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Process Development for ZnO-based Devices. Kelly Ip PhD Defense ~ July 1, 2005 ~ University of Florida ~ Materials Science and Enginering. Outline. Introduction Inductively-coupled plasma (ICP) etching Hydrogen in ZnO Contact metallization Ohmic contacts Schottky contacts

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process development for zno based devices

Process Development for ZnO-based Devices

Kelly Ip

PhD Defense ~ July 1, 2005

~ University of Florida ~

Materials Science and Enginering

outline
Outline
  • Introduction
  • Inductively-coupled plasma (ICP) etching
  • Hydrogen in ZnO
  • Contact metallization
    • Ohmic contacts
    • Schottky contacts
  • p-n junction diode
  • Conclusions
introduction
Introduction

GaNZnO

Bandgap (eV)3.43.3

µe (cm2/V-sec)220200

µh (cm2/V-sec)105-50

me0.27mo0.24mo

mh0.8mo0.59mo

Exciton binding28 60

energy (meV)

  • 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

Potential Applications

UV/Blue optoelectronics

Transparent transistors

Nanoscale detectors

Spintronic devices

icp etching
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
ICP Etching - Etch Rates

CH4/H2/Ar ~3000 Å/min

Cl2/Ar ~1200 Å/min

icp etching etch mechanism
ICP Etching - Etch Mechanism

Ion-Assisted Etch Mechanism

ER  E0.5-ETH0.5

ETH ~ 96 eV for CH4/H2/Ar

Vapor pressure

of etch products:

(CH4)2Zn

301 mTorr at 20°C

ZnCl2

1 mTorr at 428 °C

icp etching photoluminescence
ICP Etching - Photoluminescence

Optical degradation even at the lowest rf power

icp afm
ICP - AFM

Control

100 W rf

50 W rf

200 W rf

300 W rf

Zn and O etch products removed at same rate

icp etching aes and sem
ICP Etching - AES and SEM

Control

O

Zn

CH4/H2/Ar

200W rf

O

Zn

icp etching summary
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 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
Hydrogen in ZnO - Implanted - SIMS

Removal of 2H below SIMS limit at 700°C

Thermally less stable than GaN (>900ºC)

hydrogen in zno implanted rbs c
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
Hydrogen in ZnO - Implanted - PL

Severe optical degradation even after 700ºC anneal

Point defect recombination centers dominate

hydrogen in zno plasma sims
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

hydrogen in zno plasma annealed sims
Hydrogen in ZnO - Plasma/annealed - SIMS

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

hydrogen in zno plasma cv
Hydrogen in ZnO - Plasma - CV
  • 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 zno1
Hydrogen in ZnO

Implanted

Plasma exposure

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

hydrogen in zno summary
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
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
Ohmic Contacts - Ti/Al/Pt/Au on Bulk

Cross-sectional view of circular TLM

Metals

RO

R1

Bulk n-ZnO

C = RS LT2

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

ρc lowest at 250 °C anneal ρc ~ 610-4 cm2

Severe contact degradation after 600 °C anneal

ohmic contacts growth n type zno p films
Ohmic Contacts - Growth: n-type ZnO:P Films

Hall

Hall

Post-growth

Post-growth

Carrier conc

Carrier conc

mobility

mobility

Resistivity

Resistivity

Anneal T

Anneal T

3

3

2

2

W

W

(°C)

(°C)

(#/cm

(#/cm

)

)

(

(

cm)

cm)

(cm

(cm

/Vs)

/Vs)

20

20

´

´

1.5

1.5

10

10

30

30

0.002

0.002

18.5

18.5

19

19

´

´

6

10

6

10

425

425

0.013

0.013

7.8

7.8

18

18

´

´

2.4

2.4

10

10

450

450

1.3

1.3

1.9

1.9

17

17

´

´

3.2

3.2

10

10

500

500

12.8

12.8

1.5

1.5

15

15

´

´

7.5

7.5

10

10

600

600

463

463

1.8

1.8

  • 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 site

Heo et al APL 83 1128 (2003)

ohmic contacts ti al pt au zno p films
Ohmic Contacts - Ti/Al/Pt/Au ZnO:P Films

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

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

ohmic contacts p type znmgo films
Ohmic Contacts - p-type ZnMgO Films
  • 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 films1
Ohmic Contacts - p-type ZnMgO Films

Specific contact resistance after 600 °C anneal

Au: 2.5  10-5 cm2

Au/Ni/Au: 7.6  10-6 cm2

ohmic contacts summary
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 T250°C
  • Au/Ni/Au to p-type ZnMgO: lower C than Au alone
schottky contacts
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
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
schottky contacts uv ozone afm
Schottky Contacts - UV Ozone - AFM

No Ozone Treatment

30 min Ozone Treatment

schottky contacts uv ozone xps
Schottky Contacts - UV Ozone - XPS

Desorption of surface C contaminants

schottky contacts w pt au on bulk
Schottky Contacts - W/Pt/Au on Bulk

Sputter-induced damages

  • Non-rectifying for 250 °C and 500 °C anneal
  • Rectifying after 700 °C anneal
schottky contacts w pt au aes
Schottky Contacts - W/Pt/Au - AES
  • 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 w 2 b 5 vs w 2 b
Schottky Contacts - W2B5 vs. W2B

W2B5/Pt/Au as deposited

W2B/Pt/Au as deposited

W2B5/Pt/Au 600ºC annealed

W2B/Pt/Au 600ºC annealed

schottky contacts summary
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
p-n Junction Diode - Growth and Structure

Circular ohmic contact (50 to 375 m diameter)

Zn0.9Mg0.1O: P0.02 PLD film (~1.4 m)

Buffer n-ZnO PLD film (~0.8 m)

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

Full backside ohmic contact

  • Pulsed laser deposition (PLD)
  • (0001) bulk ZnOsubstrate
  • Zn0.9Mg0.01O:P0.02 target
  • KrF excimer laser ablation source
    • Laser repetition rate: 1 Hz
    • Laser pulse energy density: 3 J-cm-2
  • Growth: 400 °C, O2 overpressure of 20 mTorr

Undoped buffer layer necessary for good rectifying behavior

  • 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
p n junction diode iv characteristics
p-n Junction Diode - IV Characteristics

Measured at room temp:

VRB –9.0 V

Js 4.610-9 A·cm-2

Vf 4.0 V

RON 14.5 m ·cm-2

p n junction diode reverse breakdown
p-n Junction Diode - Reverse Breakdown

Temperature coefficient:

Slightly negative ~ .1 to .2 V/K

Presence of defects

Non-optimized growth and processing

p n junction diode summary
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
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
Acknowledgements
  • Committee members:
    • Prof. Stephen Pearton, Chair
    • Prof. Cammy Abernathy
    • Prof. David Norton
    • Prof. Rajiv Singh
    • Prof. Fan Ren, External
  • Contributors: