Properties of Resistivity, Reflection and Absorption Related to Structure of ITO Films

J.Mater.Sci.Technol.,2012,28(4),

325–328.

Properties of Resistivity,Re?ection and Absorption Related to Structure of ITO Films

Yipeng Chao,Wu Tang ?and Xuehui Wang

State Key Laboratory of Electronic Thin Films and Integrated Devices,University of Electronic Science and Technology of China,Chengdu 610054,China

[Manuscript received July 11,2011,in revised form December 8,2011]

Indium tin oxide (ITO)?lms were fabricated on polyethylene terephthalate (PET)substrate at room tem-perature using dc magnetron sputtering technique with di?erent sputtering powers.The structural,electrical and optical properties were investigated by X-ray di?raction (XRD),Hall e?ect,re?ection and transmission,respectively.XRD patterns show gradual enhancement of crystalline quality with increasing sputtering power.Signi?cant improvement of Hall mobility due to the reduction of defects was observed though the carrier density varied slightly.Simultaneously,the mean transmission in visible light range decreased severely with increasing sputtering power.Slight move toward shorter-wavelength side of absorption peak was due to the variation of plasma wavelength.The re?ection increase of near-infrared light originated from the decrease of resistivity.Finally,band gap was obtained using Tauc s relation and it was consistent with Burstein–Moss shift.

KEY WORDS:Crystalline quality;Hall mobility;Absorption

1.Introduction

Indium tin oxide (ITO)?lm has been widely used as transparent conducting material in many electronic and optoelectronic devices such as ?at panel display,anti-re?ection coating,solar cell and heat mirror for its high transmission in visible light range and high conductivity [1–3].Flexible substrate such as poly-ethylene terephthalate (PET)is more suitable than glass substrate in the applications of electronic maps,smart cards and portable computers where ?exibil-ity,weight,and safety issues are important [4,5].One fatal disadvantage of PET substrate is the lack of re-sistance at high temperature and ITO ?lms deposited at low temperature usually show high density of de-fects which are apt to cause severe absorption in vis-ible light range.Since the sputtered atom on PET substrate usually has low mobility,the sputtering rate controls the crystalline quality of ITO ?lms.The elec-?Corresponding author.Prof.,Ph.D.;Tel.:+862883202550;E-mail address:tang@https://www.wendangku.net/doc/97773022.html, (W.Tang).

trical and optical properties of ITO ?lms are closely related to the ?lm crystalline quality,thus it is impor-tant to investigate the in?uence of sputtering power on microstructural,electrical and optical properties.In this article,ITO ?lms with di?erent sputtering powers of 60,80,100and 120W were fabricated.The in?uence of sputtering power on the microstructure,resistivity and optical property was investigated.2.Experimental

ITO ?lms were prepared on ?exible PET sub-strate using dc magnetron sputtering technique at room temperature with di?erent sputtering powers of 60,80,100and 120W.The target was a ceramic disc of 10%SnO 2and 90%In 2O 3by weight.Before deposition,the substrate was rinsed with acetone for 15min,and then cleaned with deionised water in ul-trasonic bath for 20min and dried with nitrogen of a purity of 99.99%.Before deposition the base pressure was evacuated to 7.0×10?4Pa,and then Ar gas of high purity was introduced.The ceramic target was

326

Y.P.Chao et al.:J.Mater.Sci.Technol.,2012,28(4),325–328.

6080100120

T h i c k n e s s /

m

Sputtering power / W

Fig.1Thickness variation of ITO ?lms with di?erent

sputtering powers

presputtered for 30min to remove the impurity.Dur-ing sputtering the working pressure,sputtering time and target-substrate distance were 0.2Pa,90min and 100mm,respectively.

The structure of ITO ?lms was measured by X-ray di?raction (XRD).The thickness was estimated by cross-section scanning with scanning electron mi-croscopy (SEM).The carrier concentration,Hall mo-bility and resistivity were measured by the van der Pauw method.The infrared re?ectance and trans-mission was measured by Fourier transform infrared spectroscopy (FTIR).3.Results and Discussion

The thickness of ITO ?lms with di?erent sputter-ing powers is shown in Fig.1.Film thickness corre-sponding to 60–120W is 1.46,1.60,2.75and 3.61μm,respectively.It is shown that at the initial stage of 60–80W,the deposition rate is small,and with increas-ing sputtering power,the deposition rate increases and almost keeps constant.When the ?lm growth process begins,the sputtered atoms reach the surface of PET substrate randomly and there is almost no di?usion along PET surface due to the limited mo-bility,accordingly,the current growth mode usually displays longitudinal growth and the nucleation par-ticle is small.With increasing sputtering power,the sputtered atoms become more energetic and simulta-neously the amount increases [6].The atoms reaching the surface have higher mobility and lateral growth happens.Thereby,the increase of sputtering power is apt to reduce the density of defects and promote the growth of particles [7].

