Semiconductor Nanowires April 2005 Yvonne Gawlina Semiconductor Nanowires JASS 05 Yvonne Gawlina...

Apr

il 20

05 Y

vonn

e G

awlin

a

Semiconductor Nanowires


JASS 05

Yvonne Gawlina

Technische Universität München

Apr

il 20

05 Y

vonn

e G

awlin

a


Overview

Introduction

Synthesis of Nanowires

- Pseudowires

- Free standing nanowires

Properties of Nanowires

Applications

Apr

il 20

05 Y

vonn

e G

awlin

a


IntroductionThe “first nanotechnologists” worked in the middle ages:

stained glass: nanoparticles of gold and silver in glass

Creation of Nanowire a new challenge in modern age!

Apr

il 20

05 Y

vonn

e G

awlin

a


“Pseudowires”Pseudowires: wires which are enclosed in other material

Methods of manufacturing:

- lithography and etching top down

- electrostatically induced wires

- strain induced wires

- growth on patterned surfaces

- cleaved edge overgrowth

Apr

il 20

05 Y

vonn

e G

awlin

a


“Pseudowires”

Litography and etching

Formation of 2d quantum well

Coating with resist

Create pattern Etch, until wire remains

Disadvantages: optical and electrical dead layer because of defects due to etching

Sometimes overgrown again to shield wire!

Apr

il 20

05 Y

vonn

e G

awlin

a


“Pseudowires”Electrostatically induced wires

- Creation of a Schottky contact (metal on semiconductor):

- application of voltage raises/lowers the bands

creation of “wires” for holes/electrons at certain voltages

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

Apr

il 20

05 Y

vonn

e G

awlin

a


“Pseudowires”Strain induced wires

2d quantum wire Carbon as stressor Wires through strain

Disadvantage: very small potential only for low temperatures

Apr

il 20

05 Y

vonn

e G

awlin

a


“Pseudowires”V-groove nanowires

V-shape due to different etching directions

Growth of barrier material

Growth of wire material

Growth of 2nd barrier material to sharpen groove again

wire

Apr

il 20

05 Y

vonn

e G

awlin

a


“Pseudowires”

Cleaved edge overgrowth

Growth of quantum well

rotation Growth of second quantum well

wire

Disadvantage: low temperatures needed

Apr

il 20

05 Y

vonn

e G

awlin

a


Synthesis of Nanowires

Methods of Nanowire synthesis

VLS (Vapour Liquid Solid) method

Modification of VLS

CVD (Chemical Vapour Deposition)

LCG (Laser Ablation catalytic Growth)

Low temperature VLS method

FLS (Fluid Liquid Solid) mechanism

SLS (Solution Liquid Solid) mechanism

OAG (Oxide Assisted Growth)

Apr

il 20

05 Y

vonn

e G

awlin

a


Vapour Liquid Solid method

Alloys have phase diagrams

A B

T

liquidus

solidus

mixed crystal

liquid

liquid and solid

Lever rule:

ls

totsl

gg

gga

al + as = 1

Basics about phase diagrams

gl gtot gs

Apr

il 20

05 Y

vonn

e G

awlin

a


Vapour Liquid Solid methodEutectic:

- 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

Eutectic

T

ABsolidus

liquidus

liquid

Mixed crystal

A + liquid B+ liquid

Apr

il 20

05 Y

vonn

e G

awlin

a



Growth procedure:

reactant vapour reactant vapour

Liquid catalytic nanocluster

Nanowire nucleation

Nanowire growth

- mix of semiconductor and metal - eutectic - melting point of Semiconductor with metal lower - growth of one pure material

metal as catalyst

T

A B

l

Mixed crystal

A + l B+ l

supersaturating

metal +Sc

reactant vapour

metal metal +Sc metal +Sc Sc

reactant vapour

Apr

il 20

05 Y

vonn

e G

awlin

a


Vapour Liquid Solid methodSynthesis 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

Pseudobinary phase diagram

GaAs

T

Au

liquid

Au + GaAs

Au + liquid

GaAs+ liquidE.g. Au - GaAs pseudobinary phase diagram

Apr

il 20

05 Y

vonn

e G

awlin

a



Problem:

in fluid at according temperature critical diameter about d = 0.2 m

CC

RTln

dc

4

- critical diameter, so that the liquid catalyst clusters are stable in equilibrium

