Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
2003-01-10
2004-06-08
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C257S744000, C438S602000, C438S931000
Reexamination Certificate
active
06747291
ABSTRACT:
RIGHTS OF THE GOVERNMENT
The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
BACKGROUND OF THE INVENTION
Silicon Carbide, SiC, is a semiconductor material having a wide band gap, a high saturated electron drift velocity, a high breakdown electric field, good thermal conductivity, and desirable mechanical hardness. Based on these and an additional extraordinary combination of electronic, thermal, and mechanical properties possessed, SiC-based devices have potential for performance superior to the Si or GaAs based devices [1] of current popularity. Parenthetic numbers such as these identify references listed in APPENDIX 1 at the close of this specification. There is a growing demand for electronic components operable under severe conditions for a wide range of military uses, including X-band radar, space radar, vehicle and aircraft power uses, military communications, electronic warfare, and missile applications. SiC-based electronic devices are expected to play a critical role in these applications. For examples, SiC devices have crucial applications in X-band radar for the future “National Missile Defenses” system.
Ohmic contact accomplishment with SiC is presently however a critical issue in achieving successful SiC device fabrications. Ohmic contacts carry electron current into and out of the semiconductor material, ideally with no parasitic resistivity. High performance devices require high quality ohmic contact on SiC. For example, large ohmic contact resistance limits the high frequency performance of SiC devices [2]. It has been determined that contact resistivity to p-type SiC must be reduced to about 10
−6
&OHgr;-cm
2
in order to obtain X-band (i.e., 8-12 GHz) performance [2]. The high operating frequency of an X-band radar will for example give the high resolution required for precise tracking and for discrimination of incoming warheads from other objects in the “National Missile Defenses” system. The resistance of ohmic contact on p-type SiC must therefore be reduced by one or two orders of magnitude from that currently obtained [1] and reduced via a manufacturable process.
Another consideration with respect to the use of Silicon Carbide is that ohmic contact on SiC only can be achieved with the use of high temperature annealing, annealing at temperatures near 1000° C., and with SiC having high doping concentration, concentration measuring in the upper 10
18
and 10
19
atoms/cm
3
range [3-5]. Such high temperature annealing results in difficulties in the device fabrication, and a high doping concentration can damage the SiC lattice and deteriorate device performance. A significant advance in understanding ohmic contact formation on Silicon Carbide is thus required for many useful additional applications.
P-type Silicon Carbide is available in epitaxial wafer and substrate forms from suppliers to the semiconductor field and at least one supplier of such material is identified in the paragraphs following herein. It is well known however that ohmic contact formation on p-type SiC is more difficult than on n-type SiC [3-5]. A few studies have been performed with respect to ohmic contacts on p-type SiC. Only a few materials, such as Al, Ti/Al, Mo, Ta, W, Pd, W/Pt/Al, Al/Si, CoSi
2
and metal borides, are able to form ohmic contact on p-type SiC [6-14]. For such contacts it is usually required that the doping concentration be at the upper 10
18
and 10
19
atoms per cubic centimeter level, and the annealing temperatures be at 1000° C. or above to achieve the contacts. The specific resistivities achieved in these contacts are in the range of 10
−3
to 10
−5
&OHgr;cm
2
, values greater than those achieved in n-type SiC.
Al has been considered the best material for ohmic contacts on p-type Silicon Carbide [3]. Mo, Ta, and W require the annealing temperatures at 1100-1200° C. [9]. Ta is difficult to adhere on SiC. Pd is easily oxidized, and only stable up to 350° C. in air [10]. The low melting temperature of Al (660° C.) and the oxidation characteristics of Al however make processing contacts involving this metal difficult. Al film also has a very rough surface morphology after annealing. Since the Ti/Al alloy has higher melting temperature than Al, the surface morphology improves using Ti/Al alloy. However, the addition of Ti degrades the contact achieved. Currently, the most commonly used materials for ohmic contacts on p-type Silicon Carbide are Ti/Al alloy [6], and the specific resistivity achieved is at 10
−5
&OHgr;cm
2
for SiC with the doping concentration of 1.3×10
19
cm
−3
, and at 10
−4
&OHgr;cm
2
with the doping concentration of 7×10
18
cm
−3
after annealing at 1000° C. [6]. The involved mechanism is not well understood. Clearly in this environment there is need for improvement in the state of the Silicon Carbide ohmic contact art. Such improvement is believed provided by the present invention.
SUMMARY OF THE INVENTION
The present invention provides a low electrical resistance ohmic contact with p-type Silicon Carbide semiconductor material.
It is therefore an object of the present invention to provide low electrical resistivity p-type Silicon Carbide ohmic contacts that can be achieved at a lower processing temperature than currently used contact arrangements.
It is another object of the invention to provide a usable p-type Silicon Carbide ohmic contact that is achievable with contact related processing temperatures of 900 degrees Celsius.
It is another object of the invention to provide a p-type Silicon Carbide ohmic contact enabling use of lower Silicon Carbide semiconductor material doping levels than other contact arrangements.
It is another object of the invention to provide a p-type Silicon Carbide ohmic contact in which graphitic sp
2
Carbon materials are used to an advantage rather than imposing the usually accepted detrimental effects.
It is another object of the invention to provide an improved Silicon Carbide ohmic contact that is achieved through use of a metallic catalytic agent during contact fabrication.
It is another object of the invention to provide an improved Silicon Carbide ohmic contact that is achieved through use of a aluminum or another metal catalytic agent during contact fabrication.
It is another object of the invention to provide an improved Silicon Carbide ohmic contact achievable with annealing temperatures some two hundred degrees Celsius below those used in present Silicon Carbide device ohmic contact fabrications.
It is another object of the invention to provide an improved Silicon Carbide ohmic contact achievable with semiconductor doping concentrations an order of magnitude below those used in present Silicon Carbide device ohmic contact fabrications.
It is another object of the invention to provide an improved Silicon Carbide ohmic contact through the use of graphitic carbon structures in the contact region.
It is another object of the invention to provide an improved Silicon Carbide ohmic contact achievable through the predictable conversion of an initial form of Carbon into a contact-usable different form of Carbon.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
These and other objects of the invention are achieved by the method of fabricating a graphitic sp
2
Carbon-enabled ohmic contact for a p-type Silicon Carbide semiconductor device, said method comprising the steps of:
providing a cleaned surfaced wafer sample of p-type Silicon Carbide having one of selected doping concentration less than 1×10
19
atoms/cm
3
and selected initial resistivity characteristics;
covering said sample cleaned surface with a layer of amorphous sp
2
and sp
3
Carbon mixture;
supplying a layer of Carbon conversion-accelerating catalytic elemental material over
Collins Warren E.
Landis Gerald
Lu Weijie
Mitchel William C.
Crane Sara
Hollins Gerald B.
Kundert Thomas L.
The United States of America as represented by the Secretary of
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