- Email: [email protected]
Bound and unbound exciton—electron states in II–VI quantum well structures with a 2DEG
,oum.uov CRYSTAL aROWTH
ELSEVIER
Journal
of
Crystal Growth 184/185(1998)X18-821
Bound and unbound exciton-electron states in II-VI quantum well structures with a 2DEG D.R. Yakovleva,b,*, V.P. Kochereshkob, W. Ossau”, J.X. Shen”, A. Waag”, G. Landwehra, P.C.M. Christianen”, J.C. Maanc aPhysikalisches Institut der Universitiit Wiirzburg, 97074 Wiirzburg, Germany ‘A.F. Iqfe Physico-Technical Institute Russian Academy of Sciences, 194021 St. Petersburg, Russian Federation ‘Research Institute for Materials, High Field Magnet Laboratory, University of N@egen, 6525 ED Nijmegen, The Netherlands
Abstract
Optical tuning of electron concentration in a quantum well is realized in CdTe/(Cd,Mg)Te heterostructures. Effects governed by the exciton-electron interaction at low electron density are studied by use of photoluminescence and reflectivity in magnetic fields and under microwave radiation. Experimental manifestations of bound (negatively charged excitons) and unbound (combined exciton-cyclotron resonance) exciton-electron states are discussed. ~0 1998 Elsevier Science B.V. All rights reserved. PACS:
71.35.Gg; 76.40. + b; 78.66.Hf
Keywords:
Exciton+lectron
interaction;
Excitons in quantum wells
Effects resulting from the exciton+lectron interaction in the presence of a two-dimensional electron gas of low density at n$g<< 1, where n, is the electron concentration and aa is the exciton Bohr radius, became a subject of intensive investigations very recently. This interest has been stimulated by the paper of Kheng et al., where the observation of a negatively charged exciton (X-) in CdTe/ (Cd,Zn)Te modulation-doped quantum well (QWs)
*Corresponding author. Tel: + 49 931 888 5125; + 49 931 888 5142; e-mail: [email protected].
fax:
0022-0248/98/$19.00 (0 1998 Elsevier Science B.V. All rights reserved PII SOO22-0248(97)0053 l-9
has been reported [l]. After that, negatively and positively charged excitons have been observed in GaAs/(Al,Ga)As modulation-doped QWs with the free carrier concentration tuned by a variation of the gate voltage [2]. In this paper we present an overview of effects caused by the electronPexciton interaction in QWs with a 2DEG of low density. To study the phenomena caused by the excitonelectron interaction we have chosen a CdTe/ (Cd,Mg)Te QW system which is known to exhibit strong excitonic effects. The necessary variation of the electron density was achieved by an external optical illumination exploiting a specially designed layer composition. The type I CdTe/Cd0,,4Mg,,,,Te
D.R. Yakovlev et al. /Journal
qfCyysta1
heterostructure consists of a 75 A thick QW, which is sandwiched between 1000 A thick shortperiod superlattices (SLs) (30 A/30 A), grown by molecular-beam epitaxy on a (1 0 0)-oriented Cd 0.9Jn0.03 Te substtate. The QW is separated from the SLs by 200 A thick barriers (see inset of Fig. l), which leads to a much higher probability for electrons to tunnel from the SLs into the QW, as compared to holes, due to the difference in effective mass. Consequently, the electron concentration in the QW can be varied accurately by the intensity of Ar-ion laser illumination with a radiation energy (hoin = 2.41 eV) exceeding the SL band gap. Photoluminescence (PL), PL-excitation (PLE) and reflectivity spectra were measured at 1.6 K in magnetic fields up to 20 T applied perpendicular to the QW layers. Time-resolved measurements were performed by use of an acousto-optical modulator with a time resolution of 100 ns. A dye-(Pyridine) or Ti : sapphire laser with a photon energy (hmdir) below the SL band gap was used for direct creation of excitons in the QW. Additionally, the variation of the PL intensity under microwave (MW) radiation of 70 GHz and 20 mW was analyzed. The PL spectrum of the QW, which consists of two lines, is shown in Fig. 1. The X line is due to the recombination of excitons, and has a linewidth of 1.5 meV with a Stokes shift of only 1 meV, demonstrating the high interface quality of the structure. The X- line, corresponding to the negatively charged exciton, is shifted by 3.5 meV to lower energy from the X line [1,3]. Increasing the background electron concentration by illumination with an Argon-ion laser, leads to an enhanced intensity of the X- line relative to the X line, as expected for a charged exciton. The temporal dependence of this effect (see inset of Fig. 1) shows a rise time of 1.5 us, which was found to decrease for larger illumination intensities, as it is determined by the number of electrons collected in the QW from the SLs. The decay time of the effect is equal to 2.7 us, which was found to be independent of the illumination intensity and is due to the indirect recombination of the QW electrons with holes located in the SLs. Note that the recombination time of excitons in the QW is 200 ps [4], which is much shorter than the present dynamical processes measured in the us-range. The maximal electron concentration in the QW
