Transcript Molecular Beam Epitaxy of Low Resistance Polycrystalline P-Type GaSb Y. Dong, D. Scott, Y.

Molecular Beam Epitaxy of
Low Resistance Polycrystalline
P-Type GaSb
Y. Dong, D. Scott, Y. Wei, A.C. Gossard and M. Rodwell.
Department of Electrical and Computer Engineering,
University of California, Santa Barbara
[email protected] 1-805-893-3812
University of California
Santa Barbara
15th IPRM 2003 Santa Barbara, CA
Yingda Dong
Outline
 Motivations
 Polycrystalline material for InP HBT’s extrinsic base
 Why choose GaSb
 MBE growth of Poly-GaSb
 Electrical Properties of Poly-GaSb
 Conclusions
University of California
Santa Barbara
Yingda Dong
InP Vs SiGe HBTs
Advantages of InP HBTs over SiGe HBTs
~20:1 lower base sheet resistance,
~ 5:1 higher base electron diffusivity
~ 3:1 higher collector electron velocity,
~ 4:1 higher breakdown-at same ft.
However, InP HBTs have not provided
decisive advantages over SiGe HBTs in
mixed-signal ICs.
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Santa Barbara
Yingda Dong
Strong Features of Si/SiGe HBT Process
 Highly scaled
 Very narrow active junction
areas
 Very low device parasitics
 High speed
 Low emitter resistance using
wide n+ polysilicon contact
 Low base resistance using large
extrinsic polysilicon contact
 High-yield, planar processing
 High levels of integration
 LSI and VLSI capabilities
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Polycrystalline Base Contact
SiGe HBT process: extensive use of poly-Si for base contact
The Advantages of Polycrystalline Base Contact:
 Reduce the B-C capacitance by allowing metal-to-base contact over the field
oxide
 Reduce the base resistance by highly doping the polycrystalline extrinsic base
Low CBC, RBB
High Maximum Oscillation
Frequency (Fmax), ECL logic
speed…
Can a similar technology be developed for InP HBTs ?
University of California
Santa Barbara
Yingda Dong
Polycrystalline Base Contact in InP HBTs
1) Epitaxial growth
2) Collector pedestal
etch, isolation, SiO2
planarization
N - c ol l e ct o r
N - c oll e ct o r
SiO2
N + s u b c oll e c t o r
N + s u b c ol l e ct o r
S. I . s u b s t r at e
S.I. s u b st r at e
University of California
Santa Barbara
Yingda Dong
Polycrystalline Base Contact in InP HBTs
4) Deposit base metal,
encapsulate with SiN,
pattern base and form
SiN Sidewalls
3) Base Regrowth
Extrinsic base
SiO2
SiN
Base Metal
Intrinsic base
4
N- collector
N + s u b c o ll e c t o r
S.I. s u b str at e
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Santa Barbara
N- collector
SiO2
N + s u b c oll e c t o r
S .I . s u b st r at e
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Polycrystalline Base Contact in InP HBTs
5) Regrow InAlAS/InGaAs emitter
Emitter contact
InAlAs/InGaAs emitter
SiN
Base metal
P++ extrinsic base
Collector contact
N- collector
SiO2
N + s u b c oll e ct or
S.I. s u b st r at e
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Santa Barbara
Yingda Dong
Properties of Polycrystalline Material
 Small crystallites join
together at grain
boundaries
 Inside each crystallite:
single crystal
Polycrystalline InAs
 At grain boundaries: a
large number of traps
 Fermi level pinned
Polycrystalline GaSb
University of California
Santa Barbara
Yingda Dong
Material Choices for Polycrystalline Base
Polycrysalline material choices:
 GaAs
 Wide bandgap  low hole mobility
 Fermi level pinned in mid-bandgap
Ec
Ev
Ef
Grain boundary
 large band-bending barrier
 GaSb
 Narrow bandgap  high hole mobiliy
 Fermi-level pinned on valence band
Ec
Ev
Ef
 InSb
 Narrow bandgap
 low melting point (~520 οC)
 Can not withstand emitter regrowth
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Santa Barbara
Grain boundary
Schematic diagram of suggested energy
band structure near grain boundary in ptype of GaAs and GaSb
Yingda Dong
MBE Growth of Polycrystalline GaSb
1) 3000Å SiO2 deposited on
Semi-insulating GaAs by
PECVD.
2) Poly-GaSb samples were
grown in a Varian Gen II
system.
Poly-GaSb
SiO2
GaAs
3000Å
 Sb source valved and
cracked
 CBr4 delivered through high
vacuum leak vavle
 Growth rate fixed at 0.2
μm/hr
University of California
Santa Barbara
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Influence of V/III Beam Flux Ratio
8.0
 Hole mobility
changes little with
V/III ratio
19
9x10
7.5
Growth Temperature: 440°C
19
-3
7.0
 Hole concentration
increases with
decreasing V/III ratio
6.5
19
6x10
6.0
19
5x10
5.5
19
4x10
5.0
19
3x10
4.5
19
2x10
2
(Reason: Carbon
must displace
antimony to be
effective p-type
dopant)
19
7x10
Hole Mobility (cm /vs)
Hole Concentration (cm )
8x10
0
2
4
6
8
10 12 14 16 18 20 22 24
4.0
BEP(Sb)/BEP(Ga)
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8x10
19
7
6
-3
19
5
7x10
19
4
6x10
19
3
2
 Hole mobility
decreases with
growth temperature
9x10
Hole Mobility (cm /vs)
 Hole concentration
changes little with
growth temperature
Hole Concentration (cm )
Influence of Growth Temperature
5x10
19
4x10
19
Film thickness: 1000Å
BEP(Sb)/BEP(Ga)=5
420
440
460
2
480
500
520
1
Growth Temperature (°C)
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Santa Barbara
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Grain Size’s Temperature Dependence
SEM pictures of poly-GaSb samples
Polycrystalline GaSb
Grown at 520 οC
Polycrystalline GaSb
Grown at 475 οC
Gain size: ~350nm
Grain size: ~100nm
University of California
Santa Barbara
Yingda Dong
Poly-GaSb’s Grain Size and Resistivity
-2
400
4.0x10
-2
350
3.5x10
-2
300
3.0x10
-2
250
2.5x10
-2
200
2.0x10
-2
150
1.5x10
-2
100
1.0x10
-2
5.0x10
-3
420
Film thickness
Grain size (nm)
 Resistivity increases
rapidly when grain size
exceeds the film
thickness
4.5x10

