High Performance Hall Sensors Built on Chemical Vapor Deposition-Grown Bilayer Graphene.

Graphene has been considered as an excellent channel material for constructing magnetic sensors or Hall elements with high sensitivity and linearity. Compared to intensively reported graphene Hall elements (GHEs) fabricated on monolayer graphene, the exploration on bilayer graphene-based Hall elements is very rare. Here, we first investigate the performance and potential of Hall elements built on chemical vapor deposition-grown bilayer graphene. Without applying any gate voltage, the bilayer GHEs exhibit a typical voltage sensitivity of 119 mV/VT and current sensitivity of 397 V/AT, which are higher than those in the monolayer GHEs, indicating the better performance in practical applications. Moreover, the bilayer GHEs present obviously lower noise and then the minimum detection magnetic field compared to the monolayer ones. Hall elements built on bilayer graphene show certain unique advantages and can be used as an important supplement to mainstreaming monolayer GHEs.

Introduction

Because of high carrier mobility, ultra-thin thickness, stable structure, scalable fabrication, and other properties, graphene has been considered as an excellent channel material for constructing Hall elements,[1] which is a widely used magnetic sensor. Graphene Hall elements (GHEs) exhibit remarkable properties including high sensitivity, magnetic field resolution, temperature stability, high linearity, as well as good compatibility with the Si CMOS process and have been demonstrated as one of the most promising fields for graphene to be used in practice.[2−4] Compared to intensively reported GHEs fabricated on monolayer graphene, the exploration of a bilayer graphene-based Hall element is very rare.[5] As one kind of important graphene, bilayer graphene has some advantages on electronic property to the monolayer one, including better stability and lower noise, and then shows unique performances on the bilayer GHE. In practical application, Hall devices mainly work in the low frequency band and are vulnerable to flicker noise (1/f noise), which determines the magnetic field resolution of Hall elements. According to the previous research work,[6−9] bilayer graphene devices exhibited much lower (orders of magnitude) 1/f noise than the monolayer graphene devices. Furthermore, it is difficult to avoid the occurrence of two or more layers on chemical vapor deposition (CVD)-grown graphene,[10,11] which is suitable for practical applications of GHEs owing to the advantages of scalable preparation, good uniformity, and low cost. A few related works have produced bilayer graphene Hall devices, but do not match the known properties of bilayer graphene materials.[5] Therefore, it is necessary to fabricate and systematically explore the GHEs based on CVD-grown bilayer graphene.

In this work, we fabricate Hall elements on CVD-grown bilayer graphene and then investigate their performance on magnetic sensing. The fabricated bilayer graphene devices exhibit slightly lower carrier mobility and higher residual carrier density than the devices built on monolayer graphene with the same fabrication process. However, owing to the broader Dirac cone and flatter dispersion relation, the bilayer GHEs have higher sensitivity than the monolayer GHEs at zero back gate bias, indicating the better performance in practical applications. Moreover, the bilayer GHEs present obviously lower 1/f noise and minimum detection magnetic field compared to the monolayer ones. Therefore, the Hall element built on bilayer graphene shows certain unique advantages and can be used as a supplement to mainstreaming monolayer GHE.

Results and Discussion

The GHEs are fabricated on CVD-grown bi/monolayer graphene through a standard micro–nano processing technology, as shown in Figure a and described in refs (2) and (12). Bilayer and monolayer graphene samples were grown on copper foils by the CVD method[13−15] based on different growth conditions but in the same CVD equipment. The grown bilayer graphene has a hexagonal morphology, as shown in Figure b, and is identified as the bilayer by the Raman measurements (inset in Figure b) after being transferred to the Si/SiO2 substrate in the following process. The graphene was then transferred to the Si substrate covered by a 500 nm thickness thermal-grown SiO2 layer by the well-developed bubble method. An invisible D peak in the Raman spectrum indicates the high quality of the graphene even after transfer. In addition, we also measured the height and flatness of double-layer graphene with AFM, as shown in Figure S1 and S2 of Supporting Information. The measured height of the bilayer graphene is approximately 1.5 nm. Affected by the transfer residual PMMA, the surface roughness is around 0.6 nm, which does not affect the fabrication and performance of GHEs. The Hall elements are then fabricated based on bilayer and monolayer graphene through the process described in Figure a, and a typical as-fabricated GHE is shown in Figure c. The transfer curves of monolayer and bilayer graphene FETs were measured at bias of −0.1 V using the Si substrate as the back gate (Figure d), which showed obviously different characteristics. Compared with the monolayer FET, the bilayer one exhibited higher conductance and broader minimum conductance valley owing to more conducting channels and flatter dispersion (e–k) relation of bilayer graphene. Moreover, the bilayer device presented lower carrier mobility (783 cm2/V. s), higher residue carrier density (n0 = 1.1 × 1012/cm2), and higher Dirac point voltage (26 V) than the monolayer device (1090 cm2/V s, 9.2×1011/cm2 and 10 V respectively). Statistics comparison of multiple groups devices (Figure e,f) confirms that the bilayer graphene exhibited lower carrier mobility and higher Dirac point voltage than the monolayer graphene, which is similar to the previously reported results.[16,17]