819
Growth 1841185 (1998) 818-821
provided by the external illumination is estimated to be 5 x 10” cm-‘, and is below the critical concentration for exciton screening in II-VI QWs [S]. Fig. 2 depicts the effect of microwave radiation on the exciton and negatively charged exciton PL T=1.6 K
X;. X
I 1.63
1.62
, ,0
.,
I,
10,
,
,
1.64
I
a,, Time&Is) , , , 1 6
1.65
Energy (eV) Fig. 1. Photoluminescence spectra of a 75 A thick CdTe/ Cd0.,4Mg,.,,Te QW taken under different excitation conditions: (I) direct excitation in the QW only; (2) direct excitation and illumination above the SL band gap; (3) illumination only. The inset shows temporal changes of the X-line intensity measured under pulsed illumination (open circles). 200
T=l.6 K
A-
2500
3
2000
c * .V
excitation
\ excitation
above SL gap
below SL gap
- -200
- -400 il - -600
b 2 zl E ‘Z ‘C F % .z s
C
- -600
01.
J-
”
1.625
3
.
1’.
‘.
1.630 Energy
.
1
.
.
2
.
1.635
(6v)
Fig. 2. Photoluminescence intensity of a 75 A thick CdTe/ CdO.,aMgO ,,Te QW (excitation energy exceeds the SL gap) and its modulation under microwave radiation of 70 GHz.
820
D.R. Yakovleu et al. 1 Journal of Crystal Growth 1841185 (1998) 818-821
intensity. Being effectively absorbed by charged carriers, the MW radiation heats the electrons and reduces the probability of the X- formation. Experimentally, it appears in the PL spectra as a reduction of the X- line intensity and increase of the exciton line intensity. This is shown in Fig. 2 where the change of the PL intensity under 70 GHz MW radiation is plotted. The modulation of the signal raises up to 6% of the PL intensity under the excitation above the SL gap and is strongly reduced for excitation below the SL gap when no electrons are collected from the SLs into the QW. In order to compare directly the MW effects for different excitation conditions the excitation densities were chosen in a way to have the same integral PL intensities for excitations with energy below and above the SL gap. The electron mobility in the QW of 4000 cm’/Vs was determined from the suppression of the MW effect in external magnetic fields (for details see Ref. [6]). This mobility value is in reasonable agreement with results of magnetotransport experiments for CdTe/(Cd,Mg)Te
QWs C71. Fig. 3 shows reflectivity spectra of the CdTe/ Cd0,,4Mg,,,,Te structure detected for two circular polarizations in a magnetic field of 7.5 T applied perpendicular to the QW plane. In addition to excitonic resonances (lsshh, Is-lh, 2s-hh) the lines determined by the bound (X-) and unbound (ExCR) exciton-electron states are present. The negatively charged exciton line is strongly polarized indicating a total polarization of the 2DEG at low temperatures by moderate magnetic fields. Now, we turn our attention to the ExCR line identified firstly in Ref. [3] as a combined exciton-cyclotron resonance. The ExCR line is due to the following process: an incident photon creates an exciton and simultaneously excites a background electron from the zeroth to the first Landau level. The following experimental manifestations shows up the background electron contribution in the ExCR process: (i) The intensity of the ExCR-line in a PLE spectrum increases strongly for higher illumination intensities (i.e. for larger y1,), whereas the magnetoexciton lines are insensitive to this factor. (ii) The ExCR-line shifts linearly with a magnetic field with a slope of 1.14 meV/T, which is compara-
__
T=l.BK B = 7.5 T
ExCR 15-h
I,
2s-hh
Io-
[email protected] .s Gki
4.5
2 4.0
yv
3.5
Is-hh 1.62
1s-lh
2s-hh
t t
1.63
; 1.65
1.64 Energy
1.66
WI
Fig. 3. Reflectivity spectra of a 75 A thick CdTe/ Cd0,,4Mg0,,,Te QW measured at a magnetic field of 7.5 T. Note that the trion (X-) and ExCR lines appear in different circular polarizations.
