The properties of dilute bismuthides and rare-earth containing materials for applications in thermoelectrics, optoelectronics, and terahertz technology
Date
2015
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
(In)GaAs compounds containing low concentrations of Bi are called dilute
bismuthides. When Bi is incorporated, bandgap narrowing occurs because of valence
band anticrossing (VBAC). The ability to tune the bandgap of this material is useful in
the field of optoelectronics. Dilute bismuthides, or more specifically n-InGaBiAs, is
also expected to be a good thermoelectric material because Bi is a heavy atom and the
conduction band should be similar to InGaAs. A heavy Bi atom can effectively scatter
heat phonons, which reduces the thermal conductivity. With similar conduction band
to InGaAs, a relatively high thermoelectric power factor is expected as well. This
dissertation discusses the electrical and thermoelectric properties of n-InGaBiAs. The
dissertation also briefly discusses how this material is ideal for mid-infrared
transparent contacts. Transparent contacts are useful for devices such as liquid crystal
displays (LCDs) and solar cells. However, traditional transparent contact materials
like indium tin oxide (ITO) only work at the visible to near-infrared range. At midinfrared
wavelengths, indium tin oxide is highly reflective due its plasma frequency.
Dilute bismuthides are potential candidates for mid-infrared transparent contacts
because they are transparent in mid-infrared wavelengths and have relatively low sheet
resistances.
ErAs or TbAs nanoparticles can be embedded in GaAs or InGaAs by codeposition
via molecular beam epitaxy (MBE). These materials have interesting
properties that can be applied to thermoelectrics and terahertz technologies. For
thermoelectrics, the nanoparticles have a dopant-like behavior and provide scattering
centers for phonons. ErAs nanoparticles in GaAs provides the short carrier lifetimes
and high dark resistance that is needed for a terahertz material. An interesting question
one may ask is what will happen when both Er and Tb are co-deposited. It was
hypothesized that core-shell nanoparticles would form through a strain-driven process
when both Er and Tb are co-deposited. This dissertation discusses the observation of
self-assembled rare-earth core-shell nanoparticles in InGaAs. The core-shell
nanoparticles was observed using atom probe tomography (APT). The structure
consisted of a mixed core (ErAs and TbAs) and a pure TbAs shell. A simple
energetics model, which is also discussed in this dissertation, confirms the formation
of core-shell nanoparticles. An application for InGaAs containing core-shell
nanoparticles is terahertz technologies, or more specifically, a photoconductive switch.
These nanoparticles are expected to provide the short carrier lifetimes and high dark
resistance needed for THz applications using InGaAs, but more research needs to be
performed. Unexpectedly, TbAs may appear to have a bandgap, rather than semimetal.
A discussion about the band structure of TbAs is presented in this dissertation as well.
Finally, this dissertation presents the future directions for these materials. For
dilute bismuthides, materials with high Bi concentrations are ideal for mid-IR lasers,
but growing these materials is not straightforward. This dissertation discusses some
potential solutions to their growth and fabrication. For core-shell nanoparticles, more
growths and characterization can be performed to determine how the rare-earth
concentrations (or ratios) affect the number of core-shell nanoparticles. In conclusion,
dilute bismuthides and (In)GaAs containing rare-earth, core-shell nanoparticles remain
promising for applications in thermoelectrics, optoelectronics, and terahertz
technologies.