Typical Atomic Layer Deposition Research Articles

Chemical mechanism for nucleation enhancement in atomic layer deposition of Pt by surface functionalization

Atomic layer deposition (ALD) of metals on oxide substrates often yields islandlike growth, undesirable toward conductor applications. Thus, enhancement of initial nucleation of metal ALD is of great importance. One potential solution reported is to functionalize the substrate surface with small molecules, such as trimethylaluminum (TMA) and aluminum chloride (AlCl3) before metal ALD, so that nucleation delay is reduced, and the coverage and conformality are enhanced. In this study, density functional theory calculations are utilized to elucidate the chemical mechanism toward a change of the reactivity of the surface toward metal ALD upon functionalization by TMA or AlCl3. Surface adsorption of a typical Pt ALD precursor, trimethyl(methylcyclopentadienyl)platinum (MeCpPtMe3), on an OH-terminated SiO2 surface is compared to its adsorption on AlCH3-terminated and AlCl-terminated surfaces. Considering rate-limiting steps on each surface, greater rates for adsorption of Pt are found for the organometallic-functionalized surfaces than the bare SiO2 surface, confirming the chemical enhancement effect by functionalization.

(Invited) In Situ Measurements during ALD/ALE Processes

Atomic layer deposition (ALD) and atomic layer etching (ALE) provide exquisite control over the addition and removal of thin film materials and are invaluable tools for semiconductor manufacturing and a wide range of other applications. However, it is not always clear why some ALD/ALE processes succeed while others fail. Performing in situ measurements during ALD/ALE can unravel the detailed surface chemistries underlying these processes to elucidate the reaction mechanisms and ultimately yield more robust and reliable processes. In this presentation, I will motivate the need for in situ measurements during ALD/ALE and provide detailed examples of the techniques used under typical ALD conditions including quartz crystal microbalance, mass spectrometry, infrared spectroscopy, spectroscopic ellipsometry, and optical emission spectroscopy for plasma-based processes. I will also describe some of the high vacuum in situ methods that have been exploited such as X-ray photoelectron spectroscopy and scanning probe microscopy. In addition, I will discuss synchrotron X-ray based methods for in situ study of ALD/ALE reactions.

Role of a cyclopentadienyl ligand in a heteroleptic alkoxide precursor in atomic layer deposition.

Alkoxide precursors have been highlighted for depositing carbon-free films, but their use in Atomic Layer Deposition (ALD) often exhibits a non-saturated growth. This indicates no self-limiting growth due to the chain reaction of hydrolysis or ligand decomposition caused by β-hydride elimination. In the previous study, we demonstrated that self-limiting growth of ALD can be achieved using our newly developed precursor, hafnium cyclopentadienyl tris(N-ethoxy-2,2-dimethyl propanamido) [HfCp(edpa)3]. To elucidate the growth mechanism and the role of cyclopentadienyl (Cp) ligand in a heteroleptic alkoxide precursor, herein, we compare homoleptic and heteroleptic Hf precursors consisting of N-ethoxy-2,2-dimethyl propanamido (edpa) ligands with and without cyclopentadienyl ligand-hafnium tetrakis(N-ethoxy-2,2-dimethyl propanamido) [Hf(edpa)4] and HfCp(edpa)3. We also investigate the role of a Cp ligand in growth characteristics. By substituting an alkoxide ligand with a Cp ligand, we could modify the surface reaction during ALD, preventing undesired reactions. The last remaining edpa after Hf(edpa)4 adsorption can undergo a hydride elimination reaction, resulting in surface O-H generation. In contrast, Cp remains after the HfCp(edpa)3 adsorption. Accordingly, we observe proper ALD growth with self-limiting properties. Thus, a comparative study of different ligands of the precursors can provide critical clues to the design of alkoxide precursors for obtaining typical ALD growth with a saturation behavior.

Plasma-enhanced atomic layer deposition of crystalline Ga2S3 thin films

Gallium (III) sulfide is a frontrunner for many energy storage and optoelectronic applications, which demand a deposition technique that offers a high level of control over thickness, composition, and conformality. Atomic layer deposition (ALD) is a potential technique in this regard. However, the state-of-the-art ALD processes for depositing Ga2S3 often lead to films that are amorphous and nonstoichiometric, and contain significant contaminations. Herein, we present a new plasma-enhanced atomic layer deposition (PE-ALD) process using the hexakis(dimethylamido)digallium precursor and H2S plasma coreactant to deposit high-quality Ga2S3 sulfide thin films and compare it to the thermal ALD process using the same reactants. While both cases exhibit typical ALD characteristics, substantial disparity is observed in the material properties. The PE-ALD process deposits crystalline Ga2S3 sulfide thin films at a temperature as low as 125 °C with a growth per cycle of 1.71 Å/cycle. Additionally, the PE-ALD process results in smooth and stoichiometric Ga2S3 films without any detectable carbon and oxygen contamination. Grazing incidence wide-angle x-ray scattering analysis indicates that the as-deposited Ga2S3 film crystallizes in a cubic structure with a preferred orientation along the [111] direction. The Ga2S3 film exhibits a transmittance of 70% and a bandgap of 3.2 eV with a direct transition.

