Electrical porous silicon chemical sensor for detection of organic solvents

Thông tin tài liệu

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Sensors and Actuators B 106 (2005) 347–357 Electrical porous silicon chemical sensor for detection of organic solvents M. Archer a,∗ , M. Christophersen, P.M. Fauchet a,b a Department of Biomedical Engineering, Center for Future Health, University of Rochester, 601 Elmwood Avenue, Rochester, NY 14642, USA b Department of Electrical and Computer Engineering, Center for Future Health, University of Rochester, 601 Elmwood Avenue, Rochester, NY 14642, USA Received 20 April 2004; received in revised form 17 August 2004; accepted 18 August 2004 Available online 25 September 2004 Abstract A novel electrical sensor platform containing a porous silicon (PSi) layer on a crystalline silicon substrate has been developed in which the electrical contacts are made exclusively on the backside of the substrate allowing complete exposure of the surface to the sensing molecules. The PSi layers were 20 ␮m thick with an average pore diameter of 1␮m. Real-time measurements of capacitance (C) and conductance (G) were performed and the response produced by the addition of different organic solvents was evaluated. The observed response is attributed to the combined effect of achange in dielectric constant inside the porousmatrix and amodification in the depletionlayer width inthe crystalline silicon structure. A space charge region modulation model was used to explain the effect induced by molecules of different dipole moments, dielectric constants, polarizabilities and water solubilities. © 2004 Elsevier B.V. All rights reserved. Keywords: Macroporous silicon; Electrical sensors; Organic solvent detection; Chemical sensor 1. Introduction Porous silicon(PSi) isproduced by electrochemicaldisso- lution of crystalline silicon in a hydrofluoric acid based electrolyte. The resulting structure consists of pores alternating with crystalline silicon rods attached to a crystalline silicon substrate. PSi is characterized by a large internal surface area sensitive to the presence of charged molecules, which can be exploited for sensor development. Since the discovery of the room temperature photoluminescence (PL) of PSi [1,2] a great amount of work has been devoted to establish itsorigin. The surface of PSi plays a crucial role in its electrical [3–5] and optical behavior [6,7]. The observed change in conductivity upon exposure to different organic solvents and other molecules such as oxygen, suggests the existence of at least ∗ Corresponding author. Tel.: +1 585 2731559; fax: +1 585 2732981. E-mail address: archer@ece.rochester.edu (M. Archer). two response mechanisms. The first one relies on a change in the charge distribution within the crystallites due to the alignment of polar molecules on the surface [8,9]. The second one involves charge transfer reactions mediated by surface traps during adsorption [10] and oxidation of molecules on thesurface ofporous silicon[9].Thesecond mechanismis supportedbytheobserved shiftinthePLpeak ofPSi uponex- posure to valence band and conduction band quenchers with different redox potentials [11]. Although a complete model does not exist, it is clear that PSi is sensitive to charge and that the response upon exposure to certain organic solvents is reversible [12]. Aside from its electrical and optical properties, PSi offers a wide range of morphological properties as well as possible surface modifications that are useful for sensing applications [13]. We have taken advantage of the electrical activity of porous silicon and the tunability of its morphology to de- velop a novel electrical sensor platform. Recently, we de- 0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.08.016 348 M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 tected DNA hybridization and the presence of ethanol via a change in the conductance [14] of mesoporous silicon layers (pore diameter 20–50 nm) in structures with two concentric circular electrical contacts made on the backside of the crystalline silicon substrate. We explained our findings as due to charge redistribution in the crystalline silicon substrate induced by the changes in the PSi layer. In this paper, we present an optimized design of our porous silicon sensor in which the size of the device is greatly reduced, the geometry of the backside electrodes is modified and a porous layer having a different morphology is used. Systematic experiments with different organic solvents in the liquid phase and realistic modeling of the structures are also presented. Four different geometrieshave beenusedto measure electrical properties of PSi. These include metal contacts evaporated only onthe porous layer surface [15]orin a “sandwich” configuration [8,9,16,17], the use of coplanar contacts on the porous layer [10] and interdigitated electrodes using pnjunc- tions surrounded by PSi [18,19]. In all cases the response of the device depends on the characteristics of the electrical contact with the PSi. In comparison, in our devices, the field propagates from the crystalline silicon substrate to the PSi layer. Since no electrical contact is made on the porous layer, any influence of the contact barrier and chemical reactions that may occur between the metal and the organic solvent is eliminated. The observed response is therefore related only to the presence of molecules inside the porous layer and their interaction with the surface of the layer. 2. Materials and methods 2.1. Sensor fabrication The porous layers were fabricated by electrochemical dissolution of p-type silicon (ρ ∼ 10–20  cm) under galvano- static (constant current) conditions with a current density of 4 mA/cm 2 . Theelectrolyteusedwas4 wt.%hydrofluoric acid (49 wt.