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Observation of ferromagnetism at room temperature for Cr+ ions implanted ZnO thin films
Applied Surface Science 253 (2007) 8524–8529 www.elsevier.com/locate/apsusc
Observation of ferromagnetism at room temperature for Cr+ ions implanted ZnO thin films H. Li a,b, J.P. Sang a,*, F. Mei a, F. Ren a, L. Zhang a, C. Liu a a
Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, China b State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 200050, China Received 30 November 2006; accepted 16 April 2007 Available online 22 April 2007
Abstract Single crystalline ZnO films were grown on c-plane GaN/sapphire (0 0 0 1) substrates by molecular beam epitaxy. Cr+ ions were implanted into the ZnO films with three different doses, i.e., 1 1014, 5 1015, and 3 1016 cm 2. The implantation energy was 150 keV. Thermal treatment was carried out at 800 8C for 30 s in a rapid thermal annealing oven in flowing nitrogen. X-ray diffraction (XRD), atomic force microscopy, Raman measurements, transmission electron microscopy and superconducting quantum interference device were used to characterize the ZnO films. The results showed that thermal annealing relaxed the stress in the Cr+ ions implanted samples and the implantation-induced damage was partly recovered by means of the proper annealing treatment. Transmission electron microscopy measurements indicated that the first five monolayers of ZnO rotated an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. The magnetic-field dependence of magnetization of annealed ZnO:Cr showed ferromagnetic behavior at room temperature. # 2007 Elsevier B.V. All rights reserved. PACS : 61.72.Vv; 75.50.Pp; 78.30.Fs Keywords: ZnO; Ion implantation; Thermal annealing; Ferromagnetism
1. Introduction Oxides have many excellent properties and are widely used in many fields, such as ferroelectricity, high permittivity, superconductivity, magnetism and photoelectricity. Among them, ZnO is a promising material for electronic and optoelectronic devices [1–3] due to its low dielectric constants, high chemical stabilizations, wide and direct band gap (3.37 eV), large excitonic binding energy of 60 meV, and good piezoelectric properties. Ion implantation is a powerful tool to selectively dope ZnO thin films and to change their electrical [4,5], magnetically [3], and optical [6] properties. It has been found that ZnO is significantly more resistant than other compound semiconductors including GaN, which is a main rival with similar structural and optical properties, by the irradiation with electrons [7–10], protons [11,12], and heavier
* Corresponding author. Tel.: +86 27 8422 5507; fax: +86 27 8422 5507. E-mail address: [email protected] (J.P. Sang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.028
ions [13,14]. Ion implantation has advantage to easily introduce different transition metals into ZnO, followed by thermal treatment that is usually necessary to remove the implant damage and to activate the dopants. Kucheyev et al. [13] studied the damage built-up in ZnO thin films using Au and Si ion implantation at room temperature and 77 K. Strong dynamic annealing was observed and ZnO thin film was not rendered amorphous even with high dose Au ion implantation. Jeong et al. [15] study the annealing effect of low-energy As ion implanted ZnO film at a dose of 1015 cm 2. They found an optimum annealing condition at 800 8C for 1 h to fully recover the damage. On the other hand, as a most pronounced diluted magnetic semiconductors (DMS) candidate, ZnO with magnetic doping has attracted considerable attention recently. By ab initio calculation based on the local density approximation, it is predicted that doping with transition metal atoms of V, Cr, Fe, Co and Ni into ZnO may produce ferromagnetic behavior without any additional carrier doping treatment [14]. Recent experiments approved the hypothesis [16,17].
H. Li et al. / Applied Surface Science 253 (2007) 8524–8529
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In this study, we present the experimental results on evaluation of crystal quality and interfacial microstructure of the Cr+ ions implanted ZnO samples. We also discuss the annealing effect on stress relaxation and damage recovery and the magnetic characteristics of the ZnO:Cr samples. 2. Experimental The ZnO wafer with a thickness of 1 mm used in the study was grown on a single crystalline GaN template that was grown on sapphire (0 0 0 1) plane. Both ZnO and GaN layers were deposited by using molecular beam epitaxy. The ZnO wafer was cut into four pieces. The former three were implanted with Cr+ ions under different doses of 1 1014, 5 1015, and 3 1016 cm 2 (5 at.% Cr-doped), while the last one was kept unimplanted as a reference. The implantations were conducted with the energy of 150 keV at the room temperature. After implantation, the samples were annealed in a rapid thermal annealing (RTA) oven in flowing nitrogen to recover the implant damage and to drive Cr atoms into the substitutional sites. The annealing temperature was 800 8C, and the annealing time was 30 s. During annealing, the samples were proximity capped with ZnO epilayers (grown on GaN/sapphire) to suppress the escape of oxygen. D8 Advanced X-ray diffraction (XRD) analysis was performed by employing a high-resolution diffractometer with Cu Ka radiation, operated at a voltage of 40 kV and a current of 40 mA. The surface morphological properties of all the samples were characterized by atomic force microscopy (AFM). MicroRaman spectra were measured in the back scattering geometry in a range of 200–800 cm 1, using the 514.4 nm excitation lines from an Ar+ laser and a Renishaw RM 1000 spectrometer. Transmission electron microscope (TEM) measurements were carried out using a JEM-2010FEF with an accelerating voltage of 200 kV. The specimens for the cross-sectional TEM study were prepared by traditional cutting, polishing, dimpling and milling procedures. Magnetization measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer.
