Structure and ferromagnetism of Mn+ ion-implanted ZnO thin films on sapphire

Superlattices and Microstructures 39 (2006) 41–49 www.elsevier.com/locate/superlattices

Structure and ferromagnetism of Mn+ ion-implanted ZnO thin films on sapphire G. Brauera,∗, W. Anwanda, W. Skorupaa, H. Schmidtb, M. Diaconub, M. Lorenzb, M. Grundmannb a Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, Postfach 510119,

D-01314 Dresden, Germany b Institut für Experimentelle Physik II, Fakultät für Physik und Geowissenschaften, Universität Leipzig,

Linnestr. 5, D-04103 Leipzig, Germany Available online 16 September 2005

Abstract Slow Positron Implantation Spectroscopy (SPIS), based on the generation, implantation and subsequent annihilation of mono-energetic positrons in a sample, has been used to study depth dependent vacancy-type damage in three ZnO films grown by pulsed laser deposition on c-plane sapphire. Doping was achieved by implantation of 250 keV Mn+ ions at 300 ◦ C with three different fluences—1016 , 3 × 1016 , and 6 × 1016 cm−2 , and subsequent thermal annealing in air. Evolution of the open volume damage, its depth distribution, and the magnetic behavior was investigated by SPIS and Magnetic Force Microscopy. No indication of magnetic domain formation was found in any of the three films after implantation and the first annealing at 500 ◦ C, whereas after the second annealing at 750 ◦ C the two samples having the higher fluence showed stripe-like magnetic domains. © 2005 Elsevier Ltd. All rights reserved.

1. Introduction In the past few years, diluted magnetic semiconductors have attracted considerable attention in view of their potential for development of novel magneto-optoelectronics [1–3]. ∗ Corresponding author. Tel.: +49 351 2602 117; fax: +49 351 2603 285.

E-mail address: [email protected] (G. Brauer). 0749-6036/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2005.08.030

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Among these materials, the Mn–Zn–O system is of special interest due to the recent discovery of ferromagnetism above room temperature. It has been suggested that the magnetic properties of ZnO:Mn films depend on factors such as crystalline and residual defects [4], or maybe the formation of an oxygen-vacancy-stabilized metastable phase Mn2−x Znx O3−δ [5]. A few early positron annihilation spectroscopy studies of ZnO bulk materials have already been published [6,7], and three distinct annealing stages were observed at about 200, 500, and 750 ◦C, in agreement with other studies [8]. As expected, due to the variety of possible defects in ZnO, and the fact that native defects may exist on both sub-lattices and in different charge states, the interpretation of experimental results is not trivial. In the present study, ZnO films grown by pulsed laser deposition (PLD) on c-plane sapphire were doped by ion implantation of Mn+ . The depth distribution of open volume damage, which is always connected with ion implantation, was estimated by Slow Positron Implantation Spectroscopy (SPIS). A search for ferromagnetism (magnetic domains) was performed by Magnetic Force Microscopy (MFM). The important influence of thermal annealing steps on the structural and magnetic properties of the Mn+ -implanted ZnO films is demonstrated here. 2. Experimental The ZnO thin films were deposited by PLD [9], using a KrF excimer laser LAMBDA PHYSIK LPX 305 operating at 248 nm wavelength, on c-plane sapphire (0001) substrates of 10 × 10 × 0.5 mm3 dimension. A nucleation layer of about 10 nm thickness was deposited at a reduced 1 Hz laser repetition frequency to improve the crystalline film quality according to [10]. The oxygen partial pressure was 2 × 10−3 mbar to ensure low roughness of the as-grown films. The substrate temperature was chosen between 450 and 600 ◦C to control the grain size of the ZnO thin films in the range between 80 and 350 nm. A total number of 40 000 laser pulses was applied at 10 cm target-to-substrate distance to grow about 1 µm thick films. Selected area electron diffraction patterns proved the monocrystallinity of the PLD films and revealed the epitaxial relationship ¯ ZnO[0001]  Al2 O3 [0001] and ZnO[2110]  Al2 O3 [1100] which was also confirmed by XRD pole figures. The ZnO thin films were implanted with Mn+ ions at 300 ◦C with three different fluences—1×1016, 3×1016, and 6×1016 cm−2 . The expected profiles of ions and vacancies are shown in Fig. 1 and were estimated by TRIM [11] calculations. Two subsequent annealing steps were performed at 500 and 750 ◦C, each for 30 min in air. SPIS measurements were carried out with the monoenergetic positron beam “SPONSOR” at Rossendorf [12] at which a variation of the positron energy E from 0.03 to 36 keV with a smallest step width of 50 eV is possible. The energy resolution of the Ge detector at 511 keV is 1.09 ± 0.01 keV, resulting in a high sensitivity to changes in materials properties from surface to depth. About 1 × 106 events per spectrum have been accumulated. The motion of the electron–positron pair prior to annihilation causes the Doppler broadening of the 511 keV annihilation line and can be characterized by the line-shape

