High-Performance CVD Bernal-Stacked Bilayer Graphene Transistors

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Letter

High Performance CVD Bernal-Stacked Bilayer Graphene Transistors for Amplifying and Mixing Signals at High Frequencies Mengchuan Tian, Xuefei Li, Tiaoyang Li, Qingguo Gao, Xiong Xiong, Qianlan Hu, Mengfei Wang, Xin Wang, and Yanqing Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04065 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

High Performance CVD Bernal-Stacked Bilayer Graphene Transistors for Amplifying and Mixing Signals at High Frequencies Mengchuan Tian, Xuefei Li, Tiaoyang Li, Qingguo Gao, Xiong Xiong, Qianlan Hu, Mengfei Wang, Xin Wang, Yanqing Wu* Wuhan National High Magnetic Field Center and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China E-mail: [email protected]. Abstract. Tunable bandgap can be induced in Bernal-stacked bilayer graphene by a perpendicularly electric displacement field. Here, we carry out a comprehensive study on the material synthesis of CVD Bernal-stacked bilayer graphene and devices for amplifying and mixing at high frequencies. The transistors show large output current density with excellent current saturation with high intrinsic voltage gain up to 77. Positive extrinsic forward power gain |S21|2 has been obtained up to 5.6 GHz as well as high conversion gain of -7 dB for the mixers. The conversion gain dependence on tunable on-off ratio of the transistor has also been discussed.

Keywords: Bernal-stacked BLG, amplifiers, mixers, power gain, voltage gain, conversion gain

Graphene, a gapless two-dimensional material with ultra-high carrier mobility and high velocity, has been widely investigated for wide applications ranging from optics to electronics, many of which have shown the great potential of graphene f or radio frequency (RF) transistors and circuits.1, 2 As of today, graphene RF transistors with cut-off frequency fT of more than 300 GHz and maximum oscillation frequency fmax of more than 100 GHz have been reported, most of which are based on

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monolayer graphene (MLG) either grown by chemical vapor deposition (CVD) or epitaxially on SiC.3-5 Despite the impressive progress being made in this field, the unsatisfactory gain values due to the lack of current saturation when used as amplifiers with un-matched networks and mixers have become a bottleneck for high-performance wireless communication circuits.6-10 Up to now, monolayer graphene devices with strong current saturation have been obtained mainly including the ambipolar behavior of the channel or velocity saturation of the charge carriers.

11

The other way to

obtain current saturation is to open a band gap with large on-off ratios for electrostatic pinch-off of the channel. Unlike monolayer graphene, Bernal-stacked bilayer graphene (BLG) has shown a bandgap more than 100 meV tunable by electric displacement12, and without the stringent requirement of nano-patterning as in the monolayer graphene nanoribbon approach. Previous study shows that intrinsic voltage gain obtained by high-quality mechanical exfoliated Bernal-stacked BLG is six-fold larger that of the MLG devices.13 However, Bernal-stacked BLG by mechanical exfoliation has been found to suffer from large size variations and uncontrollable layer thickness and unsuitable for large-scale applications. Large size single-crystal Bernal-stacked BLG up to millimeter size grown by CVD has been reported.14. However, the main electrical results are on dc analysis and the experimental investigation on RF transistors are still lacking. In this work, we report a comprehensive investigation of graphene RF transistors based on CVD Bernal-stacked BLG as the channel materials. Well-behaved current saturation characteristics have been demonstrated by introducing a bandgap through the application of a perpendicular electric displacement field using double gate structures with high intrinsic voltage gain up to 77. A fmax/fT ratio more than 1 provides a significant advantage for RF amplifiers. Forward power gain |S21|2 larger than 0 dB up to 5.6 GHz has


