Note: Descriptions are shown in the official language in which they were submitted.
CA 02322595 2000-10-06
Title of the Invention:
METHOD OF MAKING AN OHMIC CONTACT TO
p-TYPE SILICON CARBIDE, COMPRISING TITANIUM
CARBIDE AND NICKEL SILICIDE
Field of the Invention:
The present invention relates to a method of successfully producing an ohmic
contact to a
p-type SiC substrate by formation of nickel silicide and titanium carbide.
Background of the Invention:
Most semiconductor devices need terminal connections to carry electric current
to and
from the internal of the semiconductor device. Such terminal connections,
usually called "ohmic
contacts", must not however impair the semiconductor device itself. Thus, the
voltage drop over
the ohmic contact should be negligible compared to the voltage drop across
other areas of the
semiconductor device at the current density in question.
The formation of low specific contact resistance ohmic contacts to p-type SiC
is an open
issue in SiC-related technology because of the large band gap and electron
affinity of SiC.
Although a number of metals and metal-like compositions have been tested as
contacts to p-
doped SiC, no metals with a work-function of about 6 eV are known to form the
metal/p-type SiC
junction without potential barrier. For this reason, the fabrication of ohmic
contacts to p-type SiC
even using metals with high work function (such as osmium (Parsons et al., "Os
rectifying
Schottky and ohmic junction and W/WC/TiC ohmic contacts on SiC", U.S. Pat. No.
5,929,523,
(1999)) or platinum (Glass et al., Method of forming platinum ohmic contact to
p-type silicon
carbide, U.S. Pat. No. 5,409,859, (1995))) still requires a heavily doped
(more than 1019 cm-3)
undercontact layer of semiconductor in order to allow carrier tunneling
transport across the
interface between the metal and the semiconductor.
Aluminum has been considered as a potential contact metal because it is a
doping
impurity of p-type in silicon carbide, but its low melting point, 660 C, makes
it less convenient at
high power or high temperature operation. Another problem with aluminum is its
reactivity with
oxygen that may result in insulating oxides. Thus, the most popular ohmic
contacts to p-type 4H-
SiC material are the AUTi based metallizations (Furukawa et al. "Silicon
carbide semiconductor
device with ohmic electrode consisting of alloy", U.S. Pat. 5124779). Indeed,
the Al-Ti alloy is
preferred over pure Al due to its higher melting point (1100 C for Al/Ti 90/10
wt.% - ASM
Handbook, Vol. 3, "Alloy Phase Diagrams", Ed. H. Baker, p. 2.54) thus,
resulting in more
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thermally stable ohmic contacts. But titanium is characterized by a strong
oxygen gettering effect,
which causes small amounts of oxygen residues to be captured. These residues
later oxidize the
aluminum during the contact heat treatment. To overcome this problem, it was
proposed to
deposit aluminum, titanium and silicon layers (Kronlund et al, "Method of
producing an ohmic
contact and a semiconductor device provided with such ohmic contact" U.S. Pat.
5,877,077) that
form an aluminum-titanium-silicide after a heat treatment. During the tri-
metal aluminum-
titanium-silicide formation any bound oxygen is rejected from the SiC-metal
interface but a
necessary condition for obtaining a low contact resistance value is that the
interface has to move
inside SiC during silicide formation. Moreover, in the case of ohmic contacts
to p-type SiC with
AI-containing silicides or alloys, an additional problem to the above
mentioned of contact
oxidation, is the aluminum evaporation from the top surface of the deposited
metals (J. Crofton,
L. Beyer, J. R. Williams, E. D. Luckowski, S. E. Mohney and J. M. Delucca,
Sol. St. Electron.,
Vol 41 (1997), p. 1725). The value of the specific contact resistance is
directly related to the
quantity of aluminum, which does remain in the metallization. The above
analysis clearly shows
that, obtaining ohmic contacts with low values of specific contact resistance
is a very delicate task
for Al/Ti alloys and silicides posing serious problems of reproducibility.
Silicides and carbides are the ideal contact compounds concerning stability at
high
temperatures. Formation of silicides is more preferable comparing with
carbides, because of their
lower resistivity. Metal silicide may be formed by two ways: (1) by
interaction of the deposited
metal with deposited silicon; or (2) by interaction of the metal with silicon
carbide. The first case
was thoroughly investigated as the metal silicide contains very few
carbonaceous species like free
elemental carbon, carbides and other carbon-containing compounds that are
considered as factors
increasing the contact resistance. Stable contacts on n-type 6H-SiC have been
obtained (Tischler
et al., "Low resistance, stable ohmic contacts to silicon carbide, and method
of making the same",
U.S. Pat. No. 5,980,265, (1999)) by depositing a sacrificial silicon layer
before deposition of
various metals and forming metal silicides following annealing. In addition,
the sacrificial silicon
layer was in most cases doped with a common dopant of silicon carbide. In this
way, a thin top
layer of SiC was doped during the annealing step thus reducing the contact
resistance. However,
for this purpose a long annealing of 1 hour at temperatures higher than 1000 C
was necessary.
Another possible way to overcome the problem of the produced carbonaceous
species during
reaction with the SiC is to form simultaneously silicides and carbides with
the silicon and the
carbon, which are released from SiC during high temperature heat treatment (K.
V. Vassilevski,
K. Zekentes, G. Constantinidis, N. Papanicolaou, 1. P. Nikitina, A. I.
Babanin, Mat. Sci. For.
Vols. 338-342, p. 1017). In this way, ohmic contacts on n-type SiC have been
obtained (Bartsch
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et al. "Semiconductor device having ohmic contact and a method for providing
ohmic contact
with such a device" WO 00/14805). However, SiC decomposition by this way is
not easily
controlled and this process is usually undesirable for fabrication of
ultrahigh frequency devices
usually having shallow p-n junctions. At least, the depth of the SiC
decomposition should be well
defined and uniform over the sample area.
Summary Of The Invention:
The present invention concerns a method of producing an ohmic contact to p-
type silicon
carbide comprising of two layers the first one comprising nickel silicide and
the second one
comprising titanium carbide. The layers of titanium and nickel are deposited
on p-type SiC and
an aluminum layer is deposited preferably at the interface between the silicon
carbide and the
above metals of titanium and nickel. The deposited layers are annealed to
convert at least a part
of deposited metals to nickel silicide and titanium carbide while aluminum is
surface evaporated.
The contact is formed by reaction between the metals and the semiconductor,
and thus the in-situ
simultaneous formation of metal silicide and carbide suppress the release of
excess carbon at the
contact interface. The presence of aluminum is necessarily employed prior to
heat treatment to
lower the contact resistivity. Noble metals may be deposited preferably in
between titanium and
nickel to improve the contact morphology.
In detail, the invention provides a method of forming an ohmic contact to a p-
type SiC
substrate, comprising the steps of:
(A) depositing the layers of aluminum, titanium and nickel on the p-type SiC
substrate; and then,
(B) heat treating the resulting article for sufficient time and at sufficient
temperature
to convert at least part of it to titanium carbide and nickel silicide by
reaction of
SiC with the contact metals of titanium and nickel.
The invention particularly concerns the embodiments of such method wherein the
heat
treatment is carried out at a temperature in the range of from about 850 C to
about 1150 C.
The invention further concerns the embodiments of the above methods wherein
one or
more layers of one or more noble metals (especially platinum, gold or
palladium species) are
deposited preferably in between titanium and nickel contact metals.
The invention further concerns the embodiments of the above methods wherein
the
depositing layer(s) comprise(s) are formed through a method selected from the
group consisting
of chemical vapor deposition, sputtering and evaporation from a metal source.
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The invention particularly concerns the embodiments of the above methods
wherein the
p-type SiC substrate is a part of multilayer epitaxial structure or single
crystal of SiC. The
invention particularly concerns the embodiments of such method wherein the p-
type SiC substrate
has an acceptor concentration of equal to or greater than about 10' gcm"3,
and/or the contact has a
specific contact resistance to SiC of less than about 104ohm=cm2.
The invention further provides an ohmic contact to a p-type SiC substrate,
comprising
titanium carbide and nickel silicide, optionally containing one or more layers
of one or more
noble metals (especially wherein the noble metals are platinum, gold or
palladium species). The
invention also provides such ohmic contacts, wherein the p-type SiC substrate
is a part of
multilayer epitaxial structure or single crystal of SiC. The invention also
provides such ohmic
contacts, wherein the p-type SiC substrate has an acceptor concentration 101
gcm-3 and higher.
The invention also provides such ohmic contacts, wherein the contact has a
specific contact
resistance to SiC of less than 10-4 ohm=cm2.
