Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors

S. J. Pearton(1), W. T. Lim(1), J. S. Wright(1), R. Khanna(1), L. Voss(1), L. Stafford(1), L. C. Tien(1), H. S. Kim(1), D.P. Norton(1), J.-J. Chen(2), H.T. Wang(2), B.S. Kang(2), F. Ren(2), J. Jun (3), J. Lin(3), A.Osinsky(4) and A.Dabiran(4)

(1) MSE, (2) Chem. Engin., (3) ECE, University of Florida, Gainesville, FL 32611(4) SVT Associates, Eden Prairie, MN 55344

Supported in part by NSF DMR 0400416(Verne Hess) and DOE DE-FC26-04NT42271(Ryan Egidi)

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

available • Grown on inexpensive (glass)

substrates at low temperatures• High exciton binding energy• Heterojunction by substitution in

Zn-site – Cd ~ 3.0 eV– Mg ~ 4.0 eV

• Ease of synthesis of nanowires• 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

Zn(Mg,Cd)O alloys

The ternary system CdO-ZnO-MgO covers a large bandgap range

< Single quantum wells >

Zn0.95Cd0.05O/ZnO Heterojunction Band

Offsets by XPS (samples grown by SVT-Andrei Osinsky)

Samples grown by rf plasma assisted MBE

2.9 eV bandgap for ZnCdO

XPS performed at UF, Charles Evans and Associates

Conduction band offset 0.30 eV

Valence band offset 0.17 eV

Energy Band Diagram of Zn0.95Cd0.05O/ZnO Heterojunction

EZn 2p3ZnCdO

ECZnCdO

EVZnCdO

ECZnO

EVZnO

EC=0.30eV

EV=0.17eV

ZnCdO ZnO

EZn 2p3ZnO

(EV – EZn 2p3)ZnO

=1020.83 eV

EgZnO =3.37 eVEg

ZnCdO=2.90 eV

(EV – EZn 2p3)ZnCdO

=1020.85 eV

ΔEv = (EZn-2p-EV)thick ZnCdO-( EZn-2p-EV)ZnO- (EZn-2p-EZn-2p)ZnCdO/ZnO ZnCdO is an attractive option as the narrow bandgap active region in

ZnO-based heterojunction LEDs (ZnMgO band offset almost all in VB)

Ohmic Contacts to ZnCdO

200 300 400 500 600

10-4

10-3

10-2

Ti/Au Ti/Pt/Al/Au

c (O

hm

-cm

2 )

Annealing Temperature (oC)

The minimum contact resistivityTi/Au 2.3x10-4Ωcm2 at

450oC anneal Ti/Al/Pt/Au1.6x10-4Ωcm2

at 500oC anneal Severe degradation after 600oC

anneal

Optical Microscopy Images of Metal on ZnCdO

As annealing temperature increases, metals start to form intermetallic compounds.

Reference

600oC450oC

350oC10 µm Reference

450oC 600oC

350oC

Ti/Au to ZnCdO Ti/Al/Pt/Au to ZnCdO

Smoother morphology after annealing even at 600oC

Reacted appearance after 350oC

Ti/Au more thermally stable than Ti/Al/Pt/Au More information: AES Depth profile

AES Depth Profile of Ti/Au to ZnCdO

0 500 1000 1500 2000-8000

0

8000 Au

OC

Au Au AuAu AuAu

As received

Cou

nt/s

ec

Kinetic energy (eV)

-8000

0

8000

Au

Au

CTiTi

OZn Au Au Au Au Au

Annealing at 450oC

-8000

0

8000

Au

Au

CTi

TiO

Zn Au Au Au Au Au

Annealing at 500oC

0 500 1000 1500 2000 2500 30000

50

100

N

Au Ti

C

O Zn

Cd

Ga

As received

Ato

mic

con

cent

ratio

n (%

)

Sputter Depth ( o

A)

0

50

100

C

Au

Ti

O Zn

Cd N

Ga

Annealing at 450oC

0

50

100 C N O Ti Zn Ga Cd Au

Au

C

O

Cd

Zn

TiN

Ga

Annealing at 500oC

Zn and Ti outdiffusion to the surface by 450oC

The formation of the TiOx interfacial

region is evident after annealing improved contact resistance

AES Depth Profile of Ti/Al/Pt/Au to ZnCdO

0 500 1000 1500 2000

-80000

8000 Au

OC

Au Au Au AuAuAu

As received

Cou

nt/s

ec

Kinetic energy (eV)

-8000

0

8000

AlAu

Au

C OAu Au Au Au Au

Annealing at 450oC

-80000

8000

AlAu

Au

C O

Au Au Au AuAu

Annealing at 500oC

0 1000 2000 3000 4000 5000 60000

50

100

C N O Al Ti Zn Ga Cd Pt Au

N

AuAl

C

Pt

Ti O

As received

Ato

mic

con

cent

ratio

n (%

)

Sputter Depth ( o

A)

0

50

100Au

C

Al

PtAu

Cd

Zn

N

Ga

Annealing at 450oC

0

50

100

C

GaO

Zn

Al

Pt Au

O

Ti

Zn

Ti

N

Ga

Annealing at 500oC

Al outdiffusion to the surface by 450oC in the metallization scheme

Outdiffusion of Pt, Al, and Ti at higher

anneal temperatures and oxidation of the Ti

TI/Au Ohmic Contact to Al-doped n-ZnO

100 200 300 400 50090

100

110

120

130

140

150

160

170

As deposited

N ~ 1.32x1019 cm-3

N ~ 9.09x1018 cm-3

Rs

(Oh

m/s

qu

are)

Annealing Temperature (oC)

As deposited

100 200 300 400 500

1E-7

1E-6

As deposited

As deposited

N ~ 1.32x1019 cm-3

N ~ 9.09x1018 cm-3

Rc

(Oh

m-c

m2 )

Annealing Temperature (oC)

The as-deposited contacts are ohmic with excellent specific contact resistivity of

2.4x10-7 Ω cm2

Subsequent annealing produces a minimum value of 6x10-8 Ω cm2 after processing at 300oC

Carrier tunneling and additional annealing further reduces the specific contact resistance

Tunneling of Ti/Au Contact to Al-doped n-ZnO

1 μm ZnO:Al

TiAu 800 Å

200 Å

0.0020 0.0025 0.0030 0.0035

2x10-7

4x10-7

6x10-7

8x10-710-6

Annealed at 150oC

Spe

cific

con

tact

res

istan

ce

(Ohm

-cm

2 )

1/T (1/K)

Temperature range: 25~225oC Independence of temperature

tunneling is the dominant current transport mechanism

The relation between the specific resistivity and doping concentration:

)](2

exp[*

D

BeSSCR

N

mR

Wet Chemical Etching

Process involves either oxidation or reduction of semiconductor surface followed by removal of the soluble reaction product

High selectivity

Isotropic etch profile

Ability to remove undesirable ions and contaminants from the wafer surface

Photoresist

Film to be etched

Underlying Film

Isotropic etch profile

Ohmic ring

substrate

n+-ZnO

n-ZnMgOZnO

p-ZnMgOp-ZnO

Ohmic ring

ZnO LED cross section structure

Etching of ZnCdO (samples grown at SVT )

0.0015 0.0020 0.0025 0.0030 0.0035 0.0040

30

40

50

60

70

80

90

100 HCl H3PO4

RT

Etc

h R

ate

(n

m/m

in)

Concentration (M)

2.8 2.9 3.0 3.1 3.2 3.3 3.44

5

6 0.0031M HCl, Ea=0.37 Kcal/mol

0.0029M H3PO4, Ea=0.38 Kcal/mol

Etc

h r

ate

(n

m/m

in)

1000/T(K-1)

Using dilute HCl and H3PO4 mixtures Controllable etch rates in the range

(<100 nm min-1) for mesa formation

Solution temperature in the range of 25- 75oC

The etch rate is diffusion-limited

Selective Etching of ZnCdO over ZnO

100μm

0.0020 0.0025 0.0030 0.00350

20

40

60 HCl H3PO4

Etc

h s

elec

ivit

y

Concentration (M)

Optical microscopy minimum undercut Etch rate is independent of orientation

The selectivity with HCl/H2O was over 30

The maximum selectivity with H3PO4

/H2O was ~15

Etching of ZnMgO

0.005 0.010 0.015 0.020 0.0250

250

500

750

1000 HCl H3PO4

Etc

hing

rat

e (n

m/m

in)

Concentration (M)

Solution temperature in the range of 25-75oC The etch rate is diffusion-limited

Selective Etching of ZnMgO over ZnO

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

10

20

30

40

50 HCl H3PO4

ZnO substrate

Etc

hin

g ra

te (

nm

/min

)

Concentration (M)

0.010 0.015 0.020 0.025 0.0300

50

100

150

200

250

300

350

400

450

HCl H3PO4

Etc

h se

leci

vity

Concentration (M)

The selectivity with HCl/H2O was over 250 The maximum selectivity with H3PO4/H2O was ~30

EFFUSION CELL

SUBSTRATEHEATER

RHEED SCREEN

IONGAUGE

e-GUN

Zn

Mg

O3/O2

OZONE GENERATOR

RF PLASMA

Zn

Mg

OXYGEN

• Growth of ZnO on Ag-coated Si via MBE.