%) in N,N dimethylformamide (DMF) [20]. The use of a mildoxidizer such as DMFresults in straightandsmooth pore walls withpore diameter inthemicrometer range. These conditions were selected to increase the pore diameter and to enhance the sensitivity to changes in the space charge region. The porous layers were etched for 70min resulting in 20␮m thick layers. Fig. 1 shows a SEM picture of the layers. As will be shown in Section 3, a thin layer of surface oxide is required for proper operation of the devices. The layers were thus chemically oxidized by immersion in 30wt.% hydrogen peroxide (H 2 O 2 ) for a period of 48h at room temperature (22 ◦ C). Although the oxide layer produced by this oxidation technique is very thin [21], Fourier transform infrared (FTIR) [22] and spectroscopicellipsometry [23] studies have demonstrated the presence of vibrational modes and modifications of the dielectric function characteristic of oxidized porous silicon. The oxide is hydrophilic enough to allow the infiltration of water soluble molecules without the need of Fig. 1. Scanning electron microscopy (SEM) cross sectional view of a macroporous silicon layer produced form p-type silicon (ρ ∼ 10–20 cm) with an organic electrolyte. The bright areas correspond to c-Si rods and the dark areas to pores propagating from the surface parallel to the c-Si rods. a thicker thermally grown oxide. After oxidation the porous layers wererinsedwithdeionizedwater andethanol anddried under a stream of nitrogen. The oxide on the backside of the crystalline siliconsubstrate was strippedwith a15% HFsolu- tion (7:1, water: 49 wt.% HF) prior to the contact placement. The wafers were cleaved into sections of 4 × 7mm and two coplanar electrical contacts were placed 700␮m apart on the crystalline silicon substrate. In our approach, the PSi surface is completely exposed to the sensing molecules and no metal contacts are made to it, avoiding the introduction of foreign materials into the porous matrix. Fig. 2 shows a schematic cross-sectional view of our device and images of the front and backsides showing the electrical contacts and the actual dimensions. In order to avoid any of the solvents tested from reaching the backside contacts, the sensors were fixed on a glass slide, which ensures a horizontal surface for a uniform distribution of the solvent on the porous layer and protects the backside of the device. 2.2. Measurement setup Real-time capacitance (C) and conductance (G) measurements were performed with an inductance– capacitance–resistance (LCR) multifrequency meter. The measurement parameters (frequency and bias voltage) and the data acquisition and storage were controlled with a LabView TM routine. The dc bias voltage and the amplitude of the ac signal were selected to enhance the signal to noise ratio and the reproducibility of the C–V measurements. For this purpose we first varied the dc voltage from −10 to 10 V, the ac signal from 90 to 2 Vrms and the frequency from 100Hz to 100kHz. Above ±5Vdc and 1Vacrms the reproducibility of the results was low and the signal to noise ratio was reduced. In all further experiments, we selected a dc bias of 0 V and an ac signal of 90 mVrms M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 349 Fig. 2. (a) Schematic cross sectional view of the porous silicon sensor. The electrical contacts are placed on the back part of the layer (c-Si) by aluminum evaporation or colloidal silver paint 700␮m apart. (b) Pictures of the front and backsides of the device. Fig. 3. (a) Phase angle (Θ) and (b) impedance (Z) as a function of frequency. The measurements were done in a dry sample (no infiltration of the porous layer) with a dc bias of 0 V and an ac signal of 90 mV. and then determined the optimal frequency based on the measured values of impedance (Z) and phase angle (Θ). The results are shown in Fig. 3. At low frequencies the device behaves as a resistor and, as the frequency increases, the phase angle shifts towards a capacitive behavior and the impedance is reduced. We selected a frequency of 100 kHz to reduce the effect of parasitic capacitances and of interface states at the contact site [24]. The measurements under these experimental conditions correspond to a parallel C–G circuit. All the experiments were performed at room temperature (22 ◦ C) under a controlled humidity ambient (40–50% relative humidity). A schematic representation of the measurement setup is shown in Fig. 4. Fig. 4. Schematic of the measurement setup. An inductance–capacitance– resistance(LCR)metercontrolledbyLabView TM isusedtomeasure the real- time changes of capacitance (C) and conductance (G). The measurements are performed under a low humidity ambient. 2.3. Measurements with organic solvents To evaluate the response of our sensor to organic solvents, we exposed thedevice to moleculeswithdifferent dipolemo- ments, polarizabilities and dielectric constants. The solvents were separatedin two groups,polar andnon-polar molecules. Water was analyzed independently. The characteristics of the solvents used are shown in Table 1 [25] Four characteristics were selected to understand the response: • Dielectric constant: related tothe electric field distribution inside the porous layer. • Dipole moment:relatedto the local fields on the surface of the porous layer. • Polarizability: related to the orientation of the molecule with respect to the porous layer surface. • Bond character: related to water solubility (polar molecules are more soluble in water than non polar molecules). Prior to the addition the sensor was allowed to stabilize for at least 20 min under the temperature and humidity conditions indicated in Section 2.2. Individual experiments were performed on different layers by adding 10␮l of solvent. We have investigated the sensitivity of the sensor to volumes in the range of 