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Semiconductor Nanowires
Semiconductor Nanowires
JASS 05
Technische Universität München
April 2005 Yvonne Gawlina
Yvonne Gawlina
Semiconductor Nanowires
Overview
Introduction
Synthesis of Nanowires
- Pseudowires
Properties of Nanowires
Applications
April 2005 Yvonne Gawlina
- Free standing nanowires
Semiconductor Nanowires
Introduction
 stained glass: nanoparticles of gold and silver in glass
Creation of Nanowire a new challenge in modern age!
April 2005 Yvonne Gawlina
The “first nanotechnologists” worked in the middle ages:
Semiconductor Nanowires
“Pseudowires”
Pseudowires: wires which are enclosed in other material
Methods of manufacturing:
- lithography and etching  top down
- electrostatically induced wires
- strain induced wires
- cleaved edge overgrowth
April 2005 Yvonne Gawlina
- growth on patterned surfaces
Semiconductor Nanowires
“Pseudowires”
Litography and etching
Coating with
resist
Create pattern
Etch, until
wire remains
Sometimes overgrown again to shield wire!
Disadvantages: optical and electrical dead layer because of defects
due to etching
April 2005 Yvonne Gawlina
Formation of 2d
quantum well
Semiconductor Nanowires
“Pseudowires”
Electrostatically induced wires
- Creation of a Schottky contact (metal on semiconductor):
- application of voltage raises/lowers the bands
split gates: two slightly separated metal strips
 with voltage: potential minimum creates of wire of variable width
disadvantage: potential minima not very deep  only for low temperature
April 2005 Yvonne Gawlina
 creation of “wires” for holes/electrons at certain voltages
Semiconductor Nanowires
“Pseudowires”
Strain induced wires
Carbon as stressor
Wires through strain
Disadvantage: very small potential only for low temperatures
April 2005 Yvonne Gawlina
2d quantum wire
Semiconductor Nanowires
“Pseudowires”
V-groove nanowires
Growth of
barrier
material
Growth of
wire material
Growth of 2nd
barrier material
to sharpen
groove again
wire
April 2005 Yvonne Gawlina
V-shape due
to different
etching
directions
Semiconductor Nanowires
“Pseudowires”
Cleaved edge overgrowth
Growth of
quantum well
rotation
Disadvantage: low temperatures needed
Growth of second
quantum well
April 2005 Yvonne Gawlina
wire
Semiconductor Nanowires
Synthesis of Nanowires
Methods of Nanowire synthesis
VLS (Vapour Liquid Solid) method
Modification of VLS
CVD (Chemical Vapour Deposition)
LCG (Laser Ablation catalytic Growth)
FLS (Fluid Liquid Solid) mechanism
SLS (Solution Liquid Solid) mechanism
OAG (Oxide Assisted Growth)
April 2005 Yvonne Gawlina
Low temperature VLS method
Semiconductor Nanowires
Vapour Liquid Solid method
Basics about phase diagrams
Alloys have phase diagrams
Lever rule:
s
tot
g

g
al  s l
g g
T
liquid
solidus
mixed crystal
gl gtot
A
gs
B
al + a s = 1
April 2005 Yvonne Gawlina
liquidus
liquid and solid
Semiconductor Nanowires
Vapour Liquid Solid method
Eutectic:
- coexistence of 3 phases
- lowest temperature where system is still totally liquid
- minimum of liquidus curve
- solid in solid + liquid phase consists of only one material
liquidus
liquid
Eutectic
A + liquid
B+ liquid
Mixed crystal
A
solidus
B
April 2005 Yvonne Gawlina
T
Semiconductor Nanowires
Vapour Liquid Solid method
- mix of semiconductor and metal
- eutectic
- melting point of Semiconductor with metal lower
- growth of one pure material
