Structural, optical, magnetic and electrical properties of dilute magnetic semiconductor Cd1−xMnxTe

Solid State Sciences 13 (2011) 23e29

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Structural, optical, magnetic and electrical properties of dilute magnetic semiconductor Cd1
xMnxTe S.A. Gad*, M. Boshta, A.M. Moustafa, A.M. Abo El-Soud, B.S. Farag Solid State Physics Dept., National Research Center, El-Bohoos str., 12311 Dokki, Giza, Egypt

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 July 2010 Received in revised form 26 August 2010 Accepted 28 September 2010 Available online 27 October 2010

Cd1
xMnxTe was prepared in the powder form by direct fusion technique. The structural properties of the prepared compounds were investigated by X-ray diffraction. It was found that, the materials crystallize in the zincblende form. The lattice parameters, the bond lengths and the crystallite size decrease with increasing the Mn content while the internal microstrain increases. The energy band gap has been calculated from optical and electrical measurements and was found to increase as the Mn content increases. The electrical conductivity increases with increasing temperature in the 300e550 K range. The line width (DH) of EPR spectra increases with increasing Mn content. The magnetic susceptibility shows non-linear variation with Mn concentration. Ó 2010 Elsevier Masson SAS. All rights reserved.

Keywords: Structure Optical properties Magnetic properties Electrical properties Cd1
xMnxTe

1. Introduction

2. Preparation

Cd1
xMnxTe (CMT) is a diluted magnetic semiconducting (DMS) system that offers a wide applicability in many spintronic magnetooptic devices [1e8] due to the sped exchange interactions between the mobile spins of the conduction electrons and localized spins of the magnetic elements. Magnetic properties of diluted magnetic semiconductors Cd1
xMnxTe are mostly due to the presence of substituted divalent Mn ions which are distributed in a random way in the cation sub-lattice of the crystals. The behavior of the Cd1
xMnxTe is either paramagnetic, spinglass like or anti-ferromagnetic depending on the concentration of the divalent Mn ions [9e14]. Cd1
xMnxTe mixed crystals belong to the IIeVI Mn e based semimagnetic wide-gap semiconductors, which are currently of great interest owing to their pronounced magnetic and magneto-optical properties (giant Zeeman splittings and giant Faraday rotations) and because of new possibilities of application in spintronic devices [15]. CdMnTe bulk crystals have been exhibited a high resistivity (>1010 U cm) and a relatively high electron mobility- lifetime (ms) product (>10
6 cm2/V) [4].

Cd1
xMnxTe crystals with different concentrations of Mn (x ¼ 0.05, 0.1, 0.15 and 0.2) were prepared by placing high purity elements (99.99%) in stoichiometric proportion in a silica glass tube with graphitized walls. After sealing the tube under a pressure of 10
5 Torr, it was placed into a furnace whose temperature was raised gradually up to 1100  C and annealed for 3 days. During the synthesis, the molten material was shaken by vibration to ensure homogeneity. The temperature of the furnace was gradually cooled to 900  C and kept for one week at this temperature.

* Corresponding author. E-mail address: [email protected] (S.A. Gad). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.09.022

3. Expertimental The X-ray diffraction patterns of Cd1
xMnxTe (x ¼ 0.05, 0.1, 0.15 and 0.2) were recorded at room temperature using Bruker D8 diffractometer (Germany) with Cu kal radiation, the tube operated at 40 kV and 40 mA. The diffraction patterns were collected in the angular 2q range of 10e95 with step scanning mode of 0.0092 2q and counting time of 20 s/step. Diffuse reflection measurements were done in the wavelength range from 600 nm to 1200 nm using Jasco (V-570) spectrophotometer. Electron spin resonance was recorded as first deviation using an x-band spectrum with a magnetic field modulation of 100 kHz using (Bruker Elexsys-500), receiver gain 60, sweep width 6000 center at 4389 Oe, with a nominal microwave power 0.20 W.

24

S.A. Gad et al. / Solid State Sciences 13 (2011) 23e29

Fig. 2. X-ray powder diffractograms of Cd1
xMnxTe, 0.05  x  0.2.

Fig. 1. Representative example of EDAX for Cd1
xMnxTe.

