J. Cent. South Univ. Technol. (2008) 15: 449-453

DOI: 10.1007/s11771-008-0084-x

Photoluminescence properties of ZnO thin films prepared by DC magnetron sputtering

YANG Bing-chu(杨兵初), LIU Xiao-yan(刘晓艳), GAO Fei(高 飞), MA Xue-long(马学龙)

(School of Physics Science and Technology, Central South University, Changsha 410083, China)

Abstract: ZnO thin films were prepared by direct current(DC) reactive magnetron sputtering under different oxygen partial pressures. And then the samples were annealed in vacuum at 450 ℃. The effects of the oxygen partial pressures and the treatment of annealing in vacuum on the photoluminescence and the concentration of six intrinsic defects in ZnO thin films such as oxygen vacancy(V_O), zinc vacancy(V_Zn), antisite oxygen(O_Zn), antisite zinc(Zn_O), interstitial oxygen(O_i) and interstitial zinc(Zn_i) were studied. The results show that a green photoluminescence peak at 520 nm can be observed in all the samples, whose intensity increases with increasing oxygen partial pressure; for the sample annealed in vacuum, the intensity of the green peak increases as well. The green photoluminescence peak observed in ZnO may be attributed to zinc vacancy, which probably originates from transitions between electrons in the conduction band and zinc vacancy levels, or from transitions between electrons in zinc vacancy levels and up valence band.

Key words: ZnO thin films; photoluminescence; zinc vacancy; magnetron sputtering

1 Introduction

ZnO, as a high-efficiency and low-voltage phosphor, has recently received much interest because of its potential use in new low-voltage luminescence applications. The notable property of ZnO is its high exciton bonding energy (60 meV)^[1], much higher than that of GaN (21 meV). Therefore, it is a promising candidate for replacing GaN as the next generation of photoelectric materials.

The photoluminescence of ZnO thin films has been investigated theoretically and experimentally by many researchers. The photoluminescence peaks may appear at 380^[2-6](ultraviolet), 420^[7-9](blue) and 520 nm^{[8, 10-11]} (green) indefinitely, depending on different preparation methods and growth conditions. The ultraviolet peak is usually considered to originate from near-band- emission^[12-13], and others may originate from some intrinsic defects, which are not decided. Especially the green photoluminescence has received tremendous attention. It may be related to intrinsic defects in ZnO thin films such as antisite oxygen^[10](O_Zn), oxygen vacancy^[11-14](V_O), zinc vacancy^[15](V_Zn), antisite zinc^[16] (Zn_O), or interstitial zinc^[17](Zn_i) and so on. Therefore, the establishment of the exact mechanism for the luminescence should be probed further by combining theoretical and experimental approaches.

In this work, ZnO thin films were prepared at different oxygen partial pressures by direct current(DC) reactive magnetron sputtering and then annealed in vacuum. In addition, their photoluminescence spectra were also studied.

2 Experimental

ZnO thin films were deposited onto glass substrate at 200 ℃ by DC reactive magnetron sputtering. Zinc with a purity of 99.99% was used as a target material. And the target-to-substrate distance was 8.0 cm. A mixed gas of oxygen and argon was used as the sputtering gas with a total pressure of 1.0 Pa. The flux of oxygen was increased with different oxygen partial pressures of 0.20, 0.25 and 0.33 Pa, while the flux of argon was kept the same at 15 mL/min. The depositing time was 60 min and the sputtering power was 150 W. Sample A with an oxygen partial pressure of 0.20 Pa was annealed in vacuum at 450 ℃ for 90 min. The glass substrates were first cleaned in propanone and ethyl alcohol solutions in an ultrasonic bath for 15-20 min, rinsed with deionized water and finally dried in N₂ gas stream with high purity. In order to remove the oxide of the target surface, Zn target was pre-sputtered by using argon gas before depositing ZnO thin film.

The structural properties of ZnO thin film were characterized on an X-ray diffractometer (SIEMENS D-500, Cu K_α with a wavelength of 0.154 18 nm). The photoluminescence spectra were obtained at 260 nm of Xe laser excitation using a fluorescence spectro- photometer (F-2500). The measured wavelength range varied from 400 to 600 nm.

3 Results and discussion

3.1 X-ray diffraction

Fig.1 shows the X-ray diffraction patterns of ZnO thin films at different oxygen partial pressures. It can be seen from Fig.1 that only an (002) orientation peak at  2θ≈34? is observed in all the samples, which implies that all the ZnO thin films grow along c-axis perpendicular to substrate and are of good crystal quality.

Fig.1 X-ray diffraction patterns of ZnO thin films at different oxygen partial pressures

The structural parameters of ZnO thin films prepared at different oxygen partial pressures are shown in Table 1. It is obvious that the full wave at halt maximum(FWHM) decreases first and then increases with increasing oxygen partial pressure. The grain size is calculated by Schererr equation:

                                 (1)

where  D is the diameter of ZnO thin films along c-axis; λ is the wavelength of X-ray; β is the FWHM and θ is the half diffraction angle. By calculating it is obtained that the sample prepared at an oxygen partial pressure of 0.25 Pa has the largest grain size of 17.42 nm. It can be explained that the sample prepared at an oxygen partial pressure of 0.25 Pa has the best crystal quality.

Table 1 also indicates that with increasing oxygen partial pressure, the diffraction peak shifts right gradually. According to Bragg formula:

d=λ/(2sinθ)                                  (2)

where  d is the interplanar distance of ZnO crystal. The interplanar distance decreases correspondingly, which implies that ZnO thin films become compact with increasing oxygen partial pressure.

Table 1 Structural parameters of ZnO thin films prepared at different oxygen partial pressures

3.2 Photoluminescence

Fig.2 shows the photoluminescence spectra of ZnO thin films at different oxygen partial pressures, which reveals that only a green peek sited at 520 nm is observed in each sample, whose intensity increases with increasing oxygen partial pressure.

Fig.2 Photoluminescence spectra of ZnO thin films at different oxygen partial pressures

The formation mechanism of green photo- luminescence is discussed as follows.

The transformation of the intrinsic defects of ZnO thin films in oxygen can be expressed as

O₂＋                          (3)

O₂＋                         (4)

O₂＋＋                  (5)

O₂                                (6)

O₂＋                        (7)

＋                         (8)

where  superscript “×” means neutral; “O_O” means that oxygen atom sites at ZnO crystal; “Zn_Zn” means zinc atom sites at ZnO crystal. As shown in Eqns.(3), (5)-(7), the concentrations of oxygen vacancy and interstitial zinc decrease with increasing oxygen partial pressure, while the concentration of interstitial oxygen and antisite oxygen increase.

As shown in Eqn.(8), the concentration of antisite zinc increases with increasing oxygen vacancy and interstitial zinc concentrations, but the oxygen vacancy and interstitial zinc concentrations decrease with increasing oxygen partial pressure. Hence, with increasing oxygen partial pressure, the antisite zinc concentration decreases.

Based on Eqn.(4), the concentration of zinc vacancy increases with increasing oxygen partial pressure, while according to Eqn.(7), it decreases. The concentration of zinc vacancy depends on which of Eqns.(4) and (7) is dominant. The formation energy of zinc vacancy is similar to that of oxygen vacancy^[15], which is much less than that of antisite oxygen. So Eqn.(4) is dominant. As a result, with increasing oxygen partial pressure, the concentration of zinc vacancy increases.

The dependence of the concentration of ZnO thin films’ intrinsic defects on the variation of oxygen partial pressure is listed in Table 2.

Table 2 Intrinsic defects concentration vs variation of oxygen concentration

It is noticeable that only the concentrations of zinc vacancy, interstitial oxygen and antisite oxygen increase with increasing oxygen partial pressure. As for the photoluminescence spectra, it is found that the intensity of the peak increases with increasing oxygen partial pressure. So the green photoluminescence peak may be attributed to zinc vacancy, interstitial oxygen and antisite oxygen. In order to verify which of the three gives rise to the green peak, sample A prepared at an oxygen partial pressure of 0.20 Pa was annealed in vacuum.

According to the thermal annealing model of ZnO thin films proposed by L? et al^[18], further analysis was carried out.

Receiving enough energy by thermal annealing, the atoms (especially oxygen atom) decompose on the surface^[18], the counter reactions of Eqns.(3), (6) and (7) are shown in Eqns.(9)-(11), respectively.

