Evolution of different structural phases of TiO2 films with oxygen partial pressure and Fe doping and their electrical properties

Materials Research Bulletin 47 (2012) 2001–2007

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Evolution of different structural phases of TiO2 films with oxygen partial pressure and Fe doping and their electrical properties Komal Bapna, R.J. Choudhary *, D.M. Phase UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452001, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 October 2011 Received in revised form 3 February 2012 Accepted 5 April 2012 Available online 15 April 2012

We have studied the influence of oxygen partial pressure (OPP; 250 mTorr–1  10 5 Torr) and Fe doping (2 and 4 at.%) on structural and electrical properties of TiO2 thin films on LaAlO3 substrates. X-ray photoelectron spectroscopy suggests that Fe is not in metal cluster form. It is found that the evolution of the three phases; anatase, rutile and brookite of TiO2 as well as the magneli phase (TinO2n 1) strongly depends on the OPP and Fe doping concentration. All the films grown at 250 mTorr show insulating behavior, whereas films grown at 1  10 2 and 1  10 4 Torr reveal high temperature metallic to low temperature semiconducting transition. Interestingly, films deposited at 1  10 5 Torr reveal charge ordering, which is contributed to the magneli phase of TiO2. The present study suggests that functionality of TiO2 thin film based devices can be tuned by properly selecting the OPP and dopant concentration. ß 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Thin films A. Magnetic materials B. Laser deposition C. X-ray diffraction C. Photoelectron spectroscopy D. Electrical properties

1. Introduction A pooled semiconducting and magnetic behavior in diluted magnetic semiconducting (DMS) oxide materials has a huge prospect in the swiftly growing area of spintronics [1]. To realize DMS materials, magnetic impurity is doped in an otherwise nonmagnetic semiconducting system. The magnetic interaction between the magnetic ions via charge carriers provides a coupling between the charge and spin degrees of freedom of electrons [2]. In recent past a huge attention has been paid toward the DMS materials based on TiO2 as a host semiconducting material, owing to its several attracting properties and applications in optoelectronic devices [3,4]. However, a wide range of diverging results related to magnetic, electrical and magneto-transport properties has emerged from such studies. There have been debates regarding the origin of magnetization in these systems, whether it is due to intrinsic nature of carrier mediated type or due to some extrinsic effect [5–10]. The extrinsic effect could be due to the formation of magnetic impurity clusters or formation of secondary magnetic impurity phase in the host matrix. It is expected that the divergence of various results could be due to sensitivity of crystal structure of TiO2 on the processing conditions, which would maneuver its electrical, magnetic, optical or any other related property.

* Corresponding author. Tel.: +91 731 2463913; fax: +91 731 2462294. E-mail address: [email protected] (R.J. Choudhary). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.011

Mainly TiO2 is found in three crystalline forms: rutile, anatase and brookite. The crystal structure, lattice constant and symmetry of these phases are given in Table 1. The band gaps of anatase and rutile phases of TiO2 are 3.2 eV and 3 eV respectively. Accordingly they show insulating behavior; however, by creating oxygen vacancy its conductivity can be increased [11]. In bulk form, rutile phase of TiO2 is more stable than its anatase phase, whereas brookite is highly unstable [12]. A sufficient oxygen vacancy in TiO2 is known to create magneli phases (TinO2n 1), which exhibit miscellaneous electrical transport properties; such as Ti5O9 is a polaronic insulator with a charge ordering, while Ti4O7 shows metal to semiconducting transition upon cooling due to bipolaronic ordering [13]. In thin film form the different phases of TiO2 can be controlled by a proper choice of the substrate and deposition condition. There have been various studies related to the transition metal (TM) doped TiO2 thin films for its exploration in DMS applications [14–18]. Since the crystal field symmetry in anatase, rutile, brookite or magneli phases is different, the electron phonon coupling is different leading to difference in their electrical, optical and magnetic properties. Therefore, magnetism in TiO2 based DMS thin films is sensitive to its formation in rutile phase or anatase phase [17]. Though there are various reports available in the literature on TiO2 based DMS films, most of the reports either discuss the effect of OPP on the growth of TiO2 [19] or describe the effect of dopant concentration at a particular OPP on the structural, electrical, magnetic or magneto-transport properties [5,8,15]. In the present work, we have studied the effect of oxygen partial pressure (OPP) and Fe doping on the

Anatase Rutile Brookite

Tetragonal Tetragonal Orthorhombic

a = b = 3.784, c = 9.512 a = b = 4.593, c = 2.985 a = 9.211, b = 5.472, c = 5.171

I41/amd Pbca P42/mnm

structural and electrical properties of TiO2 films. It transpires that variation in OPP and Fe concentration brings about the evolution of TiO2 structural phases differently having different electrical properties.

