-
et
oha
Atomic force microscopy
) thsptart tovem, Aovepo
and substrate temperature of 120 C showed the lowest resistivity
(5.1210 cm on PEN substrate,ubstrate) and high average
transmittance (N90% in both substrates). AZO lms
) lmsplicationic papanels,applicadO, an
Thin Solid Films 518 (2010) 58605865
Contents lists available at ScienceDirect
Thin Soli
w.eresearch groups have studied materials, ZnO based materials,
insteadof ITO due to the toxicity and high cost problems. ZnO is an
n-typesemiconductor, with a wide band gap (3.3 eV), large free
excitonbinding energy (60 meV) and high mechanical and thermal
stabilities[3,5]. Also, it has cost advantage, high resource
availability, stability inhydrogen plasma and non-toxicity [6].
When impurity elements ofgroup III (B, Al, Ga, and In), especially
Al, are incorporated, ZnO becomesn-type material with the
improvement of electrical and opticalproperties because of the
increase in the carrier concentration and
easy to carry. Stainless steel or polymer lms can be used as
exiblesubstrates for exible devices. Although polymer substrate
haslimitation of deposition temperature for maintaining its
chemical andmechanical properties, applications of polymer
substrate draw muchattention due to lightweight, exibility and
transparency contrary tostainless steel substrate. Many researchers
deposited AZO lms onpolymer substrates such as polyimide (PI) [17],
polyethylene tere-phthalate (PET) [18], polyethylene naphtahlate
(PEN) [19], polyethy-lenesulfone [20] and polycarbonate [3]. S.
Fernandez et al. and X.T. Japmobility [7]. W. Yang et al. and Lin
et al. invZnO) lms which had low resistivity, high a4.6104cm and
90% and 8.4310
Kuroyanagi [10] and Keum [11] group also
Corresponding author.E-mail address: [email protected] (S.-W.
Rhee).
0040-6090/$ see front matter 2010 Elsevier B.V.
Adoi:10.1016/j.tsf.2010.05.098d ZnO, etc., must have a3
cm)andgoodoptical[3,4]. Recently, many
low deposition temperature, high reproducibility and good
adhesion tothe substrate [16]. Lately, many people are interested
in exible andtransparent devices with lightweight and small volume
for foldable,large bandgap(N3.2 eV), low resistivity (N10
transmittance (N80%) in the visible region1. Introduction
Transparent conducting oxide (TCOfor transparent and exible
device apdisplays, plasma display panels, electroemitting diode,
solar cells, touch poptoelectronic devices [1,2]. For
theseindium-tin oxide (ITO), In2O3, SnO2, C 2010 Elsevier B.V. All
rights reserved.
have beenwidely studiedns such as liquid crystaler displays,
organic light-gas sensor and othertions, TCO lms, such as
lms with the resistivity of 1.1103cm at 300 C and 101cmat 200 C.
