Characterization of Contact Resistivity on InAsGaSb Interface

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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
yingda_at_ece.ucsb.edu 1-805-893-3812
2003 Electronic Materials
Conference
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Motivations
Base resistance (RB) is a key factors limiting
HBTs high frequency performance.
fmax ?
RB ?
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Base Resistance
A large contribution to base resistance
Contact resistance between metal and p-type
base
E
B
C
Sub-collector
Substrate
Contact resistivity on p-type material is usually
much higher than on n-type material. Reason
holes have larger effective mass than electrons.
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Base contact on n-type material
Is it possible to make the base contact on n-type
material?

• 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

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Polycrystalline Base Contact in InP HBTs
1) Epitaxial growth
2) Collector pedestal etch, SiO2 planarization
P base
SiO2
P base
SiO2
N- collector
subcollector
N subcollector
N subcollector
S.I. substrate
S.I. substrate
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Polycrystalline Base Contact in InP HBTs
4) Deposit base metal, encapsulate with SiN,
pattern base and form SiN sidewalls
3) Extrinsic-base regrowth
Base metal
Base metal
N extrinsic base
N
N
P extrinsic base
P
P
SiO2
P base
SiO2
SiO2
P base
SiO2
subcollector
subcollector
N subcollector
N subcollector
S.I. substrate
S.I. substrate
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Polycrystalline Base Contact in InP HBTs
5) Regrow emitter

• n/p interface
• Is it rectifying or ohmic?
• If ohmic, is the interfacial contact resistivity
  low enough?

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P GaSb / N InAs Heterostructure
We propose to use p GaSb capped with n InAs as
the extrinsic base.

• InAs-GaSb heterostructure forms a broken-gap band
  lineup
• Mobile charge carriers tunnel between the p-type
  GaSbs valence band and the neighboring n-type
  InAss conduction band ? ohmic p-n junction

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Early Interests in InAs(n)/GaSb(p) Material System
InAs(n)/GaSb(p) heterostructure has been studied
in 1990s with focuses on
1x105 A/cm2

• Negative differential resistance (NDR)
• Application in high frequency tunneling diodes

Current Density
Applied Bias
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Focus of This Work

• The contact resistivity across the
  InAs(n)/GaSb(p) interface at relatively low
  current density (lt104 A/cm2).
• (No NDR at low current density)
• The dependence of contact resistivity on the
  doping concentration in InAs and GaSb layers.

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MBE Growth of Test Structures

• Samples grown in a Gen II system
• Sb source valved and cracked
• CBr4 delivered through high vacuum leak valve
• Layer structure designed for InP HBTs extrinsic
  base ? for processing reasons, total thickness
  constrained

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Measurement of Interfacial Contact Resistivity
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Measurement of Interfacial Contact Resistivity
2) Mesa defined to limit the current flow.
S.I. InP
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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
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Measurement of Interfacial Contact Resistivity
Y Axis intercept Contact resistance between
metal and InAs
1000Å n InAs
100Å p GaSb
500Å p Grading from GaAs0.51As0.49
400Å p GaAs0.51Sb0.49
S.I. InP
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Measurement of Interfacial Contact Resistivity
4) Top InGaAs layer selectively etched
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Measurement of Interfacial Contact Resistivity
Y Axis intercept Contact resistance between
metal and InAs
contact resistance between InAs and GaSb
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Contact Resistivitys dependence on p-type GaSb
layers doping

• Silicon doping in n-type InAs layer fixed at
  1x1017cm-3
• Carbon doping in p-type GaSb varied

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Contact Resistivitys dependence on n-type InAs
layers doping

• Carbon doping in p-type GaSb layer fixed at
  4x1019cm-3 and 7x1019cm-3.
• Silicon doping in p-type GaSb varied.

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Resonant Enhancement of Current Density
InAs/GaSb
EC
For the single InAs/GaSb interface, reflection
occurs due to imperfect coupling of InAs
conduction-band states and GaSb valence-band
states
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
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Experiment Result
InAs/GaSb
EC
C 7x1019 cm-3
Contact resistivity 6.0x10-7 ?-cm2
Si 1x1017 cm-3
EV
EC
EV
12Å AlSb
InAs/GaSb/AlSb/GaSb
EC
C 7x1019 cm-3
Si 1x1017 cm-3
Contact resistivity 5.4x10-7 ?- cm2
EV
EC
EV
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Comparison with metal on p InGaAs
Lowest interfacial contact resistivity obtained
4x10-7 ?-cm2 Contact resistivity of metal
on p InGaAs 1x10-6 ?-cm2
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Questions Answered

• n/p interface
• Is it rectifying or ohmic? -- YES
• If ohmic, is the interfacial contact resistivity
  low enough? -- YES

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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.

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Acknowledgement
This work was supported by the DARPATFAST program