Co-Doped TiO2 Rutile Thin Films Deposited by MOCVD Method H. Saragih, P. Arifin and M. Barmawi

Laboratory of Electronic Material Physics, Department of Physics, Bandung Institute of Technology

Jl. Ganesa 10 Bandung 40132, Indonesia

Abstract—Co-doped TiO2 thin films were grown on Si(100) substrates by MOCVD method using titanium (IV) isopropoxide [Ti(OCH(CH3)2)4] and tris (2,2,6,6-tetramethyl-3, 5-heptanedionato) cobalt (III), Co(TMHD)3 precursors. The growth parameters, crystal structure, ferromagnetic and electrical transport properties of thin films were investigated. Ferromagnetic behaviors of films were observed at room temperature. Magnetic and structural properties strongly depend on the Co concentration and the growth temperature. Epitaxial thin films with highest magnetic saturation were found on the films grown at temperature of 450 °C. Resistivity as a function of temperature measurement shows that the films have semiconducting properties.

Keywords- Ferromagnetic thin film, Co-doped TiO2 rutile and MOCVD.

I. INTRODUCTION

Diluted magnetic semiconductors (DMSs) have attracted much attention because of their potential applications in spin-electronics. Two types of DMS, i.e. II–VI, such as Mn-doped CdTe and ZnSe [1], and III–V, such as Mn-doped GaAs [2], have been intensively studied. However, most of these materials suffer from their low Curie temperature, typically lower than 110 K, thus limiting their possible applications. Room temperature ferromagnetism in Co-doped TiO2, first discovered by Matsumoto et al. [3], seems very promising for applications and has attracted much further experimental work [4–8]. TiO2 has three stable crystal structures: rutile, anatase and brookite. Ferromagnetism of Co-doped TiO2 was first observed in anatase structure which was grown on LaAlO3(001) and SrTiO3(001) substrates by laser molecular beam epitaxy (MBE) method [3]. These films showed a weak ferromagnetic behavior, having a saturation moment (Ms) of 0.32 B/Co atom. Later, the same authors reported that Ms of 1

B/Co could be obtained in Co-doped TiO2 rutile films deposited on -Al2O3 [9], while Chambers et al. found that the ferromagnetism of the anatase films could be increased up to 1.26 B/Co using oxygen-plasma-assisted MBE [4]. More recently, an Ms value of as high as 1.7 B/Co, the value of Co metal, has been reported [10–12]. Such a high Ms value was attributed to the Co segregation or clustering [8,10,11]. However, up to now, all epitaxial Co-doped TiO2 films were grown by MBE method. On the other hand, high quality of thin films can also be obtained by metal organic chemical vapor deposition (MOCVD) method.

MOCVD is currently among the most important method for the growth of thin, high purity epitaxial films suitable for applications in electronic and optoelectronics devices [6]. The metal organic precursors have excellent properties since they are completely vaporized, decomposed into oxides at relatively low temperature, commercially available with high purities and at low price [13]. Co-doped TiO2 thin films in rutile structure deposited by MOCVD method, to the best of our knowledge, have not been reported yet. In this paper, we report the growth and characterization of Co-doped rutile TiO2 thin films grown by MOCVD.

II. EXPERIMENT

The Co-doped TiO2 thin films were grown on Si(100) substrates. The substrates were cleaned by acetone for 10 minutes and then by methanol for 10 minutes, followed by an etch using 10% HF for 2 minutes. The substrates were dried under N2 jet and immediately mounted onto a molybdenum susceptor in the reactor.

Titanium (IV) isopropoxide (TTIP) [Ti(OCH(CH3)2)4] and tris (2,2,6,6-tetramethyl-3, 5-heptanedionato) cobalt (III), were used as precursors for Ti and Co, respectively. Oxygen gas was used as O source. The Co(TMHD)3 powder was dissolved in a tetrahydrofuran (THF) solvent. Solutions of precursors were stored in different bubblers. They were vaporized and transported into the reactor using argon carrier gas. Temperature of 50°C and 100°C were used to vaporize TTIP and Co(TMHD)3 solutions, respectively. The flow rates of argon carrier gas through TTIP and Co(TMHD)3 precursors were varied. O2 gas with flow rate of 60 sccm was supplied through a separate gas line into the chamber. Heating tape was wrapped around the TTIP and Co(TMHD)3 vapor-transport lines to prevent condensation.

The growth temperatures were varied between 400°C and 550°C. The growth was conducted for 120 minutes, followed by a slow cooling down to room temperature at a rate of 200°C/hour to prevent strain-induced microcracks. The crystalline structure and morphology of the films were characterized by X-ray diffraction employing Cu K radiation and scanning electron microscope (SEM), respectively. Chemical composition of the films was determined by energy dispersive x-ray analysis. Room temperature magnetic properties were characterized by a vibrating sample magnetometer (VSM), where the magnetic field was applied parallel to the film

ICONN 20061-4244-0453-3/06/$20.00 2006 IEEE 1

surface in field up to 1 Tesla. The resistivity of the films was measured by means of a Hall-van der Pauw method.

III. RESULTS AND DISCUSSION

As a host material, TiO2 thin films without Co-dopant were deposited on Si(100) substrates. The depositions parameters were: bubbler temperature of TTIP, Tb(Ti) = 50°C; substrate temperature, Ts = 450°C; bubbler pressure, Pb(Ti) = 260 Torr; flow rate of argon gas through TTIP precursor, Ar(Ti) = 100 sccm and flow rate of oxygen gas O2 = 60 sccm. These parameters were found to be the optimum deposition parameters to obtain epitaxial rutile Co-doped TiO2 thin films.

Fig. 1 shows the XRD pattern of TiO2 thin film grown at 450°C. XRD pattern shows that the grown film was in a single phase having a rutile structure with (002) orientation (R(002)). The films grown using other parameters have a mixing of anatase and rutile structures. Fig. 2 shows the XRD pattern of TiO2 thin film deposited at temperature of 550oC in which the other parameters were unchanged. The deposition parameters, in particular the total pressure and the substrate temperature, were strongly influence the film structure. This occurs due to the competition between the surface mobility of precursors and the equilibrium time required for the formation of a stable surface state. As it is well known, at higher total pressure, the precursor molecule density is increased, lead to the decrease of the surface mobility, but the equilibrium surface state might not be formed due to the high deposition rate at high total pressure. At high deposition temperature (e.g. 550oC), it is found that all the grown films show the mixing of anatase and rutile structure. This is probably due to the shorter equilibrium time required for the rutile structure growth induced by the high surface mobility at high temperatures.

Fig. 3 shows the SEM images of surface morphology and cross-section of TiO2 thin films. The cross-section micrograph reveals a columnar structure of grains with relatively similar shape. No abnormal grains were grown in the films. Surface of the films are relatively smooth. The grains are closely packed with strong coalition between them. This implied that the nucleation density on the substrate surface at the initial growth was high.

Figure 1. XRD pattern of TiO2 thin film grown on Si (100) substrate with deposition parameter: Tb(Ti) = 50oC, Ts = 450oC, Pb(Ti) = 260 Torr, flow rates of

Ar(Ti) and O2 are 100 sccm and 60 sccm, respectively.

Figure 2. XRD pattern of TiO2 thin film grown on Si (100) substrate with parameter deposition of Tb(Ti) = 50oC, Ts = 550oC, Pb(Ti) = 260 Torr, flow rates

of Ar(Ti) and O2 are 100 sccm and 60 sccm, respectively.

Figure 3. SEM images of surface (a) and cross-section (b) of TiO2 thin film with parameter depositions as described in fig. 1.

Figure 4. Dependence of Co content in TiO2 thin films versus Ar flow rates passed through Co(TMHD)3 precursor.

Co-doped TiO2 thin films were grown using additional Co(TMHD)3 precursor dissolved in tetrahydrofurant with similar deposition parameters. The vapor of Co(TMHD)3precursor was transported to the reactor by Ar gas with varied flow rates between 20 sccm and 90 sccm. The Co concentrations in the grown films were in the range of 0.4% to 11.0%, respectively. It appears that the number of atoms incorporated in the films depend on the flow rate of Ar gas. The increase of Ar gas flow rate, the number of Co atoms which could enter into TiO2 host matrix is higher. The dependence of Co concentration incorporated in TiO2 thin films on the Ar flow rates is shown in fig. 4. A limit of Co atom solubility in TiO2 material is shown. The estimate

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saturation concentration of Co incorporated in TiO2 films is around 10.4%, which corresponds to Ar flow rate of 60 sccm. Increasing Ar flow rate above 60 sccm does not increase the Co concentration significantly.

