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Jan 4, 2019 - The advanced Hall magnetic sensor using an ion-gated graphene field-effect transistor demonstrates a high current-normalized sensitivity...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Low-Voltage-Operated Highly Sensitive Graphene Hall Elements by Ionic Gating Joonggyu Kim,†,‡,∥ Junhong Na,†,∥ Min-Kyu Joo,§ and Dongseok Suh*,† †

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea § Department of Applied Physics, Sookmyung Women’s University, Seoul 04310, Republic of Korea ‡

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S Supporting Information *

ABSTRACT: The advanced Hall magnetic sensor using an iongated graphene field-effect transistor demonstrates a high currentnormalized sensitivity larger than 3000 V/AT and low operation voltages smaller than 0.5 V. From commercially available grapheneon-SiO2 wafers, large-area arrays of ion-gated graphene Hall element (ig-GHE) samples are prepared through complementary metal-oxidesemiconductor-compatible fabrication processes except the final addition of ionic liquid electrolyte covering the exposed graphene channel and the separate gate-electrode area. The enhanced carrier tunability by ionic gating enables this ig-GHE device to be extremely sensitive to magnetic fields in low-voltage-operation regimes. Further electrical characterization indicates that the operation window is limited by the nonuniform carrier concentration over the channel under high bias conditions. The drain-current-normalized magnetic resolution of the device measured using the low-frequency noise technique is comparable to the previously reported values despite its significant low power consumption. KEYWORDS: magnetic Hall sensor, low-voltage operation, ionic gating, graphene, Hall element array

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magnetic Hall sensor applications.18,19 For the graphene channel device, a high mobility with moderate suppression of carrier density enables the feature of high current-normalized sensitivity (SI). Meanwhile, TMDCs enable the precise control of carrier density under moderate carrier mobility to yield a high SI. In both cases, the direct electrical doping on a 2D active channel can be realized through a capacitive gate electric field applied in the transistor configuration. However, the previously reported devices required large electrical biases especially in the gate electrode, which is not suitable for portable, mobile, or low-power device applications. To overcome these challenges, we herein report a graphene Hall element (GHE) fabricated with a large-area graphene grown by chemical vapor deposition (CVD) and other complementary metal-oxide-semiconductor (CMOS)-compatible processes such as photolithography. Furthermore, an electrolyte is employed to control the GHE by ionic gating. It has several distinctive characteristics compared to those of a conventional solid dielectric gating system. The most wellknown feature of liquid electrolyte gating is the realization of ultrahigh gate capacitance owing to the formation of an electric double layer. A strong charge carrier modulation allows for the

INTRODUCTION In recent years, wireless sensor networks (WSNs) have been considered crucial for the realization of internet of things (IoT).1−5 The primary aspect of IoT technology is to embed electronics into any physical objects and allow them to compute, communicate, and connect to the Internet via WSNs. Hence, investigations of a new communication system with WSNs have garnered attention in both industry and academia. Considering the general features of the sensing elements in WSNs,3 however, the corresponding investigations for new sensing elements with low-cost fabrication and low-power operation should be conducted. The magnetic Hall sensor is an important device in the fields of mechatronics engineering,6,7 contactless electromagnetic detection,8,9 and state-of-the-art nano/biotechnologies.10,11 Because high carrier mobility and thin active channel formation properties are required for channel materials of high-performace Hall sensors, III−V compounds such as GaAs, InAs, InP, InSb, and Si, with which a two-dimensional (2D) electron gas system can be realized, have been employed in the active channel of the device. However, both the performance and cost effectiveness should be satisfied to fulfill the market’s high demand. Recently, graphene and monolayer transition-metal dichalcogenides (TMDCs), the representative atomically thin 2D materials,12−17 have been suggested as optimal materials for © XXXX American Chemical Society

Received: October 12, 2018 Accepted: January 4, 2019 Published: January 4, 2019 A

DOI: 10.1021/acsami.8b17869 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces significant reduction in the operating gate voltage. The results indicate a high current-normalized sensitivity of ≈3000 V/AT and a Hall mobility of ≈3000 cm2/(V s) in the operation voltage range lower than 0.5 V for both the hole and electron channels. In the 2015 International Technology Roadmap for Semiconductors (ITRS), the lowest bias voltage for smart sensors is lower than 0.8 V (in 2015) and it is predicted to be lower than 0.45 V until the year 2029. As such, the current study offers the future device requirements suggested by the ITRS.

