NANOWIRES

1
NANOWIRES

• Ongi Englander
• Advisor Prof. Lin
• Feb 11, 2002

2
Outline

• Background
• Thermoelectric materials, properties nanowires
• Quantum confinement
• Magnetic nanowires
• Silicon nanowires
• Nanowire fabrication
• Template assistance
• Electrochemical deposition
• CVD
• Our current goals

3
Why nanowires? Thermoelectric nanowires?

• Thermoelectric compounds
• Bismuth telluride (Bi2Te3) - refrigeration
• Bismuth antimony (BiSb)
• Lead telluride (PbTe)
• Silicon germanium (Si1-xGex) power generation
• These materials are used for smaller cooling
  applications
• Not highly efficient
• Environmentally friendly
• Very reliable

(http//www.peltier-info.com/photos.html)
4
Thermoelectric properties

• Measured by a materials dimensionless figure of
  merit ZT(a2s/l)T
• a - Seebeck coeff,
• s -electrical conductivity,
• l- total thermal conductivity (electronic and
  lattice))
• Best thermoelectric materials ZT 1 (upper
  limit for many years) for simple/bulk materials
• a ? s ?
• s ? l ?

5
Motivation I Increase ZT

• Desire to increase ZT for improved device
  capability and efficiency
• Need to manipulate material properties
  difficult do to on the macro scale
• Significantly easier to do on smaller scales
  where structures quantum confined
• Fabricate, produce and study 0D, 1D and 2D
  confined structures
• Discover and characterize many unique properties
  and behaviors with possible wide range
  applicability

6
Motivation II Quantum confinement

• Trap particles and restrict their motion
• Quantum confinement produces new material
  behavior/phenomena
• Engineer confinement- control for specific
  applications
• Structures

• Quantum dots (0-D) only confined states, and
  no freely moving ones
• Nanowires (1-D) particles travel only along the
  wire
• Quantum wells (2-D) confines particles within a
  thin layer

(Scientific American)
7
Why nanowires?
They represent the smallest dimension for
efficient transport of electrons and excitons,
and thus will be used as interconnects and
critical devices in nanoelectronics and
nano-optoelectronics. (CM Lieber, Harvard)

• General attributes desired properties
• Diameter 10s of nanometers
• Single crystal formation -- common
  crystallographic orientation along the nanowire
  axis
• Minimal defects within wire
• Minimal irregularities within nanowire arrays

8
Magnetic nanowires

• Important for storage device applications
• Cobalt, gold, copper and cobalt-copper nanowire
  arrays have been fabricated
• Electrochemical deposition is prevalent
  fabrication technique
• lt20 nm diameter nanowire arrays have been
  fabricated

Cobalt nanowires on Si substrate
(UMass Amherst, 2000)
9
Silicon nanowires

• Grown with the aid of a nucleating metal (gold,
  zinc, iron) and silane (SiH4) as the silicon
  source
• In many cases Vapor-Liquid-Solid (VLS) is the
  assumed growth mechanism
• Whisker/crystal growth phenomenon
• Catalyst is found at tip of nanowire
• CVD of silane (thermal decomposition of silane)
• Various methods of integrating catalyst
• Electron-beam evaporation of Au islands onto SiO2
  coated Si substrate
• Zn deposited onto an electrochemically etched
  porous Si substrate
• Porous Fe/SiO2 gel used as substrate
• Successfully grow 20 nm 30 nm diameter wire
  arrays

10
Nanowire fabrication

• Challenging!
• Template assistance
• Electrochemical deposition
• Ensures fabrication of electrically continuous
  wires since only takes place on conductive
  surfaces
• Applicable to a wide range of materials
• High pressure injection
• Limited to elements and heterogeneously-melting
  compounds with low melting points
• Does not ensure continuous wires
• Does not work well for diameters lt 30-40 nm
• CVD
• Laser assisted techniques

11
Template assisted nanowire growth

• Create a template for nanowires to grow within
• Based on aluminums unique property of self
  organized pore arrays as a result of anodization
  to form alumina (Al2O3)
• Very high aspect ratios may be achieved
• Pore diameter and pore packing densities are a
  function of acid strength and voltage in
  anodization step
• Pore filling nanowire formation via various
  physical and chemical deposition methods

12
Al2O3 template preparation

• Anodization of aluminum
• Start with uniform layer of 1mm Al
• Al serves as the anode, Pt may serve as the
  cathode, and 0.3M oxalic acid is the electrolytic
  solution
• Low temperature process (2-50C)
• 40V is applied
• Anodization time is a function of sample size and
  distance between anode and cathode
• Key Attributes of the process (per M. Sander)
• Pore ordering increases with template thickness
  pores are more ordered on bottom of template
• Process always results in nearly uniform diameter
  pore, but not always ordered pore arrangement
• Aspect ratios are reduced when process is
  performed when in contact with substrate
  (template is 0.3-3 mm thick)

13
The alumina (Al2O3) template
(T. Sands/ HEMI group http//www.mse.berkeley.edu/
groups/Sands/HEMI/nanoTE.html)
alumina template
Si substrate
100nm
(M. Sander)
14
Electrochemical deposition

• Works well with thermoelectric materials and
  metals
• Process allows to remove/dissolve oxide barrier
  layer so that pores are in contact with substrate
• Filling rates of up to 90 have been achieved

(T. Sands/ HEMI group http//www.mse.berkeley.edu/
groups/Sands/HEMI/nanoTE.html)
15
Silicon nanowire CVD growth techniques

• With Fe/SiO2 gel template (Liu et al, 2001)
• Mixture of 10 sccm SiH4 100 sccm helium, 5000C,
  360 Torr and deposition time of 2h
• Straight wires w/ diameter 20nm and length
  1mm
• With Au-Pd islands (Liu et al, 2001)
• Mixture of 10 sccm SiH4 100 sccm helium, 8000C,
  150 Torr and deposition time of 1h
• Amorphous Si nanowires
• Decreasing catalyst size seems to improve
  nanowire alignment
• Bifurcation is common
• 30-40 nm diameter and length 2mm

16

• Nucleation by TiSi2 islands on Si (Kamins et al,
  2000)
• SiH4/SiH2Cl2 _at_ 6400C 6700C for 395 sec,
• PrSiH4 .038 Torr, Ptotal 20 Torr
• Mostly curved wires, some are branched along
  their length, some are straight 25 nm in
  diameter, 1.3 mm long
• Vapor-Liquid-Solid (VLS) based reaction w/ Au as
  catalyst (Westwater et al, 1997)

• Au/Si molten alloy balls nucleate and grow to a
  specific size - transition to Si nanowire growth
  occurs
• SiH4 diluted to 10 in He flowing at 40 sccm,
  pressure range 0.110 Torr temp range of
  3207000C
• Wire quality is best in lower pressure/higher
  temperature range, w/ wire diameter as thin as
  10nm

17
Template-assisted, Au nucleated Si nanowires

• Gold evaporated (Au nanodots) into thin 200nm
  alumina template on silicon substrate
• Ideally reaction with silane will yield desired
  results
• Need to identify equipment that will support this
  process contamination, temp and press issues
• Additional concerns include Au thickness, Au on
  alumina surface, template intact vs removed

Au dots
Au
100nm
1µm
template (top)
(M. Sander)
18
Future goals

• The capability to fabricate nanowires of specific
  parameters (composition, contacting substrate,
  dimension and location)
• Nano-MEMS integration
• Devices and applications
• Can nanowires serve as a link?