Imaging the interfacial charge carrier dynamics at axial p-n junction nanowires
Internship Description
Workshop Description
To
optimize the light harvesting, multiple junctions with different band gaps can
be combined to match the solar spectrum, but lattice matching requirements
severely limit the materials available for devices based on thin film growth.
In general, nanowires (NWs) have emerged as building blocks for electronic and
photonic technologies due to their distinct advantages over its bulk and planar
counterparts. Multi-junction solar cells containing several p-n junctions which
can be used to surpass the Shockley−Queisser efficiency limit in solar cells.
However, the carrier dynamics at the interfaces of the p-n junctions are still
not well understood. Being in this regime, the classical picture of
photo-induced charge transfer at a heterojunction can be described as follows:
right after excitation, electron-hole pairs are generated. This is followed
either by electron-hole separation and drifting in opposite directions under
the influence of the electric field of the p-n junction, or the electron-hole
pairs can recombine radiatively by emitting a photon, or non-radiatively by
carrier trapping or/and Auger recombination. However, where the exciton is
localized and trapped, how the transfer occurs and how it depends upon the
local environment is an important as yet unanswered question. The processes by
which carriers transfer back across the interface and recombine, thereby
returning to the equilibrium state, are highly dependent upon the local
environment and are not fully understood and they cannot be catalogued using spectroscopic
techniques. In other words, the accessibility of these dynamical processes by
static imaging or steady-state and time-resolved spectroscopic techniques is
very limited.
The unique opportunity to visualize the carrier dynamics selectively on the
material surface can only be accessed by 4D S-UEM with fs temporal and nm
spatial resolutions. We will begin by utilizing the S-UEM technique to
investigate charge dynamics in n-InGaN/p-GaN nanowires. For the S-UEM
measurement, right after laser excitation, the time-resolved secondary electron
images arising from the first few nm of the sample surface will be recorded,
providing the image-contrast changes for mapping charge dynamics.
Straightforwardly, bright or dark image contrast correspond to increase or
decrease in the local electron density, telling us where the carriers
(electrons/holes) are localized. In other words, bright and dark image contrast
is interpreted as an increase in the local electron and hole densities,
respectively. Moreover, charge separation and charge recombination can be
directly accessed from the spreading out the bright contrast or from
diminishing the dark contrast.
Deliverables/Expectations
Deliverables
With real-space imaging, we
can examine the temporal behavior of a local surface area (i.e., pixel areas),
providing direct information about the charge carrier dynamics including
trapping and recombination on the surface and at the interface of inorganic
single crystals, different structural compositions of InGaN nanowires and the
electron-hole localization across the p-n junction based on GaN/InGaN
nanowires. The results would help us in gaining valuable insights into the
photo-physics of such materials and guide in the better designing of
optoelectronic devices
Faculty Name
Omar F. Abdelsaboor
Field of Study
Field Of Study
Electron
imaging, interface dynamics, oproelectronic applications