2–10 ␮l and our results suggest that the volume of the solvent is not related to the magnitude but rather to the duration of the response (time during which the layer remains wet). A more detailed characterization of the volu- 350 M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 Table 1 Values of dielectric constant (ε), dipole moment (µ) and electronic polarizability (α) of the organic solvents tested [25] Solvent dc dielectric constant Dipole moment (D) Electronic polarizability (×10 24 cm 3 ) Bond character Acetonitrile 37.5 3.45 4.4 Polar Acetone 20.7 2.85 6.33 Polar Ethanol 24.5 1.69 5.41 Polar Chloroform 4.81 1.15 9.5 Polar Benzene 2.27 0 10.32 Non-polar Toluene 2.83 0.43 12.26 Non-polar Water 80 1.82 1.45 Polar metric sensitivity of the device would require the handling of subnanoliter volumes. Within this range the response could besignificantlyinfluencedbythesolvent’s vaporpressureand its surface tension, which in turn would affect the infiltration of the solvent into the porous layer. In our experiments, the response of the capacitance (C) and the conductance (G)was measured as the percentage change of the signal with respect to the reference value. 3. Model 3.1. Space charge region modulation (SCRM) model In order to understand our results and to compare them with those in the literature we have developed a space charge region modulation (SCRM) model. We will first describe our sensor as a field effect device and derive an equivalent electrical circuit. The principles ofdetectionand the assumptions made in the analysis will then be discussed. Finally, the correlation of the electrical response with the electrical characteristics of the molecules will be presented in Section 4. In a field effect transistor (FET) current flows through a channel when a voltage is applied between the source and drain terminals. The conductance in the channel, which is directly proportional to its dimensions and the number of carriers can be modulated by changing either of these two variables. This modulation is done by applying an electric field through a metal gate terminalwhichcan be placed at the same plane between the source and drain (e.g., MESFET) or parallel to them (e.g., JFET). When FETs areusedaselectro- chemical transducers the metal gate is substituted by an electrolyte or a synthetic selective membrane and modulation of the conductance in the channel results from thechangeinpo- tential at the semiconductor surface when chemical species are present. In our device the gate electrode is substituted by the porous layer and the channel is the c-Si substrate. Al- though the experiments are carried out under a controlled humidity ambient, the presence on the porous layer surface of at least a monolayer of water is unavoidable. This initial condition of the porous layer surface renders it with a larger hydrophilic character, which influences its adsorption properties. When a molecule is infiltrated in the porous layer its interaction with the surface will change the field distribution in the c-Si rods. The porous layer then becomes a charged layer that can modulate the field in the c-Si channel by two mechanisms: (1) change in thespacecharge region by charge redistribution and (2) change in the width of the conductive channel. IntheSCRMmodel, adsorbedmolecules changethespace charge region ordepletion layer of thecrystalline silicon rods and even affect the crystalline silicon substrate. In lightly doped Si small changes in the surface charge produce a large change in the space charge region width. Since the oxide that covers the surface is thin the effect of electrical charges on the oxide surface can influence the underlying silicon. These two characteristics constitute the principle of detection and transductionofourPSisensor. Wenotethattheyhaverecently been exploited for the development of other devices such as cantilever field effect sensors [26]. When thesurfaceof theporous layeris exposed toa liquid, electrostatic equilibrium at the solid/liquid interface must be established. This produces an electric field that results in an electrical double layer formed by the space charge region (in the semiconductor) and the Gouy layer (equivalent to the space charge region in the electrolyte). Adsorbed ions between these two layers form the Helmholtz region, which to- gether with the semiconductor space charge region, accounts for most of the potential drop (Galvani potential) across the double layer. While the Helmholtz region is primarily sensitive to the adsorption/desorption of ions and the electrolyte composition, the role of the semiconductor space charge region is to compensate charge until equilibrium is reached [21]. The response of our device is produced by the charge compensation in the c-Si along with changes in the dielectric constant of the porous layer. The change in the space charge region will depend on the sign of the charge “felt” by the c-Si rod and its magnitude, which is also influenced by the orientation of the molecule with respect to the surface. If the effect is that of a positive charge then the width of the space charge region increases, the opposite being true for negative charges. Water soluble molecules tend to as- sociate closer to the surface therefore inducing a stronger effect. 