Journal of the Korean Physical Society, Vol. 53, No. 1, July 2008, pp. 110�114

Green Emission from ZnO Thin Films

Young Rae Jang and Keon-Ho Yoo�

Department of Physics and Research Institute for Basic Sciences, Kyung Hee University, Seoul 130-701

Seung Min Park

Department of Chemistry, Kyung Hee University, Seoul 130-701

(Received 10 September 2007)

ZnO thin �lms with di�erent thicknesses were grown using pulsed laser deposition by varyingthe deposition time. The PL spectra of these �lms showed that the green emission decreases withthe �lm thickness. The deposited single �lm was thicker at the center than at the edge and thePL spectra from di�erent positions from the center to the edge showed the same behavior. X-raydi�raction analysis show that the thicker �lm with less green emission has better crystallinity. The�lm's composition, analyzed using energy dispersive X-ray spectroscopy, revealed that the oxygen-to-zinc ratio was larger in the sample with stronger green emission. This result does not favor theprevious suggestions that oxygen vacancies or zinc interstitials are the origins of the green emission.Based on the �lm thickness dependence, we suggest that the green emission is related with thestrain due to lattice mismatch between the �lm and the substrate.

PACS numbers: 68.55.ag, 78.55.Et, 81.15.FgKeywords: ZnO, Photoluminescence, Green emission

I. INTRODUCTION

ZnO is important for short-wavelength light-emissiondevices such as ultraviolet or blue light emitting diodesand laser diodes due to its large band gap (�3.37 eV)and exciton binding energy (�60 meV) [1{3]. Thin �lmsof ZnO have been fabricated using various techniquessuch as RF sputtering [4, 5], chemical vapor deposition[6], molecular beam epitaxy [7], atomic layer epitaxy [8]and pulsed laser deposition (PLD) [4,9{13]. Of all thesetechniques, PLD has an advantage in that it allows aconvenient control of the stoichiometry of the �lm justby varying the composition of the target or the ambientgas pressure [14].A photoluminescence (PL) spectrum of ZnO usually

consists of two main peaks, near band-edge (�380 nm)and green (�530 nm) emissions. The origin of the greenemission is still debatable [15]; some authors attribute itto oxygen vacancies [16{21] while others attribute it tozinc interstitials [22,23].In this article, ZnO thin �lms with di�erent thick-

nesses were grown by PLD by varying the depositiontime. These �lms were characterized by using PL, en-ergy dispersive X-ray spectroscopy (EDX), �eld emis-sion scanning electron microscope (FESEM) and X-raydi�raction (XRD) measurements to elucidate the originof the green emission. Our results indicate that the strain

�E-mail: khyoo@khu.ac.kr; Fax: +82-2-957-8408

due to the lattice mismatch between the �lm and the sub-strate is responsible for the green emission in ZnO thin�lms.

II. EXPERIMENT

The ZnO thin �lms in this study were grown on Si(100) substrates at a substrate temperature of 600 �Cby using PLD with a commercially available ZnO target.ZnO �lms with di�erent thicknesses were obtained byvarying the deposition time from 5 to 20 min. The �lmthickness was determined by using FESEM.The PL was measured at room temperature and at 18

K by using a continuous wave He-Cd laser (� = 325 nm)as the excitation source. The power density was about0.5 W/cm2. The �lm's composition and the structuralproperties of the �lm were investigated by using EDXand XRD, respectively.

III. RESULTS AND DISCUSSION

The FESEM cross-sectional images for the ZnO thin�lms with di�erent deposition times are shown in Fig-ure 1. The thicknesses at the centers of the �lms for thegrowth times of 5, 10 and 20 min were determined to beabout 100, 200 and 400 nm, respectively. Therefore, thegrowth rate was about 20 nm/min. Figure 2 shows the

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Green Emission from ZnO Thin Films { Young Rae Jang et al. -111-

Fig. 1. FESEM cross-sectional images of the ZnO thin�lms deposited for 5, 10 and 20 minutes.

FESEM cross section of the �lm grown for 20 min at dif-ferent positions a, b and c. The �lm thickness decreasedfrom the center (a) to the edge (c) and the edge thick-ness was about 100 nm, almost the same as the centerthickness of the �lm grown for 5 min.Figure 3 shows the PL spectra measured at room tem-

perature (RT) of ZnO thin �lms deposited for 5, 10and 20 min. The thickest �lm (20 min) exhibits themost intense near-band-edge emission (�380 nm) andthe weakest green emission, indicating good crystallinity.However, the thinnest �lm (5 min) displays the weakestnear-band-edge emission and the strongest green emis-sion, which indicates poor crystallinity. These resultsreveal that the green emission has a strong correlationwith the �lm thickness. This thickness dependence ofPL is corroborated in the PL spectra of the thickest �lm(20 min) at di�erent positions as shown in Figure 4. Thegreen emission is the weakest at the center of the �lm (a)and the strongest at the edge of the �lm (c).Figure 5 shows the XRD data for the ZnO thin �lms

deposited for 5, 10 and 20 min. The X-rays were incidentat about 2� from the sample surface so that only the sur-

Fig. 2. FESEM cross-sectional images of the ZnO thin�lm deposited for 20 minutes at di�erent positions a, b andc shown in the top image of the sample.

