AVS 67 Session TF-Invited On Demand: Thin Film Invited On Demand Session

Complex oxides are of great interest for their rich variety of chemical and physical properties including magnetism, ferroelectricity, multiferroicity, catalytic behavior, and superconductivity. Up to date, the preparation of crystalline complex oxide thin films has been mainly limited on substrates that can stand high temperature thermal treatments and on single crystal substrates when epitaxial growth is pursued. These requirements dramatically limit their applicability excluding the possibility to prepare many artificial multilayered architectures to investigate emergent phenomena that arise in thin films and at their interfaces. To tackle this bottleneck, herein, we will present a facile and sustainable chemical route to prepare water-soluble epitaxial Sr₃Al₂O₆ thin films to be used as sacrificial layer for future free-standing epitaxial complex oxide manipulation. Two solution processes are put forward based on metal nitrate and metalorganic precursors to prepare dense, homogeneous and epitaxial Sr₃Al₂O₆ thin films that can be easily etched by milli-Q water. [1] Thorough investigation of the precursor-solvent compatibility, thermogravimetric analysis and film crystallization is performed. Additionally, we demonstrate the viability of Sr₃Al₂O₆to subsequently prepare and transfer a wide variety of atomic layer deposited functional oxides (Al₂O₃, Co₃O₄, Fe₂O₃, CoFe₂O₄, BiFeO₃) on arbitrary substrates ranging from flexible polymers to silicon substrate. This robust and low-cost procedure could be adopted to prepare a wider family of thin film compositions to fabricate artificial heterostructures and 2D materials with monolayer by monolayer control to go beyond the traditional electronic, spintronic, and energy storage and conversion devices.

[1] P. Salles, M.Coll et al. Adv. Mater. Interf, (2021), 8, 2001643

Titanium oxide (TiO₂) has been an attractive material with interest for various applications, including photocatalysts, optical coatings, and the high-permittivity dielectrics of DRAM capacitors. The atomic layer deposition (ALD) technique is used to deposit thin films with excellent step coverage, accurate thickness control, and excellent film quality. The most common precursors for the ALD of TiO₂ were tetrakis(dimethylamido)titanium (TDMAT) and titanium tetraisopropoxide (TTIP). However, they showed low ALD temperatures because of their insufficient thermal stability. In general, the higher the deposition temperature, the better the physical and electrical properties of the dielectric film. Therefore, Ti precursors with better thermal stability and reasonable reactivity are requested. The thermal stability of the Group 4 metal (Ti, Zr, Hf) precursors can be improved by introducing a “spectator ligand,” cyclopentadienyl (C₅H₅, Cp). Stabilizing ability of the Cp ligand can be further enhanced by Me-substitution [1]. In the present work, we compared the thermal stability and reactivity of heteroleptic Ti precursors by density functional theory (DFT) calculations and the ALD experiments. Thermolysis reactions of precursors were simulated, and the ALD temperature windows and the film densities were investigated. The surface reactions of the precursors were simulated, and the saturation dose and step coverage were investigated. Compounds with a different “spectator ligand,” such as Cp, Me₅Cp, or linked amido-Cp, were compared. Compounds with different “actor ligands,” dimethylamido or methoxy, were also compared. The introduction of a linked ligand or Me₅Cp enhanced the thermal stability compared to introducing a Cp ligand, and the precursor with methoxy ligands showed better reactivity than the precursor with dimethylamido ligands.The underlying mechanisms for better stability or reactivity were described by DFT simulation.

[1]J.-P. Niemelä et al.,Semicond. Sci. Technol. 32 (2017) 093005.

Two-dimensional materials are inherently anisotropic – with strong bonding in-plane and weak bonding out-of-plane. Consequently, their optical properties are birefringent, particularly at frequencies of the optically active infrared phonons. At these frequencies, the strong optical anisotropy results in hyperbolicity, where the permittivity tensor along at least one axis is negative, while at least one is positive. Hexagonal boron nitride (hBN) is an exemplary material in this regard. Materials which exhibit hyperbolicity can support volume-confined polaritonic modes, which have a wavelength much shorter than that of light in free space. Such spatially compressed modes have important implications for IR technologies, as they may be utilized to create planar meta-optics that are much more compact than the current state of the art. In particular, 2D materials can be used to realize metasurfaces, capable of shaping both nearfield- and far field light waves for a broad range of applications.

Remote epitaxy, i.e., epitaxial growth through a semi lattice-transparent monolayer material such as graphene, has emerged as a promising alternative that circumvents some of the limitations of conventional epitaxy directly on a crystalline substrate. However, the microscopic mechanisms for remote epitaxy remain unclear, in particular, the role of point and extended defects in graphene. In this talk, I will describe our understanding of the mechanisms based on in-situ electron diffraction, photoemission, atomic force microscopy, and photoemission electron microscopy (PEEM). I will begin with experiments on the model system GaAs on graphene/GaAs (001), grown by molecular beam epitaxy (MBE), and describe recent efforts for the growth of Heusler compounds on graphene-terminated Al2O3 (0001). I will also describe one particularly promising application: our observation of strain-induced magnetism in rippled Heusler membranes that are exfoliated from graphene [1]. This work was supported by the ARO (W911NF-17-1-0254), AFOSR (FA9550-21-1-0127), DARPA (0011940523), and NSF (DMR-1752797).

[1] D. Du, et. al. arXiv:2006.10100 (2020)