3.2. Simulations and equivalent circuit In order to evaluate the field distribution in our device we performed a simulation of the electric field using com- mercially available software (Ansoft, Maxwell 2-D). Fig. 5 M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 351 Fig. 5. Simulation of the electric field distribution in the cross section of a c-Si substrate with two parallel electrodes. An ac voltage of 1 V peak to peak at 100 kHz excitation frequency and a dc voltage of 0V were used as experimental parameters. shows the calculated electric field distribution for 15 cm p-type Si with two parallel contacts placed on the back. The applied ac voltage is 1 V peak to peak at 100 kHz excitation frequency and the dc voltage is 0 V. The field intensity is larger near the contacts and gradually decreases towards the top c-Si surface. This field distribution is very similar to the one reported by Ramos et al. [27] for dielectrophoresis applications with a similar electrode geometry. The current density distribution is also such that its magnitude is larger at the interface with the electrodes close to the gap, and gradually reduces towards the surface of the c-Si substrate. The simulation shows that the electric field can reach the top of the c-Si where the PSi layer is located. As mentioned before the response of our device is based on changes in the porous layer induced by the presence of molecules. This effect was simulated by considering a uniform sheet of charge on the top surface in the presence of a dc voltage of 1 V and an ac voltageof 0V betweenthe electrodes.As anexamplea lineof charge of 34 nC was used for this simulation. We calculated this value from the number of silanol (Si OH) groups per unit area on chemically oxidized silica surfaces as reported in theliterature [28].Thecalculations weremade considering a density of ∼3 × 10 12 silanols/cm 2 [21,28] distributed over a line of charge of 7 mm length and 1 mm width with each silanol group contributing to one elemental charge (1.6 × 10 −19 C). Although this is just an approximation under ide- Fig. 6. Simulation of the electrostatic field distribution in the cross section of the c-Si substrate with a uniform line of charge of 34nC on the top surface. A dc voltage of 1V and an ac voltage of 0 V were used as the experimental parameters. alized conditions (all silanols protonated) the same approach has been used in other sensor devices [26] with satisfactory results. Fig. 6 shows the calculated dc electric field distribution under these conditions. A modification in the charge at the top silicon surface induces a strong variation in the field distribution (1 order of magnitude) with respect to the undis- turbed condition (results not shown) and therefore a change in the measured electrical properties. The results of the simulations and the following assumptions were used in the model: • The interface states at the metal–semiconductor junction donotaffecttheresponse.Ajunction capacitanceispresent at each electrical contact and its value remains constant at the given excitation frequency [24]. • The porous layer is modeled as a composite material made of alternating pores (or void space) and crystalline silicon rods with bulk silicon properties. • The electric field (E) and the current density (J) reach the porous layer. • The infiltration of the porous layer with materials of different physical properties produces a change in the width of the depletion region of the c-Si rods and a change in dielectric constant of the void space. • The PSi layer can be considered as a charged layer in contact with thec-Si substrate. Changes in thecharge distribution within the porous layer extend into the c-Si substrate. 352 M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 Fig. 7. Schematic section of the sensor showing a PSi layer composed of void space or pores and c-Si rods, and the c-Si substrate. Each element in the porous layer is modeled as a parallel capacitor–resistor element, C pore || R pore for the pores and C rod || R rod for the c-Si rods. The c-Si substrate is represented by G subs and C junct is the metal-silicon junction capacitance at the contactthat remains constant. The fieldstrength hasa maximum value at the substrateand gradually reduces towards the porous layer. Theequivalent circuit is shown on the right. Fig. 7 shows a schematic representation of a section of the PSi device composed of pores, c-Si rods and a c-Si substrate. In the porous layer the pores and the rods are modeled as impedances composed of a parallel capacitor–resistor (RC) element. Z pore (C pore || R pore ) accounts for the pores and Z rod (C rod || R rod ) for the c-Si rods. The c-Si substrate is represented by Z subs (G subs ) and two junction impedances at the contacts sites Z junc (C junct ). The equivalent impedance is given by: Z eq = [(Z subs )(Z pore + Z rod )] Z subs + Z pore + Z rod + 2Z junct (1) In addition, C rod ∝ 1/W d , C pore ∝ ε, and G subs ∝ (aN A ), where W d represents the width of the space charge region, N A the number of carriers per cm 3 and a the conductive c-Si channel width. When the pores are empty, R pore is infinite so no current flows through. Conduction in the c-Si rods of the porous layer does not depend solely on the doping density (N A ) but also on the presence of a depletion layer, the majority carrier distribution underneath it and the orientation of the interconnected crystalline silicon rods with respect to the current paths.We have evaluatedthis effectinself-supporting layerswiththesamecontactgeometry.Inthis casetheelectric fieldwasconfined completelyintheporouslayer.A reference conductance valueof 60–100nShas beenmeasured whenthe pores are empty. In our devices, PSi is attached to the substrate, the electric field in the porous layer is thus weaker and itsinfluenceonthe measuredconductivityisminimal.Indeed, we measure a higher reference conductance between 0.1 and 0.2 mS. This change in magnitude is due to the presence of the c-Si substrate and the metal–semiconductor contact. The model can therefore be further simplified by neglecting R rod and R pore and considering only C pore and C rod in the porous layer impedance. It is also worthy to clarify that a Schot- tky barrier is considered at the metal–semiconductor contact. This is based on calculations of the difference