Physica E 10 (2001) 419–423www.elsevier.nl/locate/physe

Giant tunability of excitonic photoluminescence transitions inantiferromagnetic EuTe epilayers induced by magnetic polarons

W. Heiss ∗, G. Prechtl, G. Springholz

Institut f�ur Halbleiter- und Festk�orperphysik, Johannes Kepler Universit�at Linz, Altenbergerstrasse 69, A-4040 Linz, Austria

Abstract

EuTe layers with high purity, grown by molecular beam epitaxy, are investigated by magneto-optical spectroscopy. Lowtemperature photoluminescence (PL) experiments reveal narrow, exciton like emission peaks exhibiting a Stokes shift of300 meV. These emission peaks can be tuned over a giant range of 160 meV by applying magnetic 0elds between zero and5 T. The temperature dependence of the PL transition energies shows a kink at the antiferromagnetic-paramagnetic phasetransition shifting to lower temperatures with increasing magnetic 0eld. This unique magnetic 0eld dependence as well asthe temperature dependence of the PL transitions results from the formation of magnetic polarons, due to d–f exchangeinteractions between electrons in the conduction band and localized magnetic moments. c© 2001 Elsevier Science B.V. Allrights reserved.

PACS: 71.35.Ji; 71.35.Aa; 71.70.Ej; 75.50.Ee

Keywords: Magnetic semiconductors; Magnetic polarons; Magneto-optics; EuTe

In recent years, diluted magnetic semiconductors(DMS) have attracted tremendous interest due to theirunique physical properties, like the appearance of thehuge exciton spin (Zeeman) splitting and the giantFaraday e>ect [1]. The strong spin splitting is pro-portional to the concentration of magnetic ions in thesemiconductor host lattice as well as to the exchangeintegral. The latter was found to reach maximum

∗ Corresponding author. Tel.: +43-732-2468-9643; fax:+43-732-2468-9696.E-mail address: wolfgang.heiss@jk.uni-linz.ac.at (W. Heiss).

values for Co, Cr and Fe in II–VI semiconductors, butthe achieved spin splitting is limited by the small sol-ubility of the magnetic elements in the semiconduc-tor host lattice of a few percent [2–4]. The highestspin splitting is currently obtained in Mn-based ternaryII–VI alloys with Mn concentrations of several 10%[5–7]. The large spin splitting leads to a shrinkage ofthe fundamental absorption with magnetic 0eld andhence to a red shift of the exciton photoluminescence(PL) transitions.

Anomalous large magnetic 0eld induced redshifts of the absorption edge [8–10] and of the

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420 W. Heiss et al. / Physica E 10 (2001) 419–423

photoluminescence transition energy [11] have alsobeen observed in Eu-monochalcogenides (EuXc).Nevertheless, these materials exhibit di>erent proper-ties than DMS in several respects. First of all, thesematerials are classical Heisenberg magnets due toexchange interactions between the localized mag-netic moments of the Eu2+ ions, resulting from theirhalf 0lled 4f levels (spin 7=2). So, because of theopposite sign of the nearest and next-nearest neigh-bor exchange integral, EuTe is antiferromagnetic oftype-II structure while the other EuXc exhibit ferro-magnetic or meta magnetic behavior. Furthermore,the valence band is formed by the p-orbitals of thechalcogenes, while the conduction band is formed bythe 5d and 6s orbitals of the Eu ions. Between the0lled valence band and the empty conduction bandsenergy levels of the half 0lled 4f orbitals of the Eu2+

ions are located. Therefore, photoexcitation createsexcitons consisting of strongly localized 4f-holes andd-electrons in the conduction band. Within the Bohrradius of the photoexcited electrons the local mag-netic moments become ferromagnetically aligned dueto d–f exchange interactions, thus MPs are formed.While in DMS, the formation of exciton-MPs resultsonly in a small Stokes shift of the PL lines whichdecreases with rising magnetic 0eld [12], in EuTe,in contrast, it seriously a>ects the exciton transitionenergies observed in PL experiments.

In this work, we investigate PL transitions in epi-taxial EuTe layers with high purity. We detect nar-row emission lines due to exciton transitions exhibit-ing unique magnetic 0eld and temperature dependen-cies, caused by the formation of MPs. We 0nd, e.g.,a linear dependence of the exciton transition energyon magnetic 0eld with a giant tunable range spanningover more than 160 meV for 0elds ranging between 0and 5 T. Furthermore, the excitonic PL lines exhibita Stokes shift which is about 10 times larger than inall DMS. These PL lines were not observed so far inEuTe, because all previous PL experiments were per-formed on powders [13], polycrystalline thin layers[10], and bulk crystals [11] showing only a broad lu-minescence band at lower energies due to defect re-lated transitions.

