Showing papers by "Suzanne E. Mohney published in 2016"

Ultrathin Titanium Dioxide Nanolayers by Atomic Layer Deposition for Surface Passivation of Crystalline Silicon

Abstract: We demonstrate a low surface recombination velocity of 14 cm/s with only 1.5 nm thin titanium dioxide (TiO2 ) layers on undiffused 10 Ωcm p-type crystalline silicon. The TiO2 nanolayers were deposited by thermal atomic layer deposition at 150 °C and 200 °C substrate temperatures using tetrakis-dimethyl-amido titanium as the Ti precursor and water as the oxidant. The influence of a post-deposition anneal in forming gas at different temperatures was investigated. We have observed that a subsequent anneal in forming gas at 350 °C enhances the surface passivation quality of the TiO2 layers tremendously. Increasing the thickness of the TiO2 layers leads to a reduction of the surface passivation quality. Introducing a thin interfacial layer of silicon oxide (1.6 nm) grown by rapid thermal oxidation underneath the TiO2 layer improves the surface passivation of thicker TiO2 layers (5.5 and 15 nm). These results show that ultrathin TiO2 layers with a thickness of only 1.5 nm can be used to effectively passivate the c-Si surface.

Robustness of topological surface states against strong disorder observed in B i 2 T e 3 nanotubes

Abstract: Three-dimensional topological insulators are characterized by Dirac-like conducting surface states, the existence of which has been confirmed in relatively clean metallic samples by angle-resolved photoemission spectroscopy, as well as by anomalous Aharonov-Bohm oscillations in the magnetoresistance of nanoribbons. However, a fundamental aspect of these surface states, namely, their robustness to time-reversal-invariant disorder, has remained relatively untested. In this work, we have synthesized thin nanotubes of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ with extremely insulating bulk at low temperatures due to disorder. Nonetheless, the magnetoresistance exhibits quantum oscillations as a function of the magnetic field along the axis of the nanotubes, with a period determined by the cross-sectional area of the outer surface. Detailed numerical simulations based on a recursive Green function method support that the resistance oscillations are arising from the topological surface states which have substantially longer localization length than that of other nontopological states. This observation demonstrates coherent transport at the surface even for highly disordered samples, thus providing a direct confirmation of the inherently topological character of surface states. The result also demonstrates a viable route for revealing the properties of topological states by suppressing the bulk conduction using disorder.

Impact of Premetallization Surface Preparation on Nickel-based Ohmic Contacts to Germanium Telluride: An X-ray Photoelectron Spectroscopic Study.

Abstract: Surfaces of polycrystalline α-GeTe films were studied by X-ray photoelectron spectroscopy (XPS) after different treatments in an effort to understand the effect of premetallization surface treatments on the resistance of Ni-based contacts to GeTe. UV–O3 is often used to remove organic contaminants after lithography and prior to metallization; therefore, UV–O3 treatment was used first for 10 min prior to ex situ treatments, which led to oxidation of both Ge and Te to GeOx (x < 2) and TeO2, respectively. Then the oxides were removed by deionized (DI) H2O, (NH4)2S, and HCl treatments. Additionally, in situ Ar+ ion etching was used to clean the GeTe surface without prior UV–O3 treatment. Ar+ ion etching, H2O, and (NH4)2S treatments create a surface richer in Ge compared to the HCl treatment, after which the surface is Te-rich. However, (NH4)2S also oxidizes Ge and gradually etches the GeTe film. All treated surfaces showed poor stability upon prolonged exposure to air, revealing that even (NH4)2S does not pas...

Condensed phase diagrams for the metal–W–S systems and their relevance for contacts to WS2

Abstract: Motivated by interest in contacts to the layered semiconductor WS 2 , this study investigates condensed phase equilibria in M–W–S systems (M = transition or post-transition metal) and relates the findings to earlier work on M–Mo–W systems. Thermodynamic data were collected or estimated for the binary phases bearing M, W, and/or S; and a literature search for existing ternary phases was performed. Condensed phase M–W–S isothermal phase diagrams were calculated where sufficient data was available. These phase diagrams reflected three general forms. The early transition metals and group 13 post-transition metals were mostly reactive with WS 2 , with the resulting diagrams being either metal sulfide dominant or ternary/solid solution dominant. The late transition metals were all WS 2 dominant, where the metal is in equilibrium with WS 2 . These findings are similar to those from our earlier study on the M–Mo–S phase diagrams and can be used to guide studies on contacts to WS 2 .

Pd and Au Contacts to SnS: Thermodynamic Predictions and Annealing Study

Abstract: Tin(II) sulfide (SnS) is emerging as an attractive p-type absorber layer material for thin film photovoltaics, which motivates the search for Ohmic, low-resistance contacts for SnS. In this study, Pd and Au contacts were prepared on sputter-deposited SnS films and electrically characterized both as-deposited and after annealing of the contacts. Ternary phase diagrams were also calculated to help predict whether the chosen metals would react with the SnS film. Pd was expected to react with the SnS film, while Au was expected to be in thermodynamic equilibrium. The Pd contacts appeared reactive, and their resistance was minimized with post-deposition annealing at 400°C, while the Au contacts showed little change upon annealing and remained unreactive.