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been obtained without any external matching networks, showing its capability of power amplifying for active applications in monolithic microwave integrated circuits. Finally, active transconductance mixer with a record conversion gain up to -7 dB has also been demonstrated, surpassing all previous results, and approaching that of commercial III-V transistors.15, 16 A copper pocket formed by folding a 25 µm thick copper foil, was used for CVD growth of BLG in a 3-inch tube furnace (methods in the Supporting information). BLG grows on the exterior of the copper pocket by a diffusion mechanism, where carbon atoms diffuse from the interior to the exterior of the copper pocket via the catalytically active inner exposed copper regions.14 After growth, the BLG films outside the pocket were transferred onto the SiO2/Si substrates covered with a 14-nm atomic-layer-deposited

(ALD)

high-k

HfSiOx

dielectrics,

using

polymethylmethacrylate(PMMA)-assisted transfer method. The high-k dielectrics are expected to provide better interface quality than conventional thermally oxidized SiO2.17 After 120 min of growth, BLG with repeated domains with typical size around 100 µm can be obtained as shown in the optical image in Figure 1a, b. Larger BLG domains more than 600 µm could be obtained by further increasing the growth time (Figure S2). Raman spectroscopy with a 532-nm excitation laser was used to verify the stacking order of the BLG domain as shown in Figure 1c where typical Raman spectra of MLG and BLG are compared. The 2D peak of the BLG shown in the inset of Figure 1c can be well-fitted to four Lorentzian peaks, corresponding to the characteristics of Bernal-stacked BLG.18 Figure 1d, e shows the Raman mappings of the same area as in Figure 1b including the position of the 2D peaks and the full width at half maximum (FWHM) of the 2D peaks, respectively. In comparison with MLG regions, the 2D peaks of the BLG regions show a wider FWHM and an



up-shift of position, also in good agreement with the features of the Bernal-stacked BLG.18 Remarkably, the maps of BLG regions show uniformly distributed color, indicating the high homogeneity of Bernal-stacked BLG domains. Negligible D band (~1350 cm-1) can be observed, suggesting the high quality of the graphene films after transferring to the high-k dielectric substrate. Detailed mapping of the intensity ratio of the 2D and G bands (I2D/IG) for BLG films transferred onto SiO2 and HfSiOx substrates are shown in Figure S3a, b. Clearly, the BLG films transferred onto HfSiOx substrate show higher I2D/IG values, indicating the reduced charged impurities at the graphene-dielectric interfaces.19

Figure 1. Bernal-stacked BLG. (a) Optical images of the BLG with repeated domains. Scale bar: 100 µm. (b) Zoom in of a BLG domain with typical size around 100 µm. Scale bar: 20 µm (c) Raman characterization of MLG and BLG. Inset, peak fitting of the 2D peak of the BLG. (d) The 2D peak position and (e) 2D peak FWHM maps of the same area as in (b). (f) Schematic view of the two-finger configuration BLG transistors with top and back gate. (g) Resistivity as a function of Vtg at different Vbg for the BLG transistors with a gate length of 1 µm. The Vds is fixed at 0.1 V. (h)


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Two-dimensional contour plot of resistivity as functions of both Vtg and Vbg for devices in (g).

Two-finger configuration RF transistors with top and bottom gate (dual gated) are fabricated using standard nanofabrication processes such as electron-beam lithography and metal evaporation. Typical RF device structures are shown in Figure 1f where ground-signal-ground (GSG) coplanar pad design is used for subsequent RF signal measurements. An 80-nm-thick Al metal film was evaporated to form the top-gate electrodes, which at the same time also provided thin top-gate dielectric of alumina with high capacitive efficiency and high-quality interface from naturally oxidation of Al metal in air, and the dielectric also shows negligible leakage current within the sweeping voltage range (Figure S4a). Electrical transport measurements have been performed to investigate the electronic characteristics of the RF devices using Agilent B1500A and a network analyzer. Figure 1g shows the dc characteristics of BLG transistors with a gate length of 1 µm in the presence of a perpendicular electric field. The drain-source bias (Vds) is fixed at 0.1 V, and the top-gate voltage (Vtg) is swept from -2 to 2 V with back-gate voltage (Vbg) varying from -60 to 60 V. As shown in the contour plot of resistance as a function of Vtg and Vbg in Figure 1h, tunable peak resistance at Dirac point is observed, confirming the Bernal-stacked nature of the BLG under displacement electric field.12,

