The document summarizes the development and characterization of a tunable AlGaN-based solar-blind UV-sensitive Schottky photodiode. It discusses the background of UV-sensitive sensor materials and devices, properties of UV radiation and its applications. It also covers the fundamentals of wide-bandgap semiconductors, AlGaN semiconductors, Schottky diodes and photocurrent generation. The experimental section details the electrical and spectral characterization methodology, as well as the optoelectronic characterization system used.

1. Introduction Experimental Results Conclusions References Acknowledgements Done
Development and characterization of a
tuneable AlGaN-based solar-blind
UV-sensitive Schottky photodiode
Louwrens van Schalkwyk
Supervisor: Prof. W.E. Meyer
Co-supervisor: Prof. F.D. Auret
Electronic Materials & Thin Film Research Group
Department of Physics, University of Pretoria
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 1/44

2. Introduction Experimental Results Conclusions References Acknowledgements Done
Introduction
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 1/44

3. Introduction Experimental Results Conclusions References Acknowledgements Done
Background
Tuneable UV-sensitive Sensor Materials & Devices
Financier
(Advanced Electronics Programme)
Consortium Member
(Device development)
Consortium Member
(Research on MgZnO)
Consortium Member
(Research on AlGaN)
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 2/44

4. Introduction Experimental Results Conclusions References Acknowledgements Done
Electromagnetic radiation
Spectral solar irradiance model from the Sun at sea-level
UV VIS NIR
200 600 1000 1400 1800
0
0.5
1
1.5
2
2.5
Wavelength [nm]
SolarirradianceW·m2
·nm
−1
Extraterrestrial
Terrestrial
5770 K Blackbody
5 4 3 2 1
Photon energy (hν) [eV]
The ASTM E490-00 extraterrestrial,
ASTM G173-03 (direct normal) terrestrial
and ideal 5770 K blackbody terrestrial
spectral solar irradiance of the Sun
[NREL, 2010]1
.
1
From ASTM terrestrial reference spectra data for photovoltaic performance evaluation
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 3/44

5. Introduction Experimental Results Conclusions References Acknowledgements Done
UV radiation
Wavelength region & classification
VUV UV-C UV-B UV-A
100 150 200 250 300 350 400
0.0
0.2
0.4
0.6
Wavelength [nm]
SolarirradianceW·m2
·nm
−1
9 8 7 6 5 4
Photon energy (hν) [eV]
O2 absorption
O3 absorption
Magnified UV region of spectral
solar irradiance from the Sun.
VUV is absorbed by O2.
The Sun does radiate deleterious
UV-C, but UV-C is absorbed by O2
and stratospheric ozone.
UVR reaching the Earth’s surface
consists of 10 % carcinogenic
UV-B, partially absorbed by
stratospheric ozone, and 90 %
UV-A that stimulates photo-
synthesis and pigmentation.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 4/44

6. Introduction Experimental Results Conclusions References Acknowledgements Done
UV radiation
Applications
Fields of application
UV-sensitive sensor
application type
Civil Industrial Military Medical Scientific
Biological
UV-B
dosimeter
Greenhouse
agrotechnics
Detect organic
compounds
Blood
disinfection
Vitamin synthesis
Chemical Detect agents Resin curing Analize agents Photochemistry
Combustion
Flame
safeguard
Combustion
engineering
Plume
detection
Optical
communication
Secure non-
line-of-sight
ground-based
communication
UV-emitter calibration
Pollution
monitoring
Large area
sterilization
Plasma research
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 5/44

7. Introduction Experimental Results Conclusions References Acknowledgements Done
UV radiation
Detection – Operation of current and future electro-optic devices
Photomultiplier tubes and Si-based electro-optic systems
L
E
N
S
F
I
L
T
E
R
Image
Intensifier
Source Electronics
(intensity
loss)
(Si-based photodetector)
UV-CVIS
UV
IR
VIS
UV
IR
VIS
UV-C–sensitive sensor
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 6/44

8. Introduction Experimental Results Conclusions References Acknowledgements Done
UV radiation
Detection – Operation of current and future electro-optic devices
Photomultiplier tubes and Si-based electro-optic systems
L
E
N
S
F
I
L
T
E
R
Image
Intensifier
Source Electronics
(intensity
loss)
(Si-based photodetector)
UV-CVIS
UV
IR
VIS
UV
IR
VIS
UV-C–sensitive sensor
WBG-based photodetector electro-optic systems
L
E
N
S
Source Electronics
(WBG material-based photodetector)
VIS
UV
IR
UV-C
UV-C–sensitive sensor
VIS
UV
IR
(only UV-C
detected)
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 6/44

9. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronics
What are wide-bandgap semiconductors?
E(electronenergy)
EF
E0
EC
EV
Eg
(a)
Filled band
Bandgap
Filled
states
(in valence band)
Empty
states
(in valence band)
Bandgap
Empty
conduction
band
(b)
Filled band
Bandgap
Filled
valence
band
Empty
conduction
band
Bandgap
Empty
conduction
band
(c)
Filled band
Bandgap
Filled
valence
band
Bandgap
Empty
conduction
band
(d)
Filled
valence
band
Wide-
bandgap
Empty
conduction
band
The four possible electron band structures in solids at 0 K. Adapted from Callister [2003:616].
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 7/44

10. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronics
A direct wide-bandgap semiconductor
E
EC
EV
Eg
0
–
hν = Eg
¯p
(1)
(2)
Empty conduction band
Filled valence band
– denotes electron in conduction band
denotes hole in valence band
An energy–momentum (E–¯p ) space diagram of a direct-bandgap semiconductor together with
an illustrated threshold (1) and arbitrary (2) direct band-to-band optical transition.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 8/44

11. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronics
The AlGaN semiconductor
0.0 0.2 0.4 0.6 0.8 1.0
3.5
4.0
4.5
5.0
5.5
6.0
Al mole fraction (x)
Bandgap[eV]
360
340
320
300
280
260
240
220
200
Wavelength[nm]
Eg (AlxGa1−xN) = 3.42(1 − x) + 6.2x − 0.8x(1 − x)
λ (AlxGa1−xN) = 1240
Eg
Variation of the bandgap at 300 K for
the AlxGa1−xN semiconductor. The
relation was determined from
samples grown in AlN nucleation
(measured by PDS) as a function of
the Al fraction in the solid phase
(measured by calibrated EDS).
Adapted from Omnés et al.
[1999:5288].
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 9/44

12. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronics
The Schottky diode
E(electronenergy)
EFm
E0
E0
EC
EFs
EV
qφm
qχs
Eg
metal n-type
semiconductor
x
neutral
WD
qφB
qψbi
– – – – –
ED+ + + + +
+
+
+ + + + + +
−
−
−
−
−
−
−
Qsc
Qm
E
e−
mse−
sm
Detailed energy-level
diagram of an ideal
metal–n-semiconductor
contact at thermal equili-
brium (without an interfacial
layer and interface states).
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 10/44

13. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronics
Photocurrent generation in a Schottky photodiode
EFm
E0
E0
EC
EFs
EV
Eg
neutral
WD
qφB– – – – –
ED+ + + + +
+
+
+ + + + + +
−
−
−
−
−
−
−
E
hν ≥ Eg
–
An energy-level diagram
illustrating the photovoltaic
process, whereby a direct
band-to-band photoexcited
electron-hole pair in the
depletion region is separated
by the built-in electric field.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 11/44

14. Introduction Experimental Results Conclusions References Acknowledgements Done
Experimental
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 11/44

15. Introduction Experimental Results Conclusions References Acknowledgements Done
Front irradiated photodiodes
. . . challenges and solutions . . .
Choosing a Schottky barrier material
Irradiated on the same side as the Schottky contact.
Schottky barrier materials must be optically transparent.
Especially, high optical transmittance to UV radiation.
High work function semitransparent metals, such as Pt and Pd have high
reflectance to UV radiation.
IrO2 was effectively used by Kim et al. [2002] for GaN MSM UV-sensitive
photodetectors.
IrO2 has > 5 eV work function and low resistivity of approx. 50 µΩ·cm.
And high optical transmittance to UV radiation.
Therefore the investigation of Ir Schottky contacts on AlGaN semiconductors.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 12/44

16. Introduction Experimental Results Conclusions References Acknowledgements Done
Front irradiated photodiodes
. . . challenges and solutions . . .
Choosing a Schottky barrier material
Irradiated on the same side as the Schottky contact.
Schottky barrier materials must be optically transparent.
Especially, high optical transmittance to UV radiation.
High work function semitransparent metals, such as Pt and Pd have high
reflectance to UV radiation.
IrO2 was effectively used by Kim et al. [2002] for GaN MSM UV-sensitive
photodetectors.
IrO2 has > 5 eV work function and low resistivity of approx. 50 µΩ·cm.
And high optical transmittance to UV radiation.
Therefore the investigation of Ir Schottky contacts on AlGaN semiconductors.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 12/44

17. Introduction Experimental Results Conclusions References Acknowledgements Done
Characterization
Electrical characterization
Electrical parameters of interest
I–V measurements
Ideality factor (η)
Barrier height (φB)
Reverse leakage current
density (JR)
Series resistance (RS)
C–V measurements
Free carrier concentration
(Nd = ND − NA)
Electrical characterization system
HP4140B pA meter/DC voltage source for I–V measurements.
HP4192A LF Impedance Analyser for C–V measurements.
LabVIEW routine for measurements and analysis.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 13/44

18. Introduction Experimental Results Conclusions References Acknowledgements Done
Characterization
Spectral characterization
Spectral parameters of interest
Cut-off wavelength (λcut-off) in [nm]
Responsivity [A/W] at a specific wavelength (Rλ)
Quantum efficiency (ηλ)
UV-to-VIS/NIR rejection ratio
Spectral characterization system
EMR source(s) emitting from 200 nm to 1100 nm.
Monochromator to allow for wavelength selection.
An optical fibre that guides the EMR to the photodiode.
Optimized equipment for use in the UV region.
Calibration unit to calibrate radiating source.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 14/44

19. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Construction overview
Optical Fibre
Monochromator
(λ selection)
Filter
& Optics
Φe Φλ Eλ
Shielded Enclosure
Programmable pA meter/
DC voltage source
Schematic illustration of optoelectronic characterization system used for the electrical and
spectral characterization of UV-sensitive photodiodes.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 15/44

20. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Construction overview
A photograph of the optoelectronic characterization system.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 16/44

21. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Calibration theory and photodetector substitution method
d Φλ [W]
Source
dAs m2
s
θs
ˆns
Ls = d 2
Φλ
dAs cos θs d Ωr
W · m−2
s · sr−1
zsr [msr]
line-of-sight
Receiver
dAr m2
r
θr
ˆnr
Er = d Φλ
dAr
W · m−2
r
d2
Φλ Rλ
A · W−1
d2
Jp
Schematic illustration of a radiating source and an irradiated receiver.
After using a standard detector for calibration, the irradiance at the PD can be
calculated from
EPD = ESD
zSD
zPD
2
W/cm
2
(1)
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 17/44

22. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Calibration theory – calculating Rλ and ηλ
Responsivity is determined from
Rλ =
Jp
EPD (λ)
A · W
−1
, (2)
where EPD (λ) is the irradiance at the PD and Jp the photocurrent density at a
specific wavelength.
Quantum efficiency is then calculated from:
ηλ =
1240
λ [nm]
Rλ. (3)
UV-to-VIS/NIR rejection ratio calculation:
Jp (λs) /Jp (λ ≥ λs) =
Jp (λs)
|Jp (λi) − Jd|
. (4)
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 18/44

23. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Control and measurement
A screenshot of the Front Panel of the spectral characterization NI LabVIEW routine.
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24. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Properties of the optical fibre
Full-acceptance
angle (2θNA)
Only EMR falling in at an angle
≤ θNA (half-acceptance angle)
is guided along the optical fibre
angle ≤ confinement angle
Spatial irradiance
distribution
Ee(0)
e 2
EeOptical fibre axis
Spot-size of
radius w (z )
EMR exiting
at θin < θNA
EMR entering
at θin ≤ θNA
Cladding – n1
Core – n0
angle ≤ critical angle for
total internal reflection
θNA
θin
θout
w (0)
z [m]
w (z )
x or y
Schematic illustration of a step-index optical fibre and some of its properties.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 20/44

25. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Calibration of lamps – UV lamp
200 220 240 260 280 300 320 340 360 380 400
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Wavelength [nm]
Irradianceat90mmnW·cm−2
3.54.04.55.05.56.0
Photon energy (hν) [eV]
Calibrated UV spectral irradiance of the 30 W deuterium lamp for wavelengths ranging from
from 200 nm to 400 nm in 5 nm increments.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 21/44

26. Introduction Experimental Results Conclusions References Acknowledgements Done
Optoelectronic characterization system
Calibration of lamps – tungsten-halogen lamp
400 500 600 700 800 900 1000 1100
0.0
10
20
30
40
50
Wavelength [nm]
Irradianceat90mmnW·cm−2
Without 400 nm filter
With 400 nm filter
1.52.02.53.0
Photon energy (hν) [eV]
Calibrated VIS-to-NIR spectral irradiance of the 30 W tungsten-halogen lamp set to 3.50 A for
wavelengths ranging from 350 nm to 1100 nm in 5 nm increments, with and without the
400 nm order sorting filter.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 22/44

27. Introduction Experimental Results Conclusions References Acknowledgements Done
Results
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 22/44

28. Introduction Experimental Results Conclusions References Acknowledgements Done
Schottky photodiode fabrication
The AlGaN semiconductor samples
1 Layered ohmic structure of Ti/Al/Ni/Au
(150/2000/450/500 Å)
2 Ohmic contact was two-step annealed under
Ar ambient for five minutes at both
500°C and 700°C.
3 Two circular 0.65 mm-diameter and four
1.0 mm-diameter, 50 Å-thick Ir Schottky
contacts were deposited.
4 Ir Schottky contacts were annealed at
700°C under O2 ambient for 20 minutes.
5 Annealing resulted in the barely visible IrO2
Schottky contacts.
6 Six circular 0.30 mm-diameter, 1000 Å-thick
Au probe contact pads were deposited (and
later annealed).
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29. Introduction Experimental Results Conclusions References Acknowledgements Done
Schottky photodiode fabrication
I–V characteristics of a typical Schottky photodiode
−2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0 1.5 2.0
10−15
10−12
10−9
10−6
10−3
Voltage [V]
Current[A]
As-dep.
Anneal 700°C in O2
IrO2/Au contact
3 × 1013
He+
cm−2
Reprinted from van Schalkwyk et al. [2012:1530], Copyright (2011), with permission from Elsevier.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 24/44

30. Introduction Experimental Results Conclusions References Acknowledgements Done
Schottky photodiode fabrication
The spectral responsivities of a typical Schottky photodiode
200 220 240 260 280 300 320 340 360
0
10
20
30
40
50
60
70
Wavelength [nm]
ResponsivitymA·W−1
As-dep.
Anneal 700°C in O2
IrO2/Au contact
3 × 1013
He+
cm−2
4.04.55.05.56.0
Photon energy (hν) [eV]
Adapted from van Schalkwyk et al. [2012:1531], Copyright (2011), with permission from Elsevier.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 25/44