O₂＋                         (9)

O₂                               (10)

O₂＋                       (11)

Besides, since the sample is annealed in vacuum, the decomposition reaction occurs frequently. Accompanying with the decomposition reaction, the concentrations of interstitial oxygen and antisite oxygen decrease, while the zinc vacancy increases.

At the same time, the interstitial atoms (especially interstitial zinc), which receive certain energy, diffuse or transfer in the film and some interstitial zinc atoms composite with some vacancies^[18], as shown in Eqns.(8) and (12):

＋                       (12)

Some interstitial zinc atoms transfer to the surface and then combine with oxygen atom^[18], as shown in Eqn.(5). Therefore, the concentration of interstitial zinc decreases with the diffusion and transference of atoms.

By thermal annealing, the concentration of zinc vacancy increases with the decomposition shown in Eqn.(11), but decreases with the composition shown in Eqn.(12). In fact, the probability of the decomposition of Eqn.(11) is much larger than that of the composition of Eqn.(12). Thus the concentration of zinc vacancy increases by thermal annealing.

In a word, thermal annealing results in the decrease of the concentration of interstitial oxygen and antisite oxygen, and the increase of the concentration of zinc vacancy. The photoluminescence spectra show that the intensity of the peak related to interstitial oxygen and antisite oxygen decreases, while that related to the zinc interstitial increases.

Fig.3 shows the photoluminescence spectra of sample A before and after annealing in vacuum. It is seen that the intensity of the green peak increases after annealing. According to the above discussion, the green peak is attributed to zinc vacancy. Its origination may have two possibilities.

Fig.3 Photoluminescence spectra of sample A before and after annealing

Fig.4 shows the transmission spectrum of sample A annealed in vacuum. According to following equations:

α(hv)=A(hv-E)^1/2                            (13)

α=ln(1/T)/d                                 (14)

where  α is the absorption coefficient of ZnO thin film; hv is the photon energy; A is a constant; E is the band gap; T is the transmittance; d is the thickness of the film; the band gap is calculated to be about 3.14 eV (shown in Fig.5).

Fig.4 Transmission spectrum of ZnO thin film

Fig.5 Relationship between α²(hv)² and hv

Zinc vacancy is an accepter defect, as a result, its energy level sites above the valence band. As the energy of the wave of 520 nm is 2.39 eV and the band gap is 3.14 eV, the green peak may originate from transitions between electrons in the conduction band and zinc vacancy levels. So the zinc vacancy level may site at 0.75 eV over the valence band top (see Fig.6). This data is between 0.71 eV recited by BYLANDER^[19] and 0.80 eV calculated by KOHAN^[15] with the first principle, which implies that this speculation is reasonable.

Fig.6 Sketch map of defect levels of zinc vacancy in zinc oxide

According to the native point defect states in ZnO calculated by XU et al^[20] using a full-potential linear muffin-tin orbital method, the valence band is composed of two parts: the up valence band (originating from the  O 2p states, and changing from 0 to -4.0 eV ) and the down valence band (originating from the Zn 3d states, from -4.0 to -6.5 eV ). The zinc vacancy defect level is above the valence band, so the green peak may originate from transitions between electrons in zinc vacancy levels and up valence band (shown in Fig.7).

Fig.7 Sketch map of energy band of ZnO

4 Conclusions

1) ZnO thin films become compact with increasing oxygen partial pressure. The sample prepared at an oxygen partial pressure of 0.25 Pa is of the best crystal quality.

2) The green photoluminescence sited at 520 nm is attributed to zinc vacancy. Two possible mechanisms about the green photoluminescence are proposed. One is that the green photoluminescence may originate from transitions between electrons in the conduction band and zinc vacancy levels, the other is that the green photoluminescence probably originate from transitions between electrons in zinc vacancy levels and up valence band.

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Foundation item: Project(60571043) supported by the National Natural Science Foundation of China

Received date: 2007-11-29; Accepted date: 2008-01-08

Corresponding author: YANG Bing-chu, Professor; Tel: +86-731-8879525; E-mail: bingchuyang@126.com

(Edited by CHEN Wei-ping)