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Journal of Crystal Growth 287 (2006) 39–43

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Optical phonon modes in ZnO nanorods on Si prepared by pulsed laserdeposition

Vinay Guptaa,�, P. Bhattacharyab, Yu. I. Yuzukb, K. Sreenivasa, R.S. Katiyarb

aDepartment of Physics and Astrophysics, University of Delhi, Delhi 110007, IndiabDepartment of Physics, University of Puerto Rico, San Juan, P.O. Box 23343 PR, USA

Abstract

An array of vertically aligned c-axis-oriented zinc oxide (ZnO) nanorods were fabricated at a low substrate temperature (500 1C)

without using any catalytic template on the Si substrate by pulsed laser deposition at a high pressure (1–5Torr). The successive

deposition of ZnO nanoparticles one over the other during growth led to the formation of elongated and well-isolated nanorods with a

diameter in the range 70–350 nm and up to 14mm long. The influence of deposition parameters on the Raman phonon modes was

investigated. The polarized Raman studies were carried out in the backward geometry with incident light (i) parallel and (ii)

perpendicular to the nanorod length. The activation of lower and upper surface phonon modes were observed in the ZnO nanorods

between the transverse optical (oTO) and the longitudinal optical (oLO) phonon frequencies. The lower surface mode observed at

�475 cm�1 is found to be predominant in the X ðZZÞX configuration. The surface phonon modes were not found in the samples prepared

at 4600 1C due to the coalescence process resulting in a continuous film. The agreement of polarized Raman spectra with single crystal

data indicates the growth of good quality ZnO nanorods, and allows direct integration on Si without introducing contamination at the

interface due to any catalytic material.

r 2005 Elsevier B.V. All rights reserved.

PACS: 61.82.Rx; 62.60.+v; 78.30.�j

Keywords: A1. c-axis orientation; A1. Nanorods; A1. Raman scattering; A1. Surface phonon mode; A3. PLD; B2. Zinc oxide

1. Introduction

Recent demonstration of novel devices including ananoscale laser and electrochemically gated quantum dottransistor, and the highly efficient exciton UV lasing actionunder optical pumping from the nanoclusters and thinfilms indicate that zinc oxide (ZnO) is a very promisingmaterial for applications in modern nanoelectronics andphotonics [1–4]. The availability of large quantities of well-aligned ZnO nanorods in single crystalline form isextremely important for developing high-efficiency, short-wavelength optoelectronic nano-devices. The various pos-sible crystalline ZnO nanostructures including nanowires,nanoneedles and nanorods have been synthesized by avariety of techniques such as molecular beam epitaxy,

e front matter r 2005 Elsevier B.V. All rights reserved.

rysgro.2005.10.039

ing author. Tel.: +91 98 1156 3101; fax: +91 11 2766 7061.

ess: drvin_gupta@rediffmail.com (V. Gupta).

thermal evaporation, vapor phase transport and the laserablation technique [3–8]. However, well-aligned and c-axis-oriented nanorods were grown either on epitaxiallymatched substrates, and/or using the catalytic templates,or mostly at high processing temperatures (700–1000 1C).The mismatch in terms of the coefficient of thermalexpansion between the nanorods and the substrate mustbe reduced when they are deposited. The presence of acatalytic layer of other materials during the synthesis ofnanostructures may lead to contamination in the grownmaterial, especially at the edges of the nanostructures.Therefore, synthesis of nanostructure without any catalytictemplate, or using the self-catalytic behavior of thematerial would be of interest. Moreover, the synthesis ofvertically aligned ZnO nanorods directly on silicon couldbe exploited for the integration of the existing microelec-tronic technology with the nanoscale short wavelengthfunctional devices. Also, very little is known about the

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Fig. 1. Scanning electron micrographs of ZnO nanorods on Si (a) surface

topography; (b) cross-sectional view; and (c) single nanorod.

V. Gupta et al. / Journal of Crystal Growth 287 (2006) 39–4340

optical vibrational spectra of ZnO nanorods, and thevibrational frequencies are expected to be different fromthe bulk, since the bonding partners missing at the surfaceare in large quantity [9]. In the present work, we report thesuccessful growth of a large quantity of well-aligned c-axis-oriented ZnO nanorods directly on Si using a pulsed laserdeposition (PLD) technique at a relatively low processingtemperature (�500 1C) without using any catalytic tem-plate. The polarized Raman optical studies under differentpolarization configurations have been investigated indetail.