Fig.2shows the XRD patterns of ITO ?lms with di?erent sputtering powers.The di?raction peak at 26deg.is attributed to the PET substrate,and the reduction of intensity is due to the increase of ?lm ab-sorption with increasing thickness [8].There is no dif-fraction peaks from ITO ?lms at sputtering power of 60–80W.With increasing sputtering power to 100W,

20304050607080

I n t e n s i t y / a .u .

2 / deg.

60 W

80 W

100 W

120 W

(400)

Fig.2XRD patterns of ITO ?lms

6080100120

R e s i s t i v i t y /c m

Spu ttering pow er / W

H a l l m o b i l i t y / c m

V s Fig.3Resistivity (ρ),carrier density (n )and Hall mo-bility (μ)of ITO ?lms

(400)di?raction peak appears abruptly and the inten-sity is rather strong.With further increase of sput-tering power,(400)di?raction angle does not move and the intensity gets stronger.Therefore,the crys-talline quality of ITO ?lms improves greatly just due to increasing sputtering power.

The resistivity (ρ),carrier density (n )and Hall mobility (μ)were investigated through Hall e?ect as shown in Fig.3.With increasing sputtering power,the resistivity decreases monotonously and the mini-mum resistivity (1.2×10?3?·cm)is achieved at sput-tering power 120W.It reveals that the carrier den-sity decreases gradually except for the ?lm of 100W and the magnitude of variation is rather small ranging 1.5×1020–3.5×1020cm ?3,but the Hall mobility varies greatly ranging 5–35cm 2·V ?1·s ?1.Obviously,the re-duction of resistivity is mainly due to the increase of Hall mobility.The Hall mobility in polycrystalline semiconductors is related with grain boundary scat-tering and ionized impurity scattering [9,10].However in heavily-doped semiconductors,the potential bar-rier at grain boundary is signi?cantly reduced by the carrier density through [11]:

?B =

eQ 2

8εr ε0N d

(1)

Y.P.Chao et al.:J.Mater.Sci.Technol.,2012,28(4),325–328.

327

5001000150020002500

0.0

0.2

0.4

0.6

0.8

T r a n s m i s s i o n W avelength / nm

60 W

80 W

100 W

120 W

Fig.4Transmission spectra of ITO ?lms with sputtering

power 60,80,100and 120W,respectively

5001000150020002500

0.0

0.1

0.2

0.3

0.40.5

0.6

0.7

R e f l e c t i o n

W avelength / nm

60 W

80 W

100 W

120 W

Fig.5Re?ection spectra of ITO ?lms with sputtering

power 60,80,100and 120W,respectively

where Q is the electric charge trapped at grain bound-aries and N d is the donor concentration.The sym-bols εr and ε0represent dielectric constants.The donor concentration is approximately equal to car-rier density in heavily-doped ITO ?lms at room temperature [12],thus it is seen that with carrier den-sity over 1020cm ?3,the potential barrier at grain boundary is negligibly small.The Hall mobility is mainly a?ected by the ionized impurity scattering through [13]:

μi =4e h (3π

)1/3N ?2/3

i

(2)where h is the Planck constant and N i is the density of charged impurity.With increasing density of charged impurity,the Hall mobility is reduced and vice versa.The density of charged impurity can be assumed to be equal to the carrier density,so the increase of Hall mobility is directly related to the reduction of carrier density,and the exceptional decrease of ITO ?lm at 100W also follows the same reciprocal relationship.The transmission spectra of ITO ?lms with di?er-ent sputtering powers are shown in Fig.4.The high-est transmission in visible light range is achieved with

