Magnetic, electric and crystallographic properties of diluted magnetic InSe(1−x)Fe(Co)x semiconductor

Journal of Alloys and Compounds 530 (2012) 102–106

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Magnetic, electric and crystallographic properties of diluted magnetic InSe(1−x) Fe(Co)x semiconductor Karimat El-Sayed a,∗ , Z.K. Heiba a,b , K. Sedeek c , H.H. Hantour c a

X-Ray Dffraction Unit, Physics Department, Faculty of Science, Ain Shams University, Egypt Department of Physics, Faculty of Science, Taif University, 888 Taif, Saudi Arabia c Physics Department, Faculty of Science, Girls Branch, Al-Azhar University, Egypt b

a r t i c l e

i n f o

Article history: Received 20 December 2011 Received in revised form 19 March 2012 Accepted 22 March 2012 Available online 30 March 2012 Keywords: Electric Magnetic Diluted magnetic semiconductors InTe Microstructures

a b s t r a c t The structural, magnetic and electric properties of Fe or Co doped InSe system has been studied. The X-ray diffraction patterns of the doped samples indicate the presence of InSe0.9 Fe0.1 or InSe0.9 Co0.1 , together with a non-magnetic minor phase of In4 Se3 . The InSe0.9 Fe0.1 system is ferromagnetic with high Curie temperature of 870 K. In contrast, the InSe0.9 Co0.1 system is antiferromagnetic with different Neel temperatures. Crystallite sizes of the different phases show anisotropy along different crystallographic directions, they vary from 9 to 40 nm. The largest size is along the [0 0 l] direction normal to the staking layers planes. The random model was applied to explain the origin of ferromagnetic properties of Fe doped sample. The non-magnetic phase In4 Se3 played a major role in the high temperature ferromagnetic properties of InSe0.9 Fe0.1 and the polarization of magnetic spins. The electrical conductivity increased by an order of magnitude of 2 and 1.5 in the case of Fe and Co doped samples, respectively, suggesting that the incorporation of Fe or Co creates new band configuration and hence a modification of electronic density of states of the samples studied. The anomaly in the electrical properties after doping with Fe or Co may suggest that these doped samples may be used as spintronics materials. © 2012 Elsevier B.V. All rights reserved.

1. Introduction There is an increasing interest in the III–VI materials, which have applications in optoelectronic, photovoltaic industries, and photo electrochemical solar cell devices [1]. Some solar cell devices perform efficiency of 16.4% [2]. The materials also draw attention for their switching and memory effects [3]. InSe is an n-type semiconductor, belonging to the III–VI layered semiconductor family [4]. It has a direct optical band gaps in the range of 1.42–1.62 eV, and indirect band gaps varying from 0.83 to 1.29 eV [5]. It has a potential application as an absorber layer in photovoltaic devices [6]. Its high absorption coefficient as well as its optimum energy band gap, makes it suitable for solar energy conversion [7]. The structure has strong covalent bonding within the layer planes and weak Vander Waals bonding between planes inducing easy cleavage [8]. Diluted magnetic semiconductors (DMSs) are semiconductors to which typically small percentage of a magnetic impurity has been intentionally introduced [5]. They attracted considerable attention forming a class of materials with both semiconducting and

∗ Corresponding author. Tel.: +20 2 22601742, fax: +20 2 26842123. E-mail address: [email protected] (K. El-Sayed). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.03.099

magnetic properties. DMS materials have wide application prospects in spintronic devices, which simultaneously exploit the charge and spin properties of electrons. Accordingly, the search for room temperature ferromagnetic DMS materials is in progress in recent years, and many such materials were reported [9]. It is widely expected that new functionalities for electronics and photonics are produced if the injection, transfer, and detection of carrier spin can be controlled at room temperature [10]. A successful operation of spintronic devices requires more than ferromagnetic semiconductor. It requires the support of spinpolarized transport so that spin-polarized charge carriers can be injected into a non-magnetic semiconductor [11]. There are two major criteria for selecting the most promising semiconductor spintronic materials. First, the ferromagnetism is retained to practical temperatures (300 K), and second, it is applicable in an existing technology [12]. Until now, no work has been carried out on InSe as modified by incorporating magnetic atoms, namely Fe or Co; thus, their classification as diluted magnetic semiconductors with spintronic character is still to be determined. In the present work, structural, magnetic and electrical properties of InSe doped with Fe or Co has been investigated. The objective is twofold: (i) identify the best conditions for preparing InSe1−x Mx (M for Fe or Co and x = 0.0, 0.1), and (ii) correlate the magnetic and electrical properties of doped samples with its microstructures characterization.