Fig. 1. X-ray diffraction patterns of the as-grown ZnO film on GaN/sapphire. The inset was the X-ray u-rocking curve for the (0 0 0 2)-ZnO peak of the asgrown sample. The FWHM of the rocking curve was 0.428.
reflecting lattice disorder caused by the implantation. In addition, it has been found that the diffraction angle (u) decreased after annealing, indicating that the annealing relaxes the stress in the implanted ZnO sample. The intensity of the ZnO (0 0 0 2) peak of the annealed sample is stronger than that of the implanted sample; while the FWHM value of the annealed sample is smaller (0.4268) than that of the asimplanted sample (0.4438). The phenomena indicate that the crystallinity of the ZnO film is improved by annealing. Therefore, the annealing process can not only relax the stress in the Cr+ implanted sample, but also recover the implant damage. The thermally induced damage recovery could be also confirmed by AFM measurements. To clarify this, we studied the surface morphology of as-implanted and annealed samples (3 1016 cm 2), with the as-grown sample being a reference. Fig. 3(a)–(c) show AFM images (2 mm 2 mm) for the asgrown, as-implanted and annealed sample (3 1016 cm 2).
3. Results and discussion Fig. 1 shows a typical XRD diffraction pattern of the asgrown ZnO film grown on GaN/sapphire substrates. The spectrum shows clearly pronounced ZnO (0 0 0 2) diffraction peak located at 34.48 and ZnO (0 0 0 4) diffraction peak located at around 738, indicating that the film has a strong c-axis preferred orientation. The (0 0 0 6) diffraction peak corresponding to the sapphire (0 0 0 1) substrates is also clearly observed. The inset is XRD u-rocking curve of the ZnO (0 0 0 2) diffraction peak of the as-grown ZnO film. The fullwide at half-maximum (FWHM) value of the sample is 0.4208, which indicates that the mosaicity of the film is relatively small and the crystal quality is high. Fig. 2 shows a comparison of (0 0 0 2) rocking curves of the as-implanted sample and annealed sample (3 1016 cm 1). After Cr+ implantation into ZnO film, the FWHM value increases from 0.4208 to 0.4438,
Fig. 2. X-ray u-rocking curves for the (0 0 0 2)-ZnO peak of as-implanted sample and annealed sample (3 1016 cm 2).
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Fig. 4. Raman spectra of the as-grown ZnO sample and as-implanted samples (1 1014 cm 2, 5 1015 cm 2) measured at room temperature.
Fig. 3. (a–c) are AFM images for the as-grown ZnO sample, the as-implanted and the annealed sample (3 1016 cm 2). The surface roughness (rms) for each is about 7.5, 7.8 and 8.2 nm, respectively.
The surface of the as-grown sample is very smooth and the root mean square (rms) roughness is about 7.5 nm. After the implantation, the rms roughness increases to 7.8 nm. However, when the sample is annealed at 800 8C for 30 s in flowing N2 in a RTA apparatus, the grains grow up and the surface becomes rougher (rms roughness is 8.2 nm) than that of the as-grown and as-implanted sample as shown in Fig. 3(c). Therefore, the thermal annealing can change significantly the surface morphology, increasing the grain size and improving the crystallinity of the Cr+ implanted samples. Raman spectra were obtained in the backscattering configuration with incident light perpendicular to the ZnO 4 samples. Wurtzite ZnO belongs to the space group C6v . At the
Fig. 5. Room temperature Raman spectra of the as-implanted and annealed samples, (a): 1 1014 cm 2, (b): 5 1015 cm 2, and (c): 3 1016 cm 2.