The invention also provides an ohmic contact to a p-type SiC substrate,
comprising
titanium carbide and nickel silicide, optionally containing one or more layers
of one or more
noble metals, wherein the contact is made by the method, comprising the steps
of:
(A) depositing one or more layers of aluminum, titanium and nickel on the p-
type
SiC substrate; and then,
(B) heat treating the resulting article for sufficient time and at sufficient
temperature
to convert at least part of the deposited material to titanium carbide and
nickel
silicide by reaction of SiC with the contact metals of titanium and nickel.
The invention concerns such ohmic contacts, wherein the heat treatment is
carried out at
a temperature in the range of from about 850 C to about 1150 C, and/or wherein
one or more
layers of noble metals (especially platinum, gold or palladium species) are
deposited preferably in
between said titanium and nickel contact metals. The invention particularly
concerns such ohmic
contacts wherein at least one of said depositing layers is formed using a
method selected from the
group consisting of chemical vapor deposition, sputtering and evaporation from
a metal source.
Description Of The Figures:
The invention will be described more in detail below with reference to the
accompanying
drawings, which illustrate preferred and exemplary embodiments, and wherein:
FIG. 1 shows a SIMS profile of contact metallization after vacuum anneal
(inductive
heating) for 50 sec at 1150 C in 500 Pa of H2 at 340 sccm. The Y-axis is I in
arbitrary units; the
X-axis is time.
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FIG. 2 shows AFM photos (5 mx5 m with 100nm z-scan) of the surface: of the
contact
with Ni/Pd and with Pt/Ni cap layers respectively (from left to the right).
FIG. 3(a) and FIG. 3(b) are Auger depth profiles of an ohmic contact according
to the
present invention (a) before annealing and (b) annealed at 1000 C for 120 sec:
region 11- Ni
silicide containing contact layer; region 12- Ti carbide containing contact
layer; region 13 - p-
type SiC substrate.
FIG. 4 is a current-voltage characteristic diagram measured between two
contact pads of
the TLM structure for contacts fabricated without (line denoted 21) and with
(line denoted 22)
aluminum (50 nm) layer. For both contacts, the Ti (100 nm)/Pt (25 nm)/Ni (50
nm) deposition as
well as the subsequent annealing were performed in the same process runs.
FIG. 5 is a current-voltage characteristic diagram measured between two
contact pads of
the TLM structure for contacts before (line 31) and after (line 32) heat
treatment for contacts
similar to that of FIG. 3(a) and FIG. 3(b) respectively.
FIG. 6(a) and FIG. 6(b) are an AFM (a) 3D view and (b) cross section profile
of the
surface of 4H-SiC surface after removal of the ohmic contact according to the
invention. A -
surface after contact removal; B - free surface. The step height between the
two arrows in FIG.
6(b) is equal to 110 nm.
FIG. 7 is a Nomarski photo of the Al/Ti/Pt/Ni contact (a) before annealing and
(b), (c),
(d) annealed by RTA at 1000 C for 120 sec. Contact diameter is 40 m. The
thickness of Pt layer
is (a) 25 nm; (b) 10 nm; (c) 20 nm; and (d) 25 nm.
FIG. 8(a) and FIG. 8(b) are (a) a TLM mesa structure formed on 4H-SiC p+-n-n+
epitaxial wafer and (b) the resistance dependencies on the distance between
contact pads for
various temperatures.
FIG. 9 is a plot of contact specific transition resistance and sheet
resistivity of p-doped
4H-SiC layer on reciprocal temperature.
FIG. 10 is an x-ray phase analysis plot for the ohmic contact according to the
invention
covered by a gold overlay.
Detailed Description of the Preferred Embodiments:
In a preferred embodiment, the present invention provides a method of
producing an
ohmic contact to a p-type SiC substrate, which comprises the step of
depositing aluminum,
titanium and nickel in that order and the subsequent step of heat treatment,
for sufficient time and
at sufficient temperature, to form titanium carbide and nickel silicide by
reaction of SiC with the
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contact metals of titanium and nickel. The p-type SiC substrate is either bulk
crystal or an
epitaxial layer of SiC p-type doped.
In a preferred embodiment, the aforementioned p-type SiC substrate is of the
3C, 4H, 6H,
15 R polytype.
In a preferred embodiment, the aforementioned p-type SiC substrate has an
acceptor
concentration 1018cm-3 and higher.
In a preferred embodiment, the aforementioned heat treatment is carried out at
a
temperature of at least 850 C. Any suitable method may be employed to
accomplish such heat
treatment, including lasers, heat lamps, etc.
