Current Applied Physics 10 (2010) S278–S281

Contents lists available at ScienceDirect

Current Applied Physics

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

Electrical, structural and etching characteristics of ZnO:Al films preparedby rf magnetron

Yong Hyun Kim a,b, Kyung Seok Lee a, Taek Sung Lee a, Byung-ki Cheong a, Tae-Yeon Seong b,Won Mok Kim a,*

a Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Koreab Department of Materials Science and Engineering, Korea University, Seoul 136-701, Republic of Korea

a r t i c l e i n f o

Article history:Received 13 December 2008Accepted 29 July 2009Available online 11 November 2009

Keywords:Transparent conducting oxideAl doped ZnOMagnetron sputteringEtching

1567-1739/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cap.2009.11.061

* Corresponding author. Fax: +82 2 958 5509.E-mail address: wmkim@kist.re.kr (W.M. Kim).

a b s t r a c t

Al doped ZnO (AZO) films were prepared by radio frequency (rf) magnetron sputtering with varying sub-strate temperature, working Ar gas pressure and rf power imposed on 2-inch ZnO–Al2O3 (2 wt%) target,and their electrical and structural properties together with the corresponding etching behavior in 0.5%HCl solution were examined. The effect of rf power on the electrical and structural properties of AZO filmswas marginal, but in the case of working Ar gas pressure and substrate temperature, substantial varia-tions in the electrical and structural properties were observed. The optimum electrical properties wereobtained for AZO film deposited at 150 �C in lowest working pressure of 1.2 mTorr. The behavior of craterformation upon etching varied significantly depending on the structure of the film, and it was shown thatthe etching rate could be expressed in inversely proportional function of the crystallinity represented as(002) peak intensity. Also, for films with similar crystallinity, i.e. (002) peak intensity, dense structuredfilm deposited at high temperature had much lower etching rate than open structured films depositedunder high working Ar gas pressure.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

ZnO films doped with metallic impurities like Al (AZO) and Ga(GZO) have attracted much attention as the front electrodes in thinfilm Si solar cells due to stability under hydrogen plasma not tomention its low material cost and electrical and optical propertiescomparable to Sn doped In2O3 (ITO). In order for a transparent con-ducting oxide (TCO) film like AZO to be successfully utilized asfront electrodes of Si thin film solar cells, the morphological char-acteristics of the film surface become important for the achieve-ment of high efficiency by enhancing the light trapping in theabsorber layer [1,2]. When deposited by sputtering which is oneof the most frequently used technique to deposit doped ZnO films,film surface is rather smooth, and it was shown that such smoothsurface could be transformed to textured surface successfully bysimple wet chemical etching [2,3]. The electrical, optical and struc-tural properties of the doped ZnO films are influenced substantiallyby the deposition parameters in usual sputtering technique [4].Furthermore, these deposition parameters also affect the surfacecharacteristics of films such as etching behavior or surface mor-phology before and after etching process [5,6].

ll rights reserved.

In this report, we investigated the electrical and structuralproperties of AZO films prepared by radio frequency (rf) magnetronsputtering with varying deposition parameters, and, by performingthe surface texturing in diluted 0.5% HCl solution, examined theetching characteristics in embryo at which the effects of the as-grown structures on etching behavior would be manifested.

2. Experiments

AZO films were deposited on glass substrate (Corning eagle2000) by rf magnetron sputtering from 2-inch diameter ZnO targetcontaining 2 wt% Al2O3 with pure Ar gas. Three sets of AZO filmswere prepared by varying deposition temperature (TS), workingpressure (pAr) and rf power (Prf). In one set, TS varied from roomtemperature (RT) to 450 �C at fixed pAr of 1.2 mTorr and Prf of50 W. Second set of AZO films were grown by changing pAr from1.2 to 10 mTorr at fixed TS of 150 �C and Prf of 50 W. The otherset of AZO films were deposited with varying Prf at fixed TS of150 �C and pAr of 1.2 mTorr.

Film thickness was measured by a surface profilometer (KLA-tencor, ASIQ) after etching away part of the film with dilutedHCl solution. The film thicknesses were 277 ± 34 nm. The electri-cal properties were determined from Hall-effect measurementusing a Van der Pauw method with indium contact pads at room

Y.H. Kim et al. / Current Applied Physics 10 (2010) S278–S281 S279

temperature. The structural analysis was carried out by an X-raydiffraction (XRD, Rikaku, ATX-XRD) measurement (Cu Ka wave-length = 0.1540562 nm, h-2h scan mode). The average grain size(dG) was estimated from the full width at half maximum (FWHM)of the (002) peak according to the Sherrer formula. X-ray photo-electron spectroscopy (XPS) spectra were recorded with Phi 8000spectrometer using Al Ka (1486.6 eV) radiation. The surface mor-phology and structure of the AZO films were determined from ascanning electron microscopy (SEM).