K. El-Sayed et al. / Journal of Alloys and Compounds 530 (2012) 102–106 2. Experimental 2.1. Syntheses InSe, InSe0.9 Fe0.1 and InSe0.9 Co0.1 compounds were prepared by solid-state reaction techniques, using very highly pure elements (99.999%). The proper amounts of In, Se, Fe and Co weighed on a sensitive microbalance. The steps of the preparation process are as follows: (i) The proper amount of elements were mixed together and then grounded in a mill for 60 min in the presence of argon gas, then placed in a cleaned silica tube which was then evacuated to 10−4 Torr. (ii) Heating up at 1000 ◦ C (±5 ◦ C) for 10 h, then decreasing the heating temperature to 660 ◦ C, for 7 days. The molten mass was occasionally, shaken to ensure complete mixing of the constituents. (iii) The molten mass allowed to cool down to room temperature to get an ingot. These steps were repeated for the samples of InSe, InSe0.9 Fe0.1 and InSe0.9 Co0.1 . As the melting point of Fe and Co is very high (1535 ◦ C and 1495 ◦ C, respectively), substitution by diffusion through the as prepared binary systems are used. 2.2. Measurements X-ray diffraction data were collected using a Philips X’pert MPP diffractometer with a goniometer type PW3050/10. Rietveld’s powder diffraction profile-fitting technique was employed for refining the structure and microstructure parameters obtained from the different samples. The magnetic properties were investigated by using the squid magnetometer. Oxford Cryostat connected to an automatic temperature controller and a 617 Kiethley electrometer were used to measure the electrical conductivity. Flat samples cut from the ingot perpendicular to the axis parallel to the axis of the test tube, the flat cut was then wet polished and then prepared as disk of 1–2 mm thickness. Good contact achieved by painting the opposite faces of the sample by carbon dag. The ohmic behavior of the metallic contact was characterized by measuring the (I–V) characteristics. The DC conductivity is calculated from the formula:

  

=

1 R

L A

(1)

where  is the DC conductivity of the sample, R is its resistance, L is the distance between the two electrodes, and A is the cross-section area of the applied electrode.

3. Results and discussion 3.1. XRD and microstructure analysis Fig. 1 shows the diffraction patterns of the InSe, InSe0.9 Fe0.1 and InSe0.9 Co0.1 samples. It can be seen that the InSe sample is a single phase matching the ICDD card no (34-1431) with the hexagonal space group P63 /mmc. For the samples modified by Fe or Co, two phases are identified; the main phase (InSe) and a minor phase

Fig. 1. The diffraction patterns of InSe, InSe0.9 Fe0.1 and InSe0.9 Co0.1 samples.

103

Table 1 The refined lattice parameters (a and c) (Å), Z-fractional coordinate of In and Se, unit cell volume V (Å3 ), the anisotropic crystallite size D (nm), the microstarin e and the reliability factors: Rwp and Rp (%) obtained from Rietveld analysis of the powder XRD patterns of all the samples.

a (Å) c (Å) V (Å3 ) Z (In) Z (Se) D(h 0 0) (nm) e(h 0 0) D(0 0 l) (nm) e(0 0 l) D(1 0 l) (nm) e(10l) InSe (%) In4 Se3 (%) Rwp (%) Rp (%)

InSe

InSe0.9 Fe0.1

InSe0.9 Co0.1

4.0037 (14) 16.644 (23) 231.06 0.1679 (22) 0.0900 (32) 21 15 × 10−5 39.5 83 × 10−5 10.0 0.0172 100 0 16 13

3.9915 (15) 16.640 (15) 229.59 0.167 (20) 0.0903 (27) 19 0.003 47.5 84 × 10−4 11.0 0.019 91.5 8.5 17 13

4.0022 (12) 16.6440 (13) 230.97 0.168 (72) 0.0921 (14) 14 0.002 39.8 28 × 10−4 10.0 0.020 93.5 6.5 18 14

