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Applied Surface Science 257 (2011) 3856–3860

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

ffect of substrate temperature on the morphology, structural and opticalroperties of Zn1−xCoxO thin films

.Y. Yanga,b, B.Y. Mana,∗, M. Liua, C.S. Chena, X.G. Gaoa, C.C. Wanga, B. Hua

College of Physics and Electronics, Shandong Normal University, No. 88, East Wenhua Road, Jinan 250014, PR ChinaLaiwu Vocational and Technical College, Laiwu, Shandong 271100, PR China

r t i c l e i n f o

rticle history:eceived 3 May 2010eceived in revised form 9 November 2010ccepted 9 November 2010vailable online 10 December 2010

eywords:n1−xCoxO thin films

a b s t r a c t

Zn1−xCoxO thin films with c-axis preferred orientation were deposited on sapphire (0 0 0 1) by pulsedlaser deposition (PLD) technique at different substrate temperatures in an oxygen-deficient ambient.The effect of substrate temperature on the microstructure, morphology and the optical properties ofthe Zn1−xCoxO thin films was studied by means of X-ray diffraction (XRD), atomic force microscopy(AFM), UV–visible–NIR spectrophotometer, fluorescence spectrophotometer. The results showed that thecrystallization of the films was promoted as substrate temperature rose. The structure of the samples wasnot distorted by the Co incorporating into ZnO lattice. The surface roughness of all samples decreased assubstrate temperature increased. The Co concentration in the film was higher than in the target. Emission

LD

ubstrate temperatureorphology

tructural and optical properties

peak near band edge emission of ZnO from the PL spectra of the all samples was quenched because thedopant complexes acted as non-radiative centers. While three emission bands located at 409 nm (3.03 eV),496 nm (2.5 eV) and 513 nm (2.4 eV) were, respectively, observed from the PL spectra of the four samples.The three emission bands were in relation to Zn interstitials, Zn vacancies and the complex of VO andZni (VOZni). The quantity of the Zn interstitials maintained invariable basically, while the quantity of the

as sub

VOZni slightly decreased

. Introduction

Diluted magnetic semiconductors (DMS) have attracted exten-ive attention due to their potential application such as spin valve,pin light emitting diodes, spin field emitting transistors, non-olatile memory, optical isolators, ultra-fast optical switches anduantum computation [1]. Due to its wide band gap (3.37 eV) and

arge excitation energy (60 meV), transition-metal (TM)-doped ZnOas been investigated as a promising DMS for implementing spin-lectronics device concepts [2]. In particular, ZnO doped with Coas been the subject of intense research [3]. Much attention haseen mainly focused on magnetism and its origination of DMS,eanwhile few people have studied specially structure and optical

roperties. Thus we investigated specially these properties in theaper.

Various TM-doped DMS thin films were prepared by sputter-

ng [4–7], metal-organic chemical vapor deposition [8,9], a sol–gel

ethod [10,11], pulsed-laser deposition [12,13], and molecular-eam epitaxy [14]. The pulsed laser deposition (PLD) techniqueas some other advantages such as controllable deposition oxy-

∗ Corresponding author. Tel.: +86 531 86180804; fax: +86 531 86180804.E-mail address: byman@sdnu.edu.cn (B.Y. Man).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.11.057

strate temperature increased.© 2010 Elsevier B.V. All rights reserved.

gen partial pressure, high controllability of film composition andgrowth process, low substrate temperature (Ts) and high energyof the ablated particles in the laser produced plume [15–17]. So inthis paper, PLD technique was employed in the advantages of PLDtechnique to fabricate Zn1−xCoxO thin films.

It is well known that the thickness and properties of as-preparedfilm is related to various parameters such as laser energy density,substrate temperature, gas pressure, the distance between sub-strate and target, deposition time, etc. This paper investigated theeffects of Ts on the structural, morphological and optical propertiesof Zn1−xCoxO films grown on sapphire (0 0 0 1) by PLD at differentTs from 300 ◦C to 600 ◦C in an oxygen-deficient ambient. The vari-ation of both the lattice constant and morphology as Ts changingwas analyzed. The transmittance spectra of the samples were mea-sured. Three strong emission peaks located at 409 nm, 496 nm and513 nm, respectively, were observed, and the photoluminescence(PL) mechanisms were discussed.

