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Picosecond dynamics of magnetic polarons in Cd1−xMnxTe
Journal of Luminescence 38 (1987) 263—265 North-Holland, Amsterdam
263
Section 10. Semiconductors PICOSECOND DYNAMICS OF MAGNETIC POLARONS IN Cdi~MnrTe Y. OKA, K. NAKAMURA, I. SOUMA, M. KIDO and H. FUJISAKI Research Irutztute for Scientific Measuremenrt, Tohoku University, Katahira, Sendai 980, Japan We report the dynamical behavior of magnetic polarons (MPs) in Cd ~Mn~Te(v_0.05 0.6). Exciton luminescence in samples of v—0.05 0.1 shows the dynamics of the acceptor-bound MP, where the formation time of the bound MP is 1—2 ns. For v=0.2—0.4 the free MP is created and relaxes into deep localized states in the fluctuating band gap, forming the localized MP. For v> 0.5 the lifetime of the MP decreases to less than 250 ps, which is caused by fast-energy transfer of the exciton energy to delectrons ofthe Mn ions. The spin-glass ordering ofthe Mn spins for s >0.2 at low temperature is found to affect the MP energies.
Cd0 ~Mn0~Te
1.Introduction In semimagnetic semiconductors, Il—Vt compound semiconductors which contain Mn ions, electrons and holes in conduction and valence bands interact with the d-electrons of Mn by the exchange effect. Under band-toband optical excitation a created electron—hole pair forms a free exciton due to Coulomb interaction. Further the exchange interaction for the exciton forces alignment of the d-electron spins of the neighboring Mn ions and stabilizesthe exciton energy: this is a magnetic polaron (MP)
Cd ~Mn~Tewith various concentrations Mn ion pstime-resolved spectroscopy of the exciton luminesc-
H=~~l 1 .665 >~.
i .660
~
~ 1 .655 ~
_—
H=7T
::: .
0 2. Experimental The experiment was performed in Cd1 ~Mn~Te grown by the Bridgeman method with an Mn concentration of
x=O—0.6. lime resolved measurements were made using a mode-locked Ar ion laser and a time-correlated singlephoton counting detection system. The time resolution of the present set-up is 50 Ps by deconvolution calculation [71.A magnetic field was applied by a superconducting magnet up to 7 T.
3. Experimental results and discussion 3.1. Bound magnetic polarons
In the low Mn concentration case (x~0.1) the fluorescence spectra are dominated by the acceptor-bound exciton. The time behavior of the fluorescence for x= 0.05 is shown in fig. 1 by contour maps at H=0 T and 7 T. The horizontal arrow in fig. 1 shows the exciton energy which is determined from the minimum point of the excitonreflection spectrum. The fluorescence peak at 0 T shows a slight energy decrease as the time increases and 2 ns after 0022-2313/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
4.2K
2
4
6
8
TI ME (nsec) Fig. 1. Time characteristics of the luminescence due to the acceptor-bound exciton in Cd11 ~Mn1 1Je.
