characterization techniques and epitaxy

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Carrier Mobility and Velocity

 Mobility - the ease at which a carrier(electron or hole) moves in asemiconductor

 – Symbol: mn for electrons and mp for holes

 Drift velocity  – the speed at which acarrier moves in a crystal when an electric

field is present – For electrons: vd = mn E

 – For holes: vd = mp E

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Drift Currents

 E  pn Aq I 

 L

V  E 

 pn Aq L

V  I 

 pnq A

 L

V 

 R

V  I 

o pon

a

o pona

o pon

aa

m m 

m m 

m m 

1

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Four Point Probe

Probe tips must makean Ohmic contact

 – Useful for Si

 – Not most compoundsemiconductors

S when t2ln

Swhen t2

 I 

V t 

 I 

V 

S 

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Diffusion

When there are changes in theconcentration of electrons and/or holesalong a piece of semiconductor

 – the Coulombic repulsion of the carriers forcethe carriers to flow towards the region with alower concentration.

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Diffusion Currents

o pondiff  diff  diff  

o po pdiff  

diff  

onondiff  

diff  

 p Dn Dq J  J  A I 

dx

dpqD pqD J 

 A

 I 

dx

dnqDnqD J 

 A

 I 

 pn

 p

 p

n

n

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Relationship between Diffusivityand Mobility

q

kT  D

q

kT  D

 p

 p

n

n

m 

m 

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Mobility vs. Dopant Concentrationin Silicon

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

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 Van der Pauw

Four equidistant Ohmiccontacts

Contacts are small in

area Current is injected

across the diagonal

 Voltage is measuredacross the otherdiagonal

Top view of Van der Pauw sample

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

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Calculation

Resistance is determined with and without amagnetic field applied perpendicular to thesample.

 F  R Rt 

 R

 B

t  H 

22ln

14,2334,12

24,13

  m 

F is a correction factor that takesinto account the geometric shapeof the sample.

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Hall Measurement

See http://www.eeel.nist.gov/812/hall.html for amore complete explanation

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

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Calculation

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

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

mobility of the carriers

where A is the cross-sectional area.

 L A

 R R R

 L

 H  H  H   

  m 

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N vs. P doping

The sign of the Hall voltage, VH, and on

 R 13,24 in the Van der Pauw measurementprovide information on doping.

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Epitaxial Material Growth

Liquid Phase Epitaxy (LPE)

 Vapor Phase Epitaxy (VPE)

Molecular Beam Epitaxy (MBE) Atomic Layer Deposition (ALD) or Atomic

Layer Epitaxy (ALE)

Metal Organic Chemical Vapor Deposition(MOCVD) or Organometallic Vapor PhaseEpitaxy (OMVPE)

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MBE

Wafer is moved into the chamber using amagnetically coupled transfer rod

Evaporation and sublimation of source material

under ultralow pressure conditions (10-10 torr) – Shutters in front of evaporation ovens allow vapor to

enter chamber, temperature of oven determinesvapor pressure

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

form a crystal

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Schematic View

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

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

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

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

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 Advantages

Slow growth rates

In-situ monitoring of growth

Extremely easy to prevent introduction ofimpurities

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Disadvantages

Slow growth rates

Difficult to evaporate/sublimate somematerials and hard to prevent theevaporation/sublimation of others

Hard to scale up for multiple wafers

Expensive

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MOCVD

Growths are performed at room pressure or lowpressure (10 mtorr-100 torr)

Wafers may rotate or be placed at a slant to the

direction of gas flow – Inductive heating (RF coil) or conductive heating

Reactants are gases carried by N2 or H2 intochamber

 – If original source was a liquid, the carrier gas isbubbled through it to pick up vapor

 – Flow rates determines ratio of gas at wafer surface

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Schematic of MOCVD System

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

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

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 Advantages

Less expensive to operate

 – Growth rates are fast

 – Gas sources are inexpensive

Easy to scale up to multiple wafers

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Disadvantages

Gas sources pose a potential health andsafety hazard

 – A number are pyrophoric and AsH3 and PH3 

are highly toxic

Difficult to grow hyperabrupt layers

 – Residual gases in chamber

Higher background impurityconcentrations in grown layers

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Misfit Dislocations

Occur when the difference between thelattice constant of the substrate and theepitaxial layers is larger than the critical

thickness.

http://www.iue.tuwien.ac.at/phd/smirnov/node68.html

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Critical Thickness, tC

whereb is the magnitude of the lattice distortion caused by adislocation (Burger vector)

f is the mismatch between the lattice constants of film

and the substraten is Poisson’s ratio (transverse strain divided by the axial

strain).