Interplay of Precursor and Plasma for The Deposition of HfO2 via PEALD: Film Growth and Dielectric Properties

AbstractHfO2 thin films are appealing for microelectronic applications such as high‐κ dielectric layers, memristors, and ferroelectric memory devices. To fulfill the different requirements of each application, the properties of the deposited material need to be tuned accordingly. In this context, plasma‐enhanced atomic layer deposition (PEALD) is a powerful processing route to tailor the properties of HfO2 thin films, especially at low temperatures. Herein, a comprehensive bottom‐up approach is presented, ranging from the synthesis of molecularly engineered Hf precursors to the development of a HfO2 PEALD process and a detailed evaluation where plasma can be exploited to tune the dielectric properties. With the example of the newly synthesized bis‐(dialkylamido)‐bis‐(formamidinato) Hf(IV) precursor, [Hf{η2‐(iPrN)2CH}2(NMe2)2] which is reactive, thermally robust and volatile, successful implementation in a PEALD process for HfO2 at low temperatures is demonstrated. The typical atomic layer deposition (ALD) characteristics of precursor saturation, linearity, and ALD temperature window are demonstrated with constant growth of 0.7 Å per cycle from 125 to 200 °C, yielding high‐purity layers. The effect of plasma pulse duration on the chemical composition alongside structural, topographical, as well as dielectric properties of the films is investigated. For the latter, the films are incorporated in metal‐insulator semiconductor (MIS) structures.

A Low‐Temperature Batch Process for the Deposition of High‐Quality Conformal Alumina Thin Films for Electronic Applications

High‐quality, alumina thin films are extensively used as dielectrics, passivation layers, and barrier layers in electronics and many other applications. However, to achieve optimum stoichiometry and thus performance, the layers are often grown at elevated temperatures (>200 °C) using techniques such as atomic layer deposition (ALD). This is problematic for substrates or structures with low thermal budgets. Herein, alumina thin films are grown on 200 mm silicon substrates employing a versatile deposition method known as MVD at low deposition temperatures (35–150 °C). The chemical composition of the resulting films is investigated postdeposition using X‐ray photoelectron spectroscopy (XPS) and variable angle spectroscopic ellipsometry, with fully stoichiometric Al2O3achieved at deposition temperatures as low as 100 °C. Dielectric measurements confirm outstanding dielectric properties compared to typical thermal ALD layers deposited at much higher temperatures. This low‐temperature deposition performance by considering the MVD reactor design and the “pump‐type” regime of precursor delivery versus the “flow‐type” regime of ALD is rationalized and understood. The results clearly demonstrate that alumina thin films grown with MVD are highly versatile for electronic applications and are of particular relevance and interest for the high‐volume processing of dielectric, passivation, and barrier layers at low temperatures.

Plasma-Enhanced Atomic Layer Deposition of Molybdenum Oxide Thin Films at Low Temperatures for Hydrogen Gas Sensing.

Molybdenum oxide thin films are very appealing for gas sensing applications due to their tunable material characteristics. Particularly, the growing demand for developing hydrogen sensors has triggered the exploration of functional materials such as molybdenum oxides (MoOx). Strategies to enhance the performance of MoOx-based gas sensors include nanostructured growth accompanied by precise control of composition and crystallinity. These features can be delivered by using atomic layer deposition (ALD) processing of thin films, where precursor chemistry plays an important role. Herein, we report a new plasma-enhanced ALD process for molybdenum oxide employing the molybdenum precursor [Mo(NtBu)2(tBu2DAD)] (DAD = diazadienyl) and oxygen plasma. Analysis of the film thickness reveals typical ALD characteristics such as linearity and surface saturation with a growth rate of 0.75 Å/cycle in a broad temperature window between 100 and 240 °C. While the films are amorphous at 100 °C, crystalline β-MoO3 is obtained at 240 °C. Compositional analysis reveals nearly stoichiometric and pure MoO3 films with oxygen vacancies present at the surface. Subsequently, hydrogen gas sensitivity of the molybdenum oxide thin films is demonstrated in a laboratory-scale chemiresistive hydrogen sensor setup at an operation temperature of 120 °C. Sensitivities of up to 18% are achieved for the film deposited at 240 °C, showing a strong correlation between crystallinity, oxygen vacancies at the surface, and hydrogen gas sensitivity.

