Porous silicon

Porous Silicon (pSi) is a form of the chemical element silicon which has an introduced nanoporous holes in its microstructure, rendering a large surface to volume ratio in the order of 500m²/cm³.

History

Porous silicon was first discovered by accident in 1956 by Arthur Uhlir Jr. and Ingeborg at the Bell laboratories in US. At the time, Ulhir and Ingeborg were in the process of developing a technique for polishing and shaping the surfaces of silicon and germanium. However, it was found that under several conditions a crude product in the form of thick black, red or brown film were formed on the surface of the material. At the time, the findings were not taken further and were only mentioned in Bell’s labs technical notes. [Canham 1993 A glowing future for silicon, New Scientist. Available from: http://www.newscientist.com/article/mg13818683.800.html ]

Despite the discovery of porous silicon in the 1950’s, the scientific community was not interested in porous silicon until the late 1980’s. At the time, Leigh Canham – whilst working at the Defence Research Energy in England – reasoned that the porous silicon may display quantum confinement effects [Sailor Research Group 17 February 2003, Introduction to Porous Si, Sailor research group at UCSD, Department of Chemistry, University of California. Available from: http://chem-faculty.ucsd.edu/sailor/research/porous_Si_intro.html ] . The intuition was followed by successful lab results published in the 1990. In the published experiment, it was revealed that silicon wafers can emit light if subjected to electrochemical and chemical dissolution.

The published result stimulated the interest of the scientific community in its non-linear optical and electrical properties. The growing interest was evidenced in the number of published work concerning the properties and potential applications of porous silicon. In an article published in 2000, it was found that the number of published work grew exponentially in between 1991 and 1995 [Parkhutik V. 2000, 'Analysis of Publications on Porous Silicon: From Photoluminescence to Biology', Journal of Porous Materials, Vol 7, Issue 1-3, pp363-6.]

In 2001, a team of scientists at the Technical University of Munich inadvertently discovered that hydrogenated porous silicon reacts explosively with oxygen at cryogenic temperatures, releasing several times as much energy as an equivalent amount of TNT, at a much greater speed (an abstract of the study can be found below). Explosion occurs because the oxygen, which is in a liquid state at the necessary temperatures, is able to oxidize through the porous molecular structure of the silicon extremely rapidly, causing a very quick and efficient detonation. Although hydrogenated porous silicon would probably not be effective as a weapon, due to its functioning only at low temperatures, other uses are being explored for its explosive properties, such as providing thrust for satellites.

Fabrication of Porous Silicon

Fabrication of porous silicon may range from its initial formation through stain-etching or anodization cell, drying and storage of porous silicon and surface modification needed.

Formation of Porous Silicon by Anodization

One method of introducing pores in silicon is through the use of an anodization cell. A possible anodization cell employs platinum cathode and silicon wafer anode immersed in Hydrogen Fluoride (HF) electrolyte. Corrosion of the anode is produced by running electrical current through the cell. It is noted that the running of constant DC current is usually implemented to ensure steady tip-concentration of HF resulting in a more homogenous porosity layer although pulsed current is more appropriate for the formation of thick silicon wafers bigger than 50µm [Halimaoui A. 1997, ‘Porous silicon formation by anodization, in Properties of Porous Silicon. Canham, LT, Institution of Engineering and Technology, London, pp. 12-22.]

It was noted by Halimaoui that hydrogen evolution occurs during the formation of porous silicon.

“When purely aqueous HF solutions are used for the PS formation, the hydrogen bubbles stick to the surface and induce lateral and in-depth inhomogeneity”

The hydrogen evolution is normally treated with absolute ethanol in concentration exceeding 15%. It was found that the introduction of ethanol eliminates hydrogen and ensures complete infiltration of HF solution within the pores. Subsequently, uniform distribution of porosity and thickness is improved.