(In4 Se3 ) matching ICDD card no (83-0039) with the orthorhombic space group Pnnm. During Rietveld analysis, a preferred orientation along the [0 0 l] direction as shown in Tables 1 and 2, which could not be avoided during measurements in spite of back-loading and fine grinding the sample. This preferred orientation was mainly due to the presence of the stacking layers aligned on top of each other along the [0 0 l] direction [4]. Fig. 2 shows the staking layers of the (0 0 l) planes. Fig. 3 is a zoom-in part of the InSe pattern where anisotropic broadening is obvious for different peaks. For example the broadenings for the (1 0 l) reflections is due to the high strain (0.0175) and the smaller crystallite size (10 nm) along (1 0 l] compared with the other crystallographic direction in the structure as shown in Tables 1 and 2. The structural parameters obtained from Rietveld refinement for the InSe1−x Fex (Cox ) are shown in Table 1. Fig. 4 shows the profile fitting resulting from Rietveld refinement. During analysis, the only trial that gives reasonable good R-factors is by accommodating the magnetic atoms substituting for Se in InSe as intended during preparation. With the incorporation of Fe or Co, small changes in unit cell parameters and fractional atomic coordinates of the InSe phase are also detected. This is another proof that the Fe or Co is incorporated in the InSe phase only. Table 2 shows large anisotropy values in crystallite size and microstrains along different crystallographic directions, especially along the (1 0 l] direction. This anisotropy results in rather large

Fig. 2. The scanning electron micrograph of InSe0.9 Fe0.1 showing deformed stacking layers.

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Table 2 Apparent crystallite size DF (nm) and mean-square strain eg  from single and multiple line (h h 0) and (1 0 l) analysis of InSe, InSe0.9 Fe0.1 and InSe0.9 Co0.1 . hkl

InSe Single

002 004 006 008 110 0012 101 103 105 107

InSe0.9 Fe0.1 Multiple

InSe0.9 Co0.1

Single

Multiple

Single

Multiple

DF

DF

eg 

DF

DF

eg 

DF

DF

eg 

42.6 48.2 64.2 63.6 33.7 70.1 12.5 14.3 13.8 15.0

42.3

0.032

67.5

0.059

0.063

0.094

11.7

0.097

40.0 49.0 54.7 66.4 49.5 46.2 12.7 14.1 13.7 13.4

76.3

14.2

39.1 51.3 91.3 83.6 55.9 66.5 9.9 14.2 13.8 21.0

10.8

0.099

Fig. 3. A zoom in part of the InSe diffraction pattern.

Fig. 5. Transmission electron micrograph of InSe0.9 Co0.1 sample showing a mixture of quantum dot size and more elliptical shape particle with larger size.

3.2. Magnetic properties R-values in the Rietveld analysis. The crystallite sizes are around 10 nm along (1 0 l], around 20 nm along [h 0 0] and around 40 nm along the [0 0 l] direction. The microstrains along (1 0 l] direction are large, which results in a pronounced broadening observed in the (1 0 l) reflections shown in Fig. 3. In the presence of Fe or Co, the microstrain increases along all directions at different rates as shown in Table 2. Comparing the crystallite size obtained from Rietveld analysis with those measured by using TEM of Fig. 5 indicates some similarities. However, accurate results require many micrographs to get good distribution for 2 2 2 all the sizes contained in the samples.

The hysteresis curve of InSe0.9 Fe0.1 sample shown in Fig. 6 indicates that it has a hysteresis loop with very low coercivity (HC = 66.02 Oe) and very low reminance magnetization (Mr = 0.07 emu/g) and saturation magnetization (MS = 3.33 emu/g). This result is similar to that of In(Ga,Mn)N thin films, which showed a clear hysteresis at 300 K, with small coercivities of the order 52–85 Oe and residual magnetization of the order of

Fig. 4. Profile fitting resulting from Rieveld analysis of InSe0.9 Fe0.1 .

Fig. 6. Hysteresis curve of InSe0.9 Fe0.1 sample.

K. El-Sayed et al. / Journal of Alloys and Compounds 530 (2012) 102–106

105

Fig. 7. Temperature dependent of M (molar susceptibility) for InSe0.9 Fe0.1 . Fig. 8. Temperature dependent of M (molar susceptibility) for InSe0.9 Co0.1 .

-2

log[σ(Ω.cm)-1]