2. Experiments

A Zn1−xCoxO (x = 0.1) target was prepared by a general solid-state reaction method. ZnO (99.99%) powder and Co2O3 (99.9%)powder as raw materials were mixed for 6 h using a ball-mill, thenuniaxially pressed (200 MPa) into a disk. The corresponding target

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as continuously sintered at 800 ◦C, 1000 ◦C and 1200 ◦C for 2 h inir, continuously and, respectively, and finally quenched to roomemperature in air.

A KrF excimer laser with a wavelength of 248 nm at the repe-ition rate of 10 Hz was used as a light source for the ablation ofarget. The laser energy density was 3 J/cm2. The growth cham-er was evacuated by a turbo-molecular pump to a pressure below.5 × 10−4 Pa. The distance between substrate and target was main-ained at 40 mm. The deposition process was carried out for 10 mint 300 ◦C, 400 ◦C, 500 ◦C, 600 ◦C, respectively. Prior to deposition,he sapphire substrates were cleaned sequentially in acetone, de-onized water and alcohol by an ultrasonic cleaner, then, dried withitrogen gas. During growth, the substrate holder and target holderere rotated at 5 rpm in an inverse direction using a stepping motoruring deposition to obtain uniformly thick films.

The phase purity and crystal structure of the samples were con-rmed by X-ray diffraction (XRD, Y-2000) with Cu K� (1.5418)adiation. The thickness of as-prepared films was measured usingrofiler (Dektak 6 M). The composition was determined usingnductively coupled plasma atomic emission spectrum (PE Instru-

ents ICP-OES Optima 2000 DV). The surface morphology wasbserved using atomic force microscopy (AFM, IIIa) where topogra-hy images were taken in contact model. The optical transmittanceas measured using a UV–visible–NIR spectrophotometer (V570)

n the wavelength from 200 to 800 nm. PL measurement (F-4500)as carried out by using He–Cd laser at 325 nm at room tempera-

ure and taken over a wavelength range from 340 nm to 600 nm.

. Results and discussion

Fig. 1 shows the XRD patterns of Zn1−xCoxO thin films at variouss. The reflections were found exclusively to illustrate a strong peakorresponding to the (0 0 2) peak of ZnO, the weak (0 0 4) diffrac-ion peak of ZnO appeared as Ts up to 600 ◦C, no other peak wasetected, i.e. the trace of cobalt metal, CoO, Co2O3 phase was notetected. According to the diffraction peak, we deduced that allamples possessed a hexagonal wurtzite crystal structure with aore preferential c-axis orientation and that Co atoms probably

ncorporated into ZnO host and occupied the substitutional zincites in ZnO arrays, resulting in no chaos of the wurtzite structure bydding a small amount of Co atoms during the samples preparation.he full width at half maximum (FWHM) of the (0 0 2) diffractioneak of the samples prepared at 300 ◦C, 400 ◦C, 500 ◦C, 600 ◦C was.378, 0.356, 0.325, 0.287, respectively. The size of the crystallites D

n the grains can be estimated by the Debye–Scherrer formula [18]:

= 0.89�

B cos �

here � is the X-ray wavelength of 1.5418 A, � is the Braggiffraction angle, and B is FWHM. The mean grain size of the

ig. 1. X-ray diffraction patterns of Zn1−xCoxO thin films grown at 300 ◦C, 400 ◦C,00 ◦C and 600 ◦C, respectively.