ble to the electron cyclotron energy in CdTe/ (Cd,Mg)Te QWs (see Fig. 4a). An extrapolation of this shift to zero field meets approximately the energy of the 1s state of the heavy-hole exciton (Is-hh). It follows from the theoretical consideration of the ExCR phenomenon, that the absorption spectrum of the system in magnetic fields consists of a free exciton line and several lines shifted to higher energies [3]. The peak positions of these additional lines correspond to energy distances of ntio,,,(l + me/M) with respect to the exciton resonance M = m, + mh is the exciton mass). (n = 1, 2, 3 . . . . An estimate of the energy shift of the n = 1 state with the experimentally determined masses m, = O.llmO and mh = 0.48mo [3] gives 1.24 meV/T, close to the experimentally observed shift of the ExCR-line of 1.14 meV/T. (iii) An additional experimental evidence for the contribution of the free electrons into the ExCR comes from a resonant polaron coupling phenomenon presented in Fig. 4b. Resonant polaron coupling for free electrons occurs at the condition hw, = hOLo, where w, is the cyclotron frequency freand wLo is the longitudinal-optic-phonon quency (in CdTe hoLo = 21 meV). It can be established from the typical anticrossing behavior
D.R. Yakovlev et al. 1 Journal
1.645 -
of Cvstal
T=1.6 K
.,*d
Is-state of el-hhl exciton 0 . (a)
.--------------__-__------_______--___________9 s
.I’
2o . LO phonon energy in CdTe
:
_,.&-; 4s
E 15: *a-’ tl .o-s ‘0: a ..a*’ 6 . .x5* t 5: .’ II5 . _-*- .op-
0
UN 1
5
10
15
20
Magnetic Field (T) Fig. 4. (a) ExCR and exciton energy dependences on external magnetic fields in a 75 A thick CdTe/CdO.,,Mg,.,,Te QW (A Ih hh = 17 meV). (b) Energy difference between ExCR and exciton line in a 75 A thick CdTe/Cd0.6Mg,.,Te QW (A,,, ,,,, = 31 meV). Note deviation from a linear law when the energy difference aproaches the energy of LO phonon.
caused
by
the
mixing
of the
electron
and
phonon
[S]. Such anticrossing featur:s were observed for the ExCR line in a 75 A thick CdTe/ Cdo,hMgo,,Te multiple QW structure with 1450 A thick barriers. In this structure an energy splitting between light-hole and heavy-hole states (Alh_,,,,) is 31 meV and the ExCR line is not masked by the strong resonance of light-hole exciton. Actually, a small value of Alhmhh= 17 meV in the CdTe/ QW does not allow to follow the Cd o,,4Mgo.,,Te states
821
ExCR line at fields above 14 T (see Fig. 4a) and to reach the resonant polaron coupling region. In summary, in QWs containing a 2DEG of low density the energy spectrum of free two-dimensional excitons is supplemented by the bound state of negatively charged exciton and an unbound state corresponding to a combined exciton-cyclotron resonance. This work is supported in part by the Deutsche Forschungsgemeinschaft (OS 98/5-l), European Commission under the contract No.CHGT-CT93005 1 (Large Facility), Volkswagen Foundation, and by the Russian Foundation for Basic Research Grant No.95-02-04061a.
