ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 69 (2008) 2670–2673

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/jpcs

Optical and electronic properties of nano-Cd1�xMnxTe alloy

Pushan Banerjee, Biswajit Ghosh �

Advanced Materials and Solar Photovoltaic Division, School of Energy Studies, Jadavpur University, Kolkata 700032, India

a r t i c l e i n f o

Article history:

Received 18 November 2007

Received in revised form

4 April 2008

Accepted 30 June 2008

Keywords:

A. Multilayers

A. Nanostructures

D. Electrical properties

D. Optical properties

97/$ - see front matter & 2008 Elsevier Ltd. A

016/j.jpcs.2008.06.142

esponding author. Tel./fax: +9133 2414 6823.

ail address: goloke3@yahoo.co.in (B. Ghosh).

a b s t r a c t

Cd1�xMnxTe thin films were fabricated by thermal interdiffusion of multilayers of sputtered compound

semiconductors as well as thermally evaporated elements. Electron microscopy revealed their

nanostructures. The alloys have been investigated for evaluation of optical and electronic parameters.

Spectrophotometry helped to find out the bandgap and composition; photoluminescence was used for

observing relative transition probabilities at room temperature. Photoresponse showed the light

dependence of the resistance of the alloy films. Hall measurements and four-probe tests indicated the

influence of manganese on the room-temperature electronic properties of the alloy.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

In the present years there have been growing interests in thenew magnetoelectronic devices based on the control of thecharge as well as the spin of electrons. Intensive researchactivities have been devoted to metallic multilayers composedof alternating ferromagnetic and non-magnetic metals. Newtype of devices using the spin of carriers has also been proposedutilizing diluted magnetic semiconductors (DMS) [1], by intro-ducing magnetic elements in well-characterized semiconduc-tors, thus making it integrable with electronic devices. Carrier-induced ferromagnetism, as observed in p-type Mn-basedDMS [2], is very promising in device applications where magneticand transport properties can be combined and also magneticfield can control electronic properties. Though in general theferromagnetic transition temperatures are lower in II–VI DMSin comparison to III–V counterparts [3], II–VI materials areideal for fundamental studies because localized spins and holescan be introduced and controlled independently and dimen-sional effects can be examined in modulation-doped hetero-structures.

In the present work, thin films of Cd1�xMnxTe were studied tofind out some common optical and electronic properties of thismaterial. Already single crystalline, thin film and quantum-wellheterostructures of this material have been studied extensively byother workers to see the enhanced properties and magneticinteractions among the atoms [4–9]. In the present work,

ll rights reserved.

nanocrystalline thin-film configuration was taken for analysisbecause of the ease of fabrication of nanocrystalline thin filmscompared to single crystals or quantum wells.

2. Experimental techniques

Varying compositions of Cd1�xMnxTe alloy were fabricatedusing two methods—(a) radio-frequency magnetron-sputteringprocess and (b) thermal evaporation in vacuum. In the former,thin films of p-type manganese telluride and cadmium telluridewere stacked sequentially from the respective targets onto soda-lime glasses using a net power density of about 3.7 W/cm2 atan argon pressure of 0.0270.005 mbar. Three repetitions ofthis MnTe/CdTe sequence were carried out and the multi-layer structures were annealed in vacuum at 350 1C for an hour.In the later process, repeated sequential thermal evaporationof elemental manganese, cadmium and tellurium was carriedout. It was followed by annealing at 400 1C—a slightly highervalue than the previous one so as to provide sufficient energyfor compound formation from elements. The processing stepshave been optimized after detailed studies are reported else-where.

In both the cases, the composition was varied by adjusting thethickness of the individual members of the multilayer structures.Six sets of samples were taken for analysis, with sets 1, 3 and 5fabricated through R.F. magnetron-sputtering and sets 2, 4 and 6through thermal evaporation. The individual thicknesses of thelayers are given in Table 1 (for sputtered samples) and Table 2(for evaporated samples). No external doping of the films wascarried out.

