Scanning Gate Microscopy (SGM) of semiconductor nanostructures

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1 Scanning Gate Microscopy (SGM) of semiconductor nanostructures H. Sellier, P. Liu, B. Sacépé, S. Huant Dépt NANO, Institut NEEL, Grenoble, France B. Hackens, F. Martins, V. Bayot UCL, Louvain-la-Neuve, Belgique M. Pala IMEP, Minatec, Grenoble, France L. Desplanque, X. Wallart IEMN, Lille, France GDR 2426 Physique Quantique Mésoscopique Session thématique «Champ proche» 2-4 novembre

2 Outline 1. Description of SGM technique - context - potential - operation 2. Review of SGM experiments - contributors - microscopes - quantum point contact - quantum dot - quantum Hall effect - quantum ring 3. ANR project on electron interactions - objectives - strategy 2

3 Introduction to SGM Local probe of electron properties in semiconductor heterostructures where electrons are several tens of nanometers below the surface thus not accessible by Scanning Tunneling Microscopy 2DEG Quantum Point Contact Quantum Wire Quantum Dot Quantum Ring Quantum Hall Effect V I V 3

4 SGM versus STM STM SGM Scanning Tunneling Microscopy Scanning Gate Microscopy I e- V Φ I conducting surface surfaces, nano-objects, defects tunneling current local density of state Vtip V insulating surface high mobility 2DEG heterostructure conductance of device local gate effect 4

5 Tip induced scattering potential Low density electron gas imperfect screening of the tip potential local potential change modified electron scattering Vtip < 0 equipotential lines - - Vcontact = 0 V2DEG, local < 0 Other ingredients : Contact potential Dielectric constants Etched trenches Surface gates Charged defects 5

6 Tip induced scattering potential Examples in the SGM literature : Crook et al, Phys. Rev. B (2000) Aidala et al, Nat. Phys. (2007) Another model : based on Krcmar et al, Phys. Rev. B (2002) z0 + εr1 εr2 6

7 Tip induced scattering potential SGM = scattering method (STM = intrinsic LDOS) E = EC - e V E = EC - e V EF EF x xtip Medium electron density (N ~ 1012 cm-2) - small perturbation x xtip Low electron density (N ~ 1011 cm-2) - strong back-scattering Leroy, 2003 PhD thesis 7

SGM operation Device: High mobility 2DEG Device patterning (surface gate and/or etching) Instrument: Low temperature AFM (4He, 3He, dilution) with magnetic field Positioning: AFM topographic image to

8 SGM operation Device: High mobility 2DEG Device patterning (surface gate and/or etching) Instrument: Low temperature AFM (4He, 3He, dilution) with magnetic field Positioning: AFM topographic image to locate the device 2DEG doped barrier undoped channel buffer layer substrate I V Scanning: Tip scan at constant distance with applied voltage Vtip while measuring conductance G Result: Image of the local gate effect on the global device conductance 8

9 Outline 1. Description of SGM technique - context - potential - operation 2. Review of SGM experiments - contributors - microscopes - quantum point contact - quantum dot - quantum Hall effect - quantum ring 3. ANR project on electron interactions - objectives - strategy 9

10 SGM around the world Start Place Group 2DEG 1996 US - Harvard Westervelt, Eriksson, Topinka, Leroy, Bleszinski, Aidala,... US - Santa-Barbara 1999 US - Berkeley McEuen, Bachtold, Woodside,... US - Stanford US - Santa-Barbara 2000 UK - Cambridge Ritchie, Smith, Crook,... UK - Cambridge 2004 CH - Zürich Ensslin, Ihn, Pioda, Gildemeister, Baumgartner,... D - Regensburg US - Santa-Barbara 2005 US - Arizona Ferry, Aoki, DaCunha,... JAP? (InGaAs) 2006 F - Grenoble Huant, Bayot, Hackens, Martins, Sellier,... F - IEMN (InGaAs) 2007 US - Stanford Goldhaber-Gordon, Jura, Topinka,... US - Bell Labs 2010 I - Pisa Heun, Paradiso,... US - Bell Labs 2010 B - Louvain Bayot, Hackens, Martins,... F - IEMN (InGaAs) 10

11 SGM tips Piezoresistive AFM cantilevers Tortonese, APL (1993) ThermoMicroscopes, CA not produced any more... Example : Harvard Piezoelectric quartz tuning fork Karrai (1995) Giessibl (1996) SNOM, AFM, STM, SGM Example : Grenoble 11

12 SGM microscopes Grenoble 4 He 4K 9T Heun 3 He 400mK 9T 12

13 SGM microscopes Westervelt 3 He 400mK 7T Ensslin 3 He4He 100mK 8T 13

14 Quantum Point Contact Harvard (Westervelt) Imaging electron flow Creates interferences Topinka et al, Nature (2001) 1.7 K n = 4.5 x 1011 cm-2 µ = cm2/vs 2DEG 57 nm below surface tip at 13 nm Vtip = -3 V 14

