Aerospace & Defense Technology - February 2021 - 36

Tech Briefs

Atomistic- and Meso-Scale Computational Simulations for
Developing Multi-Timescale Theory for Radiation
Degradation in Electronic and Optoelectronic Devices
Fundamental mechanisms and knowledge gained from atomic- and meso-scale simulations can be
input into rate-diffusion theory as initial conditions to calculate the steady-state distribution of point
defects in a mesoscopic layered structured system, thus allowing the development of a multitimescale theory to study radiation degradation in electronic and optoelectronic devices.
Air Force Research Laboratory, Kirtland Air Force Base, New Mexico

I

ing, migration and interaction with microstructural features is crucial for developing a multi-scale theory to explore
radiation degradation in electronic and
optoelectronic devices.
At the most fundamental level, molecular dynamics can be used to study
defect production, migration and interaction, while kinetic Monte Carlo methods can be employed to simulate defect
evolution and their spatial distribution.
One particular semiconductor, gallium
arsenide (GaAs), has received considerable attention due to its potential electronic applications, such as GaAs-based
metal semiconductor field effect transistors and logic gates, near-infrared imaging devices and photovoltaic nodules. As
a means for fabricating compound semiconductors, the interest in using ion implantation continuously increases, which
inevitably introduces defects. The diffusion and accumulation of these defects
are critical factors controlling the dynamics of ion implantation processes and device degradation, as well as affecting
charge compensation, minority-carrier
lifetimes and luminescence efficiencies.
Recently, use of GaAs in high-power
space-energy systems and special spaceprobe applications has been proposed.
However, space radiation damage to
[001]
the GaAs may be
[[110
00
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[ 10]]
[1
a limiting factor
on interplanetary
[111]
o.
missions unless suf[010]
[010]
ficient shielding is
[11111]
[111
[110]
provided to keep
damage levels un [100]
der acceptable lim[001]
its. Con sequently,
(b)
(a)
radi ation damage
studies have been
(a) GaAs crystal structure; (b) illustration of stereographic triangles on a refmade ex peri men erence sphere.
t is well known that in a perfect crystal, the continuous free-electron states
are quantized into many Bloch bands
separated by energy gaps. These Bloch
electrons move freely inside the crystal
with an effective mass different from
the free-electron mass. In the presence
of defects, however, the field-driven
current flow of Bloch electrons in the
perfect crystal will be scattered locally
by these defects, leading to a reduced
electron mobility. Also, the photo-excited electron lifetime, due to non-radiative recombination with defects, has
been proven to be a key factor affecting
the sensitivity or the performance of
optoelectronic devices (e.g., photo-detectors and light-emitting diodes).
The dangling bonds attached to the
point defects may capture extra electrons to form charged defects. In this
case, the positively charged holes in the
system will be trapped to produce a
strong space-charge field, while the negatively charged electrons may generate
the so-called 1/f−current noise in their
bumpy motions due to the presence of
many potential minima and maxima
from randomly distributed charged defects. Therefore, understanding of defect production, stabilization, cluster-

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tally on the effects of electron, and proton and neutron irradiation, including
defect production and annealing, as well
as effects on the performance of GaAs
devices.
On the other hand, understanding the
basics of ion-solid interaction and irradiation damage has led to significant developments in state-of-the-art atomic-level,
kinetic Monte Carlo and meso-scale simulations, and these simulations have dramatically advanced the knowledge of defects and defect processes in a number of
materials, ranging from metals to semiconductors to ceramics. Recently, largescale ab initio and classical molecular dynamics (MD) methods have been
developed for the study of radiation damage in semiconductors, and these methods are used to explore the number of displacements produced, defect clustering
and disordering, as well as the effects of
charge transfer and charge-density redistribution on the dynamics and ultimate
charge-state of defect formation. These
simulations have demonstrated some
nonlinear effects taking place at low and
high energies, which can greatly modify
the number of displacements as predicted
by the simplified Kinchin-Pease model.
It has been long realized that the observed radiation damage to electronic
components made from semiconductors or semiconducting compounds is
proportional to the non-ionizing energy loss (NIEL). Consequently, the experimental use of NIEL for correlating
proton-induced displacement damage
in semiconductor devices has been
widely applied over the past decade,
which has proven useful in the study of
both Si and GaAs. Moreover, radiation
damage in other semiconductors such
as diamond and SiC, and many III-V
compounds, such as InP, has been in-

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