Co clustering and ferromagnetism in chemical vapor deposited Ti1-xCoxO2-delta thin films

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Co clustering and ferromagnetism in chemical vapor deposited Ti 1 x Co x O 2 thin films

Sueng-Hee Kang, Hoa Nguyen Thi Quynh, Soon-Gil Yoon, Eui-Tae Kim, Zonghoon Lee, and Velimir Radmilovic

Citation: Applied Physics Letters 90, 102504 (2007); doi: 10.1063/1.2711184 View online: http://dx.doi.org/10.1063/1.2711184

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/10?ver=pdfcov Published by the AIP Publishing

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Co clustering and ferromagnetism in chemical vapor deposited Ti

_1−x

Co

_x

O

₂₋_␦

thin films

Sueng-Hee Kang, Hoa Nguyen Thi Quynh, Soon-Gil Yoon, and Eui-Tae Kim^a^兲 School of Nano Science and Technology, Chungnam National University, Daeduk Science Town, Daejeon 305-764, Korea

Zonghoon Lee and Velimir Radmilovic

National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 72-150, Berkeley, California 94720

共Received 11 October 2006; accepted 30 January 2007; published online 7 March 2007兲

Stoichiometric Ti_1−xCoxO₂and oxygen-deficient Ti_1−xCoxO₂₋_␦thin films were grown on Si共001兲by plasma-enhanced metal-organic chemical vapor deposition and their microstructures and ferromagnetic properties were investigated. The stoichiometric film grown at 430 ° C showed no discernable Co metal clustering or measurable coercive field. In contrast, oxygen-deficient films fabricated without supplying O₂contained significant Co clusters of⬃10– 20 nm, which appeared to be the major reason for the observed room-temperature ferromagnetism. With increasing oxygen vacancies of the films, the coercive field and saturation magnetization values increased to⬃460 Oe and ⬃27 emu/ cm³ 共1.55␮B/ Co atom兲 approached that for bulk cobalt, respectively. © 2007 American Institute of Physics.关DOI:10.1063/1.2711184兴

Diluted magnetic semiconductors共DMSs兲have attracted considerable interest because of their potential applications to spintronic devices, such as spin transistors, nonvolatile storage, logic devices, etc.^1–8 Among the potential DMS materials, transition metal-doped TiO₂ 共Ti1−xMxO₂兲 materials have been studied extensively as one of the most prom- ising candidates,^2–8 since Ti_1−xCoxO₂ anatase grown by combinatorial laser molecular beam epitaxy 共MBE兲 was shown to be ferromagnetic and semiconducting at temperatures up to 400 K.² However, little information is available on Ti_1−xMxO₂ DMS thin films grown by chemical vapor deposition⁸ 共CVD兲 even though CVD has a number of advantages over physical vapor deposition 共PVD兲methods, i.e., laser MBE,^2,4,7 sputtering,³ etc., for mass production and increasing the integration levels in the Si process. More- over, the effects of CVD growth conditions on transition- metal clustering in TixM_1−xO₂ thin films have not been addressed so far. The origin of the room-temperature ferromagnetism 共RTFM兲 of Ti_1−xMxO₂ DMS is the most important issue but, whether it is due to an intrinsic^2–5or an extrinsic effect共transition-metal clusters兲,^6,7remains contro- versial. Transition-metal clustering in Ti_1−xMxO₂is critically dependent on growth techniques and growth conditions.^5–10 The CVD growth process is highly complex and close to thermodynamically equilibrium, while PVD is far from an equilibrium growth process. As-grown Ti_1−xCo_xO₂thin films, produced by metal-organic CVD 共MOCVD兲, have recently been reported to be highly insulating and to show no Co clustering with an exceptionally high Co concentration of up to 12 at. %.⁸ This could be due to the thermodynamically equilibrium deposition process of CVD under an oxygen-rich growth environment. In this letter, we report on significant correlations between CVD growth conditions 共oxygen rich and poor兲 and Co clustering, which eventually affects the RTFM characteristics of Co-doped TiO₂thin films.

The Co-doped TiO₂ 共Ti1−xCoxO₂兲 thin films studied in this work were grown by plasma-enhanced MOCVD. In or- der to create an oxygen-poor growth environment, O₂ was not supplied during the Ti_1−xCoxO_2−x thin film growth.

Oxygen-deficient Ti_1−xCoxO₂₋_␦ thin films were formed by plasma assistance even at a low growth temperature of 270 ° C. Such oxygen-deficient Ti_1−xCoxO₂₋_␦thin films show very different microstructures and magnetic properties from their counterpart stoichiometric Ti_1−xCoxO₂thin films grown with an O₂flow of 60 cm³/ min. The thin films were grown at various substrate temperatures共270, 430, and 510 ° C兲and a radio-frequency plasma power of 40 W under a chamber pressure of 1.2 Torr on Si共100兲 substrates for 30 min.