(Invited) Investigating Surface Reaction Thermodynamics: In Situ Calorimetry for Atomic Layer Deposition

Atomic layer deposition (ALD) is comprised of sequential self-limiting surface reactions that are inherently sensitive to the surface chemistry of the substrate and the reactivity of the chosen precursors. In order to better understand ALD processes and direct their future development we must investigate the fundamental mechanisms of ALD reactions. This is commonly done using a combination of computational modeling and experimental investigations using techniques like spectroscopic ellipsometry and IR spectroscopy. We present pyroelectric calorimetry for ALD as an in situ technique to measure the time resolved heat generation from ALD reactions. This provides a new dimension of thermodynamic and kinetic reaction data.As an experimental technique, pyroelectric calorimetry measures ALD reactions on realistic surfaces as opposed to idealized models. We have designed and constructed custom calorimeters that are compatible with typical ALD process parameters. These calorimeters have been calibrated with a thermal resolution down to 0.1 μJ/cm2 and a temporal resolution of 50ns. To put this in perspective for an ALD reaction, this corresponds to about 0.1% of the heat evolved in the trimethylaluminum (TMA) and water ALD process and about 104 faster than our measurement of the TMA half reaction. The temporal resolution of our calorimeters is orders of magnitude faster than most complementary in situ analysis techniques including ellipsometry, IR spectroscopy, quartz crystal microgravimetry, and mass spectrometry. This time resolution provides information on precursor flow dynamics and reaction kinetics.Pyroelectric calorimetry was used to investigate the TMA and water half reactions for ALD Al2O3 along with in situ spectroscopic ellipsometry thickness measurements and Rutherford backscattering spectrometry (RBS) composition analysis. This enabled comparisons of the change in thickness with the amount of heat generated as each half reaction saturates. Atomic growth rates calculated form RBS were used to calculate the heat generated on a per-atom basis. These results were then compared with proposed reaction mechanisms and energy changes from density functional theory (DFT) computational modeling.Measurements of reaction and adsorption heats can also be used to compare and evaluate different precursors and inhibitor molecules for ALD growth inhibition in area-selective ALD. We investigated how the thermodynamics of adsorption can guide our choice of precursor-inhibitor pairs when using small molecule inhibitors that rely on competitive adsorption and chemical passivation to prevent ALD growth. Pyroelectric calorimetry offers new insight into what drives and limits ALD reactions and provides many opportunities for future investigations.

Atomic Layer Deposition of Ruthenium Dioxide Thin Films Using RuO4 and Alcohols As Reactants

Ruthenium dioxide (RuO2) is the most stable oxide of Ru and has attracted tremendous attention in different applications such as supercapacitors, catalysis, and electrochemical devices. The high conductivity, good chemical stability, a work function value that is even higher than metallic Ru are few properties that make this material so demanding1. Although RuO2 is such an interesting material, atomic layer deposition (ALD) literature reports on this material are scarce. The existing ALD processes based on metalorganic Ru precursors necessitate careful tuning of the O2 partial pressure in order to deposit RuO2 films, as Ru metal is produced at lower O2 partial pressures. Furthermore, reported processes often require high temperature (>180°C) depositions and suffer from high nucleation delays, up to several hundred cycles in some cases.2 In this scenario, we present a novel ALD strategy solely for the synthesis of RuO2 films that takes advantage of the reaction between various alcohols and ruthenium tetroxide (RuO4) without any significant nucleation delay. The process, which uses methanol and RuO4 as reactants, exhibits a growth per cycle (GPC) of 1 angstrom (Å) per cycle (Figure 1a and 1b) at a substrate temperature as low as 60 °C. A constant GPC of 1Å is obtained in the temperature window of 60 °C-120 °C, while the RuO4 precursor is known to decompose above 125 °C.3 The depositions result in amorphous RuO2 films, as confirmed with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. The films can be transformed into crystalline rutile RuO2 by annealing in helium or air at around 400 °C (Figure 1c). The films are conductive, as is evident from the resistivity value of 230 µΩ.cm for a 20 nm film, and the conductivity improved slightly after the anneal.Interestingly, the reaction of other alcohol molecules such as ethanol, 1-propanol, and 2-propanol with RuO4 results in a higher GPC compared to the methanol-based process and it is concluded that the GPC can be increased by increasing the number of carbon atoms in the alcohol chain (Figure 1d). However, all these processes employing different alcohols display typical self-saturating ALD properties. The reaction mechanism of the developed ALD approach was studied in depth using in situ mass spectrometry and in vacuo XPS studies. A reaction mechanism is proposed based on these learnings, wherein during the first half-cycle, alcohol molecules are oxidized into aldehyde and H2O on a RuO2 surface. This in turn causes the partial reduction of the surface RuO2 layer into RuOx or Ru. During the next half-cycle RuO4 oxidizes the surface back to RuO2 and additional RuO2 is deposited on the surface. References Ryden, W.; Lawson, A.; Sartain, C. C., Electrical Transport Properties of Ir O 2 and Ru O 2. Physical Review B 1970, 1 (4), 1494.Austin, D. Z.; Jenkins, M. A.; Allman, D.; Hose, S.; Price, D.; Dezelah, C. L.; Conley Jr, J. F., Atomic layer deposition of ruthenium and ruthenium oxide using a zero-oxidation state precursor. Chemistry of Materials 2017, 29 (3), 1107-1115.Minjauw, M. M.; Dendooven, J.; Capon, B.; Schaekers, M.; Detavernier, C., Atomic layer deposition of ruthenium at 100° C using the RuO 4-precursor and H 2. Journal of Materials Chemistry C 2015, 3 (1), 132-137. This work is funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No.765378. Figure 1

