The document describes the vapor-liquid-solid (VLS) mechanism for the growth of semiconductor nanowires using chemical vapor deposition. In the VLS process, a metal catalyst such as gold is used, which forms a liquid alloy with the semiconductor material when heated. Precursor gases diffuse through this liquid alloy droplet where the semiconductor precipitates out at the liquid-solid interface, allowing for nanowire growth. Key parameters that influence nanowire size and morphology include the size and material of the liquid catalyst droplet as well as growth temperature and precursor gases.

2.  Introdution
 Process
 Requirements
 Various Nanowires
 Control of Size
 Precursors and catalysts

3.  VLS- Vapour Solid Liquid Growth
 VLS is a method for growth of One-Dimensional Structure like Nanowires
from Chemical Vapour Deposition.
 The name VLS mechanism reflects the pathway of Si, which coming from
the vapour phase diffuses through the liquid droplet and ends up as a
solid Si wire.

4. o Take Silicon Wafer
o Deposit a thin Au film(~1-10 nm) which act as catalyst
Via Sputter deposition or thermal evaporation
o Silicon + Gold 385 C Liquid Mixture(Droplet)
o Silicon Species evaporate
o Diffusion (Liquid-Vapour Interface )
o Condense Liquid Droplet
o Supersaturation
o Precipitate (Solid-Liquid Interface)
o Nucleation
o Crystal Growth in unidirection ( Solid-Liquid Interface)

6.  Must form Liquid solution
 Distribution coefficient must be less than unity
 Equilibrium vapour Pressure over liquid droplet
must be small
 Physically active but Chemically inert
 Solid solubility must be low

7.  Solid-Liquid interface must be well defined
crystallographically
 General Supersaturation should kept relatively low
 For Cylindrical shape:
Lateral Growth rate < Longitudinal Growth rate

8. For Si Nanowires:
 Catalyst-Mixture of silicon powder with 5 wt% Fe
 Temp-1200 C
 Nanowire Diameter ~15nm
 Nanowire Length – few 10 to several 100 micrometers
 Amorphose oxide layer of ~2nm over coated the
outside of silicon nanowires
For GaAs Nanowires:
 Catalyst- Gold, Silver & Copper

9. Size of Nanowire ~ Liquid Catalyst Droplet ~ Layer thickness of Catalyst
 For 10nm Au size~ Germanium nanowire 150nm
 For 5nm Au size~ Germanium nanowire 80nm
 Further Reduction ~ No Change
Use of Monosized catalyst colloids:
Size of Nanowire ~ Catalyst nanoclusters
 For 28.2 nm diameters Gold colloids ~ 30.2 nm GaP nanowires
 For 18.5 nm diameters Gold colloids ~ 20.0 nm GaP nanowires
 For 8.4 nm diameters Gold colloids ~ 11.4 nm GaP nanowires
 Further Reduction ~ Small Droplet ~ High Solubility ~ High Supersaturation
Results in Lateral Growth on side surface
Nanowires like conical structure

10. For Silicon Nanowires:
 Gaseous Precursors: SiCl4
For Ge Nanowires:
 Precursors: Ge(s) + GeI4(g)  2GeI2(g)
 In Liquid Catalyst: 2GeI2  Ge(l) + GeI4(g)
ZnO Nanowires:
 Catalyst: Au coated ~2-50nm
 1:1 mixture of ZnO & Graphite powder
 Temp:900-925 C
 Constant flow of argon ~ 5-30min
ZnO + C Zn + CO
 Au Coating of 50 nm ~ 80-120 nm Nanowires diameter~ 10-20 μm length
 Au Coating of 3 nm ~ 40-70 nm Nanowires diameter ~ 5-10 μm length