in the work functions between silver contact and the low-doped p-type c-Si. This assumption is difficult to address experimentally since the measured conductance includes the contribution of the c-Si substrate and not only that of the junction. When the PSi layer is infiltrated with charged molecules the electricaldouble layer changes.Charge redistributionand changes in the dielectric constant take place. Fig. 8 shows a schematic of the effect produced by positive charge on the surface and the simplified electrical equivalent circuit. The molecule’s charge on the surface changes the width of the space charge region (W d ) and the majority carrier distribution underneath it, which in turn influences C rod while the change in dielectric constant modifies C pore . Majority carrier redistribution in the c-Si substrate and a reduction of the width of the conduction channel (a) modify G subs . 4. Results and discussion 4.1. Response to polar molecules Fig. 9 shows the real-time evolution of the normalized capacitance (C) and conductance (G) of the sensor after exposure to10␮l of chloroform,acetone,ethanol, andacetonitrile at room temperature. The device’s response is different for each solvent and suggests that they can be identified by at least three parameters: magnitude, sign, and duration of the response (related to the evaporation rate of the solvent). The reversibility of the response with ethanol and acetone is con- sistent with the reported literature [12,29] for luminescent PSi. In Table 2, we present for each solvent the maximum Fig. 8. Schematic representation of the effect induced by the presence of a positive charge on the surface of the porous layer. The space charge region width (W d ) increases within the crystalline silicon rod and active carriers (N A ) are redistributed below the space charge region. The width of the conductance channel a is reduced as a result of the increased depletion region. The simplified electricalequivalent circuit ofthemodel is shownon the right along with the change of each variable. M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 353 Fig. 9. Real-time capacitance (C) and conductance (G) measurements of individual porous silicon sensors upon exposure to (a) chloroform, (b) acetone, (c) ethanol, and (d) acetonitrile. In each case 10 ␮l of the solvent was added at a time indicated by the black arrow. change in capacitance (%C) and conductance (%G) with respect to reference. A negative (positive) value corresponds to a reduction (increase) below (above) the reference value. Fig. 10 shows that the results of Table 2 are correlated with the dielectric constant. According to the SCRM model infiltration of the pores with a different material changes C pore . We need to explain why chloroform,ethanol andacetone producea negative shift with respect to reference. If the only variable involved in the response was C pore then as the solvent penetrates the pores a reduction in the capacitance would never be observed. This suggests that a mechanism other than pore filling needs to be considered and that the other physical properties (water solubility and dipole moment) of the molecules affect C rod , specifically via the electrical double layer as described ear- lier. The four polarsolvents evaluated in this partofthestudy possess different degrees of water solubility and a dielectric constant larger than three. The water solubility influences the adsorption on the surface and the value of the dielectric constant the type of polarizability. For a dielectric constant Table 2 Maximum percentage changes in capacitance (%C) and conductance (%G) with respect to the reference value for the polar molecules tested Solvent (%C)(%G) Chloroform −44 −46 Acetone −13 −21 Ethanol −7 −10 Acetonitrile 53 37 A negative sign indicates a reduction of the variable below the reference value. below 2.5, molecules exhibit only electronic polarizability and, above this value, a certain degree or orientation polarization (for example water). The polarizability plays a role in the response since it defines the orientation of the molecule with respect to the electric field and the surface and therefore its effect in the space charge region. In order to explain the effects in both the pores and the rods, we will first consider the simplified model presented in Fig. 8. The porous silicon capacitance (C PSi ) consists of a series arrangement of C pore and C rod , therefore: C PSi = C rod C pore C pore + C rod (2) Fig. 10. Measured change in capacitance (%C) and conductance (%G) with respectto thereference value as a function of the dielectric constant for chloroform, acetone, ethanol and acetonitrile. 354 M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 If C rod  C pore then C PSi ≈ C rod and, since C rod decreases as a result of the increase in W d , then the change will be negative (−C PSi ). If C rod  C pore then C PSi ≈ C pore and the change will be positive (+C PSi ). Chloroform, acetone and ethanol produce a decrease in capacitance. Out of these three molecules chloroform is the less water soluble with the lowest dipole moment, which suggests that it is interacting weakly with the surface and that the orientation of the dipole array is inducing an effect “felt” as a large positive charge that depletes the c-Si rod making C rod  C pore . As the water solubility increases along with the dipole moment and the dielectric constant, the positive charge-like effect is reduced making C rod ≈ C pore . Acetonitrile, which has a larger dipole moment,largerdielectricconstantanda highwatersolubility, induces a positive shift by making C rod  C pore . The modulation of the space charge region width in the c-Si rods is similar to what has been reported for silicon nanowires [29] and is strongly dependent on the surface charge. Fig. 