Elucidating the Surface Chemistry of Atomic Layer Deposition on Complex Metal Oxide Cathode Materials

Current research in the field of lithium-ion batteries (LIBs) is geared towards stabilizing the electrode-electrolyte interfaces to improve the LIB cycling stability and storage capacity. In particular, irreversible reactions between non-aqueous electrolytes and metal oxide cathode surfaces can corrode the active material and reduce LIB performance. Protective coatings on the cathode surface can mitigate these issues, and atomic layer deposition (ALD) is well-suited to prepare ultrathin, conformal, corrosion-resistant barrier layers on the surface of the electrodes. While there are reports that some metal oxide and metal fluoride barrier coatings on select cathodes can improve LIB performance, there is no general understanding of why certain ALD coating/cathode combinations are beneficial while others are not. This problem is complicated by the move towards complex LIB cathodes such as lithium nickel manganese cobalt oxide (NMC) materials that may have multiple phases present on the surface including hydroxides and carbonates. Understanding how the ALD precursors react on these materials to create protective coatings is challenging and requires a systematic study to elucidate the surface chemistry of the ALD precursors with each component of the complex oxide surface. Therefore, this work aims at understanding the effects of ALD precursor exposures and ALD coatings on the surface electronic structure of LIB cathode materials.Al2O3 and AlF3 were selected as prototypical metal oxide and metal fluoride surface coatings as they are widely reported in literature due to their ability to act as barrier coatings on cathodes. Additionally, the ALD precursor vapors used for these coatings, i.e., trimethyl aluminium (TMA) and hydrogen fluoride pyridine (HFpy), were adsorbed on the surfaces of these cathode materials under typical ALD conditions to observe their effect on the surface composition and electronic state. The cathode materials chosen for this study included all of the component oxides for NMC cathodes (i.e. Li, Ni, Mn, and Co) in all common oxidation states (e.g., MnO, MnO2, and Mn2O3) as well as selected ternary, quaternary, and quinary forms. In each case, the substrate surface was characterized using X-ray photoelectron spectroscopy (XPS) before and after surface treatment to elucidate changes in the surface composition and charge state induced by the individual ALD precursors and the ultrathin coating. Overall, this fundamental study will assist in the design and synthesis of novel ALD barrier coatings that demonstrate the highest sensitivity on multi-element cathode materials, ultimately leading to robust electrode-electrolyte interfaces for high-performance LIBs.

Stoichiometry controlled homogeneous ternary oxide growth in showerhead atomic layer deposition reactor and application for ZrxHf1−xO2

Atomic layer deposition (ALD) processes for binary oxide (AOy or BOz) growth consist of a sequential introduction of metal precursor (precursor-A or precursor-B) and oxidant-O such that the respective surface reactions are self-limiting with respect to precursor and oxidant exposure times (tA or tB and tO). This approach has been further extended for ternary oxide AδB1−δOλ deposition with (i) super-cycle ALD method (where each super-cycle comprises of m-cycles of AOy ALD followed by n-cycles of BOz ALD), (ii) precursor co-dosing method (where precursor-A and precursor-B are simultaneously pulsed followed by an oxidant-O pulse), and (iii) 3-step ALD (where precursor-A, precursor-B, and oxidant-O are sequentially pulsed). In this Letter, we present a subsaturation pulse initiated 3-step process with ApBO… pulsing sequence for ternary oxide AδB1−δOλ deposition in showerhead ALD reactors. Here, the pulse-Ap reaction step is controlled in the subsaturation regime, while both pulse-B and pulse-O reaction steps are allowed to reach saturation as in a typical ALD. From kinetic simulations, we show that the chemisorbed –Ache surface coverage [Ache] could be controlled below its saturation limit [Ache]sat with exposure time tA and precursor impingement rate kAin in the pulse-Ap reaction step. Furthermore, with precursor transport model, we show that kAin could be varied with a better control using ampoule temperature TampA and precursor-A carrier gas flow FiA together than using TampA alone. As example, we report ZrpHfO… pulsed deposition of ZrxHf1−xO2 ternary oxide samples ZHO1–ZHO4 in a showerhead ALD reactor, and from quantitative XPS analysis, we show that the Zr-fraction (x) could be varied in the range of 0.094 ≤ x ≤ 0.159 with Zr-carrier gas flow FArZr.

