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Multiscale characterization of defects in 4H-SiC high-power devices and the effect on MOSFET reliability
El Hageali, Sami A.
El Hageali, Sami A.
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2022
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Abstract
Silicon carbide (4H-SiC) has been accepted as an optimal semiconductor that can substitute for silicon
for fabricating advanced power devices for high temperature, power, and frequency applications, owing to
its outstanding physical properties. Despite the major progress in SiC process technology, SiC-based
devices suer from bipolar degradation. As an example, SiC p-i-n diodes have previously suered from an
increase in the forward voltage drop under forward conduction stress. It was discovered that the basal
plane dislocations (BPDs) in epitaxial layers result in the formation of stacking faults (SFs) that can
expand through a mechanism called \recombination enhanced glide mechanism" whenever the p-i-n diode
is forward biased. The SFs represent regions of poor lifetime and are regions with poor conductivity
modulation. Indeed, SFs not only act as recombination centers but also impede the
ow of majority
carriers. The occurrence of SFs must therefore be prevented, the expansion behavior needs to be
understood and correlated to SiC-based device performance.
For this reason, the central focus of this dissertation is to provide a full understanding of specic types
of extended defects found in commercial wafers through a multiscale analysis and study the eect on
4H-SiC metal{oxide{semiconductor eld-eect transistors (MOSFETs) reliability.
The rst study highlights the power of a multiscale luminescence characterization approach to studying
extended defects in epitaxial 4H-SiC semiconducting materials using two complimentary techniques,
photoluminescence (PL) and cathodoluminescence (CL). The BPD network generated from strain around a
down-fall particle indicated the presence of dierent structures, such as Shockley-type and Frank-type SFs.
Ultraviolet-PL imaging allowed for a rapid identication of the inner structure of the defect by revealing
the BPD network and the presence of various SFs, and CL was used to provide better spatial and spectral
information. This detailed optical analysis provides a pathway for the fundamental understanding of the
impact of defects on device performance and provides a better understanding of their formation and
development during epitaxial growth. In this work, an inclusion was selected as an example, but this
method applies to any heterostructures or areas that show BPDs.
The nature, origin and behavior under device operation of so called "trapezoidal defects" were revealed
using a complete multiscale characterization study and the results were correlated with degradation of
MOSFETs having this defect. The correlation between the luminescence and microscopy results allowed us
to precisely identify the nature of these SFs as: Single Shockley, Extrinsic Frank type (2,3)n and,
Multilayer Frank type (4,2), 8H. The optoelectronic study showed that expansion of SFs within the
trapezoidal defect is greatly hampered by sessile dislocations and that trapezoidal defects are spread on multiple basal planes; Electron beam induced current (EBIC) imaging showed that dislocations within this
defect act as strong sites of carrier recombination which is likely to have an impact on the on-state transfer
characteristics of SiC devices. EBIC and Transmission electron microscopy (TEM) revealed that
trapezoidal defects come from the substrate and propagate into the epilayer. Furthermore, device electrical
measurements showed that as the percentage coverage of trapezoidal defects increases within the active
area of a MOSFET device, the on-state resistance increases. Body diode stressing measurements are in
agreement with the statement that expansion of SFs within the trapezoidal defect is greatly hampered by
sessile dislocations which in part is benecial to body diode degradation. Indeed, the results showed that
trapezoidal defects do not degrade devices as much as SFs that can freely expand. Overall, our conclusions
nd that trapezoidal defects should still be considered non-killer at low percentage coverage in the active
are, but eorts by substrate and epilayer manufacturers need to be made to erase their occurrence.
The origin, formation mechanism and behavior of "Star-defect" under device operation were
investigated. This study shows that the in-grown star-shaped defect originates in the substrate from
on-axis grown boule and propagates until the epilayer/substrate interface. It has a highly strained center
core, with primary arms and novel secondary dislocation arrays that were found to be emanating from the
primary arms. The secondary arrays are found to be prismatic faults formed from intersection of BPDs.
Star-defect has BPDs aggregate present along its core as well as at intersections between primary
arms/secondary arrays. These aggregates are nucleation points of SFs expansion that propagate from
multiple depths until reaching the epilayer. The total impact on device yield is critical for a single
star-defect with secondary arrays as the defect spans a large wafer area (5 by 5 cm-2). A wafer having
multiple star-defects will pass initial MOSFETs screening-tests making the devices available on the market
but a rapid electrical degradation of the MOSFET is expected.
The results presented are crucial to industrial manufacturers in order to assess device reliability. As a
result of this work, SiC devices' electrical yield and the potential degradation can be sampled in real time
during fabrication.
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