If you stepped up and suggested that eliminating the substrate was the future of semiconductor manufacturing, nine times out of 10 (or 10 out of 10) you would be dismissed with a wave of the hand. That’s not too different than the initial reactions Lars Samuelson of Lund University in Sweden received when he presented that possibility to his colleagues.
“When I first suggested the idea of getting rid of the substrate, people around me said ‘you’re out of your mind, Lars; that would never work,'” Samuelson relates in a release describing his latest research. But it did work and the process for doing it, which was published in the journal Nature ("Continuous gas-phase synthesis of nanowires with tunable properties"), looks like it could reach the commercial stage in applications for solar cells in as little as two to four years.
The process consists of putting freely suspended gold nanoparticles in a gas flow. These gold nanoparticles serve as a substrate on which semiconductor nanowires can grow.
Research in the area of growing nanowires with “seed” particles (metal nanoparticles) in a gas flow has already enjoyed some breakthroughs this year. In February, researchers at MIT demonstrated that by controlling the amount of gas you could actually change the properties of the resulting nanowires.
However, the Lund University research team still saw that the field of fabricating semiconductor nanowires lacked a method by which nanowires could be mass-produced “with perfect crystallinity, reproducible and controlled dimensions and material composition, and low cost.”
So Samuelson and his colleagues experimented with a process they dubbed “aerotaxy”--a name based on the process known as epitaxy, in which a crystal layer is grown on crystal substrate. Aerotaxy is essentially an aerosol-based growth method that proved successful in continuously producing nanowires with controlled dimensions. The trick to getting it to work properly was carefully controlling the temperature, the timing of the process, along with the dimensions of the seed particles—in this case, the gold nanoparticles.
“In addition, the process is not only extremely quick, it is also continuous. Traditional manufacture of substrates is batch-based and is therefore much more time-consuming,” adds Samuelson in the release.
The research team has gone so far as to actually build a prototype manufacturing system consisting of a series of ovens that will cure the nanowires to create variants such as p-n diodes. With this focus on engineering the fabrication techniques, the researchers seem to be really pushing for a solar cell prototype in two years.