Single and Multilayer Atomic Layer Deposition on and into Mesoporous Silicon Layers

Surface functionalization and pore filling of porous layers are essential key aspects for successful application of complex porous matrices. Atomic layer deposition (ALD) in deep macroporous trenches with high aspect ratio using different coating materials was investigated for a wide field of applications, such as energy storage, catalysis, and membrane technology1-3. Experimental data for ALD into mesopores, especially in the low mesoporous range (up to 20 nm) are rare4, 5. ALD on and into mesoporous silicon layers has different functions: 1st Surface passivation (sealing) of the underlying mesoporous structures, such as electrical barrier, diffusion barrier or corrosion protection. In this case complete pore filling is necessary to avoid transport processes through the passivated mesoporous interface. 2nd Pore size reduction (narrowing), i.e. adjustment of pore size for mesoporous membranes or single pores, e.g. for universally applicable, label-free, and nanopore-based sensor systems for the detection of small molecules in the nanofiltration range. Additionally, pore narrowing can be used for fine tuning of porous silicon-based sensors or optical filter properties5. 3rd Surface functionalization, e.g. in biomimetic or bioinspired systems or in fuel cells as catalytic layer.This contribution gives an extensive investigation of single and multi-layer depositions on and into mesoporous silicon layers and analyzes the material distribution and penetration profile of the corresponding atomic compositions.Mesoporous silicon layers were fabricated using highly doped (0.01-0.02 Ωcm) p-type substrates. Electrochemical etching (anodization) was performed in a mixture of 50 m% HF and absolute ethanol in volume ratio 1:1, to generate a 5 µm thick mesoporous layer. A current density of 70 mA/cm2 was used. 48 % porosity was measured by gravimetric technique. Nitrogen adsorption-desorption isotherms were measured in a Belsorp-mini from Rubotherm/Bel GmbH at 77 K using native porous silicon layers. Pore radius (rp) and pore radius distribution were evaluated using Barrett-Joyner-Halenda (BJH) theory. An average pore size radius of 4.68 nm, and pore radius range up to 12 nm were measured. However, surface oxidation will change the layer morphology, especially the pore size, porosity, and specific surface area.The layer deposition process was carried out with different ALD single layers, such as thermal Al2O3, plasma TiO2, thermal HfO2, thermal ZnO, and layer combinations as multilayer ALD, such as plasma Al2O3/TiO2 in an Oxford FlexAl reactor. Typical ALD cycle consists of two half-cycles, sequential precursor and co-reactant doses, which are separated by purge and pump steps, leading to self-limited layer growth6. Co-reactants and oxidizers are typically oxygen sources (H2O or oxygen plasma). When using an oxygen-containing medium, vacuum oxidation occurs, which leads to partial oxidation of the huge inner surface of the mesoporous layer5.Characterization of ultrathin layer deposited in complex 3D mesoporous structures is challenging and demands special analysis methods. In our experiments the penetration depth and depth distributions of atomic compositions for different ALD single and multilayers were measured using secondary neutral mass spectrometry (SNMS). Fig. 1 shows one example of the material distribution for Al2O3 atomic compositions on silicon substrate (Fig. 1a) and on and into mesoporous silicon substrate (Fig. 1b) with the same nominal Al2O3 layer thickness of 96.89 nm, which was measured on the silicon substrate with a Woollan M-2000FTM ellipsometer. The influence of both half-cycles of the ALD process must be considered at the evaluation, both the deposition effect on and into mesoporous silicon layers, and the transformation of the inner pore surface by oxidation. In the mesoporous layer the Al atomic composition profile follows the initial Al atomic profile up to 110 nm and subsequently shows a decreasing tendency up to 225 nm. The O atomic composition shows a decreasing tendency from 125 nm to 225 nm, and a saturation at 3.5 atomic percent. The existence of O in the deeper (>250 nm) mesoporous layers (Fig. 1b) indicates the partial oxidation of the underlying mesoporous layer occurring mainly during the H2O dose half-cycle process sequence. Penetration depths and profiles will be presented for the investigated single ALD layer. Further experimental results show that for multilayer ALD, e.g. with Al2O3/TiO2 layer combination, the first ALD-layer of the multilayer and its specific properties are dominating in the pore filling of the mesoporous layer.

