Most of us are familiar with soldering in plumbing – for example, when joining two pieces of metal piping together. The technique involves introducing a filler metal (the solder) in between the two metals to be joined. The solder has a lower melting point than the two metals, and acts as a sort of “glue”.

The method is of course also routinely employed in microelectronics but unlike metals, there are no real techniques to join semiconductor pieces together without disrupting the semiconductors’ properties at the soldered junction.

The electronic properties of semiconductor interfaces are much more sensitive to impurities and structural defects than are those at metal-metal junctions. Metal-metal contacts continue to behave as Ohmic conductors but Schottky barriers form at semiconductor-semiconductor interfaces because of trapped charged carriers or when Fermi levels misalign. Impurities are introduced when making electrical contacts on nanostructures using techniques like electron beam lithography, for instance. As well as being expensive and time-consuming, the resists and solvents used in such processes leave residues that contaminate samples.

Chalcogenidometallates could join II-VI, IV-VI and V-VI semiconductors

To avoid these problems, a “bottom-up” approach, like soldering, would be better. But, the structure and composition of an ideal semiconductor solder needs to very closely match that of the semiconductors it is bonding. A team led by Dmitri Talapin of the University of Chicago is now saying that chalcogenidometallates based on cadmium, lead and bismuth might just fit the bill here, and that these materials could be used to join many technologically important II-VI, IV-VI and V-VI semiconductors.

In one experiment, the researchers showed that a Na2Cd2Se3 solder could bond CdSe nanocrystals without destroying the semiconductor’s electronic properties. They confirmed this by making field-effect transistors (FETs) from soldered CdSe nanocrystals and measuring their electron mobilities – which they found to be as high as 210 cm2/Vs. This value is the best yet for any solution-processed inorganic semiconductor FET and proves that the grain boundaries in the materials do not contain many charge transport “bottlenecks”. In another experiment, the team succeeded in soldering CdTe crystals with Na2CdTe2.

New solders can facilitate semiconductor processing, so we can handle these materials in the same way as we do ceramics and polymers, Talapin tells For example, the solders allow us to consolidate semiconductor powders into three-dimensional shapes using moulds and could thus help us make better printable electronics and optoelectronics, as well as mouldable or 3D printable photovoltaic and thermoelectric materials.

Full details about the research are reported in Science DOI: 10.1126/science.1260501.