluoride‐Mediated SiO2 Fluorination/Passivation for Self‐Aligned Molybdenum/Metal Oxide Nanoribbons

AbstractArea‐selective deposition (ASD) is a forefront nanopatterning technique gaining substantial attention in the semiconductor industry. While current research primarily addresses single‐material ASD, exploring multi‐material ASD is essential for mitigating complexity in advanced nanopatterning. This study describes molybdenum hexafluoride (MoF6)‐mediated fluorination/passivation of the hydroxylated SiO2 (SiO2‒OH) at 250 °C as a new method to pacify nucleation during subsequent ZnO and TiO2 atomic layer deposition (ALD). In contrast, Al2O3 ALD is not passivated on the fluorinated SiO2 (SiO2‒F). The study further shows that Mo ALD using MoF6 and silane (1 wt% SiH4 in Ar) selectively proceeds on hydrogen‐terminated Si (Si‒H), whereas SiO2‒OH becomes fluorine‐passivated without observable Mo deposition. This enables subsequent ZnO and TiO2 ASD on Mo versus SiO2‒F, as confirmed by X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). Proposed growth and inhibition mechanisms highlight the importance of the metal oxide precursor in achieving selectivity. Taken together, self‐aligned Mo/ZnO and Mo/TiO2 nanoribbons are demonstrated on coplanar nanoscale Si‒H/SiO2‒OH patterns by sequentially integrating two individual ASD processes: 1) Mo ASD on Si‒H versus SiO2‒OH; and 2) ZnO or TiO2 ASD on Mo versus SiO2‒F. This work highlights the potential for this approach in new ASD systems.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon.

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Comparison of three titanium-precursors for atomic-layer-deposited TiO2 for passivating contacts on silicon

Atomic layer-deposited (ALD) TiO2 thin films on silicon were deposited using titanium tetrachloride (TiCl4), titanium tetraisopropoxide (TTIP), and tetrakis(dimethylamino)titanium (TDMAT) together with water vapor as the oxidant at temperatures ranging between 75 and 250 °C. The Si surface passivation quality of as-deposited and isothermally annealed samples was compared using photoconductance lifetime measurements in order to calculate their effective surface recombination velocities Seff. A low Seff of 3.9 cm/s (J0s=24fA/cm2) is achieved for as-deposited TiCl4-TiO2 at 75 °C when a chemically grown (i.e., from RCA cleaning) SiOx interface layer is present. Depositing TTIP-TiO2 at 200 °C on a chemically grown SiOx interface layer yields equivalent Seff values; however, in this case, TTIP-TiO2 requires a 5–15 min postdeposition forming gas anneal at 250 °C. In contrast, TDMAT-TiO2 was not found to provide a similar level of passivation with/without a chemically grown SiOx interface layer and postdeposition anneal. Modeling of the effective lifetime curves was used to determine the magnitude of the effective charge densities Qf in the TiO2 films. In all cases, Qf was found to be of the order of ∼1011 q cm−2, meaning field-effect passivation arising from ALD TiO2 is relatively weak. By comparing the material properties of the various TiO2 films using ellipsometry, photothermal deflection spectroscopy, Raman spectroscopy, elastic recoil detection analysis, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, we find experimental support for the role of Cl (in conjunction with hydrogen) playing a beneficial role in passivating dangling bond defects at the Si surface. It is concluded that low deposition temperature TiCl4 processes are advantageous, by providing the lowest Seff without any postanneal and a comparatively high growth per cycle (GPC).

Inhibition of atomic layer deposition of TiO2 by functionalizing silicon surface with 4-fluorophenylboronic acid

As the size of the components in electronic devices decreases, new approaches and chemical modification schemes are needed to produce nanometer-size features with bottom-up manufacturing. Organic monolayers can be used as effective resists to block the growth of materials on non-growth substrates in area-selective deposition methods. However, choosing the appropriate surface modification requires knowledge of the corresponding chemistry and also a detailed investigation of the behavior of the functionalized surface in realistic deposition schemes. This study aims to investigate the chemistry of boronic acids that can be used to prepare such non-growth areas on elemental semiconductors. 4-Fluorophenylboronic acid is used as a model to investigate the possibility to utilize the Si(100) surface functionalized with this compound as a non-growth substrate in a titanium dioxide (TiO2) deposition scheme based on sequential doses of tetrakis(dimethylamido)titanium and water. A combination of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry allows for a better understanding of the process. The resulting surface is shown to be an effective non-growth area to TiO2 deposition when compared to currently used H-terminated silicon surfaces but to exhibit much higher stability in ambient conditions.

