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Andreas Markwitz: Nanowhiskers

On the left of Andreas is an Atomic Force Microscope (AFM) image of silicon nanowhiskers. Thex scale goes from 0 to 10 micrometers. On his right is a vacuum chamber similar to the one nanowhiskers are grown in.

Some might say that the age of the explorer has long passed away. Hardly a spot on the Earth remains that has not been subjected to the analytic human gaze. Some look to outer space to seek the excitement of unknown worlds but Andreas Markwitz has found the frontier of scientific understanding right underneath his nose. It was eleven years ago in 1993, while studying for his PhD at Frankfurt University that Andreas discovered the existence of Silicon nanowhiskers. When a wafer of silicon is annealed (slowly heated) under high vacuum conditions, the structure of the silicon rearranges itself. Tiny little pillars of silicon around 10nm high self-assemble all over the surface so that one square cm might have four billion nanowhiskers. The tips of the nanowhiskers are very sharp so it only takes a small field to induce the whiskers to emit electrons and the incredible density of features on these materials makes them perfect for using in low field electron emitters that would be small and light enough to be used in portable devices. The goal of research at GNS is to use the nanowhiskers as cathodes in portable ‘sniffers' ( mass spectrometers for analyzing gases ). It is applications such as this combined with a passion for discovery that propels Andreas in his long voyage into the un-chartered nano-worlds.

Andreas emphatically insists that research in this area is a very long and complicated process. The thing about nanotechnology is that the electrical, optical and mechanical properties of these ultra small materials are entirely different from anything studied before. Interactions between individual atoms and electrons and quantum effects come into play with extraordinary and unpredictable outcomes. This makes research in the area exciting but longwinded. Both in understanding and equipment they have had to start from scratch. After having found the nanowhiskers at the beginning of the research, it is just in the last few months that the team have proved they work as electron emitters.

How are Silicon Nanowhiskers made?

Below is a brief outline of the process they use at GNS to fabricate nanowhiskers:

  • Cleaning: The Si(100) substrate (either p-type or n-type) is cleaned to remove any surface dust or dirt. The native oxide layer is not removed, so this stage is very simple and quick.
  • Ion Implantation: In some cases ions are implanted into the surface (the effect of ion implantation is discussed later in the article).
  • Annealing: The substrate is inserted into a high vacuum chamber and heated using a raster scanned electron beam that scans over the substrate at high speed, heating it in a homogeneous way. The sample is slowly raised to a peak temperature of 900- 1100蚓 and held there for a few seconds then cooled again. The substrate is removed from the vacuum at room temperature and Voila! It is covered with nanowhiskers.

But how do nanowhiskers form? This is a question Andreas and the team spent 5 years wrestling with. In very simple terms this is what they came up with.

Nanowhisker formation occurs in a two-stage process. Firstly, decomposition of the native oxide layer: Voids form in the oxide film exposing clear silicon underneath. Silicon from the voids reacts with the oxide layer and the voids grow laterally until they coalesce. This causes a roughening of the surface and an uneven surface potential energy. After complete oxide adsorption Si species begin to migrate across the surface to kinetically favoured sites or nucleating islands. Island number and size grows as the annealing continues resulting in the growth of crystalline pillars.

On the left is an AFM image of silicon nanowhiskers from above. The image on the right shows a profile of whiskers in the circled regions.

Ion Implantation

It was discovered about three years ago that implanting nitrogen ions into the silicon surface prevented the formation of nanowhiskers. The reason for this was a total mystery. They've tried implanting other inert gases, thinking that the effect might be the same. It wasn't! When neon ions were implanted they formed gas bubbles underneath the surface that exploded to form wide shallow craters up to 10痠 in diameter. Implanting oxygen had a similar effect. Next they tried implanting carbon ions. To their surprise half the carbon came to the surface, bonded with silicon and formed SiC nanoboulders. A whole branch of research has sprung up to investigate nanoboulders. These little critters could also have huge potential in technology. Andreas's team has been researching the effects of ion implantation and annealing in collaboration with the University of Frankfurt/M, Germany.

The GNS Nanofabrication facility

Andreas and his team of six scientists at GNS (Geological and Nuclear Sciences) have designed and created a new fabrication tool for the production of nanocrystalline materials. Their technology meets a growing need for more efficient nanofabrication methods. It is simple, cheap, fast, compatible with conventional methods and materials and allows for precise control of geometries of nanostructured chips. When it is completed the set-up will include two high vacuum/ultra high vacuum ion implanters (so that two different ion beams can be applied simultaneously) and the 3 MV particle accelerator at GNS for ion beam analysis (IBA) that can be used to determine the ion implantation depth and concentration. This new facility will allow for more penetrating research into the effects of ion implantation. The substrates are annealed in a high vacuum chamber with a raster electron beam. This is all set up and working. Because the research at GNS is forging into new territory they have had to design and make all their equipment from raw materials.

Parts of the the nanofabrication facility. Unfortunately it is not as photogenic as it is functional.

GNS and the world nanotechnology scene

GNS is leading the world with its new nanofabrification methods and facilities. They have five patents; three on the fabrication of nanowhiskers, one on the fabrication of nanoboulders and one unrelated. Andreas has published 130 papers. GNS works in collaboration with research groups in other universities; Frankfurt/M, Cambridge and Canterbury (in New Zealand). A team from FZ Rossendorf in Germany does high resolution X-TEM (cross sectional transition electron microscopy) on nanowhiskers made at GNS to analyse their structure. Once the GNS team have reached their goal of creating a portable sniffer they have the option of either setting up a spin off company for further production (as Jeff Tallon and his team at IRL have done with superconductors) or sell the patent, either of which will be a healthy boost for the New Zealand economy.