Quantum behavior of graphene transistors near the scaling limit.

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Quantum behavior of graphene transistors near the scaling limit. Wu,Y
Title Quantum behavior of graphene transistors near the scaling limit.
Authors Y Wu,V Perebeinos,YM Lin,T Low,F Xia,P Avouris
Journal Nano letters
Issue 3
Issn 1530-6992
Isbn
Doi 10.1021/nl204088b
PMID 22316333
Volume 12
Pages 1417-23
Keywords
Website
Publication Year Mar 2012

Abstract


The superior intrinsic properties of graphene have been a key research focus for the past few years. However, external components, such as metallic contacts, serve not only as essential probing elements, but also give rise to an effective electron cavity, which can form the basis for new quantum devices. In previous studies, quantum interference effects were demonstrated in graphene heterojunctions formed by a top gate. Here phase coherent transport behavior is demonstrated in a simple two terminal graphene structure with clearly resolved Fabry-Perot oscillations in sub-100 nm devices. By aggressively scaling the channel length down to 50 nm, we study the evolution of the graphene transistor from the channel-dominated diffusive regime to the contact-dominated ballistic regime. Key issues such as the current asymmetry, the question of Fermi level pinning by the contacts, the graphene screening determining the heterojunction barrier width, the scaling of minimum conductivity, and of the on/off current ratio are investigated.


Acronyms

Acronyms
ALD => atomic layer deposition
BN => boron nitride
CT => charge transfer
CVD => chemical vapor deposition
DLC => diamond-like carbon
DOS => density-of-states
E F ∝ √ n => energy is changed by the gate, the density-of-states (DOS) and correspondingly the carrier density
FLG => few-layer graphene
Fig- 8d => Fig- 8c provides a significantly increased photoresponse and allows photodetection with full surface illumination
GFETs => graphene field effect transistors
LEED => low energy electron diffraction
ML => monolayer
RF => radio-frequency
SP => surface plasmons
SPP => surface phonon polariton modes
T => temperature
at the gate => as the ratio of the output voltage (at the drain) to the input voltage

  • low energy bandstructure graphene involves π electrons.
  • first bandstructure calculations performed 1947 by P.R. Wallace 1 bandstructure is shown in Fig. 1a.
  • valence band is formed by bonding π states, conduction band is formed by anti-bonding π* states.
  • states  ; no avoided crossing, valence and conduction bands touch at six points, so-called Dirac points.
  • Two points Fig. 1a as K K points.
  • energies 1 eV, most electrical transport properties, bandstructure approximated by two cones representing valence and conduction bands touching at Dirac point.
  • Electron dispersion energy region extent linear, similar light unlike other conventional 2D systems dispersion 2-8 .
  • linear dispersion implications regarding properties of graphene.
  • unlike conventional 2D electron systems,

  • formed semiconductor interfaces, graphene atomic layer observation, perturbations interact π-electron system.

  • unit cell graphene contains two carbon atoms graphene lattice formed by two sub-lattices, A B, evolving two atoms ( Fig. 1b).
  • low energy structure graphene can form a Dirac Hamiltonian H=v F σ h - k, σ a spinor-like wavefunction, v F Fermi velocity graphene, k wavevector electron 2-8 .
  • character graphene wavefunction arises not spin, fact that two atoms unit cell.
  • define a pseudo-spin same direction group velocity electron population A B sites.
  • spin, pseudo-spin reversal not allowed carrier interactions.
  • underlies inhibition

  • graphene 2-8 .
  • inhibition backscattering, electron-phonon coupling high optical phonon frequencies responsible transport properties of graphene.

Graphene synthesis

  • Graphene has synthesized ways different substrates.
  • following, summarize synthesis methods, maturity, advantages disadvantages, targeted applications.

  • Graphene first from graphite 2004 9 .
  • low-budget technique growth interest graphene.
  • Graphene flakes to the study elucidation graphene properties.
  • size several-microns ( tens microns best), shapes, azimuthal orientation not controlled.
  • applications that take advantage graphene's transport properties require graphene on large scale (e.g., wafer-scale), arrays graphene flakes positioned unique azimuthal orientation substrate.
  • structures not demonstrated flakes technology expected limited relevance high-end applications.
  • focus on graphene synthesis techniques that shown promise limitations.