References
.
0
Growth 1841185 (1998) 818-821
Cl1 K. Kheng, R.T. Cox, Y. Merle d’Aubigne, F. Bassani, K. Saminadayar, S. Tatarenko, Phys. Rev. Lett. 71 (1993) 1752. PI G. Finkelstein, H. Shtrikman, I. Bar-Joseph, Phys. Rev. Lett. 74 (1995) 976. R.A. Suris, W. Ossau, A. 131 D.R. Yakovlev, V.P. Kochereshko, Waag, G. Landwehr, P.C.M. Christianen, J.C. Maan, Proc. 23 Int. Conf. Physics of Semiconductors, Berlin, 1996, World Scientific, Singapore, 1996, p. 2071. A. Waag, T. Litz, r41 W. Ossau, U. Zehnder, B. Kuhn-Heinrich, G. Landwehr, R. Hellmann, E.O. Giibel, Superlattices Microstruct. 16 (1994) 5. M.G. Tyazhlov, A.F. Dite, A.I. Filin, A. c51 V.D. Kulakovskii, Forchel, D.R. Yakovlev, A. Waag,G. Landwehr, Phys. Rev. B 54 (1996) 4981. H. Schenk, W. Ccl J.X. Shen, D.R. Yakovlev, V.P. Kochereshko, Ossau, A. Waag, G. Landwehr, Proc. 12th Int. Conf. on the Application of High Magnetic Fields in Semiconductor Physics, Wiirzburg, 1996, World Scientific, Singapore, 1997, p. 741. [71 S. Scholl, J. Gerschiitz, F. Fischer, A. Waag, K. Schiill, H. Schifer, G. Landwehr, Solid State Commun. 94 (1995) 935. PI R.J. Nicholas, S. Sasaki, N. Miura, F.M. Peeters, J.M. Shi, G.Q. Hai, J.T. Devreese, M.J. Lawless, D.E. Ashenford, B. Lunn, Phys. Rev. B 50 (1994) 7596.
ELSEVIER
Journal
of
Crystal Growth 184/185(1998)X18-821
Bound and unbound exciton-electron states in II-VI quantum well structures with a 2DEG D.R. Yakovleva,b,*, V.P. Kochereshkob, W. Ossau”, J.X. Shen”, A. Waag”, G. Landwehra, P.C.M. Christianen”, J.C. Maanc aPhysikalisches Institut der Universitiit Wiirzburg, 97074 Wiirzburg, Germany ‘A.F. Iqfe Physico-Technical Institute Russian Academy of Sciences, 194021 St. Petersburg, Russian Federation ‘Research Institute for Materials, High Field Magnet Laboratory, University of N@egen, 6525 ED Nijmegen, The Netherlands
Abstract
Optical tuning of electron concentration in a quantum well is realized in CdTe/(Cd,Mg)Te heterostructures. Effects governed by the exciton-electron interaction at low electron density are studied by use of photoluminescence and reflectivity in magnetic fields and under microwave radiation. Experimental manifestations of bound (negatively charged excitons) and unbound (combined exciton-cyclotron resonance) exciton-electron states are discussed. ~0 1998 Elsevier Science B.V. All rights reserved. PACS:
71.35.Gg; 76.40. + b; 78.66.Hf
Keywords:
Exciton+lectron
interaction;
Excitons in quantum wells
Effects resulting from the exciton+lectron interaction in the presence of a two-dimensional electron gas of low density at n$g<< 1, where n, is the electron concentration and aa is the exciton Bohr radius, became a subject of intensive investigations very recently. This interest has been stimulated by the paper of Kheng et al., where the observation of a negatively charged exciton (X-) in CdTe/ (Cd,Zn)Te modulation-doped quantum well (QWs)
*Corresponding author. Tel: + 49 931 888 5125; + 49 931 888 5142; e-mail: [email protected].
