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, 1999, 41, . 5 Exciton-Electron Interaction in Quantum Wells with a Two Dimensional Electron Gas of Low Density W. Ossau, D.R. Yakovlev, C.Y. Hu, V.P. Kochereshko, G.V. Astakhov, R.A. Suris, P.C.M. Christianen, J.C. Maan Physikalisches Institut der Universitt Wrzburg, Am Hubland, 97074 Wrzburg, Germany A.F. Ioffe Physico-technical institute, of Russian Academy of Sciences, 194021 St. Petersburg, Russia Research Institute for Materials, High Field Magnet Laboratory, University of Nijmegen, 6525 ED Nijmegen, The Netherlands IIVI quantum well structures containing a 2DEG of low density have been investigated by means of polarized photoluminescence, photoluminescence excitation and reflectivity in external magnetic fields up to 20 T. The spin splittings of the excition X and the negatively charged exciton X- are measured as a function of the magnetic field strength. The behavior of the magnetic-field-induced polarization degree of the luminescence line related to Xdemonstrates the formation process of negatively charged excitons from excitons and free carriers polarized by the external magnetic field. We have determined the binding energies of the trion formed either with the heavy-hole or the light-hole exciton. The optically detected magnetic resonance (ODMR) technique was for the first time applied to study the optical transition processes in a nanosecond timescale. The electron ODMR was observed with the detection on either the direct exciton or the negatively charged exciton X. Further evidence for the interaction of excitons with the electrons of the two dimensional gas are demonstrated by a combined exciton-cyclotron resonance line observed in reflectivity and luminescence excitation, shake-up processes observed in photoluminescence as well as inelastic and spin-dependent scattering processes.

Effects resulting from the exciton-electron interaction in barriers, which leads to a much higher probability for the the presence of a two-dimensional electron gas (2DEG) electrons to tunnel from the SL to the QW, as compared to of low density at neaB 1, where ne is the electron holes, due to the difference in the effective masses. Conseconcentration and aB is the exciton Bohr radius, became a quently, the electron concentration in the QW can be varied subject of intensive investigations very recently. This interest accurately by the intensity of illumination with a radiation has been stimulated by the observation of a negatively energy exceeding the SL band gap. For the experiments charged exciton X- in CdTe/(Cd,Zn)Te modulation-doped with additional microwave illumination we used a back wave quantum well (QW) structures [1]. oscillator, which frequency can be tuned from 55 GHz to In this paper we review several new effects observed in 80 GHz by the application of different dc voltages. For structures where the exciton interacts with a 2DEG of low these ODMR experiments the microwaves were chopped carrier density. In detail we discuss: the spin splitting and at 45 Hz and the synchronous changes of the PL intensities polarization dependence of X and X- in high magnetic were recorded by a two-channel photoncounter.

field [2]; the combined exciton-cyclotron resonance [3];

the optically detected magnetic resonance (ODMR) on 2. Magneto-Optical Study of the Trion X- [4]; the shake-up process [5,6] and the spin-dependent broadening of excitonic states [7].

2.1. Zeeman spl i t t i ng and pol ari zat i on deg r e e. In the first part of this paper we concentrate on 1. Sample Structures and Experimental the properties of negatively charged exciton states in the magnetic field range extended up to 20 T. PL was excited Details with a Ti: sapphire laser at energies below the band gap of In this study we performed measurements on two dif- the barriers and the external magnetic fields were applied ferent types of structures. We used modulation-doped perpendecular to the QW layers (Faraday geometry).

CdTe/(Cd,Mg)Te or ZnSe/(Zn,Mg)(S,Se) QWs with a In Fig. 1, a the PL and PLE spectra detected for a 8 nm2DEG of low density of about 1.5 1010 cm-2. The thick CdTe/Cd0.7Mg0.3Te QW at a temperature of 1.6 K structures were grown by molecular-beam epitaxy on (100) and at zero magnetic field are shown. The exciton line X oriented GaAs substrates and are selectively doped with dominates in the PLE spectrum but is much weaker than iodine or chlorine separated by a spacer layer from the QW. the negatively charged exciton line X- in the PL spectrum, Furthermore, we investigated specially designed structures, which reflects the strong probability for excitons to be bound where we varied the carrier concentration by external optical in the X- complex. This situation is changed under applied illumination [3]. To achieve this the heterostructures are magnetic fields when the 2DEG is polarized (Fig. 1, b). At sandwiched between short period superlattices, where the 7 T the - polarized PL component of the exciton line QWs are separated from the superlattice (SL) by 20 nm thick increases strongly in intensity and becomes comparable with 832 W. Ossau, D.R. Yakovlev, C.Y. Hu, V.P. Kochereshko, G.V. Astakhov, R.A. Suris, P.C.M. Christianen...

Figure 1. a photoluminescence and PLE spectra of an 8 nm-thick CdTe/Cd0.7Mg0.3Te modulation-doped SQW structure. X and Xlabel the heavy-hole exciton and the negatively charged exciton lines; b PL spectra taken in the magnetic field of 7 T are shown for two circular polarizations + (dashed) and - (solid) lines, respectively; c magnetic field dependence of the PL line positions (upper panel) and the Zeeman splitting (lower panel). The electron spin splitting calculated for ge = -1.46 is plotted by a solid line. The heavy-hole splitting is calculated from the electron and the X and X- splitting respectively. For details see text.

the X- line intensity. We also stress here that at 7 T the X The magnetic-field-induced polarazation degree for X and the X- PL lines are polarized with opposite signs and and X- PL lines in QWs with and without 2DEG is for the X- line the high energy component is stronger in displayed in lower part of Fig. 3. In undoped QWs the intensity than the lower one. Detailed dependencies of X X- line is unpolarized and the X line polarization increases -and X- PL intensities on the magnetic field strength are linearly with a slope of 0.04T only, which is considerably -smaller than the thermal equilibrium value of 0.3T. In plotted in the upper panel of Fig. 3 and will be discussed modulation-doped QWs the polarization degree of the Xbelow.

line is nonmonotonic and alters its sign at 12 T and the For a detailed analysis of the observed PL polarization a exciton line polarization is strongly enhanced. It is obvious precise knowledge about the spin splitting of the exciton and that the presence of a strongly polarized 2DEG induces the free carrier states is essential first. The experimentally such strong modification. One can see in Fig. 2 that in determined spin splittings for excitons and X- are very close to each other (Fig. 1, c, upper panel). The excitonic g factor has a positive sign (lower part of Fig. 1). The electron g factor at the bottom of the conduction band in an 8 nm thick CdTe/Cd0.7Mg0.3Te QW (ge = -1.46) is known with high accuracy [4,8]. The heavy-hole spin-splitting was deduced by subtracting the electron splitting from the excitonic one.

The hole splitting is zero at magnetic fields below 12 T and increases at higher fields giving rise to a positive g factor value. There is no significant difference observable whether the hole g factor is determined by the splitting of X or X-.

Schematically the spin splitting for 2DEG elecrons, excitons and X- are presented in Fig. 2. Note that the X- state is splitted due to the heavy-hole contribution only, but the spin splitting of the X- optical transitions is also determined by the splitting of the conduction band states (ge), as the 2D electron is the final state after X- recombination (for details Figure 2. Schematical presentation of an X- formation process in see referance [9]).

external magnetic fields from excitons and 2DEG electrons.

, 1999, 41, . Exciton-Electron Interaction in Quantum Wells with a Two Dimensional Electron Gas of Low Density 2.2. The ef f ect of mi crowave radi at i on o n t h e t r i o n. The formation process of the trion is also reflected in the PL spectra which are taken with and without additional microwave illumination. These experiments have been performed on an 8 nm-thick QW structures with optical tuning of the 2DEG density [3,4].

At B = 0 T the microwaves decrease the X- emission and increase the X emission, whereas at magnetic fields above 3 T the microwave radiation increase the X- emission and decrease the X emission in - polarization and have no obvious influence on X- and X emission in + polarization.

Figure 3. Magnetic field variation of the PL line intensities detected for different circular polarizations for an 8 nmthick CdTe/Cd0.7Mg0.3Te modulation-doped QW (upper panel).

Magnetic-field-induced circular polarization degree of the exciton and negatively charged exciton PL lines with (circles) and without (triangles) modulation doping (lower pannel).

magnetic fields the X- formation process is limited to the creation of a (+1/2, -1/2, +3/2) state constructed from a (+1/2) electron and a (-1/2, +3/2) exciton. The (+1/2, -1/2, -3/2) X- state is populated by relaxation from the (+1/2, -1/2, +3/2) state. In magnetic fields below 12 T these states are not splitted and the state which contribute to the + polarized transition is preferably populated. At higher fields, when the state contributing to the - polarized Figure 4. ODMR (70 GHz) signals detected from an 8 nm thick transition becomes the lowest one, its thermal occupation CdTe/Cd0.7Mg0.3Te QW with optical tuning of 2DEG density on leads to the rise of the negative polarization. The exciton polarization under these conditions is determined by the X- X emission and X- emission for (a) - and (b) + polarization.

