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Current Applied Physics 12 (2012) 1606e1610

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Current Applied Physics

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Spectroscopic ellipsometry modeling of ZnO thin films with various O2 partialpressures

Edward Namkyu Cho, Suehye Park, Ilgu Yun*

Department of Electrical and Electronic Engineering, 262 Seongsanno, Seodaemun-gu, Yonsei University, Seoul 120-749, Republic of Korea

a r t i c l e i n f o

Article history:Received 5 March 2012Received in revised form7 May 2012Accepted 16 May 2012Available online 27 May 2012

Keywords:ZnODC sputteringO2 partial pressureSpectroscopic ellipsometry

* Corresponding author. Tel.: þ82 2 2123 4619; faxE-mail address: iyun@yonsei.ac.kr (I. Yun).

1567-1739/$ e see front matter � 2012 Elsevier B.V.doi:10.1016/j.cap.2012.05.030

a b s t r a c t

ZnO films were deposited on thermally oxidized SiO2/p-type Si (100) substrates and glass substrates byDC magnetron sputtering using a metal Zn target. Three types of samples were prepared with variousO2/(Ar þ O2) ratios (O2 partial pressure) of 20%, 50%, and 80%. The properties of these ZnO thin films wereinvestigated using X-ray diffraction (XRD), optical transmittance, atomic force microscopy (AFM), andspectroscopic ellipsometry in the spectral region of 1.7e3.1 eV. The structural and optical properties ofZnO thin films were affected by O2 partial pressure. Relationships between crystallinity, the ZnO surfaceroughness layer, and the refractive index (n) were investigated with varying O2 partial pressure. It wasshown that the spectroscopic ellipsometry extracted parameters well represented the ZnO thin filmcharacteristics for different O2 partial pressures.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, ZnO thin films have attracted much attentiondue to their potential applications in thin-film transistors, surfaceacoustic wave filters, and transparent conductive oxides [1e3]. ZnOis a wide direct bandgap semiconductor of around 3.4 eV witha large exciton binding energy of 60 meV [4]. ZnO thin films havebeen deposited by magnetron sputtering [1e4], atomic layerdeposition (ALD) [5], pulsed laser deposition (PLD) [6], spraypyrolysis [7], and evaporation [8]. Among these methods, magne-tron sputtering has been the most popular technique to depositZnO thin films.

In general, there have been many investigations of the proper-ties of ZnO thin films using ZnO ceramic targets for magnetronsputtering techniques [1e3,9]. However, there have been fewexperimental results using a metallic Zn target for depositing ZnOthin films [4,10]. In this paper, a metallic Zn target is used to depositZnO thin films with various O2/(Ar þ O2) ratios (O2 partial pres-sure). The advantage of using a metallic Zn target compared toa ZnO ceramic target is its ability to more easily control stoichi-ometry by changing O2 partial pressure [11]. The structural andoptical properties of the ZnO thin films are investigatedwith the aidof X-ray diffraction (XRD), optical transmittance, atomic forcemicroscopy (AFM), and spectroscopic ellipsometry measurements

: þ82 2 313 2879.

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in the spectral region of 1.7e3.1 eV. Spectroscopic ellipsometry hasbeenwidely used to determine optical properties of thin films suchas ZnO [3,10,12], In2O3 [13], and NiO [14]. Here, spectroscopicellipsometry is employed to extract parameters which can depictthe influence of O2 partial pressure on the ZnO thin films.

2. Experiments

ZnO films were deposited on p-type Si (100) substrates coveredwith 100 nm thermal SiO2 layers and glass substrates by DCmagnetron sputtering using a metal Zn (99.95% purity) target. Priorto deposition, the substrates were ultrasonically cleaned in acetone,isopropanol, de-ionized water and were subsequently dried innitrogen gas. The chamber was initially evacuated to 5 � 10�5 Torr.A metallic Zn target was pre-sputtered for 3 min to removecontamination on the target surface. ZnO films were deposited for15min usingmixtures of Ar and O2 at aworking pressure of 6mTorrwith the DC power fixed at 100 W. Three types of samples wereprepared with varying O2 partial pressure: 20% (S1), 50% (S2), and80% (S3). The sum of Ar and O2 flows was 10 standard cubiccentimeters per minute (sccm). All deposition processes wereperformed at room temperature.

The structural properties of the films were examined with XRDwhere Ni-filtered Ka (l¼ 1.54056 Å) radiationwas used. The opticaltransmittance was measured in the range of 300e900 nm usinga UVeVIS spectrophotometer. The surface morphology of the filmswas observed by AFM with a scan area of 1 mm � 1 mm. Spectro-scopic ellipsometry was used to extract the film thickness and

E.N. Cho et al. / Current Applied Physics 12 (2012) 1606e1610 1607

refractive index in the photon energy range of 1.7e3.1 eV. Ellips-ometry parameters, delta (D) and psi (J), were recorded at anincidence angle of 70.2�.

In order to verify extracted spectroscopic ellipsometry param-eters for S1, S2, and S3, the two additional samples were depositedby varying deposition time with O2 partial pressures of 50% (S4)and 80% (S5) which having similar total film thickness to S1.

Fig. 2. Plot of (ahv)2 versus hv for S1 to S3 while the inset shows the optical trans-mittance spectra.

3. Results and discussion

Fig. 1 illustrates the XRD patterns for S1 to S3 with different O2

partial pressures. No sharp (002) diffraction peak is observed in S1,indicating that S1 is nearly amorphous. Due to the shortage of O2 asa reactive gas, metal Zn ions and particles have not been fullyoxidized [10]. With increasing O2 partial pressure, a diffractionpeak is observed at 34� corresponding to the (002) orientation.However, the strongest peak intensity with the lowest full width athalf maximum (FWHM) is observed in S2, indicating that anO2/(Ar þ O2) ratio of 50% is a critical O2 partial pressure in ourexperiment [9]. The grain size (D) for S2 and S3 is estimated fromthe Scherrer formula as follows [10]:

D ¼ 0:9lbcosq

(1)

where l, b, and q are the X-ray wavelength (1.54056 Å), Braggdiffraction angle, and the FWHM of the diffraction peak, respec-tively. Grain sizes of S2 and S3 were calculated to be 7.22 nm and3 nm, respectively. The grain size of S1 cannot be calculated due tothe broad diffraction peak.

Optical transmittance is measured to determine optical bandgap(Eg) for samples S1 to S3. The Eg can be calculated by the equationbelow [15]:

ahv ¼ A�hv� Eg

�n (2)

where a is the absorption coefficient, hv is the photon energy, and Ais a constant. Previous work showed that n ¼ 1/2 is more suitablefor ZnO thin films due to their direct Eg [15]. Fig. 2 shows a plot of(ahv)2 versus hv for samples S1 to S3 while the inset shows theoptical transmittance spectra. Eg can be obtained from the x-intercept in the linear region. As shown, Eg increases from 3.29 eVto 3.32 eV as O2 partial pressure increases from 20% to 80% (S1 to

Fig. 1. XRD patterns for S1 to S3 with different O2 partial pressures.

S3). When O2 partial pressure is decreased, the ZnO thin films showmore metallic behavior due to the shortage of O2 [16].

To determine the optical constants of ZnO thin films, a Cauchymodel, which is suitable for semiconductors and dielectrics, is used.In the Cauchy model, the refractive index (n) and extinction coef-ficient (k) as a function of wavelength (l) can be determined by thefollowing formulas [3]:

nðlÞ ¼ Aþ 106B

l2þ 1012C

l4(3)

kðlÞ ¼ a

�b

�1239:8

l� g

��(4)

where A, B, and C are the fitting parameters, a is the extinctioncoefficient amplitude, b is the exponent factor, and g is the bandedge.

