ᣂቇⴚ㗔 䇸䉮䊮䊏䊠䊷䊁䉞䉪䉴䈮䉋䉎 䊂䉱䉟䊮䋺ⶄว 㑐䈫㕖ᐔⴧ䉻䉟䊅䊚䉪䉴䇹 ᐔᚑ22ᐕᐲ ળ䇮2011ᐕ3 4ᣣ䇮 ੩ᄢቇᧄㇹ䉨䊞䊮䊌䉴 㗄 A02 ኒᐲ 㑐ᢙᴺℂ 䈮ၮ䈨䈒㕖ᐔⴧ 䊅䊉䉴䉬䊷䊦㔚 વዉ䉻䉟䊅䊚䉪䉴 ઍ ᄢᎿ ㆺ

Transcription

1 A02

2 GCOE

3 Si device (further downsizing) Novel nanostructures (such as atomic chain) Nanoscale multi-terminal resistance measurement Carbon nanotube transistor Atomic switch

4 Interplay:l Dynamics: Realisticmodeling:

5 / //2 4

6 Motivation In devices, switching is often important Teraheltz technology is in progress time scale of electron dynamics Gate-length dependence of FET cutoff frequency Exp.: THz electron dynamics THz technology Carbon Nanotubes Zhong et al., Nature Nanotech. 3, 201 (2008). Burke, Solid-State Electronics 48, 1981 (2004)

7 SimulationMethod Hamiltonian: + Nearest-neighbor -orbital tight-binding model No Coulomb interaction & electron-phonon interaction AC response: + Nonequilibrium Green s function (NEGF) method Wide-band limit approximation (= constant density of states for electrodes)

8 CONTACTEFFECTSONACTRANSPORT Yamamotoetal.,PRB82,205404(2010).

9 Contacteffectonsusceptance Susceptance : Inductive Susceptance : Capacitive Susceptance 1.5 (9,0) zigzag SWNT (5,5) 5) armchair SWNT L=any positive integer L=3m-23 2 L=3m-1 L=3m

10 g=2.0g 0 g=2.0g 0 g=1.0g 0 g=1.0g 0 g=0.25g 0 g=0.25g 0

11 Susceptance Yamamotoetal.,PRB82,205404(2010). vs v.s. Conductance (9,0) zigzag SWNT Capacitive Inductive (5,5) armchair SWNT Capacitive Inductive G [e 2 /h] 1.3 L=3m-2 L=3m-1 L=3m G [e 2 /h] TheinductivecapacitivesusceptancetransitionoccursatG=e 2 /h.

12 SimulationMethod D1

13 Results Metallic CNT without defects conductance vs. frequency susceptance vs. frequency

14 CNT StoneWales : 2.214nm(9unitcells) : 2.214nm(9unitcells) : : 10 : :

15 Electrochemical resistive switch (atomic switch) Cu/Ta 2 O 5 /Pt Energy dispersive x-ray spectrometry T. Sakamoto et al., Appl. Phys. Lett. 91, (2007)

16 Cu/Ta 2 O 5 /Pt: Electronic Transport of Ta 2 O 5 with interstitial Cu or O defect (by ATK) 1) Pure Ta 2 O 5 2) O vacancies 3) Cu dopants Cu Cu1 Cu2 Cu3 Cu4 V V V Pt ent t (A) (A) Curren Ta 4 O 10 Ta 4 O 9 Ta 4 O 10 Cu(+) Trans smission Transm Ta O 4 10 Ta 4 O 9 Ta 4 O 10 Cu (V) E-E F (ev) Voltage (V) EE F (ev) Cu dopants- Apparently enhance the transport of Ta 2 O 5 O vacancies - Hardly affects the transport of Ta 2 O 5 Gu et al., ANS Nano 4, 6477 (2010).

17 Cu/Ta 2 O 5 examined by ESM (& SIESTA) Charge induced by the applied electric field Cu Ta 2 O 5 Vacuum Cu Ta 2 O 5 Cu Vacuum Ta 2 O 5 x Potential change due to the applied field Small field near the interface

18 Comparison on the electric field between een two models Model A Model B Electr ric field (ev/) (ev/ ) Model 4B Model 3 A Cu O() 4Cu() Origin of the small field near the interface: Interface states? Surface screening? Interface (mid-gap) states Applied bias does not induce motions of Cu at the interface!! Cu oxide formation at the interface may be essential

19 orbitalpartitionmethod D1Prof.S.W.Han(SeoulNat lu) ConsideranMIMcapacitor it invacuum NeartheFermilevel,the orbitalsoftheleftandright electrodesarecompletely decoupled d Partitiontheorbitalsinto leftandrightelectrodesand electrodes occupythemdifferently Applydipolecorrectionin p vacuumtosimulatean isolatedslabinvacuum E F E F

20 Orbitalpartitionapproach approach Obtainasetoforbital eigenvalues andfill orbitalsfrombelowto obtainthefermilevelthe 7

21 Orbitalpartitionapproach approach Takeorbitalswithina presetwindow aroundthefermi level 7

22 Orbitalpartitionapproach approach Separatetheminto leftandright electrodes 7

23 Orbitalpartitionapproach approach Setdifferentfermi levelsintheleftand rightelectrodes 7

24 Orbitalpartitionapproach approach Calculatechargedensity, calculatev KS from, andcontinuescfloop 7

25 Electrostaticpotential(Au/MgO/Au)

26 Energy&capacitancevs.voltage voltage Dielectric relaxation further charging of capacitor Dielectricrelaxation furtherchargingofcapacitor increaseinenergy

