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Characterization of Contact Resistivity on InAs/GaSb Interface. Y. Dong , D. Scott, A.C. Gossard and M.J. Rodwell. Department of Electrical and Computer Engineering, University of California, Santa Barbara. - PowerPoint PPT Presentation
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University of CaliforniaSanta Barbara Yingda Dong
Characterization of Contact Resistivity on InAs/GaSb Interface
Y. Dong, D. Scott, A.C. Gossard and M.J. Rodwell.
Department of Electrical and Computer Engineering,
University of California, Santa Barbara
yingda@ece.ucsb.edu 1-805-893-3812 2003 Electronic Materials Conference
University of CaliforniaSanta Barbara Yingda Dong
Motivations
Base resistance (RB) is a key factors limiting HBT’s high frequency performance.
8maxB BC
ff
R C
Sub-collector
Substrate
E
C
B
RB fmax
University of CaliforniaSanta Barbara Yingda Dong
Base Resistance
Sub-collector
Substrate
E
C
B
A large contribution to base resistance:
Contact resistance between metal and p-type base
Metal
Ec
Ev
Ef
+
Tunneling
Contact resistivity on p-type material is usually much higher than on n-type material.
Reason: holes have larger effective mass than electrons.
University of CaliforniaSanta Barbara Yingda Dong
Base contact on n-type material
Is it possible to make the base contact on n-type material?
S.I. substrate
N+ subcollector
SiO2 N- collector
P+ base
SiO2
P+N+
Base metal
P+N+
Base metal
Emitter
Emitter contact metal
CollectorMetal
Collector Metal
Base metal contact on n-type extrinsic base RB could be reduced
Metal to base contact over field oxide CBC can be reduced
Large emitter contact area RE can be reduced
High ft , fmax , ECL logic speed…
University of CaliforniaSanta Barbara Yingda Dong
S.I. substrate
Polycrystalline Base Contact in InP HBTs
1) Epitaxial growth 2) Collector pedestal etch, SiO2 planarization
N+ subcollector
N- collector
P+ base
S.I. substrate
N+ subcollector
SiO2 subcollector
P+ base
SiO2
University of CaliforniaSanta Barbara Yingda Dong
Polycrystalline Base Contact in InP HBTs
3) Extrinsic-base regrowth 4) Deposit base metal, encapsulate with SiN, pattern base and form SiN sidewalls
S.I. substrate
N+ subcollector
SiO2 subcollector
P+ base
SiO2
P+ extrinsic base
N+ extrinsic base
S.I. substrate
N+ subcollector
SiO2 subcollector
P+ base
P+
N+
Base metal
SiO2
P+
N+
Base metal
University of CaliforniaSanta Barbara Yingda Dong
Polycrystalline Base Contact in InP HBTs
5) Regrow emitter
S.I. substrate
N+ subcollector
SiO2 N- collector
P+ base
SiO2
P+
N+
Base metal
P+
N+
Base metal
Emitter
Emitter contact metal
CollectorMetal
Collector Metal
n+/p+ interface Is it rectifying or ohmic?
If ohmic, is the interfacial
contact resistivity low
enough?
University of CaliforniaSanta Barbara Yingda Dong
P+ GaSb / N+ InAs Heterostructure
We propose to use p+ GaSb capped with n+ InAs as the extrinsic base.
EC
EV
P+ GaSb
N+ InAs EC
EV
Ef
InAs-GaSb heterostructure forms a
broken-gap band lineup
Mobile charge carriers tunnel between
the p-type GaSb’s valence band and
the neighboring n-type InAs’s
conduction band ohmic p-n junction
University of CaliforniaSanta Barbara Yingda Dong
Early Interests in InAs(n)/GaSb(p) Material System
InAs(n)/GaSb(p) heterostructure has been studied in 1990s with focuses on:
Applied Bias
Cur
rent
Den
sity Negative differential resistance (NDR)
Application in high frequency tunneling
diodes
1x105 A/cm2
University of CaliforniaSanta Barbara Yingda Dong
Focus of This Work
The contact resistivity across the InAs(n)/GaSb(p) interface at
relatively low current density (<104 A/cm2).
(No NDR at low current density)
The dependence of contact resistivity on the doping
concentration in InAs and GaSb layers.
University of CaliforniaSanta Barbara Yingda Dong
MBE Growth of Test Structures
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
1000Å n+ InAs
Carbon doped
Silicon doped
Samples grown in a Gen II
system
Sb source valved and
cracked
CBr4 delivered through high
vacuum leak valve
Layer structure designed
for InP HBT’s extrinsic
base for processing
reasons, total thickness
constrained
University of CaliforniaSanta Barbara Yingda Dong
Measurement of Interfacial Contact Resistivity
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
1000Å n+ InAs
1) Transmission line patterns defined, Ti/Pt/Au contact metal deposited and lifted-off.
University of CaliforniaSanta Barbara Yingda Dong
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
1000Å n+ InAs
2) Mesa defined to limit the current flow.
Measurement of Interfacial Contact Resistivity
University of CaliforniaSanta Barbara Yingda Dong
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
1000Å n+ InAs
3) Contact resistivity between metal and n+ InAs layer measured.
