Silicon carbide:metal carbide alloy semiconductor and method of making the same

A new type of semiconductor material is disclosed which consists of a .beta.-SiC:metal carbide alloy having the general formula Si.sub.w (metal 1).sub.x (metal 2).sub.y (metal 3).sub.z C, where w+x+y+z=1 and 1>w>0. The metals are selected from the group consisting of Ti, Hf, Zr, V, Ta, Mo, W and Nb, with Ti, Hf, and Zr preferred. By selecting appropriate proportions of metal carbide and SiC, the alloy's bandgap may be tailored to any desired level between the bandgaps of the metal carbide and SiC. Semiconductor devices are preferably formed by epitaxially growing a layer of the new alloy upon a substrate having a .beta.-SiC or TiC type crystal structure. In addition to retaining the benefits of single-bandgap .beta.-SiC with certain advantages, the new alloys make it possible to implement various electrical devices that cannot be achieved with .beta.-SiC, and also have a potential for bandfolded superlattices for infrared detectors and lasers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to semiconductor materials and methods of forming 
the same, and more particularly to .beta.-silicon carbide semiconductors. 
2. Description of the Related Art 
Silicon has gained acceptance as the basic semiconductor material, with 
other semiconductors such as GaAs used for limited applications such as 
extra high speed operations. However, both silicon and GaAs have 
significant limitations in terms of voltage breakdown levels, saturated 
electron drift velocity and the density of devices that can be implemented 
with these materials. Furthermore, it is not practical to make 
heterostructure devices with silicon, and both silicon and GaAs have fixed 
bandgaps that cannot be changed for different applications. 
A new type of semiconductor referred to as beta silicon carbide(.beta.-SiC) 
has been developed recently which generally retains the operating features 
of more conventional semiconductors, but has significant advantages. As 
compared with both silicon and GaAs, .beta.-SiC exhibits a very high 
breakdown voltage, a high saturated electron drift velocity which makes it 
useful for extremely high frequencies, and a high thermal conductivity 
which aids in heat dissipation and a consequent capability for a very high 
density of devices. In addition, the wide bandgap and superior high 
electric field properties of .beta.-SiC, in conjunction with advances in 
submicron semiconductor processing technology, offer the possibility of 
significant breakthroughs in size, power, speed, operating temperature and 
radiation resistance of solid state semiconductor devices and integrated 
circuits. This new semiconductor and its potential applications are 
described in J. D. Parsons, R. F. Bunshah and O. M. Stafsudd "Unlocking 
the Potential of Beta Silicon Carbide", Solid State Technology, November 
1985, pages 133-139. 
While .beta.-SiC has very significant advantages that make it a potential 
replacement for silicon as the basic semiconductor, it also has some 
limitations. Like silicon, its bandgap is fixed and prevents it from being 
used for several types of devices such as high electron mobility 
transistors (HEMT), avalanche photo diodes (APD), heterojunction bipolar 
transistors, impact avalanche transit time diodes (IMPATT) with narrow 
bandgap avalanche regions, and bandfolded superlattices for infrared 
detectors and lasers. Furthermore, .beta.-SiC has a low low-field mobility 
which reduces maximum operating frequency of a metal-semiconductor-field 
effect transistor (MESFET) near the source and drain contact. 
SUMMARY OF THE INVENTION 
In view of the above problems with the related art, the object of the 
present invention is to provide a family of semiconductor materials which 
retain the advantages of .beta.-SiC over more conventional semiconductors, 
and yet is applicable to a greater range of devices, has a bandgap which 
can be readily adjusted and tailored to the desired application, and has a 
higher low-field mobility. 