3. Results and discussion

Fig. 1 summarizes the variations in electrical properties of AZOfilms with varying TS, pAr and Prf. The minimum resistivity of 3.89 x10�4 Xcm was obtained for film deposited at TS = 150 oC,pAr = 1.2 mTorr and Prf = 50 W. Above this temperature, both theHall mobility and the carrier concentration decreased with increas-ing deposition temperature, resulting in very high resistivity forAZO film deposited at 450 oC. The resistivity of AZO films increasedmonotonically with increasing pAr. On the other hand, the effect ofPrf on the electrical properties was marginal.

For all the films, XRD profiles showed only strong peak from(002) plane indicating a typical hexagonal wurzite structure with(002) preferred orientation. In Fig. 1, the variations of the grain size(dG) and the c-axis lattice parameter (cLP) are also shown. The bestcrystallinity was observed for the AZO film grown at TS = 150 �Cand pAr = 1.2 mTorr. dG decreased rapidly with increasing TS andpAr, while showing relatively constant level for varying Prf. In thecase of cLP, the influence of varying pAr and Prf was relatively small,while TS exhibiting large influence.

The relatively constant level of the electrical and the structuralproperties observed in AZO films deposited with increasing Prf is

Fig. 1. Electrical (resistivity (q), Hall mobility (lH) and carrier concentration (n))and structural (grain size (dG) and c-axis lattice parameter (cLP)) properties of AZOfilms deposited with varying (a) TS, (b) pAr, and (c) Prf.

attributed to the competitive nature between the decreased ionto neutral ratio of incoming particles and the increased thermalloading because of increased deposition rate [4]. As for the workingpressure, high pAr will cause decrease in mean free path of sput-tered particles, leading to small diffusion energy which will even-tually result in poor crystallinity with open structure and poorelectrical properties [7,8].

The deterioration of the electrical properties as well as struc-tural properties observed in AZO films prepared at elevated TS

was also reported in previous studies of ZnO films doped with Si[9], Al [9,10] and Ga [11] films, although the optimum TS varieddepending upon the materials system and deposition method.Fig. 2 represents XPS spectra of Zn 2p3/2 and Al 2p3/2 of AZO filmsdeposited at 150 and 450 �C. The intensity of Al 2p3/2 spectrum forAZO film deposited at 450 �C is much larger than that deposited at150 �C, indicating the higher Al content for film deposited at highertemperature due to desorption of Zn during deposition because ofhigh vapor pressure of Zn. Also, close examination of the atomicconcentration of O and Zn revealed that oxygen to zinc ratio inAZO film deposited at 450 �C had slightly higher than that depos-ited at 150 �C. From these observations, the degraded electricaland structural properties in AZO films grown at elevated TS couldbe ascribed to (1) Zn deficiency due to desorption of Zn from thegrowing surface in this film, and (2) possible formation of Al2O3

segregated at the grain boundaries due to large up-take of oxygenas was stated previously [9]. Both cases will result in reduction ofgrain size.

In an attempt to improve the structural and the resulting elec-trical properties of the AZO films deposited at elevated tempera-ture, an AZO film with intentional Zn addition during sputterdeposition was prepared by co-sputtering of AZO and Zn targetsat 450 �C. Although Zn-added AZO films showed much improvedelectrical resistivity of 6.0 x 10�4 Xcm, and grain size of 27 nmwhen compared with AZO film deposited without Zn addition atthe same temperature, but the properties were still not parallelsto the AZO film deposited at optimized condition.

In Fig. 3, SEM micrographs of etched surface after 7 s etching in0.5% HCl for selected samples are shown. Insets are the corre-sponding surface after prolonged etching in the same magnifica-tion. AZO films deposited at TS = 150 �C and pAr = 1.2 mTorr withdifferent Prf (sample A, B) showed differences in crater size anddensity. For film at high Prf, large craters with small number den-sity were formed. However, after prolonged etching, the surfacemorphology of these samples was close to each other. Withincreasing pAr, the surface was regularly and smoothly etcheddue to high porosity of film, and practically no craters were ob-served for film deposited at highest pAr of 10 mTorr (D). AZO film(Sample E) deposited at TS = 450 �C and pAr = 1.2 mTorr exhibitedrather uniformly textured surface with fine craters. For AZO filmdeposited with intentional Zn addition at 450 �C (sample F), the

Fig. 2. The XPS spectra of Zn 2p3/2 and Al 2p3/2 of AZO films deposited at 150 and450 �C.

