Highly Conducting Transparent Indium-Doped ZincOxide Thin Films

BUDHI SINGH1 and SUBHASIS GHOSH1,2

1.—School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India.2.—e-mail: subhasis.ghosh.jnu@gmail.com

Highly conducting transparent indium-doped zinc oxide (IZO) thin films havebeen achieved by controlling different growth parameters using radio fre-quency magnetron sputtering. The structural, electrical, and optical proper-ties of the IZO thin films have been investigated for varied indium content andgrowth temperature (TG) in order to find out the optimum level of doping toachieve the highest conducting transparent IZO thin films. The highestmobility and carrier concentration of 11.5 cm2/V-s and 3.26 9 1020 cm�3,respectively, have been achieved in IZO doped with 2% indium. It has beenshown that as TG of the 2% IZO thin films increase, more and more indiumatoms are substituted into Zn sites leading to shift in (002) peaks towardshigher angles which correspond to releasing the stress within the IZO thinfilm. The minimum resistivity of 5.3 9 10�4 X-cm has been achieved in 2%indium-doped IZO grown at 700�C.

Key words: IZO, transparent conducting electrode, stress

INTRODUCTION

Zinc oxide (ZnO) has stimulated strong researchinterest in recent years1,2 due to its multi-functionalapplications, environmental friendliness, and rela-tive abundance. As grown, ZnO exhibits n-typeconductivity due to uncontrolled intrinsic defects,mostly by O vacancies and Zn interstitials.3 Then-type conductivity of ZnO can be increased andcontrolled by doping with group III elements,4–8

whereas p-type ZnO still remains a challenge. ZnO-based, highly transparent and conducting thin filmsare anticipated to have wide applications fortransparent electronics, and transparent electrodeswill replace indium tin oxide. Of three group IIIdopants, aluminum (Al), gallium (Ga), and indium,Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) havebeen studied most extensively.4–9 Mostly, heavydoping in ZnO has been achieved by engineeringgrowth parameters or by post-growth annealing. Itis difficult to tailor and reproduce material proper-ties using post-growth annealing7,10 as there are

several uncontrolled factors, which are in mostcases undesirable for device processing. The stress-induced band structure engineering and enhance-ment of carrier mobility have been investigated indetail in Si, Ge, and III-V semiconductors.11,12 It hasbeen shown that a two-fold increase in hole mobilityand drive current in p-channel silicon transistors11

and a four-fold increase in carrier mobility III–Vsemiconductors12 can be achieved by inducinguniaxial/biaxial compression intrinsically orextrinsically on the channel materials. Similarstress-induced band structure engineering andmodification of electrical properties in II–VI semi-conductors, in particular ZnO, have not been fullyexplored. Nevertheless, variation of energy gap inZnO has been recently reported using densityfunctional-based first principle calculations.13,14

The structural properties and band structure can betuned by applying stress externally or inducingstrains through lattice mismatch in epitaxial films.The lattice mismatch generally gives rise to eitherbiaxial strains along the interfacial plane or uniax-ial strains normal to the interface plane. Thesemethods are not suitable for polycrystalline thinfilms. However, the stress can also be inducedintrinsically by engineering chemical pressure(Received September 4, 2013; accepted May 16, 2014)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-014-3256-5� 2014 TMS

while growing the thin films. To incorporate stressin ZnO and simultaneously make it highly con-ducting by heavy doping, it is required to replace Znwith a group III element whose size is more thanthat of Zn. Indium would be most suitable for thispurpose. Moreover, it has been realized thatindium-doped ZnO (IZO) has several advantagesover other group III-doped ZnO, such as (1) it could beused as transparent conducting electrode for fullytransparent and flexible thin film transistors,15 (2)IZO could be a potential candidate for ZnO basedspintronics devices,16 (3) indium is less reactive thanAl and more resistive to oxidation during the thinfilm growth, and (4) it has been shown that amor-phous IZO-based field effect transistors could bepotential candidates for future oxide-based flexibleelectronics. In this paper, we show that high mobility(l), low resistivity (q) and high optical transmissioncan be achieved in IZO by engineering two parame-ters: (1) stress introduced by chemical pressure and(2) growth temperature (TG).

