Characterization of Contact Resistivity on InAs/GaSb Interface Y. Dong, D. Scott, A.C.
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Transcript Characterization of Contact Resistivity on InAs/GaSb Interface Y. Dong, D. Scott, A.C.
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
[email protected] 1-805-893-3812
University of California
Santa Barbara
2003 Electronic Materials Conference
Yingda Dong
Motivations
Base resistance (RB) is a key factors limiting HBT’s high
frequency performance.
f
f max
8 RBCBC
E
B
RB
fmax
C
Sub-collector
Substrate
University of California
Santa Barbara
Yingda Dong
Base Resistance
A large contribution to base
resistance:
E
Contact resistance between metal
and p-type base
B
C
Sub-collector
Substrate
Contact resistivity on p-type material
is usually much higher than on ntype material.
Reason: holes have larger effective
mass than electrons.
Ec
Ef
Ev
University of California
Santa Barbara
+
Metal
Tunneling
Yingda Dong
Base contact on n-type material
Is it possible to make the base contact on n-type
material?
Base metal contact on ntype extrinsic base RB
could be reduced
Emitter contact metal
Emitter
Metal to base contact over
field oxide CBC can be
reduced
Base metal
Base metal
N+
P+
N+
P+
P+ base
Large emitter contact area
RE can be reduced
SiO2
Collector
Metal
N- collector
SiO2
Collector
Metal
N+ subcollector
High ft , fmax , ECL logic speed…
University of California
Santa Barbara
S.I. substrate
Yingda Dong
Polycrystalline Base Contact in InP HBTs
1) Epitaxial growth
2) Collector pedestal etch,
SiO2 planarization
P+ base
N- collector
P+ base
SiO2
subcollector
N+ subcollector
N+ subcollector
S.I. substrate
S.I. substrate
University of California
Santa Barbara
SiO2
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
Base metal
N+ extrinsic base
P+ extrinsic base
P+ base
SiO2
subcollector
Base metal
N+
N+
P+
P+
P+ base
SiO2
SiO2
subcollector
N+ subcollector
N+ subcollector
S.I. substrate
S.I. substrate
University of California
Santa Barbara
SiO2
Yingda Dong
Polycrystalline Base Contact in InP HBTs
5) Regrow emitter
n+/p+ interface
Emitter contact metal
Is it rectifying or ohmic?
Emitter
If ohmic, is the interfacial
contact resistivity low
Base metal
Base metal
N+
P+
enough?
N+
P+
P+ base
SiO2
Collector
Metal
N- collector
SiO2
Collector
Metal
N+ subcollector
S.I. substrate
University of California
Santa Barbara
Yingda Dong
P+ GaSb / N+ InAs Heterostructure
We propose to use p+ GaSb capped with n+ InAs as the extrinsic base.
InAs-GaSb heterostructure forms a
EC
broken-gap band lineup
P+ GaSb
Mobile charge carriers tunnel between
the p-type GaSb’s valence band and
EC
the neighboring n-type InAs’s
conduction band ohmic p-n junction
University of California
Santa Barbara
EV
Ef
N+ InAs
EV
Yingda Dong
Early Interests in InAs(n)/GaSb(p) Material System
InAs(n)/GaSb(p) heterostructure has
been studied in 1990s with focuses on:
Negative differential resistance (NDR)
Application in high frequency tunneling
diodes
Current Density
1x105 A/cm2
Applied Bias
University of California
Santa 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 California
Santa Barbara
Yingda Dong
MBE Growth of Test Structures
Samples grown in a Gen II
system
1000Å n+ InAs
Silicon doped
cracked
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
400Å p+ GaAs0.51Sb0.49
S.I. InP
Sb source valved and
CBr4 delivered through high
vacuum leak valve
Carbon doped
Layer structure designed
for InP HBT’s extrinsic base
for processing reasons,
total thickness constrained
University of California
Santa Barbara
Yingda Dong
Measurement of Interfacial Contact Resistivity
1)
Transmission line patterns defined,
Ti/Pt/Au contact metal deposited and lifted-off.
1000Å n+ InAs
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
400Å p+ GaAs0.51Sb0.49
S.I. InP
University of California
Santa Barbara
Yingda Dong
Measurement of Interfacial Contact Resistivity
2) Mesa defined to limit the current flow.
1000Å n+ InAs
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
400Å p+ GaAs0.51Sb0.49
S.I. InP
University of California
Santa Barbara
Yingda Dong
Measurement of Interfacial Contact Resistivity
3) Contact resistivity between metal and n+ InAs layer measured.
