Fast Flexible Bottom-Gated Hydrogen Sensor based on Silicon

Nanomembrane

Minkyu Cho1,2, Incheol Cho1, Kyuyoung Kim1, and Inkyu Park1,*.

1Department of Mechanical Engineering, Korea Advanced Institute of Science and Techno

logy (KAIST), Daejeon, 34141, Republic of Korea

2School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlant

a, Georgia, 30332, USA

*Corresponding Author: inkyu@kaist.ac.kr

Keywords: Flexible Hydrogen Sensor, Silicon Nanomembrane, Palladium Nanoparticle,

Bottom-Gated Sensor, Body Effect Sensing

Abstract

High performance flexible hydrogen sensor using a silicon nanomembrane (Si NM) coated

with palladium nanoparticles (Pd NPs) was developed. After the formation of gate structure on

a released Si NM, selectively pre-doped Si NM was flip-transferred onto a plastic substrate.

Along with Pd NPs deposited on top of the Si channel, the bottom gate structure allows the

sensor to operate in a sub-threshold regime maximizing the response and recovery speed. A

device simulation study revealed that the current change caused by shifting the threshold

voltage upon H2 exposure is the main operating mechanism of the sensor. The fabricated sensor

showed high response (up to 250% @ 0.7% H2 concentration), short response time (τ10-90 =

10s), and short recovery time (τ90-10 = 10s). In addition, the sensor showed low detection limit

(50 ppm) and high mechanical robustness.

Introduction

Hydrogen (H2) is a next generation clean energy source that does not leave carbon footprints.

However, H2 is colorless, odorless, but highly explosive with a concentration above 4%.

Therefore, several types of H2 sensors such as optical[1-7], chemoresistive[8-18], catalytic

combustion type sensors[19-21] have been developed. Specifically, resistive-type H2 sensors are

widely studied among researchers due to their low cost and high sensing performance. Among

many types of chemoresistive H2 sensors, palladium (Pd) nanostructure-based H2 sensors use

the resistance change of Pd as it transforms to palladium hydride (PdHx) upon the absorption

of H2 molecules. Others use the semiconductor-based sensor with Pd decoration, where the

work function of Pd is changed and the carrier distribution in the semiconductor body is

modulated upon the exposure to H2 gas. Flexible gas sensors are light-weight and mechanical

shock-resistive, which make them useful for portable electronics and automobile/aerospace

applications[22]. Various flexible H2 sensors have been reported up to date with a goal of

achieving mechanically robust, highly sensitive, quickly responding/recovering, and highly

selective sensors. Flexible gas sensors based on Pd-functionalized metal oxide H2 sensors

showed high responses to H2. However, they suffer from low response at room temperature and

thus require high temperature for proper sensor operation[23-27]. Flexible H2 sensors based on

low-dimensional materials, such as graphene, transition metal dichalcogenide (TMD), and

carbon nanotube have shown moderate responses to H2 and good mechanical robustness[28-30].

However, further improvement of H2 sensing is needed in terms of response and recovery time.

In this paper, a flexible H2 sensor was demonstrated using Pd nanoparticle (Pd NP)-

functionalized silicon nanomembrane (Si NM) transistor. The sensor structure and operating

mechanism are described in Scheme 1. The sensor body is mainly composed of the Si NM,

source/drain, and gate. As the sensor is exposed to H2, the H2 molecules are diffused into Pd

forming Pd hydride (PdHx), which changes the potential at the bulk substrate modulating the

inversion current across the Si channel. The selectively pre-doped Si NM is released from

silicon-on-insulator (SOI) substrate and transferred to a plastic substrate to form bottom-gated

MOSFET structure. After the transfer, Pd nanoparticles (Pd NP) are evenly deposited on the Si

channel region. The fabricated sensor showed high response (up to 250% @ 0.3V gate bias),

and fast response (τ10-90 = 10s) and recovery time (τ10-90 = 10s). The sensor retained its sensing

characteristics after repeated bending cycles. The sensor tests with various gases showed that

the sensor has a high H2 selectivity over other gases.

