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Phys. Status Solidi C 7, No. 6, 1580–1582 (2010) / DOI 10.1002/pssc.200983168

Transport properties of

microstructured MF-sputtered

Zn0.98Al0.02O

M. Piechotka*,1, M. T. Elm1, T. Henning1, B. Szyszka2, B. K. Meyer1, and P. J. Klar1

1 I. Physikalisches Institut, Justus-Liebig-Universitat Giessen, Germany2 Fraunhofer-Institut fur Schicht- und Oberflachentechnik, Braunschweig, Germany

Received 4 September 2009, revised 30 December 2009, accepted 5 January 2010Published online 17 March 2010

Keywords ZnAlO, sputtering, microstructure, electrical transport, etching, lithography

∗ Corresponding author: e-mail markus.piechotka@exp1.physik.uni-giessen.de, Phone: +49-641-9933147, Fax: +49-641-9933139

We studied the effect of microstructuring on the electrictransport properties of Zn0.98Al0.02O thin films (AZO)grown by reactive mid-frequency magnetron sputtering.A series of AZO wire arrays was prepared by pho-tolithography followed by wet-chemical etching. Thenominal wire width b′ was varied between 8 and 32 μm.The wire arrays were characterized by scanning electron

microscopy, atomic force microscopy, and temperature-dependent resistivity measurements. The extension z ofthe surface-layer affected by the microfabrication pro-cess and its effect on the electronic transport through thewire were assessed. z is determined by the grain struc-ture of the sputtered layer and is independent of wirethickness and degree of under-etching.

1 Introduction ZnO is a II-VI semiconductor, whichcrystallizes in the wurtzite structure, with a direct bandgapof about 3.4 eV at room temperature. The electrical con-ductivity can be varied from semiconducting to metallic bycontrolled n-type doping. These properties lead to the con-siderable technological potential of ZnO-based materialsfor application in UV light-emitters, varistors, transpar-ent high power electronics, surface acoustic wave devices,piezoelectric transducers, gas-sensors, and as window ma-terial for displays and solar cells. AZO is a possible al-ternative for indium-tin-oxide as a transparent conductiveoxide for silicon based thin film solar cells due to its com-petitive price and high stability [1–6].Miniaturized electronic devices require predictable trans-port properties. Therefore, it is important to study theeffects of microfabrication on the electronic transport inmaterials as a function of structure size. Modifications ofthe electronic states at the surface as well as structuraldamage may be caused by the pattern transfer into the thinfilm as essential part of the microfabrication process. Theextent of the damaged region will not only depend on theetching process and conditions, but also on the structuralproperties of the film itself.

Figure 1 SEM-images of the surface of the as-grown sample(left) and of a wire with nominal width b′ = 8 μm (right).

2 Microfabrication of wire arrays A Zn0.98Al0.02Ofilm was grown on a glass substrate by reactive mid-frequency magnetron sputtering. The film has a layer thick-ness of about 700 nm and consists of crystalline columnargrains with typical diameters of 50 nm as can be seen inFig. 1a. The c-axis of the grains is oriented perpendicularto the substrate plane. The free electron concentration isabout 1021 cm−3 at room temperature.Pieces of this sample were structured by photolithographyinto arrays of wires between two contact pads. The wires

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2 AFM images of structured samples with a nominalwire width b′ = 32 μm (left) and 8 μm (right). The heightscales are given in nm.

Table 1 Nominal and actual number of wires, N ′ and N , andnominal and actual wire width, b′ and b, and the standard devi-ation σb of b determined from the analysis of the AFM images.

N ′ N b′ (μm) b (μm) σb (μm)

31 30 32 29.26 0.29

31 30 32 26.56 0.33

62 60 16 13.25 0.23

62 60 16 11.31 0.28

125 105 8 4.23 0.29

125 125 8 2.16 0.24

are of length L. The nominal wire width b′ and the gap be-tween two wires are equal, while the number of wires perpattern N ′ is varied such that the total wire cross-sectionA′ = N ′b′h is the same for all patterns, where h is thefilm thickness. The pattern was transferred into the sam-ple by wet-chemical etching using H3PO4:C2H4O2:H2O(1:10:100 by volume). The contact pads were made bythermal evaporation of Al in a second microfabricationprocess.

3 Structural properties of the wires The wirearrays were investigated by AFM, SEM, and optical mi-croscopy. The AFM images in Fig. 2 reveal that the actualwidth b of the wires is smaller than the nominal width b′.Moreover, it can be clearly seen that the sidewalls of thewires are not ideally smooth, but exhibit a significant sur-face roughness. A statistical analysis of the AFM imageswas carried out to quantify the structural properties: actualwidth and sidewall roughness as mean wire width b andstandard deviation σb, respectively. In addition, optical in-spection yielded the actual number of unbroken wires Nfor each sample, while the analysis of the SEM images isused to verify the AFM results. A comparison of the actualand the nominal parameters of the microstructured arraysis given in Table 1. The actual wire width b is smaller thanthe nominal wire width b′ for all wires, due to the under-etching of the resist mask caused by the almost isotropicwet-chemical etching process. The standard deviation ofthe wire width σb is nearly constant and shows almost nodependence on the degree of under-etching characterized