 Grain size increases
steadily with growth
temperature
Resistivity

-cm

50
0
440
460
480
500
520
Growth Temprature (°C)
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Santa Barbara
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Small Grain Vs. Large Grain
Ec
Ev
Ef
Small grain:
Grain boundary
Large grain:
 More grain boundaries for
carriers to cross
 Fewer grain boundaries for
carriers to cross
 Larger total boundary areas
connecting crystallites
 Smaller total boundary areas
connecting crystallites
Small band bending barrier  Total connecting boundary area more important
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Yingda Dong
Grain Size Vs Film Thickness
SiO2
University of California
Santa Barbara
Yingda Dong
Grain Size Vs Film Thickness
SiO2
University of California
Santa Barbara
Yingda Dong
Grain Size Vs Film Thickness
SiO2
University of California
Santa Barbara
Yingda Dong
Grain Size Vs Film Thickness
When the film thickness approaches the grain size, the
total connecting boundary area will be significantly reduced
Rapid resistivity increase
SiO2
University of California
Santa Barbara
Yingda Dong
Thickness Dependence
-2
1.6x10
Growth Temperature: 440ºC
Resisitivity

-cm
Bulk resistivity has
strong dependence on
film thickness
-2
1.4x10
-2
1.2x10
-2
1.0x10
-3
8.0x10
-3
6.0x10
1000
1500
2000
2500
3000
Layer Thickness (Å)
Sheet resistivity
increases very fast with
decreasing thickness
University of California
Santa Barbara
Poly GaSb
Thickness
(Ǻ)
Hole
Concentration
Ns (cm-3)
Mobility
 (cm2/Vs)
Bulk
Resistivity
 (cm))
Sheet
resistivity
S (/•)
3000
8.2e19
10.2
7.5e-3
240
2000
8.0e19
8.6
9.1e-3
450
1500
8.1e19
5.8
1.3e-2
900
1000
7.8e19
5.1
1.6e-2
1550
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Comparison Between Poly-GaSb and Poly-GaAs
With similar carbon doping
level, grain size and film
thickness, the resistivity of
poly-GaSb’s resistivity is
more than one order of
magnitude lower than that
of poly-GaAs.
University of California
Santa Barbara
Poly-GaSb
by MBE
(This work)
Poly-GaAs
by GSMBE
(N.Y. Li et
al, 1998)
Carbon doping
density (cm-3)
8x1019
8x1019
Grain Size (Å)
~700
400~2000
Film Thickness
(Å)
3000
4000
Bulk Resistivity
(-cm)
7.5x10-3
~1x10-1
Yingda Dong
Conclusions
 Poly-GaSb proposed to be used as extrinsic base
material for InP HBTs
 Low resistance poly-GaSb films can be achieved by
MBE growth using CBr4 doping
 The resistivity of poly-GaSb has strong dependence
on film’s thickness and grain size, particularly when
the film thickness is comparable with the grain size.
University of California
Santa Barbara
Yingda Dong
Acknowledgement
This work was supported by the DARPA—TFAST program
University of California
Santa Barbara
Yingda Dong