Lecture 24 - nanowiresweb.eecs.umich.edu/~peicheng/teaching/EECS598_06... · Semiconductor Nanowires EECS 598-002 Winter 2006 Nanophotonics and Nano-scale Fabrication P.C.Ku. ...

Lecture 24 –Semiconductor Nanowires

EECS 598-002 Winter 2006Nanophotonics and Nano-scale Fabrication

P.C.Ku

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Size dependence (quantum confinement)

g(E) = Density of states

Eg E

g(E)

Eg E

g(E)

Eg E

g(E)

Eg E

g(E)

bulk sheet wire dot

3D 2D 1D 0D

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Why nanowires?

Usually easier to fabricate than QD’sMore variety of materials available.Large surface area may be useful for some applications.Can also tolerate large lattice mismatch along the interface.More easily to be made uniformNew physics: exciton diffusion, …

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Exciton diffusion

Science 273 (1996) 1351.

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VLS growth

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VLS growth (cont.)

Vapor supply combining with the catalyst (e.g. Au) to form an eutectic melt (at a much lower temperature compared to the melting point of the source materials.)

Supersaturation within the melting droplet leads to crystallization.

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Phase diagram for Si nanowires growth

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Nanowire heterostructure

J. Appl. Phys. 97 (2005) 114325.

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Atomically abrupt interface

Gas sourceMBE

Nano Lett 2 (2002)87.

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Si/SiGe

Wu et al., Nano Lett. 2 (2002) 83.

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Multishell structure

Nature 420 (2002) 57.

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Hybrid assembly

Langmuit-Blodgett method

Nano Lett3 (2003) 1255.

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Hybrid assembly

Flow assembly

Science 291 (2001) 630.

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Growth on patterned substrates

Adv. Mat. 17 (2005) 2098.

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Appl. Phys. A 80 (2005) 1133.

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Nanotubes

Other materials can also form tubular structures. Examples are BC2N and BC3 tubes, …

Carbon nanotubes Boron nitride nanotubes

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Rolling

Zigzag face Armchair face

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Cap structure

Nanotube descends from the fullerene (C60, C70, C78, …) family.

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SWNT vs. MWNT

MWNT’s have more defectsMWNT’s are easier and less expensive to produce

SWNT facility (Carbon Nanotechnology Inc)

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Fabrication

Requires plasma state and metal catalystTo create the plasma state:

Arc discharge of a graphite rodLaser ablation of a graphite rodChemical vapor deposition

Metal catalyst: Cobalt, iron, or nickel keep the tube open and ensures the formation of hexagonal structure

To get SWNT, high pressure CO flow has been developed.Major difficulties:

Impurity can be removed from acid treatment can add other impuritiesDifferent tubes produced at the same time electrical heating can remove metallic MWNT; however, no way for SWNT.

30V, 100A

He

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Electronic properties – Carbon tubes

SWNT: Either metal or semiconductor:Armchair : metaln-m = 3k (k ≠ 0) : semiconductor with tiny Eg

all others : semiconductor with Eg inversely proportional to tube diameter

MWNT: similar to SWNT due to weak interaction between walls

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C vs BN nanotubes

BN tubes are electrically uniformAll BN nanotubes are semiconductors and have same Egregardless of tube diameter, chirality, and number of walls.

Bulk BN is insulator cannot be used as an electrode

Bulk BN

Tungstun electrodeanode

Cu as cathode

He 650 torr

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Review article

Law et al., Ann. Rep. Mat. Res. 34 (2004) 83.