nanostructural properties of zinc oxide thin films grown on non-planar substrates

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 2006; 38: 51–55Published online in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/sia.2200

Nanostructural properties of zinc oxide thin filmsgrown on non-planar substrates

Everett Lee, Jennifer J. Russell and Robert N. Lamb∗

School of Chemistry, University of NSW, Sydney, 2052, Australia

Received 24 February 2005; Revised 30 March 2005; Accepted 27 May 2005

The crystallographic structure of zinc oxide thin films grown on optical fibres using single source chemicalvapour deposition (SSCVD) was analysed using near edge X-ray absorption fine structure (NEXAFS).Zinc diethyl carbamate was used as a precursor for the growth of highly conformal films in a one-stepdeposition process without substrate rotation and at substrate temperatures of 400–575 °C. It was foundthat the growth temperatures greatly affected the crystallographic structure of the film with no preferredcrystallographic orientation and negligible crystallinity at low temperatures and very high crystallinitywith pure c-axis orientation at high temperatures. Cross-sectional analysis of the films by scanningelectron microscopy (SEM) showed the presence of a film at all points around the fibre. These filmsgenerally consisted of densely packed columns that bore a strong resemblance to c-axis-oriented filmsgrown on planar substrates. Copyright 2005 John Wiley & Sons, Ltd.

KEYWORDS: zinc oxide; ZnO; chemical vapour deposition; thin films; curved substrate

INTRODUCTION

Conformal deposition of semiconductor films on non-planar substrates allows a variety of new devices to bedeveloped. Of particular interest is the application to anall-fibre acousto-optic modulator.1 – 3 For such a device, apiezoelectric film is deposited on an optical fibre. Zincoxide is an ideal material for this application, as ithas a wurtzite structure and possesses a relatively highpiezoelectric coefficient of 12 pm/V (d33).4 In order to exhibitthe maximum piezoelectric coefficient, the ZnO film mustpossess a c-axis orientation. This requires that the c-axis mustbe perpendicular to the optical fibre at all points around thefibre. When a ZnO film is deposited on a planar substrate, thefilm has a natural tendency to become c-axis oriented. This isbecause the (0001) plane has the lowest surface free energyand it is therefore more energetically favourable for the filmto be c-axis oriented. However, the dual tasks of growingand characterising c-axis-oriented ZnO films on optical fibrespresent unique challenges due to the highly curved natureof the substrate.

The films in this study were grown using single sourcechemical vapour deposition (SSCVD). In this technique, theprecursor used for deposition contains the components ofthe film bonded together and surrounded by organic ligandsdesigned to increase volatility. Zinc diethylcarbamate wasused as the precursor for this study and was preparedas described in a previous paper.5 Using the SSCVD

ŁCorrespondence to: Robert N. Lamb, School of Chemistry,University of NSW, Sydney, NSW 2052, Australia.E-mail: [email protected]/grant sponsor: Australian Synchrotron ResearchProgram.

growth technique, conformal films have been deposited onoptical fibres without the need for substrate rotation.6 In arelated study in which planar Si(100) substrates were tiltedwith respect to the precursor flux, it was found that thecrystallographic orientation of the film is independent ofthe column morphology. Although the columns within thefilm were found to grow perpendicularly to the direction offlux, the c-axis orientation of the crystallites was maintainedperpendicularly to the substrate.7 This implies that a filmdeposited using SSCVD should remain c-axis orientedregardless of the geometry of the substrate and the filmmorphology.

Several growth parameters affect the quality of thedeposited films. The most easily controlled of these is thetemperature of the substrate. It was found previously thatin order to achieve c-axis orientation in SSCVD ZnO filmson planar substrates, an optimum substrate temperature of450 °C must be used.5 At lower temperatures the crystallitesdo not achieve a preferred orientation and the film consists ofall three orientations (a-, b- and c-axis). In the present paper,the effect of substrate temperature on crystallite orientationaround optical fibres is investigated.

