ARTICLES

Highly (002)-oriented ZnO film grown by ultrasonic spraypyrolysis on ZnO-seeded Si (100) substrate

Jun-Liang ZhaoState Key Laboratory of High-Performance Ceramics and Superfine Microstructures, ShanghaiInstitute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China;and Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Xiao-Min Lia)

State Key Laboratory of High-Performance Ceramics and Superfine Microstructures, ShanghaiInstitute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

Sam Zhangb)

School of Mechanical and Aerospace Engineering, Nanyang Technological University,Singapore 639798

Chang YangState Key Laboratory of High-Performance Ceramics and Superfine Microstructures, ShanghaiInstitute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China;and Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Xiang-Dong Gao and Wei-Dong YuState Key Laboratory of High-Performance Ceramics and Superfine Microstructures, ShanghaiInstitute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

(Received 12 December 2005; accepted 23 January 2006)

ZnO films are grown by the ultrasonic spray pyrolysis method on ZnO seeding layerdeposited on Si (100) by pulsed laser deposition. The resultant film possesses acolumnar microstructure perpendicular to the substrate and exhibits smooth, dense, anduniform morphology. The preferred orientation along the c-axis of the film issignificantly enhanced compared to that without the seeding layer. ZnO film grown onZnO-seeded silicon exhibits higher hall mobility, lower resisitivity, and higherphotoluminescence intensity.

I. INTRODUCTION

ZnO film has received extensive attention because ofits notable properties1–3 such as a direct wide band gap of3.37eV and a high exciton bonding energy of 60 meVat room temperature (which is much higher than the20 meV of ZnSe or 21–25 meV of GaN). Furthermore,ZnO can grow at lower temperatures than GaN andZnSe—a preferred property in realizing integration ofZnO-based optoelectronic devices into a silicon-basedprocess. As such, ZnO is expected to be a promisingcandidate for replacing GaN in blue and ultraviolet(UV) optoelectronic applications, such as UV laser di-odes, blue-to-UV light emitting diodes, and UV detec-tors.4,5

Many techniques have been used to deposit ZnO films,including pulsed laser deposition (PLD),5–7 metalorganicvapor-phase epitaxy,8 magnetron sputtering,9 chemicalvapor deposition,10 sol-gel,11 and ultrasonic spray py-rolysis (USP).12–15 USP is a simple and inexpensivemethod for large-area deposition. The atmosphericgrowth environment of USP also improves stoichi-ometry, thus reducing intrinsic defects such as oxygenvacancies that, in turn, improve luminescence properties.This has been demonstrated in preparation of p-type ZnOfilms.13–15 However, ZnO films of high crystallinity aredifficult to obtain via the USP technique owing to itsinsufficient atomic kinetic energy for proper crystalgrowth.

PLD is an effective method for depositing high-qualityfilms with complex composition at relatively low sub-strate temperatures.7 To circumvent the USP technologyinherited low-energy problem, this article deposited ahigh-quality ZnO seed layer first with PLD followed byUSP growth. The induction mechanism of the seedinglayer was studied. The relationships between the electri-cal and luminescence properties of films and their crystalstructure were also investigated.

a)Address all correspondence to this author.e-mail: lixm@mail.sic.ac.cn

b)This author was an editor of this journal during the review anddecision stage. For the JMR policy on review and publication ofmanuscripts authored by editors, please refer to http://www.mrs.org/publications/jmr/policy.html.

DOI: 10.1557/JMR.2006.0291

J. Mater. Res., Vol. 21, No. 9, Sep 2006 © 2006 Materials Research Society 2185

II. EXPERIMENTAL

A ZnO seeding layer was deposited by PLD. For thispurpose, a sintered ZnO (99.99% purity) pellet was usedas the ablation target for a KrF excimer laser (wavelengthof 248 nm, energy density of 5 J/cm2, and repetition rateof 5 Hz; COMpex; Lambda Physik, Acton, MA). Si(100) was used as the substrate, which was etched withdiluted hydrofluoric acid (HF) (10%) for 3 min beforeloading into the deposition chamber. The substrate tem-perature was 500 °C and the oxygen partial pressurewas 1 × 10−2 Pa. The seed layer deposition was carriedout for 15 min to form a layer of about 20–30 nm thick.In USP deposition, an aqueous solution of zinc acetate[Zn(CH3COO)2·2H2O, AR, 0.5 mol/L] was selected asthe precursor, and the aerosol of the precursor solutionwas generated by a commercial ultrasonic nebulizer(with a frequency of 1.65 MHz) and was transportedto the heated substrate at temperatures rangingfrom 400 °C–500 °C. The growth rate was controlled at2–20 nm/min for total thickness of about 150 nm. Forcomparison, the ZnO film without a seeding layer wasalso directly grown on Si (100) substrate by USP.