  • Graphene and few-layer graphene (FLG) grown by chemical vapor deposition (CVD) C-containing gases on metal surfaces and/or by surface segregation C dissolved bulk metals.
  • Depending solubility C metal, latter growth process, .

  • first evidence single layer graphite on metals found low energy electron diffraction (LEED) patterns Pt surfaces 10 .

  • patterns attributed surface carbon surface bulk Pt crystals 11 , first single layer graphite by J. W. May 1969 12 .
  • lot work devoted to the study formation few layer graphite by surface segregation C during annealing C-doped metals, including Ni 13 , Fe 14 , Pt 15 , Pd 15 , Co 15 .
  • example coexistence CVD surface segregation processes found graphitization Ni CH 4 -H 2 mixture 1000 °C 16,17 , production carbon species Ni surface by decomposition CH 4 creates concentration gradient between surface and the bulk, causing carbon atoms metal and form solution.
  • After saturation, graphite forms on the surface.
  • cooling, C atoms dissolved metal at high temperature precipitate metal surface, forming more layers graphene.
  • This process control, on polycrystalline metal foils, grain boundaries behave than grains 18 .
  • single crystal Ni (111), surface and the absence grain boundaries produce more thinner FLG, polycrystalline Ni, grain boundaries serve graphene nucleation sites favoring growth.
  • Different cooling rates lead different C segregation behaviors, affecting thickness quality of graphene films 19 .
  • Ni surface H 2 before graphene formation thickness uniformity graphene layer 19 .
  • H 2 eliminates impurities S P cause variations carbon solubility, affecting graphene thickness 17 .
  • Reina al. 20 used ambient-pressure CVD synthesize 1-- 12 layer graphene films on polycrystalline Ni films, ethylene decomposition Pt(111) surfaces, resulted in formation single layer graphite 21 .

  • deposition of graphene on Cu surfaces provides example surface-mediated CVD process 22 .
  • solubility C Cu (less than 0.001 atom% at 1000 °C 23 vs. 1.3 atom% at 1000 °C Ni 24 ), graphene can form on Cu only by direct decomposition

  • C containing gas on Cu surface.
  • graphene growth process self-limiting 23 , stopping one monolayer (ML).
  • control case graphene grown on Ni, solubility C Ni.
  • Graphene has grown Ru(0001) by surface segregation 25,26 Ir(111) by low-pressure CVD 27,28 .

  • electrical properties CVD graphene cannot tested metal substrates.
  • processes transfer graphene on insulating substrate developed 19,20,22 .
  • ability select host substrate growth substrate advantage graphene grown on metals.
  • At the same time, transfer process affects graphene's integrity, properties, performance.
  • Wrinkle formation, impurities, graphene , other defects, occur transfer.
  • Graphene growth (not ) substrates sizes limited only by size reactor, roll-to-roll process 29 , enables graphene production large scale lowers cost unit area.
  • enable large area applications of graphene future (e.g., electrodes large area electronics and cells as ITO replacement) 30 .

  • graphene grows most metals, on polycrystalline metal substrates polycrystalline structure 2D, i.e., same graphene layer single crystal domains graphene rotated neighboring domains and domain boundaries, alternating pentagon-heptagon structures 31 .
  • CVD graphene grown on Cu advantage one ML growth, discussed , grown single crystal Cu, structure, disorder between domains and grain boundaries 32,33 .
  • relation between graphene and Cu lattices proven ( disproven) 34 .
  • appropriate conditions, single crystals graphene can grown, shown Fig. 2a 35 .

  • CVD graphene exhibit electrical transport properties similar graphene flakes (e.g., high mobility) measurement takes place single graphene domain, interdomain measurements show effects grain boundaries.
  • variability mobility CVD graphene.
  • comes high-end applications, mobility maximization uniformity required wafer scale, techniques that produce graphene with unique azimuthal orientation wafer pursued.
  • following discuss approach based on SiC.