The changes are normalized by the respective total intensities of formation process: (-1/2, +3/2) excitons are washed out X and X- emissions. In - polarization sharp ODMR lines were for X-, but (+1/2, -3/2) excitons are preserved as the observed on the broad ODMR background signals. In the inset the (-1/2) electron states are empty. As a result the exciton sharp electron ODMR (70 GHz) line detected on X emission in PL increases strongly for the - polarized component (see polarization is shown. The resonance position lies at Bres = 3.424 T Fig. 3, upper panel).

with a linewidth B = 39 mT.

6 , 1999, 41, . 834 W. Ossau, D.R. Yakovlev, C.Y. Hu, V.P. Kochereshko, G.V. Astakhov, R.A. Suris, P.C.M. Christianen...

The ODMR (70 GHz) spectrum detected on X and X- observed for the (-3/2) X- in + polarization although emission are shown in Fig. 4, a for - polarization and the microwave resonant absorption induces a decrease of Fig. 4, b for + polarization. The changes are normalized the (+1/2) electron population.

by their respective total intensities of X and X- emission, The microwave radiation pumps electrons from the low and the positive and negative signs represent the increase spin state (+1/2) to the upper spin state (-1/2) continuand decrease of PL intensity, respectively. The maximal ously until the electrons reach a new steady population. The change is about 8% of the total intensities of X and X- change of the electron population by the magnetic resonant emission. With the increase of the magnetic field strength, absorption results in the change of the formation probability the - ODMR signals decline fast to zero and then change of X-. This is the reason why the magnetic resonance of their signs, whereas the + ODMR signals decline slowly electrons can be detected on the X- or the X emission.

to zero. In - polarization a sharp positive and negative From the descussion above it is obvious that electron spinODMR line (at B 3.42 T) appears respectively on the dependent and the electron spin-conserving formation and broad ODMR background signals detected on X- and X recombination processes of X- makes the electron ODMR emission. However, no sharp ODMR lines was observed in detectable. This formation mechanism of X- can be further + polarization.

supported by the broad ODMR background (at B > 3T) The details of the sharp ODMR lines are shown in the shown in Fig. 4, a, b. Besides the microwave resonant absorpinset of Fig. 4, a. For 70 GHz microwaves the resonant lines tion, the heating of electrons in the microwave field [10,11] lie at Bres = 3.424 T with the linewidth B = 39 mT. From also induced an increase of the (-1/2) electron population the resonant magnetic field strength for different microwave and a decrease of the (+1/2) electron population due to frequencies g = -1.461 0.002 was obtained very the raising of the excess electron temperature [12]. This precisely. We identify the sharp lines as the ODMR of the enhances the formation probability of the (-3/2) X- just as electrons of the 2D gas, whereas the broad background and the resonance case discussed above. The broad background the polarization dependence is determined by the formation signals (at B > 3T) decrease with the magnetic field process of X-.

strength due to the suppression of the microwave heating The scheme of the formation of X- under magnetic field by magnetic field [13].

is shown in Fig. 2 where the g factor of the electron is We could say the above microwave heating effect is negative and that of heavy hole is very small for low fields polarised since the background (at B > 3T) occur only and positive for higher field strength. The formation and the in - polarization. At low magnetic fields (B < 1T) where recombination of X- from the optically active exciton can the two electron spin states have nearly the same population, be written as another microwave heating effect occurs and we call it nonpolarized because the background occur in both circular e(-1/2) + X(-1) X(-3/2) photon (-) +e(-1/2) (1) polarizations. The increase of the electron kinetic energy by microwave heating decreases the probability for an exciton e(+1/2) + X(+1) X(+3/2) photon (+) +e(+1/2). (2) trapping an electron. This is in accord with the temperature experiments on X- [14]. So we observed a decrease of Under the applied excitation conditions in this structure X- emission and an increase of X emission in both circular the electron density is estimated to be less than 41010 cm-2.

polarization at B < 1 T shown in Fig. 4, a, b.

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