The influence of ambient conditions on properties of MgxZn1−xO films by sputtering

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Vacuum 82 (2008) 459–462 www.elsevier.com/locate/vacuum

The influence of ambient conditions on properties of MgxZn1xO films by sputtering Hui Li, Xiaojun Pan, Min Qiao, Yongzhe Zhang, Tao Wang, Erqing Xie School of Physical Science and Technology, Lanzhou University, 222 South Tianshui Road, Lanzhou 730000, Gansu, People’s Republic of China Received 27 March 2007; received in revised form 1 June 2007; accepted 20 June 2007

Abstract The MgxZn1xO films were prepared in different Ar–O2 mixture ambience by magnetron sputtering. According to the X-ray diffraction (XRD) patterns and the energy dispersive X-ray spectroscopy (EDS) results, it was found that the Mg contents in the films varied with the different ratios of O2/O2+Ar, and the crystal quality of the films improved with the increasing of Mg contents. Meanwhile, the ultraviolet and visible (UV–vis) absorption spectroscopy indicated that the band gap of the films also increased. Moreover, it could be seen that the photoluminescence (PL) spectrum was different from that of undoped Zinc oxide (ZnO) films or the results in other reports on the MgxZn1xO films: there was no blueshift effect happening for the near-band-edge (NBE) emission in MgxZn1xO films with different Mg contents. r 2007 Published by Elsevier Ltd. Keywords: MgxZn1xO films; Band gap; PL spectra

1. Introduction As a wide and direct band gap (3.37 eV) semiconductor, zinc oxide (ZnO) material has already been used for a variety of applications such as gas sensors, surface acoustic wave devices, and transparent contacts. Since ZnO has a larger exciton binding energy (60 meV) than other wide band gap materials such as GaN or ZnSe [1,2], it shows excellent excitonic effects from room temperature up to 590 K [3]. Therefore, many researchers have made some efforts primarily aiming at the use of this semiconductor for optoelectronic application in the blue and UV region, e.g., light emitting and laser diodes. In designing ZnO-based optoelectronic devices, two requirements are to be satisfied: one is the preparation of p-type ZnO and the other is modulation of the band gap. While it has been made progress in p-type doping of

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E-mail address: [email protected] (E. Xie). 0042-207X/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2007.06.003

ZnO [4–6], the latter is focused on development of MgxZn1xO or CdyZn1yO alloy. Especially, the different Mg compositions in MgxZn1xO films could regulate the band gap in a wide range (3.4–4.0 eV), which could be used as a barrier layer in device heterostructures [7]. However, there exist some problems in the growth of high-quality MgxZn1xO films, such as the different lattice symmetries of ZnO (wurtzite) and MgO (rocksalt), and the unstable thermodynamic solid solubility of MgO in ZnO. So far, some high-quality MgxZn1xO films have been prepared by some complicated methods involving pulsed laser deposition (PLD), molecular-beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) [8–10]. As for the growth of MgxZn1xO films by sputtering method, some other factors in processing, such as sputtering gas and substrate temperature, would produce some effects on their properties [11,12]. Therefore, we tend to focus on the ambient conditions of MgxZn1xO films by sputtering. In our present work, the growth of MgxZn1xO films prepared in different ambiences (the different ratios of O2/O2+Ar) by magnetron sputtering was reported. The effects of Mg in ZnO films on the structural and optical properties of the films have been discussed in detail.

