Platinum ohmic contact to p-type silicon carbide

A method and resulting ohmic contact structure between a high work function metal and a wide bandgap semiconductor for which the work function of the metal would ordinarily be insufficient to form an ohmic contact between the metal and the semiconductor. The structure can withstand annealing while retaining ohmic characteristics. The ohmic contact structure comprises a portion of single crystal wide bandgap semiconductor material; a contact formed of a high work function metal on the semiconductor portion; and a layer of doped p-type semiconductor material between the single crystal portion and the metal contact. The doped layer has a sufficient concentration of p-type dopant to provide ohmic behavior between the metal and the semiconductor material.

FIELD OF THE INVENTION 
The present invention is a method of successfully forming ohmic contacts to 
p-type wide bandgap semiconductor materials using high work function 
metals, as well as the resulting ohmic contact structures. In a particular 
embodiment, the invention is a method for forming platinum ohmic contacts 
on silicon carbide and the resulting ohmic contact structures. 
BACKGROUND OF THE INVENTION 
In typical applications, semiconductor devices are operated by, and are 
used to control, the flow of electric current within specific circuits to 
accomplish particular tasks. Accordingly, because of their high 
conductivity, the most useful and convenient materials for carrying 
current from device to device are metals. 
In order to connect devices into useful circuits, appropriate contacts must 
be made between semiconductor devices and the metals--e.g. printed 
circuits, wires, or any other appropriate metal elements--used to carry 
current. Often, the most appropriate contacts are also formed of metal. 
Such metal contacts should interfere either minimally or preferably not at 
all with the operation of either the device or the current carrying metal. 
The contact must also be physically and chemically compatible with the 
semiconductor material. 
In this regard, the term "ohmic contact" is used to define such an 
appropriate metal-semiconductor contact. Specifically, an ohmic contact 
can be defined as a metal-semiconductor contact that has a negligible 
contact resistance relative to the bulk or spreading resistance of the 
semiconductor, Sze, Physics of Semiconductor Devices, Second Edition, 
1981, page 304. As further stated therein, an appropriate ohmic contact 
will not significantly change the performance of the device to which it is 
attached, and it can supply any required current with a voltage drop that 
is appropriately small compared with the drop across the active region of 
the device. 
An ohmic contact can also be qualified using other characteristics of both 
a metal and a semiconductor. In most cases, and in addition to the other 
necessary physical and chemical characteristics, in order to act as an 
ohmic contact to a particular p-type semiconductor, the metal must have a 
work function greater than the work function of the semiconductor. 
Ideally, the work function of the metal should be greater than the 
electron affinity and the bandgap of the semiconductor. 
As known to those familiar with this art and this terminology, the work 
function is defined in terms of the Fermi energy of the material. In turn, 
the Fermi energy of a material is the effective average energy of the 
electrons in thermal equilibrium with the surroundings of the material. 
Alternatively, the Fermi energy can be defined as the energy at which half 
the available states are on the average actually populated with electrons 
at thermal equilibrium. The work function of the material is the energy 
required to remove an electron having the Fermi energy from the material 
to an infinite distance away from the material. 
Summarized somewhat differently, if the work function of the metal is 
greater than that of the p-type semiconductor, it may qualify as an ohmic 
contact material to that p-type semiconductor. If, however, the work 
function of the semiconductor is greater than that of the metal, it will 
be difficult or impossible to establish ohmic behavior and rectifying 
behavior may be demonstrated instead. 
One material for which great semiconductor potential has long been 
recognized is silicon carbide (SiC). Silicon carbide has well known 
advantageous semiconductor characteristics: a wide bandgap, a high thermal 
conductivity, a high melting point, a high electric field breakdown 
strength, a low dielectric constant, and a high saturated electron drift 
velocity. Taken together, these qualities potentially give electronic 
devices formed from silicon carbide the capability to operate at higher 
temperatures, higher device densities, higher speeds, higher power levels, 
and even under higher radiation densities, as compared to other 
semiconductor materials. Accordingly, attempts to produce appropriate 
devices from silicon carbide, as well as attempts to produce device 
quality silicon carbide itself, have been of interest to scientists and 
engineers for several decades. As stated above, one aspect of device 
manufacture in any semiconductor material, and specifically including 
silicon carbide, is the ability to produce appropriate ohmic contacts. 
