A Comparison of ZnO Nanowires and Nanorods Grown Using MOCVD and Hydrothermal Processes

A Comparison of ZnO Nanowires and Nanorods Grown UsingMOCVD and Hydrothermal Processes

ABDIEL RIVERA1, JOHN ZELLER,2 ASHOK SOOD,2

and MEHDI ANWAR1,3

1.—Electrical and Computer Engineering, University of Connecticut, Storrs, CT 06269, USA.2.—Magnolia Optical Technologies, Woburn, MA 01801, USA. 3.—e-mail: [email protected]

A comparison of ZnO nanowires (NWs) and nanorods (NRs) grown usingmetalorganic chemical vapor deposition (MOCVD) and hydrothermal syn-thesis, respectively, on p-Si (100), GaN/sapphire, and SiO2 substrates isreported. Scanning electron microscopy (SEM) images reveal that ZnO NWsgrown using MOCVD had diameters varying from 20 nm to 150 nm andapproximate lengths ranging from 0.7 lm to 2 lm. The NWs exhibited cleantermination/tips in the absence of any secondary nucleation. The NRs grownusing the hydrothermal method had diameters varying between 200 nm and350 nm with approximate lengths between 0.7 lm and 1 lm. However, theNRs grown on p-Si overlapped with each other and showed secondary nucle-ation. x-Ray diffraction (XRD) of (0002)-oriented ZnO NWs grown on GaNusing MOCVD demonstrated a full-width at half-maximum (FWHM) of 0.0498(h) compared with 0.052 (h) for ZnO NRs grown on similar substrates usinghydrothermal synthesis, showing better crystal quality. Similar crystal qual-ity was observed for NWs grown on p-Si and SiO2 substrates. Photolumines-cence (PL) of the NWs grown on p-Si and SiO2 showed a single absorption peakattributed to exciton–exciton recombination. ZnO NWs grown on GaN/sap-phire had defects associated with oxygen interstitials and oxygen vacancies.

Key words: ZnO, nanowires, nanorods, MOCVD, hydrothermal,photoluminescence

INTRODUCTION

ZnO has a direct energy band gap of 3.37 eV, arelatively large exciton energy of 60 meV, and lon-gitudinal optical (LO) phonon energy of 72 meV,making it suitable for optoelectronic applications1

including quantum cascade lasers (QCLs),2 ultravi-olet (UV) light-emitting diodes (LEDs),3 and UVdetectors.4 ZnO nanowires (NWs) and nanorods(NRs) have been found to exhibit large spontaneousand strain-induced piezoelectric polarizations,enabling them to be harnessed for energy harvestingapplications.5 In addition, the thermoelectric prop-erty of ZnO NWs has been utilized by fabricating

nanoscale devices for energy scavenging.6 Further-more, ZnO is chemically stable and biocompatible,making ZnO NW/NR-based devices suitable for bio-sensing applications.7

ZnO NWs have been grown using various methods,including molecular-beam epitaxy (MBE), metalor-ganic chemical vapor deposition (MOCVD), andhydrothermal synthesis. MBE allows monitoring ofthe structural quality while the NWs are beinggrown. However, this type of growth requires the useof catalysts such as gold as a seed layer,8 which canintroduce undesired defects into the structure,affecting the crystal quality.9 MOCVD provides con-trol over the morphology and orientation of NWs byallowing adjustment of temperature, gas flow, andpressure without requiring the use of catalysts.10–12

The effect of growth temperature on MOCVD using(Received August 15, 2012; accepted December 28, 2012;published online February 8, 2013)

Journal of ELECTRONIC MATERIALS, Vol. 42, No. 5, 2013

DOI: 10.1007/s11664-012-2444-4� 2013 TMS

894

dimethylzinc adduct and O2 as the zinc and oxygensources, respectively, has been studied by Blacket al., who found that only a small window (between475�C and 525�C) allowed the growth of NWs whilepreventing the growth of other nanostructures.13 Karet al.14 reported MOCVD-assisted growth of ZnONWs on Si (100) substrates using ZnO thin films asseed layers, demonstrating high-quality single-crys-talline growth along the (002) direction. Photolumi-nescence (PL) emission peaks were centered at both377 nm and �430 nm.14

