This shows nano-scale rods made of germanium doped with tin. Photo: TU Wien.
This shows nano-scale rods made of germanium doped with tin. Photo: TU Wien.

When you bake a cake, you can combine the ingredients in almost any proportions, and they will still always be able to mix together. Things are a little more complicated in materials chemistry.

Often, the aim is to change the physical properties of a material by adding a certain proportion of an additional element, a process known as doping, but it isn't always possible to incorporate the desired element into the crystal structure of the material. Scientists at TU Wien in Austria have now created a new method for doping germanium with desired foreign atoms, creating new materials with significantly altered properties. They describe the method in a paper in ACS Nano.

"Incorporating foreign atoms into a crystal in a targeted manner to improve its properties is actually a standard method," says Sven Barth from the Institute of Materials Chemistry at TU Wien. Modern electronics are based on semiconductors with certain additives, such as silicon crystals doped with atoms of phosphorus or boron.

In theory, the semiconductor germanium is supposed to fundamentally change its properties and behave like a metal when doped with a sufficient amount of tin. In practice, however, this has proved difficult to achieve.

One possible option is simply to melt the two elements, thoroughly mix them together in liquid form and then let them solidify, as has been done for thousands of years to produce simple metal alloys. "But in our case, this simple thermodynamic method fails, because the added atoms do not efficiently blend into the lattice system of the crystal," explains Barth. "The higher the temperature, the more the atoms move inside the material. This can result in these foreign atoms precipitating out of the crystal after they have been successfully incorporated, leaving behind a very low concentration of these atoms within the crystal."

Barth's team has therefore developed a new approach that links particularly rapid crystal growth with very low temperatures. In this process, the correct proportion of foreign atoms is continuously incorporated as the crystal grows.

The crystals grow in the form of nano-scale threads or rods, and at considerably lower temperatures than before, in the range of just 140–230°C. "As a result, the incorporated atoms are less mobile, the diffusion processes are slow, and most atoms stay where you want them to be," says Barth.

Using this method, Barth and his team have been able to incorporate up to 28% tin and 3.5% gallium into germanium. This is considerably more than was previously possible with the conventional thermodynamic combination of these materials – by a factor of 30 to 50.

This opens up new possibilities for microelectronics. "Germanium can be effectively combined with existing silicon technology, and also the addition of tin and/or gallium in such high concentrations offers extremely interesting potential applications in terms of optoelectronics," says Barth. The materials could be used to produce infrared lasers, photodetectors or innovative LEDs that emit in the infrared range, since these additives significantly change the physical properties of germanium.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.