When used as a semiconductor material, particularly p-type, silicon carbide 
presents special challenges with respect to ohmic contacts because of its 
relatively large bandgap which results in a relatively large work function 
as well. For example, in the ideal case, disregarding any effects of band 
bending caused by Fermi level pinning, p-type alpha (6H) silicon carbide 
has a work function ranging from about 5.7 to 7.2 electron volts (eV) 
depending upon the carrier concentration. For those well skilled in the 
art, the work function is defined as the sum of the electron affinity plus 
the energy level between the conduction band and the Fermi level. As a 
result, finding metals with a work function greater than that of silicon 
carbide has to date proved difficult and troublesome. The same problems 
hold true for other wide bandgap semiconductor materials such as zinc 
selenide (ZnSe), gallium nitride (GaN), diamond, boron nitride (BN), 
gallium phosphide (GaP), and aluminum nitride (AlN). 
As a result, ohmic contacts to p-type silicon carbide are generally formed 
of alloys such as aluminum-titanium and aluminum-silicon which have to be 
annealed at relatively high temperatures (e.g. 900.degree. C.) in order to 
form an appropriate ohmic contact. As known to those familiar with such 
devices, the repeated exposure of a device to such high temperatures will 
eventually generally change its character. Furthermore, the presence of 
aluminum or aluminum alloys on silicon carbide-based semiconductor devices 
limits the later treatment or processing of such devices because of the 
limitations of the physical and chemical properties of the alloys and the 
alloying metals, particularly their somewhat lower melting points. 
Platinum (Pt), because of its characteristics as a noble metal, is a 
desirable candidate for ohmic contacts to all sorts of semiconductor 
materials. Unfortunately, and with respect to the parameters discussed 
earlier, the work function of platinum is 5.65 eV; i.e. generally about 
0.6 eV less than that of p-type 6H silicon carbide. Thus, approached from 
a bandgap theory, platinum should not provide an ohmic contact to silicon 
carbide. 
Therefore, the need exists for ohmic contacts to p-type silicon carbide 
that can use metals more preferable than those presently most commonly 
incorporated. There is a further need for a contact that does not require 
high temperature annealing in order to perform as an ohmic contact, and 
for an ohmic contact which maintains its ohmic character after any 
necessary or desired subsequent annealing of the device or silicon 
carbide. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a p-type ohmic contact 
structure and a method of forming it between high work function metals 
such as platinum and wide bandgap semiconductors such as silicon carbide 
for which the work function of the metal would ordinarily be insufficient 
to form an ohmic contact between the metal and the p-type semiconductor 
and wherein the structure can withstand annealing while retaining ohmic 
characteristics. The invention meets this object with an ohmic contact 
structure formed of a portion of single crystal wide bandgap semiconductor 
material such as silicon carbide, a contact formed of a high work function 
metal on the semiconductor portion, and a layer of doped p-type 
semiconductor material between the single crystal portion and the metal 
contact in which the doped layer has a sufficient concentration of p-type 
dopant to provide ohmic behavior between the metal and the semiconductor 
material.

DETAILED DESCRIPTION 
The present invention is an ohmic contact structure between a high work 
function metal and a p-type wide bandgap semiconductor for which the work 
function of the metal would ordinarily be insufficient to form an ohmic 
contact between the metal and the semiconductor, and wherein the structure 
can withstand annealing while retaining ohmic characteristics. The ohmic 
contact structure comprises a portion of single crystal wide bandgap 
p-type semiconductor material, a contact formed of a high work function 
metal on the semiconductor portion, and a layer of doped p-type 
semiconductor material between the single crystal portion and the metal 
contact. The doped layer has sufficient concentration of p-type dopant to 
provide ohmic behavior between the metal and the semiconductor material. 
It will be understood by those familiar with wide bandgap semiconductors 
such as silicon carbide and semiconductor devices formed therefrom that 
the invention is most useful in making an ohmic contact to p-type silicon 
carbide, as the doping technique described herein, if used to implant 
p-type dopants into n-type SiC, would produce a p-n junction rather than 
an ohmic contact to p-type SiC. 
As noted herein, in the preferred embodiment, the wide bandgap 
semiconductor material comprises silicon carbide, but can also be selected 
from the group consisting of zinc selenide (ZnSe), gallium nitride (GaN), 
diamond, boron nitride (BN), gallium phosphide (GaP), and aluminum nitride 
(AlN). 
Further to the preferred embodiments, the high work function material 
preferably comprises platinum, but can also be selected from the group 
consisting of platinum silicide (Pt.sub.x Si.sub.l-x, where x is an 
integer to form various combinations of platinum and silicon), gold (Au), 
nickel (Ni), palladium (Pd), silver (Ag), chromium (Cr), and tungsten (W). 
As these metals demonstrate, the high work function metal is often a low 
reactivity or noble metal. 
