The team claim to have developed a method for the controlled manufacturer of porous silicon carbide, which they say is suitable for biological applications – without an additional coating.
To demonstrate the technology’s potential, a mirror – which selectively reflects different colours of light – has been integrated into a SiC wafer. This has been done by creating thin layers with a thickness of approximately 70nm each and with different degrees of porosity.
Markus Leitgeb developed the new material processing technology as part of his dissertation with Professor Ulrich Schmid in cooperation with CTR Carinthian Tech Research and sponsored by the Competence Centers for Excellent Technologies programme.
Leitgeb from TU Wien, said: “There is a whole range of exciting technical possibilities available to us when making a porous structure with countless nano holes from a solid piece of a semiconductor material.
“The porous structure influences the manner in which light waves are affected by the material. If we can control the porosity, this means we also have control over the optical refractive index of the material.”
The TU Wien team believe this could be useful within sensor technology. For example, the refractive index of tiny quantities of liquid can be measured using a porous semiconductor sensor, allowing a reliable distinction between different liquids.
The team are also working on producing microstructures and nanostructures by making certain areas of the SiC wafer porous in a highly localized manner, before depositing a new SiC layer over these porous areas and then causing the latter to collapse in a controlled manner.
The colour-selective mirror works by first cleaning the surface and partially covering it with a layer of platinum. The silicon carbide is then immersed in an etching solution and exposed to UV light. This causes a thin porous layer – initially 1µm thick – to form in these areas that are not coated with platinum. An electrical charge is then applied to set the porosity and the thickness of the subsequent layers. The first porous layer promotes the formation of the first pores when the electrical charge is applied.
“The porous structure spreads from the surface further and further into the interior of the material,” Leitgeb explained. “By adjusting the electrical charge during this process, we can control what porosity we want to have at a given depth.”
According to the team, this made it possible to create a complex layered structure of silicon carbide layers with higher and lower levels of porosity, which was finally separated from the bulk material by applying a high voltage pulse.
The thickness of the individual layers can be selected in a way that allows the layered structure to reflect certain light wavelengths particularly well, or to enable light wavelengths to pass through, resulting in an integrated, colour-selective mirror.
Schmid said: “We have demonstrated that our new method can be used to reliably control the porosity of silicon carbide on a microscopic scale. This technology promises many potential applications, from anti-reflective coatings, optical or electronic components and special biosensors, through to resistant supercapacitors.”
