Vacuum 83 (2009) Contents lists available at ScienceDirect. Vacuum. journal homepage:

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1 Vacuum 83 (2009) Contents lists available at ScienceDirect Vacuum journal homepage: Femtosecond laser deposited zinc oxide film and its optical properties Yifa Yang a,b, Hua Long a, Guang Yang a, *, Aiping Chen a, Qiguang Zheng a, Peixiang Lu a, ** a Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan , China b Department of Physics, Hubei Normal University, Huangshi 435, China article info abstract Article history: Received 11 March 2008 Received in revised form 25 September 2008 Accepted 7 October 2008 Keywords: Zinc oxide Pulse laser deposition X-ray diffraction PACS: Dz Fg Et Zinc oxide (ZnO) thin films have been grown on Si () substrates using a femto-second pulsed laser deposition (fspld) technique. The effects of substrate temperature and laser energy on the structural, surface morphological and optical properties of the films are discussed. The X-ray diffraction results show that the films are highly c-axis oriented when grown at 80 C and (103)-oriented at 500 C. In the laser energy range of 1.0 mj 2.0 mj, the c-axis orientation increases and the mean grain size decreases for the films deposited at 80 C. The field emission scanning electron microscopy indicates that the films have a typical hexagonal structure. The optical transmissivity results show that the transmittance increases with the increasing substrate temperature. In addition, the photoluminescence spectra excited with 325 nm light at room temperature are studied. The structural properties of ZnO films grown using nanosecond (KrF) laser are also discussed. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Zinc oxide (ZnO) has a direct and wide band gap of 3.37 ev and a high exciton binding energy of 60 mev at room temperature, which gives it great potential for use as blue and ultraviolet light emitting diodes and laser diodes [1 3]. Recently, ZnO has attracted much attention due to its potential for p-type and dilute magnetic films [4,5]. Meanwhile, in order to obtain high-quality ZnO films, many methods, such as metal-organic chemical vapour deposition (MOCVD) [6], sol-gel [7], molecular beam epitaxy (MBE) [8], rf magnetic sputtering (RFMS) [9] and pulse laser deposition (PLD) [10,11] have been used. Among these methods, the PLD technique has shown to have advantages compared with other techniques. These include the requirement for simple experimental apparatus, a capability to create high-energy source particles, and high quality film growth at low temperatures [12]. Especially recently, femtosecond (fs) laser systems have opened a new way for PLD applications. In contrast to the nanosecond (ns) laser, the interaction between the fs laser and materials is the result of a multiphoton ionization process with weak thermal diffusion effects [13]. Perriere et al. [14] and Okoshi et al. [15] have studied the effects of laser * Corresponding author. ** Corresponding author. addresses: gyang@mail.hust.edu.cn (G. Yang), lupeixiang@mail.hust. edu.cn (P. Lu). pulse width, laser wavelength on the properties of ZnO films. However, it is necessary and important to make more detailed studies on the relationship between ZnO film characteristics and the experimental parameters when using a fs laser. Sapphire has been widely used as the substrate for the growth of ZnO films and high-quality films have been obtained. Compared with sapphire, Si is not only important for the integration of optoelectronic devices, but also cheaper and easier to cleave. In this work, we report the fabrication of ZnO films on Si () using the fspld technique and investigate the effects of substrate temperature and laser pulse energy on the structural and surface morphological properties of the films. The optical transmittance in the wavelength range from 200 nm to 900 nm and the photoluminescence (PL) spectra excited with 325 nm at room temperature were studied. In comparison, the structural properties of ZnO films using nanosecond (KrF) PLD were also discussed. 2. Experimental Experiments were carried out using a Ti:sapphire laser system (SPITFIED 50FS-1 K-HP), which provided the laser beam with pulse duration of 50 fs and a repetition rate of 1 khz at the wavelength of 800 nm. The ZnO target was sintered from high-purity ZnO powder (99.999%) at 800 C for 5 h. The laser beam was focused onto the target, 50 mm from the Si substrates, which were put onto a rotating holder to improve the uniformity of the films. The target was also mounted onto a rotating holder to avoid fast drilling. Before the X/$ see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.vacuum

