Graduate Thesis Or Dissertation

 

Synthesis and Analysis of Heterostructural Semiconductor Alloy Sn1-xCaxCh (Ch=S, Se) and Nitrides Zn1-xWxN and Zn1-x(W1-yMoy)xN Public Deposited

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  • This work explores the synthesis and characterization of the metastable alloys Sn1-xCaxCh (Ch= S, Se) and nitride compounds Zn-W-Mo-N, which have recently been predicted by theorists. Single phase thin films of Sn1-xCaxS are prepared by pulsed laser deposition and radio-frequency magnetron sputtering and of Sn1-xCaxSe are prepared by pulsed laser deposition. A transition from the rock salt cubic structure at high Ca concentrations to the orthorhombic structure at lower Ca is observed at x = 0.25 for the sulfide and x = 0.18 for the selenide. Optical bandgaps for both Sn1-xCaxCh alloys follow nonlinear trends with a discontinuity near the transition composition range from 1.5-2.45 eV and 0.6-3.5 eV for Sn1-xCaxS and Sn1-xCaxSe respectively. Sn1-xCaxS samples were highly resistive, yielding resistivities of 〖10〗^2 -〖 10〗^4 Ωcm at low Ca concentrations (x < 0.1) and increasing rapidly with added Ca. Sn1-xCaxSe samples were much more conductive with resistivities 〖10〗^(-2) - 5×〖10〗^1 Ωcm over the range of compositions (0 < x < 1) and exhibited excellent thermoelectric properties with a power factor PF = 2 μWcm-1K-2 at x = 0.16. The phase decomposition of these alloys is also explored using STEM EDS (scanning transmission electron microscopy energy dispersive x-ray spectroscopy). Thin films of nitride compounds Zn-W-N and Zn-Mo-W-N are deposited by RFMS and the range of compositions for stabilization of the predicted wurtzite phase (at Zn3WN4) is tested. Single phase crystalline WZ samples were obtained between 60-87% and 50-84% Zn cation percent for the Zn-W-N and Zn-Mo-W-N systems respectively. Optical absorption for samples is calculated from measured transmission and reflectance for 800 < λ < 1100 nm, and samples at low Zn percentages are more absorbing in the IR than at higher Zn. Resistivities are measured for all films and found to be 〖10〗^3- 〖10〗^5 and 〖10〗^(-1) -〖 10〗^5 Ωcm for Zn-W-N and Zn-W-Mo-N respectively. Composition uniformity and crystalline uniformity are explored by STEM EDS and TEM respectively. WZ phase films were found to be compositionally uniform, but films at lower Zn concentrations (around 30% Zn) tended to phase separate. The experimental work presented here shows the viability of computational predictions of metastable materials and materials searches in alternative materials space to expand the number of known materials and understanding of metastable materials.
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  • This work was primarily funded as part of the Center for Next Generation Materials by Design an Energy Frontier Research Center funded by the Department of Energy Office of Science.
  • This work was partly funded by the Whiteley Materials Fellowship for the summer of 2017.
  • This work, particularly the nitride section, was funded by the 2017 Department of Energy Office of Science Graduate Student Research Award.
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