H. Li et al. / Applied Surface Science 253 (2007) 8524–8529
G-point of the Brillouin zone, optical phonons have Gopt = A1 + 2B1 + E1 + 2E2 where, A1 and E1 modes belong to polar symmetries and can have different transverse (TO) and longitudinal (LO) optical phonon frequencies, all being Raman active, while the B1 modes are silent. Fig. 4 presents the room temperature Raman spectra of the as-grown sample and the asimplanted samples with different doses exited by a Ar+ laser working at 514.4 nm. Note that the highly doped ZnO sample (3 1016 cm 2) did not exhibit any significant Raman signals. For all samples, peaks around 327, 380 cm 1 are assigned to the vibrational modes of E2L , A1 (TO), respectively. Based on the reported zone-center optical phonon frequencies in ZnO [18], the Raman peak at 435 cm 1 should be assigned to the E2H mode. The observation of these modes indicates that all samples have the wurtzite structure according to the wellknown selection rules. The peak located at 567 cm 1 is assigned to A1 (LO) mode. This mode is associated with the defects of O-vacancy, Zn-interstice, or their complexes. It is found that the peak of A1 (LO) mode in each spectrum is sharp and intense, suggesting that the ZnO film was grown in Zincrich condition, which induced that many lattice defects of Ovacancy, Zn-interstice, or their complexes existed in the ZnO sample. These high density lattice defects enhance the vibration of the A1 (LO) mode. From Fig. 4, we can see clearly that with increasing dose of the Cr+ ions, a significant increase of intensities and broadening of the bands at 567 cm 1 are noticed in the as-implanted samples. This change is related to the surface damage caused by the energetic Cr+ implantation. Therefore, the surface damage strongly affects the Raman intensity and the width of the peaks. Besides, we also observed
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the A1 (LO) mode of GaN located at 733 cm 1. Fig. 5 shows the Raman spectra of the as-implanted and annealed samples (1 1014, 5 1015 and 3 1016 cm 2). As shown in Fig. 5(a) and (b), the intensity of the A1 (LO) decreases and the intensity of E2H increases while its FWHM decreases after annealing at 800 8C for 30 s. It is worthy to emphasize that the peaks of E2H and A1 (LO) recur after thermal annealing as indicated in Fig. 5(c). This means that the crystallinity of the surface damage was partly recovered by means of the annealing treatment. Fig. 6 shows the TEM images of annealed sample (3 1016 cm 2). Fig. 6(a) is the cross-sectional TEM image of the as-grown ZnO film grown on a GaN/sapphire substrate. It is clear to see the top ZnO preferentially oriented film, the transitional layer, GaN layer and the sapphire substrate. Some dark fringes in the film show the existence of threading dislocations which are perpendicular to the interface. Fig. 6(b) is a selected area electron diffraction (SAED) pattern from the transitional layer including both ZnO film and the GaN film, in which 1 and 2 are GaN (01–10) and (0 0 0 1) reflections, and a and b are ZnO (01–10) and (0 0 0 1) reflections. The electron beam parallel to the [2–1–10] zone axis of ZnO or GaN. From Fig. 6(b), we can conclude the perfect main epitaxial relationship of the sample: ZnO (0 0 0 1)[2–1–10]//GaN (0 0 0 1) [2–1–10]. The atomic arrangement in the c-plane of ZnO and GaN is shown in Fig. 6(c). No in-plane rotation of ZnO lattice with respect to the GaN buffer layer was found. A cross-sectional high-resolution TEM study was carried out to determine the structure of ZnO/GaN interface. Fig. 7 shows the high-resolution TEM images of the sample taken in
Fig. 6. (a) The cross-sectional TEM image of the annealed sample. (b) The SAED pattern of transitional layer of annealed sample with electron beam parallel to the [2–1–10] zone axis of ZnO or GaN, in which 1 and 2 are GaN (01–10) and (0 0 0 1) reflections and a and b are ZnO (01–10) and (0 0 0 1) reflections. (c) Illustration of the main domain orientation relationship of ZnO on GaN buffer layer.
