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Erasure Of Floating Gates in the Natural Radiation Environments in Space 1
Displacement Damage-induced catastrophic
second breakdown in silicon carbide Schottky
power diodes

Leif Scheick, Member, IEEE, Luis Selva, Heidi Becker
1

AbstractA novel catastrophic breakdown mode in reverse
biased silicon carbide diodes has been seen for particles that
are too low in LET to induce SEB, however SEB-like events
were seen from particles of higher LET. The low LET
breakdown mechanism correlates with second breakdown in
diodes due to increased leakage and assisted charge injection
from incident particles. Percolation theory was used to predict
some basic responses of the devices.

I. I
NTRODUCTION

Silicon carbide devices are becoming more attractive
for harsh environments due to the inherent toughness of the
silicon carbide substrate. Due to the wide energy band gap,
silicon carbide devices can operate at extremely high
temperatures without intrinsic conduction effects. Silicon
carbide can endure an electric field about eight times greater
than silicon or GaAs before exhibiting avalanche breakdown.
High breakdown electric fields allow for very high-voltage,
high-power devices such as power diodes. Also, devices can
be scaled aggressively, providing ULSI options for integrated
circuits. Silicon carbide has a high thermal conductivity. Heat
will flow more freely through silicon carbide than other
semiconductor materials and most metals at room
temperature. This property allows extremely high power level
operation and the dissipation of the generated energy. The
wide band gap also gives silicon carbide good resistance to
lattice damage, especially from displacement damage. Also,
the wide band gap of silicon carbide corresponds to unique
optical properties, which have been utilized in the fabrication
of blue and green LEDs. 4H-SiC has a band gap energy of
3.26 eV and 6H-SiC has a band gap energy of 3.03 eV. In
comparison, GaAs has a band gap energy of 1.43 eV and
silicon has a band gap energy of 1.12 eV. Table I compares
these and other material properties.
Silicon carbide devices are used in a wide variety of
applications and have been studied for radiation effects and
reliability in a wide spectrum of environments [1]-[3]. The
innate robustness of silicon carbide gives it a high breakdown
voltage and resistance to stress induced breakdown [4]-[7].

1
The research in this paper was carried out at the Jet Propulsion Laboratory,
California Institute of Technology under contract with the National
Aeronautics and Space Administration (NASA) Code AE, under the
Electronic Parts and Packaging Program (NEPP).
L. Scheick is with the Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109 USA (telephone: 818-354-3272, e-mail:
leif.scheick@jpl.nasa.gov).
H. Becker is with the Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109 USA (telephone: 818-353-5491, e-mail:
heidi.becker@jpl.nasa.gov).

L. Selva is with the Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109 USA (telephone: 818-354-5751, e-mail:
luis.selva@jpl.nasa.gov).


Silicon carbide has recently been studied for total ionizing and
displacement damage effects [8]-[12]. Silicon carbide has
shown innate hardness to displacement damaging and ionizing
radiation. Discrete silicon carbide devices have shown a
similar robustness in performance during and after irradiation.
This paper presents and discusses catastrophic failure modes
in silicon carbide power diodes due to proton and heavy ion
radiation that contradicts the current body of results.

T
ABLE
I.
C
OMPARISON OF
S
EMICONDUCTOR
P
ARAMETERS
.
4H-SiC
6H-SiC
GaAs
Silicon
Band gap energy [eV] 3.26
3.03
1.43
1.12
Breakdown electric
field [V/cm]
2.2E+06
2.4E+06 3.0E+05 2.4E+05
Thermal Conductivity
[W/cm K @ RT]
3.45
3.45
0.5
1.5
Saturates electron drift
velocity [cm/sec (@ E
2 x 10
5