Atomic layer deposited conformal ceramic coatings for anti-corrosion of Ag nanoparticles

The goal of this work is to explore atomic layer deposition (ALD) as a thin film coating technique to inhibit corrosion of metal nanoparticles, given the technique’s ability to form a conformal coating on high aspect ratio substrates. Various coating materials, including Al2O3, ZrO2, HfO2, and TiO2 of different thicknesses were coated on Ag nanoparticles (NPs) in a fluidized bed ALD reactor. To avoid sintering of Ag NPs during the ALD process, the reaction temperature was set to 100 °C, lower than typical ALD reaction temperatures for these metal oxides. The ultrathin coatings (up to ~10 nm thick) were conformally deposited on the surface of Ag NPs. During the corrosion tests in a neutral 3.5 wt% NaCl solution, the corrosion current density was lowered by more than one order of magnitude; in an acid 0.5 M H2SO4 solution, the stabilities of different thin coatings were compared. Ultraviolet–visible spectra of coated Ag NPs showed retention of surface plasmonic resonance.

Atomic Layer Deposition of Al2O3 with Alcohol Oxidants for Impeding Substrate Oxidation

Atomic layer deposition (ALD) is a vapor phase thin film deposition technique based on self-limited surface reaction. ALD processes consist of two (or more than) half-reactions. The first half-reaction is the self-limiting adsorption of precursor molecules which contain core metal atoms; the second half reaction is the self-limiting reaction between surface adsorbed precursor molecules and reactants. Since ALD could deposit thin films with high quality, good uniformity, high conformality, and sub-nanometer thickness controllability, [1-3] ALD has been regarded as one of the most suitable deposition technologies for semiconductor device fabrication. Since thin films of alumina (Al2O3) have wide range of applications such as high-k dielectric material for electronic devices, mechanical and chemical protective coatings, diffusion barriers, and optical coatings, [4-7] ALD of Al2O3 process is the mostly and thoroughly studied. For the deposition of Al2O3, trimethylaluminum (TMA) have been the most widely used for Al precursor due to its high vapor pressure and reactivity. And water (H2O) is widely utilized as the oxygen source in ALD Al2O3 processes, because since H2O often shows facile ligand exchange reaction during ALD. However, ALD Al2O3 processes with H2O reactant showed undesirable substrate oxidation issue. For example, there was an unwanted interface oxide between ALD deposited Al2O3 film and Si substrate. The interface oxide could reduce the dielectric constant of the deposited thin films and increase leakage current density. [8-9] Especially, the oxidation of substrate is critical issue for 2 dimensional (2D) transition-metal dichalcogenides (TMDCs) based field effect transistors (FETs). From our previous results, the oxidation of MoS2 by H2O considerably degraded device performance. [10-11] To avoid the oxidation of substrates and improve device performance, it is necessary to develop a new ALD process by using oxidants with lower oxidation potential than that of H2O. Despite its technical importance, ALD Al2O3 processes with weaker oxidants such as alcohols have rarely been investigated. [8] For this reason, the chemical reaction mechanism between surface adsorbed precursor and reactant has not been clearly identified. In this work, we fundamentally investigated ALD process Al2O3 on Si substrate, using TMA and various alcohol oxidants (methanol (MeOH), ethanol (EtOH), and n-propanol (n-PrOH)). Furthermore, we investigate the reaction mechanism of various alcohol oxidants during ALD of Al2O3 with TMA. Density functional theory (DFT) calculations at B97D3 level of theory were performed using Gaussian 09 suite of programs. Our developed ALD processes showed typical ALD growth characteristics. The saturated growth rates with MeOH, EtOH and n-PrOH were 0.10, 0.96, and 0.74 Å/cycle, respectively. From the calculation results, we revealed that the beta-hydrogen transfer reaction of EtOH and n-PrOH could easily oxidize surface methyl group into surface hydroxyl. The results could be applicable to highly integrated semiconductor devices fabrication and 2D TMDC based FET fabrication processes.

Atomic Layer Deposition of Nickel Using a Heteroleptic Ni Precursor with NH3 and Selective Deposition on Defects of Graphene.