The geometry of the optical fibre substrate limits themethods of characterisation that can be used to determinecrystal orientation on optical fibres. While a conventionaltechnique such as X-ray diffraction is useful for confirmingc-axis orientation of a film grown on a planar substrate,it cannot be used effectively on a c-axis-oriented film onan optical fibre. This is because the curvature of the fibreand the size of the X-ray beam mean that a small numberof crystallites with localised c-axis orientation will alwayssatisfy the Bragg condition; and the resulting XRD willresemble a powder pattern. To determine the orientation of

Copyright 2005 John Wiley & Sons, Ltd.

52 E. Lee, J. J. Russell and R. N. Lamb

crystallites around the fibre, we used near edge X-ray finestructure (NEXAFS) spectroscopy.8

The NEXAFS spectrum is derived from transitions of corelevel s-state electrons to partially filled and empty p statesin the molecular orbitals. The probability of s!p transitionsis maximum when the electric field vector of the incomingX-rays is parallel to the final p state. Only transitions parallelto the direction of polarisation of the X-rays will contribute tothe NEXAFS spectrum. By examining the angle dependenceof the NEXAFS spectrum, it is possible to determine theorientation of crystals within the film, as the transitions forc-axis will be different from those for a- and b-axis films.

EXPERIMENTAL

Film growth occurred with simultaneous deposition onboth optical fibres and planar silica wafers using a custom-designed sample holder. Planar silica wafers were attached tothe sample holder using screws. The optical fibres, 0.25 mmin diameter, were first stripped of their polymer claddingusing dichloromethane. They were then threaded throughtight-fitting copper tubes with an open section to allowfilm deposition onto the fibres. Depositions occurred withsubstrate temperatures of 400, 450, 500, and ¾575 °C fortwo hours. In the case of films grown at 400, 450, and 500 °C,the growth temperature was maintained within 5 °C duringthe entire deposition. Because of the limitations in the heatingsystem, it was not possible to maintain the same degree oftemperature control for films grown at ¾575 °C, and a largetemperature fluctuation of š20 °C occurred.

The conformal nature of the film and the film morphologywere studied by examining cross-sectional samples ina Hitachi 4500 SEM. The cross-sectional samples wereprepared by placing the fibre on a soft surface andapplying pressure with a razor blade until the fibre cleaved.Angle dependent NEXAFS was performed at the AustralianNational Beamline Facility (Photon Factory, Tsukuba, Japan)on beamline 20B. The NEXAFS spectra were recorded usingthe Zn K-edge (9.62 to 9.72 keV) with a step size of 0.25 eVfor the planar samples using monochromatised radiation.The step size was halved to obtain higher resolution for thefilms deposited on optical fibres. A 10-element geraniumfluorescence detector was used to collect the NEXAFSdata.

RESULTS AND DISCUSSION

The film morphology and the conformal nature of the filmcan be studied using scanning electron microscopy (SEM).Figure 1(a) is a cross-sectional image of a fibre that has beencleaved. Figure 1(b) shows a higher-magnification imageof a typical section of the fibre. It can be seen that thefilm is columnar and dense with the columns orientedperpendicular to the substrate. From our previous results7

based on films grown on planar substrates, it would beexpected that the columns within the film should be alignedwith the incoming flux and hence the columns should beperpendicular to the substrate only in a very small area. Thisis not the case when the film is deposited on a fibre possibly

(a)

(b)

Figure 1. (a) An SEM cross-sectional image of a film grown onan optical fibre that has been subsequently cleaved and (b) ahigher-magnification image of a typical section of film locatedon the surface of the fibre.

because of a decrease in local flux due to the curvature of thefibre. This issue will be explored further in a future paper.

The crystallite orientation within the films was examinedusing angle dependent NEXAFS. In order to correctlyinterpret the data, a series of reference films were grownon planar substrates and those were examined usingXRD to confirm their crystal orientations. The referencesamples were deposited on silica substrates so that substratechemistry did not differ between planar and non-planarsamples. One randomly oriented film deposited on aplanar substrate was also examined as a reference. Filmswere deposited on both planar and non-planar substratesat temperatures ranging between 400 and 575 °C. Thereference (planar) samples were examined by NEXAFSin two geometries; normal to the incident X-ray beamand at a grazing angle of ¾25° to the X-ray beam. TheNEXAFS spectrum of films in each of these positions isshown in Fig. 2. The spectrum of the normal incidencegeometry shows an intense peak at ¾9667 eV followedby a smaller peak at ¾9678 eV and a small shoulder at¾9660 eV.

