Article pubs.acs.org/JPCC
Elucidating the Surface Reaction Mechanisms During Atomic Layer Deposition of LixAlySizO by in Situ Fourier Transform Infrared Spectroscopy Jea Cho, Taeseung Kim, Trevor Seegmiller, and Jane P. Chang* Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States S Supporting Information *
ABSTRACT: Reaction mechanisms in the atomic layer deposition (ALD) of Li x Al y Si z O (LASO) using a LiOC(CH 3 ) 3 /H 2 O−Al(CH 3 ) 3 /H 2 O−Si(OCH2CH3)4/H2O chemistry were studied via in situ Fourier transform infrared spectroscopy (FTIR) at 225 °C. ALD deposition of Al2O3 using an Al(CH3)3/H2O chemistry and LiOH using a LiOC(CH3)3/H2O chemistry demonstrated ideal ALD growth. ALD deposition of SiO2 alone by Si(OCH2CH3)4/H2O chemistry was unsuccessful; however, incorporation of the same Si(OCH2CH3)4/H2O chemistry in between ALD processes of Al2O3 and LiOH resulted in ALD deposition of LixAlySizO. The as-deposited ALD LASO film with a composition of Li0.40Al0.32Si0.28O on a Si(100) substrate was amorphous but crystallized into β-LiAlSiO4 upon rapid thermal annealing (RTA) at 900 °C under N2. The ionic conductivity of as-deposited, amorphous ALD Li0.40Al0.32Si0.28O was as high as 1.62 × 10−6 S/cm at 361 °C with an activation energy of Ea = 0.70 eV.
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INTRODUCTION There is a strong scientific and technological interest for improving Li ion battery (LIB) technology for next-generation microelectronics, especially to power on-chip devices with a significantly reduced footprint. In order for smaller LIBs to power these devices without excessive sacrifice of energy and power density, an extensive redesign of the current twodimensional (2D) planar electrode geometry into novel threedimensional (3D) electrodes is required.1−6 The major bottleneck that currently prohibits the effective implementation of 3D architecture lies in the lack of an appropriate electrolyte with the desired physicochemical properties.3,4 Consequently, a major breakthrough in LIB fabrication is required, which can only be realized by a well-engineered, solid-state thin-film electrolyte in a 3D architecture. Atomic layer deposition (ALD) is a deposition technique based on saturated surface half-reactions, allowing the deposition of pinhole-free, conformal, uniform, and homogeneous films.7,8 It is the only thin film deposition technique that can provide a high level of thickness control and uniformity on nonplanar, complex structures. ALD has been implemented for next-generation metal−insulator metal capacitors in dynamic random-access memory (DRAM) and the gate oxides in complementary metal oxide semiconductor (CMOS) transistors, demonstrating the technological relevance of ALD, especially in nanoscale manufacturing.9 ALD processes have been developed for various classes of materials including metals, metal oxides, nitrides, sulfides, phosphates, silicides, tellurides, and selenides.10−14 As such, ALD is well-recognized as the © XXXX American Chemical Society
technology essential for breakthrough advancements in nextgeneration LIBs, that is, if a thin-film solid-state Li ion conductor can be synthesized with desirable physicochemical properties for electrolyte applications. Such an electrolyte should have a high ionic conductivity, depending on the thickness of the film, in the range 10−3−10−7 S/cm while also being electronically insulating.13 With the aim of developing a solid-state Li ion conductor, studies on ALD of Li-containing films have been expanding recently. While there are studies that report on ALD of LiOH, the results from these studies have been rather inconsistent and controversial. The reported growth rates for ALD of LiOH vary significantly from 0.8715,16 to 6.5 Å/cycle17 under similar experimental conditions. Additionally, studies of ALD LiOH and Li-containing complex oxides have been largely on material characterizations of the deposited materials; the underlying surface mechanisms in deposition processes were often neglected. As a result, the published studies not only reported such drastically different growth rates, but also various nonidealities observed during the growth of Li-containing complex oxides: Miikkulainen et al. has reported that the composition and thickness of ALD lithium titanate (LixTiyO, ∼97 nm) did not correlate closely to the ratio of ALD subcycles of LiOH and TiO2 with lithium tert-butoxide (LTB) as the Li precursor, and titanium tetrachloride (TiCl4) or titanium Received: September 21, 2015 Revised: April 18, 2016
