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Copyright © 2016 American Scientific Publishers All rights reserved Printed in the United States of America

Vol. 16, 4901–4905, 2016 www.aspbs.com/jnn

The Analysis of Characteristics in Dry and Wet Environments of Silicon Nanowire-Biosensor Hyoun Mo Choi1 , Dong Jae Shin1 , Jung Han Lee2 3 , Hyun-Sun Mo1 , Tae Jung Park4 , Byung-Gook Park2 3 , Dong Myong Kim1 , Sung-Jin Choi1 , Dae Hwan Kim1 2 ∗ , and Jisun Park1 ∗ 1 School of Electrical Engineering, Kookmin University, Seoul 136-702, Republic of Korea Inter-University Semiconductor Research Center (ISRC), Seoul 151-742, Republic of Korea 3 Department of Electrical Engineering and Computer Science (EECS), Seoul National University (SNU), Seoul 151-744, Republic of Korea 4 Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea 2

Our study investigates differences in sensitivity of dry and wet environment in the field of biosensing experiment in detail and depth. The sensitivity of biosensing varies by means of surrounding conditions of silicon nanowire field effect transistor (SiNW FET). By examining charged polymer reaction in the silicon nanowire transistor (SiNW), we have discovered that the threshold voltage (VT ) shift and change of subthreshold slope (SS) in wet environment are smaller than that of the air. Furthermore, we analyzed the sensitivity through modifying electrolyte concentration in the wet condition, and confirmed that VT shift increases in low concentration condition of phosphate buffered saline (PBS) due to the Debye length. We believe that the results we have found in this study would be the cornerstone in contributing to advanced biosensing experiment in the future.

Keywords: SiNW FET, Capacitance, Debye Length, Phosphate Buffered Saline (PBS), Dry Environment, Wet Environment, VT Shift, Sensitivity, Biosensing.

1. INTRODUCTION Nano-structure devices, such as silicon nanowire (SiNW),1 2 Si nano-ribbon3 and carbon nanotube,4 5 have been in center of attention because of their promising usability of a low-cost6 and mass production.7 These devices have been considered as strong candidates for a portable biosensor, which is structured with a lab-ona-chip8 form that is based on the integrated circuit due to their availability for real-time diagnosis and label-free function.9 Particularly, it should be paid attention that the sensitivity and the surface to volume ratio of the silicon nanowire (SiNW) are much better than that of other devices10 as many research groups have reported with a focus on the surface chemistry in silicon nanowire (SiNW). Thus, the silicon nanowire (SiNW) is important and suitable to be utilized for biosensors. However, the silicon nanowire FET demonstrates unstable current ∗

Authors to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 5

characteristics11 when undergoing repeated measurement and there is low signal to noise (SNR).12 Moreover, it is generally known that the biosensor operates within the dry environment, yet there has not been sufficient studies.13 In order to cope with aforementioned problems, it is significant to investigate the sensitivity under various conditions, such as dry and wet environments. In this study, the main mechanism of sensing sensitivity, which is depending on the wet and dry environments of gate region reacting by a target in the silicon nanowire device, is analyzed with a stable method for obtaining high sensitivity.

2. EXPERIMENTAL DETAILS Figure 1 demonstrates the structure of SiNW and its top view. Table I represents structural parameters of the measured device employed in this study. The chemicals affects only the sensing area as shown in Figure 1(c) since the channel region of SiNW was etched and the other area were covered by passivation. In addition the gate induced

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doi:10.1166/jnn.2016.12247

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The Analysis of Characteristics in Dry and Wet Environments of SiNW-Biosensor

Choi et al.

