Solid State Communications 149 (2009) 2177–2180

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Al-doping effect on structural, transport and optical properties of ZnO films bysimultaneous RF and DC magnetron sputteringJ.J. Lu a,∗, S.Y. Tsai b, Y.M. Lu c, T.C. Lin a, K.J. Gan aa Nano-Technology R&D Center, Kun-Shan University, Tainan, Taiwanb Department of Materials Science & Engineering, National Cheng-Kung University, Tainan, Taiwanc Graduate Institute of Electro-Optical Engineering, National University of Tainan, Tainan, Taiwan

a r t i c l e i n f o

Article history:Received 16 July 2008Received in revised form24 July 2009Accepted 14 September 2009 by J.A. BrumAvailable online 18 September 2009

PACS:78.20. Ci78.66. Hf

Keywords:A. Al-doped ZnO filmsD. Semiconductor–metal transition

a b s t r a c t

Transparent and conductive Al-doped ZnO films have been prepared by simultaneous RF and DCmagnetron sputtering. In order to study the properties of the Al-doped ZnO films, we performed X-ray diffraction, X-ray absorption spectroscopy, temperature dependence of electrical resistance and Hallmeasurements, as well as optical transmission spectroscopy. The results revealed that all the sampleswere polycrystalline with a strong preferential c-axis orientation. A minimum resistivity of 7.13 ×10−3 � cmwas obtained, and ametallic-type conducting behavior was observed for the film at 50W. Ourpresent work suggests that the electrical transport property of the Al-doped ZnO films is closely related tothe crystallinity. A large number of defects due to poor crystallinity and the induced stress field are ableto immobilize the free carrier thereby reducing the conductivity.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The transparent conducting ZnO film presents many remark-able characteristics due to its large piezoelectric constant, wideband gap energy, stability, non-toxicity and low cost. It can bewidely used in many technological domains, such as flat panel dis-play, solar cell, acoustic wave device, and piezoelectric transducer.Recently, a number of ZnO films doped with various metallic

ions have been studied extensively for the manipulation of theiroptical and electrical properties for wide applications [1–4]. InAl-doped ZnO (AZO) powder systems, a metallic-type conductingbehavior was observed by Mantas et al. [5]. It is generally believedthat for N-type doping ZnO, the enhancement of conductivityis ascribed to the increase in carrier concentration due to morefree electrons and more oxygen vacancies. However, only a fewstudies on the conducting behavior have been reported, and themechanism that leads to the semiconductor–metal transition isstill under investigation.The AZO film can be fabricated by the sol–gel [6–8], evapora-

tion [9,10], deposition [11–13], and sputtering methods [14–17],etc. Among these methods, magnetron sputtering is considered to

∗ Corresponding author. Tel.: +886 6 2052118; fax: +886 6 2727175.E-mail address: jjlu@mail.ksu.edu.tw (J.J. Lu).

0038-1098/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2009.09.021

be a suitable technique due to inherent ease with which the fab-rication conditions can be controlled. In this study, the AZO filmsat various doping levels were fabricated by simultaneous RF mag-netron sputtering of Zn and DCmagnetron sputtering of Al. Severalmeasurements, including X-ray diffraction (XRD), temperature de-pendence of electrical resistivity, X-ray absorption spectroscopy(XAS), optical transmission spectrum, and Hall measurementswere carried out in order to investigate the effect of Al-doping onstructural, transport and the optical properties.

2. Experimental details

The pure ZnO and AZO films were deposited onto heatedglass substrates (Corning 1737F) by magnetron co-sputtering inamixture of oxygen and argon gases. The targets used in this studyare metal Zn (99.99% purity, 76.2 mm diameter), and metal Al(99.999% purity, 76.2mmdiameter). The RF power of Zn targetwaskept constant at 300 W, and a DC power of 0–200 W was appliedto the Al target.The XRD patterns of the deposited films were obtained by an

X-ray diffractometer (Rigaku, RINT 2000) using Cu Kα radiation(λ = 1.54056 Å). Zn K-edge EXAFS (Extended X-ray AbsorptionFine Structure) X-ray absorption spectroscopy was carried outat the NSRRC (National Synchrotron Radiation Research Center,Taiwan) on X-ray wiggler beamline BL17C. The EXAFS data were

2178 J.J. Lu et al. / Solid State Communications 149 (2009) 2177–2180

collected from the fluorescence signal. General EXAFS data analysisis described in the literature [18].The temperature dependence of electrical resistance of the

films was measured by four-point probe method on an LR-700 ACbridge in a system fully automated for temperature stability anddata acquisition. The resistivity of the films at room temperaturewas measured by a Hall measurement system (LakeShore,Model 7662). The optical transmission spectra were obtained byusing a UV spectrophotometer (MFS-630, Multi-Purpose OpticalCharacteristics Measurement System) with a continuous waveHe–Cd laser in a wavelength range of 300–800 nm.

3. Results and discussions

The atomic percentages of Al in the ZnO films, which weresemi-quantitatively determined by the EDS (Energy DispersionSpectrum) of a field emission scanning microscope (FE-SEM, JEOL,JSM-6700F), are listed in Table 1. The Al content increases as thepower of Al target increases from 3.5% (50 W) to 16.2% (200 W).Fig. 1 displays the XRD patterns of the deposited films. The

XRD patterns reveal all of the films are polycrystalline witha hexagonal lattice structure and a strong preferential c-axisorientation. Compared with pure ZnO film, the intensity and thehalf-width of the peaks are similar for 50 W. The peaks becomeless intense and broader as the DC power increases. At 200 W,the structure might be weakly crystalline or even amorphous fromthe spectra presented. The inset depicts the position of the (002)peak between 32◦ and 36◦. Compared with pure ZnO film, thereis a tendency for the peaks to shift towards larger angles. Similarbehavior was also observed by Kim et al. [17], who prepared AZOfilms by RF magnetron sputtering. They proposed a substitutionalreplacement of Zn2+ by Al3+, because the ionic radius of Al3+(0.53 Å) is smaller than Zn2+ (0.72 Å). This proposal can alsoaccount for the degradation of crystallinity at higher doping levelsas follows. When a higher power is applied, more Al atoms canincorporate into the lattice of ZnO, whereas the induced residualstress increases aswell. The increasing stress is likely to deterioratethe crystal structure of ZnO lattice. However, some previous worksreported a reverse shift in (002) peak [7,19]. Whether the Al ionscan replace Zn sites is still controversial. It seems that variousdeposition methods or different doping levels might cause thisdisagreement.To further verify the local structure, EXAFS analysis was

realized. Fig. 2 displays the k3-weighted Fourier transformedEXAFS spectra which were obtained by multiplying the weightingscheme (k3) after background subtraction and normalization. Twomajor peaks around 1.5 and 2.9 Å can be observed. The formercorresponds to the nearest oxygen atoms, and the latter is dueto the Zn atoms in the second shell. Data fitting was performedusing FEFFIT from UWXAFS3.0 in combination with FEFF8.0. TheEXAFS spectrum of 50 W and the best fitting are depicted in theinset. For 50 W, the obtained Zn–O and Zn–Zn bond distancesare 1.98 and 3.20 Å with the corresponding Debye–Waller factors(σ 2) 0.005 and 0.009 Å respectively. The obtained Zn–O and Zn–Znbond lengths are all around 2.0 and 3.2 Å respectively, and thecoordination numbers (CN) for the Zn–O shell are all near 4. Theseresults are in good accordance with those reported ZnO films inthe würtzite-type structure. When a higher Al power is applied,the intensity of the second peak decreases, and it almost smearsout at 200 W, revealing the degradation in crystal structure. Theresult is also consistent with the XRD measurement.The grain size D representing longitudinal coherence length of

the crystal was deduced from the (002) peak by using Scherrer’sformula [20]:

D =0.9λβ cos θB

(1)

Inte

nsity

( a

rb. u

nit)

Al power (W)

30 40 50 602θ (degree)

2θ (

degr

ee)

200 W

150 W

100 W

50 W

(002)

