Investigation of capacitive humidity sensing behavior of silicon nanowires

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

Investigation of capacitive humidity sensing behavior of silicon nanowires Huilin Li, Jian Zhang à , BaiRui Tao, LiJuan Wan, WenLi Gong Department of Electronic Engineering, State Key Laboratory of Transducer Technology, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China article info Article history: Received 14 July 2008 Received in revised form 21 October 2008 Accepted 28 October 2008 Available online 18 November 2008 PACS: 71.15.Pd 71.20.Mq 73.21.Hb 73.63.Rt Keywords: Silicon nanowires Humidity sensor Relative humidity Capacitance–frequency conversion abstract In this paper, the fabrication and the sensing characteristics of the humidity sensors based on the electroless chemical deposition-etched silicon nanowires had been studied. The humidity sensors were constructed by the selectively electrochemically etched silicon nanowires. The sensing mechanism is based on the capacitance variations due to the adsorption/desorption of water vapor of silicon nanowires. The frequency–capacitance conversion circuit had been set up to convert the capacitance variation into the frequency shift. Labview system had been employed to monitor and record the frequency. The study indicated that the humidity sensors had the simple structure and the high performance such as the high sensitivity, the wide humidity detection range, the good stability and repeatability. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Recently, silicon nanowires (SiNWs) had attracted more and more attention due to their potential applications in nanosensors and nanoelectronics [1–5]. The studies had indicated that the SiNWs had some favored qualities such as the big surface-to- volume areas and the superior electrical properties which can be modulated [6–8]. For example, SiNWs are a good candidate sensing materials for gas sensors [9]. Besides the advantages mentioned above, the fabrication process of SiNWs is also compatible with an ordinary silicon production process [10].So the integration of the SiNWs-based sensors and the integrated circuits are possible. All these will greatly improve the sensor performance. Although SiNWs are the potential materials for sensing application, the research about the SiNWs-based humid- ity sensor, to our knowledge, is seldom found. It has been demonstrated that water adsorption increases the conductance and the capacitance of porous silicon (PS) [11–15]. This is the basic sensing mechanism for PS humidity sensors. A change in dielectric constant, dipole moment and possible chemisorption or physi- sorption on the surface of PS had been proposed to explain the response [16]. Therefore, we postulated that the SiNWs should exhibit the good humidity sensing behavior just as PS. In this paper, the humidity sensing characteristics of SiNWs prepared by the electrochemically etched method were studied. And a novel capacitive humidity sensor based on the SiNWs was fabricated. The sensing properties of SiNWs were studied. 2. Experiment 2.1. Preparation of silicon nanowire The silicon nanowire was fabricated according to Ref. [17] using a chemical etching procedure. The detailed process is as follow: 1.19 g AgNO 3 was dissolved in 100 ml distilled water under the ultrasonic agitation. Then, 100 ml HF (20%) was added at room temperature. The mixed solution was used as the etchant for SiNWs preparation. The chemically cleaned silicon wafers were put into the etchant. The etching time was kept $60 min in this study. Fig. 1(a) is the top-view SEM picture of silicon nanowires as- received and (b) is the cross-section SEM image. It is observed that the silicon nanowires have been prepared on the substrate. They are aligned perpendicularly to the bulk silicon substrate and their average length is about 80 m m. And the length of SiNWs can be adjusted by controlling the proper etching time. 2.2. Sensor configuration The humidity sensors were prepared from the silicon nano- wires. Fig. 2 is the schematic diagram of the sensor. The humidity sensors were constructed by glued two copper leading wires into ARTICLE IN P RESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.10.016 à Corresponding author. E-mail address: jzhang@ee.ecnu.edu.cn (J. Zhang). Physica E 41 (2009) 600–604 the surface of SiNWs structure. For all sensors, the distance between two leading wires was kept at $3 mm. Fig. 3 is the equivalent circuit model of the humidity sensor constructed. In principle, the sensor can be simplified into the parallel connection of a resistor R b , and lots capacitors C xy (1oxom and 1oyon). The reisistor R b represents the influence of the silicon substrate. The substrate resistance is a constant because it is hardly affected by outer moisture. And the capacitors C xy represent the capacitances between two adjacent charged individual nanowires. The total capacitance C H can be equivalent to the sum of lots of capacitors (with capacitance C xy ) in serial and parallel connection: Cn ¼ Cn1kCn2k Cnm (1) C H ¼ C1 þ C2 þ þ Cn (2) If the capacitance between any two nanowires, C xy , is simplified as a parallel plate capacitor, the capacitance can be expressed as C xy ¼ e 0 e r (A/d), where e 0 is the vacuum dielectric constant, e r the relative dielectric constant between the nanowires, d the distance between two nanowires, A the aligned area between two nanowires, respectively. In our study, d and A are fixed. The water vapor adsorbption onto the SiNWs can cause the variation of e r and lead to the change of the capacitance since the relative dielectric constant of water ($60) is larger than that of the air ($1). 2.3. System for humidity sensing detection Fig. 4 shows the schematic diagram of the humidity detection system. The system consisted of three parts: the data recording system, the standard humidity generation and the 555 capaci- tance–frequency conversion circuit. The LabView virtual instru- ments DAQ PCI6221 (PCI6221, NI, USA) were used to collect the output frequency of the 555 IC multivibrator circuit in real-time. The controlled humidity environments were achieved using the saturated aqueous solutions in a closed glass vessel at an ambient temperature of 25 1C [18]. The 555 capacitance–frequency conversion circuit can change the capacitance variations of the sensors into the frequency shifts. In the testing process, the humidity capacitive sensor was incorporated into the 555 time-based circuit and acted as a capacitor component. The capacitance variation of humidity sensors due to water adsorption can be transformed to the frequency shift. The fÀC H transformation equation is as follows: F o ¼ 1:43 RC H ðHzÞ (3) where C H is equivalent capacitance of the sensor, R the total equivalent resistance and f the output frequency, respectively. 3. Results and discussion The developed humidity sensors were tested in the home- made system as previously mentioned. The performances of sensors were characterized. 3.1. Humidity measurement In this study, four humidity sensors based on SiNWs prepared under different condition, denoted as samples 1–4, were tested. For samples 1–3, the etching time for the SiNWs was 60, 50, and 45 min, implying the different dimension of SiNWs resulted, respectively. After etching process, these three samples were annealed at 100 1C for several times in order to form the native oxide layer. For comparison, sample 4 was prepared under the etching time of 60 min without further annealing process. Fig. 5 is the frequency response curves of samples 1–4 with the corresponding relative humidity level. The initial frequency (at humidity of 11.3%RH) values are not different for four samples, implying that the initial capacitance values of these sensors are different. We can see that the output frequency values of the sensors tended to decrease when the humidity level increased from 11.3% to 98%. Nonlinear responses can be found for all sensors. The sensitivity of the sensor can be denoted as the slope for the response curves. We can find that the sample 1, with the longer etching time, exhibited the bigger slope, i.e., the higher sensitivity, À133.29 Hz/RH while for the unannealing sample 4, the sensitivity was low, À71.15 Hz/RH. The negative sensitivity ARTICLE IN P RESS Fig. 1. SEM pictures of silicon nanowires (a) top-view image and (b) cross-section image. Fig. 2. Principle model of humidity sensor developed. H. Li et al. / Physica E 41 (2009) 600 –604 601 values indicated that the output frequency decreased with the increasing humidity. The results indicated that the