Volatile Crystalline Semiconductor Core Fibers

Author

Thomasina ZaengleFollow

Date of Award

5-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Committee Chair/Advisor

John Ballato

Committee Member

Thomas Hawkins

Committee Member

Ursula Gibson

Committee Member

Philip Brown

Committee Member

Mark Johnson

Abstract

Optical fibers play critical roles across many facets of everyday life from communications to e-commerce to sensing and security. The ubiquity of optical fibers arises from their intrinsic clarity and, as glasses, their ability to be thermally drawn at high speeds over long distances when suitably heated about their glass transition temperature. Sixteen years ago, the first thermally drawn crystalline core fibers were fabricated using the molten core method, whereby a melt is confined within a glass capillary tube that is then drawn to fiber. This opened the door to crystalline semiconductor core fibers, which are now the backbone of many in-fiber optoelectronic devices and wearable electronics.^1,2 With this realization that crystalline semiconductor cores could be fabricated using an industrially scalable manufacturing method (thermal drawing) / (the molten core method), materials development into other optoelectronically interesting materials took off. Starting with simple unary systems,^3,4 followed by binary line compounds,⁵ isomorphic,⁶ and eutectic systems^7,8 which demonstrate excellent optoelectronic properties for a wide range of in-fiber applications. With demonstrations of sensing,⁹ detection,¹⁰ amplification,¹¹ signal processing,¹² light generation,¹³ etc., what could be left?

Materials that incongruently solidify or exhibit volatility as they melt, have been precluded from molten core fabrication to-date due to the buildup of vapor pressure during fiber draw, causing the glass cladding to rupture. However, materials with these attributes, such as GaAs, ZnSe, and InP, are of considerable interest for photonic and optoelectronic devices. The work presented in this dissertation breaks with conventional wisdom and reports the first fabrication and evaluation of volatile and incongruently melting semiconductor core fiber, (GaAs, ZnSe, InP), using the “flux assisted molten core method.” Through the introduction of a flux phase, the melt temperature of the volatile semiconductor is decreased, greatly reducing its volatility. For this reason, the fabrication of volatile semiconductor species is possible using the molten core thermal draw process enabling the study of these fiber’s crystalline formation and properties.

Volatile and incongruently melting core phases (GaAs, ZnSe, and InP) are shown to impact the thermodynamic / kinetic balance following both the thermal drawing and CO₂ laser post-processing to create, in some cases, metastable phases. During these thermal processes, many of the resulting fibers had reactive chemistry taking place, with the majority of the phases differing from what was expected from the equilibrium phase diagrams of the initial semiconductor:flux system. Thus, the energetic stability of the phases was found through their respective phase diagrams, and if necessary, formation enthalpy diagrams. The flux phases would change depending on the amount of, e.g., As, volatilizing and the rate of cooling induced from fiber draw and CO₂ laser post-processing. However, the semiconductor phase, i.e., GaAs, ZnSe, and InP were always present in the as-drawn and laser annealed fiber, except for the ZnSe:In₂Se₃ fiber.

The semiconductor:flux cores underwent CO₂ laser post-processing in order to segregate and recrystallize the semiconductor phase. The volatility and incongruency impact the ability to create a consistent melt zone within the core during CO₂ laser processing, thus creating a non-optimal environment for crystal growth. However, the GaAs and ZnSe fibers demonstrated segregation from the flux phases in the lateral, longitudinal, and radial directions within the core. This represents the first demonstration for in-fiber metal-semiconductor junctions containing volatile species.

In summary, this dissertation opens new doors by providing a path to the direct fiberization of volatile and incongruently melting semiconductor core phases. The as-drawn and laser annealed phases are, in some cases, unexpected based upon the semiconductor:flux equilibrium phase diagram in the literature. Many of the phases were deemed thermodynamically stable; however, some were metastable, based on the resulting materials equilibrium phase diagram and formation enthalpy diagram. The as-drawn fibers were post-processed using a CO₂ laser in order to segregate the semiconductor from the flux and recrystallize the semiconductor phase. It was realized that the glass cladding, volatility, and inhomogeneity of the core prohibited single crystal growth, however segregation in the lateral, longitudinal, and radial directions was achieved.

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Recommended Citation

Zaengle, Thomasina, "Volatile Crystalline Semiconductor Core Fibers" (2024). All Dissertations. 3640.
https://open.clemson.edu/all_dissertations/3640

Author ORCID Identifier

0000-0002-2637-8664