Impurity Segregation

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Impurity SegregationImpurity Segregation

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

Float ZoneFloat Zone

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

www.mrsemicon.com/crystalgrowth.htm

Impurity SegregationImpurity Segregation

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

Impurity SegregationImpurity SegregationAtom 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

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

Crystalline DefectsCrystalline Defects Point DefectsPoint Defects

– VacanciesVacancies– ImpuritiesImpurities– Antisite DefectsAntisite Defects

Line DefectsLine Defects– DislocationsDislocations

EdgeEdge LoopLoop

Volume DefectsVolume Defects– VoidsVoids– Screw DislocationsScrew Dislocations

Edge DislocationEdge Dislocation

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

Screw DislocationScrew Dislocation

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

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”

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

Dislocation CountDislocation Count When you purchase a wafer, one of the When you purchase a wafer, one of the

specifications is the EPD, Etch Pit specifications is the EPD, Etch Pit DensityDensity– Dislocations etch more rapidly in acid than Dislocations etch more rapidly in acid than

crystalline materialcrystalline 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 a means that there is a

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

Wafer ManufacturingWafer Manufacturing Boules are polished into cylindersBoules are polished into cylinders Aligned using an x-ray diffraction systemAligned using an x-ray diffraction system 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 dislocations defects that can coalesce into edge dislocations if not removed if not removed

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

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)

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

Schematic ViewSchematic View

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

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

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

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

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

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

Schematic of MOCVD Schematic of MOCVD SystemSystem

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

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

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

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

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.

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

ResistanceResistance

Resistivity and ConductivityResistivity and Conductivity Fundamental material propertiesFundamental material properties

ipnopon nqpnq

ResistivityResistivity

n-type n-type semiconductorsemiconductor

p-type p-type semiconductorsemiconductor

Drift CurrentsDrift Currents

EpnAqILVE

pnAqLVI

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.

Diffusion CurrentsDiffusion Currents

opondiffdiffdiff

opopdiff

onondiff

pDnDqJJAI

dxdpqDpqDJ

IdxdnqDnqDJ

Relationship between Relationship between Diffusivity and MobilityDiffusivity and Mobility

qkTDqkTD

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

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

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

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

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.

22ln14,2334,12

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.

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

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.

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

IVtIVS

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.

Impurity Segregation

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