position and lifetime of photoluminescence in cd1−xmnxte and zn1−xmnxte. exchange dependent...

E. MULLER and W. GEBHARDT: Position and Lifetime of Photoluminescence 259

phys. stat. sol. (b) 137, 259 (1986)

Subject classification: 78.55; S8.15

Fakultat fizr Physik der Universitat Regensburgl)

Position and Lifetime of Photoluminescence in Cdl-,Mn,Te and Znl-,Mn,Te Exchange Dependent Effects

BY E. MULLER and W. GEBHARDT

Data on position and lifetime of the photoluminescence band in Cdl-,Mn,Te and Znl-,Mn,Te are reported for different manganese concentrations 5. The luminescence is due to intraatomic transitions within the manganese ions. The blue shift at low temperatures shows the typical behavior of an exchange dependent parameter. The lifetimes are in the microsecond range. At low temperatures the decay is dominated by the radiative lifetime with weak temperature dependence. In Znl-,Mn,Te the lifetime varies with manganese concentration, whereas in Cdl-,Mn,Te the concentration dependence is insignificant.

Messungen zur Bandenlage un$ebensdauer der Photolumineszenz in Cdl -,Mn,TeundZnl -,Mn,Te werden fur verschiedene Mangankonzentrationen vorgestellt. Die Blauverschiebung bei tiefen Temperaturen zeigt das typische Verhalten eines austauschabhangigen Parameters. Die Lebens- dauer liegt im Mikrosekundenbereich. Bei tiefen Temperaturen hat das Abklingen der Lumineszenz nur eine schwache Temperaturabhangigkeit und wird von der radiativen Lebensdauer bestimmt. In Znl -,Mn,Te andert sich die Lebensdauer mit der Mangankonzentration, wahrend die Konzen- trationsabhangigkeit in Cdl -,Mn,Te nicht signifikant ist.

1. Introduction

Semimagnetic semiconductors are formed when a substantial part of the cations in II-VI semiconductors is replaced by magnetic ions, usually manganese. These substances combine the properties of well known semiconductors with those of diluted magnetic materials [l, 21. A prototype of this group of substances is Cdl-,Mn,Te in which the red photoluminescence was first studied [3, 41. This luminescence which peaks a t about 2 eV is attributed tointra-atomic transitions 4T, + 6A, within the 3d5 configura- tion of Mn2+-ions [3, 5 to 81. The observed broad band is a multi-phonon side band which exhibits an additional broadening due to local fluctuations of the crystal field. This is a result of the random occupation of cation sites by Mn2+ ions. The width of the distribution of 4T1 states has been estimated from time resolved emission spectra [5].

The subject of the present paper is the measurement of band position and lifetime of the luminescence as a function of temperature and concentration. The luminescence band was measured with either cw excitation or pulse excitation. The shift was derived from the first moment of the band, the lifetime from the decay of the integrated intensity described in Section 2. The results are presented in Section 3. It will be shown in the discussion (Section 4) that the band shift shows a low temperature behavior which is typical for an exchange dependent energy. Several mechanisms are discussed. The lifetime has only a weak temperature dependence a t low temperatures, but above 60 K it strongly decreases with temperature due to trapping.

l) Universitatsstr. 31, D-8400 Regensburg, FRG. li'

260 E. MULLER and W. GEBHARDT

2. Experimental

Single crystals of Znl-,Mn,Te (with x = 0.1, 0.3, 0.7) and Cldl-,Mr!,Te (with x = = 0.45, 0.55, 0.65) were used. The Zn compounds were grown by W. Giriat, IVIC Caracas (Venezuela), the Cd compounds by J. K. Furdyna, Purdue University (USA). The crystal growth technique was a Bridgeman method used in both laboratories. The luminescence band was excited with the 514.5 nm line of an argon laser. The luminescence was collected and focussed on the entrance slit of a 1 m monochromator. The output was directed to a photomultiplier. For the time-resolved spectra we used an excitation pulse length of 64 ps. The decay was recorded in the dark period which was in most experiments also chosen to be 64 ps. In each of the 64 time windows a spectrum was recorded with a multichannel scaler, so that the time window was 2 ps. Each spectrum consists of 100 data points. The spectral resolution was 0.2 nm. The integrated intensity or zeroth moment was calculated from the luminescence spectra J ( E , t ) , by