Goal:

finding methods to get smaller metal clusters to start NW growth

= surface free energy = molar Volume R = gas constant T = absolute temperature C = concentration of semiconductor component in liquid alloy

Cequilibrium concentration

Apr

il 20

05 Y

vonn

e G

awlin

a


Chemical Vapour DepositionE.g. growth of GaN nanowires in CVD reactor

- Ni catalyst on Si substrate with 0.5 M Ni(NO3)26H2O

drying in oven

- formation of Ni islands on Si substrate

- Ga and GaN powder in inner reactor

- Hydrogen in outer tube to minimise side reactions until 700 °C

- Ammonia gas into inner reactor

start of nanowire growth

- Nitrogen gas during cooling phase

Apr

il 20

05 Y

vonn

e G

awlin

a


Chemical Vapour Deposition

1. Vertical tubular furnance

2. Gas inlet line

3. Ni-coated Si substrate

4. Gas outlet line

5. Outer reactor tube

6. Inner reactor tube

CVD reactor

Apr

il 20

05 Y

vonn

e G

awlin

a


Laser Ablation Catalytic Growth nanometer sized cluster with laser ablation

h

M, SC

M SC

SCSC SC

SCSC

M

SC

Laser ablation

Vapour condenses in cluster

Supersaturation until start of wire growth

Transport from growth zone

Apr

il 20

05 Y

vonn

e G

awlin

a


Laser Ablation Catalytic Growth

Laser

Focus

Target in quartz tube

Tube furnaceCold finger

Gas: in Gas: out

LCG reactor

Apr

il 20

05 Y

vonn

e G

awlin

a


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)

- [111] growth direction

Nanowire diameter depends on nanocluster catalyst diameter:

Nanocluster nm 4.9 +/- 1.0 9.7 +/- 1.5 19.8 +/-2.0 30.3 +/- 3.0

Nanowire nm 6.4 +/- 1.2 12.3 +/- 2.5 20.0 +/- 2.3 31.1 +/- 2.7

Apr

il 20

05 Y

vonn

e G

awlin

a


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)Ga

Apr

il 20

05 Y

vonn

e G

awlin

a


Low temperature VLS method

CC

RTln

dc

4

E.g. T = 400°C and 1% of Si

d = 6nm

usually with Si conc. of about 20-30 % d= 0.2 m

E.g. Ge with Ga forms eutectic at only 30 °C!

= surface free energy = molar Volume R = gas constant T = absolute temperature C = concentration of semiconductor component in liquid alloy

Cequilibrium concentration

Apr

il 20

05 Y

vonn

e G

awlin

a


Fluid Liquid Solid mechanismE.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)

Au

SiO2

Si

Au

SiO2

Si

Si Si

Si

Au

SiO2

Si

Si Si

SiSi

Apr

il 20

05 Y

vonn

e G

awlin

a


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

FLS reactor

Apr

il 20

05 Y

vonn

e G

awlin

a


Solid Liquid Solid mechanismE.g. amorphous Si nanowires with SLS

Si substrate

Ni

Heat Heat

Si Si Si Si

Si nanowires

Heat

Si Si Si Si

Ni coated Si substrate

Heat diffusion of Si into Ni

Supersaturating of Ni

Growth of Si nanowires

Si - Ni alloy

Apr

il 20

05 Y

vonn

e G

awlin

a


Oxide Assisted Growth- Oxides as catalyst instead of metal

- production of Ge nanowires, Si nanowires, carbon nanowires, silicon and SnO2 nanoribbons, Group III - V and II - VI compound semiconductor nanowires

Ge nanowires Silicon nanoribbons

Apr

il 20

05 Y

vonn

e G

awlin

a


Oxide Assisted GrowthProcess 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

- Si takes places of oxide start of nanowire growth and outer layer of SiOx

Apr

il 20

05 Y

vonn

e G

awlin

a


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

- growth surpressed in certain directions

Triangle [110]

Rough circle [110]

Rough rectangle [112]

Pentagon [001]

yield

[001]

[110]

Apr

il 20

05 Y

vonn

e G

awlin

a


OAG, VLS and temperatureFor 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

