Implanted p+n-Junctions in Silicon Carbide
1
1
A. Hallén, 1M.S. Janson, 1J. Osterman, 1U. Zimmermann, 1M. Linnarsson, 2A.
Kuznetsov, 3Y. Zhang, 4P.O.Å. Persson, and 2B.G. Svensson
Dept. of Microelectronics and Information Technology, Royal Inst. of Technology, P.O. Box Electrum 229, SE 164
40 Kista, Sweden
2
Physics Dept., Physical Electronics, University of Oslo, PB 1048, Blindern, N 0316 Oslo, Norway
3
Div. of Ion Physics, Ångström Laboratory, Uppsala University, P.O. Box 534, SE 751 21 Uppsala, Sweden
4
Thin Film Physics Div., Dept. of Physics, Linköping University, SE 581 83 Linköping, Sweden
Abstract. Ion implantation is considered a key technology for the realisation of silicon carbide electronic devices. Here
we will give an overview of the field and present some recent results of ion implanted 4H SiC epitaxial layers. Mainly
Al ions of keV energies have been used at different fluence, flux and target temperature. The samples have been
investigated by secondary ion mass spectrometry (SIMS), channeling Rutherford backscattering (RBS-c) and
transmission electron microscopy (TEM), both as-implanted and after annealing up to 1900 °C. Also the electrical
activation of Al-implanted and annealed material has been investigated by scanning spreading resistance microscopy
(SSRM). The damage accumulation, monitored by RBS-c, is linear with ion fluence but depends strongly on
implantation temperature and ion flux. Annealing at temperatures above 1700 °C is needed to remove the damage and to
electrically activate implanted Al ions. At these high annealing temperatures, however, dislocation loops are formed that
have a negative influence on device performance.
thermal diffusion is practically impossible due to the
small diffusion coefficients of common n- and p-type
dopants. A few promising attempts have been reported
where implantation of donor [2,3] and acceptor ions
[4-8] is utilized to make n+p- or p+n-junctions,
respectively. Although these junctions qualitatively
show the expected reverse blocking and forward
conducting characteristics, there is still a long way to
go before optimized implantation and annealing
conditions are developed that result in ideal forward
injection and reverse leakage properties. Here we will
focus on the p+n-junction, which is typical for high
power bipolar devices, where a thick low doped nregion supports a large reverse bias and a very
efficient hole emitter is needed in order to establish a
high plasma level. There are also two other reasons for
directing our attention towards the acceptor ions, such
as B and Al, implanted in a low doped epitaxial nlayer to form a p+n-junction. One being that these ions
seem to require higher temperatures than donors to
become activated, i.e. occupy the correct substitutional
sites. Temperatures over 1600 °C are commonly used
and these high temperatures can lead to surface
INTRODUCTION
Recent successfull development in silicon carbide
crystal growth has triggered an intense activity among
manufacturers of high power and high frequency
electronic semiconductor devices [1]. The reasons why
SiC attracts such an interest from this device
community are, firstly, that the dielectric strength is 10
times higher than that of silicon and, secondly, that the
thermal conductivity is 2-3 times higher than for
silicon. This means that devices of SiC can be made
smaller, switch faster, loose less energy, work at
higher temperatures, and that expensive cooling
peripherals become redundant. The material properties
of SiC enables new devices to be realised which by far
out-performs
state-of-the-art
devices,
mostly
manufactured in Si and to some extent GaAs.
Silicon carbide is, however, a very difficult
material to process and new technology is needed in
order to realise these future components. Ion
implantation is considered to be a key technology for
area selective introduction of doping atoms, since
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
653
decomposition, outdiffusion of dopants through the
surface, transient enhanced diffusion (TED) of dopants
to larger depths, and also to the growth of extended
defects in the end-of-range region. The second reason
is that the ionization energies are typically higher for
acceptors, leading to a small hole concentration at
room temperature. A simple remedy for this is to
implant more acceptors, but higher doses lead to
amorphisation and precipitation if the concentration of
dopants exceeds the solubility limit. Taking these
problems into account, it is clear that we need to treat
the acceptor implantation process with special care.