M

I ( t ) = J J ( E , t ) d E . 0

In order to obtain the band shift the center of mass energy or first moment was calculated from the data points by

The decay of the total intensity I ( t ) deviates from a pure exponential. But an excellent fit of all measured data was found with the function

I ( t ) = C exp (-at") . (2.3)

Using (2.3) we calculated an averaged lifetime in the following way:

Note that for a pure exponential decay (12 = 1) (2.4) gives the decay time - z = .-I.

Time-resolved spectra are only measured over a limited time range of a t most 128 ps. However, by using a fit with (2.3) the integration of (2.4) can be carried out over an unlimited time interval.

3. Results

Luminescence spectra recorded a t 2 K are displayed in Fig. 1 for different concentrations. The luminescence in Znl -,Mn,Te is visible within the whole concentration range (0 < x < 0.75) where a stable mixed phase exists. In Cdl-,Mn,Te the luminescence cannot be observed below x = 0.45 because the direct band-to-band transitions cover the emission. The luminescence band is shifted in both systems to higher energies with increasing x. This shift is very small in the Cd system but i t is substantial in the Zn system.

Position and Lifetime of Photoluminescence in Cdl -,Mn,Te and Znl -,MnzTe 261

a- energy /eV/ 21 20 19 18 21 20 19 18

I - -" 5

A 05 B

E

El 3

P

0 570 630 690 570 630 690

wavelength Inm) - Fig. 1. Luminescence intensity versus wavelength. The intensity was corrected for the spectral response of the detector. The temperature was 2 K. a) Cdl -,Mn,Te: D z = 0.45, 0 0.55, + 0.65. b) Znl-,Mn,Te: x = 0.10,O 0.30, + 0.70

In Fig. 2 the peak position given by the first moment is plotted versus temperature. The temperature variation is similar for both materials. When the temperature is low- ered the band first undergoes a red shift. At about 60 K the peak energy reaches a minimum. At still lower temperatures the band shifts to the blue. A t LHeT the peak energy saturates. This peculiar low temperature shift decreases with concentration x in Znl-,Mn,T and is probably zero for an isolated Mn2+ impurity.

Fig. 3 shows the averaged lifetime determined with (2.4). The variation with temperature reveals different regions : a strong decrease with increasing temperature above 60 K and only a weak variation below this temperature. The sample of Zno,gMno.lTe seems to be an exception as there is a monotonous decrease o f t from 2 up to 100 K. Table 3 shows a comparison of lifetimes obtained in this work with those from other authors in similar systems.

1.90 0 40 80 120

I I

I I

I r i

L I I b i

T f K l - 0 10 80 120

Fig. 2. Position of the photoluminescence given by the first moment plotted versus temperature. a) Cdl-,Mn,Te: z = 0.45, x 0 . 5 5 , ~ 0.65. b) Znl-.,Mn,Te: 0 x = 0.10, x 0.30, 0 0.70


-

b

o x 0

I X 1 a n

I I

Zn,-, Mnx Te

1 x x x x 0 I I

0 4G 80 120 0 40 80 I20 T I K ) -

Fig. 3. Mean lifetime of total luminescence intensity versus temperature. a) Cdl-,Nn,Te: x = 0.45, x 0.55, 0 0.65. b) Znl-,Mn,Te: 0 x = 0.10, x 0.30, 0 0.70

4. Discussion

4.1 Lou. temperature band shift

In this section we demonstrate that exchange interaction is the possible origin of the anomalous change of E(T). Several processes will be discussed. In Fig. 4 a and b the anonialous band shift normalised to the value a t T = 0 K is plotted versus temperature. The full linc is the calculated square of sublattice magnetisation for a diluted antiferrornagnet with S = 512 within a mean field approximation. The fit is quite good although T, is the only adjustable parameter. One obtains T, = (45 5 10) K for both the Cd and Zn compounds which is practically independent of the manganese concentration x. One would expect, however, that T, depends on x since the distribution of cluster dimensions changes with concentration. This is obviously not the case, for reasons which are not yet clear. Table 1 contains NBel temperatures from different experiments.