The crystallinity and morphology of ZnO films werecharacterized by x-ray diffraction (XRD; D/MAX-2550V, CuK�), atomic force microscopy, field emissionscanning electron microscopy (FESEM, JSM-6700F),and reflective high-energy electron diffraction (RHEED).The electrical properties, including resistivity, carrierconcentration, and hall mobility, were measured by thevan der Pauw method using a Hall effect measurementsystem (HL5500PC) with magnetic field strength of0.326 T. Silver spot electrodes were made on ZnO filmsand the Ohmic contact between electrodes and films wasconfirmed before the electrical measurements. Photolu-minescence measurements were performed at room tem-perature using a 325-nm line of a He-Cd laser as anexcitation source. The illuminated area on the samplesurface was about 1 mm2 and the maximum power den-sity of the laser used was 2.5 W/cm2.

III. RESULTS AND DISCUSSION

A. Structural analysis

Figure 1 shows the XRD patterns of the ZnO seedinglayer and the ZnO films grown with and without a seed-ing layer. The film grown without the seeding layer ex-hibits a polycrystalline structure with random orientation[Fig. 1(a)]. The ZnO seeding layer by PLD is highlytextured in a c-axis orientation (002) [Fig. 1(b)]. TheZnO film grown on the seeded wafer exhibits the samesingle orientation (002) [Fig. 1(c)]. To evaluate the in-duction effect of the seed layer, we introduced a factor of(I(002)

ZS−I(002)S)/I(002)

Z, where I(002)Z, I(002)

S, and I(002)ZS

are the strength of the (002) peak for the ZnO film

without a seed layer, the ZnO seed layer, and the filmwith the seed layer, respectively. The factor calculatedfrom Fig. 1 is up to 200, which means that the strengthof (002) peak for the film by USP has been enhanced200 times because of the induction effect of the seedlayer. The degree of preferred orientation change can bequantitatively represented through a coefficient of tex-ture, T(hkl), defined as

T�hkl� =Im�hkl�

I0�hkl��

1

n �1

n Im�hkl�

I0�hkl�, (1)

where Im(hkl) is the measured relative intensity of thereflection from the (hkl) plane, I0(hkl) is that from thesame plane in a standard reference sample (JCPDS 36-1451), and n is the total number of reflection peaks fromthe film. In the present analysis, n � 4 because fourmajor directions are involved (002, 101, 102, and 103).The calculated coefficient of (002) texture T(002) for ZnOfilm without a seeding layer is 2.5, whereas the T(002) forthe film with seeding layer is 4, indicating that the (002)preferred orientation of ZnO film is significantly en-hanced by the seeding layer.

The RHEED patterns from the surface of ZnO filmswith and without a seed layer are illustrated in Fig. 2.RHEED for the film without a seed layer presents a ringpattern, indicating a polycrystalline structure with no pre-ferred orientation. The pattern for the film with a seedlayer shows a well-aligned spotty pattern, revealing ahigh (002) textured structure, which is in good agreementwith XRD analysis.

The growth conditions in the USP process, such as thesubstrate temperature and film growth rate, have an im-portant effect on the crystallinity of ZnO film on theseeded layer. Figure 3 shows XRD patterns for ZnOfilms grown on the seed layer with different substratetemperatures. Eq. (1) is used to calculate the coefficient

FIG. 1. XRD spectra of (a) ZnO films grown by USP on bare Si (100),(b) ZnO seed layer grown by PLD, and (c) ZnO films grown by USPon PLD seed layer at substrate temperatures of 500 °C.

J-L. Zhao et al.: Highly (002)-oriented ZnO film grown by ultrasonic spray pyrolysis on ZnO-seeded Si (100) substrate

J. Mater. Res., Vol. 21, No. 9, Sep 20062186

of (002) texture as function of deposition temperature,and n � 2 is assumed here because only two reflectionpeaks are involved (002 and 101). It can be seen that thedegree of (002) preferred orientation increases with thesubstrate temperature, and the film presents single (002)orientation at 500 °C. Figure 4 shows the XRD patternsfor ZnO films grown with different deposition rates. Thefilm grown at a higher rate exhibits (002) and (101)peaks, whereas only the (002) peak is observed in thefilm grown at a lower rate. Therefore, a higher sub-strate temperature and a lower deposition rate favorsthe growth of textural growth of ZnO films in USPdeposition on a seeded layer. In our experiments, the

optimum conditions for film growth are given at thesubstrate temperature of 500 °C and the deposition rateof 2 nm/min.

