Nicholas Becker

Low temperature atomic layer deposition of highly photoactive hematite using iron(III) chloride and water

Nanostructured hematite (α-Fe2O3) has been widely studied for use in a variety of thin film applications including solar energy conversion, water oxidation, catalysis, lithium-ion batteries, and gas sensing. Among established deposition methods, atomic layer deposition (ALD) is a leading technique for controlled synthesis of a wide range of nanostructured materials. In this work, ALD of Fe2O3 is demonstrated using FeCl3 and H2O precursors at growth temperatures between 200 and 350 °C. Self-limiting growth of Fe2O3 is demonstrated with a growth rate of ∼0.6 A per cycle. As-deposited, films are nanocrystalline with low chlorine impurities and a mixture of α- and γ-Fe2O3. Post-deposition annealing in O2 leads to phase-pure α-Fe2O3 with increased out-of-plane grain size. Photoelectrochemical measurements under simulated solar illumination reveal high photoactivity toward water oxidation in both as-deposited and post-annealed films. Planar films deposited at low temperature (235 °C) exhibit remarkably high photocurrent densities ∼0.71 mA cm−2 at 1.53 V vs. the reversible hydrogen electrode (RHE) without further processing. Films annealed in air at 500 °C show current densities of up to 0.84 mA cm−2 (1.53 V vs. RHE).

Thermal conductivity of Er+3:Y2O3 films grown by atomic layer deposition

Cross-plane thermal conductivity of 800, 458, and 110 nm erbium-doped crystalline yttria (Er+3:Y2O3) films deposited via atomic layer deposition was measured using the 3ω method at room temperature. Thermal conductivity results show 16-fold increase in thermal conductivity from 0.49 W m−1K−1 to 8 W m−1K−1 upon post deposition annealing, partially due to the suppression of the number of the -OH/H2O bonds in the films after annealing. Thermal conductivity of the annealed film was ∼70% lower than undoped bulk single crystal yttria. The cumulative interface thermal resistivity of substrate-Er+3:Y2O3-metal heater was determined to be ∼2.5 × 10−8 m2 K/W.

Tunneling Spectroscopy of Superconducting MoN and NbTiN Grown by Atomic Layer Deposition.

A tunneling spectroscopy study is presented of superconducting MoN and Nb0.8Ti0.2N thin films grown by atomic layer deposition (ALD). The films exhibited a superconducting gap of 2 meV and 2.4 meV, respectively, with a corresponding critical temperature of 11.5 K and 13.4 K, among the highest reported Tc values achieved by the ALD technique. Tunnel junctions were obtained using a mechanical contact method with a Au tip. While the native oxides of these films provided poor tunnel barriers, high quality tunnel junctions with low zero bias conductance (below ∼10%) were obtained using an artificial tunnel barrier of Al2O3 on the films surface grown ex situ by ALD. We find a large critical current density on the order of 4 × 106 A/cm2 at T = 0.8Tc for a 60 nm MoN film and demonstrate conformal coating capabilities of ALD onto high aspect ratio geometries. These results suggest that the ALD technique offers significant promise for thin film superconducting device applications.