kelly ip phd defense ~ july 1, 2005 ~ university of florida ~ materials science and enginering...

Kelly IpPhD Defense ~ July 1, 2005

~ University of Florida ~Materials Science and Enginering

Process Development for ZnO-based Devices

~ University of Florida ~ Materials Science and Engineering ~

Outline

Introduction Inductively-coupled plasma (ICP) etching Hydrogen in ZnO Contact metallization

Ohmic contacts Schottky contacts

p-n junction diode Conclusions


Introduction Direct, wide bandgap Bulk ZnO (n-type) commercially

available Grown on inexpensive substrates at low

temperatures High exciton binding energy Heterojunction by substitution in Zn-site

Cd ~ 3.0 eV Mg ~ 4.0 eV

Nanostructures demonstrated Ferromagnetism at practical Tc when

doped with transition metals Obstacle: good quality, reproducible p-

type

GaN ZnOBandgap (eV) 3.4 3.3µe (cm2/V-sec) 220 200µh (cm2/V-sec) 10 5-50me 0.27mo 0.24mo

mh 0.8mo 0.59mo

Exciton binding 28 60energy (meV)

Potential Applications

UV/Blue optoelectronics

Transparent transistors

Nanoscale detectors

Spintronic devices


ICP Etching Wet etching

HCl, HNO3, NH4Cl, and HF Generally isotropic with limited resolution and selectivity

High-density plasma etching Anisotropic with high resolution Favored by modern manufacturing environment

Bulk, wurtzite (0001) ZnO from Eagle-Picher Gas chemistry:

Cl2/Ar (10/5 sccm) & CH4/H2/Ar (3/8/5 sccm) Constant ICP source power at 500W and process pressure

at 1 mTorr Varied rf chuck power: 50 – 300W


ICP Etching - Etch Rates

50 100 150 200 250 300

0

1000

2000

3000

4000

5000

Cl2/Ar etch rate

CH4/H

2/Ar etch rate

Etc

h R

ate

(Å/m

in)

RF Power (W)

50

100

150

200

250

300

350 Cl

2/Ar dc bias

CH4/H

2/Ar dc bias

DC

Bia

s (-

V)

CH4/H2/Ar ~3000 Å/min

Cl2/Ar ~1200 Å/min


ICP Etching - Etch Mechanism

10 12 14 16 18 200

500

1000

1500

2000

2500

3000

Etc

h R

ate

(Å/m

in)

Square Root (25+Vb)

Cl2/Ar Chemistry

CH4/H

2/Ar Chemisty

Ion-Assisted Etch Mechanism

ER E0.5-ETH0.5

Vapor pressure of etch products:

(CH4)2Zn

301 mTorr at 20°C

ZnCl2

1 mTorr at 428 °C

ETH ~ 96 eV for CH4/H2/Ar


ICP Etching - Photoluminescence

2.0 2.5 3.0 3.50.0

0.5

1.0

1.5

2.0

2.5

3.0

50W rf100W rf300W rf200W rf

Control

PL

In

ten

sit

y (

Arb

.)

Energy (eV)

2.0 2.5 3.0 3.5

0.01

0.1

1Control50W rf

100W rf300W rf200W rf

Lo

g P

L In

ten

sity

(A

rb.)

Energy (eV)

Optical degradation even at the lowest rf power


ICP - AFM

Control

50 W rf 100 W rf

200 W rf 300 W rf

0 50 100 150 200 250 300

2

4

6

8

Control

ZnOCH

4/H

2/Ar

500W ICP Power

RM

S R

ough

ness

(nm

)

rf Power (W)

Zn and O etch products removed at

same rate


ICP Etching - AES and SEM

Kinetic Energy (eV)

dN(E)

Min: -9203Max: 6112

50 250 450 650 850 1050 1250 1450 1650 1850 2050

Kinetic Energy (eV)

dN(E)

Min: -9742Max: 5973

50 250 450 650 850 1050 1250 1450 1650 1850 2050

CH4/H2/Ar

200W rfO

Zn

Zn

OControl


ICP Etching - Summary

Dry etching is possible with practical etch rates using CH4/H2/Ar

Surface is smooth and stoichiometric Anisotropic sidewalls Optical quality is sensitive to ion energy

and flux


Hydrogen in ZnO

Hydrogen Predicted role as shallow donor Introduced from growth ambient Present in optimal plasma etch chemistry