Figure 5. XRD patterns of Co-doped TiO2 thin films with Co content up to 5.77%.

Figure 6. XRD patterns of rutile (002) TiO2 thin film and rutile (002) Co-doped TiO2 thin film with Co content of 5.77%..

The XRD patterns of Co-doped TiO2 thin films with Co content up to 5.77% are shown in fig. 5. It shows that the crystal structure of the films does not change for Co concentration up to 5.77%. The films remain in rutile structure with single orientation of (002) plane.

Fig. 6 shows XRD pattern of rutile (002) structure of TiO2and Co-doped TiO2 films deposited at temperature of 450oC.The peaks are clearly separated and (002) peak of Co-doped TiO2 slightly shifts toward a higher angle, which indicates a shorter lattice constant along c-axis. This result suggest that a good solid solution of rutile Co-doped TiO2 was formed. At low deposition temperature, the intensity of rutile (002) peak become weaker and broader (not shown), and disappear when substrate temperature was lower than 350oC.

The dependence of Co concentration on the magnetic properties of Co-doped TiO2 thin films at room temperature were investigated by a VSM measurement. The magnetization (M) versus field (H) curves of the films with Co concentration

up to 5.77% are shown in Fig. 7. Hysteresis behavior was observed, indicating that the Co-doped TiO2 rutile thin films are ferromagnetic at room temperature. The hysteresis behaviors are characterized by coercive (Hc) and saturation (Ms) magnetic field which describe the anisotropy properties and magnetic permeability of the film, respectively. As seen on Fig. 7, both Hc and Ms depend on the Co concentration. Film with Co concentration of 1.83% has higher coercive field, i.e. Hc=100 Oe compared to the others, and therefore has higher anisotropy property. Meanwhile the highest Ms of 3.3 emu/cm3

is found in the film with Co concentration of 5.77%.

Figure 7. The magnetization (M) hysteresis loops of Co-doped TiO2 rutile thin films with Co contents of (a) 0.41%, (b) 1.83%, (c) 2.97%, and (d)

5.77%.

Figure 8. Temperature dependence of the resistivity of Co-doped TiO2 thin films.

In order to determine the conduction mechanism in Co-doped TiO2 thin films, the temperature dependence of the electrical resistivity was studied (Fig. 8). The films with Co concentration up to 11 % indicate semiconductor behavior as shown by the increases of electrical resistivity as the temperature decreases. The plots suggest that there are two types of conduction mechanism contribute to the resistivity in the two temperature ranges. As seen from Fig. 8 it is possible

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(c) M(e

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to find the best-fitting function in the low (up to 140K) and high temperature (from 140K up to 300K) ranges. In the low temperature ranges, the resistivity characteristic obeys T-1/4, [

T1/2 exp(T0/T)1/4] [14]. According to the Mott model, this characteristic describes a three-dimensional (3D) variable range hopping of conductivity between localized states. Whereas, at high temperature ranges, the best fit for the conduction characteristic was obtained by ~ exp( E/T) with

E is an activation energy, [ exp( E/kT)]. Thus, at high temperatures ranges, the resistivity exhibits a thermally activated process. The conductivity mechanism is mainly determined by hopping of carriers thermally activated into the band tails as mono-energetic trap state and becomes thermodynamically accessible at higher temperatures.

IV. CONCLUSSION

The growth of Co-doped TiO2 thin films by MOCVD method has been investigated. Using the optimum deposition parameters, rutile TiO2 and Co-doped TiO2 films were grown. In this method, the solubility of Co dopant in TiO2 thin films is found at around 10.4 %. It is also found that the crystal structure of TiO2 remain the same as it is doped by Co up to a concentration of 5.77%. Co-doped TiO2 thin films exhibit ferromagnetic behaviors at room temperature. The films with Co content up to 11.0% indicate semiconductor behavior which characterized by two types of conduction mechanism. At low temperature the conduction mechanism is a three-dimensional (3D) variable range hopping of conductivity between localized states, whereas at higher temperatures the conductivity mechanism is mainly determined by thermally activated hopping of carriers.

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