Measurement of Interfacial Contact Resistivity
University of CaliforniaSanta Barbara Yingda Dong
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
1000Å n+ InAs
0 2 4 6 8 10 12 14 160.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
R=0.09+0.24LR
sh=24 Ohm/Square
RC=1.0E-8 Ohmcm2
R (
Ohm
)
Gap Spacing (m)
Y Axis intercept = Contact resistance between metal and InAs
Measurement of Interfacial Contact Resistivity
University of CaliforniaSanta Barbara Yingda Dong
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
n+InAs
n+InAs
n+InAs
n+InAs
4) Top InGaAs layer selectively etched
Measurement of Interfacial Contact Resistivity
University of CaliforniaSanta Barbara Yingda Dong
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
n+InAs
n+InAs
n+InAs
n+InAs
0 5 10 15 20 25 300
10
20
30
40
50
60
70
R=2.3+2.16LR
sh=216 Ohm/Square
R (
ohm
)Gap Spacing (m)
Y Axis intercept = Contact resistance between metal and InAs + contact resistance between InAs and GaSb
Measurement of Interfacial Contact Resistivity
University of CaliforniaSanta Barbara Yingda Dong
Contact Resistivity’s dependence on p-type GaSb layer’s doping
2x1019 3x1019 4x1019 5x1019 6x1019 7x1019
5.0x10-7
1.0x10-6
1.5x10-6
2.0x10-6
2.5x10-6
3.0x10-6
Si doping in InAs layer: 1x1017cm-3
InA
s-G
aSb
Inte
rfac
e C
onta
ct R
esis
tivity
(-
cm2 )
Carbon Doping Density in GaSb Layer (cm-3)
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
n+InAs
n+InAs
n+InAs
n+InAs
Silicon doping in n-type InAs layer
fixed at 1x1017cm-3
Carbon doping in p-type GaSb
varied
University of CaliforniaSanta Barbara Yingda Dong
Contact Resistivity’s dependence on n-type InAs layer’s doping
S.I. InP
400Å p+ GaAs0.51Sb0.49
500Å p+ Grading from GaAs0.51As0.49
100Å p+ GaSb
n+InAs
n+InAs
n+InAs
n+InAs
Carbon doping in p-type GaSb layer
fixed at 4x1019cm-3 and 7x1019cm-3.
Silicon doping in p-type GaSb
varied.
1017 1018 1019 10202.0x10-7
4.0x10-7
6.0x10-7
8.0x10-7
1.0x10-6
1.2x10-6
1.4x10-6
1.6x10-6
1.8x10-6
2.0x10-6
C doping in GaSb layer: 4x1019cm-3
C doping in GaSb layer: 7x1019cm-3
InA
s-G
aSb
Inte
rfac
e C
onta
ct R
esis
tivity
(-
cm2 )
Silicon Doping Density in InAs Layer (cm-3)
University of CaliforniaSanta Barbara Yingda Dong
Resonant Enhancement of Current Density
EC
EV
EC
EV
InAs/GaSb
EC
EV
EC
EV
InAs/GaSb/AlSb/GaSb
Formation of a quantum well layer
between the InAs/GaSb interface and
an AlSb barrier resonant
enhancement of the current density
For the single InAs/GaSb interface,
reflection occurs due to imperfect
coupling of InAs conduction-band
states and GaSb valence-band
states
University of CaliforniaSanta Barbara Yingda Dong
Experiment Result
EC
EV
EC
EV
InAs/GaSb
EC
EV
EC
EV
InAs/GaSb/AlSb/GaSb
Si: 1x1017 cm-3C: 7x1019 cm-3
Si: 1x1017 cm-3C: 7x1019 cm-3
12Å AlSb
Contact resistivity: 6.0x10-7 -cm2
Contact resistivity: 5.4x10-7 - cm2
University of CaliforniaSanta Barbara Yingda Dong
Comparison with metal on p+ InGaAs
Doping Density of p-GaSb (cm-3)
Doping Density of n-InAs (cm-3)
Contact Resistivity (Ω-cm2)
2x1019 1x1017 2.8x10-6
2x1019 6x1017 3.0x10-6
4x1019 1x1017 1.3x10-6
4x1019 1x1019 1.6x10-6
4x1019 5x1019 9.0x10-7
7x1019 1x1017 6.0x10-7
7x1019 1x1019 8.2x10-7
7x1019 5x1019 4.2x10-7
Lowest interfacial contact
resistivity obtained: ~ 4x10-7 -cm2
Contact resistivity of metal on p+
InGaAs: ~1x10-6 -cm2
University of CaliforniaSanta Barbara Yingda Dong
Questions Answered
S.I. substrate
N+ subcollector
SiO2 N- collector
P+ base
SiO2
P+
N+
Base metal
P+
N+
Base metal
Emitter
Emitter contact metal
CollectorMetal
Collector Metal
n+/p+ interface
Is it rectifying or ohmic? -- YES
If ohmic, is the interfacial contact
resistivity low enough? -- YES
University of CaliforniaSanta Barbara Yingda Dong
Conclusions
Propose to use InAs(n)/GaSb(P) as extrinsic
base of InP HBT
Investigate the contact resistivity between
InAs(n)/GaSb(p) interface and its dependence
on doping densities on both sides of the
heterojunction.
Compare the InAs(n)/GaSb(p) interfacial contact
resistivity with that of metal on p+ InGaAs.
University of CaliforniaSanta Barbara Yingda Dong
Acknowledgement
This work was supported by the DARPA—TFAST program
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