1000Å n+ InAs
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
400Å p+ GaAs0.51Sb0.49
S.I. InP
University of California
Santa Barbara
Yingda Dong
Measurement of Interfacial Contact Resistivity
Y Axis intercept = Contact resistance between metal and InAs
4.0
3.5
R=0.09+0.24L
Rsh=24 Ohm/Square
3.0
2
RC=1.0E-8 Ohmcm
R (Ohm)
2.5
2.0
1.5
1000Å n+ InAs
1.0
100Å p+ GaSb
0.5
500Å p+ Grading from GaAs0.51As0.49
0.0
0
400Å p+ GaAs0.51Sb0.49
2
4
6
8
10
12
14
16
Gap Spacing (m)
S.I. InP
University of California
Santa Barbara
Yingda Dong
Measurement of Interfacial Contact Resistivity
4) Top InGaAs layer selectively etched
n+
InAs
n+
InAs
n+
InAs
n+
InAs
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
400Å p+ GaAs0.51Sb0.49
S.I. InP
University of California
Santa Barbara
Yingda Dong
Measurement of Interfacial Contact Resistivity
Y Axis intercept = Contact resistance between metal and InAs
+ contact resistance between InAs and GaSb
70
R=2.3+2.16L
Rsh=216 Ohm/Square
60
R (ohm)
50
40
30
20
n+
InAs
n+
InAs
n+
InAs
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
n+
InAs
10
0
0
5
10
15
20
25
30
Gap Spacing (m)
400Å p+ GaAs0.51Sb0.49
S.I. InP
University of California
Santa Barbara
Yingda Dong
Contact Resistivity’s dependence on p-type GaSb layer’s doping
Silicon doping in n-type InAs layer
2
InAs-GaSb Interface Contact Resistivity ( -cm )
fixed at 1x1017cm-3
Carbon doping in p-type GaSb
varied
n+
InAs
n+
InAs
n+
InAs
n+
InAs
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
400Å p+ GaAs0.51Sb0.49
3.0x10
-6
2.5x10
-6
2.0x10
-6
1.5x10
-6
1.0x10
-6
5.0x10
-7
17
Si doping in InAs layer: 1x10 cm
2x10
19
3x10
19
4x10
19
5x10
19
6x10
19
-3
7x10
19
-3
Carbon Doping Density in GaSb Layer (cm )
S.I. InP
University of California
Santa Barbara
Yingda Dong
Contact Resistivity’s dependence on n-type InAs layer’s doping
Silicon doping in p-type GaSb
varied.
n+
InAs
n+
InAs
n+
InAs
n+
InAs
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
2
fixed at 4x1019cm-3 and 7x1019cm-3.
InAs-GaSb Interface Contact Resistivity ( -cm )
Carbon doping in p-type GaSb layer
2.0x10
-6
1.8x10
-6
1.6x10
-6
1.4x10
-6
1.2x10
-6
1.0x10
-6
8.0x10
-7
6.0x10
-7
4.0x10
-7
2.0x10
-7
19
-3
C doping in GaSb layer: 4x10 cm
19
-3
C doping in GaSb layer: 7x10 cm
10
17
10
18
10
19
20
10
-3
400Å p+ GaAs0.51Sb0.49
Silicon Doping Density in InAs Layer (cm )
S.I. InP
University of California
Santa Barbara
Yingda Dong
Resonant Enhancement of Current Density
InAs/GaSb
For the single InAs/GaSb interface,
reflection occurs due to imperfect
coupling of InAs conduction-band
states and GaSb valence-band
states
EC
EV
EC
EV
InAs/GaSb/AlSb/GaSb
EC
Formation of a quantum well layer
between the InAs/GaSb interface
and an AlSb barrier resonant
enhancement of the current density
EV
EC
EV
University of California
Santa Barbara
Yingda Dong
Experiment Result
InAs/GaSb
Contact resistivity:
6.0x10-7 -cm2
Si:
1x1017 cm-3
EC
C: 7x1019 cm-3
EV
EC
EV
12Å AlSb
InAs/GaSb/AlSb/GaSb
EC
Si:
Contact resistivity: 5.4x10-7 - cm2
1x1017 cm-3
C: 7x1019 cm-3
EV
EC
EV
University of California
Santa 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 California
Santa Barbara
Yingda Dong
Questions Answered
n+/p+ interface
Is it rectifying or ohmic? -- YES
Emitter contact metal
Emitter
Base metal
If ohmic, is the interfacial contact
resistivity low enough? -- YES
Base metal
N+
N+
P+
P+
P+ base
SiO2
Collector
Metal
N- collector
SiO2
Collector
Metal
N+ subcollector
S.I. substrate
University of California
Santa 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 California
Santa Barbara
Yingda Dong
Acknowledgement
This work was supported by the DARPA—TFAST program
University of California
Santa Barbara
Yingda Dong