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Fig. 7. High-resolution TEM images of (a) the ZnO layer and (b) ZnO/GaN interface along [2–1–10] zone axis of ZnO. (c) Fourier filtered image corresponding to rectangle region of (b).
the [2–1–10] zone axis of ZnO. The high crystalline quality of the ZnO layer can be directly reflected in the high resolution image of Fig. 7(a), from which we could see the atomic arrangement clearly. Lattice mismatch between the ZnO film and the GaN buffer layer results in a high density of misfit dislocation at the interface. Fig. 7(b) shows the high-resolution TEM image of ZnO/GaN interface as indicated by arrows. Lots of dislocations can be observed in the high-resolution TEM image. The Fourier filtered image of the rectangle region of Fig. 7(b) gives a closer look at the interfacial area as shown in Fig. 7(c). Along the interface (indicated by the white arrow), the atom positions are highly displaced, suggesting the existence of a high strain in the film. As can be seen in Fig. 7(c), the first five monolayers’ atoms in the interfacial layer rotate an angle off the
[0 0 0 1]-axis of the GaN in the interfacial layer. Kong et al. [19] had investigated the interface structural properties of the ZnO/GaN interface. They confirmed the interfacial layer was the monoclinic Ga2O3 phase by a comparison of the experimentally obtained diffraction pattern with a simulated diffraction pattern. In our work, we conjecture that the first five atomic layers may form another phase (such as monoclinic Ga2O3) or just be the lattice faults. Whether the interfacial layer formed another phase or not and the mechanism of formation of the first five atomic layers are interesting topics for future study. The magnetic properties of the annealed ZnO:Cr sample (3 1016 cm 2) were studied using SQUID magnetometer. The measurements were carried out at room temperature (300 K). Fig. 8 shows room temperature hysteresis present in
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of the interfacial layer and the corresponding Fourier filtered image, we can observe that the first five monolayers’ atoms in the interfacial layer rotate an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. We conjecture that the first five atomic layers may form other phase (such as monoclinic Ga2O3) or just be the lattice faults. The result of the magnetic measurement indicates that the annealed ZnO:Cr sample with the dose of 3 1016 cm 2 exhibit ferromagnetic behavior at room temperature. Acknowledgements
Fig. 8. Hysteresis curve for the annealed ZnO:Cr sample with the dose of 3 1016 cm 2 at room temperature.
This work was supported by the NSFC for the grants under Nos. 10345006, 10475063, and by HBSF under No. 2004ABA079, as well as by NCET and SRF for ROCS, Ministry of Education. References
the 5 at.% Cr-implanted and annealed ZnO sample. The background contribution to the magnetization originated from the GaN buffer layer and the diamagnetism of the sapphire substrate were subtracted. The saturation magnetization and residual magnetization are 1.699 10 5 and 5.060 10 6 emu, respectively. The coercive force in the sample is 100.564 Oe. These results confirm that the ZnO:Cr sample is ferromagnetic at room temperature. The magnetic properties of the as-implanted and annealed samples with lower implanted doses were also measured. But they did not exhibit any ferromagnetic behavior. The reasons are that the low-dosage implantation did not make the Cr+ ions concentration reach the critical point of ZnO-based DMS and the annealing treatment partly removed the implant damage and activated the magnetic dopants. 4. Conclusions In summary, high quality ZnO film grown by MBE was implanted with Cr+ ions under three different doses. Then, anneal experiments have been conducted to induce the lattice damage recovery. The XRD, AFM and Raman measurements show that the crystallinity of the Cr-implanted sample was improved by thermal treatment. TEM image and SAED pattern show that the main epitaxial relationship is perfect, i.e. ZnO (0 0 0 1)[2–1–10]//GaN(0 0 0 1)[2–1–10]. High-resolution TEM image of the ZnO layer confirms the high crystalline quality of the ZnO film. From the high-resolution TEM image