Result and discussion

The sensor fabrication started with the selective doping of source and drain regions with

phosphorous (P) ion implantation on a p-type silicon-on-insulator (SOI) wafer as shown in

Figure 1 (a). After the ion implantation, the top Si layer was selectively patterned and etched

with reactive-ion-etching (RIE) to form etch holes followed by immersion in hydrofluoric (HF)

solution. During the immersion, the HF solution was infiltrated to etch holes and buried-oxide

(BOX) layer underneath the top Si layer as shown in Figure 1 (b). After the BOX removal, the

top Si layer, called silicon nanomembrane (Si NM), was released from the substrate and bonded

to the substrate with weak Van der Waals force as shown in Figure 1 (c). After the Si NM

release, gate dielectric/metal stack and source/drain metal were deposited on the Si NM as

shown in Figure 1 (d). Then, the Si NM was transferred upside down to SU-8 coated plastic

substrate and later permanently cured with flood UV exposure as shown in Figure 1 (e). It

should be noted that the gate dielectric/metal stacks and source/drain metal are transferred

together with SiNM because they are deposited on top of the released SiNM. After the transfer,

devices were isolated with selective RIE process and 1 nm of Pd was deposited to form Pd

nanoparticles (Pd NPs) on top of the Si channel by electron beam evaporation as shown in

Figure 1 (f). Detailed fabrication process can be found in Methods section. Figure 1 (g) shows

a photographic image of the sensor. Figure 1 (h) is an optical microscope image of the sensor

with a bottom-gated structure.

Figure 2 (a) presents the I-V characteristics of the bottom-gate thin-film-transistor before Pd

deposition, which presents both linear and saturation regions of typical MOSFET. After the Pd

deposition, threshold voltage was increased from 0.8 V to 0.9 V which is due to the body

effect31. The H2 sensing mechanism can be explained based on the following equation:

∆𝑉𝑉𝑡𝑡ℎ = 𝛾𝛾(2∅𝐹𝐹 + 𝑉𝑉𝑆𝑆𝑆𝑆 − 2∅𝐹𝐹) (1)

where ∆𝑉𝑉𝑡𝑡ℎ is the change of the threshold voltage, 𝛾𝛾 is the body-effect parameter, 2∅𝐹𝐹 is

the surface potential, and 𝑉𝑉𝑆𝑆𝑆𝑆 is the source-body voltage. As Pd NPs are deposited on the Si

channel region, VSB increases and, therefore, the threshold voltage increases as well. Figure 2

(b) shows the transfer curves of the sensor before and after Pd NP deposition. The graph shows

that Vth was shifted from 0.82 V to 0.92 V after Pd NP deposition. Atomic force microscope

(AFM) scan image of Pd-coated Si surface in Figure 2 (c) indicates that the diameter of Pd NP

size varies from 5 to 30 nm according to the image analysis and they are densely distributed.

The scanning electron microscope (SEM) image in Figure 2 (d) clearly shows the presence of

densely-packed Pd NPs. To investigate the effect of the Pd NPs on the sensor characteristics,

the device structures were simulated in Figure 2 (e). Silvaco ATLASTM simulation software

was used to generate meshes of the sensor structure. The detailed simulation structure is

presented in Figure S2. According to the simulation result, the increase of the current density

in the channel region is clearly shown with the addition of Pd NPs. The formation of the built-

in electric field at the Pd/Si interface resulted in the shift of threshold voltage and the current

increase at zero gate bias.

The sensor response (%) is defined as:

𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 (%) = 𝐼𝐼𝐻𝐻2−𝐼𝐼𝑎𝑎𝑎𝑎𝑟𝑟𝐼𝐼𝑎𝑎𝑎𝑎𝑟𝑟

× 100 (2)

where IH2 is the current during H2 exposure and Iair is the current in air environment. Upon H2

exposure, Pd NPs are transformed to PdHX lowering the built-in electric field at the Pd/Si

interface, and VSB is changed. This results in the threshold voltage shift toward negative side

increasing the drain current. Figure 3 shows the sensor response versus different H2

concentrations and gate bias. The result showed reproducible sensor response to different H2

concentrations in the range of 0.1 – 0.7%. The response starts to reduce with a gate bias more

than 0.3V. However, the response and recovery time became much shorter at the gate bias of