Fig. 3. SEM micrographs of etched AZO films. A(150 �C, 1.2 mTorr, 50 W), B(150 �C, 1.2 mTorr, 100 W), C(150 �C, 5 mTorr, 50 W), D(150 �C, 10 mTorr, 50 W), E(450 �C,1.2 mTorr, 50 W) and F(450 �C, 1.2 mTorr, 60 W + Zn 50 W).

Fig. 4. Plot of etching rate as a function of (002) peak intensity.

S280 Y.H. Kim et al. / Current Applied Physics 10 (2010) S278–S281

initial surface morphology was similar to that of AZO film depos-ited at 150 �C (sample A). But prolonged etching did not causeany further change in surface morphology, only reduction of thick-ness was observed.

In Fig. 4, the etching rates are shown as a function of (002) peakintensity measured from XRD. The etching rate increased graduallyfrom 5.4 to 7.8 nm/s with increasing TS from 150 to 450 �C, show-ing the effect of increased grain boundary area due to reducedgrain size. Also, the etching rate of the film deposited at high pAr

jumped up abruptly, reaching as high as 31.4 nm/s for film depos-ited at pAr of 10 mTorr, which manifests the effect of open structurefor films deposited at high pAr. When deposited at 150 �C, the filmsdeposited with high Prf had lower etching rate, but the effect of Prf

on the etching rate of the films deposited at 450 �C was negligible.Also, it is noted that Zn-added AZO film deposited at 450 �C exhib-ited the lowest etching rate among all the films. These observations

reveal that grain boundary area competes with the densification ofthe structure at high temperature.

Y.H. Kim et al. / Current Applied Physics 10 (2010) S278–S281 S281

4. Conclusion

The electrical, structural properties and corresponding etchingcharacteristics of AZO films prepared by using rf magnetron sput-tering with varying deposition temperature, working Ar gas pres-sure and rf power were investigated. The optimum electricalproperties were observed for films deposited at 150 �C in1.2 mTorr. The effect of rf power on electrical and structural prop-erties was relatively small, but increased working Ar gas pressureand substrate temperature resulted in deterioration of the filmproperties due to formation of open structure and Zn deficiency,respectively. The resulting film structure depending on sputterparameters had substantial effect on etching behavior such asthe crater size and number density as well as the etching rate.The etching rates were shown to be inversely proportional to thestructural parameter expressed by (002) peak intensity, and, forfilms with similar crystallinity, structurally dense films yieldedlower etching rate.

Acknowledgments

This study was supported partially by New and Renewable En-ergy R&D program (2008-N-PV08-P-09) under the Korea Ministry

of Commerce, Industry and Energy (MOCIE), and partially by theKorea Science and Engineering Foundation (KOSEF) grant fundedby the Korean Government (MEST) (20090064868).

References

[1] J. Springer, B. Rech, W. Reetz, J. Müller, M. Vanecek, Sol. Energy Mater. Sol. Cells85 (2005) 1.

[2] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schöpe, C. Beneking, H. Wagner, A.Löffl, H.W. Schock, Thin Solid Films 351 (1999) 247.

[3] J. Müller, B. Rech, J. Springer, M. Vanecek, Sol. Energy 77 (2004) 917.[4] K. Ellmer, J. Phys. D: Appl. Phys. 33 (2000) R17.[5] J. Hüpkes, B. Rech, O. Kluth, T. Repmann, B. Zwaygardt, J. Müller, R. Drese, M.

Wuttig, Sol. Energy Mater. Sol. Cells 90 (2006) 3054.[6] O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, Thin Solid Films 442

(2003) 80.[7] Y. Igasaki, H. Kanma, Appl. Surf. Sci. 169 (2001) 508.[8] K.H. Kim, K.C. Park, D.Y. Ma, J. Appl. Phys. 81 (1997) 81.[9] T. Minami, H. Sato, T. Sonoda, H. Nanto, S. Takata, Thin Solid Films 171 (1989)

307.[10] B. Szyszka, Thin Solid Films 351 (1999) 164.[11] T. Yamada, A. Miyake, S. Kishimoto, H. Makino, N. Yamamoto, T. Yamamoto,

Surf. Coat. Technol. 202 (2007) 973.