Anal. Chem. 1995, 67,3727-3732
Gas Sensing Properties of Porous Silicon Israel &hecMer*
Department of Chemistry, Technioplsrael Institute of Technology, Haifa 32 000,Israel
Moshe Ben-Chorinand Andreas Kux Department of Physics-€16, Technical University of Munich, 0-85747Garching, Gennany
Conductivity of porous silicon layers (ptype) has been investigated for oxganic vapor sensing. A many orders of magnitude increase in conductivity in response to a vapor pressure change from 0 to 100%has been measured for some compounds. The conductivity (at a constant pressure) varies exponentially with the compound's dipole moment. The temporal response of the porous silicon layers is in the seconds range, and the recovery is much slower (minutes). However, due to the tremendous conductivity changes and the low background noise, a complete recovery is not needed for sensing purposes. The mechanism of conductivity enhancement has been studied us@ several methods. It is attributed to an increase in the density of charge carriers. An additional mechanism based on increased diffusMty may take place in microporous silicon. The observed characteristics suggest the application of porous silicon to future chemical sensors. The sensors have the potential to be integrated monolithidly with other silicon devices using current technologies. Porous silicon CpS) is becoming an increasingly important electronic material in current fabrication technology. It has been suggested as a potential optical material for new silicon light emitting The fabrication of this material is carried out by a very simple electrochemical etching of silicon, which creates a network of nanometer-sized Si structures. Extensive studies have already been carried out on the electro- and photoluminescence properties of this material. Several models have been proposed to explain the origin of light emission, including quantum codnement effects,1s2J0J radiative recombination via (1) Canham, L.T. Appl. PhF. Left. 1 9 9 0 , 5 7 , 1046-1048. (2) Lehmann, V.; Gosele, U. Appl. Phys. Left. 1991, 58, 856-858. (3) Richter, A; Steiner; P.; Kozlowski, F.; h g , W. IEEE Electron Device Left. 1991, 12, 691-692. Steiner, P.; Kozlowski, F.; Lang, W. Appl. Phys. Left. 1 9 9 3 , 62, 2700-2702. (4) Koshida, N.; Koyama, H. Appl. Phys. Left. 1992, 60,347-349. (5) Halimaoui, A; Oules, C.; Bomchil, G.; Bsiesy, A; Gaspard, F.; Herino, R ; Ligeon, M.; Muller, F. Apfil. Phys. Left. 1991, 59, 304-306. (6) Bressers, P. M. M. C.; Knapen, J. W. J.; Meulenkamp, E. A; Kelly, J. J. APPl. Phy. Left. 1992, 61, 108-110. (7) Canham, L. T.; Leong, W. Y.; Beale, M. I. J.; Cox, T. I.; Taylor, L. Appl. Php. Left. 1992, 61, 2563-2565. (8) Bomchil, G.; Halimaoui, A; Sagnes, I.; Badoz, P. A; Berbezier, I.; Perret, P.; Lambert, B.; Vincent, G.; Garchery, L.; Regolini, J. L. Appl. Sue Sci. 1993, 65, 394-407. (9) Canham, L. T. MRS Bull. 1 9 9 3 , 18 (July), 22-28. (10) Calcott, P. D. J.; Nash, K J.; Canham, L. T.; Kane, M. J.; Brumhead, D. J. Phys. C 1 9 9 3 , 5 , L91.
