State-of-the-art Graphene Transistors on Hexagonal Boron Nitride, High-k, and Polymeric Films for GHz Flexible Analog Nanoelectronics
1
Jongho Lee,1 Kristen N. Parrish,1 Sk. Fahad Chowdhury,1 Tae-Jun Ha,1 Yufeng Hao,2 Li Tao,1 Ananth Dodabalapur,1 Rodney S. Ruoff,2 and Deji Akinwande1*
Electrical and Computer Engineering, 2Mechanical Engineering and the Materials Science and Engineering Program, The University of Texas, Austin, Texas 78712, USA. *
[email protected] Abstract
We report graphene field-effect transistors on hexagonal boron nitride, high-k, and polymeric films featuring state-of-the-art electrical and mechanical properties on flexible substrates. The record electrical performance includes the highest ON current (~0.3mA/µm), the first demonstration of current saturation on flexible films and intrinsic gain, and the highest conversion gain flexible graphene frequency doubler. Extrinsic transit frequency of 2.23GHz, and maximum frequency of 1.15GHz are also achieved. In addition, robust electrical response down to 0.7mm mechanical bending radius is realized. Introduction The outstanding charge transport in graphene field-effect transistors (GFETs) offer attractive prospects for high-speed electronics. These outstanding features including high-mobilities (~10,000cm2/V-s) (1), (2), sub-THz cutoff frequencies at moderate channel lengths (3), (4), and electron-hole symmetry, afford linear and non-linear analog signal processing with the simplicity of a single transistor (5), (6). In addition, graphene is substrate agnostic and mechanically flexible. Hence, it is an ideal material for flexible electronics on polymeric substrates. In this paper, we investigate the realization and performance comparison of GFETs on flexible substrates featuring state-of-the art DC, AC, RF, and mechanical properties for the first time. The record DC performance includes the highest ON current (~0.3mA/µm), and the first demonstration of current saturation on flexible films. The AC properties include the first demonstration of intrinsic gain (gm/gds), and the highest power and conversion gain (CG) flexible graphene frequency doubler. Extrinsic (not deembedded) transit frequency (fT)~2.23GHz, and maximum frequency (fMAX) ~1.15GHz is achieved. In addition, robust electrical response down to 0.7mm bending radius for more than 20 bending cycles is realized, a record for GFET mechanical properties. The outstanding properties are made possible by the first realization of the ideal GFET structure on flexible polyimide (PI) films, featuring the integration of i) multi-finger embedded gates to enable arbitrary number of fingers and scalable gate dielectrics, ii) hexagonal boron nitride (h-BN) gate dielectric for impurity free graphene interface, and iii) robust Si3N4 passivation as demonstrated
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by immersion tests in water, tea, coffee, and milk. These results indicate that optimized GFETs can enable high-performance analog and RF flexible nanoelectronics. GFET Device Realization on Flexible Substrates T hree device str uctures, namely, top -gate GFET (TG-GFET), embedded-gate GFET with h-BN dielectric (EG-hBN/GFET), and embedded-gate GFET with high-k dielectric (EG-highk/GFET) were fabricated on PI and evaluated in this work. Fig. 1 illustrates the 3-D images of each structure and their optical images after complete f ab r i ca tio n. T he f ab r i c ati o n wa s r ea li zed u s i n g microelectronic and electron-beam lithography following our prior work (7). Due to uneven surface topography of polymeric films, additional treatments are necessary to achieve the best lithographic resolution and registration (8). In brief, PI films were spin-coated with liquid polyimide (PI2574 from HD Microsystems) and cured. TG-GFETs were prepared directly on these substrates, while others require more processing steps before graphene-transfer; atomic layer deposited Al2O3 or exfoliated h-BN films were prepared as gate dielectrics. Optimized annealing was performed at 300ºC to improve the surface smoothness of h-BN films under nitrogen ambient. Fig. 2 (a) is an atomic force microscope image of an h-BN flake on PI with root mean square (RMS) roughness ~0.4nm, comparable to the inter-layer spacing and indicative of an almost perfect
Fig. 1 Illustration and optical images of the three GFET device structures on PI evaluated in this work. (a) Top-gate GFET (TG-GFET). (b) Embedded-gate GFET with h-BN gate dielectric (EG-hBN/GFET). The inset shows the device. (c) Embedded-gate GFET with high-k Al203 gate dielectric (EG-highk/GFET). The embedded-gate devices feature multi-fingers up to 18 fingers, while the top gate is limited to 2 fingers.