EXPERIMENTAL PROCEDURES

The IZO sputtering targets with different indiumcontent from 0.1% to 10% were prepared by solidstate route. High pure ZnO (99.999%) and In2O3

(99.999%) powder procured from Sigma Aldrichwere mixed in stoichiometric ratio, ground for 24 hand sintered at 850�C for 24 h for preparing targetsof 1 inch in diameter and 0.25 inches thick. IZO thinfilms were deposited on a transparent quartz sub-strate using radio frequency (RF) magnetron sput-tering. Table I shows the stoichiometric ratio andresulting composition of IZO thin films. The quartzsubstrate used for film deposition was ultrasonicallycleaned for 5 min in trichloroethylene, acetone,isopropyl alcohol, and de-ionized water, and thendried in nitrogen gas atmosphere. The target to thesubstrate distance was kept fixed at 7 cm. Argonand oxygen gases were used as sputtering gases in aratio of 6:4. These parameters have been optimizedfor obtaining the highest quality ZnO films ofthicknesses from 100 nm to 300 nm. The TG wasvaried from 100�C to 700�C. The backgroundpressure was 10�6 mbar and the chamber wasmaintained at 0.01 mbar during growth. The qmeasurement of IZO films was carried out usingthe Van der Pauw method. The l and electron

concentration (n) in IZO films were determined byHall measurements under a magnetic field of 1 Te-sla with Keithley 6221 current source and 2182Ananovoltmeter. The thickness of the samples wasmeasured by a SOPRA GES5E spectroscopic ellips-ometer and also by Rutherford backscatteringspectroscopy (RBS). Grazing incidence x-ray dif-fraction (XRD) was performed using the PanalyticalXpert Pro system. A Shimadzu UV-2401PC spec-trophotometer was used for absorption spectroscopyin the wavelength range of 200 nm to 1000 nm. TheRBS system consists of an evacuated chamber con-taining He ion generator, accelerator, and detector.RBS measurements were performed in a vacuum of10�6 mbar using a 2.0 MeV He+ ion beam in aGeneral Ionex Tandetron accelerator. Sample anddetector were kept in the Cornell geometry17 suchthat the backscatter detector is directly below theincident beam. The samples were titled to 60� offnormal to increase the depth resolution. Energyspectra were obtained using a surface barrierdetector and were analyzed using the RUMP com-puter simulation program.18

RESULTS AND DISCUSSIONS

Figure 1 shows the transmission and absorptionspectra for IZO thin films grown at 600�C, withdifferent indium content. The transmittance valueof the IZO thin films vary significantly between 75%and 95% as a function of wavelength and dopingconcentration. This variation in transmission is dueto interference resulting from the multiple reflec-tion of light between the lower surface in contactwith the quartz substrate and the free surface of theIZO film. This wavy nature of the IZO thin filmsdepends upon the thickness of the sample; thethicker sample (>200 nm) shows more interferencefringes. The inset of Fig. 1 shows the absorptionspectra for the same samples. The high quality ofthe samples can be judged from three observations:(1) the presence of an excitonic peak in all IZOsamples, (2) the absence of mid-gap and band tailstates, and (3) the presence of the interferencepattern. The thicknesses of the IZO thin filmsobtained from the interference pattern in absorptionspectra exactly match the values obtained inde-pendently by spectroscopic ellipsometery and RBSmeasurements described in the next section.

Table I. The stoichiometric ratio and resulting composition of IZO

Indiumcontent (%)

Molecularformula

Wt of ZnO(gm)

Wt of In2O3

(gm)Wt of ZnO/Wt

of In2O3 (wt ratio)Resulting

composition of In (%)

10 Zn0.90In0.10O 5.884 1.115 5.277 5.54 Zn0.96In0.04O 6.535 0.464 14.08 2.32 Zn0.98In0.02O 6.765 0.235 28.74 1.21 Zn0.99In0.01O 6.881 0.118 58.06 0.60.1 Zn0.999In0.001O 6.988 0.012 582.33 0.1

Singh and Ghosh

Figure 2 shows the RBS spectra of IZO thin filmswith different indium contents varying from 0.1% to10% grown at 600�C. Zn and indium peaks can beresolved unambiguously in IZO thin films at lowerthickness. Figure 2 also shows the fitting of RBSspectra with the RUMP computer program18 whichsimulates the contribution of individual peaks cor-responding to the various elements in IZO thinfilms. The height of the indium peak in RBS spectraincreases as dopant concentration varies from 0.1%to 10%. From the RBS analysis, it has been foundthat not all the indium atoms incorporate into theIZO thin films during deposition. The amounts ofindium in the IZO films are 0.1%, 0.6%, 1.2%, 2.3%,and 5.5% for actual doping of 0.1%, 1%, 2%, 4%, and10%, respectively. Kim et al.4 have also observed asimilar trend regarding the difference between theactual and resulting composition. The thicknesses ofall IZO thin films obtained from RBS simulationmatch exactly with the values obtained from theellipsometer. The thicknesses of the IZO films usedfor optical, structural, and electrical characteriza-tion are between 300 nm and 350 nm. But, in thecase of higher thickness samples (>200 nm), the Znand indium peaks overlap in the RBS spectra whichcan only be resolved in samples with thicknessesless than 200 nm.

