in-situ ohmic contacts to p-ingaas - ucsb...300 350 400 450 sb t (substrate temp. (oc) tendency to...

In-situ Ohmic Contacts to p-InGaAs

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

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

Mark WisteyfElectrical Engineering, University of Notre Dame, IN

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

Outline• MotivationMotivation

– Low resistance contacts for high speed HBTs– Approachpp

• Experimental details– Contact formation– Fabrication of Transmission Line Model structures

• Results– Doping characteristics– Effect of doping on contact resistivity– Effect of annealing

• Conclusion



– Low resistance contacts for high speed HBTs– Approachpp



• Conclusion


Device Bandwidth Scaling Laws for HBT

To double device bandwidth:

• Cut transit time 2xeW

• Cut RC delay 2x

Scale contact resistivities by 4:1*

bcWcTbT

Scale contact resistivities by 4:1

HBT: Heterojunction Bipolar TransistorRC

f in 21

effcbbb CRff

8max


*M.J.W. Rodwell, CSICS 2008

InP Bipolar Transistor Scaling Roadmap256 128 64 32 nm width

Emitter256 128 64 32 nm width

8 4 2 1 Ω·µm2 access ρ

Base175 120 60 30 nm contact width

Base10 5 2.5 1.25 Ω·µm2 contact ρ

Collector106 75 53 37.5 nm thick

9 18 36 72 mA/µm2 currentµ4 3.3 2.75 2-2.5 V breakdown

fτ 520 730 1000 1400 GHzf 850 1300 2000 2800 GHzfmax 850 1300 2000 2800 GHz

Contact resistivity serious barrier to THz technologyy gy

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


simultaneous THz ft and fmax

Approach

To achieve low resistance, stable ohmic contacts• Higher number of active carriers

- Reduced depletion widthE h d t li t l- Enhanced tunneling across metal-semiconductor interface

B tt f ti t h i• Better surface preparation techniques- For efficient removal of oxides/impurities


• Scaled device thin base

Approach (contd.)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

Need a refractory metal for thermal stability

15 Pd/Ti diff i

100 nm InGaAs grown in MBE

15 nm Pd/Ti diffusion


100 nm InGaAs grown in MBE


– Low resistance contacts for high speed HBTs and FETs– Approachpp



• Conclusion


E il th b S lid S M l l B E it

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

- Semi insulating InP (100) substrateSemi insulating InP (100) substrate- Un-doped InAlAs buffer- CBr4 as carbon dopant source4

- Hole concentration determined by Hall measurements

Semi insulating InP Substrate

100 nm In0.52Al0.48As: NID buffer100 nm In0.53Ga0.47As: C (p-type)


Semi-insulating InP Substrate

In-situ contacts

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

Why Mo? Refractory metal (melting point 2620 oC)- Refractory metal (melting point ~ 2620 oC)

- Easy to deposit by e-beam technique- Easy to process and integrate in HBT process flow

20 nm in-situ Mo





TLM (Transmission Line Model) fabrication• E-beam deposition of Ti, Au and Ni layers

• Samples processed into TLM structures by photolithography d lift ffand liftoff

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

50 nm ex-situ Ni

20 nm in-situ Mo20 nm ex-situ Ti

500 nm ex-situ Au





• Resistance measured by Agilent 4155C semiconductor parameter

Resistance Measurementy g p

analyzer

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

scanning electron microscope (SEM)

• TLM Width ~ 25 µm

4

5

)

R22

3

Res

ista

nce

(W

RR ShC

C

22

0

1

0 0.5 1 1.5 2 2.5 3 3.5

R


Gap Spacing (m)

• Extrapolation errors:Error Analysis

• Processing errors:p– 4-point probe resistance

measurements on Agilent 4155C

g– Variable gap spacing along

width (W)– Overlap resistance4155C

– Resolution error in SEM3.5

Overlap resistance

2

2.5

3

ce (

)

R

d

Variable gap along width (W)1.10 µm 1 04 µm

0 5

1

1.5

2

Res

ista

nc d

Lgap

1.10 µm 1.04 µm

0

0.5

0 1 2 3 4 5 6Gap Spacing (m)

Rc W

Overlap Resistance



– Low resistance contacts for high speed HBTs and FETs– Approachpp



• Conclusion


Doping Characteristics-IHole concentration Vs CBr4 flux

1020

70

80

3 )

4

60

70hole concentration

ty (c

m2 -V

s)

ntra

tion

(cm

-3

Tsub = 460 oC

40

50mobility

Mob

ilit

Hol

e C

once

n

1019

40

0 10 20 30 40 50 60CBr

4 foreline pressure (mtorr)

