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Thin Film and Nanowires of Transparent Conducting Oxides for Chemical Gas Sensing and LED Applications
1. Thin Film and Nanowires of Transparent
Conducting Oxides for Chemical Gas
Sensing and LED Applications
Raj Kumar
1
2. 2
Conclusion and Future prospective
Preparation of Thin Film and Characterization
Device Fabrication
Overview on Sensing Materials & Sensors
Deposition Techniques
Objective
Transparent Conducting Oxides (TCOs) Materials
3. Objective
• It aims towards new classes of materials, the p- and n-type Active
Semiconductor Oxides (ASO), passive n-type amorphous Transparent
Conducting Oxides (a-TCOs) and p-n type junctions based on the above.
These materials will be certified for device concepts highlighting the
potential of oxides as electronic materials in the automotive industry.
• To model, synthesize, characterize and certify oxides for electronic
applications to overcome the limits of Silicon based electronics and to
catapult the electronics industry into a new era of growth.
• This is a highly competitive environment of high importance for Europe’s
economy and with challenging demands on information technology such
as Sensors, Displays and Energy sources to provide sustainable
mobility for the European society.
3
5. Overview on sensing materials & sensors
• p-type metal oxides exhibit a wide
range of catalytic activities and
sensing capabilities and can be used
for the fabrication of gas sensors that
exhibit various novel functionalities.
• Binary metal oxides were widely
investigated in recent years, in
contrast to the p-type ternary
compounds.
• Composite oxides (CuO/SnO2 NWs,
NiO/ZnO NWs etc.) and Nanowires for
sensors
5
H.-J. Kim, J.-H. Lee / Sensors and Actuators B 192 (2014) 607– 627
Ist
Generation
Sensors Thick Films
2nd
Generation
Sensors Thin Films
3rd
Generation
Sensors
Nanowires,
nanomaterials
etc. Diagram illustrating requirements to gas sensors, [G. Korotcenkov /
Materials Science and Engineering R 61 (2008) 1–39
6. Sensing mechanism in n-type and p-type oxides
• A surface depletion region, which is having a
large electrical resistance, and an unaffected
bulk, which is having a low resistance
• The electrical current will flow through the grain
or bulk “in perpendicular ”
• Ionosorption of molecules at the surface leads
to a bending of the bands and the building of
dipoles at the surface changes the electron
affinity. Both changes determine the change in
work function
• Appearance of surface accumulation layer,
which has a higher concentration of free charge
carriers (holes) and thus a lower resistivity
surface
• The electrical current will flow through the
space charge layer “parallel” to the surface
• Resistance increase (reducing gases) and
resistance decrease (oxidising gases)
6
*M. Hubner, C.E. Simion, A. Tomescu-Stanoiu, S. Pokhrel, N. Barsan, U. Weimar, Sensors and Actuators B 153 (2011) 347–353.
*Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors, N. Barsan & C. Simion & T. Heine & S. Pokhrel & U. Weimar
CO(gas) + O-
(ads) + h+ → CO2 + S
O3 → O2 + OCO(gas) + O- = CO2(des)
-
CO2
-
(des) = CO2(gas) + e-
7. Deposition Techniques
• Thin film deposition by RF Sputtering
• Advantages:
– Industrially scalable technique
– Growth patterning, via shadow
masking
• Disadvantages:
– Strong influence of growth substrate
(Al2O3, Si, Glass)
• Nanowires Growth by Vapour Phase
Growth
7
9. Delafossite (CuAlO2)
• 2” target (CuAlO2, Cu : Al = 1 : 1) from
JSI
• Pressure 10*10-3mbar, P=100W
• O2 / Ar = 0/100-10/90-25/75-50/50
• T= RT to 400°C
9
2” CuAlO2 (CAO)
target when plasma
started
K. Vojisavljevi´c et al. Journal of
the European Ceramic Society
33 (2013) 3231–3241
• p-type conductivity up to
1 S cm-1 at room tem.
H. Kawazoe, et al. p-Type electrical conduction in
transparent thin films of CuAlO2, Nature 389
(1997)939–942.
S. K. Misra and A. C. D. Chaklader, “System
Copper Oxide-Alumina,”J. Amer. Ceram.
SOC., 46 [lo] 509 (1963).
