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Oscillatory behavior of magneto-optical interband emissions in asymmetric quantum well structures
Superlattices and Microstructures, Vol. 21, No. 4, 1997
Oscillatory behavior of magneto-optical interband emissions in asymmetric quantum well structures ˜ es, M. V. B. Moreira A. R. Alves†, L. A. Cury, P. S. S. Guimara Departamento de F´ısica, Instituto de Ciˆencias Exatas, Universidade Federal de Minas Gerais, Caixa Postal 702, 30161-970, Belo Horizonte, MG, Brazil
(Received 15 July 1996) We report on the oscillatory behavior of the photoluminescence intensity from asymmetric AlGaAs/InGaAs/GaAs quantum well structures in the presence of a perpendicular magnetic field. Two distinct photoluminescence peaks originating from transitions from the ground (e1 ) and the first excited (e2 ) electronic states to the heavy hole state (hh1 ) are observed. The opposite phase of the oscillations shows clearly the competitive process between the transitions from the ground and first excited states. Electron transfer mechanisms cannot explain the origin of these oscillations. The optical oscillations emerge from changes in the effective electron–hole interaction. c 1997 Academic Press Limited
Key words: magneto-optical oscillation, asymmetric quantum well, magnetophotoluminescence.
1. Introduction The oscillatory behavior with magnetic field of the interband photoluminescence peak intensity in semiconductor asymmetric quantum wells has been widely investigated in the past few years [1–12]. Of particular interest is the correlation between these optical oscillations and the ‘electrical oscillations’, i.e. the quantized Hall plateaus in the transverse resistivity ρxy and the zero-resistance states in the longitudinal resistivity ρxx . When the interband photoluminescence (PL) peak in an asymmetric quantum well is monitored as the magnetic field B is increased, its intensity is found to oscillate with a 1/B period which is close or identical to the period of the Shubnikov–de Haas oscillations. Investigating one-side doped GaAs quantum wells with just one conduction subband populated, Heiman et al. [3] and Goldberg et al. [6] observed minima in the PL intensity at Landau level (LL) integral filling factors ν. They suggest that the minima in the PL could be attributed to changes in the screening when the Fermi energy is located at the localized states between Landau levels. The changes in the screening would affect the interaction between the two-dimensional electron gas and the holes in the valence Landau level. Further results [4, 7, 9], obtained from specially designed quantum wells with a second electron subband lying close to the Fermi energy, have provided new insights to explain the origin of the PL oscillations. Chen et al. [4] have found pronounced oscillations in the intensity of the PL line arising from recombination of electrons of the second subband with heavy holes. They found minima in .
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† Permanent address: Departamento de F´ısica, Universidade Federal de Vi¸cosa, 36570-000, Vi¸cosa MG, Brazil.
0749–6036/97/040591 + 05 $25.00/0
sm960209
c 1997 Academic Press Limited
592
Superlattices and Microstructures, Vol. 21, No. 4, 1997
the PL intensity when the Fermi level lay within localized states. They interpreted the optical oscillations as being caused by many-body interactions of the Fermi-edge electrons with the exciton of the second subband (Fermi edge singularity). Another possible interpretation was given by Skolnick et al. [12] in terms of the crossing between the LL of the first subband with the second electron subband energy e2 . This crossing would lead to a maximum population in e2 , which in turn would cause the enhancement of the PL intensity by the greater wavefunction overlap. At ν = 1, Turberfield et al. [5] and Dahl et al. [8] observed minima in the intensity of the PL arising from electron–hole recombination from the ground state electron subband, e1 -hh1 , which are accompanied by maxima in the intensity of the PL from electron–hole recombination from the second electron subband, e2 -hh1 . Dahl et al. point out that the increase in the e1 -hh1 recombination time at T < 1 K should also account for the increase in the intensity from other PL decay channels competing with the ground state recombination. In the present work we provide clearer evidence of the competitive relation of the e1 -hh1 and e2 -hh1 recombination processes. An increase in the PL intensity from the e1 -hh1 transition will necessarily lead to a decrease in the e2 -hh1 transition (and vice versa) since the intensity of both PL lines is determined by the relatively low number of photoexcited holes in the valence band. Our results do not corroborate the interpretation by Skolnick et al. [12]. We show that the processes responsible for the PL oscillations are due to an enhancement of the interaction between the holes in the valence band and the electrons in the first electronic subband which occurs when the extended states of a LL of e1 cross the Fermi energy. This enhancement is not the Fermi edge singularity proposed by Chen et al. [4]. .