T = 1100 °C : Coexistence of OAG and VLS

Apr

il 20

05 Y

vonn

e G

awlin

a


Nanowires

Summary of some single crystal nanowires synthesised

Material

GaAs

GaP

GaAs0.6P.0.4

InP

InAS

InAs0.5P0.5

Zns

ZnSe

CdS

CdSe

Si1-xGex

Minimum d in[nm]

3

3-5

4

3-5

3-5

3-5

4-6

3-5

3-5

3-5

3-5

structure

Zinkblende

Zinkblende

Zinkblende

Zinkblende

Zinkblende

Zinkblende

Zinkblende

Zinkblende

Wurtzite

Wurtzite

Diamant

Ratio of components

1.00 : 0.97

1.00 : 0.98

1.00 : 0.58 : 0.41

1.00 : 0.98

1.00 : 1.19

1.00 :0.51 : 0.51

1.00 : 1.08

1.00 : 1.01

1.00 : 1.04

1.00 : 0.99

Si1-xGex

Apr

il 20

05 Y

vonn

e G

awlin

a



PL dependence on direction: parallel “on”

perpendicular “off”

- intensity uniform along wire

- periodic cos2dependence

PL characterisation

Apr

il 20

05 Y

vonn

e G

awlin

a


Properties of NanowiresSize Dependant PL

- Shift to higher energies with decreasing diameter

- Quantum confinement effects below d = 20nm

- T - dependant shift

Apr

il 20

05 Y

vonn

e G

awlin

a



Theory:

particle in an infinite cylinder

)L

z)sin(π

R

r(αNJ)z,Ψ(r he,he,

010he,he,

Wave function:

)( ehhe

2

e

22

01*

2

(x)Ψ(x||xx|ε

e|))Ψ(x

L

π

R

α

2mΔΕ

hx

Energy shift

Size Dependant PL

Apr

il 20

05 Y

vonn

e G

awlin

a


Properties of NW

Polarised excitation and emission

Excitation emission

Solid line: parallel

dashed line: perpendicular

0.070.91ΙΙ

ΙΙρ

ΙΙ

ΙΙ

Polarisation rate:

Most nanowires = 0.96Theory: infinite dielectric cylinder in vacuum and laser is constant

e0

0i E

εε

2εE

with = 12.4 for InP

= 0.96

Apr

il 20

05 Y

vonn

e G

awlin

a


Properties of NW

Polarised Photodetection

photodetector

Conductance vs. power density:

upper branch: light parallel polarised

lower branch: light perpendicular polarised

Conductance vs. polarisation angle

Apr

il 20

05 Y

vonn

e G

awlin

a


Properties of NW

Thermal conductivity

Alteration of phonon transport in nanowires:

- more boundary scattering

- changes in phonon dispersion relation

- quantization of phonon transport

vlc3

1κ v

Cv= specific heat

v = velocity of phonons

l = mean free path

Mean free path for phonons in solids in the nm range

Apr

il 20

05 Y

vonn

e G

awlin

a



Thermal conductivity

Deviation from the Debye T3 law

Si NW thermal conductivity 2 orders of magnitude smaller than in bulk Si

Apr

il 20

05 Y

vonn

e G

awlin

a


Properties of NW

Doping

- possible to dope nanowires, e.g silicon: boron doped p-type

phosphor doped n-type

many new exciting possibilities for application of nanowires

Lightly doped Heavily doped metallic

Apr

il 20

05 Y

vonn

e G

awlin

a


Applications- Nanowire heterostructures

+ axial heterostructures, e.g GaP-GaAs heterojunction

+ radial heterostructures, e.g. Si-Ge

+ Nanowire superlattices

Apr

il 20

05 Y

vonn

e G

awlin

a


Applications- Sensors

+ pH sensors

+ gas sensors (e.g. Ammonium, Water)

- Single mode optical wave guides

Gas in Gas out

Apr

il 20

05 Y

vonn

e G

awlin

a


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

- Lasers (electrically driven)

- nanoFETs

etc.

Apr

il 20

05 Y

vonn

e G

awlin

a


Summary

Synthesis

Pseudowires

Free standing Nanowires

Properties

PL

Thermal

Doping

Applications

Semiconductor Nanowires April 2005 Yvonne Gawlina Semiconductor Nanowires JASS 05 Yvonne Gawlina...

Documents

Transcript of Semiconductor Nanowires April 2005 Yvonne Gawlina Semiconductor Nanowires JASS 05 Yvonne Gawlina...