Electrical Activation Processes in Ion Implanted SiC
Device Structures
K.A. Jones1, M.H. Ervin1, P.B. Shah1, M.A. Derenge1,
R.D. Vispute2, T. Venkatesan2, and J.A. Freitas3
1
3
Army Research Lab - SEDD, 2800 Powder Mill Rd, Adelphi, MD 20873
2
University of Maryland, Physics Dept., College Park, MD 20742
Naval Research Lab, Code 6877, 4555 Overlook Ave., Washington, DC 20375
Abstract: Electrically activating implanted dopants requires annealing temperatures above that where the Si
evaporates preferentially. Methods for preventing this are discussed, and it is shown that AlN can be used as an
annealing cap for temperatures as high as 1600°C. For the higher temperatures required to activate acceptors, a dual
BN/AlN cap can be used. Defects such as the DI defect, however, cannot be annealed out, and it is shown that this
defect is probably a deep donor that can act as a hole trap, and it is most likely an extended defect such as a dislocation
loop.
is impossible to create an atmosphere that is in
equilbrium with both the exposed implanted and
unimplanted regions. We have demonstrated
that AlN can behave as an effective annealing
cap; it can survive annealing temperatures as
high as 1600°C for times as long as 30 min, it
does not react with the SiC, and it can be
removed selectively with a warm KOH etch [2].
Although 1600°C seems to be sufficient for
activating the n-type dopant, N, much evidence
exists suggesting that higher temperatures are
required to activate most of the p-type implants.
We attempted to use an amorphous graphite cap,
which can be removed easily by oxidizing it, but
it crystallizes and can only be removed by ion
milling it off. When we deposited it or Al2O3 on
AlN to prevent the AlN from decomposing
thermally, they were not strong enough to
prevent the evaporation of N and/or Al, and
blow-holes were created in them [3]. We have
shown that BN films are strong enough to stop
the AlN from decomposing, and they can be ion
milled off, and then the AlN film can again be
preferentially removed in warm KOH [4].
In this paper we will show that much can be
learned about the activation processes in N, Al,
and Al/C implanted samples as they can be
annealed for long times at high temperatures
without the preferential of Si and the subsequent
degradation it causes.
Introduction
Because the rate of the diffusion of dopants is
too low to be technologically useful even at
temperatures as high as 1800°C [1], SiC can only
be doped by ion implantation. Thus, to create an
isolated p-n junction in an n-type film and at the
same time retain the planar technology, the pregions must be fabricated by localized ion
implantation. This is also true for the fabrication
of junction barrier Schottky (JBS) diodes and
double diffused MOSFET's. Even when diodes
are made from p-n layered structures by etching
away the surrounding p-region, the areas around
them are often implanted to decrease the electric
field in the vicinity of the junction to prevent
premature breakdown. This is known as junction
termination extension or JTE.
The dopants must not only be implanted, they
have to be activated by thermally annealing the
sample. However, the temperatures required to
activate the dopants are above temperatures
where an appreciable amount of Si evaporates
preferentially. Si3N4 capping layers cannot be
used as they are to prevent the preferential
evaporation of As from GaAs that is being
annealed to activate implanted ions because it,
too, evaporates at the high temperatures used to
activate the implanted dopants in SiC, typically ~
1600°C. Some have tried to prevent the Si from
evaporating by covering the device wafer with
another SiC wafer, but a hermetic seal is not
formed, and SiC is expensive. Others place SiC
powder in the annealing chamber or introduce
SiH4 to create a Si overpressure, but this only
increases the Si deposition rate and does nothing
to the rate of Si evaporation. Moreover, the
implanted region is in a higher energy state so it
Experimental Procedure
After lightly doped p-type films were implanted
approximately uniformly to 3x1019 cm-3 to a
depth of ~ 0.3 µm, AlN films were deposited on
them to a thickness of ~200 nm using pulsed
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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laser deposition (PLD). The samples were
annealed at various temperatures, TA, for 30 min
up to a maximum temperature of 1600°C, and
then the sheet resistance, RSH, was determined as
a function of the measurement temperature, TM.
The same procedure was followed for the
implantation of Al or Al and C into a lightly
doped n-type film. They were implanted with
1020 cm-3 Al or 1020 cm-3 Al and 1020 cm-3 C to a
depth of ~ 0.5 µm. In addition, the
catholdoluminescence spectra were recorded.
Sheet resistivity (Ohm/square)
1000000
Resistivity [Ω-cm]
0.02
Room Temperature
0
1500
1550
1600
EA = 250 meV
10000
Calculated
EA = 191 meV
50
100
150
200
250
300
at the same temperature. RSH decreases with
increasing TM because the Al acceptor is so deep,
but it is not clear just how deep. When a room
temperature mobility of 50 cm2/V⋅s is assumed
[5], it appears that the best fit for the RSH vs TM
curves in Fig. 2 for the samples that have been
completely activated after a 1650°C anneal is
250 meV. This is substantially larger than the
generally accepted value of 192 meV [6], and the
best fit depth for the model would be even
deeper if, as expected, the mobility is lower in
implanted layers than it is in films that are grown
epitaxially.
