Structural and infrared properties of zinc oxide film and nanowires

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Structural and infrared properties of zinc oxide film and nanowires

S.Y. Pung a,n, K.L. Choy a, Evgeny A. Vinogradov 1,b, Nadezhda N. Novikova b, Vladimir A. Yakovlev b

a Faculty of Engineering, Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, UKb Institute for Spectroscopy of RAS, 142190 Troitsk, Moscow reg., Russia

a r t i c l e i n f o

Article history:

Received 13 January 2010

Received in revised form

17 April 2010

Accepted 24 April 2010

Communicated by R. FornariAvailable online 5 May 2010

Keywords:

A3. Chemical Vapor Deposition

B1. ZnO

B2. Semiconducting II–VI materials

a b s t r a c t

The structural and infrared properties of the highly (00.2) oriented ZnO film, randomly grown

Au-catalyzed ZnO nanowires (NWs) and vertically aligned self-catalyzed ZnO NWs were compared. In

the XRD analysis, (0 0 2) diffraction intensity of self-catalyzed ZnO NWs was enhanced mainly

attributed to the preferential growth of NWs in [0 0 0 1] as compared to the ZnO film and the randomly

grown Au-catalyzed ZnO NWs. The high UV-to-green emission ratio of self-catalyzed ZnO NWs in room

temperature PL measurement indicates that they had a better crystal quality as compared to

Au-catalyzed ZnO NWs and ZnO film. Infrared spectroscopy has been used to characterize these films

and nanowires too. The phonon peak 407 cm�1 which related to the transverse optical (TO) vibrations

perpendicular to the optical axis was observed in the IR reflectivity measurements on the highly

c-oriented ZnO film. The IR peaks that appeared in the 550–580 cm�1 region of the spectra of the

specimens could be assigned to the ZnO NWs as it was not observed in the ZnO film. These peaks were

observed in the 550–580 cm�1 region in both s- and p-polarized light for the randomly grown Au-

catalyzed ZnO NWs. In contrast, the IR peak at 580 cm�1 was clearly shown in p-polarized light but not

in the s-polarized light for vertically aligned ZnO NWs. This indicated that the vibration was polarized

along the vertically aligned ZnO NWs. The (00.2) orientation of the ZnO specimens could be identified

by comparing the p- and s-polarized IR spectra.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

ZnO thin films and nanowires have received increasing researchfocus since 1990s due to their unique properties such as widebandgap energy (3.37 eV) and large exciton binding energy (60 meV)at room temperature. These unique properties have made ZnO apromising material for photonic devices in ultraviolet to bluewavelength range. It is expected that ZnO based devices such aslight-emitting diodes (LEDs) and exciton laser will overwhelm theGaN-related materials in near future as far as the excitonic-stimulatedemission and optically pumped laser action in high-quality ZnO films[1,2] and NWs [3] have been observed at room temperature.Nevertheless, high quality of ZnO is needed in the excitonic lasingprocess so as the optical losses in non-radiative recombinationcenters and/or traps of excitons can be minimized [3,4].

Synthesis of good crystal quality ZnO NWs, particularly invertically aligned fashion, is of great interest because it is animperative step to realize nanophotonic devices such as light emittingdiodes and laser diodes. Vertically aligned ZnO NWs can besynthesized by homeoepitaxy growth or heteroepitaxy growth. In

homeoepitaxy growth, highly (00.2) oriented ZnO films are used asseed layers to self-catalyze the ZnO NWs [5–10]. The self-catalyzedZnO NWs are governed by Vapor–Solid (VS) mechanism as there is noforeign catalyst being used to grow the NWs. In heteroepitaxygrowth, substrates with a small lattice misfit with ZnO such as SiC(5.5%) [11], GaN (1.9%) [12], and a-sapphire (0.08%) [13], arecommonly used to grow vertically aligned ZnO NWs. Foreigncatalysts, such as Au [11,13,14], are normally used to catalyze ZnONWs growth on these substrates. The growth of ZnO NWs catalyzedby Au is governed by Vapor–Liquid–Solid (VLS) mechanism.

