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Intrinsic defects responsible for the anomalous Raman peaks and the room-temperature ferromagnetism in nitrogen-doped ZnO thin films
Surface & Coatings Technology 231 (2013) 307–310
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Intrinsic defects responsible for the anomalous Raman peaks and the room-temperature ferromagnetism in nitrogen-doped ZnO thin films Ching-Chung Wang a, Chao-Ming Fu a, Yu-Min Hu b,⁎ a b
Department of Physics, National Taiwan University, Taipei 10617, Taiwan, ROC Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan, ROC
a r t i c l e
i n f o
Available online 23 November 2012 Keywords: Zinc oxide Nitrogen Intrinsic defect Photoluminescence Ferromagnetism
a b s t r a c t This work aims to investigate the intrinsic defects responsible for the anomalous Raman peaks and the room-temperature ferromagnetism in the nitrogen-doped ZnO (ZnO:N) thin films deposited using a magnetron sputtering method. The X-ray diffraction results indicate that all of the films are grown with a highly c-axis-oriented wurtzite structure. All photoluminescence spectra exhibit an intensive broad band centered at approximately 3.2 eV as well as a weak band at approximately 2.2 eV. By using a multi-peak fitting method, we deduce that the zinc vacancies, the zinc interstitials, and the substituted nitrogen at the oxygen site are responsible for the emissions embedded in the intensive broad band, while the oxygen vacancies and the oxygen interstitials are responsible for the weak band. These defects in the ZnO:N films may be associated with the four anomalous Raman peaks at approximately 277, 511, 584 and 644 cm −1. Room-temperature ferromagnetism has been observed for all of the ZnO:N films. With an increase in film thickness, the saturation magnetization first decreases rapidly and then increases. A close relationship between the saturation magnetization and the relative change in the c-axis lattice constant is observed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the room-temperature ferromagnetism (RT-FM) in ZnO-based diluted magnetic semiconductors has received much attention regarding fundamental research and possible applications in spintronic devices. Despite considerable theoretical and experimental efforts, both the origin and coupling mechanism of the magnetic moments in the transition metal (TM)-doped ZnO are still matters of debate [1–3]. Meanwhile, several researchers have reported RT-FM in undoped ZnO films [4], nitrogen (N)-doped ZnO (ZnO:N) films [5], carbon (C)-doped ZnO films [6] and nano-needles [7], implying that the magnetic moments may arise solely from the native defects irrespective of the TM ions. Wang et al. [8] performed a comprehensive theoretical investigation on the vacancy-induced magnetism in ZnO thin films and argued that the magnetic moments stem from the unpaired 2p electrons at the oxygen sites surrounding the zinc vacancies (VZn). Based on the ab initio calculations, Zuo et al. [9] predicted that the VZn and interstitial oxygen (Oi) in ZnO may result in the formation of 1.77 and 2.0 μB local moments, respectively. In addition to the intrinsic defects mentioned above, the substitution of oxygen by nitrogen (NO) has been predicted to introduce a magnetic moment of 1.0 μB in the ZnO:N film [10,11]. The local magnetic moments can mainly arise from the 2p orbitals of N and couple ferromagnetically with each other. Qualitatively, these theoretical results may explain ⁎ Corresponding author. E-mail address: [email protected] (Y.-M. Hu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.11.044
