Porous Silicon

surface science / reaction dynamics / laser photochemistry / laser-surface interactions / nanotechnology / physical chemistry / chemical physics

Photochemical & Chemical Modification of Si and Porous Si

HF+Si Rxn Mechanism

Porous silicon is silicon with lots of holes in it and has properties that are much different than that of bulk crystalline or polycrystalline silicon. There is great interest in obtaining light from nanoscale Si [see links ( 1 ), ( 2 ) and ( 3 )] structures for optoelectronic, nanoelectronic and biomedical applications, even fuel cells. I've been very interested in several aspects of porous silicon formation including the mechanism by which HF attacks the silicon and how etching conditions affect the optical and structural properties of the resulting thin film. We done work on the mechanism of etching, the photoluminescence of porous silicon, the structure of the film after laser-assisted etching or stain etching, deposition of other photoluminescent material during etching, and new methods to measure etch rates and porosity. To understand etching in acidic fluoride solutions it is also important to consider the nature of the highly non-ideal fluoride solutions and their composition.

HF (hydrofluoric acid) is rather unreactive with a silicon surface because once it removes any oxide layers, it leaves the surface hydrogen terminated with one H atom bound to each surface Si atom. To initiate further reactivity and etching this H-terminated surface has to be activated in one of three ways: (i) electrochemically by applying a sufficient voltage (anodic etching), (ii) electrochemically by adding an oxidant with a sufficient standard electrode potential (stain etching), or (iii) photoelectrochemically by irradiating the silicon with a laser or lamp with a sufficiently short wavelength to excited electrons to the conduction band. In all three cases, a vacancy is formed in the silicon valence band. A vacancy is also called a hole. In other words, an electron is removed from near one of the Si–H bonds. This activates the previously passivated Si–H bond, making it susceptible to attack by the fluoride solution. After initiation, the etching of one silicon atom proceeds very rapidly according to the mechanism outlined to the right.

We've also investigated reactions that occur either during or after porous silicon formation. Etching results in the formation of the hexafluorosilicate anion, (SiF₆)^2–. When etching is performed in the presence of cations that create an insoluble salt such as K⁺, Rb⁺ and Cs⁺, a hexafluorosilicate salt can precipitate out on the porous silicon surface. When such a layer is irradiated with ultraviolet light, the layer emits light. This process is called photoluminescence. After a porous silicon layer is formed it is initially H-terminated. Irradiation of porous silicon with an ultrafast 120 fs duration infrared laser pulse in the presence of ethanol leads to a reaction between the por-Si surface and the ethanol. The attachment of ethanol to the porous silicon changes the optical properties of the por-Si layer. The photoluminescence goes away but the reacted thin film is very efficient at facilitating the combination of two ultrafast infrared photons into a new visible photon - a process known as sum frequency generation.

Stain etching in action

To make microporous (also sometimes called nanoporous) photoluminescent porous silicon we primarily use stain etching. In stain etching an oxidant is mixed with fluoride to form an aqueous solution that spontaneously produces porous silicon once a silicon crystal has been dipped in it. We have already shown that the fluoride can be provided not only by HF but also by NH₄HF₂. Together with Maggie Dudley, we are now investigating the role of the oxidant and how it can be used to control both the photoluminescence spectrum and the morphology of the por-Si film. In addition, Maggie is studying whether surfactants can be added to avoid the formation of bubbles. Bubbles cause problems in that they cover the surface. This blocks access to the surface by the solution and results in inhomogeneities in the porous thin film.

MED at paintball

Working with porous silicon has its advantages. Not only is there a lively community of researchers in the field but we also like to meet at the beach in Tenerife or Catalonia or sometimes on a volcano such as El Teide in Tenerife. In the tradition of "porous silicon researchers only have their photos taken outdoors," Maggie steadfastly refuses to have a lab photo taken but I did manage to get a photo during paintball. For which she repaid me by shooting me between the eyes.

For further information on related topics, try these sites:

Labs working in Surface Science, Nanotechnology and Catalysis

Porous Semiconductors Science & Technology

Porous silicon as a biomaterial

Optoelectronics with porous silicon at Univ of Bath Centre for Photonics and Photonic Materials

Sailor Research Group and an introduction to porous silicon

Porous silicon as a bionanomaterial at the Univ of Rochester

Fauchet Research group

Biosilicon form pSivida

Go to the West Chester University Chemistry Home Page

Go to the West Chester University Home Page

Maintained by kkolasinski@wcupa.edu

Last Modified: 28 November 2007