27 4 D3M2 4 4 [B. Gao et al., Phys. Rev. Lett. 95, ] 4 4

28 (SCC-DFTB) [PRB 58, 7260 (1998), Phys. Stat. Sol. (b) 217, 41 (2000).] (NEGF) (NEGF) [PRB, 44,, 8017 (1991). PRB, 72,, (2005).] )] Mulliken

29 d p ev p d p di 2 =di 3 =0 I 2 =0 I 3 =0 I 1 =I I 4 =I = ev 14 /2 =ev 14 /2 p ev

30 4 p V 14 p (b), (c) d 23 p [ev V] d s-p d s-p V 14 [V] ()d (a) d 23 = nm, d s-p = nm 0.30 V] p [e p [e ev] V 14 [V] (b) d 23 =2.10 nm, d s-p = 0.22 nm V 14 [V] (c) d 23 =2.80 nm, d s-p = 0.18 nm

31 4(1) (1) I 2,I 3 (a) 2 3 I 1,I 4 V 14 =0.3 V 0.3~0.5 [a.u] I 1,II 4 I 2,II 3 I 2 (c)i 2,I 3 (b) (c) I 2 I 2 I 3 I 3 I 3

32 T. Frederiksen et al., Phys. Rev. B 75, (2007). 42mV

33 dv/di :2 : 2 : 4 : mv75 mv 42 mv 75 mv 242

34 V A-D 75 mv V A-C V B-D mv V A-B V C-D 42 mv100mv

35 A01G Generalized shifted COCG method (G) E G (0) E G (E) E

36 (E) E ~ 1. G (0) Generalized shifted COCG method 2. G (0) NeumannDyson m: Iteration

37 (CNT) () ()??? Nagatsuetal: PRL105,157403(2010).

38 sp2sp3 Asp3 A= (meV) 400! 4.0(meV)

39 [&]+BetheSalpeter Konabe&Watanabe: PRB 83,045407(2011).

40 H.Yanagisawaetal,PRL103,257603(2009) Field emission Photo field emission Optical field emission e - E F e - e - metal vacuum weak laser field strong laser field

41 Effect of width modulation in semiconducging armchair GNR N(=7) Energy [ev] N=5 N=5A-GNR AGNR k z a/ Armchair-edged GNR (A-GNR) Metallic when N=3m-1 (m:integer) Energy [ev] N=6A-GNR N=6 AGNR Our question: How does the presence of narrow ribbon region influences the transport characteristics in originally semiconducting armchair GNR? k z a/ N vac 7 (Number of edge lattice vacancies) Wide (N=6) Narrow (N=5) Wide (N=6)

42 Ener rgy [ev] V V Switching behavior in the absence of width modulation (N=6 Armchair GNR) S Source Drain V dope =1[V] Doping DS ka/ 1[ V ] 0.1[ V ] V GS =0[V] V GS =1[V] D T(E) Electrode Gate Source Electrode Gate ka/ GNR Ener rgy [ev] Top Gate Insurator 16 I D /I 0 Bottom Gate N=6 A-GNR Perfect tedge M ch = Electrode Gate t ins 2[nm] Electrode Gate Drain I V GS [V] e h 1eV A 0 E0 Side view Top view N=6 L ch 153a0 6.4nm

43 Local density of states in the absence of width modulation 3 2 Source Drain Local Density of States 3 2 Lch=6.4 nm En nergy [ev] S D V dope =1[V] ka/ V V V Doping GS DS T(E) 1[ V ] 0[ V ] 0.1[ V ] ka/ Source Electrode Gate Electrode Gate GNR z [nm] Top Gate Insurator 16 Bottom Gate t ins Electrode Gate 2[nm ] Electrode 43 Gate Drain

44 Effect of width modulation (appearance of resonant peak in transmission) Energy [ev] S Source Drain N vac =1 V dope =1[V] D Energy [ev] I /I D N=6A-GNR M ch =15 N vac =1 N vac =0 (Perfect Edge) ka/ ka/ T(E) V GS [V] N I0 E0 1eV 38.74A h vac 1 e L ch 153a0 6.4nm Appearance of resonant peak in the transmission influences the current

45 Effect of Edge Lattice Vacancy En nergy [ev] ] S Source -1 V dope =1[V] -2 N vac =2 Drain En nergy [ev] ] En nergy [ev V] S Source 0 0 D -1-1 V dope =1[V] N vac =7 Drain D ka/ T(E) ka/ ka/ T(E) ka/ N vac 2 En nergy [ev V] N 6 N 5 N vac 7

46 Local density of states in the presence of width modulation 3 2 Source Drain 3 2 Local Density of States Narrow region [ev] En nergy S D 0-1 N =7 vac V dope =1[V] N= N=6 N=5 N=6 N=6 V Doping ka/ T(E) ka/ 1[ V ] VGS 0[ V ] VDS 0.1[ V ] z [nm] Discrete levels in narrow region continuum energy levels in narrow region) Our conclusion: In contrast to ordinary semiconductor nanostructure, transport in armchair GNR can be influenced significantly even by a small width modulation (or edge vacancies)!