High magnetic field optical studies of 2DEG in modulation-doped ZnSe quantum wells

Physica E 12 (2002) 512 – 515

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High magnetic eld optical studies of 2DEG in modulation-doped ZnSe quantum wells W. Ossaua; ∗ , D.R. Yakovleva; b , G.V. Astakhova; b , A. Waaga , C.J. Meinigc , H.A. Nickelc , B.D. McCombec , S.A. Crookerd a Physikalisches

Institut der Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany Io$e Physico-Technical Institute, Russian Academy of Sciences, 194017, St.Petersburg, Russia c Department of Physics, State University of New York at Bu$alo, Bu$alo, NY 14260, USA d National High Magnetic Field Laboratory, Los Alamos, 87545 New Mexico, USA

b A.F.

Abstract Properties of a two-dimensional electron gas with a density of 5 × 1011 cm−2 in a ZnSe=(Zn; Be; Mg)Se quantum well structure have been investigated by means of polarized magneto-luminescence in elds up to 47 T and by an optically detected resonance (ODR) technique. With increasing magnetic eld, the optical spectra of the electronic system changes its appearance from Landau-level-like with a linear shift of lines, to excitonic-like with a diamagnetic shift. The transition occurs at lling factor 2. Pronounced oscillatory features at even lling factors are found for the luminescence intensity and polarization degree, the energy of optical transitions, and in ODR signal. ? 2002 Published by Elsevier Science B.V. PACS: 71.10.Ca; 71.35.−y; 78.66.Hf Keywords: ZnSe quantum wells; Two-dimensional electron gas; Excitons

Electronic systems of high density and their properties in high magnetic elds have been studied in great detail for III–V heterostructures. II–VI heterostructures, due to strong Coulombic interaction, are attractive for investigation of Coulombic correlation eAects in dense two-dimensional electron gases (2DEGs). Recently, several reports for modulation-doped CdTe-based quantum wells (QWs) have been presented (see e.g. Refs. [1,2] and references therein). ZnSe-based QWs provide even stronger Coulombic interactions (exciton binding energy in bulk ∗

Corresponding author. Tel.: +49-931-888-5125; fax: +49-931-888-5142. E-mail addresses: [email protected] (W. Ossau), [email protected] (D.R. Yakovlev).

ZnSe is 20 meV compared with 5 meV in GaAs and 10 meV in CdTe). However, the published data on ZnSe-based QWs with a dense 2DEG are limited to (Zn; Cd)Se=ZnSe structures with a ternary alloy in the well layer [3,4]. In these structures, inhomogeneous broadening due to alloy Iuctuations imposes strong limitations on the sensitivity of the optical methods. We report here magneto-optical studies of a 2DEG in ZnSe=(Zn; Be; Mg)Se QWs with an inhomogeneous broadening of the luminescence line as low as 2 meV. A modulation-doped QW structure and an undoped reference structure were grown by molecular-beam epitaxy on an (1 0 0)-oriented GaAs substrate. Each J structure consists of a 67-A-thick ZnSe single quanJ tum well embedded between 1000-A-thick Zn0:82 Be0:08 Mg0:10 Se barriers, which provide connement

1386-9477/02/$ - see front matter ? 2002 Published by Elsevier Science B.V. PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 3 4 7 - 2

W. Ossau et al. / Physica E 12 (2002) 512 – 515

σ

+

σ

_

T= 1.6 K

A

PL In tensity

potentials in the conduction and valence bands of 190 and 50 meV, respectively. To prevent the loss of carriers escaping into the substrate and recombining at the surface, the structure was conned by Zn0:71 Be0:11 Mg0:18 Se barriers. Both barrier materials are lattice matched to the GaAs substrate. The modulation-doped layer with Iodine donors is J separated from the QW by a 100-A-thick spacer layer. Optical methods reported here give us the 2DEG density in the QW ne = 5 × 1011 cm−2 . Polarized magneto-luminescence has been measured at T = 1:6 K in long-pulse magnetic elds up to B = 47 T applied perpendicular to the QW plane (Faraday geometry). Details of the setup are published in Ref. [5]. Optically detected resonance (ODR) experiments based on the detection of photoluminescence (PL) intensity varied under far-infrared illumination (184:3 m) were performed at T = 4:2 K [6]. The PL spectrum of the undoped sample consists of two narrow lines with a full-width at a half-maximum (FWHM) of 1:2 meV. One, positioned at 2:826 eV (see arrows in Fig. 1), is due the recombination of the heavy-hole excitons (X), while the other has a maximum shifted by 5:5 meV to lower energy and is caused by recombination of negatively charged excitons (trions, T) [7]. The diamagnetic shift of these lines with increasing magnetic elds (see solid lines in Fig. 2) is well tted with the electron and in-plane heavy-hole eAective masses me = 0:15m0 and mhh = 0:5m0 . The PL spectrum of the doped QW shows a broad band with a FWHM of about 7 meV, which is close to the value of the 2DEG Fermi energy 7:7 meV estimated for ne = 5 × 1011 cm−2 . The maximum of the PL band has an energy 2:8095 eV and is red-shifted by 16:5 meV with respect to the X energy in the undoped sample, presumably due to band gap renormalization. This value is in reasonable agreement with the data reported for (Zn; Cd)Se=ZnSe QWs: 19 meV for ne = 1:1 × 1012 cm−2 and 43 meV for ne =1:9 × 1012 cm−2 [4]. With increasing magnetic eld the broad band transforms into a narrow line (“A”-line) with width of 2 meV (eld dependence of FWHM is presented in Fig. 3a). This value is mainly determined by inhomogeneous broadening due to QW width- and barrierheight Iuctuations. One can clearly see in Fig. 1 the strong modication of the PL spectra, which is typical for modulation-doped QWs. The PL band splits into a set