L

A

L

L

A

2. Experimental

20

1x 1 0 -2 T o rr L

A

R

1 x 1 0 -4 T o r r

A /M

1 x 1 0 -5 T o rr

L

L

2 5 0 m T o rr

A(008)

Symmetry

L(002)

Lattice constant (A˚)

L(001)

Crystal structure

Intensity (Arb. Units)

L

Phases

T iO 2

L

30

40

L

A

LAO

50

60

A /M

L (003)

(a )

L

R(210)

Table 1 Crystal structure, lattice constant and symmetry of various phases of bulk TiO2.

A(004)

K. Bapna et al. / Materials Research Bulletin 47 (2012) 2001–2007

2002

70

80

3. Results and discussion We performed the XRD measurements at high power (4 kW, current 100 mA and voltage 40 kV) of undoped and Fe doped TiO2 thin films deposited under various OPP values. In Fig. 1(a) and (b) we show the XRD patterns obtained for undoped and 2% Fe doped TiO2 films respectively, deposited at different OPP values. The pattern taken at high power and plotted in log scale will clearly reveal the presence of any impurity such as Fe clusters or formation of FeO or Fe2O3 or Fe3O4 phase if they form in TiO2 or co-occurrence of different phases of TiO2 in these films, if they are present in crystalline form and having a detectable size. The XRD patterns of the films are plotted along with that of the LAO substrate to distinguish the films XRD peaks and substrate XRD peaks. Since the lattice parameter of LAO (0.3788 nm) matches more closely with the anatase phase of TiO2 (0.3785 nm) than any other phase, one would expect a favorable anatase phase nucleation on LAO substrate. It is important to mention here that the nucleation of the anatase phase of TiO2 is independent of the

(b )

L

L

T i 0 .9 8 F e 0 .0 2 O 2 2 5 0 m T o rr

L

A

L

L

-2

A

1x1 0 T o rr L

L

A

L A

-4

1x1 0 T o rr L -5

1x1 0 T o rr

30

40

A L(002)

L(001)

20

L

A L(002)

B (020)

L

A (008)

L

Intensity (Arb. Units)

The targets used for the deposition were sintered pellets of Ti1 xFexO2 (x = 0, 0.02, 0.04) prepared by solid-state reaction route. 200 nm thick films (measured by Stylus Profilometer) were grown by the pulsed laser deposition (PLD) technique using KrF excimer laser (248 nm) on (0 0 1) LaAlO3 (LAO) substrates heated at 600 8C. Deposition was carried out at different OPP values varying in the range of 250 mTorr–1  10 5 Torr. Before the deposition, substrates were chemically cleaned in ultrasonic bath sequencely with tri-chloro ethylene, acetone and methanol for 5 min each to remove any oil or hydroxyl adsorbates on the substrate surface. The laser repetition rate and energy density at the target surface for the deposition were kept at 10 Hz and 2 J/cm2 respectively. After the deposition, substrates were cooled down to room temperature in the same OPP as used during the deposition, except for the film deposited at 250 mTorr. For the film deposited at OPP of 250 mTorr, the chamber was filled with 400 Torr of O2 after deposition and then cooled. Structural properties of these films are studied with X-ray diffraction (XRD) (Cu Ka source, Rigaku diffractometer) and Raman spectroscopy measurements (model HR-800, Jobin Yvon) using He–Ne laser (l = 632 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using Omicron Energy Analyzer (EA 125) instrument with Al Ka (1486.6 eV) X-ray source. The pressure of the analyzer chamber was in order of 1  10 10 Torr during the XPS measurement. The analysis of different oxidation states of ions was performed by deconvolution of unresolved peaks. The value corresponding to C 1s peak was used as a reference for spectrum analysis. Electrical properties were studied using four-probe resistivity technique. Magnetic measurments have been performed using superconducting quantum interference device (SQUID)-vibrating sample magnetometer (SVSM; Quantum Design Inc., USA).