0003-2700/95/0367-3727$9.00/0 6 1995 American Chemical Society
surface states,12and silicon-based compounds like SiH,,13 siloxene,14and amorphous silicon.15 Only a few researches have been interested in chemical effects on the emitted light.16-18 This study is focused on the chemical sensing properties of PS. Silicon-based chemical sensors would be of extreme practical i m p o h c e : a number of new devices are possible. The sensing activity of PS is based on its large internal surface area of -500 m2 cmW3, which implies possible adsorbate effects. Characterization of these surfaces and measurements of their speci6c area have been carried out by means of several technique^.^^-^^ The quenching of PS photoluminescence by chemicals has been already suggested for application in chemical s e n s ~ r s . ~Nev~J~ ertheless, sensors based on photoluminescence changes are unlikely to be realized, due to their high cost: such devices include complex and expensive (time-resolved) spectroscopic setup. We suggest a different chemical approach, based on the conductivity of PS rather than on the photoluminescence quenching. This study is based on our discovery of the tremendous changes in the electrical conductivity of PS exposure to various chemicals in the gas phase. Chemical sensors based on the electrical conductivity of other semiconductors are well known (e.g., ref 221, and their contribution has been rec0gnized.2~The advantages of PS sensors are the simplicity of fabrication, low cost, and a possibility for integration with other silicon-based devices. Actually, a similar application has already been proposed by Anderson et al.24 They investigated capacitance variations of PS layers under exposure to vapors. The reported capacitance variations were within 1 order of magnitude only (factor of 4.4), and the temporal responses were in the 5-10 min range. Here (11) Suemoto, T.; Tanaka, K; Nakajima, A; Itakura, T. Phys. Rev. Left. 1993, 70, 3659-3662. (12) Koch, F.; Petrova-Koch, V.; Muschik, T.; Nikolov, A; Gavrilenko, V. MRS P ~ o c1993,283, . 197-202. Koch F. MRS Proc. 1993,298,319. (13) Prokes, S. M.; Glembocki, 0.J.; Bermudez, V. M.; Kaplan, R; Friedersdotf, L. E.; Searson, P. C. Phys. Rev. E 1992,45,13788. Prokes, S. M.; Carlos, W. E.; Bermudez, V. M. APjV. Phys. Left. 1992, 61, 1447-1449. (14) Brandt, M. S.; Fuchs, H. D.; Stutzmann, M.; Weber, J.; Cardona, M. Solid State Commun. 1992, 81, 307-312. (15) Perez, J. M.; Villalobos, J.; McNeill, P.; Prasad, J.; Cheek, R; Kelber, J.; Estrera, P.; Stevens, P. D.; Glosser, RAppl. Phys. Left. 1992,61.563-565. (16) Lauerhaas, J. M.; Credo, G. M.; Heinrich, J. L.; Sailor, M. J. 1.Am. Phys. SOC.1992, 114, 1911-1912. (17) Lauerhaas, J. M.; Sailor, M. J. Science 1993,261, 1567-1568. (18) Ben-Chorin, M.; Kux, A; Schechter, I. Appl. Phys. Lett. 1994,64, 481. (19) Herino, R; Bomchil, G.; Barla, K; Bertrand, C.; Ginoux, J. L]. Electrochem. SOC.1987, 134, 1994. (20) Canham, L. T.; Groszek, A J. J. Appl. Phys. 1992, 72, 1558-1565. (21) Bard, A J. J. Electrochem. SOC.1994, 141, 402-409. (22) Bott, B.; Jones, T. A Sew. Actuators 1984,5, 43-53. (23) Janata, J. Anal. Chem. 1992, 64, 196R-218R (24) Anderson, R C.; Muller, R S.; Tobias, C. W. Sem. Achtators 1990,A23, 835-839.
Analytical Chemistry, Vol. 67,No. 20, October 15, 7995 3727
we compare these figures with the conductivity responses. Capacitance variations were attributed to condensation of liquid in the pores. In the following, we show that the mechanism of conductance increase is different and is not based on condensation at all. Furthermore, the dynamic range of conductivity is much larger than that of the capacitance. An attempt to develop a humidity sensor based on the hydrophobic and hydrophilic properties of PS has also been carried out;25however, no other effects have been tested.
30
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O~~~~~~~~o~n~o~nonnna~non0~8ooooooooooo EXPERIMENTAL SECTION
The microporous silicon (micro-PS) layers were prepared by anodization of a (100) ptype, B-doped Si wafer with resistivity of 5 Q cm in a 1:l volume solution of HF (4% in water)-ethanol. The etch current density was 30 mA cm-2. The anodization time was chosen to form layers of 10-20 pm thickness. For conductivity measurements, aluminum contacts (area of -4 mm2,thickness of a few micrometers) were evaporated on top of the porous silicon layer and were connected to metal wires. Current was measured using a Keithley 236 source-measure unit in a two-terminal sandwich configuration. The source unit provided a stabilized voltage that was applied between the evaporated contact and an ohmic contact on the back of the Si wafer. The measured currents were in the range of nano- to microamperes. The instrument was equipped with an IEEE-488 interface, and all data were transferred to a microcomputer for analysis. For some of the studies, meso-porous silicon (meso-PS) layers were used. These layers were prepared from a p+ (100) substrate with a resistivity of 10 mQ cm. The etching conditions were the same as those used for the micro-PS samples. Following anodization, the meso-PS layers were separated from the underlying silicon substrate by an eledropolishing step (same acid concentration, but 0.3 A cm-2 anodization current). In-plane conductivity of the freestanding meso-PS layers was measured in the twoterminal configuration, between two evaporated aluminum contacts (a few micrometers thick). Exposure to chemical gases was carried out in a small vacuum chamber. Samples were kept in ambient conditions and were introduced into the vacuum chamber just for the chemical exposure measurements. The sample was first flushed with clean nitrogen and evacuated using a rotary pump. Proper gas mixtures, containing the desired partial pressure of the organic gases and clean and dry nitrogen, were prepared in a large container and introduced into the sample chamber by a valve. The valve was closed when the desired gas pressure in the chamber was reached. Nitrogen was then added (in a few seconds) in order to perform the measurements at a fixed total pressure of -1 bar and in order to keep a well-defined gas composition and to prevent penetration of other gases from the outside. RESULTS AND DISCUSSION Current-Voltage Prose. The current-voltage profile of our
PS samples is approximately symmetrical with respect to zero voltage, as shown in Figure 1, for both vacuum or nitrogen atmosphere. Nitrogen has no iduence on conductivity and photoluminescence properties, so it can be used for dilution. For low methanol partial pressures, the current-voltage curve remains symmetrical, and only the current amplitude increases. Exposure (25) Yamana, A. J. Electrochem. Soc. 1995, 137, 2925.
3728 Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
0 I
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Table 1
compound benzene cyclohexane fluorobenzene toluene trichloroethylene chloroform diethyl ether 2-propanol
methanol ethanol
n-propanol acetone
methyl ethyl ketone Nfldimethylformamide
dipole moment/D
0 0
1.6 0.36 0.8 1.01 1.25
1.66 1.7 1.69 1.68 2.88
2.7
3.82
vapor pressure/Torr
110 104 70 29 77 210 587 43 128 59 17 233 105 4
to higher pressures has a remarkable effect and introduces rectifying characteristics. Analysis of these characteristics,for various gas concentrations and for various thicknesses of PS layers,'* suggests that the current in forward bias is a correct measure of the PS resistance. The change from symmetrical to rectifying behavior with increasing methanol pressure is due to the electrical structure of the device, which may be described as a serial combination of a resistor (PS layer) with a diode (Schottky barrier at the interface between Si and PS). At low methanol concentration, the PS resistance is so high that it limits the current, resulting in symmetrical current-voltage curves. At higher concentrations, the resistance is much reduced and the current in the reverse bias is limited by the interface, giving rise to rectifying behavior. However, in forward bias, the limiting element is the PS resistance, and thus, the sensing characteristics of the PS were measured with a forward voltage bias. Vapor Effects. We measure the effect of 14 compounds, as listed in Table 1. The relative current changes (at 5 V) caused by exposure to these compounds (compared to clean nitrogen) are given in Figure 2, for 10 mbar and for half of the vapor pressure at room temperature. Following Lauerhaas et al.,16who showed a correlation of the luminescence quenching with the dipole moment of the adsorbate, we plot the current increase on a logarithmic scale as a function of the dipole moment of the molecules. While the effect on the luminescence was linear,16 the conductivity increase is exponential. A many orders of
