- Email: [email protected]
Tuning the composition and optical band gap of pulsed laser deposited ZnO1−xSx alloy films by controlling the substrate temperature
Journal of Alloys and Compounds 617 (2014) 413–417
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tuning the composition and optical band gap of pulsed laser deposited ZnO1xSx alloy films by controlling the substrate temperature Lei Zhang a, Lei Li a, Liangheng Wang a, Mingkai Li a, Yinmei Lu a,b, Bruno K. Meyer b, Yunbin He a,⇑ a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory of Green Preparation and Application for Functional Materials, Ministry of Education, Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China b I. Physikalisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany
a r t i c l e
i n f o
Article history: Received 29 April 2014 Received in revised form 2 July 2014 Accepted 4 August 2014 Available online 11 August 2014 Keywords: ZnO1xSx thin films Pulsed laser deposition Substrate temperature Solid solubility Band gap energy
a b s t r a c t High-quality ZnO1xSx thin films were grown on (0 0 1) sapphire substrates in the temperature range of 300–800 °C by pulsed laser deposition (PLD) with a ZnS ceramic target and O2 as reactive gas. By increasing the substrate temperature, the crystalline quality of the films is enhanced. The S content in the single-phase ZnO1xSx films can be systematically adjusted from 0.556 to 0.202 via changing the substrate temperature. The maximum S content in the film grown at 300 °C reaches 0.556 without phase separation, which is significantly higher than the solid solubility limits reported previously for the ZnOS alloys. The narrowed band gap of the ZnO1xSx film (2.63 eV) grown at the low substrate temperature will extend the application of ZnO-based optoelectronic devices to the blue light region. As the composition, structure, and band gap energy of the ZnOS films were found to depend critically on the growth temperature, this work suggests a simple and flexible means of tuning the composition and optical band gap of ZnOS alloy films by controlling the substrate temperature during the PLD process. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction With a wide band gap of 3.3 eV and a large exciton binding energy of 60 meV at room temperature, ZnO is important for blue and ultraviolet optoelectronic devices, transparent electronics, spintronic devices, and sensor applications [1]. For a semiconductor to be useful, particularly in reference to optoelectronic devices, band gap engineering is a crucial step toward the device development. By alloying a semiconductor with compounds from the same group having different band gaps, the band gap of the resultant alloy can be tuned, thereby affecting the wavelength of exciton emissions. In the case of ZnO, alloying with MgO, CdO and ZnS are effective means for increasing or decreasing the band gap energy respectively [2–5]. It is important to note that all these compounds are not isostructural to wurtzite ZnO, and the ionic radius of Mg2+ is relatively close to that of Zn2+, whereas Cd2+ has a much larger radius. Therefore, limited solubility and a variation in lattice parameters with deformation in the structure are expected in the ternary alloys. Alloying of MgO and CdO with ZnO has been studied extensively in comparison to the anionsubstituted compound of ZnO1xSx (ZnOS). Heterostructures made with ZnO and MgZnO or CdZnO have already been demonstrated ⇑ Corresponding author. Tel./fax: +86 27 88661803. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.jallcom.2014.08.024 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
[6,7], and are expected to play key roles in future ZnO-based optoelectronic devices. Variations in the composition and solubility limit of the ZnO alloys reported in the literature are due to the variation in processing conditions such as the substrate temperature and the oxygen partial pressure. For the later one, by controlling the oxygen partial pressure one can tune the composition of the ZnOS ternary alloy accurately, and it is an efficient way for the band gap engineering of ZnO [8,9]. For the former one, however, there are so far only limited reports concerning the substrate temperature effects. The dependence of Mg content on the growth temperature has been utilized successfully to tune the composition (and the band gap) over a wide range (0.5 6 x 6 1) for epitaxial MgxZn1xO alloy films grown by pulsed laser deposition (PLD) using a single ceramic target of specific composition [10]. The dependence of the band gap on the substrate temperature was attributed to the change of Mg/Zn ratio in the films due to different vapor pressures of Mg and Zn species at different temperatures. At higher substrate temperatures, the desorption rate of Zn from the substrate surface is much higher than that of Mg due to the higher vapor pressure of Zn relative to Mg, and results in Mg-rich films with larger band gaps [11]. Compared to the cation-substituted MgZnO and CdZnO alloy systems, only limited experimental work focused on the synthesis methods has been reported to date for the anion-substituted ZnOS alloys, and the information available is both incomplete and not
414
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
well verified. In the recent years, Yoo et al. [5] first succeeded in doping S into ZnO with the PLD technique at 700 °C by alternatively ablating a ZnO and a ZnS target. They found that the S concentration in the hexagonal ZnOS alloys was limited to 0.13. Meyer et al. [12] synthesized single-phase wurtzite ZnO1xSx alloys in the whole composition range of 0 6 x 6 1 by RF sputtering at a low substrate temperature of 340 °C. The band gap energy can be adjusted down to (2.6 ± 0.1) eV for x = (0.45 ± 0.05), and the composition dependence of the band gap energy for the ternary system was determined with an optical bowing parameter of about 3.0 eV. Pan et al. [13] deposited ZnO1xSx alloy films with wurtzite and zincblende structures occurred in the O-rich (0 6 x 6 0.23) and the S-rich (0.77 6 x 6 1) films, respectively, by RF magnetron sputtering at substrate temperature of 300 °C followed by annealing at 500 °C. However, a systematical study on effects of the substrate temperature in the growth of ZnOS alloy films is still lacking to date. The synthesis of ZnOS at thermodynamic equilibrium state is quite difficult and with rather limited solid solubility of ZnO and ZnS due to the different crystalline symmetry of both compounds and the inferior stability of sulfur [14]. However, with nonequilibrium processes, such as PLD and sputtering, the alloying of ZnO and ZnS can be achieved with a large solid solubility that is well beyond the thermodynamic limit [9,13]. In the PLD process, the laser ablated species have certain kinetic energy and can thus nucleate and grow into film at relatively low temperature, different from the equilibrium growth where only thermal energy is supplied by high temperature heating. In such a non-equilibrium process, the substrate temperature can play a crucial role in the formation of meta-stable phases. As reported previously [10], epitaxial and single-phase MgZnO films having a composition range of 0.5 6 x 6 1 can be obtained by controlling the growth temperature. In the present work, we used a ZnS ceramic target and O2 as reactive gas to grow ZnOS alloy films by the PLD technique. The substrate temperature during the deposition process was varied systematically in the range of 300–800 °C in order to study the influence of the growth temperature on properties of the resulting ZnOS alloy films. 2. Experimental details ZnO1xSx thin films were grown on single-crystal sapphire (0 0 1) substrates by the PLD technique using a KrF excimer laser (Lambda Physik COMPEX PRO 205 F, k = 248 nm) as the ablation source. A high-purity ZnS (99.99%) ceramic was employed as the ablation target, and high-purity O2 (99.999%) was introduced as reactive gas. The sapphire substrates were cleaned ultrasonically in organic solvents and de-ionized water, and then blown dry by nitrogen. The target-substrate distance was set at 5.5 cm, and the laser beam energy was fixed at 200 mJ/pulse with a pulse repetition rate of 5 Hz. Prior to film deposition, the vacuum chamber was evacuated to 3.0–5.0 104 Pa. The O2 partial pressure was kept constant at 2 Pa during the deposition, while the substrate temperature was varied in the range of 300–800 °C. All films were grown for 30 min with thicknesses of 300–400 nm. The evolution of the surface morphologies was characterized by atomic force microscopy (AFM). X-ray diffraction (XRD) was performed using a powder diffractometer (Rigaku, D/Max-IIIC) with Cu Ka radiation (k = 0.154184 nm) for the structural characterization of the films. The optical properties of the films were measured by UV–VIS scanning spectrophotometer (DU 800). The phonon properties of the films were measured by Raman spectroscopy (HORIBA, JOBIN YVON HR800 UV) with the focal length of 800 mm at room temperature in back scattering geometry. The 514 nm line of a He–Ne laser was used for excitation, and the spectra were collected in the range of 200–800 cm1 using a multichannel charge-coupled device (CCD) detector. Composition and chemical state analyses of the films were accomplished by X-ray photoelectron spectroscopy (XPS, PHOIBOS 150, SPECS) at photon energy of 1486.6 eV (Al Ka radiation).
