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History |
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Silicon carbide is the only stable compound in
the Si-C equilibrium system at atmospheric pressure. SiC was first observed in
1824 by Jöns Berzelius. The properties and potential of the material were,
of course, not understood at the time. The growth of polycrystalline SiC with
an electric smelting furnace was introduced by Eugene Acheson around 1885. He
was also the first to recognize it as a silicide of carbon and gave it the
chemical formula SiC. The only occurrence of SiC in nature is found in
meteorites. Therefore, SiC cannot be mined but must be manufactured with
elaborate furnace techniques. In its polycrystalline forms, silicon carbide has
long been a well proven material in high-temperature, high-strength and
abrasion resistant applications. Silicon carbide as a semiconductor is a more
recent discovery. In 1955, Jan Antony Lely proposed a new method for growing
high quality crystals which still bears his name. From this point on, the
interest in SiC as an electronic material slowly began to gather momentum; the
first SiC conference was held in Boston in 1958. During the 60's and 70's SiC
was mainly studied in the former Soviet Union. Year 1978 saw a major step in
the development of SiC, the use of a seeded sublimation growth technique also
known as the modified Lely technique. This breakthrough led to the possibility
for true bulk crystal preparation. The first blue LED was fabricated already in
1979 and in 1987, Cree Research Inc., the first commercial supplier of SiC
substrates, was founded. Today, there are a few companies and many active
research groups in the field, but SiC industry still remains a small business.
In the beginning, the SiC industry was concentrated around the blue LEDs that
are impossible to manufacture using conventional silicon technology. The
potential of the power electronics industry is much larger and its interest has
grown only in recent years as the progress of silicon devices have began to
stagger. This also largely explains the rapidly increasing research on SiC
growth. |
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The most common polytypes are 3C and 6H; 4H,
15R and 2H have also been identified, but are much more rare. All other
polytypes are combinations of these basic sequences. The only cubic polytype
(3C) is also referred as -SiC. |
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The thermal conductivity of SiC is larger
than that of copper. |
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While the polytype 6H is the easiest to
grow, 4H would be favored by the power electronics industry. In the field of
SiC, the problems with micropipes and polytypes dominate to such a degree that
the research of dislocations, vacancies and impurities still remains an
academic activity. When the material develops |
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Growth from Melt Most commercially utilized
single crystal semiconductor boules are grown from a melt or solution, but this
is not a feasible option for SiC growth. SiC does not have any liquid phase in
normal engineering conditions. Calculations have indicated that stoichiometric
melting is possible only under pressures exceeding bar at temperatures higher
than 3200 C. Even if the solubility of carbon in silicon melt ranges from 0.01%
to 19% in the temperature interval from 1412 to 2830 C, at high temperatures
the evaporation of silicon makes the growth unstable. The solubility of carbon
can be increased by adding certain metals to the melt (e.g., praseodymium,
terbium, scandium). This would, in principle, enable the use of crystal pulling
techniques, such as Czochralski growth. Unfortunately there is no crucible
material available that would be stable with these melts. It is also speculated
that the solubility of the added metals in the growing crystal is too high to
be acceptable in semiconductor materials [10, 20]. In spite of all the
problems, SiC was grown from melt at 2200 C and 150 bar in a recent study. The
crucible was made of graphite and it also acted as the carbon source. A
1.4-inch crystal was demonstrated [21]. The technology is very expensive and
might never be economically feasible. However, growing from a solution would
avoid many of the problems related to the growth techniques from gas phase.
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Chemical Vapor Deposition A well established
method for growing thin crystalline layers directly from gas phase is chemical
vapor deposition (CVD) [14]. In the process a mixture of gases is injected to
the growth chamber. The higher the temperature the larger is the probability
that the initial bonds will crack and the radicals will attach to the surface
thus leading to epitaxial growth. When temperature is increased the probability
of sticking increases but also the etch rate from the surface increases. The
growth rate is therefore determined by the desorption of the reaction products
and by the etch rate of the surface, and by the diffusion dominated mass
transport of the source molecules. Generally, the growth rates in CVD are too
low to allow boule production, usually tens of micrometers an hour [22]. By
increasing the temperature the growth rate increases, but at the same time
problems related to the controlling of the growth become more severe, and
problems such as homogeneous nucleation in the gas phase may occur. These
problems might be overcome by a very careful control of the thermal and
thermodynamic conditions. This technology is not yet available, even though
research on high temperature CVD (HTCVD) is under way [23, 24]. |
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Lely Growth The Lely growth method is used
even nowadays to grow the crystals of the highest quality. A schematic Lely
geometry is presented in Figure 3.1. In CVD the growth is driven by the initial
gas composition, whereas in the Lely method the growth is due to temperature
gradients within the system. The system is close to chemical equilibrium and
the partial pressures of the SiC forming species are higher where also the
temperature is higher. This leads to a pressure gradient that results in mass
transport from the hot parts of the crucible to the cooler parts of the
crucible [25, 26]. Figure 3.1: The Lely method for growth of SiC crystals In
the Lely growth the temperature distribution is such that in the cylindrical
crucible the temperature minimum is at the origin. Therefore the gases travel
towards the origin. The porous graphite holding the source provides nucleation
centers for infinitely small seed crystals. They will eventually grow larger
and usually obtain an energetically favorable hexagonal form. Unfortunately,
the Lely grown crystals are limited and random in size, in average they are
about the size of a nail. Because the Lely growth method does not require any
seed crystals it is the method which all the other SiC crystals originate from.