Figure 1

Characterization and device manufacturing of graphene. (a) Fabrication steps of graphene devices. (b) SEM photos of graphene on copper and the Raman spectrum of bilayer graphene. (c) Photos of the prepared graphene Hall device. (d) Comparison of transition characteristic curves between monolayer and bilayer graphene. (e) Mobility distribution of 10 pairs of monolayer and bilayer graphene bottom gate devices. (f) Dirac point voltage distribution of 10 pairs of monolayer and bilayer graphene bottom gate devices.

We then characterize the performance of GHEs. A typical bilayer GHE shows a voltage sensitivity (SV) of 119 mV/VT at a fixed input voltage (3.3 V) and a current sensitivity (SI) of 398 V/AT at a fixed input current (10 μA), which is superior to the reported bilayer GHEs[5] and the conventional silicon-based Hall devices (∼100 V/AT).[18] As a comparison, a typical monolayer GHE shows a voltage sensitivity (SV) of 79 mV/VT and a current sensitivity (SI) of 158 V/AT under the same conditions. Statistics experimental results shown in Figure c demonstrate that the bilayer GHEs exhibited higher current sensitivity and higher voltage sensitivity than the monolayer GHEs. It is well known that SI and SV are directly determined by the key transport parameters, that is, and (n and μ are respectively density and mobility of carriers, W and L are the width and length of the channel, and q is the elementary charge). There is contradiction in this work: the bilayer GHEs exhibit higher carrier density and lower mobility than the monolayer ones, but higher SI and SV. Different from only one kind of carrier working in conventional semiconductor-based Hall elements, both electron and hole carriers work in GHEs, especially near the Dirac point[19,20] and then the sensitivity needs to be corrected toandwhere nh and ne are density of hole and electron, respectively, and we assumed that electrons and holes present the same mobility. Considering that the carrier density and mobility in the graphene channel strongly depend on the Fermi level which is modulated by gate voltage, the Hall sensitivities can be further given byand

Figure 2

(a) Hall voltage output of the bilayer graphene Hall device and monolayer graphene Hall device with invariable input current (10 μA). (b) Hall voltage output of the bilayer graphene Hall device and monolayer graphene Hall device with invariable input voltage (3.3 V). (c) Current sensitivity and voltage sensitivity of five pairs of Hall devices based on different materials. (d) Voltage sensitivity of the hall device under different gate voltages (described by Hall mobility). In the actual test, the gate voltage is 0 V.

If the GHE works without additional gate voltage (Vg = 0), which is the simplest and low-cost mode, the current sensitivity in eq can be rewritten as

According to eq , there are two approximate limits for the SI in GHEs. First, the carrier density is dominated by n0 near the Dirac point, and SI is approximately proportional to VDirac. Second, the carrier density is dominated by gate-induced carriers far from the Dirac point, and then SI is inversely proportional to VDirac. Almost all our GHEs are close to the first limits owing to low back gate efficiency of the fabricated devices, and then the bilayer GHEs with higher VDirac present higher SI than the monolayer GHEs.