A vibrating sample magnetometer model 9600-1-VSM was used for the magnetic properties measurements. The A.C. conductivity was measured using a computerized LCR circuit type (Hioki 3532-50 LCR Hi-Tester, Japan) with frequency range 42 Hze5 MHz and over the temperature range from room temperature to 526 K. 4. Results and discussion 4.1. Composition analysis The compounds of Cd1
xMnxTe with compositions (x ¼ 0.05, 0.1 0.15 and 0.2) were determined by energy dispersive X-ray analysis (EDXA). The calculated contents of Cadmium, Manganese and Tellurium (wt%) were comparable with (wt%) of the starting materials indicating that the prepared bulk nearly stoichiometric. The results are recorded as shown in the representative example in Fig. 1 and Table 1. 4.2. Structure Preliminary X-ray diffraction patterns are shown in Fig. 2. It seems that all the patterns have the same features, but the peak intensity of the reflecting planes varies and the peak positions shift slightly on passing from one composition to the other. The prepared compounds consist of single phase CdTe (zincblende) structure, the

Table 1 Chemical analysis data (EDAX) of Cd1
xMnxTe solid solutions with indicated x in powder form compared to the calculated one. Element

Elemental composition (at %) x ¼ 0.05

Cd Mn Te

x ¼ 0.1

x ¼ 0.15

x ¼ 0.2

Exp

Cal

Exp

Cal

Exp

Cal

Exp

Cal

46.30 1.12 52.58

45.1 1.16 53.81

42.94 2.39 54.96

43.18 2.3 54.469

43.1 2.96 53.94

41.3 3.56 55.14

41.26 3.8 54.94

39.351 4.81 55.84

diffraction lines are completely matched with ICDD card number (15-0770) with the absence of the characteristic X-ray diffraction lines corresponding to any precipitation of elements or binary alloys with other stoichiometry. This may indicate the complete solubility of the constituent elements of all synthesized compounds Cd1
xMnxTe. In order to study the effect of Mn substitution on the CdTe compounds, we carry out a structure refinement applying the whole diffraction pattern fitting by means of Rietveld method [16]. For the refinement, FULLPROF program [17,18] was used. The required positional parameters used for Rietveld method has been taken from the reported structure of CdTe [19] which has a zincblende configuration. So the space group has been taken as F-43 m with Te occupies the tetrahedral sites 4c of coordinates (¼¼¼) and Cd takes the site 4a of coordinates (000). The background variation was described by a polynomial with 5th order. The angular dependence of the peak full width at half maximum (FWHM) was described by Caglioti’s formula [20]. Peak shapes were described by the modified Thompson Cox-Hasting pseudo-Voigt profile function [21] and preferential orientation was modeled using March’s function [22,23]. The isotropic thermal motion of the atoms was considered, a scattering factor for neutral atoms was used. The thermal parameters and occupancy of both of Cd and Mn were refined using the same code number. In the first step of the refinement the global parameters (profile asymmetry, background, and specimen displacement) were refined. In the next step, the structural parameters (atomic coordinates, specimen profile breadth parameters, lattice parameters, temperature factors, preferred orientation and site occupancy factors) were refined in sequence modes. In the last cycle when the discrepancy factor Rwp has reached minimum value, all the parameters (global and

Table 2 The refined unit cell parameters and agreement factors of Cd1
xMnxTe solid solutions with indicated x. Manganese atomic content x ¼ 0.05 Crystal System a (Å) Volume (Å3) S. G. Rp (%) Rwp (%) Rexp (%) Chi2

Cubic 6.47300(4) 271.217(3) F-43 m 17.2 15.8 11.8 1.79

x ¼ 0.1

x ¼ 0.15

x ¼ 0.2

6.45183(0) 6.44618(5) 268.565(3) 267.859(3)

6.44589(6) 267.823(5)

18.8 17.5 10.35 2.85

21.1 19.3 11.6 2.8

16.7 15.6 13.1 1.42

S.A. Gad et al. / Solid State Sciences 13 (2011) 23e29

25

Table 3 The atomic positions of Cd1
xMnxTe solid solutions with indicated x. Property Atomic positions