T
l
 metal as catalyst
A+l
B+ l
Mixed crystal
B
A
reactant vapour
metal
Liquid
catalytic
nanocluster
reactant vapour
metal +Sc
supersaturating
reactant vapour
metal +Sc
Nanowire
nucleation
reactant vapour
metal +Sc
Nanowire
growth
Sc
April 2005 Yvonne Gawlina
Growth procedure:
Semiconductor Nanowires
Vapour Liquid Solid method
Synthesis of multicomponent semiconductor, like
binary III-V materials (GaAs, GaP,InAs, InP)
ternary III-V materials (GaAs/P, InAs/P)
binary II-VI materials ( ZnS, ZnSe, CdS, CdSe)
binary Si Ge alloys
T
liquid
E.g. Au - GaAs pseudobinary
phase diagram
GaAs+ liquid
Au +
liquid
Au + GaAs
Au
GaAs
April 2005 Yvonne Gawlina
Pseudobinary phase diagram
Semiconductor Nanowires
Vapour Liquid Solid method
- critical diameter, so that the liquid catalyst clusters are stable in equilibrium
Problem:
in fluid at according temperature  critical diameter about d = 0.2 mm
Goal:
finding methods to get smaller metal clusters to start NW growth
April 2005 Yvonne Gawlina
4
dc 
 C 

RTln 
 C 
 = surface free energy
 = molar Volume
R = gas constant
T = absolute temperature
C = concentration of semiconductor component in
liquid alloy
C equilibrium concentration
Semiconductor Nanowires
Chemical Vapour Deposition
E.g. growth of GaN nanowires in CVD reactor
- Ni catalyst on Si substrate with 0.5 M Ni(NO3)26H2O
 drying in oven
- formation of Ni islands on Si substrate
- Ga and GaN powder in inner reactor
- Ammonia gas into inner reactor
start of nanowire growth
- Nitrogen gas during cooling phase
April 2005 Yvonne Gawlina
- Hydrogen in outer tube to minimise side reactions until 700 °C
Semiconductor Nanowires
Chemical Vapour Deposition
CVD reactor
1. Vertical tubular furnance
2. Gas inlet line
3. Ni-coated Si substrate
4. Gas outlet line
6. Inner reactor tube
April 2005 Yvonne Gawlina
5. Outer reactor tube
Semiconductor Nanowires
Laser Ablation Catalytic Growth
nanometer sized cluster with laser ablation
hn
SC
SC
M
SC
SC
SC
M, SC
Laser
ablation
Vapour
condenses in
cluster
SC
SC
Supersaturation
until start of wire
growth
Transport from
growth zone
April 2005 Yvonne Gawlina
M
Semiconductor Nanowires
Laser Ablation Catalytic Growth
LCG reactor
Cold finger
Focus
Tube furnace
Gas: in
Target in
quartz tube
Gas: out
April 2005 Yvonne Gawlina
Laser
Semiconductor Nanowires
Laser Ablation Catalytic Growth
Results with LCG:
with Si:
- uniform Diameter down to 3 nm.
- Amorphous coating, consisting of SiO2
- Nanocluster at the end of the wire, consisting of metal and Si (e.g. FeSi2)
Nanowire diameter depends on nanocluster catalyst diameter:
Nanocluster nm 4.9 +/- 1.0 9.7 +/- 1.5
Nanowire nm
19.8 +/-2.0
30.3 +/- 3.0
6.4 +/- 1.2 12.3 +/- 2.5 20.0 +/- 2.3 31.1 +/- 2.7
April 2005 Yvonne Gawlina
- [111] growth direction
Semiconductor Nanowires
Low temperature VLS method
- metal with low melting point (e.g. Ga ,In, Bi..)
- eutectic with very low semiconductor content
- silane decomposition by atomic hydrogen
e.g. SiHx(g) + xH(g)  Ga-Si(l) + xH2(g)
April 2005 Yvonne Gawlina
Ga
Semiconductor Nanowires
Low temperature VLS method
4
dc 
 C 

RTln 
 C 
 = surface free energy
 = molar Volume
R = gas constant
T = absolute temperature
C = concentration of semiconductor component in
liquid alloy
C equilibrium concentration
 d = 6nm
usually with Si conc. of about 20-30 %  d= 0.2 mm
E.g. Ge with Ga forms eutectic at only 30 °C!
