Ultrathin transition-metal oxide and dichalcogenide layers ranging from monolayer (2D) thickness to a few nanometers, exhibit a wide range of distinct physical properties when compared to their bulk counterparts. There is growing interest in understanding the crystallization behavior of these materials when approaching the 2D regime, in order to optimize the growth of already observed phases and realize the synthesis of predicted ones. Information on nucleation rates and critical nucleus size in homogeneous nucleation processes, the relative importance of various defects in heterogeneous nucleation, grain growth rates, and interface evolution in the 2D regime is highly sought to enable interface engineering in 2D materials. Tungsten trioxide (WO3) is a particularly interesting system to study in ultrathin configurations because it exhibits a wealth of crystallization-dependent properties. In addition to its catalytic and ion-intercalation derived properties, such as charge storage and electrochromic behavior, it also shows evidence of a superconducting phase, an insulator-to-metal transition that may be achieved via electrostatic gating, and a transition from the conventional monoclinic, orthorhombic, and hexagonal polytypes observed in bulk and nanophases, to a stable free-standing, 2D single-layer honeycomb-like structure that is semiconducting and predicted to have a very high cohesive energy (10.5 eV) as well as one of the highest in-plane stiffness among metal oxide and dichalcogenide 2D materials (250 N/m). In this work we investigate the crystallization behavior of ultrathin WO3 formed after oxidation of tungsten (W), using X-ray diffraction (XRD), atomic force microscopy (AFM), and impedance spectroscopy (IS). Polycrystalline W films of 10-nm thickness, deposited by atomic layer deposition (ALD) on n-type (001)-oriented silicon substrates, were annealed in oxidizing environment in a tube furnace at temperatures ranging from 275°C to 600°C. XRD on films annealed at 600°C shows the formation of the monoclinic phase of WO3. The XRD patterns for annealed films are dominated by the sharp monoclinic (200) reflection at 2θ = 24.45°, indicating that the WO3 crystallites exhibit preferential orientation with their  axis aligned perpendicular to the substrate. AFM shows surface morphology comprising distorted cubic-like features consistent with the underlying monoclinic structure observed by XRD. The AFM scans clearly reveal that the films are dominated by out-of-plane interfaces normal to the  direction of the WO3 crystallites. At temperatures above 500°C, large crystallites are observed in the AFM scans, with heights in the 10-30 nm range, indicating significant 3D restructuring due to crystal growth after oxidation. The crystallite number density exhibits an Arrhenius relationship with the annealing temperature with an activation energy of 110 kJ/mol. Impedance spectroscopy measurements between 1 Hz and 7 MHz on thin film samples after the deposition of top nickel contacts, show Nyquist impedance arcs that depend on the annealing temperature and ambient. The impedance of samples with various WO3 thicknesses, degrees of 3D restructuring, and crystallite number densities are analyzed in terms of equivalent circuits that yield the “grain interface” and “grain interior” contributions to the resistance and capacitance of the samples. The dependence of these electrical characteristics is compared with the structural features obtained by XRD and AFM to identify distinct kinetic regimes present during W oxidation, as well as crystal nucleation and growth of the WO3 ultrathin layers.
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E