The crystal quality and optical phonon of ZnO can becharacterized by numerous techniques. One of them is by Ramanscattering technique. The Raman active optical phonon modes ofZnO specimens, i.e., A1+E1+2E2 are measured and compared[15,16]. However, according to the group theory, single-crystal-line ZnO has eight sets of optical phonon modes at G point of theBrillouin zone. The remaining 4 sets of optical phonon modes are2B1 modes (Raman silent) and A1+E1 modes [infrared (IR) active].In this work, the IR active ZnO optical phonon will be studiedusing IR technique. A better understanding of the IR active opticalphonon properties may be helpful in the development of ZnObased opto-electronic devices [17,18].

The structural and optical properties of highly c-orientedZnO film, randomly grown Au-catalyzed ZnO NWs, and verticallyaligned self-catalyzed ZnO NWs are reported in this paper.The highly c-oriented ZnO film was deposited using Atomic Layer

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

Journal of Crystal Growth

0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.jcrysgro.2010.04.050

n Corresponding author. Tel.: +44 1159514031; fax: +44 1159513800.

E-mail addresses: [email protected] (S.Y. Pung),

[email protected] (K.L. Choy), [email protected]

(E.A. Vinogradov).1 Tel.: +74 967510579; fax: +74 967510886.

Journal of Crystal Growth 312 (2010) 2220–2225

Author's personal copyARTICLE IN PRESS

Deposition (ALD) technique, whereas the ZnO NWs weresynthesized by Chemical Vapor Deposition (CVD) technique. Thevertically aligned ZnO NWs were grown on the highly (00.2)oriented ZnO film without using foreign catalyst. In contrast,randomly grown Au-catalyzed ZnO NWs were synthesized onp-type Si (1 0 0). The crystal and structural properties of ZnOspecimens have been analyzed using a combination of XRD, SEM/EDX, PL, TEM, SAED, and HR-TEM. Furthermore, we will discuss theinteresting optical features of these ZnO specimens measured usingthe IR reflectivity and transmittance measurements.

2. Experimental procedure

Highly (00.2) oriented ZnO film (160 nm) was deposited onSi substrate [p-type, (1 0 0)] using ALD technique. The detailsof the deposition procedures and conditions were presented anddiscussed elsewhere [5]. The Si substrate pre-deposited withhighly (00.2) oriented ZnO film was used as a seed layer to self-catalyze vertically aligned ZnO NWs in the subsequent CVDprocess. In addition, a thin Au film (5 nm thick) was deposited ona Si substrate as catalyst for the randomly grown ZnO NWs. Thegrowth of ZnO NWs was carried out in a horizontal tube reactorusing a low pressure CVD technique for 30 min. Zn powder(99.99%) was loaded in the middle of quartz tube furnace. Both Sisubstrates pre-coated with ZnO thin film and Au film were loadedin the downstream of the reactor. The reactor was heated to650 1C under a constant flow of argon gas. After reaching thedesired temperature, oxygen (5 sccm) flowed into the furnacefor the growth of NWs. The pressure during the growth processwas kept at 2 kPa.

The crystalline structures of ZnO film and NWs werecharacterized using X-ray Diffractometer (Bruker, CuKa radiationwith a wavelength of 1.5406 A) and transmission electronmicroscopy (JEOL, JEM-2100F). The surface morphology of thefilm was examined using a scanning electron microscope (SEM,Phillips XL30 ESEM-FEG). Photoluminescence (PL) studies wereperformed on these ZnO specimens with N2 laser excitation

source (337 nm, pulse rate 20 Hz). In addition, infrared (IR)spectroscopy was used to characterize the optical properties ofthe ZnO specimens. IR reflectivity and transmittance measure-ments in the spectral range 50–7500 cm�1 were performed usingIFS 66v/s (BRUKER) infrared Fourier-transform spectrometer withreflection units at the near normal incidence and at the obliqueangles of incidence up to the grazing angles [reflection–absorp-tion spectroscopy (RAS) in polarized light]. The spectral resolutionwas 4 cm�1.