In a preferred embodiment, a noble metal layer is deposited preferably in
between the
titanium and the nickel layers.
In a further preferred embodiment, the aforementioned noble metal layer is of
platinum,
or palladium or gold material.
Thus, the invention described herein makes possible the objectives of (1)
producing
ohmic contacts which comprise titanium carbide and nickel silicide thereby
attaining a thermally
stable ohmic contact; (2) producing ohmic contacts comprising titanium carbide
and nickel
silicide which exhibit reproducibly low typical contact resistivity values for
silicon carbide
substrates having an acceptor concentration 1018cm-3 and higher; (3) producing
ohmic contacts
comprising titanium carbide and nickel silicide for which decomposition of the
SiC under the
contact is limited to a uniform depth of about 100 nm (i.e. sufficiently small
comparing to typical
thickness of epitaxial layers for ultra high frequency devices); (4) producing
ohmic contacts
comprising titanium carbide and nickel silicide which has a good adhesion for
deposition of a
metal overlay preferably gold for packaging purposes.
Having now described the invention in detail, the same may be better
understood and its
numerous objectives and advantages become more apparent to those familiar in
the art by
reference to the following Examples which are not intended to restrict or
limit the subject matter
of the invention.
EXAMPLE 1
The most popular ohmic contacts to p-type 4H-SiC material are Al/Ti based
metallizations (Porter et al., Mater. Sci. Eng. B 34, 83 (1995); Crofton, J.
et al, Sol. St. Electron.,
Vol 41(1997), p. 1725). However, aluminum has a high driving force for
oxidation that can
deteriorate the quality of the ohmic contact. In addition, reproducibility
problems occur when the
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SiC doping does not exceed 1x1019 cm'3 whatever the metallization scheme
(Vassilevski, K.V. et
al., Proc. ECSCRM'98 Conference, Sept. 1998, Montpellier, France).
In order to overcome the above problems in fabricating low resistivity ohmic
contacts,
three processing approaches were combined and successfully applied. More
precisely, vacuum
annealing coupled with metal cap layers (Pd/Ni and Ni/Pt) resulted in non-
oxidized contacts
independent of the heating method (Rapid Thermal Annealing-RTA, resistive and
inductive). In
addition, a heavily doped p-type 4H-SiC layer with an aluminum concentration
of approximately
1.5x1020 cm-3 was grown (TDI Inc., 8660 Dakota Dr., Gaithersburg, MD 20877,
USA) by low
temperature Liquid Phase Epitaxy (LPE) prior to metal contact deposition. The
above approach
resulted in reproducible and of low specific contact resistance (z1x10-4
S2.cm2) ohmic contacts
(Vassilevski, K.V. et al., Proc. ECSCRM'98 Conference, Sept. 1998,
Montpellier, France).
Auger Electron Spectroscopy (AES) and Secondary Ion Mass Spectroscopy (SIMS)
depth profiles were performed to determine the structural and morphological
characterization of
the above contacts, while X-Ray Diffraction (XRD) was used for phase analysis.
Surface
morphology was investigated by Nomarski, Scanning Electron (SEM) and Atomic
Force
Microscopy (AFM) methods. The investigation was conducted on Circular
Transmission Line
Model (CTLM) patterns also used for specific contact resistance measurements.
The first results of the AES analysis showed evidence of oxidation from the
surface to the
interface with the SiC, for annealing in neutral gas environment. Only the
samples annealed
under vacuum conditions were in-depth oxygen-free. Moreover, there is an
evident correlation of
the AES profiles with the SIMS profiles (see FIG. 1) concerning the Ti, Si, C
and Ni inter-
diffusion. XRD spectra showed the presence of non-reacted Al while, the AES
profile showed
that the composition of Al in the metallization is less than 10%. However, it
is evident from the
SIMS profile that Al forms the contact metal to SiC and there is an
interdiffusion with Ti. In
FIG. 2, the characteristic roughness for the two different cap schemes is
shown. The contact with
Pt/Ni cap has a higher roughness but it does not exceed 50nm in all cases.
EXAMPLE 2
In this case, the p-type SiC substrate is a 4H-SiC polytype layer grown by
Liquid Phase
Epitaxy (LPE) (K. V. Vassilevski, S. V. Rendakova, I. P. Nikitina, A. I.
Babanin, A. N. Andreev,
and K. Zekentes, Semiconductors, 33, 1206-1211, (1999). The acceptors
concentration, which in
this case were Al atoms, was measured by Secondary Ion Mass Spectrometry
(SIMS) and it was
around 1.5x1020 cm-3.