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RESULTS AND DISCUSSION A commercially available graphene-on-SiO2-substrate sample (purchased from Graphene Square Inc.) was used to obtain more generality in the sample’s quality. The graphene channel was defined as a six-probe Hall bar shape with the channel length and width of 260 and 20 μm, respectively, using photolithography and reactive-ion etching. Electrical contacts were patterned using a Cr/Au (5:50 nm thick) bimetal layer. Subsequently, a 30 nm thick Al2O3 film was deposited on the entire substrate by atomic layer deposition. To allow ions in an electrolyte to enter the graphene channel, certain areas of the Al2O3 passivation film were opened selectively on the graphene channel and planar gate electrode using wet etching. After adding a droplet of an ionic liquid electrolyte, i.e., 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI), thermal annealing was applied to remove water molecules on the graphene surface to hinder both hysteresis during the conductance measurement and the reduction in carrier mobility.20,21 The corresponding schematic images of the device fabrication process are represented in Figure S1. A chipset image of a large-area GHE with an electrolyte, an optical image of the channel part of the single GHE, and a schematic cross-sectional view of the complete electrolytegating device (not scaled) are presented in Figure 1a−c, respectively. The area of the planar gate electrode is approximately 100 times larger than the graphene channel area. It is noteworthy that all patterns in the process were formed by conventional photolithography techniques that are compatible with the standard semiconductor device fabrication process. The basic electrical characterization results of the ion-gated GHE (ig-GHE) device without applying the magnetic field are summarized in Figure 2. The drain voltage (Vds) curves as a function of ionic gate voltage (Vig) with different constant Ids biases (100 nA, 300 nA, 500 nA, 700 nA, 1 μA, 3 μA, 5 μA, 7 μA, and 10 μA) are obtained, as shown in Figure 2a. The typical ambipolar characteristics of graphene field-effect transistors are also shown in the ig-GHE device with the maximum points of Vds that correspond to the charge neutrality point of graphene (VCNP). As the constant Ids bias increases, however, the VCNP tends to move toward the positive region. A similar trend in VCNP shift is also observed in Figure 2b, which shows the Ids curves as a function of Vig at different Vds biases (1, 2, 5, 10, 20, 50, 100, 200, and 500 mV). This phenomenon can be ascribed to the local doping effect on the graphene channel by a high Vds bias correlated with the applied Vgs bias.22−25 To demonstrate the Vds dependence of the charge neutrality point more clearly, VCNP as a function of Vds is plotted and displayed in Figure 2c on the basis of the Vds maximum and Ids minimum points of Figure 2a,b, respectively. The two sets of the data shown by the VCNP values exhibit the same tendency

Figure 1. (a) Photograph of the GHE array chipset with droplets of the ionic liquid electrolyte (EMIM-TFSI). (b) Optical microscope image of the single GHE with the Hall bar geometry. “S”, “D”, and “V+” and “V−” denote the source, drain, and positive and negative polarities of the voltmeter, respectively. (c) Schematic illustration of the cross-sectional view (not scaled) of ion-gated GHE (ig-GHE).

depending on Vds, confirming the reliability of the VCNP shift behavior. In the low drain-bias regime, the VCNP values saturate with a constant value of ≈0.35 V, which should be the actual charge neutrality point of the graphene. In the high drain-bias regime, however, the VCNP values increase continually toward the positive region as the drain bias increases. This drainvoltage-induced VCNP shift can be explained by simple band diagrams of the graphene channel, as shown in Figure 2d. Assuming that the charge neutrality point is achieved at the zero gate voltage, the Fermi level is located at the Dirac point of graphene when the zero gate voltage is applied, as shown in the bottom band diagram of Figure 2d. As the drain voltage increases toward the positive region, the gate−drain potential difference becomes negatively larger; therefore, the local Fermi level near the drain side is lowered to the Dirac point (see the middle and top band diagrams of Figure 2d). In this case, the drain-induced local doping effect is predominant in the graphene channel with partial p-doping near the drain side. Additionally, it is necessary to apply a higher positive gate voltage to increase the Fermi level up to the Dirac point of graphene. That is, the VCNP shifted positively as a result of applying the high-positive-drain bias. Considering that the electron and hole injection from the source and drain electrodes is balanced with each other at the charge neutrality point of the graphene device, as well as the drain-induced local doping effect, the relationship between the VCNP shift (ΔVCNP) and drain voltage change (ΔVds) follows the equation |(1/ 2)ΔVds| ≈ |ΔVCNP|.23 In Figure 2c, the ΔVCNP values from the two different data sets are 0.33 and 0.24, corresponding to the (1/2)ΔVds values of 0.324 and 0.250, thus supporting that the charge neutrality point shift is caused solely by the draininduced local doping effect. B

DOI: 10.1021/acsami.8b17869 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Electrical transport characterization of the ig-GHE device without applying the magnetic field. (a) Drain voltage and (b) drain current curves as a function of ionic gating with constant drain current and drain voltage biases, respectively. (c) Charge neutrality points as a function of drain voltage extracted from (a) and (b). (d) Simple band diagrams of the graphene channel describing the drain-voltage-induced Fermi level change in the system. (e) Output characteristics of an ig-GHE device.

