impurity segregation

43
Impurity Segregation Impurity Segregation 1 ) 1 ( o k o o S L S o f k C C C C k Where Co is the initial concentration of th impurity in the melt

Upload: garrett-potts

Post on 01-Jan-2016

55 views

Category:

Documents


1 download

DESCRIPTION

Impurity Segregation. Where Co is the initial concentration of th impurity in the melt. Float Zone. www.mrsemicon.com/crystalgrowth.htm. www.tms.org/pubs/journals/JOM/9802/Li/. Impurity Segregation. - PowerPoint PPT Presentation

TRANSCRIPT

Page 1: Impurity Segregation

Impurity SegregationImpurity Segregation

1)1(

okooS

L

So

fkCC

C

Ck

Where Co is the initial concentration of th impurity in the melt

Page 2: Impurity Segregation

Float ZoneFloat Zone

www.tms.org/pubs/journals/JOM/9802/Li/

www.mrsemicon.com/crystalgrowth.htm

Page 3: Impurity Segregation

Impurity SegregationImpurity Segregation

L

xk

ooS

o

ekCxC )1(1)(

Where Co is the initial concentration of the impurity in the solid and L is the width of the melted region within RF coil

Page 4: Impurity Segregation

Impurity SegregationImpurity Segregation

Atom Cu Ag Au C Ge Sn As

ko 4 · 10–4 10–6 2.5 · 10–5 7 · 10–2 3.3 · 10–2 1.6 · 10–2 0.3

Atom O B Ga Fe Co Ni Sb

ko 0.5 0.8 8 · 10–3 8 · 10–6 8 · 10–6 4 · 10–4 2.3 · 10–2

Page 5: Impurity Segregation

BridgemanBridgeman

Used for some compound Used for some compound semiconductorssemiconductors– Particularly those that have a high vapor Particularly those that have a high vapor

pressurepressure– Produced “D” shaped boulesProduced “D” shaped boules

Page 6: Impurity Segregation

Crystalline DefectsCrystalline Defects

Point DefectsPoint Defects– VacanciesVacancies– ImpuritiesImpurities– Antisite DefectsAntisite Defects

Line DefectsLine Defects– DislocationsDislocations

EdgeEdge LoopLoop

Volume DefectsVolume Defects– VoidsVoids– Screw DislocationsScrew Dislocations

Page 7: Impurity Segregation

Edge DislocationEdge Dislocation

http://courses.eas.ualberta.ca/eas421/lecturepages/mylonite.html

Page 8: Impurity Segregation

Screw DislocationScrew Dislocation

http://focus.aps.org/story/v20/st3

Page 9: Impurity Segregation

Strain induced DislocationsStrain induced Dislocations

The temperature profile across the The temperature profile across the diameter of a boule is not constant diameter of a boule is not constant as the boule cools as the boule cools – the outer surface of the boule contracts the outer surface of the boule contracts

at a different rate than the internal at a different rate than the internal regionregion

– Thermal expansion differences produces Thermal expansion differences produces edge dislocations within the bouleedge dislocations within the boule Typical pattern is a “W”Typical pattern is a “W”

Page 10: Impurity Segregation

Strain due to ImpuritiesStrain due to Impurities

An impurity induces strain in the An impurity induces strain in the crystal because of differences incrystal because of differences in– ionic radius as compared to the atom it ionic radius as compared to the atom it

replacedreplaced Compressive strain if the ionic radius is largerCompressive strain if the ionic radius is larger Tensile strain if the ionic radius is smallerTensile strain if the ionic radius is smaller

– local distortions because of Coulombic local distortions because of Coulombic interactionsinteractions

Both cause local modifications to EgBoth cause local modifications to Eg

Page 11: Impurity Segregation

Dislocation CountDislocation Count

When you purchase a wafer, one of When you purchase a wafer, one of the specifications is the EPD, Etch Pit the specifications is the EPD, Etch Pit DensityDensity– Dislocations etch more rapidly in acid Dislocations etch more rapidly in acid

than crystalline materialthan crystalline material– Values for EPD can run from essentially Values for EPD can run from essentially

zero (FZ grown under microgravity zero (FZ grown under microgravity conditions) to 10conditions) to 1066 cm cm-2-2 for some materials for some materials that are extremely difficult to grow.that are extremely difficult to grow. Note that EPD of 10Note that EPD of 1066 cm cm-2-2 means that there is means that there is

a dislocation approximately every 10a dislocation approximately every 10ms.ms.

