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
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 max
f

8 RB CBC
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
RC=1.0E-8 Ohmcm
2
R (Ohm)
2.5
2.0
1.5
1000Å n+ InAs
1.0
100Å p+ GaSb
500Å p+ Grading from GaAs0.51As0.49
0.5
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
10
20
-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