Graphene FETs EE439 FINAL PROJECT Yiwen Meng Su Ai
Introduction What is Graphene? An atomic-scale honeycomb lattice made of carbon atoms Before 2004, Hypothetical Carbon Structure Until 2004, physicists Andre Geim and Konstantin Novoselov separated Graphene from Graphite successfully
Background In 1918, V. Kohlschütter and P. Haenni described the property of graphite oxide paper 1948, G. Ruess and F. Vogt has captured image of few layer grapheme (approximate 3 to 10 layer) by Transmission electron microscope 2004, Andre Geim and Konstantin Novoselov produced grapheme by tape
Property Extremely thin, absorb only 2.3% of light Heat Transfer Coefficient = 15000cm 2 /m*k Room Temperature, Electron Mobility = 15000cm 2 /V*s Resistance = 10-6 Ω*cm The strength of graphene is 200 times higher than steel.
Production Techniques Exfoliation Epitaxy Silicon carbide Metal substrates Metal-carbon melts Reduction Sodium ethoxide pyrolysis Nanotube slicing Graphite sonication Carbon dioxide reduction
Exfoliation Splitting single layers of graphene from multi-layered graphite. Achieving single layers typically requires multiple exfoliation steps, each producing a slice with fewer layers, until only one remains. Geim and Novosolev used adhesive tape to split the layers in 2004 Disadvantage: the size of the layer is hard to control so it is unriliable to produce the graphene sample whose length meets the requirement for application Cost: cost $1000 to produce a sample as the size of cross section of human hair
Epitaxy The deposition of a crystalline overlayer on a crystalline substrate, where there is registry between the two. In some cases epitaxial graphene layers are coupled to surfaces weakly enough (by Van der Waals forces) to retain the two dimensional electronic band structure of isolated grapheme. Graphene monolayers grown on SiC and Ir are weakly coupled to these substrates and the graphene substrate interaction can be further passivated.
Epitaxy Silicon carbide Heating silicon carbide (SiC) to high temperatures (>1,100 C) under low pressures (~10 6 torr) reduces it to graphene. Size of graphene: Dependent upon the size of the wafer The face of the SiC : Highly influences the thickness, mobility and carrier density of the resulting graphene The electronic band-structure (so-called Dirac cone structure) was first visualized in this material
Silicon carbide (SiC) The energy band of Graphene Electron states in conduction and valence bands have opposite chirality An electron at graphene s Fermi energy EF carries with it a fluctuating polarization cloud that gives rise to both intraband and interband transitions. Conduction Band Valence Band
Epitaxy Metal substrates Graphene grown on iridium(ir) is very weakly bonded, uniform in thickness and can be highly ordered. Graphene on iridium is slightly rippled. Due to the long-range order of these ripples, minigaps in the electronic band-structure (Dirac cone) become visible High-quality sheets of few-layer graphene exceeding 1 cm 2 in area have been synthesized via chemical vapor deposition on thin nickel(ni) films with methane as a carbon source At very low pressure, the growth of graphene automatically stops after a single graphene layer forms Copper foil: Atmospheric pressure CVD growth produces multilayer graphene
Metal-carbon melts This process dissolves carbon atoms inside a transition metal melt at a certain temperature and then precipitates the dissolved carbon at lower temperatures as single layer graphene (a) melting nickel in contact with graphite as carbon source (b)dissolution of carbon inside the melt at high temperatures (c) reducing the temperature for growth of grapheme (d) temperature-time diagram of the process
Experiment on Graphene FETs High On/Off Current Ratio and Large Transport Band Gap at Room Temperature Typical on/off current ratio typically around 5 in top-gated graphene FETs On/off current ratio of around 100 and 2000 at room temperature and 20 K, respectively Band gap of few hundred millielectronvolts: be created by the perpendicular E-field in bilayer graphenes Dual-gate bilayer graphene FETs: measured electrical band gap of >130 and 80 mev at average electric displacements of 2.2 and 1.3 V/nm Epitaxial-Graphene RF FETs on Si-Face 6H-SiC Substrates
High On/Off Current Ratio and Large Transport Band Gap at Room Temperature (a)schematic view of bilayer graphene in Bernal stacking. A1 and B2 are equivalent without vertical E field shown by the green arrow. This symmetry is broken under E-field (b) Three-dimensional schematic view of the dualgate bilayer graphene FET (c) The layer structure within this bilayer graphene FET channel The room temperature output characteristics of the bilayer graphene FET in (a) at Vbg ) -100 V and Vtg from -2 to 6 V
High On/Off Current Ratio and Large Transport Band Gap at Room Temperature On/off current ratio (room temperature) =1uA/0.01uA =100 Size: 1.6 um * 3 um Vtg: -2.6 ~ 6.4 V Vbg: -120 ~ -80V, step: 20 source GND, drain bias: 1mV
Epitaxial-Graphene RF FETs on Si-Face 6H-SiC Substrates Measured common-source current voltage characteristics of 2 12 μm graphene FETs are shown while stepping the gate-to-source (Vgs)voltage from 5 V (top curve) with a step of 2.5 V. The schematic of Epitaxial-Graphene FET is shown in the inset Drain source space: 3 um Gate length: 2 um On state current: 0.2 A/mm at Vds=1V,Vgs=5V Peak extrinsic DC gm: 55mS/mm at Vds= 5V
Epitaxial-Graphene RF FETs on Si-Face 6H- SiC Substrates Measured common-source current voltage characteristics of 2 12 μm graphene FETs are shown while stepping the gate-to-source (Vgs)voltage from 5 V (top curve) with a step of 2.5 V. The schematic of Epitaxial-Graphene FET is shown in the inset Drain source space: 1 um Gate length: 2 um On state current: 1.18A/mm at Vds=1V,Vgs=5V Peak extrinsic DC gm: 140 ms/mm at Vds= 5V On/off Current ratio: 3~4