31. Introduction Experimental Results Conclusions References Acknowledgements Done
Schottky photodiode fabrication
Parameters obtained from optoelectronic characterization
Average electrical parameters. Adapted from van Schalkwyk et al. [2012:1530], Copyright
(2011), with permission from Elsevier.
Fabrication η φB0
+− 15 % J R at −1 V R S
+− 25 % Nd
step [eV] nA · cm−2
[Ω] 1018
cm−3
As-dep. 1.63 1.34 0.18 +− 0.23 1200 1.1
Annealed 1.38 1.52 5.2 +− 8.4 250 0.87
IrO2/Au 1.57 1.19 49 +− 84 100 2.3
He+ 1.86 1.17 0.080 +− 0.036 310 0.94
Average spectral parameters.
Fabrication λcut-off R275 η275
step [nm] mA · W−1
[ % ]
As-dep. 295 48 23
Annealed 295 40 18
IrO2/Au 295 52 23
He+ 295 64 29
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32. Introduction Experimental Results Conclusions References Acknowledgements Done
Schottky photodiode fabrication
Parameters obtained from optoelectronic characterization
Average electrical parameters. Adapted from van Schalkwyk et al. [2012:1530], Copyright
(2011), with permission from Elsevier.
Fabrication η φB0
+− 15 % J R at −1 V R S
+− 25 % Nd
step [eV] nA · cm−2
[Ω] 1018
cm−3
As-dep. 1.63 1.34 0.18 +− 0.23 1200 1.1
Annealed 1.38 1.52 5.2 +− 8.4 250 0.87
IrO2/Au 1.57 1.19 49 +− 84 100 2.3
He+ 1.86 1.17 0.080 +− 0.036 310 0.94
Average spectral parameters.
Fabrication λcut-off R275 η275
step [nm] mA · W−1
[ % ]
As-dep. 295 48 23
Annealed 295 40 18
IrO2/Au 295 52 23
He+ 295 64 29
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 26/44

33. Introduction Experimental Results Conclusions References Acknowledgements Done
Schottky photodiode fabrication
Conclusions
Electrical characterization – as-dep. compared to after dep. of Au probe contact
Ideality factor improved slightly from 1.63 to 1.57.
Schottky barrier height decreased from (1.34 to 1.19) eV.
Reverse leakage current density increased from approx. (0.2 to 50) nA · cm−2
.
Series resistance decreased from (1200 to 100) Ω.
Nd has decreased by at least 30 % after He+
irradiation.
Spectral characterization – as-dep. compared to after dep. of Au probe contact
λcut-off = 295 nm (4.2 eV), corresponding to bandgap of 35 % mole fraction Al.
Spectral responsivity increased from (48 to 52) mA/W at 275 nm.
Quantum efficiency stayed at 23 % at 275 nm.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 27/44

34. Introduction Experimental Results Conclusions References Acknowledgements Done
Schottky photodiode fabrication
Conclusions (continued)
Electrical characterization of an annealed Au probe contact
Ideality factor was 1.42 +− 0.07.
Schottky barrier height of 1.33 (1 +− 10 %) eV.
Reverse leakage current density was (3.3 +− 3.4) nA · cm−2
.
Series resistance of 66 (1 +− 45 %) Ω.
Nd of 4.8 (1 +− 30 %) × 1018
cm−3
.
Spectral characterization of an annealed Au probe contact
Spectral responsivity (58 +− 30) mA/W at 275 nm.
Quantum efficiency of (26 +− 14) % at 275 nm.
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35. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Photograph of the first fabricated four-quadrant detector
1 Two large 0.80 mm-diameter contacts on either
side were used for mask alignment.
2 Four outer strips are optimized layered ohmic
structures of Ti/Al/Ni/Au (300/1800/400/1500 Å).
3 Ohmic contacts were multi-step annealed
under Ar ambient for three minutes at 400°C,
20 s at 700°C, 30 s at 830°C and 30 s at
900°C.
4 Four square (0.90 × 0.90) mm2
, 100 Å-thick
Ir Schottky contacts were deposited.
5 Schottky’s were two-step annealed under O2
ambient for 30 minutes at both 600°C and
730°C to form the more UV transmissive
IrO2.
6 Four 0.80 × 10−3
cm2
, 1500 Å-thick
triangular Au probe contact pads were
deposited on top of the outer-most corners of
the square IrO2 contacts.
7 Au probe contacts were annealed under O2
ambient at 500°C for ten minutes.
8 Final photosensitive area of each quadrant
was 7.3 × 10−3
cm2
.
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36. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Photograph of the second fabricated four-quadrant detector
Wires were bonded to the (3.0 × 5.0) mm2
sample by starting from either an ohmic contact or
Au probe contact pad and then bonded to a strip on the microchip carrier.
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37. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Photograph of the second fabricated four-quadrant detector
A closer view of the four-quadrant detector using a different lighting technique and rotated
90° clockwise with respect to the previous photograph.
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38. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
The I–V characteristics of the second four-quadrant detector
−2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0 1.5 2.0
10−15
10−12
10−9
10−6
10−3
Voltage [V]
Current[A]
B2-1
B2-2
B2-3
B2-4
Adapted from van Schalkwyk et al. [2014:94], Copyright (2013), with permission from Elsevier.
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39. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
The spectral responsivities of the second four-quadrant detector
200 220 240 260 280 300 320 340 360
0
5
10
15
20
25
30
35
40
Wavelength [nm]
ResponsivitymA·W−1
B2-1
B2-2
B2-3
B2-4
4.04.55.05.56.0
Photon energy (hν) [eV]
Adapted from van Schalkwyk et al. [2014:95], Copyright (2013), with permission from Elsevier.
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40. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
The UV-to-VIS/NIR rejection of the second four-quadrant detector
400 500 600 700 800 900 1000 1100
102
103
104
105
106
Wavelength [nm]
UV-to-VIS/NIRRejectionRatio[−]
B2-1
B2-2
B2-3
B2-4
1.52.02.53.0
Photon energy (hν) [eV]
Adapted from van Schalkwyk et al. [2014:95], Copyright (2013), with permission from Elsevier.
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41. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Electrical parameters from optoelectronic characterization
Adapted from van Schalkwyk et al. [2014:94], Copyright (2013), with permission from Elsevier.
η φB0 J R at −1 V R S Nd
Quadrant [eV] nA · cm−2
[Ω] 1018
cm−3
B2-1 1.92 1.30 0.46 90 2.0
B2-2 2.09 1.15 1.1 150 1.7
B2-3 1.91 1.24 0.38 140 1.3
B2-4 1.95 1.18 6.5 110 1.5
Avg.: 1.97 1.22 2.1 120 1.6
Std. dev.: 0.09 0.08 3.3 30 0.3
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 35/44

42. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Spectral parameters from optoelectronic characterization
Adapted from van Schalkwyk et al. [2014:95], Copyright (2013), with permission from Elsevier.
λcut-off R250 η250 UV-to-VIS/NIR
Quadrant [nm] mA · W−1
[%] [Jp (λs) /Jp (λ ≥ 400)]
B2-1 275 28 14 102
to 105
B2-2 275 28 14 102
to 104
B2-3 275 30 15 102
to 105
B2-4 275 26 13 102
to 105
Avg.: 275 28 14 102
to 105
Std. dev.: 1.0 0.5
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 36/44

43. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Conclusions
• Contacts successfully deposited using three different physical contact masks.
• Detector mounted and epoxy wire bonded onto commercial microchip carrier.
Higher η (1.97) possibly a result of 500°C annealing of Au probe contact.
Schottky barrier height (φB) was (1.22 +− 0.08) eV.
Although Nd was high (1018
cm−3
), J R was less than 65 nA · cm−2
and either
attributed to the proper formation of the IrO2 Schottky contact or by annealing the
properly deposited Au probe contact pads.
R S of (120 +− 30) Ω was low, but higher than (66 +− 28) Ω and possibly due to the
addition of the silver-filled epoxy and Al wires.
λcut-off = (275 +− 5) nm (4.51 eV) ≡ bandgap of approx. 46 % mole fraction Al.
R250 = (28 +− 1.0) mA · W−1
and ηλ = (14 +− 0.5) % was relatively low.
UV-to-VIS/NIR rejection ratio between 103
and 105
proved detector solar-blind.
Uniformity of characteristics allowed for use in demonstrating a working solar-blind
UV-sensitive electro-optic device.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 37/44

44. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Conclusions
• Contacts successfully deposited using three different physical contact masks.
• Detector mounted and epoxy wire bonded onto commercial microchip carrier.
Higher η (1.97) possibly a result of 500°C annealing of Au probe contact.
Schottky barrier height (φB) was (1.22 +− 0.08) eV.
Although Nd was high (1018
cm−3
), J R was less than 65 nA · cm−2
and either
attributed to the proper formation of the IrO2 Schottky contact or by annealing the
properly deposited Au probe contact pads.
R S of (120 +− 30) Ω was low, but higher than (66 +− 28) Ω and possibly due to the
addition of the silver-filled epoxy and Al wires.
λcut-off = (275 +− 5) nm (4.51 eV) ≡ bandgap of approx. 46 % mole fraction Al.
R250 = (28 +− 1.0) mA · W−1
and ηλ = (14 +− 0.5) % was relatively low.
UV-to-VIS/NIR rejection ratio between 103
and 105
proved detector solar-blind.
Uniformity of characteristics allowed for use in demonstrating a working solar-blind
UV-sensitive electro-optic device.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 37/44

45. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Conclusions
• Contacts successfully deposited using three different physical contact masks.
• Detector mounted and epoxy wire bonded onto commercial microchip carrier.
Higher η (1.97) possibly a result of 500°C annealing of Au probe contact.
Schottky barrier height (φB) was (1.22 +− 0.08) eV.
Although Nd was high (1018
cm−3
), J R was less than 65 nA · cm−2
and either
attributed to the proper formation of the IrO2 Schottky contact or by annealing the
properly deposited Au probe contact pads.
R S of (120 +− 30) Ω was low, but higher than (66 +− 28) Ω and possibly due to the
addition of the silver-filled epoxy and Al wires.
λcut-off = (275 +− 5) nm (4.51 eV) ≡ bandgap of approx. 46 % mole fraction Al.
R250 = (28 +− 1.0) mA · W−1
and ηλ = (14 +− 0.5) % was relatively low.
UV-to-VIS/NIR rejection ratio between 103
and 105
proved detector solar-blind.
Uniformity of characteristics allowed for use in demonstrating a working solar-blind
UV-sensitive electro-optic device.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 37/44

46. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
Conclusions
• Contacts successfully deposited using three different physical contact masks.
• Detector mounted and epoxy wire bonded onto commercial microchip carrier.
Higher η (1.97) possibly a result of 500°C annealing of Au probe contact.
Schottky barrier height (φB) was (1.22 +− 0.08) eV.
Although Nd was high (1018
cm−3
), J R was less than 65 nA · cm−2
and either
attributed to the proper formation of the IrO2 Schottky contact or by annealing the
properly deposited Au probe contact pads.
R S of (120 +− 30) Ω was low, but higher than (66 +− 28) Ω and possibly due to the
addition of the silver-filled epoxy and Al wires.
λcut-off = (275 +− 5) nm (4.51 eV) ≡ bandgap of approx. 46 % mole fraction Al.
R250 = (28 +− 1.0) mA · W−1
and ηλ = (14 +− 0.5) % was relatively low.
UV-to-VIS/NIR rejection ratio between 103
and 105
proved detector solar-blind.
Uniformity of characteristics allowed for use in demonstrating a working solar-blind
UV-sensitive electro-optic device.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 37/44

47. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
The UV-C–sensitive electro-optic demonstration device
The working solar-blind UV-sensitive electro-optic demonstration device (left) with LED
indicators at the back (right). Photos with the courtesy of the CSIR MSM unit.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 38/44

48. Introduction Experimental Results Conclusions References Acknowledgements Done
The four-quadrant detector
The UV-C–sensitive electro-optic demonstration device
The epoxy wire-bonded four-quadrant detector mounted on a commercial microchip carrier
inside the housing behind a protective quartz glass (courtesy of the CSIR MSM unit).
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 39/44

49. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 39/44

50. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
General
• An optoelectronic characterization system was constructed and calibrated against
standards traceable to the NIST, using the photodetector substitution method.
• An UV-sensitive Schottky PD fabrication procedure was established.
• An optimized tuneable front-irradiated AlGaN-based solar-blind UV-sensitive Schottky
PD would consist of
1 a Ti/Al/Ni/Au (300/1800/400/1500 Å) ohmic contact that was multi-step annealed
under Ar ambient for three minutes at 400°C, 20 s at 700°C, 30 s at 830°C and
30 s at 900°C,
2 an UV transmissive IrO2 Schottky contact, formed when 50 Å-thick Ir was
two-step annealed under O2 ambient for 30 minutes at both 600°C and 730°C,
3 and a 1500 Å-thick Au probe contact on top of the Schottky contact that was
annealed under O2 ambient at 350°C for ten minutes, instead of 500°C.
• Lastly, a four-quadrant detector was developed and used to prove the feasibility of
such a detector in future solar-blind UV-sensitive electro-optic systems.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 40/44

51. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
General
• An optoelectronic characterization system was constructed and calibrated against
standards traceable to the NIST, using the photodetector substitution method.
• An UV-sensitive Schottky PD fabrication procedure was established.
• An optimized tuneable front-irradiated AlGaN-based solar-blind UV-sensitive Schottky
PD would consist of
1 a Ti/Al/Ni/Au (300/1800/400/1500 Å) ohmic contact that was multi-step annealed
under Ar ambient for three minutes at 400°C, 20 s at 700°C, 30 s at 830°C and
30 s at 900°C,
2 an UV transmissive IrO2 Schottky contact, formed when 50 Å-thick Ir was
two-step annealed under O2 ambient for 30 minutes at both 600°C and 730°C,
3 and a 1500 Å-thick Au probe contact on top of the Schottky contact that was
annealed under O2 ambient at 350°C for ten minutes, instead of 500°C.
• Lastly, a four-quadrant detector was developed and used to prove the feasibility of
such a detector in future solar-blind UV-sensitive electro-optic systems.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 40/44

52. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
General
• An optoelectronic characterization system was constructed and calibrated against
standards traceable to the NIST, using the photodetector substitution method.
• An UV-sensitive Schottky PD fabrication procedure was established.
• An optimized tuneable front-irradiated AlGaN-based solar-blind UV-sensitive Schottky
PD would consist of
1 a Ti/Al/Ni/Au (300/1800/400/1500 Å) ohmic contact that was multi-step annealed
under Ar ambient for three minutes at 400°C, 20 s at 700°C, 30 s at 830°C and
30 s at 900°C,
2 an UV transmissive IrO2 Schottky contact, formed when 50 Å-thick Ir was
two-step annealed under O2 ambient for 30 minutes at both 600°C and 730°C,
3 and a 1500 Å-thick Au probe contact on top of the Schottky contact that was
annealed under O2 ambient at 350°C for ten minutes, instead of 500°C.
• Lastly, a four-quadrant detector was developed and used to prove the feasibility of
such a detector in future solar-blind UV-sensitive electro-optic systems.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 40/44

53. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
General
• An optoelectronic characterization system was constructed and calibrated against
standards traceable to the NIST, using the photodetector substitution method.
• An UV-sensitive Schottky PD fabrication procedure was established.
• An optimized tuneable front-irradiated AlGaN-based solar-blind UV-sensitive Schottky
PD would consist of
1 a Ti/Al/Ni/Au (300/1800/400/1500 Å) ohmic contact that was multi-step annealed
under Ar ambient for three minutes at 400°C, 20 s at 700°C, 30 s at 830°C and
30 s at 900°C,
2 an UV transmissive IrO2 Schottky contact, formed when 50 Å-thick Ir was
two-step annealed under O2 ambient for 30 minutes at both 600°C and 730°C,
3 and a 1500 Å-thick Au probe contact on top of the Schottky contact that was
annealed under O2 ambient at 350°C for ten minutes, instead of 500°C.
• Lastly, a four-quadrant detector was developed and used to prove the feasibility of
such a detector in future solar-blind UV-sensitive electro-optic systems.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 40/44

54. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
General
• An optoelectronic characterization system was constructed and calibrated against
standards traceable to the NIST, using the photodetector substitution method.
• An UV-sensitive Schottky PD fabrication procedure was established.
• An optimized tuneable front-irradiated AlGaN-based solar-blind UV-sensitive Schottky
PD would consist of
1 a Ti/Al/Ni/Au (300/1800/400/1500 Å) ohmic contact that was multi-step annealed
under Ar ambient for three minutes at 400°C, 20 s at 700°C, 30 s at 830°C and
30 s at 900°C,
2 an UV transmissive IrO2 Schottky contact, formed when 50 Å-thick Ir was
two-step annealed under O2 ambient for 30 minutes at both 600°C and 730°C,
3 and a 1500 Å-thick Au probe contact on top of the Schottky contact that was
annealed under O2 ambient at 350°C for ten minutes, instead of 500°C.
• Lastly, a four-quadrant detector was developed and used to prove the feasibility of
such a detector in future solar-blind UV-sensitive electro-optic systems.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 40/44

55. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
General
• An optoelectronic characterization system was constructed and calibrated against
standards traceable to the NIST, using the photodetector substitution method.
• An UV-sensitive Schottky PD fabrication procedure was established.
• An optimized tuneable front-irradiated AlGaN-based solar-blind UV-sensitive Schottky
PD would consist of
1 a Ti/Al/Ni/Au (300/1800/400/1500 Å) ohmic contact that was multi-step annealed
under Ar ambient for three minutes at 400°C, 20 s at 700°C, 30 s at 830°C and
30 s at 900°C,
2 an UV transmissive IrO2 Schottky contact, formed when 50 Å-thick Ir was
two-step annealed under O2 ambient for 30 minutes at both 600°C and 730°C,
3 and a 1500 Å-thick Au probe contact on top of the Schottky contact that was
annealed under O2 ambient at 350°C for ten minutes, instead of 500°C.
• Lastly, a four-quadrant detector was developed and used to prove the feasibility of
such a detector in future solar-blind UV-sensitive electro-optic systems.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 40/44

56. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
General
• An optoelectronic characterization system was constructed and calibrated against
standards traceable to the NIST, using the photodetector substitution method.
• An UV-sensitive Schottky PD fabrication procedure was established.
• An optimized tuneable front-irradiated AlGaN-based solar-blind UV-sensitive Schottky
PD would consist of
1 a Ti/Al/Ni/Au (300/1800/400/1500 Å) ohmic contact that was multi-step annealed
under Ar ambient for three minutes at 400°C, 20 s at 700°C, 30 s at 830°C and
30 s at 900°C,
2 an UV transmissive IrO2 Schottky contact, formed when 50 Å-thick Ir was
two-step annealed under O2 ambient for 30 minutes at both 600°C and 730°C,
3 and a 1500 Å-thick Au probe contact on top of the Schottky contact that was
annealed under O2 ambient at 350°C for ten minutes, instead of 500°C.
• Lastly, a four-quadrant detector was developed and used to prove the feasibility of
such a detector in future solar-blind UV-sensitive electro-optic systems.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 40/44

57. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
Further research
• Quality of proposed layered ohmic structure needs to be quantified in terms of contact
and specific contact resistances and can be obtained through either CTLM or TLM.
• Research in characterizing AlGaN-based UV-sensitive Schottky PDs, using the
recommended fabrication procedures for the ohmic, Schottky and Au probe contacts.
• Certain applications in the 250 nm to 280 nm wavelength range, require the detection
of signals typically lower than 20 fW · cm−2
at 270 nm or incident photon-flux densities
of lower than 3.0 × 104
q · s−1
· cm−2
.
1 Research on fabricating back-irradiated PDs with smaller Schottky contacts
while increasing the quantum efficiency and improving the signal-to-noise ratios.
2 Also, operation with or without gain can be considered.
3 For detecting even lower signals, the understanding of defects within the
material and their effects will be important and, therefore, require defect
characterization studies.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 41/44

58. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
Further research
• Quality of proposed layered ohmic structure needs to be quantified in terms of contact
and specific contact resistances and can be obtained through either CTLM or TLM.
• Research in characterizing AlGaN-based UV-sensitive Schottky PDs, using the
recommended fabrication procedures for the ohmic, Schottky and Au probe contacts.
• Certain applications in the 250 nm to 280 nm wavelength range, require the detection
of signals typically lower than 20 fW · cm−2
at 270 nm or incident photon-flux densities
of lower than 3.0 × 104
q · s−1
· cm−2
.
1 Research on fabricating back-irradiated PDs with smaller Schottky contacts
while increasing the quantum efficiency and improving the signal-to-noise ratios.
2 Also, operation with or without gain can be considered.
3 For detecting even lower signals, the understanding of defects within the
material and their effects will be important and, therefore, require defect
characterization studies.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 41/44

59. Introduction Experimental Results Conclusions References Acknowledgements Done
Conclusions
Further research
• Quality of proposed layered ohmic structure needs to be quantified in terms of contact
and specific contact resistances and can be obtained through either CTLM or TLM.
• Research in characterizing AlGaN-based UV-sensitive Schottky PDs, using the
recommended fabrication procedures for the ohmic, Schottky and Au probe contacts.
• Certain applications in the 250 nm to 280 nm wavelength range, require the detection
of signals typically lower than 20 fW · cm−2
at 270 nm or incident photon-flux densities
of lower than 3.0 × 104
q · s−1
· cm−2
.
1 Research on fabricating back-irradiated PDs with smaller Schottky contacts
while increasing the quantum efficiency and improving the signal-to-noise ratios.
2 Also, operation with or without gain can be considered.
3 For detecting even lower signals, the understanding of defects within the
material and their effects will be important and, therefore, require defect
characterization studies.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 41/44

60. Introduction Experimental Results Conclusions References Acknowledgements Done
References
Calaprice, A. 2011. The Ultimate Quotable Einstein. Princeton University Press.
Callister, Jr., W.D. 2003. Materials Science and Engineering: An Introduction. 6th edn. John Wiley & Sons, Inc.
Kim, Jong Kyu, Jang, Ho Won, Jeon, Chang Min, & Lee, Jong Lam. 2002. GaN metal–semiconductor–metal ultraviolet
photodetector with IrO2 Schottky contact. Appl. Phys. Lett., 81(24), 4655–4657.
NREL. 2010. Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS). [Online] Available at:
www.nrel.gov/rredc/smarts/ [Accessed: 2013-09-26].
Omnés, F., Marenco, N., Beaumont, B., de Mierry, Ph., Monroy, E., Calle, F., & Muñoz, E. 1999. Metalorganic vapor-phase
epitaxy-grown AlGaN materials for visible-blind ultraviolet photodetector applications. J. Appl. Phys., 86(9), 5286–5292.
van Schalkwyk, L., Meyer, W.E., Auret, F.D., Nel, J.M., Ngoepe, P.N.M., & Diale, M. 2012. Characterization of AlGaN-based
metal–semiconductor solar-blind UV photodiodes with IrO2 Schottky contacts. Phys. B: Condens. Matter, 407(10),
1529–1532. (Permission granted under CCC licence no. 3420300755774 for both print and electronic reuse of full
article in thesis or dissertation).
van Schalkwyk, L., Meyer, W.E., Nel, J.M., Auret, F.D., & Ngoepe, P.N.M. 2014. Implementation of an AlGaN-based solar-blind
UV four-quadrant detector. Phys. B: Condens. Matter, 439(Apr.), 93–96. (Permission granted under CCC licence no.
3420660708760 for both print and electronic reuse of full article in thesis or dissertation).
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 42/44

61. Introduction Experimental Results Conclusions References Acknowledgements Done
Acknowledgements
My sincere gratitude to the following people and institutions
My parents Jasper & Hannelie, and my brother André.
My supervisor Prof. W.E. Meyer, co-supervisor Prof. F.D. Auret and the rest of
the staff in the Electronic Materials and Thin Film Research Group.
Mr. J. Wallis, Mr. R. Stolper, Mr. C. Coetzer and Mr. W. van der Westhuizen of
the SST competence area of the CSIR MSM unit.
To the Technology Innovation Agency (TIA) and National Research
Foundation (NRF).
NRF disclaimer: Any opinion, findings and conclusions or recommendations expressed in this material are those of
the author(s) and therefore the NRF does not accept any liability in regard thereto.
My friends, family as well as the rest of the staff and students in the Physics
Department.
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 43/44

62. Introduction Experimental Results Conclusions References Acknowledgements Done
Thank you for your
attendance and attention
F 8 f
Concern for man himself and his fate must always form the
chief objective of all technological endeavors . . . in order
that the creations of our minds shall be a blessing and not
a curse to mankind. Never forget this in the midst of your
diagrams and equations.
b From an address entitled “Science and Happiness,”
presented at the California Institute of Technology,
Feb. 16, 1931. Quoted in the New York Times,
Feb. 17 and 22, 1931. Einstein Archives 36-320
f 8 F
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 44/44

63. Introduction Experimental Results Conclusions References Acknowledgements Done
Development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode 44/44