Fig. 1. AFM images of the ZnO1xSx films deposited at substrate temperatures of (a) 300 °C, (b) 500 °C, (c) 700 °C, and (d) RMS roughness of the films vs. the deposition temperature.
grow in a three-dimensional mode, having a smooth, dense and uniform surface, and the grain size grows with increasing the substrate temperature. The surface root mean square (RMS) roughness of the films vs. the deposition temperature is shown in Fig. 1(d). The RMS roughness increases from 1.163 to 1.832 nm when the substrate temperature rises from 300 to 700 °C. The AFM study reveals that, increasing the substrate temperature during the PLD process enhances the crystallinity of the film, leading to larger grains while increasing slightly the surface roughness. This is in line with the general observations on the PLD growth of thin films, in which the nucleation process is mainly governed by the substrate temperature while the surface mobility of the condensing species defines the crystallinity of the film. Generally a high substrate temperature favors rapid and defect free growth of crystallites due to optimum surface diffusion of the species, whereas a low substrate temperature results in the growth of a disordered or a poorly crystallized structure with a relatively small RMS roughness [15]. Fig. 2 shows XRD spectra measured in the h–2h scanning mode of the ZnO1xSx films deposited at different substrate temperatures. It is well known that wurtzite ZnO (0 0 2) plane should induce a diffraction peak around 2h = 34.42° as denoted in the figure by a dotted red line. Considering that the ionic radius of S
3. Results and discussion The surface morphology of the ZnO1xSx films deposited at different substrate temperatures was characterized by AFM, and the images are shown in Fig. 1(a)–(c). It can be seen that, all the films
Fig. 2. XRD spectra of ZnO1xSx films deposited at different substrate temperatures.
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
is larger than that of O, the diffraction angle of the (0 0 2) peak would decrease when S substitutes for part of O in the O sublattice of ZnO. Therefore, the peaks between 30° and 34.42° in the XRD spectra are identified as the (0 0 2) diffraction peaks of wurtzite ZnOS containing various amounts of S. The spectra show no signs of polycrystalline or second phases, demonstrating that S completely dissolved into the O sublattice and pure-phase wurtzite ZnOS films with preferential c-axis orientation were deposited. Upon increase of the substrate temperature, the (0 0 2) peak shifts towards higher angles, indicating shrinking of the lattice along caxis. Note that the calculated bond length d (Zn-anion) is 2.34 Å for the Zn–S bond vs. 1.98 Å for the Zn–O bond [16]. Therefore, the decreasing lattice constant c indicates declining S content in the O sublattice of ZnOS films. With the peak shifting from 31.64° to 34° by increasing the temperature from 300 to 800 °C, the lattice constant c decreases from 5.592 to 5.247 Å, corresponding to a decrease of S content in the ZnOS alloys from 0.556 to 0.202 (cf. Fig. 3(a)). Apparently, S can easily desorb at higher substrate temperatures thus yielding O-rich films. For the film deposited at 800 °C, one observes a diffraction peak corresponding to the hexagonal ZnOS (0 0 4) plane in addition to the (0 0 2) peak appeared in the films deposited at lower temperatures. This demonstrates that the out-of-plane ordering of the ZnOS films along caxis improves at higher substrate temperatures due to enhanced surface mobility and migration of species on the substrate surface. It is worth to note that, at a substrate temperature above 750 °C, epitaxial growth of high-quality ZnOS films could be achieved, as reported in our previous work [9]. To analyze the chemical states of Zn, S and O, and determine the S contents in the ternary ZnOS thin films, XPS measurements were carried out. All spectra shown in Fig. 3 were obtained after Ar-ion sputter etching to remove contaminants and adsorbates on the film surfaces and calibrated with the C 1s hydrocarbon peak at
Fig. 3. (a) Survey-scan XPS spectra of ZnO1xSx films deposited at different substrate temperatures. (b) Narrow-scan XPS spectra of Zn 2p3/2, O 1s and S 2p core-level states of the ZnO1xSx film deposited at 500 °C.