  • graphitization hexagonal SiC crystals during annealing at high temperatures vacuo reported by Badami 1961 36 .
  • annealing conditions top layers of SiC crystals undergo thermal decomposition, Si atoms desorb carbon atoms remaining on the surface re-bond form epitaxial graphene layers 37-39 .
  • kinetics graphene formation resulting graphene structure properties depend reactor pressure type gas atmosphere 40-47 .
  • Growth on the Si-face hexagonal SiC wafers, i.e., h-SiC(0001), appropriate conditions exhibits growth kinetics (contrary C-face growth) allowing better control number of graphene layers.
  • fact that the azimuthal orientation epitaxial graphene on Si-face determined by crystal structure of the substrate 37,38 , provides pathway uniform coverage structural coherence at wafer-scale.
  • addition, graphene grown on semi-insulating SiC be used without transfer insulating substrate.

  • Graphene formation starts top surface layers of SiC proceeds 48,49 .
  • three Si-C bilayers (ca. 0.75 nm) form one graphene layer (ca. 0.34 nm).
  • C-rich (6√3×6√3)R30° surface reconstruction ( buffer layer ) forms , C atoms arranged to graphene no sp 2 structure and covalent bonds underlying Si atoms exist 48 .
  • layer not exhibit

  • properties of graphene.
  • buffer layer forms one converted to graphene.
  • buffer layer responsible n type doping graphene on SiC (0001).
  • second graphene layer grow same way.
  • rate graphene formation slows after second layer 41 .
  • attributed inhibition Si removal SiC decomposition front.
  • Si atoms defect graphene (e.g., pinhole grain boundary), SiC terrace edge, sample edge, order escape.
  • More graphene layers form when graphene defectivity increases.
  • high vacuum, graphenization starts at low temperatures (1100 -- 1200 °C), where C atoms not graphene films with thickness 6 layers formed on SiC surface 50,51 .
  • gas atmosphere (e.g., Ar) pressures 1 bar, sublimation rate Si reduced , graphenization starts at temperatures higher than 1450 -- 1500 °C, where C atoms more form higher

  • quality graphene films with thickness limited only 1 --2 layers 44 .
  • lower thickness result concentration graphene defects, acted escape routes Si atoms.

  • structural coherence at wafer-scale epitaxial graphene on h-SiC(0001) demonstrated Fig. 2b 52 , shows five identical LEED patterns, different wafer areas.
  • sample grown using optimized growth process comprising surface preparation disilane graphenization Ar 52,53 .
  • Samples grown this process exhibit Hall mobilities approaching 5000 cm 2 /Vs carrier density n 4 × 10 11 cm −2 .
  • 54

  • bunching ( increased step heights Fig. 3d vs. Fig. 3c).
  • results link process graphene formation step bunching.
  • H 2 surface (Figs. 3a 3c) shows step graphene is not allowed form, H 2 etching took place 20 °C graphenization step.
  • Raman spectroscopy map Fig. 3f two-dimensional plot graphene's Raman 2D peak intensity distribution.
  • majority sample (red/pink area) exhibits intensity half stripes associated terrace edges.
  • Based plots Fig. 3e, peak corresponds red/pink pixel in Fig. 3f wider, higher intensity peak pixel in Fig. 3f, areas correspond one graphene layer, areas correspond two graphene layers.
  • growth second graphene layer initiated SiC terrace edges, agreement earlier observations 44 .

  • surface morphology properties of graphene grown on h-SiC(0001) depend miscut angle wafer surface 53 .
  • Graphene grown on wafers miscut angle 0.28° narrower terraces than graphene on surfaces miscut angles 0.1°, shows lower Hall mobility than .
  • Steps carrier transport 57 samples consisting , step-free areas dimensions larger than carrier path enable attainment highest mobility particular graphene film surroundings 53 .
  • Riedl al. 58 , other groups, demonstrated annealing epitaxial graphene on SiC(0001) at temperatures 700 °C H 2 breaks covalent bonds between buffer layer top bilayer SiC, converting to graphene.
  • reduces n type doping of graphene on SiC(0001) 58 , dependence of the mobility graphene on temperature 59 .