共C₁₁H₁₉O₂兲2共C₃H₇O兲2Ti and Co共C₁₁H₁₉O₂兲3 were bubbled with an Ar flow of 150 cm³/ min at 200 ° C and 50 SCCM 共sccm denotes cubic centimeter per minute at STP兲 at 190 ° C, respectively. The total flow rate was fixed at 300 cm³/ min. The resistivity of the thin films was measured by a four-point probe CMT-SR 1000 electrometer. Magnetic properties were characterized using a Lake Shore 7400 series vibrating sample magnetometer共VSM兲at room temperature.

To investigate the origin of ferromagnetic properties, microstructures, and Co distribution, we carried out systematic transmission electron microscopy 共TEM兲 studies. Conven- tional TEM characterization was performed using JEOL 200CX microscope at 200 kV. Selected-area diffraction 共SAD兲 patterns were used to identify the phases. Electron diffraction patterns were simulated using the Desktop Mi- croscopist commercial TEM simulation package. Energy dis- persive spectroscopy共EDS兲microchemical analysis was carried out using a Philips CM200 field emission gun microscope at 200 kV equipped with an Oxford ultrathin window silicon EDS detector. EDS line profiles were ac- quired using probe sizes of 1.4 nm. For the quantitative EDS analysis, the Cliff-Lorimerkfactors of Ti, O, and Co atoms were set at 1.299, 1.980, and 1.576, respectively.

Figure 1 shows the room-temperature magnetization characteristics of the as-grown oxygen-deficient

a兲Author to whom correspondence should be addressed; electronic mail:

[email protected]

APPLIED PHYSICS LETTERS90, 102504

共

2007

兲

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Ti_1−xCo_xO₂₋_␦ thin films grown at various growth tempera- tures共270, 430, and 510 ° C兲without supplying O₂gas. The thin film grown at 270 ° C does not show a measurable coercive field共Hc兲within the VSM resolution limit. At growth temperatures above 430 ° C, the Ti_1−xCoxO₂₋_␦thin films ex- hibit a hysteresis loop with nonzero Hc indicating RTFM.

The Ti_1−xCo_xO₂₋_␦ thin film grown at 430 ° C shows an H_c value of⬃200 Oe and a saturation magnetization value共Ms兲 of ⬃12 emu/ cm³ 共⬃0.73␮B/ Co atom兲. When the growth temperature was increased to 510 ° C, theH_candM_s values increase to⬃460 Oe and⬃27 emu/ cm³共1.55␮B/ Co atom兲, respectively. The inset in Fig.1shows the resistivity values for the films. The resistivity of films decreases almost expo- nentially with increasing growth temperature. The resistivity values of films grown at 270, 430, and 510 ° C are⬃54 000,

⬃100, and⬃0.4⍀cm, respectively. The decreased resistivity can be attributed to an increase in the number of oxygen vacancies in the films. It is clear that the enhanced magnetic characteristics are strongly correlated to the magnitude of oxygen vacancies of the films. It was recently reported that oxygen vacancies not only plays a critical role in realizing intrinsic RTFM共Refs. 5, 11, and 12兲but also enhances Co clustering.⁹It should be noted that theMsvalue共1.55␮B/ Co兲 of the film grown at 510 ° C approaches that of bulk cobalt 共1.75␮B/ Co兲.

To gain insights into the effect of oxygen vacancies on Co clustering and RTFM, a stoichiometric Ti_1−xCo_xO₂ thin film was grown at 430 ° C with an O₂ flow of 60 cm³/ min.

Figure2shows the VSM magnetic characteristics of the stoichiometric film and an oxygen-deficient film grown at 430 ° C without supplying O₂ gas. The stoichiometric film does not show a measurableHc while the oxygen-deficient film shows ferromagnetic characteristics. As expected, the stoichiometric film was insulating, with a high resistivity of ⬃40 k⍀cm. To investigate the microstructures of the films, systematic TEM studies were carried out. Figures3共a兲 and3共b兲show cross-sectional dark-field images of stoichiometric and oxygen-deficient films, respectively. Both films have interesting microstructures consisting of a nanocrystalline bottom region and a well-developed columnar crystal- line upper region. As seen in the SAD patterns of Fig.3共a兲, anatase polycrystallites are dominant in both the bottom and upper regions. Considering the diffraction patterns and dark-

field images, the bottom region 共0 –⬃270 nm兲 of the stoichiometric film seems to be nanocrystalline, with a size of

⬃10 nm. Meanwhile, the oxygen-deficient film has a thinner nanocrystalline layer 共⬃100 nm兲 having slightly larger grains共⬃10– 20 nm兲. In the earlier growth stage, randomly oriented nanocrystallines may be formed due to limited Ti adatom migration at the low growth temperature used 共430 ° C兲. The most important feature is the numerous pre- cipitates共⬃10– 20 nm size兲 in the oxygen-deficient film, as seen in the scanning TEM image of the inset of Fig. 3共b兲. The precipitates in the oxygen-deficient film turned out

FIG. 1. Magnetization hysteresis loops for Ti_1−xCoxO_2−␦thin films grown at various temperatures共270– 510 ° C兲. The inset shows the resistivities of the films.

