Presentation
Graphene and new 2D materials: Opportunities for High Frequencies applications April 21th, 2015 H. Happy, E. Pallecchi, B. Plaçais, D. Jiménez, R. Sordan, D. Neumaier Graphene Flagship – WP4 HF electronic [email protected] Outline • Graphene FET: State of the art • GFET optimization • Gate contact • Current saturation exploring velocity saturation • Ballistic devices • Graphene flagship WP4 –High frequency electronic: Some achievements • Summary and outlook [email protected] 2 Progress in fabrication process GFET on SiC substrate Devices on Substrate Optical Microscope Image Full Device SEM Image FIB cross section of the active part of device 3 Study of parametric variation of performances 8 Devices (Number) 7 30 25 20 5 4 3 2 1 10 0 ft_intr 14 21 28 34 41 48 ft_DUT 5 / (GHz) fmax 0 -5 -10 -15 1 10 frequency (GHz) M.S. Khenissa et al.; EuMIC 2014 – Roma – Italy, doi: 10.1109/EuMIC.2014.6997796 (IEMN) 100 Devices (Number) Gains (dB) 15 6 8 7 6 5 4 3 2 1 0 9 12 15 18 21 24 27 fmax (GHz) 4 GFET – State of the art Figures of merit: ft, fmax ft,: Current gain cut-off frequency fmax Maximum oscillation frequency (over this frequency, there is no power gain) F. Schwierz, Proceedings of the IEEE, vol. 101, no 7, 2013 5 How these figures of merit (FOM) are defined ? Conventional GFET structure RF probes for measurements Top view of dual-gated graphene FET Device under test (DUT) How to extract performances of GFET ? (De-embedding procedure) 6 Extraction of extrinsic/intrinsic performance G Rg VGS RGD RGS CGS CGD gm VGS 1/ gd Rd D CDS 50 Gains (dB) 40 H21 30 20 Rs H21 S fmax 10 ft = 30 GHz ft fmax = 20 GHz 0 -10 Ft_probes = 12.5 GHz ft_Probes_plan -20 0.1 Frequency (GHz) 1 10 100 • Pads structures are capacitive • fmax remains constant with capacitive transform 7 How to define figures of merit (FOM) Small signal equivalent circuit of GFET without access lines cdsp cgdp cgsp Source Rs Rg G Cgs R gs cgdp Gate Cgd gm VGS R gd Drain Rd Rg RGD RGS cgsp VGS CGS CGD 1/gd Rd CDS gm VGS D cdsp Rs C ds 1/ gd S Ft < Ft < Ft = Fc = Gm/(2 πCgs) Fmax = Fmax < Fmax_intr 17 < 30 < 80 (GHz) 23 = 23 < 55 GHz 8 Some drawbacks of GFET o Contact and access resistance o No pinch-off o Lack of current saturation Compare to the typical HEMT devices Av ≈ gm gd Low Voltage gain High value of access resistance Main objective: Fmax # 100 GHz 9 Outline • Graphene FET for High Frequency (HF): State of the art • GFET optimization • Gate contact • Current saturation exploring velocity saturation • Ballistic devices • Graphene flagship WP4 –High frequency electronic: Some achievements • Summary and outlook [email protected] 10 Metal-Graphene Contact Resistance - Modeling Components for contact resistance Device Electrostatics Specific contact resistivity (3Dà2D) Lateral resistance 11 Metal-Graphene Contact Resistance Graphene etched below pure Au contacts Rc < 90 Ωµm 1400 1200 1000 R (Ω) • 800 600 R at V0 (Dirac point) 400 500 nm Au (100 nm) Rc W = 86 Ω µm 200 0 Rc W = 87 Ω µm 0 200 400 600 800 1000 1200 1400 L (µm) R. Sordan (WP4 Flagship) 12 Velocity saturation - Requirements Low contact resistance High mobilty graphene layer and low doping (phonon saturation) 50 Ωµm < Rc < 150 Ωµm Ft extr = 20 GHz Fmax = 13 GHz (Wg = 2x50 µm, Lg = 1 µm). O. Habibpour et al.; EuMW 2014 – Roma – Italy (WP4 Flagship) 13 Velocity saturation – impact of 2D materials G-FET GoBN Y. Wu et al. / Nano Letters 12 (2012) 3062 L=0.6 µm I Meric et al. / IEDM (2011) I Meric et al. IEEE, Vol. 101, No. 7, (2013) "↓$%& ~34 ()* 14 Towards tunable contact and ballistic GFET Artist view of device SEM picture (L=200 nm) contact gate Drain Drain 16 n Source SEM m hB N Channel gate Source Gated Pd-contacts, 20 nm thick W-gates Contact - - - - - - - contact junctions + + + + + + Contacted graphene Free graphene Contact gate (2) Channel gate (1) h-BN(20nm) SiO2 Contact - - - - - - - + + + + + + Contacted graphene Contact gate (2) Contact gate (2) Vg2 Si++ Q. Wilmart et al. (unpublished) WP4 Flagship 15 Towards tunable contact and ballistic GFET Vcont G2 Drain S D SEM Sourc e Differential resistance Q. Wilmart thesis Vds Drain 100µm G1 ~ Vch Sourc e Pulsed contact gating RF-gain switching High impact of BN – good dielectric compared to Al2O3 16 Towards tunable contact and ballistic GFET L=500nm, W=1.5 µm Velocity saturation ! Vg=-3...0V Vg=0...3V 17 Outline • Graphene FET for High Frequency (HF): State of the art • GFET optimization • Gate contact • Current saturation exploring velocity saturation • Ballistic devices • Graphene flagship WP4 –High frequency electronic: Some achievements • Summary and outlook [email protected] 18 From devices to circuits Graphene-based Inverters Av = - 11.3 High voltage gain Av = 11,3 with L = 2 µm 19 From devices to circuits – Ring oscillator VDD VDD VDD The highest oscillation frequency 1 2 3 4 was fo = 4.3 GHz at the gate length L = 0.9 µm L = 0.8 µm and W = 5 µm E. Guerriero et al., ACS Nano 7, 5588 (2013) WP4 Flagship 20 Summary and outlook • Progress in graphene processing is made • H-BN show a great impact on the GFET performances • Looking for large scale material • Expertise already transfer also to WP8 (Flexible electronics) Recent results using CVD Graphene from Graphenea C. Sire et al, Nano Le0. 12, 1184 (2012) (CEA – IEMN -‐ Northwestern U.) fmax= 7 GHz and ft= 20 GHz on flex substrate 21 22 23 WP4 – HF electronics • Nanostructuration of GFET channel (graphene nanoribbonds) SEM image of nanoribbons and nanomesh obtained by e-beam lithography Nanoribbonds width: from 10 nm to 50 nm 24 Nanostructuration of GFET channel GFET on SiC with Hydrogen intercalation Mobility: 2300 cm2/V.s @ Vds = 300 mV U_ext, U_intr, H21_DUT, H21_ext, H21_intr (dB) 60 50 40 30 20 10 Ft_probes = 17 GHz 0 -10 0.01 Lg = 100 nm 0.1 Frequency 1 (GHz) 10 100 fmax = 30 GHz fmax = 70 GHz ft = 50 GHz ft = 140 GHz WP4 Flagship (IEMN) (Without access resistance) 25 De-embedding structures Measurement plan Metal Graphene FET [Ymeasured] Pad Oxide “open” structure Extrinsic performance « Intrinsic » performance [Yextr] = [Ymeasured] [Ypad] [Yintr] = [Ymeasured] - [Yopen] Only parasitic capacitances are removed 26
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