(a)

TEOS

(b)

Al

PBS solution

ILD SiNW Buried oxide

TSiNW

Air

TBOX

p-type substrate PBS solution

(c) Sensing area Pad

Sensing area

S

Wch Source

Drain

Figure 1. (a) A schematic view of the device structure. (b) Top view of the SiNW device. (c) The enlarged image of the sensing area.

drain leakage (GIDL) property appears in the transfer curves because the sensing area includes some parts of the edge of source and drain. The process of experiment carried out in this study is as follows. In order to fabricate an isolated channel on a chip, polydimethylsiloxane (PDMS) and SiNW device were firstly exposed to UV ozone for 180 seconds, and then the device with PDMS was thermally treated on a hot plate at 120 C for 10 minutes. Also, the SiNW device was exposed to UV ozone for 330 seconds since the native oxide surface for reacting to 3-aminopropyltriethoxy-silane (APTES) has to have the hydrophilic property. Then, the SiNW device was immersed in 1% APTES, which allows reacting to the charged polymers in 99% ethanol at room temperature for 30 minutes. After that, it was rinsed out with ethanol, and finally the device was blown by N2 gas several times. Then, it underwent a heat treatment on a hot plate at 120 C for 10 minutes to remove unreacted APTES. Figures 2(a) and (b) show the SiNW devices with PDMS in both wet and dry environments, respectively. As you can see, the PBS solution was injected into PDMS reservoir in the wet environment whereas not a single action was taken intentionally in the dry environment to check if there is any difference. Also, we applied the voltage at back gate under the condition of constant drain bias as Table I. Structure parameters. Symbol

Type Width of SiNW Length of SiNW Thickness of native oxide Thickness of SiNW Thickness of buried oxide Doping concentration of SiNW Doping concentration of S/D

Wch Lch Tnox TSiNW TBOX NNW NSD

4902

Value

PAH PSS S

|VD| = 1 V

APTES layer SiNW D Buried oxide Si-substrate

VBG

Lch

Structure parameter

APTES layer SiNW D Buried oxide Si-substrate

(d)

Unit

Accumulation 10∼130 nm 2∼30 m 3 nm 80 nm 375 nm 2 × 1018 cm−3 1 × 1020 cm−3

|VD| = 1 V

VBG

Figure 2. The chips used in (a) wet (PBS), and (b) dry environments. The schematic view of measurement in (c) wet (PBS), and (d) dry environments after reacting to PAH.

shown in the Figures 2(c) and (d) to obtain and analyze the initial property of the SiNW device in the wet and dry environments. In order to evaluate the property by using negatively charged polymer, diluted polysodium-styrenesulfonate (PSS) solution of 50 M with deionized (DI) water was dropped on a SiNW surface for an hour at room temperature. By doing so, any chemical reactivity can be observed. Then, the surface was rinsed with DI water and dried with N2 gas. Then, transfer curves were measured in both the dry and wet environment under the same conditions as aforementioned in the previous section. Subsequently, the device was reacted to diluted polyallylamine hydrochloride (PAH) which has positively charged solution of 50 M with DI water at room temperature for an hour. After treating with DI water and N2 gas, the I –V curves was measured in dry and wet environments.

3. RESULTS AND DISCUSSION The VT was shifted to the positive direction in both the dry and wet environments and this can also be confirmed through the reaction of PSS whereas the VT was negatively shifted in both the dry and wet environments under the PAH conditions as shown in Figure 3. Moreover, as the curves in Figures 3(a) and (b) show that the decrease in SS was observed as the process of PSS and PAH was implemented. With the equivalent capacitance model3 proposed in our group, the SS and capacitance can be expressed as C SS = 23 · Vth · m = 23 · Vth 1 + total (1) CA −1 −1 + Csub CA−1 = CBOX

(2)

−1 −1 Ctotal dry = 1 − CPSS + CPAH −1 −1 −1 + CAPTES + Cnox + CSD

(3)

and all the parameters are listed in Table II.3 J. Nanosci. Nanotechnol. 16, 4901–4905, 2016

Choi et al. 10–5 10–6 –7

Drain current, ID [A]

10

10–8

(b)

n-type SiNW FET

10–5 10–6

Wch/Lch = 130 nm/10 µm Dry environ.

