Control of preferred 222 crystalline orientation of sputtered indium tina Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh, Viet Nam b Fa
Trang 1Control of preferred (222) crystalline orientation of sputtered indium tin
a
Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh, Viet Nam
b
Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh, Viet Nam
c
Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 18 February 2014
Received in revised form 30 July 2014
Accepted 29 August 2014
Available online 9 September 2014
Keywords:
Indium tin oxide
Thin films
Texture
Conductivity
Surface roughness
Sputtering
We report a two-step growth process for the fabrication of (222)-plane textured indium tin oxide (ITO)films A thin ITO seed layer was grown in mixed Argon + Oxygen gases, followed by a thick ITO deposited in Argon gas X-Ray diffraction shows that the sputtered ITOfilms exhibit strongly preferred (222) crystalline orientation The (222)-plane textured ITOfilms have high transmittance above 80% in the visible range and carrier concentration, mobility and resistivity in the range of 1021cm−3, 40 cm2/Vs and 10−4Ω·cm, respectively The surface rough-ness of our (222) textured ITOfilms is 1.4 nm, which is one of the smallest value obtained from sputtered ITO thinfilms
© 2014 Elsevier B.V All rights reserved
1 Introduction
Tin-doped indium oxide (ITO) has been known as a transparent
electrode in several optoelectronic devices such as liquid crystal
dis-plays, solar cells, organic light-emitting diodes (OLEDs), smart window,
touch screen, and otherflat panel displays due to its high optical
trans-mittance and low electrical resistivity[1–6] Recently, the organic
light-emitting diodes (OLEDs), which are one of the most promising
candi-dates forflat panel displays, demand a very flat surface of ITO film[7]
for improving electroluminescence efficiency and display lifetime[8]
In general, homogeneity and surface roughness are very important for
the reliability of devices since the organic layers in the OLEDs have
thicknesses of only about 100 nm[9] In particular, the peak-to-valley
roughness of ITOfilm has a linear relationship with the reverse leakage
current of devices[8] Also, the surface morphology of ITOfilms
consid-erably affects the patterning properties during the fabrication process of
flat panel displays[10] Many published literatures show that electrical
and optical properties of ITO thinfilms strongly depend on its
preferen-tial crystallographic orientation The ITOfilms with (400)
crystallo-graphic orientation have smaller optical band-gap, less effective“Sn”
doping and larger grain size than the (222) texturedfilms[11] Nakaya
et al proposed that the ITOfilms with the (222) preferred orientation
experience little deterioration at its interface with an over-lyingfilm, thereby improving the light emission characteristics and lifetime of de-vices[12] In addition, since there is a small lattice mismatch between the neighboring oxygen-oxygen (O-O) distance on the close-packed ITO (222) and ZnO (002) planes, it benefits the initial nucleation and subsequent growth of high quality ZnO materials on (222) ITO sub-strates[13,14], probably leading to a good contact for carrier transport
in solar cells based on ZnO substance materials
Kim et al have found that the preferential orientations of the ITO thinfilms depend on the oxygen partial pressure An ITO film grown with pure Ar gas shows a preferential (400)-plane orientation parallel
to the substrate surface while the preferential orientation offilms changed from (400) to (222) plane when even a small amount of O2
was added to the Ar sputtering environment It was also observed that the diffraction intensity of the (222) peak decreased as the oxygen par-tial pressure increased[15] Moreover, most publications indicate that the (222) textured ITOfilms grown in mixed Ar + O2gases have poor conductivity compared with the (400) textured ITOfilms grown in Ar gas environment The reason for this is the reduction density of oxygen vacancies, which is the main contributor of electrical carriers in the ITO film
In this paper, we report the procedure to prepare (222) textured ITO films with high conductivity grown in Ar gas environment instead of mixed Ar + O2 gases The proposed procedure is the two-step sputtering process, in which a thin oxygen seed layer of indium tin
⁎ Corresponding author.
E-mail address: tcvinh@hcmus.edu.vn (C.V Tran).
http://dx.doi.org/10.1016/j.tsf.2014.08.041
Contents lists available atScienceDirect Thin Solid Films
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / t s f
Trang 2oxide (O-ITO) was sputtered in the mixed Ar + O2gases prior to the
de-position of the thick overhead ITOfilms in Ar gas
2 Experiments
The ITO thinfilms were prepared on soda-lime glass substrate by dc
magnetron sputtering The target was commercial ceramic target with
10 wt.% SnO2(99.99% purity) impurity The substrate was kept at a
dis-tance of 5 cm from the target The substrate temperature and sputtering
power were maintained at 350 °C and 50 W during the deposition,
re-spectively In order to deposit an oxygen seed layer of indium tin
oxide (O-ITO), the vacuum chamber was evacuated down to pressure
5.3 × 10−4Pa prior to deposition Then the oxygen reactive gas was
introduced into the chamber and the required pressure, for example
4.2 × 10−1Pa, was set Argon gas was introduced thereafter till the
pre-set pressure reached 5.3 × 10−1Pa Both argon inert gasflow and
oxy-gen reactive gasflow were controlled by a mass flow controller The thin
O-ITO seed layer wasfirstly grown on glass in mixed (O2+ Ar) gases at
5.3 × 10−1Pa The thickness of O-ITO seed layer is about 2 nm Then, the
vacuum chamber was evacuated down to pressure 5.3 × 10−4Pa again
for the following deposition of the 300-nm thick ITO layer in pure Ar gas
at 5.3 × 10−1Pa The thickness offilms was monitored by using the
Quartz oscillator (XTM/2-INFICON (USA)) The crystalline phases of
thefilms were characterized in the θ–2θ mode by using a D8 Advance
(Bruker) X-ray diffractometer (XRD) with Cu Kα radiation (λ =
0.154 nm) Electrical properties offilms were carried out using Hall
measurements (Ecopia HMS-3000) The optical transmittance spectra
were measured using a UV–vis (Jasco V-530) in the wavelength range
from 200 nm to 1100 nm The surface morphology was investigated
by Atomic force microscopy (5500 AFM SYSTEM-AGILENT, Tapping
mode) and scanning electron microscopy (SEM, JEOL JSM-7401F,
oper-ating voltage is 30 kV) The work function was measured by Ultraviolet
Photoelectron Spectroscopy (UPS) using a Model AC-2 instrument
(RIKEN KEIKI)
3 Results and discussion
Fig 1shows X-ray diffraction patterns of the ITO thinfilms prepared
with and without the O-ITO seed layer It is obvious that the ITO thin
film without O-ITO seed layer reveals polycrystalline structure with
dif-ferently orientated crystalline planes such as (400), (222), (211), (440),
and (622) Among these planes, it has been found that there is
preferen-tial growing competition between (222) and (400) planes, the (400)
plane preferential orientation This structural characteristic has been
attained by other authors by growing ITO thinfilms in pure Ar gas and using not only magnetron sputtering but also other methods[16–19]
In contrast, the ITOfilm with O-ITO seed layer, shows a prominently strong (222) peak, which can be understood that grain growth in the (222) direction is obviously favored against growth in other directions
[20] This indicates that the thin O-ITO seed layer has significant effect
on the crystal grain orientation of an overhead ITOfilm
In addition, SEM images reveal the significant influence of the O-ITO seed layer on surface morphology of the overhead ITO layer The visible difference of surface morphology of the ITO thinfilms prepared with and without the O-ITO seed layer is shown inFig 2 It can be seen that the ITOfilm with O-ITO layer reveals “grain structure” (Fig 2a) while thefilm without O-ITO layer shows “domain structure” (Fig 2b) This
“domain structure” is also called a “grain–subgrain” structure[10]or
“domain–grain” structure[7] of conventionally sputtered ITO thin films in an oxygen-deficient environment, which has been obtained
by other authors[7,10,21] The SEM images strongly show that there
is transformation from“domain structure” into “grain structure” corre-sponding to the (400) into (222) texture, respectively, due to an intro-duction of the initial O-ITO seed layer prior to conventionally deposited ITO layer only in pure Ar gas
Fig 3exhibits the estimated surface roughness (RMS) obtained from AFM analysis There are distinct differences in surface roughness be-tween the two samples with and without O-ITO layer of 1.4 nm and 3.7 nm in a scan area of 5μm × 5 μm, respectively Jung et al.[7]reported that the ITO samples prepared by dc magnetron sputtering have an RMS roughness in the range 2–4 nm Raoufi et al showed AFM images of as-deposited and annealed ITO thinfilms revealing the formation of a po-rous granular surface with surface roughness values in the range of 0.847–3.846 nm[22] Hotovy et al reported that ITO thinfilms grown
films with and without O-ITO layer.