2 Y. Yang et al. / Vacuum 83 (2009) C 350 C 200 C C C 200 C 150 C 50 C deposition, the chamber was evacuated to a base pressure of Pa, followed by introducing oxygen gas to obtain the working pressure. The crystallinity and orientation of ZnO/Si films were determined by x Pert PRO X-ray diffraction (XRD). The surface morphologies of the unannealed and annealed films were observed using a field emission scanning electron microscope (FESEM). The optical transmittance spectra of the annealed films were measured by Model U-3310 double-beam ultraviolet visible (UV vis) spectrophotometer, and the PL properties of the films were examined by FP-6500 spectrofluorometer. 3. Results and discussion 3.1. XRD analysis Fig. 1. XRD patterns of ZnO/Si films deposited by fspld at different substrate temperature. In order to investigate the influence of substrate temperature, the laser energy was fixed to be 1.5 mj and oxygen pressure was maintained to be Pa, which is the optimal pressure for fspld deposition (different from the typical value of 0.5 Pa using nspld). The Si substrates were heated in the range of 50 C 500 C and the XRD patterns observed for the ZnO/Si films are shown in Fig. 1. The results show that all the ZnO/Si films under different temperatures are highly c-axis oriented, the () diffraction peak is dominant below 350 C and other peaks such as (), (101), (103) are also found. When the temperature is 80 C, the films are almost purely c-axis oriented. Above 80 C, the () diffraction peak starts to decrease and the (103) peak increases greatly and becomes almost the same as that of () peak for the films grown at 350 C. When the temperature increases to 500 C, the ZnO films are highly (103) oriented. This means that the optimized temperature for the growth of c-axis oriented ZnO films is as low as 80 C in the fs system, which is different from the results of ZnO films deposited by nspld (where it is higher than 600 C) and is also lower than the lowest temperature (230 C) reported by others using a femto-second laser [15]. The difference may be caused by laser parameters such as the higher laser energy density focused on Table 1 Relationship between c-axis orientation degree and substrate temperature for ZnO/ Si films deposited by fs laser Temperature ( C) c Orientation degree (%) Fig. 2. XRD patterns of annealed ZnO/Si films deposited by fspld at different substrate temperature. the target and higher repetition rate in this system, which is helpful for the low-temperature growth of ZnO films. The relationship between c-axis orientation degree (P c ) and growth temperature is shown in Table 1. Here the value of P c was calculated by the equation [16], P c ¼ P Ið00kÞ= P I ðhlkþ, and one k hlk can see that P c is maximum for the films deposited at 80 C and decreases with higher temperature. To study the effect of the heat treatment, all the films were annealed at 600 C for 10 h in air and the XRD results are shown in Fig. 2, from which one can see that, compared with Fig. 1, allthe diffraction peaks become sharper, indicating the better crystallinity after annealing. Meanwhile, the position of ZnO () diffraction peak is located at C for the as-deposited ZnO film (for example, the film deposited at 80 CinFig.1). After annealing, it shifts to higher angle side and approaches the angle of C, which is close to the peak position of ZnO bulk, indicating the tensile stress existing along the c-axis of the ZnO films is relaxed after annealing. From the XRD results, the mean grain size of the films can be calculated from Scherrer s formula, D t ¼ 0.9l/Bcosq, where D t indicates the mean grain size in the direction of c-axis, B is the full width at half-maximum (FWHM) of () peak, l is X-ray wavelength of Cu ka radiation, and q is the diffraction angle. The results are shown in Table 2 and suggest that the grain sizes obviously become larger (about 42 nm) after annealing. In addition, the effects of laser energy on the orientation of ZnO/ Si films were studied and the laser energy was changed from 1.0 mj to 2.0 mj. The temperature and pressure were maintained to be 80 C and Pa, respectively. The results are shown in Table 3. It can be found that the c-axis orientation increases and the mean grain size decreases with the increasing of laser energy, and comes to be saturated when laser energy is above 1.5 mj FESEM analysis Fig. 3(a) and (b) provides the FESEM results of the samples at 80 C before and after annealing, respectively. As shown in Fig. 3(a), Table 2 The grain size of ZnO/Si films deposited by fs laser at different substrate temperature Temperature ( C) D t (unannealed) (nm) D t (annealed) (nm)