Formation of Porous Silicon by Stain Etching

It is possible to obtain porous silicon through stain-etching with hydrofluoric acid, nitric acid and water. A publication in 1957 revealed that stain films can be grown in dilute solutions of nitric acid in concentrated hydrofluoric acid. [Archer RJ. 1960 ‘Stain Films on Silicon’. The Journal of physics and chemistry of solids. Vol 14. pp. 104-10.] Porous silicon formation by stain-etching is particularly attractive because of its simplicity and the presence of readily available corrosive reagents; namely Hydrogen Nitride (HNO3) and Hydrogen Fluoride (HF). Furthermore, stain-etching is useful if one needs to produce a very thin porous Si films. [Coffer JL.1997, ‘Porous silicon formation by stain etching, in Properties of Porous Silicon, Canham, LT, Institution of Engineering and Technology, London, pp. 23-28.] A publication in 1960 by R.J. Archer revealed that it is possible to create stain films as thin as 25Å through stain-etching with HF-HNO3 solution.

Drying of Porous Silicon

Porous silicon is systematically prone to presence of cracks when the water is evaporated. The cracks are particularly evident in thick or highly porous silicon layers. [Bellet D. 1997, ‘Drying of porous silicon’, in Properties of Porous Silicon, Canham, LT, Institution of Engineering and Technology, London, pp. 38-43.] The origin of the cracks has been attributed to the large capillary stress due to the minute size of the pores. In particular, it has been known that cracks will appear for porous silicon samples with thickness larger than a certain critical value. Bellet concluded that it was impossible to avoid cracking in thick porous silicon layers under normal evaporating conditions. Hence, several appropriate techniques have been developed to minimize the risk of cracks formed during drying.

Supercritical Drying
Supercritical drying is reputed to be the most efficient drying technique but is rather expensive and difficult to implement. It was first implemented by Canham in 1994 and involves superheating the liquid pore above the critical point to avoid interfacial tension.

Freeze Drying
Freeze drying was first implemented by Gruning and Yelon in 1995. After the formation of porous silicon, the sample is frozen at a temperature of around -500F and sublimed under vacuum.

Pentane Drying
The technique uses pentane as the drying liquid instead of water. In doing so the capillary stress is reduced because pentane has a lower surface tension than water.

Slow Evaporation Rate
Slow evaporating technique can be implemented following the water or ethanol rinsing. It was found that slow evaporation decreased the trap density

Surface Modification of Porous Silicon

The surface of porous silicon may be modified to exhibit different properties. Often, freshly etched porous silicon may be unstable due to the rate of its oxidation by the atmosphere or unsuitable for cell attachment purposes. Therefore, it can be surface modified to improve stability and cell attachment

Surface Modification Improving Stability

Following the formation of porous silicon, its surface is covered with covalently bonded hydrogen. Although the hydrogen coated surface is sufficiently stable when exposed to inert atmosphere for a short period of time, prolonged exposure render the surface prone to oxidation by atmospheric oxygen. The oxidation promotes instability in the surface and is undesirable for many applications. Thus, several methods were developed to promote the surface stability of porous silicon.

An approach that can be taken is through thermal oxidation. The process involves heating the silicon to a temperature above 1000 C to promote full oxidation of silicon. The method reportedly produced samples with good stability to ageing and electronic surface passivation [Chazalviel JN. Ozanam F. 1997, ‘Surface modification of porous silicon’, in Properties of Porous Silicon, Canham, LT, Institution of Engineering and Technology, London, pp. 59-65.]

Porous silicon exhibits a high degree of biocompatibility. The large surface area enables bio-organic molecules to adhere well. It degrades to silicic acid, which causes no harm to the body. This has opened potential applications in medicine such as a framework of the growth of bone.

Surface Modification Improving Cell Adhesion

Surface modification can also affect properties that promote cell adhesion. One particular research in 2005 studied the mammalian cell adhesion on the modified surfaces of porous silicon. The research used rat PC12 cells and Human Lens Epithelial (HLE) cells cultured for four hours on the surface modified porous silicon. Cells were then stained with vital dye FDA and observed under fluorescence microscopy. The research concluded that ‘amino silanisation and coating the pSi surface with collagen enhanced cell attachment and spreading’ [Low SP. Williams KA. Canham LT. Voelcker NH. 2006. ‘Evaluation of mammalian cell adhesion on surface-modified porous silicon’, Biomaterials, Vol 27. pp. 4538-46.]