20

Temperature-dependent measurements are used to evaluate the bandgap. As shown in the Figure S5b and c, the vertical displacement field introduces an electrically effective gap of over 40 meV in the bilayer graphene. The extrinsic low-field electron mobility extracted from the peak transconductance method is more than 2000 cm2/(V·s) (Figure S4b, c). The contact resistance of around 100 Ω·µm is obtained by the transmission-line method.



a

b 2

Vbg = 60 V

0.4

Ids (mA/µ µ m)

gm(mS/µm)

0.8

Vds(V)

0.0

1 1.5

-0.4

Ron = 550 Ω⋅µm

1

Vbg from -40 V to 60 V step 10 V

c 2

0

Vtg(V)

1

1

0

0.0

Vds(V)

0

Vds (V)

1.0

Vbg = 60 V

1.8 0.0

0.9

1.8

0.0 0.0

Vds(V)

2

gm

80

gds

60

0.5

1

0.9

1

d

2

Vbg = 0 V

0 0

2

gm/gds

-1

gm,gds (mS/µ µ m)

-0.8 -2

Ids(mA/µ µ m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

20

0.6

1.2

Vds (V)

0 1.8

Figure 2. DC characteristics of the BLG transistor with a gate length of 1 µm. (a) Transconductance curves at Vbg = 60 V. (b) Back-gate output characteristics at different Vbg. (c) Top-gate output characteristics at Vbg = 0 V (left) and Vbg = 60 V (right). Vtg varies from -2 to 1 V in 0.5 V steps with Vds varying from 0 to 1.8 V. (d) Measured gm, gds, and gm/gds across a range of drain voltage at Vgs = -2 V. Figure 2a shows the transconductance characteristics gm of a BLG transistor with a gate length of 1 µm at Vbg = 60 V and Vds at 1 and 1.5 V, respectively. A high extrinsic gm of 0.78 mS/µm is obtained owing to the high capacitive efficiency of top-gate dielectric and the high mobility of the BLG channel. The Dirac point voltage is around 0 V at Vbg = 0 V, indicating minimal doping effects from both the HfSiOx substrate and the alumina top gate dielectric (Figure S6a). The back-gate output characteristics with floating top-gate are shown in Figure 2b, where output drain current of more than


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2 mA/µm is obtained at Vbg = 60 V. A low on-resistance (Ron) of 550 Ω·µm indicates small contact resistance and low sheet resistance at on state. Figure 2c shows the top-gate output characteristics at two different back gate biases Vbg = 0 (left) and 60 V (right). We also measure the hysteresis in the transfer characteristics and output characteristics under high bias (Figure S6a and b). The small hysteresis indicates low trap density of the bilayer graphene device. Strong displacement electric field created by the back gate voltage induces a bandgap in BLG electrostatically which in turn improves the pinch-off of the graphene channel with better current saturation characteristic.13 The current saturation characteristics of dual-gate Bernal-stacked BLG transistors will result in low output conductance gds, which is essential for high intrinsic voltage gain and power gain. As plotted in Figure 2d, high intrinsic voltage gain defined as gm/gd of 77 is obtained, which is a critical factor for realizing more generalized circuit functions, such as signal amplifiers and mixers where devices based on MLG typically lack unless extremely thin oxide is used. To evaluate the high-frequency performance of BLG transistors, high-frequency scattering parameters (S-parameters) of the transistors were measured up to 30 GHz using standard GSG probes. Figure. 3a, b shows the as-measured short-circuit current gain (|h21|), and Mason’s unilateral power gain (U) extracted from S-parameters for devices with Lg = 2 µm and Lg = 1 µm, respectively. As a result, the cut-off frequency fT and maximum oscillation frequency fmax can be obtained to be 6.7 GHz and 9.6 GHz for the 2 µm device and 13 GHz and 16 GHz for the 1 µm device. The Vtg is biased at the peak transconductance while the Vbg is set to 60 V for all small signal measurements. These results for fT and fmax are among the highest reported as-measured RF performance for CVD graphene (Table S2 and Table S3) and comparable to the results reported by the epitaxial graphene RF devices



with similar gate lengths.21, 22 The RF characteristics after de-embedding to remove parasitic effects are shown in Figure S7, Figure S8 and Table S1 a

b Lg = 2 µ m

|h21|

Gain

U1/2

f

MAX

Lg = 1 µm

10

U1/2

f

= 9.6 GHz

1

= 16 GHz

T

1

10

Frequency (GHz)