Unfolding an Elusive Area-Selective Deposition Process: Atomic Layer Deposition of TiO2 and TiON on SiN vs SiO2.

Area-selective atomic layer deposition (AS-ALD) processes for TiO2 and TiON on SiN as the growth area vs SiO2 as the nongrowth area are demonstrated on patterns created by state-of-the-art 300 mm semiconductor wafer fabrication. The processes consist of an in situ CF4/N2 plasma etching step that has the dual role of removing the SiN native oxide and passivating the SiO2 surface with fluorinated species, thus rendering the latter surface less reactive toward titanium tetrachloride (TiCl4) precursor. Additionally, (dimethylamino)trimethylsilane was employed as a small molecule inhibitor (SMI) to further enhance the selectivity. Virtually perfect selectivity was obtained when combining the deposition process with intermittent CF4/N2 plasma-based back-etching steps, as demonstrated by scanning and transmission electron microscopy inspections. Application-compatible thicknesses of ∼8 and ∼5 nm were obtained for thermal ALD of TiO2 and plasma ALD of TiON.

Ultrathin TiO2 Coatings via Atomic Layer Deposition Strongly Improve Cellular Interactions on Planar and Nanotubular Biomedical Ti Substrates.

This work aims to investigate the chemical and/or structural modification of Ti and Ti-6Al-4V (TiAlV) alloy surfaces to possess even more favorable properties toward cell growth. These modifications were achieved by (i) growing TiO2 nanotube layers on these substrates by anodization, (ii) surface coating by ultrathin TiO2 atomic layer deposition (ALD), or (iii) by the combination of both. In particular, an ultrathin TiO2 coating, achieved by 1 cycle of TiO2 ALD, was intended to shade the impurities of F- and V-based species in tested materials while preserving the original structure and morphology. The cell growth on TiO2-coated and uncoated TiO2 nanotube layers, Ti foils, and TiAlV alloy foils were compared after incubation for up to 72 h. For evaluation of the biocompatibility of tested materials, cell lines of different tissue origin, including predominantly MG-63 osteoblastic cells, were used. For all tested nanomaterials, adding an ultrathin TiO2 coating improved the growth of MG-63 cells and other cell lines compared with the non-TiO2-coated counterparts. Here, the presented approach of ultrathin TiO2 coating could be used potentially for improving implants, especially in terms of shading problematic F- and V-based species in TiO2 nanotube layers.

Comparison of H2O2 and H2O oxidations on TDMAT absorbed on silicon(100) surface during reaction step of ALD–TiO2 process: A DFT study

Atomic layer deposition (ALD) is a thin film technology with high potential in nano fabrication. There is industrial demand to produce an ALD–TiO2 thin film using a low temperature process for applications of bending electronic devices and biomaterials. TDMAT has gained the most attention as a suitable titanium source to grow an ALD–TiO2 film at extremely low temperatures. In this process, almost all published reports have focused on metal-oxide precursors. The current work investigates the effects of oxidizing agents. For the first time, a theoretical investigation was carried out on the oxidation of H2O2 and H2O on TDMAT absorbed on a silicon (100) surface during the reaction step of the ALD–TiO2 process. We simulated the oxidation process, which continued from the adsorption step of the ALD–TiO2 process, where the TDMAT was used as a titanium precursor. The cluster of the Si9H12 =O2 =Ti[N(CH3)]2 surface model was used to represent the remaining TDMAT molecule absorbed on the Si(100) substrate. The reaction mechanisms of H2O2 oxidizing on this surface were evaluated based on their reaction pathways. The reaction pathway of H2O oxidizing on this surface was calculated at the same level to compare the effect of oxidizers. The DFT at the B3LYP 6–311 G+ (2d,p) and 6–31 G(d,p) theoretical levels were used to calculate the potential energy surface and to optimize the electronic structures in this process. It was found that the reaction mechanism depended on the nature of chemical bond dissociation within the oxidizer molecules during the oxidation process. In the oxidation of H2O2, the H2O2 molecule contained O–O and O–H bonds, which were dissociated during the oxidation process, resulting in different product structures. Only the O–O bond dissociation pathway produced the TiO2 structure. The two species of TDMAT ligands at the surface were completely oxidized using a single H2O2 molecule. The H2O2 oxidation produced more –OH groups (double) compared to H2O oxidation. In H2O oxidation, the H2O molecule contained only the O–H bond within its molecule and required activation energies significantly less than for all pathways of H2O2. The HOMO–LUMO properties and IR frequency characteristics (cm−1) of the plausible product structures were discussed. The results should be useful in guiding the selection of suitable precursors for use in the co–reactant system of the ALD–TiO2 process.