• Nominal Ag film thickness: 20 ~ 200 Å.

(Coalesce into islands at growth temp.)

• Oxygen source: ozone/oxygen mixture

• Growth Temperature: 300°C ~ 600 °C.

• Site-selective growth of ZnO nanorods possible using a catalysis-driven molecular beam epitaxy method.

Zn flux O2/O3 flux

Ag catalystparticles

ZnO

hexagonal

wurtzite st.

(Mg,Zn)O

cubic

rock salt st.(Zn1-xMgx)O/(Zn1-xMgx)O hexa. / hexa.wurtzite / wurtzite

Radial heterostructured (Zn,Mg)O

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = none

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = 8 × 10-7

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = 2 × 10-7

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = 4 × 10-7

[unit: mbar]Tg= 400C

core / sheath(Zn1-xMgx)O / (Mg,Zn)O

hexa. / cubicwurtzite / rock salt st.

core / sheath

Nanowires vs Zn, Mg pressures

I II

Fabrication of ZnO nanowire device

Insulator

Electrode (Al/Pt/Au) Al/Pt/Au

ZnO Nanowire

-. Fundamental understanding of transport

-. Nano sensors (UV, chemical, bio.)

-. Nanoelectronics

Motivation

-. Electrode : Al/Pt/Au by sputtering

-. Diameter of ZnO nanowire : 130 nm

-. Channel Length : 3.5 m

Structure of Nanodevice

ZnO Nanorod MOS FET

Source Drain

Gate Oxide

Nanowire

• Apply the stable oxide((Ce, Tb)MgAl11O19 ) for each device

• Can be used as passive layer in gas, humidity, chemical sensor

0 2 4 6 8 10

0

2x10-8

4x10-8

6x10-8

8x10-8

I DS(A

)

VDS

(V)

VG=0 V

VG=-0.5 V

VG=-1 V

VG=-1.5 V

VG=-2 V

VG=-2.5 V

Si

Insulator (SiO2)

Source(Al/Pt/Au)

Drain(Al/Pt/Au)

Gate(Al/Pt/Au)

Nanowire

Gate oxide((Ce,Tb)MgAl11O19)

2 3 4 5 6 7 8 9 10 11 12

0

50

100

150

200

250

300

Con

duct

anc

e(nS

)

pH

non UV UV(365nm)

Microchannel

Insulator

electrode(Al/Pt/Au)

Nanowire

Si

Insulator (SiO2)

pH sensor with gateless nanorod FET

8.5 nS/ pH in the dark20 nS/ pH under UV(365nm)

Appl. Phys. Lett., 86, 112105 (2005)

pH

0 100 200 300 400 500 6000.0

4.0x10-8

8.0x10-8

1.2x10-7

1.6x10-7

1211109876

543

2

I DS(A

)

Time(sec)

non UV UV(365nm)

ZnO Nano-Rods for Hydogen Sensing

• ZnO currently used for detection of humidity, UV light and gas detection

• Easy to synthesize on a plethora of substrates

• Bio-safe characteristics• Large chemically sensitive

surface to volume ratio• If coated with Pt or Pd, can

increase device’s sensitivity to hydrogen

• High compatibility to microelectronic devices

S D

ZnO M-NRs

Al2O3 Substrate

Al/Pt/Au

a) b)

S D

ZnO M-NRs

Al2O3 Substrate

Al/Pt/Au

a) b)

Schematic of Multiple ZnO Nano-Rods

Close-Up of Packaged ZnO Nano-Rod Sensor

Single nanorod hydrogen gas sensor

0 30 60 90 120 150

-0.04

-0.03

-0.02

-0.01

0.00

0.01

H2

H2

H2

H2

ZnO nanorod with Pd

AirAirAirAirO2

500ppm250ppm

100ppm

10ppm

N2

ΔR

/R (

Sen

sitiv

ity)

Time(min)

Insulator

Pt-ZnO Nanorod

Electrode (Al/Pt/Au) Al/Pt/Au

1. Nitrogen doping [ Tsukazaki et al. Nat. Mater. 4, 42 (2005) ]

• Growth method

: L-MBE (repeated-temperature-modulation epitaxy)

• Structure

: p-ZnO:N / i-ZnO / n-ZnO:Ga LED on a ScAlMgO4 substrate

Current status of ZnO LED research

(b) Current-voltage (c) Electroluminescence(a) Structure

Current status of ZnO LED research

2. Phosphorus doping [ Lim et al. Adv. Mater. 18, 2720 (2006) ]

• Growth method

: Sputtering system

• Structure

: p-ZnO:P / n-ZnO:Ga LED on a sapphire substrate

: Mg0.1Zn0.9O energy barrier layer

(a) Current-voltage (b) Electroluminescence

Current status of ZnO LED research

3. Arsenic doping [ Ryu et al., Appl. Phys. Lett. 88, 241108 (2006) ]

• Growth method

: Hybrid beam deposition (HBD)