Although the inventors do not wish to be bound by any particular theory, it 
appears most likely that the doped layer must include a sufficient 
concentration of p-type dopant to reduce the width of the depletion region 
sufficiently at the interface between the metal contact and the 
semiconductor material to allow carrier tunneling transport across the 
interface. As will be understood by those familiar with such devices, a 
depletion region is established between a metal and a semiconductor 
material when the metal has a work function that would ordinarily cause it 
to have rectifying characteristics rather than ohmic characteristics with 
respect to that particular semiconductor. 
"Tunneling" refers to a quantum mechanical process, well-understood by 
those familiar with this art, in which a carrier makes a transition 
through or across an energy barrier in a manner forbidden by classical 
mechanics. 
In selecting a combination of wide bandgap semiconductor material and high 
work function metal, the high work function metal is preferably selected 
to have: the thermal chemical potential to form compounds with the 
semiconductor material; diffusional stability with the semiconductor 
material; the ability to form solid solutions with the semiconductor 
material; and stability against environmental degradation. 
The most preferred embodiment of the present invention is illustrated in 
FIG. 1 which comprises an ohmic contact structure between platinum and 
silicon carbide and that can withstand annealing while retaining ohmic 
characteristics. The ohmic contact structure first comprises a portion of 
single crystal silicon carbide 10. The portion 10 can comprise a bulk 
crystal or an epitaxial layer. The silicon carbide is preferably selected 
of the group consisting of the 3C, 2H, 4H, 6H, 8H, and 15R polytypes, and 
is most preferably the 6H polytype. 
A platinum contact 11 is on the silicon carbide portion 10. As further 
illustrated in FIG. 1, a layer 12 of doped p-type silicon carbide is 
between the single crystal portion 10 and the platinum contact 11, with 
the doped layer 12 having a sufficient concentration of p-type dopant to 
provide ohmic behavior between the silicon carbide portion 10 and the 
platinum contact 11. 
Preferably, the doped layer 12 has a sufficient concentration of p-type 
dopant to reduce the width of the depletion region sufficiently at the 
interface between the platinum contact 11 and the doped layer 12 to allow 
carrier (electron) tunneling transport across the interface. 
In preferred embodiments, the doped layer 12 comprises an implanted region 
of the single crystal portion 10. The implantation is carried out 
consistently with the high temperature ion implantation set forth in U.S. 
Pat. No. 5,087,576 to Edmond et al. for "High Temperature Implantation of 
Silicon Carbide" which is incorporated entirely herein by reference. 
Alternatively, the doped layer 12 can comprise an epitaxial layer should 
the use of such a layer be more advantageous or convenient than the ion 
implantation technique. 
The preferred dopant for the p-type layer 12 is aluminum, although boron is 
also acceptable. The concentrations should be at least 1.times.10.sup.17 
cm.sup.-3, with concentrations of at least about 5.times.10.sup.18 
cm.sup.-3 preferred, and concentrations of at least 1.times.10.sup.19 
cm.sup.-3 most preferred. 
The doped p-type layer has an actual thickness of at least 50 angstroms 
(.ANG.) and preferably more than 100 .ANG.. 
Again, although the inventors do not wish to be bound by any particular 
theory, it appears useful for the platinum contact 11 to have formed a 
platinum silicide composition between the doped p-type layer 12 and the 
platinum contact 11 in order to obtain the ohmic behavior of the present 
invention. 
Thus, in another embodiment, the invention comprises the method of forming 
an ohmic contact between a wide bandgap p-type semiconductor material and 
a high work function metal to produce an ohmic contact that can withstand 
annealing. In this embodiment, the invention comprises forming a layer of 
doped wide bandgap semiconductor material on a single crystal portion of 
the same semiconductor material in which the doped layer has a sufficient 
concentration of p-type dopant to provide ohmic behavior between the 
semiconductor material and the high work function material. A layer of the 
metal is then deposited on the doped layer of the semiconductor material 
to provide the ohmic contact to the semiconductor material. 
In a preferred embodiment, the step of forming the doped layer comprises 
forming a layer of doped silicon carbide on a portion of single crystal 
silicon carbide and then depositing a layer of platinum on the doped layer 
to provide an ohmic contact to the silicon carbide. 
As set forth with respect to the structure, the preferred polytype of 
silicon carbide is the 6H polytype, although the silicon carbide can also 
be selected from the group consisting of the 3C, 2H, 4H, 6H, 8H, and 15R 
polytypes. 