11presentsasimplesimulationofthe modelvariables over time for molecules acting as a positive and a negative charge on the surface. Both the time scale and the values shown are arbitrary values for illustrative purposes only. In these simulationswe havefurther assumedthat theequivalent capacitance (C eq ) is given by the porous silicon capacitance (C PSi ) in series with a fixed junction capacitance (C junct )at each contact and that the equivalent conductance (G eq )is given by the substrate conductance (G subs ). The conductance is considered to be directly proportional to the number of carriers in the c-Si substrate (N A ) as well as the width of the channel (a). Starting with a dry device (air in the pores) three stages in the response are identified: exposure (addition of the solvent), stabilization (complete infiltration of the pores) and evaporation (drying of the pores). According to these simulations the effect of acetonitrile is that of a negatively charged molecule, which produces the response in Fig. 11b whilechloroform, ethanolandacetone actasa positivecharge (Fig. 11a). As it can be seen in Fig. 9, neither chloroform nor acetonitrile return to the reference value after the solvent has evaporated, even after flushing the chamber with nitrogen. This suggests that a chemical reaction has taken place, in- Fig. 11. Effect of the modification in the model parameters C pore , C rod , and C PSi in the equivalent capacitance (C eq ) and conductance (G eq ) when molecules acting as a positive charge (a) and a negative charge (b) interact with the surface. The exposure phase corresponds to the initial contact with the organic solvent that infiltrates into the layer. The stabilization period corresponds to the complete infiltration of the layer. As the solvent evaporates the signal returns to the reference value. troducing states that act as a charged layer at the surface. The effect of chloroform on oxidized PSi is not documented but in hydrogen-terminated silicon (no oxide) other chlori- nated hydrocarbons produce a reversible effect in its lumi- nescence [30]. Thedifference in reversibility suggeststhat an irreversible modification of the oxidized surface takes place. Similar considerations apply for acetonitrile, which has not been studied on oxidized surfaces but can be chemically adsorbed on clean silicon surfaces [31]. The relevance of the oxide properties on the sensitivity of PSi has been widely investigated by Sailor and coworkers [13]. It is also interesting to notice that acetonitrile and chloroform have the highest ionization potential of the four solvents tested (12.194 and 11.37 eV, respectively). The energy of chemisorption is the difference between the work function of the semiconductor and the ionization energy of the molecule [32]. If chemisorption is taking place then changes in the surface charge influence the response of the device. When the samples initially exposedtoacetonitrileweresubsequentlyexposedtoethanol, no response was observed which confirms that exposure to acetonitrile produced a permanent surface modification. This wasfurther confirmedwhenthe sensorswerereexposedto the solventfor severaltimes allowingtherecovery ofthe baseline between additions. For ethanol and acetone, in which the response was reversible, the variation of the maximum change in capacitance (%C) andconductance(%G) was no more than ±1% with a maximum shift in the baseline of 5% over a 20min timeline. For acetonitrile and chloroform, the response is not reversible therefore the same sensor could not be tested but the reproducibility of the response in different sensors was good (±5%). 4.2. Influence of the channel width (a) in the response It was mentioned before that the value of G subs depended on the change in the space charge region width (W d )by changing the majority carrier distribution underneath and the width of the conduction channel (a). Neither of these two variables can be probed directly or their independent contribution extracted from the experimental value of conductance. Nevertheless the channel width can be also M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 355 Table 3 Maximumpercentagechanges inconductance(%G)with respecttotheref- erence value for PSi layers of different thicknesses upon addition to ethanol PSi thickness (␮m) (%G) 10 −6 20 −7 40 −10 60 −15 80 −13 100 −12 changed by increasing the porous silicon thickness, which in turn reduces the thickness of the c-Si substrate. Although this is a geometric modification of the channel rather than a space charge width related in can provide some insight into the relevance of this variable in the response. For this purpose, PSi layers of different thicknesses varying from 10 to 100 ␮m were tested using ethanol. The experiments were done as explained in Section 2.2 and the percentage change in conductance (%G) was measured with respect to the reference value. The results are presented in Table 3. These results show a correlation of a decreasing conductance (larger – %G) as the channel becomes narrower (thicker PSi layers) up to 80 ␮m. This supports our assumption that G subs is affected by the channel width (a). It is likelythatthe maximumresponse occurswithin thefirst 50or 60 ␮m PSi thickness and after this depth has been infiltrated the sensitivity of W d to any further charge compensation is reduced. At this point the effect of W d and the carrier distribution may be defining the limits of sensitivity rather than the geometry changes. 4.3. Response to water It has been demonstrated that water increases the conductance [8,33] and capacitance [15,34] of PSi. This is the basis for PSi humidity sensors. A change in dielectric constant, dipole moment and possible chemisorption or physioadsorp- tion on the surface of porous silicon has been proposed to explain the response. An additional characteristic is the in- trinsic dipolemoment of thewater moleculethat confers pure orientation polarizability. Fig. 12. Real-time capacitance (C) and conductance (G) measurements of the porous silicon sensor upon exposure to water. (a) The first part of the response with a coupled behavior in capacitance (C) and conductance (G) is observed over the first minutes of the response. (b) Over a large period of time the two responses are different. TheexperimentalresultsshowninFig. 12canbeexplained with the SCRM model and the factors previously mentioned. We identified two phases intheresponse. The first one shown in Fig. 12a is characterized by a reduction in conductance coupled with a reduction in capacitance. In the second phase of the response the capacitance increases slowly above the reference value while the conductance remains uncoupled from this behavior as shown in the complete response in Fig. 12b. The firstpart oftheresponseshown inFig. 12aisproduced by apositivechargeon the surface. Thisis in accordancewith the observations made by Moeller et al. [33], which suggest that during water adsorption states behaving as acceptors can be introduced changing the surface charge. In our model this in turn changes the characteristics of the electrical double layer and therefore the space charge region. Charge redistribution in the c-Si rods decreases C rod and G subs is modified as majority carriers accumulate below the space charge region and the conductive channel width (a) is reduced. After adsorption on the surface has reached a steady state C rod and G subs remain constant and the increase in capacitance is produced by C pore . Since the permittivity of molecules with orientation polarization decreases at high frequencies [35] C pore changes in such way that C rod  C PORE , making C PSi ≈ C pore and producing a positive signal (+C PSi ). 4.4. Response to non-polar molecules Schechter and coworkers [8] reported an enhancement of PSi conductivity withexposure tomoleculeswith zero dipole moment. They suggested that the conductivity enhancement could be related to other factors aside from the dipole moment. To investigate this possibility we performed experiments using benzene (µ =0D) and toluene (µ = 0.43D). Both exhibit a very low water solubility and given the value of their dielectric constant (ε = 2.27 for benzene, ε = 2.38 for toluene) the polarization of these molecules is purely electronic. The characteristic response in capacitance and conductance along with the measured change in these variables is shown in Fig. 13. Since we do no have an independent way to probe the influence of the field on the molecule inside the pores the 356 M. Archer et al. / Sensors and Actuators B 106 (2005) 347–357 Fig. 13. Real-time capacitance (C) and conductance (G) measurements of porous silicon sensors upon exposure to (a) benzene and (b) toluene. In each case 10 ␮l of the solvent was added as indicated by the black arrow. results will be explained based on the predictions of the SCRM model. The hydrophobicity (low water solubility) of toluene and benzene and their low dielectric constant explain the small change in capacitance and conductance, and the sign of the maximum change (indicative of a positive charge). Assuming that these molecules interact weakly with the surface, do not produce permanent chemical modifications (reversibility of the response) and do not have a permanent dipole moment the only parameter left to influence the response is the electronic polarizability (α). The propagation of the field inside the structures is orienting the molecules perpendicular to the pore wall surface and the lack of orientation polarizability eliminates any counteracting effect. The orientation of the dipole array is “felt” as a positive charge. According totheSCRMmodel,thiseffectandtheweakinter- action of the molecule with the surface produce a very small increase in the space charge region width (W d ) therefore decreasing slightly C rod , which impacts on the charge redistribution atthesubstrate (G subs ). A small changeinC pore is also expecteddue to thelowdielectric constant.These results suggest that the interaction of the molecule with the surface and the orientationofthe dipoleplaya crucialrolein the response of the device. Reversibility of toluene and benzene has also been reported in as anodized luminescent PSi [12,30]. 5. Conclusions The large surface area of porous silicon and the sensitivity of its surface to charge molecules make it an ideal can- didate in sensor development. We have evaluated the use of a new electrical sensing device based on macroporous silicon (pore diameter 1-2 ␮m) layers in which the contacts are made on the backside of the substrate. This approach allows complete exposure of the surface without the presence of metallic contacts on the surface. The sensitivity of this device is not only related to the dipole moment and the dielectric constant of the molecules but also to their interaction with the surface and the alignment of their dipole. Molecules with different electrical and chemicalcharacteristicsproduce a change in magnitude and sign in the capacitance and conductance. To explain our results, we proposed a space charge region modulation (SCRM) model that considers the effect of changes in the dielectric constant of the porous silicon matrix along with the interaction of different molecules with the surface. The simulations performed consider the simul- taneous change in dielectric constant and charge distribution induced bymoleculeswithdifferent propertiesand theresults obtained are in accordance with our experimental results. In this paper, we demonstrated the use of our device as a chemical sensor capable of producing a different response upon exposure to