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DOI: 10.1021/acs.jpcc.5b09212 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C tetraisopropoxide (Ti(OiPr)4) as the Ti precursor.18 ALD of LixAlyO (LAO, 67−400 nm) using LTB and trimethylaluminum (TMA) as Li and Al precursors has reported similar nonideal growth, where the composition and growth rate diverged significantly from the projected behaviors.19,20 While some of these studies did investigate the growth mechanisms of LiOH and Li-containing complex oxides via in situ quartz crystal microbalance (QCM) measurements, an accurate determination of surface species and surface reactions is still limited; it does not offer detailed information on the surface chemistries of the deposition process. As a result, there still exists a critical need for a comprehensive mechanistic study to understand the underlying surface reactions during ALD of Licontaining multicomponent films to enable improved thickness and composition control. The present study unveils some of the growth mechanisms in ALD synthesis of lithium aluminosilicate (LixAlySizO, LASO), a solid-state Li ion conductor with ionic conductivity ranges in the range 10−3−10−9 Ω/cm at 300 °C depending on composition and crystallinity.21−25 While ALD of LixAlySizO has been reported previously,21 it did not provide insight on the mechanistic details during ALD of LixAlySizO. Additionally, ALD depositions of SiOx containing films is known to be challenging,26−28 yet the detailed mechanistic studies have been limited despite the importance of complex surface chemistries understood.26,27,29,30 In this in-depth mechanistic study, ALD of LixAlySizO is studied via in situ Fourier transform infrared spectroscopy (FTIR). Upon a detailed understanding of the underlying surface chemistries of the constituent oxides LiOH, Al2O3, and SiO2, studies on the synthesis and material properties of ALD deposited LixAlySizO are followed.
vibrational spectroscopy studies. The in situ FTIR spectroscopy allows monitoring of surface species after each half-cycle of ALD. The use of high surface area nanoparticles is essential for transmission FTIR spectroscopy to obtain sufficient signals. The ZrO2 NPs (Nanostructured & Amorphous Materials, Inc., 99+% purity) used in this study had an average particle diameter of 20 nm with a specific surface area of ∼25 m2/g. The LixAlySizO thin films were deposited by alternating ALD deposition cycles of LiOH, Al2O3, and SiO2 with a global cycle sequence of b(Al−O)−a(Li−O)−c(Si−O)−a(Li−O), where a, b, and c represent the local cycle number for lithium oxide, aluminum oxide, and silicon oxide, respectively. Multiple global cycles were performed until the desired film thicknesses were achieved (Supporting Information, Table S1). Al2O3 was chosen to be the first layer as it is known to have excellent adhesion to hydroxylated surfaces.12 The NPs were supported by a corrosion-resistant stainless steel grid (McMaster-Carr, Type 316) with 100 lines per inch. The particles were pressed into the grid using polished stainless steel dies and a hydraulic press. A tantalum foil was spot-welded to the edges of the grid to facilitate resistive heating. A type-K thermocouple was spot-welded to the sample holder for an accurate temperature reading, which was PID controlled to maintain a deposition temperature of 225 °C. The vibrational spectroscopic studies were performed with a Nicolet Nexus 670 FTIR spectrometer with a deuterated triglycine sulfate (DTGS) detector. All spectra were obtained at a deposition temperature of 225 °C. Before reactant exposures, the KBr windows on the chamber were isolated by gate valves to prevent contamination on windows. The spectrometer setup was purged with dry, CO2-free (
