- Email: [email protected]
BeMnTe multiple quantum wells with a type II band alignment
ARTICLE IN PRESS
Journal of Luminescence 108 (2004) 65–68
Magnetic field enhanced luminescence in ZnSe/BeMnTe multiple quantum wells with a type II band alignment A. Nakamuraa,*, N. Katoa, I. Yamakawaa, R. Akimotob b
a Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Photonics Research Institute, AIST, Tsukuba Central 2-1, Tsukuba 305-8568, Japan
Abstract Magneto-optical properties of ZnSe/Be1xMnxTe multiple quantum wells with a type II band alignment have been investigated with magnetic fields up to 7 T. We have observed a large Zeeman shift and an increase in intensity of the interface recombination band. The Zeeman shift is analyzed by a phenomenological model based on a strong p–d exchange interaction of the valence band in the Be1xMnxTe layer. The increase in intensity is interpreted in terms of suppression of potential fluctuations due to the localized-spin alignment of Mn ions with the magnetic field. r 2004 Elsevier B.V. All rights reserved. PACS: 78.66.Hf; 78.20.Ls; 75.50.Pp; 73.20.r Keywords: Diluted magnetic semiconductors; Type II semiconductor superlattices; Magneto-optics
1. Introduction Diluted magnetic semiconductors (DMS) are known for their giant magneto-optical effects due to a strong sp–d exchange interaction between band electrons and magnetic ions. In quantum well (QW) structures of DMS, we can modify this giant splitting as well as the confinement energies of electrons and holes [1–3]. Zn(Mn)Se/Be(Mn)Te multiple quantum well (MQW) structures have recently attracted attention because of their type II band alignment with large band offsets [4–6]. Electrons and holes are separately confined in the Zn(Mn)Se and Be(Mn)Te layers, respec*Corresponding author. Tel.: +81-52-789-4450; fax: +8152-789-5316. E-mail address: [email protected] (A. Nakamura).
tively. We can investigate the magneto-optical properties solely arising from the p–d exchange interaction (s–d interaction) in Be1xMnxTe MQW structures (Zn1xMnxSe/BeTe MQW structures) [6,7]. Since the electron–hole recombination takes place at hetero-junctions in type II MQWs, the luminescence properties are sensitive to interfaces between the ZnSe and Be1xMnxTe layers and to potential fluctuations due to the Mn ions. The inplane anisotropy of hidden interfaces in QW structures has been studied by polarized spectroscopy of spin-oriented carriers in Zn(Mn)Se/BeTe MQWs [8]. In this paper we report on the luminescence properties and dynamics of the electron–hole recombination in type II ZnSe/ Be1xMnxTe MQWs with magnetic fields up to 7 T. A large Zeeman shift and an increase in intensity of the interfacial recombination band
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.012
ARTICLE IN PRESS 66
A. Nakamura et al. / Journal of Luminescence 108 (2004) 65–68
have been interpreted in terms of a strong p–d exchange interaction.
ZnSe/Be1xMnxTe-MQW samples were grown on (0 0 1)-oriented GaAs substrates by molecularbeam epitaxy (MBE) in a dual-chamber system [5]. After a 250-nm thick GaAs buffer layer was grown in a III–V chamber to improve the quality of the GaAs surface, the substrate was transferred to a II–VI chamber for the growth of ZnSe/ Be1xMnxTe-MQW structures. Growth conditions were adjusted to form Zn–Te chemical bonds at interfaces. The MQW structure is composed of 20 periods of 5.1 nm (18 monolayers (MLs))-wide ZnSe layers and 2.5 nm (9 ML)-wide Be1xMnxTe layers. The Mn compositions x of the samples used are 0.05 and 0.029. An ultraviolet diode laser (403 nm) and second-harmonic light output of a femtosecond Ti:sapphire laser (399 nm) were used for photoexcitation, and generation of these carriers in the ZnSe layers of the MQW sample. A magnetic field up to 7 T was applied in the Faraday configuration, parallel to the structure growth axis. Sample temperatures were varied over the range 2.2–10 K.
3. Results and discussion Fig. 1 shows the PL spectra of ZnSe/ Be0.95Mn0.05Te-MQWs with magnetic fields B of 0–7 T at 4.2 K. Two principal bands (A and X) and a phonon replica (X-LO) were observed in the spectra. The A band is assigned to the interface recombination of electrons and holes confined in each well, and the X band to the interface recombination of electrons and holes localized at terraces with a thickness of 1 ML in both ZnSe and Be0.95Mn0.05Te layers [7]. This assignment is based on the calculation of energy levels of confined states using a one-dimensional square well potential model. A change in energy due to 1 ML fluctuations of both layers corresponds to the energy splitting between the A and X bands.
Intensity
2. Experimental
A
7T
4.2K
X
0T X-LO
1.75
1.80 1.85 Photon Energy (eV)
1.90
Fig. 1. Luminescence spectra of ZnSe/Be0.95Mn0.05Te-MQWs for magnetic fields B between 0 and 7 T at 4.2 K.
With increasing magnetic field, both the A and X bands exhibit a redshift due to Zeeman splitting of the valence band in the Be1xMnxTe layer and a significant increase in intensity. Before discussing the enhancement of the PL intensity with magnetic field, we analyze the observed redshift. The magnetic-field dependence of the redshift at 4.2 K is shown by open circles in Fig. 2(a). The shifts measured at 2.2 and 10 K are also shown by closed circles and triangles, respectively. The phenomenological model studied by Gaj et al. [9] describes the giant Zeeman effect due to the strong p–d exchange interaction associated with localized spins of Mn ions. Using a modified Brillouin function, the Zeeman shift DE of the electron–hole recombination energy is given by the following equation: 1 5gmB B DE ¼ xjN0 bjSeff B5=2 : ð1Þ 2 2kB ðT þ T0 Þ Here N0b is a p–d exchange constant, while the effective Mn spin Seff and the effective temperature T0 account phenomenologically for the d–d interaction of the Mn ions. Solid curves in Fig. 2(a) show the best-fit curves for the Zeeman shifts measured at different temperatures. The derived parameter values are N0b=0.3470.01 eV, Seff=1.51 and T0=2.1 K for x=0.05. In Fig. 2(b) we plot normalized PL intensities measured at different temperatures against B/ (T+T0). We find a scaling relation between the
ARTICLE IN PRESS A. Nakamura et al. / Journal of Luminescence 108 (2004) 65–68
recombination is enhanced by the Mn-spin alignment with the magnetic field. We measured decay curves with a magnetic field in ZnSe/Be0.971Mn0.029Te-MQWs. As shown in Fig. 3, the decay curves exhibit two-component decay, and the slow component increases with increasing magnetic field. The decay curves were analyzed using the following equation:
0
-5
-10
IðtÞ ¼ b1 expðt=t1 Þ þ b2 expðt=t2 Þ:
The decay times t1 and t2 obtained are 19 and 460 ns, respectively, for all the decay curves measured at different magnetic fields. With increasing magnetic field, the fraction of the slow component b2 =ðb1 þ b2 Þ increases. The slow component is ascribed to the recombination process resulting from overlapping of electron and hole
-15 0 (a)
2
4 Magnetic Field (T)
6
3
2
7T 104
0
Intensity
1
0
1 B/(T+T0) (T/K)
3T 103
0T
2
Fig. 2. (a) Peak-energy shifts of the A band as a function of magnetic field in ZnSe/Be0.95Mn0.05Te-MQWs measured at different temperatures. Closed circles for 2.2 K, open circles for 4.2 K and triangles for 10 K. The solid curves indicate the results fitted by Eq. (1). (b) Normalized intensities of the A band measured at different temperatures as a function of B=ðT þ T0 Þ: The solid curves indicate the results fitted by Eq. (2).
normalized intensity and B/(T+T0), which suggests that the Mn spin alignment affects the electron– hole recombination process. Therefore, we analyze the normalized intensity at T using the Brillouin function IðB; TÞ 5gmB B ¼ aB5=2 þ 1; ð2Þ Ið0; TÞ 2kB ðT þ T0 Þ where a is an adjustable parameter. As shown in Fig. 2(b) the observed dependence can be well reproduced by Eq. (2) using the same parameters as those giving the best fit to the Zeeman-shift data. This result suggests that the interfacial
102 0 (a)
Fraction of slow component
Normalized Intensity
4
(b)
ð3Þ
0.2
0.4 0.6 Time (µs)
0.8
1.0
3
3
2
2
1
1
Normalized Intensity
Shift (meV)
67
0 0 (b)
0.5 B/(T+T0) (T/K)
1.0
Fig. 3. (a) Decay curves of the A band intensity with B=0, 3 and 7 T in ZnSe/Be0.971Mn0.029Te-MQWs. Dashed curves indicate the fitted curves of the two-component exponential function. (b) Fraction of the slow-decay component (triangles) and normalized intensity (closed circles) as a function of B=ðT þ T0 Þ: The solid curve indicates the slow-component fit to Eq. (2).
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A. Nakamura et al. / Journal of Luminescence 108 (2004) 65–68
wave functions at the interface. In Fig. 3(b) we plot the fraction b2 =ðb1 þ b2 Þ against B=ðT þ T0 Þ together with the normalized PL intensity. The observed dependence of the fraction (triangles) is in good agreement with that of the normalized intensity (closed circles), and the dependence is well reproduced by the chosen Brillouin function. We now discuss a mechanism that enhances the electron–hole recombination with the magnetic field. In type II MQWs, electrons and holes are separately confined in the ZnSe and Be1xMnxTe layers, respectively. There are two contributions to the hole mobility in the Be1xMnxTe layers at low temperatures: alloy fluctuations due to Mn-ion substitutions and potential fluctuations associated with random orientations of localized Mn spins. The potential variation from the alloy fluctuations is not large because the Mn contents of the samples studied here are less than 5%. The Mn spins are aligned along an applied magnetic field and the spatial fluctuations of the p–d exchange energy between the hole and the localized spin are suppressed. As a result, the holes become more mobile in the lateral direction of the Be1xMnxTe layer. For recombination of an electron and a hole at the interface, the hole in the Be1xMnxTe layer and the electron in the ZnSe layer must exist at the same location at the interface between both the layers. As the probability of both electron and hole being found at the same location becomes higher with increasing magnetic field, the interfacerecombination component increases.
4. Summary Enhancement of interfacial recombination with a magnetic field in type II ZnSe/Be0.95Mn0.05TeMQWs has been investigated under CW and femtosecond pulse-laser excitations. We have observed a redshift and an increase in PL intensity associated with the giant Zeeman effect of the valence-band state in the Be0.95Mn0.05Te layer.
The phenomenological model which describes the giant Zeeman shift as being from the strong p–d exchange interaction explains well the increase in PL intensity. The Mn spin alignment with the magnetic field suppresses potential fluctuations due to localized spins in the Be1xMnxTe layer, and as a result holes can migrate along the interface to recombine with electrons in the ZnSe layer.
Acknowledgements This work is supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] M. von Ortenberg, Phys. Rev. Lett. 49 (1982) 1041. [2] M. Dai, H. Luo, W.C. Chou, F.C. Zhang, N. Samarh, M. Dobrowolska, J.K. Furdyna, Phys. Rev. Lett. 67 (1991) 3824. [3] I.A. Merkulov, D.R. Yakovlev, A. Keller, W. Ossau, J. Geurts, A. Waag, G. Landwehr, G. Karczewski, T. Wojtowicz, J. Kossut, Phys. Rev. Lett. 83 (1999) 1431. [4] A. Waag, F. Fischer, K. Schull, T. Baron, H.J. Lugauer, Th. Liz, U. Zehnder, W. Ossau, T. Gerhardt, M. Keim, G. Reuscher, G. Landwehr, Appl. Phys. Lett. 70 (1997) 280. [5] R. Akimoto, Y. Kinpara, K. Akita, S. Kobayashi, Appl. Phys. Lett. 78 (2001) 580. [6] D.R. Yakovlev, C. Sas, B. Konig, L. Hansen, W. Ossau, G. Landwehr, L.W. Molenkamp, Appl. Phys. Lett. 78 (2001) 1870. [7] A. Nakamura, N. Hori, M. Sakai, N. Kato, K. Akita, R. Akimoto, in: A.R. Long, J.H. Davis (Eds.), Proceedings of International Conference on Physics of Semiconductors, Institute of Physics Conference Series No. 171, Institute of Physics Publishing, Edinburgh, 2002, pp. D99. [8] D.R. Yakovlev, A.V. Platonov, E.L. Ivchenko, V.P. Kochereshko, C. Sas, W. Ossau, L. Hansen, A. Waag, G. Landwehr, L.W. Molenkamp, Phys. Rev. Lett. 88 (2002) 257401. [9] J.A. Gaj, R. Planel, G. Fishman, Solid State Commun. 29 (1979) 435.
Journal of Luminescence 108 (2004) 65–68
Magnetic field enhanced luminescence in ZnSe/BeMnTe multiple quantum wells with a type II band alignment A. Nakamuraa,*, N. Katoa, I. Yamakawaa, R. Akimotob b
a Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Photonics Research Institute, AIST, Tsukuba Central 2-1, Tsukuba 305-8568, Japan
Abstract Magneto-optical properties of ZnSe/Be1xMnxTe multiple quantum wells with a type II band alignment have been investigated with magnetic fields up to 7 T. We have observed a large Zeeman shift and an increase in intensity of the interface recombination band. The Zeeman shift is analyzed by a phenomenological model based on a strong p–d exchange interaction of the valence band in the Be1xMnxTe layer. The increase in intensity is interpreted in terms of suppression of potential fluctuations due to the localized-spin alignment of Mn ions with the magnetic field. r 2004 Elsevier B.V. All rights reserved. PACS: 78.66.Hf; 78.20.Ls; 75.50.Pp; 73.20.r Keywords: Diluted magnetic semiconductors; Type II semiconductor superlattices; Magneto-optics
1. Introduction Diluted magnetic semiconductors (DMS) are known for their giant magneto-optical effects due to a strong sp–d exchange interaction between band electrons and magnetic ions. In quantum well (QW) structures of DMS, we can modify this giant splitting as well as the confinement energies of electrons and holes [1–3]. Zn(Mn)Se/Be(Mn)Te multiple quantum well (MQW) structures have recently attracted attention because of their type II band alignment with large band offsets [4–6]. Electrons and holes are separately confined in the Zn(Mn)Se and Be(Mn)Te layers, respec*Corresponding author. Tel.: +81-52-789-4450; fax: +8152-789-5316. E-mail address: [email protected] (A. Nakamura).
tively. We can investigate the magneto-optical properties solely arising from the p–d exchange interaction (s–d interaction) in Be1xMnxTe MQW structures (Zn1xMnxSe/BeTe MQW structures) [6,7]. Since the electron–hole recombination takes place at hetero-junctions in type II MQWs, the luminescence properties are sensitive to interfaces between the ZnSe and Be1xMnxTe layers and to potential fluctuations due to the Mn ions. The inplane anisotropy of hidden interfaces in QW structures has been studied by polarized spectroscopy of spin-oriented carriers in Zn(Mn)Se/BeTe MQWs [8]. In this paper we report on the luminescence properties and dynamics of the electron–hole recombination in type II ZnSe/ Be1xMnxTe MQWs with magnetic fields up to 7 T. A large Zeeman shift and an increase in intensity of the interfacial recombination band
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.012
ARTICLE IN PRESS 66
A. Nakamura et al. / Journal of Luminescence 108 (2004) 65–68
have been interpreted in terms of a strong p–d exchange interaction.
ZnSe/Be1xMnxTe-MQW samples were grown on (0 0 1)-oriented GaAs substrates by molecularbeam epitaxy (MBE) in a dual-chamber system [5]. After a 250-nm thick GaAs buffer layer was grown in a III–V chamber to improve the quality of the GaAs surface, the substrate was transferred to a II–VI chamber for the growth of ZnSe/ Be1xMnxTe-MQW structures. Growth conditions were adjusted to form Zn–Te chemical bonds at interfaces. The MQW structure is composed of 20 periods of 5.1 nm (18 monolayers (MLs))-wide ZnSe layers and 2.5 nm (9 ML)-wide Be1xMnxTe layers. The Mn compositions x of the samples used are 0.05 and 0.029. An ultraviolet diode laser (403 nm) and second-harmonic light output of a femtosecond Ti:sapphire laser (399 nm) were used for photoexcitation, and generation of these carriers in the ZnSe layers of the MQW sample. A magnetic field up to 7 T was applied in the Faraday configuration, parallel to the structure growth axis. Sample temperatures were varied over the range 2.2–10 K.
3. Results and discussion Fig. 1 shows the PL spectra of ZnSe/ Be0.95Mn0.05Te-MQWs with magnetic fields B of 0–7 T at 4.2 K. Two principal bands (A and X) and a phonon replica (X-LO) were observed in the spectra. The A band is assigned to the interface recombination of electrons and holes confined in each well, and the X band to the interface recombination of electrons and holes localized at terraces with a thickness of 1 ML in both ZnSe and Be0.95Mn0.05Te layers [7]. This assignment is based on the calculation of energy levels of confined states using a one-dimensional square well potential model. A change in energy due to 1 ML fluctuations of both layers corresponds to the energy splitting between the A and X bands.
Intensity
2. Experimental
A
7T
4.2K
X
0T X-LO
1.75
1.80 1.85 Photon Energy (eV)
1.90
Fig. 1. Luminescence spectra of ZnSe/Be0.95Mn0.05Te-MQWs for magnetic fields B between 0 and 7 T at 4.2 K.
With increasing magnetic field, both the A and X bands exhibit a redshift due to Zeeman splitting of the valence band in the Be1xMnxTe layer and a significant increase in intensity. Before discussing the enhancement of the PL intensity with magnetic field, we analyze the observed redshift. The magnetic-field dependence of the redshift at 4.2 K is shown by open circles in Fig. 2(a). The shifts measured at 2.2 and 10 K are also shown by closed circles and triangles, respectively. The phenomenological model studied by Gaj et al. [9] describes the giant Zeeman effect due to the strong p–d exchange interaction associated with localized spins of Mn ions. Using a modified Brillouin function, the Zeeman shift DE of the electron–hole recombination energy is given by the following equation: 1 5gmB B DE ¼ xjN0 bjSeff B5=2 : ð1Þ 2 2kB ðT þ T0 Þ Here N0b is a p–d exchange constant, while the effective Mn spin Seff and the effective temperature T0 account phenomenologically for the d–d interaction of the Mn ions. Solid curves in Fig. 2(a) show the best-fit curves for the Zeeman shifts measured at different temperatures. The derived parameter values are N0b=0.3470.01 eV, Seff=1.51 and T0=2.1 K for x=0.05. In Fig. 2(b) we plot normalized PL intensities measured at different temperatures against B/ (T+T0). We find a scaling relation between the
ARTICLE IN PRESS A. Nakamura et al. / Journal of Luminescence 108 (2004) 65–68
recombination is enhanced by the Mn-spin alignment with the magnetic field. We measured decay curves with a magnetic field in ZnSe/Be0.971Mn0.029Te-MQWs. As shown in Fig. 3, the decay curves exhibit two-component decay, and the slow component increases with increasing magnetic field. The decay curves were analyzed using the following equation:
0
-5
-10
IðtÞ ¼ b1 expðt=t1 Þ þ b2 expðt=t2 Þ:
The decay times t1 and t2 obtained are 19 and 460 ns, respectively, for all the decay curves measured at different magnetic fields. With increasing magnetic field, the fraction of the slow component b2 =ðb1 þ b2 Þ increases. The slow component is ascribed to the recombination process resulting from overlapping of electron and hole
-15 0 (a)
2
4 Magnetic Field (T)
6
3
2
7T 104
0
Intensity
1
0
1 B/(T+T0) (T/K)
3T 103
0T
2
Fig. 2. (a) Peak-energy shifts of the A band as a function of magnetic field in ZnSe/Be0.95Mn0.05Te-MQWs measured at different temperatures. Closed circles for 2.2 K, open circles for 4.2 K and triangles for 10 K. The solid curves indicate the results fitted by Eq. (1). (b) Normalized intensities of the A band measured at different temperatures as a function of B=ðT þ T0 Þ: The solid curves indicate the results fitted by Eq. (2).
normalized intensity and B/(T+T0), which suggests that the Mn spin alignment affects the electron– hole recombination process. Therefore, we analyze the normalized intensity at T using the Brillouin function IðB; TÞ 5gmB B ¼ aB5=2 þ 1; ð2Þ Ið0; TÞ 2kB ðT þ T0 Þ where a is an adjustable parameter. As shown in Fig. 2(b) the observed dependence can be well reproduced by Eq. (2) using the same parameters as those giving the best fit to the Zeeman-shift data. This result suggests that the interfacial
102 0 (a)
Fraction of slow component
Normalized Intensity
4
(b)
ð3Þ
0.2
0.4 0.6 Time (µs)
0.8
1.0
3
3
2
2
1
1
Normalized Intensity
Shift (meV)
67
0 0 (b)
0.5 B/(T+T0) (T/K)
1.0
Fig. 3. (a) Decay curves of the A band intensity with B=0, 3 and 7 T in ZnSe/Be0.971Mn0.029Te-MQWs. Dashed curves indicate the fitted curves of the two-component exponential function. (b) Fraction of the slow-decay component (triangles) and normalized intensity (closed circles) as a function of B=ðT þ T0 Þ: The solid curve indicates the slow-component fit to Eq. (2).