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Table 1Thicknesses of the individual layers for R.F. sputtered films

Set no. 1 3 5

MnTe thickness (nm) 19 35 55

CdTe thickness (nm) 206 170 155

Table 2Thicknesses of the individual layers for thermally evaporated films

Set no. 2 4 6

Mn thickness (nm) 3 8 13

Cd Thickness (nm) 80 70 65

Te thickness (nm) 132 130 136

Fig. 1. FESEM image of Cd1�xMnxTe alloy film.

2.2E+11

2.0E+11

1.8E+11

1.6E+11

1.4E+11

1.2E+11

1.0E+11

8.0E+10

6.0E+10

4.0E+10

2.0E+10

0.0E+00

(ααhν

)2 (eV

-2 c

m-1

)

1 1.2 1.4 1.6 1.8 2 2.2 2.4

Energy (eV)

Set-1 Set-2 Set-3 Set-4 Set-5 Set-6

Fig. 2. (ahn)2 vs. energy plot for Cd1�xMnxTe films.

P. Banerjee, B. Ghosh / Journal of Physics and Chemistry of Solids 69 (2008) 2670–2673 2671

3. Results and discussion

The films obtained after thermal interdiffusion were nanocrys-talline in nature, with crystal sizes of a few tens of nanometers.This is evident from the FESEM image (taken using Jeol 6700F fieldemission scanning electron microscope) of set-3, shown in Fig. 1.Other sets of films had similar crystal sizes and are not shownhere.

3.1. Optical properties of the DMS films

The transmission characteristics of the samples were deter-mined using a Perkin–Elmer Lambda-35 UV–visible spectro-photometer and the absorption coefficients (a) were evaluatedfrom the data. Cd1�xMnxTe being a direct bandgap material, theplots of (ahn)2 vs. hn has been shown in Fig. 2 for calculation ofdirect bandgap from the intercepts of the tangents (drawn athigher energy side) on the energy axis.

The change in the bandgap of Cd1�xMnxTe is linear withx—increasing from the value of CdTe (x ¼ 0) up to the solubilitylimit of Mn in CdTe matrix (x ¼ 0.77). Several workers observedthe nature of variation of the bandgaps of Cd1�xMnxTe withcomposition [10] at a range of temperatures through differentmethods of measurement. Out of them we have employed thefollowing relation (Eq. (1)) observed using ellipsometry at 300 K:

ECd1�xMnxTeg ¼ 1:53þ 1:26x ðin eVÞ (1)

The direct bandgaps were calculated from intercepts by drawingtangents on the curves in Fig. 2 and have been presented inTable 3. It also displays the value of ‘‘x’’ calculated using Eq. (1).

The luminescence of the films were studied using a Perkin-Elmer LS-55 luminescence spectrometer with 450 nm excitationand have been presented in Fig. 3.

Vecchi et al. [11] carried out PL measurements on Bridgman-grown Cd1�xMnxTe at 76 K over a range of compositions. Theyidentified three specific emission peaks: one corresponding toexcitonic recombination across the fundamental gap, one fortransition from the first-excited 4G level to the ground state 6S ofthe localized Mn2+ ion and the third related to transition from thefirst-excited 4G level of Mn2+ ion to the top G8 valence band. In ourwork, we found the presence of two peaks only for each set. Thehighest peaks near 1.82 eV, as observed from Fig. 3, originatedfrom the transitions across the bandgap and the broad peaks near

2 eV, as described in Ref. [11], were due to transitions from first-excited level 4G to the ground state 6S of localized Mn2+ ions, theenergy value being independent of composition. However, in allthe sets, the primary peaks were near 1.82 eV. This is because, incase of interdiffusion of multilayer, the topmost layer (from whichthe PL emission occurred) remains almost in the same composi-tion like the bottommost layer. This is in accordance of Fick’s lawof interdiffusion. So in all the sets, the topmost layer emissionacross the bandgap occurred almost in the same energy.