by Δb. The edge of the wires looks somewhat frazzled.The value of σb ≈ 0.3 μm corresponds to the extent ofthe frazzled region. The results of the statistical analysis ofthe AFM images are fully corroborated by the exemplarySEM image in Fig. 1b. The ‘frazzled character’ of the side-walls of the wire becomes clear now. The sidewall regionconsists of free-standing ZnO pillars, only the center of thewire still exhibits the closed surface consisting of mergedZnO grains as depicted in Fig. 1a for the as-grown sample.The observed behavior can be understood as follows: Theetch rate R1 along the grain boundaries is higher than R2

of the microcrystalline grains leading to the frazzling ofthe sidewalls. The extension of the frazzled region is de-termined by the interplay of the difference of the two etchrates and the mean grain size. The time τ for etching a free-standing ZnO grain of diameter G is approximately givenby G = 2R2τ . During τ the etchant works its way alongthe grain boundaries to the distance R1τ into the layer. As-suming that this is equal to the depth D of the damagedlayer at the wire sidewall, one obtains D ≈ R1G/(2R2).Thus, D depends on the two etch rates and the grain sizeonly. Therefore, the frazzling of the sidewalls of the mi-crostructure can indeed be considered as an intrinsic fea-ture of the sputtered film on the samples studied.

4 Electronic properties of the wires Ideally theelectric resistance of a wire is solely determined by its ge-ometric extensions (length L, width b, height h) and thespecific resistivity ρ of the wire material. As seen in theprevious section, the actual physical wire width is alreadyhard to define because of the surface roughness of the wiresidewalls whose impact on the electronic transport is not apriori clear. In addition, new electronic states are createdat the wire sidewalls in the microfabrication process. Suchsurface states lead to a band bending of the volume statesin the surface region, causing a depletion region [7]. Usingliterature values [3,8,9] one obtains an estimated width dof the depletion region of 0.45 nm.In order to determine the influence of the microfabricat-ion-induced changes of the wire sidewalls on the transportbehaviour we measured the temperature-dependent resis-tance for the sample series. The transport properties of thereference samples were measured in the Van der Pauw ge-ometry while the resistances of the wire samples were in-vestigated in a four-point probe geometry. The measure-ment temperature varied from 1.5 K to 280 K. The Ar-rhenius plots of the temperature-dependent resistances ofall samples in Fig. 3 reveal metallic transport behaviour,as expected for the high free carrier concentration n ≈1021 cm−3. Furthermore, it is found that the sample resis-tance increases with decreasing wire width. The observeddifferences for the resistances R of the wire arrays can beanalysed on the basis of the following equation for the re-sistance of a wire array:

R =ρL

A=

ρL

Nhbeff=

ρL

Nh(b − 2z), (1)

Phys. Status Solidi C 7, No. 6 (2010) 1581

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where the total cross-sectional area A is given as the prod-uct of the actual number N of wires, the layer thicknessh, and an effective wire width beff . The resistivity ρ is thesame as for bulk and is determined from the reference sam-ple. We assume that ρ is not altered by the microfabricationprocess. The width z contains the effect of the microfab-rication process on the electric transport of the electronsthrough the wire. It is determined by the structural andelectronic changes of sidewall regions. The smaller thevalue of z the lesser is the effect of the microstructuringon the electronic transport. We assume that z is constantwithin a sample series, i.e. depends on the original layerand the etching process only, but not on wire width. UsingEq. (1) one can define the quantity Y ≡ − ρL

2NhR = − b2 +z

which is linear in b. Plotting Y versus b allows one todetermine the width z of the microfabrication-induced re-gion as the ordinate intercept (Fig. 3). We obtain a widthz = (325 ± 115) nm which agrees well with the corre-sponding structural quantity, the mean standard deviationof the wire width σb = (277 ± 36)nm, but is consider-ably larger than the corresponding electronic quantity, theextension d of the depletion zone. The good agreementbetween z and σb can be understood considering that theregion of this width really consists of almost free-standinggrains which cannot contribute to the electronic transport.The following design rule can be given: The width b of themicrostructure is effectively reduced to beff due to the re-

gions consisting of free-standing grains at sidewalls of theetched microstructure. In our case, 2z is almost 700 nm,showing that this effective reduction needs to be takeninto account in the design of micro and nanostructures;in particular, that actual lateral dimensions below 700 nmwill not provide a continuous electronic transport pathanymore.

5 Conclusions Series of wire arrays of differentwire width in the micrometer range were prepared byphotolithography followed by wet-chemical etching froman AZO thin-film grown by reactive mid-frequency mag-netron sputtering. Correlating the structural and electronicresults allows one to assess the extension of the sidewalllayer which is affected by the microfabrication process.The sidewall layer affecting the transport consists of free-standing grain columns, i.e. is structurally limited and iscaused by the interplay of the grain structure of the sput-tered layer and the wet-chemical etching process used totransfer the pattern. Other etching processes for the patterntransfer, such as dry etching by ion bombardment or re-active ion etching, may lead to significantly better resultsas the problem of the different etch rates can be over-come. However, it will not be possible to entirely avoidmicrofabrication-induced damage of the wire sidewalls byusing other etching techniques. These considerations needto be accounted for when employing these materials inmicroelectronic devices.

Acknowledgements We are grateful for funding bythe Deutsche Forschungsgemeinschaft in the framework ofSPP 1386.

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1582 M. Piechotka et al.: Transport in microstructured AZOp

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