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Dipole Moment I Debye Figure 2. Current changes (relativeto nitrogen) induced by several compounds, at 10 mbar and at half the vapor pressure. The applied voltage was 5 V. The point on the right, at 3.82D, was measured at 4 Torr only (itsvapor pressure at room temperature),which accounts for the deviation. Note the many orders of magnitude effects of both the dipole moment and pressure. magnitude effect was observed for acetone and for methyl ethyl ketone, which have large dipole moments, while benzene, with zero dipole moment, has a very low influence on the conductivity of PS. This exponential dependence of the conductivity change (at the same gas pressure) on the dipole moment is of considerable importance for sensor development. However, the spread of the data in Figure 2 shows that the dipole moment is not the only physical factor that affects the conductivity. Water vapor, for example, has only a negligible effect, although the dipole moment is very large. This is attributed to the hydrophobic nature of the hydrogen-passivated surface of PS crystallites. After we removed this surface coverage by means of a 400 "C annealing step (in air), a large increase in the conductivity was observed following exposure to water. In a similar mannen, Lauerhaas and Sailor" reported enhancement of the photoluminescence quenching by water vapor following a chemical treatment, which makes the hydrophobic surface more hydrophilic. The insensitivity of the hydrated surface to water vapor is an additional advantage for sensor applications. Furthermore, the possibility to control the device selectivity by surface treatment allows for opthizing the sensitivityto speciiic chemical compounds. Pressure Dependence. In addition to the abovementioned exponential dependence on the dipole moment, there is another exponential dependence: the current as a function of the gas pressure for each of the separate compounds. This is shown in Figure 3 for methanol (on the left axis) and for other compounds with lower dipole moments (right axis). Again we see that the change in the conductivity is not directly related to the dipole moment, since even for benzene, which has zero dipole moment, an increase in the conductivity as a function of concentration is observed. This suggests that there exists a mechanism of conductivity enhancement which is not related to the dipole moment. TWOdifferent routes have also been proposed for luminescence quenching due to polar and nonpolar adsorbate~.~~ Temporal Response. An additional important feature is the time response and recovery of the PS conductivity following gas exposure. In order to study these characteristics, we exposed a PS sample to methanol in the followingway. The sample chamber was evacuated and exposed to clean nitrogen. The chamber was
."0.0
0.2
0.4
0.6
0.8
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PIP, Figure 3. Exponential dependence of current changes (relativeto vacuum) on the gas pressure (relativeto vapor pressure) for several solvents. The curves of benzene and toluene do not cross when presented against absolute pressure. 150
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2000
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Time I s Figure 4. Temporal response and recovery of the porous silicon
conductivity on exposure to methanol. Due to the intense response, there is no practical need to wait for full recovery. The inset shows a schematic drawing of the PS sensor and its electric connections. then evacuated again and exposed to a mixture of nitrogen with 10 mbar of methanol, at the same total pressure (-1 bar). After a defined time of -2 min, the chamber was evacuated again for a few minutes and then filled with a new mixture of nitrogen and methanol, at a higher concentration. This concentration was kept for exactly the same period inside the chamber before the chamber was evacuated again. The procedure described above was repeated for several cycles. The results are shown in Figure 4. An increase in the current immediately after the exposure is observed, followed by a recovery after evacuation. The response and the recovery times, as a function of the methanol concentration, can be obtained from this figure. A complete recovery of the signal is not fast,18 probably due to the fractal nature of the micro-PS surface. The long full-recoverytime gives rise to a slight elevation of the baseline during measurements. However, the results of Figure 4 demonstrate that the main part of the recovery occurs on a quite short time scale. Usually, 10%recovery is obtained in 30 s, and 9096 recovery takes -100 s. The derivative of the signal, for instance, would indicate the change in the methanol concentration. Therefore, for practical purpose, both the response and the recovery can be considered fast. The measured temporal characteristics can be rationalized by a simple model. The adsorption rate is generally given byz6 (26) Morrison, S.R ?he Chemical Physics of Sufuces; Plenum Press: New York,
1977.