Classification of Porous Silicon

Porosity

‘The porosity is defined as the fraction of void within the PS layer and can be determined easily by weight measurement’.⁴ During formation of porous silicon layer through anodisation, the porosity of a wafer can be increased through increasing current density, decreasing HF concentration and thicker silicon layer. The porosity of porous silicon may range from 4% for macroporous layers to 95% for mesoporous layers. A study by Canham in 1995 found that ‘a lum thick layer of high porosity silicon completely dissolved within a day of in-vitro exposure to a simulated body fluid’ [Canham LT.1995, 'Bioactive Silicon Structure Through Nanoetching Techniques', Advanced Materials, Vol 7, No. 12, pp. 1033-7.] It was also found that a silicon wafer with medium to low porosity displayed more stability. Hence, the porosity of porous silicon is varied depending on its potential application areas.

Pore Size

The porosity value of silicon is a macroscopic parameter and doesn’t yield any information regarding the microstructure of the layer. It is proposed that the properties of a sample are more accurately predicted if the pore size and its distribution within the sample can be obtained. Therefore, porous silicon has been divided into three categories based on the size of its pores; macroporous, mesoporous, and microporous.

Key Characteristic of Porous Silicon

Highly Controllable Properties

Porous silicon studies conducted in 1995 showed that the behaviour of porous silicon can be altered in between ‘bio-inert’, ‘bioactive’ and ‘resorbable’ by varying the porosity of the silicon sample. ¹⁰The in-vitro study used simulated body fluid containing ion concentration similar to the human blood and tested the activities of porous silicon sample when exposed to the fluids for prolonged period of time. It was found that high porosity mesoporous layers were completely removed by the simulated body fluids within a day. In contrast, low to medium porosity microporous layers displayed more stable configurations and induced hydroxyapatite growth.

Bioactive

The first sign of porous silicon as a bioactive material was found in 1995. In the conducted study, it was found that hydroxyapatite growth was occurring on porous silicon areas. It was then suggested by author L.T. Canham that ‘hydrated microporous Si could be a bioactive form of the semiconductor and suggest that Si itself should be seriously considered for development as a material for widespread in vivo applications’¹⁰ Another paper published the finding that porous silicon may be used a substrate for hydroxyapatite growth either by simple soaking process or laser-liquid-solid interaction process [Pramatarova L. Pecheva E. Dimova-Malinovska. Pramatarova R. Bismayer U. Petrov T. Minkovskis N. 2004, ‘Porous silicon as a substrate for hydroxyapatite growth’, Vacuum, Vol. 76. pp 135-8]

Since then, in-vitro studies have been conducted to evaluate the interaction of cells with porous silicon. One particular study in 1995 studied the interaction of B50 rat hippocampal cells with porous silicon and found that B50 cells have clear preference for adhesion to porous silicon over untreated surface. The study indicated that porous silicon can be suitable for cell culturing purposes and can be used to control cell growth pattern [Sapelkin AV. Bayliss SC. Unal B. Charalambou A. 2005. ‘Interaction of B50 rat hippocampal cells with stain-etched porous silicon’. Biomaterial, Vol. 27, pp. 842-6.]

Non Toxic Waste Product

Another positive attribute of porous silicon is the degradation of porous silicon into monomeric silicic acid (SiOH4). Silicic acid is reputed to be the most natural form of element in the environment and is readily removed by kidneys.

The human blood plasma contains monomeric silicic acid at levels of less than 1mg Si/l, corresponding to the average dietary intake of 20-50mg/day. It was proposed that the small thickness of silicon coatings presents minimal risk to a toxic concentration being reached. The proposal was supported by an experiment involving volunteers and silicic-acid drinks. It was found that concentration of the acid rose only briefly above the normal 1mg Si/l level and was efficiently expelled by urine excretion [Canham, L T. 2001, 'Will a chip every day keep the doctor away?' Physics World ]

ee also

* Silicon
* Porosity
* Quantum wire
* Etching (microfabrication)

External links

* [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11497868&dopt=Abstract Abstract of the study of porous silicon as an explosive]

References

* Porous Silicon, edited by Z C Feng & R Tsu, World Scientific (Singapore), 1994 ISBN 981-02-1634-3
* Properties of Porous Silicon, edited by L.T. Canham, Inspec, IEE, London, 1997