Most of us are familiar with the infamous saying, “you can’t teach an old dog new tricks.” While that may be true for many scenarios, it doesn’t necessarily hold true for lab processing. It is possible to get your old equipment to turn out new materials by tweaking process parameters on existing tools.
Currently, there is renewed interest in the academic world around the vapor-liquid-solid (VLS) growth mechanism for nano-scale semiconductors.
This mechanism was first described by researchers at Bell Labs in the 1960s who used it to explain the strange formation which was sometimes seen in thin film deposition reactors of thin wispy strands of semiconductors. These researchers understood that the deposition was brought about by the presence of metal particles in the reactors which would act as catalysts to the growth of the strands.
Trends towards building and understanding components on the nanometer scale, particularly at the confluence of molecular biology and integrated circuit fabrication, has re-kindled interest in the VLS growth mechanism. Labs around the world are attempting to harness this method of growing high purity, single crystal semiconductor “nanowires” for diverse applications ranging from biomolecule identification to the growth of transistors.
Recently, WTC’s Microfabrication Lab has developed VLS growth techniques to form Silicon nano-cloth films. In this method, a silicon wafer is densely coated with tiny crystals of catalyst. The catalyst is formed through an ultra-thin film deposition followed by an annealing process to form discrete nano crystals of catalysis. After this, the VLS growth mechanism is performed inside a chemical vapor deposition reactor. The result is a dense cloth of silicon wires or fibers randomly intertwined. By selectively depositing a catalyst, it is possible to control where on the wafer the nanowires form, even to the point where a single wire can be grown. Work is being done to characterize this material as grown in the lab, and to develop a robust selective growth techniques.
The material has incredibly diverse potential applications that take advantage of the incredible surface area to volume ratio, such as bio sensors. One of the problems in biomolecule detection is the ability to generate a sufficient signal to detect the presence of low concentrations of substances in small sample sizes. By having a higher surface area in a smaller space it is possible to increase signal while keeping noise constant, thus lowering the detection limit, even for incredibly small sample sizes.
Researchers have begun looking into the use of these nano-semiconductors in the fabrication of high efficiency solar cells. Through in situ doping, it is possible to grow nanowires to form the pn-junctions which are at the heart of the solar cell. Together with the very high optical absorption of this material and the ability to fabricate high densities of junctions tailored to the full spectrum of solar radiation,, photovoltaic researchers are intrigued about the possibility of forming new types of high-efficiency, cost-effective solar cells with nanowires.
There is also great interest in using this method to grow atomically sharp nanoneedles to be used as very fine instruments to probe the components of cells which are still living, or for imaging materials at the atomic scale. It may be possible to form conducting nanowires long enough to puncture a living cell, but small enough that they would be able to probe a single organelle, or image a single molecule.
Ultimately, the WTC Microfab Lab’s goal is to provide our customers with the tools they need to perform cutting-edge research. By developing new processes on our existing systems, we expand the breadth of what the lab has to offer and get maximum value from the equipment and processes we’ve already invested in. We keep costs low and avoid capital expenses of new equipment. So, in this case, it may just be possible to teach that old dog new tricks after all.
Related WTC links: