Characterization of Contact Resistivity on InAs/GaSb Interface

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

[email protected] 1-805-893-3812 2003 Electronic Materials Conference


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


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.


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…


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



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



5) Regrow emitter

S.I. substrate

N+ subcollector

SiO2 N- collector

P+ base

SiO2

P+

N+

Base metal

P+

N+

Base metal

Emitter


CollectorMetal

Collector Metal

n+/p+ interface Is it rectifying or ohmic?

If ohmic, is the interfacial

contact resistivity low

enough?


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


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


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.


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


Measurement of Interfacial Contact Resistivity

S.I. InP

400Å p+ GaAs0.51Sb0.49


100Å p+ GaSb

1000Å n+ InAs

1) Transmission line patterns defined, Ti/Pt/Au contact metal deposited and lifted-off.


S.I. InP

400Å p+ GaAs0.51Sb0.49


100Å p+ GaSb

1000Å n+ InAs

2) Mesa defined to limit the current flow.



S.I. InP

400Å p+ GaAs0.51Sb0.49


100Å p+ GaSb

1000Å n+ InAs

3) Contact resistivity between metal and n+ InAs layer measured.



S.I. InP

400Å p+ GaAs0.51Sb0.49


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



S.I. InP

400Å p+ GaAs0.51Sb0.49


100Å p+ GaSb

n+InAs

n+InAs

n+InAs

n+InAs

4) Top InGaAs layer selectively etched



S.I. InP

400Å p+ GaAs0.51Sb0.49


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



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


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


Contact Resistivity’s dependence on n-type InAs layer’s doping

S.I. InP

400Å p+ GaAs0.51Sb0.49


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)


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


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


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


Questions Answered

S.I. substrate

N+ subcollector

SiO2 N- collector

P+ base

SiO2

P+

N+

Base metal

P+

N+

Base metal

Emitter


CollectorMetal

Collector Metal

n+/p+ interface

Is it rectifying or ohmic? -- YES

If ohmic, is the interfacial contact

resistivity low enough? -- YES


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.


Acknowledgement

This work was supported by the DARPA—TFAST program

Documents

Characterization of Contact Resistivity on InAs/GaSb Interface