Process Optimization and Development for

ZnO Optoelectronics and Photodiodes

Jon Wright Dept. of Materials Science and Engineering,

Univ. of Florida, Gainesville, FLJan 18, 2007

Outline

• Introduction & Motivation• Background

– Contacts (Ohmic + Schottky)– Ion Implantation (Group V)

• Project Objectives• Methodology• Preliminary Results

– Ir/Au Ohmic Contacts– Surface Treatment Analysis

• Conclusions & Timeline

ZnO – Basic (Electrical) Properties

• Direct, wide bandgap• High excitonic binding

energy – 60 meV

• Inexpensive growth

• Easily etched– (acids and alkalis)

• Radiation stability

Property Value

Lattice parameters at 300 K (nm)a0: 0.32495

c0: 0.52069

Density (g cm-3) 5.606

Stable phase at 300 K Wurtzite

Melting point (ºC) 1975

Thermal conductivity 0.6, 1-1.2

Linear thermal expansion coefficienta0: 6.5 10-6

c0: 3.0 10-6

Static dielectric constant 8.656

Refractive index 2.008, 2.029

Energy bandgap (eV) Direct, 3.37

Intrinsic carrier concentration (cm-3)

<106 max n-type doping: n ~

1020

max p-type doping: p ~ 1017

Exciton binding energy (meV) 60

Electron effective mass 0.24

Electron Hall mobility, n-type at 300 K (cm2V-

1s-1)200

Hole effective mass 0.59

Hole Hall mobility, p-type at 300 K (cm2V-1s-1) 5 - 50

ZnO vs. GaN• Bulk ZnO (n-type) commercially

available • Grown on inexpensive substrates at

low temperatures• Lower exciton energy for GaN• 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 3.2µe (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

Motivation

ZnO-based electronic devices

• UV light-emitting diodes• Optoelectronics• Transparent thin-film transistors

– Flat panel displays– Solar cells

• Piezoelectric transducers• Gas-sensors

• Photonic devices– High density data storage

Ohmic contacts to n-ZnO

• Earlier Metallizations– Ti/Au, Zn/Au, Al/Pt

Re/Ti/Au, Ru, Pt/Ga– ρsc 10-3 – 10-7 Ω.cm2

• c-TLM reduces steps• Au ↓ sheet resistance • Surface carrier ↑

annealing– Adv: oxygen loss– Disad: surface degradation

• Surface cleaning ↓ b

• Limited info w/ p-ZnOK. Ip et al. AIP (2004).

Schottky Contacts to ZnO

• Schottky Obstacles– Surface states– Defects @ surface layer– Metal/ZnO intermixing

• Typically Au, Ag, Pd, Pt– Φb ~ 0.6-0.84 eV

– n > 1 (~1-2+)– Poor thermal stability

• High n factor– Tunneling– Interface layer– Surface conductivity– Deep recomb. centers

Element Work Function (eV) Ideal Barrier Height (eV)

B 4.45 0.35

Cr 4.5 0.4

Pt 5.64 1.54

Ti 4.33 0.23

W 4.55 0.45

Zr 4.05 -0.05

1expexp2**

nkT

qV

kT

qTAJ b

p-type Doping in ZnO

• Several deposition methods– Group V: N, P, As, Sb – all on O sites– MBE requires low temp for high dopant conc.

• Crystal quality poor below 500°C

– Post-deposition annealing results inconsistent

• Hole conc. ~ 1015-1017 cm-3

• Limitations in band edge electroluminescence– Deep traps: non-radiative recombination centers– Low density of holes at junction– Diffusion of carriers away from active region

p-type Ion Implantation for ZnO

• Dopant beam makes vacancies for acceptors

• Questions:– Correct ion dosage– Limiting residual damage– Maximizing acceptors

• Need for understanding– Damage accumulation– Thermal stability of defects

Project Objectives

The goals of this project are three fold:1. Optimization of Ohmic contacts to ZnO

– Ir, Re, WNx, TiNx, ZrNx, and TaNx

2. Optimization of Schottky contacts to ZnO– Ir, Re, WNx, TiNx, ZrNx, and TaNx

3. Investigation of electrical properties for implanted Group V dopants in ZnO

Aim: Develop processes for ZnO devices– Specifically for UV optoelectronics and LEDs– Realization of p-type ZnO nanowire devices

Why Use These Materials?

• Nitrides have excellent electrical properties– Highly conductive– High melting temperature– Strong bonds lead to low diffusivity probability– Thermally stable – some Nitrides up to 800°C on GaN

• Ir, Re successful novel metallizations for GaN– Superb thermal stability

• Group V elements most promising p-type dopants– Difficulty with shallow acceptor levels due to defect states– Group I elements tend to occupy interstitial sites (act as donors)

Methodology – Ohmic Contacts Processing

• Surface Treatment/Cleaning• Photolithography – c-TLM pattern if possible

[J. Chen thesis]

• Sputter deposit metallization scheme– Novel metallizations include Au overlayer

• Lift-off• Anneal (300°C-1000°C, 1 min, N2 or O2)

Methodology – Schottky Contacts Processing

• Sample Treatment/Cleaning• Photolithography for Ohmic contact (outer ring)

• Sputter deposit Ti/Au (basic Ohmic contact)• Lift-off• RTA anneal 450°C , 30 sec N2 ambient• Schottky photolithography realignment• Sputter deposit metallization scheme

– Novel metallizations include Au overlayer• Lift-off

• Anneal contacts (300°C-1000°C, 1 min, N2 or O2)