2. Experimental procedure

ZnO nanorods were fabricated using a XrF Excimerpulsed-laser ablation of a sintered ceramic ZnO targetprepared using the standard ceramic processing. Thegrowth was performed under a high background oxygenpressure (1–7Torr) and low substrate to target distance(25mm). The surface morphology and microstructure wasstudied by scanning electron microscopy (Jeol JSM-5800LV) and atomic force microscopy (AFM) (DigitalInstruments, Nanoscope IIIa). The crystalline orientationwas analyzed using X-ray diffractometer (Siemens, D5000).Raman spectra were excited using the polarized light forma coherent INNOVA 99 Ar+ laser (l ¼ 514:5 nm) andanalyzed using a Jobin Yvon T64000 spectrometerequipped with a charge coupled device in a backscatteringgeometry. An optical microscope with 80� objective wasused to focus the incident light on a spot of about 2 mm indiameter on the sample.

3. Results and discussion

The scanning electron micrographs (SEM) of theprepared ZnO nanorod on Si at a substrate temperatureof 500 1C and under a high pressure of 5 Torr are shown inFigs. 1(a)–(c). The surface micrograph showed the forma-tion of well-separated and uniformly distributed hexagonalgrains with an average diameter 120–200 nm. The growthof isolated ZnO nanorods on the Si surface without usingcatalytic template was clearly revealed from the cross-sectional microstructure (Fig. 1b). However, the possibilityof assistance from self-catalytic behavior of the thin ZnOfilm deposited on Si substrate during the initial phase of thegrowth process may not be ruled out. These nanorods werevertically well-aligned with uniform length, diameter anddistribution density. The average diameter of the nanorodswas found to be strongly dependent on the growth time,and increased from about 70 to 250 nm while increasing thedeposition time from 2 to 20min. The length of thenanorods was found to depend on the deposition time and,in the present work, successful growth of 14-mm-long rodsat an ablation time of �20min with an aspect ratio of�40–70 was achieved. The SEM image of a single isolatedZnO nanorod of about 200 nm diameter and 5 mm length isshown in Fig. 1(c). The edge of the nanorods is sharp and

fine because of the self-nucleation of ZnO nanoparticlesalong the preferred c-axis growth direction on Si at theoptimum temperature without any catalytic template. TheZnO nanorods fabricated at 500 1C remain well-isolatedand vertically aligned in the entire pressure range of1–7Torr. However, the diameter of the rods was found tobe lowest at an optimum oxygen pressure of 5Torr. A largevariation in the diameter of the nanorods (70–350 nm) wasobserved at a substrate temperature of 600 1C. However,no formation of ZnO rods was evident at a higher substratetemperature (4600 1C). Also, at low substrate tempera-tures (o 450 1C) a ZnO film having a very rough andporous microstructure is formed. It is important to note

ARTICLE IN PRESSV. Gupta et al. / Journal of Crystal Growth 287 (2006) 39–43 41

that the key to the realization of a well-aligned array ofZnO nanorods directly on an Si substrate is optimumsubstrate temperature (450–600 1C), which is essentiallyrequired to settle the nanoparticles of ZnO one over theother during the growth process resulting in the formationof nanorods with a hexagonal cross-section. The atomicforce 3D micro-image of the array of ZnO nanorodsfabricated on an Si substrate at the substrate temperature500 1C and oxygen pressure 5Torr is shown in Fig. 2. Thegrowth of well-isolated and vertically aligned nanorodshaving diameters ranging from 100 to 250 nm wasconfirmed. The results are in good agreement with theSEM analysis. The growth mechanism of ZnO nanorodson Si using PLD is attributed to the ablation of the oxidetarget under high oxygen pressure (1–7Torr) and optimumsubstrate temperatures (450–600 1C). The significant ad-vantage of PLD is the ability to change oxygen pressureeasily during the deposition up to a higher value of 7Torrenabling control of crystalline size of the fabricatednanorods. The plume length is reduced with the increasein processing pressure and, therefore, the Si substrate in thepresent study is placed at a smaller target-to-substratedistance (25mm). Collisions between the ablated speciesfrom the target and the oxygen species increases withincrease in oxygen pressure. If oxygen pressure is too high(1–7Torr), the vapor species can undergo enough collisionsthat nucleation and growth of these vapor species formnanoparticles before their arrival at the Si substrate. TheZnO nanoparticles under high pressure starts nucleating onthe substrate surface along the preferred growth directionsi.e c-axis of the crystallographic wurtzite structure andsettled down one over the other to form long and well-seperated nanorods. The possibility of a self-nucleationcenter due to thin ZnO film grown during the initial phaseof processing may not be ruled out. At high substratetemperature, the migration effects on the substrate surfaceis activated, and the process of coalescence of nucleatingnanoparticles is expected [10], resulting in the growth of a