Fig 2 SEM images of a) the (222) textured ITO film with O-ITO layer and b) the ITO
Trang 3with-by RF sputtering deposition have a surface roughness value in the range
of 2.3–8.2 nm[23] The RMS surface roughness of ITO samples from 1.7
to 3.8 nm grown at different substrate temperatures was reported by
Malathy et al.[24] The low surface roughness of 3.7 nm obtained
from our ITOfilm without O-ITO layer is consistent with the above
pub-lished values With the presence of the O-ITO seed layer, the surface of
overhead ITOfilm becomes smoother with lower roughness (RMS =
1.4 nm) In conjunction with the SEM images shown inFig 2, this can
be attributed to the difference in height of“domain–grains”, which
seems to be a factor in inducing different surface roughness in ITO
films For the ITO film without O-ITO layer, the domains with different
morphologies have different protrusions on the surface of thefilm,
and thus create steps and edges leading to high surface roughness In contrast, the ITOfilms grown on the O-ITO seed layer with (222) pre-ferred orientation do not show significant difference in the height of the domains or grains, thus showing a smoother surface
Hall measurement results of ITO thinfilms prepared with and with-out the O-ITO seed layer are listed inTable 1, in which the O-ITO seed layer was grown at various partial oxygen gas pressures in the range from 1.3 × 10−4Pa to 4.2 × 10-1Pa The results show that the electrical properties are identical and stable with carrier concentration, mobility and resistivity in the range of 1021cm−3, 40 cm2/Vs and 10−4Ω·cm, re-spectively In addition, not only electrical properties but also the charac-teristic of transmittance spectra of samples in the wavelength range from 200 nm to 1100 nm, as shown inFig 4, is also identical The result shows that the average transmittance in visible range of all samples is the same and over 80% Furthermore, bothfilms have the same work function (4.7 eV and 4.77 eV) The high value of work function is re-quired for efficient hole injection in OLEDs From consideration of the data, it is preliminarily concluded that the changing preferential orien-tation to prominently (222) orientated ITOfilm due to O-ITO seed layer has a trivial effect on the low resistivity while maintaining high transmittance in the visible range and high work function, which are necessary for using the ITOfilms as transparent conducting electrodes
in devices
It has been known that the usualfilm growth process can be simply expressed in two steps: 1) the nucleation process; and 2) subsequently, the growth process The effect of the oxygen partial pressure on chang-ing the preferential orientation in the correspondchang-ing literature was con-sidered in terms of two overall steps in thefilm growth process In our opinion, the preferential orientation development of crystalline grains mainly depends on the initial orientations during the nucleation pro-cess As proved in our study, the changing preferential orientation to (222) prominent plane is caused only by the partial oxygen gas pressure
in the initial nucleation process.Fig 5shows XRD patterns of ITOfilms with the O-ITO seed layer grown at various partial oxygen gas pressures
in the range from 1.3 × 10−4Pa to 4.2 × 10-1Pa It can be obviously seen
Fig 3 AFM images of a) the (222) textured ITO film with initial O-ITO layer and b) the ITO
film without O-ITO layer.
Table 1
Electrical properties of ITO films with O-ITO seed layer deposited at various partial oxygen gas pressures from 1.3 × 10 −4 Pa to 4.2 × 10 -1 Pa.
