Carbon based Nanoscale Electronics

Transcription

1 Carbon based Nanoscale Electronics ME class

2 Outline driving force for the carbon nanomaterial electronic properties of fullerene exploration of electronic carbon nanotube gold rush of graphene

3 How far we can push Si?

4 Beyond Si C sheets III IV V B C N? Al Si P Ga Ge As Si ingots and wafers? Ge ingots and wafers

5 Forms of bonding in Carbon sp 2 Graphite (hexagonal crystal) sp 3 Cubic crystal Diamond (cubic crystal) Very flexible bonding: richness of structures

6 Our main personage Graphene

7 Building block of graphene -benzene molecule p z states π state (1957)

8 Graphene Allotropes

9 The first well-defined carbon nanomaterials

10 Electronic property Strong intra-molecular interaction

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14 The discovery of CNT Rolling up graphene

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18 semiconducting

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20 Fermi-level of Pd electrode aligns well with the valence band of the CNT, affording near ballistic p-channel conductance CNT FET inverter Fermi-level of Sc electrode aligns well with the conductance band of the CNT, affording near ballistic n-channel conductance

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26 Graphene: easy to make, hard to find. 5 µm

27 The displacement of atoms caused by thermal fluctuation become The displacement of atoms caused by thermal fluctuation become comparable to interatomic distance at any finite temperature

28 Simple Yet Efficient Mechanical Extraction by scotch tape

29 From graphite to graphene Graphene multi layers Graphene single layer

30 What makes graphene stable, against the theory? Graphene has a tendency to be rough and have locally a finite curvature: rippling J. C. Meyer et al., Nature 446, 60 (2007)

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32 Indirect evidence Scanning Tunneling Microscope image E. Stolyarova et al., cond-mat/

33 special electronic properties of graphene originating from the crystal symmetry t ~ 0.1 ev A A t ~ 2.7 ev B Unit cell Next Nearest neighbors Nearest neighbors

34 Electronic properties of graphene Dirac cone Momentum space Relativistic, massless Dirac Ferminos

35 What makes graphene special from conventional 2DEG semi-metal metal massless carrier Ultra relativistic solid state t at low speed of light 2 2 E ( p ) = ± v p + p = ± v ± F x y F p E v F ± ( p, m) = ± m 3ta = c / v 4 F + v 2 F p 2 with m = 0

36 Charge transport in graphene In the presence of magnetic field Quantized Hall conductance σ xy (4e 2 /h) 7 / 2 5 / 2 3 / / / / / / 2-4 σ xy (4e 2 /h) quantized ρ xx (kω) 6 4 T =4K B =12T n (10 12 cm -2 ) Novoselov et al., Nature 438, 197 (05) Y.Zhang et al., Nature 438, 201 (05)

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38 Half integer quantum Hall effect

39 Room temperature QHE Historically Efforts to extend the QHE temperature range by, for example, using semiconductors with small effective masses of charge carriers have so far failed to reach temperatures above 30 K Mechanism due to the highly unusual nature of charge carriers in graphene, which behave as massless relativistic particles (Dirac fermions) and move with little scattering under ambient conditions

40 Physical mechanisms of Quantum Hall Effect: impurities and/or particular states (e.g., edge currents) seem to be important for the 'integer' ' effect in the fractional quantum um Hall effect the Coulomb interaction is considered as the main reason. Graphene opens a new door for Quantum Physicist s, To explore the fundamental knowledge of 2D system

41 Charge transport in graphene without magnetic field Novoselov et al, Science 306, 666 (2004)

42 What s more in the charge transfer Sub-linear conductivity short-range range scatters: ripples, atomic defects Minimum conductivity the impurity issue is the hot-debate in the current research it remains one of the challenges for future device application make graphene clean? Long-range scatters: charged impurities

43 An alternative to make it clean Suspended graphene Approaching ballistic transport T=100k model Reaching high mobility: 200,000cm 2 /v/s And long free mean path

44 For graphene, N=4 (2 from spin and 2 from K and K ) Constant value for certain device Constant value for certain device, independent of external electric field

45 Mobility comparison Mobility of suspended graphene: 200,000cm 2 /v/s!!

46 Possible Applications High carrier mobility even at highest electric field induced concentrations, largely unaffected by doping, ballistic electron transport over sub µm distances at 300K May lead to ballistic room temperature transistors. GaTech group made proof of concept transistor leaks electrons, but it s a start.

47 Graphene nanoelectronics All graphene Single Electron Transistor Coulomb blockade oscillation at 4K Coulomb blockade resonance at 0.3K

48 Room Temperature SET E (ev) αħν F /d 1/d (nm) Compared with other semiconductors, and it requires ribbons with width d of about 10 nm for room-temperature operation E >3κT 10nm constriction Quantum dot Challenge: reproducibility?

49 Size dependency of energy gap Random distribution is due to the quantum confinement on the 2D plane

50 Graphene nanoribbon electronics High on/off ratio ~10nm width of graphene, resulting in the quantum confinement on the plane semiconductor FET feature Dai HJ, Science, 2008

51 top-down micro/nano fabrication bottom-up Chemical tailor & self-assembly Graphene is promising for molecular electronics design the shape with desirable size engineering the energy gap via tuning the size energy gap dependent FET performance

52 May be a new toy for semiconductor engineers