XRD pattern (a) and SEM micrograph (b) of the sintered CuAlO2 sample.
P-type copper aluminum oxide thin films for gas-sensing applications, Sensors and Actuators B 209 (2015), 287–296,
C. Baratto, R. Kumar, G. Sberveglieri, K.Vojisavljevic, B.Malic.
Transparent films after annealing
10. Functional Characterization
• Thin films were deposited on 2x2 mm Al2O3 substrate with IDC contacts and Pt meander
(heater)
• Sensing film were aged, by in situ heater at 500°C for two weeks prior to tests
Inert Ar atmosphere
10
Images of sensing film after Aging and Testing
P-type copper aluminum oxide thin films for gas-sensing applications, Sensors and Actuators B 209 (2015), 287–296,
C. Baratto, R. Kumar, G. Sberveglieri, K.Vojisavljevic, B.Malic.
Dep @ RT Dep @ 200°C Dep @ 400°C
Dep @ 200°C +
annealed at 1000°C
11. Structural Characterization
• XRD patterns of samples A, B and C,
annealed at 500°C for 36 h in air.
11
20 30 40 50
*
*
* ***
Intensity(a.u.)
A
B
C
ann. at 500
o
C
C uAl2
O 4
*
s ubs trate
20 40 60
0
100
200
300
400
500
C uO
C uAlO 2
2q (
o
)
CuO+ CuAlO2+CuAl2O4
Weight quantities of crystalline phases present in aged samples A, B, C and sample D annealed at 1000 °C and crystallite
size of the copper oxide determined by the Rietveld refinement method. Numbers in parenthesis indicate the error in
the last significant digit.
P-type copper aluminum oxide thin films for gas-sensing applications, Sensors and Actuators B 209 (2015), 287–296,
C. Baratto, R. Kumar, G. Sberveglieri, K.Vojisavljevic, B.Malic.
10 20 30 40 50 60
D
*
*
*
*
*******
Intensity(a.u.)
C uAl2
O 4
*s ubs trate
20 40 60
C uO
2q (
o
)
Spinel phase CuAl2O4+CuO traces
• XRD pattern of the sample D.
Peaks corresponding to the
substrate are denoted with the
stars, while the peaks from
different phases are denoted with
the Miller indices.
12. Ozone sensing
• A,B,C showed p-type behaviour (predominant CuO)
• D samples showed n-type behaviour (predominant spinel phase)
• The experimental data can be well fitted by the typical power law for
metal oxide sensors: Response = A*[C]B
12
A-dep@RT
B-dep@200°C
C-dep@400°C
D-B annealed @1000°C
P-type copper aluminum oxide thin films for gas-sensing applications, Sensors and Actuators B 209 (2015), 287–296,
C. Baratto, R. Kumar, G. Sberveglieri, K.Vojisavljevic, B.Malic.
Zheng et al. Appl. Phys.
Lett. 85, 1728 (2004); doi:
10.1063/1.1784888
13. Response to Reducing and Oxidizing gases
13
• Bar plot of relative response of sensor A deposited at room temperature to all the
gases at 300°C and 400°C. Note the logarithmic scale
P-type copper aluminum oxide thin films for gas-sensing applications, Sensors and Actuators B 209 (2015), 287–296,
C. Baratto, R. Kumar, G. Sberveglieri, K.Vojisavljevic, B.Malic.
• Optimum response to ozone (70ppb) at 300°C working temperature
• Optimum response to CO (100ppm), ethanol (100ppm) and acetone
(25ppm) at 400°C working temperature
15. p-type NiO thin films
• Two fold aim:
• p-type thin films to be used in heterojunction with ZnO nanowires
• Testing of Sensing film for pollutant gases
• Resistivity: 0.012 Ωcm on fused silica/ Si-wafer at RT, 50% O2 concentration
• Two different approach:
15P-Type NiO Thin Films Prepared by Sputtering for Detection of Pollutants; Sensors and Microsystems, Proceedings of the 17th National Conference, Brescia, Italy, 5-7
February 2013, (Pages 121-125); R. Kumar, C. Baratto, G. Faglia, G. Sberveglieri, E. Bontempi, L. Borgese.