April 2005 Yvonne Gawlina
E.g. T = 400°C and 1% of Si
Semiconductor Nanowires
Fluid Liquid Solid mechanism
E.g growth of Si nanowires
- alkanethiol coated Au nanocrystals (d = 6.7 +/- 2.6 nm) tethered on Si
substrate
- diphenysilane (C12H12Si) decomposes in supercritical cyclohexane (C6H12)
Si
Si
Si
Si
Au
Au
SiO2
Si
Si
Au
SiO2
SiO2
Si
Si
April 2005 Yvonne Gawlina
Si
Si
Semiconductor Nanowires
Fluid Liquid Solid mechanism
manipulation of NW:
- metal seed density and size
- Diphenylsilane rate
- Temperature
- T small: few nanoparticles but nanowires curled
- T high: straight nanowires but more nanoparticles
April 2005 Yvonne Gawlina
FLS reactor
Semiconductor Nanowires
Solid Liquid Solid mechanism
E.g. amorphous Si nanowires with SLS
Si nanowires
Si - Ni alloy
Ni
Si
Si
Heat
Ni coated Si
substrate
Si
Si Si
Si
Si
Si
Heat
Heat diffusion of Supersaturating
of Ni
Si into Ni
Heat
Growth of Si
nanowires
April 2005 Yvonne Gawlina
Si substrate
Semiconductor Nanowires
Oxide Assisted Growth
- Oxides as catalyst instead of metal
Ge nanowires
Silicon nanoribbons
April 2005 Yvonne Gawlina
- production of Ge nanowires, Si nanowires, carbon nanowires, silicon
and SnO2 nanoribbons, Group III - V and II - VI compound
semiconductor nanowires
Semiconductor Nanowires
Oxide Assisted Growth
Process of OAG for Si:
- SiO2 powder added to Si (SiO2 needed throughout process)
- ablation of powder
- silicon sub-oxides form bonds with Si substrate
- “dangling bonds” act as nuclei
April 2005 Yvonne Gawlina
- Si takes places of oxide  start of nanowire growth and outer layer of SiOx
Semiconductor Nanowires
Oxide Assisted Growth
- kinds of silicon oxide cluster:
+ oxygen rich cluster
+ silicon rich cluster
+ silicon monoxide like clusters (Si : O = 1:1)
- highest reactivity in Si rich cluster
yield
[110]
Triangle
[110]
Rough circle
[110]
Rough
rectangle
[112]
Pentagon
[001]
[001]
April 2005 Yvonne Gawlina
- growth surpressed in certain directions
Semiconductor Nanowires
OAG, VLS and temperature
For Si:
T = 1100 °C - 1200 °C : d gets smaller with decreasing T, metal found in
wire  VLS mechanism with [111] as favoured growth direction
T = 850 °C - 1050 °C : no metal in wire  OAG region, diameter not
dependant on T
April 2005 Yvonne Gawlina
T = 1100 °C : Coexistence of OAG and VLS
Semiconductor Nanowires
Nanowires
Minimum d
in[nm]
structure
Ratio of
components
GaAs
3
Zinkblende
1.00 : 0.97
GaP
3-5
Zinkblende
1.00 : 0.98
GaAs0.6P.0.4
4
Zinkblende
1.00 : 0.58 : 0.41
InP
3-5
Zinkblende
1.00 : 0.98
InAS
3-5
Zinkblende
1.00 : 1.19
InAs0.5P0.5
3-5
Zinkblende
1.00 :0.51 : 0.51
Zns
4-6
Zinkblende
1.00 : 1.08
ZnSe
3-5
Zinkblende
1.00 : 1.01
CdS
3-5
Wurtzite
1.00 : 1.04
CdSe
3-5
Wurtzite
1.00 : 0.99
Si1-xGex
3-5
Diamant
Si1-xGex
Material
April 2005 Yvonne Gawlina
Summary of some single crystal nanowires synthesised
Semiconductor Nanowires
Properties of Nanowires
PL characterisation
PL dependence on direction: parallel