The XRD data of IZO thin films grown at 600�Cshowed a pronounced c-axis orientation resulting ina strong (002) peak corresponding to the wurtzitestructure, as shown in Fig. 3. In addition tothe (002) peak, a small peak corresponds to (101)orientation, which is due to deviation from c-axisorientation can only be observed in the sample withhighest (10%) indium content. The absence of In2O3

phase in the XRD of IZO films indicates homoge-nous mixing of In2O3 with ZnO resulting in single

phase wurtzite ZnO. As indium content increases inIZO thin films, the diffraction peak (002) shiftstowards a lower angle. This behavior indicates thatthe incorporation of indium into ZnO increases thec-axis lattice constant from 5.189 A

´(ZnO thin film)

to 5.35 A´

in 10% IZO thin film, and this results inresidual stress in IZO films. This is due to a largerionic radius of In3+ (0.094 nm) than that of Zn2+

(0.074 nm). Similar behavior has been reported byLiu et al.19 for IZO nanowire. For hexagonal crys-tals, the inplane stress r can be calculated using abiaxial strain model and is given by20

r ¼ 2C13 � C33 C11 þ C12ð ÞC13

� �c� c0

c0

� �; (1)

where c is the lattice constant obtained from the(002) diffraction peak, co (=5.206 A

´) is the lattice

constant in undoped ZnO, and C11, C12, C13, and C33

are elastic stiffness constants with their respectivevalues of 2.1 9 1011 N/m2, 1.2 9 1011 N/m2,1.05 9 1011 N/m2, and 2.1 9 1011 N/m2, respec-tively.6 Using these values, Eq. 2 results followingthe relationship for the stress in the thin films,

r ¼ �4:5� 1011 c� c0

c0

� �N�

m2: (2)

The calculated values of stress for differentindium contents are given in Fig. 4. It has beenfound that both undoped ZnO and highly doped(>2%) IZO show maximum stress. In undoped ZnO,the stress is mainly due to a thermal and latticemismatch between the quartz substrate and theZnO thin films. In low-doped IZO samples, the

Fig. 1. Transmission spectra of the IZO thin films with differentindium content grown at 600�C. Inset the absorption spectra of thesame samples. Spectra shifted upward for clarity. Arrow indicatesthe position of excitonic peak. Connecting lines are a guide for eyes.

(a) (b)

(c) (d)

Fig. 2. RBS spectra fitted with the RUMP computer simulation pro-gram (solid line) of (a) 0.1%, (b) 1%, (c) 2%, and (d) 10% doped IZOthin film. Empty circle represents experimental data. Steps at 600and 900 channel numbers are due to Si and O.

Highly Conducting Transparent Indium-Doped Zinc Oxide Thin Films

stress gets released and attains a value the same asthat in undoped ZnO powder. The inbuilt chemicalpressure due to the difference in sizes of indium andZn atoms has been anticipated to be the reason forreleasing stress in indium-doped ZnO. This conjec-ture is supported by the data presented in Fig. 4which shows monotonic dependence of stress on theindium content. The stress eventually becomes zeroat a critical indium content which is little more than1%. After this critical level of doping, indium atomsare pushed into interstitial sites leading to theexpansion of the hexagonal lattice and thusenhancing the compressive strain in the IZO filmsindicated by the negative sign. Figure 4 also shows then and l as a function of dopant concentration. As theindium content increases, the n of IZO thin film firstincreases, becomes maximum (�3.26 9 1020 cm�3) atthe 2% doping level, and then decreases and becomes7.88 9 1019 cm�3 at the 10% doping level. Hence, nincreases with indium concentration from 0.1% to 2%,but after the 2% doping level the indium atoms eithergo to interstitial sites or segregate into the grainboundaries and are not activated as dopants. Similarbehavior has been observed4 in AZO thin films. Themaximum value of l (�11.5 cm2/V-s) has beenobtained in 2% indium-doped IZO thin films. It hasalso been observed that both l and n follow similarnonmonotonic dependence on the indium content.This indicates that grain boundary scattering6 is thedominant mechanism for deciding the charge carriermobility in IZO thin films.

The XRD data of the IZO thin film doped with 2%indium, grown at different TG, show a pronouncedc-axis orientation, as shown in Fig. 5. As TG of theIZO films is increased, the intensity of the (002)

peak increases, becomes sharper, and shifts towardhigher angles. In addition to the (002) peak, a smallpeak corresponding to (101) orientation is seen at36.2�, which is due to deviation from the c-axisorientation. The shift of the diffraction peak can beassociated with the release of tensile strain devel-oped within the film. At higher TG, large numbers ofindium atoms are substituted into Zn sites, and thisleads to a shift in the 2h value of the IZO filmstoward higher angles. Similar shifts have beenreported for post-growth annealing.21 Hence, it canbe emphasized from XRD data that, at higher TG,larger numbers of indium atoms are incorporatedinto the IZO films as compared to films grown atlower TG. The intensity of the (002) peak increaseswith TG and this can be attributed to enhancedtexturing of the films. Hence, the XRD data pre-sented in Figs. 3 and 5 indicate that the 2% indium-doped IZO thin film grown at ‡600�C has theoptimum electrical and optical properties (Fig. 4)which are essential for the development ofIZO-based transparent electronics.

Figure 6 shows how the residual stress varieswith TG in IZO thin films with 2% indium. Thedecreasing stress with TG indicates that high TG

improves the crystal quality of the IZO thin films.Hence, the cumulative effect of high TG and residualstress release by indium in IZO thin films leads tothe highest conducting thin films. Figure 6 furthershows how q and l of 2% indium-doped IZO thinfilms vary with TG. As expected, Fig. 6 shows themonotonic increase of l and decrease of q with TG.

Fig. 3. XRD profile of IZO films prepared with different indium con-tent varying from 0.01% to 10%. The 2h (002) peak around 34�shows the pronounced c-axis orientation in the film. Upper arrowindicates the possible position of the In2O3 XRD peak. Their absenceindicates the monophasic nature of thin films. The lower arrow indi-cates the position of the (002) peak of the ZnO powder.

(a)

(b)

Fig. 4. (a) The dependence of the (002) 2h peak position on theindium content. Variation of stress within the IZO film as a function ofindium content is also shown. The stress value for undoped ZnO thinfilm is 0.14 9 1010 N/m2. Horizontal line the zero residual stress. Thevertical arrow indicates the estimated value of indium content atwhich the (002) peak of the IZO film matches with the (002) peak ofthe ZnO powder and the stress of the IZO film becomes almost zero.(b) Carrier concentration (n) and mobility (l) of IZO thin films as afunction of indium content in IZO thin films. Connecting lines areguides for eyes.

Singh and Ghosh

The decrease of q with TG signifies that, as TG

increases, more indium atoms incorporate into theIZO thin films, as corroborated by the XRD data pre-sented in Fig. 5. The minimum value of 5.3 9 10�4 X-cm for q has been obtained in 2% indium-doped IZOthin films grown at 700�C. An almost similar value of1.2 9 10�4 X-cm was observed in AZO thin filmsgrown by RF sputtering.7,22

CONCLUSIONS

In conclusion, we have grown IZO thin filmsby RF sputtering with a wide range of In contents

ranging from 0.1% to 10%. It has been found that TG

and indium content have a strong influence on thestructural, electrical, and optical properties of theIZO thin films. Minimum q has been observed inIZO thin films grown at 700�C with an indiumcontent of 2% and grown at ‡600�C without post-growth annealing.

ACKNOWLEDGEMENTS

We acknowledge Advanced InstrumentationResearch facility of JNU and IUAC for providingexperimental facilities for this work. B. S. thanksCouncil of Scientific & Industrial Research, India,for the financial support through fellowship.

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Fig. 5. XRD spectra of IZO films grown at different temperatures(TG). The 2h (002) peak at around 34� shows the pronounced c-axisorientation in the thin films. A small peak at 36.2 is due to the (101)orientation, which has minimum intensity in sample grown atTG = 700�C. The right inset shows the deviation of the 2h (002)peaks from that of undoped ZnO powder.

Fig. 6. Variation of stress and resistivity (q) of 2% IZO film as afunction of TG. Inset shows the variation of mobility (l) of 2% IZO thinfilms as a function of TG. Connecting lines are guides for eyes.

Highly Conducting Transparent Indium-Doped Zinc Oxide Thin Films