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residue-free interface. Fig. 2 (b) compares the roughness of the surfaces evaluated in this work. The surface topography of the as-received PI film is over 100nm, which can be substantially improved (RMS 1~2nm) by curing the polymer film and subsequent deposition of a high-k dielectric over the cured surface. While the surface of as-exfoliated h-BN film is comparable to that of high-k on PIs, it is further improved by annealing as shown in the figure. Large-area monolayer graphene was grown by chemical vapor deposition (CVD) and transferred onto the PI films (9). A gate last process and patterned gate first process were used for the TG-GFET and embedded gate devices respectively. Embedded-gate devices with 2, 6, 10, and 18 fingers have been measured. Finally, Si3N4 was deposited by plasma-enhanced CVD on the embedded gate devices to passivate and protect the underlying structures.
IDID/ /width width (uA/um) (µA/µm)
A 2.5
A
10µm
100 4
3.5 2.5
VD=10mV
-2
Drain Current (uA/um)
300 B 300
-1
0
1
2
3
EG-hBN/GFET
250 250
Symbols – Data 200 Line – Model 200 150 150 100 100 50 50
L=1µm
00
0
0.2
0.4
0.6
0.8
1
1.2
Drain Voltage (V) Fig. 3 Electrical measurements of graphene devices. (a) EG-hBN/GFET offers the highest gate control owing to the clean impurity free surface even though the device had a longer channel and thicker EOT compared to the other two GFETs. (b) Output characteristics showing GFET current saturation on flexible substrates.
quantum capacitance, electron-hole asymmetry, and velocity saturation (11). In addition, we achieved GFET intrinsic gain(~|3.5|) on flexible substrates (Fig. 4) partly enabled by current saturation. This is a key milestone affording GFET analog/RF amplifiers for flexible electronics. Temperature measurements on a EG-highk/GFET showed weak dependence likely due to grain boundary and defect scattering in the graphene layer. High frequency gain data (Fig. 5) show embedded-gate devices feature record extrinsic (not deembedded) GHz fT up to 2.23GHz with an fMAX~1.15GHz from a 10-finger device, more than 2x higher than the 0.55GHz fMAX from a recent flexible GFET report (12). It is expected that a 10x increase is readily attainable with smaller overlap capacitance and higher μ. Table. I
Anneal
1 0.5 h-BN
Polyimide
Fig. 2 Surface roughness of dielectrics. (a) Surface of the smooth h-BN on flexible PI by AFM. The vertical scale bar of 200nm is shown on the right. (b) Comparison of the surface roughness of different dielectrics.
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0.5
VG - VDIRAC (V)
High-K ALD
2
0
1
Cure
3
1.5
1.5
-3
gm / gds
RMS Surface Roughness (nm)
B
2
0
DC, AC, and RF Properties DC transport measurements (Fig. 3 (a)) reveal that the EG-hBN/GFET offers the largest gate control, with strong electron-hole symmetry. The TG-GFET suffered from high charge impurity (nimp) and contact resistance (RC) which limited the field-effect. Peak low-field mobilities (μ) of ~1,000cm2 /V-s was extracted for the TG-GFET and EG-highk/GFET, and ~2,300cm2/V-s for the EG-hBN/GFET using a validated drift model (10). Higher μ is expected with further improvement in the graphene transfer process (7). GFET current saturation and record current density of ~0.3mA/µm is observed on flexible films (Fig.3(b)). The experimental output characteristics are in strong agreement with a rigorous drift-diffusion model that accounts for the
TG-GFET, L=0.25µm, Top-gate GFET EOT=17.2nm EG-highk/GFET, L=0.5µm, EOT=8.6nm MEGFET EG-hBN/GFET, L=1µm, EOT=18.5nm hBN/EGFET
0.5 0 -0.5 -1 -1.5 -2 -2.5 -3 -3.5 -4
EG-hBN/GFET
-2
-1
0
1
L=0.5µm
2
3
4
5
Gate Voltage (V) Fig. 4 First demonstration of greater than unity intrinsic gain (gm/gds) in graphene transistors realized on flexible substrates. VD=0.51V.
