2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 1

In-situ Ohmic Contacts to p-InGaAs

Ashish Baraskar, Vibhor Jain, Evan Lobisser, Brian Thibeault, Arthur Gossard and Mark Rodwell

ECE and Materials Departments, University of California, Santa Barbara, CA

Mark WisteyElectrical Engineering, University of Notre Dame, IN

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 2

Outline• Motivation

– Low resistance contacts for high speed HBTs

– Approach• Experimental details

– Contact formation

– Fabrication of Transmission Line Model structures

• Results

– Doping characteristics

– Effect of doping on contact resistivity

– Effect of annealing

• Conclusion

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 3

Outline• Motivation

– Low resistance contacts for high speed HBTs

– Approach• Experimental details

– Contact formation

– Fabrication of Transmission Line Model structures

• Results

– Doping characteristics

– Effect of doping on contact resistivity

– Effect of annealing

• Conclusion

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 4

Device Bandwidth Scaling Laws for HBT

To double device bandwidth:

• Cut transit time 2x

• Cut RC delay 2x

Scale contact resistivities by 4:1*

*M.J.W. Rodwell, CSICS 2008

bcWcTbT

eW

HBT: Heterojunction Bipolar TransistorRC

f in 2

1

effcbbb CR

ff

8max

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 5

InP Bipolar Transistor Scaling Roadmap

Emitter256 128 64 32 nm width

8 4 2 1 Ω·µm2 access ρ

Base175 120 60 30 nm contact width

10 5 2.5 1.25 Ω·µm2 contact ρ

Collector

106 75 53 37.5 nm thick

9 18 36 72 mA/µm2 current

4 3.3 2.75 2-2.5 V breakdown

fτ 520 730 1000 1400 GHz

fmax 850 1300 2000 2800 GHz

Contact resistivity serious barrier to THz technology

Less than 2 Ω-µm2 contact resistivity required for simultaneous THz ft and fmax*

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 6

Approach

To achieve low resistance, stable ohmic contacts

• Higher number of active carriers

- Reduced depletion width

- Enhanced tunneling across metal-

semiconductor interface

• Better surface preparation techniques

- For efficient removal of oxides/impurities

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 7

• Scaled device thin base

(For 80 nm device: tbase < 25 nm)

• Non-refractory contacts may diffuse at higher temperatures through

base and short the collector• Pd/Ti/Pd/Au contacts diffuse about 15 nm in InGaAs on annealing

Approach (contd.)

100 nm InGaAs grown in MBE

15 nm Pd/Ti diffusion

Need a refractory metal for thermal stability

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 8

Outline• Motivation

– Low resistance contacts for high speed HBTs and FETs

– Approach• Experimental details

– Contact formation

– Fabrication of Transmission Line Model structures

• Results

– Doping characteristics

– Effect of doping on contact resistivity

– Effect of annealing

• Conclusion

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 9

Semi-insulating InP Substrate

100 nm In0.52Al0.48As: NID buffer

100 nm In0.53Ga0.47As: C (p-type)

Epilayer growth by Solid Source Molecular Beam Epitaxy (SS-MBE)– p-InGaAs/InAlAs

- Semi insulating InP (100) substrate

- Un-doped InAlAs buffer

- CBr4 as carbon dopant source

- Hole concentration determined by Hall measurements

Epilayer Growth

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 10

100 nm In0.52Al0.48As: NID buffer

100 nm In0.53Ga0.47As: C (p-type)

20 nm in-situ Mo

Semi-insulating InP Substrate

In-situ contacts

In-situ molybdenum (Mo) deposition - E-beam chamber connected to MBE chamber- No air exposure after film growth

Why Mo? - Refractory metal (melting point ~ 2620 oC)- Easy to deposit by e-beam technique- Easy to process and integrate in HBT process flow

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 11

TLM (Transmission Line Model) fabrication

• E-beam deposition of Ti, Au and Ni layers

• Samples processed into TLM structures by photolithography and liftoff

• Contact metal was dry etched in SF6/Ar with Ni as etch mask, isolated by wet etch

100 nm In0.52Al0.48As: NID buffer

100 nm In0.53Ga0.47As: C (p-type)

20 nm in-situ Mo20 nm ex-situ Ti

50 nm ex-situ Ni500 nm ex-situ Au

Semi-insulating InP Substrate

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 12

• Resistance measured by Agilent 4155C semiconductor parameter

analyzer

• TLM pad spacing (Lgap) varied from 0.5-26 µm; verified from

scanning electron microscope (SEM)

• TLM Width ~ 25 µm

Resistance Measurement

W

RR ShC

C

22

0

1

2

3

4

5

0 0.5 1 1.5 2 2.5 3 3.5

Gap Spacing (m)

Res

ista

nce

()

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 13

• Extrapolation errors:– 4-point probe resistance

measurements on Agilent 4155C

– Resolution error in SEM

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6

Res

ista

nce

()

Gap Spacing (m)

R

d

Rc

Error Analysis• Processing errors:

Lgap

W

Variable gap along width (W)

1.10 µm 1.04 µm

– Variable gap spacing along width (W)

