Magnetooptical study of semimagnetic alloys Cd1−xMnxTe

Phymca 117B& 1lab (1983) 452-454 North-HollandPubhshatgCompany

452

MAGNETOOPTICAL STUDY OF SEMIMAGNETIC ALLOYS Cdl_xMnxTe

G. Rebmann ~, C. Rlgaux ~, G. Bastard m, M. Menant m, R. Trlboulet

, W. Girlat °ma

Groupe de Physique des Solides de l'Ecole Normale Sup6rieure, Paris, France. ~m

Laboratoire de Physique des S o l i d e s , CNRS, Meudon, France.

*~IVIC, Centro de Fislca, Caracas, Venezuela. Tl~e splitting of the exciton state in magnetic field is studied by reflectance experiment on concentrated Cdl_xMnxTe alloys. A linear decrease of the splitting is observed as a function of x, inferring a zero magnetization for x 0 ~ 0.61 corresponding to the antiferromagnetic phase transition. Absorption spectra of concentrated alloys show several absorption bands resulting from the excitation of manganese 3d states.

I.

INTRODUCTION

Cdl_xMnxTe mixed crystals, large gap semlconductors, display unusual optical and magnetooptical phenomena originating from half-filled d shells of Mn. Giant splittings of the exciton state under external magnetic field, observed on dilute alloys ~I-4], are induced by s-d and p-d exchange interactions. Measurements of the exclton splittings provide an optical determination of the localized d electrons magnetization [I-4~. Magnetooptical methods offer interesting posslbilities to study magnetic phase transitions in these semlmagnetic semiconductors [5]. An anomalous behavior of the absorption in CdMnTe alloys was previously reported [6-~ . An absorption edge near 2.2 eV was attributed to the onset of transitions inside 3d shells of manganese ~]. Correlatively an intense photolumlnescence band, at 1.98 eV, was ascribed to Stokes-shifted hT 1 + 6A1 transitions of the ~,~+ ions ~,lOj. We report magnetooptical experiments performed on Cdl_xMnxTe mixed crystals in the range of composition 0.3 - 0.7 covering the reglon of the spin glass-antiferromagnetic phase transltion IIII. Absorption measurements are also carrled out on ultra-thin samples of concentrated alloys to study the absorption spectrum originating from the excltatlon of locahzed manganese 3d states. II.

MATERIALS

The Cdl_xMnxTe single crystals were grown by the 8ridgman method from Cd, Mn and Te [12] or from CdTe and MnTe ~ . The composltion of the samples was checked from electron microprobe measurements [12,1~ and also from density and photoluminescence measurements [I~ . The x values agreed quite well with the technological composltion, as expected from the particular CdTe-~InTe pseudobinary phase diagram [12]. The Cdl_xMnxTe solid solutions possess the zinc-blende structure, and all the as-grown crystals were of p-type, or semi-insulating [12,14].

0 378-4363/83/0000-0000/$03 00 © 1983 North-Hollap,I

III.

EXPERIMENTS

Reflectlvity and absorption measurements were performed on concentrated Cdl_xMnxTe alloys, at liquid and pumped helium temperatures in magnetic field up to 70 kG, using circularly and linearly polarized light in Faraday and Voxgt configuration respectively. A strong structure in reflectlvity spectra corresponds to the free exciton state associated with the valence-to-conduction band transitions (Fig. l) The energy of this structure, observed up to x = 0.7, increases linearly with x and is in good quantitative agreement with the data previously reported EI-4] for alloys of lower Mn content. Transmission measurements carried out on thin samples show the absorption edge associated wlth the fundamental transition (Fig.6). Its poeitlon follows the same composltion dependence as the exclton structure for x ~ 0.4. Around the compositlon 0.50, the energy gap becomes larger than the transltlon energy ( ~ % 2.2 eV) due to the excitation of localized Mn states. Experimental data in the high concentratlon range are reported and dlscussed in § 2. I. Exciton exchange-induced spllttings in concentrated alloys We have studled the splitting of the exciton state under external magnetlc field by magnetoreflectxon measurements : - for alloys of x < 0.35, slx Zeeman-like components are observed similarly as in the case of dilute alloys ~]. Two symmetrical components appear for ~ // ~. A strong and a weak structure are observed for each of circular polarization o + and ~-. With increasing alloying, one notices a broadening of the structures and an important reduction of the splittings ; - magnetoreflectance spectra of concentrated alloys exhlbit only the strong o+ and ~- components which are separated up to x = 0.55. The F1g.1 111ustrates magnetoreflection spectra of concentrated alloys ;