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2. Experimental The samples used in this study were modulation doped n-type Al0.3 Ga0.7 As/In0.15 Ga0.85 As/GaAs asymmetrical single quantum wells grown by molecular beam epitaxy on Cr-doped semi-insulating (100)-oriented GaAs substrate. The epitaxial layers of the structure consist of the following sequence: a 0.6 µm GaAs buffer ˚ non-intentionally doped Al0.3 Ga0.7 As spacer ˚ In0.15 Ga0.85 As undoped quantum well; a 50 A layer; the 200 A 18 ˚ layer; a 500 A Al0.3 Ga0.7 As Si-doped layer (n = 3.8 × 10 cm−3 ). Finally, the structure was covered by a ˚ GaAs n + = 7 × 1018 cm−3 cap layer. The electrically measured sheet density and mobility of the sample 50 A are, respectively, 1.5 × 1012 cm−2 and 5 × 104 V cm−2 s, at 2 K. The magnetophotoluminescence measurements were performed at 2 K in a 17 T superconducting magnet. The excitation light produced by an Ar-laser emitting at 514.5 nm was guided to the sample by a single optical fiber. The PL emitted light, collected by a beam of optical fibers, was detected by a Ge detector after dispersion from a Spex 1702/04 monocromator. The excitation light intensity was kept constant at 10 W cm−2 in all measurements.
3. Results and discussion Figure 1 shows representative photoluminescence spectra at three different magnetic fields. At B = 0 T, two PL lines, which we identify as the transitions e1 -hh1 and e2 -hh1 , are seen. For our structures, the selection rule 1l = 0 (where l is the subband quantum number) does not apply because of the strongly asymmetrical band profile due to charge accumulation in the well. In these conditions the e2 -hh1 PL line becomes observable even if the e2 subband is slightly above the Fermi energy EF [9]. The inset in Fig. 1 shows the result of a self-consistent calculation for the conduction band profile of our samples. The e2 subband is located 6 meV above EF and 50 meV above the e1 subband. The only input parameters in our self-consistent calculations are the structural characteristics of the sample, such as layer thicknesses and doping concentration. The carrier concentration in the well, n S , comes out as a result of the calculation and its predicted value agrees very well with the experimental value given by the Shubnikov–de Haas period. As shown in Fig. 1, the PL peak intensity for the e2 -hh1 transition is about six times greater than that for the e1 -hh1 transition at B = 0 T. .
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Superlattices and Microstructures, Vol. 21, No. 4, 1997
593
T=2K
E2 EF
PL intensity (a.u.)
(c) B = 15 T
E1
EF x5 (b) B = 11.5 T
LL2
(a) B = 0 T
e1 – hh1 1.350
1.365
e2 – hh1 1.380
1.395 E (eV)
1.410
1.425
Fig. 1. Three representative photoluminescence spectra showing the evolution of the e1 -hh1 and e2 -hh1 peak intensities with the magnetic field applied perpendicular to the plane of the layers. .
.
