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Growth and Electronic Structure of Heusler Compounds for Use in Electron Spin Based Devices


Spintronic devices, where information is carried by the quantum spin state

of the electron instead of purely its charge, have gained considerable interest

for their use in future computing technologies. For optimal performance, a pure

spin current, where all electrons have aligned spins, must b e generated and

transmitted across many interfaces and through many types of materials. While

conventional spin sources have historically been elemental ferromagnets, like Fe

or Co, these materials pro duce only partially spin polarized currents. To increase

the spin polarization of the current, materials like half-metallic ferromagnets,

where there is a gap in the minority spin density of states around the Fermi

level, or topological insulators, where the current transport is dominated by

spin-locked surface states, show promise. A class of materials called Heusler

compounds, with electronic structures that range from normal metals, to half metallic ferromagnets, semiconductors, sup erconductors and even top ological

insulators, interfaces well with existing device technologies, and through the

use of molecular beam epitaxy (MBE) high quality heterostructures and films

can b e grown. This dissertation examines the electronic structure of surfaces and

interfaces of both top ological insulator (PtLuSb– and PtLuBi–) and half-metallic

ferromagnet (Co2MnSi– and Co2FeSi–) III-V semiconductor heterostructures.

PtLuSb and PtLuBi growth by MBE was demonstrated on AlxIn1−xSb (001)

ternaries. PtLuSb (001) surfaces were observed to reconstruct with either (1x3)

or c(2x2) unit cells depending on Sb overpressure and substrate temperature.


The electronic structure of these films was studied by scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission sp ectroscopy. STS measurements as well as angle resolved photoemission spectropscopy (ARPES) suggest that PtLuSb has a zero-gap or semimetallic band structure. Additionally,

the observation of linearly dispersing surface states, with an approximate crossing point 240meV above the Fermi level, suggests that PtLuSb (001) films are

topologically non-trivial. PtLuBi films also display a Fermi level position approximately 500meV below the valence band maximum.

Co2MnSi and Co2FeSi were also grown by MBE on GaAs (001) for use as

spin injectors into GaAs lateral spin valve devices. By the growth of the quaternary alloy Co 2FexMn1−xSi and varying x, electron doping of the full Heusler

compound was demonstrated by observation of a crossover from a majority spin

polarization of Co 2MnSi to a minority spin polarization in Co2FeSi. Co2MnSi

films were studied as a function of the nucleation sequence, using either Co–

or MnSi– initiated films on c(4x4) GaAs. Studies using x-ray photoemission

spectroscopy (XPS), STM/STS, and transmission electron microscopy (TEM)

suggest that the bulk of the Co2MnSi films and the interfacial structure between

Co2MnSi and GaAs is not modified by the nucleation sequence, but a change

in spin transport characteristics suggests a modification of semiconductor band

structure at the Co2MnSi/GaAs interface due to diffusion of Mn leading to compensation of the Schottky barrier contact. Diffusion of Mn into the GaAs was

confirmed by secondary ion mass spectrometry (SIMS) measurements. The proposed mechanism for the modified spin transport characteristics for MnSi initiated films is that additional diffusion of Mn into the GaAs, widens the Schottky

barrier contact region. These studies suggest that the ideal initiation sequence

for Co2MnSi/GaAs (001) lateral spin valve devices is achieved by deposition of

Co first.

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