In this study, we used a four-layer model to represent the filmsystem, i.e., air/rough ZnO surface layer/ZnO film/SiO2/Si substrate,where the Si substrate is treated as infinite and SiO2 is fixed at100 nm. The rough ZnO surface layer is modeled through theBruggeman effective medium approximation (EMA) with 50% ZnObulk and 50% void. The film system is illustrated in Fig. 3.

Fig. 4 (a), (b), and (c) shows the experimental and fitted ellips-ometry parameters, D andJ, as a function of the photon energy forS1, S2, and S3, respectively. It is shown that good agreementbetween experimental and fitted spectra is obtained. The photonenergy is in the range of 1.7e3.1 eV in our experiment. Consideringthat all the samples have larger Eg (extracted from the opticaltransmittance) than our measured ellipsometry photon energyrange, the fitting parameters related with k in eq. (4) are neglected.Fitted parameters for the film system are summarized in Table 1. T1and T2 are the thicknesses of the rough ZnO surface layer and ZnOfilm, respectively. The total film thickness of ZnO can be obtainedfrom T1 þ T2. As shown, the total film thicknesses of ZnO for S1, S2,and S3 are 181.4 nm, 156.1 nm, and 93.7 nm, respectively. Thedeposition rate is decreased as O2 partial pressure increasesbecause O2 has a lower sputtering yield than Ar [17]. The thick-nesses of the rough ZnO surface layer for S1, S2, and S3 are 9.2 nm,11.3 nm, and 7.2 nm, respectively. Previous work showed thatsurface roughness increased as film thickness increased [5,18].However, as can be seen from our fitting parameters, the rough ZnO

Fig. 3. Four-layer film system used for spectroscopic ellipsometry modeling.

Fig. 4. Experimental and fitted ellipsometry parameters (D and J) as a function of thephoton energy for (a) S1, (b) S2, and (c) S3.

Table 1Spectroscopic ellipsometry extracted best-fit parameters for S1, S2, and S3.

Sample T1 (nm) T2 (nm) A B (nm2) C (nm4)

S1 9.2 172.2 1.91 0.0059 0.0058S2 11.3 144.8 2.01 0.015 0.0049S3 7.2 86.5 1.95 0.0052 0.0058

T1: the thickness of ZnO surface rough layer, T2: the thickness of ZnO film.

E.N. Cho et al. / Current Applied Physics 12 (2012) 1606e16101608

surface layer thickness is lower for S1 than S2. This result can beattributed to different crystallinity obtained from XRD measure-ments. Since S1 is nearly amorphous while S2 is polycrystalline, S1has a thinner rough surface layer than S2 even though the total filmthickness of S1 is thicker than S2 due to its amorphous state [5,19].

To further examine the surface layer, AFM is used tomeasure thesurface roughness. Although the root mean square (RMS) surfaceroughness is not an exact value of the thickness of the rough surfacelayer, AFM analysis can be used to verify the fitted parameter [14].Fig. 5 (a), (b), and (c) shows the measured surface morphology forS1, S2, and S3, respectively. RMS surface roughnesses for S1, S2, andS3 are measured to be 1.7 nm, 2.2 nm, and 1.2 nm, respectively. TheRMS surface roughness for S2 is the largest among the samples,resulting from the largest grain size, as measured from the XRDanalysis [1]. S3 has a smaller measured RMS surface roughness thanthat of S1 due to its smaller total film thickness, even though S3 ispolycrystalline. From the AFM analysis, these results support thefitted surface rough layer thickness trends observed as O2 partialpressure varies.

Fig. 6 shows the extracted optical constant (n) as a function ofphoton energy for S1 to S3. The n values are in the range of 1.9e2.3for all samples, which are similar to those previously reported forZnO films [10,12]. It can be seen that the n values are significantlyaffected by the O2 partial pressure. S2 has the highest n value fol-lowed by S3 and S1. It is considered that n is related to the packingdensity of the film [20]. As the packing density increases, the nvalue increases. According to the XRD analysis, S2 has a larger grainsize than S3 while S1 has no grain due to its amorphous state. Fromthese results, it can be concluded that S2 has the largest packingdensity followed by S3 and S1 because a larger grain size corre-sponds to a higher packing density [21].