Hydrothermal synthesis is a simple, low-cost, andlow-temperature process used to grow verticallyoriented NWs and NRs.15,16 Yang et al.,17 using thelow-temperature (85�C) hydrothermal method,reported the growth of lateral self-assembled single-crystal ZnO NWs around the edge of a ZnO seed layer,with a maximum x-ray diffraction (XRD) peak at34.8� (2h) and c-lattice constant of 5.2 A. Dense andlong, vertically aligned arrays of ZnO NWs have beenreportedly synthesized on Si substrates by Tianet al.18 using a similar hydrothermal process butincorporating ammonium hydroxide to enhance thegrowth rate, producing single-crystalline NW arrays.Due to the low growth temperatures (below 100�C)possible with the hydrothermal method, substratessuch as flexible materials which could not otherwisebe used with other methods can be utilized to growZnO NRs and NWs.5,19 However, a standard proce-dure for NW/NR samples grown using low-tempera-ture hydrothermal methods is to anneal them at hightemperature (e.g., 800�C) to improve their crystalquality and optical properties, which thus precludesthe use of flexible substrates.20 In this paper wereport a comparison of the morphology and crystalstructure of ZnO NWs grown using MOCVD and ZnONRs grown using hydrothermal synthesis on p-Si(100), GaN/sapphire (001), and SiO2/p-Si (001) sub-strates. We also include an analysis of the opticalproperties of the NWs grown using MOCVD.

GROWTH

ZnO NWs were grown utilizing a First NanoEasyTube 3000 MOCVD at a constant pressure of70 Torr. Diethylzinc (DEZn) was used as the Znsource, N2O as the oxygen source, and N2 as thecarrier gas. Prior to growth, the samples wereultrasonically cleaned in acetone and methanol for5 min each, rinsed with deionized (DI) water, andthen dried in air in a laboratory oven. Initially, aZnO thin-film epilayer was grown at 400�C for2 min under a constant flow of 100 standard cubiccentimeter per minute (sccm) DEZn and 50 sccmN2O carried by 1 standard liter per minute (slm) N2.The epilayer was then annealed at 625�C in N2

atmosphere. The NWs were grown at 625�C for20 min with DEZn and N2O flow rates of 20 sccmand 50 sccm, respectively.

Dumont et al.21 have suggested the occurrence ofhomolytic fission in the decomposition of DEZn,

resulting in the formation of the hydrocarbons eth-ylene and ethane. Thiandoume et al.22 reportedthe decomposition of DEZn versus temperature,observing a total dissipation at 360�C, which limitsthe minimum MOCVD growth temperature for thegrowth of ZnO NW. The lowest growth temperaturereported using MOCVD is 200�C, involving thegrowth of ZnO thin films using DEZn and O2 as thezinc and oxygen sources, respectively.23 The use ofN2O as the oxygen source in the growth process mayreduce premature oxidation of DEZn.24

The low-temperature hydrothermal growth wasperformed in two steps. Initially, 90 mg zinc acetateand 120 mg potassium hydroxide were dissolved in50 ml methanol and then constantly stirred for5 min at 60�C. Using the zinc acetate solution as aseed layer, the samples were then spin coated. Theepilayer was grown by immersing the samples inaqueous Zn(NO3)2 solution of 0.1 M and 0.1 M ofhexamethylenetetramine (HMTA) at 90�C in a lab-oratory oven for 1 h.25 Finally, NR growth wasaccomplished in a water bath at 70�C using an equi-aqueous solution of 25 mM Zn(NO3)2 and HMTA for3 h to 7 h.26

RESULTS AND DISCUSSION

Figure 1a–c shows scanning electron microscopy(SEM) images of ZnO NWs grown using MOCVD onp-Si, GaN/sapphire, and SiO2/p-Si substrates. Thediameters and approximate lengths of the NWsvaried from 90 nm to 150 nm and 1 lm to 2 lm onthe p-Si substrate, 20 nm to 40 nm and 0.7 lm to1.0 lm on GaN, and 70 nm to 90 nm and 1 lm to2 lm on SiO2/p-Si, respectively. The dimensions ofthe NWs along with those for the NRs grown usingthe hydrothermal method are summarized inTable I. The orientation of the nanowires variedfrom being perpendicular to the basal plane in thecase of GaN substrates, tilted 60� to 80� toward thec-axis for p-Si substrates, and randomly orientedfor SiO2 substrates. The small lattice mismatchbetween GaN and ZnO (1.8%) promoted the growthof ZnO NWs oriented along the c-axis.27 Also, the useof a ZnO thin film as a seed layer may have reducedthe lattice mismatch and could result in verticallyaligned NWs for other substrates such as Si withlarger lattice mismatch.28 To explore the stability ofthe NWs at different temperatures, SEM imageswere captured when increasing the temperature upto 800�C in 100�C increments. The morphology of theNWs remained unchanged after being exposed to atemperature of up to 800�C for 1 h.

Figure 1d–f shows SEM images of ZnO NRsgrown on p-Si, GaN/sapphire, and SiO2/p-Si sub-strates using the hydrothermal process. The NRshad hexagonal shapes and were mostly orientedalong the (002) plane. The NRs grown on p-Si(Fig. 1d) had diameters varying between 200 nmand 300 nm, with approximate lengths between0.7 lm and 0.9 lm. Secondary nucleations were

A Comparison of ZnO Nanowires and Nanorods Grown Using MOCVD and Hydrothermal Processes 895

observed, where the tips of NRs functioned as theseed layer. In the hydrothermal method, thedecomposition of the precursors Zn(NO3)2 + 6H2O,HMTA, and DI water forms formaldehyde, ammo-nium, and hydroxide (C6H12N4 + H2O M 6HCHO +4NH4 + 4OH). HMTA acts as a source of OH todrive the precipitation reaction. Hydroxide ionsare released as the HMTA decomposes and controlsthe acidity of the solution by negating the effectof hydrogen ions. pH effects on the growth of ZnOhave been noted by Ashford et al.29 Zinc ions origi-nating from the zinc nitrate are combined withthe hydroxide to form zinc oxide and water(Zn2 + 2OH M ZnO + H2O). The concentration ofZn ions in the solution during the hydrothermalprocess is limited and decreases with increasinggrowth time.

The secondary nucleations observed in the insetof Fig. 1d may be controlled by varying the growthtime and replacing the solution with a new solution

during the growth process. As explained by Leet al.,30 the degree of supersaturation influenceswhether the growth of NWs (or NRs in this case)continues along the vertical axis or if secondarygrowth from laterally oriented faceted ZnO ([0101])occurs. A low NH4

+/Zn2+ ratio promotes the precipi-tation of ZnO resulting in high-density nucleation.Conversely, a high NH4

+/Zn2+ ratio causes dissolu-tion of ZnO and suppresses nucleation. To ensurecontinuous growth of vertically aligned NRs, afresh supply of reagent was needed. In the case ofMOCVD NW synthesis, the precursor quantity wascontrolled during the growth to significantly reducethe probability of secondary nucleation.

Figure 1e and f show vertically aligned NRsgrown on GaN and SiO2 substrates, respectively.The NRs had similar dimensions, with diametersthat varied between 200 nm and 350 nm and lengthof approximately 1 lm. The energy-dispersive x-rayspectroscopy (EDS) results shown in Fig. 2 confirm

Fig. 1. SEM images of ZnO NWs grown using MOCVD on (a) p-Si (100), (b) GaN/sapphire, and (c) SiO2/p-Si. SEM images of ZnO NRs grownusing hydrothermal synthesis on (d) p-Si (100), (e) GaN/sapphire, and (f) SiO2/p-Si.

Table I. Summary of dimensions and crystal quality of the NWs and NRs grown using MOCVD andhydrothermal processes

Dimensions and CrystalStructure

p-Si (001) GaN/Sapphire (001) SiO2/p-Si (001)

NW-MOCVD

NR-Hydrother.

NW-MOCVD

NR-Hydrother.

NW-MOCVD

NR-Hydrother.

