Elevated Deposition Temperatures Research Articles

RF magnetron sputtering of [formula omitted] thin films: Analysis of thermal annealing induced tuning of structural, optical characteristics, and energy band alignments

This correspondence showcases the deposition of ultra-wide bandgap β−Ga2O3 thin films via the utilization of RF magnetron sputtering at an elevated deposition temperature of 400 °C, followed by a comprehensive analysis of their structural, surface and optical properties using X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Raman, Ultraviolet–Visible Spectroscopy (UV–Vis), and Atomic Force Microscopy (AFM) at 800 °C, 900 °C, and 1000 °C annealed sample. Annealing up to 1000 °C led to a significant enhancement in the film’s crystallinity by 10%, accompanied by a remarkable decrease in dislocation density of up to 93.5%. The study also observed several key trends upon annealing, including a decreasing lattice constant from 12 to 11.97 Å, and an increase in the bandgap from 4.8 to 5.3 eV. Concurrently, crystallite size was consistently increased from 4.07 to 15.77 nm. The conduction band offset (CBO) decreased with higher annealing temperatures. Additionally, the increase in bandgap led to an enhancement in the critical electric field from 8.9 to 11.2 MV/cm. The Electron Density Mapping (EDM) results depicted a covalent bond between the atoms for different annealing processes along with the fundamental structural change in atomic coordinates and unit cell parameters, which has not been reported yet. These findings underline the remarkable material properties of β−Ga2O3 at 1000 °C temperatures. This establishes it as a compelling substitute for materials like SiC and GaN, showcasing its potential across various high-power applications due to a superior Baliga figure of merit and robust sensitivity even under elevated temperatures, positioning it as a promising candidate for optoelectronic applications.

Phase separation paths in metastable Zr1-xAlxN monolithic layers compared to multilayers with TiN: Growth versus annealing temperatures

Metastable super-saturated Zr1−xAlxN alloys tend to phase separate into the equilibrium cubic (c) ZrN and wurtzite (w) AlN due to a deep miscibility gap. Transformation is shown here to follow distinctly different paths depending on if Zr1−xAlxN (x = 0.3 and 0.6) is sputter deposited as a single layer or multi-layered with TiN, and further varied by post-deposition annealing. Using in situ high-energy synchrotron wide-angle X-ray scattering and analytical transmission electron microscopy, surface segregation effects are compared to secondary phase transformations occurring in as-deposited layers during thermal annealing up to 1000°C. For the primary phase transformation from the vapor phase, w-AlN nucleates and an AlN-ZrN labyrinthine structure evolves at elevated deposition temperature with semi-coherent interfaces over several nanometers, where the higher Al content narrows the structure in both single and multilayers. Transformation in thinner alloy layers is governed by epitaxial forces which stabilize single-phase c-Zr0.4Al0.6N, which enables c-Zr0.4Al0.6N/TiN superlattice growth at temperatures as low as 350°C. Regardless of the decomposition route, the formation of c-AlN is impeded and w-AlN instantaneously forms during transformation. In contrast, isostructural decomposition into w-AlN and w-Zr(Al)N occurs in w-Zr0.4Al0.6N alloys during annealing.

Controlled Crystallinity of TiO2 Layers Grown by Atmospheric Pressure Spatial Atomic Layer Deposition and their Impact on Perovskite Solar Cell Efficiency

Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD) is an upcoming deposition technique suitable for a variety of materials and combines the benefits of a regular atomic layer deposition with a significantly increased deposition rate at ambient conditions. In this work, amorphous and anatase TiO2 layers are fabricated by AP-SALD via systematic variation of process conditions such as temperature, reactant (H2O and O3), and posttreatment. The formed layers are characterized for their structural and optoelectronic properties and utilized as a hole-blocking layer in hybrid perovskite solar cells. It is found that TiO2 layers fabricated at elevated deposition temperatures possess strong anatase character but expose an unfavorable interface to the perovskite layer, promoting recombination and lowering the shunt resistance and open circuit voltage of the solar cells. Therefore, the interface is essential for efficient charge extraction, which can be significantly improved by controlling the process parameters.

Enhanced medium-range order in vapor-deposited germania glasses at elevated temperatures.