The step of forming the layer of doped silicon carbide most preferably 
comprises forming a layer doped with a sufficient concentration of p-type 
dopant to reduce the width of the depletion region sufficiently at the 
interface between the platinum contact and the silicon carbide portion to 
allow carrier tunneling transport across the interface. As noted with 
respect to the structure, the doped p-type layer has a carrier 
concentration of at least about 1.times.10.sup.17 cm.sup.-3, more 
preferably 5.times.10.sup.-3, and most preferably at least 
1.times.10.sup.19 cm.sup.-3. 
Both aluminum and boron can be used as dopants, and in certain embodiments, 
it is preferable to include aluminum on an atomic percentage basis of 
between about 0.1 and 5%. 
One of the advantages of the invention is that the platinum does not need 
to be annealed to the silicon carbide in order to form the ohmic contact 
structure. If desired, however, the contact structure can be annealed 
without destroying its ohmic character. Thus, the flexibility offered by 
the present invention offers significant advantages in device design and 
manufacture. 
With respect to annealing, there are two potential annealing steps that can 
be used in the method of the present invention. The first is the annealing 
of the heavily doped layer following ion implantation, and the other is 
the optional annealing of the metal contact after it has been deposited. 
As another advantage, when the doping of the layer is accomplished through 
ion implantation, the step of forming the doped silicon carbide layer can 
either precede or follow the step of depositing the platinum. 
In carrying out ion implantation, and as set forth in the experimental 
section herein, the step of high temperature ion implantation preferably 
comprises implanting with an energy of about 10 kilovolts (kv), a dose of 
about 6.times.10.sup.15 cm.sup.-2, with the silicon carbide maintained at 
about 600.degree. C., a peak concentration of 5.times.10.sup.-3, and to a 
depth of about 100 .ANG.. 
As alluded to earlier, the ion implanted layer can be annealed prior to the 
step of depositing the layer of platinum and such an anneal can be carried 
out at temperatures of at least about 1000.degree. C. 
Furthermore, the ohmic contact structure can be annealed following the step 
of depositing the layer of platinum, either before or after ion 
implantation. The annealing of the platinum is generally carried out at a 
temperature of at least about 400.degree. C., and preferably as high as 
900.degree. C. 
When the platinum is annealed, it appears that it is most advantageous to 
anneal the platinum to form platinum silicide. Alternatively, the step of 
depositing a layer of platinum can comprise the step of depositing a layer 
of platinum silicide which can be done by any appropriate technique 
including direct deposition, sputter deposition, or any other appropriate 
technique. 
In yet another embodiment, the invention comprises an active semiconductor 
device which comprises a silicon carbide active portion, a platinum 
contact to the active portion, and a layer of doped p-type silicon carbide 
between the silicon carbide active portion and the platinum contact. The 
doped layer has a sufficient concentration of p-type dopant to provide 
ohmic behavior between the silicon carbide active portion and the platinum 
contact. Any such semiconductor device would include at least the 
structure shown in FIG. 1 and as described in the specification with 
respect to FIG. 1. Typical devices could include a junction diode, a 
bipolar junction transistor, a capacitor, a light emitting diode, a 
photodetector, a field effect transistor (MOSFET, JFET, or MESFET), and 
devices such as thyristors, IMPATT diodes, resistors and sensors. 
EXPERIMENTAL 
The Pt films were deposited on the Si (0001) face of single crystal, p-type 
.alpha.(6H)-SiC epilayers grown on n-type substrates. Depositions were 
conducted on Al implanted (10 keV, 6.times.10.sup.15 cm.sup.2 or 50 keV, 
2.times.10.sup.16 cm.sup.2 dosages at approximately 600.degree. C.), doped 
(approximately 10.sup.17 cm.sup.-3) epilayers. The implanted samples were 
annealed in vacuum at 1500.degree. C. prior to metallization. Carrier 
concentrations of these samples were difficult to obtain, however, due to 
the high doping levels. Prior to metallization, all of the substrates were 
precleaned in 10% hydrofluoric acid to remove the native oxide. Upon entry 
into the vacuum chamber, the substrates were thermally desorbed over a 
resistive graphite heater for 15 minutes at 700.degree. C. to remove 
hydrocarbon contaminants on the surface. 
Deposition was conducted n an ultra-high vacuum (UHV) electron beam 
evaporation system, which is discussed in detail along with the 
experimental procedures in Glass et al., "Low Energy Ion Assisted 
Deposition of Titanium Nitride Ohmic Contacts on Alpha (6H) Silicon 
Carbide," Applied Physics Letters, Vol. 59(22), pages 2868-2870 (1991); 
and Glass "Interface Chemistry and Structure Resulting From Low Energy Ion 
Assisted Deposition of Titanium Nitride on Ceramic Substrates", Thesis, 
N.C. State University, Raleigh, N.C. (1991). Base pressures were 
approximately 1-2.times.10.sup.-10 torr, and deposition pressures were 
approximately 8.times.10.sup.-8 torr. Approximately 1000 .ANG. of Pt was 
deposited at an evaporation rate of 10 .ANG./min., with substrate 
rotation. The wafers were one inch (1") in diameter. 