From the XRD results, it is known that the planar filmshave c-axis orientation. The a-, b-axes are, therefore, parallelto the electric field vector, and hence the normal incidencespectrum is characteristic of the 1s!4sp3

a,b-axes transition.9 – 11

The orientation of the crystallites with respect to the incomingX-rays is depicted schematically in Fig. 2.

Copyright 2005 John Wiley & Sons, Ltd. Surf. Interface Anal. 2006; 38: 51–55DOI: 10.1002/sia

Structure of zinc oxide thin films grown on fibres 53

Grazing

NormalIncidence

GrazingIncidence

c-axis oriented film

Direction of polarization

(a) (b)

Polarized X-rays

0

0.5

1

1.5

2

2.5

9.62 9.64 9.66 9.68 9.70 9.72Energy (keV)

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Normal

Figure 2. A schematic showing the orientation of the film relative to the X-ray source (a) and the resulting angle dependent NEXAFSspectra of a c-axis-oriented planar reference film (b) taken from each of these orientations.

When the reference film is in grazing incidence, theresulting spectrum is quite different. In this spectrum, thereare three distinct peaks of similar intensity at 9664, 9670and 9678 eV. Furthermore, the shoulder that was presentin the spectrum in normal incidence at ¾9660 eV is nolonger evident. The grazing spectrum was attributed to the1s!4sp3

c-axis transition since the electric field polarisation isparallel to the c-axis and perpendicular to the a-, b-axes.9 – 11

Examining the films on optical fibre substrates adds aslight complication in that the curvature of the fibre preventscomplete angle dependency in the spectrum. Figure 3 showsa schematic of a c-axis-oriented film on a fibre in twopositions relative to the X-ray polarisation: parallel andperpendicular. Remembering that the spectrum consistsof transitions occurring parallel to the direction of X-raypolarisation, the a-, b-axes will contribute to the NEXAFSspectrum when the fibre is in parallel geometry. There willbe no contribution from the 1s!4sp3

c-axis transition, as thisdirection is perpendicular to the direction of polarisationof the X-ray beam. The line shape of the NEXAFS spectrumshould resemble the pure 1s!4sp3

a,b transition observed fromthe c-axis-oriented planar substrates examined at normalincidence. When the fibre is perpendicular to the incomingX-rays, contributions from the c-axis as well as the a-, b-axeswill be seen because of the curvature of the substrate. Hencethe resultant NEXAFS spectrum will have contributions fromboth the 1s!4sp3

c-axis and 1s!4sp3a,b-axes transitions and the

resultant line shape should contain features observed in theNEXAFS spectra for the reference film on both the positionsshown in Fig. 2, i.e. the spectrum will represent a randomlyoriented film.

Direction of polarization

PerpendicularOrientation

Polarized X-rays Parallel Orientation

c-axis parallel to x-raypolarization

→ no 1s – 4sp3c

transition, only1s – 4sp3

a,b transition

c-axis perpendicular tox-ray polarization

→1s – 4sp3c and

1s – 4sp3a,b transitions

OpticalFiber X-Section

Figure 3. A schematic showing the two positions of the fibrefilm relative to the X-ray source.

NEXAFS spectra obtained from films deposited on opticalfibres at different substrate temperatures are shown in Fig. 4.The NEXAFS spectra obtained from the film grown at400 °C in parallel and perpendicular geometries is shownin Fig. 4(a). Overlaying these two spectra shows no obviousdifferences, so the film shows no angle dependency. Thisindicates that there was no preferred orientation of thecrystallites in the film, i.e. the film was randomly oriented. Asimilar spectrum has been observed in a previous study in


54 E. Lee, J. J. Russell and R. N. Lamb

0

1

2

3

4

5

6

9.615 9.635 9.655 9.675 9.695 9.715 9.735

Energy (keV)