3.2. Quantification of electronic properties

Among the electronic properties, the response of the developedDMS to light was studied using a tungsten–halogen lamp, with anintensity of 100 mW/cm2. The light was made incident on the

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Table 3Bandgaps and ‘‘x’’ calculated from spectrophotometry

Set 1 2 3 4 5 6

Direct bandgap from transmission spectra (eV) 1.75 1.825 1.90 1.98 2.025 2.07

‘‘x’’ For direct bandgap from transmission spectra 0.17 0.23 0.29 0.35 0.39 0.43

350

300

250

200

150

100

50

0

Inte

nsit

y (a

.u.)

1.7 1.8 1.9 2 2.1 2.2 2.3Energy of Emitted Rediation (eV)

Set-1 Set-3 Set-4 Set-5 Set-6Set-2

Fig. 3. Luminescence properties of Cd1�xMnxTe films.

110000

100000

90000

80000

70000

60000

50000

40000

Res

ista

nce

(Ohm

)

0 20 40 60 80 100 120 140 160 180 200

Time of Illumination (s)

Set 3 Set 2 Set 1

Fig. 4. Photoresponse for sets 1–3.

8000

7000

6000

5000

4000

3000

2000

1000

Res

ista

nce

(Ohm

)

0 20 40 60 80 100 120 140 160 180 200

Time of Illumination (s)

Set 6 Set 5 Set 4

Fig. 5. Photoresponse for sets 4–6.

P. Banerjee, B. Ghosh / Journal of Physics and Chemistry of Solids 69 (2008) 2670–26732672

films of size 2�1 cm2. The DMS films were, in turn, kept overa cold metallic plate at a temperature of 20–22 1C, to avoidthe effect of thermally generated carriers. The correspondingchanges in resistances were noted as a function of time ofillumination and have been shown in Figs. 4 and 5. Thecontact used for the electronic measurements was nickel–silver.In this contacting process, nickel was evaporated first on the filmsowing to its high work function (resulting in ease of contactformation) followed by the application of silver paste (owing toits higher conductivity and resulting ease of charge flow to theouter circuit).

Each of the films exhibited a fall in resistance after illuminationand finally showed a tendency of saturation when the generationof carriers reached towards their maximum. From the plots, it canbe easily seen that the rate of fall of resistances graduallydiminished from set-1 to set-6. Thus, with rise in Mn-fraction(as shown for the sets in Table 3), the response to light (i.e. thepercentage change of resistance after 3 min) gradually reduced, asshown in Table 4.

The electronic parameters of the DMS films, as shown inTable 4, were evaluated using vander Pauw method of Hallmeasurement at room temperature. The measurements werecarried out at a magnetic field of 5.0 kG using nickel–silvercontacts at the corners of square-shaped samples (1 cm2 in size).Four-probe analysis using nickel–silver contacts helped to calcu-late the activation energies of the Cd1�xMnxTe alloy films withvarious values of x. The values of activation energies has also beenshown in Table 4 while the ln(r) vs. 1/T plot obtained from four-probe measurement has been displayed in Fig. 6.

The resistivity values thus decreased with increase in Mn-fraction. Also the resistivity of sputtered films are higher than theevaporated films due to the presence of more free elemental Te inthe later. Evaporated films always contained some free Te (orTeO2) at the top, because Te was the topmost layer and inaccordance to Fick’s law of interdiffusion all of the Te did notinterdiffuse completely in the bulk of the film. Thus, evaporatedfilms were more as p-type due to the presence of free Te (or TeO2)at the top compared to sputtered films (where telluridecompounds were deposited). The activation energies slightlyincreased with Mn-fraction.

4. Conclusions

Nanocrystalline thin films of Cd1�xMnxTe prepared by twoprocesses of deposition have been studied here. Though thebandgaps of nanostructured films generally change abruptlybelow the crystal size of 10 nm, here the crystallites were higherin dimension than 10 nm. So, the pattern of change in bandgapwith composition for such nanocrystals can be approximated asthat for polycrystals or single crystals, as described in Eq. (1). Thephotoluminescence spectra at room temperature were similar tothose reported for lower temperature by others, with theexception that one particular transition was absent. Increase inMn-content has resulted in rise in bandgap and correspondingrise in activation energy but fall in the reduction of resistanceunder illumination.