A (004)

2 θ (D e g r e e )

LAO

50

60

2 θ (D eg ree )

70

80

Fig. 1. XRD patterns of (a) undoped; and (b) 2 at.% Fe doped TiO2 films on LAO substrate deposited at various OPP values. Symbols L, A, R, B and M represent LAO, anatase, rutile, brookite and magneli phases respectively.

nature of termination of LAO substrate, whether it is terminated with LaO layer or AlO2 layer. Though, the different termination may affect the interface between TiO2 and LAO substrate and its electronic properties [20,21]. We observe from the pattern that in most of the films, though the most intense peak is due to (0 0 4) reflection of anatase phase of TiO2, there are signatures due to brookite and rutile phases also in some of the films. The obtained different phases of TiO2 in these films are tabulated in Table 1. Interestingly, single anatase phase is observed in all the films deposited at OPP of 1  10 2 Torr. Besides this, we note that evolution of different phases of either undoped or Fe doped samples grown at any other OPP does not follow a systematic trend. The undoped and Fe doped films grown at 1  10 5 Torr show that (0 0 4) reflection is shifted toward higher 2u value of 38.18 as compared to 37.98 observed in other films. Since some of the oxygen deficient magneli phases also reveal reflection close to 38.18, therefore, it is difficult to confirm from XRD pattern whether this reflection is due to oxygen deficient anatase phase or magneli phase. Hong et al. in their work on Fe and Ni doped TiO2 films grown on LAO and SrTiO3 (STO) substrates showed that films with various Ni contents on STO were anatase, and the films of Ni 4.3% and 5.2% grown on LAO were anatase as well, however, the films of Ni 3.6% and 4.6% on LAO were rutile [22]. Close to this observation, for pure and Fe doped TiO2 films, we notice different XRD patterns for films grown at any OPP value (except at OPP of 1  10 2 Torr). These results emphasize that stabilization of any pure phase of TiO2, though, critically depends on the OPP used during deposition, the coexistence of the different phases or occurrence of any of the pure phase will also hugely depend on the nature and concentration of dopants.

K. Bapna et al. / Materials Research Bulletin 47 (2012) 2001–2007

250mTorr -2 1x10 Torr -4 1x10 Torr -5 1x10 Torr

Intensity (Arb. Units)

Fe 2p

TiO2

Intensity (Arb. Units)

2003

Fe 2p

-2 1x10 Torr Ti0.98Fe0.02O2 Ti0.96Fe0.04O2 729 720 711 2+ B. E. (eV)

Satellite

730

Fe

725

2p

Fe

2p

Satellite

1/2

Fe

2+

Fe

3+

720

715

3/2

710

3+

705

B.E. (eV) Fig. 3. XPS spectra for Fe 2p core level for Ti0.96Fe0.04O2 thin film deposited at OPP of 1  10 4 Torr. The inset shows the spectra for Fe 2p core level for Ti0.98Fe0.02O2 and Ti0.96Fe0.04O2 thin films deposited at OPP of 1  10 2 Torr.

Ti0.98Fe0.02O 2

Ti0.96Fe0.04O

14

16

18

20

22

24

θ (degrees) Fig. 2. Rocking curves for the main peak at 37.98 of undoped and Fe doped TiO2 thin films deposited at various OPP values.

To further study the effect of OPP on the crystallinity, we carried out the rocking curve of the undoped and Fe doped samples for the (0 0 4) reflection of anatase TiO2 phase. In Fig. 2, we show the rocking curves of (0 0 4) reflection obtained for pure and Fe doped (2 and 4 at.%) films (please note the log scale of y-axis) and the full width at half maxima (FWHM) for all the films are given in Table 1. We note that films of only anatase phase show minimum FWHM values, suggesting their excellent crystalline quality. As expected, broader FWHM is observed in films having mixed phases of TiO2 as compared to the pure anatase phase film. Interestingly, all the 2% Fe doped TiO2 films reveal a lower FWHM in rocking curve, suggesting that 2% Fe doping further improves the crystallinity of the anatase phase. The FWHM of pure anatase phase observed in 2% Fe doped TiO2 film is 0.208, which is very close to that observed for FWHM of rocking curve (0.148) for single crystal LAO (0 0 2) reflection, indicative of the excellent crystalline nature of the film. We also note the largest value of FWHM of rocking curves of the undoped and Fe doped films deposited at 1  10 5 Torr, suggesting that these films are of very poor crystallinity. By looking at the XRD data cards it transpires that the (0 0 4) reflection of anatase phase of TiO2 is very close to either (2 0 0) reflection of magneli Ti5O9 phase (PCPDF #761690) or ( 2 2 0) reflection of Ti4O7 phase (PCPDF #771391). If such magneli phase is grown on LAO substrate, one would expect a larger value of FWHM in rocking curve due to huge lattice mismatch between LAO and TinO2n 1 phase, resulting in poor crystallinity of the film. Therefore, the observed broad FWHM in rocking curves for films deposited at 1  10 5 Torr suggests that the observed peaks in normal u–2u scan could be due to emergence of magneli phase Ti5O9 or Ti4O9 rather than oxygen deficient anatase TiO2.

