Journal of Crystal Growth 252 (2003) 275–278

Effect of post-thermal annealing on properties of ZnO thin filmgrown on c-Al2O3 by metal-organic chemical vapor deposition

Xiaotian Yanga, Guotong Dua,*, Xinqiang Wanga, Jinzhong Wanga, Boyang Liua,Yuantao Zhanga, Dan Liua, Dali Liua, H.C. Ongb, Shuren Yanga

aState Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130023,

People’s Republic of ChinabDepartment of Electrical Engineering, Chinese University of Hong Kong, Hong Kong

Received 9 November 2002; accepted 16 January 2003

Communicated by R. Kern

Abstract

In this paper, high-quality ZnO film was grown by metal-organic chemical vapor deposition. Post-thermal annealing

was performed on ZnO film in vacuum and oxygen condition. We could find only the X-ray diffraction pattern of (0 0 2)

ZnO film indicating strong c-oriented growth. The quality of ZnO film was improved by thermal annealing in vacuum

as confirmed by XRD and photoluminescence measurement. Raman scattering on as-grown ZnO film indicated that the

quality of ZnO film was improved by thermal annealing in oxygen. The intensity of deep-level emission increased much

after annealing in oxygen, while it nearly did not change after annealing in vacuum. We believed that the deep-level

emission was related to zinc vacancy. The resistivity increased from 2.4 to 1100O cm after thermal annealing in oxygen.

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 81.05.Dz; 81.15.Gh; 73.61.Ga

Keywords: A3. Metalorganic chemical vapor deposition; B1. Oxides; B2. Semiconducting II–VI materials

1. Introduction

Much attention has been paid recently to ZnOfor short-wavelength photonic device applicationssince it has direct band gap energy of 3.37 eV atroom temperature [1]. Due to its large excitonbinding energy of 60meV [2], ZnO has strongexcitonic emission in the ultraviolet (UV) rangeeven at room temperature. The exciton bindingenergy is twice as large as that of GaN (24meV),

the most popular material for blue LED and LDs.In addition, ZnO can be grown at 500�C,hundreds of degrees lower than GaN. Further-more, it has potential uses in optoelectronicdevices such as photodetectors [3], surface acousticwave devices [4], electroluminescence devices andsolar cells [5].

ZnO thin films can be grown by variousdeposition techniques such as MBE [6], pulsedlaser deposition (PLD) [7], magnetron sputtering[8], metalorganic chemical vapor deposition(MOCVD) [9] and so on. Among them, MOCVDprovides the advantage of growing high-quality

*Corresponding author. Tel.: +86-431-8499371.

E-mail address: laserlab@mail.jlu.edu.cn (G. Du).

0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0022-0248(03)00898-4

films due to its versatility in controlling the variousthermodynamic interactions. It also offers highgrowth rate and growth efficiency, large areauniformity and different doping processes. Ther-mal annealing is a useful method to increase thequality of epitaxy film. In this paper, ZnO film wasgrown on c-Al2O3 by MOCVD. Thermal anneal-ing was performed in vacuum and oxygen condi-tion. Its effect on properties of ZnO film wasstudied by X-ray diffraction spectrum, Ramanspectrum and photoluminescence spectrum.

2. Experimental procedure

ZnO films are deposited on c-Al2O3 by MOCVDwith rotating disk vertical reactor. The substrate isplaced onto a susceptor, which can be rotated at aspeed with a maximum of 1000 rpm. The bestvacuum of our chamber is about 1� 10�3 Pa. Aresistive heater is placed below the wafer carrierand the heating is accomplished by radiation. Acontinuous flow of carrier gas (high-purity N2) isintroduced uniformly into chamber from the top.The precursors are diethyl zinc (DEZn) and O2.Ultrahigh-purity Ar passes through the DEZnbubbler which is maintained at a temperaturewithin the �8�C to 10�C range. Since DEZn reactsvigorously with O2 even at room temperature,DEZn and O2 are introduced into chamber by twoinjectors, respectively, to reduce the pre-reaction.Each injector comprises two tubes as shown inFig. 1. The one inside is arranged with small holesso that DEZn can flow out uniformly. The oneoutside is placed horizontally, with a straitrectangle facing the substrate. In this method,uniform and high-quality ZnO film can beobtained. The growth conditions of our sampleare as follows: The chamber pressure is about

1 Torr. The N2 flow is 600 sccm, and the O2 flow is90 sccm. Ar passes through the bubbler at 2 sccm.An additional Ar flow of 6 sccm is introduced intothe DEZn line immediately at the exit of thebubbler to reduce the transit time of DEZn to thereactor. The temperature of DEZn bubbler is 0�C.The growth temperature is 600�C. The thicknessof ZnO film is 1 mm. Thermal annealing isperformed at 700�C in vacuum for 1 h and at800�C in oxygen for 1 h, respectively.

We used SIEMENS D08 discover X-ray dif-fractometer to investigate the crystal quality. Atroom temperature in back-scattering configurationby using 514.5 nm Ar+ laser line excitation, we useRENISHAW Ramascope micro-Raman system totake Raman measurement. The scattered light isdetected by a water-cooled charge-coupled device(CCD) detector. PL spectrum is measured by325 nm He–Cd laser. The PL signal from thesample is filtered by a monochromator and pickedup by a computer.