ence 257 (2011) 3856–3860 3857

sample deposited at 300 ◦C, 400 ◦C, 500 ◦C and 600 ◦C was 25 nm,25 nm, 29 nm, 33 nm, respectively. The grain sizes of the samplesincreased and FWHM decreased as Ts increasing, implying that thecrystallization degree was improved with the rising Ts because therising Ts improved the mobility of the deposited atoms. The valueof the c-axis lattice was 5.460 A (Ts = 300 ◦C), 5.284 A (Ts = 400 ◦C),5.269 A (Ts = 500 ◦C) and 5.232 A (Ts = 600 ◦C), respectively, slightlylarger than the ones of bulk ZnO (c = 5.220 A). The increase of c-axislattice constant could attribute to interstitial atoms and the resid-ual stress in the films [19]. The c-axis lattice of Zn1−xCoxO filmswas larger than that of the bulk ZnO, so all the samples showedcompressive strain. The stress was generated during depositionbecause of the freezing of structural defects at low temperature,the increase of Ts promoted the atomic mobility and reduced thestructural defects [20], thus the c-axis lattice decreased close to theones of bulk ZnO with the increasing Ts. In the following UV–visibleand PL results, the divalent octahedral cobalt, with larger 0.65 A(low spin) and 0.745 A (high spin), did not introduced into thesamples, while Co2+ (0.58 A, in high-spin state) in tetrahedral coor-dination smaller than that of Zn2+ (effective ionic radius of 0.60 A)[21] incorporated into ZnO arrays. So the compressive strain of theZn1−xCoxO thin films was not ascribed to the stress derived fromthe different ionic radius between the Co ions and Zn ions. Both Zninterstitials and O interstitials could lead to increasing the valueof c-axis lattice, but only Zn interstitials existed in the samples, asdemonstrated in later PL. Therefore, the presence of Zn interstitialswas one of the causes that gave rise to the increasing c-axis lattice.

The thickness of as-prepared films was measured using Pro-filer. The thickness of as-prepared films deposited was at 300 ◦C,400 ◦C, 500 ◦C, 600 ◦C, when other parameters were kept constantas mentioned in Section 1, was 270 nm, 275 nm, 281 nm, 298 nm,respectively, which manifested the growth rate of the films waspromoted with the increasing Ts under the condition of invariableother parameters.

The composition of as-prepared films was determined usinginductively coupled plasma atomic emission spectrum. The molarratio of Zn to Co of as-grown films prepared at 300 ◦C, 400 ◦C,500 ◦C and 600 ◦C was, respectively, 89.6:10.4, 85.9:14.1, 85.6:14.4,85.4:14.6, 85.1:14.9. In all cases, the Co concentration in the filmwas higher than in the target because the number of Zn atomsof re-vaporization was more than of Co atoms. The substrate wasbombarded by the laser produced plasma, which gave rise to highsubstrate temperature, so the re-vaporization of Zn and Co atomsfrom the film surface occurred. The heat of vaporization of Zn(115.30 kJ mol−1 [22]) was bigger than of Co (373.3 kJ mol−1 [23]),so Zn atoms in afterglow plasma body were deposited easily onsubstrate than Co. At the same time the bond energy of Zn–O inwurtzite ZnO was smaller than of Co–O since the bond length ofZn–O in wurtzite ZnO was longer than of Co–O [24], thus, the num-ber of Zn atoms of re-vaporization was more than of Co atoms.Consequently, the Co concentration in the film is higher than in thetarget. The higher the substrate temperature, the more Zn atomsthan Co atoms revaporized from the film surface. As a result, the Coconcentration in the film rose with the increasing Ts.

Fig. 2 displays three-dimensional surface morphology imagesof Zn1−xCoxO thin films deposited at different Ts. The scanningarea was 2 �m × 2 �m. The root-mean-square (RMS) of all sam-ples deposited at 300 ◦C, 400 ◦C, 500 ◦C and 600 ◦C, determinedby the AFM measurements, was 1.56 nm, 1.06 nm, 0.60 nm and0.57 nm, respectively. The surfaces were dense and smooth, alsoshowed preferred c-axis orientation. Moreover, we could see that

the films deposited at various Ts showed different surface rough-ness. In the as-deposited films, island-like grains, which are due tothe 3D columnar growth [25], completely covered the whole filmsurface as shown in Fig. 2. The surface roughness of all thin filmsshowed distinct decrease when Ts changed from 300 ◦C to 600 ◦C,