(102)undoped

34.8

34.7

34.6

34.5

34.40 40 80 120 160 200

Fig. 1. X-ray diffraction patterns of the AZO films. The inset shows the positions of(002) peak at different powers.F

.T. A

mpl

itude

( a

rb. u

nit )

50 W

Fittingundoped

50 W

100 W

150 W

200 W

0 1 2 3 4

r (Å)

r (Å)

0 1 2 3 4

Fig. 2. The k3-weighted Fourier transformed EXAFS spectra of the pure and the AZOfilms. The inset shows the EXAFS spectrum of 50 W and the best fitting.

where β is the FWHM of (002) peak corrected from instrumentalbroadening of themonocrystalline Si diffraction peak, and θB is theBragg angle. The deduced data are presented in Table 1. The grainsize for 200W is not listed, because the diffraction peak is too broadto determine the FWHM. It is noticed that the grain size attains amaximum value of 30.9 nm at 50 W.The room-temperature resistivity and the carrier concentration,

which were obtained from the Hall measurement at roomtemperature, are also listed in Table 1. Resistivity (ρ) is relatedby the free carrier concentration (n) and the mobility (µ) by theequation:

ρ = 1/neµ. (2)

J.J. Lu et al. / Solid State Communications 149 (2009) 2177–2180 2179

Table 1Measured Al composition, grain size D, resistivity, carrier concentration, and optical energy gap (Eg ) of the AZO films at different Al target powers.

Power (W) Al content (%) Grain size D (nm) Resistivity (� cm) Carrier concentration (cm−3) Eg (eV)

0 0 29.7 1.12× 10−1 1.54× 1019 3.3550 3.5±0.5 30.9 7.13× 10−3 2.93× 1020 3.40100 6.1±0.9 25.6 9.32× 10−2 1.39× 1019 3.42150 11.4±1.7 21.8 5.82× 10−1 3.49× 1018 3.45200 16.2±2.7 – 7.84× 101 8.95× 1017 3.54

3

2

1

R(T

)/R

(300

K)

60 100 140 180 220 260 300Temperature (K)

3.0 3.2 3.4 3.6Energy ( eV )

1.5

1.0

0.5

0.0

α (1

05 c

m-1

)

undoped50 W100 W150 W200 W

Fig. 3. Temperature dependence of the normalized electrical resistances(R(T )/R(300 K)) of the AZO films. The inset depicts the absorption coefficient (α)vs. photon energy (hν).

The increase in carrier concentration might originate fromthe following two mechanisms: (1) Substituting Al3+ for Zn2+produces one extra free electron that contributes to the electricalconductivity.(2) The point defects (oxygen vacancies) increase dueto a lower atomicO/Zn ratiowith doping [1]. Themobility is limitedby several scattering processes: lattice (phonon) scattering, grainboundary scattering, (ionized, neutral, and magnetic) impurityscattering, and scattering from defects or dislocations. From thedata, the AZO film at 50 W can have the largest grain size withoutdeteriorating the crystallinity. Due to higher carrier concentrationand less scattering, its room-temperature resistivity therefore canachieve aminimumvalue. However, the resistivity increases as theDC power is further increased. This can be explained as follows.When the doping level is increased, the dopant atoms in the crystalgrains and grain boundaries tend to saturation. In such a situation,a higher doping level will result in an increase in ionized impurity.According to Conwell–Weisskopf formulawhen degenerate chargecarriers are scattered by impurity ions, the energy dependence ofmobility is [9,21]