Glasses are nonequilibrium solids with properties highly dependent on their method of preparation. In vapor-deposited molecular glasses, structural organization could be readily tuned with deposition rate and substrate temperature. Here, we show that the atomic arrangement of strong network-forming GeO2 glass is modified at medium range (<2 nm) through vapor deposition at elevated temperatures. Raman spectral signatures distinctively show that the population of six-membered GeO4 rings increases at elevated substrate temperatures. Deposition near the glass transition temperature is more efficient than postgrowth annealing in modifying atomic structure at medium range. The enhanced medium-range organization correlates with reduction of the room temperature internal friction. Identifying the microscopic origin of room temperature internal friction in amorphous oxides is paramount to design the next-generation interference coatings for mirrors of the end test masses of gravitational wave interferometers, in which the room temperature internal friction is a main source of noise limiting their sensitivity.

Hierarchical morphologies in co-sputter deposited thin films

Co-depositions of immiscible alloy films at specific processing conditions have yielded hierarchical microstructures which consist of distinct features at multiple length scales, often agglomerates and concentration modulations on the submicrometer scale and fine nanoprecipitates in a matrix on the nanoscale. The present work examined a series of immiscible alloy systems: Cu-Mo, Cu-Ag, Cu-Fe, Cu-Ta, Mo-Ag, Cu-Mo-Ag, to determine the kinetic conditions favorable for hierarchical organization and the formation mechanism of such structures. Thin films of six immiscible systems were sputter co-deposited over a range of deposition rates from 0.12 to 2 nm/s and various temperatures from 400 to 800\ifmmode^\circ\else\textdegree\fi{}C. The resulting microstructures indicate that hierarchical structures form with sufficient disparity in kinetic energy between the constituent atoms, one species being highly mobile ($A$) and the other relatively immobile ($B$). This condition arises typically at elevated deposition temperatures and reduced deposition rates but is also alloy dependent. The hierarchical structures form during deposition via phase separation and self-organization processes across the multiple length scales. The adatoms diffuse on the film surface with the highly mobile species swiftly agglomerating into $A$-rich domains within which $B$-rich nanoprecipitates form, often self-organizing into periodic arrays. The smallest $B$-rich nanoprecipitates in the $A$-rich domains are found to be coherent and in a metastable crystal structure ($B$ taking the structure of $A$), but coarser precipitates exhibit the equilibrium structure of the $B$ element. The $A$-rich domains are surrounded by a $B$-rich matrix that phase-separates into a concentration modulated structure. The observations are interpreted via a model incorporating material properties and process parameters.

Investigation of temperature-dependent ferroelectric properties of Y-doped HfO2 thin film prepared by medium-frequency reactive magnetron co-sputtering

The crystal structure as well as electrical properties of Y-doped HfO2 thin films were evaluated. The thin films were produced by co-sputtering with Y and Hf metal targets in an argon/oxygen atmosphere. A 10 nm-thick HfO2 thin film with 1.5 at.% yttrium content deposited at 200 °C and annealed at 850 °C for 40 s in an inert nitrogen atmosphere shows excellent ferro-electricity with a significant remnant polarization (Pr~20 μC/cm2) and a low leakage current density (about 10−6 A/cm2 at 1 MV/cm). The crystal structure and ferroelectric properties of Y-doped HfO2 thin films exhibit strong temperature dependence. The ferroelectric phase (orthorhombic phase) fraction was observed to be suppressed in favor of the para-electric phase (monoclinic phase) with increasing deposition temperature through analyses grazing incidence X-ray diffraction and high resolution transmission electron microscopy. Through the analysis of polarization and current density characteristics, a correlation between decreasing remnant polarization and a greater leakage current density was revealed as the deposition temperature was increased. The origin of the degradation of the ferro-electricity of Y-doped HfO2 films with elevated deposition temperature was attributed to the formation of m-phase which was unfavorable for promoting transition to the ferroelectric o-phase during post-deposition annealing.

