Energy behavior with magnetic field of negatively charged magnetoexcitons in quantum wells and heterojunctions
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321 Mat. Res. Soc. Symp. Proc. Vol. 588 © 2000 Materials Research Society
about 35T. As a result, the triplet state will become the fundamental state beyond this field. In the case of a large QW (300A or more), such a crossing should not happen at least for magnetic fields up to 50T. We present here the results of circularly polarized magneto-photoluminescence experiments performed on two modulation doped GaAs/AlGaAs heterostructures. The measurements were performed in high magnetic fields (up to 60T) and at low temperatures (between 0.37 and 1.5K). Our data allowed us to identify the formation of the X states and we followed their behavior up to the highest magnetic field. We found that the X, and the Xt states will merge (and possibly cross) at a field of about 40T in the case of a 200A QW. In the case of a single heterojunction (SHJ) no such crossing occured up to 60T. EXPERIMENT The samples that we investigated were a 200A GaAs/A10 .55Ga0 .45As single QW and a GaAs/Alo. 3Gao.7 As single heterojunction (SHJ). Both were modulation doped, with a dark electron density of 1.2x10 11 cm-2 and had mobilities higher than 3x10 6 cm 2/Vs. With constant laser illumination (at 632.8nm) during the measurements, the 2DEG density increased to 1.58x10 1cm for the QW and to 2.2x10 1 cm-2 for the SHJ. The width of the Al0 .3Ga 0 .7As spacer was 700A in both samples. The experimental layout for the PL measurements has been described previously [16]. Using a quasi-continuous magnet, the field was varied from 0 to 60 T, while the temperature was changed from 1.5K to 370mK. EXPERIMENTAL RESULTS AND DISCUSSION In Fig. 1 we show the results obtained from the 200A QW at a temperature of 1.5K in both RCP (a+) and LCP (T-) polarization for two different magnetic fields (6T and 16T). The spectra at B=OT were identical for both polarizations. X0
xO
a
H
.
HX
b
\.
xs-
RCFP
X.H
ca
T=1.5K
_
F=1.5Kj v1
v--0.41 .1
LCP 1526
1528
1530
1532
15
1534
Energy (meV)
1534
1536
1535
1540
Energy (meV)
FIGI. The (T+(RCP) and Y-(LCP) polarized spectra at B=6T (a) and B=16T (b) taken at T=1.5K on a 200A QW. The intensities are all normalized to the B=OT field data.
322
At B=6T, in both polarizations, three peaks can be distinguished. The middle peak derived directly from the 2DEG recombinations at B=OT is ascribed to the neutral excitons X0 which, due to the wide undoped AlO. 3Gao. 7As spacer (700A), will be non-localized. The higher energy peak labeled H will be discussed elsewhere. We shall focus our attention on the lower energy peak which we believe to be generated by the triplet state of the charged exciton Xt-. In order to understand this assignment, we note that the lowest energy spin wave-functions for the Xt" state are [10]: In a- (LCP) polarization: 4'LCP(-1/ 2 ) =e el" hNl,
and
VI' LCP(- 3 /2 ) =
and
VltRCp(+3/2) = I (eiel,+el, el")hl"
In o+ (RCP) polarization: 2 NtRCP(+I/ ) = e~l-e, hi"
1 (eTe~l+el e")hJ
(1)
(2)
For the (T-(LCP) polarization, the formation of the triplet state requires the presence of either
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