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2. Experiment The MgxZn1xO films were prepared by reactive magnetron sputtering equipped with a round Zn target (the purity was 99.99%, d ¼ 50 mm), which was in the bottom and facing up the substrate. A slice of Mg (the purity was 99.9%, S ¼ 100 mm2) was placed on the target. Meanwhile, for observing the effects of Mg alloyed ZnO films on different properties, the substrates we used were pSi single-crystal wafers, and glass wafers for structural and optical measurement, which were cleaned with a standard cleaning procedure before preparation, i.e. ultrasonically cleaned in acetone, rinsed in alcohol, and then dried in hot air. Before the preparation, the target was sputtered in argon ambience at 3.0 Pa for 15 min for elimination of the contamination. Then, the base pressure of the chamber was pumped down to 2.7  103 Pa and the mixture of Ar and O2 with different ratio was introduced into the vacuum chamber. In the growth process, the working pressure was maintained at 3.0 Pa. The preparation time of the sample was 60 min. The crystal structures of synthesized samples were investigated by X-ray diffraction (XRD; Rigaku D/MaxIIIC Cu Ka ray) and the energy dispersive X-ray spectroscopy (EDS; NORAN System Six) showed the element ratios of Mg/Mg+Zn in the samples; the Ultraviolet and visible (UV–vis) absorption spectra were measured to evaluate the band gap of MgxZn1xO films and photoluminescence (PL) spectra of the films were performed in the range of 350–600 nm using 325 nm He–Cd LASER as the excitation source.

the (0 0 2) peak indexed from the crystallographic data of hexagonal ZnO. No peaks relating to other materials such as MgO appear in the graph, which reveals that the introduction of Mg into the films could not change the wurtzite structure of the films by sputtering method, and Mg tends to lodge the position of Zn because of the similar radius of Mg2+ (0.57 A˚) and Zn2+(0.60 A˚). When R ¼ 0.2, the intensity of (0 0 2) peak was weak and the full-width at half-maximum (FWHM) was broad; as the R increased, the intensity of (0 0 2) peak became stronger and the FWHM narrower, indicating that the crystal quality of MgxZn1xO films improved with the increasing oxygen in ambient conditions. It was considered that the amount of defects related to oxygen (such as oxygen vacancies VO) in the films was reduced as the oxygen partial pressure increased, resulting in the crystal quality of the films being improved [13]. From the inset in Fig. 1, it could be found that the position of (0 0 2) peak shifted toward a lower angle compared with that of the undoped ZnO films (2y ¼ 34.42 indexed from the standard XRD data): for the samples a–c, which was at 2y ¼ 34.38 or 34.36 or 34.23, respectively. Otherwise, when R ¼ 0.8, the FWHM of (0 0 2) peak broadened and shifted to a higher angle compared with that of sample c, which was located at 2y ¼ 34.30. By following the Bragg law nl ¼ 2d sin y, where d is the inter-planar spacing, y is the diffraction angle; it could be understood that the decrease of the diffraction angle y resulted in the increase of d. For the (0 0 2) orientation, the lattice constant c was calculated by

3. Results and discussion c¼ Fig. 1 showed the XRD patterns of MgxZn1xO films prepared at different ratios of O2/O2+Ar (R) by magnetron sputtering at room temperature. The crystal quality of the sample is good and one strong peak is found distinctly:

Fig. 1. The XRD patterns of MgxZn1xO films at different R (O2/ O2+Ar): (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, and the position change of (0 0 2) peak in inset figure.

l . sin y

For the samples a–d, the lattice constant c calculated were 0.5216, 0.5219, 0.5239 and 0.5229 nm, respectively. It indicated that the MgxZn1xO films prepared by magnetron sputtering had a larger lattice constant than that of undoped ZnO films (0.5210 nm), and the sample c had the largest constant lattice c. According to the results above, it was thought that not only did oxygen partial pressure have some effects on the structure and lattice constant c of the films, but also some more important factors caused by oxygen partial pressure should have an effect on them at the same time, which we would deal with later. Let us have a look at the absorption spectrum measured at room temperature by UV–vis spectrophotometer Fig. 2 depicted the results of absorption spectra, from which it could be seen that the optical transmittance of MgxZn1xO film was not good in visible range and the absorption edge was below the wavelength of 380 nm. There was a similar change trend of the absorption edge with the XRD results: as the R increased, the absorption edge of three samples shifted toward short wavelength except that of sample d,

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Fig. 2. The absorption spectra of MgxZn1xO films.