3 894 Y. Yang et al. / Vacuum 83 (2009) Table 3 Effect of laser pulse energy on the crystal parameters of the films Laser energy (mj) c Orientation ratio (%) Mean grain size (nm) d-space (nm) ZnO films without annealing are composed of small grains with the size of about 30 nm, which is in agreement with the calculated results from XRD patterns (see Table 2). After annealing, the mean grain size of ZnO/Si films becomes obviously larger, as shown in Fig. 3(b), indicating better crystallinity. In addition, for the annealed ZnO films, the grain of hexagonal shape due to the typical ZnO structure can be observed. Moreover, the FESEM images of the annealed films grown at different laser energy were shown in Fig. 4. It is found that the grain size decreases with the increasing of laser energy, which is in agreement with the calculated results from XRD results (see Table 3) and the films become denser and more uniform, indicating the laser with higher energy (above 1.5 mj) is benefit to the improvement of the crystallinity in the films. In order to compare the ZnO films deposited using nanosecond laser, the films were also grown by KrF laser (248 nm, 20 ns, 5 Hz, 2J/cm 2 ). During the deposition, the oxygen pressure was maintained to be 0.5 Pa. It is found that the optimal temperature is 700 C, which is much higher than that using fspld. The structural properties and surface images of nspld-grown ZnO films (without annealing) are shown in Fig. 5. From Fig. 5(a), the XRD pattern shows that the nspld-grown film, compared with the XRD results by fspld in Fig. 1, has a higher degree of c-axis orientation and bigger grain size (about 50 nm nm). From Fig. 5(b), the crosssectional FESEM image indicates that the film deposited by nspld has a smooth surface and uniform thickness. From Fig. 5(c), one can see that the films have a perfect homogeneous surface and no particles or holes (>500 nm) are found, which is the disadvantage in the nspld-grown films, indicating that the ZnO films with good crystallinity can be obtained under the optimal condition in nspld system. Fig. 3. FESEM images of ZnO/Si films deposited by fspld at 80 C. (a) Unannealed, (b) annealed. Fig. 4. FESEM images at different laser pulse energy (a) 1.0 mj, (b) 1.2 mj, (c) 1.5 mj and (d) 2.0 mj.

4 Y. Yang et al. / Vacuum 83 (2009) a ZnO films deopsited by nspld 004 Transmittance ( ) C 150 C 300 C 50 C b Fig. 6. Optical transmittance of the ZnO/Si annealed films grown by fspld. quartz plate was used as the substrate. The laser energy was set to be 1.5 mj and the oxygen pressure was maintained at Pa. The UV vis optical properties in the range from 200 nm to 900 nm for the annealed films at temperature from 50 Cto300 Care shown in Fig. 6. The steep absorption at the wavelength of about c 500 nm a PL Intensity (arb. units) 382 nm 395 nm 410 nm Experimental result Gauss fit Gauss fit peak 500 nm Fig. 5. (a) XRD pattern, (b) cross-sectional FSEM image, and (c) surface FSEM image of the ZnO/Si film deposited by nspld. From the results of XRD and FESEM, the ZnO films fabricated by fspld are composed of most of ZnO nanocrystallities, 30 nm in size, which is different from the films fabricated by nanosecond laser. In the latter case, almost single-crystalline ZnO films with larger grain size can be obtained (see Fig. 5 and Ref. [14]). These differences may be related to phenomena taking place during the growth, explicitly to the differences in the kinetic energy values of the species reaching the surface of the films. For fs laser deposition, the kinetic energy emitted by the target during laser irradiation is much higher than that by nanosecond laser, which may lead to a comparatively poorer crystallinity, and the films can be actually considered as formed from ZnO nanocrystals [14]. b PL Intensity (arb. units) Experimental result 410 nm Gauss fit Gauss fit peak 433 nm 392 nm 3.3. Optical properties The optical properties of the samples deposited by fspld were measured by UV vis spectrophotometer and PL spectrofluorometer, respectively. In order to measure the optical transmittance, the Fig. 7. Room temperature PL spectra for ZnO/Si films prepared at 80 C and oxygen pressure at Pa. (a) Unannealed film, and (b) annealed film.