10

Drain current, –ID [A]

(a)


VD = 1 V 1st

10–9 2nd

10–10 3rd 10–11

APTES

10–12

APTES

Wch/Lch = 20 nm/5 µm

PSS

Dry environ.

PAH

VD = –1 V 10–8 2nd

10–9

1st

10–10

3rd

10–11 10–12

PSS

10–13

–7

p-type SiNW FET

10–13

PAH 10–14 –20

–15

–10

–5

0

10–14 –25

5

–20

Back gate voltage, VBG [V]

–8

10

10–6

Wet environ.

10–7

0.1 × PBS VD = 1 V

10–9

10

–9

10

–11

10–12

1st APTES

10–10

10–14 –1.0

PAH –0.5

3rd 10–11 0.0

–0.2

Wet environ. 0.1 × PBS VD = –1 V

10–8 10–9 10–10

10–9

10–11 10–10

10–13 0.0

0.5


10–12

2nd

PSS 10–13

0

p-type SiNW FET


10–10

–5

10–5

n-type SiNW FET

10–7


(d)

10–5 10–6

–10



(c)

–15

1.0


1st

2nd 3rd

10–11

10–14 –2.0

–1.2 –1.5

APTES PSS

–1.0 –1.0

PAH –0.5

0.0


Figure 3. Transfer characteristics depending on surrounding environments of SiNW. (a) n-type and (b) p-type SiNW in dry environment. (c) n-type and (d) p-type SiNW in wet environment (0.1 × PBS).

By utilizing those equations, we were able to figure out that the SS decreases by the decrease in Ctotal , which is due to the added reactants. Also, the SS after the PSS reaction, which is indicated in Figures 3(a) and (c), was extracted in order to compare the characteristics of SS in the dry and wet environments. The SS of PSS shown in Figures 3(a) and (c) were 2.02 V/dec and 0.16 V/dec, respectively. As seen in Eqs. (3) and (4), the reason for good SS characteristics was due Table II.

Equation parameters.

Parameter

Symbol

Value

Unit

Diffusion capacitance (0.1 × PBS) Stern layer capacitance APTES capacitance Native oxide capacitance Thermal voltage

Cdiff CSTERN CAPTES Cnox Vth

301 20 452 115 259

F/cm2 F/cm2 F/cm2 F/cm2 mV

J. Nanosci. Nanotechnol. 16, 4901–4905, 2016

to the smaller Ctotoal wet , which is smaller than Ctotoal dry by the added capacitance components, such as Cstern and Cdiff in the wet environment. −1 −1 Ctotal wet = 1 − CPSS + CPAH −1 −1 −1 −1 −1 + CAPTES + Cnox + CSD + Cdiff + Cstern (4)

On the other hand, the change in SS shown in Figures 3(c) and (d) is relatively smaller in the wet environment. This explicates that the added capacitance components in the wet environment, i.e., value of CStern and Cdiff , are higher than the neighboring capacitance components by high permittivity of water, and it signifies that the SS change is small as the process of Ctotal is implemented. As a consequence of our investigation on the charged polymer reaction of SiNW (Fig. 4), it was ascertained that the VT shift in the wet environment is smaller than that of the air. This trait was the same for both of the PSS and 4903

The Analysis of Characteristics in Dry and Wet Environments of SiNW-Biosensor n-type SiNW Dry environ. Wet environ. 15 0.1 × PBS

0.6

p-type SiNW

Sensitivity, ⎜∆VT⎜[V]

Sensitivity, ∆VT [V]

20

0.3 0.5

10

0.2

0.4 0.3

0.1

0.2

5

0.1

0.4

0.2

(a)

Sensitivity, ⎜∆VT⎜[V]

n-type SiNW

p-type SiNW 0.0

0.0

–2

–0.1

–4

– 0.1

–0.2 – 0.2

–0.3

–6

→

PAH

– 0.3

–0.4

0.4

0.2

(b) →

APTES

–8

(b ) –12

Figure 4. The changes of VT before and after the chemical reaction of (a) PSS and (b) PAH in accordance with environments of SiNW. (Left: n-type SiNW, Right: p-type SiNW).