the observed RT-FM in the undoped or the nonmagnetic-elementdoped ZnO films. In view of the defect-induced magnetism, one may suspect that the saturation magnetization (Ms) is positively proportional to the determinant defect concentration. The correlation between the Ms and the defect concentration has already been proposed by several other researchers [12–14]. Note that the determinant defect concentration may vary with the film thickness [12] and depend not only on the deposition method and the ambient growth but also on the dopant [15]. In this study, we prepared ZnO:N films with the radiofrequency (r.f.) magnetron sputtering method under N2 atmosphere and examined the thickness dependence of the defect-related RT-FM. In particular, we report a correlation between the Ms and the relative change in the c-axis lattice constant of ZnO:N films. Possible defects responsible for the RT-FM in ZnO:N films will be discussed in detail. 2. Experimental details The ZnO:N films were deposited using a ceramic ZnO (99.99% purity) target onto the c-plane (0001) sapphire (Al2O3) substrates at 400 °C by r.f. magnetron sputtering in an ambient mixture of Ar and N2 in an ultra-high vacuum system (Model: Pspu-100HC, Advanced System Technology Co.) at a pressure of ~ 2.66 Pa. The mixture of ultra-pure (99.999%) Ar and N2 gasses was introduced via two independent mass flow controllers, each with a flow rate of 30 sccm (sccm denotes cubic centimeters per minute at standard temperature and pressure). The r.f. power of the ZnO target was maintained at
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100 W, and the growth time of the film was selected to be 2, 4 and 6 h, producing films with thicknesses of 352, 715 and 1100 nm, respectively. The deposition rate of all of the films was approximately 3 nm per minute. The crystal structure was determined by X-ray diffraction (XRD, Rigaku Ultima IV) with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 VersaProbe spectrometer with a monochromatized Al Kα X-rays at hν = 1486.6 eV. Room-temperature photoluminescence (RT-PL) measurement was performed on a micro-Raman system (HORIBA HR800) using the 325 nm line of a He–Cd laser. The Raman spectra were recorded in the backscattering configuration using a Jobin-Yvon LabRam-HR spectrometer with an incident 633-nm He–Ne laser. The magnetic properties were probed by a superconducting quantum interference device (Quantum Design).
3. Results and discussion Fig. 1 shows the XRD θ–2θ scans of the ZnO:N films on a logarithmic scale. All of the films are preferentially c-axis-oriented with the wurtzite structure, as evidenced by the predominant ZnO (002) peak. No other peaks corresponding to either Zn metal or the Zn3N2 phase can be observed within the detection limit of XRD. The inset of Fig. 1 displays the variation in the c-axis lattice constant (cfilm) of the films with different thicknesses. The cfilm values were calculated directly from the position of the ZnO (002) peak using the Bragg law. All of the cfilm values are slightly larger than that of bulk ZnO. One may expect that the cfilm value would decrease with the film thickness due to strain relaxation; however, the cfilm value of 1100-nm thick film is slightly larger than the cfilm value of 715-nm thick film. According to the PL results shown below, we suggest that the abnormal increase in the cfilm value of 1100-nm thick film is due to an increase in the amounts of Oi. Fig. 2 displays the RT-PL spectra of the ZnO:N films. All of the films exhibit an intense broad band and a weak band at approximately 3.2 eV and 2.2 eV, respectively. The results of multi-peak-fitting are shown in the inset of Fig. 2, where the energy levels of the point defects used in the Gaussian fitting are based on a recent review article on ZnO [16]. The intense broad emission band is well fitted by five peaks. Peak 1 is located at 3.27 eV and is associated with the free exciton (FX) [17]. Peak 2 is located at 3.20 eV, with an energy difference of 70 meV from peak 1, and can be assigned to the first longitudinal phonon replica of the FX [18]. The other peaks may be attributed to defect-related emissions. Meyer et al. [19] proposed that NO may
Fig. 1. XRD θ–2θ scans of the ZnO:N films on a logarithmic scale. The inset illustrates the variation in the calculated c-axis lattice constant with different film thicknesses.
Fig. 2. RT-PL spectra of the ZnO:N films. The insets show the multi-peak fitting results.