3. Results and discussion

X-ray diffraction measurement was performedon as-grown ZnO film and annealed in vacuumZnO film (figure not shown). We observed thesharp diffraction of (0 0 2) ZnO films. This showedthat the films were of strongly c-oriented growth.The diffraction peak of (0 0 2) annealed ZnO filmwas stronger and shaper than that of as-grownone. This indicated that the quality of ZnO filmwas improved after thermal annealing in vacuum.

Fig. 2 shows the PL spectra of as-grown sampleand annealed sample in vacuum. Strong UVemission can be observed at 3.28 eV in as-grownZnO film. We could not find the deep-levelemission in the PL spectrum. The full-width athalf-maximum (FWHM) is 110meV, which iscomparable to that of ZnO film grown by MBE[10]. From the figure, we can find that UVemission becomes stronger after annealing. Andthe center wavelength of UV emission blue-shiftedto 3.29 eV. Considering the exciton binding energyof 60meV of ZnO, the UV emission of annealedsample coincides better with that of ZnO bulk than

Fig. 1. Schematic representation of separate injectors including

outer and inner tubes.

X. Yang et al. / Journal of Crystal Growth 252 (2003) 275–278276

that of as-grown sample. These indicate that thequality of ZnO film is improved after annealing.

Figs. 3 and 4 are the Raman spectra of as-grownZnO film and annealed ZnO film in oxygen. ZnOhas a hexagonal wurtzite structure and belongs tothe C6n symmetry group. The group theorypredicts two A1, two E1, two E2, and two B1

modes. Among them, one A1 and one E1 and twoE2 modes are Raman active. In crossed-polariza-tion back-scattering configuration, E2 is Ramanactive. As shown in Fig. 2, the scattering peak at437.6 cm�1 is ascribed to E2 vibration mode. Theother scattering peak is due to the sapphiresubstrate. We could clearly find that the intensityof E2 scattering peak becomes stronger after

annealing in oxygen. In addition, the observationof E2 vibration mode shows that ZnO film is inhigh quality according to group theory.

PL spectra of as-grown ZnO film and annealedZnO film in oxygen are performed in roomtemperature as shown in Fig. 5. From the spec-trum of as-grown ZnO film, we could hardly findthe deep-level emission. Strong UV emission canbe observed. However, the deep-level emission at2.43 eV dominated the PL spectrum with wideFWHM after post-thermal annealing in oxygen.And the intensity UV emission decreases afterannealing. In general, this deep green emission isbelieved to come from oxygen vacancies, inter-stitial zinc or zinc vacancies [11–13]. In oxygen

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.60

50

100

150

200

250

300

350

400

450

Non-annealed

Annealed

Inte

nsity

(a.

u.)

Wavelength (eV)

Fig. 2. Room-temperature PL spectra of as-grown ZnO film

and annealed ZnO film in vacuum.

400 500 600 700 800

1000

2000

3000

E2

Inte

nsity

(a.

u.)

Wavenumber (cm-1)

Fig. 3. Raman spectra of as-grown ZnO film.

400 500 600 700 800

1000

2000

3000

Inte

nsity

(a.

u.)

Wavenumber (cm-1)

E2

Fig. 4. Raman spectra of annealed ZnO film in oxygen.

3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.00

500

1000

1500

2000

2500

3000

3500

4000

Non-annealed

Annealed

Inte

nsity

(a.

u.)

Wavenumber (eV)

Fig. 5. Room-temperature PL spectra of as-grown ZnO film

and annealed ZnO film in oxygen.

X. Yang et al. / Journal of Crystal Growth 252 (2003) 275–278 277

condition, the pressure of oxygen is relatively high,thus the following reactions will occur:

12O2 þ V�

O ¼ O�O; ½V�

O�pP�1=2O2

;

12O2 ¼ O�

O þ V�Zn; ½V�

Zn�pP1=2O2

;

Zni þ 12O2 ¼ ZnZn þOO; ½Zni�pP

�1=2O2

:

Here, ½V�Zn� and ½V�

O� are the concentration of thenon-ionized vacancy of oxygen and zinc, respec-tively.

According to the above reactions, the oxygenvacancies and interstitial zinc decrease while thezinc vacancies increase with the increase of thepressure of oxygen. This indicates that the deep-level emission is related to zinc vacancies. Theconcentration of zinc vacancies increases so muchthat the optical-produced carriers are mostlytrapped by vacancies. Therefore, the intensity ofUV emission decreases.

In addition, the resistivity of ZnO film nearlydoes not change after annealing in vacuum butincreases from 2.4 to 1100O cm after thermalannealing in oxygen.

4. Conclusion

High-quality ZnO film is grown on c-Al2O3 bymetal-organic chemical vapor deposition(MOCVD). Thermal annealing of ZnO film isperformed in vacuum and oxygen condition. UVemission becomes stronger after thermal annealingin vacuum. In Raman spectrum, the intensity of E2

vibration mode increases after thermal annealingin oxygen. The deep-level emission dominates thePL spectrum after annealing in oxygen while it

nearly does not change after annealing in vacuum.By analyzing, we believe that the deep-levelemission is related to zinc vacancy. The resistivityincreases from 2.4 to 1100O cm after thermalannealing in oxygen while it nearly does notchange after thermal annealing in vacuum.

Acknowledgements

This work was supported by NSFC-RGC (No.59910161983) and NSFC (No. 60177007,60176026) and 863 Project under Contract No.2002AA311130.

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