Sample Base pressure
(Pa)
Deposition pressure (Pa)
Partial oxygen pressure (Pa)
Carrier concentration
×10 21
cm−3
Carrier mobility
cm 2
/V·s
Resistivity
×10−4Ω·cm
Fig 4 The optical transmittance of ITO samples with and without O-ITO layer.
Trang 4that the (222) oriented crystalline plane grows strongly and becomes
the preferential orientation in the overhead ITOfilms as the partial
ox-ygen gas pressures in the O-ITO seed layer preparation increased As a
result, it can be inferred that high partial oxygen gas pressure in the
O-ITO layer deposition process is favorable to create dominant (222)
crystal seed grains, from which (222) oriented crystal grains grow
ex-tensively The growth mechanism can be explained as follows: The
(222) nucleation is a primary nucleation due to the natural structure
of indium atom, a face-centered tetragonal structure, in which the
(111) plane is the lowest energy plane At thefirst stage of the
deposi-tion process, the indium atoms arrived and aggregated on the surface
substrate to make the (111) plane, where oxygen atom was absorbed
to generate the (222) nucleation In poor oxygen partial pressure and
the high substrate temperature (350 °C), the (400) nucleation can be
formed competitively with the (222) nucleation because of the longer
diffusion length and higher mobility of metal adatoms In the rich
oxy-gen partial pressure at the same substrate temperature of 350 °C, the
(222) nucleation is more favorable Since the thin seed O-ITO layer
has a preferred (222) orientation, the following ITO deposition in Ar
at-mosphere grows directly on the (222) nucleation Consequently, the
overhead ITO thinfilm grows uniquely in the (222) orientation
4 Conclusions
With the initial thin oxygen rich seed layer indium tin oxide (O-ITO)
grown in mixed Ar + O2gases, we can grow the overhead ITOfilms in
pure Ar gas with strongly preferred (222) crystalline orientation The
(222) textured ITOfilms have the same optical and electrical properties
compared to the published (400) textured ITOfilms with high
transmit-tance above 80% in the visible range with carrier concentration, mobility
and resistivity in the range of 1021cm−3, 40 cm2/Vs and 10−4Ω·cm,
respectively In addition, the surface roughness of our (222) textured
ITOfilms is 1.4 nm, which is one of the smallest value obtained from
sputtered ITO thinfilms A very flat surface of (222) textured ITO films
can be valuable in optoelectronic devices
Acknowledgments
We would like to thank Professor Derrick Mott (Japan Advanced Institute of Science and Technology— JAIST) for your assistance with our manuscript Your proofreading and editing greatly helped the read-ability of our work
References
[1] P.K Song, Y Shigesato, M Kamei, I Yasui, Electrical and structural properties of tin-doped indium oxide films deposited by DC sputtering at room temperature, Jpn J Appl Phys 38 (1999) 2921.
[2] H Kim, J.S Horwitz, G Kushto, A Pique, Z.H Kafafi, C.M Gilmore, D.B Chrisey, Effect
of film thickness on the properties of indium tin oxide thin films, J Appl Phys 88 (2000) 6021.
[3] H Izumi, F.O Adurodija, T Kaneyoshi, T Ishihara, H Yoshioka, M Motoyama, Elec-trical and structural properties of indium tin oxide films prepared by pulsed laser deposition, J Appl Phys 91 (2002) 1213.
[4] Y.S Jung, S.S Lee, Development of indium tin oxide film texture during DC magne-tron sputtering deposition, J Cryst Growth 259 (2003) 343.
[5] V Senthilkumar, P Vickraman, M Jayachandran, C Sanjeeviraja, Structural and op-tical properties of indium tin oxide (ITO) thin films with different compositions pre-pared by electron beam evaporation, Vacuum 84 (2010) 864.
[6] B Houng, S.L Lin, S.W Chen, A Wang, Influence of an In2O3 buffer layer on the properties of ITO thin films, Ceram Int 37 (2011) 3397.