0 20 40 60

Fig. 4. Xormalised low temperature shift of the luminescence band position (E( T ) - x(65 K))/ ( E ( 0 ) - E(65 K)). The full line was calculated from the square of the normalised sublattice magnetisation. M 2 ( T / T ~ ) / M z ( 0 ) . Data for a) Cdl-,Mn,Te: 5 = 0.45, x 0.55,O 0.65. b) Znl-,Mn,Te: 0 5 =- 0.10, x 0.55, 0 0.70

Posit,ion and Lifetime of Photoluminescence in Cdl -,Mn,Te and Znl -,Mn,Te 263

Tab le 1 NBel temperatures of antiferromagnetic Cdl -,Mn,Te obtained with different experimental methods

experimental method TN (K)

specific heat for z = 0.7 [9] 3 8 + 2 magnetic susceptibility for x = 0.7 [9] 35 5 2 neutron scattering [lo, 111 49 shift of luminescence 45 + 10

We will now compare the observed band shift with similar effects observed in semimagnetic semiconductors and other manganese compounds (see also Table 2). In a recent paper of Diouri et al. [19] the temperature dependence of the fundamental absorption edge in Cdl-,Mn,Te was studied and an additional exchange induced blue shift was observed a t temperatures below 50 K. The magnitude of this effect increases with x2 and reaches 45 meV a t x = 0.73. In the model used by the authors the band shift is explained by the s-d and p-d exchange interaction first proposed by Galazka and Kossuth [22] which shifts valence and conduction bands in second order perturbation theory.

Seehra and Groves [17] measured the shift of the 4T1, 4T2, and 4A, + *E absorption bands in MnO. They find a behavior analogous t o that shown in Fig. 2 a and b. The low temperature shift has the same sign and is of the same order of magnitude as that found in Cdl-,Mn,Te and Znl-,Mn,Te at high manganese concentrations (see Table 2). The authors relate the observed effects in MnO to exchange striction. Unfortunately i t is not known if similar magneto-elastic effects are present in semimagnetic semiconductors. However, the spin-glass behavior which occurs within a large range of concentration prohibits the formation of domains with axial anisotropy, although volume striction is possible and should be discussed. We may assume an upper limit of volume striction of A V / V = 0.01 and calculate the corresponding pressure. Using the bulk modulus of ZnTe to be B, = 51 GPa [20], we obtain P = = 0.5 GPa. The pressure shift of the manganese 4Tl absorption (-62 meV/GPa) and the intrinsic absorption edge (+82 meV/GPa for x = 0.7) have recently been measured [21]. With these values the expected shifts in Zno,3Mno.,Te would be -31 and $41 meV, respectively.

These are large effects which have the correct order of magnitude. The estimate indicates that magneto-striction, if present, must be considered. Even in this case the temperature dependence would be given by H2(T) as in Fig. 4. However, the expected shift of the 4T, band has a negative sign in contrast to the experimental results, which

Table 2 A comparison of the absorption band shift in MnO [17] and a-MnS [18] with the emission band shift in Cdo~,,Mno,,,Te and Zno,,Mn0,,Te. J,, are the nearest and J,,, the second nearest neighbor exchange constants. The data of Jnn and J,,, for &In0 are taken from [12, 131, for a-MnS from [13, 141, for Cdl-,Mn,Te from [15], and Znl-,Mn,Tefrom[16]

substances MnO a-MnS Cd,,.,,Mrb.,Te z110.3Afn0.7Te

Jnnn (K) 11 12.5 2 Jnn (K) 10 7 10 11.8

E(O) - Emin of 4Tlg (meV) abs. 18.2 em. 38.6 em. 28.9 E(O) - Enijn of 4 T ~ g (meV) abs. 32.4 @(O) - Emin of 4A1g, 4E, (meV) abs. 26.8 17.3


give equal sign for both, the shift of the fundamental absorption and that of the *T1 -+ 6Al luminescence.