  • Growth on the C-face hexagonal SiC, e.g., 4H(0001 - ), exhibits faster kinetics, thickness control .
  • number of graphene layers resulting film not , surface , LEED patterns variability azimuthal orientation graphene film components (layers domains).
  • Fig. 4 shows LEED patterns taken two different samples graphene on 4H--SiC(0001 ).
  • Fig. 4a film with average thickness four graphene layers 60 .
  • Figs. 4b 4c same spot sample similar average thickness 61 .
  • strong graphene spots corresponding graphene unit cell rotation of 30° SiC cell, graphene on SiC(0001), disorder films, demonstrated by diffuse arches attributed to graphene.
  • disordered graphene has azimuthal orientation, , , different two samples.
  • Fig. 4a shows maximum intensity diffuse arches ± 2.2° SiC [1000] direction (rotation 30° ± 2.2° strong graphene spot), Fig. 4b angle between 8° 9° (rotation of 30° ± 8° 9°).
  • preferred twist orientations depend sample preparation conditions.
  • Theory shown twist angles 30° ± 8.2° 30° ± 2.2° produce

  • structures smallest unit cells ( refs.
  • 62 63, references ).

  • addition high quality graphene synthesis, material issue fabrication graphene devices involves finding gate insulator and substrate.
  • high constant film needed.
  • Graphene .
  • insulators (such as SiO 2 , HfO 2 , Al 2 O 3 , etc.) form quality, non-uniform films on .
  • Most importantly, interaction between graphene, insulator surface, charged defects near interface reduce mobility of carriers graphene 64 .
  • number different approaches have been used address problem, including first depositing , buffer layer surface 64 followed by atomic layer deposition (ALD) film, plasma-assisted deposition Si 3 N 4 65 , deposition metal film ( Al) seed layer, oxidized ALD insulator film 66 .
  • approach involves deposition of graphene on thin layers of exfoliated, single crystal, hexagonal boron nitride (BN) flakes, be used both as the gate insulator and as the substrate 67 .
  • approach provides perturbation graphene properties.
  • form BN cannot adapted technological use.

Graphene electronics

  • Graphene has some properties appealing applications in electronics.
  • high carrier mobility, μ, received most attention.
  • Mobilities excess 100 000 cm 2 /V ⋅ s 68,69 saturation velocities 5 10 7 cm ⋅ s -1 reported 70 .
  • addition, thinness, strength, flexibility material, fact that high carrying capacity ( 10 9 A/cm 2 ), high conductivity ( 5000 Wm -1 K -1 ) 71 , all contribute appeal.
  • most record properties material conditions.
  • technology, graphene part more structure, conditions by application .
  • example, electrical transport variety scattering interactions 72-77 .
  • include scattering long-range interactions with charged impurities on graphene more supporting insulator substrate, short-range interactions with neutral defects adsorbates by roughness phonons.
  • mechanism dominates scattering depends on the quality graphene characteristics environment graphene exists 78 .
  • instance, Coulomb scattering charged impurities dominates low temperatures when graphene contact with polar substrates such as SiO 2 Al 2 O 3 74 .
  • presence absorbates, scattering present substrate removed graphene suspended.
  • graphene heated, adsorbates , phonon scattering becomes , high mobilities 68,69 .
  • graphene

  • scattering point defects dominate carrier transport 79 .
  • type scatterer dominates particular graphene sample magnitude carrier mobility (μ), dependence on temperature (T) carrier density (n) 78 .
  • mobilities greater than 100 000 cm 2 /Vs scattering dominated by acoustic phonons, μ AC ∝ 1/nT 68,69,74 .
  • Long-range Coulomb scattering results in mobilities order 1000 -- 10 000 cm 2 /Vs n 73,75,76 .
  • Neutral defects become either highly samples high carrier densities μ ∝ 1/n 72,76,80,81 .