the excitation the peak energy is shifted 0.4 meV to lower energy. The peak of the fluorescence is 6—7 meV lower than the exciton energy~this shift is considerably larger thanthebindingenergyoftheacceptor-boundexciton (5.5 meV) in CdTe [7]. Therefore the bound exciton state in the sample of ~=0.05 is much more stable than the case of CdTe. However, at 7 T the peak energy does not show such a temporal shift in energy. We note that the Mn spins in this case are oriented by the intense external magnetic field. The peak energy is decreased by 28 meV to the lower energy side due to the giant Zeeman effect [1] while the energy separation at 7 T from the exciton is decreased to 5.0 meV. The lifetime at 7 T increases significantly as we can see from a long wing of the luminescence in the contour map with regard to time, which is evidence for external-field-induced localization. The temporal decrease of the energy of the bound cxciton state at 0 T, and its absence at 7 T, shows the existence of the MP state of the acceptor bound exciton, the bound MP (BM P). Therefore fig. 1 shows the dynamics
(7ka
264
£ I
of Pico ~cc 1)11(1 di nain ii s of niayn~’IioIs 1/a, Ofl iti I d
of the formation of the bound MP from the free exciton by gradual alignment of the surrounding Mn ions at 0 1. while such a gradual formation process of the MP does not arise at 7 T owing to the initial alignment of the Mn spins by the magnetic field. Similar dynamics of the BMP can be seen for c<0.l. 3.2. 1 ree and localized
magnetic polarons
At higher Mn concentration a significant effect of the fluctuation of the band gap arises due to the inhomogeneous distribution of the cation atoms. The optically crc ated free exeiton then relaxes by phonon emission to localized exeiton states produced by the fluctuating band edge. During the localization process the formation of the MP by exchange also takes place. To distinguish these two energy relaxation processes. time resolsed measurements have been made for samples of s—0.2 0.4. Figure 2 shows the temporal energy shift of the fluorescence peak for —0.2, where the peak shift occurs even at a magnetic field of 7 T. contrary to the case of the BMP shown in fig. 2. The time constant for the low energy shift of the peak is 1.5 2.0 ns. which does not de pend on the magnetic-field strength. This fact indicates that the formation of the MP in each exciton site in the fluctuating band gap is faster than the subsequent relaxalion to more localized states: the excitonic MP is therefore formed in each lattice site by aligning the Mn spins distributed around the site. Then the energy further re laxes to fall into the lower localized states. Thus in the intermediate Mn concentration range of ~—0.2 0.4. free MPs (FMPs) are created from the free excitons in the early stage after the optical excitation. The localized MPs (LMPs) arise during the relaxation of the localized cxcitons. In this region the transient fluorescence spectra are dominated by the FMP and the LMP.
LdQ8MnQ2Te
lin h
The lifetime of the MP can be determined bs the decay time constant of the integrated intensity of the lurninescence for the Mn mole fraction c. The lifetime shows, at II 1) T. a gradual decrease from 2.1) to 1 ‘S ns during the increase of x from 0.05 to 0.4. At 7 T the lifetime in the sample s 0.05 increases remarkably to 4.0 ns. which shows the magnetically induced localization of the BMP ( cI. fig. I 3.3 Fnergi’ lions0 r For \ >0.5 the lifetime of the exciton fluorescence decreases steeply to 250 ps and the dominant fluorescence changes from that of the excitons to emission by the d electrons. The band-gap energies at these Mn concentralions exceed the d d transition energs of the Mn ions. The remarkable decrease of the exeiton lifetime is produced by energy transfer ofthe exciton energs to the d-electrons of Mn ions. ?.
4. tlagnetic polaromis in spin glus s
The existence of the spin-glass phase in Cd Mn, Ic has been elucidated by Galaika et al. for s>0.2 at low temperature from a characteristic cusp appearing in ternperature dependence ofthe de susceptibility [9]. The spinglass state of the Mn spins must affect the MP energy as well as the energy relaxation of the MP. Figure 3 shows the temperature dependence of the fluorescence-peak energy’ at 0 and 7 T for s_0.3. At UT the peak energy grad1.97
Cd0 7Mn031e 1
.
1.95
~1.94 u..i
1,82
H=7T
H 7T
1.84 0~
1.82
1.93
-
/
~.B0 0
2
4
8
8
TIME (n3ec) Fig. 2. Time characieristics of the luminescence due io the free and localized exciions in C’d ~Mn
Te
1.92
0
20
40
60
TEMPERATURE (K) Fig Temperature dependence of the peak energy of ihc lumi ncscence in I d Mn Ic.
Y Oka ci al/Picosecond dynamics of magnetic polarons in Cd
1 ~Mn~Te
ually decreases as the temperature decreasees to 20 K, which is evidence for the MP in the paramagnetic Mn
265
from the Special Research Project on High-Melting-Ternperature Materials of the same Ministry.
spins. The peak energy increases, however, for a further decrease of the lattice temperature. The energy minimum is at 16 K which is comparable to the temperature of the
spin-glass transition reported by Galazka. The dependence of the peak energy shift on temperature is the inverse ofthat of the susceptibility. This is considered to be the effect of the spin-glass magnetization on the binding energy of the MP. We can confirm this effect of the spin-glass by applying high magnetic fields, where the Mn spins are forced to align to the field direction. At 7 T the exciton peak decreases monotonically to the lower energy side as the temperature decreases. Here the magnetic field destroys the spin-glass phase and the MP is now in the paramagnetic phase. The exciton state in semimagnetic semiconductors is thus strongly affected by the spin-glass phase of Mn ions [10]. Study of the dynamics of the MP in the spin-glass is under progress.