Figure 3. Hall measurement analysis of the ig-GHE device. (a) Hall voltage as a function of ionic gating measured at (a) +0.5 and (b) −0.5 T external magnetic fields. (c) Hall voltage as a function of the external magnetic field when different drain current biases are applied. Note that each Hall voltage in (c) corresponds to the peak Hall voltage obtained in (a) and (b). (d) Drain-current-normalized sensitivity as a function of ionic gating.

Another sign of the drain-induced local doping effect is a kink in the output characteristics.22−25 In Figure 2e, the output

characteristics of an ig-GHE device fabricated on the same substrate are represented. The kink behavior is observed in the C

DOI: 10.1021/acsami.8b17869 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Low-frequency noise analysis. (a) Voltage spectral density of hole- (Vgs = −0.8 V) and electron (Vgs = +1 V)-dominant regimes as a function of frequency at various drain current biases. The legends of the different drain current biases are represented in (b). (b) Voltage spectral density values extracted from the thermal noise limit and the relatively high frequency point at 5 kHz depending on ionic gating under varying drain current biases. (c) Derived magnetic resolution depending on ionic gating under different drain current biases. (d) Drain-current-normalized magnetic resolution as a function of ionic gating. The error bars are based on the different drain current biases.

3a,b. Thus, the maximum values of the absolute Hall sensitivity SA (=∂VH/∂B) at different drain current biases can be obtained from the linear slopes in Figure 3c. Slight deviations in the linearity of the Hall voltage as a function of the magnetic field could be associated with the geometrical error in the Hall bar structure, which can be improved by optimizing the device structure.27 The drain-current-normalized Hall sensitivity values SI (=SA/Ids) can also be defined and plotted as a function of ionic gating with corresponding drain current biases in Figure 3d. The maximum value of SI is obtained as ≈3000 V/AT, which is larger than the other reported values of graphene Hall sensors with solid gate dielectrics.18,28−33 When the relatively high drain current biases are applied, the drain-induced local doping effect can also affect the SI values. First, the maximum points of SI shift positively in the gate voltage as the applied drain current is increased. This can be explained by the draininduced VCNP shift because the maximum points of SI are supposed to appear near the charge neutrality points.18 Furthermore, the maximum values of SI reduce as the applied drain current is increased. This is attributable to the lowered VH because of the large difference in the carrier density along the lateral channel associated with the spatially distributed Fermi level induced by a high drain voltage, as shown in Figure

curves at +0.4 and +0.5 V ionic gating, which is explained by the band diagrams in Figure 2d as well. When the draininduced local doping occurs at the positive gate voltage, the Fermi level of the drain side reaches the Dirac point of graphene with the minimized carrier density compared to that of other channel parts. At this moment, the device becomes less sensitive to the drain bias and the kink behavior appears consequently. The same kink phenomenon also appeared in the ig-GHE device, as shown in Figure S2. The detailed Hall voltage analysis of the ig-GHE device is necessary to characterize the sensitivity values, which are important parameters of magnetic Hall sensors.26 Figure 3a,b shows the Hall voltages as a function of ionic gating at different drain current biases when +0.5 and −0.5 T external magnetic fields are applied, respectively. Note that the legend of the applied drain current biases is expressed in Figure 3d. At the +0.5 T magnetic field, the positive (negative) maximum Hall voltage points for the hole (electron) regime are observed near the charge neutrality points. When the magnetic field is reversed to −0.5 T, the signs of the Hall voltages for each carrier are also reversed. Figure 3c represents the Hall voltage values as a function of magnetic field at different drain current biases. Each Hall voltage value is the maximum value obtained from the data where ionic gating is swept, as shown in Figure D

DOI: 10.1021/acsami.8b17869 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

values are observed as ≈0.3 G/Hz0.5 near the charge neutrality points that are larger than the other reported values with solid gate dielectrics.18,28,29 These relatively large Bmin values are caused by the large SV values with respect to the applied drain current biases, implying that the noise sources that originated from the interface between the graphene and electrolyte are more pronounced in the ig-GHE device than those in the other GHE devices with solid gate dielectrics. Nevertheless, the significantly lowered operation current and voltage compared to those of other GHE devices are still useful in magnetic Hall sensors.18,28,29 Additionally, we have performed a deeper quantitative analysis on the device performance with respect to the Bmin values. According to previous reports,28,30,35 the Bmin values are supposed to be independent of the applied drain current because the magnetic resolution is dominated by the 1/f noise of the device. In other words, assuming that the SV exhibits the 1/f dependence, the Bmin can be expressed as28