[1] D.C. Look, Mater. Sci. Eng., B 80 (2001) 383. [2] D.C. Look, B.Ya. Claflin, I. Alivov, S.J. Park, Phys. Status Solidi A 201 (2004) 2203. [3] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J. Vac. Sci. Technol. B 22 (2004) 932. [4] T.S. Jeong, M.S. Han, C.J. Youn, Y.S. Park, J. Appl. Phys. 96 (2004) 175. [5] F. Reus, C. Kirchner, Th. Gruber, R. Kling, S. Maschek, W. Limmer, A. Waag, P. Ziemann, J. Appl. Phys. 95 (2004) 3385. [6] E. Alves, E. Rita, U. Wahl, J.G. Correia, T. Monteiro, J. Soares, C. Boemare, Nucl. Instrum. Methods Phys. Res. B 206 (2003) 1047. [7] D.C. Look, J.W. Hemsky, J.R. Sizelove, Phys. Rev. Lett. 82 (1999) 2552. [8] F. Tuomisto, V. Ranki, K. Saarinen, D.C. Look, Phys. Rev. Lett. 91 (2003) 205502. [9] Yu.V. Gorelkinskii, G.D. Watkins, Phys. Rev. B: Condens. Matter 69 (2004) 115212. [10] C. Coskun, D.C. Look, G.C. Farlow, J.R. Sizelove, Semicond. Sci. Technol. 19 (2004) 752. [11] F.D. Auret, S.A. Goodman, M. Hayes, M.J. Legodi, H.A. van Laarhoven, D.C. Look, Appl. Phys. Lett. 79 (2001) 3074. [12] A.Y. Polyakov, N.B. Smirnov, A.V. Govorkov, E.A. Kozhukhova, V.I. Vdovin, K. Ip, M.E. Overberg, Y.W. Heo, D.P. Norton, S.J. Pearton, J.M. Zavada, V.A. Dravin, J. Appl. Phys. 94 (2003) 2895. [13] S.O. Kucheyev, J.S. Williams, C. Jagadish, J. Zou, C. Evans, A.J. Nelson, A.V. Hamza, Phys. Rev. B: Condens. Matter 67 (2003) 094115. [14] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39 (2000) L555. [15] S. Jeong, M.S. Han, J.H. Kim, C.J. Youn, R.Y. Ryu, H.W. White, J. Cryst. Growth 275 (2005) 541. [16] H. Saeki, H. Tabata, T. Kawai, Solid State Commun. 120 (2001) 439. [17] M. Sugiura, K. Uragou, M. Tachiki, T. Kobayashi, J. Appl. Phys. 90 (2001) 187. [18] C.A. Arguello, D.L. Rousseau, S.P.S. Porto, Phys. Rev. 181 (1969) 1351. [19] S.K. Hong, H.J. Ko, Y. Chen, T. Hanada, T. Yao, J. Vac. Sci. Technol. B 18 (2000) 2313.
Observation of ferromagnetism at room temperature for Cr+ ions implanted ZnO thin films H. Li a,b, J.P. Sang a,*, F. Mei a, F. Ren a, L. Zhang a, C. Liu a a
Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, China b State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 200050, China Received 30 November 2006; accepted 16 April 2007 Available online 22 April 2007
Abstract Single crystalline ZnO films were grown on c-plane GaN/sapphire (0 0 0 1) substrates by molecular beam epitaxy. Cr+ ions were implanted into the ZnO films with three different doses, i.e., 1 1014, 5 1015, and 3 1016 cm 2. The implantation energy was 150 keV. Thermal treatment was carried out at 800 8C for 30 s in a rapid thermal annealing oven in flowing nitrogen. X-ray diffraction (XRD), atomic force microscopy, Raman measurements, transmission electron microscopy and superconducting quantum interference device were used to characterize the ZnO films. The results showed that thermal annealing relaxed the stress in the Cr+ ions implanted samples and the implantation-induced damage was partly recovered by means of the proper annealing treatment. Transmission electron microscopy measurements indicated that the first five monolayers of ZnO rotated an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. The magnetic-field dependence of magnetization of annealed ZnO:Cr showed ferromagnetic behavior at room temperature. # 2007 Elsevier B.V. All rights reserved. PACS : 61.72.Vv; 75.50.Pp; 78.30.Fs Keywords: ZnO; Ion implantation; Thermal annealing; Ferromagnetism
1. Introduction Oxides have many excellent properties and are widely used in many fields, such as ferroelectricity, high permittivity, superconductivity, magnetism and photoelectricity. Among them, ZnO is a promising material for electronic and optoelectronic devices [1–3] due to its low dielectric constants, high chemical stabilizations, wide and direct band gap (3.37 eV), large excitonic binding energy of 60 meV, and good piezoelectric properties. Ion implantation is a powerful tool to selectively dope ZnO thin films and to change their electrical [4,5], magnetically [3], and optical [6] properties. It has been found that ZnO is significantly more resistant than other compound semiconductors including GaN, which is a main rival with similar structural and optical properties, by the irradiation with electrons [7–10], protons [11,12], and heavier
* Corresponding author. Tel.: +86 27 8422 5507; fax: +86 27 8422 5507. E-mail address: [email protected] (J.P. Sang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.028
ions [13,14]. Ion implantation has advantage to easily introduce different transition metals into ZnO, followed by thermal treatment that is usually necessary to remove the implant damage and to activate the dopants. Kucheyev et al. [13] studied the damage built-up in ZnO thin films using Au and Si ion implantation at room temperature and 77 K. Strong dynamic annealing was observed and ZnO thin film was not rendered amorphous even with high dose Au ion implantation. Jeong et al. [15] study the annealing effect of low-energy As ion implanted ZnO film at a dose of 1015 cm 2. They found an optimum annealing condition at 800 8C for 1 h to fully recover the damage. On the other hand, as a most pronounced diluted magnetic semiconductors (DMS) candidate, ZnO with magnetic doping has attracted considerable attention recently. By ab initio calculation based on the local density approximation, it is predicted that doping with transition metal atoms of V, Cr, Fe, Co and Ni into ZnO may produce ferromagnetic behavior without any additional carrier doping treatment [14]. Recent experiments approved the hypothesis [16,17].