It has been suggested that co-implanting C
assists in the activation because it improves the
chances that the Al will occupy a Si site rather
than a C site [7]. Another explanation is that the
Al is more easily incorporated into the lattice
because it can more easily surround itself with
four C atoms when the C is implanted and not
already bonded to four Si atoms [8]. This is
consistent with the fact that the C implants do
not noticeably reduce RSH for the 1650°C anneal
as at this high TA kinetic factors no longer affect
the incorporation of the Al.
We have also noted in our studies of SiC coimplanted with Si that RSH is larger than for
those implanted only with Al and annealed at the
same temperature. The difference is larger at the
lower TA 's, and there is little difference at the
higher TA's. This can be explained by the
increased difficulty the Al has in surrounding
itself with four C atoms at lower temperatures,
and that the kinetic effects are not much of a
factor at the higher TA's.
The catholdoluminescence spectrum for a coimplanted sample annealed at 1300°C is shown
in Fig. 3. This spectrum and all the other spectra
are qualitatively the same indicating that a
125 °C
1450
Al / C 1650 ºC
Figure 2. The sheet resistance of Al and Al and C
implanted layers plotted as a function of the
measurement temperature for layers annealed for 30
min at 1500, 1600, or 1650°C.
Room Temperature
50 °C
75 °C
1400
Al 1650 ºC
Measurement temperature (Celsius)
0.01
1350
Al / C 1600 ºC
0
0.05
0.03
Al / C 1500 ºC
100000
1000
Results and Discussion
That N implants are almost completely
activated by a 1600°C anneal is demonstrated in
Fig. 1 where RSH is plotted as a function of the
measurement temperature for samples annealed
different annealing temperatures. It is seen that
RSH changes very little between TA = 1500°C
and TA = 1600°C, and the RSH vs TM curve is
very close to what one would predict for
complete activation. In addition RSH decreases
0.04
Al 1500 ºC
1650
Anneal Temperature [°C]
Figure 1. The sheet resistance of N implanted layers
plotted as a function of the annealing temperature for
layers annealed for 30 min and measured at room
temperature, 50, 75, 125°C.
with increasing TM for these two TA's reflecting
the decrease in the mobility with increasing TM.
For the lower TA's of 1300 and 1400°C, RSH
increases with increasing TM implying that some
of the carriers are excited from levels below that
of the N donor level. These levels could be
defect levels that have not yet been annealed out.
The increase in carrier density with increasing
TM more than offsets the decrease in the
mobility.
However, the activation of the p-type implant,
Al, is quite different as shown in Fig. 2 where
RSH is measured as a function of TM for TA =
1500, 1600, and 1650°C. It is seen that RSH in all
instances decreases with increasing TM, and the
samples co-implanted with C have a smaller RSH
than those implanted only with Al and annealed
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Egilisson et al [9] have convincingly shown
that the DI defect is a deep donor 0.35 eV above
the valence band. The holes that are trapped out
by it are now deeper. This can also explain why
our RSH curves appear to be associated with an
acceptor that is deeper than that attributed to Al
in epitaxially grown material. As noted above,
the modeled curve would require an acceptor
depth greater than 0.25 eV if the carrier mobility
is lower in the implanted than in an epitaxially
grown layer, as is expected.
The DI defect has been associated with a
divacancy [10], but we believe it is a more
extensive defect such as a dislocation loop, and
the dislocation loop increases in size as TA
increases [8]. This is consistent with our EPR
results showing that the implanted Al acceptor
does not behave in the same way one in an
epitaxial film does, and a new peak emerges that
is associated with an extended defect as it has
little anisotropy and is broad. It is also
consistent with our Rutherford backscattering
spectroscopy (RBS) results that show the χmin
initially decreases as TA increases, but it begins
to increase at the larger TA's. We attribute this
increase to the nucleation and growth of
dislocation loops.
Figure 3. Cathodoluminescence spectrum of a coimplanted sample annealed at 1300°C for 30 min.
1300°C anneal is sufficient to optically activate
the implants. It is not well understood why
significantly higher temperatures are required to
electrically activate them.
There are, however, subtle differences
particularly in the relative peak heights of the
peaks associated with a free electron
recombining with a hole bound to an Al acceptor
near 3.0 eV, and the two peaks associated with
the DI defect near 2.9 eV [7]. As shown in Fig.
4, the peak near 3.0 eV decreases in intensity as
TA increases. One explanation is that the hole is
trapped out by the DI defect and the ability for
the DI defect to act as a hole trap increases with
TA. Also, its ability to trap the holes is impeded
slightly by the presence of implanted C as the
relative CL peak height of the co-implanted
samples is a little higher for the same annealing
temperature.
1650
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1650
Al Implanted
1500
1400
1600
1300
1500
RT
1400
1300
RT
Al & C Implanted
Figure 4. Cathodoluminescence spectra in the vicinity
of 2.9 and 3.0 eV for Al implanted and co-implanted
samples annealed at various temperatures.
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