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Fig. 3. The change of Mg content and band gap of films with the different ratio of O2/O2+Ar (R) (towards the left to Mg content and towards the right to band gap).

which moved to long wavelength. The results could be used to evaluate the band gap of MgxZn1xO films as follows. The absorption coefficient a(hn) could be obtained from T ¼ A exp(at), where A is a constant, and t is the thickness of the films approximately 800 nm in our samples and by the following formula for a direct semiconductor: aðhvÞ ¼ Aðhv  E g Þ1=2 , where A and Eg are the constant and the band gap of films, respectively. The calculated results were listed in Fig. 3. It could be seen that the band gap energies were 3.50, 3.52, 3.83 and 3.58 eV for samples a, b, c and d, respectively. Compared with other reports on the band gap, it was supposed that the Mg composition had some effects on the change of band gap of MgxZn1xO films [14,15]. To further investigate the effects, we carried out the EDS measurements on all samples. The analytical method of Proza (Phi–Rho–Z) was used to evaluate the contents of different elements. It was found that the elements of film were composed of O, Zn and Mg. The calculated ratios content of Mg/Mg+Zn (x) in the films were listed in Fig. 3. It could be seen that the Mg contents in films were different with the changed ratios of O2/O2+Ar (R): in samples a, b, c and d, the Mg/Mg+Zn were 0.11, 0.12, 0.21 and 0.14, respectively. The changed shape of the band gap was similar to that of Mg composition in the films, which was consistent with our opinion above. Meanwhile, all the Mg compositions in the films were less than the solubility limit of Mg in MgxZn1xO films, while some papers reported that the solubility limit was 33% [7,16]. Only when x is larger than 33%, some new XRD peaks about other materials would appear. This was revealed in our XRD results. The fewer x could also be used to explain that the intensity of (0 0 2) peak became weaker and its position shifted to a high angle in sample d. It was thought that because the sputtering rate of Mg is bigger than that of Zn, as the ratios of O2/O2+Ar increased, the contents of Mg in

Fig. 4. The PL spectra of MgxZn1xO films measured at room temperature.

the films increased, improving the crystal quality of films and the preferred orientation shifted towards a lower angle. When R ¼ 0.6, the film had best crystal quality and the band gap was biggest with the highest ratio of Mg/Mg+Zn in MgxZn1xO film. As the R increased to 0.8, the ratio of Mg/Mg+Zn became smaller. It was considered that the increasing oxygen in ambient conditions caused some oxidation of surface in the target, which was not helpful to reactive sputtering method. Because the oxidation of Mg is easier than that of Zn and the ratio area of Mg/Zn in target (about 1/20 in our experiment) was small, the effect of oxidation on Mg surface was dominant compared with that of Zn target, which reduced Mg content in the MgxZn1xO film. So the Mg content in sample d was smaller than that of sample c. Fig. 4 showed the PL spectra of MgxZn1xO films measured at room temperature. It was shown that the spectra contained two peaks: the near-band-edge (NBE)