0.08–0.77 emu/g [11]. Fig. 7 shows the temperature dependent of M (molar susceptibility) for InSe0.9 Fe0.1 . The calculated blocking temperature TB (at which the temperature started to decrease) and the Curie temperatures TC are 733 K and 870 K, respectively. This might suggest that the magnetic effect is produced from magnetic domain in the nanosize range. Similar properties and characterizations have been already achieved but at low Curie temperatures, in materials such as; (Ga,Mn)As (T = 110 K), In0.5 Ga0.5 Mn0.07 (T = 110 K) and (In,Mn)As (T = 35 K) [13]. Ferromagnetism is rarely observed in semiconductors that is due to both; low density of carriers and the prevalence of antiferromagnetic super exchange interaction among local moments [14], consequently most ferromagnetic semiconductors have relatively low Curie temperature. Few materials only for which room temperature ferromagnetism has been also reported [15]. The spintronic phenomena for DMSs arise from the sp–d interaction between the magnetic and the electronic subsystems. Any perturbation of these magnetic and electronic subsystems may create anomaly [7] in the magnetic and electric properties. Two basic approaches have emerged for the mechanism of ferromagnetism in DMSs; the first is the random model by Zener [16] and the other is the cluster models [17], one of them could be valid in the present work. Nevertheless, the probability of the random model may be higher in this case, since XRD analysis showed that Fe ions prefer to occupy InSe phase rather than In4 Se3 phase. On the other hand, the nonmagnetic semiconducting In4 Se3 phase may act as a spin-polarized transport [11]. This spin polarization may be the cause of producing ferromagnetic properties in the present case at room temperature; Pearton et al. [10] saw similar features in other materials. Furthermore, from the hysteresis loop, the calculated relative permeability for InSe0.9 Fe0.1 is >3.364, confirming the ferromagnetic properties of this sample. The small coercivity and small residual flux present in InSe0.9 Fe0.1 may allow these materials to be used in optoelectronic and photovoltaic devices. Fig. 8 shows the temperature dependence of the magnetic susceptibility (M ) for InSe0.9 Co0.1 . As shown, the sample undergoes antiferromagnetic transitions at Neel temperature 563 K. Perhaps there may be another additional transition at higher temperature, higher than the range allowed for the experimental temperature range used in this experiment. One can suggest that with an increase of temperature higher than the first Neel temperature may be another antiferromagnetic transition is taking place. Some other samples containing transition elements

InSe0.9Fe0.1 -3

InSe0.9Co0.1 -4

-5

-6

InSe

2

2.5

3

3.5

3

-1

10 /T(K )

Fig. 9. log  versus 103 /T plot for InSe samples.

showed also successive transitions such as La0.5 Sr0.5 MnO3 and Nd0.5 Sr0.5 MnO3 [18]. 3.3. The electric properties 3.3.1. InSe sample The log  versus 103 /T plots for InSe, InSe0.9 Fe0.1 and InSe0.9 Co0.1 samples is shown in Fig. 9. The results of the measured room temperature electrical conductivity,  RT and activation energy E obtained from the slope using Arrhenius equation are tabulated in Table 3. InSe relation for the conductivity showed the occurrence of only one conduction mechanism over the whole temperature Table 3 Values of the DC conductivity  RT and the activation energy (E) for InSe, InSe0.9 Co0.1 and InSe0.9 Fe0.1 samples. Sample

 RT ( cm)−1

E1 (eV)

E2 (eV)

InSe InSe0.9 Co0.1 InSe0.9 Fe0.1

1.171 × 10−5 5.870 × 10−4 1.497 × 10−3

0.43 0.10 0.22

– 0.032 0.083

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K. El-Sayed et al. / Journal of Alloys and Compounds 530 (2012) 102–106

range, suggesting that only one band of localized state is situated at the top of the extended states (E1 = 0.44 eV). Earlier works on In1−x Sex films reported values of E equal 0.103–0.115 eV over the temperature range 200–310 K. These values were attributed to thermoionic emission [19]. Non-linear variation of resistivity with 1/T of InSe thin films was also reported [20]. The electrical measurements of InSe films showed two values for the activation energy according to the temperature range; over 30–450 K, E = 0.18 eV related to extrinsic conduction and over 450–600 K, E = 0.61 eV due to intrinsic conduction. The discrepancies in the results in different works attributed to the presence of different percentage of excess Se in InSe and also the different temperature range at which the conductivity was measured [19–21]. 3.3.2. InSe0.9 Fe0.1 and InSe0.9 Co0.1 The incorporation of Fe or Co atoms in InSe seems to create new band configurations and hence a modification of its electronic density of states. The conductivity measured at room temperature  RT increased by about 2 orders of magnitudes (200 times) and by about 1.5 order of magnitudes (150 times) by incorporating Fe and Co atoms, respectively in InSe (Table 3). This means that both are electrically active. They create excess of free charge carriers in accordance to the relation: n ≈ n0 e−E/RT . Moreover these charge carriers were assisted by the ordered magnetic spin in the ferromagnetic InSe0.9 Fe0.1 phase [15] and partially ordered magnetic spin of InSe0.9 Co0.1 . The activation energy E2 due to charge transfer inside the created Fe or Co impurity bands equals 0.083 and 0.03 eV, respectively in the considered samples, also a more higher temperatures charge transfer to bands situated at 0.22 and 0.10 eV are taking place in InSe0.9 Fe0.1 and InSe0.9 Co0.1 , respectively. The result is that interactive transition-metal orbital configurations yields electron spins that cooperate with the electron charge producing higher carrier density that will induce a smaller barrier width than that of intrinsic InSe. Consequently, a higher electrical conductivity [22]. 4. Conclusion In this work, by introducing typically 10% of Fe or Co, respectively into InSe, it was possible to prepare nanodiluted magnetic semiconductors (DMSs), having magnetic and electric properties at room temperature. Incorporation of interactive transition-metal Fe or Co into InSe yields spins that cooperates with the electron charge to increase the conductivity of InSe samples into many order of magnitudes. It is found that 10% of Fe changes InSe phase into ferromagnetic,

