Fabrication of tio 2 based transparent conducting oxide on glass and polyimide substrates

Amorphous Ti0.96Nb0.04O2films were deposited at room temperature by using sputtering, and were then crystallized through annealing under reducing atmosphere.. In order to achieve low temp

Fabrication of TiO 2 -based transparent conducting oxide on glass and

polyimide substrates

T Hitosugia,b,⁎ , N Yamadab, N.L.H Hoangc, J Kasaib, S Nakaob, T Shimadab,c, T Hasegawab,c

aAdvanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai, Japan

bKanagawa Academy of Science and Technology (KAST), Kanagawa, Japan

cDepartment of Chemistry, University of Tokyo, Tokyo, Japan

a b s t r a c t

a r t i c l e i n f o

Available online 20 November 2008

Keywords:

Titanium oxide

Transparent conducting oxide

Sputtering

Titanium dioxide

We report on preparation and properties of anatase Nb-doped TiO2transparent conducting oxide films on glass and polyimide substrates Amorphous Ti0.96Nb0.04O2films were deposited at room temperature by using sputtering, and were then crystallized through annealing under reducing atmosphere Use of a seed layer substantially improved the crystallinity and resistivity (ρ) of the films We attained ρ = 9.2 × 10− 4

Ω cm and transmittance of ~70% in the visible region on glass by annealing at 300 °C in vacuum The minimum ρ of 7.0 × 10− 4

Ω cm was obtained by 400 °C annealing in pure H2

© 2008 Elsevier B.V All rights reserved

1 Introduction

Transparent conducting oxides (TCOs) are materials realizing high

optical transmittance and high electrical conductivity at the same

time They are indispensable in devices that require electrical contact

and optical access, such as flat panel displays (FPDs), light-emitting

diodes (LEDs), and solar cells[1,2] Currently, Sn-doped indium oxide

(ITO) is the most widely used TCO, because of its excellent transparent

conducting properties[3]and the ease of film growth However, rapid

progress in opto-electronic devices requires TCOs with additional

characteristics For example, the emission intensity of GaN-based LEDs

is expected to be raised by using a TCO with a high refractive index In

solar cell applications, TCOs with higher infrared transparency are

desired in order to elevate the energy conversion efficiency These

situations have motivated us to develop alternative TCOs with unique

properties unattainable from existing TCO materials, such as ITO, ZnO

and SnO2[4]

Recently, we have reported on pulsed laser deposition (PLD)

growth of anatase Ti1−xNbxO2(TNO) transparent conductor[5,6] This

material is characterized by a wide band gap (3.2 eV)[7]and relatively

low effective mass ~ m0 (m0: free electron mass) [8], and shows

electrical and optical properties comparable to those of ITO Moreover,

TNO exhibits other remarkable features, i.e., high refractive index,

high transmittance in the infrared region, and high chemical stability

in a reducing atmosphere The report on the anatase TNO has

stimulated studies on growth, mechanism and application of this TCO, [9–13]

In this paper, we present the fabrication of TNO polycrystalline films on glass and plastic (polyimide) substrates Amorphous thin films were deposited at first and then annealed to obtain transparent conductive TNO films In order to achieve low temperature processing not exceeding 300 °C and high electrical conductivity, we used a seed layer, from which nucleation was initiated during the annealing

2 Experimental details Sputter-deposited amorphous films deposited on unheated non-alkali glass (Corning 1737) or polyimide plastic substrates were crystallized to obtain transparent conductive TNO films[14,15] The temperature of the unheated substrate was in a range of 70–80 °C during deposition A sintered Ti1 − xNbxO2 − δ (x = 0.037 or 0.06) disks

(diameter: 2 in.), annealed in reducing atmosphere in order to introduce oxygen vacancies, were used as a target The base pressure of deposition chamber was maintained at ~5 × 10− 5Pa Deposition was conducted in a mixture of Ar and O2with various ratios f(O2) = [O2/(Ar + O2)] under a total pressure of 1.0 Pa The RF power (13.56 MHz) applied to the target was kept constant at 120 W during sputtering Before the film deposition, the target surface was sputter-cleaned by pure Ar for

10 min in order to remove surface oxide layers and contamination, and was subsequently pre-sputtered for 5 min under the same conditions as for film growth The as-deposited amorphous films were annealed in a rapid thermal annealing furnace, where the annealing temperature was raised at a rate of 100 °C/min Deposition condition and annealing conditions are summarized in Table 1 Carrier transport properties were measured using the standard Hall

Thin Solid Films 517 (2009) 3106–3109

⁎ Corresponding author Advanced Institute for Materials Research (WPI-AIMR),

Tohoku University, Sendai, Japan.

E-mail address:hitosugi@wpi-aimr.tohoku.ac.jp (T Hitosugi).

0040-6090/$ – see front matter © 2008 Elsevier B.V All rights reserved.

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

bar geometry The Nb content x of the film sputtered using x = 0.037

target was determined to be x = 0.040 ± 0.001 by Rutherford

back-scattering spectrometry (RBS) The value of x tends to show slightly

larger than the composition of the target, and composition difference

between seed and top layers was not detected Structural properties

were characterized by X-ray diffraction and cross-sectional

transmis-sion electron microscopy (TEM)

3 Results and discussions Fig 1(a) schematically shows the structure of the presently fabricated TNO films A seed layer with a thickness of 30 nm was

first deposited on the unheated substrate at f(O2) = 5%, and,

subse-quently, a main film layer was grown at f(O2) = 0.05%

Fig 1(b) is a plot of the resistivity (ρ) of the annealed TNO films against annealing temperature These films were subjected to 60 min annealing in pure H2(1 × 105Pa) or vacuum (3 × 10− 3Pa) and were confirmed to be in the anatase polycrystalline phase, except for the film prepared at 250 °C, from X-ray diffraction measurements The film annealed at 250 °C was still amorphous, and thus, we determined the optimal annealing temperature to be 300–400 °C The single–layer film without seed layer does not crystallize when annealed at 300 °C,

Table 1

Summary of deposition and annealing conditions and resistivity of the samples

Sample

number

Composition

Ti 1− x Nb x O 2

conditions

Resistivity

Fig 1 (a) Schematic structure of double-layer film (b) Resistivity as a function of

annealing temperature.

Fig 2 Temperature dependence of (a) resistivity, (b) carrier density, (c) Hall mobility of anatase Ti Nb O polycrystalline film on glass substrate (sample #2).

T Hitosugi et al / Thin Solid Films 517 (2009) 3106–3109

showing the importance of seed layer As the f(O2) increases,

crystallization temperature is reduced (not shown in figure), that is,

crystallization temperature of seed layer is lower than that of main

layer We speculate that when the seed layer crystallizes at 300 °C, the

main film is crystallized due to the initiation from seed layer However,

it is still unclear about the origin of the link between crystallization

temperature and oxygen stoichiometry

Notably, the values of vacuum-annealed films are almost identical

to those of the H2-annealed ones The TNO film annealed at 300 °C in

vacuum showed ρ as low as 9.2 × 10− 4Ωcm (sample #1,Table 1) The

lowest resistivity ρ = 7.0 × 10− 4Ωcm (carrier density ne= 1.2 × 1021cm− 3

and Hall mobility μH= 8 cm2V− 1s− 1) was obtained by annealing at

400 °C in pure H2 (sample #2), while as-grown films showed

5 × 101

Ωcm at room temperature Without seed layer, an annealed

single-layer film (f(O2) = 0.05%) exhibited 9.8 × 10− 4Ωcm (sample #3),

which is larger than the value of the film with seed layer Reason of this

improvement in ρ is still in debate By applying a similar double-layer

structure to Ti0.94Nb0.06O2, we attained ρ = 6.4 × 10− 4Ωcm (sample #4),

while the film without seed layer exhibited 7.6× 10− 4Ωcm (sample #5)

These films show metallic temperature dependence of electron

transport properties at low temperature.Fig 2(a), (b) and (c) show ρ,

ne, and μH, respectively, as functions of temperature (T) for the

double-layer Ti0.96Nb0.04O2 film prepared by 400 °C annealing in H2

atmosphere (sample #2) The ρ–T curve shows a metallic temperature

dependence, dρ/dT N 0, and neis almost independent of temperature,

clearly indicating that the present polycrystalline TNO film can be

regarded as a degenerated semiconductor From the ne value of

~1.2 × 1021cm− 3, it is estimated that doped Nb atoms are activated

with an efficiency of N90% This result strongly suggests that the Nb

dopants are substituted for Ti sites in anatase TiO2without segrega-tion High carrier activation efficiency is a unique characteristic of TNO

in both epitaxial and polycrystalline films, in sharp contrast to those of ITO, typically b50%[16] The μHincreases with decreasing tempera-ture, implying that the room temperature ρ is dominated by phonon scattering In other words, grain boundary scattering is not a dominant factor in determining ρ

Transmittance (Tr) and reflectance (R) spectra of a TNO film after

annealing (thickness ~ 200 nm) are shown inFig 3(a) The Trvalues in

a wavelength region of 400–800 nm are 60–80%, while R ranges from

10 to 40% The large R is due to the relatively high refractive index of anatase TNO, approximately 2.4 at 500 nm The absorbance (A) in the visible region, evaluated from the formula A = 1−(Tr+ R), is as low

as 10%, indicating excellent transparency of the present TNO films (Fig 3(b))

By using the above-mentioned low temperature process, we fabricated TNO films on plastic film substrate.Fig 4(a) is an X-ray diffraction pattern of a TNO film deposited on polyimide (sample #6), clearly indicating an anatase (101) peak.Fig 4(b) compares transport properties between TNO films deposited on polyimide and glass The

Fig 3 (a) Transmittance, reflectance, and (b) absorbance, of double-layer Ti 0.94 Nb 0.04 O 2

polycrystalline film on a glass substrate (sample #2).

Fig 4 (a) X-ray diffraction pattern of anatase Ti 0.96 Nb 0.04 O 2 thin film on polyimide film.

“A(101)” denotes anatase (101) peak Amorphous film on polyimide was annealed in H 2

atmosphere (1 × 10 5 Pa) at 300 °C (sample #6) (b) Comparison of transport properties between films on non-alkaline glass (sample #2) and on polyimide (sample #6).

T Hitosugi et al / Thin Solid Films 517 (2009) 3106–3109

former exhibits slightly higher ρ of 1.9 × 10− 3Ωcm, reflecting lower ne

and lower μH This might be due to surface roughness of the polyimide

substrate and contamination from polyimide

4 Conclusion

We have established a low temperature preparation procedure

(~300 °C) using seed layers for transparent-conducting anatase

Nb-doped TiO2 (TNO) polycrystalline films on glass and polyimide

substrates We achieved the lowest resistivity of ρ = 6.4 × 10− 4Ωcm and

excellent optical transparency (transmittance ~70%, absorptionb10%) in

the visible region on glass The optimal TNO film showed high carrier

activation efficiency of N90% and metallic temperature dependence of ρ

These results highlight anatase TNO as a promising candidate for

next-generation TCOs

Acknowledgment

This work was supported by the Global COE Program for Chemistry

Innovation, NEDO, MEXT Elements Science and Technology Project,

and Grain-in-Aid for Young Scientists (B) 19760475, 2007

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