Transcript Outline - Prof. Stephen J. Pearton's Research Group

Process Optimization and Development
for
ZnO Optoelectronics and Photodiodes
Jon Wright
Dept. of Materials Science and Engineering,
Univ. of Florida, Gainesville, FL
Jan 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
Property
Lattice parameters at 300 K (nm)
• Direct, wide bandgap
• High excitonic binding
energy – 60 meV
Density (g cm-3)
Stable phase at 300 K
Melting point (ºC)
Thermal conductivity
Linear thermal expansion coefficient
• Inexpensive growth
• Easily etched
– (acids and alkalis)
Static dielectric constant
a0: 0.32495
c0: 0.52069
5.606
Wurtzite
1975
0.6, 1-1.2
a0: 6.5  10-6
c0: 3.0  10-6
8.656
Refractive index
2.008, 2.029
Energy bandgap (eV)
Direct, 3.37
Intrinsic carrier concentration (cm-3)
Exciton binding energy (meV)
• Radiation stability
Value
<106
max n-type doping: n ~
1020
max p-type doping: p ~
1017
60
Electron effective mass
0.24
Electron Hall mobility, n-type at 300 K (cm2V1s-1)
200
Hole effective mass
0.59
Hole Hall mobility, p-type at 300 K (cm2V-1s-1)
5 - 50
ZnO vs. GaN
Bandgap (eV)
µe (cm2/V-sec)
µh (cm2/V-sec)
me
mh
Exciton binding
energy (meV)
GaN
3.4
220
10
0.27mo
0.8mo
28
ZnO
3.2
200
5-50
0.24mo
0.59mo
60
• 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
Potential Applications
UV/Blue optoelectronics
Transparent transistors
Nanoscale detectors
Spintronic devices
• Nanostructures demonstrated
• Ferromagnetism at practical Tc
when doped with transition
metals
• Obstacle: good quality,
reproducible p-type
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-ZnO
K. Ip et al. AIP (2004).
Schottky Contacts to ZnO
• Schottky Obstacles
– Surface states
– Defects @ surface layer
– Metal/ZnO intermixing
•
  qb
J  A T exp
 kT
**
2
  qV  
exp
  1
  nkT  
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
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
– Correct ion dosage
– Limiting residual damage
– Maximizing acceptors
+
0.03 N implanted ZnO
Current(A)
• Questions:
0.04
0.02
0.01
0.00
-0.01
-0.02
-0.03
-0.04
-15
• Need for understanding
– Damage accumulation
– Thermal stability of defects
600C, O2, 2 mins.
800C, O2, 2 mins.
950C, O2, 2 mins.
-10
-5
0
5
Voltage(V)
10
15
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
k
 qb 
 C  ** exp

qA T
 kT 
 2  m*
S
b
 C  exp
  N
D





Methodology – Ion Implantation
2.0x10
19
1.5x10
19
1.0x10
19
5.0x10
18
-2
140 keV; 2.4e14 cm
-2
65 keV; 9e13 cm
-2
30 keV; 5e13 cm
-2
10 keV; 2e13 cm
Sum
-3
•
N, P, As dopants @ doses 1013-1014
cm-2
Implantation temp varied RT – 300°C
Concentration(cm )
•
•
Annealed between 600 – 950°C
– RTA
– PLD chamber, O2 ambient (in-situ)
•
Hall measurements used to calculate:
– Carrier type
– Carrier density
– Acceptor ionization energy
0.0
0
•
Use of Oxygen to reduce vacancies
•
Depth Profiles by AES/SIMS
1000
2000
3000
Depth(A)
4000
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
2
-4
10
10
8
4
-5
10
2
-6
10
0
200
400
600
800
Annealing Temperature (C)
1000
 square)
Specific Contact Resistance (
6
Sheet Resistance (
 cm )
Prelim Research – Ir/Au Ohmic Contacts
Ir/Au Contacts – AES Profiles
As-Deposited
100
Atomic Concentration (%)
Ir
80
70
60
Zn
O
50
40
30
10
70
60
Zn
50
O
40
30
C
10
100 200 300 400 500 600 700 800
0
0
0
80
C
20
20
Sputter Depth (Å )
Ir
Au
90
Atomic Concentration (%)
Au
90
1000 C Anneal
100
0
100 200 300 400 500 600 700 800
Sputter Depth (Å )
Only slight intermixing btw Au and Ir layers until 800°C(+)
2
-4
100
-4
80
-4
60
5x10
4.5x10
-4
3.5x10
40
-4
3x10
20
-4
2.5x10
-4
2x10
0
3
6
 /square)
Specific Contact Resistance (
4x10
Sheet Resistance (
 cm )
Ir/Au Contacts – Thermal Stability
Pre-anneal
9 12 15 18 21 24 27 30
Days Annealed @ 350C
No change to Rsh after 30 days
30 Days
5
1
 /square)
10
Nitrogen Anneal
Oxygen Anneal
Specific Contact Resistance (
0.1
1E-3
1E-4
0
4
10
Nitrogen Anneal
Oxygen Anneal
3
0.01
200
400
600
800
Sheet Resistance (
2
 cm )
Ir/Au Contacts – N2 vs. O2 Anneal
Annealing Temperature (C)
Resistance increased w/ O2 anneal – IrO2 layer
10
2
10
1
10
0
200
400
600
Annealing Temperature (C)
800
Ir/Au Contacts – N2 vs. O2 Anneal
O2 Anneal
Au
Ir
70
60
Zn
50
O
40
30
Atomic Concentration (%)
Atomic Concentration (%)
80
Ir
Au
90
90
80
70
60
Zn
50
O
40
30
20
20
10
0
N2 Anneal
100
100
10
C
0
0
100 200 300
400 500
600 700 800 900 1000
Sputter Depth (Å )
C
0
100
200
300
400
500
600
700
Sputter Depth (Å )
AES can not detect IrO2 layer, however more interdiffusion of Ir w/ N2 anneal
800
Prelim Research – Surface Treatment
7
6
Intensity (a.u.)
5
AsDep
Anneal
Ozone
H3PO4
O2
BCl3
Ar
4
3
2
1
0
1.5
2.0
2.5
Energy (eV)
3.0
3.5
Surface Treatment – IV Character
Surface
Treatment
0.04
Current (A)
0.02
0.00
Ar
Ozone
O2
BCl3
Anneal
H3PO4
-0.02
-0.04
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Voltage (V)
0.4
0.6
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
0.8
All treatments result in Ohmic contacts except for Oxygen plasma.
Investigation Timeline
Plan
2005
Fall
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
2006
Spring
Summer
2007
Fall
Spring
Summer
2008
Fall
Spring
Summer
2009
Fall
Spring
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