1

10

Frequency (GHz)

c

d 10

Gain (dB)

15 10

MAX

f = 13 GHz

fT = 6.7 GHz 1

|h21|

Gain

10

Gain (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Voltage gain (Z21/Z11) Lg = 2 µm

5 0 -5 0.01

Power gain (|S21|2) 0.1

1

Frequency (GHz)

10

Voltage gain (Z21/Z11) 5

0

Lg = 1 µ m

5.6 GHz

Power gain (|S21|2)

-5 0.01

0.1

1

10

Frequency (GHz)

Figure 3. As-measured RF characteristics at Vbg = 60 V. (a), (b) shows measured small-signal S-parameter gains for the BLG transistors with Lg = 2 µm and Lg = 1 µm, respectively. (c), (d) shows the measured frequency response of AC open-circuit voltage gain (Z21/Z11) and forward power gain (|S21|2) for the BLG transistors with Lg = 2 µm and Lg = 1 µm, respectively.

Both devices have a fmax/fT ratio about 1.5 before de-embedding and a fmax/fT about 2 after de-embedding. The high ratio is mainly attributed to the superior current saturation characteristics, as indicated in the formula , fmax=(fT/2)/[gds(Rg+Rs)+2πfTCgdRg]1/2 where gds is the output conductance, Rg is the gate resistance, Rs is the source resistance, and Cgd is the gate to drain capacitance. A larger


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fmax indicates the potential application of the proposed devices for high-frequency amplifiers. Figure 3c, d shows forward power gain |S21|2 and AC open-circuit voltage gain |Z21|/|Z11| as a function of frequency for devices with Lg = 2 µm and Lg = 1 µm, respectively. The AC open-circuit voltage gain stays above 0 dB for the entire frequency range up to 30 GHz. Moreover, |S21|2, which reflects the real power amplification of a two-port network is around 1.1 dB at 1 GHz for the 1 µm channel length device, and stays above positive up till 5.6 GHz, which is among the highest value reported in literatures for CVD graphene amplifiers (Table S4).23, 24 This demonstration of the well-behaved amplifiers which can amplify signals and provide power gain at high frequencies above GHz shows great promise of the Bernal-stacked BLG transistors for realistic advanced RF circuitry.

Figure 4. Active transconductance mixer. (a) Measurement setup for the gate driven active mixer. (b) Measured conversion gain at Vds = 1.5 V as a function of input RF power. LO power = 2 dBm,



LO frequency = 2 GHz, input RF signal = 2.2 GHz. (c) Conversion gain and on/off ratio at different back-gate voltage bias. (d) Conversion gain and power gain benchmark for different groups’ work.

