Transcript Schottky Barrier Heights

Integrated Circuit Devices
Professor Ali Javey
Summer 2009
MS Junctions
Reading: Chapter14
Metal-Semiconductor Contacts
Two kinds of metal-semiconductor contacts:
• metal on lightly doped silicon –
rectifying Schottky diodes
• metal on heavily doped silicon –
low-resistance ohmic contacts
 : electron affinity
s    ( EC  EF ) FB
Metal Work Function
The work function of metals depend strongly on the
“environment.”
Workfunction, M is an invariant property of metal. It is the
minimum energy required to free up electrons from metal.
Question: how can you change S ?
Ideal MS Contact
Assumptions:
M and S are in intimate contact, on atomic scale
No oxides or charges at the interface
No intermixing at the interface
Question: how are Vbi and Bn related?
Schottky Barrier Heights: Metal on Si
fBn tends to increase with increasing metal work function
Schottky Barrier Heights: metal silicide on Si
Silicide ErSi1.7 HfSi MoSi2 ZrSi2
f Bn (V) 0.28 0.45 0.55 0.55
0.55 0.49
f Bp (V)
TiSi2 CoSi2 WSi2
0.61 0.65 0.67
0.45 0.45 0.43
NiSi2 Pd2Si
0.67 0.75
0.43 0.35
PtSi
0.87
0.23
Silicide-Si interfaces are more stable than metal-silicon
interfaces. After metal is deposited on Si, an annealing step is
applied to form a silicide-Si contact. The term metal-silicon
contact includes silicide-Si contacts.
• Depletion Width:
Does this depletion width equation look familiar?
Question
How can you measure Bn and Bp?
Using CV Data to Determine fB
Bn
qVbi
Ec
Ef
Cdep  A
s
Wdep
Ev
Bn
q(Vbi + V)
qV
Ec
Ef
Ev
Question:
How should we plot the CV
data to extract fB?
Using CV Data to Determine fB
1/C 2
2(Vbi  V )
1

2
C
qN d  s A2
V
Vbi
Once Vbi is known, B can be determined using:
Nc
qVbi   Bn  ( Ec  E f )   Bn  kT ln
Nd
Carrier Injection at the MS Contacts
For each MS junction, 3 components contribute to
the overall injection current:
– Thermionic emission current
– Tunneling current
– Thermally activated tunneling current
What parameters affect each component?
Is the net current zero? Why?
Question:
How do p+n junctions differ from MS
junctions under a forward bias?
Question
• How does the band diagram look for a MS
junction with a Schottky barrier height of
zero?
Applications of Schottky Diodes
I I
Schottky
Schottky diode
ffBB
PN junction
junction
PN
diode
V
V
• I0 of a Schottky diode is 103 to 108 times larger than a PN
junction diode, depending on fB .
• A Schottky diode is the preferred rectifier in low voltage,
high current applications.
Ohmic MS Contacts
Two ways to achieve ohmic MS contacts:
– Reduce the Schottky barrier height. How???
– Reduce the Schottky barrier width (depletion
width). How????
How would each approach give you an ohmic contact?
Schottky Barriers and Fermi Level Pinning
In actual fabricated metal-Si junctions, Fermi level pinning
prevents us from ever getting zero Schottky barrier height.
Two tricks for reducing Fermi level pinning:
1. thin interfacial oxide/nitride
2. 1D semiconductors
1D nanotube
Diameter ~ 1 nm