5. A.W. Snyder, inThe Compound Eye and Vision of Insects,G.A.
Horridge, ed., Oxford University Press, London, 1975.
6. R. Menzel, D.F. Ventura, H. Hertel, J.M. de Souza, and U.
Greggers,J. Comp. Physiol. A108: 165–177 (1986).
7. C.G. Bernhard, and W.H. Miller,Acta. Physiol. Scand.56:
385–386 (1962).
8. C.G. Bernhard, W.H. Miller, and A.R. Moller,Acta. Physiol.
Scand.63: 1–79 (1965).
9. A. Yoshida, M. Motoyama, A. Kosaku, and K. Miyamoto,Zool.
Sci.14: 737–741 (1997).
10. H. Ghiradella,J. Morphol.202: 69–88 (1989).
11. R.E. Silberglied, inThe Biology of Butterflies,R.I. Vane-Wright and P.R. Ackery, eds., Academic Press, London, 1985.
12. F.U. Hartl,Science381: 571–580 (1996).
13. T. Yura, H. Nagai, and H. Mori,Annu. Rev. Microbiol.47: 321–
350 (1993).
14. B.L. Stoddard, H.P. Biemann, and D.S. Koshlan, Jr., Cold Spring Harbor Symp. Quant. Biol.57: 1–15 (1992).
15. A. Lupas,Trends Biochem. Sci.21: 375–382 (1996).
16. T. Nara, I. Kawagishi, S.-O. Nishiyama, M. Homma, and Y.
Imae,J. Biol. Chem271: 17932–17936 (1996).
17. R. Hurme, K.D. Berndt, S.J. Normark, and M. Rhen,Cell90:
55–64 (1997).
18. G.K. Noble, and A. Schmidt,Proc. Am. Philos. Soc.77: 263–288 (1937).
19. T.H. Bullock, and R.B. Cowles, Science 115: 541–543 (1952).
20. T.H. Bullock, and F.P.J. Diecke, J. Physiol. 134: 47–87 (1956).
21. E. Newman, and P. Hartline,Sci. Am.246: 98–107 (1982).
22. S.-I. Terashima, and R.C. Goris, Infrared Receptors and the Trigeminal Sensory System. Harwood Academic Press, Amsterdam, 1998.
23. A.L. Campbell, T.J. Bunning, M.O. Stone, D. Church, and M.S.
Grace,J. Struct. Biol.126: 105–120 (1999).
24. F. Amemiya, T. Ushiki, R.C. Goris, Y. Atobe, and T. Kusunoki, Anat. Rec.246: 135–146 (1996).
25. D.H. Theodoratus, D. Chiszar, and H.M. Smith,Psychol. Rec.
47: 461–472 (1997).
26. T. de Cock Buning,Acta Biotheoretica34: 193–206 (1985).
27. T. de Cock Buning,Am. Zool.23: 363–375 (1983).
28. T. de Cock Buning, S. Terashima, and R.C. Goris,Cell. Mol.
Neurobiol.1(1): 69–85 (1981).
29. B.S. Jones, W.F. Lynn, and M.O. Stone,J. Theo. Bio.209: 201–
211 (2001).
30. H. Schmitz, and H. Bleckmann,J. Comp. Physiol. A182: 647–
657 (1998).
31. E.G. Linsley,J. Econ. Entomol.36: 341–342 (1943).
32. W.G. Evans,Nature202: 211 (1964).
33. W. Gronenberg, and H. Schmitz,Cell. Tissue Res.297: 311–318 (1999).
34. W.G. Evans,Ann. Entomol. Soc. Am.59: 873–877 (1966).
35. R.D. Hudson, Infrared System Engineering. J Wiley, NY, 1969.
36. M. Sfakiotakis, D.M. Lane, and J.B.C. Davies,IEEE J. Oceanic Eng.24(2): 237–252 (1999).
37. X. Shen, A.M. Belcher, P.K. Hansma, G.D. Stucky, and D.E.
Morse,J. Biol. Chem.272: 32472–32481 (1997).
38. M. Sarikaya,Micros. Res. Tech.27: 360–375 (1994).
39. N. Kroger, R. Deutzmann, and M. Sumper,Science286: 1129–
1132 (1999).
40. J.N. Cha, K. Shimizu, Y. Zhou, S.C. Christiansen, B.F.
Chmelka, G.D. Stucky, and D.E. Morse,Proc. Natl. Acad. Sci.
USA96: 361–365 (1999).
