A brief overview of
Sept. 29, 2006
Pictures from http://www.ghuth.com/vision/?p=7, http://bios.ewi.utwente.nl/research/micronanofluidics/solvingmicromachining.doc/index.html and
James Gole displays a treated silicon wafer illuminated by
ultraviolet (purple) light. The orange-red photoluminescence from
the porour silicon is clearly visible.
• What is porous silicon?
• Why is it interesting?
• How is it made?
• Features and controllability
What is porous silicon?
• Electrochemically etched silicon
– Pores : nm µm sized
– Pore nucleation – many models [1,2]
Suggests defects near surface play primary
• Discovered in 1956 by Arthur Uhlir at
Bell Labs, doing some electro-
Type of porous Si
Corresponding size regime
for dominant porosity (nm)
George Marsh, Materials Today January 2001
Why is it interesting?
• CMOS compatibility
– light emission in silicon!
– Large surface area to volume ratio
• Surface area ~200-1000 m2/cm3 [4,7]
– nanometer size effects
• Increases bandgap [2,3]
Peak wavelength (nm)
B. Delley and E. F. Steigmeier, Size dependence of band gaps in
silicon nanostructures, Appl. Phys. Lett. 67 (16), 16 October 1995,
As ‘d’ decreases, Eg increases
* For fun & comparison, 1g activated carbon ~ 400-1500 m2 surface area
Why is it interesting? (cont’d)
• Optoelectronic properties a function of skeleton size
• Feature sizes are dynamically controllable
• Seems to be biologically compatible
The range of tunable pore sizes (2 to 2000 nm) in porous silicon, size of a small DNA fragment ~10-30 nm nanometers, proteins
~100-nm, and bacteria and cells ~several microns diameter.
slow release of drugs
sensitive biosensor for proteins, antigens, and DNA, and it can be modified w