3858 S.Y. Yang et al. / Applied Surface Science 257 (2011) 3856–3860

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ig. 2. Three-dimensional surface morphology images of Zn1−xCoxO thin films depo

ndicating that the increasing atom mobility at higher Ts reducedhe surface roughness, because the larger mobility ions diffusedast and easily arrived at equilibrium position. At the same time,e observed that the tips on top of crystallites increased at quan-

ity and length as Ts increased because the c-axis orientation wasreferred when the growth temperature was higher.

UV–visible spectroscopy was employed to study the opticalroperties of electronic structure. Fig. 3 shows the transmittancepectra of Zn1−xCoxO thin films deposited at various temperatures.he different absorption peaks were observed at 565 nm, 610 nm,

nd 655 nm, which were due to the d–d electron transitions of high-pin Co2+ ions in a tetragonal crystal field. The 3d levels in Co2+ ionsndergo splitting due to tetrahedral crystal field formed by oxygen

ons presenting ZnO, and these absorption peaks at 565 nm, 610 nm,

ig. 3. Optical transmission spectra for Zn1−xCoxO thin films deposited at 300 ◦C,00 ◦C, 500 ◦C, and 600 ◦C, respectively.

at various substrate temperatures: (a) 300 ◦C; (b) 400 ◦C; (c) 500 ◦C; (d) 600 ◦C.

and 655 nm correspond to 4A2(F) → 2A1(G), 4A2(F) → 4T1(P), and4A2(F) → 2E(G), respectively, in high spin Co2+(d7) [26]. So this fur-ther confirmed that the substitution of Co2+ for Zn2+ ions in ZnOlattice, in good agreement with the result of XRD above.

Room-temperature photoluminescence (PL) spectra of all theZnO samples are shown in Fig. 4. Three emission bands in thespectra, located at about 409 nm (3.03 eV), 496 nm (2.5 eV) and513 nm (2.4 eV) were observed, but the emission peak at about380 nm attributed to ultraviolet (UV) near band edge emission ofZnO was not. The emission band located at about 409 nm (3.03 eV)

was observed in ZnO thin film or Cr-doped ZnO nanoparticles inthe literature [27–30]. The emission band located at about 496 nm(2.5 eV) was discerned in ZnO thin film or Al-doped ZnO thin film[28,31,32]. The emission band located at about 513 nm (2.4 eV) was

Fig. 4. Room temperature photoluminescence spectra of Zn1−xCoxO thin filmsdeposited at 300 ◦C, 400 ◦C, 500 ◦C, and 600 ◦C, respectively.

S.Y. Yang et al. / Applied Surface Sci

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ig. 5. The positions of the intrinsic point defect state levels in the energy band ofnO obtained by using FP-LMTO calculation.

etected in ZnO thin film, Al-doped ZnO nanoparticles or Ge-dopednO compound [27,33,34]. Hence it was concluded that emissionands located at 409 nm (3.03 eV), 496 nm (2.5 eV) and 513 nm2.4 eV) were not ascribed to the presence of cobalt but the defectsuch as Zn vacancy (VZn), O vacancy (VO), Zn interstitial (Zni), Onterstitial (Oi), etc. Xu et al. [35] had calculated the energy lev-ls of defects in ZnO thin films by using FP-LMTO calculation. Andhey showed that the energy intervals between the Zn interstitialevels and the top of the valence band, between the Zn intersti-ials and the Zn vacancies and between the complex of VO and ZniVOZni) and the top of the valence band were, respectively, about.9 eV, 2.6 eV and 2.4 eV, as shown in Fig. 5 [35], where oc means anctahedrally positioned defect, te means a tetrahedrally positionedefect. Three photoluminescence centers (409 nm, 3.0 eV; 496 nm,.5 eV; 513 nm, 2.4 eV) observed in this work agreed quite well withu’s theoretical calculation. Furthermore, according to the analysisbove and the oxygen-deficient ambient of the thin films growth,e could conclude that the oxygen vacancies and Zn interstitials