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


Number of di carbon defects as CBr flux

Doping Characteristics-IIHole concentration Vs V/III flux

10

9 ) cm

-3

Hole concentration Vs V/III flux

6

8

ratio

n (1

019CBr = 60 mtorr

4

e C

once

ntr

210 20 30 40 50 60

Group V / Group III

Hol

As V/III ratio hole concentration

h pothesis As deficient s rface dri es C onto gro p V sites


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

Doping Characteristics-IIIH l t ti V b t t t t

2 1020

)

Hole concentration Vs substrate temperature

1.2 1020

1.6 1020

tratio

n (c

m-3

)

19

8 1019

Hol

e C

once

nt

CBr = 60 mtorr4 1019

300 350 400 450

H

S b T (oC)Substrate Temp. (oC)

Tendency to form di-carbon defects as Tsub *


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

y

Doping Characteristics-IIIH l t ti V b t t t t

2 1020

)

Hole concentration Vs substrate temperature

)

1.2 1020

1.6 1020

tratio

n (c

m-3

)

1020

ratio

n (c

m-3

)

19

8 1019

Hol

e C

once

nt

CBr = 60 mtorr1019

Tsub = 460 oCTsub = 350 oC

Hol

e C

once

ntr

4 1019

300 350 400 450

H

S b T (oC)

10

0 20 40 60 80 100CBr

4 foreline pressure (mtorr)

HSubstrate Temp. (oC)

Tendency to form di-carbon defects as Tsub *


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

y

Results: Contact Resistivity - I

Metal Contact ρc (Ω-µm2) ρh (Ω-µm)In-situ Mo 2.2 ± 0.8 15.4 ± 2.6

20

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

10

15

tanc

e (

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

ρ lower than the best reported contacts to

5Res

ist

WR

R ShCC

22

(100 nm thick film)

ρc lower than the best reported contacts to pInGaAs (ρc = 4 Ω-µm2)[1,2] 0

0 1 2 3 4Gap Spacing (m)

1 G iffith t l I di Ph hid d R l t d M t i l 2005

W


1. Griffith et al, Indium Phosphide and Related Materials, 2005.2. Jain et al, IEEE Device Research Conference, 2010.

Results: Contact Resistivity - II

10

(-

m2 )

*

1exp

pCTunneling

sist

ivity

, c

pTunneling

ThermionicE i i ~ constant*

onta

ct R

es

Data suggests tunneling

Emission c ~ constant

15 10 15

Co

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

gg g

High active carrier concentration is the key to low resistance contacts


* Physics of Semiconductor Devices, S M Sze

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

Thermal Stability - IMo contacts annealed under N2 flow for 60 mins. at 250 C

Before annealing After annealing

Molybdenum C substrate

ρc (Ω-µm2) 2.2 ± 0.8 2.8 ± 0.9

• ρc increases on annealing

• Mo reacts with residual

interfacial carbon?* Kiniger et. al.*


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

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

Thermal Stability - IIMo contacts annealed under N2 flow for 60 mins. at 250 C

TEM of Mo-pInGaAs interfacep

- Suggests sharp interface

- Minimal/No intermixing

Au

- Minimal/No intermixing Ti

InGaAsMo

200 nm InAlAs


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)( ρc µ )

Contacts s itable for TH transistors Contacts suitable for THz transistors


Thank You !

Questions?

AcknowledgementsONR, DARPA-TFAST, DARPA-FLARE


Extra SlidesExtra Slides


Correction for Metal Resistance in 4-Point Test Structure

WL /R W/)( 2/1 WLsheet /metalR Wcontactsheet /)( 2/1

WI

V

L

W V

V

I

xRWLW t lh tt th t ///)( 2/1

Error term (R /x) from metal resistance

xRWLW metalsheetcontactsheet ///)(


Error term (Rmetal/x) from metal resistance

Random and Offset Error in 4155C

0.6646

0.6647• Random Error in

resistance

0.6645

0.6646

nce

() measurement ~ 0.5

m• Offset Error < 5 m*

0.6644

Res

ista

n • Offset Error < 5 m

0.6642

0.6643

0 5 10 15 20 25Current (mA)

*4155C datasheet


Accuracy Limits

• Error Calculations• Error Calculations– dR = 50 mΩ (Safe estimate)

dW 1– dW = 1 µm– dGap = 20 nm

2• Error in ρc ~ 40% at 1.1 Ω-µm2


in-situ ohmic contacts to p-ingaas - ucsb...300 350 400 450 sb t (substrate temp. (oc) tendency to...

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