Optimization of Plasma-Sprayed Lanthanum Chromite Interconnector Through Powder Design and Critical Process Parameters Control

For tubular solid oxide fuel cells (SOFCs), individual cells are connected in series by the interconnector to obtain desired voltage and power output. As the most widely used ceramic interconnector for high-temperature SOFCs, lanthanum chromite-based material is quite difficult to sinter to high density due to the high vapor pressure of the chromium component. In this paper, dense strontium-doped lanthanum chromite (LSC) membranes with enhanced lamellar interface bonding were achieved at an elevated deposition temperature over the critical bonding temperature by atmospheric plasma spraying (APS). The effects of molten droplet particle size on the chemical composition and phase structure of LSC deposits were studied. The preferential vaporization of Cr during plasma spraying was of the greatest interest here. When the particle size of molten droplet was smaller than 30 μm, the vaporization loss of Cr increased remarkably with the decrease in particle size, resulting in impurity phases. When the particle size was larger than 30 μm, the vaporization loss of Cr was greatly reduced. The electrical conductivity of plasma-sprayed LSC deposits reached 8.7 and 2.1 S/cm at 850 °C in oxidizing and reducing atmospheres, respectively. The optimized LSC deposits exhibited good stabilities of phase structure and properties under operation conditions, which is adequate for SOFC interconnector application.

Hillock formation in co-deposited thin films of immiscible metal alloy systems

Hillocks are protruding sections of a surface that can influence the electronic and mechanical properties of a thin film. In monolithic films, these hillocks are formed due to thermally induced stress gradients during deposition. In this study, the surface morphology of co-sputtered immiscible Cu-X (X is a body centered cubic (BCC) group V or VI metal) thin films is characterized to elucidate the conditions that lead to hillock formation. Cu-Ta and Cu-Mo-Ag films were co-deposited with physical vapor deposition (PVD) magnetron sputtering at 25, 400, 600, and 800 °C. A significant number of homogeneously distributed surface hillocks were observed only in the 600 and 800 °C films. High-angle annular dark field (HAADF) cross-sectional imaging revealed significant Cu agglomerations underneath the protruding features. For the Cu-Ta films, the Cu was enveloped in a Cu-Ta nanocrystalline matrix. For Cu-Mo-Ag, the Cu was surrounded by Mo-Ag concentration modulations. While thermal stress gradients arise during deposition of immiscible metal films, biaxial stress calculations and literature cases reveal that they are not solely responsible for hillock formation. The observed morphologies align with a surface diffusion kinetic model that evaluates diffusion length of adatoms during deposition as a function of deposition temperature. This indicates a phase separation driving force paired with the constituent elements’ dissimilar mobilities at elevated deposition temperatures contribute to the presence of hillocks.

Fabricating Fe nanocrystals via encapsulation at the graphite surface

In this paper, the authors describe the conditions under which Fe forms encapsulated nanocrystals beneath the surface of graphite, and they characterize these islands (graphite + Fe) thoroughly. The authors use the experimental techniques of scanning tunneling microscopy (STM) plus x-ray photoelectron spectroscopy (XPS) and the computational technique of density functional theory (DFT). Necessary conditions for encapsulation are preexisting ion-induced defects in the graphite substrate and elevated deposition temperature of 875–900 K. Evidence of encapsulation consists of atomically resolved STM images of a carbon lattice, both on top of the islands and on the sloping sides. The nature of the images indicates that this carbon lattice corresponds to a graphene blanket consisting of more than one graphene sheet that drapes continuously from the top of the island to the graphite substrate. The formation of iron carbide is not observed based on XPS. Shapes of the island footprints are consistent with metallic Fe, predominantly in the hcp or fcc form, though larger islands tend toward bcc. Island structures with hexagonally close-packed lateral hcp or fcc planes are stabilized by their excellent lattice match with the graphite substrate. Evolution of island density with prolonged deposition time provides evidence of coarsening, perhaps via Smoluchowski ripening. The encapsulated Fe clusters are stable in air at room temperature, protected by smaller Fe clusters that decorate defect sites and block permeation of gases. DFT shows that two configurations of Fe are more stable within the gallery than adsorbed on top of the surface: a single atom of Fe and a film (slab) of metallic Fe. Comparison with other metals shows that encapsulated Fe is similar to Cu but dissimilar to Ru or Dy, leading the authors to conclude that carbon dissolution in the metal does not play a role in encapsulation.

