Porous silicon

from Wikipedia, the free encyclopedia

Porous silicon (abbreviated pSi ) is a form of the chemical element silicon . The eponymous specialty is the nanoporous structure, i.e. the pores have a size in the range from 10 −9 to 10 −7  m. This results in a particularly high surface-to-volume ratio in the range of up to 500 m 2 / cm 3 . Due to its special optical and electrical properties, porous silicon is suitable for the manufacture of solar cells and batteries .

history

Porous silicon was discovered in 1956 by Arthur Uhlir Jr. and Ingeborg Uhlir, who were then working at Bell Laboratories in the USA on a process that could be used to shape and polish the surface of silicon and germanium . They discovered that under suitable conditions a thick black, red or brown film forms on the surface of the material. However, these results were only mentioned in a laboratory note and not pursued further.

Three decades later suspected Leigh Canham , who at this point in the Defense Research Agency worked in England, in porous silicon quantum - confinement effects, which in 1990 could be verified experimentally. Only then was the interest of science in the non-linear optical and electrical properties of the material aroused.

Manufacturing

Anodizing

One possibility of producing porous silicon is anodic oxidation . Platinum is typically used as the cathode material, silicon as the anode and hydrogen fluoride (HF) as the electrolyte. While the application of a direct current leads to a more homogeneous layer of porous silicon, alternating current is more suitable for the formation of silicon wafers with a thickness of over 50 μm. The formation of hydrogen gas can result in greater inhomogeneities in this process. To counteract this, ethanol (at least 15%) is added to the electrolyte . This can significantly increase the homogeneity.

etching

In addition, porous silicon can be made by etching with hydrofluoric acid (HF), nitric acid (HNO 3 ) and water. This process is particularly attractive because of its simplicity and the wide availability of the necessary materials. This method is also very useful in the production of particularly thin pSI films; Layer thicknesses of only 25  Angstroms can be produced in this way.

dry

In the case of simple drying through evaporation, the capillary tension, which is proportional to the curvature of the interface, causes cracks to appear above a certain layer thickness. For this reason, processes have been developed which are intended to minimize the risk when drying pSi. Supercritical drying is considered to be the most effective drying technique, since in the course of this the interface disappears completely, but it is relatively expensive. With pentane drying, the water is first replaced by pentane , which has a lower surface tension than water. During the subsequent drying process, only slight tension occurs.

properties

Explosiveness

In 2001 a working group at the Technical University of Munich discovered by chance that hydrogenated pSi soaked with liquid oxygen is highly explosive and that its explosive power exceeds that of TNT . Other oxidizing agents avoid the need for very low temperatures and make handling safer.

Optical properties

The refractive index and the resulting optical properties of a material depend, among other things, on the porosity and the medium within the pores. The refractive index of porous silicon can therefore differ significantly from that of other types of silicon.

Individual evidence

  1. Porous silicon in battery technology . Heise.de. Retrieved September 3, 2010.
  2. Porous silicon in the photovoltaic industry ( Memento from August 18, 2011 in the Internet Archive ). Institute for Solar Energy Research Hameln. Retrieved September 3, 2010.
  3. LT Canham: A glowing future for silicon . In: New Scientist . 1993.
  4. ^ Friedemann Völklein, Thomas Zetterer: Practical knowledge of microsystem technology . 2nd Edition. Vieweg + Teubner, 2006, ISBN 3-528-13891-2 , pp. 17 .
  5. Patent EP588296A1 : Method for manufacturing a component with porous silicon. Published March 23, 1994 .
  6. JL Coffer: Porous silicon formation by stain etching. In: Properties of Porous Silicon. Canham, LT, Institution of Engineering and Technology, London 1997, pp. 23-28.
  7. M. Thönissen, Forschungszentrum Jülich: Spectroscopic characterization of layers and layer systems made of porous silicon with regard to optical and optoelectronic applications , 1999.
  8. Munich researchers accidentally find super-explosives made of porous silicon. On: Wissenschaft.de from August 7, 2001.
  9. refractive index of pSi . Refractive Index Database. Retrieved September 3, 2010.