415
284.5 eV. As can be seen from the survey spectra (Fig. 3(a)), the peak intensities of the S 2p and 2s states decrease gradually while those of the O 1s state and O KLL Auger-lines increase accordingly with increasing substrate temperature. Quantifications on the composition of the ZnOS films were done using the Zn 2p, S 2p and O 1s core-level signals, and the results are given in the legend in Fig. 3(a). As shown, by tuning the substrate temperature in the PLD process the S concentration in the ZnOS films can be adjusted from 0.556 to 0.202. It is remarkable that, the S content in the ZnOS film deposited at 300 °C can reach 0.556, which is the highest S concentration achieved so far for single-phase ZnOS alloy films deposited by PLD, and is far beyond the thermal-equilibrium solubility limit of S in the ZnOS alloys. Locmelis et al. [14] grew bulk ZnO1xSx crystals by chemical transport reactions at 900 °C (close to the thermal-equilibrium status), and determined S solubility limit of about 5 mol% in the ZnO1xSx crystals. With the PLD technique, Yoo et al. [5] doped S into ZnO by alternatively ablating a ZnO and a ZnS target, and found that the S content in the hexagonal ZnOS alloy films was limited to 0.13. In our previous work [9], by ablating a ZnS target under different oxygen partial pressures, we have achieved S solubility of 23% in the PLD grown single-phase ZnOS epitaxial films. The present work suggests that it is a more efficient way to incorporate S into ZnO and further extend the S solubility by changing the substrate temperature during the PLD process. The extended solubility limit is due to the nonequilibrium process of PLD, especially at low substrate temperatures, which is far away from the thermal equilibrium state. This is well in line with a previous work [12], in which sputtering was used to deposited ZnOS films at a low temperature of 340 °C, achieving single-phase ZnO1xSx alloys in the entire composition range of 0 6 x 6 1. The largely extended solid solubility of S in ZnO will offer more freedom to the band gap engineering of ZnO (see following sections). Detailed XPS spectra of Zn 2p3/2, O 1s and S 2p of the ZnOS film deposited at 500 °C are shown in Fig. 3(b). The typical O 1s spectrum of the ZnOS films can be consistently fitted with two Gaussian-like peaks, centered at 529.9 eV and 531.5 eV, which correspond respectively to the bulk and surface oxygen in the ZnOS films [17]. The optical transmission spectra of the ZnO1xSx films measured at room temperature are shown in Fig. 4(a). It can be seen that the average transmittance of the films in the visible region is about 80%. The sharp absorption edge of the optical transmission spectra shifts towards shorter wavelengths with increasing substrate temperature, which indicates that the band gap energy of the films increases gradually. As mentioned above, the films deposited at higher substrate temperatures (700 and 800 °C) possess enhanced crystalline quality, which reduces the absorption and scattering of the light at the grain boundaries and results in higher transmittances. For the films deposited at lower temperatures, the transmission spectra show obvious oscillations which are the result of the interference between light reflected at the film surface and at the film/substrate interface, indicating a much smoother surface and sharper interface for the films deposited at lower temperatures. The absorption coefficients (a) were determined based on the transmission measurements. Fig. 4(b) shows a2 vs. photon energy hv plots of the films, based on which the optical band gaps of the films were determined assuming an allowed direct transition [18]. The dependences of the band gap energy and the S content on the substrate temperature are shown in Fig. 5(a). It is seen that both the band gap energy and S content show approximately a linear dependence on the substrate temperature for the ZnOS films, consistent with the observations in MgZnO and ZnCdO thin films [4,10]. The band gap energy can be adjusted down to 2.63 eV, which is coincident with the report of Meyer et al. for the ZnOS films sputtered at a low substrate temperature [12]. It is worth
416
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
noting that the broadened range (from 2.63 to 3.08 eV) of the band gap energy for the single-phase ZnOS alloys will extend the application of ZnO-based alloys for advanced optoelectronic devices to the blue light region. Interestingly, the substrate temperature, a simple processing parameter of PLD, can be effectively used for the band gap engineering of ZnOS alloy films. The changing of band gap energy with the substrate temperature results essentially from the composition variation in the films caused by the different vapor pressures of O and S species at different substrate temperatures [10]. Sulfur species has a higher vapor pressure and desorbs easily at higher substrate temperatures leading to the decrease of S content and the increase of band gap energy. Thus, optimal temperature window must be followed for the PLD process to achieve both good crystalline quality and desired optical characteristics. Fig. 5(b) shows the experimental data and fitting curve of the band gap energy as a function of the S content x in the ZnO1xSx films. Different from previous reports [8,12] that showed a nonlinear relationship between the band gap energy and the composition with a bowing parameter, in this work, the band gap energy shows approximately a linear dependence on the S concentration, which can be described by the following equation:
EZnOS ½xðTÞ ¼ E a xðTÞ;
Fig. 4. (a) Optical transmission spectra and (b) the absorption coefficient square (a2) vs. photon energy hv plots of ZnO1xSx films deposited at different substrate temperatures.
ð1Þ
where x(T) is the S content in the film deposited at a substrate temperature of T. Linear fitting results in a slope (a) of 1.10 and an intercept parameter E of 3.28, which is coincident with the band gap energy of the binary ZnO films we grew with PLD [8]. Therefore, for ZnO1xSx films grown by PLD at different temperatures with any given S content x, the band gap Eg can be determined according to
EZnOS ½xðTÞ ¼ 3:28 1:1 xðTÞ:
ð2Þ
This equation indicates that, the band gap energy of ZnOS alloy films grown by PLD at states far away from the thermal equilibrium favors a linear dependence on the S content rather than the bowing behavior typically observed for ZnOS alloy films [8,12]. In general, the band gap of a ternary alloy depends nonlinearly on the concentration of its constituents with a bowing parameter b that is mostly non-zero and positive. For a solid solution ABxC1x, the band gap is generally given by
Eg ðABx C1x Þ ¼ xEg ðABÞ þ ð1 xÞEg ðACÞ bxð1 xÞ
Fig. 5. (a) Band gap energy and S content of ZnO1xSx films as a function of the substrate temperature. (b) Experimental data and fitting curve of the band gap energy of ZnO1xSx films as a function of the S content x.
