Image: Cross-sectional scanning electron microscope image of porous Si. This is an n-type, luminescent sample. The pores propagate primarily in the <100> direction of the crystal. The porous layer in this image is approximately 30 microns thick.
Porous silicon was accidentally discovered by Uhlir at Bell Laboratories in the mid 1950s. He was trying to develop a means to electrochemically machine silicon wafers for use in microelectronic circuits. He found that under the appropriate conditions of applied current and solution composition, the silicon did not dissolve uniformly but instead fine holes were produced, which propagated primarily in the <100> direction in the wafer. Since this did not provide the smooth polish desired, these curious results were reported in a Bell labs technical note and the material was pretty much ignored. In the 1970s and 1980s a moderate level of interest arose because the high surface area of porous Si was found to be useful as a model of the crystalline Si surface in spectroscopic studies,[2-5] as a precursor to generate thick oxide layers on Si, and as a dielectric layer in capacitance-based chemical sensors. Then, in the late 1980s, Leigh Canham at the Defense Research Agency in England reasoned that the diaphanous Si filaments generated when the pores become large and numerous enough to overlap might display quantum confinement effects. This intuition turned out to be correct, and the electrochemically etched material was reported to fluoresce with a bright red-orange color.[7, 8] As expected from the quantum confinement relationship, this color is at an energy that is significantly larger than the bandgap energy for bulk Si, which occurs in the infrared region of the spectrum.
With the discovery of efficient visible light emission from porous Si came an explosion of work focused on creating Si-based optoelectronic switches, displays, and lasers. Problems with the materials chemical and mechanical stability and its disappointingly low electroluminescence efficiency led to a waning of interest by the mid 1990s. In the same time period, the unique features of the material- its large surface area within a small volume, its controllable pore sizes, its convenient surface chemistry, and its compatibility with conventional silicon microfabrication technologies-inspired research into applications far outside of optoelectronics. Of particular relevance to our research are the biomedical[10-18] and sensor properties of the material.
Image: Electrochemical etching cell used to prepare porous Si from single crystal Si wafers. The electrolyte used is typically a 3:1 mixture of 48% aqueous HF and ethanol.
click here to access the engineering diagrams for this cell.
|Synthesis of Nanocrystalline Porous Silicon Layers. Porous silicon is generated by etching crystalline silicon in aqueous ethanolic hydrofluoric acid (HF) electrolytes. The open pore structure and large specific surface area (a few hundred m2 per cm3, corresponding to about a thousand times the surface of the polished silicon wafer) make porous Si a convenient material for sensitive detection of liquid and gaseous analytes. The ability to electrochemically tune the pore diameters and to chemically modify the surface[21, 22] provides control over the size and type of molecules adsorbed.
Typical Preparation. This is the preparation used to prepare a Fabry-Perot layer or a photonic crystal. We usually use highly doped p-type, boron doped, polished (100) silicon wafers, with a resistivity of between 0.0005 and 0.001 Ohm-cm and 400 microns thick. We have found Siltronix to be a consistent, high quality manufacturer of these materials. A square approximately 1 cm on an edge is cleaved, and the chip is placed in a Teflon etch cell shown at left (for schematics click here), using a piece of aluminum foil as a back contact and a Viton O-ring to seal the cell. The cell is filled with a 3:1 (v/v) mixture of 48% aqueous HF and absolute ethanol. A loop of platinum wire is immersed in the solution as the counter electrode. An anodic current of approx. 40 mA/cm2 is passed between the aluminum back contact and the platinum counter electrode for 5-15 minutes. For a photonic crystal such as a rugate filter, the current density is modulated between approx 10 and 90 mA/cm2 with a period of approx 10 sec. After completion of the etching, the cell and sample are washed several times with ethanol and dried under a stream of nitrogen. This produces a film that is a few microns thick with pores on the order of several nm in diameter.
Image: Microscopic image of porous Si "Smart Dust" particles encoded with two different colors. These particles, each roughly the size of a human hair, can be used as sensors for chemical or biological compounds, or in applications designed to rapidly screen for new drugs or genetic markers for disease.
|Chemical Sensors from Porous Si. The first use of porous Si as a sensor was demonstrated by Tobias in the late 1980s. This sensor used capacitance changes of the porous Si layer upon adsorption of chemical species as a transduction method. There are now many physical properties of porous Si that have been harnessed for sensor applications, including capacitance,[6, 23] resistance, photoluminescence and optical reflectivity. Detection of toxins,[27, 28] volatile organic compounds,[29-31] polycyclic aromatic hydrocarbons (PAHs), explosives,[33, 34] DNA,[14, 35] and proteins have all been demonstrated,[36-38] and detection limits of at least a few ppb have been demonstrated for some of these. Sensing can be accomplished based on changes in either the photoluminescence or the optical reflectivity spectrum, and these two transduction modes are studied extensively in our group.
Sensing using Optical Reflectivity from Fabry-Pérot Layers and Multilayers of Porous Si. The determining physical parameter for the sensor devices based on optical reflectivity is the optical thickness of the films, which is the product of the refractive index (n) and the thickness (L). These are both determined and well controlled by the electrochemical parameters used in the synthesis, and thin films with given optical parameters can be etched in a reproducible way. Figure 2 gives an example of a high quality Fabry-Pérot interferometer film made from porous Si. The high contrast observed in the optical fringes is a signature of the formation of two planar and parallel interfaces. Shifts in these fringes occur upon analyte binding in the pores, and provides a sensitive transduction modality. We have used this phenomenon as a means of assessing the bulk composition of a porous layer and to determine the chemical stability of a given type of surface modification.
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|Designed by Andrea Tao.
Main address: Department of Chemistry, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0358 (858) 534-0227
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Last modified Monday, February 17, 2003