Advances in silicon transistor technology have sustained Moore's Law for decades, but the physical limits of silicon will require alternative transistor materials for progress to continue. Group III-V semiconductors show potential, but they are rarer than silicon, and cannot be mixed as easily to form tunable alloys. Two researchers in the US are predicting how new tricks might be coaxed from an old alternative—silicon–germanium.

SiGe has been studied since the 1950s, and has already been adopted in mixed-signal and analogue circuits due to its narrower band gap and shorter response time than silicon. Furthermore, says lead researcher Gregory Guisbiers, "SiGe can be easily integrated into the existent Si technology because SiGe is totally miscible with Si all over its composition range. This allows the mismatch stress with the silicon wafer to be controlled by tuning the composition of the alloy."

But if SiGe is to take over where pure silicon transistors finish, engineers will need a better understanding of how the alloy behaves at the scales at which future generations of chips will operate. To achieve this, Guisbiers and his student at the University of Texas at San Antonio, Brandon Bonham, used the principles of nanothermodynamics to predict how particle size and shape affect SiGe's thermal and optical properties, as well as the miscible–immiscible phase transition. As Guisbiers explained to nanotechweb.org: "Having the phase diagram in hand will tell you which temperature range should be used during the synthesis or post-annealing treatment in order to get a randomly alloyed nanoparticle or a Janus nanoparticle."

The shape controls the size effect

One finding from the study was expected: moving from the bulk material to a nanoparticle causes the phase diagram to shift downwards, and the smaller the particle, the greater the effect. This is because nanoparticles, with their greater surface-to-volume ratios, are dominated by surface effects. One additional consequence is that smaller particles of SiGe have a wider band gap for a given composition.

Less expected was the discovery that the magnitude of the size effect was controlled strongly by the shape of the nanoparticle, and in particular by the number of facets. Cubic and tetrahedral nanoparticles showed a much greater downward shift in their phase diagrams as particle size decreased—and a correspondingly large increase in their band gap. More sub-spherical shapes, however, were less affected by particle size.

Yet more surprising was that the miscible–immiscible phase transition stayed largely constant across all nanoparticle shapes and sizes, barely differing from that of the bulk material. "Consequently," says Guisbiers, "the region where the SiGe (randomly mixed) can be synthesized is reduced at the nanoscale compared to the bulk scale. People may think that the phase diagram was just going to be shifted downward to a lower temperature in the same manner for all the phase transitions, but this is not the case."

Band-gap tuning

In the near term, Guisbiers and Bonham's new results will help researchers synthesize SiGe nanoparticles with sizes, shapes and compositions relevant to their experiments. Eventually, transistors consisting of single SiGe nanoparticles with specially tuned electrical properties may allow Moore's Law to continue to hold, even when the possibilities of pure-silicon have been exhausted.

Full details of the work can be found in Nanotechnology.