Microstructure and photoluminescence properties of

Applied Surface Science 239 (2005) 176–181

Microstructure and photoluminescence properties of ZnO thin films grown by PLD on Si(1 1 1) substrates X.M. Fan, J.S. Lian*, Z.X. Guo, H.J. Lu The Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130025, China Received in revised form 24 May 2004; accepted 24 May 2004 Available online 25 June 2004

Abstract ZnO thin films on Si(1 1 1) substrate were deposited by laser ablation of Zn target in oxygen reactive atmosphere; Nd-YAG laser with wavelength of 1064 nm was used as laser source. X-ray diffraction and atom-force microscopy were applied to characterize the structure and surface morphology of the deposited ZnO films. The optical properties of the ZnO thin films were characterized by photoluminescence with an Ar ion laser as a light source. It was found that ZnO film with a majority of c-axis growth grains can be obtained under the condition of substrate temperature 450550 8C. Corresponding to the c-axis growth structure, intense UV emission with narrow FWHM was obtained from the ZnO films grown at substrate temperature 500 8C. The green deep level PL emission centering about 518 nm can be attributed to the electron transitions from the bottom of the conduction band to the antisite oxygen OZn defect levels. # 2004 Elsevier B.V. All rights reserved. Keywords: ZnO; PLD; UV photoluminescence; X-ray diffraction

1. Introduction One important advantage of ZnO is that it is a II–VI semiconductor of wurtzite structure with a wide direct-band-gap of 3.2 eV [1] at room temperature. Wide and direct band gap semiconductors are of interest for blue and ultraviolet optical devices, such as light-emitting diodes and laser diodes [2–4]. The other notable advantage of ZnO is its high exciton binding energy (60 meV) at room temperature, which is much higher than that of ZnS (20 meV) and GaN (21 meV). This makes ZnO an ideal material to realize *

Corresponding author. Tel.: þ86-431-5095875; fax: þ86-431-5095876. E-mail address: [email protected] (J.S. Lian).

room temperature excitonic devices. Recently, many methods are being used to obtain ZnO thin films, including metal-organic chemical vapor deposition (MOCVD) [5], molecular beam epitaxy (MBE) [6], rf magnetron sputtering [7] and pulse laser deposition (PLD) [8]. Pulsed laser deposition (PLD) technique is more useful in obtaining high quality thin films of metal oxide materials. The plasma fabricated by the pulsed laser ablation is very energetic, and its mobility can be easily controlled by changing processing parameters. For these practical reasons, PLD technique has been wildly applied for the formation of the high quality thin films. However, in general the excimer lasers with shorter wavelength and high purity ZnO target (99.99% purity) have been widely used to grow ZnO thin films

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.144

X.M. Fan et al. / Applied Surface Science 239 (2005) 176–181

ZnO(101)

Fig. 1 shows the XRD of the ZnO thin films on Si(1 1 1) substrate obtained by laser ablation of Zn target in oxygen reactive atmosphere at different substrate temperature (300550 8C) with a fixed oxygen pressure of 11 Pa. Three main diffraction peaks of ZnO thin films appear at 2y ¼ 31:818, 34.468 and 36.278, which are corresponding to the (1 0 0) plane, the (0 0 2) plane and the (1 0 1) plane of ZnO thin films, respectively. It is seen that the films obtained with the substrate temperature at 300350 8C have random polycrystalline structure without preferred orientation. However, when the substrate temperature increased to 500 8C, only two diffraction peaks of (0 0 2) and (1 0 1) existed and (0 0 2) peak became the peak with maximum intensity. This XRD result indicates that a structure with main (0 0 2) orientation or caxis growth formed at substrate temperature of 500 8C. In Fig. 2, two micrographs of atomic force microscopy (AFM) show the surface grain structures of the samples grown at the substrate temperature of 350 and 500 8C, respectively. The surface structure grown at 500 8C is mainly composed of homogeneous columZnO(002)