Sputtering Deposition
•Gas pressure 10*10-3mbar
•O2 / Ar = 0/100-10/90-25/75-50/50
•T= 200°C
•P=100W
Sputtering Deposition
•Gas pressure 10*10-3mbar
•O2 / Ar = 0/100
•T= RT, 100°, 200° and 400°C
•P=100W
• Minimum resistivity reported in literature: 0.14 Ωcm and a hole
concentration of 1.3 × 1019 cm−3 ; H. Sato, et al. Thin Solid Film, Vol. 236, Issues 1–2, 1993, Pages 27–31
16. • NiO phase was confirmed by XRD analysis
in each thin film, deposited at different
temperature
• Crystallite dimension increased along
with preferred orientation as substrate
temperature increases
16
Courtesy of Elza Bontempi Dept. of Mechanical and Industrial Engineering, Univ. of Brescia
Crystallographic study of thin films
• Thin film thickness varied from ~75
nm -300 nm (30 min dep. time) and
thickness decreases as conc. of O2
increases from 0% to 50%
Deposited @ 0% O2
Temperature
17. Optical and Raman study of thin films
• Deposition temperature have a minor impact
on band gap (3.6-3.8eV) and transmittance
• After annealing transmittance increases and
thin film becomes transparent
17
Deposited @ 0% O2
• Raman spectra of NiO samples deposited on Si substrate as a
function of the deposition temperature. The modes of NiO were
observed in all samples. Modes of Si substrates are labelled with *.
18. Functional characterization
• Working Temp. = 200°C to 500°C; RH=50%
• Response for oxidizing gases (NO2, Ozone), R =(Ggas-Gair) / Gair
• Response for reducing gases (CO, ethanol), R =(Gair-Ggas) / Ggas
• The optimum response to ethanol and acetone was observed at 400°C working
temperature; a low response to CO was observed. NH3 does not show any
response
• NO2 optimum working temperature is 300°C, at 200°C recovery times is too long.
18
Dynamic response to CO,
Ethanol and Acetone
Basic design of Sensor
Sensor
19. Sensing response towards Ozone & Impact of
Crystalline dimension
• Ozone optimum working temperature is 200°C
while the recovery times is too long, at 500°C
it seems that response is constant
• Film deposited at RT in inert atmosphere shows
the highest response with 70ppb concentration
of O3
19Tailoring of porous nanostructured thin film of NiO with texture surface by varying the ambient condition for the detection of pollutant gases. R. Kumar, C. Baratto,
G. Faglia, G. Sberveglieri, E. Bontempi, L. Borgese, under review in Thin Solid Films Journal.
Decrease
of grain
size
Increase of
sensor
response
Decrease of
response
time
21. Sensor preparation
21
Name Sensing layer
ZnO nws ZnO nws
ZnO nws/NiO1 ZnO nws+15’deposition NiO
ZnO nws/NiO2 ZnO nws+30’ deposition NiO
Source =ZnO powder at 1200°
Substrates=Pt catalyst
Temp. range=510-570°C
Sputtering target =NiO
P=10-2 mbar (50%Ar/50%O2)
W=100W
time=15’-30’
ZnO nws
NiO layer
Gas sensing study of ZnO nanowire heterostructured with NiO for
detection of pollutant gases
C. Baratto*, R. Kumar, E. Comini, G. Faglia, G. Sberveglieri; Procedia
Engineering 87 ( 2014 ) 1091 – 1094
22. SEM and EDX
• ZnO nws: very thin NWs with diameter in the
range of 10-20 nm + crystalline platelets
dispersed over the nanowire forest.
• ZnO nws/NiO1 : increased diameters of the
wires (30-40 nm), that seems to be covered by a
porous layer of NiO.
• ZnO nws/NiO2 sample shows similar
morphology to the previous sample, but the
porous layer of NiO appears to be thicker on
wires and on platelets
22
Sample Name Zn
Atomic %
Ni
Atomic %
ZnOnws/NiO1 36.1 14.1
ZnOnws/NiO2 32.7 22.0
ZnO NWs
ZnO NWs/NiO1
ZnO NWs/NiO2
23. Sensing characterization
• Ethanol, Acetone, CO and NO2
• Response to NO2 was observed for
temperatures lower than 300°C
• Response to ethanol and acetone increases
with optimum working temperatures at
500°C
• A 3-fold enhancement in sensing response
for ZnO NWs/NiO1
23
Ethanol Acetone Y. Nakamura, Photo-Assisted Organic Pollutants Sensing by a
Wide Gap pn Heterojunction www.intechopen.com
Y. Liu et al. Sensors and Actuators B 191 (2014)
537– 544.