PAH reactions. The coupled gate capacitance of the wet environment is much larger than that of the dry environment due to the dielectric permittivity difference. Therefore, when SiNW is in the dry environment, the effect on the charged targets appear to be more efficient. What was observed carefully is a huge variance in sensitivity of the dry environment. The results elucidate that the sensitivity in the dry environment is significantly higher than that of the wet environment. On the contrary, it was verified that the sensitivity is more efficient but irregular and unstable due to the target layers, which were not chemically uniformed, in the dry environment. 10–5 10–6 –7


10

–8

10

2nd

10–11

10

–13

10–14 –1.0

1

4 lB

PSS

(5)

i z2i

Wch/Lch = 20 nm/5 µm Wet environ.

10

0.01 × PBS VD = –1 V

10–8 10–9 1st

2nd

10–10 3rd

10–11

10

PAH

i

p-type SiNW FET

10–12

APTES

–0.5

10–5

–7

3rd 10–12

PAH

As shown in Figure 6, the PAH reactivates were smaller than that of PSS, and it is because the location of PAH layer is farther away from the SiNW rather than the location of PSS layer, hence it will undergo less influence of Debye length. In other words, the PAH layer can gradually go beyond the Debye length.

10–6

10–9 10–10

D =

(b)

n-type SiNW FET Wch/Lch =130 nm/10 µm Wet environ. 0.01 × PBS VD = 1 V

1st

→

In addition, the two ion concentrations in the wet environment were compared as shown in Figures 3(c), (d), and 5. The VT shift variation by the PSS reactions shown in Figures 3(c) and 5(a) were 0.1 V and 0.53 V, respectively. It elucidates that the VT shift becomes more sensitive at low concentration condition since Debye length ( D ) is inversely proportional to the square root of the ion concentration as indicated in Eq. (5).14


(a)

PSS

PSS

Figure 6. VT changes in accordance with PBS solution concentration in wet environment. (a) n-type, (b), p-type SiNW.

Dry environ. Wet environ. 0.1 × PBS

–10

0.01 × PBS 0.1 × PBS

p-type SiNW

0.0

APTES PSS

–13

PAH 0.0

0.5


4904

PSS

PSS

0.6

0

Sensitivity, ∆VT [V]

→

APTES

0.0

Figure 5.

0.01 × PBS 0.1 × PBS

n-type SiNW

0.0

0.0

(a) 0

Choi et al.

1.0

10–14 –2.0

–1.5

–1.0

–0.5

0.0


Transfer characteristics. (a) n-type and (b) p-type SiNW in wet environment (0.01 × PBS).


Choi et al.


4. CONCLUSION

References and Notes

We have analyzed the various VT shifts and SS changes based upon the dry and wet environments using SiNW FET, and figured out that the sensitivity is dependent upon the concentration of buffer solution in wet the environment. As a result of comparing the dry and wet environments, it was verified that the sensitivity is higher in the dry environment, yet there are unstable drawbacks. The results derived from the ion concentration in buffer solution in the wet environment were more sensitive at 0.01 × PBS. Therefore, more reliable and sensitive biosensing guideline is possible based upon our results that have been obtained by comparing various conditions, such as wet and dry environments.

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Acknowledgment: This work was supported by National Research Foundation of Korea through the Ministry of Science, ICT and Future Planning (Grant No. 2013R1A1A2065339), in part by BK+ with the Educational Research Team for Creative Engineers on MaterialDevice-Circuit Co-Design under Grant 22A20130000042. Authors would like to thank Ms. Seowoo Nam in Hankuk Academy of Foreign Studies, Yongin, Korea, for a useful discussion on the charged polymer and measurement setup.

Received: 20 March 2015. Accepted: 20 April 2015.


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