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What do we need to know about Ge? |
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There is only one thing to be aware of: Ge was the material for the very
first transistors in the 60ties but has not been used for many years (with a few marginal exceptions) until 2000 and later.
It is, however, experiencing a sort of "come back" now - but in a tricky way. |
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Just a short list of why Ge is of interest again: |
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- You can alloy it with Si - up to a point - and thus change the band gap, carrier mobilities and the lattice constant
in potentially beneficial ways.
- You can use Ge single crystals as a substrates for growing certain layers better than on other substrates. This
is enticing, e.g., for GaAs based space solar cells.
- You can use Ge single crystals for any uses where a low band gap is beneficial (detectors, sensors).
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What do we need to know about SiC? |
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That it has quite interesting basic properties but is very difficult to produce
as nearly perfect single crystal. The reason for that is that it comes in many different lattice types - the word is polytypes - meaning that there are many different stacking sequences of the Si-C
base in a basically hexagonal lattice |
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We will use a couple of pictures from a different Hyperscript at this point that illustrate what happens. |
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| Polytypes of SiC |
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All we do is to stack the building unit - SiC - in more tricky variants than the
fcc and hex structure shown under "3C" and "2H"; the nomenclature used for
SiC. Look up the original page for details,
but you don't have to know this. |
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What you should know is that SiC actually exists in all those polytypes (there are
many more) and that given crystals may even be mixes of several polytypes. |
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It is not easy at all to grow a good single SiC crystal in a defined polytype; unfortunately
the properties depend on the polytype: |
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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]
| 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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You get the drift: SiC would be great for certain uses (high power, high
speed, ...), but there are hardly any real products out there - despite major (usually military inspired) efforts. |
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We thus will not go into SiC much more but wait and see. Just one little innovation
- a cheap way of making good single crystals of one polytype - would generate a new technology and a new market. |
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As a last use of SiC it should be mentioned that it often serves as substrate for producing
thin layers of GaN.. |
© H. Föll (Semiconductor Technology - Script)
