Materials Transactions, Vol. 51, No. 3 (2010) pp. 557 to 560 #2010 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

Room-Temperature in and Aluminum Co- Tin Dioxide Diluted Magnetic

Xingzhi Ning1;*, XiaoFang Liu1;*, Ronghai Yu1, Ji Shi2 and Yoshio Nakamura2

1Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China 2Department of Metallurgy and Ceramics, Tokyo Institute of Technology, Tokyo 152-8550, Japan

Sn0:897Co0:10Al0:003O2 diluted magnetic films were grown on (100) silicon substrate by magnetron sputtering and then annealed at 300C and 500C for 1.5 h, respectively. Room temperature ferromagnetism was observed for the as-deposited film. The saturation magnetization (Ms) decreases as annealing temperature increases, and the value of Ms for the films annealed at 500 C is about half of the value 6 for the as-deposited films. The electrical resistivity of Sn0:897Co0:10Al0:003O2 drops in a range of 10 to 10 m with increasing the annealing temperature. The XRD and HRTEM investigation illustrates that no impurity phase and Co clusters exist in the films, indicating the ferromagnetism of the films is intrinsic. The F-center-mediated exchange model is adopted to explain the room-temperature ferromagnetism of the films. [doi:10.2320/matertrans.M2009373]

(Received November 10, 2009; Accepted December 7, 2009; Published February 3, 2010) Keywords: diluted magnetic semiconductor, tin dioxide, ferromagnetism, microstructure

1. Introduction and Fe doped SnO2 films with different sputtering parameters to vary the oxygen vacancies,18–21) and observed the doped Diluted magnetic semiconductors (DMSs), which could SnO2 films show RTFM. However, these films are insulated combine charges and spins of electrons into a single nature so that they are not appropriate for identifying two substance, have attracted considerable attention since they main models for RTFM of DMS: carrier inducement model6) exhibit a potential application in the field of applications,1–4) and F-center model.12) Therefore, we are proposed to as -valve transistors, and spin-emitting diodes etc. DMSs fabricate Al and Co co-doped SnO2 films to increase the fabricated by doping 3d (TM) elements conductivity and then studied. into oxide semiconductors such as TM-(TiO2, ZnO, SnO2 In present work, Al and Co co-doped SnO2 films have been etc.)5,6,16) have been extensively studied because they exhibit fabricated and investigated. The resistivity of the films room temperature ferromagnetism (RTFM), which is as- fabricated is in the semiconductor region. After annealing the sumed in an intrinsic nature. resistivity could be adjusted by thermal annealing due to the For the study of diluted magnetic semiconductors, a change of the defects. The Al and Co co-doped SnO2 films considerable research has been focused on ZnO5–7,11–13) as a were observed to exhibit RTFM and could be explained by host semiconductor because of its wide band-gap in the the F-center model. ultraviolet region (about 3.36 eV), which is promising for the development of spin-optoelectronic devices. High epitaxial 2. Experiments Co doping ZnO films on SiO2 (101) substrate were fabricated by magnetron sputtering at a low temperature (200 C) and Co and Al co-doped SnO2 films were grown on Si exhibit room temperature ferromagnetism.2,6) Kim et al. have substrate (100) by magnetron sputtering. The sputtering fabricated Co and Al co-doping ZnO films by magnetron target was fabricated by placing Co and Al strips (99.99% sputtering and observed that the doped ZnO films exhibit purity) on sintered SnO2 disk to control the doping concen- RTFM.7) With increasing of Co concentration in the ZnO tration. The base pressure of the sputtering deposition 4 films, magnetization firstly increases, and then decreases at chamber was kept to 6 10 Pa. A ratio of Ar : O2 ¼ 5:1 Co concentration of 30%. In their experiments, no evidence mixture gas with total pressure of 0.8 Pa was used during shows that Al is related to the origin of RTFM. However, it deposition process. The power of sputtering is 180 w and has been reported7) that Al atoms are considered to occupy sputtering time is 1.5 h. After sputtering the as-deposited the interstitial sites in the lattice atoms, and they are films were annealed at 300C and 500C for 1.5 h, respec- suggested to be related to the formation of defects and result tively. in RTFM observed. The composition of the films was determined by X-ray SnO2 has been received considerable interest for its wider fluorescence and could be expressed as Sn0:897Co0:10- 16) (3.7 eV, wider than ZnO). As one of wide-band- Al0:003O2. The crystallographic structure of the films was gap oxides, SnO2 has been proposed for various applications, investigated by XRD (Rigaku 2500X, Cuk, ¼ 0:15406 such as liquid crystal display transistors, gas sensors, and nm). Room temperature magnetic properties were measured ferroelectric transparent thin-film transistors, etc. More by vibration sample magnetometer (VSM, H ¼1:5T recently, it is reported Co doped SnO2 exhibits RTFM in 1:5T). The electrical properties were obtained by aixACCT both film and particle forms.9,14–17) We have fabricated Co TF 2000 analyzer. High-resolution transmission electron microscopy (HRTEM) observation was carried out to inves- *Graduate Student, Tsinghua University tigate the microstructure of the films. 558 X. Ning, X. Liu, R. Yu, J. Shi and Y. Nakamura

Fig. 1 XRD patterns of as-deposited SCAO films and films thermally annealed.