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Table 4Electronic parameters calculated from Hall analysis, photoresponse and four-probe test

Set % Change in resistance after 3 min Resistivity (O cm) Carrier density (cm�3) Hall mobility (cm2/V s) Activation energy (eV)

1 59.1 275.19 1.66�1015 13.61 0.474

2 58.3 262.25 1.388�1015 17.17 0.483

3 57.0 138.57 1.43�1015 31.42 0.533

4 41.6 105.82 1.74�1015 33.83 0.565

5 35.0 21.30 7.35�1015 39.86 0.596

6 32.7 6.34 1.48�1016 66.23 0.636

8

7

6

5

4

3

2

1

0

In (

ρρ)

0.0026 0.0028 0.003 0.0032 0.0034

1/T (K-1)

Set-1 Set-2 Set-3 Set-4 Set-5 Set-6

Fig. 6. ln(r) vs. 1/T calculated from four-probe analysis.

P. Banerjee, B. Ghosh / Journal of Physics and Chemistry of Solids 69 (2008) 2670–2673 2673

Acknowledgments

The authors are thankful to Council of Scientific and IndustrialResearch (CSIR), Govt. of India for funding this work in the form offellowship to the first author. The help extended by Dr. Abhijit

Saha of UGC-DAE CSR, Kolkata towards taking luminescencemeasurements is also hereby acknowledged.

References

[1] I. Zutic, J. Fabian, S. Das Sarma, Spintronics: fundamentals and applications,Rev. Mod. Phys. 76 (2004) 323–410.

[2] T. Dietl, A. Haury, Y.M. d’Aubign’e, Free carrier-induced ferromagnetism instructures of diluted magnetic semiconductors, Phys. Rev. B. 55 (1997)R3347–R3350.

[3] T. Dietl, H. Ohno, F. Matsukura, Hole-mediated ferromagnetism in tetra-hedrally coordinated semiconductors, Phys. Rev. B 63 (2001) 195205–195225.

[4] L. Kowalczyk, A. Mycielski, L. Sniadower, R.R. Galazka, CdMnTe single crystalsfor room temperature optical isolator, Phys. Stat. Sol. (c) 1 (2004) 985–988.

[5] R. Rey-de-Castro, D. Wang, X. Zheng, A. Verevkin, R. Sobolewski, M. Mikulics,R. Adam, P. Kordo, A. Mycielski, Subpicosecond Faraday effect in Cd1�xMnxTeand its application in magneto-optical sampling, Appl. Phys. Lett. 85 (2004)3806–3808.

[6] A.S. Cross, D. Wang, G. Guarino, S. Wu, A. Mycielski, Roman. Sobolewski,Studies of coherent acoustic phonons in CdMnTe diluted magnetic singlecrystals, J. Phys.: Conf. Ser. 92 (2007) 012015–012018.

[7] M. Godlewski, Dynamics of exciton recombination in low-dimensionalheterostructures of II–VI semiconductors (CdTe/CdMnTe, ZnCdSe/ZnSe),J. Lumin. 76–77 (1998) 174–180.

[8] V.F. Aguekian, D.E. Ashenford, B. Lunn, A.V. Koudinov, Yu.G. Kusrayev, B.P.Zakharchenya, Photoluminescence of ‘‘dark’’ excitons in CdMnTe quantumwell, embedded in a microcavity, Superlattice. Microst. 41 (2007) 386–391.

[9] V.F. Aguekian, D.E. Ashenford, B. Lunn, A.V. Koudinov, Yu.G. Kusrayev, B.P.Zakharchenya, Photoluminescence spectra of CdTe/CdMnTe quantum wellstructures in an external magnetic field, Phys. Stat. Sol. (b) 195 (2006)647–652.

[10] J.K. Furdyna, J. Kossut (Eds.), Diluted Magnetic Semiconductors, in: Semi-conductors and Semimetals, vol. 25, Academic Press Inc, New York, 1988(Chapter 1).

[11] M.P. Vecchi, W. Giriat, L. Videla, Photoluminescence studies of the Mn2+

d-levels in Cd1�xMnxTe, Appl. Phys. Lett. 38 (1981) 99–101.