References
[1]J.K. Furdyna, J. AppI. Phys. 53 (1982) 7637 and refs. therein. (2] A. Golnik,J. GinterandJ.A. Gaj, J. Phys. C 16(1983)6073. [3] J. Warnock, RN. Kershaw. D. Ridgely, K. Dwight. A. Wold and R.R. Galazka, J. Lumin. 34 (1985) 25. 14] Y. Oka. K. Nakamura. I. Souma and H. Fujisaki, Proc. mt. Conf. on the Physics of Semiconductors, Stockholm (1986) p. i77i. [5] J.J. Zayhowski. C. Jagannath, R.N. Kershaw, D. Ridgley, K. Dwight and A. Wold, Sol. St. Commun. 55 (1985) 941; Phvs. Rev. B (1987). [6] J.H. Harris and A.V. Nurmikko, Phys. Rev. B 28(1983) 1181. [7] Y. Oka, K. Nakarnura and H. Fujisaki. Phys. Res. Lett. 57
Acknowledgement The work was partially supported by the Grant-in-Aid for Scientific Research on New Functionality Materials from the Ministry of Education, Science and Culture of Japan under No. 62604512. Support was also received
(1986) 285. [8] P. Heisinger, S. Suga, F. Willman and W. Dreybrodt. Phys.
Stat. Sol. (b) 67 (1975) 641. [9] R.R. Galazka, S. Nagata and PH. Keesom. Phys. Rev. B 22 (1980) 3344. [10] Y, Oka, K. Nakamura, I. Souma, M. Kido and H. Fujisaki, to be published.
263
Section 10. Semiconductors PICOSECOND DYNAMICS OF MAGNETIC POLARONS IN Cdi~MnrTe Y. OKA, K. NAKAMURA, I. SOUMA, M. KIDO and H. FUJISAKI Research Irutztute for Scientific Measuremenrt, Tohoku University, Katahira, Sendai 980, Japan We report the dynamical behavior of magnetic polarons (MPs) in Cd ~Mn~Te(v_0.05 0.6). Exciton luminescence in samples of v—0.05 0.1 shows the dynamics of the acceptor-bound MP, where the formation time of the bound MP is 1—2 ns. For v=0.2—0.4 the free MP is created and relaxes into deep localized states in the fluctuating band gap, forming the localized MP. For v> 0.5 the lifetime of the MP decreases to less than 250 ps, which is caused by fast-energy transfer of the exciton energy to delectrons ofthe Mn ions. The spin-glass ordering ofthe Mn spins for s >0.2 at low temperature is found to affect the MP energies.
Cd0 ~Mn0~Te
1.Introduction In semimagnetic semiconductors, Il—Vt compound semiconductors which contain Mn ions, electrons and holes in conduction and valence bands interact with the d-electrons of Mn by the exchange effect. Under band-toband optical excitation a created electron—hole pair forms a free exciton due to Coulomb interaction. Further the exchange interaction for the exciton forces alignment of the d-electron spins of the neighboring Mn ions and stabilizesthe exciton energy: this is a magnetic polaron (MP)
Cd ~Mn~Tewith various concentrations Mn ion pstime-resolved spectroscopy of the exciton luminesc-
H=~~l 1 .665 >~.
i .660
~
~ 1 .655 ~
_—
H=7T
::: .
0 2. Experimental The experiment was performed in Cd1 ~Mn~Te grown by the Bridgeman method with an Mn concentration of
x=O—0.6. lime resolved measurements were made using a mode-locked Ar ion laser and a time-correlated singlephoton counting detection system. The time resolution of the present set-up is 50 Ps by deconvolution calculation [71.A magnetic field was applied by a superconducting magnet up to 7 T.
