Fine, porous structures with tiny holes look like a nano-sized sponge can be generated in semiconductors. These structures open up new possibilities for the realization of tiny sensors or unusual optical and electrical components.
Experiments with porous structures made of silicon have already been conducted. Researchers from TU Wien have successfully developed a method for the controlled manufacture of porous silicon carbide. Silicon carbide is significantly better than silicon. Silicon carbide has greater chemical resistance, giving it the ability to be used in biological applications without any additional coating needed.
The researchers used a special mirror that selectively reflects different colors of light. The mirror, integrated into a SiC wafer, created thin layers with a thickness of 70 nm and different degrees of porosity. This demonstrated the potential of the new silicon carbide semiconductors.
“There is a whole range of exciting technical possibilities available to us when making a porous structure with countless nanoholes from a solid piece of a semiconductor material," said Markus Leitgeb from the Institute of Sensor and Actuator Systems at TU Wien.
Leitgeb developed the new material processing technology as part of his dissertation with Professor Ulrich Schmid in cooperation with CTR Carinthian Tech Research AG and sponsored by the Competence Centers for Excellent Technologies (COMET) program.
"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," said Leitgeb.
This can be useful in sensor technology. For example, the refractive index of tiny quantities of liquid can be measured by using a porous semiconductor sensor allows for a reliable distinction between different liquids.
Another option from a technical and application-oriented perspective is to make certain areas of the SiC wafer porous in a highly localized way before depositing a new SiC layer over the porous areas and causing the latter to collapse in a controlled manner. This technique produces microstructures and nanostructures which can play a key role in sensor technology.
In all of these techniques, it is crucial that the appropriate starting material is selected.
"Until now, silicon has been used for this purpose, a material with which we already have a lot of experience," said Professor Schmid.
Silicon has significant drawbacks. Under harsh environmental conditions, like extreme heat or alkaline solutions, structures made of silicon are attacked and rapidly destroyed. Because of this, silicon sensors are not suitable for biological or electrochemical applications.
Attempts have been made at TU Wein to achieve something similar with the semiconductor silicon carbide, which is biocompatible and considerably more robust from a chemical view. Some special tricks are required in order to produce structures from silicon carbide.
First, the surface is cleaned and partially covered with a thin layer of platinum. The silicon carbide is immersed in an etching solution and exposed to UV light to initiate oxidation processes. This causes a thin porous layer to form in the areas that are not coated in platinum. An electrical charge is also applied in order to precisely 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," explains Leitgeb. "By adjusting the electrical charge during this process, we can control what porosity we want to have at a given depth."
This allowed the researchers to produce a complex, layered structure of silicon carbide layers with higher and lower levels of porosity, which separated from the bulk material by applying a high voltage pulse. The thickness of the individual layers can be selected in a way that the layered structure reflects certain light wavelengths particularly well or allows certain light wavelengths to pass through. This resulted in an integrated color-selective mirror.
"We have thus demonstrated that our new method can be used to reliably control the porosity of silicon carbide on a microscopic scale," said Ulrich Schmid. "This technology promises many potential applications, from anti-reflective coatings, optical or electronic components and special biosensors, through to resistant supercapacitors."
The paper on this research was published in the journal APL Materials.