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Fig. 1. Ion and vacancy distribution in ZnO films calculated by TRIM [11].

parameter S. In brief, the value of S is defined by the ratio of counts in the central region of the annihilation gamma peak and the total number of counts in the peak. It is common to define the central region for a certain defect-free sample to obtain a reference value of Sref ∼ 0.5. In the present study, a Czochralski-grown Si(100) (n-type, 6–10  cm) sample having a size of about 10 mm × 10 mm served as a reference. The same region is then used to calculate the values of S for every other sample studied. For an easier discussion of changes in S it is furthermore common to normalize the parameter S to the bulk value Sbulk obtained for the reference sample. The depth distribution of rapidly thermalized positrons at a given positron energy E, which can be described by a Makhovian profile, may become distorted due to positron diffusion and trapping prior to annihilation. It is understood that each point of a measured S(E) dependence represents an integral over this final positron distribution at a given E. The derivation of physical information from S(E) is performed by application of the program code VEPFIT [13]. Thereby, it is assumed that the sample may consist of a certain number of homogeneous layers which are characterized each by a given width (corresponding to a given depth), height (represented by a given S), and positron diffusion length L + —for a recent example on 6H-SiC see Ref. [14]. The films were analyzed with respect to grain formation by Atomic Force Microscopy (AFM) and with respect to magnetic domain formation by MFM using a Dimension 3100 Scanning Probe Microscope operated at 50 nm lift-of distance with a low-coercivity MFM tip. 3. Results and discussion From the results shown in Fig. 1 it is obvious that the vacancy distribution differs from the ion distribution, and the maximum of open-volume damage should be located at a

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Fig. 2. S(E) curves for an as-grown ZnO film deposited on c-plane sapphire, and of an untreated c-plane sapphire substrate.

depth of ∼80 nm. In Fig. 2 an experimental S(E) spectrum for a thin ZnO film obtained by PLD on sapphire is shown. Its analysis gives three distinct layers: a first ‘surface’ layer of about 450 nm thickness which is characterised by a relatively low damage level, as indicated by an S value 2% higher than that measured for a bulk monocrystalline ZnO sample assumed to be ‘defect free’. A second layer, located from 450 nm depth down to the sapphire substrate, is of a higher damage level, as indicated by an S value 4.6% higher than that of the bulk ZnO S value. However, according to the TRIM calculations (see Fig. 1), the implantation region should not extend deeper than 250 nm, so that the implantation was always done into the less-damaged layer on top of the ZnO films. The third layer starts at ∼800 nm to ‘infinite’ depth and represents the sapphire bulk. From the results presented in Fig. 3 it may be learned in general—as expected—that the implantation creates vacancy-type damage. The damage level increases with increasing fluence, and the thickness of the first damaged layer decreases from about 100 nm to about 60 nm and to about 40 nm, respectively. This experimental finding differs from the results of TRIM calculations (see Fig. 1) and is connected with the implantation temperature being chosen at 300 ◦C, i.e. already above a first annealing step of vacancy-type damage found at 200 ◦C [6–8]. Thus, there is an increased mobility of vacancy-type defects above 200 ◦C which results in the formation of larger agglomerates with increasing fluence closer and closer to the film surface during ion implantation. Annealing at 500 ◦C results in a continuation of the processes already going on during implantation, i.e. the movement of more deeply located vacancy-type damage towards the surface and further agglomeration there (see Fig. 4). Further annealing at 750 ◦C is not sufficient to remove the vacancy-type damage formed in the films (see Fig. 5). The large agglomerates previously formed near the film surface are nearly annealed out and thus differences between the three fluences become negligible. However, some deep reaching damage up to a depth of about 750 nm becomes visible

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Fig. 3. Comparison of the S(E) curves of samples implanted with 250 keV Mn+ ions at 300 ◦ C and different fluences.