in contrast, the same percentage of Co changes it into antiferromagnetic. The non-magnetic phase, In4 Se3 , plays a major role in the high temperature ferromagnetic InSe0.9 Fe0.1 , and antiferromagnetic InSe0.9 Co0.1 , as it may act as a polarizer for the magnetic spins. Moreover its presence as impurities helped in creating defects of many types such as; deformed textures, cleavage of staked layers. More over defects produced by constrained lattice and atomic positions due to incorporation of Fe or Co, create new band configurations and hence a modification of its electronic density of states. This will induce a smaller barrier width than that present in InSe, and consequently an increase in conductivity .We may conclude that by incorporating Fe or Co into InSe, changes it into diluted semiconductor with spintronic Characters. These types of semiconductors have wide application in optoelectronic and spintronic devices. References [1] K. Hara, K. Sayama, H. Arakawa, Sol. Energy Mater. Sol. Cells 62 (2000) 441–447. [2] S. Balvicius, A. Cesnys, Phys. Status Solidi 35 (1976) K41–K43. [3] A.I. Lebedev, A.V. Michurin, I.A. Sluchinska, V.N. Demin, I.H. Munro, J. Phys. Chem. Solids 61 (2007) 2012. [4] S. Pal, D.N. Bose, Solid State Commun. 97 (8) (1996) 725–729. [5] H. Ohno, Science 281 (1998) 951–956. [6] B. Beschoten, P.A. Crowell, I. Malajovich, D.D. Awschalom, F. Matsukura, A. Shen, H. Ohno, Phys. Rev. Lett. 83 (15) (1999) 3073–3076. [7] F. Matsukra, H. Ohno, T. Dietl, Hand Book of Magnetic Materials, vol. 14, Elsevier, New York, 2002, pp. 1–87. [8] A. Segura, C. Martinz-Tomas, A. Casanovas, A. Cantareo, A. Martinez-Pastor, A. Chevy, Appl. Phys. A 45 (1989) 445–450. [9] S. Duman, B. Gürbulak, S. Do˘gan, A. Türüt, J. Microelectron. Eng. 86 (2009) 106–110. [10] S.J. Pearton, Y.D. Park, C.R. Abernathy, M.E. Overberg, G.T. Thaler, K. Tihyun, F. Ren, Electron. Mater. 82 (2003) 288–293. [11] K. Leeor, J. Manish, R.J. Chelikowsky, Phys. Rev. B 66 (2002) 041203–041207. [12] S.J. Pearton, C.R. Abernuthy, D.P. Norton, A.F. Hebard, Y.D. Park, L.A. Boatner, J.D. Budai, Mater. Sci. Eng. R40 (2003) 137–168. [13] A. Oiwa1, Y. Mitsumori, R. Moriya, T. Słupinski, H. Munekata, Phys. Rev. Lett. 88 (2000) 137202–137206. [14] B. Beschoten, P.A. Crowell, I. Malajovich, D.D. Awschalom, F. Matsukura, A. Shen, H. Ohno, Phys. Rev. Lett. 83 (1999) 3073–3077. [15] G.A. Medvedkin, T. Ishibashi, T. Nishi, K. Hiyata, Jpn. J. Appl. Phys. 39 (2000) L949–L954. [16] C. Zener, Phys. Rev. B 81 (1951) 440–444. [17] M. Van Schilfgaade, O.N. Myrasov, Phys. Rev. B 63 (2001) 195205–195209. [18] H. Fujishiro, Physica B 307 (2001) 57–63. [19] M. Parlak, C. Ercelebi, I. Gunal, Z. Salaeva, K. Allakhverdiev, Thin Solid Films 258 (1995) 79–86. [20] H.M. Pathan, S.S. Kulkarni, R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 39 (2005) 16–27. [21] H.H. Hantour, Al-Azhar University, Faculty of Science, Girls Branch Physics Department, Ph.D. (2010). [22] Q. Liu, J. Dai, Z. Liu, X. Zhang, G. Zhu, G. Ding, J. Phys. D: Appl. Phys. 43 (2010) 455401–455412.