H. Li et al. / Applied Surface Science 253 (2007) 8524–8529
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In this study, we present the experimental results on evaluation of crystal quality and interfacial microstructure of the Cr+ ions implanted ZnO samples. We also discuss the annealing effect on stress relaxation and damage recovery and the magnetic characteristics of the ZnO:Cr samples. 2. Experimental The ZnO wafer with a thickness of 1 mm used in the study was grown on a single crystalline GaN template that was grown on sapphire (0 0 0 1) plane. Both ZnO and GaN layers were deposited by using molecular beam epitaxy. The ZnO wafer was cut into four pieces. The former three were implanted with Cr+ ions under different doses of 1 1014, 5 1015, and 3 1016 cm 2 (5 at.% Cr-doped), while the last one was kept unimplanted as a reference. The implantations were conducted with the energy of 150 keV at the room temperature. After implantation, the samples were annealed in a rapid thermal annealing (RTA) oven in flowing nitrogen to recover the implant damage and to drive Cr atoms into the substitutional sites. The annealing temperature was 800 8C, and the annealing time was 30 s. During annealing, the samples were proximity capped with ZnO epilayers (grown on GaN/sapphire) to suppress the escape of oxygen. D8 Advanced X-ray diffraction (XRD) analysis was performed by employing a high-resolution diffractometer with Cu Ka radiation, operated at a voltage of 40 kV and a current of 40 mA. The surface morphological properties of all the samples were characterized by atomic force microscopy (AFM). MicroRaman spectra were measured in the back scattering geometry in a range of 200–800 cm 1, using the 514.4 nm excitation lines from an Ar+ laser and a Renishaw RM 1000 spectrometer. Transmission electron microscope (TEM) measurements were carried out using a JEM-2010FEF with an accelerating voltage of 200 kV. The specimens for the cross-sectional TEM study were prepared by traditional cutting, polishing, dimpling and milling procedures. Magnetization measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer.
Fig. 1. X-ray diffraction patterns of the as-grown ZnO film on GaN/sapphire. The inset was the X-ray u-rocking curve for the (0 0 0 2)-ZnO peak of the asgrown sample. The FWHM of the rocking curve was 0.428.
reflecting lattice disorder caused by the implantation. In addition, it has been found that the diffraction angle (u) decreased after annealing, indicating that the annealing relaxes the stress in the implanted ZnO sample. The intensity of the ZnO (0 0 0 2) peak of the annealed sample is stronger than that of the implanted sample; while the FWHM value of the annealed sample is smaller (0.4268) than that of the asimplanted sample (0.4438). The phenomena indicate that the crystallinity of the ZnO film is improved by annealing. Therefore, the annealing process can not only relax the stress in the Cr+ implanted sample, but also recover the implant damage. The thermally induced damage recovery could be also confirmed by AFM measurements. To clarify this, we studied the surface morphology of as-implanted and annealed samples (3 1016 cm 2), with the as-grown sample being a reference. Fig. 3(a)–(c) show AFM images (2 mm 2 mm) for the asgrown, as-implanted and annealed sample (3 1016 cm 2).
3. Results and discussion Fig. 1 shows a typical XRD diffraction pattern of the asgrown ZnO film grown on GaN/sapphire substrates. The spectrum shows clearly pronounced ZnO (0 0 0 2) diffraction peak located at 34.48 and ZnO (0 0 0 4) diffraction peak located at around 738, indicating that the film has a strong c-axis preferred orientation. The (0 0 0 6) diffraction peak corresponding to the sapphire (0 0 0 1) substrates is also clearly observed. The inset is XRD u-rocking curve of the ZnO (0 0 0 2) diffraction peak of the as-grown ZnO film. The fullwide at half-maximum (FWHM) value of the sample is 0.4208, which indicates that the mosaicity of the film is relatively small and the crystal quality is high. Fig. 2 shows a comparison of (0 0 0 2) rocking curves of the as-implanted sample and annealed sample (3 1016 cm 1). After Cr+ implantation into ZnO film, the FWHM value increases from 0.4208 to 0.4438,
Fig. 2. X-ray u-rocking curves for the (0 0 0 2)-ZnO peak of as-implanted sample and annealed sample (3 1016 cm 2).
