Fundamental reflectivity spectra of Hg1−xMnxTe mixed crystals in the 1.7 to 3.5 eV energy range

Solid State Communications,Vol. 25, pp. 579—581, 1978.

Pergamon Press.

Printed in Great Britain

FUNDAMENTAL REFLECTIVITY SPECTRA OF Hg1 ~Mn~TeMIXED CRYSTALS IN THE 1.7 TO 3.5 eV ENERGY RANGE T. Kendelewicz and E. Kierzek.Pecold Institute of Physics, Polish Academy of Sciences, Warsaw (Received 14 July 1977 byM. Cardona) The room temperature reflectivity coefficient R(E) for the mixed crystals Hg1 _~Mn~Te (x up to 0.57) in the energy range 1.7 to 3.5 eV was investigated. Two distinct maxima E1 and E1 + i~ connected with the transitions in the critical points on the [111] direction of the Brilouin zone for the samples with x up to 0.3 and the more diffused structure of R(E) for the samples with x > 0.3 was observed. A quadratic dependence of E1 and E1 + ~ transition energy vs alloy composition with x up to 0.3 was found, with bowing coefficient c = 1.21 ±0.02 and 1.06 ±0.02 respectively. The energy variation of an additional shoulder probably connected with the e1 transitions at L point of the Brillouin zone is also reported. IN THIS PAPER we present the results of the room technological difficulties arise, when x exceeds the value temperature reflectivity measurements performed on 0.3. The presence of MnTe2 inclusions in this range of Hg1 _~Mn~Te solid solutions for x from 0.0 to 0.57, in compositions was reported in the earlier technological the spectral range 1 .7 to 3.5 eV. For the first time the work of Delves [1]. The presence of the second phase dependence of the interband transition energies on inclusions seems to be confirmed by recent ESR composition x in Hg1 _~Mn~Te mixed crystals for measurements [6] performed on the crystals investienergies higher than fundamental energy gap was gated in our optical measurements. established. The Hg1 _~Mn~Te mixed crystals have been preThe solid solution Mn~Hg1_~Tewith zinc-blendepared by the modified Bridgeman method. MnTe type lattice can be prepared in the range of composition inclusions with hexagonal NiAs structure were found from x = 0.0 to x = 0.8 [1]. One of components MnTe by means of X-ray measurements analysis for the is an antiferromagnetic semiconductor with Néel manganese rich samples x 0.5. No traces of MnTe2 temperature TN = 307 K [2] and crystallizes in hexastructure were found in our crystals. gonal NiAs structure. This alloy system has been recently The average composition x was determined by of great interest because of its anomalous physical density measurements. The surfaces of the samples used properties deduced from magnetoabsorption [3] and in the reflectivity measurements were polished, and Shubnikov—de Haas [4] measurements in the compshortly before using, etched with 20% solution of Br2 osition range near semimetal—semiconductor transition in methyl alcohol. point x = 0.07 at 4.2 K, the anomalous behaviour being The room temperature normal incidence reflectivity due to the presence of half filled d shell and electrons spectra inofFig. HgTe of investigated some Hg1 _~Mn~Te 2~ions. From transport optical shown 1. and In the spectral samples range weare localized on Mn measurements the fundamental F 6 —I’s energy gap was observe two distinct maxima. In some samples we found to vary almost linearly with composition in the observe also a shoulder near 2 eV. The similarity of the range up to x = 0.2 [3—5]. reflectivity spectra of Hg1 _~Mn~Te and some ternary The object of our research was to investigate the zinc-blende-type alloys such as Hg1 _~Cd~Te and optical properties of crystalline samples of Hg1 _~Mn~Te Hg1 _~Zn~Te [7—91 as well as smooth variation of the with x up to 0.3 in the energy range above the fundaenergy of the observed respectivity peaks with compmental energy gap. We have also measured samples osition, beginning with the zinc-blende-type constituent with x up to 0.57 for the sake of completeness and HgTe enables us to interpret the experimental comparison, results. The observed two maxima designated E1 and Although it is possible to prepare fairly homoE1 + ~ according to the notation defined by Cardona genous Hg1 _~Mn~Te monocrystals for x 0.1 and [10] for zinc-blende-type materials, are connected with single phase zinc-blende-type crystals for x up to 0.3 the direct transitions in the critical points in the [111] ,