  • graphene channel electrical properties, transport graphene device affected dominated by other parts .
  • carriers have be injected into the graphene channel collected metal contacts.
  • Contacts generate potential energy barriers carriers affect device performance.
  • Graphene and metal different workfunctions, causes charge transfer (CT) between .
  • resulting dipole layer leads doping of graphene metal and band-bending.
  • more metals significant modification graphene bandstructure.
  • carrier injected graphene pass dipole barrier formed by CT metal channel graphene (p-n junction-like) barrier 82-84 .
  • Experiments show gate-dependent metal-graphene contact resistance ranges couple hundred Ω kΩ μm 84 .
  • contact resistances resistance graphene channel importance affecting transport amplified channel length scaled 85 .
  • consequence CT contacts introduction asymmetry between electron hole transport 86,87 .

Graphene transistors

  • carriers have been injected into the graphene channel transport controlled by gate-induced electric field.
  • bias applied gate raises electron energy, bias lowers .
  • graphene, E F neutrality point E NP ( Dirac point), transport involves holes, E F >E NP electrons transported.
  • Fermi energy is changed by the gate, the density-of-states (DOS) and correspondingly the carrier density (E F √ n) changed.
  • basis switching graphene field effect transistors (GFETs).
  • unlike transistors semiconductors bandgap, GFET does not turn , DOS =0 neutrality point.
  • conductivity order G min 4e 2 /πh remains 2 .
  • factor determines role graphene electronics.
  • ratio achieved by gating graphene transistor order 10, number depending on the quality graphene and the effectiveness gating.
  • Digital transistors utilized logic applications, other hand, require on/off

  • ratios higher than 10 4 .
  • 2D graphene is not digital switch.
  • research efforts gap graphene, addressing topic scope article.

  • lack of a band-gap not support use digital switch, carrier mobility, high transconductance graphene devices, thinness and stability material candidate fast analog electronics, radio-frequency (RF) transistors.
  • analog RF operation ability switch-off device, , not essential.
  • example, signal amplifiers, application devices, transistor on-state RF signal amplified DC gate bias.
  • Figs. 5a 5b illustrate structure GFET devices.

  • performance metric RF transistors cut-off frequency, T , defined frequency at which the current gain becomes one drain short-circuited source.
  • behaved device T given by T =g m /2πC , g m (g m = dI/dV g ) C dc transconductance capacitance device, 70,88 .
  • optimization T GFETs involves increasing g m minimizing capacitance.
  • cut-off frequency provides indicator potential channel material, voltage gain demanded devices.
  • voltage gain transistor defined as the ratio of the output voltage (at the drain) to the input voltage ( gate).
  • given by ratio of the transconductance g m output conductance g d (g d = dI/dV d ).
  • applications power gain required metric frequency,

  • max , which is the frequency gain becomes unity.
  • Unlike T , metric not only material (graphene), structure of the device.

  • Graphene from graphite been used DC 89,90 RF 91-98 GFETs.
  • first efforts utilizing graphene synthesized (wafer) scale based thermal decomposition SiC 92,95 .
  • highly result came 2010 wafer scale RF GFET produced T values 100 GHz 95 .
  • lack of a band-gap high optical phonon frequency graphene (~200 meV) current saturation achieve low.
  • fact that the graphene used mobility ~1500 cm 2 V -1 s -1 gate length , by today industry standards, (240 nm), suggested improvements performance achieved by material and device optimizations.
  • second generation SiC-based GFETs reached T values exceeding 300 GHz 40 nm channel lengths 97 .
  • better current saturation and thinner gate oxides produced ~40 GHz.
  • Fig. 5c show frequency dependence of the current gain (⎜h 21 ⎜) two different types GFETs.
  • One based CVD graphene DLC T = 155 GHz 98 and other based epi-graphene T = 300 GHz 97 .