25. ZnO NWs as a heterojunction for LED application
• Band offsets diagram of GaN, NiO and CuAlO2 semiconductors with respect to ZnO
25
THE NOBEL PRIZE IN PHYSICS 2014 THE ROYAL SWEDISH ACADEMY OF SCIENCES HTTP://KVA.SE
26. Heterojunction study of ZnO NWs/p‐GaN
• Removal of p-type GaN on the back contact (courtesy of UNIGiessen,
Germany)
• Deposition of catalyzer by evaporation to reduce the sputtering damage on p-
type GaN (courtesy of OSRAM, Germany)
• Tailor PMMA removal and plasma etching procedure to enhance UV emission
from ZnO nanowires
26
27. LED Structure of p-GaN + Pt catalyzer + ZnO NWs
• NWs arrangements depends on the thin film on which they are growing:
• Thin aligned ZnO NWs deposited on p-GaN
• Polymer layer (PMMA/Toluene ) deposited on ZnO NWs
• Plasma etching was performed in Ar
27
ZnO nanowire covered with PMMA
• PL spectra of ZnO and GaN acquired in the
respective region; with this type of growth the
coverage area is not complete and some signal from
GaN can be observed also in the ZnO region
28. PL, EL and I-V characterization
• Electroluminescence spectra of n- ZnO NWs/p-GaN
heterojunction (LED structure: n-ZnO NWs + Pt
catalyzer + p-GaN) at different voltage in visible region.
28
Au contacts on n-ZnO NWs-n-ZnO NWs
(n-n junction)
I-V- characteristics curve of Au contacts
on n-ZnO/p-GaN heterojunction
Au contacts on p-GaN-p-GaN
(p-p junction)
29. Heterojunction study of p-NiO/ ZnO NWs
• Thin film of NiO deposited by sputtering on top of ZnO NWs
• PL was recorded from ZnO NWs
• No EL was recorded from the p-n heterojunction
29
Au cont. / NiO film/PMMA etching in Ar/
ZnO nanowires (Pt catalyzer) / Al2O3
NiO thin film on ZnO
nanowire-cracks on
the surface-highly
resistive layer
ZnO nws after
plasma etching
Embodiment diagram of LED device
Au contact on NiO thin film
(p-p junction)
p-n heterojunction
Au contact on ZnO NWs
(n-n junction)
30. Heterojunction study of ZnO NWs/p-Cu-Al-O thin film
• PL was recorded from ZnO NWs
• A broad spectrum was observed in visible region
• Non ohmic and non linear behaviour were recorded at Cu-Al-O
and ZnO nanowires
• No EL was recorded from p-n heterojunction
• Nearly rectifying nature was recorded at p-n heterojunction
30
Schematic diagram of n-ZnO NWs/p-
type Cu-Al-O LED device
I-V curve (p-n hetero junction)
31. Conclusion
• Minimum resistivity of NiO thin film was recorded ~0.012 Ωcm at RT
and 50 %O2
• Energy band gap of NiO lies in between 3.6 - 3.8 eV
• p-NiO and p-Cu-Al-O thin film show good potential towards ozone.
They can be utilized as a sensing film in chemical gas sensors
• NiO layer deposited on ZnO NWs enhanced the sensing response
and decrease the response time towards ethanol and acetone
• Au contact shows the ohmic nature at n-ZnO NWs and p-GaN or p-
NiO
• PL and EL were recorded at ZnO NWs/p-GaN while I-V curve on p-n
junctions shows nearly rectifying nature
• EL was recorded for ZnO NWs/p-GaN while no EL was recoded in
case of n-ZnO NWs/p-NiO and n-ZnO NWs / Cu-Al-O heterojunction
31
32. Acknowledgments
EU 7th Framework programme and “ORAMA” consortium member for the financial
support during my research work http://orama-fp7.eu/en/project/
Prof. Barbara Malic
OSRAM Germany
UNIGiessen, Germany
Coordinator: Prof. Theonis Riccò
32
Thank you for your attention