AZO lms can be deposited with several depositiontechniques such as
R.F and D.C magnetron sputtering [12], chemicalvapor deposition
[6], pulsed laser deposition [13], spray pyrolysis [14],and solgel
method [15]. Magnetron sputtering is most widely usedamong all of
them. Compared with other deposition techniques,magnetron
sputtering technique can deposit a lm on large areas atestigated
AZO (Al-dopedverage transmittance of3cm and 86% [8,9].deposited 100
nm AZO
et al. deposited Aand 8.5104possible to depothermal
stabilitycolors from yellosolubility [21]. Inexible substratevalues
(13 ppm/
ll rights reserved.deposited on PEN substrate showed similar
electrical and optical properties like AZO lms deposited on
glasssubstrates.Optical properties 3.85103 cm on glass sDeposition
of Al-doped ZnO lms on polyfrequency magnetron sputtering
Jung-Min Kim, P. Thiyagarajan, Shi-Woo Rhee System on Chip
Chemical Process Research Center, Department of Chemical
Engineering, P
a b s t r a c ta r t i c l e i n f o
Available online 1 June 2010
Keywords:Transparent conducting oxidesAl-doped zinc
oxidePolyethylene naphthalateFlexible substratesRadio frequency
magnetron sputteringX-ray diffraction
100 nm Al-doped ZnO (AZOradio frequency magnetronincluding
sputtering power,the sputtering power, targedecreased due to the
improincreased from 25 to 120 Cdue to the signicant imprPEN
substrates at sputtering
j ourna l homepage: wwhylene naphthalate substrate with
radio
ng University of Science and Technology (POSTECH), Pohang
790-784, Republic of Korea
in lms were deposited on polyethylene naphthalate (PEN)
substrates withuttering using 2 wt.% Al-doped ZnO target at various
deposition conditionsget to substrate distance, working pressure
and substrate temperature. Whensubstrate distance and working
pressure were decreased, the resistivity wasent of crystallinity
with larger grain size. As the substrate temperature was
ZO lms showed lower electrical resistivity and better optical
transmittancement of the crystallinity. 2 wt.% Al-doped ZnO lms
deposited on glass andwer of 25 W, target to substrate distance of
6.8 cm, working pressure of 0.4 Pa
3
d Films
l sev ie r.com/ locate / ts fZO lmswhich had low resistivity of
1.1103cmcm on PET and PI substrate [17,18]. Although it issit at
higher substrate temperature due to higherof PI substrate, it is a
highly colored polymer withw to brown and shows poor
processability, and lowthe case of PEN substrate, it is a strong
candidate fors due to the lowest coefcient of thermal expansionC),
high transparency (N85%), low water absorption
-
(0.14%), the highest tensile strength (275 MPa), high Young's
modulus(6.1 GPa) and higher barrier property for oxygen and carbon
dioxide[22]. Y.M. Chung et al. investigated the properties of 200
nm AZO lms,the resistivity of 1.2103cm and 80% of transmittance,
depositedon PEN substrates with non-reactive pulsed D.C. CHUBM
co-magnetronsputtering in order to observe the effect of working
pressure [19].
In this study, 100 nmAZOlmwas deposited on PEN substrate
usingR.F. magnetron sputtering in order to observe several
depositionparameter effects such as sputtering power, target to
substrate distance
glass substrate was ultrasonically cleaned in acetone, ethanol,
isopropylalcohol, and rinsed in distilled water for 10 min in each
case to removeimpurities on the substrate surface. Al-doped ZnO
(AZO) lms weredeposited with R.F. magnetron sputtering system using
AZO ceramictarget (ZnO 98 wt.%: Al2O3 2 wt.%, 2 in diameters,
99.995%, TASCO). Thebase chamber pressure was 1.3103 Pa obtained by
a rotary vacuumpump and a turbomolecular pump. After the evacuation
of the chamber,pre-sputtering was executed at 100W, 0.4 Pa for 10
min in order toremove impurities on the target surface. The Ar ow
rate was kept at
Fig. 1. AFM images of bare glass substrate (a) and bare PEN
substrate (b).
5861J.-M. Kim et al. / Thin Solid Films 518 (2010) 58605865and
substrate temperature includingworking pressure. AZO lms werealso
deposited on glass substrate for reference to judge properties
ofAZO lms deposited on PEN substrate for applications of exible
andtransparent devices. To optimize deposition condition of the AZO
thinlm as a TCO, their structural, electrical and optical
properties wereinvestigated using various analytical tools.
2. Experimental details
Glass and polyethylene naphthalate (PEN) substrates were used
assubstrates to compare the material properties on both substrates.
TheFig. 2. XRD patterns of AZO lms deposited on glass substrate
((a), (c)) and PEN substrate ((bof 25 W, substrate temperature of
25 C).50 sccm throughout the deposition condition. The substrate
was rotatedwith 15 rpm to get uniform AZO lms during deposition.