‘~

579

580

REFLECTIVITY SPECTRA OF Hg1 .~Mn~Te

x~0.w

Vol. 25, No. 8

-~

35 X0340

.~

~~0530-~

I

_____

~a~o

__

30 40

~25

~154

~

~

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~ 0115

=

______—

_~

0

______

~

______

________

~0

2,0

Photon 3,0 energy 40 /e~1 5,0 40

Fig 3 Reflectivity of MnTe vs photon energy at room temperature.

direction.ofThe splitting the doublet valence nature band. TheE1 being due peaks to the are spin-orbit pronounced and easy to observe in the whole range of

X=0.056

____________________

200

________ 300

250

Photon energy (eVI—’ Fig. I. Room temperature reflectivity as a function of energy for some samples of Hg1 _~Mn~Te alloys in arbitrary units.

composition, while E1 + i~ maxima are in general less distinct and entirely diffused in manganese rich samples. In Fig. 2 the variation of the energies corresponding to the observed reflectivity peaks has been plotted against alloy composition. The energies of the E1 maxima were determined with an accuracy 0.02 eV. Figure 2 shows the results for theE1 + ~i energy only for these

180 ~,2,70

samples in which this energy can be determined with an accuracy better than 0.03 eV. It was impossible to determine the energies of E1 + ~i transitions with sufficient accuracy in the case of other samples because of the broadening of the corresponding peaks. The numerical fitting ofE1 and E1 + ~i vs x up to 2 gives X 0.3 assuming the relation: Ex = a + bx + cx the= following results:

~ 2,60

E

E1~ 3,00 ~

2,90

1(x) = 2.11 (±0.02) 2 + 0.47(±0.02)x + 1.21 (±0.02)x (E 1 +L~1)(x)= 2.74(±0.02)+0.46(±0.02)x

2,50 2,110

2,30

2. +1.06±O.02x From the relations (1) we obtain E

2.20 2,10

1 = 3.79 eV and E1 + ‘~ = 4.26 eV for hypothetical MnTe with the zinc-blende lattice. The bowing coefficient c is constant with the 20% accuracy in the case of the samples with x up to 0.3. If the experimental values of E1 andE1 + ~

200 1,90

oJo~

A,

0,60~~

,,

0,5C

a

0:10

0,20

0,30

(1)

0,110

0,50 X

Fig. 2. Variation of the energy of theE1 E1 + ~ and e1 transitions as well as J.~ spin-orbit splitting as a function of x for Hg1 _~Mn~Te mixed crystals. ,

for the samples with x above 0.3 were included in the numerical analysis the value of c was strongly decreased. This fact seems to be related to a non-homogeneity of the manganese-rich samples with Mn content above 30%. In Fig. 2 we also present the values of the spin-orbit splitting ~i as a function of composition, together with

Vol. 25, No. 8

REFLECTIVITY SPECTRA OF Hg1 ,~Mn~Te

581

least squares fit to the linear dependence. The slow sition energy dependence on the composition is similar decrease of A~with x could be connected with the fact to that of the E1 peak. These two facts favour also the that the spin-orbit splitting of the investigated mixed second alternative explanation. crystals is determined by the spin-orbit splitting of the The fact that HgTe and MnTe have different crystal Sp electrons of Te. and band structures is the main difficulty in choosing An additional shoulder on the R(E) curve at energies the theoretical model which could explain our results. lower than E1 observed for samples with x up to 0.3 Our measurements of reflectivity coefficient ofhexamay be caused by a Such transition between therecently d shell inner gonal MnTe presentedofinR(E) Fig.with 3 exhibit quitemaximum different 2~ions. transitions were spectral dependence one main states of Mn reported at similar energies in the transmission experenergy at 2.2 eV. Since in all existing theories of the iments on Cd 1 _~Mn~Te mixed crystals [II]. A mixed crystal band structure the knowledge of the possible alternative explanation is that observed structure band parameters of the pure constituents is required, we may be connected with the e1 transitions at L point of are not able to apply any theory to our results because the Brillouin zone. This latter explanation seems to be the band parameters of MnTe are not known well more probable since the linear extrapolation of the enough. shoulder energy to x = 0 gives the value e1 = 1.96 ± 0.03 eV, which is in good agreement with the value of e1 for HgTe see Fig. 2 [10, 121. Moreover, we do not Acknowledgement The authors wish to express their observe any substantial composition dependence of the gratitude to Dr. R.R. Gal~zkafor stimulating comments intensity of the transition in question, while the tranand for his constant interest in this work. —

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