To produce contacts for current-voltage (I-V) measurements, the Pt was 
deposited through a molybdenum shadow mask containing 0.76 and 0.50 mm 
diameter contact pads. One half of the one inch wafer was unmasked to 
provide a large area contact for the surface-to-surface I-V measurements. 
Electrical measurements were conducted on the as-deposited contacts and on 
those successively annealed from 450.degree. to 850.degree. C. in 
100.degree. increments under UHV. Measurements were conducted ex-situ. on 
a Rucker-Kolls electrical probe station. All I-V curves were taken between 
the smallest diameter pads and the unmasked region. To study the reaction 
interface, Auger spectroscopy (AES) was performed as a function of depth 
on separate as-deposited and annealed samples with a JEOL JAMP-30. 
FIG. 2 displays the room temperature linear-linear I-V plots of 
as-deposited Pt contacts on SiC samples grown with p-type dopant 
concentrations of 2.times.10.sup.18 cm.sup.-3. FIG. 3 displays the same 
for the annealed contacts. These contacts displayed rectifying 
characteristics, with a soft breakdown beginning at approximately 1 volt, 
with a leakage current of 0.5 .mu.A/mm.sup.2 at 2 V. After the 550.degree. 
C. anneal, the linear-linear plot indicates a decrease in forward 
resistance. After the 650.degree. C. anneal, the linear-linear I-V curves 
develop close to linear character in the forward and reverse bias 
direction. After the 750.degree. C. anneal and higher, the log-log curves 
are the most linear, with a short and low sloped SCL region. It was, 
however, not possible to entirely remove this region, despite better than 
950.degree. C. anneals. 
Chemical analysis of the annealed films confirmed the data provided in the 
studies by Chou, "Anomalous Solid State Reaction Between SiC and Pt"; 
Journal of Materials Research, Vol. 5(3), pages 601-08 (1990); and 
Bermudez, et al., "Investigation of the Structure and Stability of the 
Pt/SiC (001) Interface", Journal of Materials Research, Vol. 5(12), pages 
2882-2893 (1990). Chemical profile versus depth for an ohmic contact 
according to the invention is shown in FIG. 4. This data, in addition to 
chemical mapping, indicated that carbon (C) was uniformly distributed 
throughout the interface and surrounding regions, while the silicon (Si) 
had diffused to the surface and segregated into islands within the Pt 
film. Specifically, FIG. 4 is an Auger profile of Pt on a non-implanted 
sample and FIG. 7 is an Auger profile of an implanted sample. The profiles 
are exemplary of the difusional behavior of the contact with and without 
the implant. 
The salient point is that although the contact I-V characteristics of 
conventional Pt-SiC contacts did become mostly linear as the anneal took 
place, they never were completely indicative of ohms law character near 
the origin. This was interpreted to indicate a small barrier between the 
reacted film and the SiC substrate. 
THE INVENTION 
FIG. 5 shows the linear I-V characteristics of an ohmic contact structure 
according to the present invention; a Pt contact to a SiC substrate 
implanted with 10 keV Al+ions at a dosage of 6.times.10.sup.15 /cm.sup.2. 
This curve was typical of the as-deposited contact, and changed little 
after annealing up to 850.degree. C. The log-log plot of these contacts 
(FIG. 6) is a straight line with slopes of unity. This indicates the ohmic 
character. 
It is not clear at this time why I-V curves from the Pt contacts to the 
non-implanted substrates were non-linear through the origin. Due to the 
extensive intermixing seen in the depth profiles of FIG. 4, it is 
difficult to apply theoretical interface models to this work. It is clear, 
however, that the implantation and annealing processes promote the ohmic 
properties regardless of the anneal state of the contact. This in turn 
indicates that the formation of a degenerate (p+) region is probable 
although further testing with other metals may be required to confirm 
this. 
In summary, an ohmic contact has been produced using Pt deposited via 
electron beam evaporation onto a p+Si terminated (0001) face of p-type 
.alpha. (6H)-SiC. This layer was created by implantation of Al ions on 
600.degree. C. substrates at dosage levels of 6.times.10.sup.15 /cm.sup.2. 
In the drawings and specification, there have been disclosed typical 
preferred embodiments of the invention and, although specific terms have 
been employed, they have been used in a generic and descriptive sense only 
and not for purposes of limitation, the scope of the invention being set 
forth in the following claims.