  • development CVD graphene, transferred any substrate provided opportunity selection substrates.
  • used SiO 2 containing charged defects, degrade value reproducibility mobility GFETs.
  • One substrate

  • used diamond-like carbon (DLC).
  • graphene, , , non-polar.
  • first results yielded GFETs T = 155 GHz 98 and more improvements led T =300 GHz 97 .
  • study of the temperature dependence GFETs DLC showed performance remains same 300 K 4K absence carrier freeze-out demonstrating that graphene electronics be used environments space.
  • Thin layers of exfoliated, single crystal, hexagonal boron nitride (BN) been used most both as the gate insulator and as the substrate graphene transistors 67 .
  • BN isomorph graphite bandgap ( 6 eV) choice insulator.
  • Single layer graphene on material shows enhanced carrier mobilities ( 60 000 cm 2 V -1 s -1 n 1× 10 12 cm -2 ) reduced doping 67 .
  • material build graphene devices provided fabrication process other than BN single crystal exfoliation, CVD BN, developed.

  • Efforts address integration issues on-going.
  • number issues need addressed, involving adapting deposition technologies developed silicon technology to graphene.
  • first integrated graphene circuit involved GHz frequency mixer based grown graphene 99 .
  • Mixing achieved by channel resistance modulation applied ( ) graphene samples.
  • structure and performance frequency mixer are shown in Fig. 6.
  • IC demonstrated advantage that graphene offers applications insensitivity resulting mixer performance temperature variations.
  • Frequency mixing based behavior graphene has demonstrated 101 .
  • application analog transistors involves RF signal amplification.
  • Graphene-based IC voltage amplifiers fabricated 101 only power gains (3 dB) achieved 101 .
  • Efforts achieve better current saturation and increase gain focus effort.

Graphene photonics

  • addition electrical transport properties, graphene has optical properties wavelength range.
  • wavelength light properties less defects technological use graphene easier.

  • Theory based particle description graphene predicts absorption incidence photons energy range dispersion of the Dirac cone , i.e. near , value πα, α structure constant ( 2 /h - c 1/137) 102,103 .
  • studies verified absorption 2.3 % 103-105 .
  • More , energy range studies graphene absorption 106,107 reveal richer behavior.
  • Fig. 7 106 , energies absorption increases , peaks 4.6 eV asymmetric lineshape.

  • range where π−π* interband transitions saddle-point singularity near the M point graphene Brillouin zone expected.
  • ab-initio GW calculations predict transition at 5.2 eV 108 .
  • observations accounted by invoking interactions form saddle-point exciton red-shifts excitation energy.
  • asymmetric lineshape develops Fano-type interaction between exciton continuum interband transitions near the M point 106-109 .

  • states graphene decay fast.
  • ultrashort pulse generates e-h pairs highly non-equilibrium state.
  • interactions, i.e., carrier-carrier interactions provide fast energy redistribution mechanism 110,111 .
  • Time-resolved measurements show after 200 -- 300 fs Fermi-Dirac distribution attained electronic temperature T 109 .
  • expected, decay rate function electron density (doping).
  • energy redistribution processes, energy dissipation takes place phonon emission.
  • decay optical phonons fast, taking place a few picosecond time scale.
  • excitation energy fallen optical phonon energy ( 200 meV), acoustic phonon emission slow (nanoseconds) leads formation energy dissipation bottleneck 111 .
  • uncoupling lattice and electronic temperatures, resulting electrons persist nanoseconds.
  • Coupling electrons polar substrate surface phonon polariton modes (SPP) become decay path case 112 .

  • presence field gradient, photoexcitation graphene produces photocurrent, be used number applications.
  • advantages using graphene photodetector wide absorption range, high mobility of carriers, thinness and low cost material and ability operate temperature.
  • electric field produced by applying voltage bias.
  • graphene not band-gap, produce , leading heating and shot noise.
  • reasons, use fields .
  • fields present graphene p-n junctions.
  • junctions formed by, example, fabricating split-gate devices, formed at metal-graphene contacts difference between work functions resulting charge transfer ( Fig. 8a).
  • carriers driven by potential gradient p-n junction 113-115 by photothermal effects 116-118 , arise laser heating and difference Seebeck coefficients two sides, V PTE = (S 2 S 1 )ΔT.
  • photothermal effects enhanced acoustic phonon bottleneck leading carriers.
  • suggested generated signal case fast, T >T lattice and heat transport involving carriers, lower heat capacity than phonons 117,118 .