Optoelectronic Properties of Hydrogenated Amorphous Substoichiometric Silicon Carbide with Low Carbon Content Deposited on Semi‐Transparent Boron‐Doped Diamond

Hydrogenated amorphous substoichiometric silicon carbide (a‐Si1−xCx:H, x &lt; 0,1) thin films and diodes with low carbon content are prepared from a mixture of H2, SiH4, and CH4 by plasma‐enhanced chemical vapor deposition at a relatively high temperature of 400 °C on semi‐transparent boron‐doped nanocrystalline diamond (B‐NCD) electrodes with an underlying Ti grid. Vibration spectra indicate that CH4 prevents Si crystallization at elevated deposition temperatures and confirm an increasing carbon content up to x = 0.1 for samples grown with SiH4/CH4 flows up to 1:3. Dark current–voltage characteristics of B‐NCD/a‐Si1−xCx:H diodes show a rectifying ratio of about four orders at ±1 V. However, under white light illumination, an energy conversion efficiency of 4% is limited by a high serial resistivity of the B‐NCD electrode and S‐shaped photocurrent near the open‐circuit voltage.

Investigation on the interfacial stability of multilayered Cu–W films at elevated deposition temperatures during co-sputtering

Cu–W nano-multilayered films show great potential applications in microelectronic, plasma and nuclear fields due to the unique interfacial structure. In this paper, Cu–W films with different microstructures were synthesized by co-sputtering at different deposition temperatures. At room temperature, a nano-multilayered structure with the modulation period (λ) of 5 nm appears with alternatively arrayed W-rich and Cu-rich regions. With the increase of the deposition temperature, the periods of the nano-layers become larger and fuzzy, and finally the totally separated phases of Cu and W grains can be identified at temperature higher than 300 °C. The dominant mechanism of such microstructural evolution can be ascribed by the surface diffusion ability of the deposited atoms as evidenced by temperature as well as deposition time. The hardness of the films declines with the elevated temperatures and by analyzing the hardening mechanisms, the interfacial strengthening effect was verified due to the high hardness in nano-multilayered Cu–W films.

Deposition of thin tungsten carbide films by dual ion beam sputtering deposition

Hard materials coating fabricated by physical vapor deposition techniques methods are wildly used because of their excellent mechanical strength, stability and chemical resistivity. In this article, compact and high Young's modulus tungsten carbide films with ∼100 nm thickness were prepared on Si (100) substrates by dual ion beam sputtering deposition method, and elucidates the influence of deposition temperature and assistant ion source on the adhesion strength and Young's modulus of tungsten carbide thin films. Morphological analysis, composition, and structure of tungsten carbide films were investigated by Atomic force microscope, X-ray photoelectron spectrometry, Scanning electron microscope. The X-ray diffraction analysis shows that the thin films comprise of hexagonal structure W2C. The adhesion strength is improved due to ion beam source bombardment and elevated deposition temperature. The Young's modulus of the film is codetermined by the density, microstructure and crystal structure. The influence of density, microstructure and crystal structure on Young's modulus was discussed in detail. The Young's modulus is improved due to the more compact structure caused by appropriate assistant source bombardment and elevated deposition temperature, and gets degradative due to the disorganized, coarse and granular structure caused by overlarge assistant source bombardment.

Suppression of Sulfur Desorption of High-Temperature Sputtered MoS2 Film By Applying DC Bias