We calculated the voltage-dependent SV of the monolayer and bilayer GHEs using eq with the parameters (n0, Cg, VDirac, and μ) retrieved from the transfer curves in Figure d. To eliminate the influence of geometry, equivalent Hall mobility μH is used here to describe voltage sensitivity, as shown in Figure d. It is obvious that μH of the bilayer GHE is lower than that of the monolayer one at very large range of Vgs, while it exceeds at the small region of Vgs around 0 V. Although the maximum sensitivity can be obtained in monolayer GHEs by applied a gate voltage as reported by many previous works,[19,20] the additional power supply will increase complexity of peripheral circuit and cost. Therefore, the high sensitivity without gate bias is available to GHEs in practical applications.

In addition to sensitivity, minimum detection magnetic field Bmin and linearity are also important metrics for the Hall element.[12,18] The Bmin at voltage mode is obtained bywhere Vnoise is the equivalent noise voltages of the Hall element. We measured the low-frequency noise of five pairs of mono/bilayer GHEs and compared their Vnoise, as shown in Figure a. The bilayer GHEs exhibit much lower noise than the monolayer ones since the lower layer in bilayer graphene is helpful in partially screening scattering from charged centers on the substrate.[5,7,21] The statistical comparison of Bmin between mono- and bilayer GHEs are shown in Figure b, which demonstrates that the bilayer GHEs exhibit an obvious advantage on the detection limit to the monolayer GHEs.

Figure 3

(a) Standard deviation of each group of sampling data to measure the stability of device output and noise during this period. (b) Taking the standard deviation as the noise amplitude, the minimum measured magnetic field of the device under DC output is obtained (c,d) Linearity of the bilayer graphene Hall device, which is tested in constant current and constant voltage mode.

The linearity of bilayer GHE is characterized at current mode (Figure c) and voltage mode (Figure d). Except that the relative value of deviation near zero magnetic field, the linearity deviation at current or voltage mode remains within 4%, which is among the best results of reported monolayer GHEs[2,22] and meets the linearity requirements of commercial Hall sensors even without any additional correction circuit.[18]

Conclusions

In conclusion, we investigated the performance and potential of Hall elements built on CVD-grown bilayer graphene. The fabricated bilayer graphene devices exhibited lower carrier mobility and higher residual carrier density than the devices built on monolayer graphene with the same fabrication process. However, owing to the broader Dirac cone and flatter dispersion relation, the bilayer GHEs have higher sensitivity than the monolayer GHEs at zero substrate bias, indicating the better performance in practical applications. Specifically, the bilayer GHEs exhibited a typical voltage sensitivity of 119 mV/VT and current sensitivity of 397 V/AT without applying any voltage on the substrate. Moreover, because of the bilayer structure, the scattering effect of the substrate on the carriers is weakened. This makes the bilayer GHEs present obviously lower 1/f noise and then smaller minimum detection magnetic field than the monolayer ones. As a result, the Hall element built on bilayer graphene shows certain unique advantages and can be used as a supplement to mainstreaming monolayer GHE.

Methods

Growth of Bilayer and Monolayer Graphene

Two kinds of graphene was grown by CVD with a mixture of methane and hydrogen as the gaseous precursor and copper foil as the metal substrate. Bilayer graphene was fabricated under a two-step growth process. First, 2 SCCM methane and 20 SCCM hydrogen were introduced at 1030 °C for 2 min. Then the flow of methane was lowered to 1 SCCM with hydrogen flow maintained at 20 SCCM for 10 min. Monolayer graphene was fabricated with 5 SCCM methane and 20 SCCM hydrogen at 1030° for 5 min.

Fabrication of Graphene Devices

Graphene was first transferred from the copper foil to the silicon oxide substrate. The transfer method we used was bubble transfer.[23] Copper foil with graphene was coated with PMMA A4 950k at a speed of 2000 rpm, connected to the negative pole of 3 V DC power supply. The same copper foil was connected as the positive pole. Then copper foil was electrolyzed in 0.1 mol/L NaOH solution. The negative pole was hydrogen gas, and graphene and PMMA would be peeled off the copper foil. Graphene was bonded to the silicon wafer, and PMMA was dissolved with acetone. Device fabrication used electron beam exposure as a graphical method, graphene channels were fabricated by ICP oxygen etching, and excess graphene was removed. Ti/Au contact and test electrodes are fabricated by electron beam evaporation coating.