Si(1 1 1) substrate was used as the underlay. Si substrates were rinsed three times in acetone with ultrasonic vibration, each rinse for 15 min; and then rinsed in ethanol for 15 min before they were put into the deposition chamber. The experiments were performed in a deposition system. Before deposition, the vacuum chamber of the deposition system was evacuated by turbo-molecular pump down to 5 104 Pa and then filled with oxygen (99.99% purity) at a working pressures of 11 Pa. The laser energy density was measured to be about 31 J/cm2. Zn (99.99% purity) targets were ablated by a Nd-YAG laser (wavelength of 1064 nm, with pulse duration of 100 ns, frequency of 10 Hz). The target–substrate distance was kept at 2.5 cm. The deposition time of 20 min was maintained. The film thickness measured by the cross section image of SEM was about 0.8–1.3 mm, which depends on the substrate temperature. In the present experiment, the heater block temperature is considered as the substrate temperature, which was kept in the range from 300 to 550 8C. After deposition, film crystal structure was investigated by X-ray diffraction (XRD, Rigaku Dymax) with a Cu target and a monochronmator at 50 kV and 300 mA. Atomic force microscopy (Q350) was used to characterize the surface morphology of the film. The optical properties of the ZnO thin films were characterized by photoluminescence with an Ar ion laser as a light source using an excitation wavelength of 325 nm. All spectra were measured at room temperature.

3.1. The structure of ZnO film

ZnO(100)

2. Experimental

3. Results and discussion

o

550 C

o

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Intensity

[9–11]. Few reports were published on laser (pulse duration of 100 ns, wavelength of 1064 nm) and Zn target (99.99% purity) applied by our knowledge. It was reported [12] that the films (with many droplets) were obtained by lasers with wavelength of 1064 nm. However, in our experiment it is possible to obtain relatively high quality ZnO thin films through control the process parameters including laser energy density, oxygen pressure and substrate temperature. In this paper we study the structural and optical properties of ZnO thin films obtained at different substrates temperatures by laser (wavelength of 1064 nm) ablation of Zn target in oxygen active atmosphere.

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o

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o

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34 35 36 2 th eta

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Fig. 1. XRD (2y–I) patterns of ZnO thin films on Si(1 1 1) grown by PLD at different substrate temperatures.

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Fig. 2. AFM of ZnO thin films on Si(1 1 1) grown by PLD at substrate temperature: (a) 350 8C; (b) 500 8C.

nar grains (with a average grain diameter of about 240 nm). Several large tower-like grains with parallel layer structure grown vertical to the substrate surface can be clearly seen (Fig. 2 (b)). A rather flatness

surface was obtained for the ZnO film with (0 0 2) orientation structure (the peak-to-tail roughness is about 100 nm, see Fig. 2 (b)). The film grown at 350 8C has smaller average grain size (160 nm) and


a relatively rough surface (the peak-to-tail roughness is about 400 nm, see Fig. 2 (a)). From the above results, it is known that (0 0 2) preferred orientation of ZnO film is formed likely at relatively high temperature of substrate. During PLD, the kinetics of atomic arrangement is mainly determined by substrate temperature and the energy of deposition atoms. It was reported [13] that the (0 0 2) orientation has the lowest surface energy among these orientations. Therefore, at a relatively high temperature, atoms on the surface have high mobility, there should be enough time for adatoms to move on surface to look for the lowest energy sites before these adatoms are covered by the next layer of atoms. Otherwise, low substrate temperature results in low adatom mobility, which limits the formation of lower energy structure. Also, at too high substrate temperature (for example, 550 8C or higher), on one hand the interdiffusion between ZnO and Si substrate can take place, and Si atoms were easier to capture oxygen atoms from ZnO to form SiO2 in the interface; on the other hand high temperature promoted the decomposition of ZnO [14]. Both reactions resulted in the formation of Zn crystals in the film. It can be found from Fig. 1 that Zn(1 0 1) peak (43.238) at substrate temperature 550 8C is higher than that at 500 8C. As a result, in the present experiment, 500 8C or so may be optimal growth temperature for the formation of (0 0 2)-orientation ZnO thin films deposited by laser (wavelength of 1064 nm) ablation of Zn target in oxygen active atmosphere.

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Fig. 3. Room temperature (298 K) PL spectra of ZnO thin films on Si(1 1 1) grown by PLD at different substrate temperatures.

that the FWHM is the narrowest at 500 8C. The FWHM at 500 8C was 140 meV, which is narrower than that (187 meV) reported by [15] obtained by PLD with eximer laser. Therefore, both the intensity and FWHM of the UV PL spectra of ZnO film depend strongly on the microcrystalline structure. And our experimental results showed that (0 0 2) orientation or c-axis ZnO film has favorable UV PL. As to the deep level emission of PL spectra, a series of broad deep level emissions for different substrate temperature were shown in Fig. 3. The main center of these peaks is 518 nm, or 2.386 eV. It was reported that [16–19] the deep level emission is probably

3.2. Photoluminescence characteristics of film

45

Fig. 3 shows the room temperature photoluminescence (PL) spectra obtained from the ZnO thin films on Si(1 1 1) substrate deposited at different substrate temperature (300550 8C) with a fixed oxygen pressure of 11 Pa. It can be seen that all samples show a typical luminescence behavior with the two emissions of a narrow UV peak centering around 382 nm and a broad deep level peak centering around 510540 nm. The intensity of narrow UV emission increased markedly with the increase of the substrate temperature in the range of 300500 8C and then decreased sharply at the substrate temperature of 550 8C. Fig. 4 shows the measured FWHM of the UV emission from the PL spectra varying with substrate temperature. It is found

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FWHM

35 30 25 20 15

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350

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450

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Fig. 4. The FWHM of UV emission of room temperature (298 K) PL spectra of ZnO thin films on Si(1 1 1) grown by PLD varying with the substrate temperature.

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Fig. 5. The draft of calculated defect’s levels in ZnO film [20].

relative to the variation of the intrinsic defects in ZnO films. There are five kinds intrinsic defects in ZnO film, such as zinc vacancy VZn, oxygen vacancy VO, interstitial zinc Zni, interstitial oxygen Oi, and antisite oxygen OZn. Sun had calculated [20] the energy levels of the intrinsic defects in ZnO by applying the fullpotential linear muffin-tin orbital method. The calculation results were shown in Fig. 5. It is seen that the energy interval from the bottom of the conduction band to the OZn level, 2.38 eV, is in coincidence with the energy of the deep level emission observed in our experiment. That is, the deep level emission originated mainly from OZn defects. The concentration of the antisite defect OZn in ZnO films deposited at a fixed oxygen pressure depended on the substrate temperature. For one hand, metal Zn cannot be oxidized completely at low substrate temperature, which results in the increase of interstitial zinc Zni; for another hand, too high substrate temperature will cause the interdiffusion between ZnO and Si substrate and the formation of SiO2, which results in the increase of oxygen vacancy VO in film. It is known that more interstitial zinc Zni and more oxygen vacancy VO will lower the concentration of the antisite defect OZn in ZnO films. Therefore, high and low substrate temperature can all depress the concentration of the antisite defect OZn in ZnO films, which results in low intensity of broad deep level emission as can been seen from Fig. 3. There is also a minor peak centering at 533 nm or 2.318 eVat 450 and 500 8C, which, according to Fig. 5, should be attributed to the electronics transition from the bottom of the conduction band to the interstitial

oxygen Oi level (2.28 eV). The other defects could not be related to the deep level emission, because these energy intervals (1.62, 2.9 and 3.06 eV) are either too small or too large. According to the analysis of the experimental phenomena and the calculation of the defect levels in the ZnO films, it is believed that the deep level emission of ZnO films is relative to the electronics transition from the bottom of conduction band to the antisite OZn and the interstitial oxygen Oi levels. For the green emission, different explanations were proposed in recent years. Vanheusden et al. [21] believed that the ionized vacancies were responsible for the green emission. Zhang et al. [22] deduced, from their experimental result of the green emission with 490 nm wavelength, that the green emission originate from the electron transition from the level of the ionized oxygen vacancies to the valence band. Lin et al. [16] observed the green emission centering at 2.38 eV from the ZnO film deposited on silicon substrate and suggested that green emission should correspond to the electron transition from the bottom of the conduction band to the antisite defect OZn level. Therefore, in our films the green emission centering at 2.386 and 2.318 eV is similar to Lin et al.’s results.

4. Conclusion ZnO film on Si substrate was obtained by ablation of Zn target in oxygen atmosphere, with a Nd-YAG laser of 1064 nm wavelength as the pulse laser source. The influences of the substrate temperature on the structure and optical properties of ZnO film were studied.


1. ZnO film with mainly (0 0 2) texture formed at the substrate temperature 500 8C, the average grain diameter parallel to the surface of the films was about 240 nm. 2. All samples show a typical luminescence behavior. The intensity and FWHM of UV PL are strongly dependent on the microcrystalline structure and high optical quality of ZnO films was observed in the films grown at substrate temperature 500 8C. 3. The electron transitions from the bottom of the conduction band to the antisite oxygen OZn and the interstitial oxygen Oi defect levels should mainly contribute to the deep level emission of PL spectra of ZnO films.

Acknowledgements The authors would like to thank Prof. Y.C. Liu for many discussions and The Advanced Center for Optoelectronics Functional Materials Research, Northeast Normal University for PL measurement. References [1] L.I. Berger, Semiconductor Materials, CRC Press, New York, 1997.

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