As the research field of 2D layered materials expands and progresses, especially for the transition metal dichalcogenaides (TMD), the demand for high quality thin films is also rising. There have been many techniques proposed to fabricate TMD films [1-2]. Among the fabrication techniques, RF magnetron sputtering is expected to realize MoS2 device operating in normally-off mode due to large-area uniformity and suppression of impurity [3]. On the other hand, during sputtering deposition using compound MoS2 target, it is known that the film suffers from sulfur desorption especially when the substrate is heated to obtain sufficient crystallization of the film. We have reported the 2-step technique with the post-deposition sulfurization (PDS) to achieve the S implementation [4-5], however the crystallization is still insufficient due to the lack of migration of Mo atoms. Therefore prevention of sulfur desorption with 1-step method is desired in order to fabricate high quality MoS2 thin film. In this study, we proposed applying positive DC bias to the substrate during high-temperature sputter deposition in order to prevent sulfur desorption and obtain superior crystallinity by the enhanced Mo migration. MoS2 was deposited on SiO2 substrate by RF sputtering. The substrate temperature was varied from 250°C to 600°C. Ar partial pressure was also varied from 0.09 to 1.0 Pa with RF power of 50 to 225 W. The sputtering duration was fixed to 600 s. In order to investigate the effect of the sputtering parameters, the sputter-deposited MoS2 flms were evaluated with X-ray electron spectroscopy (XPS) to confirm Mo-S bond formation. The S/Mo ratio of the film was calculated from the ratio of Mo 3d and S 2p spectra. The samples with elevated deposition temperature were evaluated with Raman spectroscopy and compared to the as-depo. sample and the PDS sample [4]. As a result of applying bias voltage, higher positive bias voltage yielded films with higher S/Mo. Stoichiometric S/Mo ratio of 2.0 was achieved by applying positive bias voltage even with high-temperature sputtering. This is considered to be the attracting effect of S2- to the substrate due to the positive bias. When the Ar partial pressure was increased, the S/Mo ratio decreased. This result shows that the mean free path of the sputtered particles have large effect on the amount of S2- reaching the substrate determining S/Mo ratio of the film. As the sputter RF power was increased S/Mo also increased, which may also provide larger Mo migration energy, therefore result in the better crystallinity. By optimizing each sputtering parameters, S/Mo of 2.0 was achieved at 400-600°C. This work was partly supported by JST CREST Number JPMJCR16F4, Japan. This work was also partly supported by JSPS KAKENHI Grant Number 16J11377. REFERENCE D. Sarkar, X. Xie, W. Liu, W. Cao, J. Kang, Y. Gong, S. Kraemer, P. M. Ajayan, K. Banerjee, Nature 526, 91 (2015)S. Larentis, B. Fallahazad, E. Tutuc, Appl. Phys. Lett. 101, 223104 (2012)T. Ohashi, K. Suda, S. Ishihara, N. Sawamoto, S. Yamaguchi, K. Matsuura, K. Kakushima, N. Sugii, A. Nishiyama, Y. Kataoka, K. Natroi, K. Tsutsui, H. Iwai, A. Ogura, and H. Wakabayashi, Jpn. J. Appl. Phys. 54, 04DN08 (2015)S. Ishihara, Y. Hibino, N. Sawamoto, K. Suda, T. Ohashi, K. Matsuura, H. Machida, M. Ishikawa, H. Sudoh, H. Wakabayashi and A. Ogura, Jpn. J. Appl. Phys. 55,04EJ07 (2016)S. Ishihara, Y. Hibino, N. Sawamoto, K. Suda, T. Ohashi, K. Matsuura, H. Machida, M. Ishikawa, H. Sudoh, H. Wakabayashi and A. Ogura, Jpn. J. Appl. Phys. 55,06GF01 (2016) Figure 1

Mechanical properties and tribological behaviour of Mo-N coatings deposited via high power impulse magnetron sputtering on temperature sensitive substrates

Hard coatings represent a suitable resource to protect structural materials against oxidation and wear phenomena. Physical Vapour Deposition techniques usually require elevated deposition temperatures to accomplish desired performance, becoming incompatible with sensitive substrates. High Power Impulse Magnetron Sputtering technology allowed obtaining high quality 8 μm Mo-N coatings at low temperature on a commercial Al-Si alloy. Produced films exhibited good adhesion, noteworthy mechanical characteristics (Hardness ≥ 22 GPa, Modulus ≥ 300 GPa) and notable tribological properties. Indeed, they could withstand over 33000 revolutions in dry conditions (track radius 26.5 mm, sliding speed 1.25 m/s, initial Hertzian contact pressure 1 GPa) without evident damage, while the substrate could not overcome a few dozen revolutions in the same operating conditions.