After briefly describing the sample preparation and
measurements, we will highlight various aspects of the
implantation process. Starting with the as-implanted
profiles and how the damage levels can be kept low, to
the post-implant annealing, which restores the lattice
and positions the dopants at substitutional sites, to the
final test of the implantation, namely to record the
resistivity.
RESULTS AND DISCUSSION
As-implanted distributions
Implantation in a crystal leads to channeling of the
implanted ions and this is normally an unwanted
effect. To minimize channeling in silicon the wafer is
therefore tilted 7° during implantation. If the same
procedure is applied when implanting 4H SiC one may
in the worse case align the sample exactly in the
<0001> direction, since this polytype is grown about
8° off axis in the <1120> direction. This results in ion
distributions that are much deeper than expected from
a random implantation. Channeling can also be used
beneficially to implant the ions with a minimum of
damage done to the lattice, but this procedure is
technologically difficult to implement and it also
leaves some of the shaping of the dopant profile to the
lattice. A demonstration of the important effect of
channeling in the <0001> direction is shown in Fig. 1,
where 1.5 MeV 27Al ions have been implanted in 6H
epitaxial SiC to doses ranging from 6.6×1011 to
7.1×1014 cm-2 [11]. The larger peak at 1.3 µm is the
random peak and the figure also shows the dynamic
build up of damage that for higher doses causes the
ions to be dechanneled earlier. For doses above 6×1013
cm-2 the channels become filled with interstitial
defects and the concentration of ions in the channeled
tail has saturated. Studying the random peak in greater
detail, it can be seen that it is shifted towards greater
depths and eventually the maximum peak appears
around 1.7 µm instead.
EXPERIMENTAL
Most of the studies are performed on high quality
chemical vapour deposited (CVD) epitaxial 4H SiC
grown at Linköping University [9]. The doping in
these n-layers is typically around 2×1015 cm-5. In some
of the Rutherford backscattering (RBS) experiments
we have used higher doped n-type substrate material
from Cree, also of 6H polytype, but the Rutherford
backscattering (RBS) results does not indicate that
there is a difference in damage levels between
epitaxial materials and substrates, or between
polytypes. Implantations and RBS channeling
measurements have been performed at several tandem
accelerators in Kista, Stockholm, at the Australian
National University, Canberra and at the Pacific
Northwest National Laboratory, Richmond, WA,
USA. Transmission electron microscopy was done at
Linköping University using a 200 keV Philips CM20
UT microscope equiped with a LaB6 filament.
Annealing of the samples has been done with an
Epigress RF heated furnace and secondary ion mass
spectrometry (SIMS) was done with a Cameca 4f
instrument, using oxygen (O2) as primary ion. The
electrical activation of the implanted profiles was
measured by a Nanoscope III atomic force microscope
equipped with a spreading resistance module [10].
12
3-
-2
Fluence (10 cm )
19
) 10
710
m
c( 1018
n
oti
ar 1017
t
n
e
c
n 1016
o
C
27
+
1.5 MeV Al <0001>
160
56
26
11
2.6
0.66
1015
0
1
2
3
4
5
6
7
Depth (µm)
FIGURE 1. Implantations along the <0001> direction of
1.5 MeV Al in SiC for various doses.
By coincidence, the qualitative shape of the highest
dose is roughly what device designers aim at with a
high concentration at the surface, to allow for a good
ohmic contact and high injection efficiency, and a
654
softer profile at larger depths to ackomodate the
reverse bias without field crowding.