The microstructure of ZnO film grown at optimumconditions is investigated with FESEM. Figure 5 showsSEM images of the ZnO films grown with and withoutthe seeding layer. Without the seeding layer [Fig. 5(a)],the crystalline grains of the ZnO film are irregular ag-gregates with a characteristic dimension of approxi-mately 100 nm. With the seeding layer [Fig. 5(b)], thefilm exhibits smoother and more uniformly orientedgrains with larger size of approximately 200 nm. Fromthe cross-sectional morphologies [Figs. 5(c) and 5(d)], itcan be seen that the film without the seeding layer[Fig. 5(c)] consists of loosely packed grains with randomorientation, whereas the film with the seeding layer

FIG. 2. RHEED from the surface of ZnO films grown by USP at thesubstrate temperature of 500 °C and the growth rate of 2 nm/min: (a)without ZnO seed layer deposited by PLD and (b) with ZnO seed layerdeposited by PLD.

FIG. 3. XRD spectra of ZnO films grown by USP with PLD seedlayer at the deposition rate of 2 nm/min with different substrate tem-peratures of (a) 400 °C, (b) 450 °C, and (c) 500 °C. Inset is the cal-culated coefficient of texture as function of substrate temperature.

FIG. 4. XRD spectra of ZnO films grown by USP with PLD seedlayer at the substrate temperature of 500 °C with various depositionrates of (a) 20 nm/min and (b) 2 nm/min. Inset is the calculatedcoefficient of texture as function of film deposition rate.

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J. Mater. Res., Vol. 21, No. 9, Sep 2006 2187

[Fig. 5(d)] grows as a densely packed columnar structureperpendicular to the substrate, showing highly preferredorientation, which is in good agreement with the XRDresults.

B. Induction mechanism of the seeding layer

It is well known that the physical and chemical prop-erties of substrate have a significant influence on nuclea-tion and grain evolution, especially in the initial stages ofgrowth, which determines the further evolution of filmmorphology and texture.16 When a film is grown on bareSi (100) substrate in an ambient atmosphere, the sub-strate is always covered with a thin amorphous SiOx

layer of about 2–3 nm in thickness. When ZnO film isdeposited on a bare silicon wafer, ZnO crystals are ef-fectively forced to self-nucleate on this amorphous SiOx

layer, and, naturally, the first few atomic layers of ZnOfilm are randomly oriented because of the amorphousnature of the nucleating surface. When enough energy isoffered for further growth, thermodynamics prevail and apreferred orientation along [002] direction becomesdominant. That is also why (002)-oriented ZnO film canbe easily grown on Si (100) substrate by PLD, sputtering,

etc., in which high kinetic energy of the plasma facilitatesthe atoms to transport to the positions with the lowestfree energy, resulting in the (002) textured structure.However, in the USP process, because of the low kineticenergy of atoms and the relatively low substrate tempera-ture, there is not enough energy for such atomic transportduring film growth, and as a result, a randomly orientedpolycrystalline structure is produced.

By introducing a ZnO seeding layer, the substrate sur-face states change significantly. The seeding layer is wellcrystallized with highly preferred (002) orientation and itshows an atomic scale smooth surface (with rms of only0.3 nm as determined by atomic force microscopy analy-sis). Therefore, the surface of the ZnO-seeded Si sub-strate exhibits a hexagonal array of atoms well alignedaccording to the (002) plane of wurtzite ZnO. Whengrowing ZnO film on the seed layer by USP, the first fewatomic layers are induced to nucleate along the [002]direction. This is known as the seed layer, or substrate-induced nucleation texture, which has been discussed indetail in Ref. 17. Based on the seed layer-induced nu-cleation mechanism, the ZnO film can evolve into a(002) texture by USP even at a relatively low film growthenergy.

FIG. 5. FESEM morphologies for ZnO films grown by USP at the substrate temperature of 500 °C and the growth rate of 2 nm/min. (a) Surfacemorphology without PLD seed layer, (b) surface morphology with seed layer, (c) cross-sectional morphology without seed layer, and(d) cross-sectional morphology with seed layer.