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Fig. 4. Raman spectra of the as-grown ZnO sample and as-implanted samples (1 1014 cm 2, 5 1015 cm 2) measured at room temperature.
Fig. 3. (a–c) are AFM images for the as-grown ZnO sample, the as-implanted and the annealed sample (3 1016 cm 2). The surface roughness (rms) for each is about 7.5, 7.8 and 8.2 nm, respectively.
The surface of the as-grown sample is very smooth and the root mean square (rms) roughness is about 7.5 nm. After the implantation, the rms roughness increases to 7.8 nm. However, when the sample is annealed at 800 8C for 30 s in flowing N2 in a RTA apparatus, the grains grow up and the surface becomes rougher (rms roughness is 8.2 nm) than that of the as-grown and as-implanted sample as shown in Fig. 3(c). Therefore, the thermal annealing can change significantly the surface morphology, increasing the grain size and improving the crystallinity of the Cr+ implanted samples. Raman spectra were obtained in the backscattering configuration with incident light perpendicular to the ZnO 4 samples. Wurtzite ZnO belongs to the space group C6v . At the
Fig. 5. Room temperature Raman spectra of the as-implanted and annealed samples, (a): 1 1014 cm 2, (b): 5 1015 cm 2, and (c): 3 1016 cm 2.
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G-point of the Brillouin zone, optical phonons have Gopt = A1 + 2B1 + E1 + 2E2 where, A1 and E1 modes belong to polar symmetries and can have different transverse (TO) and longitudinal (LO) optical phonon frequencies, all being Raman active, while the B1 modes are silent. Fig. 4 presents the room temperature Raman spectra of the as-grown sample and the asimplanted samples with different doses exited by a Ar+ laser working at 514.4 nm. Note that the highly doped ZnO sample (3 1016 cm 2) did not exhibit any significant Raman signals. For all samples, peaks around 327, 380 cm 1 are assigned to the vibrational modes of E2L , A1 (TO), respectively. Based on the reported zone-center optical phonon frequencies in ZnO [18], the Raman peak at 435 cm 1 should be assigned to the E2H mode. The observation of these modes indicates that all samples have the wurtzite structure according to the wellknown selection rules. The peak located at 567 cm 1 is assigned to A1 (LO) mode. This mode is associated with the defects of O-vacancy, Zn-interstice, or their complexes. It is found that the peak of A1 (LO) mode in each spectrum is sharp and intense, suggesting that the ZnO film was grown in Zincrich condition, which induced that many lattice defects of Ovacancy, Zn-interstice, or their complexes existed in the ZnO sample. These high density lattice defects enhance the vibration of the A1 (LO) mode. From Fig. 4, we can see clearly that with increasing dose of the Cr+ ions, a significant increase of intensities and broadening of the bands at 567 cm 1 are noticed in the as-implanted samples. This change is related to the surface damage caused by the energetic Cr+ implantation. Therefore, the surface damage strongly affects the Raman intensity and the width of the peaks. Besides, we also observed
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the A1 (LO) mode of GaN located at 733 cm 1. Fig. 5 shows the Raman spectra of the as-implanted and annealed samples (1 1014, 5 1015 and 3 1016 cm 2). As shown in Fig. 5(a) and (b), the intensity of the A1 (LO) decreases and the intensity of E2H increases while its FWHM decreases after annealing at 800 8C for 30 s. It is worthy to emphasize that the peaks of E2H and A1 (LO) recur after thermal annealing as indicated in Fig. 5(c). This means that the crystallinity of the surface damage was partly recovered by means of the annealing treatment. Fig. 6 shows the TEM images of annealed sample (3 1016 cm 2). Fig. 6(a) is the cross-sectional TEM image of the as-grown ZnO film grown on a GaN/sapphire substrate. It is clear to see the top ZnO preferentially oriented film, the transitional layer, GaN layer and the sapphire substrate. Some dark fringes in the film show the existence of threading dislocations which are perpendicular to the interface. Fig. 6(b) is a selected area electron diffraction (SAED) pattern from the transitional layer including both ZnO film and the GaN film, in which 1 and 2 are GaN (01–10) and (0 0 0 1) reflections, and a and b are ZnO (01–10) and (0 0 0 1) reflections. The electron beam parallel to the [2–1–10] zone axis of ZnO or GaN. From Fig. 6(b), we can conclude the perfect main epitaxial relationship of the sample: ZnO (0 0 0 1)[2–1–10]//GaN (0 0 0 1) [2–1–10]. The atomic arrangement in the c-plane of ZnO and GaN is shown in Fig. 6(c). No in-plane rotation of ZnO lattice with respect to the GaN buffer layer was found. A cross-sectional high-resolution TEM study was carried out to determine the structure of ZnO/GaN interface. Fig. 7 shows the high-resolution TEM images of the sample taken in
Fig. 6. (a) The cross-sectional TEM image of the annealed sample. (b) The SAED pattern of transitional layer of annealed sample with electron beam parallel to the [2–1–10] zone axis of ZnO or GaN, in which 1 and 2 are GaN (01–10) and (0 0 0 1) reflections and a and b are ZnO (01–10) and (0 0 0 1) reflections. (c) Illustration of the main domain orientation relationship of ZnO on GaN buffer layer.