This objective is realized with a new class of semiconductors consisting of 
metal-carbide:SiC alloys having a TiC or beta SiC crystal structure, 
approximately the same lattice parameter, and the general formula Si.sub.w 
(metal 1).sub.x (metal 2).sub.y (metal 3).sub.z C, where w+x+y+z=1 and 
1&gt;w&gt;0. .beta.-SiC and TiC both have cubic crystal structures, but their 
space groups are somewhat different; .beta.-SiC has a diamond II-type 
structure, whereas TiC has space groups similar to NaCl. The metals are 
selected from the group consisting of Ti, Hf, Zr, V, Ta, Mo, W and Nb, and 
preferably from Ti, Hf, and Zr. The metal component should be selected so 
that the metal-carbon bonds in the alloy are predominantly covalent, only 
electrons in the outermost orbitals of the outermost electron shell 
contribute to bonding, and only the outermost d orbital electrons are 
available for conduction. 
The new semiconductor can be specially tailored for applications requiring 
a specific bandgap energy level, between the bandgap energy levels of the 
metal carbide and the SiC, by selecting the relative proportions of metal 
carbide and SiC to yield the desired bandgap. In this manner, several 
devices can be implemented which are not presently available with either 
silicon or .beta.-SiC. 
The invention also includes a method for forming a semiconductor structure 
with the new material by epitaxially growing the alloy upon a substrate 
having a NaCl or .beta.-SiC lattice structure and a lattice parameter 
matching substrate, preferably by chemical vapor deposition from a gaseous 
mixture of the alloy's constituent materials. The relative proportions of 
metal carbide and SiC are selected to yield the desired bandgap energy 
level for the alloy. The substrate upon which the semiconductor structure 
is grown preferably comprises single crystal TiC. 
Further features and advantages of the present invention will be apparent 
to those skilled in the art from the following detailed description of 
preferred embodiments, taken together with the accompanying drawings, in 
which:

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention involves a new class of semiconductors consisting of 
Group IV Alloys of .beta.-SiC and one or more metal carbides. .beta. is 
the conventional nomenclature for the 3C cubic lattice structure of SiC. 
The relative proportions of SiC and metal carbide are selected to yield a 
semiconductor having any desired bandgap energy level between the bandgaps 
of the SiC and the metal carbide. The band structure of the metal carbide 
should be similar to that of .beta.-SiC to permit the growth of an alloy 
combining the two materials. The following metal carbides have been 
determined to have band structures with conduction band minimums and 
valence band maximas at K space positions near those of .beta.-SiC, and 
are therefore candidates for use in forming the new semiconductor: Ti, Hf, 
Zr, Va, Ta, Mo, W and Nb. Their lattice parameters are all within about 5% 
of that of .beta.-SiC, thus permitting the formation of an alloy rather 
than phase separation. 
The particular metal carbide selected should meet several constraints to 
permit bandgap control, and also to permit electrical conduction by 
doping. The metal must combine with carbon to form a monocarbide; 
otherwise there may be too few electrons in the outer shell which will 
result in a self-doping effect. The metal-carbon bond should be 
predominantly covalent, rather than ionic, to form better semiconductor 
bonds and to prevent polarity effects in the bonding. Only the electrons 
in the outermost orbitals of the metal's outermost shell should contribute 
to the bonding; it is harder to dope the resulting material effectively if 
inner electrons contribute to the bonding. Finally, only the outermost d 
orbital electrons should be available for conduction, since this produces 
better bonding characteristics and facilitates conduction when doped. 
The band structures of the carbides of the metals identified above are 
similar, and in some cases nearly identical, to .beta.-SiC. In addition, 
the conduction band minimum and valence band maximums for these metal 
carbides are at or near the same values in K space as .beta.-SiC. K space 
refers to momentum rather than distance space, and is also referred to as 
the reciprocal lattice. It is discussed, for example, in Sze, "Physics of 
Semiconductor Devices", Chap. 2, John Wiley & Sons, Inc., 1969. The 
location of conduction band minimums and valence band maximums in K space 
is discussed in L. A. Hemstreet and C. Y. Fong, "Recent Band Structure 
Calculations of Cubic and Hexagonal Polytypes of Silicon Carbide", 
Silicon-Carbide 1973, Proceedings of the Third International Conference on 
Silicon Carbide, Sep. 17-20, 1973, edited by R. C. Marshall, J. W. Faust, 
Jr. and C. E. Ryan. 