2d. To maintain the maximum SI value, it is recommended to apply the proper drain current bias up to ≈1 μA and ionic gating of ≈0.4 V. This relatively low operation current and voltage are the advantages of this ig-GHE device in pursuing the low-power operation of magnetic Hall sensors. Additionally, the Hall mobility (μH) can be calculated from the following equation: μH = SIσ, where σ is the electrical conductivity. Considering that a mobility enhancement layer for graphene such as hexagonal boron nitride is not employed, the device shows reasonable carrier mobility values exceeding ≈3000 cm2/(V s), as shown in Figure S3. Based on the two-carrier model and general Hall effect theory, the SI in graphene can be written as below when the Hall factor is unity.34 SI =

2 2 1 nhμ h − neμe q (nhμ h − neμe )2

(1)

where q, nh and ne, and μh and μe denote the electronic charge, the carrier density of the hole and electron, and the carrier mobility of the hole and electron, respectively. Assuming that the carrier mobilities of the hole and electron are the same and assuming proper carrier density between the hole and electron, the SI can be approximated to 1/(qn2D), which is a function of net carrier density, n2D (=nh − ne).28,34,35 At this time, in the graphene channel, the SI can be determined solely by the carrier density. Therefore, a decrease of the carrier density would be the first approach to increase the SI value using other semiconductor channels with proper band gaps. In the semiconductor Hall elements, however, it is worth noting that significantly reduced and imbalanced carrier mobility values arising from the Schottky contact and/or different charge scattering cross sections could make the approximation of the SI value unavailable. Another important parameter for magnetic Hall sensors is the minimum magnetic resolution defined as Bmin (=SV0.5/SA), where SV is the voltage spectral density. To obtain the precise values of Bmin based on the ig-GHE device, the low frequencynoise (LFN) analysis is conducted, as shown in Figure 4. The SV data as a function of frequency are plotted at different drain current biases for the hole (Vgs = −0.8 V) and electron (Vgs = +1 V) regimes in Figure 4a,b, respectively. It is noteworthy that the Hall voltage fluctuations are recorded as a function of time and Fourier transform, while the constant drain current biases are applied across the channel in this LFN measurement, which is a special case for magnetic Hall sensors. The 1/f trends of the SV are observed in relatively low frequency regimes, but these SV values are saturated to certain values corresponding to the thermal noise of 4kTR, where k, T, and R are the Boltzmann constant, temperature, and resistance, respectively. Both the measured SV values at 5 kHz and the calculated thermal noise values are plotted as a function of ionic gating in Figure 4b. The noise levels near the charge neutrality point are larger than those in the hole- and electrondominant regimes. This phenomenon may be ascribed to the lowered screening effect of Coulomb scattering and exposed trap sites by a reduction in the charge carrier density. These SV and thermal noise values matched well with each other, implying that the noise level of the ig-GHE device at a relatively high frequency is limited only by the thermal noise. The minimum magnetic resolution (Bmin) values at 5 kHz are also plotted as a function of ionic gating for different drain current biases, as shown in Figure 4c. The minimum Bmin

Bmin =

SV SA

=

αHIds2 fn2DSch Ids qn2D

αHq2n2D fSch

=

(2)

where αH, f, n2D, Sch, and q are the noise parameter, frequency, carrier density, channel area, and electronic charge, respectively. In our device, however, the Bmin values at 5 kHz do not appear to be independent of the applied drain current biases, as shown in Figure 4c. A similar phenomenon has been already observed in a previous report of the GHE device with a solid gate dielectric.18 As discussed above, eq 2 holds under the assumption that the dominant noise mechanism follows the 1/f dependence. Therefore, if the voltage fluctuation does not follow the 1/f noise dependence but is dominated by the thermal noise in the relatively high frequency regime, the expression of Bmin values must be modified with the thermal noise of 4kTR as follows Bmin =

4kTR = SA

4kTR Ids qn2D

=

4kTR qn2D Ids

(3)