Isotropic temperature factor

Occupancy

Te-Cd/Mn (Cd/Mn)e(Cd/Mn) TeeCd/MneTe

Te Cd Mn Te Cd Mn Te Cd Mn

x ¼ 0.05

x¼1

x ¼ 0.15

x¼2

0.25, 0.25, 0.25 0, 0, 0 0, 0, 0 0.069(3) 0.320(4) 0.320(4) 0.999 (2) 0.935(4) 0.036(4) 2.8029 Å 4.5771 Å 109.471

0.25, 0.25, 0.25 0, 0, 0 0, 0, 0 0.502 0.767(22) 0.767(22) 0.999(5) 0.863(2) 0.065(2) 2.7937 Å 4.5621 Å 109.471

0.25, 0.25, 0.25 0, 0, 0 0, 0, 0 1.318(43) 1.203(54) 1.203(54) 0.998(2) 0.850(4) 0.150(4) 2.7913 Å 4.5581 Å 109.471

0.25, 0.25, 0.25 0, 0, 0 0, 0, 0 0.321(3) 0.425(5) 0.425(5) 0.999(4) 0.785(3) 0.185(3) 2.7912 Å 4.5579 Å 109.471

structural parameters) were refined simultaneously till minimum goodness of fit index, c2. The results of the Rietveld refinement agreement factors for all compounds are summarized in Table 2. The atomic coordinates, isotropic temperature factor, occupancy, bond distances and angles for each compound are shown in Table 3. Fig. 3 shows the observed, calculated, and difference profile of Cd0.85Mn0.15Te compound as a representative one. It is clear from the variation of the lattice constant with x parameter given in Table 2 that the lattice constant decreases with increasing Mn content this may be due to the difference in the ionic radius Cd¼ 0.78 and Mn¼ 0.66 [24] this result agree with the results reported by Hawng et al. [25]. Since the zincblende structure of Cd1
xMnxTe compounds is cubic and both ions are tetrahedrally coordinated as shown in Fig. 4, the local environment of the tetrahedrons are formed by like atoms. Therefore, it is meaningful to consider the cationecation distance dcec, cationeanion distance dcea over the entire range of existence of Cd1
xMnxTe. The quantity dcec, dcea and tetrahedral angles result from the refinement are tabulated in Table 3. From this table it is clear, that both values of dcec and dcea decreases with increasing the x parameter, while the tetrahedral angle is kept unchanged, the decrease in the bond lengths may be due to the difference in the ionic radius between Cdþ2 and Mnþ2. The decrease in the lattice constant and bond lengths with Mn concentration strongly show that Mn ions are incorporated into the host Cd sites substitutionally. The same effect of the Mn substituted a wide band gap material has been obtained [26].

Fig. 4. CdMnTe unit cell.

The variation of the microstrain with the Mn atomic content shown in Fig. 5. From this figure it is clear, that the microstrain tends to increase as the Mn content increases. This increase does not only arises from the difference in the ionic radius of Cd and Mn but also may be due to the deficiency of the occupancies of both of Cd and Mn (see Table 3) than that of the proposed values, which

0.5

Microstrain

0.4

0.3

0.2

Fig. 3. Final rietveld refinement for Cd0.85Mn0.15Te sample as representative one. The observed (closed circles) and calculated (solid line) X-ray diffraction profiles and the difference between them (on the bottom). Vertical bars refer to calculated Bragg peak positions.

0.1 0

0.05

0.1

0.15

0.2

Mn atomic content Fig. 5. The relation between microstrain and Mn atomic content.

0.25

S.A. Gad et al. / Solid State Sciences 13 (2011) 23e29 1600

1.80

1500

1.75

1400

1.70

1300

1.65

Y =1.462+1.504 X

Energy gap (Eg)

Crystallite size Å

26

1200 1100 1000

1.60 1.55 1.50

900

1.45 800 0

0.05

0.1

0.15

0.2

0.25

1.40 0.00

Mn atomic content

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

Composition (x) Fig. 6. The relation between crystallite size and Mn atomic content. Fig. 8. The optical energy band gap as a function of Mn.

consequently lead to increase the number of vacant sites in the lattice and consequently enhance the internal microstrain. Fig. 6 shows the relation between the crystallite size of the investigated compounds and Mn atomic content. It is clear that disregarding the value of the sample x ¼ 0.15 the crystallite size decreases as the Mn content increases which means that as the number of the created vacancies increases the crystallite size decreases. 4.3. Optical properties The diffuse reflectance (R) was measured as a function of wave length (l) from 600 to 1200 nm. The spectra within the entire frequency range are shown in Fig. 7. The energy band gap Eg can be determined from the onset of the linear increase in the diffuse reflectance [27]. The obtained values of Eg show a reasonable linear behavior as a function of the atomic fraction of Mn are shown in Fig. 8 and follow the relation:

Eg ¼ 1:462 þ 1:504x

(1)

The values of Eg are in agreement with the published data by Kosyachenko et al. are shown in Table 4 [28]. The increase of the energy gap with x may be due to a decrease in the lattice constant.

Table 4 The calculated values of the activation energy and energy gaps from the electrical conductivity and the optical energy gaps of Cd1
xMnxTe. Composition

DE1

DE2

Eg (elec.)

Eg (opt.)