Fig. 2. Three-dimensional atomic force microscopy image of ZnO

nanorods.

continuous film having densely packed large-size grainsinstead of isolated nanorods.Fig. 3 shows the X-ray diffraction (XRD) spectra of the

vertically aligned array of ZnO nanorods on Si, depositedat 5Torr and 500 1C. Only two reflections corresponding to(0 0 2) and (0 0 4) planes were observed in the XRD profile,indicating that the vertically aligned ZnO rods had apreferred c-axis orientation along their length, and werenormal to the Si substrate. It is important to point out thatthe samples prepared at higher substrate temperature(4450 1C) and higher oxygen pressure (1–7Torr) remainedhighly c-axis-oriented. However, at lower temperatures(o450 1C), XRD peaks corresponding to other reflectionsof ZnO were noted, indicating the polycrystalline nature ofthe deposited material. A minimum energy is required forthe nucleation of grains along the preferred direction.Therefore, 450–600 1C is found to be the optimumsubstrate temperature required to settle the nanoparticlesof ZnO one over the other along the preferred c-axisgrowth direction under higher oxygen pressures (1–7Torr)and, thereby, resulting in the formation of an array ofvertically aligned nanorods.Raman scattering studies were carried out on ZnO

nanorods to obtain information about the effects ofincrease in surface area in comparison to a continuousfilm or single crystal on the optical phonons and the latticedefect modes. Raman spectra were obtained in the back-scattering configuration with incident light both theparallel and perpendicular to the length of the ZnOnanorods. ZnO with a wurtzite structure belongs to theC6n symmetry group. At the C-point of the Brillouin zone,optical phonons have Copt ¼ A1 þ 2B1 þ E1 þ 2E2, whereA1 and E1 modes belong to polar symmetries and can havedifferent transverse (TO) and longitudinal (LO) opticalphonon frequencies, all being Raman active, while the B1

modes are silent.

30 40 50 60 70

(004

)

Si

(002

)

Inte

nsity

(a.

u.)

2θ (degrees)

Fig. 3. X-ray diffraction of the arrays of ZnO nanorods.

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Table 1

Phonon modes observed in ZnO samples prepared on Si at different substrate temperature (150–850 1C) by PLD

Phonon modes 150 1C 300 1C 500 1C 600 1C 4600 1C

E2 (low) 98 98 98 98 98

E1 (TO) 410 410

E2 (high) 438 438 438 438 438

LM 475 475

UM 572 571

0 200 400 600 800 1000 12000

3000

6000

9000

12000

15000

ZnO nanorods on Si

E2(high)

E2(low)

Si

LM

UM

Inte

nsity

(a.

u.)

Raman shift (cm-1)

XX

XZ

ZX

ZZ

0 200 400 600 800 1000 12000

2000

4000

6000

8000

10000

12000ZnO single crystal

Inte

nsity

(a. u

.)

Raman shift (cm-1)

XX

XZ

ZX

ZZ

(a)

(b)

Fig. 4. Polarized Raman spectra under different polarization configura-

tions for (a) ZnO nanorods on Si; (b) ZnO single crystals, incident and

scattered beam directions being Y and Y , respectively.

V. Gupta et al. / Journal of Crystal Growth 287 (2006) 39–4342

Strong Raman bands at 98 and 438 cm�1 were observedin all the samples prepared under different substratetemperatures and were attributed to the non-polar E2

vibration modes corresponding to the wurtzite phase[9,11,12]. The different optical phonon modes observed inthe prepared ZnO samples under different condition arelisted in Table 1. A broad and weak E1 (TO) mode at about410 cm�1 was observed in the sample deposited at lowsubstrate temperatures (o500 1C), and may be attributedto the contribution of the polarization due to mixedoriented crystallites and the breaking of selection rules forpropagation in a direction other than the Z crystal-lographic axis [12]. These results are in agreement withthe findings from the XRD analysis. The absence of E1

(TO) mode in samples deposited at 4450 1C indicates theformation of preferred c-axis-oriented ZnO nanorods withtheir length perpendicular to the Si surface.

