午夜福利1000集合

GRAVITY is great to have around for activities like walking or skydiving. But
try growing the huge crystals or 鈥渨afers鈥� of semiconductor from which
microprocessors and memory chips are fashioned, and it becomes a serious
nuisance. Now a team of Chinese researchers has underlined the point by showing
that electronic devices made from crystals grown in space can outperform those
made from crystals grown here on Earth. The discovery may even provide a
commercial raison d鈥櫭猼re for the International Space Station鈥攚hose
purpose remains a mystery to many commentators.

The team, from the Chinese Academy of Sciences and the Hebei Semiconductor
Research Institute, sent a furnace containing a chunk of the semiconductor
gallium arsenide (GaAs) into orbit aboard a retrievable satellite. The furnace
melted the bulk of the material, leaving a small solid piece, or seed, in the
centre. As the molten material solidified around the seed, it formed a crystal
20 millimetres in diameter by 100 millimetres long.

Back on Earth, the team used the crystal to build transistors and integrated
circuits, which they compared with devices made from an ordinary GaAs wafer. The
devices which had been made from the space crystal performed much better: single
transistors were far less prone to random voltage 鈥渘oise鈥�, and the integrated
circuits were less likely to suffer current leakage, when electrons stray from
their intended path. Better still, they found that optical receivers鈥攌nown
as photodiodes鈥攈ad higher sensitivity than normal devices. They produced
more current for each incident photon, so optical networks could benefit from
the discovery.

鈥淭heir numbers look very good,鈥� says David Matthiesen, a materials engineer
at Case Western Reserve University in Ohio who works with space-grown crystals.
He says this is the first time devices have been built using space crystals.
GaAs crystals grown in space by other researchers have been too small for
this.

On Earth, the researchers say, convection can cause atoms of arsenic to end
up in spots that should, in a perfect crystal lattice, contain gallium atoms.
These slight errors can affect the performance of devices.

In the near-zero gravity of Earth orbit, lighter liquid doesn鈥檛 rise so
convection is impossible. Simple diffusion therefore dominates the way molecules
move through molten material and, hence, it dominates crystal growth. The team鈥檚
theory is that this allows crystals to grow in a more ordered fashion.

The International Space Station will have room to grow much larger crystal
ingots, Chen says, which could mean that the ISS partners might eventually be
able to commercialise the technology. Before that happens, his work could lead
to improvements in chip-making techniques on Earth. 鈥淣ow that they have better
devices,鈥� says Matthiesen, 鈥渢hey have to figure out why they are better and take
a good look at what it is they can improve.鈥