induce a shallow acceptor level at approximately 165 meV above the valence band. Look et al. [20] suggested that the trapping of zinc interstitial (Zni) near a NO, forming the Zni–NO donor complex, is a shallow donor at approximately 30 meV below the conduction band. Accordingly, peak 3, which is weak and located at 3.14 eV, may be attributed to a donor-acceptor pair recombination that occurs between the Zni–NO shallow donor and NO acceptor levels [21]. Additionally, peak 4 is located at 3.07 eV and may be assigned as a transition from the conduction band to the VZn level. A theoretical calculation has predicted that the VZn level is located 0.3 eV above the valence band [22]. Moreover, peak 5, which is broad and located at 3.02 eV, might originate from the electron transition between the band tail state levels of the surface defects and/or the lattice imperfections [23]. In addition, peak 6 located at 2.24 eV and peak 7 at 2.01 eV may be associated with VO [24] and Oi [25], respectively. The Oi-related emission intensity of the 1100-nm thick film is larger than that of the 715-nm thick film, indicating a greater amount of Oi in the 1100-nm thick film. Fig. 3 shows the Raman spectra of the ZnO:N films with different thicknesses. The peaks indicated by asterisks are due to scatterings from the sapphire substrate. The peaks at 333 cm−1 and 438 cm−1 have been assigned to the optical phonon 2E2(M) mode and E2(high) mode of the ZnO crystal lattice [26], respectively. Notably, four anomalous Raman peaks at frequencies of ~277, 511, 584 and 644 cm−1 are present in all of the films and their intensities are positively proportional to the film thickness. The presence of four anomalous Raman peaks is associated with the existence of point defects. The Raman peak at 584 cm−1 is assigned to the E1(LO) mode, which may be related to intrinsic host defects, such as VO and Zni. Adding N2 gas during growth may then stabilize these defects in the films. In addition, the Raman peaks at 277 cm−1 and 511 cm−1 have been suggested to originate from the complex defects of Zni–NO and Zni–Oi [27], respectively, while the peak at 644 cm−1 may be due to a complex defect of Zni coupled with N dopants [28]. It is clear that the intrinsic defects deduced from the Raman results are in agreement with those from the PL results.
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Fig. 5. The saturation magnetization and the relative change in the c-axis lattice constant as a function of film thickness for the ZnO:N films. Fig. 3. Raman spectra of the ZnO:N films. Peaks from the sapphire substrate are indicated by *.
The purity of samples has been characterized by means of XPS, which provides a detection limit down to 0.1 at.%. In the XPS survey scans (not shown here) of the ZnO:N films, detectable elements are Zn, O, and carbon. Within the XPS detection limit, no magnetic impurities can be observed in the films. Fig. 4(a) shows the M–H curves of ZnO:N films at RT, wherein the diamagnetic contribution from the sapphire substrate has been subtracted from the raw data. All of the films exhibited a dominant paramagnetic (PM) response. After subtracting a positive slope (PM response) for each curve, we obtained a weak FM signal for all of the films [see Fig. 4(b)]. The weak FM with low remanence and coercivity (66–86 Oe), as displayed in the inset of Fig. 4(b), is often observed in ZnO-based diluted magnetic oxides. As previously mentioned, theoretical investigations have suggested that the VZn, Oi and NO in ZnO may result in the formation of a local magnetic moment. In our PL and Raman results, the corresponding emissions and vibrational peaks have been observed. We argue that the RT-FM in ZnO:N film is due to the existence of VZn, Oi and NO. It should be pointed out that the c-sapphire substrate also exhibits a weak ferromagnetic signal (not shown here) with a Ms value of ~ 4.8 × 10 −4 emu/cm 3. We use a reference density of 3.97 g/cm 3 of
single crystal sapphire to convert the unit of Ms from emu/cm 3 to emu/g. The calculated value of 12 × 10 −5 emu/g is very close to that reported in the literature (~ 14 × 10 −5 emu/g) [29]. Nevertheless, all the ZnO:N samples have stronger ferromagnetic signals than that of the pure substrate. After the subtraction of the saturation moment from the contribution of the substrate, the Ms values of the ZnO:N films with thicknesses of 352, 715, and 1100 nm are 1.34, 0.06, and 1.15 emu/cm 3, respectively. From the PL result shown in Fig. 2, we observed that the peak intensity of the VZn-related emission (peak 4) for the 352-nm thick film is three-fourths the emission intensity for the 715-nm thick film, while the peak intensity of the Oi-related emission (peak 7) for the 352-nm thick film is nearly equal to the peak intensity for the 715-nm thick film. The significant decrease in the Ms value with respect to an increase in the film thickness from 352 to 715 nm is possibly due to the decrease in VZn concentration [8,12]. Zubiaga et al. [30] observed that the concentration of VZn in ZnO film decreases with an increase in the film thickness. It should be noted that the Ms value for the 1100-nm thick film is larger than the Ms value for the 715-nm thick film. The PL results show that the peak intensity of the Oi-related emission for the 1100-nm thick film is approximately three times the peak intensity for the 715-nm thick film. Although the greater amount of Oi in the 1100-nm thick film is unknown at this time, it may be the cause for the increase in the Ms value.