513

B

46 T

6T T 2. 80

2. 81

2. 82

X

0T 2. 83

Energy ( eV) J Fig. 1. Photoluminescence spectra of a 67-A-thick ZnSe=Zn0:82 Be0:08 Mg0:10 Se SQW detected at diAerent magnetic elds. Exciton (X) and trion (T) energies in an undoped QW are shown by arrows.

of lines, which shift to higher energies with increasing magnetic eld. Energetic positions of these lines vs. magnetic eld are plotted in Fig. 2. All plotted lines show a nonmonotonic behavior with changing slopes. The lowest energy peak reveals a linear shift with a slope of 0:38 meV=T in elds below 10:3 T, which converts at higher elds into a diamagnetic shift typical for excitonic states. Such a behavior has been reported recently for GaAs-based QWs [8,9]; these authors demonstrate that the transition occurs at a lling factor  = 2; and at high magnetic elds the emission is indistinguishable from trions. In our structure the Landau level fan shown by dashed lines corresponds to the pure electron eAective mass me = 0:15m0 . Up to now it is not very clear why the hole cyclotron motion does not contribute to this shift, however this property is rather general and has been noticed in

W. Ossau et al. / Physica E 12 (2002) 512 – 515 -0.8

-2

B

2.825

0.47 meV/T LL0

2.820

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-0.2 8

v =1

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4 3

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0.38 meV/T LL0

A,σ A,σ B,σ

_ + +

=1

2

0

T=1.6 K

3 10

20

30

40

50

Magnetic Field (T ) Fig. 2. Energies of PL maxima vs. magnetic eld strength in a J 67-A-thick ZnSe=Zn0:82 Be0:08 Mg0:10 Se SQW detected in + (◦) and − (•) polarizations. Exciton (X) and trion (T) energies in an undoped QW at B = 0 T are shown by arrows. Their diamagnetic shifts shown by solid lines are moved to lower energies by 7:7 meV for comparison with A and B lines of the doped structure.

GaAs QWs as well [9]. The Landau level with a slope of 0:47 meV=T, which accounts for a reduced mass with the electron and hole (mhh = 0:5m0 ) contribution, becomes observable in the eld range between 10 and 18 T, i.e. for 1 ¡  ¡ 2 (see solid line in Fig. 2). In magnetic elds above 10:3 T ( ¡ 2); the lowest energy peak, which is labeled as A-line, shifts similarly to the trion line in the undoped QW. However its energy does not approach the trion position, as it was observed in GaAs QWs, and is about 8 meV below it. We suggest that the diAerence is caused by the strong Coulombic interaction in ZnSe QWs. A new B-line appeared in + polarization for  ¡ 1: Its diamagnetic shift and the energy distance from the A-line resembles the excitonic line in the undoped structure. The nature of this line has been discussed in Ref. [10]. The energy shifts of the lowest PL maximum is nonmonotonic. It shows an upward cusp at integer lling factors from 1 to 4, and downward convex curves between them. The behavior is qualitatively similar to the reported results for GaAs and CdTe structures. The most pronounced feature at 10:3 T corresponds to

Ener gy (eV)

4

2.816

0 -2

2.814

-4 -6

2.812

v =1

6

2.810

3

(c)

4

0

-8

2

10

20

ODR signal

2

2.810

0

(b)

LL2

LL1

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0.0

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2.815

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PL Intensity

Ener gy ( e V)

T

6

-0.6

FWHM (meV)

11

5x10 cm

X

Polarization degree

514

-10 -12 30

Magnetic Field (T) Fig. 3. Oscillatory behavior at the integer lling factors of diAerent parameters of the doped QW: (a) Polarization degree of PL and PL linewidth of A line; (b) PL peak intensities for A and B lines; (c) Energy shift of − -polarized A line in PL and FIR induced changes of PL intensity (i.e. ODR signal). All data were measured at T = 1:6 K, except ODR signal which was detected at 4:2 K.

 = 2. From this value we get the 2DEG density in the doped structure ne = 5 × 1011 cm−2 and calculate the expected elds for the set of integer lling factors corresponding to this concentration. These elds are marked by arrows in Fig. 2 and by closed triangles and vertical lines in Fig. 3. Fig. 3 collected diAerent experimental data with the aim to stress the nonmonotonic behavior in the vicinity of integer lling factors. Very pronounced features are observed for the even  = 2; 4; 6 and 8 for the PL polarization degree, which has a minimum when the Landau level is fully occupied (Fig. 3a). Also PL intensity of the A-line in − polarization shows dips at =1; 2; and 4 (Fig. 3b). The ODR signal is induced by nonresonant heating of the 2DEG-induced by the far-infrared illumination [6]. It is minimal for =2 and 4 when the electron heating inside the same Landau level is not possible as all the states are occupied.

W. Ossau et al. / Physica E 12 (2002) 512 – 515

To conclude, oscillatory features of the magnetoluminescence at integer lling factors of a 2DEG were observed in a modulation-doped ZnSe-based quantum well. Modulation-doped, ZnSe-based QWs, due to their small inhomogeneous broadening and strong Coulombic interaction, provide a reliable model system to investigate excitonic eAects and Coulombic correlation eAects in the presence of a dense 2DEG. The authors are thankful to V.P. Kochereshko for useful discussions. The work was supported by the Deutsche Forschungsgemeinschaft (Grant Nos. Os98=6, 436RUS113=557 and SFB 410), NSF Grant DMR-9722625, as well as by the Russian Foundation for Basic Research (Grant No. 01-02-16990). References [1] Y. Imanaka, T. Takamasu, G. Kido, G. Karczewski, T. Wojtowicz, J. Kossut, J. Crystal Growth 214=215 (2000) 240.

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