To know the valence state of Fe in TiO2 matrix, we performed XPS measurements for Fe 2p core level as shown in Fig. 3 for 4% Fe doped TiO2 film grown at 1  10 4 Torr as an example. Features of Fe 2p core level spectrum were fitted well with combined Gaussian–Lorentzian functions. A doublet feature occurs corresponding to Fe 2p3/2 and Fe 2p1/2, which is further decomposed into two doublets. The binding energy positions corresponding to Fe 2p3/2 and Fe 2p1/2 for Fe2+ are observed at 709.2 eV and 722 eV, along with a satellite at 715.5 eV. Whereas, the binding energy positions corresponding to Fe 2p3/2 and Fe 2p1/2 for Fe3+ are observed at 710.8 eV and 724.4 eV along with a satellite at 720 eV. These values are consistent with the previously reported binding energy positions of Fe 2p3/2 and Fe 2p1/2 levels of Fe2+ and Fe3+ and their respective satellite features [23]. The main peak of Fe 2p3/2 in case of Fe metal clusters occurs at 706 eV. Therefore XPS data clearly rules out the possibility of formation of Fe metal clusters in TiO2 matrix. Similar pattern has been observed for the other films (grown with different OPP and Fe concentration, for reference, core level spectra of 2% and 4% Fe doped samples grown at OPP of 1  10 2 Torr are shown in the inset of Fig. 3). To further understand the structural properties of these films, we performed Raman spectroscopy measurements at room temperature of these films as shown in Fig. 4. Primarily we observe Raman modes at 197, 399, 516 (515 and 519 cm 1 bands are convoluted) and 639 cm 1 corresponding to anatase phase [24] in all the samples except undoped TiO2 film grown at 1  10 5 Torr. This confirms that most of the films have anatase phase as main character, consistent with the XRD findings. In undoped TiO2 film grown at 1  10 5 Torr, no mode of anatase TiO2 phase is observed. This is in line with our XRD analysis of the films that the grown film may not be due to anatase TiO2 but some magneli phase. We cannot compare our observed Raman spectra of these films with standard Raman spectra of Ti4O7 and Ti5O9, since their room temperature Raman spectra are not available in the literature to the best of our knowledge. Also in 4% Fe doped film grown at 1  10 5 Torr, besides the Raman modes corresponding to the TiO2 anatase phase, a Raman mode is observed close to that obtained for undoped TiO2 film grown at 1  10 5 Torr (shown by (M) in Fig. 4(a) and (c)), which we suspect to be due to magneli phase. This suggests that in 4% Fe doped film grown at 1  10 5 Torr, we have mixed phases of anatase TiO2 and the magneli phase. The FWHM of its rocking curve also revealed much lower value as compared to the undoped TiO2 film grown at same pressure, suggesting that while undoped film grown at 1  10 5 Torr has only Ti5O9 phase, 4% Fe doped film has anatase as well as some magneli phase. We also observe some Raman

K. Bapna et al. / Materials Research Bulletin 47 (2012) 2001–2007

2004

(a)

A

A

A

TiO2

S

250 mTorr

S

A

Intensity (Arb. Units)

-2

1x10 Torr -4

1x10 Torr

R R

M -5

1x10 Torr

200

M

300

400

500

600

-1

Raman shift (cm )

A

A

(b)

Ti0.98Fe0.02O2

A

Intensity (Arb. Units)