Analytical Chemistty, Vol. 67, No. 20, October 15, 1995
3729
sample in methanol eV. o,=102 (ncm).'
. .---.E,=0.49
where T1is the density of adsorption sites, p is the pressure, and r is the adsorbate density (per area or volume unit). The first term describes the desorption process, and the second one describes the adsorption. At relatively low adsorption, kl and kz are pressure independent, and we can derive the steady state (drldt = 0) adsorbate concentration,
r = rg/@+ k,/kd
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where PO is adsorbate dependent. By eqs 3 and 4, we get
Our pumping procedure starts at surface concentration UO), given by the steady state condition r(0)= Tt(kz/kl)P (t = O), and we pump down to low pressures. Thus, the second term in eq 1 vanishes, and the solution of this equation (at p FZ 0) is
r(t)= r(0)e-kl'
(6)
Provided that the conductivity instantaneously responds to the concentrations, eqs 5 and 6 give
Equation 7 has been fitted to our data, and good results have been obtained for the conductivity as a function of time, after a fast pumping down of the vapors. Deviations have been observed at longer times, probably due to contribution of the second term in eq 1,which was neglected here. At longer times (>lo00min), the readsorption cannot be neglected. Mechanism of Conductivity Enhancement. We discuss now the origin of the enhancement of PS conductivity by the adsorbed molecules. We first examine one suggestion made by Anderson et alF4 They have argued that the change in the electrical properties of PS following adsorption (capacitance in their study) is due to liquid condensation inside the microcapillaries of the PS. This would lead to an increased capacitance as well as an extra current flowing through the liquid, resulting in an "artificial" conductivity enhancement. In order to check this mechanism, we have measured the refractive index of our PS layers, using FT-IR spectroscopy. The existence of liquid inside the pores should increase the effective refractive index. Such an ,
3730 Analytical Chemistry, Vol. 67, No. 20, October 75, 1995
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Figure 5. Temperature dependence of meso-PS conductivity (at 5 V) in vacuum and in methanol vapor. The activation energy is lowered in the presence of the methanol vapor, while the conductivity prefactor is not changed.
The exponential characteristics of Figure 3 can be expressed in the following form: = oP'$,
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increase was observed only above methanol pressure of 80 mbar, suggesting that below this pressure no condensation occurs. Nevertheless, conductivity enhancement was observed at much lower methanol pressures. Moreover, the recovery of the refractive index following evacuation was faster than the current relaxation. A full understanding of the mechanism of the conductivity enhancement requires a knowledge of both the electrical transport properties of the PS and the charge transfer reaction that occurs during the adsorption at the silicon surface. Unfortunately, the conductivity in micro-PS is influenced by a variety of parameters. The material is a fractal network of nanometer-sized crystallites. Quantum confinement causes spatial fluctuations of the effective band gap. As a result, internal energy barriers exist between the crystallites, which electrons have to overcome. Electrons move through the system on the most favorable route. Even without the presence of adsorbate molecules, the conductivity mechanism is not trivial, and their addition further complicates the situation. In order to shed some light on the question of the conductivity enhancement, we simplified the problem by examining meso-PS layers. The silicon structures in these samples are larger (1020 nm),217 and therefore quantum size effects are negligible. They also lack the fractal topology possessed by the micro-PS.28This allowed us to treat mesePS as a usual semiconductor.B The main difference between a meso-PS layer and a piece of bulk crystalline silicon is the large surface area of the first. On the other hand, this is the common feature it shares with micro-PS. Thus, one might expect to learn about the conductivity enhancement problem by looking at meso-PS, since the charge transfer processes in both materials are probably similar, because they depend mostly on the local environment of the adsorption sites. Figure 5 is an Arrhenius plot of the conductivity as a function of temperature. When the sample is kept in vacuum, the activation energy is 0.49 eV, and the conductivity prefactor is -102 C2 cm-l. When methanol is introduced into the chamber, the activation energy decreases (to 0.4 ev), but the conductivity prefactor is (27) Cullis, A G.; Canham, . L T. Nature 1991,253, 335-338. (28) Goudeau, P.; Naudon, A; Bomchil, G.; Herino, R J. Appl. Phys. 1989,66, 625. (29) Schwarz, R; Wang,F.; Ben-Chorin, M.; Grebner, S.; Nikolou, A; Koch, F. Thin Solid Films 1995,255, 23-26.