AZO lmsweredeposited at various sputtering powers, target to
substrate distances,working pressures and substrate temperatures to
nd the optimizeddeposition condition of R.F. magnetron sputtering.
The thickness of AZOlms was xed at 100 nm, regardless of substrates
and depositionconditions. AZO lms were deposited on glass and PEN
substrates atvarious sputtering powers of 25W, 50W, 75W, 100W and
150W,target to substrate distance of 6.8 cm, 9 cm and 11 cm,
working pressureof 0.4 Pa, 0.9 Pa, 1.5 Pa, 2.0 Pa and2.7 Pa and the
substrate temperatureof25 C, 60 C, 90 C and 120 C. The substrate
temperature was limited to), (d)) at various target to substrate
distance andworking pressure (at sputtering power
-
reduction of the collisions. This leads to the improvement of
crystallinityand the increase of grain size as shown in XRD result,
thereby, the carrierconcentration and mobility can be increased
because of the decrease ofimpurity scattering and grain boundary
scattering [24,26]. Fig. 4 showsXRD patterns of AZO lms deposited
on the glass and PEN substrates atdifferent sputtering powers with
xing target to substrate distance of6.8 cm, working pressure of 0.4
Pa and substrate temperature of 25 C.AZO lms have the strong (002)
peaks around 234.36 on glasssubstrate and 234.3 on PEN substrates
at 25W. Thismeans that AZOthin lms are hexagonal structures and
have preferred orientation withthe c-axis perpendicular to the
substrate [24]. There are no metallicaluminum ormetallic zinc peaks
from the XRD patterns. The (002) peakposition was continuously
shifted to lower angle with the increase ofsputtering power because
of the stress of AZO lms. Generally, a peakshift to a smaller angle
indicates an increase in the lattice d-spacingwithcompressive
stress, while a shift to a larger angle indicates a decrease inthe
lattice d-spacing with tensile stress [27]. Therefore, AZO
lmsdeposited on glass and PEN substrate got compressive stress
assputtering power increases. In the case of AZO lms deposited on
PENsubstrate, the variation of peak intensity and shift was larger.
AZO lmsdeposited on PEN substrate probably get higher damage at
highsputtering power because PEN substrate is softer than glass
substrate.When the sputtering power increases, the (002) peak
intensity of AZOlms deposited on glass and PEN substrates becomes
lower. This meansthat the crystallinity becomes poorwith increasing
sputtering power. Asthe sputtering power increased from25W to 150W,
the deposition rate
Fig. 3. The resistivity of AZO lms deposited on glass and PEN
substrates at varioustarget to substrate distance (a) and working
pressure (b).
5862 J.-M. Kim et al. / Thin Solid Films 518 (2010) 58605865120
C because Tg of PEN substrate was 122 C. AZO lms deposited onglass
and PEN substrates were investigated with several analysismethods.
The thickness of AZO lm was measured using
scanningelectronmicroscope (JEOL JSM-840A). The resistivity of
theAZO thinlmwasmeasured using 4-point probe (KEITHLEY 2400). The
crystallinity ofthe AZO thin lm was analyzed with X-ray diffraction
(XRD, modiedPhilips-1880) using CuK radiation. The surface
morphology and theroughness of the AZO thin lm were analyzed using
Atomic ForceMicroscope (AFM, DimensionTM 3100). The transmittance
of the AZOthin lm was measured using UVVis-Spectroscopy (JASCO,
V-530).