Influence of deposition temperature on the microstructure of thin-film electrolyte for SOFCs with a nanoporous AAO support structure

In this paper we investigate the influence of deposition temperature on the microstructure of yttria stabilized zirconia thin-films based on an anodized aluminum oxide (AAO) support structure. The yttria-stabilized zirconia (YSZ) electrolytes were deposited on a Pt anode/AAO support using DC magnetron reactive sputtering under deposition temperatures of RT and 500 °C. Elevating the deposition temperature led to enhanced surface mobility in the sputtered adatoms, which helped prevent pinhole generation and minimized the thickness of the electrolyte. A thin-film fuel cell with a YSZ electrolyte only 300 nm thick was successfully fabricated by elevating the deposition temperature. This cell exhibited an open circuit voltage (OCV) of 0.97 V, which is significantly higher than the OCV values of 0.3 V for a cell deposited at RT. However, in spite of the thin electrolyte, the performance of the cell deposited at the higher temperature showed limited value due to its relatively high polarization resistance. Through further investigation into the grain morphology, we verify that the increasing deposition temperature can affect not only the film density but also increase the grain size of the electrolyte, which is related to oxygen incorporation for ORR kinetics. Electrochemical impedance spectroscopy (EIS) results indicate that the grain size change caused by the elevated deposition temperature adversely affected the polarization resistance and the cell performance. These results indicate that careful adoption of elevated electrolyte deposition temperatures are required to optimize fuel cell performance.

Effects of non-magnetic phase and deposition temperature on magnetic properties of FePt–MgO granular thin films on single-crystal MgO substrate

(FePt)100−x(MgO)x granular thin films were fabricated by using co-sputtering on single crystal MgO substrate at different deposition temperatures followed by a high-temperature annealing. The adding MgO was found to have the following effects: 1) high fraction of MgO phase (70vol%) degraded the L10 phase transformation; 2) FePt grains embedded in MgO matrix were well separated, leading to minimized exchange interaction between the FePt grains; 3) 55vol% fraction of MgO was optimal for the L10-FePt phase formation with highest coercivity, due to the formation of granular structure, which was also verified by comparing the structure and properties of FePt/MgO bilayer films. Deposition temperature has also effect on the magnetic properties of FePt–MgO films. Room temperature deposition did not result in the L10 phase even after annealing at 800°C. However, the magnetic properties of the films deposited at 200°C were better than those deposited at 300°C. The low coercivity resulted from elevated deposition temperature was attributed to the thermal expansion and competitively growth among FePt and MgO grains. The results revealed that adding appropriate amount of MgO (55vol%) can be achieved by finding a proper deposition temperature (200°C), well-defined granular L10-FePt films with fine grain distribution, high coercivity and small domain size.

Ionic conductivity of bias sputtered lithium phosphorus oxy-nitride thin films

This manuscript reports a study of the ionic conductivity of radiofrequency sputter deposited, lithium phosphorous oxy-nitride (LIPON), solid state electrolyte amorphous thin films. Films having compositions near Li2.5PO3.5N0.5 were deposited under varying conditions of process gas, substrate bias, and deposition temperature. To understand the variations in ionic conductivity observed, the films were extensively characterized to examine structural and compositional differences, including examination by x-ray photoelectron spectroscopy (XPS), inductively coupled plasma–optical emission spectroscopy, and spectroscopic ellipsometry. For the XPS study, depth profiling was used to allow a critical examination of the role of triply coordinated nitrogen in the ionic conductivity of LIPON. The highest ionic conductivity of 9.8×10−6S/cm was obtained at an elevated deposition temperature and is correlated to a reduced density of defects, as indicated from the optical characterization.