250 mTorr

S

A

-2

1x10 Torr

S

R

-4

1x10 Torr -5

1x10 Torr 200

R

S S

300

400

500

600

-1

Raman shift (cm )

(c)

Ti0.96Fe0.04O2

Intensity (Arb. Units)

250 mTorr A

B

B

B

B

R

A

B

R

A

-2

1x10 Torr -4

1x10 Torr

A

-5

1x10 Torr M 200

300

400

500

600

-1

Raman Shift (cm ) Fig. 4. Raman spectra of (a) undoped; (b) 2 at.% and (c) 4 at.% Fe doped TiO2 thin films on LAO substrate deposited at various OPP values. Symbols A, R and M represent anatase, rutile and magneli phases respectively.

modes corresponding to rutile (at 445, 612 cm 1) [25] or brookite phase (at 247, 322, 367, 543 cm 1) [26] in a few films, confirming their presence in the respective films as shown in Fig. 4. Thus Raman spectra corroborate with the findings of XRD results that the dependence of OPP on the phase of TiO2 is different for Fe doped samples. Now we discuss the possible reasons behind the dependence of OPP and Fe concentration on the occurrence of different phases of TiO2. We would like to mention here that though the respective bulk targets of undoped or Fe doped TiO2 had mixed crystalline phases of rutile and anatase, the dominant phase was anatase in undoped TiO2 bulk target and rutile phase in Fe doped TiO2 bulk targets. However, during the film growth by PLD, the constituent species or ions of the target material ejected from the target after ablation form a plasma plume, which condense on the heated substrate. Then the film crystalline structure is controlled by the growth parameters like nature of substrate, substrate temperature, OPP, target to substrate distance, laser energy, repetition rate and energy of the ad-atoms. Hence, evolution of different phases in thin film form (Table 2) cannot be understood by the target structure history in the present study. Thus now we look into the other aspects of the film growth in the present case. The (0 0 1) plane of the anatase phase of TiO2 has a good lattice matching to the (0 0 1) LAO surface and the similarity in the arrangements of the oxygen sub-lattice with that of the LAO substrate seeds the formation of anatase phase [27]. However, if we vary oxygen concentration in the film from its optimal value, the oxygen sub-lattice will be modified and its interface with the LAO substrate will be strained. In an earlier report Schuisky et al. suggested that rutile crystallites grow from the surface of the anatase films at grain boundaries and dislocations [28]. It is known that in thin films dislocations occur to accommodate the strain present in the films, which is created due to mismatch with the substrate or due to defects. Thus a strained interface, due to variation in OPP during deposition, would lead to the formation of TiO2 rutile or brookite phase also. The difference in crystal structures of Fe doped films with the undoped film could also be accounted to the modified oxygen sublattice in Fe doped samples. Since Fe charge state is lesser than 4+ state of Ti in TiO2, the doping of Fe in TiO2 would induce oxygen vacancy to maintain charge neutrality and accordingly modify the oxygen sublattice. This will lead to difference in the evolution of various phases at different OPP as compared to the undoped film as revealed by XRD. It is also known that energetically anatase and rutile phases are very close to each other at high temperature 600 8C [29], which happens to be the substrate temperature during deposition. Therefore, it turns out that at this deposition temperature there is always a likelihood of co-occurrence of rutile and anatase phases. Also LAO substrate produces strain on the films 0.5 kbar. Looking into the phase diagram of TiO2, we find that such a strain provides a small window for coexistence of anatase, rutile and brookite phases [29]. Though at 600 8C, the conversion between brookite and rutile needs rather higher pressure, Fe doping can play a crucial role in controlling its phase by inducing additional strain effect (due to ionic radii mismatch between Fe2+/3+ and Ti4+), besides the strain effect due to OPP and substrate. Hence in such a scenario, variation of OPP, doping of transition metal ions and its

Table 2 Occurrence of various phases of TiO2 films at different OPP for undoped and Fe doped TiO2 films and their rocking curve FWHM values (8). 250 mTorr