1.0
Normalised RBS Yield
Using a Monte Carlo programme for simulation of
ion distributions in crystalline material [12] we have
investigated channeling in various directions. To
calibrate this program we have used over 150 profiles
of ions ranging in mass from H to Ga and in energies
from 20 keV to 3 MeV recorded by secondary ion
mass spectrometry (SIMS). Fig. 2 shows simulated
implantations in the <0001> and the <1120> axial
channels. The random peak is seen at slightly smaller
value than 0.1 µm and, for the latter channel, the ions
travels over 30 times as long as randomly implanted
ions.
temperature. Recently it has been shown that also the
flux, or the rate of incoming ions, is an important
parameter, which can by utilized to lower the level of
damage in implanted SiC [13]. The dynamic annealing
occuring during implantation is not only stimulated by
higher temperatures, but also seems to be enhanced by
performing the implantations at a lower flux.
0.80
0.60
0.40
0.20
RT
150 K
0.0
0
1 1022
2 1022
3 1022
Total Vacancy Concentration (cm -3)
FIGURE 3. Relative backscatter yield as a function of
fluence, or the vacancies produced by the incoming ions, for
Al implantations done at 150 K and RT.
Annealing
Post implant annealing is used to reduce the
damage further. However, a substantial amount of the
self-interstitials (silicon and/or carbon) form circular
planar defects in the basal plane, surrounded by
dislocation loops. These disks have been extensively
investigated by transmission electron microscopy
[14,15], and it has been shown that the area covered by
disks increases in size with increasing annealing time
and temperature until saturation is reached. After this
saturation, which presumably means that the
interstitials are either bound in these basal plane
defects or has recombined, the smaller disks tend to
become smaller while the larger disks grow. This is a
highly undesirable from a device point of view and
leads to lattice strain and inhomogeneous emitter
properties. In Fig 4 we show the evolution of the area
covered by disks as a function of annealing
parameters. The saturation value is around 1.8% for
the 1700 ºC anneal is clearly seen after 120 minutes.
Annealing at 1800, 1900 and 2000 ºC for 30 minutes
all yield the same saturation level. After this saturation
the large platelets continue to grow at the expense of
the smaller. While surface decomposition can be coped
with by carefully optimizing the ambient conditions
during anneal or by capping the surface by for instance
AlN, the growth of these extended defects is a problem
FIGURE 2. Monte Carlo simulations using binary collision
approximation of implantations of 60 keV Al ions in two
different channeling directions.
Damage
The damage caused by the implanted ions is
another major obstacle for an ideal implanted p+
emitter. Performing the implantations at high
temperatures is widely recognized technique to reduce
the damage during the implantations. Typically
temperatures around 700 ºC are used, which reduce the
damage (as monitored by RBS) considerably
compared to a room temperature implant. SiC is
considered to be a very inert material with low
diffusivities for most elements. Fig. 3 shows, however,
that there is thermally stimulated motion of
implantation induced point defects already at room
temperature. The figure compares the damage from Al
implantations at 150 K with room temperature Al
implantation as a function of the ion fluence. About
2/3 of the damage is annealed already at this
655
that sets an upper bound for post implant annealing
process.
may dramatically change the electrical properties of
this junction. It has recently been shown [19] that the
diffusion of the long boron tail occurs by a transient
enhanced diffusion (TED) process, very similar to
what has been observed in Si. The TED and
outdiffusion of boron are unwanted effects that are not
easily controlled. In addition to this redistribution of
boron perpendicular to the surface, it has been
suggested that boron spreads laterally upon annealing
[20]. All together these effects tend to make device
manufacturing using boron to an artform rather than a
science.
20
10
3-
Out-diffusion
19
) 10
11
B
Dose 5e14 cm
m
c( 1018
c.
n
o 1017
C
n
or
16
o 10
B
FIGURE 4. Growth of the planar defects as a function of
annealing conditions. The total area covered by the defects
saturates at about 1.8% of the total surface area studied by
TEM.