41. M. Banish, and R.F. Crouse, SY Technology, Huntsville, AL, 1999.
42. P.S. Callahan,Tuning in to Nature: Solar energy, Infrared ra- diation, and the Insect Communication System.The Devin- Adair Company, Old Greenwich, CT, 1975.
43. S. M. Kirkpatrick, J.W. Baur, C.M. Clark, L.R. Denny, D.W.
Tomlin, B.A. Reinhardt, R. Kannan, and M.O. Stone,Appl.
Phys. A69: 461–464 (1999).
44. M.O. Stone, J.W. Baur, L.A. Sowards, and S.M. Kirkpatrick, Photonics West, SPIE Proc.San Jose, 2000.
45. C. Wang, R.J. Stewart, and J. Kopecek,Nature397: 417–420 (1999).
BIOSENSORS, POROUS SILICON
ANDREASJANSHOFF
Johannes-Gutenberg-Universit ¨at Mainz, Germany
CLAUDIASTEINEM Universit ¨at Regensburg Regensburg, Germany
INTRODUCTION
Biosensors consist of a biologically active layer that re- sponding to an analyte in solution and a powerful trans- ducer that transforms and amplifies the reaction into a measurable signal. Biosensors can constantly measure the presence, absence, or concentration of specific organic or in- organic substances in short response time and ultimately at low cost. They are used commercially in health care, biotechnological process control, agriculture, veterinary medicine, defense, and environmental pollution monitor- ing. A common requirement of all of these applications is on-site chemical information—preferably in real time—on some dynamic or rapidly evolving process. Most biosensors are based on molecular events as they take place at the cellular membrane or inside the cell involving enzyme cas- cades. Their perceived advantages over existing technolo- gies include the ability to monitor broad or narrow spec- tra of analytes continuously in real time, and their weak- ness is the instability of the biological molecules outside their natural environment, which results in a restricted lifetime for the device. The challenge is to find a matrix for biomolecules that provides high compatibility of the mate- rial with biological substances, low-cost fabrication, and special optical and electrical properties to generate a signal that measures the interaction between analytes in solution and the receptive biological layer. It is also desirable that it be compatible with conventional microfabrication tech- niques to miniaturize the device or to build individually addressable arrays.
The high surface area in conjunction with its unique optical and electrical properties and its compatibility with silicon microelectronics fabrication techniques has led to the proposal that porous silicon may be a suitable material for building sensor devices. Several different transducer
122 BIOSENSORS, POROUS SILICON
schemes have evolved based on thin film interference, capacitance changes, and the photoluminescent properties of porous silicon.
HISTORICAL OVERVIEW
Porous silicon is not a newly discovered material. Ulhir reported 45 years ago that porous silicon (PSi) is gener- ated during the electropolishing of silicon under anodic polarization in a hydrofluoric acid containing electrolyte if the current density falls short of a critical value (1).
Since its first discovery, the material has been studied ex- tensively because it was considered suitable for electronic applications (local insulation, gettering of impurities, sac- rificial layers, etc.). However, the impact of PSi increased far more than expected in 1990 when Canham unexpect- edly discovered a red bright photoluminescence from PSi at room temperature (2). The emission of visible light from PSi produced a sensation because the energy gap of silicon (1.1 eV) corresponds to the infrared region and does not explain the occurrence of photoluminescence in the visible regime. Within months after this observation, several labs reportedly detected visible light emission from PSi by pass- ing an electric current through it (electroluminescence) (3). This was a vital discovery because any optoelectronic device that might use PSi will probably operate by conven- tional electroluminescence. Inspired by this unique prop- erty of PSi, the efforts of the scientific community during the last 10 years led to much useful information about aspects of PSi formation and its physical and chemical properties. Despite these efforts, several aspects of PSi for- mation and even some of the physical and chemical prop- erties are still a matter of discussion.
POROUS SILICON FORMATION
PSi layers can be prepared chemically or electrochemically (4). The electrochemical route starting from boron (p-type) or phosphorus (n-type) doped silicon is mostly employed.
