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Stanford Breakthrough Could Make Better Chips Cheaper
angry tapir writes: Researchers at Stanford University have come up with a new way to make chips and solar panels using gallium arsenide, a semiconductor that beats silicon in several important areas but is typically too expensive for widespread use. "[I]t can cost about $5,000 to make a wafer of gallium arsenide 8 inches in diameter, versus $5 for a silicon wafer, according to Aneesh Nainani, who teaches semiconductor manufacturing at Stanford. The new Stanford process (abstract) seeks to lessen this thousand-to-one cost differential by reusing that $5,000 wafer. Today the working electronic circuits in a gallium arsenide device are grown on top of this wafer. Manufacturers make this circuitry layer by flowing gaseous gallium arsenide and other materials across the wafer surface. This material condenses into thin layer of circuitry atop the wafer. In this scenario, the wafer is only a backing. The thin layer of circuitry on top of this costly platter contains all of the electronics."
From what I understand of it (which is very little) it's relatively easy to coax a crucible of pure, molten Si to grow into a single crystal - those long grey sausage-like boules are a single crystal of silicon, so are incredibly pure with a consistent crystalline structure. It's a lot harder to get gallium arsenide to do the same thing.
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You need a wafer with the same crystal structure and lattice constant. If there is a mismatch
between the inter-atomic distance (aka lattice parameter or lattice constant), the atoms
deposited on this wafer will try to adjust to this lattice.
If the layer is thin, the deposited crystal will in effect be compressed or expanded. While this is OK from a mechanical and
crystal point of view, the electronic properties of the grown semiconductor will change. E.g. the bandgap (energy distance
between filled and empty energy levels) will shift, which will change the electronic properties of the material.
If/when the layer becomes thick (~5 atomic layers), the grown crystal will (try to) revert to its native lattice constant. However, there's
no ordered way to do this, so the grown crystal will contain lots of defects, or on the worst case, become amorphous
(loose its crystal structure alltogether). Defects destroy the material from an electronic point; it provides ample opportunities
for electrones and holes to recombine. It increases the leakage current and power dissipation and alse change the electronic band structure.
I haven't read the article, but what I don't understand how they intend to separate the circuitry from the wafer... it's not exactly
a tape that you can peel off, or a thick slab which you can hammer away.
To get the single-crystal purity of the surface layers, they need to perfectly match the crystal dimensions of the substrate. Making it out of the same thing is aa easy way to achieve that.
There may be the issue of demand. PV cells apparently require a lot of material compared to a lot of other potential applications of GaAs (RF? Optoelectronics?). If you really started mass-producing them from GaAs, you'd start straining the global supply (200 tonnes per year or so?) long before you'd reach anything close to current global production of silicon-based PV cells.
Ezekiel 23:20