Inte

nsi

ty

Parallel Perpendicular

4a

4b

4c

4d

Reference Planar Film

Figure 4. Angle dependent NEXAFS spectra of the referenceplanar film (top) and ZnO films grown on optical fibres 400,450, 500 and 575 °C in both orientations.

a NEXAFS spectrum of a randomly oriented ZnO thin filmgrown on a planar substrate.8 It is interesting to note thatthis spectrum more closely resembles that of the referencefilm in normal incidence rather than the reference film atgrazing incidence. This is because the bonds in the a-, b-axes outnumber those in the c-axis 3 to 1 and hence theNEXAFS spectrum of a random film will be predominantlythat of a 1s!4sp3

a,b-axes transition with a small proportion of1s!4sp3

c-axis transition.Figure 4(b) is an angle dependent NEXAFS spectrum of

a film grown at 450 °C. In this case, the spectra taken in the

two different geometries show some differences. The dropof intensity of the main peak at 9665 eV in perpendicularorientation compared to parallel orientation suggests thatthere is a large contribution from the 1s!4sp3

c-axis transition inperpendicular orientation. This indicates that there is a largeproportion of crystallites with preferred c-axis orientationin the film. In a purely c-axis-oriented film, there is a well-defined shoulder at ¾9660 eV, as seen in the reference sampleat normal incidence. Although the two spectra in Fig. 4(b)are different, the shoulder is not well defined in the parallelgeometry spectrum. This suggests that there is still a smallamount of randomly oriented crystallites within the film inaddition to c-axis-oriented crystallites.

A NEXAFS spectrum of a film grown at a temperatureof 500 °C is shown in Fig. 4(c). In this spectrum, there is ahigher degree of angle dependency as shown by the greaterdifference in peak heights in the spectrum between the twopositions. Furthermore, the NEXAFS spectrum of the filmin parallel orientation resembles the a, b spectrum of thereference film, including the shoulder at ¾9660 eV. Thiswould suggest that the film is highly c-axis oriented withvery low proportions of other orientations present in thecrystallites. The most significant difference between the twospectra occurs for films grown at 575 °C as shown in Fig. 4(d).The NEXAFS spectrum of this fibre film in parallel orientationbears a strong resemblance to the pure a, b spectrum of thereference film, including the strong shoulder at ¾9660 eV. Inperpendicular orientation, an additional feature is present inthe strong shoulder at 9670 eV. A comparison of the NEXAFSspectra of films grown on non-planar substrates and thegrazing-incidence spectrum from the reference film grownat 575 °C is shown in Fig. 5. The broad shoulders in the fibrepeak at 9664 and 9670 eV correspond very strongly with thepeaks observed in the reference film. Given that the NEXAFSspectrum of the film in parallel orientation shows only thefeatures of a pure a, b spectrum and the NEXAFS spectrumof the same film at perpendicular orientation contains bothfeatures from the pure a-, b- and pure c-axis spectra, theresults conclusively prove that the crystallites within the filmare c-axis oriented at 575 °C, and furthermore, no significantamounts of other orientations are present.

0.5

1

1.5

2

2.5

9.62 9.63Energy/keV

Inte

nsi

ty (

arb

itra

ry u

nit

s)

09.64 9.65 9.66 9.67 9.68 9.69 9.70 9.71 9.72

Fibre ParallelFibre Perpendicular

Reference film(grazing)

Figure 5. Comparison between the reference ZnO film in parallel orientation and the fibre film grown at 575 °C measured in bothorientations.


Structure of zinc oxide thin films grown on fibres 55

CONCLUSIONS

We have shown that c-axis-oriented films can be depositedon non-planar substrates without the need for complicatedapparatus to rotate the substrates. This ability opens the wayfor fabrication of devices on a variety of substrates of differentgeometries and has particular application to the all-fibreacousto-optic phase modulator. The higher temperaturesrequired to achieve c-axis orientation on the non-planarsubstrates are most likely due to substrate curvature andthermal conductivity effects.

AcknowledgementsThe authors wish to acknowledge the assistance of Dr Foran andDr Hester at the ANBF and the financial support by the AustralianSynchrotron Research Program.

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nanostructural properties of zinc oxide thin films grown on non-planar substrates

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