The EuTe layers were grown by molecular beamepitaxy using elemental e>usion sources for Eu andTe. Because of its comparatively large lattice constantof 6:598 JA; (1 1 1) oriented BaF2 was used as sub-

strate exhibiting a relatively small lattice mis0t of 6%and a thermal expansion coeKcient which is essen-tially equal to that of EuTe. This is particularly impor-tant because epitaxial growth and optical measurementinvolve temperature cycling over as much as 600◦C.From in situ reLection high-energy electron di>rac-tion (RHEED) studies we 0nd that the EuTe (1 1 1)surface has a very strong tendency for (1 0 0) faceta-tion due to the resulting lowering of the free surfaceenergy. Therefore, the growth conditions have to becontrolled in a very precise way in order to obtain highquality and smooth epitaxial layers. In particular, 2Dlayer-by-layer growth can be maintained only underthe condition that the EuTe (1 1 1) surface is very closeto the borderline between the Eu- and Te-stabilizedsurface states, which can be easily distinguished bytheir di>erent surface reconstructions (see Ref. [14]).For the constant Te-to-Eu beam Lux ratio of 2 and theEuTe growth rate of 0:6 monolayers=s (=0:82 �m=h),this transition takes place at a substrate temperatureof 260◦C, which has to be maintained to better than±10◦C during growth. All epitaxial 0lms grown un-der such conditions show sharply streaked RHEEDdi>raction patterns throughout growth.

Magneto-optical experiments were performed inFaraday con0guration using a split coil magnet cryo-stat giving 0elds in the range of 0–8 T. The photoluminescence (PL) was excited by the 488 nm lineof an Ar laser focused on the sample to a spot sizeof about 100 �m and detected by a photomultiplierwith a spectral cut o> at 1:4 eV. Prior to the PLexperiments, the samples were characterized by trans-mission measurements using a 150 W halogen lampmonochromized by a 1

4 m spectrometer. A transmis-sion spectrum of a 3 �m thick EuTe epilayer is shownin Fig. 1, measured at a temperature of 1:7 K. It ex-hibits Fabry-Perot interference fringes due to multiplereLections at the epilayer-substrate interface and thesample surface. The band gap energy of 2:25 eV atzero 0eld, determined from the absorption edge de-picted by the arrow in Fig. 1, is in good agreementwith that obtained from magneto-optical Kerr rotationexperiments [15]. With increasing magnetic 0eld Bup to 1 T the absorption edge slightly shifts to higherenergies, whereas increasing B further results in ashrinking of the band gap, as shown in detail in theinset of Fig. 1. At a magnetic 0eld of 6 T the total redshift of the absorption edge amounts 30 meV. This

W. Heiss et al. / Physica E 10 (2001) 419–423 421

Fig. 1. Transmission spectra of the 3 �m thick EuTe epilayerfor zero 0eld and 6 T, measured at 1:7 K. The inset shows themagnetic 0eld dependence of the gap energy determined from thefundamental absorption edge.

Fig. 2. 1:7 K photoluminescence spectra of the EuTe sample beforeand after annealing. The dashed line indicates the detection cuto> of the used photomultiplier.

red shift corresponds to the exciton Zeeman splittingobserved in DMS and is about as high as that ofCd1−xMnxTe with a concentration of 4% at the same0eld. At zero 0eld, a shrinking of the absorption edgeof 30 meV with an almost linear slope is observedalso when the temperature rises from 2 to 60 K.

Typical 1:7 K PL spectra of our EuTe epilayer areshown in Fig. 2. The luminescence spectra can be di-vided into two regions: a more than 200 meV broadluminescence band around 1:5 eV, which corresponds

Fig. 3. Excitonic emission spectra for various magnetic 0eld, mea-sured at 1.7 and 15 K, exhibiting a giant magnetic 0eld tunability.