The resulting crystals have low defect and micropipe densities.
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Seeded Sublimation Growth The seeded
sublimation growth, also known as physical vapor transport (PVT), is the method
of the present study. It is historically referred to as the modified Lely
method. The geometry was initially quite similar to the Lely geometries but the
difference is the use of a seed crystal which results in a more controlled
nucleation. The seeded sublimation process is nowadays the standard method for
growing bulk monocrystalline silicon carbide [10, 18, 19, 27, 28, 29, 30, 31].
In the process polycrystalline SiC at the source sublimes at a high
temperature, (1800-2600 C) and low pressure. The resulting gases travel through
natural transport mechanisms to the cooler seed crystal where crystallization
due to supersaturation takes place. The seed crystal is usually situated at the
top of the crucible in order to prevent contamination by falling particles.
There are different flavors of sublimation growth. The most important factor
that can be varied, is the crucible design and the temperature distribution
related to it. The crystals tend to grow along isotherms and therefore the
shape of the temperature distribution must be carefully designed. Many
different designs have been tested over the years. Two different designs are
presented in Figure 3.2. One way to classify them is to look at the placement
of the source material. Figure 3.2: Two versions on the seeded sublimation
growth geometry, the original modified Lely on the left and the modern version
on the right. The most common design is to put the source at the bottom of the
crucible so that the surface of the source is facing the growing surface [32,
33, 34]. This minimizes the source-to-seed distance, but in large systems it
makes the uniform heating of the source material difficult. A partial solution
to this problem is to set the source material in a ring-like configuration near
the crucible walls. The vapors are transported through porous graphite walls.
This design enables improved temperature control of the source and cooling of
the seed. The design has shown its potential in growing large crystals [25, 29,
35, 36, 37]. The drawback is a lower growth rate due to larger source-to-seed
distance, contamination by the porous graphite and possible problems in
controlling the shape of the growing crystal. Due to these reasons this design
seems to be decreasing in popularity. The so called sublimation sandwich method
was proposed initially in 1970 to grow thin epitaxial layers of monocrystalline
SiC. The design is partially open and the environment may be used to control
the gas-phase stoichiometry [38, 39, 40, 41, 42]. The method has a high growth
rate, more parameters to control the system, but it has not yet been shown that
it can be used to grow large boules. The high growth rate is mainly achieved
with a small source-to-seed distance and a large heat flux enabled by a small
amount of source material. These facts make the growth of large boules quite
difficult. The quality of the crystals grown with this method is, however,
quite promising. The crucibles are usually heated up by electromagnetic
induction as depicted in Figure 3.3. Typically, the frequency is in the range
of 10 to 100 kHz. Resistive heating might give improved temperature control,
but it requires more engineering efforts and is more expensive. Also the
resistive elements may be quickly worn out at the high temperatures required.
The sublimation growth systems are controlled during the growth process mainly
by changing the pressure of the inert gas (usually argon) and coil position
[33, 43, 44]. The chemical composition of the system may be affected by the
selection of the source and crucible material. Since the selection of crucible
materials is quite limited at the high temperatures required, carbon materials
are usually employed. At least two kinds of graphite are used: dense graphite
that is a good conductor of heat and electricity as well, and porous graphite
that has significantly smaller conductivities in both respects. As chemically
inert crucibles have some advantages, tantalum has recently been proposed as a
crucible material [40]. The material choices may have a decisive effect on the
temperature and impurity control of the crystal growth process. Figure 3.3: A
schematic model of an induction heated seeded sublimation geometry. The seed
crystals for sublimation growth are produced with the Lely method or taken from
previous sublimation growths. Unfortunately, the good properties of the
original Lely-grown crystals are difficult to maintain and therefore the
resulting crystals may have considerably higher defect and micropipe densities.
The sublimation growth is an evolutionary process. By selecting the seed among
the best wafers it may gradually be possible to get rid of micropipes. The size
of the crystal may be increased only gradually by radial growth. This is very
different from the growth of silicon where the quality and size of seed
crystals are not that critical. The processing of SiC crystals is a much more
difficult task than the processing of crystalline silicon. The grown boules
must be cut and polished before device manufacturing. Even though technological
problems still exist in these fields, they will probably not hinder the advance
of SiC industry. The availability of high quality bulk material is the real
bottleneck. SiC wafers are already commercially available. The market leader
has introduced a 3'' wafer but only 2'' wafers are commercially available.
There are also some other producers of SiC wafers that provide crystals up to
two inches [10, 45]. Although the seeded sublimation growth is the most
promising method for producing SiC crystals, and has been known for more than
twenty years, there are still some major difficulties involved in the process.
The polytype formation and growth shape are poorly controlled and the doping is
non-uniform. There are also severe defects, such as micropipes and dislocations
in the grown crystals. Many of the problems of the sublimation method are
inherent but the technological limits may though be pushed further. |
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