Diameter (nm) 90–150 200–300 20–40 200–350 70–90 200–350Length (lm) 1–2 0.7–0.9 0.7–1.0 �1 1–2 �1ZnO (002) (2h) (�) 34.4798 34.4868 34.5784 34.51 34.3808 34.5c-Lattice const. (A) 5.1982 5.1971 5.1838 5.1942 5.2127 5.19c-Lattice strain (%) 0.11 0.13 0.39 0.19 0.17 0.28FWHM (h) 0.0498 0.0591 0.0497 0.052 0.0588 0.0776FWHM (arcsec) 179 213 178 187 212 279

Rivera, Zeller, Sood, and Anwar896

that both the NWs and NRs exhibit clean surfaceswithout any trace of metal residue, which may beattributed to the growth being performed in theabsence of metal catalysts.

A Bruker D-8 Advance x-ray diffractometer with awavelength k = 1.5406 A corresponding to the CuKa line was used to investigate the crystal structureof the grown NWs. Figure 3 shows the XRD patternfor the ZnO NWs grown on p-Si, GaN, and SiO2

substrates using MOCVD. The inset of Fig. 3 showsdominant peaks related to ZnO (002). For ZnOgrown on p-Si and SiO2 substrates using MOCVD,the peak at 34� (2h) incorporated the overlappingZnO NWs (002) and ZnO thin film (002). In the caseof the GaN/sapphire substrate, an additional dif-fraction peak associated with GaN was present.These peaks were resolved by using a Lorentzian fit:using Eq. 1, a peak resulting from a combination ofmultiple peaks due to diverse crystal planes and/ormaterials may be decomposed to identify the con-stituent peaks, thereby providing the maximum andthe full-width at half-maximum (FWHM) for each ofthe peaks.

y ¼ y0 þ 2A

pw

4ðx� xcÞ2 þw2: (1)

Here, y is the x-ray intensity, y0 is the intensityoffset, A is the area, w is the width, and xc is thecenter peak angle. ZnO NWs oriented along the(002) direction had maximum intensity at 34.4798�(2h) and full-width at half-maximum (FWHM) of0.0498� (h) (179 arcsec) for p-Si, 34.5784� (2h) andFWHM of 0.0497� (h) (178 arcsec) for GaN, and34.3808� (2h) and FWHM of 0.0588� (h) (212 arcsec)for SiO2. Lee et al.31 reported, for similar MOCVDgrowth conditions, FWHMs of 11.57� (41,652 arcsec)and 0.053� (190.8 arcsec) for Si (100) and GaN/sap-phire (001) substrates, respectively.

The c-lattice constants for the NWs grown usingMOCVD were estimated to be 5.1982 A, 5.1838 A,

and 5.2127 A for p-Si, GaN, and SiO2, respectively,using Eq. 2:32

sin2h ¼ k2

4

4

3

h2 þ hkþ k2

a2

� �þ l2

c2

� �; (2)

where h corresponds to the angle of diffraction, k isthe x-ray wavelength (1.5406 A), and (h, k, l) are theMiller indices. The c-lattice constant reflected out-of-plane strains of 0.11%, 0.39%, and 0.17% for p-Si,GaN, and SiO2, respectively, compared withunstrained ZnO (5.204 A). Strain was estimatedusing: �? ¼ ðcgrown � cunstrainedÞ=cunstrained; where c isthe c-lattice constant for ZnO. Figure 4 shows thestrain associated with the c-lattice constant of theZnO NWs for various MOCVD-grown NW diameterranges. The strain of the c-lattice constant of theNW is increasing with decreasing NW diameter,which is consistent with the observations reportedby Tsao et al.33

Inte

nsit

y (a

.u.)

Energy (keV)1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

ZnO

Zn

Fig. 2. Energy-dispersive x-ray spectroscopy (EDS) of ZnO NWsgrown on p-Si (100) using MOCVD, showing zinc and oxygen as theonly two elements present in the structure.

40 45 50 55 60 65

0

2

4

6

GaN

SiO2

p-Si GaN SiO2

Inte

nsity

(a.

u.)