ruly existed in the samples. Therefore, the emission band centeredt 409 nm (3.0 eV) was assigned to the electron transition from then interstitials to the top of the valence band, which is in goodccordance with the literature [28,36]. The emission band centeredt 496 nm (2.5 eV) was ascribed to the electron transition from then interstitials to the Zn vacancies, which was in agreement withhe literature [32]. The emission band centered at 513 nm (2.4 eV)as ascribed to the electron transition from the complex of VO and

ni (VOZni) to the top of the valence band, which agree very wellith the literature [32,37]. The intensity and position of emissioneak at about 409 nm (3.03 eV) maintained invariable, implyinghat the quantity of the Zn interstitials kept basically unchangeableith Ts in fixed atmosphere. While the intensity of the emission at

bout 513 nm (2.4 eV) decreased as the Ts decreasing, suggestinghat the quantity of the complex of VO and Zni (VOZni) decreasedlightly as the Ts increased. The emission peak at about 380 nmttributed to ultraviolet (UV) near band edge emission of ZnO wasuenched when x ≥ 0.116 in Zn1−xCoxO [38] because the dopantomplexes acted as non-radiative centers [39], which was also theeason that the near band edge photoluminescence in all the sam-

les were quenched for x > 0.104 in all the samples of Zn1−xCoxOhin films as mentioned above. It is pointed out that a homoge-ous film thickness was expected and that it will be worth to probehe electrical properties, e.g. by Hall effect measurements in van

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ence 257 (2011) 3856–3860 3859

der Pauw geometry. We will make a further study to identify thedefects by probing electrical properties.

4. Conclusion

Zn1−xCoxO thin films with c-axis preferred orientation weredeposited on sapphire (0 0 0 1) by pulsed laser deposition (PLD)technique at different Ts from 300 ◦C to 600 ◦C in an oxygen-deficient ambient. The effect of substrate temperature on themicrostructure, morphology and the optical properties of theZn1−xCoxO thin films was studied by means of X-ray diffrac-tion (XRD), atomic force microscopy (AFM), UV–visible–NIRspectrophotometer, fluorescence spectrophotometer. The crystal-lization of the films was promoted with increasing Ts and thesurface roughness of all samples decreased as substrate temper-ature increased because the rising Ts improved the mobility of thedeposited atoms. The cobalt incorporated into ZnO lattice and didnot disturb the structure of the samples. The Co concentration inthe film was higher than in the target. Three emission bands inthe spectra, located at about 409 nm (3.03 eV), 496 nm (2.5 eV) and513 nm (2.4 eV) were observed, but the emission peak at about380 nm attributed to ultraviolet (UV) near band edge emission ofZnO was not. Three photoluminescence centers (409 nm, 3.0 eV;496 nm, 2.5 eV; 513 nm, 2.4 eV) observed in this work agreed quitewell with Xu’s theoretical calculation. The emission band centeredat 409 nm (3.0 eV) was assigned to the electron transition from theZn interstitials to the top of the valence band. The emission bandcentered at 496 nm (2.5 eV) was ascribed to the electron transitionfrom the Zn interstitials to the Zn vacancies. The emission band cen-tered at 513 nm (2.4 eV) was ascribed to the electron transition fromthe complex of VO and Zni (VOZni) to the top of the valence band.The quantity of the Zn interstitials remained basically unchangedwith Ts in fixed atmosphere. While the quantity of the complex ofVO and Zni (VOZni) decreased slightly as the Ts increased. The emis-sion peak at about 380 nm attributed to ultraviolet (UV) near bandedge emission of ZnO was quenched because the dopant complexesacted as non-radiative centers.

Acknowledgments

The authors are grateful for the financial support by the NationalNatural Science Foundation of China (10874103), the ProvincialNatural Science Foundation of Shandong (Y2007A05) and TheProject-sponsored by SRF for ROCS, SEM, PR China.

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