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-25 -20 -15 -10 -5
Conversion Gain (dB)
H21 , MAG (dB)
Transit Frequency (MHz)
B 35
EG-highk/GFET
30 H 21 H21 25 MAG MAG 20 15 fT =2. 23GHz 10 5 0 fT =1.15GHz -5 0.05 0.5 5
Fig. 5 (a) Extrinsic fT of EG-hBN/GFET showing a linear dependence on drain voltage. (b) Current gain (H21) and maximum available gain (MAG) obtained from S-parameters. VD=0.7V. These metrics are from raw device measurements and reflect the actual circuit speed that the current GFETs can afford for flexible nanoelectronics.
10
-30 -35 -40 -45 -50 -55
Input Power (dBm)
Fig. 6 Record experimental GFET frequency doubler circuit on flexible PI. (a) Circuit schematic for frequency doubler characterization. Zin=ZL=50Ω, typical of RF circuits. The GFET is biased at the Dirac point. The L and C network are needed to route DC and AC signals separately. (b) GFET conversion gain on flexible substrates. The line is a visual guide of the maximum CG of -29.5dB. fin=10MHz. VD=0.5V. TABLE II Comparison of GFET doublers Metrics Max CG Max POUT Size Circuits (dB) (dBm) (µm2) GFET on -35.0 -23.4 50×0.5 Quartz (15) GFET on SiO2 -30.8 -23.2 25×1.6 (16) GFET on SiO2 -25 -25 N/A (17) EG-hBN/GFET -29.5 -22.2 20×1 (this work)
compares the merits of the three device structures with the conclusion that while embedded-gates with h-BN dielectric is ideal (13), (14), EG-highk/GFET is the best practical device structure for now because further synthesis progress is needed for the integration of h-BN/graphene layers. Doubler Circuit Performance and Performance Prospects EG-hBN/GFET doubler circuit (Fig. 6(a)) showed high spectral purity, and high CG and output power (Fig. 6(b)) compared to prior GFET doublers (Table II). This work outperforms most recently reported graphene frequency doublers and achieves the highest CG on flexible films. The performance (CG of -29.5dB and output power of -22.2dBm) was enabled by the strong electron-hole symmetry afforded by h-BN dielectrics. Using our validated model, a non-linear circuit simulation of GFET frequency doublers (Fig.7) was performed to assess the ultimate CG limits. The gate and drain fields were restricted to avoid dielectric and graphene air breakdown, therefore
5
-25
-60
Frequency (GHz)
0
-20
resulting in a predicted realistic CG of ~-5dB with moderate μ and negligible nimp and Rc. Mechanical Deformation and Immersion Tests Mechanical tests (Fig. 8) on EG-highk/GFETs showed robust electrical response against stretching and bending down to a record 0.7mm bending radius. Repeated bending measurements at 0.7mm radius did not degrade the normalized resistance. We introduce the concept of
TABLE I Comparison of 3 flexible GFET device structures EG-hBN/ GFET
EG-highk/ GFET
Finger scalability in a single layer
No
Yes
Yes
Gate dielectric seed layer
Needed
None
None
Gate dielectric scalability
No
Yes
Yes
Automated gate dielectric process
Yes
No
Yes
Expected GFET performance
Good
Excellent
Very good
Comments Realizing large # of fingers is straight -forward without the need for 2nd interconnect level. Seed layer is needed for even coating of high-k on graphene. Not needed for embedded-gate. For top-gate devices, seed layer prevents ultra-thin dielectrics (