V/cm)] 2E+07 2E+07 1E+07 1E+07

II. T
HEORY

Silicon carbide Schottky diodes have been available
commercially for some time. Schottky diodes have been
shown to be simple to manufacture and the Schottky
architecture has been shown to be very compatible with the
silicon carbide substrate. Silicon carbides lattice structure
allows for unique doping structure. Nitrogen doping takes a
carbon site and aluminum takes the silicon site. This
characteristic allows for compensated doping and more
defined intrinsic regions [4]-[6].
Figure 1 juxtaposes a pn diode structure and a
Schottky diode structure. The primary difference is that the pn
diode is a minority carrier device and the Schottky diode is a
majority carrier device. The rectifying structure in the pn
diode is the pn depletion region, while the Schottky diode
uses a metal semiconductor contact as the rectifying junction.
The breakdown characteristics are essentially the same for pn
and Schottky diodes. Silicon Schottky diodes have lower
breakdown voltages compared to similar silicon pn diodes due
to high curvature in depletion region layers, silicon surface
effects, and ohmic contact issues [4]-[6]. Device architecture
can compensate for some of the weaknesses of Schottky
structures. Silicon carbides wide band gap, and therefore
higher breakdown voltage, allows for a more robust Schottky
device.
Silicon carbide has many traits that lead to reliability
challenges. First, silicon carbide lattices continue to suffer
from a variety of defect producing impurities [13]. Silicon
carbide can experience burnout due to defects in the lattice
and breakdown characteristics continue to be a limiting factor
in silicon carbide devices [13]. Screw plane defects can cause
microfilaments that lead to device breakdown. Large screw
defects, with Burg vectors over two lattice sites, are called
micro-pipes and are a major failure mode in silicon carbide
substrates [13], [14]. Micropipes and voids in silicon carbide
are very conductive, which causes silicon carbide power
devices to be less reliable. Silicon carbide also has a higher 2
propensity to negative temperature coefficient breakdown in
the presence of defects. Silicon and silicon carbide have a
20% lattice mismatch. The transition from silicon to silicon
carbide, as in poorly made crystals with areas of only silicon,
can cause defects and stresses on the substrate [15].
Second, silicon carbide exhibits polytypism, which
makes fabrication and process control a uniquely hard
challenge for silicon carbide devices. Multiple types in a
substrate can lead to plane, point and screw defects in a
substrate [7], [13]. These defects decrease the breakdown
voltage of silicon carbide. Neutron irradiation has been shown
to induce defects in silicon carbide substrates [15]. The
physical structure and response of cascades from energetically
displaced Si atoms have been investigated [16]-[17]. These
investigations have shown that irradiated silicon carbide can
exhibit clusters of defects tens of nanometers in size.
Silicon carbide has exhibited several reliability
problems that have seriously impacted yield. Figure 2, taken
from [18], plots the distribution of breakdown voltages of
virgin Schottky barrier diodes. These diodes were free of
micropipe defects, which are a major cause of failure in
silicon carbide. Lesser defects are present and presumably
contribute to the breakdown variation. Since defect density
has been correlated with lower breakdown voltages, defects
induced by radiation are expected to lower breakdown voltage
and the determination of this response is a primary focus of
this study.
All diodes exhibit a high current, low voltage
condition after avalanche breakdown that is related to the
thermal breakdown of the device. This is called second
breakdown or snap back. The second breakdown condition
onsets when the intrinsic carrier concentration, n
i
, is equal to
the dopant concentration, N
n,p
, or

p
n
i
N
n
,
=

and

(1)

kT
E
v
c
i
g
e
N
N
n =

(2)

where, N
c
is the effective electron density of states in the
conduction band, and N
v
is the effective density of states in
the valence band [19]. Obviously from (2), high temperature
in a local region, whether caused by ions and/or defects will
drive n
i
up until (1) becomes true. Second breakdown occurs
at this point through a microfilament path. Defects have been
linked to the formation of microplasmas that can trigger a
temperature rise that initiates second breakdown [18].
Silicon diodes experience the breakdown described
by (1) and (2). In general, these devices will experience the
aforementioned breakdown at very high reverse biases or
under irradiation from high LET (>20 MeV.cm
2
/mg) ions.
Proton irradiation in silicon d