2θ Angle (deg)

Sapp.

p-Si

34.4 34.8 35.20

50

100

150

200

250

300

Inte

nsity

(a.

u.)2θ Angle (deg)

ZnO (002)

Fig. 3. XRD of ZnO NWs grown using MOCVD on p-Si (solid line),GaN/sapphire (squares), and SiO2 (triangles). The inset shows theZnO peak associated with ZnO oriented along (002) and GaN.

Fig. 4. Strain on the c-lattice constant of ZnO NWs versus diameterof NWs on GaN/sapphire (square), SiO2/p-Si (triangle), and p-Sisubstrates (circle). The NWs were grown using MOCVD.


The quality of the ZnO epilayer utilized as theseed layer to grow ZnO NWs using MOCVD wasalso characterized. ZnO thin films were orientedalong (002) and had maximum at 34.5784� andFWHM of 0.0697� (h) (251 arcsec) for p-Si, maxi-mum of 34.5784� and FWHM of 0.0684� (h) (246arcsec) for GaN, and maximum of 34.4308� andFWHM of 0.0557� (h) (201 arcsec) for SiO2. Thec-lattice constants for the ZnO thin films wereestimated to be 5.1838 A, 5.1624 A, and 5.1982 Awith strains of 0.39%, 0.80%, and 0.02% for p-Si,GaN, and SiO2 substrates, respectively. The ZnOthin film worked as a buffer layer, reducing thestress of the grown NWs, and furthermore limitingthe formation of threading dislocations and defectsat the heteroepitaxial interface. Additional shallowdiffraction peaks were observed for NWs grown onp-Si and SiO2, which are attributed to ZnO (100,101, 102, and 110) as illustrated in Fig. 3. For ZnONWs grown on GaN/sapphire, only peaks associatedwith the substrate and background were observed,as the NWs were uniformly vertically aligned.

Figure 5 shows the XRD patterns for ZnO NRsgrown on p-Si, GaN, and SiO2 substrates using thehydrothermal process. Compared with the samplesgrown using MOCVD, the peak intensities associ-ated with ZnO along (002) were weaker, with max-imum at 34.4868� (2h) and FWHM of 0.0591� (h)(213 arcsec) for p-Si, maximum at 34.510� (2h) andFWHM of 0.052� (h) (187 arcsec) for GaN, andmaximum at 34.50� (2h) and FWHM of 0.0776� (h)(279 arcsec) for SiO2; the XRD results for the NRsand NWs are summarized in Table I. The FWHMsof the samples grown using MOCVD were smaller incomparison with the FWHMs of the NRs grown onthe same substrates using the hydrothermal pro-cess, which could indicate higher crystal quality.The c-lattice constants of the NRs grown usinghydrothermal were estimated to be 5.1971 A,5.1942 A, and 5.1900 A, reflecting a strain ofapproximately 0.13%, 0.19%, and 0.28% for p-Si,GaN, and SiO2, respectively. Additional shallowdiffraction peaks were observed related to ZnO (100,101, 102, 103, 200, and 110) and the correspondingsubstrates. The growth method for the ZnO epilayerused as the seed layer for the NRs does not neces-sarily provide uniformity, which could contribute tothe randomness reflected in Fig. 1d. Alternatively,the ZnO seed layer could be grown on MOCVD priorto the growth of ZnO NRs using hydrothermalsynthesis. Fragala et al. reported the growth of ZnONRs by chemical bath deposition at 70�C on a ZnOseed layer grown on MOCVD with the growthtemperature varied over the 300�C to 600�C rangeand deposition time in the 10 min to 60 minrange.34 The discontinuous film structure grown at300�C resulted in slanted and geometrically notwell-defined ZnO NRs, while geometrically definedgrains grown in the 400�C to 600�C temperaturerange resulted in hexagonally shaped NRs with

homogeneous distribution, being vertically alignedfor growth temperatures above 500�C.

Figure 6 shows the PL spectra for ZnO nanowiresgrown on p-Si, GaN, and SiO2 substrates. The PLwas performed at room temperature using a xenonlight source centered at wavelength of 280 nm.Single peaks located at 380 nm having FWHM of14.6932 nm and at 378 nm having FWHM of 15 nmwere observed for p-Si and SiO2 substrates,

40 45 50 55 60 65

0

5

10

SiO2

GaN

ZnO (103)ZnO (110)In

tens

ity (

a.u.