fax:
0022-0248/98/$19.00 (0 1998 Elsevier Science B.V. All rights reserved PII SOO22-0248(97)0053 l-9
has been reported [l]. After that, negatively and positively charged excitons have been observed in GaAs/(Al,Ga)As modulation-doped QWs with the free carrier concentration tuned by a variation of the gate voltage [2]. In this paper we present an overview of effects caused by the electronPexciton interaction in QWs with a 2DEG of low density. To study the phenomena caused by the excitonelectron interaction we have chosen a CdTe/ (Cd,Mg)Te QW system which is known to exhibit strong excitonic effects. The necessary variation of the electron density was achieved by an external optical illumination exploiting a specially designed layer composition. The type I CdTe/Cd0,,4Mg,,,,Te
D.R. Yakovlev et al. /Journal
qfCyysta1
heterostructure consists of a 75 A thick QW, which is sandwiched between 1000 A thick shortperiod superlattices (SLs) (30 A/30 A), grown by molecular-beam epitaxy on a (1 0 0)-oriented Cd 0.9Jn0.03 Te substtate. The QW is separated from the SLs by 200 A thick barriers (see inset of Fig. l), which leads to a much higher probability for electrons to tunnel from the SLs into the QW, as compared to holes, due to the difference in effective mass. Consequently, the electron concentration in the QW can be varied accurately by the intensity of Ar-ion laser illumination with a radiation energy (hoin = 2.41 eV) exceeding the SL band gap. Photoluminescence (PL), PL-excitation (PLE) and reflectivity spectra were measured at 1.6 K in magnetic fields up to 20 T applied perpendicular to the QW layers. Time-resolved measurements were performed by use of an acousto-optical modulator with a time resolution of 100 ns. A dye-(Pyridine) or Ti : sapphire laser with a photon energy (hmdir) below the SL band gap was used for direct creation of excitons in the QW. Additionally, the variation of the PL intensity under microwave (MW) radiation of 70 GHz and 20 mW was analyzed. The PL spectrum of the QW, which consists of two lines, is shown in Fig. 1. The X line is due to the recombination of excitons, and has a linewidth of 1.5 meV with a Stokes shift of only 1 meV, demonstrating the high interface quality of the structure. The X- line, corresponding to the negatively charged exciton, is shifted by 3.5 meV to lower energy from the X line [1,3]. Increasing the background electron concentration by illumination with an Argon-ion laser, leads to an enhanced intensity of the X- line relative to the X line, as expected for a charged exciton. The temporal dependence of this effect (see inset of Fig. 1) shows a rise time of 1.5 us, which was found to decrease for larger illumination intensities, as it is determined by the number of electrons collected in the QW from the SLs. The decay time of the effect is equal to 2.7 us, which was found to be independent of the illumination intensity and is due to the indirect recombination of the QW electrons with holes located in the SLs. Note that the recombination time of excitons in the QW is 200 ps [4], which is much shorter than the present dynamical processes measured in the us-range. The maximal electron concentration in the QW
819
Growth 1841185 (1998) 818-821
provided by the external illumination is estimated to be 5 x 10” cm-‘, and is below the critical concentration for exciton screening in II-VI QWs [S]. Fig. 2 depicts the effect of microwave radiation on the exciton and negatively charged exciton PL T=1.6 K
X;. X
I 1.63
1.62
, ,0
.,
I,
10,
,
,
1.64
I
a,, Time&Is) , , , 1 6
1.65
Energy (eV) Fig. 1. Photoluminescence spectra of a 75 A thick CdTe/ Cd0.,4Mg,.,,Te QW taken under different excitation conditions: (I) direct excitation in the QW only; (2) direct excitation and illumination above the SL band gap; (3) illumination only. The inset shows temporal changes of the X-line intensity measured under pulsed illumination (open circles). 200
T=l.6 K
A-
2500
3
2000
c * .V
excitation
\ excitation
above SL gap
below SL gap
- -200
- -400 il - -600
b 2 zl E ‘Z ‘C F % .z s
C
- -600
01.
J-
”
1.625
3
.
1’.
‘.
1.630 Energy
.
1
.
.
2
.