to the PL emission also observed by other groups[10,11], and two sharp emission lines close to 1:9 eV,labeled X1 and X2 in Fig. 2. Within the broad PL bandat 1:5 eV the spectrum exhibits several maxima andminima with a regular spacing of 77 meV due to mul-tiple interference of the PL light within the 3 �m thicklayer. For this PL band, post growth thermal anneal-ing at 400◦C for 5 min results in a decrease of thePL intensity as well as a narrowing of the spectralwidth, as shown in Fig. 2. The intensity of the narrowemission line X1, in contrast, increases by a factor ofthree upon annealing. In EuTe epilayers doped by Biwith a concentration of 1020 cm−3, in comparison, weobserve only the broad band luminescence at 1:5 eV.This behavior shows that the PL spectra in EuTe sen-sitively depends on the crystalline quality of the sam-ples. Furthermore, it indicates that the broad PL bandat 1:5 eV is caused by self-activated emission associ-ated with deep centers, as observed also in other widegap semiconductors [16], whereas the emission peaksat 1:9 eV clearly correspond to excitonic transitions.The di>erence between the energy of these excitonicPL peaks and that of the absorption edge (Stokes shift)at 2:25 eV amounts 300 meV, which is about 10 timeslarger as compared to the Stokes shift observed inII–VI DMS [12].

The most striking feature of the excitonic PL emis-sion lines X1 and X2 is their huge red-shift in appliedexternal magnetic 0elds (H), as demonstrated in Figs.3 and 4. With increasing 0eld both excitonic PL linesshift rapidly to lower energies. At 1:7 K, applying a

422 W. Heiss et al. / Physica E 10 (2001) 419–423

Fig. 4. Transition energy of the X1 exciton peak as function ofmagnetic 0eld (a) and as function of temperature (b). The opencircles in (b) are obtained at zero 0eld by the use of a 0ve timessmaller laser excitation power.

magnetic 0eld of 5 T results in total red shift of the X1

line of more than 160 meV. This PL red shift exceedsby far that of the absorption edge shown in Fig. 1 andit is also considerably larger as compared to all PLshifts observed in any DMS compound. In the DMS,the PL red shift is almost equal to half of the giantexciton spin splitting reaching values at 5 T of, e.g.,60 meV for Cd1−xMnxTe with x=16% [5]. For com-parison, in a Zn0:80Cd0:20Se=MnSe digital alloy, thisvalue amounts to about 50 meV at the same 0eld [17],while in Cd0:952Co0:048Se [2] and in Zn0:94Fe0:06Se [3]shifts smaller than 40 meV are observed.

Fig. 3 shows a comparison of the exciton emis-sion spectra at 1:7 K to that at 15 K. It demonstratesclearly, that the energy splitting between the X1 andX2 emission peak decreases with rising magnetic 0eldas well as with increasing temperature. In detail, wefound a magnetic 0eld dependence of this splittingwhich is proportional to 1=B. Furthermore, the split-ting monotonously shrinks with rising temperature ex-hibiting a step-like decrease at a temperature of 9:6 K,where the antiferromagnetic-paramagnetic phase tran-sition occurs. With rising excitation power we observealso a relative increase of the X2 emission intensity inrespect to that of the X1 line. A comparison of these ex-

perimental dependencies with luminescence data fromantiferromagnetic insulators like BaMnF4 [18] indi-cates that the X2 emission peak represents a magnonside-band of the X1 peak.

The magnetic 0eld and temperature dependenceof the X1 emission line is summarized in Fig. 4. Inparticular, the energy of the X1 peak shows a lin-ear dependence as function of magnetic 0eld with aslope of −34 meV at 1:7 K and −26 meV at 15 K.This linear dependence is in contrast to the magnetic0eld dependence of the PL energy observed in DMS,where this dependence is related to the giant Zeemansplitting and, therefore, shows a dependence propor-tional to a Brillouin function of B. When the temper-ature is increased from 2 K to the NPeel temperature(TN), the transition energy of the X1 peak slightlydecreases, while increasing the temperature furtherresults in a considerably blue shift of the X1 peak. Atthe antiferromagnetic-paramagnetic phase transitiona kink is observed in the temperature dependence ofthe X1 transition, which shifts to lower temperaturesT with increasing magnetic 0eld. Therefore, map-ping the kink in the dependence of the X1 transitionenergy versus T for various applied 0elds allows todetermine the magnetic phase diagram of EuTe. Toobtain the correct value of TN = 9:6 K, however, thePL experiments have to be performed using rathersmall excitation intensities to avoid sample heating,as demonstrated for zero 0eld by the open circles inFig. 4(b).

The temperature and magnetic 0eld dependence ofthe PL transition energies has been calculated recentlyby Umehara [19]. In these numerical calculations thespontaneous magnetic ordering of the 4f spins and theMP formation induced by d–f exchange interactionsis taken into account, as well as the Coulomb inter-action between photoexcited electron hole pairs andthe electron (hole)-optical-phonon interactions. Thesecalculations predict a distinct temperature dependenceof the PL transition energy due to the d–f interaction,with a constant emission energy for temperatures upto TN, and a subsequent blue shift of 20 meV when thetemperature rises from TN to 2 × TN, in good agree-ment with the behavior shown in Fig. 4(b) for zero0eld. Furthermore, the calculation predicts a linear redshift of the PL transition energy in increasing mag-netic 0elds, dominated by the d–f interactions, simi-lar as shown by the experimental results in Fig. 4(a).