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Prior to contact fabrication, the SiC sample was cleaned sequentially by (1)
degreasing in
organic solvents, (2) bathing in De-lonized water (DI) and, finally, through
the use of a standard
RCA cleaning procedure (D. C. Burkman, D. Deal, D. C. Grant, C. A. Peterson,
Aqueous
Cleaning Processes, in Handbook of semiconductor wafer cleaning technology:
science,
technology, and applications, ed. W. Kern, Noyes Publication, USA (1993)
p.120). Dipping of
the sample in 10% HF for 2 min at room temperature was performed in between
the above steps.
The circular Transmission Line Model (TLM) geometry of the contacts to p+
epitaxial layer was
defined by contact UV lithography. Immediately prior to placing the sample in
the vacuum
chamber, the sample patterned with AZ 5214 photo resist was immersed in 10% HF
for 10 sec at
room temperature, followed by rinsing in deionized (DI) water and blow drying
with nitrogen.
The metal deposition was made by e-beam evaporation at 'a residual pressure <
5x10-7mbarr
without heating of the substrate during the process. The metals were deposited
in a single run in
the following sequence: A1(50 nm)/ Ti(100 nm)/ Pt(25 nm)/ Ni(50 nm). After
excess metal
removal by lift-off in acetone, the rapid thermal annealing (RTA) process was
performed by
lamps at about 1000 C for 120 sec in a commercial RTA chamber pumped down to
4x10-5Torr.
The contact composition was investigated by Auger electron spectroscopy (AES)
depth
profiling. FIG. 3(a) shows the Auger depth profile of the deposited metals
prior to annealing.
The annealing process (FIG. 3(b)) led to formation of layered contact
structure having two
clearly separated regions: (a) close to the surface where the curves
corresponding to Ni and Si are
parallel as well as of higher intensity and (b), close to the interface with
SiC where the curves
corresponding to Ti and C are parallel as well as of higher intensity. The
parallel curves are a
clear indication of stoichiometric material formation. Indeed, nickel silicide
is formed by
reaction of nickel with the silicon of SiC and the titanium carbide is formed
by the reaction of
titanium with the carbon of SiC. The results of X-Ray Diffraction (XRD)
analysis reported in
Example 4 corroborate this conclusion. Thus, the creation of this ceramic
containing metal
(cermet) contact layer is progressing simultaneously with the decomposition of
SiC. Al was not
detected by the AES analysis of the annealed samples and an Al/Ti alloy is not
formed in the case
of the ohmic contact proposed by the present invention. However, Al is
necessary for lowering
the resistance of the ohmic contact, as it is obvious from FIG. 4 in which
case the Al presence
was necessary for obtaining an ohmic behavior. Although the inventors do not
wish to be bound
by any particular theory, it appears most likely that Al decreases the
potential barrier existing at
the interface metal-semiconductor, that carriers have to overcome for
traversing this interface.
FIG. 5 shows the current-voltage (I-V) characteristics of the sample measured
between
two contact pads of the TLM structure before (line denoted 31) and after
annealing (line denoted
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32). The linear behavior of the line 32 shows the ohmic character of the
contact while before
annealing no current is passing through. The value of the specific contact
resistance extracted
from the current-voltage measurements performed on the circular TLM patterns
was slightly less
than 10-4 ohm-cm2.
To estimate the depth of SiC that is decomposed during ohmic contact
formation, the
contacts were removed by sequential dipping of the sample in aqua regia and
HNO3:HF (3:1) for
several times. The morphology of the 4H-SiC after contact removal was
investigated by Atomic
Force Microscopy (AFM). FIG. 6(a) and FIG. 6(b) show the AFM image (3D view
(a) and
cross-section profile (b)) of the sample in the region of the contact edge. It
is clearly seen, that
the contact formation caused a SiC decomposition to a depth of about 100 nm.
The steps along
the y-axis are due to the SiC epitaxial growth process. The mean roughness of
the recess floor
(i.e. where the contact structure was present before the etch) was estimated
to be about 30 nm,
while the mean roughness of the free surface (i.e. without any contact
formation) measured
between growth steps was no more than 3 nm.
Introducing the platinum, gold or palladium layer preferably in the interface
between Ni
and Ti led to reduction of contact surface roughness (see FIG. 7) and to the
improvement of the
gold overlay adhesion.