Effect of Adhesion Layer at the Pt/TiO2 Interface on the Degradation Pathways of TiO2-Protected III-V Water-Splitting Photocathodes

Photoelectrochemical (PEC) water splitting has been identified as a promising route towards the production of green hydrogen as a solar fuel to meet the global energy demands [1]. Highly efficient photoelectrodes made of III-V compounds are promising candidates to reach the goal of >15% solar-to-hydrogen efficiency [2]. However, the main bottle neck of these materials is their poor stability in direct contact with acidic electrolytes, which is needed to ensure efficient mass transport and an efficient hydrogen evolution reaction (HER). Atomic layer deposited (ALD) titanium dioxide (TiO2) is commonly utilized as a protective layer to shield the absorber from the electrolyte [3]. To reduce HER overpotentials, the TiO2 film needs to be modified with a catalyst layer. In a demonstration of a TiO2 protected III-V PEC device, Cheng et al. proposed that the detachment of the catalyst layer induced the degradation of the protective TiO2 film in a pH 0 electrolyte which eventually led to the corrosion of the photoabsorber [4].In this work, GaInP/GaAs/Ge photocathodes for water reduction were protected with ALD TiO2 layers deposited from tetrakis(dimethylamido)titanium (TDMAT) and water precursors with varying thicknesses. A 2 nm catalytic Pt layer was deposited via electron beam evaporation (EBE) to reduce the overpotential of the HER. The conformal coverage of the absorber with TiO2 was confirmed with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). None of the TiO2 films were able to stabilize the illuminated III-V electrodes for 60 min at 0 V versus the counter electrode (CE) in a pH 0 electrolyte (1 M H2SO4). Characterization of the electrochemical potential at the electrode’s front side revealed an unstable regime for the TiO2 protection layer. ICP-MS of the electrolytes, SEM/EDX and electrochemical analyses revealed the detachment of the catalytic Pt layer that led to the dissolution of the TiO2 and the III-V degradation (Figure 1A). The adhesion of Pt on TiO2 was improved by depositing a 2 nm Ti layer via EBE at the Pt/TiO2 interface (Figure 1B). The adhesion layer acts as a wetting layer for Pt, reducing the nucleation energy barrier and increasing the number of nucleation sites compared to Pt/TiO2. The enhanced wetting is due to the formation of Ti−Pt chemical bonds [5] and the overpotential @ 10 mA/cm2 to perform HER was 60 mV, similar to that of a pristine EBE Pt layer. The Ti layer greatly improves the stability of TiO2/III-V photoelectrodes, confirming the results of Cheng et al. and illustrating the importance of a stable catalyst layer.Figure 1. A) SEM micrograph of degradation of Ge/GaAs/GaInP/TiO2/Pt photocathode after 60 min. of photocurrent measurements at 0 V vs. CE in 1 M H2SO4. B) Accelerated stability test in dark for 2 nm Pt/15 nm TiO2 on FTO with (red curve) and without (black curve) a 2 nm Ti adhesion layer in between the Pt and TiO2 layers.

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.