ð3Þ
where Eg(ABxC1x), Eg(AB) and Eg(AC) are the band gap energies of the ternary alloy ABxC1x, binary constituents AB and AC, respectively. Theoretical analyses have shown that the bowing is due to combined effects of (i) volume deformation of the band structure with the alloy lattice constants, (ii) charge exchange in the alloy with respect to the binary constituents, and (iii) a structural contribution due to the relaxation of the cation–anion bond lengths in the alloy [19]. In the specific case of ZnOS, previous experimental reports including our work on ZnOS films grown epitaxially by PLD under various O2 pressures have revealed that, the alloy band gap can be well fitted with Eq. (3), resulting in a bowing parameter b of 2.9–3.0, and it tends to decrease with increasing S content on the O-rich side [8,12,13]. However, for the ZnOS films grown at fixed O2 pressure but different temperatures in this work, the band gap variation deviates strongly from the bowing behavior, favoring instead a linear dependence on the S content as present above. In addition to the aforementioned three factors commonly considered by the theory [19], the giant stress related to the extraordinarily high S contents and structural defects such as grain boundaries and lattice disorders in the polycrystalline films grown at low temperatures may have played important roles in the band gap tuning of ZnOS alloy films in the present study. Fig. 6 shows the Raman spectra of the ZnO1xSx thin films deposited at different substrate temperatures. As known, the peaks
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
417
gap energy of 2.63 eV. The band gap energy shows a linear dependence on the S content which is in turn determined by the substrate temperature of the PLD process. The present work demonstrates that, controlling the substrate temperature during the PLD process offers further freedom in addition to adjusting the O2 (S) partial pressure for flexible tuning of the composition and thus the optical band gap of ZnOS alloy films. The largely extended S solubility in ZnO (up to 55.6%) at low growth temperature can narrow the band gap of ZnOS alloys to 2.63 eV, which will extend the application of ZnO-based optoelectronic devices to the blue light region. Acknowledgements Fig. 6. Raman spectra of ZnO1xSx films deposited at different substrate temperatures.
located at 378 cm1 and 574 cm1 correspond to A1 (TO) and A1 (LO) modes of wurtzite ZnO, respectively. Considering single-phase wurtzite ZnOS films were obtained at different substrate temperatures in this study, i.e., S dissolved completely into the O-lattice, the two peaks that have somewhat shifts with respect to the ZnO A1(TO) and A1(LO) modes are assigned, respectively, to A1(TO) and A1(LO) modes of ZnOS. With increasing the substrate temperature, the ZnOS A1 (LO) mode shifts towards higher frequencies and the peak intensity decreases gradually. However, the peak position of ZnOS A1 (TO) mode has nearly no change and the peak intensity also decreases. As the ZnOS films deposited at different substrate temperatures have various compositions, i.e., different O and S concentrations, this observation can well be interpreted according to our previous report on ZnOS films deposited under varying O2 partial pressure that had also different compositions [8]. The upward shift of the A1 (LO) vibration mode with increasing substrate temperature can again be well reconciled with the XRD results. The A1 (LO) mode is known to be polarized and propagated along the c axis. As presented above, the ZnOS films deposited at higher temperatures contain less S and have reduced c-axis length as revealed by XRD. This implies shortening of respective bonds along the c-axis, leading to stiffened mode of the phonons with higher vibration frequencies. The Raman scattering measurements confirm that the PLD-grown ZnOS films all have a wurtzite structure, and the lattice constant c as well as the composition of the films can be tuned to large extent by varying the substrate temperature during the PLD process. 4. Conclusions In summary, we have grown high-quality ZnO1xSx films on c-sapphire substrates by PLD at different substrate temperatures using a ZnS ceramic target and O2 as the reactive gas. It turns out that the band gap energy as well as the S content of the films can be well adjusted by controlling the substrate temperature. At a low temperature of 300 °C, 55.6% S can be introduced into ZnOS film without phase separation, which is far above the thermodynamic solubility limit of S in ZnO, resulting in narrowed band
This work was supported by the National Natural Science Foundation of China (Grant Nos. 50972041, 51002049, 61274010), Program for New Century Excellent Talents in University (NCET09-0135), Research Fund for the Doctoral Program of Higher Education of China (20124208110005, 20124208120006), Scientific Research Foundation for Returned Scholars, Ministry of Education of China, the Natural Science Foundation of Hubei Province (2011CDA81), the Science Foundation from Hubei Provincial Department of Education (D20131001), Wuhan Municipal Academic Leaders Program (200951830550), and the Open Fund from Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, China. References [1] Ü. Özgür, Ya I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [2] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [3] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237. [4] W.F. Yang, B. Liu, R. Chen, L.M. Wong, S.J. Wang, H.D. Sun, Appl. Phys. Lett. 97 (2010) 061911. [5] Y.Z. Yoo, Z.W. Jin, T. Chikyow, T. Fukumura, M. Kawasaki, H. Koinuma, Appl. Phys. Lett. 81 (2002) 3798. [6] H.C. Jeon, S.H. Park, S.J. Lee, T.W. Kang, T.F. George, Appl. Phys. Lett. 96 (2010) 101113. [7] W.F. Yang, L.M. Wong, S.J. Wang, H.D. Sun, C.H. Ge, Y.S. Lee, H. Gong, Appl. Phys. Lett. 98 (2011) 121903. [8] Y.B. He, L. Zhang, L.H. Wang, M.K. Li, X.Z. Shang, X. Liu, Y.M. Lu, B.K. Meyer, J. Alloys Comp. 587 (2014) 369. [9] Y.B. He, L.H. Wang, L. Zhang, M.K. Li, X.Z. Shang, Y.Y. Fang, C.Q. Chen, J. Alloys Comp. 534 (2012) 81. [10] S. Choopun, R.D. Vispute, W. Yang, R.P. Sharma, T. Venkatesan, H. Shen, Appl. Phys. Lett. 80 (2002) 1529. [11] W. Yang, S.S. Hullavarad, B. Nagaraj, I. Takeuchi, R.P. Sharma, T. Venkatesan, R.D. Vispute, H. Shen, Appl. Phys. Lett. 82 (2003) 3424. [12] B.K. Meyer, A. Polity, B. Farangis, Y. He, D. Hasselkamp, Th Krämer, C. Wang, Appl. Phys. Lett. 85 (2004) 4929. [13] H.L. Pan, T. Yang, B. Yao, R. Deng, R.Y. Sui, L.L. Gao, D.Z. Shen, Appl. Surf. Sci. 256 (2010) 4621. [14] S. Locmelis, C. Brünig, M. Binnewies, A. Börger, K.D. Becker, T. Homann, T. Bredow, J. Mater. Sci. 42 (2007) 1965–1971. [15] P.W. Wilmott, J.R. Huber, Rev. Mod. Phys. 72 (2000) 315. [16] C. Persson, C. Platzer-Björkman, J. Malmström, T. Törndahl, M. Edoff, Phys. Rev. Lett. 97 (2006) 146403. [17] R.R. Thankalekshmi, A.C. Rastogi, J. Appl. Phys. 112 (2012) 063708. [18] A. Polity, B.K. Meyer, T. Krämer, C.Z. Wang, U. Haboeck, A. Hoffmann, Phys. Status Solidi (a) 203 (2006) 2867. [19] James E. Bernard, Alex Zunger, Phys. Rev. B 36 (1987) 3199.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tuning the composition and optical band gap of pulsed laser deposited ZnO1xSx alloy films by controlling the substrate temperature Lei Zhang a, Lei Li a, Liangheng Wang a, Mingkai Li a, Yinmei Lu a,b, Bruno K. Meyer b, Yunbin He a,⇑ a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory of Green Preparation and Application for Functional Materials, Ministry of Education, Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China b I. Physikalisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany
a r t i c l e
i n f o
Article history: Received 29 April 2014 Received in revised form 2 July 2014 Accepted 4 August 2014 Available online 11 August 2014 Keywords: ZnO1xSx thin films Pulsed laser deposition Substrate temperature Solid solubility Band gap energy