annealing process is benefic to enhance the sensor sensitivity. This can be explained by the fact that the annealing process under high temperature is beneficial to form the native silicon oxide layer and the oxidized SiNWs surface tends to be more hydrophilic. The capacitance values of the sensors under different humidity levels also can be calculated from Eq. (3). Fig. 6 is the relationship between the calculated capacitance values of samples 1–3 at different relative humidity. For samples 1–3, since the etching time, t, is different (t 1 4t 2 4t 3 ), the length of SiNWs resulted, L,is also different (L 1 4L 2 4L 3 ). From Fig. 6, we can see the capacitance values increase with the humidity level increasing. And sample 1 had the biggest capacitance change in these three samples. The longer SiNWs will lead to the larger capacitance and the increased sensitivity. With the length of the SiNWs increasing, the nonlinear degree of the response curve tends to increase. From the equivalent capacitance equation C xy ¼ e 0 e r (A/d), the longer nano- wires will lead to the increased electrode area, and thus the increased capacitance values. ARTICLE IN P RESS Fig. 4. Schematic diagram of testing platform of humidity characteristic for silicon nanowires sensor. 10 0 2000 4000 6000 8000 10000 12000 Frequency (Hz) Relative humidity (RH%) Sample 1 Sample 2 Sample 3 Before annealing 20 30 40 50 60 70 80 90 100 Fig. 5. The relationship between the frequency shift and the corresponding relative humidity. Fig. 3. Equivalent capacitance model of silicon nanowires sensor. 0 0 5 10 15 20 25 30 35 40 Capacitance (pF10 3 ) Relative humidity (RH%) Sample 1 Sample 2 Sample 3 20 40 60 80 100 Fig. 6. The relationship between the calculated capacitance variations and the corresponding relative humidity. H. Li et al. / Physica E 41 (2009) 600–604602 3.2. Reproducibility Fig. 7 shows the frequency behavior of sample 1 as a function of time for different relative humidity. This sample works under a humidity cycle of high-to-low and low-to-high step. From the figure, we can see that the ascending curves are quite similar with the descending ones. It is indicated that the sensor has good frequency reproducibility or low humidity hysteresis. And we can also see that the sensor absorption time is less than 180 s and the desorption time is less than 100 s. In this study, the sample 2 was used to cycling test between RH 11.3% and 85%. The consequence shows as Fig. 8. The test result indicates that the average frequency floating at RH ¼ 85% and 11.3% is only 70.5% and 71.1%, respectively. A slight floating frequency can be seen when the relative humidity comes back to the same value. So the silicon nanowires humidity sensors can work repeatedly. 3.3. Stability In this study, the silicon nanowires humidity sensor was measured in different relative humidity circumstance. Fig. 9 shows the long-time frequency stability at four different kinds of RH level. They are 11.3%, 43%, 75% and 85%RH, respectively. The frequency was measured every 5 min for 3 h and the frequency data were recorded by the computer. Slight variation in frequency float is observed over the time range. In all measurements, the variations of frequency float are less than 160 ppm. It is indicated that the sensors have a good stability characteristics under the same RH level. 3.4. Discussion On the silica surface, there are three different groups: siloxane bridges (QSi–O–SiQ), hydroxyl groups (–OH) and unsaturated Si atoms. The siloxane bridges are somewhat hydrophobic, while hydroxyl groups (–OH) and unsaturated Si atoms are absolutely hydrophilic. At low temperature, water vapor is absorbed on the silica surface by physisorption; at high temperature, it becomes chemisorbed by reacting with the siloxanes. Since the hydro- phobicity of silica surface increases with the decreasing amount of hydroxyl groups, the hydrothermal stability of silica can be improved by increasing the sintering temperature or by modifying with some organic or inorganic groups to substitute the hydroxyl groups. However, the organic groups on the silica surface themselves are not very stable at elevated temperatures. The variations of the capacitance were related to the amount of water vapor adsorbed. If we assumed that the capacitance variation, D C, is