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There is no such thing as plain
SiC! |
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Instead, whenever you look in the
literature, you will find names like 3C-SiC, 6H-SiC,
4H-SiC, or 2H-SiC. In other words: There are many different
polytypes of SiC. |
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Polytypism is a special case of
Polymorphism, which means that a given
element or compound can assume more than one crystal structure. Polytypism
simply is the one-dimensional variant of polymorphism. |
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SiC (unfortunately) is sort of
the paradigmatic material for polytypism. The always identical hexagonal
two-dimensional SiC layers can form many crystal structures by different
ways of stacking the layers on top of each other - that's why it is
one-dimensional. See "MatWiss I" (in
German) for the basics of how crystals can be formed by stacking atomic
layers. |
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SiC does not just have a few
polytypes, it has more than 200! Now you have a problem: which one is
the best for the application you have in mind, and if you know that, can you
actually make it all by itself (and not in a mix with all the others)? |
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SiC is also the only stable
group IV - IV compound
semiconductor. No other combination of the elements C, Si,
Ge, Sn exists in a defined lattice (and not just as mixed crystal
like Si-Ge). |
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All polytypes have a rather large indirect bandgap and other properties, which
make SiC a very interesting material for many applications. |
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As you may have guessed, SiC
is devilishly difficult to grow as a (large) single crystal of one polytype with low defect density. SiC
actually boasts a particular (and very bad) lattice defect all of its own -
so-called micropipes - the likes of which have not (yet) been found in other
crystals. |
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The formal way of identifying
polytypes, ie.e. the nomenclature
of the polytypes, is explained in the link (basic module); here we just
look at the more important variants in terms of the
familiar
"ABC" stacking definition. The basic building block (not
necessarily a unit cell) is highlighted in light blue. |
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Upon contemplation,
you should be able to notice that the "3C" structure is
nothing but the ABC stacking sequence of a close-packed fcc
lattice; the 2H is the corresponding simple hcp structure
resulting from an AB stacking sequence. |
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Your guess then that C stands
for "cubic"; H for hexagonal, is correct. If an
"R" comes up in variants not shown here, it stands for
rhombohedral. |
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In some older nomenclature, cubic
SiC is also known as b-SiC; the
hexagonal phase (6H-SiC more or less) is the a-SiC |
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Not only the structure of SiC
polytypes is different, but so are their electronic properties. |
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The always indirect band gap varies from 2.4 eV for the
cubic 3C-SiC to 3,3 eV for the simple hexagonal 2H-SiC
variant. Other relevant parameters like carrier mobility might be quite
different, too. Some values (mostly adpated from the publications or
presentations of Erlangen (Germany) SiC group) are shown below |
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4H-SiC |
6H-SiC |
15R-SiC |
3C-SiC |
| Band Gap [eV] |
3.265 |
3.023
3.03 |
2.986 |
2.390 |
| Lattice Constant [Å] |
a |
3.08
3.073 |
3.08 |
3.08 |
4.36 |
| c |
10.05 |
15.12 |
37.70 |
- |
| Effective Mass [mc]
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me |
0.37 |
0.69 |
0.53 - 0.28 |
0.68 - 0.25 |
| mh |
0.94 |
0.92 |
- |
- |
| Mobility (@ 300K) [cm2/Vs] |
µe |
500 |
300 |
400 |
900 |
| µh |
50 |
50 |
- |
20 |
Thermal conductivity (RT)
[W/cm · K] |
3.0 - 3.8 |
3.0 - 3.8 |
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If you look at the table long enough, you should
now actually have a
question! |
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Anyway, besides the rather large
bandgap, the effective masses and the mobilities are not so remarkable
compared to the
more standard semiconductors. |
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However, if you compare on a
more specific level, there
are definite advantages. Activate the link if you are interested. |
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Here we will only look at the basics;
details are left to an advanced
module. |
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In most cases, large single rystals
are grown from a melt (e.g.
Silicon) or some
solution (e.g. quartz, or sugar if you leave you coffee cup around too long),
but this is not a feasible option for SiC single crystal growth since
SiC does not have a liquid phase under normal conditions (i.e. without
applying a large pressure). SiC is also extremely hard (close to
diamond) and therefore has a high melting point (or is it the other way
around?). |
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There is also, in principle, no
crucible material that could contain molten SiC at is nominal melting
point temperature of < 2.500 oC. Nevertheless, SiC
was grown from a melt at 2200 oC and 150 bar in a
recent study, but this is probably not a commercially viable process. |
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We need a basically new method of
crystal growth. Some "older" techniques are described
in the link, the main
method used nowadays is
physical vapor
transport (PVT) also known as
seeded sublimation growth or modified Lely method. |
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A piece of SiC is heated to
(1800-2600) oC at low pressure. Due to the high sublimation
rate, SiC vapor forms and deposits itself on a cooler single-crystalline
seed crystal. |
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Straightforward and basically simple, as shown in
the schematic picture on the right. |
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However, pondering the situation,
some questions should come to mind: |
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What materials can you use for the
crucible and everything else that gets hot? After all, not many materials can
cope with temperatures above 2000 oC! |
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Well, you are basically stuck with graphite, and
maybe a bit of Ta here and there. That means, of course, that you are
forming SiC also on your crucible walls and everywhere else. If it
flakes off, you will have a defect problem. |
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What kind of growth rate can your get? |
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Well, as you would expect: Not much! Growth rates
depend on many parameters, but are in the range of 0.2 - 2 mm/hr. That's
about a factor of 50 slower than the growth rates for Si crystal
pulling and that makes SiC crystal growing automatically expensive |
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What polytype will you get (hoping that it will not be a
mixture)? What determines what you get? Can you control it and, if yes, how?
