Semiconductor Nanowires. Stefan Heun NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Pisa, Italy

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1 Semiconductor Nanowires Stefan Heun NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Pisa, Italy

2 Leaning Tower in Pisa Pisa and Hangzhou: Twin towns since 2008

3 Institute of Nanoscience- National Research Council NEST (Pisa) NNL (Lecce) S3 (Modena) Adm. Genova

4 Research Activities at NEST SAW-driven electronic dynamics, lab-on-chip THz quantum cascade lasers Semiconductor Nanowires Quantum Hall physics, solid-state interferometry Hybrid and superconducting structures Quantum dots Graphene and artificial graphene Nanobiotechnology

5 Research Activities at NEST SAW-driven electronic dynamics, lab-on-chip THz quantum cascade lasers Semiconductor Nanowires Quantum Hall physics, solid-state interferometry Hybrid and superconducting structures Quantum dots Scanning gate Microscopy AFM at 300 mk at 9 Tesla Graphene and artificial graphene Nanobiotechnology

6 Research Activities at NEST SAW-driven electronic dynamics, lab-on-chip THz quantum cascade lasers Semiconductor Nanowires Quantum Hall physics, solid-state interferometry Hybrid and superconducting structures Quantum dots Graphene and artificial graphene Compositional Mapping of QDs XPEEM Nanobiotechnology with synchrotron radiation

7 Research Activities at NEST SAW-driven electronic dynamics, lab-on-chip THz quantum cascade lasers Semiconductor Nanowires Quantum Hall physics, solid-state interferometry Hybrid and superconducting structures Quantum dots Graphene and artificial graphene Nanobiotechnology Hydrogen on Graphene UHV variable temperature STM

8 Research Activities at NEST SAW-driven electronic dynamics, lab-on-chip THz quantum cascade lasers Semiconductor Nanowires Quantum Hall physics, solid-state interferometry Hybrid and superconducting structures Quantum dots Graphene and artificial graphene Nanobiotechnology

9 Outline III-V Nanowires at NEST: Growth and Research Pd-assisted Growth of InAs NWs

10 Outline III-V Nanowires at NEST: Growth and Research Pd-assisted Growth of InAs NWs

11 People CBE Growth: D. Ercolani, Ang Li, and L. Lugani (NEST-SNS), Lucia Sorba (NANO-CNR) NWs Devices: S. Roddaro, A. Pescaglini, A. Pitanti, F. Beltram (NEST-SNS) and A. Tredicucci (NANO-CNR) Hybrid Devices: P. Spathis, S. Biswas and F. Giazotto (NANO- CNR) TEM: F. Rossi, L. Nasi, G. Salviati (IMEM-CNR), V. Grillo (NANO-CNR), M. Gemmi Pd:InAs: S. Heun (NANO-CNR), B. Radha and G. Kulkarni (JNCASR, Bangalore)

12 CBE Facility Riber Compact-21 CBE system Group III : TMIn, TEGa, TMAl Group V : TBAs, TBP, TDMASb, TMSb n-doping: TBSe

13 Hybrid nanodevices InAs NW Josephson Junctions InAs NW - Vanadium SQUID S. Roddaro et al., Nano Res. 4 (2011) 259. F. Giazotto et al., Nature Physics, in press, arxiv:

14 Probing surface potential using RTN In collaboration with Harry Ruda s group from U Toronto, Canada Single trap Two traps Electron occupation of defects on the surface of NWs is a sensitive measure of the local surface potential. J. Salfi et al., ACS Nano 5 (2011) 2191.

15 InAs/InP axial heterostructured NWs

16 High-T single-electron devices Tuning of dot energy spectrum with electric dipole moment InAs/InP NW QD CB up to 50K S. Roddaro et al., Nanoletters 11 (2011) 1695.

17 AlAs nanowires TEM Analysis Ang Li et al., Crystal Growth & Design, submitted. Very fast oxydation

18 AlAs-GaAs CS NWs Ang Li et al., Crystal Growth & Design, submitted.

19 InSb/InAs NWs HR TEM Analysis InSb: <110> zone axis, InAs: <2-1-10> zone axis D. Ercolani el al. Nanotechnology 20, (2009)

20 InAs-InSb NWs as RT n-n diodes Broken band gap alignment (type III) leads to rectifying behaviour. A. Pitanti et al., Phys. Rev. X, submitted.

21 InAs-InP-InSb NWs HRTEM Insertion of an InP layer strongly enhances the rectification. Room T A. Pitanti et al., Phys. Rev. X, submitted.

22 Outline III-V Nanowires at NEST: Growth and Research Pd-assisted Growth of InAs NWs

23 Motivation NWs have a high potential for electronic, optoelectronic and sensor applications Au is the commonly used catalyst: inert and stable Pd is an attractive candidate: good ohmic contacts to semiconductors Pd has a high molecular connectivity useful for biosensing application Pd shows a high hydrogen response Pd nanoparticles: direct-write electron beam resist

24 Experimental details Pd catalysts: few nm-thick spin coated Pd(SC 8 H 17 ) 2 and Pd(SC 16 H 33 ) 2 films and thermolysis at 300 o C for 30 min -> 5-15 nm sized Pd particles Deoxidation at 520 o C with TBA -> Pd particles d avg =35nm InAs NWs are grown by CBE with TMIn ( Torr) and TBAs (1-3Torr) MO precursors T growth : o C Growth time: 2 and 4 hrs

25 InAs substrate orientation (100) NWs density is 14±2 mm 2 (111)A (111)B <011> directions under 55.5±2.3 => <111> S. Heun et al., Crystal Growth & Design 10 (2010) 4198.