TiO2 Fe0.02Ti0.98O2 Fe0.04Ti0.96O2

1  10

2

Torr

1  10

4

Torr

1  10

5

Torr

Phases

FWHM

Phases

FWHM

Phases

FWHM

Phases

FWHM

A+R A A+R+B

0.34 0.20 0.47

A A A

0.30 0.29 0.34

A+R A A+B

0.89 0.33 0.41

A/M A+B A/M

2.40 0.35 0.66

K. Bapna et al. / Materials Research Bulletin 47 (2012) 2001–2007

concentration would closely maneuver the phase of undoped and doped TiO2 films. Recently Li et al. showed the effect of doping and asserted that when Fe is used as a dopant, there was a minor difference in the formation energy of rutile and anatase phases [30]. This further confirms the possibility of the formation of partial rutile phase in Fe doped TiO2 even if it is grown on LAO substrate, which provides an excellent lattice matching with the anatase phase of TiO2. To understand the effect of variation in OPP on the electrical properties of the present films, we performed four-probe resistivity measurements. In Fig. 5(a)–(c), we show resistivity behavior of films deposited at OPP values of 1  10 2, 1  10 4 and 1  10 5 Torr for undoped and Fe doped samples. Films deposited at 250 mTorr were highly resistive and could not be measured with the available experimental set-up. It is known that conductivity in TiO2 is due to oxygen vacancy, which forms a shallow donor level [15,31]. In the absence of oxygen vacancies, it is expected to be highly resistive, as in the case of films (undoped or Fe doped) grown at OPP of 250 mTorr. From Fig. 5(a) and (b), it is clear that all the films grown at 1  10 4 Torr and 1  10 2 Torr OPP values show a transition from high temperature metallic behavior to low temperature semiconducting behavior. Such behavior is expected

-2

1x10 Torr

(a) Ti0.98Fe0.02O2

ρ (Ω−cm)

1

Ti0.96Fe0.04O2

0.1 TiO2 0

50

100

150 T (K)

200

250

300

-4 (b) 1x10 Torr

0.1

ρ(Ω−cm)

Ti0.96Fe0.04O2

TiO2

ln ρ

TiO2-d

0.0016

0.0

8E-4 0

0.1 -1 0.2 1/T (K )

50

100

Ti0.98Fe0.02O2 150

200

250

300

T (K)

-5

0.020

1x10 Torr

0.018

TiO2 Ti0.98Fe0.02O2

0.016

100 10 1

Ti0.96Fe0.04O2

0.014

ρ (Ω-cm)

ρ (Ω-cm)

1000

(c)

0.022

0.1

0.012

0.01

0.010 0

50

100

150

200

250

1E-3 300

T (K) Fig. 5. (a) Resistivity behavior with temperature for undoped and Fe doped TiO2 films grown at OPP at (a) 1  10 2 Torr; (b) 1  10 4 Torr; and (c) 1  10 5 Torr. The inset in (b) shows the deviation from thermally activated Arrhenius behavior [r = r0 exp( EA/kT)] in undoped TiO2 film deposited at OPP value of 1  10 4 Torr.