Fig. 4. (a) M–H curves of the ZnO:N films at the RT. After subtracting a PM response from each curve, the hysteresis loops are displayed in (b). The inset presents the magnified data at low fields.
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It should be noticed that hydrogen is an inevitable and unintentional impurity in ZnO and its contribution to FM cannot be ruled out [31]. Besides, we observed a close relationship between the Ms value and the relative change in the c-axis lattice constant (relative to cbulk, 5.206 Å), as depicted in Fig. 5. Based on the PL and Raman results, we deduce that the increasing amount of Oi is responsible for the abnormal increase in the cfilm value as the film thickness increases from 715 nm to 1100 nm. Recently, Schoofs et al. [14] observed a parabola-like dependence of the Ms value on the relative change in the c-axis lattice constant in the pure and low Mn-doped ZnO films and ascribed the variation in the c-axis lattice constant to a change in the concentration of the determinant native defects. The results of this study support the argument that the lattice strain caused either by a variation in the amounts of determinant native defects [12–14] or by a dopant [15] could modulate the Ms of the ZnO film. However, further studies on the relationship between the defects and the lattice parameters in ZnO films and nanostructures are necessary, and the relevant investigations are ongoing. 4. Conclusion
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17]
We investigated the thickness dependence of the defect-related room-temperature ferromagnetism in ZnO:N films deposited using a magnetron sputtering method under N2 atmosphere. The PL and Raman spectra results reveal that the VZn, Oi and NO defects are present in the ZnO:N films. The abnormal increase in the cfilm value results from an increase in the amount of Oi as the film thickness increased from 715 nm to 1100 nm. Additionally, there is a close relationship between the Ms and the relative change in the c-axis lattice constant in the ZnO:N films. Based on the results, we conclude that the observed RT-FM in ZnO:N films is mainly due to VZn, Oi and NO defects, and the variation in the Ms value is associated with the relative change in the c-axis lattice constant.
[18] [19]
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
Acknowledgments [30]
The authors would like to thank the National Science Council for financial support (Grant No. NSC-99-2112-M-390-006).
[31]
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Intrinsic defects responsible for the anomalous Raman peaks and the room-temperature ferromagnetism in nitrogen-doped ZnO thin films Ching-Chung Wang a, Chao-Ming Fu a, Yu-Min Hu b,⁎ a b
Department of Physics, National Taiwan University, Taipei 10617, Taiwan, ROC Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan, ROC
a r t i c l e
i n f o
Available online 23 November 2012 Keywords: Zinc oxide Nitrogen Intrinsic defect Photoluminescence Ferromagnetism
a b s t r a c t This work aims to investigate the intrinsic defects responsible for the anomalous Raman peaks and the room-temperature ferromagnetism in the nitrogen-doped ZnO (ZnO:N) thin films deposited using a magnetron sputtering method. The X-ray diffraction results indicate that all of the films are grown with a highly c-axis-oriented wurtzite structure. All photoluminescence spectra exhibit an intensive broad band centered at approximately 3.2 eV as well as a weak band at approximately 2.2 eV. By using a multi-peak fitting method, we deduce that the zinc vacancies, the zinc interstitials, and the substituted nitrogen at the oxygen site are responsible for the emissions embedded in the intensive broad band, while the oxygen vacancies and the oxygen interstitials are responsible for the weak band. These defects in the ZnO:N films may be associated with the four anomalous Raman peaks at approximately 277, 511, 584 and 644 cm −1. Room-temperature ferromagnetism has been observed for all of the ZnO:N films. With an increase in film thickness, the saturation magnetization first decreases rapidly and then increases. A close relationship between