Effects of atomic layer deposition on the optical properties of two-dimensional transition metal dichalcogenide monolayers

Two-dimensional semiconducting transition metal dichalcogenides (TMDs) have attracted significant interest due to their unique optoelectronic properties. More often, these materials are enclosed inside a dielectric layer that can work as an insulator for field-effect transistors. The insulating layer is typically grown with atomic layer deposition (ALD). Here, we study the effects on bare and hBN-covered monolayer MoS2 and WSe2 flakes with ALD TiO2 films. Our results reveal a significant shift and decrease in intensity in photoluminescence and Raman signals of the monolayer TMDs. Further analysis suggests that these changes are caused by chemical doping, strain, and dielectric screening after the ALD. Our study not only sheds light on the impact of ALD on the properties of TMDs, but also indicates ALD can be an alternative method to engineer the doping, strain and dielectric environment for potential optoelectronics and photonics applications.

(Invited) Enhanced Photocatalysis on TiO2-Passivated III-V Compound Semiconductors and 2D Material Heterostructures for Water Splitting and CO2 Reduction

We report enhanced photocatalysis for H2 evolution and CO2 reduction using TiO2-passivated InP and GaAs photocathodes.1-4 The TiO2 layer makes the InP semiconductor photochemically stable. This represents a major step forward in photocatalysis, which has typically been limited to metal oxide materials. In addition to making these surfaces stable, the TiO2 film, deposited by atomic layer deposition (ALD), also provides a substantial enhancement in the efficiency of H2 evolution. We find that passivating GaAs with just a few nm of TiO2 produces a shift in the onset potential of H2 evolution by +0.35 V at 1 mA/cm2 and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE). Here, thinner TiO2 films produce a larger enhancement than thicker films, which correlates with the higher density of O-vacancies (i.e., Ti3+ surface states) observed in these thinner films using X-ray photoemission spectroscopy (XPS). While TiO2 films 1-5nm thick produce large enhancements, no enhancement is observed for TiO2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO2 films.Using transient reflectance spectroscopy (TRS), we measure the photoexcited carrier dynamics in a GaP/TiO2 photoelectrode, as well as the electrostatic field dynamics at this semiconductor-liquid interfaces in situ under various electrochemical potentials.5 Here, the electrostatic fields at the surface of the semiconductor are measured via Franz−Keldysh oscillations (FKO). These spectra reveal that the nanoscale TiO2 protection layer enhances the built-in field and charge separation performance of GaP photoelectrodes.We also demonstrated plasmon-resonant enhancement of photocatalysis on monolayer WSe2.6 In addition, we measured the stacking dependence and resonant interlayer excitation of monolayer WSe2/MoSe2 heterostructures for photocatalytic energy conversion.7 Qiu, J., G. Zeng, M. Ge, S. Arab, M. Mecklenburg, B. Hou, C. Shen, A.V. Benderskii and S.B. Cronin, Correlation of Ti3+ states with photocatalytic enhancement in TiO2-passivated p-GaAs. Journal of Catalysis, 337, 133 (2016).Qiu, J., G.T. Zeng, M.A. Ha, M.Y. Ge, Y.J. Lin, M. Hettick, B.Y. Hou, A.N. Alexandrova, A. Javey and S.B. Cronin, Artificial Photosynthesis on TiO2-Passivated InP Nanopillars. Nano Letters, 15, 6177-6181 (2015).Qiu, J., G.T. Zeng, M.A. Ha, B.Y. Hou, M. Mecklenburg, H.T. Shi, A.N. Alexandrova and S.B. Cronin, Microscopic Study of Atomic Layer Deposition of TiO2 on GaAs and Its Photocatalytic Application. Chemistry of Materials, 27, 7977-7981 (2015).Wang, Y. and S.B. Cronin, Performance Enhancement of TiO2-encapsulated Photoelectrodes Based on III–V Compound Semiconductors, in Ultrathin Oxide Layers for Solar and Electrocatalytic Systems. 2022, Royal Society of Chemistry. p. 103-134.Xu, Z.H., B.Y. Hou, F.Y. Zhao, Z. Cai, H.T. Shi, Y.W. Liu, C.L. Hill, D.G. Musaev, M. Mecklenburg, S.B. Cronin and T.Q. Lian, Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes. Nano Letters, 21, 8017-8024 (2021).Chen, J., C.S. Bailey, Y. Hong, L. Wang, Z. Cai, L. Shen, B. Hou, Y. Wang, H. Shi, J. Sambur, W. Ren, E. Pop, and S.B. Cronin., Plasmon-Resonant Enhancement of Photocatalysis on Monolayer WSe2. ACS Photonics, 6, 787 (2019).Chen, J., C.S. Bailey, D. Cui, Y. Wang, B. Wang, H. Shi, Z. Cai, E. Pop, C. Zhou and S.B. Cronin, Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion. ACS Applied Nano Materials, DOI:10.1021/acsanm.9b01898 (2020).