The spatial overlap for electron and hole wavefunctions for electrons in e2 is much stronger than the overlap integral for holes and e1 electrons, which explains the large difference in PL intensity. Our calculation of the overlap integrals confirms the observed ratio in the PL intensities. As the magnetic field increases a dramatic change in the relative intensities of the two PL lines is observed, apart from the diamagnetic shifts. The variation in the intensities is apparent in the curves for 11.5 T and 15 T in Fig. 1. At B = 11.5 T, which is very close to filling factor ν = 5, the intensities for the two PL lines are nearly the same. The peak labeled as LL2 comes from electron–hole recombination from the second Landau level of e1 , and its difference in energy to the e1 -hh1 peak gives the value of the separation between Landau levels, h¯ ωc . The PL peak coming from recombination from the third Landau level of e1 , LL3, is located 2h¯ ωc above e1 -hh1 . This peak is not clearly seen in the figure due to its vicinity to the e2 -hh1 transition. Reckoning from both the self-consistent calculations and by comparison with the position of the electrical Shubnikov–de Haas maxima (shown on the top of Fig. 2), the LL3 peak at B = 11.5 T is in near resonance with EF . The oscillations in the intensities of the PL lines are more clearly revealed in Fig. 2. This shows the intensities (IPL ) of the PL peaks as a function of the magnetic field. Also shown in Fig. 2 is the magnetoresistance (Rxx ) of the sample, which was measured simultaneously with the magnetophotoluminescence. We observe a good correspondence between the electrical Shubnikov–de Haas oscillations in Rxx and the oscillations in the e1 -hh1 PL peak intensity. Below approximately 7 T, the maxima and minima in the e1 -hh1 PL line are almost exactly in phase with the maxima and minima in Rxx . It is also striking to see that the oscillations in the intensity for the second PL line, e2 -hh1 , are strictly 180◦ out of phase with those oscillations in the IPL for e1 -hh1 . Chen et al. [4] have also observed oscillations in the intensity of the PL lines in samples that are similar to ours. .
.
Superlattices and Microstructures, Vol. 21, No. 4, 1997 Resistance (a.u.)
594 T=2K
Rxy Rxx
IPL (a.u.)
e1 – hh1
e2 – hh1
0
2
4
6
8 10 12 14 B (T)
Fig. 2. In the upper figure we show the Shubnikov–de Haas oscillations (Rxx ) and Hall plateaus (Rxy ). Below are depicted the magneto-photoluminescence oscillations for: the e1 -hh1 and e2 -hh1 transitions. .
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However, the oscillations in e2 -hh1 seen by Chen et al. are in-phase with the oscillations in Rxx , opposite to the results shown in Fig. 2. Our results cannot be explained by the model suggested by Skolnick et al. [12], which proposes that the variations in the intensity of the PL lines are due to electron transfer mechanisms between the Landau levels and the e2 subband, each time a Landau level of e1 crosses the e2 energy level. This model could only explain an increase in the e2 -hh1 PL peak intensity as the Landau levels pass through the Fermi energy, as seen by Chen et al., and not a pronounced decrease as demonstrated in Figs 1B and 2. Our results imply that the magneto-optical oscillations can only be caused by an enhancement of the net effective electron–hole interaction. Figure 2 also exhibits a direct evidence of the competitive process between the e1 -hh1 and e2 -hh1 transitions, with the pronounced oscillations from e2 -hh1 clearly in opposite phase from those of e1 -hh1 . Since the photoexcited holes have a relatively low and practically fixed number in the valence band, it is evident that the enhanced recombination of holes with electrons in the e2 subband (resulting in a maxima for the e2 -hh1 PL line) will necessarily produce an intensity minima in the e1 -hh1 PL and vice versa. .
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4. Conclusion We measured the photoluminescence intensity oscillations with magnetic field for the two interband transitions in an asymmetric AlGaAs/InGaAs/GaAs quantum well and showed clearly the competitive process between them. We show that the PL oscillations could not be attributed to an electron transfer mechanism. An alternating enhancement of the electron–hole interaction with the magnetic field appears to be responsible for these oscillations. The basis of this mechanism is not yet understood. In order to understand the phenomenon better we are planning new experiments using samples with different Fermi energies at different temperatures and laser excitation powers.
Superlattices and Microstructures, Vol. 21, No. 4, 1997
595
Acknowledgements—This work is supported by CNPq and FAPEMIG. A. R. Alves is on leave from UFV, Vi¸cosa, MG, supported by CAPES.
References .