G Rebrnann et aL /Magnetooptical study ofsemimagnetic alloys Cdl.xMnxTe

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Figure l : Exciton reflectance spectra of Cd0.46Mn0.54Te , T = 1,9 K. - for x ~ 0.6, the reflectance spectra do not show any measurable splitting of the exciton state. The observed structures are identified according to the simplified scheme described in Ref.2, Including only effect of exchange on the F 8 valence (J = 3/2) and r 6 conduction (J = I/2) bands, and neglecting effect of magnetic field on delocalized electron states. The stronger components observed for o polarization correspond to (3/2, I/2) ~ u- and (-3/2,-I/2) : o + transitions. For // H, the two structures are attributed to (I/2,1/2) and (-I/2,-I/2) transitions. The energy difference between the strong components are : AE~ = (u - B) NO x

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where ~, 8 are exchange integrals for r 6 and r 8 bands respectively. denotes the thermal average of the Mn 2+ spln component along the magnetlc field. N O Is the number of unit cells per unit volume. The experimental splittings of the o and ~ components as a function of the magnetic field are reported on Fig.2 (x = 0.35). The ratio AE~/AE u Is independent of magnetic field. Its experimental value is in excellent agreement with the theoretical prediction (8/3 - ~)/(8 - =), using the exchange integrals (a/B = -0.25) deduced by Gaj et al [1~ In the lower concentration range. This fact supports the validity of the simplified scheme for alloys of higher Mn content. We have studied the evolution of the splitting AE o with increasing the Mn content. Experimental data are reported on Fig.3, for 0.3 ~ x ~ 0.55. The results show a strong reduction of the

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splitting and consequently of the d electrons magnetization wlth increasing the Mn content, putting in evidence the increasing influence of antiferromagnetic interactions between Mn 2+ ions.

The field dependence of the magnetization exhibits only weak s i i i i i i curvatures for 1 2 3 4 5 6 7 x ~ 0.35 (Flg.3). MAGNETIC FIELD I T I Magnetization curves of concentrated Figure 3 : Energy splittings alloys are linear of strong components of ex- in the investigated field range and citon spectra vs magnetic field for alloys of various become practically independent of t e m Mn contents. perature between 1.5 and 4.4 K. Cai i i i racterlzing the splitting by its slope in the region of linearity, plots of AEo/H versus the w Mn molar fraction x I0 are reported in -r Flg.4. One observes b a linear decrease W of AEo/H as a function of x~ extrapolating to zero for x o = 0.61 ± 0.02. i This composition 03 O.A O~ 0.5 coincides wlth the X critical concentrabJ

Figure 4 : Composition depen-~°:h:°rresp°nding appearance dence of the slopes AEo/H.

of an antiferromagnetic ordered~hase observed by neutron diffraction [15].

G Rebmann et aL / Magnetoopttcal study of semlmagnetm alloys Cdl.xMnxTe

454

2. Optzcal excitatlon of manganese 3d state CdMnTe alloys exhlblt an absorptlon edge 16,7,81 occurring near 2.2 eV (T = 4.2 K) whatever the Mn composition for x exceeding 0.5. The crystal opaclty above 2.2 eV have prevented the observation of the fundamental interband transitlon in concentrated alloys. The exlstence of an optical transition is corroborated by the reflectlvity data. The Fig.5 shows a strong reflectance

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Acknowledgements : we wlsh to thank Dr. R. Planel for fruitful discussions.