Highly regular laser-induced periodic silicon surface modified by MXene and ALD TiO2 for organic pollutants degradation

The co-catalytic ability of surface groups in MXene renders MXene-based composites efficient photocatalysts for organic dye photodegradation. In this study, we introduce a novel approach combining highly-regular laser-induced periodic surface Si structures (Si nanoripples - SiNR) with MXene and TiO2, deposited through atomic layer deposition (ALD), to achieve stable, repeatable, and highly efficient photocatalytic materials. The ternary system, SiNR/MXene/TiO2, exhibits high light absorption across the entire spectrum. Furthermore, the photodegradation efficiency of SiNR/MXene/TiO2 is approximately 2.6 times higher than that of SiNR/TiO2 samples. This remarkable enhancement in photocatalytic activity can be attributed to two main factors: increased separation of photogenerated charge carriers and enhanced light absorbance through the light trapping effect. The synergistic combination of SiNRs, MXene, and TiO2 in the ternary nanocomposite structure enables efficient separation of photogenerated charge carriers, thereby minimizing recombination and enhancing the degradation of organic dyes. Moreover, the introduction of a laser-patterned surface and its ALD modification significantly enhances light absorbance, ensuring effective utilization of a wider range of the solar spectrum for photocatalysis. Therefore, SiNR/MXene/TiO2 ternary nanocomposites hold great promise for developing high-performance photocatalysts for water purification applications, offering improved stability, repeatability, and enhanced photocatalytic efficiency.

Ti doped hematite photoanodes: Protective coverage by titania overlayers

In this work, the photoresponse of Ti doped hematite electrodes was optimized by adjusting the thickness and the level of doping. The electrodes were then covered by a TiO2 overlayer by dip coating (DC) or by atomic layer deposition (ALD) to improve chemical stability. Coverage of hematite by a dip-coated sol–gel TiO2 film of thickness up to 135 nm resulted only in a small decrease in photocurrent (by about 30 %). A similar decrease in photocurrent (about 25 %) was observed after the coverage by a very thin (2 nm) ALD TiO2 film while the increase of the thickness to 8 nm led to almost complete suppression of photocurrent. This behaviour can be explained by the different morphology and structure of the overlayers. Although the surface morphology of sol–gel TiO2 films was very smooth and dense, penetration of such films by the electrolyte occurred even for a thickness of 135 nm. On the other hand, much thinner ALD TiO2 films (8 nm) exhibited almost no electrolyte penetration. Such different blocking properties of the two TiO2 overlayers are in direct agreement with the observed dissolution rate in acidic media – the better the blocking properties of an overlayer the better the chemical stability of stratified hematite/titania photoelectrode.

A systematical investigation of layer growth rate, impurity level and morphology evolution in TiO2 thin films grown by ALD between 100 and 300 °C

TiO2 thin films prepared by atomic layer deposition (ALD) have attracted great attention due to the widespread application of the oxide as a promising charge storage material for lithium or proton batteries. In this work, we study TiO2 film grown on Si substrates by atomic layer deposition with tetrakis (dimethylamino) titanium as metal precursor (TDMAT) and water vapour as an oxidant. The chemical composition and impurity content of the film as a function of growth temperature is studied by Ion Beam Analysis (IBA). D2O (99.8%) was used as oxidant to study the film growth to distinguish between hydrogen atoms originating from the water oxidant or the metal precursor. Combining ellipsometry with Rutherford Backscattering Spectrometry (RBS), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection Analysis (ERDA) reveals film density as a function of growth temperature. Atomic force microscopy (AFM) was used to characterize the film surface structure. By investigating the structural and compositional range of ALD TiO2 films will open the new opportunities of application.