Page 12: Impurity Segregation

Wafer ManufacturingWafer Manufacturing

Boules are polished into cylindersBoules are polished into cylinders Aligned using an x-ray diffraction Aligned using an x-ray diffraction

systemsystem Cut into slices using a diamond edged Cut into slices using a diamond edged

sawsaw– Slices are then polished smooth using a Slices are then polished smooth using a

colloidal gritcolloidal grit Mechanical damage from sawing causes point Mechanical damage from sawing causes point

defects that can coalesce into edge defects that can coalesce into edge dislocations if not removed dislocations if not removed

Page 13: Impurity Segregation

http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/backbone/r6_1_2.html#_dum_1

Page 14: Impurity Segregation

Epitaxial Material GrowthEpitaxial Material Growth

Liquid Phase Epitaxy (LPE)Liquid Phase Epitaxy (LPE) Vapor Phase Epitaxy (VPE)Vapor Phase Epitaxy (VPE) Molecular Beam Epitaxy (MBE)Molecular Beam Epitaxy (MBE) Atomic Layer Deposition (ALD) or Atomic Layer Deposition (ALD) or

Atomic Layer Epitaxy (ALE)Atomic Layer Epitaxy (ALE) Metal Organic Chemical Vapor Metal Organic Chemical Vapor

Deposition (MOCVD) or Organometallic Deposition (MOCVD) or Organometallic Vapor Phase Epitaxy (OMVPE)Vapor Phase Epitaxy (OMVPE)

Page 15: Impurity Segregation

MBEMBE

Wafer is moved into the chamber using a Wafer is moved into the chamber using a magnetically coupled transfer rodmagnetically coupled transfer rod

Evaporation and sublimation of source Evaporation and sublimation of source material under ultralow pressure conditions material under ultralow pressure conditions (10(10-10-10 torr) torr)– Shutters in front of evaporation ovens allow vapor Shutters in front of evaporation ovens allow vapor

to enter chamber, temperature of oven determines to enter chamber, temperature of oven determines vapor pressurevapor pressure

Condensation of material on to a heated waferCondensation of material on to a heated wafer– Heat allows the atoms to move to appropriate sites Heat allows the atoms to move to appropriate sites

to form a crystalto form a crystal

Page 16: Impurity Segregation

Schematic ViewSchematic View

http://web.tiscali.it/decartes/phd_html/III-Vms-mbe.png

Page 17: Impurity Segregation

http://www.mse.engin.umich.edu/research/facilities/132/photo

http://ssel-front.eecs.umich.edu/Projects/proj00630002.jpg

Page 18: Impurity Segregation

AdvantagesAdvantages

Slow growth ratesSlow growth rates In-situ monitoring of growthIn-situ monitoring of growth Extremely easy to prevent Extremely easy to prevent

introduction of impuritiesintroduction of impurities

Page 19: Impurity Segregation

DisadvantagesDisadvantages

Slow growth ratesSlow growth rates Difficult to evaporate/sublimate some Difficult to evaporate/sublimate some

materials and hard to prevent the materials and hard to prevent the evaporation/sublimation of othersevaporation/sublimation of others

Hard to scale up for multiple wafersHard to scale up for multiple wafers ExpensiveExpensive

Page 20: Impurity Segregation

MOCVDMOCVD Growths are performed at room pressure or Growths are performed at room pressure or

low pressure (10 mtorr-100 torr)low pressure (10 mtorr-100 torr) Wafers may rotate or be placed at a slant Wafers may rotate or be placed at a slant

to the direction of gas flowto the direction of gas flow– Inductive heating (RF coil) or conductive heatingInductive heating (RF coil) or conductive heating

Reactants are gases carried by NReactants are gases carried by N22 or H or H22 into into chamberchamber– If original source was a liquid, the carrier gas is If original source was a liquid, the carrier gas is

bubbled through it to pick up vaporbubbled through it to pick up vapor– Flow rates determines ratio of gas at wafer Flow rates determines ratio of gas at wafer

surfacesurface

Page 21: Impurity Segregation

Schematic of MOCVD Schematic of MOCVD SystemSystem

http://nsr.mij.mrs.org/1/24/figure1.gif

Page 22: Impurity Segregation

http://www.semiconductor-today.com/news_items/2008/FEB/VEECOe450.jpg

Page 23: Impurity Segregation

AdvantagesAdvantages

Less expensive to operateLess expensive to operate– Growth rates are fastGrowth rates are fast– Gas sources are inexpensiveGas sources are inexpensive

Easy to scale up to multiple wafersEasy to scale up to multiple wafers

Page 24: Impurity Segregation

DisadvantagesDisadvantages

Gas sources pose a potential health Gas sources pose a potential health and safety hazardand safety hazard– A number are pyrophoric and AsHA number are pyrophoric and AsH33 and and

PHPH33 are highly toxic are highly toxic

Difficult to grow hyperabrupt layersDifficult to grow hyperabrupt layers– Residual gases in chamberResidual gases in chamber

Higher background impurity Higher background impurity concentrations in grown layersconcentrations in grown layers

Page 25: Impurity Segregation

Misfit DislocationsMisfit Dislocations

Occur when the difference between Occur when the difference between the lattice constant of the substrate the lattice constant of the substrate and the epitaxial layers is larger than and the epitaxial layers is larger than the critical thickness. the critical thickness.

