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

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Department of Electronic Engineering, State Key Laboratory of Transducer Technology, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China

a r t i c l e i n f o

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

a b s t r a c t

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

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 AgNO3 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 80mm 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 2is the schematic diagram of the sensor The humidity sensors were constructed by glued two copper leading wires into

Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/physe

Physica E

Corresponding author.

E-mail address: jzhang@ee.ecnu.edu.cn (J Zhang).

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the surface of SiNWs structure For all sensors, the distance

between two leading wires was kept at 3 mm

Fig 3is the equivalent circuit model of the humidity sensor

constructed In principle, the sensor can be simpliﬁed into the

parallel connection of a resistor Rb, and lots capacitors Cxy

(1oxom and 1oyon) The reisistor Rbrepresents the inﬂuence

of the silicon substrate The substrate resistance is a constant

because it is hardly affected by outer moisture And the capacitors

Cxy represent the capacitances between two adjacent charged

individual nanowires The total capacitance CHcan be equivalent

to the sum of lots of capacitors (with capacitance Cxy) in serial and

parallel connection:

If the capacitance between any two nanowires, Cxy, is simpliﬁed

as a parallel plate capacitor, the capacitance can be expressed as

Cxy¼e0er(A/d), wheree0is the vacuum dielectric constant,erthe 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 ofer

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 4shows 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 fCHtransformation equation is as follows:

Fo¼1:43

RCH

where CH 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 5is 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, 133.29 Hz/RH while for the unannealing sample 4, the sensitivity was low, 71.15 Hz/RH The negative sensitivity

Fig 1 SEM pictures of silicon nanowires (a) top-view image and (b) cross-section

image.

Fig 2 Principle model of humidity sensor developed.

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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 6is 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 (t14t24t3), the length of SiNWs resulted, L, is also different (L14L24L3) FromFig 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 Cxy¼e0er(A/d), the longer nano-wires will lead to the increased electrode area, and thus the increased capacitance values

Fig 4 Schematic diagram of testing platform of humidity characteristic for silicon nanowires sensor.

10

0

2000

4000

6000

8000

10000

12000

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

3)

Sample 1 Sample 2 Sample 3

Fig 6 The relationship between the calculated capacitance variations and the corresponding relative humidity.

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3.2 Reproducibility

Fig 7shows 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 asFig 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, DC, is proportional to the water vapor adsorbed,Dm The capacitance variations also can be regarded approximately as the amount of water vapor adsorbed (DCpDm) The relative humidity is in fact the relative pressure of water vapor compared

to the saturated pressure The relationship between the capaci-tance variations,DC, and the humidity, RH are shown in Fig 10

Fig 10can 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(DC) 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 11is the linear ﬁtting curves for samples 1–3 following the Freundlich adsorption model The parameters for ﬁtting curves in detail are summarized inTable 1 From this table, for all sensor

0

2000

4000

6000

8000

10000

12000

75%

(relative humidity)

85%

57%

43%

11.3%

Time (sec)

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%)

Time (Sec)

Sample 2 RH%(11.3%)

0 0 1 2 3 4 5 6 7

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

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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 K14K24K3, 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 r1or2or3, 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

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0

5

10

15

20

25

30

35

40

Sample 1 Sample 2 Sample 3

3)

Fig 10 Capacitance isothermal–adsorption curves of sensors at 25 1C.

-1.6

1

2

3

4

5

6

7

8

9

10

11

ln (RH)

Sample 1

Sample 2

Sample 3

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

Fig 11 Linearly ﬁtting curve for the Freundlich adsorption.

Table 1

Results for isothermal adsorption equation for samples 1–3.

ln(DC) ¼ 1/n(RH)+ln K n K r

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

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