Quantified Uniformity and Selectivity of TiO2 Films in 45‐nm Half Pitch Patterns Using Area‐Selective Deposition Supercycles

AbstractArea‐selective deposition (ASD) shows great promise for sub‐10 nm manufacturing in nanoelectronics, but significant challenges remain in scaling to ultrasmall dimensions and understanding feature‐dependent nonuniformity and selectivity loss. This work addresses these problems by simultaneously quantifying uniformity and selectivity for passivation/deposition/etch supercycles in 45 nm half‐pitch TiN/SiO2 line/space patterns. This work employs three selective processes that are uniquely suited for supercycle processing: dimethylamino‐trimethylsilane (DMA‐TMS) inhibition, TiO2 atomic layer deposition (ALD), and HBr/BCl3 plasma etch. The DMA‐TMS inhibition selectively passivates the SiO2 nongrowth surface without affecting deposition on the TiN and TiO2 growth surfaces. The plasma etch removes TiO2 defect particles at a faster rate than the conformal TiO2 film or SiO2 lines. Using three supercycles of this process, this work demonstrates 8 nm of TiO2 with 88% uniformity and ≈100% selectivity according to transmission electron microscopy (TEM), a 2×improvement in film thickness from previous reports in similar nanoscale patterns. Integrated consideration of uniformity and selectivity at specific feature scales will facilitate the effective design of selective deposition processes for nanoscale electronic devices.

In Vitro Study of Zirconia Surface Modification for Dental Implants by Atomic Layer Deposition.

Zirconia is a promising material for dental implants; however, an appropriate surface modification procedure has not yet been identified. Atomic layer deposition (ALD) is a nanotechnology that deposits thin films of metal oxides or metals on materials. The aim of this study was to deposit thin films of titanium dioxide (TiO2), aluminum oxide (Al2O3), silicon dioxide (SiO2), and zinc oxide (ZnO) on zirconia disks (ZR-Ti, ZR-Al, ZR-Si, and ZR-Zn, respectively) using ALD and evaluate the cell proliferation abilities of mouse fibroblasts (L929) and mouse osteoblastic cells (MC3T3-E1) on each sample. Zirconia disks (ZR; diameter 10 mm) were fabricated using a computer-aided design/computer-aided manufacturing system. Following the ALD of TiO2, Al2O3, SiO2, or ZnO thin film, the thin-film thickness, elemental distribution, contact angle, adhesion strength, and elemental elution were determined. The L929 and MC3T3-E1 cell proliferation and morphologies on each sample were observed on days 1, 3, and 5 (L929) and days 1, 4, and 7 (MC3T3-E1). The ZR-Ti, ZR-Al, ZR-Si, and ZR-Zn thin-film thicknesses were 41.97, 42.36, 62.50, and 61.11 nm, respectively, and their average adhesion strengths were 163.5, 140.9, 157.3, and 161.6 mN, respectively. The contact angle on ZR-Si was significantly lower than that on all the other specimens. The eluted Zr, Ti, and Al amounts were below the detection limits, whereas the total Si and Zn elution amounts over two weeks were 0.019 and 0.695 ppm, respectively. For both L929 and MC3T3-E1, the cell numbers increased over time on ZR, ZR-Ti, ZR-Al, and ZR-Si. Particularly, cell proliferation in ZR-Ti exceeded that in the other samples. These results suggest that ALD application to zirconia, particularly for TiO2 deposition, could be a new surface modification procedure for zirconia dental implants.