a b s t r a c t High-quality ZnO1xSx thin films were grown on (0 0 1) sapphire substrates in the temperature range of 300–800 °C by pulsed laser deposition (PLD) with a ZnS ceramic target and O2 as reactive gas. By increasing the substrate temperature, the crystalline quality of the films is enhanced. The S content in the single-phase ZnO1xSx films can be systematically adjusted from 0.556 to 0.202 via changing the substrate temperature. The maximum S content in the film grown at 300 °C reaches 0.556 without phase separation, which is significantly higher than the solid solubility limits reported previously for the ZnOS alloys. The narrowed band gap of the ZnO1xSx film (2.63 eV) grown at the low substrate temperature will extend the application of ZnO-based optoelectronic devices to the blue light region. As the composition, structure, and band gap energy of the ZnOS films were found to depend critically on the growth temperature, this work suggests a simple and flexible means of tuning the composition and optical band gap of ZnOS alloy films by controlling the substrate temperature during the PLD process. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction With a wide band gap of 3.3 eV and a large exciton binding energy of 60 meV at room temperature, ZnO is important for blue and ultraviolet optoelectronic devices, transparent electronics, spintronic devices, and sensor applications [1]. For a semiconductor to be useful, particularly in reference to optoelectronic devices, band gap engineering is a crucial step toward the device development. By alloying a semiconductor with compounds from the same group having different band gaps, the band gap of the resultant alloy can be tuned, thereby affecting the wavelength of exciton emissions. In the case of ZnO, alloying with MgO, CdO and ZnS are effective means for increasing or decreasing the band gap energy respectively [2–5]. It is important to note that all these compounds are not isostructural to wurtzite ZnO, and the ionic radius of Mg2+ is relatively close to that of Zn2+, whereas Cd2+ has a much larger radius. Therefore, limited solubility and a variation in lattice parameters with deformation in the structure are expected in the ternary alloys. Alloying of MgO and CdO with ZnO has been studied extensively in comparison to the anionsubstituted compound of ZnO1xSx (ZnOS). Heterostructures made with ZnO and MgZnO or CdZnO have already been demonstrated ⇑ Corresponding author. Tel./fax: +86 27 88661803. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.jallcom.2014.08.024 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
[6,7], and are expected to play key roles in future ZnO-based optoelectronic devices. Variations in the composition and solubility limit of the ZnO alloys reported in the literature are due to the variation in processing conditions such as the substrate temperature and the oxygen partial pressure. For the later one, by controlling the oxygen partial pressure one can tune the composition of the ZnOS ternary alloy accurately, and it is an efficient way for the band gap engineering of ZnO [8,9]. For the former one, however, there are so far only limited reports concerning the substrate temperature effects. The dependence of Mg content on the growth temperature has been utilized successfully to tune the composition (and the band gap) over a wide range (0.5 6 x 6 1) for epitaxial MgxZn1xO alloy films grown by pulsed laser deposition (PLD) using a single ceramic target of specific composition [10]. The dependence of the band gap on the substrate temperature was attributed to the change of Mg/Zn ratio in the films due to different vapor pressures of Mg and Zn species at different temperatures. At higher substrate temperatures, the desorption rate of Zn from the substrate surface is much higher than that of Mg due to the higher vapor pressure of Zn relative to Mg, and results in Mg-rich films with larger band gaps [11]. Compared to the cation-substituted MgZnO and CdZnO alloy systems, only limited experimental work focused on the synthesis methods has been reported to date for the anion-substituted ZnOS alloys, and the information available is both incomplete and not
414
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
well verified. In the recent years, Yoo et al. [5] first succeeded in doping S into ZnO with the PLD technique at 700 °C by alternatively ablating a ZnO and a ZnS target. They found that the S concentration in the hexagonal ZnOS alloys was limited to 0.13. Meyer et al. [12] synthesized single-phase wurtzite ZnO1xSx alloys in the whole composition range of 0 6 x 6 1 by RF sputtering at a low substrate temperature of 340 °C. The band gap energy can be adjusted down to (2.6 ± 0.1) eV for x = (0.45 ± 0.05), and the composition dependence of the band gap energy for the ternary system was determined with an optical bowing parameter of about 3.0 eV. Pan et al. [13] deposited ZnO1xSx alloy films with wurtzite and zincblende structures occurred in the O-rich (0 6 x 6 0.23) and the S-rich (0.77 6 x 6 1) films, respectively, by RF magnetron sputtering at substrate temperature of 300 °C followed by annealing at 500 °C. However, a systematical study on effects of the substrate temperature in the growth of ZnOS alloy films is still lacking to date. The synthesis of ZnOS at thermodynamic equilibrium state is quite difficult and with rather limited solid solubility of ZnO and ZnS due to the different crystalline symmetry of both compounds and the inferior stability of sulfur [14]. However, with nonequilibrium processes, such as PLD and sputtering, the alloying of ZnO and ZnS can be achieved with a large solid solubility that is well beyond the thermodynamic limit [9,13]. In the PLD process, the laser ablated species have certain kinetic energy and can thus nucleate and grow into film at relatively low temperature, different from the equilibrium growth where only thermal energy is supplied by high temperature heating. In such a non-equilibrium process, the substrate temperature can play a crucial role in the formation of meta-stable phases. As reported previously [10], epitaxial and single-phase MgZnO films having a composition range of 0.5 6 x 6 1 can be obtained by controlling the growth temperature. In the present work, we used a ZnS ceramic target and O2 as reactive gas to grow ZnOS alloy films by the PLD technique. The substrate temperature during the deposition process was varied systematically in the range of 300–800 °C in order to study the influence of the growth temperature on properties of the resulting ZnOS alloy films. 2. Experimental details ZnO1xSx thin films were grown on single-crystal sapphire (0 0 1) substrates by the PLD technique using a KrF excimer laser (Lambda Physik COMPEX PRO 205 F, k = 248 nm) as the ablation source. A high-purity ZnS (99.99%) ceramic was employed as the ablation target, and high-purity O2 (99.999%) was introduced as reactive gas. The sapphire substrates were cleaned ultrasonically in organic solvents and de-ionized water, and then blown dry by nitrogen. The target-substrate distance was set at 5.5 cm, and the laser beam energy was fixed at 200 mJ/pulse with a pulse repetition rate of 5 Hz. Prior to film deposition, the vacuum chamber was evacuated to 3.0–5.0 104 Pa. The O2 partial pressure was kept constant at 2 Pa during the deposition, while the substrate temperature was varied in the range of 300–800 °C. All films were grown for 30 min with thicknesses of 300–400 nm. The evolution of the surface morphologies was characterized by atomic force microscopy (AFM). X-ray diffraction (XRD) was performed using a powder diffractometer (Rigaku, D/Max-IIIC) with Cu Ka radiation (k = 0.154184 nm) for the structural characterization of the films. The optical properties of the films were measured by UV–VIS scanning spectrophotometer (DU 800). The phonon properties of the films were measured by Raman spectroscopy (HORIBA, JOBIN YVON HR800 UV) with the focal length of 800 mm at room temperature in back scattering geometry. The 514 nm line of a He–Ne laser was used for excitation, and the spectra were collected in the range of 200–800 cm1 using a multichannel charge-coupled device (CCD) detector. Composition and chemical state analyses of the films were accomplished by X-ray photoelectron spectroscopy (XPS, PHOIBOS 150, SPECS) at photon energy of 1486.6 eV (Al Ka radiation).