Cd 0.8Mn0.2Te Cd 0.85Mn0.15Te Cd 0.9Mn0.1Te Cd0..95Mn 0.05Te CdTe (from extrapolation of the curve)

0.057 0.056 0.0554 0.0551 e

0.873 0.838 0.80 0.773 e

1.746 1.68 1.591 1.547 e

1.77 1.67 1.6 1.54 1.462

4.4. Magnetic properties Fig. 9 shows the electron paramagnetic resonance spectra for Cd1
xMnxTe samples (x ¼ 0.05, 0.1 0.15 and 0.2). The spectra exhibit only one broad resonance line due to Mn2þ. The broadening is proportional to the Mn concentration. The line width (peak to peak width) was found to increase with increasing Mn concentration as shown in Table 5 and the g values are around 2.0 [29e34] which is in good agreement with the values reported by Addachi et al. [35]. This line broadening can be attributed to inhomogneities in the internal field which arise due to random distribution of Mn2þ ions. Similar line broadening is also reported in Zn1
x Mnx Te crystals with x  0.6.

60 55

45

15000

S1

S3( Cd0.85Mn0.15Te)

10000

S1 (Cd0.95Mn0.05Te) S2(Cd0.9Mn0.1Te)

5000

S3(Cd0.85Mn0.15Te) S4(Cd0.8Mn0.2Te)

S2

S2( Cd0.9Mn0.1Te) S1(Cd0.95Mn0.05Te)

S3 S4

40

Intensity(%)

Diffuse Reflection (R%)

50

S4( Cd0.8Mn0.2Te)

35 30 25

0 -5000

20

-10000

15 10 600

-15000 700

800

900

1000

1100

1200

Wavelength (nm) Fig. 7. The diffuse reflectance (R) versus the wavelength l for Cd1
xMnxTe.

1000

2000

3000

4000

5000

Field(G) Fig. 9. EPR spectra for Cd1
xMnxTe samples at room temperature.

6000

S.A. Gad et al. / Solid State Sciences 13 (2011) 23e29 Table 5 The line width of the Cd1
xMnxTe.

27

Table 6 The values of magnetic induction Bs and Coercivities Hcof Cd1
xMnxTe.

Composition (x)

ΔH (G)

X

BS(E.M.U./G)

HC(OE)

BR (E M.U./G)

HK (OE)

MRAG

0.05 0.1 0.15 0.2

100 150 200 240

0.05 0.1 0.15 0.2

0.05847 0.1006 0.08166 0.1098

681.9 403 91.9 88.28

0.02065 0.00753 0.001721 0.001232

3495 9646 3755 9785

0.01 0.01 0.01 0.01

The splitting of the resonance line is only observed if the width of the individual lines is smaller than their separation. An important mechanism for line broadening is the dipolar interaction between the ions. This causes the molecular field acting on the ions to vary from site to site with shifting the resonance frequency of the different ions. This will broaden the line (inhomogeneous broadening). Due to this broadening mechanism, the six line pattern is transformed into a single structureless line if the manganese concentration is increased above 0.002 [36]. The magnetic hysteresis loops of Cd1
xMnxTe powder have been measured as a function of Mn concentrations (x ¼ 0.05, 0.1, 0.15 and 0.2) at room temperature are shown in Fig. 10. Table 6 confirmed that the magnetic saturation induction Bs depends on Mn concentration. Moreover, the coercivities, Hc of Cd1
xMnxTe decrease with increasing Mn concentration. The magnetic susceptibility is shown in Fig. 11 as a function of Mn concentration. It is clear from the figure that the susceptibility shows non-linear variation with the composition (x). Veera Brahmam et al. [37] explained that non-linear variation of susceptibility with composition as follows. At lower concentration of Mn, the “Sp” electrons with spin opposite to the spin of the atom in ZnTe lattice interact with the spin of Mn atom, which causes a decrease in susceptibility. At higher concentrations of Mn, the ded exchange interaction between Mn atoms dominates over the sped exchange interaction resulting in an abrupt increase in susceptibility [37].

4.5. Electrical conductivity The most fundamental aspects of a solid are its band structure and the transport mechanisms of the carrier. Transport measurements are studied by electrical conductivity to cast more light on the value of the activation energy and band gap. In spite of that no previous efforts have been done to measure the electrical conductivity of the different composition Cd1
xMnxTe except those, lastly on Cd0.96Mn0.04Te [38], we have measured the electrical conductivity. The electrical conductivity of a semiconductor at a temperature T is given by:

s ¼ s0 $expðDE=kTÞ

(2)

Where s0 and DE represent the pre-exponential value and the activation energy, respectively and k is Boltzmann’s constant. The temperature dependence of the a.c. electrical conductivity for bulk Cd1
xMnxTe, at a relatively high frequency (at which sa.c ¼ sd.c), with different compositions (x ¼ 0.05, 0.1, 0.15 and 0.2) are shown in Fig. 12. It is obvious from Fig. 12 that the conductivity of the investigated samples over the whole temperature range indicating normal non degenerate semiconducting behavior. Moreover the linear variation of line (s) with 1/T for all samples indicates that the conduction in these samples is through thermally activated process.