Overlap Resistance

– Overlap resistance

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 14

Outline• Motivation

– Low resistance contacts for high speed HBTs and FETs

– Approach• Experimental details

– Contact formation

– Fabrication of Transmission Line Model structures

• Results

– Doping characteristics

– Effect of doping on contact resistivity

– Effect of annealing

• Conclusion

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 15

1019

1020

40

50

60

70

80

0 10 20 30 40 50 60

hole concentration

mobility

Mob

ility

(cm

2 -Vs)

CBr4 foreline pressure (mtorr)

Hol

e C

once

ntra

tion

(cm

-3)

Doping Characteristics-IHole concentration Vs CBr4 flux

– Hole concentration saturates at high CBr fluxes– Number of di-carbon defects as CBr flux

Tsub = 460 oC

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 16

2

4

6

8

10

10 20 30 40 50 60

Group V / Group III

Hol

e C

once

ntra

tion

(10

19)

cm-3

As V/III ratio hole concentration

hypothesis: As-deficient surface drives C onto group-V sites

Doping Characteristics-IIHole concentration Vs V/III flux

CBr = 60 mtorr

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 17

4 1019

8 1019

1.2 1020

1.6 1020

2 1020

300 350 400 450

Hol

e C

once

ntra

tion

(cm

-3)

Substrate Temp. (oC)

CBr = 60 mtorr

Doping Characteristics-IIIHole concentration Vs substrate temperature

*Tan et. al. Phys. Rev. B 67 (2003) 035208

Tendency to form di-carbon defects as Tsub *

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 18

4 1019

8 1019

1.2 1020

1.6 1020

2 1020

300 350 400 450

Hol

e C

once

ntra

tion

(cm

-3)

Substrate Temp. (oC)

CBr = 60 mtorr

Doping Characteristics-IIIHole concentration Vs substrate temperature

*Tan et. al. Phys. Rev. B 67 (2003) 035208

1019

1020

0 20 40 60 80 100

Tsub = 460 oC

Tsub = 350 oC

CBr4 foreline pressure (mtorr)

Hol

e C

once

ntra

tion

(cm

-3)

Tendency to form di-carbon defects as Tsub *

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 19

ρc lower than the best reported contacts to

pInGaAs (ρc = 4 Ω-µm2)[1,2]0

5

10

15

20

0 1 2 3 4

Re

sist

an

ce (

)Gap Spacing (m)

1. Griffith et al, Indium Phosphide and Related Materials, 2005.

2. Jain et al, IEEE Device Research Conference, 2010.

W

RR ShC

C

22

• Hole concentration, p = 1.6 x 1020 cm-3

• Mobility, µ = 36 cm2/Vs• Sheet resistance, Rsh = 105 ohm/ (100 nm thick film)

Results: Contact Resistivity - I

Metal Contact ρc (Ω-µm2) ρh (Ω-µm)

In-situ Mo 2.2 ± 0.8 15.4 ± 2.6

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 20

1

10

5 10 15

Con

tact

Res

istiv

ity, c (

-

m2 )

[p]-1/2 (x10-11 cm-3/2)

*

1exp

pCTunneling

Data suggests tunneling

ThermionicEmission c ~ constant*

High active carrier concentration is the key to low resistance contacts

* Physics of Semiconductor Devices, S M Sze

Results: Contact Resistivity - II

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 21

Mo contacts annealed under N2 flow for 60 mins. at 250 oC

Thermal Stability - I

• ρc increases on annealing

• Mo reacts with residual

interfacial carbon?*

*Kiniger et. al., Surf. Interface Anal. 2008, 40, 786–789

Kiniger et. al.*

Molybdenum C substrate

Before annealing After annealing

ρc (Ω-µm2) 2.2 ± 0.8 2.8 ± 0.9

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 22

Mo contacts annealed under N2 flow for 60 mins. at 250 oC

TEM of Mo-pInGaAs interface

- Suggests sharp interface

- Minimal/No intermixing

200 nm InAlAs

InGaAsMo

Ti

Au

Thermal Stability - II

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 23

Summary

• Maximum hole concentration obtained = 1.6 x1020 cm-3 at a substrate temperature of 350 oC

• Low contact resistivity with in-situ metal contacts (lowest ρc=2.2 ± 0.8 Ω-µm2)

Contacts suitable for THz transistors

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 24

Thank You !

Questions?

Acknowledgements

ONR, DARPA-TFAST, DARPA-FLARE

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 25

Extra Slides

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 26

Correction for Metal Resistance in 4-Point Test Structure

WLsheet /metalR Wcontactsheet /)( 2/1

Error term (Rmetal/x) from metal resistance

L

W

I

I

V

V

xRWLW metalsheetcontactsheet ///)( 2/1

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 27

Random and Offset Error in 4155C

0.6642

0.6643

0.6644

0.6645

0.6646

0.6647

0 5 10 15 20 25

Res

ista

nce

(

)

Current (mA)

• Random Error in resistance measurement ~ 0.5 m

• Offset Error < 5 m*

*4155C datasheet

2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 28

Accuracy Limits

• Error Calculations– dR = 50 mΩ (Safe estimate)– dW = 1 µm– dGap = 20 nm

• Error in ρc ~ 40% at 1.1 Ω-µm2