µi =

(2m∗

)1/2ε1/2E3/2Fπe3NiZ2

1

ln(1+ εEF

N1/3i Ze2

) , (3)

where ε is the dielectric constant of the film,Ni is the concentrationof the ionized impurity, and m∗ is the effective mass. From theabove equation one can see that larger impurity concentrationswill reduce the mobility. At the same time, the induced stress field

due to the deterioration in the crystallinity is able to reflect freecarriers, which also leads to a decrease in mobility.Fig. 3 shows temperature dependence of the normalized elec-

trical resistance (R(T )/R(300 K)) of the samples between 60 and300 K. The pure ZnO and AZO films with higher Al-doping levelsexhibit a typical semiconductor-type conduction behavior as ex-pected. However, the resistance of 50 W decreases as the tem-perature decreases in a temperature range of 200–300 K andremains almost constant below 200 K, i.e., the conduction behav-ior is prone to a metallic-type conduction behavior. Conductionbehavior also closely relates to the energy band. To estimate theoptical energy gaps (Eg), we carry out optical transmission mea-surement. Absorption coefficient (α) can be calculated by the re-lation T = exp(−αt), where T is the transmittance which is theratio of the intensity of transmitted light to the intensity of incidentlight, and t is the thickness of the film [17,19]. The optical energyband gap Eg at zero temperature for a direct transition can be de-termined from α using the relation for parabolic bands [14,19,22]:

α = A(hν − Eg)1/2 (4)

where A is a constant depending on the semiconductor material,hν is the incident photon energy. Taking into account the thermalbandwidth-broadening effect (δ) at high temperature, the Eg canbe determined by the equation [3,23]:

α = A{(hν − Eg)+ [(hν − Eg)2 + δ2]1/2

}1/2. (5)

Fig. 3 inset displays the absorption coefficient vs. photonenergy. From Eq. (5), the Eg can be calculated from Eg = (3hν1 +hν2)/4. The hν1 is the energy where the maximum of ∂α/∂(hν)occurs, and the tangent to α at hν1 intercepts the energy axis athν2 (see Fig. 3 inset). The obtained Eg data are listed in Table 1.Compared with pure ZnO, the Eg value at lower power dose notshift significantly, and all the AZO samples exhibit a blue-shift.Such a shift is attributed to the Burstein–Moss effect [24,25],stemming from the conducting band edge being filled by excessivecarriers donated by the doped impurity.A semiconductor–metal transition can be achieved by some

external parameters, whichmay be pressure, doping, or amagneticfield [26]. The estimated critical concentration, at which thistransition is expected to occur, changes from∼0.01 to∼0.05mol%depending on various theoreticalmodels used [5,23]. FromTable 1,the concentrations are all beyond these critical values. Whenthe power is further increased, most previous reports revealedthat substitutional incorporation of Al3+ ions is not proportional[6,16,17]. On the other hand, the kinetic energies of sputteringparticles increase as larger DC powers are applied. This effectwill enhance the ion bombardment effect and thus increase theresidual stress, consequently deteriorating the crystallinity. Sinceconducting behavior also closely relates to the crystallinity, themetallic-type conducting behavior can only be observed at 50 Win our present study.The incorporation of Al3+ ions into ZnO lattice also results in

extra weakly bound electrons. From the aspect of energy band,the overlapping of the wave functions of these weakly boundelectrons is able to form an impurity band near the band gap. Theimpurity band becomes broader as the free carrier concentration(n) increases. Once the impurity band becomes broad enough toreach the edge of theconducting band, the effective ionization

2180 J.J. Lu et al. / Solid State Communications 149 (2009) 2177–2180

energy of these shallow donors tends to vanish, hence resulting ina metallic-type conducting behavior [26].

4. Conclusions

The XRD and XAS measurements reveal that all the samplesare essentially polycrystalline with a hexagonal würtzite-typestructure. The XRD result suggests a possible substitutionalincorporation of Al3+ into Zn2+ sites in our present work. Whileapplying a higher power, the crystallinity of the films becamedeteriorated. The degradation of the lattice is probably causedby the increase of induced residual stress due to more Al3+incorporating into the zinc sites and the enhancement of ionbombardment effect.The resistivity of the films reached a minimum value of 7.13×

10−3 � cm, and a semiconductor–metal transition was observedat 50W. The improvement of conductivity can be attributed to theincrease in carrier concentration and the decrease in scattering.The increase of carrier concentration along with the merging ofthe donor and conducting band are likely to lead to the observedsemiconductor–metal transition.

Acknowledgement

This work was supported by National Science Council of theRepublic of China under contract No. NSC 97-2112-M-168-001-MY2.

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