[1] }C. H. Perry, J. M. Worlock, M. C. Smith, and A. Petrou, in High Magnetic Fields in Semiconductor Physics, edited by G. Landwehr, Springer Series in Solid-State Sciences, Vol. 71, Springer-Verlag, Berlin, Heidelberg: p. 202 (1986). } V. Kukushkin, K. von Klitzing, and K. Ploog, Phys. Rev. B 37, 8509 (1988). [2] I. } Heiman, B. B. Goldberg, A. Pinczuk, C. W. Tu, A. C. Gossard, and J. H. English, Phys. Rev. Lett. 61, [3] D. 605 (1988). } Chen, M. Fritze, A. V. Nurmikko, D. Ackley, C. Colvard, and H. Lee, Phys. Rev. Lett. 64, 2434 (1990). [4] W. [5] A. } J. Turberfield, S. R. Haynes, P. A. Wright, R. A. Ford, R. G. Clark, J. F. Ryan, J. J. Harris, and C. T. Foxon, Phys. Rev. Lett. 65, 637 (1990). } B. Goldberg, D. Heiman, A. Pinczuc, L. Pfeiffer, and K. West, Phys. Rev. Lett. 65, 641 (1990). [6] B. } Chen, M. Fritze, A. V. Nurmikko, M. Hong, and L. L. Chang, Phys. Rev. B 43, 14738 (1991). [7] W. } Dahl, D. Heiman, A. Pinczuk, B. B. Goldberg, L. N. Pfeiffer, and K. W. West, Phys. Rev. B 45, 6957 [8] M. (1992). } Chen, M. Fritze, W. Walecki, A. V. Nurmikko, D. Ackley, J. M. Hong, and L. L. Chang, Phys. Rev. [9] W. B 45, 8464 (1992). } K¨uchler, P. Hiergeist, G. Abstreiter, J. P. Reithmaier, H. Riechert, and R. L¨osch, in High Magnetic [10] R. Fields in Semiconductor Physics III, edited by G. Landwehr, Springer Series in Solid-State Sciences, Vol. 101, Springer-Verlag, Berlin, Heidelberg: p. 514 (1986). } A. J. M. Driessen, S. M. Olsthoorn, T. T. J. M. Berendschot, H. F. Pen, L. J. Giling, G. A. C. Jones, [11] F. D. A. Ritchie, and J. E. F. Frost, Phys. Rev. B 45, 11823 (1992). } S. Skolnick, P. E. Simmonds, and T. A. Fisher, Phys. Rev. Lett. 66, 963 (1991). [12] M. .
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Oscillatory behavior of magneto-optical interband emissions in asymmetric quantum well structures ˜ es, M. V. B. Moreira A. R. Alves†, L. A. Cury, P. S. S. Guimara Departamento de F´ısica, Instituto de Ciˆencias Exatas, Universidade Federal de Minas Gerais, Caixa Postal 702, 30161-970, Belo Horizonte, MG, Brazil
(Received 15 July 1996) We report on the oscillatory behavior of the photoluminescence intensity from asymmetric AlGaAs/InGaAs/GaAs quantum well structures in the presence of a perpendicular magnetic field. Two distinct photoluminescence peaks originating from transitions from the ground (e1 ) and the first excited (e2 ) electronic states to the heavy hole state (hh1 ) are observed. The opposite phase of the oscillations shows clearly the competitive process between the transitions from the ground and first excited states. Electron transfer mechanisms cannot explain the origin of these oscillations. The optical oscillations emerge from changes in the effective electron–hole interaction. c 1997 Academic Press Limited
Key words: magneto-optical oscillation, asymmetric quantum well, magnetophotoluminescence.
1. Introduction The oscillatory behavior with magnetic field of the interband photoluminescence peak intensity in semiconductor asymmetric quantum wells has been widely investigated in the past few years [1–12]. Of particular interest is the correlation between these optical oscillations and the ‘electrical oscillations’, i.e. the quantized Hall plateaus in the transverse resistivity ρxy and the zero-resistance states in the longitudinal resistivity ρxx . When the interband photoluminescence (PL) peak in an asymmetric quantum well is monitored as the magnetic field B is increased, its intensity is found to oscillate with a 1/B period which is close or identical to the period of the Shubnikov–de Haas oscillations. Investigating one-side doped GaAs quantum wells with just one conduction subband populated, Heiman et al. [3] and Goldberg et al. [6] observed minima in the PL intensity at Landau level (LL) integral filling factors ν. They suggest that the minima in the PL could be attributed to changes in the screening when the Fermi energy is located at the localized states between Landau levels. The changes in the screening would affect the interaction between the two-dimensional electron gas and the holes in the valence Landau level. Further results [4, 7, 9], obtained from specially designed quantum wells with a second electron subband lying close to the Fermi energy, have provided new insights to explain the origin of the PL oscillations. Chen et al. [4] have found pronounced oscillations in the intensity of the PL line arising from recombination of electrons of the second subband with heavy holes. They found minima in .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
† Permanent address: Departamento de F´ısica, Universidade Federal de Vi¸cosa, 36570-000, Vi¸cosa MG, Brazil.