3. Results and discussion

3.1. Structural analysis of ZnO films and ZnO nanowires

The XRD results in Fig. 1 show a comparison of the crystallinityof the as-synthesized ZnO film and NWs. As shown in Fig. 1(a),several ZnO diffraction peaks such as (10.0), (00.2), (10.1), (10.2),(11.0), etc. were observed on the Au catalyzed NWs. Thesediffraction peaks patterns were matched with the standarddiffraction pattern of wurtzite ZnO (JCP2.2CA. 01-079-2205).In contrast, both ZnO film (Fig. 1(b)) and self-catalyzed ZnONWs (Fig. 1(c)) exhibited strong (00.2) diffraction peaks. Noother ZnO and Zn metal diffraction peaks were observed. Theenhancement of (00.2) diffraction intensity of self-catalyzed ZnONWs was attributed to the preferential growth of NWs in [0 0 0 1]and might be an indirect indication of better crystal quality ascompared to ZnO film and Au-catalyzed NWs.

Fig. 2(a) shows the surface morphology of Au-catalyzed ZnONWs. The growth of ZnO NWs was random as expected due to thelarge lattice mismatch between ZnO and Si. The randomly grownZnO NWs contributed to the bulk-like XRD diffraction pattern asindicated in Fig. 1(a). Nanoparticles were observed at the tipsof NWs (as highlighted in the circles of Fig. 2(a and b)). Thenanoparticle composes of Au–Si–Zn–O elements when it wasanalyzed using TEM equipped with EDS as shown in Fig. 2(d).This result indicates that the growth of these NWs was governedby VLS mechanism. The Cu element of the EDS analysis was

30

2θ (degree)

35 40 45 50 55 60 65

01000200030004000500060007000

01000200030004000500060007000

0100000200000300000400000500000600000700000

Au

(2

00

)

Au

(1

11

)

(10

.1)

(11

.0)

(10

.2)

(10

.0)

Au-catalyzed ZnO NWs

Self-catalyzed ZnO NWs

ZnO film

Inte

nsity (

a.u

.)

(00

.2)

Fig. 1. XRD results of (a) Au-catalyzed ZnO NWs, (b) ZnO thin film, and (c) self-catalyzed ZnO NWs.

S.Y. Pung et al. / Journal of Crystal Growth 312 (2010) 2220–2225 2221


contributed from the TEM copper grid. The average length anddiameter of the Au-catalyzed ZnO NWs were 371.2717.14 and59.873.89 nm, respectively. Fig. 3 shows ZnO film with smoothsurface. The calculated crystal size of the film from Scherrer’sequation was 17.7 nm. In addition, the ZnO polycrystal grainswere highly (00.2) oriented, with c-axes perpendicular to thesubstrate surface.

Vertically aligned ZnO NWs were synthesized on this ZnO filmas shown in Fig. 4(a). The highly (00.2) oriented polycrystal grainsof ZnO film, facilitate the growth of ZnO NWs in c-axis directionvia VS mechanism. This highly aligned NWs enhanced the (00.2)diffraction peaks in XRD analysis as demonstrated in Fig. 1(c). NW

tips with hexagonal shape were clearly observed from the topview as shown in Fig. 4(b), indicating that the ZnO NWs had awurtzite crystalline structure. The average length and diameter ofself-catalyzed ZnO NWs were 503.2722.78 and 98.375.57 nm,respectively. No nanoparticle could be found at the tips of NWs.Further structural characterization of self-catalyzed ZnO NW wasperformed by HR-TEM as displayed in Fig. 4(c). The result revealsa clear lattice spacing of 0.52 nm corresponding to the interplanarspacing of the wurtzite ZnO (00.2), indicating that the growth ofZnO NWs occurred preferentially along the [0 0 0 1] direction.Furthermore, the SAED pattern of a single ZnO NW in Fig. 4(d)shows that the corresponding NW was single-crystal in structure.

cps

cps

1 μm

Tip of NW

Fig. 2. (a) Randomly grown ZnO NWs catalyzed by Au film, (b) TEM image of Au-catalyzed ZnO NWs, (c) EDS analysis at the NW, and (d) EDS analysis at the tip of NW.

S.Y. Pung et al. / Journal of Crystal Growth 312 (2010) 2220–22252222


Room temperature PL measurement was carried out on theseZnO specimens. A dominant UV emission peak (377 nm) and aweak green emission (520 nm) were observed on these samplesas shown in Fig. 5. The UV emission peak of ZnO was attributed to

Fig. 3. ALD-deposited ZnO film, consisting of tiny polycrystal grains (calculated

crystal size using Scherrer’s equation is 17.7 nm).

