New material could act as a "nanofridge" for microchips

October 9, 2008: Researchers at Spain's UAB Department of Physics and the Barcelona Institute of Materials Science (ICMAB-CSIC) have developed a new material based on supernets formed with two alternative layers, one made of silicon (Si) and the other of germanium (Ge) nanocrystals (quantum dots). The discovery could pave the way for the creation of nanofridges for common semiconductor devices, given that the structure is compatible with silicon technology.
The project proposes to place the quantum dots in an uncorrelated fashion on consecutive layers by introducing a small sub-layer of carbon between each layer of silicon and Ge nanodots. The dots on one layer would not be vertically aligned with those of the lower layer, thus hiding the information of the quantum dots found on the lower levels.

Sketch of the samples containing ordered and disordered nanocristal structures. The represented materials are silicon (blue), germanium (orange), and carbon (black). (Source: UAB)

The result of the uncorrelation between consecutive layers is the reduction in thermal conductivity, since it becomes more difficult to transport heat perpendicularly from the multilayers. Researchers were able to prove that this reduction reached a factor in excess of two when compared to structures with a vertical correlation of dots. This could greatly influence the design of new materials with improved thermoelectric characteristics.
Along with creating nanofridges, Ge-based structures also could be used in high-temperature applications, such as in recovering heat generated in combustion processes and converting it to electrical energy. "The results of this work have implications for the development of highly efficient thermoelectric materials and on-chip nanocooling devices," the scientists claim.
The research appeared in the July 7 issue of Applied Physics Letters.

Thermical conductivity for the ordered sample (blue) and for the disordered one (red). Straight line corresponds to the computation following the theoretical model. The one on the right corresponds to the acoustic phonons Raman spectrum that take part on the heat transportation, measured on the same samples. (Source: UAB)