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The good agreement between the calculated depen-dencies and the experimental data con0rms that themagnetic 0eld and temperature dependence of the ex-citonic PL transitions is dominated by the formationof MPs, whose size shrinks above the NPeel tempera-ture with rising T . The strong e>ect of the MP for-mation on the PL transition energies is caused by thelarge magnetic moment of the MPs. It is much largerin EuTe compared to that in Mn based II–VI DMS be-cause of (1) the localized nature of the photoexcitedexcitons, and (2) the higher density of magnetic ions,and (3) the larger spin of the single magnetic ion (s= 7

2for Eu2+ compared to s = 5

2 for Mn2+), and (4) the6 times smaller antiferromagnetic-exchange couplingbetween the magnetic ions, which has to be broken upto form MPs.

We reported on magneto-optical investigations onepitaxial EuTe layers with high purity. We detect nar-row, excitonic luminescence transitions at an energyof 1:9 eV, about 300 meV below the energy of the ab-sorption edge. In contrast to the absorption edge, theenergy of these PL transitions can be linearly tunedover a giant range of more than 160 meV by applyingmagnetic 0elds between 0 and 5 T. The temperaturedependence of the transition energies shows a kinkat the NPeel temperature which shrinks with magnetic0eld. The magnetic 0eld and temperature dependenceof the PL transitions is strongly governed by magneticpolarons with large magnetic moments, formed due toexchange interactions between photoexcited electronsin the conduction band and electrons localized in thehalf 0lled 4f states of the magnetic Eu2+ ions.

Acknowledgements

This work is supported by the FWF, the GME, andthe Austrian Academy of Sciences.

References

[1] J.K. Furdyna, J. Kossut, Diluted Magnetic Semiconductors,in: R.K. Willardson, A.C. Beer, (Eds.), Semiconductor andSemimetals, Vol. 25, Academic Press, New York, 1988.

[2] F. Hamdami, J.P. Lascaray, D. Coquillat, A.K. Battacharjee,M. Nawrocki, Z. Golacki, Phys. Rev. B 45 (1992) 13298.

[3] A. Twardowski, P. Glod, P. Pernambusco-Wise, J.E. Crow,M. Demianiuk, Phys. Rev. B 46 (1992) 7537.

[4] W. Mac, A. Twardowski, M. Demianiuk, Phys. Rev. B 54(1996) 5528.

[5] J.A. Gaj, R. Planel, G. Fishman, Solid State Commun. 29(1979) 435.

[6] M. Arciszewska, M. Nawrocki, J. Phys. Chem. Solids 47(1986) 309.

[7] A. Twadowski, P. Swiderski, M. von Ortenberg, R. Pauthenet,Solid State Commun. 50 (1984) 509.

[8] J. Schoenes, Z. Phys. B 20 (1975) 345.[9] T. Kasuya, Critical Rev. Solid State Sci. 3 (1972) 131.

[10] P. Wachter, Critical Rev. Solid State Sci. 3 (1972) 189.[11] R. Akimoto, M. Kobayashi, T. Suzuki, J. Phys. Soc. Japan

63 (1994) 4616.[12] G. Mackh, D.R. Yakovlev, W. Ossau, H. Heinke, T. Litz, F.

Fischer, A. Waag, G. Landwehr, R. Hellmann, E.O. Gobel,Phys. Rev. B 50 (1994) 14069.

[13] G. Busch, P. Streit, P. Wachter, Solid State Commun. 8(1970) 1759.

[14] G. Springholz, G. Bauer, Appl. Phys. Lett. 62 (1993) 2399.[15] H. Krenn, W. Herbst, H. Pascher, Y. Ueta, G. Springholz,

G. Bauer, Phys. Rev. B 60 (1999) 8117.[16] A.N. Georgobiani, S.I. Radautsan, I.M. Tiginyanu, Sov. Phys.

Sem. 19 (1985) 121.[17] S.A. Crooker, D.A. Tulchinsky, J. Levy, D.D. Awschalom,

R. Garcia, N. Samarth, Phys. Rev. Lett. 75 (1995) 505.[18] T. Tsuboi, W. Kleemann, Phys. Rev. B 27 (1983) 3762.[19] M. Umehara, Phys. Rev. B 52 (1995) 8140.