Towards the 3D Thin-Film Li-Ion Battery: A Novel Solution-Based Process for the Deposition of (Multi-)Metal Oxide Coatings on High Aspect Ratio Features

Many types of rechargeable batteries are on the market of which lithium ion chemistries capture the main part of the portable electronics. Conventional Li-ion battery technology typically uses slurries (based on powders) for the active electrode component in combination with a liquid electrolyte. In order to overcome the relatively high risk of shorts and hence thermal runaway or explosions, due to the high inflammability of the electrolyte, membrane separators and a bulky casing are necessary. As an alternative, all-solid-state Li-ion batteries and in particular thin film based battery technology is being explored. It is obvious that the use of compact thin films as components in these commercially available devices has advantages regarding safety, especially when it comes to the electrolyte. Furthermore, they are characterized by a higher resistivity against (electro)chemical degradation, resulting in a longer cycle life. The typical lower conductivity of solid electrolytes, compared to liquid electrolytes, is compensated by the thinness of the films. However, planar thin film electrodes limit the maximum energy density of these devices. Therefore, 3D thin-film batteries, consisting of electrode and electrolyte thin films, coated on 3D structured surfaces, creating a large surface area and thus an higher energy density, are proposed. One can understand that for this new type of batteries, which are currently in an exploratory stage, there is a strong need for coating of various oxides (electrodes and electrolyte) on 3D surfaces.[1] Conformal coverage of high surface area 3D substrates is typically done via atomic layer deposition (ALD). However, ALD suffers from high running and equipment costs, low throughput (especially for substrates with a strongly increased surface area) and a limited applicability towards complex, multi-metal oxides.[2] In view of commercial 3D battery applications and upscaling, there is a strong need to overcome these disadvantages. We present a novel chemical solution deposition (CSD) process for the deposition of (multi-)metal oxide coatings on 3D substrates, i.e. silicon micropillars coated with 20 nm TiN.[3] The CSD method has been developed first for TiO2, a candidate anode material. As starting point, an aqueous citrate-peroxo Ti precursor, which had been successfully applied for the deposition of uniform, good quality TiO2on planar substrates via spin coating [4], was used. This precursor system was optimized, in particular by adding an additive, to allow coating on high aspect ratio features. The whole was spray coated at 180°C and by controlling all levels of the process, complete coverage of the silicon pillars with the Ti precursor was obtained. Since the deposition was performed at elevated temperature, the solution was instantly transformed into a gel, consisting of coordinated complexes in which Ti ions were immobilized. The deposited gel, which was already partly decomposed due to the elevated deposition temperature, is shown in the SEM images in Figure 1. Further heat treatment of the pillars lead to the complete combustion of the gel and thus to titanium oxide formation, it also triggered the crystallization of the deposited layer. A thorough crystallization study indicated that phase-pure TiO2 anatase films could be obtained by a well-defined post deposition anneal at 500°C. Lithium-ion insertion and extraction behavior was observed for the anatase film deposited on TiN-coated silicon pillars, indicating its application potential for 3D Li-ion batteries. The presented technique was also successfully applied to deposit Li4Ti5O12 on the microstructured substrate. Because of its negligible volume change upon charge/discharge, this anode material seems best suited for thin-film applications.[1] On-going research further shows that the reported novel method, which implies operational simplicity, low cost and relatively short deposition times, can be applied to achieve a series of multi-metal oxide coatings (e.g. LLTO) on 3D substrates, bringing commercial thin-film 3D Li-ion batteries one step closer to reality. [1] P. M. Vereecken et al., ECS Transactions, 58(10), 111-118 (2013). [2] (a) T. R. B. Foong et al., Adv. Funct. Mat., 20, 1390-1396 (2010) / (b) S. M. George, Chem. Rev., 110, 111-131 (2010). [3] S. Gielis et al., EP/PCT patent, submitted. [4] (a) A. Hardy et al., J. Sol-Gel Sci. Technol., 44, 65-74 (2007) / (b) I. Truijen et al., J. Sol-Gel Sci. Technol., 41, 43-48 (2007). Figure 1

The Structure of Electrodeposited Aluminum Alloys from Chloroaluminate Ionic Liquids: Let’s Not Ignore the Temperature

Electrodeposited alloys are rarely in thermodynamic equilibrium, and several unique microstructural features such as extended solid solubility, metastable crystalline phases, and metallic glasses are generally observed. Because the excess free energy of the deposit is inversely proportional to the atomic mobility, phase constitution depends strongly on deposition temperature. This paper examines the influence of deposition temperature on the crystal structure of electrodeposited Al-Mn and Al-Ti alloys electrodeposited from AlCl3-NaCl and AlCl3-EMImCl ionic liquids from temperatures ranging from 80 °C to 425 °C. Deposition at low temperature favors the preferential deposition of amorphous and crystalline phases which appear as high temperature variants in the equilibrium phase diagram. These structures generally have high symmetry, and for this reason are relatively easy to nucleate. The room temperature equilibrium phases, which generally have a more complicated structure, can only be obtained at elevated deposition temperatures.