A First-Principles Investigation of the Driving Forces Defining the Selectivity of TiO2 Atomic Layer Deposition

Area-selective deposition (ASD) is a technique to deposit material only on a defined area of a prepatterned surface, while no deposition occurs on adjacent surface areas. It is the subject of intense investigations by the scientific and engineering communities as it offers the prospect to simplify and improve patterning processes for fabrication of nanoelectronic devices as well as to reduce the manufacturing costs. Numerous efforts have been dedicated to identify process conditions for highly selective ASD processes. Still, the search for optimal conditions is often an empirical process due to the limited understanding of the mechanisms that take place at the atomic scale. Understanding the links between precursor reactivity, surface treatments, and the reactor operating conditions could greatly contribute to the development of highly selective ASD processes. In this paper, we therefore combine first-principles calculations with statistical thermodynamics to understand the role of the precursors in area-selective TiO2 atomic layer deposition (ALD). First, we investigate the selectivity loss mechanisms for TiCl4/H2O ALD on SiO2 nongrowth surfaces with different surface terminations (e.g., OH groups and trimethylsilyl groups). We link the resulting thermodynamic driving forces to experimental reports. Subsequently, we extend the investigation to a total of 26 commercially available titanium precursors and to three different oxygen sources and rank their potential for TiO2 ASD for the SiO2 surfaces with different surface terminations (OH groups and trimethylsilyl groups). We find that the combination of TiCl4 with H2O offers the best performance in terms of selectivity. The theoretical approach proposed here is expected to greatly assist and accelerate the design of precursors for different ASD approaches.

Band alignment of TiO2/SiC and TiO2/Si heterojunction interface grown by atomic layer deposition

Silicon carbide (SiC) is regarded as a promising semiconductor owing to its wide band gap and high thermal conductivity. Meanwhile, it possesses issues such as interface properties, which may affect the performance of SiC substrate power devices (e.g. MOSFET), especially when compared with similarly structured silicon appliances. Given that the development of SiC semiconductor devices has a number of commonalities with conventional silicon-based semiconductors, titanium dioxide (TiO2), a material that has a great track record in Si-based semiconductor devices, has been chosen for investigation in this work. Although TiO2 is not capable of being a gate dielectric alone on the SiC substrate because of its relatively narrow band gap, it can be adopted into composite or multilayer gate dielectrics to reach satisfying characteristics. As such, the interfacial state and heterostructure between TiO2 and SiC remain worthy being researched. In the present study, the properties of atomic layer deposited (ALD) TiO2 films on silicon substrates were compared with those on 4H-SiC substrates via x-ray photoelectron spectroscopy, atomic force microscopy, and x-ray reflectometry. It is shown that the interface state between ALD TiO2 film and both types of substrates as-deposited have similar chemical conditions, whereby TiO2 layer barely react with substrates, containing great amount of oxygen vacancies. According to band alignment calculations, heterostructure of both samples are type-II heterojunctions with negatively shifted conduction band. Although the large bandwidth of 4H-SiC hinders the use of TiO2 as a gate dielectric in power devices, this structure has the potential for other semiconductor products.

White and black anodic TiO2 nanotubes: Comparison of biological effects in A549 and SH-SY5Y cells

In this work, the cytotoxicity of black TiO2 nanotubes in comparison to their white counterparts is exploited for the first time. To provide a comprehensive overview, the toxicity is also assessed in comparison to multi-walled carbon nanotubes (MWCNTs) using two different cell lines. The TiO2 nanotube layers were prepared by anodic oxidation of Ti foils and investigated in the as-prepared white form, and in the black form, achieved by reduction via comproportionation reaction of Ti and TiO2. Additionally, a part of the black TiO2 nanotube layers was coated with a thin TiO2 coating with a nominal thickness of ~0.275 nm by Atomic Layer Deposition (ALD). Pulmonary A549 cells and neural SH-SY5Y cells were exposed to a wide range of TiO2 nanotube concentrations (1–500 μg/mL) for 24 and 48 h. Then, the cytotoxicity was evaluated using cell viability and glutathione assessments. Essentially, no reduction in cell viability was observed after incubating the cells with TiO2 nanotubes up to 100 μg/mL, while MWCNTs decreased the cell viability at a similar concentration significantly. In addition, ALD TiO2 coating on the black TiO2 nanotubes caused disappearance of mild toxic effects of white and black TiO2 nanotubes due to the shading of carbon and fluorine species incorporated within the TiO2 nanotube walls.