This indicates that not the Bmin values but the product of Bmin and Ids is independent of the applied drain current. For our igGHE device, the drain-current-normalized Bmin values (BminIds) are plotted as a function of ionic gating in Figure 4d. The error bars associated with the different drain current biases are relatively small, implying that the Bmin values are normalized with the applied drain current. In this case, the BminIds values in the ig-GHE device are comparable to those of other reported GHE devices with solid gate dielectrics, which are in the BminIds range of 10−7−10−6 AG/Hz0.5 at high drain current biases of 0.01−1 mA.18,28,35 Therefore, the Bmin must be distinguished carefully in terms of the operation frequency. Even though the low-power operation of the graphene Hall element by ionic gating has been successfully demonstrated in this work, the additional gate terminal in the Hall element could add another complexity in the practical use of the sensor. Nevertheless, detailed analyses of the important parameters in the Hall element such as the sensitivity and magnetic resolution could have been conducted by the gate-dependent I−V measurement with the magnetic field and low-frequency measurement. Obtaining an optimum device operation point by finding the maximum sensitivity with the gate-dependent carrier density is also important to analyze the performance of E

DOI: 10.1021/acsami.8b17869 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces the ig-GHE device. With the negligible hysteresis in the transfer curves, the GHE device with ionic gating could pave the way to realize the low-power-operating magnetic Hall element based on graphene.

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CONCLUSIONS A significantly low-power operation of the ig-GHE device fabricated using CMOS-compatible techniques such as CVD, photolithography, and ionic gating has been demonstrated. The operation points of the ig-GHE device were lower than 1 μA drain current, corresponding to a 0.2 V drain voltage with ≈0.35 V ionic gating. At this operation point, the maximum drain-current-normalized Hall sensitivity (SI) and minimum magnetic resolution (Bmin) were obtained as ≈3000 V/AT and ≈0.3 G/Hz0.5, respectively. Even though the drain-induced local doping effect occurred in the high drain-bias regime, it was not significant under the ig-GHE device operation points owing to the low drain current and voltage applied. Additionally, the drain-current-normalized Bmin was newly defined with the thermal noise in the relatively high frequency regime. This low-voltage-operated ig-GHE device demonstrated a possible approach for the adoption of atomically thin two-dimensional materials in practical magnetic sensors suitable for the IoT environment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junhong Na: 0000-0002-5314-2868 Dongseok Suh: 0000-0002-0392-3391 Author Contributions ∥

J. Kim and J. Na contributed equally to this work.

Author Contributions

All authors contributed to the writing of the manuscript and approved its final version. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS-R011-D1) and the National Research Foundation of Korea (NRF-2016R1A2B2012336 and NRF2018R1D1A1B07050608), funded by the Ministry of Science and ICT, Republic of Korea.

MATERIALS AND METHODS

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Graphene Hall Elements Fabrication. The ig-GHE devices were fabricated using a large-area CVD-grown graphene transferred onto a 300 nm SiO2/Si substrate purchased from Graphene Square Inc., Korea.36 A six-probe Hall bar shape of graphene with an active channel area (L × W) of 260 μm × 20 μm was patterned and dryetched using O2 plasma (flow rate: 20 sccm; power: 20 W; and time: 60 s) in a reactive-ion etching system (AFS-R4T, All-For-System, Korea). Subsequently, bimetal electrodes of Cr/Au (5:50 nm) were patterned and deposited. It is noted that all patterning processes were performed using conventional photolithography techniques (mask aligner: MA6, Karl Suss; photoresist: AZ-GXR-601). To passivate the metal electrode parts except the graphene channel, a 30 nm thick Al2O3 layer was first deposited by an atomic layer deposition system with a source of trimethyl aluminum (CH3)3Al and water, followed by additional photolithography and wet etching processes with a buffered oxide etchant (6:1). The 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) purchased from Sigma-Aldrich was drop-casted only on the graphene channel and gate pad. To achieve a better interface quality between the graphene and electrolyte, a post-thermal annealing process was conducted for 4 h under 500 sccm Ar flow at 180 °C in a horizontal quartz tube furnace. The reduced hysteresis after the thermal annealing process was observed in the transfer curves, as shown in Figure S5. Electrical and Magnetic Property Measurement. Electrical transport and Hall measurements were performed at room temperature under a high-vacuum chamber equipped with a DC Hall measurement system (model 8425, Lake Shore Cryotronics, Inc.) and a semiconductor characterization system (4200-SCS, Keithley Instruments, Inc.). Noise Property. The low-frequency noise (LFN) characteristics are measured using a customized LFN measurement system consisting of a metal shielding box with direct current (DC) batteries, a voltage amplifier (SR5113, Signal Recovery), and a data acquisition system (DAQ-4431, National Instruments).37

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Device fabrication process, electrical properties of the igGHE device without applying the magnetic field, Hall mobility calculation, and double-sweep transfer characteristics of the ig-GHE device (PDF)

REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b17869. F

DOI: 10.1021/acsami.8b17869 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b17869 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX