- Email: [email protected]
ZnTe multiple quantum wells
Journal of Luminescence 87}89 (2000) 393}395
Time-resolved optical spectroscopy in Cd\V MnV Te/ZnTe multiple quantum wells R. Pittini *, M. Takahashi , J.X. Shen , Y. Oka RISM, Tohoku University, Katahira 2-1-1, Sendai, 980-8577, Japan CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract Time-resolved photoluminescence (PL) and optical pump}probe experiments were carried out in Cd\V MnV Te/ZnTe multiple quantum wells to study the dynamics of the excitonic magnetic polaron in this material. In the pump}probe data, three signals were detected. The main signal is negative and monitors the energy relaxation of the excitons created by the pump pulse. The other two pump}probe signals are positive and their intensity grows while the excitons created by the pump pulse recombine. The low-energy positive pump}probe signal is found to track the average Mn spin polarization in the sample. Instead, the high-energy positive peak indicates the enhancement of the density of states due to correlation e!ects. 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Excitonic magnetic polaron; Pump}probe; Diluted magnetic semiconductors
Diluted magnetic semiconductors (DMSs) are currently attracting wide interest for the occurrence of a strongly enhanced Zeeman splitting of the band states, large Faraday rotations and for the formation of magnetic polarons. These properties arise from the exchange interaction between the carriers and the localized moments of the magnetic ions. The con"nement of the carriers in quantum well structures further enhances the interaction between the band electrons and the localized Mn moments. The evolution of magnetic polarons in Cd\V MnV Se and Cd Mn Te has already been studied by several \V V authors. Bound magnetic polarons were observed in nCd Mn Se by pump}probe experiments [1]. Time-resolved Faraday rotation measurements [2] were carried out in Cd\V MnV Te to map the time evolution of the interaction between the electron spin and the magnetic moments of diluted impurity spins. Time-resolved PL experiments were performed in semimagnetic quantum wells [3,4] to study the dynamics of the exciton magnetic polarons. In this work, we studied the energy
* Corresponding author: Tel.: #81-22-217-5361; fax: #8122-217-5363. E-mail address: [email protected] (R. Pittini)
relaxation and the exciton magnetic polaron formation in Cd Mn Te/ZnTe multiple quantum wells (MQWs) \V V with x"0.10. For this purpose, we carried out timeresolved PL and optical pump}probe experiments. In a PL experiment, the carriers couple to excitons and relax to the band edge forming magnetic polarons. Therefore, optical emission contains information on the time evolution of the magnetic polarons. In contrast, the magnetic polaron formation cannot be observed directly in an optical absorption experiment, because the Mn> spins begin to be polarized at the moment of detection [5]. Cd Mn Te/ZnTe MQWs with x between 0 and 0.2 \V V were grown by hot wall epitaxy on a GaAs (0 0 1) substrate. The samples consist of 35 to 100 quantum wells with the magnetic well layers alternating with the ZnTe barrier layers. The time-resolved PL signal was detected with a streak camera with a time resolution of 5 ps placed behind a spectrometer. For the excitation we used the frequency doubled output pulses (120 fs) from a Ti : sapphire laser. Transient absorption experiments were performed using an optical parametric ampli"er (OPA) seeded and pumped by 120 fs short pulses from a Ti : sapphire laser. The linearly polarized output of the OPA was used as pump beam for the pump}probe experiments, with the pulse energy varying between 1 and
0022-2313/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 1 4 - 7
394
R. Pittini et al. / Journal of Luminescence 87}89 (2000) 393}395
2000 lJ/cm on the sample. To probe the time variation of the optical absorption after the pump excitation, we used pulses with a white spectrum generated with a sapphire platelet. All the optical experiments presented here were performed at 4.2 K. In Fig. 1(a), we show the optical density of Cd Mn Te/ZnTe MQWs with 100 repetitions of 22 As wide quantum wells alternating with 58 As wide barriers. The onset of optical absorption is characterized by the Gaussian peak (marked as a gray area) of the heavy-hole (hh) excitons in the quantum wells centered at 2.0 eV with a line width of 84 meV. In the PL spectrum (also shown in Fig. 1a) the hh exciton is observed as a 36 meV wide peak centered at 1.909 eV. The large Stokes shift of 91 meV indicates pronounced band edge #uctuations for the wells. In Fig. 1(b), we show the variation of the optical density induced by the pump pulse after several time delays after pumping the sample with 1.0 lJ/cm pulses, su$cient to saturate the excitons in the wells. Already a few ps (2}20 ps) after the pump excitation, one observes a deeply negative pump}probe signal, which marks the energy position of the hh excitons created by the pump pulse. The energy of these excitons is found to relax to the localized states at band edge and "nally stabilize within 100 ps (Fig. 1b), in good agreement with the relaxation to the mobility edge in comparable samples [6]. For longer time delays, the intensity of this main negative peak decreases re#ecting the recombination of the hh excitons created by the pump pulse. Furthermore, two positive peaks are observed to develop after longer time delays in the pump}probe spectra on both, the low-energy and high-energy sides of this negative pump}probe peak. In Fig. 2, we plot the transient absorption spectrum taken at 420 ps after the pump excitation. The two positive peaks are clearly recognized in Fig. 2. In particular, the energy of the &positive peak 2' is close to the exciton peak observed in the optical density (Fig. 1a). One can notice that the positive pump}probe signal is peculiar because it indicates an enhancement of the density of states achieved with the pump pulse. To study these two positive peaks further, we look at the time variation of the integrated intensity for each pump}probe peak (Fig. 3). The intensity of the negative pump-probe signal drops exponentially with a decay time of 409 ps. This is faster compared to the decay time of 505 ps observed for the PL intensity. But it con"rms the general scenario that the excitons contributing to the PL signals are more localized than the excitons observed in the pump}probe signal. The intensity of both positive peaks is observed to increase monotonically with time in the experimental window. The high-energy positive peak (&positive peak 2') increases very sharply until 70 ps and then the raise becomes slower. The time dependence of the energy of &positive peak 2' is found to follow the time variation of the negative pump}probe signal. Therefore, &positive
Fig. 1. (a) Optical density (full line and left scale) and photoluminescence spectra (dashed line and right scale) of Cd Mn Te/ZnTe MQWs with well width ¸ "22 As and barrier width 58 As . The "t of the heavy-hole exciton peak in the optical density is marked as a gray area; (b) Pump}probe spectra taken for several time delays. The curves are shifted on the vertical scale for clarity. The arrow indicates the position of the low-energy positive peak (discussed in the text).
Fig. 2. Lineshape of the pump}probe spectrum of Cd Mn Te/ZnTe MQWs (22 As /58 As ) at 420 ps after the pump excitation.
peak 2' is closely related to the energy relaxation to the band edge of the excitons created by the pump pulse. Considering the high density of carriers created in the wells by the pump pulse, we attribute the &positive peak 2' to the enhancement of the density of states appearing at the high-energy side of the band edge as a consequence of the Coulomb screening. The e!ect of the energy renormalization sharpens the band edge while keeping the total density of states constant. This yields an enhanced density of states on the high-energy side of the average band edge. Therefore, &positive peak 2' is characteristic of samples with signi"cant band edge #uctuations. It is interesting to note that the intensity of &positive peak 2' increases with time in spite of the fact that the exciton
R. Pittini et al. / Journal of Luminescence 87}89 (2000) 393}395
395
Fig. 3. Time dependence of the integrated pump}probe signal for the three peaks de"ned in Fig. 2.
Fig. 4. Time dependence of the energy di!erence between the negative and the low-energy positive pump}probe signals.
density decreases due to recombinations after the pump excitation. This can be explained as follows. The band edge renormalization e!ects are believed to be mainly due to the free carriers, as the total charge of the excitons is zero. Free carriers are known to coexist with the excitons in the wells of our samples for some time after the pump excitation [7]. Furthermore, the enhancement of the density of states appears at the high-energy side and is probed by new excitons created by the probe pulse. Therefore, the pump}probe signal connected with &positive peak 2' is proportional to both, (a) the density of free carriers at the band edge and (b) the availability of unoccupied exciton states in the free exciton region where the density of states is enhanced. In this sense, the intensity of &positive peak 2' is specularly symmetric to the intensity of the negative pump}probe signal (as is indeed observed in Fig. 3). But, "nally, at long delay times after the pump excitation, the intensity of &positive peak 2' must decrease again in concomitance with the disappearance of the band edge renormalization e!ects, when most of the carriers have recombined already. The sharp kink observed at 70 ps (Fig. 3) indicates the time when most of the carriers and excitons have relaxed to the band edge and corresponds well to the results of transient PL spectroscopy [6]. The other positive pump}probe signal, &positive peak 1' appearing on the low-energy side of the negative peak, has a much smaller intensity and is detected only after a signi"cant number of excitons have already decayed after polarizing the Mn spin. Then, new excitons can be absorbed by the magnetic polarons where the original exciton has already recombined. Thus, the creation of a new exciton in the magnetic environment polarized by the original exciton does not violate Pauli's exclusion principle. In this sense, the new excitons created by the probe pulse enter a new state (a magnetic polaron) induced by the pump pulse thus yielding a positive pump}probe signal. For longer time delays, the intensity of this positive peak grows further because increasingly more spin polarized regions are left free for the new excitons. Therefore, the energy di!erence between the negative pump}probe peak and the low-energy positive
pump}probe peak (Fig. 4) is a direct measure of the Mn spin polarization in the sample. Time-resolved Faraday rotation measurements performed in similar samples showed that the Mn spin polarization is long-living and relaxes with a decay time of 400 ps about [8]. In our experiment, the Mn spins are not released at the time zero, but are kept polarized until the initial excitons created by the pump pulse have decayed. Therefore, the time dependence shown in Fig. 4 is not a simple exponential decay, but shows that the decay of the Mn spin polarization becomes faster at about 400 ps after the pump excitation, i.e. after a signi"cant number of excitons created initially by the pump pulse have decayed already. In conclusion, we have shown that a negative and two positive pump}probe signals are characteristic of Cd Mn Te/ZnTe MQWs. The negative signal tracks the relaxation of the heavy-holes excitons created by the pump pulse, the high-energy positive peak arises from the energy renormalization of the #uctuating band edge and the low-energy positive signal monitors the time dependence of the Mn spin polarization.
References [1] J.H. Harris, A.V. Nurmikko, Phys. Rev. Lett. 51 (1983) 1472. [2] D.D. Awschalom, J.-M. Halbout, S. von Molnar, T. Siegrist, F. Holtzberg, Phys. Rev. Lett. 55 (1985) 1128. [3] D.R. Yakovlev, in: U. RoK ssler (Ed.), FestkoK rperprobleme Advances in Solid State Physics 3, Viewweg, Braunschweig, 1992, p. 251. [4] D.R. Yakovlev, K.V. Kavokin, I.A. Merkulov, G. Mackh, W. Ossau, R. Hellman, E.O. GoK bel, A. Waag, G. Landwehr, Phys. Rev. B 56 (1997) 9782. [5] R. Pittini, H. Mitsu, M. Takahashi, J.X. Shen, Y. Oka, J. Appl. Phys. 85 (1999) 5938. [6] J.X. Shen, M.C. Debnath, I. Souma, E. Shirado, T. Saitou, T. Sato, R. Pittini, Y. Oka, proceedings ICL99. [7] R. Pittini, J.X. Shen, M. Takahashi, Y. Oka, J. Crystal Growth, to be published. [8] S.A. Crooker, D.D. Awschalom, J.J. Baumberg, F. Flack, N. Samarth, Phys. Rev. B 56 (1997) 7574.
Time-resolved optical spectroscopy in Cd\V MnV Te/ZnTe multiple quantum wells R. Pittini *, M. Takahashi , J.X. Shen , Y. Oka RISM, Tohoku University, Katahira 2-1-1, Sendai, 980-8577, Japan CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract Time-resolved photoluminescence (PL) and optical pump}probe experiments were carried out in Cd\V MnV Te/ZnTe multiple quantum wells to study the dynamics of the excitonic magnetic polaron in this material. In the pump}probe data, three signals were detected. The main signal is negative and monitors the energy relaxation of the excitons created by the pump pulse. The other two pump}probe signals are positive and their intensity grows while the excitons created by the pump pulse recombine. The low-energy positive pump}probe signal is found to track the average Mn spin polarization in the sample. Instead, the high-energy positive peak indicates the enhancement of the density of states due to correlation e!ects. 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Excitonic magnetic polaron; Pump}probe; Diluted magnetic semiconductors
Diluted magnetic semiconductors (DMSs) are currently attracting wide interest for the occurrence of a strongly enhanced Zeeman splitting of the band states, large Faraday rotations and for the formation of magnetic polarons. These properties arise from the exchange interaction between the carriers and the localized moments of the magnetic ions. The con"nement of the carriers in quantum well structures further enhances the interaction between the band electrons and the localized Mn moments. The evolution of magnetic polarons in Cd\V MnV Se and Cd Mn Te has already been studied by several \V V authors. Bound magnetic polarons were observed in nCd Mn Se by pump}probe experiments [1]. Time-resolved Faraday rotation measurements [2] were carried out in Cd\V MnV Te to map the time evolution of the interaction between the electron spin and the magnetic moments of diluted impurity spins. Time-resolved PL experiments were performed in semimagnetic quantum wells [3,4] to study the dynamics of the exciton magnetic polarons. In this work, we studied the energy
* Corresponding author: Tel.: #81-22-217-5361; fax: #8122-217-5363. E-mail address: [email protected] (R. Pittini)
relaxation and the exciton magnetic polaron formation in Cd Mn Te/ZnTe multiple quantum wells (MQWs) \V V with x"0.10. For this purpose, we carried out timeresolved PL and optical pump}probe experiments. In a PL experiment, the carriers couple to excitons and relax to the band edge forming magnetic polarons. Therefore, optical emission contains information on the time evolution of the magnetic polarons. In contrast, the magnetic polaron formation cannot be observed directly in an optical absorption experiment, because the Mn> spins begin to be polarized at the moment of detection [5]. Cd Mn Te/ZnTe MQWs with x between 0 and 0.2 \V V were grown by hot wall epitaxy on a GaAs (0 0 1) substrate. The samples consist of 35 to 100 quantum wells with the magnetic well layers alternating with the ZnTe barrier layers. The time-resolved PL signal was detected with a streak camera with a time resolution of 5 ps placed behind a spectrometer. For the excitation we used the frequency doubled output pulses (120 fs) from a Ti : sapphire laser. Transient absorption experiments were performed using an optical parametric ampli"er (OPA) seeded and pumped by 120 fs short pulses from a Ti : sapphire laser. The linearly polarized output of the OPA was used as pump beam for the pump}probe experiments, with the pulse energy varying between 1 and
0022-2313/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 1 4 - 7
394
R. Pittini et al. / Journal of Luminescence 87}89 (2000) 393}395
2000 lJ/cm on the sample. To probe the time variation of the optical absorption after the pump excitation, we used pulses with a white spectrum generated with a sapphire platelet. All the optical experiments presented here were performed at 4.2 K. In Fig. 1(a), we show the optical density of Cd Mn Te/ZnTe MQWs with 100 repetitions of 22 As wide quantum wells alternating with 58 As wide barriers. The onset of optical absorption is characterized by the Gaussian peak (marked as a gray area) of the heavy-hole (hh) excitons in the quantum wells centered at 2.0 eV with a line width of 84 meV. In the PL spectrum (also shown in Fig. 1a) the hh exciton is observed as a 36 meV wide peak centered at 1.909 eV. The large Stokes shift of 91 meV indicates pronounced band edge #uctuations for the wells. In Fig. 1(b), we show the variation of the optical density induced by the pump pulse after several time delays after pumping the sample with 1.0 lJ/cm pulses, su$cient to saturate the excitons in the wells. Already a few ps (2}20 ps) after the pump excitation, one observes a deeply negative pump}probe signal, which marks the energy position of the hh excitons created by the pump pulse. The energy of these excitons is found to relax to the localized states at band edge and "nally stabilize within 100 ps (Fig. 1b), in good agreement with the relaxation to the mobility edge in comparable samples [6]. For longer time delays, the intensity of this main negative peak decreases re#ecting the recombination of the hh excitons created by the pump pulse. Furthermore, two positive peaks are observed to develop after longer time delays in the pump}probe spectra on both, the low-energy and high-energy sides of this negative pump}probe peak. In Fig. 2, we plot the transient absorption spectrum taken at 420 ps after the pump excitation. The two positive peaks are clearly recognized in Fig. 2. In particular, the energy of the &positive peak 2' is close to the exciton peak observed in the optical density (Fig. 1a). One can notice that the positive pump}probe signal is peculiar because it indicates an enhancement of the density of states achieved with the pump pulse. To study these two positive peaks further, we look at the time variation of the integrated intensity for each pump}probe peak (Fig. 3). The intensity of the negative pump-probe signal drops exponentially with a decay time of 409 ps. This is faster compared to the decay time of 505 ps observed for the PL intensity. But it con"rms the general scenario that the excitons contributing to the PL signals are more localized than the excitons observed in the pump}probe signal. The intensity of both positive peaks is observed to increase monotonically with time in the experimental window. The high-energy positive peak (&positive peak 2') increases very sharply until 70 ps and then the raise becomes slower. The time dependence of the energy of &positive peak 2' is found to follow the time variation of the negative pump}probe signal. Therefore, &positive
Fig. 1. (a) Optical density (full line and left scale) and photoluminescence spectra (dashed line and right scale) of Cd Mn Te/ZnTe MQWs with well width ¸ "22 As and barrier width 58 As . The "t of the heavy-hole exciton peak in the optical density is marked as a gray area; (b) Pump}probe spectra taken for several time delays. The curves are shifted on the vertical scale for clarity. The arrow indicates the position of the low-energy positive peak (discussed in the text).
Fig. 2. Lineshape of the pump}probe spectrum of Cd Mn Te/ZnTe MQWs (22 As /58 As ) at 420 ps after the pump excitation.
peak 2' is closely related to the energy relaxation to the band edge of the excitons created by the pump pulse. Considering the high density of carriers created in the wells by the pump pulse, we attribute the &positive peak 2' to the enhancement of the density of states appearing at the high-energy side of the band edge as a consequence of the Coulomb screening. The e!ect of the energy renormalization sharpens the band edge while keeping the total density of states constant. This yields an enhanced density of states on the high-energy side of the average band edge. Therefore, &positive peak 2' is characteristic of samples with signi"cant band edge #uctuations. It is interesting to note that the intensity of &positive peak 2' increases with time in spite of the fact that the exciton
R. Pittini et al. / Journal of Luminescence 87}89 (2000) 393}395
395
Fig. 3. Time dependence of the integrated pump}probe signal for the three peaks de"ned in Fig. 2.
Fig. 4. Time dependence of the energy di!erence between the negative and the low-energy positive pump}probe signals.
density decreases due to recombinations after the pump excitation. This can be explained as follows. The band edge renormalization e!ects are believed to be mainly due to the free carriers, as the total charge of the excitons is zero. Free carriers are known to coexist with the excitons in the wells of our samples for some time after the pump excitation [7]. Furthermore, the enhancement of the density of states appears at the high-energy side and is probed by new excitons created by the probe pulse. Therefore, the pump}probe signal connected with &positive peak 2' is proportional to both, (a) the density of free carriers at the band edge and (b) the availability of unoccupied exciton states in the free exciton region where the density of states is enhanced. In this sense, the intensity of &positive peak 2' is specularly symmetric to the intensity of the negative pump}probe signal (as is indeed observed in Fig. 3). But, "nally, at long delay times after the pump excitation, the intensity of &positive peak 2' must decrease again in concomitance with the disappearance of the band edge renormalization e!ects, when most of the carriers have recombined already. The sharp kink observed at 70 ps (Fig. 3) indicates the time when most of the carriers and excitons have relaxed to the band edge and corresponds well to the results of transient PL spectroscopy [6]. The other positive pump}probe signal, &positive peak 1' appearing on the low-energy side of the negative peak, has a much smaller intensity and is detected only after a signi"cant number of excitons have already decayed after polarizing the Mn spin. Then, new excitons can be absorbed by the magnetic polarons where the original exciton has already recombined. Thus, the creation of a new exciton in the magnetic environment polarized by the original exciton does not violate Pauli's exclusion principle. In this sense, the new excitons created by the probe pulse enter a new state (a magnetic polaron) induced by the pump pulse thus yielding a positive pump}probe signal. For longer time delays, the intensity of this positive peak grows further because increasingly more spin polarized regions are left free for the new excitons. Therefore, the energy di!erence between the negative pump}probe peak and the low-energy positive
pump}probe peak (Fig. 4) is a direct measure of the Mn spin polarization in the sample. Time-resolved Faraday rotation measurements performed in similar samples showed that the Mn spin polarization is long-living and relaxes with a decay time of 400 ps about [8]. In our experiment, the Mn spins are not released at the time zero, but are kept polarized until the initial excitons created by the pump pulse have decayed. Therefore, the time dependence shown in Fig. 4 is not a simple exponential decay, but shows that the decay of the Mn spin polarization becomes faster at about 400 ps after the pump excitation, i.e. after a signi"cant number of excitons created initially by the pump pulse have decayed already. In conclusion, we have shown that a negative and two positive pump}probe signals are characteristic of Cd Mn Te/ZnTe MQWs. The negative signal tracks the relaxation of the heavy-holes excitons created by the pump pulse, the high-energy positive peak arises from the energy renormalization of the #uctuating band edge and the low-energy positive signal monitors the time dependence of the Mn spin polarization.
References [1] J.H. Harris, A.V. Nurmikko, Phys. Rev. Lett. 51 (1983) 1472. [2] D.D. Awschalom, J.-M. Halbout, S. von Molnar, T. Siegrist, F. Holtzberg, Phys. Rev. Lett. 55 (1985) 1128. [3] D.R. Yakovlev, in: U. RoK ssler (Ed.), FestkoK rperprobleme Advances in Solid State Physics 3, Viewweg, Braunschweig, 1992, p. 251. [4] D.R. Yakovlev, K.V. Kavokin, I.A. Merkulov, G. Mackh, W. Ossau, R. Hellman, E.O. GoK bel, A. Waag, G. Landwehr, Phys. Rev. B 56 (1997) 9782. [5] R. Pittini, H. Mitsu, M. Takahashi, J.X. Shen, Y. Oka, J. Appl. Phys. 85 (1999) 5938. [6] J.X. Shen, M.C. Debnath, I. Souma, E. Shirado, T. Saitou, T. Sato, R. Pittini, Y. Oka, proceedings ICL99. [7] R. Pittini, J.X. Shen, M. Takahashi, Y. Oka, J. Crystal Growth, to be published. [8] S.A. Crooker, D.D. Awschalom, J.J. Baumberg, F. Flack, N. Samarth, Phys. Rev. B 56 (1997) 7574.