3. Experimental results and discussion 3.1. Bound magnetic polarons
In the low Mn concentration case (x~0.1) the fluorescence spectra are dominated by the acceptor-bound exciton. The time behavior of the fluorescence for x= 0.05 is shown in fig. 1 by contour maps at H=0 T and 7 T. The horizontal arrow in fig. 1 shows the exciton energy which is determined from the minimum point of the excitonreflection spectrum. The fluorescence peak at 0 T shows a slight energy decrease as the time increases and 2 ns after 0022-2313/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
4.2K
2
4
6
8
TI ME (nsec) Fig. 1. Time characteristics of the luminescence due to the acceptor-bound exciton in Cd11 ~Mn1 1Je.
the excitation the peak energy is shifted 0.4 meV to lower energy. The peak of the fluorescence is 6—7 meV lower than the exciton energy~this shift is considerably larger thanthebindingenergyoftheacceptor-boundexciton (5.5 meV) in CdTe [7]. Therefore the bound exciton state in the sample of ~=0.05 is much more stable than the case of CdTe. However, at 7 T the peak energy does not show such a temporal shift in energy. We note that the Mn spins in this case are oriented by the intense external magnetic field. The peak energy is decreased by 28 meV to the lower energy side due to the giant Zeeman effect [1] while the energy separation at 7 T from the exciton is decreased to 5.0 meV. The lifetime at 7 T increases significantly as we can see from a long wing of the luminescence in the contour map with regard to time, which is evidence for external-field-induced localization. The temporal decrease of the energy of the bound cxciton state at 0 T, and its absence at 7 T, shows the existence of the MP state of the acceptor bound exciton, the bound MP (BM P). Therefore fig. 1 shows the dynamics
(7ka
264
£ I
of Pico ~cc 1)11(1 di nain ii s of niayn~’IioIs 1/a, Ofl iti I d
of the formation of the bound MP from the free exciton by gradual alignment of the surrounding Mn ions at 0 1. while such a gradual formation process of the MP does not arise at 7 T owing to the initial alignment of the Mn spins by the magnetic field. Similar dynamics of the BMP can be seen for c<0.l. 3.2. 1 ree and localized
magnetic polarons
At higher Mn concentration a significant effect of the fluctuation of the band gap arises due to the inhomogeneous distribution of the cation atoms. The optically crc ated free exeiton then relaxes by phonon emission to localized exeiton states produced by the fluctuating band edge. During the localization process the formation of the MP by exchange also takes place. To distinguish these two energy relaxation processes. time resolsed measurements have been made for samples of s—0.2 0.4. Figure 2 shows the temporal energy shift of the fluorescence peak for —0.2, where the peak shift occurs even at a magnetic field of 7 T. contrary to the case of the BMP shown in fig. 2. The time constant for the low energy shift of the peak is 1.5 2.0 ns. which does not de pend on the magnetic-field strength. This fact indicates that the formation of the MP in each exciton site in the fluctuating band gap is faster than the subsequent relaxalion to more localized states: the excitonic MP is therefore formed in each lattice site by aligning the Mn spins distributed around the site. Then the energy further re laxes to fall into the lower localized states. Thus in the intermediate Mn concentration range of ~—0.2 0.4. free MPs (FMPs) are created from the free excitons in the early stage after the optical excitation. The localized MPs (LMPs) arise during the relaxation of the localized cxcitons. In this region the transient fluorescence spectra are dominated by the FMP and the LMP.
LdQ8MnQ2Te
lin h
The lifetime of the MP can be determined bs the decay time constant of the integrated intensity of the lurninescence for the Mn mole fraction c. The lifetime shows, at II 1) T. a gradual decrease from 2.1) to 1 ‘S ns during the increase of x from 0.05 to 0.4. At 7 T the lifetime in the sample s 0.05 increases remarkably to 4.0 ns. which shows the magnetically induced localization of the BMP ( cI. fig. I 3.3 Fnergi’ lions0 r For \ >0.5 the lifetime of the exciton fluorescence decreases steeply to 250 ps and the dominant fluorescence changes from that of the excitons to emission by the d electrons. The band-gap energies at these Mn concentralions exceed the d d transition energs of the Mn ions. The remarkable decrease of the exeiton lifetime is produced by energy transfer ofthe exciton energs to the d-electrons of Mn ions. ?.