1.635
(6v)
Fig. 2. Photoluminescence intensity of a 75 A thick CdTe/ CdO.,aMgO ,,Te QW (excitation energy exceeds the SL gap) and its modulation under microwave radiation of 70 GHz.
820
D.R. Yakovleu et al. 1 Journal of Crystal Growth 1841185 (1998) 818-821
intensity. Being effectively absorbed by charged carriers, the MW radiation heats the electrons and reduces the probability of the X- formation. Experimentally, it appears in the PL spectra as a reduction of the X- line intensity and increase of the exciton line intensity. This is shown in Fig. 2 where the change of the PL intensity under 70 GHz MW radiation is plotted. The modulation of the signal raises up to 6% of the PL intensity under the excitation above the SL gap and is strongly reduced for excitation below the SL gap when no electrons are collected from the SLs into the QW. In order to compare directly the MW effects for different excitation conditions the excitation densities were chosen in a way to have the same integral PL intensities for excitations with energy below and above the SL gap. The electron mobility in the QW of 4000 cm’/Vs was determined from the suppression of the MW effect in external magnetic fields (for details see Ref. [6]). This mobility value is in reasonable agreement with results of magnetotransport experiments for CdTe/(Cd,Mg)Te
QWs C71. Fig. 3 shows reflectivity spectra of the CdTe/ Cd0,,4Mg,,,,Te structure detected for two circular polarizations in a magnetic field of 7.5 T applied perpendicular to the QW plane. In addition to excitonic resonances (lsshh, Is-lh, 2s-hh) the lines determined by the bound (X-) and unbound (ExCR) exciton-electron states are present. The negatively charged exciton line is strongly polarized indicating a total polarization of the 2DEG at low temperatures by moderate magnetic fields. Now, we turn our attention to the ExCR line identified firstly in Ref. [3] as a combined exciton-cyclotron resonance. The ExCR line is due to the following process: an incident photon creates an exciton and simultaneously excites a background electron from the zeroth to the first Landau level. The following experimental manifestations shows up the background electron contribution in the ExCR process: (i) The intensity of the ExCR-line in a PLE spectrum increases strongly for higher illumination intensities (i.e. for larger y1,), whereas the magnetoexciton lines are insensitive to this factor. (ii) The ExCR-line shifts linearly with a magnetic field with a slope of 1.14 meV/T, which is compara-
__
T=l.BK B = 7.5 T
ExCR 15-h
I,
2s-hh
Io-
[email protected] .s Gki
4.5
2 4.0
yv
3.5
Is-hh 1.62
1s-lh
2s-hh
t t
1.63
; 1.65
1.64 Energy
1.66
WI
Fig. 3. Reflectivity spectra of a 75 A thick CdTe/ Cd0,,4Mg0,,,Te QW measured at a magnetic field of 7.5 T. Note that the trion (X-) and ExCR lines appear in different circular polarizations.
ble to the electron cyclotron energy in CdTe/ (Cd,Mg)Te QWs (see Fig. 4a). An extrapolation of this shift to zero field meets approximately the energy of the 1s state of the heavy-hole exciton (Is-hh). It follows from the theoretical consideration of the ExCR phenomenon, that the absorption spectrum of the system in magnetic fields consists of a free exciton line and several lines shifted to higher energies [3]. The peak positions of these additional lines correspond to energy distances of ntio,,,(l + me/M) with respect to the exciton resonance M = m, + mh is the exciton mass). (n = 1, 2, 3 . . . . An estimate of the energy shift of the n = 1 state with the experimentally determined masses m, = O.llmO and mh = 0.48mo [3] gives 1.24 meV/T, close to the experimentally observed shift of the ExCR-line of 1.14 meV/T. (iii) An additional experimental evidence for the contribution of the free electrons into the ExCR comes from a resonant polaron coupling phenomenon presented in Fig. 4b. Resonant polaron coupling for free electrons occurs at the condition hw, = hOLo, where w, is the cyclotron frequency freand wLo is the longitudinal-optic-phonon quency (in CdTe hoLo = 21 meV). It can be established from the typical anticrossing behavior
D.R. Yakovlev et al. 1 Journal
1.645 -
of Cvstal
T=1.6 K
.,*d
Is-state of el-hhl exciton 0 . (a)
.--------------__-__------_______--___________9 s
.I’
2o . LO phonon energy in CdTe
:
_,.&-; 4s
E 15: *a-’ tl .o-s ‘0: a ..a*’ 6 . .x5* t 5: .’ II5 . _-*- .op-
0
UN 1
5
10
15
20
Magnetic Field (T) Fig. 4. (a) ExCR and exciton energy dependences on external magnetic fields in a 75 A thick CdTe/CdO.,,Mg,.,,Te QW (A Ih hh = 17 meV). (b) Energy difference between ExCR and exciton line in a 75 A thick CdTe/Cd0.6Mg,.,Te QW (A,,, ,,,, = 31 meV). Note deviation from a linear law when the energy difference aproaches the energy of LO phonon.