3. Results and discussion
The surface morphology of the glass and PEN substrate
wascompared to observe the effect of the substrate roughness to
theproperties of the lm. Fig. 1 shows AFM images of bare glass and
barePEN substrate. Bare glass surface shows smooth surface with
itsroughness of about 0.28 nm. On the other hand, bare PEN
substratesurfacemorphology reveals the crest and troughwith its
average surfaceroughness of 1.94 nm.Rougher surface substrate leads
to lower electricalproperties of the lms by carrier scattering
[18], therefore, the lmsdeposited on PEN substrates show to be more
resistive than thosedeposited on glass substrate following our
results. A. Miyake et al. groupalso reported thatGZOlmsdeposited on
cyclo-olenpolymer substrateshowed lower resistivity and
transmittance than that on glass substratebecause of the
deterioration in the crystallinity of GZO lm [23]. Aftercomparison
of the substrate surface morphologies, AZO lms weredeposited on
glass and PEN substrates at various target to substratedistances
and working pressures in order to observe their effect.
Otherconditions were xed at sputtering power of 25W and
substratetemperature of 25 C.When the target to substrate
distanceandworkingpressure decrease from 11 cm to 6.8 cm and from
2.7 Pa to 0.4 Pa, thedeposition rates increase from 1.46 nm/min to
2.81 nm/min and1.94 nm/min to 2.81 nm/min. When the target to
substrate distanceand working pressure decrease, the collisions of
between Ar ions andsputtered atoms decrease. This leads to the
sputtered atoms havingenough kinetic energy for arriving at the
substrate, and diffusive abilityof the atoms increases [24],
thereby, the deposition rate increased withdecreasing target to
substrate distance and working pressure. XRDpatterns of AZO lms
deposited on glass and PEN substrates of differenttarget to
substrate distance andworking pressure are shown in Fig. 2. Inall
cases, strong (002) peak of AZO lms is observed at 2=34.36 onglass
substrate and 2=34.3 on PEN substrate. This indicates that AZOlm
deposited on glass and PEN substrates is the hexagonal
wurzitestructure and has a preferred orientation with the c-axis
perpendicularto the substrate, regardless of substrate materials
[24]. There is no peakshiftwith changing target to substrate
distanceandworkingpressure. Asthe target to substrate distance and
working pressure decrease, the(002) peak intensity of AZO lms
improves. The indicates that thecollision decreases between Ar ions
and sputtered atoms withdecreasing target to substrate distance and
working pressure, thereby,improving the crystallinity and making
larger grain with higher kineticenergy [25,26]. L. Chen et al. and
Y.K. Moon et al. showed similar resultswith changing target to
substrate distance andworkingpressure [25,26].When the peak
intensity and peak variation of AZO lms deposited onglass andPEN
substratewere compared, those of AZOlms deposited onglass substrate
are larger. The reason probably is that glass substrate isharder
and smoother which means glass substrate can support to formgood
crystallinity of AZO lms than PEN substrates. Fig. 3 shows
theresistivity of AZO thin lms deposited on glass and PEN
substrates atvarious target to substrate distance and working
pressure. When AZOlms were deposited on both substrates at the
target to substratedistance of 6.8 cmandworking pressure of 0.4 Pa,
AZOlms show lowerresistivity, 1.8102cm on glass substrate and
3.2102cm onPEN substrate. When the target to substrate distance and
working
pressure decrease, the sputtered atoms can get higher energy due
to the also increased from 2.81 nm/min to 14.74 nm/min, because the
number
-
5863J.-M. Kim et al. / Thin Solid Films 518 (2010) 58605865of
atoms is sputtered from the target and gets a higher energy
thatcontributes to the lm growth with increasing sputtering
power.However, when the deposition rate is too fast at higher
sputteringpower, there is not enough time to form good
crystallinity and compactAZO lms [28]. In addition, sputtered atoms
having higher energy athigher sputtering power impact to form the
lms, thereby, causing thebombardment electron to lose energy and
promoting non-uniform lmgrowth because of the creation of stress
[28]. High sputtering powereffect also was conrmed with AFM images.
Fig. 5 shows the
Fig. 4. XRD patterns of AZO lms as a function of sputtering
powers on glass substrate(a) and PEN substrate (b) (at target to
substrate distance of 6.8 cm, working pressure of0.4 Pa, substrate
temperature of 25 C).morphologies of AZO lms deposited on PEN
substrate at varioussputtering powers.Morphology of AZO lms becomes
poor and rougherwith lowerlmdensity as sputteringpower increases.