0.10

0.15

S4 ( Cd0.8Mn0.2Te)

0.08

S3 (Cd0.85Mn0.15Te)

0.06

0.10

0.04

B(e.m.u/g)

0.05

0.02 0.00

0.00

-0.02

-0.05

-0.04 -0.06

-0.10

-0.08 -10000

-5000

0

5000

10000

A

0.10

S2 (Cd0.9Mn 0.1 Te)

0.08

-10000

-5000

0 A

0.06

S1 (Cd 0.95Mn 0.05Te)

5000

10000

5000

10000

0.04

B(e.m.u/g)

0.05 0.02 0.00

0.00

-0.02

-0.05

-0.04 -0.06

-0.10

-10000

-5000

0 H(Oe)

5000

10000

-10000

-5000

0 H(Oe)

Fig. 10. The magnetic hysteresis of cd1
xMnxTe samples at room temperature.

28

S.A. Gad et al. / Solid State Sciences 13 (2011) 23e29

present conductivity data could not be done since there is no reported data of the electrical properties of Cd1
xMnxTe. Also, it is remarkable to notice that the conductivity decreases as Mn content increases.

1.2x10-5

At Magnetic Field = 10030(Oe)

Susceptibility

1.1x10-5

1.0x10-5

5. Conclusion

9.0x10-6

The prepared Cd1
xMnxTe consists of single phase (zincblende) structure. The lattice constant and the bond lengths decrease with increasing Mn concentration, this indicate that Mn ions are incorporated into the host Cd sites substitutionally, but the microstrain increases as the Mn content increases. The energy gap calculated from the optical and electrical measurements was found to increase with the increase of the Mn content. From the EPR measurements, the spectra exhibit only one broad resonance line due to Mnþ2. The broadening is proportional to Mn concentration and can be attributed to inhomogneities in the internal field due to random distribution of Mnþ2 ions. The magnetic susceptibility shows non-linear variation with composition (x).

8.0x10-6

7.0x10-6

6.0x10-6 0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

Composition (x) Fig. 11. Magnetic susceptibility versus Mn concentration.

The measured electrical conductivity, of each composition indicated two straight lines with different slopes both could fit relation (2). The first lines are in the low temperature region from 300 to 400e455 K, while the other lines are in the high temperature region from 400 to 455e550 K indicating a transition occur near 400e455 K. The activation energies DE1 and DE2 have been calculated from the slope of the lines in Fig. 12. In the low temperature range, the values of DE1 were found to nearly constant ¼ 0.055 eV. This may indicate that Mn substitute Cd in the sub-lattice and not interstitial sites, so it is not affect any how the extrinsic conduction, this agrees well with x-ray data. Whereas, the activation energy, DE2, in the high temperature range were found to be half of the determined optical band gap, indicating that the conduction mechanism in Cd1
xMnxTe at high temperature range (T > 400) is intrinsic, while the extrinsic conduction dominates in the low temperature range (T < 450). The calculated activation energies and electrical energy gaps together with the obtained optically energy gaps are given in Table 4 for comparison. It can be seen that the evaluated thermal energy gaps are very close and agree with those evaluated optically. This suggests the validity of the experimental measurements. A comparison of the

-2

S4 ( Cd0.8Mn0.2Te) S3 (Cd0.85Mn0.15Te)

-3

S2 (Cd0.9Mn0.1Te) S1( Cd0.95Mn0.05Te) log(σ)

-4

-5

s

-6

ss s

-7 1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

1000/T(K ) Fig. 12. ac conductivity versus the temperature for Cd1
xMnxTe.

3.4

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S.A. Gad et al. / Solid State Sciences 13 (2011) 23e29 [35] N. Addachi, G. Kido, Y. Nakagawa, Y. Oka, J. Magn. Magn. Mater. 90-91 (1990) 778. [36] Paulus, Joannes Theodorus Eggenkamp, PHD. Thesis, “Carrier concentration dependence of the magnetic properties of Sn1
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[37] K. Veera Brahmam, D. Raja Reddy, B.K. Reddy, Spectrochim. Acta Part A 60 (2004) 741. [38] L.A. Kosyachenko, O.L. Maslyanchuk, I.M. Rarenko, V.M. Sklyarchuk, Phys. State Solid C 1 (4) (2004) 925.