Understand diffusion behavior and thermal stability Bulk, wurtzite (0001) ZnO, undoped (n~1017cm-3)

from Eagle-Picher Hydrogen incorporation

Ion Implantation of 2H or 1H (100keV, 1015 - 1016 cm-2) 2H plasma exposure in PECVD at 100-300°C, 30 mins

Post-annealing: 500 - 700°C


Hydrogen in ZnO - Implanted - SIMS

0.5 1.0 1.5 2.01015

1016

1017

1018

1019

1020

2H 1015 cm-2 ZnO 5 min anneals

700°C

600°C

500°C

As Implanted

H C

on

cen

tra

tion

(a

tom

s/cm

3 )

Depth (m)

Removal of 2H below SIMS limit at 700°C

Thermally less stable than GaN (>900ºC)


0.4 0.6 0.8 1.0 1.2 1.4 1.60

200

400

600

RB

S Y

ield

Energy (MeV)

virgin

11016 cm2

Depth (Å)

100 keV 1H ZnO

10000 7500 5000 2500 0

Hydrogen in ZnO - Implanted - RBS/C

Minimal affect on BS yield near surface

Small increase in scattering peak (6.5% of the random level before implantation and 7.8% after implantation) the

nuclear energy loss profile of 100keV H+ is max


Hydrogen in ZnO - Implanted - PL

2.0 2.5 3.0 3.510-3

10-2

10-1

100

101

Control 500 °C 600°C 700°C As-implanted

PL I

nten

sity

(A

rb.)

Energy (eV)

2.0 2.5 3.0 3.5

0.004

0.006

0.008

0.010

0.012

0.014

700°C Anneal600°C Anneal500°C AnnealAs-implanted

PL

Int

ensi

ty (

Arb

.)

Energy (eV)

Severe optical degradation even after 700ºC anneal

Point defect recombination centers dominate


Hydrogen in ZnO - Plasma - SIMS

Large diffusion depth 2H diffuses as an interstitial, with little trapping by the lattice elements or by defects or impurities

0 5 10 15 20 25 30

1015

1016

1017

1018

200°C100°C 300°C

2H plasma treatment

Con

cent

ratio

n (a

tom

s/cc

)

Depth (m)


Hydrogen in ZnO - Plasma/annealed - SIMS

0 5 10 15 20 25

1015

1016

1017

1018

No anneal

2H plasma 200°CPost treatment anneal

500°C, 5min

400°C, 5min

Con

cent

ratio

n (a

tom

s/cc

)

Depth (m)

2H completely evolve out of the crystal at 500°C


Hydrogen in ZnO - Plasma - CV

0.0 0.1 0.2 0.31x1017

2x1017

3x1017

4x1017

Do

pa

nt C

on

ce

ntr

atio

n (

cm

-3)

Depth (µm)

2H Plasma exposure at 200°C & 600°C Anneal

2H Plasma exposure at 200°C As Grown

Effects 2H plasma treatment

Passivate the compensating acceptor

impurities

Induces a donor state and increases the free electron concentration

Suggest H from growth process

n-type conductivity probably arises from multiple impurity sources


Hydrogen in ZnO

0 100 200 300 400 500 600 700

0

20

40

60

80

100

0 200 400 600

1

10

100

% 2 H

Rem

aini

ng

Anneal Temperature (°C)

% 2 H

Rem

aini

ng


5 min Anneal

Implanted 2H

Plamsa 2H

Implanted 2H is slightly more thermally stable: trapping at residual damage in the ZnO by the

nuclear stopping process

Implanted

Plasma exposure


Hydrogen in ZnO - Summary Thermal stability and diffusion behavior of hydrogen in

ZnO T 700 °C completely evolved the implanted H from

ZnO Residual implant-induced defects severely degrade

optical properties and minimal affect crystal structure Plasma: incorporation depths of about 30 m for 0.5 hr

at 300°C T 500 °C to remove H introduced by plasma exposure Thermal stability of the hydrogen retention :

direct implantation > plasma exposure Trapping at residual implant damage


Ohmic Contacts

Require low specific contact resistance Surface treatments

As-received Organic solvents (trichloroethylene, methanol,

acetone, 3 mins each) H plasma

Ti/Al/Pt/Au metal scheme on n-type ZnO Bulk PLD films

Au/Ni/Au and Au on p-type ZnMgO


Ohmic Contacts - Ti/Al/Pt/Au on Bulk

250 300 350 400 450

2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3

1.0x10-2

Spec

ific

Con

tact

Res

isti

vity

(·

cm2 )