This result relatedto XRD patterns as shown in Fig. 4. For
increasing the kinetic energy ofsputtered atoms to form good
crystallinity, the substrate temperaturewas increased until 120 C
at sputtering power of 25W, target tosubstrate distance of 6.8
cmandworking pressure of 0.4 Pa. Fig. 6 showsthe XRD patterns of
AZO lms deposited on glass and PEN substrate atvarious substrate
temperatures. The limitation of substrate temperaturewas 120 C,
because the glass transition temperature (Tg) of PEN was122 C. All
AZO lms have strong (002) peak which indicates that AZO
Fig. 5. AFM images (1 m1 m) of AZO lms deposited
onlmsarepolycrystallinewith thehexagonalwurzite structure
andhaveapreferred orientation with the c-axis perpendicular to the
substrates[24].When the substrate temperature increases,
the(002)peakbecomes
PEN substrates at various sputtering power ((a)(c)).
Fig. 6. XRD patterns and grain size of AZO lms deposited at
various substratetemperatures on glass substrate (a), PEN substrate
(b) and grain size (c) (at sputteringpower of 25 W, target to
substrate distance of 6.8 cm, working pressure of 0.4 Pa).
-
more intense and sharper, obviously. This means that the
crystallinityand grain size are signicantly improved due to
increasing surfacediffusion of sputtered atoms which have higher
kinetic energy from thesubstrate [19]. On the glass substrate, the
enhancement of thecrystallinity of AZO lm was better than that on
PEN substrates. Grainsize of AZO lms at various substrate
temperatures was calculated usingDebyeScherrer formula as shown in
Fig. 6(c). Scherrer's formula [28] is:
D =0:9B cos
where 0.9 is correction factor, is thewavelength of X-ray
source, is theBragg diffraction angle in degree, and B is the
diffraction peak widthproportional to FWHM. When the substrate
temperature increases, thegrain size becomes larger from 4.54 nm to
6.01 nm and from 3.57 nm to5.09 nm on glass and PEN substrates,
respectively. This result isconsistent with the XRD observations.
As the substrate temperatureincreases, the (002) position shifts to
higher angle from 2=34.36 to34.48 on glass substrate, from2=34.3 to
34.48 on PEN substrate. The
shows the lower resistivity due to the reduction in the
scattering of thecarriers at the grain boundaries and crystal
defects, which increased theapparent carriermobility [30]. Grain
boundary scattering is reducedwithimproving crystallinity and grain
size, the carrier concentration andmobility increase [17].
Therefore, the resistivity of AZO lms deposited athigher substrate
temperature becomes low. From this electricalresistivity results,
the structural property is a powerful effect to improveelectrical
properties. Kim's group also reported that the electricalproperties
strongly depend on the crystallinity of AZO lms [31].Comparison of
AZO lms deposited on glass and PEN, AZO lmsdeposited on glass
substrates have lower resistivity than AZO lmsdeposited on PEN
substrate due to better crystallinity and grain sizes.Fig. 8 shows
the transmittance of AZO thin lms deposited on glass andPEN
substrate as a function of substrate temperature. All of AZO
lmsdeposited on glass and PEN substrate exhibited a transmission of
higherthan 80% in the visible region. The average transmittance of
AZO lmsdeposited on glass substrate increases from 87.46% to
90.23%, and thatdeposited on PEN substrate also increases from
87.36% to 90.12% withincreasing substrate temperature from 25 C to
120 C. It is probably thatthe improvement of crystallinity at
higher temperature as XRD results. S.Fernandez et al. also got the
similar trend result [18]. The highest averagetransmittance of
AZOlms deposited on glass and PEN substrate is above90% at 120 C,
regardless of substrate materials. The absorption edge ofAZO lms
deposited on glass substrate slightly shifted to shortwavelength
than that of AZO lms deposited on PEN substrate. Itmeans that the
band gap of AZO lms deposited on glass substrate iswider [18].