Next we discuss the exchange interaction between the Mn2+ ions as a source of the shift. It leads to a correction of the transition energy E(4T1) - E(6A1) which is propor- tional to the magnetic energy

Here U,(T) is the internal magnetic energy

U,(T) = C 2Jij(S&) (4.2) which can be obtained from the specific heat [9] by integration

U,(T) = f cm(T’) dT’ , 0

(Y is the ratio of exchange constants in the excited state and ground state

(4.3)

In (4.1) the magnon energy has been neglected. For a < 1 we expect

AE(T) < Um(T) * With the results of Galazka et al. [9] we obtain an upper limit which is two orders of magnitude smaller than the observed band shift. Therefore it seems to be unlikely that the antiferromagnet exchange interaction between Mn2+ ions is responsible for the low Lemperature shift.

The exchange interaction of the Galazka/Kosuth type [22] allows much larger effects but would need a strong hybridisation of Mn-3d and Te-5p functions. There is indeed evidence for considerable hybridisation from both experimental and theoretical work [21 to 251. In this case a crystal field state has to be written as

y = (1 - C Z ’ ) ~ / ~ y(3d) + ay(5p). (4.5)

The value of u2 for W ( ~ A ~ ) and y(4T1) is not known, but is likely to be around 0.20 [25]. This does not necessarily mean that the process considered here gives only 2076 of the observed magnitude. A detailed analysis which cannot be given here has to take into account not only the “rigid spin” terms s,S,i but also terms like 8,S-j + s-S+j. This would give rise to an energy shift due to spin fluctuations induced by exchange interaction with valence band electrons. Again the temperature dependence is determined by spin correlation functions as in (4.2) which leads to a N 2 ( T ) dependence in M F approximation as shown in Fig. 4.

4.2 Lozo temperature lifetimes

The observed luminescence band is a multiphonori side band. The electronic transitions are spin and parity forbidden. The parity selection rule which prohibits electric dipole transitions between crystal field states of even parity is broken by the point group Td of the cation site which lacks inversion symmetry. The effectiveness of the spin selection rules is released when pair transitions are considered. The spin change by a Erenkel exciton a t one manganese ion is compensated by an inverse spin change a t another manganese ion which is in the electronic ground state (see Fig. 5 ) . The theory of these magnon assisted electric dipole transitions is well known and has been worked out for stoichiometric manganese compounds more than ten years ago [26] to

Position and Lifetime of Photoluminescence in Cdl -,Mn,Tc and Znl-,Mn,Te 265

cold band processes 5= 3/2, M = 3/2

T M-412

S - 5/2, M =5/2 -I- s=5/z, M=5/z I a

S=5/2, M=@ + b

S=3/2, M=3/2

M=3/2 S= $2,

hot band processes

M=3/z I S ~ 5/2, M= 5/2

T 5 -51’2, M =3/2 1 S=3/2, M=1/2

sife 2 C site I

Fig. 5. Three pair transitions which contribute to the phonon side band of the 4T1 - 6A, emission. The processes a) and b) have maximum probability a t T = 0 and decrease with increasing temperature. The probability of process c) is zero a t T = 0 but increases with temperature

[29]. However, the concept of pair transitions is not restricted to stoichiometric compounds but may equally well be applied to diluted magnetic systems.

Two effects have to be considered when total lifetimes are discussed : the radiative lifetime zR and the trapping time zt. In a pure exponential decay they are related by

(4.6)

However, as mentioned above the decay is non-exponential and therefore Z cannot be decomposed as done in (4.6). Instead one should use an averaged lifetime as defined by (2.4). In spite of this complication two different temperature regions may be

1 1 1 _ _ - - f-. z zR zt

distinguished, as discussed in Section 3. We suggest that the low temperature region is dominated by the radiative lifetime and the high temperature region by trapping. In Cdl -,Mn,Te there is evidence for radiationless energy transfer to traps which re-emit in the near infrared at 1.2 eV [30, 311. This transfer becomes very effective above 60 K.