Fig. 4. Influence of annealing at 500 ◦ C on S(E).

which was already present after ion implantation but masked (see Fig. 6). This damage is stable against annealing up to 750 ◦C, and therefore should be of a different type compared with the damage observed near the film surface. It may be guessed that due to the high implantation temperature the implantation region of the Mn+ ions extends much deeper than indicated by TRIM. In other words, Mn atoms at interstitial positions may cause a lattice expansion and this is seen as an increase in S. Further investigations are necessary to underline this assumption.

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Fig. 5. Change of S(E) caused by an annealing at 750 ◦ C.

Fig. 6. Influence of the preparation steps of a ZnO film on sapphire implanted with 250 keV Mn+ ions at 300 ◦ C to a fluence of 6 × 1016 cm−2 .

Further annealing at still higher temperatures was impossible because the ZnO films started to defoliate from the substrate. Therefore, ZnO single crystals might be a better choice for future studies at higher annealing temperatures. Fig. 7(a) shows an AFM image taken from an as-grown PLD ZnO thin film; its granular structure (with grain size around 260 nm) remains unchanged after implantation of 250 keV Mn+ ions with a fluence of 3×1016 cm−2 . The corresponding MFM image of the as-grown film is shown in Fig. 7(b), and after implantation the MFM image was also unchanged, thereby demonstrating the negligible effect of implantation on the surface morphology

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Fig. 7. AFM and MFM images of the as-grown ZnO thin film ((a), (b)), after Mn+ ion implantation (fluence of 3 × 1016 cm−2 ) and first annealing at 500 ◦ C ((c), (d)), and after 750 ◦ C annealing ((e), (f)). After the second annealing at 750 ◦ C, the film surface became more rough (e), and the MFM image shows long-ranging stripe-like magnetic domains (f).

and magnetic properties. The AFM image in Fig. 7(c) shows the film surface after the first annealing step at 500 ◦C, which again shows the granular structure of the as-grown film surface (Fig. 7(a)). The corresponding MFM phase image in Fig. 7(d) taken after the 500 ◦C annealing shows a weak contrast of the granular surface structure and not any additional contrast due to magnetic domains. Therefore, no indication of magnetic domain formation was found in the as-grown, as-implanted and 500 ◦C-annealed samples by MFM. After the second annealing step at 750 ◦C, as shown in Fig. 7(e), the surface roughness increased considerably and a secondary phase segregated. Interestingly, magnetic domains were detected by MFM for the two samples implanted at the higher fluences after annealing at 750 ◦C, as illustrated in Fig. 7(f) for the sample implanted with Mn+ at a fluence of 3 × 1016 cm−2 . Here are seen stripe-like magnetic domains; such domains have been observed also by Sato et al. in CdGeP2 :Mn [15]. We expect the two films with magnetic domain formation to be soft ferromagnetic, because the domain formation was only observable using low-coercivity MFM tips. The higher-coercivity tips are probably influencing the sample magnetization. The domain width of a periodicity of 1.2–1.6 µm in the ferromagnetic samples is comparable to the stripe domain width observed on Ni thin films [16] and on ZnO:Mn thin films PLD grown from ZnO:Mn targets [17], where magnetic domains of about 1.3 µm width were clearly observed on 1 µm thick films.

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No indication of long-range magnetic domains was found in any of the three implanted films after implantation or after the first annealing step at 500 ◦C (compare the SPIS results in Fig. 4), whereas after the second annealing step at 750 ◦C (Fig. 5) the two samples having the higher fluence showed magnetic domains. The observed domain formation can be related to the results of theoretical investigations by Sluiter et al. (Ref. [18]) pointing out the crucial role of defects in the weak and preparation-sensitive ferromagnetism in ZnO:Mn. In the implanted samples the vacancy-type damage formed in the films becomes visible only after annealing at 750 ◦C (Figs. 5 and 6). According to the SPIS (Figs. 2–6) and AFM measurements, defect formation and the grain size in the sample implanted with the lower fluence does not differ from those in the two samples with the higher fluence. Therefore, the detectability of magnetic domain formation seems to be clearly dependent on the amount of implanted Mn, with a threshold implantation fluence between 1 × 1016 and 3 × 1016 Mn+ ions cm−2 . 4. Conclusions The implantation of 250 keV Mn+ ions into virgin ZnO films generates open-volume defects which become visible after annealing at 750 ◦C. We observed the importance of vacancy generation accompanying the Mn+ ion implantation for ferromagnetic ordering in Mn+ -implanted ZnO. A possible application would be the creation of ZnObased nanomagnets as proposed by Esquinazi et al. [19] for irradiated carbon-based nanomagnets. The SPIS technique offers a unique possibility to prove the presence of vacancies for optimizing defect mediated ferromagnetism in ZnO films. Acknowledgement This work is supported by the German BMBF within the Young Scientists Group ‘NanoSpintronics’ at the University of Leipzig, Grant No. FKZ 03N8708. References [1] [2] [3] [4] [5] [6]