the saturation magnetization and the relative change in the c-axis lattice constant is observed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the room-temperature ferromagnetism (RT-FM) in ZnO-based diluted magnetic semiconductors has received much attention regarding fundamental research and possible applications in spintronic devices. Despite considerable theoretical and experimental efforts, both the origin and coupling mechanism of the magnetic moments in the transition metal (TM)-doped ZnO are still matters of debate [1–3]. Meanwhile, several researchers have reported RT-FM in undoped ZnO films [4], nitrogen (N)-doped ZnO (ZnO:N) films [5], carbon (C)-doped ZnO films [6] and nano-needles [7], implying that the magnetic moments may arise solely from the native defects irrespective of the TM ions. Wang et al. [8] performed a comprehensive theoretical investigation on the vacancy-induced magnetism in ZnO thin films and argued that the magnetic moments stem from the unpaired 2p electrons at the oxygen sites surrounding the zinc vacancies (VZn). Based on the ab initio calculations, Zuo et al. [9] predicted that the VZn and interstitial oxygen (Oi) in ZnO may result in the formation of 1.77 and 2.0 μB local moments, respectively. In addition to the intrinsic defects mentioned above, the substitution of oxygen by nitrogen (NO) has been predicted to introduce a magnetic moment of 1.0 μB in the ZnO:N film [10,11]. The local magnetic moments can mainly arise from the 2p orbitals of N and couple ferromagnetically with each other. Qualitatively, these theoretical results may explain ⁎ Corresponding author. E-mail address: [email protected] (Y.-M. Hu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.11.044
the observed RT-FM in the undoped or the nonmagnetic-elementdoped ZnO films. In view of the defect-induced magnetism, one may suspect that the saturation magnetization (Ms) is positively proportional to the determinant defect concentration. The correlation between the Ms and the defect concentration has already been proposed by several other researchers [12–14]. Note that the determinant defect concentration may vary with the film thickness [12] and depend not only on the deposition method and the ambient growth but also on the dopant [15]. In this study, we prepared ZnO:N films with the radiofrequency (r.f.) magnetron sputtering method under N2 atmosphere and examined the thickness dependence of the defect-related RT-FM. In particular, we report a correlation between the Ms and the relative change in the c-axis lattice constant of ZnO:N films. Possible defects responsible for the RT-FM in ZnO:N films will be discussed in detail. 2. Experimental details The ZnO:N films were deposited using a ceramic ZnO (99.99% purity) target onto the c-plane (0001) sapphire (Al2O3) substrates at 400 °C by r.f. magnetron sputtering in an ambient mixture of Ar and N2 in an ultra-high vacuum system (Model: Pspu-100HC, Advanced System Technology Co.) at a pressure of ~ 2.66 Pa. The mixture of ultra-pure (99.999%) Ar and N2 gasses was introduced via two independent mass flow controllers, each with a flow rate of 30 sccm (sccm denotes cubic centimeters per minute at standard temperature and pressure). The r.f. power of the ZnO target was maintained at
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100 W, and the growth time of the film was selected to be 2, 4 and 6 h, producing films with thicknesses of 352, 715 and 1100 nm, respectively. The deposition rate of all of the films was approximately 3 nm per minute. The crystal structure was determined by X-ray diffraction (XRD, Rigaku Ultima IV) with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 VersaProbe spectrometer with a monochromatized Al Kα X-rays at hν = 1486.6 eV. Room-temperature photoluminescence (RT-PL) measurement was performed on a micro-Raman system (HORIBA HR800) using the 325 nm line of a He–Cd laser. The Raman spectra were recorded in the backscattering configuration using a Jobin-Yvon LabRam-HR spectrometer with an incident 633-nm He–Ne laser. The magnetic properties were probed by a superconducting quantum interference device (Quantum Design).