J-L. Zhao et al.: Highly (002)-oriented ZnO film grown by ultrasonic spray pyrolysis on ZnO-seeded Si (100) substrate

J. Mater. Res., Vol. 21, No. 9, Sep 20062188

In seeding layer-induced growth of ZnO film by USPdeposition, the substrate temperature and the film growthrate are the key factors affecting development of the filmstructure. A higher substrate temperature offers higherenergy for film growth, and a lower growth rate providesenough time for atomic diffusion to their dynamicallyfavorable positions. Therefore, in a reasonable range, ahigh temperature and low growth rate favor the evolutionof preferred (002) orientation.

C. Electrical properties

The electrical properties of the ZnO films with andwithout the seeding layer are summarized in Table I.Both films exhibit n-type conductivity, which originatesfrom the intrinsic donor defects, such as zinc interstitialsand oxygen vacancies,18,19 or unintentionally doped de-fects, such as hydrogen.20,21 However, there exists a dif-ference for the electrical properties between the twotypes of films. The film with a seeding layer exhibitshigher carrier concentration, higher hall mobility, andlower resistivity than that without. With a seeding layer,the crystal structure of the film is significantly improvedand the grain size is increased, leading to a reduced con-centration of structural defects such as dislocations andgrain boundaries. These structural defects act as the car-rier recombination center, carrier transportation barrier,or carrier scattering center, and traps for free carriers.Thus, the decrease of the concentration of crystal defectsreleases trapped carriers, resulting in the increase of freecarrier concentration. The improvement of crystal qualityreduces the carrier scattering from structural defects,leading to higher hall mobility.

D. Photoluminescence

To investigate the optical properties of ZnO films,photoluminescence (PL) measurements were performedon the ZnO films as shown in Fig. 6. A strong near-band-edge UV emission peak at 380 nm and a weak deep-levelemission centered at about 500 nm can be observed forboth samples. The green emission band is attributed tooxygen-related defects,22,23 which form a deep donorlevel in the band gap. The relatively weak deep-levelemission confirms that the films obtained by USP arewell close to stoichiometric ZnO and of optically high

quality. With the seed layer, UV and green emissionbecome noticeably stronger. Structural defects, such asdislocations and grain boundaries, can trap photo-generated carriers into a nonradiative recombinationprocess before the near-band-edge and deep-level radia-tive recombination occur. This nonradiative relaxationprocess decreases the PL intensity.24 Because the ZnOfilms with the seeding layer have a much-improved crys-tallinity and a reduced concentration of nonradiative re-combination centers, UV and deep-level emission arenoticeably enhanced.

IV. CONCLUSION

Highly textured ZnO films (along the c-axis or 002orientation) were deposited via the USP method on ZnO-seeded Si (100) substrate. By introducing the seedinglayer, the nucleation mechanism changes from self-nucleation to seeding layer-induced nucleation. As a re-sult, the crystallinity is markedly enhanced and the co-efficient of texture is significantly improved in a c-axisorientation. The ZnO films grown at a high temperatureand low deposition rate exhibit a smooth, uniform, anddense columnar structure perpendicular to the substrate.Owing to the improved crystalline quality and reducedconcentration of structural defects, ZnO film with a seed-ing layer demonstrates lower resisitivity, higher carrierconcentration, and higher hall mobility than that without.PL spectrum for the ZnO film with a seeding layer alsogives stronger near-band-edge and deep-level emission.These results provide a good starting point for the growthof high-quality, p-type, doped ZnO film by USP.

ACKNOWLEDGMENTS

This work was supported by the Ministry of Scienceand Technology of China through the 973-Project under

TABLE I. Electrical properties for undoped ZnO films grown by USPat the substrate temperature of 500 °C and the growth rate of 2 nm/minwith and without ZnO seed layer deposited by PLD.

Resistivity��cm

Hallmobility

cm2�V−1�s−1

Carrierconcentration

cm−3

Hallcoefficient

m2�C−1

Without seed layer 8.4 × 10−1 3.38 −2.2 × 1018 −14.2With seed layer 8.3 × 10−2 12.1 −6.22 × 1018 −5.02

FIG. 6. Photoluminescence spectra of ZnO films grown by USP at thesubstrate temperature of 500 °C and the growth rate of 2 nm/min:(a) without ZnO seed layer deposited by PLD and (b) with ZnO seedlayer deposited by PLD.

J-L. Zhao et al.: Highly (002)-oriented ZnO film grown by ultrasonic spray pyrolysis on ZnO-seeded Si (100) substrate

J. Mater. Res., Vol. 21, No. 9, Sep 2006 2189

Grant No. 2002CB613306 and by the National NaturalScience Foundation of China under Grant No. 90401010.

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