As Received H

2 Plasma

Organic Solvents

Bulk n-ZnO

Metals

Cross-sectional view of circular TLM

R1

RO

ρc lowest at 250 °C anneal ρc ~ 610-4 cm2

Severe contact degradation after 600 °C anneal

)/(

)/(

)/(

)/(ln

2 11

1

11

1

T

TOT

TO

TOO

O

T

O

ST LRK

LRK

R

L

LRI

LRI

R

L

R

RRR

C = RS LT2

Marlow and Das, Solid-State Electron. 25 91 (1982)


Ohmic Contacts - Ti/Al/Pt/Au on Bulk - AES


Ohmic Contacts - Ti/Al/Pt/Au on Bulk - SEM


Ohmic Contacts - Growth: n-type ZnO:P Films N-type phosphorus-doped

ZnO film on (0001) Al2O3 grown by PLD

Post-growth annealing Increase anneal temperature

Decrease carrier concentration and Hall mobility

Increase resistivity Reduction of shallow state

density P dopants activation as

acceptors in O site1016 1017 1018 1019 1020

0

5

10

15

20

Carrier Mobility Resistivity

Carrier Concentration (cm-3)

Car

rier

Mob

ilit

y (c

m2 /V

-s)

10-3

10-2

10-1

100

101

102

103

600 °C

500 °C

450 °C

425 °C

As-grown

Resistivity (

-cm)

Post-growth Anneal T

(°C)

Carrier conc

(#/cm3)Resistivity

(cm)

Hall mobility

(cm2/Vs)

30 1.5 1020 0.002 18.5425 6 1019 0.013 7.8450 2.4 1018 1.3 1.9500 3.2 1017 12.8 1.5600 7.5 1015 463 1.8

Heo et al APL 83 1128 (2003)

Post-growth

Anneal T

(°C)

Carrier conc

(#/cm3)

Resistivity

( cm)

Hall

mobility

(cm2 /Vs)

30 1.5 1020 0.002 18.5

425 6 1019 0.013 7.8

450 2.4 1018 1.3 1.9

500 3.2 1017 12.8 1.5

600 7.5 1015 463 1.8


Ohmic Contacts - Ti/Al/Pt/Au ZnO:P Films

1015 1016 1017 1018 1019 1020 1021

10-7

10-6

10-5

10-4

10-3

Spec

ific

Con

tact

Res

ista

nce

(-c

m2 )

Carrier Concentration (cm-3)

As-deposited 200°C anneal,

measured at 30 °C 200°C anneal,

measured at 200°C

Nonalloyed:

n = 1.5 1020 cm-3

c = 8.7 10-7 -cm2

Annealed:Measured at RT:

n = 6.0 1019 cm-3

c = 3.9 10-7 -cm2

Measured at 200 °C

n = 2.4 1018 cm-3

c = 2.2 10-8 -cm2

Ti/Al/Pt/Au (200/800/400/800)Å on PLD ZnO:P films


Ohmic Contacts - p-type ZnMgO Films

-0.50 -0.25 0.00 0.25 0.50-0.50

-0.25

0.00

0.25

0.50

Ni/Au (200/800 Å)

No anneal

250 oC

400 oC

500 oC

Cur

rent

(m

A)

Bias (V)

-0.50 -0.25 0.00 0.25 0.50-5.0

-2.5

0.0

2.5

5.0

Ti/Al (200/800 Å)

No anneal

250 oC

400 oC

500 oC

600 oC

Cur

rent

(m

A)

Bias (V)

0 100 200 300 400 500 6000

4000

8000

12000

16000

20000

Ti/Au Ohmic contact Ni/Au Ohmic contact

Res

ista

nce

()

Annealing Temperature (oC)

• Ohmic behavior after annealing 500 °C

• Ti/Au more thermally stable than Ni/Au contacts

• Severe degradation of Ni/Au after 600 °C anneal

S. Kim et al APL 84 1904 (2004)


Ohmic Contacts - p-type ZnMgO Films

-5.0 -2.5 0.0 2.5 5.0-0.04

-0.02

0.00

0.02

0.04

600 °C Annealed

As-deposited

Au/Zn0.9

Mg0.1

O:P0.02

Cur

rent

(A

)

Bias (V)

