Spectro-microscopy in the field emission regime of scanning tunneling microscopy
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Date
2020
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Doctoral Thesis
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Abstract
In the Fowler-Nordheim (FN or field-emission) regime of Scanning Tunneling
Microscopy (STM), the tip-target distance is from a few to tens of nanometers.
In this situation, the quantum mechanical tunneling of electrons between tip
and target is entirely suppressed. Instead, electrons can be field-emitted out
of the tip to the vacuum by a process first described at the dawn of quantum
mechanics by Fowler and Nordheim. Their energy is typically in the range of
a few tens of electronvolts. They build the primary beam in this imaging technology
that we call SFEM – standing for Scanning Field Emission Microscopy
– to distinguish it from STM.
The primary beam’s impact on the target leads to the excitation of electrons off
the surface. Under favorable electrostatic circumstances, the excited electrons
escape the tip-target junction and build a new electronic system of secondary
electrons (SE), absent in the tunneling regime of STM. A dissertation by L. De
Pietro [1] discovered that secondary electrons are spin-polarized [2]. D. Zanin
[3] recorded secondary electrons’ energy spectra characterized mainly by two
peaks. The former corresponding to the elastically scattered electrons. The
latter corresponding to the ”true” secondary electrons, i.e., those with kinetic
energy just above the vacuum level at the sample, resulting from the cascade
of inelastically scattering processes. A clear signature for the appearance of
characteristic energy losses or energy gains was missing in Zanin’s dissertation.
In SFEM, not only scattering processes such as those leading to the production
of elastic and inelastic electrons are relevant, but also the behavior of the
electrons excited off from the surface in the presence of the strong electric
field existing in the vicinity of the tip apex must be taken into account. As a
consequence of this complexity, the mechanism of contrast formation and
spatial resolution in images acquired by SFEM has yet to be deciphered.
This Dissertation reports experimental results that shed light on the physical
processes involved in primary electrons and secondary SFEM imaging,
performed with and without energy resolution. In particular
The standard view of the current-voltage characteristic measured in
field emission foresees a diode-like law that reflects the existence of
a potential barrier surrounding the tip through which electrons can
iv
tunnel. Such a barrier depends on the work function of the tip and
the electric field at the tip apex. Here we observe a yet undetected
dependence of the field emitted (primary) current from the material
residing on the target side separated several nanometers from the tip.
The contrast, observed upon imaging dual systems such as domains
of W and Carbidic-W (WC) on a W(110)-surface, is about one hundred
times larger than the contrast one expects from the vertical surface
corrugation and about ten times larger than the contrast expected from
the work function differences between W and WC. Other dual systems
(such as W versus Fe/W and W versus Fe/WC or W versus Au/W)
show a similar contrast in the primary electron channel of detection.
This dissertation records similar figures of contrast and lateral spatial
resolution along the various channels available in SFEM-imaging, i.e.,
absorbed current imaging, emitted current imaging, total electron imaging,
and energy-resolved SFEM images, as demonstrated by imaging
of surfaces with dual systems, including semiconducting Ge versus
Au-covered Ge. These systematic observations suggest SFEM as a
suitable spectro-microscopy technology not only for metallic surface
investigation but also for semiconductor surfaces.
We observe a clear signature for characteristic energy gains within
the ”sea” of inelastically scattered electrons residing between the ”true”
secondary and the elastically scattered electrons. We are able to assign
one specific energy gain (at about 22 eV) to those electrons that are
excited upon the decay of a W-plasmon. In virtue of this result, SFEM is
established as a technology that can be used to detect energy-resolved
features with nanoscale spatial resolution.
Almost all electrons field-emitted from the tip are absorbed by the target,
and only less than 1% typically escape the tip-sample junction. This
dissertation detects for the first time minute but significant difference
between field-emitted current and absorbed current at selected voltages,
opening the possibility to performing spectro-microscopy directly in the
primary beam channel, thus avoiding the necessity of detecting any SE.
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Examiner: Pescia, Danilo
Examiner : Gürlü, Oğuzhan
Examiner : Xanthakis, J.P.
Examiner: Vaterlaus, Andreas
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ETH Zurich
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Subject
Microscopy; Spectroscopy; Field emission; Scanning tunneling microscopy (STM); Low Energy Electron microscopy
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03351 - Pescia, Danilo (emeritus) / Pescia, Danilo (emeritus)