For most electrochemical preparations of PSi (2–6), single- crystal silicon [(100)- or (111)-oriented wafers] is anodized in an aqueous or ethanolic HF solution under constant current conditions. The exact dissolution chemistry of sili- con is still in question, although it is generally accepted that holes are required in the initial oxidation steps. This means that for n-type material, significant dissolution occurs only under illumination, high electric fields, or other hole-generating mechanisms. A couple of facts have been gathered about the course of pore formation: (1) hydrogen gas evolves in a 2:1 atomic ratio to silicon; (2) current ef- ficiencies have been measured at approximately two elec- trons per dissolved silicon atom and (3) the final, stable end product for silicon in HF is H2SiF6(4,5). Though the reac- tion mechanism is still unclear and several different mech- anistic variants for the anodization of silicon surfaces have been proposed, a simplified sum equation can be written for the dissolution process:
Si(s)+6HF(aq)+2h+→H2SiF6+H2+2H+(aq)
One mechanistic model presented by Lehmann and G¨osele comprises an entirely surface-bound oxidation scheme of hole capture and subsequent electron injection to produce the divalent silicon oxidation state (7). The silicon surface continuously vacillates between hydride and fluoride coverage at each pair of electron/hole ex- changes. It appears that, despite the thermodynamic sta- bility of the Si–F bond, it does not remain on the silicon surface in any stable, readily measured form. The present consensus is that hydrogen exists on the silicon surface in at least two different forms, Si–H and Si–H2and possibly a third form, Si–H3. For both n- and p-type silicon, low current densities are essential in PSi formation (Fig. 1).
Low current densities ensure a sufficient amount of HF molecules (or F−ions) at the silicon–electrolyte interface.
Because holes from the bulk silicon phase reach the bot- toms of the pores first, silicon at the pore bottoms is pref- erentially dissolved. This is, however, a very simple ex- planation. Several other aspects of pore propagation are discussed in the literature, such as image force effects, hole diffusion, crystallography, charge transfer, quantum confinement, and surface tension (5). Higher current den- sities result in an excess of holes at the silicon–electrolyte interface, and the corrosion reaction becomes limited by diffusion of HF molecules (or F− ions). This leads to a preferential reaction of the upper parts of the silicon surface that results in smooth electropolishing (Fig. 1).
Because electropolishing does not occur in organic solu- tions, it appears to depend on the formation of an oxide layer atop the silicon surface.
The formation of pores results from the complex inter- play of chemical kinetics, charge distribution, and differ- ing crystal face reactivities, so it is obvious that the issue of PSi films comprises rather different porous structures ranging from those holding micron-sized pores to sponge- like layers that contain nanometer-sized pores. Pore struc- tures and dimensions are determined by a large number of preparative conditions: doping level and type, crystal orientation, composition of electrolyte, construction of the electrolytic cell, anodization regime, sample precondition- ing, and postanodization processing (5). In fact, samples produced by different research groups are hardly compa- rable, even if the preparative conditions are apparently the same. No wonder great controversy exists over the mecha- nism of PSi formation.
CHARACTERIZATION OF POROUS SILICON
The body of knowledge about PSi formation has been ob- tained from current–voltage characteristics, as described earlier (5). Besides electrochemistry, several other methods have been employed to study the morphology of PSi. Among them, transmission electron microscopy (TEM) has con- tributed a large amount of information about structural de- tails on individual pore propagation and silicon microcrys- tals because it is the only method to visualize microporous silicon directly (4). Scanning electron microscopy (SEM) is used mainly for macroporous silicon obtained from n-type or heavily doped p-type silicon etched at high current densities. Scanning probe techniques such as atomic force microscopy (AFM) are especially useful for
BIOSENSORS, POROUS SILICON 123
+ +
+
−
−
−
− −
+
+
p -type p -type
n -type n -type
p -type p -type
n-type
p -type
CB CB
EF
EF
EF VB
VB
VB
Solution Solution
Solution Solution
Solution Solution
Solution
Solution
EF qV VB
qV CB
CB
CB
CB
CB
CB
Forward bias
EF
EF
EF
EF VB
VB
VB
W
W
VB
Reverse bias
n -type
n -type CB
CB EF
EF VB
VB
Solution
Solution (2)
(1)
(3)
hv B
Figure 1. (a) Typical current–voltage relationships for n- and p-type silicon. The solid line is the dark response, and the dashed line indicates the response under illumination. The first (lower) current peak corresponds to a surface anodic oxide formed during and required for electropolishing. The second (higher) current peak marks the beginning of stable current oscillations and the possible formation of a second type of anodic oxide (5). (b) (1) The semiconductor–electrolyte interface before (left) and in thermal equilibrium (right), (2) at forward and reverse bias, and (3) during anodic etching.n-type PSi has to be illuminated to provide holes for the etching process. CB: conductance band, EF: Fermi energy, VB: valence band, W: width of the depletion layer.