3. Results and discussion Fig. 1 shows the XRD θ–2θ scans of the ZnO:N films on a logarithmic scale. All of the films are preferentially c-axis-oriented with the wurtzite structure, as evidenced by the predominant ZnO (002) peak. No other peaks corresponding to either Zn metal or the Zn3N2 phase can be observed within the detection limit of XRD. The inset of Fig. 1 displays the variation in the c-axis lattice constant (cfilm) of the films with different thicknesses. The cfilm values were calculated directly from the position of the ZnO (002) peak using the Bragg law. All of the cfilm values are slightly larger than that of bulk ZnO. One may expect that the cfilm value would decrease with the film thickness due to strain relaxation; however, the cfilm value of 1100-nm thick film is slightly larger than the cfilm value of 715-nm thick film. According to the PL results shown below, we suggest that the abnormal increase in the cfilm value of 1100-nm thick film is due to an increase in the amounts of Oi. Fig. 2 displays the RT-PL spectra of the ZnO:N films. All of the films exhibit an intense broad band and a weak band at approximately 3.2 eV and 2.2 eV, respectively. The results of multi-peak-fitting are shown in the inset of Fig. 2, where the energy levels of the point defects used in the Gaussian fitting are based on a recent review article on ZnO [16]. The intense broad emission band is well fitted by five peaks. Peak 1 is located at 3.27 eV and is associated with the free exciton (FX) [17]. Peak 2 is located at 3.20 eV, with an energy difference of 70 meV from peak 1, and can be assigned to the first longitudinal phonon replica of the FX [18]. The other peaks may be attributed to defect-related emissions. Meyer et al. [19] proposed that NO may
Fig. 1. XRD θ–2θ scans of the ZnO:N films on a logarithmic scale. The inset illustrates the variation in the calculated c-axis lattice constant with different film thicknesses.
Fig. 2. RT-PL spectra of the ZnO:N films. The insets show the multi-peak fitting results.
induce a shallow acceptor level at approximately 165 meV above the valence band. Look et al. [20] suggested that the trapping of zinc interstitial (Zni) near a NO, forming the Zni–NO donor complex, is a shallow donor at approximately 30 meV below the conduction band. Accordingly, peak 3, which is weak and located at 3.14 eV, may be attributed to a donor-acceptor pair recombination that occurs between the Zni–NO shallow donor and NO acceptor levels [21]. Additionally, peak 4 is located at 3.07 eV and may be assigned as a transition from the conduction band to the VZn level. A theoretical calculation has predicted that the VZn level is located 0.3 eV above the valence band [22]. Moreover, peak 5, which is broad and located at 3.02 eV, might originate from the electron transition between the band tail state levels of the surface defects and/or the lattice imperfections [23]. In addition, peak 6 located at 2.24 eV and peak 7 at 2.01 eV may be associated with VO [24] and Oi [25], respectively. The Oi-related emission intensity of the 1100-nm thick film is larger than that of the 715-nm thick film, indicating a greater amount of Oi in the 1100-nm thick film. Fig. 3 shows the Raman spectra of the ZnO:N films with different thicknesses. The peaks indicated by asterisks are due to scatterings from the sapphire substrate. The peaks at 333 cm−1 and 438 cm−1 have been assigned to the optical phonon 2E2(M) mode and E2(high) mode of the ZnO crystal lattice [26], respectively. Notably, four anomalous Raman peaks at frequencies of ~277, 511, 584 and 644 cm−1 are present in all of the films and their intensities are positively proportional to the film thickness. The presence of four anomalous Raman peaks is associated with the existence of point defects. The Raman peak at 584 cm−1 is assigned to the E1(LO) mode, which may be related to intrinsic host defects, such as VO and Zni. Adding N2 gas during growth may then stabilize these defects in the films. In addition, the Raman peaks at 277 cm−1 and 511 cm−1 have been suggested to originate from the complex defects of Zni–NO and Zni–Oi [27], respectively, while the peak at 644 cm−1 may be due to a complex defect of Zni coupled with N dopants [28]. It is clear that the intrinsic defects deduced from the Raman results are in agreement with those from the PL results.
C.-C. Wang et al. / Surface & Coatings Technology 231 (2013) 307–310
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Fig. 5. The saturation magnetization and the relative change in the c-axis lattice constant as a function of film thickness for the ZnO:N films. Fig. 3. Raman spectra of the ZnO:N films. Peaks from the sapphire substrate are indicated by *.