4. tlagnetic polaromis in spin glus s
The existence of the spin-glass phase in Cd Mn, Ic has been elucidated by Galaika et al. for s>0.2 at low temperature from a characteristic cusp appearing in ternperature dependence ofthe de susceptibility [9]. The spinglass state of the Mn spins must affect the MP energy as well as the energy relaxation of the MP. Figure 3 shows the temperature dependence of the fluorescence-peak energy’ at 0 and 7 T for s_0.3. At UT the peak energy grad1.97
Cd0 7Mn031e 1
.
1.95
~1.94 u..i
1,82
H=7T
H 7T
1.84 0~
1.82
1.93
-
/
~.B0 0
2
4
8
8
TIME (n3ec) Fig. 2. Time characieristics of the luminescence due io the free and localized exciions in C’d ~Mn
Te
1.92
0
20
40
60
TEMPERATURE (K) Fig Temperature dependence of the peak energy of ihc lumi ncscence in I d Mn Ic.
Y Oka ci al/Picosecond dynamics of magnetic polarons in Cd
1 ~Mn~Te
ually decreases as the temperature decreasees to 20 K, which is evidence for the MP in the paramagnetic Mn
265
from the Special Research Project on High-Melting-Ternperature Materials of the same Ministry.
spins. The peak energy increases, however, for a further decrease of the lattice temperature. The energy minimum is at 16 K which is comparable to the temperature of the
spin-glass transition reported by Galazka. The dependence of the peak energy shift on temperature is the inverse ofthat of the susceptibility. This is considered to be the effect of the spin-glass magnetization on the binding energy of the MP. We can confirm this effect of the spin-glass by applying high magnetic fields, where the Mn spins are forced to align to the field direction. At 7 T the exciton peak decreases monotonically to the lower energy side as the temperature decreases. Here the magnetic field destroys the spin-glass phase and the MP is now in the paramagnetic phase. The exciton state in semimagnetic semiconductors is thus strongly affected by the spin-glass phase of Mn ions [10]. Study of the dynamics of the MP in the spin-glass is under progress.
References
[1]J.K. Furdyna, J. AppI. Phys. 53 (1982) 7637 and refs. therein. (2] A. Golnik,J. GinterandJ.A. Gaj, J. Phys. C 16(1983)6073. [3] J. Warnock, RN. Kershaw. D. Ridgely, K. Dwight. A. Wold and R.R. Galazka, J. Lumin. 34 (1985) 25. 14] Y. Oka. K. Nakamura. I. Souma and H. Fujisaki, Proc. mt. Conf. on the Physics of Semiconductors, Stockholm (1986) p. i77i. [5] J.J. Zayhowski. C. Jagannath, R.N. Kershaw, D. Ridgley, K. Dwight and A. Wold, Sol. St. Commun. 55 (1985) 941; Phvs. Rev. B (1987). [6] J.H. Harris and A.V. Nurmikko, Phys. Rev. B 28(1983) 1181. [7] Y. Oka, K. Nakarnura and H. Fujisaki. Phys. Res. Lett. 57
Acknowledgement The work was partially supported by the Grant-in-Aid for Scientific Research on New Functionality Materials from the Ministry of Education, Science and Culture of Japan under No. 62604512. Support was also received
(1986) 285. [8] P. Heisinger, S. Suga, F. Willman and W. Dreybrodt. Phys.
Stat. Sol. (b) 67 (1975) 641. [9] R.R. Galazka, S. Nagata and PH. Keesom. Phys. Rev. B 22 (1980) 3344. [10] Y, Oka, K. Nakamura, I. Souma, M. Kido and H. Fujisaki, to be published.