Fig. 1. AFM images of the ZnO1xSx films deposited at substrate temperatures of (a) 300 °C, (b) 500 °C, (c) 700 °C, and (d) RMS roughness of the films vs. the deposition temperature.
grow in a three-dimensional mode, having a smooth, dense and uniform surface, and the grain size grows with increasing the substrate temperature. The surface root mean square (RMS) roughness of the films vs. the deposition temperature is shown in Fig. 1(d). The RMS roughness increases from 1.163 to 1.832 nm when the substrate temperature rises from 300 to 700 °C. The AFM study reveals that, increasing the substrate temperature during the PLD process enhances the crystallinity of the film, leading to larger grains while increasing slightly the surface roughness. This is in line with the general observations on the PLD growth of thin films, in which the nucleation process is mainly governed by the substrate temperature while the surface mobility of the condensing species defines the crystallinity of the film. Generally a high substrate temperature favors rapid and defect free growth of crystallites due to optimum surface diffusion of the species, whereas a low substrate temperature results in the growth of a disordered or a poorly crystallized structure with a relatively small RMS roughness [15]. Fig. 2 shows XRD spectra measured in the h–2h scanning mode of the ZnO1xSx films deposited at different substrate temperatures. It is well known that wurtzite ZnO (0 0 2) plane should induce a diffraction peak around 2h = 34.42° as denoted in the figure by a dotted red line. Considering that the ionic radius of S
3. Results and discussion The surface morphology of the ZnO1xSx films deposited at different substrate temperatures was characterized by AFM, and the images are shown in Fig. 1(a)–(c). It can be seen that, all the films
Fig. 2. XRD spectra of ZnO1xSx films deposited at different substrate temperatures.
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
is larger than that of O, the diffraction angle of the (0 0 2) peak would decrease when S substitutes for part of O in the O sublattice of ZnO. Therefore, the peaks between 30° and 34.42° in the XRD spectra are identified as the (0 0 2) diffraction peaks of wurtzite ZnOS containing various amounts of S. The spectra show no signs of polycrystalline or second phases, demonstrating that S completely dissolved into the O sublattice and pure-phase wurtzite ZnOS films with preferential c-axis orientation were deposited. Upon increase of the substrate temperature, the (0 0 2) peak shifts towards higher angles, indicating shrinking of the lattice along caxis. Note that the calculated bond length d (Zn-anion) is 2.34 Å for the Zn–S bond vs. 1.98 Å for the Zn–O bond [16]. Therefore, the decreasing lattice constant c indicates declining S content in the O sublattice of ZnOS films. With the peak shifting from 31.64° to 34° by increasing the temperature from 300 to 800 °C, the lattice constant c decreases from 5.592 to 5.247 Å, corresponding to a decrease of S content in the ZnOS alloys from 0.556 to 0.202 (cf. Fig. 3(a)). Apparently, S can easily desorb at higher substrate temperatures thus yielding O-rich films. For the film deposited at 800 °C, one observes a diffraction peak corresponding to the hexagonal ZnOS (0 0 4) plane in addition to the (0 0 2) peak appeared in the films deposited at lower temperatures. This demonstrates that the out-of-plane ordering of the ZnOS films along caxis improves at higher substrate temperatures due to enhanced surface mobility and migration of species on the substrate surface. It is worth to note that, at a substrate temperature above 750 °C, epitaxial growth of high-quality ZnOS films could be achieved, as reported in our previous work [9]. To analyze the chemical states of Zn, S and O, and determine the S contents in the ternary ZnOS thin films, XPS measurements were carried out. All spectra shown in Fig. 3 were obtained after Ar-ion sputter etching to remove contaminants and adsorbates on the film surfaces and calibrated with the C 1s hydrocarbon peak at
Fig. 3. (a) Survey-scan XPS spectra of ZnO1xSx films deposited at different substrate temperatures. (b) Narrow-scan XPS spectra of Zn 2p3/2, O 1s and S 2p core-level states of the ZnO1xSx film deposited at 500 °C.