ARTICLE IN PRESS 68
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wave functions at the interface. In Fig. 3(b) we plot the fraction b2 =ðb1 þ b2 Þ against B=ðT þ T0 Þ together with the normalized PL intensity. The observed dependence of the fraction (triangles) is in good agreement with that of the normalized intensity (closed circles), and the dependence is well reproduced by the chosen Brillouin function. We now discuss a mechanism that enhances the electron–hole recombination with the magnetic field. In type II MQWs, electrons and holes are separately confined in the ZnSe and Be1xMnxTe layers, respectively. There are two contributions to the hole mobility in the Be1xMnxTe layers at low temperatures: alloy fluctuations due to Mn-ion substitutions and potential fluctuations associated with random orientations of localized Mn spins. The potential variation from the alloy fluctuations is not large because the Mn contents of the samples studied here are less than 5%. The Mn spins are aligned along an applied magnetic field and the spatial fluctuations of the p–d exchange energy between the hole and the localized spin are suppressed. As a result, the holes become more mobile in the lateral direction of the Be1xMnxTe layer. For recombination of an electron and a hole at the interface, the hole in the Be1xMnxTe layer and the electron in the ZnSe layer must exist at the same location at the interface between both the layers. As the probability of both electron and hole being found at the same location becomes higher with increasing magnetic field, the interfacerecombination component increases.
4. Summary Enhancement of interfacial recombination with a magnetic field in type II ZnSe/Be0.95Mn0.05TeMQWs has been investigated under CW and femtosecond pulse-laser excitations. We have observed a redshift and an increase in PL intensity associated with the giant Zeeman effect of the valence-band state in the Be0.95Mn0.05Te layer.
The phenomenological model which describes the giant Zeeman shift as being from the strong p–d exchange interaction explains well the increase in PL intensity. The Mn spin alignment with the magnetic field suppresses potential fluctuations due to localized spins in the Be1xMnxTe layer, and as a result holes can migrate along the interface to recombine with electrons in the ZnSe layer.
Acknowledgements This work is supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] M. von Ortenberg, Phys. Rev. Lett. 49 (1982) 1041. [2] M. Dai, H. Luo, W.C. Chou, F.C. Zhang, N. Samarh, M. Dobrowolska, J.K. Furdyna, Phys. Rev. Lett. 67 (1991) 3824. [3] I.A. Merkulov, D.R. Yakovlev, A. Keller, W. Ossau, J. Geurts, A. Waag, G. Landwehr, G. Karczewski, T. Wojtowicz, J. Kossut, Phys. Rev. Lett. 83 (1999) 1431. [4] A. Waag, F. Fischer, K. Schull, T. Baron, H.J. Lugauer, Th. Liz, U. Zehnder, W. Ossau, T. Gerhardt, M. Keim, G. Reuscher, G. Landwehr, Appl. Phys. Lett. 70 (1997) 280. [5] R. Akimoto, Y. Kinpara, K. Akita, S. Kobayashi, Appl. Phys. Lett. 78 (2001) 580. [6] D.R. Yakovlev, C. Sas, B. Konig, L. Hansen, W. Ossau, G. Landwehr, L.W. Molenkamp, Appl. Phys. Lett. 78 (2001) 1870. [7] A. Nakamura, N. Hori, M. Sakai, N. Kato, K. Akita, R. Akimoto, in: A.R. Long, J.H. Davis (Eds.), Proceedings of International Conference on Physics of Semiconductors, Institute of Physics Conference Series No. 171, Institute of Physics Publishing, Edinburgh, 2002, pp. D99. [8] D.R. Yakovlev, A.V. Platonov, E.L. Ivchenko, V.P. Kochereshko, C. Sas, W. Ossau, L. Hansen, A. Waag, G. Landwehr, L.W. Molenkamp, Phys. Rev. Lett. 88 (2002) 257401. [9] J.A. Gaj, R. Planel, G. Fishman, Solid State Commun. 29 (1979) 435.