0.5V and above. This may be attributed to the switching time delay from off-state to sub-

saturation upon exposure to H2. Further investigation is needed to understand the reason of

slow response speed in off-state. More detailed analysis for the gas sensing result is presented

in Figure 4 (a-c). Figure 4 (a) shows the response versus H2 concentration at different gate bias

(0.3V, 0.5V, 0.6V, 0.7V, 0.8V and 0.9V). At the gate bias of 0.3V, the average sensor responses

were 38%, 98%, 169% and 250% to 0.1%, 0.3%, 0.5% and 0.7% of H2 concentrations,

respectively. The relationship between sensor response and concentration can be defined by

the equation according to Langmuir isotherm absorption theory[32]: ∆I/𝐼𝐼𝑎𝑎𝑎𝑎𝑎𝑎 ∝ 𝜃𝜃 =

𝑘𝑘1/𝑘𝑘−1𝑃𝑃𝐻𝐻2 where 𝜃𝜃 is the absorption coverage, 𝑘𝑘1 is absorption constant, 𝑘𝑘−1 is

desorption constant, and 𝑃𝑃𝐻𝐻2 is H2 partial pressure. The sensor response is linearly

proportional to the square root of the partial pressure at 0.5V bias with R2 ~ 0.99. Figure 4 (b)

shows a graph of response time versus H2 concentration with different gate bias. The response

time was defined as the time between 10% and 90% of the current change after H2 exposure.

At 0.3V gate bias, the response times were 169 s, 168 s, 204 s and 287 s to 0.1%, 0.3%, 0.5%

and 0.7% of H2 concentration respectively. However, for > 0.5V, the response time became

much faster. At 0.5V gate bias, the response times were 28 s, 13 s, 11 s and 10 s to 0.1%, 0.3%,

0.5% and 0.7% of H2 concentration respectively. Figure 4 (c) shows the recovery time versus

H2 concentration with different gate bias. The recovery time is defined as the time between 90%

and 10% of the current change during the air purge cycle. Similar to the response time, the

recovery times at 0.5V and above were much faster than those at 0.3V. At 0.3V gate bias, the

recovery times were 68 s, 39 s, 34 s and 42 s to 0.1%, 0.3%, 0.5% and 0.7% of H2 concentration

respectively. At 0.5V gate bias, the recovery times were 35 s, 19 s, 12 s and 10 s to 0.1%, 0.3%,

0.5% and 0.7% of H2 concentration respectively. Figure 4 (d-f) show the response, response

and recovery time under different H2 concentrations. Figure 4 (d) indicated that the responses

at 0.3V gate bias showed highest responses. However, the response recovery time was much

slower than those at 0.5V bias and above as shown in Figure 4 (e, f). This discernable

differences in response/recovery time between 0.3 V and 0.5 V voltage bias indicate that the

sensor speed is fastest when it is operated in sub-threshold regime. In this regime, the inversion

layer is not fully formed, therefore, it is less sensitive to the Schottky barrier change at the

interface between Pd and Si NM upon H2 exposure. The limit of detection (LOD) of the sensor

was 50 ppm as shown in Figure S3. To show the sensor uniformity, we randomly chose seven

sensors and plotted ID vs. VG curves. Overall, the sensors show uniform I-V characteristics as

shown in Figure S4. The eight repeated cycle tests at 0.7% H2 concentration in Figure S5 shows

a base current drift was with an average - 3.67 nA/cycle and 117% of response with standard

deviation of 2.24 % demonstrating reliability of the sensor.

For reliable sensor operation, it is indispensable to consider the temperature effects on the sensing

characteristics. The ambient temperature change may affect the sensor characteristics by two major

factors: 1) solubility change and 2) threshold voltage change of MOSFET. According to Sievert’s law,

H2 solubility is proportional to the inverse of the temperature. Absorbed H2 molecules gain enough

kinetic energy to desorb from Pd surface as temperature increases. This phenomenon was demonstrated

with experiments in our previous work[13, 33]. The second major temperature effect is the threshold

voltage shift due to the surface potential change (∅𝐹𝐹 ), which is proportional to the temperature.