Methodology – Contact Measurements

• Electrical Characterization– Contact resistance

• 4-probe TLM measurement• 2-probe C-TLM measurement

– Δ Annealing temperature– Δ Annealing time– Variation in measurement

temperature (RT – 300°C)– Schottky Diode parameter

measurements

• Auger Electron Spectroscopy• Scanning Electron Microscopy• Thermal stability measurements

kT

q

TqA

k bC

exp

**

D

bSC

N

m

*2exp

Methodology – Ion Implantation• N, P, As dopants @ doses 1013-1014

cm-2

• Implantation temp varied RT – 300°C

• Annealed between 600 – 950°C– RTA– PLD chamber, O2 ambient (in-situ)

• Hall measurements used to calculate:– Carrier type– Carrier density– Acceptor ionization energy

• Use of Oxygen to reduce vacancies

• Depth Profiles by AES/SIMS

Ion Implantation → ZnO Nanowires

• Ability to create pn junction is paramount– Acceptor implantation + characterization

• Why Nanowires?– FETs, photodetectors, gas sensors, nano-cantilevers– Allow investigation of carrier transport properties (1-D)– Surface quality, ambient environment critical to character of device

• ZnO nanorods (d ~130 nm) grown by MBE– p-type nanowires by injection of acceptors– Contacts on wires using p-type Ohmic metals

• Nanowire pn junctions– Masked implantation OR focused ion beam– Determination of EA, ρ – activation kinetics

Prelim Research – Ir/Au Ohmic Contacts

0 200 400 600 800 100010-6

10-5

10-4

Annealing Temperature (C)Spec

ific

Con

tact

Resi

stan

ce (

cm2 )

2

4

6

810 S

heet Resistan

ce (square)

Ir/Au Contacts – AES Profiles

0 100 200 300 400 500 600 700 800 0

10

20

30

40

50

60

70

80

90

100

Sputter Depth (Å )

C

O

Ir Au

Zn

Ato

mic

Con

cent

ratio

n (%

)

As-Deposited

Only slight intermixing btw Au and Ir layers until 800°C(+)

0 100 200 300 400 500 600 700 800 0

10

20

30

40

50

60

70

80

90

100 1000 C Anneal

Sputter Depth (Å ) A

tom

ic C

once

ntra

tion

(%)

C

O

Ir Au

Zn

Ir/Au Contacts – Thermal Stability

0 3 6 9 12 15 18 21 24 27 302x10-4

2.5x10-4

3x10-4

3.5x10-4

4x10-4

4.5x10-4

5x10-4

Days Annealed @ 350CSpec

ific

Con

tact

Res

ista

nce (

cm2 )

20

40

60

80100S

heet R

esistance (/sq

uare)

30 Days

Pre-anneal

No change to Rsh after 30 days

Ir/Au Contacts – N2 vs. O2 Anneal

Resistance increased w/ O2 anneal – IrO2 layer

0 200 400 600 8001E-4

1E-3

0.01

0.1

1

Spe

cific

Con

tact

Res

ista

nce

( c

m2 )

Annealing Temperature (C)

Nitrogen Anneal Oxygen Anneal

0 200 400 600 800101

102

103

104

105

She

et R

esis

tanc

e (

/squ

are)

Annealing Temperature (C)

Nitrogen Anneal Oxygen Anneal

Ir/Au Contacts – N2 vs. O2 Anneal

0 100 200 300 400 500 600 700 800 0

10

20

30

40

50

60

70

80

90

100 N2 Anneal

Sputter Depth (Å )

Ato

mic

Con

cent

rati

on (

%)

C

O

Ir Au

Zn

Sputter Depth (Å )

0 100 200 300 400 500 600 700 800 900 1000 0

10

20

30

40

50

60

70

80

90

100

O2 Anneal

Ato

mic

Con

cent

ratio

n (%

)

C

O

Au Ir

Zn

AES can not detect IrO2 layer, however more interdiffusion of Ir w/ N2 anneal

Prelim Research – Surface Treatment

1.5 2.0 2.5 3.0 3.50

1

2

3

4

5

6

7

Inte

nsi

ty (a.u

.)

Energy (eV)

AsDep Anneal Ozone H3PO4 O2 BCl3 Ar

Surface Treatment – IV Character

All treatments result in Ohmic contacts except for Oxygen plasma.

Surface Treatment Rsh ρsc

(Ohm/□) (Ohm cm2)

Argon 66.17459 0.0040942

Ozone 51.29278 0.0019744

Oxygen 102.2857 0.1094889

BCl3 45.55315 0.0016784

Anneal 57.57407 0.0028027

H3PO4 57.96292 0.0016462

As-Dep (LTLM) 51.39528 0.0005177

Investigation TimelinePlan 2005 2006 2007 2008 2009

  Fall Spring Summer Fall Spring Summer Fall Spring Summer Fall Spring

Literature Review                      

Fabrication (Ohmic & Schottky)                      

Measurements of Contact Resistance                      

Measurements of Contact Intermixing & Second Phases                      

Thermal Stability Measurements of ZnO Contacts                      

Long-term Aging Studies                      

Ion Implantation (Doping)                      

Thermal Stability Measurements for Annealed ZnO                      

Activation Study for Different Acceptor Dopants                      

Diffusivity Measurements                      

Oral qualifier & defense                      

Dissertation & defense                      

Acknowledgements

• Advisory Committee– Prof. S.J. Pearton (Chair)– Prof. C.R. Abernathy– Prof. D.P. Norton– Prof. R. Singh– Prof. F. Ren

• Contributors– Dr. L. Stafford, Dr. B.P. Gila, L.F. Voss, R.

Khanna, H-T. Wang, S. Jang