water, ethanol, acetone, chloroform, acetonitrile, benzene and toluene. The sensitivity to charged molecules can however extend the use of these devices to biological applications. We have also demonstrated the use of oursensor forselectiveand reproducibledetectionof DNA hybridization in real-time [36]. Acknowledgements This work was supported in part by a grant from the In- fotonics Center of Excellence and the sponsors of the Center for Future Health. M.A. was the recipient of a dermatology training grant. References [1] L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. (1990) 57. [2] V. Lehmann, U. Gosele, Porous silicon formation: a quantum wire effect, Appl. Phys. Lett. (1991) 58. [3] C. Cadet, D. Deresmes, D. Villaume, D. Stievenard, Influence of surface defects on the electrical behavior of aluminum-porous silicon junctions, Appl. Phys. Lett. 64 (1994) 2827–2829. [4] V. Lehmann, F. Hofmann, F. Moeller, U. Gruning, Resitivity of porous silicon: a surface effect, Thin Solid Films 255 (1995) 20–22. [5] L.A. Balagurov, D.G. Yarkin, E.A. Petrova, Electronic transport in porous silicon of low porosity made on a p+ substrate, Mater. Sci. Eng., B 69/70 (2000) 127–131. [6] J. Rehm, G. McLendon, L. Tsybeskov, P.M. Fauchet, How methanol affects the surface of blue and red emitting porous silicon, Appl. Phys. Lett. 66 (1995) 3669–3671. [...]... Christophersen, P.M Fauchet, Macroporous silicon electrical sensor for DNA hybridization detection, Biomedical Microdevices 6 (3) (2004) 203–224 Biographies Marie Archer received her Masters Degree in Biomedical Engineering from the University of Rochester in May 2003 The topic of her Ph.D research was the characterization of electrical porous silicon based sensors for their use in biodetection She carried out... Cunin, J.R Link, M.J Sailor, Standoff detection of chemicals using porous silicon “Smart Dust” particles, Adv Mater 14 (2002) 1270–1272 [13] T Gao, J Gao, M.J Sailor, Tuning the response and stability of thin film mesoporous silicon vapor sensors by surface modification, Langmuir 18 (2002) 9953–9957 [14] M Archer, P.M Fauchet, Electrical sensing of DNA hybridization in porous silicon layers, Phys Stat Sol... Q Wei, H Park, C.M Lieber, Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species, Science 293 (2001) 1289–1292 357 [30] M.J Sailor, in: L.T Canham (Ed.), Properties of Porous Silicon, Institute of Electrical Engineers, London, UK, 1997 [31] F Tao, Z.H Wang, M.H Quiao, Q Liu, W.S Sim, G.Q Xu, Covalent attachment of acetonitrile on Si(1 0 0) through Si C... Schechter, M Ben-Chorin, A Kux, Gas sensing properties of porous silicon, Anal Chem 67 (1995) 3727–3732 [10] S Green, P Kathirgamanathan, Effect of oxygen on the surface conductance of porous silicon: towards room temperature sensor applications, Mater Lett 52 (2002) 106–113 [11] J Rehm, G.L McLendon, P.M Fauchet, Conduction and valence band edges of porous silicon determined by electron transfer, J Am Chem... A Satou, Gas identification by a single gas sensor using porous silicon as the sensitive material, Jpn J Appl Phys., Part 1 34 (1995) 5840–5843 [18] K Watanabe, T Okada, I Choe, Y Sato, Organic vapor sensitivity in a porous silicon device, Sens Actuat B 33 (1996) 194–197 [19] S.-J Kim, S.-H Lee, C.-J Lee, Organic vapour sensing by current response of porous silicon layer, J Phys D: Appl Phys 34 (2002)... Christophersen, J Carstensen, G Hasse, Formation and application of porous silicon, Mater Sci Eng R39 (2002) 93–139 [21] H.H Okorn-Schmidt, Characterization of silicon surface preparation processes for advanced gate dielectrics, IBM J Res Dev 43 (1999) 351–365 [22] A Nakajima, T Itakura, S Watanabe, N Nakayama, Photoluminescence of porous Si, oxidized then deoxidized chemically, Appl Phys Lett 61 (1992)... postdoctoral fellow at the Center for Future Health His research at University of Rochester involved the use of porous semiconductors for photonic crystals, MEMS structuring and biosensor applications Professor Philippe Fauchet has 20 years of experience in semiconductor optoelectronics, ultrafast phenomena and lasers, nanoscience and nanotechnology with silicon, biosensors, electroluminescent materials... (2003) 503–507 [15] R.C Anderson, R.S Muller, C.W Tobias, Investigation of porous silicon for vapor sensing, Sens Actuat A A21/A23 (1990) 835–839 [16] A Motohashi, M Ruike, M Kawakami, H Aoyagi, A Kinoshita, A Satou, Identification of water molecules in low humidity and possibility of quantitative gas analysis using porous silicon gas sensor, Jpn J Appl Phys., Part 1 35 (1996) 4253–4256 [17] A Motohashi,... University of Florida under the supervision of Professor Dr Hummel He received his Ph.D degree in engineering (Doctor in Engineering) on June 2002 and was awarded “summa cum laude” for his research work Marc Christophersen is a co-author of over 40 publications in scientific journals and holds three German patents for semiconductor structuring He worked for Professor Dr Fauchet’s group at the University of. .. Ben-Chorin, F Koch, Post-treatment effects on electrical conduction in porous silicon, Thin Solid Films 255 (1995) 16–19 [34] Z.M Rittersma, A Splinter, A Boedecker, W Benecke, A novel surface-micromachined capacitive porous silicon humidity sensor, Sens Actuat B 68 (2002) 210–217 [35] S.-J Kim, J.-Y Park, S.-H Lee, S.-H.J Yi, Humidity sensors using porous silicon with mesa structure, Phys D: Appl Phys . Sensors and Actuators B 106 (2005) 347–357 Electrical porous silicon chemical sensor for detection of organic solvents M. Archer a,∗ ,. Macroporous silicon; Electrical sensors; Organic solvent detection; Chemical sensor 1. Introduction Porous silicon( PSi) isproduced by electrochemicaldisso- lution

Ngày đăng: 16/03/2014, 15:23

Xem thêm: Electrical porous silicon chemical sensor for detection of organic solvents