The purity of samples has been characterized by means of XPS, which provides a detection limit down to 0.1 at.%. In the XPS survey scans (not shown here) of the ZnO:N films, detectable elements are Zn, O, and carbon. Within the XPS detection limit, no magnetic impurities can be observed in the films. Fig. 4(a) shows the M–H curves of ZnO:N films at RT, wherein the diamagnetic contribution from the sapphire substrate has been subtracted from the raw data. All of the films exhibited a dominant paramagnetic (PM) response. After subtracting a positive slope (PM response) for each curve, we obtained a weak FM signal for all of the films [see Fig. 4(b)]. The weak FM with low remanence and coercivity (66–86 Oe), as displayed in the inset of Fig. 4(b), is often observed in ZnO-based diluted magnetic oxides. As previously mentioned, theoretical investigations have suggested that the VZn, Oi and NO in ZnO may result in the formation of a local magnetic moment. In our PL and Raman results, the corresponding emissions and vibrational peaks have been observed. We argue that the RT-FM in ZnO:N film is due to the existence of VZn, Oi and NO. It should be pointed out that the c-sapphire substrate also exhibits a weak ferromagnetic signal (not shown here) with a Ms value of ~ 4.8 × 10 −4 emu/cm 3. We use a reference density of 3.97 g/cm 3 of
single crystal sapphire to convert the unit of Ms from emu/cm 3 to emu/g. The calculated value of 12 × 10 −5 emu/g is very close to that reported in the literature (~ 14 × 10 −5 emu/g) [29]. Nevertheless, all the ZnO:N samples have stronger ferromagnetic signals than that of the pure substrate. After the subtraction of the saturation moment from the contribution of the substrate, the Ms values of the ZnO:N films with thicknesses of 352, 715, and 1100 nm are 1.34, 0.06, and 1.15 emu/cm 3, respectively. From the PL result shown in Fig. 2, we observed that the peak intensity of the VZn-related emission (peak 4) for the 352-nm thick film is three-fourths the emission intensity for the 715-nm thick film, while the peak intensity of the Oi-related emission (peak 7) for the 352-nm thick film is nearly equal to the peak intensity for the 715-nm thick film. The significant decrease in the Ms value with respect to an increase in the film thickness from 352 to 715 nm is possibly due to the decrease in VZn concentration [8,12]. Zubiaga et al. [30] observed that the concentration of VZn in ZnO film decreases with an increase in the film thickness. It should be noted that the Ms value for the 1100-nm thick film is larger than the Ms value for the 715-nm thick film. The PL results show that the peak intensity of the Oi-related emission for the 1100-nm thick film is approximately three times the peak intensity for the 715-nm thick film. Although the greater amount of Oi in the 1100-nm thick film is unknown at this time, it may be the cause for the increase in the Ms value.
Fig. 4. (a) M–H curves of the ZnO:N films at the RT. After subtracting a PM response from each curve, the hysteresis loops are displayed in (b). The inset presents the magnified data at low fields.
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It should be noticed that hydrogen is an inevitable and unintentional impurity in ZnO and its contribution to FM cannot be ruled out [31]. Besides, we observed a close relationship between the Ms value and the relative change in the c-axis lattice constant (relative to cbulk, 5.206 Å), as depicted in Fig. 5. Based on the PL and Raman results, we deduce that the increasing amount of Oi is responsible for the abnormal increase in the cfilm value as the film thickness increases from 715 nm to 1100 nm. Recently, Schoofs et al. [14] observed a parabola-like dependence of the Ms value on the relative change in the c-axis lattice constant in the pure and low Mn-doped ZnO films and ascribed the variation in the c-axis lattice constant to a change in the concentration of the determinant native defects. The results of this study support the argument that the lattice strain caused either by a variation in the amounts of determinant native defects [12–14] or by a dopant [15] could modulate the Ms of the ZnO film. However, further studies on the relationship between the defects and the lattice parameters in ZnO films and nanostructures are necessary, and the relevant investigations are ongoing. 4. Conclusion
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We investigated the thickness dependence of the defect-related room-temperature ferromagnetism in ZnO:N films deposited using a magnetron sputtering method under N2 atmosphere. The PL and Raman spectra results reveal that the VZn, Oi and NO defects are present in the ZnO:N films. The abnormal increase in the cfilm value results from an increase in the amount of Oi as the film thickness increased from 715 nm to 1100 nm. Additionally, there is a close relationship between the Ms and the relative change in the c-axis lattice constant in the ZnO:N films. Based on the results, we conclude that the observed RT-FM in ZnO:N films is mainly due to VZn, Oi and NO defects, and the variation in the Ms value is associated with the relative change in the c-axis lattice constant.
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The authors would like to thank the National Science Council for financial support (Grant No. NSC-99-2112-M-390-006).
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