124 BIOSENSORS, POROUS SILICON
detecting topographical features in conjunction with ma- terial properties such as friction, elasticity, conductance, and energy dissipation. Quantitative data about poros- ity and poreradii distribution may be inferred from low- temperature adsorption and desorption of gases. The most prominent technique, the BET (Brunauer–Emmett–
Teller) method, is based on measuring the gas volume adsorbed by a material as a function of pressure; the BJH (Barret–Joyner–Halendra) method uses the Kelvin equa- tion toinfer the pore radius from gas condensation inside the pores (8). Simple gravimetric analysis and profilometer measurements of pore nucleation and propagation have provided valuable information about the anodization of sil- icon (9). Optical properties and morphological details are studied by spectroscopic techniques such as UV-vis, Raman and IR spectroscopy, as well as spectroscopic ellipsome- try (10). Ellipsometry reveals information about porosity and the dielectric function of the material and is particu- larly useful for determining changes in the refractive index and thickness of the material. Details of pore morphology can also be obtained from X-ray crystallography measure- ments, as demonstrated by grazing angle experiments us- ing X rays and synchrotron radiation.
Key parameters that describe the overall properties of porous material are porosity and pore radius, which de- pend mainly on the composition and temperature of the electrolyte, the dopant concentration, and the current den- sity (5). Pore sizes can vary over a wide range from macro- pores (pores>50 nm wide) and mesopores (2–50 nm) to micropores (<2 nm). Generally, an increase in pore ra- dius accompanies an increase in the anodization poten- tial or current density for both n- and p-type silicon. At low current densities, the pores are randomly oriented and filamentlike. In contrast, the pores “pipe” at high current densities close to the electropolishing regime. The effect of dopant concentration on pore morphology is well explored.
The pore diameters and interpore spacings of lightly doped p-type silicon are between 1 and 5 nm and exhibit a net- worklike appearance. Increasing the dopant concentration results in forming clear channels that have larger pore diameters and directed pore growth. Although the n-type silicon is more complex, increased dopant concentration is characterized by decreasing pore diameter and interpore spacing. The pore diameters in n-type PSi are considerably larger than those of the p-type silicon at low dopant con- centration (3,5). Electrochemical etching of lightly doped n-type silicon wafers in the dark results in forming low porosity materials that exhibit macropores whose radii are in the micrometer range. Under illumination, much larger porosities can be obtained and micro- to macropores are found. Using p-type silicon of low resistivity, the porous texture is always thin, and the pore size distribution is in the 1 to 5-nm regime.
The results of a systematic study of porous layers formed in heavily doped substrates has been published by Herino (11). Generally, the porosity increases as HF con- centration decreases in p-type silicon, whereas the influ- ence of the HF concentration on the pore size of the n-type is not very pronounced. The specific surface area is in the range of 180–230 m2/cm3 in p-type silicon and 90–
230 m2/cm3in n-type silicon and is not very sensitive to the forming parameters.
80 75 70 65 Porosity
[%]
Wavelength [nm]
500 600
700
800 PL-Intensity
Figure 2. Photoluminescent spectra of lightly doped p-type PSi layers of various porosities. The layer is about 1µm thick, and the specific resistivity of the silicon 0.2cm in all cases [reprinted with permission from (10)].
OPTICAL PROPERTIES OF POROUS SILICON
The demand for visible light-emitting devices made en- tirely from silicon is enormous because silicon is the domi- nant material for electronic and optical devices such as waveguides, detectors, and modulators. However, silicon is an indirect semiconductor, and thus light emission is inef- ficient. A direct photon transition at the energy of the min- imum band gap does not meet the requirement of conser- vation of momentum in silicon. Therefore, electrons at the minimum of the conduction band need a significant amount of time to receive the necessary momentum transfer to recombine with holes in the valence band. Conse- quently, nonradiative recombination reduces the quantum efficiency considerably and results in emission of weak in- frared wavelength light due to its small indirect band gap of 1.1 eV (12). In 1990, Canham announced the discovery of photoluminescence from PSi electrochemically etched at room temperature (2). Figure 2 shows typical photolumi- nescent spectra of p-type PSi that depend on porosity.
Tunable photoluminescence from anodically etched sili- con is expected to have great impact on the development of optoelectronics, filters, chemical and biological sensors, and optical data storage, to name just a few applications.
The mechanism of luminescence, however, is still a matter of controversial discussions. Available models can be grouped into four classes: those based on quantum con- finement alone, nanocrystal surface states, specific defects or molecules, and structural disordered phases (13).