Atomic layer deposition (ALD) of Ni was demonstrated by introducing a novel oxygen-free heteroleptic Ni precursor, (η3-cyclohexenyl)(η5-cyclopentadienyl)nickel(II) [Ni(Chex)(Cp)]. For this process, non-oxygen-containing reactants (NH3 and H2 molecules) were used within a deposition temperature range of 320–340 °C. Typical ALD growth behavior was confirmed at 340 °C with a self-limiting growth rate of 1.1 Å/cycle. Furthermore, a postannealing process was carried out in a H2 ambient environment to improve the quality of the as-deposited Ni film. As a result, a high-quality Ni film with a substantially low resistivity (44.9 μΩcm) was obtained, owing to the high purity and excellent crystallinity. Finally, this Ni ALD process was also performed on a graphene surface. Selective deposition of Ni on defects of graphene was confirmed by transmission electron microscopy and atomic force microscopy analyses with a low growth rate (∼0.27 Å/cycle). This unique method can be further used to fabricate two-dimensional functional materials for several potential applications.

Redox Layer Deposition of Thin Films of MnO2 on Nanostructured Substrates from Aqueous Solutions

In this work, we report a new method for depositing thin films of MnO2 on planar and complex nanostructured surfaces, with high precision and conformality. The method is based on repeating cycles of adsorption of an unsaturated alcohol on a surface, followed by its oxidation with aqueous KMnO4 and formation of thin, solid MnO2. The amount of manganese oxide formed in each cycle is limited by the quantity of the adsorbed alcohol; thus, the growth exhibits the self-limiting characteristics of atomic layer deposition (ALD). Contrary to the typical ALD, however, the new redox layer deposition is performed in air, at room temperature, using common chemicals and simple laboratory glassware, which greatly reduces its cost and complexity. We also demonstrate application of the method for the fabrication of a nanostructured MnO2/Ni electrode, which was not possible with thermal ALD because of the rapid decomposition of the gaseous precursor on the high surface-area substrate. Thanks to its simplicity, the conformal deposition of MnO2 can be easily upscaled and thus exploited for its numerous (electro)chemical applications.

Improvement of the electrical and interfacial propertie of TiN/ZrO2 by a modulated atomic layer deposition process with controlled O3 dosing

In this work, we have studied the atomic layer deposition (ALD) of ZrO2 under various O3 dosing conditions and systematically investigated the interfacial properties of ZrO2/TiN during ALD fabrication using the CpZr(N(CH3)2)3 and O3 reactant combination. In a typical ZrO2 ALD process, the oxidation condition fundamentally determines the interface oxidation and bulk film properties. High oxidation power is required to enhance the crystallinity of the ZrO2 film with low defect densities but inevitably causes the growth of the interfacial oxide, which reduces the dielectric constant of the resulting film. In the current study, we propose a modulated ALD process with controlled O3 dosing to realize the high performance of a metal-insulator-metal (MIM) capacitor device. The initial ALD cycles proceeded with low O3 dosing to suppress the oxidation of the TiN substrate, and the subsequent bulk film was grown with high O3 dosing to improve the crystallinity of the ZrO2. As a result, we achieved a high dielectric constant ZrO2 material with a reduced defect density and a minimum interface capacitance equivalent thickness (CET) of the TiN/ZrO2/TiN MIM capacitor under modulated oxidation conditions. This work provides a useful method to control the interface and bulk properties of dielectric layers at the atomic level.

Multi-walled carbon nanotube forests covered with atomic-layer-deposited ruthenium layers for high-performance counter electrodes of dye-sensitized solar cells

Abstract On account of the high surface area and good catalytic activity of multi-walled carbon nanotube (MWNT) forests, they have great potential for use as alternative materials to conventional Pt/fluorine-doped tin oxide (FTO) counter electrodes (CEs). Nevertheless, the inferior charge collection efficiency of MWNT forest CEs is a drawback that needs to be overcome. Here, we uniquely introduce a Ru layer on MWNT forests by a typical atomic layer deposition (ALD) process. Under an optimal ALD cycle number of 600, Ru layers directly connect vertically grown MWNTs by acting as their bridges while also retaining the microporous structure of the MWNTs. As a result, deposition of the Ru layer under the optimal conditions causes a decrease in the resistances of as-synthesized MWNT forests (charge transfer resistance Rct: from 155.3 Ω to 4.12 Ω; series resistance Rs: from 205.1 Ω to 12 Ω); these decreased resistances are better than even those of Pt/FTO CEs (Rct = 14.2 Ω and Rs = 17.6 Ω). Dye-sensitized solar cells with the optimally deposited Ru/MWNT forest CEs show performances equivalent to those of devices with Pt/FTO CEs on account of the excellent Rct and Rs values of the former CEs.