TiC, ZrC, and HfC meet all of the above constraints and have lattice 
parameters which most closely match that of .beta.-SiC, and therefore are 
the preferred metal carbides for use in the invention. Va, Ta, Mo, W and 
Nb can all have unpaired (unfilled) d bands, and thus can have d electrons 
which may not be sufficiently localized to prevent them from acting as 
conduction electrons. The implementation of the invention with a metal 
carbide consisting of TiC is illustrated in FIGS. 1-3. FIG. 1 illustrates 
the constant 2.35 eV bandgap energy of .beta.-SiC, which is equal to the 
energy gap between the conduction and valence bands. FIG. 2 illustrates 
the bandgap of TiC in K space. It exhibits an indirect bandgap, meaning 
that the conduction minimum is displaced from the valence maximum in K 
space. The bandgap is approximately -0.1 eV, indicating that the 
conduction minimum is less than the valence maximum and the material is 
therefore a metal. 
FIG. 3 illustrates, for an alloy consisting of .beta.-SiC and the metal 
carbide, the dependence of the alloy's bandgap energy with the relative 
proportions of SiC and TiC. A semiconductor with 100% SiC is represented 
by the left hand vertical axis, while a metal with 100% TiC is illustrated 
by the right hand vertical axis. The bandgap energy for alloys of these 
two constituents will vary in a manner generally indicated by the curve 2, 
beginning at 2.35 eV on the .beta.-SiC axis, and decreasing to -0.1 eV on 
the TiC axis. The curve for any particular metal carbide will generally be 
non-linear, and must be determined empirically. However, the curve will be 
non-varying and monotonic, i.e., it will be continuous with no 
discontinuities. 
In general, the bandgap energy curve for any particular .beta.-SiC-metal 
alloy will be monotonic if the k locations of the conduction band minimum 
and valence band maximum are approximately the same for the metal as for 
SiC; this condition is satisfied with the metals identified above. A 
monotonic bandgap curve is very helpful in reliably engineering a 
semiconductor material within the present new class of semiconductors to 
have a desired bandgap. A semiconductor with any desired bandgap between 
the 2.35 eV and -0.1 eV limits for .beta.-SiC can be constructed simply by 
selecting the appropriate proportions of .beta.-SiC and TiC which yield 
the desired bandgap from the curve. 
While the invention has been illustrated by referring to a semiconductor 
consisting of an alloy of .beta.-SiC and a single metal carbide, more than 
one metal may be employed. For example, a quaternary alloy having the 
formula Si.sub.0.5 Ti.sub.0.24 Zr.sub.0.26 C would be suitable for 
applications requiring its corresponding bandgap. Although theoretically 
any number of the metals identified above could be employed, a practical 
limit of three different metals will prevail if they are limited to the 
preferred group of Ti, Zr and Hf. A general formula for an alloy of this 
type is: 
##EQU1## 
A ternary alloy with two different metals is achieved by setting z equal 
to zero, while a binary allow with only one metal carbide is achieved by 
setting both y and z to zero. 
It is theoretically desirable that the alloy employ direct bandgap 
materials, so that it can be used with lasers. A direct bandgap 
semiconductor is defined as one in which the conduction band minimum is 
generally aligned in K space with the valence band maximum, and is 
illustrated in FIG. 4. It requires only a single photon to excite an 
electron up from the valence to the conduction band. An indirect bandgap 
semiconductor, illustrated in FIG. 5, has a conduction band minimum which 
is offset in K space from the valence band maximum. It requires both a 
photon to excite the electron up from the valence to the conduction band, 
and a phonon to excite the electron laterally in K space. Since SiC is an 
indirect bandgap material, the metal carbide SiC alloy should similarly be 
indirect bandgap. 
Bandgap variation by the selection of a particular alloy composition has 
previously been accomplished in a different context with AlGaAs. For 
example, see "Semiconductor Lasers and Heterojunction LEDA" by Henry 
Kressel & J. K. Butler, Chap. 11, pages 357-96, Academic Press, 1977. 
However, AlGaAs does not have the advantageous environmental thermal 
stability, high electric field and bandgap range properties of the metal 
carbide .beta.-SiC alloys described herein. 