proportional to the water vapor adsorbed, D m. The capacitance variations also can be regarded approximately as the amount of water vapor adsorbed ( D Cp D m). The relative humidity is in fact the relative pressure of water vapor compared to the saturated pressure. The relationship between the capaci- tance variations, D C, and the humidity, RH are shown in Fig. 10. Fig. 10 can also be regarded as the isotherm curves for the sensors simultaneously. Further, according to the adsorption theory, the relationships between the capacitance and the humidity level were linearly fitted using Freundlich adsorption model, ln( D C) ¼ 1/n(RH)+ln K. Here, N is a constant which relate to water vapor (absorbent) and SiNWs (adsorbate). And K is a parameter which reflects to the adsorption capability of SiNWs. The bigger K indicates that the nanowires can adsorb water vapor more easily. Fig. 11 is the linear fitting curves for samples 1–3 following the Freundlich adsorption model. The parameters for fitting curves in detail are summarized in Table 1. From this table, for all sensor ARTICLE IN P RESS 0 0 2000 4000 6000 8000 10000 12000 75% (relative humidity) 85% 57% 43% 11.3% Frequency (Hz) Time (sec) 2000 4000 6000 8000 Fig. 7. Time-dependent frequency responses for the sensor under one cycle (with humidity level descending from 85% to 11.3%, then ascending to 85%). 0 2000 4000 6000 8000 10000 12000 14000 RH%(85%) Frequency (Hz) Time (Sec) Sample 2 RH%(11.3%) 500 1000 1500 2000 2500 Fig. 8. Reproducibility curve of silicon nanowires humidity sensor. 0 0 1 2 3 4 5 6 7 Frequency (KHz) Time / min RH(11.3%) RH(43%) RH(75%) RH(85%) Sample 3 20 40 60 80 100 120 140 160 180 Fig. 9. The long-time frequency stability testing at four different humidity circumstances. H. Li et al. / Physica E 41 (2009) 600 –604 603 samples, the correlation coefficient r is near to 1 which demonstrates that the Freundlich adsorption isotherms are suitable for our sensor adsorption. K values are much bigger than 1, indicating that the nanowires have a superior adsorption capability to the water vapor. For samples 1–3, the K values satisfied K 1 4K 2 4K 3 , implying that the longest SiNWs for sample 1 have the largest adsorption capability. This also had been verified by the highest sensitivity values of sample 1. In addition, the correlation coefficient r, satisfying r 1 or 2 or 3 , which indicates that the sample 3, with the shortest SiNWs, is most suitable for the Freundlich adsorption isotherm description, which has the best linearity. According to the fitting results, it was demonstrated that the samples adsorbing water vapor can be described by the Freundlich isotherm. So, it is concluded that the sensor humidity response can be attributed to both chemisorption and physisorp- tion. 4. Conclusions We have prepared silicon nanowires array using chemical etching. These nanowires arrange regularly and have high-specific surface area. The SiNWs have been used as a simple low-cost humidity sensor. Some properties like accuracy, reproducibility and stability of the sensor had been discussed in this paper. It is demonstrated that SiNWs is a useful humidity-sensitive nanos- tructured material. Because SiNWs can be fabricated easily as well as can be compatible with the latest silicon technology, silicon nanowires humidity sensors have great potential in actual applications. Acknowledgements The project is supported by National Natural Science Founda- tion of China (60672002), Shanghai Pujiang Project (06PJ14037) and Shanghai Leading Academic Discipline Project, Project Number: B411. References [1] C.L. Dai, M.C. Liu, F.S. Chen, C.C. Wu, M.W. 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Table 1 Results for isothermal adsorption equation for samples 1–3. ln( DC) ¼ 1/n(RH)+ln K nKr Sample 1 ln( DC) ¼ 5.0938ln(RH)+10.53172 0.1963 37485 0.9932 Sample 2 ln(DC) ¼ 4.7909ln(RH)+9.49948 0.2087 13352 0.994 Sample 3 ln( DC) ¼ 4.6385ln(RH)+8.54525 0.2156 5142 0.9984 H. Li et al. / Physica E 41 (2009) 600–604604 . Investigation of capacitive humidity sensing behavior of silicon nanowires Huilin Li, Jian Zhang à , BaiRui Tao, LiJuan Wan, WenLi Gong Department of. etched silicon nanowires. The sensing mechanism is based on the capacitance variations due to the adsorption/desorption of water vapor of silicon nanowires.