Specific contact resistance after 600 °C anneal

Au: 2.5 10-5 cm2

Au/Ni/Au: 7.6 10-6 cm2

-5.0 -2.5 0.0 2.5 5.0-0.04

-0.02

0.00

0.02

0.04

As-deposited

600 °C Annealed

Au/Ni/Au/Zn0.9

Mg0.1

O:P0.02

Cur

rent

(A

)Bias (V)


Ohmic Contacts - Au/ZnMgO

0 500 1000 1500 2000 2500 3000 35000

20

40

60

80

100Au/Zn

0.9Mg

0.1O:P

0.02 As-deposited

O

Zn

MgC

Au

A

tom

ic C

once

ntra

tion

(%

)

Sputter Depth (Å)

0 500 1000 1500 2000 2500 3000 35000

20

40

60

80

100Au/Zn

0.9Mg

0.1O:P

0.02 600ºC Annealed

C

Zn

Mg

O

Au

Ato

mic

Con

cent

rati

on (

%)

Sputter Depth (Å)


Ohmic Contacts - Au/Ni/Au/ZnMgO

0 500 1000 1500 2000 25000

20

40

60

80

100Au/Ni/Au/Zn

0.9Mg

0.1O:P

0.02 As-deposited

Mg

Ni

C

O

Zn

Au

A

tom

ic C

once

ntra

tion

(%

)

Sputter Depth (Å)

0 500 1000 1500 2000 25000

20

40

60

80

100

Ni

O

Zn

CMg

Au

Au/Ni/Au/Zn0.9

Mg0.1

O:P0.02

600ºC Annealed

Ato

mic

Con

cent

rati

on (

%)

Sputter Depth (Å)


Ohmic Contacts - Summary

Ti/Al/Pt/Au to n-type ZnO (bulk, thin film) No significant improvement from H2

plasma treatment or organic solvent cleaning

AES revealed Ti-O interfacial reactions and intermixing between Al and Pt layers T250°C

Au/Ni/Au to p-type ZnMgO: lower C than Au alone


Schottky Contacts

Previous Works Metals: Au, Ag, Pd Schottky barriers heights ~ 0.6-0.8 eV Barrier heights not following the difference in the

work function value interface defect states determine contact characteristics

Au is unstable even at 60°C This Work Investigate the effect of UV surface cleaning Metal schemes:

PLD n-type film: Pt Bulk: Pt, W, W2B, W2B5, CrB2


Schottky Contacts - Pt/Au on Bulk

No ozone treatment: Linear I-V Ozone treatment:

B = 0.696 eV

= 1.49 Js = 6.17 10-6 A-cm-2

-0.10 -0.05 0.00 0.05 0.10-1.0

-0.5

0.0

0.5

1.0

No ozone

Cur

rent

(m

A)

Bias (V)

0.0 0.1 0.2 0.3 0.4 0.50.000

0.005

0.010

0.015

0.020

B = 0.696 eV

= 1.49

Js = 6.17 10-6 A-cm-2

30 min ozone

Cur

rent

(A

)

Bias (V)

-10 -8 -6 -4 -2 0-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

30 min ozone

Cur

rent

(m

A)

Bias (V)


Schottky Contacts - UV Ozone - AFM

No Ozone Treatment

30 min Ozone Treatment


Schottky Contacts - UV Ozone - XPS

1000 800 600 400 200 0-60000

-40000

-20000

0

20000

40000

60000No Ozone

ZnCO

O

Zn

Zn

N(E

)

Energy (eV)

C 1s peak No ozone (at. %) 30 min (at.%)

No Ar+ sputter 29.5 5.8

1 min Ar+ sputter 5.3 1.1

2 min Ar+ sputter 2.6 0.1

1000 800 600 400 200 0

-100000

-50000

0

50000

100000

150000

CO Zn

Zn

Zn

30 min Ozone

N(E

)

Energy (eV)

Desorption of

surface C

contaminants


Schottky Contacts - W/Pt/Au on Bulk

-0.4 -0.2 0.0 0.2 0.4-0.1

0.0

0.1

0.2

0.3

Cur

rent

(m

A)

Bias (V)

700 °C, 1 min anneal 30 min ozone No ozone

-0.010 -0.005 0.000 0.005 0.010

-1.0

-0.5

0.0

0.5

1.0

Cur

rent

(m

A)

Bias (V)