Referring now to FIG. 6, a two-dimensional representation of the lattice 
structure for .beta.-SiC is illustrated, with S representing a silicon 
atom and C a carbon atom. The atoms are arranged in a regular cubic .beta. 
pattern, the carbon atoms alternating with silicon atoms. Each atom of one 
material forms bonds with the atoms of the other material in each of the 
three axial directions. 
FIG. 7 is a two-dimensional representation of an Si.sub.0.5 Ti.sub.0.5 C 
lattice structure. In this case, every other silicon atom is replaced with 
a titanium atom. Referring back to FIG. 3, the resulting alloy's bandgap 
will be somewhere between the 2.35 eV .beta.-SiC and -0.1 eV TiC levels, 
although probably not exactly half-way between since the bandgap curve is 
generally not linear. 
FIG. 8 illustrates the crystal lattice structure of an alloy which is 
further up and to the left along the bandgap curve of FIG. 3, specifically 
Si.sub.0.75 Ti.sub.0.25 C. Again, the representation is two-dimensional, 
but it should be understood that the crystal lattice structure will also 
extend into and out of the page in a similar fashion. This alloy will have 
a greater bandgap than that of FIG. 7, since it is closer to the 
.beta.-SiC axis of FIG. 3. 
Thus, any particular bandgap level within the limits of the metal carbide 
SiC alloy can be achieved by selecting the appropriate proportions for the 
constituents. To date no method of predicting a bandgap curve for a 
particular type of alloy has been devised, and the curve must be 
determined empirically. 
Alloys can be constructed within the present invention which retain the 
advantages of .beta.-SiC, and which yield exceptionally stable, high 
temperature, high power, high frequency, radiation hard semiconductor and 
optical devices. The present alloys generally have a higher low-field 
mobility than .beta.-SiC, and can be used as the active layer in MESFETs. 
Virtually any electronic device structure employing homostructures and/or 
heterostructures can be synthesized using the new alloys. In addition to 
the potential for displacing semiconductor and Group III-V semiconductors 
for electronic devices, the new alloys have potential applications for 
replacing lasers and infrared detector structures, as well as HgCdTe 
infrared detectors. 
A preferred fabrication technique for forming a device with the new alloys 
is illustrated in FIGS. 9 and 10. Referring first to FIG. 9, the alloy 
atoms are provided in a gaseous phase 4 and deposited by chemical vapor 
deposition onto a suitable substrate 6. The various alloy materials are 
each introduced in a gaseous phase and mixed together in an empirically 
determined ratio. The relative proportion of materials in the gaseous 
phase is generally not the same as their relative proportions when 
deposited as an alloy on the substrate. The relationship between the 
proportions of materials in the gaseous and deposited stages depends on 
factors such as pressure, temperature and flow rate, and is determined 
empirically. The chemical vapor deposition yields an epitaxial growth of 
the alloy material 4 upon the substrate 6. The substrate should 
accordingly have a lattice parameter which is similar to that of the alloy 
to facilitate epitaxial growth. A preferred substrate material is single 
crystal TiC. 
FIG. 10 illustrates a finished semiconductor device, which in this case is 
an FET. A semiconductor layer, which for this example consists of 
.beta.-SiTiC with a desired proportion between Si and Ti, has been 
epitaxially grown over the substrate 6. After suitable doping to form a 
channel region, source and drain contacts 10, 12 and gate contact 14 are 
formed to complete the device. 
While the formation of a rather simple device has been illustrated, it 
should be realized that many types of different devices are possible with 
the new alloy semiconductor. These include devices which could not 
previously been implemented with .beta.-SiC, such as high electron 
mobility transistors, superior avalanche photo diodes, heterojunction 
bipolar transistors, implant avalanche transit time diodes with narrow 
bandgap avalanche regions, and band folded superlattices for infrared 
detectors and lasers. 
Thus, while specific embodiments of the invention have been described and 
illustrated, it should be understood that numerous variations and 
alternate embodiments will occur to those skilled in the art. Accordingly, 
it is intended that the invention be limited only in terms of the appended 
claims.