[7] [8] [9] [10]

H. Ohno, Science 281 (1998) 951. G.A. Prinz, Science 282 (1998) 1660. S. Das Sarma, Am. Sci. 89 (2001) 516. Y.W. Heo, M.P. Ivill, K. Ip, D.P. Norton, S.J. Pearton, J.G. Kelly, R. Rairigh, A.F. Hebard, T. Steiner, Appl. Phys. Lett. 84 (2004) 2292. D.C. Kundaliya, S.B. Ogale, S.E. Lofland, S. Dhar, C.J. Metting, S.R. Shinde, Z. Ma, B. Varughese, K.V. Ramanujachary, L. Salamanca-Riba, T. Venkatesan, Nat. Mater. 3 (2004) 709. S. Brunner, W. Puff, P. Mascher, A.G. Balogh, in: S.J. Zinkle, G.E. Lucas, R.C. Ewing, J.S. Williams (Eds.), Microstructural Processes in Irradiated Materials, in: Mater. Res. Soc. Symp. Proc., vol. 540, Materials Research Society, Warrendale, PA, 1999, p. 207. S. Brunner, W. Puff, A.G. Balogh, P. Mascher, Mater. Sci. Forum 363–365 (2001) 141. D.C. Look, D.C. Reynolds, J.W. Hemsky, R.L. Jones, J.R. Sizelove, Appl. Phys. Lett. 75 (1999) 811. M. Lorenz, E.M. Kaidashev, H. von Wenckstern, V. Riede, C. Bundesmann, D. Spemann, G. Benndorf, H. Hochmuth, A. Rahm, H.-C. Semmelhack, M. Grundmann, Solid State Electron. 47 (2003) 2205. E.M. Kaidashev, M. Lorenz, H. von Wenckstern, A. Rahm, H.-C. Semmelhack, K.-H. Han, G. Benndorf, C. Bundesmann, H. Hochmuth, M. Grundmann, Appl. Phys. Lett. 82 (2003) 3901.

G. Brauer et al. / Superlattices and Microstructures 39 (2006) 41–49

49

[11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [12] W. Anwand, H.-R. Kissener, G. Brauer, Acta. Phys. Polonica A 88 (1995) 7. [13] A. vanVeen, H. Schut, J. deVries, R.A. Hakvoort, M.R. Ijpma, in: P.J. Schultz, G.R. Massoumi, P.J. Simpson (Eds.), Positron Beams for Solids and Surfaces, AIP, New York, 1990, p. 171. [14] G. Brauer, W. Anwand, P.G. Coleman, W. Skorupa, Vacuum 78 (2005) 131. [15] K. Sato, G.A. Medvedkin, K. Hayata, Y. Hasegawa, T. Nishi, R. Misawa, T. Ishibashi, Joint Magneto-Optical Recording International Symposium/Asia Pacific Data Storage Conference 2000, MORIS/APDSC2000, Nagoya, 31.10.–02.11.2000, Presentation TU-D07. [16] R. Naik, S. Hameed, P. Talagala, L.E. Wenger, V.M. Naik, J. Appl. Phys. 91 (2002) 7550. [17] M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.-W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117. [18] M.H.F. Sluiter, Y. Kawazoe, P. Sharma, A. Inoue, A.R. Raju, C. Rout, U.V. Waghmare, Phys. Rev. Lett. 94 (2005) 187204-1. [19] P. Esquinazi, D. Spemann, R. Höhne, A. Setzer, K.-H. Han, T. Butz, Phys. Rev. Lett. 91 (2003) 227201-1.