Chemistry 140a

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Chemistry 140a

• Lecture 5
• Jan, 29 2002

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Fermi-Level Equilibration

• When placing two surfaces in contact, they will
  equilibrate just like the water level in a canal
  lock.
• The EF of the semi-conductor will always lower to
  the EF of the metal or the solution. This can be
  understood by looking at the density of states
  for each material/soln.

Semi-Conductor
Metal/Soln.
Initial EF
Eq. EF
Initial EF
Eq. EF
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Fermi-Level Equilibration

• Charge comes from the easiest thing to ionize,
  the dopant atoms. This leads to a large region of
  () charges within the semi-conductor.
• In the metal all of the charge goes to the
  surface. (Gausss Law)
• The more charge transferred the more band
  bending.

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Depletion Approximation

• All donors are fully ionized to a certain
  distance, W, from the interface.
• WW(ND,Vbi)

ND W
Vbi W
- - - - -
- - - - -

X
W
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Final Picture
E
E
EVac
EVac
?m
?sc
ECB
Vbi
EF
ECB
Vbi
EF
EF
- - -
Eg
EVB
EVB


x
x
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Useful Equations
E(x) Electric Field (V/cm)
?(x) Electric Potential (V)
?(x) Electric Potential Energy (J)
E(x)

• Poissons Eqn

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Electric Potential (V)

• Integrate Poissons Eqn.
• B.C.s
• Result

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Depletion Width

• Rearranging for W
• As expected, W increases w/ Vbi and decreases w/
  ND
• If one accounts for the free carrier
  distributions tail around xW

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Typical Values
Vbimax (V) ND (cm-3) W (?m) Q (C/cm2)
1 1013 11 1010
1 1016 0.36 31011
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Electric Potential Energy

• E(x) -q?(x)
• ?(0) -Vbi
• qVbi (EF,SC-EF,M)
• ?B Vbi Vn
• Barrier height
• Independent of doping
• Vbi and Vn are doping dependent

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Electric Field (V/cm)
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I-V Curve
No Band Bending
I
Low Band Bending
High Band Bending
V
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Review

• N-type P-type

E
E
EVac
EVac
?m
?sc
?sc
Vbi
?m
__
ECB
ECB
-
EF
Vbi
- - -

EF
Eg
Eg
EVB
EVB


x
x
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Solution Contact

• 1017 atoms in 1mL of 1mM solution
• D.O.S. argument holds
• Difference in exchange current across the
  interface

A- A- A- A- A- A-
Li Li Li Li Li Li

Significantly less than typical W 10nm
5-10 Angstroms
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Semiconductor Contacting Phase

• No longer 1-Sided Abrupt Jxn. as the
  semi-conductor doesnt have infinite capacity to
  accept charge
• Assume ND(n-type)NA(p-type), then WnWp

p-type
n-type
e-
Diode directionalized current
h
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Degenerate Doping

• Dope p-type degenerately
• NAgtgtND --gt 1-sided Abrupt Jxn.

P-N Homojunction
B
N-type
B
B
N-type
P-type
Wn
Wp
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Heterojunctions

• 2 different semiconductors grown w/ the same
  cyrstal structure (difficult)
• Ge/GaAs ao5.65 angstroms

Broken
Normal
Staggered
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LASERs

• 3 Pieces --gt 2 Heterjunctions
• p-(Al,Ga)As GaAs n-(Al, Ga) As

e-
h?
h
Traps electrons and holes
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Fermi-Level Pinning

• Ideal Case
• (only works for very ionic semiconductors like
  TiO2 and SnO2)

??
1
EF,M
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Whats Missing?

• Fermi-Level pinning hurts
• Hinders our ability to fine tune Vbi
• Vbi/NiVbi/PtVbi/Au
• Why does this happen?

Solution contact for GaAs sees Fermi-level
pinning, while the barrier height correlates well
with the electro-chemical potential for
solution contact to Si
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Devious Experimenter

• Given a Si sample with a magic type of metal on
  the surface X
• Thus the Fermi-level will alwaysequilibrate to
  the Fermi-level of X
• Thin interface --gt e-s tunnel through it and no
  additional potential drop is observed

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What is X?

• Any source or sink for charge at the interface
• Dangling bonds
• Surface states
• etc.

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Questions

• Questions
• Abrupt 1-sided junction
• (What is it?)
• Sign of Electric P.E. and Electric Potential
• (Are they correct? I put them as they were in the
  notes, but this doesnt seem to agree with the
  algebra to me)