The Structure of Electrodeposited Aluminum Alloys from Chloroaluminate Ionic Liquids: Let’s Not Ignore the Temperature

The electrodeposition of aluminum alloys from chloroaluminate electrolytes has a rich history, beginning with the high temperature alkali halide-based melts, such as AlCl3-NaCl, and transitioning into the ambient temperature analogs, such as AlCl3-1-ethyl-3-methylimidazolium chloride ([EMIM]Cl). Although still very much moisture sensitive, these organic halide systems became widely accepted due to their extremely low vapor pressure, which made them experimentally less challenging. More recently with the introduction of the air- and water-stable systems that do not necessarily require handling in an inert atmosphere, a significant portion of the current electrodeposition research is being conducted at or near ambient temperature.Electrodeposited alloys are rarely in thermodynamic equilibrium, and several unique microstructural features such as extended solid solubility, metastable crystalline phases, and metallic glasses are generally observed. While exact correlations between electrodeposition and vapor deposition have not been developed, the excess free energy possible in alloys produced by these two techniques is similar and on the order of 50 kJ mol-1. Because the excess free energy of the deposit is inversely proportional to the atomic mobility, phase constitution depends strongly on deposition temperature. This talk will focus on the influence of deposition temperature on the crystal structure of electrodeposited aluminum-transition metal alloys.As an example, Figure 1 shows a diagram of the structures observed over a large range of compositions and deposition temperatures for Al-Mn alloys electrodeposited from Lewis acidic AlCl3-NaCl containing controlled additions of MnCl2. There are basically two temperature regimes where the phases have varying degrees of excess free energy with respect to those expected from the equilibrium phase diagram. At lower temperatures (150 to 250 °C), no equilibrium structure other than the fcc Al solid solution is observed. The dominant phases present are a metallic glass at low Mn composition and a number of metastable crystalline phases such as γ-brass Al8Mn5, B2 AlMn and L10 AlMn, which appear as high-temperature variants on the equilibrium phase diagram. These structures generally have high symmetry, and for this reason are relatively easy to nucleate. Notice that all of the crystalline phases deposited at low temperature have cubic or near-cubic symmetry. In the temperature range of 250 to 425 °C, some of the phases predicted by the equilibrium phase diagram at these temperatures, such as Mn4Al11 and γ2 Al8Mn5, are deposited. The formation of these phases is thermally activated at elevated deposition temperatures. The metallic glass turns into a multi-phase structure at elevated deposition temperature, comprised primarily of equilibrium MnAl6 and the icosahedral phase, showing 5-fold rotational symmetry. Figure 1 clearly shows that the phase constitution depends strongly on alloy composition and deposition temperature. Variations in current density may influence alloy composition, but appear to have a negligible effect on the structure of the electrodeposit. For instance, two alloys of identical composition that have been deposited at the same temperature, but at different current densities from electrolytes containing different amounts of Mn2+, have essentially the same crystal structure.Another alloy system showing interesting temperature dependence is Al-Ti. Here we compare two alloys of similar composition, one deposited from AlCl3-[EMIM]Cl at 80 °C and the other from AlCl3-NaCl at 150 °C. The electron diffraction pattern from the AlCl3-[EMIM]Cl deposit (Fig. 2(a)) is consistent with single phase, disordered fcc Al-Ti. Figure 2(b) shows the diffraction pattern from the AlCl3-NaCl deposit, which in addition to the fcc structure, shows the superlattice reflections indicative of L12 ordering. The appearance of antiphase boundaries in the ordered alloys (not shown) suggests that the deposit may be disordered initially and then orders in the solid state, subsequent to the charge transfer step and adatom incorporation into the lattice. This is very similar to the disorder trapping observed in rapidly solidified alloys. The measured domain size is consistent with a mechanism of diffusion-controlled domain growth at the examined deposition temperatures and times.