5 896 Y. Yang et al. / Vacuum 83 (2009) nm is observed in all the spectra and the corresponding band gap is determined to be 3.27 ev, which is close to that of bulk ZnO. The oscillatory curve of transmittance implies that the grown ZnO films have a flat surface and uniform thickness. For the annealed films deposited at 80 C, above 95% optical transmittance can be obtained. The PL spectra for ZnO/Si films (80 C, Pa, 1.5 mj) are shown in Fig. 7, and the broken curve is analyzed by gauss fitting. As shown in Fig. 7(a), it shows three major peaks: two UV near bandedge emission peak around 382 nm (3.25 ev) and 395 nm (3.15 ev), and a violet emission peak around 410 nm (3.03 ev). After annealing, the emission peak at 382 nm disappears, see Fig. 7(b), and the peak at 433 nm (2.87 ev) is observed. The calculation by Fan [17] indicates that the emission peak at 382 nm is due to the direct recombination of excitons, and the violet emission peak at about 433 nm is originated from the transition of electrons from the shallow donor level of the oxygen vacancies or Zn interstitial to the top of valence band. The above results can be explained as follows. After annealing, the size of ZnO grains becomes larger and the exciton concentration becomes lower, leading to a decrease in the UV emission intensity, which arises due to the free exciton recombination. Meanwhile, the defect concentration of oxygen vacancies or Zn interstitials will increase because of oxygen loss during the annealing, probably leading to an increase in the free exiton non-radiative recombination, which results in the decrease of the UV emission peak. In addition, the violet emission intensity at 433 nm is also related to oxygen vacancies or Zn interstitials. Fu et al. [18] reported that the band-gap located at 2.86 ev is due to the transition from shallow donor level to the top valence band, corresponding to the PL peak at 433 nm observed in our experiment. 4. Conclusion Highly c-axis oriented ZnO films were grown on Si () substrates by fs laser deposition. The influence of temperature and laser energy on the structural properties of ZnO films was discussed. The XRD results showed that the (00l)- and (103)- oriented ZnO films can be controlled by changing the substrate temperature. It is also found that the value of mean grain size decreases with the increasing laser energy from 1.0 mj to 2.0 mj. In addition, the optical transmittance and the PL spectra of the films were also studied. Acknowledgements The authors acknowledge the Analytical and Testing Center in Huazhong University of Science and Technology for XRD, SEM and spectra measurements. The finance support from the National Natural Science Foundation of China ( ) and Specialized Research Fund for the Doctoral Program of Higher Education ( ) are also acknowledged. References [1] Dong BZ, Fang GJ, Wang JF, Guan WJ, Zhao XZ. J Appl Phys 2007;101: [2] Shan FK, Liu GX, Lee WJ, Shin BC. J Appl Phys 2007;101: [3] Tang ZK, Yu P, Wang GL, Kawasaki M, Ohtomo A, Koinuma H, et al. Solid State Commun 1997;103:459. [4] Zhao JL, Li XM, Krtschil A, Krost A, Yu WD, Zhang YW, et al. Appl Phys Lett 2007;90: [5] Zhang YB, Liu Q, Sritharan T, Gan CL, Li S. Appl Phys Lett 2006;89: [6] Park JH, Jang SJ, Kim SS, Lee BT. Appl Phys Lett 2006;89: [7] Zhang Y, Lin BX, Sun XK, Fu ZX. Appl Phys Lett 2005;86: [8] Sun JW, Lu YM, Liu YC, Shen DZ, Zhang ZZ, Li BH, et al. Appl Phys Lett 2006;89: [9] Abouzaid M, Ruterana P, Liu C, Morkoc H. J Appl Phys 2006;99: [10] Duclère JR, Doggett B, Henry MO, McGlynn E, Rajendra Kumar RT, Mosnier JP, et al. J Appl Phys 2007;101: [11] Craciun V, Elders J, Gardeniers JGE, Boyd IW. Appl Phys Lett 1994;65:2963. [12] Li MY, Anderson W, Chokshi N, DeLeon RL, Tompa G. J Appl Phys 2006;: [13] Christoph M, Volker E. Phys Rev Lett 1994;73:3207. [14] Perriere J, Millona E, Seiler W, Leborgne CB, Cracium V, Albert O, et al. J Appl Phys 2;91:690. [15] Okoshi M, Higashikawa K, Hanabusa M. Jpn J Appl Phys 2001;40:1287. [16] Takahashi J, Kawano S, Shimada S. Jpn J Appl Phys 1999;38:493. [17] Fan XM, Lian JS, Guo ZX, Lu HJ. Appl Surf Sci 2005;239:176. [18] Fu ZX, Guo CX, Lin BX, Liao GH. Chin Phys Lett 1998;15:457.