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Three-Dimensional Nanoscale Composition Mapping of Semiconductor NanowiresD.E. Perea, J.E. Allen, S.J. May, B.W. Wessels, D.N. Seidman, L.J. Lauhon

Nano Letters 6(2). Feb 2006, p181-185.

Joanne YimEE C235/NSE C203

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Nanowire composition

• Compositional flucuations especially critical at nanoscale– Dopants, catalyst, at interface, etc…

• Composition mapping tools:– SIMS = secondary ion mass spectroscopy: resolution

limited to >100nm– TEM = transmission electron microscopy (Energy

Dispersive X-ray Spectroscopy and Electron Energy Loss Spectroscopy): not for volumetric, low concentration.

• Averaged over electron beam spot size – Cross sectional Scanning Tunneling Microscopy (XSTM)

• Requires making a cross section• 2D

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LEAP Microscope• Local Electrode Atom Probe microscope• Sub-nanometer

resolution for atom tomography (imaging by sectioning)

• Create 3D mapping of composition

• First data published from commercial instrument in 2002

http://www.imago.com/imago/

3 easy payments of$999,999.95

(prices and participation may vary)

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• microtip sample: ~100nm radius of curvature in UHV chamber

• + bias sample, makes positive ions on sample tip– negatively pulsed local

electrode tip to induce positive ions to leave

• Position sensitive detector collects ions– Lighter mass -> travel faster– Mass-to-charge -> element– Relative composition using

counts– Straight line travel -> relative

positions

http://www.imago.com/imago/html/technology/technology.jsp

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Experimental

• Au seed-catalyzed InAs nanowire growth on GaAs(111)B by MOVPE– Epitaxial, vertical array of nanowires 140um

long, spaced 500um apart as defined by initial Au e-beam/liftoff

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(curvature is software artifact)

Cross section shows lines corresponding to growth planes (0001) ┴ growth direction

spacing of 0.35nm

Au atoms(98% In, As removed for picture)

SEM

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Catalyst-nanowire interface

10 nm dia X 1 nm thick slices

23x14x14 nm3

www.nsf.gov/mps/dmr/highlights/05highlights/imr/0449933.ppt

Interface width <0.5nm

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Summary

• 3D composition mapping can be achieved

• LEAP microscope sample geometry ideal for nanowire morphology

• Found Au catalyst incorporated along length of nanowire