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Good question! First, you might get mixtures as
shown in the picture (courtesy of the Erlangen group). Otherwise, the following
parameters are essential:
- Polytype of the seed crystal (as you might have guessed).
- "Face" of the seed crystal; i.e if the surface is a C- or
a Si layer. If you start with a 4H-SiC seed crystal, for example,
you tend to get 4H-SiC if you have a C-face, and 6H-SiC if
you have a Si face. Why? Nobody really knows.
- Temperature difference and - gradient betwen SiC source and seed.
Small values tend to favor 4H-SiC, larger values 6H-SiC growth.
- Gas composition. Whatever gas you add will influence the polytype you
obtain. C-rich gases, for example, promote 4H-SiC growth
- The pressure, oddly enough, seems not to have a large influence on
polytypie.
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Note that while the polytype
6H is the easiest to grow, 4H would be favored by the power
electronics industry. |
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Last not least: What kind of crystal
quality do you get? What is the dislocation density? |
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The bad news is: the dislocation
density is high. The good news is, you do not worry too much about that - you
worry about something weird called "micropipe (and mixtures of polytypes, and
all kinds of stacking faults or special boundary faults, and carbon inclusion,
or Si inclusion, or big voids, ...). |
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To quote an Internet source:
"Problems with micropipes and polytypes dominate to such a degree that
the research of dislocations, vacancies and impurities still remains an
academic activity". |
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What are micropipes?
Well, micropipes are hollow channels running through the lattice; the diameter
of these pipes is (0.1 - 5) µm. |
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It is not totally clear
what micropipes are, how they are formed, and why they exist at all. The
probably best way to think about these defects is to consider them to be screw
dislocations with a "giant" Burgers vector (violating the rather
general
rule that Burges vectors always are the shortest possible lattice vectors)
and a hollow core. |
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The hollow core actually makes sense.
If you accept the "giant" Burgesvector bit, it is energetically far
more favorable to have the dislocation core hollow instead of extremely
strained. What you pay in terms of surface energy, you easily gain in avoided
elastic energy. |
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But this is not gospel yet.
Micropipes are at present simply not completely understood. |
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Micropipes are also somehow connected
to the growth mechanism of the crystal. This is neatly illustrated in the
picture on the right (taken with a scanning force microscope, courtesy of H.
Strunk; Uni Erlangen) where typical growth spirals are visibly centered around
a micropipe. |
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Micropipes also will definitely kill
any device that contains one of them. They thus must be avoided as much as
possible! |
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Let's look at the state of the art of
what is around. To quote from the product sheet of the major SiC
supplier Cree, Inc. (located somewhat ironically in Silicon Drive 4600 in
Durham, North-Carolina, USA): |
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At present, wafer
diameters are 50.8 mm or 76.2 mm; doping (usually with N
for n-type and Al for p-type) at high levels produces
resistivities in the 0.0x mWcm region. Or
there is no doping for semi-insulating stuff. |
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4H- and
6H-SiC polytypes are sold; for a
more detailed look of some
of the products that are available use the link. |
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The 2003 state of
the art (mostly in the laboratories and not necessarilly on the market) is
summarized in the following table: |
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| Diameter |
100 mm
"Four-inch" |
For Si, 100 mm was the standard back in the late
70ties/early 80ties). |
| Defects |
Micropipes |
< 1 cm–2 for 3''
< 30 cm–2 for 100 mm |
Increasing wafer size usually dramatically increases micropipe density |
| Dislocations |
3 · 103 cm–2 achieved |
Factor 10 reduction |
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Of course, in the many laboratories
(university and industrial) devoted to SiC, some data might be even
better. |
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The basic electronic properties were
listed above and in an
illustration module, here
we briefly consider doping
and optical
properties. |
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First let's ask ourselves a question
that should have come up by now: Why is SiC interesting for
optoelectronic applications? How
could Siemens make
light-emitting diodes back in 1977 from an indirect
semiconductor? |
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Well, maybe there are bound excitons
as in the case of
GaP? Right - maybe! To quote one of the recommended
Books (published
1995):
"The emission (of a-SiC; i.e.
probably 6H-SiC) occurs in a wide band from about 400 nm - 600 nm
with a maximum at 480 nm (blue). So far it is not clear what kind of
transition causes the SiC emission". |
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Now you should be glad: There is something left
to do - for you! |
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The situation becomes a bit clearer,
maybe, by pondering another quote (from a very good source in
Sveden): |
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"The viewpoint of a crystal grower
differs largely from that of a spectroscopist. The work of a crystal grower is
often to provide material of very high quality and sometimes also of high
purity. A
PL spectrum which may
look excellent for a crystal grower (i.e. shows nothing for an indirect
semiconductor), may perhaps not create any higher emotional feelings for a
spectroscopist. Indeed, samples which for a crystal grower may be the outcome
of failed experiments will be the samples of greatest interest for the
spectroscopist". |
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In other words: SiC crystals usually are
full of defects with some energy level in the band gap. Besides the levels form
the (usually heavy) doping, and all kinds of exciton levels, there are all
kinds of atomic defects, spanning the range from simple vacancies to impurity
atoms and clusters of atomic defects with levels in the big and roomy band gap
of SiC. |
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There are thus many possible transitions or
recombination
channels for electron and holes, and some of those transitions might will
emit light. |
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Some more information about the photo
luminescence properties of SiC can be found in an
advanced module, here we
simply note that the light emission properties of SiC are not so much a
property of the ideal (doped) perfect material, but of crystal lattice defects
in a general sense. |
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However: Whatever recombination
events produce light - the quantum efficiency is never very good - the over-all
efficiency of the early LEDs was < 1%. Nevertheless, before
the advent of GaN in the nineties, SiC LEDs were the only ones
emitting in the blue. |
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© H. Föll (Semiconductor - Script)