Page 26: Impurity Segregation

Carrier Mobility and VelocityCarrier Mobility and Velocity

MobilityMobility - the ease at which a carrier - the ease at which a carrier (electron or hole) moves in a (electron or hole) moves in a semiconductorsemiconductor– Symbol: Symbol: nn for electrons and for electrons and pp for holes for holes

Drift velocityDrift velocity – the speed at which a – the speed at which a carrier moves in a crystal when an carrier moves in a crystal when an electric field is presentelectric field is present– For electrons: vFor electrons: vdd = = n n EE

– For holes: For holes: v vdd = = p p EE

Page 27: Impurity Segregation

H

L

W

Va

Va

Page 28: Impurity Segregation

ResistanceResistance

A

L

WH

LR

Page 29: Impurity Segregation

Resistivity and ConductivityResistivity and Conductivity

Fundamental material propertiesFundamental material properties

1

11

ipnopon nqpnq

Page 30: Impurity Segregation
Page 31: Impurity Segregation

ResistivityResistivity

dn

d

ipdn

opon

Nq

N

nNq

pnq

1

1

1

2

n-type n-type semiconductorsemiconductor

p-type p-type semiconductorsemiconductor

ap

apa

in

opon

Nq

NNn

q

pnq

1

1

1

2

Page 32: Impurity Segregation

Drift CurrentsDrift Currents

EpnAqIL

VE

pnAqL

VI

pnqAL

V

R

VI

opon

a

opona

opon

aa

1

Page 33: Impurity Segregation

DiffusionDiffusion

When there are changes in the When there are changes in the concentration of electrons and/or concentration of electrons and/or holes along a piece of semiconductorholes along a piece of semiconductor– the Coulombic repulsion of the carriers the Coulombic repulsion of the carriers

force the carriers to flow towards the force the carriers to flow towards the region with a lower concentration.region with a lower concentration.

Page 34: Impurity Segregation

Diffusion CurrentsDiffusion Currents

opondiffdiffdiff

opopdiff

diff

onondiff

diff

pDnDqJJA

Idx

dpqDpqDJ

A

Idx

dnqDnqDJ

A

I

pn

p

p

n

n

Page 35: Impurity Segregation

Relationship between Relationship between Diffusivity and MobilityDiffusivity and Mobility

q

kTD

q

kTD

p

p

n

n

Page 36: Impurity Segregation

Mobility vs. Dopant Mobility vs. Dopant Concentration in SiliconConcentration in Silicon

http://www.ioffe.ru/SVA/NSM/Semicond/Si/electric.html#Hall

Page 37: Impurity Segregation

Wafer CharacterizationWafer Characterization

X-ray DiffractionX-ray Diffraction– Crystal Orientation Crystal Orientation

Van der Pauw or Hall MeasurementsVan der Pauw or Hall Measurements– ResistivityResistivity– MobilityMobility

Four Point ProbeFour Point Probe– ResisitivityResisitivity

Hot Point ProbeHot Point Probe– n or p-type materialn or p-type material

Page 38: Impurity Segregation

Van der PauwVan der Pauw

Four equidistant Four equidistant Ohmic contactsOhmic contacts

Contacts are small Contacts are small in areain area

Current is injected Current is injected across the diagonalacross the diagonal

Voltage is measured Voltage is measured across the other across the other diagonaldiagonal Top view of Van der Pauw sample

http://www.eeel.nist.gov/812/meas.htm#geom

Page 39: Impurity Segregation

CalculationCalculation

Resistance is determined with and Resistance is determined with and without a magnetic field applied without a magnetic field applied perpendicular to the sample.perpendicular to the sample.

FRRt

R

B

tH

22ln14,2334,12

24,13

F is a correction factor that F is a correction factor that takes into account the takes into account the geometric shape of the geometric shape of the sample.sample.

Page 40: Impurity Segregation

Hall MeasurementHall Measurement

See See http://www.eeel.nist.gov/812/hall.htmlhttp://www.eeel.nist.gov/812/hall.html for a more complete explanationfor a more complete explanation

http://www.sp.phy.cam.ac.uk/SPWeb/research/QHE.htmlhttp://www.sp.phy.cam.ac.uk/SPWeb/research/QHE.html

Page 41: Impurity Segregation

CalculationCalculation

Measurement of resistance is made while Measurement of resistance is made while a magnetic field is applied perpendicular a magnetic field is applied perpendicular to the surface of the Hall sample.to the surface of the Hall sample.– The force applied causes a build-up of carriers The force applied causes a build-up of carriers

along the sidewall of the samplealong the sidewall of the sample The magnitude of this buildup is also a function of The magnitude of this buildup is also a function of

the mobility of the carriersthe mobility of the carriers

where A is the cross-sectional area.where A is the cross-sectional area.

L

A

R

RR

L

HHH

Page 42: Impurity Segregation

Four Point ProbeFour Point Probe

Probe tips must Probe tips must make an Ohmic make an Ohmic contactcontact– Useful for SiUseful for Si– Not most compound Not most compound

semiconductorssemiconductors

S when t 2ln

S when t 2

I

VtI

VS

Page 43: Impurity Segregation

Hot Point ProbeHot Point Probe

Simple method to determine whether Simple method to determine whether material is n-type or p-typematerial is n-type or p-type– Note that the sign of the Hall voltage, Note that the sign of the Hall voltage,

VVHH, and on , and on R R13,2413,24 in the Van der Pauw in the Van der Pauw measurement also provide information measurement also provide information on doping.on doping.