Toshiba nanocrystals will light up LED technology

Toshiba nanocrystals will light up LED technologySteve Bush
Exotic compound semiconductors have been needed to produce LEDs ever since General Electric used gallium arsenide (GaAs) to make the first practical one in 1962.
Since then, the materials and structures have got ever more complex and today’s LEDs often also include aluminium, nitrogen, indium and phosphorus and owe more to laser manufacture than to older LED processes.
All this complexity adds to cost and for some time researchers have been hunting for a way to make LEDs using silicon on standard silicon processes.  
  Hot technology… Baking thin amorphous silicon layers produces the essential nanocrystals.
There have been some successes, although none come close to matching compound semiconductor devices for brightness or efficiency – and this situation is unlikely to change for a while yet.
Porous silicon, for instance, has been shown to emit light, but making it requires processes not compatible with chip manufacture.
Silicon nanocrystals can also be persuaded to emit light. however, according to LED maker Toshiba, they have, up to now, required between 10 and 25V to operate; excessive for today’s chips.
The latest announcement comes from the Advanced Research Laboratory of Toshiba in Japan, where it has pushed nanocrystal device operating voltage below 5V while using formation methods broadly compatible with current integrated circuit production.
Making Toshiba’s devices involves depositing a thin (around 2nm)layer of amorphous silicon onto a silicon wafer, then heating it to between 700 and 850?C in an oxygen or nitrogen atmosphere for a few minutes.
During this heating, silicon nanocrystals grow on the wafer surface inside the amorphous silicon.
Once cool, the nanocrystals emit visible light at room temperature when reverse biased through a Schottky contact.
The crystals are hemispherical and, by altering the amorphous silicon layer thickness, processing temperature, atmosphere and heating duration, the researchers have made different sizes.
Orange emission came from samples processed at 700?C in nitrogen or oxygen.
Red came from 800?C processing in nitrogen (at 3.5-4V), but an oxygen atmosphere at these high temperatures caused the nanocrystals to oxidise away. Similar destruction of the nanocrystals happened with nitrogen above 850?C where they were converted to polysilicon.
The experiments have narrowed down the source of the electroluminescence, but not identified it.
It is not due to defects in the amorphous silicon, or defects at either of its surfaces, or the amorphous silicon – nanocrystal boundary. However, it only happens in nanocrystals less than 2nm across and 1.5nm high, and wavelength is related to crystal size.
The team’s best guess at the moment is that the light is coming from within the nanocrystals themselves and that emission is due to “the quantum confinement effect” within them. More specifically, “the recombination of confined electrons and trapped holes”.
The team’s report concludes that efficiency, at the moment very poor, can be improved by increasing the number of nanocrystals and strengthening the carrier confinement and that:”The LEDs that have been demonstrated are promising devices for realising monolithic optoelectronic ICs”.


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