Atomic Layer Deposition of Intermetallic Co3Sn2 and Ni3Sn2 Thin Films

AbstractIntermetallics form a versatile group of materials that possess unique properties ranging from superconductivity to giant magnetoresistance. The intermetallic Co–Sn and Ni–Sn compounds are promising materials for magnetic applications as well as for anodes in lithium‐ and sodium‐ion batteries. Herein, a method is presented for the preparation of Co3Sn2 and Ni3Sn2 thin films using diamine adducts of cobalt(II) and nickel(II) chlorides, CoCl2(TMEDA) and NiCl2(TMPDA) (TMEDA = N,N,N′,N′‐tetramethylethylenediamine, TMPDA = N,N,N′,N′‐tetramethyl‐1,3‐propanediamine) combined with tributyltin hydride. The films are grown by atomic layer deposition (ALD), a technique that enables conformal film deposition with sub‐nanometer thickness control. The Co3Sn2 process fulfills the typical ALD qualifications, such as self‐limiting growth, excellent film uniformity, and conformal coverage of a trench structure. X‐ray diffraction (XRD) shows reflections characteristic to the hexagonal Co3Sn2 phase, which confirms that the films are, indeed, intermetallic instead of being mere alloys of Co and Sn. The films are extremely pure with impurity levels each below 1.0 at.%. Ni3Sn2 films similarly exhibit the expected XRD reflections for the intermetallic phase and are of high purity. The Co3Sn2 film show magnetic hysteresis with high coercivity values exceeding 500 Oe, indicating great potential in terms of applicability of the films.

(Invited) ALD Metal Fluorides for Ultraviolet Filter and Reflective Coating Applications

We report on the development of atomic layer deposition (ALD) processes for integrating optical filter coatings directly onto back-illuminated silicon sensors for ultraviolet applications. These devices, including CCD imaging arrays and large-area avalanche photodiodes, are modified with a two-dimensional doping method utilizing molecular beam epitaxy to provide high internal quantum efficiency in the UV. Detector integrated coatings are then required to recover additional external losses due to natural silicon reflectance at these short wavelengths. This includes the development of broadband AR coatings, multilayer narrowband coatings, and metal-dielectric filters for solar-blind devices. The use of ALD is attractive for this purpose because the low deposition temperature and thermal surface reactions do not degrade the 2D doping layers, and therefore do not result in reduced detector quantum efficiency. Applications implementing these coated detectors related to terrestrial and space astronomy, astrophysics missions, and particle physics experiments will be discussed. Cosmology is experiencing a very exciting time in exploration. Communities of scientists and technology innovators are studying four concepts for flagship future missions selected and supported by NASA. Two of these four missions are Earth-orbiting large aperture telescope observatories that will include UV capabilities. Currently, these ongoing studies have identified high efficiency detectors and high reflectivity mirror coatings as high priority technologies, particularly those that can extend and enhance system performance for UV wavelengths shorter than those previously explored by missions like the Hubble Space Telescope. At wavelengths below 200 nm, typical ALD dielectric oxide materials like TiO2, HfO2, SiO2, and Al2O3 exhibit large optical losses; we have pursued the development of new processes for ALD metal fluorides to extend coating performance into this far UV wavelength range. Thin films of MgF2, AlF3, and LiF are demonstrated using anhydrous hydrogen fluoride (HF) as the fluorine-containing precursor. This is in contrast to alternate ALD methods for fluoride materials which often utilize reactive metal fluorides as the fluorine precursors, and can result in thin films with components of residual metal oxide contamination. The use of HF has allowed for films with good UV optical properties at deposition temperatures as low 100 °C. This enables compatibility with a variety of 2D-doped silicon sensor formats as well as optical components like polymeric diffraction gratings. The deposition rate of these fluoride materials has a more significant substrate temperature dependence than other common ALD chemistries that often display a ‘window’ of uniform film growth per ALD cycle. Characterization of these coatings is performed with measurements of far UV reflectance and transmittance, x-ray photoelectron spectroscopy, x-ray diffraction, ellipsometry, and atomic force microscopy. These ALD fluoride materials are also effective protective coatings for reflective aluminum optics. Protected UV Al mirrors with ALD AlF3 overcoats are shown to be competitive with state-of-the-art commercial mirrors made with conventional physical vapor deposition (PVD) methods. The use of ALD in these components holds promise in reducing the thickness of the overcoat relative to PVD approaches. This has significant performance implications for far UV mirrors and the ultimate short wavelength cutoff of Al mirrors for potential future NASA astrophysics missions. The large scale coating uniformity, and corresponding reflectance uniformity of ALD overcoats is also attractive for the same applications. Additionally, the use of HF-based ALD chemistries can also allow for the thermal atomic layer etching (ALE) of a variety of materials including aluminum oxide. By combining these ALE methods with ALD encapsulation, it is possible to chemically strip the oxidation that occurs on metallic aluminum and effectively replace it with thin fluoride layers. Stable Al mirrors with protective fluoride coatings approximately 3 nm thick are demonstrated with this combined ALE/ALD approach. The environmental stability of these coatings will be described in the context of resistance to high-humidity conditions. Figure 1