The down-conversion mixer has been demonstrated using gate mixing of the local oscillator (LO) and RF signals with an external power combiner, as shown in Figure 4a, where the LO frequency is 2 GHz, and the input RF signal is 2.2 GHz. The conversion gain of the active transconductance mixers can be calculated by Gc=g12rds/[4wRF2Cgs2(Rg+Rs+Ri)], where g1 is the first order coefficient of the Fourier series of gm(t), rds is output resistance, Ri is the channel resistance and Cgs is the gate-source capacitance.25, 26 As the formula shows, a high-gain mixer can be realized by increasing the output resistance rds, which can be obtained by biasing the transistors in the strong current saturation region. The device with a gate length of 2 µm was used to perform the high-gain mixer due to the relatively smaller gate resistance. Figure 4b shows the conversion gain dependence on the input RF power where a record high conversion gain of -7 dB at Vbg = 40 V is achieved, which is the highest conversion gain at 2-4 GHz frequency range reported for all graphene mixers to date.6-10, 26-30 The output spectrum of the mixer with input signals fRF = 2.2 GHz and fLO = 2 GHz is displayed in Figure S9a where the IF signal is observed at 200 MHz as expected. The conversion gain relationship with the LO signal is shown in the Figure S9b where it increases with LO power. As expected for better current saturation case, both the conversion gain and on/off ratio increase with the back-gate voltage. In fact, a significant value of over 10 dB in conversion gain can be continuously tuned using the dual gate voltages, which is unattainable previous on monolayer graphene devices. Figure 4d shows the benchmark of power gain and active mixer conversion gain with previous work where detailed data


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can be seen from Table 1 with the key figures of merit for graphene active transconductance mixers and amplifiers. Our work represents the highest power gain and conversion gain obtained simultaneously of all graphene RF transistors reported so far, owing to the unique current saturation characteristics of the Bernal-stacked BLG. Table 1: Benchmark parameters for active mixers and power amplifiers Ref

L/W µ gm Rc fT*Lg PLO (µm/µm) (cm2/V•s) (µS/µm) (Ω•µm) (GHz•µm) (dBm)

fLO (GHz)

Conversion gain (dB)

|S21|2 (dB)

6

0.17/10

2000

360

--

5.78

7

4

-55

-15

7

2/150

1200

5.5

2000

--

0

0.01

-35

--

8

0.6/20

3300

--

130

9

15

4

-31

--

9

0.75/20

3000

330

100

3.75

0

4

-14

-8

10

0.2/80

3000

600

--

6.4

0

2

-12

-3

This work

2/20

2300

600

100

13.4

2

2

-7

-0.1

This work

1/20

2300

780

100

13

2

2

-8.8

1.1

In conclusion, we have demonstrated high-performance RF transistors using CVD-grown Bernal-stacked BLG as the channel material. The transistors exhibited strong current saturation characteristics with high intrinsic voltage gain. In RF transistors, high forward power gain and AC open-circuit voltage gain were obtained at GHz frequency range. Moreover, when the devices worked as an active transconductance mixer, a record conversion gain of -7 dB was obtained. Our work represents an important step toward practical implementation of high-gain devices and circuits using CVD Bernal-stacked BLG.



AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Yanqing Wu: 0000-0003-2578-5214 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank He Zhang, Xiangjie Zhao, Ge Gao and Wei Xu in the Center of Micro-Fabrication and Characterization of Wuhan national laboratory for optoelectronics for the support in e-beam lithography and metal deposition. This project was supported by the Natural Science Foundation of China (Grant Nos. 61574066 and 61390504) and technology innovation project of Hubei Province (Grant No. 2017AAA127). ASSOCIATED CONTENT Supporting Information Bernal-stacked BLG growth and transfer, atomic layer deposition, device fabrication and measurement; Larger BLG domains growth; Mapping of I2D/IG for BLG films transferred onto different substrates; Breakdown characteristics of the top-gate dielectric and mobility of the BLG device; Low temperature measurements; The hysteresis under high bias operation; De-embedding structures and de-embedding ratio; Component parameters for the 1 µm gate length device; RF characteristics after de-embedding; Active transconductance mixers for the device of Lg = 2 µm;


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from

Mini-Circuits;

the

datasheet

is

available

at

available

at

www.minicircuits.com/pdfs/MACA-63H+.pdf. 16. ADL5350

from

Analog

Devices;

the

datasheet

is

www.analog.com/media/en/technical-documentation/data-sheets/ADL5350.pdf.


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TOC Graphic 2D FWHM (cm-1)

R (Ω Ω)

2

65.40

228.0

1

Vtg (V)

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0 -1

29.00

-2 -60

28.00

-30

0

Vbg (V)


30

60

High-Performance CVD Bernal-Stacked Bilayer Graphene Transistors

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