PEG-coated gold nanorod monoclonal antibody conjugates in ...
Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors S. J. Pearton (1), W. T. Lim (1), J....
-
Upload
annabel-bishop -
Category
Documents
-
view
217 -
download
0
Transcript of Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors S. J. Pearton (1), W. T. Lim (1), J....
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
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