26 RHEED patterns Exerimental observed patterns Calculated diffraction patterns Zincblende NWs Wurzite NWs S. Heun et al., Crystal Growth & Design 10 (2010) 4198.

27 HRSEM NWs: smooth and zigzagged sidewalls Triangular base with {112}A side facets Most of the triangles are oriented along [-211] Few oriented [2-1-1] : insertion of rotation stacking fault 2D growth but not close to the NWs (MO capture ) Tapered wires => small diffusion constant S. Heun et al., Crystal Growth & Design 10 (2010) 4198.

28 HAADF TEM Triangular section with {211}-type side facets S. Heun et al., Crystal Growth & Design 10 (2010) 4198.

29 EDX: catalyst particle study Both catalyst particles are crystalline : BCC B2 structure Composition Pd:In 1:1 No Pd in the NWs Pd In S. Heun et al., Small 6 (2010) 1935.

30 Growth on patterned substrates Pd(SC 16 H 33 ) 2 is negative-tone directwrite e-beam resist S. Heun et al., Crystal Growth & Design 10 (2010) 4198.

31 HRTEM of zigzagged NWs [2-1-1] direction FFT [1-10] direction Extended defects: stacking faults S. Heun et al., Small 6 (2010) 1935.

32 Geometrical Phase Analysis X= [01-1] Y=[111] Zigzagged NWs Rotation maps Smooth NWs r o Radial axial strain maps S. Heun et al., Crystal Growth & Design 10 (2010) 4198.

33 HRSEM NWs: smooth and zigzagged sidewalls Triangular base with {112}A side facets Most of the triangles are oriented along [-211] Few oriented [2-1-1] : insertion of rotation stacking fault 2D growth but not close to the NWs (MO capture ) Tapered wires => small diffusion constant S. Heun et al., Crystal Growth & Design 10 (2010) 4198.

34 NW distribution 24nm Bimodal distribution 30nm Growth rates: 3.23nm/min 3.82nm/min 40nm 330C 32nm 57nm dl dt 2 1 w r w for L w S. Heun et al., Small 6 (2010) 1935.

35 NW distribution 24nm Bimodal distribution 30nm Growth rates: 3.23nm/min 3.82nm/min 40nm 330C 32nm 48nm 340C 57nm dl dt 2 1 w r w for L w S. Heun et al., Small 6 (2010) 1935.

36 NW distribution 24nm Bimodal distribution 30nm Growth rates: 3.82 nm/min 3.23 nm/min 40nm 330C 32nm 48nm 340C 57nm dl dt 2 1 w r w for L w S. Heun et al., Small 6 (2010) 1935.

37 NW distribution 24nm Bimodal distribution 30nm Growth rates: 3.82 nm/min 3.23 nm/min 40nm 330C 32nm 48nm 340C 57nm w = 17 ± 3 nm; = 1.2 nm min -1 Model by J. Johansson et al. J. Phys. Chem 109, (2005) S. Heun et al., Small 6 (2010) 1935.

38 HRTEM: catalyst particle study Particle is more faceted and 0.5 aspect ratio In Particle is smooth and 0.6 aspect ratio S. Heun et al., Small 6 (2010) 1935.

39 Zigzagged NWs Periodic sawtooth faceting already observed for Si and GaAs NWs. For not stable facets parallel to the growth direction => Sawtooth faceting (Ross et el. PRL 95, (2005) ) Thermodynamic arguments suggest that inward (outward) force induced by the liquid catalyst particles favour the introduction of the other facet = > zigzagged NWs For III-V: faceting of sidewalls in alternating {111} and {002} planes observed (Zou et al, Small 3 (2007) 389).

40 Zigzagged vs. smooth NWs Model of Ross suggests VLS growth mode for zigzagged NWs. Period and amplitude of sawtooth should scale with NW diameter Here: critical diameter Faceting of catalyst particles is generally associated with the VSS growth mode. Smooth NWs grow in VSS mode.

41 Zigzagged vs. smooth NWs Zigzagged NWs from liquid catalyst particles Solid catalyst particles => smooth sidewalls Two different growth mechanisms: VLS (zigzagged) versus VSS (smooth) Size dependence of the melting T: small particles are liquid while large are solid

42 Zigzagged vs. smooth NWs This model naturally explains the observed bimodality of the tipdiameter distribution Variation temperature of 10K => 20nm of the critical diameter 24nm 32nm 30nm 40nm 48nm 57nm

43 Summary Pd-assisted growth of InAs NWs. Two distinct classes of NWs: smooth and zigzagged. Bimodal NWs distribution: above (below) a critical diameter NWs are smooth (zigzagged) Zigzagged NWs grow from liquid (VLS) particles while smooth NWs grow from solid (VSS) particles. InAs NWs on patterned substrates by employing Pd(SC 16 H 33 ) 2 as a direct-write e-beam resist.