caused
by
the
mixing
of the
electron
and
phonon
[S]. Such anticrossing featur:s were observed for the ExCR line in a 75 A thick CdTe/ Cdo,hMgo,,Te multiple QW structure with 1450 A thick barriers. In this structure an energy splitting between light-hole and heavy-hole states (Alh_,,,,) is 31 meV and the ExCR line is not masked by the strong resonance of light-hole exciton. Actually, a small value of Alhmhh= 17 meV in the CdTe/ QW does not allow to follow the Cd o,,4Mgo.,,Te states
821
ExCR line at fields above 14 T (see Fig. 4a) and to reach the resonant polaron coupling region. In summary, in QWs containing a 2DEG of low density the energy spectrum of free two-dimensional excitons is supplemented by the bound state of negatively charged exciton and an unbound state corresponding to a combined exciton-cyclotron resonance. This work is supported in part by the Deutsche Forschungsgemeinschaft (OS 98/5-l), European Commission under the contract No.CHGT-CT93005 1 (Large Facility), Volkswagen Foundation, and by the Russian Foundation for Basic Research Grant No.95-02-04061a.
References
.
0
Growth 1841185 (1998) 818-821
Cl1 K. Kheng, R.T. Cox, Y. Merle d’Aubigne, F. Bassani, K. Saminadayar, S. Tatarenko, Phys. Rev. Lett. 71 (1993) 1752. PI G. Finkelstein, H. Shtrikman, I. Bar-Joseph, Phys. Rev. Lett. 74 (1995) 976. R.A. Suris, W. Ossau, A. 131 D.R. Yakovlev, V.P. Kochereshko, Waag, G. Landwehr, P.C.M. Christianen, J.C. Maan, Proc. 23 Int. Conf. Physics of Semiconductors, Berlin, 1996, World Scientific, Singapore, 1996, p. 2071. A. Waag, T. Litz, r41 W. Ossau, U. Zehnder, B. Kuhn-Heinrich, G. Landwehr, R. Hellmann, E.O. Giibel, Superlattices Microstruct. 16 (1994) 5. M.G. Tyazhlov, A.F. Dite, A.I. Filin, A. c51 V.D. Kulakovskii, Forchel, D.R. Yakovlev, A. Waag,G. Landwehr, Phys. Rev. B 54 (1996) 4981. H. Schenk, W. Ccl J.X. Shen, D.R. Yakovlev, V.P. Kochereshko, Ossau, A. Waag, G. Landwehr, Proc. 12th Int. Conf. on the Application of High Magnetic Fields in Semiconductor Physics, Wiirzburg, 1996, World Scientific, Singapore, 1997, p. 741. [71 S. Scholl, J. Gerschiitz, F. Fischer, A. Waag, K. Schiill, H. Schifer, G. Landwehr, Solid State Commun. 94 (1995) 935. PI R.J. Nicholas, S. Sasaki, N. Miura, F.M. Peeters, J.M. Shi, G.Q. Hai, J.T. Devreese, M.J. Lawless, D.E. Ashenford, B. Lunn, Phys. Rev. B 50 (1994) 7596.