1 μm 1 μm

(00.2)

(00.2)-

(11.0)- -

(11.0)

2 nm

[0001]

Fig. 4. Vertically aligned ZnO NWs catalyzed by highly (00.2) oriented ZnO thin film in (a) 301 view, (b) top view, (c) HR-TEM image, and (d) SAED image.

350

Self-catalyzed ZnO NWs Au-catalyzed ZnO NWs ZnO film

Inte

nsity

(a.

u.)

Wavelength (nm)400 450 500 550 600

Fig. 5. Room temperature PL spectrum of self-catalyzed ZnO NWs, Au-catalyzed

ZnO NWs and ZnO film.



the near-band edge (NBE) emission of wide-band gap ZnOwhereas the green emission was generally assumed to berelated to defects such as oxygen deficiency [19,20]. Therefore,the presence of a strong UV emission and a weak green emissionin the PL measurement indicated that the self-catalyzed ZnO NWshad a good crystallinity as compared to the Au-catalyzed ZnONWs and ZnO film. ZnO film had the lowest UV-to-green emissionratio. This could be attributed to the presence of OH species orfragments of hydrocarbon in the film as a result of incompletedecomposition of precursors as discussed in our previouswork [5].

3.2. IR reflectivity and transmittance measurements

Fig. 6(a and b) shows the IR reflectivity and transmittancespectra at the near normal incidence in the spectral range 300–700 cm�1. The dispersion analyses of the reflectivity andtransmittance spectra of Fig. 6 were used to obtain filmparameters. The experimental spectra were compared with thecomputer simulations calculated using Fresnel equations forreflectivity and transmittance of a structure under consideration.The frequency dependence of the film dielectric function wasrepresented as a sum of high frequency dielectric constant eN anda set of Lorentz oscillators as shown in Eq. (1)

eðnÞ ¼ e1þX fTOi

2

nTOi2�n2�igTOin

þnp

2

�n2�intnð1Þ

where nTOi is the ith vibrational mode frequency, fTOi its oscillatorstrength, gTOi its width, np the free carrier plasma frequency, nt thecarrier collision frequency in plasma, and eN the high frequencydielectric constant. Spectra fitting, varying both film and substrateparameters, was done using SCOUT—SpectrosCopic Objects andUTilities, a WINDOWS application written by Theiss [21]. We hadchosen the starting parameters for the structure: film thicknessesand dielectric function model parameters in Eq. (1). The programhad calculated the reflectivity and transmittance spectra of thestructure and compared them with the experiment. Then it hadvaried the adjusting parameters, changing them step by step tominimize the difference between the experimental and thecalculated spectra.

The best fit ZnO film and NWs dielectric functions are shownin Fig. 7. In the case of Au film (5 nm), the best fit obtained for thecomplex dielectric function is constant in the spectral range(300–700 cm�1) of the fitting, i.e., e¼10+60i. The positive realpart indicates the nanoparticles (island structure) of Au film andits value is consistent with the dielectric functions of the reportedvalue in [22] for the 1100–1150 cm�1 spectral range.

The IR reflectivity properties of ZnO films and NWs were alsoexplored at 451 angle of incidence in s- and p-polarized light asshown in Fig. 8(a and b), respectively. For the highly c-orientedZnO film, only one phonon peak 407 cm�1, which is related tothe transverse optical (TO) vibrations perpendicular to the opticalaxis (E1 mode) was observed in s-polarized light (Fig. 8(a)). Inp-polarized light a low frequency shoulder appears (A1 mode)

300

0.4

0.6

0.8

Reflecta

nce

Wavenumbers (cm-1)

ZnO film (highly c-oriented)

Au-catalyzed ZnO NWs (randomly growth)

Self-catalyzed ZnO NWs (vertically aligned)

Si: 620 cm-1

0.0

0.2

0.4

0.6 ZnO film (highly c-oriented)



Tra

nsm

itta

nce

400 500 600 700

300 400 500 600 700

Wavenumbers (cm-1)

Si: 620 cm-1

Fig. 6. IR spectra of ZnO film and ZnO NWs measured at the near normal incidence

in (a) reflectivity mode and (b) transmittance mode.