Therefore, the temperature effect should be absolutely considered for palladium/semiconductor gas

sensors. This is a common practice for every chemo-resistive gas sensor. In order to resolve this problem,

temperature should be also measured simultaneously, and temperature effect should be compensated by

temperature calibration methods such as machine learning algorithm.

Various gases were tested for the sensor to investigate the gas selectivity as shown in Figure

5. The gas concentration for each gas type was set according to the permissible exposure limits

(PELs) by Occupational Safety and Health Administration (OSHA). The result shows that the

sensor does not respond or negligibly respond to nitrogen dioxide (NO2) and carbon monoxide

(CO) since intrinsic Si without any surface functionalization is inert to these gases. The sensor

showed response to hydrogen disulfide (H2S) as well as H2 because both H2S and H2 are

dissociated into H atoms, and these atoms are diffused into Pd and changing its work function.

The low gas selectivity of hydrogen to hydrogen disulfide or water vapor is one of common

problems to solve in room temperature hydrogen sensor. According to the literature, Pd itself

has low sensitivity to hydrogen radicals[34]. However, it has been reported MOS structure with

Pd shows some sensitivity to hydrogen radicals due to impurities in the metal / at the interfaces.

We believe that, with adoption of the micro heaters, filters, sensor array, different sensor

operating regime with various reaction material, and pattern recognition, the gas selectivity

issue can be improved.

To investigate the mechanical robustness of the sensor, the sensor has gone through several

cycles of repeated bending and the sensing test afterward. The sensor response was measured

after 1000 and 5000 bending cycles with curvature radii varying from infinite to 20 mm. The

mechanical strain of the sensor during the bending is 0.5%[8]. The sensor was bent along the Si

channel direction. After 1000 bending cycles, the sensor did not show significant degradation

of its sensing capability. Even after 5000 bending cycles, the sensor still retained the responses

as compared to those before bending. However, the response time was increased after 5000

bending cycles. This slow response after 5000 bending is similar to what is observed when the

sensor was operated in transistor off regime meaning at Vg << Vth when inversion current is not

formed along the Si channel. The increased response time after 5000 bending is attributed to

the threshold voltage shift due to increased electron traps in SiO2 [35]. Although the response

time was increased after 5000 bending cycles, the sensor was still able to detect H2 with

different concentrations. The sensor responses under static bending were also presented in

Figure S6. The result shows that the sensor retained its sensing performance under static

bending. This robustness after repeated bending may be due to the thin SiNM layer which

corroborates the flexural rigidity of the sensor. In addition, the sensor response under various

humidity conditions was tested (RH=0%, 20%, and 80%) as shown in Figure S7. The test result

shows that the response time is increased in high humidity condition. The increased response

time in high humidity condition is due to the condensed water molecules that occupy the

absorption sites and prevent the diffusion of H atoms into Pd lattice. The response also changed

with humidity which may be attributed to the base current change in high humidity conditions.

This issue may be resolved by using water absorbing filters36, humidity sensors and machine

learning algorithm to compensate for the H2 sensing data.

Conclusion

In this work, flexible H2 sensor was developed based on Pd NP coated Si NM enabled by

flip-transfer method. The prompt built-in electric field change of Pd NP upon the intake of H2

and bottom-gated Si NM transistor that operates in sub-threshold regime result in fast response

and recovery, which is explained by body effect-induced threshold voltage shift. Both

simulation and measurement result suggested that the decoration of Pd NP and its

transformation into PdHx changes the channel current by shifting the threshold voltage. The H2

gas response test demonstrated high response (up to 250% @ 0.7% H2 concentration) and fast

response (10s) and recovery (10s) time under the operation in sub-threshold regime. Moreover,

the sensor showed excellent linearity to tested H2 concentration range with R2 > 0.99. Gas

selectivity test showed that the sensor has high selectivity to other gas such as CO and NH3.

Repeated bending test was performed and showed the mechanical robustness of the sensor. The

fabrication process is easily applicable to wafer scale and can be integrated with electronic

circuitry and other silicon-based sensors. The presented flexible sensor would be useful for

wearable electronics and IoT that requires detection of H2 in hazardous environment.

Acknowledgement

This research was supported by Basic Science Research Program through the National

Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT & Future

Planning (2018R1C1B6006338).