As-sputtered 30 min ozone no ozone

Sputter-induced damages Non-rectifying for 250

°C and 500 °C anneal Rectifying after 700 °C

anneal

No ozone 30 min ozone

B (eV) 0.45 0.49

4.5 3.2

Js (A-cm-2) 8.4310-2 2.1110-2


Schottky Contacts - W/Pt/Au - AES

0 200 400 600 800 10000

5000

10000

15000

20000

25000

30000

O

Zn

W

PtAu

C

30 min ozone, as-sputtered

Inte

nsit

y (a

rb. u

nit)

Time (s)

0 200 400 600 800 10000

5000

10000

15000

20000

25000

Zn

Zn

W

AuPt

C O

30 min ozone, 700 °C anneal

Inte

nsit

y (a

rb. u

nit)

Time (s)

• Post-deposition annealing 500 °C: no detectable intermixing

• 700 C anneal: Zn diffused out to the Au-Pt interface, independent of whether the samples had been exposed to ozone


Schottky Contacts - W2B5 vs. W2BW2B5/Pt/Au as deposited

W2B5/Pt/Au 600ºC annealed

W2B/Pt/Au as deposited

W2B/Pt/Au 600ºC annealed


Schottky Contacts - Summary

Ozone treatment removes surface C contamination Pt contacts: ozone treatment produces transition

from ohmic to rectifying behavior W contacts: require annealing T 700°C to repair

sputter-induced damages AES revealed intermixing of metal layers and out-

diffusion of Zn to Au-Pt interface Low barrier heights for boride contacts W2B showed good thermal stability high

temperature ohmic contacts


p-n Junction Diode - Growth and Structure

Full backside ohmic contact

Bulk ZnO (0.5 mm, n ~ 1017 cm-3)

Buffer n-ZnO PLD film (~0.8 m)

Zn0.9Mg0.1O: P0.02 PLD film (~1.4 m)

Circular ohmic contact (50 to 375 m diameter)

Pulsed laser deposition (PLD)

(0001) bulk ZnO substrate

Zn0.9Mg0.01O:P0.02 target

KrF excimer laser ablation sourceLaser repetition rate: 1 Hz

Laser pulse energy density: 3 J-cm-2

Growth: 400 °C, O2 overpressure of 20 mTorr

Ohmic contacts: p-ZnMgO: Pt/Au (200/800Å)

n-ZnO: Ti/Al/Pt/Au 200/400/200/800Å)

Annealed at 200 °C, 1 min, N2 ambient

Undoped buffer layer necessary for good rectifying behavior


p-n Junction Diode - IV Characteristics

-10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0-0.04

-0.02

0.00

0.02

0.04

Pt/Au 50 m Diode

Cur

rent

(A

)

Bias (V)

Measured at room temp:

VRB –9.0 V

Js 4.610-9 A·cm-2

Vf 4.0 V

RON 14.5 m ·cm-2


p-n Junction Diode - Reverse Breakdown

-8 -6 -4 -2 0-0.04

-0.03

-0.02

-0.01

0.00

Cur

rent

(A

)

Bias (V)

Pt/Au 210 m diodeMeasurement Temperatures

30 °C 50 °C 100 °C 150 °C 200 °C

0 50 100 150 200 2501

2

3

4

5

6

7

Pt/Au

Rev

erse

Bre

akdo

wn

Vol

tage

(V

)

Measurement Temperature (°C)

)](1[ 00 TTVV RBRB

Temperature coefficient:Slightly negative ~ .1 to .2 V/K

Presence of defects

Non-optimized growth and processing


p-n Junction Diode - Summary

Demonstrated ZnO-based p-n junctions ZnMgO/ZnO heterostructure system n-type ZnO buffer on the ZnO substrate is critical

in achieving acceptable rectification in the junctions

Important step in realizing minority carrier devices in the ZnO system


Conclusions ICP etching

Methane-based chemistry Practical etch rate but optical degradation

H in ZnO Much less thermally stable than GaN Completely evolve out by 700°C anneals

Ohmic contacts to ZnO Straightforward n-type Rapidly improving for p-type

Schottky contacts to ZnO Low B for both n-type and p-type Surface states dominate transport mechanism

p-n junction diode using ZnMgO/ZnO demonstrated


Acknowledgements

Committee members: Prof. Stephen Pearton, Chair Prof. Cammy Abernathy Prof. David Norton Prof. Rajiv Singh Prof. Fan Ren, External

Contributors:Y.-W. Heo Y. Li B. Luo

B.P. Gila E.S. Lambers K.H. Baik

A.H. Onstine M.E. Overberg J.R. LaRoche

kelly ip phd defense ~ july 1, 2005 ~ university of florida ~ materials science and enginering...

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