not affected. Assuming semiconductor behavior, the conductivity can be approximated by30
where N, is the effective density of states at the band edge, e is the electron charge, ,u is the free electron mobility, and E, and Ef are the band edge and Fermi energies, respectively. We assume that only the majority carriers contribute to the conductivity and that N, and ,u are only weakly dependent on the temperature. In a doped semiconductor, 4 might be a function of the temperature. However, in our case, the activation energy is constant over the whole range of the measurement, and therefore the Fermi level is pinned at a k e d point with respect to the band edge. In vacuum, the activation energy is approximately half the band gap of crystalline silicon (-1.1 ev), suggesting that the Fermi level is near mid-gap position. Therefore, we can conclude that the material behaves like a compensated semiconductor. When methanol is introduced, the Fermi level shifts to a new position and is fixed closer to the band edge. The meaning of this result is that methanol injects extra carriers into the PS. The sign of these carriers was determined by thermopower measurements. A small temperature gardient was applied to the sample, and the sign of the induced voltage indicated that the majority carriers are electrons for this material prepared from highly pdoped substrate. Thus, adsorbed methanol (as well as other vapors discussed here) behaves like an electron donor. There are two possible mechanisms to account for the methanol effect. It might be that during adsorption, the methanol is oxidized, thus giving an extra electron to the silicon skeleton. Since oxidation of methanol (and of ketones, which also show a large conductivity effect) is questionable, another mechanism has to be considered. It is possible that the existence of methanol with its high polarizabilityat the vicinity of the PS internal surfaces give rise to local fields, shifting the energy position of other surface states. Thus, the energy distribution of density of states inside the gap changes, shifting the Fermi level position with respect to the conduction band edge. The mechanism suggested above is based on a strong change in the density of charge carriers, while their mobility (diffusivity) is kept constant. This seems to be the case for meso-PS; however, for micro-PS, another mechanism might contribute as well. As we have stated before, conduction in micro-PS is limited by barriers between particles. A preferential adsorption of polarizable molecules on such sites might reduce the barriers, increasingthe effective diffusivity of the carriers and thus the conductivity. It is probable that both mechanisms contribute to the conductivity enhancement in micro-PS. Finally, it is interesting to discuss the changes observed in photoluminescence properties of micro-PS and their relation to the conductivity enhancement. A shift to the Fermi level, caused by addition of carriers, will increase the nonradiative recombination rate and therefore quench the conductivity. It has been already shown that PS luminescence can be quenched by the application of voltage, due to an injection of extra carriers into the crystallite^.^^^ This process increases the probability for Auger recombination. However, it has been observed that this process (30) Seeger, K. Semiconductor Physics, 5th ed.; Springer: New York, 1988.