EXAMPLE 3
In this case, the p-type SiC substrate is a layer being part of a 4H-SiC p+-n-
n+ epitaxial
structure. This structure was grown on Si-face of n-type 4H-SiC substrate with
orientation 8
degrees off basal plane. The thickness and the doping levels of the layers
were verified by
secondary ion mass spectrometry (SIMS). The aluminum atoms concentration in
the I m thick
p+ layer was found to be of - 1.5x1019cm-3, while the acceptor concentration
in thep+ layer was
equal to (6-8)x1018cm-3, according to Hg-probe capacitance-voltage
measurements. Therefore,
about -50% of aluminum atoms are electrically activated in the p+ layer, as it
was deduced from
the comparison of the Hg-probe measurement results with the data obtained by
SIMS.
The contact fabrication procedure was the same as that reported in the above
Example 2.
In addition, a gold overlay of 200 nm in thickness was deposited for improving
the cunent
spreading in the contact metallization and the electrical contact between the
contact pads and the
tips of the probe station. A commercial plasma system was used to form the
mesa structures for
the measurement of the contact resistivity by linear TLM method (pads size
40x80 m2 with
distances 4, 8, 12, and 16 m between them). A 200 nm aluminum mask was
deposited on the p-
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type epitaxial layer. The chamber was pumped down to 2x10-5mbar before the
etch process.
Uncovered SiC was etched away down to the n+ epitaxial layer. After the SiC
etching, the Al
mask was selectively removed in KOH solution.
Electrical characterization of the contacts was performed by linear
transmission line
model (TLM) (G. S. Marlow, M. B. Das, Solid-State Electronics, 25, 91-94,
(1982)) in the
temperature range from 21 to 186 C. TLM mesa structure formed on 4H-SiC p'-n-
n+ epitaxial
wafer is shown in FIG. 8(a). The I-V characteristics measured between two
contact pads were
linear up to the current value of about 6 mA, at which the voltage drop was
exceeding 3 V and
current spreading in the n- layer was occurred. The contact resistivity at
room temperature was
measured before and after sample heating and was found to be the same. The
measurements were
carried out in the air ambient. The resistance dependencies on the distance
between contact pads
for various temperatures are shown in FIG. 8(b). The contacts revealed
specific contact
resistance of 9- 10-5 ohm=cm2 at 21 C decreasing to 3.1 = 10-5 ohm=cm2 at 186
C. The strong
dependence of the contact resistivity on the temperature is shown in FIG. 9.
Although the
inventors do not wish to be bound by any particular theory, this dependence is
characteristic for
contacts formed to relatively low doped semiconductors, where thermionic
emission is the
predominant current transport mechanism (E. D. Marshall, M. Murakami,
"Contacts to
Semiconductors, Fundamentals and Technology", Ed. L.J. Bril.lson, Noyes
Publications, USA
(1993) pp. 8-9). The dependence of the sheet resistivity of p-type SiC
epitaxial layer on
reciprocal temperature is also shown in FIG. 9. The slope of the best fit of
experimental data
gives the value of acceptor ionization energy about 200 meV. This value is
very close to the
published experimental data (I. Nashiyama, in: "Properties of Silicon
Carbide", Ed. G. L. Harris,
EMIS Data Reviews Series, No. 13. INSPEC, IEE, London, UK Chapter 4.1 (1995)
pp. 87-92)
and it confirms the accuracy of the performed measurements.
EXAMPLE 4
In this case, the SiC substrate is a 6H-SiC single crystal grown by the Lely
method. The
contacts were fabricated in an identical way as that described in the above
Example 2. The only
modification in the whole process was that metal layers were deposited all
over the SiC substrate
surface. In this way, the resulting sample is optimized for structural
investigation.
Phase analysis of the ohmic contacts was performed by x-ray diffraction.
Results of x-ray
phase analysis are shown in FIG. 10. It was found that the contact layer
contained Ni2Si, Ni3Si2
and TiC components after annealing. The introduction of the platinum layer in
the contact metal
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composition had no effect on the formation of the cerrnet structure containing
nickel silicides and
titanium carbide. Indeed, creation of Ni2Si and TiC was also discovered when
Pd was used
instead of Pt layer.
While the invention has been described herein with reference to specific
aspects, features,
and embodiments, it will be apparent that other variations, modifications, and
embodiments are
possible, and all such variations, modifications, and embodiments therefore
are to be regarded as
being within the spirit and scope of the invention.