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emission in UV area and the broad visible emission related to deep level defects. The NBE emission of MgxZn1xO films was located at around 3.48 eV, which was larger than that of undoped ZnO films (3.3 eV) because of the introduction of Mg into the film. Furthermore, because the NBE emission had some relation with exciton emission, the energies of NBE of all samples were lower than the band gap energies (for sample a, the NBE was located at around 3.48 eV, which was smaller than the band gap 3.50 eV). Different from other papers reported on the NBE emission shifting to higher energy with increasing Mg contents in MgxZn1xO films [17], the position of NBE emission changed so slightly that it could not be found out at all. As to the results of the above-mentioned NBE emission, it was considered from two sides: on one side, as the changing Mg content in MgxZn1xO films was of a low concentration (some reported the value being lower than 10%), the NBE emission energy changed obviously between 3.2 and 3.5 eV; when the Mg content varied to a high concentration (higher than 12%), the shift of NBE emission became smaller [17,18]. On the other side, the shifting effects happened evidently at low temperature, while the NBE emission was usually too weak at room temperature to distinguish different emission mechanisms (such as free exciton or bound exciton recombination) [10,12,19]. Except for the reasons above, the NBE emission also depended on the crystal quality as the results in ZnO films by different methods. The better the crystal quality, the stronger the NBE emission. Therefore, no distinct change was found on the position of NBE emission in our experiment, and it seemed to be that the intensity of UV emission improved with the crystal quality of films. As for the visible emission at 520 nm, it was ascribed to the oxygen defects (VO) or impurity level as that of undoped ZnO films. The introduction of Mg into the films could not change the level position in the forbidden band. Therefore, the corresponding position of visible emission did not change except the intensity. The intensity of visible emission decreased as the ratio of O2/O2+Ar increased: the intensity of sample c or d was weaker than that of sample a or b because of the higher crystal quality of the films in the oxygen-rich ambience.

4. Conclusion It was studied the properties of MgxZn1xO films deposited under different ambient conditions by sputtering. It was observed that the different ambient conditions of O2/O2+Ar resulted in the different Mg contents in MgxZn1xO films, and the crystal quality of films improved with the increasing of Mg contents. The widening of band gap energy by increasing Mg composition was found in the UV–vis absorption spectra. However, all the positions of NBE emission in PL spectra were located at around 3.48 eV, and did not change with increasing of Mg composition because of the higher Mg contents and the measurement at room temperature. References [1] Bagnall DM, Chen YF, Zhu Z, Yao T, Shen MY, Goto T. Appl Phys Lett 1998;73:1038. [2] Sun Y, Ketterson JB, Wong GKL. Appl Phys Lett 2000;77:2322. [3] Ohotomo A, Kawasaki M, Sakurai Y, Ohkubo I, Shiroki R. Mater Sci Eng B 1998;56:263. [4] Yamamoto T. Phys Stat Sol 2002;193:423–33. [5] Kim K-K, Kim H-S, Hwang D-K, Lim J-H. Appl Phys Lett 2003;83:63. [6] Yang HS, Li Y, Norton DP, Ip K, Pearton SJ. J Appl Phys Lett 2005; 86:192103. [7] Makino T, Segawa Y, Kawasaki M, Ohtomo A, Shiroki R, Tamura K. Appl Phys Lett 2001;78:1237. [8] Schmidt-Grund R, Schubert M, Rheinla¨nder B, Fritsch D, Schmidt H. Thin Solid Films 2004;455–456:500–4. [9] Koike K, Hama K, Nakashima I, Takada G-Y. J Crystal Growth 2005;278:288–92. [10] Liu W, Gu S, Zhu S, Ye J. J Crystal Growth 2005;277:416–21. [11] Hwang D-K, Jeong M-C, Myoung J-M. Appl Surf Sci 2004;225: 217–22. [12] Choi C-H, Kim S-H. J Crystal Growth 2005;283:170–9. [13] Jeong S-H, Kim B-S, Lee B-T. Appl Phys Lett 2003;82:2625. [14] Park WI, Yi GC, Jang HM. Appl Phys Lett 2001;79:2022. [15] Choopun S, Vispute RD, Yang W, Sharma RP, Venkatesan T, Shen H. Appl Phys Lett 2002;80:1529. [16] Muthukumar S, Chen Y, Zhong J, Cosandey F, Lu Y, Siegrist T. J Crystal Growth 2004;261:316–23. [17] Terasako T, Shirakata S, Kariya T. Thin Solid Films 2002; 420–421:13–8. [18] Li JH, Liu YC, Shao CL, Zhang XT, Shen DZ. J Colloid Interface Sci 2005;283:513–7. [19] Gruber Th, Kirchner C, Kling R, Reuss F. Appl Phys Lett 2004; 84:5359.