1. 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
(Verne Hess) and DOE DE-FC26-04NT42271
(Ryan Egidi)

2. Potential Applications UV/Blue optoelectronics Transparent transistors
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 ZnO
Bandgap (eV)
µe (cm2/V-sec)
µh (cm2/V-sec)
me mo 0.24mo
mh mo 0.59mo
Exciton binding 28 60
energy (meV)
Potential Applications
UV/Blue optoelectronics
Transparent transistors
Nanoscale detectors
Spintronic devices

3. Zn(Mg,Cd)O alloys
The ternary system CdO-ZnO-MgO covers a large bandgap range
< Single quantum wells >

4. 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

5. 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
= eV
EgZnO =3.37 eV
EgZnCdO=2.90 eV
(EV – EZn 2p3)ZnCdO
= 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)

6. Ohmic Contacts to ZnCdO
The minimum contact resistivity
Ti/Au  2.3x10-4Ωcm2 at 450oC anneal
Ti/Al/Pt/Au1.6x10-4Ωcm2 at 500oC anneal
Severe degradation after 600oC anneal

7. Optical Microscopy Images of Metal on ZnCdO
As annealing temperature increases, metals start to form intermetallic compounds.
Ti/Au to ZnCdO
Ti/Al/Pt/Au to ZnCdO
Reference
600oC
450oC
350oC
10 µm
Reference
450oC
600oC
350oC
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

8. AES Depth Profile of Ti/Au to ZnCdO
Zn and Ti outdiffusion to the surface by 450oC
The formation of the TiOx interfacial region is evident after annealing
 improved contact resistance

9. AES Depth Profile of Ti/Al/Pt/Au to ZnCdO
Al outdiffusion to the surface by oC in the metallization scheme
Outdiffusion of Pt, Al, and Ti at higher anneal temperatures and oxidation of the Ti

10. TI/Au Ohmic Contact to Al-doped n-ZnO
The as-deposited contacts are ohmic with excellent specific contact resistivity of x10-7 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

11. Tunneling of Ti/Au Contact to Al-doped n-ZnO
1 μm ZnO:Al Ti Au
800 Å
200 Å
Temperature range: 25~225oC
Independence of temperature
tunneling is the dominant current transport mechanism
The relation between the specific resistivity and doping concentration:

12. Wet Chemical Etching High selectivity Isotropic etch profile
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
Ohmic ring
substrate
n+-ZnO
n-ZnMgO
ZnO
p-ZnMgO
p-ZnO
Photoresist
Film to be etched
Underlying Film
Isotropic etch profile
ZnO LED cross section structure

13. Etching of ZnCdO (samples grown at SVT )
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 oC
The etch rate is diffusion-limited

14. Selective Etching of ZnCdO over ZnO
100μm
Optical microscopy
minimum undercut
Etch rate is independent of orientation
The selectivity with HCl/H2O was over
The maximum selectivity with H3PO /H2O was ~15

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

16. Selective Etching of ZnMgO over ZnO
The selectivity with HCl/H2O was over 250
The maximum selectivity with H3PO4/H2O was ~30

17. Site-selective growth of ZnO nanorods possible using a catalysis-driven molecular beam epitaxy method.
EFFUSION CELL
SUBSTRATE
HEATER
RHEED SCREEN
ION
GAUGE
e-GUN Zn Mg
O3/O2
OZONE
GENERATOR
RF PLASMA
OXYGEN
Zn flux
O2/O3 flux
Ag catalyst
particles
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.

18. Nanowires vs Zn, Mg pressures
ZnO
Radial heterostructured (Zn,Mg)O
(Mg,Zn)O I II
hexagonal
core /
sheath
core /
sheath
cubic
wurtzite st.
(Zn1-xMgx)O/(Zn1-xMgx)O
(Zn1-xMgx)O / (Mg,Zn)O
rock salt st.
hexa. / hexa.
hexa. / cubic
wurtzite / wurtzite
wurtzite / rock salt st.
 Zn = 3 × 10-6
 O3/O2 = 5 × 10-4
 Mg = none
 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
 Zn = 3 × 10-6
 O3/O2 = 5 × 10-4
 Mg = 8 × 10-7
Tg= 400C
[unit: mbar]

19. Fabrication of ZnO nanowire device
Electrode (Al/Pt/Au)
 Motivation
Al/Pt/Au
-. Fundamental understanding of transport
-. Nano sensors (UV, chemical, bio.)
-. Nanoelectronics
Insulator
 Structure of Nanodevice
-. Electrode : Al/Pt/Au by sputtering
-. Diameter of ZnO nanowire : 130 nm
-. Channel Length : 3.5 m

20. ZnO Nanorod MOS FET
Source
Drain
Gate
Oxide
Nanowire Si
Insulator (SiO2)
Source
(Al/Pt/Au)
Drain
Gate(Al/Pt/Au)
Nanowire
Gate oxide
((Ce,Tb)MgAl11O19)
Apply the stable oxide((Ce, Tb)MgAl11O19 ) for each device
Can be used as passive layer in gas, humidity, chemical sensor

21. pH sensor with gateless nanorod FET
Nanowire
Microchannel
electrode
(Al/Pt/Au)
Insulator Si
Insulator (SiO2)
8.5 nS/ pH in the dark
20 nS/ pH under UV(365nm)
Appl. Phys. Lett., 86, (2005)

22. 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
Schematic of Multiple ZnO Nano-Rods
Close-Up of Packaged ZnO Nano-Rod Sensor

23. Single nanorod hydrogen gas sensor
Insulator
Pt-ZnO Nanorod
Electrode (Al/Pt/Au)
Al/Pt/Au

24. Current status of ZnO LED research
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
(a) Structure
(b) Current-voltage
(c) Electroluminescence

25. 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

26. Current status of ZnO LED research
3. Arsenic doping [ Ryu et al., Appl. Phys. Lett. 88, (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
(a) Structure
(c) Electroluminescence

27. Device Fabrication 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)
ZnO substrate
N+ implanted ZnO (300nm)
Au (80nm)
Ni (20nm)
Au (200nm)
Ti (20nm)
This picture is a device wire-bonded for EL test. The size is 2 by 4 mm. These circles are the contact metal for implanted ZnO. Rectangular gray area is the back contact metal for ZnO substrate. We use a commercialized ZnO substrate from Cermet. It’s undoped, I grade with electron concentration of 1017, the mobility is ~190cm2/V.s. This company also achieved pn LED in Dr. Pan will give us a nice talk today, too!
The substrate was implanted by the four doses, following by thermal activation using either RTA or furnace. The annealing temperature is from 600C to 1000C. After N activation, the back contact metal is Ti/Au and the front contact metal is Ni/Au using e-beam evaporation.

28. Diode I-V Characteristics
Leakage -6V
Ideality factor~11

29. Light emission from ZnO pn homojunction device
Device fabrication
Light emission from ZnO pn homojunction device

30. Electroluminescence at 120K

31. Vertical ZnO NWs/PEDOT LED Nanowire Array
The cross section schematic of ZnO nanowires LED

32. 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

33. Conclusions
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
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