Methods

Sensor Fabrication

20 nm of SiO2 was grown on n-type SOI wafer with resistivity of 1-20 Ω‧cm (200 nm / 375

nm thicknesses for top silicon and buried oxide layer, respectively). After the thermal oxidation,

source/drain regions were selectively opened with a photoresist patterning. For the ion

implantation, phosphorous (P) ion was ion-implanted with 20 keV energy and 5×1015 cm-2 dose.

After the ion implantation, the photoresist and thermal oxide was removed by organic solvents,

piranha solution and hydrofluoric acid (HF) to remove photoresist and thermal oxide. Etch

holes were formed on the top silicon layer with a photoresist patterning and reactive-ion etching

(RIE). After the etch hole formation, the sample was immersed in HF. During the HF immersion

process, the buried oxide layer was etched away, and silicon nanomembrane (Si NM) was

released from the substrate and bonded to the substrate by Van der Waals force. After the Si

NM release, gate dielectric/metal stack and source drain metal were selectively deposited. After

the deposition, the Si NM was flip-transferred to SU-8-coated poly (ethylene terephthalate)

substrate followed by flood UV exposure. After the transfer, 1 nm Pd was deposited on the

channel region by an e-beam evaporator to form Pd nanoparticles on the Si channel.

Gas Sensing Test

The flexible bottom-gate H2 sensor was tested inside a sealed chamber with gas inlets and

outlets. Gate and source-drain voltages were applied by a DC voltage supply and current source,

while the current across the source and drain of the sensor was monitored in real time. A gas

sensing test was performed in a dry air environment, and the gas flows were controlled by mass

flow controllers (MFCs) that were connected to a computer with a LabVIEW interface. The

total gas flow rate was kept at 500 sccm. The H2 test was conducted in synthetic air base by

mixing N2 and O2 in 4:1 ratio. For the humidity test, the relative humidity (RH) was adjusted

by changing the proportion of the humid air generated by saturated salt solution.

Mechanical Bending Test

The mechanical bending test was performed with a linear stage. The sensor was repeatedly

pressed and released at two end points of the sensor while the center point was fixed at a

cylindrical fixture, similar to three points bending. The radius of curvature was 20 mm during

the bending state.

Scheme 1. Graphic illustrations of flexible bottom gate thin-film-transistor H2 Sensor

Figure 1. (a-f) Illustrations of the sensor fabrication process, (g) photographic picture of the

complete sensor (scale bar: 1 cm), (h) optical microscope image of the sensor (scale bar: 50µm)

Figure 2. (a) Measured I-V characteristics of the flexible bottom-gated MOSFET, (b) measured

transfer characteristic before and after Pd decoration, (c) atomic force measurement (AFM)

scan images of Pd nanoparticles on silicon, (d) scanning electron microscope (SEM) image of

Pd nanoparticles (scale bar: 30 nm), (e) device simulation of the sensor without (up) and with

(bottom) Pd layer

Figure 3. Real-time H2 sensing characteristics with different H2 concentration and gate bias.

The right axis and the brown bar graphs indicate H2 concentrations. The base current is

increased as higher gate bias was applied which corresponds to ID-VG curve. The response

(∆I/Iair × 100) when VG = 0.3V is the highest because of low base current. However, the

response time is lowest due to slow saturation of the current after H2 exposure.

Figure 4. (a-c) response (%), response time (τ10-90) and recovery time (τ90-10) vs. H2 concentration,

(d-f) response (%), response time (τ10-90) and recovery time (τ90-10) vs. gate bias (V). the response

time is clearly reduced as the gate bias ≥ 0.5V.

Figure 5. The sensor response to various gases: (a) Hydrogen Sulfide (H2S), (b) Carbon

Monoxide (CO), and (c) Nitrogen Dioxide (NO2). The concentrations for each test gases are

based on permissible exposure limits (PELs) by Occupational Safety and Health

Administration (OSHA).

Figure 6. H2 response after repeated bending: (a) before bending, (b) after 1000 times, and (c)

after 5000 times. The sample was bent in linear stage with 20mm of radius curvature. The

repeated bending test was performed on a linear stage with two tips mounted at the end that

press both end of the sensor while the middle part of the sensor was fixed at a cylindrical chuck

with 40 mm diameter.

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