In Znl -.Mn,Te the low temperature behavior of the lifetime is concentration dependent. The sample with z = 0.1 reaches a relatively large lifetime of 43 ps at T = 2 K, which decreases nionotonously with increasing temperatures. This may be explained by the fact that we are mainly concerned with pair transitions. But we may find in low concentration samples still a considerable number of single manganese ions which contribute to a very slow decay and thus increasing the lifetime (see Table 3). The luminescence lifetime also decreases with concentration (z = 0.3 and 0.7) and remains constant a t low temperatures.

T a b l e 3 Low temperature lifetime of the manganese luminescence 4T1 - 6Al in various mixed compounds. Note that the lifetime of this luminescence is a t least two orders of magnitude shorter than the lifetime measured in stoichiometric compounds in which the manganese site has a center of inversion symmetry [32, 33, 351

t (p) 52 & 5 [34] 43 20 500 [36] 140 [36] T (K) 10 2 2 28 28


In the Cdl-,Rln,Te samples the low temperature lifetime is independent of x for reasons which are not yet clear. There is also a slight increase o f t with temperatures. This is probably due to an increase of the radiative lifetime. We mention here another mechanism which also leads to an increase of Z and which works as follows. The occupation of 4T1 states under cw excitation changes with increasing temperatures favoring higher lying states. If these states possess a longer lifetime we would obtain the observed effect. However, there are two experimental reasons why this mechanism must be excluded. Firstly the experimental data show that the lifetime becomes shorter with increasing luminescence energy, i.e. higher lying states have shorter lifetimes [5]. Secondly the increase of the lifetime with temperature is even more pronounced when the luminescence a t the short wavelength side of the band is recorded [37].

Lifetimes in several stoichiometric manganese halides were discussed by Kambli and Giidel [32, 331. Their data of RbMnC1, and CsMnBr, show great similarities with our data of Fig. 3. But there are important structural differences between these halides and semimagnetic semiconductors. RbMnC1, and CsMnBr, have two inequivalent manganese sites with probably different lifetimes. Furthermore a large portion of the emission in manganese halides comes from shallow traps formed by Mn2+ ions in a distorted environment. The excitons localised at the traps have a considerably longer lifetime than the intrinsic excitons. On the other hand, in Cdl-,Mn,Te and Znl-,Mn,Te all *TI states are well localised and only a slow hopping motion is possible. Fuji- warn et al. [29] have compared the strength of the absorption bands in MnF2 and RbMnF, with the theory of magnon assisted transitions. A 20% increase was found for the 6A1g -. 4T1, bands when the temperature was raised to T,. This increase is in contrast to the decrease of the transition probability in Cdl-,Mn,Te as indicated by Z(T). We hare therefore no conclusive evidence that the radiative lifetime in the mixed systems exhibits an exchange induced temperature dependence. All what can be said is that a weak temperature dependence is in accordance with the theory of pair transitions.

5. Summary and Conclusion

We have measured the temperature dependence of band position and lifetime of the 4T1 -+ 6Al manganese luminescence in Cdl-,Mn,Te and Znl-,Mn,Te. It is suggested that the low temperature band shift and the behavior of the radiative lifetime is due to exchange effects. Several models are discussed to explain the band shift. Antiferro- magnetic exchange interaction between the Mn2+ is too small t o give the observed effects. Magnetostriction would give the wrong sign. Exchange interaction between Mn2+ ions and band electrons might give the right order of magnitude if the restriction to a rigid spin lattice is relaxed.

A t low temperatures total lifetimes are dominated by the radiative lifetime due to electric dipole transitions which involve pairs of manganese ions. The expected decrease of lifetime with concentration is only observed in Znl-,MnxTe.

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position and lifetime of photoluminescence in cd1−xmnxte and zn1−xmnxte. exchange dependent...

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