200 keV
-2
TED inward diffusion
As implanted
Annealed 15 min/1700 C
15
10
Boron has earlier been favoured as a suitable
candidate for acceptor doping due to its smaller mass
that causes less damage. Several recent findings have
now shown that aluminum is a better choice in many
cases, despite the larger mass. One reason is that the
solubility limit for Al is 2×1020 cm-3, which seems to
be at least 3 times more than for B [16]. It has
furthermore been established that the ionization level
of Al is lower than that for B, around 0.20 eV
compared to about 0.25 eV above the valence band
maximum for Al and B, respectively. This is of course
very important for acceptors where the ionization
energy already is large compared to donors. In
addition, B forms a deeper trap, having an ionization
level of about EV+0.6 eV. The identity of this level is
still under discussion and BSi-SiC [17] and BSi-VC [18]
pairs has been proposed. The fact that B under some
conditions forms this deep state, is of course also a
disadvantage. Boron diffuses more easily than
aluminum and this could, of course, be an advantage.
But as long as these diffusion processes are not
understood, it is better to avoid boron. In Fig. 5 we
show the diffusion of B implanted with an energy of
200 keV and a dose of 5×1014 cm-2 in the as-implanted
case and after a 15 minute anneal at 1700 °C. Under
such annealing conditions a similar Al-profile would
not change, but a redistribution of the B profile occurs
both outwards, through the surface, and inwards, to
large depths. The concentration for the inward
diffusing boron may seem low, but remembering that
the material is epitaxial n-type SiC doped with
nitrogen to 3×1015 cm-3, one realises that this long tail
14
10
0
1
2
3
4
Depth (µm)
5
6
FIGURE 5. An implanted boron profile shown before and
after annealing.
Hall measurements have frequently been used to
establish the electrical activation of implanted species.
This technique has the advantage of giving both the
mobility and charge carrier concentration, but it also
has some limitations, for instance it requires a
homogeneous doping concentration. We have utilized
a novel technique to establish the activation of Al
implantation, namely scanning spreading resistance
microscopy (SSRM). This measurement technique is
based on a combination of the traditional spreading
resistance method, which have been used for
semiconductors for a very long time, and an atomic
force microscopy (AFM). In Figure 6 we show the
SIMS profile of an Al multiple energy implantation
(right scale) and also the SSRM measured current. By
calibrating the SSRM signal with samples of known
hole concentration we conclude that the activation of
this particular implant is about 2%. However,
difference in mobility and trap concentration can affect
this value. Two SSRM curves are shown from samples
annealed at 1550 and 1650 ºC for 30 minutes each.
The difference is quite small, but a slight improvement
in activation can be seen for the higher temperature
anneal. This result is in reasonable agreement with
other reports on activation in implanted 4H SiC [21].
656
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3-
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-4
10
10
)
18
m
c( 10
n
oi
t
ar
t
n
e 1016
c
n
o
c
l
A
S 1014
M
I
S
SIMS
10-6
-8
10
-10
10
SSRM 1650 C
S
S
R
M
c
u
r
er
n
t
A(
)
SSRM 1550 C
12
-12
10
10
0
0.5
1
Distance (µm)
FIGURE 6. Scanning spreading resistance measurement of
an Al (multiple energy) implanted 4H SiC epi layer. For
comparison the SIMS profile is also included.
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12. J. Wong-Leung, M.S. Janson and B.G. Svensson,
accepted for J. Appl. Phys. (2003)
We have given examples of problems and possible
improvements of the implanted p+n-junction in 4H
SiC. Aluminum is shown to be a more favourable
acceptor specie than boron, despite the larger mass that
gives rise to more lattice damage. Furthermore, there
exist a trade-off for the annealing, where higher
temperatures and longer anneal times result in a higher
degree of electrically activation, but also gives rise to
extended basal plane defects that can be very harmful
to devices.
13. A.Yu. Kuznetsov, J. Wong-Leung, A. Hallén, C.
Jagadish, and B.G. Svensson, subm. to J. Appl. Phys.
(2002)
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Yakimova, D. Panknin, and W. Skorupa, J. Appl. Phys.
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Schöner, M.S. Janson, C. Jagadish and B.G. Svensson,
Appl. Surface Sci. 9283, (in press, 2002)
ACKNOWLEDGMENTS
Financial support was given by the Swedish
Foundation for Strategic Research.
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and N.T. Son, Mat. Sci. Forum, 353-356, p. 455 (2001)
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