• Structure

: p-ZnO:As / active layer / ZnO substrate

: BeZnO/ZnO active layer (seven quantum wells)

(b) Current-voltage (c) Electroluminescence(a) Structure

Device Fabrication

ZnO substrate

N+ implanted ZnO (300nm)

Au (80nm)

Ni (20nm)

Au (200nm)Ti (20nm)

Cermet: (0001) undoped, I grade n0=1017 cm-3; μe=190 cm2/V·s

Proc. of SPIE, Vol.5941, 59410D-1(2005) Implantation dose 1: 10keV, 2×1013 cm-2

dose 2: 30keV, 5×1013 cm-2

dose 3: 65keV, 9×1013 cm-2

dose 4: 140keV, 2.4×1014 cm-2

Thermal activation (RTA, furnace; T=600~1000°C)Backside metal: Ti/Au(20/200nm)Front-side metal: Ni/Au(20/80nm)

Diode I-V Characteristics

-15 -10 -5 0 5 10 15-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04N+ implanted ZnO

600C, O2, 2 mins.

800C, O2, 2 mins.

950C, O2, 2 mins.

Cur

rent

(A)

Voltage(V)-10 -8 -6 -4 -2 0 2 4

1x10-10

1x10-8

1x10-6

1x10-4

1x10-2

1x100

800C O2 RTA

-------- linear fit, slope=1.4

Cur

rent

(A)

Voltage(V)

Leakage current~10-4A @ -6V

Ideality factor~11

Light emission from ZnO pn homojunction device

Device fabrication

Electroluminescence at 120K

350 400 450 500 550 600

0

2000

4000

6000

8000

10000

12000

T= 120 K T= 298 K

I= 30 mA

EL

inte

nsity

(ar

b. u

nit)

wavelength (nm)

350 400 450 500 550 6000

50000

100000

150000

200000

250000

300000

350000

400000

450000T= 298 K

Un-implanted ZnO Implanted ZnO

PL

inte

nsity

(ar

b. u

nit)

wavelength (nm)

Vertical ZnO NWs/PEDOT LED Nanowire Array

350 400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

PL

Int

ensi

ty (

a. u

. )

Wavelength ( nm )

PL at RT

(378)

The cross section schematic of ZnO nanowires LED

0.000 0.005 0.010 0.015 0.020 0.025

0

2

4

6

8

I-V curve L-I Curve

Current (A)

Vo

ltag

e (V

)

2.50x10-8

5.00x10-8

7.50x10-8

1.00x10-7

Lig

ht in

tensity (m

W)

Summary Valence and conduction band offsets of the Zn0.95Cd0.05O/ZnO

material system are 0.17 eV and 0.30 eV, respectively. In the ZnMgO, the band offset is mainly in the valence band

Ohmic contacts fairly simple on n-and p-ZnO, but Schottky contacts are difficult (low barrier height, leaky).

The etch selectivity of ZnCdO/ ZnO with HCl/H2O >30

Some rudimentary LEDs demonstrated by groups worldwide-need to show robust bandedge EL on cheap, large area substrates if there is any chance of finding a niche relative to the nitrides

Functional nanowires with excellent structural and optical quality-many types of sensors demonstrated-Electrical transport properties of single ZnO nanowires, Pt/ZnO nanowire Schottky Diode, depletion-mode ZnO nanowire field-effect transistor, UV, pH, & gas sensor

Lots of room to study transport/functionality in radial and longitudinal wires

Site-selective growth of ZnO nanowires using catalyst, Ag, by molecular Beam Epitaxy

Bimodal growth of cored ZnO/(Zn,Mg)O heterostructured nanowires.

Type I -. Core : Zn1-xMgxO (x < 0.02) , Hexagonal wurtzite structure

-. Sheath : Zn1-xMgxO (x >> 0.02), Hexagonal wurtzite structure

Type II -. Core : Zn1-xMgxO (x < 0.02), Hexagonal wurtzite structure

-. Sheath : (Mg,Zn)O, Cubic rock salt structure

(Mg,Zn)O nanowires having cubic rock salt structure

Conclusions

Nano-devices using ZnO nanowires

Electrical transport properties of single ZnO nanowire

Pt/ZnO nanowire Schottky Diode

Depletion-mode ZnO nanowire field-effect transistor

UV, pH, & gas sensor