[7] Y.S Jung, D.W Lee, D.Y Jeon, Influence of dc magnetron sputtering parameters on surface morphology of indium tin oxide thin films, Appl Surf Sci 221 (2004) 136.
[8] Y.-H Tak, K.-B Kim, H.-G Park, K.-H Lee, J.-R Lee, Criteria for ITO (indium-tin-oxide) thin film as the bottom electrode of an organic light emitting diode, Thin Solid Films 411 (2002) 12.
[9] C.H Jonda, A.B.R Mayer, U Stolz, Surface roughness effects and their influence on the degradation of organic light emitting devices, J Mater Sci 35 (2000) 5645.
[10] M Kamei, Y Shigesato, S Takaki, Origin of characteristic structure of tin-doped indium oxide films, Thin Solid Films 259 (1995) 38.
[11] P Thilakan, C Minarini, S Loreti, E Terzini, Investigations on the crystallization properties of RF magnetron sputtered indium tin oxide thin films, Thin Solid Films
388 (2001) 34.
[12] K Nakaya, Y Kayaga, M Codama, O Onitsuka, Organic Electroluminescent Device and Preparation Method with ITO Electrode (111) Orientation, United States Patent (2001) No 6188176.
[13] X Teng, H Fan, S Pan, C Ye, G Li, Abnormal photoluminescence of ZnO thin film on ITO glass, Mater Lett 61 (2007) 201.
[14] X.W Sun, L.D Wang, H.S Kwok, Improved ITO thin films with a thin ZnO buffer layer by sputtering, Thin Solid Films 360 (2000) 75.
[15] J.-H Kim, J.-H Lee, Y.-W Heo, J.-J Kim, J.-O Park, Effects of oxygen partial pressure
on the preferential orientation and surface morphology of ITO films grown by RF magnetron sputtering, J Electroceram 23 (2007) 169.
[16] H.D Jang, C.M Seong, H.K Chang, H.C Kim, Synthesis and characterization of indium-tin oxide (ITO) nanoparticles, Curr Appl Phys 6 (2006) 1044.
[17] R Das, K Adhikary, S Ray, The role of oxygen and hydrogen partial pressures on structural and optical properties of ITO films deposited by reactive rf-magnetron sputtering, Appl Surf Sci 253 (2007) 6068.
[18] N Manavizadeh, F.A Boroumand, E Asl-Soleimani, F Raissi, S Bagherzadeh, A Khodayari, M.A Rasouli, Influence of substrates on the structural and morphological properties of RF sputtered ITO thin films for photovoltaic application, Thin Solid Films 517 (2009) 2324.
[19] A Salehi, The effects of deposition rate and substrate temperature of ITO thin films
on electrical and optical properties, Thin Solid Films 324 (1998) 214.
[20] Y.-L Lee, K.-M Lee, Effect of ambient gases on the characteristics of ITO thin films for OLEDs, Trans Electr Electron Mater 10 (2009) 203.
[21] Y Han, D Kim, J.-S Cho, S.-K Koh, Ultraflat indium tin oxide films prepared by ion beam sputtering, Thin Solid Films 473 (2005) 218.
[22] D Raoufi, Morphological characterization of ITO thin films surfaces, Appl Surf Sci.
255 (2009) 3682.
[23] J Hotovy, J Hüpkes, W Böttler, E Marins, L Spiess, T Kups, V Smirnov, I Hotovy, J Kovac, Sputtered ITO for application in thin-film silicon solar cells: relationship be-tween structural and electrical properties, Appl Surf Sci 269 (2013) 81.
[24] V Malathy, S Sivaranjani, V.S Vidhya, T Balasubramanian, J.J Prince, C Sanjeeviraja,
M Jayachandran, Role of substrate temperature on the structural, optoelectronic and morphological properties of (400) oriented indium tin oxide thin films depos-ited using RF sputtering technique, J Mater Sci Mater Electron 21 (2010) 1299.
Fig 5 XRD diffraction pattern of ITOfilms with O-ITO layer deposited at various partial
ox-ygen gas pressures from 1.3 × 10 −4 Pa to 4.2 × 10 -1 Pa.