International News
NEW SILICON NANOWIRES
CAN REALLY TAKE THE HEAT
By Berkeley Lab
Scientists have demonstrated a new material that conducts heat 150 % more efficiently than conventional materials . The method – an ultrathin silicon nanowire – could enable smaller , faster microelectronics with a heat-transfer-efficiency that surpasses current technologies . Electronic devices powered by microchips that efficiently dissipate heat would in turn consume less energy – an improvement that could help mitigate the consumption of energy produced by burning carbon-rich fossil fuels that have contributed to global warming , as well as the associated cooling requirements .
“ By overcoming silicon ’ s natural limitations in its capacity to conduct heat , our discovery tackles a hurdle in microchip engineering ,” says Junqiao Wu , the scientist who led the Physical Review Letters study reporting the new device . Wu is a faculty scientist in the Materials Sciences Division and professor of materials science and engineering at UC Berkeley .
HEAT ’ S SLOW FLOW THROUGH SILICON Our electronics are relatively affordable because silicon – the material of choice for computer chips – is cheap and abundant . But although silicon is a good conductor of electricity , it is not a good conductor of heat when it is reduced to very small sizes – and when it comes to fast processing , that presents a big problem for tiny microchips .
Within each microchip resides tens of billions of silicon transistors that direct the flow of electrons in and out of memory cells , encoding bits of data as ones and zeroes , the binary language of computers . Electrical currents run between these hard-working transistors , and these currents inevitably generate heat .
Heat naturally flows from a hot object to a cool object . But heat flow gets tricky in silicon .
In its natural form , silicon is made up of three different isotopes – forms of a chemical element containing an equal number of protons but different number of neutrons ( hence different mass ) in their nuclei .
About 92 % of silicon consists of the isotope silicon-28 , which has 14 protons and 14 neutrons ; around 5 % is silicon-29 , weighing in at 14 protons and 15 neutrons ; and just 3 % is silicon-30 , a relative heavyweight with 14 protons and 16 neutrons , explained co-author Joel Ager , who holds titles of senior scientist in Berkeley
Junqiao Wu / Berkeley Lab
Optical microscopy image of a microdevice consisting of two suspended pads bridged by a silicon nanowire .
Lab ’ s Materials Sciences Division and adjunct professor of materials science and engineering at UC Berkeley .
As phonons , the waves of atomic vibration that carry heat and wind their way through silicon ’ s crystalline structure , their direction changing when they bump into silicon-29 or silicon-30 , whose different atomic masses “ confuse ” the phonons , slowing them down .
“ The phonons eventually get the idea and find their way to the cold end to cool the silicon material ,” but this indirect path allows waste heat to build up , which in turn slows your computer down , too , Ager says .
A BIG STEP TOWARD FASTER , DENSER MICROELECTRONICS For many decades , researchers theorised that chips made of pure silicon-28 would overcome silicon ’ s thermal conductivity limit , and therefore improve the processing speeds of smaller , denser microelectronics .
“ But purifying silicon down to a single isotope requires intense levels of energy which few facilities can supply – and even fewer specialise in manufacturing market-ready isotopes ,” Ager says . Fortunately , an international project from the early 2000s enabled Ager and leading semiconductor materials expert Eugene Haller to procure silicon tetrafluoride gas – the starting material
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