Using quantum-well nano-apertures to probe hot-electron motion
in metal films Jonathan Pelz, Ohio State University, DMR-0505165
Unique cleaved-quantum well (QW) nanoapertures (with width down to
1 nanometer) are used in combination with Scanning Tunneling
Microscopy (STM) to visualize and quantify how hot electrons move
and scatter in thin metal films. Understanding hot electron motion
in metal interconnects is increasingly important as the spacing
between transistors in integrated circuits (such as PC processors)
shrinks below 100 nanometers, and will become even more critical if
circuits made from nanotubes and nanowires become commercially
viable. In the experiment, an STM tip injects a cloud of hot
electrons into the metal (whose size and shape distribution is not
known), and is scanned over a QW aperture [see Fig. (a)]. By
measuring how the hot electron current entering a QW changes with
tip position [data points in Fig. (b)] the hot-electron
distribution was determined to have a simple exponential shape
[blue curve in (c)] with a lateral size this increases with metal
film thickness. This rules out several common assumptions about hot
electron transport in metals, and will lead to better predictions
of how hot electrons enter and scatter in ultra-small metal
interconnects. Manuscript submitted to Applied Physics Letters. See
also Tivarus, Pelz, Hudait, and Ringel, Appl. Phys. Lett. 87,
182105 (2005).
Interdisciplinary Training:With NSF support, two undergraduate
REU students (Corey Campbel and Steven Stone) and three graduate
students (Camelia Marginean, Cristian Tivarus, and Yulu Che)
received highly interdisciplinary training using advanced
techniques in technologically important topics in
nanoscience.Collaboration with Undergraduate Institutions:Professor
Pelz collaborates with Professor Susan Lehman and her students of
the College of Wooster to study nanometer scale electronic behavior
of cleaved quantum dots (top figure) resulting so far in a March
Meeting presentation for Prof. Lehman and a manuscript in
preparation.Outreach:The PI regularly presents physics
demonstrations in elementary school classrooms (1st 6th Grade), and
has worked with elementary school teachers to help prepare for the
Ohio State Proficiency Test. This year, the PI is also advising a
local middle school teacher and student team for the National
Engineers Week Future City Competition (see
http://www.futurecity.org). This years essay topic is Keep our
Future Citys Infrastructure Healthy: Using Nanotechnology To
Monitor City Structures and Systems. Interdisciplinary Training,
Collaborations, and Outreach Jonathan Pelz, Ohio State University,
DMR-0505165
Who: Professor Jonathan Pelz (Ohio State University), with
graduate students Camelia Marginean and Cristian Tivarus, and in
collaboration with Professor Steven Ringel (also at Ohio State
University).
Acronyms/background info: - Scanning Tunneling Microscopy (STM).
The technique of Ballistic Electron Emission Microscopy (BEEM) is
based on STM. It uses an STM tip to inject hot electrons into a
thin metal film as shown in Fig. (a).- Quantum Well (QWs) are made
by sandwiching a thin film of a material with small bandgap [such
as gallium arsenide (GaAs)] between larger bandgap barrier layers
[such as aluminum gallium arsenide (AlGaAs)].- Nano-apertures. We
make these by cleaving a QW heterostructure to expose the cleaved
QW openings, then depositing a thin metal film [which in our case
is gold (Au)].
What/So-what: Over the past 20 years, there have been a number
of conflicting reports about how a nanometer-sized metal tip
tunnel-injects hot electrons into metal films, and how hot
electrons scatter and cool (lose energy) as they move from the top
surface to the buried metal/substrate interface. A measurement of
how the electrons spread laterally away from the injection point
would help resolve some to the conflicts, in particular related
whether the electrons enter the metal as a forward focused beam,
and how quickly their directions are randomized by elastic
scattering. The PIs group and collaborators have pioneered the use
of Schottky contacts on cleaved quantum well apertures in
combination with applied the STM-related technique of Ballistic
Electron Emission Microscopy (BEEM). In this highlight, BEEM
profiles [Fig. (b)] measured by scanning the tip over the QW
apertures were analyzed to make the first quantitative estimates of
the shape and later extent of the hot electron distribution. It was
found that the profiles could be explained very well by a
hot-electrons distribution (at the buried metal/semiconductor
interface) that decayed exponentially with distance from the
location directly below the tip, with a decay length that varied
from ~6 nm for a 4 nm-thick Au film to ~11 nm for a 15 nm-thick Au
film. By measuring how the electrons spread sideways AND how they
attenuate when the metal film thickness is varied, stringent tests
can be made of proposed models. By comparing with Monte Carlo
simulations of electron motion, it was concluded that the injected
electron beam could not be forward focused, but rather must enter
the metal with a wide range of angles, or quickly scatter to large
angles before reaching the buried interface. This was not the
prevailing belief. This has potential relevance for device
simulations of how electrons move between nm-sized field effect
transistors (FETs) through ultra-small metal interconnects. Future
studies will address how the scattering and spreading is affected
by temperature (scattering is expected to reduce at lower
temperature), metal type, and metal film structure.
Who: Professor Jonathan Pelz (Ohio State University), with
graduate students Camelia Marginean and Cristian Tivarus, and in
collaboration with Professor Steven Ringel (also at Ohio State
University).
Acronyms/background info: - Scanning Tunneling Microscopy (STM).
The technique of Ballistic Electron Emission Microscopy (BEEM) is
based on STM. It uses an STM tip (held at a certain voltage) to
inject hot electrons (with an energy that depends on the tip
voltage) into a thin metal film as shown in Fig. (a). BEEM works by
measuring how many of these electrons can cross the metal film and
enter the substrate, as the tip voltage and/or the tip position is
varied. The local conduction band energy of the substrate can be
determined from the minimum threshold voltage the tip must have to
see current in the substrate, and by scanning the tip regions with
higher transmittance (i.e. over a pinhole) can be identified.
What/So-what: A great many things are still not well understood
at all about how electrons (and holes) move across molecule/metal
and molecule semiconductor interfaces. Two extremely important
issues are (1) how the energy levels in the molecular materials
line up with the energy bands in the inorganic materials, and (2)
are the molecular films continuous and uniform. BEEM can give
valuable nm-scale information about these issues that is not
possible to measure with other techniques.
Both the 4th grade and 6th grade Ohio proficiency tests have (or
had) questions about Newtons laws. So in discussions with the
elementary school teachers, we decided some demonstrations and
explanations would be the most useful.