Experimental data, however, are most consistent with the so-called smart quantum confinement model that comprises the quantum confinement model, including contributions from surface states (14). The general fea- tures of light emission from PSi may be explained in terms of reduced nonradiative recombination, as deduced from time development of photoluminescent intensity after short laser pulse excitation. The rather slow decay provides evidence that reduction of nonradiative recom- bination, rather than an increased amount of radiative
BIOSENSORS, POROUS SILICON 125 transitions, is the reason for the enhanced quantum effi-
ciency, compared to bulk silicon. Significant light emission is observed only for microporous silicon, and the band gap widens (1.4–2.2 eV) as crystal size decreases, essentially identical to the particle in the box phenomenon in quan- tum mechanics. The increased path length of electrons in larger crystals renders recombination with surface defects or other mechanisms very likely. Consequently, light emission from larger structures is poor, whereas bright luminescence occurs in microporous material and is accompanied by a shift to higher photon energies from the near IR to the visible region. There is a correlation between porosity as an indirect measure of particle size and emissive energy. The smallest particle size is obtained from lightly doped p-type PSi etched at low current densities. Moreover, evidence for nanocrystallinity of the porous material from ESR analysis, TEM measurements, and phonon-assisted luminescence strongly support the quantum confinement model (13). All three primary colors were obtained, and the consequences are important for future display applications (13,15). Many chemical sensors based on PSi use luminescent reduction and thus provide a transducing mechanism for quantifying adsorption of analytes on the surface. Examples of chemical sensors employing photoluminescent reduction are given later.
Because recent biosensor developments are based on the dielectric function of PSi films, it is instructive to re- view briefly the reflectance and transmission of PSi layers and emphasize interference patterns and suitable effec- tive medium approximations. The dielectric functionε(ω) connects the dielectric displacementDto the electric field vector E(12). The polarization P represents the part of D that arises due to polarization of the dielectric mate- rial induced by an external electrical field. The total po- larizability of matter is usually separated into three parts:
the electronic, ionic, and dipolar. The dielectric constant at optical wavelengths (UV-vis) arises almost entirely from electronic polarizability, and the dipolar and ionic contri- butions are small at high frequencies (Fig. 3).
The dielectric function is not a constant but depends strongly on the frequency of the external electrical field.
The frequency dependence of the dielectric function arises from relaxation processes, vibrations of the electronic sys- tem and atomic cores, that are accompanied by macroscopic polarization. At certain wavelengths, however, it is reason- able to assume a constant value. The dielectric function of a solid can be inferred from measuring the reflectivity, re- fractive index, and absorbance, all functions that are ac- cessible by optical spectroscopy. The real and imaginary function of the dielectric function can be accessed from reflectivity measurements. The refractive indexn(ω) and the extinction coefficientK(ω) are related to the reflectiv- ityr(ω) at normal incidence in vacuum by (16)
r(ω)= n+iK−1
n+iK+1. (1) By definition,nandKare related to the dielectric function by
ε(ω)=n+iK. (2)
The measured quantity is the reflectance R(ω), which is related to the reflectivityr(ω) by its complex product:
R(ω)= Eref∗ Eref
Einc∗ Einc
=r∗r, (3)
where Erefis the electric field vector of the reflected light and Einc that of the incoming light. The following de- scription of thin film interference provides the necessary foundation to understand the functioning of most popu- lar biosensors based on the shift of interference fringes that arise from reflections at thin transparent PSi layers.
Because PSi can be described as a film of a particular di- electric function different from that of bulk silicon, it is instructive to look at wave propagation in thin films on solid supports. Assumption of transparency due to the high porosity of PSi simplifies the treatment. At the interface between two media that have different refractive indices (n1andn2), an incident wave is partially transmitted in the medium and reflected (Fig. 4).
This follows from the boundary conditions for elec- tric and magnetic fields. Reflectivity and transmission coefficients can be obtained from the Fresnel equations which can be simplified for normal incidence and ideal transparent media by takingK=0.
In the visible range, the dielectric function of PSi may be described by an effective medium approximation (EMA).
Porous silicon consists of two media, the pore filling and the pore walls. The geometry of the pores determines the way the dielectric functions of these two media can be combined to give an effective dielectric function between that of silicon and the pore filling medium. The following section briefly summarizes the most prominent effective
Infrared
Frequency Total polarizability (real part)
UHF to microwaves
Ultra- violet α-dipolar
α-ionic
α-electronic
Figure 3. Frequency dependence of the different contributions to polarizability (12).