selectively suppresses the red part of the photoluminescence band, because injection into the larger particles that are responsible for this light is easier. The other mechanism we have suggested for the micro-PS conductivity effect, the lowering of barriers between particles, would also give rise to luminescence quenching, since excited carriers have easier access to nonradiative centers. However, in this case, one would expect that a similar decrease in the photoluminescence intensity would take place all over the spectrum, as observed in the experiments. Further investigation is needed to clariry these points. Comparison to Other Measurements. Anderson et alez4 have examined PS devices for vapor sensing, mainly as a humidity sensor. They have found a 440% increase in the capacitance of PS layers in response to a humidity change from 0 to 100%. In addition, they have found only weak sensitivity to methanol and acetone. This is not in disagreement with our results, since their samples were prepared in such a way that the internal surfaces were oxidized and therefore hydr0philic.2~Another batch of their samples, prepared in a special way which restored the original hydrophobic nature, showed no response to water vapor, but a large effect was induced by methanol. As we have discussed before, the humidity response can be easily changed by simple pretreatment or aging of the samples. We chose samples that do not respond to water vapor, since we were interested in sensing the organic materials. Water vapor is expected to influence the conductivity of hydrophobic PS only at a high humidity level and prolonged exposure. Mares et d.3l reported on conductivity changes as a function of humidity for relative humidities larger than 25%. Even in that case, the wetting of the inner surfaces occurred only after exposure for several days. We believe that the prolonged exposure also results in partial oxidation, which allows wetting to occur. This effect is negligible in our samples, due to the relatively short exposure times we used. The vapor condensation-based mechanism, proposed by Anderson et alez4and by Mares et a1.,3l was eliminated in our case by measuring the refractive index of the PS layer, carried out using FT-IR spectroscopy. It should be mentioned that even in the capacitance study, the internal condensation is not the only possible explanation, since capacitance may be changed by adsorption of molecules with a high dipole moment, even under submonolayer coverage. CONCLUDING REMARKS
PS has many of the characteristics required for chemical sensor development. First, its electrical response varies by many orders of magnitude for different gases at the same partial pressure. In addition, for each of the gases, an exponential current enhancement has been observed as a function of partial pressure. This means that the chemical nature of the adsorbates has a considerable influence on conductivity, which is favorable for sensor applications. Preparation of PS samples is very simple, much simpler than for other semiconductor- and metal oxidebased sensors. Since this layer is constructed on a standard silicon wafer, the final sensors can be integrated with other siliconbased devices. The proposed PS device is easily handled, since it is not affected by exposure to oxygen and other ambient gases. Actually, very similar results were obtained when pure nitrogen was (31) Mares, J. J.; Kristofic, J.; Hulicius, E. Thin Solid Films 1995, 255,272275.
Analytical Chemistry, Vol. 67,No. 20, October 15, 1995
3731
replaced with air. (Nitrogen was used just to ensure a stable and well-defined environment.) The PS device is rather stable, although variations could be observed after a few months (probably due to a slow process of internal oxidation). The mechanism of conductivity enhancement is not completely clear. In the case of meso-PS, where no quantum effects are expected, it is probably based on carriers activation, while in the case of micro-PS, an additional mechanism of increased diffusMty may take place. However, it is clear from the construction of the device that the conductivity takes place vertically (across the surface). Experiments with photoconductivity of these samples support this concept. Probably, the organic gases penetrate into the PS layer under the aluminum contacts and contribute to the observed conductivity enhancement. Nevertheless, more experimental evidence is needed for a final conclusion on the mechanism and the active sites. Further investigation of the performance of n-type porous silicon, which is known to possess a different microstructure, is of interest. The effects of preparation procedure on the final
3732 Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
sensitivity and response have still to be studied. This procedure consists of many variables, such as etching current and time or composition of the etching solution, that probably should influence the final characteristics. Further understanding of the physical effects involved in the current enhancement may contribute to sensor development, regarding its selectivity, temporal response, and concentration sensitivity. ACKNOWLEWMENT
We thank S. Grebner and J. Diener for their assistance. Fruitful discussions with V. Lehmann, R Schwarz, and F. Wang are acknowledged. This study was supported in part by the Koebner-Klein Endowment Fund and by the Jewish Communities of Germany Research Fund. Received for review March 28, 1995. Accepted July 10, 1995.e AC950303P @
Abstract published in Advance ACS Abstracts, August 15, 1995.