  • focused properties of graphene arising interband transitions.
  • far-IR terahertz regions,

  • free carrier absorption dominates.
  • frequency dependence free carrier response graphene can by Drude model absorption conductivity given by σ(ω)=iD/[π(ω+iΓ)], D Drude weight Γ -1 carrier scattering rate 119 .
  • Fig. 9 shows - - spectra CVD graphene and the different effect chemical ( ) doping two regions graphene spectrum.
  • Analysis light extinction (1 T/T 0 ) Drude regime provide conductivity, degree doping of graphene carrier mobility 119 .

  • addition, particle excitations graphene, excitations, i.e., surface plasmons (SP), importance.
  • Unlike dispersion of the quasi-particles graphene, SPs dispersion not by light 120 .
  • SP excitation breaking symmetry graphene.
  • by patterning graphene , generate 1D 0D structures 120 .
  • plasmonics

  • based metal nanoparticles highly advancing field , graphene offers opportunities advantages plasmonics.
  • associated confinement graphene, significant longer SP lifetimes 121 propagation distances , most importantly, ability carrier density graphene .
  • patterned graphene structures light-graphene coupling be used manipulate optical properties.

Optoelectronic applications

  • first demonstration graphene photodetector based fields generated at metal-graphene contacts.
  • segments graphene (~200 nm) contacted with Pd by light IR 123 .
  • Fig. 8b shows measured AC photoresponse junction detector 1.55 μm modulated light beam.
  • constant response 40 GHz, which was the frequency limit measurement system.
  • Modeling suggested detector response limited by RC constant devices 0.6 THz.
  • device, illumination contacts produces equal polarity currents no photocurrent.
  • design shown in Fig. 8c provides a significantly increased photoresponse and allows photodetection with full surface illumination (Fig. 8d).
  • device utilizes inter-digitated metal electrodes two different metals, one high workfunction other low workfunction.
  • two different workfunctions produce different doping band-bending graphene allows photodetection area device.
  • Photodetectors design shown detect data streams 1.55 μm light pulses rate 10 GBits/s 124 .

  • increase photoresponse graphene detectors, number other approaches have employed.
  • one case, enhanced light absorption photocurrent generation achieved excitation plasmons gold nanoparticles deposited on graphene 125 .
  • involves incorporating graphene Fabry-Pérot microcavity increasing absorption 60 % 850 nm 126 .

  • ability modulate Fermi level graphene by gate field leads application fast electro-absorption modulator 127 .
  • High speed, footprint high bandwidth modulators highly on-chip interconnects.
  • lightgraphene interaction , absorption light beam by single graphene layer .
  • M. Liu al. 127 graphene with silicon waveguide increase absorption and achieved modulation by 0.1 dBμm -1 1.35 -- 1.60 μm light frequencies 1 GHz.
  • case photodetectors, advantage graphene-based modulators III-V based devices ability integrated Si-CMOS electronics.

  • property of graphene saturable absorption.
  • Saturable absorption situation light absorption decreases increasing light intensity.
  • Most materials show some saturable absorption , , high light intensities (close damage).
  • absorbers are used laser cavities convert wave output laser train ultrashort light pulses.
  • Graphene with wide absorption range, fast decay high stability application used produce picosecond laser pulses 128,129 .

  • review presented some progress graphene synthesis applications in electronics photonics.
  • research area development stage more work needed graphene's potential.
  • applications of graphene radio-frequency electronics, panel displays, cells as electrodes, high density conductors .
  • graphene photodetectors imaging systems, light modulators switches, mode devices, rf radiation screening systems, 2D graphene plasmonics applications .
  • At the same time, quality area graphene improving.
  • expect other applications that exploit properties of graphene coming years.
  • Success require , research effort funding.

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