In order to verify the fitness of the n values with various O2partial pressures, the two additional experiments are performed. Toexclude the effect of total film thickness, the total film thicknessesof the films, S4 (O2 partial pressure: 50%) and S5 (O2 partial pres-sure: 80%), are controlled to w180 nm by varying deposition timewhich having similar total film thickness to S1. Spectroscopicellipsometry modeling for the additionally deposited samples, S4and S5, is performed. Each n values for S4 and S5 is fixed as theextracted values from S2 and S3 due to same O2 partial pressures,respectively. Fig. 7 (a) and (b) shows the experimental and fittedellipsometry parameters as a function of the photon energy for S4

Fig. 5. Measured surface morphology for (a) S1, (b) S2, and (c) S3 by AFM with a scanarea of 1 mm � 1 mm.

Fig. 6. Extracted optical constant (n) as a function of photon energy for S1 to S3.

Fig. 7. Experimental and fitted ellipsometry parameters as a function of the photonenergy for (a) S4, and (b) S5 having similar total film thickness to S1.

E.N. Cho et al. / Current Applied Physics 12 (2012) 1606e1610 1609

Table 2Spectroscopic ellipsometry extracted best-fit parameters for S1, S4, and S5.

Sample T1 (nm) T2 (nm)

S1 9.2 172.2S4 19.6 154.9S5 11.2 168.5

T1: the thickness of ZnO surface rough layer, T2: the thickness of ZnO film.

E.N. Cho et al. / Current Applied Physics 12 (2012) 1606e16101610

and S5, respectively. As can be seen from Fig. 7, the good agreementbetween experimental and fitted spectra is obtained. Fittedparameters for S4 and S5 are summarized in Table 2. The S1 fittedparameters are also shown for comparison. From the Table 2, it isshown that the modeled total film thickness for S4 and S5 aresimilar to the total film thickness of S1. The thicknesses of the roughZnO surface layer for S1, S4, and S5 are 9.2 nm, 19.6 nm, and11.2 nm, respectively. Previous research reported that surfaceroughness increased as grain size increased [1]. From the XRDanalysis in Fig. 1, it is shown that the grain size of ZnO films is thelargest for O2 partial pressure of 50% followed by 80% while thegrain size of ZnO film for O2 partial pressure of 20% is not calculateddue to its amorphous state. By excluding the effect of total filmthickness, the thickness of rough ZnO surface layer for S4 ismodeled to be the largest followed by S5 and S1 which is consistentwith the XRD analysis. From the result, it is concluded that theextracted refractive index values from S2 and S3 well fit to S4 andS5, respectively. The extracted n values can well represent theproperties of ZnO thin films with different O2 partial pressures.

4. Conclusion

Structural and optical properties of ZnO thin films with varyingO2 partial pressure have been investigated. XRD results revealedthat a critical O2 partial pressure exists for the ZnO thin films. Egvalues for ZnO thin films were obtained from the optical trans-mittance measurements. It was shown that the Eg decreases as O2partial pressure decreases because the ZnO thin films showedmoremetallic behavior due to the shortage of O2. The Cauchy model was

used to determine the optical constants of the ZnO thin films. Fromthe spectroscopic ellipsometry modeling, the thicknesses of ZnOthin films decreased as O2 partial pressure increased for the samedeposition time due to a lower sputtering yield of O2 than that of Ar.The thickness of the rough surface layer is highest when the O2partial pressure is 50%, as this produced the samplewith the largestgrain size. This result was also verified by AFM analysis. The n valuewas the highest for a 50% O2 partial pressure followed by 80% and20% O2 partial pressures.

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