A well-defined peak appeared at around 572 cm�1 in theRaman spectra of ZnO nanorods prepared at 500–600 1C,and is at a lower frequency in comparison to the reportedE1 (LO) mode in bulk ZnO at 591 cm�1 [9]. To understandthe origin of the observed mode at 572 cm�1, polarizedRaman studies were performed on ZnO nanorods grown at500 1C in the backward geometry, with the incident light (i)perpendicular and (ii) parallel to the length of thenanorods, and the spectra recorded under differentconfigurations are shown in Fig. 4a. The polarized Ramanspectra recorded on the ZnO single crystal under similarconfigurations are shown in Fig. 4(b) for comparison. Thespectra recorded in the different directions of polarizations(XX , XZ, ZX and ZZ) of incident and scattered beambeing along Y and Y , respectively, were in good agreementwith the single crystal data. In the present study Y directionis in the XY crystallographic plane because of the difficultyin ascertaining the exact a and b lattice directions in thedeposited nanorods. It is known from the previous studieson a ZnO single crystal that both E1 (LO) at 591 cm�1 andA1 (LO) at 579 cm�1 modes are considerably weaker ascompared to their respective TO phonons [11,12]. There-fore, the broad and well-define peak observed around572 cm�1 (Table 1) may not be attributed to the bulk E1 orA1 LO phonons. Instead, this band may be assigned to thesurface phonon scattering, which is expected in the Ramanspectra when the crystallite size is much smaller than thewavelength of the incident light [13]. The SEM and AFManalysis clearly indicate the formation of ZnO nanostruc-

tures at 300–600 1C having small and isolated crystallitesand satisfying the above condition. It was reported that thesurface phonon modes in wurtzite nanocrystals havefrequencies intermediate between A1 (LO) and E1 (TO),and the corresponding observed modes in bulk ZnO are at579 and 413 cm�1, respectively [9]. In the present work, theobserved mode at 572 cm�1 is between the LO and TOfrequencies, where the surface phonon modes are expecteddue to breaking of the translation symmetry, and it ishighly localized near the boundary of the crystallites [14].

ARTICLE IN PRESSV. Gupta et al. / Journal of Crystal Growth 287 (2006) 39–43 43

Since the surface area is considerably increased due to thefabrication of ZnO nanorods atp600 1C, the interaction oflight with the surface should activate the surface phononmodes to the observable limits. This should give rise to twobranches of surface modes between the transverse optical(oTO) and the longitudinal optical (oLO) phonon frequen-cies of ZnO [15]. The frequency of the upper surfacephonon mode (UM) is normally below the correspondingbulk LO value and, therefore, the observed mode of572 cm�1 is assigned to UM. The corresponding lowersurface phonon mode (LM) is clearly observed at around475 cm�1 in the highly c-axis oriented and well-isolatedvertically aligned ZnO nanorods predominantly in theexpected Y ðZZÞY configuration (Fig. 4a). Here therespective direction of the incident and the scattered beamsare Y and reverse Y (i.e.Y ), whereas Z is the direction ofboth incident and the scattered beams’ polarization. Theabsence of these surface modes in the samples deposited athigher temperatures (4600 1C) indicates that the ZnOnanorods coalesce and form a continuous film. Theagreement of the polarized Raman spectra of the ZnOnanorods with the corresponding single crystal spectraindicate the growth of good quality and vertically well-aligned ZnO nanorods, and allow direct integration on Siwithout catalytic template of any other material, thereby,avoiding the possibility of any contamination at theinterface. Furthermore, the growth of these rods at arelatively low processing temperature (500 1C) is compa-tible with the microelectronic technology without introdu-cing defects at the interfaces.

4. Conclusions

An array of single crystalline vertically aligned ZnOnanorods on Si with hexagonal cross-section without usingany catalyst have been grown successfully at 500 1C by thepulsed laser deposition. The Raman spectra confirmed theformation of c-axis-oriented and well-aligned ZnO nanor-ods. The activation of two branches of surface modes(upper and lower) between the TO and LO phononfrequencies have been found at 572 and 475 cm�1,

respectively. Polarized Raman spectra in various config-urations were in good agreement with the single crystaldata, and the additional peaks at 475 and 572 cm�1

observed in the ZnO nanorods in the Y ðZZÞY configura-tion were assigned to the lower and upper surface phononmodes, respectively.

Acknowledgements

The authors acknowledge partial financial supports fromNASA#NCC3-1034 and DAAD 19-03-1-0084 grants.Authors (VG and KS) are also thankful to Departmentof Science and Technology (DST), India, for financialassistance. Authors are also thankful to MCC-UPR forXRD and SEM measurements.

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