)

2 Angle(deg)

Sapp.

ZnO (101)

ZnO (102)

p-Si

34.3 34.4 34.5 34.6 34.7 34.8

0

300

600

900

1200

1500

1800

ZnO (002)

p-Si

GaN

SiO2

GaN

θ

Fig. 5. XRD of ZnO NRs grown using hydrothermal synthesis on p-Si, GaN/sapphire, and SiO2. The inset shows the ZnO peak asso-ciated with ZnO oriented along 0002 and GaN (squares), SiO2 (tri-angles), and p-Si (solid line). Shallow diffraction peaks associatedwith ZnO oriented along (101), (102), (110), and (103) are observed.

360 380 400 420 440 460 480 500

0

2

4

6

8

10

12

14

16

18

20

22 p-Si

GaN

SiO2

*

**

378nmFWHM = 18.1nm

380nmFWHM = 14.7nm

p-Si

SiO2

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

GaN

378nmFWHM = 15.2nm

*

Fig. 6. PL of ZnO NWs grown using MOCVD on p-Si (100) (solidline) with a single peak at 380 nm, GaN (squares) with a strongerpeak at 378 nm, and SiO2 (triangles) with a single peak at 378 nm.Peaks are associated with exciton–exciton recombination.


respectively. These peaks correspond to recombi-nation of excitons through an exciton–exciton colli-sion process.35 No apparent defects related to Zn orO vacancies were observed, which can be attributedto the confinement of defects at the ZnO thin film/substrate interface. Room-temperature PL of ZnONWs has been reported with excitation in the visiblelight spectrum (425 nm £ k £ 600 nm) regardless ofthe substrate.12,36–38 For ZnO NWs grown on GaNusing MOCVD, a predominant peak was located at378 nm with FWHM of 18.18 nm. The high stress ofZnO NWs grown on GaN, observed in Fig. 3, cancontribute to the broadening of the peak in com-parison with p-Si and SiO2, for the same growthmethod. A shallow peak was identified at 480 nmthat, upon decomposition using a Lorentzian fit,resulted in observed peaks at 474 nm and 490 nm,which correlated to oxygen interstitials and oxygenvacancies, respectively.39

CONCLUSIONS

ZnO NWs and NRs were grown on p-Si (100),GaN, and SiO2 substrates using both MOCVD andhydrothermal synthesis. The NWs had largerlength-to-diameter ratios in comparison with theNRs. In addition, the MOCVD-grown NWs exhib-ited a clean surface in the absence of secondarynucleations, while some of the NRs grown on p-Siusing the hydrothermal method overlapped witheach other and showed secondary NRs nucleatedfrom the tips of the primary NRs. As observed inSEM images and confirmed by XRD, the only trulyvertically aligned growth using MOCVD wasachieved for the NWs grown on GaN, in contrast tothe NRs grown using hydrothermal synthesis whichwere mostly perpendicular to the surface regardlessof the substrate. This could suggest that the orien-tation of NWs grown by MOCVD was directlyaffected by the smaller lattice mismatch of GaN(1.8%). For NRs grown using hydrothermal syn-thesis, the orientation appeared to be related to theseed layer condition instead. The NWs demon-strated higher crystal quality than the NRs basedon the XRD data, as the FWHMs of the XRD peakcorresponding to ZnO (002) were 0.0498�, 0.0497�,and 0.0588� h, in contrast with the NR FWHMsof 0.0591�, 0.052�, and 0.0776� h for p-Si, GaN/sapphire,and SiO2/p-Si substrates, respectively. Photolumines-cence of the NWs grown on p-Si and SiO2 showeda single absorption peak associated with exciton–exciton recombination, while ZnO NWs grown onGaN/sapphire also had defects attributed to oxygeninterstitials and oxygen vacancies.

ACKNOWLEDGEMENTS

This research was funded by Magnolia OpticalTechnologies, Inc. and NAVAIR. The authors wouldlike to acknowledge the support of Dr. TariqManzur (NUWC) and Mr. Bud Holloway (NAVAIR).

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