 “on”
perpendicular  “off”
- intensity uniform along wire
April 2005 Yvonne Gawlina
- periodic cos2dependence
Semiconductor Nanowires
Properties of Nanowires
- Shift to higher energies with decreasing diameter
- Quantum confinement effects below d = 20nm
- T - dependant shift
April 2005 Yvonne Gawlina
Size Dependant PL
Semiconductor Nanowires
Properties of Nanowires
Size Dependant PL
Theory:
particle in an infinite cylinder
Wave function:
re,h
z e,h
Ψ(re,h , z e,h )  NJ 0 (α01
)sin(π
)
R
L
2
ΔΕ 
2m*
  α01 2  π  2 

  

 R   L  


e2
 Ψ(x e ) ( xh ) |
| Ψ(x h )(xe )
ε| x e  x h |
April 2005 Yvonne Gawlina
Energy shift
Semiconductor Nanowires
Properties of NW
Polarised excitation and emission
Polarisation rate:
ΙΙΙ  Ι
ρ
 0.91 0.07
ΙΙΙ  Ι
Most nanowires r= 0.96
Excitation
emission
2ε 0
Ei 
Ee
ε  ε0
Solid line: parallel
dashed line: perpendicular
 with e= 12.4 for InP
 r= 0.96
April 2005 Yvonne Gawlina
Theory: infinite dielectric cylinder in
vacuum and laser is constant
Semiconductor Nanowires
Properties of NW
Polarised Photodetection
photodetector
Conductance vs. power density:
lower branch: light perpendicular polarised
Conductance vs. polarisation angle
April 2005 Yvonne Gawlina
upper branch: light parallel polarised
Semiconductor Nanowires
Properties of NW
Thermal conductivity
1
κ  c v vl
3
Cv= specific heat
v = velocity of phonons
l = mean free path
Alteration of phonon transport in nanowires:
- more boundary scattering
- changes in phonon dispersion relation
- quantization of phonon transport
April 2005 Yvonne Gawlina
Mean free path for phonons in solids in the nm range
Semiconductor Nanowires
Properties of Nanowires
Thermal conductivity
Deviation from the Debye T3 law
April 2005 Yvonne Gawlina
Si NW thermal conductivity 2 orders of
magnitude smaller than in bulk Si
Semiconductor Nanowires
Properties of NW
Doping
- possible to dope nanowires, e.g silicon: boron doped  p-type
Lightly doped
Heavily doped metallic
 many new exciting possibilities for application of nanowires
April 2005 Yvonne Gawlina
phosphor doped  n-type
Semiconductor Nanowires
Applications
- Nanowire heterostructures
+ axial heterostructures, e.g GaP-GaAs heterojunction
+ radial heterostructures, e.g. Si-Ge
April 2005 Yvonne Gawlina
+ Nanowire superlattices
Semiconductor Nanowires
Applications
- Sensors
+ pH sensors
+ gas sensors (e.g. Ammonium, Water)
- Single mode optical wave guides
Gas out
April 2005 Yvonne Gawlina
Gas in
Semiconductor Nanowires
Applications
- Nanophotonics
+ nanoLEDs (p and n type nanowires in crossed nanowire device
 light from crossing point at forward bias)
- Nanoprobes
+ Tips for Atomic Force Microscopy
- High temperature, high current superconductors
- nanoFETs
etc.
April 2005 Yvonne Gawlina
- Lasers (electrically driven)
Semiconductor Nanowires
Summary
Synthesis
Pseudowires
Free standing Nanowires
Properties
PL
Doping
Applications
April 2005 Yvonne Gawlina
Thermal