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Fig. 7. High-resolution TEM images of (a) the ZnO layer and (b) ZnO/GaN interface along [2–1–10] zone axis of ZnO. (c) Fourier filtered image corresponding to rectangle region of (b).
the [2–1–10] zone axis of ZnO. The high crystalline quality of the ZnO layer can be directly reflected in the high resolution image of Fig. 7(a), from which we could see the atomic arrangement clearly. Lattice mismatch between the ZnO film and the GaN buffer layer results in a high density of misfit dislocation at the interface. Fig. 7(b) shows the high-resolution TEM image of ZnO/GaN interface as indicated by arrows. Lots of dislocations can be observed in the high-resolution TEM image. The Fourier filtered image of the rectangle region of Fig. 7(b) gives a closer look at the interfacial area as shown in Fig. 7(c). Along the interface (indicated by the white arrow), the atom positions are highly displaced, suggesting the existence of a high strain in the film. As can be seen in Fig. 7(c), the first five monolayers’ atoms in the interfacial layer rotate an angle off the
[0 0 0 1]-axis of the GaN in the interfacial layer. Kong et al. [19] had investigated the interface structural properties of the ZnO/GaN interface. They confirmed the interfacial layer was the monoclinic Ga2O3 phase by a comparison of the experimentally obtained diffraction pattern with a simulated diffraction pattern. In our work, we conjecture that the first five atomic layers may form another phase (such as monoclinic Ga2O3) or just be the lattice faults. Whether the interfacial layer formed another phase or not and the mechanism of formation of the first five atomic layers are interesting topics for future study. The magnetic properties of the annealed ZnO:Cr sample (3 1016 cm 2) were studied using SQUID magnetometer. The measurements were carried out at room temperature (300 K). Fig. 8 shows room temperature hysteresis present in
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of the interfacial layer and the corresponding Fourier filtered image, we can observe that the first five monolayers’ atoms in the interfacial layer rotate an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. We conjecture that the first five atomic layers may form other phase (such as monoclinic Ga2O3) or just be the lattice faults. The result of the magnetic measurement indicates that the annealed ZnO:Cr sample with the dose of 3 1016 cm 2 exhibit ferromagnetic behavior at room temperature. Acknowledgements
Fig. 8. Hysteresis curve for the annealed ZnO:Cr sample with the dose of 3 1016 cm 2 at room temperature.
This work was supported by the NSFC for the grants under Nos. 10345006, 10475063, and by HBSF under No. 2004ABA079, as well as by NCET and SRF for ROCS, Ministry of Education. References
the 5 at.% Cr-implanted and annealed ZnO sample. The background contribution to the magnetization originated from the GaN buffer layer and the diamagnetism of the sapphire substrate were subtracted. The saturation magnetization and residual magnetization are 1.699 10 5 and 5.060 10 6 emu, respectively. The coercive force in the sample is 100.564 Oe. These results confirm that the ZnO:Cr sample is ferromagnetic at room temperature. The magnetic properties of the as-implanted and annealed samples with lower implanted doses were also measured. But they did not exhibit any ferromagnetic behavior. The reasons are that the low-dosage implantation did not make the Cr+ ions concentration reach the critical point of ZnO-based DMS and the annealing treatment partly removed the implant damage and activated the magnetic dopants. 4. Conclusions In summary, high quality ZnO film grown by MBE was implanted with Cr+ ions under three different doses. Then, anneal experiments have been conducted to induce the lattice damage recovery. The XRD, AFM and Raman measurements show that the crystallinity of the Cr-implanted sample was improved by thermal treatment. TEM image and SAED pattern show that the main epitaxial relationship is perfect, i.e. ZnO (0 0 0 1)[2–1–10]//GaN(0 0 0 1)[2–1–10]. High-resolution TEM image of the ZnO layer confirms the high crystalline quality of the ZnO film. From the high-resolution TEM image
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