300

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

Com

ple

x d

iele

ctr

ic function (

ε)

Re ε οf ZnO film (highly c-oriented)

Imε of ZnO film (highly c-oriented)

-30

-20

-10

0

10

20

30

40

50

60

70

80

Co

mp

lex d

iele

ctr

ic f

un

ctio

n (

ε)

Re ε for Au-catalyzed ZnO NWs (randomly growth)

Im ε for Au-catalyzed ZnO NWs (randomly growth)

Re ε for self-catalyzed ZnO NWs (vertically aligned)

Im ε for self-catalyzed ZnO NWs (vertically aligned)

Wavenumbers (cm-1)

400 500 600 700

300 400 500 600 700

Wavenumbes (cm-1)

Fig. 7. The complex dielectric functions of (a) highly c-oriented ZnO films and

(b) ZnO NWs. Re e and Im e stand for the real part and imaginary part of the

complex dielectric functions of the ZnO specimens, respectively.

S.Y. Pung et al. / Journal of Crystal Growth 312 (2010) 2220–22252224


(Fig. 8(b)). Hence the film is highly c-oriented, and its optical axisis normal to the surface. This correlates well with the XRD resultas shown in Fig. 1(b). The band close to 620 cm�1 can be assignedto the silicon substrate (second order) (Figs. 6 and 8).Furthermore, the peaks that appeared in the region of550–600 cm�1, as shown in Fig. 8, can be assigned only to ZnONWs (regardless of Au-catalyzed or self-catalyzed), because theyare not present in the spectra of ZnO film. For vertically alignedZnO NWs, the peak at 580 cm�1 is clearly present inp-polarization mode (Fig. 8(b)) but not in the s-polarizationmode. This indicated that this vibration is polarized along thealigned ZnO NWs. In contrast, some peaks are observed in 550–600 cm�1 region in both polarization modes for the Au-catalyzedZnO NWs. Thus, there is no preferred growth direction in thisspecimen. This result agrees well with the XRD analysis (Fig. 1(a))and the SEM image (Fig. 2(a)).

4. Conclusions

The structural and crystal quality of the highly c-oriented ZnOfilm, randomly grown Au-catalyzed ZnO NWs and verticallyaligned self-catalyzed ZnO NWs were compared. The very strong

(00.2) diffraction intensity of self-catalyzed ZnO NWs in XRDanalysis can be mainly attributed to the preferential growth ofNWs in [0 0 0 1] as compared to the ZnO film and Au-catalyzedZnO NWs. In addition, self-catalyzed ZnO NWs have a bettercrystal quality as the PL results show that it has the highest UV-to-green emission ratio as compared to Au-catalyzed ZnO NWsand ZnO film. In the IR spectra, only one phonon peak 407 cm�1,which is related to the transverse optical (TO) vibrationsperpendicular to the optical axis was observed on the highlyc-oriented ZnO film. For vertically aligned ZnO NWs, the IR peak at580 cm�1 is clearly present in the p-polarization mode but not inthe s-polarization mode when measured at the 451 angle. Thisindicates that this vibration is polarized along the aligned ZnONWs. In contrast, some peaks are observed in the 550–580 cm�1

region in both polarization modes due to the random orientationof Au-catalyzed ZnO NWs. In summary, the polarized infraredspectra allow us to understand the orientation of the ZnOspecimens, which can be contributed to the development of ZnObased opto-electronic devices.

Acknowledgements

This work was supported by Universiti Sains Malaysia,Ministry of Higher Education Malaysia, EU-FP6 EXCELL projectand programs of the RAS Presidium ‘‘The fundamental opticalspectroscopy and its applications’’, ‘‘Bases of the fundamentalresearches of nanotechnology and nanomaterials’’.

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19 (1977) 1352.

Re

fle

cta

nce

Re

fle

cta

nce

0.4

0.6

0.8

0.2

0.4

0.6

407 cm-1

(TO)

407 cm-1

(TO)

550 - 600 cm-1

550 - 600 cm-1

Si:620 cm-1

ZnO

ZnO

E=Ey

E

Z

Z

NWs

NWs

X

X

Ex

Ez

300 400 500 600 700

Wavenumbers (cm-1)

300 400 500 600 700

Wavenumbers (cm-1)

ZnO film(higghly c-oriented)

Au-catalyzed ZnO NWs ((randomly growth)


ZnO film (highly c-oriented)



Fig. 8. The reflectivity of ZnO film and NWs at 451 angle of incidence in (a) s-

polarized light and (b) p-polarized light.


Structural and infrared properties of zinc oxide film and nanowires

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