Fig. 3. PL spectra of the ZnO thin �lms deposited for 5,10 and 20 minutes.

face of the sample contributed to the data. Since thereis no substrate peak in the XRD spectrum of the 5-minsample, the e�ective depth of the region contributing toXRD is less than the thickness of the 5-min sample. Thesample deposited for 20 min shows clear peaks character-istic of a wurtzite structure whereas the sample depositedfor 5 min shows no characteristic structures. This impliesthat, when the �lm thickness is small, the ZnO wurtzitestructure becomes defective. This may be due to thestrain originating from the lattice mismatch between the�lm and the substrate.The crystallinity of the sample can also be checked by

using the low-temperature PL spectra. Figure 6 showsthe PL spectra at 18 K for the ZnO thin �lms depositedfor 5, 10 and 20 min and normalized so that they have

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Fig. 4. PL spectra of the ZnO thin �lm deposited for 20min at the di�erent positions shown in the inset as a, b and c.

Fig. 5. XRD curves of the ZnO thin �lms deposited for 5,10 and 20 minutes.

the same height for the band-edge PL peak. As displayedin the RT PL spectra, the green emission is almost zeroin the 20-min sample whereas it is clearly observed inthe 5-min sample.Figure 7 is a magni�ed image of Figure 6 in the near-

band-edge emission range (3.1 � 3.5 eV). In the 20-min�lm, the longitudinal optic (LO) phonon replicas of thefree exciton (FX), such as FX-1LO, FX-2LO and FX-3LO, are found [25] whereas no phonon replicas are no-ticed in the 5-min �lm. These results are consistent withthe XRD results. Namely, when the �lm thickness issmall, the wurtzite structure is imperfectly formed andno LO phonon replicas are found. On the other hand,the presence of phonon replicas in the thick samples in-dicates that there is a part with good crystallinity. Itleads one to believe that some portion of the �lm nearthe substrate acts as a bu�er and that a good wurtzitestructure is formed in the upper part of �lm above thebu�er layer.Considering the results of the thickness-dependent PL

Fig. 6. PL spectra at 18 K of the ZnO thin �lms depositedfor 5, 10 and 20 minutes. The spectra are normalized so thatthey have the same peak height for the band edge emission.

Fig. 7. Magni�ed image of Figure 6 in the near-band-edgeemission range (3.1 � 3.5 eV).

and XRD, we may conclude that the green emission orig-inates from a defective structure, which is caused by thestrain due to lattice mismatch between the �lm and thesubstrate, near the substrate. Some researchers have re-ported that the use of a bu�er layer such as GaN for re-ducing the strain can reduce the green emission in ZnOthin �lms [24,25].The chemical composition of the �lms was analyzed

using EDX, as shown in Figure 8 and summarized in Ta-ble 1. In all the �lms, the ratio of oxygen to zinc (O/Zn)is larger than 1 and this ratio becomes closer to 1 for the�lms with longer growth times with better crystallinity.Therefore, the oxygen-to-zinc ratio is larger for the �lmwith stronger green emission. This result does not favorprevious suggestions that oxygen vacancies or zinc inter-stitials may be the origin of the green emission [16{23].In our earlier experiments, we found that the peak po-

sition of the green emission varied with the surface mor-phology of the �lm. This implies that not only the strain

Green Emission from ZnO Thin Films { Young Rae Jang et al. -113-

Fig. 8. EDX analysis of the ZnO �lms with di�erentgrowth time of 5, 10 and 20 minutes.

Table 1. Oxygen-to-zinc ratio as measured by EDX.

sample 5 min 10 min 20 min 20 min 20 minand position position a position b position cO/Zn ratio 1.8 1.5 1.2 1.3 1.5

due to the lattice mismatch between the �lm and the sub-strate but also the strain caused by di�erent distributionof grains can cause the defect emissions. Earlier, vari-ous ZnO nanostructures were reported to show di�erentdefect emissions, such as green, yellow and orange [15].ZnO structures a�ected by strains with di�erent magni-tudes and directions, could have di�erent deep-level elec-tronic states, yielding di�erent defect emissions, such asgreen, yellow and orange.

IV. CONCLUSIONS

We have studied ZnO �lms with di�erent thicknessesby varying the growth time or by sampling di�erent po-sitions of a single �lm. The green PL emission has a

clear correlation with the �lm thickness and increaseswith decreasing thickness. XRD data and LO phononreplicas in the low-temperature PL spectra show thatthe crystallinity signi�cantly deteriorated with decreas-ing thickness, probably caused by the strain due to latticemismatch between the �lm and the Si substrate. In con-clusion, we propose that the green emission is correlatedwith strain-related structural defects.The EDX data also showed that the �lm with stronger

green emission had a larger oxygen-to-zinc ratio. This re-sult does not favor previous studies ascribing the originof the green emission to oxygen vacancies or zinc inter-stitials.

ACKNOWLEDGMENTS

This work was supported in part by the Brain Korea21 project.

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