3. Results and Discussions

Figure 1 shows the XRD patterns of Sn0:897Co0:10Al0:003O2 (SCAO) films grown on Si (100) substrates as-deposited and thermally annealed. Similar to un-doped SnO2, the Sn0:897Co0:10Al0:003O2 (SCAO) films are with the same structure of spathic phase. No impurity phases and Co clusters were detected with the resolution of XRD for either the as-deposited film or the annealed samples. For the as-deposited SCAO films, the (110) peak position is 26.36, which is lower than that of the standard SnO2 (110) peak (26.61). According to the Bragg’s Law, the distance between (110) planes for the as-deposited SCAO films was calculated to be about 0.347 nm, which is larger than that of the un-doped SnO2 (d110 ¼ 0:335 nm). The expansion of the lattice parameters is due to that the ionic radius of Co (0.082 nm), which is larger than that of Sn (0.074 nm) in the Fig. 2 M-H loops at room temperature of as-deposited and vacuum- octahedral coordination. After annealing the (110) peak annealed SCAO films. intensity increases and the full width at half maximum (FWHM) decreases, indicating the crystallinity of SCAO film is improved and the grain size increases. A visible right shift internal stress introduced during deposition, i.e. the improve- of (110) peak during annealing process was observed, which ment of the crystallinity. could be associated with the lattice shrinkage and the stress Figure 3 show the -V curves for the films in the voltage relaxation. range of 520 V. The resistivity of all the samples is in the Figure 2 gives the illustration of the magnetic properties region of 10106 m, which is located in the semiconductor (M-H loops) for the films as-deposited and annealed. A clear region. The resistivity of Co-doped SnO2 films is usually magnetic behavior is observed for all the samples. It is about 108 m due to the wide band gap (Eg) nature of 19) obviously that the magnetization decreases with increasing SnO2. It thus indicates that Al doping would increase the the annealing temperature. For as-deposited sample, satura- number of carriers and therefore improve the electrical tion magnetization Ms is approximately 0.30 emu/cc, and conductivity. A sharp decrease in the resistivity is seen as the this value decreases to about 0.15 emu/cc. The coercivity Hc films were annealed at 500 C. The increase of current density for the as-deposited film is about 600 Oe and it reduces and decrease of resistivity are suggested to be related to the to 113 Oe after thermal annealing at 300C, and further improvement of the crystallinity during thermal annealing. deceases to about 23.0 Oe after annealing the films at It reported that in Co, Al co-doped ZnO systems, free carrier 8,10) 500 C. The decrease of Ms and Hc is supposed to relate is mainly related to the presence of Al atoms. Similarly, to the decease of defects, increase of grain size, relaxing of in SnO2 system, Al atoms would play the same role as to Room-Temperature Ferromagnetism in Cobalt and Aluminum Co-Doping Tin Dioxide Diluted Magnetic Semiconductors 559

0 Fig. 5 Scheme of F-center model, Vos are represented by squares, Al interstitials are represented by black arrows and Co magnetizations are represented by red arrows. (a) as-deposited SCAO film (b) annealed SCAO film Fig. 3 R-V curves of as-deposited and vacuum-annealed SCAO films.

Fig. 4 HRTEM image of SCAO film (cross-section) with different annealing conditions. (a) as-deposited film (b) 300C annealed film (c) 500C annealed film increase the density of the carriers. As the crystallinity is seen that the crystallinity of SCAO films annealed at 300Cis improved, the carrier scattering at the grain boundaries nearly the same as compared with that of as-deposited films. becomes weak. This makes the carrier (mainly from Al The grain size of the films dramatically increases as they atoms) transportation become easier, and thus increases the were annealed at 500C, which is consistent with the XRD conductivity of SCAO films. results. Figure 4 shows typical HRTEM cross-section images of It is assumed that for the films with semiconducting nature, as-deposited and annealed SCAO films. No Co clusters or the ferromagnetic behavior could be understood based on other impurity phases, which were suspected to induce two models: free carrier inducement and F-center-mediated ferromagnetic behavior and could not detected by XRD, were exchange. Figure 5 shows the scheme of as-deposited film found from the observation. From HRTEM images it can be and vacuum-annealed film. 560 X. Ning, X. Liu, R. Yu, J. Shi and Y. Nakamura

In as-deposited film, the Al atoms are mostly bounded annealing temperature increases, the saturation magnetiza- in the crystal lattice, and with high density of defects the tion decreases and resistivity decreases. The XRD patterns crytallinity is poor, resulting in high electrical resistivity. and HRTEM images indicate that neither impurity phase nor After thermal annealing, the density of the lattice defects Co cluster exist in the samples. The concentration and is reduced, and the crystallinity is improved. Therefore, distribution of oxygen vacancies change which results in the the electrical conductivity increases. However, in the free decrease of exchange interactions, which is agreement with carrier inducement mechanism, with increase of electrical the F-center-mediated exchange model. conductivity, exchange interactions between carriers and local magnetic moment would be enhanced, leading to the Acknowledgements increase of saturation magnetization. This assumption is somehow controversial with the experimental results as The work is supported by the National Science Founda- shown above. tion of China (No. 50525101, 50729101 and 50771058), If we consider the F-center-mediated exchange mechanism and National Basic Research Program of China (No. (FC), it could explain the RTFM in SCAO films. Chang 2010CB934602). et al.8) and Kim et al.7) indicated that in the Co and Al co-doped ZnO DMSs fabricated by pulse laser deposition REFERENCES (PLD), the oxygen vacancies are relatively difficult to be formed. For the as-deposited film, a number of oxygen 1) T. Dietl, H. Ohno, F. Matsukura, J. Cibert and D. Ferrand: Science 287 0 (2000) 1019–1022. vacancies (Vo) are introduced to compensate for the charge imbalance induced by replacing Sn4þ with Co2þ. In addition, 2) F. Pan, C. Song, X. J. Liu, Y. C. Yang and F. Zeng: Mater. Sci. Eng. R 62 (2008) 1–35. non-equilibrium growth process during magnetron sputtering 3) K. Sato and Y. Katayama: Jpn. 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