sputtering power 60W and the mean transmission is about 75%which is lower than the reported result [14].With increasing sputtering power,the transmission decreases greatly.Generally the sum of transmission,re?ection and absorption is equal to 100%.The re-?ection spectra are also shown in Fig. 5.It is seen that the re?ection in visible light range changes very slightly,so the absorption is considered as the main source for transmission reduction.The absorption of ?lms is generally related to absorption coe?cient αand ?lm thickness t through exp(-αt ),and the trans-mission,re?ection and ?lm thickness are used to ob-tain the absorption coe?cient [15].The absorption co-e?cient in visible light range decreases with increasing sputtering power,which is consistent with the XRD results.The absorption of ITO ?lms in visible light range is related to all kinds of defects which usually in-troduce density of state into the band gap,therefore,the decrease of absorption indicates the reduction of density of defects.It is concluded that the increase of sputtering power helps to improve the crystalline quality and reduce the density of defects,and the re-duction of transmission is attributed to the increase of ?lm thickness.

Slight move toward shorter-wavelength side of re-?ection minimum is also observed with increasing sputtering power.This is due to the variation of car-rier density and electron e?ective mass.The electron e?ective mass m *is obtained through classic Drude model [16]:

m ?

=ne 2λ2p 4π2ε0ε∞c 2(3)where n ,c and λp are the carrier density,speed of

light in vacuum and plasma wavelength,respectively,and ε0and ε∞are the free space dielectric constant and high frequency dielectric constant,respectively.The plasma resonance wavelength is related to the re?ection minimum wavelength λmin as [17]:

λp =λmin

ε∞

ε∞?1(4)

The minimum of re?ection is directly obtained from re?ection spectrum.The calculation of m ?/m 0(where m 0is the free electron mass)with sputtering power of 60,80,100and 120W is 0.17,0.11,0.22and 0.09,respectively.The minimum m ?is obtained with sputtering power 120W corresponding to the largest Hall mobility,and the variation law of electron e?ec-tive mass is quite opposite with Hall mobility.Clas-sic collision model μ=e τ/m ?is employed to interpret the relationship between Hall mobility and e?ective mass,and the reduction of e?ective mass helps to increase the Hall mobility [18].The shift of infrared absorption peak (not shown here)is due to the vari-ation of plasma wavelength,and simultaneously the re?ection enhancement of near-infrared light with in-creasing sputtering power is mainly attributed to the reduction of resistivity [19].

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Y.P.Chao et al.:J.Mater.Sci.Technol.,2012,28(4),325–328.

6080100120

B a n d g a p / e V

Sputtering pow er / W

Fig.6Band gap of ITO ?lms vs sputtering power

The variation of band gap vs sputtering power is shown in Fig.6.The band gap is calculated through Tauc s relation using direct transition method [20].It is noted that the variation laws of band gap and car-rier density are the same.The shift of band gap with carrier density is due to the well-known Burstein–Moss shift [21].When the semiconductor is so highly-doped that Fermi level exceeds the conduction band and the semiconductor enters degenerate state,the activated electron at the top of valance band cannot jump to the bottom of conduction band because it is already ?lled.The activated electron can only jump to higher energy and thus the band gap is increased.4.Conclusions

(1)(400)peak of ITO ?lm appears and the peak intensity increases with increasing sputtering power,therefore,it is clear that the increase of sputtering power is bene?cial to improve the crystalline quality.(2)With increasing sputtering power,the resistiv-ity decreases though carrier density changes slightly.Besides,great improvement for Hall mobility is ob-served.Ionized impurity scattering is proposed to in-terpret the variation of Hall mobility.

(3)Though the mean absorption of ITO ?lms in visible light range increases with increasing sputtering power,the absorption coe?cient still decreases.This phenomenon is consistent with the increase of Hall mobility and both are related to the improvement of crystalline quality.

(4)Slight change of optical band gap for ITO ?lms is observed and related to the variation of car-rier density.Well known Burstein–Moss shift is pro-posed to interpret the correlation.The applicability

of Burstein–Moss shift con?rms that the ITO ?lms are highly-doped and display degenerate property.

Acknowledgements

This work was supported by the National Natural Sci-ence Foundation of China (No.51071038),Program for New Century Excellent Talents in University (NCET-09-0265),Sichuan Province Science Foundation for Youths (No.2010JQ0002),and State Key Laboratory for Me-chanical Behavior of Materials,Xi an Jiaotong University (No.201011005),respectively.

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