2005

in oxygen deficient TiO2 films, as reported earlier. It is envisaged that in these films, donor band due to oxygen vacancies overlap with the conduction band, giving rise to high temperature metallic behavior. However, there are debates regarding the low temperature semiconducting behavior in such films. Different electric transport behaviors such as thermally activated [15], variable range hopping [32] and small polaron hoping (SPH) [33] behaviors are proposed to explain the semiconducting behavior of TiO2. Recently Yildiz et al. proposed that Fe doping in TiO2, further increases the magnitude of SPH coupling [33]. In the present study, we attempted to fit the low temperature semiconducting behavior with all the models mentioned above. However, none of these models gives a proper fitting in the semiconducting regime of any of these samples. An example of deviation from thermally activated Arrhenius behavior [r = r0 exp( EA/kT)] in undoped TiO2 film deposited at OPP value of 1  10 4 Torr is shown in inset of Fig. 5(b). It is quite possible that in such films oxygen vacancy is not uniformly distributed, which would cause non-Arrhenius behavior. The electrical transport behavior of such semiconducting state was explained by Michel et al. [34]. They suggested that in a semiconducting material non-Arrhenius resistivity behavior could also occur in a band transport model, provided the energy distribution of reservoir states (donor state in the present case) is sufficiently broadened or the density of band states exhibits exponential tails. Both the possibilities lead to non-linear shift of the Fermi level as a function of temperature, resulting in a non-Arrhenius temperature dependence of the resistivity [34]. Therefore, absence of thermally activated behavior in our films does not rule out the possibility of band transport model in these films. However, a further study is needed to elaborate this aspect in these films. (Recently we have shown that undoped and 4% Fe doped TiO2 films reveal Kondo like scattering at low temperature [35].) The resistivity behavior of films deposited at 1  10 5 Torr is quite different from rest of the films, as is shown in Fig. 5(c). Undoped TiO2 shows semiconductor behavior in the studied temperature range of 300–4 K. The resistivity behavior follows thermally activated behavior in the temperature range of 300– 175 K. At 145 K, we note an abrupt change in slope of the resistivity behavior and resistivity increases sharply below this temperature. The overall change in resistivity of this film is from 3 mV cm at room temperature to 400 V cm at 4 K. The sharp increase in resistivity below 145 K can be attributed to the charge ordering of Ti3+–Ti3+ ions as suggested in the case of oxygen deficient Ti5O9 phase. Indeed, the XRD pattern also confirms the magneli phase of this film. Interestingly, 2% Fe doped film deposited at OPP value of 1  10 5 Torr reveals a high temperature metallic behavior, which turns abruptly into an insulating behavior below 150 K. There is further change in slope of the resistivity behavior below 100 K. This observation is interesting in the sense that either XRD pattern or Raman spectra of 2% Fe doped sample did not indicate the presence of magneli phase in the film. However, the resistivity behavior is similar to those reported for Ti4O7. Since these films are grown at lowest OPP value of 1  10 5 Torr in the present study, the possibility of occurrence of magneli phase is quite likely to happen. From XRD pattern it could not be distinguished since the (0 0 4) peak of anatase TiO2 matches closely with ( 2 2 0) peak of Ti4O7 (PCPDF #771391). Chakraverty and Schlenker suggested that ordered bipolaron and liquid bipolaron states were the roots for different trends of resistivity below 150 K [36]. It is recalled here that in TiO2 d, one oxygen vacancy is associated with two electrons. It may happen that one of these electrons would convert neighboring Ti4+ ion into Ti3+ ion and another would be delocalized in TiO2. This delocalized electron can give rise to metallicity behavior observed at high temperature. This itinerant electron, if trapped between two Ti

K. Bapna et al. / Materials Research Bulletin 47 (2012) 2001–2007

2006

at 1  10 2 Torr, exhibiting single phase anatase structure and having highest magnetic dopant concentration among all Fe doped films. The magnetization versus magnetic field behavior of the sample at room temperature is shown in the inset of Fig. 7, which clearly reveals its magnetic hysteresis behavior. The drop in magnetization at higher field is due to the diamagnetic contribution of the LAO substrate. To get an estimation of the Curie temperature (TC), we performed magnetization versus temperature measurement at magnetic field value of 300 Oe using SQUIDVSM Oven option, as shown in Fig. 7. The TC is observed to be 620 K. The overall magnetic properties suggest the system to exhibit magnetic ordering, however further work is required to strike a relation between the electrical and magnetic property of such samples. 4. Conclusions

Fig. 6. Different phases obtained in pulsed laser deposited TiO2 films on (0 0 1) oriented LaAlO3 substrate as a function of oxygen partial pressure (OPP) and Fe doping concentration. Symbols A, R, B and M represent anatase, rutile, brookite and magneli phases respectively. Subscript symbols indicate the presence of the phases in very small amount. A schematic of different electrical states of these films are also represented; CO stands for charge ordering obtained in the films deposited at OPP value of 1  10 5 Torr.

ions, can cause formation of a bipolaron, subsequently, increasing the resistivity below a certain temperature. The resistivity behavior of 4% Fe doped TiO2 film deposited at OPP value of 1  10 5 Torr is also consistent with magneli phase as reported by Nakajima et al. [13]. An overview of the above results is graphically presented in Fig. 6. These results strongly suggest that the oxygen partial pressure and TM dopant concentration enormously affect the phase and structure of TiO2 vis-a-vis their electrical transport properties. It is known that most of the applications of TiO2 are susceptible to its structure. Therefore, while exploring TiO2 based system for DMS applications or for that reason any other application such as photo-chromatic or catalytic, one has to judicially select the three important parameters such as oxygen partial pressure, nature of dopant and dopant concentration, which will enormously affect the functionalities of devices. As for exploring these TiO2 based systems for DMS applications, the magnetic properties of these systems are also very crucial. To see the magnetization property, we have performed magnetization measurements for 4% Fe doped TiO2 sample grown

Ti0.96Fe0.04O2

4

-2

3 50

2

M (emu/cc)

M (emu/cc)

(1x 10 Torr)

0

-50

1

-2000

0

2000

H (Oe)

340

440

540

640

Temperature (K) Fig. 7. Magnetization (M) versus temperature (T) curve for Ti0.96Fe0.04O2 grown at 1  10 2 Torr OPP. Inset shows M versus magnetic field (H) curve of the same sample at room temperature.