415
284.5 eV. As can be seen from the survey spectra (Fig. 3(a)), the peak intensities of the S 2p and 2s states decrease gradually while those of the O 1s state and O KLL Auger-lines increase accordingly with increasing substrate temperature. Quantifications on the composition of the ZnOS films were done using the Zn 2p, S 2p and O 1s core-level signals, and the results are given in the legend in Fig. 3(a). As shown, by tuning the substrate temperature in the PLD process the S concentration in the ZnOS films can be adjusted from 0.556 to 0.202. It is remarkable that, the S content in the ZnOS film deposited at 300 °C can reach 0.556, which is the highest S concentration achieved so far for single-phase ZnOS alloy films deposited by PLD, and is far beyond the thermal-equilibrium solubility limit of S in the ZnOS alloys. Locmelis et al. [14] grew bulk ZnO1xSx crystals by chemical transport reactions at 900 °C (close to the thermal-equilibrium status), and determined S solubility limit of about 5 mol% in the ZnO1xSx crystals. With the PLD technique, Yoo et al. [5] doped S into ZnO by alternatively ablating a ZnO and a ZnS target, and found that the S content in the hexagonal ZnOS alloy films was limited to 0.13. In our previous work [9], by ablating a ZnS target under different oxygen partial pressures, we have achieved S solubility of 23% in the PLD grown single-phase ZnOS epitaxial films. The present work suggests that it is a more efficient way to incorporate S into ZnO and further extend the S solubility by changing the substrate temperature during the PLD process. The extended solubility limit is due to the nonequilibrium process of PLD, especially at low substrate temperatures, which is far away from the thermal equilibrium state. This is well in line with a previous work [12], in which sputtering was used to deposited ZnOS films at a low temperature of 340 °C, achieving single-phase ZnO1xSx alloys in the entire composition range of 0 6 x 6 1. The largely extended solid solubility of S in ZnO will offer more freedom to the band gap engineering of ZnO (see following sections). Detailed XPS spectra of Zn 2p3/2, O 1s and S 2p of the ZnOS film deposited at 500 °C are shown in Fig. 3(b). The typical O 1s spectrum of the ZnOS films can be consistently fitted with two Gaussian-like peaks, centered at 529.9 eV and 531.5 eV, which correspond respectively to the bulk and surface oxygen in the ZnOS films [17]. The optical transmission spectra of the ZnO1xSx films measured at room temperature are shown in Fig. 4(a). It can be seen that the average transmittance of the films in the visible region is about 80%. The sharp absorption edge of the optical transmission spectra shifts towards shorter wavelengths with increasing substrate temperature, which indicates that the band gap energy of the films increases gradually. As mentioned above, the films deposited at higher substrate temperatures (700 and 800 °C) possess enhanced crystalline quality, which reduces the absorption and scattering of the light at the grain boundaries and results in higher transmittances. For the films deposited at lower temperatures, the transmission spectra show obvious oscillations which are the result of the interference between light reflected at the film surface and at the film/substrate interface, indicating a much smoother surface and sharper interface for the films deposited at lower temperatures. The absorption coefficients (a) were determined based on the transmission measurements. Fig. 4(b) shows a2 vs. photon energy hv plots of the films, based on which the optical band gaps of the films were determined assuming an allowed direct transition [18]. The dependences of the band gap energy and the S content on the substrate temperature are shown in Fig. 5(a). It is seen that both the band gap energy and S content show approximately a linear dependence on the substrate temperature for the ZnOS films, consistent with the observations in MgZnO and ZnCdO thin films [4,10]. The band gap energy can be adjusted down to 2.63 eV, which is coincident with the report of Meyer et al. for the ZnOS films sputtered at a low substrate temperature [12]. It is worth
416
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
noting that the broadened range (from 2.63 to 3.08 eV) of the band gap energy for the single-phase ZnOS alloys will extend the application of ZnO-based alloys for advanced optoelectronic devices to the blue light region. Interestingly, the substrate temperature, a simple processing parameter of PLD, can be effectively used for the band gap engineering of ZnOS alloy films. The changing of band gap energy with the substrate temperature results essentially from the composition variation in the films caused by the different vapor pressures of O and S species at different substrate temperatures [10]. Sulfur species has a higher vapor pressure and desorbs easily at higher substrate temperatures leading to the decrease of S content and the increase of band gap energy. Thus, optimal temperature window must be followed for the PLD process to achieve both good crystalline quality and desired optical characteristics. Fig. 5(b) shows the experimental data and fitting curve of the band gap energy as a function of the S content x in the ZnO1xSx films. Different from previous reports [8,12] that showed a nonlinear relationship between the band gap energy and the composition with a bowing parameter, in this work, the band gap energy shows approximately a linear dependence on the S concentration, which can be described by the following equation:
EZnOS ½xðTÞ ¼ E a xðTÞ;
Fig. 4. (a) Optical transmission spectra and (b) the absorption coefficient square (a2) vs. photon energy hv plots of ZnO1xSx films deposited at different substrate temperatures.
ð1Þ
where x(T) is the S content in the film deposited at a substrate temperature of T. Linear fitting results in a slope (a) of 1.10 and an intercept parameter E of 3.28, which is coincident with the band gap energy of the binary ZnO films we grew with PLD [8]. Therefore, for ZnO1xSx films grown by PLD at different temperatures with any given S content x, the band gap Eg can be determined according to
EZnOS ½xðTÞ ¼ 3:28 1:1 xðTÞ:
ð2Þ
This equation indicates that, the band gap energy of ZnOS alloy films grown by PLD at states far away from the thermal equilibrium favors a linear dependence on the S content rather than the bowing behavior typically observed for ZnOS alloy films [8,12]. In general, the band gap of a ternary alloy depends nonlinearly on the concentration of its constituents with a bowing parameter b that is mostly non-zero and positive. For a solid solution ABxC1x, the band gap is generally given by
Eg ðABx C1x Þ ¼ xEg ðABÞ þ ð1 xÞEg ðACÞ bxð1 xÞ
Fig. 5. (a) Band gap energy and S content of ZnO1xSx films as a function of the substrate temperature. (b) Experimental data and fitting curve of the band gap energy of ZnO1xSx films as a function of the S content x.