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Figure 5 : Reflectance structure near 2.2 eV in Cd.46Mn.54Te. structure observed at 2.]8 eV in Cd0.46Mn0,54Te, T = 1.9 K. The position of this structure is independent of magnetic fleld. Transmlss~on measurements carried out on ultrathin (few mzcrons thick) samples of Cd]_xMnxTe alloys in the high concentration range reveal the existence of several absorption bands and provide observation of the onset of the fundamental absorptlon edge. Absorption spectra obtained at T = 4.4 K, for x = 0.64 and 0.70, are reported on Fig • 6 " The spectrum of Cd 0 • 30Mn 0 .7 Te consists of two absorption bands, centered at ~.43 and 2.63 eV. Above 2.7 eV, the absorption increases evidencing the onset of the interband

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The llneshape of the absorptlon spectrum observed in concentrated CdMnTe can be explained by the excitation of localized Mn 3d states, by referring to the absorption data in Mn-doped ZnS []6]. In these cubic crystals, transitions between the ground state 6A I and the sequence of excited states 4TI, 4T2, 4AI, 4E glve rise to a series of absorption bands involving in each band zere-phonon and phonon lines. The positions of the absorptzon structures observed in CdMnTe are practically coincident with the energies of the lines identified to 6A 1 ÷ 4T I (2.21 eV), 6A I ÷ 4T 2 (2.44 eV), 6A 1 + bE and/or 6AI_+ 4A 1 (2.63 eV) transitions in Mn-doped ZnS L16J. These experimental features provide a direct evidence for the optlcal excitation of manganese 3d states in CdMnTe.

2 20 ENERGY

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transitions. In Cdo•36Mn0.64Te, the first absorptlon band with the maximum at 2.43 eV is observed, but the second maximum is screened by the absorptlon edge. The absorption spectra of concentrated alloys are not affected by applying magnetic field (up to 70 kG).

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REFERENCES []j Gaj, J.A., Planel, R. and Flshman, G,, Solid State Commun. 29, 435 (1979). [21 Ga3, J.A., Ginger, J. and Galaska, R.R., Phys. Status Solidi (b) 89, 655 (1978). [3] Twardowskl, A., NawrockiT-M. and Ginter, J., Phys. Status Solidl (b) 96, 497 (1979). [4] Bastard, G., Gaj, J.A., P'[anel, R. and R1gaux, C., Journal de Physique 41, C5, 247 (1980) ; and references included. [5] Kett, H., Gebhardt, W., Krey, U. and Furdyna, J.K., Journal of Magnetism and Magnetic Materials 25, 215 (1981)• Nguyen The--Khoi and Gaj, J.A., Phys. Status Solidi (b) 83, K133 (1977). Diourl, J.,--Lascaray, J.P., Trlboulet, R., Solid State Conamun. 42, 231 (1982). [8] Abreu, R.A., G1rlat,--W. and Vecchi, M.P., Phys. Lett. 85A, 399 (1981). [97 Vecchl, M.P.~--dlrlat, W. and Videlo, L., Appl. Phys. Left• 38 (2), 99 (1981). []0] Morlwskl, M.M., Bec-ker, W.M., Gebhardt, W., and Galaska, R.R., Solid State Commun. 39, 367 (1981). Ill] Galaska, R.R., Nagata, S. and Keesom, P.M., Phys. Rev. B 22, 3344 (1980). [12] Triboulet, R.--and Dldier, G , J. Cryst. Growth 52, 614 (1981). [13] Stanklew-~cz, J., Bottka, N. and G1riat, W., J. Phys. Soc. Japan 49 Suppl.A, 827 (1980). [14] Stankiewicz, J. and A--~ay, A., J. Appl. Phys. 53 (4), 3117 (1982). [15] G-{ebultowicz, T., Keps, H., Buras, B., Clausen, K. and Galaska, R.R., Solid State Commun. 40, 499 (1981) ~6~ Langer, D--?and Ibukl, S. Phys. Rev. 138, A 809 (1965).

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