Upload
ramakrishnan-ram
View
237
Download
0
Embed Size (px)
Citation preview
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 1/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 2/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 3/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 4/25
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.
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 5/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 6/25
Relationship between Diffusivityand Mobility
q
kT D
q
kT D
p
p
n
n
m
m
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 7/25
Mobility vs. Dopant Concentrationin Silicon
http://www.ioffe.ru/SVA/NSM/Semicond/Si/electric.html#Hall
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 8/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 9/25
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.
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 10/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 11/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 12/25
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.
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 13/25
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)
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 14/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 15/25
Schematic View
http://web.tiscali.it/decartes/phd_html/III-Vms-mbe.png
http://ssel front eecs umich edu/Projects/proj00630002 jpg
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 16/25
http://www.mse.engin.umich.edu/research/facilities/132/photo
http://ssel-front.eecs.umich.edu/Projects/proj00630002.jpg
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 17/25
Advantages
Slow growth rates
In-situ monitoring of growth
Extremely easy to prevent introduction ofimpurities
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 18/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 19/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 20/25
Schematic of MOCVD System
http://nsr.mij.mrs.org/1/24/figure1.gif
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 21/25
http://www.semiconductor-today.com/news_items/2008/FEB/VEECOe450.jpg
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 22/25
Advantages
Less expensive to operate
– Growth rates are fast
– Gas sources are inexpensive
Easy to scale up to multiple wafers
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 23/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 24/25
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
8/10/2019 Characterization Techniques and Epitaxy
http://slidepdf.com/reader/full/characterization-techniques-and-epitaxy 25/25
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).