In conclusion, we have deposited undoped and Fe doped (2 and 4 at.%) TiO2 films on (0 0 1) oriented LaAlO3 substrate at various OPP values and studied the evolution of different phases of TiO2. Pure anatase phase is observed in undoped and Fe doped TiO2 films only at OPP of 1  10 2 Torr. Other than this OPP value, the phases obtained in undoped TiO2 films at different OPP are not followed in the same fashion in Fe doped films. Films grown at lower OPP show formation of some magneli phase of TiO2 which illustrate charge ordering at low temperature. XPS studies confirm that Fe is not in metal cluster form in the doped films. The Curie temperature for 4% Fe doped TiO2, showing single phase of anatase TiO2 is found to be 620 K. Our results divulge that stabilization of any pure phase of TiO2, though, critically depends on the substrate and OPP used during deposition, the coexistence of the different phases or occurrence of any of the pure phase will also hugely depend on the nature and concentration of dopants as well. Acknowledgments We are thankful to Dr. N.P. Lalla for XRD, Dr. V. Sathe for Raman spectroscopy and Dr. R. Rawat for four-probe resistivity measurements. References [1] I. Zˇutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76 (2004) 323–410. [2] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488–1495. [3] N.R. Mathews, M.A.C. Jacome, E.R. Morales, J.A.T. Antonio, Phys. Status Solidi C 6 (2009) S219–S223. [4] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, J. Phys. Chem. Solids 63 (2002) 1909– 1920. [5] S.B. Ogale, Adv. Mater. 22 (2010) 3125–3155, and references therein. [6] T. Jungwirth, J. Sinova, J. Masˇek, J. Kucˇera, A.H. MacDonald, Rev. Mod. Phys. 78 (2006) 809–864. [7] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173–179. [8] Z. Wang, W. Wang, T. Jinke, L.D. Tung, L. Spinu, W. Zhou, Appl. Phys. Lett. 83 (2003) 518–520. [9] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019– 1022. [10] J.M.D. Coey, P. Stamenov, R.D. Gunning, M. Venkatesan, K. Paul, New J. Phys. 12 (2010) 053025–053038. [11] B.-S. Jeong, D.P. Norton, J.D. Budai, Solid-State Electron. 47 (2003) 2275–2278. [12] S.-D. Mo, W.Y. Ching, Phys. Rev. B 51 (1995) 13023–13032. [13] T. Nakajima, T. Tsuchiya, T. Kumagai, J. Solid State Chem. 182 (2009) 2560– 2565. [14] S. Duhalde, M.F. Vignolo, F. Golmar, C. Chiliotte, C.E. Rodrı´guez Torres, L.A. Errico, A.F. Cabrera, M. Renterı´a, F.H. Sa´nchez, M. Weissmann, Phys. Rev. B 72 (2005) 161313R–161316R. [15] S.R. Shinde, S.B. Ogale, S. Das Sarma, J.R. Simpson, H.D. Drew, S.E. Lofland, C. Lanci, J.P. Buban, N.D. Browning, V.N. Kulkarni, J. Higgins, R.P. Sharma, R.L. Greene, T. Venkatesan, Phys. Rev. B 67 (2003) 115211–115214. [16] W. Prellier, A. Fouchet, B. Mercey, J. Phys.: Condens. Matter 15 (2003) 1583R– 1601R. [17] K.J. Kim, Y.R. Park, J.Y. Park, J. Korean Phys. Soc. 48 (2006) 1422–1426. [18] M.S. Dabney, M.F.A.M. van Hest, C.W. Teplin, S.P. Arenkiel, J.D. Perkins, D.S. Ginley, Thin Solid Films 516 (2008) 4133–4138.

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