ThebandgapsofAZOlmswere calculatedby theTaucmodel
5864 J.-M. Kim et al. / Thin Solid Films 518 (2010)
58605865reason is that the compressive stress of AZO lms becomes
reduced.When the AZO lms were deposited at lower substrate
temperature, thestress is generated due to the freezing in the
structure defects [19].However, the increasing substrate
temperature in a suitable rangeincreases the atomic mobility and
reduces the structural defects, thus arelaxation of AZO lms is
observed [19]. Compared with the lmsdeposited on PEN substrates,
the intensity of diffraction peaks of AZOlms deposited on glass
substrates is more intense and sharper becauseof better
crystallinity and grain sizes of AZO lms deposited on
glasssubstrates. Glass substrate is probably harder and smoother
than PENsubstrate, it can support well to growAZO lm on the
substrate, thereby,AZO lms deposited on glass substrate can have
better crystallinity. Asthe substrate temperature increased, the
surface roughness of lm onboth substrates, glass and PEN, also
increased from 1.47 nm to 2.62 nmand from1.87 nmto2.70 nmbecauseof
largegrain size, respectively.Ourlaboratory already reported the
effect of substrate temperature on AZOlms elsewhere [29]. Fig. 7
shows the resistivity of AZO lms depositedonglass andPEN substrates
as a functionof substrate temperature.Whenthe substrate temperature
increases from 25 C to 120 C, the resistivityof AZO lm deposited on
glass substrate decreases from1.8102 cmto 3.85103 cm, and the
resistivity of AZO lm deposited on PENsubstrate decreases from
3.2102 cm to 5.12103 cm due tosignicant improvement of
crystallinity and grain size as shown in XRDresults. The
crystallinity and grain size are improved with increasingsubstrate
temperature because the sputtered atoms can get higherenough energy
from the substrate temperature. The higher crystallinity
Fig. 7. The resistivity of AZO lms deposited on glass and PEN
substrates at various
substrate temperatures.Fig. 8. The transmittance of AZOlms
deposited on glass (a) and PEN substrate (b) at various
substrate temperatures.
-
[32]. The band gaps of AZO lms deposited changed from 3.47 eV
to3.51 eV on glass substrate and 3.40 eV to 3.43 eV on PEN
substrate assubstrate temperature increases from 25 C to 120 C. The
improvementof crystallinity of AZO lms with increasing substrate
temperature leadsto the increase of carrier concentration
andmobility due to the reductionof grain boundary scattering [30].
As the carrier concentration increases,the Fermi level in the
conduction band of AZO lms increases, thereby,the band gap becomes
wider with the lowest resistivity according toBursteinMoss effect
[3,33].
4. Conclusion
To optimize deposition condition of AZO lm as a TCO, we
havestudied AZO lms deposited on glass and PEN substrates using
R.F.magnetron sputtering systemwith various deposition parameters
suchas sputtering power, target to substrate distance, working
pressure andsubstrate temperature.When the AZO lms deposited on
glass and PENsubstrates at low sputtering power of 25W, short
target to substratedistance of 6.8 cm, low working pressure of 0.4
Pa and the highsubstrate temperature 120 C, they were shown to have
the bestcrystallinity, the lowest resistivity (3.85103 cm on glass
substrateand 5.12103 cm on PEN substrate) and the highest
transmittance(N90%). Besides, AZO lms deposited on PEN substrates
showcomparable properties like lms deposited on glass substrates,
andAZO lms deposited at optimized condition can be used for the
exibledevices using transparent and exible substrates.
Acknowledgments
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Deposition of Al-doped ZnO films on polyethylene naphthalate
substrate with radio frequency magnetron
sputteringIntroductionExperimental detailsResults and
discussionConclusionAcknowledgmentsReferences