15 Quantum Point Contact Harvard (Westervelt) Imaging electron flow and cyclotron orbit under magnetic field Aidala et al, Nat. Phys. (2007) 4.2 K n = 3.8 x 1011 cm-2 µ = cm2/vs 2DEG 47 nm below surface 15

16 Quantum Point Contact Arizona (Ferry) Universal Conductance Fluctuations DaCunha, Aoki, et al, Appl. Phys. Lett. (2006) 280 mk 16

17 Quantum Point Contact Cambridge (Ritchie) Tuning QPC conductance «0.7 anomaly» Erasable Electrostatic Lithography Crook et al, Science (2006) 150 mk n = 3.1 x 1011 cm-2 µ = cm2/vs 2DEG 97 nm below surface 17

18 Quantum Dot ETH Zürich (Ensslin) Coulomb blockade resonances Analysis of tip potential (AFM nanolithography) Pioda et al, Phys. Rev. Lett. (2004) 300 mk n = 5 x 1011 cm-2 µ = cm2/vs 2DEG 34 nm below surface 18

19 Quantum Dot Harvard (Westervelt) Spectroscopy of single electron dot Tip potential width >> dot size... Fallahi et al, NanoLetters (2005) 1.7 K n = 3.8 x 1011 cm-2 µ = cm2/vs 2DEG 52 nm below surface 19

20 Quantum Dot Harvard (Westervelt) Image Coulomb blockade centers InAs nanowire with Ti/Al contacts Bleszinski et al, NanoLetters (2007) 4.2 K 20

21 Quantum Dot Berkeley (McEuen) Carbon nanotube with kinks Image Coulomb blockade centers + Single Electron Force Microscopy Woodside et al, Science (2002) 6 K 0.6 K 21

22 Quantum Hall Effect SGM at high magnetic field : - Berkeley (McEuen) - ETH Zürich (Ensslin) Transmission by edge states : no back scattering SGM images only at the transition between plateaus See talk by B. Hackens, Louvain-la-Neuve (Belgium) 22

Theory and simulation (IMEP) Ns ~ 2 x 1012 cm-2 µ ~ 100

23 Quantum Rings SGM experiments (NEEL) Grenoble2 + Louvain + Lille MBE growth (IEMN) E-beam lithography (UCL) 600 nm Theory and simulation (IMEP) Ns ~ 2 x 1012 cm-2 µ ~ cm2/vs [4K] Le ~ 2 µm [4K] ballistic Lφ µm [4K] coherent 23

effect Δϕ = 2π e ( V1 V2 ) dt h Dephasing by magnetic field magnetic

24 Quantum Rings Aharonov-Bohm interferences by SGM B. Hackens et al., Nature Physics (2006) Dephasing by tip potential electrostatic A-B effect Δϕ = 2π e ( V1 V2 ) dt h Dephasing by magnetic field magnetic A-B effect iso-phase lines = information on electron wave function interferences 24

Quantum Rings Local Density of State by SGM

: Analytical model for single channel : M.

25 Quantum Rings Local Density of State by SGM Experiment Simulation (Marco Pala, IMEP, Grenoble) F. Martins et al, Phys. Rev. Lett. (2007) Influence of defects and magnetic field : Analytical model for single channel : M. Pala et al., Phys. Rev. B. (2008), Nanotechnology (2009) 25

26 Outline 1. Description of SGM technique - context - potential - operation 2. Review of SGM experiments - contributors - microscopes - quantum point contact - quantum dot - quantum Hall effect - quantum ring 3. ANR project on electron interactions - objectives - strategy 26

27 0.7 anomaly in QPC 27

28 ANR ITEM ITEM = Interaction et Transport à l'echelle Mésoscopique ITEM-Th (2008) J.L. Pichard, CEA, Saclay R. Jalabert, D. Weinmann, IPCMS, Strasbourg 1D chain U=0 U = 1.7 Freyn et al, Phys. Rev. Lett. (2008) 28

29 ANR ITEM ITEM = Interaction et Transport à l'echelle Mésoscopique ITEM-Exp (2010) H. Sellier et al, Néel, Grenoble M. Sanquer et al, CEA, Grenoble A. Ouerghi et al, LPN, Marcoussis 29

Final Research Report: Electronic and spin properties of one-dimensional semiconductor systems

Final Research Report: Electronic and spin properties of one-dimensional semiconductor systems Researcher: Sébastien Faniel Advisor: Sorin Melinte 1. Initial objectives The purpose of our project was to