0749–6036/97/040591 + 05 $25.00/0
sm960209
c 1997 Academic Press Limited
592
Superlattices and Microstructures, Vol. 21, No. 4, 1997
the PL intensity when the Fermi level lay within localized states. They interpreted the optical oscillations as being caused by many-body interactions of the Fermi-edge electrons with the exciton of the second subband (Fermi edge singularity). Another possible interpretation was given by Skolnick et al. [12] in terms of the crossing between the LL of the first subband with the second electron subband energy e2 . This crossing would lead to a maximum population in e2 , which in turn would cause the enhancement of the PL intensity by the greater wavefunction overlap. At ν = 1, Turberfield et al. [5] and Dahl et al. [8] observed minima in the intensity of the PL arising from electron–hole recombination from the ground state electron subband, e1 -hh1 , which are accompanied by maxima in the intensity of the PL from electron–hole recombination from the second electron subband, e2 -hh1 . Dahl et al. point out that the increase in the e1 -hh1 recombination time at T < 1 K should also account for the increase in the intensity from other PL decay channels competing with the ground state recombination. In the present work we provide clearer evidence of the competitive relation of the e1 -hh1 and e2 -hh1 recombination processes. An increase in the PL intensity from the e1 -hh1 transition will necessarily lead to a decrease in the e2 -hh1 transition (and vice versa) since the intensity of both PL lines is determined by the relatively low number of photoexcited holes in the valence band. Our results do not corroborate the interpretation by Skolnick et al. [12]. We show that the processes responsible for the PL oscillations are due to an enhancement of the interaction between the holes in the valence band and the electrons in the first electronic subband which occurs when the extended states of a LL of e1 cross the Fermi energy. This enhancement is not the Fermi edge singularity proposed by Chen et al. [4]. .
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.
.
.
.
.
.
.
.
2. Experimental The samples used in this study were modulation doped n-type Al0.3 Ga0.7 As/In0.15 Ga0.85 As/GaAs asymmetrical single quantum wells grown by molecular beam epitaxy on Cr-doped semi-insulating (100)-oriented GaAs substrate. The epitaxial layers of the structure consist of the following sequence: a 0.6 µm GaAs buffer ˚ non-intentionally doped Al0.3 Ga0.7 As spacer ˚ In0.15 Ga0.85 As undoped quantum well; a 50 A layer; the 200 A 18 ˚ layer; a 500 A Al0.3 Ga0.7 As Si-doped layer (n = 3.8 × 10 cm−3 ). Finally, the structure was covered by a ˚ GaAs n + = 7 × 1018 cm−3 cap layer. The electrically measured sheet density and mobility of the sample 50 A are, respectively, 1.5 × 1012 cm−2 and 5 × 104 V cm−2 s, at 2 K. The magnetophotoluminescence measurements were performed at 2 K in a 17 T superconducting magnet. The excitation light produced by an Ar-laser emitting at 514.5 nm was guided to the sample by a single optical fiber. The PL emitted light, collected by a beam of optical fibers, was detected by a Ge detector after dispersion from a Spex 1702/04 monocromator. The excitation light intensity was kept constant at 10 W cm−2 in all measurements.