ð3Þ
where Eg(ABxC1x), Eg(AB) and Eg(AC) are the band gap energies of the ternary alloy ABxC1x, binary constituents AB and AC, respectively. Theoretical analyses have shown that the bowing is due to combined effects of (i) volume deformation of the band structure with the alloy lattice constants, (ii) charge exchange in the alloy with respect to the binary constituents, and (iii) a structural contribution due to the relaxation of the cation–anion bond lengths in the alloy [19]. In the specific case of ZnOS, previous experimental reports including our work on ZnOS films grown epitaxially by PLD under various O2 pressures have revealed that, the alloy band gap can be well fitted with Eq. (3), resulting in a bowing parameter b of 2.9–3.0, and it tends to decrease with increasing S content on the O-rich side [8,12,13]. However, for the ZnOS films grown at fixed O2 pressure but different temperatures in this work, the band gap variation deviates strongly from the bowing behavior, favoring instead a linear dependence on the S content as present above. In addition to the aforementioned three factors commonly considered by the theory [19], the giant stress related to the extraordinarily high S contents and structural defects such as grain boundaries and lattice disorders in the polycrystalline films grown at low temperatures may have played important roles in the band gap tuning of ZnOS alloy films in the present study. Fig. 6 shows the Raman spectra of the ZnO1xSx thin films deposited at different substrate temperatures. As known, the peaks
L. Zhang et al. / Journal of Alloys and Compounds 617 (2014) 413–417
417
gap energy of 2.63 eV. The band gap energy shows a linear dependence on the S content which is in turn determined by the substrate temperature of the PLD process. The present work demonstrates that, controlling the substrate temperature during the PLD process offers further freedom in addition to adjusting the O2 (S) partial pressure for flexible tuning of the composition and thus the optical band gap of ZnOS alloy films. The largely extended S solubility in ZnO (up to 55.6%) at low growth temperature can narrow the band gap of ZnOS alloys to 2.63 eV, which will extend the application of ZnO-based optoelectronic devices to the blue light region. Acknowledgements Fig. 6. Raman spectra of ZnO1xSx films deposited at different substrate temperatures.
located at 378 cm1 and 574 cm1 correspond to A1 (TO) and A1 (LO) modes of wurtzite ZnO, respectively. Considering single-phase wurtzite ZnOS films were obtained at different substrate temperatures in this study, i.e., S dissolved completely into the O-lattice, the two peaks that have somewhat shifts with respect to the ZnO A1(TO) and A1(LO) modes are assigned, respectively, to A1(TO) and A1(LO) modes of ZnOS. With increasing the substrate temperature, the ZnOS A1 (LO) mode shifts towards higher frequencies and the peak intensity decreases gradually. However, the peak position of ZnOS A1 (TO) mode has nearly no change and the peak intensity also decreases. As the ZnOS films deposited at different substrate temperatures have various compositions, i.e., different O and S concentrations, this observation can well be interpreted according to our previous report on ZnOS films deposited under varying O2 partial pressure that had also different compositions [8]. The upward shift of the A1 (LO) vibration mode with increasing substrate temperature can again be well reconciled with the XRD results. The A1 (LO) mode is known to be polarized and propagated along the c axis. As presented above, the ZnOS films deposited at higher temperatures contain less S and have reduced c-axis length as revealed by XRD. This implies shortening of respective bonds along the c-axis, leading to stiffened mode of the phonons with higher vibration frequencies. The Raman scattering measurements confirm that the PLD-grown ZnOS films all have a wurtzite structure, and the lattice constant c as well as the composition of the films can be tuned to large extent by varying the substrate temperature during the PLD process. 4. Conclusions In summary, we have grown high-quality ZnO1xSx films on c-sapphire substrates by PLD at different substrate temperatures using a ZnS ceramic target and O2 as the reactive gas. It turns out that the band gap energy as well as the S content of the films can be well adjusted by controlling the substrate temperature. At a low temperature of 300 °C, 55.6% S can be introduced into ZnOS film without phase separation, which is far above the thermodynamic solubility limit of S in ZnO, resulting in narrowed band
This work was supported by the National Natural Science Foundation of China (Grant Nos. 50972041, 51002049, 61274010), Program for New Century Excellent Talents in University (NCET09-0135), Research Fund for the Doctoral Program of Higher Education of China (20124208110005, 20124208120006), Scientific Research Foundation for Returned Scholars, Ministry of Education of China, the Natural Science Foundation of Hubei Province (2011CDA81), the Science Foundation from Hubei Provincial Department of Education (D20131001), Wuhan Municipal Academic Leaders Program (200951830550), and the Open Fund from Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, China. References [1] Ü. Özgür, Ya I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [2] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [3] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237. [4] W.F. Yang, B. Liu, R. Chen, L.M. Wong, S.J. Wang, H.D. Sun, Appl. Phys. Lett. 97 (2010) 061911. [5] Y.Z. Yoo, Z.W. Jin, T. Chikyow, T. Fukumura, M. Kawasaki, H. Koinuma, Appl. Phys. Lett. 81 (2002) 3798. [6] H.C. Jeon, S.H. Park, S.J. Lee, T.W. Kang, T.F. George, Appl. Phys. Lett. 96 (2010) 101113. [7] W.F. Yang, L.M. Wong, S.J. Wang, H.D. Sun, C.H. Ge, Y.S. Lee, H. Gong, Appl. Phys. Lett. 98 (2011) 121903. [8] Y.B. He, L. Zhang, L.H. Wang, M.K. Li, X.Z. Shang, X. Liu, Y.M. Lu, B.K. Meyer, J. Alloys Comp. 587 (2014) 369. [9] Y.B. He, L.H. Wang, L. Zhang, M.K. Li, X.Z. Shang, Y.Y. Fang, C.Q. Chen, J. Alloys Comp. 534 (2012) 81. [10] S. Choopun, R.D. Vispute, W. Yang, R.P. Sharma, T. Venkatesan, H. Shen, Appl. Phys. Lett. 80 (2002) 1529. [11] W. Yang, S.S. Hullavarad, B. Nagaraj, I. Takeuchi, R.P. Sharma, T. Venkatesan, R.D. Vispute, H. Shen, Appl. Phys. Lett. 82 (2003) 3424. [12] B.K. Meyer, A. Polity, B. Farangis, Y. He, D. Hasselkamp, Th Krämer, C. Wang, Appl. Phys. Lett. 85 (2004) 4929. [13] H.L. Pan, T. Yang, B. Yao, R. Deng, R.Y. Sui, L.L. Gao, D.Z. Shen, Appl. Surf. Sci. 256 (2010) 4621. [14] S. Locmelis, C. Brünig, M. Binnewies, A. Börger, K.D. Becker, T. Homann, T. Bredow, J. Mater. Sci. 42 (2007) 1965–1971. [15] P.W. Wilmott, J.R. Huber, Rev. Mod. Phys. 72 (2000) 315. [16] C. Persson, C. Platzer-Björkman, J. Malmström, T. Törndahl, M. Edoff, Phys. Rev. Lett. 97 (2006) 146403. [17] R.R. Thankalekshmi, A.C. Rastogi, J. Appl. Phys. 112 (2012) 063708. [18] A. Polity, B.K. Meyer, T. Krämer, C.Z. Wang, U. Haboeck, A. Hoffmann, Phys. Status Solidi (a) 203 (2006) 2867. [19] James E. Bernard, Alex Zunger, Phys. Rev. B 36 (1987) 3199.