FIG. 2. Magnetization curves of stoichiometric Ti_1−xCoxO₂ and oxygen- deficient Ti_1−xCoxO_2−␦thin films grown at 430 ° C.

FIG. 3. Cross-sectional dark-field TEM images of 共a兲 stoichiometric Ti_1−xCoxO₂ and 共b兲 oxygen-deficient Ti_1−xCoxO_2−␦ thin films grown at 430 ° C.

102504-2 Kanget al. Appl. Phys. Lett.90, 102504共2007兲

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to be⬃10– 20 nm Co clusters, as seen in the nanoprobe EDS results 关Fig. 4共d兲兴. In contrast, our careful TEM studies confirmed that Co clusters do not exist in the stoichiometric film. Under an oxygen-deficient growth environment without supplying O₂ gas, a lack of oxygen around the Co adatoms and oxygen vacancies in the film can enhance the clustering of Co metal atoms.^5,6,8 Significant amounts of Co clusters were found in the oxygen-deficient film grown at 430 ° C;

the observed RTFM withMsof⬃0.73␮B/ Co can mainly be attributed to extrinsic characteristics 共Co clusters兲 even though the possibility of intrinsic RTFM cannot be com- pletely ruled out. A higher growth temperature can create a more oxygen-poor growth environment on the growth front surface, and thus enhance Co clustering. Some large Co clus- ters共⬃0.1– 1␮m兲can be observed, even on the surface of the oxygen-deficient film grown at 510 ° C, which results in a larger M_s of ⬃1.55␮B/ Co as well as a larger H_c of

⬃460 Oe, compared to a film grown at 430 ° C, as seen in Fig.1.

It should be emphasized that there is a strong correlation between Co distribution共clustering兲and the microstructures of thin films as well. The Co distribution in both stoichiometric and oxygen-deficient thin films was investigated by nanoprobe EDS line profiling 共Fig. 4兲. The EDS results were achieved from the Si/film interface 共position: 0 nm兲 to near the film surface 共position: ⬃600 nm兲. Figure 4共b兲 shows the ratios of the number of counts in the TiK␣ and the CoK␣ peaks, indicating the distribution of Co in the stoichiometric Ti_1−xCoxO₂film. Nonuniform Co distribution is distinguished along the growth direction. The Co concentration continues to increase in the nanocrystalline region 共0 –⬃280 nm兲 with increasing film thickness. The Co concentration then begins to decrease in a well-developed columnar anatase structure 共⬎280 nm兲 and becomes uniform beyond⬃350 nm. A quantitative Cliff-Lorimer EDS analysis revealed that the mole fractions of Co in the middle of the nanocrystalline and the well-developed polycrystalline regions were 2.93 and 0.98 at. %, respectively. Figures 4共c兲 and 4共d兲 shows EDS results for the oxygen-deficient Ti_1−xCoxO₂₋_␦film. The oxygen-deficient film shows a similar

behavior, having a slightly higher Co concentration in the nanocrystalline region than in the well-developed anatase structure. Nevertheless, as seen in Fig.3共b兲, Co clusters are rarely formed in the nanocrystalline region. Co clusters are observed more densely in the well-developed anatase structure 共⬃100– 300 nm兲. Co atoms may spread in the grain boundary area of the nanocrystalline region and, thus, discernable Co clustering can be prohibited. When the film was developed into the microcrystalline structure, segregated Co atoms from the nanocrystalline region appear to enhance Co clustering. As seen in Fig.4共d兲, the Co metal cluster size was

⬃10– 20 nm. In addition, Figs. 4共a兲 and 4共c兲 confirms that the film grown without supplying O₂gas is far more oxygen- deficient than the film grown with O₂.

In conclusion, the solubility of Co in anatase polycrystalline TiO₂ thin films is significantly low 共⬍⬃3 at. %兲 at CVD growth conditions and far closer to equilibrium than when PVD is used. The Co metal clustering of excess Co atoms was critically affected by the CVD growth environment used and the microstructure of the films. While, under an oxygen-rich environment, excess Co atoms were segregated onto the growth front surface without discernable Co metal clustering at 430 ° C, an oxygen-poor growth environment enhanced the formation of Co clusters which appears to be the main reason for the observed RTFM of oxygen- deficient Ti_1−xCo_xO₂₋_␦thin films. However, Co clusters were rarely formed in the nanocrystalline region, even though Co atoms accumulated at higher concentrations.

This work is supported by research fund of Chungnam National University in 2005 and the Korea Research Foun- dation Grant funded by the Korean Government共MOEHRD兲共KRF-2006-331-D00260兲. The authors acknowledge support of the National Center for Electron Microscopy, Lawrence Berkeley Laboratory, which is supported by the U.S. Depart- ment of Energy under Contract No. DE-AC02-05CH11231.

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FIG. 4. EDS nanoprobe line profiling results.关共a兲and共b兲兴Stoichiometric Ti_1−xCoxO₂ and 关共c兲 and 共d兲兴 oxygen-deficient Ti_1−xCoxO_2−␦ thin films grown at 430 ° C.

102504-3 Kanget al. Appl. Phys. Lett.90, 102504共2007兲