3. Results and discussion Figure 1 shows representative photoluminescence spectra at three different magnetic fields. At B = 0 T, two PL lines, which we identify as the transitions e1 -hh1 and e2 -hh1 , are seen. For our structures, the selection rule 1l = 0 (where l is the subband quantum number) does not apply because of the strongly asymmetrical band profile due to charge accumulation in the well. In these conditions the e2 -hh1 PL line becomes observable even if the e2 subband is slightly above the Fermi energy EF [9]. The inset in Fig. 1 shows the result of a self-consistent calculation for the conduction band profile of our samples. The e2 subband is located 6 meV above EF and 50 meV above the e1 subband. The only input parameters in our self-consistent calculations are the structural characteristics of the sample, such as layer thicknesses and doping concentration. The carrier concentration in the well, n S , comes out as a result of the calculation and its predicted value agrees very well with the experimental value given by the Shubnikov–de Haas period. As shown in Fig. 1, the PL peak intensity for the e2 -hh1 transition is about six times greater than that for the e1 -hh1 transition at B = 0 T. .
.
Superlattices and Microstructures, Vol. 21, No. 4, 1997
593
T=2K
E2 EF
PL intensity (a.u.)
(c) B = 15 T
E1
EF x5 (b) B = 11.5 T
LL2
(a) B = 0 T
e1 – hh1 1.350
1.365
e2 – hh1 1.380
1.395 E (eV)
1.410
1.425
Fig. 1. Three representative photoluminescence spectra showing the evolution of the e1 -hh1 and e2 -hh1 peak intensities with the magnetic field applied perpendicular to the plane of the layers. .
.
The spatial overlap for electron and hole wavefunctions for electrons in e2 is much stronger than the overlap integral for holes and e1 electrons, which explains the large difference in PL intensity. Our calculation of the overlap integrals confirms the observed ratio in the PL intensities. As the magnetic field increases a dramatic change in the relative intensities of the two PL lines is observed, apart from the diamagnetic shifts. The variation in the intensities is apparent in the curves for 11.5 T and 15 T in Fig. 1. At B = 11.5 T, which is very close to filling factor ν = 5, the intensities for the two PL lines are nearly the same. The peak labeled as LL2 comes from electron–hole recombination from the second Landau level of e1 , and its difference in energy to the e1 -hh1 peak gives the value of the separation between Landau levels, h¯ ωc . The PL peak coming from recombination from the third Landau level of e1 , LL3, is located 2h¯ ωc above e1 -hh1 . This peak is not clearly seen in the figure due to its vicinity to the e2 -hh1 transition. Reckoning from both the self-consistent calculations and by comparison with the position of the electrical Shubnikov–de Haas maxima (shown on the top of Fig. 2), the LL3 peak at B = 11.5 T is in near resonance with EF . The oscillations in the intensities of the PL lines are more clearly revealed in Fig. 2. This shows the intensities (IPL ) of the PL peaks as a function of the magnetic field. Also shown in Fig. 2 is the magnetoresistance (Rxx ) of the sample, which was measured simultaneously with the magnetophotoluminescence. We observe a good correspondence between the electrical Shubnikov–de Haas oscillations in Rxx and the oscillations in the e1 -hh1 PL peak intensity. Below approximately 7 T, the maxima and minima in the e1 -hh1 PL line are almost exactly in phase with the maxima and minima in Rxx . It is also striking to see that the oscillations in the intensity for the second PL line, e2 -hh1 , are strictly 180◦ out of phase with those oscillations in the IPL for e1 -hh1 . Chen et al. [4] have also observed oscillations in the intensity of the PL lines in samples that are similar to ours. .
.
Superlattices and Microstructures, Vol. 21, No. 4, 1997 Resistance (a.u.)
594 T=2K
Rxy Rxx
IPL (a.u.)
e1 – hh1
e2 – hh1
0
2
4
6
8 10 12 14 B (T)
Fig. 2. In the upper figure we show the Shubnikov–de Haas oscillations (Rxx ) and Hall plateaus (Rxy ). Below are depicted the magneto-photoluminescence oscillations for: the e1 -hh1 and e2 -hh1 transitions. .
.
However, the oscillations in e2 -hh1 seen by Chen et al. are in-phase with the oscillations in Rxx , opposite to the results shown in Fig. 2. Our results cannot be explained by the model suggested by Skolnick et al. [12], which proposes that the variations in the intensity of the PL lines are due to electron transfer mechanisms between the Landau levels and the e2 subband, each time a Landau level of e1 crosses the e2 energy level. This model could only explain an increase in the e2 -hh1 PL peak intensity as the Landau levels pass through the Fermi energy, as seen by Chen et al., and not a pronounced decrease as demonstrated in Figs 1B and 2. Our results imply that the magneto-optical oscillations can only be caused by an enhancement of the net effective electron–hole interaction. Figure 2 also exhibits a direct evidence of the competitive process between the e1 -hh1 and e2 -hh1 transitions, with the pronounced oscillations from e2 -hh1 clearly in opposite phase from those of e1 -hh1 . Since the photoexcited holes have a relatively low and practically fixed number in the valence band, it is evident that the enhanced recombination of holes with electrons in the e2 subband (resulting in a maxima for the e2 -hh1 PL line) will necessarily produce an intensity minima in the e1 -hh1 PL and vice versa. .
.
4. Conclusion We measured the photoluminescence intensity oscillations with magnetic field for the two interband transitions in an asymmetric AlGaAs/InGaAs/GaAs quantum well and showed clearly the competitive process between them. We show that the PL oscillations could not be attributed to an electron transfer mechanism. An alternating enhancement of the electron–hole interaction with the magnetic field appears to be responsible for these oscillations. The basis of this mechanism is not yet understood. In order to understand the phenomenon better we are planning new experiments using samples with different Fermi energies at different temperatures and laser excitation powers.
Superlattices and Microstructures, Vol. 21, No. 4, 1997
595
Acknowledgements—This work is supported by CNPq and FAPEMIG. A. R. Alves is on leave from UFV, Vi¸cosa, MG, supported by CAPES.
References .
[1] }C. H. Perry, J. M. Worlock, M. C. Smith, and A. Petrou, in High Magnetic Fields in Semiconductor Physics, edited by G. Landwehr, Springer Series in Solid-State Sciences, Vol. 71, Springer-Verlag, Berlin, Heidelberg: p. 202 (1986). } V. Kukushkin, K. von Klitzing, and K. Ploog, Phys. Rev. B 37, 8509 (1988). [2] I. } Heiman, B. B. Goldberg, A. Pinczuk, C. W. Tu, A. C. Gossard, and J. H. English, Phys. Rev. Lett. 61, [3] D. 605 (1988). } Chen, M. Fritze, A. V. Nurmikko, D. Ackley, C. Colvard, and H. Lee, Phys. Rev. Lett. 64, 2434 (1990). [4] W. [5] A. } J. Turberfield, S. R. Haynes, P. A. Wright, R. A. Ford, R. G. Clark, J. F. Ryan, J. J. Harris, and C. T. Foxon, Phys. Rev. Lett. 65, 637 (1990). } B. Goldberg, D. Heiman, A. Pinczuc, L. Pfeiffer, and K. West, Phys. Rev. Lett. 65, 641 (1990). [6] B. } Chen, M. Fritze, A. V. Nurmikko, M. Hong, and L. L. Chang, Phys. Rev. B 43, 14738 (1991). [7] W. } Dahl, D. Heiman, A. Pinczuk, B. B. Goldberg, L. N. Pfeiffer, and K. W. West, Phys. Rev. B 45, 6957 [8] M. (1992). } Chen, M. Fritze, W. Walecki, A. V. Nurmikko, D. Ackley, J. M. Hong, and L. L. Chang, Phys. Rev. [9] W. B 45, 8464 (1992). } K¨uchler, P. Hiergeist, G. Abstreiter, J. P. Reithmaier, H. Riechert, and R. L¨osch, in High Magnetic [10] R. Fields in Semiconductor Physics III, edited by G. Landwehr, Springer Series in Solid-State Sciences, Vol. 101, Springer-Verlag, Berlin, Heidelberg: p. 514 (1986). } A. J. M. Driessen, S. M. Olsthoorn, T. T. J. M. Berendschot, H. F. Pen, L. J. Giling, G. A. C. Jones, [11] F. D. A. Ritchie, and J. E. F. Frost, Phys. Rev. B 45, 11823 (1992). } S. Skolnick, P. E. Simmonds, and T. A. Fisher, Phys. Rev. Lett. 66, 963 (1991). [12] M. .
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