Reactive sputtering of mixed-valent oxides: a route to tailorable optical absorption

Date of Award

2015

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Advisor: P. Terrence Murray

Abstract

This work details two specific research thrusts exploring the deposition and characterization of mixed valent oxide systems. The first of these thrusts investigated the effect of the oxygen content, during reactive sputter deposition, on the optical, chemical, and structural properties of oxides of molybdenum, germanium, and rhenium. Exploration of the Mo-O system was conducted using a technique known as modulated pulse power magnetron sputtering (MPPMS), while the Ge-O and Re-O systems were deposited via direct current magnetron sputtering (DCMS). Films deposited under poisoned mode conditions were shown to be highly transparent with refractive index (n) values of n₅₅₀=1.60 for GeO₂, and n₅₅₀=1.97 for MoO₃, similar to values reported for bulk constituents. The Re-O system, unlike Ge-O and Mo-O, displayed a significantly high sensitivity to ambient moisture. Chemical analysis via XPS indicated the presence of instability as a result of the moisture induced decomposition of Re₂O₇ into HReO₄, and catalytic disproportionation of Re₂O₃ into Re and hydrous ReO₂. The second research thrust within this project was focused on the deposition of three component mixed oxide systems with multiple valence states. This effort, which utilized the results from individual material depositions mentioned previously, required the use of stable and thermodynamically compatible material systems, namely Mo-O and Ge-O ΔfH°(MoO2)= -588 kJ/mol and ΔfH°(GeO2)= -580 kJ/mol). Note that Re-O was not explored as part of the ternary deposition effort due to the aforementioned chemical instability. To achieve the goal of depositing mixed valent thin films with tailorable optical absorption, an industrially scalable co-deposition method was devised in order to deposit molybdenum cations within a dielectric GeO₂ matrix. The high power densities associated with the MPPMS process were systematically varied in order to control the oxygen partial pressure via gettering, allowing for control over the oxidation state and concentration of Mo⁴⁺ (MoO₂) and Mo³⁺ (Mo₂O₅) cations within a transparent GeO₂ matrix. In addition, this work devised a modification to the Berg model for reactive sputtering that is capable of predicting the resulting oxidation states of Mo and Ge within a reasonable degree of accuracy. The co-deposition procedure devised within this work allowed for the optical gap of mixed MoxGeyOz films to be tailored between 3.4 eV and 0.4 eV, spanning useful ranges for devices operating in the visible and near-infrared.

Keywords

Sputtering (Physics), Chemical vapor deposition, Oxides Effect of oxygen on, Thin films Optical properties, Materials Science, magnetron sputtering, thin film deposition, ellipsometry, optical coatings, x-ray photoelectron spectroscopy, XPS, optical absorption, band gap tailoring, molybdenum oxide, germanium oxide, Mo-Ge-O, mixed oxide coatings, reactive sputtering

Rights Statement

Copyright © 2015, author

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